  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.
