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In this paper, we review ongoing progress in the development of novel on-chip, low loss planar molecular sensors that address the emerging need in the field of biochemical sensing. Chalcogenide glasses were identified as the material of choice for sensing due to their wide infrared transparency window. We report the details of manufacturing processes used to realize novel high-index-contrast, compact micro-disk resonators. Our findings demonstrate that our device can operate in dual modalities, for detection of the infrared optical absorption of a binding event using cavity enhanced spectroscopy, or sensing refractive index change due to surface molecular binding and extracting micro-structural evolution information via cavity enhanced refractometry.

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.