Dynamics of Excited States in DNA Strands and DNA-Silver Nanoclusters

银离子(Ag+)可以与DNA中的氮和氧原子结合，产生不寻常和有趣的分子结构。DNA就像一个脚手架，精确地控制着单个银离子的位置。当加入另一种化学试剂为Ag+离子提供电子时，DNA中的Ag+离子阵列变成不超过几十个金属原子的小团簇。这些微小的团簇具有显著的光学性质，包括例如荧光。荧光是一个分子吸收一种波长的光，然后不久就发出另一种颜色的光的过程。这是宏观银器所不具备的特性。它们的荧光性质使这些银团簇在传感和成像应用方面具有吸引力。然而，尽管这些星系团引起了广泛的兴趣，但人们对是什么控制了它们的光发射知之甚少。在这个由化学系化学结构动力学和机制(CSDM-A)计划资助的项目中，俄亥俄州立大学的Bern Kohler教授和在他指导下工作的学生正在研究由短激光脉冲(几百飞秒，其中飞秒是一万亿分之一秒)激发的DNA-金属组装。科勒团队观察到银团发出的光的颜色如何取决于入射激光脉冲的颜色和时间间隔。在某些情况下，根本不会发出任何光。这些研究揭示了光能如何被微小的金属星团吸收，并以光的形式再次发射或消散到星系团周围的奥秘。参与这个项目的本科生和研究生正在获得最先进的激光技术的经验，以及解决光谱学和纳米科学接口问题所需的各种分析。这种多学科的培训对于维护国家的科学家队伍非常重要。这个项目的目标是对DNA-金属络合物和纳米结构的紫外光和可见光激发形成的激发电子态有基本的了解。银离子与碱基的氮和氧原子配位，形成金属介导的碱基对和丰富的自组装结构。结合银离子的还原产生了致密的金属纳米结构，其中含有少量的银原子，具有显著的但鲜为人知的光物理性质。利用飞秒光谱，在单核苷酸-金属离子络合物及其自组装纳米结构中，以及在以DNA为载体的银纳米团簇中，观察和表征了非辐射和辐射衰减路径。利用时间域测量方法建立了银纳米团簇的综合光物理模型。对DNA和金属之间电子相互作用的基本了解可以为有机-无机杂化界面提供新的见解，这些界面对于控制光生载流子和氧化还原等价物的分离和复合具有吸引力。利用超快光谱技术研究少原子贵金属纳米团簇有望促进社会有益的科学技术进步。控制和操纵金属纳米粒子激发态的能力可以推动化学成像和可持续能源生产领域的发展。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Silver ions (Ag+) can bind to the nitrogen and oxygen atoms in DNA giving rise to unusual and interesting molecular structures. DNA acts as a scaffold that precisely controls the positions of individual silver ions. When another chemical reagent is added to provide electrons to the Ag+ ions, the array of Ag+ ions in the DNA become small clusters containing no more than a few dozen metal atoms. These miniscule clusters have remarkable optical properties, including for example fluorescence. Fluorescence is a process by which a molecule absorbs light at one wavelength and shortly afterwards emits light of another color. This is a property that macroscopic silver objects do not possess. Their fluorescence properties makes these silver clusters attractive for sensing and imaging applications. However, despite widespread interest in these clusters, very little is known about what controls their light emission. In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Bern Kohler of The Ohio State University and students working under his direction are investigating DNA-metal assemblies that are excited by short laser pulses (a few hundred femtoseconds, where a femtosecond is one quadrillionth of one second). The Kohler team observes how the color of the light emitted from the silver clusters depends on the color and time interval of the impinging laser pulse. Under some conditions, no light is emitted at all. These studies are revealing the mysteries of how light energy is absorbed by small metal clusters, and either emitted again as light or dissipated into the clusters' surroundings. The undergraduate and graduate students involved in this project are gaining experience in state-of the-art laser techniques, and the kinds of analysis needed to solve problems at the interface of spectroscopy and nanoscience. This multidisciplinary training is important for upholding the nation's workforce of scientists. This project targets fundamental understanding of excited electronic states formed by UV and visible excitation of DNA-metal complexes and nanostructures. Silver ions coordinate to the nitrogen and oxygen atoms of the nucleobases, giving rise to metal-mediated base pairs and a rich variety of self-assembled structures. Reduction of the bound silver ions produces compact metal nanostructures containing small numbers of silver atoms with remarkable, but poorly understood photophysical properties. Using femtosecond spectroscopy, nonradiative and radiative decay pathways are being observed and characterized in mononucleotide-metal ion complexes and their self-assembled nanostructures, and in DNA-hosted silver nanoclusters. Time-domain measurements are employed to develop a comprehensive photophysical model of silver nanoclusters. Fundamental understanding of the electronic interactions between DNA and metals can provide new insights into hybrid organic-inorganic interfaces that are attractive for controlling the separation and recombination of photogenerated carriers and redox equivalents. The study of few-atom noble metal nanoclusters by ultrafast spectroscopy promises to accelerate societally beneficial advances in science and technology. The ability to control and manipulate the excited states of metal nanoparticles could advance the fields of chemical imaging and sustainable energy production.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.