Application of fluorescence correlation spectroscopy: a study of flocculation of rigid rod-like biopolymer (schizophyllan) and colloidal particles

The flocculation between the rod-like biopolymer Schizophyllan and two types of colloidal particles (latex with diameter 40 nm and alumina with diameter 60 nm) has been investigated by means of fluorescence correlation spectroscopy (FCS). The concentration ratio of Schizophyllan/particle q was varied in the range 0.1 approximately 20. Under conditions of pH about 5.7, 1 mmol.L(-1) NaCl, and room temperature (22+/-0.5 degrees C), the particles are strongly charged (alumina particles positively charged, latex negatively), while Schizophyllan is neutral. We observed that Schizophyllan chains flocculate with both types of particles, which suggests that the charge neutralization does not play a decisive role in these interactions. The ratio of fluorescence intensity of one floc over that of one particle, Q(f)/Q(p), and the corresponding hydrodynamic radius (r(h)) of the flocs have been measured. For a Schizophyllan-latex system, Q(f)/Q(p) reached a maximum value of 5 for q=3 indicating that the flocs contained five particles on average. The corresponding value of r(h) was r(h)=455 nm. The flocculation kinetic of latex particles with Schizophyllan was too fast to be measurable by FCS. For the Schizophyllan-alumina system, Q(f)/Q(p) was stable at about 1 in the whole studied range of q but r(h) increased with q suggesting that many Schizophyllan chains are adsorbed on individual particles. The flocculation kinetic of this system was studied by FCS and the obtained results were compatible with those of photon correlation spectroscopy.