Ha Jeong-Myeong, Solovyov Andrew, Katz Alexander
Department of Chemical Engineering, University of California, Berkeley, California 94720-1462, USA.
Langmuir. 2009 Sep 15;25(18):10548-53. doi: 10.1021/la9013174.
Use of organic ligands to partially passivate nanoparticles against sintering yet retain a degree of small molecule accessibility to the metal surface has been a lofty goal in functional materials synthesis, which in principle also enables the design of preferred electronic and steric environments on a nanoparticle surface. Catalysis using gold in particular requires donor ligands that facilitate an electron-rich metal surface and generalizable strategies for dealing with deactivation due to sintering. Here, synthesis and characterization of gold nanoparticles postsynthetically modified with the chelating ligand cone-5,11,17,23,29,35-hexa(tert-butyl)-37,39,41-tris(diphenylphosphinomethoxy)-38,40,42-trimethoxycalix[6]arene (1) is reported. In solution as well as when supported on the surface of TiO2, nanoparticles modified with tripodal calix[6]arene phosphine ligand 1 demonstrate enhanced protection against sintering relative to unmodified, tetraoctylammonium bromide-surfactant-stabilized gold nanoparticles. In between adsorbed calixarene ligands, there is accessible gold surface area in these nanoparticles, and this is measured quantitatively for the first time for a calixarene-modified nanoparticle, using a newly developed fluorescence methodology involving 2-naphthalenethiol as a relevant chemisorption probe molecule. Ligand steric bulk critically influences amount of accessible surface on the metal nanoparticle since the use of a smaller calix[4]arene ligand (MBC) results in a 7-fold lower accessible surface area relative to using 1 under otherwise similar conditions. In addition, surface coverage of 1 controls accessible surface area in an unintuitive fashion: a 4-fold increase in accessible metal surface area is observed upon increasing the surface coverage of 1 to be 1.5-fold higher than the minimum required for surface saturation. This is presumably the result of a more open ligand packing of 1 at higher surface coverages, which allows greater accessibility to 2-napthalenethiol.
使用有机配体部分钝化纳米颗粒以防止烧结，同时保留一定程度的小分子与金属表面的可及性，这一直是功能材料合成中的一个崇高目标，原则上这也能够在纳米颗粒表面设计出理想的电子和空间环境。特别是使用金进行催化需要供体配体来促进富电子的金属表面，并需要通用策略来处理由于烧结导致的失活问题。在此，报道了用螯合配体锥-5,11,17,23,29,35-六（叔丁基）-37,39,41-三（二苯基膦基甲氧基）-38,40,42-三甲氧基杯[6]芳烃（1）对金纳米颗粒进行后合成修饰的合成及表征。在溶液中以及负载在TiO₂表面时，用三脚架杯[6]芳烃膦配体1修饰的纳米颗粒相对于未修饰的、由溴化四辛基铵表面活性剂稳定的金纳米颗粒，表现出增强的抗烧结保护作用。在这些纳米颗粒中，吸附的杯芳烃配体之间存在可及的金表面积，并且首次使用涉及2-萘硫醇作为相关化学吸附探针分子的新开发的荧光方法对杯芳烃修饰的纳米颗粒进行了定量测量。配体的空间体积对金属纳米颗粒上可及表面的量有至关重要的影响，因为在其他条件相似的情况下，使用较小的杯[4]芳烃配体（MBC）导致可及表面积比使用1时低7倍。此外，1的表面覆盖率以一种不直观的方式控制可及表面积：当将1的表面覆盖率提高到比表面饱和所需的最小值高1.5倍时，观察到可及金属表面积增加了4倍。这可能是由于在较高表面覆盖率下1的配体堆积更开放，从而使2-萘硫醇的可及性更高。
