Total Internal Reflection Absorption Spectroscopy (TIRAS) for the Detection of Solvated Electrons at a Plasma-liquid Interface

Summary

This article presents a total internal reflection absorption spectroscopy (TIRAS) method for measuring short-lived free radicals at a plasma-liquid interface. In particular, TIRAS is used to identify solvated electrons based on their optical absorbance of red light near 700 nm.

Abstract

The total internal reflection absorption spectroscopy (TIRAS) method presented in this article uses an inexpensive diode laser to detect solvated electrons produced by a low-temperature plasma in contact with an aqueous solution. Solvated electrons are powerful reducing agents, and it has been postulated that they play an important role in the interfacial chemistry between a gaseous plasma or discharge and a conductive liquid. However, due to the high local concentrations of reactive species at the interface, they have a short average lifetime (~1 µs), which makes them extremely difficult to detect. The TIRAS technique uses a unique total internal reflection geometry combined with amplitude-modulated lock-in amplification to distinguish solvated electrons' absorbance signal from other spurious noise sources. This enables the in situ detection of short-lived intermediates in the interfacial region, as opposed to the bulk measurement of stable products in the solution. This approach is especially attractive for the field of plasma electrochemistry, where much of the important chemistry is driven by short-lived free radicals. This experimental method has been used to analyze the reduction of nitrite (NO₂^-_(aq)), nitrate (NO₃^-_(aq)), hydrogen peroxide (H₂O_2(aq)), and dissolved carbon dioxide (CO_2(aq)) by plasma-solvated electrons and deduce effective rate constants. Limitations of the method may arise in the presence of unintended parallel reactions, such as air contamination in the plasma, and absorbance measurements may also be hindered by the precipitation of reduced electrochemical products. Overall, the TIRAS method can be a powerful tool for studying the plasma-liquid interface, but its effectiveness depends on the particular system and reaction chemistry under study.

Introduction

Plasma-liquid interactions represent an area of growing interest in the plasma science and engineering community. The complex interface between plasmas and liquids, which contains a variety of highly reactive free radicals, has found applications in many areas including analytical chemistry, plasma medicine, water and wastewater treatment, and nanomaterial synthesis^1,2,3,4,5,6. While there are various configurations that can be used to bring a plasma in contact with a liqu....

Protocol

1. Constructing the Experimental Setup

Note: To run this experiment, assemble a system consisting of a plasma reactor where the reaction will take place, optical components for absorbance measurements, and the electronic lock-in amplification system to process the signal.

1. Construct the plasma electrochemical cell.
   1. Manufacture a reactor cell consisting of a transparent glass vessel, 50.8 mm (2 in) in diameter, with two optical windows at angles of approximately 20° down from the normal plane.
   2. Construct a non-permeable lid containing four orifices, which will be used to introduce the plasma ....

Results

As mentioned in step 5 of the procedure, this experiment measures the cosine and sine components of the absorbance signal, the phase angle between them, and the magnitude of the signal. A plot of the magnitude of the signal and its two components is shown in Figure 4.

Occasionally, there will be measurements which may be not optimal or even unusable. This may be due to a misalignment of the laser wi.......

Discussion

The results show that the measurement of absorbance of light at the plasma-liquid interface is an effective method to detect and measure the concentration of plasma-solvated electrons in an aqueous solution. The subsequent measurement at different wavelengths results in the measurement of the absorption spectrum. Though this experiment was done in an aqueous NaClO₄ solution, the methodology should be valid for a great variety of other liquids, provided that electrons can solvate in the liquid.

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Function Generator	Protek	B8055
Lock-in Amplifier	Stanford Research Systems	SR830
High-Voltage Power Supply	Stanford Research Systems	PS325
Photodetector	Self-built
Flowmeter	Key Instruments	60310 R5
Flow controller	Omega Engineering	FMA 5400A/5500A
Camera	Dino-lite	Dinocapture 2.0
Voltmeter	Amprobe	AM-510
Optical Cage System	Thorlabs		30 mm cage system
Goniometers	Thorlabs	RP01 - Ø2	Manual rotation stage
Diode lasers	Thorlabs
Electrochemical cell	Adams & Chittenden Scientific Glass	Custom-made product
Stainless steel capillary	Restek		0.007 in. ID
SHV Coax Cable	SRS	Custom-made product
Sodium Perchlorate	Sigma-Aldrich		ACS reagent, ≥98.0%
Argon	Airgas	AR UHP300	Ultra-high purity
LabVIEW	National Instruments		Software used to generate in-house program used to collect data