Recent developments have greatly improved the sensitivity of optical sensors based on metal nanoparticle arrays and single nanoparticles. We introduce the localized surface plasmon resonance (LSPR) sensor and describe how its exquisite sensitivity to size, shape and environment can be harnessed to detect molecular binding events and changes in molecular conformation. We then describe recent progress in three areas representing the most significant challenges: pushing sensitivity towards the single-molecule detection limit, combining LSPR with complementary molecular identification techniques such as surface-enhanced Raman spectroscopy, and practical development of sensors and instrumentation for routine use and high-throughput detection. This review highlights several exceptionally promising research directions and discusses how diverse applications of plasmonic nanoparticles can be integrated in the near future.

Biosensing with plasmonic nanosensors

Localized surface plasmon resonance (LSPR) spectroscopy of metallic nanoparticles is a powerful technique for chemical and biological sensing experiments. Moreover, the LSPR is responsible for the electromagnetic-field enhancement that leads to surface-enhanced Raman scattering (SERS) and other surface-enhanced spectroscopic processes. This review describes recent fundamental spectroscopic studies that reveal key relationships governing the LSPR spectral location and its sensitivity to the local environment, including nanoparticle shape and size. We also describe studies on the distance dependence of the enhanced electromagnetic field and the relationship between the plasmon resonance and the Raman excitation energy. Lastly, we introduce a new form of LSPR spectroscopy, involving the coupling between nanoparticle plasmon resonances and adsorbate molecular resonances. The results from these fundamental studies guide the design of new sensing experiments, illustrated through applications in which researchers use both LSPR wavelength-shift sensing and SERS to detect molecules of chemical and biological relevance.

https://www.photonics.ethz.ch/fileadmin/user_upload/Courses/NanoOptics/willets.pdf

Localized Surface Plasmon Resonance Spectroscopy and Sensing

In 1978 it was discovered, largely through the work of Fleischmann, Van Duyne, Creighton, and their coworkers that molecules adsorbed on specially prepared silver surfaces produce a Raman spectrum that is at times a millionfold more intense than expected. This effect was dubbed surface-enhanced Raman scattering (SERS). Since then the effect has been demonstrated with many molecules and with a number of metals, including Cu, Ag, Au, Li, Na, K, In, Pt, and Rh. In addition, related phenomena such as surface-enhanced second-harmonic generation, four-wave mixing, absorption, and fluorescence have been observed. Although not all fine points of the enhancement mechanism have been clarified, the majority view is that the largest contributor to the intensity amplification results from the electric field enhancement that occurs in the vicinity of small, interacting metal particles that are illuminated with light resonant or near resonant with the localized surface-plasmon frequency of the metal structure. Small in this context is gauged in relation to the wavelength of light. The special preparations required to produce the effect, which include among other techniques electrochemical oxidation-reduction cycling, deposition of metal on very cold substrates, and the generation of metal-island films and colloids, is now understood to be necessary as a means of producing surfaces with appropriate electromagnetic resonances that may couple to electromagnetic fields either by generating rough films (as in the case of the former two examples) or by placing small metal particles in close proximity to one another (as in the case of the latter two). For molecules chemisorbed on SERS-active surface there exists a "chemical enhancement" in addition to the electromagnetic effect. Although difficult to measure accurately, the magnitude of this effect rarely exceeds a factor of 10 and is best thought to arise from the modification of the Raman polarizability tensor of the adsorbate resulting from the formation of a complex between the adsorbate and the metal. Rather than an enhancement mechanism, the chemical effect is more logically to be regarded as a change in the nature and identity of the adsorbate.

Surface-enhanced spectroscopy

Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

Surface raman spectroelectrochemistry: Part I. Heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver electrode

st ra te the applicability of the TFV-ICP system to the analysis of samples over a n extended concentration range. T h e curve covers the four-decade concentration range of 0.001 to 10 fig Be/ml or from 2.5 X 10-11 t o 2.5 x 10-7 gram of Be. Discussion. A comparison of the TFV-ICP detection limits reported in Table I1 with t h e best values so far reported for the introduction of nebulized solution into the plasma shows t h a t the TFV-ICP system is superior by 1 to 2 orders of magnitude. This appears to be the result of the increased concentration of the analyte, already desolvated and vaporized by the filament, passing through the axial channel of the plasma per unit t ime. T h e same mechanism is utilized in AAS and AFS when filament vaporization is performed, bu t for these techniques, the free a toms must be produced a t the filament surface whereas for the plasma system it is only necessary to vaporize the analyte elements; the dissociation and excitation occur in the plasma. t /i' 1 0 U d oooo1 0001 0 01 01 10 10 0 Concentration (+Q B c l m l )

Mode-locked laser Raman spectroscopy. New technique for the rejection of interfering background luminescence signals

A new spectroelectrochemical technique based on the resonance Raman effect is described whereby high resolution vibrational spectra can be obtained for electrogenerated intermediates and products. Both controlled potential coulometric electrolysis in bulk solution and cyclic potential step electrolysis under semiinfinite diffusion mass transport conditions have been tested as electrogeneration modes. Resonance Raman spectra of the tetracyanoethylene anion radical (TCNE.-), which exhibits perfectly reversible electrogeneration behavior, have been obtained in both of these electrogeneration modes with 4880, 4765, and 4579 A Ar+ laser excitation. In addition resonance Raman intensity (RRI) vs. time transients produced by single-shot, double-potential steps at a microelectrode have been obtained for the 2194 cm-I q(Ag) CEN and 1421 cm-I u2(Ag) C=C stretching modes. The Raman transients correlated exactly in time with the corresponding chronoamperometric and transmission chronoabsorptometric signals indicating that the Raman transients and spectra obtained under cyclic potential step conditions result from TCNEmin the electrochemical diffusion layer. The elucidation of electrochemical reaction pathways invariably requires the detection, identification, and kinetic monitoring of intermediates and/or products generated either on the electrode surface or in the adjacent solution. It has been demonstrated that spectroscopic monitoring of electrogenerated species (spectroelectrochemistry) is extremely useful for purposes of identification and structural characterization as well as providing superior molecular specificity for detection and kinetic monitoring relative to the usual electrochemical parameters current, potential, time, etc.' Restricting discussion to electrogenerated species in solution, the spectroscopic techniques which have made the greatest impact on electrochemistry to date are electron spin resonance2 (ESR) and electronic absorption spectroscopy' (uv-visible). ESR is routinely used for identification of paramagnetic species in bulk solution produced by controlled potential electrolysis and more recently has been applied to quantitative kinetic studies of electrogenerated radical ions in the diffusion layer following current step elect r ~ l y s i s . ~ ~ Electronic absorption spectroscopy has proven extremely versatile in that i t can be interfaced to electrochemistry in transmission and internal reflection modes at optically transparent electrodes (OTE's) as well as in the specular reflection mode at bulk metal electrodes. In addition uv-visible spectroscopy is readily applied to the most common mass transport conditions including bulk electrolysis,6 finite diffusion (thin layer electrochemi~try),~ semiinfinite diffusion,' and hydrodynamics.8 The utility of electronic absorption spectroscopy for purposes of species identification is unfortunately limited due to the relatively low information content of uv-visible spectra in fluid solution. However, one finds that because of the high sensitivity of uv-visible spectrophotometric measurements and the large values of electronic absorption extinction coefficients, electronic absorption spectroscopy is most useful for following the kinetics of electrochemical transformations over a wide dynamic range of time from hours to microseconds. Several attempts have been made to couple vibrational spectroscopy to e l ec t r~chemis t ry .~ -~ '~ The motivation for this is the greater degree of molecular specificity and sensitivity to subtle environmental factors inherent in vibrational spectroscopy relative to electronic absorption spectroscopy. The attempts made to use infrared spectroscopy (ir) to study electrogenerated species in solution have met with only limited success because of the following experimental limitations: (1) small free spectral window caused by competitive absorption from the solvent/supporting electrolyte system as well as the electroactive substrate; (2) relatively low spectrophotometric sensitivity due to the small values of infrared extinction coefficients; and (3) rapid time response is limited as a result of the high electrical resistance of infrared transparent electrode materials. Recently it has been suggested that Raman spectroscopy might be more readily applied to electrochemical systems than ir from the viewpoint of free spectral window, ease of cell construction since borosilicate glass will work as a window material, and ease of application to aqueous solutions since water is a poor Raman scatterer.' 5*16 The problem with this approach is the lack of sensitivity inherent in the normal spontaneous Raman scattering process. Typically samples must be 0.1 1.0 M in order to get Raman spectra with acceptable signal-to-noise (SN) ratios even for strong Raman scatterers. This lack of sensitivity would appear to preclude the use of ordinary Raman spectroscopy to observe spectra or kinetic transients in solution at the concentration levels of electroactive substrate normally used in electrochemical experiments. Fleischmann and coworkersI7 have, nevertheless, demonstrated that Raman spectroscopy will be very useful for observing strong Raman scatterers adsorbed on high surface area electrodes as a function of applied electrode potential. In this paper we wish to demonstrate that vibrational spectra of electrogenerated species in bulk solution or in the electrochemical diffusion layer can be readily obtained from solutions originally ca, 1 X 10-3 hf in electroactive substrate by utilizing the resonance Raman effect.Is The primary characteristic of resonance Raman spectroscopy that we capitalize on for application to electrochemistry is that laser excitation within an electronic absorption band results in enhancements of the Raman scattering cross section by factors as large as 105-106 for certain vibrational modes of the electrogenerated species. In addition we will illustrate various advantageous features of resonance Raman spectroelectrochemistry (RRSE) such as: ( I ) use of low resistance bulk metal electrodes for in situ studies in the electrochemical diffusion layer; (2) cell design criteria are relatively unrestrictive from both the Raman spectroscopic and electrochemical points of view; (3) there is essentially no spectral interference from the solvent, supporting electrolyte, or electroactive substrate (unless it is colored); (4) the only requirements for observation of an electrogenerated species by RRSE are that it have a lifetime compatible with its mode of electrogeneration and have an extinction coefficient greater than ca. 1000 M-I cm-' at one (or Van Duyne et al. / Resonance Raman Spectroelectrochemistry

Resonance Raman spectroelectrochemistry. I. Tetracyanoethylene anion radical

Resonance Raman spectroelectrochemistry. 2. Scattering spectroscopy accompanying excitation of the lowest 2B1u excited state of the tetracyanoquinodimethane anion radical

Resonance raman spectroelectrochemistry: V. Intensity transients on the millisecond time scale following double potential step initiation of a diffusion controlled electrode reaction

David L. Jeanmaire

Papers

Surface raman spectroelectrochemistry: Part I. Heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver electrode

Mode-locked laser Raman spectroscopy. New technique for the rejection of interfering background luminescence signals

Resonance Raman spectroelectrochemistry. I. Tetracyanoethylene anion radical

Resonance Raman spectroelectrochemistry. 2. Scattering spectroscopy accompanying excitation of the lowest 2B1u excited state of the tetracyanoquinodimethane anion radical

Resonance raman spectroelectrochemistry: V. Intensity transients on the millisecond time scale following double potential step initiation of a diffusion controlled electrode reaction