Helical poly-L-glutamic acid templated nanoporous aluminium oxides

Article abstract of DOI:10.1039/b416113k

The synthesis of porous aluminium oxide made in the presence of helical poly-L-glutamic acid is reported. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b416113k

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Helical poly-L-glutamic acid templated nanoporous aluminium oxides{
Jeng-Shiung Jan and Daniel F. Shantz*
Received (in Columbia, MO, USA) 1st November 2004, Accepted 10th February 2005
First published as an Advance Article on the web 4th March 2005
DOI: 10.1039/b416113k
Fig. 1 shows an FE-SEM image and as plot11 for the PLGA
The synthesis of porous aluminium oxide made in the presence
templated alumina after calcination. The surface area of the
aluminium oxide is 340 m2 g21. Using non-porous aluminium
oxide C as the reference material,12 the as analysis for the alumina
sample shows the total pore volume to be 0.19 cm3 g21. The
tangent line of the as curve taken from 1.0 , as , 1.2, yields a
micropore volume of 0.1 cm3 g21. The isotherm does not exhibit
a hysteresis loop excluding the presence of pores . 4.0 nm. While a
lack of suitable reference materials makes calculating the pore size
distribution ambiguous at best from the adsorption data, it is clear
from the as analysis that, if the PLGA remains in a helical
conformation (see below), the pores formed are due to individual
PLGA chains or PLGA dimers. The supporting information also
contains the as plots for the analysis performed using a non-porous
a-alumina as the reference material.{ BJH analysis of the
adsorption isotherm shows two maxima at approximately
of helical poly-L-glutamic acid is reported.
Developing new synthetic approaches to fabricate porous oxides
with unique and controllable structural features is currently of
great interest. In this regard the use of biological molecules such as
amino acids, peptides, and proteins has drawn considerable
attention.1–6 Materials such as silica diatoms and nacre are held
out as the ultimate examples of the complex structures nature can
assemble at ambient conditions. In many regards these goals have
energized the field of inorganic materials chemistry and have been
summarized in several excellent reviews.7–9
Recent work in our laboratory has taken a different approach,
namely using the folded state of a polypeptide chain (e.g. a-helix)
or polypeptide aggregate (e.g. b-sheet) as a means to tailor pore
architectures. In our initial report10 we describe the synthesis of
silicas wherein the pore size and shape are a consequence of the
folded state of the poly-L-lysine used as the template. Here we
demonstrate, using aluminium oxide, that this is also possible for
the synthesis of oxides at low pH (,3) using helical poly-L-
glutamic acid. These materials, while amorphous by X-ray
diffraction and transmission electron microscopy (TEM), are
porous after the removal of the polypeptide by extraction or
calcination. In situ circular dichroism measurements demonstrate
that the polypeptide observable by CD does not unfold during
synthesis, and IR spectroscopy indicates that the polypeptide in the
as-made composite retains its folded state. This work demonstrates
the general applicability of the concept that the folded state of a
polypeptide chain, here in a helical state, can be used to achieve a
templating effect.
Poly-L-glutamic acid (PLGA, Na+-salt, MW y 17,000) and
AlCl3?6H2O were used as received from Aldrich. Oxides were
synthesized by using 0.1 N HCl solution to adjust a 0.5 mg ml21
solution of PLGA to pH 4. After 15 minutes of mixing this
solution a 0.1 M metal chloride solution was added. Precipitates
were formed within 30 minutes, which were collected by
centrifugation, filtered with copious quantities of deionized water,
and then air dried overnight. The polypeptide was removed from
the oxide by calcination at 673 K for 8 hours. FE-SEM images of
the samples after removal of the peptide indicate that small
particles, of approximately 50 nm in size, are formed. Both powder
X-ray diffraction and TEM verify that the materials are
amorphous and possess no long-range structural order.
{ Electronic supplementary information (ESI) available: FE-SEM, TEM,
in situ solution CD, IR, nitrogen adsorption, and as-plot of PLGA-
templated iron oxide phase. as-plot of PLGA-templated alumina using
a-alumina as the reference material. See http://www.rsc.org/suppdata/cc/
b4/b416113k/
Fig. 1 (Top) FE-SEM image and (bottom) as plot for PLGA templated
alumina. Scale bar in the FE-SEM image is 200 nm.
*shantz@che.tamu.edu
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1.5–2.0 nm and 3.0–3.5 nm depending on the form of the
expression for the statistical film thickness, t. The validity of the
BJH formalism is questionable for pores in this size range,11
however the results are qualitatively in agreement with the
as-analysis.
The nitrogen adsorption data clearly shows the materials are
porous, however it cannot provide quantitative information
regarding the pore size distribution. Hence we performed two
additional sets of experiments to probe the conformation of the
poly-L-glutamic acid during oxide synthesis and the polypeptide
conformation inside the as-made hybrid material. Fig. 2 shows
in situ circular dichroism (CD) measurements. Upon addition of
the metal chloride to the PLGA solution there is a decrease in
the signal intensity, however the transition from an a-helix A
random coil is clearly not observed. In all spectra the double
minima indicative of an a-helix are clearly observable; by
contrast the positive ellipticity at 220 nm indicative of a
random coil is not observed. The decrease in signal intensity is
consistent with the observation that large oxide particles form
very rapidly. Hence one would expect a decrease in the overall
signal intensity given that the path length (i.e. cuvette size) is
the same for all spectra shown. These measurements show that
the polypeptide chains observable by CD are in a helical
conformation.
Fig. 3 IR spectrum for alumina–PLGA composite material.
retaining a helical conformation inside the oxide. The CD
results show clearly that the PLGA observable in solution is
helical. The IR results for the alumina samples support that the
helical conformation of the PLGA is retained in the hybrid
material.
Fig. 3 shows the IR spectra of the as-made hybrid material.
While the bands are very broad compared to solution spectra of
PLGA, the IR of the as-made PLGA–alumina composite material
is consistent with helical PLGA. The bands at 1654 cm21 and
1543 cm21 are consistent with previous work13 and indicate a
helical conformation, and the shoulder at 1720 cm21 is also
consistent with the side-chain carboxylate group being
protonated. The spectrum is consistent with the polypeptide
In addition to the aluminas shown above, we also explored the
synthesis of porous iron oxides in the presence of poly-L-glutamic
acid. The interpretation of these experiments is much more
ambiguous than the results shown above for the aluminas. The
supporting information contains the adsorption data, CD, and IR
results for the iron oxide samples.{ These materials, analogous to
the aluminas, are amorphous. Given the lack of suitable
reference iron oxide materials for use in as-analyses, making
quantitative conclusions about pore size distributions and micro-
pore/mesopore volumes based on adsorption data is not
warranted.14 The materials have large surface areas by BET
analysis (200 m2 g21) but appear to possess low porosity. However
this is inconsistent with the fact that it is very difficult to
remove the PLGA from the composite material, and that in
fact the sample still contains PLGA by IR after two extractions
overnight in refluxing ethanol. The FE-SEM shows the particles
are small (,50 nm) but TEM indicates they are porous. CD
analysis and IR are also less conclusive, as the IR indicates
that both protonated and deprotonated PLGA are occluded
in the as-made composite material. Given the iron phase
formed is likely an iron hydroxyoxide, the presence of deproto-
nated side chains groups may not indicate an unfolded state as
compared to what is usually observed in solution. While the
details of the iron hydroxy oxide systems are less clear than the
aluminas, they are unusual and potentially very interesting
materials.
In summary, here we show that poly-L-glutamic acid can be
used to synthesize porous aluminium oxide. These materials, which
do not possess any long range order, have appreciable porosity.
In situ CD measurements indicate that the PLGA in solution
remains folded during synthesis. IR measurements of the
composite materials show that in the alumina the PLGA retains
Fig. 2 In situ circular dichroism measurements of PLGA + aluminium
chloride solutions. Times given in figure are in minutes.
2138 | Chem. Commun., 2005, 2137–2139
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its helical conformation. The results as a whole show that PLGA
has a clear templating effect in the aluminas while the results
are more ambiguous in the case of the iron materials. However
in both cases unique materials are formed. This work expands on
our previous work in the silica–poly-L-lysine system, indicating
that using folded helical polypeptide as templates for
fabricating porous oxides is a concept that appears to be generally
applicable.
Notes and references
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This work was supported by Texas A&M University and the
Texas A&M University Life Science Task force. The authors
gratefully acknowledge Professor Marty Scholtz for access to the
circular dichroism instrumentation, Seungju Lee for acquiring the
FE-SEM images, and Shane Carr for acquiring the TEM images.
The FE-SEM acquisition was supported by the National Science
Foundation under Grant No. DBI-0116835.
Jeng-Shiung Jan and Daniel F. Shantz*
Department of Chemical Engineering, Texas A&M University, TAMU
3122, College Station, TX 77843-3122, U. S. A.
E-mail: shantz@che.tamu.edu; Fax: +1 979-845-6446;
Tel: +1 979-845-3492
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 2137–2139 | 2139