Embedding Functionalities Into The Walls Of Polyelectrolyte Capsules

The distinct advantage of using the capsule-based systems described above lies in the potential of combining a variety of substances with different functionalities in one unique system. Modification of capsule walls by colloidal nanoparticles and biological macromolecules is described further in this section. (fig 2) shows how, based on the principle of the LbL assembly, different types of charged nanoparticles can be simultaneously embedded in the capsule wall by employing electrostatic forces between the nanoparticles and the oppositely charged layer.

Fig. 2. Schematic showing addition of multiple functionalities into the walls of polyelectrolyte (PE) capsules. (i) Loading of charged nanoparticles (NPs) such as fluorescent (green), magnetic (black), and metal (yellow) NPs into the PE wall. The NPs shown are negatively charged and thus stick to the positively charged polymer layers. (ii) Stabilization of the capsule by subsequent LbL assembly. (iii) Core dissolution. The capsules are not drawn to scale. Only one layer of NPs is shown for sake of clarity.

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By adding luminescent colloidal semiconductor nanoparticles such as CdTe and CdSe quantum dots, the capsules can be easily detected by measuring their fluorescence signal with noninvasive optical techniques. The use of fluorescent nanoparticles as fluorophores for bio-labeling applications presents two main advantages compared with commercial organic fluorophores 43 T. Pellegrino et al., Small 1 (2005), p. 48. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (91). Firstly, they are characterized by nearly continuous excitation spectra with narrow emission bands, located at different wavelengths depending on the nanoparticle size. This allows for simultaneous excitation of probes of different colors by light of a single wavelength. Secondly, their reduced photobleaching makes them suitable for measurements over long periods of time. On the other hand, the Cd-based quantum dots, which are still most frequently used, can cause cytotoxic effects by the release of Cd ions.

By embedding magnetic nanoparticles such as Fe₃O₄ into the capsule walls, the movement of capsules in a desired direction can be controlled by applying an external magnetic field gradient. In addition, heating of the nanoparticles upon application of radio frequency fields increases the permeability of the capsule walls.

Noble metal nanoparticles such as gold and silver are known to strongly absorb light. Upon irradiation, the major part of the absorbed light energy by each nanoparticle is transformed into heat. The collective effect of several nanoparticles in a unique system is to amplify the heating effect, resulting in an increase in temperature in the surrounding areas. In this way, by embedding metal nanoparticles into the walls of polyelectrolyte capsules, the integrity/permeability of the wall of individual capsules can be selectively perturbed.

Another possibility for polyelectrolyte capsule functionalization is through conjugation of biological molecules to their walls. Biomolecules could be added to the outermost layers of the capsule walls by electrostatic adsorption or covalent binding, depending on their charge features (fig .3). Such biofunctionalized capsules might fully mimic the features of bioactive molecules, thus improving the biocompatibility of the capsules. Using this concept, the interaction of capsules with living cells could be greatly improved. Furthermore, based on the molecular recognition system of cells, specific interactions between capsules and cells can be achieved by adding ligand molecules to the capsule surface to specifically bind to receptor molecules present on cells.

Fig. 3. Schematic of bioconjugation of capsule surfaces. (i) Electrostatic adsorption. The biological molecules have a positively charged domain and thus stick to negatively charged polymer layers. (ii) Covalent binding of bimolecular. The capsules are not drawn to scale.