Nanostructured Surfaces and Electrical Trapping

In order to create a patterned photoluminescence signal that can be used for an optofluidic microscope, we must have a method to selectively trap different species of particles in precise locations. Electrical fields can be used to trap small charged particles suspended in liquids. Quantum dots of different colors have been trapped on patterned ITO surface using so called layer by layer deposition [11]. By alternating the sign of the charge on an electrode, the surface can be made to either accept or repel the charged dots.

The specificity and flexibility of this approach may be extended by targeted de- livery of quantum dots to electroactive nanowells. An electroactive well is simply a nanohole defined in an insulator that is sandwiched between two conducting layers; typically the substrate is ITO on glass and the top electrode is gold or aluminum de- posited by evaporation. When a voltage is applied between the two conducting layers, strong electrical are present inside the well and extending into the space above it. As a suspension of nanoparticles (or dye molecules) flows over the activated nanowell, the suspended particles feel a trapping force due to the influence of the electric field distribution, ~ FE =q ~E+ 2πv∇ ~ E2 · ℜ ǫp−ǫm ǫp+ 2ǫm ! , (5.6)

where q is the charge on the particle, E~ is the electric field v is the volume of the particle and ǫp and ǫm are the dielectric constants of the particle and medium re-

QDs Nanowells 100nm diameter QDs Nanowells 100nm diameter (B) (A) (C) Glass Substrate PMMA ITO Electrodes Nanoparticle delivery via

PDMS nanochannels Glass Substrate PMMA ITO Electrodes Nanoparticle delivery via

PDMS nanochannels

PMMA

ITO Electrodes Nanoparticle delivery via

PDMS nanochannels QDs Nanowells 100nm diameter QDs Nanowells 100nm diameter (B) (A) (C) Glass Substrate PMMA ITO Electrodes Nanoparticle delivery via

PDMS nanochannels Glass Substrate PMMA ITO Electrodes Nanoparticle delivery via

PDMS nanochannels

PMMA

ITO Electrodes Nanoparticle delivery via

PDMS nanochannels

Figure 5.9: (a) Delivery of QD solutions to nanowells using PDMS nanochannels. (b) QDs are deposited on the substrate due to the electric field applied between the ITO electrodes. The nanochannels are removed, revealing a complete OFM substrate. (c)Finite element simulation of filling an electrostatically active nanowell with charged QDs ! ! ! (a) Attraction (c) Rejection (b) Storage/Detection FE FE

Strong field gradients (dielectrophoresis and electrophoresis)

Low field gradients (electrophoresis)

spectively. Examination of equation 5.6 that the dielectrophoretic force is directed in the direction of strongest field gradients with a sign dependent on the dielectric constant of the particle relative to that of the medium; whereas the direction of the electrophoretic force is aligned with the direction of the applied field, with a sign de- pendent on the sign of the charged particle. Reversing the applied voltage will cause the electrophoretic force to change direction, which can be used to eject a charged particle. Figure 5.1.4 illustrates the electrokinetics of particle interaction with this type of well.

For electrostatic confinement to work in this structure, the applied potential must be sufficiently strong to overcome thermal diffusion of the nanoparticles. A funda- mental limitation of any localized trapping scheme, such as that in Figure 5.1.4, is that as the applied potential between the upper and lower electrodes must be less than 1V to remain below the activation energy for electrolytic bubble generation and degradation of the ITO surface. We perform an order of magnitude analysis, comparing the amount of energy required to move a conjugated quantum dot out of the well against the applied potential field, with energy of the thermal diffusion,kbT.

Wtrap

Wtherm

= qV

kbT

(5.7)

Estimating the total charge from the electrophoretic mobility of the conjugated system reported by Gao et al. [11] yields anWtrap/Wtherm ratio of 455 for an applied voltage

of 1V. This result shows that the trapping potential should dominate the thermal diffusion.

We constructed a prototype of an electroactive nanowell by e-beam lithography. The device was manufactured using a relatively simple process which begins by spin coating an initial layer of PMMA, which serves as an insulation layer between the optically transparent Indium Tin Oxide (ITO) layer and a 100nm thick gold layer

Figure 5.11: Fluorescence imaging of electrokinetic trapping and rejection of a dye solution in a 1 micron “nanowell”. The dotted line illustrates the nanowell boundary, and the scale bar is 500 micron.

evaporated on top. A second PMMA layer is then spun on the top layer and the trapping wells defined by electron beam lithography. Following exposure the upper PMMA layer is developed and the hole pattern is etched in the gold layer. The upper PMMA layer is then washed off and the bottom layer is developed creating the wells. PDMS fluidics are then used to deliver the quantum dot cocktails to the general area of the targeted storage site and the electric field applied between the liquid in the channel and ITO layer is used to attract the quantum dots to the appropriate storage well (See figure fig:dye). Erasing is done by reversing the polarity and rejecting the cocktail in to the bulk. Readout is accomplished though an inverted epi-fluorescence scheme whereby both the excitation light is sent through the same high N.A. objective and the signal collected and sent through the fiber spectrometer. The gold layer serves to significantly reduce background noise.

The feasibility of trapping charged particles was tested by initial experiments using Rhodamine 110, a common fluorescent dye. Rhodamine 110 was chosen because it has an electrophoretic mobility comparable to that of thioglycolic acid stabilized CdTe quantum dot nanoparticles[11]. A dye–water solution was flowed over an electroactive nanowell structure with a diameter of 1 micron, while a trapping voltage of ˜0.5V

was applied. The excitation illumination is a Argon Ion laser at 488 nm, which is suppressed by a holographic notch filter before detection. Figure 5.1.4 shows the clear visibility of the dye trapped within the nanowell when a positive potential is applied. Reversing this potential causes the trapped dye to rejected from the well, and the measured fluorescence is virtually indistinguishable from the background fluorescence.