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(a) The absorption and emission spectra of QDot525 are measured at 40 nM concentration in pH 9.5 borate buffer. The intensity of the emission spectrum is different with excitation wavelengths 280 and 500 nm. (b) Repetitive scans of the emission spectrum between 450 and 600 nm at scan speed and 280 nm excitation light reveal a decrease in emission intensity over time. To illustrate photodarkening the relative area under each spectrum is plotted. In the second sequence the spectra are recorded during excitation with 500 nm light showing the time dependence of photobrightening. The cycle of photobrightening during 280 nm excitation and photodarkening during 500 nm excitation is repeated in the third and fourth sequence, respectively. (c) Scaling the emission spectra obtained by excitation at 280 nm and excitation at 500 nm to the same peak height shows that the emission spectral profile is not altered.
(a) Quantum dots QDot525 are embedded in a sol gel matrix at 40 nM concentration. To ease observation an area adjacent to an impurity (air bubble) in the gel is chosen and irradiated at 488 nm at 3 mW laser output for 3 min. to induce photobrightening. (b) When the area is imaged six days later the photobrightened region is still clearly visible. (c) The photobrightening or photodarkening depends on the laser power applied. Eight square areas (A–H) are irradiated at increasing intensity. (d) The luminescence lifetime is highest for the maximum photobrightened area A at 4.5 ns and decreases to 3 ns. Plotting the lifetime distributions (e) reveals a shift of the peak lifetime but no widening of the lifetime distributions (lifetime distributions for squares F and G not shown for clarity). (f) Photobrightening is independent of the initial state of the quantum dots.
Time evolution of the luminescence for different excitation intensities during two-photon excitation at 700 nm. The photon count rate was recorded at 3 s intervals from the TAC rate of the PMC-100 detector. The excitation intensity is shown next to each trace. Note that the time of excitation per pixel is much larger than in image scanning. Subsequently the relative increases in luminescence intensity far exceed those shown in Fig. 2.
Different illumination intensities and times applied to squares A–H.
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