Singlet Oxygen Generation by Laser Irradiation of Gold Nanoparticles

Abstract

The formation of singlet oxygen by irradiation of gold nanoparticles in their plasmon resonance band with continuous or pulsed laser light has been investigated. Citrate-stabilized nanoparticles were found to facilitate the photogeneration of singlet oxygen, albeit with low quantum yield. The reaction caused by pulsed laser irradiation makes use of the equilibrated hot electrons that can reach temperatures of several thousand degrees during the laser pulse. Although less efficient, continuous irradiation, which acts via the short-lived directly excited primary "hot" electrons only, can produce enough singlet oxygen for photodynamic cancer therapy and has significant advantages for practical applications. However, careful design of the nanoparticles is needed, since even a moderately thick capping layer can completely inhibit singlet oxygen formation. Moreover, the efficiency of the process also depends on the nanoparticle size.

Figure 1

Photobleaching of the DPBF absorbance upon CW irradiation at 532 nm, 1 W (37 W cm^–2), in a 50/50 (v/v) mixture of water and ethanol: (a) in the absence of NPs and (b) in the presence of 15 nm citrate-stabilized spherical gold NPs. Shown are absorbance spectra taken at intervals of 10 min from before the irradiation up to a maximum irradiation time of 60 min; the arrows indicate the direction of change.

Figure 2

(a) Time dependence of the photobleaching of the DPBF absorbance at 412 nm, A₄₁₂(DPBF), upon CW irradiation at 532 nm, 1 W (37 W cm^–2), in the absence and presence of citrate-stabilized spherical NPs with 15 and 46 nm diameter; shown here are the results from several individual experiments (dashed lines) and the average (solid lines), after subtraction of the NP absorbance and normalization to 1 at time zero; see the Experimental Section for details of data treatment and analysis. (b) Effect of the NPs alone, calculated by subtracting the photobleaching effect of DPBF in the absence of NPs from the results obtained in the presence of NPs; the solid lines in (b) are linear fits of the data in the range 20–60 min.

Figure 3

Gradient of the time-dependent DPBF absorbance photobleaching in the irradiation time window 20–60 min for different samples under CW irradiation at 532 nm, 1 W (37 W cm^–2). For experiments in the presence of NPs, the NP concentration was adjusted to yield an absorbance of 0.4 at 532 nm. Experiments for DPBF in the absence of NPs and in the presence of 15 nm spherical NPs were also undertaken after bubbling the sample with nitrogen for 10 min, as indicated (+N2). The error bars correspond to the standard deviation of several repeat experiments, and ∗∗∗ indicates statistically significant differences with respect to the experiment on DPBF only, as determined by the ANOVA F-test at p < 0.001; it should be noted that the results for 15 or 46 nm spherical NPs without nitrogen bubbling or a PEG capping layer were found to be different to all other results at this statistical significance level. No repeat experiment was undertaken for irradiation of nanorods at 532 nm, but the same result (no additional bleaching in the presence of nanorods) was obtained for irradiation at 800 nm (Figure S5).

Figure 4

Photobleaching of the DPBF absorbance upon laser irradiation with 5 ns laser pulses at 532 nm, 0.15 W, 10 Hz repetition rate (corresponding to a power density of 1.5 W cm^–2 and a pulse energy density of 0.15 J cm^–2). (a) Absorbance spectra in the presence of citrate-stabilized spherical gold NPs with 15 nm diameter, taken at intervals of 5 min from before the irradiation (gray) up to a maximum irradiation time of 30 min; the arrow indicates the direction of change. (b) Gradient of the time-dependent DPBF photobleaching (measured at 412 nm) in the irradiation time window of 20–30 min in the absence and presence of NPs.

Figure 5

Time-dependent temperatures of the conduction band electrons (red), lattice (black), and first solvent layer (blue), calculated for our experiments using nanosecond-laser pulse excitation (15 nm spherical NPs in 50/50 EtOH/water, 5 ns laser pulses with 0.15 J cm^–2 intensity, solid lines) and for the experiments described in ref (24) (40 nm spherical NPs in 80/20 EtOH/water, 7 ns laser pulses with 0.03 J cm^–2 intensity, dashed lines) using the “two-temperature model” for the electron and phonon heat baths, coupled to finite-element heat transfer and diffusion simulations in the surrounding solvent (see Supporting Information for details); time zero corresponds to the center of the laser pulse.

Figure 6

Schematic diagram showing the population probability f(E) for a NP electron state at energy E near the Fermi level, E_F, under different conditions: (a) in equilibrium at room temperature, (b) at an electron temperature of T_e = 2100 °C after electron–electron equilibration (“hot electrons”), and (c) immediately after the absorption of photons by single electrons (“primary hot electrons”, with population changes highly exaggerated to make them visible). Also shown are the energies of the ground-state triplet (³Σ) and lowest-excited singlet state (¹Δ) of oxygen as well as the next singlet state (¹Σ) under the assumption that E_F is equidistant from the ³Σ and ¹Δ energies. Excitation of an oxygen molecule to ¹O₂ requires the simultaneous transfer of an electron from the oxygen to a hole at the energy of the ³Σ state and of a hot electron with the opposite spin and an energy at the ¹Δ (or ¹Σ) level to the oxygen molecule.