Sara A. Bilmes

Silica-alginate-fungi biocomposites for remediation of polluted water

Here we introduce an assembly for bioremediation of polluted water based on the immobilization of alginate beads loaded with filamentous fungus Stereum hirsutum inside nanoporous silica hydrogels. The resulting hybrid device exhibits good physical, chemical and biological stability, being effective in the removal and degradation of malachite green (MG), even in solutions with a high concentration of the dye. This fact is a consequence of adsorption and regulated transport of the dye, as well as the retention of dye degradation enzymes inside the hydrogel. The optimal structure of the hydrogel for an efficient dye–enzyme encounter resulted from the fine adjustment of synthesis conditions in order to achieve a suitable porosity. The results presented here open the possibility of bioremediation without dissemination of exotic organisms to the environment, and can be extended to a vast variety of strains due to the inherent high biocompatibility of the present procedure.

Development of a Biosensor for Environmental Monitoring Based on Microalgae Immobilized in Silica Hydrogels

A new biosensor was designed for the assessment of aquatic environment quality. Three microalgae were used as toxicity bioindicators: Chlorella vulgaris, Pseudokirchneriella subcapitata and Chlamydomonas reinhardtii. These microalgae were immobilized in alginate and silica hydrogels in a two step procedure. After studying the growth rate of entrapped cells, chlorophyll fluorescence was measured after exposure to (3-(3,4-dichlorophenyl)-1,1-dimethylurea) (DCMU) and various concentrations of the common herbicide atrazine. Microalgae are very sensitive to herbicides and detection of fluorescence enhancement with very good efficiency was realized. The best detection limit was 0.1 μM, obtained with the strain C. reinhardtii after 40 minutes of exposure.

Silver Nanoparticle-Mesoporous Oxide Nanocomposite Thin Films: A Platform for Spatially Homogeneous SERS-Active Substrates with Enhanced Stability

We introduce a nanoparticle-mesoporous oxide thin film composite (NP-MOTF) as low-cost and straightforward sensing platforms for surface-enhanced Raman Spectroscopy (SERS). Titania, zirconia, and silica mesoporous matrices templated with Pluronics F-127 were synthesized via evaporation-induced self-assembly and loaded with homogeneously dispersed Ag nanoparticles by soft reduction or photoreduction. Both methods give rise to uniform and reproducible Raman signals using 4-mercaptopyridine as a probe molecule. Details on stability and reproducibility of the Raman enhancement are discussed. Extensions in the design of these composite structures were explored including detection of nonthiolated molecules, such as rhodamine 6-G or salicylic acid, patterning techniques for locating the enhancement regions and bilayered mesoporous structures to provide additional control on the environment, and potential size-selective filtration. These inorganic oxide-metal composites stand as extremely simple, reproducible, and versatile platforms for Raman spectroscopy analysis.

Thermodecomposition synthesis of porous β-Bi2O3/Bi2O2CO3 heterostructured photocatalysts with improved visible light photocatalytic activity

Novel porous β-Bi2O3/Bi2O2CO3 p–n heterostructures were synthesized by partially decomposing porous Bi2O2CO3 at 300–375 °C. The structures, morphologies, optical properties, and specific surface areas of the as-synthesized samples were characterized by means of thermogravimetry and differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, UV-Vis spectroscopy, and N2 gas adsorption. Two types of dyes, methyl orange (MO) and methylene blue (MB), were chosen as model organic pollutants to evaluate the photocatalytic activity of the as-synthesized samples. The porous β-Bi2O3/Bi2O2CO3 p–n heterostructures exhibited much higher photocatalytic activity than β-Bi2O3 and Bi2O2CO3 and MO and MB could be completely degraded within 24 and 50 min, respectively. In addition, phenol as a colorless organic pollutant was also chosen to further study the photocatalytic activity of Bi2O2CO3, β-Bi2O3 and β-Bi2O3/Bi2O2CO3. The β-Bi2O3/Bi2O2CO3 heterostructures also showed much higher photocatalytic activity for the photodegradation of phenol than β-Bi2O3 and Bi2O2CO3. The obtained results indicated that the formed p–n heterojunction in the porous β-Bi2O3/Bi2O2CO3 composite significantly contributed to the improvement of electron–hole separation and the enhancement of photocatalytic activity. The mechanisms for the enhanced photodegradation of selected organic pollutants over the β-Bi2O3/Bi2O2CO3 composite are discussed in this study.

Improving silica matrices for encapsulation of Escherichiacoli using osmoprotectors

The development of sol–gel based processes for the encapsulation of living cells is a challenging task where the host material must be optimized both in terms of intrinsic properties and impact on cell viability. Here, the mechanical stability of silica hosts obtained via a mixed aqueous route based on sodium silicate–silica nanoparticle mixtures could be improved by increasing the relative content of molecular precursors over colloidal silica. The specific surface area, pore volume and diffusion properties of the host were also modified. Moreover, this resulted in an increase in the ionic strength, inducing high osmolarity that is detrimental to encapsulated Escherichia coli bacteria. Glycine betaine, a well-known E. coli osmoprotector, could be successfully used as an additive to the sol–gel formulation to limit this osmotic stress. Compared to the previously studied glycerol additive, glycine betaine was found more efficient to preserve the bacterial viability. Moreover, this efficiency was obtained with much lower amounts of glycine betaine, whose addition has therefore no detrimental impact on the silica host structure. These data provide new information about the different cellular stresses resulting from the sol–gel encapsulation process and demonstrate the importance of combining chemical and biological approaches to design more robust functional “living” materials.

The influence of applied bias potential on the photooxidation of methanol and salicylate on titanium dioxide films

Abstract The mechanism of the photoelectrochemical oxidation of methanol and salicylic acid on anatase film electrodes was studied as a function of the applied potential and pollutant concentration at pH 3. The dependencies of the steady state photocurrents on substrate concentration reflect the type of surface interaction: weak in the case of methanol, that leads to a simple saturation curve, and strong in the case of salicylate, that shows a steady state photocurrent peaking at intermediate concentrations. At 0.6 V vs. SCE the oxidation rate is largely enhanced as compared to open circuit conditions ( E oc =−0.3 V). Even under nitrogen, the reaction proceeds at an appreciable rate, and the ratio of circulated charge to the number of oxidized salicylate ions approaches 28 electrons per mol at low salicylate concentration: oxidized salicylate mineralizes almost totally, and the intermediates are rapidly destroyed. At higher substrate concentrations, the ratio decreases, and uv spectral evidence suggests the formation of some undefined oxidation products. Under oxygen at 0.6 V, the radicals generated in the initial photoelectrochemical step are mostly oxidized by O 2 , increasing the amount of salicylate destroyed for a given total circulated charge; at sufficiently high substrate concentration, the above ratio decreases to values below 4. No evidence of the presence of traces of partially oxidized molecules is found. Adequate control of the experimental conditions permits therefore to achieve substantially increased efficiencies of salicylate destruction per absorbed photon, and the build up of uncontrolled intermediates can be prevented. The results are discussed in terms of the oxidation length Y , defined as the number of oxidation steps that are triggered by a single hole transfer event, and of the oxidation efficiency ϵ , defined as the ratio of the oxidation length to the maximum possible oxidation length (the length achieved when one hole transfer suffices to trigger total mineralization).

Sol-gel silica platforms for microalgae-based optical biosensors

An advanced hybrid biosensing platform with improved optical quality is developed based on the acidic encapsulation of microalgi in silica matrices synthesized by TAFR (tetraethoxysilane derived alcohol free route). The three microalgi (Chlorella vulgaris, Pseudokirchneriella subcapitata and Chlamydomonas reinhardtii) were previously immobilized in alginate following the two-step procedure. Tuning the alginate protecting function with the aid of Tris-HCl buffer, the sol-gel synthesis was conducted at pH 4.0 well below the tolerance limit imposed by the encapsulated microalgae. The acidic condensation of Si(IV) generates silica matrices with outstanding optical properties that suit the requirements of biosensors based on optical detection methods.

Improving bacteria viability in metal oxide hostsvia an alginate-based hybrid approach

A two-step process relying on cell encapsulation in alginate beads followed by inorganic gelation from colloidal metal oxides was successfully applied to the immobilization of Escherichia coli bacteria in the presence of boehmite and zirconium oxyhydroxide particles. In the case of the Al-based gel, the alginate bead obtained at low biopolymer content provides an efficient barrier against the encapsulation stress. In contrast, an increase in the alginate concentration together with the phosphate-induced mineralization of the biopolymer bead is found necessary to maintain the viability of entrapped bacteria in Zr-based gels. Diffusion studies using model molecular and colloidal species put in evidence that positively charged Zr-oligomers and ZrO2 nanoparticles may be involved in the cytotoxicity of the precursor solution. This optimization of the encapsulation process allows the first observation of E. coligrowth within such metal oxide–alginate hybrid gels. Results presented in this work give a clear evidence that sol–gel based cell encapsulation can now be envisioned within a wide variety of metal oxide hosts through the optimization of the pre-encapsulation environment.

New method for the simultaneous determination of diffusion and adsorption of dyes in silica hydrogels

The fine tuning of porosity in sol gel based devices makes possible the design of novel applications in which the transport of molecules through the oxide gel plays a crucial role. In this work we develop a new method for the simultaneous analysis of diffusion and adsorption of small diffusing probes, as anionic and cationic dyes, through silica mesoporous hydrogels synthesized by sol-gel. The novelty of the work resides in the simplicity of acquisition of the experimental data (by means of a desk scanner) and further mathematical modeling, which is in line with high throughput screening procedures, enabling rapid and simultaneous determination of relevant diffusion and adsorption parameters. Net mass transport and adsorption properties of the silica based hydrogels were contrasted to dye adsorption isotherms and textural characterization of the wet gels by SAXS, as well as that of the corresponding aerogels determined by Field Emission Scanning Electron Microscopy (FESEM) and N2 adsorption. Thus, the validation of the results with well-established characterization methods demonstrates that our approach is robust enough to give reliable physicochemical information on these systems.

Functional nanocomposites based on the infusion or in situ generation of nanoparticles into amphiphilic epoxy gels

The production of nanocomposites with functional properties via the infusion of preformed nanoparticles (NPs) or their in situ generation inside an amphiphilic epoxy gel is reported. The gel was synthesized by the reaction of a diepoxy monomer based on diglycidyl ether of bisphenol A with an n-alkylamine, followed by annealing the resulting linear polymers above their glass transition temperatures to produce physical gelation through tail-to-tail association of pendant alkyl chains. Some of the advantages of these polymer gels are: (a) they have a low crosslink density and can therefore be significantly swollen by several organic solvents, (b) the presence of pendant alkyl chains provides a convenient chemical environment for the stabilization of NPs coated with alkyl chains, (c) the presence of secondary hydroxyls and tertiary amine groups in the polar backbone of polymer chains can be used to coordinate and reduce different precursors of NPs. Preformed NPs could be successfully infused into the gels keeping their optical properties (e.g., CdSe quantum dots) or magnetic behavior (e.g., γ-Fe2O3@oleic acid NPs) in the resulting nanocomposite. In situ generation of Au and Ag NPs (average size close to 10 nm) inside the amphiphilic gels was produced by infusing HAuCl4 or AgNO3 followed by reduction to the corresponding metals with secondary alcohols present in the polymer backbone, at 100 °C. Amphiphilic gels were employed as hosts for the in situ precipitation of gold(I)-dodecanethiolate leading to films exhibiting a red emission (638 nm) when excited with UV light (300 nm).