Environmental safety has become a major concern for the safety of organisms due to the presence of a large number of harmful contaminants in the atmosphere. Different approaches have been adopted to control and manage the threat of environmental pollution, but this is still a worldwide challenge. Therefore, there is a need to develop sensitive, fast and selective techniques for detecting and screening pollutants for effective bioremediation. From this perspective, enzymes or enzyme-producing biological systems can be used to detect, quantify and degrade or convert pollutants into non-polluting compounds to restore ecological balance. Biosensors are a reliable, special, and sensitive way to detect and measure environmental pollution.

A biosensor is a self-sufficient integral tool that provides accurate, quantitative, and analytical information using biochemical receptor that is in direct contact with a transduction element. The biosensor consists of three main components, including a biological recognition element, a transducer and a signal processing system. Key features of enzyme-based biosensors include accuracy and precision in measurement, speed, sensitivity, specificity and measurement range, test reliability, calibration and long-term stability, robustness, size, safety and portability analysis cost and user acceptability. Enzymes can detect the presence of certain analytes by measuring the consumption or product of certain compounds (such as CO₂, NH₃, H₂O₂, H⁺, O₂), so the sensor can identify pollutants and correlate their presence with the substrate. Enzymes are mainly divided into six categories, including oxidoreductases, transferases, hydrolases, isomerases, lyases and ligases, in which oxidoreductases and hydrolases are mostly used as biosensors to detect a variety of harmful heavy metals and metalloids.

Oxidoreductase

Oxidoreductases are a class of enzymes that transfer electrons from one molecule to another. Molecules that transfer electrons are called electron donors or reductants, while molecules that accept electrons are called electron acceptors or oxidants. Many oxidoreductases have been successfully used to detect and quantify heavy metals or metalloids. For example, tyrosinase, polypyrroleglucose oxidase (PPy-GOx), nitrate reductase, and so on, have been used for detection Cd²⁺, Cr³⁺, Cr⁶⁺, Cu²⁺, Ni²⁺, Pb²⁺, Zn²⁺ and so on.

Hydrolase

Hydrolases are a class of enzymes that hydrolyze chemical bonds. An example of hydrolase for detecting heavy metal contamination is the detection of Cu²⁺, Hg²⁺, Cd²⁺, Zn²⁺ and Ag²⁺ by alkaline phosphatase.

Isomerase

Isomerases are a class of enzymes that convert molecules from one isomer to another to aid in intramolecular rearrangement. Isomerase was successfully used as a biosensor to detect Hg²⁺.

Enzymes-based Biosensor in the Detection of Pollutants

The pollutants in the environment are mainly classified into inorganic compounds (heavy metals), organophosphates, and phenolic compounds. Heavy metals including cadmium, chromium, copper, nickel, led, zinc, and so on can be detected by inhibition of nitrate reductase. Pyrroloquinoline quinone-dependent glucose dehydrogenase (PPQ-GDH) and horseradish peroxidase can be used to detect Pb²⁺, Cd²⁺, Hg⁺, and Hg²⁺. Alkaline phosphatase biosensors can successfully detect Cd²⁺, Ni²⁺, Pb²⁺, Co²⁺, and Zn²⁺. Glucose oxidase can be used to detect heavy metal ions such as Ag⁺, Cd²⁺, Cu²⁺, Cr³⁺, etc. Urease can be used to detect Ag+, Pb²⁺, Sb³⁺, Cu²⁺, Cd²⁺, Co²⁺, Ni²⁺, and so on. Tyrosinase can be used to detect Cu²⁺ ions.

The use of organophosphates (pesticides, insecticides, and herbicides) poses serious threats to the environment, human health, and ecosystems. Commonly used organophosphates include parathion, methyl parathion, malathion, diazinon, chlorpyrifos, fenitrothion, dichlorvos, and phosmet. These compounds have serious hazardous to humans and animals, so their detection and quantification are critical. A variety of enzymes were used to construct biosensors for the detection of organophosphates, including acetylcholinesterase (AChE), butyrycholinesterase (BChE), glucose oxidase (GOx), urease, organophosphorus acid anhydrolase and hydrolase (OPAA and OPH).

Phenols and their derivatives are considered to be toxic compounds and are found in various industrial wastewaters associated with the production and synthesis of plastics, dyes, polymers, pharmaceuticals, detergents, insecticides, disinfectants, etc. Chloro- and nitrophenols are the major degradation products of organophosphate insecticides and chlorinated phenoxyacids. These compounds show severe toxicity in plants and animals, cause genotoxicity and mutagenicity, and reduce other life processes such as photosynthesis, respiration and enzyme-catalyzed reactions at low concentrations. Enzymes used to detect phenolic compounds and their degradation products include laccase, tyrosinase, and peroxidases.

References

1. Nigam V K, Shukla P. Enzyme based biosensors for detection of environmental pollutants. [J]. J Microbiol Biotechnol, 2015, 25(11):1773–178.
2. Sarkar A, et al. An overview of enzyme-based biosensors for environmental monitoring. [B]. Tools, Techniques and Protocols for Monitoring Environmental Contaminants, 2019, 307-329.
3. Rao M A, et al. Enzymes as useful tools for environmental purposes. Chemosphere, 2014, 107:145-162

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