Nafion and Multiwall Carbon Nanotube Modified Ultrananocrystalline Diamond Microelectrodes for Detection of Dopamine and Serotonin

At a Glance

Metadata	Details
Publication Date	2021-05-06
Journal	Micromachines
Authors	An‐Yi Chang, Shabnam Siddiqui, Prabhu U. Arumugam
Institutions	Louisiana Tech University, Louisiana State University in Shreveport
Citations	9
Analysis	Full AI Review Included

Executive Summary

This research details the development and characterization of a highly sensitive and selective microelectrode for long-term neurochemical detection, leveraging modified boron-doped ultrananocrystalline diamond (BDUNCD).

Core Achievement: Fabrication of a Nafion-Multi-Walled Carbon Nanotube (MWCNT)-BDUNCD hybrid microelectrode demonstrating superior performance metrics for Dopamine (DA) and Serotonin (5-HT) detection.
Enhanced Sensitivity: The modified electrode achieved a 166-fold increase in DA sensitivity (6.75 µA µM^-1 cm^-2) and a 16-fold increase in 5-HT sensitivity (4.55 µA µM^-1 cm^-2) compared to unmodified BDUNCD.
High Selectivity: The Nafion layer effectively rejected anionic interferents, specifically Ascorbic Acid (AA), enabling clear, distinguishable DA and 5-HT peaks in a complex ternary mixture (DA, 5-HT, and excess AA).
Extended Lifetime: The modified surface exhibited significantly reduced fouling, allowing for stable neurochemical monitoring for up to 9 hours, a substantial improvement over standard carbon electrodes.
Fast Kinetics: Demonstrated a rapid response time (2.0 ± 0.16 s) and an ultra-low Limit of Detection (LOD) for DA (5.4 ± 0.40 nM), suitable for studying fast neurotransmitter dynamics in vivo.
Methodology: Utilized Electrophoretic Deposition (EPD) for precise, controlled coating of MWCNTs and Nafion, and integrated the sensor into a droplet-based microfluidic platform for accurate long-term testing.

Technical Specifications

Parameter	Value	Unit	Context
BDUNCD Film Thickness	2	µm	Deposited via HFCVD
BDUNCD Resistivity	~0.08	Ω·cm	As-deposited film
Microelectrode Diameter	250	µm	Disk geometry
Microelectrode Geometrical Area	~0.05	mm²	3 x 3 array format
MWCNT Coating Thickness	~200	nm	Applied via EPD
Nafion Coating Thickness	~50	nm	Applied via EPD
DA Sensitivity (Nafion-MWCNT)	6.75	µA µM^-1 cm^-2	Highest recorded
5-HT Sensitivity (Nafion-MWCNT)	4.55	µA µM^-1 cm^-2	Highest recorded
DA Limit of Detection (LOD)	5.4 ± 0.40	nM	Calculated via 3 * SD/slope
DA Limit of Quantification (LoQ)	18.9 ± 1.78	nM	Measured concentration range
Response Time (Nafion-MWCNT)	2.0 ± 0.16	s	Time to reach 2^nd oxidation peak
Clearance Rate (Nafion-MWCNT)	3.35 ± 1.81	s	Time for signal decay (T₂₀ to T₆₀)
Long-Term Stability Duration	9	h	Tested in ternary mixture (DA, 5-HT, AA)
DA Oxidation Potential (Ternary Mix)	-12.97 ± 0.02	mV	Measured via DPV
5-HT Oxidation Potential (Ternary Mix)	173.34 ± 2.52	mV	Measured via DPV

Key Methodologies

The fabrication process involved microfabrication of the BDUNCD base, followed by two distinct Electrophoretic Deposition (EPD) steps for surface modification, and final integration into a microfluidic platform.

BDUNCD Microelectrode Fabrication:
- Substrate: 4-inch Si wafers coated with 1 µm thermal SiO₂.
- Diamond Deposition: 2 µm thick BDUNCD film deposited using Hot Filament Chemical Vapor Deposition (HFCVD).
- Patterning: Optical microlithography and etching used to define 250 µm diameter disk microelectrodes in a 3 x 3 array.
MWCNT Coating via EPD (Achieving ~200 nm thickness):
- Suspension Preparation: 1 mg/mL -COOH functionalized MWCNTs in deionized water.
- Charging: 5 µM MgCl₂·6H₂O salt solution added and sonicated for 30 min to impart a positive charge to the MWCNTs.
- Deposition: Stepwise voltage scan of -6 V applied to the BDUNCD working electrode for 10 min (using a Pt microwire counter electrode).
- Curing: Cured at 70 °C for 10 min.
Nafion Coating via EPD (Achieving ~50 nm thickness):
- Solution: 5 wt % Nafion solution.
- Deposition: Stepwise voltage scan of +0.5 V applied to the microelectrode for 2 min (using an Ag/AgCl counter/reference electrode).
- Curing: Rinsed in deionized water and cured at 70 °C for 10 min.
Microfluidic Platform Integration:
- PDMS Fabrication: Two-layer PDMS chip constructed (100 µm thick 1^st layer, 65 µm thick 2^nd layer defining the microchannel).
- Bonding: Oxygen plasma treatment (40 W for 20 s) used to bond the PDMS layers to the BDUNCD chip, ensuring excellent sealing.
- Testing Protocol: Droplet-based flow used: 100 µM DA droplets (0.02 mL volume) introduced every 2 min, carried by a continuous 1X PBS background flow (0.1 mL/min).

Commercial Applications

The Nafion-MWCNT-BDUNCD microelectrode technology is highly relevant for applications requiring robust, long-term, and selective electrochemical sensing in complex biological environments.

Chronic Neurochemical Monitoring:
- Development of implantable sensors for continuous, long-duration tracking of neurotransmitter fluctuations (DA, 5-HT) in animal models or clinical settings.
- Research tools for studying the progression of neurodegenerative diseases (e.g., Parkinson’s disease) and drug addiction mechanisms in vivo.
Drug Discovery and Pharmacology:
- High-throughput screening platforms (using the microfluidic integration) to evaluate the effects of pharmaceutical agents on neurotransmitter release and uptake kinetics.
High-Fidelity Biosensing:
- Applications requiring high resistance to biofouling, leveraging the chemical inertness and dimensional stability of the BDUNCD base, making it superior to traditional carbon fiber electrodes.
Analytical Electrochemistry:
- Creation of highly selective sensors for detecting electroactive species in complex mixtures (e.g., biological fluids, environmental samples) where high concentrations of interferents (like AA) are present.

View Original Abstract

Neurochemicals play a critical role in the function of the human brain in healthy and diseased states. Here, we have investigated three types of microelectrodes, namely boron-doped ultrananocrystalline diamond (BDUNCD), nafion-modified BDUNCD, and nafion-multi-walled carbon nanotube (MWCNT)-modified BDUNCD microelectrodes for long-term neurochemical detection. A ~50 nm-thick nafion-200-nm-thick MWCNT-modified BDUNCD microelectrode provided an excellent combination of sensitivity and selectivity for the detection of dopamine (DA; 6.75 μA μM−1 cm−2) and serotonin (5-HT; 4.55 μA μM−1 cm−2) in the presence of excess amounts of ascorbic acid (AA), the most common interferent. Surface stability studies employing droplet-based microfluidics demonstrate rapid response time (<2 s) and low limits of detection (5.4 ± 0.40 nM). Furthermore, we observed distinguishable DA and 5-HT current peaks in a ternary mixture during long-term stability studies (up to 9 h) with nafion-MWCNT-modified BDUNCD microelectrodes. Reduced fouling on the modified BDUNCD microelectrode surface offers significant advantages for their use in long-term neurochemical detection as compared to those of prior-art microelectrodes.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi12050523

References

2015 - Ventral striatal dopamine reflects behavioral and neural signatures of model-based control during sequential decision making [Crossref]
1962 - The pathogenesis of Parkinson’s disease: A new hypothesis
2015 - Jejunal Infusion of Levodopa-Carbidopa Intestinal Gel Versus Oral Administration of Levodopa-Carbidopa Tablets in Japanese Subjects with Advanced Parkinson’s Disease: Pharmacokinetics and Pilot Efficacy and Safety [Crossref]
2009 - Reward-learning and the novelty-seeking personality: A between- and within-subjects study of the effects of dopamine agonists on young Parkinson’s patients [Crossref]
1994 - Continuousin vivo monitoring of evoked dopamine release in the rat nucleus accumbens by amperometry [Crossref]
2012 - Technological Barriers in the Use of Electrochemical Microsensors and Microbiosensors for in vivo Analysis of Neurological Relevant Substances [Crossref]
2003 - Detecting Subsecond Dopamine Release with Fast-Scan Cyclic Voltammetry in Vivo [Crossref]
2017 - Hitchhiker’s Guide to Voltammetry: Acute and Chronic Electrodes for in Vivo Fast-Scan Cyclic Voltammetry [Crossref]
2011 - Carbon nanofiber electrode array for electrochemical detection of dopamine using fast scan cyclic voltammetry [Crossref]