Supporting Information
Location Deterministic Biosensing from Quantum-Dot-Nanowire Assemblies
Chao Liu1, Kwanoh Kim2, D. L. Fan1,2,*
1.
Materials Science and Engineering Program, Texas Materials Institute, The University of
Texas at Austin, Austin, Texas, 78712, USA;
2.
Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas,
78712, USA
*Email: dfan@austin.utexas.edu
1. Fabrication of Au nanowires
Gold nanowires (NWs) were fabricated by electrodeposition into nanoporous anode alumina
disks.1,2 The diameters of the NWs are determined by the pore size of the templates. By adjusting
the amount of electric charge passing through the circuit at a voltage of -0.92 V (vs Ag/AgCl),
the lengths of the Au NWs can be readily controlled.
2. Optical Detection Setup
Mechanisms for manipulation of QDs:
An upright fluorescent microscope (BX 51, Olympus Inc.) equipped with a high-speed
ultrasensitive CCD camera (ProEM 512B, Princeton Instruments Inc.) was employed for
characterizing the motions of QDs. A Lumen 200 photoluminescence illumination system (Prior
Scientific, Inc.) combined with cube filter sets were used for imaging QDs.
Biosensing by the QD-NW assemblies:
A 532 nm laser (Coherent, Inc.) was used as the excitation light source. An invert microscope
(IX 71, Olympus, Inc.) equipped with a spectrograph (Acton 2500, Princeton Instruments, Inc.)
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and a high-speed ultrasensitive CCD camera (ProEm 512B, Princeton Instruments, Inc.) were
employed to record both images and spectra from the nanosensors.
3. Microelectrode fabrication process
Two types of microelectrodes (parallel and circular microelectrodes, Figure 1 and Figure 4,
respectively) were fabricated. First, a Cr/Au (5 nm/ 95 nm) bi-layer film was deposited on a thin
glass slide by the electron-beam evaporation at a base pressure of 5×10-6 Torr. Then, the
microelectrodes were patterned by the standard photolithography followed by the wet etching.
As a result, both the parallel (the gap size of 135 µm) and the circular electrodes (the inner radius
of 65 µm and the outer radius of 200 µm) were obtained as shown in Figure 1 and Figure 4,
respectively.
In order to clearly demonstrate biosensing process, microelectrodes were patterned on ITO
coated slide glass by the standard photolithography method followed by the wet etching process.
4. Au/SiO2 core-shell nanowires fabrication
The core-shell Au/SiO2 nanowires were fabricated by coating the surface of Au nanowires
with a uniform SiO2 layer via the hydrolysis of tetraethyl orthosilicate (TEOS). Specifically, the
whole batch of Au nanowires was dispersed and sonicated in a mixed solution of TEOS (0.8 ml),
ammonia (0.2 ml), deionized water (3.6 ml) and ethanol (6 ml) for 2 hours. The as-grown
Au/SiO2 nanowires were collected by centrifugation after washing by D.I. water and ethanol
alternatively for three times.
5. Estimation of QDs’ velocity under the DEP effect
2
In the manipulation process, a spherical particle with a radius of r in a liquid with a viscous
coefficient of η is subjected to two forces, the Stokes drag force and the DEP force. The
movement of particles is governed by3
𝑑𝑢
𝐹𝐷𝐸𝑃 − 𝐹𝐷𝑟𝑎𝑔 = 𝑚 𝑑𝑡 ,
(1)
According to the Stokes’ law, 𝐹𝐷𝑟𝑎𝑔 = 6𝜋𝜂𝑟(𝑢 − 𝑣). Where u is the particle velocity, v is
the fluid velocity, and m is the mass of particles. For a spherical particle, the DEP force is given
2
by4 𝐹𝐷𝐸𝑃 = 2𝜋𝜀𝑚 𝑟 3 𝑅𝑒(𝐾)∇𝐸𝑟𝑚𝑠
.
In a low Reynold number solution, the DEP force and Drag force balance each other. In
addition, on the circular electrode, the electric field is given by 𝐸𝑟𝑚𝑠 = 𝑉𝑟𝑚𝑠 /𝜋𝑅, Where R is the
distance from the center of the inner electrode to the spherical particle. Then the magnitude of
the particle velocity can be calculated as:
𝑢=
2
2𝜀𝑚 𝑟 2 𝑅𝑒(𝐾)𝑉𝑟𝑚𝑠
3𝜂𝜋 2 𝑅 3
,
(2)
Where 𝜀𝑚 = 7.09 × 10−10 𝐹/𝑚 , 𝑟 = 10 𝑛𝑚 , 𝑉𝑟𝑚𝑠 =
20
√2
𝑉 , 𝜂 = 8.9 × 10−4 𝑃𝑎 ∙ 𝑠 . Use R=80
m and assuming Re(K)=1, of typical order of magnitude of that of semiconductor materials, the
velocity of QDs can be estimated as
𝑢 = 2 𝑛𝑚/𝑠
3
Figure S1: Au nanowires (5 µm in length, 300 nm in diameter) attracted and aligned on parallel
microelectrodes at frequencies of (a) 50kHz, 20V, (b) 300kHz, 20V, (c) 500kHz, 20V, and (d)
800kHz, 20V. (e) The average chaining length as a function of the applied AC frequency.
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Figure S2. Simulation of the distribution of electric fields when nanowires are attracted to the
edge of microelectrodes. The parallel electrodes are in yellow, the nanowires are in white, and
the electric field lines are in red.
Java applet by Michael J. McGuffin, November 2004, University of Toronto. The simulation was
obtained by the electrostatic model where arrays of connected conducting spheres were
approximated as wires and electrodes.
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Figure S3 (a) (c) At 90 kHz and 20V, displacement of different QDs (relative to the center of the
inner electrode) versus time. (b) (d) Velocities of QDs increase linearly with 1/R3.
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Figure S4 Direct attraction of Cy5-biotin in suspension (20 pM) on the tips of the Au nanowires.
The fluorescent signals (in blue) of Cy5-biotin were detected from the tips of Au nanowires in an
E field of 700 KHz, 20 V. The fluorescent signals of the background Cy5-biotin solution (20
pM) is in magenta. Inset: the fluorescent image of Cy5-biotin on the tips of Au nanowires; scale
bar: 5 µm.
References
1
X. B. Xu, K. Kim, H. F. Li, and D. L. Fan, Adv Mater 24, 5457 (2012).
2
D. L. Fan, F. Q. Zhu, R. C. Cammarata, and C. L. Chien, Nano Today 6, 339 (2011).
3
A. Castellanos, A. Ramos, A. Gonzalez, N. G. Green, and H. Morgan, J Phys D Appl
Phys 36, 2584 (2003).
4
T. B. Jones, Electromechanics of particles. (Cambridge University Press, Cambridge ;
New York, 1995), pp.xxii.
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