Chemical Gardens Research Articles

Cover Feature: Tubular Structures of Calcium Carbonate: Formation, Characterization, and Implications in Natural Mineral Environments (Chem. Eur. J. 65/2021)

Calcium carbonate chemical gardens are tubular mineral structures precipitated upon the addition of sodium carbonate solution to calcium chloride pellets. The tubes are composed of texturally distinct bilayers of aggregates of rhombohedral and cone-shaped calcite on their exterior and interior surfaces, respectively. These highly crystalline and dense bilayers prohibit the development of significant electrochemical potential differences across the tube walls. More information can be found in the Full Paper by M. Kellermeier, J. M. García-Ruiz et al. (DOI: 10.1002/chem.202101417).

Tubular Structures of Calcium Carbonate: Formation, Characterization, and Implications in Natural Mineral Environments.

Chemical gardens are self-assembled tubular precipitates formed by a combination of osmosis, buoyancy, and chemical reaction, and thought to be capable of catalyzing prebiotic condensation reactions. In many cases, the tube wall is a bilayer structure with the properties of a diaphragm and/or a membrane. The interest in silica gardens as microreactors for materials science has increased over the past decade because of their ability to create long-lasting electrochemical potential. In this study, we have grown single macroscopic tubes based on calcium carbonate and monitored their time-dependent behavior by in situ measurements of pH, ionic concentrations inside and outside the tubular membranes, and electrochemical potential differences. Furthermore, we have characterized the composition and structure of the tubular membranes by using ex situ X-ray diffraction, infrared and Raman spectroscopy, as well as scanning electron microscopy. Based on the collected data, we propose a physicochemical mechanism for the formation and ripening of these peculiar CaCO3 structures and compare the results to those of other chemical garden systems. We find that the wall of the macroscopic calcium carbonate tubes is a bilayer of texturally distinct but compositionally similar calcite showing high crystallinity. The resulting high density of the material prevents macroscopic calcium carbonate gardens from developing significant electrochemical potential differences. In the light of these observations, possible implications in materials science and prebiotic (geo)chemistry are discussed.

Controlled self-assembly of chemical gardens enables fabrication of heterogeneous chemobrionic materials

Chemical gardens are an example of a chemobrionic system that typically result in abiotic macro-, micro- and nano- material architectures, with formation driven by complex out-of-equilibrium reaction mechanisms. From a technological perspective, controlling chemobrionic processes may hold great promise for the creation of novel, compositionally diverse and ultimately, useful materials and devices. In this work, we engineer an innovative custom-built liquid exchange unit that enables us to control the formation of tubular chemical garden structures grown from the interface between calcium loaded hydrogel and phosphate solution. We show that systematic displacement of phosphate solution with water (H2O) can halt self-assembly, precisely control tube height and purify structures in situ. Furthermore, we demonstrate the fabrication of a heterogeneous chemobrionic composite material composed of aligned, high-aspect ratio calcium phosphate channels running through an otherwise dense matrix of poly(2-hydroxyethyl methacrylate) (pHEMA). Given that the principles we derive can be broadly applied to potentially control various chemobrionic systems, this work paves the way for fabricating multifunctional materials that may hold great potential in a variety of application areas, such as regenerative medicine, catalysis and microfluidics.

The Copper Chemical Garden as a Low Cost and Efficient Material for Breaking Down Air Pollution by Gaseous Ammonia

AbstractChemical garden (CG) from copper(II) sulfate, nitrate and chloride (CG CuSO4, CG Cu(NO3)2, CG CuCl2) were grown, and characterized from the structural and compositional point of view by using scanning electron microscopy, X‐ray powder diffraction, elemental analysis, thermogravimetric analysis coupled with mass spectrometry, and DR (diffuse reflectance) UV‐Vis‐NIR spectroscopy. The main crystalline phases, controlled by the anion of the starting salt, were brochantite and kobyashevite for CG CuSO4, gerhardtite, rouaite and anthonyite for CG Cu(NO3)2, and atacamite for CG CuCl2. The materials were then exposed to ammonia vapors to test the effectiveness of their entrapping property. All materials proved to be very efficient and rapid in the uptake of ammonia, which invariably results in the formation of a Cu(II)/NH3 complex. However, after a few tens of minutes, CG Cu(NO3)2 and CG CuCl2 release water and get wet, thereby resulting unsuitable for applications. Only CG CuSO4 remains dry for at least 25 hours. This makes it a valid candidate for building devices for trapping ammonia, and possibly other gases capable of interacting with Cu(II). The entrapment of ammonia by this material was also characterized by 1H and 29Si MAS‐NMR XAS spectroscopies.

Experimental Approaches for Testing the Hypothesis of the Emergence of Life at Submarine Alkaline Vents.

Since the pioneering experimental work performed by Urey and Miller around 70 years ago, several experimental works have been developed for approaching the question of the origin of life based on very few well-constructed hypotheses. In recent years, attention has been drawn to the so-called alkaline hydrothermal vents model (AHV model) for the emergence of life. Since the first works, perspectives from complexity sciences, bioenergetics and thermodynamics have been incorporated into the model. Consequently, a high number of experimental works from the model using several tools have been developed. In this review, we present the key concepts that provide a background for the AHV model and then analyze the experimental approaches that were motivated by it. Experimental tools based on hydrothermal reactors, microfluidics and chemical gardens were used for simulating the environments of early AHVs on the Hadean Earth (~4.0 Ga). In addition, it is noteworthy that several works used techniques from electrochemistry to investigate phenomena in the vent–ocean interface for early AHVs. Their results provided important parameters and details that are used for the evaluation of the plausibility of the AHV model, and for the enhancement of it.

Chemical Gardens Under Mars Conditions: Imaging Chemical Garden Growth In Situ in an Environmental Scanning Electron Microscope

AbstractWe have performed and visualized chemical garden growth experiments directly in situ in an environmental scanning electron microscope. The microscope chamber simulates the surface of Mars. We demonstrate that chemical gardens can form under the conditions of temperature and pressure existing on Mars in the presence of liquid water, silicates, and metal salts. Using chemical gardens from aluminum salts, tubes were formed where the external surface is formed principally by silicate and the inner surface is predominantly aluminum oxide‐hydroxide. It should thus be expected that similar growths of metal salts may be found in geological explorations of water‐rich environments during the search for life on Mars.

Dubiofossils from a Mars-analogue subsurface palaeoenvironment: The limits of biogenicity criteria.

The search for a fossil record of Earth's deep biosphere, partly motivated by potential analogies with subsurface habitats on Mars, has uncovered numerous assemblages of inorganic microfilaments and tubules inside ancient pores and fractures. Although these enigmatic objects are morphologically similar to mineralized microorganisms (and some contain organic carbon), they also resemble some abiotic structures. Palaeobiologists have responded to this ambiguity by evaluating problematic filaments against checklists of "biogenicity criteria". Here, we describe material that tests the limits of this approach. We sampled Jurassic calcite veins formed through subseafloor serpentinization, a water-rock reaction that can fuel the deep biosphere and is known to have occurred widely on Mars. At two localities ~4km apart, veins contained curving, branched microfilaments composed of Mg-silicate and Fe-oxide minerals. Using a wide range of analytical techniques including synchrotron X-ray microtomography and scanning transmission electron microscopy, we show that these features meet many published criteria for biogenicity and are comparable to fossilized cryptoendolithic fungi or bacteria. However, we argue that abiotic processes driven by serpentinization could account for the same set of lifelike features, and report a chemical garden experiment that supports this view. These filaments are, therefore, most objectively described as dubiofossils, a designation we here defend from criticism and recommend over alternative approaches, but which nevertheless signifies an impasse. Similar impasses can be anticipated in the future exploration of subsurface palaeo-habitats on Earth and Mars. To avoid them, further studies are required in biomimetic geochemical self-organization, microbial taphonomy and micro-analytical techniques, with a focus on subsurface habitats.

Deep‐sea corrosion rusticles from iron‐hulled shipwrecks

AbstractVertical tube‐shaped iron‐oxide accumulations, named rusticles, obtained from the wrecks of the HMAS Sydney II and the HSK Kormoran at 2480 m ocean depth were chemically analysed along with surrounding seawater. Rusticles consisted of a porous aggregation of iron oxides and high levels of toxic metals and metalloids. Their growth rate is approximately 1 cm per year, predominantly over the areas of the hulls that remained underwater during their service years. A connection between the quality of antifouling paints and rusticle growth and composition was found. A mechanism explaining the formation of rusticles is proposed based on corrosion of carbon steel in deep‐seawater, water chemistry, surface complexation of iron oxides and chemical garden formation mechanisms. This study provides a complete rationalisation of the process of rusticle formation and deep‐water corrosion that is applicable to the long‐term integrity of offshore infrastructure.

Exploring the Formation of Calcium Orthophosphate‐Pyrophosphate Chemical Gardens

AbstractChemical gardens are characterised by the self‐assembly of mineralised abiotic architectures. Utilising the fundamental building blocks of bone mineral, namely calcium and orthophosphate ions, chemical gardens that recapitulate microstructural and compositional features of hard tissue can be grown. Interplay between orthophosphate and pyrophosphate species is highly relevant to natural mineral deposition processes, though this has yet to be explored in the context of generating biologically relevant chemical gardens. Here, tubular calcium orthophosphate‐pyrophosphate chemical gardens were grown from the interface between calcium loaded hydrogels ([Ca2+]=1 M) layered with different orthophosphate‐pyrophosphate solutions ([Pi]+[PPi]=0.7 M). We determine the effect of solution pyrophosphate content on chemical garden morphology and growth rate. Extracted structures were analysed by means of powder X‐ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and X‐ray fluorescence spectroscopy (XRF), revealing orthophosphate‐pyrophosphate solution dependent differences in precipitated mineral crystallinity, composition and microstructure, respectively. Lastly, the potential application of the structures is discussed in the context of tissue engineering and regenerative medicine.

Iron‐Silicate Chemical Garden Morphology and Silicate Reactivity with Alpha‐Keto Acids

AbstractChemical gardens, which are self‐organizing, abiotic, inorganic precipitates in far‐from‐equilibrium systems, are of interest for prebiotic chemistry/origin of life research and, under certain reaction systems, can be considered hydrothermal vent analogs. While the presence of different additives to chemical gardens, including phosphate and amino acids, have been explored, the reactivity of organic molecules in chemical garden systems is not well understood. Here we explored the reactivity of two metabolically important alpha‐keto acids (pyruvic acid and glyoxylic acid) and ammonia in the presence of iron‐silicate chemical gardens. While reactivity was not observed in the case of pyruvic acid, we found that glyoxylic acid formed alpha‐hydroxy acids and amino acids. We found that the observed organic reactivity can be attributed to the silicate in the exterior solution as opposed to the solid iron‐containing chemical garden structure. These results have implications for the reactivity of hydrothermal vent chimneys and their surrounding environments in iron‐ and silica‐rich systems.

Chemical Garden Membranes in Temperature-Controlled Microfluidic Devices.

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 coflowing 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.

The Growth of an Electrochemical Garden on a Zinc Electrode

AbstractThe far‐from equilibrium precipitation reaction of chemical gardens can lead to the formation of biomimetic and complex structures providing a new route for the rational architectural design of functional materials. Inspired by recent developments in the field of chemical gardens, we put forward a new scientific question: “Is it possible to create an electrochemical garden?” By implementing state‐of‐the‐art electrochemical techniques and using the phenomenon of metal corrosion, we sculpture self‐organized structures on a zinc disc‐electrode surface by a mechanism similar to that of chemical gardens. A deeper search in the formation mechanism reveals that ion‐selective membranes are the driving force for the growth of an electrochemical garden. At last, electrochemical instabilities, introduced under proper conditions, result in the emergence of current oscillations in the region where electrochemical gardens were discovered. Current oscillations sculpture the electrode surface with a variety of self‐organized precipitate structures.

Zinc Borate Chemical Garden and Zinc Borate Powders from Tincal Mineral and Zinc Sulfate Heptahydrate

The formation of the first membrane, the swelling of the crystal by incoming water from the semipermeable membrane and the formation of irregular shaped branches were observed by optical microscopy, when zinc sulfate heptahydrate crystals were immersed in saturated borax solution. The powders obtained by mixing dilute aqueous borax and zinc sulfate solutions had B, O, Na, S and Zn elements. Presence of Na Zn 1/2B4O7.xH2O was indicated by EDX analysis. The molar ratio of B 2O3/ZnO in powders was around 2. FTIR analysis indicated the ratio of absorbance values of asymmetric stretching vibrations of B(3)-O at 1351 cm-1 to that of B (4)-O at 1026 cm-1 increased with their heating time at 90ºC during their preparation. X-ray diffraction patterns indicated the presence of Zn(OH)2 and Zn 4(OH)6(SO4)·4H2O. The zinc borate compounds in the powders were not crystalline since no sharp peaks related to zinc borates were present in x-ray diffraction diagram. There were two mass loss steps in TG curves of the powders. The first step at 150-350ºC and the second step at 700-950ºC were due to elimination of water and due to decomposition of sulfate ions respectively. The submicron powders were a mixture of zinc borate, Zn(OH)2, Zn4(OH)6(SO4)·4H2O and Na Zn 1/2B4O7.xH2O and they could be used as lubricant additive due to their small particle size of 600 nm.

Oscillatory budding dynamics of a chemical garden within a co-flow of reactants

The oscillatory growth of chemical gardens is studied experimentally in the budding regime using a co-flow of two reactant solutions within a microfluidic reactor. The confined environment of the reactor tames the erratic budding growth and the oscillations leave their imprint with the formation of orderly spaced membranes on the precipitate surface. The average wavelength of the spacing between membranes, the growth velocity of the chemical garden and the oscillations period are measured as a function of the velocity of each reactant. By means of materials characterization techniques, the micro-morphology and the chemical composition of the precipitate are explored. A mathematical model is developed to explain the periodic rupture of droplets delimitated by a shell of precipitate and growing when one reactant is injected into the other. The predictions of this model are in good agreement with the experimental data.

Filament dynamics in planar chemical gardens.

Filaments in a planar chemical garden grow following tortuous, erratic paths. We show from statistical mechanics that this scaling results from a self-organized dispersion mechanism. Effective diffusivities as high as 10-5 m2 s-1 are measured in 2D laboratory experiments. This efficient transport is four orders of magnitude larger than molecular diffusion in a liquid, and ensures widespread contact and exchange between fluids in the chemical-garden structure and its surrounding environment.

Self‐Assembled Structures from Solid Cadmium(II) Acetate in Thiol/Ethanol Solutions: A Novel Type of Organic Chemical Garden

AbstractCadmium(II) acetate in tablet form, immersed in ethanol solutions with a high concentration of heptanethiol (30–80 mM), was able to develop self‐assembled sail‐shaped structures. The solution in which the self‐assembled structure is formed is totally organic (both solute and solvent), thus representing a unique case among organic Chemical Gardens. The constituting material for this new Chemical Garden (CG) is made of a single phase, namely a cadmium heptanethiolate with a microcrystalline structure consisting of a central cluster [CdnSn]. The morphological and structural features were studied using different techniques (SEM‐EDS, elemental analysis, ATR‐FTIR, 113Cd MAS NMR, XRPD). A crystalline cell for the Cd(II) heptanethiolate cluster was obtained that completely differs from that of the solid phase precipitated by mixing an ethanol solution of Cd(II) acetate with heptanethiol. It follows that the conditions under which the formation of Cd(II) heptanethiolate occurs (slowly from solid, quickly from solution) play a critical role in determining the nature and structure of the precipitating phase.

Systems Chemistry Looking Forward

Systems Chemistry Looking Forward

Chemical Gardens as Electrochemical Systems: In Situ Characterization of Simulated Prebiotic Hydrothermal Vents by Impedance Spectroscopy.

In an early earth or planetary chimney systems, hydrothermal fluid chemistry and flow durations play a large role in the chimney's ability to drive electrochemical reactions for the origin of life. We performed continuous electrochemical impedance spectroscopy (EIS) characterization on inorganic membranes representing prebiotic hydrothermal chimney vents in natural seafloor systems, by incorporating an electrode array into a chimney growth experiment. Localized potential and capacitances profiles in the chimney reveal a dynamic system where redox processes are driven by transport phenomena, increasing rapidly due to disequilibrium until achieving equilibrium at about 100 mV and 1000 μF/cm2 . The impedance in the chimney interior is three orders of magnitude lower (100 Ohms/cm2 vs 100 KOhms/cm2 ) than at the ocean or the ocean/chimney interface. The calculated peak dissipation factor (DF) values are more than ten times higher (40.0 vs 3.0) and also confirm the elevated chemical reactivity in the chimney interior.

Nanoscale Anatomy of Iron‐Silica Self‐Organized Membranes: Implications for Prebiotic Chemistry

AbstractIron‐silica self‐organized membranes, so‐called chemical gardens, behave as fuel cells and catalyze the formation of amino/carboxylic acids and RNA nucleobases from organics that were available on early Earth. Despite their relevance for prebiotic chemistry, little is known about their structure and mineralogy at the nanoscale. Studied here are focused ion beam milled sections of iron‐silica membranes, grown from synthetic and natural, alkaline, serpentinization‐derived fluids thought to be widespread on early Earth. Electron microscopy shows they comprise amorphous silica and iron nanoparticles of large surface areas and inter/intraparticle porosities. Their construction resembles that of a heterogeneous catalyst, but they can also exhibit a bilayer structure. Surface‐area measurements suggest that membranes grown from natural waters have even higher catalytic potential. Considering their geochemically plausible precipitation in the early hydrothermal systems where abiotic organics were produced, iron‐silica membranes might have assisted the generation and organization of the first biologically relevant organics.

Nanoscale Anatomy of Iron‐Silica Self‐Organized Membranes: Implications for Prebiotic Chemistry

Iron‐silica self‐organized membranes, so‐called chemical gardens, behave as fuel cells and catalyze the formation of amino/carboxylic acids and RNA nucleobases from organics that were available on early Earth. Despite their relevance for prebiotic chemistry, little is known about their structure and mineralogy at the nanoscale. Studied here are focused ion beam milled sections of iron‐silica membranes, grown from synthetic and natural, alkaline, serpentinization‐derived fluids thought to be widespread on early Earth. Electron microscopy shows they comprise amorphous silica and iron nanoparticles of large surface areas and inter/intraparticle porosities. Their construction resembles that of a heterogeneous catalyst, but they can also exhibit a bilayer structure. Surface‐area measurements suggest that membranes grown from natural waters have even higher catalytic potential. Considering their geochemically plausible precipitation in the early hydrothermal systems where abiotic organics were produced, iron‐silica membranes might have assisted the generation and organization of the first biologically relevant organics.