Mesoporous TiO2 Monoliths Impregnated with CdS and CuO Nanoparticles for Airborne Bacteria Inactivation Under Visible Light.

In this work, macroscopic TiO2 monoliths are proposed to serve simultaneously as support and co-catalyst in a continuous flow photoreactor. The impregnation via one-pot of mesoporous TiO2 with CdS (m-TiO2/CdS) and CuO (m-TiO2/CuO) nanoparticles enabled the formation of photocatalytic heterojunctions retaining high specific surface area (~ 100 m2/g). The impregnated monoliths of 2-3 mm in size were employed as photocatalysts to inactivate airborne bacteria under blue light, reducing the emission of living airborne bacteria up to 0.1% and 37.7% when using m-TiO2/CdS and m-TiO2/CuO, respectively. Bacteria were characterized and quantified by flow cytometry and cell lysis was confirmed by SEM, detecting collapsed bacteria. Along 96 h of continuous photocatalysis at a flow rate of 2.2 L/min, the cell concentration presented maxima and minima due to the adsorption-desorption stages of bioaerosols over the catalysts, in concordance with thermal gravimetric analysis. The reactivation of catalysts was achieved by calcination at 400 °C, however, after a third re-cycle, the photocatalytic activity for all monoliths was practically negligible because the physicochemical surface changes hinder the adequate bioaerosol adsorption. These porous systems could emerge as promising gas-phase catalysts since the mass transport is facilitated by porosity and the release of catalyst nanoparticles is avoided by the active support, providing a safe and viable model for bioaerosols inactivation to improve indoor air quality with the use of interior lighting. Supplementary Information: The online version contains supplementary material available at 10.1007/s10562-021-03659-9.

TiBALDH as a precursor for biomimetic TiO2 synthesis: stability aspects in aqueous media.

Titanium(iv) bis(ammonium lactate)dihydroxide (TiBALDH) is a commercially available reagent frequently used to synthesize TiO2. Particularly, for the biomimetic synthesis of TiO2, TiBALDH is the preferred precursor because it can be mixed in aqueous solutions with no apparent hydrolysis or condensation reactions. Thus, proteins or other biomolecules can be used as a template in aqueous systems for the synthesis of TiO2 from TiBALDH. Nevertheless, there is evidence that TiBALDH is in equilibrium with TiO2, and even, the principal structure of the complex has been suggested as [Ti4O4(lactate)8]8-. Since that chemical equilibrium depends on the polarity of the solvent, in this work we explored a diversity of media to test the chemical stability of TiBALDH and its equilibrium with TiO2 at room temperature. TiBALDH (2.078 M) contains particles of 18.6 ± 7.3 nm in size, if it is diluted with deionized water, the particles reach a size of 5.2 ± 1.7 nm, which suggest that intermolecular interactions form polymers of titanium lactate complexes reversibly, reaching equilibrium after 10 hours. Typical buffer systems were tested; TiBALDH reacted rapidly only with phosphate groups, even if the source came from DNA. Therefore, phosphate buffer must be avoided in biomineralization TiO2 synthesis. In solutions of TiBALDH at basic pH, condensation reactions are promoted to form a gel containing anatase nanoparticles, but if the solutions are acidic, monodisperse anatase nanoparticles of ∼5 nm were observed. The results show that the commercial reagent TiBALDH contains many species of titanium lactate complexes in equilibrium with TiO2, and it is affected by the concentration, time, pH, and several ions. This peculiar behavior must be taken into account when this precursor is used and it could be useful to develop novel synthesis routes of macrostructures with biomolecules in aqueous systems.

Using the M13 Phage as a Biotemplate to Create Mesoporous Structures Decorated with Gold and Platinum Nanoparticles.

By taking advantage of the physical and chemical properties of the M13 bacteriophage, we have used this virus to synthesize mesoporous silica structures. Major coat protein p8 was chemically modified by attaching thiol groups. As we show, the resulting thiolated phage can be used as a biotemplate able to direct the formation of mesoporous silica materials. Simultaneously, this thiol functionality acts as an anchor for binding metal ions, such as Au(3+) and Pt(4+), forming reactive M13-metal ionic complexes which evolve into metal nanoparticles (NPs) trapped in the mesoporous network. Interestingly, Au(3+) ions are reduced to Au(0) NPs by the protein residues without requiring an external reducing agent. Likewise, silica mesostructures decorated with Au and Pt NPs are prepared in a one-pot synthesis and characterized using different techniques. The obtained results allow us to propose a mechanism of formation. In addition, gold-containing mesoporous structures are tested for the reduction of 4-nitrophenol (4-NP) and methylene blue (MB) in the presence of NaBH4. Although all of the gold-containing catalysts exhibit catalytic activity, those obtained with thiolated phages present a better performance than that obtained with M13 alone. This behavior is ascribed to the position of the Au NPs, which are partially embedded in the wall of the final mesostructures.

Biomimetic sol-gel synthesis of TiO2 and SiO2 nanostructures.

We report the heptapeptide-mediated biomineralization of titanium dioxide nanoparticles from titanium alkoxides. We evaluated the influence of pH on the biomineralized products and found that nanostructured TiO2 was formed in the absence of external ions (water only) at pH ~ 6.5. Several variants (mutants) of the peptides with different properties (i.e., different charges, isoelectric points (pIs), and sequences) were designed and tested in biomineralization experiments. Acid-catalyzed experiments were run using the H1 (HKKPSKS) peptide at room temperature, which produced anatase nanoparticles (~5 nm in size) for the first time via a heptapeptide and sol-gel approach. In addition, the peptide H1 was used to synthesize SiO2 nanoparticles. The influence of the pH and the added ions were monitored: at higher pH levels (8-9), SiO2 nanoparticles (20-30 nm in size) were obtained. In addition, whereas borate and Tris ions allowed the formation of colloidal systems, phosphate ions were unable to produce sols. The results presented here demonstrate that biomineralization depends on the sequence and charge of the peptide, and ions in solution can optimize the formation of nanostructures.