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...

Two dimensional arrays of monodispersed Ag-nanoparticles separated by different gaps with sub-10 nm precision are fabricated on anodic alumina substrates with self-organized pores. Light scattering spectra from the arrays evolve with the gaps, revealing plasmonic coupling among the nanoparticles, which can be satisfactorily interpreted by analytical formulae derived from generic dipolar approximation. The general formulism lays down a foundation for predicting the Q factor of an array of metallic nano-particles and its geometric characteristics.

Recently, specific biomarkers in the surface-enhanced Raman scattering (SERS) spectra of bacteria have been successfully exploited for rapid bacterial antibiotic susceptibility testing (AST) - dubbed SERS-AST. The biomolecules responsible for these bacterial SERS biomarkers have been identified as several purine derivative metabolites involved in bacterial purine salvage pathways (W. R. Premasiri, J. C. Lee, A. Sauer-Budge, R. Theberge, C. E. Costello and L. D. Ziegler, Anal. Bioanal. Chem., 2016, 408, 4631). Here we quantified these metabolites in the SERS spectra of Staphylococcus aureus and Escherichia coli using ultra-performance liquid chromatography/electrospray ionization-mass spectrometry (UPLC/ESI-MS). The time dependences of the concentrations of these molecules were measured using C- orC-purine derivatives as internal and external standards respectively in UPLC/ESI-MS measurements. Surprisingly, a single S. aureus and an E. coli cell were found to release millions of adenine and hypoxanthine into a water environment in an hour respectively. Furthermore, simulated SERS spectra of bacterial supernatants based on the mixtures of purine derivatives with measured concentrations also show great similarity with those of the corresponding bacterial samples. Our results not only provide a quantitative foundation for the emerging SERS-AST method but also suggest the potential of exploiting SERS for in situ monitoring the changes in bacterial purine salvage processes in response to different physical and chemical challenges.

Among the strategies for growing one-dimensional nanostructures such as nanotubes and nanowires, a very viable approach is deposition of the desired material into a template with arrays of well-aligned nanochannels.[1±7] A competitive template with such characteristics is the porous anodic aluminum oxide (AAO) film, whose nanochannels can even laterally self-organize into hexagonally close-packed (hcp) domains exhibiting short-range order provided it is grown under specific anodization conditions. [8,9] It has been demonstrated that the range of the order can be further extended by several orders of magnitude using lithographic-guiding techniques, [4,10±12] and that the pore size distributions of such guided arrays are much narrower than those of the self-organized ones. The successful fabrication of such long-range-ordered nanochannel arrays, hereafter referred to simply as ordered arrays, has not only broadened the potential applications of AAO films but also opened possibilities for the fabrication of arrays of nanostructures arranged according to a custom-designed geometry. For example, one can envision an array with only part of it covered by nanodots or nanowires while the rest of the surface area remains empty. Depending on its geometry, such an array, with designed optical and electronic properties, could be used as a photonic crystal and/or a waveguide.[13±16] One of the viable approaches for fabricating such a custom-designed array is to grow the desired material into a template with a partially closed nanochannel array. Herein, we demonstrate a focused ion beam (FIB) directwrite lithographic method for selectively closing part of the channels of an ordered array on an AAO film in order to create a custom-designed nanochannel array. The initial ordered arrays were fabricated by FIB lithographic guiding techniques where the closure of the nanochannels within a certain area was achieved by raster scanning the FIB over the area, thus directly bombarding the AAO film. The successful fabrication of such a template with custom-designed nanochannel arrays opens up numerous possibilities for the creation of nanowire or nanodot arrays with desired geometric patterns. The fabrication process always starts from growing an ordered array by anodizing a finely polished aluminum sample that has been patterned with a guiding lattice on its surface. The lattice is a two-dimensional array of hcp concave pits created by FIB direct-write lithography. An ordered array is achieved when the lattice constant of the guiding lattice is carefully matched with the electrolyte and anodization voltage.[17] For the present work, we set the lattice constant, and therefore the spacing, of the ordered array to be 100 nm, and grew the nanochannels to a typical aspect ratio of larger than » 50. Figure 1a shows a micrograph of a typical ordered array on an AAO film taken by scanning with a 50 keV gallium FIB with a beam current of 1.1 pA and a diameter of » 10 nm over the sample while collecting the secondary electron signal to provide th...