Peptide‐Directed Synthesis and Assembly of Hollow Spherical CoPt Nanoparticle Superstructures

Controlling the directed assembly of nanoparticles into welldefined nanoparticle superstructures is a significant challenge, and our goal is to develop a general methodology to address this issue. The success and broad applicability of a particular nanoparticle-assembly methodology should be assessed according to the following important criteria: 1) diverse structural scope, 2) ability to tune and tailor superstructure metrics (e.g., nanoparticle size, nanoparticle shape, interparticle distances, and superstructure diameter), and 3) diverse compositional scope. Only through rational control of structure, metrics, and composition can diverse collections of nanoparticle superstructures with highly specific and tailorable properties ultimately be designed and prepared. We recently introduced and developed a peptide-based methodology for directing the synthesis and assembly of gold nanoparticles. This methodology relies on carefully designed peptide-conjugate molecules that control the synthesis of individual nanoparticles and direct their assembly into complex nanoparticle superstructures. We successfully demonstrated that this methodology addresses criteria 1 and 2 listed above. Specifically, we used this methodology to prepare a diverse set of complex gold nanoparticle superstructures, including double helices, linear belts, and hollow spherical structures, and to tune the metrics of the superstructures. In this contribution, we begin to address criterion 3 listed above. Peptide conjugates are the centerpiece molecules in this methodology. They consist of two components: an inorganicbinding peptide and an organic moiety tethered to the peptide terminus. The peptide portion binds to the nanoparticle surface while the organic moiety influences the assembly of the peptide and therefore the assembly of the nanoparticles. Numerous peptides exist which have been evolved and selected, naturally or unnaturally, to adhere to specific inorganic surfaces. Thus far, we have only utilized a gold-binding peptide in this methodology. In principle, however, we could choose any inorganic-binding peptide, which would allow us to target and prepare nanoparticle superstructures of variable composition. Here, we introduce, describe, and demonstrate how our peptide-based methodology can be adapted to target and prepare CoPt nanoparticle superstructures. It is established that CoPt nanoparticles are potentially useful in nanomedicine as magnetic resonance imaging (MRI) contrast agents and as electrocatalysts. For these applications, individual CoPt nanoparticles are typically employed. We sought to assemble CoPt nanoparticles into hollow spherical sub100 nm superstructures, reasoning that they may ultimately be useful as multifunctional bionanomaterials capable of serving both as MRI imaging and delivery agents (e.g., drug, biomolecule, labeling, etc.) or as high surface area electrocatalysts. As a first step in the latter direction, we show herein that the assembled CoPt superstructures can serve as electrocatalysts for the oxidation of methanol. To prepare hollow spherical CoPt nanoparticle superstructures, we first selected the Co-binding peptide HYPTLPLGSSTY (Co1-P10), which was isolated by Naik et al. and is hereafter referred to as PEPCo. PEPCo was used previously by Naik et al. to prepare both Co nanoparticles and CoPt alloy nanoparticles. The reported syntheses are straightforward and are performed in aqueous media at a neutral pH value. In order to prepare hollow spherical superstructures, we decided to utilize the conjugate BP-PEPCo (C12H9CO-HYPTLPLGSSTY, BP= biphenyl; see the Supporting Information for synthetic details), because our previous results with biphenyl-based PEPAu conjugates resulted in hollow spherical gold nanoparticle superstructures. BP-PEPCo, upon dissolution and subsequent incubation in HEPES buffer (0.1m, HEPES= 4-(2-hydroxyethyl)piperazineethanesulfonic acid), assembles into well-defined spherical structures ((30.9 4.5) nm), as evidenced by transmission electron microscopy (TEM) studies (Figure S3 in the Supporting Information). Encouraged by these results, we next adapted the reported synthesis for CoPt nanoparticles by replacing PEPCo with BP-PEPCo to determine whether BP-PEPCo could serve the dual purpose of directing both the synthesis of CoPt nanoparticles and their assembly into spherical superstructures (Scheme 1). Specifically, a 6.5:1 mixture of cobalt acetate and BP-PEPCo was incubated for four hours at room temperature in HEPES buffer to give a colorless solution. Thereafter, aliquots of sodium borohydride [*] C. Song, Y. Wang, Prof. N. L. Rosi Department of Chemistry, University of Pittsburgh 219 Parkman Avenue, Pittsburgh, PA 15260 (USA) E-mail: nrosi@pitt.edu Homepage: http://www.pitt.edu/~nrosi/nathanielrosi.htm