Au-Coated Polystyrene Nanoparticles with High-Aspect-Ratio Nanocorrugations via Surface-Carboxylation-Shielded Anisotropic Etching for Significant SERS Signal Enhancement

Abstract

Increasing the hot-spot area with high enhancement ability on SERS-active particles is generally acknowledged as one of the efficient ways to significantly improve the average SERS signal of nanoparticles. A method to create roughness on the surface of nanoparticles was proposed by oxygen plasma etching noncarboxylated polystyrene beads. However, the mechanism of nanocorrugation formation was not clear. Thus, in this paper, we employ argon-based reactive ion etching (RIE) incorporated with carboxylated polystyrene nanoparticles to investigate the roles of nanocorrugations’ morphologies for SERS signal enhancement. The formation mechanism of the nanocorrugations has been investigated thoroughly through a comparison with those formed by oxygen-based RIE processes from their high resolution X-ray photoelectron spectra and surface morphologies with or without hydrazine reduction treatment. Moreover, polystyrene beads with more intrinsic carboxyl groups and etched by argon plasma produce higher nanocorrugations. It is suggested that carbonyl groups with high bond energy become nanomasks on polystyrene bead surfaces and provides high selectivity between carboxyl and polystyrene surfaces under RIE. Raman intensity enhancement on a 20-nm gold coated nanocorrugated polystyrene bead array is summarized by three factors: (1) the effect of plasmonic coupling among neighboring particles, (2) the nanocorrugation-contributed roughness, and (3) the pitch size of nanocorrugations, through the analysis of SEM images, AFM height images, and LSPR signals. Among these factors, the pitch size of nanocorrugations (ranging from ∼6 nm to ∼12 nm on the surface of polystyrene beads) dominates the SERS enhancement. The 870 nm/120s oxygen plasma etched polystyrene beads (OPSBs) with a minimum pitch size of 6 nm provides the highest Raman intensity enhancement (measured by 632.8-nm He–Ne laser), which is 12 times greater than the intensity of nontreated (870 nm/0s) polystyrene beads (while the Au/Ti coating is 20 nm/5 nm).

A Nanoporous Cytochrome c Film with Highly Ordered Porous Structure for Sensing of Toxic Vapors

“…The typical thickness of the monolayered PS film is similar to the radius of the PS beads. The fabricated films show nicely packed and highly organized PS beads that are uniformly distributed and well interconnected with small interstitial channels on the silicon wafers . The highly organized packing and uniform coating are attributed to the interparticle forces, namely, the lateral capillary force, convection force, and flotation force that govern their packing geometry …”

Surface enhanced Raman scattering (SERS) based biomicrofluidics systems for trace protein analysis

“…To further enhance the SERS effect other than the nanoparticle arrays with limited pairing regions, increasing hot spot regions on individual nanoparticles became a practical idea, for example, Au nanoflowers, 27 Ag coated nest-like Zn networks, 28 Ag nanoparticle decorated ZnO nanotapers, 29 and Au coated nanomushrooms. 30 These nanostructures not only generated plasma coupling on a plane as in an array but also induced more hot spot regions from the threedimensional surfaces on individual nanoparticles. For nanoflowers, Xie et al employed 4-(2-Hydroxyethyl) piperazine-1-ethanesulfonic acid (HEPES) as a weak capping agent to reduce Au ions (AuCl 4 À ) into Au nanocrystals.…”

“…The narrow gaps among the fan-out nanoextrusions on each nanoflower induced a plasma coupling effect, and the SERS signal increased by about 7 folds, which is higher than that of 2D nanoparticle arrays. The randomly distributed nanoextrusions on nanoflowers were further improved into more regular, directional, and longer finger type nanostructures on nanomushrooms by Hsieh et al 30 Nanomushrooms were fabricated by directional oxygen/argon plasma etching on top of noncarboxylated polystyrene beads, as shown in Fig. 5(a).…”

“…Among these factors, the pitch size of nanocorrugations (ranging from $6 nm to $12 nm on the surface of polystyrene beads) dominates the SERS enhancement and the minimum pitch size of 6 nm provides the highest Raman intensity enhancement to more than 12 folds of the 2D nanoparticle arrays. 30 This significant SERS signal amplification is attributed to the parallel finger-shape pitches, inducing "hot lines," which provide more plasmonic coupling regions than that of nanoparticle pairs, 19 nanoparticle arrays, 25 and nanoflower particles 27 with only "hot spots." Attributed to the plasmonic coupling regions in the unit space of the different strategies of nanostructure arrangements, the SERS signal relation of nanoparticle pairs, nanoparticles arrays, nanoflower particles, and nanomushrooms are 1 fold, 4 folds, 7 folds, and 12 folds, respectively, as shown in Fig.…”

“…Particles with 3D nanostructures for SERS enhancement: (a) mechanism and structures of nanomushrooms for SERS enhancement and (b) schematics of SERS enhancement by different strategies of nanostructure arrangements. 19,25,27,30…”