A Nonconventional Model of Protocell-like Vesicles: Anionic Clay

Article pubs.acs.org/Langmuir

A Nonconventional Model of Protocell-like Vesicles: Anionic Clay Surface-Mediated Formation from a Single-Tailed Amphiphile Na Du, Ruiying Song, Haiping Li, Shue Song, Renjie Zhang, and Wanguo Hou* Key Laboratory of Colloid and Interface Chemistry (Ministry of Education), Shandong University, Jinan 250100, P.R. China S Supporting Information *

ABSTRACT: We report a novel model system of precursor cellular membranes, self-assembled from micellar solution of a common anionic single-tailed amphiphile (STA), including sodium dodecyl sulfate (SDS) and sodium dodecylbenzenesulfonate (SDBS). The self-assembly process was mediated with solid surfaces of Mg2Al-CO3 hydrotalcite-like compound (HTlc), an anionic clay, in the absence of cosurfactants or any additives. The resultant STA vesicles were characterized using negative−staining and cryogenic transmission electron microscopies, as well as dynamic light scattering and steady state fluorescence techniques. Interestingly, the obtained STA vesicles displayed good stability even after the removal of the anionic clay surface (ACS), and a self-reproduction phenomenon was observed for the “preformed” STA vesicles when mixing with corresponding STA micellar solutions. More importantly, the micelle-to-vesicle transition for SDS could be still arisen in high-salinity artificial seawater under the ACS mediation. Instead of conventional fatty acid scenario, our finding provides another novel possible model for protocell-like vesicles, which are easily formed under the plausible prebiotic conditions.

1. INTRODUCTION The bilayer membranes that are similar to modern cellular boundaries are postulated to have been abiotically formed and originated in the spontaneous self-assembly of amphiphlic molecules on the prebiotic Earth, providing compartmentalization for the origin of life.1−3 In contrast to the phospholipidbased membranes of all modern cells, early cell membranes are thought to be composed of simple single-tailed amphiphiles (STAs).4 Most studies on protocell structures formed from potentially prebiotic amphiphiles have so far focused on fatty acid vesicles.1,5,6 This is because fatty acids are readily synthesized under prebiotic natural conditions,7 in particular, those found in Martian meteorites.8−10 Nevertheless, the hypothesis favoring primordial vesicle formation by fatty acids faces important difficulties,11 i.e., their apparent lack of stability to changes in pH and osmotic pressure,12 as well as their relatively high critical aggregation concentrations (cac).13 In fact, owing to the presence in Martian meteorites as a significant piece of evidence, alkyl phosphates, alkyl sulfates, and alkyl sulfonates have also been proposed as possible constituents of early membranes.8−10 Except for the alkyl phosphates case,14 no reports have focused on the spontaneous formation of vesicles from these simple STAs to explore the formation mechanism of protocell structures in the absence of cosurfactants or additives. There have been a few reports involving STA vesicle formation, but they either introduced photocatalytic reactions15 or employed ionic liquids16 in the systems, or used STAs with special structures.17 Commonly, simple STAs, such as sodium dodecyl sulfate (SDS) and sodium dodecylbenzenesulfonate (SDBS), are considered to be © 2015 American Chemical Society

unable to form vesicles in aqueous solution in the absence of cosurfactants or any other additives, even in chemical textbooks.18 More recently, we found an interesting phenomenon that rough glass surfaces can promote an STA spontaneously forming vesicles without any additives.19,20 This inspired us to explore the probability of STA vesicle formation mediated by clay mineral surfaces, which played an active role in the abiotic origin of life.21 In this work, we chose two common anionic STAs, SDS and SDBS, as model constituents of early membranes rather than fatty acids, to establish a tentative model for protocell structures. The anionic clay Mg2Al-CO3 hydrotalcite-like compound (HTlc)22 which immobilized on a solid (aluminum and glass) substrate was chosen to simulate as clay minerals. It was found that clay minerals may play an active role in the abiotic origin of life. Under the mediation of anionic clay surfaces (ACSs), STA molecules (certainly simpler than the glycerophospholipids found in cell membranes) can selfassemble into closed bilayers, providing a possible precursor of present-day cellular membranes. In particular, these STA vesicles could be formed even in high-salinity artificial seawater. Indeed, the STA vesicles could be a good model to explore the origin of life, without disregarding other potential applications such as in medical, chemical and biotechnological domains. Received: May 11, 2015 Revised: October 26, 2015 Published: November 2, 2015 12579

DOI: 10.1021/acs.langmuir.5b03477 Langmuir 2015, 31, 12579−12586

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used for the measurements. The obtained data were analyzed by CONTIN.27 Steady State Fluorescence Measurements. Steady state fluorescence readings were performed on a fluorescence spectrometer (PerkinElmer LS 55, USA) with 25 μM Laurdan. General polarization values were calculated from emission intensities at 500 and 430 nm upon excitation at 374 nm. All measurements were taken at 25 ± 0.5 °C.

2. EXPERIMENTAL SECTION 2.1. Chemicals. SDS (≥99.0% purity, Sigma) and SDBS (>95.0% purity, TCI) were recrystallized three times from ethanol/acetone and methyl alcohol before use,23 respectively. The fluorescent probe Laurdan (6-dodecanoyl-N,N-dimethyl-2-naphthylamine, ≥ 97.0% HPLC, Sigma) was used as received. Ultrapure water was obtained using a Hitech-Kflow water purification system (Hitech, China). The ACSs were prepared by the in situ growth of HTlc crystals on aluminum sheets with a size of ca. 4 cm × 4 cm × 1 mm and on glass pieces with a size of 4 cm × 3 cm × 1.5 mm (Figure S1 in the Supporting Information (SI)),24,25 noted as ACS-Al and ACS-glass, respectively. The artificial seawater was prepared according to ref 26, with a salinity of 36.5 g/L, containing 488 mM sodium chloride, 19.0 mM magnesium chloride, 7.2 mM magnesium sulfate, 7.6 mM calcium sulfate, 4.8 mM potassium sulfate, 0.86 mM potassium bromide, and 0.45 mM boric acid. 2.2. ACS-Mediated Formation of Vesicles. Separate micellar solutions (20 mM) of SDS and SDBS were prepared by dissolving the surfactants in ultrapure water. An ACS-substrate was immersed in 50 mL of each micellar solution, in a well-sealed plastic tube. All of the samples were shaken in a thermostatic water bath shaker (Jiangsu Medical Instrument Factory, China) for a given time, at 25 ± 0.5 °C. The vesicle formations in the aqueous solutions were examined before and after the removal of the ACS. 2.3. Characterization of Samples. Scanning Electron Microscope (SEM) Observations. The morphology of the ACSs was observed using a JEOL JSM-6700F field emission SEM (JEOL, Japan) operating at 15 kV. The specimens were coated with a layer of gold before SEM observations. Atomic Force Microscope (AFM) Observations. The morphology of the ACSs was observed using a Nanoscope IIIa multimode AFM (Digital Instruments Corp., USA). X-ray Diffraction (XRD) Determinations. XRD patterns were recorded with a D/max-rA model diffractometer (Bruker AXS, Co., Ltd., Germany) with Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 40 mA in the 2θ range of 10−70° with a scanning step of 0.08°/min. Water Contact Angle (θw) Measurements. The θw on the ACSs and the substrate surfaces were measured using a DSA10 contact angle goniometer (Krüss, Germany). A droplet of ultrapure water (∼30 μL) was carefully placed on the solid surface using a syringe. Photographs of the droplet were then recorded after equilibrium had been reached (∼2 min) to obtain the θw values. Each θw value was the average of three measurements on different locations of a given surface. Transmittance (Tr) Measurements. An ultraviolet−visible (UV− vis) spectrometer (Hewlett-Packard 8453, Germany) was employed for Tr measurements, at 25 ± 0.5 °C and a wavelength of 510 nm. Sample was placed in a cell covered with a plastic cap to prevent evaporation. The reference solution is water or artificial seawater, depending on the composition of the studied systems. Negative-Staining (NS) TEM Observations. The negative-staining (NS, with uranyl acetate ethanol solution) technique was used to prepare the TEM samples. Observations of the vesicle morphology were performed using a JEM-1011 TEM (JEOL, Japan). Cryogenic (Cryo-) TEM Observations. Aggregate structures in the surfactant solution were determined by cryo-TEM. The samples were prepared in a controlled-environment vitrification system (Cryoplunge TM3, USA) at 25 °C at 95% relative humidity. A micropipette was used to load a 1 mL aliquot of solution onto a copper grid coated with carbon film. The excess solution was blotted with two pieces of filter paper, resulting in the formation of thin films suspended on the mesh holes. After 10 s, the grid, with sample, was quickly plunged into liquid ethane cooled by liquid nitrogen. The vitrified sample was transferred to a cryogenic specimen holder (Gatan 626), and examined on a JEOL JEM 1400 TEM operating at 120 kV. Dynamic Light Scattering (DLS) Measurements. DLS was carried out at scattering angle of 90°. A standard laser light scattering spectrometer (Brookhaven, England) equipped with a coherent radiation 200 mW diode pumped solid-state 488 nm laser and a Brookhaven Instruments Corporation (BI-9000AT) correlator were

3. RESULTS AND DISCUSSION 3.1. Feature of ACSs. SEM observations reveal that the surface of original aluminum and glass substrates both have quite smooth surfaces (Figure S2 in the SI). As expected, a large amount of hexagonal plate-like HTlc crystals with a lateral size and a thickness of ∼700 and 75 nm respectively, are observed on the ACS-Al and ACS-glass by SEM (Figure 1a, b).

Figure 1. (a,b) SEM and (c,d) AFM images of (a,c) ACS-Al and (b,d) ACS-glass.

The HTlc structure of the crystals is confirmed by XRD for their powders scraped from the substrates (Figure S3 in the SI). The coated amounts of HTlc on an aluminum sheet and a glass piece were determined to be ∼9.5 and 1.1 mg, respectively. Owing to the fact that they are perpendicularly attached to the substrate surfaces via their edges (Figure S4 in the SI), the hexagonal anionic clay platelets are assumed to be in situ grown onto the substrates via a strong chemical interaction.25 The HTlc crystals are interlaced with each other, producing a rough solid surface (or HTlc film). The root-mean-square roughness (Rq) of the ACS-Al and ACS-glass, estimated from AFM images (Figure 1c, d), are 80.5 and 72.9 nm, respectively. Their θw values were measured to be 0° (Figure S5 in the SI), indicating their strong hydrophilic feature. 12580

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Figure 2. Negative-staining TEM images of vesicles induced by (a,c) ACS-Al and (b,d) ACS-glass in (a,b) SDS and (c,d) SDBS solutions. Scale bar: 500 nm.

Figure 3. Cryo-TEM images of vesicles induced by (a,b,e) ACS-Al and (c,d,f) ACS-glass in (a−d) SDS and (e,f) SDBS solutions.

3.2. ACS-Mediated Formation of Vesicles. After the ACSs were immersed in 20 mM SDS and SDBS micellar solutions at ∼25 °C for 4 days, significant formation of vesicles was observed by NS- and cryo-TEM, as shown in Figures 2 and 3. This indicates that a micelle-to-vesicle transition occurred in the STA solutions under the mediation of the ACSs. The mean sizes of the so-formed SDS and SDBS vesicles are ∼150 and 450 nm, respectively, measured from the TEM images. The change of the Tr of the STA systems upon ACSmediated time (tm) was monitored, as shown in Figure 4. The Tr values for all the studied systems decrease rapidly during the initial 3 days, then remain in equilibrium. Different from the

ACS-mediated systems, no change in Tr for the STA solutions in the absence of ACSs was observed. These indicate that, under the mediation of the ACSs, the STA vesicles are formed gradually until a vesicle−micelle phase-equilibrium is reached. The DLS data also confirm the ACS-mediated formation of vesicles in the simple STA solutions (Figure 5). Before addition of the ACSs, the DLS plots of the SDS and SDBS solutions appear as single peaks at hydrodynamic diameters (Dh) of 3.8 and 2.7 nm, respectively, which are typical values for a micellar system. After the ACSs were added into the STA solutions for 7 days, a new peak at a Dh value of 150−210 nm for the SDS system and 440−485 nm for the SDBS system was observed, 12581

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intensities (excitation 374 nm) at 430 and 500 nm, respectively.26−32 A decrease in the surface curvature of aggregates, such as a micelle-to-vesicle transition, is commonly accompanied by a decreased GP value, arising from a decreased solvated state of the dye. Figure 6 shows the fluorescence emission spectra of the STA solutions before and after the ACS meditation, and a characteristic change in Laurdan emission intensities was observed. The GP values of the two STA solution systems were calculated, both showing a decrease over the mediation of ACS; the GP value for the SDS solution decreases from 0.88 to 0.72 and that for the SDBS solution decreases from 0.54 to 0.47. This is indicative of the micelle-tovesicle transition that occurred in the STA solutions under the ACS meditation. Evidently, the above results indicate that, through the mediation of ACSs, STA vesicles can be formed in their micellar solutions without any additives. Indeed, this is contrary to the conventional understanding of the STA alone being unable to form vesicles in water. To understand whether the substrates themselves or the soluble substances leached from the ACSs might induce the vesicle formation, two verification tests were performed. In the first one, we immersed the original substrates (aluminum sheets and glass pieces) in the STA micellar solutions for 6 days. For the other one, we prepared the STA micellar solutions using ultrapure water in which the ACSs had been soaked for 6 days. No vesicle aggregates were detected in these systems by turbidity measurements (Figure S6 in the SI), TEM, and DLS. In addition, after being immersed in the STA solutions over 10 days, no obvious change in the morphology of the ACS-Al and ACS-glass was observed by SEM (Figure S7 in the SI). These results prove that the formation of the STA vesicles is induced solely by the ACSs, not by the substrates or any possibly leached substances. Notably, the so-obtained STA vesicles exhibit good stability; even after the removal of ACSs, no significant change in the vesicle phase were determined by TEM during a storage period of six month at room temperature (Figure S7 in the SI). 3.4. STA Vesicles in Artificial Seawater. As suggested by Knauth,33 the sea of the prebiotic Earth may be 1.5−2.0 times

Figure 4. Turbidity behavior of 20 mM STA solutions upon time in the presence of ACS (λ = 510 nm).

along with a significant decrease in the micelle−peak intensities. The vesicle size determined by DLS is comparable to those obtained by TEM observation. Notably, both micelle and vesicle peaks coappear in the DLS plots, indicating that the micelles and vesicles coexisted in the solutions. Such solutions containing both micelles and vesicles simultaneously have been reported in the literature.28 In addition, no significant difference in the vesicle formation meditated by the ACS-Al and the ACS-glass were observed, indicating that the substrates have no obvious influence on the vesicle formation. This is because both of them were fully covered with HTlc crystals and have close Rq values. To further verify the micelle-to-vesicle transition occurring at the microscopic level, steady state fluorescence measurements were performed for the STA solutions before and after the ACS meditation, using Laurdan as the probe. Laurdan is a C12 fatty acid analogue with a fluorescent naphthalene derivative, and its emission spectrum is sensitive to the polarity of its environment.29,30 Laurdan can be used to monitor structural changes in aggregates,29,31 and its emission intensities may vary with the aggregate structures changing, which can be identified by using a unitless Generalized Polarization (GP) parameter: GP = (I500 − I430)/(I500 + I430), where I430 and I500 are the emission

Figure 5. Hydrodynamic diameter distributions of (a,b) SDS and (c,d) SDBS solutions induced by (a,c) ACS-Al and (b,d) ACS-glass. 12582

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Figure 6. Laurdan emission spectra of (a) SDS and (b) SDBS solutions before and after the ACS meditation. Asterisks indicate peaks whose emission intensities are used to calculate GP.

strongly sensitive to the ionic content of the aqueous solution where they form, precipitating at salt concentration values well below those estimated for primitive ocean conditions.13 Therefore, SDS vesicles seem to be a better alternative model for exploring the origin of life. 3.5. Possible Formation Mechanism of STA Vesicles. There remains a basic question needed to be answered: why can the ACSs mediate the formation of STA vesicles? We cannot give an accurate explanation now, but without doubt, it should be related to aggregation of STAs on the solid surface. A possible mechanism for this phenomenon is discussed as follows. It is well-known that the self-assembly morphology of amphiphilic molecules is determined by their geometric parameters, which is commonly described using the wellknown molecular packing parameter (P) defined as35 P = v0/ asl0, in which v0 is the amphiphile tail volume, l0 is the tail length, and as is the occupied area per amphiphile at the aggregate surface. As a general rule, spherical micelles are favored when P ≤ 1/3, cylindrical micelles when 1/3 ≤ P ≤ 1/ 2, and bilayers or vesicles are favored only when 1/2 ≤ P ≤ 1.36 Simple STAs in aqueous solutions commonly exhibit large as values, resulting in their P values being less than 1/2. Therefore, the idea that the STA alone cannot form vesicles has been spawned. The large as values of simple STAs arise from the hydration of the head groups and the electrostatic repulsion between the head groups. Strong adsorption of solid surfaces for STAs via electrostatic interaction, hydrogen bonding, and/ or van der Waals force can significantly decrease the hydration of the head groups and obstruct the electrostatic repulsion between the head groups, resulting in a lower as value in the surface aggregates of STAs. For example, the as values of SDS and SDBS have been reported to be ∼0.53−0.66 and 0.59− 0.60 nm2, respectively, for their assemblies at gas/liquid and

more salty than now. So, it is not representative of the early oceans to simply use ultrapure water or sodium chloride solutions. Therefore, we explored the formation of STA vesicles in an artificial seawater26,34 to simulate the prebiotic chemistry environment. Interestingly, we found that, with the mediation of the ACSs, SDS vesicles can also form in the artificial seawater, as evidenced by turbidity and NS-TEM determinations (Figures 7 and 8). Different from SDS, precipitation of

Figure 7. Turbidity behavior of 20 mM SDS artificial seawater solutions upon time in the presence of ACS (λ = 510 nm).

SDBS appeared in the artificial seawater. Comparing to the SDS vesicles in ultrapure water, the quantity of them formed in the artificial seawater seems decreased evidently. Although the exact reason for this result is not clear now, we speculate that it might be because the high salinity of the artificial seawater inhibits the adsorption or enrichment of SDS molecules on the ACSs, which reduces the transition from micelles to vesicles. As we know, the conventional protocell model fatty acid vesicles are

Figure 8. Negative-staining TEM images of vesicles induced by (a) ACS-Al and (b) ACS-glass in SDS solution. Scale bar: 200 nm. 12583

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Figure 9. Schematic illustration of the ACS-mediated formation of STA vesicles.

Figure 10. Hydrodynamic diameter distributions of the mixtures of the preformed STA vesicle solutions and the STA micellar solutions with a volume ratio of 1:3 after standing for 1 and 40 days. (a,b) SDS system, (c,d) SDBS system; the preformed vesicles induced by (a,c) ACS-Al and (b,d) ACS-glass.

liquid/liquid interfaces.37−40 SDS and SDBS have the same l0 and v0 values of 1.67 nm and 0.35 nm2.41 The P values of SDS and SDBS are thus estimated to be ∼0.40−0.32 and 0.36−0.35, respectively, at gas/liquid and liquid/liquid interfaces. These low P values suggest that both SDS and SDBS alone in water are favorable for forming micelles. However, the as values of SDS and SDBS in adsorption bilayers on HTlc surfaces have been determined to be ∼0.41 and 0.34 nm2,39,40 yielding P values of 0.51 and 0.61, respectively. This suggests that the adsorbed bilayers of STAs on solid surfaces are most likely the precursors of vesicle formation. Based on the above discussion, we propose a possible mechanism for the ACS-mediated formation of STA vesicles (Figure 9). After ACSs are immersed into STA solutions, the STA micelles and monomers adsorb on the rough ACSs (or the surface of HTlc), forming curved bilayers.42 The borders of such bilayers are normally of higher energy, which can be decreased when the ends of the bilayers merge to form vesicles.43 Thus, the curved bilayers are detached from the ACSs, subsequently, can close to form vesicles to decrease the interfacial energy of the aggregates. With the repetition of this process, the micellar phase is gradually transformed into a vesicle phase, until a dynamic equilibrium is achieved between the two phases. It is most likely that the transition from STA micelles to vesicles without external interferences poses a high free-energy barrier, but the mediation of solid surfaces can overcome this barrier and then make the transition happen.

Once the STA vesicles are formed, they can coexist with the micelles stably in the solution. As we know, fatty acid vesicles exhibit a self-reproduction phenomenon,44 which may play a crucial role in the abiotic origin of life. The possible self-reproduction for the SDS and SDBS vesicles formed by ACS mediation was explored. After removing the ACSs, the “preformed” STA vesicle solutions were mixed with the corresponding STA micellar solutions, with a 1:3 volume ratio of vesicle solutions to micellar solutions. The micelle-to-vesicle transition possibly occurring in the systems was determined by DLS. Surprisingly, after standing for 40 d at room temperature, we found that the vesicle peak intensity of the mixed systems increases significantly, accompanied by an obvious decrease in the micelle peak intensity (Figure 10). This indicates that the selfreproduction of the STA vesicles occurred in the mixed systems, just like that observed in fatty acid vesicle systems.44 Additionally, six months after the removal of ACSs, NS-TEM observations for the STA vesicle systems seemingly exhibit growth and division phenomena (Figure S8 in the SI), which is considered to be a typical self-reproduction procedure. However, the reason for this self-reproduction phenomenon remains to be addressed.

4. CONCLUSION Cellular life must have started with a much simpler organization than that found in even the simplest extant organisms. Studying 12584

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(7) Simoneit, B. R.; Rushdi, A. I.; Deamer, D. W. Abiotic formation of acylglycerols under simulated hydrothermal conditions and selfassembly properties of such lipid products. Adv. Space Res. 2007, 40, 1649−1656. (8) Cronin, J. R. Clues from the origin of the solar system: meteorites. Molecular Origins of Life 1998, 119−146. (9) Ourisson, G.; Nakatani, Y. Addendum: Origins of cellular life: Molecular foundations and new approaches. Tetrahedron 1999, 55, 3183−3190. (10) Pizzarello, S. The chemistry of life’s origin: A carbonaceous meteorite perspective. Acc. Chem. Res. 2006, 39, 231−237. (11) Thomas, J. A.; Rana, F. The influence of environmental conditions, lipid composition, and phase behavior on the origin of cell membranes. Origins Life Evol. Biospheres 2007, 37, 267−285. (12) Zhu, T. F.; Szostak, J. W. Exploding vesicles. J. Syst. Chem. 2011, 2, 4. (13) Ruiz-Mirazo, K.; Briones, C.; de la Escosura, A. Prebiotic Systems Chemistry: New Perspectives for the Origins of Life. Chem. Rev. 2014, 114, 285−366. (14) Sakai, T.; Miyaki, M.; Tajima, H.; Shimizu, M. Precipitate deposition around CMC and vesicle-to-micelle transition of monopotassium monododecyl phosphate in water. J. Phys. Chem. B 2012, 116, 11225−11233. (15) Griffith, E. C.; Rapf, R. J.; Shoemaker, R. K.; Carpenter, B. K.; Vaida, V. Photoinitiated Synthesis of Self-Assembled Vesicles. J. Am. Chem. Soc. 2014, 136, 3784−3787. (16) Wang, H.; Zhang, L.; Wang, J.; Li, Z.; Zhang, S. The first evidence for unilamellar vesicle formation of ionic liquids in aqueous solutions. Chem. Commun. 2013, 49, 5222−5224. (17) Roy, A.; Maiti, M.; Roy, S. Spontaneous Formation of Vesicles by Sodium 2-Dodecylnicotinate in Water. Langmuir 2012, 28, 12696− 12703. (18) Alberty, R. A. Physical Chemistry; Wiley: New York, 1987. (19) Du, N.; Song, R.; Zhu, X.; Hou, W.; Li, H.; Zhang, R. Vesicles composed of one simple single-tailed surfactant. Chem. Commun. 2014, 50, 10573−10576. (20) Zhu, X.; Du, N.; Song, R.; Hou, W.; Song, S.; Zhang, R. Rough Glass Surface-Mediated Formation of Vesicles from Lauryl Sulfobetaine Micellar Solutions. Langmuir 2014, 30, 11543−11551. (21) Brack, A. Clay Minerals and the Origin of Life. In Developments in Clay Science, Faïza, B., Gerhard, L., Eds.; Elsevier: Amsterdam, 2013; Vol. 5, Chapter 10.4, pp 507−521. (22) Duan, X.; Evans, D. G. Layered Double Hydroxides; Springer Science & Business Media: Berlin Heidelberg New York, 2006; Vol. 119. (23) Zhai, L.; Zhao, M.; Sun, D.; Hao, J.; Zhang, L. Salt-Induced Vesicle Formation from Single Anionic Surfactant SDBS and Its Mixture with LSB in Aqueous Solution. J. Phys. Chem. B 2005, 109, 5627−5630. (24) Lü, Z.; Zhang, F.; Lei, X.; Yang, L.; Xu, S.; Duan, X. In situ growth of layered double hydroxide films on anodic aluminum oxide/ aluminum and its catalytic feature in aldol condensation of acetone. Chem. Eng. Sci. 2008, 63, 4055−4062. (25) Guo, X.; Zhang, F.; Xu, S.; Evans, D. G.; Duan, X. Preparation of layered double hydroxide films with different orientations on the opposite sides of a glass substrate by in situ hydrothermal crystallization. Chem. Commun. 2009, 6836−6838. (26) Farias, A. P. S.; Tadayozzi, Y. S.; Carneiro, C. E.; Zaia, D. A. Salinity and pH affect Na+-montmorillonite dissolution and amino acid adsorption: a prebiotic chemistry study. Int. J. Astrobiol. 2014, 13, 259−270. (27) Provencher, S. W. CONTIN: A general purpose constrained regularization program for inverting noisy linear algebraic and integral equations. Comput. Phys. Commun. 1982, 27, 229−242. (28) Yin, H.; Lei, S.; Zhu, S.; Huang, J.; Ye, J. Micelle-to-Vesicle Transition Induced by Organic Additives in Catanionic Surfactant Systems. Chem. - Eur. J. 2006, 12, 2825−2835.

the formation of bilayer structures from simple amphiphiles helps to understand the origin of life. The current work shows that, under the mediation of ACSs, a simple STA, such as SDS and SDBS, can form vesicles from its micellar solution without any additives. Importantly, the STA vesicles show good stability even after the removal of the ACSs. Especially, the SDS vesicles can form even in high-salinity artificial seawater. Furthermore, the preformed STA vesicles can exhibit self-reproduction when mixing with corresponding STA micellar solutions. These results demonstrate that SDS vesicles may be a better alternative model of precursor cellular membranes in comparison with the conventional model fatty acid vesicles, and that clay mineral surfaces may play an active role in the abiotic origin of life. This work may shed some light on the establishment of the model systems of early cell membranes for exploring the origin of life, but many essential questions concerning the nature of these protocell-modeled STA vesicles, such as why the STA vesicles can exist stably and what the mechanism of self-reproduction of the STA vesicles is, still need to be addressed.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03477. Experimental procedures, characterization of samples, TEM and SEM images, XRD pattern and water contact angle measurements (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel: +86-531-88564242; Fax: +86-531-88564750. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported financially by the National Natural Science Foundation of China (Nos. 21173135, 21273135 and 21403128), and the Foundation for Outstanding Young Scientist in Shandong Province (No. 2014BSE27153).

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b03477 Langmuir 2015, 31, 12579−12586

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DOI: 10.1021/acs.langmuir.5b03477 Langmuir 2015, 31, 12579−12586