Stimulus-responsive colloidal sensors with fast holographic readout

TL;DR

This paper introduces a holographic microscopy method to measure the swelling of polymer colloidal spheres, enabling in situ detection of monomer concentration with high spatial resolution.

Contribution

It demonstrates a novel combination of optical micromanipulation and holographic characterization for precise, real-time sensing of chemical concentrations at the single-particle level.

Findings

Swelling degree correlates with monomer concentration.

Holographic readout provides nanometer-scale precision.

Method allows spatially resolved chemical sensing.

Abstract

Colloidal spheres synthesized from polymer gels swell by absorbing molecules from solution. The resulting change in size can be monitored with nanometer precision using holographic video microscopy. When the absorbate is chemically similar to the polymer matrix, swelling is driven primarily by the entropy of mixing, and is limited by the surface tension of the swelling sphere and by the elastic energy of the polymer matrix. We demonstrate though a combination of optical micromanipulation and holographic particle characterization that the degree of swelling of a single polymer bead can be used to measure the monomer concentration in situ with spatial resolution comparable to the size of the sphere.