Characterization of flexibility of ultrathin protein films by optical sensing

Films consisting of three cross-linked molecular layers of human serum albumin (HSA) or three HSA layers alternating with three heparin layers were immobilized on surfaces of grating coupler sensors and hydrophobized glass. Optical responses of the coated sensors to pH changes were observed. A decrease in the effective refractive index, Delta N-eff, measured by a grating coupler was observed when the pH of buffers contacting the films was increased. The process was quite reversible, which indicated that the mass of the film remained constant when the buffers were repeatedly exchanged. Theoretical treatment of the grating coupler optics related the decrease in Delta N-eff to expansion of the films. The interpretation of sensor measurements was confirmed by atomic force microscopy which revealed reversible changes in the thickness of NSA/heparin film induced by pH changes of the buffers.