Self-Assembly at nano-Scale Binary Nanoparticles Superlattices

1. Christopher B. Murray et al. Nature439, 55-59 (5 January 2006) C. B. Murray and S. O'Brien et al. Nature423, 968-971 (26 June 2003) Self-Assembly at nano-Scale Binary Nanoparticles Superlattices Student: Xu Zhang

2. Why Binary Nanoparticle Superlattice (BNSL)? • BNSL offers new collective properties based on the distinct properties of the resulting nanometre-scale building blocks. An example. • To synthesize metamaterials—materials with properties arising from the controlled interaction of the different nanocrystals in an assembly—with programmable physical and chemical properties. • It is the fundamental step toward the development of assemblies of three or more different nanocomponents and the further study of the 'bottom up' machanism that runs through chemistry, biology and material science.

3. Example: The BNSL of magnetic nanocrystals and semiconductor quantum dots

4. Two steps to prepare the BNSL • Placing a substrate in a colloidal solution of two types of nanoparticles • Evaporating the solvent in a low-pressure chamber enabled the nanoparticles to self-assemble into the ordered structures.

5. Program the assembly(1)Tuning the charge state of the nanoparticles • Coulomb energy determines the stoichiometry of the growing BNSL. • An extended three-dimensional BNSL can form only if the positive and negative charges compensate each other. • Tuning the charge state of the nanoparticles can be achived by adding surfactant molecules like carboxylic acids, tri-n-octylphosphine oxide (TOPO) or dodecylamine to solutions of the nanoparticles. See the electrophoretic mobility of nanocrystals. • Reproducible switching between different BNSL structures has been achieved by adding small amounts of different kinds of surfactant molecules, an example.

6. Adding surfactant molecules to tune the charge state of the nanoparticles a–d, Distribution of electrophoretic mobility for 7.2 nm PbSe nanocrystals. a, PbSe nanocrystals washed to remove excess of capping ligands. The grey bars show mobilities predicted for nanocrystals with charges of -1, 0, 1 and 2 (in units of e). b–d, Electrophoretic mobility of PbSe nanocrystals in the presence of b, 0.02 M oleic acid, c, 0.06 M oleic acid and d, 0.05 M tri-n-octylphosphine oxide. e, f, Comparison of electrophoretic mobilities of 7.2 nm PbSe and 4.8 nm Au nanocrystals in the presence of e, 0.02 M oleic acid and f, 0.05 M tri-n-octylphosphine oxide, respectively. a.u., arbitrary units.

7. Tuning the charge state of the nanoparticles allows to direct the self-assembly process a, 6.2 nm PbSe and 3.0 nm Pd nanoparticles self-assembled into orthorhombic AB- and AlB2-type BNSLs, and b, into NaZn13-type BNSL. c, d, 7.2 nm PbSe and 4.2 nm Ag nanoparticles self-assembled into orthorhombic AB and cuboctahedral AB13 BNSLs, respectively. e, f, 6.2 nm PbSe and 5.0 nm Au nanoparticles self-assembled into CuAu-type and CaCu5-type BNSLs, respectively.

8. Other methods to Program the assembly • Adjusting the relative concentrations of the particle components. • The nanocrystal shape can also be used as a powerful tool to engineer the structure of the self-assembled BNSLs. • Choosing of substrate can be used to control the BNSL structure. • Of course varying combinations of the building blocks is another way to control the BNSL structure. • An example.

9. Engineer the structure of the BNSLs by nanocrystal shape a, b, Self-assembled from LaF3 triangular nanoplates (9.0 nm side) and 5.0 nm Au nanoparticles; c, self-assembled from LaF3 triangular nanoplates and 6.2 nm PbSe nanocrystals. The insets show a, a magnified image, and b, c, proposed unit cells of the corresponding superlattices. The structure shown in a forms on silicon oxide surfaces, while structures shown in b and c form preferentially on amorphous carbon substrates

10. Limitation of the method The non-equilibrium nature of the evaporative self-assembly process adds additional complexity

11. Conclusions • It is specifically at the nanoscale that the van der Waals, electrostatic, steric repulsion and the directional dipolar interactions can contribute to the interparticle potential with comparable weight. These, together with the effects of particle substrate interactions and space-filling (entropic) factors, combine to determine the BNSL structure • Precise control of nanoparticle size, shape and composition allows to engineer electronic, optical and magnetic properties of nanoparticle building blocks. Assembling these nanoscale building blocks into a wide range of BNSL systems provides a powerful modular approach to the design of 'metamaterials' with programmable physical and chemical properties.

12. Development of Nanoparticle Libraries for Biosensing Eric Yi Sun, Lee Josephson, Kim A. Kelly, and Ralph Weissleder Center for Molecular Imaging Research, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts 02129-2060 Bioconjugate Chem.,ASAP Article Web Release Date:December 30, 2005

13. Modify the surface of magnetofluorescent nanoparticles with small molecules

14. Microarray feasibility experiments

15. Reasons for recommendation • The idea to achieve specificity through deriving the nanoparticles with small molecules is inspirational and deserve attention. • The methods they developed are widely applicable and easy to follow. • The style of writing of the paper is clear, concise and meaningful.