NANO LETTERS
Preparation of Mesoscale and Macroscale Silica Nanotubes Using a Sugar-Appended Azonaphthol Gelator Assembly
2002 Vol. 2, No. 1 17-20
Jong Hwa Jung,† Seiji Shinkai,§,# and Toshimi Shimizu*,†,‡,# CREST, Japan Science and Technology Corporation (JST), Nanoarchitectonics Research Center, National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Centeral 4, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan, Nanoarchitectonics Research Center (NARC), National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Centeral 5 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, and Chemotransfiguration Project, Japan Science and Technology Corporation (JST), 2432 Aikawa Kurume, Fukuoka 839-0861, Japan Received September 5, 2001; Revised Manuscript Received October 26, 2001
ABSTRACT Sugar-appended azonaphthol derivatives 1 possessing an aromatic hydroxyl group and 2 possessing the n-butanoyloxy group were synthesized and proved to be capable of gelating water/ethanol mixtures. Gels formed by gelator 1 displayed different morphologies depending on the gelator concentration. The aqueous gel formed by 3.0 wt % of 1 consisted of crystalline fibers with thicknesses ranging from 400 to 600 nm, whereas at lower a concentration of gelator 1 (0.1 wt %) the gel consisted of a three-dimensional network of fibers with thicknesses of 20 to 30 nm. These gels were subjected to the sol−gel polymerization process, using tetraethoxysilane (TEOS). Interestingly, the obtained silica consisted of nanotubular structures with inner diameters of 20−25 nm and 450−600 nm using two different gels of 1, respectively. These results prove that the diameter size of silica nanotubes can be controlled by varying the gelator concentration.
Recently, much attention has been given to the development of novel porous materials1,2 because of their potential wideranging applications in catalysis, adsorption, nanotechnology, etc. Alongside the synthesis of porous materials with integrated structures, the preparation of single, isolated nanotubes is another major challenge. Unlike the well-known carbon nanotubes,3 which are usually synthesized by hightemperature techniques, most inorganic (oxide) single nanotubes have been efficiently prepared by using templates such as DNA,4 proteins,5 organic crystals,6 and phospholipids.7 The exploitation of new organic gelators that can gelate various organic solvents has become an active area of research.8,9 Particularly, the use of cholesterol- and cyclohexane-based organogel templates has enabled the formation of various hollow silica or titanium oxide fibers with linear,10 helical,11 and multilayered morphologies,12 through transcription caused by electrostatic or hydrogen-bonding interactions. On the other hand, only one example exists in which a sugar* Corresponding author: Fax no. +81-298-61-2659;
[email protected] † CREST. ‡ Nanoarchitectonics Research Center. § Chemotransfiguration Project. # These authors contributed equally to this work. 10.1021/nl0100623 CCC: $22.00 Published on Web 11/15/2001
© 2002 American Chemical Society
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appended gel possessing the benzylidene and a aminophenyl groups was used as the template for sol-gel transcription.13 In this case, tubular as well as spherical silica structures were formed as a result of the hydrogen-bonding interaction between the amino group present in the organic gelators molecule and negatively charged silica particles.13 However, since most of the sugar-appended gelators reported up to now consist of a sugar moiety and either a long alkyl chain or an aromatic moiety, they lack effectively available functional moieties for electrostatic or hydrogen bonding interactions. Therefore, the superstructures they form through selfassembly in aqueous solutions (e.g., nanosized fibers, helicalribbons, nanotubular structures14) are generally very difficult to transcribe into silica structures. Recently, Whitten and co-workers found that by increasing the temperature, rigid or bundled gel strands become increasingly mobile as a result of solvent-induced swelling of the internal fibril bundles and can even disaggregate into smaller, more flexible fiber bundles.9a-c These results suggest that transcription of this system could result in the formation of tubular silica with a variety of inner diameters by varying the organogel concentration and the reaction temperature. These facts persuaded us to transcribe the nanofiber structure
Figure 1. SEM and TEM pictures of the aqueous gels of (a) 0.1 and (b) 3.0 wt % concentrations of 1, respectively.
of an aqueous gel into its inorganic silica analogue, which would possess either meso- or macroscale inner diameters. It has been shown that the presence of a cationic charge or hydrogen-bonding site (amine) is indispensable for successful transcription of the organogel template into a silica structure;10 however, we have noticed that the introduction of a cationic group into a sugar-appended gelator molecule is very difficult. To overcome this dilemma, we prepared compound 1 possessing a sugar group that shows moderate hydrophility and gel-forming ability and an azonaphthol group which, through azohydrazone tautomerism, results in the presence of a secondary amino group that can act as a driving force for successful sol-gel transcription. Transcription of the aqueous gel 1 in which both the azo and hydrazone form were present resulted in the exclusive formation of uniform silica nanotubes possessing either meso- or macroscale inner diameters, depending on the concentration of the gelator. In this paper we report a new method and high-yield approach for the silica nanotube formation using these sugar-appended azonaphthol gelators.
Compounds 1 and 2 were synthesized according to a method reported previously15 and were characterized by IR and 1H NMR spectroscopy, as well as by elemental analysis. Gelation tests for compounds 1 and 2 were carried out in various organic solvents as well as water.15 Both gelators formed translucent gels in various water/ethanol mixtures,15 indicating that the gelation condition also influences the particular gel properties. SEM and TEM pictures (Figure 1) of gels of gelator 1 show different morphologies depending on the weight percentage of the gelator. At relatively a high concentration of 1 (3.0 wt %) an aqueous gel shows a crystalline fiber structure with diameters ranging from 400 18
to 600 nm and with fiber lengths of a few micrometers, whereas at a low concentration of 1 (0.10 wt %) the gel shows the typical three-dimensional network fiber structure with diameters of 20-30 nm and lengths exceeding 1 µm. An aqueous gel of 2 (3.0 wt %) was shown to consist of fibers with diameters of 200-500 nm and lengths of several micrometers. To transcribe the superstructures present in the aqueous gels into their silica structures, sol-gel polymerization of TEOS was carried out using 1 and 2 according to methods described previously.8-10 In a typical experiment, the gelator (0.1-5.0 wt %) was dissolved in a medium for sol-gel polymerization: the medium consisted of gelator (1.0-5.0 mg), ethanol (10-200 mg), TEOS (20-50 mg), water (1001000 mg), and either benzylamine (10-20 mg) or 0.1 M NaOH (10-20 mg) as a catalyst. The sample was sealed in a glass tube and left at room temperature for 1-2 days. After calcination the obtained silica was investigated with SEM (Figure 2). The silica nanotubes obtained from the aqueous gel 1 (low concentration: 0.1 wt %) display a fibrous structure with outer diameters of ca. 80-85 nm and lengths of a few micrometers. Remarkably, the yield for the silica transcription was almost 100%. Identical tubular silica was obtained when using either benzylamine or strong bases such as NaOH and TEAH (tetraethylmmonium hydroxide). The hydrazone species of compound 1 still remained after treatment of 0.1 M NaOH by UV observation. Therefore, we propose that probably the aromatic hydroxyl group of compound 1 partially protonated and changed into a phenolate ion in gel state. More interestingly, the silica nanotube with 1000 nm outer diameter was created using 3.0 wt % of an aqueous gel 1 (Figure 2b),16 indicating that rigid or large-sized (bundled) gel strands were accurately transcribed into the relatively large-sized silica. The corresponding SEM picture revealed a nanotube with a relatively smooth surface. Therefore, the synthetic conditions also influence the fine morphologies of materials. What is the actual driving force that allows transcription of this particular aqueous gel superstructure into its silica analogue? Interestingly, the UV-vis observation showed that Nano Lett., Vol. 2, No. 1, 2002
Figure 2. SEM pictures of the silica nanotube obtained from (a) the low and (b) the high concentrations of an aqueous gel 1 after calcination.
Figure 3. TEM pictures of the silica nanotube obtained from (a) the low and (b) the high concentrations of an aqueous gel 1 after calcination. Scheme 1
in polar solvents gelator 1 actually exists as two separate species: the azo and the hydrazone form (Scheme 1).15 Hence, the hydrogen-bonding interaction of the negatively charged silica particles with the amino group of the hydrazone species most likely acts as a driving force for silica transcription. To confirm this, we carried out experiments using gelator 2. Although possessing a similar structure to gelator 1, compound 2 cannot adopt the hydrazone form. Indeed, the silica obtained after transcription of an aqueous gel 2 proved to be granular. This strongly confirms the aforementioned assumption that the amino group of the hydrazone species is crucial in providing a driving force for silica transcription. From previous work it is known that gelator 1 preferentially adopts the azo form in the gel state.15 To further confirm that the gel superstructure really acts as the template for the growth of the silica nanotubes, the Nano Lett., Vol. 2, No. 1, 2002
silica obtained by transcription of the low weight percentage aqueous gel 1 was calcined (500 °C, several hours) to remove the organic template. Subsequent TEM investigation (Figure 3a) showed that no change in morphology had taken place. Furthermore, it clearly showed that the center part of the tubes is light and that both edges are dark, confirming the well-defined nanotube structure and the previous existence of an aqueous gel inside the silica fibers. Average inner and outer diameters are 20-25 nm and 80-85 nm, respectively, making the thickness of the silica wall ca. 30 nm. Furthermore, the TEM picture clearly shows that both sides of the channel are open. The average value observed for the inner diameter of the silica nanotube is in good agreement with the thickness of aqueous gel fibers observed independently (Figure 1a). These results further support the view that oligomeric silica particles are adsorbed onto the surface 19
of fibers by hydrogen-bonding interaction and that aqueous gel fibers indeed act as a template.8-12 On the other hand, the silica material obtained from the high concentration aqueous gel of compound 1 shows nanotubes with internal diameters of ca. 450-600 nm and wall thicknesses ranging from 200 to 400 nm (Figure 3b). Powder XRD patterns of these silica nanotubes showed only one broad peak at 22°24°, indicating that the silica framework is amorphous. In conclusion, the work presented here has demonstrated a new method that provides control over the inner diameter of silica nanotubes through variation of the concentrations of the sugar-appended azonaphthol gelators used. Welldefined silica nanotubes possessing either meso- or macroscale inner diameters have been formed through hydrogenbonding interactions between the fiber structures and the silica precursor. We believe that this concept will be more generally applicable for the preparation of various silica structures with different sizes and shapes. Potential applications of these silica nanotubes, involving the inclusion of suitable materials in the cavity, as well as their use in delivery systems, are now being investigated.
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Nano Lett., Vol. 2, No. 1, 2002
