Glycine Oligomerization by Pulsed Discharge Plasma over Aqueous Solution under Atmospheric Pressure

Abstract

Chemical reactions of amino acids induced by discharge plasma are important for understanding the mechanism of biological effects of discharge plasma in biomedical applications. In this study, we generated a nano-second pulsed discharge plasma under atmospheric pressure over an aqueous solution containing glycine. The reaction products after the pulsed discharge plasma treatments were analyzed by high-performance liquid chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy. The oligomerization reaction of glycine was induced in aqueous solution and produced glycine oligomers at the beginning of the discharge plasma. However, the glycine oligomers were decomposed into products with low molecular weight by excessive pulsed discharge plasma. According to comparative experiments, physical force of the plasma is believed to induce the glycine reaction. Moreover, the reactions depended on the pH, but not the conductivity, of the glycine solution. Glycine in aqueous solution was reacted by the discharge plasma only at neutral pH because the reaction proceeded only when glycine ions were in the zwitterionic state. Anions and cations of glycine reacted very little under the discharge plasma.

Pulsed Discharge Plasma in High-Pressure Environment for Water Pollutant Degradation and Nanoparticle Synthesis

“…Due to these phenomena, the carbon materials generation from the decomposition of glycine compound also increased with increasing operating pressure. Even though the reason for this is not clear yet, it explains why the carbon material was formed only at the high operating pressures [40][41][42][43].…”

“…The gaps between the atomic layers in these carbon products were around 0.34 nm, where the 0.34 nm distance can be associated with the lattice spacing of graphene layers [111][112][113][114]. This revealed that the carbon materials could be generated by applying the high-voltage discharge plasma on the glycine solution through deamination and decarboxylation reactions [40][41][42][43]98]. Li et al conducted experiments for graphene nanosheet synthesis by applying pulsed arc discharge plasma to petroleum asphalt with a liquid water medium [111].…”

“…The main apparatus was the batch reactor (SUS-316, AKICO, Tokyo, Japan) with a volume of 15 mL and a pulsed DC power supply (MPC; Suematsu Electronics MPC2000S, Kumamoto, Japan) to generate an electrical discharge plasma. The experimental procedure was presented elsewhere [37][38][39][40][41][42][43]. To start the experiment, the SUS reactor was manually filled with around 5 to 7 mL of the starting material.…”

“…As mentioned above, there are two phenomena occurring when applying high-voltage discharge plasma in the gas-liquid water system containing an organic substance, namely, electrode erosion and organic substance decomposition. The electrode erosion may generate metal nanoparticles [129][130][131][143][144][145], and the decomposition reaction of an organic substance, e.g., glycine compound, may generate carbon materials [37][38][39][40][41][42][43]. Hence, during the high-voltage discharge plasma treatment, the interaction between the carbon and titanium atoms produced from the decomposition reaction of the glycine compound and the erosion of titanium rod electrode occurred and could result in titanium carbide (TiC) nanoparticles (see Figure 14b).…”