(177e) Engineered Assembly of Uniform Hierarchically Porous Patches from Metal Nanoparticles

Colloidal assemblies with controlled and reproducible structure could be an important element in sensors, microreactors, nanooptical and nanoelectronic devices. We demonstrated that a combination of convective assembly and latex templating allows assembly of gold nanoparticles into hierarchically porous substrates for sensors based on surface-enhanced Raman spectroscopy (SERS). At present, SERS substrates made by us or others are in the form of coatings and stripes of size from millimeters to centimeters. There is a range of important advantages that can be derived from making substrates in the form of arrays of sub-millimeter patches. The simplest approach to depositing gold nanoparticles and latex template spheres in small spots would be to dry small droplets of suspension on a surface, however this leads to non-uniform "coffee ring" deposits. We developed a new technique that would allow making uniformly thick and well structured dots. The method is based on controlled drying of droplets from concentrated suspension on surfaces with intermediate hydrophobicity. We found that droplets of microsphere suspensions deposited on partially hydrophobic polymer (of contact angle ca. 40-80 deg) do not dry by the accepted "coffee ring" mechanism. Instead, they begin shrinking, the particles inside begin crystallizing and the final result is a flat or slightly bulging patch of crystallized latex. If metallic nanoparticles are added to the droplet, latex crystal - gold nanoparticle patches of round flat shape, equivalent in structure to the SERS materials precursors are formed. The structure-dependent performance of such SERS substrates was systematically characterized with cyanide in continuous flow millifluidic chambers. A matrix of experiments was designed to isolate the SERS contributions arising from nano- and microscale porosity, long range ordering of the micropores, and the thickness of the nanoparticle layer. The SERS results were compared to the substrate structure observed by scanning electron microscopy and optical microscopy to correlate substrate structure to SERS performance. The Raman peak intensity was consistently highest for nanoporous substrates with three-dimensionally ordered micropores, and decreases if the micropores are not ordered, or not templated. Removing the nanoscale porosity by fusion of the nanoparticles (without removing the large micropores) leads to drastic plunge in substrate performance. The peak intensity does not strongly correlate to the thickness of the nanoparticle films.