Basic Principle of SLS/DLS Measurements in Fluorescent/Phosphorescence Solutions

As a typical elastic scattering technique,laser light scattering results cannot precisely describe the solution structure when the samples absorb incident light. Therefore, commercial laser light scattering instrument has been modified and upgraded to utilize four independent lasers, e. g. 785 nm,632.8 nm,532 nm and 457 nm, respectively as the light sources. By switching the wavelengths of incident lasers to avoid the sample absorption, the instrument can be widely used in fluorescent/phosphorescence solution systems. Spherical polystyrene nanoparticles with and without fluorescent dyes are used to calibrate the modified instrument. It is found that the height of the normalized intensity-intensity time correlation function decreases with the increase of the scattering angle when the sample absorbs the incident laser light. Such a correlation function fails to describe the sample solution structure, because the corresponding hydrodynamic radius distribution shows two modes and the average characteristic line width versus the square of scattering vector q(2) does not fit a good linear relationship any more. Therefore, another laser which is not absorbed by the sample is used to do the calibration again. The corresponding intensity-intensity time correlation functions and the fitting results can precisely describe the solution structure. Fluorescence correlation spectroscopy (FCS) is also conducted to further prove the results obtained from the modified laser light scattering instrument are precise and accurate.