Chemosensors, Volume 10, Issue 7 (July 2022) – 56 articles

There is an increasing need for the development of low-cost and highly sensitive gas sensors for environmental, commercial, and industrial applications in various areas, such as hazardous gas monitoring, safety, and emission control in combustion processes. Considering this, resistive-based gas sensors using metal oxide semiconductors (MOSs) have gained special attention owing to their high sensing performance, high stability, and low cost of synthesis and fabrication. The relatively low final costs of these gas sensors allow their commercialization; consequently, they are widely used and available at low prices. This review focuses on the important MOSs with different morphologies, including quantum dots, nanowires, nanofibers, nanotubes, hierarchical nanostructures, and other structures for the fabrication of resistive gas sensors. Full article

Optical multisensor systems are easy-to-use and inexpensive analytical devices. In this work, we propose an optical multisensor system based on the luminescence of Nd(III) and Yb(III) complexes in the near-infrared (NIR) spectral region. The observed emission bands play the role of secondary light sources for further analysis of milk—for the determination of fat content and for the recognition of adulteration. The samples for analysis were prepared by putting a drop of milk upon a thin glass covering the powdered mixture of lanthanide complexes, which were excited by a light-emitting diode (LED) in the ultraviolet region (the maximum intensity at 365 nm). The diffuse-reflectance spectra of samples were acquired in the short-wave NIR range 750–1100 nm using a portable NIR spectrometer. The developed optical system was tested using two sets of milk samples with varying concentration levels of fat and added urea. The obtained spectral data were analyzed using a number of multivariate prediction and classification methods of chemometrics and the results were statistically compared. The regression and classification model performances achieved in this proof-of-concept study illustrate the feasibility of the optical multisensor analysis based on luminescent light sources in the short-wave NIR range, in particular, for their application in the dairy. Full article

Rapid and early diagnosis of lethal coronavirus disease-19 (COVID-19) caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an important issue considering global human health, economy, education, and other activities. The advancement of understanding of the chemistry/biochemistry and the structure of the SARS-CoV-2 virus has led to the development of low-cost, efficient, and reliable methods for COVID-19 diagnosis over “gold standard” real-time reverse transcription-polymerase chain reaction (RT-PCR) due to its several limitations. This led to the development of electrochemical sensors/biosensors for rapid, fast, and low-cost detection of the SARS-CoV-2 virus from the patient’s biological fluids by detecting the components of the virus, including structural proteins (antigens), nucleic acid, and antibodies created after COVID-19 infection. This review comprehensively summarizes the state-of-the-art research progress of electrochemical biosensors for COVID-19 diagnosis. They include the detection of spike protein, nucleocapsid protein, whole virus, nucleic acid, and antibodies. The review also outlines the structure of the SARS-CoV-2 virus, different detection methods, and design strategies of electrochemical SARS-CoV-2 biosensors by highlighting the current challenges and future perspectives. Full article

A highly sensitive self-powered photoelectrochemical (spPEC) sensing platform was constructed for Hg²⁺ determination based on the g-C₃N₄-CdS-CuO co-sensitized photoelectrode and a visible light-induced redox cycle for signal amplification. Through successively coating the single-layer g-C₃N₄, CdS, and CuO onto the surface of an electrode, the modified electrode exhibited significantly enhanced PEC activity. The microstructure of the material was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). However, the boost in photocurrent could be noticeably suppressed due to the consumption of hole-scavenging agents (reduced glutathione) by the added Hg²⁺. Under optimal conditions, we discovered that the photocurrent was linearly related to the Hg²⁺ concentration in the range of 5 pM–100 nM. The detection limit for Hg²⁺ was 0.84 pM. Moreover, the spPEC sensor demonstrated good performance for the detection of mercury ions in human urine and artificial saliva. Full article

In 2008, the modified European Restriction of Hazardous Substances (RoHS) directive prohibited the use of hazardous substances such as lead, cadmium, and mercury. As such, an urgent need for lead-free components emerged in Europe. In this frame, we have decided to study the microstructure influence of zinc oxide thin films on the detection of hydrogen sulfide (H₂S). Zinc oxide thin films were deposited by PLD on silicon substrates under different conditions to modify the microstructure. In order to compare our demonstrators to current commercial semiconductor gas sensors, measurements under H₂S were also performed with sensors from Figaro and Winsen corporations. Gas sensors were therefore implemented by using commercial cases in view to test them with Simtronics gas detector DG477. The good sensitivity values measured at T = 400 °C under 100 ppm H₂S, and response times as low as 30 s, definitely confirm that ZnO thin films could be developed for commercial sensors. Full article

Gram-negative bacteria are difficult to kill due to their complex cell envelope, including the outer membrane (OM) and cytoplasmic membrane (CM). To monitor the membranolytic action of antimicrobials on Gram-negative bacteria would facilitate the development of effective antimicrobials. In this paper, an aggregation-induced emission luminogen (AIEgen) with microenvironment-sensitive properties was employed to indicate the interaction of antimicrobials with the OM and CM of Gram-negative bacteria. The damaged extent of OM and CM caused by antimicrobials with the change of dosage and incubation time can be visually captured based on the variation of two emission colors of IQ-Cm responding to OM-defective (green) and CM-disruptive bacteria (orange). Meanwhile, the activity assessment of antimicrobials can be easily realized within 1~2 h based on the distinct response of IQ-Cm to live and dead E. coli, which is much faster than the agar plate culture. This probe may shed light on the understanding of the interaction between the membrane-active antimicrobials and cell envelope of Gram-negative bacteria and contribute to the future development of antimicrobials. Full article

The focus of this work was to develop a simple electrochemical method for the determination of vitamin C (VitC) by using a specially constructed microelectrode made from pyrolytic graphite sheet (PGS). A procedure for quantifying VitC in a real sample was established. VitC shows a single quasi-reversible reaction. The method was optimized, and analytical determination was performed by using cyclic voltammetry and square wave voltammetry for electroanalytical purposes. The obtained results show a linear response of the PGS electrode in a wide concentrations range. For the lower concentration range, 0.18–7.04 µg L⁻¹, the sensitivity is 11.7 µAcm⁻²/mgL⁻¹, while for the higher concentration range, 10.6–70.4 µg L⁻¹, the sensitivity is 134 µAcm⁻²/mgL⁻¹, preserving the linearity of 0.998 and 0.999. The second objective was to determine the effect of the addition of five different types of “green” biowaste on plant growth, VitC content, and antioxidant activity in arugula (Eruca sativa L.) using the developed method. After three weeks of cultivation, small differences in growth and large differences in certain nutritional characteristics were observed. The addition of black coffee makes the soil slightly alkaline and causes a significant increase in VitC content and antioxidant activity. Full article

Ultraviolet-visible absorption spectroscopy is widely used to monitor water quality, and rapid optical signal detection is a key technology in the process of spectrum measurement. In this paper, an ultrafast spectrophotometer system that can achieve spectrum data acquisition in a single flash of the xenon lamp (within 200 ns) is introduced, and a real-time denoising method for the spectrum is implemented on a field programmable gate array (FPGA) to work cooperatively with the nanosecond spectrum acquisition system, in order to guarantee the quality of the spectrum signals without losing running speed. The hardware of the data acquisition and processing system are constructed on a Xilinx Spartan 6 FPGA chip and its peripheral circuit, including an analog to digital converter and a complementary metal-oxide-semiconductor transistor (CMOS) sensor’s diver circuit. An oversampling method that is suitable for the CMOS sensor’s output is proposed, which works on the CMOS sensor’s dark current noise and readout noise. Another moving-average filter method is designed adaptively, which works on the low-frequency component to filter out the residual spectrum noise of the spectrum signal. The implementation of the filter on the FPGA has been optimized by using a pipelined structure and dual high-speed random-access memory (RAM). As a result, the CMOS linear image sensor successfully captured the spectrum of xenon flash light at the readout clock frequency of 500 kHz and the processing manipulation to the full UV-Vis spectrum data was accomplished at a sub-microsecond speed performance. After the digital filter and oversampling technology were implemented, the coefficient of variation of the measurements reduced from 9.57% to 1.74%, while the signal noise ratio (SNR) of the absorption spectrum increased nine times, compared to the raw data of the CMOS sensor’s output. The tests towards different analyte samples were conducted, and the system shows good performance on distinguishing different concentrations of different analyte solutions on both ultra-violet and visible spectrum bands. The present work showcases the potential of the CMOS sensor’s technique for the fast detection of contaminated water containing nitrate and organic compounds. Full article

Gene therapy is a technique that is currently under expansion and development. Recent advances in genetic medicine have paved the way for a broader range of therapies and laid the groundwork for next-generation technologies. A terminally substituted difluorene-diester Schiff Base calix[4]arene has been studied in this work as possible nanovector to be used in gene therapy. Changes to luminescent behavior of the calixarene macrocycle are reported in the presence of ct-DNA. The calixarene macrocycle interacts with calf thymus DNA (ct-DNA), generating changes in its conformation. Partial double-strand denaturation is induced at low concentrations of the calixarene, resulting in compaction of the ct-DNA. However, interaction between calixarene molecules themselves takes place at high calixarene concentrations, favoring the decompaction of the polynucleotide. Based on cytotoxicity studies, the calixarene macrocycle investigated has the potential to be used as a nanovehicle and improve the therapeutic efficacy of pharmacological agents against tumors. Full article

Diabetes has become one of the most prevalent endocrine and metabolic diseases that threaten human health, and it is accompanied by serious complications. Therefore, it is vital and pressing to develop novel strategies or tools for prewarning and therapy of diabetes and its complications. Fluorescent probes have been widely applied in the detection of diabetes due to the fact of their attractive advantages. In this report, we comprehensively summarize the recent progress and development of fluorescent probes in detecting the changes in the various biomolecules in diabetes and its complications. We also discuss the design of fluorescent probes for monitoring diabetes in detail. We expect this review will provide new ideas for the development of fluorescent probes suitable for the prewarning and therapy of diabetes in future clinical transformation and application. Full article

Using the first-principles theory, this study explored the electronic behavior and adsorption effect of SO₂ on an InP₃ monolayer doped with transition metal atoms (Pd, Pt, Au, Fe, Co and Mo). Through calculation and analysis, the optimum doping sites of TM dopants on the InP₃ monolayer were determined, and the adsorption processes of SO₂ by TM-InP₃ monolayers were simulated. In the adsorption process, all TM-InP₃ monolayers and SO₂ molecules were deformed to some extent. All adsorption was characterized as chemical adsorption, and SO₂ acted as an electron acceptor. Comparing E_ad and Q_t, the order of the SO₂ adsorption effect was Mo-InP₃ > Fe-InP₃ > Co-InP₃ > Pt-InP₃ > Pd-InP₃ > Au-InP₃. Except for the Au atom, the other five TM atoms as dopants all enhanced the adsorption effect of InP₃ monolayers for SO₂. Furthermore, the analysis of DCD and DOS further confirmed the above conclusions. Based on frontier orbital theory analysis, it is revealed that the adsorption of SO₂ reduces the conductivity of TM-InP₃ monolayers to different degrees, and it is concluded that Pd-InP₃, Pt-InP₃, Fe-InP₃ and Mo-InP₃ monolayers have great potential in the application of SO₂ resistive gas sensors. This study provides a theoretical basis for further research on TM-InP₃ as a SO₂ sensor. Full article

The determination of nucleic acids has become an analytical diagnostic method with many applications in fields such as biomedical sciences, environmental monitoring, forensic identification, and food safety. Among the different methods for nucleic acid analysis, those based on the polymerase chain reaction (PCR) are nowadays considered the gold standards. Isothermal amplification methods are an interesting alternative, especially in the design of chip-based architectures. Biosensing platforms hold great promise for the simple and rapid detection of nucleic acids since they can be embedded in lab-on-a-chip tools to perform nucleic acid extraction, amplification, and detection steps. Electrochemical transduction schemes are particularly interesting in the design of small and portable devices due to miniaturization, low-energy consumption, and multianalyte detection capability. The aim of this review is to summarize the different applications of isothermal amplification methods combined with electrochemical biosensing techniques in the development of lab-on-a-chip tools and wearable sensors. Different isothermal amplification methods are revised, and examples of different applications are discussed. Finally, a discussion on patented devices is also included. Full article

TiO₂-based ultraviolet photodetectors have drawn great attention and are intensively explored. However, the construction of TiO₂-based nanocomposites with excellent ultraviolet responses remains challenging. Herein, a TiO₂ nanorod array was successfully prepared on fluorine-doped tin oxide (FTO) conductive glass by a one-step hydrothermal method. Then, polypyrrole (PPy)-TiO₂ nanorod array composites were designed via subsequent in situ oxidative polymerization. The morphologies, structures, and photocurrent responses of the nanocomposites were systematically investigated. The results demonstrated that polypyrrole-TiO₂ exhibited a stronger photocurrent response than pure TiO₂ due to the p-n junction formed between n-type TiO₂ nanorod arrays and p-type polypyrrole. The PPy-TiO₂ composite obtained by deposition three times had the best photoelectric properties, exhibiting good performance with a sensitivity of 41.7 and responsivity of 3.5 × 10⁻³ A/W. Finally, the mechanism of the photoelectrical response of PPy-TiO₂ composites was discussed, guiding the design of high-performance TiO₂-based ultraviolet photodetectors. Full article

The abnormal expression of miRNA is closely related to the occurrence of pancreatic cancer. Herein, a programmable DNAzyme amplifier for the universal detection of pancreatic cancer-related miRNAs was proposed based on its programmability through the rational design of sequences. The fluorescence signal recovery of the DNAzyme amplifier showed a good linear relationship with the concentration of miR-10b in the range of 10–60 nM, with a detection limit of 893 pM. At the same time, this method displayed a high selectivity for miR-10b, with a remarkable discrimination of a single nucleotide difference. Furthermore, this method was also successfully used to detect miR-21 in the range of 10–60 nM based on the programmability of the DNA amplifier, exhibiting the universal application feasibility of this design. Overall, the proposed programmable DNAzyme cycle amplifier strategy shows promising potential for the simple, rapid, and universal detection of pancreatic cancer-related miRNAs, which is significant for improving the accuracy of pancreatic cancer diagnosis. Full article

We propose and discuss an effective tool for substance detection and identification using a broadband THz pulse that is based on frequency conversion near the substance absorption frequencies. With this aim, we analyze the evolution of spectral intensities at the doubled absorption frequencies in order to prove their similarity to those at which the absorption of THz pulse energy occurs. This analysis is provided for both artificial THz signals and the real signals reflected from the substances under consideration. We demonstrate the feasibility of the proposed approach in the detection and identification of substances with an inhomogeneous surface, which is the most difficult case for practice, by using the method of spectral dynamic analysis and integral correlation criteria. Full article

The electrical and chemical properties of the titanium dioxide (TiO₂) coated spirals grown by the atomic layer deposition (ALD) technique in two different temperatures of 150 °C and 300 °C are studied. The thickness of the TiO₂ layers studied are 20, 40, and 80 nm. A switched capacitor amplifier is used to investigate the pH response and the capacitance of the samples. It is found that the performance of the TiO₂ samples depends on either the thickness or the deposition temperature due to the differences in the physical properties of the oxide layer such as surface roughness and film density. The high temperature samples are more crystalline, whereas the low temperature samples are more amorphous. Since there is a low pass filter effect in the electrolyte–sample interface, the TiO₂ coated samples show the better response to the pH change for the high temperature samples as the sensor surface area for binding the hydrogen ions is larger and the charge transfer resistance is smaller. Furthermore, more roughness on the surface can be obtained by increasing the thickness, which reduces the charge transfer resistance. In this study, the 80 nm sample deposited at 300 °C gives the best pH response of 40 mV/pH. Full article

In recent years, wearable sensors have enabled the unique mode of real-time and noninvasive monitoring to develop rapidly in medical care, sports, and other fields. Sweat contains a wide range of biomarkers such as metabolites, electrolytes, and various hormones. Combined with wearable technology, sweat can reflect human fatigue, disease, mental stress, dehydration, and so on. This paper comprehensively describes the analysis of sweat components such as glucose, lactic acid, electrolytes, pH, cortisol, vitamins, ethanol, and drugs by wearable sensing technology, and the application of sweat wearable devices in glasses, patches, fabrics, tattoos, and paper. The development trend of sweat wearable devices is prospected. It is believed that if the sweat collection, air permeability, biocompatibility, sensing array construction, continuous monitoring, self-healing technology, power consumption, real-time data transmission, specific recognition, and other problems of the wearable sweat sensor are solved, we can provide the wearer with important information about their health level in the true sense. Full article

Lung cancer is one of the malignant tumors with the highest mortality rate, and the detection of its tumor marker carcinoma antigen 125 (CA125) is significant. Here, an electrochemical immunoassay for CA125 was described. Nitrogen-doped reduced graphene oxide (N-rGO), carboxylated multi-walled carbon nanotubes (CMWCNTs) and gold nanoparticles (AuNPs) were applied to co-modify glassy carbon electrode (GCE), after incubation with Anti-CA125, the modified electrode was employed for the specific detection of CA125. The N-rGO@CMWCNTs (Nitrogen-doped reduced graphene oxide@carboxylated multi-walled carbon nanotubes) were used as a matrix, while CS@AuNPs (Chitosan@gold nanoparticles) with high conductivity and biocompatibility was immobilized on it through the reaction between carboxyl groups from CMWCNTs and amino groups, hydroxyl groups from chitosan (CS), resulting in the effect of double signal amplification. The immunosensor demonstrated excellent electrochemical performance with a linear detection range of 0.1 pg mL⁻¹–100 ng mL⁻¹, and the detection limit was as low as 0.04 pg mL⁻¹ (S/N = 3). It had been verified that this method had good precision and high accuracy, and the immunosensor could remain stable for 10 days. This research provided a new method for the detection of CA125 in serum. Full article

Convective PCR (CPCR) can perform rapid nucleic acid amplification by inducing thermal convection to continuously, cyclically driving reagent between different zones of the reactor for spatially separate melting, annealing, and extending in a capillary tube with constant heating temperatures at different locations. CPCR is promoted by incorporating an FTA membrane filter into the capillary tube, which constructs a single convective PCR reactor for both sample preparation and amplification. To simplify fluid control in sample preparation, lysed sample or wash buffer is driven through the membrane filter through centrifugation. A movable resistance heater is used to heat the capillary tube for amplification, and meanwhile, a smartphone camera is adopted to monitor in situ fluorescence signal from the reaction. Different from other existing CPCR systems with the described simple, easy-to-use, integrated, real-time microfluidic CPCR system, rapid nucleic acid analysis can be performed from sample to answer. A couple of critical issues, including wash scheme and reaction temperature, are analyzed for optimized system performance. It is demonstrated that influenza A virus with the reasonable concentration down to 1.0 TCID₅₀/mL can be successfully detected by the integrated microfluidic system within 45 min. Full article

Acetone detection is of great significance for environmental monitoring or diagnosis of diabetes. Nevertheless, fast and sensitive detection of acetone at low temperatures remains challenging. Herein, a series of rGO-functionalized three-dimensional (3D) In₂O₃ flower-like structures were designed and synthesized via a facile hydrothermal method, and their acetone-sensing properties were systematically investigated. Compared to the pure 3D In₂O₃ flower-like structures, the rGO-functionalized 3D In₂O₃ flower-like structures demonstrated greatly improved acetone-sensing performance at relatively low temperatures. In particular, the 5-rGO/In₂O₃ sensor with an optimized decoration exhibited the highest response value (5.6) to 10 ppm acetone at 150 °C, which was about 2.3 times higher than that of the In₂O₃ sensor (2.4 at 200 °C). Furthermore, the 5-rGO/In₂O₃ sensor also showed good reproducibility, a sub-ppm-level detection limit (1.3 to 0.5 ppm), fast response and recovery rates (3 s and 18 s, respectively), and good long-term stability. The extraordinary acetone-sensing performance of rGO/In₂O₃ composites can be attributed to the synergistic effect of the formation of p-n heterojunctions between rGO and In₂O₃, the large specific surface area, the unique flower-like structures, and the high conductivity of rGO. This work provides a novel sensing material design strategy for effective detection of acetone. Full article

Fast and accurate point-of-care testing (POCT) of infectious diseases is crucial for diminishing the pandemic miseries. To fight the pandemic coronavirus disease 2019 (COVID-19), numerous interesting electrochemical point-of-care (POC) tests have been evolved to rapidly identify the causal organism SARS-CoV-2 virus, its nucleic acid and antigens, and antibodies of the patients. Many of those electrochemical biosensors are impressive in terms of miniaturization, mass production, ease of use, and speed of test, and they could be recommended for future applications in pandemic-like circumstances. On the other hand, self-diagnosis, sensitivity, specificity, surface chemistry, electrochemical components, device configuration, portability, small analyzers, and other features of the tests can yet be improved. Therefore, this report reviews the developmental trend of electrochemical POC tests (i.e., test platforms and features) reported for the rapid diagnosis of COVID-19 and correlates any significant advancements with relevant references. POCTs incorporating microfluidic/plastic chips, paper devices, nanomaterial-aided platforms, smartphone integration, self-diagnosis, and epidemiological reporting attributes are also surfed to help with future pandemic preparedness. This review especially screens the low-cost and easily affordable setups so that management of pandemic disease becomes faster and easier. Overall, the review is a wide-ranging package for finding appropriate strategies of electrochemical POCT targeting pandemic infectious disease detection. Full article

A novel enzyme-free photoelectrochemical (PEC) potential measurement system based on Dy-OSCN was designed for ascorbic acid (AA) detection. The separation and transmission of internal carriers were accelerated and the chemical properties became more stable under light excitation due to the regular microstructure of the prepared Dy-OSCN monocrystal. More importantly, the PEC potential method (OCPT, open circuit potential-time) used in this work was conducive to the reduction of photoelectric corrosion and less interference introduced during the detection process, which effectively ensured the repeatability and stability of the electrode. Under optimal conditions, the monocrystal successfully served as a matrix for the detection of AA, and the prepared PEC sensor exhibited a wide linear range from 7.94 × 10⁻⁶ mol/L to 1.113 × 10⁻² mol/L and a sensitive detection limit of 3.35 μM. Practical human urine sample analysis further revealed the accuracy and feasibility of the Dy-OSCN-based PEC platform. It is expected that such a PEC sensor would provide a new way for rapid and non-invasive AA level assessment in human body constitution monitoring and lays a foundation for the further development of practical products. Full article

The gas sensitivity of semiconductor metal oxides, such as γ-Fe₂O₃ and SnO₂, is investigated together with the synergistic effects in conjunction with grapheme. Nanoparticles of γ-Fe₂O₃, γ-Fe₂O₃/SnO₂, and γ-Fe₂O₃/SnO₂/RGO, prepared by two-step fabrication, were assembled in gas-sensing devices to assess their sensitivities; response and recovery times for the detection of ethanol, methanol, isopropanol, formaldehyde, H₂S, CO, and NO gases at different temperatures but constant concentrations of 100 particles per million (ppm); and H₂S, which underwent the dynamic gas sensitivity test in different concentrations. Each sample’s crystallinity and microscopic morphology was investigated with X-ray diffraction and a scanning electron microscope. In comparative gas sensitivity measurements, the ternary composite of γ-Fe₂O₃/SnO₂/RGO was identified as an ideal candidate, as it responds to all four tested liquids in the gas phase as well as H₂S with a response value equal to 162.6. Further, only the ternary composite γ-Fe₂O₃/SnO₂/RGO hybrid nanoparticles responded to NO gas with a sensor response value equal to 4.09 in 12 s. However, only the binary composite γ-Fe₂O₃/SnO₂ responded to CO with a corresponding sensitivity of 1.59 units in 7 s. Full article

Physical processes working in parallel with digital ones have transformed the way we view systems and have led to the creation of applications that boost the quality of people’s lives, increase security as well as decrease production costs of goods. Critical to this evolution is the cost decrease in the components of such systems, among which are gas sensors. In this work, a custom-made Co₃O₄ gas sensing element is presented, which can potentially be used as part of a cyber-physical system (CPS) for O₃ monitoring. To investigate its performance, a CPS is developed using low-cost, low-power micro-controller units (MCUs) and comparisons both with the laboratory equipment and commercial off-the-shelf (COTS) ozone sensors are provided. The experiments show that the Co₃O₄ sensor works at room temperature with low input voltage and low power consumption when used with the proposed MCUs. Moreover, an enhanced gas sensing performance against ozone is observed under low-pressure conditions due to the detection of low ozone concentrations (85.90 ppb) and good sensor response (113.1%) towards 1100 ppb O₃. However, the drawbacks that need improvement relate to the kinetics of the charge carriers, which affect the response time and recovery behavior. The effect of humidity needs to be clarified in further works. Full article

A novel, direct and simple colorimetric method employing μicroanalytical paper-based devices (μ-PADs) for the selective determination of bismuth is described. The suggested method exploits the colorimetric variation of bismuth after its rapid reaction with methyl thymol blue (MTB) in an acidic medium (pH ranging between 0.7 and 3.0), modified with nitric acid, on the surface of a paper device at room temperature. The devices are low cost, composed of chromatographic paper and wax barriers and the analytical protocol is easily applicable with minimal technical expertise and without the need for experimental apparatus. The user must add a test sample and read the color intensity of the colored Bi(III)–MTB complex formed at the sensing area using a simple imaging device such as a flatbed scanner. Various chemical variables, such as HNO₃ and MTB concentration, reaction time, ionic strength, detection zone size and photo-capture detector are optimized. A study of interfering ions such as K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻ and HCO³⁻ was also conducted. The stability of the paper devices is also studied in different maintenance conditions with particularly satisfactory results, rendering the method suitable for field analysis. The detection limits are as low as 3.0 mg L⁻¹ with very satisfactory precision, ranging from 4.0% (intra-day) to 5.5% (inter-day). Natural water samples are successfully analyzed, and bismuth percentage recoveries were calculated in the range of 82.8 to 115.4%. Full article

In this work, hydrochloric acid (HCl)-doped molybdenum ditelluride (MoTe₂) nanosheets/polyaniline (PANI) nanofiber composites are prepared by in situ chemical oxidation polymerization, and then the composites are deposited on interdigital electrodes (IDEs) to fabricate a NH₃ gas sensor. Morphological analysis of the composites reveals that the PANI fibers are deposited on 2D MoTe₂ sheets, showing a porous mesh microstructure structure with a more continuous distribution of PANI layer. FTIR spectrum analysis indicates the interaction between the MoTe₂ nanosheets and the PANI in the MoTe₂/PANI composites. The results demonstrate that the as-prepared MoTe₂/PANI composites exhibit higher response than the pure PANI, in particular, the 8 wt.% MoTe₂/PANI composites display about 4.23 times enhancement in response value toward 1000 ppm NH₃ gas compared with the pure PANI. The enhanced NH₃ gas-sensitive properties may be due to the increasing surface area of MoTe₂/PANI composite films and the possible interaction of the P-N heterojunctions formed between PANI and the 2H-MoTe₂ nanosheets. Full article

Current diagnostics call for rapid, sensitive, and selective screening of physiologically important biomarkers. Point-of-care (POC) devices for the rapid, reliable, and easy acquisition of bioinformation at, or near the patient, offer opportunities for better healthcare management. Electrochemical biosensors with high sensitivity and ease of miniaturization are advantageous for such applications. We report a photolithographically micropatterned PEDOT:PSS and silk protein-based fully organic 3-electrode sensor (O3ES) for ultralow volume (single drop—10 µL) detection of analytes in whole blood. The O3ES produces reliable electrochemical signals in whole blood from a mouse model with minimal biofouling interference. The O3ES is demonstrated as a portable device for the simultaneous detection of dopamine, ascorbic acid and uric acid using voltammetry techniques. The O3ES displays excellent sensitivity towards each analyte in whole blood, and in the presence of each other. The water-based, ambient processing of the sensors allows the immobilization of enzymes in the organic working electrode. Amperometric detection of uric acid via uricase with high sensitivity in whole blood is demonstrated. Finally, the performance of the O3ES under enzymatic degradation is studied by monitoring sensitivity over an operating lifetime of ~14 days. This work demonstrates the realization of low-cost, disposable POC sensors capable of detecting blood metabolites using ultralow sample volumes. Full article

The quartz crystal microbalance with dissipation (QCM-D) represented a substantial breakthrough in the use of the QCM sensor in diverse applications ranging from environmental monitoring to biomedical diagnostics. To obtain the required selectivity and sensitivity of a volatile organic compounds (VOC) sensor, it is necessary to coat the QCM sensor with a sensing film. As the QCM sensor is coated with the sensing film, an increase in the dissipation factor occurs, resulting in a shorter and shorter ring-down time. This decrease in ring-down time makes it difficult to implement the QCM-D method in an economical and portable configuration from the perspective of large-scale applications. To compensate for this effect, a regenerative method is proposed by which the damping effect produced by the sensing film is eliminated. In this sense, a regenerative circuit as an extension to a virtual instrument is proposed to validate the experimental method. The simulation of the ring-down time for the QCM sensor in the air considering the effect of the added sensing film, followed by the basic theoretical concepts of the regenerative method and the experimental results obtained, are analyzed in detail in this paper. Full article

A lab-made electronic nose (Enose) with vacuum sampling and a sensor array, comprising nine metal oxide semiconductor Figaro gas sensors, was tested for the quantitative analysis of vapor–liquid equilibrium, described by Henry’s law, of aqueous solutions of organic compounds: three alcohols (i.e., methanol, ethanol, and propanol) or three chemical compounds with different functional groups (i.e., acetaldehyde, ethanol, and ethyl acetate). These solutions followed a fractional factorial design to guarantee orthogonal concentrations. Acceptable predictive ridge regression models were obtained for training, with RSEs lower than 7.9, R² values greater than 0.95, slopes varying between 0.84 and 1.00, and intercept values close to the theoretical value of zero. Similar results were obtained for the test data set: RSEs lower than 8.0, R² values greater than 0.96, slopes varying between 0.72 and 1.10, and some intercepts equal to the theoretical value of zero. In addition, the total mass of the organic compounds of each aqueous solution could be predicted, pointing out that the sensors measured mainly the global contents of the vapor phases. The satisfactory quantitative results allowed to conclude that the Enose could be a useful tool for the analysis of volatiles from aqueous solutions containing organic compounds for which Henry’s law is applicable. Full article

High purity methyl tert-butyl ether (MTBE) can be used to adjust gasoline octane values. However, an isomer, methyl sec-butyl ether (MSBE), is the main by-product of its industrial production, and this affects the purity of MTBE. Pt/Au NPs@Co₃O₄ composites with a hollow dodecahedron three-dimensional structure were synthesized using ZIF-67 as a template, with Pt and Au nanoparticles (NPs) evenly distributed on the shell of the hollow structure. A CTL sensor was established for the determination of MSBE based on the specificity of Pt/Au NPs@Co₃O₄. The experimental results showed that Pt/Au NPs@Co₃O₄ had a strong specific cataluminescence (CTL) response to MSBE, with no interference from MTBE. The linear range was 0.10–90 mg/L, the limit of detection was 0.031 mg/L (S/N = 3), the RSD was 2.5% (n = 9), and a complete sample test could be completed in five minutes. The sensor was used to detect MSBE in MTBE of different purity grades, with recoveries ranging from 92.0% to 109.2%, and the analytical results were consistent with those determined by gas chromatography. These results indicate that the established method was accurate and reliable, and could be used for rapid analysis of MTBE gasoline additive. Full article