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Highly bright fluorescent gold nanoclusters (Au NCs) have been prepared by one-step reduction of aqueous precursor solution in the presence of multidentate thioether-terminated poly(methacrylic acid) (PTMP-PMAA). The fluorescence quantum yield of the resultant Au NCs is 4.8% higher than that of the similarly sized Au NCs prepared by the etching method (1.8–4.0%). These Au NCs show excellent photostability and have been successfully applied to label the hematopoietic cells first. The results show that Au NCs were endocytosed by the cancer cells significantly more than the normal cells, in comparison with control experiments labeled with fluorescent quantum dots (CdTe). The cytotoxicity experiments demonstrate the excellent biocompatibility of Au NCs, proven by a relatively lower cytotoxicity than CdTe. These robust near-infrared Au NCs show great potential in biolabeling, tracking, and imaging of other cells and diseases, especially in the diagnosis and treatment of chronic myeloid leukemia.

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The size and shape of metal particles, especially when below 100 nm, has profound influence on their physical and chemical properties. 1 Bulk metals are known electrical conductors and optical reflectors due to the free movement of electrons in the conduction band. 2 On the contrary, the metal nanoparticles (MNPs), especially those of gold, silver, and copper, exhibit a variety of colors, depending on their size and shape, due to surface plasmon resonance (SPR), which is attributed to the collective oscillation of conduction electrons upon interaction with electromagnetic radiations. 1a,b The band structure of metals becomes discontinuous and is further broken down to discrete energy levels when the size of their particles is reduced to ∼1 nm. 2 These molecule-like metal nanoclusters (MNCs) do not possess any plasmonic behavior but display other interesting optical and catalytic properties. 3 For example, such MNCs, filling the gap between the metal atoms and NPs, can still interact with light via electronic transitions between discrete energy levels resulting in intense light absorption and emission, a phenomenon known as fluorescence. 4 By controlling the number of atoms in MNCs, the gap between the discrete energy levels can be controlled, thus making it possible to control the absorption and emission wavelengths. 5 Because of these size-dependent and unique optical, electrical, and other physical/chemicals properties, 6 the MNCs are now being extensively studied for a host of applications including bioimaging, 7 sensing, 8 optoelectronics, 9 catalysis, 10 and so on.