The Application of Commercial Surface Acoustic Wave Radio Communication Filters as Transducers for DMMP Sensors.

In the present study, we used two popular radio communication SAW resonators as a base for gas sensors and tested their performance. Taking into account issues related to sensor sensitivity, the possibility of applying a sensor layer, the availability of devices, and other related issues, we selected two popular single-port resonators with center frequencies of 315 and 433 MHz (models R315 and R433, respectively) for testing purposes. Both resonators were equipped with a sensitive film of hexafluoroisopropanol-substituted polydimethylsiloxane, a material that selectively absorbs molecules with a high ability to form basic hydrogen bonds. Fabricated sensors were used to detect trace amounts of dimethyl methylphosphonate (DMMP) vapor, which has often been used in similar studies as a nerve chemical warfare agent simulant. Sensors using both devices loaded with sensor layers of an optimal thickness rapidly reacted to a gas containing DMMP at a concentration of 3 mg/m3, generating a stable analytical signal ranging from several to several dozen kilohertz. In the case of R433, the frequency signal was 20.5 kHz at 1 min from the beginning of exposure to DMMP. The obtained results showed that the used transducers exhibited good performance as a base for gas sensors. Finally, their suitability for sensing applications was confirmed by a comparison with the results obtained in previous similar studies.

Rapid colorimetric assay based on the oxidation of 2,2-azino-bis(3-ethylbenzothiazoline)-6-sulfonic acid-diammonium salt for nitrite detection in meat products.

This work developed a rapid colorimetric method for nitrite detection in meat products. The detection was based on the reaction of nitrite with 60 mM HCl to produce radicals which further oxidized ABTS (50 µM) to form a water-soluble blue-green product (ABTS•+). The absorbance was measured at a maximum absorption wavelength of 412.5 nm. Parameters such as concentration of HCl, concentration of ABTS and reaction time were evaluated. The absorbance was linearly proportional to the concentration of nitrite (0.1-20 µM) with the limit of detection of 0.34 µM. The proposed method was a time-saving assay since it required only 2 min to complete one measurement. There was no effect of the interference produced by other ions. The assay was robust with 2.5%RSD (n = 50). In meat product samples, high accuracy was observed with the recoveries between 100 ± 2.2% and 105 ± 3.7%. The amount of nitrite in meat products detected by the ABTS method was found in the range of 5.41 - 7.62 mg/kg. The conventional Griess method was applied to determine nitrite in the same meat products. There was no statistically significant difference between the two methods (P = 0.05).

Sparked ZnO nanoparticles-based electrochemical sensor for onsite determination of glyphosate residues.

Glyphosate (N-(phosphonomethyl)glycine) is well known nonselective and broad-spectrum herbicide that has been extensively used in agricultural areas around the world to increase agricultural productivity. However, the utilization of glyphosate can cause environmental contamination and health problems. Therefore, the detection of glyphosate with a fast, low-cost, and portable sensor is still important. In this work, the electrochemical sensor has been developed by modifying of working surface on the screen-printed silver electrode (SPAgE) with a mixtures solution between zinc oxide nanoparticles (ZnO-NPs) and poly(diallyldimethylammonium chloride) (PDDA) by the drop-casting process. The ZnO-NPs have been prepared based on a sparking method by using pure zinc wires. The ZnO-NPs/PDDA/SPAgE sensor shows a wide range of glyphosate detection (0µM-5 mM). The limit of detection of ZnO-NPs/PDDA/SPAgE is 2.84µM. The ZnO-NPs/PDDA/SPAgE sensor exhibits high selective towards glyphosate with minimal interference from other commonly used herbicides including paraquat, butachlor-propanil and glufosinate-ammonium. Furthermore, the ZnO-NPs/PDDA/SPAgE sensor demonstrates a good estimation of glyphosate concentration in real samples such as green tea, corn juice and mango juice.

Colorimetric Sensors for Chemical and Biological Sensing Applications.

Colorimetric sensors have been widely used to detect numerous analytes due to their cost-effectiveness, high sensitivity and specificity, and clear visibility, even with the naked eye. In recent years, the emergence of advanced nanomaterials has greatly improved the development of colorimetric sensors. This review focuses on the recent (from the years 2015 to 2022) advances in the design, fabrication, and applications of colorimetric sensors. First, the classification and sensing mechanisms of colorimetric sensors are briefly described, and the design of colorimetric sensors based on several typical nanomaterials, including graphene and its derivatives, metal and metal oxide nanoparticles, DNA nanomaterials, quantum dots, and some other materials are discussed. Then the applications, especially for the detection of metallic and non-metallic ions, proteins, small molecules, gas, virus and bacteria, and DNA/RNA are summarized. Finally, the remaining challenges and future trends in the development of colorimetric sensors are also discussed.

Synthesis and Integration of Hybrid Metal Nanoparticles Covered with a Molecularly Imprinted Polymer Nanolayer by Photopolymerization.

Interfacing recognition materials with transducers has consistently presented a challenge in the development of sensitive and specific chemical sensors. In this context, a method based on near-field photopolymerization is proposed to functionalize gold nanoparticles, which are prepared by a very simple process. This method allows in situ preparation of a molecularly imprinted polymer for sensing by surface-enhanced Raman scattering (SERS). In a few seconds, a functional nanoscale layer is deposited by photopolymerization on the nanoparticles. In this study, the dye Rhodamine 6G was chosen as a model target molecule to demonstrate the principle of the method. The detection limit is 500 pM. Due to the nanometric thickness, the response is fast, and the substrates are robust, allowing regeneration and reuse with the same performance level. Finally, this method of manufacturing has been shown to be compatible with integration processes, allowing the future development of sensors integrated in microfluidic circuits and on optical fibers.

Recent Progress in Self-Powered Sensors Based on Liquid-Solid Triboelectric Nanogenerators.

Recently, there has been a growing need for sensors that can operate autonomously without requiring an external power source. This is especially important in applications where conventional power sources, such as batteries, are impractical or difficult to replace. Self-powered sensors have emerged as a promising solution to this challenge, offering a range of benefits such as low cost, high stability, and environmental friendliness. One of the most promising self-powered sensor technologies is the L-S TENG, which stands for liquid-solid triboelectric nanogenerator. This technology works by harnessing the mechanical energy generated by external stimuli such as pressure, touch, or vibration, and converting it into electrical energy that can be used to power sensors and other electronic devices. Therefore, self-powered sensors based on L-S TENGs-which provide numerous benefits such as rapid responses, portability, cost-effectiveness, and miniaturization-are critical for increasing living standards and optimizing industrial processes. In this review paper, the working principle with three basic modes is first briefly introduced. After that, the parameters that affect L-S TENGs are reviewed based on the properties of the liquid and solid phases. With different working principles, L-S TENGs have been used to design many structures that function as self-powered sensors for pressure/force change, liquid flow motion, concentration, and chemical detection or biochemical sensing. Moreover, the continuous output signal of a TENG plays an important role in the functioning of real-time sensors that is vital for the growth of the Internet of Things.

Road Map of Semiconductor Metal-Oxide-Based Sensors: A Review.

Identifying disease biomarkers and detecting hazardous, explosive, flammable, and polluting gases and chemicals with extremely sensitive and selective sensor devices remains a challenging and time-consuming research challenge. Due to their exceptional characteristics, semiconducting metal oxides (SMOxs) have received a lot of attention in terms of the development of various types of sensors in recent years. The key performance indicators of SMOx-based sensors are their sensitivity, selectivity, recovery time, and steady response over time. SMOx-based sensors are discussed in this review based on their different properties. Surface properties of the functional material, such as its (nano)structure, morphology, and crystallinity, greatly influence sensor performance. A few examples of the complicated and poorly understood processes involved in SMOx sensing systems are adsorption and chemisorption, charge transfers, and oxygen migration. The future prospects of SMOx-based gas sensors, chemical sensors, and biological sensors are also discussed.

Microwave-assisted synthesis of copper oxide particles and its application for determination of imazapyr in environmental samples.

Imazapyr (IMA) is currently applied as pre- and post-emergence herbicide for control of weeds in crops. Because of its extensive use, IMA residues may reach water sources and soils. Consequently, its accurate measurement is demanded for timely actions with minimal involved steps and analysis time. Herein, copper oxide particles (Cu2O PS) were proposed as chemical sensor for determination of IMA residues. Cu2O PS were prepared by a facile microwave-assisted method using glucose as reducing agent and polyvinylpyrrolidone as stabilizer. The effect of main experimental parameters on the conversion rate of the Cu2O PS were analyzed by the response surface methodology. Obtained particles were thoroughly characterized in order to determine the particle size distribution, morphology, surface charge, optical and surface properties for further application. Determination of IMA was only based on the localized surface plasmon resonance band of Cu2O PS at 473 nm. Under optimal conditions, the method was evaluated in the concentration range between 80.0 and 1,000 µg L-1 showing a limit of detection about 101 µg L-1 (R2 >0.98). The applicability of the proposed methodology to determine IMA in soil and water samples was assessed with satisfactory recoveries (104-121.8%) displaying a good implementation prospect in environmental complex matrices.

Utility of 5-(2',4'-dimethylphenylazo)-6-hydroxy-pyrimidine-2,4-dione in PVC membrane for a novel green optical chemical sensor to detect zinc ion in environmental samples.

In plasticized (2-nitro-phenyloctyl ether (o-NPOE)) and polyvinyl chloride (PVC) membrane incorporating (N,N-diethyl-5-(octadecanoylimino)-5H-benzo[a] phenolxazine-9-amine (ETH 5294) and sodium tetraphenyl borate (NaTPB), an ionophore 5-(2',4'-dimethylphenylazo)-6-hydroxy-pyrimidine-2,4-dione (DMPAHPD) form an optical chemical sensor for zinc determination is ascribed. The sensor response is based on selective complexation of Zn2+ with DMPAHPD in the designed membrane phase, resulting in an ion exchange process between H+ in the membrane and Zn2+ in the sample solution. The influences of several experimental parameters, as membrane composition, pH, and type and concentration of the regenerating reagent, were demonstrated. The sensor has a response range of 5.0 × 10-9 to 2.5 × 10-5 M Zn2+ with detection and quantification limits of 1.6 × 10-9 and 4.9 × 10-9 M, respectively. The response time of 1 min at 0.1 M phosphate buffer solution of pH 5.0 with recording repeatability and sensor-to sensor reproducibility is reported. The proposed sensor signifies high selectivity for Zn2+ over various transition metal ions, alkali, and alkaline earth ions. The sensor membrane can be simply regenerated with 0.5 M HNO3. The sensor has been used to assess Zn2+ in river, waste, tap, sea, well, and spring waters samples, serum of diabetic patients, powdered milk, hair, red meat, pharmaceutical formulations, and talc powder samples.

Simultaneous In Situ Imaging of pH and Surface Roughening during the Progress of Crevice Corrosion of Stainless Steel.

Stainless steel plays an important role in industry due to its anti-corrosion characteristic. It is known, however, that local corrosion can damage stainless steel under certain conditions. In this study, we developed a novel measurement system to observe crevice corrosion, which is a local corrosion that occurs inside a narrow gap. In addition to pH imaging inside the crevice, another imaging technique using an infrared light was combined to simultaneously visualize surface roughening of the test piece. According to experimental results, the lowering of local pH propagated inside the crevice, and after that, the surface roughening started and expanded due to propagation of corrosion. The real-time measurement of the pH distribution and the surface roughness can be a powerful tool to investigate the crevice corrosion.

Non-Specific Responsive Nanogels and Plasmonics to Design MathMaterial Sensing Interfaces: The Case of a Solvent Sensor.

The combination of non-specific deformable nanogels and plasmonic optical probes provides an innovative solution for specific sensing using a generalistic recognition layer. Soft polyacrylamide nanogels that lack specific selectivity but are characterized by responsive behavior, i.e., shrinking and swelling dependent on the surrounding environment, were grafted to a gold plasmonic D-shaped plastic optical fiber (POF) probe. The nanogel-POF cyclically challenged with water or alcoholic solutions optically reported the reversible solvent-to-phase transitions of the nanomaterial, embodying a primary optical switch. Additionally, the non-specific nanogel-POF interface exhibited more degrees of freedom through which specific sensing was enabled. The real-time monitoring of the refractive index variations due to the time-related volume-to-phase transition effects of the nanogels enabled us to determine the environment's characteristics and broadly classify solvents. Hence the nanogel-POF interface was a descriptor of mathematical functions for substance identification and classification processes. These results epitomize the concept of responsive non-specific nanomaterials to perform a multiparametric description of the environment, offering a specific set of features for the processing stage and particularly suitable for machine and deep learning. Thus, soft MathMaterial interfaces provide the ground to devise devices suitable for the next generation of smart intelligent sensing processes.

Measuring Protons with Photons: A Hand-Held, Spectrophotometric pH Analyzer for Ocean Acidification Research, Community Science and Education.

Ocean Acidification (OA) is negatively affecting the physiological processes of marine organisms, altering biogeochemical cycles, and changing chemical equilibria throughout the world's oceans. It is difficult to measure pH broadly, in large part because accurate pH measurement technology is expensive, bulky, and requires technical training. Here, we present the development and evaluation of a hand-held, affordable, field-durable, and easy-to-use pH instrument, named the pHyter, which is controlled through a smartphone app. We determine the accuracy of pH measurements using the pHyter by comparison with benchtop spectrophotometric seawater pH measurements, measurement of a certified pH standard, and comparison with a proven in situ instrument, the iSAMI-pH. These results show a pHyter pH measurement accuracy of ±0.046 pH or better, which is on par with interlaboratory seawater pH measurement comparison experiments. We also demonstrate the pHyter's ability to conduct both temporal and spatial studies of coastal ecosystems by presenting data from a coral reef and a bay, in which the pHyter was used from a kayak. These studies showcase the instrument's portability, applicability, and potential to be used for community science, STEM education, and outreach, with the goal of empowering people around the world to measure pH in their own backyards.

Electrical and Low Frequency Noise Characterization of Graphene Chemical Sensor Devices Having Different Geometries.

Chemiresistive graphene sensors are promising for chemical sensing applications due to their simple device structure, high sensitivity, potential for miniaturization, low-cost, and fast response. In this work, we investigate the effect of (1) ZnO nanoparticle functionalization and (2) engineered defects onto graphene sensing channel on device resistance and low frequency electrical noise. The engineered defects of interest include 2D patterns of squares, stars, and circles and 1D patterns of slots parallel and transverse to the applied electric potential. The goal of this work is to determine which devices are best suited for chemical sensing applications. We find that, relative to pristine graphene devices, nanoparticle functionalization leads to reduced contact resistance but increased sheet resistance. In addition, functionalization lowers 1/f current noise on all but the uniform mesa device and the two devices with graphene strips parallel to carrier transport. The strongest correlations between noise and engineering defects, where normalized noise amplitude as a function of frequency f is described by a model of AN/fγ, are that γ increases with graphene area and contact area but decreases with device total perimeter, including internal features. We did not find evidence of a correlation between the scalar amplitude, AN, and the device channel geometries. In general, for a given device area, the least noise was observed on the least-etched device. These results will lead to an understanding of what features are needed to obtain the optimal device resistance and how to reduce the 1/f noise which will lead to improved sensor performance.

Modified Natural Rubber as a Simple Chemical Sensor with Smartphone Detection for Formaldehyde Content in a Seafood Sample.

A new biodegradable platform-based sensor for formaldehyde assay is proposed. Natural rubber latex was modified to polylactic acid-chloroacetated natural rubber polymer blend sheets. The polymer blend sheet was grafted using a water-based system with amine monomers as a platform, with a spot exhibiting positive polarity for immobilizing with anionic dye (Acid Red 27). The sensor was exposed to formaldehyde. The color intensity of the dye on the sensor spot would decrease. Using a smartphone with image processing (via ImageJ program), the color intensity change (∆B) could be followed. A linear calibration, ∆B intensity = 0.365 [FA] + 6.988, R2 = 0.997, was obtained for 10-150 mM FA with LOD and LOQ at 3 and 10 mM, respectively (linear regression method). The precision was lower than 20% RSD. Application to real seafood samples was demonstrated. The ready-to-use sensor with the proposed method was cost-effective, was portable for on-site analysis, and demonstrated green chemical analysis.

Olfactory flow in the sea catfish, Ariopsis felis (L.): Origin, regulation, and resampling.

The olfactory epithelium of the sea catfish, Ariopsis felis, is found on a pinnate array of lamellae (the olfactory rosette) housed within a nasal chamber. The nasal anatomy of A. felis suggests an ability to capture external water currents. We prepared models from X-ray micro-computed tomography scans of two preserved specimens of A. felis. We then used dye visualisation and computational fluid dynamics to show that an external current induced a flow of water through a) the nasal chamber and b) the sensory channels of the olfactory rosette. The factors responsible for inducing flow through the nasal chamber are common to fishes from two other orders. The dye visualisation experiments, together with observations of sea catfishes in vivo, indicate that flow through the nasal chamber is regulated by a mobile nasal flap. The position of the nasal flap - elevated (significant flow) or depressed (reduced flow) - is controlled by the sea catfish's movements. Flow in the sensory channels of the olfactory rosette can pass through either a single channel or, via multiple pathways, up to four consecutive channels. Flow through consecutive sensory channels (olfactory resampling) is more extensive at lower Reynolds numbers (200 and 300, equivalent to swimming speeds of 0.5-1.0 total lengths s-1), coinciding with the mean swimming speed of the sea catfishes observed in vivo (0.6 total lengths s-1). Olfactory resampling may also occur, via a vortex, within single sensory channels. In conclusion, olfactory flow in the sea catfish is regulated and thoroughly sampled by novel mechanisms.

Process-property correlations in laser-induced graphene electrodes for electrochemical sensing.

Laser-induced graphene (LIG) has emerged as a promising electrode material for electrochemical point-of-care diagnostics. LIG offers a large specific surface area and excellent electron transfer at low-cost in a binder-free and rapid fabrication process that lends itself well to mass production outside of the cleanroom. Various LIG micromorphologies can be generated when altering the energy input parameters, and it was investigated here which impact this has on their electroanalytical characteristics and performance. Energy input is well controlled by the laser power, scribing speed, and laser pulse density. Once the threshold of required energy input is reached a broad spectrum of conditions leads to LIG with micromorphologies ranging from delicate irregular brush structures obtained at fast, high energy input, to smoother and more wall like albeit still porous materials. Only a fraction of these LIG structures provided high conductance which is required for appropriate electroanalytical performance. Here, it was found that low, frequent energy input provided the best electroanalytical material, i.e., low levels of power and speed in combination with high spatial pulse density. For example, the sensitivity for the reduction of K3[Fe(CN)6] was increased almost 2-fold by changing fabrication parameters from 60% power and 100% speed to 1% power and 10% speed. These general findings can be translated to any LIG fabrication process independent of devices used. The simple fabrication process of LIG electrodes, their good electroanalytical performance as demonstrated here with a variety of (bio)analytically relevant molecules including ascorbic acid, dopamine, uric acid, p-nitrophenol, and paracetamol, and possible application to biological samples make them ideal and inexpensive transducers for electrochemical (bio)sensors, with the potential to replace the screen-printed systems currently dominating in on-site sensors used.

Detection of Gadolinium with an Impedimetric Platform Based on Gold Electrodes Functionalized by 2-Methylpyridine-Substituted Cyclam.

Gadolinium is extensively used in pharmaceuticals and is very toxic, so its sensitive detection is mandatory. This work presents the elaboration of a gadolinium chemical sensor based on 2-methylpyridine-substituted cyclam thin films, deposited on gold electrodes, using electrochemical impedance spectroscopy (EIS). The 2-methylpyridine-substituted cyclam (bis-N-MPyC) was synthesized in three steps, including the protection of cyclam by the formation of its CH2-bridged aminal derivative; the product was characterized by liquid 1H and 13C NMR spectroscopy. Spin-coated thin films of bis-N-MPyC on gold wafers were characterized by means of infrared spectroscopy in ATR (Attenuated Total Reflectance) mode, contact angle measurements and atomic force microscopy. The impedimetric chemical sensor was studied in the presence of increasing concentrations of lanthanides (Gd3+, Eu3+, Tb3+, Dy3+). Nyquist plots were fitted with an equivalent electrical circuit including two RC circuits in series corresponding to the bis-N-MPyC film and its interface with the electrolyte. The main parameter that varies with gadolinium concentration is the resistance of the film/electrolyte interface (Rp), correlated to the rate of exchange between the proton and the lanthanide ion. Based on this parameter, the detection limit obtained is 35 pM. The bis-N-MPyC modified gold electrode was tested for the detection of gadolinium in spiked diluted negative urine control samples.

Electrical and Electrochemical Sensors Based on Carbon Nanotubes for the Monitoring of Chemicals in Water-A Review.

Carbon nanotubes (CNTs) combine high electrical conductivity with high surface area and chemical stability, which makes them very promising for chemical sensing. While water quality monitoring has particularly strong societal and environmental impacts, a lot of critical sensing needs remain unmet by commercial technologies. In the present review, we show across 20 water monitoring analytes and 90 references that carbon nanotube-based electrochemical sensors, chemistors and field-effect transistors (chemFET) can meet these needs. A set of 126 additional references provide context and supporting information. After introducing water quality monitoring challenges, the general operation and fabrication principles of CNT water quality sensors are summarized. They are sorted by target analytes (pH, micronutrients and metal ions, nitrogen, hardness, dissolved oxygen, disinfectants, sulfur and miscellaneous) and compared in terms of performances (limit of detection, sensitivity and detection range) and functionalization strategies. For each analyte, the references with best performances are discussed. Overall, the most frequently investigated analytes are H+ (pH) and lead (with 18% of references each), then cadmium (14%) and nitrite (11%). Micronutrients and toxic metals cover 40% of all references. Electrochemical sensors (73%) have been more investigated than chemistors (14%) or FETs (12%). Limits of detection in the ppt range have been reached, for instance Cu(II) detection with a liquid-gated chemFET using SWCNT functionalized with peptide-enhanced polyaniline or Pb(II) detection with stripping voltammetry using MWCNT functionalized with ionic liquid-dithizone based bucky-gel. The large majority of reports address functionalized CNTs (82%) instead of pristine or carboxyl-functionalized CNTs. For analytes where comparison is possible, FET-based and electrochemical transduction yield better performances than chemistors (Cu(II), Hg(II), Ca(II), H2O2); non-functionalized CNTs may yield better performances than functionalized ones (Zn(II), pH and chlorine).

Chemosensitive Thin Films Active to Ammonia Vapours.

The paper presents various dispersive systems developed for sensing toxic substance-ammonia. Polycarbonate dissolved in methylene chloride was used as a polymer matrix, which was enriched with: multi-walled carbon nanotubes (MWCNs), reduced graphene oxide (rGO) and conductive polymer (polyaniline-PANi). Dispersive systems were applied to the prefabricated substrates with comb electrodes by two methods: spraying and drop-casting, forming an active chemosensitive to ammonia vapours films. The spraying method involved applying the dispersion to the substrate by an aerograph for a specific time, whereas drop-casting involves depositing of the produced dispersive systems using a precision automatic pipette. The electrical responses of the obtained films were examined for nominal concentrations of ammonia vapours. Different types of dispersions with various composition were tested, the relationships between individual compounds and ammonia were analysed and the most promising dispersions were selected. Sensor containing rGO deposited by drop-casting revealed the highest change in the resistance (14.21%).

Lead Assays with Smartphone Detection Using a Monolithic Rod with 4-(2-Pyridylazo) Resorcinol.

A monolithic rod of polyurethane foam-[4-(2-pyridylazo) resorcinol] (PUF-PAR) as a simple chemical sensor for lead assays with smartphone detection and image processing was developed. With readily available simple apparatus such as a plastic cup and a stirrer rod, the monolithic PUF rod was synthesized in a glass tube. The monolithic PUF-PAR rod could be directly loaded by standard/sample solution without sample preparation. A one-shot image in G/B value from a profile plot in ImageJ for a sample with triplicate results via a single standard calibration approach was obtained. A linear single standard calibration was: [G/B value] = -0.038[µg Pb2+] + 2.827, R2 = 0.95 for 10-30 µg Pb2+ with a limit of quantitation (LOQ) of 33 µg L-1. The precision was lower than 15% RSD. The proposed method was tested by an assay for Pb2+ contents in drinking water samples from Bangkok. The results obtained by the proposed method agree with those of ICP-OES and with 100-120% recovery, demonstrating that the method is useful for screening on-site water monitoring.