Home >  About ISSP >  Publications > Activity Report 2018 > Xxxx Group

Dynamic Light Scattering Microscope: Accessing Opaque Samples with High Spatial Resolution

Shibayama Group

Dynamic light scattering (DLS) is a technique for obtaining the size distribution of particles in solution. However, it cannot be applied to opaque samples. There are two types of opaque samples. One is a black sample, which is a strong light-absorbing material such as ink. In this case, scattered light is completely absorbed by the sample itself and one cannot obtain the signal. The other type is milky samples, which are strong light-scattering materials such as milk. In this case, multiple scattering is inevitable. In addition to this, poor spatial resolution sometimes also becomes a problem. There is a growing demand to investigate the dynamic behavior of samples with a higher spatial resolution. One such example is a tracking of material transportation in biological cells. To realize such applications, the spatial resolution needs to be near the diffraction limit. Here, we propose a new DLS technique, “DLS microscope” that mitigates the above-mentioned disadvantages (Fig. 1) [1]. The proposed DLS microscope obtains signals with a backscattering geometry. This enables us to measure opaque samples with high spatial resolution.

 

Fig. 1. Photograph of the dynamic light scattering microscope.

Fig. 2. (Left side) Concentration dependence of the size distribution of a polystyrene latex suspension. The nominal diameter of the polystyrene latex particles is 100 nm. The 1 - 0.01 wt%, as measured by the DLS microscope, is represented by the solid lines. The 0.01 - 0.0001 wt%, as measured by the typical DLS system, is represented by the dashed lines. (Right side) Concentration dependence of the size distribution of Chinese ink. The 10 - 0.05 wt%, as measured by the DLS microscope, is represented by the solid lines. The 0.05 - 0.001 wt%, as measured by the typical DLS system, is represented by the dashed lines.

By using the DLS microscope, we measured the size distribution of opaque samples such as polystyrene latex and Chinese ink with changing their concentrations. Figure 2 shows the results for the polystyrene latex suspension and Chinese ink. In the case of polystyrene latex suspension, the data show the monodisperse nature. The calculated diameter shows good agreement with the nominal diameter (100 nm). Since the result for the polystyrene latex suspension shows peaks at almost the same position in all concentrations, we can conclude that the polystyrene latex suspension shows no morphology change through varying the concentration. This is consistent with the following molecular interpretation: each particle has a negative charge on the surface, and the particles repel each other keeping their form unchanged and showing Brownian motion. This result corresponds to the result obtained by using diffusing wave spectroscopy [2]. In contrast, the result for Chinese ink seems to show that the average particle size increases and the size distribution broadens at higher concentrations. However, as Chinese ink is a protective colloidal solution, the particles are considered not to aggregate. One of the possible explanations for our result is the existence of attractive interactions between colloidal particles. In other words, colloidal particles show collective motion rather than Brownian motion at higher concentrations. This consideration is supported by the fact that the viscosity of neat ink is approximately five times higher than that of water. Taking advantage of its high spatial resolution, this technique can also be readily applied to other media such as biological cells and gels.


References
  • [1] T. Hiroi and M. Shibayama, Opt. Express. 21, 20260 (2013).
  • [2] P. Navabpour et al., Colloid Polym. Sci. 283, 1025 (2005).
Authors
  • T. Hiroi and M. Shibayama