如何指挥和控制纳米粒子的自组装是纳米技术的一个关键问题。
密歇根大学的研究人员发现了一种将流体聚集纳米晶体转为自由浮动薄片的方法，生命有机体中一些蛋白质组装的过程也是采用这种方式。
化学工程、材料科学和工程教授Sharon Glotzer说，“这在生物学和纳米科学的两个基本组成元件蛋白质和纳米粒子之间建立了一个重要衔接，这项突破对从下而上地将材料聚集起来用于药物投递和能源等应用是相当振奋人心的。”
博士后研究生Kotov说，“这项工作的重要性在于它在蛋白质世界和纳米科技世界之间建立了一个关键衔接，一旦我们知道如何控制纳米粒子与它们自组装能力之间的力量，这将使我们在一系列应用中受益，例如光捕获纳米粒子装置和像蛋白质一样但实质上却是纳米粒子的新药。”
根据纳米粒子的大小，在紫外照射之下薄片能呈现从鲜绿色到深红的颜色。薄片由碲化镉晶体（太阳能电池材料）做成，薄片的宽度大约2微米，厚度约为人头发的1/5。Glotzer说，科学家早已知道如何将纳米粒子制成薄片，但那些薄片仅仅在粒子处于两种流体之间时才能获得，从未在单流体表面上获得过。
该项工作三年前始于Kotov的实验室，他和他的团队在实验中发现了薄片，虽然得到了薄片，但是他们不能确定是如何制造出来的。
Kotov说，“我们知道某些特定蛋白质在生命体内自组装到称为S-layers的层里，”S-layers蛋白包括各种各样的细菌及其他称为archaea的单细胞原核生物的最外层的细胞被膜蛋白，他们在表面和界面能形成正方形、六角形或其他外形的第2薄片，并被困在流体中。研究团队致力于建立控制S-layer蛋白质自组装的力量与控制纳米粒子自组装的力量之间的联系。Glotzer的团队也参与到这项工作当中，专门从事计算机建模和仿真。
“很可能S-layer蛋白质之间的力量是高度异向性的，并且我们猜测这也是纳米粒子的特点”，Glotzer说，“计算机仿真允许我们进一步拓展和检验这个假设”。
Glotzer的研究团队发现，CdTe纳米晶体的独特形状提升了一种组合力量，从而导致形成异常的二维外形。Kotov研究团队随后的实验表明，其中任一力量的缺失，薄片都不能形成，这种结果证实了上述模仿预言。
“自组装是生产具有特定的几何和物理化学表面特性的生物学分子组织序列的基本自然组装原则”，Glotzer认为，“在生产功能纳米材料和设备时，如果我们可以适当地设计模块，自组装较传统制造业方法有实质的优势，这是我们设法做的。”
背景资料：
密歇根大学工程学院在全美工程学院之中处于前列，学院预算超过1.3亿美元。学院有11个系和二个NSF工程研究中心，这些系和研究中心对三个领域特别重视：纳米科技和综合性微系统、多孔与分子生物技术和信息技术。学院努力在这些领域为资本建设计划和项目寻求增加1.1亿美元，从而支持进一步研究发现。学院的目标是推进学术发展和推广尖端研究以提高公共健康和福利。
英文原文：
Researchers make nanosheets that mimic protein formation
How to direct and control the self-assembly of nanoparticles is a fundamental question in nanotechnology.
University of Michigan researchers have discovered a way to make nanocrystals in a fluid assemble into free-floating sheets the same way some protein structures form in living organisms.
"This establishes an important connection between two basic building blocks in biology and nanotechnology, that is, proteins and nanoparticles, and this is very exciting for assembling materials from the bottom up for a whole slew of applications ranging from drug delivery to energy," said Sharon Glotzer, professor of chemical engineering and materials science and engineering.
Glotzer and Nicholas Kotov, associate professor of chemical engineering, and their students who are post doctoral researchers have co-authored a paper scheduled to appear Oct. 13 in the journal Science.
"The importance of this work is in making a key connection between the world of proteins and the world of nanotechnology" Kotov said. "Once we know how to manipulate the forces between the nanoparticles and their ability to self-organize, it will help us in a variety of practical applications from light-harvesting nanoparticle devices to new drugs which can act like proteins, but are actually nanoparticles."
The sheets, which can appear colored under UV illumination from bright green to dark red depending on the nanoparticle size, are made from cadmium telluride crystals, a material used in solar cells. The sheets are about 2 microns in width, about 1/5 the thickness of a human hair.
Scientists have long known how to coax nanoparticles into forming sheets, Glotzer said. But those sheets have only been achieved when the particles were on a surface or at an interface between two fluids, never while suspended in a single fluid.
The work started in Kotov's lab three years ago, when he and his team observed the sheets in experiments. Though they created them, they weren't sure how.
"We were aware of certain proteins in living organisms that self-assemble into layers, called S-layers," Kotov said. S-layer proteins comprise the outermost cell envelope of a wide variety of bacteria and other single-celled, prokaryotic organisms called archaea, and they are able to form 2-d sheets with square, hexagonal, and other packings at surfaces and interfaces, as well as suspended in fluid. The group sought to make the connection between the forces governing S-layer protein assembly and the forces governing the nanoparticle assembly. That's when Glotzer's group, whose expertise is in computer modeling and simulation, became involved.
"It's likely that the forces between S-layer proteins are highly anisotropic, and we suspected this was also a feature of the nanoparticles," Glotzer said. "Computer simulations allowed us to further develop and test this hypothesis."
Post doctoral researcher Zhenli Zhang of Glotzer's group tried various combinations of forces based on information gleaned from experiments performed by post doctoral Zhiyong Tang of Kotov's group. The team discovered that the unique shape of the CdTe nanocrystals gave rise to a combination of forces that conspired to produce the unusual two-dimensional packing. Subsequent experiments by Kotov's group showed that if any of the forces were missing, the sheets would not form, confirming the simulation predictions.
"Self-assembly is nature's basic building principle for producing organized arrays of biomolecules with controlled geometrical and physicochemical surface properties," Glotzer said. "In the fabrication of functional nanoscale materials and devices, self-assembly offers substantial advantages over traditional manufacturing approaches, if we can design the building blocks appropriately. This is what we're trying to do."
The paper is called Self-assembly of CdTe Nanocrystals into Free-Floating Sheets. The work was partially supported by seed funds provided by the U-M College of Engineering's Nanotechnology Initiative and by the Department of Energy, the Air Force Office of Scientific Research, and the National Science Foundation.
