Microfluidic growth of Ag nanoparticles onto porous sllicon/PDMS surface for reliable SERS detection

Thanks to the miniaturized environment, microfluidics can offer an improved control over the growth of nanoparticles (NPs) particularly in terms of increased uniformity. An in-flow synthesis process can be therefore advantageous to fabricate plasmonic NPs for Surface-Enhanced Raman Scattering (SERS), which requires reproducible and uniform NPs arrangements aimed to obtain a repeatable Raman enhancement. In this study, silver NPs are synthesized in elastomeric microfluidic chips (polydimethylsiloxane, PDMS) hosting ultra-thin porous silicon (pSi) membranes. The NPs growth is investigated through the evolution of UV-Vis transmission spectra presenting Localized Surface Plasmon Resonances (LSPRs). The intrinsic reducing properties of the pSi towards the silver cations are exploited for the NPs synthesis. A motorized syringe is connected to the microfluidic chip and drives the injection of a AgNO3 solution into the reaction chamber (Fig.1a). Synthesis parameters, such as the silver nitrate concentration, the solvent, and the flow rate, are systematically varied in order to understand their influence on the NPs growth and morphology. At medium/high salt precursor concentrations (10-2 – 10-1 M) the appearance of a plasmonic dip over the pSi-PDMS spectral features is already evident after the first minute of reaction and a continuous red-shift combined with a broadening of the LSPRs is observed at increasing contact times until a saturation regime is reached at around 20 minutes (Fig1.b I-V). In order to compare the microfluidic results with static synthesis conditions, the immersion plating of an open pSi-PDMS membrane attached to a plastic cuvette is performed. In this case, a reduced growth rate is observed, as shown by the slower evolution of the plasmonic resonance (Fig1b VI), together with an altered morphology of the NPs. In fact, FESEM imaging reveals that under dynamic conditions the obtained NPs are smaller and more uniformly distributed on the pSi surface, in comparison to the larger and isolated particles obtained by static dipping, in agreement with their optical responses (Fig.b V-VII). Such differences are reflected in the SERS properties of the samples, which are tested with 4-mercaptobenzoic acid (4-MBA) as probe molecule. A high Raman signal intensity fluctuation is detected for the immersion plated substrates, while a higher average intensity and a better homogeneity characterize the in-flow synthesized ones. These results show the potentialities of the microfluidic process for the in situ fabrication of reliable SERS substrates.