As we know, cold plasma contains reactive species, ions, excited atoms, photons, free radicals, gas atoms and molecules, etc. In cold plasma, the energy from electrical power will be transferred to electrons and these high energy electrons will start subsequent chemical reactions that will result in different ions, excited atoms, free radicals, etc.
Now my question is,
1) How the initial high energy electrons are produced? Is the presence of any gas molecule needed to produce high energy electrons from it, which will start the subsequent reactions? and is there any other source to produce the initial high energy electrons in cold plasma except gas molecules? such as the production of free electrons from the surface of the electrodes!? Like in a complete vacuum condition!
2) The major effect of cold plasma on substrates is due to reactive species and free radicals. I was wondering if the free electrons and the high energy between the electrodes (in DBD) can contribute to the degradation of a chemical sample?
3) If the response to question 2 is yes, Is there any way to prevent the reaction of reactive species with our sample and so to subtract their effect (like by using a particular barrier, etc) and assess the effect of UV light, and high energy electron on the degradation of a chemical compound using a DBD cold plasma system?
Dear Ehsan,
About your second question, I know that these high energy species could cause some chemical reactions in the samples which could result in color change and lipid peroxidation. And to answer your third question, you might be able to asses the effect of long-lived and short-lived species separately by placing your samples at different distances from the voltage source, but before that you need to define the species at different distances. this might help you group the species which are responsible for the chemical reactions. As far as I know the high energy free electrons have very short life-time and they tend to react quickly after producing.
please see below paper for more information:
https://iopscience.iop.org/article/10.1088/0022-3727/47/28/285404/meta
I hope it helps.
Thank you for your response, Saeedeh Taheri.
By high energy species, do you mean high energy electrons? If yes, could you please share with me the article regarding the effect of high energy electrons on color change and lipid peroxidation?! That article could give me an idea of the reaction mechanism of high energy electrons.
Regarding your suggestion, as we are using the DBD (dielectric barrier discharge) plasma system, It is not possible to put the sample at different distances from the plasma source as we can do by Jet plasma. However, I have decided to try indirect DBD treatment at an in-package mode to be able to assess the effect of long-lived reactive species on the degradation of the chemical.
Defining the species is really complicated in plasma as many reactions are going on at the same time and these reactions and the concentrations of the reactive species could be totally different at different treatment times due to the active nature of the species. Defining the plasma chemistry requires collecting all the reactions involving all the possible species (electrons, positive and negative ions, metastables, radicals, etc). This can correspond to a hundredth of reactions and species. Moreover, a slight change in the voltage, medium gas ratio, flow rate, relative humidity, substrate physicochemical characteristics, etc can impact the production and concentration of the reactive species which makes it difficult to define the species at a particular time.
Thank you for your help.
Dear Ehsan,
By the species, I meant all the species which are present in the plasma and as far as I know, the mechanisms and the responsible species for each effect have not been fully understood. However, the colour change, for example, could be caused by singlet oxygen as it is explained in the below paper:
Article Afterglow of atmospheric non-thermal plasma for disinfection...
the effect of plasma on lipid could be found in the below paper:
Article Effect of Cold Atmospheric Pressure Plasma on Maize Seeds: E...
there are also some useful explanation about the effect of the plasma on the quality attributes of food products in the below paper that you might find interesting:
Article The Potential of Cold Plasma for Safe and Sustainable Food Production
An important point is that the high energy electrons have very high temperature (in the order of 1000K) and they react quickly (nanoseconds) with other molecules and species present in the plasma. therefore, in a non-equilibrium plasma, the high energy ions and molecules (depends on the gas), which have much lower temperature, are responsible for the decontamination and the quality change. A large difference between the temperature of the free electrons and the ions is one of the very interesting characteristics of the cold plasma which has made it suitable for food applications.
About the measurement of the reactive species (ions, atoms, molecules,... and not the electrons), an spectrometer like OES (Optical Emission Spectrometer) or FTIR might be useful to have an estimation of the species at a specific dosage of the plasma and at a specific location inside the chamber. Please see below paper:
Article Plasma inactivation of Aspergillus flavus on hazelnut surfac...
All the best.
Thank you Saeedeh Taheri for your valuable response.
Actually, in our current project, we have used different gas mixtures in DBD plasma and we have characterized the type and concentrations of reactive species using OES and Drager tubes.
However, using different gas ratios of O2 and N2 (with different reactive species concentration) did not significantly affect the degradation rate of our toxin. Hence, my hypothesis is that degradation of our particular toxin could be due to factors other than reactive species, such as the high energy electrons which can break the bonds in the chemical structure of the toxin. Hence, I am looking for a way to evaluate my hypothesis.
Using indirect DBD treatment could be helpful; however, if there is any other way to do, I will be glad to hear it.
First, you create a plasma, which means you have some electrons(which were initially bound in high energy states) with sufficient energy not to be recaptured. These free electrons have an energy distribution (Maxwellian or not). If your question is why not all of them have the same energy, the answer is collisions
The electrons that start a plasma reaction must be already free electrons and not bound to any atom. The E or B fields used to create a typical cold plasma are not sufficient to remove bounded electrons. The best answer I have heard so far is that gamma rays or other high energy natural interactions with the gas will produce free electrons (a very small amount but still something). This small density of electrons will be what the electrical sources are using to create more net charged species. Once the plasma is ignited, only a small fraction of the total free electrons will obtain high enough energies to subsequently produce charged species that sustain the plasma. The dominant generation of charged species in cold plasmas is via electron collisions. Electronegative plasmas are more complicated; some species are made simply by electron attachment and are a loss mechanism of already free electrons. Fortunately, the electrons being used for this generation of charged particles wasn't from the high energy electron population needed to generate more free electrons from the neutral gas.
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