Single-Molecule Investigation of Nanocatalysis

[851257]该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。纳米粒子在有机合成、污染物去除和能源生产中催化许多化学转化；因此，人们付出了巨大的努力来表征纳米颗粒催化剂的结构并了解其催化性能。虽然单个纳米粒子的结构可以用先进的透射电子显微镜和扫描探针显微镜研究到原子分辨率，但它们的催化性质主要是在系综水平上测量的，得到了平均性质。然而，纳米颗粒的催化性能本质上是不均匀的，这是由于它们的结构分散性、表面位置的不均匀分布和表面重组动力学。为了解决这种异质性挑战并消除测量中的集合平均，PI率先使用荧光反应的单分子荧光显微镜，在单颗粒和单周旋分辨率下研究纳米颗粒催化。重点关注胶体金纳米颗粒的氧化还原催化作用，PI的初步研究表明，在溶液中以单通量分辨率实时跟踪单个金纳米颗粒的催化通量是可能的。我们发现，在催化产物生成反应中，所有金纳米颗粒都遵循Langmuir-Hinshelwood机制，但具有非均相反应活性；对于产物解离反应，存在三个纳米粒子亚群，它们在两个平行的产物解离途径之间表现出不同的反应活性和不同的动力学。此外，单个金纳米颗粒表现出较大的时间活性波动，这是由于在表面催化和产物对接位点以不同的时间尺度发生的催化诱导和自发的表面重组动力学。在此基础上，PI建议使用单粒子单转换方法：1)探索Au纳米颗粒的非均相反应性和表面重组动力学的本质，2)确定单个Au纳米颗粒催化的尺寸依赖性，以及3)确定单个Au纳米颗粒催化的形状依赖性。获得的结果将揭示其非均相反应性的结构起源，揭示其表面重组动力学的本质，并深入了解它们在单粒子水平上的结构-活性相关性，其中许多是传统的系综测量所无法达到的。更广泛的影响本提案中描述的单分子纳米催化研究为多相/纳米催化和单分子研究开辟了新的方向。如果设计合适的荧光探针反应，单颗粒方法也可推广到其他纳米级催化剂的研究。所获得的知识将有助于理解控制纳米颗粒催化活性的基本原理，这将有助于改进现有的纳米级催化剂和设计用于多相催化的新催化剂。金属纳米颗粒被广泛用作化学转化的催化剂，特别是用于燃料和太阳能电池中的能量转换。从这些研究中获得的基础知识将有助于了解化学工业、燃料和太阳能电池中使用的纳米颗粒的活性，这将有助于确定改进它们的可能方法，从而使整个社会受益。PI的教育工作整合了他的单分子研究活动，以促进本科和研究生课程的跨学科教学和培训。倡议积极扩大妇女和少数民族学生（本科生和研究生）参与研究，并为包括四年制大学本科生在内的本科生提供培训。PI实验室的科学专业知识和技术仪器为当地社区的研究基础设施增添了力量。PI的外展活动将促进公众对前沿单分子研究的理解和支持，并促进K-12学生的科学教育。

0851257ChenThis award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Intellectual MeritNanoparticles catalyze many chemical transformations in organic synthesis, pollutant removal, and energy production; intense efforts are thus made to characterize the structure and understand the catalytic properties of nanoparticle catalysts. While structures of individual nanoparticles can be studied down to atomic resolution with advanced transmission electron microscopy and scanning probe microscopies, their catalytic properties have been mainly measured at the ensemble level, obtaining averaged properties. The catalytic properties of nanoparticles are intrinsically heterogeneous, however, due to their structural dispersions, heterogeneous distribution of surface sites, and surface restructuring dynamics. To address this heterogeneity challenge and to remove ensemble averaging in measurements, the PI has pioneered in studying nanoparticle catalysis at the single-particle and single-turnover resolution, using single-molecule fluorescence microscopy of fluorogenic reactions. Focusing on the redox catalysis by colloidal Au-nanoparticles, the PI's preliminary studies show it is possible to follow in real time in solution the catalytic turnovers of single Au-nanoparticles at single-turnover resolution. We found that for the catalytic product formation reaction, all Au-nanoparticles follow a Langmuir-Hinshelwood mechanism but with heterogeneous reactivity; for the product dissociation reaction, three nanoparticle subpopulations are present that show heterogeneous reactivity with distinct kinetics between two parallel product dissociation pathways. Moreover, individual Au-nanoparticles show large temporal activity fluctuations, attributable to catalysis-induced and spontaneous surface restructuring dynamics that occur with distinct timescales at the surface catalytic and product docking sites.Building on the achievements, the PI proposes to use the single-particle single-turnover approach to:1) probe the nature of the heterogeneous reactivity and surface restructuring dynamics of Au nanoparticles,2) determine the size dependence of single Au-nanoparticle catalysis, and 3) determine the shape dependence of single Au-nanoparticle catalysis. The results to be obtained will reveal the structural origins of their heterogeneous reactivity, unravel the nature of their surface restructuring dynamics, and gain insight into their structure-activity correlations at the single-particle level, much of which are beyond the reach of conventional ensemble measurements.Broader impactThe single-molecule nanocatalysis research described in this proposal opens new directions in both heterogeneous/nano-catalysis and single-molecule research. The single-particle methodology is also generalizable to study other nanoscale catalysts if a suitable fluorogenic probe reaction is designed. The knowledge to be gained will help understand the fundamental principles governing catalytic activities of nanoparticles, which will assist the efforts in improving current nanoscale catalysts and designing new ones for heterogeneous catalysis. Metal nanoparticles are widely used as catalysts for chemical transformations and in particular for energy conversion in fuel and solar cells. The fundamental knowledge from these studies will help understand the activity of nanoparticles used in the chemical industry and in fuel and solar cells, which will help in identifying possible means to improve them, thus benefiting the society as a whole.The PI's education efforts integrate his single-molecule research activities to promote interdisciplinary teaching and training in both undergraduate and graduate classes. The PI actively broadens the participation of women and minority students (both undergraduate and graduate) in research and offer training to undergraduate students, including those from four-year colleges. The scientific expertise and technical instrumentation of the PI's lab add strength to the research infrastructure of local community. The PI's outreach activities will advance the public understanding and support of frontier single-molecule research and promote science education in K-12 students.