Biosensors in environmental monitoring

Azmain Al Faik
February 2021

What is biosensor? How does it work?

A biosensor (biological sensor) works fundamentally by detecting the presence of chemicals. It uses a living organism or biological molecules (e.g., enzymes, antibodies) to bind variables of interest and a transducer to generate an electrical signal proportional to the resulting change in analyte concentration. In other words, it is a device that converts biological processes into a detectable electrical signal. A typical biosensor is shown in Figure 1, which consists of the following components:

Analyte: The chemical that needs to be detected. For example, glucose is an “analyte” in a glucose biosensor.

Bioreceptor: A biomolecule that can specifically recognize an analyte and binds with it. For instance, enzymes, DNA, nucleic acid, antibodies, microbes, cells, and tissues can be used as bioreceptor that can specifically bind with specific analytes.

Transducer: An element that converts a biological signal into a corresponding electrical signal (voltage or current).

Signal conditioning unit: The electrical signal from a transducer of a biosensor is usually very low. A signal conditioning unit amplifies the signal and filters out noises into a “clean” detectable signal, which is then passed to a microcontroller.

As in Figure 1, a bioreceptor molecule is attached to a transducer surface in a biosensor system. When an analyte comes in contact with the transducer, the analyte selectively binds with the bioreceptor. This binding event produces a signal in the form of light, heat, pH, charge, or mass change^[4]. The transducer detects and transforms the change into an electrical signal. A signal conditioning unit filters and amplifies the electrical signal and shows it on display. This signal is proportional to the amount of analyte concentration in the reaction.

What are the types of biosensors?

Biosensors can be categorized depending on bioreceptor molecules or transducers used. Based on the transducers, biosensors can be classified into four types as follows:

Electrochemical biosensor: A typical electrochemical biosensor transduces a biochemical event into an electrical signal. An electrode works as a signal detector and support materials for bioreceptor molecules. An example of an electrochemical biosensor is the xanthine biosensor, where xanthine oxidase (bioreceptor) is immobilized on an electrode.

Optical biosensor: It incorporates a bioreceptor molecule with an optical transducer system. PCR (polymerase chain reaction) uses optical transducers to detect amplified target DNA molecules.

Thermal biosensor: The biosensor that measures the released or absorbed heat energy of the biochemical reaction is known as a thermal biosensor. COD (Chemical oxygen demand) biosensor is based on a thermal biosensor principle, where heat energy is released due to oxidation reaction.

Piezoelectric biosensor: Piezoelectric biosensors are based on the piezoelectric effect. The piezoelectric effect is the ability to generate an electric charge in response to applied mechanical stress on piezoelectric materials, such as crystals, quartz, certain ceramics, DNA, and various proteins. A piezoelectric biosensor is used in detecting antigens in body fluid for tuberculosis diagnosis. 

Biosensors have a wide array of applications such as disease monitoring, drug discovery, food safety, pathogens detection, and detection of pollutants^[3]. The most commonly used biosensors in environmental monitoring are electrochemical and optical biosensors^[1].

Why is biosensor?

Biosensors are interesting due to their unique characteristics such as selectivity, specificity, stability, and sensitivity^[4]. Besides, they are easy to use, fast, accurate, and low-cost. Though biosensors are mostly used in disease monitoring, drug delivery, and food safety, it is gradually becoming popular in environmental pollutants detection, since it typically eliminates the necessity of sample preparation. Three distinct advantages, i.e., portability, miniaturization, and low sample requirement, made biosensors an excellent sensing tool for environmental monitoring. 

Environmental Monitoring Parameters

A healthy environment requires continuous monitoring, but the monitoring variables in the environment are vast and complex. Detecting variables, such as heavy metals, pesticides, toxins, and pathogens, are costly and time-consuming with conventional sensing technologies. Table 1 summarizes the application of biosensors for the detection of various analyte classes from the environment with different methods employed. Biosensors for measuring some of the key environmental variables are briefly described below:

Table 1: Biosensors for environmental application

Analyte Class	Bioreceptor	Transducer	Application
Heavy metals	Microbes, enzyme	Electrochemical, Optical	Water, soil
Nitrogen compounds	Enzyme	Optical, Electrochemical	Water, soil, waste water
Pesticides	Antibody, enzyme, microbes	Optical, Electrochemical	Water, soil, air
Herbicides	Antibody, enzyme, microbes	Optical, Electrochemical	Water, soil, air
Phenolic compounds	Enzyme, microbes	Optical, Electrochemical	Water, soil
Dioxins	Microbes	Optical, Electrochemical	Water, soil, air
Toxins	Microbes	Electrochemical, Optical	Water, soil, air
Pathogens	Enzyme, microbes	Optical, Electrochemical	Water, soil

Heavy Metals

Due to the high toxicity, heavy metals have become a pressing concern in recent times. They are prevalent in the air, soil, water, and industrial waste.  The most commonly found toxic heavy metals are lead, mercury, zinc, cadmium, copper, and arsenic^[1].

Lead and mercury are highly toxic and ubiquitous pollutants in environment. Several optical biosensors has been developed to detect lead (Pb⁺²) and mercury (Hg⁺²) ions. A fluorrescence based optical biosensor has been recently developed that can detect Pb⁺² ions with a detection limit of 5 nM^[2]. DNA was immobilized as a bioreceptor on a carboxylated magnetic beads support. A nucleic acid based optical biosensor was also developed for the detection of Hg⁺² ions. The detection limit was found 0.84 pM. The detection limit further improved to 3 fM by introducing an electrochemical biosensor, where nucleic acid was immobilized on a single walled carbon nanotube surface^[2]. It diplayed a large surface area, high electrical conductivity and excellant substrate binding strength.

Pesticides

Organophosphorus pesticides are extensively used in agriculture as insecticides. Due to their high toxicity, the European Community (Directive 98/83/EC) has set a pesticide residue limit to 0.5 μg/L  for consumable water^[1]. Hence, simple, sensitive, and miniaturized biosensors have been developed to detect and monitor pesticides without the need for sample preparation. Enzyme-based electrochemical biosensors are the most effective tool for detecting pesticides, where acetylcholinesterase enzyme acts as a predominantly used bioreceptor^[2].

Paraoxon and parathion are the two most commonly used insecticides in agriculture. A disposable enzymatic electrochemical biosensor was developed using a self-assembled monolayer on gold screen-printed electrodes to detect paraoxon. The detection limit of the biosensor was 2 ppb (parts per billion) with a sensitivity of 113 μA cm^-2}^[2]. Another electrochemical biosensor was proposed to detect methyl parathion. In this case, acetylcholinesterase was used as a bioreceptor and immobilized on gold nanoparticles along with chitosan. The limit of detection was 5 fg mL^{-1 [2]}. Acetamiprid was detected at a limit of 17 fm using an aptamer based electrochemical biosensors in another study^[2].

Toxins

Toxins are harmful to human health. Most of the toxins are naturally present in the environment and some are produced by algae or bacteria. Brevetoxin-2, okadaic acid, saxitoxin, and bisphenol A are common toxins found in the environment. For the early detection of these toxins, fast, reliable, and cost-effective biosensors are needed to develop.

An electrochemical biosensor was proposed for brevetoxin detection; a limit of detection was found 106 pg mL^-1 with good selectivity^[2]. An aptamer was used as a bioreceptor and immobilized on a gold nanoparticle coated electrode. An electrochemical and an optical biosensor were also proposed to detect okadaic acid; antibodies were used as a bioreceptor in both cases. In both biosensors, okadaic acid was detected at a level of 0.05 ng mL^-1[2].

Pathogens

Pathogens are usually present in drinking water. If the pathogens present in drinking water is higher than the allowable limit, it could become a potential threat to human health. Recently, an optical biosensor based on surface plasmon resonance is proposed to detect Legionella pneumophila. The detection limit of bacteria in a range of 10⁴-10⁸ CFU mL^-1[2]. An improved limit of detection (10 CFU mL^-1) was obtained by electrochemical biosensors based on plasmon resonance principle. An Escherichia coli bacteria detection technique was developed by whole cell imprinting biosensor based on optical and piezoelectric principle for for real-time monitoring. The dectection time is 1h or less. An electrochemical biosensor was also developed using antibodies as bioreceptor to detect Bacillus subtilis with a detection limit of 10² CFU mL^-1[2].

Conclusion

Although biosensors have some limitations, their usage in environmental monitoring is widespread. The reusability of a biosensor is one of the main concerns in continuous environmental monitoring. Recently, the self-assembled monolayer technique has improved the reusability of certain biosensors. By implementing this technique, a biosensor can be used up to 200 times. Another shortcoming of biosensors is the stability after six months of use since the efficiency of biomolecules, particularly of enzymes, tends to decrease over time. Considering all the drawbacks biosensor still offers significant advantages over conventional sensing techniques. Hence, companies and research organizations should put more effort into minimizing the existing drawbacks of biosensors by providing sufficient research funds; and accelerating the commercialization of biosensors.

References

1. Gieva, E., Nikolov, G. and Nikolova, B. [2014]. Biosensors for environmental monitoring, Challenges in Higher Education & Research 12: 123–127.
2. Justino, C. I., Duarte, A. C. and Rocha-Santos, T. A. [2017]. Recent progress in biosensors for environmental monitoring: a review, Sensors 17(12): 2918.
3. Karunakaran, C., Rajkumar, R. and Bhargava, K. [2015]. Introduction to biosensors, Biosensors and bioelectronics, Elsevier, pp. 1–68.
4. Nikhil, B., Pawan, J., Nello, F. and Pedro, E. [2016]. Introduction to biosensors, Essays in Biochemistry 60(1): 1–8.

Written by:

Azmain Al Faik (@FaikWin)

Edited by:

Md Abdul Halim (@BiometHalim) (www.mdhalim.com)