Nature and kinetics of paramagnetic defects in chitosan induced by beta-irradiation of chitosan
A set of chitosan samples irradiated by electrons with various doses was studied using the EPR method. Two kinds of paramagnetic defects, PC1 and PC2, initiated by this irradiation due to the breakage of bonds in positions C5 and C1 of the chitosan structure, are revealed in the “amorphous” and “cry...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2018
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| Cite this: | Nature and kinetics of paramagnetic defects in chitosan induced by beta-irradiation of chitosan / A.A. Konchits, B.D. Shanina, I.B. Yanchuk, S.V. Krasnovyd // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2018. — Т. 21, № 4. — С. 336-344. — Бібліогр.: 25 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1860479659685183488 |
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| author | Konchits, A.A. Shanina, B.D. Yanchuk, I.B. Krasnovyd, S.V. |
| author_facet | Konchits, A.A. Shanina, B.D. Yanchuk, I.B. Krasnovyd, S.V. |
| citation_txt | Nature and kinetics of paramagnetic defects in chitosan induced by beta-irradiation of chitosan / A.A. Konchits, B.D. Shanina, I.B. Yanchuk, S.V. Krasnovyd // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2018. — Т. 21, № 4. — С. 336-344. — Бібліогр.: 25 назв. — англ. |
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| description | A set of chitosan samples irradiated by electrons with various doses was studied using the EPR method. Two kinds of paramagnetic defects, PC1 and PC2, initiated by this irradiation due to the breakage of bonds in positions C5 and C1 of the chitosan structure, are revealed in the “amorphous” and “crystalline” samples of chitosan. The structure of defects, their spectroscopic parameters, and the kinetics of accumulation/decay have been established for the first time. It is found that the EPR spectrum of the “crystalline” samples consists of 10 almost equidistant lines of the super-hyperfine (SHF) structure with the splitting between them A = 7.4 G for the PC1 center, and a single wide line with a markedly different g-value, attributed to the PC2 one. Both these lines are also present in powder “amorphous” samples, but the SHF structure of the PC1 centers in them is not registered because of the broadening of the individual SHF components. Kinetics of defect accumulation with increasing dose D of the irradiation, and their gradual disappearance during prolonged storage of samples in air, were discovered and studied. Kinetic equations were solved, and the D-dependence and decay times were found from the comparison of theoretical results with the experimental ones. It has been shown that the concentration of shallow and deep traps for electrons affects the rate of the decay process. The recovery process is much slower in the samples having a more perfect crystalline structure.
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ISSN 1560-8034, 1605-6582 (On-line), SPQEO, 2018. V. 21, N 4. P. 336-344.
© 2018, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
336
Semiconductor Physics
Nature and kinetics of paramagnetic defects
induced by beta-irradiation of chitosan
А.А. Konchits, B.D. Shanina, І.B. Yanchuk, S.V. Krasnovyd
*
V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, 03680 Kyiv, Ukraine
*
E-mail: sergkrasnovyd88@gmail.com
Abstract. A set of chitosan samples irradiated by electrons with various doses were studied
using the EPR method. Two kinds of paramagnetic defects PC1 and PC2 initiated by this
irradiation due to the breakage of bonds in positions C5 and C1 of the chitosan structure are
revealed in the “amorphous” and “crystalline” samples of chitosan. The structure of defects,
their spectroscopic parameters, and kinetics of accumulation/decay have been established
for the first time. It is found that EPR spectrum of the “crystalline” samples consists of 10
almost equidistant lines of the super-hyperfine (SHF) structure with the splitting between
them A = 7.4 G for PC1 center, and a single wide line with the markedly different g-value,
attributed to the PC2 one. Both these lines are also present in powder “amorphous”
samples, but the SHF structure of the PC1 centers in them is not registered because of
broadening the individual SHF components. Kinetics of defect accumulation with
increasing dose D of the irradiation, and their gradual disappearance during prolonged
storage of samples in air was discovered and studied. Kinetic equations were solved, and
the D-dependence and decay times were found from the comparison of theoretical results
with the experimental ones. It has been shown that the concentration of shallow and deep
traps for electrons affect the rate of the decay process. The recovery process is much slower
in the samples having a more perfect crystalline structure.
Keywords: chitin, chitosan, EPR, β-irradiation, super-hyperfine splitting.
doi: https://doi.org/10.15407/spqeo21.04.336
PACS 61.82.Pv, 76.30.Rn, 82.35.Pq, 87.53.Ay, 87.80.Lq
Manuscript received 01.11.18; revised version received 00.00.18; accepted for publication
00.00.18; published online 00.00.18.
1. Introduction
Chitin and its deacetylated derivative chitosan (see
Fig. 1) possess unique physical and chemical properties.
Chitin and chitosan are non-toxic, bioactive,
biocompatible, and widely applicable, in particular, both
in the nanoparticle form and antioxidant in anticancer
therapy, in theranostics, and so on. They have
antimicrobial activity, are able to absorb heavy metals [3,
11, 14-16, 19, 22, 25] and used as a hemostatic (like to
Celox) and antimutagenic drugs [23]. In recent years,
chitosan has been used in biosensors [12], as well as in
composite chitosan-Ag NPs as SERS substrate [18].
Chitin is a linear polysaccharide with unbranched
chains composed of elementary units 2-acetamido-2-
deoxy-D-glucose, connected by the glycosides bonds.
The macromolecule of natural chitin contains a small
amount of units with the free primary amino group NH3.
The large length of the chain and its flexibility help to
create a complicated super-molecular structure of highly
oriented macromolecules of 15…25 nm in length,
consisting in turn of micro-fibrils with ~3 nm in diameter
for α-chitin [13]. The giant β-chitin crystallites (~50 nm
in diameter, ~3 µm in length) were observed for chitin
embedded into a protein matrix [5].
Chitosan is usually obtained by the treatment of
chitin with the 40…50% aqueous NaOH solution at
110…140 °C [11]. According to the nomenclature of the
European Chitin Society (EUCHIS), chitin and chitosan
are determined through the degree of insolubility (chitin)
or solubility (chitosan) in the acetic acid [8, 20].
The main methods of the chitin or chitosan
modification amount to the reduction of the molecular
weight, MW, due to destroying the amino group by γ-
and β-irradiation, ultrasonic treatment, as well as by
mixing with various chemical solvents, additives or
doping of the material with hydrogen. In the case of
ionizing radiation, it can lead to breaking the
chitin/chitosan chains, bond crossing and opening the
rings, which results in the irreversible chemical change.
The radiolysis degradation mechanism of chitosan was
studied at various temperatures by using γ-
60
Co rays with
doses of 5 to 300 kGy [2, 4]. Participation of the amino
groups in the mechanism of radiolysis is one of the
important problems of chitosan radiation stability
because this treatment can produce toxic effects.
SPQEO, 2018. V. 21, N 4. P. 336-344.
Konchits А.А., Shanina B.D., Yanchuk І.B., Krasnovyd S.V. Nature and kinetics of paramagnetic defects induced by …
337
Fig. 1. Сhemical structures of chitin (1) and chitosan (2).
The electron microscopy, FTIR, Raman
spectroscopy and electron paramagnetic resonance (EPR)
are used to study the degradation products and modified
structure of the chitosan. The EPR play a specific role,
since it is the direct method for studying the properties of
any paramagnetic centers (PC) produced under
irradiation.
In [6, 24], the EPR spectra have clearly shown a
significant effect of chitosan on the suppression of the
superoxide anion radicals and lipid radicals of the
linoleic acid. Depending on the source and production
way of the chitin and chitosan, the presence of PC with
the g-factor of 2.0012 to 2.0035 is shown. The
multicomponent EPR spectra were recorded in [4] for the
chitosan after γ- or e-beam irradiation. At doses above
200 kGy, a new signal consisting of asymmetric triplet
with an overall splitting of 68 G belonging to nitroxyl-
type radicals have been detected.
The present study is devoted to the nature and
kinetic characteristics of PC, induced in chitosan by β-
irradiation in low sterilization doses 5…35 kGy at room
temperature. Based on the highly resolved super-
hyperfine structure (SHFS), observed for the first time,
the nature of the centers designated as PC1 and PC2 has
been established. The dynamic and kinetic characteristics
of PC, and the ability of system to recover after β-
irradiation have been investigated.
2. Materials and methods
Samples of the commercial chitosan were used after
preliminary cleaning. Part of the obtained powdered
samples (hereinafter denoted as X1) was undergone to
recrystallization in the study process (hereinafter denoted
as X2). Paramagnetic properties of the samples were
studied both before and after irradiation with a beam of
fast electrons with energy of 2 MeV, which was carried
out by means of an accelerator ILA-6. EPR
measurements were carried out at room temperature
using the X-band spectrometer “Radiopan” SE/X-2244.
The concentration of PC was determined using the
reference sample MgO:Cr
3+
(g = 1.9799) with the
number of spins Ns = 3.4·10
14
. Paramagnetic properties
were also studied briefly in some subsidiary materials
that are commonly used in production of Celox (solvents,
fillers etc.). Samples were put in the silica tubes
produced by Wilmad–Lab glass firm, which have no
false EPR signals.
3. Results and discussion
3.1. General characteristics of the EPR spectra
Fig. 2 represents EPR spectra of the initial chitosan
sample X1, initial and purified celox samples and sample
of the solvent (succinic acid) before irradiation. The
chitosan and Celox samples show the intensive wide EPR
signals with g ≈ 2.55, ∆Hpp ~ 0.6 kG for chitosan, and
g ≈ 2.37, ∆Hpp ~1.3 kG for Celox sample. These signals
are similar to those observed earlier in [9], and can be
attributed to uncontrolled impurities of iron oxides. In
addition, there is a narrow line in the spectrum of Celox
sample with g ≈ 2.004, ∆Hpp ~10 G (Fig. 2, curve 2),
where characteristics of which coincide with those for the
structure defects revealed in [21]. In the spectrum of X1
samples, there is a narrow line as well (Fig. 2, insert), but
its origin is associated with nitroxyl-type radicals [4]. In
the recrystallized samples X2, the EPR signals are
absent. After purification, all the iron related signals
essentially decrease, see for example (Fig. 2, curve 3). In
purified samples X1, a signal from nitroxyl radicals is
still recorded.
After irradiation, the paramagnetic properties of the
samples X1 and X2 (further only these samples will be
studied) change significantly, demonstrating strong EPR
signals. Fig. 3a shows, for example, the ESR spectrum of
sample X1 after irradiation with the dose of 25 kGy. It is
seen that spectrum is a single line with some bend in the
center of the spectrum.
In contrast, the EPR spectrum of sample X2
(Fig. 3b) demonstrates well resolved super-hyperfine
splitting (SHFS). Preliminary analysis of this spectrum
showed that it consists of 5 low-field lines, belonging to
1500 3000 4500
-1000
0
1000
E
P
R
s
ig
n
a
l,
a
.u
.
Magnetic Field, G
2
3
4
1
3300 3375
-150
0
150
E
P
R
s
ig
n
a
l,
a
.u
.
Magnetic Field, G
Fig. 2. EPR spectra of the initial unpurified chitosan sample X1
(1), initial (2) and purified (3) Celox samples; and solvent
sample (succinic acid C4H6O4) (4). Insert in Fig. 2: part of the
EPR spectrum of sample X1, recorded with accumulation; this
small signal belongs to nitroxyl radicals [4].
SPQEO, 2018. V. 21, N 4. P. 336-344.
Konchits А.А., Shanina B.D., Yanchuk І.B., Krasnovyd S.V. Nature and kinetics of paramagnetic defects induced by …
338
3280 3320 3360 3400
-1000
0
1000
E
P
R
s
ig
n
a
l,
a
.u
.
Magnetic Field, G
MgO: Cr
3+
Sample X1
a)
PC1*
PC2
3280 3320 3360 3400
-1000
0
1000
E
P
R
s
ig
n
a
l,
a
.u
.
Magnetic Field, G
Sample X2
b)
PC1
PC2
Fig. 3. EPR spectra for the samples Х1 (a) and X2 (b) in some
days after β-irradiation with the dose 25 kGy. Dashed lines are
the result of fitting the theory with experiment. ν = 9373 MHz.
EPR transitions with nuclear spin +Iz and 5 high-field
lines for transitions with –Iz; as well as a wide central line
that belongs to another spectrum. It should be
emphasized that the observed SHFS spectrum is almost
equidistant. It was found that if the dimensions of the
crystallites in the sample X2 decrease to the submicron
ones, SHFS is presented in the spectrum with poor
resolution because of the greater width the separately
components. It was found also that possible anisotropy of
the g-factor and SHFS tensor A is not greater than the
width of individual SHF components. Indeed, the EPR
spectra recorded for a single crystallite of the sample X2
(~3×2×0.4 mm) at different orientations of the magnetic
field relative to the plane of the sample (||, ⊥, and at 45°)
did not show any noticeable changes within the limits of
measurement accuracy.
3.2. Analysis of the EPR spectra
First of all, let us address the question of the reason for
the specific shape of the observed spectrum for powder
sample X1 (Fig. 3a), which is characterized by a smaller
amplitude of the peak high-field signal and a clearly
observed bend near the center of the spectrum. There are
two possible reasons for this feature. It could be either a
single resonance line of the paramagnetic centers (PC)
with an anisotropic g-tensor, or the spectrum of two
resonance signals with slightly different isotropic g-
factors.
To test the assumption 1, the signal was presented
by the Lorentzian function with the resonance frequency
hω = βH·geff, depending on the angle θ between magnetic
field H and the main axes of g-tensor:
( ) 212222
|| sincos θ+θ= ⊥gggeff with Hgg β⋅− ⊥
22
||
comparable with the line width. The calculated
absorption signal was averaged over the angle (θ), after
that the result was differentiated for comparison with the
experimental curve. The fitting to the experimental signal
forced to abandon the first case, because the PC with an
anisotropic g-tensor gives a high-field wing with a
greater amplitude than the low-field one opposite to the
experimental curve feature. Consequently, the spectra
belong to a different paramagnetic centers (further PC1
and PC2) with the spin S1 = S2 = 1/2 and slightly different
isotropic g-factors found to be equal to gPC1 =
2.0048 ± 3·10
–4
, and gPC2 = 2.0031 ± 3·10
–4
.
The spectrum for the sample X2 (Fig. 3b) was
described by the theoretical function as a sum of eleven
Lorentzian lines. Calculation of the theoretical spectrum
and fitting to the experimental one was made in ORIGIN
program, by using the least square method. The results of
this fitting have been summarized in Table 1.
The analysis has shown that all the super-hyperfine
lines (1 to 10 in Table 1) belong to the centers, the nature
of which is below identified as being identical with the
PС1 centers in the samples X1, but the SHF structure in
which is not resolved due to greater amorphicity of the
samples X1.
To understand what a free radical is with the SHFS
structure found, let us suppose that electron spin interacts
with some non-equivalent nuclei with the spin I = 1/2.
The most probable candidates are hydrogen atoms in the
structure shown in Fig. 1b. Hamiltonian of the system is
as follows:
Table 1. Magnetic resonance field and EPR line width values
for resonance transitions in the spectrum of Fig. 3b; ν =
9373 MHz.
Resonance
line
number
Experimental
resonance
field Hres, G
Line
width
∆Hpp, G
Calculated Hres
with constants
from Table 2, G
1 3307.6 3.5 3307.6
2 3315.0 3.5 3315
3 3322.4 4.6 3322.7
4 3330.2 5.8 3330.2
5 3337.7 6.4 3337.3
6 3343.5 5.8 3344.0
7 3351.2 5.8 3351.4
8 3357.8 5.8 3359
9 3366.5 4.6 3366.5
10 3373.9 3.5 3373.9
11 (PC2) 3343.2 11.6 –
SPQEO, 2018. V. 21, N 4. P. 336-344.
Konchits А.А., Shanina B.D., Yanchuk І.B., Krasnovyd S.V. Nature and kinetics of paramagnetic defects induced by …
339
Table 2. Super-hyperfine constants for the paramagnetic
centers PC1.
Number of nucleus 1 2 3 4
|A||, j|, G 28.6 15.5 14.8 7.4
aj ≈ bj, G 9.5 5.2 4.9 2.5
( )∑ ∑ +−−+⊥ +⋅++β=
4
1
4
1
,,,,||, 5.0 jjjjzjzzz ISISAIASSHgH
(1)
where A|| = a + 2b, A⊥ = a – b with the Fermi constant
2
)0(
3
8
ψββ⋅
π
= nngga and dipole-dipole constant
3
3
2 −ββ= rgg
h
b nn . The number of paramagnetic nuclei
that interact with electron have to be larger than three,
because three non-equivalent nuclei with I = 1/2 form
only 2
3
states with the electron spin Sz. As soon as there
are ten resonance transitions for PC1, the electron spin
interacts with the minimum four non-equivalent nuclei
(2
4
is enough to form ten transitions). The procedure is to
find energies for 16 states of Hamiltonian (1), to
calculate the resonance frequencies for 16 transitions and
to compare with the experimental resonance fields from
Table 1. The central point of the spectrum Hres, 0 is found
to be equal to 3340.75 G, which corresponds to g-factor
g(PC1) = 2.0046 (±3·10
–4
). The total distance between the
highest and lowest resonance transitions in Table 1 is
equal to 66.3 G and in theory is ( ) ( )0
2
,,|| 21 ωΣ−Σ ⊥ jjjj AA .
On the other hand, the latter high-field transition is
distant from Hres,0 by
( ) ( ) ( )0
2
,,|| 2121G3.33 ωΣ+Σ−= ⊥ jjjj AA . These two
equations give ( ) GA jj 3.00
2
, =ωΣ ⊥ and consequently the
2-nd order contribution is negligible. In order to find 4
constants A||,j, it is enough to use 4 transitions from
Table 1, after that to check the remaining resonance
frequencies using the found constants. The accordance
with the experimental resonance fields is satisfied. The
calculated A||,j values are given in Table 2; |A⊥, j| is less
than the line width of SHFS component and has an order
of the experimental error.
The obtained values of Aj and g-factor of the
paramagnetic center PC1 indicate that the free radical is
localized in the carbon position C5 of the chitosan
structure. It has two nearest hydrogen atom and two
distant hydrogen ones in the neighbor hydroxyl groups.
The broader line 11 in Table 1 does not exhibit a visible
SHF structure and by its characteristics completely
coincides with the PC2 centers observed in the X1
samples: g(PC2) = 2.0031 ± 3·10
–4
; peak-to-peak line width
∆Hpp = 11.6 G. The nature of the PC2 centers is most
likely related to the carbon dangling bonds in the C1
position of the chitosan structure.
3.3. Microwave saturation experiments and spin
relaxation of the paramagnetic centers
The relationship of centers PC1 and PC2 is justified by
the saturation of the signals and the spin relaxation times,
which are estimated by means of the signal continuous
saturation method. Fig. 4a shows the measured EPR
signals for the sample X2 at different microwave powers.
Fig. 4b shows the saturation curves for the centers 1 and
2, which are in agreement with the theory of saturation of
homogeneously broadened EPR signals [17]:
( )( ) ( )
21
2
1
2
2/1
21
2
1
2
0.5
max
25.01
25.0
const=PP
TTH
TTH
I
γ+
γ
⋅ . (2)
Here, ( )2 = pppp HAI ∆⋅ , App is the signal amplitude
between extremes of the derivative of absorption, ∆Hpp –
signal width between extremes, H1 – amplitude of the
microwave field, 17s1076.1/g −⋅=β=γ h , β – Bohr
magneton, T1 and T2 – spin-lattice and spin-spin
relaxation times of PC. Adjustment (2) to the
experimental curves allows finding the spin-relaxation
times for these two PC. From Fig. 4b and formula (2), the
value of the saturation factor was found
3280 3320 3360 3400
-1500
0
1500
E
P
R
s
ig
n
a
l,
a
.u
.
Magnetic Field, G
-14 dB
-20 dB
-26 dB
-32 dB
a)
Sample X2
0.0 0.1 0.2
0
50
100
150
I P
C
1
,
I P
C
2
,
a
.u
.
(P/P
max
)
1/2
PC1
PC2
b)
Fig. 4. a) EPR spectra for the sample X2 in some days after β-
irradiation with 25 kGy at different levels of microwave power.
Dashed lines are the result of fitting the theory with experiment.
b) Signal saturation curves for the centers PC1 and PC2.
SPQEO, 2018. V. 21, N 4. P. 336-344.
Konchits А.А., Shanina B.D., Yanchuk І.B., Krasnovyd S.V. Nature and kinetics of paramagnetic defects induced by …
340
0 10 20 30
0
100
200
300
1 day
4 days
5 days
13 days
19 days
28 days
56 days
Irradiation dose D, kGy
E
P
R
i
n
te
n
s
it
y
o
f
P
C
1
,
a
.u
.
PC1
a)
0 10 20 30
0
100
200
1 day
4 days
5 days
13 days
19 days
28 days
56 days
E
P
R
i
n
te
n
s
it
y
o
f
P
C
2
,
a
.u
.
Irradiation dose D, kGy
PC2
b)
Fig. 5. Irradiation dose dependence of the integrated intensities
for the paramagnetic centers PC1 (a) and PC2 (b) for different
time intervals between the EPR recordings; symbols correspond
to the experimental values, dashed lines – theoretical ones.
21
2
max,1
225.0= TTHS γ , where H1, max ≈ 0.3 G is the
maximum value of the microwave field H1, which are the
characteristics of spectrometer. It was found S1 = 80 for
PC1 and S2 = 100 for PC2. The spin-spin relaxation time
ppHgT ∆⋅= −7
2 1031.1 , where ∆Hpp is given in Gauss
unities. It is estimated for PC1 with the minimum line
width of the lowest SHFS component 1 in Table 1 that
∆Hpp = 3.5 G, and for PC2 with ∆Hpp = 11.6 G. After that
the spin-lattice relaxation time T1 is obtained from S1, S2
values with H1, max = 0.3 G. The results are as follows:
.ms6.2,s1056.0:2PC
;ms6.0,s109.1:1PC
1
8
2
1
8
2
=⋅=
=⋅=
−
−
TT
TT
Paramagnetic spins, responsible for the broad signal
PC2, have the shorter spin-spin relaxation time, but a
longer spin-lattice relaxation time, since spin-lattice
interaction in the disordered area is weakened as
compared with that in the crystalline area. Measurements
of the relaxation times of similar free radicals in γ-
irradiated cellulose were carried out recently using the
pulse saturation method [10]. The authors of this paper
found T2 by one order of value longer, but one order of
value shorter T1. The value of T2 depends on the PC
concentration, which was not determined in [10].
3.4. Irradiation dose dependence of paramagnetic
response for samples X1
In this section, we consider the kinetics of the
accumulation/decay processes of PC, depending on the
dose of irradiation and subsequent storage of the samples
in air.
The measurement procedure was as follows. The set
of powdered chitosan samples X1 was irradiated using
the beam of fast electrons with different doses. One day
after irradiation, EPR signals were recorded for each
sample with a certain radiation dose. Then the samples
were stored for some time in air, after which recording
the EPR spectra was repeated; this procedure was
repeated many times. The results are given in Fig. 5. It is
clearly seen that the concentration of free radicals
gradually increases with increasing the radiation dose
and decreases when the irradiated samples are stored in
air. Separation of the EPR spectrum by two signals with
the above g-factors was made for the EPR signals after
each time interval. As a result, Fig. 5 demonstrates the
dependence of the integrated intensity on both the
radiation dose and time of sample storage in air. (The
integrated intensity is proportional to the PC
concentration.) As can be seen from Fig. 5, centers PC1
and PC2 show the similar dose dependences.
The experimental results shown in Fig. 5 by
symbols are described by the calculated curves as the
function of the irradiation dose D:
Table 3. Kinetic parameters that determine the dependence of N+ on the exposure time of the samples in air after irradiation with
the dose 35 kGy. The value (N0
+/N0) = 0.17 was found to be identical in all the cases.
Time, days 1 4 5 13 19 28 56
с1, 10
3
с2, 10
3
PC1
PC2
0.11
0.23
0.08
0.15
0.05
0.09
0.03
0.065
0.016
0.037
0.007
0.018
0.002
0.005
ξ1, 10
–3
ξ2, 10
–3
PC1
PC2
0.7
3
0.45
3
0.45
2.2
0.4
2
0.2
2
0.2
2
0.2
2
β1, kGy
–1
β2, kGy
–1
PC1
PC2
0.16
0.12
0.14
0.12
0.15
0.14
0.14
0.12
0.14
0.13
0.13
0.14
0.14
0.14
SPQEO, 2018. V. 21, N 4. P. 336-344.
Konchits А.А., Shanina B.D., Yanchuk І.B., Krasnovyd S.V. Nature and kinetics of paramagnetic defects induced by …
341
( ) ( )[ ]11117.0
2/1
2,12,12,12,1 −⋅β+⋅⋅ξ+⋅+= DDcI . (3)
Although the dose dependence is the same for
signals 1 and 2, values I1 and I2 are different due to
different parameters ξ, с, β given in Table 3.
Let us consider different processes that take part in
creation of the paramagnetic radicals in the chitosan
sample. Let N0 be a total concentration of molecules able
to become the free radical, both paramagnetic and non-
paramagnetic; paramagnetic fraction of them N
+
is the
concentration of free radicals; then N0 – N
+
is the fraction
of non-paramagnetic molecules, which becomes free
radicals in the process of collision with irradiation
particles; n& is the concentration of electrons emerging
after breaking the molecules and chains by collision with
the irradiation particles; G is the generation rate of
paramagnetic centers; Wt – rate of electron capture by
traps created during structure destruction under the
irradiation; R is the rate of the electron and paramagnetic
center elimination in their collision. The kinetic
equations for evolution of n and N
+
in the irradiation
process are as follows:
( ) ( )
000
,,,,
0 N
N
N
n
W
N
n
WWWWW
N
n
RteieGeieG
+
⋅⋅−⋅++−+=
&
,
( ) ( )
0000 N
N
N
n
W
N
N
WWWW
N
N
RiGiG
+++
⋅⋅−⋅+−+=
&
. (4)
where WG, WG, e are the probabilities of the birth of a free
radical and free electron, which are in proportion to the
radiation dose D: WG = α D, WG, e = αe D, moreover
αe > α; Wi, e, Wi – probabilities of thermal ionization for
atoms and molecules; Wt – probability of electron capture
by traps, WR = R N0 – probability of recombination of
electrons and free radicals. The stationary solution (at
∞→t , 0=n& , 0=+N& ) of the system (4) is accurately
and reduces to a quadratic equation for N
+
: 02 =+ cyy ,
the solution of which can be represented in the following
manner:
( )1)1(5.0
0
0
0
−β+⋅⋅+=
++
Dc
N
N
N
N
,
( ) ( ) 000 0 NDNNN == ++
. (5)
Here,
( ) ( ) ( )
( ) ( ).,1
,,1
,,
,,00
teietiReiie
Reitiei
WWWWWWW
WWWWWcDcc
+α=β<<⋅+⋅⋅α=ξ
++=ξ+=
(5a)
Comparison of (5) with (3) and Table 3 leads to the
conclusion that the concentration of PC is determined by
the rate of generation and by the ratio of capture rates on
traps and paramagnetic centers, i.e., c0 is the greatest
value and the most variable with the time duration after
irradiation. In the expression for c0 (5a), there is one
quantity only, which is the largest and it varies
appreciably as the sample return to equilibrium, it is the
rate of the electron capture by the traps; thus c0 ≈ Wt /WR.
All above processes are fast; however, after
irradiation there are also seen slow processes, which in
the measurement (see Fig. 5) can be seen as a steady
decrease in the value N
+
at any dose, after several days of
waiting between measurements. Especially brightly, this
property appears at the maximum irradiation dose.
3.5. Decay of the paramagnetic centers concentration
during storage of irradiated samples in air
Fig. 6 shows the dependences of the radical
concentration on the waiting time between successive
measurements of the EPR signal. These dependences is
described by the two-exponential function with the times
τ1 and τ2 exponential decay and shown in Fig. 6 by
dashed lines. One can see that the values of intensities
are varied by an order for fifty days. Table 3 shows that
the kinetic characteristic also changed with time an order
of magnitude which is a property of Wt. Thus, the main
feature of the material is the release of electrons from
traps that are the result of the interatomic bond breaking
under irradiation and atom shifts from the sites. These
traps are not paramagnetic; they are extended formations,
and disappear with time, which is necessary for the
recovering of the molecular system in structural
relaxation process.
0 15 30 45
0
100
200
300
5 kGy
10 kGy
20 kGy
30 kGy
35 kGy
Time interval, days
E
P
R
i
n
te
n
s
it
y
o
f
P
C
1
,
a
.u
.
PC1
a)
0 15 30 45
0
50
100
150
200
250
b)
PC2
Time interval, days
E
P
R
i
n
te
n
s
it
y
o
f
P
C
2
,
a
.u
.
5 kGy
10 kGy
20 kGy
30 kGy
35 kGy
Fig. 6. Decay of PC1 (a) and PC2 (b) concentrations with time
after irradiation of the samples X1 with different doses;
symbols – experiment, dashed lines – theory.
SPQEO, 2018. V. 21, N 4. P. 336-344.
Konchits А.А., Shanina B.D., Yanchuk І.B., Krasnovyd S.V. Nature and kinetics of paramagnetic defects induced by …
342
3250 3300 3350 3400
-200
0
200
5 kGy
E
P
R
s
ig
n
a
l,
a
.u
.
Magnetic Field, G
35 kGy
0 10 20 30
0.75
1.50
2.25
E
P
R
s
ig
n
a
l,
a
.u
.
Irrad. dose D, kGy
Fig. 7. EPR spectra of the samples X1 2 years after irradiation
with the doses 5 and 35 kGy (recorded with accumulation). The
central line is the total contribution of nitroxyl radicals and PCl
centers; the side components are due to nitroxyl radicals only
[4]. Insert in Fig. 7: the intensity of the central line of spectrum
as a function of the irradiation dose.
The dashed lines, which describe the experimental
data in Fig. 6, are calculated according to the functions:
( ) ( )2,,02,,01,0 expexp)( iiliiii tAtAItI τ−⋅+τ−⋅+= , (6)
where Ai1, Ai2 are weight factors for the exponential
functions. All the parameters in Eq. (6) are given in
Table 4. Note that the contribution of the fast exponential
function is 60% and is essentially independent of the
dose. The same dependence shows that Wt, obviously at
the expense of reducing the number of traps due to the
ionization and their decay in the process of diffusion of
atoms.
Electrons of irradiation are captured by the traps
after irradiation turning off. In the process of recovering,
the number of traps is reduced by the amount of
( ) ( )tWNtN Dt −=δ exp0 , where WD is the atom diffusion
rate, and n is increased as the n = n0 + δn with δn = δN(t).
Therefore, in this case:
( ) ( )( ) ( )tWnNRWNN DtnG −⋅−⋅=+
exp1 00 0
. (7)
Irradiation of the material leads to a large number of
traps with low potential barrier, which disappear for one
or two days. Traps with higher barriers are able to keep
electrons for 15…19 days. This property is a proper
characteristic of the material and does not depend on the
radiation dose.
It is important to note that the paramagnetic defects
in the crystalline samples Х2 are much more stable, and
their concentration decreases for one year by about 5
times, while those in the samples Х1 decrease by one
order of value for 2 to 3 months. It is related with higher
extent of the crystallinity in the samples Х2.
Finally, with regard to the behavior of nitroxyl
centers (Fig. 2, inset), one can say their EPR intensity
almost does not change upon irradiation with these small
doses that were used in our experiments. However, 2
years after irradiation, the PC2 centers completely
disintegrate, while the more stable nitroxyl radicals and
the residual spectrum of PC1 are still recorded, albeit in a
small concentration. Shown in Fig. 7 is the EPR
spectrum in the chitosan sample X1 2 years after
irradiation with the dose 35 kGy. It can be seen that only
nitroxyl radicals and residual signal of PC1 centers (main
contribution to the central line of spectrum) are recorded.
The insert to Fig. 7 shows that there is an approximately
linear relationship between the dose of chitosan samples
irradiation and the intensity of the central line 2 years
after irradiation.
4. Conclusion
The observed EPR spectra were described by the sum of
two EPR signals PC1 and PC2 with the spin S = 1/2,
Lorentz line shape, different g-factors and different line
widths. For PC1: gPC1 = 2.0047 ± 4·10
–4
, ∆Hpp,1 =
3.5…6.4 G and 9 G for separately SHF components in
the samples X2 and the envelope of the spectrum in
samples X1, accordingly. For PC2: gPC2 =
2.0031 ± 3·10
-4
, ∆Hpp,2 ≅ 12 G. Based on the first time
observed well-resolved SHF structure of the spectrum in
the “crystalline” samples, it has been ascertained that the
radicals PC1 are characterized by the electron state
located at the site C5 and has the SHF interaction with
four hydrogen atoms from the molecule CH2OH and two
neighbor hydroxyl groups with A||, j = 28.6, 15.5, 14.8,
7.4 G. The value of g-factor is found to be 2.0047, which
is close to the obtained one in [10]. The 2-nd order
Table 4. Structure recovery time and the weight factors of the respective exponential functions.
PC1 PC2 Dose,
kGy
Weight of
the 1st
exponent
τ0,1,1
days
Weight of
the 2nd
exponent
τ0,1,2 days Weight of
the 1st
exponent
τ0,2,1
days
Weight of the
2nd exponent
τ0,2,2
days
5 0.17 18 0.83 0.5 0.4 15 0.6 1
10 0.45 12 0.55 1 0.47 15 0.53 1
20 0.41 12 0.59 1 0.35 17 0.65 1
30 0.48 12 0.52 2 0.39 19 0.61 2.8
35 0.36 14 0.64 3 0.36 19 0.64 3
SPQEO, 2018. V. 21, N 4. P. 336-344.
Konchits А.А., Shanina B.D., Yanchuk І.B., Krasnovyd S.V. Nature and kinetics of paramagnetic defects induced by …
343
correction |A⊥, j|
2
has the small values and gives a
contribution only to the line width of SHFS components.
By this reason, the width of these lines increases from the
distant components to the central ones, which is observed
in the experimental spectrum of X2 samples.
The second wider signal PC2 belongs to dangling
bonds of carbon localized most probably in C1 position
of the chitosan structure and does not show the super-
hyperfine structure. It has been found the functional
dependence of EPR signals on the irradiation dose. The
achieved concentration of radicals decreases with time
after irradiation along the two-exponential law with the
characteristic times τ0,1,1 = 12…18 days, τ0,1,2 = 15…19
days, τ0,2,1 = 0.5…3 days, τ0,2,2(2) = 1…3 days, being
dependent on the irradiation dose. The long time decay
contributes only about 40% to the intensity decrease.
Higher stability of radicals in samples with a more
crystalline structure has been observed.
Thus, irradiation of chitosan with fast electrons in
sterilization doses leads to the appearance of a significant
concentration of paramagnetic defects that disintegrates
gradually, but not fully, at prolonged storage of the
samples in air.
Acknowledgement
The authors of the paper are grateful for support by
National Academy of Science of Ukraine (Project №
26/17-Н).
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Authors and CV
Andriy Andriyovich Konchits.
Dr. A.A. Konchits is a leading
scientist of the Department Optics and
Spectroscopy at the V.E. Lashkaryov
Institute of Semiconductor Physics
NAS of Ukraine.
His main research interests include
the nanostructured materials and
composites. Also, he has been working on porous
materials (porous coal), their structure, and electrical
properties.
E-mail: konchits@ukr.net
Bela Dmytrivna Shanina
Dr. Shanina B.D. is a professor and a
leading researcher of the Department
Optics and Spectroscopy at the V.E.
Lashkaryov Institute of
Semiconductor Physics NAS of
Ukraine. Her research areas are
electronic properties of
nanostructured materials.
E-mail: shanina_bela@rambler.ru
Igor Bogdanovich Yanchuk
Igor Bogdanovich Yanchuk is a PhD
of the Department Biotechnology at
the Farmak. His main research
interests include the correlation of
structural features of carbon materials
with their optical and mechanical
properties.
E-mail: i.b.yanchuk@gmail.com
Serhii Volodymyrovich Krasnovyd.
Mr. Krasnovyd S.V. is junior
researcher of the Department Optics
and Spectroscopy at the V.E.
Lashkaryov Institute of
Semiconductor Physics NAS of
Ukraine. His main research interests
are the nanostructured materials, their
electronic properties.
E-mail: sergkrasnovyd@gmail.com
|
| id | nasplib_isofts_kiev_ua-123456789-215328 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1560-8034 |
| language | English |
| last_indexed | 2026-03-23T18:47:47Z |
| publishDate | 2018 |
| publisher | Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| record_format | dspace |
| spelling | Konchits, A.A. Shanina, B.D. Yanchuk, I.B. Krasnovyd, S.V. 2026-03-12T08:56:03Z 2018 Nature and kinetics of paramagnetic defects in chitosan induced by beta-irradiation of chitosan / A.A. Konchits, B.D. Shanina, I.B. Yanchuk, S.V. Krasnovyd // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2018. — Т. 21, № 4. — С. 336-344. — Бібліогр.: 25 назв. — англ. 1560-8034 PACS: 61.82.Pv, 76.30.Rn, 82.35.Pq, 87.53.Ay, 87.80.Lq https://nasplib.isofts.kiev.ua/handle/123456789/215328 https://doi.org/10.15407/spqeo21.04.336 A set of chitosan samples irradiated by electrons with various doses was studied using the EPR method. Two kinds of paramagnetic defects, PC1 and PC2, initiated by this irradiation due to the breakage of bonds in positions C5 and C1 of the chitosan structure, are revealed in the “amorphous” and “crystalline” samples of chitosan. The structure of defects, their spectroscopic parameters, and the kinetics of accumulation/decay have been established for the first time. It is found that the EPR spectrum of the “crystalline” samples consists of 10 almost equidistant lines of the super-hyperfine (SHF) structure with the splitting between them A = 7.4 G for the PC1 center, and a single wide line with a markedly different g-value, attributed to the PC2 one. Both these lines are also present in powder “amorphous” samples, but the SHF structure of the PC1 centers in them is not registered because of the broadening of the individual SHF components. Kinetics of defect accumulation with increasing dose D of the irradiation, and their gradual disappearance during prolonged storage of samples in air, were discovered and studied. Kinetic equations were solved, and the D-dependence and decay times were found from the comparison of theoretical results with the experimental ones. It has been shown that the concentration of shallow and deep traps for electrons affects the rate of the decay process. The recovery process is much slower in the samples having a more perfect crystalline structure. The authors of the paper are grateful for the support of the National Academy of Science of Ukraine (Project №26/17-Н). en Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України Semiconductor Physics Quantum Electronics & Optoelectronics Semiconductor physics Nature and kinetics of paramagnetic defects in chitosan induced by beta-irradiation of chitosan Article published earlier |
| spellingShingle | Nature and kinetics of paramagnetic defects in chitosan induced by beta-irradiation of chitosan Konchits, A.A. Shanina, B.D. Yanchuk, I.B. Krasnovyd, S.V. Semiconductor physics |
| title | Nature and kinetics of paramagnetic defects in chitosan induced by beta-irradiation of chitosan |
| title_full | Nature and kinetics of paramagnetic defects in chitosan induced by beta-irradiation of chitosan |
| title_fullStr | Nature and kinetics of paramagnetic defects in chitosan induced by beta-irradiation of chitosan |
| title_full_unstemmed | Nature and kinetics of paramagnetic defects in chitosan induced by beta-irradiation of chitosan |
| title_short | Nature and kinetics of paramagnetic defects in chitosan induced by beta-irradiation of chitosan |
| title_sort | nature and kinetics of paramagnetic defects in chitosan induced by beta-irradiation of chitosan |
| topic | Semiconductor physics |
| topic_facet | Semiconductor physics |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/215328 |
| work_keys_str_mv | AT konchitsaa natureandkineticsofparamagneticdefectsinchitosaninducedbybetairradiationofchitosan AT shaninabd natureandkineticsofparamagneticdefectsinchitosaninducedbybetairradiationofchitosan AT yanchukib natureandkineticsofparamagneticdefectsinchitosaninducedbybetairradiationofchitosan AT krasnovydsv natureandkineticsofparamagneticdefectsinchitosaninducedbybetairradiationofchitosan |