Surface Modification Approaches for Electrochemical Biosensors

Electrochemical biosensors are transducers that convert biological information into electrical information. Electrochemical biosensors provide qualitative and quantitative information (Wang 1999) on the existence and concentration of the target compounds in the analyte in the form of current (amperometric biosensor) or voltage (potentiometric biosensor). A typical amperometric biosensor consists of three components: the analyte, the transduction element (electrode and conductive nanomaterials) and the biorecognition element (enzyme) (McLamore et al., 2010a; McLamore et al., 2010b; McLamore et al., ; Shi et al., 2010). During biosensor operation, target compound in the sample is specifically recognized by the enzymes immobilized on the electrode. Electrooxidative intermediate is produced by this enzyme-substrate interaction. The produced electrooxidative intermediate is oxidized or reduced by the voltage applied on the biosensor, and current proportional to substrate concentration is generated and recorded. By calibrating the biosensor using solutions with known concentration, the relationship between measured current and substrate concentration is obtained. The sensitivity and specificity of the sensor is ensured by the high selectivity of enzymes. Considering the functional mechanism of biosensors, surface modification of the electrode is vital to biosensor performance. The most straightforward and also widely used approach is to immobilize enzymes on the electrode with a polymer layer. However, this method has two major limitations. One is that the activity of the enzymes can be affected by structural change due to the polymer layer, and affected by the pH of the layer (Zou et al., 2008). The other is that the thickness of the polymer layer cannot be precisely controlled, so the response time and sensitivity of the biosensor could be affected (Li et al., 1996). To overcome these limitations, some groups used polymers with neutral pH such as silicate sol-gel for enzyme immobilization to preserve enzyme activity (Salimi et al., 2004) while some groups used electric methods such as cyclic voltammetry to control layer deposition (Llaudet et al., 2005; Smutok et al., 2006). Furthermore, to obtain better performance, nanomaterials including carbon nanotubes (CNTs) and metal nanomaterials are often involved in surface modification (McLamore et al., 2010a; McLamore et al., 2010b; McLamore et al., ; Shi et al., 2010). Since different modification approaches result in quite distinct biosensor performance, problems with evaluating and comparing different approaches, and sorting out the optimal ones have arisen. To solve this problem, a standardization method which evaluates the performance of biosensors constructed by different approaches is needed.