The physics and chemistry of silica-in-silicates nanocomposite hydrogels and their phycocompatibility

Abstract

aSilicates-in-silica nanocomposite hydrogels obtained from sodium silicates/colloidal silica mixtures have previously been found to be useful for bacterial encapsulation. However the extension of synthesis conditions and the understanding of their impact on the silica matrix would widen the applicability of this process in terms of encapsulated organisms and the host properties. Here the influence of silicates and the colloidal silica concentration as well as pH conditions on the gel time, the optical properties, the structural and mechanical properties of silica matrices was studied. We show that gel formation is driven by silicate condensation but that the aggregation of silica colloids also has a major influence on the transparency and structure of the nanocomposites. Three different photosynthetic organisms, cyanobacteria Anabaena flos-aquae and two microalgae Chorella vulgaris and Euglena gracilis, were used as probes of the phycocompatibility of the process. The three organisms were highly sensitive to the silicate concentration, which impacts both the gelation time and ionic strength conditions. The Ludox content was crucial for cyanobacteria as it strongly impacts the Young's modulus of the matrices.The detrimental effect of acidic pH on cell suspension was compensated by the silica network. Overall, it is now possible to select optimal encapsulation conditions based on the physiology of the targeted cells, opening wide perspectives for the design of biosensors and bioreactors.

Silica Hydrogels as Entrapment Material for Microalgae

“…Here, the metal ions occur as by-products. Commonly applied sodium silicate concentrations of 1.06–19 wt % resulted in sodium ion concentrations of 0.64 to 18.8 mol/L [ 41 , 42 , 97 , 98 , 124 , 129 , 130 , 164 , 165 ]. It has already been applied for the entrapment of the microalgae Chlorella vulgaris , Dictosphaerium chlerelloides , Scenedesmus intermedius , Scenedesmus sp., Mesotaenium sp., and Cyanidium caldarium as well as the euglenoid Euglena gracilis and the cyanobacteria Anabaena flos-aqua , Synechococcus sp., and Cyanothece [ 41 , 97 , 98 , 124 , 129 , 130 ].…”

“…Moreover, the authors argued that the nanoparticles are small enough to be potentially internalized [ 42 ]. This effect seems to be species-specific, as other microalgae and cyanobacteria show cell activity in LUDOX ® -reinforced silica hydrogels [ 97 , 98 , 124 , 129 , 130 , 164 , 165 ]. It was discussed that observed harmful effects of elevated LUDOX ® concentrations from 1 to 3 mol/L on entrapped Anabaena flos-aquae could be caused by an increased Young’s modulus [ 130 ].…”

Nanoparticle-Hydrogel Composites: From Molecular Interactions to Macroscopic Behavior

“…Depending on the composition, the transmittance of nanocomposite gels may also be sensitive to pH variations. For example, Ahmed and co-authors [ 105 ] showed that nanocomposite hydrogels obtained from sodium silicates/colloidal silica mixtures, developed for bacterial encapsulation, lost transparency when lowering the pH from 7 to 5 (as well as the ability to gel) when using Ludox silica particles, while sodium silicate NPs showed the opposite trend, with samples becoming more opaque when the pH was increased from 5 to 6. The authors concluded that the transparency variations were related to NP aggregation upon pH variation and that by varying the pH and NP concentration, whose increase resulted in an increase in optical density, it was possible to synthesize gels with a similar transparency for a wide range of silicate concentrations [ 105 ].…”

“…Nanocomposite hydrogels have been shown to exhibit an increase of both shear storage modulus ( G ’) [ 59 , 68 , 74 , 103 , 112 , 113 , 114 , 115 ] and Young’s modulus ( E ) [ 52 , 59 , 102 , 103 , 105 , 107 , 109 , 114 , 115 , 116 ] in comparison to covalently cross-linked hydrogels with a similar polymer composition prepared with some form of cross-linking agent (for example N , N ′-methylenebis(acrylamide)) [ 59 , 109 ]. However, as previously mentioned, the covalently cross-linked gels generally have a higher cross-linking density [ 109 ], which might justify, at least partially, the different behavior of the materials.…”

“…However, as previously mentioned, the covalently cross-linked gels generally have a higher cross-linking density [ 109 ], which might justify, at least partially, the different behavior of the materials. It was shown that increasing the concentration of NPs can lead to an increase in shear storage modulus and Young’s modulus for low NP concentrations, followed by a plateau [ 59 , 102 , 103 , 104 , 105 , 109 , 111 , 112 , 117 , 118 ]. This was also the case with PAAm hydrogels reinforced with graphene NSs [ 119 ].…”