ИСТИНА

Noble metal nanoparticles (NP) have been extensively studied in the processes of coagulation and aggregation caused by various low-molecular and high-molecular coagulants: proteins, artificial polymers, thiol compounds, and inorganic electrolytes. One of the most vivid manifestations of the aggregation is the surface plasmon resonance band shift that can be observed by regular spectrophotometry and even by naked eye (the red solution of gold NP turns blue, while the yellow solution of silver NP becomes pink). However, the light scattering by the solutions formed as a result of such an aggregation has been underinvestigated. Analytical methods based on this phenomenon demonstrate low detection limits, wide working ranges and a high stability of signals. Rayleigh light scattering is an effective technique for studying the aggregation processes and is quantified by measuring re-emitted light of the same wavelength as incident light using a fluorimeter. The intensity of the Rayleigh light scattering is proportional to the volume of the species and the molar concentration of the solute that is the quantitative basis for the following assay. We have studied the aggregation of silver and gold NP with polyhexamethylene guanidine hydrochloride (PHMG), a cationic polyelectrolyte with a polymerization degree of 5–90. PHMG is an efficient disinfectant with a maximum allowable concentration in natural water of 0.03 mg/L. This and other cationic polymers cause aggregation of Ag and Au NP detectable by obtaining synchronous spectra with a regular spectrofluorimeter (Cary Eclipse) and also spectrophotometrically by using a photometer. With silver NP, an increase in polyelectrolyte concentration leads to higher aggregation, more intense scattering, and a red shift in visible spectrum; a further increase in the polyelectrolyte concentration (over ~0.1 mg/L) results in the decrease of the scattering signal. This can be explained by recharging the citrate-stabilized anionic NP and their stabilization by the excess of the cationic polymer, which probably starts at the critical concentration of the polycation. This is also supported by DLS data showing that the mean aggregate size increases with PHMG concentration until its critical value and then becomes smaller. The critical concentration of the polymer can be effectively changed and the dynamic range of the analytical method widen by adjusting the concentration of NP in solution. Using this assay, PHMG can be determined in the aqueous solution within 0.01–0.1 mg/L by using silver NP and 0.05–15 mg/L with gold NP. With gold NP, 0.01 mol/L NaCl completely eliminates the scattering signal, however, the silver NP – PHMG aggregates withstand 1 mol/L NaCl. Interestingly, dilute solutions of PHMG (0.5 mg/L) are more reactive when prepared from concentrated (100 mg/L) stock solutions of PHMG, while the same 0.5 mg/L solutions prepared from ~1 mg/L stock or by serial dilution demonstrate lower aggregation activity. That might be explained by association of the polyelectrolyte in the concentrated solution. The metastable PHMG fragments likely to be present after dilution may form aggregates larger than the “aged” equilibrium solutions, which results in higher light scattering.