Optical detection and spectroscopy of single molecules and single nanoparticles have been achieved at room temperature with the use of surface-enhanced Raman scattering. Individual silver colloidal nanoparticles were screened from a large heterogeneous population for special size-dependent properties and were then used to amplify the spectroscopic signatures of adsorbed molecules. For single rhodamine 6G molecules adsorbed on the selected nanoparticles, the intrinsic Raman enhancement factors were on the order of 10 14 to 10 15 , much larger than the ensemble-averaged values derived from conventional measurements. This enormous enhancement leads to vibrational Raman signals that are more intense and more stable than single-molecule fluorescence.

http://science.sciencemag.org/content/sci/275/5303/1102.full.pdf

Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering

This feature article highlights work from the authors' laboratories on the synthesis, assembly, reactivity, and optical applications of metallic nanoparticles of nonspherical shape, especially nanorods. The synthesis is a seed-mediated growth procedure, in which metal salts are reduced initially with a strong reducing agent, in water, to produce ∼4 nm seed particles. Subsequent reduction of more metal salt with a weak reducing agent, in the presence of structure-directing additives, leads to the controlled formation of nanorods of specified aspect ratio and can also yield other shapes of nanoparticles (stars, tetrapods, blocks, cubes, etc.). Variations in reaction conditions and crystallographic analysis of gold nanorods have led to insight into the growth mechanism of these materials. Assembly of nanorods can be driven by simple evaporation from solution or by rational design with molecular-scale connectors. Short nanorods appear to be more chemically reactive than long nanorods. Finally, optical applica...

Anisotropic metal nanoparticles: Synthesis, assembly, and optical applications

12.2.2. Four-Wave Mixing (FWM) 4849 12.2.3. Dye Aggregation 4850 12.2.4. Optoelectronic Nanodevices 4850 12.3. Sensor 4851 12.3.1. Chemical Sensor 4851 12.3.2. Biological Sensor 4851 12.4. Catalysis 4852 13. Conclusion and Perspectives 4852 14. Abbreviations 4853 15. Acknowledgements 4854 16. References 4854 * Corresponding author E-mail: tpal@chem.iitkgp.ernet.in. † Raidighi College. § Indian Institute of Technology. 4797 Chem. Rev. 2007, 107, 4797−4862

Interparticle Coupling Effect on the Surface Plasmon Resonance of Gold Nanoparticles: From Theory to Applications

In the Raman effect, incident light is inelastically scattered from a sample and shifted in frequency by the energy of its characteristic molecular vibrations. Since its discovery in 1927, the effect has attracted attention from a basic research point of view as well as a powerful spectroscopic technique with many practical applications. The advent of laser light sources with monochromatic photons at high flux densities was a milestone in the history of Raman spectroscopy and resulted in dramatically improved scattering signals (for a general overview of modern Raman spectroscopy, see refs 1-5). In addition to this so-called spontaneous or incoherent Raman scattering, the development of lasers also opened the field of stimulated or coherent Raman spectroscopies, in which molecular vibrations are coherently excited. Whereas the intensity of spontaneous Raman scattering depends linearly on the number of probed molecules, the coherent Raman signal is proportional to the square of this number (for an overview, see refs 6 and 7). Coherent Raman techniques can provide interesting new opportunities such as vibrational imaging of biological samples,8 but they have not yet advanced the field of ultrasensitive trace detection. Therefore, in the following article, we shall focus on the spontaneous Raman effect, in the following simply called Raman scattering. Today, laser photons over a wide range of frequencies from the near-ultraviolet to the near-infrared region are used in Raman scattering studies, allowing selection of optimum excitation conditions for each sample. By choosing wavelengths which excite appropriate electronic transitions, resonance Raman studies of selected components of a sample or parts of a molecule can be performed.9 In the past few years, the range of excitation wavelengths has been extended to the near-infrared (NIR) region, in which background fluorescence is reduced and photoinduced degradation from the sample is diminished. High-intensity NIR diode lasers are easily available, making this region attractive for compact, low cost Raman instrumentation. Further, the development of low noise, high quantum efficiency multichannel detectors (chargecoupled device (CCD) arrays), combined with highthroughput single-stage spectrographs used in combination with holographic laser rejection filters, has led to high-sensitivity Raman spectrometers (for an overview on state-of-the-art NIR Raman systems, see ref 10). As we shall show in section 2, the nearinfrared region also has special importance for ultrasensitive Raman spectroscopy at the singlemolecule level. As with optical spectroscopy, the Raman effect can be applied noninvasively under ambient conditions in almost every environment. Measuring a Raman spectrum does not require special sample preparation techniques, in contrast with infrared absorption spectroscopy. Optical fiber probes for bringing excitation laser light to the sample and transporting scattered light to the spectrograph enable remote detection of Raman signals. Furthermore, the spatial and temporal resolution of Raman scattering are determined by the spot size and pulse length, respectively, of the excitation laser. By using a confocal microscope, Raman signals from femtoliter volumes (∼1 μm3) can by observed, enabling spatially resolved measurements in chromosomes and cells.11 Techniques such as multichannel Hadamard transform Raman microscopy12,13 or confocal scanning Fourier transform Raman microscopy14 allow generation of high-resolution Raman images of a sample. Recently, Raman spectroscopy was performed using near-field optical microscopy.15-17 Such techniques overcome the diffraction limit and allow volumes significantly smaller than the cube of the wavelength to be investigated. In the time domain, Raman spectra can be measured on the picosecond time scale, providing information on short-lived species such as excited 2957 Chem. Rev. 1999, 99, 2957−2975

Ultrasensitive Chemical Analysis by Raman Spectroscopy

The structure of the light-harvesting chlorophyll a/b-protein complex, an integral membrane protein, has been determined at 3.4 A resolution by electron crystallography of two-dimensional crystals. Two of the three membrane-spanning alpha-helices are held together by ion pairs formed by charged residues that also serve as chlorophyll ligands. In the centre of the complex, chlorophyll a is in close contact with chlorophyll b for rapid energy transfer, and with two carotenoids that prevent the formation of toxic singlet oxygen.

Atomic model of plant light-harvesting complex by electron crystallography

Fluorescence enhancement was studied on silver colloidal metal films (CMFs) using two systems:  (1) Langmuir−Blodgett monolayers of fluorescein-labeled phospholipids separated from the surface of the films by spacer layers of octadecanoic acid and (2) biotin−fluorescein conjugates captured by avidin molecules adsorbed on top of a multilayer structure formed by alternating layers of bovine serum albumin−biotin conjugate (BSA−biotin) and avidin. The dependence of fluorescence intensity on the number of lipid or protein spacer layers deposited on the surface of the CMF was investigated. The results demonstrate the requirement for adsorbate location within the region between Ag particles for maximal enhancement. The density of avidin molecules on the surface of the BSA−biotin/avidin multilayers adsorbed on the CMF was also determined. A procedure for forming a rigid, uniform silica layer around the Ag particles on the CMF is described. The layer protects the particles from undesirable chemical reactions such ...

Enhancement of Molecular Fluorescence near the Surface of Colloidal Metal Films

Laser ablation of metals is a new and very powerful method for preparation of surface-enhanced Raman scattering (SERS) active colloids. The method is characterized by its simplicity and versatility. Stable Ag, Au, Pt, Pd, and Cu colloids are prepared by ablation of the metal for ~ 10 min in water and organic solvents. An important advantage of this approach over conventional chemical procedures is that the colloids are free of organic or ionic species. Consequently, the chemical and physical effects of ions or other adsorbates can be studied under carefully controlled conditions. The SERS activity of colloidal metals prepared by laser ablation is comparable or superior to that of chemically prepared colloids.

Laser Ablation of Metals: A New Method for Preparing SERS Active Colloids

Colloidal films of gold and silver were prepared on glass or quartz slides. The slides were derivatized with (3-mercaptopropyl)trimethoxysilane and subsequently reacted with aqueous metal colloids for variable time periods. The formation of the sulfur-metal bond provides a stable colloidal film on the surface. Because of the electrostatic interaction between individual particles, a semiregular structure is produced, as can be seen from electron micrographs. The unique property of the colloidal film is that they possess the optical properties of colloidal metals and the convenience of solid substrates. The effect of the dielectric constant of solvents on the optical frequencies, as well as the specific interaction of the solvent molecules with the metal on the plasmon resonances, was examined in detail. The colloidal films exhibit strong enhancement of Raman scattering and fluorescence emission from molecules adsorbed on the surface. Enhancement of fluorescence was observed for fluorescein-labeled molecules spaced 0-200 A away from the surface. These substrates can be used in a number of analytical applications, such as surface-enhanced spectroscopies as well as for fundamental studies of plasmon resonances in small metal particles. 40 refs., 6 figs., 1 tab.

Colloidal metal films as a substrate for surface-enhanced spectroscopy

Silver colloids of 20 nm mean particle diameter were prepared by laser ablation and modified by adsorption of iodide and bromide ions. Addition of cytochrome c to this colloid resulted in the reduction of the protein, which was monitored by surface-enhanced resonance Raman scattering and absorption spectroscopies. Colloidal metal films, prepared from the same Ag colloid, were employed to minimize contributions from aggregation. Effects of surface modification on the Ag plasmon resonance were studied in both colloidal suspensions and colloidal metal films. The conclusion was made that adsorption of I- and Br- results in charging of the Ag particle as a whole and a shift of its potential to more negative values. The donated charge is delocalized in a thin surface layer and does not significantly affect the plasmon resonance frequency of the particle.

Reduction of Cytochrome c by Halide-Modified, Laser-Ablated Silver Colloids

This review summarizes the literature relating to the application of surface-enhanced Raman scattering (SERS) and surface-enhanced resonance Raman scattering (SERRS) techniques to the study of biological molecules. The emphasis is on publications that have appeared during the period from 1985 to 1991. The review is divided into six major parts. First, a brief overview of the current understanding of the mechanistic aspects of SERS/SERRS is given, with an emphasis on the relationship between theoretical predictions and experimental results. The most common experimental systems (colloids, metal island films, and electrodes) are described. Studies of biological systems are described in the second (small molecules), third (DNA and proteins) and fourth (membranes proteins and membrane preparations) sections. In the fifth or conclusion section, the potential use of SERS or SERRS as a method for obtaining spectra of native biological molecules is evaluated. Finally, the sixth section describes advances in Raman instrumentation in terms of their possible impact on future applications of SERS/SERRS techniques to biological molecules.

Therese M. Cotton

Papers

Enhancement of Molecular Fluorescence near the Surface of Colloidal Metal Films

Laser Ablation of Metals: A New Method for Preparing SERS Active Colloids

Colloidal metal films as a substrate for surface-enhanced spectroscopy

Reduction of Cytochrome c by Halide-Modified, Laser-Ablated Silver Colloids

Application of surface-enhanced Raman spectroscopy to biological systems