Metal nanoparticles: from "artificial atoms" to "artificial molecules".
Metal nanoparticles: from "artificial atoms" to "artificial molecules".
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DOI：
10.1002/anie.200701554
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发表时间：
2007-08
影响因子：
--
通讯作者：
D. Perepichka;F. Rosei
中科院分区：
文献类型：
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作者：
D. Perepichka;F. Rosei
纳米结构材料通常表现出对于相同成分的块体材料来说不寻常的特性。例如，当纳米结构的尺寸缩小到某个阈值以下时，它们可能表现为量子点，即电荷载流子被限制在类原子势中的零维系统。为了利用纳米级特性进行应用，人们必须能够控制纳米结构的尺寸以及它们组装成宏观结构（例如阵列或薄膜）。贵金属纳米粒子表现出独特的电子和光学特性，这些特性与其尺寸和形状密切相关。基于溶液的金属纳米粒子合成利用功能性表面活性剂分子存在下金属盐的化学还原，将完整的单层包裹在纳米粒子核心周围，防止聚结。这种单分子层具有保护功能，并决定颗粒与其环境之间相互作用的性质，从而控制它们在不同介质中的溶解度。长链硫醇是最广泛使用的表面活性剂分子，特别是对于金纳米粒子。它们在金属表面上的自组装基于共价金属硫键（约 50 kcalmol ）的形成以及烃链之间有吸引力的范德华相互作用，其强度随着链的长度而变化（每个亚甲基约 1.5 kcalmol 1）。在平坦的金属表面上，这些相互作用导致形成高度有序的自组装单层 (SAM)，这种现象首次在金表面（主要是 Au(111)）上被观察到并得到最广泛的研究。通常认为金纳米颗粒上的 SAM 具有非常相似的结构，尽管传统观点和实验观察表明，分子-表面相互作用（取决于表面晶面的晶体取向）在纳米颗粒多面体的顶点处特别弱。 [11-13] 纳米颗粒表面构成 SAM 的分子的功能是定义金属纳米颗粒几乎所有应用的关键因素。尽管用于合成纳米颗粒的强还原条件对硫醇配体中官能团的插入设置了严格的限制，但实际上任何官能团都可以通过随后的配体交换反应引入纳米颗粒中。附着在纳米粒子上的分子之间的共价和强非共价相互作用已被用来设计各种纳米结构，包括单层和多层，以及具有导电聚合物或低聚物的纳米复合材料。此外，金纳米颗粒已被用作“多价”核心来构建有机树枝状结构。然而，这种结合的各向同性特征给自下而上制造更复杂的基于纳米颗粒的结构带来了问题。几个小组研究了与多齿分子连接的单功能纳米颗粒，并建立了控制纳米颗粒组装的定向相互作用。所需的单官能化纳米颗粒是通过与结合到固体载体上的官能硫醇反应制备的。这种方法通常用于金纳米颗粒作为分子连接中的电极界面的应用。利用用作配体的 DNA 链之间的高保真相互作用，制备了单功能化金纳米颗粒的线性、环状和离散分支阵列。然而，单功能纳米粒子的“单价”性质使它们成为旁观者的附属群体，而不是这些纳米工程努力的构建模块。 Stellacci 小组最近的工作表明，表面曲率在分子自组装中发挥着关键作用，为该领域开辟了新的视角。该小组进行了详细的扫描隧道显微镜 (STM) 研究，结果表明，在某些纳米颗粒尺寸以下，表面 [*] Prof. D. F. Perepichka Department of Chemistry McGill University 801 Sherbrooke Street West Montr1al, QC H3A2K6 (Canada) 传真：(+1)514-398-3797 电子邮件：dmitrii.perepichka@mcgill.ca
Nanostructured materials often exhibit properties which are unusual for bulk materials of the same composition. When the size of nanostructures shrinks below a certain threshold, for example, they may behave as quantum dots, that is, as zero-dimensional systems in which the charge carriers are confined in atomic-like potentials. To harness nanoscale properties for applications, one must be able to control the size of the nanostructures as well as their assembly into macroscopic structures (such as arrays or films). Noble-metal nanoparticles exhibit unique electronic and optical properties that are critically related to their size and shape. Solution-based syntheses of metal nanoparticles exploit the chemical reduction of metal salts in the presence of functional surfactant molecules, which wrap a full monolayer around the nanoparticle core, preventing coalescence. This molecular monolayer has a protecting function and determines the nature of the interactions between the particles and their environment, hence controlling their solubility in different media, for example. Longchain thiols are the most widely used surfactant molecules, particularly for gold nanoparticles. Their self-assembly on metal surfaces is based on the formation of a covalent metal–sulfur bond (ca. 50 kcalmol ) and of attractive van der Waals interactions between the hydrocarbon chains, the strength of which scales with the length of the chain (ca. 1.5 kcalmol 1 per methylene group). On flat metallic surfaces, these interactions lead to the formation of highly ordered self-assembled monolayers (SAMs), which were first observed, and most extensively studied, on gold surfaces (predominantly Au(111)). A very similar architecture is generally presumed for SAMs on gold nanoparticles, although conventional wisdom and experimental observations suggest that molecule–surface interactions, which depend on the crystallographic orientation of the surface facets, will be particularly weak at the vertices of the nanoparticle polyhedron. 11–13] The functionality of the molecules composing the SAM at the nanoparticle surface is the key factor defining almost every application of metal nanoparticles. Although the strongly reductive conditions used for the synthesis of nanoparticles set stringent limitations on the insertion of functional groups in the thiol ligands, practically any functionality can be introduced into the nanoparticle by subsequent ligand-exchange reactions. Covalent and strong noncovalent interactions between molecules attached to nanoparticles have been used to engineer various nanostructures, including monoand multilayers, and nanocomposites with conducting polymers or oligomers. Also, gold nanoparticles have been used as “multivalent” cores to build organic dendritic structures. However, the isotropic character of such binding poses a problem for the bottomup fabrication of more complex nanoparticle-based architectures. Several groups have investigated monofunctionalized nanoparticles linked to multidentate molecules with established directional interactions that control the assembly of the nanoparticles. The required monofunctionalized nanoparticles have been prepared through reaction with a functional thiol bound to a solid support. This approach is commonly used in the application of gold nanoparticles as electrode interfaces in molecular junctions. Using the high-fidelity interaction between DNA strands employed as ligands, linear, cyclic, and discrete branched arrays of monofunctionalized gold nanoparticles have been fabricated. However, the “monovalent” nature of monofunctionalized nanoparticles renders them spectator pendant groups, rather than building blocks in these nanoengineering endeavors. Recent work by Stellacci<s group showing that surface curvature plays a key role in molecular self-assembly opened new perspectives in this field. This group performed detailed scanning tunneling microscopy (STM) investigations, which suggested that, below certain nanoparticle dimensions, surface [*] Prof. D. F. Perepichka Department of Chemistry McGill University 801 Sherbrooke Street West Montr1al, QC H3A2K6 (Canada) Fax: (+1)514-398-3797 E-mail: dmitrii.perepichka@mcgill.ca