Highly fluorescent silver nanoparticles (AgFNP) have been prepared by a facile photochemical method, yielding these materials in just a few minutes and with excellent long-term stability. The method makes use of photogenerated ketyl radicals that reduce Ag(+) from silver trifluoroacetate in the presence of amines. While as functional materials these AgFNP can be described as of nanometer dimensions, we believe that the luminescence arises from particle-supported small metal clusters (predominantly Ag(2)). The materials have been characterized by electron microscopy, fluorescence and absorption spectroscopy, fluorescence lifetime studies, and (19)F NMR spectroscopy. Exploratory work shows that the fluorescence from AgFNP can be efficiently quenched by paramagnetic quenchers, and these studies have been combined with electron paramagnetic resonance work.

Nanomedicine, defined as the application of nanotechnology in the medical field, has the potential to significantly change the course of diagnostics and treatment of life-threatening diseases, such as cancer. In comparison with traditional cancer diagnostics and therapy, cancer nanomedicine provides sensitive cancer detection and/or enhances treatment efficacy with significantly minimized adverse effects associated with standard therapeutics. Cancer nanomedicine has been increasingly applied in areas including nanodrug delivery systems, nanopharmaceuticals, and nanoanalytical contrast reagents in laboratory and animal model research. In recent years, the successful introduction of several novel nanomedicine products into clinical trials and even onto the commercial market has shown successful outcomes of fundamental research into clinics. This paper is intended to examine several nanomedicines for cancer therapeutics and/or diagnostics-related applications, to analyze the trend of nanomedicine development, future opportunities, and challenges of this fast-growing area.

The photochemistry of aromatic ketones through the Norrish type I cleavage of benzoins and via photoreduction generates ketyl radicals that readily reduce many metal ions, including silver and gold. Reduction to Au(0) and Ag(0) leads to the spontaneous formation of nanoparticles (NPs) in aqueous or micellar solutions. Careful consideration of kinetic factors to minimize triplet quenching by metal ions can lead to rapid NP generation. These materials are quite stable and have interesting reactivities due to the essentially unprotected characteristics of the surface.

The association and resulting fluorescence quenching of CdSe quantum dots by 4-amino-2,2,6,6-tetramethylpiperidine oxide (4-amino-TEMPO), a persistent nitroxide, have been examined using electron paramagnetic resonance (EPR) and fluorescence spectroscopy. EPR data suggest binding constants around (8 +/- 4) x 10(6) M(-1) for green (2.4-2.5 nm) nanoparticles, and the application of Job's method indicates that the preferred mode of binding involves one or two quencher molecules per quantum dot, although more quenchers could bind at high concentrations of 4-amino-TEMPO. Fluorescence quenching by 4-amino-TEMPO is at least 3 orders of magnitude more efficient than by TEMPO itself, reflecting the strong binding confirmed by the EPR data. Stern-Volmer plots are nonlinear and in light of the EPR data probably reflect ready accessibility of the CdSe surface to one or two 4-amino-TEMPO molecules, while additional quenchers can only bind if they displace trioctylphosphine oxide ligands. Quantum dot-4-amino-TEMPO complexes can be used as free radical sensors, since the fluorescence (quenched by the nitroxide) is readily restored when radicals are trapped to form alkoxyamines.

Quenching of quantum dot luminescence by nitroxides shows steep exponential behavior and is dependent on nanoparticle size, being most effective for the smaller particles. The interaction of CdSe nanoparticles with TEMPO (non‐binding) and 4‐amino‐TEMPO (binding) has been examined using fluorescence and electron spin resonance techniques. The data suggest that quenching involves an electron exchange mechanism. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)