A solution study of silica condensation and speciation with relevance to in vitro investigations of biosilicification

Article abstract of DOI:10.1021/jp101347q

Requiring mild synthesis conditions and possessing a high level of organization and functionality, biosilicas constitute a source of wonder and inspiration for both materials scientists and biologists. In order to understand how such biomaterials are formed and to apply this knowledge to the generation of novel bioinspired materials, a detailed study of the materials, as formed under biologically relevant conditions, is required. In this contribution, data from a detailed study of silica speciation and condensation using a model bioinspired silica precursor (silicon catechol complex, SCC) is presented. The silicon complex quickly and controllably dissociates under neutral pH conditions to well-defined, metastable solutions of orthosilicic acid. The formation of silicomolybdous (blue) complexes was used to monitor and study different stages of silicic acid condensation. In parallel, the rates of silicomolybdic (yellow) complex formation, with mathematical modeling of the species present, was used to follow the solution speciation of polysilicic acids. The results obtained from the two assays correlate well. Monomeric silicic acid, trimeric silicic acids, and different classes of oligomeric polysilicic acids and silica nuclei can be identified and their periods of stability during the early stages of silica condensation measured. For experiments performed at a range of temperatures (273-323 K), an activation energy of 77 kJ·mol^-1 was obtained for the formation of trimers. The activation energies for the forward and reverse condensation reactions for addition of monomers to polysilicic acids (273-293 ± 1 K) were 55.0 and 58.6 kJ-mol^-1, respectively. For temperatures above 293 K, these energies were reduced to 6.1 and 7.3 kJ·mol^-1, indicating a probable change in the prevailing condensation mechanism. The impact of pH on the rates of condensation were measured. There was a direct correlation between the apparent third-order rate constant for trimer formation and pH (4.7-6.9 ± 0.1) while values for the reversible first-order rates reached a plateau at circumneutral pH. These different behaviors are discussed with reference to the generally accepted mechanism for silica condensation in which anionic silicate solution species are central to the condensation process. The results presented in this paper support the use of precursors such as silicon catecholate complexes in the study of biosilicification in vitro. Further detailed experimentation is needed to increase our understanding of specific biomolecule silica interactions that ultimately generate the complex, finely detailed siliceous structures we observe in the world around us.

Full text of DOI:10.1021/jp101347q

J. Phys. Chem. B 2010, 114, 9947–9955
9947
A Solution Study of Silica Condensation and Speciation with Relevance to in Vitro
Investigations of Biosilicification
David J. Belton, Olivier Deschaume, Siddharth V. Patwardhan, and Carole C. Perry*
Nottingham Trent UniVersity, School of Science and Technology, Clifton Lane, Nottingham NG11 8NS, United Kingdom
ReceiVed: February 12, 2010; ReVised Manuscript ReceiVed: May 28, 2010
Requiring mild synthesis conditions and possessing a high level of organization and functionality, biosilicas
constitute a source of wonder and inspiration for both materials scientists and biologists. In order to understand
how such biomaterials are formed and to apply this knowledge to the generation of novel bioinspired materials,
a detailed study of the materials, as formed under biologically relevant conditions, is required. In this
contribution, data from a detailed study of silica speciation and condensation using a model bioinspired silica
precursor (silicon catechol complex, SCC) is presented. The silicon complex quickly and controllably dissociates
under neutral pH conditions to well-defined, metastable solutions of orthosilicic acid. The formation of
silicomolybdous (blue) complexes was used to monitor and study different stages of silicic acid condensation.
In parallel, the rates of silicomolybdic (yellow) complex formation, with mathematical modeling of the species
present, was used to follow the solution speciation of polysilicic acids. The results obtained from the two
assays correlate well. Monomeric silicic acid, trimeric silicic acids, and different classes of oligomeric polysilicic
acids and silica nuclei can be identified and their periods of stability during the early stages of silica condensation
measured. For experiments performed at a range of temperatures (273-323 K), an activation energy of 77
kJ·mol- was obtained for the formation of trimers. The activation energies for the forward and reverse
condensation reactions for addition of monomers to polysilicic acids (273-293 ( 1 K) were 55.0 and 58.6
1
-1
-1
kJ·mol , respectively. For temperatures above 293 K, these energies were reduced to 6.1 and 7.3 kJ · mol ,
indicating a probable change in the prevailing condensation mechanism. The impact of pH on the rates of
condensation were measured. There was a direct correlation between the apparent third-order rate constant
for trimer formation and pH (4.7-6.9 ( 0.1) while values for the reversible first-order rates reached a plateau
at circumneutral pH. These different behaviors are discussed with reference to the generally accepted mechanism
for silica condensation in which anionic silicate solution species are central to the condensation process. The
results presented in this paper support the use of precursors such as silicon catecholate complexes in the
study of biosilicification in vitro. Further detailed experimentation is needed to increase our understanding of
specific biomolecule silica interactions that ultimately generate the complex, finely detailed siliceous structures
we observe in the world around us.
Introduction
chemical reactions: the formation of dimers, and then trimers
and oligomers, that are kinetically distinct when monitored by
Silica formation in biological organisms has fascinated
scientists for many years owing to the sophistication exhibited
by organisms in structural and morphological control over the
biosilica generated. Research on plant, animal, and single-celled
organisms has shown that organic biomolecules are involved
in biosilicification,1 and associated model studies have been
performed in vitro to try to understand the mechanisms by which
molybdenum colorimetric complexation methods, such that
changes in rate and mechanism during the condensation process
5,13
can be monitored.
The formation of silica from solutions
containing orthosilicic acid is thought to be activated by the
formation of anionic silicate species that condense with un-
-4
charged monomers.1 However pK
6,17
s ranging from 9.5 to 10.7
a
18-20
for small oligomeric species through to condensed particles
5
-12
these biomolecules act.
In natural aqueous systems, mono-
and 9.8 for the monomer21 suggest that concentrations of these
anionic species are low when condensation occurs under
circumneutral pH conditions. The estimated population of
silicic acid is found at levels of a few tens to about one hundred
ppm.1
3-15
From this low precursor concentration reservoir,
silicifying organisms are able to sequester and then deposit silica
in a controlled manner. In vitro, aqueous monosilicic acid at
levels above 100 ppm and circumneutral pH will spontaneously
condense to larger silicate oligomers followed by stable particles
that will eventually form a gel network by a process occurring
-
22
3
Si(OH) O at pH 7 is 0.18%. In many studies a single reaction
order/mechanism is assumed throughout the condensation
process, but in respect of the complexity and number of
competing species suggested by modeling and speciation studies
this would appear na ¨ı ve.23 We would expect the observed
kinetics to change throughout the condensation process to reflect
the changing reactions and speciation at the molecular and
colloidal levels.13 The orders of reaction observed during
condensation have been determined for a range of precursors,
concentrations, and pH, with an initial zero order with respect
1
3
via well-defined stages: (i) polymerization of monomer to
small particles, (ii) particle growth, and finally (iii) formation
of branched particle networks resulting in gelation. The earliest
stages of condensation can be further described by distinct
*
To whom correspondence should be addressed. E-mail: carole.perry@
24,25
ntu.ac.uk.
to monomer (the so-called induction period) being observed.
1
0.1021/jp101347q  2010 American Chemical Society
Published on Web 07/22/2010

9
948 J. Phys. Chem. B, Vol. 114, No. 31, 2010
Belton et al.
4
8
Subsequent condensation has been hypothesized to follow orders
the dissolution of borosilicate glass. Where mixtures of species
have been monitored, an empirical approach has been used to
compare complexation rates during different stages of dissolu-
tion,53 and, to the best of our knowledge, no information is
available distinguishing the early stages of silica condensation.
In this paper we report on the detailed study of a model silica
precipitation system using dipotassium tris(1,2-benzenediolato-
25-30
from 1 to 5,
sometimes showing transitional behavior with
2
4,31,32
24
pH
or degree of condensation.
When silica is formed within an organism, the reaction
2
-4
environment includes biomolecules such as proteins,
poly-
amines,1 and carbohydrates.
,33
34,35
If we are to understand the
specific influence of such biomolecules on orthosilicic acid
condensation, then the effects of factors such as pH, temperature,
and precursor concentration of the model condensing system
chosen must first be understood. In addition, the silicate species
present must be known and controllable, especially if the early
stages of the condensation process are to be studied. The ideal
model system will consist exclusively of monomers at the
beginning of the condensation process. Any other oligomeric
or particulate species present will cause complications with data
interpretation as redissolution of species and condensation with
already formed silicates will exhibit varying kinetic behavior.
2
O,O′)silicate·2H O as the monosilicic acid precursor with
1
titrimetric, H NMR, and colorimetric analytical (molybdenum
yellow and blue) data being presented. The principal aims of
the investigation were (i) to confirm the rapid and controlled
dissociation of dipotassium tris(1,2-benzenediolato-O,O′)sili-
2
cate·2H O to give initially pure supersaturated solutions of
orthosilicic acid, making this complex an extremely desirable
precursor for a model experimental silicifying system, (ii) to
perform a kinetic analysis and study the effect of pH and
temperature on the condensation process (to obtain rate constants
and activation energies), and (iii) to use the rate of silicomo-
lybdic acid complexation to implicate/identify the silicate species
present at different times during the early stages of the
condensation process.
Many silicic acid precusors have been used to investigate
3
,7,9,11
silicification in vitro including alkoxysilanes,
glycol modi-
fied alkoxysilanes,1
0,36
6
8
sodium silicate solutions, silica sols,
5
,37,38
and silicon 1,2-dihydroxybenzene complexes.
However, the
suitability of such model systems and their exact chemistries
such as silicate speciation at a given pH, time, and concentration
are not always fully known. As examples, the use of alkoxysi-
lanes is problematic as the initial stage involves the hydrolysis
of alkoxysilane bonds resulting in a mixture of partially
hydrolyzed and partially condensed species in solution as well
as varying amounts of additional alcohol generated by alkoxide
hydrolysis;39 the use of stabilized silica sols limits analysis to
effects on aggregation; alkaline solutions of sodium silicate are
Results and Discussion
In order to confirm that the initial requirements of the model
system were met, i.e., the only available source of condensable
silicate present at the start of the reaction was orthosilicic acid
at supersaturated concentrations, a study of the conditions and
rate of precursor dissociation was performed.
Preparation of Orthosilicic Acid Solutions. Optimization
of Conditions. The results of a titrimetric study showed that
stoichiometric additions of acid were required to fully dissociate
the precursor complex at an acid to complex ratio of 2:1, Figure
40-43
known to contain stabilized oligomeric species,
which upon
initiation of the condensation process by pH reduction, dissociate
to monomer at the same time as existing monomers begin to
condense, thereby blurring the picture of the condensation
process.
1
1A. H NMR at any of the levels of addition showed an absence
of signals from species other than 1,2-dihydroxybenzene and
the silicon tris-1,2-dihydroxybenzene complex (SCC) indicating,
that at least in the time frame of the NMR experiment, no partial
complexes containing only one or two 1,2-dihydroxybenzene
In contrast, silicon 1,2-dihydroxybenzene complexes are
stable, forming mildly basic aqueous solutions (pH ≈ 10) that,
upon neutralization to pH ≈ 7, rapidly form solutions of
orthosilicic acid that are self-buffering above pH 6.5. In addition,
such hypervalent silicon complexes have been implicated in the
1
molecules are generated during dissociation, Figure 1B. H NMR
analysis also showed that addition of acid ranging from 0-2
mol equiv resulted in dissociation of the complex which was
both rapid and controlled based on the amount of acid added,
Figure 1C. The dissociation of the catecholate complex appears
to proceed via two stages, Figure 1D: (i) removal of the first
44,45
biosilicification process,
possibly as a means for the transport
of silicon at concentrations exceeding the solubility of mono-
silicic acid, with the presence of 1,2-dihydroxybenzene being
1,2-dihydroxybenzene moiety by acid hydrolysis requiring 2 mol
known to slightly increase the solubility of amorphous and
crystalline silica.13 In previously reported studies and those
equiv of acid that destabilizes the hypervalent six-coordinate
state followed by (ii) removal of the now labile remaining 1,2-
dihydroxybenzene moieties by hydrolysis. The overall reaction
can be written as:
described below, the concentration of free 1,2-dihydroxybenzene
remains constant during the course of the condensation reaction.
The increased solubility and any other effect of the free ligand
can thus be taken into account using control experiments,
enabling the effects of other experimental variables such as the
concentration of a chosen additive to be studied.
+
[
K Si(C H O ) ] + 2H + 4H O f Si(OH) +
2
6
4
2 3
2
4
+
Silicomolybdic acid methods, as used in this study, are
routinely used for the determination of silica content in aqueous
3C H (OH) + 2K
(1)
6
4
2
4
6,47
solutions
and also in the measurement of rates of silica
condensation6 through the disappearance of molybdenum
active species with time. A number of studies have been
conducted attempting to differentiate between different silicate
solution species with comparison to data acquired by inductively
coupled plasma (ICP) methods and have been able to show that
there are significant levels of monomeric silicic acid in seawa-
ter. Measurement of the complexation rate of samples with
the molybdenum-containing compleximetric reagents has been
used to differentiate between monomeric and particulate species
,29
This mechanism precludes the formation of any partially
hydrolyzed species, as the rate-determining step must be the
first acid hydrolysis stage so only monomeric silicic acid or
unhydrolyzed complex remains. At an acid to complex ratio
lower than 2:1, any remaining undissociated complex in solution
was also found to be stable and able to buffer the pH of the
reaction system for more than 24 h. The dissociation process
for SCC is different from that observed for alkoxysilane
precursors that require only catalytic amounts of acid for
hydrolysis, and for which condensation and hydrolysis of
4
8
4
9-52
in solutions and solids
and between molecular species in

Solution Study of Silica Condensation and Speciation
J. Phys. Chem. B, Vol. 114, No. 31, 2010 9949
1
Figure 1. (A) Results of titrimetry with hydrochloric acid with markers representing the levels of molar ratio additions used in the H NMR study.
1
(
1
B) Typical H NMR spectra of partially and fully dissociated complex (singlet at 6.65 ppm from complex, multiplet centered at 6.85 from dissociated
+
,2-dihydroxybenzene). X:Y represents the mole ratios of Si:H . (C) Plot of residual complex for samples treated with increasing amounts of acid.
(
D) Acid dissociation of silicon catecholate complex precluding the formation of partially hydrolyzed species in the time frame of the NMR
experiment.
addition. Although the level of orthosilicic acid selected is high
in the context of natural reservoirs, it does enable the complete
condensation process from initiation to equilibrium to be
observed within a 24 h period. Any observed impact of additives
compared with the control model should still be relevant to a
biological system albeit on a longer time scale.
Speciation Studies. Correlation between Molybdenum
Yellow and Molybdenum Blue Analyses. In order to generate
meaningful inferences from data generated from the model
system, it is important that species present during the condensa-
tion process are known. Here we have, for the first time (to our
knowledge), attempted to identify the silicate species present
in a mixed silicate system at various stages of the condensation
process using a combined analysis of the kinetics of condensa-
tion extracted from molybdenum blue data and the kinetics of
species dissociation when silicate species are brought into
contact with the molybdic acid reagent.
Figure 2. (A) Plot of residual complex with dissociation time for
samples treated with increasing amounts of acid. (B) Relationship
between pH and residual complex concentration. The pH values 7.2,
7.1, 6.9, 6.4, and 3.0 correspond to molar equivalents of acid to complex
of 1.4, 1.6, 1.8, 1.9, and 2.0:1, respectively.
partially and fully hydrolyzed species compete during the early
5
4
stages of silica formation.
Speciation in 30 mM Condensing Systems. The molybde-
num blue (silicomolybdous acid) colorimetric method when
applied to the study of silica condensation yields data such as
that shown in Figure 3A. From this can be extracted kinetic
information and the effect of physicochemical parameters such
as pH and temperature (data analysis from the study is presented
below) and solution additives on the early stages of silica
condensation. An alternative approach to the monitoring of
silicic acid concentrations in solution is the molybdenum yellow
(silicomolybdic acid) method where, for samples taken at time
points identified by the vertical lines in Figure 3A, the
development of the yellow color with time distinctly evolves,
with both a decrease in the final silicon concentration detected
and a visible change in the shape of the plots being observed,
Figure 3B. The evolution suggests the presence of at least two
types of molybdenum yellow-active species, together with a
much slower reacting fraction, mostly made of highly condensed
species such as silica particles. The mathematical modeling of
To measure the rate of complex dissociation upon pH
adjustment, quenching experiments were carried out wherein
after initial pH adjustment to values between 3.0-7.2, the pH
was then raised back to 10 by the addition of potassium
hydroxide solutions, limiting the time for dissociation to as little
as 20 s. A typical spectrum of a quenched dissociated complex
is given in the Supporting Information, Figure S1. The mol %
of residual complex was calculated at each sample time, and
the data were plotted as residual complex versus time after pH
adjustment, Figure 2A, or as residual complex versus pH after
initial adjustment, Figure 2B. The data show that from as little
as 20 s after mixing and pH adjustment, the complex had
dissociated to equilibrium values at all levels of acid addition.
Based on the above data, the optimum conditions for the
model system were chosen to be the addition of sufficient 2 M
hydrochloric acid to reduce the pH of a 30 mM solution of the
dipotassium tris(1,2-benzenediolato-O,O′) silicate ·2H
2
O to 6.8
2
(
0.05. This ensured that practically all the complex was
this data with a three-species model gave fits with R values
dissociated (ca. 99%) and that the pH was not subject to
significant variation due to any minor inaccuracies in acid
evolving from 0.9999 to 0.9704 during the 90 min condensation
period. As condensation proceeds, the first-order law becomes

9
950 J. Phys. Chem. B, Vol. 114, No. 31, 2010
Belton et al.
4
Figure 3. (A) The decrease in silicomolybdous acid (molybdenum blue) complex with time following pH reduction for 30 mM initial [Si(OH) ]
at pH 6.8, and (B) the formation of silicomolybdic acid yellow complex after increasing condensation times. The vertical lines in plot A represent
a selection of the sampling times used to obtain the molybdenum yellow data presented in plot B. The vertical line in plot B denotes the time at
which the blue silicomolybdic acid complex is formed from species available in solution.
4
Figure 4. Silicate speciation during the condensation process for 30 mM initial [Si(OH) ] at pH 6.8, with 3 (A), 4 (B), and 5 (C) species fits.
TABLE 1: Apparent Third-Order Rate Constants for the
from trimers.5,55 The data obtained from the molybdenum yellow
complexation experiments demonstrate that the fractions at-
tributable to trimeric species and monomers disappear over time,
with the concomitant formation of larger species having slower
kinetics of complexation.
Molybdenum Blue [Si(OH)
4
] Assay
apparent third-order rate
6
-2 -1
pH of condensation
constant × 10 /mM
s
6.8
6.4
5.7
4.7
3.4
3.4 ( 0.24
1.41 ( 0.10
0.35 ( 0.03
0.04 ( 0.003
If the molybdenum yellow data is now subject to a fitting
procedure using four or five species, the slow-complexing
species fraction appears to split between different subclasses
of slower-reacting species. As condensation proceeds, the
slower-reacting, smaller species fractions are progressively
replaced by even more insoluble fractions. For a four-species
model (Figure 4B), a species showing reversibility with respect
to the molybdic acid reagent was observed after 20 min and
increased in abundance with time to form a stable population.
Increasing the species fit to five indicated that this species is
further condensed to a slower dissociating entity (Figure 4C).
The formation of this latter species coincides with the detection
of the first silica particles based on light scattering measurements
on a similar model system.37
no measurable condensation
unable to perfectly describe species breakdown. Indeed, the silica
particles formed during the later stages of condensation, with
sizes above the critical nucleus dimensions, behave as a solid
phase as opposed to solution species and therefore adopt specific
kinetics of dissolution (Figure 4A).
In the absence of a constraint over the composition of the
system and for all the speciation models tested, the mathematical
solution converges to the presence of only one species, the
-
1
monomer, for t ) 0 min (complexation rate of 0.42 min ).
Applying a three-species fit (Figure 4A) to the early stages of
condensation (0-5 min) showed the presence of only one
species in addition to the monomer. This second species exhibits
Where these species are at low levels (post third-order stage,
6
-15 min), reversible first-order kinetics is still observed (see
5
-1
the in-depth kinetic analysis for details, Figure 7), with the
addition of monomers to trimers and larger species being the
prevailing reaction. The observed reversibility relates to silicate
dissociation during condensation and not during molybdenum
yellow complexation.
a slower complexation rate (0.03 min ) and begins to appear
rapidly after pH adjustment. Over the time period 0-6 min,
molybdenum blue data gave results consistent with a dominant
third-order process, e.g, the formation of trimeric species (Figure
6
A and Table 1). In addition, a loss of around 45% of
molybdenum blue active species from the system can be
observed. This observation can be rationalized by the formation
of trimers from the condensation of monomer and dimer, with
the second species identified from molybdenum yellow analysis
being assigned as trimers. The other possible species, dimers,
are likely to be present only as an intermediate, low concentra-
tion fraction, even at this stage of the condensation process.
From 6 to 15 min condensation time, molybdenum blue data
was found to fit to an expression for a reversible first-order
reaction (Figure 6C), suggesting that the favored reaction is for
monomer to condense on to already existing oligomers, starting
Speciation in 10 mM Condensing Systems. In order to
further differentiate molybdenum-active species of tetrameric
or greater size during the early stages of condensation, the initial
orthosilicic acid concentration was reduced to 10 mM (Figure
5), with the aim of limiting the extent and speed of the
conversion of oligomeric species to silica particulates. However,
similarly to observations made for other metal ion systems, a
lower [Si] will also reduce the proportion of such oligomers.
At 10 mM, the molybdenum blue activity remains virtually
unaltered for about 10 min before measurable condensation
occurs, Figure 5. Stable levels of total molybdenum active

Solution Study of Silica Condensation and Speciation
J. Phys. Chem. B, Vol. 114, No. 31, 2010 9951
Figure 5. (A) Formation of silicomolybdic acid complex after increasing condensation times for 10 mM initial [Si(OH)
4
] at pH 6.8. (B) Silicate
4
speciation during the condensation process for 10 mM initial [Si(OH) ] at pH 6.8-3 species. The vertical line denotes the time at which corresponding
molybdenum blue data was collected for the reactions.
Figure 6. (A) Apparent third-order regions for condensation measured by the molybdenum blue assay. (B) Plot of the log of the apparent third-
order rate constants against pH. (C) Log of the forward and reverse of the first-order rate constant variation with pH.
species were measured for 10 min after initiation of the
condensation process. However, the rates of color development
of the samples did vary, suggesting that the speciation pattern
varied too. Fitting the molybdenum yellow complex data with
three species representing monomer, trimer, and larger oligomers
is reversible, the silicic acid concentration in solution lies well
above the saturation level and the system is strongly driven
toward condensation rather than redissolution. This is not the
case for the addition of monomers to the trimer or larger
oligomers that follows first-order kinetics and rate constants for
both the forward and the reverse reaction can be measured.
The Effect of pH on the Reaction Process. The effect of
pH on the rates of the early stages of silicic acid condensation
was explored over the range 3.4-6.8 (measurements at higher
pH were not made due to the presence of significant amounts
of undissociated complex). Example raw data are given in the
Supporting Information, Figure S2. The steep change in the
apparent third-order rate constant with pH (Table 1, Figure 6)
reinforces the importance of carefully controlling acid addition
to obtain meaningful data from model silicification experiments.
For example, an error of (0.2 in pH from the model conditions
would result in a discrepancy of (20% in the measured rate
constant. The observed third-order rate constants decreased
sharply with pH, and at a pH of 3.4 no evidence of condensation
was observed in the first 24 h of measurement, although after
7 days the level of monosilicic acid had reduced by ca. 67%
(data not shown). The log of the apparent third-order rate
2
gave R values of 0.9991-0.9989. Fitting with a larger number
of species gave rise to random fluctuations in the species
concentrations, both due to the lower signal/noise ratio obtained
for this concentration and to the inadequacy of the models using
more than three species for this silicon concentration and
condensation times analyzed. The presence of a low level of
trimer was detected even during the first minute of the reaction,
the concentration of this species remaining fairly constant until
the formation of a second condensed species (t ) 30 min). For
longer condensation times, the trimer concentration decreased
practically to zero while the level of the condensed oligomer
continued to increase, this observation coinciding with the onset
of a reduction in total molybdenum active species and is
attributed to the formation of stable silicate oligomers not seen
at a comparable stage in the condensation experiments per-
5
6,57
formed at 30 mM. According to modeling studies,
the
formation of these more stable species is attributable to slow
rearrangements and further condensation that is not allowed for
in the faster condensing experiments performed at higher
concentration.
In-Depth Kinetic Analysis of the Model System: Temper-
2 6
ature and pH Effects. Upon condensation, [Si O(OH) ] dimers
+
constant shows a linear response with pH, i.e., with log [H ],
Figure 6B. Reversible first-order rates also increased (both
forward and reverse) with increasing pH but reached a limit
for the highest pH values, this limit being absent from the
apparent third-order rate trend (Figure 6C). This difference in
are formed as intermediate species but do not seem to be
observed from molybdenum yellow analysis. Moreover, these
species, being able to dissociate into 2 equiv of silicic acid
during the complexation step of the assay, are not observed in
the molybdenum blue assay. Further condensation of these
species yields a trimer that is the first species not able to break
down under the conditions of the assay. This stage is observed
as the apparent loss of 3 equiv of silicic acid and follows
behavior can be explained by considering the pK
a
s of monosi-
2
licic acid and particulate silica. With a pK
a
of 9.8, monosilicic
acid has a weak tendency to deprotonate. The deprotonation of
silanol groups on this species follows a linear trend between
pH 5 and 8, with fewer than 2% of the silanols being
deprotonated at pH 8. By comparison, colloidal silica particles
are stronger acids with a pK of 6.8, and at a pH of 8, 94% of
a
silanol groups are deprotonated. As the level of deprotonation
increases so does the difficulty in removing more protons from
‘
apparent’ third-order reaction kinetics. Although this process

9
952 J. Phys. Chem. B, Vol. 114, No. 31, 2010
Belton et al.
energies with pH32,42,44,45,61,62 and temperature29 have been
measured. Computational studies also give comparable values
TABLE 2: Apparent Third-Order Rate Constants Derived
from the Appropriate Domain Data from the Molybdenum
Blue [Si(OH)
4
] Assay
-1
-1
of 50 kJ ·mol for trimer formation and 95 kJ ·mol for dimer
64
temperature of
condensation/K
apparent third-order
formation and demonstrated the effect of water on the reaction,
with lower activation energies being obtained in excess water
rate constant × 10 /mM s-1
6
-2
-
1
(
55 kJ ·mol ) whereas higher values are obtained under
1 63
2
2
2
3
3
3
73
83
93
03
13
23
0.156
0.427
1.35
4.33
10.8
-
anhydrous conditions (84 kJ ·mol ).
The activation energies obtained respectively for the forward
and reverse processes reduce from 55 and 58.6 kJ ·mol to 6.1
and 7.3 kJ ·mol , respectively, for a temperature change
-
1
-1
30.0
between 20 and 30 °C. At higher temperatures, both the duration
and starting time of the first-order condensation reaction stage
decrease dramatically, making the transition between the
kinetic domains difficult to isolate and may be the reason
for the low activation energies found in this study. The higher
activation energy obtained for the apparent third-order domain
the remaining silanol groups due to the increased charge density
on the particles. Therefore, the increase in the rate of condensa-
tion with pH is limited until at even higher pH (above 10) the
silica will begin to redissolve. This behavior is not observed
for the third-order rate domain, as fewer than 1% of the
orthosilicic acid molecules are anionic so there is no increased
resistance to deprotonation with pH change. This data can be
used to understand why additions of a range of molecules and
ions to the model system significantly affects the reaction period
where trimer formation is the dominant reaction as opposed to
the time period where reversible first-order kinetics dominate.3
A greater change in solution chemistry is required to influence
the degree of ionization of oligomers and particulate silica
compared to monomer under the circumneutral pH conditions
used for the in vitro studies of biosilicification.
-
1
(77 kJ · mol ) fits with the lower acidity of the silanol groups
of monomers compared to equivalent groups on highly
condensed species. This observation reinforces further the
conclusions derived from the experiments performed under
different pH conditions.
7,38
Discussion. Relevance to Biosilicification
In this study we have analyzed in detail a model system based
on the hexacoordinated silicon catechol complex, which has
5
,37,38,59
been central to our bioinspired studies on silicification.
The Effect of Temperature. Calculation of Activation
Energies. Experiments following the kinetics of silicic acid
condensation were performed at temperatures between 273 and
Upon pH adjustment, the system provides rapidly and stoichi-
ometrically metastable solutions of monosilicic acid of control-
lable concentration. While the general methodology used for
our previous studies included other aspects of analysis such as
3
23 K. The increase in the apparent third-order rate constant,
with temperature highlights the need to conduct experiments
3
,38,65
k
3
aggregation phenomena and the final materials properties,
with strict temperature control (Figure 7A and Table 2).
we focused here on the reaction kinetics and solution speciation
pertinent to the early stages of the silicification process and
identified time periods over which a range of oligomeric species
were present in the condensing systems, Scheme 1.
In order to understand mechanisms of biosilicification and
to mimic biological silica formation, so-called biomimetic and
A discrepancy of only (2 °C results in approximately a
20% variation in k . The beginning and end of the apparent
3
(
third-order domain were also shifted to earlier times upon
temperature increase. The Arrhenius equation (2) was used to
derive thermodynamic parameters from kinetic constants ob-
tained at different temperatures:
bioinspired in vitro model studies have been performed and
reported in recent literature.6
,7,9,37,65-73
Research into biological
silicification has shown the involvement of catalytic biomol-
-
Ea/RT
k ) Ae
(2)
1,22,74,75
ecules such as proteins and peptides.
In vitro model
studies have used synthetic molecules (termed ‘bioinspired
additives’) which are derived from biomolecules identified to
be responsible for biosilica formation. These model studies
typically use a silica precursor and investigate the effects of
the presence of a bioinspired additive, inorganic ions (e.g.,
phosphate), reaction pH, self-assembly of additives, etc., on
silica formation.
Several aspects of silica formation have been investigated
such as particle nucleation and growth, materials morphologies,
and porosities. However, the early stages of condensation
involving monomeric, dimeric, and oligomeric silicates have
been rarely monitored. One possible reason for lack of
understanding of initial condensation mechanisms is that not
every chosen silica precursor can produce a pure solution of
orthosilicic acid (rather a mixture of oligomeric silicates and
partially hydrolyzed/condensed precursor is formed). As a result,
with k being the rate constant, T being the temperature, E
the activation energy of the process, and A being a term relating
to the collisional frequency of the reacting species and the
a
being
likelihood that they will be in the correct orientation to react.
1
The plot of ln k
3
against T- gave a linear trend (Figure 7B),
emphasizing the correct selection of the time intervals for
determining the individual rate constants. The value for E
a
-1
obtained for the third-order process was 77 kJ ·mol . Similar
treatment of the first-order domain data, Figure 7C gave
activation energies for the forward and reverse condensation
-1
reactions of 55.0 and 58.6 kJ ·mol , respectively. This is in
5
good agreement with the results obtained by Harrison and Loton
who measured an activation energy of 58 kJ ·mol- over the
first-order region but did not separate the forward and reverse
rates. Activation energies for monomer condensation onto
1
5
8
particles were also found to be of this order. Activation
energies of condensation reported elsewhere range from 13 to
in vitro studies of biosilicification have predominantly focused
on additive-silica particle aggregation.6
8,76,77
Exceptions are the
-
1 29,32,39,42-45,59-63
85 kJ·mol .
Activation energies determined for
studies of hydrolysis and condensation of monofunctional
alkylalkoxysilanes where the precursor is allowed to form dimers
early condensation stages of a first-order process have been
determined and range from 27 kJ ·mol- for an apparent first-
1
while further reaction is restricted.
66,67
-1
order reaction (using TEOS) up to 71 kJ ·mol calculated for
The use of a silicon-catecholate complex and the silicate
speciation reported herein has allowed us to study closely the
6
0
monomer condensation on particles. Variations of activation

Solution Study of Silica Condensation and Speciation
J. Phys. Chem. B, Vol. 114, No. 31, 2010 9953
Figure 7. (A) Apparent third-order rate domains at 273-323 K (B) Arrhenius plot of the apparent third-order rates. (C) Arrhenius plot for the
forward and reverse first-order rate constants.
SCHEME 1: Silicate Species Isolated by Curve Fitting of
Molybdenum Yellow (silicomolybdic acid) Data during
the Condensation of Orthosilicic Acid from an Example
relevant pHs and the impact this knowledge can have on
understanding biological silica formation.
3
0 mM Condensing System
Experimental Section
The following materials were purchased and used without
further treatment: Dipotassium tris(1,2-benzene-diolato-O,O′)-
silicate (Silicon Catechol Complex, SCC) 97% (Sigma Aldrich);
oxalic acid dihydrate 99%, ammonium molybdate tetrahydrate
9
9%, 4-(methylamino)phenol sulfate 99% (Acros organics);
hydrochloric acid 98%, sodium hydroxide 98%, sodium sulfite
9% (Fisher Scientific); 1000 ppm silica (as SiO ) standard
9
2
(
(
BDH); sodium 2,2-dimethyl-2-silapentane-5-sulfonate (DSS)
MSD isotope).
1
-3
early stages of silicic acid condensation, and selected examples
are discussed. The amino acid lysine and its oligomers/polymers
have been of great interest in silica formation, and their presence
in silica-forming systems have produced spherical particles,
Titrimetric/ H NMR Study. Aqueous 30 mM ·dm solu-
3
-
tions of SCC were titrated with freshly standardized 2 M ·dm
solutions of hydrochloric acid by addition of 10 µL aliquots,
and the pH was monitored using a Radiometer PHM 240 pH
meter fitted with a Mettler Toledo microcombination electrode.
The stoichiometry of the acid dissociation was determined and
8
,9,69,78,79
granular gels, and hexagonal plates.
However, these
published reports did not probe the effect of lysine on the early
stages of condensation and therefore concluded, based on the
data available, that lysine promotes the aggregation of small
silica particles in solution, leading to silica precipitation under
1
then used to prepare solutions of SCC for H NMR studies.
The individually prepared samples (0-2.0 mol equiv HCl) were
monitored using a JEOL JNM-EX270 FT NMR spectrometer.
The lock signal and chemical shift standard were supplied by
76
mild conditions. In view of the results presented in this article,
when the early stages of silicic acid condensation were studied
in conjunction with particle growth, it was revealed that lysine
-3
use of an insert containing 1 mg·cm 2,2-dimethyl-2-silapen-
tane-5-sulfonate (DSS) in D O. Spectra were acquired using 32
2
(
and its oligomers and polymers) increase the rates of silicic
scans and a relaxation delay of 1 s.
acid condensation (e.g., trimerization). This effect became
prominent as the chain length of lysine oligomers increased.37
In addition to the effect of lysine on condensation rates, lysine
was also found to promote aggregation of silica particles in
Molybdenum Complexation Studies. The molybdenum blue
method routinely used by us and others to study the condensa-
tion of orthosilicic acid involves the formation of a yellow
silicomolybdic acid complex which is then reduced to the more
intensely colored silicomolybdous acid complex, that is used
37
solution which supported the earlier findings reported in the
literature.76 The experimental approach described in this paper
has allowed us to develop a greater understanding at the
molecular level how polyamines inspired by those found in
9
for a more sensitive assay. The rate of formation of the yellow
complex gives additional information on the silicate species
present as the rate of dissociation of silicate oligomers to
33
13
nature are perfectly adapted to their function in the silicification
monomer is species dependent. In these studies the molyb-
process.6
5,80
Furthermore, the mechanistic understanding of
denum yellow data was used for the first time to provide
information on speciation changes with time during the con-
densation process. The molybdenum blue method was used to
obtain information on changes in residual concentrations of
orthosilicic acid with time and to obtain kinetic rate constants
and activation energies for reactions occurring during the early
stages of condensation. The required stock solutions (molybdic
acid reagent and reducing reagent) were prepared as follows:
biological and bioderived molecules in model studies of biosilica
formation have enabled researchers to tailor the chemistry of
additives and reaction conditions in order to generate materials
8
0
with specified properties. A further example is the role that
hydroxyl-containing biomolecules (e.g., glycoproteins and pro-
teins rich in amino acids such as serine and threonine) play in
biosilicification. Previous studies suggested their involvement
8
1,82
without any direct experimental evidence.
Using the ap-
2
Molybdic acid reagent stock: Ammonium molybdate ·4H O (20
3
proach outlined in this article a range of small molecules and
proteins rich in hydroxyl groups have been shown to have little
direct effect on the early stages of condensation in aqueous
media.71
g) and hydrochloric acid (60 cm ) were dissolved and made up
to 1000 cm3 with distilled and deionized water (ddH
O).
2
3
Working solutions were prepared by diluting 1.5 cm of the
3
2
stock solution with 15.0 cm of ddH O. Reducing reagent (used
These examples clearly demonstrate the importance of the
knowledge of silicate speciation in solutions with physiologically
for the molybdenum blue method): Oxalic acid (20 g), 4-me-
thylaminophenol sulfate (6.67 g), and sodium sulfite (4 g) were

9
954 J. Phys. Chem. B, Vol. 114, No. 31, 2010
Belton et al.
3
dissolved in ddH
2
O (500 cm ). Concentrated sulfuric acid (100
spectrophotometric measurement of the total digested silicon
content using a standard molybdenum blue method. At the end
of the initial fitting step, an apparent rate for each species
reaction with the reagent was calculated as a average of the
rates obtained for this species in the different samples analyzed.
To calculate this average rate for one species, the (silicon
concentration, rate of complexation) parameter pairs obtained
for the species in each sample after the first fit was used to
calculate the average rate as a linear combination of the rates
determined from different samples. New narrower boundaries
for the variation of the kinetic constants were also determined,
and the fitting/refinement operations reiterated until convergence
of the parameters.
3
mL) was added and the solution made up to 1000 cm with
2
O. A 8.0 cm volume of this solution was added after the
silicomolybdic acid complex had been allowed to develop for
3
ddH
1
5 min.
Molybdenum Yellow Speciation Studies. A 10 µL amount
of the condensing monosilicic acid solutions (prepared by
reduction of the pH of 10 and 30 mM ·dm- solutions of
3
dipotassium tris(1,2-benzenediolato-O,O′) silicate ·2H
2
O to 6.9
0.05 or where otherwise stated) was added to the working
molybdic acid reagent solution at known time intervals between
-24 h. The formation of the yellow silicomolybdic acid
(
0
complex was monitored at 370 and 410 nm over a period of 30
min from the time of addition of the 10 µL aliquot to the
molybdic acid solution. Measurements made at 410 nm gave
far better results, for although the spectrometer response at 370
nm was higher, the signal:noise ratio was much poorer due to
interfering solution species contributing to the signal at the lower
wavelength, Figure S3. Only the results of the 410 nm
measurements are presented in the main body of the paper.
Molybdenum Blue Complexation Studies. The molybde-
num blue method was used to study the silicification process
in model systems. The condensing systems used were prepared
by adjustment of 30 mM SCC solutions to the desired pH
(
3.4-6.8 ( 0.05) over a range of temperatures (0-50 °C
regulated by a thermostatically controlled water bath), and also
using 10 mM SCC solutions adjusted to pH 6.8 and maintained
at 25 °C for slower condensation studies. Then 10 µL aliquots
of the condensing silicic acid solutions were added to the
working molybdic acid reagent at known time intervals and left
for 15 min before addition of the reducing reagent. Solutions
were then left for at least 2 h for the blue silicomolybdous acid
complex to develop before measuring absorbance at 810 nm.
Levels of molybdenum active silicate species were determined
against calibration standards prepared from dilutions of a
The rate of silicomolybdic acid complexation is dependent
on the rate of silicate oligomer dissociation in the molybdic
acid reagent. Studies carried out using molybdic acid conditions
9
of concentration and pH similar to here showed the complex-
ation rate dependence on dissociation. Using calibration data
obtained from solutions of known concentration, the curve of
the silicic acid molybdenum yellow chelate concentration versus
time was calculated from the A ) f(t) curve (where A is the
UV absorbance of the solution, t the time following mixing of
the reagents, and f the relationship between A and t). The data
was fitted with a theoretical function based on the assumption
that the dissolution of all silicon species present obeys pseudo-
first-order kinetics and that the reaction of silicic acid with the
reagent also obeys such a law. The fitting parameters comprise
species concentrations together with their apparent rates of
dissolution and the apparent rate of silicic acid reaction with
the organic reagent.
2
stabilized 1000 ppm SiO standard.
Kinetic Analysis of Molybdenum Blue Data. Kinetic
analysis of the orthosilicic acid condensation process was
5
performed according to literature methods with data being fitted
2-
to [Si(OH)
for apparent third- and reversible first-order kinetics, respectively
where a is the monomer concentration at time t, and a is the
4 ∞
)] and ln([a] - [a ]) over appropriate time periods
(
∞
monomer concentration at equilibrium for an infinite condensa-
tion time a). Four repeat determinations were carried out at 293
(
1 K, giving a number of degrees of freedom of n - 1) 3.
Mathematical Modeling of Molybdenum Yellow Com-
plexation. Speciation models of up to five species were tested
during this study. When adding more species to the fitting
model, the number of species was limited by the following: (a)
species which, during the fitting procedure, see their rates of
dissolution converge to a similar value, indicating the existence
of one species rather than two; (b) the differentiation of slow-
reacting species such as highly condensed oligomers, hampered
by their very limited dissolution during the time of the assay.
This limited dissolution leads to a low signal-to-noise ratio for
such species, and random fluctuations in the speciation pattern
as a result of uncertainties affecting the fitting process. For the
fitting operation, a MatLab routine was used to limit the variation
of apparent reaction rates for identical species present in different
samples. The fitting algorithm uses a built-in nonlinear least-
squares minimization technique (MatLab curve-fitting toolbox)
for the optimization of curve-fitting parameters. The initial
values of apparent rates were chosen on the basis of literature
For a 95% confidence with three degrees of freedom, the t value
-6
is 3.182 and giving a confidence interval of 1.16 × 10
(
-8
-2 -1
8
.21 × 10 (≈9%) mM
s
for the apparent third-order rate
-3
-4
-1
constant and 1.69 × 10 ( 1.61 × 10 (≈7%) s for the
overall reversible first-order. A sample analyzed under the
conditions used has therefore only 5% probability of falling
outside of these intervals by systematic experimental error. Even
relatively small changes observed in the determined rate
constants can therefore be regarded as significant based on this
analysis when using this model system.
Supporting Information Available: Typical spectrum of
base-quenched dipotassium tris(1,2-benzenediolato-O,O′)silicate.
2H O dissociation, Figure S1, a typical condensation profile
2
measured using the molybdenum blue method, showing the
reduction in molybdenum blue active species with time, Figure
S2, and the comparison of signal-to-noise ratio for 3 and 30
mM orthosilicic acid solutions measured at 370 and 410 nm,
Figure S3. This material is available free of charge via the
Internet at http://pubs.acs.org.
5
data (available with potassium as a counterion) together with
the analysis of pure samples of some of the species (monomer
and highly condensed silica). The initial species concentrations
were set equal to a fraction (10 mM for three species) of the
total concentration (30 mM). The curves of the detected silicon
concentration against time were then fitted a first time using an
apparent first-order model. The total silicon concentration for
each sampling time was confirmed by digestion of sample
portions with concentrated KOH and subsequent standard
References and Notes
(1) Sumper, M.; Kr o¨ ger, N. J. Mater. Chem. 2004, 14, 2059–2065.
(
2) Perry, C. C.; Keeling-Tucker, T. Colloid Polym. Sci. 2003, 281,
6
52–664.
3) Cha, J. N.; Stucky, G. D.; Morse, D. E.; Deming, T. J. Nature
2000, 403, 289–292.
(

Solution Study of Silica Condensation and Speciation
J. Phys. Chem. B, Vol. 114, No. 31, 2010 9955
(
4) Cha, J. N.; Shimizu, K.; Zhou, Y.; Christiansen, S. C.; Chmelka,
(44) Hertzog, A.; Weiss, A. In Biochemistry of Silicon and Related
Problems; Bendz, G., Lindqvist, I., Eds.; Plenum Press, 1978; pp 109-
127.
B. F.; Stucky, G. D.; Morse, D. E. Proc. Natl. Acad. Sci. U.S.A. 1999, 96,
61–365.
3
(
5) Harrison, C. C.; Loton, N. J. Chem. Soc., Faraday Trans. 1995,
(45) Kinrade, S. D.; Gillson, A. M. E.; Knight, C. T. G. J. Chem. Soc.,
Dalton Trans. 2002, 307–309.
9
1, 4287–4297.
(
(
(
6) Coradin, T.; Livage, J. Colloids Surf., B 2001, 21, 329–336.
7) Patwardhan, S. V.; Clarson, S. J. Silicon Chem. 2002, 1, 207–214.
8) Coradin, T.; Durupthy, O.; Livage, J. P. Langmuir 2002, 18, 2331–
(46) Ringbom, A.; Ahlers, P. E.; Shtonen, S. Anal. Chim. Acta 1959,
2
0, 78–83.
(
47) Langmyhr, F. J.; Graff, P. R. Anal. Chim. Acta 1959, 21, 334–
3
39.
2
336.
(
(48) Rakhimova, O. V.; Tsyganova, T. A.; Antropova, T. V.; Kostyreva,
9) Sudheendra, L.; Raju, A. R. Mater. Res. Bull. 2002, 37, 151–159.
T. G. Glass Phys. Chem. 2000, 26, 303–306.
(
10) Patwardhan, S. V.; Raab, C.; Husing, N.; Clarson, S. J. Silicon
(
(
49) Dietzel, M. Geochem. Cosmochim. Acta 2000, 64, 3275–3281.
50) Bennett, P. C.; Melcer, M. E.; Siegel, D. I.; Hassett, J. P. Geochem.
Chem. 2003, 2, 279–285.
11) Kr o¨ ger, N.; Lorenz, S.; Brunner, E.; Sumper, M. Science 2002,
98, 584–586.
12) Mizutani, T.; Nagase, H.; Fujiwara, N.; Ogoshi, H. Bull. Chem.
Soc. Jpn. 1998, 71, 2017–2022.
13) Iler, R. K. The Chemistry of Silica; Plenum Press: New York, 1979.
14) Tr e´ guer, P.; Nelson, D. M.; Van Bennekom, A. J.; Demaster, D. J.;
Leynaert, A.; Queginer, B. Science 1995, 268, 375–379.
15) Gallinari, M.; Ragueneau, O.; Corrim, L.; Demaster, D. J.; Tr e´ guer,
P. Geochem. Cosmochim. Acta 2002, 66, 2701–2717.
(
Cosmochim. Acta 1988, 52, 1521–1530.
51) Wonisch, H.; G e´ rard, F.; Dietzel, M.; Jaffrain, J.; Nestroy, O.;
2
(
(
Boudot, J. P. Geoderma 2008, 144, 435–445.
(
(
97.
52) Iler, R. K. J. Colloid Interface Sci. 1980, 75, 138–148.
(
(
53) Dietzel, M.; Usdowski, E. Colloid Polym. Sci. 1995, 273, 590–
5
(
54) Harris, M. T.; Brunson, R. R.; Byers, C. H. J. Non Cryst. Solids
(
1
990, 121, 397–403.
(
55) Trinh, T. T.; Jansen, A. P. J.; van Santen, R. A. J. Phys. Chem. B
(
(
(
16) Ashley, K. D.; Innes, W. B. Ind. Eng. Chem. 1952, 44, 2857–2863.
17) Greenberg, S. A.; Sinclair, D. J. Phys. Chem. 1955, 59, 435–440.
18) Volosov, A. G.; Khodakovsky, I. L.; Ryzhenko, B. N. Geochem.
2006, 110, 23099–23106.
(56) Mora-Fonz, M. J.; Catlow, R. A.; Lewis, D. N. Angew. Chem.,
Int. Ed. 2005, 44, 3082–3086.
(57) Ng, L. V.; Mc Cormick, A. V. J. Phys. Chem. 1996, 100, 12517–
12531.
(58) Fleming, B. A. J. Colloid Interface Sci. 1986, 110, 40–64.
Int. 1972, 9, 362–377.
(
(
(
19) Seward, T. M. Geochem. Cosmochim. Acta 1974, 38, 1651–1664.
20) Bilinski, H.; Ingri, N. Acta Chem. Scand. 1967, 21, 2503–2510.
21) Belyakov, V. N.; Strazhenko, D. N.; Soltirskii, N. M.; Strelko, V. V.
(
59) Perry, C. C.; Keeling-Tucker, T. Chem. Commun. 1998, 2587–
2
2
1
588.
Ukr. Khim. Zh. 1974, 40, 236.
22) Perry, C. C.; Belton, D. J.; Shafran, K. L. In Progress in Molecular
and Submolecular Biology; Springer-Verlag: Berlin, 2003; Vol. 33, p 269.
23) Jolivet, J. P. In Metal Oxide Chemistry and Synthesis, 3rd ed.; J.
(
(
(
60) Geische, H. J. Eur. Ceram. Soc. 1994, 14, 189–204.
61) Penner, S. S. J. Polym. Sci. 1946, 1, 441–444.
(
62) Brady, A. P.; Brown, A. G.; Huff, H. J. Colloid. Sci. 1953, 8, 252–
(
76.
Wiley & Sons: 2000; pp 96-101.
(
(
(
63) Rao, N. Z.; Gelb, L. D. J. Phys. Chem. B 2004, 108, 12418–12428.
64) Garofalini, S. H.; Martin, G. J. Phys. Chem. 1994, 98, 1311–1316.
65) Belton, D. J.; Patwardhan, S. V.; Perry, C. C. J. Mater. Chem. 2005,
(
(
24) Baumann, H. Z. Anal. Chem. 1966, 217, 241–247.
25) White, D. E.; Brannock, W. W.; Murata, K. J. Geochem. Cosmo-
chim. Acta 1956, 10, 27–59.
26) Icopini, G. A.; Brantley, S. L.; Heaney, P. J. Geochem. Cosmochim.
Acta 2005, 69, 293–303.
5, 4629–4638.
66) Bassindale, A. R.; Brandstadt, F. K.; Lane, T. H.; Taylor, P. G.
J. Inorg. Biochem. 2003, 96, 401.
(
(
(
(
(
27) Goto, K. J. Phys. Chem. 1956, 60, 1007–1008.
28) Jørgensen, S. S. Acta Chem. Scand. 1968, 22, 335–341.
29) Bishop, A. D., Jr.; Bear, J. L. Thermochim. Acta 1972, 3, 399–
(67) Delak, K. M.; Sahai, N. Chem. Mater. 2005, 17, 3221.
(68) Knecht, M. R.; Wright, D. W. Langmuir 2004, 20, 4728.
(69) Patwardhan, S. V.; Mukherjee, N.; Steinitz-Kannan, M.; Clarson,
4
09.
S. J. Chem. Commun. 2003, 10, 1122.
(70) Roth, K. M.; Zhou, Y.; Yang, W.; Morse, D. E. J. Am. Chem. Soc.
2005, 127, 325.
(71) Tilburey, G. E.; Patwardhan, S. V.; Huang, J.; Kaplan, D.; Perry,
C. C. J. Phys. Chem. B 2007, 111, 4630.
(
(
(
30) Alexander, G. B. J. Am. Chem. Soc. 1954, 76, 2094–2096.
31) Kitahara, S. ReV. Phys. Chem. Jpn 1960, 30, 131–137.
32) Coudurier, M.; Baudru, R.; Donnet, J. B. Bull. Soc. Chim. Fr. 1971,
9
, 3147–3153.
(
33) Kr o¨ ger, N.; Deutzmann, R.; Bergsdorf, C.; Sumper, M. Proc. Natl.
Acad. Sci. U.S.A. 2000, 97, 14133–14138.
34) Silicon and Siliceous Structures in Biological Systems; Simpson,
T. L., Volcani, B. E., Eds.; Springer-Verlag: New York, 1981.
(72) Liang, M. K.; Patwardhan, S. V.; Danilovtseva, E. N.; Annenkov,
V. V.; Perry, C. C. Mater. Res. Bull. 2009, 24, 1700.
(73) Jin, R. H.; Yuan, J. J. AdV. Mater. 2005, 17, 885.
(74) Brutchey, R. L.; Morse, D. E. Chem. ReV. 2008, 108, 4915.
(
(
75) Patwardhan, S. V.; Clarson, S. J.; Perry, C. C. Chem. Commun.
005, 1113.
76) Lopez, P. J.; Gautier, C.; Livage, J.; Coradin, T. Curr. Nanosci.
005, 1, 73.
77) Groeger, C.; Lutz, K.; Brunner, E. Cell Biochem. Biophys. 2008,
0, 23.
78) Patwardhan, S. V.; Mukherjee, N.; Clarson, S. J. J. Organomet.
Polym. 2001, 11, 193.
79) Patwardhan, S. V.; Maheshwari, R.; Mukherjee, N.; Kiick, K. L.;
Clarson, S. J. Biomacromolecules 2006, 7, 491.
80) Belton, D. J.; Patwardhan, S. V.; Annenkov, V. V.; Danilovtseva,
E. N.; Perry, C. C. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 5963.
81) Hecky, R. E.; Mopper, K.; Kilham, P.; Degens, E. T. Mar. Biol.
973, 19, 323.
82) Lobel, K. D.; West, J. K.; Hench, L. L. Mar. Biol. 1996, 126, 353.
(
(
35) Currie, H. A.; Perry, C. C. Ann. Bot. 2007, 100, 1383–1389.
36) Brook, M. A.; Chen, Y.; Guo, K.; Zhang, Z.; Brennan, J. D. J.
2
2
5
(
Mater. Chem. 2004, 14, 1469–1479.
37) Belton, D. J.; Paine, G.; Patwardhan, S. V.; Perry, C. C. J. Mater.
Chem. 2004, 14, 2231–2241.
38) Belton, D. J.; Patwardhan, S. V.; Perry, C. C. Chem. Commun.
005, 3475–3477.
39) Brinker, C. J.; Scherer, G. W. In The Physics and Chemistry of
Sol-Gel Processing; Academic Press Inc., 1990; Chapter 3, pp 97-127.
40) McCormick, A. V.; Bell, A. T.; Radke, C. J. Zeolites 1987, 7, 183–
90.
(
(
(
(
2
(
(
(
(
1
1
1
(41) Dent Glasser, L. S.; Lachowski, E. E. J. Chem. Soc., Dalton Trans.
(
980, 393–398.
1
(
42) Dent Glasser, L. S.; Lachowski, E. E. J. Chem. Soc., Dalton Trans.
980, 399–402.
43) Engelhardt, G.; Rademacher, O. J. Mol. Liq. 1984, 27, 125.
(
(
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