unique advantages for the synthesis of AgNPs.
D. P. Perkasa et al. / Atom Indonesia Vol. 47 No. 2 (2021)
1
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Gamma Radiosynthesis of Colloidal Silver
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Nanoparticles Stabilized in ι-Carrageenan Under
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Atmospheric Gases: A Surface Plasmon
6
Resonance Based Study
7 8
D. P. Perkasa*, M. Y. Yunus, Y. Warastuti, B. Abbas
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Center for Isotopes and Radiation Application, National Nuclear Energy Agency (BATAN),
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Jl. Lebak Bulus Raya No. 49, Jakarta 12440, Indonesia
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A R T I C L E I N F O A B S T R A C T
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Article history:
Received 25 June 2019
Received in revised form 15 July 2020 Accepted 15 September 2020 Keywords:
Gamma radiosynthesis Silver nanoparticles ι-Carrageenan
Surface plasmon resonance
ι-Carrageenan is a biodegradable and biocompatible biomaterial which potentially stabilizes colloidal silver nanoparticles (AgNPs). The present study explored gamma radiosynthesis of AgNPs at varied concentration of ι-carrageenan solutions.
The reaction system contained 1.0 mM silver precursor from silver nitrate salt.
Gamma irradiation was conducted at doses up to 20 kGy under simple condition, i.e., atmospheric gases and without addition of hydroxyl radical scavenger.
The behavior of AgNPs in suspension was characterized based on their surface plasmon resonance (SPR) absorption spectra which were measured using UV-vis spectrophotometer. The results show that colloidal AgNPs were successfully radiosynthesized due to dual stabilizing/reducing activity of ι-carrageenan.
The degradation product of ι-carrageenan shows antioxidant activities, which increase the reducing condition of the reaction system. TEM micrograph reveals that the nanoparticles are spheroid in shape and monodisperse with an average particle size of below 10 nm. The SPR spectra indicate that the highest AgNPs concentration is found for irradiation at a dose of 10 kGy and ι-carrageenan concentration of 1.0 % (w/v). However, instability of AgNPs occurred a day after radiosynthesis due to oxidative dissolution and agglomeration. Further works on pH adjustment and optimization on irradiation dose and ι-carrageenan concentration are critical to improve the stability of colloidal AgNPs.
© 2021 Atom Indonesia. All rights reserved
INTRODUCTION
16
Silver exhibits unique size-dependent
17
biological, physical and chemical properties at
18
nanoscale which differ greatly from those of bulk
19
materials [1,2]. For therapeutic applications in
20
biomedical field, silver nanoparticles (AgNPs) have
21
showed potential antimicrobial [3-5], antioxidant
22
[6], and anticancer [7]. The AgNPs also exhibit
23
strong absorption band in UV-visible (UV-vis) light
24
region due to the surface plasmon resonance (SPR)
25
of conductive electron [8,9] for application in
26
bioimaging and biosensor. The characteristics and
27
behavior of AgNPs are dependent on their particle
28
size and morphology, which are affected by the
29
Corresponding author.
E-mail address: [email protected] DOI:
synthesis process [1,2]. Therefore, consideration has
30
been given to synthesis of AgNPs to control their
31
morphology and size.
32
The synthesis of AgNPs in solution usually
33
contains three main components, i.e., metal
34
precursor, reducing agent, and stabilizing agent.
35
Various routes have been developend for the
36
synthesis of AgNPs, which can be categorized as
37
chemical, physical, and biological methods [10].
38
However, despite the progress that have been made
39
in synthesis techniques, they still have some
40
drawbacks such as the use of toxic reducing agent,
41
high temperature requirement, long processing time,
42
and high energy used while the morphology and
43
particle size are still difficult to control [1,2].
44
In this context, gamma radiosynthesis offers
45 46
Firstly, Ag+ ions reduced to uncharged state of Ag0
47
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Journal homepage: http://aij.batan.go.id
by reducing radicals produced by radiolysis of
48
water. Thus, there is no issue about potential toxicity
49
of the residual reducing agent [3,11] which supports
50
the ensuring of its biocompatibility for application in
51
biomedical field. Also, gamma radiosynthesis
52
operates under simple physicochemical conditions
53
that allow the homogenous reduction and
54
nucleation of AgNPs, have the capability of
55
producing AgNPs with narrow particle size
56
distributions, offer effective control of morphology,
57
and can be used to fabricate core-shell and alloyed
58
metal nanoparticles [1,2].
59
Previous studies about radiosynthesis AgNPs
60
have been reported using different stabilizer such as
61
chitin derivative [11], chitosan [4], honey [3],
62
polyvinyl alcohol [5], and silk fibroin [12]. The use
63
of carrageenan as a stabilizer and reducing
64
agent have been reported by using green
65
processing [13], but not in gamma radiosynthesis
66
yet. The ι-carrageenan is sulphated linear
67
polysaccharides extracted from certain species
68
of red algae class (Rhodophyceae) which
69
shows biodegradability and biocompatibility for
70
potential application in biomedical field. Indeed, the
71
ι-carrageenan have high metal binding activity [14]
72
which potentially provides steric passivation of
73
radiosynthesized AgNPs. Gamma radiosynthesis of
74
AgNPs is usually conducted in the absence of
75
oxygen and in the presence of OH radical scavenger
76
(predominantly isopropanol) to prevent unwanted
77
oxidation during the radiosynthesis process [1,11].
78
The present research studied the synthesis of
79
AgNPs stabilized in ι-carrageenan solution using
80
gamma ray radiation. The gamma irradiation was
81
conducted without isopropanol addition and under
82
air atmosphere. The isopropanol was not included in
83
reaction system due to its toxicity to central nerves
84
system [15-17]. Meanwhile, the N2 purging step to
85
remove dissolved oxygen was eliminated because it
86
contributes to longer processing time and additional
87
cost. Beside of the simplicity, the N2 purging
88
facilities is not always available in lab of developing
89
countries.
90 91 92
EXPERIMENTAL METHODS
93
Materials
94
The AgNO3 (Merck, Darmstad, Germany) and
95
KBr (Merck, Darmstad, Germany) were of
96
analytical grade and used without further
97
purification. The ι-carrageenan was kindly provided
98
by Research and Development Centre for Marine
99
and Fisheries Product Processing and
100
Biotechnology, Ministry of Marine and Fisheries
101
Affair, Indonesia. Water used in this study was
102
deionized water.
103
Gamma radiosynthesis of AgNPs
104
A fresh 0.2 M AgNO3 solution was prepared
105
by dissolving the silver salt crystal in water at room
106
temperature under dark condition. The ι-carrageenan
107
solutions were prepared by dissolving ι-carrageenan
108
powder in water at 80 °C for 15 min under
109
contionous stirring. The solutions were prepared
110
at four level concentration i.e. 0.05, 0.1, 0.5,
111
and 1.0 % (w/v). The reaction system for gamma
112
radiosynthesis of AgNPs was 200 μl of the 0.2 M
113
AgNO3 solution blended with 40 ml of ι-carrageenan
114
solution in a screw-capped bottle. The reaction
115
system was homogenized by vortexing for 20 s.
116
There was no nitrogen bubbling treatment nor
117
isopropanol addition subjected to the reaction
118
system.
119
The reaction system was irradiated using a
120
Co-60 gamma ray irradiator irradiator (series
121
Gammacell 220, MDS Nordion, Ottawa, Canada) at
122
a dose rate of about 5 kGy/h Center for Application
123
of Isotopes and Radiation–BATAN, Jakarta.
124
The absorbed dose of gamma ray was varied at
125
5, 10, 15 and 20 kGy. The ι-carrageenan solution
126
without the addition of AgNO3 was also prepared in
127
the same manner and used as a control.
128 129 130
Characterization of colloidal AgNPs
131
Previous studies [8,9,18] reported that UV-vis
132
spectroscopy is one of the most developed
133
non-destructive analytical technique for AgNPs in
134
solution because the change in spectral absorbance
135
on both physical change and behavior of AgNPs in
136
suspension. Thus, the occurrence of AgNPs in
137
solution was investigated spectroscopically using
138
an ultraviolet-visible (UV-vis) spectrophotometer
139
(series Cary 100 UV-Vis spectrophotometer,
140
Agilent, Santa Clara, US) based on their surface
141
plasmon resonance (SPR). Soon after irradiation
142
process, the absorbance UV-vis spectra were
143
measured using a 1 cm pathlength quartz cuvette
144
at wavelength of 200-800 nm at 1 nm resolution.
145
The measure spectra was subjected to
146
fitting/deconvultion analysis using Gaussian
147
function.
148
The colloidal AgNPs was dropped above
149
copper grid and dried, then the morphology of
150
AgNPs were observed under a JEM-1400
151
transmission electron microscopy (JEOL, Tokyo,
152
Japan) operating at 120 kV. Further, the TEM
153
micrograph was processed and analyzed for
154
average particle size and polydispersity index (PDI)
155
using software ImageJ (ver. ImageJ 1.51i,
156
developed by Wayne Rasband, National Institutes
157
of Health, USA).
158
Prior to Fourrier transform infrared (FTIR)
159
spectroscopy, the colloids were subjected to
160
lyophilization using freeze-drying instrument
161
(series Beta I, Christ, Osterode am Harz, Germany).
162
Then, the lyophilized samples were dispersed in
163
KBr powder. The FTIR spectra of samples was
164
measured over spectral range of 4000-400 cm-1
165
using FTIR spectrophotometer (series IRPrestige21,
166
Shimadzu, Kyoto, Japan).
167
The pH change after and before irradiation
168
were measured using a pH meter (series PB-10
169
Basic Meter, Sartorius AG, Goettingen, Germany).
170 171 172
Stability of Colloidal AgNPs
173
The stability of colloidal AgNPs as a function
174
of time was characterized using particle instability
175
parameter (PIP) based on UV-Vis absorbance
176
spectroscopy [19,20]. The UV-vis spectra of samples
177
were recorded and processed as previously described
178
at several time points (3, 6, 13, and 15 day). Based on
179
the peak intensity and wavelength shift, the PIP value
180
is defined as in Eq. (1).
181 182
∗ ∗ (1)
183 184
where I* is a weighted intensity value and λ* is
185
weightened wavelength shift. The I* is defined as
186
in Eq. (2):
187 188
∗ (2)
189
190
where ΔI is a change in peak intensity from the
191
reference peak intensity (I0) to peak intensity at n
192
point in time. The weightened wavelength shift is
193
defined as in Eq. (3).
194 195
∗ (3)
196 197
where λ0 is the reference peak wavelength position,
198
λn is the peak position at n point in time and C is a
199
weighting function defined as in Eq. (4).
200
201 ∗
∗
. (4)
202
203
such that an unstable system is defined by a 10 %
204
change in intensity or a wavelength shift of 10 nm
205
[20,19].
206 207 208
RESULTS AND DISCUSSION
209
The ι-carragenan solution is colorless and
210
transparent as shown in Fig. 1(a). Interestingly,
211
the irradiated solution remains colorless and
212
transparent after irradiated at doses up to 20 kGy.
213
Previous research reported that the gamma
214
irradiation caused the browning of carrageenan
215
solution [21]. The brown color is related to
216
formation of double bond after main chain
217
degradation and/or hydrogen abstraction [22].
218
Browning due to gamma irradiation has also been
219
reported for irradiated other polysaccharides, such as
220
alginate [22,23] and chitosan [24,25].
221 222
223 224 225
(a) (b)
226 227
Fig. 1. Appearance of irradiated ι-carrageenan solution at
228
various doses and concentration (a) without and
229
(b) with addition of 1 mM AgNO3.
230 231 232
233 234 235
(a)
236 237
238 239 240 241
(b)
242 243
Fig. 2. (a) UV-vis spectra of ι-carrageenan irradiated at various
244
doses and (b) their absorbance intensity at 240-250 nm.
245 0 kGy
5 kGy 10 kGy 15 kGy 20 kGy
i-C 0.05 % i-C 0.5 % i-C 0.05 % i-C 0.5 % i-C 0.1 % i-C 1.0 % i-C 0.1 % i-C 1.0 %
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
200 250 300 350 400 450
Absorbance (A.U.)
Wavelength (nm)
i-C 0 kGy i-C 5 kGy i-C 10 kGy i-C 15 kGy i-C 20 kGy
3.0 2.5 2.0 1.5 1.0 0.5 0.0
250 248 246 244 242 8 6 4 2 Absorbance at 240-250 nm (A.U.) 0
Dose kGy
Center fo gravity (nm)
Y = 0.11462X + 0.60659 Adj. R-square = 0.99962
5 10 15 20
The formation of double bond can be
246
evidenced by the occurance of absorbance peak at
247
about 265 nm in UV-vis spectra [21,22,25].
248
Based on Fig. 2(a)-(b) (data shown in suppl. data
249
Table S1), the UV-vis spectra of irradiated samples
250
show a new peak as compared to unirradiated
251
sample, but it is not centered at about 260 nm.
252
The absorption peak is centered at about 247 nm
253
which is slightly red shifted from 243.94 to
254
249.29 nm with increasing irradiation dose. Intensity
255
of the peaks are linearly correlated with increasing
256
dose (adj. R2 = 0.996). This peak is assigned to the
257
formation of carbonyl group which is confirmed by
258
occurrence of new peak at 1721 cm-1 in FTIR
259
spectra of the irradiated samples, as shown in Fig. 3.
260
These results comfirm that double bond linkage is
261
not formed during irradiation under condition in this
262
research, thus the irradiated ι-carragenan solution
263
remains colourless and transparent.
264
The color change is known to be influenced
265
by type of gases in reaction system. Nagasawa et al.
266
[22] reported that dark brown color under gamma
267
irradiation occurs when polysaccharides solution is
268
irradiated under nitrogen gas and air atmosphere, but
269
little color change is observed on irradiation in the
270
presence of oxygen. Even more, no color change
271
observed when pH of solution is set below 3.0 at
272
sufficient amount of oxygen [25]. It seems that the
273
absent of brown color in this study is due to the
274
occurance of significant amount of dissolved oxygen
275
within ι-carragenan solution. The dissolved oxygen
276
is possibly transferred from atmosphere during
277
homogenization of solution with vigorous vortex at
278
room temperature.
279 280
281 282 283
Fig. 3. FTIR spectra of raw ι-carrageenan, irradiated
284
ι-carrageenan, and ι-carrageenan supported AgNPs.
285 286
Gamma irradiation of ι-carragenan solution
287
containing AgNO3 leads to color change from
288
colorless transparent to yellow-to-brown, as
289
shown in Fig. 1(b). The UV-vis spectra of samples
290
show a new absorption peak which is not observed
291
in unirradiated sample and irradiated samples
292
without addition of AgNO3, as shown in Fig. 4.
293
294 295 296
(a)
297 298
299 300 301
(b)
302 303
304 305 306
(c)
307 308
309 310 311
(d)
312 313
Fig. 4. UV-vis spectra of ι-carrageenan supported AgNPs
314
synthesized at dose of (a) 5, (b) 10, (c) 15 and (d) 20 kGy.
315 4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (1/cm)
Transmitttance (%)
i-C Irradiated i-C i-C stabilzed AgNPs
927.80 849.68 803.49
1742.76 1202.67 1163.12 1128.41 1093.69
1253.78 1033.89
1721.54 1728.29 Absorbance (A.U.)Absorbance (A.U.) Absorbance (A.U.)
8
6
4
2
0
200 300 400 500 600 700
8
6
4
2
0
200 300 400 500 600 700
8
6
4
2
0
200 300 400 500 600 700
8
6
4
2
0
200 300 400 500 600 700
Absorbance (A.U.)
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
i-C 0.05 % AgNPs i-C 0.1 % AgNPs i-C 0.5 % AgNPs i-C 1.0 % AgNPs
silver ion by following in Eqs. (6) and (7) [1,2]:
The light absorption gives rise to a well-defined
316
band of scattered wavelengths in the visible
317
spectrum specifically at wavelength of about
318
395 – 418 nm, in contrast to small nonresonant
319
scatterers that have monotonously increasing
320
scattering cross-section toward shorter wavelength.
321
This peak corresponds to the surface plasmon
322
resonance (SPR) band of silver particles that arise
323
from the coherent oscillation of conduction electron
324
near AgNPs surfaces [8,18].
325
The occurrence of AgNPs in solution is
326
confirmed by TEM measurement, as shown in
327
Fig. 5 (data shown in suppl. data Fig. S1).
328
The primary morphology of the AgNPs is
329
spherical, while a small portion of AgNPs are rod
330
and triangular. The average particle size is at
331
about 7.6-9.17 nm which vary slightly in related
332
to i-carrageenan concentration. Most of the
333
particles have a size of below 20 nm; however,
334
several particles at a range of 20-53 nm are
335
observed. The PDI values of colloidal AgNPs are
336
below 0.5 which indicates that particle size
337
distribution is at monodisperse system. This result
338
confirms that gamma irradiaton can be a convenient
339
means to the synthesis of AgNPs which is in
340
agreement with previous reports [3].
341
342
343 344 345 346
(a)
347
348
349 350 351 352 353
(b)
354
355 356 357
(c)
358 359
360 361 362
(d)
363 364
Fig. 5. TEM micrograph and particle size distribution of AgNPS
365
stabilized in (a) 0.05, (b) 0.1, (c) 0.5, and (d) 1.0 % (w/v)
366
ι-carrageenan
367 368
The fundamental of gamma radiosynthesis
369
relies on the radiolysis of water into radical species,
370
reduction of Ag+ ion by reducing species into
371
uncharged state Ag0 (nucleation stage), and
372
subsequent coalescent of Ag0 to form AgNPs
373
(growth stage). Radiolysis of water generates a
374
number of species with very short half-lifes (~10-10)
375
through simplified reaction in Eq. (5):
376 377
•, , •, , , (5)
378 379
where solvated electron and hydrogen radicals are
380
strong reducing agent with redox potential of
381
E°(H2O/eaq-) = -2.87 VNHE at pH 7 and E°(H+/H•) =
382
-2.3 VNHE, respectively. Both species can reduce
383 384 385
→ (6)
386 387
• → (7)
388 389
Binding energy between two Ag0 dimers
390
or between Ag0 and Ag+ is stronger than the
391
binding energy between Ag0 and the solvent.
392
Therefore, once silver atoms are synthesized, they
393 50
40 30 20 10 0
0 10 20 30 40 50 60
Frequency (%)
Diameter (nm)
30 25 20 15 10 5 0
Frequency (%)
0 5 10 15 20 25 30 35 Diameter (nm)
daverage = 8.57 + 4.31 nm PDI = 0.50
daverage = 9.17 + 2.95 nm PDI = 0.32
25 20 15 10 5 0
Frequency (%)
0 5 10 15 20 25 30 35 Diameter (nm)
daverage = 8.72 + 3.01 nm PDI = 0.35
25 20 15 10 5 0
Frequency (%)
0 5 10 15 20 25 30 35 Diameter (nm)
daverage = 7.61 + 2.25 nm PDI = 0.29
as centers for nucleation, as follows in Eq. (10) [1,2].
(8) and (9) [1,2].
tend to interact to form dimers, following in Eqs.
394 395 396
→ (8)
397 398
→ (9)
399 400
Charged dimer clusters are further reduced and act
401 402 403
→ (10)
404 405
Under continuous gamma irradiation, absorption of
406
Ag+ onto the nuclei leads to the formation of
407
clusters, as follows in Eq. (11) [1,2].
408 409
→ (11)
410 411
Finally, monodisperse AgNPs are generated by
412
coalescent process, as follows in Eq. (12).
413 414
→ (12)
415 416
where m, n, and p represent nuclearitites, while x, y,
417
and z are the numbers of associated Ag ions [1,2].
418
Various morphologies occur when AgNPs in (9)
419
grow past a critical size [1].
420
Interestingly, radiosynthesis in this research
421
was conducted under simple condition, i.e., without
422
the use of neurotoxic isopropanol (hydroxyl
423
scavenger) and under atmospheric gases. However,
424
previous researches [1,2] reported that hydroxyl
425
radicals are capable of oxidizing Ag+ and/or Ag0 into
426
higher oxidation state. Also, dissolved oxygen can
427
cause oxidation and corrosion to metal nanoparticles
428
[1,2]. The reducing condition in this research
429
is related to dual stabilizing/reducing activity of
430
ι-carrageenan which would be explained based on
431
SPR spectra of the colloidal AgNPs in Fig. 4.
432
The SPR is sensitive to changes in the local
433
dielectric environment. Among noble metal
434
nanoparticles, AgNPs has the highest efficiency of
435
plasmon excitation in the visible spectrum due to the
436
different dielectric properties originated from the
437
small overlap between the SPR and the interband
438
transition in silver from 4d electrons to the
439
hybridized 5sp band that starts at 320 nm [9,26].
440
There have been many studies that have
441
characterized the good correlation between change
442
in SPR spectral absorbance on both physical change
443
and behavior of AgNPs in suspension [8,9,18],
444
thus UV-vis spectroscopy can be a fast and easy
445
analytical tool to monitor AgNPs properties in
446
the suspension.
447
As shown in Fig. 4, the UV-vis spectra of all
448
samples have a single SPR peak due to dipolar
449
resonance mode of electron near the surface of
450
AgNPs. Dipolar plasmon resonance only occurs
451
when the dimension of the particles is much smaller
452
than wavelength of incident light, specifically for a
453
diameter of below 67 nm for AgNPs because all
454
electrons in the entire particle experience a roughly
455
uniform electric fielding [8]. Also, single SPR peak
456
is also only occurs for well-dispersed isolated silver
457
particles suspension [9]. Thus, this phenomena
458
confirm the results of TEM micrograph that silver
459
particles in this study was at nanosize and well-
460
dispersed as isolated particles within the suspension.
461
This results indicate that ι-carrageenan is a good
462
stabilizer to generate AgNPs via bottom-up
463
approach. The stabilization is related to entrapment
464
of Ag+ ions by polar OH groups and electron-rich
465
oxygen atoms on the ι-carrageenan chain when
466
AgNO3 solution is added to ι-carrageenan solution
467
[14]. During radiosynthesis, the entrapped Ag+ ions
468
along ι-carrageenan chains were reduced into Ag0 as
469
following (6,7), which sequentially act as passivated
470
nucleating point for growth of well-dispersed
471
isolated nanosilver particles as following (8-12).
472
Peak intensity increases with increasing ι-
473
carrageenan concentration at certain irradiation dose
474
of up to 15 kGy, as shown in Fig. 4(a)-(c) (data
475
shown as spectrum 0D in suppl. data Fig. S2-S5).
476
The SPR absorbance correlates to AgNPs
477
concentration and obeys the Lambert-Beer law
478
[18,27]. Thus, the result means that concentration of
479
AgNPs in suspension increases with increasing
480
i-carrageenan concentration. Peak intensity also
481
increases with increasing irradiation dose in which
482
maximum value is reached at lower irradiation
483
dose at higher ι-carrageenan (data shown as
484
spectrum 0D in suppl. data Fig. S2-S5). Among all
485
the treatments, the highest value is achieved
486
at treatment of irradiation dose of 10 kGy
487
and ι-carrageenan of 1.0 % (w/v) which was
488
measured at 7.58 A.U. with FWHM of 82.14 nm.
489
For ι-carrageenan of 0.05 and 0.1 % (w/v), the peak
490
intensity keeps increasing with increasing dose up to
491
20 kGy. This results indicate that i-carrageenan acts
492
not merely as a stabilizing agent, but also as a
493
reducing agent in the preparation of AgNPs.
494
Higher concentration of ι-carrageenan and/or
495
increasing gamma irradiation yields more
496
degradation products with reducing activity.
497
The dual stabilizing/reducing activity of carrageenan
498
has been previously reported for microwave-assisted
499
synthesis of AgNPs in which reducing activity is
500
performed by the sulphate groups of ι-carrageenan
501
[13]. Under gamma irradiation, the mechanism of
502
reduction activity seems related to antioxidant
503
activity of degradation product of ι-carrageenan.
504
Previous reports [28,29] reported that degradation
505
product of carrageenan shows hydroxyl scavenging,
506
reducing power, and DPPH radical scavenging
507
activity. Specifically for degradation by gamma
508
irradiation, those activities are related to
509
depolymerization which is accompanied by
510
increased reducing ends [28].
511
However, peak intensity decreases after
512
irradiation at 15 and 10 kGy for ι-carrageenan of
513
0.5 and 1.0 % (w/v), respectively. The decreasing
514
intensity is companied with red-shift of λSPR as well as
515
broadening of SPR spectra. The broadening and red-
516
shift of resonant bands indicates the increases in
517
particle size or agglomeration of nanoparticles.
518
Previous researches [5,8,9] have reported that a
519
red-shift and broadening of the dipolar resonance is
520
observed in large particles along with the appearance
521
of higher-order of SPR modes due to radiative losses.
522
When dimension of AgNPs increases, light cannot
523
polarize homogenously and the field is no longer
524
uniform throughout the NP, which results in phase
525
retardation effect [8,9]. Shorter ι-carrageenan chains at
526
higher irradiation doses provides lower steric
527
stabilizing activity for AgNPs. This result suggests that
528
an optimum irradiation dose need to be determined for
529
balancing the optimum trade-off between stabilizing
530
and reducing activity.
531
Previous study [21] reported that important
532
functional groups are still present in the FTIR
533
spectra of irradiated ι-carrageenan after irradiation at
534
100 kGy. In this study, functional groups related to
535
ester sulphate disappear after gamma irradiation as
536
shown in Fig. 3. The unirradiated ι-carrageenan
537
shows characteristic peaks related to ester sulphate
538
at 1253 cm-1, C-O linkage of 3,6-anhydro-D-
539
galactose at 1033 and 27 cm-1, and galactose 4-
540
sulphate at 849 cm-1. e uniradiated sample also
541
shows a peak related to 3,6-anhydro-D-galactose-2-
542
sulphate at 803 cm-1 which is the characteristic and
543
the distinctive properties of ι-carrageenan. All the
544
characteristic peaks are absent in FTIR spectra of
545
irradiated sample with and without addition of
546
AgNO3. It seems that irradiation at oxygenic
547
condition causes massive removal of sulphate
548
groups. The removal of sulphate groups changes
549
the pH of the solution from neutral to acidic,
550
which decreases with increasing irradiation doses
551
and ι-carrageenan concentration, as shown in Fig. 6.
552
The differences on degree of ι-carrageenan
553
degradation and pH suspension would alter the
554
stability of AgNPs in suspension.
555 556
557 558 559 560
Fig. 6. pH of ι-carrageenan irradiated at various doses.
561
562 563 564
(a)
565
566 567 568
(b)
569 570
571 572 573
(c)
574 575
576 577 578
(d)
579 580
Fig. 7. Particle instability parameter of ι-carrageenan
581
supported AgNPs synthesized at γ-ray dose of (a) 5, (b) 10,
582
(c) 15 and (d) 20 kGy at function of time.
583 7
6 5 4 3 2
pH
Dose (kGy)
0 5 10 15 20
i-C 0.05 % i-C 0.1 % i-C 0.5 % i-C 1.0 %
0.6 0.5 0.4 0.3 0.2 0.1
PIP
3 4 5 6 7 8 9 10 11 12 13 14 15 Time (day)
0.6 0.5 0.4 0.3 0.2 0.1
PIP
3 4 5 6 7 8 9 10 11 12 13 14 15 Time (day)
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
PIP
3 4 5 6 7 8 9 10 11 12 13 14 15 Time (day)
i-C 0.05 % AgNPs i-C 0.1 % AgNPs i-C 0.5 % AgNPs i-C 1.0 % AgNPs 1.0
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
PIP
3 4 5 6 7 8 9 10 11 12 13 14 15 Time (day)
follows in Eqs. (13) and (14).
The potential for application of AgNPs is
584
strongly dependent on their stability against
585
aggregation due to a strong correlation between
586
size and biological activity of AgNPs [30-32].
587
As shown in Fig. 7, most of colloidal AgNPs are
588
unstable as their PIP are higher than the cut-off
589
value of 0.1. Stable AgNPs are only observed for
590
3 days for all AgNPs synthesized at doses of 10 kGy
591
and for AgNPs synthesized at 5 kGy and 10 kGy
592
stabilized in 0.1 % (w/v) ι-carrageenan. For colloidal
593
AgNPs synthesized at a dose of 5 kGy, increasing
594
PIP value by time sourced is mainly from decreasing
595
intensity of SPR peak (data shown in suppl. data
596
Table S2-S5). This results indicate the decreasing
597
AgNPs concentration due to oxidative dissolution
598
of AgNPs at low pH of colloidal AgNPs as
599 600 601
4 → 2 (13)
602 603
2 → 2 (14)
604 605
where metallic silver oxidized by dissolved oxygen
606
and subsequently reacts with protons [33].
607
Meanwhile, the shape of dipolar vibration peaks are
608
invariable with small change in the λn. It seems that
609
agglomeration does not occur due to spacious
610
interparticle distant at low AgNPs concentration.
611
For colloidal AgNPs synthesized at higher
612
doses, increasing PIP value by time sourced occurs
613
from both decreasing intensity and wavelength-shift
614
of the λn (data shown in suppl. data Table S2-S5).
615
Meanwhile, the shape of SPR vibration peaks are
616
broadening accompanied with the formation of
617
shoulder peak indicating the agglomeration of
618
AgNPs [8,9]. Agglomeration seems related to
619
degradation of ι-carrageenan into shorter chains at
620
higher irradiation doses which lowering their steric
621
stabilizing activity. Low pH also contributes to the
622
increasing agglomeration rate of AgNPs [33,34].
623
Therefore, pH adjustment is one of among the
624
critical process parameters to maintain the stability
625
of colloidal AgNPs.
626 627 628
CONCLUSION
629
In summary, gamma irradiaton has been
630
used as a means to synthesize colloidal AgNPs.
631
The nanoparticles are spheroid in shape which
632
is monodisperse with average particle size below
633
10 nm. By using i-carrageenan as a stabilizing agent,
634
the radiosynthesis in this research was conducted
635
under atmospheric gases and without isopropanol
636
addition due to dual stabilizing/reducing
637
activity of i-carrageenan. Highest AgNPs
638
concentration is found at irradiation dose of
639
10 kGy and ι-carrageenan of 1.0 % (w/v). However,
640
the ι-carrageenan concentration and irradiation dose
641
need to be optimized to balance the trade-off
642
between reducing activity and stabilizing activity.
643
Also, further work is needed to improve stability of
644
radiosynthesized colloidal AgNPs stabilized in
645
ι-carrageenan solution.
646 647 648
ACKNOWLEDGMENT
649
Authors acknowledge The Research and
650
Development Center for Marine and Fisheries
651
Product Processing and Biotechnology, Ministry
652
of Marine and Fisheries Affair, Indonesia for
653
kindly providing the ι-carrageenan for this research.
654
We also acknowledge Drs. Erizal, APU, and
655
Dr. Darmawan Darwis (Center for Application of
656
Isotope and Radiation, National Nuclear Energy
657
Agency, Indonesia) for helpful discussion and
658
encouragement.
659 660 661
AUTHOR CONTRIBUTION
662
Dian Pribadi Perkasa contributed as the main
663
contributor of the paper. All Authors read and
664
approved the final version of the papers.
665 666 667
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