Highly Raman‐Enhancing Substrates Based on Silver Nanoparticle Arrays with Tunable Sub‐10 nm Gaps

Abstract

Raman spectroscopy, which is based on the inelastic scattering of photons by chemical entities, has been successfully utilized for the investigation of adsorbed molecules on surfaces, [1][2][3] although the low cross section limits its applications.Surface-enhanced Raman scattering (SERS) has drawn a lot of attention since its discovery in 1974, [4] primarily because it can greatly enhance the normally weak Raman signal and thereby facilitate the convenient identification of the vibrational signatures of molecules in chemical and biological systems. [5] Recently, the observation of single-molecule Raman scattering has further enhanced the Raman detection sensitivity limit and widened the scope of SERS for sensor applications. [6,7] Although SERS effects can be achieved simply by exploiting the electromagnetic resonance properties of roughened surfaces or nanoparticles of Au or Ag, the fabrication of reliable SERS substrates with uniformly high enhancement factors remains the focus of much research. Spraying Au or Ag colloids on a substrate leads to an extremely high SERS signal at some local 'hot-junctions'; [6][7][8] however, it is not easy to achieve a reliable, stable, and uniform SERS signal spanning a wide dynamical range using this method. Van Duyne and coworkers have used nanosphere lithography, [9] while Liu andLee exploited soft lithography, [10] in order to fabricate Ag nanoparticle arrays with high SERS activity and improved uniformity. Käll and co-workers have shown theoretically that the effective Raman cross section of a molecule placed between two metal nanoparticles can be enhanced by more than 12 orders of magnitude.[11] Such enhancement is likely to be related to the 'hot-junctions' observed in some SERS experiments. Several theoretical groups have also investigated field enhancement for SERS from metal nanoparticle arrays. [12][13][14] Specifically, García-Vidal and Pendry proposed that very localized plasmon modes, created by strong electromagnetic coupling between two adjacent metallic objects, dominate the SERS response in an array of nanostructures.[12] The interparticle-coupling-induced enhancement was attributed to the broadening of the plasmon resonance peak because the probability of the resonance covering both the excitation wavelength and the Raman peak increases with its width. They calculated the average enhancement factor over the surfaces of an array of infinitely long Ag nanorods with semicircular cross sections, and showed that significant near-field interaction occurs between adjacent nanorods when the gap between the nanorods reaches half the value of their diameter. Other groups have studied the dependence of the enhancement factor on the gap between adjacent nanoparticles on a SERS active substrate. For example, Gunnarsson et al. investigated SERS on ordered Ag nanoparticle arrays with an interparticle gap above 75 nm. [15] Lee and co-workers were able to achieve the temperature-controlled variation of interparticle gaps between Ag nanoparticles embedded in a polymer membra...

Functionalized arrays of Raman-enhancing nanoparticles for capture and culture-free analysis of bacteria in human blood

“…Ag/AAO-SERS substrates consisting of arrays of Ag-nanoparticles partially embedded in AAO nanochannels were fabricated according to the methods described previously 4,21 . To coat Van, substrates (1×1 cm 2 ) were immersed in the various concentration (100 mM-80 µM) of Van hydrochloride (Sigma) aqueous solution for 1 h, and then dried in the air for 12 h. UV-Visible spectrometry (Metertech, SP-8001) was employed to measure the amount of Van in the solution before and after the coating process.…”

“…A typical SERSactive substrate consists of arrays of nano-scaled metallic objects, for example, Ag nanoparticles and etch-pits on Ag surfaces, which can sustain surface plasmon polariton resonance and enhance the Raman signal of molecules on or near the substrate [1][2][3] . Recently, a type of SERS-active substrate with uniformly large and highly reproducible Raman-enhancing power has been developed by growing Ag nanoparticles on arrays of anodic aluminum oxide (AAO) nanochannels to take advantage of the sub-10-nm inter-particle gaps, which act as 'hot junctions' for creating the electromagnetic enhancement 4 . The high sensitivity and reproducibility of such a substrate-hereafter referred to as Ag/AAO-SERS substrate-facilitated the use of SERS for chemical/biological sensing applications [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] .…”

Plasmonic band gap structures for surface-enhanced Raman scattering

“…It has been previously shown that most favorable condition for SERS is observed when plasmonic resonance absorption lies between the exciting laser and Raman emission wavelengths [12]. With the help of the metallic nanoparticles, the enhancement factors up to 10 15 allowing single molecule detection, have been reported [11,[13][14][15][16][17]. Efforts to understand and develop SERS as an analytical tool are dependent on methods for fabricating templates with stable and reproducibly high activities.…”

Fabrication and Applications of Metal Nanowire Arrays Electrodeposited in Ordered Porous Templates

“…As a thumb of rule, the distribution in the length of electrodeposited nanowires in porous template channels increases with the wire length. Therefore, in order to have uniform length MNWs for SERS studies either the deposition time must be shortened, yielding nanodots rather than nanowires [Kartopu et al, 2006;Wang et al, 2006], or the initially long MNWs must be levelled by an appropriate post treatment, such as mechanical polishing [Kartopu et al, 2008b] and ion milling [Sauer et al, 2005]. In the latter case, a chemical etching may be applied to increase the exposed area of nanowires ( Fig.…”