Novel Amperometric Sensor Based on Glassy Graphene for Flow Injection Analysis

At a Glance

Metadata	Details
Publication Date	2025-04-13
Journal	Sensors
Authors	Ramtin E. Shabgahi, Alexander Minkow, Michael Wild, Dietmar Kissinger, A. Pasquarelli
Institutions	Universität Ulm, Diamond Materials (Germany)
Citations	4
Analysis	Full AI Review Included

Technical Documentation & Analysis: Glassy Graphene Sensors on MPCVD Diamond

Executive Summary

This research successfully demonstrates the fabrication of highly stable and sensitive glassy graphene (GG) amperometric sensors using a nickel-catalyzed Rapid Thermal Annealing (RTA) process on Polycrystalline Diamond (PCD) substrates. The findings validate the superior performance of MPCVD diamond as a robust platform for advanced carbon-based electrochemistry.

Novel Synthesis: Glassy Graphene (GG) films were synthesized from pyrolyzed photoresist films (PPFs) using RTA (up to 950 °C) and a thin Nickel (Ni) catalyst layer.
Substrate Superiority: PCD substrates provided significantly stronger graphene-substrate adhesion and structural integrity compared to $\text{SiO}_2/\text{Si}$, mitigating delamination issues observed during electrochemical cycling.
Enhanced Kinetics: Electrochemical Impedance Spectroscopy (EIS) confirmed that GG/PCD exhibited lower interfacial resistance ($\text{R}_1$) and improved charge transfer kinetics for the outer-sphere redox marker ([Ru($\text{NH}_3$)$_6$]$^{3+/2+}$).
High Sensitivity: The GG/PCD sensor achieved high sensitivity (1.029 µA $\text{cm}^{-2}/\text{µM}$) and a low limit of detection (LOD = 1.05 µM) for adrenaline detection in Flow Injection Analysis (FIA).
Commercial Viability: The RTA method offers a cost-effective, scalable, and lithography-compatible approach for developing high-performance carbon electrodes, leveraging the mechanical and electrochemical stability of MPCVD diamond.

Technical Specifications

The following table summarizes the critical material and performance parameters achieved using the Polycrystalline Diamond (PCD) substrate platform.

Parameter	Value	Unit	Context
Substrate Material	Polycrystalline Diamond (PCD)	N/A	MPCVD grown, 5 x 10 $\text{mm}^2$ chips.
RTA Annealing Temperature	950	°C	Optimal temperature for GG conversion.
RTA Annealing Time	1	min	Short processing window.
Ni Catalyst Thickness	50	nm	Used for metal-induced crystallization (MIC).
Bare PCD RMS Roughness ($\text{R}_a$)	3.47 ± 0.48	nm	Polished surface quality prior to coating.
GG/PCD RMS Roughness ($\text{R}_a$)	28.10	nm	After 1 min RTA, due to Ni migration/GG formation.
Potential Window (GG/PCD)	1.68	V	Measured in PBS (pH 7.4) at 0.1 V $\text{s}^{-1}$.
Double-Layer Capacitance ($\text{C}_{dl}$)	72.98	µF $\text{cm}^{-2}$	GG/PCD (950 °C, 1 min).
Interfacial Resistance ($\text{R}_1$)	878	Ω $\text{cm}^{2}$	GG/PCD, indicating strong adhesion (vs. 1479 Ω $\text{cm}^{2}$ for GG/$\text{SiO}_2/\text{Si}$).
$\Delta E_p$ ([Ru($\text{NH}_3$)$_6$]$^{3+/2+}$)	78.50 ± 2	mV	GG/PCD, indicating quasi-reversible kinetics.
Adrenaline Linear Range (FIA)	3-300	µM	Excellent linearity ($\text{R}^2$ = 0.99).
Adrenaline Sensitivity (FIA)	1.029	µA $\text{cm}^{-2}/\text{µM}$	Achieved at 0.8 V applied potential.
Adrenaline Limit of Detection (LOD)	1.05	µM	Calculated using LOD = (3 x $\sigma$)/S.

Key Methodologies

The fabrication of the GG microelectrodes involved a precise sequence of microfabrication and thermal processing steps:

Substrate Cleaning: PCD substrates were treated with chromosulfuric acid (80 °C, 20 min) to eliminate residual non-diamond carbon (NDC) impurities, ensuring a pristine surface for deposition.
Photoresist Application: An image reversal photoresist (AZ 5214E) was spin-coated and patterned using UV photolithography to define the microelectrode geometry.
Two-Step Pyrolysis (PPF Formation): Pyrolysis was conducted via Rapid Thermal Annealing (RTA) under a high-purity Nitrogen ($\text{N}_2$) atmosphere (3000 sccm):
- Step 1 (Curing): 350 °C for 60 min to stabilize the structure and prevent excessive material loss.
- Step 2 (Graphitization): Ramped to 950 °C (20 °C/s) and held for 1 min to convert the photoresist into pyrolyzed photoresist film (PPF), a glassy carbon precursor.
Nickel Catalyst Deposition: A 50 nm thin film of Nickel (Ni) was deposited onto the PPF microelectrodes via thermal evaporation, followed by a lift-off process.
GG Conversion (Second RTA): A second RTA process (identical to Step 2, 950 °C for 1 min) was performed, using the Ni film as a catalyst to transform the PPF into Glassy Graphene (GG) via the metal-induced crystallization (MIC) and layer exchange mechanism.
Electrochemical Analysis: The resulting GG/PCD electrodes were characterized using Cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS), and Flow Injection Analysis (FIA) for adrenaline detection.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to support the replication and advancement of this research, offering high-quality MPCVD diamond materials and custom engineering services essential for developing next-generation electrochemical sensors.

Applicable Materials

Research Requirement	6CCVD Material Solution	Technical Advantage
Robust Substrate	Polycrystalline Diamond (PCD)	High mechanical stability and chemical inertness, ensuring strong adhesion for $\text{sp}^{2}$-carbon films (GG), crucial for long-term sensor stability in FIA.
High-Performance Baseline	Boron-Doped Diamond (BDD)	The industry gold standard for electrochemistry. BDD offers the widest potential window and minimal background current, enabling superior limits of detection (LOD) compared to GG/PCD.
Structural Analysis	Single Crystal Diamond (SCD)	Optical grade SCD substrates (Ra < 1 nm) are available for fundamental studies requiring ultra-low defect density and precise epitaxial growth of carbon films.

Customization Potential

6CCVD provides the necessary flexibility to meet the exact specifications required for advanced microelectrode fabrication:

Custom Dimensions: We supply PCD plates and wafers up to 125 mm in diameter, allowing for high-throughput microfabrication and scaling beyond the 5 x 10 $\text{mm}^2$ chips used in this study.
Thickness Control: We offer PCD layers from 0.1 µm up to 500 µm, and substrates up to 10 mm thick, ensuring compatibility with various RTA systems and carrier board designs.
Advanced Metalization: We offer in-house deposition of thin-film metals (Au, Pt, Pd, Ti, W, Cu) to precisely replicate or optimize the 50 nm Nickel (Ni) catalytic layer used for the GG conversion process.
Surface Engineering: Our polishing capabilities ensure PCD surfaces meet stringent microfabrication requirements (Ra < 5 nm for inch-size PCD), critical for uniform photoresist coating and subsequent RTA processing.

Engineering Support

6CCVD’s in-house PhD material science team can assist researchers and engineers in optimizing material selection and processing parameters for similar carbon-based electrochemical sensor projects:

Process Optimization: Consultation on adapting the RTA parameters (temperature, time, gas flow) to achieve specific GG microstructures or layer thicknesses on diamond.
Material Transition: Support for transitioning from the GG/PCD platform to the superior electrochemical performance of BDD for Flow Injection Analysis (FIA) and neurotransmitter detection.
Interfacial Stability: Expertise in developing custom metalization schemes and surface treatments to maximize adhesion and minimize interfacial resistance ($\text{R}_1$) between carbon films and diamond substrates.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Flow injection analysis (FIA) is widely used in drug screening, neurotransmitter detection, and water analysis. In this study, we investigated the electrochemical sensing performance of glassy graphene electrodes derived from pyrolyzed positive photoresist films (PPFs) via rapid thermal annealing (RTA) on SiO2/Si and polycrystalline diamond (PCD). Glassy graphene films fabricated at 800, 900, and 950 °C were characterized using Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM) to assess their structural and morphological properties. Electrochemical characterization in phosphate-buffered saline (PBS, pH 7.4) revealed that annealing temperature and substrate type influence the potential window and double-layer capacitance. The voltammetric response of glassy graphene electrodes was further evaluated using the surface-insensitive [Ru(NH3)6]3+/2+ redox marker, the surface-sensitive [Fe(CN)6]3−/4− redox couple, and adrenaline, demonstrating that electron transfer efficiency is governed by annealing temperature and substrate-induced microstructural changes. FIA with amperometric detection showed a linear electrochemical response to adrenaline in the 3-300 µM range, achieving a low detection limit of 1.05 µM and a high sensitivity of 1.02 µA cm−2/µM. These findings highlight the potential of glassy graphene as a cost-effective alternative for advanced electrochemical sensors, particularly in biomolecule detection and analytical applications.

Tech Support

Original Source

DOI: https://doi.org/10.3390/s25082454

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