12194
J. Phys. Chem. C 2007, 111, 12194-12198
Nanoparticle-Free Synthesis of Fluorescent Gold Nanoclusters at Physiological Temperature Yuping Bao,† Chang Zhong,† Dung M. Vu,‡ Jamshid P. Temirov,† R. Brian Dyer,*,‡ and Jennifer S. Martinez*,† MPA-CINT, Mail Stop K771, and C-PCS, Mail Stop J567, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ReceiVed: March 2, 2007; In Final Form: May 19, 2007
Few-atom gold nanoclusters exhibit strong size-dependent fluorescence emission. For many applications, these clusters are preferable to quantum dots or organic fluorophores as fluorescent labels. However, the published synthetic procedures to create these nanoclusters involve toxic chemicals, are of low synthetic yield, and concurrently produce large nanoparticles. Here, we report two methods for high-yield synthesis of gold nanoclusters, utilizing physiological temperature and green chemistry. Blue, green, and red emissive nanoclusters with pH stability in a range of 6-8 were produced.
Introduction Single-molecule measurements have significantly advanced our understanding of the molecular movement, dynamics, and biological function of proteins and other biomolecules.1 Fluorescence sensing and imaging techniques remain two of the primary methods for in vitro detection of molecules in solution and in vivo imaging of cells and cellular processes.2-4 Ultrasensitive techniques, such as fluorescence correlation spectroscopy (FCS), have allowed the single-molecule observation of protein folding dynamics5,6 and three-dimensional tracking of single particles.7 Unfortunately, the performance and sensitivity of these laser-induced fluorescence techniques are greatly limited by the fluorescent labels attached to the DNA, proteins, or other cellular components. For instance, organic fluorophores, which are most commonly used in fluorescence spectroscopy, are easily photobleached during the time scale of observation, leading to reduced sensitivity and decreased tracking time of the targets. Large fluorescent tags can also perturb the labeled biomolecules, causing artificial movement within cells and altered protein-protein interactions. Additionally, organic fluorophores tend to have small Stokes shifts, decreasing their utility for multicolored imaging8 and fluorescence resonance energy transfer (FRET) experiments. Relative to conventional organic dyes, semiconductor quantum dots show great promise in biolabeling due to their improved photophysical properties including size-tunable narrow emissions, large Stokes shifts, and minimal photobleaching. Unfortunately, quantum dots are commonly synthesized using harsh conditions and toxic precursors, are difficult to surface passivate, have large physical size comparable to proteins, and tend to photoblink.9 Each of these attributes limits the utility of quantum dots for fluorescence imaging and sensing. All of these challenges require the development of new fluorescent reporters that meet the following requirements: (1) nontoxic to cells and other organisms; (2) smaller than the biomolecules of interest, such as proteins, so that normal biological functions, such as protein-protein interactions, are not disturbed; (3) optimized photophysical * Corresponding authors. E-mail:
[email protected] (R.B.D.);
[email protected] (J.S.M.). † MPA-CINT. ‡ C-PCS.
properties such as no photobleaching, no photoblinking, short lifetimes, and high quantum yields; and (4) facile synthesis and attachment to biomolecules. Each of these attributes is essential for the development of advanced diagnostics and the in vivo imaging of cellular processes. Few-atom gold nanoclusters are collections of small numbers of gold atoms (
