Abstract
DNA-protected silver clusters (AgN-DNA) possess unique fluorescence properties that depend on the specific DNA template that stabilizes the cluster. They exhibit peak emission wavelengths that range across the visible and near-IR spectrum. This wide color palette, combined with low toxicity, high fluorescence quantum yields of some clusters, low synthesis costs, small cluster sizes and compatibility with DNA are enabling many applications that employ AgN-DNA. Here we review what is known about the underlying composition and structure of AgN-DNA, and how these relate to the optical properties of these fascinating, hybrid biomolecule-metal cluster nanomaterials. We place AgN-DNA in the general context of ligand-stabilized metal clusters and compare their properties to those of other noble metal clusters stabilized by small molecule ligands. The methods used to isolate pure AgN-DNA for analysis of composition and for studies of solution and single-emitter optical properties are discussed. We give a brief overview of structurally sensitive chiroptical studies, both theoretical and experimental, and review experiments on bringing silver clusters of distinct size and color into nanoscale DNA assemblies. Progress towards using DNA scaffolds to assemble multi-cluster arrays is also reviewed.
Highlights
We review the properties of a compelling new class of nanoscale optical element: fluorescent silver clusters that are stabilized by single-stranded “binding pockets” in oligonucleotide strands and strand assemblies
We will focus on what our group has learned about the underlying composition and structure of pure AgN-DNA that lead to these fascinating properties (Figure 1a,b) and the prospects for realizing metal cluster nanophotonics on DNA scaffolds that self-assemble by Watson-Crick pairing of synthetic DNA strands, such as DNA origami [11], tile-based assemblies [12], and nanoscale three-dimensional shapes [13]
Using select DNA oligomers, we found that visibly fluorescent silver clusters form preferentially in single-stranded regions of DNA hosts, rather than on double-stranded regions
Summary
We review the properties of a compelling new class of nanoscale optical element: fluorescent silver clusters that are stabilized by single-stranded (ss) “binding pockets” in oligonucleotide strands and strand assemblies Such DNA-stabilized Ag clusters (AgN-DNA), first reported by Petty [1], are emerging in applications that range from biological imaging [2], to molecular logic schemes [3] and strand-exchange on-off switches [4], to sensors for single base mutations [5,6,7], microRNAs [8], and DNA target strands [9]. Several groups have already demonstrated intriguing optical phenomena from the interaction of nanoscale optical elements arranged at close proximity on DNA scaffolds, including fluorescent dyes, metal nanoparticles and semiconductor quantum dots [17,18,19,20] Due to their unique combination of metallic and molecular attributes, ligand-protected metal clusters of up to some tens of atoms in size are beginning to be examined as a distinct class of nanophotonic element. While atomic-level structures for AgN-DNA are still not established, recent work reviewed here suggests that the selection of particular fluorescence colors by the DNA template arises largely from cluster size selection, with additional color tuning by the specific base and silver cation environment
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