Silver Nanoparticle Thin Films with Nanocavities for Surface‐Enhanced Raman Scattering

Abstract

The formation of nanometer-sized gaps between silver nanoparticles is critically important for optimal enhancement in surface-enhanced Raman scattering (SERS). A simple approach is developed to generate nanometer-sized cavities in a silver nanoparticle thin film for use as a SERS substrate with extremely high enhancement. In this method, a submicroliter volume of concentrated silver colloidal suspension stabilized with cetyltrimethylammonium bromide (CTAB) is spotted on hydrophobic glass surfaces prepared by the exposure of the glass to dichloromethysilane vapors. The use of a hydrophobic surface helps the formation of a more uniform silver nanoparticle thin film, and CTAB acts as a molecular spacer to keep the silver nanoparticles at a distance. A series of CTAB concentrations is investigated to optimize the interparticle distance and aggregation status. The silver nanoparticle thin films prepared on regular and hydrophobic surfaces are compared. Rhodamine 6G is used as a probe to characterize the thin films as SERS substrates. SERS enhancement without the contribution of the resonance of the thin film prepared on the hydrophobic surface is calculated as 2 x 10(7) for rhodamine 6G, which is about one order of magnitude greater than that of the silver nanoparticle aggregates prepared with CTAB on regular glass surfaces and two orders of magnitude greater than that of the silver nanoparticle aggregates prepared without CTAB on regular glass surfaces. A hydrophobic surface and the presence of CTAB have an increased effect on the charge-transfer component of the SERS enhancement mechanism. The limit of detection for rhodamine 6G is estimated as 1.0 x 10(-8) M. Scanning electron microscopy and atomic force microscopy are used for the characterization of the prepared substrate.

Size Effect of 3D Aggregates Assembled from Silver Nanoparticles on Surface‐Enhanced Raman Scattering

“…[37,38] The assembly process can be influenced by changing the hydrophobic and hydrophilic properties of the substrate or the nanoparticle surface. [26,39] Recent studies demonstrated that the distance between two nanoparticles is very critical for higher SERS enhancement. [18] When the interparticle distance is decreased to 1 nm, SERS enhancement becomes suitable for single-molecule detection.…”

“…However, we have recently demonstrated their use as reproducible and high-performance substrates. [26] The range and position of the SPs around the noble metal nanoparticles and their influence on SERS enhancement was studied. [27] When the SPs of nanoparticles are excited, they tend to localize around the sharp edges of the metal nanoparticles, and this localization depends on shape.…”

Surface-Enhanced Raman Scattering as an Emerging Characterization and Detection Technique

“…The evidence, that SERS can be used for single-molecule detection and clear understanding of the relationship between LSPR and SERS enhancement mechanism, has inspired many recent studies focused on design and fabrication of new types of SERS substrates [149][150][151].…”

Fundamentals and applications of SERS-based bioanalytical sensing

“…They can be fabricated by assembly of NPs or vapor deposition of metal on a substrate to obtain thin film plasmonic nanostructures. Inter-particle distance, orientation, type, and size of NPs in the assembled NPs are critical factors for SERS performance [85,118,119]. Roughness and thickness of the film and type of the metals are the influencing parameters for SERS when thin films are used as SERS substrate [120].…”

“…also increases. Controlling this aggregation helps generate higher SERS enhancement due to the increased possibility of " hot-spot" formation [85,86]. One must consider, however, that very large aggregates diminish the effective formation of SPs due to the deformations and dampening of the electron cloud in the aggregate and therefore generate poor SERS activity.…”