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Biosensors are analytical devices that detect biomolecules in a complex sample by converting the physical or chemical signal to an optical signal proportional to the concentration of an analyte. A typical biosensor consists of a biorecognition element, a signal transducing unit, and a signal processing unit. The biorecognition element, also known as the bioreceptor, is the component that determines the specificity of the device and is usually a molecular species that recognizes analytes through a biochemical mechanism. The bioreceptor binds the analyte of interest to the surface of the sensor used for the reaction. The biochemical signals of the reaction are then converted by the signal transducing unit into the measurable signals. Biosensors can be classified as optical, electrochemical, thermometric, piezoelectric, or magnetic based on the means of signal transduction. Optical signal with high sensitivity, immunity to external disturbance, stability, and low noise takes advantages over other physical signals. Therefore, optical biosensors present good performance in detecting biological systems. Optical biosensors can utilize various biological materials such as enzymes, antibodies, antigens, receptors, nucleic acids, and whole cells as bioreceptors. The mainstream optical biosensors include surface plasmon resonance (SPR) based biosensor, optical waveguide-based biosensor, optical resonator-based biosensor, photonic crystal-based biosensor, and optical fiber-based biosensor. Most optical biosensors use evanescent field to sense the analyte and environmental media. In the evanescent field, the analyte is recognized by the bioreceptors immobilized on the waveguide surface, thus influencing the guiding properties of the waveguide, specifically, the change of the effective mode index. The change is corresponding to the concentration of the analyte and the biomolecular conformation, and can be assessed by the optical properties of the waveguide, such as the phase, amplitude, and resonance momentum. Without complexity in the pre-treatment and probable influence on the nature of target molecules, optical biosensors take advantages over the traditional sensors with labels.

Technological advances have made biosensors smaller, more accurate, more portable, reliable, and sensitive to analytes. Biosensors have the high accuracy quantification and low detection limits without the expensive and time-consuming issues of traditional detection. Therefore, biosensors have a wide range of applications in many fields, including environmental monitoring, disease biomarker detection, food process control, drug discovery, and others. One of the main applications of biosensors is the detection of biomolecules that are either indicators of a disease or targets of a drug. Advances in instrument sensitivity enable the direct detection of target molecule binding to immobilized receptors, greatly increasing the utility of biosensors in drug screening. Biosensors can also be used as platforms for monitoring food traceability, quality, safety, and nutritional value. Whether used for long-term monitoring or single shot analysis, biosensors can be used as technologically advanced devices in complex studies with limited resources.