Article pubs.acs.org/JPCC
Growing Inorganic Membranes in Microfluidic Devices: Chemical Gardens Reduced to Linear Walls Bruno C. Batista and Oliver Steinbock* Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, United States S Supporting Information *
ABSTRACT: The hollow precipitate tubes in chemical gardens conserve the nonequilibrium conditions present during their formation and are an important example of molecular processes causing complex macroscopic self-organization. We report a greatly simplified experimental model of these structures that is based on the formation of an inorganic membrane in a microfluidic device. Within this device, we induce the precipitation of Mn(OH)2 and other metal hydroxides at the reactive interface of steadily injected NaOH and MnCl2 solutions. The resulting precipitate wall extends along the entire length of the reactor channel and can be positioned at will, and its width increases strictly in the direction of the metal solution. These thickening dynamics obey a square root law. The corresponding effective diffusion coefficient is proportional to [OH−], shows a sigmoidal dependence on [Mn2+], and also depends on the precipitating metal ion. The precipitate wall is permeable to methylene blue and strongly adsorbs methyl orange. Electron and optical microscopy reveals decaying micrometer-sized perturbations and a 40 μm thick gel-like layer on the surface exposed to the Mn2+ solution. The wall growth is also followed by in situ Raman spectroscopy. Potential applications toward materials and origins-of-life research are discussed.
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sodium silicate solution (waterglass).22−25 The resulting, upward growing tubes can reach lengths of several centimeters and have diameters of about 1 mm.23−25 The tubes are always hollow, and their thin walls consist of metal hydroxides or oxides with a silica-rich outer layer.23 Numerous other chemical reactions produce similar structures including precipitation reactions, yielding insoluble carbonates, phosphates, sulfides, and borates as well as polyoxometalates.26−30 Tube formation based on similar mechanisms is also observed in corrosion systems, during the setting of cement, and under ice sheets in very cold seawater.31−33 Another naturally occurring example of chemical-garden-like tubes are the large chimney structures at hydrothermal (off-axis alkaline) vents that on early Earth provided an ideal environment for prebiotic chemistry and possibly the beginning of life, which in its own right justifies a thorough investigation of these precipitates.34−36 Beyond the exciting relevance to origins-of-life research, modern studies of the wall material have revealed a broad spectrum of potential applications in materials science and engineering.37,38 Examples include uses as powerful Brønsted acid catalysts, photocatalysts, inexpensive materials for the absorption of pollutants, and platforms for the inclusion of polymer microbeads, quantum dots, as well as cells.39−43 In addition, postsynthetic modifications of certain tubes have been demonstrated, which include the heating of the structures to 900 °C without loss of the tubes’ macroscopic integrity.44 Another clear target for applications is the nonlithographic
INTRODUCTION Complex systems far from the thermodynamic equilibrium reveal a wealth of spatiotemporal phenomena that range from deterministic chaos to traveling chemical waves and Turing patterns.1 For chemists and engineers these dynamic states offer tremendous possibilities for the design of materials and devices capable of intricate responses to external stimuli, self-healing, and other interesting dynamics.2−8 Moreover, they can show hierarchical spatial structures spanning many orders of magnitude from the molecular scale up to a length scale of several millimeters.9 Foremost, however, they might hold the key to a new paradigm under which materials and devices are neither engineered from bulk materials nor assembled in serial steps but rather grown in a life-like fashion.10,11 To come closer to this ambitious goal, scientists in the field of chemical selforganization have renewed their efforts to understand and control pattern formation in reactions that create solid products. These systems create permanent structures that reflect the complex disequilibrium processes long after the system has reached its equilibrium state.12 Examples include Liesegang patterns, spiral-shaped precipitation waves, polymers produced in self-propagating reaction zones, complex inorganic biomorph structures, and tubular objects in chemical gardens.13−23 Most of the latter structures are based on purely inorganic precipitation reactions. During the past decade, their investigation has gained broad interest creating a new research field called chemobrionics.22 Among its most iconic examples are chemical gardens. In the classical version of this experiment, chemical gardens are grown from macroscopic seed particles (metal salts excluding group(I) elements) that are placed into a © 2015 American Chemical Society
Received: September 9, 2015 Revised: October 25, 2015 Published: November 10, 2015 27045
DOI: 10.1021/acs.jpcc.5b08813 J. Phys. Chem. C 2015, 119, 27045−27052
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The Journal of Physical Chemistry C production of microfluidic networks in which tubes could have diameters of down to 1 μm and consist of a wealth of different substances and mesoscopic building blocks. Thouvenel-Romans and Steinbock, for instance, demonstrated the successful connection of two glass capillaries with a hollow precipitation tube.45 Later Cronin et al. developed a method for the directed growth of microtubes based on computer-controlled holography that exploits the sensitivity of the reaction zone to fluid flow. The latter authors also reported the formation of controlled junctions between different tubes.46 All of these applications clearly require sufficiently powerful means of external control capable of shaping the propagating reaction zones to yield desired radii and orientations of the individual tubes as well as tailored connectivities among them. The development of such control strategies is not only of practical importance but also crucial for improving our quantitative understanding of the involved processes. Experiments that advance this understanding typically simplify the reaction conditions as exemplified by microgravity experiments that eliminate all buoyancy-induced forces.47 In this context, an important advancement has been the substitution of the seed particle with corresponding solutions.45,48,49 The solutions are injected into the waterglass at controlled pump rates, thus eliminating transients due to the dissolution of the finite seed particle. Furthermore, the characteristics of both the inner and the outer reactant solutions are fully known (at least near the injection nozzle), which include concentrations, densities, and viscosities. Key results obtained from this approach include the identification of distinct, buoyancy-dependent growth regimes and the description of the radii of jetting tubes by a hydrodynamic model.45,48 More recently, our group also showed that tube growth does not require the presence of silicate, carbonates, or similar species but can occur when hydroxide solution is injected into various metal salt solutions.50 These experiments offer a reduced chemical complexity as they eliminate polymerization reactions and decrease the number of reactions to essentially the acid−base neutralization and the formation of metal hydroxide. In 2014, the physical aspects of this push toward simpler model systems were further developed by Haudin et al., who studied injection-grown chemical gardens in spatially confined Hele−Shaw cells.51,52 These quasi-two-dimensional chemical gardens revealed a wealth of interesting morphologies including channel-like but erratically turning filaments that resemble the original tubes and seemingly softer perhaps gel-like patterns featuring lobes and thin hair-like structures.51,52 In this article, we present a novel methodology that continues these efforts to simplify the physicochemical complexity of the precipitation patterns down to their most basic building blocka linear wall. Our experiments are carried out in microfluidic devices, provide direct insights into the thickening dynamics of the precipitation wall, and should also prove valuable for systematic studies of possible prebiotic reactions in porous precipitate walls exposed to steep concentration gradients.
Figure 1. (a) Schematics of the experimental setup. A parafilm membrane cut to the desired channel shape is assembled between two acrylic plates. Two inlets in the upper plate allow the injection of solutions of NaOH (left) and metal salts (right). (b) Photograph of the resulting microfluidic device. In this example, a Mn(OH)2 precipitate membrane is created at the interface between the solutions. See also movie 1 in the SI.
plate and four holes with a diameter of 4.0 mm are drilled into both plates. Barb fittings (NResearch Inc., FITM 331) are glued onto the smaller holes using an epoxy adhesive. The three layers are then assembled and secured using four screws and nuts (Figure 1b). The membrane is cut to the desired shape with a razor blade orin our experimentswith an inexpensive electronic cutting tool (Silhouette Portrait) connected to a personal computer. The cut membrane always consists of one connected piece and hence can be easily transferred to the acrylic support plate. The typical pattern is based on channel-like cut outs with a width of 3 mm and circular cut outs for the screws. After the device production is completed, the barb fittings are connected with tubing (Tygon, inner diameter 1/16”) to a programmable syringe pump (KD Scientific 200). In our experiments, the two syringes contain NaOH solution and MnCl2 (or other metal salt) solutions. Typical pump rates are in the range of 1−32 mL/h per syringe and correspond to average flow velocities between 0.14 and 4.6 cm/s that yield very short transit times of the solutions within the channel. Prior to the experiment, the tubing is filled with water that is pumped through the device at a rate of 32 mL/h. This approach prevents undesired effects that were observed if the reactant solution is pumped directly into the air-filled device (e.g., perturbations due to surface tension and the unsynchronized arrival of the two solutions at the channel junction). To avoid pressure variations caused by the discontinuous detachment of droplets from the device outflow, we guide the solution to a waste container via a small membrane bridge. All reaction and diffusion studies are carried out at room temperature (22 °C). The progress of the precipitation reaction within the device is observed using an inverted microscope (Leica DM IRB, 10× objective) under bright-field illumination. The microscope is connected to a charge-coupled device camera (COHU 2122), and the video signal is digitized using a PC-based frame grabber board (Data Translation DT3155) controlled by HLImage+ +97 software. Additional photos are acquired with a commercial SLR camera. The formed precipitate wall is also characterized using scanning electron microscopy (JEOL 7401 FE-SEM, 30 kV). For this purpose the sample is carefully extracted from the device, rinsed in water, dried under ambient conditions, and finally gold sputtered. In situ spectroscopic measurements of the precipitate are performed using a micro-Raman system at an excitation
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EXPERIMENTAL SECTION All reactions are performed in microfluidic devices that can be readily produced without the specialized equipment required for lithographic methods. As shown in Figure 1a, the device consists of a cut parafilm membrane (approximate thickness 130 μm) sandwiched between two plexiglas plates measuring 4 × 5 cm2 (thickness 1.5 mm). Using a drill press, two or three holes with a diameter of 1.2 mm are created in the top plexiglas 27046
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equal width, and the individual walls are in direct contact with the sharp corners of the respective channel junctions. It is obvious that this approach is not limited to two walls but could be easily extended to numerous walls if such configurations are desirable. It should also be possible to separate the outflow into different channels in a way that allows mass exchange between the resulting streams only through the precipitate membrane. To further demonstrate the performance characteristics of our methodology, we show in Figures 2d and 2e that the precipitate wall can be precisely grown at a predetermined position within the reaction channel. In the following, x denotes the wall position perpendicular to the flow direction, wc is the channel width, Q1 and Q2 are the individual pump rates within a device with two inflow ports, and QT equals Q1 + Q2. Figure 2d shows that x/wc equals Q1/QT as expected from volume conservation in incompressible fluids. The small photos in Figure 2e are representative examples of wall segments positioned by this method. We note that the thickening of the wall also causes small pressure differences between the two reactant streams that follow from Bernoulli’s principle. In addition to osmotic pressure this fluid-dynamical pressure could be used to drive or diminish mass transfer through the precipitate membrane. For this purpose flow rates would be adjusted after the formation of the precipitate. After preparation, the precipitate wall can be easily removed from the opened device and is sufficiently strong to allow us to rinse off residual solution without breakage. The samples also withstand subsequent drying and can be mounted on sample holders for further inspection by scanning electron microscopy. Figure 3 shows representative micrographs of gold-sputtered
wavelength of 633 nm. The setup couples an optical microscope to a Jobin Yvon Horiba LabRam high-resolution spectrometer. The laser spot size is approximately 10 μm. The power at the sample is about 0.6 mW, and the spectral width of the spectrometer is 7.4 cm−1.
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RESULTS AND DISCUSSION Figure 2a shows a microfluidic device with a Y-shaped channel design. In this example, the reactant solutions (MnCl2 and
Figure 2. Engineering precipitate growth. Photographs of Mn(OH)2 membranes grown inside a (a) Y-shaped, (b) curved, and (c) threeinlet channel. The graph in d and the photographs in e demonstrate the possibility of controlling the position x of the membrane along the channel’s width by adjusting the flow rate ratio.
NaOH) are delivered from the two channels in the top portion of the photo and then flow past the junction point of the device through a common channel. Due to the low Reynolds number Re = lρv/η ≈ 300 in this system, the two fluids show no turbulent mixing but rather form a sharp vertical interface that extends near the middle of the channel in the flow direction. In the absence of reactions, such interfaces lose their opposing and steep concentration gradients only very gradually due to slow diffusion fluxes, thus providing an ideal situation for the formation of geometrically simple precipitate walls. Such a wall can be easily discerned in Figure 2a as a brown line which extends from the corner of the mixing point down to the outflow port of the device. In some situations, it might be desirable to produce such precipitate walls in curved channels because such designs allow for long reaction interfaces in compact devices. A simple example of a curved channel design is shown in Figure 2b. Despite the channel curvature (and enhanced Taylor dispersion), the wall forms reliably in the middle of the channel. In addition it is possible to create multiple walls in one channel as demonstrated in Figure 2c. In this example, the device has three inflows delivering NaOH solution in the middle channel and MnCl2 solution in the top and bottom one from three syringes at identical pump rates. The resulting walls partition the reaction channel in subchannels of essentially
Figure 3. Scanning electron micrographs of Mn(OH)2 precipitate membranes grown for 4 h. Scale bars correspond to (a) 200, (b) 100, (c) 10, and (d) 100 μm.
samples revealing striking rectangular shapes of high aspect ratio (Figure 3d). The surface roughness of the samples varies from side to side. We find that the plexiglass-facing surfaces (Figure 3a) are much smoother than the solution-facing surfaces (Figure 3b). There is also evidence that the side exposed to the manganese solution is rougher than the hydroxide side. On the former surface, we typically observe small, micrometer-sized circular holes of unknown origin as shown in Figure 3c, which magnifies the boxed area in Figure 3b. Notice that these micropores do not penetrate across the much thicker wall span. 27047
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for systematic analyses. Most importantly, we observe that in our system the wall growth occurs strictly in the direction of the manganese solution. This finding is in agreement with an earlier result on silica−copper precipitation tubes that were found to grow only in the direction of the interior metal (Cu2+) solution and not into the surrounding basic (silicate) solution.49 In the latter study, the authors discussed the interesting question of whether the growth direction depends on the concentration ratio of the employed interior and exterior reactant species. This scenario follows if one simplifies the process to the simple reaction-diffusion system A + B → C (with appropriate initial conditions). This problem was analyzed theoretically by Gálfi and Rácz, who showed that product formation occurs only in the direction of the reactant species present in lower concentration; however, this possible concentration dependence was not investigated for theobviously more complex tube growth experiments.54 In our setup, precipitate walls form for a wide range of reactant concentrations and concentration ratios. We can therefore perform experiments in which the ratio of initial concentrations [OH−]/[Mn2+] equals values below one. For these conditions, we find that precipitate growth still occurs strictly in the direction of the manganese solution. For instance, for reactant concentrations of [NaOH] = 0.1 M and [MnCl2] = 1.0 M, a reaction time of 4 h resulted in the formation of a 180 μm thick wall, but we did not observe growth in the direction of the hydroxide solution. These observations clearly rule out the above model as the explanation of unidirectional wall growth and suggest that the wall has a very different permeability for hydroxide and metal ions favoring the transport of the negatively charged ion. Figure 5 describes the dependence of the wall growth kinetics on the employed reactant concentrations (see also
As already shown in Figures 1b and 2a, the precipitate wall does not show any noticeable, macroscopic width variations along the channel axis. Closer inspection with an optical microscope, however, reveals that the initial wall is not perfectly straight but slowly “outgrows” initial deviations from a perfectly linear boundary. We emphasize that these initial deformations are small and only rarely exceed an amplitude of 20 μm. A typical example is shown in Figure 4. Figure 4a shows a
Figure 4. Micrographs showing the precipitation front evolution after (a) 1 h and (c) 4 h of growth. Frame (b) magnifies a subsection of (a) revealing a gel-like interface. The frames correspond to (a,c) 500 × 300 μm2 and (b) 100 × 40 μm2. Panel (d) shows the decay of surface modulations at the reactive interface during the course of 3 h.
micrograph of the early wall surface. The bright and dark image regions correspond to the manganese solution and the precipitate, respectively. Notice that the hydroxide side of the wall is further to the left and outside of the image. Figure 4b is a magnified view of the interface, and Figure 4c shows the same area as Figure 4a but after 3 h of continuous reaction in the flow field of the microfluidic channel. The smoothening process of the wall surface is illustrated in Figure 4d, which superimposes several snapshots of the wall surface that is in contact with the manganese solution. Notice that as the wall thickens, concave (convex) perturbations disappear because they grow faster (slower) than the near planar portions. In addition, we find at the same surface a gel-like layer with a width of approximately 40 μm (Figure 4b). This spongy region persists at this thickness throughout the experiment and is not observed at the opposite surface of the precipitate wall. It seems likely that it consists of gel-like manganese hydroxide, and we note that nanostructured manganese dioxide as well as electrochemical supercapacitors can be obtained from similar materials using sol−gel techniques.53 In addition, we observe in our experiments floating submicrometer particles in the manganese-containing subchannel but never on the hydroxide side. These particles can get trapped in the gel-like layer (see movie 2 in the SI). Of particular interest is the quantitative thickening dynamics of the precipitate wall since this process is analogous to the slow radial growth of tubes in conventional chemical gardens.23,49 While the phenomenon is difficult to study in the tube system,49 the simple geometry of our walls combined with the possibility to directly monitor their thickening allows
Figure 5. (a) Time evolution of the membrane’s width as a function of [OH−] for [Mn2+] = 0.5 M = constant, and fitted square root functions (b) Corresponding effective diffusion coefficients Deff as obtained from the fits. (c and d) Same analysis but for [OH−] = 0.5 M = const and seven different [Mn2+]. Numbers at the graphs in a and c are the molar concentrations of OH− and Mn2+, respectively. 27048
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The Journal of Physical Chemistry C movie 3 in the SI). All experiments are carried out at constant pump rates of 2 mL/h. Figure 5a shows the temporal evolution of the wall width w(t) for five different hydroxide concentrations and a constant manganese concentration of 0.5 M. The width was measured at a position 5 mm distant from the point of initial contact. The experimental data are compared to fitted square root functions w(t) = (Deff t)1/2 that assume diffusion-controlled growth kinetics. In particular, for high hydroxide concentrations and the first 2−3 h of reaction, we find very good agreement between the measurements and the fits. During later stages (not considered for fitting), the growth rate slows down slightly and causes deviations. Notice that these deviations do not result from the consumption of reactants as the solutions are steadily refreshed within the microfluidic device. The values of the fitting coefficient Deff increases with the employed concentration of hydroxide ions (Figure 5b), which for equal reaction times results in precipitate walls of noticeably different thickness. This concentration dependence and the overall small values indicate that Deff is not the diffusion coefficient of a molecular species but rather an effective diffusion coefficient that involves kinetic factors. We specifically find the proportional dependence Deff = δ [OH−] with δ = 5.4 ± 0.8 × 10−7 cm2/(s M). Perhaps even more interesting is the dependence of Deff on the employed metal ion concentration. Again, our measurementsall recorded at a constant hydroxide concentration of 0.5 Mare in very good agreement with simple square root functions and only deviate downward for very thick walls. The resulting values of Deff, however, show a clear sigmoidal dependence on the manganese concentration with an inflection point near 0.25 M. Furthermore, we find that the wall growth for [Mn2+] below 0.1 M is exceedingly slow. One possible explanation of this threshold behavior is that at low metal concentrations, the slower aggregation and growth of the forming colloidal particles does not allow for an effective attachment to the precipitate wall. This interpretation is in agreement with earlier results on the growth of silica-free FeS tubes in an injection system that revealed a concentration threshold of about 0.1 M below, whose tubes give way to plumes of colloidal particles.28 Thus far our analyses of the wall growth kinetics have focused strictly on the reaction of hydroxide with manganese ions. Wall formation in our microfluidic device, however, is also observed for other metal ions including Mg2+, Fe2+, Co2+, and Cu2+. Examples of precipitate walls grown for 4 h and identical reactant concentrations are shown in Figure 6a−e. The observed wall colors are overall in agreement with the colors of the expected metal hydroxides. In some cases, however, we observe rather abrupt color changes across the wall, indicating compositional changes and possibly the formation of (typically dark) metal oxides. For all of the materials tested, we find that the membrane grows in the direction of the metal solution, with growth kinetics adequately described by square-root laws. The resulting values of Deff are shown in Figure 6f and reveal that Mg2+- and Cu2+-based walls are the fastest and slowest growing structures, respectively. As already discussed in the context of hydroxide transport (Figure 5), the precipitate wall is not an impermeable barrier but a microporous membrane. Accordingly, it is possible to have slow mass exchange between the subchannels even for species other than hydroxide ions. To demonstrate this feature and to evaluate whether the membrane has macroscopic defects, we performed experiments as shown in Figure 7a. Here
Figure 6. Micrographs of precipitate membranes prepared during the reaction of 0.5 M OH− with 0.5 M solutions of (a) Mg2+, (b) Mn2+, (c) Fe2+, (d) Co2+, and (e) Cu2+. The pH of the metal chloride solutions was adjusted to 2.0 prior to experiments. Field of view: 1.4 × 0.7 mm2. (f) Effective diffusion coefficients obtained from the width kinetics and fitting.
Figure 7. (a) Photograph of a microfluidic device containing water (left) and a solution of methylene blue (right). Red bar shows the region over which we average the transmitted light intensity. (b) Decay of transmitted light intensity (red dots) and result from numerical simulation (black line). The Mn(OH)2 membrane was grown for 4 h and had an average width of 600 μm.
we first form the membrane for 4 h and then replace the inflow of hydroxide and manganese solutions with water and methylene blue solution (10−4 M), respectively. The latter fluids are pumped for 10 min, and then the flow is stopped. During the first subsequent minutes, there is no evidence for any dye penetrating the precipitate wall. We hence conclude that the membrane is, along its entire length, free of macroscopic holes and defects. Over the course of about 3 h, however, we clearly observe the diffusion of dye into the initially clear subchannel. Figure 7b shows the corresponding intensity decrease of transmitted white light as detected 0.25 mm away from the membrane surface. The time t = 0 marks the cessation of fluid inflow. The continuous line is the result of a numerical simulation based on Fick’s second law ∂c/∂t = ∂/∂x(D∂c/∂x), where c(x,t) describes the spatiotemporal dynamics of the dye concentration and D(x) is the dye’s space-dependent diffusion coefficient. The channel walls are modeled as no-flux boundaries, and the function D(x) describes the slow diffusion in the wall and the faster (Dsol = 0.84 × 10−5 cm2/s)55 diffusion in the left and right solutions. The rectangular shape of D(x) jumping from Dsol to Dwall and back 27049
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both encompassing regions associated with manganese hydroxide modes. Peaks at 310, 390, 490, and 650 cm−1 can be associated with Mn2O3.57,58 On the basis of visual inspection (Figure 6b) and the above-mentioned spectroscopic analysis we identify the freshly prepared precipitate as being a spatially layered mixture of manganese hydroxides, oxy-hydroxides, and oxides. Our spectroscopic results show the importance of in situ characterization of the wall material, because it is likely that most if not all samples obtained from chemical garden studies undergo chemical and physical changes during drying. Most results from earlier studies must hence be interpreted carefully in the context of the postsynthetic sample preparation.
is approximated by very steep but differentiable hyperbolic tangent functions. Using a simple step function for the initial concentration profile, we then perform repeated numerical integrations of the diffusion equation to minimize the rootmean-square deviation between the simulated curve and the experimental data. The resulting curve agrees well with the experimental time trace and yields an effective diffusion coefficient of Dwall = 1.4 × 10−7 cm2/s. This value equals roughly 1% of the diffusion coefficient in aqueous solution. Accordingly, transport of this cationic species across the precipitate membrane is possible but very slow. We also performed experiments in which we replace the probe molecule methylene blue with the anionic methyl orange (3 × 10−5 M). After cessation of its inflow, we observe a continuous decrease in absorption that is most pronounced at the precipitate wall that clearly acts as a sink for methyl orange; however, no absorption increase occurs in the subchannel across the wall (see movie 4 in the SI). This finding suggests that methyl orange is strongly adsorbed by the wall material and that the adsorption capacity is sufficiently high to prevent transport across the precipitate barrier. This interpretation is also supported by a recent study that reported strong adsorption of methyl orange to manganese oxide powder.56 Lastly, we emphasize that our method allows not only for direct visual inspection of the precipitate wall but also for in situ characterization by spectroscopy. Figure 8a and 8b shows
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CONCLUSION Our study extends recent efforts of simplifying the growth conditions of chemical gardens to its logical end point. Using a microfluidic device, the inner and outer solutions of the classical experiment react in a laminar flow at a sharp and stable interface and the resulting precipitation reactions form a linear wall. This approach allowed us to collect quantitative data on the wall thickening dynamics and to analyze certain physicochemical features in situ. A major result is the direct characterization of the growth dynamics as a diffusion-controlled process, in which the wall thickness w increases only in the direction of the precipitating metal ion even if the reactant concentration ratio is adjusted to favor product formation in the opposite direction. This finding shows that the wall growth cannot be explained in terms of a simple A + B → C reaction−diffusion process but is rather controlled by the charge-dependent transport of reactant ions across the precipitate membrane.59 While our approach is powerful in addressing certain aspects of the complex structures in chemical gardens, others are excluded during the various simplification steps. Those include the osmotic pump action of the seed particle at the base of the growing tube, curvature-related effects, and mechanical motion that can be observed in some systems. However, we believe our method will allow the analysis of several other important characteristics that go beyond the wall thickening dynamics that we focused on here. Such open problems include systematic studies of chemical changes during the wall growth, the lower concentration thresholds for the two reactants, and the role of silicate. As mentioned in the Introduction, precipitation tubes are also an important research target for origins-of-life studies as life on Earth might have started in the precipitation chimneys of off-axis alkaline vents.34 Today these hollow, tower-like structures are materials containing various oxides and sulfides of iron and nickel and are subject to concentration, pH, and temperature gradients that for long periods of time are maintained by geological processes. This natural presence of spatial confinement, geochemical catalysts, and free energy sources in conjunction with the presence of H2, CO, and CH4 in the effluent might have provided an ideal environment for prebiotic chemistry.34−37,60 Related research is often carried out in chemical garden-like tubes and membranes but faces the tremendous challenge of exploring a high-dimensional parameter space in comparatively short periods of time. Our microfluidics approach is ideally suited for such analyses. Interesting first applications of our method should include the investigation of peptide bond formation in nickel and/or iron sulfide precipitate membranes. The likelihood for such
Figure 8. Raman spectra of (a) an in situ Mn(OH)2 membrane grown for 2 h, (b) the same membrane after drying for 1 day inside the microfluidic channel, and (c) a reference sample consisting of Mn3O4 powder.
Raman spectra of manganese-based walls in the microfluidic device. The spectrum in Figure 8a characterizes the wall after 2 h of growth, whereas Figure 8b was recorded after an additional waiting period of 1 day, during which no new reactants were delivered into the reactor. For comparison, we also show the Raman spectrum of a pure Mn3O4 reference sample that shows a large peak near 650 cm−1. The latter reference data are in excellent agreement with the spectrum in Figure 8b, showing that the “aged” and dried wall consists of Mn3O4. The earlier and hydrated sample ( Figure 8a), however, reveals numerous peaks. In particular, we note a broad peak spanning from 440 to 570 cm−1 and a small one at 600 cm−1, 27050
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The Journal of Physical Chemistry C
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processes to occur appears rather high as the formation of oligopeptides indeed occurs in slurries of these materials.60 Lastly, we note that the controlled formation of inorganic membranes in microfluidic reactors could also prove useful for controlling sol−gel processes and the production of metal oxide nanoparticles and films.61 We emphasize that MnOx materials, similar to the samples produced here, display excellent activity toward water splitting and oxygen reduction reactions.62,63 The compartmentalized channels of our microfluidic device as well as the possibility to easily change the reaction conditions, reagents, and pH during the growth of the membrane offer innumerous opportunities for the synthesis of materials with tailored gradient composition. In addition in situ techniques, such as the Raman characterization employed here, can be easily coupled allowing for the real-time analysis of intermediates and products.
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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.jpcc.5b08813. Text describing additional details regarding the SI movies (PDF) Movie showing the microfluidic cell operation (AVI) Movie showing unidirectional growth of the precipitate membrane (AVI) Movie showing precipitate particles transported along the Mn2+-filled subchannel (AVI) Movie showing adsorption of methyl-orange by the membrane (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +1 850-644-4824. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge financial support from the National Science Foundation (DMR-1005861). B.C.B. acknowledges the National Council for Scientific and Technological Development (CNPq, Brazil) for a postdoctoral fellowship. We thank Bert van de Burgt for assistance with the Raman measurements.
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REFERENCES
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