Detection of Microenvironmental Changes Induced by Protein Adsorption onto Self-Assembled Monolayers using an Extended Gate-Field Effect Transistor

Nonspecific protein adsorption on self-assembled monolayer (SAM) alkanethiols with various terminal groups was investigated qualitatively and quantitatively using an extended gate-field effect transistor (extended gate-FET). The SAMs were characterized by XPS, cyclic voltammogram and water contact angle measurements. Changes in gate voltage of 1 mV caused by intrinsic charges of adsorbed protein on an undecanethiol SAM in 15 mM Dulbecco's phosphate buffered saline were equivalent to 3.6 ng cm(-2), 1.3 ng cm(-2), kind 16 ng cm(-2) for bovine serum albumin (BSA), lysozyme, and bovine plasma fibrinogen (BPF), respectively, as calculated by die Debye-Huckel model. Adsorption coefficients, maximum adsorption densities, and Gibbs free energies of adsorption were successfully determined using the Langmuir equation. The isotherms depended on the surface properties of the SAMs for BSA and lysozyme adsorption. In contrast, changes in gate voltage were almost independent of SAM type for BPF adsorption. Adsorption of large proteins may not be quantitatively detected because of the large dimensions of the biomolecules compared with the Debye length. In summary, the FET measurement is a nonlabeling, highly sensitive, and quantitative method for detecting nonspecific adsorption of small proteins with dimensions that are comparable to the Debye length of a solution.