Sponsored by HORIBADec 16 2013
Today, quantum-dot photoluminescence is increasingly being used in biology, materials science, energy, and medicine. The NanoLog spectrofluorometer from HORIBA Jobin Yvon has been specifically designed for recording near-IR fluorescence from nanoparticles.
The instrument contains an imaging emission spectrograph with a selectable-grating turret, a double-grating excitation monochromator, and a wide range of detectors. The NanoLog has optimal excitation optics for single-wall carbon nanotube research or any solid sample in front-face or right-angle mirror configurations. It can even be customized to offer lifetime and steady-state information about quantum dots.
Photoluminesce Lifetimes
Figure 1. TCSPC/MCS accessory attached to the sample compartment of a NanoLog.
A time-correlated single-photon counting (TCSPC) multichannel-scaling (MCS) accessory (Figure 1) can be fitted to the NanoLog. A range of quantum-dot samples from Evident Technologies were assessed with this scheme. Figure 2 shows the sample with quantum dots (PbS + polycarbonate) dispersed in CHCl3. A pulsed laser-diode at λ = 980nm, 50kHz, and pulse-width of ≈ 450ps was used to excite the sample. Following this, emission was recorded at 1465nm with a band-pass of 64nm by means of MCS on a Hamamatsu 10330-75 near-IR photomultiplier tube. Time per channel was 100ns (with MCS and TCSPC cards, the 10330-75 is able to resolve lifetimes from 60ps to DC). Then, measurements were carried out until the peak channel reached 100000 counts. While fitting the decay, reconvolution is not necessary since the laser pulse occupied just a single channel.
Figure 2. Fluorescence decay (upper plot) of PbS- polycarbonate quantum dots in CHCl3, and residuals to the fit (lower plot).
A bi-exponential model was used to find the X2 = 1.001 and a Durbin-Watson statistic = 1.957, demonstrating a remarkable fit to the data shown in Figure 2. T1 = 242ns and T2 = 928ns were the two re-covered lifetimes for the quantum dots.
Subsequently, the NanoLog was used to examine a second sample of quantum dots i.e. PbS and polymethylmethacrylate PMMA dissolved in toluene. Experimental parameters were analogous to the earlier sample, except for the fact that emission was recorded at 1115 nm. Figure 3 shows the study results.
Figure 3. Fluorescence decay of PbS-PMMA quantum dots in toluene; the upper plot shows the decay; the lower plot is the residuals to the fit.
Using the same bi-exponential model, X2 = 1.031 and a Durbin-Watson statistic = 2.027 were identified, also indicating a good fit to the data in Figure 3. T1 = 625ns and T2 =1.12µs were the two recovered lifetimes for these quantum dots. A table of results for a different PbS quantum-dot dispersions is shown below:
Table 1
| Dispersant |
T1 (μs) |
T2 (μs) |
X2 |
| λabs = 1040 nm; λexc = 980 nm |
| Polystyrene |
1.82 |
0.69 |
0.91 |
| PMMA |
2.52 |
1.37 |
0.97 |
| Polycarbonate |
2.22 |
0.79 |
1.10 |
| Flexographic ink |
0.57 |
0.17 |
1.10 |
| λabs = 1400 nm |
| Polystyrene (λexc = 980 nm) |
1.00 |
0.61 |
1.14 |
| Polystyrene (λexc = 635 nm) |
0.93 |
0.57 |
1.19 |
| PMMA (λexc = 980 nm) |
1.12 |
0.62 |
1.03 |
| PMMA (λexc = 635 nm) |
1.11 |
0.62 |
1.20 |
| Polycarbonate (λexc = 980 nm) |
0.93 |
0.24 |
1.00 |
| Polycarbonate (λexc = 635 nm) |
0.96 |
0.40 |
1.13 |
| Flexographic ink (λexc = 980 nm) |
0.30 |
0.14 |
0.95 |
Conclusion
HORIBA’s NanoLog spectrofluorometer is an important tool for analyzing fluorescence lifetimes of samples whose luminescence is mainly in the near-IR, such as quantum dots. The NanoLog is also used in biosensing, solid-state research, and cancer studies, and comes with TCSPC multichannel-scaling options of 2ns/channel and 500ns/channel, plus a broadband 5509 photomultiplier tube that is sensitive from 300 to 1700nm with a time- transit spread of 1.5ns.
This information has been sourced, reviewed and adapted from materials provided by HORIBA.
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