A biosensor consists of a bioelement that interacts with an analyte and a transducer that converts the response into an electrical signal. The bioelement is usually an enzyme, antibody or microorganism and the transducer may be optical, acoustic, electrochemical or calorimetric.
The first step in preparing a biosensor is the application of the biological element to the surface of the sensor. The sensor may be made of a metal, a polymer or glass. The most common method for applying the bioelement is to coat the sensor with the biological element. The most commonly used bioelements include enzymes, antibodies, organelles, biological tissue and microbes. Coating of the sensor may be achieved using polylysine, aminosilane, epoxysilane or nitrocellulose to allow attachment to silicon chips or silica glass.
Coating may also be achieved by fixing the bioelement on the surface layer by layer using alternatively charged polymer coatings. Sometimes, three dimensional lattices of hydrogel or xerogel are used chemically or physically to trap the bioelement on the surface. Chemical entrapment of the bioelements refers to strong chemical bonding that keeps the element in place, while physical entrapment means the element is unable to pass through the pores in the gel’s matrix.
Sol-gel is the hydrogel that is usually employed and is a glassy silica created through the polymerization of silicate monomers in the presence of the biological elements using physical entrapment. Other hydrogels used include acrylate hydrogel, which polymerize upon radical initiation.
The bioelement and the sensor are coupled together in one of four ways:
- Membrane entrapment – A semipermeable membrane is used to separate the analyte and the bioelement. The sensor is attached to the bioelement.
- Physical adsorption – A combination of van der Waals forces, hydrophobic forces, hydrogen bonds, and ionic forces are used to attach the biomaterial to the sensor’s surface.
- Matrix entrapment – Also called porous entrapment, a porous encapsulation matrix is created around the biological element to help it bind to the sensor.
- Covalent Bonding – The sensor surface is treated as a reactive group that the bioelement binds to.