DOI: 10.1002/cphc.200700598 Encapsulation of Single Small Gold Nanoparticles by Diblock Copolymers Hong Y. Chen,* [a] Sinoj Abraham, [b] Juana Mendenhall, [a] Soazig C. Delamarre, [a] Kahli Smith, [a] Il Kim, [a, b] and Carl A. Batt [a] There has been considerable interest in recent years in using metal, semiconductor, and magnetic nanoparticles in biological applications. [1–7] A wide range of ligation and encapsulation methods have been developed to render the nanoparticles soluble in aqueous solution, to prevent aggregation, and to provide means by which functional molecules can be attached. Among these methods, encapsulation of nanoparticles by a polymer, [8,9] phospholipid, [10] or inorganic [11,12] shell is of particu- lar interest to us, since these stable shells prevent dissociation of surface ligands and provide anchor points where biomole- cules are unlikely to be lost once attached. This is a significant advantage over direct conjugation through surface ligands, since even strong thiol ligands can dissociate from or undergo exchange on gold surfaces, [13] let alone weaker ligands on the surfaces of quantum dots or magnetic nanoparticles. Stable at- tachment of biomolecules would be particularly important where only a few biomolecules are selectively attached to a nanoparticle, or when multiple types of singly functionalized nanoparticles are mixed. Stable functionalization of quantum dots remains a chal- lenge. While biomolecules have been attached to quantum dots and used for biological studies, [4,5] a nondissociable ligand shell would be required for attachment of biomolecules selec- tively and with controlled valency. Recently, Taton etal. report- ed encapsulation of gold nanoparticles (AuNPs) [14,15] and mag- netic nanoparticles (MagNPs) [16,17] by amphiphilic diblock co- polymers. The resulting nanoparticles have a stable, well-de- fined core/shell structure impermeable to ionic species in aqueous solution. Such a polymer shell would be ideal for functionalization of quantum dots if a similar encapsulation methodology could be adopted. However, it was found that in this system small (d < 10 nm) AuNPs and MagNPs act as solutes in polymer micelles and are therefore prone to multiple inclu- sion on encapsulation. [14] In contrast, large AuNPs act as sur- face templates on which polymer molecules assemble into mi- cellar shells that each encapsulate a single AuNP. Since most nanoparticles used for biological studies, particularly quantum dots, have diameters in the range of 2–9 nm, it is necessary that we develop new methods that can encapsulate single nanoparticles of sizes similar to quantum dots. Herein we report the encapsulation of single small AuNPs, in preparation for future work on quantum dots, since AuNPs are easier to handle and characterize. Diblock copolymers such as PS 108 PGA 108 , PS 132 PAA 72 , and PS 159 PAA 62 [PS: polystyrene, PGA: poly(glutamic acid), PAA: poly(acrylic acid)] were used to en- capsulate AuNPs in “hairy” micelles (Figure 1B); the resulting core/shell nanoparticles are stable in solution without chemical crosslinking. The long hydrophilic blocks of the polymers were initially chosen to help stabilize attached biomolecules, but were later found to allow encapsulation of single small AuNPs. The use of such polymers for encapsulation requires conditions that differ from existing literature methods. Previously, only highly asymmetric copolymers (PS 250 PAA 13 , PS 160 PAA 13 , PS 100 PAA 13 , and PMMA 240 PAA 13 ) [PMMA: poly(methyl methacry- late)] were used, [14] which gave “crew-cut” micellar shells (Fig- ure 1A), whereas polymers with longer hydrophilic chains (PS 159 PAA 62 and PS 49 PAA 54 ) failed to do so. [14,16,18] Figures 2B and C show transmission electron microscopy (TEM) and scan- ning electron microscopy (SEM) images of the polymer-encap- sulated AuNPs (AuNP@polymer). Despite the long hydrophilic blocks, the nanoparticles appear to be spherical with a well-de- fined core/shell structure similar to those reported by Taton and co-workers. [14] Micellization of crew-cut polymers in the lit- erature usually involves slow addition of a nonsolvent to a so- lution of polymer in a good solvent, [14,19] whereby polymer self-assembly is induced by slow and controlled exclusion of polymer from the solution. Since polymers with long hydro- [a] Prof. H. Y. Chen, + J. Mendenhall, S. C. Delamarre, K. Smith, Prof. I. Kim, Prof. C. A. Batt Department of Food Science, Cornell University Ithaca, NY 14853 (USA) E-mail: hongyuchen@ntu.edu.sg [b] S. Abraham, Prof. I. Kim Department of Polymer Science and Engineering Pusan National University, Jangjeondong, Geumjeong-gu Busan 609-735 (Korea) [ + ] Current address: Division of Chemistry and Biological Chemistry Nanyang Technological University, 637616 (Singapore) Supporting information for this article is available on the WWW under http://www.chemphyschem.org or from the author. Figure 1. Gold nanoparticles encapsulated in A) crew-cut and B) hairy mi- celles, composed of linear diblock copolymers with short and long hydro- philic polymer chains, respectively. The hydrophobic blocks in both cases form a compact shell in aqueous solution. 388 # 2008 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim ChemPhysChem 2008, 9, 388 – 392