**1. Introduction**

Water is an essential part of all the living beings on earth, but in recent times, anthropogenic activities have increased immensely, which are the major causes of water pollution, disturbing the marine biodiversity and leading to a tremendous water shortage [1–3]. Even though the chemicals and water nutrients are crucial to our day-to-day lives, the excessive

**Citation:** Hui, Y.; Huang, Z.; Alahi, M.E.E.; Nag, A.; Feng, S.; Mukhopadhyay, S.C. Recent Advancements in Electrochemical Biosensors for Monitoring the Water Quality. *Biosensors* **2022**, *12*, 551. https://doi.org/10.3390/ bios12070551

Received: 26 June 2022 Accepted: 13 July 2022 Published: 21 July 2022

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amount threatens humans, aquatic life, and animals. The pollution of water and habitat degradation are the causes of the escalating water shortage and the reasons for the deterioration in marine biodiversity. Although freshwater accessibility has deteriorated over the past decades, water demand has risen, particularly in warm areas with minimal rainfall. Recently, 71% of the world's inhabitants, equal to 4.3 billion, were dealing with water shortages for several months [4]. Although water demand sharply increased, massive water pollution increased water scarcity and declining water quality in the past decades.

The characteristics of water pollution are comprised of their physical presence, chemical parameters, and richness of microorganisms. The concentration and composition of ingredients in water differ extensively. They can be categorized into four distinct classifications, such as (i) inorganic chemicals, (ii) nutrients, (iii) microorganisms' pollution, and (iv) organic pollutants. They can bring about harmful ecological consequences, for example, the interference of internal secretion and hormone systems, stimulation of genotoxicity and cytotoxicity, and hazardous effects [5]. The strength of ingredients in water is essential for selecting, designing, and operational treatment processes and recycling waste. The variable quantity of contaminants in effluent over time also increases the attention to emerging technologies for monitoring the water and applying reasonably priced and real-time approaches [6]. This review is mainly focused on monitoring heavy metals, nutrients, organic pollutants, biochemical oxygen demand, and microorganisms. Heavy metals in soil and water are considered environmental contaminants with elevated toxicity, easy accretion, and complicated degradation [7]. Nutrients bring about water eutrophication. Organic pollutants, particularly persistent organic pollutants (POPs), have harshly harmful impacts on human health and the environment with their complex degradation and potential bioaccumulation [8]. The biochemical oxygen demand (BOD) is the essential supervisory index to measure organic water contamination and demonstrate water quality [9,10]. Water quality monitoring is critical and closely related to our life and production.

Conventional analytical techniques or laboratory-based procedures, such as gas chromatography (GC), high-performance liquid chromatography (HPLC), atomic absorption spectroscopy (AAS), atomic fluorescence spectrometry (AFS), and inductively coupled plasma mass spectrometry (ICP-MS), are sensitive, precise, and consistent. They are regularly used to measure water parameters with the help of trained operators. However, they are involved with bulky and costly instrumentation, take much time for sample preparation, and are unsuitable for in situ measurements, especially requiring trained operators' help and transporting the water samples to laboratories for assessment [11–13]. Additionally, they cannot asses the accumulative toxicity or nutrient value of multiple chemicals or pollutants in a sample, which is a crucial objective of water quality monitoring applications [14]. Many property indicators are regularly used to determine the different qualities of water for settling or recycling. Many of them are laboratory-based techniques, which require complex pretreatment, and consequently, the methods are sluggish and expensive [1,15]. These characteristics encourage developing new technologies that are more low-cost, portable, sensitive, and efficient in the on-site real-time detection of multicontaminants containing a wide variety of materials [16,17]. The significant challenges of developing a portable biosensing device are inadequate sensitivity and poor selectivity during the on-site detection. The significant level of noises can come on chemical components level from the sampling field and ambient environments can be variable due to the harsh environments or diurnal variations. These are the major obstacles where the researchers are putting lot of attentions on how to avoid these for generating a reliable and portable biosensing output signal. The portable biosensing method is successfully utilized for other applications, such as pesticide residues in fruits and vegetables [18], POC Detection for biomedical application [19], chemical and biological pollutants in water [20].

In recent years, the advancement of electrochemical biosensors for detecting environmental pollutants has received considerable attention [21–25]. Biosensors have many advantages over the conventional lab-based method, including low costs, portability, fast response time, less usage of reagents, and the capability to continuous monitor the complex

wastewater [26–28]. Such sensors significantly benefit from sensing the minimum level in polluted water, such as wastewater. Biosensors are also compact and miniaturized devices that facilitate the advancement of portable sensing systems to monitor on-site effluents [29]. Bearing in mind the wide range of bio-recognition elements (including enzymatic, immunochemical, non-enzymatic receptor, whole-cell and DNA elements, and molecularly imprinted polymer (MIP)), the various types of biosensors can be classified as (i) electrochemical [30], (iii) piezoelectric [31], (ii) optical [32], and (iv) thermal biosensors [33] based on their working principles and transducing mechanisms [34], but the current review paper will cover the topics which are related to electrochemical biosensing. An electrochemical biosensor is based on the interactions between the immobilized bio-recognition element on its surface with binding molecules (the analyte of interest) and generating the changes in electrochemical properties, further translating into a meaningful electrical signal. The electrochemical methods offer rapid detection, fabrication, excellent sensitivity, and low cost.

Moreover, by operating at a wide range of potential, it is possible to simultaneously determine multiple analytes with different electrochemical potentials. Electrochemical biosensors' efficiency in monitoring water pollutants' presence relied on bio-recognition elements, transducers, and immobilization techniques, which offer us the classification criterion. In comparison with optical methods, electrochemical transduction has advantages for analyzing turbid samples because it is non-sensitive to light. For optical sensing, they are likely to be interference from environmental effects, costly, and susceptible to physical damage.

This review provides an overview of recent progress in developing electrochemical biosensors for water quality detection, focusing on the last decade. Some older publications are cited to support and build up the critical concepts of electrochemical biosensors. We expect this critical review will help those working in ecological toxicant analysis in water, some scientists who might be unaware of electroanalytical chemistry and biosensors.
