Biosensors are biological elements aimed at detecting a biological or chemical component in a sample. Biosensors are often composed of two parts: a biological element (recognition element) and a transducer. The recognition elements can be an enzyme, antibody, or a microarray of single-stranded DNA that interact with the target analyte and result in the transducer emitting a measurable signal such as an electrical signal. Biosensors initially started in the 1960s, include various types such as:

– Enzyme-based
– Immunosensors
– DNA and protein biosensors (microarrays)
– Tissue/organelle based
– Thermal biosensors
– Optical biosensors

Microfluidics has been extensively used in biological and chemical research. A hot topic area of research both for the academia and the industry is point of care (POC) diagnostics. POC devices are portable types of equipment often used for pathogen or chemical substance detection at the point of care. POC devices heavily rely on microfluidic chips with embedded biosensors for the precise detection of these substances. A Microfluidic device can easily be integrated with various types of biosensors.

• Enzymatic biosensors: Enzymatic biosensors were first reported in 1967. They are often immobilized on the surface of the microfluidic chips adsorption, covalent binding, entrapment, encapsulation, or crosslinking and are suitable for POC diagnostic platforms. Upon reaction with an analyte, theses enzymatic biosensors convert the enzymatic reaction into a quantifiable signal such as electrochemical or optical signals. Some of the enzymatic biosensors that have been used in microfluidic chips are:

o Glucose
o Urea
o Cholesterol/lipid
o Lactate

• Immunosensors: Immunosensors rely upon the affinity of antibodies for their associated antigens. In these systems, often antibody or antigen is employed as the recognition element and is exposed to the sample to detect the target biomarker. Fabrication of Biosensors in Microfluidics is advantageous for immunoassays since they usually consume very low amounts of reagents and can reduce the cost by using less of these costly reagents.

Immunosensors are either heterogeneous or homogenous and both have integrated into microfluidic chips. In heterogeneous immunoassays, the antibodies or antigens are immobilized on the solid surface of a microchannel while in the homogenous type they form the complex freely in the solution in immunoreaction microchambers. An important aspect of the immunoassays is the immobilization strategy. Depending on the scope of the study, the antibody, etc., various methods can be employed for immobilization such as:

o Immobilization of the antibody on the microchannel surface
o Immobilization onto microparticles
o Immobilization onto the surface of the electrodes

Measurement techniques and readout systems, however, depend on the type of sensors.

o Electrochemical immunosensors:

 Cyclic voltammetry
 Amperometry
 Impedimetry
 Differential pulse voltammetry

o Optical immunosensors:

 Fluorescence
 Luminescence
 Absorbance (colorimetry)
 Surface plasmon resonance
 Surface-enhanced Raman scattering