Highly fluorescent noble-metal quantum dots

Abstract

Highly fluorescent, water-soluble, few-atom noble-metal quantum dots have been created that behave as multielectron artificial atoms with discrete, size-tunable electronic transitions throughout the visible and near infrared. These molecular metals exhibit highly polarizable transitions and scale in size according to the simple relation E(Fermi)/N(1/3), predicted by the free-electron model of metallic behavior. This simple scaling indicates that fluorescence arises from intraband transitions of free electrons, and these conduction-electron transitions are the low-number limit of the plasmon-the collective dipole oscillations occurring when a continuous density of states is reached. 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

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 concentrations, suggesting that increasing nanocluster size leads to lower energy emission. Figure 1. b. Emission from the three shortest wavelength emitting gold nanocluster solutions (from left to right) under long-wavelength UV lamp irradiation (366nm). The leftmost solution appears slightly bluer, but similar in color to Au₈ (center) due to the color sensitivity of the human eye. Green emission appears weaker due to inefficient excitation at 366 nm.

Figure 2

Figure 2. a. ESI mass spectrum of G2-OH PAMAM encapsulated gold nanodots. PAMAM-Au₈ mass spectrum (theoretical m/z of dendrimer G2-OH (3272) encapsulated Au₈ and 5H₂O is 4939) Figure 2. b. Other identified Au nanocluster species observed in a typical mass spectrum of a fluorescent gold cluster solution.

Figure 3

Fluorescent nanocluster size determinations. Correlations of mass spec abundance with each fluorescence transition intensity observed in Figure 1A over many samples yield linear correlations of only a single species with each fluorescence transition. In addition to separately identified Au₈, nanocluster sizes (emission maxima) were determined to be Au₅ (385 nm), Au₁₃ (510 nm), Au₂₃ (760 nm), and Au₃₁ (866 nm) through linear correlation of mass abundance and fluorescence intensity at each emission maximum.

Figure 4

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_fermi/N^1/3, as predicted by the jellium model (1, 2). When N is equal to 1, the energy of valence electron is equal to Fermi energy because the valence electron is at the HOMO level. Emission energies of Au₂₃ and Au₃₁ exhibit slight deviations from the E_fermi/N^1/3 scaling. Consistent with the narrow excitation and emission spectra, the potential confining the free electrons flattens slightly for Au₂₃ and Au₃₁, with anharmonicity parameter U=0.033 (42). The experimental values for the emission energies of Au₃ (82), Au₂₈ (9)and Au₃₈ (71) are 3.66, 1.55, and 1.44 eV respectively (represented by ▲), which are all consistent with the observed scaling relations. Kubo’s predicted model E_f/N (12) and the square potential box model (6/5E_f/N^2/3 (1) are also shown in the figure. Obviously these models can not accurately fit the emission energy scalings of the gold clusters.

Figure 5

Schematic of size-dependent surface potentials of gold clusters on different size scales. For the smallest gold clusters (Au₃ to Au₁₃), cluster emission energies can be well fit with the energy scaling law E_fermi/N^1/3, where N is the number of atoms in each cluster, indicating that electronic structure transitions of these small gold clusters are well-described by a spherical harmonic potential. With increasing size, small anharmonicities distort the potential well, which at larger sizes gradually distorts into a Woods-Saxon potential surface (42), and eventually becomes a square well potential characteristic of electrons in large metal nanoparticles (1).

Figure 6

Antibunched fluorescence from a single Au₂₃ cluster, excited at 632.8 nm. Photons arriving at each of two detectors show a decreased probability of two photons arriving simultaneously (at zero interphoton delay). The rise time of the antibunched signal matches the Au₂₃ lifetime in Table 1.

Figure 7

Emission spectra (excitation at 375 nm) of aqueous PAMAM dendrimer (a) encapsulating NaBH₄-reduced gold nanoclusters and (b) treated with persulfate with increasing additions of Na₂S (red curve to violet curve). Insets (c) and (d) show spectra (a) and (b), respectively, normalized to equal integrated intensity. The persulfate-treated samples show the creation of a green-emitting species apparent at as little as a 20-fold excess of Na₂S, while all but the two final Au-containing samples (500- and 1000-fold excess of Na₂S) show no change in peak shape.

Figure 8

Emission spectra of octadecanethiol encapsulated gold nanoclusters in chloroform. Blue (455-nm) emitting gold clusters with maximum excitation at 365 nm. Green (510 nm) emitting gold clusters with maximum excitation at 430 nm. Red (600 nm) emitting gold clusters with maximum excitation at 586 nm. IR (776 nm) emitting gold clusters with maximum excitation at 650 nm. These excitation and emission spectra closely match those in PAMAM scaffolds.