This work aims at developing a new technique 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 to build muscle-like actuators in which these molecules work in parallel to output large forces. 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. More specifically, we have engineered special thiolated model molecules (bisdisulfide/C_(28)H_(34)O_4S_4) and demonstrated the nano-to-micro self-assembly through flow interface using thiol-gold bonding chemistry. Our method has produced gold/molecule aggregates as big as 50 µm that are completely made of 30 nm gold nanoparticles and 3 nm model molecules

Liu, Yi

Meng, Ellis

Shih, Chi-Yuan

Stoddart, J. Fraser

Tai, Yu-Chong

Zheng, Siyang

Caltech Authors - Main

NANO-TO-MICRO SELF-ASSEMBLY USING SHEAR FLOW DEVICES 
Chi-Yuan Shih, 'Siymg Zheng, 'Ellis Meng, 'Yu-Chong Tui, 'Yi Liu and 'J. Fraser Stoddurt 
'Caltech Micromachining Laboratory 
Electrical Engineering Department MC136-93, California Institute of Technology 
Pasadena CA9 1 125 USA 
Telephone: (626)395-2227 Fax(626)584-9104 
E-mail:cyshih@caltech.edu 
'University of California Los Angeles, Department of Chemistry and Biochemistry 
I 
ABSTRACT 
This work aims at developing a new technique to precisely 
assemble nano-materials into micro- or even meso-scale 
devices. For example, OUT long-term goal is to use massively 
architected motor-molecules [l]  to build muscle-like actuators 
in which these molecules work in parallel to output large 
forces. As an important first step, we report here the 
successhl 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/CzsH?404S4) and demonstrated the nano-to-micro 
self-assembly through flow interface using thiol-gold bonding 
chemistry. Our method has produced goldmolecule 
aggregates as big as S0pm that are completely made of 30nm 
gold nanoparticles and 3nm model molecules 
INTRODUCTION 
Precision molecular assembly is the cornerstone of 
nanotechnology. It follows that the major technical challenge 
is to assemble arrays of motor molecules repeatedly and 
coherently from the nano up to micron scale. As a first step, 
we will research a new technology to manufacture micron- 
scale, muscle-like actuators from assemblies of nanoscale 
linear-motor molecules and gold nanoparticles. We propose to 
use nano/microfluidic technology to achieve this goal. 
Integrated microfluidic systems will he developed to 
manipulate strategically thiolated motor-molecules and gold 
nanoparticles. Our goal is to not only demonstrate the 
feasibility of fabricating hiomimetic muscle fibers based upon 
engineered linear motor molecules, but to do so using scalable 
technology. 
Fig. 1 shows the basic 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 flow interface for 
controlled assembly. The second is the unsaturated growth of 
aggregate because shear flows continue to supply fresh nano- 
materials to the interface, leading to large aggregates. 
Figure 1: "Shear Flow Assembly" working principle. 
FABRICATION 
To achieve molecular assembly in microfluidic devices, we 
have designed and fabricated two types of devices as described 
in the following. The first type of devices is to make PDMS 
(Sylgard 184) molded channels bound to glass substrate as 
shown in Figure 2(a). Microchannels of 10pm deep are 
molded onto PDMS surface. The PDMS pieces are then bound 
to glass slides by overnight baking at 80°C. A total of six 
designs with different number of input channels, channel 
widths and shear flow configurations are made to study their 
effects on assembly efficiency. 
I . - _  
1 -  
' -.a'. 
U 
I - 
F i q r e  2: Devices fabricated. (a) PDMS/glass device. (b) 
gl&s/silicon device.' 
Due to the fact that the proposed "muscle" molecules 
dissolve much better in organic solvents which in general are 
not compatible with PDMS, we design and fabricate a 
glassisilicon device as shown in Figure 2(b). Fluidic channels 
were formed on silicon wafer with DRIE. Anodic bonding is 
used to bond silicon chip to glass chip. This device is capable 
0-7803-8265-X/04/$17.00 02004 IEEE 422 
of handling any organic solvent including dichlomethane 
(DCM) and acetone that are not suitable for PDMS material. 
DEVICE AND REACTION SYSTEM CALIBRATION 
Shear flow device calibration 
Once devices are made, syringe pumps are used for liquid 
supply with a wide range of flow rate controllability. As 
shown in Figure 3 ,  the shear-flow width is defined to he the 
width of the central flow after the multiple inlet junctions (in 
the case of a three-input channel design). The central flow 
width calibration is done using fluorescence dye under a 
fluorescence microscope. Our results show that central flow 
width as small as lpm or smaller can he achieved, which is 
perfect for muscle fiber synthesis. 
- I@ 
Figure 3: Shear-flow width calibration. 
Reaction system calibration 
The success of the proposed “muscle” fiber assembly 
depends on the conjugation reaction between the thiol groups 
of the “muscle” molecules and gold nanoparticles. To study 
the conjugation efficiency under various conditions, we have 
engineered special thiolated “model molecules” 
(bisdisulfide/C28Hs404S~) as shown in Figure 4. With model 
molecules we then study the solvent effects, gold nanoparticle 
size effects and temperature effects to find the working 
window for gold-molecule conjugation. 
Solvent effect 
First, we find that model molecules dissolve poorly in water 
hut they dissolve much better in DCM (>IOOmM), acetone 
(about I d )  and ethanol (400pM). On the other hand, the 
colloidal gold nanoparticle solution from British Bio-cell Inc. 
(BBI) is a water-based system. We therefore need to find a 
solvent that can dissolve reasonable amount of “muscle” 
molecules and won’t destabilize gold nanoparticles when 
mixed with gold solution. 
A screening experiment is then carried out to find a suitable 
solvent system. First we find that we could do solvent 
exchange for 30nm gold nanoparticles. We centrifuge aqueous 
nano gold solution and re-suspend gold precipitate in ethanol, 
IPA and acetone. As negative controls, we mixed ethanol, 
IPA, acetone and DCM with gold solution based on water, 
ethanol, IPA and acetone. DCM is found to cause 
agglomeration of gold nanoparticles. IPA also destabilizes 
aqueous gold solution slowly. When model molecules in 
acetone are mixed with water, crash out of molecules happens 
as a result of low solubility. Finally, we choose model 
molecules in ethanol and gold nanoparticle in ethanol as the 
solvent system. As shown in Figure 5(a), upon mixing model 
molecule solution with gold solution the model molecules can 
crosslink gold nanoparticles to change solution color from red 
to purplehlack and eventually the aggregate will precipitate 
[2]. This reaction is not shown in negative controls which do 
not contain molecules meaning that aggregation of gold is 
caused by molecule cross-linking of nanoparticles. 
P 9 
Figure 4: Bisdisulfide model molecule 
l - ! ! ! L - A “ . J  I 
.- 
Figure 5: Molecule/gold-nonoparticle reaction in test tube. (a) 
molecule/gold reaction in ethanol. 0) Gold nanoparticle size 
effect. 
Gold nanoparticle size effect 
Gold nanoparticle solutions with particle sizes of 5nm, 
lOnm and 30nm are used to study the particle size effect on 
gold-molecule conjugation. Figure 5@) shows that only 30nm 
gold particle solution provides visible reaction. We believe 
this gold size effect is related to geometrical shapes of gold 
nanoparticles. Colloidal gold with a diameter larger than 25nm 
exhibit geometrical eccentricity [3]. In fact, our TEM pictures 
(Figure 6) verify that 30nm gold nanoparticles have crystalline 
nature. These crystal facets provide larger binding aredenergy 
for molecules to bridge them and result in stable aggregate. 
Figure 6: TEM of 30nm gold nanoparticles. 
Temperature effect 
Temperature treatment of the molecule-gold aggregate was 
performed to see if higher temperature can break the thiol-gold 
bonds. Reaction mixture of aggregates are heated up to 100°C 
for 2hr. There is no visible change to the aggregate from our 
experiments meaning that thiol-gold bonds do not break in 
423 
general up to 100°C. This is positive in terms of the high 
stahility of aggregates once formed. 
RESULTS AND ANALYSIS 
Self-assembly of aggregates made of gold nanoparticles and 
model molecules are camed out in a 4-chatmel input shear 
flow device as shown in Figure 7. Gold-molecule aggregates 
form right after shear flows meet. In the downstream, 
aggregates accumulate and continue to grow in size. TEM of 
the aggregates (Figure 8) clearly shows 3D cross-linking of the 
gold nanoparticles. 
ethanol 
"band" formed in 
upstream 
large pieces farmed in 
downstream 
30nm gold 
in ethanol 
ethanol 
(a) Device configuration and experimental setup. 
(6) Assembled "band" structure right afrer mixing of gold 
and moleculejow (I&) and assembled structure confined by 
ethanol sheathflow in the downstream (right). 
Figure 7: Molecule/gold-nanoparticles assembly in device 
Fizure8: TEM uictures. Pure 30nm Fold in water llefi). 
I - 
Assembled structure in device (right). 
Aggregate size distribution 
With the formation of aggregates in the downstream of 
shear flow device, we further examine the size distribution of 
the aggregate particles. Figure 9(a) shows the distribution of 
the aggregate sire over time. When the aggregation is done in 
test tuhes we find that the aggregate particle sizes is quite 
homogeneous and in general smaller than IOpm. On the other 
hand, when fluidic device is used to form the aggregates. 
significant improvement in terms of forming large size 
aggregates in a short time is achieved. First, the aggregate size 
continues to grow with time and up to 50 pm size of aggregate 
has been found. Secondly, the growth rate in microfluidic 
device is found to he much faster than in the test tube reaction 
as shown in Figure 9(h). The results confirm the validity of 
our proposed shear-flow approach. 
.,A+ 1 
20 40 60 80 100 120 
Time (min) 
(a) Time evolution ofparticle size distribution in device. 
p" 15 
g) 10 
' 0 20 40 60 80 100 12C 
Nucleation Time (min) 
(6) Aggregate growth rate comparison. 
Figure 9: Particle size and growth rate analysis. 
Electrical property of aggregates 
In order to study the electrical properties of as formed gold- 
molecule aggregates, pieces of aggregates are collected and 
carefully placed onto an electrode chip in liquid to bridge a 
pair of electrodes. A 20 hr air dry followed by a 4 br baking at 
100 'C is used to remove extra solvent and to form better 
electrical contact between the assembled material and the gold 
electrode as shown in Figure IO.  I-V measurements show the 
aggregate formed by gold-molecule conjugation has a 
resistance of 16.1GQ which corresponds to a resistivity of - 
IO6 R-cm based on the estimated size of the aggregate. In our 
experiments, we also do a control with only pure gold 
aggregate. Without surprise, pure gold aggregate has a very 
small resistivity of - R-cm because the aggregate is 
formed by gold-to-gold connection. The fact that gold- 
molecule aggregate has a resistivity 11 order-of-magnitude 
higher than the pure gold aggregate suggests that conjugation 
among gold nanoparticles and molecules is achieved in the 
assembly process. Large resistivity is due to the insulating 
molecules between gold nanoparticles. 
424 
Furthermore, experiment also shows that no electrical 
breakdown was observed for gold-molecule aggregate up to +/- 
lOOV or +/-4OkV/cm, which is encouraging for the goal of 
using electrical signal to actuate motor molecule. 
4 .40  -0.05 0.00 0.05 0.10 
Voltage (volt) 
Figure I O :  Electrical properq measurement of molecule-gold 
aggregate. 
Aggregate collector 
In order to collect and shape the aggregate formed in the 
shear flow device, an in-channel collector is built. Figure 1 1  
shows that a dense and rectangular-shaped aggregate is 
successfully collected. The leak holes around the collector are 
made to avoid aggregate clogging around the collector inlet. 
J 
Figure I I :  In-channel aggregate collector. 
Alternative assembly strategies 
The versatile use of shear-flow assembly is further 
demonstrated by 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, “static” liquid interface can be created to 
Figure 12: Gold nanoparticle assembly assisted by DNA 
hybridization. 
serve the purpose. Figure 12 shows self-assembly of 
complementary goldDNA conjugates through DNA 
hybridization [2] at the liquid interface 12 hours after self- 
assembly in the “stopped shear flow channel. 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 as shown in Figure 13. Electrostatic attraction between 
the charged particles then quickly forms a 2mm-long/12pmun- 
high fiber within seconds. 
r 
R i  pmmg amine gmup 
(a) Fiber/sheet structure formed alongflow interface 
~- A> 20pm . .  - 
___... . ~- - -  - -  - , ~ ~ 
(b) Assembled fiber pieceflushed to the downstream. 
Figure 13: Sheet/Fiber structure formed by self-assembly of 
positivelyhegatively charged gold nanoparticles. 
CONCLUSIONS 
Molecular assembly using shear flow devices and thiol- 
gold bonding chemistry is demonstrated. Gold-molecule 
aggregates as big as 50pm that are completely made of 30nm 
gold nanoparticles and 3nm model molecules can be produced 
using our approach. Currently, we are studying using 
rotaxane, the linear motor molecules, to form muscle fiber with 
the techniques developed in this work. Selective growth and 
anchoring of aggregateimuscle on electrodes for making 
actuators are also under investigation. 
ACKNOWLEDGEMENTS 
This work is sponsored by DARPA Biomolecular Motor 
(BMM) project and the NSF Center for Neuromorphic 
Engineering (CNSE) at Caltech. 
REFERENCES 
[ I ]  
[2] 
V. Balzani, A. Credi, F.M. Raymo and J.F. Stoddart, 
Angew. Chem. Int. Ed. (2000), 39,3348-3391. 
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. 
M.A. Hayat, “Collaidal Gold: Principles, Methods, and 
Applications”, Academic: San Diego, 1989; Val 1. 
[3] 
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Nano-to-micro self-assembly using shear flow devices

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Nano-to-micro self-assembly using shear flow devices

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