Tunable growth of gold nanostructures at a PDMS surface to obtain plasmon rulers with enhanced optical features

Innovative optical devices for (bio)sensing based on Localized Surface Plasmon Resonance (LSPR) transduction allow to exploit the sensitivity of traditional SPR on miniaturized spectrometers suitable for point-of-care testing. In fact, LSPR reading can be realized by transmittance through interrogation with white light. In this framework, the effective coupling of plasmonic nanomaterials to optically transparent polymers is of great interest for having cheap, versatile, and sensitive optical devices of new concept. Some effort in fabricating gold and silver spherical nanoparticles through their in situ synthesis on PDMS has been reported, displaying moderate bulk sensitivity up to 70 nm/RIU. Herein we report that simple modifications to the general preparation of these composites (AuNPs@PDMS) can result in several improvements of the optical features. In particular, a two-steps growth of AuNPs@PDMS including PDMS surface chemical treatment before the second growth results in the formation of well-exposed and densely-packed 3D conglomerates of spheroids with enhanced bulk sensitivity and plasmon rulers character. Differently from available data, the obtained structures achieve sensitivity to refractive index change of about 250 nm/RIU, 3.5 higher than spherical nanoparticles with similar protocols and near the optical performances of anisotropic NPs. Due to the strong 3D character of the obtained structures, plasmon coupling effects are realized and investigated at PDMS surface. In particular, we show that these nanocomposite substrates subjected to biological stimuli exhibit red/blue-shifts typical of induced plasmon coupling/uncoupling, providing a step forward in composite plasmonic nanomaterials for application to (bio)sensing.