Collective oscillations of massless particles in two-dimensional (2D) Dirac materials offer an innovative route toward implementing atomically thin devices based on low-energy quasiparticle interactions. Strong confinement of near-field distribution on the 2D surface is essential to demonstrate extraordinary optoelectronic functions, providing means to shape the spectral response at the mid-infrared (IR) wavelength. Although the dynamic polarization from the linear response theory has successfully accounted for a range of experimental observations, a unified perspective was still elusive, connecting the state-of-the-art developments based on the 2D Dirac plasmon-polaritons. Here, we review recent works on graphene and three-dimensional (3D) topological insulator (TI) plasmon-polariton, where the mid-IR and terahertz (THz) radiation experiences prominent confinement into a deep-subwavelength scale in a novel optoelectronic structure. After presenting general light-matter interactions between 2D Dirac plasmon and subwavelength quasiparticle excitations, we introduce various experimental techniques to couple the plasmon-polaritons with electromagnetic radiations. Electrical and optical controls over the plasmonic excitations reveal the hybridized plasmon modes in graphene and 3D TI, demonstrating an intense near-field interaction of 2D Dirac plasmon within the highly-compressed volume. These findings can further be applied to invent optoelectronic bio-molecular sensors, atomically thin photodetectors, and laser-driven light sources. Graphene and topological insulators are promising platforms for near-field nanotechnology on a subwavelength scale. Authors present the highly confined nature of Dirac plasmon-polaritons in graphene, topological insulator, and hybrid systems. Because the Bloch Hamiltonian in a unit cell is inversion symmetric, the gapless-linear band dispersion appears at the corner of the Brillouin zone, namely K and K′ points. Strikingly, the many-body interactions in graphene can be described based on the (2 + 1)-dimensional Dirac Hamiltonian. A range of optoelectronic measurements has demonstrated that graphene’s optical responses are different from those in the conventional two-dimensional semiconductor structures, primarily because of the 2D Dirac dispersion. The early studies on graphene focused on the transport measurements in a strong magnetic field2 verifying the half-integer quantum Hall (QH) conductivity σxy = (4e2/h)(N + 1/2), where N is integer. While the conventional 2D electron gas presents the standard integer QH state σxy ∝ (e2/h)N, the QH spectra in monolayer graphene are shifted by a half-integer, which can be elucidated by the (2 + 1)-dimensional Dirac Hamiltonian in a strong magnetic field. The gapless nature of graphene bandstructure also hosts the Dirac-fluid; the co-existence of electron and hole in the charge-neutral point has shown viscous charge flow5.Graphene consists of hexagonal arrangements of carbon atoms in two-dimension (2D). The unit cell of graphene contains two equivalent carbon atoms, where the nearest-neighbor hopping interaction is based on the sp9. The light-induced particle-hole polarization of graphene can be calculated using random-phase approximation (RPA)14. The calculation results show that the graphene polarization is determined by two independent contributions of the intraband and interband polarization, where the Fermi energy distinguishes the two regimes. At the long-wavelength limit, the interband polarization promises a constant absorbance ~2.3% independent of light frequency15. In contrast, the intraband polarization explains the free carrier response against electromagnetic radiation18. Notably, light coupling with dipole momentum q exceeding photon qc can excite the graphene plasmon-polariton (q/qc ~ 102). Here, the relationship of plasmon frequency ω versus the plsmon momentum q follows the conventional plasmon mode n) dependence allows us to distinguish the 2D Dirac plasmon (ω ∝ n1/4) from the conventional 2D plasmon (ω ∝ n1/2)19. A truly intriguing aspect of monolayer graphene is that the interparticle interaction is weak compared to conventional semiconductors, which is attributed to the linear dispersion of the gapless bandstructure11. The RPA result can precisely describe the 2D Dirac plasmon’s density-dependence in this weakly interacting system.Plasmonics, the collective response of light-matter interaction, also shed light on the unique properties of graphene22. Nonetheless, the scattering dynamics of Dirac fermions mainly limit the lifetime of 2D Dirac plasmon25. The momentum relaxation mechanism at low carrier density is primarily attributed to the impurity scatterings24, although the electron-phonon interaction still affects the carrier lifetime at high carrier density25. On the other hand, monolayer graphene encapsulated with hexagonal boron nitride (hBN) has shown high carrier mobility μ ~ 105 cm2V−1 s−1 because of uniform interfaces of the van der Waals heterostructure26. With the low density of interface impurity, the hBN/graphene/hBN structures provide a sensitive tool to investigate the quasiparticle interactions. For instance, 2D Dirac plasmon can be coupled with phonon-polaritons29, where the plasmon energy is split into several branches near the phonon-polariton energy. The high mobility of monolayer graphene also enables us to detect an acoustic plasmon mode34, which strongly confines the plasmon momentum into a deep-subwavelength limit. Notably, the compressed graphene polarization unravels electron-electron interactions at low carrier density31.The near-field distribution arising from the charge oscillations in monolayer graphene can be coupled with novel optoelectronic structures2 hybridization for the crystal bonding, the gapless-linear dispersion appears at the K and K′ points in the Brillouin zone. In reality, however, the spin–orbit coupling can open up the bandgap via the σ-π bond hybridizations37. The induced bandgap is a tiny perturbation ~μeV compared to the typical Fermi-energy scale observed in graphene; thereby, it is safe to use the ideal 2D Dirac dispersion to describe the light-matter interactions. Nonetheless, the bulk bandgap arising from the spin-orbit coupling produces highly non-trivial results at the graphene boundary: the bandgap is replaced by a spin-dependent one-dimensional (1D) conduction channel. A significant aspect of the 1D boundary state is that the spin direction is orthogonal to the direction of particle momentum. The spin-filtered transport promises dissipationless conduction as long as the spin-orbit coupling sustains the bulk bandgap in monolayer graphene. The possible existence of the non-trivial boundary state could be experimentally confirmed in other crystals with strong spin-orbit interaction39. Crucially, the boundary state of three-dimensional (3D) crystal can host 2D conduction at the surface, where the spin direction is orthogonal to surface normal vector and momentum direction. 3D topological insulators (TI), Bi2Se3, Bi2Te3, and Sb2Te3, are known to show the non-trivial boundary state on the surface41. Essentially, the surface state of 3D TI follows 2D Dirac dispersion similar to the case of graphene, except that the spin- and valley-degeneracy are absent on the 3D TI surface. Therefore, the 3D TI surface hosts 2D Dirac plasmon44, while the bulk inside TI spatially separates the opposite surface states, screening the near-field emanating from the plasmon-polariton.For the ideal case of graphene, which assumes spThis review provides a comprehensive analysis of 2D surface plasmon-polariton (SPP) in the gapless Dirac-band dispersions of graphene and 3D TI. The SPP in graphene and 3D TI uncover the unique light-matter interaction in the associated 2D Dirac-band. The electric field oscillations from SPP are confined up to a few atomic layers, which could amplify Coulomb interactions with neighboring materials. The following section addresses general aspects of 2D Dirac plasmon in terms of dispersion relations calculated from RPA. After presenting the dispersion relation in a monolayer 2D Dirac-band, we proceed with the interactive systems incorporating quasiparticle excitations. Furthermore, we introduce basic experimental techniques to couple electromagnetic waves with plasmon-polariton, presenting novel methods to control the 2D Dirac plasmon in a deep-subwavelength scale. The electrical and optical modulations are achieved in a graphene field-effect device and graphene-integrated 3D TI structure. As to provide future outlooks for the 2D Dirac plasmon, we list state-of-art applications of graphene plasmon at the end of this review.q → 0 of 2D Dirac plasmon dispersion follows the sublinear relationship 31 from the conventional SPP dispersion. When q is nested at the Fermi surface, additional photon energy is required to excite SPP above the Fermi energy, which results in the blue-shift in the SPP dispersion. Recent studies on graphene highlight the significance of non-local response resulting from the strongly-confined plasmon wavelength (λp)31. Figure ω and wavevector q are normalized by the Fermi energy EF and Fermi wavevector kF, respectively. Here, we employed the full description of the non-local graphene polarization P derived from the RPA (Eq. (9) in ref. 10), finding zeros of the dielectric function ε = 1−υqP, where υq = e2/2κ0κq is the Fourier-transformed 2D Coulomb interaction. Figure κ changes, where the screening effect of the surrounding medium shifts the plasmon dispersion toward the lower frequency regime. Although the local approximation ωlocal obtained from Plocal (Eq. (12) in ref. 10) follows the non-local SPP  at the long wavelength limit (q → 0), it deviates when the q is comparable to kF, implying that the SPP is nested at the Fermi surface. SPP can excite interband particle-hole pairs when entering the single-particle excitation (SPE) region. The intrinsic SPP decay rate is characterized by the Landau damping γLandau, which becomes significant at the deep-subwavelength regime. The rest this section embraces the non-local P to calculate the SPP dispersion with an average dielectric constant κ = 4 of the surrounding medium. This is because graphene is surrounded by dielectric materials in general. Besides, the RPA calculation is appropriate for weakly interacting particles, which requires the screening effect of the surrounding medium. The Wigner-Seitz radius rs is a brief measure of the interparticle interactions rs = (α/κ)(c/vF)(4/gsgv)1/2, where rs ≪ 1 implies weak particle interactions11. The screening factor κ = 4 lowers the rs value to ~0.5, where we assume the fine-structure constant α = 1/137, the graphene Fermi velocity vF = 1 × 106 m/s, and spin- and valley-degeneracies gsgv = 4.The energy and momentum dispersions of SPP characterize the longitudinal oscillations of free carriers on metallic films. The local approximation mentclass2pt{minimmentclass2pt{minimωSO)47, where the SPP dispersion is split into upper and lower branches at the vicinity of ωSO. The energy splitting is characterized by the coupling constant αsp = ε∞[(ε∞ + 1)−1−(ε0 + 1)−1], where ε∞ (ε0) is the high-frequency (static) dielectric constant. Figure 13d = 0.5 nm is the distance between the graphene and substrate. Here, we assumed the graphene EF = 0.4 eV above the silicon oxide SO-phonon energies of 100 meV and 145 meV. Clearly, αsp increases the energy splitting in the SPP dispersion. The lowest branch of the phonon-coupled SPP is damped upon entering the SPEintra region, which requires extreme confinement of λp and the large plasmon-phonon coupling. Yan et al.45 reported that the graphene optical phonon (~0.2 eV) assists the plasmon damping into intraband particle-hole pairs, where the inelastic damping manifests as the broadened SPP resonance. On the other hand, the hyperbolic phonon-polariton (HPP) can propagate inside a dielectric material with anisotropic phonon-polariton resonances49. Thin films of hBN host the HPP energy bands at 90–100 meV (type I) and 175–200 meV (type II). The plasmon-phonon coupling inside the HPP bands is different from the SO phonon-plasmon coupling because the HPP mode also disperses with the polariton wavevector. These considerations are discussed elsewhere50.Charge oscillations in an atomically thin layer experience strong Coulomb interactions when the dielectric constant rapidly changes as a function of frequency. This effect is pronounced near the surface optical (SO) phonon frequencies  along with the normal axis of the optical (acoustic) plasmon mode. At the long-wavelength limit q → 0, the dispersion of the optical mode ω ∝ q). Besides, the distinct charge distribution imposes a potential limit to the light coupling method. The far-field radiation mainly excites the optical plasmon mode since the distance of the double-layer graphene is generally far shorter than the photon wavelength in free space54. Recently, the acoustic plasmon mode in graphene has been demonstrated in a novel dual-gate structure31, where the asymmetric charge distribution is driven by the image charge reflected on the metallic gate side. As indicated in Fig. d decreases. Strikingly, the decrease in d also curbs the acoustic mode’s damping rate, presumably because the out-of-plane dipole moment becomes significant in close proximity. On the other hand, the SPP response in 3D TI is analogous to the case of the double-layer graphene because the 3D TI surface states always come in a pair, where the TI bulk spatially separates the surface states. Notably, a general expression of dynamic screening function for the double-layer structure can be found in ref. 53, where the ε includes different values of κ inside and outside of the 2D Dirac layers.The decrease in Te above 1000 K, creating the hot Dirac plasma57. Figure kBTe exceeds EF, where we introduce a characteristic constant η = kBTe/EF to compare the intrinsic plasmon to the cold plasma case (η = 0). The intrinsic plasmon dispersion can be obtained from the zeros of the dielectric function 1 − υqP = 0 with the polarization14μ(Te) is the chemical potential following the charge conservation D(E) = gsgvE/2πℏ2vF2. When η > 1, the interband thermal excitation increases the sum of the electron and hole density following ω ∝ n1/4, it is evident that the increase in ω arises from the thermal population of the 2D Dirac bands14. However, at the intermediate state (η = 0.6), ω is somewhat decreased compared to the cold plasma case, implying that the 2D Dirac plasmon non-monotonically depends on the temperature. Unlike the conventional 2D electron gas, D(E) in 2D Dirac-bands is proportional to E. As a result, the intraband thermal excitation always accompanies the decrease in μ(Te), which explains the red-shift in ω at the intermediate state. Figure ω as a function of η. Meanwhile, the SPP encounters Landau damping γLandau before entering the SPE region because of the free carriers smeared from the Fermi surface , where the strong confinement is ascribed to the slow Fermi velocity of graphene (vF/c = 1/300). One intuitive way to acquire an additional momentum q = 2π/λp is to fabricate a nanoribbon structure58, where the period P gives rise to the plasmon momentum q = 2π/P. Figure 54. The electric field perpendicular to the nanoribbon confines the polarization in the nanoribbon width W upon the laser irradiation. As shown in Fig. W (increase in q) increases the plasmon frequency, following the sublinear dispersion of the 2D Dirac plasmon This part introduces experimental techniques to excite plasmon-polariton in graphene and 3D TI. In general, the plasmon wavelength 60 and TI plasmon62, tailoring the phase and propagation of the excited SPP64. Figure λ0 = 11.28 μm), confining the photon wavelength into the deep subwavelength scale. The SPP launched by the AFM-probe spreads out away from the source, which experiences reflection at the edge of the graphene flake. A prominent light scattering occurs when the reflected SPP reaches the AFM-probe again, producing the near-field modulations depending on the AFM-tip position  allows us to acquire the near-field profiles of the graphene plasmonion Fig. . Sweepin23, electron–impurity scatterings24, electron–phonon scatterings25, and dielectric losses of environment materials, imposing an intrinsic limit to the SPP lifetime. The highest Qp value of 130 is reported in the graphene plasmon using the s-SNOM measurement65, circumventing various scattering sources by lowering the temperature (~50 K) and encapsulating the graphene with hBN flakes. At the low temperature, the electron-phonon scattering is largely reduced, remaining the dielectric loss as a residual scatterer against the graphene SPP. As reported in ref. 28, capping the graphene with hBN plays a vital role in reducing the impurity scattering, where the Qp can be increased by five times when transferring the graphene from Si/SiO2 (Qp ~ 5) to hBN substrate (Qp ~ 25). Besides, the impurity scattering can be reduced by increasing the free carrier density of graphene. On the other hand, the fabrication of graphene nanostructure may contribute to the plasmon damping, which takes place at the edge of graphene nanoribbon via the elastic processes45. Similar effects are extensively studied in metallic nanoparticles66.Understanding the dissipation pathway of graphene plasmon is essential to develop graphene-based optoelectronic applications. In general, the scattering event of graphene SPP involves a range of sources: electron–electron scatterings69. Figure fpump−fprobe = fSPP) and momentum (kpump + kprobe = kSPP) are conserved during the difference frequency generation (DFG). Here, the transverse magnetic (TM) polarizations of the pump and probe allow the efficient coupling of the evanescent electric field and the graphene SPP at the interface. The excitation of graphene SPP manifests in the transient increase of the probe-intensity since the pump-photon is converted to the probe-photon through the DFG process. Here, the extracted fSPP and kSPP from the transient change in the probe spectra follow the 2D Dirac plasmon dispersion. Proper tuning of photon energies yields the nonlinear conversion efficiency of 6 × 10−5 W−1 in the optical waveguide69  of ~50, greater than the value in graphene nanoribbons (Qp ~ 5).Counter-propagating electromagnetic waves inside a nonlinear medium can generate a difference-frequency of the input frequencies. Such a nonlinear process can be used to excite the SPP in graphenee69 Fig.  and 6 × ace Fig. 68. The s70, photovoltaic71, and bolometric effects72. Whether the plasmonic excitation affects the photoresponse of graphene is of particular interest in the field of graphene-based optoelectronic applications73. Lundeberg et al.21 reported that the 2D Dirac plasmon can be employed as a thermoelectric photodetector in a dual-gated graphene field-effect device. Figure λ0 = 10.6 μm) into the photocurrent I2. Here, the AFM-tip position (xtip and ytip) modulates the I2 amplitude, indicating that the plasmonic excitation affects the graphene photoresponse. In fact, the AFM-tip position changes the absorption cross-section of the device; the light absorption can be enhanced by the constructive interference between the AFM-tip and the SPP reflected from the graphene edge, otherwise destructive interference reduces I2. The sixfold change in the I2 sign indicates that the thermoelectric effect is responsible for the photoresponse in the dual-gate device ∆T, where SR(L) is the Seebeck coefficient of the right (left) junction, and ∆T is the light-induced increase in the junction temperature. The gate voltage independently controls the carrier density of the right and left side of the junction. A closer look into the photocurrent map reveals that the I2 signal decays away from the junction with a decay length of ~250 nm  was less than the I2 decay length (xtip-axis) at all carrier densities, indicating that the thermoelectric action is driven by the thermal diffusion rather than the direct transport of SPP into the junction.Photoresponse in doped graphene is based on thermoelectric30. For this reason, the dual-gate graphene field-effect device can launch the acoustic plasmon mode, given that the graphene-metal distance d is minimal compared to λp. The frequency dispersion extracted from the near-field modulations can verify the acoustic plasmon mode. Figure λ0 ~ 100 μm), which far exceeds the distance d = 42 nm of the device. As seen from Fig. λp/λ0 ~ 1/66) in the dual-gate structure. Previously, we have addressed that the phase velocity vp = ω/q of the acoustic plasmon mode approaches the Fermi velocity (vF = 106 m/s in graphene) when d is decreased. As demonstrated in ref. 31, vp are suppressed up to vF when the d decreases from 27 nm to 5.5 nm in the dual-gate device  deviates from the long-wavelength limit (dashed lines), explaining the non-local response at the low carrier density.The dispersion relation of 2D Dirac plasmon can be altered by surrounding electrostatic properties. In particular, an image charge of the graphene SPP can be reflected on a neighboring metallic film. As displayed in Fig. ice Fig. . The den55 reported that the electron temperature of graphene reaches 3200 K when using the ultrashort pulses (40 fs) of the near-infrared (IR) radiation (λ = 1.56 μm). In Fig. λp induced by the near-IR pump, where the mid-IR pulses (~200 fs) are resonantly coupled with the AFM-tip, leaving the near-field fringes as a function of the tip position. As displayed in Fig. s is pronounced at the temporal overlap of near-IR pump and mid-IR probe pulses, i.e. at the zero pump-probe delay. The fringe intervals of s (black dashed lines) reveal that the pump-induced λp is inversely proportional to the mid-IR frequency ω, which characterizes the 2D Dirac plasmon dispersion under the optical excitations. Notably, the derivatives of scattering signal (ds/dx) shown in Fig. s. The pump-induced signal diminishes at the probe delay of 2 ps . The large bandgap in Bi2Se3 (~0.3 eV) allows us to access the TSS response, hosting the 2D Dirac plasmon at the THz band. In Fig. 2Se3 bandstructure is projected onto the Γ point of the surface Brillouin zone, where the TSS Fermi velocity is ~6 × 105 m/s according to the angle-resolved photoemission measurement74. The molecular-beam-epitaxy (MBE) synthesizes 30 quintuple layers  of Bi2Se3 on (Bi0.5In0.5)2Se3 buffer layer, preventing the impurities arising from the lattice mismatch75. In general, pristine 3D TI films are degenerately doped because of vacancy-defect sites at the surface and crystalline defects in the bulk. With the decrease in the defects via interface engineering, a truly bulk-insulating status of 3D TI is obtained. The microribbons of Bi2Se3 provide the plasmon momentum k = π/L0, which is coupled with the THz-probe electric-field perpendicular to the ribbon direction. On the other hand, the THz-pump with an electric-field intensity ~ 0.4 MV/cm is incident parallel to the ribbon direction  from the phonon-dressed plasmon spectra (solid red line) by employing the conventional plasmon-phonon coupling model77.Having identified the thermal effect on the graphene plasmon, we now present the 2D Dirac plasmon on the topological insulator (TI) surface, where an intense terahertz (THz) radiation drives the non-equilibrium plasmonic dynamicsion Fig. . The THz2Se3 bulk bandgap, the optical response in TSS is entangled with the bulk state. The low photon energy of the THz-pump (~4 meV), on the other hand, can avoid the unwanted carrier excitation in bulk, achieving the thermal distribution of the TSS with Te above ~1400 K. As we have shown in Fig. kBTe exceeding the Fermi energy EF increases the plasmon frequency νpl due to the interband thermal excitations. Hence, the small value of Bi2Se3EF ~ 75 meV allows the interband thermal excitation when kBTe ~ 120 meV is provided. Figure L0 value decreases, similar to the result observed in graphene  decreases when Te increases, following the charge conservation law. Nonetheless, the strong interband thermal excitation increases νpl at all k values, demonstrating the intrinsic TSS plasmon under the THz excitations  can trigger photoexcitation inside bulk, increasing the free carrier density transiently. According to the time-resolved THz spectroscopy82 and angle-resolved photoemission spectroscopy83, the recombination of the photoinduced bulk carriers shows a similar timescale to the cooling dynamics of thermal energy. For this reason, THz-pump as a driving source is indispensable for measuring the intrinsic plasmon dynamics in Bi2Se3, because the low photon energy ~4 meV does not contribute to the interband photoexcitation. In contrast, when the 1.55 eV optical-pump drives the bulk photoexcitation, the time-resolved THz spectroscopy reveals a large blue-shift in the plasmon frequency84, revealing the density-dependence of the conventional 2D plasmon.As we have discussed in this section, irradiation of laser pulses can drive the ultrafast dynamics of 2D Dirac plasmons, where the transient increase of 81 showed that the TSS scattering dynamics are different from the bulk scatterings under the THz-pump excitation, where the pump-induced TSS scattering suppresses the THz conductivity. On the other hand, a resonant coupling of THz-pump and the Bi2Se3 phonon at ~2 THz displaces the lattice ions from the equilibrium85, leaving coherent oscillations in the second-harmonic-generation of probe pulses. Recently, Schmid et al.86 observed the high-harmonic (HH) generation of intense THz photon below the Bi2Te3 bulk bandgap, where the HH orders was continuously controlled by the carrier-envelop phase of the THz pulse. The non-integer multiples of the HH generation occur when the Dirac electrons traverse Dirac point ballistically, allowing to discern between the TSS dynamics and the bulk response.It is instructive to note that the nonlinear kinetics of TSS under the intense THz radiation is of general interest in the field of Dirac materials. Indeed, Luo et al.2Se3 at low frequency (κTI = 100) prevents the electrostatic doping, effectively screening the gate-induced electric field. Previously, thin layers of Bi2Se3 (~10 nm) and molecular charge transfers are used to demonstrate the ambipolar field-effect of the TSS88, otherwise the gate-voltage VG barely charges the Bi2Se3 carrier density. Nonetheless, one can electrically control the TSS plasmon by integrating a monolayer CVD-grown graphene on top of Bi2Se3 TI microribbons, where the VG can tune the graphene chemical potential μG, changing the dielectric environment of the surrounding Bi2Se3. Indeed, the dynamic screening effect of monolayer graphene modulates the SPP response in Bi2Se3  device and the field-effect, where the transparent ionic-gate controls μG. While the longitudinal resistance ρxx of Bi2Se3 remains at ~2.5 kΩ, the ambipolar field-effect on G-Bi2Se3 illuminates that the monolayer graphene is in a p-doped state before applying VG. To appreciate the SPP response in a doped G-Bi2Se3, we performed simulations using the 3D-EM simulator (High-Frequency Structure Simulator), where the impedance boundary conditions Z(ω) = 1/σ(ω) incorporate the Drude sheet conductance σ(ω) = D/(Γ − iω). Here, the Drude weight D = (e2/4πℏ2)gsgvμTSS/G and the scattering rate Γ determine the density-dependent reactance −Im[Z(ω)] .Next, we explore the electrostatic modulation of the TSS plasmon integrated with graphene. Unlike the case of graphene, the large dielectric constant of BiSe3 Fig. . Figure ω)] Fig.  with theμTSS of 250 meV and the gate-dependent μG ranging from 0 meV to 200 meV, we can reproduce the density-dependent wave extinction E(ν) = 1 − t/t0 spectrum of the G-Bi2Se3  relative to the undoped G-Bi2Se3 (VG = 1.5 V) to isolate the plasmonic resonance from the phonon resonance at ~ 2 THz, where the spectral blue-shift becomes evident at the higher carrier density. The bare plasmon extinction (gray dashed line) can also be extracted from the fit function (red line). The experimental νpl shows an excellent agreement with the simulated νpl when we assume μG = 200 meV at VG = −1 V, which corresponds to the gate capacitance ~1.2 × 1012 cm−2. Figure νpl values are almost identical at all doped status of G-Bi2Se3. One intriguing aspect of G-Bi2Se3 is that the density-dependence in νpl is different from the standard 2D Dirac plasmon μTSS is independent of VG in the device, the density-dependence in G-Bi2Se3 is written as μTSS arises from the top and bottom TSS. For this reason, the dynamic polarization of graphene on top of Bi2Se3 can efficiently control the SPP response, achieving the νpl modulation ~50% within the VG range.Using Se3 Fig. , where t VG Fig. . We plot89, gas molecules90, and polar phonons46. Figure −1 and 1660 cm−1 provide the vibrational fingerprints of the protein. The sensitive molecular detection of the graphene plasmon is ascribed to the strong confinement of near-field distribution up to a few nanometers from the surface. In contrast, the near-field distribution of gold plasmons reaches hundreds of nanometers at the same resonance frequency, resulting in a lower sensitivity to the vibrations of the molecular band.The extraordinary near-field interaction of graphene plasmon and molecular vibrations can be used to develop highly efficient molecular sensors detecting proteins51 demonstrated a mid-IR photodetector based on the graphene nanodisk stacks  of hBN enhance the mid-IR responsivity of the graphene field-effect device. Figures −1 inside the HPP band at both TM and TE pump polarizations  but intrinsically distinguished since the spin texture itself constructs the surface state in the 3D TI. The spin conversion efficiency in the TI surface is superior to the conventional Rashba state in 2DEG because the TI Fermi surface consists of a single helical spin texture. In contrast, the Fermi energy in 2DEG intersects with co-centric double Fermi contours with opposite spin helicity. Meanwhile, the light-matter interaction brings significant aspects to the Dirac surface states. The obliquely-incident photon helicity can be coupled with the in-plane spin texture of the 3D TI surface99, where the corresponding spin depletion leads to the transverse charge current. As a result, the helicity-dependent photocurrent captures the unique helical spin texture of the 3D TI surface. In a ferromagnet and 3D TI bilayer, light illumination on the ferromagnetic layer injects spin into the 3D TI surface100. The spin is efficiently converted into charge current via the inverse Edelstein effect, where the surface charge current manifests itself in the broadband THz radiation.From the viewpoint of spintronics, the 3D TI helical spin texture provides promising tools to investigate the spin-charge coupled phenomena2Se3. Integrating the graphene and Bi2Se3, we could control the Bi2Se3 plasmon frequency by changing the graphene Fermi energy, revealing the extraordinary response of near-field interactions in close proximity. The outstanding properties of 2D Dirac plasmon can be employed to develop future mid-IR photodetectors, bio-molecular sensors, THz detectors, and light sources.Versatile optoelectronic applications of 2D Dirac plasmon stem from the highly confined near-field distribution on the surface at the wide range of IR wavelength. Employing the non-local 2D Dirac polarization, we have shown that the plasmon dispersion undergoes remarkable modifications under the plasmon-phonon coupling, plasmon-plasmon coupling, and interband thermal excitations. Both far- and near-field detections of 2D Dirac plasmon have been performed at IR wavelength, allowing us to characterize the comprehensive plasmonic response for device applications. For instance, the thermoelectric current in a novel dual-gate device could detect the acoustic mode of graphene plasmon-polariton, confining the plasmon wavelength into the deep-subwavelength scale. Meanwhile, intense optical and THz pulses can trigger non-equilibrium thermal distribution at the 2D Dirac-band, changing the plasmon dispersion of graphene and 3D TI Bi


The terahertz wave modulation properties of graphene were investigated using an external 975 nm continuous wave laser with different power. Upon excitation laser, the transmission and modulation depth was measured using terahertz time-domain spectroscopy. The experimental results showed that the modulation depth of monolayer graphene and 3-layer graphene was 16% and 32% under the 1495 mW excitation power. Further, we analyzed the graphene modulation mechanism based on the Drude model and the thin-film approximation. Both theoretical analysis and calculation results showed that the terahertz wave could be modulated using graphene with different excitation laser power. Terahertz wave modulation properties of graphene are investigated using an external 975 nm continuous wave laser. The modulation depth of monolayer graphene and 3-layer graphene was 16% and 32% under 1495 mW excitation power. However, the modulation of terahertz wave technology has developed slowly and is restricting the development of terahertz technology. Thus, modifying the terahertz wave is a popular area of study.The terahertz (THz) wave is an electromagnetic wave that falls between the infrared and microwave wavelengths. Because of its strong penetration, low energy, spectral fingerprint, coherence, and large communication bandwidth,2 hybridized carbon atoms. Due to its excellent optical, electrical, and magnetic properties, graphene has crucial applications in field-effect transistors, liquid crystal displays, nanocomposites, photodetectors, light-emitting devices, microwave transistors, ultrafast lasers, photovoltaic cells, sensing and energy storage materials.17,18 Zhang et al.19 reported the first graphene light modulator; a graphene layer was placed on top of the optical waveguide in the modulator, and a driving voltage was used to control the graphene layer’s Fermi energy level. To modulate terahertz waves, Sensale-Rodriguez et al.20,21 designed a transmissive and reflective terahertz modulator. The terahertz wave was modulated by adjusting the gate voltage. The transmissive and reflective terahertz modulators had maximum modulation depths of 15% and 64%, respectively. Wu et al.22 developed a graphene-based terahertz modulator by filling an ionic gel material between two monolayers of graphene. This structure could enhance the electric field modulation effect on graphene’s Fermi energy level. The modulation depth was 83% when the voltage was 3 V. The modulation methods described above are based on the electrically-driven mode. However, the influence of the charging and discharging process severely restricts the response speed of the electrically controlled terahertz wave. The issue of constrained response speed can be resolved by modulating the terahertz wave in an optically-driven mode. Weis et al.23 reported using femtosecond laser excitation to achieve a wide band tunability of the THz transmission. They observed a maximum amplitude modulation depth of 99%. However, there was only a maximum 18% difference between the transmission through bare silicon and the sample. Compared with pulsed femtosecond lasers, continuous wave (CW) lasers are more convenient and cost-effective for implementing all-optical terahertz modulators. Wen et al.24 demonstrated an all-optical terahertz modulator based on monolayer graphene on germanium, which could be driven by a 1.55 μm CW laser. The maximum difference between the transmission through the germanium and the sample was 30%. Fu et al.25 fabricated a THz modulator consisting of monolayer graphene on a silicon substrate. The experimental results showed that the modulation depth of the transmission could reach 74% for the proposed modulator under an external 450 nm continuous-wave laser photo-excitation. And the difference in transmission between the sample and the silicon substrate can reach a maximum of 49.3%. Du et al.26 reported an active broadband THz wave modulator based on the graphene-silicon hybrid structure under photoexcitation of a 532 nm CW laser. Previous reports have discussed the optically-driven modulation properties of monolayer graphene. However, there are rare reports about optically-driven multilayer graphene.Graphene is a typical two-dimensional material that consists of spIn this paper, we analyzed the THz modulation properties of monolayer and 3-layer graphene. We used a 975 nm CW laser as the excitation source. The penetration depth of the silicon substrate is different under different exciting wavelengths. The excitation source used in this study had a larger penetration depth in the silicon substrate than in the previous reports. With 975 nm CW laser illumination, the transmission and modulation depth was measured using terahertz time-domain spectroscopy (THz-TDS) under different excitation power. We observed significant transmission attenuation and increased modulation depth when the excitation laser power was tuned. And then, we revealed the graphene modulation mechanism using the Drude model and the thin-film approximation.22.1The experimental system contained a THz-TDS and an external 975 nm CW laser as shown in The ultrafast femtosecond laser emitted a central wavelength of 780 nm pulse, of which the pulse width was less than 100 fs. Using a beam splitter (CBS), the pulse laser was divided into two paths with different energy. The laser with higher energy served as the pump beam, irradiating the GaAs photoconductive antenna and generating a terahertz beam. The generated terahertz beam was focused on a test sample through the parabolic mirror. Due to the interaction between the test sample and the terahertz beam, the terahertz wave penetrated the sample and carried the sample information. And then, the terahertz beam carrying information from the sample struck the ZnTe electro-optic crystal. On the other hand, the laser with lower energy was used as the probe beam, which met the terahertz beam on the ZnTe electro-optic crystal after the lens collimation and focus. Throughout the ZnTe electro-optic crystal, the probe beam containing the terahertz information of the sample was changed from isotropic polarization to anisotropic polarization. And then, the probe beam was divided into two perpendicular polarized beams using a Wollaston prism. A differential photodetector (PD) collected the two perpendicular polarized beams. At last, the terahertz time-domain spectrum was obtained by the PD collected signal. Moreover, a 975 nm CW laser was selected as an excitation laser.2.2The monolayer and 3-layer graphene samples were purchased from Shenzhen Six Carbon Technology Co., Ltd. We measured the spectral transmission of the graphene samples by standard THz-TDS. We used a 975 nm CW laser for photo-doping the graphene samples. The laser beam was obliquely incident on the sample (from graphene to the silicon substrate) with an angle of 45°, as shown in 33.1The time-domain spectra of silicon, monolayer graphene, and 3-layer graphene, without external photoexcitation, are shown in Taking silicon as a reference, the terahertz amplitude decreased after it penetrated the graphene. In particular, at two peaks (12 ps and 14.5 ps) and one trough (13.4 ps), the terahertz signal decreased to 86.2%, 86.5%, and 86.2% of the reference signal amplitude after passing through 3-layers graphene, respectively. This indicates that graphene absorption of the terahertz signal is subsequently enhanced with increasing layers.The THz response monolayer graphene and 3-layer graphene under an external excitation of a 975 nm CW laser irradiation is shown in 3.2Atrans(ω), Aref(ω) are the amplitude values of the sample and the reference (silicon) after Fourier transform, respectively.The frequency-domain spectra were obtained using a fast Fourier transform of the time-domain signals to quantify the transmission of terahertz waves at different frequencies. The transmission was defined as in 27,28 For silicon, the average transmission decreased from 100% without optical excitation to 96% with a pump power of 1495 mW. Upon photo-doping of graphene, the transmission of the THz wave decreased. With a pump power of 1495 mW, the average transmission of monolayer graphene decreased to 62%, and the average transmission of 3-layer graphene decreased to 38%. Compared with the previous reports, we obtained a higher maximum value difference between the transmission through the silicon and graphene using a 975 nm CW laser pump. The maximum difference between the transmission through the silicon and the monolayer graphene or 3-layer graphene, was 34% and 58%, respectively. This was because the more photo-generated carriers could be excited using a longer wavelength laser. Furthermore, the THz beam attenuation was significantly enhanced compared to silicon at equivalent power levels of the photo-doping graphene. This showed that graphene was dominant in the THz wave modulation compared with silicon.The dependence of the average transmission on the excitation laser power is shown in Aoff(ω) and Aon(ω) denote the terahertz amplitudes without and with excitation laser, respectively.Furthermore, the modulation depth was defined as follows.The modulation depth of monolayer and 3-layer graphene at 1.14 THz is depicted in 3.3When the excitation laser was incident from the graphene side, a small portion of the excitation laser was absorbed by graphene. It caused photo-generated free carriers to arise from the graphene. The nonabsorbed transmitted portion of the excitation laser penetrated the silicon substrate. Most of the energy was completely absorbed within the penetration length defined by the absorption coefficient. Because the amount of absorbed energy of silicon was larger than in graphene, the major fraction of photo-generated free carriers were generated in silicon. The illuminated area and the penetration length of the excitation laser in silicon defined the free carriers within a volume. And then, the photo-generated free carriers diffused into graphene and accumulated at the interface of the silicon substrate and graphene until a steady state was attained. These free carriers arriving in graphene experienced much higher mobility than their silicon counterparts. This resulted in a more significant change of conductivity than in silicon. The free carriers induced a change in the electric conductivity, which was inherently correlated with decreased THz transmission through the material. Thus, the THz transmission and modulation depth depended on the conductivity change.29e, ℏ, ω, vF, Γ, and Nc were the elementary charge, Planck’s constant, angular frequency, Fermi velocity, carrier scattering probability and carrier concentration, respectively. Moreover, randomly stacked multilayer graphene can be treated as multiply electronically decoupled monolayer graphene.30,31 The silicon prohibited bandwidth was 1.12 eV, while the silicon cutoff wavelength for its intrinsic leap was 1107 nm. We used a 975 nm CW laser as an excitation source to generate photo-generated carriers. The free carriers induced a change in the electric conductivity. Different laser power could excite different amounts of photo-generated charge carriers. Thus, graphene could modulate the terahertz wave with different excitation laser power.The monolayer graphene conductivity in the terahertz range complied with the Drude model, as follows.32 to describe the experimental data. The frequency-dependent equivalent conductivity of graphene could be extracted from the measured transmission spectra using the following formula,n1 is the refractive index of air, n2 is the refractive index of the graphene substrate, Z0 = 377 Ω is the vacuum impedance, and Anlay(ω), Aref(ω) are the amplitude values of the sample and the reference after Fourier transform.Because the thickness of the graphene was far smaller than the center wavelength of the THz wave, we applied the thin-film approximationThe relation between excitation power and graphene conductivity is shown in 4In this paper, we investigated the terahertz wave modulation properties of graphene with different excitation CW laser power. Under excitation from a 975 nm CW laser in 10 gradients, the monolayer and 3-layer graphene terahertz transmission and modulation depth were measured using the THz-TDS system. Different from previous reports, we use a 975 nm CW laser as an excitation source. It had a larger penetration depth in the silicon substrate than in previous reports, which excited more photo-generated carriers. Thus, a higher maximum value difference between the transmission through the silicon and graphene than in previous reports was obtained. For the graphene, the THz transmission decreased with increasing layer number or excitation power. When the excitation power was 1495 mW, the modulation depth of monolayer graphene and 3-layer graphene was above 16% and 32%, respectively. The Drude model and standard thin-film approximation were employed to analyze the graphene modulation mechanism. The variation of graphene conductivity increased with increasing laser power, which influenced the modulation depth. Therefore, the terahertz wave could be modulated using graphene with different excitation laser power. Optically tuned graphene is a potential method for realizing a terahertz wave active amplitude modulator in future applications.Conceptualization – Shaohang Chen, Binyi Qin and Jianming Liu; data curation – Binyi Qin; formal analysis – Shaohang Chen, Binyi Qin, Yizhi Liang and Jincun Zheng; funding acquisition – Binyi Qin, Ruizhao Yang, Tinghui Li and Yanni Zhou; investigation – Ruizhao Yang, Tinghui Li and Yun Li; methodology – Shaohang Chen, Binyi Qin, Yanni Zhou and Jincun Zheng; project administration – Binyi Qin; resources – Binyi Qin, Ruizhao Yang and Tinghui Li; software – Yizhi Liang and Jianming Liu; supervision – Binyi Qin and Jianming Liu; validation – Yun Li, Tinghui Li and Yanni Zhou; writing original draft – Shaohang Chen, Ruizhao Yang and Yun Li; writing review and editing – Binyi Qin, Yizhi Liang and Jianming Liu. All authors have read and agreed to the published version of the manuscript.There are no conflicts to declare.


Screen-printed graphene layers on flexible substrates are one of the most advanced printed electronics developments of recent years. Obtaining thin, flexible, highly conductive components, whose applications are increasingly directed towards biomedical engineering and even medicine, requires an in-depth understanding of the correct choice of materials and procedures. Our work was aimed at investigating the influence of homogenisation in the triple rolling process over pastes dedicated to the screen printing technology, on their rheological parameters and the properties of the prints. The effect of selecting a suitable polymer matrix and different packing of graphene flakes was evaluated. Several studies were carried out, which can provide an excellent knowledge base in the context of graphene screen-printing pastes. Paste rheology, printability, path thickness, sheet resistance and adhesion to the substrate were investigated. Selected layers were also subjected to SEM imaging. One of the fastest-growing industries in recent times is printed electronics. The possibility of obtaining components with high mechanical resistance while maintaining flexibility has led to an increase in research in this area . PrintedThe most versatile and extensively employed technology for printed electronics is screen printing. This technology is not only characterised by its high versatility as well as a good resolution but also enables printing on a large variety of substrates ,7. ScreeScreen printing technology imposes specific requirements on the materials involved. A paste form is required, i.e., a stable suspension of nano- to micro-sized particles in a polymer matrix. The properties of the pastes are adjusted to the dedicated application. However, it is not only the characteristics of the final printed product that are important but also the ease and repeatability of the printing process .−5 Ω∙m [One of the breakthroughs, in terms of the expansion of prospective screen-printing technology applications, was the implementation of printable graphene-based composites. Based on these materials, these include innovative electrodes , capacit−5 Ω∙m . A graph−5 Ω∙m . Nevertheless, it should be highlighted here that for a value-enhanced application, it is not only the material of the functional phase that is crucial but also the carrier used. The right polymer can significantly improve the uniformity of the paste and provide the right properties for the composite to be used in the printing process. The polymer serves as a matrix in the final print. It is intended to provide the best possible mechanical performance, such as surface hardness, high abrasion and scratch resistance, as well as chemical resistance. Considering printing on a flexible substrate, it is also crucial to ensure adequate flexibility. Popular and widely used polymers as matrices are plastics  or thermosetting and chemically curing resins ,18. ExamThe increasing pressure to produce increasingly efficient layers, often both highly conductive and flexible, enabling ease of printing and proper adhesion to the substrate, often entails complex and multi-step paste manufacturing processes. One of the often-mentioned paste production steps is its homogenization in the rolling process ,22. ThisFurthermore, very often engineers and scientists are faced with the dilemma of the necessity to compromise between printability and mechanical and electrical properties. Therefore, investigating the impact of the triple-rolling process on various parameters can provide precious information for many researchers and manufacturers. In this paper, we present a study on the effects of the rolling process of graphene pastes based on different carriers on their rheology, printability and electrical and adhesion parameters. Flexible substrates, which are among the most researched at the moment due to the rapid growth of flexible electronics, were investigated in this study. The investigation was dedicated to graphene pastes, which represent a strong trend in the development of screen-printing technology.®  were used. According to the datasheet, they range from 1 to 2 μm in width and 8–15 nm in thickness.In this work, graphene nanoplatelets from CheapTube5 u, which dissolved in 2-(2-butoxyethoxy) ethyl acetate to form a carrier referred to in this paper as PMMA. Another polymer tested was Laroflex M35 with a density of 1.24 · 103 kg/m3, which was dissolved in a mixture of 2-(2-butoxyethoxy) ethyl acetate and 2-butoxyethanol in a ratio of (97:3); referred to as LARO carrier. Two types of thermoplastic polyurethane (TPU) were also tested, in the form of Elastollan® soft 35AP, with a density of 1.18 · 103 kg/m3, and Elastollan® hard C80A, with a density of 1.19 · 103 kg/m3. Both were dissolved in N, N-Dimethylformamide; their designations in the paper are TPU soft and TPU hard.In this study, four different polymers were used. The solvents were selected concerning the particular polymers tested, ensuring good solubility and with consideration of their use in screen-printing technology, i.e., paying attention to an appropriately high boiling point. The first polymer used was poly(methyl methacrylate) with an average molecular weight of 3.5 · 10® , Laroflex M35, Elastollan soft 35AP and Elastollan C 80A were purchased from BASF® .Poly(methyl methacrylate) and all solvents were purchased from Sigma AldrichThe printing substrate was a 36 µm PET film purchased from MICEL Sp z o.o .® ECI 1010 ink purchased from Tekra LLC .Silver contacts were applied with LOCTITEThe carriers were obtained by mixing the weighed components for 48 h at a temperature adapted to the individual polymers comprising the carriers, 50 °C for LARO and PMMA and 30 °C for soft and hard TPU, respectively.In the next step, the carriers were combined with nanoplatelets of graphene, representing the conductive functional phase. An appropriate amount of graphene and carrier was weighed and ground for at least five minutes in an agate mortar. Each series was prepared in five different degrees of packing of the functional phase   with a sThe paths were printed using an Aurel c920 screen printer , Aurel A® R/S-CPS+ rheometer equipped with an RCT-50-2 spindle , dedicated to the viscosity ranging between 0.006–50,900 Pa·s. A shear rate from 0 to 400 s−1 for was tested for 100 s. The viscosities were analysed using the dedicated Rheo 3000 software (version 1.2.2009.1). The rheology of the pastes was measured with a Brookfield® . The values were recalculated into sheet resistance (Ohms per square).The resistance of the printed paths was measured with a UT804 multimeter manufactured by Uni-Trend Technology® profilometer , with the force of the stylus set to 3 mg.Path thickness was measured using Bruker’s DektakXT® disc knife included in the BG VF 1842 C kit. Scotch® tape  was applied to the cut sections and ripped off after approximately three minutes by uniform movement. The crumbled material was removed with a brush and the results were compared according to the ISO 2409 standard, which defines the level of adhesion on a scale of 0–5, where 0 means no material detachment at all, 1 indicates from 0–5% of the total material, 2 from 5–15%, while 3 is between 15–35% and 4 is between 35% and 65% of the total pattern. Larger material detachment is qualified as 5. Adhesion was tested using a 2 mm ANTICORRScanning electron microscopy (SEM) of three layers of best conductivity was conducted on a Hitachi SU8230 instrument  with an accelerating voltage of 5.0 kV and an upper secondary electron detector.For the obtained pathways with markedly different conductivity capabilities, their structure was additionally compared under the Keyence VHX-900F optical microscope .Firstly, the dependence of the pastes’ viscosity on the shear rate was analysed. This comparison was carried out for all the tested carriers and graphene concentrations, in all cases for both triple-rolled and non-rolled materials. For all four carriers tested, higher dynamic viscosity was observed with increasing graphene packing for both triple-rolled and non-rolled materials. A tripling of the graphene concentration increased the dynamic viscosity registered for the minimal shear rate by up to several hundred times. This is understandable, as graphene nanoplatelets are a powder additive with a highly developed specific surface area and low bulk density. Furthermore, for all pastes, irrespective of the carrier used, as the packing increased, a greater curve slope angle was recorded for shear rates below 150 1/s. For higher speeds, the correlation between viscosity and the aforementioned parameter decreased dramatically, i.e., a further increase in shear rate resulted in a slight change in viscosity.Considering the effect of the rolling process on the rheological properties, various dependencies can be observed. For PMMA pastes, an improvement in rheological properties and a decrease in dynamic viscosity were noted for all pastes. For LARO pastes, the rolling process resulted in a decrease in viscosity for the packing of graphene by 15% and an increase for the other variants. For pastes containing soft TPU, the rolling process caused a decrease in the viscosity for pastes with higher packing (12.5% and 15%). Such a result indicates that for those carriers only for larger packing, the rolling process introduces a significant deagglomeration of the functional phase and contributes to a reduction of internal friction in the material which results in lower viscosity. The hard TPU-based pastes showed worse rheological properties after the rolling process. For shear rates above 100 1/s, the dynamic viscosity values decreased rapidly and were close to zero. Comparing the results obtained for various carriers, the highest dynamic viscosity was observed for the hard TPU paste d). AnalyIn screen printing technology, the standard dynamic viscosity is between 0.5–50 Pa∙s for the shear rate achieved during squeegee work (This speed is 150–180 1/s). Therefore, despite significant differences and sometimes extremely high viscosities at the zero shear rates, almost all of the pastes examined showed adequate viscosity for the speeds relevant for use in screen printing. The exceptions are rolled pastes based on hard TPU, where viscosity was too low.Next, the electrical properties of the tested materials were compared. For this purpose, the sheet resistance of a dozen rectangular paths of different thicknesses printed on dedicated silver contacts was measured. During the process of preparing the paths, significant difficulty in the printing process was noted for pastes containing hard TPU. Their high viscosity, combined with strong adhesion to the squeegee and screen, inhibited proper deposition of the paste on the substrate, resulting in frayed and patchy paths . Such a result qualifies the TPU hard carrier as not screen-printable. For this reason, further research on this material was discontinued. In addition, non-rolled pastes containing soft TPU showed limited printability. At higher graphene packing (12.5% and 15%), the material failed to settle well on the substrate despite several passes of the squeegee. Since only part of the material was squeezed through the screen in a single printing cycle, which would not be optimal for the screen-printing process, it was decided to set the maximum graphene packing of this carrier at 10% by weight without a triple-rolling process. Adding the rolling step enabled printing and the higher tested graphene packing. The results obtained for the pastes, found to be highly screen-printable, are presented in For both rolled and unrolled pastes, with all three carriers considered, a decrease in the layer resistance of the paths was observed as the graphene packing increased. This is a predictable result because graphene is the conductive functional phase of these materials. For LARO-based pastes, with a threefold increase in graphene packing, the sheet resistance decreased by up to two orders of magnitude. When comparing the paths printed from pastes subjected to the unification process on the triple-roller with those not subjected to this process, it is worth noting that in most cases a benefit in terms of a reduction in sheet resistance is evident. The greatest changes were observed for the low graphene packing. For LARO pastes with the lowest graphene content, rolling resulted in a more than 100-fold decrease in sheet resistance. For LARO pastes containing the highest tested packing, there was a decrease in the layer resistance value of 0.12 kΩ/□. For the pastes on soft TPU, rolling not only improved the bonding of the functional phase to the polymer phase and unified the material enough to make it printable, but an improvement in electrical properties is also apparent. For PMMA pastes, the rolling process only had a positive effect on pastes with up to 10% packing. In contrast, the 12.5% and 15% pastes had a slightly higher electrical resistance after the rolling process.The reason for the diminishing effect of improving electrical parameters with increasing packing can be attributed to the fact that, with excessive amounts of graphene, the rolling process can lead not only to deagglomeration but also to fragmentation of the graphene flakes, which in turn can result in more difficult carrier tunnelling and thus poorer electrical properties.Comparing all of the carriers, it is interesting to observe that, at low graphene packing, significant differences in the obtained values of sheet resistance can be noted, while for maximum graphene packing the resistance results are very similar for all the carriers. The interactions between the filler particles and the matrix polymer seem to be more crucial for low functional phase contents. For these packings, an easy path for electron tunnelling between flakes is important, so polymer density and particle size may also be significant. At higher packing levels, the role of the polymer is more directed towards maintaining the print on the substrate and enabling the material to maintain homogeneity.An important parameter that is very often neglected in investigations, but which may vary depending on the packing of the functional phase, and also on the carrier used, is the path thickness. This parameter has great significance in the context of the obtained resistance of the layers. We examined the path thicknesses of all the paths obtained . The thiThe results obtained show that, as the packing increases, the printed paths have a higher thickness. That may be directly related to the viscosity of the pastes, as pastes with lower viscosity spread over the substrate to a greater extent. It is also noticeable that LARO pastes allow for thicker paths to be printed, which may be attributable to the amount of polymer required to produce a paste with suitable rheology, but also to the higher intermolecular interactions within this carrier. However, the influence of the carrier itself is considerably lower than the packing of graphene flakes, which is understandable, given that the amount of the added functional phase is the main determinant of paste viscosity.Non-rolled pastes have been omitted from this comparison due to their very similar outcomes, indicating that the path thickness is not significantly affected by the triple roll process.In the final stage of the study, the adhesion properties of the printed paths to the flexible PET substrate used were checked for both rolled (R) and not rolled (NR) pastes. In this work, we used flexible substrates on account of the strong development of flexible electronics. It is on difficult, flexible substrates that additional material processing, in the form of three-roll milling, can allow graphene pastes to be printed with adequate adhesion. Tests were carried out for at least three 20 mm × 20 mm square prints each time and the median of the adhesion values obtained is listed .The definite effect on adhesion to the substrate for both the polymer matrix used and the concentration of the functional phase is evident. The more graphene in the layer, the better the adhesion for all three polymer matrices tested.The poorest adhesion properties among the tested polymers were noted for PMMA-based prints. For the 5% graphene packing, 35–65% detachment was observed. However, for higher concentrations of graphene, the detached material did not exceed a value of 5% of the print. Prints on both PMMA and TPU matrixes showed similar adhesion. In the most severe cases, this resulted in the detachment of no more than 15% of the print Such dependencies can originate from several factors; from the properties of the polymers themselves, such as their elasticity and density, the interaction of the used solvent with the PET substrate and the interaction of the carrier with the functional phase, finishing with the viscosity of the paste modulated by both the carrier and the graphene packing.In addition, the effect of the rolling process of the pastes on the adhesion of the layers printed with their use is also evident. This is probably related to the homogenisation of the paste itself. Since the agglomerates of the functional phase, which are present in greater quantity in non-rolled pastes, can be the link that detaches the layer from the substrate, the deagglomeration, being part of homogenisation, leads to better adhesion.With the highest packing of graphene being 15%, for pastes based on all three carriers subjected to the unification process on a three-roller machine, the results of layer resistance, layer thickness and adhesion were found to be very similar. These layers were subjected to SEM imaging to verify to what extent the structures of these prints are alike .The analysis of the SEM images shows that, despite the differences in the polymer matrix, with sufficiently high packing of the functional phase, highly similar layers are obtained. The homogeneous and dense distribution of graphene flakes is evident. The visible bulges are the result of the screen-printing technology used.The role of both the composition and the selection of the process for producing graphene-based screen-printing pastes was outlined. The importance of the selection of the polymer and the degree of packing of the functional phase was discussed, as well as the influence of the triple-rolling process concerning rheological parameters, screen-printing potential, and properties of the obtained paths concerning sheet resistance, path thickness, and adhesion to the flexible substrate.Depending on the carriers used, different rheological, electrical and adhesion properties of the composites produced were obtained. It was shown that the results can be significantly different even within a single polymer family, as in the case of the two thermoplastic polyurethanes tested, both of which were based on polyethers, and differing in hardness and the presence of a plasticiser (in soft TPU).We have shown that graphene pastes with a higher packing of the functional phase, possessing higher dynamic viscosity values, not only exhibit better electrical performance but also have better adhesion properties to PET films.Above all, we demonstrated the significance of a correct paste preparation process. The triple-rolling process revealed the influence on the rheology and printability of the pastes, as well as the sheet resistance and adhesion of the acquired paths. The rolled materials showed lower electrical resistivity, with the greatest impact at lower graphene packing. A clear improvement in the adhesion of the rolled composites was noted for all carriers.


Since the first commercial biosensor device for blood glucose measurement was introduced in the 1970s, many “biosensor types” have been developed, and this research area remains popular worldwide. In parallel with some global biosensor research reports published in the last decade, including a great deal of literature and industry statistics, it is predicted that biosensor design technologies, including handheld or wearable devices, will be preferred and highly valuable in many areas in the near future. Biosensors using nanoparticles still maintain their very important place in science and technology and are the subject of innovative research projects. Among the nanomaterials, carbon-based ones are considered to be one of the most valuable nanoparticles, especially in the field of electrochemical biosensors. In this context, graphene oxide, which has been used in recent years to increase the electrochemical analysis performance in biosensor designs, has been the subject of this review. In fact, graphene is already foreseen not only for biosensors but also as the nanomaterial of the future in many fields and is therefore drawing research attention. In this review, recent and prominent developments in biosensor technologies using graphene oxide (GO)-based nanomaterials in the field of cancer diagnosis are briefly summarized.  Graphene, called the material of the future, consists of a single layer of carbon atoms arranged in a regular hexagonal lattice appearance , is prodSince the award of the above-mentioned Nobel Prize, many researchers have been conducting research on graphene in a wide variety of disciplines, including medicine, physics, and chemistry. As of 2022, nearly 296,500 articles based on graphene have been produced and many companies have also started to work in this field. A great deal of effort has been made by researchers on the synthesis of graphene to date, and when these studies are examined in general, it is seen that mechanical exfoliation and chemical vapor deposition (CVD) methods still maintain their popularity. High quality graphene sheets can be produced by both methods . Since g2 g−1) 3−/4−, which is the parameter especially focused on in impedimetric studies, is strongly affected by electrode modification. In line with this basic information, a Nyquist curve with a very small semi-circle was obtained in the presence of a redox probe with the glassy carbon electrode (GCE) electrode used, and then an increase in the diameter of the semi-circle was observed when the experiment was repeated with the GO modified electrode. This has been interpreted as the modification of GO, whose sp2 networks are disrupted during its synthesis, and that electron transfer to the electrode interface is prevented, and this layer acts as an insulating layer. Then, a single-stranded DNA was covalently immobilized on the GO modified electrode surface and it was observed that the Ret value increased further. It is stated that this situation occurs due to the electrostatic repulsion of the negatively charged phosphate backbone in DNA against ferrocyanide. Then, a single-stranded DNA was covalently immobilized on the GO modified electrode surface and it was observed that the Ret value increased further. This situation is evaluated as both the DNA binding to the surface and the negatively charged phosphate backbone of DNA exerting electrostatic repulsion against ferrocyanide. In the continuation of the experiment, when the electrode surface was subjected to electrochemical reduction (when scanning from 0.0 to −1.7 V) after DNA modification, it was determined that the Ret value decreased dramatically, that is, the conductivity increased clearly. It is an indication that the GO on the surface decreases as a result of reduction and this situation occurs with the formation of rGO. To summarize, the use of rGO in biosensor platforms is due to its contribution to the acceleration of electron transfer. It has been proven by many studies that it improves EIS responses in DNA biosensor designs. When hybridization occurs, the Ret value increases again because this means that [Fe(CN)6]3−/4− cannot penetrate the double helix bundles with more negatively charged phosphate backbones on the surface. Similarly, in CV experiments performed with step-by-step surface modifications and with the same redox probe, the findings obtained from the electrode-electrolyte interface are compatible with the EIS results. Electrochemical impedance spectroscopy (EIS) is a technique that allows the analysis of both the resistive and capacitive properties of physical and/or biomolecular modifications of electrode surfaces, including nanomaterials, nucleic acids, proteins structures etc. In this concept, EIS is one of the basic electrochemical methods used to determine the fundamental redox events at the electrode–electrolyte interface. In fact, it is the most widely used method in studies of this type of research. However, evaluations are usually made on the basis of complementary results obtained with both the EIS and CV method. For example, in a study carried out by Gong et al. to investigate the properties changing on the surface with graphene oxide modification, Nyquist curves obtained from the solution interface with the glacial carbon electrode using the conventional redox probe ferri/ferrocyanide were evaluated . The eleIn fact, many of the GO production methods are known to damage this structure. As a result of this damage, it was determined that some functional groups were formed in the substance or on its surface, and carbonaceous or metallic impurities were formed. However, it is seen in many reports that this situation is often turned into an advantage by electrochemical or chemical means. For example, covalent bonding procedures,  which ar6]3−/4− is used for label-free analyses as given in the following sections. CV, DPV, and SWV techniques are also used in both label-free and label-based biosensors to monitor the redox events and mechanisms occurring on the electrode surface. Of these, cyclic voltammetry (CV) is the most widely used voltammetric method to explain basic electrochemical events, such as the thermodynamics of oxidation-reduction reactions and electron-transfer kinetics occurring at the electrode-solution interface, as well as mass transport towards the electrode surface ,68. In tDifferential pulse voltammetry (DPV) is another basic electrochemical technique and in a cell containing a triple electrode system, measurements are performed at the moment when the faradaic current is highest and capacitive current is lowest. Thus, the signal-to-noise ratio increases and hence sensitivity increases. In the method, the difference in current per applied pulse (∆Ip) is recorded as a function of linearly increasing potential. The resulting differential curve is in the form of a peak and its height is proportional to the concentration .Square wave voltammetry (SWV) is also a pulse voltammetry technique that has the advantage of being extremely fast and at the same time sensitive. In this method, a potential is applied to the working electrode in the form of a symmetrical square wave. In each square wave cycle, the current is measured twice, at the end of the applied forward and reverse pulses. The difference of these currents gives the net current value. The square wave voltammogram is obtained when the net current obtained is plotted as a function of the applied potential. The SWV measurement is extremely fast, it is possible to increase the precision of the analysis by averaging several voltammetric measurements .As a result, the modification and redox events occurring on the electrode surface can be elucidated by evaluating these techniques individually or together.Cancer is among the diseases that carry a serious risk of death, and it is becoming more and more common around the world. There are about 200 different forms of cancer detected so far . AccordiSince early diagnosis in cancer is life-saving, new diagnostic technologies to be developed must provide fast and accurate analysis. In this context, cancer biomarkers are becoming increasingly attractive targets for electrochemical biosensor designs alongside classical detection methods, as they are useful tools for understanding the developmental processes of cancer disease. Thanks to advances in nanotechnology, various cancer biomarkers, i.e., cancer cells, metabolites, miRNA, surface antigens/proteins, etc., biodetection is possible . Some ofHowever, there are some difficulties in developing a biodevice with features that will allow early detection of cancer. For example, the biosensor should be able to perform sensitive and selective analysis with a small sample. If an evaluation is made in terms of the number of cancerous cells, it is expected that a biosensor with clinical validity will accurately detect a tumor in the range of 100–1000 cell counts . DespiteIn this section, prominent examples of graphene-containing electrochemical biosensors developed for cancer diagnosis will be given. It was determined that real samples  analyses were also performed in all of the studies in the review. Some other interesting designs and the basic contents of them are collected in two different tables at the end of the review  and square wave anodic stripping voltammetry (SWASV). It is understood that this methodology, which was discovered by Professor Wang and his team in 2002 and is based on the dissolution of metal ions from QDs with acidic solution, is still a popular and powerful alternative electrochemical biosensor design technique despite many years since its discovery 3−/4− redox couple was used in measurements to monitor both surface modifications and antibody-antigen binding via CV or SWV methodologies. In addition, the electrochemical signal of gold nanoparticle label was also evaluated in the determination. In this system, p53 antibodies could be detected with a detection limit of 0.088 pg/mL with a linear range of 0.1 pg/mL to 10 ng/mL.An innovative and remarkable biosensor platform developed for cancer prognosis has been reported by Elshafey et al., where gold nanoparticles provide large surface area for p53 protein modification and reduced graphene oxide is used for signal enrichment . In the Dou et al.  demonstrOn the other hand, in paper-based biosensors, another sensor platform, graphene oxide nanomaterial and various labels are used for signal amplification and enrichment of the sensing surface area in the diagnosis. In fact, biosensors based on GO-modified paper have been produced since the 2010s. Electrochemical biosensor designs in cancer diagnosis have also been a prominent research area in recent years due to paper-based electrodes. Nevertheless, it is still considered that there are some issues that need to be overcome in order to reveal point-of-care systems . A few i2) to further increase the sensor performance. The paper-based part in the microfluidic paper-based analytical device (μPAD) consists of two layers of selectively patterned square filter paper of the same size (35.0 mm × 35.0 mm), and the specially patterned parts, where the analysis will be carried out, were designed with the AutoCAD 2012 program. Eight carbon-based circular working electrode regions are printed on the paper surface using screen-printed technology  sandwich assay protocol, and (iv) a microfluidic system 3−/[Fe(CN)6]4− containing redox probe solution using CV and SWV techniques. As expected, a decrease in electrochemical response was obtained after the target analyte was bound. The detection limit of the developed LSG-AuNS-based aptasensor was found to be 0.008 ng/mL. Thanks to the 3D gold nanostructures deposited on LSG, high electron transfer rate, low detection limit, and reproducible results were achieved. Another outstanding aspect of this study is the development of an aptasensor that can accurately and sensitively analyze the amount of HER-2 added to undiluted human serum. In addition, a special software has been developed to use the presented system as a POC biosensor device, and the conversion from a laboratory aptasensor to a hand-held aptasensor containing a mobile phone has been achieved. However, with the handheld device, analysis could be performed with a higher detection limit (100 ng/mL) than the aptasensor, which was developed in this study for use only in the laboratory environment.As another interesting example of graphene-containing biosensors in cancer research, Rauf et al. reported  a laser-Analysis of cancer cells or their markers using modified graphene oxide with different metallic structures such as tungsten oxide-graphene composite  and reduBiosensor designs with 3D materials have also attracted attention in recent years. To give an example of a prominent work, Jang et al.  developeFor more information on electrochemical biosensors developed for cancer diagnosis, both containing types of graphene and with other nanomaterial design contents, some valuable review studies reported recently are available in the literature ,131,132.Graphene oxide-based nanomaterials on immunosensors or biosensors offer a range of possibilities in regard to cancer diagnosis. Two tables containing some of the other recently reported studies and the basic biosensor design elements related to these studies is presented below .In this review, we tried to present prominent graphene oxide-based electrochemical studies that have been reported in recent years and have a suitable infrastructure for clinical use for cancer diagnosis. Although significant progress has been made in the synthesis of graphene and its derivatives and their use in drug carriers/therapeutics in vitro, more progress is needed in the field of in vivo analysis. On the other hand, if an evaluation is made in terms of electrochemical studies, first of all, it can be said that while promising studies continue, there are also discussions on whether graphene and its derivatives increase the electroactivity/conductivity sufficiently. For this purpose, many elements, including N, S, P, B, etc., are integrated into graphene structures by doping . We beliAs is known, zero-dimensional graphene quantum dots GQDs, a member of the latest-generation graphene family, have also attracted great interest from researchers and industries in recent years . In lineApart from all these, comprehensive studies including extensive efforts in the field of theranostics, which combine diagnosis and treatment in a single system/platform in the medical field, are also being carried out ,173. HavIt is necessary to develop at least stable surface sensors and diagnostic kit for “open–use–dispose”-type electrochemical sensors . Much moFinally, we would like to point out that we think that, among the electrochemical biosensors examined in this review, paper-based sensors or systems that can measure with an application able to be downloaded to a mobile phone are more suitable for clinical practice. However, electrochemical biosensors  are also very promising for wearable health monitoring, thanks to their important advantages such as selectivity, sensitivity, rapidity, portability . It is c


The present paper describes the effect of the concentration of two graphene oxides (with different oxygen content) in the modifier layer on the electrochemical and structural properties of noble metal disk electrodes used as working electrodes in voltammetry. The chemistry of graphene oxides was tested using EDS, FTIR, UV–Vis spectroscopy, and combustion analysis. The structural properties of the obtained modifier layers were examined by means of scanning electron and atomic force microscopy. Cyclic voltammetry was employed for comparative electrochemical studies. Electrochemical methods are based on measurement of the current associated with the molecular properties and interfacial processes of chemical species. The recorded response results from direct transformation of the desired chemical information  into a current signal , according to the selected method. Voltammetry is considered as one of the most sensitive electroanalytical methods, suitable for the determination of trace amounts of many metals and compounds in clinical, industrial, and environmental samples ,2,3,4. V2O. Seven years later, Ruess [2SO4 along with: (1) sodium nitrate for in situ production of nitric acid in the presence of KMnO4 (Hummers’ method); (2) fuming nitric acid and a KClO3 oxidant (Staudenmaier method); (3) concentrated phosphoric acid with KMnO4 (Tour method); or (4) concentrated nitric acid and a KClO3 oxidant (Hoffmann method) [3 as an oxidant (Staudenmaier’s and Hofmann’s methods) exhibit a higher C/O ratio, whereas methods employing KMnO4 (Hummers and Tour) yield a larger proportion of oxygen-containing groups. Oxidative methods employing KClO3 result in oxides containing mostly CO groups , whereas methods using KMnO4 lead to oxides containing large amounts of carbonyl (Hummers) or both carbonyl and carboxyl groups (Tours) [Graphene, which consists of a one-atom-thick planar sheet containing an sp2-bonded carbon structure with exceptionally high crystalline and electronic quality, is a novel material that has emerged as a rapidly rising star in the field of material science ,12. Everr, Ruess  suggester, Ruess  proposedr, Ruess  presenter, Ruess , GO consr, Ruess  proposedr, Ruess ,27,28. Ir, Ruess ,32,33,34 method) ,36,37,38 (Tours) . The aim of this study was to compare the properties of noble metal electrodes modified with graphene oxide and to determine how graphene oxide chemical composition and concentration affect their electrochemical and structural  properties.−1 with a resolution of 4 cm−1. Electroanalytical measurements were carried out using a µAutolab instrument  controlled by GPES 4.9 electrochemical software. The three-electrode electrochemical cell employed in the study consisted of a reference electrode, an auxiliary electrode (platinum wire), and a working disk electrode .The surface topography of the studied electrodes was investigated using an atomic force microscope . The AFM measurements with the scan size of 5 µm × 5 µm were performed in the tapping mode using silicon scanning probe  with a nominal spring constant of 42 N/m and resonance frequency of 320 kHz. The roughness parameters Ra and Rq were defined on the basis of AFM topography images . The surface morphology and elemental composition of electrodes was investigated with high-resolution scanning electron microscopy  equipped in a Schottky field emission electron emitter. The surface morphology measurements were performed using CBS detector, enabling the observation of surface using signal of backscattered electrons (BSE). The images were recorded at 3 kV with electron beam deceleration 4 kV, spot size 2.5, and working distance 6.7 mm. The chemical elemental analysis of GO I- and GO II-modified gold electrodes was determined with EDAX Roentgen spectrometer (EDS) with Octane Pro Silicon Drift Detector (SDD) . Attenuated total reflection Fourier transform infrared (ATR–FTIR) spectra were recorded on a Nicolet iS50 spectrometer equipped with an MCT detector and the GATR accessory with Ge crystal over the spectral region from 600 to 4000 cmThe potential of the working electrode was measured vs. an Ag/AgCl electrode. Double-distilled water was used throughout the experiments. All the chemicals, including graphene oxide (GO I), reduced graphene oxide (GO II), and hexacyanoferrate system were purchased from Sigma-Aldrich and used as received.The GO I/GO II suspension was prepared weekly by dispersing an appropriate amount of GO I/GO II powder in DMF (dimethylformamide) in a 5 mL volumetric flask. The resultant solution was kept in a refrigerator at 4 °C in the dark. Spectrophotometric measurements were made using a Cary 100 Bio UV–Vis spectrophotometer .The general procedure used to obtain voltammograms was as follows: Ten mL of supporting electrolyte was placed in the voltammetric cell and the solution was purged with argon for 10 min (if necessary). After recording the initial blank, the required volumes of the analyte were added by means of a micropipette. Then, the solution was deoxygenated for 10 s (if necessary), and a voltammogram was recorded. All electrochemical measurements were carried out at ambient temperature. Each measurement was repeated three times and a mean was calculated.−1 were prepared. Afterwards, 20 µL of each solution was deposited on alumina foil by sessile drop technique and, subsequently, the samples were allowed to evaporate the solvent. Final spectra were received by the addition of 64 scans at a resolution of 4 cm−1.In order to obtain FTIR spectra of both graphene derivatives, DMF solutions of GO I and GO II at a concentration of 20.0 g L−1 were prepared for each oxide. Prior to modification with GO I or GO II, the working electrode surface was sonicated in ethanol for 60 s. Next, working electrode surface was mechanically polished with 0.05 μm Al2O3 slurry on a polishing cloth to a mirror finish. Then, it was ultrasonically treated in ethanol for 180 s and washed with double-distilled water. The modifier suspension (3 µL) was dropped onto the surface of the cleaned electrode and dried at ambient temperature. A new modifier film was prepared before each series of measurements.Modifying solutions containing graphene oxides were prepared with dimethylformamide. Suspensions at concentrations ranging from 1.0 to 50.0 g L63−/Fe(CN)64− redox couple and suspensions of graphene oxides (GO I and GO II) in a concentration range from 1.0 to 50.0 g L−1. 63−/Fe(CN)64− redox couple recorded on bare and modified gold (A) and platinum (B) electrodes. Both working electrodes were modified with graphene oxide suspensions at a concentration of 1.0 g L−1.Voltammetric measurements provide a wealth of information concerning the properties and characteristics of electrochemical processes. Cyclic voltammetry is an important and widely used technique determining analyte electrochemical behavior, including the formal redox potential, thermodynamic and transport properties, electron transfer kinetics, and adsorption processes. The hexacyanoferrate(II)/(III) redox couple undergoes a nearly reversible electrode reaction without any complications of preceding or post-chemical reactions; so, it has been a popular choice as a redox standard in CV. Electrochemical measurements were performed at modified working electrodes using the Fe(CN)−1 and lower. The relationships between hexacyanoferrate anodic peak current and the surface concentration of GO I recorded at both Au and Pt electrodes are shown in −1 GO I suspension. An analogous study using GO II suspensions also revealed the strongest signals of the Fe(CN)63−/Fe(CN)64− redox couple for the 20.0 g L−1 concentration at both Pt and Au electrodes. 63−/Fe(CN)64− voltammograms recorded at the GO II-modified platinum electrode.The strongest signals at both electrodes were observed for the electrode surface-modified with GO I. Significant difference may be observed for GO II suspension, which influence tested working electrodes in the opposite way. As can be seen in −1 GO II  were much lower than those obtained at electrodes modified with 20.0 g L−1 GO I . Since GO II was purchased as reduced graphene oxide and RGO has fewer oxygen groups (than GO), which improves its electrical conductivity, the opposite results were expected (oxygen content in both GOs will be thoroughly discussed in It is worth noting that the highest currents recorded at electrodes modified with 20.0 g L63−/Fe(CN)64− is a chemically reversible system, in which the oxidized form of the solution species can be regenerated from the reduced form (and vice versa), and both forms are stable on the time scale of the voltammetric experiment. However, after preliminary studies, a negative impact of modifications on electrochemical reversibility could not be excluded. If this was the case, the electron transfer would be so slow that the peak potential would not reflect the equilibrium activity of the redox couple at the electrode surface. To determine whether GO modification had a positive or negative effect, cathodic and anodic signal separation  was analyzed. In the theoretical model, the difference between the anodic and cathodic peak potentials  should be equal to 59 mV/n for fully electrochemically reversible systems. However, the majority of redox systems used are quasi-reversible, with anodic/cathodic separation being much higher than 59 mV, even for fast quasi-reversible redox pairs. The separations measured in the present study are shown in The model redox pair Fe(CN)−1 concentration) was only one of several factors influencing peak separation. At the platinum electrode, the lowest separation values were observed for the lowest modifier concentrations, while at the gold electrode, the highest concentrations afforded the best reversibility. This shows again that GO I and GO II act like two totally different compounds. Another important diagnostic characterizing an electrode reaction is peak potential (pE). At fast electron transfer rates, Ep is independent of the scan rate, indicating a reversible electrode reaction. The influence of the scan rate was tested in the range of 10–500 mV s−1. It was found that the scan rate did not have any effect on peak potential , indicating a quasi-reversible electrode reaction. The next very important diagnostic tool consists of the relationships log Ip vs. log v and Ip vs. v1/2. For both graphene oxides, linear plots of log Ip vs. log v were obtained with slopes of approx. 0.5, indicating an electrode reaction with the rate governed by diffusion of the electroactive species to an electrode surface. A linear relationship (for both electrodes with both graphene oxides) was also observed for Ip vs. υ1/2, which confirmed that the mass transport rate of the electroactive species to the electrode surface occurred across the concentration gradient. In such a case, the peak current Ip is governed by the Randles–Sevcik equation: Ip = k n3/2 A D1/2 C* υ1/2, where the constant k = 2.72 × 105; n is the number of moles of electrons transferred per mole of the electroactive species; A is electrode area; D is the diffusion coefficient; C* is solution concentration; and v is the scan rate. The above formula was used to calculate the electroactive area of the Au and Pt electrodes modified with both graphene oxides  are shown in It is well known that the properties of a broad range of materials and the performance of a large variety of devices depend strongly on their surface characteristics . TherefoAs mentioned above, the modified electrodes were also characterized by atomic force microscopy (AFM) to give insight into their surface topography. Three-dimensional (3D) views of the graphene-oxide-modified gold electrodes are given in −1 and a broad peak of very low intensity at 3210 cm−1 can be assigned to the stretching mode of the O–H bond, which reveal the presence of hydroxyl groups in graphene oxide [−1 can be designated to the stretching and bending vibration of water molecules adsorbed on graphene oxide [−1  [−1 (epoxide C-O-C or phenolic C-O-H stretch) [−1 confirms the presence of the C=O from carboxyl and/or carbonyl groups in the GO II structure [With the aim of determining whether the chemical structure affects the electrochemical properties of the prepared graphene-based electrodes, spectroscopic measurements were conducted. ne oxide ,48. Morene oxide . The FTIoxy C-O) ,46,49,50stretch) ,46,49,50tructure . The FTIThis was also confirmed by UV–Vis spectroscopy . UV–Vis In the last step, modified electrodes were examined with Energy Dispersive X-ray Spectrometry. The EDS measurements revealed changes in the carbon-to-oxygen ratio in GO I and GO II samples . The quaIn this paper, the electrochemical and structural characteristics of two graphene oxides used as electrodes modifiers were analyzed. AFM and SEM results showed that the topographies of the investigated electrodes/modifications do not significantly differ. However, the EDS elemental analysis revealed changes in the oxygen content. This was also confirmed by combustion analysis, FTIR, and UV–Vis spectroscopy. Unexpectedly, more oxygen content was found in the case of GO II (declared as RGO) compared with GO I (declared as GO). From the electrochemical point of view, the presence of RGO  should improve the electrochemical conductivity; however, the opposite results were obtained. The detailed electrochemical studies with model redox system showed that purchased graphene oxides act as utterly different modifiers. Although basic analytical characteristics are similar—for example, the diffusional nature of registered signals remained unchanged—other more important properties are completely different . As a result, we may obtain two completely different sensors that could, but should not, be called uniformly “graphene-oxide-modified”.Based on the obtained results, we can generally conclude that GO I is more reduced than GO II, which is the exact opposite of the general conception of graphene oxide structure. Hence, the following question arises: is there a chance to achieve good reproducibility among the scientific community using the not well-established uniform structure of GO? Undoubtedly, we may find some positive aspects of this graphene oxide case because by using graphene oxides of slightly changed chemical composition we can develop sensors with tailored properties towards a given analyte. On the other hand, we have to be aware of the negative aspect, i.e., the lack of reproducibility between “graphene-oxide-modified sensors” when being developed from diverse research teams. In conclusion, the authors believe that there is an urgent need to standardize such popular nanomaterials by assigning a CAS Registry Number to them. The orderliness in graphene oxides nomenclature will greatly shorten the search for appropriate graphene oxide and will contribute to improvements in electrochemical analysis.