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.

We review the syntheses, optical properties, and biological applications of cadmium selenide (CdSe) and cadmium selenide-zinc sulfide (CdSe-ZnS) quantum dots (QDs) and gold (Au) and silver (Ag) nanoparticles (NPs). Specifically, we selected the syntheses of QDs and Au and Ag NPs in aqueous and organic phases, size- and shape-dependent photoluminescence (PL) of QDs and plasmon of metal NPs, and their bioimaging applications. The PL properties of QDs are discussed with reference to their band gap structure and various electronic transitions, relations of PL and photoactivated PL with surface defects, and blinking of single QDs. Optical properties of Ag and Au NPs are discussed with reference to their size- and shape-dependent surface plasmon bands, electron dynamics and relaxation, and surface-enhanced Raman scattering (SERS). The bioimaging applications are discussed with reference to in vitro and in vivo imaging of live cells, and in vivo imaging of cancers, tumor vasculature, and lymph nodes. Other aspects of the review are in vivo deep tissue imaging, multiphoton excitation, NIR fluorescence and SERS imaging, and toxic effects of NPs and their clearance from the body.

Bioconjugated nanomaterials offer endless opportunities to advance both nanobiotechnology and biomedical technology. In this regard, semiconductor nanoparticles, also called quantum dots, are of particular interest for multimodal, multifunctional and multiplexed imaging of biomolecules, cells, tissues and animals. The unique optical properties, such as size-dependent tunable absorption and emission in the visible and NIR regions, narrow emission and broad absorption bands, high photoluminescence quantum yields, large one- and multi-photon absorption cross-sections, and exceptional photostability are the advantages of quantum dots. Multimodal imaging probes are developed by interfacing the unique optical properties of quantum dots with magnetic or radioactive materials. Besides, crystalline structure of quantum dots adds scope for high-contrast X-ray and TEM imaging. Yet another unique feature of a quantum dot is its spacious and flexible surface which is promising to integrate multiple ligands and antibodies and construct multi-functional probes for bioimaging. In this critical review, we will summarize recent advancements in the preparation of biocompatible quantum dots, bioconjugation of quantum dots, and applications of quantum dots and their bioconjugates for targeted and nonspecific imaging of extracellular and intracellular proteins, organelles and functions (181 references).

The purpose of this tutorial review is to provide a comprehensive explanation of plasmon-enhanced spectroscopies, such as plasmon-enhanced Raman scattering, fluorescence, absorption, Rayleigh scattering, and hyper Raman scattering. Plasmon-enhanced spectroscopy implies the spectroscopy of enhanced optical responses of molecules in close proximity to plasmonic nanostructures, resulting in a strong enhancement in sensitivity. In this review, we explain the enhancement in plasmon-enhanced spectroscopy as an optical response of a molecule interacting with an optical resonator, which represents a plasmonic nanostructure, in analogy to cavity quantum optics to easily understand all types of plasmon-enhanced spectroscopy in the same manner. The keys to understanding the enhancement factor of each plasmon-enhanced spectroscopy are a quality factor and a mode volume of plasmonic resonators, which are well-known parameters in the Purcell effect of standard optical cavity resonators.

The electromagnetic ͑EM͒ enhancement in surface-enhanced resonance Raman scattering ͑SERRS͒ is quantitatively evaluated for rhodamine molecules adsorbed on Ag nanostructures. Polarization dependence of the plasma resonance ͑plasmon resonance͒ and the SERRS spectra from single isolated Ag nanostructures was evaluated to determine one-to-one relationship between optical anisotropy of plasma resonance, that of SERRS, and the morphology of the nanostructures. Experimental observations were compared with finitedifference time-domain calculations of the EM field induced by plasma resonance using individual morphology of the nanostructures. The experimental enhancement factor of SERRS ϳ10 9 was consistent with that of the calculations within a factor of ϳ2 for three excitation wavelengths. We conclusively fortify the indispensible importance of SERRS-EM theory with our results to design metal nanostructures generating strong EM enhancement.