A new technique is proposed to transport and further classify nanoparticles of different sizes. A graphene sheet is used as the substrate; a nanoparticle is placed on the substrate and a sliding block is located below the substrate. As the sliding block moves under the graphene substrate, a driving force is yielded from the van der Waals interaction between the sliding block and the nanoparticle. The effects of the pre-tension of the graphene substrate, size and number of layers of the nanoparticle, slip velocity, the interface commensurability and temperature on nanoparticle transportation are systematically investigated. It is found that a pre-tensioned graphene substrate could provide easier nanoparticle transport. The initial movement of the nanoparticle depends on the competition between the in-plane force and the driving force, while the subsequent transport depends on the slip velocity of the sliding block and the viscous damping force. Based on such a new transport mechanism, a novel nano-sieve can be designed, with which nanoparticles of different sizes can be screened and classified spontaneously. Our findings may be useful for promising designs of transportation, manipulation and classification of nanoparticles. To realize directional transport of micro- or nano-objects on a substrate, the substrate should have an inhomogeneous and gradient field, including the non-uniform properties of surface wetting9, surface curvature10, body temperature11, and material stiffness12. A non-uniformly tensile strain in the substrate induced by an external force can cause a small object to move or roll on its surface13. Sometimes, an external magnetic, electric, or even optical field can help to manipulate small objects. For example, an electric field can be used to trap or release charged gold nanoparticles onto or from the surface of gold electrodes14. Fennimore et al.4 constructed a fully synthetic electromechanical actuator by incorporating a rotatable metal plate with a multi-walled carbon nanotube serving as the key motion-enabling element. Alternating current dielectrophoresis could be used to control the deposition of single-walled carbon nanotubes15. A micro-electromagnet is an appropriate tool to precisely control and manipulate magnetic nanoparticles16. An optical tweezer is often adopted to trap dielectric nanoparticles, metal nanoparticles, or even DNA molecules18.Different interesting mechanisms or technologies have been proposed or used for nanoscale manipulation. Atomic force microscopy, scanning tunneling microscopy, frictional force microscopy, and micro-grippersSome of these technologies are still at evolution and development phase and can only be performed at laboratory level, which have still a long way for industrial applications. Precisely manipulating small objects or particles remains a challenge, not to mention the classification problem of nanoparticles of different sizes.Inspired by the free movement of small iron pieces on a flat piece of paper using a magnet lying under the paper, we propose a similar technique to manipulate nanoparticles. We adopt a similar system in which nanoparticles lie on a graphene sheet with a sliding block underneath. The movement of sliding block under the graphene sheet causes the movement of particles in the same direction. The difference is that the driving force in the first case is the well-known magnetic force, but the driving force in the second case is the familiar van der Waals interactions at a small scale. Based on this proposed manipulation technique, a novel nano-sieve is designed to screen and classify nanoparticles of different sizes.19. Techniques to manipulate a graphene sheet have also been studied, for example, by tailoring a graphene sheet to a certain shape and size20, stretching or bending a graphene sheet21, and connecting a graphene sheet to a carbon nanotube22 or a metal block23. Researchers have studied the motion of nanoflakes on graphene, including superlubric sliding24 and self-retracting motion25. All these skills enable the construction of a more complex system to realize various functions.Such a system could be set up experimentally considering the related skills and methods. The synthetic methods for producing a single-layer graphene sheet are relatively matureThe outline of this paper is as follows. We first establish the manipulation system and find the transport phenomenon studied in the present paper. Then, the effects of the particle size, pre-tension strain in the graphene sheet, slip velocity of the sliding block, damping force, temperature and initial interface commensurability between the nanoparticle and substrate on the driving mechanism are investigated thoroughly. Based on such a driving mechanism, a novel sieve is then designed to classify nanoparticles of different sizes.AB staking structure. The length (d1) and width (d2) of the nanoparticle can be changed in detailed simulations, whose effects on the transportation mechanism are investigated. The side length (d) of a nanoparticle is further defined as the average value of its length and width for an approximately square nanoparticle. A sliding block is placed under the graphene sheet substrate. The sliding block is simulated using a copper material as an example. Considering the possible resistance exerted by the substrate in real applications , we assume a viscous damping force γ = 0.03 eVps/nm2.A molecular dynamics simulation method is used in this paper to study the transport of nanoparticles driven by van der Waals forces. Figure In the present paper, the static friction between the nanoparticle and graphene substrate is called an in-plane force. The dynamic friction between the nanoparticle and the graphene substrate is simulated by a viscous damping force, which is proportional to the transport velocity. As the sliding block moves under the graphene substrate, a driving force is yielded due to the van der Waals interactions between the sliding block and the nanoparticle. The particle on the graphene substrate only moves when the driving force is larger than the in-plane force; as the particle moves, the damping force is spontaneously produced and replaces the in-plane force as a resistance barrier of the particle. Many simulations show that when the sliding block moves underneath the substrate and passes the nanoparticle, sometimes the nanoparticle on the graphene substrate can be driven to move forward with the sliding block, while sometimes it remains static. Typical movies are provided as Supplementary Movies First, we consider the case of a nanoparticle consisting of a rectangular three-layered graphene sheet, which is fixed on the graphene sheet substrate. Similar to the typical example shown in Fig. d2 of the cross section is maintained at 2.09 nm and the length d1 is changed from 0.71 nm to 4.96 nm, the maximum driving force is hardly affected by d1, as shown in Fig. d1 is maintained at 1.99 nm and d2 is changed from 0.86 nm to 5.03 nm, the maximum force increases, as shown in Fig. When the width 27. It shows that the absolute value of the in-plane force generally increases with the increase in length and width of nanoparticles.The in-plane force between the nanoparticle and the graphene substrate will resist the initiation of the nanoparticle’s movement, which can be obtained by moving the nanoparticle on the graphene substrate. Figure d2 of the nanoparticle, which is due to the proportional increase of in-plane force and driving force with the nanoparticle width d2 as shown in Fig. The initial movement of nanoparticles depends on the difference between the in-plane force and driving force. Figure d1 hardly influences the maximum driving force but significantly influences the in-plane force. The in-plane force of a nanoparticle with a relatively large length would be greater than its driving force, which, as a result, would prevent the movement of the nanoparticle. However, the nondimensionalized in-plane force by the corresponding maximum driving force is hardly influenced by the nanoparticle width d2, although both of them increase with the increasing particle width d2. Therefore, the driving behavior of a nanoparticle on a graphene substrate without pre-tension strain is mainly affected by its length d1.In summary, the nanoparticle length d is defined as the average value of its length d1 and width d2.In the following text, we mainly focused on an approximately square nanoparticle, whose side length Figure The influence of nanoparticle thickness and sliding block length on both forces have also been considered, both of which have no obvious effect on the maximum driving force and in-plane force. Detailed discussions can be found in Figs 28, we adopt the novel idea to study the possible motion of large particles. Cases of graphene substrate with and without pre-tension are analyzed. If a relatively large nanoparticle is put on a graphene substrate without pre-tension, both initial and final states of the nanoparticle on the graphene substrate is similar to Fig. As discussed above, the in-plane force of a large nanoparticle is always larger than the driving force, preventing the initial movement of nanoparticle. The question of how to drive a relatively large nanoparticle is put forward. Inspired by a study demonstrating that the tension strain in a graphene sheet could change the wettability of a surface based on a change in the atomistic latticeThe pre-tension strain in the graphene substrate does not show any influence on the driving force between the nanoparticle and the sliding block, details of which can be found in Fig. 29, whereby a commensurate or incommensurate state was achieved by placing a graphene flake on a graphene substrate with an AB stacking orientation or with a graphene flake rotation of 90°. In the present model, an analogous incommensurate state is attained by stretching the graphene substrate. When the pre-tension strain in the graphene substrate vanishes, the amplitude of the in-plane force reaches the maximum and becomes larger than the maximum driving force. As a result, the nanoparticle cannot be driven and remains still. When the pre-tension strain in the graphene substrate is larger than 0.1, the amplitude of the in-plane force becomes smaller than the maximum driving force. Consequently, the nanoparticle is driven to move. A series of simulations demonstrate that a pre-tension strain above ~0.1 is needed in the graphene substrate to ensure transport of nanoparticles.Figure Figure Based on the comparison of the driving force and in-plane force, we find that only small nanoparticles could be moved on the graphene substrate without pre-tension, while almost all nanoparticles could be moved on the pre-tensioned graphene substrate with a pre-tension strain of 0.2. Further simulations show that transport is also influenced by the sliding velocity of the sliding block, the number of layer and the average side length of nanoparticle, which is briefly illustrated in Fig. Figure Further simulations show that the driving behavior is also influenced by the side length and thickness of the nanoparticles. The driving behavior of single-layered and three-layered nanoparticles is typically considered, the initial state and the final state of which are illustrated in Fig. Similar to the effect of the thickness of the nanoparticle, the driving behavior is also influenced by the side length of the nanoparticles, as shown in Fig. The critical sliding velocity of the sliding block, below which nanoparticles of different side lengths and with different numbers of layers could be driven by the sliding block, is given in Fig. The transport behavior of nanoparticles without viscous damping force is also considered, in which the kinetic energy of the nanoparticle induced by the sliding block is difficult to be dissipated. The captured nanoparticle will move back and forth around the sliding block as shown in Fig. 30. Furthermore, at a high velocity, experimental observations also show that the friction value may be higher31. In our simulation, the substrate is assumed to be atomically smooth, which may obviously reduce the dissipation of kinetic energy. The “hade-make” viscous damping force is adopted in order to approach cases in real applications. With the damping force, the kinetic energy of the nanoparticle can be quickly dissipated. As a result, the moving nanoparticle will be tightly captured by the sliding block or the nanoparticle left behind could stop at the equilibrium position without large vibration back and forth.For possible applications in the future, we should consider the pre-existing wrinkles or defects in the substrate surfaces, which would increase the friction forceγ = 0.03 eVps/nm2, which should influence the damping force of a moving nanoparticle. Different damping coefficients are adopted in the simulations . The transport behavior is similar to that when the damping coefficient is 0.03 eVps/nm2; the only difference is the permitted critical velocity. Figure γ = 0.05 and 0.07 eVps/nm2, where the critical velocity decreases with an increasing damping coefficient.In all the above simulations, the viscous damping coefficient is et al.32. A small nanoparticle on a commensurate interface may have a much larger in-plane force than a large nanoparticle on an incommensurate interface if the substrate is not pre-tensioned. However, when the substrate is stretched with pre-tension strain, the interface will be always incommensurate. The effect of interface commensurability on the transport behavior of nanoparticles is much weaker than the effect of pre-tension.All the above simulations are carried out at 1 K, which can also be done at a room temperature (300 K). In addition, the effect of the initial interface commensurability on the transport behavior is also investigated. Details of both kinds of effects can be found in Supplementary Material. It shows that the general transport mechanism of nanoparticles is not influenced by the system temperature, however, nanoparticles would perform a small thermal vibration around the equilibrium position in the system at room temperature, similar to the Brownian motion in Jafary-Zadeh Copper material is used for the sliding block in all simulations. Similar driving behaviors would also occur using gold, nickel, or other metal sliding blocks.33. Two perpendicular sliding blocks under the graphene substrate are used to enable more efficient separation.The above technique is further adopted to study the separation of nanoparticles of different sizes but with the same shape. The initial configuration and typical snapshots of the nano-sieve are illustrated in Fig. When the two sliding blocks move reciprocally at 100 m/s several times, the smaller nanoparticles with a side length of 2.04 nm are more easily removed, as shown in Fig. The effect of the sliding velocity of the block on the separation of nanoparticles of different sizes is shown in Fig. Nanoparticles have increasing applications. Transporting and separating nanoparticles have attracted many interesting investigations. In this paper, a new transport technique is proposed based on the van der Waals interaction between nanoparticles and a sliding block. The results show that if the in-plane force between nanoparticles and the substrate is less than or equal to the driving force between the nanoparticles and the sliding block, nanoparticles can begin to move. The subsequent movement of nanoparticles depends mainly on the competition between the driving force and the damping force. The pre-tension in the graphene substrate could reduce the in-plane force, which would be helpful for nanoparticle transport. Effects of nanoparticle sizes and thickness, as well as the speed of the sliding blocks on the transport behavior are investigated in detail. The nanoparticle transport would be easier if the speed of the sliding block is lower. A critical velocity is put forward as a key parameter to describe the transport behavior, below which the nanoparticle can be driven. Generally, the critical velocity decreases with the increase of nanoparticle size and thickness, as a result, transport is easier for smaller and thinner nanoparticles. Furthermore, the transport would also be easier with a lower viscous damping force and a higher temperature. The initial interface commensurability has no obvious effect on the transport behavior, especially in the case with a pre-tensioned substrate. The present technique can be further applied to design a novel nano-sieve, with which nanoparticles of different sizes can be selected.34 is used to describe two different types of interactions: (1) interactions between carbon (C) atoms in the graphene nanoparticle, and (2) interactions between carbon atoms in the graphene substrate. The interactions between copper (Cu) atoms in the sliding block abide by the EAM potential35. The interactions between C atoms in the graphene nanoparticle and C atoms in the graphene substrate abide by the Lennard-Jones potential ε = 2.968 meV and σ = 0.3407 nm36. Similarly, the interaction between C atoms in the graphene nanoparticle and Cu atoms in the sliding block also abides by the Lennard-Jones potential but with ε = 19.996 meV and σ = 0.3225 nm37. The cut-off distance and the time step in all the simulations are set as 1 nm and 1 fs, respectively. We used the open-source Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) software for all simulations, and the temperature was fixed at 1 K with a NVT ensemble38.In Fig. In Fig. Supplementary MaterialSupplementary Movie 1Supplementary Movie 2Supplementary Movie 3Supplementary Movie 4Supplementary Movie 5Supplementary Movie 6Supplementary Movie 7


Onychophorans, commonly known as velvet worms, display a remarkable diversity of reproductive strategies including oviparity, and placentotrophic, lecithotrophic, matrotrophic or combined lecithotrophic/matrotrophic viviparity. In the placentotrophic species, the embryos of consecutive developmental stages are attached to the uterus via a placental stalk, suggesting they might be transported passively towards the vagina due to proximal growth and distal degeneration of tissue. However, this assumption has never been tested using specific markers. We therefore analyzed the patterns of cell proliferation and apoptosis in the genital tracts of two placentotrophic peripatids from Colombia and a non-placentotrophic peripatopsid from Australia.All three species show a high number of apoptotic cells in the distal portion of the genital tract near the genital opening. In the two placentotrophic species, additional apoptotic cells appear in ring-like vestigial placentation zones of late embryonic chambers. While moderate cell proliferation occurs along the entire uterus in all three species, only the two placentotrophic species show a distinct proliferation zone near the ovary as well as in the ring-like implantation zone of the first embryonic chamber. In contrast to the two placentotrophic species, the non-placentotrophic species clearly does not show such regions of high proliferation in the uterus but exhibits proliferating and apoptotic cells in the ovarian stalks. While cell proliferation mainly occurs in stalks carrying maturating oocytes, apoptosis is restricted to stalks whose oocytes have been released into the ovarian lumen.Our results confirm the hypothesis that the uterus of placentotrophic onychophorans grows proximally but is resorbed distally. This is supported by the detection of a proximal proliferation zone and a distal degenerative zone in the two placentotrophic species. Hence, cell turnover might be responsible for the transport of their embryos towards the vagina, analogous to a conveyor belt. Surprisingly, the distal degenerative zone is also found in the non-placentotrophic species, in which cell turnover was unexpected. These findings suggest that the distal degenerative zone is an ancestral feature of Onychophora, whereas the proximal proliferation zone might have evolved in the last common ancestor of the placentotrophic Peripatidae.The online version of this article (10.1186/s12983-019-0317-x) contains supplementary material, which is available to authorized users. Onychophorans, also called “velvet worms” Fig. a–c, are sensu ref. [Mesoperipatus tholloni from tropical Africa, as the reproductive mode and embryonic development are unknown from the latter [Even though the anatomy of onychophorans has changed little since the Early Cambrian , they hansu ref. ), which nsu ref. , 14–23. nsu ref. . The remnsu ref. , 24–30. nsu ref. , 31–33, nsu ref. –38, and nsu ref. , 31. By Among these types of nourishment supply, placentotrophic viviparity is unique, as a similar developmental mode has not been described from any other invertebrate taxon so far , 20. ThiEuperipatoides rowelli from Australia. To detect cell proliferation, we used an antibody directed against phospho-histone H3 as a mitosis marker. Apoptotic cells were detected using the terminal deoxinucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) technique, which labels free 3′-OH ends of the degrading DNA of apoptotic cells [E. rowelli and the placentotrophic viviparous peripatid Principapillatus hitoyensis [However, this hypothesis has never been tested by using specific markers for cell proliferation and controlled cell death –43. In tic cells . Additioic cells . The comtoyensis .E. rowelli and the placentotrophic Peripatidae might provide insights into the evolution of reproductive strategies in onychophorans.The results of this study might help to clarify whether the complementary growth and resorption of uterine tissue is an ancestral feature of all onychophorans or a derived feature of the placentotrophic Peripatidae, which has evolved along with the placental attachment of embryos to the uterine wall. Hence, the comparison of cell proliferation and apoptosis patterns in the genital tracts of sensu ref. [Drosophila melanogaster [E. rowelli, this succession might occasionally be interrupted due to the occurrence of embryos that have not developed beyond stage I . Each hsensu ref. [sensu ref. [The paired tubular ovaries of the placentotrophic Peripatidae have a smooth external surface without externally visible oocytes sensu ref. [E. rowelli bears externally visible oocytes of different sizes, the largest of which are raised above the ovarian surface via stalks  of E. rlks Fig. c. Additinsu ref. ) are absrus Fig. c. Even tE. rowelli  of largIn addition to cell proliferation, we detected apoptosis using TUNEL and anti-caspase-3 labeling Table . In contE. rowelli, except for individual apoptotic cells appearing in the empty stalks of the ovaries , while the tissue is resorbed distally  , 20, thuE. rowelli, whereas a distinct proliferation zone is missing. This indicates that the uterus of E. rowelli is not growing in a defined, proximal region but rather along its entire length, similar to the distal half of the uteri of the placentotrophic species in which the embryos have lost their connection to the uterus and lie freely within its lumen  indicates the presence of a distinct distal degeneration zone.Interestingly, our results further revealed cell proliferation along the entire proximo-distal extent of the paired uteri in the lecithotrophic/matrotrophic peripatopsid men Fig. b [14, 17In summary, our data revealed that cell proliferation occurs along the entire genital tract of all species investigated whereas apoptosis is restricted to a distal degeneration zone. In contrast to this, a distinct proximal proliferation zone is only present in the placentotrophic Peripatidae. Thus, our results indicate that the distal degenerative zone of the uterus is an ancestral feature of Onychophora, while the proximal proliferation zone might have evolved in the last common ancestor of the placentotrophic Peripatidae . SimilarIn addition to the distinct patterns in the uterus, the cell proliferation marker used yielded signal in the ovaries of all investigated species. This signal might originate from oogonia situated within the germinal epithelium, which undergo mitosis and eventually give rise to cells that begin meiosis and become early maturing oocytes . Since tE. rowelli  of the Based on an outgroup comparison with arthropods, some of which display exogenous ovaries –71, it hE. rowelli, in which the embryos are not attached to the uterine wall and thus cell turnover was unexpected. These findings suggest that the distal degenerative zone might be an ancestral feature of onychophorans, whereas the proximal proliferation zone might be a derived feature of the placentotrophic Peripatidae. To test this hypothesis, future studies should focus on the existence of the distal degenerative zone in additional representatives of Peripatopsidae and South-East Asian Peripatidae as well as the proximal proliferation zone in the peripatid Mesoperipatus tholloni from tropical Africa. The occurrence of a distinct pattern of cell proliferation and apoptosis in the stalks of the exogenous ovary of E. rowelli indicates that these processes are involved in the formation and degeneration of stalks. The absence of a corresponding pattern in the endogenous ovary of the placentotrophic Peripatidae confirms the proposed [Our results confirm a previous assumption that the genital tracts of the placentotrophic viviparous Peripatidae is constantly growing at the proximal end while tissue is resorbed at the distal end near the genital opening, thus retaining a constant length of the uterus while the embryos are transported towards the genital opening in a conveyor-like fashion. Surprisingly, we also detected a similar distal degenerative zone  in the genital tract of the lecithotrophic/matrotrophic viviparous species proposed  loss of Euperipatoides rowelli Reid, 1996 (Peripatopsidae) from Australia and two undescribed placentotrophic viviparous species of Peripatidae (Neopatida) from Colombia, gen. sp. 1 and gen. sp. 2, were studied , those stored in PBS with sodium azide were rinsed several times in 1 mL PBS (5 × 5 min). Afterwards, they were incubated in 1 mL of 0.1 M sodium citrate (pH 6.0) for 30 min at 70 °C and rinsed in 1 mL PBS containing 0.3% Triton X-100 (PBS-Tx) at room temperature. Apoptotic cells were detected using the In Situ Cell Death Detection Kit  according to the manufacturer’s protocol. The genital tracts were incubated in 450 μL labeling solution containing 50 μL enzyme solution for 2 h at 37 °C in a hybridization oven with gentle shaking. Positive controls were performed by treating the tissue with 0.25 mg/ml DNAse I (Roche) for 30 min at room temperature prior to the TUNEL labeling procedure. In these controls, all nuclei were TUNEL positive  and pre-incubated in 1 mL 5% normal goat serum  in PBS-Tx for 3 h at room temperature. They were then incubated with the primary rabbit polyclonal anti-phospho-histone H3 (Ser10) mitosis marker  overnight at room temperature with gentle shaking. After several washes with 1 mL PBS-Tx , a secondary antiserum  was applied overnight at room temperature in the dark with gentle shaking. The samples were washed several times with 1 mL PBS-Tx , counterstained with the DNA-selective fluorescent dyes SYBR®Green  or 4′,6-Diamidin-2-phenylindol  for 1 h and mounted between two cover slips in Vectashield® Mounting Medium .For anti-caspase-3 immunolabeling, genital tracts stored in methanol were rehydrated stepwise in PBS at 4 °C , washed several times with PBS-Tx (3 × 5 min) and incubated in 5% NGS for 10 min at room temperature. After replacing the blocking solution with fresh 5% NGS, the genital tracts were incubated in the blocking solution for 3 h at room temperature. Incubation with the primary antiserum  was carried out for three nights at 4 °C with gentle shaking. After several washing steps with 1 mL PBS-Tx , the secondary antiserum  was applied for three nights at 4 °C in the dark with gentle shaking. The samples were washed several times with 1 mL PBS-Tx , counterstained with DAPI (diluted 1:1000 in PBS) for 1 h at room temperature and mounted in Vectashield® mounting medium. The resulting signal was similar to the patterns detected with the TUNEL technique, indicating that the detected signals are specific to apoptotic cells.The samples were analyzed with the confocal laser scanning microscopes Leica TCS 5 STED  and LSM 880 , and the stereomicroscope Axio Zoom V16 equipped with an Axiocam 503 color digital camera (Carl Zeiss MicroImaging GmbH). Confocal optical sections were taken at intervals ranging from 1 to 5 μm and the resulting image stacks were merged digitally into maximum projection micrographs. The colors were adjusted using Leica AS AF, v2.3.5 (Leica Microsystems), and ZEN 2012 SP1 software packages . The stereomicrographs were taken at different focal planes and merged to single projections using the ZEN 2012 blue edition software version 1.1.2.0 (Carl Zeiss MicroImaging GmbH) or the Auto-Blend Layers function in Photoshop version CS 5.1 . All micrographs were adjusted for brightness and contrast using Photoshop CS 5.1. Final panels and diagrams were designed using Illustrator CS 5.1 (Adobe Systems Inc.).Additional file 1:A) or SYBR®Green  are represented in gray. Proximal is up in all images. A Proximal part of the genital tract including the first five uterine chambers containing embryos of subsequent developmental stages. B Distal part of one uterus horn. C Detail of the common duct/vagina. Abbreviations: cd, common duct/vagina; ch1–ch5 and ch7, uterine chambers containing embryos of subsequent developmental stages; d, distal; p, proximal; pz, proximal proliferation zone; ut, uterus. Scale bars: A, B: 500 μm; C: 200 μm. (TIF 7280 kb)Apoptosis in the genital tract of the placentotrophic peripatid gen. sp. 1. Nuclei of TUNEL-positive cells are marked in orange, nuclei stained with DAPI  are marked in cyan. Proximal is up in all images. A Overview of a genital tract of E. rowelli. B Proximal portion of the genital tracts of gen. sp. 1 (left and middle) and distal portion of the genital tract of gen. sp. 2 (right). Abbreviations: cd, common duct/vagina; ch1–ch5 and ch7, uterine chambers containing embryos of subsequent developmental stages; d, distal; dz, distal degeneration zone; oc, oocytes; ov, ovary; p, proximal; pz, proximal proliferation zone; sr, seminal receptacle; ut, uterus. Scale bars: 500 μm. (TIF 5269 kb)Apoptosis and cell proliferation in the genital tracts of Additional file 3:E. rowelli detected with an antibody directed against cleaved caspase-3 (Asp175). Caspase-positive cells are illustrated in orange, cell nuclei stained with DAPI are represented in gray. Proximal is up in all images. A Ovary with stalked oocytes. B Common duct/vagina. Abbreviations: cd, common duct/vagina; oc, oocyte; ov, ovary. Scale bars: A: 500 μm; B: 200 μm. (TIF 2090 kb)Apoptosis in the genital tract of Additional file 4:TUNEL control in the proximal uterus near the implantation zone of the placentotrophic peripatid gen. sp. 1. Note that after the DNAse treatment, all cell nuclei are TUNEL-positive (orange). Abbreviations: ut, uterus. Scale bars: 200 μm. (TIF 1682 kb)


The sinusoidal graphene layer supports continuous plasmonic resonances, forming a broad electric-tuning absorbing band. Bandpass FSS constructs a transmission window outside the absorbing band. The simulation results indicate that the absorption from 0.5 THz to 1 THz can be tuned continuously from 0.4 to 0.9 with angle and polarization independence. A transparent window peaking at 1.65 THz maintains high transmittance over 0.7. The metamaterial absorber has potential applications for detection, stealth, filtering, and electromagnetic compatibility.A graphene-based controllable broadband terahertz metamaterial absorber with transmission band is presented in this paper. It consists of a graphene-SiO Terahertz (THz) absorbers have aroused the interest of more and more researchers due to potential applications for detection, sensing, trapping, communication, and stealth ,2,3,4,5.To sustain the SPPs, the operating bandwidth is generally narrow, which limits the application scope of TMAs. Therefore, the methods to widen the bandwidth have received considerable attention. One means of overcoming this limitation is by integrating several resonators with similar shapes and different sizes into a unit cell so that adjacent resonance peaks are overlapped, thus, forming a broad absorbing band ,15. Anot2 dielectric layer backed with a bandpass frequency selective surface (FSS), which achieves a controllable broad absorbing band with a transmission window. Compared with our previous works in Reference [FE via a voltage bias, the absorption can be regulated continuously. Moreover, adopting a bandpass FSS to substitute the gold ground allows the signals in the passband to transmit with low attenuation and reflects transmitted waves in the absorbing band without affecting the performance of TMA.Here we study the electromagnetic response of a composite structure consisting of a single sinusoidal-patterned graphene layer deposited on a SiOeference , the TMAThe device can be used as an amplitude modulator. When the incident wave operating at the absorbing band impinges obliquely on TMA, the amplitude of the reflected wave can be regulated continuously via tuning the voltage between graphene and FSS to control the absorptivity. The angle independence makes the separation of incident and reflected waves more convenient. In addition, in the telecommunication system, the TMA can function as a powerful spatial filter. It not only absorbs the low-frequency signal with high penetration but also ensures the normal operating signal passes with little loss, which contributes to the anti-interference, electromagnetic compatibility, and signal separation. Moreover, the power of incident wave can be collected by probe and transformed into voltage between the graphene and FSS. Meanwhile, the absorption changes accordingly, and the power of the incident wave is sensed by the TMA. It should be noted that the design scheme can be applied to other frequency regions due to the scalability of metamaterial and the idea of replacing metal ground with FSS is applicable for other kinds of TMA to expand the practical scope.2 spacer in the middle, and a gold bandpass FSS at the bottom. The pattern of graphene consists of two intersecting sinusoidal curves, the functions of which are expressed as follows:p represents the period of the unit cell. The gradually-changed widths support continuous plasmonic resonances excited by the incident wave, thus, forming a broad absorbing band f1. The plasmonic resonances of graphene have polarization independent property and the same period in both x and y directions ensures the polarization independence. The SiO2  parallel resonance so as to construct a passband [p, a, b, and s, respectively. Gold with thickness gt is modeled as a lossy metal material with a conductivity of 4.56 × 107 S/m [f1 is regulated since the chemical potential FE tuned by the voltage changes the conductivity of the graphene layer.As indicated in iO2 = 4, ) with thpassband . The per 107 S/m . ControlBk is the Boltzmann’s constant, e represents the charge of an electron and ℏ = h/2π denotes the reduced Planck’s constant, respectively. FE is the chemical potential (Fermi energy), τ is the relaxation time set as 0.3 ps, T is room temperature set as 300 K and H(Ω) is defined as:FE is regulatable via controlling the bias voltage between patterned graphene and FSS, thus, changing the conductivity of graphene, resulting in the variation of resonant depth of TMA. The reflectance and transmittance are obtained by R = |S11|2, T = |S21|2 . And the absorption is calculated by A = 1 – R − T. All the numerical simulations are carried out by CST Microwave Studio in this paper.The control of absorption should be contributed to the tunability of graphene. Graphene is modelled as infinitely thin layer described by surface impedance ,25. The FE = 0.3 eV, the effective absorption band (absorption > 0.9) ranges from 0.5 THz to 1.0 THz, with a normalized bandwidth of 66.7%. The maximum absorption drops to less than 0.4 as the FE decreases to 0 eV so that a high switching intensity (>50%) can be achieved by electrical control of the graphene. The operating band of TM polarization almost coincided with that of TE polarization since the width variation ranges, which depend on the resonance length and operating band, are the same in both x and y directions. But there are still some small, inconsistent fluctuations in the effective absorption band, which is mainly caused by the different curvature variation of the unit cell in x and y directions. Moreover, the absorption spectrums as a function of the frequency and FE for both TE and TM polarizations have been calculated and displayed in FE increases from 0 eV to 0.5 eV, the absorption is improved gradually and then shows saturate tendency with operating band unchanged. The simulation results show excellent polarization independence and controllable absorption property.First, we simulate the broadband TMA backed with gold film instead of the FSS to better understand the principle and simply the analysis. To better understand the principle of broadband absorption, the distributions of surface current intensity are plotted in Except the tunability, the angle independence is also one of the important characteristics for TMAs exploiting plasmon resonance of graphene . Figure 2 substrate, and the spectrum response is displaced at the top of FE = 0 eV, a partial absorbing band with 0.4 absorption peaks at around 0.63 THz and the passband locating at 1.65 THz still sustains transmittance over 0.9. As FE increases to 0.3 eV, there is a broad effective absorption band ranging from 0.5 THz to 1 THz, but the transmission of the passband decreases to 0.7. As illustrated in Based on the angle and polarization independent TMA, we adopt the bandpass FSS to replace the gold ground so that an extra transmission band takes shape, thus, expanding fields of application. In the integration design, the substrate in front of the FSS leads to the drop of resonant frequency . TherefoFE = 0.3 eV. As shown in 2-FSS sandwich structure.Further, for TE polarization, the electric field intensity distributions along the propagation direction at 0.6 THz and 1.65 THz are given in 2 substrate. The angle-independent broadband absorption is formed by continuous plasmonic resonances supported by the gradient widths, and the same period in both x and y directions ensures the polarization independence. The absorptivity can be tuned from 0.4 to 0.9 over 0.5 THz to 1 THz via electrostatically controlling chemical potentials of graphene. Substituting the traditional metal ground with bandpass FSS, a transmission band appears at 1.65 THz without affecting the performance of absorbing band. As FE increases from 0 eV to 0.3 eV, the passband sustains high transmission over 0.7. Due to the controllable absorption along with the transmission window, the design can widen the application of TMA such as electromagnetic compatibility, stealth, and filtering relying on the anti-jamming capability and low loss for the normal signals. Although the transmission of the window decreases slightly with the increase of chemical potential, it can be avoided by further optimizing the structure to reduce the loss caused by the graphene layer. Our following work will focus on increasing the number of channels and explore the method to control the switch of the transmission band.In summary, we demonstrate a tunable broadband metamaterial absorber with a transmission window, which is constituted by a sinusoidal graphene layer and a gold FSS separated by a SiO


In this sensing system, the graphene woven mesh is embedded inside ionic polymer membrane to interact with mobile ions and to trace their movements. The migration of mobile ions inside the membrane induces an electrical signal on the mesh and provides the information regarding ion distribution, which is proven to be highly correlated with the bending deformation of the actuator. Using this integrated self‐sensing system, the responses of an ionic actuator to various input stimulations are precisely estimated for both direct current and alternating current inputs. Even though the generated displacement is extremely small around 300 µm at very low driving voltage of 0.1 V, high level accuracy (96%) of estimated deformations could be achieved using the self‐sensing actuator system.Here, inspired by mechanoreceptors in the human body, a self‐sensing ionic soft actuator is developed that precisely senses the bending motions during actuating utilizing a 3D graphene mesh electrode. The graphene mesh electrode has the permeability of mobile ions inside the ionic exchangeable polymer and shows low electrical resistance of 6.25 Ω Sq A graphene mesh electrode having high ion permeability and electrical conductivity is adopted as a self‐sensing ionic soft actuator inspired by mechanoreceptors in the human body. The graphene mesh electrode provides electrical signals sensitive to ion migration inside the polymer electrolyte without blocking their movements and is used to predict the bending deflection of the actuator. The main drawback of ionic soft actuators is their unpredictable responses and highly nonlinear behavior.In fundamental studies, the mechanical displacement of ionic actuators is usually measured through external sensors, such as laser sensors or strain gauges. However, parallel implementation of external sensors with an actuator is not possible in real applications because of cost and space limitations. Accordingly, some research groups are focusing on the fabrication of self‐sensing actuators, in which sensing and actuation are integrated within a single component for real‐time monitoring of the actuator movement. Most of the currently available mechanisms of self‐sensing actuators require the simultaneous measurement of the electrical resistance along the actuator electrode during actuation. When the actuator is stimulated, one of the electrodes will be stretched while the other is compressed. Since the electrical resistance varies with the effective length of the electrodes, one is able to measure the bending deformation in the form of curvature or tip displacement.However, inferring the resistance values by measuring the voltage drop along an electrically active component is an intricate task.In principle, various kinds of mechanoreceptors in the human body work by changing the ion equilibrium in receptor cells.A 3D graphene woven mesh is a perfect candidate for the design of an ion‐permeable sensing electrode due to its liquid permeability and high electrical conductivity. An established way to monolithically assemble graphene woven mesh at a large scale is to grow graphene by chemical vapor deposition (CVD) method on metallic meshes.In this study, we propose a self‐sensing ionic soft actuator utilizing 3D graphene woven mesh as a signal collector. This graphene mesh electrode is composed of a monolithic network of hollow graphene tubes assembled in a mesh‐type architecture. The high electrical conductivity of graphene, along with its chemical stability, facilitates a neat and stable data acquisition during the deformation of the actuator. In addition, the wavy pattern of graphene tubes in the mesh‐type structures renders this electrode flexible and stretchable, which are crucial requirements for implementation in ionic soft actuators. The proposed graphene mesh is a plane weaved mesh that contains multiple arrays of large holes. While collecting data, the graphene mesh electrode does not impede ion motion, such that ions can freely migrate under the imposed electric field. Not only is this measurement technique beneficial in estimating the mechanical deformation of soft ionic actuators, but it could also be employed for measuring the ionic conductivity of polymer membranes and for real‐time monitoring of ion migration within ionically conductive polymers.22.1Scheme+) into the neuron cell. The presence of extra Na+ ions alternates the potential of the neuron cell and provides the action potential for firing the neuron (Scheme The sensing mechanism of the proposed self‐sensing actuator is inspired by the reception function of human mechanoreceptors. As shown in n Scheme a.m hydrochloric acid (HCl) solution. Through layer‐by‐layer casting, the as‐prepared graphene mesh was later embedded inside a Nafion/1‐Ethyl‐3‐methylimidazolium tetrafluoroborate (EMIM‐Bf4) membrane. Using drop casting, the graphene‐embedded membrane was then sandwiched between two poly polystyrene sulfonate (PEDOT:PSS) layers as actuator electrodes. More details about material synthesis and fabrication steps can be found in the Experimental Section and Supporting Information. In the aforementioned layout, the embedded graphene mesh acts as a sensing electrode that apprehends ion movements within the polymer electrolyte membrane, without hindering the ions' migration. Indeed, large holes (≈160 µm) in the graphene mesh provide a path for ion migration, such that ions can freely move in response to the applied electric field  woven mesh was selected as a template for growing the graphene layer. The CVD method is based on catalytic growth of graphene; among a variety of transition metals, Ni has been shown to be a promising substrate for growing multilayer graphene because it allows for growth of large‐scale crack‐free graphene.FigureThe as‐prepared graphene mesh was first characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images in GI/I2D) in Raman spectra. Based on the literature, the intensity of the G and 2D peaks of graphene can be used for determining the number of layers.−1) to that of the 2D peak (at around 2750 cm−1) increases. As illustrated in Figure GI/I2D ratio does not notably vary through the surface, signifying that the thickness of graphene remains similar on the whole surface. These mapping data indicate uniform graphene growth through CVD, which results in enhanced electro‐mechanical properties of the sample. Figure −1) at all four points. Since a D peak in Raman spectra is an indication of defects in graphene sheets, the absence of this peak at all points reaffirms the growth of high quality graphene.To further examine the uniformity of the synthesized graphene, Raman spectra were captured from a 10 µm × 10 µm area of the sample. Figure Figurem HCl solution (a highly oxidative medium). As explained earlier, the sample was then subjected to 1000 CV cycles in EMIM‐Bf4/Acetonitrile solution (0.5 m) to inspect the chemical stability of the graphene mesh in the electrochemical reactions that might be involved in the actuation process. The elemental mapping of this sample did not show a meaningful change in the oxygen content when compared with the sample before CV  on three samples: graphene coated Ni mesh right after CVD, pristine graphene mesh after etching with Ni, and graphene mesh after being subjected to 1000 cycles of cyclic voltammetry (CV). The results of these analyses are presented in V Figure c. This cFigure−1 even at 180° bending deformation. This resistance value is prominent when compared to other previously reported graphene electrodes, pointing at a high electrical conductivity of the proposed electrode.Thereafter, employing layer by layer casting of the Nafion/EMIM‐Bf4 5:3 wt) solution and drying, the graphene mesh electrodes were embedded inside electrolyte membrane. To study the effect of the graphene mesh on the mechanical properties of the electrolyte membrane, a tensile test was conducted. As illustrated in Figure  wt solutFollowing the characterization of the proposed composite membrane, the actuator was fabricated by drop‐casting of PEDOT:PSS electrodes on both sides of the membrane. Details about actuator fabrication steps and wiring of the electrodes are provided in the Experimental Section, as well as Supporting Information. The SEM image taken from the cross‐sectional view of the actuator is displayed in Figure 2.2FigureAfter preparation, the actuator was tested for sensing‐actuation performance by applying an input electric potential across its electrodes. While performing the actuation test, we measured the absolute potential at the embedded graphene mesh electrode and the tip displacement of the actuator by a laser sensor to provide a reference value. To better illustrate the sensing mechanism, a schematic diagram of the proposed self‐sensing actuator is provided in Vin), one of the actuator electrodes experiences an absolute potential of zero (anode), while the other electrode experiences an absolute potential of Vin (cathode). This potential difference generates an electric field through the membrane, which induces an absolute potential on the graphene mesh electrode, located between the actuator electrodes. Such an induced potential is a fraction of the input voltage, and it depends on the location of the mesh with respect the anode. At the same time, the electric field inside the electrolyte membrane causes the differential migration of the cations and anions, which, in turn, generates a uneven charge distribution along the thickness of the membrane that manifests into a varying electrical potential on the graphene mesh electrode . As sketched in Figure By applying a constant input potential  and ions‐induced potential (Vion). These signals are also shown in Figure Consequently, the sensing signal can be decomposed into its contributions: the electrodes‐induced potential :A closer look at Figure By substituting Equation SM is the estimated tip displacement of the self‐sensing actuator. Beyond supporting evidence by the circuit model, the proposed explanation can be grounded in the distributed model of Nemat‐Nasser and Li,By multiplying the ions‐induced signal by another constant, one can estimate the actuator displacement.C1 and C2 can be estimated from one set of signals acquired from a specific input stimulus and then utilized to explore other inputs. Figure Employing linear regression, the coefficients SM) and only use it for DC inputs. To adjust the model for dynamic inputs, a phase shifting term could be added, by considering a differential form of the static model (see Supporting Information)DM and δSM are the tip displacements of the actuator estimated through the dynamic and static Models, respectively. In this equation, ω is the frequency of the harmonic input potential, while φ is the phase delay of the static model. As in the static model, the coefficients C1 and C2 along with the phase shift φ in the dynamic model can be estimated from regression for a specific input potential and then used for other inputs.While the proposed model works adequately for DC inputs, it shows sluggish performance for dynamic inputs such as AC voltage. As shown in Figure FigureAs shown in C2 and phase shift φ needed to be modified for each case, implying the frequency‐dependency of these parameters. As illustrated in Figure C2 and φ are described by logarithmic functions with respect to the frequency of the input stimulation. This could be expected due to interplay of damping and inertia effects in the actuator, which is modulated by the excitation frequency.The other influential parameter affecting the estimation model is the frequency of the input voltage. Figure In addition to sine wave inputs, the as‐prepared self‐sensing actuator was also tested for square wave and triangle wave inputs Figure e,f. SimiIn order to inspect the life‐time stability of the proposed self‐sensing actuator, the samples were again tested after 2 months to check if their performance is degraded or not. However, as presented in Figure S17, Supporting Information, no meaningful change was observed neither in output displacement of the actuator nor in sensing signal. After 2 months, the sensing signal was still very neat and clean without any noise or disturbance.3Inspired by mechanoreceptors in the human body, we developed an innovative self‐sensing ionic soft actuator using graphene woven mesh electrode. The proposed 3D graphene electrode showed outstanding multi‐functional attributes, such as flexibility, high electrical conductivity, and ion permeability. Embedding a high‐quality graphene mesh inside the electrolyte membrane could afford a novel self‐sensing ionic actuator that could precisely measure the displacement during actuation. In the suggested bio‐inspired sensing mechanism, the sensing signals were generated by mobile ions inside an electrolyte membrane, similar to various types of mechanoreceptors in the human body. The signals, collected by graphene mesh, contained the information regarding the distribution of ions within the membrane and was properly correlated with the bending deflection to estimate the tip displacement of the actuator. Using the sensing data, semi‐empirical models were proposed to estimate the tip displacement for both static and dynamic electrical stimulations. These static and dynamic models properly predicted the tip displacement of the actuator at a variety of input potentials.The obtained results offer compelling evidence in favor of this newly designed self‐sensing actuator. Not only can the proposed graphene mesh electrode and sensing mechanism be used for new types of self‐sensing actuators, but also for many other applications, such as measuring ionic conductivity of polymer membranes or monitoring ion movement inside solid‐liquid electrolytes in real‐time. Further investigation of this approach could pave the way for rigorous electrochemical studies of electrolytic membranes that could improve our understanding of ions' interactions.4Graphene Mesh Synthesis: A plane woven Ni mesh with a pore size of 155 µm was selected as a template for graphene growth. Ni mesh substrates were cut into 1 cm × 4 cm samples and washed with ethanol. The substrates were lunched in a long quartz tube (1.5 m) and heated to 1000 °C in ambient pressure and Ar atmosphere (1000 s.c.c.m.). After reaching the reaction temperature, the samples were kept in this condition to remove the oxides and other impurities from the surface. In order to initiate the growth, reaction gases  were flowed into the tube while decreasing Ar flow rate to 400 s.c.c.m. After 10 min, the H2 and CH4 flows were stopped and the furnace was removed from the samples to swiftly cool them to room temperature. The Ni substrate was etched from graphene coated samples using 3 m HCl solution while keeping samples in the oven (80 °C) for 24 h. Afterward, floating graphene samples were washed with DI water three times to remove the acid and then dried by freeze‐drying technique at −50 °C and 14 Pa pressure.Fabrication of Self‐Sensing Actuator: First, Cu foil was attached to one end of graphene mesh using silver paste. Then, the electrolyte solution was prepared by dissolving 3 g Nafion and 1.8 g EMIM‐Bf4 in 30 mL Dimethylacetamide (DMAc). To achieve a well‐dissolved solution, the mixture was stirred in an oil bath (80 °C) for 24 h. Then, 2 mL of the electrolyte solution was casted in a circular glass Petri dish (diameter: 55 mm) and dried in the oven to remove the DMAc. Afterward, the as‐prepared graphene electrode was placed on the dried membrane and 0.5 mL of the electrolyte solution was drop‐casted on the graphene mesh and dried to fix the mesh in that position. Another 4 mL of electrolyte solution was casted on the dried membrane to fully embed the graphene mesh electrode inside the membrane. After obtaining the membrane with embedded graphene mesh, it was sandwiched between actuator electrodes by drop‐casting DMSO/PEDOT:PSS (5 vol%) on both sides of the membrane. Finally, the actuators were cut into favorable sizes and the sensing‐actuating measurements proceeded.Material Characterization: Micro‐scale surface morphology of the graphene mesh and cross‐sectional view of the actuator were obtained using a Field Emission Scanning Electron Microscope (Magellan400) made by the FEI Company. This microscope was equipped with an Energy‐dispersive X‐ray spectroscopy system that was used for elemental mapping of the samples. Nano‐scale surface morphology and cross‐sectional views of graphene sheets were obtained using a Field‐emission transmission electron microscope (300 kV Tecnai G2 F30 S‐Twin) made by the FEI Company. High‐resolution images of the graphene mesh embedded in the electrolyte membrane were captured using an optical microscope (ECLIPSE LV100D‐U) made by the Nikon Company. Raman spectra analysis was carried out using a High‐Resolution Raman Device (LabRAM HR Evolution Visible‐NIR) made by HORIBA. Mechanical properties of electrolyte membrane were determined using a Universal/Tensile Test Machine (AGS‐X) made by SHIMADZU Corporation. Two‐probe resistance measurement of the graphene mesh was executed on a Keithley Multimeter (DMM7510).Actuation Measurements: To perform the sensing‐actuating measurements, the as‐prepared actuator was clamped at one end using two electrodes that were connected to the power supply in order to transfer the driving input voltages to the actuator electrodes. The sensing signal was collected from the graphene mesh electrode at the same end. The tip displacement of the actuator was recorded at the free end using a High‐Accuracy Laser Displacement Sensor (LK‐H080) produced by KEYENCE Corporation. All signals were simultaneously acquired using a Data Acquisition System  made by National Instruments.The authors declare no conflict of interest.SupplementaryClick here for additional data file.


In this study, a label-free graphene-based fluorescence probe used for detection of volatile organic liquids was fabricated by a simple, efficient and low-cost method. To fabricate the probe, a bio-based β-cyclodextrin (β-CD) was firstly grafted on reduced graphene surfaces effectively and uniformly, as evidenced by various characterization techniques such as Ultraviolet/Visible spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and transmission electron microscopy. The subsequent inclusion of Rhodamine B (RhB) into the inner cavities of the β-CD grafted on the graphene surfaces was achieved easily by a solution mixing method, which yielded the graphene-based fluorescent switch-on probe. In addition, the gradual and controllable quenching of RhB by Fluorescence Resonance Energy Transfer from RhB to graphene during the process of stepwise accommodation of the RhB molecules into the β-CD-functionalized graphene was investigated in depth. A wide range of organic solvents was examined using the as-fabricated fluorescence probe, which revealed the highest sensitivity to tetrahydrofuran with the detection limit of about 1.7 μg/mL. Some insight into the mechanism of the different responsive behaviors of the fluorescence sensor to the examined targets was also described. This is owing to its unique physical and chemical properties , such as2+ , Hg2+ [12+ [2+ ,18, Pb2+ [2+ [2+  and K+ [+ [2+ [+ ,21, and + [2+ [+ , RNA [12+ [2+ [+ ,14,23,24+ [2+ [+ , cystein+ [2+ [+ , biotin + [2+ [+ , thrombi+ [2+ [+ , matrix + [2+ [+  and chol+ [2+ [+ . Despite+ [2+ [+ . The wid+ [2+ [+ . On the + [2+ [+ , indicatThe widespread use of GO as the basic platform is due to its high yielding synthesis  and its stable colloid nature in aqueous solution favored by hydrophilicity and electrostatic repulsions of the abundant oxygen functionalities . This waTo maintain the main advantages of GO in the fabrication of a graphene-based fluorescence probes, such as low cost, high yield and water solubility, it is expected that the graphene will be prepared from GO by a reduction/deoxygenation process. However, removal of the oxygen functional groups from GO surfaces via various reduction approaches to prepare reduced graphene results in an irreversible aggregation and agglomeration, and even restacking to the bulk graphite structure, which severely limits the applications of the thus-reduced graphene . This cai.e., the resulting fluorescence probe or denoted as CD-G-R. A wide spectrum of organic liquids, including water-soluble and -insoluble organic solvents, were subsequently employed as the target molecules to attack CD-G-R in order to examine the sensitivity and selectivity of CD-G-R to these molecules. To the best of our knowledge, this is the first report of the use of a graphene-based sensing platform for detection of the small organic molecules in liquid form, even though a large body of work has been dedicated to the detection of various metal ions and biomolecules using GO- or graphene-based sensing systems, as mentioned before. The present strategy for fabrication of the sensing platform CD-G-R is very simple, efficient, and cost-effective. Moreover, high fluorescence turn-on sensitivity and fair selectivity to tetrahydrofuran (THF) can be observed. THF is a polar ether and is a widely used solvent for many applications such as processing solvent, polymer production, and component in stripping fluids, cleaning fluids, functional fluids, coatings, lacquers and corrosion inhibitors [In this work, a bio-based β-cyclodextrin (β-CD) is utilized as the covalent modifier to fabricate a functionalized graphene with high water solubility by a one-pot functionalization and reduction process. The β-CD-modified and functionalized graphene will hereafter be called CD-G. The as-prepared CD-G can be used directly as the sensing platform that can quench the fluorescence dye efficiently by inclusion of the dye molecules into the CD-G and hence provide close contact of the dye with the graphene moiety, a super nanoquencher. By selectively driving the dye molecules out of the CD-G cavities based on host-guest competition interactions with given target molecules, a sensing probe can be constructed using CD-G as the basic platform. In the present work, Rhodamine B (RhB) was adopted as the fluorescence reporting molecule for CD-G to form an inclusion complex of CD-G and RhB, hibitors . A notabhibitors . Acute thibitors  and carchibitors  have beehibitors ,46 and ahibitors  have alsThe photo images and schematic illustrations given in −1 (C–O in C–OH/C–O–C) [−1 can be indexed to O–H, C–H, O–H , and C–O–C groups, respectively [The structure characterization results for GO and CD-G are shown in H/C–O–C) . This inectively . −1 (which can be attributed to the enhanced benzene ring vibrations of CD-G), suggesting the effective deoxygenation and restoration of conjugation structure of graphene. In addition, the absorption peak stemming from water becomes much smaller and blue-shifts to higher wavenumber . This is likely due to the high coverage of β-CD on the graphene surfaces, thus changing the water adsorption behavior of graphene to that of β-CD. Because of the disturbance effect of the graphene moiety on the β-CD grafted on CD-G planes, this absorption peak turns to be smaller as compared to that for pure β-CD. It is worth mentioning that the absorption corresponding to O–H group is blue-shifted from about 3,333 (for GO) and 3,399 cm−1 (for β-CD) to 3,429 cm−1 for CD-G, probably caused by the intermolecular interactions between GO and β-CD such as hydrogen bonding [d001/spacing of 0.83 nm, being much larger than that of graphite (0.34 nm) due to the incorporation of abundant oxygen functionalities) [After chemical reduction with hydrazine in the presence of β-CD, the absorptions assigned to carboxyl group, disappears completely and a new absorption band arises at approximately 1,556 cm bonding . The X-ralities) . It is nalities) . This alalities) . This isThe surface morphologies of GO and CD-G are observed by scanning electron microscopy (SEM), with the images shown in v/v). To completely quench the RhB fluorescence monitored by fluorescence spectroscopy, the mixing ratio was further increased to 35/100.The CD-G is used firstly as the nanoquencher to quench the fluorescence of the RhB solution and yield the inclusion complex of CD-G and RhB. The results are presented in Using the completely quenched inclusion complex of CD-G and RhB as the starting sensing platform, the subsequent detection of different organic liquids are performed. A range of water-soluble organic solvents was firstly detected, with the results shown in The C1-C3 monohydric alcohols, that is methanol, ethanol, and isopropanol, can turn on the fluorescence to a decent extent observable by the naked eye . The C2 dihydric (ethylene glycol) and C3 trihydric (glycerol) alcohols show a lower fluorescence switch-on efficiency as compared to the C1-C3 monohydric alcohols. This can be attributed to the fact that the increased hydroxyl groups impart higher polarity to the ethylene glycol and glycerol, which reversely reduce the competition with RhB to accommodate into CD-G because of its higher affinity to the hydrophilic outside wall (with abundant hydroxyl groups) than to the hydrophobic inner cavity of β-CD. Acetone and acetonitrile have a comparable turn-on efficiency relative to C2 and C3 monohydric alcohols, in this case ethanol and isopropanol, respectively. In addition, DMF and DMA have a similar turn-on capacity for the quenched RhB to the C1 monohydric alcohol, namely methanol (DMF shows a little higher capacity than DMA according to the fluorescence spectra). DMSO exhibits a parallel turn-on effect as compared with C2 dihydric and C3 trihydric alcohols. Moreover, THF shows an overwhelming performance over all the other examined water-soluble organic solvents, as well as over all the examined water-insoluble organic solvents . The best match with the molecular structure of inner cavity can be used to explain for the best performance of THF among the examined organic liquids since the inner surface structure of β-CD lined with the cyclic ether-like anomeric oxygen atoms and the C3-H and C5-H hydrogen atoms see  is closeTo summarize the fluorescence switch-on effect of all the organic solvent investigated in the present study, a comparison histogram of the emission intensity at about 577 nm is plotted against the mixing system with different organic liquids, with the result presented in Considering that the graphene-based sensing platform CD-G-R has a selectivity for THF among the organic liquids examined in the present study at the given excitation wavelength and scanning range. The subsequent calculation of LOD is carried out by monitoring the fluorescence spectra of CD-G-R system upon the addition of increasing concentration of THF (0–44.5 mg/mL), with the result shown in 0) for detection of THF is given as the inset of 0 and F are the fluorescence intensity in the absence and presence of THF, respectively. Error bars in the calibration curve were obtained from five parallel measurements. The pristine CD-G-R shows nearly no emission in the detected range, probably indicating that all the RhB molecules have accommodated in CD-G and thus lead to a close contact with graphene moiety and highly effective quenching of the fluorescence by FRET from RhB to the graphene. The CD-G-R showed a good linear response of the relative fluorescence intensity with respect to the THF concentration in the range of 0.218–17.5 mg/mL (r2 = 0.98816) as shown in the inset. The LOD measured as three times the signal of the blank is found to be about 1.7 μg/mL.The calibration curve of the relative fluorescence  was supplied by Oriental Chem. and Lab. Supplies Ltd. . β-cyclodextrin  was purchased from Aladdin Chemistry Co. Ltd. . Rhodamine B  was obtained from Sigma-Adrich. All other chemicals were purchased from Oriental Chem. and Lab. Supplies Ltd.  and used as received.Graphite fine powder (E.P.) was obtained from Tianheng Technology Co. Ltd. . Hydrazine hydrate of A.R grade (N3 (1.0 g) were placed in a flask (250 mL), followed by addition of concentrated H2SO4 (50 mL) in small portions under magnetic stirring and cooling by an ice bath. The mixture was kept stirring at 5 °C for 2 h, and then KMnO4 (7.3 g) was added portionwise under cooling and continuously stirring conditions to maintain the temperature of the suspension lower than 20 °C. Afterwards, the temperature was increased to 35 °C, and the reaction mixture was stirred continuously at this temperature for 30 min. DI water (90 mL) was then added gradually, and the mixture was stirred for another 15 min. Additional DI water (55 mL) was added, followed by slow addition of H2O2  for the purpose of reducing the residual permanganate to soluble manganese ions. Finally, the mixture was filtered and washed thoroughly with diluted HCl , DI water (150 mL) and methanol (150 mL), and then vacuum-dried at 50 °C for 24 h.GO was synthesized by a modified Hummers method as described elsewhere . BrieflyAs-prepared GO powder (30.0 mg) and DI water (60.0 mL) were placed in a beaker, followed by sonication treatment for 1 h, which yielded a homogeneous yellow-brown suspension of graphene oxide (0.5 mg/mL). Separately, β-cyclodextrin (1.6 g) was added to a flask loaded with DI water (40.0 mL), and then magnetically stirred at 50 °C to dissolve the β-cyclodextrin. Afterwards, a portion of graphene oxide suspension (24 mL), ammonia (0.8 mL) and hydrazine hydrate (25.0 μL) were homogenized into the β-cyclodextrin solution, followed by a reaction at reflux under constant stirring at 80 °C for 3.5 h, leading to a black dispersion. The as-formed dispersion was charged into a dialysis bag for dialysis treatment against flowing DI water for 3 days, further washed repeatedly with DI water (50 mL) followed by ethanol (50 mL), vacuum-filtered using a nylon membrane with 0.22 um pore size, and finally dried under vacuum at 50 °C for 24 h. The resulting black powder was stored in a desiccator prior to use.The as-synthesized CD-G powder (26.5 mg) was homogenously dispersed into DI water (100 mL) by ultrasonication for 1 h. The obtained CD-G dispersion (0.265 mg/mL) was employed as the fluorescence nanoquencher. The fluorescence solution of RhB (50 nM) was prepared by dissolving the fluorescence dye RhB (0.012 mg) in DI water (500 mL). The CD-G dispersion (0.265 mg/mL) was mixed into the RhB solution  in small portions by mild stirring each for 3 min to allow the sufficient interactions between RhB molecules and CD-G sheets. The gradual quenching process can allow monitoring of the RhB fluorescence quenching behavior by taking the digital images and recording the fluorescent spectra. The final inclusion complex of CD-G and RhB, with completely quenched fluorescence determined by the fluorescence spectroscopy, was used as the fluorescence switch-on probe for detection of a wide range of organic liquids.2 atmosphere at a heating rate of 10 °C/min. The surface morphologies were observed by a field emission scanning electron microscopy . TEM images and SAED patterns were obtained by a Jeol JEM-2011 TEM facility at an acceleration voltage of 100 kV. The specimens for the TEM observation were prepared by dropping a diluted ethanol dispersion of the GO and graphene specimens onto a silicon wafer, followed by drying at room temperature. Fluorescence spectra were recorded on a Perkin-Elmer Luminescence spectrometer (LS50B), which was equipped with a motor-driven linear polarizer on the detection side, at room temperature under isotropic excitation.UV/Vis spectra were measured on a Lambda 18 UV/Vis spectrometer. FTIR spectra were recorded on a FTIR spectrometer (PerkinElmer System 2000) in KBr pellet mode. The powder XRD patterns were collected by a Bruker D8 Advance X-ray diffractometer . TGA tests were performed on a Mettler Toledo TGA/SDTA851 under NAn organic liquids-responsive graphene-based fluorescence probe has been fabricated in the present study by a simple, efficient and cost-effective method. To fabricate the probe, a one-pot chemical reduction and functionalization of GO with hydrazine in the presence of β-CD has been firstly conducted, which leads to the β-CD-modified and functionalized graphene. The β-CD has been demonstrated to be grafted on the graphene surfaces effectively and uniformly, as evidenced by various characterization techniques such as UV/Vis, FTIR, XRD TGA, SEM and TEM. The subsequent fabrication of the fluorescence probe has been achieved by inclusion of RhB molecules into CD-G using a facile and economical solution mixing method. The RhB fluorescence can be quenched controllably based on FRET from RhB to graphene through changing the mixing ratio of CD-G to RhB. A wide range of organic solvents including water-soluble and -insoluble ones has been tested using the fluorescence probe, which shows the highest sensitivity to THF, with the LOD of about 1.7 μg/mL. Some insight into the mechanism for explaining the different fluorescence responses of the probe to the examined targets has been provided. This work thereby shows a great significance for sensing and detection applications in a relatively unexplored field of detecting organic liquids using graphene-based platform, especially for the waste water containing THF contaminant, e.g., solvent leakage detection of tanks and piping systems. Based on the fluorescence spectroscopy, quantitative analysis of organic solvent concentration in water can also be realized using our developed graphene-based sensor. This work on the fabrication of the label-free graphene-based fluorescence probe by a simple, efficient and cost-effective strategy has opened up new avenues for development of other effective graphene-based probes for various sensing, monitoring and detection applications.


Three kinds of multifunctional graphene metasurfaces based on Pancharatnam–Berry (PB) phase cells are proposed and numerically demonstrated to control a reflected wave’s spin angular momentum (SAM) and orbital angular momentum (OAM) in the terahertz (THz) regime. Each proposed metasurface structure is composed of an array of graphene strips with different deviation angles and a back-grounded quartz substrate. In order to further help readers have a deeper insight into the graphene-based metasurfaces, a detailed design strategy is also provided. With the aid of the designed graphene elements, the proposed metasurfaces can achieve the full 360° range of phase coverage and provide manipulation of SAM and OAM of a circularly polarized (CP) wave at will. More importantly, simultaneous control of these two momentums can also be realized, and in order to demonstrate this function, a THz spin-controlled OAM beam generator with diverse topological charges is created, which can provide one more degree of freedom to improve the channel capability without increasing the bandwidth compared to a linearly polarized (LP) OAM beam. Numerical results verify the proposed graphene metasurfaces, which pave the way for generating spin OAM vortex waves for THz communication systems. In recent years, controlling spin angular momentum (SAM) and orbital angular momentum (OAM) of the electromagnetic waves at will has attracted considerable attention from the scientific community due to the unprecedented potential of polarization multiplexing ,2 for coFortunately, graphene, a honeycomb-like structure of monolayer carbon atoms, has been found and viewed as a promising material for THz components and applications. Due to the unique monolayer structure, graphene has a weak interaction with incident waves and supports the spreading of surface plasmon polaritons (SPPs), which leads to a smaller surface loss compared with metallic metasurfaces in the THz regime. Furthermore, different from metallic cells depending on geometric variations, the electromagnetic responses of graphene elements can be tuned by chemical doping or electrical gating . In virtIn this paper, we propose three kinds of multifunctional graphene metasurfaces based on the PB phase to arbitrarily control the SAM and OAM of a THz reflected wave, as shown in This paper is organized as follows: in 2/(V·s) are assumed [The conductivity of graphene is characterized by the sum of intra-band conductivity de model :(1)σintr 0.64 eV  and the  assumed . The surPhase and polarization are important features for controlling electromagnetic waves, and they directly affect the SAM and OAM of radiated beams, respectively. Based on the PB phase principle, metasurfaces can allow variations in polarizations and phases only by rotating the angles of the elements. The PB phase  cells caa) and a width of 3.2 μm (b), deposited on the square element with sides of 14 μm (s). The thickness of the back-metalized quartz substrate is 26 μm (h), the relative permittivity is 3.75 (2) [Different from the traditional meta-atoms of metasurfaces, which introduce abrupt phase jumps by changing the geometric parameters of cells, the proposed graphene element can easily achieve the desired phase of 2.75 (εr)  and the wn in 2)  is used On the basis of the graphene and PB phase method, in order to realize the manipulation of the incident CP wave, each element designed with graphene should possess an approximately 180° phase difference between x and y linearly polarized waves. Meanwhile, considering the influence of graphene surface losses, reflection coefficients for two polarized waves should be controlled to within an acceptable range. As depicted in In order to verify the results of the unit cell in HFSS, another modeling tool CST has also been used to simulate the same unit cell, as shown in In this proposal, although normal illumination is used as the main way, an oblique incidence is also allowable. However, due to the strict condition of co-polarization conversion, which requires that the phase difference between x-polarized wave and y-polarized wave approach 180° ± 10°, the bandwidth will reduce greatly as the oblique incident angle increase. Through the simulation validation, we found that the range of oblique incidence was only within 10° for the graphene-based unit cell, while the operation bandwidth that approaches 180° ± 10° phase difference decreases to 0.1 THz (1.3 ~ 1.4), as shown in In order to observe the variation of reflection amplitude and phase when the incident wave changes from 0° to 10°, the related simulation results are provided as follows. The phase differences and reflection amplitudes between two line-polarized waves under the different incident angles are depicted in Not only can the way of normal illumination provide a wider bandwidth (1.36 ~ 1.62 THz), but it also ensures the co-polarization conversion of the reflected wave. On the other hand, some references suggest that an offset feeding configuration should be avoided, because an amplitude null is in the center of the vortex beam of OAM. This is the smallest blocking effect on the reflective OAM vortex wave using normal incidence ,52. Cons(1)Firstly, the operation frequency band and the characteristics of the graphene should be considered. The conductivity of graphene includes two parts, i.e., the intra-band part and the inter-band part, and the influence of these on graphene is different in the low-frequency band and the high-frequency band. In our design, a low-frequency band (1.36–1.62 THz) is chosen; therefore, the conductivity of graphene is only determined by the intra-band part. By using Equation (1), a complex conductivity(2)Secondly, a unit cell composed of the graphene strip and a quartz substrate backed with a metal ground should be modeled in the HFSS software. In the simulation environment, a strip with an impedance boundary condition is adopted to represent the graphene strip. Due to the fact that the graphene’s impedance can be expressed by its conductivity, i.e.,(3)After that, parameter optimization should be executed during the simulation. The size parameters of the unit cell will not be adjusted until the phase difference between the x-polarized wave and the y-polarized wave is approximate 180° ± 10°, which is a necessary condition for realizing co-polarized conversion of reflected waves. Meanwhile, the reflection amplitude is also considered during the optimization so as to ensure the reflection efficiency. Once the aforementioned conditions have been created, the unit cell can act as the basic element for the graphene metasurface.(4)Once the unit cell has been designed, the phase distribution of the whole metasurface should be taken into account. The function of the metasurface is mainly determined by the phase distribution of the unit cells, and the values of phase distribution can be calculated by Matlab.(5)According to the phase distribution of the metasurface, the required rotation angles of graphene strips in corresponding positions can be obtained with the aid of the PB phase method.(6)Finally, the target is to build up the objective metasurface using graphene strips with specific rotation angles. In the design process, the utilization of scripts and the application programming interface (API) in HFSS can greatly accelerate the speed of establishing models.In order to further provide readers a deeper insight into graphene-based metasurfaces and provide more people with guidelines for the design of our proposals, a design strategy will be given in more detail. The design steps are as follows.The structure topology and electromagnetic properties of the proposed graphene element have been comprehensively discussed; in addition, a design strategy was also provided to guide researchers in effectively designing a graphene-based metasurface. Using this kind of reflective-type cell, multifunctional graphene metasurfaces with the capability of controlling the SAM and OAM of reflected beams at will can be built up. To further elaborate the function and physical mechanism of the proposed graphene metasurfaces, in the following parts, three functions, including arbitrary control of SAM and OAM for a reflective wave and generation of a spin-controlled OAM helical wave, will be discussed in detail.The SAM is a momentum that characterizes the spin state of a spreading wave and is directly associated with the polarization of the wave. Circularly polarized modes carry a spin angular momentum of r photon . In ordeAfter the basic elements are designed, the phase distribution of the whole graphene metasurface should be determined by its function. Here, we enable the metasurface to have the capability of controlling the SAM of a reflected wave in any expected directions. According to generalized Snell’s law, abrupt phase gradients are introduced on the metasurface to realize the arbitrary beam deflection. The surface reflection phase distribution  can be cAccording to Equation (2), the compensation phase distribution of each cell can be obtained. For a phase 2It is noted that the metasurface is different from the reflect-array antenna because the metasurface has no port. Therefore, we only consider its aperture efficiency, and the aperture efficiency of the metasurface approaches 44% by calculation. Because the aperture area of graphene metasurface is small and comparable to the wavelength, other kinds of metasurface aperture efficiencies are basically over 40%.The metasurface depends on the abrupt phase shifts provided by unit cells to manipulate the incident waves and reflected waves. Given that the discrete amplitude distribution is analogous to a non-blazed grating, it is not surprising that the angular spectrum may include one or more diffracted orders . The resOAM, according to the knowledge of optics, is in relation to a helical wavefront of a spreading wave that dependents on the phase factor LG) beam  is a typLG) beam  and commLG) beam . For theLG) beam  of the mTo verify the output beam carrying OAM with desired topological charges, a plane RHCP (right-handed circularly polarized) wave as an incident wave is illuminated normally on the proposed metasurface with 21 × 21 elements. For the case of L = 1 in Due to the capability of controlling the SAM and OAM of a reflected wave arbitrarily by using the proposed multifunctional graphene metasurfaces, manipulating the two momentums simultaneously becomes possible. Therefore, a THz spin-controlled OAM beam generator with varying topological charges can also be created. To perform this function, the required phase distribution can be obtained by adding the two kinds of phase distributions described above, and can be expressed byBased on the designed graphene elements, two graphene metasurfaces composed of 21 × 21 cells are proposed and analyzed, which can both convert a circularly polarized incident wave into a spin-controlled vortex wave that possesses SAM and OAM simultaneously in desired directions. In order to demonstrate the functions of the proposed metasurfaces, the RHCP plane waves, with excitations 15 wavelengths away from the metasurface, are illuminated normally on the surfaces. In In summary, a new multifunctional graphene metasurface based on the principle of PB phase to control the SAM and OAM of the THz reflected waves has been proposed and numerically demonstrated. Composed of a three-layer graphene structure by rotating orientations of graphene elements, the proposed graphene element not only achieves 360° phase coverage, but also possesses the capability of polarization maintenance for the CP wave. In order to verify the results of the unit cell in HFSS, another modeling tool, CST, was also used to simulate the same unit cell; meanwhile, the differences of the results were also analyzed. Then, a detailed design strategy was also provided to further help readers obtain a deeper insight into graphene-based metasurfaces. To demonstrate the performance of our proposals, three kinds of graphene metasurfaces with various functions, including independent control of SAM, OAM or both of them, i.e., a THz spin-controlled OAM beam generator, were designed and numerically demonstrated. Compared with a linearly polarized OAM beam, it is noted that the spin-controlled beam produced by our designs provides an extra degree of freedom for THz wireless communication and improves the capability of the channel without increasing the bandwidth, which has great potential for future high-speed THz communication systems.