The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

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Arrays of nanoholes in a gold film were used to monitor the binding of organic and biological molecules to the metallic surface. This technique is particularly sensitive to surface binding events because it is based upon the resonant surface plasmon enhanced transmission through the array of nanoholes. The sensitivity was found to be 400 nm per refractive index unit, which is comparable to other grating-based surface plasmon resonance (SPR) devices. The array of nanoholes is well suited for dense integration in a sensor chip. Furthermore, the optical geometry is collinear, which simplifies the alignment with respect to the traditional Kretschmann (reflection) arrangement for SPR sensing.

Periodic arrays of sub-wavelength apertures (nanoholes) in ultrathin Au films were used as substrates for enhanced-Raman spectroscopy in the optical range. Nanohole-enhanced (resonance) Raman scattering from oxazine 720 (oxa) adsorbed on arrays of different periodicities (distance between the center of the holes) was obtained. The overall Raman intensity of the adsorbed molecule was dependent on the periodicity of these arrays. The enhancement factor reached a maximum for the array that presented the largest transmission at the excitation wavelength of the laser. This shows that the enhancement of the Raman signal is provided by surface plasmon (SP) modes excited at the array of nanoholes. SP excitations lead to spatial localization of the electromagnetic fields in nanometric regions close to the surface. This field localization, allied to the unique vibrational signature of the Raman scattering and the simplified optical arrangement from the transmission optics, suggests that arrays of nanoholes should be useful for the fabrication of dense biochips for the detection of Raman-labeled analytes with high sensitivity and selectivity.Surface plasmons (SP) are electromagnetic waves that propagate along a metal-dielectric interface. 1 The excitation of SP modes leads to a strong concentration of light at the surface on the sub-wavelength scale. Advances in nanofabrication allow the creation of organized structures that can take full advantage of this spatial localization. The periodic arrays of sub-wavelength apertures (nanoholes) in metallic thin films are among the most promising of these structures for applications in photonic circuits and light manipulation at the sub-wavelength range. 2,3 The arrays of nanoholes enable an increase in the transmission of light by several orders of magnitude when the SP resonance condition is achieved. 2 This phenomenon is being explored for possible applications in several relevant fields, ranging from quantum information processing 4 to nanolithography.5 In addition, it has been demonstrated that the polarization properties of the transmitted light can also be manipulated by tailoring the shape of the nanoholes. 6,7 This level of control provides the basis for the development of sub-wavelength polarizers and switches, which are essential elements in useful nanophotonics architectures.The main application of SPs in biochemical and biomedical sciences is in chemical sensing. 8 Surface plasmon resonance (SPR) is among the most used techniques for monitoring binding events in biological systems. 9 Traditionally, an SPR measurement consists of the excitation of extended SP modes through prism coupling, using a reflection geometry (Kretschmann configuration). These angleresolved SPR devices are sensitive to surface processes at the submonolayer level. 10 Periodic arrays of nanoholes in thin gold films can also be used as SPR sensors that operate in transmission mode. 11 In this case, the surface processes are monitored by the shift in the wavelength of the maximum transmission...

[Reaction: see text]. Plasmonic-based chemical sensing technologies play a key role in chemical, biochemical, and biomedical research, but basic research in this area is still attracting interest. Researchers would like to develop new types of plasmonic nanostructures that can improve the analytical figures of merit, such as detection limits, sensitivity, selectivity, and dynamic range, relative to the commercial systems. They are also tackling issues such as cost, reproducibility, and multiplexing with the goal of providing the best plasmonic-based platform for chemical analysis. In this Account, we will describe recent advances in the optical and spectroscopic properties of nanohole arrays in thin gold films and their applications for chemical sensing. These nanostructures support the unusual phenomenon of "extraordinary optical transmission" (EOT), that is, they are more transparent at certain wavelengths than expected by the classical aperture theory. The EOT is a consequence of surface plasmon (SP) excitations; hence, the resonance should respond to the adsorption of organic molecules. We explored this effect and implemented the integration of the arrays of nanoholes as sensing elements in a microfluidic architecture. We then demonstrated how these devices could be applied in biochemical affinity tests. Arrays of nanoholes offer a small sensing footprint and operate at normal transmission mode, which make them more suitable for miniaturization. This new approach for SPR sensing is more compatible with the lab-on-chip concept and offers the possibility of high-throughput analysis from a single sensing chip. We explored the field localization properties of EOT for surface-enhanced spectroscopy. We could control the enhancement factors for SERS and SEFS by adjusting the geometry of the arrays. The shape of the individual nanoholes offers another handle to tune the enhancement factor for surface-enhanced spectroscopy and SPR sensitivity. Apexes in shaped nanostructures function as optical antennas, focusing the light at extremely small regions at the tips. We observed additional surface enhancement by tuning the apexes' properties. The extra enhancement in these cases originated only from the small number of molecules in the apex regions. The arrays of nanoholes are an exciting new substrate for chemical sensing and enhanced spectroscopy. This class of nanomaterials has the potential to provide a viable alternative to the commercial SPR-based sensors. Further research could exploit this platform to develop nanostructures that support high field localization for single-molecule spectroscopy.

Further progress in the applications of self-assembled nanostructures critically depends on developing a fundamental understanding of the relation between the properties of nanoparticle ensembles and their time-dependent structural characteristics. Following dynamic generation of hot-spots in the self-assembled chains of gold nanorods, we established a direct correlation between ensemble-averaged surface-enhanced Raman scattering and extinction properties of the chains. Experimental results were supported with comprehensive finite-difference time-domain simulations. The established relationship between the structure of nanorod ensembles and their optical properties provides the basis for creating dynamic, solution-based, plasmonic platforms that can be utilized in applications ranging from sensing to nanoelectronics.

We combine nanofluidics and nanoplasmonics for surface-plasmon resonance (SPR) sensing using flow-through nanohole arrays. The role of surface plasmons on resonant transmission motivates the application of nanohole arrays as surface-based biosensors. Research to date, however, has focused on dead-ended holes, and therefore failed to harness the benefits of nanoconfined transport combined with SPR sensing. The flow-through format enables rapid transport of reactants to the active surface inside the nanoholes, with potential for significantly improved time of analysis and biomarker yield through nanohole sieving. We apply the flow-through method to monitor the formation of a monolayer and the immobilization of an ovarian cancer biomarker specific antibody on the sensing surface in real-time. The flow-through method resulted in a 6-fold improvement in response time as compared to the established flow-over method.