Electrochemical sensors play a significant role in detecting chemical ions, molecules, and pathogens in water and other applications. These sensors are sensitive, portable, fast, inexpensive, and suitable for online and in-situ measurements compared to other methods. They can provide the detection for any compound that can undergo certain transformations within a potential window. It enables applications in multiple ion detection, mainly since these sensors are primarily non-specific. In this paper, we provide a survey of electrochemical sensors for the detection of water contaminants, i.e., pesticides, nitrate, nitrite, phosphorus, water hardeners, disinfectant, and other emergent contaminants (phenol, estrogen, gallic acid etc.). We focus on the influence of surface modification of the working electrodes by carbon nanomaterials, metallic nanostructures, imprinted polymers and evaluate the corresponding sensing performance. Especially for pesticides, which are challenging and need special care, we highlight biosensors, such as enzymatic sensors, immunobiosensor, aptasensors, and biomimetic sensors. We discuss the sensors’ overall performance, especially concerning real-sample performance and the capability for actual field application.

In this paper, the relative humidity sensor properties of graphene oxide (GO) and graphene oxide/multiwalled nanotubes (GO/MWNTs) composites have been investigated. Composite sensors were fabricated by direct laser scribing and characterized using UV-vis-NIR, Raman, Fourier transform infrared, and X-ray photoemission spectroscopies, electron scanning microscopy coupled with energy-dispersive X-ray analysis, and impedance spectroscopy (IS). These methods confirm the composite homogeneity and laser reduction of GO/MWNT with dominant GO characteristics, while ISresults analysis reveals the circuit model for rGO-GO-rGO structure and the effect of MWNT on the sensor properties. Although direct laser scribing of GO-based humidity sensor shows an outstanding response (|ΔZ|/|Z| up to 638,800%), a lack of stability and repeatability has been observed. GO/MWNT-based humidity sensors are more conductive than GO sensors and relatively less sensitive (|ΔZ|/|Z| = 163,000%). However, they are more stable in harsh humid conditions, repeatable, and reproducible even after several years of shelf-life. In addition, they have fast response/recovery times of 10.7 s and 9.3 s and an ultra-fast response time of 61 ms when abrupt humidification/dehumidification is applied by respiration. All carbon-based sensors’ overall properties confirm the advantage of introducing the GO/MWNT hybrid and laser direct writing to produce stable structures and sensors.

The transition of electrochemical sensors from lab-based measurements to real-time analysis requires special attention to different aspects in addition to the classical development of new sensing materials. Several critical challenges need to be addressed including a reproducible fabrication procedure, stability, lifetime, and development of cost-effective sensor electronics. In this paper, we address these aspects exemplarily for a nitrite sensor. An electrochemical sensor has been developed using one-step electrodeposited (Ed) gold nanoparticles (EdAu) for the detection of nitrite in water, which shows a low limit of detection of 0.38 µM and excellent analytical capabilities in groundwater. Experimental investigations with 10 realized sensors show a very high reproducibility enabling mass production. A comprehensive investigation of the sensor drift by calendar and cyclic aging was carried out for 160 cycles to assess the stability of the electrodes. Electrochemical impedance spectroscopy (EIS) shows significant changes with increasing aging inferring the deterioration of the electrode surface. To enable on-site measurements outside the laboratory, a compact and cost-effective wireless potentiostat combining cyclic and square wave voltammetry, and EIS capabilities has been designed and validated. The implemented methodology in this study builds a basis for the development of further on-site distributed electrochemical sensor networks.

Biocharcoal (BioC), a cost-effective, eco-friendly, and sustainable material can be derived from various organic sources including agricultural waste. However, to date, complex chemical treatments using harsh solvents or physical processes at elevated temperatures have been used to activate and enhance the functional groups of biochar. In this paper, we propose a novel easy and cost-effective activation method based on electrochemical cycling in buffer solutions to enhance the electrochemical performance of biocharcoal derived from almond shells (AS-BioC). The novel electrochemical activation method enhanced the functional groups and porosity on the surface of AS-BioC, as confirmed by microscopic, spectroscopic characterizations. Electrochemical characterization indicated an increase in the conductivity and surface area. A modified SPCE with activated AS-BioC (A.AS-BioC/SPCE), shows enhanced electrochemical performance towards oxidation and reduction of nitrite and paraxon ethyl pesticide, respectively. For both target analytes, the activated electrode demonstrates high electrocatalytic activity and achieves a very LOD of 0.38 µM for nitrite and 1.35 nM for ethyl paraxon with a broad linear range. The sensor was validated in real samples for both contaminants. Overall, the research demonstrates an innovative technique to improve the performance of AS-BioC to use as a modifier material for electrochemical sensors.

An iron oxide/reduced graphene oxide (ION-RGO) nanocomposite has been fabricated to functionalize a low-cost electrochemical nitrite sensor realized by light-scribed reduced graphene oxide (LRGO) electrodes on a PET substrate. To enhance the stability and adhesion of the electrode, the PET substrate was modified by RF oxygen plasma, and a thin layer of the cationic poly (diallyl dimethyl ammonium chloride) was deposited. Raman spectroscopy and scanning electron microscopy coupled to energy-dispersive X-ray spectroscopy (SEM-EDX) reveal that the light-scribing process successfully reduces graphene oxide while forming a porous multilayered structure. As confirmed by cyclic voltammetry, the LRGO electrochemical response to ferri-ferrocyanide and nitrite is significantly improved after functionalization with the ION-RGO nanocomposite film. Under optimized differential pulse voltammetry conditions, the LRGO/ION-RGO electrode responds linearly (R2 = 0.97) to nitrite in the range of 10–400 µM, achieving a limit of detection of 7.2 μM and sensitivity of 0.14 µA/µM. A single LRGO/ION-RGO electrode stands for 11 consecutive runs. The novel fabrication process leads to highly stable and reproducible electrodes for electrochemical sensors and thus offers a low-cost option for the rapid and sensitive detection of nitrite.