Graphene for BioSensor Applications

Biosensor technology can detect a biological event by the production of a measurable signal. The process of detection combines a recognition element for a type of biomolecule or chemical reaction with a transducer which provides the signal.

Biosensors can be utilized for the identification of biological analytes such as antibodies, enzymes, organelles and microorganisms. Graphene is a carbon material in a honeycomb structure with one-atom thickness that is successfully being employed in the development of new biosensors.

The Benefits of Applying Graphene to Biosensor Applications

Single-layer graphene sheets display the following properties that are particularly suitable for use as a biosensor:

• A large surface area and high mechanical strength
• Superior elasticity and thermal conductivity
• Produces a quantum Hall effect at room temperature (a quantum-mechanical version of the Hall effect where an electron system, at low temperatures and in the presence of a strong magnetic field, displays an expected voltage difference)
• High room temperature electron mobility
• A tunable band gap where no electron states exist and tunable optical properties

The properties of graphene allow for the development of biosensors with improved performance. The relatively inexpensive production costs, along with a low environmental impact, also make graphene an attractive material choice. To use graphene for biosensor applications, a chemical functionalization process must occur to enable a biocompatible surface. Graphene can be functionalized by both the covalent and non-covalent bonding of small molecules to provide compatibility for the successful immobilization of biological receptors.

Graphene-based Enzymatic Electrochemical Biosensors

Graphene can promote electron transfers between enzymes and electrode because of its excellent electrical conductivity and high surface area properties. Enzymatic electrochemical biosensors work by immobilizing enzymes onto the electrode surface in order to detect biological analytes, measured by the electrochemical redox (the loss or gain of electrons through oxidation and reduction reactions) process that occurs.

This method is particularly studied for practical use in glucose detection in diabetes management. The enzyme glucose oxidase forms hydrogen peroxide through a series of redox reactions, which can then be sensitively detected by the electrochemical signal produced. Graphene-based electrodes have been found to display high repeatability and stability in comparison to carbon nanotube-based electrodes as a result of the high enzyme loading capacity and biocompatibility of graphene.

Graphene-based Electrochemical Immunosensors

Electrochemical immunosensors are used for detecting antigen-antibody complexes, particularly useful for the diagnosis of diseases in remote environments where laboratory testing is not practical. The sensor works by employing enzyme-conjugated antibodies which bind to their associated antigen on an electrode surface, producing electrochemical activity in the enzyme that can be measured as a signal. Graphene is being utilized for the development of high performance electrochemical immunosensors.

An improved method of detecting the cancer biomarker, alpha-fetoprotein (AFP), was achieved by forming an immunosensor on a graphene surface with the addition of a gold nanoparticle-functionalized interface. The graphene surface enhanced performance by providing a large area for nanoparticle immobilization leading to a better pathway for electron transfer when the AFP immunocomplex was formed and the enzyme chemical change occurred.

Graphene-based Field Effect Transistor (FET) Biosensors

Field effect transistors (FET) apply an electric field to control the electrical activity of a device. Their employment in biosensing applications is as a result of the full electronic detection ability that can be produced on electronic chips. These types of biosensors work by the biorecognition event that occurs between a probe and target biomolecules at the gate of the FET.

During the biorecognition event, the electric charge produced changes in the conductivity of the channel between source and drain. Field effect transistor biosensors are particularly useful for identifying charged biomolecules such as the charged phosphate backbone of DNA. Graphene is a good material for this type of biosensor because it allows for band gap tuning through surface modification and can detect gate voltage change formed from small amounts of single stranded DNA.

Image Credit:

GiroScience

Sources:

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