Mapping nanoparticles in hydrogels: A comparison of preparation methods for electron microscopy

Abstract

The distribution of noble metal nanoparticles (NMNPs) in hydrogels influences their nanoplasmonic response and signals used for biosensor purposes. By controlling the particle distribution in NMNP-nanocomposite hydrogels, it is possible to obtain new nanoplasmonic features with new sensing modalities. Particle positions can be characterized by using volume-imaging methods such as the focused ion beam-scanning electron microscope (FIB-SEM) or the serial block-face scanning electron microscope (SBFSEM) techniques. The pore structures in hydrogels are contained by the water absorbed in the polymer network and may pose challenges for volume-imaging methods based on electron microscope techniques since the sample must be in a vacuum chamber. The structure of the hydrogels can be conserved by choosing appropriate preparation methods, which also depends on the composition of the hydrogel used. In this paper, we have prepared low-weight-percentage hydrogels, with and without gold nanorods (GNRs), for conventional scanning electron microscope (SEM) imaging by using critical point drying (CPD) and hexamethyldisilazane (HMDS) drying. The pore structures and the GNR positions in the hydrogel were characterized. The evaluation of the sample preparation techniques elucidate new aspects concerning the drying of hydrogels for SEM imaging. The results of identifying GNRs positioned in a hydrogel polymer network contribute to the development of mapping metal particle positions with volume imaging methods such as FIB-SEM or SBFSEM for studying nanoplasmonic properties of NMNP-nanocomposite hydrogels.