This is a copy of the slides presented at the meeting but not formally written up for the volume.

Single-molecule manipulation and detection of biomolecules has significantly advanced our understanding of the molecular movement, dynamics, and biological function of proteins. Fluorescence microscopy currently serves as one of the primary noninvasive techniques for the sensitive detection of molecules in solution and on surfaces. However, the performance and sensitivity of this laser-induced fluorescence technique is strongly influenced by the fluorescent labels attached to DNA, proteins or cells. Organic dyes have most commonly been used as fluorescent biolabels; however, they quickly photobleach, limiting the time scale over which molecular events can be followed. Quantum dots show great promise for fluorescence measurements due to their improved photophysical properties, such as size-tunable narrow emissions, large Stokes shifts and minimal photobleaching. However, problems still exist for the use of quantum dots as biolabels, including: photoblinking, toxic synthetic approaches, surface passivation issues and relatively large physical sizes that are comparable to proteins. Here, we report the synthesis of a new class of fluorescent labels, gold nanoclusters, which consist of several gold atoms (<1 nm in size) with strong fluorescence emission. These small fluorescent gold nanoclusters are synthesized at physiological temperature using poly(amidoamine) dendrimer as a template. Small blue emissive gold nanoclusters were produced without the use of a reductant and without concurrent nanoparticle formation. Gold nanoclusters with green and red emissions were synthesized using a mild reductant, also without nanoparticle formation. The studies of pH-dependent stability suggest that these fluorescent nanoclusters are very stable in a pH range of 6 - 8. These new approaches produce gold nanoclusters with a much higher yield and eliminate the toxicity of the previously reported process, resulting in a biologically compliant approach. This work is the first known report of fluorescent gold nanoclusters via a green-chemistry approach and without the formation of gold nanoparticles. This work is funded by Los Alamos Laboratory Directed research and Development program.