HORIBA
Application Note
Measuring Size of Light Absorbing Materials
AN272
Introduction
Samples that absorb certain light wavelengths can pose challenges for dynamic light scattering (DLS) systems. DLS relies on measuring scattered light, not absorbed light, from nanoparticles. Nanogold, for instance, exhibits strong absorbance due to surface plasmon resonance, which significantly reduces the scattering signal, making it difficult to measure.
This application note demonstrates the measurement of a ~26 nm nanogold sample using the HORIBA SZ-100V2 nanoparticle analyzer. Two different experimental setups were tested: one with a 90° detector and cell center position, and another with a 173° detector and cell wall position. The absorption spectrum of the nanogold was also verified using a HORIBA Duetta Fluorescence and Absorbance Spectrometer.
Analytical Test Method
- Dispersion medium: Water
- Measurement mode settings: General mode
- Scan Settings: Manual 120 seconds
- Detector Angle: 90° or 173°
- Detector Position: Cell center for 90°, Cell wall for 173°
- Form of distribution: Monodisperse/Standard
Test Procedure
- Add sample as-is to a quartz cuvette.
- Insert cuvette into the instrument and allow the temperature to equalize for 2-3 minutes at 25°C.
- Perform 3 measurements.
Experimental Results
Autocorrelation Functions
Figure 1a: Autocorrelation function with 90° detector and cell center position, showing poor signal. This setup yielded low signal and unstable data.
Figure 1b: Autocorrelation function with 173° detector and cell wall position, showing strong signal. This setup provided strong, stable data.
Z-average and Polydispersity Index Data
| Measurement | Z-average | Polydispersity index |
|---|---|---|
| 1 | 1923.1 | 0.351 |
| 2 | --- | --- |
| 3 | 1067.0 | 0.223 |
| Measurement | Z-average | Polydispersity index |
|---|---|---|
| 1 | 26.2 | 0.463 |
| 2 | 26.2 | 0.468 |
| 3 | 25.9 | 0.483 |
Absorption Spectrum
Figure 2. Absorption Spectrum for nanogold taken with HORIBA Duetta, showing a 530 nm absorption peak. This peak aligns with the green laser wavelength of the SZ-100V2 (532 nm).
Discussion
The data from the 90° detector at the cell center position exhibited very low and unstable signals, as shown in Figure 1a and Table 1a. In contrast, the 173° detector at the cell wall position yielded strong, stable autocorrelation functions (Figure 1b) and Z-average values that closely matched the expected size of the nanogold (Table 1b).
The improved data quality when using the 173° detector and cell wall position can be attributed to minimizing the laser pathlength through the sample. This reduces the opportunity for the laser light to be absorbed, especially due to the surface plasmon resonance effect in nanogold. Conversely, the 90° detector at the cell center position involves a longer pathlength, leading to greater absorption of the laser signal and less scattered light reaching the detector.
This optimized technique, using the 173° detector with the cell wall position, effectively maximizes the scattering signal while minimizing laser absorption. It can also be applied to fluorescent samples, provided a filter is used to remove incoherent fluorescent signals. The absorption spectrum confirms the nanogold's peak absorption at 530 nm, which is consistent with the SZ-100V2's 532 nm laser, explaining the significant difference observed between the two measurement configurations.
