Size-Tunable Assembly of Gold Nanoparticles Using Competitive AC

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Interface-Rich Materials and Assemblies

Size-tunable assembly of gold nanoparticles using competitive AC electrokinetics Meenal Goel, Akshay Singh, Ashwin Bhola, and Shalini Gupta Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03963 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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Size-tunable assembly of gold nanoparticles using competitive AC electrokinetics Meenal Goel, Akshay Singh#, Ashwin Bhola#, Shalini Gupta* Dept. of Chemical Engineering, Indian Institute of Technology Delhi (IITD), New Delhi, 110016, India *Corresponding #Equal

author: [email protected]

contribution

Abstract. Alternating current (AC) electrokinetics is a facile way of patterning colloidal particles into advanced structures. We demonstrate the combined use of AC dielectrophoresis (AC-DEP) and AC electrohydrodynamics (AC-EHD) in a microwell electrode geometry for size-tunable assembly of gold nanoparticles (AuNPs) into one-dimensional (1D) microwires and 2D films. The AC-DEP force scales with both particle size and field frequency whereas, the AC-EHD force depends only on the field frequency. So, a critical particle diameter (dc) exists below which the EHD phenomenon becomes more important and beyond which, the DEP force is dominating. We performed theoretical and experimental studies to determine ‘dc’ and how it gets affected by operating parameters like field frequency, voltage, particle number, electrolyte concentration, electrode size and geometry. Our results show that the morphologies of the colloidal structures formed, transition from films to microwires as the NP diameters vary from nanometers (< dc) to microns (> dc) and no assembly takes place at intermediate sizes (d ~ dc). While the film formation is governed purely by surface EHD flows, microwire synthesis is a result of EHD-assisted DEP phenomenon. Also, a minimum particle number, a low salt concentration and an optimum frequency range is required for the assembly to initiate. Keywords: Electrokinetics, microwire, thin film, electrohydrodynamics, dielectrophoresis, gold nanoparticle

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INTRODUCTION Colloidal manipulation and assembly into higher order architectures has been an area of active research for the past several decades due to the applications of these materials in sensors, coatings, photonics and electrical devices.1–9 There are many routes by which colloids can be assembled. The most notable ones include sedimentation, forced convection, template-assisted accumulation, the Langmuir-Blodgett method and use of external fields (electrical, magnetic, optical etc.).10-24 Of these approaches, electric field (EF) manipulation via AC electrokinetics is a far more superior technique than others due to its ease of implementation, generality of approach and higher throughput.25–27 AC electrokinetics involves EF-driven particle motion and/or fluid flow via two main phenomena: AC-DEP and AC-EHD. Both these phenomena can be controlled somewhat independently and have been studied under different frameworks until now. For instance, AC-DEP has been extensively applied to a wide range of particle types including latex microspheres, metal NPs, carbon nanotubes, quantum dots (QDs), biomolecules and live cells for their assembly into 1D microwires or 2D arrays.28–37 For a coplanar electrode geometry and particles more polarizable than the media, this particle assembly always takes place in the gaps between the electrodes starting from the electrode edges. AC-EHD, on the other hand, results in particle pumping away from the coplanar gaps giving rise to particle collection on top of the electrodes. When the electrode design layout is changed from a coplanar to a microwell array format (also referred to as conductive corrals, islands etc. in the literature; see Figure S1), there exist many examples of systems where particle collection takes place on top of the microwells. However, when we carefully looked for cases of particle assembly between the circular disclike electrodes, we found no such analogues irrespective of the particle types (e.g. AuNPs, latex, yeast, mammalian cells)36,38–40 or particle sizes (5 nm to 25 m) used. More interestingly, while some groups attributed this assembly behavior to be DEP-assisted,36,38 others claimed it to be EHD-driven.39,40 We found this aspect rather intriguing, as for the microwell electrode geometry, EHD and positive DEP forces act in opposite directions and should thus in turn, either cancel out (depending on their magnitude) or co-exist in such a way that they may be relatively tuned against each other to obtain a rich class of microstructures, a research space that currently lies unexplored. Further, as the EHD phenomenon depends only on the field frequency whereas, DEP scales with both field frequency and particle size, one can expect a critical particle diameter to exist across which either of the two forces becomes more prominent.

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To prove our hypothesis, we used AuNPs as a model system in our study as they were easy to synthesize and modulate in the lab. The critical particle diameter required for EHD to DEP transition was first determined by finite element analysis (FEA) and based on our results, AuNPs of different sizes were prepared experimentally. The particle suspensions were then exposed to AC fields in our microwell electrode setup and their assembly was monitored via optical microscopy. Our results showed a distinct morphology change in the microstructures formed below and above the critical particle size suggesting that competitive DEP and EHD forces were at interplay as hypothesized. A detailed investigation was thus carried out to understand the role of governing parameters like particle size, field frequency, voltage, electrolyte concentration, electrode size and electrode geometry on the observed behavior. The outcomes of our study are outlined in the sections below. While the work seems relevant for generating a rich class of materials, the insights gained into the mechanisms of competitive AC-electrokinetics cannot be ignored and may be extended to other particle types or mixtures. Theory. EF manipulation requires detailed knowledge of the particle and fluidic response to an external field. Any surface when suspended in a polar liquid medium such as water tends to generate an electrical double layer (EDL) at the interface due to the mobile counterions that originate from the surface itself and those already present in the bulk liquid.41 When an EF is applied, these counterions move along the tangential direction of the field, dragging the liquid along with them and creating a shear at the interface. This shear force results in the movement of the surface in the opposite direction due to momentum balance in a largely viscous environment (at small length scales). If the surface is small, i.e., it belongs to a micron or nanosized particle, the particle movement is “visible” resulting in a phenomenon commonly known as electrophoresis. If the surface is large, like that of a glass substrate, only the movement of liquid is observed in a process known as electroosmosis. The electroosmotic flow in case of colloidal suspensions can also carry particles along with the liquid assuming the electrophoretic force acting directly on the particles is less dominating than the electroosmotic force. The velocity of the particles in this case is equal to the velocity of the fluid. Needless to say, both phenomena lead to particle assembly in case of DC fields and can be tuned depending on the zeta-potentials (ζ) of the particle and the electrode surface. The electroosmotic force experienced by the liquid is given by eq. 1,42

𝐹𝐸𝑂

=

( )𝐸𝑡

𝜎𝐸𝑡 = 8𝜀𝑚𝑘𝑇𝑐𝑜𝑠𝑖𝑛ℎ

3

𝑧𝑒ζ

2𝑘𝑇


(1)

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where, 𝜎 is the effective charge density in the diffuse layer, 𝐸𝑡 is the tangential EF, 𝜀𝑚 is the absolute permittivity of the medium, 𝑘 is the Boltzmann constant, 𝑇 is the temperature, 𝑐𝑜 is the bulk counterion concentration, 𝑒 is the electronic charge and 𝑧 is the valency of the ions. The above equation holds true for symmetric electrolytes and regions of small surface charge densities where effects of the Stern layer can be ignored. The net movement cancels out in AC fields (in a frequency-dependent manner) resulting in no net movement along the tangential EF direction. This happens for an EDL formed on an EF-unresponsive surface (e.g. glass) activated by external electrodes. However, if the underlying surface is field-response, e.g. the electrode itself, then another important phenomenon known as AC-EHD or AC induced charge electroosmosis (AC-ICEO), similar to DC electroosmosis, becomes relevant. This happens especially in cases where electrodes are micropatterned, or in other words, where there exists considerable electrode-liquid interface area with strong EF gradients. The presence of a gradient in the electric field normal to a surface leads to a tangential electric field, which drives EHD flow by inducing a body force on the induced charge near the surface. In this case, the net motion of the ions in the two half-cycles doesn’t cancel out but rather becomes additive, leading to directed liquid pumping from the electrode edges to their center.39,42-44 The time-averaged force experienced by the liquid in this case is given by eq. 2, 𝐹𝐸𝐻𝐷(𝜔) = 𝜎𝑖𝑛𝑑𝐸𝑡 = ― 𝜀𝑚𝜅(𝜉𝑖𝑛𝑑 ― 𝜑𝑏)𝐸𝑡

(2)

where, 𝜎𝑖𝑛𝑑 is the induced charge density in the diffuse layer, 𝜅 ―1 is the Debye length of the suspension, 𝜉𝑖𝑛𝑑 = 𝜉 ± 𝑏𝑉 is the induced zeta potential in the positive and negative half-cycle, respectively, 𝑉 is applied potential, 𝑏 is a random variable whose value lies between 0 and 1, and 𝜑𝑏 is the potential in the bulk solution. Eq. 2 assumes voltage drop across the induced double layer to be a constant in the first limit of approximation.45 The fluid velocity is zero at the outer of edge of the shear plane due to the no slip boundary condition and maximum in the slip plane (approximately extending between 𝜅 ―1 to 4𝜅 ―1). Since EDL formation is a dynamic process, the net fluidic velocity also depends on the frequency of the applied field (counterions require finite time to build up) in addition to the medium viscosity and electrolyte concentration. Other than EHD, particle assembly can also take place due to the direct motion of the particles in the direction perpendicular to the tangential vector of the applied EF. This happens

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only in case of spatially non-uniform DC or AC EFs and is known as DEP. The direction of the particle movement depends on the net polarizability of the particle with respect to the medium. The time averaged DEP force in AC fields depends on the gradient of the square of applied EF and radius of the particle (𝑟) (eq. 3)

𝐹𝐷𝐸𝑃(𝜔) = 2𝜋𝜀𝑚𝑟3𝑅𝑒

Here, 𝑅𝑒

(

𝜀𝑝∗ ― 𝜀𝑚∗ 𝜀𝑝∗ + 2𝜀𝑚∗

(

𝜀𝑝∗ ― 𝜀𝑚∗ 𝜀𝑝∗ + 2𝜀𝑚∗

)|∇𝐸 (𝑅𝑀𝑆)| 2

) is the Clausius-Mossotti (CM) factor and, 𝜀

∗ 𝑝 and

(3)

𝜀𝑚∗ are the complex

permittivities of the particle and medium, respectively. Particles that are more polarizable than the media are pulled toward the high EF regions (positive DEP). Otherwise, they are pushed away to the low EF regions (negative DEP). The net dipole formed in the particle is contributed by both the polarization of its intrinsic core as well as the surface EDL (in cases of highly charged dielectric particles like latex) both of which have different relaxation frequencies.46 In this way, even uncharged particles can be made to move using DEP, overcoming the limitations imposed by DC electrophoresis.

EXPERIMENTAL SECTION Materials. Chloroauric acid (HAuCl4.3H2O) and sodium citrate (Sigma-Aldrich, India); Tannic

acid,

acetone

and

isopropyl

alcohol

(IPA)

(Fisher

Scientific,

India);

Polydimethylsiloxane (PDMS) elastomer kit (Dow Corning, USA); Silicone elastomer (Metroark, India); Indium tin oxide (ITO) coated conductive glass slides (Macwin, India); SU8-2 negative photoresist (PR) and developer (Microchem ,USA); 30 kDa MWCO centrifuge filters and ultrapure Milli-Q (MQ) water of resistivity ~ 18 MΩ.cm (Millipore, India); Platinum on glass interdigitated electrodes (IDEs) with finger width and gap sizes between 10 to 50 µm (Micrux Technologies, Spain and CEERI, Pilani, India). All reagents were of analytical grade and used as received. Methods. (i) AuNP suspensions. Citrate-capped 17 nm diameter AuNPs were synthesized using a standard protocol available in the literature.47,48 These NPs were then used to prepare the larger sized particles. For this, 15 mL of 2 nM AuNP suspension (of 17 nm size) was equally divided into two falcon tubes and centrifuged at 2713g for 15 min. The pellets formed at the bottom were transferred to a new falcon tube and the supernatants were further centrifuged up

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to 18990g with a step size of 2713g until they became colorless. The pellets formed after each centrifugation step were pooled together and resuspended in 1.2 mL of MQ water to obtain a 50 nM stock that was stored at 4 oC. The controlled centrifugation led to an increase in the particle size from nanometer to the micron range. Prior to each experiment, the 50 nM stock was used as such or diluted to 5 and 2 nM to obtain particles of sizes 10 m, 130 nm and 17 nm, respectively (the particles showed size-reversibility upon dilution). 5 m size particles were also separately prepared by centrifuging the 17 nm particles directly at 10,375g for 15 min using 30 kDa MWCO filters. The particle sizes, morphology and concentrations were characterized using a number of methods including transmission electron microscopy (TEM) (JEOL JEM 1400), dynamic light scattering (DLS) (Malvern Zetasizer ZS09), UV–visible spectroscopy (Shimadzu UV-2600) and optical microscopy (Olympus BX-53). The particle sizes were analyzed using the free ImageJ software (version 1.48). (ii) Electrode microfabrication. The microwell array electrodes were prepared on ITO-coated glass substrates. First, the ITO was cleaned by sequentially dipping it in acetone, IPA and deionized (DI) water for 15 min each followed by drying at 120°C in a standard lab oven. A 2.5 m thick layer of PR was then spin-coated on the substrates and soft baked at 95 °C for 15 min. The spin-coated epoxy layer was UV exposed (~ 40 mJ/cm2) through a mask plate for 5 s in a hard contact mode using a double-sided aligner (DSA) (EVG-620) and immediately hard baked at 95 °C for 15 min to complete the crosslinking process. Microwell arrays with feature sizes of 50 µm well diameter (D) and 50 µm edge-to-edge spacing (S) were prepared by keeping the above treated ITO substrate in an SU8 developer for 1 min. The developed microelectrodes were then cured for 30 min at 120 °C to allow complete adherence of the film to the substrate. The stability of the PR film was checked by washing the electrodes multiple times with IPA and DI water. The height of the PR layer was confirmed by a profilometer (Ambios XP2). The electrodes were freshly cleaned with IPA, DI water and 70 % v/v ethanol before each experiment. (iii) Experimental setup. The device configuration used in this study was similar to that reported in ref. 39. Briefly, a 5 µL volume of AuNP suspension was sandwiched between a patterned and a non-patterned ITO-coated glass electrode using a 500 µm thick cylindrical PDMS chamber (diam. ~ 2 mm). The electrode assembly was properly sealed to minimize evaporation. Both electrodes were connected to a waveform function generator (33500 B

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Series, Agilent) to apply a sinusoidal AC EF of strength 1 to 20 Vpp and 50 Hz to 10 kHz across the AuNP suspension. The circuit included a 1 µF capacitor in series to block any DC component produced by the generator and the strength of the applied EF was monitored using a multimeter. The NP assembly was continuously monitored in brightfield with a long-distance 50x air-objective using an optical microscope and the images were recorded using an Olympus E3 CCD color camera mounted on top. All the experiments were performed at room temperature (~ 25 oC) and repeated at least thrice before reporting the final results. Each experimental data point was collected with a fresh sample of AuNP suspension and electrode. (iv) Electrostatic simulations. The EF distribution inside the microelectrode chip was simulated using COMSOL Multiphysics® version 5.2. As the microwell geometry was symmetric, it was sufficient to simulate the EF distribution across only one microwell. Also the simulation was carried out for only one positive half-cycle of the AC field. The system geometry was drawn to scale and the boundary conditions were defined as follows: bottom electrode 20 Vpp, top electrode grounded, side walls zero charge. Water was taken as the dielectric medium (w = 80.1 and Sw = 5.5 S/m) and the PR (PR = 2 and SPR = 0 S/m) and formed a perfect insulator. The solution space was then divided into extrafine triangular meshes such that the physical properties remained constant across each mesh and number of grid points for x and y coordinates were taken as 2L+1 and L+1, respectively, where L is the total length of the geometry in the x and y directions. The EF values were calculated using the electrostatics module by solving for the potential energy variation throughout the domain. The equations used were (eqs. 4 to 6), Charge conservation:∇.𝐽 = 𝑄𝑗 (4) Current density: 𝐽 = 𝜎𝐸 + 𝐽𝑒

(5)

EF: 𝐸 = ― ∇𝑉

(6)

From COMSOL, the EF values and their gradients were obtained for calculating the EHD and DEP forces.

RESULTS Our experimental setup comprised of a high-density microarray of conductive and circular microwells embedded inside a thin insulating PR layer coated on top of an ITO-coated glass 7


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(Figure 1). This micropatterned surface served as the bottom electrode. The top electrode was a plain ITO-coated glass, separated from the bottom electrode using a non-conductive spacer. The EF distribution inside the chamber was calculated using COMSOL as shown in Figure 2a. The results showed that the magnitude of the EF was maximum just above the electrodes, in direct proportionality to the length of the field vector shown in the figure and decreased exponentially away from it. The EF lines were also highly distorted near the electrode edges but relatively uniform elsewhere including in the center of the conductive microwells. It was precisely this spatially varying non-uniform AC field inside the chip that gave rise to the ACDEP and AC-EHD electrokinetic phenomena, and thus, particle collection. To understand how these electrokinetic phenomena manifested into particle assembly, we first investigated the variation of these forces with particle size using FEA. For this, the center of the particle was placed at 𝜅 ―1 ~ 1 µm above the microwell edge (0, 1 coordinate) where it was expected to experience the maximum force. The EF strength and gradients were determined from our model for a representative set of operating conditions (20 Vpp, 100 Hz) and applied to eqs. 2 and 3 to calculate the x-component of the EHD and DEP forces, respectively. Assuming the EHD force to be independent of particle size, its dependence on the particle diameter resulted in a flat line (Figure 2c). DEP force, on the other hand, had a 𝑑3 dependence. By doing this, the critical particle diameter (𝑑𝑐) at which both the forces became equal was found to be ~ 6 µm. Below this critical size, EHD controlled the assembly process whereas thereafter, DEP became the more dominating force. More importantly, both of these two forces were many orders of magnitude larger than the Brownian force at any time and could allow facile colloidal deposition (see Supplementary Information). The order of magnitude of the critical particle size remained more or less constant at different locations around the chip (Figure S2). We also estimated the magnitudes of the two forces on a 6 m particle at various locales. A qualitative map of these forces is depicted in Figure 2b and a quantitative variation in the x-direction is shown in Figure 2d (see SI for detailed calculations). As expected, both the forces increased exponentially near the microwell edge and were comparable in their order of magnitude. However, while the EHD force always remained positive (pointing toward the center of the microelectrode), the DEP force changed direction from positive to negative as we moved from the PR toward the electrode. Manifestation of particle size on colloidal structure morphology. Based on the above theoretical estimates, we synthesized four different particle sizes to carry out our experimental

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studies – (i) 17 ± 4 nm 𝑑𝑐. The smallest sized particles were produced using a wet synthesis approach and the three larger sizes were prepared by modulating the size of the smaller particles via centrifugation followed by dilution to a desired concentration. This approach was not only simple but also allowed changing the physical state of the particles without introducing any chemical modifications in their surface properties. This was critical as the role of surface charges is important in the overall polarizability of the particles.41,42,49 The particle size distribution and morphology were confirmed by TEM and optical microscopy. While the smaller particles were spherical in shape, the larger ones appeared as random clusters (Figure 3). The clusters also displayed size reversibility upon dilution indicating that they were loosely bound rather than tightly fused together (Figure S3). When a drop of AuNP suspension was injected between the electrodes and an AC field was applied, this led to the rapid accumulation of the particles inside or around the microwells depending on their size. In case of smallest sized suspensions (𝑑 ~ 17 nm), the particles collected into films on top of the microelectrodes while in case of the largest ones (𝑑 ~ 10 m), the particles assembled into microwires at the electrode periphery. No significant particle collection was observed for the two intermediate sizes. Unlike what was reported previously for micron sized latex beads, we did not find any direct evidence of particle motion or fluid flow during the assembly process as the collection proceeded quite rapidly.39 During this time, the microstructures continued to grow laterally as well as vertically until a steady state was reached around 4 to 5 min (Figure S4 and videos S1, S2). The assemblies were found to be permanent when the EF was switched off. Also, a minimum concentration was required to initiate the collection process; limited assembly took place with a 0.5 nM suspension (Figure S5a). To ensure this was truly a concentration and not particle size effect (since particle sizes were concentration-dependent), we compared the particle sizes in 2 and 0.5 nM suspensions and they were found to be quite similar (Figure S5b). Effect of frequency and voltage. Next, we studied the effect of applied frequency and voltage as these two parameters are well known to impact the strength of the induced dipole and hence, the particle collection process. First the voltage of the applied field was varied from 1 to 20 Vpp keeping the frequency constant at 100 Hz. The experiments were performed with the smallest and largest particle sizes for which deposition was observed earlier. At 1 Vpp, no particle collection took place anywhere on the chip in both cases implying that the field was too weak

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for particle or fluidic movement (Figure 4). When the voltage was raised to 10 or 15 Vpp, the particles began to collect slowly. In particular, radial patterns were formed in case of 17 nm particles at 15 Vpp along the streamlines taken by the particles during their inward movement. At 20 Vpp, fully grown films or surface microwires were obtained. The effect of frequency at constant voltage (of 20 Vpp) was found to be more dramatic. For the 17 nm size, the particles deposited as thick rings at the outer periphery of the microwells at 50 Hz while the centers of the electrodes were devoid of any particles (Figure 5). As the frequency was raised to 100 Hz, the particle deposition became more uniform and took place selectively on top of the microelectrodes yielding disk-like, conductive films of the size of the microwells. At 500 Hz, the same films became visibly thicker as compared to 100 Hz. Finally, at 1 kHz and above, no particle assembly was seen to occur anywhere on the chip indicating that the particles failed to respond to the applied field at these conditions. The net effect of EF on intermediate particle sizes of 130 nm and 5 µm was significantly reduced as hypothesized due to the competitive nature of EHD and DEP forces (see transition region in Figure 5). In case of 10 m size particles, these slight frequency alterations led to even more complex variations in the structural morphology. At 50 Hz, dense networks of random particle aggregates were seen in the bulk phase. At 100 Hz, these random particle networks transitioned into highly dense and branched surface microwires that originated from the inner periphery of the microwells and rapidly grew outward in the gaps between the electrodes. This was accompanied by the formation of coronas in the middle region between adjacent microwells where the particles appeared to be highly depleted (video S2). Interestingly and unlike previous reports, the wires never short-circuited as they all carried the same potential.32,33,50 As a result, as the two ends approached each other, the wires either bent sideways or transformed into bulk microwires (Figure S4b). Upon increasing the frequency further to 500 Hz, surface microwires completely gave way to bulk wires that could only be visualized by adjusting the microscope stage to an elevated plane of focus. The cross-sectional thickness of the bulk microwire was also significantly larger than the surface microwire and so, the total number of bulk microwires formed throughout the chip was considerably less. At 1 kHz, the particle response to EF became highly muted similar to the 17 nm case, giving rise to very few thin and short surface microwires. And finally, no assembly took place at 10 kHz. Control experiments. (i) Effect of salt. To strengthen our hypothesis about size-dependent interplay of DEP-EHD forces, we performed many sets of control experiments. In the first set,

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thin films were deposited from 17 nm particles same as earlier but now in the presence of varying electrolyte concentrations. Since salt suppresses zeta-potential, we expected the effect of EHD to progressively reduce as the amount of electrolyte was gradually increased in the system. This is indeed what we observed in our experiments. At 1 mM NaCl, films were formed with a uniform core and distinct outer rings at the electrodes’ outer periphery (Figure 6a). At 10x higher salt concentration, the outer rings became darker at the cost of a thinner inner core, reminiscent of the 50 Hz case (Figure 6b). Collection at both these salt concentrations was also accompanied by rampant bubbling (data not shown). Finally, at 20 mM electrolyte, no particle deposition took place (Figure 6c) confirming that the results were truly an effect of the potential drop at the double layer and hence, the inability of the tangential EF to drag ions from the solution, resulting in the loss of EHD flow. To ensure this was not a particle size effect (since salt can also induce particle aggregation), UV-visible spectroscopy was performed on the AuNP suspensions. Our results confirmed that there was no significant change in the gold extinction spectra with and without salt (Figure S6). (ii) Effect of electrode size. The second set of control experiments was performed by varying the electrode dimensions. When we reduced the size of the conductive microwells by more than half (keeping all other factors constant), the EF gradients became steeper resulting in two outcomes. First, the minimum concentration required to start the assembly process reduced which meant that unlike before, particle collection now took place even in a 0.5 nM (or, a 4x diluted) suspension. Second, a new balance was attained between the DEP and EHD forces which led to an increase in the critical particle diameter to ~ 8 m (Figure 7b). As a result, even the 130 nm size particles now formed films and the transition was seen to shift to around the 5 and 10 m particle size (Figure 7a). The reason why the inside of the electrodes appeared dark in case of 10 m particles was simply an optical effect due to the increased size of the particles. (iii) Effect of electrode geometry. Next, we carried out our experiments on IDEs with a bid to investigate the effect of the electrode design layout. When AC fields of 5 Vpp and 100 Hz were applied across the 17 nm and 10 µm size suspensions, only microwires were formed and no films were observed in either case (Figure S7a). Since AuNPs are hard to visualize against the opaque background of the electrodes, repeat experiments were performed with 3.5 nm fluorescent cadmium telluride (CdTe) QDs to increase contrast. As expected, our results

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revealed that the majority of the particles were deposited on top of the electrodes while the remaining ones existed as QD microwires in the gaps between the electrodes (Figure S7c). (iv) Effect of particle concentration. Our final control experiments involved studying the effect of particle concentration on the mode of assembly. Since our protocol followed for particle size modulation involved centrifugation and resuspension of particles into smaller liquid volumes, the total particle volume fraction (p) changed from one particle size study to another. For e.g., p increased 25-fold on going from 2nM (in case of 17 nm particle size suspensions) to 50 nM concentration (for 10 m particle size suspensions) even though the total particle number concentration went down from 1.2 x 1012 to 2.7 x 104 particles/mL. Since, p is the main parameter that dictates phase transitions in colloidal suspensions (not particle size), one may argue that our experimental results may be an artefact of the change in volume fraction, not particle size as claimed. To reject this argument, we performed experiments in which two different sized particle suspensions were prepared at the same volume fraction. This was done by aggregating the 17 nm AuNPs in the presence of salt such that their particle size grew but did not reverse upon dilution (Figure S8). The maximum particle size attained using this approach was approx. 6 m. When an EF was applied across these equal volume fraction but different particle size suspensions, the film formation was only evident in case of smaller size particles (Figure S9), this reconfirmed that the morphology was a particle size-driven phenomenon, not a volume fraction-mediated effect. To further exclude the effect of salt, we also prepared larger sized particles using filter-assisted centrifugation. When the saltaggregated and filter-based particles were taken at the same particle size and volume fraction, both sets gave similar outcomes (Figure S10) further strengthening our claim of size-tunable competitive assembly using DEP and EHD. Microstructural characterization. A profilometric analysis of the metallic films formed at 20 Vpp, 500 Hz showed that they had a relatively uniform thickness of ~ 200 nm throughout the chip (Figure S11). The films were also quite stable and remained tightly bound to the ITO surface against several gentle washes of MQ water, IPA and ethanol (Figure S12). The mechanical integrity of the films could be further increased in the presence of electrolyte or by heating, rendering them more suitable for applications in biological sensing and bionanodevices. The microwires, on the other hand, were unstable. The bulk microwires collapsed immediately upon drying while the surface microwires could only retain their

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structure just after removing water (as they were mechanically supported by the substrate) but got completely washed away after two successive washes with DI water (Figure S13).

DISCUSSION The liquid motion due to interaction of the ions in EDL with the applied EF dictates the direction of particle collection by EHD. In the positive half-cycle of the AC field, the EDL on the microwell comprises of negative counterions as shown in Figure 2a. Since the PR acts as a capacitor, it also carries negative counterions in this half-cycle to maintain electrostatic balance. All the negative counterions experience a positive force towards the center of the microwell because the tangential vector of the applied EF points radially outward. In the second half-cycle, both the charges in the EDL and the direction of the applied tangential EF get inverted simultaneously, giving the same net directional movement of the fluid flow. Since the EF gradient is maximum at the PR-electrode interface, the particles present here experience the highest force. The positive DEP force is also strongest here. In the vertical direction, the particles present along the Debye length get most affected by EHD as the number of diffuse ions are maximum here. The DEP force drops exponentially from the bottom electrode surface to the top. For particles just above the PR, the tangential vectors of both EHD and DEP forces point in the same direction leading to particle collection toward the microwell whereas, for particles right above the microwell, the two forces oppose each other. It is the more dominant force that ultimately dictates the final pattern of assembly. It is important to note that the FEMLAB model used to calculate dc in our case was derived from eqs. 2 and 3 which are only valid for single particle-field interactions. However, we know that interparticle distance plays a crucial role in the dipole formation process and can significantly offset the results obtained between dilute vs concentrated suspensions. We believe that our system lies in the low dilution limit (~ 104 particles/mL for 10 m particles) where the particle-particle interactions don’t matter as much and even though the larger particles are prepared by concentrating the smaller size suspensions, their particle number fraction actually decreases as we increase the particle size. To check if this was indeed true, we performed a simple calculation using eq. 7,46

3

𝐹𝑑𝑖𝑝 = 4𝜋𝜀𝑚𝑟2𝐶𝑀2𝐸2𝑅𝑀𝑆

2𝑟 4

( ) [(3𝑐𝑜𝑠2𝜃 ― 1)𝑎 + (sin 2𝜃)𝜃] 𝑎

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(7)

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where, 𝑎 is the centre to centre interparticle distance and 𝜃 is the angle between the particles and the direction of the EF applied. Our results showed that the critical interparticle distance (𝑎𝑐 ) beyond which 𝐹𝐵 exceeds 𝐹𝑑𝑖𝑝 leading to no particle alignment is around 140 µm (Figure S14) whereas, the actual interparticle distance in the 10 m size AuNP suspension was ~ 420 µm which was three times greater than 𝑎𝑐 (see SI for details). Thus, our system lay in the dilute limit and the single particle calculations used in Figure 2 seem to be justified. Another aspect of this study was that while the EHD flow velocity remains constant and FEHD is particle size-independent, the equal and opposite drag force on a particle does increase linearly with particle size following Stokes law. This effect of increased EHD drag force with increasing particle size is experimentally observed in Figure 4, where it takes much larger voltage to form films of small nanoparticles compared to small voltages need to assemble 10 micron sized colloids. So, in reality, the theoretical critical transition suggested in Figure 2c is expected to occur at a smaller particle size than the one predicted by the model, which is what we also find in our experiments. The wide range of particle sizes over which this transition is seen, however, can be attributed to the high polydispersity index of our larger size suspensions (see SI for calculations) which is an intrinsic artefact of the experimental procedure used for particle preparation. When the particles aggregate, what likely happens is that the water gets squeezed out of their core, pushing the counterions also to the outer edge of the cluster. This presence of counterions in the higher dielectric medium where they can remain hydrated is a more thermodynamically stable configuration for the system. Thus, the particle cluster can be assumed to be a colloid with a higher effective core diameter and a counterionic layer whose polarization under EF continues to display the same behavior as that of an unaggregated particle. But since this cluster is of a dynamic nature and not a “true aggregate”, it can result in slightly different morphologies under different field conditions. For instance, the particles may slide and rearrange against each other under the effect of external EF such that the intraparticle alignment within the cluster may start along or before the intercluster arrangement, yielding thinner microwires. The reason to believe this is also apparent from the optical micrographs of the surface microwires where the cross-sectional areas of the wires appear much smaller than 10 m. While the original literature on microwire formation suggests counterionic polarization to be the dominating mechanism for induced dipole formation in AuNPs, more recent reports have debunked this idea. These new reports claim that the value of the CM factor, that determines the direction of particle collection due to DEP, stays close to 1 (for spherical

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particles) up to very large frequencies in the GHz range. So, the frequency-dependent morphology observed during microwire formation below 10 kHz is likely due to EHD-assisted flows.51 We too believe that the EHD flow sweeps particles from the bulk liquid to the areas of high field intensity (microwell edges in our case) where they experience a DEP force of strength depending on their size. This is especially true for initially dilute suspensions (𝑎 > 𝑎𝑐). If the size is small, the particles continue to be carried forward by the flow and result into films. If the size is larger than the critical diameter, the DEP force tends to hold them at the electrodePR interface and the assemblies begin to grow outward as more particles accumulate on top of the existing ones. Why DEP-directed assemblies take the form of microwires is already described earlier.33,50 Moreover, as the particles accumulate, local field distortions caused by the particles themselves can accelerate the DEP process, which can further push down the value of dc. Therefore, in our experiments, we see transitions as low as 130 nm average particle sizes and no microwire formation beyond 1 kHz with particles as large as 10 m. The variation in the EHD flows with frequency can be explained on the basis of EDL formation. At the lower frequency of 50 Hz, counter ions have sufficient time to redistribute themselves and form a double layer. Consequently, there is maximum potential drop in the double layer and not enough potential available to drag the counter ions from solution. This results in less movement of the fluid and particles thereby forming incomplete films (or, rings). On the other hand, ions do not have sufficient time to redistribute themselves with the change in polarity of the field above 1 kHz. As a result, no EDL forms and particle collection tends to zero. However, for frequency ranges between 100 to 500 Hz, EHD increases with the frequency resulting in the formation of denser films. Variations in microwire formation with frequency are analogous. As mentioned above, the microwire formation is an EHD-assisted DEP-driven phenomenon and so, the mode of microwire assembly is most influenced around the frequency where the EHD motion is highest. As a result, the surface microwires get lifted into bulk microwires at 500 Hz and then back to surface microwires. The effects of time and voltage are more straightforward. Both of them lead to greater extent of particle collection and hence, denser morphologies. The layout of the electrode also plays an important role in the types of morphology formed. Unlike the microwells, the EHD force in a coplanar electrode always pulls the particles away from the gaps onto the electrode surface whereas, DEP always pulls them toward the electrode edge (Figure S7b). In each half-cycle, an EDL with negative counterions is formed on one electrode while the one with positive counter ions is formed on the counter electrode.

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We speculate that at the start of the collection process, FEHD is more dominating resulting in the deposition of particles on the electrode surface and once the electrode surface saturates, surface zeta potential drops in turn decreasing the EHD force. At the same time, FDEP increases both due to FEHD-assisted aggregation of NPs in the bulk52 and a higher distortion of the EF caused by the continuous deposition of the particles near the electrode edges which leads to the growth of microwires in between the electrode gaps.

CONCLUSIONS In summary, we have successfully demonstrated that AC-DEP and AC-EHD electrokinetic techniques can be modulated in a competitive fashion to yield tunable assemblies from AuNPs in micropatterned electrodes. The size of the particles and the design layout of the electrodes play a crucial role in this aspect; standard IDEs failed to show this competitive behavior. The critical AuNP diameter for EHD to DEP transition was found to be 6 m in case of D 50 S 50 and 8 m in case of D 22 S 45 electrodes. While the surface EHD flows in case of smaller size particles led to 2D film deposition in the submicron thickness range, the DEP forces were responsible for 1D microwire growth in case of larger systems. The microwires transitioned from surface to bulk and back to surface in a frequency dependent manner. Maximum assembly took place around 100 to 500 Hz while no deposition was seen above 1 kHz or at 20 mM salt NaCl concentration indicating that EHD was a major contributing force in all EF-driven assembly processes.33,53 The single step, low volume requirements make our technique attractive for a wide range of applications. In addition, our process does not involve any chemical additives and operates at room temperature and pressure making it highly scalable, low cost and suitable for “green” fabrication of nanostructures. The nanofilms can be used in the areas of molecular characterization and surface-enhanced Raman scattering (SERS)-based devices for chemical and biosensing.54–56 The high mechanical integrity of the films also makes them facile platforms for pathogen capture and analysis. On the other hand, the surface microwires that never short circuit can be used for producing nanogap-sized electrodes desirable for nanoelectronic devices.

SUPPORTING INFORMATION Included with fourteen figures, two tables, and two videos.

ACKNOWLEDGEMENTS 16


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We would like to thank Centre for Excellence in Nanoelectronics (CEN), IIT Bombay for helping with microelectrode fabrication and Dr. Sameer Sapra, Chemistry, IITD for giving us the QDs. We are also highly indebted to Dr. Gaurav Goel, Chemical Engineering, IITD for the helpful technical discussions. MG acknowledges IITD for scholarship.

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Figures

Figure 1. Schematic of the experimental setup used for our AuNP microwire and film assembly: (a) 2D and (b) 3D side view.

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Figure 2. (a) Charge and EF distribution for positive half-cycle of the applied AC field. (b) The direction of particle collection due to EHD (FEHD) and DEP (FDEP) forces. (c) The effect of particle size on the DEP and EHD forces assuming no size dependence on FEHD. (d) Spatial variation in the x-components of DEP and EHD forces exerted on a 6 µm size particle kept 1 µm above the bottom surface.

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Figure 3. TEM (a & b) and optical (c & d) micrographs of AuNPs used in our experimental study with their respective particle size distribution curves below. Average particle sizes were (a) 17 ± 4 nm, (b) 130 ± 170 nm, (c) 5 ± 3 µm and (d) 10 ± 6 µm. Insets in Figs. 3b & 3d show TEM images of the corresponding particles at higher magnification.

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Figure 4. Effect of applied voltage on the rate of assembly of (a) films using 17 nm sized AuNPs and (b) microwires using 10 µm sized Au suspension. All images were taken at 100 Hz, 5 min after the start of the experiment.

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Langmuir

Figure 5. Size and frequency dependent phase chart of film to microwire transition. The EF applied was 20 Vpp for 5 min.

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Figure 6. Film deposition in the presence of NaCl for 17 nm AuNPs: (a) 1 mM, (b) 10 mM and (c) 20 mM. The EF was 100 Hz and 20 Vpp.

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Langmuir

Figure 7. Effect of electrode dimension. (a) AuNP assemblies obtained for different particle sizes using electrodes with well diameter (D) equal to 22 m and edge-to-edge spacing (S) 45 m. The EF applied was 20 Vpp and 100 Hz. (b) The new curves showing upward shift in critical particle diameter to ~ 8 m.

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TOC

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Fig. 1. Schematic of the experimental setup used for our AuNP microwire and film assembly: (a) 2D and (b) 3Dside view. 213x35mm (150 x 150 DPI)


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Fig. 2.(a) Charge and EF distribution for positive half-cycle of the applied AC field. (b) The direction of particle collection due to EHD (FEHD) and DEP (FDEP)forces.(c)The effect of particle size on the DEP and EHD forces assuming no size dependence on FEHD.(d) Spatial variation in the x-components of DEP and EHD forces exerted on a 6 µm size particle kept 1 µm above the bottom surface. 247x187mm (150 x 150 DPI)


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Fig.3. TEM (a&b) and optical (c&d) micrographs of AuNPs used in our experimental study with their respective particle size distribution curves below. Average particle sizes were (a) 17 ± 4 nm,(b) 130 ± 170nm,(c) 5 ± 3 μm and (d) 10 ± 6 μm. Insets in Figs. 3b&3d show TEM images of the corresponding particles at higher magnification. 242x115mm (150 x 150 DPI)


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Fig. 4. Effect of applied voltage on the rate of assembly of (a) films using 17 nm sized AuNPs and (b) microwires using 10 μm sized Au suspension. All images were taken at 100 Hz, 5 min after the start of the experiment. 276x145mm (96 x 96 DPI)


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Fig. 5. Size and frequency dependent phase chart of film to microwire transition. The EF applied was 20 Vpp for 5 min. 242x138mm (150 x 150 DPI)


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Fig. 6. Film deposition in the presence of NaCl for 17 nm AuNPs: (a) 1 mM,(b) 10 mM and (c) 20 mM. The EF was 100 Hz and 20 Vpp. 55x143mm (150 x 150 DPI)


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Fig. 7. Effect of electrode dimension. (a) AuNP assemblies obtained for different particle sizes using electrodes with well diameter (D) equal to 22 micron and edge-to-edge spacing (S) 45 micron. The EF applied was 20 Vpp and 100 Hz. (b) The new curves showing upward shift in critical particle diameter to ~ 8 micron. 149x112mm (150 x 150 DPI)


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TOC 243x95mm (150 x 150 DPI)


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Size-Tunable Assembly of Gold Nanoparticles Using Competitive AC

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