precipitation. The resulting silica-coated pollen grains were filtered, washed
with distilled deionized water, and dried under ambient conditions. Hollow
silica replicas of individual pollen grains were obtained by heating
mineralized samples from ambient temperature to 600 °C at a rate of 1 °C
min21 overnight.
Calcium carbonate and calcium phosphate replicas were produced by
soaking pollen grains (0.5 g) in calcium chloride (5 mL, 1 M) for 18 h,
followed by centrifugation and removal of excess aqueous CaCl2, and re-
suspension in Na2CO3 (3 h, 5 mL, 1M, pH 12) or Na2HPO4 (18 h, 5 mL, 1
M, pH = 9.04), respectively. The coated pollen grains were filtered, washed
with distilled deionized water, and calcined at 400 °C overnight to produce
hollow inorganic replicas.
1 (a) S. A. Davis, S. L. Burkett, N. H. Mendelson and S. Mann, Nature,
1997, 385, 420; (b) B.-J. Zhang, S. A. Davis, N. H. Mendelson and S.
Mann, Chem. Commun., 2000, 781.
2 Z. Li, S.-W. Chung, J.-M. Nam, D. S. Ginger and C. A. Mirkin, Angew.
Chem., Int. Ed., 2003, 42, 2306.
Fig. 2 TEM image of fractured silver-doped silica replica. Scale bar, 200
nm.
3 (a) T. Douglas and M. Young, Nature, 1998, 393, 152; (b) W. Shenton
T. Douglas, M. Young, G. Stubbs and S. Mann, Adv. Mater., 1999, 11,
253; (c) T. Douglas and M. Young, Adv. Mater., 1999, 11, 679; (d) E.
Dujardin, C. Peet, G, Stubbs, J. N. Culver and S. Mann, NanoLett., 2003,
3, 413.
4 (a) C. E. Fowler, W. Shenton, G. Stubbs and S. Mann, Adv. Mater.,
2001, 13, 1266; (b) S. W. Lee, S. K. Lee and A. M. Belcher, Adv. Mater.,
2003, 15, 689.
5 G. Cook, P. L. Timms and C. Goeltner-Spickermann, Angew. Chem.,
Int. Ed., 2003, 42, 557.
6 (a) E. L. Mayes, F. Vollrath and S. Mann, Adv. Mater., 1998, 10, 801;
(b) Y. Yushan, L. Huang, H. Wang, C. Y. Hayashi, B. Tian, D. Zhao and
Y. Yan, J. Mater. Chem., 2003, 13, 666.
7 A. Dong, Y. Wang, Y. Tang, N. Ren, Y. Zhang, Y. Yue and Z. Gao, Adv.
Mater., 2002, 14, 926.
8 V. Valtchev, M. Smaihi, A.-C. Faust and L. Vidal, Angew. Chem., Int.
Ed., 2003, 42, 2782.
9 (a) M. W. Anderson, S. M. Holmes, N. Hanif and C. S. Cundy, Angew.
Chem., Int. Ed., 2000, 39, 2707; (b) Y. Wang, Y. Tang, A. Dong, X.
Wang, N. Ren and Z. Gao, J. Mater. Chem., 2002, 12, 1812.
10 B. Jones, M. R. Rosen and R. W. Renaut, J. Sediment Res., 2001, 71,
190.
Fig. 3 UV-vis spectra showing release of ibuprofen from (:) silica, ( < )
calcium carbonate and (-) calcium phosphate replicas.
11 PXRD results (d spacings (Å) and {hkl} values): Calcite; 3.841 (102),
3.028 (104), 2.839 (006), 2.651 (105), 2.516 (110), 2.805 (113), 2.090
(202), 1.924 (204), 1.873 (116), 1.602 (212). Brushite; 3.050 (111),
2.920 (221), 2.688 (220). Monetite; 3.399 (020), 3.330 (220), 2.754
(230), 2.533 (122), 2.156 (013), 2.221 (112).
12 Silver-containing silica replicas were prepared by soaking the calcined
particles in aqueous AgNO3 (10 mL, 4 M) overnight at room
temperature in the dark. Samples were filtered and allowed to
photoreduce in natural light for 24 hrs. For TEM studies, samples were
ground and redispersed in ethanol, and air-dried onto TEM grids.
13 Magnetic silica replicas were prepared by incubation of calcined
particles with a magnetite (Fe3O4) sol, synthesized according to
previous methods. [ R. Massart, IEEE Trans. Magnetics, 1981, 17,
1247] The sol consisted of crystalline nanoparticles with roughly
spherical morphology and mean size of 10 nm.
and monitored the corresponding release profiles in simulated
body fluid (SBF).14 The high surface area silica replicas
adsorbed significantly increased levels of the drug compared
with the calcium-containing minerals. However, in each case,
the pollen-templated replicas released ca. 50% of the entrapped
drug within the first 10 minutes of immersion, after which the
rate of release decreased significantly, such that 70 to 80% of
ibuprofen was transferred into the SBF after 3 hours (Fig. 3),
suggesting that the materials could be useful in controlled
release applications.
Notes and references
†
Silica replication was achieved by soaking ca. 0.5 g of pollen for 18 h
14 For drug release studies, 0.5 g of the silica, calcium carbonate or calcium
phosphate pollen grain replicas were immersed in an ibuprofen solution
(10 ml, 4.5 M) for 3 days. The suspension was then filtered and washed,
and subsequently immersed in an aqueous solution with ionic
composition corresponding to a simulated body fluid ( T. Kokubo et al.,
J. Mater. Sci. Mater. Med., 1992, 3, 79). Aliquots of the solution were
taken at regular intervals and the amount of release ibuprofen
determined by UV-vis spectroscopy. Percentage data are based on 100%
being the absorbance of the ibuprofen solution without pollen present.
in a solution of silicic acid (Si(OH)4, 5 mL, pH = 7). The silicic acid was
prepared by passing a sodium silicate solution (27% wt SiO2) through an
acidified cation-exchange column that was charged by flushing with hot
distilled H2O, 2M HCl (aq.) and then cold distilled H2O, followed by
addition to the eluate of a few drops of the silicate stock solution to raise the
pH to 7. The soaked pollen was centrifuged (Eppendorf 5415D centrifuge,
5 min, 13.5 k rpm), and excess silicate removed, and the silicate-coated
grains resuspended in 10 mL of ethanol for 30 min to induce silica
CHEM. COMMUN., 2003, 2784–2785
2785