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Entrance effects induced rectified ionic transport in a nanopore/channel Yu Ma, Jinxiu Guo, Laibing Jia, and Yanbo Xie ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00793 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017
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ACS Sensors
Entrance effects induced rectified ionic transport in a nanopore/channel Yu Ma, †, ‡ Jinxiu Guo, †, ‡ Laibing Jia, § and Yanbo Xie*, †, ‡ †
Joint Lab of Nanofluidics and Interfaces, School of Science, Northwestern Polytechnical University, Xi’an, 710072, China Key Laboratory of Space Applied Physics and Chemistry, School of Science, Northwestern Polytechnical University, Xi’an, 710100, China § School of Marine Science and Technology, Northwestern Polytechnical University, Xi’an, 710100, China ‡
Supporting Information electrical detection effects on the translocation of biological nanopore.26 Besides, the polarization of ions concentration crossing the nanopore is also expected to be significant by the exterior surface properties of the pore, inducing different electrical detection on DNA translocation event.27 For electrokinetics energy conversion, the entrance effects (discussed in form of access resistance) has an opposing effect on the energy conversion efficiency.24 Hence, the entrance effects can have major impacts on the relevant phenomena of electrokinetics. However, the entrance effects dominated the ion conduction are still not clearly discussed, as a pore mechanically becomes a channel by increasing the length to radius ratio (L/r). Moreover, few studies discussed the entrance effects of a nanopore/channel on the rectified ion transport phenomena as far as we know.28 In this paper, we simulate the ionic transportation through a nanopore/channel by using Comsol Multiphysics, which has been widely used in calculating nanofluidics and relevant phenomena.26, 29 Our results show the boundary between the entrance effects and channel dominated regimes as a function of geometry and double layers, respectively. By taking these two effects into account, we could further achieve the nanofluidic diode by oppositely charging the exterior surfaces. To distinguish the key factor of the entrance effects, we investigated the ion conductance in three models: oppositely Exterior Charged Surface with a neutral inner wall (ECS model), oppositely Inner Charged Surface with neutral exterior surfaces (ICS model), and oppositely charged inner/exterior surfaces (ACS model).30 Our results clearly indicate the region where the exterior charged surface dominated the ion conductance and rectification. These findings help to optimize the design of nanofluidic diode and a nanofluidic based biosensor.
ABSTRACT: The nanofluidic diode, as one of the emerging nanofluidic logic devices, has been used in many fields such as biosensors, energy harvesting and so on. However, the entrance effects of the nanofluidic ionic conductance were less discussed, which can be a crucial factor for the ionic conduction. Here we calculate the ionic conductance as a function of the length-to-pore ratio (L/r), which has a clear boundary between nanopore (surface dominated) and nanochannel (geometry dominated) electrically in diluted salt solution. This entrance effects are even more obvious in the rectified ionic conduction with oppositely charged exterior surfaces of a nanopore. We build three models – Exterior Charged Surface model (ECS), Inner Charged Surface model (ICS) and All Charged Surface model (ACS) to discuss the entrance effects on the ionic conduction. Our results demonstrate, for a thin nanopore, the ECS model has a larger ionic rectification factor (Q) than that of ICS model, with a totally reversed tendency of Q compared to the ICS and ACS models as L/r increases. Our models predict an alternative option of building nanofluidic biosensors that only need to modify the exterior surface of a nanopore, avoiding the slow diffusion of molecules in the nanochannel. KEYWORDS: Nanofluidics, Entrance effects, Current rectification, Nanopore, Nanochannel
With the narrow confined geometry (1nm, with considerable value of 229 at lDu=519nm. surface (ICS model) and all charged surface model (ACS model) with all sketches shown in the inset figure of Fig. 3a, to find out the influence of entrance effects on the nanofluidic diode device. The nanofluidic diode was simulated in the same model with same boundary condition, except the oppositely charged exterior surface. The pore/channel radius kept at 10nm, with variation of n ranging from 0.1 to 1000, and salt (KCl) concentration from 0.1mM to 1M. The absolute value of surface charge density was kept as 10mC/m2 but can change polarity in different models, unless it was specified. As in above studies, the current rectification factor Q is defined to identify the rectification factor, Q=I(+1V)/I(-1V), where the I(+1V) and I(-1V) are the current at corresponding bias voltage. Herein, the ion conductance rectification occurs when Q>1, disappearing when Q=1, and reversed when Q1nm to 148 at lDu=260nm, representing an excellent figure of merit as a nanofluidic diode. One of the applications can be seen beforehand is to be used as a nanopore biosensor. By immobilizing molecules, like antigen, on the exterior surfaces of a thin nanopore, the specific binding can induce additional surface charge variation on the base of nonspecific binding. Once the specific binding occurs, the surface charge density at one side surface of nanopore can be changed, inducing the current rectifications of the nanopore. Here we discussed the sensitivity of electrical resistance sensor majorly by surface charge effects45 and surface charge induced nanofluidic diode sensor proposed in this paper. We set the nanopore geometry and initial surface charge density being in the same conditions for two models. The sensitivity of electrical resistance detection was obtained by the resistance variation normalized by the initial state (green, n=0.1, σext1=σext2 =△σ, σin1=σin2=0), while the diode sensor was calculated by the current rectification factor with variation of surface charge density on one side of nanopore (red, n=0.1, σext1-σext2 =△σ, σin1=σin2=0). Finally, we found the diode sensors is always more sensitive than the resistance model, and increases faster than of the resistance sensor with variation of surface charge density from 0-0.06C/cm2, as process in the nano-confined space.46-47 However, here we proposed an entrance effects induced nanopore possibly provides an alternative option of making the electrochemical/biological sensors, by only modifying the exterior surface of the nanopore. The possible way to achieve this type of nanofluidic diode, was to spin a sacrificial layer to protect one side of the thin membrane (eg. SiN) and treat surface with positively charged monolayers. As rapid developing of 2D materials, it is able to fabricate such a thin nanopore that has negligible thickness comparing to the dimension of pores. By the small inter-distance of the carbon atoms and pore size, the surface coating chemicals are difficult to transport to the other side of the membrane, enabling to fabricate a thin nanopore with bipolar charged surface. Our results can possibly explain the current rectifications found in a synthetic nanopore,48 besides the recent discovery of entrance effects in biological nanopore system can also indirectly prove this concept.49
Figure 4. The sensitivity of nanofluidic diode biosensor (red) is always higher than the resistance biosensor (green) as variation of surface charge density.
Conclusions In conclusion, we simulate the ion conductance of a nanofluidic pore/channel as a function of length to pore ratio. To quantitatively distinguish the entrance effects, we separated the entrance resistance and channel resistance, and our results show that the entrance effects starts to dominate the ionic conductance at n
