Tapan K. Sau

Properties and Applications of Colloidal Nonspherical Noble Metal Nanoparticles

Nanoparticles of noble metals belong to the most extensively studied colloidal systems in the field of nanoscience and nanotechnology. Due to continuing progress in the synthesis of nanoparticles with controlled morphologies, the exploration of unique morphology-dependent properties has gained momentum. Anisotropic features in nonspherical nanoparticles make them ideal candidates for enhanced chemical, catalytic, and local field related applications. Nonspherical plasmon resonant nanoparticles offer favorable properties for their use as analytical tools, or as diagnostic and therapeutic agents. This Review highlights morphology-dependent properties of nonspherical noble metal nanoparticles with a focus on localized surface plasmon resonance and local field enhancement, as well as their applications in various fields including Raman spectroscopy, fluorescence enhancement, analytics and sensing, photothermal therapy, (bio-)diagnostics, and imaging.

Nonspherical Noble Metal Nanoparticles: Colloid-Chemical Synthesis and Morphology Control

Metal nanoparticles have been the subject of widespread research over the past two decades. In recent years, noble metals have been the focus of numerous studies involving synthesis, characterization, and applications. Synthesis of an impressive range of noble metal nanoparticles with varied morphologies has been reported. Researchers have made a great progress in learning how to engineer materials on a nanometer length scale that has led to the understanding of the fundamental size- and shape-dependent properties of matter and to devising of new applications. In this article, we review the recent progress in the colloid-chemical synthesis of nonspherical nanoparticles of a few important noble metals (mainly Ag, Au, Pd, and Pt), highlighting the factors that influence the particle morphology and discussing the mechanisms behind the nonspherical shape evolution. The article attempts to present a thorough discussion of the basic principles as well as state-of-the-art morphology control in noble metal nanoparticles.

Label-free Biosensing Based on Single Gold Nanostars as Plasmonic Transducers

Gold nanostars provide high sensitivity for single nanoparticle label-free biosensing. The nanostars present multiple plasmon resonances of which the lower energy ones, corresponding to the nanostar tips and core-tip interactions, are the most sensitive to environmental changes. Streptavidin molecules are detected upon binding to individual, biotin-modified gold nanostars by spectral shifts in the plasmon resonances. Concentrations as low as 0.1 nM produce a shift of the tip related plasmon resonances of about 2.3 nm (5.3 meV).

Single gold nanostars enhance Raman scattering

Raman scattering from single gold nanostars coated with self-assembled monolayers of 4-mercaptobenzoic acid is observed. The Raman signal can be detected without the need of forming particle aggregates or resonant electronic excitation of the analyte. A total Raman enhancement factor of 107 for individual nanostars is estimated. These features make gold nanostars promising for Raman imaging in cells or cellular membranes.Raman scattering from single gold nanostars coated with self-assembled monolayers of 4-mercaptobenzoic acid is observed. The Raman signal can be detected without the need of forming particle aggregates or resonant electronic excitation of the analyte. A total Raman enhancement factor of 107 for individual nanostars is estimated. These features make gold nanostars promising for Raman imaging in cells or cellular membranes.

Selective Excitation of Individual Plasmonic Hotspots at the Tips of Single Gold Nanostars

Plasmonic hotspots in single gold nanostars are located at the tips and can be excited selectively by laser light as evidenced by photoelectron emission microscopy. Selectivity is achieved through wavelength and polarization of the excitation light. Comparing photoelectron emission intensity and dark-field scattering spectra of the same individual nanostars reveals differences in terms of observable plasmon resonance wavelengths and field enhancements. Differences are explained with the underlying near- and far-field processes of the two techniques.

Complex-shaped Metal Nanoparticles: Bottom-Up Syntheses and Applications

Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, UK Email: ashfi l@matthey.com Introduction “Complex-shaped Metal Nanoparticles: Bottom-Up Syntheses and Applications” offers a comprehensive review of shaped metal nanoparticles through synthetic strategies, theoretical modelling of growth, discussion of properties and present and future applications. The book is brought together by editors Tapan K. Sau (International Institute of Information Technology, Hyderabad, India) and Andrey L. Rogach (Department of Physics and Materials Science at the City University of Hong Kong). Between them, they draw on their considerable expertise in the synthesis of metal and semiconductor nanoparticles, spectroscopy, photonics and applications of nanomaterials, to combine 16 chapters from a large number of specialist authors. This review will cover the majority of the book, which refers in the main to noble metal particles, with the exception of a few chapters which are specifi cally related to non-platinum group metal (pgm) materials and are therefore beyond the scope of this review. The fi eld of nanoparticle preparation has enjoyed an explosion in interest in the last decade as new applications exploiting the novel physical, electronic and optical properties of the particles have been discovered. The properties of nanoparticles are highly dependent on their morphology and thus, a vast number of academic articles have been published tackling the subject of the synthesis of specifi c shapes of nanomaterials. “Complex-shaped Metal Nanoparticles: Bottom-Up Syntheses and Applications” aims to bring together this research in one volume giving a sound understanding of the general principles, with copious references to more detailed research papers if required and looking towards potential future applications.

Plasmonic Properties of Single Multispiked Gold Nanostars: Correlating Modeling with Experiments

Gold nanostars, possessing multiple sharp spikes, have emerged as promising plasmonic particles in the field of ultrasensitive sensing. We have developed a water-based method for high-yield synthesis of size-tunable anisotropic gold nanoparticles with a varying number of spiky surface protrusions, and performed systematic experimental and theoretical analyses of the optical properties of the single gold nanostars by characterizing them simultaneously with scanning electron microscopy and dark-field scattering spectroscopy. The morphologies and corresponding scattering spectra of the individual gold nanostars have been compared with electromagnetic simulations of the plasmonic resonances utilizing the finite-difference time-domain (FDTD) method. The study provides a correlation between the experimental and calculated scattering spectra and charge distributions of the different plasmon modes in the individual gold nanostars with varying numbers and relative orientations of surface protrusions. Our results provide guidelines for choosing gold nanostars with a proper number of spikes and appropriate dimensions of the core and arms for particular plasmonic applications as well as for further developing preparation methods of multispiked metal nanoparticles.