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

Vadim V. Annenkov *^a, Elena N. Danilovtseva ^a, Viktor A. Pal'shin ^a, Ol'ga N. Verkhozina^a, Stanislav N. Zelinskiy ^a and Uma Maheswari Krishnan ^b
aLimnological Institute of the Siberian Branch of the Russian Academy of Sciences, 3, Ulan-Batorskaya St., P.O. Box 278, Irkutsk, 664033, Russia. E-mail: annenkov@lin.irk.ru; annenkov@yahoo.com; Tel: +7-9148982577
^bCentre for Nanotechnology & Advanced Biomaterials (CeNTAB), School of Chemical and Biotechnology, SASTRA University, Thanjavur – 613401, Tamil Nadu, India

First published on 12th April 2017

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Vadim V. Annenkov

Vadim Annenkov was graduated from Irkutsk State University, Russia, in 1984. He received Ph.D. in Macromolecular Science from Irkutsk Institute of Chemistry in 1989 and D.Sc. degree from Irkutsk State University in 2001. He is a Deputy Director of the Limnological Institute SB RAS, Irkutsk, Russia. He has near 120 scientific papers and 18 patents. Research interests include synthesis and properties of hydrophilic polymers; physical chemistry of polymer solutions; mathematical simulation of macromolecular systems; study of the molecular mechanisms of biomineralization; biomimetic methods for nanoparticles synthesis; design systems for immune and genetic diagnosis.

Elena N. Danilovtseva

Elena Danilovtseva was graduated from the Department of Chemistry, Irkutsk State University, Russia in 1984. She received her Ph.D. in Macromolecular Science from the Institute of Chemical Sciences, Almaty, Kazakstan in 1991. She is a Senior researcher in Limnological Institute SB RAS, Irkutsk, Russia. She is author of near 75 scientific papers and 12 patents. Her scientific interests include synthesis and properties of water-soluble polymers; complexes of polymers with metal ions and oxide particles; molecular mechanisms of biosilicification; biomimetic methods for the preparation of nanoparticles and hybrid materials; design of functionalized coatings.

Viktor A. Pal'shin

Viktor Pal'shin was graduated from the Department of Chemistry, Irkutsk State University, Russia in 2008. He received his Ph.D. in Macromolecular Science from the Institute of Problems of Chemical Physics, Chernogolovka, Russia in 2012. He is a researcher in Limnological Institute SB RAS, Irkutsk, Russia. He is the author of 7 scientific articles and 1 patent. Scientific interests: synthesis and properties of organo-silica nanoparticles; colloidal systems; biosilicification; dynamic and static light scattering.

Stanislav N. Zelinskiy

Stanislav Zelinskiy was graduated from the Department of Chemistry, Irkutsk State University, Russia in 1998. He received his Ph.D. in Catalysis from the Irkutsk State University in 2002. He is a researcher in Limnological Institute SB RAS, Irkutsk, Russia. He is author of 25 scientific articles and 5 patents. His scientific interests include synthesis, structure and reactivity of organic compounds; metal-complex catalysis; synthesis of fluorescent dyes and labels.

Uma Maheswari Krishnan

Uma Maheswari Krishnan is a Professor at the Centre for Nanotechnology and Advanced Biomaterials (CeNTAB) and School of Chemical and Biotechnology, SASTRA University, India. She received Ph.D. degree in Applied Chemistry from Bharathiar University, India in 2000 and has post-doctoral experience from the Southwestern Medical Centre, University of Texas, USA. She received training on thin film processing and clean room processes at University of Arkansas, USA. In 2003 she joined as a Lecturer at SASTRA University and is currently the Associate Dean of Bioengineering and Chemistry. Her research interests include smart drug delivery systems, nanobiosensors, electrophysiology and mesoporous materials. She has about 150 publications in these areas.

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Introduction

Silicon is among the most abundant elements on the Earth, it occurs in many minerals and plays an important role in biochemical processes. Some living organisms use silicon dioxide as a structural material in their bodies.¹ Diatom algae are unicellular organisms which provide more than 20% of photosynthetic oxygen and contribute to a corresponding amount of primary organic production.² The diatom cell lives in a “glass house”³ – siliceous frustule built from silicon dioxide in a form similar to molten quartz glass (Fig. 1A). Sponges belong to a class of ancient and interesting invertebrates which play an important role in ecosystems as filtering organisms. They also serve as a residence for an enormous number of symbionts. Sponge bodies are inexhaustible source of biologically active compounds.⁴ Siliceous sponges represent more than 90% of sponges⁵ and their skeleton is based on siliceous spicules – composite needle-like constructions (Fig. 1B). Rice (Oryza sativa) is a staple crop consumed predominantly by the Asian population. It has been found that the xylem sap in these plants are rich in silicic acid. The role of silicon in augmenting disease resistance,⁶ chlorophyll a content and growth⁷ of rice plants has been demonstrated by different research groups. Silica deposits have been found in the cell walls of rice plants and it has been postulated that these silica deposits enhance the stability of the cell wall structure during mitotic cell division.⁸ Similarly, silicon deposits have also been identified in the stomata of rice plants and it is believed that these deposits tend to reduce water loss through transpiration.⁹ The nano-dimensional silica particles in rice are believed to be formed using hemi-cellulose callose as a template.¹⁰ Biogenic silica and composites are a great challenge to material science specialists because these materials are highly ordered at various levels and a lot of them, e.g. diatom frustules consists of a material similar to melting quartz glass¹¹ which artificial analogs are produced at temperatures above 1000 °C only. Siliceous organisms put many questions to biologists and biochemists: how living organisms capture silicon from the environment? How they store and transport silicon in their bodies? How they build highly ordered constructions from silicon dioxide without high temperatures and hazardous chemicals?

Silicon is present in natural water mainly in the form of monomeric silicic acid. Evidently, transformation of Si(OH)₄ to solid SiO₂ or siliceous composites proceeds through condensation in the presence of some organic substances, probably biopolymers. The nature and action mechanism of these substances is yet not thoroughly deciphered. The main approach to discovering novel biosilicification agents involves isolation and separation of organic substances from biosilica, structure identification, study of silicification activity using in vitro models, monitoring the transformation of the molecule of interest in the organism and finally formulation of the hypothesis on the mechanism of biosilicification. This strategy had resulted in the discovery of silaffins – proteins with phosphate and polyamine post-translation modifications^12,13 and silicateins – proteins capable of catalyzing hydrolysis of Si–O–C bonds.¹⁴ The growing interest in biosilicification during the last few decades has resulted in concerted research efforts devoted to aqueous phase silicic acid condensation in the presence of organic polymers bearing functional groups similar to groups in the hypothesized natural silicification agents. Such studies have at least two objectives – to understand biological processes with simpler and available models and to obtain new bioinspired material, including nanoparticles and bulk composites with the intricate and complex morphologies commonly encountered in the biological systems.

This review summarizes information about Si(OH)₄ condensation in aqueous medium in the presence of oligomeric or polymeric molecules capable of interacting with monomeric or polymeric silicic acid. Biological importance and possible applications of these materials are also discussed. Siliceous materials are intensively studied in various fields and we limit this review with water-based systems where free silicic acid is evidently present and plays an important role.

Precursors of silicic acid

Silicic acid is not stable in the free state and hence it is produced in situ from various precursors. The most simple and cheap precursors are inorganic silicates like sodium, potassium silicates and liquid glasses of variable composition, e.g. Na₂O·xSiO₂.¹⁵ Silicic acid is a very weak acid (pK_a = 9.7–9.9)^16,17 and addition of acid to a solution of inorganic silicates results in formation of Si(OH)₄. The other convenient way to obtain silicic acid from inorganic silicates is through the action of cation-exchange resin in H-form. This procedure facilitates the formation of Si(OH)₄ solution free from inorganic salts. Siliceous sols were prepared by acidification of sodium silicate solution with HCl (pH = 1–2) following with action of a cation-exchange resin which removes Na⁺ ions.¹⁸ The obtained solutions were not analyzed and it seems that monomeric silicic acid is the main component of these solutions because Si(OH)₄ is stable in acidic area.¹⁹

There are also several organic derivatives of silicic acid which are easily hydrolyzable in aqueous medium. Tetramethyl orthosilicate Si(OMe)₄ (TMOS) interacts with water and gives rise to silicic acid and methanol in approximately ten seconds. TMOS is usually added to 1–2 mM HCl solution which prevents Si(OH)₄ from condensation during hydrolysis.^20,21

Silicon catecholate salts are obtained by the reaction between 1,2-dihydroxybenzene (catechol) salt and tetraethyl orthosilicate (TEOS).²² These compounds are readily hydrolysable at neutral pH values as shown below:

The first study on silicon catecholate salt hydrolysis²³ showed that the concentration of the complex reduces from 50 mM to 20–28 mM during the first 2 min of the reaction and these levels remained virtually unaltered even after 24 h. These data however are in contradiction with the results²⁴ reported for potassium silicon catecholate salt (K2-SiKat) which showed a decrease of K2-SiKat concentration from 30 mM to 0.1–0.7 mM immediately after the pH was adjusted to 7.

Tetrakis(2-hydroxyethyl)orthosilicate (THEOS) was obtained by the reaction between TMOS and ethylene glycol²⁵ as follows:

This is a water soluble compound which on hydrolysis gives rise to silicic acid. THEOS was recommended as the precursor for synthesis of unshrinkable silica gels in aqueous medium.²⁶ There is however, no information about the hydrolysis rate for THEOS and hence participation of partially hydrolyzed forms in the condensation reaction cannot be excluded.

Inorganic silicates produce silicic acid immediately after neutralization with acid and the only byproduct formed is inorganic salt, e.g. NaCl. Hence these compounds are most suitable for study of mechanism of silicic acid condensation and for simulating biochemical reactions. On the other hand, the fast hydrolysis rate and alkaline nature of the silicates complicates experiments and can result in irreproducibility. Organic precursors do not have these disadvantages and can be applied for design of siliceous materials. But the stepwise hydrolysis and formation of organic byproducts decrease the utility of these compounds to serve as precursors of Si(OH)₄.

Silicic acid condensation in aqueous medium

Condensation of silicic acid has been extensively studied for many decades and it is described in several books and reviews.^19,27–31 We describe here the main peculiarities of the reaction in aqueous medium.

Silicate ions SiO₄⁴⁻ are stable at high pH values but at pH 13 formation of oligosilicate particles containing several silicon atoms was observed.³² The condensation proceeds as a nucleophilic substitution reaction (S_N2) between ∼Si–OH and ∼Si–O⁻ moieties through a pentacoordinated transition state:^27,33–37

This reaction requires ionized silanol and neutral ∼Si–OH groups. The first ionization constant of Si(OH)₄ (pK_a1) is 9–10^{16,17,38–45} and the subsequent pK_a values are estimated as 11.7–13.4.^17,38–45 While discussing quantitative values concerning silicic acid, we must remember that Si(OH)₄ is highly unstable and exists at low pH or in very diluted solutions only. In other cases, various condensed forms appear in several seconds which poses a challenge to the investigators. So, we have some intervals in the measured values. Condensation of silicic acid is a reversible process and the minimal concentration of monomeric Si(OH)₄ in equilibrium with solid silica or condensed soluble siliceous particles is estimated as 2–3 mM.¹⁹

Assuming pK_a1 as 10, one can calculate the dependence of the concentration of unionized Si(OH)₄ species on pH (Fig. 2A). The ∼Si–OH moieties occur at relatively high pH and according to our observations, 100 mM aqueous solutions of sodium silicate contain >50% of oligosilicates after several weeks of storage. A decrease in pH to 9.5–10.5 results in formation of considerable amounts of Si(OH)₄ and Si(OH)₃O⁻ molecules which leads to fast condensation of the silicic acid monomers. Further decrease of pH to 7 and below results in almost completely unionized silicic acid and the condensation rate drops.

The other participant of the condensation reaction is oligosilicate molecules or poly(silicic acid) (PSA): dimers, trimers and larger particles. PSA contains more acidic groups than monomeric Si(OH)₄. The pK₀ for ionization of a first ∼Si–OH group on the surface of PSA particles is estimated as 6.8.^19,46 Thus, after formation of PSA particles we have ∼ Si–O⁻ moieties even at neutral and acidic conditions (Fig. 2B). This results in two peculiarities of silicic acid condensation:

(a) Condensation at pH 7 and below proceeds with an incubation period which is necessary for formation of primary PSA particles.^19,47

(b) The condensation rate depends not only on pH but also on prehistory of the solution, e.g. if monomeric silicic acid is maintained at pH 5, it will be stable for some time, but if the solution is stored at pH 9 for several minutes prior to incubation at pH 5, PSA particles will appear and we will observe condensation on the surface of these particles. This unique characteristic opens up a wide range of possibilities for design of siliceous materials. However, it also poses difficulties in data reproducibility.

Thus, the first stage of silicic acid condensation results in formation of soluble PSA particles accompanied by a decrease of Si(OH)₄ concentration to 2–3 mM. Further fate of these particles depends mainly on pH and total silicon concentration. There are two possibilities for transformation of the primary PSA particles: aggregation and Ostwald ripening.^19,27 Aggregation is observed at pH < 7 because primary particles are almost uncharged in this condition and therefore can interact with each other giving rise to 3D gel structures at concentration greater than 50–100 mM.⁴⁸ Ostwald ripening^49,50 is a process of growing large particles at the expense of small particles and this is more characteristic at pH > 7 where PSA particles bear negative charge that prevents aggregation through electrostatic repulsion.

Study of the silicic acid condensation in aqueous medium requires methods for precisely measuring the concentration of monomeric silicic acid. The widely used procedure for quantification of silicic acid involves the reaction of silicic acid with molybdate in acidic medium resulting in the formation of yellow molybdosilicate complexes, the absorbance of which can be measured using a spectrophotometer.^51–53 Sensitivity of the method increases when the yellow complex is reduced to blue products with N-methylaminophenol (metol).^54,55 The procedure is optimized to allow only monomers and dimers of silicic acid to react with the molybdate reagent.^24,31,56 The molybdate method is not applicable in the presence of phosphates, e.g. when a phosphate buffer is used. An alternate method to measure monomeric silicic acid and its various condensed forms is through ²⁹Si NMR spectrometry.⁴⁸ Unfortunately, the natural content of this isotope (4.7%) does not permit the study of fast reactions at relatively low concentrations (tens of mmol). Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy is also must be mentioned as a convenient and quick method which allows to distinguish monomer and oligomer siliceous species using absorbance in the wavenumber region 1250–850 cm⁻¹.⁵⁷

Influence of organic polymers on condensation of monomeric silicic acid

A scan of literature reveals many reports that have described silicic acid condensation in the presence of organic polymers. These have been summarized in Table 1. Study of Si(OH)₄ condensation using the molybdate method demonstrated the influence of some polymers on kinetics of the reaction. Acceleration^{58,59,63,64,73,82,96,99,108,112,136,158,159} and inhibition^{24,72,83–85,102,124,133–135,137} of the reaction monitored as consumption of Si(OH)₄ has been reported. As mentioned earlier, the condensation of monomeric silicic acid near neutral pH values proceeds mainly through its reaction with PSA particles. Polymers can influence the process by interaction with Si(OH)₄ or with PSA molecules. The interaction between PSA and functional organic polymers proceeds similar to the interpolymer reactions between two organic polymers. These reactions have several peculiarities.¹⁶⁸

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(a) The interaction is caused by relatively weak bonds which cannot result in new compounds in the case of monomeric analogs. For example, a weak poly(acrylic acid) can interact with a very weak base poly(1-vinylpyrrolidone) in water solution giving rise to insoluble compound of 1:1 stoichiometry:¹⁶⁹

(b) The interpolymeric reactions are favorable from an entropy point of view comparing with a reaction between small molecules and unusual reactions are possible with polymers, e.g. displacement of strong acid with weak acid:

(c) The cooperative character of the interpolymeric reactions appears in the presence of a minimal critical length of the interacting sequences after which the interaction becomes almost irreversible. The value of critical length depends on the nature of the interacting groups and possible supramolecular effects. For instance, substitution of poly(acrylic acid) with poly(methacrylic acid) in the reaction with polyethylene glycol (PEG) decreases the critical length from ≈100 to ≈20 because of the stabilizing effect arising due to the hydrophobic interactions of the methyl groups from the methacrylic acid moiety.¹⁷⁰

(d) The stoichiometry of the interacting polymeric chains is also important, especially in the case of rigid chain polymers and PSA particles which are strengthened due to cross-linking through Si–O–Si bonds.

Thus, organic polymers which bear basic groups can interact with PSA by two ways:

1. Ionic bonds in the case of relatively strong polymeric bases or polymeric cations:

This kind of interaction increases the amount of silanol anions which are active in reaction with unionized monomeric silicic acid and acceleration of Si(OH)₄ condensation is observed in neutral and acidic conditions.^{99,112,118,158,171,172}

2. Polymers with weak basic groups interact with PSA particles by forming hydrogen bonds. Multiplicity of these interactions enhances binding and decrease in the number of silanol anions as seen in the silicic acid condensation in the presence of poly(1-vinylimidazole) shown below:

The decrease in ∼Si–O⁻ concentration results in a decrease in the condensation rate.^84,85,102 The same effect is probably dominant in the case of poly(N-methacryloyl-L-histidine),¹⁰² poly(1-vinylpyrrolidone)¹³⁶ and PEG.^134,135,137

There is a series of reports which contradict these schemes.^{131,137,173,174} For instance, inhibition of Si(OH)₄ condensation is observed for a wide range of polymeric cations and amines. The reason for this contradiction is possibly due to the different procedures employed for measurement of monomeric silicic acid concentration. K. Demadis et al.¹⁷³ have used the silicomolybdate method provided by the HACH Company¹⁷⁵ which allows the measurement of a “soluble” (“reactive”) silica, i.e. short oligomers of unknown length apart from the monomer and dimer molecules. In other reports,^{124,132–134,176,177} the molybdate procedure employed also has variations from the recommended protocol for measuring monomeric and dimeric forms of silicic acid exclusively.⁵⁶ Thus, it appears that the visible inhibition of Si(OH)₄ condensation with polycations can be explained by the stabilization of oligomeric forms of PSA through their complexation with polymers that prevents further aggregation of these oligomers.¹⁷⁸ On the other hand, when a blue molybdenum procedure, elaborated to measure only monomer and dimer of silicic acid is employed,¹³⁶ the same team reported acceleration of the condensation with polymeric bases. These findings imply the importance of the quantification method employed.

The formation of organic polymer – short PSA oligomer complexes also explain dissolution of silica in the presence of polymers capable of interacting with charged silicate species.^74,134,179 According to our view, the same concepts are involved in the inhibition action of adipic acid/amine-terminated polyethers D230/diethylenetriamine copolymer.¹⁸⁰ This polymer contains many groups capable of forming hydrogen bonds. Moreover, the addition of polyepoxysuccinic acid increases the inhibition, possibly due to neutralization of amine groups in the copolymer. We must also mention that study of the kinetics of Si(OH)₄ condensation requires a strict observance of all protocols, including Si(OH)₄ preparation from a precursor. For example, in ref. 24 the authors showed that long neutralization of sodium silicate (during 10 min) can result in formation of primary PSA oligomers and the system becomes more complex with presence of entities other than the desired silicic acid–polymer.

Most reports devoted to the kinetics of Si(OH)₄ condensation in the presence of water-soluble polymers demonstrate the influence of polymers on the condensation rate only and the equilibrium Si(OH)₄ concentration was 2–3 mM in the siliceous solutions without additives. These values were also observed as when the condensation proceeded with precipitation of silica–polymer composite. We observed¹⁸¹ a decrease of the equilibrium concentration when silicic acid was condensed in the presence of poly(vinyl amine) (PVA) gel containing Zn²⁺ ions. This gel was able to capture 20% of silicon from 0.1 mM solution which was attributed to the formation of highly stable PVA–PSA complex with participation of zinc ions.

Interaction of polymeric silicic acid with organic polymers in solution

Condensation of silicic acid in the presence of polymers capable of interacting with PSA results in solutions containing composite nanoparticles or various insoluble products. As mentioned in the earlier sections, PSA–polymer complexes are similar to interpolymer complexes of organic polymers. Interaction between two polymers bearing hydrophilic groups gives rise to so-called stoichiometric or non-stoichiometric complexes.^182–184 In the first case, all reactive groups, e.g. ∼COOH and ∼NH₂ react with each other and all hydrophilic groups become blocked from interaction with water medium which often results in water insolubility. The non-stoichiometric complexes are obtained when one of the components is in deficiency or when some peculiarities of the main polymeric chains do not allow stoichiometric interaction. The free polymeric segments provide solubility of these complexes. Polyampholytes often gives soluble interpolymeric complexes because only one group (basic or acidic) can interact with another polymer. Both variants are observed in PSA–polymer complexes but siliceous systems differ from organic interpolymer complexes because condensation and/or dissociation reactions can proceed during interaction with organic polymer. Thus, Si(OH)₄ condensation in the presence of organic polymer includes several parallel processes:

1. Silicic acid condensation and growth of siliceous oligomers.

2. Interaction of the siliceous oligomers with organic polymer when the oligomer size will be sufficient for this interaction.

3. Further transformations of the polymer–PSA complex, including sintering of the silica nanoparticles and conformational changes of the organic macromolecules.

Several main factors influence interaction between growing siliceous particles and organic polymers in aqueous medium.

Nature of the polymeric functional groups, pH and ionic strength

As mentioned earlier, the ability of the organic polymer to interact with PSA by means of hydrogen and/or ionic bonds depends on the basicity of functional groups. Certainly, pH, ionic strength and temperature must influence this reaction by changing the critical length of the interacting sequences. This effect was observed with polymeric amines.^64,90 Greater protonation of the amine groups at pH 7 compared with pH 9–10 resulted in enhancement of polymer–PSA interactions, larger compaction of the composite nanoparticles in the solution and increase in the precipitation probability. Control of pH in the reaction medium is often achieved using a buffer. In this case, we must take into account that phosphate, citrate and other multivalent anions influence the silicic acid condensation in the presence of polymeric amines resulting in precipitation of composites.^65,71 The ability of citrate ion to give aggregates with poly(allylamine) was utilized⁷⁷ to synthesize monodisperse 40–100 nm composite particles.

Length and flexibility of the polymer chain

Growth of siliceous particles during silicic acid condensation proceeds at the same time with interaction with organic polymer and depending on the polymer length at least two situations are possible:

1. Polymer chain is long and flexible enough to capture siliceous particle into polymeric coil (Fig. 3A). This results in encapsulation of siliceous particles in organic macromolecules giving rise to soluble composite nanoparticles similar to non-stoichiometric complexes between organic polymers. This behavior was observed for amine containing polymers,¹¹² polymers with imidazole moieties^82–84 and polyampholytes.⁸⁵ Irrespective of the nature of the functional group and ability of the polymer to accelerate or to inhibit Si(OH)₄ condensation, composite soluble nanoparticles can be obtained if the polymer units can interact with ∼Si–OH or ∼Si–O⁻ and the chain is sufficiently long and flexible. Clusters of 50–180 nm particles built up from regularly sized particles 7–8 nm in diameter were formed from silicic acid condensation in the presence of horsetail extract (Equisetum telmateia).¹⁵⁸ Insoluble cellulose macromolecules can also stabilize 4 nm diameter silica particles.¹⁸⁵

2. When the polymer chain is relatively short, a single polymer coil can not consume a siliceous particle and various aggregates are formed (Fig. 3B–D). Depending on the nature of the polymer and the reaction conditions, we can obtain large multiparticle aggregates solubilized with free segments of the polymeric chains (Fig. 3B) or insoluble material with siliceous particles serving as cross-linker for the organic polymer (Fig. 3C). In the latter case, further fusion of the siliceous particles is possible when organic polymer is deficient (Fig. 3D).

Polymer:Si ratio

Interaction between growing PSA nanoparticles and macromolecular chains often results in stable soluble nanoparticles^{63,64,85,90–93,95,96,115,116,120,131,132} or in a product consisting of uniform spherical composite particles.^{24,61,62,65–71,75–77,98,99,106–108,111,112,114,117,121,124,134,135,137,139,143,149,153,155,157} These variants exist in the case of relatively low silicon content in the system, when all PSA nanoparticles can find chains of organic polymer. Further increase in the silicic acid content results in formation of insoluble non-uniform products^64,83 because of cross-linking by means of silicic acid which was condensed independent of the polymer control.

Biosilicification: how living cell can control silica formation?

Discovery of silaffins^{12,13,140–142} – proteins with post-translational phosphate and polyamine modifications (Table 1, # 73) intensified research focused on investigating silicic acid condensation in the presence of polymeric bases and ampholytes. Silaffins are associated with siliceous frustules of diatoms and the hypothesis of biogenic silica growth around matrix of silaffins was formulated.^140,141,143 Unfortunately, silaffins can be isolated from diatom frustules in microgram quantities only which is not sufficient for study of their properties including their influence on silicic acid condensation. Hence, synthetic organic polymers can serve as a good model of silaffins. On the other hand, introduction of organic polymers into nonaqueous sol–gel systems is a well-known strategy to obtain interesting composite materials.^27,186–188 Thus Si(OH)₄ condensation regulated by water soluble polymers is expected to result in new bioinspired materials.

On analyzing the reported data on silicic acid–polymer systems (Table 1), we found two peculiarities which distinguish synthetic products from diatom frustules (Fig. 1A and B). The precipitates obtained during silicic acid condensation in the presence of organic polymer do not have regular organization at the level of several microns. The usual spherical morphology of these products are often found associated as a continuous structure. The observations of organized structures in some reports^144,189 can be attributed as a sampling artifact because the samples for microscopy were prepared by air-drying of wet precipitates. We showed⁸⁵ that air-drying of composite precipitates can result in self-organization of the particles into highly ordered structures in which the morphology is very distinct from the initial particles precipitated from solution. Self-association under slow drying gives highly organized hexagonal lattices from monodisperse 100–200 nm siliceous particles.⁹⁷ Composite precipitates obtained using an organic polymer contain the polymer in significant amount (≥10%) but the siliceous frustules of diatoms contain <5% of organic compounds,^190,191 and this organic material are not only from silaffins. Moreover, thorough cleaning of diatom frustules decreases carbon content below 0.2%.¹⁹⁰

PSA nanoparticles or oligomers stabilized with biopolymers are the most probable form of silicon in diatoms and similar silicifying organisms.^24,64,144 So, a mechanism of conversion of these composite nanoparticles to almost clear silica must exist. This mechanism must be genetically controlled with the object to provide a large diversity of species-specific frustules. A possible mechanism for such control from the cell is desiccation under the action of aquaporines.¹¹ This hypothesis includes transport of aquaporines to specified places on the membrane of silica deposition vesicle (SDV) by means of cytoskeleton. SDV contains the precursor of silica (composite nanoparticles) and the aquaporines function as specific pores which pump water out of SDV. Removal of water results in increase of silica precursor concentration which then initiates condensation of soluble composite nanoparticles to solid silica. The question that arises here is: can this desiccation-induced condensation produce silica without large content of organic substances? We have done a model experiment with polyampholyte containing polyamine sidechains.⁹⁶ Desiccation was simulated by centrifugation at 50000g which concentrated the composite organo-silica particles at the bottom of the centrifuge tube. We found formation of precipitate from 100–200 nm sintered particles. The precipitate contained 4% of organic polymer which is closer to biosilica than usual composites obtained from silicic acid and organic polymers. Thus, we hypothesize that silaffins function as stabilizing agents of PSA in diatoms and during desiccation-induced condensation a major part of organic polymer is eliminated from the solid material. A possible scheme of the biosilica synthesis is presented on Fig. 4.

Application areas of the new knowledge in silicic acid condensation

Study of silicic acid condensation in the presence of water-soluble polymers is related to processes in solution and to formation of solid substances. Both areas provide valuable information for practical applications.

The ability of organic polymers to stabilize PSA in an easily hydrolysable form is the basis of water treatment applications. Polymers of silicic acids are the cause of detrimental precipitates in industrial and domestic water flow systems. The addition of various polymers capable of interacting with PSA prevents the precipitation.^{131,134,192–194}

The soluble composite nanoparticles could be considered as new biomimetic precursors for the synthesis of more complex siliceous structures. These nanoparticles bear negative or positive charge depending on the nature of the organic polymer and can interact with oppositely charged objects. For instance, poly(allyl amine)-based composite nanoparticles formed hexagonal structures when phosphate ions were added to the solution.¹⁴⁴ Positively charged nanoparticles obtained by Si(OH)₄ condensation in the presence of poly(vinyl amine) reacted with negative composite nanoparticles obtained with poly(1-vinylimidazole-co-acrylic acid) giving rise to a fibrous material containing 40–50% of SiO₂.⁶⁴

The ability of PSA particles to interact with polymeric amines was employed^85,195 to create silicified materials by means of direct ink writing with 3D-printers. These studies are based on bioinspired concepts using matrix-assisted condensation of silicic acid.

A major application of silicification in aqueous medium is the encapsulation of biologically active substances,^{76,95,115,116,120,196} including drug delivery.¹⁹⁷ The siliceous shell is usually formed around a complex of polymeric cation and encapsulated moiety, e.g. nucleic acid in transfection applications. The obtained particles with sizes typically below 200 nm serve as delivery agents, and can release the drug or internalize into the cell following the destruction of the siliceous shell. Silicic acid condensation in the presence of positively charged surfaces opens new vistas for silicification of relatively large objects such as living cells^104,105,148 which retain viability longer than native cells. Islet cell encapsulation in a siliceous layer has been found to be beneficial to extend the survival of the cells. Enzyme encapsulation in siliceous shells is another promising strategy to design highly effective catalysts. Enzymes in free state are homogenous catalysts with disadvantages of low life time, poor stability and large difficulties in separation from the reaction products. Silicic acid condensation in the presence of polymeric bases and enzymes gives rise to particles containing the enzyme^80,100,122 which retains its catalytic activity. The siliceous layers protect the enzyme from undesirable factors and helps to separate the catalyst from the reaction medium through sedimentation. This approach is protected with an extensive patent.⁷⁹ Particles obtained from silicic acid and polymeric bases can also capture other substances, e.g. soybean oil.¹²³

Silicic acid condensation around cationic spherical matrix is a convenient way to obtain hollow particles of various sizes, from hundreds of nanometers to micrometers. The hollow siliceous particles can be applied as containers in drug delivery, as plastic fillers and in design of complex microdevices. Assembly of these particles can commence from organic beads obtained by emulsion polymerization of styrene following with amination of the beads, silicic acid condensation around beads and calcination to remove the organic polymer (Fig. 5A).^97,101 A solid matrix in the form of porous membrane was applied for synthesis of silica microtubes.¹¹⁰ The alternative one-step approach to obtain hollow particles involves silicic acid condensation around self-organized low molecular polyamines (Fig. 5B).⁵⁸ The latter method avoids the calcination stage and hence facilitates recycling of the matrix. A similar approach was applied for assembly of core–shell and hollow particles from zein (a major storage protein from corn Zea mays).¹⁹⁸

Composite submicrometer particles obtained in aqueous medium can be loaded with a fluorescent dye and applied as liquid flow tracers.⁹⁷

The main part of siliceous composite materials is prepared from organic precursors of silicic acid. Sodium silicate is attractive because it is a cheap and ecology-friendly substance comparing to alkoxysilanes. Mesoporous forms of silica based on an amine surfactant as the structure-directing porogen were prepared and used as reinforcing and toughening agents for epoxy polymers.¹⁹⁹ The material obtained from sodium silicate showed smaller framework pore sizes (4–5 nm) comparing with TEOS-based silica (6–21 nm) and provided exceptional strength and toughness.

Introduction of organic polymers, e.g. poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolymer into siliceous gel on the condensation stage results in highly ordered mesoporous structures and pore sizes in the range from 7.5 to 10 nm.²⁰⁰

Silicic acid and its oligomers act as cross-linking agents during polymerization of acrylic acid or acrylamide.²⁰¹ The obtained material can be applied as environmentally-friendly superabsorbent because the crosslinking bonds Si–O–(O)C– are hydrolysable and the sorbent dissolves after several days after application.

Porous materials were obtained by condensation in the presence of PEG^88,164 or similar surfactants and emulsifiers such as Span-80 and Tween-80.⁸⁹ Condensation in the presence of water-insoluble substance, e.g. octadecane, results in the substance encapsulation with a smooth and compact silica surface¹⁶⁵ and in the case of hexane siliceous mesocellular foams were obtained.¹⁶⁶ Organo-silica composites can be applied for synthesis of highly ordered carbon materials by dissolution of silica in hydrofluoric acid. This approach allowed to obtain highly nitrogen doped mesoscopic carbons which show high electrocatalytic activity.^202,203

Conclusion: future prospects in Si(OH)₄–polymer systems

Silicic acid condensation in the presence of water-soluble polymers is a dynamically developing area and several important points come to the fore:

(a) Development of methods to study silicic acid condensation in solution where the traditional molybdate method must be re-examined and standardized with the objective to ensure that the obtained data correspond to monomeric and dimeric forms of Si(OH)₄. This is possible with the use of ²⁹Si NMR spectrometry and isotope-enriched silicic acid. A promising and widely-spread method which allows to distinguish various forms of condensed Si(OH)₄ is attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, including multiple reflection approach. This method requires a special study devoted to the band assignment in water medium in comparison with ²⁹Si NMR data. Soluble nanoparticles formed under silicic acid condensation are difficult systems for study. Light scattering methods are often not sensitive enough for siliceous phase while electron microscopy is complicated and associated with aggregation during sample preparation. So, new approaches are needed in this area, including unusual techniques such as laser ablation combined with aerosol spectrometry.⁶⁴

(b) Simulation of biosilicification where the reaction in aqueous medium is an adequate model to study behavior of silicic acid in living organisms. The accumulated knowledge allows to approach synthesis of artificial silica with structures similar to biogenic silica. On the other hand, mimicking the silica biosynthesis must include simulation of the cell structures participating in this process on a micrometer level, e.g. membranes bearing active domains, because the transition from partially condensed silicic acid coordinated with organic polymer to organized solid silica structures is an important but poorly understood process. The in vitro experiments do not exclude study of living cells like diatoms, sponges, cell cultures of higher organisms. Further progress in this area is expected from coupling of modern equipment with specially designed tracers including fluorescent or spin probes, e.g. tagged silicic acid.

(c) New materials for encapsulation of biologically active compounds into silica or composite particles that opens new possibilities for drug delivery and design of prolonged action preparations. Self-organized water soluble nanoparticles are an attractive option that has emerged from the investigations on silicic acid condensation processes. These particles combine rigidity of siliceous structure with variable functionality of organic polymer. Reversible drying of this nanoparticles is a large challenge for chemists because the presence of active silanol groups provokes aggregation and condensation under usual freeze drying. Elaboration of the novel drying techniques will result in new drug formulations and can solve the problem of the cold chain with preparations containing protein or nucleic acid.

(d) New organic polymers can be developed as further progress in construction of siliceous nanocomposites requires new functional polymers with easily tunable structure, including functionality and the chain length. We can expect involvement of “smart” polymers showing thermo- and pH-sensitivity for the design of “smart” composites.

Acknowledgements

We acknowledge financial support from a joint grant of the Russian Science Foundation (# 16-45-02001) and the Department of Science Technology of the Ministry of Science and Technology of the Republic of India (# INT/RUS/RSF/10).

Notes and references

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