Characterizing individual Au 25 (SG) 18 clusters within a nanopore detector
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Characterizing individual Au25(SG)18 clusters within a nanopore detector Christopher E. Angevine1, Nuwan Kothalawala2, Amala Dass2 and Joseph E. Reiner1 Physics Department, Virginia Commonwealth University, 701 W. Grace St., Richmond, VA 23284, U.S.A. 2 Department of Chemistry and Biochemistry, University of Mississippi, University, MS 38677, U.S.A.
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ABSTRACT Metallic quantum clusters are stable structures that can exhibit many useful properties. Clusters can be ligand stabilized in aqueous environments to expand their usefulness as biosensors. There are some limitations in characterizing the physical and chemical properties of individual water soluble clusters. This report describes initial results of a new approach for detecting and characterizing individual gold nanoclusters (Au25(SG)18) in an aqueous solution with nanopore-based resistive pulse sensing. Here the nanopore is a single alpha hemolysin from Staphylococcus aureus. Clusters that enter through the cis side of the pore (large vestibule) usually create shallow current blockades with a mean residence time of several milliseconds. Clusters that enter through the trans side of the pore (narrow lumen) create deeper blockades that are either very short (~200 µs), long lived (~50 ms) or trapped (>10s). The short and long lived blockades yield sufficient statistics to help characterize the clusters and the trapped state events may allow for additional analysis and controlled delivery of individual clusters. We demonstrate the possibility of this additional analysis by performing I-V measurements on individually trapped clusters. These show an optimal voltage for confining a cluster within the pore. INTRODUCTION The synthesis of highly stable gold nanoclusters [1] and subsequent improvements in the technique to synthesize them has led to the ability to create ligand-protected clusters with a known and controllable number of gold atoms [2]. One of the most important features of these clusters is their size (10 s) and we had to reverse the polarity of the applied potential to remove them from the nanopore. These long lived blockades were associated with two separable blockade depths at i/io ≈ 0.13 and 0.20. There are a number of explanations for these two peaks (i.e. cluster orientation, binding differences) and further analysis using both numerical simulation and greater experimental statistics are needed to understand the cause of these two deeper blockades.
Figure 3. Current blockade and residence time distributions for clusters entering the cis side of the nanopore under 100 mV applied potential. (left) Blockades give rise to a large peak with shallow blockades at i/io ≈ 0.75 and two smaller peaks corresponding to deeper blockades at i/io ≈ 0.13 and 0.20. The large number of shallow blockades may provide an accurate means for sizing the clusters while the long lived states at 0.13 and 0.20 may be indicative of different orientations of the cluster within the nanopore. (right) The residence time distribution is well fit with a single exponential function h
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