Chemosensors, Volume 12, Issue 6 (June 2024) – 12 articles

Proton conductors are ceramic materials with a crystalline or amorphous structure, which allow the passage of an electrical current through them exclusively by the movement of protons: H⁺. Recent developments in proton-conducting ceramics present considerable promise for obtaining economic and sustainable energy conversion and storage devices, electrolysis cells, gas purification, and sensing applications. So, proton-conducting ceramics that combine sensitivity, stability, and the ability to operate at low temperatures are particularly attractive. In this article, the authors start by presenting a brief historical resume of proton conductors and by exploring their properties, such as structure and microstructure, and their correlation with conductivity. A perspective regarding applications of these materials on low-temperature energy-related devices, electrochemical and moisture sensors, is presented. Finally, the authors’ efforts on the usage of a proton-conducting ceramic, polyantimonic acid (PAA), to develop humidity sensors, are looked into. Full article

Ion-selective electrodes for tetrachloroaurate(III) have been developed for potentiometric monitoring of the reduction reaction of tetrachloroaurate(III). Three different plasticized polyvinyl chloride membranes containing tridodecymethylammonium chloride as an anion exchanger were investigated. These membranes differ in the plasticizer used, either 2-nitrophenyl octyl ether (NPOE) or tricresyl phosphate (TCP) or bis-(2-ethylhexyl) sebacate (DOS). The potentiometric response of the electrodes to the tetrachloroaurate(III) concentration was studied by two methods. In the first method, commonly used in the calibration of ion-selective electrodes, successive tetrachloroaurate(III) concentration increments were used and the potential was allowed to stabilize after each concentration step. The second method was developed to mimic the tetrachloroaurate(III) reduction reaction in which there is a continuous decrease in the concentration of tetrachloroaurate(III). This was achieved by continuously diluting an initial concentration of tetrachloroaurate(III) by pumping a diluent solution while keeping the sample volume constant. This method gave an excellent linear response to the tetrachloroaurate(III) concentration. The calibrated electrodes were used for the potentiometric monitoring of the kinetics of a newly observed reaction: the reduction of tetrachloroaurate(III) by hydroxylamine catalyzed by iodide. A mechanism for this reaction is proposed on the basis of the experimental results obtained. Full article

In recent years, in the field of bioanalysis, the use of saliva as a biological fluid for the determination of biomarkers has been proposed. Saliva analysis stands out for its simplicity and non-invasive sampling. This paper proposes a method for the dual determination of ammonium and hydrogen sulfur in saliva using two colorimetric chemosensors. The ammonia reacts with 1,2-Naftoquinone 4 sulphonic acid (NQS) entrapped in polydimethylsiloxane (PDMS) and the hydrogen sulfide with AgNPs retained in a nylon membrane. The color changed from orange to brown in the case of ammonia chemosensors and from yellow to brown in the H₂S. The experimental conditions to be tested have been established. Both analytes have been determined from their gaseous form; these are ammonia from ammonium and hydrogen sulfur from hydrogen sulfur. Good figures of merit have been obtained by using both measuring strategies (reflectance diffuse and digitalized images). The acquired results show that both sensors can be used and provide good selectivity and sensitivity for the determination of these biomarkers in saliva. Both measurement strategies have provided satisfactory results for the real saliva samples (n = 15). Recoveries on spiked samples were between 70 and 100%. This methodology can lead to possible in situ diagnosis and monitoring of certain diseases and pathologies related with NH₄⁺ and/or H₂S, in a fast, simple, cheap and non-invasive way. Full article

Functionalization by noble metal catalysts and the construction of heterojunctions are two effective methods to enhance the gas sensing performance of metal oxide-based sensors. In this work, we adopt the porous ZIF-8 as a catalyst substrate to encapsulate the ultra-small Pt nanoparticles. The Pt/ZnO-In₂O₃ hollow nanofibers derived from Pt/ZIF-8 were prepared by a facile electrospinning method. The 25PtZI HNFs sensor possessed a response value of 48.3 to 100 ppm HCHO, 2.7 times higher than the pristine In₂O₃, along with rapid response/recovery time (5/22 s), and lower theoretical detection limit (74.6 ppb). The improved sensing properties can be attributed to the synergistic effects of electron sensitization effects and catalytic effects of Pt nanoparticles, and the high surface O⁻ absorbing capability of heterojunctions. The present study paves a new way to design high performance formaldehyde gas sensors in practical application. Full article

The need for performant analytical methodologies to assess mycotoxins is vital, given the negative health impact of these compounds. Biosensors are analytical devices that consist of a biological element for recognizing the analyte and a transducer, which translates the biorecognition event into a signal proportional to the analyte concentration. The biorecognition elements can be enzymes, antibodies, or DNA fragments. The modalities of detection can be optical, electrochemical, thermal, or mass-sensitive. These analytical tools represent viable alternatives to laborious, expensive traditional methods and are characterized by specificity given by the biorecognition element, sensitivity, fast response, portability, multi-modal detection, and the possibility of in situ application. The present paper focuses on a comprehensive view, enriched with a critical, comparative perspective on mycotoxin assay using biosensors. The use of different biorecognition elements and detection modes are discussed comparatively. Nanomaterials with optical and electrochemical features can be exploited in association with a variety of biorecognition elements. Analytical parameters are reviewed along with a broad range of applications. Full article

Herein, we report the synthesis and characterization of a novel Schiff base ligand, (Z)-5-((pyren-1-ylmethylene)amino)-2,4-dihydro-3H-pyrazol-3-one (PMDP). The characterization of ligand PMDP was carried out using ESI-MS, ¹H NMR, and UV–Visible spectroscopic techniques. As a probe, PMDP displayed a detectable, colorimetric colour shift in the presence of Cu²⁺ and Fe²⁺ ions. The solution was seen to have a light brown colour and to exhibit a fluorometric “turn off” response when Cu²⁺ and Fe²⁺ ions were present in a DMSO solution (HEPES 0.01 M, pH = 7.4) at room temperature. Job’s plot revealed that the PMDP binding ratio to Cu²⁺ and Fe²⁺ ions was in 1:2 ratio. In contrast to the other metal ions (Cd²⁺, Mn²⁺, Co²⁺, Na⁺, Ni²⁺, Cu⁺, Fe³⁺, Hg²⁺, Mg²⁺, Zn²⁺, K⁺, and V⁵⁺), the synthesised probe showed exceptional sensitivity and selectivity for detecting Cu²⁺ and Fe²⁺ metal ions. The results indicate that the detection limits for Cu²⁺ and Fe²⁺ are 0.42 μM and 0.51 μM, respectively. Furthermore, PMDP was efficiently utilised for the quantitative analysis of Cu²⁺ and Fe²⁺ in real water samples, RGB colour values in smart phones, logic gate construction, and cell imaging in HeLa cells. Full article

Hydrogen (H₂) gas, with its high calorimetric combustion energy and cleanness, is a green source of energy and an alternative to fossil fuels. However, it has a small kinetic diameter, with high diffusivity and a highly explosive nature. Hence, the reliable detection of H₂ gas is essential in various fields such as fuel cells. Herein, we decorated ZnO nanoparticles (NPs) with Pd noble metal NPs, using UV irradiation to enhance their H₂ gas-sensing performance. The synthesized materials were fully characterized in terms of their phases, morphologies, and chemical composition. Then, the sensing layer was deposited on the electrode-patterned glass substrate to make a transparent sensor. The fabricated transparent gas sensor was able to detect H₂ gas at various temperatures and humidity levels. At 250 °C, the sensor exhibited the highest response to H₂ gas. As a novelty of the present study, we successfully detected H₂ gas in mixtures of H₂/benzene and H₂/toluene gases. The enhanced H₂ gas response was related to the catalytic effect of Pd, the formation of heterojunctions between Pd and ZnO, the partial reduction of ZnO to Zn in the presence of H₂ gas, and the formation of PdH_x. With a high performance in a high response, good selectivity, and repeatability, we believe that the sensor developed in this study can be a good candidate for practical applications where the detection of H₂ is necessary. Full article

Oxidative stress refers to the overproduction of reactive oxygen species and is often associated with numerous pathological conditions. Superoxide dismutase (SOD) is a widely used enzyme for evaluating oxidative stress, with numerous methods being developed for its detection in biological specimens like blood, urine, and saliva. In this study, a simple metal-assisted chemical etching method was employed for the fabrication of nanostructured silicon surfaces decorated with either silver dendrites or silver aggregates. Those surfaces were used as substrates for the immunochemical determination of SOD in synthetic saliva through surface-enhanced Raman spectroscopy (SERS) and surface-enhanced fluorescence (SEF). The immunoassay was based on a 3-step competitive assay format, which included, after the immunoreaction with the specific anti-SOD antibody, a reaction with a biotinylated secondary antibody and streptavidin. Streptavidin labeled with peroxidase was used in combination with a precipitating tetramethylbenzidine substrate for detection through SERS, whereas for SEF measurements, streptavidin labeled with the fluorescent dye Rhodamine Red-X was utilized. Both immunoassays were sensitive, with a detection limit of 0.01 μg/mL and a linear dynamic range from 0.03 to 3.3 μg/mL, enabling the evaluation of the oxidative stress status of an organism. Full article

Food safety inspections are an essential aspect of food safety monitoring. Rapid, accurate, and low-cost food analysis can considerably increase the efficiency of food safety inspections. The lateral flow immunochromatographic assay (LFIA) technique has recently grown in popularity due to its ease of use and high efficiency. It is currently commonly utilized in food inspection. In this review, we briefly introduce the principle and classification of LFIA, critically discuss the recent application status of LFIA in food contaminantion detection, and finally propose that artificial intelligence and information technology will further advance the development of LFIA in the field of food safety monitoring. Full article

Silver nanoparticles (AgNPs) possess unique characteristics ideal for enhancing device sensitivity, primarily due to their high surface-to-volume ratio facilitating heightened interaction with analytes. Integrating AgNPs into polymers or carbon-based materials results in nanocomposites with synergistic properties, enabling the detection of minute changes in the environment across various applications. In this study, we investigate the adsorption kinetics of AgNPs within multilayered layer-by-layer (LbL) structures, specifically examining the impact of AgNPs concentration in the LbL film formation that is further explored as sensing units in an impedimetric microfluidic e-tongue. Although absorption kinetic studies are infrequent, they are crucial to optimize the AgNPs adsorption and distribution within LbL structures, significantly influencing upcoming applications. Through systematic variation of AgNPs concentration within identical LbL architectures, we applied the films as sensing units in a microfluidic e-tongue capable of distinguishing food enhancers sharing the umami taste profile. Across all tested scenarios, our approach consistently achieves robust sample separation, evidenced by silhouette coefficient, principal component analyses, and long-term stability. This work contributes to exploring controlled nanomaterial-based developments, emphasizing the importance of precise parameter control for enhanced sensor performance across diverse analytical applications. Full article

Graphene oxide (GO) was used as an ion-to-electron transducer for all-solid-state nitrate electrodes based on an alkyl ammonium salt as the sensing element. Commercially available carbon screen-printed electrodes modified with GO were used as conductive substrates, whose morphology and distribution along the surface were evaluated by scanning electron microscopy and Raman spectroscopy. The potentiometric performance of the GO-based electrodes revealed a Nernstian slope of −53.5 ± 2.0 mV decade⁻¹ (R² = 0.9976 ± 0.0015) in the range from 3.0 × 10⁻⁶ to 10⁻² M and a lower limit of detection of 1.9 × 10⁻⁶ M. An impressive reproducibility between equally prepared electrodes (n = 15) was demonstrated by a variation of <6% for the calibration parameters. Constant current chronopotentiometry and water layer tests were used to evaluate the potential signal stability, providing similar performance to previously published works with graphene-based ion-selective electrodes. Notably, the GO-based sensors showed the absence of a water layer, a long-term drift of 0.3 mV h⁻¹, and a stable performance (LOD and sensitivity) over 3 months. The applicability of the proposed sensors was demonstrated in determining nitrate levels in water samples with great accuracy, yielding recovery values from 87.8 to 107.9%, and comparable (p > 0.05) results to a commercial nitrate probe. These findings demonstrate the use of GO as an alternative ion-to-electron transducer for the fabrication of all-solid-state potentiometric electrodes. Full article

Since the discovery of innovative two-dimensional (2D) materials, significant efforts have been dedicated to exploring their intriguing properties and emerging applications. Among all candidates, transition metal dichalcogenides (TMDs) have proven to be exceptional for gas sensing, while defects engineering has been introduced to modify the pristine TMDs for better gas sensing performances. In this review, we systematically summarize types of defects, advanced characterization techniques, and state-of-the-art controllable synthetic methods. Various types of defects in TMDs can induce diverse changes in chemical and electron structures, which are closely correlated with gas sensing ability. Therefore, connections between defects and gas sensing mechanisms and performances have been addressed based on both defect categories and electron affinity of gases. This review will be a guide for researchers in defective materials and open up the field of precisely synthesis chemistry and deepen the understanding of the underlying effects of defects in other 2D materials. Full article