A solution study of silica condensation and speciation with relevance to in vitro investigations of biosilicification

The Physiological Inorganic Polymers Biosilica and Polyphosphate as Key Drivers for Biomedical Materials in Regenerative Nanomedicine

There is a need for novel nanomaterials with properties not yet exploited in regenerative nanomedicine. Based on lessons learned from the oldest metazoan phylum, sponges, it has been recognized that two previously ignored or insufficiently recognized principles play an essential role in tissue regeneration, including biomineral formation/repair and wound healing. Firstly, the dependence on enzymes as a driving force and secondly, the availability of metabolic energy. The discovery of enzymatic synthesis and regenerative activity of amorphous biosilica that builds the mineral skeleton of siliceous sponges formed the basis for the development of successful strategies for the treatment of osteochondral impairments in humans. In addition, the elucidation of the functional significance of a second regeneratively active inorganic material, namely inorganic polyphosphate (polyP) and its amorphous nanoparticles, present from sponges to humans, has pushed forward the development of innovative materials for both soft (skin, cartilage) and hard tissue (bone) repair. This energy-rich molecule exhibits a property not shown by any other biopolymer: the delivery of metabolic energy, even extracellularly, necessary for the ATP-dependent tissue regeneration. This review summarizes the latest developments in nanobiomaterials based on these two evolutionarily old, regeneratively active materials, amorphous silica and amorphous polyP, highlighting their specific, partly unique properties and mode of action, and discussing their possible applications in human therapy. The results of initial proof-of-concept studies on patients demonstrating complete healing of chronic wounds are outlined.

POM@PMO plastic electrode for phosphate electrochemical detection: a further improvement of the detection limit

The development of a highly sensitive electrochemical sensor (E-sensor) is described based on stand-alone plastic electrodes (PE) for phosphate detection, being an essential nutrient in the marine environment. The detection mechanism is based on the chemical affinity between polyoxomolybdate anions (POM) and orthophosphate to form an electroactive phosphomolybdate complex. The custom-made E-sensor was formulated with an organic octamolybdate derivative (TBA₄Mo₈O₂₆) incorporated with periodic mesoporous organosilica (PMO) to obtain a significant improvement in the analytical performances of phosphate determination. This POM@PMO combination was found to be advantageous in the determination of low concentrations of phosphate in standard solutions ranging from 1 to 500 nM, using square wave voltammetry as the detection technique. This sensitivity enhancement can be attributed to the effect of hydrophobic PMO in loading more POM moieties, owing to its highly porous structure and charged shell. Consequently, the POM@PMO-PE sensor achieved a competitive sensitivity of 4.43 ± 0.14 μA.nM^-1.cm^-2 and a limit of detection of 0.16 nM with good selectivity against silicates. Finally, seawater and treated wastewater samples have been tested to validate the sensor response in comparison to the official method of phosphate determination.

Catalytic Behavior of Hydrogen Bonded Water in Oligomerization of Silicates

Even though much research has been done to demonstrate the oligomerization of zeolites and silicates, there has been almost no study that investigates the role of hydrogen bonds in these reaction pathways. This study demonstrates the catalytic activity of hydrogen bonds in the silicate oligomerization reaction pathway. The presence of hydrogen bonding has been shown to enhance the energetic favorability of the anionic-I mechanism. Catalysis is prevalent in the Si-OH rupture process of the reaction pathway. Simultaneously, the dependence of the activation barrier on the equatorial or apical nature of the cleaving hydroxyl group has also been shown. The preceding steps such as condensation and fluxional influence the strength of hydrogen bonds. An increase in hydrogen bond strength enhances its catalytic effect, leading to a higher extent of reduction in the activation barrier of the particular reaction step. Even though the quantum study focuses on the oligomerization of calcium silicate as a test case, it can be anticipated that such similar effects can be perceived in general for the oligomerization of silicates containing metallic ions in sol-gel chemistry and zeolite synthesis.

Recent Advances in Enabling Green Manufacture of Functional Nanomaterials: A Case Study of Bioinspired Silica

Global specialty silica production is over 3 million tonnes per annum with diverse applications across sectors and an increasing demand for more complex material structures and surface chemistries. Commercial manufacturing of high-value silica nanomaterials is energy and resource intensive. In order to meet market needs and mitigate environmental impacts, new synthesis methods for these porous materials are required. The development of the bioinspired silica (BIS) product system, which is the focus of this review, provides a potential solution to this challenge. BIS is a versatile and greener route with the prospect of good scalability, attractive process economics and well controlled product materials. The potential of the system lies not only in its provision of specific lead materials but also, as itself, a rich design-space for the flexible and potentially predictive design of diverse sustainable silica nanomaterials. Realizing the potential of this design space, requires an integrative mind-set, which enables parallel and responsive progression of multiple and dependent research strands, according to need, opportunities, and emergent knowledge. Specifically, this requires development of detailed understanding of (i) the pathways and extent of material diversity and control, (ii) the influences and mechanisms of scale-up, and (iii) performance, economic and environmental characteristics and sensitivities. Crucially, these need to be developed for the system overall, which sits in contrast to a more traditional research approach, which focuses initially on the discovery of specific material leads at the laboratory scale, leaving scale-up, commercialization, and, potentially, pathway understanding to be considered as distinctly separate concerns. The intention of this review is to present important recent advances made in the field of BIS. Specifically, advances made along three research themes will be discussed: (a) particle formation pathways, (b) product design, and (c) scale-up and manufacture. These advances include first quantitative investigation of synthesis-product relationships, first structured investigation of mixing effects, preparation of a broad range of functionalized and encapsulated silica materials and continued industrial engagement and market research. We identify future challenges and provide an important foundation for the development of new research avenues. These include the need to develop comprehensive and predictive product design models, to understand markets in terms of product cost, performance and environmental considerations, and to develop capabilities enabling rapid prototyping and scale-up of desired nanomaterials.

Silica dimerization in the presence of divalent cations

The presence of monovalent cations and organic tetraalkylammonium ions is known to affect the reaction pathway and chemical kinetics of the silica oligomerization reaction which is important for sol-gel chemistry studies. A detailed theoretical study focusing on the chemical reaction pathway for the dimerisation process in the presence of a divalent cation is presented in this study. Different condensation pathways such as neutral, anionic-I and anionic-II along with their relative possibilities in dimerization have been explored. It has been demonstrated that with an increase in the pH of solution, manifested through the presence of deprotonated ions (as in the anionic cases with or without the presence of divalent cations), the activation activation barrier of the dimerization reaction is lowered. It has also been demonstrated that the addition of divalent cations raises the activation barriers for the reaction and delays the overall dimerisation reaction. The stability and bond characteristics of the bridging Si-OH bond of the resulting dimer products have also been determined. Activation energy barriers for the anionic case have also been observed to vary based upon the dihedral arrangement of the hydroxyl group bonded with the silicon and the orientation of the nucleophilic attack.

In Situ Sol-Gel Synthesis of Unique Silica Structures Using Airborne Assembly: Implications for In-Air Reactive Manufacturing

Optical trapping enables the real-time manipulation and observation of morphological evolution of individual particles during reaction chemistry. Here, optical trapping was used in combination with Raman spectroscopy to conduct airborne assembly and kinetic experiments. Micro-droplets of alkoxysilane were levitated in air prior to undergoing either acid- or base-catalyzed sol-gel reaction chemistry to form silica particles. The evolution of the reaction was monitored in real-time; Raman and Mie spectroscopies confirmed the in situ formation of silica particles from alkoxysilane droplets as the product of successive hydrolysis and condensation reactions, with faster reaction kinetics in acid catalysis. Hydrolysis and condensation were accompanied by a reduction in droplet volume and silica formation. Two airborne particles undergoing solidification could be assembled into unique 3D structures such as dumb-bell shapes by manipulating a controlled collision. Our results provide a pipeline combining spectroscopy with optical microscopy and nanoscale FIB-SEM imaging to enable chemical and structural insights, with the opportunity to apply this methodology to probe structure formation during reactive inkjet printing.

First Evidence of Tris(catecholato)silicate Formation from Hydrolysis of an Alkyl Bis(catecholato)silicate

The hydrolysis of 3-ammoniumpropylbis(catecholato)silicate 1, giving two different silica-based materials containing different amounts of tris(catecholato)silicate, is reported. The latter species can be formed through an attack of catechol to the silicon atom in the pentacoordinate complex, in which the silicon-carbon bond is further activated toward electrophilic proton cleavage. The Knoevenagel reaction was used as a probe in order to test the availability of functional groups on the surface of such materials.

Inorganic Polymeric Materials for Injured Tissue Repair: Biocatalytic Formation and Exploitation

Two biocatalytically produced inorganic biomaterials show great potential for use in regenerative medicine but also other medical applications: bio-silica and bio-polyphosphate (bio-polyP or polyP). Biosilica is synthesized by a group of enzymes called silicateins, which mediate the formation of amorphous hydrated silica from monomeric precursors. The polymeric silicic acid formed by these enzymes, which have been cloned from various siliceous sponge species, then undergoes a maturation process to form a solid biosilica material. The second biomaterial, polyP, has the extraordinary property that it not only has morphogenetic activity similar to biosilica, i.e., can induce cell differentiation through specific gene expression, but also provides metabolic energy through enzymatic cleavage of its high-energy phosphoanhydride bonds. This reaction is catalyzed by alkaline phosphatase, a ubiquitous enzyme that, in combination with adenylate kinase, forms adenosine triphosphate (ATP) from polyP. This article attempts to highlight the biomedical importance of the inorganic polymeric materials biosilica and polyP as well as the enzymes silicatein and alkaline phosphatase, which are involved in their metabolism or mediate their biological activity.

Azamacrocycles and tertiary amines can be used to form size tuneable hollow structures or monodisperse oxide nanoparticles depending on the 'M' source

We show that the azamacrocycle 'cyclam' (1,4,8,11-tetraazacyclodecane) in conjunction with a silicon catecholate ion generates novel hollow tetragonal tube-like crystalline materials [(C6H4O2)3Si][C10H26N4]·H2O, whose dimensions can be tuned according to the pH of the reaction medium. The synthesis approach was successful for both silicon and germanium and we hypothesise that a range of other catecholate precursors of elements such as iron could be used to generate a large array of inorganic materials with interesting morphologies. The synthesis approach can be extended to tertiary diamines with functional group spacing playing an important role in the efficacy of complexation. Of the molecules explored to date, a C2 spacing (N,N,N',N'-tetramethylethylenediamine (4MEDAE)), leads to the most efficient structure control with hollow hexagonal tube-like structures being formed. In addition, we show that azamacrocycles, in the presence of unbuffered tetramethoxysilane (TMOS) solutions can be used to manipulate silica formation and provide a fast (ca. 10 minutes) synthesis route to particles whose diameter can be tuned from ca. 20 nm to several hundreds of nm under reaction conditions (no extremes of pH) that make the sols suitable for direct use in biotechnological applications.

Kinetics of Alkoxysilanes and Organoalkoxysilanes Polymerization: A Review

Scientists from various different fields use organo-trialkoxysilanes and tetraalkoxysilanes in a number of applications. The silica-based materials are sometimes synthesized without a good understanding of the underlying reaction kinetics. This literature review attempts to be a comprehensive and more technical article in which the kinetics of alkoxysilanes polymerization are discussed. The kinetics of polymerization are controlled by primary factors, such as catalysts, water/silane ratio, pH, and organo-functional groups, while secondary factors, such as temperature, solvent, ionic strength, leaving group, and silane concentration, also have an influence on the reaction rates. Experiments to find correlations between these factors and reaction rates are restricted to certain conditions and most of them disregard the properties of the solvent. In this review, polymerization kinetics are discussed in the first two sections, with the first section covering early stage reactions when the reaction medium is homogenous, and the second section covering when phase separation occurs and the reaction medium becomes heterogeneous. Nuclear magnetic resonance (NMR) spectroscopy and other techniques are discussed in the third section. The last section summarizes the study of reaction mechanisms by using ab initio and Density Functional Theory (DFT) methods alone, and in combination with molecular dynamics (MD) or Monte Carlo (MC) methods.

Bioavailability of a novel form of silicon supplement

In this study, we assessed uptake and potential efficacy of a novel, pH neutral form of silicon supplement in vitro and using broiler chickens as a model species. In vitro bioavailability of this supplement was significantly higher than other commercial supplements tested, all of which claim available silica content. To confirm bioavailability of the new supplement in vivo, a broiler chick feeding trial reported blood uptake that was significantly higher than a Bamboo-derived silicon supplement. We assessed dose response of the novel supplement in a further study with increased dose related levels of silicon being detected in the blood and tibia. We found tibia and foot ash residue as a percentage of dry mass was higher with inclusion of the novel supplement in the diet, particularly in young birds and that this was followed by significant increase in tibia breaking strength. This novel supplement may therefore have applications in the improvement of bone integrity, with implications for the reduction of lameness in broilers. These results indicate the novel silica supplement is readily absorbed in chicks, and transported in the blood supply to sites such as the skeleton due to it being present in a non-condensed, monomeric form. There is potential for wider application of this silica supplement in other species where bone breakages are a problem, including high performance sport.

Synthesis, Polymerization, and Assembly of Nanosilica Particles below the Isoelectric Point

The particle growth of silica below the isoelectric point plays a key role in oil well cements, production of silica gels and production of nanosilica by dissolving silicates. In this article, we study the particle growth of silica below the isoelectric point using olivine, a silicate mineral, and sodium silicate solutions as silica sources in acid, where the particle size, soluble silica concentration, specific surface area and gelling time were measured. The size of the primary particles detected by laser light scattering is 3 nm in the experiments with sodium silicate solutions. These particles grow then by aggregation forming linear chains which in time will start to branch. The particle growth follows a quadratic polynomial function and particles as large as 100 and 500 nm are detected in the final stages of experiments using sodium silica solutions and olivine, respectively. Based on these findings, a comprehensive model describing the silica particle development below the isoelectric point is proposed. This model gives fundamental information about the growth mechanism and the properties of silica (e.g., particle size of the primary particles, size of the aggregates) at the different growth stages.

Secondary structure and dynamics study of the intrinsically disordered silica-mineralizing peptide P5 S3 during silicic acid condensation and silica decondensation

The silica forming repeat R5 of sil1 from Cylindrotheca fusiformis was the blueprint for the design of P5 S3 , a 50-residue peptide which can be produced in large amounts by recombinant bacterial expression. It contains 5 protein kinase A target sites and is highly cationic due to 10 lysine and 10 arginine residues. In the presence of supersaturated orthosilicic acid P5 S3 enhances silica-formation whereas it retards the dissolution of amorphous silica (SiO2 ) at globally undersaturated concentrations. The secondary structure of P5 S3 during these 2 processes was studied by circular dichroism (CD) spectroscopy, complemented by nuclear magnetic resonance (NMR) spectroscopy of the peptide in the absence of silicate. The NMR studies of dual-labeled (13 C, 15 N) P5 S3 revealed a disordered structure at pH 2.8 and 4.5. Within the pH range of 4.5-9.5 in the absence of silicic acid, the CD data showed a disordered structure with the suggestion of some polyproline II character. Upon silicic acid polymerization and during dissolution of preformed silica, the CD spectrum of P5 S3 indicated partial transition into an α-helical conformation which was transient during silica-dissolution. The secondary structural changes observed for P5 S3 correlate with the presence of oligomeric/polymeric silicic acid, presumably due to P5 S3 -silica interactions. These P5 S3 -silica interactions appear, at least in part, ionic in nature since negatively charged dodecylsulfate caused similar perturbations to the P5 S3 CD spectrum as observed with silica, while uncharged ß-d-dodecyl maltoside did not affect the CD spectrum of P5 S3 . Thus, with an associated increase in α-helical character, P5 S3 influences both the condensation of silicic acid into silica and its decondensation back to silicic acid.

Impact of Amino Acids on the Isomerization of the Aluminum Tridecamer Al13

The stability of the Keggin polycation ε-Al₁₃ is monitored by ²⁷Al NMR and ferron colorimetric assay upon heating aluminum aqueous solutions containing different amino acids with overall positive, negative, or no charge at pH 4.2. A focus on the effect of the amino acids on the isomerization process from ε- to δ-Al₁₃ is made, compared and discussed as a function of the type of organic additive. Amino acids such as glycine and β-alanine, with only one functional group interacting relatively strongly with aluminum polycations, accelerate isomerization in a concentration-dependent manner. The effect of this class of amino acids is also found increasing with the pK_a of their carboxylic acid moiety, from a low impact from proline up to more than a 15-fold increased rate from the stronger binders such as glycine or β-alanine. Amino acids with relatively low C-terminal pK_a, but bearing additional potential binding moieties such as free alcohol (hydroxyl group) moiety of serine or the amide of glutamine, speed the isomerization comparatively and even more than glycine or β-alanine, glutamine leading to the fastest rates observed so far. With aspartic and glutamic acids, changes in aluminum speciation are faster and significant even at room temperature but rather related to the reorganization toward slow reacting complexed oligomers than to the Al₁₃ isomerization process. The linear relation between the apparent rate constant of isomerization and the additive concentration points to a first-order process with respect to the additives. Most likely, the dominant process is an accelerated ε-Al₁₃ dissociation, increasing the probability of δ isomer formation.

Biocatalysis in Silicon Chemistry

The application of biocatalytic or chemoenzymatic techniques in silicon chemistry serves two roles: it provides a greater understanding of the processing of silicon species by natural systems, such as plants, diatoms, and sponges, as well opening up avenues to green methodologies in the field. In the latter case, biocatalytic approaches have been applied to the synthesis of small-molecule systems and polymeric materials. Often these biocatalytic approaches allow access to molecular structures under mild conditions and, in some cases, molecular structures that are not accessible through traditional chemical methodologies. A review of recent advances in the applications of biocatalysis in silicon chemistry is presented.

Silicic acid condensation under the influence of water-soluble polymers: from biology to new materials

Silicic acid condensation under the influence of functional polymers is reviewed starting from biology to new materials.

Hierarchical architecture of sponge spicules: biocatalytic and structure-directing activity of silicatein proteins as model for bioinspired applications

Since the first description of the silicateins, a group of enzymes that mediate the formation of the amorphous, hydrated biosilica of the skeleton of the siliceous sponges, much progress has been achieved in the understanding of this biomineralization process. These discoveries include, beside the proof of the enzymatic nature of the sponge biosilica formation, the dual property of the enzyme, to act both as a structure-forming and structure-guiding protein, and the demonstration that the initial product of silicatein is a soft, gel-like material that has to undergo a maturation process during which it achieves its favorable physical-chemical properties allowing the development of various technological or medical applications. This process comprises the hardening of the material by the removal of water and ions, its cast-molding to specific morphologies, as well as the fusion of the biosilica nanoparticles through a biosintering mechanism. The discovery that the enzymatically formed biosilica is morphogenetically active and printable also opens new applications in rapid prototyping and three-dimensional bioprinting of customized scaffolds/implants for biomedical use.

Silica-assisted incorporation of polydopamine into the framework of porous nanocarriers by a facile one-pot synthesis

Mussel-inspired polydopamine (PDA), with its advanced bio-adhesive properties, has shown great potential in drug delivery based on host–guest interaction.

Involvement of aquaporin channels in water extrusion from biosilica during maturation of sponge siliceous spicules

Aquaporins are a family of small, pore-forming, integral cell membrane proteins. This ancient protein family functions as water channels and is found in all kingdoms (including archaea, eubacteria, fungi, plants, and animals). We discovered that in sponges aquaporin plays a novel role during the maturation of spicules, their skeletal elements. Spicules are synthesized enzymatically via silicatein following a polycondensation reaction. During this process, a 1:1 stoichiometric release of water per one Si-O-Si bond formed is produced. The product of silicatein, biosilica, is a fluffy, soft material that must be hardened in order to function as a solid rod. Using the model of the demosponge species Suberites domuncula Olivi, 1792, which expresses aquaporin, cDNA was cloned and the protein was heterologously expressed. The sponge aquaporin is grouped with the type 8 aquaporins. The function of the sponge aquaporin can be blocked by Mn-sulfate (MnSO4) and mercury chloride (HgCl2). Microscopic and functional studies suggest that aquaporin is involved in removal of the reaction water at the site where siliceous spicules are formed. Another molecule that is likely to be involved in biosilica maturation is the mucin/nidogen-like polypeptide. cDNA has also been cloned from S. domuncula. Experimental studies suggest that water extrusion/suctioning from biosilica after enzymatic synthesis during spicule formation involves both aquaporin-mediated water channeling and "polymerization-induced phase separation" facilitated by the mucin/nidogen-like polypeptide.

Molecular diversity in phenolic and polyphenolic precursors of tannin-inspired nanocoatings

The strong interfacial properties of selected plant polyphenols were recently exploited in forming functionally versatile nanocoatings via dip-coating. Here, we screened a library of ~20 natural and synthetic phenols and polyphenols, identifying eight catechol-, gallol- and resorcinol-rich precursors capable of forming coatings. Several newly identified compounds expand the molecular diversity of tannin-inspired coatings.

Magnesiothermic conversion of the silica-mineralizing golden algae Mallomonas caudata and Synura petersenii to elemental silicon with high geometric precision

Chrysophyceae, also known as golden algae, contain characteristic, three-dimensional biomineralized silica structures. Their chemical composition and microscopic structure was studied. By high-temperature conversion of the skeleton of Mallomonas caudata and Synura petersenii into elementary silicon by magnesium vapour, nanostructured defined replicates were produced which were clearly seen after removal of the formed magnesium oxide with acid.

Pre-nucleation clusters as solute precursors in crystallisation

Crystallisation is at the heart of various scientific disciplines, but still the understanding of the molecular mechanisms underlying phase separation and the formation of the first solid particles in aqueous solution is rather limited. In this review, classical nucleation theory, as well as established concepts of spinodal decomposition and liquid-liquid demixing, is introduced together with a description of the recently proposed pre-nucleation cluster pathway. The features of pre-nucleation clusters are presented and discussed in relation to recent modifications of the classical and established models for phase separation, together with a review of experimental work and computer simulations on the characteristics of pre-nucleation clusters of calcium phosphate, calcium carbonate, iron(oxy)(hydr)oxide, silica, and also amino acids as an example of small organic molecules. The role of pre-nucleation clusters as solute precursors in the emergence of a new phase is summarized, and the link between the chemical speciation of homogeneous solutions and the process of phase separation via pre-nucleation clusters is highlighted.

The effect of silica on polymorphic precipitation of calcium carbonate: an on-line energy-dispersive X-ray diffraction (EDXRD) study

Calcium carbonate is the most abundant biomineral and a compound of great industrial importance. Its precipitation from solution has been studied extensively and was often shown to proceed via distinct intermediate phases, which undergo sequential transformations before eventually yielding the stable crystalline polymorph, calcite. In the present work, we have investigated the crystallisation of calcium carbonate in a time-resolved and non-invasive manner by means of energy-dispersive X-ray diffraction (EDXRD) using synchrotron radiation. In particular, the role of silica as a soluble additive during the crystallisation process was examined. Measurements were carried out at different temperatures (20, 50 and 80 °C) and various silica concentrations. Experiments conducted in the absence of silica reflect the continuous conversion of kinetically formed metastable polymorphs (vaterite and aragonite) to calcite and allow for quantifying the progress of transformation. Addition of silica induced remarkable changes in the temporal evolution of polymorphic fractions existing in the system. Essentially, the formation of calcite was found to be accelerated at 20 °C, whereas marked retardation or complete inhibition of phase transitions was observed at higher temperatures. These findings are explained in terms of a competition between the promotional effect of silica on calcite growth rates and kinetic stabilisation of vaterite and aragonite due to adsorption (or precipitation) of silica on their surfaces, along with temperature-dependent variations of silica condensation rates. Data collected at high silica concentrations indicate the presence of an amorphous phase over extended frames of time, suggesting that initially generated ACC particles are progressively stabilised by silica. Our results may have important implications for CaCO3 precipitation scenarios in both geochemical and industrial settings, where solution silicate is omnipresent, as well as for CO2 sequestration technologies.

Small-angle X-ray scattering for imaging of surface layers on intact bacteria in the native environment

Crystalline cell surface layers (S-layers) represent a natural two-dimensional (2D) protein self-assembly system with nanometer-scale periodicity that decorate many prokaryotic cells. Here, we analyze the S-layer on intact bacterial cells of the Gram-positive organism Geobacillus stearothermophilus ATCC 12980 and the Gram-negative organism Aquaspirillum serpens MW5 by small-angle X-ray scattering (SAXS) and relate it to the structure obtained by transmission electron microscopy (TEM) after platinum/carbon shadowing. By measuring the scattering pattern of X rays obtained from a suspension of bacterial cells, integral information on structural elements such as the thickness and lattice parameters of the S-layers on intact, hydrated cells can be obtained nondestructively. In contrast, TEM of whole mounts is used to analyze the S-layer lattice type and parameters as well as the physical structure in a nonaqueous environment and local information on the structure is delivered. Application of SAXS to S-layer research on intact bacteria is a challenging task, as the scattering volume of the generally thin (3- to 30-nm) bacterial S-layers is low in comparison to the scattering volume of the bacterium itself. For enhancement of the scattering contrast of the S-layer in SAXS measurement, either silicification (treatment with tetraethyl orthosilicate) is used, or the difference between SAXS signals from an S-layer-deficient mutant and the corresponding S-layer-carrying bacterium is used for determination of the scattering signal. The good agreement of the SAXS and TEM data shows that S-layers on the bacterial cell surface are remarkably stable.

Construction of silica-enhanced S-layer protein cages

The work presented here shows for the first time that it is possible to silicify S-layer coated liposomes and to obtain stable functionalized hollow nano-containers. For this purpose, the S-layer protein of Geobacillus stearothermophilus PV72/p2 was recombinantly expressed and used for coating positively charged liposomes composed of dipalmitoylphosphatidylcholine, cholesterol and hexadecylamine in a molar ratio of 10:5:4. Subsequently, plain (uncoated) liposomes and S-layer coated liposomes were silicified. Determination of the charge of the constructs during silicification allowed the deposition process to be followed. After the particles had been silicified, lipids were dissolved by treatment with Triton X-100 with the release of previously entrapped fluorescent dyes being determined by fluorimetry. Both, ζ-potential and release experiments showed differences between silicified plain liposomes and silicified S-layer coated liposomes. The results of the individual preparation steps were examined by embedding the respective assemblies in resin, ultrathin sectioning and inspection by bright-field transmission electron microscopy (TEM). Energy filtered TEM confirmed the successful construction of S-layer based silica cages. It is anticipated that this approach will provide a key to enabling technology for the fabrication of nanoporous protein cages for applications ranging from nano medicine to materials science.

An overview of the fundamentals of the chemistry of silica with relevance to biosilicification and technological advances

Biomineral formation is widespread in nature, and occurs in bacteria, single-celled protists, plants, invertebrates, and vertebrates. Minerals formed in the biological environment often show unusual physical properties (e.g. strength, degree of hydration) and often have structures that exhibit order on many length scales. Biosilica, found in single-celled organisms through to higher plants and primitive animals (sponges), is formed from an environment that is undersaturated with respect to silicon, and under conditions of approximately neutral pH and relatively low temperatures of 4-40 °C compared to those used industrially. Formation of the mineral may occur intracellularly or extracellularly, and specific biochemical locations for mineral deposition that include lipids, proteins and carbohydrates are known. In most cases, the formation of the mineral phase is linked to cellular processes, an understanding of which could lead to the design of new materials for biomedical, optical and other applications. In this contribution, we describe the aqueous chemistry of silica, from uncondensed monomers through to colloidal particles and 3D structures, that is relevant to the environment from which the biomineral forms. We then describe the chemistry of silica formation from alkoxides such as tetraethoxysilane, as this and other silanes have been used to study the chemistry of silica formation using silicatein, and such precursors are often used in the preparation of silicas for technological applications. The focus of this article is on the methods, experimental and computational, by which the process of silica formation can be studied, with an emphasis on speciation.

Chemistry of aqueous silica nanoparticle surfaces and the mechanism of selective peptide adsorption

Control over selective recognition of biomolecules on inorganic nanoparticles is a major challenge for the synthesis of new catalysts, functional carriers for therapeutics, and assembly of renewable biobased materials. We found low sequence similarity among sequences of peptides strongly attracted to amorphous silica nanoparticles of various size (15-450 nm) using combinatorial phage display methods. Characterization of the surface by acid base titrations and zeta potential measurements revealed that the acidity of the silica particles increased with larger particle size, corresponding to between 5% and 20% ionization of silanol groups at pH 7. The wide range of surface ionization results in the attraction of increasingly basic peptides to increasingly acidic nanoparticles, along with major changes in the aqueous interfacial layer as seen in molecular dynamics simulation. We identified the mechanism of peptide adsorption using binding assays, zeta potential measurements, IR spectra, and molecular simulations of the purified peptides (without phage) in contact with uniformly sized silica particles. Positively charged peptides are strongly attracted to anionic silica surfaces by ion pairing of protonated N-termini, Lys side chains, and Arg side chains with negatively charged siloxide groups. Further, attraction of the peptides to the surface involves hydrogen bonds between polar groups in the peptide with silanol and siloxide groups on the silica surface, as well as ion-dipole, dipole-dipole, and van-der-Waals interactions. Electrostatic attraction between peptides and particle surfaces is supported by neutralization of zeta potentials, an inverse correlation between the required peptide concentration for measurable adsorption and the peptide pI, and proximity of cationic groups to the surface in the computation. The importance of hydrogen bonds and polar interactions is supported by adsorption of noncationic peptides containing Ser, His, and Asp residues, including the formation of multilayers. We also demonstrate tuning of interfacial interactions using mutant peptides with an excellent correlation between adsorption measurements, zeta potentials, computed adsorption energies, and the proposed binding mechanism. Follow-on questions about the relation between peptide adsorption on silica nanoparticles and mineralization of silica from peptide-stabilized precursors are raised.

Bioinspired silicification of silica-binding peptide-silk protein chimeras: comparison of chemically and genetically produced proteins

Novel protein chimeras constituted of "silk" and a silica-binding peptide (KSLSRHDHIHHH) were synthesized by genetic or chemical approaches and their influence on silica-silk based chimera composite formation evaluated. Genetic chimeras were constructed from 6 or 15 repeats of the 32 amino acid consensus sequence of Nephila clavipes spider silk ([SGRGGLGGQG AGAAAAAGGA GQGGYGGLGSQG](n)) to which one silica binding peptide was fused at the N terminus. For the chemical chimera, 28 equiv of the silica binding peptide were chemically coupled to natural Bombyx mori silk after modification of tyrosine groups by diazonium coupling and EDC/NHS activation of all acid groups. After silica formation under mild, biomaterial-compatible conditions, the effect of peptide addition on the properties of the silk and chimeric silk-silica composite materials was explored. The composite biomaterial properties could be related to the extent of silica condensation and to the higher number of silica binding sites in the chemical chimera as compared with the genetically derived variants. In all cases, the structure of the protein/chimera in solution dictated the type of composite structure that formed with the silica deposition process having little effect on the secondary structural composition of the silk-based materials. Similarly to our study of genetic silk based chimeras containing the R5 peptide (SSKKSGSYSGSKGSKRRIL), the role of the chimeras (genetic and chemical) used in the present study resided more in aggregation and scaffolding than in the catalysis of condensation. The variables of peptide identity, silk construct (number of consensus repeats or silk source), and approach to synthesis (genetic or chemical) can be used to "tune" the properties of the composite materials formed and is a general approach that can be used to prepare a range of materials for biomedical and sensor-based applications.

Interactions of amines with silicon species in undersaturated solutions leads to dissolution and/or precipitation of silica

The biogeochemical silicon cycle is the focus for many researchers studying the dissolution of silicon species from quartz, amorphous, and biogenic silica. Furthermore, the precipitation of biogenic silica by diatoms, radiolarian, sponges, and plants is also a popular focus for research. The ornate silica structures created by these species has attracted interest from biomaterial scientists and biochemists who have studied mineral formation in an attempt to understand how biogenic silica is formed, often in the presence of proteins and long chain polyamines. This article is at the interface of these seemingly distinct research areas. Here we investigate the effect of a range of amines in globally undersaturated silicon environments. Results are presented on the effect of amine-containing molecules on the formation of silica from undersaturated solutions of orthosilicic acid and globally undersaturated silicon environments. We sought to address two questions: can silica be precipitated/harvested from undersaturated solutions, and can we identify the silicon species that are most active in silica formation? We demonstrate that none of the bioinspired additives investigated here (e.g., poly(allylamine hydrochloride), pentaethylenehexamine, and propylamines) have any influence on orthosilicic acid at undersaturated concentrations. However, under globally undersaturated silicon concentrations, small molecules and polymers containing amine groups were able to interact with oligomers of silicic acid to either generate aggregated materials that can be isolated from solution or increase rates of oligomer dissolution back to orthosilicic acid. Additional outcomes of this study include an extended understanding of how polyelectrolytes and small molecules can promote and/or inhibit silica dissolution and a new method to explore how (bio)organic molecules interact with a forming mineral phase.

Subtleties of biomineralisation revealed by manipulation of the eggshell membrane

Biocalcification of collagen matrices with calcium phosphate and biosilicification of diatom frustules with amorphous silica are two discrete processes that have intrigued biologists and materials scientists for decades. Recent advancements in the understanding of the mechanisms involved in these two biomineralisation processes have resulted in the use of biomimetic strategies to replicate these processes separately using polyanionic, polycationic or zwitterionic analogues of extracellular matrix proteins to stabilise amorphous mineral precursor phases. To date, there is a lack of a universal model that enables the subtleties of these two apparently dissimilar biomineralisation processes to be studied together. Here, we utilise the eggshell membrane as a universal model for differential biomimetic calcification and silicification. By manipulating the eggshell membrane to render it permeable to stabilised mineral precursors, it is possible to introduce nanostructured calcium phosphate or silica into eggshell membrane fibre cores or mantles. We provide a model for infiltrating the two compartmental niches of a biopolymer membrane with different intrafibre minerals to obtain materials with potentially improved structure-property relationships.

Calcium and silicon mineralization in land plants: transport, structure and function

Plant biomineralization involves calcium and silicon transport and mineralization. Respective analytical methods and case studies are listed. Calcium carbonate is deposited in cystoliths, calcium oxalate in idioblasts. Silicon is deposited in phytoliths. Biomineralization is a coordinated process.

Biomimetic and bioinspired silica: recent developments and applications

In a previous review of biological and bioinspired silica formation (S. V. Patwardhan et al., Chem. Commun., 2005, 1113 [ref. 1]), we have identified and discussed the roles that organic molecules (additives) play in silica formation in vitro. Tremendous progress has been made in this field since and this review attempts to capture, with selected examples from the literature, the key advances in synthesising and controlling properties of silica-based materials using bioinspired approaches, i.e. conditions of near-neutral pH, all aqueous environments and room temperature. One important reason to investigate biosilicifying systems is to be able to develop novel materials and/or technologies suitable for a wide range of applications. Therefore, this review will also focus on applications arising from research on biological and bioinspired silica. A range of applications such as in the areas of sensors, coatings, hybrid materials, catalysis and biocatalysis and drug delivery have started appearing. Furthermore, scale-up of this technology suitable for large-scale manufacturing has proven the potential of biologically inspired synthesis.