Probing plasmonic nanostructures by photons and electrons.

We discuss recent developments for studying plasmonic metal nanostructures. Exploiting photons and electrons opens up new capabilities to probe the complete plasmon spectrum including bright and dark modes and related local optical fields at subnanometer spatial resolution. This comprehensive characterization of plasmonic properties of metal nanostructures provides new basic insight into the fundamental physics of "surface enhanced" spectroscopy in hottest hot spots and enables us to optimize plasmon supported processes and devices.

Surface-enhanced Raman scattering on aluminum using near infrared and visible excitation.

We observed strong surface-enhanced Raman scattering on discontinuous nanostructured aluminum films using 785 nm excitation even though dielectric constants of this metal suggest plasmon supported spectroscopy in the ultraviolet range. The excitation of SERS correlates with plasmon resonances in the 1.3-2.5 eV range identified in electron energy loss spectra.

Probing the plasmonic near-field by one- and two-photon excited surface enhanced Raman scattering.

Strongly enhanced and spatially confined near-fields in the vicinity of plasmonic nanostructures open up exciting new capabilities for photon-driven processes and particularly also for optical spectroscopy. Surface enhanced Raman signatures of single molecules can provide us with important information about the optical near-field. We discuss one- and two-photon excited surface enhanced Raman scattering at the level of single molecules as a tool for probing the plasmonic near-field of silver nanoaggregates. The experiments reveal enhancement factors of local fields in the hottest hot spots of the near-field and their dependence on the photon energy. Also, the number of the hottest spots and their approximate geometrical size are found. Near-field amplitudes in the hottest spots can be enhanced by three orders of magnitudes. Nanoaggregates of 100 nm dimensions provide one hot spot on this highest enhancement level where the enhancement is confined within less than 1nm dimension. The near-field enhancement in the hottest spots increases with decreasing photon energy.

Surface enhanced Raman scattering on Tardigrada--towards monitoring and imaging molecular structures in live cryptobiotic organisms.

Tardigrades are microscopic metazoans which are able to survive extreme physical and chemical conditions by entering a stress tolerant state called cryptobiosis. At present, the molecular mechanisms behind cryptobiosis are still poorly understood. We show that surface enhanced Raman scattering supported by plasmonic gold nanoparticles can measure molecular constituents and their local distribution in live tardigrades. Surface enhanced Raman signatures allow to differentiate between two species and indicate molecular structural differences between tardigrades in water and in a dry state. This opens new avenues for exploring cryptobiosis by studying molecular changes in live cryptobiotic organisms.

Novel optical nanosensors for probing and imaging live cells.

This review introduces multifunctional optical nanosensors based on surface-enhanced Raman scattering (SERS) and demonstrates their application in live cells. The novel nanosensors have the potential to improve our understanding of cellular processes on the molecular level. The hybrid sensor consists of gold or silver nanoparticles with an attached reporter species. The sensor can be detected and imaged based on the SERS signature of the reporter. This results in several advantages, such as high spectral specificity, multiplex capabilities, improved contrast, and photostability. SERS sensors not only highlight cellular structures, based on enhanced Raman spectra of intrinsic cellular molecules measured in the local optical fields of the gold nanoparticles, they also provide molecular structural information on their cellular environment. Moreover, the SERS signature of the reporter can deliver information on the local pH value inside a cell at subendosomal resolution. SERS sensors are suitable for one- and two-photon excitation. FROM THE CLINICAL EDITOR: This review introduces multifunctional optical nanosensors based on surface enhanced Raman scattering (SERS) and demonstrates their application in live cells. These hybrid sensors consist of gold or silver nanoparticles with an attached reporter species. The sensor can be detected and imaged based on the SERS signature of the reporter. SERS sensors highlight cellular structures and provide molecular structural information on their cellular environment. They can also deliver information on the intracellular pH-value at subendosomal resolution.

Gold nanolenses generated by laser ablation-efficient enhancing structure for surface enhanced Raman scattering analytics and sensing.

Nanoaggregates formed by metal spheres of different radii and interparticle distances represent finite, deterministic, self-similar systems that efficiently concentrate optical fields and act as "nanolenses". Here we verify experimentally the theoretical concept of nanolenses and explore their potential as enhancing nanostructures in surface enhanced Raman scattering (SERS). Self-similar structures formed by gold nanospheres of different sizes are generated by laser ablation from solid gold into water. These nanolenses exhibit SERS enhancement factors on the order of 10(9). The "chemically clean" preparation process provides several advantages over chemically prepared nanoaggregates and makes the stable and biocompatible gold nanolenses potent enhancing structures for various analytical and sensing applications.

SERS--a single-molecule and nanoscale tool for bioanalytics.

Surface enhanced Raman scattering (SERS) at extremely high enhancement level turns the weak inelastic scattering effect of photons on vibrational quantum states into a structurally sensitive single-molecule and nanoscale probe. The effect opens up exciting opportunities for applications of vibrational spectroscopy in biology. The concept of SERS can be extended to two-photon excitation by exploiting surface enhanced hyper-Raman scattering (SEHRS). This critical review introduces the physics behind single-molecule SERS and discusses the capabilities of the effect in bioanalytics (100 references).

One- and two-photon excited optical ph probing for cells using surface-enhanced Raman and hyper-Raman nanosensors.

We demonstrate spatially resolved probing and imaging of pH in live cells by mobile and biocompatible nanosensors using surface-enhanced Raman scattering (SERS) of 4-mercaptobenzoic acid (pMBA) on gold nanoaggregates. Moreover, we also show that this concept of pH nanosensors can be extended to two-photon excitation by using surface-enhanced hyper-Raman scattering (SEHRS). In addition to the advantages of two-photon excitation, the SEHRS sensor enables measurements over a wide pH range without the use of multiple probes.

Two-photon vibrational spectroscopy for biosciences based on surface-enhanced hyper-Raman scattering.

Two-photon excitation is gaining rapidly in interest and significance in spectroscopy and microscopy. Here we introduce a new approach that suggests versatile optical labels suitable for both one- and two-photon excitation and also two-photon-excited ultrasensitive, nondestructive chemical probing. The underlying spectroscopic effect is the incoherent inelastic scattering of two photons on the vibrational quantum states called hyper-Raman scattering (HRS). The rather weak effect can be strengthened greatly if HRS takes place in the local optical fields of gold and silver nanostructures. This so-called surface-enhanced HRS (SEHRS) is the two-photon analogue to surface-enhanced Raman scattering (SERS). SEHRS provides structurally sensitive vibrational information complementary to those obtained by SERS. SEHRS combines the advantages of two-photon spectroscopy with the structural information of vibrational spectroscopy and the high-sensitivity and nanometer-scale local confinement of plasmonics-based spectroscopy. We infer effective two-photon cross-sections for SEHRS on the order of 10(-46) to 10(-45) cm4 x s, similar to or higher than the best "action" cross-sections (product of the two-photon absorption cross-section and fluorescence quantum yield) for two-photon fluorescence, and we demonstrate HRS on biological structures such as single cells after incubation with gold nanoparticles.

In vivo molecular probing of cellular compartments with gold nanoparticles and nanoaggregates.

Surface-enhanced Raman (SERS) signatures were measured from single living cells at different times after the uptake of gold nanoparticles. The spectra are indicative of chemical changes in the environment of the nanostructures over time. The increase of the SERS signal strength and parallel TEM studies indicate the formation of nanoaggregates providing optimum SERS enhancement for ultrasensitive probing inside the endosomal compartment. The results have implications for medical and biotechnology applications of SERS nanosensors in cells.

Surface-enhanced Raman scattering in local optical fields of silver and gold nanoaggregates-from single-molecule Raman spectroscopy to ultrasensitive probing in live cells.

This Account discusses surface-enhanced Raman scattering at extremely high enhancement levels that can occur for molecules attached to silver and gold nanoclusters. Strongly enhanced and highly confined local optical fields enable surface-enhanced Stokes and anti-Stokes Raman spectroscopy of single molecules even under nonresonant excitation conditions as well as extremely large effective cross sections in two-photon excited Raman spectroscopy. The ability for very sensitive and spatially confined molecular structural probing makes gold and silver nanoclusters very promising tools for studies of small structures in biological materials, such as cellular compartments.

SERS signals at the anti Stokes side of the excitation laser in extremely high local optical fields of silver and gold nanoclusters.

Surface-enhanced anti-Stokes Raman scattering from pumped excited vibrational levels and surface-enhanced hyper Raman scattering show a quadratic dependence on the excitation intensity and are discussed as incoherent two-photon excited Raman processes performed in strongly enhanced local optical fields of silver- or gold nanoclusters, where both effects can experience very similar electromagnetic enhancement conditions.

Surface-enhanced Raman optical activity on adenine in silver colloidal solution.

We report the collection of Raman optical activity (ROA) spectra of adenine in silver colloidal solution, that is, surface-enhanced Raman optical activity (SEROA) using considerably shorter data acquisition times, reduced excitation power, and lower concentration, as compared to classical ROA measurements on molecules of biological interest so far reported in the literature. These improvements in experimental parameters for ROA measurements can be explained by enhanced Raman signals in the local optical fields of the silver nanoparticles and by at least 1 order of magnitude higher values for circular intensity differences (CIDs), as compared to classical ROA that has been suggested before and theoretically discussed in terms of large field gradients near a metal surface. The measured ROA effect for adenine can be understood in terms of adsorption-induced chirality in the prochiral molecules on the silver nanoparticles. Surface-enhanced Raman optical activity offers potential capabilities for sensitive, rapid, stereochemical characterization of basic building blocks of biopolymers, such as amino acids and nucleosides, as well as biologically active molecules, in particular, also for probing organization and self-assembling of such molecules on metal surfaces.

Optical probes for biological applications based on surface-enhanced Raman scattering from indocyanine green on gold nanoparticles.

We report surface-enhanced Raman scattering (SERS) studies on indocyanine green (ICG) on colloidal silver and gold and demonstrate a novel optical probe for applications in living cells. In addition to its own detection by the characteristic ICG SERS signatures, the ICG gold nanoprobe delivers spatially localized chemical information from its biological environment by employing SERS in the local optical fields of the gold nanoparticles. The probe offers the potential to increase the spectral specificity and selectivity of current chemical characterization approaches of living cells and biomaterials based on vibrational information.

Surface-enhanced Raman scattering on single-wall carbon nanotubes.

Exploiting the effect of surface-enhanced Raman scattering (SERS), the Raman signal of single-wall carbon nanotubes (SWNTs) can be enhanced by up to 14 orders of magnitude when the tubes are in contact with silver or gold nanostructures and Raman scattering takes place predominantly in the enhanced local optical fields of the nanostructures. Such a level of enhancement offers exciting opportunities for ultrasensitive Raman studies on SWNTs and allows resonant and non-resonant Raman experiments to be done on single SWNTs at relatively high signal levels. Since the optical fields are highly localized within so-called "hot spots" on fractal silver colloidal clusters, lateral confinement of the Raman scattering can be as small as 5 nm, allowing spectroscopic selection of a single nanotube from a larger population. Moreover, since SWNTs are very stable "artificial molecules" with a high aspect ratio and a strong electron-phonon coupling, they are unique "test molecules" for investigating the SERS effect itself and for probing the "electromagnetic field contribution" and "charge transfer contribution" to the effect. SERS is also a powerful tool for monitoring the "chemical" interaction between the nanotube and the metal nanostructure.