Gold colloids have fascinated scientists for over a century and are now heavily utilized in chemistry, biology, engineering, and medicine. Today these materials can be synthesized reproducibly, modified with seemingly limitless chemical functional groups, and, in certain cases, characterized with atomic-level precision. This Review highlights recent advances in the synthesis, bioconjugation, and cellular uses of gold nanoconjugates. There are now many examples of highly sensitive and selective assays based upon gold nanoconjugates. In recent years, focus has turned to therapeutic possibilities for such materials. Structures which behave as gene-regulating agents, drug carriers, imaging agents, and photoresponsive therapeutics have been developed and studied in the context of cells and many debilitating diseases. These structures are not simply chosen as alternatives to molecule-based systems, but rather for their new physical and chemical properties, which confer substantive advantages in cellular and medical applications.

The cellular internalization of oligonucleotide-modified nanoparticles is investigated. Uptake is dependent on the density of the oligonucleotide loading on the surface of the particles, where higher densities lead to greater uptake. Densely functionalized nanoparticles adsorb a large number of proteins on the nanoparticle surface. Nanoparticle uptake is greatest where a large number of proteins are associated with the particle.

Polyvalent oligonucleotide gold nanoparticle conjugates have unique fundamental properties including distance-dependent plasmon coupling, enhanced binding affinity, and the ability to enter cells and resist enzymatic degradation. Stability in the presence of enzymes is a key consideration for therapeutic uses; however the manner and mechanism by which such nanoparticles are able to resist enzymatic degradation is unknown. Here, we quantify the enhanced stability of polyvalent gold oligonucleotide nanoparticle conjugates with respect to enzyme-catalyzed hydrolysis of DNA and present evidence that the negatively charged surfaces of the nanoparticles and resultant high local salt concentrations are responsible for enhanced stability.

We report the synthesis and characterization of polyvalent RNA-gold nanoparticle conjugates (RNAAu NPs), nanoparticles that are densely functionalized with synthetic RNA oligonucleotides and designed to function in the RNAi pathway. The particles were rationally designed and synthesized to be free of degrading enzymes, have a high surface loading of siRNA duplexes, and contain an auxiliary passivating agent for increased stability in biological media. The resultant conjugates have a half-life six times longer than free dsRNA, readily enter cells without the use of transfection agents, and demonstrate a high gene knockdown capability in a cell model.Over the past decade, researchers have designed, synthesized, studied, and applied polyvalent DNA-functionalized gold nanoparticles (DNA-Au NPs). 1 These efforts have resulted in a new fundamental understanding of hybrid nanostructures,2 important and in certain cases commercially viable detection and diagnostic assays,3 and the ability to program materials assembly through the use of DNA synthons.1 , 4 Polyvalent DNA-Au NPs have several unique properties, such as sharp and elevated melting temperatures,2b enhanced binding properties 2c (as compared with free strands of the same sequence), and distance-dependent optical properties. 5 In agreement with research on polyvalent molecular systems,6 the high surface DNA density and the ability of the nanoparticles to engage in multidentate interactions are the proposed origin of these unique properties.Recently, we demonstrated the utility of the polyvalent DNA-Au NP for antisense gene regulation, where the unique ensemble properties of the conjugate confer several important advantages in the context of intracellular target recognition and binding. 7 These properties include resistance to nuclease degradation and high cellular uptake as a result of their oligonucleotide functionalization. Although antisense DNA is an important way of regulating genes, an even more promising route is through the use of siRNA. 8 However, no methods have been developed for utilizing polyvalent particles and their unusual properties to load and transport RNA across cell membranes. Indeed, one must develop synthetic routes and materials that overcome one of the most challenging problems associated with RNA, most notably its chemical instability.Based upon our observations with DNA-modified particles, we hypothesized that gold nanoparticles densely functionalized with synthetic RNA oligonucleotides would take advantage of the ensemble properties that result from the dense surface functionalization of oligonucleotides, increase the stability and efficacy of the bound RNA, while retaining the ability to act in the highly potent and catalytic RNA interference pathway. While others have chadnano@northwestern.edu. Supporting Information Available: Experimental conditions, sequences, and materials synthesized. This material is available free of charge via the Internet at http://pubs.acs.org. We determined that treatment of citrate-capped Au NPs...

Glioblastoma multiforme (GBM) is a neurologically debilitating disease that culminates in death 14 to 16 months after diagnosis. An incomplete understanding of how cataloged genetic aberrations promote therapy resistance, combined with ineffective drug delivery to the central nervous system, has rendered GBM incurable. Functional genomics efforts have implicated several oncogenes in GBM pathogenesis but have rarely led to the implementation of targeted therapies. This is partly because many “undruggable” oncogenes cannot be targeted by small molecules or antibodies. We preclinically evaluate an RNA interference (RNAi)–based nanomedicine platform, based on spherical nucleic acid (SNA) nanoparticle conjugates, to neutralize oncogene expression in GBM. SNAs consist of gold nanoparticles covalently functionalized with densely packed, highly oriented small interfering RNA duplexes. In the absence of auxiliary transfection strategies or chemical modifications, SNAs efficiently entered primary and transformed glial cells in vitro. In vivo, the SNAs penetrated the blood-brain barrier and blood-tumor barrier to disseminate throughout xenogeneic glioma explants. SNAs targeting the oncoprotein Bcl2Like12 (Bcl2L12)—an effector caspase and p53 inhibitor overexpressed in GBM relative to normal brain and low-grade astrocytomas—were effective in knocking down endogenous Bcl2L12 mRNA and protein levels, and sensitized glioma cells toward therapy-induced apoptosis by enhancing effector caspase and p53 activity. Further, systemically delivered SNAs reduced Bcl2L12 expression in intracerebral GBM, increased intratumoral apoptosis, and reduced tumor burden and progression in xenografted mice, without adverse side effects. Thus, silencing antiapoptotic signaling using SNAs represents a new approach for systemic RNAi therapy for GBM and possibly other lethal malignancies.

Mammalian cells have been shown to internalize oligonucleotide-functionalized gold nanoparticles (DNA-Au NPs or siRNA-Au NPs) without the aid of auxiliary transfection agents and use them to initiate an antisense or RNAi response. Previous studies have shown that the dense monolayer of oligonucleotides on the nanoparticle leads to the adsorption of serum proteins and facilitates cellular uptake. Here, we show that serum proteins generally act to inhibit cellular uptake of DNA-Au NPs. We identify the pathway for DNA-Au NPs entry in HeLa cells. Biochemical analyses indicate that DNA-Au NPs are taken up by a process involving receptor-mediated endocytosis. Evidence shows that DNA-Au NPs entry is primarily mediated by scavenger receptors, a class of pattern-recognition receptors. This uptake mechanism appears to be conserved across species as blocking the same receptors in mouse cells also disrupted DNA-Au NP entry. Polyvalent nanoparticles functionalized with siRNA are shown to enter through the same pathway. Thus, scavenger receptors are required for cellular uptake of polyvalent oligonucleotide functionalized nanoparticles.

We demonstrate a composite nanomaterial, termed an aptamer nano-flare, that can directly quantify an intracellular analyte in a living cell. Aptamer nano-flares consist of a gold nanoparticle core functionalized with a dense monolayer of nucleic acid aptamers with a high affinity for adenosine triphosphate (ATP). The probes bind selectively to target molecules and release fluorescent reporters which indicate the presence of the analyte. Additionally, these nanoconjugates are readily taken up by cells where their signal intensity can be used to quantify intracellular analyte concentration. These nanoconjugates are a promising approach for the intracellular quantification of other small molecules or proteins, or as agents that use aptamer binding to elicit a biological response in living systems.

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