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The magnetite nanoparticles were functionalized with silica shells bearing mercaptopropyl (monofunctional) and mercaptopropyl-and-alkyl groups (bifunctional) by single-step sol-gel technique. The influence of synthetic conditions leading to increased amounts of active functional groups on the surface and improved capacity in the uptake of Ag(I), Cd(II), Hg(II), and Pb(II) cations was revealed. The physicochemical properties of obtained magnetic nanocomposites were investigated by FTIR, Raman, XRD, TEM, SEM, low-temperature nitrogen ad-/desorption measurements, TGA, and chemical microanalysis highlighting the efficiency of functionalization and mechanisms of the preparation procedures. The removal of the main group of heavy metal cations was studied in dependence from the pH, contact time and equilibrium concentration to analyze the complexes composition for the large scale production of improved adsorbents. It was demonstrated that introduction of the alkyl groups into the surface layer prevents the formation of the disulfide bonds between adjacent thiol groups. The obtained adsorbents were employed to treat real wastewater from Ruskov, Slovakia with concentration of Fe 319 ng/cm3, Cu 23.7 ng/cm3, Zn 36 ng/cm3, Mn 503 ng/cm3, Al 21 ng/cm3, As 34 ng/cm3, Pb 5.8 ng/cm3, Ni 35 ng/cm3, Co 4.2 ng/cm3, Cr 9.4 ng/cm3, Sb 6 ng/cm3, Cd 5 ng/cm3. These materials proved to be highly effective in the removal of 50% of all metal ions, espeсially Zn, Cd, and Pb ions from it and turned recyclable, opening for their sustainable use in water purification.

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 Ellman's 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.

Iron-mediated synthesis of alkali metal iodides was quite unexpectedly demonstrated to be able to serve as a cost-efficient and reliable source of spherical single crystalline near-stoichiometric magnetite (Fe3O4) nanoparticles as revealed by TEM and XRD studies and also by XANES spectroscopic quantification of the Fe2+-content. Using the particles as nuclei for the Stoeber synthesis of silica nanoparticles, core-shell magnetic material has been produced. The nature of the magnetic component was probed by XANES spectroscopy. The size of the particles is dependent on the synthesis conditions and Si : Fe ratio but can be kept below 100 nm. It is the Si : Fe ratio that determines the stability of the particles in acidic medium. The latter was investigated spectrophotometrically as leaching of Fe3+-cations. Considerable stability was observed at Si : Fe > 10, while at Si : Fe >= 20 no measurable leaching could be observed in over 10 days. Magnetic nanoparticles with improved stability in acidic medium provide an attractive basis for creation of adsorbent materials for applications in harsh media.

Tubular ceramic membranes based on α-Al2O3 were functionalized with 3-mercaptopropyl groups using the sol-gel method. The formation of the polysiloxane layer on the inner surface of the membranes was confirmed by SEM images and FTIR spectroscopy data. The functionalized membranes (identically to the original one) retain the high performance typical of highly efficient microfiltration membranes (320–690 L/m2 h); however, they can be used to remove silver(I) ions from water due to the formation of complexes between thiol groups of the active layer and silver(I) ions. It was also shown that the decreasing concentration of functionalizing sol produces a thinner surface polysiloxane layer with more active sites available for the sorption of Ag+ ions. Thus, fourfold dilution of the starting sol results in a tenfold increase in the sorption of Ag+ ions calculated per mass unit of the functionalizing layer (from 23 to 203 mg/g).

The surface of magnetite nanoparticles was coated with functional polysiloxane layers using reaction of hydrolytic copolycondensation of tetraethoxysilane and 3-aminopropyltriethoxysilane (or N-[3-trimethoxysilylpropyl]ethylendiamine), and also that of tetraethoxysilane, 3-aminopropyltriethoxysilane and methyltriethoxysilane (or n-propyltriethoxysilane). It was shown that these functionalized magnetically controllable particles (about 60–150 nm in size as aggregates), as opposed to magnetite, adsorb urease well from aqueous solutions (up to 1 g/g), and that the level of residual activity of adsorbed layers is up to 84 % in the case of a bifunctional sample. It was established that the activity of immobilized urease is normally gradually reduced during storage of the samples, but in the case of ethylenediamine functional group is not decreased for 45 days. The synthesized samples are promising for use as magnetically directed biocatalysts.

A sol-gel method of covalent immobilization of urease on polysiloxane matrices is developed which uses glutaraldehyde or Ellman’s reagent as binding agents. We show that the urease covalently bound to the poly(3-mercaptopropyl)siloxane matrix retains 67–84% of its activity and is stable, losing only 10% of its activity after 300 days. The urease adsorbed on the poly(3-mercaptopropyl)siloxane matrix was more active than the native urease. On the basis of the literature, we suggest that, in this case, the 3-mercaptopropyl groups of the polysiloxane matrix are brought into close proximity to the active site of urease, where they possibly act as proton donors, which results in an accelerated enzymatic reaction. Covalent immobilization of urease on the polysiloxane matrix containing 3-aminopropyl was less efficient, because the immobilized enzyme was significantly less active. At the same time, the urease adsorbed on the same matrix showed high activity (60–86%).

The possibility of “double immobilization” of urease on silica gel, involving enzyme incorporation into the polysiloxane polymer prepared by sol-gel procedure on the surface of an inorganic support was studied. The optimal composition and thickness of the polysiloxane film, providing relatively high activity of urease, were determined.

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.

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.

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.