Optical Fibre Sensors Using Graphene-Based Materials: A Review

The State of the Art on Graphene-Based Sensors for Human Health Monitoring through Breath Biomarkers

The field of organic-borne biomarkers has been gaining relevance due to its suitability for diagnosing pathologies and health conditions in a rapid, accurate, non-invasive, painless and low-cost way. Due to the lack of analytical techniques with features capable of analysing such a complex matrix as the human breath, the academic community has focused on developing electronic noses based on arrays of gas sensors. These sensors are assembled considering the excitability, sensitivity and sensing capacities of a specific nanocomposite, graphene. In this way, graphene-based sensors can be employed for a vast range of applications that vary from environmental to medical applications. This review work aims to gather the most relevant published papers under the scope of "Graphene sensors" and "Biomarkers" in order to assess the state of the art in the field of graphene sensors for the purposes of biomarker identification. During the bibliographic search, a total of six pathologies were identified as the focus of the work. They were lung cancer, gastric cancer, chronic kidney diseases, respiratory diseases that involve inflammatory processes of the airways, like asthma and chronic obstructive pulmonary disease, sleep apnoea and diabetes. The achieved results, current development of the sensing sensors, and main limitations or challenges of the field of graphene sensors are discussed throughout the paper, as well as the features of the experiments addressed.

Structure-Property-Processing Correlations of Graphene Bioscaffolds for Proliferation and Differentiation of C2C12 Cells

Graphene - an atomically thin layer of carbon atoms arranged in a hexagonal lattice - has gained interest as a bioscaffold for tissue engineering due to its exceptional mechanical, electrical, and thermal properties. Graphene's structure and properties are tightly coupled to synthesis and processing conditions, yet their influence on biomolecular interactions at the graphene-cell interface remains unclear. In this study, C2C12 cells were grown on graphene bioscaffolds with specific structure-property- processing-performance (SP3) correlations. Bioscaffolds were prepared using three different methods - chemical vapor deposition (CVD), sublimation of silicon carbide (SiC), and printing of liquid phase exfoliated graphene. To investigate the biocompatibility of each scaffold, cellular morphology and gene expression patterns were investigated using the bipotential mouse C2C12 cell line. Using a combination of fluorescence microscopy and qRT-PCR, we demonstrate that graphene production methods determine the structural and mechanical properties of the resulting bioscaffold, which in turn determine cell morphology, gene expression patterns, and cell differentiation fate. Therefore, production methods and resultant structure and properties of graphene bioscaffolds must be chosen carefully when considering graphene as a bioscaffold for musculoskeletal tissue engineering.

Utilization of smartphones for the evaluation of Gr/Ni nanostructures magnetically controlled based on optical fibers surface plasmons

In the suggested optical fiber-based magnetoplasmonic system, we investigated the magnetic properties of graphene/nickel nanostructures. The plasmonic mode changes under the magnetic field observed in the intensity diagrams over time. To be accessible, cheap, and portable, we used a smartphone as a detector and processor. Considering the ambient noise and the light source, it was reported that the intensity of the changes improved up to 5 times. Further, the clad corrosion experiment carried out by pure dimethyl ketone in an intensity modulation by a smartphone camera and 10 seconds suggested removing fluorine polymer clad.

Graphene Incorporated Electrospun Nanofiber for Electrochemical Sensing and Biomedical Applications: A Critical Review

The extraordinary material graphene arrived in the fields of engineering and science to instigate a material revolution in 2004. Graphene has promptly risen as the super star due to its outstanding properties. Graphene is an allotrope of carbon and is made up of sp²-bonded carbon atoms placed in a two-dimensional honeycomb lattice. Graphite consists of stacked layers of graphene. Due to the distinctive structural features as well as excellent physico-chemical and electrical conductivity, graphene allows remarkable improvement in the performance of electrospun nanofibers (NFs), which results in the enhancement of promising applications in NF-based sensor and biomedical technologies. Electrospinning is an easy, economical, and versatile technology depending on electrostatic repulsion between the surface charges to generate fibers from the extensive list of polymeric and ceramic materials with diameters down to a few nanometers. NFs have emerged as important and attractive platform with outstanding properties for biosensing and biomedical applications, because of their excellent functional features, that include high porosity, high surface area to volume ratio, high catalytic and charge transfer, much better electrical conductivity, controllable nanofiber mat configuration, biocompatibility, and bioresorbability. The inclusion of graphene nanomaterials (GNMs) into NFs is highly desirable. Pre-processing techniques and post-processing techniques to incorporate GNMs into electrospun polymer NFs are precisely discussed. The accomplishment and the utilization of NFs containing GNMs in the electrochemical biosensing pathway for the detection of a broad range biological analytes are discussed. Graphene oxide (GO) has great importance and potential in the biomedical field and can imitate the composition of the extracellular matrix. The oxygen-rich GO is hydrophilic in nature and easily disperses in water, and assists in cell growth, drug delivery, and antimicrobial properties of electrospun nanofiber matrices. NFs containing GO for tissue engineering, drug and gene delivery, wound healing applications, and medical equipment are discussed. NFs containing GO have importance in biomedical applications, which include engineered cardiac patches, instrument coatings, and triboelectric nanogenerators (TENGs) for motion sensing applications. This review deals with graphene-based nanomaterials (GNMs) such as GO incorporated electrospun polymeric NFs for biosensing and biomedical applications, that can bridge the gap between the laboratory facility and industry.

Printing Polymeric Convex Lenses to Boost the Sensitivity of a Graphene-Based UV Sensor

Ultraviolet (UV) is widely used in daily life as well as in industrial manufacturing. In this study, a single-step postprocess to improve the sensitivity of a graphene-based UV sensor is studied. We leverage the advantage of electric-field-assisted on-demand printing, which is simply applicable for mounting functional polymers onto various structures. Here, the facile printing process creates optical plano-convex geometry by accelerating and colliding a highly viscous droplet on a micropatterned graphene channel. The printed transparent lens refracts UV rays. The concentrated UV photon energy from a wide field of view enhances the photodesorption of electron-hole pairs between the lens and the graphene sensor channel, which is coupled with a large change in resistance. As a result, the one-step post-treatment has about a 4× higher sensitivity compared to bare sensors without the lenses. We verify the applicability of printing and the boosting mechanism by variation of lens dimensions, a series of UV exposure tests, and optical simulation. Moreover, the method contributes to UV sensing in acute angle or low irradiation. In addition, the catalytic lens provides about a 9× higher recovery rate, where water molecules inside the PEI lens deliver fast reassembly of the electron-hole pairs. The presented method with an ultimately simple fabrication step is expected to be applied to academic research and prototyping, including optoelectronic sensors, energy devices, and advanced manufacturing processes.

Sensors for Volatile Organic Compounds

This paper provides an overview of recent developments in the field of volatile organic compound (VOC) sensors, which are finding uses in healthcare, safety, environmental monitoring, food and agriculture, oil industry, and other fields. It starts by briefly explaining the basics of VOC sensing and reviewing the currently available and quickly progressing VOC sensing approaches. It then discusses the main trends in materials' design with special attention to nanostructuring and nanohybridization. Emerging sensing materials and strategies are highlighted and their involvement in the different types of sensing technologies is discussed, including optical, electrical, and gravimetric sensors. The review also provides detailed discussions about the main limitations of the field and offers potential solutions. The status of the field and suggestions of promising directions for future development are summarized.

Progress on Optical Fiber Biochemical Sensors Based on Graphene

Graphene, a novel form of the hexagonal honeycomb two-dimensional carbon-based structural material with a zero-band gap and ultra-high specific surface area, has unique optoelectronic capabilities, promising a suitable basis for its application in the field of optical fiber sensing. Graphene optical fiber sensing has also been a hotspot in cross-research in biology, materials, medicine, and micro-nano devices in recent years, owing to prospective benefits, such as high sensitivity, small size, and strong anti-electromagnetic interference capability and so on. Here, the progress of optical fiber biochemical sensors based on graphene is reviewed. The fabrication of graphene materials and the sensing mechanism of the graphene-based optical fiber sensor are described. The typical research works of graphene-based optical fiber biochemical sensor, such as long-period fiber grating, Bragg fiber grating, no-core fiber and photonic crystal fiber are introduced, respectively. Finally, prospects for graphene-based optical fiber biochemical sensing technology will also be covered, which will provide an important reference for the development of graphene-based optical fiber biochemical sensors.

Micromachined Optical Fiber Sensors for Biomedical Applications

Optical fibers revolutionized the rate of information reception and transmission in telecommunications. The revolution has now extended to the field of physicochemical sensing. Optical fiber sensors (OFSs) have found a multitude of applications, spanning from structural health monitoring to biomedical and clinical measurements due to their unique physical and functional advantages, such as small dimensions, light weight, immunity to electromagnetic interference, high sensitivity and resolution, multiplexing, and remote operation. OFSs generally rely on the detection of measurand-induced changes in the optical properties of the light propagating in the fiber, where the OFS essentially functions as the conduit and physical link between the probing light waves and the physicochemical parameters under investigation. Several advanced micromachining techniques have been developed to optimize the structure of OFSs, thus improving their sensing performance. These techniques include fusion splicing, tapering, polishing, and more complicated femtosecond laser micromachining methods. This chapter discusses and reviews the most recent developments in micromachined OFSs specifically for biomedical applications. Step-by-step procedures for several optical fiber micromachining techniques are detailed.

Graphene oxide nanocomposites based room temperature gas sensors: A review

Over the last few decades, various volatile organic compounds (VOCs) have been widely used in the processing of building materials and this practice adversely affected the environment i.e. both indoor and outdoor air quality. A cost-effective solution for detecting a wide range of VOCs by sensing approaches includes chemiresistive, optical and electrochemical techniques. Room temperature (RT) chemiresistive gas sensors are next-generation technologies desirable for self-powered or battery-powered instruments utilized in monitoring emissions that are associated with indoor/outdoor air pollution and industrial processes. In this review, a state-of-the-art overview of chemiresistive gas sensors is provided based on their attractive analytical characteristics such as high sensitivity, selectivity, reproducibility, rapid assay time and low fabrication cost. The review mainly discusses the recent advancement and advantages of graphene oxide (GO) nanocomposites-based chemiresistive gas sensors and various factors affecting their sensing performance at RT. Besides, the sensing mechanisms of GO nanocomposites-based chemiresistive gas sensors derived using metals, transition metal oxides (TMOs) and polymers were discussed. Finally, the challenges and future perspectives of GO nanocomposites-based RT chemiresistive gas sensors are addressed.

Fabrication of defective graphene oxide for efficient hydrogen production and enhanced 4-nitro-phenol reduction

Hydrogen has been considered as one of the most promising alternative energy source to solve the future energy demands due to its high energy capacity and emission-free character. The generation of hydrogen from non-fossil sources is necessary for the sustainable development of human life on this planet. The hydrolysis of sodium borohydride can quickly produce a large amount of hydrogenin situand on-demand in the presence of the catalyst, which can be used as an alternative energy source. So, it is crucial to fabricate the highly efficient, robust, and economical catalyst for the production of hydrogen via hydrolysis of sodium borohydride. Herein, a facile and efficient approach for the synthesis of metal-functionalized reduced graphene oxide for the production of hydrogen at room temperature was used. Moreover, the synthesized catalyst has also been tested in the field of environmental catalysis for the reduction of toxic 4-nitrophenol to valuable 4-aminophenol in the presence of sodium borohydride. The enhanced activity of prepared metal-functionalized reduced graphene oxide is ascribed to a strong affinity between Fe-N_Xand reduced graphene oxide which facilitates electron transfer as well as synergistic effect. Overall, this work presents a crucial procedure for green chemistry reactions when a carbonaceous material is selected as a catalyst.

Immunosensing Based on Optical Fiber Technology: Recent Advances

The evolution of optical fiber technology has revolutionized a variety of fields, from optical transmission to environmental monitoring and biomedicine, given their unique properties and versatility. For biosensing purposes, the light guided in the fiber core is exposed to the surrounding media where the analytes of interest are detected by different techniques, according to the optical fiber configuration and biofunctionalization strategy employed. These configurations differ in manufacturing complexity, cost and overall performance. The biofunctionalization strategies can be carried out directly on bare fibers or on coated fibers. The former relies on interactions between the evanescent wave (EW) of the fiber and the analyte of interest, whereas the latter can comprise plasmonic methods such as surface plasmon resonance (SPR) and localized SPR (LSPR), both originating from the interaction between light and metal surface electrons. This review presents the basics of optical fiber immunosensors for a broad audience as well as the more recent research trends on the topic. Several optical fiber configurations used for biosensing applications are highlighted, namely uncladded, U-shape, D-shape, tapered, end-face reflected, fiber gratings and special optical fibers, alongside practical application examples. Furthermore, EW, SPR, LSPR and biofunctionalization strategies, as well as the most recent advances and applications of immunosensors, are also covered. Finally, the main challenges and an outlook over the future direction of the field is presented.

Silica optical fiber integrated with two-dimensional materials: towards opto-electro-mechanical technology

In recent years, the integration of graphene and related two-dimensional (2D) materials in optical fibers have stimulated significant advances in all-fiber photonics and optoelectronics. The conventional passive silica fiber devices with 2D materials are empowered for enhancing light-matter interactions and are applied for manipulating light beams in respect of their polarization, phase, intensity and frequency, and even realizing the active photo-electric conversion and electro-optic modulation, which paves a new route to the integrated multifunctional all-fiber optoelectronic system. This article reviews the fast-progress field of hybrid 2D-materials-optical-fiber for the opto-electro-mechanical devices. The challenges and opportunities in this field for future development are discussed.

2D materials coated on etched optical fibers as humidity sensor

In this investigation, etched-fibers are coated by 2D layers such as Molybdenum disulfide (MoS2), Molybdenum diselenide (MoSe2) and composition of graphene and graphene oxide (G/GO) to modify humidity sensing. The relative differentiation of attenuations (RDA) in presence of relative humidity (RH) is measured by Optical Loss Test Set at two standard-wavelengths-telecommunication (1310 nm and 1550 nm). Results show that the etched single-mode fiber (ESMF) coated with G/GO has relatively high and one by one function for RDA versus RH (more than 30%). Also, its sensitivity and variance are reasonable. The MoSe2 based sensor is applicable at humidity below 30% because of higher RDA. However, it is not useful at humidity more than 30% due to the absence of one by one function for RDA versus RH. Besides, ESMF coated with MoS2 has indistinctive behavior and is not useful as a humidity sensor.

Long period grating in double cladding fiber coated with graphene oxide as high-performance optical platform for biosensing

In this work, the development and testing of a novel fiber-optic based label-free biosensor is presented, whose performance were verified through the detection of C-reactive protein (CRP) in serum. The device is based on a long period grating fabricated in a double cladding fiber with a W-shaped refractive index (RI) profile. As a result, the working point of the device was tuned to the mode transition region by chemical etching of the outer fiber cladding, obtaining a significant enhancement of the RI sensitivity and an excellent visibility of the grating resonances due to the mode transition in an all-silica structure. The fiber transducer was coated with a nanometric thin layer of graphene oxide in order to provide functional groups for the covalent immobilization of the biological recognition element. A very low limit of detection of about 0.15 ng/mL was obtained during the detection of CRP in serum, and a large working range (1 ng/mL - 100 μg/mL) of clinical relevance has been also achieved.

Wavelength Dependent Graphene Oxide-Based Optical Microfiber Sensor for Ammonia Gas

Ammonia detection in ambient air is critical, given its implication on the environment and human health. In this work, an optical fiber tapered to a 20 µm diameter and coated with graphene oxide was developed for absorbance response monitoring of ammonia at visible (500-700 nm) and near-infrared wavelength regions (700-900 nm). The morphology, surface characteristics, and chemical composition of the graphene oxide samples were confirmed by a field emission scanning electron microscope, an atomic force microscope, X-ray diffraction, and an energy dispersion X-ray. The sensing performance of the graphene oxide-coated optical microfiber sensor towards ammonia at room temperature revealed better absorbance response at the near-infrared wavelength region compared to the visible region. The sensitivity, response and recovery times at the near-infrared wavelength region were 61.78 AU/%, 385 s, and 288 s, respectively. The sensitivity, response and recovery times at the visible wavelength region were 26.99 AU/%, 497 s, and 192 s, respectively. The selectivity of the sensor towards ammonia was affirmed with no response towards other gases.

Optical Biosensors for the Detection of Rheumatoid Arthritis (RA) Biomarkers: A Comprehensive Review

A comprehensive review of optical biosensors for the detection of biomarkers associated with rheumatoid arthritis (RA) is presented here, including microRNAs (miRNAs), C-reactive protein (CRP), rheumatoid factor (RF), anti-citrullinated protein antibodies (ACPA), interleukin-6 (IL-6) and histidine, which are biomarkers that enable RA detection and/or monitoring. An overview of the different optical biosensors (based on fluorescence, plasmon resonances, interferometry, surface-enhanced Raman spectroscopy (SERS) among other optical techniques) used to detect these biomarkers is given, describing their performance and main characteristics (limit of detection (LOD) and dynamic range), as well as the connection between the respective biomarker and rheumatoid arthritis. It has been observed that the relationship between the corresponding biomarker and rheumatoid arthritis tends to be obviated most of the time when explaining the mechanism of the optical biosensor, which forces the researcher to look for further information about the biomarker. This review work attempts to establish a clear association between optical sensors and rheumatoid arthritis biomarkers as well as to be an easy-to-use tool for the researchers working in this field.

Evanescent Wave Optical Trapping and Sensing on Polymer Optical Fibers for Ultra-Trace Detection of Glucose

Graphene sensitization of glucose-imprinted polymer (G-IP)-coated optical fiber has been introduced as a new biosensor for evanescent wave trapping on the polymer optical fiber to detect low-level glucose. The developed sensor operates based on the evanescent wave modulation principle. Full characterization via atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), and N₂ adsorption/desorption of as-prepared G-IP-coated optical fibers was experimentally tested. Accordingly, related operational parameters such as roughness and diameter were optimized. Incorporating graphene into the G-IP not only steadily promotes the electron transport between the fiber surface and as-proposed G-IP but also significantly enhances the sensitivity by acting as a carrier for immobilizing G-IP with specific imprinted cavities. The sensor demonstrates a fast response time (5 s) and high sensitivity, selectivity, and stability, which cause a wide linear range (10-100 nM) and a low limit of detection (LOD = 2.54 nM). Experimental results indicate that the developed sensor facilitates online monitoring and remote sensing of glucose in biological liquids and food samples.

Carbon Allotrope-Based Optical Fibers for Environmental and Biological Sensing: A Review

Recently, carbon allotropes have received tremendous research interest and paved a new avenue for optical fiber sensing technology. Carbon allotropes exhibit unique sensing properties such as large surface to volume ratios, biocompatibility, and they can serve as molecule enrichers. Meanwhile, optical fibers possess a high degree of surface modification versatility that enables the incorporation of carbon allotropes as the functional coating for a wide range of detection tasks. Moreover, the combination of carbon allotropes and optical fibers also yields high sensitivity and specificity to monitor target molecules in the vicinity of the nanocoating surface. In this review, the development of carbon allotropes-based optical fiber sensors is studied. The first section provides an overview of four different types of carbon allotropes, including carbon nanotubes, carbon dots, graphene, and nanodiamonds. The second section discusses the synthesis approaches used to prepare these carbon allotropes, followed by some deposition techniques to functionalize the surface of the optical fiber, and the associated sensing mechanisms. Numerous applications that have benefitted from carbon allotrope-based optical fiber sensors such as temperature, strain, volatile organic compounds and biosensing applications are reviewed and summarized. Finally, a concluding section highlighting the technological deficiencies, challenges, and suggestions to overcome them is presented.

Applications of Graphene and Its Derivatives in Chemical Analysis

In this review, the applications of graphene and its derivatives in the chemical analysis have been described. The properties of graphene materials which are essential for their use in chemical and biochemical analysis are characterized. The materials are used in sensors and biosensors, in electrochemistry, in chromatography and in the sample preparation techniques. Chemical and electrochemical sensors containing graphene materials are useful devices for detecting some chemical and biochemical compounds. Chromatographic columns for HPLC with graphene containing stationary phases may be used for separation of polar and nonpolar components of some specific mixtures. Graphene materials could be successfully employed during sample preparation for analysis with SPE, magnetic SPE, and SPME techniques. HighlightsThe review of the applications of graphene (G) and its derivatives, graphene oxide (GO) and reduced graphene oxide (rGO), in chemical and biochemical analysis is proposed.The electron donor-acceptor and proton donor-acceptor interactions for the graphene based materials - analytes systems and their impact on the analysis results are discussed, particularly:    i) in electrochemistry,ii) in chromatography,iii) in modern sample preparation techniquesiv) in sensors of different types.The essence of the thermal stability and the nomenclature of the graphene based materials in their different applications in chemical systems of different classes was discussed (and suggested).The benefits of using SPME fibers with immobilized graphene materials have been presented in detail.

Graphene-based nanocomposites for sensitivity enhancement of surface plasmon resonance sensor for biological and chemical sensing: A review

Surface plasmon resonance (SPR) offers exceptional advantages such as label-free, in-situ and real-time measurement ability that facilitates the study of molecular or chemical binding events. Besides, SPR lacks in the detection of various binding events, particularly involving low molecular weight molecules. This drawback ultimately resulted in the development of several sensitivity enhancement methodologies and their application in the various area. Among graphene materials, graphene-based nanocomposites stands out owing to its significant properties such as strong adsorption of molecules, signal amplification by optical, high carrier mobility, electronic bridging, ease of fabrication and therefore, have established as an important sensitivity enhancement substrate for SPR. Also, graphene-based nanocomposites could amplify the signal generated by plasmon material and increase the sensitivity of molecular detection up to femto to atto molar level. This review focuses on the current important developments made in the potential research avenue of SPR and fiber optics based SPR for chemical and biological sensing. Latest trends and challenges in engineering and applications of graphene-based nanocomposites enhanced sensors for detecting minute and low concentration biological and chemical analytes are reviewed comprehensively. This review may aid in futuristic designing approaches and application of grapheneous sensor platforms for sensitive plasmonic nano-sensors.

Cross-Linking Stabilizes Electrical Resistance of Reduced Graphene Oxide in Humid Environments

Reduced graphene oxide (rGO) is an excellent candidate for many sensor applications, since its electrical properties can be tailored to become sensitive to temperature, humidity, strain, and chemicals. However, the wide use of rGO may be limited by its susceptibility to humidity changes. Here, for the first time, we report experimental evidence that (i) the interlayer spacing between rGO sheets can swell upon exposure to humid environments due to the intercalation of water and (ii) the expanded interlayer spacing increases electrical resistance. As a novel means of mitigating this instability, ethylenediamine was used as a covalent cross-linker to anchor rGO sheets to limit interlayer expansion and stabilize electrical resistance under humid environments.

Advances on Photonic Crystal Fiber Sensors and Applications

In this review paper some recent advances on optical sensors based on photonic crystal fibres are reported. The different strategies successfully applied in order to obtain feasible and reliable monitoring systems in several application fields, including medicine, biology, environment sustainability, communications systems are highlighted. Emphasis is given to the exploitation of integrated systems and/or single elements based on photonic crystal fibers employing Bragg gratings (FBGs), long period gratings (LPGs), interferometers, plasmon propagation, off-set spliced fibers, evanescent field and hollow core geometries. Examples of recent optical fiber sensors for the measurement of strain, temperature, displacement, air flow, pressure, liquid-level, magnetic field, and hydrocarbon detection are briefly described.

Sensitive Electrochemical Detection of Caffeic Acid in Wine Based on Fluorine-Doped Graphene Oxide

We report here a novel electrochemical sensor developed using fluorine-doped graphene oxide (F-GO) for the detection of caffeic acid (CA). The synthesized graphene oxide (GO) and F-GO nanomaterials were systematically characterized with a scanning electron microscope (SEM), and the presence of semi-ionic bonds was confirmed in the F-GO using X-ray photoelectron spectroscopy. The electrochemical behaviours of bare glassy carbon electrode (GCE), F-GO/GCE, and GO/GCE toward the oxidation of CA were studied using cyclic voltammetry (CV), and the results obtained from the CV investigation revealed that F-GO/GCE exhibited the highest electrochemically active surface area and electrocatalytic activity in contrast to the other electrodes. Differential pulse voltammetry (DPV) was employed for the analytical quantitation of CA, and the F-GO/GCE produced a stable oxidation signal over the selected CA concentration range (0.5 to 100.0 μM) with a low limit of detection of 0.018 μM. Furthermore, the acquired results from the selectivity studies revealed a strong anti-interference capability of the F-GO/GCE in the presence of other hydroxycinnamic acids and ascorbic acid. Moreover, the F-GO/GCE offered a good sensitivity, long-term stability, and an excellent reproducibility. The practical application of the electrochemical F-GO sensor was verified using various brands of commercially available wine. The developed electrochemical sensor successfully displayed its ability to directly detect CA in wine samples without pretreatment, making it a promising candidate for food and beverage quality control.

Natural Source-Based Graphene as Sensitising Agents for Air Quality Monitoring

Natural carbon powder has been used as a precursor to prepare two main types of sensitising agents of nitrogen-doped carbon nanoparticles (N-CNPs) and nitrogen-doped graphene quantum dots coupled to nanosheets (N-GQDs-NSs) by using simple treatments of chemical oxidation and centrifugation separation. Characterization based on FTIR, XPS, XRD, Raman spectroscopy, FE-SEM, HR-TEM, AFM, UV-Vis and FL, revealed successful doping carbon nanoparticle with nitrogen with an average plane dimension of 50 nm and relatively smooth surface. The versatility of the prepared samples as sensitising agents was developed and established by exploiting its ability for detection of volatile organic compounds via simple optical fibre based sensing configuration. The comparative experimental studies on the proposed sensor performance indicate fast response achieved at a few tens of seconds and excellent repeatability in exposure to the methanol vapour. The low limit of detection of 4.3, 4.9 and 10.5 ppm was obtained in exposure to the methanol, ethanol and propanol vapours, respectively, in the atmosphere condition. This study gives insights into the chemical/physical mechanism of an enhanced economic optical fibre based gas sensor and illustrates it for diverse sensing applications, especially for chemical vapour remote detection and future air quality monitoring.

Structure of graphene and its disorders: a review

Monolayer graphene exhibits extraordinary properties owing to the unique, regular arrangement of atoms in it. However, graphene is usually modified for specific applications, which introduces disorder. This article presents details of graphene structure, including sp2 hybridization, critical parameters of the unit cell, formation of σ and π bonds, electronic band structure, edge orientations, and the number and stacking order of graphene layers. We also discuss topics related to the creation and configuration of disorders in graphene, such as corrugations, topological defects, vacancies, adatoms and sp3-defects. The effects of these disorders on the electrical, thermal, chemical and mechanical properties of graphene are analyzed subsequently. Finally, we review previous work on the modulation of structural defects in graphene for specific applications.

Synergy between nanomaterials and volatile organic compounds for non-invasive medical evaluation

This article is an overview of the present and ongoing developments in the field of nanomaterial-based sensors for enabling fast, relatively inexpensive and minimally (or non-) invasive diagnostics of health conditions with follow-up by detecting volatile organic compounds (VOCs) excreted from one or combination of human body fluids and tissues (e.g., blood, urine, breath, skin). Part of the review provides a didactic examination of the concepts and approaches related to emerging sensing materials and transduction techniques linked with the VOC-based non-invasive medical evaluations. We also present and discuss diverse characteristics of these innovative sensors, such as their mode of operation, sensitivity, selectivity and response time, as well as the major approaches proposed for enhancing their ability as hybrid sensors to afford multidimensional sensing and information-based sensing. The other parts of the review give an updated compilation of the past and currently available VOC-based sensors for disease diagnostics. This compilation summarizes all VOCs identified in relation to sickness and sampling origin that links these data with advanced nanomaterial-based sensing technologies. Both strength and pitfalls are discussed and criticized, particularly from the perspective of the information and communication era. Further ideas regarding improvement of sensors, sensor arrays, sensing devices and the proposed workflow are also included.

Optical Graphene Gas Sensors Based on Microfibers: A Review

Graphene has become a bridge across optoelectronics, mechanics, and bio-chemical sensing due to its unique photoelectric characteristics. Moreover, benefiting from its two-dimensional nature, this atomically thick film with full flexibility has been widely incorporated with optical waveguides such as fibers, realizing novel photonic devices including polarizers, lasers, and sensors. Among the graphene-based optical devices, sensor is one of the most important branch, especially for gas sensing, as rapid progress has been made in both sensing structures and devices in recent years. This article presents a comprehensive and systematic overview of graphene-based microfiber gas sensors regarding many aspects including sensing principles, properties, fabrication, interrogating and implementations.

Graphene Oxide in Lossy Mode Resonance-Based Optical Fiber Sensors for Ethanol Detection

The influence of graphene oxide (GO) over the features of an optical fiber ethanol sensor based on lossy mode resonances (LMR) has been studied in this work. Four different sensors were built with this aim, each comprising a multimode optical fiber core fragment coated with a SnO₂ thin film. Layer by layer (LbL) coatings made of 1, 2 and 4 bilayers of polyethyleneimine (PEI) and graphene oxide were deposited onto three of these devices and their behavior as aqueous ethanol sensors was characterized and compared with the sensor without GO. The sensors with GO showed much better performance with a maximum sensitivity enhancement of 176% with respect to the sensor without GO. To our knowledge, this is the first time that GO has been used to make an optical fiber sensor based on LMR.

Sulfophenyl-Functionalized Reduced Graphene Oxide Networks on Electrospun 3D Scaffold for Ultrasensitive NO₂ Gas Sensor

Ultrasensitive room temperature real-time NO₂ sensors are highly desirable due to potential threats on environmental security and personal respiratory. Traditional NO₂ gas sensors with highly operated temperatures (200-600 °C) and limited reversibility are mainly constructed from semiconducting oxide-deposited ceramic tubes or inter-finger probes. Herein, we report the functionalized graphene network film sensors assembled on an electrospun three-dimensional (3D) nanonetwork skeleton for ultrasensitive NO₂ sensing. The functional 3D scaffold was prepared by electrospinning interconnected polyacrylonitrile (PAN) nanofibers onto a nylon window screen to provide a 3D nanonetwork skeleton. Then, the sulfophenyl-functionalized reduced graphene oxide (SFRGO) was assembled on the electrospun 3D nanonetwork skeleton to form SFRGO network films. The assembled functionalized graphene network film sensors exhibit excellent NO₂ sensing performance (10 ppb to 20 ppm) at room temperature, reliable reversibility, good selectivity, and better sensing cycle stability. These improvements can be ascribed to the functionalization of graphene with electron-withdrawing sulfophenyl groups, the high surface-to-volume ratio, and the effective sensing channels from SFRGO wrapping onto the interconnected 3D scaffold. The SFRGO network-sensing film has the advantages of simple preparation, low cost, good processability, and ultrasensitive NO₂ sensing, all advantages that can be utilized for potential integration into smart windows and wearable electronic devices for real-time household gas sensors.

Flexible Graphene-Based Wearable Gas and Chemical Sensors

Wearable electronics is expected to be one of the most active research areas in the next decade; therefore, nanomaterials possessing high carrier mobility, optical transparency, mechanical robustness and flexibility, lightweight, and environmental stability will be in immense demand. Graphene is one of the nanomaterials that fulfill all these requirements, along with other inherently unique properties and convenience to fabricate into different morphological nanostructures, from atomically thin single layers to nanoribbons. Graphene-based materials have also been investigated in sensor technologies, from chemical sensing to detection of cancer biomarkers. The progress of graphene-based flexible gas and chemical sensors in terms of material preparation, sensor fabrication, and their performance are reviewed here. The article provides a brief introduction to graphene-based materials and their potential applications in flexible and stretchable wearable electronic devices. The role of graphene in fabricating flexible gas sensors for the detection of various hazardous gases, including nitrogen dioxide (NO₂), ammonia (NH₃), hydrogen (H₂), hydrogen sulfide (H₂S), carbon dioxide (CO₂), sulfur dioxide (SO₂), and humidity in wearable technology, is discussed. In addition, applications of graphene-based materials are also summarized in detecting toxic heavy metal ions (Cd, Hg, Pb, Cr, Fe, Ni, Co, Cu, Ag), and volatile organic compounds (VOCs) including nitrobenzene, toluene, acetone, formaldehyde, amines, phenols, bisphenol A (BPA), explosives, chemical warfare agents, and environmental pollutants. The sensitivity, selectivity and strategies for excluding interferents are also discussed for graphene-based gas and chemical sensors. The challenges for developing future generation of flexible and stretchable sensors for wearable technology that would be usable for the Internet of Things (IoT) are also highlighted.

Micro and Nanostructured Materials for the Development of Optical Fibre Sensors

The measurement of chemical and biomedical parameters can take advantage of the features exclusively offered by optical fibre: passive nature, electromagnetic immunity and chemical stability are some of the most relevant ones. The small dimensions of the fibre generally require that the sensing material be loaded into a supporting matrix whose morphology is adjusted at a nanometric scale. Thanks to the advances in nanotechnology new deposition methods have been developed: they allow reagents from different chemical nature to be embedded into films with a thickness always below a few microns that also show a relevant aspect ratio to ensure a high transduction interface. This review reveals some of the main techniques that are currently been employed to develop this kind of sensors, describing in detail both the resulting supporting matrices as well as the sensing materials used. The main objective is to offer a general view of the state of the art to expose the main challenges and chances that this technology is facing currently.

Graphene-Based Materials for Biosensors: A Review

The advantages conferred by the physical, optical and electrochemical properties of graphene-based nanomaterials have contributed to the current variety of ultrasensitive and selective biosensor devices. In this review, we present the points of view on the intrinsic properties of graphene and its surface engineering concerned with the transduction mechanisms in biosensing applications. We explain practical synthesis techniques along with prospective properties of the graphene-based materials, which include the pristine graphene and functionalized graphene (i.e., graphene oxide (GO), reduced graphene oxide (RGO) and graphene quantum dot (GQD). The biosensing mechanisms based on the utilization of the charge interactions with biomolecules and/or nanoparticle interactions and sensing platforms are also discussed, and the importance of surface functionalization in recent up-to-date biosensors for biological and medical applications.