A Closed-Type Wireless Nanopore Electrode for Analyzing Single Nanoparticles

Electrochemically measuring the intrinsic feature of single nanoparticle is of great importance in nanoscience. This method demonstrate a simple but highly reproducible way to build a wireless nanopore electrode for rapid single nanoparticle analysis. The size of the nanoelectrode can reach as much as 30 nanometers by this simple fabrication method.

The current resolution and the temporal resolution during the analysis are 0.6 picoamps and 0.01 milliseconds, respectively. It is anticipated that this wireless nanopore electrode will be utilized for in vivo and non-invasive secular analysis because of the nanoscale size of the nanoelectrode tip. The localized surface plasmonic resonance property of the gold nano tip and the perfect optical vision of the cross nanopipettes could enable electric optical detection at nano-scale.

Researchers interested in the fabrication of wireless nanopore electrode should master the nanopipette pouring process. Because this is a crucial step in the procedure. They should pay attention to the environmental temperature and the humidity when pouring the pipette.

First add 4.8 milliliters of chloroauric acid with a mass fraction of 1%to 40 milliliters of deionized water with vigorous stirring. Then heat the solution to a boil. Quickly add 10 milliliters of a trisodium citrate solution with a mass fraction of 1%into the solution.

Heat the solution for an additional 15 minutes until the final solution is red in color. Place quartz capillaries in a 15 milliliter centrifuge tube filled with acetone and clean in an ultrasonic cleaner for ten minutes. When finished, remove the acetone and add ethanol.

Then place the centrifuge tube in the ultrasonic cleaner for an additional ten minutes of cleaning. Next, place the quartz capillaries into another 15 milliliter centrifuge tube with deionized water for removal of the ethanol, and perform ultrasonic cleaning for 10 minutes. Dry the quartz capillaries using a nitrogen gas stream and store them in a clean centrifuge tube.

Following this, turn on a carbon dioxide laser puller and pre-heat for 15-20 minutes to ensure a steady laser power. Install a clean quartz capillary in the pre-heated carbon dioxide laser puller. Set the pulling parameters of heat, filament, velocity, delay, and pulling force on the panel of the carbon dioxide laser puller for a specific diameter.

Fix the prepared nanopipette on a petri dish with a reusable adhesive for further characterization. Inject 10 microliters of the prepared chloroauric acid solution into the nanopipette with a microloader. Centrifuge the nanopipette for five minutes at around 1, 878 times G for the removal of air bubbles in the nanopipette.

Following centrifugation, fix the nanopipette on a cover slip with previously prepared silicone rubber, and define the area inside the nanopipette as the cis side and outside as the trans side. After waiting five minutes for the rubber to cure, place the integrated ensemble on the objective table of an inverted microscope. Turn on and adjust the dark field illumination to focus the nanopipette tip under a 10x microscope objective.

Change to a 40x objective for higher spacial resolution. Next place one silver silver chloride electrode inside the nanopipette. Then place a second grounded silver silver chloride electrode on the trans side.

Connect the silver silver chloride electrodes to a pre amplifier. Turn on the current measurement system and the corresponding software for ionic current recording. Then set the applied potential to 300 millivolts.

Now slowly add 150 microliters of sodium borohydride solution to the trans side to trigger the reaction between the chloroauric acid and the sodium borohydride. Simultaneously, electrically and optically record the current trace and the dark field image scattering spectra using the current measurement and dark field detection systems. Turn off the applied potential after the ionic current tracing returns to zero picoamps.

Wash the prepared closed type WNE with flowing deionized water from the bottom to the tip. Change the solution in the trans and cis sides to a potassium chloride solution after fabrication of the closed type WNE. Transfer 50 microliters of the 30 nanomolar gold nanopartical solution to the trans side.

Then record the current signal of single nanoparticle collision events at a potential of 300 millivolts. Finally, change the applied voltage to monitor the frequency, amplitude, and shape change of the current signal. The fabrication of a nanopipette includes three main steps.

A microcapillary with an inner diameter of 0.5 millimeter and and outer diameter of 1 milliliter is fixed in the puller, and a laser is then focused on the center of the capillary to melt the quartz. By applying forces to the terminals of the capillary, it finally separates and forms two parts with nanoscale conical tips. The procedure of generating a gold nanotip inside the nanopipette tip, after the pulling process is shown here.

An in situ characterization system was used to monitor the fabrication process of the closed type WNE by simultaneous recording of the current response and dark field images. Top view SEM images of the bare nanopipette and closed type WNE are shown here after focus ions beam splitting a side view SEM image provides the morphology of the gold nanotip inside the closed type WNE. In the single nanoparticle collision experiments, the gold nanoparticles are added to the trans side of the WNE.

The outstanding noise performance of this CNE uncovers the hidden signals with a high signal frequency. When generating the gold nano tip, a low applied potential should be used to generate the electrochemical interface. A high applied potential could speed up the generation of gold and result in defective structures in the nano tip.

The high contrast resolution and the high special resolution of this method can help researchers further understand the electron transfer process at the nano scale. Sodium borohydride is dangerous and will react violently with water. Please be careful when preparing the Sodium borohydride solution.