Electron transfer-induced blinking in Ag nanodot fluorescence

Abstract

Various single-standed DNA-encapsulated Ag nanoclusters (nanodots) exhibit strong, discrete fluorescence with solvent polarity-dependent absorption and emission throughout the visible and near-IR. All species examined, regardless of their excitation and emission energies, show similar µs single-molecule blinking dynamics and near IR transient absorptions. The polarity dependence, µsec blinking, and indistinguishable µsec-decaying transient absorption spectra for multiple nanodots suggest a common charge transfer-based mechanism that gives rise to nanodot fluorescence intermittency. Photoinduced charge transfer that is common to all nanodot emitters is proposed to occur from the Ag cluster into the nearby DNA bases to yield a long-lived charge-separated trap state that results in blinking on the single molecule level.

Figure 1

Lippert-Mataga plot of the 660nm emitting species. The solvent dielectric function was changed by the variation of the ethanol-water ratio. The absorption and emission spectra were measured in each solvent and the slope of the fitted line was used to calculate the dipole change from the ground state to the excited state, according to the Lippert-Mataga equation. The change was calculated to be 16.0 ± 1.1 Debye.

Figure 2

Femtosecond transient absorption kinetic traces. The wavelengths shown for each emitter reflect the transient absorption (black) and the ground-state depletion (red). The depletion shown appears at negative ΔOD, but is plotted in its absolute value. It has been corrected for spectral overlap by subtracting out the contribution from the transient absorption, which is based on the kinetics at 775 nm calibrated to the expected value based on the peak curve fittings. The data was collected by exciting with a 100 fs Ti-sapphire laser at 1 kHz, then probing with a white-light continuum generated from the same laser. The excitation wavelength was tuned to the peak of the ground-state absorption for each emitter.

Figure 3

(A–C) Femtosecond and nanosecond transient absorption spectra (normalized) of a series of silver clusters, labeled by the maximum emission wavelength. All of the species were excited by 100 fs pulsed excitation, except for the long delay time curve, which was generated from excitation by a 7ns pulsed laser. The dip in the spectrum around 800 nm is an instrumental artifact resulting from the white-light generation used as the probe. (D) The transient absorption and spectral fit of the Ag660 emitter, which was fitted to the 20ns delay absorption curve, revealing a broad peak centered at 650nm.

Figure 4

Energy diagram of the photoinduced charge transfer scheme. Energy values listed for the transitions were taken from ground-state and transient absorption spectra. The Ag_n-DNA complex is initially excited to the excited state of the silver cluster, from which either a decay to the radiative state or the transition to the charge-transfer (CT) state occurs. The CT state can then either relax non-radiatively back to the ground state or transition into a longer-lived CT state, from which photo-assisted reverse charge transfer can occur.

Figure 5

Nanosecond transient absorption spectra of the Ag660 nanodot at varying time delays. The curves clearly show an isosbestic point with delay at 648 nm, suggesting that depletion of the ground state at 595nm leads to population of a state responsible for the positive change in absorbance at higher wavelengths. The nanodot was pumped with a several ns pulse at 590 nm.

Figure 6

Transient absorption decay measurements. The decays were recorded at 770nm using an oscilloscope recorded photoreceiver response, whose instrument response time was 10 ns. Each decay was fitted biexponentially.

Figure 7

(A–C) Fluorescence correlation spectroscopy time trace autocorrelations for the three Ag nanodots. The species were excited (594nm, 633nm HeNe) by a focused laser (40x water objective, 1.2 NA) at the specified powers, and time traces were collected by an avalanche photodiode through a 50µm optical fiber. Two decay components were fitted, the longer time scale corresponding to diffusion through the laser focus, while the shorter time decay was attributed to the charge separated dark state.