Functionalization of Bolalipid Nanofibers by Silicification and

Publication Date (Web): July 11, 2012. Copyright © 2012 American Chemical Society. *(S.D.) Tel: +49-345-5525196. Fax: +49-345-5527026. E-mail: simon...
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Functionalization of Bolalipid Nanofibers by Silicification and Subsequent One-Dimensional Fixation of Gold Nanoparticles Simon Drescher,*,†,‡ Günter Hempel,§ Wolfgang H. Binder,∥ Bodo Dobner,‡ Alfred Blume,† and Annette Meister*,†,⊥ †

Institute of Chemistry, Physical Chemistry, Martin-Luther-University Halle-Wittenberg, von-Danckelmann-Platz 4, 06120 Halle Saale, Germany ‡ Institute of Pharmacy, MLU Halle-Wittenberg, Wolfgang-Langenbeck-Str. 4, 06120 Halle Saale, Germany § Institute of Physics, MLU Halle-Wittenberg, Betty-Heimann-Str. 7, 06120 Halle Saale, Germany ∥ Institute of Chemistry, Macromolecular Chemistry, MLU Halle-Wittenberg, Heinrich-Damerow-Str. 4, 06120 Halle Saale, Germany ⊥ ZIK HALOmem, Biophysical Chemistry of Membranes, MLU Halle-Wittenberg, Kurt-Mothes-Str. 3, 06120 Halle Saale, Germany S Supporting Information *

ABSTRACT: In the present work, we describe the successful stabilization of bolalipid nanofibers by sol−gel condensation (silicification) of tetraethoxysilane (TEOS) or 3-mercaptopropyltriethoxysilane (MP-TEOS), respectively, onto the nanofibers. The conditions for an effective and reproducible silicification reaction were determined, and the silicification process was pursued by transmission electron microscopy (TEM). The resulting bolalipid−silica composite nanofibers were characterized by means of differential scanning calorimetry (DSC), TEM, 13C, and 31P NMR spectroscopy. Finally, the novel silicified bolalipid nanofibers were used as templates for the fixation of 5 and 2 nm AuNPs, respectively, resulting in one of the rare examples of one-dimensional AuNP arrangements in aqueous suspension.

gold nanoparticles (AuNPs).19 The 1-D assembly of these AuNPs is anticipated to play an important role in many technical fields, e.g., optics, catalysis, sensing, and electronics.20−28 The fixation of AuNPs (and also other NPs) can thereby be implemented by physical entanglement, hydrophobic interactions,19,29 hydrogen bonding,30−33 coordination coupling,34 specific molecular interactions,35 and also by grafting metal−thiol bonds.36 However, the formation of 1-D arrays in aqueous solution is very demanding since the clustering of NPs into two- or three-dimensional aggregates interfere with 1-D aggregation processes. Up to now, a limited number of NP arrangements in aqueous solution were described being realized by implementation of variable organic templates, such as biopolymers (e.g., DNA) or lipid tubes.37−42 As mentioned above, bolalipid nanofibers can be successfully applied as templates for the 1-D fixation of AuNPs with a diameter of 5 nm in aqueous solution.19 This loading of the nanofibers with AuNPs can be increased by the use of sulfur-containing bolalipids.43,44 In contrast, the fixation of AuNPs with a diameter of 2 nm, showing quantum dot behavior, was not successful due to the breakdown of the nanofibers. Transmission electron microscope (TEM) investigations demonstrated the ability of these smaller NPs to access the inner core of the nanofibers leading to a

1. INTRODUCTION Sol−gel condensation of tetraethoxysilane (TEOS), called herein silicification, emerged as a reliable method to stabilize organic materials. Those organic−inorganic hybrid composites are applied in fields of, e.g., catalysis or controlled release. The synthesis of these silica-based composites, which has been the subject of numerous investigations,1,2 is based on the hydrolysis of neutral silicon alkoxide precursor, such as TEOS or further modified alkoxysilanes, and the subsequent assembly and condensation on the templates’ surface. Applying this strategy, diverse nanostructures of silica have been synthesized using various organic templates, including surfactants,3,4 peptidic lipids,5−8 peptide fibrils,9 phospholipids,10−13 and diblock copolymers.14 Besides this template approach, Shi et al. used hybrid nanofibers prepared by electrospinning from TEOS and polyvinylpyrolidone (PVP).15 Here, in this work, we describe the silicification of nanofibers that are composed of self-assembled bolaphospholipids. These bipolar phospholipids (bolalipids) are built up by one long, even-numbered methylene chain and two polar phosphocholine headgroups attached at the ends.16 The self-assembly of this class of amphiphilic molecules is exclusively driven by hydrophobic interactions of the long alkyl chains and leads to the formation of nanoscopic fiber structures.17,18 In a previous work, nonsilicified bolalipid nanofibers were used as a template for the one-dimensional (1-D) fixation of © 2012 American Chemical Society

Received: June 10, 2012 Revised: July 8, 2012 Published: July 11, 2012 11615

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2.2. Methods. 2.2.1. Sample Preparation for DSC. Homogenous dispersions of the bolalipids in water were prepared by heating the aqueous mixture to 80 °C and vortexing. 2.2.2. Sample Preparation for NMR Experiments. For 31P NMR measurements, sample BL-1.25 was centrifuged after the silicification reaction, and the obtained white and gel-like pellet was transferred into the NMR-tubes without further drying. For 13C NMR measurements, samples BL-1.25 and BL-2.1 were centrifuged; the pellets were dried and rehydrated in water to give a concentration of c = 30 mg/mL. 2.2.3. Silicification Reaction. An appropriate amount of PC-C32PC (BL-1) or HEPC-C35-HEPC (BL-2), respectively, was suspended in water, and the mixture was heated to 80 °C until a clear solution was obtained. The bolalipid suspensions were allowed to stand for 24 h at room temperature to induce the formation of nanofibers. Afterward, an excess (10-fold, 20-fold, 40-fold, or 60-fold with respect to the molar amount of bolalipid) of the silicification reagent (TEOS, MP-TMOS, MP-TEOS, or mixtures) was added in one portion to the bolalipid suspension. The silicification reaction was started by the addition of aqueous 2 M ammonia (ca. 200 μL) to obtain a pH value of 9−10. The mixture was then stirred with a magnetic stirrer with different velocities: gentle stirring with 50 rpm, moderate stirring with 300 rpm, or rapid stirring with 800 rpm. After 30, 60, or 180 min, the silicification reaction was either stopped by the addition of 2 M hydrochloric acid (100−150 μL) changing the pH to 7 (checked with pH meter) or the pH was left unchanged. Afterward, the samples were directly imaged by TEM. For the preparation of mixtures of bolalipid− silica composite nanofibers with AuNPs, the appropriate volume of the AuNP solution was added to the silicified bolalipid nanofiber suspension and agitated for 30 min. 2.2.4. Differential Scanning Calorimetry. DSC measurements were performed using a MicroCal VP-DSC differential scanning calorimeter (MicroCal Inc. Northampton, MA, USA). Before the measurements, the sample suspension and the reference were degassed under vacuum while stirring. A heating rate of 20 K/h was used, and the measurements were performed in the temperature interval from 2 to 95 °C. To check the reproducibility, three consecutive scans were recorded for each sample. The water−water baseline was subtracted from the thermograms of the samples, and the DSC scans were evaluated using MicroCal Origin 8.0 software. Samples BL-1.12 and BL-1.24 were used directly after the silicification reaction for the DSC measurements. 2.2.5. Transmission Electron Microscopy. If not indicated otherwise, samples were prepared by spreading 5 μL of the suspension onto a Cu grid coated with a Formvar-film. After 1 min, excess liquid was blotted off with filter paper, and 5 μL of 1% aqueous uranyl acetate solution was placed onto the grid and drained off after 1 min. The dried specimens were examined with a Zeiss EM 900 transmission electron microscope. For the preparation of unstained samples, the incubation with uranyl acetate solution was omitted. 2.2.6. 13C and 31P NMR Measurements. The experiments were performed using a BRUKER AVANCE-II spectrometer at a field of 9.4 T corresponding to resonance frequencies of 161.98 MHz (31P) and 100.02 MHz (13C). The 31P spin system was excited by π/2 pulses of 4.5 μs duration. The 13C signals were obtained by the cross-polarization method. During data acquisition, the protons were decoupled at their Larmor frequency of 400 MHz. The rf field strengths during decoupling as well as during cross-polarization corresponded to a mutation frequency of 50 kHz. One thousand scans and 5000 scans were accumulated for the 31 P and 13C spectra, respectively.

disruption of the van der Waals contacts between the bolalipid molecules, resulting in a break-up of the fibers.19 To overcome this drawback of nanofiber destruction, the stabilization of the bolalipid nanofibers by silicification seemed to be a promising approach and hence formed the decisive motivation of this work. In the present article, we (i) elaborated the appropriate conditions of the silicification reaction by variation of concentrations and reagent ratios and (ii) investigated the fixation of AuNPs with diameters of 5 and 2 nm, respectively, on the silicified nanofibers. For the formation of nanofiber templates, we used the bolalipid dotriacontane-1,32-diyl-bis[2-(trimethylammonio)ethylphosphate] (PC-C32-PC, BL-1) as well as the newly synthesized bolalipid pentatriacontane-1,35-diyl-bis{2-[N,Ndimethyl-N-(2-hydroxyethyl)ammonio]ethylphosphate} (HEPC-C35-HEPC, BL-2). As precursors for the silicification reaction, we examined TEOS and two types of sulfur-modified alkoxysilanes: 3-mercaptopropyltrimethoxysilane (MP-TMOS) and 3-mercaptopropyltriethoxysilane (MP-TEOS) (Figure 1).

Figure 1. Chemical structure of the long-chain bolalipids PC-C32-PC (BL-1) and HEPC-C35-HEPC (BL-2), as well as silicification reagents tetraethoxysilane (TEOS), 3-mercaptopropyltrimethoxysilane (MPTMOS), and 3-mercaptopropyltriethoxysilane (MP-TEOS).

The latter ones were used to improve the loading of AuNPs on the silicified bolalipid fiber surface due to strong interactions of AuNPs with thiol moieties.36 The silicified bolalipid nanofibers as well as their complexes with AuNPs were characterized by means of transmission electron microscopy (TEM). In addition, selected samples were further investigated by differential scanning calorimetry (DSC), 31 P NMR, and 13C NMR spectroscopy. To our knowledge, the combination of both methodologies, silicification of water-soluble templates resulting in bolalipidsilica composite nanofibers and the subsequent fixation of AuNPs, is described for the first time in the literature.

2. EXPERIMENTAL SECTION

3. RESULTS AND DISCUSSION 3.1. Silicification of Bolalipid Nanofibers. 3.1.1. Nanofiber Templates Formed by PC-C32-PC and HEPC-C35-HEPC. Besides the well-characterized bolaphospholipid PC-C32-PC,17,18 we used nanofibers of the novel bolalipid HEPC-C35-HEPC as templates for the silicification procedure. Compared to PCC32-PC (Figure 1, BL-1), HEPC-C35-HEPC has a slightly longer alkyl chain and an additional hydroxy moiety within the phosphocholine headgroup (Figure 1, BL-2). This hydroxy

2.1. Materials. Aqueous ammonia (2 M) and hydrochloric acid (2 M) were purchased from Sigma-Aldrich Co. Tetraethoxysilane (TEOS), 3-mercaptopropyltrimethoxysilane (MP-TMOS), and 3-mercaptopropyltriethoxysilane (MP-TEOS) were purchased from ABCR GmbH (Karlsruhe, Germany). Aqueous solutions of citrate stabilized AuNPs (with a mean diameter of 2 and 5 nm, respectively) were obtained from PLANO GmbH (Wetzlar, Germany). PC-C32-PC was synthesized as described previously.16 The synthesis of HEPC-C35-HEPC is described in the Supporting Information. 11616

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Table 1. Experimental Conditions for the Silicification of Bolalipid Nanofibersa silicification reagent sample number

concentration of bolalipid [mg/mL]

PC-C32-PC (BL-1) BL-1.1 BL-1.2 BL-1.3 BL-1.4 BL-1.5 BL-1.6 BL-1.7 BL-1.8 BL-1.9 BL-1.10 BL-1.11 BL-1.12 (for DSC) BL-1.13 BL-1.14 BL-1.15 BL-1.16

0.330 0.330 0.330 0.330 0.330 0.330 0.330 0.330 0.330 0.330 0.330 0.330 0.330 0.330 0.330 0.330

BL-1.17 BL-1.18 BL-1.19

0.100 0.100 0.100

BL-1.20 BL-1.21 BL-1.22

0.033 0.033 0.033

BL-1.23

0.033

BL-1.24 (for DSC) BL-1.25 (for NMR) HEPC-C35-HEPC (BL-2) BL-2.1 (for NMR)

typeb

silicification reaction x-fold excessc

duration [min]

stirring [rpm]

stoppedd

40 40 40 40 40 40 10 20 40 40 40 40 40 20 10 + 10 20

30 60 180 30 60 180 60 60 60 30 30 + 180e 30 30 + 180e 60 60 + 60 60

800 800 800 50 50 50 300 300 300 300 0 300 0 300 300 300

− − − − − − − − − − − + + − − −

40 40 40

30 180 180

300 300 300

− + +

40 40 40

30 30 180

300 300 300

− + −

40

180

300

+

1.000 0.150

TEOS TEOS TEOS TEOS TEOS TEOS TEOS TEOS TEOS TEOS TEOS TEOS TEOS MP-TMOS TEOS + MP-TMOS (stepwise addition) TEOS/MP-TMOS (1/1, n/n; mixed before addition) MP-TEOS MP-TEOS TEOS/MP-TEOS (10/1, n/n; mixed before addition) TEOS TEOS TEOS/MP-TEOS (5/1, n/n; mixed before addition) TEOS/MP-TEOS (5/1, n/n; mixed before addition) TEOS TEOS

40 60

60 100

50 300

− +

0.150

TEOS

60

100

300

+

a

All reactions were performed at room temperature and under argon atmosphere. Silicification reaction: the appropriate amount of BL-1 and BL-2, respectively, was suspended in water and heated to 80 °C until a clear solution was obtained. After 24 h, an excess of the silicification reagent was added. The silicification reaction was started by the addition of aqueous ammonia (2 M, ca. 200 μL) to obtain a pH value of 9−10, and the mixture was stirred at different velocities. After 30, 60, or 180 min, the silicification reaction was either stopped by the addition of hydrochloric acid (2 M, 100−150 μL) or the pH was left unchanged. Afterward, the samples were directly imaged by TEM. bSilicification reagents: TEOS, tetraethoxysilane; MP-TMOS, 3-mercaptopropyltrimethoxysilane; MP-TEOS, 3-mercaptopropyltriethoxysilane. cMolar excess with respect to the amount of bolalipid. dThe silicification reaction was either stopped (+) by adding hydrochloric acid (2 M) until a pH value of 7 was reached or not stopped (−). eBL-1.10 and BL-1.12 were allowed to stand for further 3 h without stirring.

3.1.2. Silicification Reaction. For the silicification reaction, dried powders of the bolalipids PC-C32-PC (BL-1) or HEPCC35-HEPC (BL-2) were suspended in water up to a concentration of 0.330 mg/mL (∼0.4 mM). The resulting suspensions were heated to temperatures above the fiber−micelle transition indicated by the formation of a clear solution. The solutions were then cooled and allowed to stand for at least 24 h at room temperature without stirring in order to induce the formation of nanofibers. These nanofibers were subsequently used as templates for the polymerization of tetraalkoxysilanes. The silicification reaction was carried out under a variety of concentrations and reaction conditions as shown in Table 1. Yin et al. previously reported that the thickness of silica coating on templates depends on the time of the silicification reaction as well as the concentration of the alkoxysilane used.46 In our study, we varied the concentration of BL-1 between 0.330 mg/mL and 0.033 mg/mL (BL-1.1 to BL-1.20), raised the excess of the silica precursors from 10-fold up to 40-fold

group offers the possibility to be covalently bound to the silica precursors, e.g., TEOS. HEPC-C35-HEPC was synthesized according to a recently described synthetic strategy for symmetrical singlechain bolalipids using tetrahydropyranyl protected 15-bromopentadecanol and 1,5-dibromopentane as starting materials16 (see Supporting Information). HEPC-C35-HEPC shows nearly the same aggregation behavior in water as its shorter chain analogue HEPC-C32-HEPC,43 the difference being a splitting of the first endothermic transition where the fibers dissociate into small micellar aggregates and a general shift of the transitions to higher temperatures (Figure S1, Supporting Information). This splitting can also be observed comparing the unmodified PC-C32PC with its longer chain analogues PC-C34-PC and PC-C36-PC, respectively.45 TEM images of negatively stained samples of highly diluted HEPC-C35-HEPC suspensions show the characteristic nanofiber morphology (Figure S2, Supporting Information). The nanofibers of HEPC-C35-HEPC and PC-C32-PC were used afterward as templates for the silicification reaction. 11617

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compared to the elements of the bolalipid. The silicified nanofibers have a cross-section of about 10−12 nm, nearly twice the length of a bolalipid molecule. This provides an approximate estimation of the silica shell thickness of about 2−3 nm. Further increase of the reaction time (samples BL-1.2 and BL-1.3) led to no obvious improvement of the silicification (Figure 2b). Decreasing the speed of stirring during the silicification reaction from 800 to 50 rpm (BL-1.4−BL-1.6) resulted in an inhomogeneous silicification so that the cross-section of the silicified fibers varied widely from 9 to 15 nm. In addition, the formation of amorphous silica could be observed; recognizable as clumpy dots between the nanofibers (Figure 2c). In contrast, diminishing the excess of the silica precursor (BL-1.7−BL-1.9) led to an insufficient silicification. TEM images showed the fibrous morphology only after staining the samples with uranyl acetate, indicating that no significant condensation of silica has taken place (Figure 2d). In the absence of the bolalipid template (BL-1 and BL-2, respectively) we observed only the formation of amorphous silica; no silica fibers of any type were observed (Figure S3, Supporting Information). Taking into account that the hydrolysis of alkoxysilanes (leading to siloxane anions subjected to subsequent siloxane condensation) is feasible under both basic and acidic conditions, the question arose if the hydrolysis of alkoxysilanes and hence silicification would proceed in a measurable extent after the mixture was adjusted to a neutral pH value or whether the silicification reaction would be slowed down significantly or, at best, would be terminated. To clarify this question, we added either hydrochloric acid after 30 min of silicification until a pH value of 7 was reached (about 100 to 150 μL), or we left the reaction mixture unperturbed (BL-1.10−BL-1.13). Afterward, both mixtures were allowed to stand for another 3 h without stirring. TEM images taken prior to the additional equilibration time showed already the presence of silicified nanofibers (Figure 3a). However, it seemed that two or more fibers of BL1 were adhering in a parallel fashion. This was more apparent after staining with uranyl acetate (Figure 3d) where single fiber strands within one silicified fiber packet are observable. Possibly, several single and densely packed nanofibers are enveloped during the silicification process and the resulting silica shell is not very thick because the uranyl acetate staining can image the single bola nanofiber within the shell. TEM images of unstained samples of BL-1.11 taken after an equilibration time of 3 h show that the silicified nanofibers are still intact (Figure 3b). Additionally, the formation of amorphous silica in-between the fibers can be observed, clearly visible after staining of the sample with uranyl acetate (Figure 3e). One can see that the additional equilibration time in combination with the basic pH value caused a continuation of the silicification process and the silica did not condense solely on the nanofibrous bolalipid template but also in solution leading to various unspecific silica aggregates. This undesirable unspecific mineralization process can be inhibited by adding HCl after the silicification reaction until a neutral pH is reached. TEM images of BL-1.13 (after 3 h equilibration time) show silicified fibers with a thickness comparable to those prior to the equilibration (Figure 3c,f). The formation of amorphous silica is clearly reduced but not completely suppressed. Again, the silicified nanofibers are adhering, which is possibly due to the relative high concentration of the bolalipid in the mixture (c = 0.33 mg/mL). The hydroxy-modified bolalipid BL-2 was also tested as template for the silicification process. However, an enhancing

with respect to the bolalipid (BL-1.7 to BL-1.9), and chose different silicification times from 30 min up to 3 h (BL-1.1 to BL-1.6). Moreover, we changed the speed of stirring during the silicification reaction (BL-1.1 to BL-1.6) and the termination of the silicification reaction by the addition of aqueous hydrochloric acid until a neutral milieu was achieved (BL-1.12 and BL-1.13). The sulfur-modified precursors MP-TMOS and MP-TEOS were used in pure form (BL-1.14, BL-1.17, and BL-1.18) or in a mixture with TEOS (BL-1.15, BL-1.1.19, BL-1.22, and BL-1.23) in order to adjust the amount of AuNPs fixed on the silicified nanofibers. In a representative experiment, an appropriate amount of silicification reagent was added to the bolalipid suspension (BL-1 or BL-2) under argon atmosphere and the hydrolysis of the silica precursors was started by the addition of small amounts (∼200 μL) of aqueous ammonia, 2 M (pH 9−10). Ethanol, which is generated during the silicification reaction, has no influence on the aggregation behavior of the bolaamphiphiles.49 Stirring during the silicification reaction should ensure appropriate mixing since the solubility of the silica precursors in water is limited. However, the stirring speed should be as low as possible in order to preserve the nanofiber structure of the bolalipids. The sol−gel condensation of silica was allowed to proceed for different periods of time, and afterward, samples were taken for TEM investigations. 3.1.3. Silicification Reaction and Characterization of Silicified Nanofibers by TEM. In a first attempt, we used BL-1 nanofibers as the template for the mineralization of the silica precursor TEOS. After 30 min of silicification with a 40-fold excess (n/n, with respect to the bolalipid) of TEOS (BL-1.1), TEM images showed that the condensation of hydrolyzed TEOS on the nanofiber surface has already started (Figure 2a).

Figure 2. TEM images of bolalipid−silica composite nanofibers: (a) BL-1.1; (b) BL-1.3; (c) BL-1.4; (d) BL-1.8. Samples in images a−c were prepared without staining, sample in image d with uranyl acetate staining.

The sample for the TEM image shown in Figure 2a was prepared without a staining agent but, the silicified fibers could be visualized due to the higher electron density of silicon 11618

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Figure 3. TEM images of bolalipid−silica composite nanofibers: (a,d) BL-1.10; (b,e) BL-1.11; (c,f) BL-1.13 (reaction determined by HCl); (g) BL1.14; (h) BL-1.19; (i) BL-1.22. Samples in images a−c were prepared without and samples in images d−i with uranyl acetate staining. The insets show images with higher magnification.

experiments with lower concentrations of the bolalipid, namely with 0.100 and 0.033 mg/mL. Furthermore, we tried a mixture of TEOS with MP-TEOS (10/1 and 5/1, n/n), which resulted in a more effective silicification compared to pure MP-TEOS. Figure 3h shows the TEM image of BL-1.19 after 3 h of silicification clearly indicating the formation of silicified nanofibers. However, some bundles of fibers sticking together are still present. With further decreasing BL-1 concentration (c = 0.033 mg/mL), we obtained single, silica coated nanofibers (BL-1.22, Figure 3i). Afterward, BL-1.20−BL-1.23 were used for further fixation of AuNPs (see section below). In summary, we found out that the most favorable conditions for the silicification process were (a) a low concentration of the bolalipid (between c = 0.100 and 0.033 mg/mL), (b) a 40-fold molar excess of the silicification reagent with respect to the bolalipid, (c) a reaction time of 30 min for TEOS and 180 min for MP-TEOS or its mixtures, (d) a moderate to fast stirring during the silicification, and (e) the addition of HCl (to pH 7) to significantly slow down the hydrolysis of the silica precursor and hence the sol−gel condensation. 3.1.4. Characterization of Silicified Nanofibers by DSC and NMR Measurements. DSC measurements were performed in order to investigate the influence of the silica shell on the

effect of the additional hydroxyl moiety connected to the choline headgroup on the mineralization reaction could not be obtained, as no difference between both bolalipids in the silicification reaction was observed (BL-2.1, for TEM image see Figure S4, Supporting Information). The bolalipid BL-2 was further used for 13C NMR investigations in comparison to BL-1. Further experiments were performed using sulfur-modified alkoxysilanes (MP-TMOS and MP-TEOS) since sulfur modification is known to be capable to improve the loading of AuNPs on the nanofibers.19 The first attempt (BL-1.14) with pure MP-TMOS was not successful due to the low miscibility of MP-TMOS with the aqueous bolalipid suspension and the rapid dissociation of the methoxy groups resulting in a faster, unspecific condensation in solution (Figure 3g). Even if the silicification with MP-TMOS was carried out after initial TEOS mineralization (BL-1.15) or if the MP-TMOS was mixed with TEOS prior to the reaction (BL-1.16), a silicification could not be detected. With the application of MP-TEOS instead of MP-TMOS, the mineralization was successful. However, with MP-TEOS, the silicification time had to be extended to at least 3 h in order to obtain silica shell formation (BL-1.17 and BL-1.18). To avoid the aggregation of the silicified nanofibers, we performed these 11619

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micelle−micelle transition. Since the silica shell may not be entirely closed around the nanofibers (see BL-1.4−BL-1.6, Figure 2c) it is conceivable that, at high temperature after the first heating, the bolalipid molecules can escape the silicified nanofibers and accumulate in the dispersion. In the following heating cycles, we find, in consequence, both transitions close to the original transition temperatures of pure BL-1. The broadening of the peaks, which is still present, can be explained by the silica disturbing the self-assembly of the bolalipid molecules to nanofibers and micelles, respectively. A different situation is found in the DSC curve of BL-1.12. In this sample, a lower bolalipid concentration (c = 0.33 mg/mL) in combination with a faster stirring during the silicification was used in order to get a consistent and more uniform silica shell around the bolalipid nanofiber (see TEM images in section above). In the first heating scan (black solid line in Figure 4b), the first transition is shifted to higher temperatures and splits into two peaks at 53.0 and 56.4 °C, respectively. The second transition becomes very broad and is also shifted to higher temperatures (78−82 °C). In the second heating scan (gray dashed line), we observe only a small and broad transition at 64−65 °C. Comparing this behavior to the results mentioned above (BL-1.24, Figure 4a), we believe that this is due to the following two effects: (a) the nearly completely closed silica shell around the bolalipid nanofibers leads to a stabilization of the fibers and therefore a shift of the fiber−micelle transition to higher temperature; (b) since the bolalipid molecules cannot escape the silicified nanofibers (sample BL-1.12), they may stay in the micellar aggregate form induced by the first heating, and the reassembly into nanofibers during the cooling scan is nearly completely inhibited resulting in only one transition, possibly micelle−micelle transition, observed in the second heating scan. TEM investigations of the samples after the DSC measurement show that the bolalipid−silica composite nanofibers are still intact (Figure 5).

temperature dependent aggregation behavior of BL-1 and to detect possible changes of the aggregate structure. Without silica shell, the DSC thermogram of pure BL-1 shows two endothermic transitions: a first, sharp peak at 48.0 °C representing the transformation of the bolalipid nanofibers into small micelles followed by a second, broad micelle−micelle transition at 73−74 °C, which is associated with a further increase of the mobility of the long alkyl chains.18 Figure 4a shows the DSC thermograms of sample BL-1.24 where BL-1 (c = 1.0 mg/mL) was silicified with a 40-fold excess

Figure 4. DSC heating curves of bolalipid-silica composite nanofibers: (a) BL-1.24; (b) BL-1.12. The DSC heating curve of BL-1 (c = 1.0 mg/mL) is presented with a dotted line for comparison. The heating rate was 20 K/h.

of TEOS slowly stirred over 60 min. The first heating curve (black solid line) indicates a shift of the first transition to higher temperatures (50.3 °C), whereas the second transition becomes more broadened and is shifted to lower temperatures (66−67 °C) compared to the pure BL-1. The situation changes with the second and third heating curve (gray dashed lines) where the first transition becomes broadened and split into several peaks between 48.8 and 49.9 °C (with a low-temperature shoulder) and the second transition shifts again to higher temperatures between 72−73 °C, slightly below the temperature of the second transition of pure BL-1. This behavior could be explained as follows: the silica shell that encloses the bolalipid nanofibers hinders the fiber−micelle transformation resulting in a higher transition temperature. The packing of the bolalipid molecules within these micelles is disturbed to some extent, which leads to a decrease and an additional broadening of the

Figure 5. TEM images (different magnifications) of bolalipid−silica composite nanofibers (BL-1.12) after the DSC measurement. The sample was prepared without a staining agent.

To get information about the mobility of the bolalipid molecules alone and within the silicified nanofiber, 13C and 31 P solid-state NMR measurements of aqueous suspensions were performed. At first, we investigated the hydroxy modified bolalipid BL-2 without silica shell showing nearly the same behavior (see Figure S5 and description in Supporting Information for more details) as the unmodified BL-1 reported previously.47 A different situation is found after the silicification of the bolalipid nanofibers. For the NMR measurements, sample BL-1.25 was centrifuged after the silicification reaction and the 11620

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white and gel-like pellet obtained was transferred into the NMR tubes without further drying in order to ensure comparability between the NMR and DSC measurements. Figure 6 shows

Figure 6. 31P NMR spectra of BL-1.25 at different temperatures. 31

P spectra of BL-1.25 at different temperatures: At room temperature (black solid line), the 31P line shape is much broader with Δσ ≈ 43 ppm compared to the nonsilicified sample (Δσ ≈ 20 ppm).47 Also, the line shape has a different asymmetry and seems to be indicating a superposition of different line shapes. The explanation of this line shape is difficult; it indicates, however, a motion of the phospholipid headgroup, which is more hindered due to the restrictions of the silica shell in the bolalipid−silica composite nanofibers. After heating the sample to 55 °C (black dotted line), the peak of the 31P resonance becomes narrower and more symmetric indicating a more pronounced isotropic motion of the bolalipid molecule compared to the silicified sample at 25 °C. However, the width of this peak (∼20 ppm) is still broader compared to the 31P resonance of the nonsilicified sample at 55 °C (∼2 ppm).47 This can be explained by a slower isotropic rotational motion of the bolalipid micelles within the bolalipid− silica composite nanofibers. After cooling the sample to 25 °C, the signal of the 31 P resonance (gray dashed lines in Figure 6 after 5 h and 1 day of equilibration, respectively) broadened again, and the peak is nearly identical to the one before heating. Probably, the micelles, which were formed within the bolalipid−silica composite nanofibers during the heating, reassembled again into fibrous structures again leading to the broad signal. In contrast to the DSC measurements of sample BL-1.12, the bolalipid fibers are reformed within the silica shell after the heating cycle, but this is probably due to different experimental conditions. Similar spectra were found for the 31P resonance of the hydroxy modified bolalipid BL-2 (data not shown), leading to the conclusion that there is no difference in silica shell formation between both bolalipids. To clarify the question whether the additional hydroxy moiety within the headgroup of the bolalipid BL-2 has an impact on the mineralization of silica and a covalent C−O−Si bond is formed to the headgroup, we performed 13C solid-state NMR measurements on a BL-2 sample in comparison to a BL-1 sample where no covalent bond of the silica shell to the headgroup is possible. To enhance the signal-to-noise ratio, we centrifuged the samples BL-1.25 and BL-2.1 after the

Figure 7. 13C NMR spectra of the headgroup region of (a) BL-1.25 and (c) BL-2.1 (silicified samples) and (b) BL-1 and (d) BL-2 (nonsilicified samples). Measurements were performed at room temperature. Inset shows the chemical structure of the two different headgroups with numbers indicating the corresponding 13C NMR signals.

silicification process, dried the pellets, and rehydrated them until a concentration of c = 30 mg/mL was reached. Figure 7 depicts the 13C NMR spectra of the headgroup region of the silicified samples (Figure 7a,c: BL-1.25 and BL-2.1) and the nonsilicified ones (Figure 7b,d: BL-1 and BL-2; for complete spectra, see Figure S6, Supporting Information). Comparison of the spectra of the pure bolalipids with those in the silicified samples shows that the peaks of the 13C resonances got broader after the silicification reaction. However, the position of the peaks is only marginally changed. The broadening of the peaks is due to the more restricted mobility of the bolalipid molecules and the headgroups within the bolalipid−silica composite nanofibers. The only slight change in the chemical shift reflects the lack of influence of the silica shell on the chemical environment of the atoms in the headgroup of the bolalipid. For instance, the resonance of the carbon atom next to the hydroxy group of BL-2 at 56 ppm (number 6 in Figure 7c,d) shifted only by ∼0.5 ppm to a higher resonance frequency indicating that no C−O−Si bond is formed connecting the bolalipid with the silica shell. 3.2. Fixation of Gold Nanoparticles. We then investigated the fixation of AuNPs to silicified nanofibers composed of BL-1 as template. The commercially available citrate-stabilized AuNPs had a mean diameter of either 5 or 2 nm and were added 11621

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nanofibers. Also, a replacement of citrate groups by surface Si−OH group is possible. In a second experiment, we used the sample BL-1.22 where BL-1 was silicified with a mixture of TEOS and MP-TEOS (5:1, n/n). TEM images prior to the addition of AuNPs show single, silicified nanofibers (see Figure 3i). After the addition of 5 nm AuNPs, these AuNPs are able to align two silica coated fibers in a parallel fashion (Figure 8c), and the AuNPs are well ordered between these two nanofibers (Figure 8d). Although the additional sulfur moiety within the MP-TEOS should enhance the binding of AuNPs to the silica nanofibers, we found no significant higher loading compared to the sample BL-1.21 prepared without MP-TEOS. An increase of the MPTEOS to TEOS ratio (samples BL-1.19 and BL-1.22) did not result in a higher loading. However, the usage of MP-TEOS/ TEOS mixtures clearly prevented the clustering of 10 or more fibers compared to samples silicified with pure TEOS. The same bolalipid−silica composite nanofibers of sample BL-1.22 were used to fix 2 nm AuNPs. Previous studies19 have shown that these small NPs disturbed the van der Waals contacts between the bolalipid molecules leading to a break-up of the nanofiber assemblies. Because of the limit of resolution of our electron microscope, 2 nm AuNPs are difficult to see in TEM images. However, the TEM images of the sample with silicified fibers after the addition of 2 nm AuNPs clearly show that the silica coated nanofibers are still intact (Figure 8e) and that the NPs are embedded between two fiber strands similar to the fixation of 5 nm AuNPs (Figure 8f).

to the suspension of silicified nanofibers realizing a ratio of bolalipid: AuNP = 1000:1 and agitated for 30 min. The samples were investigated by TEM using negative staining with uranyl acetate. Figure 8a shows a TEM image of BL-1.21 after 30 min of

4. SUMMARY AND CONCLUSIONS We could show in our study that nanofibers composed of selfassembled bolalipids can be stabilized using tetraalkoxysilanes resulting in single-strand, silica-coated nanofibers. For a successful silicification reaction, the following conditions are favorable: a very low concentration of the bolalipid (c = 0.033 mg/mL), a 40-fold excess of the silicification reagent compared to the bolalipid, a reaction time of 30 min for TEOS and 180 min for MP-TEOS mixtures, respectively, and the addition of HCl to significantly slow down the sol−gel condensation. The final bolalipid−silica composites were characterized by means of TEM, DSC, 13C NMR, and 31P NMR measurements. We could show that the silica shell around the bolalipid nanofibers was almost completely closed. The silicified nanofibers could successfully be used as a template for the 1-D fixation of 5 nm and 2 nm gold nanoparticles, respectively, in aqueous suspension. The silicified nanofibers considerably improved the loading with gold nanoparticles compared to nonsilicified nanofibers. Calcination of silicified nanofibers, a method for the formation of stable nanotubes with a specific cross-section, is currently under way.

Figure 8. TEM images of bolalipid−silica composite nanofibers (PC-C32-PC, c = 0.033 mg/mL) with gold nanoparticles (bolalipid−AuNP = 1000:1) at different magnifications: (a,b) BL-1.21 with 5 nm AuNP; (c,d) BL-1.22 with 5 nm AuNP; (e,f) BL-1.22 with 2 nm AuNP. The samples were prepared with uranyl acetate staining.

silicification with pure TEOS, neutralization with HCl, and subsequent addition of 5 nm AuNPs. Figure 8b shows a magnified image of Figure 8a where 10 and up to 20 fibers lie in a parallel fashion, and the 5 nm AuNPs are in between two fiber strands. The images clearly show the fixation of AuNPs on long strands of the silicified fiber template. Compared to our previous studies19 using the same ratio of bolalipid to AuNP (1000:1), where the binding of AuNPs to nonsilicified bolalipid nanofibers was demonstrated, the bolalipid−silica composite nanofibers show a significantly higher binding of AuNPs. The binding of AuNPs to pure bolalipid fibers was driven by hydrophobic effects, as the binding probably occurred to the exposed hydrophobic pockets present on the nanofibers.19 In the case of the silicified fibers of BL-1, the hydrophobic pockets resulting from the helical superstructure of self-assembled bolalipid molecules48 are covered, and the nanofibers have a hydrophilic surface. It is conceivable that now hydrogen bonds between Si−OH moieties at the surface and citrate groups stabilizing the AuNPs are responsible for the fixation of the AuNPs to bolalipid−silica composite

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ASSOCIATED CONTENT

S Supporting Information *

DSC graphs, TEM images, NMR measurements, and synthesis and analytical data of HEPC-C35-HEPC. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(S.D.) Tel: +49-345-5525196. Fax: +49-345-5527026. E-mail: [email protected]. (A.M.) Tel: +49-3455524935. Fax: +49-345-5527048. E-Mail: annette.meister@ chemie.uni-halle.de. 11622

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants (projects Bl 182/19-3 and Do 463/4-2; to S.D., B.D., A.B., and A.M.) and by grants within the Forschergruppe FOR 1145 (to W.H.B. and A.B.) from the Deutsche Forschungsgemeinschaft. The support of Dr. Gerd Hause (Biocenter, Martin-Luther-University Halle-Wittenberg) by providing us access to the electron microscope facility is greatly appreciated. S.D. thanks Ms. Marika Schulze and Mr. Volkmar Tell for their help in synthesizing the bolalipids. Finally, we thank Dr. Robert Sachsenhofer for technical support.

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