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Resistive-Pulse Sensing and Surface Charge Analysis of Single Nanoparticle Collision at a Conical Glass Nanopore Ya Zhou, Dandan Wang, Chuanping Li, Ping Hu, and Yongdong Jin Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b00553 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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Analytical Chemistry

Resistive-pulse sensing and surface charge analysis of single nanoparticle collision at a conical glass nanopore Ya Zhou†, §,#, Dandan Wang†, §, Chuanping Li†, ‡, Ping Hu†, §, Yongdong Jin†,‡,§,* †State Key Lab of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun, 130022, China. ‡University of Chinese Academy of Sciences, Beijing, 100049, China. §University of Science and Technology of China, Hefei, 230026, China

ABSTRACT: In this study, the collision of monodispersed single Au@SiO2 nanoparticles (NPs, ~ 83.5 ± 6.5 nm) at the outer orifice of a smaller-sized (~ 40±8 nm) conical glass nanopore was systematically investigated by the resistive-pulse sensing technique, under three different surface charge states. When NPs approach the nanopipette orifice and collide, the ionic current changes dramatically, and the collision current amplitude showed a linear relationship with the potential bias applied. In our system the electroosmotic flow is found the main driving force for the collision, and the resistive-pulse collision signal depends on the relative forces of electroosmotic, electrophoretic and electrotatic forces being applied, which in turn can reflect the surface charge properties of the tested NP and nanopore wall used. By fixing the size of the nanopore and NPs, the surface charge information of the NPs and nanopore walls can be discriminated by the magnitude and duration of the collision ionic current signals. This study manifested the feasibility of exploring the nanopore-based single NPs collision platform for surface charge analysis of the colloidal nanoparicle system.

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Collision and migration of nanoparticles (NPs) are common phenomenon and processes in bulk colloidal solutions. These processes often occur at extremely high frequencies which make them hard to be detected individually under normal circumstances. Therefore, it is quite difficult to obtain surface physicochemical properties of single NPs in solution from the motion behaviors. Traditional methods, such as transmission electron microscopy (TEM) and dynamic light scattering (DLS), cannot get surface charge information of individual NPs in solution as TEM provides only morphology and some elemental information of the particles while DLS achieves merely statistical results1. In 2007, the Bard research group first reported on the observation of single NP collisions at an ultramicroelectrode by electrocatalytic amplification2. Since then, researchers have been persuing nano-impact experiments and systems and extended the nano-impact technique for versatile electrochemical analyses and characterizations of single nanoobjects3-8. Herein, we explored the emerging resistive-pulse nanopore sensing tactic for the observation of the nano-impact events of single NPs at the outer orifice of a conical glass nanopore and the related surface charge analysis. The past few decades have witnessed the rapid development of nanopore analytics for biosensing at the single molecule level9-11. Its detection principle derives from the concept of Coulter counter9. When a molecular (or nanoobject) passes through a nanopore, the ionic current signal changes sharply owing to the molecular occupation of ion volume in the sensing zone of the nanopore12. Glass nanopipette-based nanopore as a member of artificial nanopores has recently aroused an increasing attention in the field13. It has unique advantages such as simple fabrication, mechanical stiffness14, and chemical and temperature stability, and by shrinking the orifice size of glass nanopore down to 10 nm it has been successfully exploited for single molecule level DNA detection and structural analysis15,16. Glass nanopipette nanopore has also been explored for small unilamellar phospholipid vesicle17 and metal NPs translocation studies18, and NPs collision experiment19. For example, Mao et al. have observed single particle collision events at the orifice of a nanopipette by the ionic current variations19. However, the deep relationship of the physicochemical properties of the NPs with collision signals was not investigated yet. In this study, the collision of single monodisperse Au@SiO2 NPs (~ 83.5 ± 6.5 nm in diameter) at the outer orifice of a smaller sized conical glass nanopore (~ 40±8 nm), under three different surface charges, was systematically investigated by resistive-pulse sensing technique. By analyzing the ionic current signals, we found that the resistive-pulse signal of single particle collision at the outer orifice of glass nanopore depends on the relative forces of electroosmotic, electrophoretic and electrostatic forces being applied, which in turn can reflect the surface charge properties of the tested NP and nanopore wall used. The collision current amplitude showed a linear relationship with the potential bias applied. By fixing the size of the nanopore and NPs, the surface charge information of the NPs and nanopore can be easily discriminated by both the magnitude and duration of the resulting collision ionic current signals. The result provides significant guides for physicochemical and kinetics study of single NPs in solution.

EXPERIMENTAL SECTION

Chemicals and Reagents. Hydrogen peroxide (30% H2O2), sulfuric acid (H2SO4), monosodium phosphate (NaH2PO4), HAuCl4·4H2O and disodium phosphate (Na2HPO4) were purchased from Beijing Chemicals. Potassium chloride (KCl) was purchased from Aladdin, and aminopropyltrimethoxysilane (APTMS), NH2OH·HCl, Na2SiO3, tetraethoxysilane (TEOS) were purchased from Sigma-Aldrich. (Triethoxysilyl)propylsuccinicanhydride (TPSH) was purchased from J&K Scientific LTD. All aqueous solutions were prepared from deionized water (Milli-Q, Millipore Co.). Both carboxylated and amino-modified Au@SiO2 NPs with mean diameter of ~ 83.5±6.5 nm (Figure S1) were used in the experiments. The Au@SiO2 NPs were synthesized by the previously reported method20. The carboxylated and aminomodified Au@SiO2 NPs were then prepared by surface modification of Au@SiO2 NPs with APTMS and TPSH, respectively. The detailed synthetic procedures are described in the Supporting Information. Before use, the stock solutions of particles (~ 4.0 × 1010 particles/mL) were diluted to the desired concentration with 10 mM KCl and 10 mM PBS (PH= 7.0). A concentrated solution of NPs (~ 8.0 × 1011 particles/mL) was used to perform concentration- and zeta potentionl-dependent collision experiments. Fabrication of Glass Nanopipette-based Nanopore. The glass nanopores were fabricated from quartz glass capillaries (QF100-70-10, Sutter Instrument Co.) with outer diameter of 1.0 mm and inner diameter of 0.7 mm by a laser puller. Before laser pulling, the glass capillaries were cleaned by piranha solution (3:1 98% H2SO4/30% H2O2) for 2 h. Then, the capillaries were cleaned by deionized water to remove excess acid and dried at 80 °C. The cleaned glass capillaries were rich in silica hydroxyl groups on the interior wall. The glass nanopipette-based conical nanopores were then fabricated by P2000 puller (Sutter Instruments Co.) with a program including the following parameters: heat = 700, fil = 4, vel = 60, del = 170, pull = 180. The inner diameters of the as-prepared nanopores were typically about 40 nm (40 ± 8 nm), which could be estimated by TEM measurement. The freshly prepared glass nanopores were then modified by the APTMS. First, 0.5% (volume ratio) ethanol solution was backfilled in the glass nanopipette and incubated for 30 min at room temperature to react with the pore wall. After removal of excess APTMS by ethanol, the nanopipette was baked at 120 °C for 30 min to stabilize the APTMS coating. The success of APTMS monolayer coating could be revealed from the polarity change in the corresponding ionic current rectification (ICR) behaviors of the nanopore due to the surface charge change after the treatment (Figure S2). Data Recording and Analysis. The buffer solution was injected into the prepared quarts nanopipette with a microinjector and then remove the remained air in the tips applying negative pressure. All the buffer solutions were prepared with 10 mM KCl and 10 mM PBS (PH=7.0) unless specifically stated. Then, one Ag/AgCl electrode was inserted in the nanopipette to serve as a working electrode; the other Ag/AgCl electrode as a reference electrode was immersed in the bulk solution. The current signals were recorded with amplifier Axopatch 200B (Molecular Devices) and 1440A Digidata (Molecular Devices). The collected data were analyzed with Clamfit 10.7. The histogram was plotted with origin9.0 and the curves were obtained using Gaussian fitting.

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Analytical Chemistry RESULTS AND DISCUSSION Resistive-pulse sensing of inward single nanoparticle collision at the outer orifice of a glass nanopore. As schematically illustrated in Figure 1a, experimentally a glass nanopore backfilled with supporting electrolyte solution was immersed in a colloid solution. Upon applying a bias between two electrodes, the migration of NPs through the nanopore leads to the collision of particles with the outer orifice of the nanopore. Since the orifice (~ 40 ± 8 nm) of the glass nanopipette nanopore used in this study was much smaller than the diameter of the Au@SiO2 NPs (~ 83.5 ± 6.5 nm) (cf. Figure S1), the NPs cannot translocate through the nanopore but instead the inward collision of particles at the outer orifice of glass nanopore. It is worth mentioning that the “collisions” mean here may probably not only a direct "hard" physical collision (between NPs and nanopore wall) but also a "soft" collision caused by ionic current disturbance when NPs approaching very close to the pipette opening, resulting in abrupt change in the ionic resistance21. Such collisions can be sensed by resistive pulse detection of transient ionic currents. There are some features of the signals which we should discuss. We knew that the contour of a resistive pulse response contains the complex behavior of particle collisions on a nanopore22. Each pulsed ionic peak or noisy spikes implies a collision event of single NP (Figure 1b)23. Analogous to the molecular translation event, the time elapsed from the start to the end of a collision is defined as collision time; while the time between two adjacent collision events is

Figure 1. (a) Schematic diagram of experimental principle and setup. (b) the typical resistive pulse current trace of the Au@SiO2COOH NP (~ 4.0×1010 particles/mL) collisions at -0.8 V in the 10 mM KCl and 10 mM PBS. (c) the scatter plot of current drop versus collision time and (d) the histogram of current drop distribution of the NP collision events.

called interval (Figure S3). The continuous collisions of suspended particles are recorded by multipeak of the current24. Figure 1b shows a typical 500 ms ionic current trace of the carboxylated-modified Au@SiO2 (Au@SiO2-COOH) NPs (~ 4.0×1010 particles/mL) collision where the inner chamber of the glass nanopore was held at a potential of -0.8 V vs Ag/AgCl. As seen from Figure 1c and 1d, the distribution of particle collision signals mainly fall into two types: one has a large current drop (centered at ~ 750 pA) and long collision time (on the time scale of hundreds of milliseconds); the other has a small current drop (centered at ~ 180 pA) and short collision time (at tens of

millisecond). The former may be caused by main collision of individual NPs, while the latter may be induced by the subsequent multiple sencondary collisions of the NP, as reported by others24,25. Due to the regularity of the diminishing peaks responses, the involvement of several-particle collision events in the process is less impossible. Although not very clear, the main factors that determine when the particle finally escapes from the outer orifice of the nanopore would be the Brownian motion of the particle and the electrostatic repulsion caused by the next colliding particle. The typical effective collision distance between the NP and the pipette opening for the former (main collision) was rougly estimated to be ~ 1.5 nm in our system, obtained by calculation of the blocking ratio of ionic current using a model depicted in Scheme S1 (in Supporting Information). It is imaginable that under a certain condition, after the first collision, some particles that very close to the outer orifice of the nanopore may fail to escape from it because of the inward combination force of electroosmotic, electrophoretic and electrostatic forces (See detailed explanation below), and therefore bounce back to participate multiple successive collisions, resulting in longer overall collision time. Such multiple successive collisions of an individual NP typically displayed a main current drop, followed by a several diminishing peaks, as clearly shown in Figure 1b. It could be clearly seen in Figure S4 that the ionic current decline led to a maximum at first, then gradually reduced and finally returned to the baseline. According to previous reports, the magnitude of the current change (drop) mainly depends on the volume of orifice blocked by particles26. The observed resistive pulse current traces were further confirmed to be caused by collisions of single NPs, rather than the translocation events of particles through the nanopore, since there was no translocation/collision signals observed when we switched the applied bias to 1 V (Figure S5). To further provide evidence that the current spikes are indeed due to colliding particles, we also performed collision experiments using Au@SiO2-OH NPs with different particle concentrations. As expected, the collision frequency increases with increasing the concentration of NPs, showing a approximate linear relationship (Figure S6). Direct contact between the NP and the pipette opening was also observed occasionally when performing Au@SiO2-COOH NPs collision with the APTMS-modified nanopipette. As shown in Figure S7, direct contact between NP and pipette opening was evidenced by the occurrence of a long-lasting ionic blockage due to the sticking of the negatively-charged NPs to the positively-charged pipette walls, after the approaching and collisions of the NPs, which is very similar to the previous report.21 Effects of Surface Charges of Pore Wall and nanoparticles on Collision. Usually when individual NPs translocate through a nanopore with an orifice bigger than the NPs, the direction of particle transport is a combination of electrophoresis, electroosmosis and pressure-driven flow as demonstrated in the previous reports27,28. In this study, monodispersed Au@SiO2 NPs with size bigger than the orifice of glass nanopore were set in the outside solution of the glass nanopipette. In a typical collision experiment, we used both surface negatively-charged bare glass nanopore and Au@SiO2COOH NPs (denoted as case 1 thereafter); when applied a negative potential in the nanopore chamber, the NPs migrated gradually towards the mouth of nanopipette and the collision current transients appeared, as shown in Figure 1b. The occurrence of the resistive-pulse current response is

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unexplainable by electrophoresis force (if it was the main driving force) since under the negative bias (-0.8 V) it is repulsive for the inward collision of Au@SiO2-COOH NPs with the orifice of glass nanopipette. But this can be easily explained if considered the combination force of electroosmotic, electrophoretic and electrostatic forces, as depicted below.

Figure 2. The representative raw resistive-pulse current responses of one NP collision with glass nanopore under three different experimental conditions: (a) (-/-) bare glass nanopore + Au@SiO2-COOH NPs (case 1) at the bias potential of -1V; (b) (+/-) APTMS-modified glass nanopore + Au@SiO2-COOH NPs (case 2) at the bias potential of 1V; (c) (-/+) bare glass nanopore + Au@SiO2-NH2 NPs (case 3) at the bias potential of -1V; (d, e) Typical TEM images of the Au@SiO2 NPs and glass nanopore used in this study. To check further the possible charge effects of both the pore wall and NPs on collision signals, collision experiments were comparatively studied by using the bare or APTMS-modified glass nanopipette nanopores, along with two kind of Au@SiO2 NPs with different surface charges (Au@SiO2-COOH NPs and Au@SiO2-NH2 NPs, respectively). In the experiments, as shown in Figure 2 (a-c), when a -1V voltage was applied, there was an upward current signal in case 1 (Figure 2a) and case 3 (Figure 2c). However, as shown in Figure S5, there was no resistive pulse signals observed when a positive voltage of 1 V was applied to the inner chamber of glass nanopore. But when a positive voltage of 1 V was applied, there occurred a downward current signal in case 2 (Figure 2b). Experimentally, we clearly observed plenty of collision events when a negative bias of -1 V was applied (Figure S8a). Since the two phenomena cannot be explained by the electrophoretic force (EPF), so we assume that in our cases the electroosmotic flow (EOF) is the main driving force in the collision process (case 1). In our cases, we used both monodispersed Au@SiO2 NPs with sizes (~ 83.5 ± 6.5 nm) bigger than the orifice of nanopore (inner diameter: 40 ± 8 nm) and the conical glass nanopipette-based nanopore with extremely small angle, as shown by transmission electron microscopy (TEM) in Figures 2d & 2e. The structure of the nanopipette is approximately two-dimensional with a quite long sensing zone29. As the length of the tapered nanopore channel increases, the electric field will be distributed in a larger

Figure 3. The analysis of NP collisions with glass nanopore under three different surface charge conditions: (a) (-/-) bare glass nanopore + Au@SiO2-COOH NPs (case 1); (b) (+/-) APTMSmodified glass nanopore + Au@SiO2-COOH NPs (case 2); (c) (/+) bare glass nanopore + Au@SiO2-NH2 NPs (case 3); (d) The bar chart for comparison of collision time; the red represent different charge on the NPs and the green represent different charge on the nanopore wall.

sensing area, which will result in a decrease in the EPF. In contrast, the EOF increases significantly 30. In addition to the nanopore geometry, low ionic strength could also be responsible for the signals. At lower salt concentration, the Debye length increases which generates larger EOF along the pore surface31. Since the EPF is mainly related to the surface charge of NPs while the EOF relies on the surface charge of the pore wall, the NPs collision signals were closely related to the both factors. The collision experiments of Au@SiO2 NPs with glass nanopore under the three different surface charge conditions were therefore conducted. Figure S8 shows the ionic current responses of the Au@SiO2-COOH NPs collision with the bare glass nanopore where the surfaces of NPs and pore wall are both negatively charged (case 1); whereas Figures S9 & S10 show the collision current responses of the other two cases (denoted as case 2 and case 3, respectively) where the surfaces of Au@SiO2 NPs and nanopore wall have invertible opposite charges. Figure S8b displays the scatter plot of ΔI (ΔI is representative of the current blockade) versus the collision time in case 1. Statistical analysis of the NPs collision events in case 1 exhibits collision time at tens of milliseconds level (~ 25.1 ms) (Figure 3a). By changing the surface charge of the nanopore wall to a positive state with the modification of APTMS (case 2), the collision time was reduced to a few milliseconds (~ 5.2 ms) (Figure 3b). Figure S9b shows the scatter plot of ΔI versus the collision time in the case. We also tested the collision current responses of the surface amino-modified NPs (Au@SiO2-NH2 NPs) with the bare nanopipette (case 3), in which the collision time further be reduced to ~ 2.6 ms (Figure 3c). As schematically illustrated in the insets of Figure 3 (a-c), in case 1 (Figure 3a), since the electroosmotic force is opposite to the electrophoretic and electrostatic forces on NPs at -1 V, the relatively stronger inward electroosmotic flow offsets partially the outward electrophoretic migratation and electrostatic repulsion of NPs, leading to weak (and multiple

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Analytical Chemistry successive) collisions with longer collision time (cf. Figure 1b). While in case 2, the electrophoretic, electroosmotic and electrostatic forces were all inversed, but the electroosmotic flow turns weak due to the asymmetry structure of the conical nanopore structure, which resulting in a relatively stronger collision of the NPs with a shortened collision time as compared to case 1. Unsurprisingly, in case 3 since the directions of the three forces are the same (inward), the collision time was further shortened (Figure 3c). This is reasonable since the action and reaction forces are mutually reinforced, so the rebound force (escape force) which made NPs leaving nanopipette opening turns bigger when the collision force is stronger, resulting in shortened collision time. These results verify the combination role of the electrophoretic, electroosmotic and electrostatic forces in the collision behavior of single NPs with nanopores, which in turn can be exploited to discriminate the surface charge information of single NPs and nanopore during the collision process (Figure 3d). To further verify that our nanopore collision method also has the capacity to quantify zeta potential of particles, we performed collision experiments using Au@SiO2 NPs of same sizes but with three different zeta potentials by changing the modified surface groups (-OH, -COOH, -NH2) (cf. Figure S11). As shown in Figure S12, the collision time of the particles showed an almost linear relationship with the zeta potential of the particles. It shows that the developed method is promising for quantifying the zeta potential of nanoparticles. Due to its simplicity and operation convenience, our method would be better than the previously reported technique, which involves recording passage of particles through a nanopore at different voltages32. Influence of Applied Potential on Collision Signal. We further investigated the influence of applied potential on NP collision signals by varying applied voltages and using the same glass nanopipette nanopore. As shown in Figure 4a & 4b, when an arbitrary negative voltage was applied, the dense collisions of individual NPs occurred at the outer orifice of the nanopipette, resulting in intermittent current drops (close to zero). We defined the current value inbetween two adjucent NP collision events as the baseline. The magnitude of ionic current change is mainly related to the baseline. According to previous reports, the baseline mainly related with electrolyte, aperture, voltage and others33. Collision current signals from individual NPs can be characterized by the magnitude of the current drop (ΔI) and collision frequency. Figure S13 shows the amplitude distribution of the collision events at each voltage, fitted by a Gaussian mixture model. With the increase of applied voltage from 400 to 1000 mV at 100 mV increment, the magnitude of the current drop increased from 260, 410, 540, 620, 740, 830 to 1020 pA, respectively. A good linear relationship (Figure 4c) between the current drop and the voltage in the collision was obtained. The consistent current profiles at varied voltages verified that the NPs experienced an elastic collision and the motion of nearby NPs was not severely affected. Meanwhile, the collision frequency of the NPs has been analyzed. Figure S14 displays the linear dependence of the mean collision frequency of the NPs, obtained by calculating the frequency based on the total number of recorded events in 5 s, as a function of voltages.

Figure 4. Carboxyl-modified NP collisions with bare nanopipette at different applied negative potentials. (a-b) Current traces recorded from NPs collisions with NPs concentration of ~ 4.0×1010 particles /mL at different voltages. (c) Current drop versus applied collision potential.

CONCLUSION In this study, the collision of single monodisperse Au@SiO2 NPs (~ 83.5 ± 6.5 nm in diameter) at the outer orifice of a smaller-sized conical glass nanopore (~ 40±8 nm), under three different surface charges, was systematically investigated by the resistive-pulse sensing technique. Due to the tapered channel structure and small size of the nanopore used, the electroosmotic force in the system turns stronger and becomes the main driving force for the NP collision. The resistive-pulse collision signals (duration time) were found depending on the relative forces of electroosmotic, electrophoretic and electrotatic forces being applied, which is dominated by the surface charge properties of the NPs and nanopore wall used. By fixing the size of the nanopore and NPs, the surface charge information of the NPs and nanopore walls can be discriminated by the magnitude and duration of the collision ionic current signals. This study manifested the feasibility of exploring the nanopore-based single NPs collision platform for surface charge analysis of the colloidal nanoparicle system. Owing to the simplicity and robustness of this methodology, we believe that it will be of significant interest to the wide-ranging scientific communities.

ASSOCIATED CONTENT Supporting Information Supporting Information included Figure S1-S15

AUTHOR INFORMATION Corresponding Author *[email protected]

Present Addresses †State Key Lab of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun, Jilin, 130022, P.R. China.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (No. 21675146, 21175125), the Instrument Developing Project of the Chinese Academy of Sciences (Grant No. YZ201666) and Natural Science Foundation of Jilin Province 20160101296JC.

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