Self‐Assembly of Peptide‐Inorganic Hybrid Spheres for Adaptive Encapsulation of Guests

Adv. Mater. 2010, 22, 1283–1287 2010 WILEY-VCH Verlag G T IO N Self-assembly, a common process at all scales, is emerging as a powerful, bottom-up approach for the fabrication of novel functional nanoor biomaterials. It is ubiquitous in nature. By learning from nature or imitating the self-assembly process in biological systems, one can delicately design or extract molecular building blocks for the creation of biomimetic or bioinspired nanostructural materials. Many bioactive building blocks for self-assembly are derived with inspiration from a pathogenic process. A known example is that of the diphenylalanine peptide (L-Phe-L-Phe) (FF) which is extracted from Alzheimer’s b-amyloid polypeptide as the core recognitionmotif for molecular self-assembly. Such peptides are a sort of versatile, selfassembling building block in the construction of defined supramolecular structures, owing to their ease of synthesis, facile chemical and biological modification, and biocompatibility. However, improved properties and novel functions for such nanoor biomaterials are needed to arrive at potential applications in nanotechnology. The fabrication of hybrid, supramolecular systems based on the combination of peptide or protein building blocks and inorganic components is an effective strategy to achieve the integration of functions. Herein, polyoxometalates (POMs), a well-known class of anionic oxide nanoclusters of transition metals, are used as possible inorganic components for the fabrication of such hybrid materials, owing to their potential applications in catalysis, electronics, optics, magnetic materials, medicine, and biology. We selected a Keggin-type POM, phosphotungstic acid (PTA) as a polyoxoanion model molecule, and combined it with the cationic dipeptide (CDP), H-Phe-Phe-NH2 HCl, which is derived from the FF peptide to assemble the expected hybrids. Figure 1 shows the suggested process of the formation of such functional hybrid colloidal spheres from the coassembly of PTA and CDP in water. To our knowledge, this is the first time that a stable, spherical structure has been obtained in water, based on the coassembly of a bioactive peptide and a polyoxoanion. The as-prepared colloidal spheres not only display stimuli-responsive properties to pH or temperature, but also lead to a novel function: enabling adaptive encapsulation for a wide variety of guest materials ranging from small molecules to nanoscale materials during self-assembly. The supramolecular assembly in the form of colloidal spheres based on PTA and CDP was initially investigated and prepared by the addition of an aqueous solution of PTA to a 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) solution of CDP (in a 1:5 charge ratio) at room temperature. Such a mixing resulted in an immediate, opalescent, cloudy suspension (Fig. S1a in the Supporting Information), indicating there was some assembly taking place in the system. The precipitates, which were separated from the bulky solution, were imaged using scanning electron microscopy (SEM). An SEM image (Fig. 2a) shows that the assemblies are colloidal spheres with diameters ranging from about 100 to 250 nm. Energy dispersive X-ray (EDX) spectroscopy attached to the SEM (inset in Fig. 2a) indicates that such hybrid colloidal spheres are composed of both PTA and CDP, as evidenced by the presence of tungsten and carbon elements throughout the assemblies. Transmission electron microscopy (TEM) images (Fig. 2b–c) also demonstrate the formation of regular spherical structures with average diameters of 150 nm. The high-resolution TEM (HR-TEM) studies indicate that the hybrid colloidal spheres contain basic structural units consisting of many dark objects approximately 1 nm in size (attributable to single PTA clusters) surrounded by a peptide shell with lower electron contrast (Fig. 2d). The average size of the peptide-encapsulated clusters (PECs) is about 1.4 nm. It is noted that the diameter of each POM molecule with a Keggin structure is around 1 nm. In a dynamic light scattering (DLS) measurement