Currently, semiconductor gas sensors are among the most common types of sensing devices for detecting dangerous and toxic gases in the atmosphere. However, the problem of improving their characteristics should be solved for practical application. In this work, techniques have been developed to improve the response of sensors based on zinc oxide nanowires. The first technique is to modify the chemical composition of the nanowires by forming a shell of ternary Zn-Sn-O and Zn-Fe-O systems on their surface. Another approach is to control the surface concentration of oxygen vacancies by adding sodium bromide during the synthesis of zinc oxide nanowires. The surface chemical composition and the sensor properties of the samples were studied. It was found that the sensor responses of samples of ternary oxide systems and zinc oxide sample with a high content of oxygen vacancies exceed the one of the initial zinc oxide nanowires. The results are analyzed in the terms of the interaction of reducing gases with metal oxides.

The air quality in the modern cities and urban areas is strongly affected by chemical pollutants such as toxic gases, volatile organic compounds and particulate matter. They are monitored by governmental agencies using regulatory monitoring stations which are highly accurate, but also very expensive, bulky, and maintenance demanding. There is a compulsory need to monitor air quality at high spatio-temporal resolution in smart cities for public health protection and environmental sustainability. The low-cost and low-accuracy sensors, properly calibrated, are usually deployed in stationary and mobile nodes for urban air quality monitoring. A simple indicator of the current status of urban air pollution is the Air Quality Index (AQI) used to communicate the pollution level under time-changing trend of a specific pollutant. In this study, continuous measurements have been performed in the city of Bari (Southern Italy) by electrochemical gas sensors (NO₂, O₃, CO), optical particle counter (OPC) for particulate matter (PM₁₀), NDIR infrared sensor (CO₂), including microsensors for temperature and relative humidity. The sensors have been installed in stationary nodes located in urban sites and in a mobile node mounted on a public bus moving in the urban routes. AQI data gathered by the low-cost sensors have been compared with reference instrumentations as a case-study of citizen science.

The detection of ammonia is very crucial for the welfare of modern society because of its hazardous effect on the environment and human beings. High response time is one of the serious concerns of most of the ammonia detector reported so far in the literature. This issue has been comprehensively addressed in the present investigation. Herein, the solvothermally synthesized Cu-BTC was combined with the 5 wt%, 10 wt% and 20 wt% of partially reduced graphene oxide (rGO). The structural, spectroscopic, morphological and electrical studies of as-synthesized CuBTC@rGO-5wt%, CuBTC@rGO-10wt% and CuBTC@rGO-20wt% were done by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, atomic force microscopy and current-voltage (I-V) characterization. The chemiresistive sensor based on Cu-BTC@rGO was developed on a copper-coated glass electrode via the shadow mask technique. It shows excellent sensing properties for CuBTC@rGO-10wt% in a range of 10 ppm to 80 ppm with high stability up to 30 days, good linearity and excellent response/recovery time, i.e., 84 sec and 125 sec, respectively. The limit of detection has been established as 10 ppm, which is below the maximum residue limit established by OSHA (Occupational Safety and Health Administration). Keywords: Cu-BTC, Graphene, Ammonia, Chemiresistive.

Single nanowires (NWs) are promising structures to improve sensitivity and selectivity of metal-oxide (MOX) gas sensors. Self-made MnO₂ nanowires were hydrothermally synthesized and electrically characterized in different ambient. The nanowires were approximately 4-10 µm long and about 100 nm in diameter. The nanowires were suspended in water and deposited on two parallel gold electrodes having separation distance of 4 µm. Single nanowires were aligned perpendicularly across the gold electrodes by dielectrophoresis (DEP) technique. The number of the NWs was determined by scanning electron microscopy. The conductivity was measured in synthetic air, nitrogen, and in NO₂ ambient. The tests consisted in measuring resistivity of the NWs in relation to temperature of the bottom-placed heater under the chip. The temperature went from room temperature up to 300°C. The resistivity changes were observed accounting for oxygen reduction on the NWs surface as the electrons were moving from the NWs to the oxygen. The resistivity was explored at a constant current arrangement test. Based on resistivity changes, electrical properties, such as activation energy and a type of semiconductor (p-type in case of MnO₂), were evaluated. Mott-Schottky analysis was applied to estimate acceptor concentration, as well as NWs permittivity. In future experiments, we plan to study other types of NWs (e.g., WO₃ NWs) and/or explore an effect of other gases on the NW’s electrical properties, e.g., ethanol or H₂.

A flexible humidity sensor operating at room temperature was produced by direct drop casting of a graphene oxide (GO) water solution on a substrate of bimatted polyester, previously coated with inkjet-printed interdigitated electrodes in silver. The GO was synthesized by using a modified Hummers’ method, followed by an alkaline treatment with a water solution of KOH. Changes in the device resistance were measured for varying relative humidity in the range from 15% to 70% in a sealed stainless cell. The device showed high sensitivity, good repeatability, and fast recovery time. Being flexible and robust, the proposed sensors could be easily integrated into wearable equipment.

The aim of this work was the synthesis of functional hydrogel materials obtained by photopolymerization. The resulting systems were characterized in terms of chemical structure using Fourier transform infrared spectroscopy. Subsequently, their sorption capacity and surface morphology were determined along with roughness analysis. The resulting materials have been modified with fluorescein dye and can be used in many branches of industry and medicine for example in innovative diagnostic systems.

In this work, we explore the analytical potential of a simple inexpensive sensor device based on the evolution of the high-frequency contactless conductometry method. This method was developed in the middle of the 20th century as one of the option to assess the electrical conductivity of the samples and employed electrical signal registered at a certain single AC frequency. The method did not find a wide application since the analytical signal in the developed systems was a complex function of many factors (sample conductivity, capacitive characteristics, dielectric permittivity, magnetic properties), which was difficult to be mathematically processed. We came back to this technology having the following in mind: 1) modern electronic components enable the design of such measuring devices in a very low-cost manner and allow registering the response signal in a whole range of AC frequencies; 2) application of modern machine learning tools to process these signals allows extraction of qualitative and quantitative information about the samples. It was found that the detector has numerous capabilities such as: quantification of inorganic salts in individual aqueous solutions and in complex mixtures; quantification of dielectric constants of organic solvents; distinguishing the cultures of various bacteria and cancer cells.

A virtual experiment conducted using the ICP-MS TuneSim software served as the basis for studies that compared the applicability of the central composite design with the Box-Behnken design in analytical chemistry applications. The insights gathered from these virtual experiments were used in real-life electroanalytical tests, including the determination of germanium and the studies of the antioxidant properties of herbal infusions. The experimental design and interpretation of the results were performed using Statistica software.

Deep eutectic solvents (DESs) have unique physical and chemical properties, such as low vapor pressure, ease of synthesis, stability and non-toxicity. Although they have found application in areas of research such as organic synthesis, electrochemistry, biocatalysis, and the development of biosensors, their use as sensitive coatings for chemical sensors has not been previously considered. The present paper examines the fundamental principles of generating sensitive coatings for piezoelectric quartz sensors utilizing hydrophilic deep eutectic solvents (choline:polyalcohols). Thin films from DESs with a melting point above 50 C, including those in the composite coatings with amorphous silicon oxide and chitosan, have been studied. The sorption characteristics of the coatings were thoroughly examined, along with the structures and morphologies of their surfaces. It has been demonstrated that the limits of detection and determination of volatile organic compounds in aqueous solutions by films based on DESs exhibit lower limits compared to other polymer coatings. A novel approach is proposed for processing the kinetic curve of the sorption of volatile substances by films based on DES in order to improve the reliability and detection of volatile compounds in the gas phase above aqueous solutions. The use of DES-based piezoelectric quarts sensors has been demonstrated for assessing microbiological indicators of milk.

The spread of invasive pests is accelerated by globalization and changes in climate conditions and poses a significant threat to agricultural and forest ecosystems. Advances in electronic nose sensors (e-noses) have opened new avenues for monitoring and detecting plant diseases and pests through the analysis of emitted volatile organic compounds (VOCs). The current work reviews the most recent developments in e-nose sensors and their application in plant disease and pest detection over the past five years. It also explores the challenges associated with VOC detection in agricultural settings where field sampling has a focal role in monitoring and management.