Dynamic light scattering (DLS) (also known as photon correlation spectroscopy) is one of the most widely used techniques for the study of colloidal systems. It is a fast, convenient and relatively simple technique, enabling absolute estimates of particle size for a wide range of colloidal suspensions including suspended metal nanoparticles. In this technique a laser beam is passing through a small volume of solution which contains colloidal particles, and the scattered light is collected over a small solid angle, in a direction that can be moved around the sample. Figure 1 on the top displays the setup at NanoOpitcs in Odense.
The phase and polarization of the scattered light is determined by the size, shape, and composition of scatterers. The random Brownian motion of the nanoparticles causes time-dependent fluctuations of the total intensity at the detector, with the time scale of the fluctuation being determined by the time scale of the Brownian motion of the particles. In a DLS measurement, an autocorrelation function is generated from the intensity fluctuations, and this function contains information also about the particle size distribution, which is obtained after mathematical treatment of the autocorrelation function.
Owing to their polycrystalline nature, round metallic nanoparticles have an inhomogeneous internal structure and are not perfectly spherical. These imperfections are strong enough to result in a small but highly relevant optical anisotropy.
On the other hand, when excited by electromagnetic waves, silver nanoparticles support coherent oscillations of the surface conduction electrons. This phenomenon, i.e., the confined oscillations of the charge density, is referred to as localized surface plasmon resonance. It has been shown that upon scattering, as localized surface plasmon resonance along with such optical anisotropy, it results in a depolarized speckle pattern. Investigating depolarization effect on the temporal fluctuation within the DLS measurements will highly increase information on the Ag NP size and aggregation. Initially, the commercial setup from the Brookhaven Instruments we owed was working at wavelength of 660nm (red light), and first experiments for 50 and 100 nm particles, and mixed 50 and 100 nm, were carried out at this wavelength (see figure 2).
Fig. 2 DLS signals for 100 nm, 50 nm and a mixture of both sizes in solution.
Nevertheless, the scattering from nanoparticles depends on the wavelength, for example for spherical nanoparticles the scattering cross section is inversely proportional to the fourth order of the wavelength, which means that shorter wavelengths will provide larger scattered signals.
On the other hand, the localized surface plasmon resonance for silver nanoparticles is met at wavelengths near 400nm. With all this in mind there was a need to change the laser and shift to blue light, which allows us to work with smaller nanoparticles as well as doing upgrade with depolarization measurements.