This project developed a fluorescence spectroscopy method to quantify OH radicals produced by different plasma reactors. Sodium salicylate was used as a fluorescent probe to scavenge OH radicals. Three plasma reactors - liquid discharge, gas discharge, and gas discharge with bubbling - were compared. The fluorescence intensity of sodium salicylate decreased over time in each reactor, indicating OH radical scavenging. However, differences between reactors were not observed, likely because sodium salicylate is too hydrophilic to concentrate at plasma-liquid interfaces where most OH radicals form. This method provides a basis for comparing OH production between reactors but requires a more hydrophobic probe.

1. Fluorescence Spectroscopy for Quantifying OH Radicals
Produced by Electrical Discharge Plasmas
Stephanie Hernandez1,2 and Selma Mededovic Thagard3
1ASSETs to Server Humanity REU Program, Clarkson University
2Washington and Lee University, Department of Physics & Engineering
3Clarkson University, Department of Chemical and Biomolecular Engineering
Symposium on Undergraduate Research Experiences (SURE), July 30, 2015
This project was supported in part by the National Science Foundation under Grant No. EEC-
1359256.
1
Gas Discharge Plasma

2. Plasma:
A “soup” of electrons, ions, radicals and neutral molecules
in an ionized gas which can be produced by electrical
discharge in a liquid or gas.
2
Motivation and Objectives
Motivation:
Plasma can be used for drinking and waste water treatment. As an
Advanced Oxidation Process (AOP), its efficiency is proportional to the
rate of ·OH radical production.
Objective:
 Development of a new method for quantifying ·OH radicals based on
fluorescence spectroscopy that uses sodium salicylate as a reagent.
 Comparison of ·OH radical production rates in three different plasma
reactors: liquid discharge, gas discharge, and gas discharge with
bubbling.

3. Fluorescence: The radiation emitted by certain
substances when excited by a wavelength of light. The
excitation and emission occur at different wavelengths.
3
Fluorescence Spectroscopy
Advantages:
 Sensitive
 Quantitative
 Quick
 Safe
Fluorescence Spectrophotometer
Ocean Optics Cuvettes

4.  ·OH scavenger - good selectivity
 Very soluble in water
 Affordable compared to other fluorescent probes
 High quantum efficiency
 Linear response over a wide spectral region
4
Sodium Salicylate (NaSCL)

5. 5
Experimental: Plasma Reactor
Experimental Parameters
Volume 100mL
voltage 20 kV
Repetition Frequency 52 Hz
Salicylate 3.5mg/L or 10 mg/L
Argon Flow Rate 440 mL/min
Liquid Discharge Plasma Gas Discharge Plasma Gas Discharge Plasma
with Bubbling
Experimental Parameters
Volume 1500mL
voltage 20.6 kV
Repetition Frequency 52 Hz
Salicylate 3.5mg/L or 10 mg/L
Argon Flow Rate 605 mL/min
Experimental Parameters
Volume 1500mL
voltage 20.6 kV
Repetition Frequency 52 Hz
Salicylate 3.5mg/L or 10 mg/L
Argon Flow Rate 605 mL/min

6. Methods & Data Collection
 Prepare a calibration curve.
 Run 3.5 mg/L and 10 mg/L
Solutions of NaSCL in plasma
reactors.
 Collect 3 mL samples every
10 min for fluorescence
spectroscopy analysis.
6
Experimental: Analytical Method
Salicylate Emission Spectra
Intensity(a.u)
10min
20min
30min
40min
0min
Wavelength (nm)

7. 7
Results: Fluorescence
0
100
200
300
400
500
600
700
800
0 10 20 30 40 50
Intensity(a.u)
Time (min)
Fluorescence Quenching of
NaSCL in Liquid Discharge
Plasma Reactor
10mg/L NaSCL
3.5 mg/L NaSCL
0
100
200
300
400
500
600
700
800
0 10 20 30 40 50
Intensity(a.u)
Time (min)
Fluorescence Quenching of
NaSCL in Gas Discharge
Plasma Reactor
3.5 mg/L Gas Discharge
3.5 mg/L Gas Discharge with Bubbling
10 mg/L Gas DIscharge

8. Data Presentation/Findings
8
Results: Hydrogen Peroxide 2 2
OH OH H O
OH salycilate products
 
 
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 10 20 30 40 50
Concentration(mM)
Time (min)
Hydrogen Peroxide
Concentration vs Time
0 mg/L NaSCL
3.5 mg/L Liquid Discharge
10mg/L Liquid Discharge
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 10 20 30 40 50
Concentration(mM)
Time (min)
Hydrogen Peroxide
Concentration vs Time
0 mg/L NaSCL Gas Discharge
3.5 mg/L Gas Discharge
3.5 mg/L Gas Discharge with Bubbling
10 mg/L Gas Discharge

9. OH
9
Cathode
Direction of
e- movement
Plasma interior
Interface
Bulk liquid
Anode
Direction of
e- movement
e-
aq
Small surfactant (PFBA)
Polar head
Nonpolar tail
Non-surfactant
(Salicylate)
Large surfactant
(PFOA, PFOS, Gemfibrozil)
Nonpolar tail
Polar head
OH

10.  Fluorescence spectroscopy appears to be a good technique for
relative comparison of OH radicals produced by electrical
discharge plasmas.
 Salicylate is not hydrophobic enough (i.e., it does not lower the
surface tension of the solution) so its concentration at the
plasma-liquid interface is not high enough. Not all ·OH radicals
can be scavenged.
 Because salicylate is hydrophilic, not a significant difference
between the three plasma reactors was observed.
Data Presentation/Findings
10
Conclusions

11. References
1. M.F. Al-Kuhaili, “A study of the Fluorescent Properties of spin-coated Sodium Salicylate Films,” Journal of
Luminescence, vol. 117, iss.2, pp. 209-216, 2006.
2. Rahmi and H. Itagaki “Application of 2,5 Dihydroxybenzoic Acid as a fluorescent probe to the Clarification
of Microevniorment in Hydrogels of Biopolymers,” Journal of Photopolymer Science and Technology vol. 24,
no.5, pp.517-521, 2011.
3. M. Karima, H.Leeb, Y. Kimb, H. Baeb, S. Lee “Analysis of salicylic acid based on the fluorescence
enhancement of the As(III)–salicylic acid system” vol. 576, iss 1, pp. 136-139, 2006.
S. Kanazawa, T. Furuki, T. Nakaji, R. Ichiki, “Application of Chemical dosimetry to hydroxyl radical
measurement during underwater discharge.,” Journal of Physics: Conference Series, 2013.
5. A. Gomes, E. Fernandes, J. Lima “Fluorescent Probes used for detection of reactive oxygen species,”
Journal of Biochemical and biophysical methods 2005
6. S. Kanazawa, T. Furuki, T. Nakaji, S. Akamine, R. Ichiki “Measurement of OH Radicals in Aqueous Solution
Produced by Atmospheric LF Plasma Jet,” 2005.
7.R. Joshi, S.Mededovic Thagard, “Streamer-Like Electrical Discharges in Water: Part II Environmental
Applications,” 2013.
11
References

12. Data Presentation/Findings
12
Acknowledgments
Mentors
Selma Mededovic Thagard
Special thanks to
Gunnar Stratton
Joshua Franclemont
Fei Dai
Xiangru Fan