Name: a closed-type wireless nanopore electrode for analyzing single nanoparticles
Uploaded: 2019-03-20T13:00:00.000+00:00
Duration: 8 min 31 s
Description: Watch this Scientific Journal Video about A Closed-Type Wireless Nanopore Electrode for Analyzing Single Nanoparticles at JoVE.com

This research was supported by the National Natural Science Foundation of China (61871183&#65292;21834001), Innovation Program of Shanghai Municipal Education Commission (2017-01-07-00-02-E00023), the &#8220;Chen Guang&#8221; Project from the Shanghai Municipal Education Commission and Shanghai Education Development Foundation (17CG27).

1. Preparation of Solutions
NOTE: Pay attention to general safety precautions for all chemicals. Dispose of chemicals in a fume hood, and wear gloves, goggles, and a lab coat. Keep flammable liquids away from fire or sparks. All aqueous solutions were prepared using ultrapure water (18.2 M&#937; cm at 25 &#176;C). The prepared solutions were filtered using a 0.22 &#956;m pore-size filter.
<ol>
	<li>Preparation of KCl solution
	<ol>
		<li>Dissolve 0.074 g of potassium chloride in 100 mL of deionized water.</li>
	</ol>
	</li>
	<li>Preparation of NaBH4 solution 
	<ol>
		<li>Dissolve 0.018 g of sodium borohydride in 10 mL of ethanol.</li>
	</ol>
	</li>
	<li>Preparation of HAuCl4 solution
	<ol>
		<li>Dissolve 0.010 g of potassium chloride in 1 mL of 1% chloroauric acid solution.</li>
	</ol>
	</li>
	<li>Preparation of silicone rubber
	<ol>
		<li>Mix silicone rubber containing Part A and Part B (see Table of Materials) at a ratio of 1:1 by volume.</li>
		<li>Use the mixed silicone rubber to paint a reaction area at the slide immediately during the pot time of 1 min.</li>
		<li>Cure the prepared silicone rubber on the slide for 5 min.</li>
	</ol>
	</li>
	<li>Preparation of gold nanoparticles30
	<ol>
		<li>Add 4.8 mL of chloroauric acid with a mass fraction of 1% into 40 mL of deionized water with vigorous stirring.</li>
		<li>Heat the solution to a boil.</li>
		<li>Quickly add 10 mL of a trisodium citrate solution with a mass fraction of 1% into the solution.</li>
		<li>Heat the solution for an additional 15 min until the final solution is red in color. 
		NOTE: In our case, the chloroauric acid solution was rapidly reduced by trisodium citrate, and it was observed that the solution rapidly changed from clear-yellow to dark-black.</li>
	</ol>
	</li>
</ol>
2. Preparation of Experimental Setup
<ol>
	<li>Preparation of current measurement system
	<ol>
		<li>Turn on the current measurement system containing the current amplifier (see Table of Materials) and low-noise data acquisition system (see Table of Materials)</li>
		<li>Switch on the voltage-clamp mode.</li>
		<li>Set the filter bandwidth to 10 kHz and sampling rate to 100 kHz.</li>
		<li>Assemble a self-designed specific homemade copper cage to shield external noise for experimental cells and the pre-amplifier on the inverted microscope (see Table of Materials).</li>
		<li>Ground the shell of the Faraday cage, shells of the amplifier, and inverted microscope system.</li>
	</ol>
	</li>
	<li>Setup of the dark-field detection system 
	<ol>
		<li>Generation of the gold nanotip inside the nanopipette is monitored by the dark-field microscope. 
		NOTE: An inverted microscope system (see Table of Materials) is used to take images and scatter spectra. A true-color digital CCD camera is employed to take images of the nanopipette and nanopore electrode. A dark-field condenser [numerical aperture (NA) = 0.8&#8211;0.95)] is utilized to form a dark-field illumination. 10X (NA = 0.3), 20X (NA = 0.45), and 40X (NA = 0.6) objectives are used to collect images of the closed-type WNE. Fluorescence detection is used to further verify whether there are gaps between the nanotip and inner wall of the nanopipette. This experiment is performed by another EMCCD (see Table of Materials) also integrated on the inverted microscope, and the excitation light is a built-in mercury lamp with a band-pass filter of 450&#8211;490 nm.</li>
	</ol>
	</li>
</ol>
3. Fabrication of Closed-Type WNE
<ol>
	<li>Fabrication of nanopipettes
	<ol>
		<li>Put the quartz capillaries (see Table of Materials) in a 15 mL centrifuge tube filled with acetone for 10 min of ultrasonic cleaning.</li>
		<li>Pour off&#160;acetone, then add ethanol into the same centrifuge tube.</li>
		<li>Put the centrifuge tube in an ultrasonic cleaner for 10 min of cleaning.</li>
		<li>Put the capillaries into another 15 mL centrifuge tube with deionized water for removal of the ethanol, with 10 min of ultrasonic cleaning.</li>
		<li>Continually ultrasonic clean the capillaries three times with deionized water to remove the residual ethanol.</li>
		<li>Dry the capillaries using a nitrogen gas stream.</li>
		<li>Keep the capillaries in a new, clean centrifuge tube.</li>
		<li>Turn on the CO2 laser puller (see Table of Materials)</li>
		<li>Preheat the puller for 15-20 min to ensure a steady laser power.</li>
		<li>Install the cleaned capillary in the puller.</li>
		<li>Set the pulling parameters of heat, filament, velocity, delay, and pulling force on the panel of the CO2 laser puller for a specific diameter. The detailed parameter for pulling a 30 nm diameter nanopipette in this protocol is shown in Table 1 (Figure 1).</li>
		<li>Fix the prepared nanopipette on a Petri dish with the reusable adhesive (see Table of Materials) for further characterization.</li>
	</ol>
	</li>
	<li>Fabrication of closed-type WNE
	<ol>
		<li>Inject 10 &#956;L of the prepared HAuCl4 solution into the nanopipette with a microloader.</li>
		<li>Centrifuge the nanopipette for 5 min at around 1878 x g for the removal of air bubbles in the nanopipette. 
		NOTE: For this step, we placed the nanopipette with the tip facing down into a homemade holder within a 2 mL centrifuge tube.</li>
		<li>Fix the nanopipette on a coverslip with the prepared silicone rubber (see step 1.4) and define the area inside the nanopipette as the &#8220;cis&#8221; side and outside as the &#8220;trans&#8221; side.</li>
		<li>Wait 5 min until the rubber is cured.</li>
		<li>Put the integrated ensemble on the objective table of the inverted microscope.</li>
		<li>Turn on and adjust the dark-field illumination to focus the nanopipette tip under a 10X microscope objective.</li>
		<li>Change to 20X and 40X objectives for a higher spatial resolution.</li>
		<li>Place one Ag/AgCl electrode inside the nanopipette.</li>
		<li>Place the other grounded Ag/AgCl electrode into the trans side.</li>
		<li>Connect a pair of Ag/AgCl electrodes to the pre-amplifier.</li>
		<li>Turn on the current measurement system and the corresponding software (see Table of Materials) for ionic current recording.</li>
		<li>Set the applied potential to 300 mV.</li>
		<li>Slowly add 150 &#956;L of NaBH4 solution into the trans side to trigger the reaction between HAuCl4 and NaBH4 (Figure 2). 
		NOTE: The reduction of NaBH4 in aqueous solution takes place at a violent reaction rate. Therefore, the generation of H2 from the reduction of NaBH4 may induce a defective structure of the nanotip by the generation of cavities during the gold nanotip growth.</li>
		<li>Simultaneously, electrically and optically record the current trace and the dark-field image/scattering spectra using the current measurement and dark-field detection systems (Figure 3). 
		NOTE: The ethanol solution is volatile under dark-field illumination. Pay attention to the volume of ethanol during the fabrication process.</li>
		<li>Turn off the applied potential after ionic current tracing back to 0 pA.</li>
		<li>Wash the prepared closed-type WNE with flowing deionized water from the bottom to the tip.</li>
	</ol>
	</li>
	<li>Characterization of closed-type WNE
	<ol>
		<li>Characterize closed-type WNE with a scanning electron microscope (SEM), which is a general method for characterization of nanopipettes22,31,32,33,34.</li>
		<li>Use a calcium ion fluorescent experiment to verify the sealing condition of the gold nanotip inside the nanopipette.
		<ol>
			<li>Inject 10 &#956;L of CaCl2 solution into the cis side of the closed-type WNE and Fluo-8 solution into the trans side.</li>
			<li>Connect the Ag/AgCl electrodes to the headstage.</li>
		</ol>
		</li>
		<li>Apply a 400 mV bias potential and use the EMCCD (see Table of Materials) to monitor the fluorescence response at the tip area. Use focus ions beam (FIB) to sculpt the closed-type WNE from the tip to the bottom, then determine the length of the interior metal layer or nanotip with SEM characterization.</li>
	</ol>
	</li>
	<li>Single nanoparticle collision with closed-type WNE
	<ol>
		<li>Change the solution in the trans and cis sides to a KCl solution after fabrication of the closed-type WNE.</li>
		<li>Transfer 50 &#956;L of 30 nm gold nanoparticle solution into the trans side. Record the current signal of single nanoparticle collision events at a potential of 300 mV (Figure 5).</li>
		<li>Change the applied voltage to monitor the frequency, amplitude, and shape change of the current signal.</li>
	</ol>
	</li>
</ol>

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The authors declare no conflicts of interests.

Fabrication of a well-defined nanopipette is the first step in the closed-type WNE fabrication process. By focusing a CO2 laser onto the center of the capillary, one capillary separates into two symmetrical nanopipettes with nanoscale conical tips. The diameter is easily controlled, ranging from 30-200 nm, by adjusting the parameters of the laser puller. It is noted that the parameters for pulling can vary for different pipette pullers. The environmental temperature and humidity can also influence the final di...

Nanoparticles have attracted tremendous attention due to diverse features such as their catalytic ability, particular optical features, electroactivity, and high surface-to-volume ratios1,2,3,4. Electrochemical analysis of single nanoparticles is a direct method for understanding the intrinsic chemical and electrochemical processes at the nanoscale level. To achieve highly sensitive measurements of single nanoparticles, two electrochemical approaches have been previously applied to read out nanoparticle information from current responses5,6,7. One of these approaches involves immobilizing or capturing an individual nanoparticle on the interface of the nanoelectrode for the study of electrocatalysis8,9. The other strategy is driven by single nanoparticle collision with the surface of an electrode, which generates a transient current fluctuation from the dynamic redox process.
Both of these methods require a nanoscale ultrasensitive sensing interface that matches the diameter of single nanoparticles. However, traditional fabrication of nanoelectrodes has mainly incorporated the micro-electromechanical systems (MEMS) or laser pulling techniques, which are tedious and undisciplinable10,11,12,13. For example, MEMS-based fabrication of nanoelectrodes is expensive and requires the use of a clean room, restricting the massive production and popularization of nanoelectrodes. On the other hand, laser pulling fabrication of nanoelectrodes relies heavily on experience of the operators during the sealing and pulling of a metal wire inside the capillary. If the metal wire is not well-sealed in the capillary, the gap between the inner wall of the nanopipette and wire can dramatically introduce excess background current noise and enlarge the electroactive sensing area. These drawbacks largely decrease the sensitivity of the nanoelectrode. On the other hand, the existence of a gap can enlarge the electrode area and reduce sensitivity of the nanoelectrode. As a consequence, it is hard to guarantee a reproducible performance due to the uncontrollable electrode morphologies in each fabrication process14,15. Therefore, a general fabrication method of nanoelectrodes with excellent reproducibility is urgently needed to facilitate electrochemical exploration of the intrinsic features of single nanoparticles.
Recently, the nanopore technique has been developed as an elegant and label-free approach for single molecule analysis16,17,18,19,20. Owing to its controllable fabrication, the nanopipette provides a nanoscale confinement, with a uniform diameter ranging from 30-200 nm by a laser capillary puller21,22,23,24. Moreover, this simple and reproducible fabrication procedure ensures the generalization of the nanopipette. Recently, we proposed a wireless nanopore electrode (WNE), which does not require the sealing of a metal wire inside the nanopipette. Through a facile and reproducible fabrication process, the WNE possesses a nanoscale metal deposition within the nanopipette to form an electroactive interface25,26,27,28. Since the WNE possesses a well-defined structure and uniform morphology of its confinements, it achieves high current resolution as well as low resistance-capacitance (RC) time constant for performing high temporal resolution. We previously reported two types of WNEs, open-type and closed-type, for realizing single entity analysis. The open-type WNE employs a nanometal layer deposited on the inner wall of a nanopipette, which converts the faradic current of a single entity to the ionic current response26. Usually, the diameter of an open-type WNE is around 100 nm. To further decrease the diameter of WNE, we presented the closed-type WNE, in which a solid metal nanotip fully occupies the nanopipette tip through a chemical-electrochemical approach. This method can rapidly generate a 30 nm gold nanotip inside a nanopore confinement. The well-defined interface at the tip area of a closed-type WNE ensures a high signal-to-noise ratio for electrochemical measurements of single nanoparticles. As a charged gold nanoparticle collides with the closed-type WNE, an ultrafast charging-discharging process at the tip interface induces a capacitive feedback response (CFR) in the ionic current trace. Compared to a previous single nanoparticle collision study via a nanoelectrode with metal wire inside29, the closed-type WNE showed a higher current resolution of 0.6 pA &#177; 0.1 pA (RMS) and higher temporal resolution of 0.01 ms.
Herein, we describe a detailed fabrication procedure for a closed-type WNE that has highly controlled dimensions and outstanding reproducibility. In this protocol, a simple reaction between AuCl4- and BH4- is designed to generate a gold nanotip that completely blocks the orifice of a nanopipette. Then, bipolar electrochemistry is adopted for continuous growth of a gold nanotip that reaches the length of several micrometers inside the nanopipette. This simple procedure enables the implementation of this nanoelectrode fabrication, which can be carried out in any general chemistry laboratory without a clean room and expensive equipment. To determine the size, morphology, and inner structure of a closed-type WNE, this protocol provides a detailed characterization procedure with use of a scanning electron microscope (SEM) and fluorescence spectroscopy. One recent example is highlighted, which directly measures the intrinsic and dynamic interactions of gold nanoparticles (AuNPs) colliding towards the nanointerface of a closed-type WNE. We believe that the closed-type WNE may pave a new path for future electrochemical studies of living cells, nanomaterials, and sensors at single-entity levels.

<table><tbody><tr><td>Acetone</td><td>Sigma-Aldrich</td><td>650501</td><td>Highly flammable and volatile</td></tr><tr><td>Analytical balance</td><td>Mettler Toledo</td><td>ME104E</td><td></td></tr><tr><td>Axopatch 200B amplifier</td><td>Molecular Devices</td><td></td><td></td></tr><tr><td>Blu-Tack reusable adhesive</td><td>Bostik</td><td></td><td></td></tr><tr><td>Centrifuge tube</td><td>Corning Inc.</td><td></td><td>Centrifuge Tubes with CentriStar Cap, 15 ml</td></tr><tr><td>Chloroauric acid</td><td>Energy Chemical</td><td>E0601760010</td><td>HAuCl&lt;sub&gt;4&lt;/sub&gt;</td></tr><tr><td>Clampfit 10.4 software</td><td>Molecular Devices</td><td></td><td></td></tr><tr><td>Digidata 1550A digitizer</td><td>Molecular Devices</td><td></td><td></td></tr><tr><td>DS Fi1c true-color CCD camera</td><td>Nikon</td><td></td><td></td></tr><tr><td>Ecoflex 5 Addtion cure silicone rubber</td><td>Smooth-On</td><td>17050377</td><td></td></tr><tr><td>Eppendorf Reference 2 pipettes</td><td>Eppendorf</td><td>492000904</td><td>10, 100 and 1000 &amp;micro;L</td></tr><tr><td>Ethanol</td><td>Sigma-Aldrich</td><td>24102</td><td>Highly flammable and volatile</td></tr><tr><td>Faraday cage</td><td></td><td></td><td>Copper</td></tr><tr><td>iXon 888 EMCCD</td><td>Andor</td><td></td><td></td></tr><tr><td>Microcentrifuge tubes</td><td>Axygen Scientific</td><td></td><td>0.6, 1.5 and 2.0 mL</td></tr><tr><td>Microloader</td><td>Eppendorf</td><td>5242 956.003</td><td>20 &amp;micro;L</td></tr><tr><td>Microscope Cover Glass</td><td>Fisher Scientific</td><td>LOT 16938</td><td>20 mm*60 mm-1 mm thick</td></tr><tr><td>Milli-Q water purifier</td><td>Millipore</td><td>SIMS00000</td><td>Denton Electron Beam Evaporator</td></tr><tr><td>P-2000 laser puller</td><td>Sutter Instrument</td><td></td><td></td></tr><tr><td>Pipette tips</td><td>Axygen Scientific</td><td></td><td>10, 200 and 1,000 &amp;micro;L</td></tr><tr><td>Potassium chloride,+D25+A2:F2+A2:F25</td><td>Sigma Aldrich</td><td>P9333-500G</td><td>KCl</td></tr><tr><td>Quartz pipettes</td><td>Sutter</td><td>QF100-50-7.5</td><td>O.D.:1.0 mm, I.D.:0.5 mm, 75 mm length</td></tr><tr><td>Refrigerator</td><td>Siemens</td><td></td><td></td></tr><tr><td>Silicone thinner</td><td>Smooth-On</td><td>1506330</td><td></td></tr><tr><td>Silver wire</td><td>Alfa Aesar</td><td>11466</td><td></td></tr><tr><td>Sodium borohydride,</td><td>Tianlian Chem. Tech.</td><td>71320</td><td>NaBH&lt;sub&gt;4&lt;/sub&gt;</td></tr><tr><td>Ti-U inverted dark-field microscope</td><td>Nikon</td><td></td><td></td></tr></tbody></table>

a closed-type wireless nanopore electrode for analyzing single nanoparticles

Measuring the intrinsic features of single nanoparticles by nanoelectrochemistry holds deep fundamental importance and has potential impacts in nanoscience. However, electrochemically analyzing single nanoparticles is challenging, as the sensing nanointerface is uncontrollable. To address this challenge, we describe here the fabrication and characterization of a closed-type wireless nanopore electrode (WNE) that exhibits a highly controllable morphology and outstanding reproducibility. The facile fabrication of WNE enables the preparation of well-defined nanoelectrodes in a general chemistry laboratory without the use of a clean room and expensive equipment. One application of a 30 nm closed-type WNE in analysis of single gold nanoparticles in the mixture is also highlighted, which shows a high current resolution of 0.6 pA and high temporal resolution of 0.01 ms. Accompanied by their excellent morphology and small diameters, more potential applications of closed-type WNEs can be expanded from nanoparticle characterization to single molecule/ion detection and single-cell probing.

We demonstrate a facile approach to fabricate a well-defined 30 nm wireless nanopore electrode based on a quartz conical nanopipette. The fabrication of a nanopipette is demonstrated in Figure 1, which includes three main steps. A microcapillary with an inner diameter of 0.5 mm and outer diameter of 1.0 mm is fixed in the puller, then a laser is focused on the center of the capillary to melt the quartz. By applying forces to the terminals of the capillary, it...

Watch this Scientific Journal Video about A Closed-Type Wireless Nanopore Electrode for Analyzing Single Nanoparticles at JoVE.com

A Closed-Type Wireless Nanopore Electrode for Analyzing Single Nanoparticles

Here, we present a protocol for fabrication of a closed-type wireless nanopore electrode and subsequent electrochemical measurement of single nanoparticle collisions.

Measuring the intrinsic features of single nanoparticles by nanoelectrochemistry holds deep fundamental importance and has potential impacts in ...

a-closed-type-wireless-nanopore-electrode-for-analyzing-single

Universidad San Sebastian

Research

JoVE Journal

Chemistry

7.4K Views.  East China University of Science and Technology. Here, we present a protocol for fabrication of a closed-type wireless nanopore electrode and subsequent electrochemical measurement of single nanoparticle collisions.

Video: A Closed-Type Wireless Nanopore Electrode for Analyzing Single Nanoparticles

Fluorescent Nanoparticles for the Measurement of Ion Concentration in Biological Systems

Tightly regulated ion homeostasis throughout the body is necessary for the prevention of such debilitating states as dehydration.1 In contrast, rapid ion fluxes at the cellular level are required for initiating action potentials in excitable cells.2 Sodium regulation plays an important role in both of these cases; however, no method currently exists for continuously monitoring sodium levels in vivo 3 and intracellular sodium probes 4 do not provide similar detailed results as calcium probes. In an effort to fill both of these voids, fluorescent nanosensors have been developed that can monitor sodium concentrations in vitro and in vivo.5,6 These sensors are based on ion-selective optode technology and consist of plasticized polymeric particles in which sodium specific recognition elements, pH-sensitive fluorophores, and additives are embedded.7-9 Mechanistically, the sodium recognition element extracts sodium into the sensor. 10 This extraction causes the pH-sensitive fluorophore to release a hydrogen ion to maintain charge neutrality within the sensor which causes a change in fluorescence. The sodium sensors are reversible and selective for sodium over potassium even at high intracellular concentrations.6 They are approximately 120 nm in diameter and are coated with polyethylene glycol to impart biocompatibility. Using microinjection techniques, the sensors can be delivered into the cytoplasm of cells where they have been shown to monitor the temporal and spatial sodium dynamics of beating cardiac myocytes.11 Additionally, they have also tracked real-time changes in sodium concentrations in vivo when injected subcutaneously into mice.3 Herein, we explain in detail and demonstrate the methodology for fabricating fluorescent sodium nanosensors and briefly demonstrate the biological applications our lab uses the nanosensors for: the microinjection of the sensors into cells; and the subcutaneous injection of the sensors into mice.

Fluorescent nanoparticles produced in our lab are used for imaging ion concentrations and ion fluxes in biological systems such as cells during signaling and interstitial fluid during physiological homeostasis.

Tightly regulated ion homeostasis throughout the body is necessary for the prevention of such debilitating states as dehydration.1 In contrast, rapid ion ...

Bioengineering

Fine-tuning the Size and Minimizing the Noise of Solid-state Nanopores

Solid-state nanopores have emerged as a versatile tool for the characterization of single biomolecules such as nucleic acids and proteins1. However, the creation of a nanopore in a thin insulating membrane remains challenging. Fabrication methods involving specialized focused electron beam systems can produce well-defined nanopores, but yield of reliable and low-noise nanopores in commercially available membranes remains low2,3 and size control is nontrivial4,5. Here, the application of high electric fields to fine-tune the size of the nanopore while ensuring optimal low-noise performance is demonstrated. These short pulses of high electric field are used to produce a pristine electrical signal and allow for enlarging of nanopores with subnanometer precision upon prolonged exposure. This method is performed in situ in an aqueous environment using standard laboratory equipment, improving the yield and reproducibility of solid-state nanopore fabrication.

A methodology for preparing solid-state nanopores in solution for biomolecular translocation experiments is presented. By applying short pulses of high electric fields, the nanopore diameter can be controllably enlarged with subnanometer precision and its electrical noise characteristics significantly improved. This procedure is performed in situ using standard laboratory equipment under experimental conditions.

Solid-state nanopores have emerged as a versatile tool for the characterization of single biomolecules such as nucleic acids and proteins1. However, the ...

Engineering

Monitoring Protein Adsorption with Solid-state Nanopores

Solid-state nanopores have been used to perform measurements at the single-molecule level to examine the local structure and flexibility of nucleic acids 1-6, the unfolding of proteins 7, and binding affinity of different ligands 8. By coupling these nanopores to the resistive-pulse technique 9-12, such measurements can be done under a wide variety of conditions and without the need for labeling 3. In the resistive-pulse technique, an ionic salt solution is introduced on both sides of the nanopore. Therefore, ions are driven from one side of the chamber to the other by an applied transmembrane potential, resulting in a steady current. The partitioning of an analyte into the nanopore causes a well-defined deflection in this current, which can be analyzed to extract single-molecule information. Using this technique, the adsorption of single proteins to the nanopore walls can be monitored under a wide range of conditions 13. Protein adsorption is growing in importance, because as microfluidic devices shrink in size, the interaction of these systems with single proteins becomes a concern. This protocol describes a rapid assay for protein binding to nitride films, which can readily be extended to other thin films amenable to nanopore drilling, or to functionalized nitride surfaces. A variety of proteins may be explored under a wide range of solutions and denaturing conditions. Additionally, this protocol may be used to explore more basic problems using nanopore spectroscopy.

A method of using solid-state nanopores to monitor the non-specific adsorption of proteins onto an inorganic surface is described. The method employs the resistive-pulse principle, allowing for the adsorption to be probed in real-time and at the single-molecule level. Because the process of single protein adsorption is far from equilibrium, we propose the employment of parallel arrays of synthetic nanopores, enabling for the quantitative determination of the apparent first-order reaction rate constant of protein adsorption as well as and the Langmuir adsorption constant.

Solid-state nanopores have been used to perform measurements at the single-molecule level to examine the local structure and flexibility of nucleic acids ...

Fabrication of Carbon Nanotube High-Frequency Nanoelectronic Biosensor for Sensing in High Ionic Strength Solutions

The unique electronic properties and high surface-to-volume ratios of single-walled carbon nanotubes (SWNT) and semiconductor nanowires (NW) 1-4 make them good candidates for high sensitivity biosensors. When a charged molecule binds to such a sensor surface, it alters the carrier density5 in the sensor, resulting in changes in its DC conductance. However, in an ionic solution a charged surface also attracts counter-ions from the solution, forming an electrical double layer (EDL). This EDL effectively screens off the charge, and in physiologically relevant conditions ~100 millimolar (mM), the characteristic charge screening length (Debye length) is less than a nanometer (nm). Thus, in high ionic strength solutions, charge based (DC) detection is fundamentally impeded6-8.
We overcome charge screening effects by detecting molecular dipoles rather than charges at high frequency, by operating carbon nanotube field effect transistors as high frequency mixers9-11. At high frequencies, the AC drive force can no longer overcome the solution drag and the ions in solution do not have sufficient time to form the EDL. Further, frequency mixing technique allows us to operate at frequencies high enough to overcome ionic screening, and yet detect the sensing signals at lower frequencies11-12. Also, the high transconductance of SWNT transistors provides an internal gain for the sensing signal, which obviates the need for external signal amplifier.
Here, we describe the protocol to (a) fabricate SWNT transistors, (b) functionalize biomolecules to the nanotube13, (c) design and stamp a poly-dimethylsiloxane (PDMS) micro-fluidic chamber14 onto the device, and (d) carry out high frequency sensing in different ionic strength solutions11.

We describe the device fabrication and measurement protocol for carbon nanotube based high frequency biosensors. The high frequency sensing technique mitigates the fundamental ionic (Debye) screening effect and allows nanotube biosensor to be operated in high ionic strength solutions where conventional electronic biosensors fail. Our technology provides a unique platform for point-of-care (POC) electronic biosensors operating in physiologically relevant conditions.

The unique electronic properties and high surface-to-volume ratios of single-walled carbon nanotubes (SWNT) and semiconductor nanowires (NW) 1-4 make them ...

Flow-assisted Dielectrophoresis: A Low Cost Method for the Fabrication of High Performance Solution-processable Nanowire Devices

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of nanowires. DEP is used for nanowire analysis, characterization, and for solution-based fabrication of semiconducting devices. The method works by applying an alternating electric field between metallic electrodes. The nanowire formulation is then dropped onto the electrodes which are on an inclined surface to create a flow of the formulation using gravity. The nanowires then align along the gradient of the electric field and in the direction of the liquid flow. The frequency of the field can be adjusted to select nanowires with superior conductivity and lower trap density. 
In this work, flow-assisted DEP is used to create nanowire field effect transistors. Flow-assisted DEP has several advantages: it allows selection of nanowire electrical properties; control of nanowire length; placement of nanowires in specific areas; control of orientation of nanowires; and control of nanowire density in the device. 
The technique can be expanded to many other applications such as gas sensors and microwave switches. The technique is efficient, quick, reproducible, and it uses a minimal amount of dilute solution making it ideal for the testing of novel nanomaterials. Wafer scale assembly of nanowire devices can also be achieved using this technique, allowing large numbers of samples for testing and large-area electronic applications.

In this paper, flow assisted dielectrophoresis is demonstrated for the self-assembly of nanowire devices. The fabrication of a silicon nanowire field effect transistor is shown as an example.

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of ...

High Resolution Physical Characterization of Single Metallic Nanoparticles

Individual molecules can be detected and characterized by measuring the degree by which they reduce the ionic current flowing through a single nanometer-scale pore. The signal is characteristic of the molecule&#39;s physicochemical properties and its interactions with the pore. We demonstrate that the nanopore formed by the bacterial protein exotoxin Staphylococcus aureus alpha hemolysin (&#945;HL) can detect polyoxometalates (POMs, anionic metal oxygen clusters), at the single molecule limit. Moreover, multiple degradation products of 12-phosphotungstic acid POM (PTA, H3PW12O40) in solution are simultaneously measured. The single molecule sensitivity of the nanopore method allows for POMs to be characterized at significantly lower concentrations than required for nuclear magnetic resonance (NMR) spectroscopy. This technique could serve as a new tool for chemists to study the molecular properties of polyoxometalates or other metallic clusters, to better understand POM synthetic processes, and possibly improve their yield. Hypothetically, the location of a given atom, or the rotation of a fragment in the molecule, and the metal oxidation state could be investigated with this method. In addition, this new technique has the advantage of allowing the real-time monitoring of molecules in solution.&#160;

Here, we present a protocol to detect discrete metal oxygen clusters, polyoxometalates (POMs), at the single molecule limit using a biological nanopore-based electronic platform. The method provides a complementary approach to traditional analytical chemistry tools used in the study of these molecules.

Individual molecules can be detected and characterized by measuring the degree by which they reduce the ionic current flowing through a single ...

Iron Nanowire Fabrication by Nano-Porous Anodized Aluminum and its Characterization

Magnetic nanowires possess unique properties that have attracted the interest of different fields of research, including basic physics, biomedicine, and data storage. We demonstrate a fabrication method for iron (Fe) nanowires&#160;via electrochemical deposition into anodic&#160;alumina oxide (AAO) templates. The templates are fabricated by anodization of aluminum (Al) discs, and the pore length and diameter are controlled by changing the anodizing conditions. Pores with an average diameter of around 120 nm are created using oxalic acid as the electrolyte. Using this method, cylindrical nanowires are synthesized, which are released by dissolving the alumina using a selective chemical etchant.

In this work, we describe a&#160;protocol to fabricate iron nanowires, including the formation of the porous alumina membrane that is used as the template, electrodeposition into templates using electrolyte solution, and the release of the nanowires into the solution.

Magnetic nanowires possess unique properties that have attracted the interest of different fields of research, including basic physics, biomedicine, and ...

Determination of Zeta Potential via Nanoparticle Translocation Velocities through a Tunable Nanopore: Using DNA-modified Particles as an Example

Nanopore technologies, known collectively as Resistive Pulse Sensors (RPS), are being used to detect, quantify and characterize proteins, molecules and nanoparticles. Tunable resistive pulse sensing (TRPS) is a relatively recent adaptation to RPS that incorporates a tunable pore that can be altered in real time. Here, we use TRPS to monitor the translocation times of DNA-modified nanoparticles as they traverse the tunable pore membrane as a function of DNA concentration and structure (i.e., single-stranded to double-stranded DNA).
TRPS is based on two Ag/AgCl electrodes, separated by an elastomeric pore membrane that establishes a stable ionic current upon an applied electric field. Unlike various optical-based particle characterization technologies, TRPS can characterize individual particles amongst a sample population, allowing for multimodal samples to be analyzed with ease. Here, we demonstrate zeta potential measurements via particle translocation velocities of known standards and apply these to sample analyte translocation times, thus resulting in measuring the zeta potential of those analytes.
As well as acquiring mean zeta potential values, the samples are all measured using a particle-by-particle perspective exhibiting more information on a given sample through sample population distributions, for example. Of such, this method demonstrates potential within sensing applications for both medical and environmental fields.

Here we use a polyurethane tunable nanopore integrated into a resistive pulse sensing technique to characterize nanoparticles surface chemistry via the measurement of particle translocation velocities, which can be used to determine the zeta potential of individual nanoparticles.

Nanopore technologies, known collectively as Resistive Pulse Sensors (RPS), are being used to detect, quantify and characterize proteins, molecules and ...

﻿Analyzing the Heterogeneous Performance of Single Nanoparticles with Electrochemical Methods

Tracking Electrochemistry on Single Nanoparticles with Surface-Enhanced Raman Scattering Spectroscopy and Microscopy

Studying electrochemical reactions on single nanoparticles is important to understand the heterogeneous performance of individual nanoparticles. This nanoscale heterogeneity remains hidden during the ensemble-averaged characterization of nanoparticles. Electrochemical techniques have been developed to measure currents from single nanoparticles but do not provide information about the structure and identity of the molecules that undergo reactions at the electrode surface. Optical techniques such as surface-enhanced Raman scattering (SERS) microscopy and spectroscopy can detect electrochemical events on individual nanoparticles while simultaneously providing information on the vibrational modes of electrode surface species. In this paper, a protocol to track the electrochemical oxidation-reduction of Nile Blue (NB) on single Ag nanoparticles using SERS microscopy and spectroscopy is demonstrated. First, a detailed protocol for fabricating Ag nanoparticles on a smooth and semi-transparent Ag film is described. A dipolar plasmon mode aligned along the optical axis is formed between a single Ag nanoparticle and Ag film. The SERS emission from NB fixed between the nanoparticle and the film is coupled into the plasmon mode, and the high-angle emission is collected by a microscope objective to form a donut-shaped emission pattern. These donut-shaped SERS emission patterns allow for the unambiguous identification of single nanoparticles on the substrate, from which the SERS spectra can be collected. In this work, a method for employing the SERS substrate as a working electrode in an electrochemical cell compatible with an inverted optical microscope is provided. Finally, tracking the electrochemical oxidation-reduction of NB molecules on an individual Ag nanoparticle is shown. The setup and the protocol described here can be modified to study various electrochemical reactions on individual nanoparticles.

Author Spotlight: Tracking Electrochemistry on Single Nanoparticles with Surface-Enhanced Raman Scattering Spectroscopy and Microscopy  

The protocol describes how to monitor electrochemical events on single nanoparticles using surface-enhanced Raman scattering spectroscopy and imaging.

Studying electrochemical reactions on single nanoparticles is important to understand the heterogeneous performance of individual nanoparticles. This ...

Monitoring Conformational Dynamics of Single Unmodified Proteins using Plasmonic Nanotweezers

Current single-molecule techniques to characterize proteins typically require labels, tethers, or the use of non-native solution conditions. Such changes can alter protein biophysics and reduce the usefulness of the data acquired. Plasmonic nanotweezers is a technique that uses localized surface plasmon resonance (LSPR) on gold nanostructures to enhance the electric field within a confined hotspot region. This field enhancement allows for the use of low laser powers to trap single nanoparticles far smaller than conventional optical tweezers, down to only a few nanometers in diameter, such as single proteins. Trapping of single protein molecules within the hotspot region induces a shift in the local refractive index (nprotein &#62; nwater), altering light scattering as a product of the molecule&#39;s polarisability, which is affected by its volume, shape anisotropy, and refractive index. An avalanche photodiode (APD) collects the subsequent changes in light scattering. These alterations can then be analyzed to determine changes in the trapped molecule, including its size, global conformation, and dynamics of conformational change over time. The incorporation of microfluidics within the system allows for controlled environmental changes and real-time monitoring of their subsequent effects on the molecule. In this protocol, we demonstrate the steps to trap single protein molecules, alter their environmental solution conditions, and monitor their corresponding conformational changes using a plasmonic nanotweezers system.

Plasmonic nanotweezers use localized surface plasmon resonance in gold nanostructures to trap single nanoparticles, including proteins, within a nanometer-scale optical field. Changes in the scattered signal reveal protein presence and conformational dynamics, enabling monitoring without fluorophore modifications or surface tethering.

Current single-molecule techniques to characterize proteins typically require labels, tethers, or the use of non-native solution conditions. Such changes ...