Development of piezoelectric crystal based microgravimetric immunoassay for determination of insulin concentration

A microgravimetric, piezoelectric crystal based immunoassay for the quantification of insulin concentration is described. The method utilizes a modified piezoelectric crystal device having an antibody specific to insulin bound to its surface. The antibody to insulin was immobilized on the surface of crystal electrode by using either 3-aminopropyltriethoxy silane (3-APTES), polyethyleneimine (PEI) or covalently coupled protein A-gold immobilization method. Coating an electrode with a cross linked protein A-antibody complex gave better results in terms of sensitivity and stability. Using the system described, the insulin concentration up to 1 ng ml-1 could be detected. The stability and reusability of the system was further improved by using a mild eluting reagent which successfully removed the bound insulin molecules from the antibody-coated crystal without affecting the immobilized insulin antibody. Scanning tunneling microscopic (STM) study was also done to confirm the surface coverage and orientation of insulin and antibody molecules on the modified piezoelectric crystal electrode surface. A comparison between the present study and the well-established radioimmunoassay technique (RIA) revealed that the described microgravimetric immunoassay technique (MIA) could successfully be developed as an alternative of RIA.