Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Carbon Nanotubes <s> The challenge on carbon nanotubes is still the subject of many research groups. While in the first years the focus was on the new synthesis methods, new carbon sources and support materials, recently, the application possibilities are the principal arguments of the studies. The three main synthesis methods discussed in this review are the arc discharge, the laser ablation and the chemical vapour deposition (CVD) with a special regard to the latter one. In the early stage of the nanotube production the first two methods were utilized mainly for the production of SWNTs while the third one produced mainly MWNTs. The principle of CVD is the decomposition of various hydrocarbons over transition metal supported catalyst. Single-walled (SWNT), multi-walled (MWNT) and coiled carbon nanotubes are produced. In some case, interesting carbonaceous materials are formed during the synthesis process, such as bamboo-like tubes, onions, horn-like structures. In this paper, we refer to the progresses made in the field of the synthesis techniques of carbon nanotubes in the last decade. <s> BIB001 </s> Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Carbon Nanotubes <s> We have developed carbon nanotube-based dual-mode biosensors with a metal semiconductor field effect transistor structure on a quartz substrate. DNA hybridization occurring on the Au top gate can be detected by simultaneously measuring the change in the electrical conductance and the surface plasmon resonance (SPR). Since electrical and SPR measurements offer high sensitivity and reliability, respectively, this dual-mode biosensor is expected to provide both of these features. <s> BIB002 </s> Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Carbon Nanotubes <s> We describe a method of amplifying the biosensing signal in surface plasmon resonance (SPR)-based immunoassays using an antibody-carbon nanotube (CNT) conjugate. As a model system, human erythropoietin (EPO) and human granulocyte macrophage colony-stimulating factor (GM-CSF) were detected by sandwich-type immunoassays using an SPR biosensor. For the amplification of the SPR signal, the CNT was conjugated with a polyclonal antibody, and then the conjugates were reacted with antibodies coupled with the target proteins. This amplification strategy increases the dynamic range of the immunoassays and enhances the detection sensitivity. The SPR immunoassays, combined with the CNT-assisted signal amplification method, provided a wide dynamic range over four orders of magnitude for both EPO and GM-CSF (0.1-1,000 ng/ml). The CNT amplification method is expected to realize the detection of picogram levels and a wide dynamic detection range of multiple proteins, enabling it to offer a robust analysis tool for the development of biopharmaceutical production. <s> BIB003 </s> Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Carbon Nanotubes <s> There has been an explosion of research into the physical and chemical properties of carbon-based nanomaterials, since the discovery of carbon nanotubes (CNTs) by Iijima in 1991. Carbon nanomaterials offer unique advantages in several areas, like high surface-volume ratio, high electrical conductivity, chemical stability and strong mechanical strength, and are thus frequently being incorporated into sensing elements. Carbon nanomaterial-based sensors generally have higher sensitivities and a lower detection limit than conventional ones. In this review, a brief history of glucose biosensors is firstly presented. The carbon nanotube and grapheme-based biosensors, are introduced in Sections 3 and 4, respectively, which cover synthesis methods, up-to-date sensing approaches and nonenzymatic hybrid sensors. Finally, we briefly outline the current status and future direction for carbon nanomaterials to be used in the sensing area. <s> BIB004 </s> Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Carbon Nanotubes <s> Current discoveries of different forms of carbon nanostructures have motivated research on their applications in various fields. They hold promise for applications in medicine, gene, and drug delivery areas. Many different production methods for carbon nanotubes (CNTs) have been introduced; functionalization, filling, doping, and chemical modification have been achieved, and characterization, separation, and manipulation of individual CNTs are now possible. Parameters such as structure, surface area, surface charge, size distribution, surface chemistry, and agglomeration state as well as purity of the samples have considerable impact on the reactivity of carbon nanotubes. Otherwise, the strength and flexibility of carbon nanotubes make them of potential use in controlling other nanoscale structures, which suggests they will have a significant role in nanotechnology engineering. <s> BIB005 </s> Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Carbon Nanotubes <s> Polypyrrole multi-walled carbon nanotube composite layers were used to modify the gold layer to measure heavy metal ions using the surface plasmon resonance technique. The new sensor was fabricated to detect trace amounts of mercury (Hg), lead (Pb), and iron (Fe) ions. In the present research, the sensitivity of a polypyrrole multi-walled carbon nanotube composite layer and a polypyrrole layer were compared. The application of polypyrrole multi-walled carbon nanotubes enhanced the sensitivity and accuracy of the sensor for detecting ions in an aqueous solution due to the binding of mercury, lead, and iron ions to the sensing layer. The Hg ion bonded to the sensing layer more strongly than did the Pb and Fe ions. The limitation of the sensor was calculated to be about 0.1 ppm, which produced an angle shift in the region of 0.3° to 0.6°. <s> BIB006 </s> Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Carbon Nanotubes <s> The synergism of excellent properties of carbon nanotubes and gold nanoparticles is used in this work for bio-sensing of recombinant bovine growth hormones (rbST) by making Multi Wall Carbon Nanotubes (MWCNT) locally optically responsive by augmenting it optical properties through Localized Surface Plasmon Resonance (LSPR). To this purpose, locally gold nano particles decorated gold–MWCNT composite was synthesized from a suspension of MWCNT bundles and hydrogen chloroauric acid in an aqueous solution, activated ultrasonically and, then, drop-casted on a glass substrate. The slow drying of the drop produces a “coffee ring” pattern that is found to contain gold–MWCNT nanocomposites, accumulated mostly along the perimeter of the ring. The reaction is studied also at low-temperature, in the vacuum chamber of the Scanning Electron Microscope and is accounted for by the local melting processes that facilitate the contact between the bundle of tubes and the gold ions. Biosensing applications of the gold–MWCNT nanocomposite using their LSPR properties are demonstrated for the plasmonic detection of traces of bovine growth hormone. The sensitivity of the hybrid platform which is found to be 1 ng/ml is much better than that measuring with gold nanoparticles alone which is only 25 ng/ml. <s> BIB007 </s> Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Carbon Nanotubes <s> We investigate the modification of the optical properties of carbon nanotubes (CNTs) resulting from a chemical reaction triggered by the presence of a specific compound (gaseous carbon dioxide (CO2)) and show this mechanism has important consequences for chemical sensing. CNTs have attracted significant research interest because they can be functionalized for a particular chemical, yielding a specific physical response which suggests many potential applications in the fields of nanotechnology and sensing. So far, however, utilizing their optical properties for this purpose has proven to be challenging. We demonstrate the use of localized surface plasmons generated on a nanostructured thin film, resembling a large array of nano-wires, to detect changes in the optical properties of the CNTs. Chemical selectivity is demonstrated using CO2 in gaseous form at room temperature. The demonstrated methodology results additionally in a new, electrically passive, optical sensing configuration that opens up the possibilities of using CNTs as sensors in hazardous/explosive environments. <s> BIB008 </s> Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Carbon Nanotubes <s> A novel approach for the detection of sulfamethaxazole (SMX) based on fibre optic surface plasmon resonance (SPR) technique and functionalized carbon nanotubes (CNTs) is proposed for 0 to 200 μM concentration range. The probe is prepared by immobilizing functionalized CNTs over silver coated unclad core of the fibre. The sensor’s performance is compared with a fibre optic enzyme-based SPR sensor. In the enzyme-based sensor, layer of functionalized CNTs is replaced by the layer of tyrosinase enzyme embedded polyacrylamide gel. It is found that the resonance wavelength in case of enzyme-based probe and the peak absorbance wavelength in the case of CNT-based probe show a red shift as the concentration of SMX is increased. Both the probes are characterized and optimized in terms of sensitivity, limit of detection, selectivity, pH and repeatability. The maximum sensitivities for enzyme and CNT-based sensors are found to be 0.29 and 0.37 nm/μM, respectively. Further, the corresponding limits of detection (LODs) are 1.137 and 0.8918 μM, respectively. The values of LOD are compared with the values reported in the literature for SMX sensors based on other methods/techniques. The proposed sensor utilizing CNTs is found to be the best in terms of LOD. The other advantages of proposed sensor are low cost, high sensitivity and better selectivity in addition to the advantages of remote sensing, real time monitoring, miniaturized probe and ease of handling which are due to the use of optical fibre as substrate for probe fabrication. <s> BIB009 </s> Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Carbon Nanotubes <s> A facile approach is presented for the detection of bovine serum albumin (BSA), based on fiber optic surface plasmon resonance (FOSPR) combined with molecular imprinting (MI). The probe is fabricated by exploiting the plasmonic property of silver thin film and vinyl-functionalised carbon nanotube-based MIP platform. BSA template molecules are imprinted on the MIP layer coated over multi-walled carbon nanotubes to ensure high specificity of the probe in the interfering environments. In addition, FOSPR endorses the sensor capability of real-time and remote sensing along with very high sensitivity due to the use of nanostructured MI platform. The response of the probe is considered in terms of the absorbance spectrum recorded for various concentrations of BSA. The sensor shows a wide dynamic range of 0–350 ng l−1 with a considerably linear response up to 100 ng l−1 in the peak absorbance wavelength with BSA concentration. A highest sensitivity of 0.862 nm per ng l−1 is achieved for the lowest concentration of BSA and it decreases with the increase in BSA concentration. The performance of the present sensor is compared with those reported in the literature in terms of the limit of detection. It is found that the probe possesses a lowest LOD of 0.386 ng l−1 in addition to other advantages such as real-time online monitoring, high sensitivity, high specificity, and remote sensing. <s> BIB010 </s> Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Carbon Nanotubes <s> Abstract A fiber optic SPR (FOSPR) based probe with coatings of silver and nanocomposite of CNT/Cu-nanoparticles over unclad core of the fiber is proposed for the sensing of nitrate. The sensing is based on the reduction of nitrate on interaction with copper nanoparticles and the formation of ammonium ions. The adsorption of ammonium ions on CNTs changes the dielectric properties of CNT/Cu-NPs nanocomposite which is measured as the shift in resonance wavelength. The composition ratio and the thickness of the CNT/Cu-NPs nanocomposite layer along with the pH of the nitrate solution are optimised for the best performance of the sensor. The probe operates for the concentration range of 10 −6 M–5 × 10 −3 M and offers a high sensitivity of 80.62 × 10 6 nm/M. Due to the operation in this wide range, the probe paves the way for a novel means to detect nitrate in real samples and highlights the feasibility of CNT/Cu-NPs nanocomposite based SPR sensors. Moreover, the sensor possesses 4 nM as the limit of detection. This is the lowest value reported in the literature. The entire sensing procedure could be completed within 30 s with only 20 ml of analyte that are the main demands of effective and fast detection in real sample. <s> BIB011 </s> Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Carbon Nanotubes <s> Abstract In this paper, the carbon nanotubes (CNTs)-deposited Au film photonic crystal fiber (PCF) surface plasmon resonance (SPR) sensor (CNTs/Au-PCF sensor) and CNTs-deposited Ag film PCF SPR sensor (CNTs/Ag-PCF sensor) were developed and utilized to conduct a series of experiments for the refractive index sensing characteristics study of the CNTs-deposited SPR sensors. The PCF, spliced between two sections of multimode fibers (MMFs), was coated with a metal (Au or Ag) film and then deposited with CNTs for further sensing. CNTs coating can enhance the confined electric field intensity surrounding the sensing layer, making the SPR sensor more sensitive to the changes in the ambient medium. Compared with conventional Au film PCF SPR sensor (Au-PCF sensor), the sensitivity of CNTs/Au-PCF sensor increases by 1016.09 nm/RIU. Compared with conventional Ag film PCF SPR sensor (Ag-PCF sensor), the sensitivity of CNTs/Ag-PCF sensor increases by 709.22 nm/RIU. Therefore, we find that CNTs have a more significant effect on the Au-PCF sensor than the Ag-PCF sensor. The experimental measurements results agreed well with the simulation results. Furthermore, CNTs have high surface-to-volume ratio and extremely excellent biocompatibility. Bovine serum albumin (BSA) was employed as the target analyte to evaluate the feasibility of the CNTs/Au-PCF sensor for the detection of biomolecules, and the sensor exhibits higher sensitivity (8.18 nm/(mg/mL)), lower limit of detection (LOD) (2.5 µg/mL), and faster response time (8 s) than the Au-PCF sensor. Such CNTs-deposited SPR sensors with high sensitivities and fast response present highly promising potential for application in the field of biochemistry. <s> BIB012 </s> Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Carbon Nanotubes <s> In this study, we developed a surface plasmon resonance (SPR) sensor chip based on 2,4,6-trinitrotoluene (TNT) recognition peptide-modified single-walled carbon nanotubes (SWCNTs). The carboxylic acid-functionalized SWCNTs were immobilized on a 3-aminopropyltriethoxysilane (APTES)-modified SPR Au chip surface. Through π-stacking between the aromatic amino acids and SWCNTs, the TNT recognition peptide TNTHCDR3 was immobilized onto the surface of the SWCNTs. The peptide⁻SWCNTs-modified sensor surface was confirmed and evaluated by atomic force microscope (AFM) observation. The peptide⁻SWCNTs hybrid SPR sensor chip exhibited enhanced sensitivity with a limit of detection (LOD) of 772 ppb and highly selective detection compared with commercialized carboxymethylated dextran matrix sensor chips. <s> BIB013 </s> Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Carbon Nanotubes <s> Abstract Reduced graphene oxide-polyaniline (rGO-Pani) nanocomposite based highly sensitive fiber optic pH sensor utilizing SPR technique is fabricated and characterized. The rGO-Pani nanocomposite is successfully synthesized using in-situ method. The sensing probe is fabricated by coating a thin film of silver over the unclad core of the optical fiber which is further coated by a film of rGO-Pani nanocomposite. The preparation of rGO, Pani and rGO-Pani nanocomposite are confirmed using different characterization tools such as X-ray diffraction (XRD), Raman and Fourier-transform infrared spectroscopy (FTIR). The morphology of the rGO-Pani nanocomposite is characterized by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The operating pH sensing range of the present sensor is checked by performing experiments on samples of pH lying from 2.4 to 11.35. The sensor's performance is excellent for low and high pH and it possesses the maximum sensitivity of 75.09 nm/pH at pH 11.35. The high sensitivity of the sensor is because of the change in the optical band gap of the rGO-Pani nanocomposite at the low and high pH values of the samples which is confirmed by using Tau plot. The sensor has many advantages such as compact size, low cost, high sensitivity, stability, repeatability and capability of remote sensing. <s> BIB014 </s> Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Carbon Nanotubes <s> Abstract Surface plasmon resonance (SPR) based dopamine sensor is realized using the state-of-art technique of molecular imprinting over an optical fiber substrate. Polypyrrole (PPy) is depicted as an effective polymer for the imprinting of dopamine through a green synthesis approach. Sensitivity of the probe is enhanced by the augmenting effect of surface imprinting of dopamine in polypyrrole over multiwalled carbon nanotubes (MWCNTs). To ensure the permselectivity of the probe towards dopamine molecules, a cation exchange polymer, nafion, is utilized as a membrane over imprinted sites to reduce the interference from anionic analytes like ascorbic acid and uric acid at physiological pH. The probe is characterized for a wide range of dopamine concentration from 0 to 10 -5 M in artificial cerebrospinal fluid. Various probe parameters are varied to maximize the sensitivity of the sensor. The sensor possesses 18.9 pM as the limit of detection (LOD) which is lowest of those reported in the literature. The manifestation of sensing probe over an optical fiber along with the improved LOD makes the approach highly advantageous in terms of stability, repeatability, online remote monitoring, fast response, and miniaturization for its in vivo/in vitro applications in clinical sensing of dopamine. <s> BIB015
Three main commercially recognized production techniques for CNTs are arc discharge, laser ablation, and chemical vapor deposition (CVD) BIB005 BIB001 . The arc discharge method takes place at low pressure (30-150 torr) , where two high purity graphite electrodes (diameter 6-12 mm) separated by 1-2 mm are utilized as carbon sources in a chamber generally filled with helium (hydrogen and methane atmospheres are also used). The anode is sometimes mixed with metals such as Fe, Y, Mo, Co, and Ni to produce variable nature CNTs, such as graphitic, SWCNTs, metal filled CNTs, and metal nanoparticles decorated CNTs. A direct current is passed through the chamber to produce arching, where the temperature reaches up to 4000 K. Under these conditions, carbon sublimes from the anode solidify on the cathode, where a soot is formed at a typical rate of 1 mm/min. The cathode soot, chamber soot, and inner core generally consist of single and multi-walled carbon nanotubes, depending on the use of a catalyst in the anode or pure anode. Various parameters, such as reaction environment, gas pressure inside the chamber, and the type of arc discharge, control the properties of synthesized tubes. The arc discharge method is good for the large-scale production of CNTs, however it provides a very little control over its chirality BIB005 . The second method employed for the synthesis of CNTs is laser ablation, which can be divided into classical laser ablation and ultrafast laser ablation. Here, a metal-graphite composite rod is placed in a high temperature furnace and scanned through a high-power laser. The soot produced due to target vaporization by the laser is swept by Ar gas in the chamber and is deposited on a cooled copper target outside the furnace. The properties of the produced tubes are governed by target compositional ratio, furnace temperature, the nature of ambient gas in the furnace, and laser parameters. Laser ablation produces high purity and better graphitized CNTs than the arc discharge method, however, the mass production is limited in this technique BIB005 . The most important and standard method for the large-scale production of high purity carbon nanotubes is the chemical vapor deposition (CVD) method. Various modifications have been made in the standard CVD technique to produce CNTs with desired properties, such as catalytic CVD (CCVD), plasma enhanced CVD, microwave plasma assisted CVD, water assisted CVD, and radio frequency CVD. CCVD is the most extensively utilized method for the production of CNTs. Here, a catalyst is placed in a quartz boat, typically, in a horizontal flow furnace at atmospheric pressure. The reaction mixture, consisting of a hydrocarbon and an inert gas, is passed over the bed of the catalyst at temperatures ranging from 500-1100 • C. The CNT growth in such a procedure involves the dissociation of hydrocarbon in the presence of transition metal catalysts and the saturation of carbon atoms in metal particles in an sp 2 structure in a tubular carbon state. The properties depend on various parameters, like temperature, pressure, type and concentration of hydrocarbons and metal catalysts, and the reaction time BIB001 . Some other less popular methods also employed for the production of CNTs involve the hydrothermal method and electrolysis.  Carbon nanotubes have also been explored as plasmonic materials for sensing in numerous applications. In a localized surface plasmon resonance (LSPR)based gas sensor, CNTs are utilized as plasmons, as well as an affinity material for the detection of carbon dioxide (CO 2 ) gas BIB008 . CNTs are believed to show a change in their electrical properties, which alters their optical properties upon chemical interaction with CO 2 . This property is exploited to fabricate an optical fiber Bragg grating-based LSPR sensor. The authors successfully showed a sensitive response with the observation of a direct change in optical response of CNTs upon chemical interaction with a molecule. A refractive index sensitivity (∆λ/∆n) of 6200 nm/RIU in the gaseous regime was observed, which contributes to a detection limit of 150 ppm at atmospheric pressure, making the approach highly suitable for practical CO 2 sensor application. The response of the probe was tested for other similar hydrocarbon gases, such as methane and ethane, for the specificity of the approach and the results were found to be highly satisfactory, with a change in SPR response only in the case of CO 2 BIB008 . In another approach, the π plasmon absorption of carbon-based nanomaterials in the UV-region was utilized for SWCNTs for a sensitive and selective binding of ions  . It was observed that the strong π plasmon absorption of SWCNTs in the UV region is far more sensitive to plasmonic properties in visible and NIR regimes. The method, based on UV-Vis absorption, was found to be simple and effective as compared to previously reported field effect transistor (FETs) and fluorescence-based methods, which require complex device fabrication, NIR source, and the separation of semiconducting and metallic CNTs. It was also found to be highly specific for the detection of metal ions by tailoring the surface modification of CNTs through molecular legends specific to ion attachment. The model was tested to detect Fe 3+ ions in an approach based on the surface modification of SWCNTs by a natural bacterial siderophore, Deferoxamine (DFO), for the specific attachment. The CNT surfaces were first modified with carboxyl terminated phospholipid-polyethylene glycol (PL-PEG-COOHs) for stable aqueous dispersions and further attachment of siderophore. These PL-PEG-COOHs modified CNTs show two absorption peaks in the UV region, at 246 and 275 nm, due to the π-plasmon absorption by the graphitic structure, as shown in Figure 3 . The detection limit reaches the pM range, with the sensing mechanism based on the charge transfer between Fe-DFO complex and SWNTs, as obtained from resonant Raman scattering study. The UV-Vis sensing results are shown in Figure 4 . The testing was performed in aqueous and rain water samples, with a specific response towards Fe 3+ ions, as tested in an interfering environment from other ions, such as Al 3+ , Zn 2+ , Ni 2+ , and Cu 2+ ( Figure 4D ). Reprinted with permission from  . Copyright 2016 Royal Society of Chemistry. Carbon nanotubes have also been used to enhance the sensitivity of a biosensor. In a sensitivity enhancement approach, a CNT-antibody conjugate was employed to amplify the SPR signal for a biosensing platform BIB003 . Human erythropoietin (EPO) and human granulocyte macrophage Apart from their large surface area, GO nanosheets have various oxygen functional groups, such as -OH, -COOH, which help to bind the enzyme. The total shift in resonance wavelength and sensitivity for probe III were around 50 nm and 5.06 nm/mM, respectively, which were the maximums among all three probes. In probe III, the shift was due to an enzymatic reaction, as well as the silver nanoparticles, which decompose the hydrogen peroxide, due to which the effective refractive index changes more in comparison to the probe I and II, which results in the maximum shift and sensitivity. From all three probe configurations, it was concluded that GO enhances the sensitivity of the biosensor. Carbon nanotubes have also been used to enhance the sensitivity of a biosensor. In a sensitivity enhancement approach, a CNT-antibody conjugate was employed to amplify the SPR signal for a biosensing platform BIB003 . Human erythropoietin (EPO) and human granulocyte macrophage colony-stimulating factor (GM-CSF) were tested as model systems to see the role of CNTs in the enhancement of SPR signal. Biocompatibility, fast electron transfer, and large molecular mass were reported to be key factors for sensitivity enhancement for the detection of very low levels of EPO and GM-CSF in real systems. All SPR measurements were performed on the commercially available BIAcore X SPR biosensor system (GE Healthcare, Sweden) at a flow rate of 5 µl/min, at 25 • C. Gold chips were modified with a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA) and then activated by ethyl (dimethylaminopropyl) carbodiimide /N-hydroxysuccinimide (EDC/NHS) chemistry for the capture of respective antibodies. A schematic of the detailed mechanisms of CNT assisted and without CNTs SPR detection is shown in Figure 12 . Sensors 2019, 19, x FOR PEER REVIEW 14 of 25 colony-stimulating factor (GM-CSF) were tested as model systems to see the role of CNTs in the enhancement of SPR signal. Biocompatibility, fast electron transfer, and large molecular mass were reported to be key factors for sensitivity enhancement for the detection of very low levels of EPO and GM-CSF in real systems. All SPR measurements were performed on the commercially available BIAcore X SPR biosensor system (GE Healthcare, Sweden) at a flow rate of 5 µl/min, at 25 °C. Gold chips were modified with a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA) and then activated by ethyl (dimethylaminopropyl) carbodiimide /N-hydroxysuccinimide (EDC/NHS) chemistry for the capture of respective antibodies. A schematic of the detailed mechanisms of CNT assisted and without CNTs SPR detection is shown in Figure 12 . The enhancement in SPR signal with the CNT assisted approach can be seen in Figure 13 , through the real-time SPR response and the SPR angle shift in the case of direct and CNT assisted detection. The CNT-based amplification method was reported to be sensitive for a wide dynamic range, from 0.1 to 1000 ng/mL, with the detection limit reaching 0.1 ng/mL, enabling its application in real scenarios. In a similar approach, a photonic crystal fiber (PCF)-based SPR sensor was reported with enhanced refractive index sensitivity, due to CNTs deposited on the Au and Ag film as compared to a bare Ag/Au thin film BIB012 . The sensitivity of the CNT-Au film PCF SPR sensor was reported to The enhancement in SPR signal with the CNT assisted approach can be seen in Figure 13 , through the real-time SPR response and the SPR angle shift in the case of direct and CNT assisted detection. The CNT-based amplification method was reported to be sensitive for a wide dynamic range, from 0.1 to 1000 ng/mL, with the detection limit reaching 0.1 ng/mL, enabling its application in real scenarios. colony-stimulating factor (GM-CSF) were tested as model systems to see the role of CNTs in the enhancement of SPR signal. Biocompatibility, fast electron transfer, and large molecular mass were reported to be key factors for sensitivity enhancement for the detection of very low levels of EPO and GM-CSF in real systems. All SPR measurements were performed on the commercially available BIAcore X SPR biosensor system (GE Healthcare, Sweden) at a flow rate of 5 µl/min, at 25 °C. Gold chips were modified with a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA) and then activated by ethyl (dimethylaminopropyl) carbodiimide /N-hydroxysuccinimide (EDC/NHS) chemistry for the capture of respective antibodies. A schematic of the detailed mechanisms of CNT assisted and without CNTs SPR detection is shown in Figure 12 . The enhancement in SPR signal with the CNT assisted approach can be seen in Figure 13 , through the real-time SPR response and the SPR angle shift in the case of direct and CNT assisted detection. The CNT-based amplification method was reported to be sensitive for a wide dynamic range, from 0.1 to 1000 ng/mL, with the detection limit reaching 0.1 ng/mL, enabling its application in real scenarios. In a similar approach, a photonic crystal fiber (PCF)-based SPR sensor was reported with enhanced refractive index sensitivity, due to CNTs deposited on the Au and Ag film as compared to a bare Ag/Au thin film BIB012 . The sensitivity of the CNT-Au film PCF SPR sensor was reported to In a similar approach, a photonic crystal fiber (PCF)-based SPR sensor was reported with enhanced refractive index sensitivity, due to CNTs deposited on the Au and Ag film as compared to a bare Ag/Au thin film BIB012 . The sensitivity of the CNT-Au film PCF SPR sensor was reported to increase by 1016 nm/RIU as compared to the conventional Au film PCF SPR sensor, whereas the sensitivity of the CNT-Ag film PCF SPR sensor increased by 709 nm/RIU compared to the Ag film SPR sensors. The experimental results were found to be in accordance with simulated data for such sensors, clarifying the role of CNTs in the sensitivity enhancement of SPR sensors. The results are shown in Figure 14 . increase by 1016 nm/RIU as compared to the conventional Au film PCF SPR sensor, whereas the sensitivity of the CNT-Ag film PCF SPR sensor increased by 709 nm/RIU compared to the Ag film SPR sensors. The experimental results were found to be in accordance with simulated data for such sensors, clarifying the role of CNTs in the sensitivity enhancement of SPR sensors. The results are shown in Figure 14 . A gold nanoparticle-decorated CNTs platform was developed recently for the LSPR based sensing of bovine growth hormone BIB007 . Due to the high surface to volume ratio and favorable electron transfer reactions, CNTs were augmented with Au nanoparticles to provide synergistic effects to LSPR sensing. The properties of the unique nanocomposite were explored to absorb and sense the biomolecular interaction with increased sensitivity, as compared to individual material. A concentration as low as 1 ng/mL of rbST was detected with the composite based LSPR platform, providing a new window for ultrasensitive plasmonic detection of polypeptides and proteins. The last way of utilizing CNTs in plasmonic sensors is as sensing material. The ease and variety of possible functionalization on CNTs side walls and edges make them suitable for the sensing of various species  BIB006 BIB002 BIB004 . A 2,4,6-trinitrotoluene (TNT) SPR sensor was reported, with peptide modified SWCNTs as sensing material over Au-coated SPR chips. The standard gold coated chips were firstly modified with APTES (3-aminopropyltriethoxysilane) to generate amine groups for the attachment of carboxyl terminated CNTs BIB013 . After this, TNTHCDR3 peptide was immobilized non-covalently on CNTs through a π-π interaction. The SPR measurements were carried out on a standard Biacore system. TNT samples were tested for a wide dynamic concentration range, from 0.8 ppm to 100 ppm. The real-time SPR response of peptide modified SWCNTs based SPR sensors for various concentrations of TNTs is shown in Figure 18a . The response was compared with a conventional dextran chip (CM7) and it was revealed that CNTs offer promising properties for the detection of low molecular weight analytes at low concentration levels in SPR sensors, with a significantly improved response (Figure 18b,c) . The response of the CNT modified chip is shown in Figure 18d for various concentrations of TNT. The specificity of the chip was tested for various analytes, which can interfere or are analogous to TNTs, such as DNP-glycine, 2,6-DNT, RDX, and 4-nitrobenzoyl-glycyl-glycine, showing a negligible response from all the other analytes ( Figure  18e ). The stability and reversibility of surface reactions on CNTs and the chemical stability of the gold chip are visible through the stable response of the sensor over 20-30 days (Figure 18f ). Reprinted with permission from BIB014 . Copyright 2019 Elsevier. Figure 17b shows the SPR curves corresponding to the basic region from 7 pH to 11.35, and in this case also a red shift in the resonance wavelength was observed with the increase in pH of the sample (increase in alkanity). Again, the reason for the red shift is the interaction between the pH sample and rGO-Pani nanocompoite. The emeraldine form of Pani changes to a leucoemeraldine base after the interaction with the basic sample through the reduction process. The samples with pH from 7 to 11 have an excess of OH − ions. The rGO nanosheets get converted to p-doped material via OH − ions. Therefore, the overall band gap of the nanocomposite changes, which results in a change in the effective refractive index of the sensing layer. The maximum sensitivity was found to be around 75.09 nm/pH at the pH value 11.35. In this study, the behavior of rGO nanosheets changed when they came in contact with acidic/basic samples, resulting in a change in the effective refractive index. Thus, rGO nanosheets act as a sensing matrix for the measurement of the pH. The last way of utilizing CNTs in plasmonic sensors is as sensing material. The ease and variety of possible functionalization on CNTs side walls and edges make them suitable for the sensing of various species  BIB006 BIB002 BIB004 . A 2,4,6-trinitrotoluene (TNT) SPR sensor was reported, with peptide modified SWCNTs as sensing material over Au-coated SPR chips. The standard gold coated chips were firstly modified with APTES (3-aminopropyltriethoxysilane) to generate amine groups for the attachment of carboxyl terminated CNTs BIB013 . After this, TNTHCDR3 peptide was immobilized non-covalently on CNTs through a π-π interaction. The SPR measurements were carried out on a standard Biacore system. TNT samples were tested for a wide dynamic concentration range, from 0.8 ppm to 100 ppm. The real-time SPR response of peptide modified SWCNTs based SPR sensors for various concentrations of TNTs is shown in Figure 18a . The response was compared with a conventional dextran chip (CM7) and it was revealed that CNTs offer promising properties for the detection of low molecular weight analytes at low concentration levels in SPR sensors, with a significantly improved response (Figure 18b,c) . The response of the CNT modified chip is shown in Figure 18d for various concentrations of TNT. The specificity of the chip was tested for various analytes, which can interfere or are analogous to TNTs, such as DNP-glycine, 2,6-DNT, RDX, and 4-nitrobenzoyl-glycyl-glycine, showing a negligible response from all the other analytes (Figure 18e In another study, an optical fiber-based SPR sensor was reported, where MWCNTs were utilized as sensing material for the detection of sulfamethaxazole (SMX), opening the potential sensing applications of CNTs in pharmaceuticals BIB009 . The response of the sensor was compared with a similar approach based on enzyme entrapped gel as a sensing layer for a 0 to 200 µM concentration range of SMX. The CNT-based platform offers an improved sensitivity over the desired concentration range, along with long time stability and cost effectiveness as compared to the enzymatic approach. The CNT-based probes were prepared by depositing Ag thin film on the fiber core surface, functionalizing the Ag surface with APTES, and then attaching carboxylic acid functionalized CNTs on amino group-modified fiber substrate. The other enzyme-based probes were also prepared by the coating of polyacrylamide gel entrapped with tyrosinase enzyme for the sensing of SMX. The interaction mechanism governing the SPR response due to physicochemical interaction of SMX with CNTs and tyrosinase is shown in Figure 19 . In another study, an optical fiber-based SPR sensor was reported, where MWCNTs were utilized as sensing material for the detection of sulfamethaxazole (SMX), opening the potential sensing applications of CNTs in pharmaceuticals BIB009 . The response of the sensor was compared with a similar approach based on enzyme entrapped gel as a sensing layer for a 0 to 200 µM concentration range of SMX. The CNT-based platform offers an improved sensitivity over the desired concentration range, along with long time stability and cost effectiveness as compared to the enzymatic approach. The CNT-based probes were prepared by depositing Ag thin film on the fiber core surface, functionalizing the Ag surface with APTES, and then attaching carboxylic acid functionalized CNTs on amino group-modified fiber substrate. The other enzyme-based probes were also prepared by the coating of polyacrylamide gel entrapped with tyrosinase enzyme for the sensing of SMX. The interaction mechanism governing the SPR response due to physicochemical interaction of SMX with CNTs and tyrosinase is shown in Figure 19 . Reprinted with permission from BIB009 . Copyright 2018 Springer. The detection limit improved from 1.137 µM in the case of the enzyme entrapped gel as a sensing layer to 0.8918 µM in the non-enzymatic CNT-based approach. The corresponding response in the two cases, with sensitivity variation along with the desired concentration range, is shown in Figure 20 . The maximum sensitivity at the lowest concentration also increased from 0.29 nm/µM for the enzyme-based platform to 0.37 nm/µM for CNTs as sensing material. In another biosensor development, CNTs are employed as a matrix for the molecular imprinting (MIP) of dopamine (DA), providing a generic platform for the sensing of neurotransmitters BIB015 . The MIP on the surface of CNTs encompasses the advantages of high sensitivity and selectivity due to the easy removal and uptake of the analyte during sensing, large surface to volume ratio providing increased sensing hotspots, and a stable composite with imprinting conducting polymer (polypyrrole; PPy) due to binding in a de-localized π-electron cloud Reprinted with permission from BIB009 . Copyright 2018 Springer. The detection limit improved from 1.137 µM in the case of the enzyme entrapped gel as a sensing layer to 0.8918 µM in the non-enzymatic CNT-based approach. The corresponding response in the two cases, with sensitivity variation along with the desired concentration range, is shown in Figure 20 . The maximum sensitivity at the lowest concentration also increased from 0.29 nm/µM for the enzyme-based platform to 0.37 nm/µM for CNTs as sensing material. Reprinted with permission from BIB009 . Copyright 2018 Springer. The detection limit improved from 1.137 µM in the case of the enzyme entrapped gel as a sensing layer to 0.8918 µM in the non-enzymatic CNT-based approach. The corresponding response in the two cases, with sensitivity variation along with the desired concentration range, is shown in Figure 20 . The maximum sensitivity at the lowest concentration also increased from 0.29 nm/µM for the enzyme-based platform to 0.37 nm/µM for CNTs as sensing material. In another biosensor development, CNTs are employed as a matrix for the molecular imprinting (MIP) of dopamine (DA), providing a generic platform for the sensing of neurotransmitters BIB015 . The MIP on the surface of CNTs encompasses the advantages of high sensitivity and selectivity due to the easy removal and uptake of the analyte during sensing, large surface to volume ratio providing increased sensing hotspots, and a stable composite with imprinting conducting polymer (polypyrrole; PPy) due to binding in a de-localized π-electron cloud In another biosensor development, CNTs are employed as a matrix for the molecular imprinting (MIP) of dopamine (DA), providing a generic platform for the sensing of neurotransmitters BIB015 . The MIP on the surface of CNTs encompasses the advantages of high sensitivity and selectivity due to the easy removal and uptake of the analyte during sensing, large surface to volume ratio providing increased sensing hotspots, and a stable composite with imprinting conducting polymer (polypyrrole; PPy) due to binding in a de-localized π-electron cloud of CNTs and PPy. Polypyrrole/CNTs composites were used in the development of several SPR-based sensors due to their unique conductivity, making them favorable for sensitivity enhancement of SPR sensors  BIB006 . A permselective nafion membrane was also employed over an MIP layer to minimize the cross-signaling from interfering anions, such as ascorbic acid and uric acid. The morphological characterization of the nanocomposite was confirmed through SEM and TEM images, as shown in Figure 21a -d. The removal of DA for the formation of imprinted sites in MIP layer over MWCNTs was confirmed by UV-Vis spectroscopy, as shown in Figure 21e . of CNTs and PPy. Polypyrrole/CNTs composites were used in the development of several SPR-based sensors due to their unique conductivity, making them favorable for sensitivity enhancement of SPR sensors  BIB006 . A permselective nafion membrane was also employed over an MIP layer to minimize the cross-signaling from interfering anions, such as ascorbic acid and uric acid. The morphological characterization of the nanocomposite was confirmed through SEM and TEM images, as shown in Figure 21a -d. The removal of DA for the formation of imprinted sites in MIP layer over MWCNTs was confirmed by UV-Vis spectroscopy, as shown in Figure 21e . The SPR response was recorded after optimizing various probe parameters for 0 to 10 −5 M DA concentration in artificial cerebrospinal fluid, as shown in Figure 22a . A red shift of 69 nm was observed for the whole concentration range, with a non-linear response of the resonance wavelength with DA concentration (Figure 22b ). A similar MIP and SPR-based platform was also reported for the sensing of complex structures, like proteins (BSA). The surface imprinting on CNTs provides an improved platform as compared to bulk imprinting methodologies, where the removal of such complex molecules is cumbersome BIB010 . Moreover, the catalytic properties of MWCNTs/Cu nanoparticles have been employed in a fiber optic SPR sensor for the effective and fast detection of nitrates in real samples, like soil and river water analysis BIB011 . The sensing methodology was based on the reduction of nitrate ions to ammonium ions by the catalytic properties of copper nanoparticles and the adsorption of ammonium ions on the CNT surface, bringing a change in SPR response for 10 −6 M to 5 × 10 −3 M nitrate concentration.