Supporting Information
Location Deterministic Biosensing from Quantum-Dot-Nanowire Assemblies
Chao Liu1, Kwanoh Kim2, D. L. Fan1,2,*
- Materials Science and Engineering Program, Texas Materials Institute, The University of Texas at Austin, Austin, Texas, 78712, USA;
- Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas, 78712, USA
*Email:
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.) 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 Figure4, respectively) were fabricated. First, a Cr/Au (5 nm/ 95 nm) bi-layer film was deposited on a thinglass 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 Figure4, 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 uniformSiO2layer 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
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
,(1)
According to the Stokes’ law, .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 by4.
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 , 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:
,(2)
Where , , , .Use R=80 m andassuming Re(K)=1, of typical order of magnitude of that of semiconductor materials, the velocity of QDs can be estimated as
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.
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.
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.
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
1X. B. Xu, K. Kim, H. F. Li, and D. L. Fan, Adv Mater 24, 5457 (2012).
2D. L. Fan, F. Q. Zhu, R. C. Cammarata, and C. L. Chien, Nano Today 6, 339 (2011).
3A. Castellanos, A. Ramos, A. Gonzalez, N. G. Green, and H. Morgan, J Phys D Appl Phys 36, 2584 (2003).
4T. B. Jones, Electromechanics of particles. (Cambridge University Press, Cambridge ; New York, 1995), pp.xxii.
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