Gallium arsenide waveguides as a platform for direct mid-infrared vibrational spectroscopy
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-03-31 |
| Journal | Analytical and Bioanalytical Chemistry |
| Authors | Julian Haas, Robert Stach, Claudia Kolm, Rudolf Krska, Boris Mizaikoff |
| Institutions | UniversitÀt Ulm, BOKU University |
| Citations | 11 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThe research validates Gallium Arsenide (GaAs) as a high-performance, semiconductor-based alternative to traditional materials (Si, Diamond) for mid-infrared (MIR) Attenuated Total Reflection (ATR) spectroscopy, enabling highly integrated sensor platforms.
- High Efficiency: GaAs ATR waveguides demonstrated a normalized efficiency (E) of 221 for sodium acetate detection, approximately 40% higher than the Diamond reference (E = 155).
- Broad Transparency: GaAs offers a transparency window throughout the entire analytically useful MIR range (10,000-667 cm-1), superior to Si (limited near 1200 cm-1) and Diamond (limited near 2400 cm-1).
- Integrated Platform Potential: As a III/V semiconductor, GaAs facilitates the integration of light sources (QCLs/ICLs), detectors, and the waveguide interface onto a single chip, enabling next-generation portable MIR sensors.
- Robust Surface Functionalization: The GaAs surface supports robust chemical modification via thiol-terminated self-assembled monolayers (SAMs) (HS-PEG-SH) after native oxide removal (HCl etch).
- SEIRA Signal Amplification: The platform successfully immobilized Gold Nanostars (AuNSts) to achieve Surface-Enhanced Infrared Absorption (SEIRA) effects, yielding a signal amplification factor of approx. 4 for Aflatoxin B1 (AFB1).
- Trace Detection Demonstrated: Trace-level detection of AFB1 was achieved, with the SEIRA enhancement leading to a 14% improvement in the Limit of Detection (LOD), reducing it from 4000 ppb (bare GaAs) to 3500 ppb.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| GaAs Wafer Thickness | 300 | ”m | ATR Disk Waveguide |
| GaAs Crystal Orientation | <100> | - | Undoped, DSP |
| GaAs Resistivity | 2 | MΩ cm | - |
| GaAs Internal Reflections (N) | 6 | - | 300 ”m thickness, 30° incidence |
| GaAs Refractive Index (n1) | 3.3 | - | At 1000 cm-1 |
| GaAs Utilizable Wavelength Range | 10,000-667 | cm-1 | Full MIR analytical window |
| GaAs Penetration Depth (dp) | 0.73 | ”m | At 6 ”m wavelength |
| GaAs Normalized Efficiency (E) | 221 | - | Sodium Acetate (1600-1460 cm-1 band) |
| Diamond Normalized Efficiency (E) | 155 | - | Comparative reference |
| SEIRA Amplification Factor | Approx. 4 | - | AFB1 detection vs. non-enhanced counterpart |
| AuNSt Average Feret Diameter | 55 ± 30 | nm | Immobilized Gold Nanostars |
| AuNSt Surface Coverage | Approx. 30 | % | Measured via SEM on 1.5 ”m2 area |
| AFB1 LOD (SEIRA-enhanced) | 3500 | ppb | Trace detection on AuNSt-modified GaAs |
| AFB1 LOD (Bare GaAs) | 4000 | ppb | Trace detection without SEIRA |
| Etched GaAs Contact Angle | 61 ± 7 | ° | Indicating hydrophobic surface after oxide removal |
| HS-PEG-SH Contact Angle | 19 ± 5 | ° | Indicating hydrophilic surface after SAM formation |
Key Methodologies
Section titled âKey Methodologiesâ- Wafer Preparation and Cleaning: GaAs ATR crystals (300 ”m thick, <100> orientation) were laser-cut and cleaned via sonication in acetone, IPA, and water to remove cutting debris.
- Native Oxide Removal (Activation): The GaAs surface was activated by depositing 40 ”L of concentrated HCl (32%) for 60 s, followed by rinsing with dry, degassed ethanol to suppress immediate oxide regrowth.
- Linker Immobilization (SAM Formation): The activated GaAs surface was immersed in 40 ”L of 2 mM HS-PEG-SH ethanolic solution for 12 h under argon atmosphere to form a self-assembled monolayer (SAM).
- In Situ Monitoring: IR difference spectroscopy tracked the C-O-C band (1134 cm-1) of the HS-PEG-SH polymer during immobilization, confirming adsorption kinetics consistent with a Freundlich isotherm model.
- Gold Nanostar (AuNSt) Synthesis: AuNSts were fabricated using a modified seed-mediated growth protocol involving HAuCl4, trisodium citrate, AgNO3, and L(+)-ascorbic acid (AA).
- AuNSt Immobilization: The HS-PEG-SH modified surface was immersed in the AuNSt suspension for 12 h, followed by thorough rinsing with water to remove unbound particles.
- Mycotoxin Detection: Aflatoxin B1 (AFB1) was dissolved in a 2:1 methanol/chloroform mixture (chosen for AFB1 stability and low toxicity residuals) and added to the modified GaAs surface. Spectra were recorded after complete solvent evaporation (approx. 10 min).
- Detoxification: All surfaces exposed to AFB1 were cleaned using sodium hypochlorite solution to ensure detoxification.
Commercial Applications
Section titled âCommercial ApplicationsâThe use of GaAs as an ATR waveguide material, combined with integrated photonics and SEIRA enhancement, targets several high-value engineering and analytical sectors:
- Integrated MIR Sensor Systems: GaAs is the material system of choice for integrating light sources (Quantum Cascade Lasers, QCLs), detectors, and the sensing waveguide onto a single chip, enabling highly miniaturized, portable FT-IR/ATR devices for field use.
- Food and Agriculture Safety: Direct, rapid, and sensitive detection of trace contaminants like mycotoxins (e.g., AFB1) in food and feed matrices, providing a faster, cheaper alternative to laboratory-based LCMS/HPLC methods for regulatory compliance screening.
- Biomedical Diagnostics: The robust SAM functionalization scheme allows for the immobilization of biorecognition elements (antibodies, aptamers) for non-destructive, real-time sensing of biomarkers or pathogens in aqueous (bio)medical specimens.
- Surface-Enhanced Spectroscopy (SEIRA): Deployment of the AuNSt-decorated GaAs platform for enhanced sensitivity in probing small molecules, DNA hybridization, or membrane proteins, overcoming the inherent low sensitivity of standard ATR for trace analytes.
- Chemical Process Control: Real-time, in situ monitoring of chemical reactions, polymerization, or surface modification kinetics in industrial settings, leveraging the chemical resilience and broad transparency of the GaAs waveguide.
View Original Abstract
Abstract During recent years, mid-infrared (MIR) spectroscopy has matured into a versatile and powerful sensing tool for a wide variety of analytical sensing tasks. Attenuated total reflection (ATR) techniques have gained increased interest due to their potential to perform non-destructive sensing tasks close to real time. In ATR, the essential component is the sampling interface, i.e., the ATR waveguide and its material properties interfacing the sample with the evanescent field ensuring efficient photon-molecule interaction. Gallium arsenide (GaAs) is a versatile alternative material vs. commonly used ATR waveguide materials including but not limited to silicon, zinc selenide, and diamond. GaAs-based internal reflection elements (IREs) are a new generation of semiconductor-based waveguides and are herein used for the first time in direct spectroscopic applications combined with conventional Fourier transform infrared (FT-IR) spectroscopy. Next to the characterization of the ATR waveguide, exemplary surface reactions were monitored, and trace-level analyte detection via signal amplification taking advantage of surface-enhanced infrared absorption (SEIRA) effects was demonstrated. As an example of real-world relevance, the mycotoxin aflatoxin B1 (AFB1) was used as a model analyte in food and feed safety analysis.