Reduced Graphene Oxide Nanostructures Research Articles

Bimetallic Au-Ag Reduced Graphene Oxide Nanostructures for Enhanced Detection of Dengue Virus E Protein

The development of highly sensitive and specific diagnostic tools for early-stage detection of dengue virus (DENV) is critical for effective outbreak management, particularly in resource-limited settings. In this study, we report a novel electrochemical immunosensor based on bimetallic gold silver (Au-Ag) nanoparticles integrated with reduced graphene oxide (rGO) for the detection of dengue virus envelope (E) protein. The Au-Ag bimetallic nanostructures exhibit superior electron transfer kinetics and enhanced electrocatalytic activity, while rGO serves as an excellent platform due to its large surface area and high conductivity. This synergistic combination improves antigen-antibody interactions and significantly boosts sensor performance. The immunosensor demonstrated a broad linear detection range of 100 ag ml−1 to 10 ng ml−1, with a high correlation coefficient (R2 = 0.98519). It achieved an ultra-low limit of detection (LOD) of 4.959 ag ml−1 for DENV E protein, outperforming existing detection methods. These findings highlight the potential of the Au-Ag- rGO-based immunosensor as a promising tool for point-of-care diagnosis, enabling rapid and cost-effective disease management and control.

Electrochemistry at 2D and 3D nanoelectrodes: The interplay between interface kinetics and surface density of states

Heterogenous Electron Transfer (HET) at electrode-electrolyte interfaces depends strongly on the morphological features or geometry of the nanostructure used for modifying the electrode surface. A swift HET results in faster interface kinetics, which has significant impact on the development/calibration of electrochemical devices like biomolecular sensors, supercapacitors, batteries and electrochromic platforms. The interface electrochemistry depends strongly on the electronic Density of States (DOS) of electrode materials. Over the past years, the 2D electron gas nanomaterials - primarily graphene, Graphene Oxide (GO) and Reduced Graphene Oxide (RGO), have garnered significant interest in electrochemical applications due to promising DOS features. However, the electroanalytical dependency on DOS of 3D nanostructures such as Zinc Oxide Tetrapods (ZnOT) is yet unexplored. The current work focusses on a comparative electrochemical analysis of interface kinetics at RGO (2D) and ZnOT (3D) coated screen printed electrodes, with the intention of selecting the suitable material geometry. The analyses were performed using Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). While the dependence of HET on DOS of RGO and ZnOT nanomaterials were studied using both DFT analysis and impedance-derived capacitance spectroscopy – the latter giving insights on quantum capacitance. It was observed that, the 2D RGO nanostructures exhibit higher surface DOS near the Fermi level, along with a high quantum capacitance (∼345 nF) as compared to 3D ZnOT (∼276 nF). This results in enhanced HET at the former, thereby indicating its suitability in developing futuristic electrochemical devices for various applications as desired.

Mesoporous ZnS-Sb/C Reduced Graphene Oxide Nanostructures as Anode Materials for Sodium-Ion Batteries

Mesoporous ZnS-Sb/C Reduced Graphene Oxide Nanostructures as Anode Materials for Sodium-Ion Batteries

Ultrasensitive Electrochemical Sensor Based on SnO2 Anchored 3D Porous Reduced Graphene Oxide Nanostructure Produced via Sustainable Green Protocol for Subnanomolar Determination of Anti-Diabetic Drug, Repaglinide

Herein, we have reported on a simple, environmentally friendly, and ultra-sensitive electrode material, SnO2@p-rGO, used in a clean sustainable manner for rapid electrochemical determination of an anti-diabetic agent, repaglinide (RPG). Three-dimensional porous reduced graphene oxide nanostructure (p-rGO) was prepared via a low-temperature solution combustion method employing glycine. The aqueous extract of agricultural waste “cotton boll peel” served as stabilizing and reducing agents for the synthesis of SnO2 nanoparticles. The structural and morphological characterization was carried out by XRD, Raman, SEM, EDX, FTIR, absorption, and TGA. The oxidation process of RPG was realized under adsorption controlled with the involvement of two protons and electrons. The sensor displayed a wider linearity between the concentration of RPG and oxidation peak current in the ranges of 1.99 × 10−8–1.45 × 10−5 M and 4.99 × 10−8–1.83 × 10−5 M for square-wave voltammetric and differential pulse voltammetric methods, respectively. The lower limit of detection value of 0.85 × 10−9 M was realized with the SWV method. The proposed sensor was applied for the quantification of RPG in fortified urine samples and pharmaceutical formulations. Furthermore, the sensor demonstrated reproducibility, long-term stability, and selectivity in the presence of metformin and other interferents, which made the proposed sensor promising and superior for monitoring RPG.

Facile preparation of silver nanoparticle decorated RGO nanostructures with enhanced simultaneous detection property for hydroquinone and catechol

Well-dispersed reduced graphene oxide (RGO) nanostructures with uniform decoration of silver nanoparticles (AgNPs) on them have been prepared through a hydrothermal process followed by the freeze-drying method. The obtained graphene-layer-like structure with uniform distribution of AgNPs have then been applied on the glassy carbon electrodes (GCE) to detect different concentration of hydroquinone (HQ) and catechol (CC) in their mixed solutions. Because of such well-dispersed structure and uniform distribution of AgNPs, the AgNPs/RGO/GCE illustrated enhanced simultaneous detection performance for HQ and CC. In the HQ and CC mixed solution with pH of 6.0, the AgNPs/RGO/GCE electrode exhibits a wide linear range of HQ (0.18–130 μM) and CC (0.07–120 μM), and accurate limit of detection (2.37 × 10−2 μM for HQ and 5.8 × 10−2 μM for CC). Thus, it is believed that the AgNPs/RGO nanostructures shows a great potential of applying as possible probes for simultaneous detection of HQ and CC in real environment.

Effects of Graphene Oxide and Reduced Graphene Oxide Nanostructures on CD4+ Th2 Lymphocytes.

The activation of T helper (Th) lymphocytes is necessary for the adaptive immune response as they contribute to the stimulation of B cells (for the secretion of antibodies) and macrophages (for phagocytosis and destruction of pathogens) and are necessary for cytotoxic T-cell activation to kill infected target cells. For these issues, Th lymphocytes must be converted into Th effector cells after their stimulation through their surface receptors TCR/CD3 (by binding to peptide-major histocompatibility complex localized on antigen-presenting cells) and the CD4 co-receptor. After stimulation, Th cells proliferate and differentiate into subpopulations, like Th1, Th2 or Th17, with different functions during the adaptative immune response. Due to the central role of the activation of Th lymphocytes for an accurate adaptative immune response and considering recent preclinical advances in the use of nanomaterials to enhance T-cell therapy, we evaluated in vitro the effects of graphene oxide (GO) and two types of reduced GO (rGO15 and rGO30) nanostructures on the Th2 lymphocyte cell line SR.D10. This cell line offers the possibility of studying their activation threshold by employing soluble antibodies against TCR/CD3 and against CD4, as well as the simultaneous activation of these two receptors. In the present study, the effects of GO, rGO15 and rGO30 on the activation/proliferation rate of these Th2 lymphocytes have been analyzed by studying cell viability, cell cycle phases, intracellular content of reactive oxygen species (ROS) and cytokine secretion. High lymphocyte viability values were obtained after treatment with these nanostructures, as well as increased proliferation in the presence of rGOs. Moreover, rGO15 treatment decreased the intracellular ROS content of Th2 cells in all stimulated conditions. The analysis of these parameters showed that the presence of these GO and rGO nanostructures did not alter the response of Th2 lymphocytes.

Thermal energy storage system with a high-temperature nanoparticle enhanced phase change material: Charging and discharging characteristics upon integration with process preheating

Nanoparticle enhanced Phase Change Materials (NePCMs) are modified forms of phase change materials containing nanoparticles such that the thermophysical properties are improved. Solar salt, a phase change material comprising a eutectic mixture of sodium nitrate and potassium nitrate has a thermal conductivity in the range of 0.5–1 W/m K and the addition of nanoparticles to solar salt (nanoparticle enhanced solar salt) has been demonstrated to improve its thermophysical properties. This work evaluates the charging and discharging characteristics of solar salt and nanoparticle enhanced solar salt, with copper nanoparticles (at 0.5 wt%) and reduced Graphene Oxide nanostructures (at 0.5 wt%) individually as additives. Experiments were performed simulating a thermal energy storage system (TES) for process preheating. Silicone oil and Therminol® 55 were used as heat transfer fluids during charging and discharging cycles respectively. The overall heat transfer coefficients of TES containing copper nanoparticle enhanced solar salt and reduced Graphene Oxide nanostructure enhanced solar salt as an energy storage medium were greater than that containing solar salt during both charging and discharging cycles. The enhancements in overall heat transfer coefficients were due to respective enhancements in PCM side heat transfer coefficients caused by thermal conductivity and specific heat increase brought about by the addition of copper nanoparticles/reduced Graphene Oxide nanostructures to solar salt. Considering >10 % enhancements in overall heat transfer coefficient in both charging and discharging cycles with these two NePCMs, latent heat thermal energy storage systems employing them as phase change material will be suitable for integration with a process preheating application.

POROUS SILICON NANOSYSTEMS FOR ETHANOL SENSOR

Sensory elements based on hybrid films containing porous silicon and reduced graphene oxide nanostructures have been created. A decrease in electrical resistance and an increase in the capacitance of sensor elements in the AC mode due to the adsorption of ethanol molecules have been registered. To assess the sensory properties of hybrid films, the concentration dependences of the adsorption ability in the 0–4.5% range were determined and the dynamic characteristics of ethanol sensors based on them were studied. It was found that sensor films with different ratios of porous silicon and reduced graphene oxide nanoparticles have the maximum sensitivity to ethanol in different concentration ranges. The functional properties of hybrid films can be controlled by changing the proportions of their components. The reaction time of sensory elements to changes in the concentration of ethanol molecules is 40–50 s. The obtained results expand the perspective of the application of nanosystems based on porous silicon in sensor devices.

Ruthenium dioxide anchored on reduced graphene oxide nanocomposite for 1.2 V symmetric supercapacitor devices

In this work, the high-performance symmetric electrochemical capacitor based on ruthenium dioxide anchored on reduced graphene oxide nanostructures was developed. The created system consists of the negative and positive electrodes with Na2SO4 aqueous electrolyte. The symmetric capacitor devices electrodes were tested in the potential range of 0 to 1.2 V. As a result, at 2 A/g,the rGO/RuO2 and RuO2 electrodes gained higher energy (41 Wh kg−1 and 31 Wh kg−1) and power densities (1.2 kW kg−1 and 0.98 kW kg−1). After the 2000th cycle,the rGO/RuO2 electrode has such a high cyclic efficiency of 96%, which is considerably higher than the pure RuO2 (88%).This is due to the anchoring of high conducting rGO in rGO/RuO2 nanocomposite. These findings reinforce the idea of graphene-based nanocomposites having as a safe aqueous electrolyte in high-voltage energy storage devices.

A simple method for functionalization of polypyrrole-coated cotton fabrics by reduced graphene oxide for UV screening

Here, the modified Hummer’s method was used to prepare reduced graphene oxide (RGO) nanostructures. The functional properties of the prepared RGO nanostructures were studied by using the X-ray diffraction method (XRD), and scanning electron microscopy (SEM). Using the in situ polymerization process, polypyrrole (PPy) was prepared. During polymerization, an ultrasound-assisted coating process was used to coat the cotton fabrics. In addition, the Nafion@RGO composite mixture was deposited on the surface of PPy- coated cotton fabric by dip and dry method. The presence of cellulose crystal structure was confirmed by conducting a structural study of the coated and uncoated (UC) cotton fabrics. The presence of a characteristic functional group was confirmed by the Fourier transform infrared spectroscopy–Attenuated total reflection (FTIR–ATR) study of the fabrics. Wrinkled-like morphology with a proper binding of PPy with RGO was revealed by SEM analysis. Our results showed that the RGO coated fabrics could be distributed on the cotton fabric and possessed better thermal stability and wettability properties than uncoated cotton fabric. Ultraviolet (UV)-screening properties of the coated and uncoated fabrics confirmed the better performance of the coated cotton fabrics in the order of Nafion/RGO coated fabrics (RGPC) > PPy/Cotton (PC) > UC. The transmission and screening efficiency of RGO coated fabrics were 0.02% and 99.98%, respectively. Thus the Nafion/RGO/PPy fabrics is a safe and promising UV screening and have great potential application in the all wearable textile sector.

MoS2/Ni3S2/Reduced Graphene Oxide Nanostructure as an Electrocatalyst for Alcohol Fuel Cells

The development of active and stable catalysts is essential for the commercialization of direct alcohol fuel cells. In this work, we introduce a MoS2/Ni3S2/rGO catalyst as a cost-effective, stable, and high-performance catalyst for application in alcohol fuel cells. MoS2/Ni3S2 and its hybrid with reduced graphene oxide (rGO) are synthesized and evaluated as nanocatalysts in the alcohol (methanol and ethanol) electro-oxidation process in alkaline media. The effects of temperature and scanning rate are investigated. Voltammetry results show that MoS2/Ni3S2/rGO has good catalytic efficiency and excellent stability of 106 and 104% after 200 consecutive CV cycles for MOR and EOR, respectively. The synergic effect of starfish Ni3S2 and the coated porous MoS2 facilitates the absorption of hydroxyl ions and alcohols on the surface of the catalyst, while rGO enlarges the specific surface area and the electrical conductivity of the electrocatalyst.

Designing of cerium phosphate nanorods decorated reduced graphene oxide nanostructures as modified electrode: An effective mode of dopamine sensing

In this study, we have developed a voltametric and amperometric sensor by using cerium phosphate nanorods (CPNR) decorated reduced graphene oxide (RGO) (CPRGO) modified glassy carbon electrode for sensitive and selective detection of dopamine (DA). These nanostructures have been characterized by PXRD, FT-IR, FESEM, HR-TEM, Raman and XPS spectroscopy. Electrochemical performance of electrodes was investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The sensor demonstrated the linear range 1 – 10 μM by voltametric and 1–13 μM by amperometric with the detection limit of 0.118 μM (S/N = 3) and 0.109 μM, respectively. Selective detection of DA against glucose and ascorbic acid (AA) was performed by amperometry. In present sensing method of DA in real biological fluid blood serum separated from human blood sample was applied to get recovery between 99.4 and 100.66 %. Our results show that electrode modified with these nanostructures can serve as the most sensitive and selective detection for DA in neurological environment.

Heterostructured SmCoO3/rGO composite for high-energy hybrid supercapacitors

A supercapacitor is an efficient energy storage system that acts as an excellent booster to deliver high power density required for batteries and fuel cells. Recently, composite material–based supercapacitors have attracted much more interest as promising greener and more capable candidates in energy-saving use. In this work, samarium cobalt oxide–decorated reduced graphene oxide (SmCoO3/rGO) was prepared employing solvothermal route and used as reliable electrode material. The maximum specific capacity achieved was 30.80 mAh/g for 1 A/g of SmCoO3/rGO nanocomposite with capacity retention of 86.95%@5A/g over 5000 charge discharge cycles. Better electrochemical performance of samarium and reduced graphene oxide nanostructures prevent the transfer of electrons through electrochemical active sites, creating electronic and structural diversity of electro active material. In addition, SmCoO3/rGO/AC hybrid supercapacitor device that delivered good energy and power density of 52 W h/kg and 752 W/kg at 1 A/g was designed. 74.28% capacitive retention and 98.26% coulombic efficiency was maintained over 15,000 cycles.

Fe3O4@Au/reduced graphene oxide nanostructures: Combinatorial effects of radiotherapy and photothermal therapy on oral squamous carcinoma KB cell line

Combination therapy including radiotherapy (RT) and photothermal therapy (PTT), as an alternative treatment for cancer treatment, recently has generated substantial interest. Graphene-based nanocarbons and gold content metallic nanoparticles, as one of the most attractive materials with their extraordinary physical and chemical properties, are extremely useful in this regard. We herein reported the effects of combination of cancer therapy including RT (doses of 2 and 4 Gy) and PTT (808 nm laser irradiation, NIR irradiation) as radio-photothermal therapy (RPTT) of KB oral squamous carcinoma cell line in the presence of Fe3O4@Au/reduced graphene oxide (rGO) nanostructures (NSs) at different concentrations. Fe3O4@Au/rGO NSs with different rGO contents have been synthesized by hydrothermal reaction method. They characterized by XRD, FE-SEM, TEM, EDS, FTIR and TGA. Cell viability for cytotoxicity, RT, PTT and RPTT were studied by MTT (3-[4,5-dimethylthiazol-2-yl]-2, 5-diphenyltetrazolium bromide) assay. High photothermal conversion efficiency of Fe3O4@Au/rGO NSs reaches to 61%. The resulting Fe3O4@Au NPs with approximate size of about 10–60 nm covered the rGO nanosheets with 40 wt% rGO at concentration of 20 μg ml−1, exhibited satisfactory cytotoxicity. They provided significant cell destruction under RT, PTT and specially RPTT in dose of 4 Gy. Furthermore, they have good biocompatibility on the healthy cells. Our results show that Fe3O4@Au/rGO NSs integrated with radiosensitization, photothermal therapy and their combination promising for nanomedicine and clinical applications.

Lowered total solidification time and increased discharge rate of reduced graphene oxide-solar salt composites: Potential for deployment in latent heat thermal energy storage system

Solar salt is a useful energy storage medium and a heat transfer fluid for concentrated solar thermal applications. The present work explores the use of reduced graphene oxide (rGO) nanostructures as an additive to solar salt, to improve its thermophysical properties. Reduced graphene oxide nanostructures at weight fractions of 0.125 wt% and 0.5 wt% were added to the solar salt, leading to rGO-solar salt composites. The thermophysical characterization experiments revealed that the specific heat in the 0.5 wt% rGO-solar salt nanocomposite was elevated by 6% over the temperature range of 50–270 °C. The time required for complete solidification of 0.125 wt% and 0.5 wt% was reduced by 43% and 49%. In the discharge cycle, the heat transfer rate was amplified by 90% and 138% respectively for 0.125 wt% and 0.5 wt% rGO-solar salt nanocomposites, attributable to thermal conductivity enhancement, specific heat augmentation and heterogeneous nucleation. Thus, rGO-solar salt composites are suitable for use as latent heat thermal energy storage medium in solar thermal applications.

Effect of pH-Responsive Charge-Conversional Polymer Coating to Cationic Reduced Graphene Oxide Nanostructures for Tumor Microenvironment-Targeted Drug Delivery Systems.

Tumor tissue represents a slightly acidic pH condition compared to normal tissue due to the accumulation of lactic acids via anaerobic metabolism. In this work, pH-responsive charge-conversional polymer (poly(ethylene imine)-poly(l-lysine)-poly(l-glutamic acid), PKE polymer) was employed for endowing charge-conversional property and serum stability to poly(ethylene imine) conjugated reduced graphene oxide-based drug delivery system (PEI-rGO). Zeta-potential value of PEI-rGO coated with PK5E7 polymer (PK5E7(PEI-rGO)) was −10.9 mV at pH 7.4 and converted to 29.2 mV at pH 6.0, showing pH-responsive charge-conversional property. Sharp-edged plate morphology of PEI-rGO was transformed to spherical nanostructures with vague edges by PK5E7 coating. Size of PK5E7(PEI-rGO) was found to be smaller than that of PEI-rGO in the serum condition, showing its increased serum stability. Loaded doxorubicin (DOX) in PK5E7(PEI-rGO) could be released rapidly in lysosomal condition (pH 5.0, 5 mM glutathione). Furthermore, DOX-loaded PK5E7(PEI-rGO) showed enhanced anticancer activity in HeLa and A549 cells in the tumor microenvironment-mimicking condition (pH 6.0, serum), which would be mediated by non-specific cellular interaction with decorated serum proteins. These results indicate that the pH-responsive charge-conversional PKE polymer coating strategy of cationic rGO nanostructures possesses a potential for acidic tumor microenvironment-targeted drug delivery systems.

One-Pot Electrochemical Synthesis of Lead Oxide- Electrochemically Reduced Graphene Oxide Nanostructures and Their Electrocatalytic Applications

In this paper, a new electrochemical method for the cathodic electrodeposition of lead oxide-electrochemically reduced graphene oxide (PbO-ERGO) from an aqueous solution was carried out in one-pot in the same solution containing Pb2+ and graphene oxide, leading to the direct formation of crystalline thin films at near-room temperature. XRD was employed to determine the crystallinity index of the PbO-ERGO nanostructures. SEM, XPS, EDS, and UV–visible spectroscopy techniques were employed to analyze the morphological, structural, and optical characteristics of the composite materials. Owing to the rapid charge transport in the composite materials of PbO-ERGO, rapid, and uniform photocurrent responses were observed. In addition, the PbO-ERGO composite electrode exhibited a 40 and 130 fold increase in the photocatalytic performance compared to PbO and ERGO electrodes, respectively. Then, the nanocomposite-modified electrode was applied for the non-enzymatic sensing of H2O2. A linear amperometric response to H2O2 was observed at concentrations in the range from $1 \times 10^{-5}$ to $10\times 10^{-3}$ mol L−1. The sensitivity and detection limit of the PbO-ERGO electrode were estimated as $2.26~\mu \text{A}$ mM−1 cm−2 and $2\times 10^{-7}$ mol L−1, respectively, at a signal-to-noise ratio of 3.0.

Enhanced antibacterial and photocatalytic activities of silver nanoparticles anchored reduced graphene oxide nanostructure

The nanocomposites with sliver (Ag) nanoparticle and Reduced Graphene Oxide (r-GO) has shown promising potential application for wastewater treatment due to their outstanding chemical and biological properties. In the present work, the Ag nanoparticles anchored r-GO (r-GO/Ag) nanocomposite was prepared by one-pot in situ chemical reduction technique. The synthesized composites were characterized by powder x-ray diffraction (XRD), x-ray photoelectron microscopy (XPS), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and UV-visible spectroscopy techniques. The result revealed that the dispersion of Ag nanoparticle with an average size of 24 nm on r-GO and it confirms the formation of nanocomposites. With the synergetic effect of Ag nanoparticles on r-GO displayed the significant antibacterial activity against both Gram-positive Staphylococcus aureus (S. aureus) and Gram-negative Escherichia coli (E. coli) bacteria using agar well diffusion method. Moreover, the photocatalytic performance of the nanocomposites was evaluated under visible light irradiation using Methylene Blue (MB) as well as Rhodamine B (RhB) as model organic pollutants which showed the superior catalytic activities. It is suggested that the (r-GO/Ag) nanocomposite would be the material for wastewater treatment with combined antibacterial and photocatalytic activities.

Rapid and efficient synthesis of reduced graphene oxide nano-sheets using CO ambient atmosphere as a reducing agent

Graphene oxide (GO) and reduced graphene oxide (RGO) nanostructures were synthesized using a novel method of CO gas flow under ambient pressure and at several temperatures. The produced samples of GO and RGO were structurally, chemically and optically characterized and the results were analyzed using the techniques of UV–Vis spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction, field-effect scanning electron microscopy (FE-SEM), and sheet resistance measurements. Thermo-gravimetric analysis, and FTIR indicated the successful preparation of GO and RGO. FE-SEM was used to demonstrate the layer structure of GO and RGO nanostructures. The band gap energy (Eg) of the samples was estimated through the optical absorption spectra of GO and RGOs recorded between 200 and 1100 nm wavelengths using UV–Vis spectroscopy. The results are in good agreement with the data determined by other workers. Sheet resistance of RGO shows a decreasing trend versus annealing reduced temperature. This behavior is in accordance with variation of c-axis parameter with temperature which can be suggested to be due to the removal of water molecules and oxygen-containing functional groups between the carbon layers of the GO. Removing of the latter components may results in decreasing the distance between the graphene nano-layers.

Reduced Graphene Oxide Nanostructures by Light: Going Beyond the Diffraction Limit

Graphene oxide (GO) offers excellent possibilities that are recently demonstrated in many applications ranging from biological sensors to optoelectronic devices. The process of thermal annealing aids in removing the oxygen-containing groups in GO, making GO more graphene-like, or the so-called reduced graphene oxide (rGO). Thermal reduction can also be achieved by intense light. Here, we demonstrate a scalable, inexpensive, and environmentally friendly method to pattern graphene oxide films beyond the diffraction limit of light using a conventional laser. We show that contrary to previous reports, non-linear effects that occur under high intensity conditions of laser irradiation allow the fabrication of highly conductive carbon nanowires with dimensions much smaller than the laser spot size. The potential of this method is illustrated by the fabrication of several devices on flexible and transparent substrates, including hybrid plasmonic/rGO sensors.