Characterization of flexibility of ultrathin protein films by optical sensing

Films consisting of three cross-linked molecular layers of human serum albu min (HSA) or three HSA layers alternating with three heparin layers were im mobilized on surfaces of grating coupler sensors and hydrophobized glass. O ptical responses of the coated sensors to pH changes were observed. A decre ase in the effective refractive index, Delta N-eff, measured by a grating c oupler was observed when the pH of buffers contacting the films was increas ed. The process was quite reversible, which indicated that the mass of the film remained constant when the buffers were repeatedly exchanged. Theoreti cal treatment of the grating coupler optics related the decrease in Delta N -eff to expansion of the films. The interpretation of sensor measurements w as confirmed by atomic force microscopy which revealed reversible changes i n the thickness of NSA/heparin film induced by pH changes of the buffers.