Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Graphene and Its derivatives <s> A surface plasmon resonance (SPR) based graphene biosensor is presented. It consists of a graphene sheet coated above a gold thin film, which has been proposed and experimentally fabricated recently [ChemPhysChem 11, 585 (2010)]. The biosensor uses attenuated total reflection (ATR) method to detect the refractive index change near the sensor surface, which is due to the adsorption of biomolecules. Our calculations show that the proposed graphene-on-gold SPR biosensor (with L graphene layers) is (1 + 0.025 L) x gamma (where gamma > 1) times more sensitive than the conventional gold thin film SPR biosensor. The improved sensitivity is due to increased adsorption of biomolecules on graphene (represented by the factor gamma) and the optical property of graphene. <s> BIB001 </s> Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Graphene and Its derivatives <s> Abstract A surface plasmon resonance based biomolecules sensor using silicon and graphene layers coated over the base of the high index prism sputtered with gold has been analyzed. The graphene layer has been used to enhance the adsorption of biomolecules while the addition of silicon layer between gold and graphene increases the sensitivity. The thicknesses of gold and silicon layers along with the number of graphene layers have been optimized to achieve the best performance of the sensor in terms of sensitivity and Full Width at Half Maximum (FWHM). To see the effect of wavelength of the light source, simulations have been carried out for three different wavelengths. The best performance is obtained for 633 nm wavelength with optimized thicknesses of gold and silicon layers as 40 nm and 7 nm respectively while the optimum number of graphene layers is 2. The sensitivity obtained with optimized parameters and additional silicon layer, is more than twice the value reported in the literature. <s> BIB002 </s> Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Graphene and Its derivatives <s> Abstract A sensitivity enhanced surface plasmon resonance (SPR) immunosensor based on Graphene oxide (GO) composite and staphylococcal protein A (SPA) co-modified photonic crystal fiber (PCF) is developed for human IgG detection. The PCF spliced between two section of multimode fibers (MMFs) is sputtered with gold film, which was then modified with graphene oxide and SPA for further immunosensing. Graphene oxide with abundant functional groups has a large surface area for more biomolecules immobilization and can enhance the interaction between Au film and external medium, leading to the sensitivity increased greatly. SPA can specifically bind the Fc region of the antibodies and has a high degree of orientation for capturing antibodies. Experimental results indicated that the refractive index (RI) sensitivity of graphene oxide modified Au-PCF SPR sensor reaches 4649.8 nm/RIU, which is about 1888 nm/RIU higher than that without graphene oxide film and the human IgG detection limit reaches as low as 10 ng/mL. Such a sensitivity enhanced immunosensor based on graphene oxide and SPA co-modified PCF shows a great promise in biosensing. <s> BIB003 </s> Carbon-Based Nanomaterials for Plasmonic Sensors: A Review <s> Graphene and Its derivatives <s> Fabrication and characterization of a highly sensitive fiber-optic cholesterol sensor utilizing cholesterol oxidase (ChOx) have been carried out. The operation of the sensor is based on combined phenomenon of localized and propagating surface plasmons. Two different immobilization schemes of enzyme ChOx have been used. In one, the enzyme is entrapped in the hydrogel and in second the enzyme is immobilized over the graphene oxide (GO) nanosheets. The synthesis of GO is confirmed by XRD, TEM, and Raman spectroscopy and the binding between ChOx and GO is verified through the Raman spectroscopy and Fourier transform infrared spectroscopy techniques. Following three types of probes having different configurations are fabricated and compared: 1) enzyme entrapped gel layer over silver coated unclad fiber; 2) enzyme immobilized over silver and GO coated unclad fiber; and 3) enzyme immobilized over silver, GO, and silver nanoparticles coated unclad fiber. The performance of all these probes have been compared in terms of operating range, sensitivity and limit of detection. The sensors have many advantages, such as compact size, modest cost, high sensitivity, high selectivity, and capability of remote sensing. <s> BIB004
In recent years, a number of plasmonic sensors have been proposed using graphene and its derivatives, with plasmonic metal layers for the purpose of sensitivity enhancement. A theoretical study of surface plasmon resonance-based biomolecules sensors using a graphene layer has also been reported. In this study, the graphene layer was used over the gold and silicon layer to enhance the sensitivity of the sensor BIB002 . The schematic of a prism-based SPR sensor is shown in Figure 5 . The sensor model consisted of coatings of gold, silicon, graphene, and biomolecule layers over the base of the prism. The gold was used as a plasmonic layer, while the silicon and graphene layers were used for sensitivity enhancement. Apart from sensitivity enhancement, the graphene layer also helps in the enhancement of the immobilization of the biomolecules. The thicknesses of gold, silicon, and graphene layers were optimized via simulation. The best sensitivity of the sensor was achieved for a 40 nm thick gold layer, 7 nm thick silicon layer, and two layers of graphene at 633 nm wavelength of the light source. UV-Vis-NIR spectrum. Reprinted with permission from  . Copyright 2016 Royal Society of Chemistry. In recent years, a number of plasmonic sensors have been proposed using graphene and its derivatives, with plasmonic metal layers for the purpose of sensitivity enhancement. A theoretical study of surface plasmon resonance-based biomolecules sensors using a graphene layer has also been reported. In this study, the graphene layer was used over the gold and silicon layer to enhance the sensitivity of the sensor BIB002 . The schematic of a prism-based SPR sensor is shown in Figure 5 . The sensor model consisted of coatings of gold, silicon, graphene, and biomolecule layers over the base of the prism. The gold was used as a plasmonic layer, while the silicon and graphene layers were used for sensitivity enhancement. Apart from sensitivity enhancement, the graphene layer also helps in the enhancement of the immobilization of the biomolecules. The thicknesses of gold, silicon, and graphene layers were optimized via simulation. The best sensitivity of the sensor was achieved for a 40 nm thick gold layer, 7 nm thick silicon layer, and two layers of graphene at 633 nm wavelength of the light source. Another experimental and theoretical study utilizing graphene layer over the gold layer has been reported for the sensitivity enhancement of an SPR biosensor. In this study, the prism was coated with a 50 nm thick gold layer, followed by coatings of graphene and biomolecules (DNA) layers BIB001 . The layer configuration is shown in Figure 6a . For the simulation of a graphene-based SPR sensor, an N-layer model was used and the results were compared with the experiments. In the simulations, the complex refractive index of the graphene (n) was taken as: Another experimental and theoretical study utilizing graphene layer over the gold layer has been reported for the sensitivity enhancement of an SPR biosensor. In this study, the prism was coated with a 50 nm thick gold layer, followed by coatings of graphene and biomolecules (DNA) layers BIB001 . The layer configuration is shown in Figure 6a . For the simulation of a graphene-based SPR sensor, an N-layer model was used and the results were compared with the experiments. In the simulations, the complex refractive index of the graphene (n) was taken as: where λ is the wavelength of light and c is a constant. The thickness of a single graphene was taken as 0.34 nm. Figure 7a shows the surface plasmon resonance curve for the conventional biosensor and monolayer graphene biosensor. From the figure, it is clear that the SPR angle shifts more in the case of the monolayer graphene biosensor. Figure 7b shows the sensitivity enhancement with the number of graphene layers. The improvement in sensitivity was due to two reasons. First, the graphene strongly absorbs the biomolecules with a carbon-based ring structure, which enhances the absorption efficiency of the biomolecules, and second, the optical properties of the graphene modify the SPR curves and provide a large shift for the refractive index change. In summary, this study shows that a graphene-based biosensor enhances the sensitivity 2.5 times in comparison to a conventional biosensor. An experimental study on the enhancement of sensitivity of an SPR-based sensor using graphene oxide and biomolecules attachments has also been reported. In this study, the sensitivity of an SPR-based photonic crystal fiber (PCF) refractive index sensor was compared with an Figure 7a shows the surface plasmon resonance curve for the conventional biosensor and monolayer graphene biosensor. From the figure, it is clear that the SPR angle shifts more in the case of the monolayer graphene biosensor. Figure 7b shows the sensitivity enhancement with the number of graphene layers. Figure 7a shows the surface plasmon resonance curve for the conventional biosensor and monolayer graphene biosensor. From the figure, it is clear that the SPR angle shifts more in the case of the monolayer graphene biosensor. Figure 7b shows the sensitivity enhancement with the number of graphene layers. The improvement in sensitivity was due to two reasons. First, the graphene strongly absorbs the biomolecules with a carbon-based ring structure, which enhances the absorption efficiency of the biomolecules, and second, the optical properties of the graphene modify the SPR curves and provide a large shift for the refractive index change. In summary, this study shows that a graphene-based biosensor enhances the sensitivity 2.5 times in comparison to a conventional biosensor. An experimental study on the enhancement of sensitivity of an SPR-based sensor using graphene oxide and biomolecules attachments has also been reported. In this study, the sensitivity of an SPR-based photonic crystal fiber (PCF) refractive index sensor was compared with an The improvement in sensitivity was due to two reasons. First, the graphene strongly absorbs the biomolecules with a carbon-based ring structure, which enhances the absorption efficiency of the biomolecules, and second, the optical properties of the graphene modify the SPR curves and provide a large shift for the refractive index change. In summary, this study shows that a graphene-based biosensor enhances the sensitivity 2.5 times in comparison to a conventional biosensor. An experimental study on the enhancement of sensitivity of an SPR-based sensor using graphene oxide and biomolecules attachments has also been reported. In this study, the sensitivity of an SPR-based photonic crystal fiber (PCF) refractive index sensor was compared with an immunosensor with and without a graphene oxide layer. The probe was designed by splicing the PCF fiber between the two multimode fibers BIB003 . To fabricate the sensor, the 5 mm sensing length of the PCF fiber was first coated with a 5 nm thin chrome layer and then with a 50 nm thin film of gold. After this, an over-layer of graphene oxide was deposited over the gold layer through various steps, as shown in the Figure 8a- immunosensor with and without a graphene oxide layer. The probe was designed by splicing the PCF fiber between the two multimode fibers BIB003 . To fabricate the sensor, the 5 mm sensing length of the PCF fiber was first coated with a 5 nm thin chrome layer and then with a 50 nm thin film of gold. After this, an over-layer of graphene oxide was deposited over the gold layer through various steps, as shown in the Figure 8a- The SPR spectra for the gold coated PCF fiber (Au-PCF) and gold and graphene oxide coated PCF fiber (Au/GO-PCF) are shown in Figure 9a immunosensor with and without a graphene oxide layer. The probe was designed by splicing the PCF fiber between the two multimode fibers BIB003 . To fabricate the sensor, the 5 mm sensing length of the PCF fiber was first coated with a 5 nm thin chrome layer and then with a 50 nm thin film of gold. After this, an over-layer of graphene oxide was deposited over the gold layer through various steps, as shown in the Figure 8a- The SPR spectra for the gold coated PCF fiber (Au-PCF) and gold and graphene oxide coated PCF fiber (Au/GO-PCF) are shown in Figure 9a The change in the resonance wavelength for the Au-PCF and Au/GO-PCF sensors were reported to be 114 nm and 181 nm for the refractive index range from 1.33 to 1.37, respectively. For the case of the Au-PCF and Au/GO-PCF sensors, the values of sensitivity obtained were 2761.7 nm/RIU and 4649.8 nm/RIU, respectively. Here, the graphene oxide film enhances the interaction between the Au film and analyte which increases the refractive index sensitivity. For the SPR immunosensor, the attachment steps of SPA are represented in Figure 10 . Figure 10 shows the wavelength shift curve of anti-human IgG immobilized on the Au/GO-SPA and Au-SPA sensor. When the IgG sample comes in contact with the sensor surface, the refractive index changes and a red shift is observed. The change in the resonance wavelength for the Au-SPA sensor is around 7.4 nm, while for the Au/GO-SPA sensor it is 15.2 nm. This enhancement shows that due to the large surface area and biocompatibility, GO allows for more immobilization of the antibody than the gold surface, which leads to the larger resonance wavelength shift. This study shows that GO has a great potential in sensitivity enhancement for SPR-based refractive index sensors or biosensors. The change in the resonance wavelength for the Au-PCF and Au/GO-PCF sensors were reported to be 114 nm and 181 nm for the refractive index range from 1.33 to 1.37, respectively. For the case of the Au-PCF and Au/GO-PCF sensors, the values of sensitivity obtained were 2761.7 nm/RIU and 4649.8 nm/RIU, respectively. Here, the graphene oxide film enhances the interaction between the Au film and analyte which increases the refractive index sensitivity. For the SPR immunosensor, the attachment steps of SPA are represented in Figure 10 . Figure 10 shows the wavelength shift curve of anti-human IgG immobilized on the Au/GO-SPA and Au-SPA sensor. When the IgG sample comes in contact with the sensor surface, the refractive index changes and a red shift is observed. The change in the resonance wavelength for the Au-SPA sensor is around 7.4 nm, while for the Au/GO-SPA sensor it is 15.2 nm. This enhancement shows that due to the large surface area and biocompatibility, GO allows for more immobilization of the antibody than the gold surface, which leads to the larger resonance wavelength shift. This study shows that GO has a great potential in sensitivity enhancement for SPR-based refractive index sensors or biosensors. In another study, an SPR-based fiber optic cholesterol sensor with three different approaches was discussed BIB004 . In this study, three different types of probes were fabricated, as shown in Figure  11 . In probe I, the gel entrapment method was used for the detection of cholesterol, while in probe II, the enzyme cholesterol oxidase (Chox) was immobilized over the graphene oxide nanosheets, and in probe III, polyvinyl alcohol (PVA) embedded silver nanoparticles were decorated over the GO nanosheets. For the synthesis of GO, the modified Hummer's method was used. In probe I, the probe was fabricated by coating a thin silver layer over the core of an optical fiber, followed by the enzyme entrapped hydrogel. Probe II consisted of a GO layer over the silver coated unclad core of the fiber, and a Chox enzyme was immobilized over it. Finally, probe III consisted of layers of silver and GO over the core of an optical fiber, followed by layers of PVA embedded silver nanoparticles and enzyme Chox. The SPR curves for probe I for a cholesterol concentration range of 0 to 10 mM shifted by around 18 nm, which gave 1.789 nm/mM as the sensitivity of the sensor. The reason behind the shift in resonance wavelength is that when the cholesterol sample interacts with the cholesterol oxidase, the enzymatic reaction takes place and it gets converted into Cholest-4en-3-one and hydrogen peroxide. Due to this enzymatic reaction, the effective refractive index of the sensing layer changes, which results in the shift in resonance wavelength. For probe II, the total shift in the resonance wavelength was around 32 nm, which was greater than that in the case of probe I. In In another study, an SPR-based fiber optic cholesterol sensor with three different approaches was discussed BIB004 . In this study, three different types of probes were fabricated, as shown in Figure 11 . In probe I, the gel entrapment method was used for the detection of cholesterol, while in probe II, the enzyme cholesterol oxidase (Chox) was immobilized over the graphene oxide nanosheets, and in probe III, polyvinyl alcohol (PVA) embedded silver nanoparticles were decorated over the GO nanosheets. For the synthesis of GO, the modified Hummer's method was used. In probe I, the probe was fabricated by coating a thin silver layer over the core of an optical fiber, followed by the enzyme entrapped hydrogel. Probe II consisted of a GO layer over the silver coated unclad core of the fiber, and a Chox enzyme was immobilized over it. Finally, probe III consisted of layers of silver and GO over the core of an optical fiber, followed by layers of PVA embedded silver nanoparticles and enzyme Chox. The SPR curves for probe I for a cholesterol concentration range of 0 to 10 mM shifted by around 18 nm, which gave 1.789 nm/mM as the sensitivity of the sensor. The reason behind the shift in resonance wavelength is that when the cholesterol sample interacts with the cholesterol oxidase, the enzymatic reaction takes place and it gets converted into Cholest-4en-3-one and hydrogen peroxide. Due to this enzymatic reaction, the effective refractive index of the sensing layer changes, which results in the shift in resonance wavelength. For probe II, the total shift in the resonance wavelength was around 32 nm, which was greater than that in the case of probe I. In probe II, GO provided a large surface area, and therefore the available surface area for the enzyme was increased and the shift in resonance wavelength was increased. probe II, GO provided a large surface area, and therefore the available surface area for the enzyme was increased and the shift in resonance wavelength was increased. 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.