We describe the development of multifunctional nanoparticle probes based on semiconductor quantum dots (QDs) for cancer targeting and imaging in living animals. The structural design involves encapsulating luminescent QDs with an ABC triblock copolymer and linking this amphiphilic polymer to tumor-targeting ligands and drug-delivery functionalities. In vivo targeting studies of human prostate cancer growing in nude mice indicate that the QD probes accumulate at tumors both by the enhanced permeability and retention of tumor sites and by antibody binding to cancer-specific cell surface biomarkers. Using both subcutaneous injection of QD-tagged cancer cells and systemic injection of multifunctional QD probes, we have achieved sensitive and multicolor fluorescence imaging of cancer cells under in vivo conditions. We have also integrated a whole-body macro-illumination system with wavelength-resolved spectral imaging for efficient background removal and precise delineation of weak spectral signatures. These results raise new possibilities for ultrasensitive and multiplexed imaging of molecular targets in vivo.

Multicolor optical coding for biological assays has been achieved by embedding different-sized quantum dots (zinc sulfide-capped cadmium selenide nanocrystals) into polymeric microbeads at precisely controlled ratios. Their novel optical properties (e.g., size-tunable emission and simultaneous excitation) render these highly luminescent quantum dots (QDs) ideal fluorophores for wavelength-and-intensity multiplexing. The use of 10 intensity levels and 6 colors could theoretically code one million nucleic acid or protein sequences. Imaging and spectroscopic measurements indicate that the QD-tagged beads are highly uniform and reproducible, yielding bead identification accuracies as high as 99.99% under favorable conditions. DNA hybridization studies demonstrate that the coding and target signals can be simultaneously read at the single-bead level. This spectral coding technology is expected to open new opportunities in gene expression studies, high-throughput screening, and medical diagnostics.

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.

Enhanced fluorescence from carbon nanotubes and advances in near-infrared cameras have opened up a new wavelength window for small animal imaging.Near-infrared light (700-2,500 nm) can penetrate biological tissues such as skin and blood more efficiently than visible light because these tissues scatter and absorb less light at longer wavelengths. Simply hold your hand in sunlight and your fingers will glow red owing to the preferential transmission of red and near-infrared light. At wavelengths longer than 950 nm, however, this effect diminishes owing to increased absorption by water and lipids. Nonetheless, a clear window exists at wavelengths between 650 nm and 950 nm for optical imaging of live animals 1,2 (Fig. 1). In practice, however, this window is not optimal because tissue autofluorescence produces substantial background noise and the tissue penetration depth is limited to between 1 and 2 cm (ref. 3 ).In 2003, simulations and modelling studies 4 of optical imaging in turbid media such as tissue or blood suggested that it would be possible to improve signal-to-noise ratios by over 100-fold by using quantum dot fluorophores that emit light at 1,320 nm (instead of 850 nm). However, the lack of biocompatible fluorescent probes in this second near-infrared window between 1,000 and 1,350 nm has prevented the use of this highly sensitive spectral range for in vivo imaging.On page 773 of this issue, Hongjie Dai and colleagues from Stanford University and Soochow University in China report a way to generate biocompatible fluorescent single-walled carbon nanotubes that emit between 950 and 1,400 nm (ref. 5 ). These bright nanotubes allowed deep and highly sensitive in vivo imaging of blood vessels immediately beneath and through the deep layers of skin.Single-walled carbon nanotubes are grown on solid substrates, so they must be detached, debundled and coated with a suitable hydrophilic layer before they can be used as soluble fluorophores. Dai and co-workers observed that the frequently used sonochemical method for coating the tubes with a layer of biocompatible phospholipids shortens the tubes and creates defects that quench the fluorescence. However, they found that the tubes remained fluorescent and intact if they were first coated with sodium cholate (a biological lipid) before replacing the cholate with the phospholipid-polyethylene glycol coating in a second step. This two-step process generated nanotubes that had a biocompatible surface coating and emitted stable and strongly enhanced fluorescence at near-infrared wavelengths.When the exchanged nanotubes were injected into the bloodstream of normal and tumourbearing mice, a detailed map of blood vessels (including the smaller ones inside the tumour) fluoresced brightly through the skin without any interference from background autofluorescence. Furthermore, a 15-times lower dose of nanotubes was needed to achieve such detailed signals when compared with nanotubes prepared using the direct sonochemical NIH Public Access