Imprinting Chirality in Silica Nanotubes by N-Stearoyl-serine Template

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Imprinting chirality in silica nanotubes by N-stearoyl-serine template Gila Levi, Yosef Scolnik, and Yitzhak Mastai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06540 • Publication Date (Web): 17 Aug 2016 Downloaded from http://pubs.acs.org on August 18, 2016

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ACS Applied Materials & Interfaces

Imprinting chirality in silica nanotubes by Nstearoyl-serine template Gila Levi,† Yosef Scolnik,‡ and Yitzhak Mastai†,* †

Department of Chemistry and Institute for Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan 5290002, Israel ‡

IYAR, The Israeli Institute for Advanced Research

KEYWORDS Chirality, silica nanomaterials, enantioselectivity, chiral surfactant, porous materials

ABSTRACT In this article, we describe the synthesis of imprinted chiral silica nanotubes based on the use of a chiral N-stearoyl L-serine (C18Ser) anionic surfactant as the chiral template. The resulting chiral silica nanotube structures were characterized by electronic microscopy (transmission electron microscopy (TEM) and scanning electron microscopy (SEM)) and nitrogen isotherms that proved the formation of well-ordered silica nanotubes. A C18Ser surfactant template was used for the preparation of the silica nanotubes, due to its effective molecular organization within the silica network. After chemical extraction of the chiral template, the enantioselectivity feature of the silica nanotubes was confirmed by selective adsorption of the enantiomers using circular dichroism (CD) and isothermal titration calorimetry (ITC) measurements. Although these measurements show a relatively low chiral selectivity of the silica nanotubes (ca. 6% enantiomeric excess), the system described here offers new approaches for the application of chiral porous materials in chirality.

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1. INTRODUCTION In recent decades, the use of porous inorganic materials is increasing in many industrial applications.1,2 The possibility of including chiral recognition and chiral separation alongside other useful properties of porous inorganic materials is highly relevant for a wide variety of enantioselective processes, ranging from catalysis to separation science and from chromatography to the synthesis of enantiomerically pure drugs, etc. Structural chirality can be found in many porous materials such as organic-based materials, inorganic materials and natural-product zeolites. Due to recent interest in chirality, various synthetic methods have been developed for the preparation of chiral mesoporous silicas (CMS), mostly based on the chiral template approach.3,4 For example the group of Coronas described in two articles a simple and effective synthetic route for the preparation of chiral ordered mesoporous silica (COMS) with high enantioselective nature.5,6 Their approach was based on the combination of tetraethyl orthosilicate (TEOS) and quaternized aminosilane as the silica sources and the presence of amino acid (proline) as the chiral template that resulted in a chiral ordered mesoporous silica (COMS) with structural and textural features of MCM-41-type silica. Although many advances have been made in the field of chiral mesoporous materials, the vast majority of these chiral mesostructures are still based on silica-imprinted materials. This approach facilitates the difficult process of imbedding chirality into inorganic materials using a simple preparation, with large availability of functional silanes that can be used to impart chiral properties in the hard-template model. In several studies, various types of chiral templates led to the formation of CMS. In one such study, Alvaro et al. proved that the combination of binaphthyl and cyclohexadiyl organic template groups with a tetraethyl orthosilicate precursor induces chiral porous silica formation.7 Avnir et al. demonstrated in several papers that various chiral template


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molecules, such as propranolol, 2,2,2-trifluoro-1-(9-anthryl) ethanol or tyrosine, can be used for the synthesis of chiral mesoporous silica using sol-gel matrix production.8–10 In addition, Mastai et al. reported various chiral block copolymers as templates for CMS formation.11–13 Oda et al. investigated the self-assembly organization of chiral amphiphilic molecules by controlling several parameters and studied the mechanism of formation of chiral mesoscopic molecular assemblies.14,15 Investigation of these systems had shown that the chirality transfer from chiral counterions to achiral membranes can form supramolecular chirality of the inorganic silica assemblies, such as ribbons and tubules.16,17 Recently, anionic surfactant-templated mesoporous silica (AMS) materials have yielded particularly well-ordered structures.18–22 In typical procedures, co-structure-directing agents (CSDA) induce high-ordered mesostructure formation through self-assembly of the anionic surfactant template and inorganic species.23 These CSDA molecules are composed of two sections: an alkoxysilane part than can be condensed with a silica precursor (such as TEOS), and an organic part able to interact with the head groups of the anionic surfactant template. A protonated amino group of 3-aminopropyltrimethoxysilane (APS) or a quaternary ammonium group of Ntrimethoxylsilylpropyl-N,N,N-trimethyl ammonium chloride (TMAPS) can serve as a CSDA to produce a uniform distribution of mesostructures after the extraction of the template.24 Some AMS systems have generated a helical rod-like morphology with twisted, two-dimensional hexagonal mesochannels, due to the interactions between suitable anionic surfactants and the CSDA.23,25,26 To the best of our knowledge, anionic surfactant templates imprinted in silica with a CSDA have consistently produced mesoporous or rod-like structures.18–26 In most previous studies on chiral templated silica, the final structures are typical mesoporous structures but not in nanotube morphology. Moreover, the chiral template molecule is usually used for chiral templating, and the


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other surfactants are used for the control and formation of the mesoporous structures. In the current study, we report our results on the formation of chiral-imprinted silica nanotubes using an Nstearoyl-L-serine anionic surfactant. The novelty of this study is in the combination of two chemical processes in which the chiral surfactant self-assembly produces the nanotube structures and the chiral-imprinting process yields chiral surfaces. We chose to use TEOS as the silica precursor, co-condensed with APS, and the N-stearoyl Lserine (C18Ser) as the chiral template. We examined the chiral nature of the templated silica nanotubes by performing chiral adsorption experiments using circular dichroism spectroscopy and isothermal titration calorimetry. The work presented in this article may lead to new uses of chiral porous materials in processes such as chiral catalysis and separation, where the control of porosity and enantioselectivity are significant. Moreover, the procedure developed here can be applied to the synthesis of other porous materials with nanotube structures, using a variety of chiral anionic surfactants. 2. EXPERIMENTAL SECTION 2.1. Chemicals. Stearic acid (Fluka), chloroform (Carlo erba), di-tert-butyl dicarbonate (Fluka), Nhydroxysuccinimide (Aldrich), triethylamine (Fisher Chemical), 4-dimethylaminopyridine (Fluka), L-serine (Fluka), tetrahydrofuran (Acros Organics), and hydrochloric acid (Bio-Lab) were used for the synthesis of the C18Ser surfactant. Butanol (Fluka), ethanol (Bio Lab), potassium hydroxide (Sigma), tetraethyl orthosilicate (Aldrich), and (3-aminopropyl)trimethoxysilane (Fluka) were purchased for the preparation of silica nanotubes in the template-based approach. Ethanolamine (Alfa Aesar) was used to extract the template.


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Silica nanotubes were formed using the C18Ser surfactant template, synthesized in a two-step procedure, and their chiral nature was examined by adsorption experiments with valine (Alfa Aesar) and serine (Fluka) solutions. 2.2. Synthesis of N-stearoyl-L-serine surfactant. Preparation of stearic acid N-hydroxysuccinimide ester: Stearic acid (11.4 g, 40 mmol) was dissolved in chloroform (120 mL) and, while stirring, di-tert-butyl dicarbonate (12.0 g, 56 mmol) and N-hydroxysuccinimide (4.60 g, 40 mmol) were added consecutively. Triethylamine (4.04 g, 40 mmol) and 4-(dimethylamino)pyridine (0.980 g, 8 mmol) were added, leading to slow bubbling of the carbon dioxide. The mixture was stirred at room temperature overnight and then washed with 2 M hydrochloric acid/aq. The organic layer was dried and evaporated, and crystallization from ethanol gave 9.12 g of the product as a white solid in 60% yield. Preparation of N-stearoyl-L-serine: Sodium bicarbonate (0.647 g, 7.7 mmol) was added to an aqueous solution (50 mL) of L-serine (0.810 g, 7.7 mmol). A solution of stearic acid Nhydroxysuccinimide ester (2.67 g, 7.0 mmol) in tetrahydrofuran (50 mL) was added to the serine solution and the reaction was stirred overnight at 40 °C. The solution was acidified to pH 2 and the evaporation of the tetrahydrofuran gave a white precipitate. Filtration and recrystallization from acetone afforded 1.70 g of N-stearoyl-L-serine in 65% yield. Characterization: 1H and 13C NMR spectra of the N-hydroxysuccinimide ester and synthesized chiral surfactant were obtained by an AM-300 spectrometer (Bruker). Mass spectrometry (MS) and high-resolution mass spectrometry (HRMS) were performed by an Autoflex III smartbeam MALDI spectrometer (Bruker) and on a 6545 Q-TOF instrument (Agilent) respectively. Circular dichroism (CD) spectra were recorded by a Chirascan CD spectrometer (Applied Photophysics, UK) using a bandwidth of 3.1 nm, from 240 to 196 nm. The step size and time were set at 1 nm


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and 3 sec, respectively. A solution of 0.1 M N-stearoyl-L-serine in a mixture of butanol and ethanol (1:1) was prepared. Then 18 μL of the solution were added to 1.8 mL of 0.01 M potassium hydroxide (KOH) in double-distilled water and immediately mixed. CD of the sample in a 1 mm quartz cuvette was checked over time. 2.3. Preparation of silica nanotubes using a C18Ser template in a sol-gel process. Synthesis of silica nanotubes: In a typical synthesis, N-stearoyl-L-serine (37.1 mg, 0.1 mmol) was dissolved in butanol:ethanol (0.5:0.5 mL). The solution was stirred with an aqueous solution of potassium hydroxide (100 mL, 0.01 M). After 15 min, TEOS (341 μL, 1.5 mmol) and APS (106 μL, 0.60 mmol) were added simultaneously. The system was gently stirred at room temperature for 30 min and then kept at 60 °C for 48 h. The precipitate was recovered by filtration, washed with deionized water and ethanol, and dried in air affording ca. 70 mg. Calcination of C18Ser: 140 mg of As-SiO2-C18Ser was calcined at 550 °C for 6 h, at a heating rate of 10 °C/min. The calcined silica Cal-SiO2-C18Ser obtained (ca. 65 mg) was characterized by various techniques. Extraction of C18Ser: 100 mg of As-SiO2-C18Ser was dispersed in 16 mL of ethanol and 4 mL of ethanolamine. The mixture was stirred and refluxed at 90 °C for 17 h. The Ex-SiO2-C18Ser solid (ca. 45 mg) was recovered by filtration, washed with ethanol, dried in air, and then characterized. Characterization: As-SiO2-C18Ser, Cal-SiO2-C18Ser, and Ex-SiO2-C18Ser were characterized by various techniques. Environmental scanning electron microscope (ESEM) images were obtained with a Quanta FEG instrument (FEI, USA) at acceleration voltages of 15 kV and 20 kV. Highresolution transmission electron microscope (HR-TEM) and TEM images were taken with JEM2100 and JEM-1400 instruments (JEOL, USA), at an accelerating voltage of 200 kV. The samples for the TEM measurements were dispersed in an ultrasonic bath, and a drop of the dispersion was


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placed onto a 400-mesh copper grid coated with an amorphous carbon film. Energy dispersive spectroscopy (EDS) chemical analysis was performed with a Thermo Electron Group system. Brunnauer-Emmett-Teller (BET) surface area measurements were performed by a Nova 3200E instrument (Quantachrome, USA). The samples were pre-degassed at 120 °C for two hours, under vacuum of 30 mmHg. FTIR Spectra were collected by Fourier transform infrared spectroscopy (FTIR) with an iS5 portable spectrometer (Thermo Scientific). The silica powder was ground with a pestle in an agate mortar, and mixed with KBr (IR-grade) before being pressed into a pellet. The measurement range was 4000-400 cm−1, and 32 scans were collected at a resolution of 4 cm−1. Thermogravimetric Analysis (TGA) measurements were performed on a Pyris 1 TGA instrument (Perkin Elmer). The samples were heated from 30 °C to 500 °C at 20 °C/min. Elemental analysis was achieved on FlashEA 1112 CHNS-O device (Thermo scientific). 2.4. Chiral adsorption experiments. In adsorption experiments, the chiral recognition of the Ex-SiO2-C18Ser was investigated by CD spectroscopy and isothermal titration calorimetry (ITC). For adsorption experiments investigated by CD, 2 mL of 25 mM D and L-serine aqueous solutions were added to 8 mg Ex-SiO2-C18Ser and the mixture was sonicated for 5 sec. For dynamics adsorption experiments, the CD signal in solution was probed with time after centrifugation. For chiral investigation using CD, the vials were centrifuged after 48 hours and the optical activity of the solutions was probed. All the CD measurements were performed in a 10 mm quartz cuvette. CD spectra were recorded using a bandwidth of 1 nm, from 260 to 190 nm. The step size and time were set at 0.5 nm and 0.5 sec, respectively. For adsorption experiments examined by ITC, each solution of D/L-serine (100 mM) and D/Lvaline (200 mM) was injected into the ITC cell containing 1.442 mL of 3 mg/mL Ex-SiO2-C18Ser


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suspension in water. ITC measurements were performed with a VP-ITC calorimeter (MicroCal) and the adsorption enthalpies were determined at 30 °C. A titration run consisted of consecutive injections of 5 μL lasting 8.5 sec, with an interval of 300 sec between injections. The ITC cell was stirred at a rate of 300 rpm. Before each measurement, all the solutions were degassed under vacuum using a ThermoVac accessory (MicroCal) for 5 min. 3. RESULTS AND DISCUSSION In the first stage of the work, we synthesized the chiral C18Ser. First, stearic acid was activated as an N-hydroxysuccinimide ester using di-tert-butyl dicarbonate and 4-dimethylaminopyridine.27 This reaction was carried out in chloroform at room temperature overnight, producing a stearicacid N-hydroxysuccinimide ester with a yield of 60% after recrystallization from ethanol. In the following step, the C18Ser surfactant was obtained by the reaction between the active ester and serine.28,29 Characterization details of NMR and mass spectrometry of the final C18Ser surfactant are shown in the Supporting Information. Next, we studied the self-assembly behavior of the C18Ser, namely, the formation of ordered micelle structures of C18Ser in water, using CD measurements. Briefly, 18 μL of a C18Ser solution (0.1 M) in a mixture of butanol and ethanol was stirred with 1.8 mL aqueous KOH (0.01 M) and the CD was measured. Figure 1 presents the CD kinetic results of the sample probed after 16 and 60 min. After 16 min, a CD peak with an intensity of 45.0 mdeg was obtained at a 204-nm wavelength. This intensity peak value was decreased to 43.7 mdeg after 60 min. This decrease is probably due to the rearrangement of many small C18Ser micelles into larger ones. Our CD results fit with previous studies by the group of Shinitzky,30 who showed that C18Ser undergoes a self-assembly process leading to the formation of ordered chiral micelles. The assumption is that N-stearoyl-L-serine micelles organize in such a way that there is a repetitive arrangement of the amide planes on the micellar surface.


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Wavelength (nm) Figure 1. Circular dichroism spectra of N-stearoyl-L-serine a. in the first 16 min and b. after 60 min. In the next stage, we used C18Ser as the template for the formation of silica nanotubes. Briefly, N-stearoyl-L-serine was dissolved in a butanol-ethanol mixture. The solution was stirred with a potassium hydroxide aqueous solution. After 15 min, TEOS and APS were added simultaneously and the system was stirred for 30 min and kept at 60 °C for 48 h. The silica obtained in the synthesis was characterized by various techniques. The electron-microscope images in Figure 2 depict silica nanotubes, in contrast to previous reports of mesoporous or rod-shaped silica imprinted with anionic surfactant templates. The length of the nanotubes is ca. 1-1.5 µm and their diameter ranges from 200 to 250 nm. The HR-TEM image displays a well-defined structure of a single nanotube ca. 1.5 µm long (Figure 2b). The nanotubes exhibit a hollow structure with a diameter of ca. 220 nm and a wall thickness of ca. 20 nm. Furthermore, EDS measurements of the sample confirm the formation of silica (Figure S4 of Supporting Information).


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Figure 2. a. ESEM and b. HR-TEM images of the As-SiO2-C18Ser. After the calcination of the template at 550 °C, the structure of the nanotubes was preserved (Figure 3). The calcined silica nanotubes (Cal-SiO2-C18Ser) exhibit a hollow structure with diameters ranging from 200 to 250 nm. Also, spherical nanoparticle aggregations can be seen on the surface of the silica nanotubes, indicating the formation of additional silica nanoparticles during the calcination process. The HR-TEM image confirms the structure of the Cal-SiO2-C18Ser, with a clear shell of ca. 25 nm (Figure 3b).


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Figure 3. a. ESEM and b. HR-TEM images of the Cal-SiO2-C18Ser. It is known that the calcination of the chiral template from silica at a high temperature can cause the loss of the chiral material features. Therefore, in order to protect and preserve the chirality of the silica nanotubes, we decided to extract the C18Ser template from As-SiO2-C18Ser by chemical extraction at a low temperature. To extract the template, As-SiO2-C18Ser was dispersed in ethanol and ethanolamine, and the mixture was refluxed at 90 °C. The morphology of the Ex-SiO2-C18Ser nanotubes was well maintained after the extraction of the organic template (Figure 4).


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Figure 4. a. ESEM and b. TEM images of the Ex-SiO2-C18Ser. In conclusion, we have shown that N-stearoyl-L-serine can be used as a template for the formation of well-ordered silica nanotubes. The structure of the silica nanotubes is dictated by the C18Ser surfactant. This ordered formation can be explained by the spontaneous formation of tubule surfactants, as reported by Fuhrhop et al., who used N-dodecanoyl D and L-serine as the surfactants.31 We performed BET surface-area measurements to characterize the porosity of the As-SiO2C18Ser and Ex-SiO2-C18Ser silica nanotubes. N2 isotherms of silica, before and after extraction of the template, are shown in Figure S4. A hysteresis loop between the adsorption and desorption branches of both type III isotherms can be seen, proving the porous nature of the silica adsorbent. The multi-point BET surface area of silica after the template extraction of Ex-SiO2-C18Ser was double that of the As-SiO2-C18Ser (from 45.9 m²/g to 100.6 m²/g, Table S1). This observation confirms the proper extraction of the C18Ser template leading to more porous silica nanotubes. FTIR performed on the As-SiO2-C18Ser and Ex-SiO2-C18Ser confirmed the successful extraction of the C18Ser template by the disappearance of the peaks attributed to the alkane group of the


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template at 2850-2980 cm-1 (Figure S5). TGA spectra (Figure S9) and C, H, N elemental analysis (Table S2) of the samples before and after extraction of the template also proved the proper extraction of C18Ser. In the next phase of this research we focused on the study of chiral nature of the silica. We first performed a series of CD experiments to evaluate the dynamics and capacities of L-serine adsorption process onto the silica. In figure 5, the adsorption dynamics of L-serine onto the ExSiO2-C18Ser is shown as measured by CD. The kinetics of the L-serine adsorption was found to be nearly exponential; the adsorption process was quite fast at the first 20 hours descending with time to reach the final equilibrium value after 210 hours.

Figure 5. L-Ser concentration change in solution (data points) as a function of the adsorption time. The red curve displays the exponential fit of the data. Equation: y= a*exp(-b*x)+c, with a= 9.997 mM, b= 0.01328 1/h, c= 5.788 mM, and R2= 0.986. Moreover we performed a series of CD adsorption measurements of L-Ser with different amounts of silica nanotubes in order to determine the adsorption capacities of the silica nanotubes (shown in Figure S10). Based on those adsorption measurements, the Ex-SiO2-C18Ser nanotubes were shown to be capable of adsorbing 2.0 mmol L-Ser per gram silica. This value is in agreement with that found for MCM-41 and SBA-15, as reported in the literature.6,32


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Finally In order to evaluate the chiral recognition of Ex-SiO2-C18Ser, contributed by the chiral C18Ser template, we performed adsorption experiments of serine enantiomeric solutions on ExSiO2-C18Ser using a CD instrument. In typical experiments, Ex-SiO2-C18Ser nanotubes were added to aqueous solutions of both D- and L-serine (25 mM). After 48 hours, the CD of the solutions before and after adsorption on Ex-SiO2-C18Ser nanotubes was measured (Figure 5a). In order to quantify the enantioselectivity factor of the Ex-SiO2-C18Ser nanotubes, the difference between the adsorption of the D- and L-serine enantiomers on the Ex-SiO2-C18Ser nanotubes was investigated. After interaction with the nanotubes, the decrease in L-serine concentration was greater than in Dserine (compared to the pure solutions) (Figure 5a). The nanotubes adsorbed 47.6% of the L-serine enantiomer solution and only 45.0% of the D-serine solution (Figure 5b). The enantiomeric excess (ee) in favor of L-serine was found to be ca. 6% [ee= (R-S)/(R+S) × 100%], where R and S are the CD intensity differences in the L-serine and D-serine reference solutions, respectively, before and after adsorption). This demonstrates the preferential adsorption of L-serine on Ex-SiO2-C18Ser. 60

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Figure 6. a. CD spectra of D-serine (bottom) and L-serine (top) solutions as references (red and blue spectra), and after adsorption on Ex-SiO2-C18Ser (green and red spectra). b. Adsorption percentages of D and L-serine solutions on Ex-SiO2-C18Ser.


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In order to confirm the enantioselectivity of Ex-SiO2-C18Ser, we performed ITC experiments to measure the adsorption enthalpy of pure enantiomers on the silica nanotubes. We have previously reported the usefulness of ITC experiments for determining the enantioselectivity of various materials, such as zeolites,33 and confirming enantioselective interactions in crystals34 and mesoporous carbon.35 In typical adsorption experiments performed by ITC, each solution of D and L-amino acid enantiomers was titrated in a sample cell containing silica nanotubes, and the difference in heat (ΔH) was monitored over time. Each ITC peak represents the heat change for each injection of the enantiomer solution into the silica nanotube suspension. The total heat of these interactions is calculated according to the area under the peaks. The enthalpy of chiral binding between the enantiomers and the silica nanotubes greatly contributes to the total enthalpy change H. The enantioselectivity factor was determined by the ratio between the total heat of the D- and L-enantiomers on the silica. In our ITC experiments, we used D and L-valine injection solutions because of their low heat of dilution in water, allowing accurate measurement of the adsorption enthalpy. The difference in the adsorption enthalpy between D and L-valine injection on the silica nanotubes is shown in Figure 7a. Both enantiomers displayed negative ITC peaks. The adsorption enthalpy (ΔHabs) for L-valine (-3.42 µcal/s) was clearly higher than that of D-valine (-4.30 µcal/s) per injection. The average enthalpy per injection was calculated by integration (Figure 7b) and was found to be -60.1 cal/mol for the L-valine enantiomer, and -86.2 cal/mol for D-valine. Using this difference in the chiral enthalpy of adsorption, caused by the different chiral molecular interactions between the enantiomers and the silica nanotubes, we evaluated the enantioselectivity of the silica nanotubes. The enantioselectivity factor was found to be ca. 1.43 in favor of D-valine, and was calculated according to the ratio between the average adsorption enthalpy for each enantiomer.


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Figure 7. ITC of 200 mM D/L-valine solution in Ex-SiO2-C18Ser nanotubes. a. Heat release per injection. b. Total heat release per injection (integrated areas of the ITC peaks). The ITC experiments carried out with the D/L-serine solutions are shown in Figure 8. Here, the two enantiomers displayed positive ITC peaks and in this case the enantioselectivity factor, calculated by the ratio of slopes of the average adsorption enthalpy, was found to be ca. 1.17 in favor of the L-serine enantiomer. This preferential adsorption of L-serine on the silica nanotubes is in agreement with that obtained in CD experiments. 8

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Figure 8. ITC of 100 mM D/L-serine solution in Ex-SiO2-C18Ser nanotubes. a. Heat release per injection. b. Total heat release per injection (integrated areas of the ITC peaks).


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4. CONCLUSIONS We have synthesized well-ordered silica nanotubes by imprinting an N-stearoyl-L-serine template in a simple controllable low-cost sol-gel process. The porous structure of the silica nanotubes, with a typical diameter of ca. 200 nm, was confirmed by SEM, TEM and BET characterization techniques. Furthermore, the silica nanotubes showed enantioselectivity after the extraction of the chiral N-stearoyl-L-serine template, as demonstrated by the selective adsorption of the serine and valine enantiomers in the CD and ITC experiments. These chiral silica nanotubes, having a high surface area and high mechanical and thermal stability, are suitable for various chiral chemistry uses, such as stationary phases in chromatography. The impact of chirality on almost every chemical process is well known and has significant consequences in many fields of chemistry. Therefore, there is a need for efficient strategies for the preparation of new chiral materials. In this work, we have described a new method for the preparation of chiral silica with nanotube morphology and demonstrated its enantioselectivity. Although the enantioselectivity parameters of the chiral silica nanotubes are not very high (ca. 6% ee), we have demonstrated a general principle for the preparation of chiral silica nanotubes, and future studies focusing on the optimization of our process can lead to significant improvement in the enantioselectivity parameters. Silica with nanotubes morphology can offer some interesting advantages relative to common spherical silica used in chiral HPLC. For example, the large inner volumes of silica nanotubes which can be filled with any desired chiral small molecules or peptides and proteins increasing the chiral separation capacity. In addition, silica with nanotubes morphology has distinct inner and outer surfaces that can be functionalized with different chirality. It is clear that, in the future, a variety of new chiral porous materials will be developed, and our work is part of the effort to develop this methodology. Apart from the chiral nature of the silica,


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the sol-gel process described in this article may be applied to the preparation of other materials with nanotube structure, leading to many applications in other fields of chemistry, such as catalysis, surface science and nanomaterials science. ASSOCIATED CONTENT Supporting information Synthetic schemes and NMR results of stearic acid N-hydroxysuccinimide ester and C18Ser, and characterization of As-SiO2-C18Ser and Ex-SiO2-C18Ser (TEM image, EDS, nitrogen isotherms and FTIR). AUTHOR INFORMATION Corresponding author *E-mail: [email protected]. Fax: +972 37384053. Tel: +972 3 5317681. ACKNOWLEDGEMENT G. Levi would like to acknowledge Dr. Yochai Basel for his help and support during the research project. G. Levi acknowledges the Bar-Ilan Department of Chemistry for the funding. This research was supported by the Israel Science Foundation (grant No. 660/07) and by the Ministry of Science, Technology and Space. The authors dedicate this article in memory of our colleague, Prof. Meir Shinitzky, Weizmann Institute of Science, who passed away in December 2015. REFERENCES (1)

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For Table of Contents Only


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Overall synthesis of imprinted chiral silica nanotubes based on the use of a chiral N-stearoyl L-serine anionic surfactant as the chiral template. 133x74mm (300 x 300 DPI)


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Imprinting Chirality in Silica Nanotubes by N-Stearoyl-serine Template

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