Crystalline silica prepared at room temperature from aqueous solution in the presence of intrasilica bioextracts

Article abstract of DOI:10.1039/a807404f

Silica, exhibiting crystalline texture has been prepared from aqueous solution at pH ca. 7 in the presence of intrasilica biomolecules extracted from the primitive plant Equisetum telmateia; some of the silica shows interplanar lattice spacings of 3.5 A and is present in lath-like objects up to ca. 20-30 nm wide and more than 100 nm in length; an electron diffraction pattern obtained from this material also shows interplanar spacings of 3.46, 2.13 and 1.70 A suggesting that the material might be quartz.

Full text of DOI:10.1039/a807404f

Crystalline silica prepared at room temperature from aqueous solution in the
presence of intrasilica bioextracts
Carole C. Perry* and Tracey Keeling-Tucker
Department of Chemistry and Physics, The Nottingham Trent University, Clifton Lane, Nottingham, UK NG11 8NS.
E-mail: Carole.Perry@ntu.ac.uk
Received (in Bath, UK) 23rd September 1998, Accepted 20th October 1998
Silica, exhibiting crystalline texture has been prepared from
aqueous solution at pH ca. 7 in the presence of intrasilica
biomolecules extracted from the primitive plant Equisetum
telmateia; some of the silica shows interplanar lattice
spacings of 3.5 Å and is present in lath-like objects up to ca.
20–30 nm wide and more than 100 nm in length; an electron
diffraction pattern obtained from this material also shows
interplanar spacings of 3.46, 2.13 and 1.70 Å suggesting that
the material might be quartz.
colorimetric molybdenum blue method.11 Kinetic analysis of
the solution data was carried out in accordance with our
previous studies.12‡
The effect of the biosilica extracts on both the early
oligomerization reactions (e.g. the formation of dimers and
trimers of orthosilicic acid) and on particle growth by
precipitation/dissolution reactions was studied. Electron mi-
croscopy studies of the precipitates gave information on the
effect of solution additives on nucleation, particle growth and
aggregation.
In the field of biomineralization where biological organisms are
able to regulate the formation of both crystalline and amorphous
composite mineral phases there is much interest in under-
standing how mineralization occurs.1 The process requires the
concentration of selected elements, the nucleation, growth and
moderation of growth of the mineral phase in predefined
locations at specific times during the lifetime of a particular
organism. Biominerals are usually composite phases containing
organic components such as membranes and/or proteins and
carbohydrates together with the mineral phase. The relationship
between the two is thought to be important in the regulation of
mineral composition, crystallographic phase and morphology.2
The organic phase is largely found external to the mineral phase
but low levels (ca. 0.03% by weight) of acidic proteinaceous
biopolymers can also be found intercalated within the mineral
phase, perhaps at crystal domain boundaries.3 The intercalated
biopolymers may have fundamental roles to play in nucleation
and/or crystal growth and their presence has also been shown to
have dramatic effects on the mechanical properties of crystal-
line biominerals.4 For amorphous biominerals such as silica,
biopolymers are also found in intramineral locations,5–7 how-
ever, their role in nucleation, particle growth and aggregation
remains unclear.
We are interested in extending our understanding of how
biosilicas with specific form8,9 are laid down and additionally
we are interested in the preparation of novel silica phases using
knowledge gained from the study of biological systems. To this
end we have extracted protein-containing biomolecules from
intrasilica locations in the branches of Equisetum telmateia5 and
used them in the study of silica precipitation at circumneutral
pH.† The biopolymer extract used in this study was released by
solubilization of the siliceous phase with buffered solutions of
HF following treatment of plant materials with a mixture of
concentrated nitric and sulfuric acids. The amino acid composi-
tion of this extract was rich in serine, glutamine/glutamic acid
and glycine and had associated with it a carbohydrate
component enriched in glucose and xylose. The ‘model’ system
used to study particulate silica precipitation utilized a cate-
cholato complex of silicon, K2[Si(C6H4O2)3]·xH2O as the
source of soluble silicon.10 At ca. physiological pH the complex
partially decomposes to yield orthosilicic acid which im-
mediately undergoes polycondensation reactions to relieve any
resultant supersaturation.
Statistical analysis (students t-test and Mann Whitney U-
test)13 of the kinetic data‡ showed that the addition of 1% w/w
of the Equisetum telmateia intrasilica extract to the silica
oligomerization medium results in a ca. 22% increase in the rate
of trimer formation (the second reaction in the oligomerization
process). The rate constant for the addition of a monomer to an
oligomer larger than a trimer is statistically no different for
samples including the proteinaceous extract but the rate of
removal of monomers from oligomers is reduced by ca. 33%.
The effect of these changes is to increase the rate at which
orthosilicic acid is removed from solution and to reduce the
amount of silicon left in solution at the end of the experiment.
Fig. 1 shows the increase in the levels of oligomerized silica at
all sampling points. Electron microscopy of the precipitated
silica shows evidence for aggregates built up from small
particles ca. 1–2 nm in diameter, much smaller than is expected
for the ‘blank’ system (data not shown, see ref. 10 for an
example). Other structures present include ribbon-like (lath-
like) objects and curved loops which often show characteristic
fringes of ca. 3.50 Å. Energy dispersive X-ray analysis of all
such areas indicates that the material contains silicon and
oxygen.† The ribbon-like structures and loops, Fig. 2(a)–(c), are
found in all samples from 1 h after initiation of the precipitation
reaction. Samples taken at 48 h after the start of the experiment
occasionally exhibit order over distances > 600 nm in length
and ca. 20–30 nm in width. The images obtained suggest a ‘soft’
or perhaps ‘layered’ material (for an example, see ref. 14) but
the electron diffraction patterns obtained from some areas with
d-spacings of 3.46, 2.13 and 1.70 Å are compatible with quartz
[PDF 5-490], Fig. 2(d). X-Ray diffraction data could not be
obtained from silica collected by centrifugation 7 days after
initiation of the reaction (even after an extended scan over 8 h)
indicating that the silica sample prepared in the presence of the
50 mM solutions of potassium silicon catecholate were used
with/without the biopolymer extract at 1% w/w, the pH of the
solution was lowered to ca. 7.0 by the addition of a
predetermined quantity of HCl and the concentrations of
orthosilicic acid measured as a function of time by a
Fig. 1 Plot of % of oligomerized silica (not detectable by the molybdate
method) vs. time.
Chem. Commun., 1998, 2587–2588
2587

biopolymer extract contains only low levels of crystalline
material. No additional information was obtained from diffrac-
tion studies at low values of 2q suggesting that the siliceous
materials are not mesoporous, lamellar phases.
The presence of the protein-containing extract in the
oligomerizing mixture clearly had some effect on nucleation
with the formation of (a) smaller particles of silica (the majority
of fundamental particles are smaller than 2 nm in diameter as
opposed to particles up to 4 nm in diameter for the blank
system) and, (b) silica with a crystalline appearance. The
presence of the latter material from the earliest analysis point (as
assessed by electron microscopy) suggests that this material is
formed very early in the reaction profile and is not the result of
structural rearrangement with time.
Biopolymers can be removed from the silica precipitated in
the ‘model’ reaction system utilizing the same methods used for
their initial production. The levels of proline are decreased and
the levels of glycine are ca. doubled in the proteinaceous
material extracted from the ‘model’ silica precipitated materi-
als. The amino acid composition of the biopolymer extracts is
rich in amino acids known to form b-sheet or b-turn secondary
structures. For such structures, the spacing between successive
layers is a minimum of 3.5 Å when only glycine is involved but
may be 5.7 Å for chains rich in alanine as is found in the silk
protein fibroin.15 It is possible that the observed silica structures
are either crystalline silica formed de novo from aqueous
solution, or more likely, the observed structures are generated
by epitaxial matching of the organic and inorganic matrices
with the silica structure continuing to develop from the initial
biopolymer-controlled nucleation event. It is evident that in the
biological environment additional controls must be exerted
during the process of silica precipitation in order to prevent the
formation of crystalline phases as they are much more difficult
to mould into the macroscopic structures produced by living
organisms.
absence of multicharged cations, conditions which would not be
expected to yield crystalline silica.11,16
Notes and references
† Precipitation experiments were conducted using biopolymer extracts from
two separate extractions. The amino acid compositions were measured
using an Applied Biosystems 420a amino acid analyser operated by A. C.
Willis of the MRC Immunochemistry Unit, Oxford University. For the
kinetic measurements, four sample runs were completed for each experi-
ment on the same day using a temperature controlled reaction vessel set at
23 ± 0.1 °C. Samples for electron microscopy studies were taken from
parallel experiments by dipping carbon coated formvar covered copper
electron microscope grids into the reaction vessel at 1, 4, 24, 48 h and 7 days
after the experiment was initiated and allowing the grids to air dry. Samples
for electron microscopy were investigated using a JEOL 2010 analytical
electron microscope (Link ISIS system) fitted with a LaB6 filament
operating at 200 keV. Magnifications of 200 000 3 were necessary to see
the lattice fringing present and areas of interest were subjected to energy
dispersive X-ray analysis to identify the elements with atomic number !6
present in the sample. A minimum of 10 analyses for each sample area
showed that the precipitated material contained Si and O together with
traces of K and Cl (N.B. the grids were not washed after sampling by
dipping). Crystalline structures as presented in this paper have not been
observed by transmission electron microscopy in any of our other model
precipitation experiments performed in the presence of a range of singly and
multiply charged metal ions, carbohydrates and proteins such as bovine
serum albumin, zein, concanavalin A and cytochrome c. Silica samples
were also analysed by powder X-ray diffraction; Siemens D500 dif-
fractometer operating in the range 2q = 1–80° by Professor Mark Weller
and Dr Adam Whitehead of Southampton University. The Visual Services
Department at The Nottingham Trent University are thanked for printing of
the electron micrographs for publication. BBSRC and Crosfield Chemicals
are thanked for their funding.
Amino acid composition of bioextract used in the precipitation
experiment (mol%); Asx; 8.9, Glx; 15.0, His; 3.5, Lys; 4.94, Arg, 1.54, Ser;
14.24, Thr; 3.91, Tyr; 1.30, Gly; 16.67, Pro; 9.60, Ala; 8.42, Val; 4.07, Leu;
4.3, Ile; 2.64, Phe; 0.98 Amino acid composition of biopolymers extracted
from ‘model’ system precipitated silica (mol%); Asx; 7.56, Glx; 12.3, His;
5.7, Lys; 2.73, Arg, 3.4, Ser; 12.92, Thr; 4.41, Tyr; 2.35, Gly; 30.46, Pro;
4.06, Ala; 6.95, Val; 2.94, Leu; 4.23, Ile; nd, Phe; nd. nd = not detected.
‡ Rate constants:13 blank system; k3 = 4.91 3 1026 mmol22 dm6 s21, k+
= 5.67 3 1024 s21, k2 = 1.19 3 1025 s21. With 1% biomolecule extracts;
k3 = 6.29 3 1026 mmol22 dm6 s21, k+ = 4.73 3 1024 s21, k2 = 7.97 3
Further work will involve identification of the biopolymer
component(s) (preliminary studies have shown that the extracts
contain both high molecular weight proteins and low molecular
weight glycoproteins) which are most effective in the sponta-
neous generation of crystalline silica structures from super-
saturated solutions at room temperature, neutral pH and in the
1026 s21
.
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Fig. 2 (a)–(c) Transmission electron microscopy data for unusual silica
structures precipitated in the presence of 1% w/w biopolymer extracts. All
images shown are from microscope grids prepared by the ‘dipping’ method.
Scale bars represent 10 nm; (d) electron diffraction pattern from (b). Pattern
was recorded using a camera length of 100 cm.
Communication 8/07404F
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Chem. Commun., 1998, 2587–2588