Radiolysis of confined water: Production and reactivity of hydroxyl radicals

Full text of DOI:10.1002/anie.200460284

Communications
[
1,2]
tivity towards desired functionality.
Confinement and
interfacial phenomena are expected to have dramatic effects
[
3]
on reaction rates and mechanisms. However, studies on
these effects are still very limited, as most kinetic methods are
[
2,4]
difficult in a nanometer-confined environment.
Radiolysis
of liquids is an efficient tool which can be used to investigate
chemical reactivity in complex environments. As ionizing
radiation induces reactions throughout the entire sample,
whatever its complexity, information on the chemical reac-
tivity at the microscopic scale can be obtained from macro-
scopic analytical strategies. Such studies of radiation-induced
chemical processes in nanostructured materials have mostly
[
5]
been concerned with the effects on the solid material itself
or the reactivity of molecules adsorbed at the solid surface.
[
6]
The present study aims to establish how radiation affects the
reactivity of radiolytically produced radicals within nano-
metric water-filled pores in comparison to that in homoge-
neous solutions.
Radiation chemists have thus typically sought a target that
can trap radicals in order to generate a change in some easily
[
7]
measurable property, as in chemical dosimeters. Such traps
are useful since they can probe cavities of any size and shape.
High sensitivity can be achieved by choosing a system in
which radiolysis results in the formation of a fluorescent
[
8,9]
compound and thus can be detected by spectrofluorimetry.
One of these profluorescent compounds is coumarin, which
has the advantage of only measuring the formation of the HOC
radical whilst disregarding other primary radiolysis species. In
aqueous solution, nonfluorescent coumarin is converted on
irradiation into fluorescent 7-hydroxycoumarin (7-
[
10–14]
OHC).
The fluorescence intensity is proportional to the
number of 7-OHC molecules formed, and this in turn is
Chemistry in Porous Materials
proportional to the locally available HOC concentration down
À8
À3
to 10 moldm . Such a selectivity and sensitivity with
respect to HOC can not be achieved by measuring the
bleaching of a dye under irradiation.
Radiolysis of Confined Water: Production and
Reactivity of Hydroxyl Radicals**
We have determined the concentration of 7-OHC pro-
duced in a variety of controlled pore glasses (CPGs) with pore
Sarah Foley, Patricia Rotureau, Serge Pin,
GØrard Baldacchino, Jean-Philippe Renault,* and
Jean-Claude Mialocq
[
15,16]
sizes ranging from 8 to 300 nm
to ascertain the influence
of pore size on radiolytic radical production and reaction in a
confined space. Figure 1 shows the fluorescence of 7-OHC
As technology progresses to the nanoscale, the question of
how chemical reactions are perturbed by the size of the
environment needs further consideration. Such supramolec-
ular steric effects provide new strategies to orientate reac-
[
*] Dr. S. Foley, Dr. P. Rotureau, Dr. S. Pin, Dr. G. Baldacchino,
Dr. J.-P. Renault, Dr. J.-C. Mialocq
CEA/Saclay
DSM/DRECAM/SCM/URA 331 CNRS
91191 Gif-sur-Yvette, Cedex (France)
Fax: (+33)1-6908-3466
E-mail: jrenault@drecam.cea.fr
[
**] The authors thank the Direction de L’Energie NuclØaire (CEA/
Saclay) for financial support and HervØ Coffigny of the Direction des
Sciences du Vivant (CEA/Fontenay aux Roses) for use of the gamma
irradiator.
Figure 1. Fluorescence spectra of 7-OHC extracted from irradiated
CPG of 200 nm pore size (excitation wavelength332 nm, excitation
137
À1
and emission slits 0.5 mm, gamma irradiation with Cs, 2 Gymin ).
The inset shows the concentration of 7-OHC produced as a function of
dose.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
1
10
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DOI: 10.1002/anie.200460284
Angew. Chem. Int. Ed. 2005, 44, 110 –112

Angewandte
Chemie
produced when 1 mm coumarin was irradiated in a CPG with
a pore size of 200 nm. To ensure that all the 7-OHC produced
in the CPG was extracted, control experiments were per-
formed in which aqueous solutions of coumarin containing
varying concentrations of 7-OHC were mixed with the CPGs.
The amount of 7-OHC that could be extracted from these
glasses was measured by using a fluorescence calibration
curve down to 2 ” 10 moldm ; 95 Æ 5% of the 7-OHC
introduced could be recovered, irrespective of the CPG used.
The concentration of 7-OHC produced in the pore was
determined from the extraction coefficient of 95% and the
fluorescence calibration curve. The results for a CPG with a
pore size of 200 nm are shown in the inset of Figure 1. The
concentration of 7-OHC produced remains proportional to
the dose, as in aerated homogeneous solutions. Similar results
were obtained for all the CPGs. The concentration of 7-OHC
produced was also measured for CPGs with pore sizes of 8, 50,
enhanced surface reaction of HOC at the water/silica inter-
[18]
face. To identify a possible role of this interface, CPGs with
pore sizes of 8, 50, and 300 nm were filled with various
amounts of 1 mm coumarin solution and irradiated as
described above. In hydrophilic porous materials such as
CPGs and Vycor glasses, when the amount of solution
decreases, first the center of the pore empties leaving a
layer of water on the silica surface, after which the thickness
À8
À3
[
19]
of this layer gradually diminishes. The concentration of 7-
OHC produced per Gy absorbed by water was determined as
described above. The radiolytic yield of 7-OHC was found to
increase when the amount of solution in the pores was
decreased but the coumarin concentration was kept constant,
that is, the thickness of the water layer on the silica decreased
and the relative amount of silica surface available for a
possible capture reaction of the HOC radical on the surface
increased (Figure 3a). The increase in the 7-OHC radiolytic
yield shows that the capture of HOC radicals at the surface is
not efficient. At constant coumarin concentration, the only
explanation for this increase is thus enhancement of radiolytic
production of HOC radicals.
and 300 nm filled with solutions flushed by N O. Under these
2
À
conditions, the reducing hydrated electrons e are converted
aq
into HOC radicals. Thus, in homogeneous solution, N O
2
[
11]
induces a 40% increase in the production of 7-OHC.
After exposure of the porous material to N O, the concen-
Figure 3b shows the 7-OHC production per gram of glass
and per Gray when the degree of pore saturation is increased.
This value increases linearly with respect to the amount of
water confined in the pores, but has a positive intercept. The
linear increase with respect to the filling of the pore is due to
2
tration of 7-OHC is increased by a factor of 1.5 relative to that
obtained under Ar, a value which is in good agreement with
[
11]
that obtained by Makrigiorgos et al., and which shows that
HOC radicals are responsible for the formation of 7-OHC. The
similarities between the behavior of coumarin in homoge-
neous solutions and in porous glasses and the possibility to
efficiently quantify the 7-OHC produced allowed us to apply
the capture of HOC radicals by coumarin to the case of
confined water.
Figure 2 shows how the concentration of 7-OHC in CPGs
varies as a function of the pore size of the glass. The amount of
solution added was equal to the available pore volume. The
concentration of 7-OHC produced per Gy absorbed by water
(
the radiolytic yield) increases with increasing pore size of the
CPGs. When the pore diameter decreases from 100 to 8 nm,
the radiolytic yield of 7-OHC is divided by three, and thus we
can assume that the radiolytic yield of HOC radicals decreases
by the same amount. Above a pore diameter of 150 nm, the
behavior of confined coumarin solutions is similar to that of
[
10,17]
homogenous solutions.
Such results can be attributed either to a cagelike effect
[
2]
which favors radical recombination in the pores or to an
Figure 3. a) Yield of 7-OHC as a function of the ratio of introduced
coumarin solution volume withrespect to t he wetted silica surface
area for CPGs withpore sizes of 8 (
), 50 (*), and 300 nm (~).
&
Figure 2. Effect of pore size on the yield of 7-OHC in CPGs. The empty
circle refers to the yield in solution taken from Louit et al.
b) Total amount of 7-OHC produced per gram of CPG and per Gray
directly received by the glasses as a function of this ratio.
[
10]
Angew. Chem. Int. Ed. 2005, 44, 110 –112
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111

Communications
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[
To explain this effect, we must take into account that the
quantity of 7-OHC produced is the result of competition
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[
[
[
2]
61.
9
Š, respectively). This acceleration of the reaction induced
[
[
15]W. Haller, Nature 1965, 206, 693.
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provided by the model of Barzykin and Tachiya which was
[
23]
[
developed for gels.
In porous media, reactions on long
timescales are expected to be slowed down by local depletion
and impaired diffusion, especially for low concentrations of
reactants. We assume that this slowing down of the capture of
HOC radicals by coumarin can explain part of the decrease in
radiolytic yield of 7-OHC in porous glasses.
These results show that the effects of confinement, clearly
identified at the nanopore scale, are important even at the
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[
21]A simple way to understand the confinement effect of 8 nm
pores is to evaluate whether HOC reacts with coumarin before
À9
2 À1
reaching the pore wall. Taking D = 2.3 ” 10 m s for the
diffusion constant of HOC (G. V. Buxton, C. L. Greenstock, W. P.
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can estimate the average time needed for a hydroxyl radical to
reach the pore wall as 5 ns. If the reaction constant between
hydroxyl radical and coumarin is taken as k = 8.2 ”
Received: April 9, 2004
Revised: September 6, 2004
9
À1
3 À1
1
0 mol dm s (K. Gopakumar, U. R. Kini, S. C. Ashawa,
N. S. Bhandari, G. U. Krishnan, D. Krishnan, Radiat. Eff. 1977,
Keywords: confinement effects · fluorescence spectroscopy ·
mesoporous materials · radical reactions · radiolysis
À3
3
.
32, 199) the half-life of HOC in 10 moldm coumarin solution is
about 85 ns. According to this analysis, confinement in 8 nm
pores can effectively prevent diffusion of HOC out of their
production site and increase their recombination with their
parent radicals prior to reaction with coumarin.
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1
12
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angew. Chem. Int. Ed. 2005, 44, 110 –112