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Graphene has amazing applications for sensors due to its excellent performances like high strength and good conductivity, but the transfer issue is in the way of its application perspective. Direct growth of spherical graphene films (SGFs) on cemented carbide may offer a good avenue for various applications in sensor technology, especially for electrochemical sensors. Four common methods for graphene preparation are chemical stripping, chemical vapor deposition (CVD), metal catalysis, and laser fabrication; and subject to transfer issues during usage. In order to overcome this limitation, the fabrication of in-situ growth of SGFs on carbide is proposed as a solution for constructing sensor matrices. This review explores various in-situ SGFs and their potential applications in sensors. The findings presented here shed light on transfer-free graphene with controllable structures that can serve as excellent candidates for sensor matrices.

In this work, the potential of tin (IV) 2,3-napthalocyanine dichloride (SnNcCl₂) has been studied for sensing applications due to its hydrophobic nature. The multipurpose sensor was fabricated by depositing 50-nm silver (Ag) electrodes on a glass substrate through vacuum thermal evaporation at pressure of $\sim 10^{-5}$ mbar. With the help of masking, a 40-micron inter-electrode gap between Ag electrodes was developed and then 80-nm film of SnNcCl₂ was thermally deposited in the inter-electrode gap resulting in a surface type Ag/SnNcCl₂/Ag multipurpose sensor and was studied for humidity and temperature sensing. The humidity characterization was carried out at two different frequencies, i.e. 120 and 1kHz in the relative humidity range 35–85% RH and 5.5 and 1.3 times increase was recorded with respect to initial capacitance for both frequencies, respectively. The temperature sensing was studied within a temperature range of 15–80^∘C at 120Hz frequency and 1.3 times increase in capacitance was observed with respect to initial capacitance. The sensor’s important parameters, i.e. response time and recovery time were measured to be 8 and 3s at 120Hz for humidity measurements. The morphology of the SnNcCl₂ thin film was measured by atomic force microscope (AFM) and scanning electron microscope (SEM) showing rough surface favorable for sensing applications. The amorphous structure of the film was confirmed by X-ray diffraction (XRD) while optical bandgap was calculated from ultraviolet-visible (UV-vis) spectroscopy.

The adsorption of cytosine on graphene surface is studied using density functional theory with local density approximation. The cytosine is physisorbed onto graphene through π–π interaction, with a binding energy around -0.39 eV. Due to the weak interaction, the electronic properties of graphene show little change upon adsorption. The cytosine/graphene interaction can be strongly enhanced by introducing metal atoms. The binding energies increase to -0.60 and -2.31 eV in the presence of Li and Co atoms, respectively. The transport behavior of an electric sensor based on Co-doped graphene shows a sensitivity one order of magnitude higher than that of a similar device using pristine graphene. This work reveals that the sensitivity of graphene-based bio-sensors could be drastically improved by introducing appropriate metal atoms.

Thermodynamically stable molybdenum trioxide nanorods have been successfully synthesized by a simple hydrothermal process. The product exhibits high-quality, single-crystalline layered orthorhombic structure (α-MoO₃), and aspect ratio over 20 by characterizations of X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and Fourier transform infrared (FT-IR). The growth mechanism of α-MoO₃ nanorods can be understood by electroneutral and dehydration reaction, which is highly dependent on solution acidity and hydrothermal temperature. The sensing tests show that the sensor based on MoO₃ nanorods exhibits high sensitivity to NO₂ and is not interferred by CO and CH₄, which makes this kind sensor a competitive candidate for NO₂ detection. The intrinsic sensing performance of MoO₃ maybe arise from its nonstoichiometry of MoO₃ owing to the presence of Mo⁵⁺ and oxygen vacancy in MoO₃ lattice, which has been confirmed by X-ray photoelectron spectroscopy (XPS) analysis. The sensing mechanism of MoO₃ for NO₂ is also discussed.

We have successfully synthesized ZnO nanoparticles (NPs) from solution combustion method using combustible fuel (Green gram). XRD pattern confirms that the prepared compound is composed of wurtzite hexagonal zinc-oxide. FTIR spectrum of ZnO NPs shows the band at ~ 417 cm^-1 associated with the characteristic vibration of Zn-O. The UV-Vis spectrum shows a strong absorption band at ~ 365 nm which is blue shifted due to quantum confinement effect. TEM images show the average sizes of the nanoparticles are found to be almost ~ 15–30 nm. The as-synthesized product shows good electrochemical sensing of dopamine. Furthermore the antibacterial properties of ZnO NPs were investigated by their bactericidal activity against four bacterial strains using the agar well diffusion method.

Mesoporous silica monoliths are an attractive area of research owing to their high specific surface area, uniform channels and mesoporous size (2–30nm). This paper deals with the direct templating synthesis of a mesoporous worm-like silica monolithic material using F127 — a triblock copolymer, by micro-emulsion technique using trimethyl benzene (TMB), as the solvent. The synthesized silica monolith is characterized using SEM-EDAX, XRD, BET, NMR and FT-IR. The monolith shows an ordered worm-like mesoporous structure with tuneable through pores, an excellent host for the anchoring of chromo-ionophores for the naked-eye metal ion-sensing. The mesoporous monoliths were loaded with 4-dodecyl-6-(2-pyridylazo)-phenol (DPAP) ligand through direct immobilization, thereby acting as solid-state naked-eye colorimetric ion-sensors for the sensing toxic Pb${}^{\mathrm{\text{(II)}}}$ ions at parts-per-billion (ppb) level in various industrial and environmental systems. The influence of various experimental parameters such as solution pH, limiting ligand loading concentration, amount of monolith material, matrix tolerance level, limit of detection and quantification has been studied and optimized.

In this paper, we report the preparation and characterization of a sensitive and reusable nonenzymatic glucose (NEG) sensor based on copper nanowires (CuNWs)/polyaniline (PANI)/reduced graphene oxide (rGO) nanocomposite ink. The CuNWs/PANI/rGO nanocomposite ink was prepared by solvothermal mixing of CuNWs, PANI, rGO and binders. The X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Fourier Transform Infra-Red (FT-IR) spectroscopy techniques were used to assess the structural and morphological parameters of prepared nanocomposite ink. The cyclic voltammetry (CV) technique was used to estimate the electrochemical behavior of prepared NEG sensor. The structural, morphological and spectroscopy results confirmed the change in morphological and oxidation state of CuNWs to CuO nanostructures as a constituent of nanocomposite ink. The CuO nanostructures supported on PANI/rGO demonstrated good electrochemical stability and great electrocatalytic activity toward glucose oxidation. At a glucose oxidation potential of 0.64V, the prepared NEG sensor exhibited great electrocatalytic ability by offering a high sensitivity of 843.06$\mu$AmM${}^{-1}$cm${}^{-2}$ in the linear glucose range 0–4mM with a lower detection limit of 1.6mM. In addition to these outstanding performance characteristics, CuNWs/PANI/rGO nanocomposite ink-based NEG sensor has the advantages of ease of fabrication, low cost and reusability.

Ozone sensing properties of mixed oxides of In₂O₃, ZnO, and SnO₂ in the form of thin films are explored. Exposure to ozone causes defects in the materials, and subsequently causes changes in the materials properties. In this work, a cost-effective, room temperature, real-time ozone monitoring device has been developed. The fabricated sensors are capable of detecting threshold ozone safety levels proposed by the World Health Organization (WHO) while operating at room temperature. Room temperature operation offers many advantages over high temperature operation, such as reduced power consumption, reduced fabrication costs, and ease of implementation into portable devices, such as laptops and mobile phones. The fabrication of these sensors was carried out by means of an Edwards E306A Coating System. Various mixtures of In₂O₃, ZnO, and snO₂ were deposited in a rectangular pattern on top of copper interdigitated electrodes. X-ray Photo Spectroscopy (XPS) analysis showed that there were levels of impurities in the sensor samples, which were dependant on the fabrication process and parameters. XPS analysis also gave a detailed account of the shifts in binding energies of the thin oxide layers. The results presented show that the highest response to environmentally relevant ozone concentrations is achieved with a very thin sensing layer and a high deposition rate. The performance of the sensors has been investigated and compared.

In this work, nanocrystalline hexagonal tungsten oxide was prepared by acidic precipitation from sodium tungstate solution. TEM studies of nanopowders showed that the average size of the hexagonal nanoparticles is 30–50 nm. Novel nanocomposites were prepared by embedding a low amount of gold-decorated carbon nanotubes into the hex-WO₃ matrix. The addition of MWCNTs lowered the temperature range of sensitivity of hex-WO₃ nanocomposites to NO₂ hazardous gas. The sensitivity of hex-WO₃ with Au-decorated MWCNTs to NO₂ is at the temperature range between 25°C and 250°C.

The nanoporous Co₃O₄ thin films were prepared on indium tin oxide (ITO) glasses by an electrodeposition method. The surface morphology and composition of the nanoporous Co₃O₄ films were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDS) and X-ray photoelectron spectroscopy (XPS). The results show that the as-deposited nanoporous Co₃O₄ film is constructed by many interconnected nanoflakes with thickness of about 40 nm. The cyclic voltammetry (CV) measurement indicates that the nanoporous Co₃O₄ films exhibit remarkable electrocatalytic activities for the hydrogen peroxide (H₂O₂) reduction which shows that it is a good candidate to be employed as electrode materials for electrochemical sensing of H₂O₂. Further analysis indicated that the detection sensitivity of the sensor was 1.357 mA mM^-1 cm^-2 and the detection limit was estimated to be about 0.2 mM.

A novel electrochemical sensing platform was constructed based on a facile self-assembly procedure synthetic laminar molybdenum trioxide dihydrate (MoO${}_3\cdot 2$H₂O)-graphene composite. Field emission scanning electron microscopy (FESEM), X-ray spectroscopy, X-ray diffraction (XRD) and Raman spectroscopy were employed to characterize the morphology and composition of the MoO${}_3\cdot 2$H₂O-graphene composite. As a model molecule, thiourea was utilized to investigate the electrochemical behaviors of the MoO${}_3\cdot 2$H₂O-graphene composite modified glass carbon electrode. The results show that the composite modified electrode has higher electron transfer rate than that of graphene modified electrode and bare glass carbon electrode meanwhile the peak currents of it has a good linear relationship with thiourea concentrations in the range of $2.40\times 10^{-3}-19.3\times 10^{-3}\mathrm{\text{M}}$ ($R=0.998$) with detection limit of 4.99$\mu$M ($\mathrm{\text{S/N}}=3$). This novel electrochemical sensor exhibits a higher absorption capacity ($3.87\times 10^{-8}$mol/cm²), a good reproducibility (1.41% relative standard deviation (RSD)), excellent anti-interference and a high stability. These excellent electrochemical properties of the MoO${}_3\cdot 2$H₂O-graphene composite are attributed to the loose and porous structure and the synergistic effects between graphene and MoO${}_3\cdot 2$H₂O, which make this composite material hold great potential applications for electrochemical sensor.

In this study, two hydrogen sensors with Pd/SiO₂/Si and Ni/SiO₂/Si structures have been fabricated. Palladium nanoparticles are synthesized and then deposited on the oxide surface using spin coating. Capacitance–voltage curves for the Pd/SiO₂/Si sensor at room temperature and for the Ni/SiO₂/Si sensor at 140${}^{\circ }$C in pure nitrogen and 1% H₂–N₂ mixture are described. The time required for reaching 90% of the steady-state signal magnitude ($t_{90\%}$) for Pd/SiO₂/Si capacitor was 1.4s and for Ni/SiO₂/Si capacitor was 90 s. The time interval for recovery from 90% to 10% of steady-state signal magnitude ($t_{10\%})$ for Pd/SiO₂/Si capacitor was 14s and for Ni/SiO₂/Si capacitor was 40min. For the Pd/SiO₂/Si capacitor, the response is 88% and for Ni/SiO₂/Si capacitor the response is 29%. Comparison of Pd nanoparticles capacitive- and resistance-based sensors shows that the metal-oxide-semiconductor capacitive is faster and more sensitive than the resistance-based hydrogen gas sensors.

An integrative electroanalytical method was developed for detecting Cd${}^{2+}$ and Pb${}^{2+}$ ions in aqueous solutions. Polysulfide/graphene (RGO-S) nanocomposites were prepared and their performance as electrochemical sensors for Cd${}^{2+}$ and Pb${}^{2+}$ was evaluated. The RGO-S nanocomposite was carefully characterized by scanning electron microscopy with energy-dispersive X-ray spectrometry, transmission electron microscopy, and X-ray photoelectron spectroscopy. The as-prepared RGO-S was incorporated into a pyrolytic graphite electrode (RGO-S/PGE) and used for detecting trace amount of Cd${}^{2+}$ and Pb${}^{2+}$ by differential pulse anodic stripping voltammetry. Under optimal conditions, the stripping peak current of RGO-S/PGE varies linearly with heavy metal ion concentration in the ranges 2.0–300$\mu$g L${}^{-1}$ for Cd${}^{2+}$ and 1.0–300$\mu$g L${}^{-1}$ for Pb${}^{2+}$. The limits of detection for Cd${}^{2+}$ and Pb${}^{2+}$ were estimated to be about 0.67$\mu$g L${}^{-1}$ and 0.17$\mu$g L${}^{-1}$, respectively. The prepared electrochemical heavy-metal-detecting electrode provides good repeatability and reproducibility with high sensitivity, making it a suitable candidate for monitoring Cd${}^{2+}$ and Pb${}^{2+}$ concentrations in aqueous environmental samples.

Phenolic compounds, especially 4-nitrophenol (4-NP), and chromium (VI) are highly toxic environmental pollutants. Thus, it is significantly important to establish a rapid and sensitive sensor for 4-NP and Cr(VI) monitoring in environment. In this paper, a novel approach has been adopted where E. coli-derived CDs (CDs-WT) prepared by one step hydrothermal method previously and sensitized by ampicillin (turn-on) were used as fluorescence nanoprobe (CDs-WT-Amp) for 4-NP and Cr(VI) determination based on the inner filter effect quenching mechanism (turn-off). Then, the quenching fluorescence was reversed by addition of ampicillin (turn-on). The linear ranges of 4-NP and Cr(VI) detection were 0–50$\mu$M and 0–25$\mu$M, with the limits of detection 88.89nM and 64.75nM, respectively. The “turn-on-on-off-on” fluorescent nanosensor system was successfully used to determine Cr(VI) and 4-NP in water with satisfactory results. It shows that the “turn-on-off-on” nanoprobe has great promising potential for application to environmental pollutant detection.

A new type of multifunctional metal-organic frameworks (MOFs) was synthesized by encapsulating gold nanoparticles (AuNPs) into the Cu-hemin MOFs, and first applied to an electrochemical sensor to detect catechol (CT) with the aid of electrochemically reduced graphene oxide (ERGO) for signal amplification. First, ERGO was electrochemically deposited on a bare glass carbon electrode (GCE), followed by casting Cu-hemin MOFs on an ERGO-modified electrode, and then growing AuNPs in situ on Cu-hemin MOFs/ERGO/GCE by electrochemical deposition. Cyclic voltammetry (CV), scanning electron microscopy (SEM) and current–time ($I$–$t)$ were utilized to characterize the electrochemical performance and surface characteristics of the as-prepared sensor. The results demonstrated that Cu-hemin MOFs have not only been a matrix to avoid the aggregation of AuNPs but also an ideal loading platform for the adsorption of CT due to its large surface area and porosity. In addition, the ERGO also has the advantage of fast electron transfer, which can make synergy with AuNPs@Cu-hemin MOFs nanocomposites to amplify the electrical signal. The AuNPs/Cu-hemin MOFs/ERGO/GCE exhibited an excellent electrocatalytic activity with increased electrochemical signals towards the oxidation of CT. Under the optimum experimental conditions, the sensor shows a wide linear relationship over the range of $2.0\times 10^{-6}$M to $1.692\times 10^{-3}$M with a detection limit of $2.0\times 10^{-7}$M. Moreover, the sensor presented the good reproducibility and the excellent anti-interference performance. This work would broaden the application of MOFs material in constructing more novel electrochemical sensing platform.

It is of great significance to prepare electrochemical glucose sensors with high selectivity and stability via effective and rapid methods. In this work, the self-support electrode with copper and nickel-based oxide is prepared by chemical-etching reaction which occurred under the property of electrochemical potential difference. In this processing, nickel foam is etched selectively by Cu${}^{2+}$ ions and they not only act as self-supporting electrode substrate, but also as nickel ions precursor of NiO. Moreover, the reaction can be completely satisfied on 30 min at room temperature. As a self-supporting electrode nonenzymic glucose electrochemical sensor, the electrode exhibited a wide linear range (0.04–3.00mM), low detection limit (0.02mM) with high sensitivity of 1096$\mu A\cdot {\mathrm{\text{mM}}}^{-1}\cdot {\mathrm{\text{cm}}}^{-2}$ and good selectivity, repeatability and stability. Furthermore, the application of the prepared sensor provides an avenue for the application of the transition metal materials in the field of electrochemical sensing.

Multi-walled carbon nanotube (MWCNT)-modified MoS₂/BiVO₄ was manufactured and used for the photoelectrochemical (PEC) detection of hydrogen peroxide (H₂O₂) and hypochlorite (ClO⁻). A solvothermal method was used to synthesize an MWCNT/MoS₂/BiVO₄ composite that showed perfect PEC properties because the MWCNTs and MoS₂/BiVO₄ heterostructures increased the composite’s stability against photocorrosion. Compared with the same signal of MWCNT/MoS₂/BiVO₄, the photocurrent signal of other composites was much smaller upon irradiation by visible light. According to this PEC sensor, the linear range of the H₂O₂ concentration was 1–200$\mu$M and 280–1560$\mu$M at $\mathrm{\text{pH}}=7.4$ based on the same pH when detecting ClO⁻ concentrations between 0.5–10$\mu$M and 20–340$\mu$M in a bleach sample. As a result, this sensor can be used to detect reactive oxygen species (ROS) in practical samples.

Photoelectrochemical (PEC) sensor is an important type of biosensor widely used in glutathione (GSH) sensing. The PEC properties of the photoanode present in the sensor are critical to its sensing performance. Zinc oxide (ZnO) is an excellent semiconductor with a suitable band gap and light absorption ability for photoanode applications. Meanwhile, the interfacial layer is also important in the separation and transportation process of the excitons. In this work, high-quality ZnO nanorods were grown on the indium tin oxide (ITO) substrates. An interfacial layer consisting of reduced graphene oxide (RGO) or MXene (a two-dimensional transition metal carbide)-derived TiO₂ was introduced. Our results show that the introduction of the RGO/TiO₂ hybrid interfacial layer can promote both the high-quality growth of ZnO nanorods and also provides suitable band gap grading for efficient excitons separation and transportation. The GSH sensing performance of the PEC sensor based on the ZnO nanorods grown on the RGO/TiO₂ hybrid layer-coated ITO photoanode can dramatically improve the photocurrent strength and linearity.