Adsorption Kinetics

The adsorption kinetics of oligonucleotides onto latexes is essential to evaluate the incubation time needed to reach the adsorption equilibrium. Both positively and negatively charged latexes (latex bearing sulfate groups and latex bearing amine and amidine groups, respectively) were tested [9].

The adsorption of ODN onto positively charged polymer particles was found to be rapid as expected due to their opposite charges, whereas the adsorption rate on negatively charged latex particles was low. The observed kinetics results have mainly been discussed in terms of attractive or repulsive electrostatic forces, though in some cases weaker hydrophobic interactions play a nonnegligi-ble role in the adsorption process, especially on negatively charged latexes [10]. In addition, complementary experiments were also carried out on coated cationic polystyrene particles, e.g., where a noncharged surfactant was first adsorbed on

FIG. 7 Structure of Triton X-405 surfactant [p-(1,1,3,3-tetramethylbutyl)phenoxypoly-(oxyethylene glycol)] with an average n value of 40.

the particles to modify the surface polarity and cationic charge accessibility (Triton X-405; see Fig. 7) [11]. In this case, the adsorption rate was low compared to bare cationic polystyrene latex particles because of the reduction in attractive forces involved in the adsorption process (Fig. 8). In addition, the noncharged layer on the particle surface acts as an adsorption barrier responsible for the decrease of both the adsorption rate and the amount of adsorbed oligonu-cleotides.

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