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

## TL;DR

This paper introduces a simplified method to create and purify silver-decorated polystyrene microspheres for use in sensing applications like SERS.

## Contribution

A novel acoustofluidic purification method is developed for PS-AgNPs, using isotropic wet etching to create a microfluidic device.

## Key findings

- PS-AgNPs microspheres show uniform SERS responses with a coefficient of variation of around 10%.
- The LOD for adenine and thiamine was 100 nM and 1 μM, respectively.
- An acoustophoretic microfluidic device effectively separates and purifies PS-AgNPs.

## Abstract

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.