Role of the Micro- and Nanostructure in the Performance of Surface-Enhanced Raman Scattering Substrates Assembled from Gold Nanoparticles

Abstract

Highly active and stable substrates for surface-enhanced Raman scattering (SERS) can be fabricated by using colloidal crystals to template gold nanoparticles into structured porous films. The structure-dependent performance of these SERS substrates was systematically characterized with cyanide in continuous flow microfluidic 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 (SEM) 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 a drastic plunge in substrate performance. The peak intensity does not strongly correlate to the thickness of the nanoparticle films. The results make possible the efficient controlled fabrication of stable, reproducible, and highly active substrates for SERS based chemical sensors with continuous sampling.

Nanostructures and nanostructured substrates for surface—enhanced Raman scattering (SERS)

“…The gold nanoparticles then aggregate in the voids surrounding the micro-spheres and form a structured metallic film that is preserved when the polymer template is dissolved. 40 Alternatively, these structures can be fabricated by a continuous deposition process. These procedures often produce structures with short-to medium-range order with repeated features equal to the size of the templating structure.…”

Controlled assembly of SERS substrates templated by colloidal crystal films

“…18,19 By varying these parameters we fabricated latex templates that were polycrystalline with single crystal dimensions in the 5-20 mm range, or randomly packed with no apparent long-range crystal order (Fig. 2).…”

“…All experimental measurements were performed using a point sampling protocol in a microfluidic flow cell. 16,19 Such online flow cells could be directly used as a basis for SERS sensors for continuous monitoring of water or gas fluxes. Post-deposition treatment of the nanoparticle layers with dimethyldichlorosilane vapor hydrophobized the glass substrates, enhancing the adhesion of the gold films, and improving their stability to the extent that they can be subjected to prolonged exposure to shear flows in the experimental cells.…”

“…16 A matrix of experiments was designed to isolate individual contributions to the Raman enhancement arising from the nano-and microstructure. 19 We collected multiple data points from randomly chosen spots on each SERS substrate inside the microfluidic chamber filled with liquid. This simulates application in real sensors where no drying and alignment to find ''hot spots'' is usually possible.…”

“…43 In order to reliably characterize the effect of the nanoporosity on SERS performance we developed a procedure for making nanoporosity gradient substrates by imposing a monotonically varying thermal gradient onto the backside of the SERS support. 19 This technique produced substrates that had fully fused gold nanoparticles on one end and unchanged discrete aggregated gold nanoparticles on the other, with a variable degree of fusion in the middle. The degree of nanoporosity along the sample surface after heating could be correlated to the spatially varying color change.…”