Deposition and meniscus alignment of DNA–CNT on a substrate

Journal of Colloid and Interface Science 330 (2009) 255–265

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Deposition and meniscus alignment of DNA–CNT on a substrate C.Y. Khripin a , M. Zheng b , A. Jagota a,∗ a b

Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015, USA CR&D, The DuPont Company, Wilmington, DE 19880, USA

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 25 June 2008 Accepted 24 October 2008 Available online 6 November 2008

We present a study of deposition and meniscus alignment of DNA-single walled carbon nanotube hybrids (DNA–CNT) on a silicon wafer coated with an alkyl-silane monolayer. We show that this process occurs in two stages: adsorption of DNA–CNT onto the hydrophobic surface and subsequent alignment by a passing meniscus. We study how pH, ionic strength, and time affect the density of nanotubes deposited on the surface. Experimental results are interpreted using models for the kinetics of deposition and for forces that affect alignment by the meniscus. We show that this method can be used to produce uniform global alignment of controlled density as well as controlled patterns, results that may be useful for applications such as CNT-based device construction. © 2008 Elsevier Inc. All rights reserved.

Keywords: Carbon nanotube DNA Placement Meniscus Alignment Self-assembled monolayer Kinetics

1. Introduction Single-walled carbon nanotubes (CNTs) are nano-scale carbon fibers exhibiting a variety of useful electronic properties [1] and potential applications [2]. As manufactured, CNT’s are a mixture of different types. Important steps for their use are purification and placement on a target site. Purification involves isolation of CNTs from other forms of carbon and sorting them according to their properties. Often, purification is done in a liquid phase, i.e., by first suspending the CNTs in solvent, then separating them using density differentiation [3], chromatography [4,5], or electrophoresis [6]. The purified CNTs must often be deposited on a surface in order to be useful, say at some target site and aligned in a certain direction. Various methods have been used to do this. Some researchers have used AC and/or DC electrostatic fields to align the nanotubes, placing them spanning two electrodes [7]. Carbon nanotubes have also been deposited on gold electrodes and subsequently combed using the meniscus of a water droplet on the surface [8,9]. Similar combing of DNA has been studied, particularly on hydrophobic surfaces [10–12]. Meniscus combing has also been used in layer-by-layer [13,14] assembly of carbon nanotubes and to shape nanotubes into ropes and rings [15]. One precise method of depositing carbon nanotubes is by patterning the surface with different monolayers to influence where the CNT deposits on the surface [16–18]. A similar method patterns a PDMS substrate, then

*

Corresponding author. Fax: +1 610 758 5057. E-mail address: [email protected] (A. Jagota).

0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.10.073

©

2008 Elsevier Inc. All rights reserved.

transfer-prints CNTs from the substrate [19] onto a target surface. In some of the work cited above, the CNTs were first deposited onto the surface, and then aligned by the meniscus as the liquid from which they deposited was withdrawn [14,15]. In some cases, combing of CNTs has been used to produce electronic devices [8,9]. The meniscus alignment method, also called molecular combing, is appealing for several reasons. It is done under mild conditions, important for biological applications. It can be cheaply done on a large scale, and it requires no special surface preparation beyond the deposition of a SAM. However, it generally produces a low-density film of well oriented objects (CNTs or DNA), and works only over a narrow range of conditions [8–12]. For example, DNA combing on hydrophobic surfaces is effective only over a narrow range of pH. Too low a value causes copious adsorption without alignment while too high a value results in good alignment but low surface density of adsorption [10,12]. This study aims to provide a detailed theoretical and experimental study of the process, hopefully allowing for better control in meniscus alignment. Here we report on the deposition and meniscus alignment of DNA-wrapped CNTs. By exploring a range of deposition conditions, we show that this process is robust and reproducible. The DNA– CNT complex is made by dispersing single-walled carbon nanotubes in water with single-stranded DNA, which wraps helically around the nanotube [4,5]. Wrapping with DNA is a reversible process which forms a stable complex that is soluble in water and possesses electronic properties like unmodified CNT [20,21]. DNA–CNT can be conveniently sorted by CNT chirality and length [4,5]. DNA–CNT deposits onto alkyl-monochlorosilane-modified silica surfaces from suspension in a process which is modulated by solution and surface properties [22]. The silica surface, even when

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functionalized, is negatively charged in aqueous solution [23,24]. Negatively charged DNA–CNT are normally repelled electrostatically and deposit only slightly. However, if the solution pH is decreased, silanol groups on the silica surface become protonated, lose their negative charge, and deposition increases. Similarly, if the ionic strength of the solution is increased, the electrostatic repulsion is screened and deposition increases. We study the effect of these two factors on the deposition with the expectation that better understanding would lead to better control for any researcher wishing to use similar methods for CNT deposition. In the experiments we report here, the deposited DNA–CNT are aligned when the suspension is removed and the receding meniscus combs the surface, and we consider how this process works. We demonstrate that for DNA–CNT the process produces global alignment and works over a broad range of solution conditions (pH and ionic strength), in contrast to previous molecular combing experiments [8–12]. In a previous study of DNA–CNT deposition and alignment under nominally similar conditions [22], we observed somewhat different phenomena and arrived at a different interpretation. Specifically, in that previous set of experiments we found that alignment was independent of the direction of meniscus motion and identical in different drops on the same substrate. To explain those results, we proposed that the DNA–CNT concentrate parallel to the surface in a secondary minimum in the interaction potential, forming a 2D liquid-crystalline phase [22,25]. They then deposit on the surface by thermal hopping over the potential barrier, maintaining their alignment. We will return to a comparison of current and previous results in Section 3. First (Section 2), we discuss experimental methods which are used to produce the DNA–CNT suspensions, the functionalized surfaces and meniscus alignment. Then, in Section 3, results of several experiments are presented. The first one demonstrates the nature of meniscus alignment. Then, a kinetics experiment shows that DNA–CNT are deposited everywhere in the drop, then aligned by the passing meniscus, rather than being deposited only at the edge of the receding meniscus. Experiments demonstrating the dependence of deposition density on suspension pH and ionic strength follow. In Section 4 we interpret these data using a kinetics model that captures the dependence of deposition on solution pH and ionic strength. Finally, we consider how the liquid–air meniscus exerts a force on the DNA–CNT that aligns it perpendicular to the meniscus. 2. Materials and methods Dispersion and sorting of CNTs was carried out using previously described procedures [4,22,26]. CoMoCaT nanotubes were purchased from Southwest Nanotechnologies (Norman, OK). The DNA used was (G T )30 , purchased from Integrated DNA Technologies, Inc. (Coralville, IA). To disperse the tubes with DNA, 3 ml of solution containing: 0.1% by mass CNT, 0.1% DNA, 10 mM NaH2 PO4 at pH 7 and 0.2 mM EDTA was sonicated for 2 h surrounded by ice, at 8 W, with the tip of the sonicator close to the bottom of the test tube. The resulting suspension was centrifuged at 13,000g in 100 μl aliquots for 90 min, and the pellet was discarded. The resulting suspension was purified by size-exclusion chromatography (SEC). SEC has been previously used to sort nanotubes into groups of narrow length distribution [27]. The fraction used in the experiments here was verified by atomic force microscopy (AFM) to contain nanotubes of length 560 ± 50 nm. This fraction was also free of unbound DNA and other foreign material because these impurities are removed by SEC. Fig. 1 shows an absorption spectrum of the purified suspension. The strong peaks at 990 nm (E11 transition for (6,5) CNTs) and at 575 nm (E22 transition of the same), and the relative weak-

Fig. 1. The absorption spectrum of SEC-purified DNA–CNT suspension. The strong peak at 990 nm indicates a (6,5) nanotube-enriched suspension.

ness of other peaks, suggest that the suspension contains mostly (6,5) nanotubes coated by DNA. The concentration of the suspension is estimated from gravimetric analyses to be approximately 7 μg CNT per ml. The buffer of the DNA–CNT suspension was changed as follows. The starting solution was concentrated by centrifuging over a ym-100 100 kD membrane filter until only 5% of the original solution remained. The new buffer was then introduced, re-suspending the nanotubes, and the solution was again centrifuged down to 5%. This remnant was again diluted with the new buffer. The details of each buffer composition are given with each experiment. To avoid errors due to centrifugation, all the experiments (with the exception of the meniscus alignment demonstration experiment) are from a single buffer change process. In a typical experiment, a silicon wafer was coated with an alkyl monolayer. A suspension of DNA–CNT was then placed onto the hydrophobic surface and left for some time to allow the DNA– CNT to deposit. The drop was then withdrawn, and the surface imaged with AFM. This procedure is explained in detail below. The surface was treated with octyl-dimethyl-chlorosilane following a previously described procedure [23]. The silicon wafers (3 in diameter, 500 um thick, 1–20 ohm cm resistivity, N-type Phos., 1-0-0 from Silicon Quest International, Santa Clara, CA) were immersed in 70% H2 SO4 /30% H2 O2 (here and below %volume) solution for 30 min. They were rinsed with DI water and placed into a reagent mixture containing 90% heptane, 9% octyldimethyl-chlorosilane, and 1% butylamine, which is reported to act as a catalyst [23], at 60 ◦ C. The mixture was refluxed, while stirring gently, for 3 h. The wafer was then rinsed with isopropanol and placed into an oven at 110 ◦ C, under N2 , for 2 h. It was found that this baking step stabilized the monolayer. The substrate was characterized by contact angle. A drop of DI water was placed on the surface and pumped using a syringe. The motion of the edge was recorded on video. A polynomial was fitted to the still images from the video to determine the contact angle as a function of velocity. A linear extrapolation was then used to find the advancing and receding contact angles at zero velocity. Two methods were used to deposit the DNA–CNT and align them with the meniscus. The first method is to place a drop on the surface and allow it to sit for 15 min. DNA–CNT deposit onto the surface while the drop sits in place. The drop is then gradually pulled to one side with a pipette. The normal to the meniscus front varies in a known fashion as the liquid is withdrawn, and is generally different from the vector along which a fluid material particle at the interface moves. This experiment is useful for demonstrating that alignment is with the meniscus normal, not the fluid-flow direction. A better controlled method was used for more quantitative studies of kinetics. In this method a hydrophobic glass slide is sus-

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Fig. 2. A drop of suspension containing the DNA–CNT is confined between the hydrophobic substrate and a hydrophobic glass slide. After a period of incubation that allows the DNA–CNT to deposit on the surface, the substrate is moved relative to the slide as shown by the arrows. The straight meniscus edge passes over the surface and aligns DNA–CNT parallel to the direction of motion of the substrate, all in the same direction. This figure is not drawn to scale; drop size typically is 20 μl.

pended at a 45◦ angle to the substrate 100 μm above the surface. The drop with the suspension to be deposited is then placed beneath the slide on the substrate, as shown in Fig. 2. The substrate is then moved as shown by arrows in the figure, using an electric motor under constant potential. The rate of motion is ∼1 mm/s. The receding meniscus is a straight line, as defined by the straight edge of the slide. DNA–CNT were thus aligned on the surface perpendicular to the meniscus and parallel to the direction of motion of the substrate. By placing several drops under the same slide on the same substrate it is possible to ensure that all the samples in an experiment experience similar meniscus alignment, and all deposited nanotubes on the substrate are aligned in the same direction. It should be noted that in this procedure it is assumed that the DNA–CNT are deposited onto the surface during drop incubation and are subsequently aligned by the passing meniscus. This is in contrast to other experimental procedures (see, for example, the work of Ko et al. [28]) which resulted in a drying phenomenon at the meniscus and the deposition of a thick mat of aligned DNA– CNT there. In our case the conditions were such that no significant drying was possible at the meniscus, and the contact angle during alignment was high, >80◦ . Deposition of DNA–CNT was examined using a Digital Instruments DI 3000 atomic force microscope (AFM). Several scans were

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taken of the footprint of each drop and the length of deposited DNA–CNT in the image was determined. Each AFM image was loaded into a program written in the MATLAB® environment, and the location of each nanotube end was selected via cursor. Length was used to quantify the amount of DNA–CNT deposited because in some experiments with a larger deposition the DNA–CNT became braided together; this can be seen in the AFM images in Fig. 3c and for longer deposition times in Fig. 5. When roping occurs, there is therefore some ambiguity in measurement of deposition densities from AFM images. Later we present some data that show the range of deposition densities over which such roping occurs. 3. Results and discussion We begin by reporting results from an experiment that provides strong evidence that the DNA–CNT are indeed aligned by a passing liquid meniscus. Drops containing a dilute suspension of DNA–CNT (∼1 μg/ml DNA–CNT with 0.1 mM NaH2 PO4 , 0.1 mM EDTA, 10 mM NaCl at pH 3.5) were placed onto a hydrophobic surface prepared using SigmacoteTM , a commercial silating agent. The drops were allowed to incubate for 15 min. As explained below, DNA–CNT rods deposit onto the surface in this period. Then, one drop was removed by drawing from the bottom left (Fig. 3a), a second one was removed by drawing off the liquid from the bottom right (not shown), and others were removed by rinsing with DI water. We obtained a series of AFM scans along the ‘x’ axis. Nanotubes had a nearly uniform alignment in each scan, but the mean alignment changed with distance along the ‘x’ axis, as shown in Fig. 3b. The alignment in the two drops was different. As the liquid was withdrawn using the pipette, the meniscus retained its circular shape but reduced its perimeter, one point of which remained pinned by the pipette (Fig. 3a). It is simple with this observation to calculate the local normal to the meniscus at the point where it intersects the ‘x’ axis for the two drops. The corresponding angle is plotted in Fig. 3b for both drops, and it is evident that the alignment of DNA–CNT corresponds to the normal of the passing meniscus. Examples of AFM images for the second drop (b, triangles) are given in Fig. 3c. Note that in this case the

Fig. 3. (a) Schematic drawing of a drop being withdrawn using a pipette. The alignment of DNA–CNT deposited onto a substrate from a circular drop changes from one end of the drop to the other. (b) When the drop is pulled from the bottom left, the DNA–CNT angle (squares) follows the normal to the meniscus. When the drop is pulled from the bottom right the alignment angle (triangles) follows the normal to the meniscus for that case, represented by the dashed line. (c) Some example AFM images, taken along the x-axis, for when the drop was removed from the bottom right.

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(a)

(b) Fig. 4. A drop of suspension is shown schematically from above. In order to obtain globally uniform alignment, we straighten the meniscus on one side of the drop by contacting it with a suspended glass slide (bold horizontal line; see also Fig. 2). After incubating for 15 min, we moved the meniscus down, as indicated by arrows. The resulting DNA–CNT alignment, observed by AFM, is indicated by the bold vertical lines. Two thin lines are paired in a bouquet with each bold line; these span ±6σ in nanotube alignment, where σ is the standard deviation. In (b) a sample AFM image of aligned nanotubes is shown. Although the majority of the nanotubes appear as single CNTs of 0.5 μm length, some appear to have been joined near their ends by the alignment process.

nanotubes became braided into ropes 1–5 nanometers thick. Assuming DNA–CNT are ∼1.3 nm thick [27] this could correspond to bundles of up to 20 DNA–CNT. Such extensive braiding did not occur in the subsequent experiments, presumably due to the different surface treatment. When the drops were removed by rinsing with water instead of being drawn off by the pipette, the number of deposited DNA– CNT decreased and, although the DNA–CNT were still aligned in local AFM scans, the long-range systematic change in alignment observed in Fig. 3b was lost. Presumably, the remaining DNA–CNT ended up being aligned in whatever direction the last meniscus during rinsing happened to pass. This finding differs from results of previous experiments on a similar system in which direction of drop removal did not affect alignment [22]. Because DNA–CNT align perpendicular to the meniscus, this experiment suggests that alignment is due to surface tension forces, as opposed viscous drag forces. Note that the direction of fluid motion is not generally normal to the meniscus. For example, when the drop is removed from the bottom left (Fig. 3a and squares in Fig. 3b), the vector drawn at x = 0 shows the direction of motion of a material particle on the meniscus, which is not along its normal. From these observations, we can conclude that DNA–CNT align perpendicular to the meniscus regardless of the actual direction of meniscus motion. In Supporting information (Section S3) we show that estimated meniscus forces far exceed hydrodynamic forces. 3.1. Globally uniform meniscus alignment of DNA–CNT Here we demonstrate that the meniscus alignment method can be used to create globally uniform nanotube alignment on a sur-

face. In the experiment described above, the meniscus was curved, resulting in a range of nanotube alignments. In this case, we employ a straight meniscus. One end of the drop is secured by a hydrophobic glass slide inclined 45◦ into the drop, as shown in Fig. 2. As a result, when the drop is removed its trailing end is a straight line. Fig. 4a shows a diagram of the drop, viewed from above. The bold line indicates the region of straightened meniscus. A drop was allowed to incubate for several minutes, and was then removed at ∼1 mm/s by pushing the substrate with an electric motor so that the meniscus moved in the direction indicated by arrows in Fig. 4a. After the drop was removed, the sample was examined by AFM. The angle of the aligned nanotubes was uniformly perpendicular to the meniscus, as indicated by the angle of the short bold lines. The thin lines grouped with each bold line in a bouquet formation span ±6σ in nanotube alignment, where σ is the standard deviation. Thus we conclude that DNA–CNT alignment is globally uniform. A sample of alignment nanotubes is shown in Fig. 4b. This AFM image is from the x = 1 mm, y = 1 mm data point. Most nanotubes are individual tubes ∼0.5 μm in length. However, some of the tubes appear to have been joined near their ends by the process of alignment. We will discuss this phenomenon in greater depth later in the paper. The question remains as to whether the meniscus aligns DNA– CNT deposited on the surface during the incubation period, or whether the deposition and alignment both occur at the meniscus. We will show in the following section that deposition occurs prior to alignment and so we may regard the process as having two sequential steps: deposition and meniscus alignment. Then we

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Fig. 5. The amount of DNA–CNT deposited increases with time allowed for deposition.

consider how the deposition is affected by time, pH, and salt concentration. 3.2. Variation of deposition with time and surface properties The remaining experiments were performed using the mechanical set-up described in Section 2 (see Fig. 2). This setup gives reproducible experimental conditions and globally uniform alignment of the DNA–CNT. All the drops in each experiment were incubated on the same substrate and removed simultaneously by a single glass slide. Several AFM scans were made on the surface over which each drop passed, the number and alignment of the nanotubes were tabulated, and the results were averaged to obtain a 95% error interval which is indicated by the error bars in figures below. Smooth lines are fits of the kinetics model, described in detail in Section 4. 3.2.1. Kinetics Kinetics of the deposition process were studied by varying the incubation time for a drop on the substrate prior to initiation of meniscus motion. The substrate was prepared using the liquidphase silation method described in Section 2. The drops contained DNA–CNT at a concentration of 1 μg/ml in buffer composed of 10 mM NaCl, 0.1 mM NaH2 PO4 and 0.1 mM EDTA at pH 4.5. The number of DNA–CNT, measured in μm deposited per μm2 of substrate, varied with incubation time as shown in Fig. 5. Localized deposition near the meniscus during meniscus motion has been reported for general colloidal particles [29] and for CNT’s [28]. In those studies particles deposit chiefly at the meniscus, where drying effects increase their local concentration. Here, kinetics data show that deposition increases with time and that there is negligible deposition for very small incubation times. This latter fact indicates that in our case no significant deposition occurs at the meniscus. Furthermore, on examining areas on the substrate where the drop had a total residence time of less than

5 s we found that there was only an occasional deposited nanotube, at a concentration of less than 0.05 μm/μm2 . Because our measurements of surface concentration of deposited DNA–CNT are made after the passage of the meniscus, it is possible that some fraction of DNA–CNT are removed from the surface by it. We propose provisionally that DNA–CNT rods deposited directly on the surface are not removed by the meniscus whereas those that lie on previously deposited rods are removed. Later we establish the plausibility of this argument. A reference deposition flux (number depositing per unit time per unit surface area) can be estimated for conditions when there is a negligible potential barrier based on the idea that it is limited only by diffusion [30]. To obtain a conservative estimate for this flux, we can use the relation J d = D ρb / L where J d is the flux, D is diffusivity, ρb is the bulk density of nanotubes (1 nM) and L is some length scale. We use a depletion argument to find L; assuming a linear concentration profile, we define L ≡ 2ρs /ρb where we take ρs = 2 μm/μm2 as a high observed surface density of nanotubes. Finally as an estimate of diffusivity we take the diffusivity of the tomato bushy stunt virus, which is an object roughly ten times heavier with an aspect ratio of 5:1, D = 1E − 7 cm2 /s [31]. Thus we arrive at flux J d = 1 μm/(μm2 s). The fact that this quantity is much larger than measured deposition rates strongly suggests that deposition is barrier-limited. Experiments on the pH and salt concentration dependence of deposition, discussed next, establish that the barrier is electrostatic in nature. 3.2.2. pH dependence of deposition Deposition of DNA–CNT depends on many factors, including pH, salt concentration and time. These factors are useful for controlling the deposition, and are also useful to investigate its mechanism. We propose that deposition is limited by a repulsive interaction between the negatively charged surface and the negatively charged DNA–CNT. If the DNA–CNT can overcome this force, it can stick due to strong van der Waals attractive forces that dominate closer to

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Fig. 6. The amount of nanotubes deposited onto the surface decreases with increasing pH. At higher pH the charge of the surface becomes more negative, inhibiting deposition of the negatively charged DNA–CNT.

Fig. 7. The concentration of observed DNA–CNT on the surface increases with the concentration of NaCl in the solution. At high salt concentrations the observed concentration saturates to a limiting value.

the surface. The magnitude of the repulsive force depends on the amount of negative charge on the surface, which in turn depends on the pH of the solution. The surface charge of the silica surface depends on the pH of the solution in which it is immersed [23]. At higher pH values the hydroxyl groups on the surface dissociate, leaving the surface negatively charged, and thus more repulsive to DNA–CNT. Fig. 6 shows how density of deposited DNA–CNT depends on solution pH. The DNA–CNT solution used here was 10 mM NaCl, 1 μg/ml DNA–CNT, 0.1 mM NaH2 PO4 and 0.1 mM EDTA with pH in the range 3.6–8.1. We assumed that the variation in pH was not great enough to alter the ionic strength, which was set by the 10 mM NaCl present in solution. The 0.1 mM phosphate buffer served to maintain the pH. All conditions other than the pH remained the same. In previous studies, it has been concluded that the hydroxyl groups on a bare silica surface have two different pK a values [24]. Some 81% are paired OH groups which are hydrogen bonded and have a pK a value of 8.5. The remaining 19% are isolated OH groups with a pK a of 4.9. A surface coated by organosilanes is altered strongly, in that it appears there is a gradual change in surface charge density between pH 2 and 8 [23]. Our data are qualitatively consistent with this idea since there is no particular pH range at which deposition changes markedly.

3.2.3. Salt concentration dependence of deposition The pH of the solution affects the electrostatic repulsion between DNA–CNT and the surface. Repulsion can also be modulated by changing the salt concentration, and thus the Debye screening length, of the DNA–CNT suspension. In order to capture the full range of the screening effect, we begin with a suspension at pH 8 and increase the salt concentration from 10 mM to 500 mM of NaCl. All other buffer components are the same as in the pH dependence experiment. The density of deposited nanotubes varies with the concentration of salt in the solution as shown in Fig. 7. At high salt concentrations the observed density of DNA–CNT reaches a limiting value. This experiment is significant because it shows that there is a maximum deposition density that can be achieved. This maximum could not be reached in the kinetics experiment because of the very long time period that would be required, and in the pH experiment because at pH below 3 the DNA–CNT coagulate. Thus, increasing the salt concentration provides a simple way of maximizing deposition density while still producing aligned DNA–CNT. 3.3. Braiding of DNA–CNT into ropes At higher deposition densities DNA–CNT are sometimes drawn together into ropes. This is evident in Fig. 5. At higher deposition

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ical average separation distance between the nanotubes, meniscus forces can bring adjacent rods together. The data here is taken from the kinetics, pH dependence, and salt dependence experiments presented above. The critical separation distance seems to coincide for these experiments. 3.4. Meniscus deposition in patterns

Fig. 8. Average length of observed DNA–CNT clusters as a function of surface density. Data have been taken from three independent experiments. Below a characteristic surface density (∼0.8–1.0 μm/μm2 ), the measured length agrees well with the known length of DNA–CNT rods. At this surface density, it increases sharply; this corresponds to a transition from singly resolved DNA–CNT to ropes.

Fig. 9. Nanotubes aligned in a criss-cross pattern using 2-step meniscus alignment.

times, the AFM images show not only an increase in the density of deposited nanotubes, but an increase in the apparent length of individual nanotubes (actually, nanotube clusters). This increase in length occurs at a characteristic surface density, as shown in Fig. 8. The average length of a nanotube cluster is plotted as a function of the total length of DNA–CNT deposited. The lower dotted line is close to the actual length of the DNA–CNT. At a density of just over one nanotube per μm2 , we observe that the DNA–CNT begin to occur mostly as ropes; below this density they are mostly individual rods. We suggest that at a crit-

Although the main goal of meniscus alignment is to obtain uniform global alignment, at times it may be desirable to obtain varying alignment. Here we present a brief demonstration of how several alignment angles can be achieved in the same location and of how to make alignment vary with location on the sample. An important possible application of CNTs is as transparent conductors and semiconductors (e.g., Refs. [1–4]). Due to their high charge-carrying capacity, they might make a good conductor when arrayed on glass. If the array is sparse enough it may be transparent. As a first step we present a criss-cross pattern of nanotubes in Fig. 9. First, DNA–CNT were deposited aligned in a single direction. The sample was then dried in an oven at 225 ◦ C for 45 min and nanotubes were deposited and aligned in the perpendicular direction. By varying experimental conditions, it might be possible to increase the density of deposited nanotubes past the percolation threshold to produce a conductor or semiconductor. Note that what makes the criss-cross pattern possible is the drying step between meniscus alignments in different directions. As discussed in Section 4.3 in more detail, we believe the friction between the DNA–CNT and the substrate to be much greater in the dry state than in the wet state. After the first meniscus alignment step the nanotubes dry and bond to the surface; the bonding is even stronger if the samples are baked in an oven. Subsequent deposition and meniscus alignment of new DNA–CNT in the second direction do not affect the original DNA–CNT which had dried and bonded to the surface. The meniscus is very flexible, its shape depending on surface features. Thus meniscus alignment can be used to produce uniform alignment or shapes as diverse as the shapes of meniscus one can create. To demonstrate this possibility, we have created variation of deposited DNA–CNT alignment by >π /2 radians over 500 μm (Fig. 10a) using a curved meniscus. The meniscus was curved using the following method. Parallel hydrophilic gold electrodes were deposited onto a hydrophobic surface with a 500 μm gap between them and a drop of DNA–CNT suspension was placed on top spanning the gap (Fig. 10a) When the drop was withdrawn, the meniscus formed the shape shown

Fig. 10. A straight meniscus would yield uniform alignment, while a curved meniscus yields spatially varying alignment. In this case, two hydrophilic gold electrodes 500 μm apart on a hydrophobic surface were used to produce a curved meniscus in a drop of DNA–CNT solution, (a). When the drop was removed, dragging the meniscus over the surface, DNA–CNT were aligned in a splayed pattern perpendicular to the meniscus (b). The asymmetry in alignment about the mid-point between electrodes evident in (b) is due to an asymmetry in meniscus curvature.

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in the figure, although variations in hydrophobicity and sample topography resulted in a somewhat skewed shape. DNA–CNT which had deposited onto the surface from the drop while it was stationary were aligned normal to the meniscus as the drop passed. The asymmetry evident in Fig. 10b is due to the imprecise control of meniscus angle in this simple proof-of-concept experiment. This experiment again confirms that DNA–CNT align to the meniscus normal and not to the flow direction since in this experiment the flow direction is the same for all points on the meniscus. We have thus shown that in these experiments the DNA–CNT deposit during the incubation period, as opposed to during meniscus transit. We have shown that deposition increases with increase in time, salt concentration and decreases with increasing pH. This behavior implies that there is an electrostatic energy barrier to deposition which depends on pH and salt concentration. We now consider the kinetics of nanotube deposition across this barrier.

(a)

4. Models for deposition kinetics and alignment Meniscus alignment of DNA–CNT on a hydrophobic surface, we propose, is composed of two connected phenomena: the timedependent deposition of DNA–CNT onto a surface from a drop, and the alignment of deposited DNA–CNT by a passing meniscus. To further understand these processes, we have developed models for the kinetics of deposition and for the way the meniscus aligns the DNA–CNT. (b)

4.1. Interaction potential between the DNA–CNT and the surface The kinetics of deposition can be modeled as an activated process, and the rate of deposition will depend on the energy barrier height,  E. To examine the functional dependence of  E on various experimental parameters, we consider that the interaction potential between the charged DNA–CNT and the substrate consists of two components: electrostatic repulsion and van der Waals attraction, as in DLVO theory [32]. The interaction potential depends on separation between, and relative orientation of, the DNA– CNT and surface. The potential barrier is lowest when the axis of the DNA–CNT is perpendicular to the surface. This has been confirmed numerically. We have previously examined deposition by thermal hopping over an electrostatic energy barrier of DNA–CNT rods aligned parallel to the substrate, in order to explain a different set of experiments [22]. We will compare the two situations in Section 5. The interaction energy e (in units of kb T ) per unit length of DNA–CNT with the surface is, in the Debye–Huckel approximation, e=

φ0 λ kb T

exp(−x/ld ) −

Arc t c 3x3 kb T

,

(1)

where the first term represents electrostatic repulsion and the second term represents van der Waals attraction; φ0 is the surface potential, x is the distance from the surface, λ is the effective linear charge density of the DNA–CNT (which includes the counterion condensation effect [33]), A is the Hamaker constant, rc is the tube radius, t c is the tube wall thickness, kb is the Boltzmann constant and T is the temperature. Finally, ld is the Debye length given by



ld =

εw ε0kb T 2N a e 2 c

,

(2)

where εw is the relative dielectric constant of water, ε0 is the permittivity of free space, N a is the Avogadro number, e is the charge of the electron and c is the concentration of a 1-1 salt. We integrate (1) along the length of a DNA–CNT rod aligned perpendicular to the substrate to find the interaction energy of the entire nanotube. Because the DNA–CNT is long (100’s of nm) compared to the decay lengths of the forces involved we can assume

Fig. 11. Interaction potential for a DNA–CNT perpendicular to the substrate (insert) shows several distinct features. Near the surface there is a deep potential well which corresponds to DNA–CNT sticking to the surface. Further away there is a potential barrier corresponding to  E, which needs to be overcome.  E increases approximately linearly with surface potential (a) and the Debye length of the solution (b).

that it extends from the surface to infinity. The tip of the DNA– CNT is a distance x away from the surface, and integration variable x = 0 at that point. The integral becomes

∞ E (x) = 0

=

φ0 λ kb T

φ0 λld kb T





exp −(x + x )/ld +

exp(−x/ld ) −

Arc t c 6x2 kb T

.

Arc t c 3(x + x )3 kb T



dx

(3)

In Fig. 11 we plot the interaction potential using values for parameters relevant to experimental conditions (see Supporting information, Section 1). As expected, there is an electrostatic potential barrier,  E, which the DNA–CNT must overcome in order to reach the deep potential well near the surface where the DNA–CNT will presumably zip into contact and stick. Note that the height of this barrier varies approximately linearly both with (a) surface potential, and (b) Debye length. We further prove this in the Supporting information. In Fig. 11 parameters were chosen to approximately reflect experimental conditions. For example, surface potential of 80 mV at ld = 3.1 nm in (a) corresponds to a silica surface at pH 8 and 10 mM salt and 20 mV to a surface at pH 3.5. In (b) a Debye length of 3.1 nm corresponds to 10 mM NaCl solution and 1 nm to 100 mM NaCl solution. 4.2. Deposition kinetics In Section 3 we have seen how the amount of DNA–CNT deposited is affected by solution pH and ionic strength. Based on Eq. (3), these factors also influence the potential barrier,  E (Fig. 11). We now develop kinetics models incorporating pH and ionic strength dependence.

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263

Assuming Arrhenius kinetics, the flux onto the surface, J , is given by J = J d exp(− E ),

(4)

where J d is the flux in the absence of a barrier, which is proportional to the bulk density of DNA–CNT [30]. That is, the stability ratio, which determines the reduction in deposition rate compared to the reference flux increases exponentially with the potential barrier height [34]. We consider two models for deposition kinetics. The first model (Case I), described below, assumes that the surface has a maximum number of sites that the DNA–CNT can occupy and that deposition gradually slows as sites are filled. Note that these sites are not of nanotubes aligned parallel to each other, as they are after the meniscus passes. They are of nanotubes deposited randomly on the surface. The alternate model (Case II) assumes that depositing nanotubes increase surface charge homogeneously. The corresponding surface potential and potential barrier,  E, also increase. As  E increases, deposition rate reduces compared to its initial value. We find that the first model explains our observations better, and is described in the main text. The second model is described in Supporting information. Assuming that depositing DNA–CNT occupy discrete sites on the surface, the deposition process will be limited by the number of available deposition sites,

Fig. 12. When a meniscus moving in direction of vector F m (from position a to b to c to d) encounters a hydrophilic nanotube it will experience some distortion. If adhesion is greater on the wet side, force F m will cause the nanotube to rotate about the filled circle. If adhesion is greater on the dry side, the nanotube will rotate about the hollow circle and become aligned perpendicular to the meniscus.

With two adjustable parameters, J 0 and ρt , we are able to fit the experimental data on deposition kinetics very well, as seen in Fig. 5. We can also use this model to fit the data on pH dependence of deposition. For this purpose, we first establish how pH affects surface charge density. It has been shown in experiments with alkyl-dimethyl-chlorosilane modified silica [23] that there are no clear pK a values for the silanol groups on the silica surface. Instead, the surface potential φ0 varies approximately linearly with pH from nearly 0 mV at pH 2 to about 80 mV at pH 8. Because the energy barrier  E varies linearly with surface potential, we can model it using the expression

for the data in Fig. 7 the fit was made with a different value of ρt (2.5 μm/μm2 rather than 1.9 μm/μm2 ), because the deposition appears to reach a clearly defined limit, which is slightly greater than the one in the other experiments, and which provides a ready estimate of ρt . The quantity ρt reflects this maximum density of surface sites. Its value was determined to be 1.9–2.5 μm/μm2 . Let us consider the possibility that these surface sites are a result of electrostatic exclusion by the deposited nanotubes. As a conservative estimate, suppose that each nanotube excludes an area 2ld wide around itself. Then, knowing ld = 3.1 nm (at low salt concentration, less at higher concentrations) and the nanotube length is 560 nm, the nanotube can be considered as a rectangle with 40:1 aspect ratio. Each of these rectangles deposits on the surface and occupies some amount of space, preventing other rectangles from depositing in that area. According to a study of random sequential adsorption (RSA) of rectangles [35], this aspect ratio gives a jamming density of ρt = 27 μm/μm2 . At higher salt concentrations, the aspect ratio would be even higher and the jamming density would be even greater. Thus our measured limiting site density is an order of magnitude smaller than would be expected by ordinary site exclusion, and we propose another mechanism, “thwarted RSA” or “TRSA.” This model [36] assumes that nanotubes are allowed to intersect during deposition, but during the alignment phase all nanotubes that landed on top of at least one other nanotube are removed. Using a numerical model [36], we obtain the limiting deposition density of ρt = 3.1414 ± 0.0001 μm/μm2 , which is much closer to experimental results than the conventional random parking prediction.

 E = C 1 pH + C 2 ,

4.3. Model for alignment of DNA–CNT by the meniscus

J=

    E ρs = 1− J d exp − , dt ρt kb T

 

d ρs

(5)

J0

where

ρt is the number of available surface sites. This yields 



ρs = ρt 1 − exp

− J 0t

ρt

 .

(6)

(7)

where C 1 and C 2 are constants. The differential equation becomes d ρs



ρs J= = 1− dt ρt



  C 1 pH + C 2 J d exp − . kb T

 

(8)

J0

To fit the model to experimental data (Fig. 6) we express J 0 as J 0 = J d exp(−C 1 pH) in Eq. (6) and use the number of available sites determined in the fit to kinetics data (Fig. 5), ρt = 1.9 μm/μm2 . Thus the fit is made with two unknown constants. As in the case of kinetics, we find that the model is able to capture the experimental dependence of deposition on pH. Note that this model predicts exponential decay of ρs with pH for small J 0 t /ρt . The energy barrier  E likewise increases linearly with Debye length. Thus, we can say that  E = D 1 ld + D 2 , and fit the model to the data (Fig. 7) in much the same way as in Fig. 6. Note that

Our experiments show that the DNA–CNT are aligned when the meniscus of the drop passes over them. We now consider what forces must exist in order to create this alignment. The DNA–CNT are exposed to meniscus and fluid forces. As suggested by the experiments, the alignment is due to forces applied by the meniscus on DNA–CNT rods, not due to fluid friction. We justify this further in Supporting information, by estimating these two forces to show that meniscus forces are far greater in magnitude than fluid forces. Let us consider how the meniscus force aligns the DNA– CNT. When the meniscus encounters a DNA–CNT on the surface (Fig. 12), it will be initially pinned by it because the DNA–CNT is hydrophilic while the surface is hydrophobic (Fig. 12, meniscus position b). At some point, the meniscus will reach the position of greatest distortion (Fig. 12, meniscus position c). The meniscus will then exert a force, F m , on the rod. If this force is greater than some static frictional force F f , the DNA–CNT will be moved.

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If, on the other hand, F m is less than F f , the nanotube will remain stationary. As argued by Gerdes et al. [8], alignment requires the friction between the DNA–CNT and the surface to be neither too high nor too low. We point out that this is a paradox, because this simplified argument implies that the DNA–CNT is either washed away or unmoved. The paradox can be overcome by a simple modification of the argument. Suppose that when the meniscus reaches position b in Fig. 12 thermal vibration (of the DNA–CNT or meniscus), evaporation at the meniscus, or some other factor causes the tip of the DNA–CNT to emerge into the dry region (Fig. 12, d) before the meniscus force can move the nanotube, or perhaps while the nandry

otube is moving. If the dry frictional force F f is much stronger than the wet frictional force F fwet , which is a reasonable expectation, the nanotube will not be swept away, although it might move slightly. At this point the force exerted by the meniscus will rotate the DNA–CNT about the hollow circle in Fig. 12. Bensimon et al. explain meniscus alignment of DNA in a similar manner by arguing that the ends of DNA have higher adhesion than the rest of the molecule [10–12]. This may also be true for DNA–CNT, if the hydrophobic end of the nanotube is partially free of DNA. We now demonstrate this more quantitatively. We assume that the friction between DNA–CNT and the substrate is characterized by a coefficient, η , defined such that the friction force per unit length, f, is f = −ηv,

(9)

where v is the local velocity of the DNA–CNT with respect to the substrate. Consider two limiting cases: (a) η is uniform everywhere, and (b) η is high in the ‘dry’ region of the rod above the meniscus and is much smaller in the ‘wet’ region (Fig. 12, d). As expected from the discussion in the previous paragraph, we will find that in case (a) the motion of the DNA–CNT rod would reduce alignment whereas in case (b) the rod would move to align itself normal to the meniscus. From force balance of the rod (neglecting inertial terms) we conclude that

νcx = 0, νc y = F /(ηl),

(10)

where l is the length of the rod and νcx , νc y are the ‘x’ and ‘ y’ components of the velocity of the center of mass of the region that has uniform friction. For case (a), the center of mass is the filled circle in Fig. 12; for case (b) it is the unfilled circle. Moment balance of the rod (again neglecting inertia) requires

ηω I = ∓ F (s/2) cos θ,

(11)

where I is the moment of inertia of the region of the rod that has non-zero friction and s is the length of this region. The ‘−’ sign applies in case (a); the ‘+’ sign applies in case (b). That is, in case (a) the moment about the center of mass is clockwise and Eq. (11) tells us that the rod will rotate with rate ω = dθ/dt, which is negative so that θ will reduce. This would be a mis-aligning tendency. In case (b) where the friction in the dry region is considerably greater than in the wet region the rod will rotate to align with the force. One can integrate the moment equation to determine how the angle will change in time:



ln

sec θ + tan θ sec θ0 + tan θ0



=

Fs 2η I

t,

(12)

where θ0 is the initial orientation of the rod. Our model for alignment of rods relies essentially on forces due to distortion of the meniscus. In this regard it is similar to examples of colloidal ordering at a liquid–vapor interface due to distortion of the interface [37,38], or ordering of membrane-bound DNA due to distortion of the membrane [39].

5. Summary We have studied a set of experiments in which dispersed DNA– CNT rods deposit onto a hydrophobic surface and are aligned by a passing meniscus. We have shown that (a) Meniscus alignment can be used to obtain uniform alignment over large areas. It can also be used to obtain non-uniform alignment as by controlling meniscus shape. (b) DNA–CNT deposit onto a hydrophobic silicon surface by thermally activated hopping over an electrostatic barrier. (c) Deposited DNA–CNT are aligned by surface tension forces exerted by a passing meniscus. (d) Deposition kinetics can be captured by models that combine electrostatic repulsion with van der Waals attraction. Deposition can be modulated by pH and salt concentration, and increases with time. (e) The maximum deposition density appears to depend on a thwarted random sequential adsorption process in which only those DNA–CNT that are in direct contact with the substrate remain and the rest are washed away by the meniscus. (f) Alignment by the meniscus requires that a small portion of the DNA–CNT provide high enough friction to anchor it so that surface tension forces can rotate the rod normal to the meniscus. In a previous study of DNA–CNT deposition and alignment under nominally similar conditions [22], we observed that alignment was independent of the direction of meniscus motion and identical in different drops on the same substrate. To explain those observations, we proposed that the DNA–CNT concentrate parallel to the surface in a secondary minimum in the interaction potential, forming a 2D nematic liquid-crystalline phase [22,23]. Thermal hopping over the potential barrier, as proposed here, transfers this alignment to the substrate. The overall model for DNA–CNT surface interaction was similar, but different in detail because the rods were assumed to reside in a configuration parallel to the surface prior to hopping into contact. It is likely that details of the surface preparation affect which type of process is observed. The meniscus alignment results reported in the present paper are robust and reproducible over a broad range of pH and ionic strength, although details, such as surface deposition density, are quite sensitive to surface preparation. In previous experiments combing of CNTs [8,9] and DNA [10–12] generally worked only over a narrow range of conditions. We have proposed, and it has been previously understood, that combing of an object relies on the ability of the object to adhere strongly by one end while the remaining part remains relatively free to slide. For DNA, this differential adhesion to the substrate generally occurs over only a narrow range of pH [12]. At lower pH, the entire molecule adheres too strongly whereas at high pH the entire molecule is repelled. It has been proposed that the mechanism for end-specific adhesion is due to partial denaturation of DNA at its ends, which exposes hydrophobic bases that can adhere to the substrate [12]. Possibly in the case of DNA–CNT, a partially exposed CNT tip serves as a sticky surface that is always available due to dangling pieces of ssDNA at CNT ends, thus making the process viable over a broader range of pH and ionic strength. Consistent with our observations, it has previously been reported that once DNA or CNT adsorb on the surface they cannot be removed by a subsequent rehydration [11,12]. This is also consistent with our interaction model, which has long range repulsion and a short range attraction. Our method for meniscus alignment should be applicable to a broad range of experimental situations. Longer CNTs have been combed by other researchers [8,9,15] and our method should be applicable. Different substrates could also be used, provided that

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