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

We report quenching and chemical degradation of polymer-coated quantum dots by reactive oxygen species (ROS), a group of oxygen-containing molecules that are produced by cellular metabolism and are involved in both normal physiological and disease processes such as oxidative signaling, cancer, and atherosclerosis. A major new finding is that hypochlorous acid (HOCl) in its neutral form is especially potent in degrading encapsulated QDs, due to its small size, neutral charge, long half-life, and fast reaction kinetics under physiologic conditions. Thus, small and neutral molecules such as HOCl and hydrogen peroxide (H2O2) are believed to diffuse across the polymer coating layer, leading to chemical oxidation of sulfur or selenium atoms on the QD surface. This “etching” process first generates lattice structural defects (which cause fluorescence quenching), and then produces soluble metal (e.g., cadmium and zinc) and chalcogenide (e.g., sulfur and selenium) species. We also find that significant fluorescence quenching occurs before QD dissolution, and that localized surface defects can be repaired or “annealed” by UV light illumination. These results have important implications regarding the long-term fate and potential toxicity of semiconductor nanocrystals in-vivo.

Surgery is one of the most effective and widely used procedures in treating human cancers, but a major problem is that the surgeon often fails to remove the entire tumor, leaving behind tumor-positive margins, metastatic lymph nodes, and/or satellite tumor nodules. Here we report the use of a handheld spectroscopic pen device (termed SpectroPen) and near-infrared contrast agents for intraoperative detection of malignant tumors, based on wavelength-resolved measurements of fluorescence or surface-enhanced Raman scattering (SERS) signals. The SpectroPen utilizes a near-infrared diode laser (emitting at 785 nm) coupled to a compact head unit for light excitation and collection. This pen-shaped device effectively removes silica Raman peaks from the fiber optics and attenuates the reflected excitation light, allowing sensitive analysis of both fluorescence and Raman signals. Its overall performance has been evaluated by using a fluorescent contrast agent (indocyanine green, or ICG) as well as a surface-enhanced Raman scattering (SERS) contrast agent (pegylated colloidal gold). Under in vitro conditions, the detection limits are approximately 2–5 × 10−11 M for the indocyanine dye and 0.5–1 × 10−13 M for the SERS contrast agent. Ex vivo tissue penetration data show attenuated but resolvable fluorescence and Raman signals when the contrast agents are buried 5–10 mm deep in fresh animal tissues. In vivo studies using mice bearing bioluminescent 4T1 breast tumors further demonstrate that the tumor borders can be precisely detected preoperatively and intraoperatively, and that the contrast signals are strongly correlated with tumor bioluminescence. After surgery, the SpectroPen device permits further evaluation of both positive and negative tumor margins around the surgical cavity, raising new possibilities for real-time tumor detection and image-guided surgery.

Quantum-dot (QD) nanocrystals are promising fluorescent probes for multiplexed staining assays in biological applications. However, nonspecific QD binding to cellular membranes and proteins remains a limiting factor in detection sensitivity and specificity. Here we report a new class of hydroxyl (-OH)-coated QDs for minimizing nonspecific cellular binding and for overcoming the bulky size problems encountered with previous surface coatings. The hydroxylated QDs are prepared from carboxylated (-COOH) dots via a hydroxylation and cross-linking process. With a compact hydrodynamic size of 13-14 nm (diameter), they are highly fluorescent (>60% quantum yields) and stable under both basic and acidic conditions. By using human cancer cells, we have evaluated their superior nonspecific binding properties against that of carboxylated, protein-coated, and poly(ethylene glycol) (PEG)-coated QDs. Quantitative cellular staining data indicate that the hydroxylated QDs result in a dramatic 140-fold reduction in nonspecific binding relative to that of carboxylated dots and a still significant 10-20-fold reduction relative to that of PEG- and protein-coated dots.