92
S. Le Cae¨r et al. / Chemical Physics Letters 450 (2007) 91–95
Table 1
The hydrogen peroxide concentrations are then measured
using a luminometer (TD20/20, Turner Design) and a
chemiluminescence reagent. It is a mixture of 6-amino-2,3-
dihydro-1,4-phtalazinedione (isoluminol, Sigma, 60 lmol
Characteristics of the CPG glasses obtained from CPG Biotech Inc. under
study
Glass
Mean pore Pore
Pore volume Specific area
diameter
(nm)
distribution
(cm3 gꢀ1
)
(m2 gꢀ1
)
dmꢀ3), microperoxidase (MP 11, Sigma, 30 lmol dmꢀ3
)
(
%)
and sodium hydroxide (Aldrich, 99.99%) ensuring a pH
13. The corresponding reaction mechanism can be found
in Ref. [20]. Each measure is the average of five readings.
Last, let us point out that the chemiluminescence method
gives only relative values. For each glass, known concen-
trations of hydrogen peroxide were prepared from 30%
hydrogen peroxide standard solution (Prolabo) and intro-
duced in the porous materials, and the calibration was then
performed under the same conditions as the ones used for
the irradiations. A special attention was taken so that the
solution used for calibration and the irradiated solution
were in contact during the same time with the silica glasses.
The concentration of hydrogen peroxide in 10 MeV elec-
trons irradiated hydrated glasses is determined using the
Ghormley method in which Iꢀ is oxidized to I3ꢀ by H2O2
[1,21]. The extraction procedure is the same as the one used
for chemiluminescence. Absorbance of the triiodide anion
is measured at 350 nm with a Cary 500 spectrophotometer.
The molar extinction coefficient of Iꢀ3 at 350 nm is taken as
25500 molꢀ1 dm3 cmꢀ1 [5]. For each glass, a calibration
curve was performed in the same conditions as the ones
used for irradiations.
CPG00075C
CPG00240C
CPG00350C
CPG00500C
8
25
34
50
9
6.8
5
2.9
5.9
0.49
0.96
0.97
1.09
1.1
197
82
67.5
44
CPG03000C 300
8.4
200 ꢁC for 1 h to further remove any moisture. Degassed
ultrapure water with a conductivity of 18.2 MX and a very
low total organic carbon (TOC < 10 ppb) from a Millipore
Alpha-Q apparatus containing 10ꢀ2 mol dmꢀ3 NaNO3 and
4 · 10ꢀ5 mol dmꢀ3 methanol was introduced in the porous
materials by capillary wetting under inert atmosphere.
NaNO3 and methanol were of the highest grade commer-
cially available. Aqueous solutions were introduced into
the porous glasses by capillary wetting. A mixing of 1 h
was then performed in order to homogeneously distribute
the solution within the pores. The volume of the solution
was fixed to 250 lL and the weight of the glass was calcu-
lated to saturate the pore volume (i.e. 510 mg for the glass
of 8 nm pore diameter for example).
2.2. Irradiations
In all cases, error bars in the measurements of hydrogen
peroxide concentrations are estimated to be at the most of
20%.
Irradiations were performed using either a pulsed elec-
tron beam produced by an electron accelerator or c-rays
in an IBL 637 137Cs source (CEA/Fontenay-aux-Roses).
In the present experiments, 10 ns pulses of 10 MeV elec-
trons produced by our linear accelerator at a repetition rate
of 10 Hz were used. The dose rate of 1.6 Gy nsꢀ1 was cal-
culated using the dose given by the Fricke dosimeter
[16,17]. For c-irradiations the dose rate was 1.8 Gy/min
as determined using the Fricke dosimeter [16].
3. Results and discussion
NaNO3 was added in aqueous solutions in order to pre-
vent destruction of H2O2 by hydrated electrons and H
atoms [3] and methanol to prevent destruction of hydrogen
peroxide by OH radicals. The concentrations we have used
(10ꢀ2 mol dmꢀ3 NaNO3 and 4 · 10ꢀ5 mol dmꢀ3 methanol)
were found to lead to large yields of H2O2 in the c-radio-
lysis of methanol- and nitrate-containing solutions [7].
It is worth pointing out that all the experimental cares
together enable us to detect the hydrogen peroxide pro-
duced in irradiated hydrated nanoporous glasses, even
though hydrogen peroxide is well known to decompose at
water-ceramic oxide interfaces [22]. For example, we can
estimate that, for the 50 nm pore size glass, after a
10 MeV electrons irradiation, roughly 10% of the initial
H2O2 produced remains one hour after irradiation which
is the typical duration between the end of irradiation and
the H2O2 measurement. An example of H2O2 detection is
presented in Fig. 1 in the case of a 50 nm pore size glass
exposed to c-radiations. The chemiluminescence intensity
increases linearly with the irradiation time. The slope of
the line together with the slope of the calibration curve
enables us to deduce the H2O2 radiolytic yield.
The H2O2 radiolytic yields are then calculated with
respect to the energy deposited in water.
2.3. Hydrogen peroxide concentration measurements
Two H2O2 detection methods were used: the Ghormley
method for the solutions irradiated with the 10 MeV elec-
trons and the chemiluminescence method [18,19] for the
c-irradiations. As a matter of fact, in this latter case, the
low dose rate together with irradiation times ranging from
0 to 2 h make it necessary to use chemiluminescence which
is a highly sensitive detection method: concentrations of
hydrogen peroxide as low as 10ꢀ8 mol dmꢀ3 can be
detected.
In order to extract H2O2 created upon irradiation, 3 mL
of ultra pure water are added to the irradiated hydrated
glasses. The solution is stirred for 10 min. Five minutes
later, the resulting supernatant is centrifuged at
15000 rpm using a Biofuge Stratos (Heraus) centrifuge.
All the H2O2 radiolytic yields measured for the five
hydrated CPGs exposed to c- or 10 MeV electrons irradia-