A multiple reflection attenuated total reflectance sensor incorporating a glass–indium tin oxide surface modified via direct attachment or film encapsulation of colloidal gold nanoparticles

This paper outlines improvements in sensitivity and detection limit for an optical sensor based on measurement of change in the surface plasmon resonance (SPR) absorbance of substrate-confined colloidal Au nanoparticles. Two routes were taken to obtain these improvements. First, a thin-layer, multiple internal reflection flow cell that allows for simultaneous potential control of the sensing surface was employed. This cell geometry yielded a ca. 18-fold increase in sensitivity to changes in bulk refractive index compared to a similar, previously reported sensor employing a single pass incident beam. Our second improvement involves preconcentration and encapsulation of colloidal Au nanoparticles into a quaternized poly(4-vinylpyridine) (QPVP) cationic exchange film spin-coated onto the glass–indium tin oxide (ITO) substrate surface. We demonstrate that the encapsulated nanoparticles retain their SPR activity; this approach therefore represents a novel and simple method of preparing a highly sensitive nanoparticle-based SPR sensing surface.