Wavelength-interrogated surface plasmon resonance sensor with mesoporous-silica-film-enhanced sensitivity to small molecules.

Sol-gel copolymer-templated mesoporous silica films with a thickness of 70 nm and interpore spacing of 4.34 nm were fabricated on gold layer covered glass substrates for application as a wavelength-interrogated surface plasmon resonance (SPR) sensor. The resonance wavelength (λ(R)) of the sensor with a solution sample was determined by absorptiometry at a given incident angle. A comparison between the experimental data obtained with the coated and uncoated SPR chips demonstrated that the mesoporous silica film effectively enhanced sensor response to individual adsorption of cysteamine molecules and lead(II) ions. An approximate proportional relationship between the resonance-wavelength shift of the sensor and the volume fraction of analyte molecules adsorbed in the mesoporous silica film was obtained by numerical simulation. Porosities of 0.865 and 0.785 for the two silica films used as well as the volume fractions of 0.048 and 0.116 for adsorbed lysozyme and cysteamine molecules were determined by fitting the simulation results to the experimental data. The adsorbed amount of cysteamine (∼0.5 nm) is equivalent to more than 16 full monolayers on the geometric surface of the mesoporous silica film used. In contrast, an equivalence of less than 2 full monolayers for adsorbed lysozyme molecules (3 nm × 3 nm × 4.5 nm) suggests that the mesoporous silica film has good size-selective adsorption capability due to its uniform pore size distribution. Cysteamine modification of the mesoporous silica film renders the SPR sensor able to detect lead(II) ions at concentrations as low as 1 nM.