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Rapid Bioenabled Formation of Ferroelectric BaTiO3 at Room Temperature
from an Aqueous Salt Solution at Near Neutral pH

Nature provides spectacular examples of organisms that form
intricate inorganic (bioclastic) assemblies under ambient conditions
with precise control of structure at multiple length scales and over
three dimensions.1 While mechanistic analyses of these biological
processes may lead to new insights in the ambient syntheses of
hierarchical 3D structures, the known range of biomineral chemistries
is rather limited, with the majority of bioclastic structures
composed of CaCO3 or SiO2.2 Several authors have used combinatorial
cell surface display or phage display (¡°biopanning¡±)
methods to identify peptides that bind strongly to,3 and in some
cases induce the precipitation of,4 synthetic inorganic materials.
Here we demonstrate that peptides identified by phage display
biopanning are capable of inducing the rapid, room-temperature
formation of tetragonal barium metatitanate, BaTiO3, from an
aqueous precursor solution at near neutral pH. BaTiO3-based
ceramics can exhibit attractive dielectric, ferroelectric, pyroelectric,
optical, and electrochemical properties for capacitors, displays,
thermistors, sensors, and other devices.5 A variety of chemical
approaches (mixed salt, sol-gel, vapor-diffusion sol-gel, microemulsion,
co-precipitation, polymeric precursor, etc.) have been used
to synthesize BaTiO3 powder.5,6 However, the formation of ferroelectric
(tetragonal) BaTiO3 by such processes has required heat
treatment at g500 ¡ãC for g1 h. Tetragonal BaTiO3 has been
produced via hydrothermal synthesis at 240 ¡ãC, but only after
prolonged annealing (g9 h) under highly alkaline conditions.7
Nuraje et al.8 have recently demonstrated that a synthetic peptidelike
bolaamphiphile, bis(N-R-amidoglycylglycine)-1,7-heptane dicarboxylate,
can induce the room-temperature precipitation of
ferroelectric BaTiO3. While an important result, such precipitation
occurred after exposure for 1-4 days to an alcohol-based alkoxide
solution. The rapid room-temperature synthesis of ferroelectric
BaTiO3 from a stable aqueous precursor solution at near neutral
pH has yet to be accomplished.
A M13 phage-displayed 12-mer peptide library (Ph.D.-12 Kit,
New England Biolabs, Beverly, MA) was used to search for peptidebearing
phage that bind to tetragonal BaTiO3. After five rounds of
selection, two unique phage clones were isolated. The 12-mer
peptides carried by these phage are labeled as BT1 and BT2 in
Table 1. In order to determine whether the peptides BT1 and BT2
could induce the room-temperature precipitation of BaTiO3, 20 ¨ªL
of a 20 mg mL-1 aqueous solution of the BT1 or BT2 peptide was
added to 200 ¨ªL of an aqueous precursor solution composed of
1.25 mM barium acetate (Ba(OOCCH3)2) and 1.25 mM potassium
bis(oxalato) oxotitanate(IV) (K2[TiO(C2O4)2]â2H2O) at pH 6.8. After
incubation for 2 h, the resulting precipitates were washed in water
and then methanol and then dried for 0.5 h under vacuum.
Secondary electron (SE) and transmission electron (TE) images
indicated that the precipitates generated in the presence of the BT1
and BT2 peptides contained fine (50-100 nm) faceted particles
present within 0.3-0.5 ¨ªm aggregates (Figures 1 and S1). (Note:
some amorphous precipitate material was also detected.) Selected
area electron diffraction (SAED) analyses (Figures 1c and S1c) and
X-ray diffraction (XRD) analyses (Figures 2 and S2) of these
faceted particles yielded patterns consistent with crystalline BaTiO3.
The XRD pattern of the BT2-induced precipitate exhibited distinct
splitting of the (002) and (200) peaks (see the inset in Figure 2),
which was consistent with tetragonal BaTiO3. Rietveld analyses
of the XRD patterns from the BT1- and BT2-induced precipitates
yielded good fits to tetragonal crystal structures with lattice
parameters of a ) 4.0086 Å and c ) 4.0246 Å (for BT1 BaTiO3)
and a ) 4.0064 Å and c ) 4.0328 Å (for BT2 BaTiO3).
While the BT1 and BT2 peptides possessed various types of
amino acids (nonpolar, aromatic, hydroxyl-bearing, noncharged
polar, charged), 8 of the 12 amino acids were common to both
peptides. A variety of control peptides (CON1-CON6 in Table
S1) that shared some similarities with the BT1 and BT2 peptides,
but were not isolated via specific binding to BaTiO3, were selected
for additional precipitation experiments. The pI values of the CON1,
CON2, and CON3 peptides spanned the pI range of the BT1 and
BT2 peptides. The CON3, CON4, and CON5 peptides possessed
2, 5, and 4 hydroxyl-bearing residues, respectively (note: the BT1
and BT2 peptides both possessed serine, tyrosine, and threonine
residues). Replacement of the arginine and histidine residues of
the BT2 peptide with glycine residues yielded the CON6 peptide.
(Note: Dissolution of the CON6 peptide required the use of a 1:3
solution of N,N-dimethylformamide in water. Precipitation trials
for the CON6 peptide were otherwise conducted under the same
conditions as for the BT1, BT2, and other control peptides.) SAED
analyses of small amounts of precipitates formed upon exposure
to some of these control peptides (Figure S3) did not reveal the
presence of crystalline BaTiO3 within such precipitates. These
observations suggest that the combination of conserved amino acids
(hydroxyl-bearing, amide-bearing, charged, hydrophobic) in the BT1
and BT2 peptides was important for the formation of crystalline
(tetragonal) BaTiO3.
Because the BaTiO3 that formed in the presence of the BT2
peptide exhibited more distinct tetragonal character than the BT1-
induced BaTiO3 (compare Figures 2 and S2), additional BT2-
induced BaTiO3 was generated for ferroelectric testing. BT2 BaTiO3
particles were dispersed by vortexing in water and then dropcasting
with a pipet onto a platinized, high resistivity (>10 k¿âcm) silicon
wafer, so as to obtain a layer with an average thickness of about
16 ¨ªm. Gold was then sputter deposited as a top electrode on the
BaTiO3 layer. The electric field-induced polarization of the BT2
BaTiO3 was evaluated with a Radiant Technologies¡¯ Precision LC
tester. As shown in Figure 3, the BaTiO3 particle layer exhibited
polarization hysteresis, which is a well-known characteristic of
ferroelectric materials. The slope of the P-E curve at low field
yielded a relative permittivity value of 2200, which is comparable
to values reported for BaTiO3 of 100-300 nm crystal size.9
The present work demonstrates, for the first time, that a phage
display isolated peptide can induce the room-temperature formation
of ferroelectric (tetragonal) BaTiO3 within 2 h from an aqueous
precursor solution at near neutral pH. The ability of peptides to
promote the rapid formation of functional crystalline multicomponent
ceramics under ambient conditions provides new opportunities
for the integration of such functional materials with low-temperature
or reactive materials and substrates (e.g., with polymers, bioorganics,
or silicon).
Acknowledgment. Financial support was provided by the Air
Force Office of Scientific Research (Dr. Joan Fuller, Dr. Hugh C.
DeLong, program managers) and the Office of Naval Research (Dr.
Mark S. Spector, program manager). Helpful discussions with Nils
Kro¡§ger (School of Chemistry and Biochemistry, Georgia Institute
of Technology) are gratefully acknowledged.
Supporting Information Available: Experimental procedures and
control peptide sequences, along with XRD, SAED patterns and SE,
TE images of the BT1, CON2, CON3, and CON6 precipitates. This
material is available free of charge via the Internet at http://pubs.acs.org.
References
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JA0744302
Figure 1. BaTiO3 precipitates formed at room temperature upon exposure
of the BT2 peptide to an equimolar (Ba:Ti ) 1:1) aqueous precursor solution
for 2 h. (a) SE and (b) TE images of the precipitates. (c) SAED pattern,
consistent with crystalline BaTiO3, obtained from the faceted particles.
Figure 2. XRD pattern, consistent with crystalline BaTiO3, obtained from
precipitates formed at room temperature upon 2 h exposure of the BT2
peptide to an aqueous precursor solution. The inset reveals peak splitting
consistent with the (002) and (200) reflections of tetragonal BaTiO3.
Figure 3. Polarization versus applied electric field at 1 kHz for the BT2
peptide-induced BaTiO3.
C O M M U N I C A T I O N S
J. AM. CHEM. SOC. 9 VOL. 130, NO. 1, 2008 5
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