Reproducibility of 'COST reference microplasma jets'

Abstract

Atmospheric pressure plasmas have been ground-breaking for plasma science and technologies, due to their significant application potential in many fields, including medicinal, biological, and environmental applications. This is predominantly due to their efficient production and delivery of chemically reactive species under ambient conditions. One of the challenges in progressing the field is comparing plasma sources and results across the community and the literature. To address this a reference plasma source was established during the 'biomedical applications of atmospheric pressure plasmas' EU COST Action MP1101. It is crucial that reference sources are reproducible. Here, we present the reproducibility and variance across multiple sources through examining various characteristics, including: absolute atomic oxygen densities, absolute ozone densities, electrical characteristics, optical emission spectroscopy, temperature measurements, and bactericidal activity. The measurements demonstrate that the tested COST jets are mainly reproducible within the intrinsic uncertainty of each measurement technique.

Keywords: COST reference microplasma jet; atmospheric pressure plasma jet; biomedical applications of plasmas; capacitively coupled radio frequency discharge; plasma medicine; power measurements.

Figure 1.

(a) Measured characteristics of plasma power versus effective voltage, and (b) measured effluent gas temperature at 3 mm distance from the nozzle versus plasma power, both for standard feed gas of 1 slm He with 0.5% oxygen admixture. Black dots indicate the result obtained from one of the COST jets. The coloured areas are the standard deviation between the four COST jets, explanation provided in text. The red horizontal line in (b) indicates the maximum permissible temperature for regular treatment of biological samples.

Figure 2.

(Left) thermal image taken for 16 mm distance between nozzle and sample surface, standard feed gas (1 slm He with 0.5% oxygen admixture), and reduced plasma power (0.3 W). (Right) schematic of the corresponding experimental setup.

Figure 3.

(a) Measured maximum central surface temperature as a function of the distance between the jet nozzle and sample. (b) Measured lateral surface temperature profiles at different nozzle to sample distances. Both for standard feed gas (1 slm He with 0.5% oxygen admixture and at reduced plasma power (0.3 W).

Figure 4.

Intensity ratios from optical emission measurements as a function of plasma power for standard feed gas of 1 slm He with 0.5% oxygen admixture. The considered atomic lines, He 706 nm, O 777 nm, and O 844 nm, are labelled according to their wavelength.

Figure 5.

Measured ozone density in the far effluent of the COST jets as a function of the plasma power for standard feed gas of 1 slm He and 0.5% oxygen admixture.

Figure 6.

Measured atomic oxygen density at 1 mm distance from the COST jet nozzle as a function of the plasma powers for standard feed gas of 1 slm He with 0.5% oxygen admixture.

Figure 7.

Representative images of bacterial plates treated by COST jets. 100 μl of E. coli MG1655 at approximately 8 × 10⁶ cfu ml⁻¹ were plated onto LB agar plates and exposed to a 2 min treatment by COST jet. The plasma power was kept at 0.3 W, and the feed gas was 1 slm helium with 0.5% oxygen admixture. The top panel shows a representative plate for each jet, plates following 2 min COST jet treatment and overnight incubation. Representative control plates are shown in the bottom panel. The gas-only control was also treated for 2 min but the plasma power was turned off (therefore only the helium/oxygen gas was incident on the sample), and the untreated control was plated identically to all the other treatment plates, but did not receive a COST jet treatment. For each jet, treatments were carried out in triplicate.

Figure 8.

Figure showing AOI and bacterial log reduction following treatment with the different COST jets. The red points show the average AOI induced by each jet, with the error bars showing the standard deviation. The blue points show the mean log reduction in E. coli MG1655 cfu following treatment with each jet, with the error bars showing standard deviation. For each jet, treatments were carried out in triplicate. Conditions were as stated in figure 7.