Streamlined Fabrication and Acoustofluidic Purification of Silver-Decorated Polystyrene Microspheres (PS-AgNPs) for SERS Applications

Composite microspheres of polystyrene and silver (PS-AgNPs) are highly valuable materials for catalysis, sensing, and antibacterial applications, yet their fabrication and subsequent purification remain challenging. This work presents a streamlined pathway for PS-AgNPs production, initiated by our finding that commercially available polystyrene (PS) microspheres (diameters ≥5 μm) anchor residual stabilizing polymeric structures that spontaneously facilitate the firm attachment of premade silver nanoparticles. The purified PS-AgNPs microspheres were evaluated as potential surface-enhanced Raman spectroscopy (SERS) substrates using adenine and thiamine as probe molecules, showing uniform SERS responses (coefficient of variation ≈ 10%) and limits of detection (LOD) of 100 nM and 1 μM, respectively. This demonstrates strong plasmonic activity that is suitable for sensing applications. While the synthetic approach is highly straightforward, it inherently creates a need to remove free and weakly attached nanoparticles, a critical step for PS-AgNPs practical application. We innovatively address this challenge by developing an acoustophoretic-based glass microfluidic device. Notably, the microchip was fabricated by using isotropic wet etching, a highly accessible method traditionally considered unsuitable for the precise geometries required for acoustophoresis. The separation principle relies on differential acoustophoretic migration, where larger PS-AgNPs microspheres are redirected into a collection outlet, while loose nanoparticles continue into the waste output, ensuring a high-purity final product.