This paper reported the first experimental observation on the three-dimensional (3D) orientation and aggregation of nanocrystals into ordered monocrystalline structures. In particular, it addresses several basic questions in nanomaterials synthesis and structures, including how to build 3D monocrystalline structures from nanoparticle blocks, what occurs in the oriented aggregation process of nanoparticles, and why the stepwise orientation can drive the formation of ordered structures.

“Nanoparticle-based materials have a wide range of application in catalysts, electronic devices, and biological technologies...”

Furthermore, an understanding of 3D-oriented aggregation will be helpful in controlling the formation of ordered nanostructured materials and will provide new insights into the mineralization mechanism of nanoparticles in biological systems. These might be among the reasons why this paper has drawn a lot of attention and is highly cited.

Although the concept of aggregation-driven crystal growth originated from its discovery in a biomineralization system which was experimentally confirmed by the one-dimensional aggregation growth of nanoparticles, the 3D-oriented aggregation of nanoparticles had not been demonstrated in previous researches.

This paper describes the first experimental example in controlling the 3D-oriented aggregation of nanoparticles in order to yield ordered-crystal structures by the use of organic molecules. This stepwise 3D-oriented process has been demonstrated, from the formation of primary nanoparticles, to the preferential one-dimensional orientation of nanoparticles, and eventually, transforming a 3D-oriented aggregation into a monocrystalline structure built from nanoparticles.

Comprehensive insights into the aggregation-driven growth of nanoparticles will form the basis of a novel strategy for reconstructing nanoparticle assemblies into ordered structures, and also for exploring the formation mechanism of complex biominerals.

Many researchers have been exploring the assembly of nanoparticles into highly ordered structures in a reproducible way. This paper is a part of the key to understanding why and how nanocrystals can be organized into highly ordered 3D monocrystalline structures, and it also provides new insight into the aggregation mineralization mechanism of nanoparticles in biological systems.

Over the past five years, we have focused our interests primarily on the assembly mechanism of nanoparticles and the synthesis of nanoparticle materials. In 2002, we found that small silver nanocrystals can assemble into monocrystalline-like aggregates or twin crystals through the use of a surfactant template (Chem. Phys. Lett., 374: 91-94, 2003). However, this work did not also show an experimental observation on the formation process of oriented aggregation, due to the complex nature of surfactant in the solution phase.

Afterward, we realized that strong organic ligands of biological molecules can promote the formation of highly-ordered structures by selective adsorption on different crystallographic planes in biomineralization. Thus, we chose a simple model molecule of peptides for controlling the orientation and aggregation of nanoparticles, and both these particle-built monocrystalline structures and 3D-oriented aggregation processes can be clearly observed.

Up to now, we have successfully expanded this method to include the low-temperature growth of oriented nanorod arrays (Nanotechnology, 17: 2994-2997, 2006) and the biomimetic assembly of polypeptide-stabilized CaCO3 nanoparticles (J. Phys. Chem. B, 110: 8613-8618, 2006). In fact, the paper in J. Phys. Chem. B has recently been added to the list of "Most-Accessed Articles" in this journal.

Dr Zhong-ping Zhang, the first author of the paper, has done most of the experimental works discussed in this article. He is currently working at the Institute of Intelligent Machines, Chinese Academy of Science, Hefei, Anhui.

Nanoparticle-based materials have a wide range of application in catalysts, electronic devices, and biological technologies, which have attracted considerable research efforts in global institutions. The understanding of the fundamental properties of nanoparticle assembly is a key step toward the discovery of future applications for nanoparticle materials.