Highly Fluorescent, Water-Soluble, Size-Tunable Gold Quantum Dots

Highly fluorescent, water-soluble, few-atom Au quantum dots have been created that behave as multielectron artificial atoms with discrete, size-tunable electronic transitions throughout the visible and near IR. Correlation of nanodot sizes with emission energies fits the simple relation, $E_{\mathrm{F}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}}/N^{1/3}$, predicted by the jellium model. Providing the “missing link” between atomic and nanoparticle behavior in noble metals, these emissive, water-soluble Au nanoclusters open new opportunities for biological labels, energy transfer pairs, and light emitting sources in nanoscale optoelectronics.

Figure 1

(a) Excitation (dashed) and emission (solid) spectra of different gold nanoclusters. Emission from the longest wavelength sample was limited by the detector response. Excitation and emission maxima shift to longer wavelength with increasing initial Au concentration [15], suggesting that increasing nanocluster size leads to lower energy emission. (b) Emission from the three shortest wavelength emitting gold nanocluster solutions (from left to right) under long-wavelength UV lamp irradiation (366 nm). The leftmost solution appears slightly bluer, but similar in color to ${\mathrm{A}\mathrm{u}}_8$ (center) [14] due to the color sensitivity of the human eye. Green emission appears weaker due to inefficient excitation at 366 nm.

Figure 2

Fluorescent nanocluster size determinations. In addition to separately identified ${\mathrm{A}\mathrm{u}}_8$ (455 nm) [14], nanocluster sizes (emission maxima) were determined to be ${\mathrm{A}\mathrm{u}}_5$ (385 nm), ${\mathrm{A}\mathrm{u}}_{13}$ (510 nm), ${\mathrm{A}\mathrm{u}}_{23}$ (760 nm), and ${\mathrm{A}\mathrm{u}}_{31}$ (866 nm) through linear correlation of mass abundance and fluorescence intensity at each emission maximum. The mass abundances of all gold nanocluster solutions were measured by electrospray ionization (ESI) mass spectrometry with identical ionization conditions. The reproducible and gentle ionization procedure yielded only one mass species linear in emission intensity for each emission maximum. This one-to one correspondence between mass abundance and fluorescence enabled unambiguous assignments of fluorescent gold nanocluster sizes.

Figure 3

Correlation of the number of atoms, $N$, per cluster with emission energy. Emission energy decreases with increasing number of atoms. The correlation of emission energy with $N$ is quantitatively fit with $E_{\mathrm{F}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}}/N^{1/3}$, as predicted by the spherical jellium model [2, 4]. Emission energies of ${\mathrm{A}\mathrm{u}}_{23}$ and ${\mathrm{A}\mathrm{u}}_{31}$ exhibit slight deviations from the $E_{\mathrm{F}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}}/N^{1/3}$ scaling. Consistent with the narrow excitation and emission spectra, the potential confining the free electrons flattens slightly for ${\mathrm{A}\mathrm{u}}_{23}$ and ${\mathrm{A}\mathrm{u}}_{31}$, with an anharmonicity of $U=0.033$. The one-electron artificial atom (Bohr model) scaling ($N^{-2/3}$) of semiconductor quantum dots (scaled to the metallic limit of the Fermi energy) is also shown to delineate the different scaling laws of quantum-confined metals and semiconductors. Other measurements of ${\mathrm{A}\mathrm{u}}_{28}$ [6] emission (▲) and the measured (nonfluorescent) transition energies of ${\mathrm{A}\mathrm{u}}_{11}$ [21] and ${\mathrm{A}\mathrm{u}}_{20}$ [20] are all consistent with the observed scaling relations.