Biosensor: An Overview
A biosensor is an analytical device that combines a biological recognition element with a physicochemical transducer to detect and quantify a specific analyte. In simple terms, a biosensor recognizes a target molecule through a biological component and converts this recognition event into a measurable signal. Biosensors are widely used in medical diagnostics, food safety, environmental monitoring, agriculture, biotechnology, and pharmaceutical research.
A typical biosensor consists of three major components: a bioreceptor, a transducer, and a signal-processing system. The bioreceptor provides the specificity of the sensor by recognizing the target analyte. It may be an enzyme, antibody, nucleic acid, aptamer, receptor protein, microorganism, whole cell, or molecularly imprinted material. For example, urease can be used as the biological recognition element for detecting urea, while glucose oxidase is commonly used for glucose detection.
The second component is the transducer, which converts the biological interaction into a measurable physical or chemical signal. Depending on the type of signal generated, biosensors can be classified as electrochemical, optical, piezoelectric, thermal, or semiconductor-based sensors. Electrochemical biosensors are particularly important because they are generally sensitive, rapid, relatively inexpensive, and suitable for miniaturization.
Electrochemical biosensors can further be divided into amperometric, potentiometric, conductometric, impedimetric, and voltammetric biosensors. Amperometric sensors measure current generated by an electrochemical reaction, whereas potentiometric sensors measure changes in potential. Conductometric sensors detect changes in electrical conductivity, while impedimetric sensors measure changes in impedance. Voltammetric sensors measure current as a function of an applied potential. FET-based biosensors, such as ISFETs, detect changes in channel conductance or surface potential resulting from biological interactions.
The general working principle of a biosensor can be represented as:
Analyte → Biological recognition → Biochemical/chemical reaction → Transduction → Measurable signal → Quantification
For example, in a urea biosensor, the enzyme urease specifically recognizes urea and catalyzes its hydrolysis:
Urea + H₂O → 2NH₃ + CO₂
The ammonia produced during this reaction changes the local chemical environment, including pH and ionic composition. An appropriate transducer detects this change and produces an electrical signal. After calibration, the magnitude of the signal can be correlated with the concentration of urea in the sample.
The performance of a biosensor is evaluated using several important parameters, including sensitivity, limit of detection (LOD), linear detection range, selectivity, response time, reproducibility, repeatability, stability, and recovery. A good biosensor should provide a strong and reproducible response to the target analyte while showing minimal interference from other substances present in real samples.
Biosensors have numerous applications. In healthcare, they are used for detecting glucose, cholesterol, cancer biomarkers, pathogens, hormones, and nucleic acids. In food analysis, they can detect urea, pesticides, toxins, pathogens, and food adulterants. Environmental biosensors are used for monitoring pollutants, heavy metals, and microorganisms.
