A detailed EIS study of boron doped diamond electrodes decorated with gold nanoparticles for high sensitivity mercury detection
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2021-05-04 |
| Journal | Scientific Reports |
| Authors | Maeve McLaughlin, Alexander C. Pakpour‐Tabrizi, Richard B. Jackman |
| Institutions | University College London, London Centre for Nanotechnology |
| Citations | 17 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This study details the optimization and characterization of Boron Doped Diamond (BDD) electrodes decorated with Gold Nanoparticles (AuNPs) for ultra-sensitive mercury (Hg) detection using Electrochemical Impedance Spectroscopy (EIS).
- Enhanced Sensitivity: AuNP decoration significantly improves the efficiency of Hg ion pre-concentration from the bulk electrolyte onto the electrode surface, enabling detection at concentrations as low as 1 pM.
- Improved Reactivity: Electrodes decorated with AuNPs exhibit lower capacitance and higher electron transfer rates (ko) compared to bare BDD and polished BDD (pBDD) electrodes, indicating faster reaction kinetics.
- Material Dominance: The mercury detection process is dominated by the diamond electrode characteristics. The same equivalent circuit model (modified Randles circuit) was successfully used for both bare and AuNP-decorated electrodes of the same substrate type.
- Surface Quality Influence: At high Hg concentrations (> 500 µM), the relative proportion of sp2 non-diamond carbon (higher in pBDD) has a greater influence on sensitivity than the presence of AuNPs.
- Low Capacitance Achievement: The AuNP-decorated BDD electrodes achieved a low effective capacitance (Ceff) of 0.36 µF/cm2, which is crucial for achieving low detection limits in electrochemical sensing.
- Commercial Viability: The exceptional robustness, low ko values, and high sensitivity reported confirm that these optimized diamond electrodes are ideal candidates for developing commercial, portable mercury sensors for aquatic environments.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| BDD Boron Doping Concentration | > 1020 | cm-3 | Required for quasi-metallic conductivity |
| BDD Surface Roughness (RA) | ~ 50 | µm | Unpolished polycrystalline BDD substrate |
| pBDD Surface Roughness (RA) | ~ 50 | nm | Mechanically polished BDD substrate |
| AuNP Average Diameter (BDD) | 30 ± 14 | nm | Formed by annealing 5 nm sputtered film |
| AuNP Surface Coverage (BDD) | 44 | % | Calculated via SEM/ImageJ analysis |
| AuNP Average Diameter (pBDD) | 30 ± 11 | nm | Formed by annealing 5 nm sputtered film |
| AuNP Surface Coverage (pBDD) | 42 | % | Calculated via SEM/ImageJ analysis |
| Electrolyte | 0.1 | M | HNO3 (used for all tests) |
| Mercury Concentration Range | 1 pM to 1 | mM | Hg(NO3)2 tested via sequential doping |
| EIS Frequency Range | 50 kHz to 50 | MHz | Measurement range (8 points per decade) |
| EIS Amplitude | 10 | mV | AC signal amplitude |
| Deposition Potential (Edep) | 0.35 | V | Used for Hg pre-concentration (10 min duration) |
| Stripping Potential (Estrip) | 1.0 | V | Used to strip Hg from electrode surface (10 min duration) |
| Highest Electron Transfer Rate (ko) | 1.24 ± 0.03 | cm s-1 | Achieved by pBDD + AuNP (Open Circuit, 1 pM Hg) |
| Lowest Effective Capacitance (Ceff) | 0.36 | µF/cm2 | Achieved by BDD + AuNP (0 M control) |
| BDD Crystalline Quality (FWHM) | 8.39 ± 0.11 | cm-1 | Full Width at Half Maximum of 1332 cm-1 sp3 peak |
| pBDD Crystalline Quality (FWHM) | 12.10 ± 0.18 | cm-1 | Full Width at Half Maximum of 1332 cm-1 sp3 peak |
Key Methodologies
Section titled “Key Methodologies”The experimental procedure involved substrate preparation, gold nanoparticle decoration, and electrochemical characterization using EIS.
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Substrate Cleaning (Piranha Clean):
- BDD and pBDD substrates (Element Six Ltd) were cleaned to remove organic contaminants.
- Solution: 3:1 v/v 98% HCl and 30% H2O2 (Piranha).
- Duration: 10 min.
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Hydrogen Termination:
- Substrates were hydrogen-terminated to promote stronger adherence of AuNPs (hydrophobic surface).
- Reactor: AX5010 Seki Technotron Inc.
- Plasma: H-plasma.
- Temperature: 700 °C (platen).
- Power/Pressure: 800 W, 40 Torr.
- Duration: 10 min.
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Gold Film Deposition:
- A non-continuous 5 nm gold film was sputtered onto the hydrogen-terminated BDD/pBDD surfaces.
- Equipment: Emscope SC500 gold sputter coater.
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AuNP Formation (Annealing/De-wetting):
- The sputtered gold film was annealed to segregate into discrete AuNPs (~30 nm average diameter).
- Equipment: Solaris 150 Rapid Thermal Processing System.
- Atmosphere: Nitrogen (N2).
- Temperature: 400 °C.
- Duration: 5 min.
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Electrochemical Impedance Spectroscopy (EIS):
- Setup: Three-electrode system (BDD/pBDD working electrode, Ag/AgCl reference, Platinum counter electrode).
- Electrolyte: 0.1 M HNO3, sequentially doped with 1 pM to 1 mM Hg(NO3)2.
- Cleaning Step: 150 current pulses (alternating 10 mA cm-2 and -10 mA cm-2, 100 ms duration) between Hg additions.
- Measurement Potentials (10 min application each):
- Open Circuit Potential (Control).
- Deposition Potential (0.35 V).
- Stripping Potential (1.0 V).
- Data Fitting: Equivalent circuit modeling (modified Randles circuit with Constant Phase Elements, Q) using EIS Spectrum Analyser software (x2 < 0.1 fit quality).
Commercial Applications
Section titled “Commercial Applications”The robust nature, chemical stability, and enhanced sensitivity of the AuNP-decorated BDD electrodes make them highly suitable for deployment in several demanding commercial and industrial sectors.
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Environmental Monitoring Systems:
- Development of commercial, portable sensors for high-sensitivity, real-time monitoring of mercury contamination in natural aquatic environments (rivers, lakes, oceans).
- Ideal for long-term, in-situ measurements due to BDD’s resistance to fouling and stability in harsh conditions.
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Industrial Process Control:
- Continuous monitoring of heavy metal discharge (specifically Hg) in industrial wastewater streams to ensure regulatory compliance.
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Food and Water Safety:
- Deployment in water treatment facilities and agricultural settings to monitor water quality, preventing mercury accumulation in the food chain (e.g., fish).
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Sensor Technology Development:
- The demonstrated ability to optimize electrode kinetics (low Ceff and high ko) provides a pathway for developing next-generation electrochemical sensors for other heavy metals and analytes.