Bromide-doped polypyrrole microcapsules modified with gold nanoparticles

Abstract

Micrometer sized polypyrrole capsules were prepared through the photochemical deposition of polymer onto the surface of aqueous droplets dispersed in bromoform. Scanning electron microscopy and transmission electron microscopy analyses demonstrate that the microcapsules are ca. 550 nm in diameter and contain an empty hollow region surrounded by a ca. 40 nm thick polymer shell. The XPS data reveal that the polymeric shell consists of polypyrrole doped with bromide anions, likely originating from the decomposition of bromoform during the photopolymerization reaction. The incorporation of gold nanoparticles inside the capsule allows for the ultrasound-triggered rupture of the structure. This phenomenon was applied to liberate encapsulated Rhodamine 6G to the external solution. This finding is very promising for possible applications in controlled drug delivery.

Introduction

Hollow polymer microcapsules have received considerable attention in recent years due to their important applications in medicine and chemical analysis. The most exciting feature of these structures is their ability to encapsulate guest materials inside the void regions, which can be released in a controlled manner into the surrounding medium. Polymeric hollow microcapsules have been successfully employed as delivery vehicles that transport drugs through the body to the targeted tissue. The role of the capsule's shell is either to shield the encapsulated material from degradation or to protect the human body from the harmful effects of the drug and restrict the release to only a desired therapeutic volume [1], [2], [3], [4], [5], [6]. Hollow microspheres have also been used as micrometer-sized sensors that allow for the determination of the local concentration of analytes under in situ conditions. The diffusion of the analyte to the interior of the microcapsule yields an analytical signal that is subsequently detected by a confocal microscope. For example, microsphere-based sensors were used to monitor the pH and virus-specific antibodies [4], [6]. The application of microcapsules as transducer layers in all-solid state ion selective electrodes has also been demonstrated [7].

Polymeric microcapsules are prepared from a variety of materials, which include but are not limited to polyelectrolytes [8], polysaccharides [9], proteins and conducting polymers. Although all of these materials have specific advantages, conducting polymers have recently attracted considerable attention due to their unique features [10]. They have been shown to exhibit a pH dependent permeability. In addition, they can be easily oxidized or reduced, which modifies their optical, electronic and mechanical properties [11]. Moreover, they are easily prepared through the chemical or electrochemical polymerization of the corresponding monomer. Recently, photochemical preparation of conducting polymers has attracted considerable attention. One of the most promising examples is photopolymerization of pyrrole which is carried out in chlorinated solvents [12], [13], [14], [15], [16], [17], [18]. Even though the detailed mechanism of the reaction is not known, it is believed that the photoexcitation of the monomer results in electron transfer to the solvent molecule. This generates radical cations that couple to produce the polymer. The possible mechanism of pyrrole photopolymerization in chloroform is shown below [19]:

Py + CHCl₃ + hν → Py^+· + ·CHCl₂ + Cl⁻

Py^+· + Py^+·→ (Py)₂ + 2H⁺

We have demonstrated recently that photopolymerization of pyrrole in water–chloroform emulsion yields micrometer-sized capsules [19], [20], [21]. The polymer preferentially deposits onto the surface of aqueous droplets resulting in formation of hollow structures. As polypyrrole is a biocompatible material [22], [23], [24] such structures are promising as smart drug carriers. Here, the problem of controlled release of the encapsulated material is crucial. There are, in general, two ways to achieve this task: passive or stimulated release. In passive release, the encapsulated species permeate through the capsule's shell due to a chemical potential gradient. For stimulated release, an external stimulus is required to rupture or increase permeability of the capsule's wall. Several stimuli have been demonstrated to be effective to achieve this task, e.g. temperature, light or ultrasound [25]. A key feature of ultrasound from the point of view of medical applications is that this is a non-invasive, safe and painless transmission of energy into the body. Ultrasound can be used to transmit energy at precise locations and thus may be applied to damage the capsule walls. To enhance the ultrasonic effects the incorporation of solid particles into the capsules' walls has been demonstrated. The exact mechanism of nanoparticle-mediated destruction of polymer capsules is not clear, however, it is anticipated that the main effect is due to the fact that the nanoparticle-modified polymer is more rigid thus it is more susceptible to mechanical damage [26], [27], [28].

Herein, we report a facile method for the fabrication of polymer microcapsules through the photochemical polymerization of pyrrole onto the surface of aqueous droplets dispersed in a bromoform solution. Using microscopy techniques, we show that the resulting structures contain a hollow region surrounded by a polymer wall. The encapsulation of guest species within the microcapsules was demonstrated using gold nanoparticles and a Rhodamine 6G (R6G) dye. The release of the encapsulated material was achieved through the nanoparticle-mediated ultrasound rupture of the capsule's shell. Our findings show that polypyrrole-based hollow structures are a promising material that could find medical applications in controlled drug delivery.