The present paper studies the effect of metal NPs (gold and
silver) on the fate of methyl-radicals formed by irradiating
aqueous solutions of (CH ) SQO (DMSO).
3
2
Experimental
Materials
All chemicals were of A.R. grade and were used without
further purification. The water used was deionized and was
further purified by a Millipore Milli-Q setup with a final
resistivity of >10 MO.
Fig. 1 UV-Vis spectra of colloidal silver solutions. (one day after
preparation)
Instrumentation
over a year. The gold NPs were prepared similarly. An ice-cold
ꢁ3
UV-Vis measurements were carried out using a Hewlett-
Packard Diode Array spectrophotometer model 8452A, which
enables measurements in the range of 190–820 nm and resolu-
aqueous solution containing NaBH (30 mL, 2.0 ꢀ 10 M)
4
was added at once, with vigorous stirring, to an aqueous
ꢁ3
solution of HAuCl
4
(10 mL, 1.0 ꢀ 10 M). The solution
tion of ꢄ2 nm. TEM analyses were performed using a Tecnai
turned red, and was stirred until it reached room temperature.
2
2 G TWIN TEM (FEI) acc. Volt. 200 kV of Philips
1
The pH of this solution was 8.0 ꢄ 0.1 and the wavelength of
company. The NP solutions were dried on special grids. The
grids were Lacey Formvar/Carbon, 300 mesh, Copper of Ted
Pella company. Electrophoresis analyses were performed using
a homemade U-shaped glass tube with a third ‘‘arm’’ in the
center. Copper electrodes are attached to the side arms of the
U and the central arm can be sealed with a stopcock. This
instrument is used in the teaching laboratory of the Univer-
maximum absorption was 518 ꢄ 2 nm. The excess of NaBH
4
reacts with water to yield borate, which does not react with the
radicals (see blank in Table 1).
It can be seen from Fig. 1 that upon addition of NaCl, the
band-width of the plasmon absorption band of the silver NPs
is narrower and blue shifted relative to the absorption band
without the NaCl addition. The spectral shifts are attributed
1
5
60
sity. Irradiations were performed in a Co gamma source of
ꢁ
to Cl adsorption to the particles thus increasing the electron
Noratom Gammacell, which emits g rays of 1.1 MeV. The
dose rate delivered to the sample by the Co source as
density on the surface which is expected to induce the observed
18
spectral changes according to the Mie theory. The precision
6
0
1
determined by the Fricke dosimetry was 3.0 Gy min .
6
ꢁ1
of the radiolytic results was higher when NaCl was added to
the silver NPs.
The gases were analyzed using a HP 5890 GC fitted with a
FID detector (Poropaq QS GC column 10 Ft 1/8 in, Supelco).
The NPs obtained are negatively charged according to
electrophoresis analyses. The electrophoresis analysis is per-
formed as follows: the U-shaped tube of the apparatus is filled
with tap water; NP suspension is added gently (so that the NPs
will not mix with the water) through the middle arm stopcock;
ꢁ1
The carrier gas was He (30 mL min , T = 70 1C).
Preparation of the metal NPs
NP solutions are irradiated and are thus in contact with a
variety of radicals, which are highly reactive species. Therefore
the solutions should be as ‘‘clean’’ as possible from any
reagent and especially from entities containing C–H, N–H
and S–H bonds (that exist in common stabilizers), which can
react with those radicals. For that reason the silver NPs
suspension was prepared according to Creighton’s proce-
Table 1 The effect of metal NPs on the CH
4
and C
2
H
6
yields
G
( CH
total
G(CH
G(C H )
2 6
4
)/
a
ꢂ
b
)
Experiment
G(CH
4
)
G(C
2
H
6
)
3
c
Blank
Borate blank
3.5
3.7
3.1
2.3
1.3
2.2
1.6
1.2
0.4
1.0
1.1
1.2
1.4
2.4
1.8
2.2
2.6
3.0
5.5
5.9
5.5
5.1
6.0
5.8
6.1
6.5
6.4
3.5
3.4
2.6
1.7
0.55
1.2
0.75
0.45
0.15
1
7
d
dure, by adding at once an ice-cold aqueous solution con-
ꢁ
3
e
taining NaBH
to an aqueous solution of Ag
4
(30 mL, 2.0 ꢀ 10 M), with vigorous stirring,
[Ag]NP/3
[Ag]NP/2
Ag]NP
Au]NP/10
[Au]NP/7
ꢁ
3
2
SO
4
(10 mL, 1.0 ꢀ 10 M of
[
[
+
Ag ). Very fast color changes were observed immediately
after the NaBH4 solution was added (black - orange -
yellow). Since the resulting NPs suspension was somewhat
unstable (the UV-Vis spectrum of the solution was broadened
and red shifted after a while), an aqueous solution of NaCl
f
[
[
Au]NP/5
Au]NP
a
All solutions contained 0.1 M (CH
or 9.5 (for silver solutions) and were N
3 2
) SO at pH 8 (for gold solutions)
2
O-saturated. The solutions
ꢂ
(
0.4 mL, 0.01 M) was added after the solution turned yellow,
b
were irradiated by a dose of 360 Gy.
G
total ( CH
3
) = G(CH ) +
4
in order to stabilize the silver NPs. The solution was stirred
until it reached room temperature. The UV-Vis spectrum of
the colloidal silver solution with and without the NaCl is
shown in Fig. 1. The pH of this solution was 9.6 ꢄ 0.1, and its
wavelength of maximum absorption was 394 ꢄ 2 nm. The
c
2
G(C
there were no detectable amounts of CH
the organic gases is only DMSO, and the composition of the products
2
H
6
). Other blanks of each of the materials were measured and
4
and C , i.e. the source of
2 6
H
d
4
wasn’t changed unless metal NPs were present. NaBH solution,
with the same concentration as at the NPs solutions, which was kept
overnight so that decomposition into borate was completed.
+
NaCl was added also to verify that no Ag is left in the
e
ꢁ4
+
solution. If Ag is present in the solution, precipitation of
All silver NPs are with 1.0 ꢀ 10 M NaCl before the dilution.
ꢁ9
f
[
Ag]NP = (7 ꢄ 2) ꢀ 10 M. [Au]NP = (1.7 ꢄ 0.5) ꢀ 10ꢁ7 M.
AgCl is observed. The colloids thus obtained are stable for
This journal is ꢃc the Owner Societies 2006
Phys. Chem. Chem. Phys., 2006, 8, 3552–3556 | 3553
Zidki, Tomer
