Inna V. Melnyk

Spray-dried mesoporous silica microspheres with adjustable textures and pore surfaces homogenously covered by accessible thiol functions

Micrometric mesoporous spheres with textures possessing a relative high degree of ordering and incorporating up to 0.1 thiol function per siloxane unit have been synthesised through sol–gel, self-assembly and—for the first time—spray-drying processes. The sol preparation was optimised as a function of the final properties: morphology, texture, and proportion of thiol functions. A two step synthesis allows the positioning of these functions at the pore surface as proven in particular by 1H solid state NMR. From Ag+ sorption experiments, we have shown that the thiol functions are also homogenously distributed in the spheres volume and fully accessible to external chemical species. So these materials could be used for environmental remediation or metallic nano-particle syntheses. In order to increase the stability and applicability of the materials, different post-synthesis treatments have been studied. A thermal treatment under mild conditions is enough to preserve most of the properties. From the hydrothermal treatments tested, the use of ammonia is shown to be quite interesting for the modulation of the properties. In particular, hierarchical porosities and high specific surface areas have been created. The high degree of ordering observed for the smaller pores is accompanied by a re-ordering of the surfactant polar head groups as deduced from 14N NMR. Lastly, the synthesis can be extended under some conditions to other functions or to higher proportions of thiol functions (up to 0.2).

Immobilization of urease on magnetic nanoparticles coated by polysiloxane layers bearing thiol- or thiol- and alkyl-functions

Magnetically retrievable formulations of urease potentially promising for biomedical and environmental applications were constructed by immobilization of the enzyme on the surface of magnetite nanoparticles functionalized by siloxane layers with active thiol or thiol-and-alkyl moieties. The latter were deposited using a hydrolytic polycondensation reaction of tetraethoxysilane with either 3-mercaptopropyltrimethoxysilane, or with 3-mercaptopropyltrimethoxysilane and methyltriethoxysilane, or alternatively n-propyltriethoxysilane. Immobilization of urease was carried out in different ways for comparison: by adsorption, by entrapment during the hydrolytic polycondensation reaction, or by covalent bonding. For entrapment the enzyme was introduced into solution before functionalization of the magnetite. Entrapment bound high amounts of enzyme (more than 700 mg per g of carrier), but its activity was decreased compared to the native form to between 18 and 10%. In the case of covalent binding of urease using Ellmans Reagent, binding of the enzyme was almost as efficient as in the case of entrapment but its residual activity was 75%. The residual activity of urease immobilized by adsorption on the surface of thiol-functionalized particles was truly high as compared to that of the native enzyme (97%), but binding was significantly less efficient (46%). Introduction of alkyl functions permitted increase of the amounts of the adsorbed enzyme but its activity was somewhat decreased.

Molecular insight into the mode-of-action of phosphonate monolayers as active functions of hybrid metal oxide adsorbents. Case study in sequestration of rare earth elements

The insight into the molecular aspects of ligand grafting and potential maximal capacity of hybrid organic–inorganic adsorbents bearing phosphonate ligand monolayers as active functions was obtained by single crystal X-ray studies of ligand-functionalized titanium alkoxide complexes. The attachment of molecules occurs generally in the tripodal vertical fashion with the minimal distance between them being about 8.7 A, resulting in 0.19 nm2 as the minimal surface area per function. In the present experimental work the theoretical loading capacity could almost be achieved for functionalization of mesoporous nanorods of anatase with imino-bis-methylphosphonic acid (IMPA, NH(CH2PO3H2)2) or aminoethylphosphonic acid (AEPA, H2NC2H4PO3H2). The products had the same morphology as the starting material, as was established by SEM and optical microscopy. The size and structure of the individual nanoparticles of the constituting inorganic component of the material were preserved and practically unchanged through the surface modification, as established by powder XRD and EXAFS studies. The surface area of the inorganic–organic hybrids decreased somewhat from the initial ∼250 m2 g−1, on adsorption of AEPA (0.21 mmol g−1) to ∼240 m2 g−1, and on adsorption of IMPA (0.17 mmol g−1) to ∼190 m2 g−1. The ligands were bound effectively to the surface according to TGA, EDS and FTIR analyses and remained in the mono-deprotonated form. The produced hybrid adsorbents had for the selected pH (3.5) high capacities towards adsorption of Rare Earth Element (REE) cations, but with equilibria achieved relatively slowly. The composition of the surface complexes was determined as M : L = 1 : 1 for IMPA, but varied for the AEPA from 1 : 3 to 1 : 1 dependent on the REE, which can be interpreted in terms of charge compensation as the major driving force behind binding. The cation desorption in strongly acidic media for recuperation of the adsorbed REE and the relative capacity of the re-used adsorbent have been quantified.

Enzyme immobilization on a nanoadsorbent for improved stability against heavy metal poisoning

Magnetic nanoparticles modified with siloxane layers bearing amino and thiol functions have been used for immobilization of urease either by adsorption or via surface grafting. The activity of the immobilized enzyme in the hydrolysis of urea extended to the levels typical of the native enzyme, while its long-term stability in combination with magnetic retraction opened for its repeated use in both analysis and detoxification of bio-fluids. The immobilized urease revealed strongly enhanced stability and 65% activity in the presence of 0.1mmol/l of Hg(2+) or 0.3mmol/l of Cu(2+) while the native urease did not retain any activity at all. The enzyme grafting was shown to be a potentially perspective tool in alleviation of heavy metal poisoning and to be providing an opportunity for use of the developed adsorbents as both biosensors and bio-reactants for removal of urea from biofluids.

Mesoporous silica containing ≡Si(CH2)3NHC(S)NHC2H5 functional groups in the surface layer.

One-step synthesis technique of mesoporous SBA-15 type silica with thiourea ≡Si(CH(2))(3)NHC(S)NHC(2)H(5) groups in the surface layer was developed. According to elemental analysis, the content of surface groups is 1.25 mmol/g, which is consistent with TGA data. FT-IR spectra of the obtained sample contain characteristic absorption bands of thiourea groups (-NH-C(S)-NH-), as well as polysiloxane network (SiOSi). The synthesized sample was studied by XRD, TEM, SEM, and adsorption method. It was demonstrated that the sample features porous microspheres ~0.5 μm with well-ordered internal spatial structure of the hexagonal lattice type due to the usage of template P123 during synthesis. According to XRD and TEM, the diameter of pores is 4.2-5 nm and the wall thickness between them is 2.6 nm. These data are consistent with the structural-adsorption characteristics calculated from nitrogen adsorption-desorption isotherms: S(sp.)=510 m(2)/g, V(s)=0.47 cm(3)/g, and d=4.3 nm. Equilibrium is established within 60 min during sorption of silver(І) and mercury(ІІ) ions from acidified aqueous solutions for this sample, and with the complexes are formed 1.1/1 for Ag(+) and 0.8/1 for Hg(2+) at metal/ligand ratio.

Study of Hg2+ Sorption from Water Solutions by Mesoporous Silica with Thiourea Functional Groups

The mesoporous silicas with thiourea functional group =Si(CH2)3NHC(S)NHC2H5 were synthesized using sol-gel and template methods (with cetylpyridinium chloride as template). It has been found that the sorption properties of these mesoporous silicas are influenced both by the functional group chapter and the character of the sorbent’s porous structure. At low density of ligand groups all of them are accessible for Hg2+ sorption with formation, as a rule, of the simplest complexes (1:1 composition). The rising density of functional groups in the surface layer of sorbents results in the reduction of their accessibility and complicates the complex formation process. This, causes reducetion of static sorption capacity of such materials (from 357 to 46 mg/g). It was shown that mesoporous materials with thiourea groups synthesized by template method and possessing highly ordered structures have superior kinetic characteristics compared to xerogels with the same functional groups is stable.

Sol-Gel Derived Adsorbents with Enzymatic and Complexonate Functions for Complex Water Remediation

Sol-gel technology is a versatile tool for preparation of complex silica-based materials with targeting functions for use as adsorbents in water purification. Most efficient removal of organic pollutants is achieved by using enzymatic reagents grafted on nano-carriers. However, enzymes are easily deactivated in the presence of heavy metal cations. In this work, we avoided inactivation of immobilized urease by Cu (II) and Cd (II) ions using magnetic nanoparticles provided with additional complexonate (diethylene triamine pentaacetic acid or DTPA) functions. Obtained nanomaterials were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). According to TGA, the obtained Fe3O4/SiO2-NH2-DTPA nanoadsorbents contained up to 0.401 mmol/g of DTPA groups. In the concentration range Ceq = 0–50 mmol/L, maximum adsorption capacities towards Cu (II) and Cd (II) ions were 1.1 mmol/g and 1.7 mmol/g, respectively. Langmuir adsorption model fits experimental data in concentration range Ceq = 0–10 mmol/L. The adsorption mechanisms have been evaluated for both of cations. Crosslinking of 5 wt % of immobilized urease with glutaraldehyde prevented the loss of the enzyme in repeated use of the adsorbent and improved the stability of the enzymatic function leading to unchanged activity in at least 18 cycles. Crosslinking of 10 wt % urease on the surface of the particles allowed a decrease in urea concentration in 20 mmol/L model solutions to 2 mmol/L in up to 10 consequent decomposition cycles. Due to the presence of DTPA groups, Cu2+ ions in concentration 1 µmol/L did not significantly affect the urease activity. Obtained magnetic Fe3O4/SiO2-NH2-DTPA-Urease nanocomposite sorbents revealed a high potential for urease decomposition, even in presence of heavy metal ions.

Tailoring bifunctional hybrid organic–inorganic nanoadsorbents by the choice of functional layer composition probed by adsorption of Cu2+ ions

Spherical silica particles with bifunctional (≡Si(CH2)3NH2/≡SiCH3, ≡Si(CH2)3NH2/≡Si(CH2)2(CF2)5CF3) surface layers were produced by a one-step approach using a modified Stöber method in three-component alkoxysilane systems, resulting in greatly increased contents of functional components. The content of functional groups and thermal stability of the surface layers were analyzed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, and 13C and 29Si solid-state NMR spectroscopy revealing their composition and organization. The fine chemical structure of the surface in the produced hybrid adsorbent particles and the ligand distribution were further investigated by electron paramagnetic resonance (EPR) and electron spectroscopy of diffuse reflectance (ESDR) spectroscopy using Cu2+ ion coordination as a probe. The composition and structure of the emerging surface complexes were determined and used to provide an insight into the molecular structure of the surfaces. It was demonstrated that the introduction of short hydrophobic (methyl) groups improves the kinetic characteristics of the samples during the sorption of copper(II) ions and promotes fixation of aminopropyl groups on the surface of silica microspheres. The introduction of long hydrophobic (perfluoroctyl) groups changes the nature of the surface, where they are arranged in alternately hydrophobic/hydrophilic patches. This makes the aminopropyl groups huddled and less active in the sorption of metal cations. The size and aggregation/morphology of obtained particles was optimized controlling the synthesis conditions, such as concentrations of reactants, basicity of the medium, and the process temperature.

Silica-Coated Magnetite Nanoparticles Modified with 3-Aminopropyl Groups for Solid-Phase Extraction of Pd(II) Ions from Aqueous Solutions

Here we report a simple two-step synthesis of magnetite nanoparticles (NPs) coated with silica shell functionalized with 3-aminopropyl groups. The content of surface amino groups within the range of 0.7–2.8 mmol g−1 was established by elemental analysis and acid–base titration. The materials obtained are hydrolytically stable in acidic medium and are rapidly separated from the solution using an external magnetic field. It was shown that the prepared NPs efficiently extract palladium(II) ions in the form of [PdCl4]2– from the solution. The kinetics of adsorption was well described by the second-order model with the rate constant of approximately 0.76 g mmol−1 minute−1. The isotherm of adsorption is fitted by the Langmuir model with the maximum [PdCl4]2– adsorption capacity of 0.158 mmol g−1.

Application of Sol-Gel Method for Synthesis of a Biosensitive Polysiloxane Matrix

Sol-gel method was used to synthesize polysiloxane hydrogel with encapsulated urease (immobilization degree in the range of 79–88%) which preserved the enzymatic activity at the level of 56–84%. The nature of functional groups was shown to influence the pore structure parameters. Immobilization degree and preservation of adsorbed urease activity depend on the structural-adsorption characteristics of matrices. The possibility of “double immobilization” of urease on silica gel by sol-gel method and the opportunity of reuse of the synthesized formulations was investigated. Urease immobilization on the surface of magnetite (FeO·Fe2O3) was also studied by adsorption method.