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View in (B) Highlights the View Window for the Optical Microscope )ORULGD6WDWH8QLYHUVLW\/LEUDULHV 2021 Chemical Garden Membranes in Temperature-Controlled Microfluidic Devices Qingpu Wang and Oliver Steinbock This is the final accepted manuscript, and the publisher's version of record can be found at https://doi.org/10.1021/acs.langmuir.0c03548 Follow this and additional works at DigiNole: FSU's Digital Repository. For more information, please contact [email protected] Chemical Garden Membranes in Temperature- Controlled Microfluidic Devices Qingpu Wang and Oliver Steinbock* Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, USA ABSTRACT: Thin-walled tubes that classically form when metal salts react with sodium silicate solution are known as chemical gardens. They share similarities with the porous, catalytic materials in hydrothermal vent chimneys and both structures are exposed to steep pH gradients that, combined with thermal factors, might have provided the free energy for prebiotic chemistry on early Earth. We report temperature effects on the shape, composition, and opacity of chemical gardens. Tubes grown at high temperature are more opaque indicating changes to the membrane structure or thickness. To study this dependence, we developed a temperature-controlled microfluidic device which allows the formation of analogous membranes at the interface of two co-flowing reactant solutions. For the case of Ni(OH)2, membranes thicken according to a diffusion-controlled mechanism. In the studied range of 10-40 °C, the effective diffusion coefficient is independent of temperature. This suggests that counteracting processes are at play (including an increased solubility) and that the opacity of chemical garden tubes arises from changes in internal morphology. The latter could be linked to experimentally observed dendritic structures within the membranes. 1 INTRODUCTION Chemical gardens are life-like precipitate structures which grow at the interface of two solutions if those reactants rapidly form an insoluble product.1-3 In the classic version of this process, a salt grain is placed into waterglass—essentially dissolved sodium metasilicate—and subsequently produces one or several hollow tubes as well as bulbous or vesicle-like structures. The thin wall of these structures consists of an outer layer of amorphous silica and an inner layer of metal hydroxide or oxide. Due to the buoyancy of the interior metal salt solution, tubes typically grow upwards reaching heights of several centimeters. While the latter process is driven by osmotic pressure and the resulting inflow of water, tube growth also occurs if the corresponding metal salt solution is directly injected into the waterglass.4 This injection method allowed the identification of distinct growth regimes as well as systematic measurements of shape parameters and growth speeds in terms of well-defined parameters.4 It also formed the basis of recent studies that investigated chemical garden growth under spatial confinement created by Hele-Shaw cells5-9, glass capillaries10, and microfluidic channels11-14. Y-shaped microfluidic devices are ideally suited to study the slow, secondary thickening of the chemical garden walls. This increase in the width of the metal-hydroxide precipitate membrane occurs typically in the direction of the metal salt solution and was first studied by measuring the weight increase of tubes.15 Later microfluidic studies11,16 created essentially linear membranes along the reactive interface of co-flowing, laminar streams of sodium hydroxide and metal salt solutions. These membranes increase their width w according to the simple time dependence 1/2 w ∝ (Deff t) , where Deff is an effective diffusion coefficient much smaller than the molecular diffusion coefficients of small chemical species in aqueous solution. One notable exceptions of 2 this diffusion-controlled growth law is a type of hybrid membrane that shows bidirectional and rhythmic thickening dynamics17. We also note that flow over wavy membrane surfaces can enhance diffusive transport, which has been discussed as a possible factor in enhancing the bioenergetics of prebiotic chemistry and early life.18 The links between chemical gardens and the origins of life relate to possible scenarios in which life started in hydrothermal vent systems.19-24 These systems entail tall, chimney-like precipitate structures that form when hot, mineral rich water surges into the cold ocean. The dissolved minerals as well as simple hydrocarbon species, CO, and H2 result from the serpentinization of the rock underneath the ocean floor and determine the complex composition of the precipitates which include insoluble Ca, Ba, Fe, and Ni compounds.21,25 Many of these substances are catalytically active and might have facilitated the formation of more complicated molecules including sugars,26 27,28 29,30 31,32 33 amino acids, nucleobases, and pyrophosphate as well as CO2 reduction and the carbon fixation34. In addition, the porous precipitates provided spatial confinement without the need for lipid membranes and were subjected to steep gradients in pH and temperature. The combination of this geological source of free energy with a plethora of catalytic microreactors and appropriate reactants might have provided suitable conditions for the emergence of life on Earth four billion years ago and possibly on other planetary objects with a rock-water interface such as Enceladus and early Mars.35-38 The precipitate membranes of chemical gardens are useful models for the study of specific processes in hydrothermal vents and applications as biomimetic structures39-41. They spontaneously generate gradients in pH and redox potential27,42 and are porous as well as catalytically active43-45. Various aspects of these common features have been studied through the lens of origins-of-life research such as the production of pyrophosphates in iron mineral 3 membranes31,32 and morphological similarities between chemical gardens and disputed microfossils that were found in ancient rocks associated with hydrothermal vents46. However, to date, the relevance of temperature effects has attracted little attention.47 To start closing this knowledge gap, we here report an experimental study of the effects of temperature on the growth of precipitate membranes based on the aforementioned microfluidic methodology. EXPERIMENTAL SECTION Chemicals and Materials. Nickel chloride (NiCl2·6H2O, Fisher Chemical), cobalt chloride (CoCl2·6H2O, Sigma-Aldrich), cupric sulfate (CuSO4·5H2O, Fisher Chemical), zinc sulfate (ZnSO4, Fisher Chemical), and sodium hydroxide (NaOH, Macron Fine Chemicals) are used as received. All solutions are prepared with nanopure water filtered by a Barnstead Easypure UV system (resistivity 18 M cm). Pump Injection Experiments. For the experiments performed below room temperature, reactant solutions are kept in an ice bath for 30 min. For the experiments above room temperature, reactant solutions are thermostated in a 65 °C water bath at for 30 min. After thermal equilibration, 1.0 mL of 0.1 M NaOH solution is injected through a glass capillary (inner diameter: 1.0 mm) into a glass cylinder (inner diameter: 31 mm, outer diameter: 35 mm, height: 9.0 cm) containing 40 mL of 0.05 M metal salt solution (NiCl2, CoCl2, CuSO4, or ZnSO4). The pump rate is 8 mL/h in all the experiments. We record the temperature of the solution in the cylinder by using a thermocouple data logger (Pico Technology, JKE35/277) connected to a PC and PicoLog 6 software. The sensor was positioned about 1 cm below the meniscus of the metal salt solution and about 3 mm from the container wall. 4 Microfluidic Experiments. The microfluidic device (Figure 1) consists of a cut parafilm membrane (approximate thickness: 130 µm) sandwiched between two plexiglass plates measuring 8 cm × 10 cm (thickness 1.5 mm). Two holes with a diameter of 1.6 mm are drilled onto the top plate and barb fittings (NResearch Inc., FITM 331) are secured onto the holes using an epoxy glue. The three layers are then assembled, fastened with the help of several binder clips and glass slides for even pressure distribution. The assembly is then heated on a hot plate at medium-low heat for 5 min to slightly melt the parafilm membrane. After cooling under ambient conditions, the resolidified parafilm acts as an adhesive that holds the two plates firmly together without requiring external fastening. Figure 1. (a-b) Schematics showing the experimental setup of the temperature-controlled microfluidic device. The side view in (b) highlights the view window for the optical microscope. This window is positioned right next to the temperature-regulated region. (c) Photograph of the microfluidic device and the resulting precipitate membrane (thin vertical line in the lower half of 5 the photo). The large, gray dashed box shows the position of the metal block during the experiment and the very small red box indicates the microscopic view window. We use a PC-controlled cutting tool (Silhouette Portrait) to cut the parafilm membrane into the desired pattern: a Y-shape17 with branches extending to both sides along serpentine tracks that consist of semicircular-ring segments connected by straight parts. The stem of the Y-shaped pattern is 45 mm long, 2 mm wide, and approximately 130 µm high. Two syringes containing 0.50 M NaOH and 0.25 M NiCl2 solutions are connected to the barb fittings with plastic tubing (Tygon, inner diameter 1/16’’). Reactant solutions are prepared in water degassed by prior boiling to avoid undesired bubble formation in the microfluidic channel during heating. The syringes are slowly discharged using a programmable syringe pump (New Era Pump Systems, NE 2000) at a constant pump rate of 1 mL/h. Prior to the injection, we manually fill the microfluidic channel with water to avoid surface-tension induced perturbations. We also preheat or precool the system by placing a hollow aluminum block containing circulating water on top of the microfluidic device for 30 min. The water circulation is controlled by a refrigerated bath/circulator (NESLAB RTE- 111). Notice that for the investigated temperature range of 10 to 40 °C, small but noticeable pH 48 changes occur (e.g.
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