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Nano-to-Micro Self-Assembly Using Shear Flow Devices
Chi-Yuan Shih, Siyang Zheng, Ellis Meng, Yu-Chong Tai Yi Liu and J. Frazer Stoddart

It will be extremely useful if there’s a way to precisely assemble nano-materials into micro- or even meso-scale devices. For example, our long-term goal is to use massively architected motor-molecules [1] to build muscle-like actuators, in which these molecules work in parallel to output large forces. Unfortunately, the lack of such an assembly method is still the major barrier in the whole bottom-up nanotechnology field. This work aims at attacking this problem and as an important first step, we report here the successful development of a much improved shear-flow-enhanced self-assembly method over the baseline spontaneous assembly method in test tubes [2]. More specifically, we have engineered special thiolated model molecules (bisdisulfide/C28H34O4S4) and demonstrated the nano-to-micro self-assembly using thiol-gold bonding chemistry. Our method has produced gold/molecule aggregates as big as 50_m that are completely made of 30nm gold nanoparticles and 3nm model molecules. Fig.1 shows the idea of our shear-flow assembly. The interface of two shear flows is where gold nanoparticles meet with the thiolated molecules, herein the aggregation happens. The important advantages of this approach are twofold. The first is to limit the assembly only at interface for controlled assembly. The second advantage is the unsaturated growth of aggregate because shear flows continue to supply fresh nano-materials to the interface, leading to large aggregates. To implement this design, we fabricate two types of shear flow devices (Fig.2). For water or ethanol solvent system, PDMS/glass devices are used for easy plumbing and observation. For non-polar solvents like acetone and dichloromethane, glass/silicon devices are used to avoid PDMS swelling.

The success of self-assembly depends on the conjugation reaction between molecule thiol-groups and gold nanoparticles. To study the conjugation efficiency, bisdisulfide model molecules are first synthesized (Fig.3) and tested in tubes to determine the optimal solvent and gold nanoparticle system for self-assembly performance (Fig.4). It’s found that only 30nm gold nanoparticles show observable conjugation reaction with model molecules. Supported by our TEM study, we believe this is due to the gold nanoparticle’s polygonal surfaces (for nanoparticles larger than 25nm [3]), which enables a larger bonding surface and energy. Next, we apply the optimal assembly experiments in shear-flow devices. Fig.5 shows a typical channel configuration for conjugation reaction. In the upstream, a band of self-assembled aggregates forms along flow interface. In the downstream, pieces of assembled material accumulate and continue to grow in size. TEM of the aggregates (Fig.6) clearly shows 3D cross-linking of the gold nanoparticles. To confirm that these aggregates are not due to pure particle-particle affinity, we have performed electrical measurements on the aggregates. The high gold-molecule aggregate resistivity (~10⁶ _-cm) compared to ~10-⁵ _-cm of salt-induced pure gold aggregate (Fig.7) confirms the coating of gold particle with thiolated molecules, hence indirectly proves the [gold-molecule]n linkage. Next, conjugation efficiency in the shear-flow devices and tubes are studied. Fig.8 shows that self-assembly in our shear-flow device has much larger growth rate (~0.3_m/min) and aggregate size than those from test tube (~0.063_m/min). The results validate our approach to synthesize micro devices from nano-materials.

Finally, the versatile use of shear-flow assembly is further demonstrated using two other examples. First, when longer or stiffer motor molecules are used to cross-link gold nanoparticles, conjugation reaction will need much longer time. In this case, slow-changing or even “static” liquid interface can be created to serve the purpose. Fig.9 shows self-assembly of complementary gold/DNA conjugates through DNA hybridization [2] at the liquid interface, 12 hours in the “stopped” shear flow. The second demonstration is to assemble high-aspect-ratio fibers. To do so, separate positively and negatively charged gold nanoparticle streams are flown into the two-input shear-flow device. Electrostatic attraction between the charged particles then quickly forms a 2mm-long/12_m-high fiber within seconds (Fig.10).

Submitting author: Chi-Yuan Shih, California Institute of Technology, MS 136-93, Pasadena, CA 91125. Tel: (626) 395-2227, Fax: (626) 584-9104, email: cyshih@caltech.edu

References
[1] V. Balzani, A. Credi, F.M. Raymo and J.F. Stoddart, Angew. Chem. Int. Ed. (2000), 39, 3348-3391.

[2] J.J. Storhoff, A.A. Lazarides, R.C. Mucic, C.A. Mirkin, R.L. Letsinger and G.C. Schatz, J. Am. Chem. Soc. (2000), 122, 4640-4650.

[3] M.A. Hayat, “Colloidal Gold: Principles, Methods, and Applications”, Academic: San Diego, 1989; Vol 1.

(a) Device configuration and experimental setup

(b) Assembled “band” structure right after mixing of gold and molecule flow (left) and assembled structure confined by ethanol sheath flow in the downstream (right).
Figure 5: Molecule/gold-nanoparticles assembly in device.

Figure 5: Molecule/gold-nanoparticles assembly in device.

Figure 6: TEM pictures.



Figure 7: Electrical property of molecule/gold self-assembly material.

(a) Time evolution of particles size distribution in device

(b) Aggregate growth rate comparison.

Figure 8: Particle size and growth rate analysis.

Figure 9: Gold nanoparticle assembly assisted by DNA hybridization.

(a) Fiber/sheet structure formed along flow interface.

(b) Assembled fiber piece at downstream.

Figure 10: Sheet/Fiber structure formed by self-assembly of positively/negatively charged gold nanoparticles.