- Short Communication
- Open Access
Volume discrimination of nanoparticles via electrical trapping using nanopores
© The Author(s) 2019
- Received: 15 June 2018
- Accepted: 5 March 2019
- Published: 14 March 2019
Electrophoretic capture of an oversized object on a solid-state nanopore is a useful approach for single-particle analyses via post electrical and optical measurements. Here we report on nanoparticle discriminations by the volume through combining this nanopore trap method with the cross-membrane ionic current measurements. We investigated ion transport through a pore channel being partially occluded by an electrophoretically-drawn nanoparticle at the orifice. We found distinct difference in the amount of current blockage by particles of different sizes. Multiphysics simulations revealed dominant contribution of particle volume over the other properties. We also demonstrated single-particle discriminations of two different sizes in a mixture solution. The present results demonstrate that this electrical capturing is a promising technique to immobilize a target at a single particle level that concomitantly offer wealth of information concerning their volume.
- Resistive pulse measurement
Nanopore analyses are a simple and strong method for a particle characterization that enables evaluations of various parameters such as the shape, volume, and surface charge density [1–8]. In the measurement, electrophoretic entering of analytes into the nanoscale conduit causes a short-time decreasing of the cross-pore ionic current, and the associated blockade current is used for studying the physical characteristics of individual analytes [9–12]. This method, however, cannot be used for repetitive measurements of single-particles unless additional probes are incorporated to regulate the fast translocation motions such as dielectrophoresis , optical tweezer , and a magnetic force .
The fabrication process of a nanopore is described elsewhere [8, 17]. Briefly, 20 mm × 20 mm sized silicon chips constructed with three layers, SiN/Si/SiN = 50 nm/0.5 mm/50 nm, were used as substrates. Through a reactive ion etching (RIE) for removing partial area of SiN layer on one side of the surfaces and following anisotropic wet etching of Si in KOH aq., a 50 nm-thick SiN membrane was prepared. After forming metal patterns (thickness: Cr/Au = 2 nm/30 nm) by photolithography, radio-frequency magnetron sputtering, and lift off in N, N-dimethylformamide, a 600 nm-sized pore was excavated using electron-beam lithography and RIE (Fig. 1b) in the thin membrane through using the metal patterns as markers.
For the nanopore measurements, polydimethylsiloxane (PDMS) blocks having microchannels were attached on both sides of the chip. Suspension of target particles was then injected through inlet and outlet holes penetrated in the polymeric blocks. After the injection, Ag/AgCl electrodes were set on the both blocks for application of the electrophoretic voltage Vb and measuring the ionic current Iion using Keithley 6487 picoammeter/source (Tektronix, Inc.) under the particle trap control by the handmade program using Visual Basic 6.0.
As target analyte, two carboxylated-polystyrene particles (PS-COOH) with diameter 780 nm and 900 nm (Fig. 1c, d, Thermo Fisher Scientific, Inc.) are utilized after dispersion into TE buffer (10 mM Tris–HCl, 1 mM EDTA). Their ζ-potentials were measured using Zetasizer Nano ZS (Malvern Panalytical Ltd., Zetasizer software ver. 7.12). For each particle, we obtained the values of − 73.8 mV and − 68.6 mV, respectively. Note that these oversized particles are not capable of translocating through a 600 nm pore.
As shown in Fig. 1a, the principle of a nanopore trapping method is based on a physical blocking of ion transport through a pore channel. The presence or absence of the particles at the pore can be checked by monitoring temporal changes in Iion: when a particle is captured, the current rapidly decreases due to a partial blocking of ion transport through the pore. In trapping, a particle is floating at a vicinity of the channel as the result of balance between electrophoretic force and drag force of electroosmotic flow. Both of these forces are proportional to the amplitude of voltage. The resistance of nanopore system can be described as a sum of two elements; R = Rpore + 2Racc. In this equation, Rpore = 4ρL/πd pore 2 and Racc = ρ/2dpore with electrical resistivity ρ, pore diameter dpore, and thickness L are pore resistance which means the component from inside of a pore and access resistance from electrodes to pore orifice, respectively [23–28]. In the measurement of Iion using a low aspect ratio nanopore, the factor of L/d pore 2 in Rpore would approach to zero and the total resistance could be approximated as Racc. Therefore, the amplitude of current blockades in trapping strongly concerns with the volume and the surface charge density of entering particles.
For further evaluation of relationship between the amplitude of ionic current suppression and blocking factor; volume and surface charge density, we applied nanopore trapping method to various particles (dPS = 0.78, 0.90 μm (Thermofisher Scientific Inc.) and 0.79, 0.99 μm (Polyscience, Inc.). If the surface charge density is the dominant factor of ionic current suppression, 0.99 μm PS-COOH (ζ-potential: − 81.9 mV) should show the smallest Itrap. However, the comparison showing in Additional file 1: Figure S2 reveals smaller particle which has relatively weak charge (dPS = 0.78 and 0.79 μm) can block ionic flow more effectively. This means that the certain superior of volume in ionic blockade. In addition, we employed a smaller pore sized 300 nm to trap two particles of dPS = 0.49 and 0.52 μm (ζ-potentials: − 61.4 and − 62.7 mV, respectively) in 0.4× PBS buffer (Additional file 1: Figure S3a). Despite the similarity of their ζ-potentials, the smaller particle demonstrates larger suppression. This result proves our assumption; the dominant cause of target volume on trapping current. Furthermore, we also attempted to capture the large particles (dPS = 0.78, 0.79, 0.90, and 0.99 μm) using this smaller pore (Additional file 1: Figure S3b). In spite of the larger ζ-potentials than two particles trapped steady by this pore, the ionic currents in capturing shows great fluctuations indicating an incomplete immobilization since the effective electrical field is smaller and the contribution of electroosmotic force become large relatively. It reveals that the limitation of nanopore trapping is depend on the size of analyte.
In conclusion, it was revealed that a nanopore trapping method has ability of volume-specific discrimination with similarity of surface charge. In the particle trapping process using a pore, the factor determining ionic suppression is mixing of surface charge and particle size and their priority would appear in the similarity of another. The amplitude of ionic flow is reflected a particle properties of analytes representing a volume and the usefulness as status diagnostic method for single-particle is demonstrated. Besides, contradistinction between similarity of ionic blockade and dissimilarity of trajectory indicated detail electrokinetic factor in nanoscale. This result also suggests the possibility to serve as position modulator of micro-nano scale by a simple control of applied voltage in liquid condition and provide extensibility of sensing in such conditions.
MTs and MTa planned and designed the experiments. AA performed the experiments and analysed the data. AA and MTs co-wrote paper. All the authors discussed the data and reviewed the final manuscript. All authors read and approved the final manuscript.
We thank Prof. Yuhui He for his help in the multiphysics simulations.
The authors declare that they have no competing interests.
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All data generated or analysed during this study are included in this published article [and its additional file].
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This work was supported by Grant-in-Aid for JSPS fellows Grant Number 15J05282 and Grant-in-Aid for Young Scientists (B) Grant Number 17K14098.
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