Label-free biosensors based on optically responsive nanocomposite layers: sensitivity and dynamic range

Core-shell nanoparticle layers have proven to be a promising tool for the label-free detection of binding events. Upon reflection of white light, they exhibit pronounced extinction peaks in the UV/vis and NIR regime of the electromagnetic spectrum, which shift to higher wavelengths when molecules are adsorbed. Beside drastic simplification of the instrumentation and related reduction in cost, a significantly stronger response toward alkanethiol adsorption has been observed in previous experiments than in conventional surface plasmon resonance (SPR). However, as the amount of molecules deposited onto the nanoparticle films was unknown, no quantitative relationship could be established between the measured wavelength shifts and the surface mass density of the adsorbate. In order to facilitate quantitative molecule detection, self-assembled monolayers (SAMs) of simple and ethylene glycol (EG) terminated alkanethiols with various chain lengths were prepared on the nanoparticle-coated substrates. The measured red-shift of the extinction spectrum upon molecule adsorption was related to the amount of adsorbate as determined by X-ray photoelectron spectroscopy (XPS). For the whole range of film thicknesses studied, a linear relationship is found yielding a sensitivity factor of 0.027 nm/(ng/cm (2)). As proven by enzyme-linked immunosorbent assay (ELISA), such determined sensitivity factor can also be used to correctly predict the amount of surface-bound protein in immunoreactions from the measured wavelength shifts. It is concluded that the decay length of the evanescent electric field associated with the nanoparticle sensors is more than 100 nm and, thus, significantly larger than that observed for localized surface plasmons excited in small isolated metal clusters.