Letter Cite This: Anal. Chem. XXXX, XXX, XXX−XXX
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Characterization of the Dynamic Growth of the Nanobubble within the Confined Glass Nanopore Yong-Xu Hu, Yi-Lun Ying,* Rui Gao, Ru-Jia Yu, and Yi-Tao Long* Key Laboratory for Advanced Materials & School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China
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ABSTRACT: The evolution of the nanobubble from gas molecules in liquid phase is of fundamental interest. However, the lack of sensitive tools hinders the study of growth dynamic of the bubble at nanoscale. Here, we employed a confined glass nanopore to realtime monitor the dynamics behavior of a single nanobubble generated by the reaction between NaBH4 and H2O. By analyzing the characteristic ionic current signal, the formation time and growth time of a single nanobubble could be estimated as 200 and 21 ms, respectively. Further, the nanopore size has been altered to modulate the growth behavior of the nanobubble. The results demonstrate the capability of the nanopore for sensitively tracking the behavior of single nanobubbles in liquid phase, which provides a powerful method for further understanding nanobubble evolution.
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approach for achieving single analyte discrimination on size, shape, redox properties, and charge density.20−23 Take advantage of confined space and charged interior, the glass nanopore permits the sensitive measurement of the ionic current changes induced by volume exclusion or surface charge of the analyte at nanoscale. For example, the compression and partial dehydration of negatively charged microgel through the nanopore could be measured by dynamic changes in the conductivity.24 In addition, the glass nanopore could serve as a nanoreactor to monitor the formation and dissolution of nanoprecipitation at the tip of the nanopore by small current oscillation.25,26 These results demonstrate the capacity of the glass nanopore as a powerful approach for single molecule/ nanoparticle detection with high spatial and temporal resolution. To electrochemically monitor the growth of nanobubbles, we employed here the confined glass nanopore as a nanoreactor to generate the nanobubble at its tip by inducing a chemical reaction between NaBH4 and H2O (Figure 1a,b). The reaction between NaBH4 and H2O is shown in Figure 1c. The growth of nanobubbles could be realtime visualized into the biphasic pulse with ionic current enhancement followed by a decrease in the current. Therefore, the evolution of the nanobubbles and its dynamic behavior in nanoscale space could be investigated with high temporal and spatial resolution. A diagram of our setup is shown in Figure 1a. A cylindricalshaped glass nanopore was fabricated with a radius of ∼45 nm
anoscale bubbles are closely related to applications in many fields of biomedical engineering, nanomaterials, and industrial processes such as the design of biosensor/ biochip fluidic microchannels, ultrasound diagnostics, local drug delivery, and nanodevice fabrication.1,2 For example, the existence of small bubbles on the surface of a DNA chip will affect the hybridization process and lower the accuracy of the DNA tests.3 Therefore, the measurement and understanding of the dynamics behavior of nanobubbles are of fundamental interests. The visualization of micro- or nano-gas bubbles formed on hydrophobic solid surfaces have been reported by using direct imaging such as atomic force microscopy (AFM)4,5 and optical technology.6,7 Recently, the electrochemical method has also been demonstrated as a sensitive approach for monitoring the generation and nucleation process of the nanobubble on the solid electrode surface.8−11 Nevertheless, for the unpinned nanobubble in bulk solution, the growth dynamics have not been fully understood due to the inferior spatial and time resolution of present measurement methods.6,12 Traditional dynamic light scattering (DLS) can only give the average size distribution of particles in the bulk. The long scanning time of the AFM method prevents the study of short time dynamics of nanobubbles.13 Optical techniques with less effect on the morphology of nanobubbles cannot probe the gas evolution at sufficient small length scale. Therefore, there is an urgent need to develop the proper tools to characterize nanobubbles in the liquid phase with high temporal and spatial resolution. Nanopore is a promising technique that offers label-free detection of single analytes in liquid environments.14−19 With asymmetrical conical shape and easy controlled nanoscale diameter, the glass nanopore has been utilized as a simple © XXXX American Chemical Society
Received: August 28, 2018 Accepted: October 8, 2018 Published: October 8, 2018 A
DOI: 10.1021/acs.analchem.8b03923 Anal. Chem. XXXX, XXX, XXX−XXX
Letter
Analytical Chemistry
Therefore, it can be suggested that the nanobubble is generated at the tip sensing zone of the glass nanopore in our experiment. An expanded biphasic signal in Figure 2a and b shows that the ionic current through the nanopore initially experiences a rapid increase (i−ii) and then rapidly decreases to a value slightly lower than the baseline current (ii−iii). Then, the ionic current slowly increases back to the baseline (iii−iv, Figure 2a,b). To better show the degree of current increase and decrease, we use ΔIC and ΔIR to denote the peak heights (with respect to baseline current I0) for the first and second phases of a biphasic signal, respectively. This biphasic pulse usually suggests competing effects between surface charge-induced ion accumulation and volume exclusion on the nanopore current.29 It has been reported that the nanobubble carries negative charges under neutral conditions due to the absorption of OH−.30 In our detection system, the chemical reaction between NaBH4 and H2O leads to a surplus of BO2−. Thus, the absorption of BO2− on the gas/water interface would result in a negative surface charge of nanobubbles, which could increase the ion density inside the pore. As a consequence, the conductive ionic current could be attributed to the ion accumulation at the tip of the nanopore induced by negative surface charge carried by the nanobubble. With the growth of the nanobubble, volume exclusion of the nanobubble to the electrolyte in the nanopore gradually becomes dominate and effectively hinders the transport of electrolyte ions, thereby resulting in the resistive ionic current. Therefore, the surface charge and volume information on the nanobubble are reflected by the ionic current event. The scatter plot of ΔIC/ I0 versus ΔIR/I0 and histogram in Figure 2c indicate the conductive degree is concentrated at 1.26, whereas the resistive pulse degree presents a much lower value of 0.17. To support our speculation on the origin of biphasic pulselike signals, we performed the finite element simulation at 500 mV to investigate the effect of the growth process on the ionic current modulation. The bubble radius-current curve was simulated by introducing the nanobubble with various surface charge densities into the tip of the nanopore. We assume the nanobubble has a spherical geometry and carries constant surface charge density during the growth process. According to a previous study,10 a gas bubble nucleus would form in solution first; then, the increasing supply of hydrogen results in the growth of nanobubbles. The simulation result reveals the small nanobubble (
