Colloidal objects are uibiquitous in any natural environment areosols, waste water, latexes; carriers for drugs or cosmetics; in ceramics and cements; even bacteria and viruses in water have a colloidal behaviour.
For a quality control of their production, for the analysis of pollutants or for the study of colloids’ behaviour in physiological conditions it is necessary to characterize these systems, with techniques that possibly are a) non-destructive, b) highly sensitive and c) widely applicable, providing data about size, size distribution and surface chemical composition and interactions.
Most analytical techniques are limited in the output. Moreover, they often require ad hoc prepared samples, thus are not suitable for real quality control. Last, but not least, some of them show a size-dependent sensibility, complicating the analysis output.
Our target is to develop an analytical method (PGSE-NMR) that is a) chemically selective, thus can study multi-component samples, analyzing each chemically-different species one at the time, b) robust and c) amenable to the study of equilibria or interactions, having particularly in mind those with biomolecules.
What is special about the project?
A technique for the complete characterisation of colloidal objects is of high significants in a variety of different fields: For example in pharmaceutics, where the interactions of colloids with proteins in body fluids can determine a foreign body reaction to a drug carrier; or in waste water treatment, where colloids coagulation is a fundamental step.
Summary: The analytical method (PGSE-NMR) has been developed to an optimal level. The developed analytical method has revealed as very useful for both the characterization of colloidal species an of their interactions too.
We have set the first target of our research in the development of a technique for the colloidal characterization in a chemical composition-dependent fashion: this allows for a separate analysis of objects that, even if present in the same sample, are characterized by a different composition.
Our second aim (was then to use the chemical sensitivity of this technique for studying interactions and equilibria between the target colloids and other species (colloidal or not), which are present in water at the same time.
We have first focussed our attention on two forms of colloids that our group has been studying in recent years: polymer vesicles and nanoparticles. In PGSE-NMR experiments we have used the 1H-NMR signals of the hydrophilic portions of polymers that form vesicular aggregates or are present on the surface of nanoparticles, in both cases composed of poly(ethylene glycol) (PEG) blocks. Tracking these signals, it was possible to monitor the diffusion coefficients, and from here the dimensions, of any species containing PEG.
PGSE-NMR has shown that
- the two processes of vesicle production (exposure of polymer films to water, sonication, extrusion through porous substrates) and of nanoparticle preparation (emulsion polymerization) produced two populations of colloidal species, both containing PEG, but characterized by markedly different dimensions
- the smaller colloids are in both cases micelles: for polymeric vesicles, the micelles are formed by the same block copolymers that produce vesicles too; for nanoparticles, they are composed by the emulsifier used in emulsion polymerization.
After having used PGSE-NMR for characterizing colloidal mixtures, we have employed it for probing equilibria on the surface of the vesicles and nanoparticles. The relevance of this study derives from the fact the physico-chemical stability of colloidal particles and their interactions with a biological environment mainly depend on surface phenomena. However, if there is a big need of experimental techniques for studying these phenomena, their development is still at the infancy.
The concept at the basis of our research is that the strong interactions between a smaller object and a bigger aggregate always determines a difference in the diffusion properties of the object, generally a decrease in diffusion coefficient and thus an increase in apparent size (it “sticks” on the surface of the bigger colloid).
As a first model case for the application of PGSE-NMR, we have studied the systems previously presented (polymer vesicles and nanoparticles): we have addressed the question whether the different populations of colloidal species (micelles and vesicles or nanoparticles, respectively) are in equilibrium. The presence of an equilibrium has a profound relevance for the stability of the bigger colloids (vesicles or nanoparticles): if they can be separated from the smaller colloids (micelles) and an equilibrium exists, polymers may leave the surface of the vesicle or nanoparticle, undermining their stability, in order to form new micelles and determine again an uncertain composition of the suspension.
We have indeed demonstrated the absence of an equilibrium with micelles for both vesicles and nanoparticles, by observing that the micellar component can be effectively removed (dialysis, ultrafiltration) and is not re-formed for periods up to one year.
M. Valentini, A. Vaccaro, A. Rehor, A. Napoli, J.A. Hubbell, N. Tirelli, “Diffusion NMR spectroscopy for the characterization of size and interactions of colloidal matter: the case of vesicles and nanoparticles”, Journal of the American Chemical Society, 6(7) (2004) 2142-2147 [full paper]
A. Napoli, M. Valentini, N. Tirelli, M. Müller, J.A. Hubbell, “Oxidation-Sensitive Polymeric Vesicles”, Nature Materials, 3(3) (2004) 183-189 [full paper]
Last update to this project presentation 17.10.2018