NANO LETTERS
Transverse Field Effects on DNA-Sized Particle Dynamics
2009 Vol. 9, No. 4 1659-1662
Makusu Tsutsui,† Masateru Taniguchi,*,†,‡ and Tomoji Kawai*,† The Institute of Scientific and Industrial Research, Osaka UniVersity, Ibaraki, Osaka 567-0047, Japan, and PRESTO, Japan Science and Technology Agency, Honcho, Kawaguchi, Saitama 332-0012, Japan Received January 19, 2009; Revised Manuscript Received February 16, 2009
ABSTRACT We report the development of microfluidics-integrated mechanically controllable break junction device and its applications to electrical characterizations of DNA-sized particle dynamics in a microfluidic channel. It is found that the electrostatic electrode-particle interaction slows down the particle flow through the electrode nanogaps. The present results suggest the useful capability of transverse electric field for controlling DNA translocations through a nanopore.
The past few years have witnessed emerging interest in exploiting the solid-state nanoscale-pore for label-free highspeed DNA sequencing at the single molecule level.1-8 It mimics the transmembrane channels in biological systems such as R-hemolysin residing in a lipid bilayer and yet combines pore-size controllability and practical durability. Electrical sensing of single-molecule DNA translocations through the artificial nanopores has been accomplished by monitoring the concomitant ion current changes: DNA blockades the ion conduction pathway upon traversing the pore, thereby causing momentary decrease in the current.1,2 The state of the art experiments9,10 have even demonstrated manipulations of individual DNA molecules in the nanopore by incorporating the optical trapping technique. However, the single molecule sequencing remains to be an ongoing issue. An alternative approach proposed for the fast DNA sequence read off is scanning the transverse electric current in a DNA molecule while it flows through a nanopore.11-16 The transverse electron transport in DNA is characterized by the energy alignment at the DNA base-electrode interface. Recent theoretical calculations predict the possibility of distinguishing each individual base from the difference in their HOMO-LUMO gaps providing distinct tunneling conductance.11-13 Experimental verification of this detection mechanism requires two closely spaced nanoelectrodes with the gap size precisely the same as the DNA diameter, for the already low transverse current in the subpicoampere range11-13 would further decrease exponentially with DNA-electrode tunneling distance.16 * Corresponding authors: tel, +81-6-6879-4289; fax, +81-6-6879-4289. † Osaka University. ‡ Japan Science and Technology Agency. 10.1021/nl900177q CCC: $40.75 Published on Web 03/03/2009
2009 American Chemical Society
The technical requirement can be fulfilled by exploiting nanofabricated mechanically controllable break junctions (MCBJs), which allow fine-tuning of the electrode gap size at subpicometer scale resolutions.17-19 Our scheme here is to incorporate a fluidic channel into the nano-MCBJ system for proof-of-principle demonstration of the transverse electron transport method. The present Letter reports the electrical detection of individual DNA-sized Au nanopartilcles (NPs) flowing through the nanospace in the electrode gap region using the microfluidics-integrated MCBJs. Scanning electron micrograph images of the microfluidicsMCBJ device are presented in Figure 1. The device consists of free-standing Au nanojunctions embedded in a fluidic channel (Figure 1a). The two electrodes, E1 and E2, at both ends of the channel are for introducing electric-field-driven flow of Au NPs through the channel. Fabrication procedures of this nanostructure are detailed in Figure 2. The first step is to pattern Au nanojunctions on a polyimide-coated phosphor-bronze substrate using standard electron beam lithography and radio frequency magnetron sputtering processes. The narrowest part of the nanoconstriction at the center has a cross section of 50 nm × 100 nm (thickness × width). Subsequently, a polyimide layer with thickness of about 500 nm is spin-coated. Then, a lithographically defined 30 nm thick Pt etch mask is formed, which configures the shape of the fluidics. Finally, the exposed polyimide regions are isotropically etched via a reactive ion etching method. The resulting configuration is a microfluidic channel of 2.5 µm × 2 µm (height × width) flow passage area with freestanding Au junctions embedded at a vertical position 0.5 µm below from the top of the fluidics. In the experiments, the Au junctions are broken mechanically by bending the sample substrate (Figure 1b,c) in
Figure 1. (a) Scanning electron microscopy images of the microfluidics-integrated MCBJ device showing three Au nanobridge electrodes with a 2 µm wide fluidic channel fabricated in the middle. Inset is a magnified view showing the free-standing configuration of the Au junction. (b) A nanojunction before and (c) after mechanical breaking. The sharp apex at the left electrode after the breakdown can be used as an optimal probe for electrical detection of molecular-sized particles.
Figure 2. Fabrication procedures of the microfluidics-integrated MCBJ samples. (i) A phosphor-bronze substrate is spin-coated with a 4 µm thick polyimide layer. (ii) Subsequently, Au nanojunctions are formed by standard electron beam lithography and radio frequency magnetron sputtering processes. (iii) After that, the sample is coated with a 0.5 µm thick polyimide. (iv) A Pt mask is then patterned, (v) followed by isotropic reactive ion etching.
ultrapure water containing Au NPs (British BioCell, Cardiff, U.K.) at a concentration of approximately 2 nM. The NPs are citrate-stabilized particles possessing a surface negative charge. The pH of the solution is 7.83. The size of the particles employed is 2 nm unless otherwise stated, which is close to the DNA diameter. As a consequence, two closely spaced nanoelectrodes with a gap size of dgap are formed. It is noticed that necking deformations during mechanical breaking of the junctions create a sharp apex, which is desirable for addressing the translocation events of a molecular-sized particle (Figure 1c). dgap is controllable in a range of 0.5 nm < dgap < 10 nm by further manipulation of the bending beam deflection. The displacement ratio of the nano-MCBJ is r ) ∆dgap/∆z ∼ 2 × 10-4, where ∆z denotes the displacement of the pushing rod that bends the substrate.20 This small r enables fine control of dgap with subpicometer resolution and also provides the nano-MCBJ electrodes with 1660
Figure 3. (a) I-t curves measured under various environments. (b) Examples of the pulselike signals observed in the solution. (c) A close-up view of the left signal in (b), revealing the asymmetry of the I-t characteristic. Ip and tf define the height and the fall time of the pulselike signal.
an outstanding mechanical stability. After dgap is adjusted to a specific size, a constant dc bias voltage of Ve is applied between E1 and E2 to drive Au NPs to flow through the fluidics. Simultaneously, current across the nanogap electrodes I is recorded at 2 kHz under a dc bias voltage of Vb. Figure 3a shows I-t curves (shifted vertically for clarity) acquired under various environments at Vb ) 0.2 V, Ve ) 0.2 V, and dgap ∼ 2 nm. In a vacuum, the current is quite stable with the offset current Ioff ∼ 0 pA and the noise NI ∼ (2 pA, which is close to the current resolution of our measurement system. In contrast, we obtained Ioff ∼ 2 pA and NI ∼(10 pA in the solvent. The higher Ioff suggests contribution of leakage current in the liquid environment, which also accounts for the enhanced noise level. When measured in the particle solution, both parameters further increase in proportion to the NP concentration. This manifests the ionic nature of the Au NPs used, the larger leakage current associated with high ion (NP) content. The most characteristic feature found in the I-t traces is the pulselike signals appeared in the data for the NP solution. These momentary current changes emerge at random in time Nano Lett., Vol. 9, No. 4, 2009
typically at a frequency of 10-50 events per hour. The fact that the I pulse is detected only when the dgap is adjusted to ∼2 nm ()the nominal NP size) whereas it turns out to be completely absent when dgap > 5 nm signifies that the temporal current change is related to translocation events of the NPs through the nanogaps. Two consecutively detected signals are shown in Figure 3b as an example. A magnified view (the first one in Figure 3b) reveals that the line shape of the pulselike signal is not symmetric; the current rises sharply to a maximum and thereafter declines gradually to the original base level (Figure 3c). Similar asymmetric structures are also present in the other hundreds of signals obtained. The peculiar I-t characteristics of the pulsed signals are ascribable to the NP translocation processes through the nanogaps. A single NP approaching the electrode gap region feels the transverse electric-field-induced attractive forces and eventually becomes trapped. The resulting formation and electron conduction through the electrode-NP-electrode structure leads to the observed sudden increase in I. Subsequently, however, the particle tends to escape the electrostatic trap via the Ve-induced forces. This gradually extends the tunneling distance at the electrode-NP interface and diminishes I continuously until it reaches the original current level. The fact that NPs are capable of escaping the transverse trap field instead of becoming immobile is reasonable considering the relatively weak electrostatic binding energy W, which is given as W ) (CNP - Cgap)Vb2/2, where CNP and Cgap are the capacitance of Au particles and electrode gaps, respectively.21 CNP is roughly estimated from the selfcapacitance of a NP of radius r, 4πε0r, which is ∼1.1 × 10-19 F for 2 nm sized particles. On the other hand, Cgap is calculated to be ∼0.25 × 10-19 F assuming atomically sharp electrode apex. Consequently, we obtain W ∼ 11 meV at Vb ) 0.2 V, which is as low as the thermal energy at a room temperature. Despite the little difference in the line shape, the width and height of the pulse signatures vary extensively. We have performed statistical analyses of the data by extracting the amplitude Ip and the fall time tf from each of 100 current spikes acquired. Here, tf is defined as the time duration required for I to decrease from the peak value by 90% of Ip, as explained schematically in Figure 3c. Scatter plots of Ip versus tf at three different Vb conditions are displayed in Figure 4. It is first noted that Ip distributes for more than an order of magnitude. This expansive scattering originates presumably from the size variations of NPs utilized in the experiments. Ip represents the electrical conductivity of electrode-NP-electrode junctions, which is exponentially dependent on the electrode-NP tunneling distance involved. This contact length should scale linearly with the particle size assuming that NPs are trapped exactly at the middle of the nanogaps, and hence Ip ∼ exp(r). In more realistic sense, however, the actual flow paths of the particles can deviate from the ideal situation because of the weak trapping energy comparable to thermal fluctuations. This extrinsic uncertainty in the exact trajectory of NPs translocating the electrode gap region may contribute to further expansion of the Ip variation. Nano Lett., Vol. 9, No. 4, 2009
Figure 4. Scatter plots of Ip versus tf acquired at Vb of 0.1 (red), 0.2 (black), and 0.3 V (green).
Moreover, we notice that, although data scatter over a wide range, Ip demonstrates a linear correlation with tf. This mutual correlation is not an unexpected one. As mentioned in the preceding paragraph, the high (low) Ip states are ascribable to large (small) size of NPs trapped. Large (small) size particles, at the same time, possess high (low) CNP and, hence, are prone to be subjected to strong (weak) electrostatic trapping, which necessitates longer (shorter) escape time or equivalently tf. On the other hand, the Ve-induced forces exerted on the particles are predicted to be not so different at constant Ve. The Ip-tf relation can thus be interpreted qualitatively as reflecting stronger electrode-particle bindings for larger size NPs with high Ip that necessitate longer escape time. To analyze this in more detail, we estimate the particle size distribution from Ip. When assuming Ip ∼ exp(r), the 2 orders of magnitude variation in Ip yields about a factor of 2 distribution in the particle size involved in the electrical detection experiments. This also predicts a 2-fold variation in W, which seems to be too small to explain the wide range of tf that extends for more than 2 orders of magnitude. It is perhaps the aforementioned particle flow path variability that makes it hard to quantitatively explain the experimental Ip-tf dependence. Next we discuss the effects of the transverse electric field on the particle dynamics. There is little qualitative difference in the scatter plots constructed with Ip and tf data obtained in the bias range of Vb ) 0.1-0.3 V; they share the same up sloping trend. Quantitatively, however, we find more than 2-fold difference in the average fall time, which is 0.031, 0.089, and 0.147 s at Vb ) 0.1, 0.2, and 0.3 V, respectively. The longer tf at higher Vb is consistent with a factor of 9 (4) larger W at Vb ) 0.3 V (0.2 V) compared to that at Vb ) 0.1 V, which manifests the non-negligible role of the transverse field to slow down the NP flow in the fluidic channel via the electrostatic electrode-particle attraction. These results indicate the possibility of hindering DNA translocations through a nanopore intentionally by transverse electric field control.12 Finally, we report the I-Vb characteristics of the rigidly trapped Au particles. The I spikes emerge less frequently when dgap is smaller than the particle size. Instead, I-t curves often show stepwise increase of I; i.e., current jumps and stays at that level showing staircase-like structure without 1661
the scope of this paper to seek further insight into the switching mechanism. To summarize, we have developed the microfluidics-integrated nano-MCBJ device and applied it to electrical characterizations of the dynamics of DNA-sized particles in a microfluidic channel. When the tunneling current across the nanogap electrodes is monitored, a wide dynamic range of pulse-like signals were detected, which were attributable to the particle translocation through the electrode gap region. It was found that the high electric field between the nanogaps functions to slow down the particle. The present results suggest the capability of transverse electric field control for manipulating DNA translocations through a nanopore. Acknowledgment. This research was partially supported by Grant-in-Aid for Scientific Research on Innovative Areas 20200025. References Figure 5. I-Vb characteristics of single (a) 2 nm and (b) 10 nm sized particle bridging over the nanogap electrodes. The particle conductance demonstrates bistable switching that resembles those of memory resistors.
returning to the original current state. This signifies direct contact of the particles to the nanogap electrodes and formation of stable Au-NP-Au junctions. After confirming a current step, we evacuate the sample chamber and exhibited I-Vb measurements. Figure 5a displays the I-Vb curve obtained for 2 nm NP solution under dgap ∼ 1 nm. It demonstrates a peculiar switching behavior. The transitions from the low to the high conductance states occur sharply at Vb ∼ (0.5 V upon the forward sweeps, while high-to-low transitions proceed smoothly during the backward sweeps. Similarly, flowing 10 nm sized NPs under dgap ∼ 2 nm resulted in the robust trapping. The obtained I-Vb characteristics again reveal bistable switching between two distinct conductance states. There are many common features in the I-Vb curves; for example, high(low)-to-low(high) switching only in the forward (backward) sweep. These switching characteristics are in excellent qualitative accordance with that reported recently for single Au NP junctions,22 thus evidencing stable trapping of NPs in the case when dgap < (particle size). What is the physical origin of the bistable conductance switching? We point out that the switching characteristics closely resemble those of “memristors” (memory resistors). Memristive electrical characteristics have been found in various nanostructures having metal-insulator-metal (MIM) configuration such as Ag/Ag2S/Pt23 and Pt/TiO2/Pt.24 In our case, Au nanoparticles are citrate-functionalized, and hence the particles are coated with an insulating layer. Therefore, Au-nanoparticle-Au junctions measured in the present work can also be considered as MIM systems. Nevertheless, the underlying physics of the memristive characteristics itself is still under debate. Hence, though interesting, it is beyond
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NL900177Q
Nano Lett., Vol. 9, No. 4, 2009
