A batch microfabrication of a self-cleaning, ultradurable electrochemical sensor employing a BDD film for the online monitoring of free chlorine in tap water
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-04-08 |
| Journal | Microsystems & Nanoengineering |
| Authors | Jiawen Yin, Wanlei Gao, Weijian Yu, Yihua Guan, Zhenyu Wang |
| Institutions | State Key Laboratory of Transducer Technology, Shanghai Institute of Microsystem and Information Technology |
| Citations | 17 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Core Innovation: Development of a highly integrated, silicon-glass structured electrochemical sensor chip for online free chlorine monitoring, utilizing MEMS batch microfabrication.
- Electrode System: Employs a three-electrode configuration: Boron-Doped Diamond (BDD) Working Electrode (WE), Platinum (Pt) Counter Electrode (CE), and a liquid-conjugated Ag/AgCl Reference Electrode (RE).
- Self-Cleaning Capability: The BDD WE enables electrochemical self-cleaning by generating highly oxidative hydroxyl radicals (âąOH) at a fixed potential (+2.5 V), effectively digesting organic matter and biofilms.
- Durability and Recovery: Fouled sensors recovered from 50.2% to 94.1% of their initial sensing performance after only 30 minutes of self-cleaning, proving suitability for long-term, in situ deployment.
- High Performance: Achieved an acceptable Limit of Detection (LOD) of 0.056 mg/L and excellent linearity (R2 = 0.998) in Flow Injection Analysis (FIA) mode.
- Batch Consistency: The MEMS fabrication process yielded high consistency across multiple sensors, showing a low Relative Standard Deviation (RSD) of less than 4.05%.
- Environmental Robustness: Demonstrated predictable linear dependency on key hydrologic parameters (pH, temperature, and flow rate), allowing for accurate compensation in real-world applications.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Limit of Detection (LOD) | 0.056 | mg/L | FIA mode |
| Linearity (R2) | 0.998 | N/A | FIA mode (0.1 to 1 mg/L NaClO) |
| Optimal Sensing Potential | -0.35 | V | vs. Ag/AgCl RE (for ClO- reduction) |
| Optimal Self-Cleaning Potential | +2.5 | V | vs. Ag/AgCl RE (for âąOH generation) |
| Sensing Sensitivity (BDD) | 9.108 | ”A/mgL-1/cm2 | Average of five sensors (CV mode) |
| Sensor Consistency (RSD) | < 4.05 | % | For NaClO concentrations 5-200 mg/L |
| Self-Cleaning Recovery | 94.1 | % | After 30 min cleaning time |
| Long-Term Stability (Air) | 91.4 | % | Response retained after 50 days |
| BDD Film Thickness | 3-4 | ”m | Fabricated by HFCVD |
| BDD Square Resistance | 0.5 | Ω | Measured |
| BDD Resistivity | 1.8e5 | Ω/cm | Calculated |
| BDD Surface Roughness (Ra) Increase | 2.08 | times | After O-RIE etching (175 nm to 365 nm) |
| CV Scan Rate (Selected) | 100 | mV/s | Optimized for stability and response |
| Wafer Size | 4 | inch | Silicon-glass structure |
Key Methodologies
Section titled âKey MethodologiesâThe sensor chip was fabricated using Micro-Electro-Mechanical System (MEMS) techniques on a 4-inch silicon wafer, integrating three distinct electrodes:
-
BDD Film Preparation (Working Electrode, WE):
- Substrate: Oxidized silicon wafer (2 ”m SiO2).
- Seeding: Substrate mechanically ground (W28 paste) and ultrasonicated in nanodiamond powder suspension (60-100 nm) to enhance nucleation density.
- Deposition: BDD film (3-4 ”m thick) grown via Hot-Filament Chemical Vapor Deposition (HFCVD).
- Parameters: Filament temperature 2200-2400 °C, reaction pressure 1.0-3.5 kPa, C/H ratio 2-4%, deposition time 4.5 h.
- Patterning: BDD WE defined using an aluminum mask (~400 nm) followed by Oxygen Reactive Ion Etching (O-RIE) for 40 min.
-
Counter Electrode (CE) Fabrication:
- A Platinum (Pt) thin film (~0.4 ”m thick) was deposited onto the silicon/BDD wafer surface via sputtering and patterned using a lift-off process.
-
Reference Electrode (RE) Fabrication:
- Structure: A cavity and groove were etched into the silicon wafer using 30% KOH solution (40 °C) to house the saturated KCl solution and lead wire.
- Electrode: Silver (Ag) film (~0.4 ”m) sputtered onto a Pyrex 7740 glass substrate. AgCl layer deposited electrochemically (0.25 M HCl at 4 V).
-
Final Integration:
- The patterned silicon wafer and the glass substrate (with the Ag/AgCl film) were bonded using silicon-glass bonding (35 min, 350 °C, 1200 V) to create the integrated, liquid-conjugated three-electrode sensor chip.
Commercial Applications
Section titled âCommercial Applicationsâ- Municipal Water Distribution: Online, in situ monitoring of residual free chlorine levels at critical nodes within tap water pipe networks to ensure compliance with safety standards (e.g., Chinaâs standard >0.05 mg/L).
- Industrial Water Treatment: Real-time monitoring and control of disinfection processes in industries such as food and beverage, pharmaceuticals, and cooling water systems, where precise chlorine dosing is required.
- BDD Sensor Mass Production: The successful batch microfabrication methodology (MEMS) enables cost-effective, high-volume manufacturing of BDD-based electrochemical sensors for various analytes beyond chlorine.
- Ultra-Durable Sensing Platforms: Deployment in environments prone to biofouling or organic passivation (e.g., river water, wastewater effluent) due to the integrated, highly effective electrochemical self-cleaning mechanism.
- Environmental Monitoring: Use as a robust, low-maintenance sensor for long-term environmental studies requiring continuous data collection in complex aqueous matrices.
View Original Abstract
Abstract Free chlorine is one of the key water quality parameters in tap water. However, a free chlorine sensor with the characteristics of batch processing, durability, antibiofouling/antiorganic passivation and in situ monitoring of free chlorine in tap water continues to be a challenging issue. In this paper, a novel silicon-based electrochemical sensor for free chlorine that can self-clean and be mass produced via microfabrication technique/MEMS (Micro-Electro-Mechanical System) is proposed. A liquid-conjugated Ag/AgCl reference electrode is fabricated, and electrochemically stable BDD/Pt is employed as the working/counter electrode to verify the effectiveness of the as-fabricated sensor for free chlorine detection. The sensor demonstrates an acceptable limit of detection (0.056 mg/L) and desirable linearity ( R 2 = 0.998). Particularly, at a potential of +2.5 V, hydroxyl radicals are generated on the BBD electrode by electrolyzing water, which then remove the organic matter attached to the surface of the sensor though an electrochemical digestion process. The performance of the fouled sensor recovers from 50.2 to 94.1% compared with the initial state after self-cleaning for 30 min. In addition, by employing the MEMS technique, favorable response consistency and high reproducibility (RSD < 4.05%) are observed, offering the opportunity to mass produce the proposed sensor in the future. A desirable linear dependency between the pH, temperature, and flow rate and the detection of free chlorine is observed, ensuring the accuracy of the sensor with any hydrologic parameter. The interesting sensing and self-cleaning behavior of the as-proposed sensor indicate that this study of the mass production of free chlorine sensors by MEMS is successful in developing a competitive device for the online monitoring of free chlorine in tap water.