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Heterogeneous Solvatochromism of Fluorescent DNA-Stabilized Silver Clusters Precludes Use of Simple Onsager-Based Stokes Shift Models Stacy M Copp, Alexis Faris, Steven Swasey, and Elisabeth G. Gwinn J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02777 • Publication Date (Web): 01 Feb 2016 Downloaded from http://pubs.acs.org on February 3, 2016
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The Journal of Physical Chemistry Letters
Heterogeneous Solvatochromism of Fluorescent DNAStabilized Silver Clusters Precludes Use of Simple OnsagerBased Stokes Shift Models 1
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Stacy M. Copp , Alexis Faris , Steven M. Swasey , Elisabeth G. Gwinn 1 Department of Physics, University of California – Santa Barbara, Santa Barbara, CA 93106-9530 2 Department of Chemistry, University of California – Santa Barbara, Santa Barbara, CA 93106-9530
ABSTRACT: The diverse optical and chemical properties of DNA-stabilized silver clusters (AgN-DNA) have challenged the development of a common model for these sequence-tunable fluorophores. Although correlations between cluster geometry and fluorescence color have begun to shed light on how the optical properties of AgN-DNA are selected, the exact mechanisms responsible for fluorescence remain unknown. To explore these mechanisms, we study four distinct purified AgNDNA in ethanol-water and methanol-water mixtures and find that the solvatochromic behavior of AgN-DNA varies widely among different cluster species and differs markedly from prior results on impure material. Placing AgN-DNA within the context of standard Lippert-Mataga solvatochromism models based on the Onsager reaction field, we show that such nonspecific solvent models are not universally applicable to AgN-DNA. Instead, alcohol-induced solvatochromism of AgN-DNA may be governed by changes in hydration of the DNA template, with spectral shifts resulting from cluster shape changes and/or dielectric changes in the local vicinity of the cluster. KEYWORDS: Silver cluster, DNA, Lippert-Mataga model, solvatochromism, fluorescence, Stokes Shift DNA-stabilized silver clusters (AgN-DNA) are powerful 1 tools for sensing, promising fluorescent markers for 2 bioimaging, and have precisely controllable sizes for DNA3 based photonic arrays. Yet despite a growing number of applications for AgN-DNA, the mechanisms behind their sequence-tunable fluorescence are not fully understood. Most properties of AgN-DNA are highly heterogeneous: different DNA sequences select widely varying photostabilities, chemical stabilities, quantum yields, and fluorescence colors from blue-green into the near IR. These wide-ranging attributes have hindered the development of a common model for AgN-DNA. One commonality that fluorescent AgN-DNA do appear 4 to share is a rod-like cluster core. While the detailed structure of AgN-DNA has not been solved, there is considerable evidence for elongated cluster geometries, 5–9 with cluster length selecting color. By isolating AgN-DNA 10,11 previous studies showed that with monodisperse sizes, the distinctive form of optical absorbance spectra, dependence of peak absorbance wavelengths on silver 4,5 7 content, non-spherical values of magic cluster sizes, 8 circular dichroism spectral features, and strongly 9 polarization-dependent emission all point to a rod-like 5 cluster geometry. Another feature shared by all fluorescent AgN-DNA is excitation via the UV absorbance band of the
stabilizing DNA strand, in addition to the visible or IR 12 wavelength excitation band selected by cluster length. These commonalities raise the question of whether processes governing fluorescence might also be universal across distinct AgN-DNA colors and stabilizing DNA strands. For ligand-stabilized metal clusters in general, the factors controlling fluorescence quantum yields (QY) remain obscure. The nature of the initial excited state, non-radiative pathways, and roles of ligands versus cluster structure are all open questions. Recent studies that varied ligand composition while preserving the same cluster structure found that the QY of weakly fluorescent thiol-protected Au25 -5 -4 clusters increased from 2x10 to 1x10 upon addition of 13 ligand substituents with higher electron donating power. This implicated charge transfer from the ligands to the cluster upon excitation, consistent with long (∼µs) fluorescence lifetimes arising from low spatial overlap between a cluster-centered ground state and excited states with high weight on ligands. Contrasting studies of phosphine-protected Au25-xAgx clusters used fixed ligand composition and cluster geometry to investigate effects of altering cluster composition. Increasing the silver content x from 12 to 13 atoms increased the QY dramatically, from -3 ∼1x10 to 0.4. This was attributed to reaching nearly complete Ag 5s character in ground and excited states upon
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addition of the 13 Ag atom and greater electron 14 delocalization over the cluster. Such an increase to a QY approaching unity would be difficult to understand for an excited state with significant charge transfer to the ligands, due to low overlap with a cluster-centered ground state. Thus, the nature of excited states may differ between ligand-stabilized clusters with high QY ∼ 1 and low QY
