Most recently, polymeric nanocapsules have attracted more and more attention because of their specific properties and applications, such as drug delivery 1 2 3 4, catalysts 5 6, light-emitting diodes 7 , probing single-cell signaling 8 , self-healing materials 9 , and so on.

By now, a variety of physical and chemical strategies have been developed for the preparation of polymeric nanocapsules such as template method 10 11 12 13 14 15 16 17, micelle method 18 19 20 21, emulsion polymerization 22 23 24, interfacial polymerization 25 26 27, and other methods 28 29 30 31. In most of the polymeric nanocapsules reported, the single voided cavum in the nanocapsules were globose. Furthermore, the surface-modification procedure is important to introduce some functional groups onto their inner and/or outer surfaces so that the functional polymeric nanocapsules were achieved.

In the present work, we developed a facile strategy for the crosslinked polymeric multi-lacuna nanocapsules (CP(St–OA) nanocapsules) about 40 nm with carboxylic groups on their inner and outer surfaces. The spindle-like α-Fe₂O₃nanoparticles were organo-modified with oleic acid (OA) and the oleic acid modified spindle-like α-Fe₂O₃nanoparticles (OA–Fe₂O₃) formed the small conglomerations in water. Then the small conglomerations were used as the templates for the soapless microemulsion polymerization. The small conglomerations of OA–Fe₂O₃were encapsulated in the crosslinked polymer nanoparticles obtained. Then the crosslinked polymeric multi-lacuna nanocapsules (CP(St–OA) nanocapsules) with carboxylic groups on their inner and outer surfaces were achieved after the templates were etched with HCl in tetrahydrofuran (THF).

Oleic acid (OA) with a formula of C₁₈H₃₄O₂ (or CH₃(CH₂)₇CH=CH(CH₂)₇COOH) is a monounsaturated omega-9 fatty acid found in various animal and vegetable sources. It is widely used as surfactant for the soapless seeds emulsion polymerization in the recent years 32 33 34 35 36. It could form mono-molecular or bi-molecular layer on the surfaces of the inorganic nanoparticles via its carboxyl groups and the C=C group could copolymerize with the vinyl monomers. So it acts as an interlinkage between the inorganic cores and the polymer shells.

In the present work, oleic acid was used for the surface modification of the spindle-like α-Fe₂O₃nanoparticles. It could be found that the oleic acid modified α-Fe₂O₃nanoparticles (OA–Fe₂O₃) had dispersed better than the bare α-Fe₂O₃nanoparticles (Fig. 1a and b). The α-Fe₂O₃nanoparticles formed the small conglomerations composed with several pieces of α-Fe₂O₃nanoparticles in water. The OA formed bi-molecular layers on the surfaces of the small conglomerations as shown in Scheme 1.

After the polymerization of the monomer St and the crosslinker DVB added, the crosslinked polymer shells were obtained to encapsulate the small conglomerations as templates. The carboxyl groups were decorated onto the surfaces of the α-Fe₂O₃nanoparticles encapsulated in the crosslinked polymer nanoparticles (Fe₂O₃/CP(St–OA)) via the copolymerization of OA. The small conglomerations of the α-Fe₂O₃nanoparticles were found in the Fe₂O₃/CP(St–OA) (Fig. 1c) and the electron diffractometry (ED) of the α-Fe₂O₃nanoparticles in the (Fe₂O₃/CP(St–OA) confirmed it (Fig. 2). Its average particle size was found to be 163.5 nm by the LLS analysis.

Then the Fe₂O₃/CP(St–OA) nanoparticles were dispersed in THF and treated with HCl to remove the small conglomerations of the α-Fe₂O₃nanoparticles. The products were near white after the etching procedure. The characteristic IR bands of α-Fe₂O₃nanoparticles at 449 and 532 cm⁻¹disappeared in the Ft-IR spectrum of the CP(St–OA) nanocapsules (Fig. 3). And the electron diffractometry (ED) of the α-Fe₂O₃could not be observed in the CP(St–OA) nanocapsules. They indicated that the α-Fe₂O₃nanoparticles had been removed completely.

The TEM images of the CP(St–OA) nanocapsules were given in Fig. 1d. The multi-lacuna structures were found in the CP(St–OA) nanocapsules with particle size of about 40 nm. They were near mono-dispersed. Compared with the structures and particle size of the Fe₂O₃/CP(St–OA) nanoparticles, the voided cavum were in the center of the nanocapsules. It could be predicated that the crosslinked polymer shells had reset and shrank in the etching period in THF.

The effects of pH value on the zeta potentials of the crosslinked polymeric multi-lacuna nanocapsules (CP(St–OA) nanocapsules) are illustrated in Fig. 4. In the studied pH range, the CP(St–OA) nanocapsules showed the negative zeta potential. This indicated that the surfaces of the silica nano-sheets were negative charged in the pH range. Continuously increasing the pH value to the basic condition, the absolute value of its zeta potential increased. It validated the presence of the carboxyl groups on the surfaces of the CP(St–OA) nanocapsules, as illustrated in Scheme 1. The surface functional groups are expected to extend the applications of the polymeric nanocapsules.

In summary, we developed a facile strategy for the preparation of the mono-dispersed crosslinked polymeric multi-lacuna nanocapsules with functional groups on the inner and outer surfaces via the soapless microemulsion polymerization technique. Their structures, surface functional group and particle size could be altered by changing the inorganic oxide nano-cores, the surfmer (polymerizable surfactant) used and the formula of the polymerization. The technique is expected to extend the applications of the polymeric nanocapsules.