The reversible swelling process of poly(amidoamine)-PAMAM dendrimers examined by the surfaceplasmon resonance (SPR) and quartz crystal microbalance (QCM) techniques.
Dendrimers are specific class polymers. They have highly branched, symmetric and spherical structuresproduced by means of either divergent or convergent methodologies of synthesis which lead to formation ofmonodispersed nanoparticles of controlled dimension, dependent on the generation number and the specificsurface functionalisation. Study of dendrimer’ properties is still in its infancy, as compared to classical polymers,and the large gap in our knowledge must be filled to enable potential applications of these new materials, e.g., asdrug delivery vesicles. Structure of dendrimer molecules is described in the literature as “fuzzy”. This structurefavours, that, upon contactwith water, a gel-like layer is formed on surface of a hydrophilic dendrimer which thelayer is of a much lower density than the core region. Water also fills internal voids of dendrimer. As a result, theeffective volume of the molecule in aqueous solution is larger than under dry conditions. Furthermore, at lowpH, the molecule volume increases as a result of adopting extended conformations which minimize electrostaticrepulsion between the protonated functional groups. The opposite effect occurs with increasing the ionic strengthwhich forces the molecule to adopt back-folded conformations, as a result of. the charge compensation. Thesephenomena are illustrated in Fig.. which shows molecular models of G-6 PAMAM dendrimer obtained bymolecular dynamics simulations under different pH (acidic, neutral and alkaline, respectively).
The volume increase of PAMAM dendrimers may be interpreted as swelling the molecule due tointeractions between the solvent and the primary and tertiary amine groups. For PAMAM dendrimers at high pH(>=12), none of amine groups are protonated. At neutral pH, only the primary amines are protonated, and at lowpH (<4) – both the primary and tertiary amines are protonated. Protonation of amine groups (in particular tertiaryamines) inside dendrimer attracts hydration water into internal voids, leading to a greater swelling. Themolecular dynamics simulations show that, at low pH, the PAMAM dendrimer’ structure is open, allowing forpenetration of solvent molecules into the interior. Maiti et al.estimated that, at high pH, three water moleculesare bound per tertiary amine and this ratio increases to six at low pH.
The QCM and SPR techniques will be jointly used to characterize the chosen PAMAM dendrimersadsorbed on gold and silica surfaces – the films formed in situ in aqueous solutions and after drying them. Bycombining these techniques, it is possible to evaluate content of water trapped in dendrimers, by the followinganalysis.
Concluding, the major advantage of the project is that the QCM and the SPR techniques, when usedjointly, will allow for evaluation of parameters characterizing the dendrimers monomolecular films, inparticular, degree of the dendrimers hydration and increase in the molecule volume, in dependence on pH andthe ionic strength. By following time changes in the film thickness, kinetics of the adsorption process may becharacterized. Comparison between the results for the three dendrimers let evaluate influence of the surfacecharge density (increasing with the generation number) on the adsorption. Performing the SPR measurements onpolarized Au slides will allow for a deeper evaluation of role of electrostatic interactions in the adsorption.Following time changes of the film thickness upon changing pH from alkaline to acidic one enables evaluationof kinetics of the swelling process.
