Chemistry Reference
In-Depth Information
enhance the infiltration of bioactive plasma compounds (e.g., short-lived ROS) into
cells. Higher electric fields cause irreversible membrane damage, and thus lysis of
cells andnanosecondpulses withhighamplitudes havethepotential toaffect transport
processes across subcellular membranes or cause apoptosis. However, electric fields
as well as heat are not considered to play a dominant role in the inactivation by means
of nonthermal plasmas.
The mechanisms of plasma interaction with MO are rather complex and can
be very specific. They are not only determined by the type of MO but also by its
environment and condition.
8.2.2.3.2 Processes of Decontamination
Quite different inactivation kinetics were observed by means of plasmas. Beside
IC with a single line (continuous effect, exponential decay of CFU), curves with
two or more consecutive lines, each with different D
i
(number of phase
i
1, 2, 3)
have been observed. The multitude of different IC disclose the high complexity of
physical and biochemical processes by means of plasmas. Often the effect of the
plasma is not continuous, i.e., several processes are involved, and indeed synergistic
effects are possible. The kinetics of inactivation depends on the type of MO, the
type of substrate, as well as the specific plasma parameters and conditions (gas or
gas mixture, plasma generation method, direct or remote exposure of samples
∗
). In
Figure 8.25, IC with two inactivation phases (with
D
1
<
=
D
2
) are shown (see figure
caption for experimental details).
In medium- and low-pressure plasmas, the UV photons are considered to play
a dominant role [102,114]. In the examples shown in Figure 8.25, the first phase
with the smaller
D
-value is due to the inactivation of genetic material of isolated MO
(endospores) or of the first layers of stacked spores by UV irradiation. The
D
1
-value in
Figure 8.25 decreases with the nitrogen content due to the optimization of UV emis-
sion that is caused by the molecular band of NO in these plasmas. This phase stops
after a certain treatment time because of the limited penetration depth of UV photons
into stacked spores or spores covered with various debris. The following, slower
phase is attributed to erosion processes of the covering material. Photon-induced
intrinsic desorption by UV leads to the formation of volatile by-products (e.g., CO,
CH
x
) from the MO material. Furthermore, reactive species from the plasma (O, OH,
O
2
) are adsorbed on the MO and subsequently undergo chemical reactions forming
VOCs and CO
2
and H
2
O (spontaneous etching). Recent experiments where spores of
Bacillus atrophaeus
were exposed to beams of hydrogen atoms and Ar
+
ions have
shown that chemical sputtering due to the simultaneous impact of H atoms and argon
ions at around 100 eV causes a very effective etching and perforation of spore coats
[120]. The etching process can be enhanced by UV photons (UV-induced etching),
and the erosion rate increases with substrate temperature. It has also been described
in the literature that UV inactivation can come into action again when the remain-
ing living MO have been sufficiently uncovered from debris. In [121] spot (about
1.4 10
7
MO/cm
2
) and spray contaminated (6.4 10
4
MO/cm
2
) samples were treated
with identical plasmas (low-pressure microwave-sustained planartron discharge).
∗
Remote exposure (or afterglow): The sample is placed at some distance from the plasma and thus charged
and short-lived species are not in contact with it.
