Atomic Force Microscopy (AFM) Tip based Nanoelectrode with Hydrogel Electrolyte and Application to Single-Nanoparticle Electrochemistry
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2024-02-23 |
| Journal | Journal of Electrochemical Science and Technology |
| Authors | Kyungsoon Park, Thanh Duc Dinh, Seongpil Hwang |
| Institutions | Korea University, Jeju National University |
| Citations | 2 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research details a novel, facile, and cost-effective method for fabricating nanoelectrodes by integrating a commercial conductive Atomic Force Microscopy (AFM) tip with a hydrogel electrolyte.
- Core Innovation: Achieved precise control of the electroactive area (down to the nanometer scale) by utilizing the oscillation amplitude set point of the AFM in non-contact mode to govern the contact interface between the conductive tip and the agarose hydrogel.
- Materials Platform: The system uses a commercially available Boron-Doped Diamond (BDD) coated AFM tip as the working electrode and agarose hydrogel, which serves dual roles as a quasi-solid electrolyte and a mechanical substrate.
- Electrochemical Performance: The fabricated structure successfully operates as an ultramicroelectrode, exhibiting characteristic radial diffusion confirmed by sigmoidal cyclic voltammograms (CVs) of ferrocenemethanol (FcMeOH).
- Area Control Demonstrated: The effective height (a) of the pyramidal nanoelectrode was calculated from the steady-state current (iss), showing control from 54.8 nm (4 nm set point) up to 231.6 nm (6 nm set point).
- Single Nanoparticle Application: The platform was used to electrochemically deposit and immobilize single copper (Cu) nanoparticles onto the BDD tip apex, with morphology (urchin-like vs. nanorod) controllable by the AFM set point.
- Catalytic Activity Confirmed: The single Cu nanoparticle demonstrated electrocatalytic activity for the two-step consecutive reduction of nitrate (NO3-), providing a tool for fundamental single-particle electrochemistry studies.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Conductive Electrode Material | Boron-Doped Diamond (BDD) | N/A | Coating on commercial AFM tip |
| Electrolyte/Substrate | Agarose Hydrogel (8.3 wt%) | N/A | Solid electrolyte medium |
| Redox Probe (FcMeOH) Concentration | 1.0 | mM | Used with 1.0 M KCl supporting electrolyte |
| FcMeOH Oxidation Potential | 0.4 | V | vs. Ag/AgCl quasi-reference electrode |
| Nanoelectrode Height (Set Point 4 nm) | 54.8 | nm | Calculated from steady-state current (iss) |
| Nanoelectrode Height (Set Point 6 nm) | 231.6 | nm | Calculated from steady-state current (iss) |
| Stable Contact Current (Set Point 12 nm) | ~2 | pA | Chronoamperometric confirmation |
| Cu Deposition Potential Stop | -0.65 | V | Potential sweep limit for deposition |
| Estimated Deposited Cu Quantity | 5.5 x 10-15 | mol | Calculated from anodic charge (oxidation) |
| Urchin-like Cu Spike Width | ca. 100 | nm | Deposited at 1 nm set point (FE-SEM) |
| Cu Nanorod Dimensions | 90 x 200 | nm | Width x Length, deposited at 13 nm set point (FE-SEM) |
| Nitrate Reduction (First Stage) Potential | -0.83 | V | Observed using Cu-modified BDD tip |
| CV Scan Rate (FcMeOH) | 0.02 | V/s | Used for nanoelectrode characterization |
Key Methodologies
Section titled âKey MethodologiesâThe fabrication and testing process relies on the precise mechanical control offered by the AFM system integrated with standard electrochemical techniques.
- Hydrogel Preparation: Agarose (8.3 wt%) was dissolved in water at 90°C, followed by microwave heating (700 W for 30 s) to ensure transparency and eliminate air bubbles, then solidified overnight.
- Electrolyte Loading: Solidified agarose pads were soaked for 8 hours in the desired aqueous electrolyte (e.g., 1.0 mM FcMeOH in 1.0 M KCl) to achieve equilibrium saturation.
- Nanoelectrode Formation: A BDD-coated AFM tip (Working Electrode) was brought into contact with the hydrogel surface using the AFM non-contact mode. The oscillation amplitude set point was used as the feedback control mechanism to define the interfacial contact area.
- Steady-State Current Measurement: Chronoamperometry was used to confirm stable electrical contact. Cyclic Voltammetry (CV) of FcMeOH was performed to verify radial diffusion behavior characteristic of an ultramicroelectrode.
- Active Area Determination: The steady-state limiting current (iss) obtained from CV was used in the equation iss = 3.238nFDCa to calculate the effective height (a) of the pyramidal nanoelectrode, correlating the AFM set point to the physical contact area.
- Cu Nanoparticle Deposition: Cu was electrodeposited onto the BDD tip apex using CV scans (0.1 V to -0.65 V) in an acidic electrolyte containing CuSO4. The morphology (urchin-like or nanorod) was controlled by varying the AFM set point during deposition (e.g., 1 nm vs. 13 nm).
- Electrocatalytic Testing: The Cu-modified BDD tip was used to perform Linear Sweep Voltammetry (LSV) in agarose containing potassium nitrate (KNO3) to confirm and characterize the two-step consecutive nitrate reduction reaction.
- Post-Deposition Analysis: The morphology of the deposited Cu nanoparticles was verified using Field Emission-Scanning Electron Microscopy (FE-SEM) after rinsing and retracting the AFM tip.
Commercial Applications
Section titled âCommercial ApplicationsâThis technology provides a robust, scalable, and low-cost alternative to traditional nanoelectrode fabrication methods, opening doors for advanced electrochemical research and sensing applications.
- Electrochemical Sensing: Development of highly localized, ultra-sensitive sensors for environmental monitoring (e.g., nitrate, heavy metals) and biomedical diagnostics, leveraging the enhanced mass-transfer rates of nanoelectrodes.
- Fundamental Catalysis Research: Studying the intrinsic electrocatalytic activity of single nanoparticles (Cu, Pt, Au, Carbon) without averaging effects, crucial for optimizing materials for fuel cells, electrolyzers, and CO2 reduction.
- Nanofabrication and Prototyping: Rapid, customizable fabrication of nanoelectrode arrays or single probes for Scanning Electrochemical Microscopy (SECM) applications, bypassing the time and expense associated with lithography or Focused Ion Beam (FIB) milling.
- Microscopy and Spectroscopy Integration: Creating advanced probes for combined AFM-electrochemical systems, allowing simultaneous topographical and electrochemical mapping at the nanoscale.
- Energy Storage Material Characterization: Investigating the electrochemical properties and degradation mechanisms of individual active particles used in next-generation batteries and supercapacitors.
View Original Abstract
<p>An unconventional fabrication technique of nanoelectrode was developed using atomic force microscopy (AFM) and hydrogel. Until now, the precise control of electroactive area down to a few nm<sup>2</sup> has always been an obstacle, which limits the wide application of nanoelectrodes. Here, the nanometer-sized contact between the boron-doped diamond (BDD) as conductive AFM tip and the agarose hydrogel as solid electrolyte was well governed by the feedback amplitude of oscillation in the non-contact mode of AFM. Consequently, this low-cost and feasible approach gives rise to new possibilities for the fabrication of nanoelectrodes. The electroactive area controlled by the set point of AFM was investigated by cyclic voltammetry (CV) of the ferrocenmethanol (FcMeOH) combined with quasi-solid agarose hydrogel as an electrolyte. Single copper (Cu) nanoparticle was deposited at the apex of the AFM tip using this platform whose electrocatalytic activity for nitrate reduction was then investigated by CV and Field Emission-Scanning Electron Microscopy (FE-SEM), respectively.</p>