Low-cost Electrochemical Sensors Research Articles

(Invited) Past, Present and Future for Electrochemical Gas Sensors in Energy Applications

The markets for modern low-cost electrochemical gas sensors have been growing for longer than sensors have been around. Energy markets for gas sensors include the oil, gas and electricity industries as well as the new developing and fast growing green energy sectors. The primary gases to detect include combustible hydrocarbons, oxygen, and toxic gases. Specifically, we are interested in methane (blue and green), ammonia, and H2 for the renewable energy sectors but include hydrocarbon fuels and CO2 as a greenhouse gas emission. The reasons for monitoring crosscut all areas of the energy business and include applications in production, transport, storage and use. Primary sensor uses include: 1) health and safety (people and assets), 2) leak detection and isolation to help mitigate product losses, 3) monitoring to protect the environment and 4) measurements for process control and efficacy. Each of these areas of sensing have special requirements and demand cost-effective and time efficient sensor performance in many and varied real world scenarios.Electrochemical gas sensors have a long and rich history starting with the commercialization of the first practical potentiometric CO2 gas sensor by Severinghaus in 1954 and the O2 amperometric sensor by Clark in 1956 that launched the modern blood gas analysis industry. Additional milestones include the introduction of the “diffusion electrode” in 1968 creating the modern amperometric sensor which has produced a large array of sensors for toxic and hazardous gases including H2, ammonia, and other energy gases. Progress has been made in the field of electrochemical gas sensors, not only in improving performance, sensitivity, selectivity, response time, and stability, but also in logistical properties including miniaturization, lower power consumption, low cost as well as communication and computation to make automated-operational systems. New high-volume production has contributed to lower costs commensurate with a chip-based sensor mentality. The evolution of one of the gas sensor technologies is given below (the room temperature AGS or amperometric gas sensor) and similar progress has been seen in mixed potential and solid state gas sensors.The growing understanding of the sensor’s fundamental electrocatalytic reactions has led to tailored designs of electrode-electrolyte combinations and packages for the various applications. Often ignored in sensor publications of performance, the fundamental electrocatalytic studies are poised to make significant advances in energy gas sensor selectivity and sensitivity. The additional implementation of intelligent algorithms (AI/ML) to make “smart” sensors and sensor arrays complements advanced nano-materials and designs for improving sensor performance. One major improvement is our understanding of the sensor response mechanisms at the electrocatalytic level. This new research will enable new electrochemical sensor advances that are poised to impact the health and wellbeing of both people and the planet. Ideas and concepts that significantly contribute to the safe and efficient rollout of the newest green energy platforms are presented. Figure 1

Convenient and Accurate Detection of Dopamine and Glucose by Modifying Carbon Fiber Electrodes

Fast, sensitive, and low-cost high-performance detectors have gradually become an indispensable tool for people to keep healthy, and sensors are the key devices of detection equipment. In this work, a ZnO nanocrystal modified carbon fiber electrode was prepared using a hydrothermal method, and on this basis, a galvanostatic point deposition method was used to load copper nanoparticles to prepare a sensor electrode. Scanning electron microscopy and X-ray diffraction were used to comprehensively analyze composition, morphology, and environmental adaptability of the prepared electrodes. The DPV test was used to verify the enhanced effect of ZnO nanorods on neurotransmitter detection. The ZnO/CF showed an obvious electrical signal (0.22 V, 4 × 10−5A) in the detection of dopamine (DA) solution, and the Cu-NPs/ZnO/CF also showed excellent detection results in the glucose detection experiment., providing two excellent examples for the development of low-cost electrochemical sensors. The electrodes can specifically detect DA in the presence of ascorbic acid and uric acid, and the detection limit of the electrode for detecting DA is about 0.4 μM. In addition, the Cu-NPs/ZnO/CF electrode successfully realized the enzyme-free detection of glucose, and the detection limit could reach 0.5 μM.

Nonlinear Regression Approach as a Correction Factor of Measurements of Low-Cost Electrochemical Air Quality Sensors

Air quality directly affects the health of humans. The health implications of poor air quality are recognized by professionals and the public alike and these concerns have driven both the proliferation of formal sensor networks, but also low-cost sensors which can be used in the home. The advancement of technology in recent years has also led to the rapid development of low-cost sensors. Given that citizens are concerned about the air quality of the environment in which they live, they are turning to the supply of low-cost sensors, as they are affordable. The question of the reliability of measurements from low-cost sensors remains an area of research. In this research work, the optimization of ozone (O3) and nitrogen dioxide (NO2) measurements of low-cost electrochemical air quality sensors is investigated by applying nonlinear regression, using a second-order polynomial equation as a correction factor. The proposed correction method is implementable in IoT devices, as it does not require high computational resources. The results show that the measurements are susceptible to correction, with the effect that the corrected values are close to the actual values obtained by the reference instruments of the Department of Environmental Pollution Control Project of Athens (PERPA), a service of the Greek Ministry of the Environment and Energy.

Electrochemistry of chloramphenicol on laser-induced graphene electrodes and its voltammetric determination in honey.

Laser scribing is a promising technology for the rapid and large-scale production of low-cost electrochemical sensors from diverse substrates. Polyimide has been the most popular so far because of its low cost, flexibility and capability of generating high-quality porous graphene films, known as laser-induced graphene (LIG). Herein we report the electrochemistry of chloramphenicol (CAP) on LIG electrodes and its determination in honey samples. LIG electrodes were fabricated by the photothermal conversion of sp3 carbon within the polymeric matrix into sp2 carbon using a CO2 laser cutter. The LIG electrode associated with differential pulse voltammetry (DPV) showed good linearity (R2 > 0.99) in the range from 10 to 160 μmol L-1 with a limit of detection of 1.0 μmol L-1 and good precision (RSD < 5%) for the electrochemical reduction of CAP species. Detection was possible free from the interference of other antibiotics, such as amoxicillin, tetracycline, sulfanilamide, and sulfamethoxazole. Spiked honey samples were analyzed by the standard-addition method and recovery values between 86 and 109% were obtained, which confirmed the absence of sample matrix effects. Therefore, the proposed sensor is an alternative, feasible, low-cost, and powerful analytical tool for the determination of CAP in honey.

Identification of the Safe Variation Limits for the Optimization of the Measurements in Low-Cost Electrochemical Air Quality Sensors

Nowadays, the study of air quality has become an increasingly prominent field of research, particularly in large urban centers, given its significant impact on human health. In many countries, government departments and research centers use official high-cost scientific instruments to monitor air quality in their regions. Meanwhile, concerned citizens interested in studying the air quality of their local areas often employ low-cost air quality sensors for monitoring purposes. The optimization and evaluation of low-cost sensors have been a field of research by many research groups. This paper presents an extensive study to identify the safe percentage change limits that low-cost electrochemical air quality sensors can have, in order to optimize their measurements. For this work, three low-cost air quality monitoring stations were used, which include an electrochemical sensor for nitrogen dioxide (NO2) (Alphasense NO2-B43F) and an electrochemical sensor for ozone (O3) (Alphasense OX-B431). The aim of this work is to explore the variance of the aforementioned sensors and how this variability can be used to optimize the measurements of low-cost electrochemical sensors, closer to real ones. The analysis is conducted by employing diagrams, boxplot and violin curves of the groups of sensors used, with satisfactory results.

Laboratory Performance Evaluation of a Low-Cost Electrochemical Formaldehyde Sensor.

Formaldehyde is a known human carcinogen and an important indoor and outdoor air pollutant. However, current strategies for formaldehyde measurement, such as chromatographic and optical techniques, are expensive and labor intensive. Low-cost gas sensors have been emerging to provide effective measurement of air pollutants. In this study, we evaluated eight low-cost electrochemical formaldehyde sensors (SFA30, Sensirion®, Staefa, Switzerland) in the laboratory with a broadband cavity-enhanced absorption spectroscopy as the reference instrument. As a group, the sensors exhibited good linearity of response (R2 > 0.95), low limit of detection (11.3 ± 2.07 ppb), good accuracy (3.96 ± 0.33 ppb and 6.2 ± 0.3% N), acceptable repeatability (3.46% averaged coefficient of variation), reasonably fast response (131-439 s) and moderate inter-sensor variability (0.551 intraclass correlation coefficient) over the formaldehyde concentration range of 0-76 ppb. We also systematically investigated the effects of temperature and relative humidity on sensor response, and the results showed that formaldehyde concentration was the most important contributor to sensor response, followed by temperature, and relative humidity. The results suggest the feasibility of using this low-cost electrochemical sensor to measure formaldehyde concentrations at relevant concentration ranges in indoor and outdoor environments.

Differential Pulse Voltammetric Tuning of the Screen-Printed Carbon Electrode Surface to Enhance the Electrochemical Performance and Multiplex Detection

Screen-Printed Carbon Electrode (SPCE) has shown tremendous scope for the miniaturization and commercialization of low-cost electrochemical sensors and biosensors devices. But the commercial SPCEs have a narrow potential window (PW) that limits their versatile applications for various analytes owing to the presence of some inherent redox peaks in the wider PW. In this work, we have explored the most effective and innovative electrochemical tuning of the SPCE surface using DPV (differential pulse voltammetry) for the removal of those inherent redox peaks that are responsible for narrowing the working PW. After that, the electrochemical performance of the tuned SPCE electrode was evaluated in terms of a range of electrochemical parameters including reproducibility by CV (Cyclic Voltammetry), DPV and EIS (electrochemical impedance spectroscopy) measurements using [Fe(CN)6]3−/4− as a model redox analyte. In addition, the suitability of the tuned electrode for multiple analytes detection was tested in presence of a mixture of Cd2+ (cadmium), Cu2+ (copper) and Hg2+ (Mercury) ions using CV and DPV techniques. The electrochemical analysis data show that after electrochemical DPV treatment, the SPCE surface exhibits excellent reproducibility and electrochemical performance within a wider PW minimizing their inherent drawback creating a wide scope of their precise applications in various fields.

Field evaluation of low-cost electrochemical air quality gas sensors under extreme temperature and relative humidity conditions

Abstract. Modern electrochemical gas sensors hold great potential for improving practices in air quality (AQ) monitoring as their low cost, ease of operation and compact design can enable dense observational networks and mobile measurements. Despite that, however, numerous studies have shown that the performance of these sensors depends on a number of factors (e.g. environmental conditions, sensor quality, maintenance and calibration), thereby adding significant uncertainties in the reported measurements and large discrepancies from those recorded by reference-grade instruments. In this work we investigate the performance of electrochemical sensors, provided by two manufacturers (namely Alphasense and Winsen), for measuring the concentrations of CO, NO2, O3 and SO2. To achieve that we carried out collocated yearlong measurements with reference-grade instruments at a traffic AQ monitoring station in Nicosia, Cyprus, where temperatures ranged from ca. 0 ∘C in the winter to almost 45 ∘C in the summer. The CO sensors exhibit the best performance among all the ones we tested, having minimal mean relative error (MRE) compared to reference instruments (ca. −5 %), although a significant difference in their response was observed before and after the summer period. At the other end of the spectrum, the SO2 sensors reported concentration values that were at least 1 order of magnitude higher than the respective reference measurements (with MREs being more than 1000 % for Alphasense and almost 400 % for Winsen throughout the entire measurement period), which can be justified by the fact that the concentrations of SO2 at our measuring site were below their limit of detection. In general, variabilities in the environmental conditions (i.e. temperature and relative humidity) appear to significantly affect the performance of the sensors. When compared with reference instruments, the CO and NO2 electrochemical sensors provide measurements that exhibit increasing errors and decreasing correlations as temperature increases (from below 10 to above 30 ∘C) and RH decreases (from &gt;75 % to below 30 %). Interestingly, the performance of the sensors was affected irreversibly during the hot summer period, exhibiting different responses before and after that, resulting in a signal deterioration that was more than twice that reported by the manufacturers. With the exception of the Alphasense NO2 sensor, all low-cost sensors (LCSs) exhibited measurement uncertainties that were much higher, even at the beginning of our measurement period, compared to those required for qualifying the sensors for indicative air quality measurements according to the respective European Commission (EC) Directive. Overall, our results show that the response of all LCSs is strongly affected by the environmental conditions, warranting further investigations on how they are manufactured, calibrated and employed in the field.

Prototype design of urban monitoring on several gaseous pollutants using low-cost electrochemical sensors

Monitoring atmospheric gaseous pollutants in urban areas is increasingly essential to anticipate poor urban air quality and the increasing number of acute respiratory infections caused by air pollution. Usually, the monitoring of air pollution is conducted by the government and scientists by using accurate and expensive instruments. However, this instrument’s high-cost installation and maintenance limited the number of monitoring stations that can be placed. It makes it hard to identify the pollutant’s local source, especially on an urban scale. Hence, this study designed alternative techniques to monitor several gaseous pollutants using the Low-cost Air Quality System (LAQS). LAQS has enormous potential to be a reference station’s supporting tool and increase the resolution of air pollution monitoring systems. A custom-made low-cost electrochemical sensor was used in this study to detect several gaseous pollutants, including ozone (O3), nitrogen dioxide (NO2), and carbon monoxide (CO), together with temperature and relative humidity measurements. The system was also equipped with a data logger and 4G module for data storage and transmission, respectively. In the first stage, two scenarios were employed, including its examination inside and outside the room before co-location.

Air Pollution Monitoring via Wireless Sensor Networks: The Investigation and Correction of the Aging Behavior of Electrochemical Gaseous Pollutant Sensors

The continuously growing human activity in large and densely populated cities pollutes air and consequently puts public health in danger. This is why air quality monitoring is necessary in all urban environments. However, the creation of dense air monitoring networks is extremely costly because it requires the usage of a great number of air monitoring stations that are quite expensive. Instead, the usage of wireless sensor networks (WSNs) that incorporate low-cost electrochemical gas sensors provides an excellent alternative. Actually, sensors of this kind that are recommended for low-cost air quality monitoring applications may provide relatively precise measurements. However, the reliability of such sensors during their operational life is questionable. The research work presented in this article not only experimentally examined the correlation that exists between the validity of the measurements obtained from low-cost gas sensors and their aging, but also proposes novel corrective formulae for gas sensors of two different types (i.e., NO2, O3), which are aimed at alleviating the impact of aging on the accuracy of measurements. The following steps were conducted in order to both study and lessen the aging of electrochemical sensors: (i) a sensor network was developed to measure air quality at a place near official instruments that perform corresponding measurements; (ii) the collected data were compared to the corresponding recordings of the official instruments; (iii) calibration and compensation were performed using the electrochemical sensor vendor instructions; (iv) the divergence between the datasets was studied for various periods of time and the impact of aging was studied; (v) the compensation process was re-evaluated and new compensation coefficients were produced for all periods; (vi) the new compensation coefficients were used to shape formulae that automatically calculate the new coefficients with respect to the sensors’ aging; and (vii) the performance of the overall procedure was evaluated through the comparison of the final outcomes with real data.

Preparation and Use of Low-Cost Electrochemical Sensors

Since the decade of the 90s when the first works of professor Joseph Wang were published, screen-printed electrodes have been proposed for many objectives. In the first works in Brazil a few years later, some authors proposed electrochemical sensors using graphite from batteries, which provide great results for the determination of several analytes.1 In another interesting work, Professor Lucio Angnes (USP) and collaborators prepared gold electrodes from CD, which presented a thin film of the noble metal.2 They obtained an extremely low-cost electrochemical sensor. After that, this research line gained notoriety in Brazil, and the number of groups and papers has increased considerably. In this regard, we can highlight some researchers that have been working on low-cost and disposable electrochemical sensors, including Lauro Kubota (UNICAMP), Luiz Humberto Marcolino Junior (UFPR), Márcio Fernando Bergamini (UFPR), Ronaldo Censi Faria (UFSCar), Éder Cavalheiro (USP), Osvaldo Novais de Oliveira Junior (USP), Sergio Spínola (USP), Murilo Santiago (CNPEM), Renato Souza Lima (CNPEM), Orlando Fatibello Filho (UFSCar), Wendell Coltro (USP), William Reis (UNICAMP), and Thiago Paixão (USP). The laboratory of Sensors, Nanomedicine, and Nanostructured Materials (LSNano) located at UFSCar Araras has also developed low-cost sensors using recyclable materials and conductive inks with simple preparation. Low-cost electrodes can be prepared with infinite possibilities by using different substrates, such as circuit boards,3 acetate sheets,4 and polyethylene terephthalate (PET) from bottles5 as well as paper6. In the last example, various types of papers have been applied in this area, including cardboard,7 waterproof,6 and adhesives8. Therefore, materials that we use daily can become important devices with great applications. There are many options to produce low-cost electrochemical sensors, including pencil drawing, stencil, laser scribing, and screen-printing techniques.9 The strategy depends on the subject and the materials that have been deposited to be used as conductors. In this context, the screen-printing technique is come from the t-shirt preparation and is an excellent alternative to produce hundreds of sensors in a few minutes. They can be applied by using different electrochemical techniques, such as potentiometry, voltammetry, and electrochemical impedance spectroscopy. For this purpose, low-cost inks, which are made of a polymer and a conductive material, are necessary. Conductive inks can be prepared with water based-polymers,10 nail polish,11 and traditional polymers based on organic solvents12. Otherwise, graphite carbon nanotubes and/or graphene and carbon dots have been used as conductive carbon-based materials.13 Also, metallic nanoparticles such as copper, platinum, and gold can be great alternatives for this intention but can be quite costly.

Electrochemical sensors on board a Zeppelin NT: in-flight evaluation of low-cost trace gas measurements

Abstract. In this work, we used a Zeppelin NT equipped with six sensor setups, each composed of four different low-cost electrochemical sensors (ECSs) to measure nitrogen oxides (NO and NO2), carbon monoxide, and Ox (NO2+O3) in Germany. Additionally, a MIRO MGA laser absorption spectrometer was installed as a reference device for in-flight evaluation of the ECSs. We report not only the influence of temperature on the NO and NO2 sensor outputs but also find a shorter timescale (1 s) dependence of the sensors on the relative humidity gradient. To account for these dependencies, we developed a correction method that is independent of the reference instrument. After applying this correction to all individual sensors, we compare the sensor setups with each other and to the reference device. For the intercomparison of all six setups, we find good agreements with R2≥0.8 but different precisions for each sensor in the range from 1.45 to 6.32 ppb (parts per billion). The comparison to the reference device results in an R2 of 0.88 and a slope of 0.92 for NOx (NO+NO2). Furthermore, the average noise (1σ) of the NO and NO2 sensors reduces significantly from 6.25 and 7.1 to 1.95 and 3.32 ppb, respectively. Finally, we highlight the potential use of ECSs in airborne applications by identifying different pollution sources related to industrial and traffic emissions during multiple commercial and targeted Zeppelin flights in spring 2020. These results are a first milestone towards the quality-assured use of low-cost sensors in airborne settings without a reference device, e.g., on unmanned aerial vehicles (UAVs).

Evaluation of low-cost gas sensors to quantify intra-urban variability of atmospheric pollutants

Low-cost electrochemical air quality sensors can provide deep insights into the intra-urban variability of different air pollutants after proper calibration using field co-location with regulatory Air Quality Monitoring stations.

Low-Cost Nitric Oxide Sensors: Assessment of Temperature and Humidity Effects.

High equipment cost is a significant entry barrier to research for small organizations in developing solutions to air pollution problems. Low-cost electrochemical sensors show sensitivity at parts-per-billion by volume mixing ratios but are subject to variation due to changing environmental conditions, in particular temperature. In this study, we demonstrate a low-cost Internet of Things (IoT)-based sensor system for nitric oxide analysis. The sensor system used a four-electrode electrochemical sensor exposed to a series of isothermal/isohume conditions. When deployed under these conditions, stable baseline responses were achieved, in contrast to ambient air conditions where temperature and humidity conditions may be variable. The interrelationship between working and auxiliary electrodes was linear within an environmental envelope of 20-40 °C and 30-80% relative humidity, with correlation coefficients from 0.9980 to 0.9999 when measured under isothermal/isohume conditions. These data enabled the determination of surface functions that describe the working to auxiliary electrode offsets and calibration curve gradients and intercepts. The linear and reproducible nature of individual calibration curves for stepwise nitric oxide (NO) additions under isothermal/isohume environments suggests the suitability of these sensors for applications aside from their role in air quality monitoring. Such applications would include nitric oxide kinetic studies for atmospheric applications or measurement of the potential biocatalytic activity of nitric oxide consuming enzymes in biocatalytic coatings, both of which currently employ high-capital-cost chemiluminescence detectors.

Calibration of SO2 and NO2 Electrochemical Sensors via a Training and Testing Method in an Industrial Coastal Environment.

Low-cost sensors can provide inaccurate data as temperature and humidity affect sensor accuracy. Therefore, calibration and data correction are essential to obtain reliable measurements. This article presents a training and testing method used to calibrate a sensor module assembled from SO2 and NO2 electrochemical sensors (Alphasense B4 and B43F) alongside air temperature (T) and humidity (RH) sensors. Field training and testing were conducted in the industrialized coastal area of Quintero Bay, Chile. The raw responses of the electrochemical (mV) and T-RH sensors were subjected to multiple linear regression (MLR) using three data segments, based on either voltage (SO2 sensor) or temperature (NO2). The resulting MLR equations were used to estimate the reference concentration. In the field test, calibration improved the performance of the sensors after adding T and RH in a linear model. The most robust models for NO2 were associated with data collected at T < 10 °C (R2 = 0.85), while SO2 robust models (R2 = 0.97) were associated with data segments containing higher voltages. Overall, this training and testing method reduced the bias due to T and HR in the evaluated sensors and could be replicated in similar environments to correct raw data from low-cost electrochemical sensors. A calibration method based on training and sensor testing after relocation is presented. The results show that the SO2 sensor performed better when modeled for different segments of voltage data, and the NO2 sensor model performed better when calibrated for different temperature data segments.

Long-Period Fiber Gratings Coated with Poly(ethylene glycol) as Relative Humidity Sensors

Relative humidity is an important parameter in controlled environments and is typically monitored using low-cost electrochemical sensors with low resolution and accuracy. This kind of sensors cannot not be implemented in harsh or explosive environments (as in pyrotechnic facilities) due to electrical discharges, or in marine structures where the oxidation of the sensing probe materials changes the sensing response). In these cases, fiber optic sensors can provide solutions due to their intrinsic properties, such as immunity to electromagnetic interference and resistance in harsh environments. This work presents preliminary results regarding the steps of the fabrication of Long-Period Fiber Gratings, the coating processes with a thin layer of poly(ethylene glycol) (PEG) and its sensing performance to relative humidity, displaying a from 60 to 100%sensitivity of 0.6 nm/%RH in the range of 80 to 100%RH.

Different approaches for fabrication of low-cost electrochemical sensors

Electrochemistry combined with economical and sustainable platforms (such as paper) provides portable, affordable, robust, and user-friendly devices. In general, techniques, such as photolithography and sputtering, are excellent alternatives for producing these platforms. However, owing to the requirement of expensive and sophisticated instrumentation, as well as cleanroom facilities, these techniques have limited access. Thus, the search for easy to use and produce approaches have been reported, using consumables, including adhesives, carbon ink, graphite, pencil, office paper, paperboard, among others. In this sense, in this mini-review, we discuss various strategies explored to fabricate low-cost electrochemical sensors, including its main applications. Different manufacturing methods, such as screen and stencil printing, laser-scribing, and pencil drawing, will be discussed here, emphasizing the performance of the obtained devices, in addition to their advantages and disadvantages. • Low-cost materials can be used in the production of versatile disposable sensors. • Technical discussions focus on different parameters of electrode production that must take into account. • Three simple and practical techniques are discussed in detail, encouraging point-of-care approaches.

Field calibration of a low-cost sensors network to assess traffic-related air pollution along the Brenner highway

This paper presents the results of a field campaign aiming at testing the ability of a network of low-cost electrochemical sensors to measure nitrogen dioxide concentration levels alongside one of the major Italian highway arteries. The results of a double on-field calibration, allowing for investigating the performance of the sensors under a broad range of weather conditions, are first shown and discussed. Different regression models are tested and their performance is widely assessed. Then, the measurements of the calibrated sensors are analyzed during a year-long field campaign, testing their performance against reference air quality stations and paying particular attention to different statistical indices. Results show a satisfactory performance of the low-cost sensors, highlighting their suitability to complement measurements from standard air quality stations, to reach a wider spatial coverage and to monitor pollutant concentrations in critical situations, when standard measurements are usually not feasible. Moreover, the dataset available from the year-long field campaign allows to extensively investigate nitrogen dioxide concentrations alongside the highway, pointing out in particular the strict relationship between pollutant concentration patterns and meteorological phenomena typical of Alpine valleys, such as daily-periodic thermally-driven wind systems.

A Novel Bike-Mounted Sensing Device with Cloud Connectivity for Dynamic Air-Quality Monitoring by Urban Cyclists.

We present a device based on low-cost electrochemical and optical sensors, designed to be attached to bicycle handlebars, with the aim of monitoring the air quality in urban environments. The system has three electrochemical sensors for measuring NO2 and O3 and an optical particle-matter (PM) sensor for PM2.5 and PM10 concentrations. The electronic instrumentation was home-developed for this application. To ensure a constant air flow, the input fan of the particle sensor is used as an air supply pump to the rest of the sensors. Eight identical devices were built; two were collocated in parallel with a reference urban-air-quality-monitoring station and calibrated using a neural network (R2 > 0.83). Several bicycle routes were carried out throughout the city of Badajoz (Spain) to allow the device to be tested in real field conditions. An air-quality index was calculated to facilitate the user’s understanding. The results show that this index provides data on the spatiotemporal variability of pollutants between the central and peripheral areas, including changes between weekdays and weekends and between different times of the day, thus providing valuable information for citizens through a dedicated cloud-based data platform.

A Low Cost Fe3O4-Activated Biochar Electrode Sensor by Resource Utilization of Excess Sludge for Detecting Tetrabromobisphenol A.

Owing to its ubiquity in natural water systems and the high toxicity of its accumulation in the human body, it is essential to develop simple and low-cost electrochemical sensors for the determination of 3,3′,5,5′-tetrabromobisphenol A (TBBPA). In this work, Fe3O4–activated biochar, which is based on excess sludge, was prepared and characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and BET analysis to analyze its basic features. Subsequently, it was used to fabricate an electrochemical sensor for the detection of TBBPA. The electrochemical test results revealed that the Fe3O4–activated biochar film exhibited a larger active surface area, a lower charge transfer resistance and a higher accumulation efficiency toward TBBPA. Consequently, the peak current of TBBPA was significantly enhanced on the surface of the Fe3O4–activated biochar. The TBBPA sensing platform developed using the Fe3O4–activated biochar composite film, with relatively a lower detection limit (3.2 nM) and a wider linear range (5–1000 nM), was successfully utilized to determine TBBPA levels in water samples. In summary, the effective application of Fe3O4–activated biochar provided eco-friendly and sustainable materials for the development of a desirable high-sensitivity sensor for TBBPA detection.