There has been considerable interest in recent years in using metal, semiconductor, and magnetic nanoparticles in biological applications. [1][2][3][4][5][6][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 particular interest to us, since these stable shells prevent dissociation of surface ligands and provide anchor points where biomolecules 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 attachment 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 challenge. 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 selectively and with controlled valency. Recently, Taton et al. reported encapsulation of gold nanoparticles (AuNPs) [14,15] and magnetic nanoparticles (MagNPs) [16,17] by amphiphilic diblock copolymers. The resulting nanoparticles have a stable, well-defined 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 inclusion on encapsulation. [14] In contrast, large AuNPs act as surface templates on which polymer molecules assemble into micellar 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 encapsulate AuNPs in "hairy" micelles (Figure 1 B); 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 po...
