Comprehensive study upon physicochemical properties of bio-ZnO NCs

Abstract

In this study, for the first time, the comparison of commercially available chemical ZnO NCs and bio-ZnO NCs produced extracellularly by two different probiotic isolates (Latilactobacillus curvatus MEVP1 [OM736187] and Limosilactobacillus fermentum MEVP2 [OM736188]) were performed. All types of ZnO formulations were characterized by comprehensive interdisciplinary approach including various instrumental techniques in order to obtain nanocomposites with suitable properties for further applications, i.e. biomedical. Based on the X- ray diffraction analysis results, all tested nanoparticles exhibited the wurtzite structure with an average crystalline size distribution of 21.1 nm (CHEM_ZnO NCs), 13.2 nm (1C_ZnO NCs) and 12.9 nm (4a_ZnO NCs). The microscopy approach with use of broad range of detectors (SE, BF, HAADF) revealed the core-shell structure of bio-ZnO NCs, compared to the chemical one. The nanoparticles core of 1C and 4a_ZnO NCs are coated by the specific organic deposit coming from the metabolites produced by two probiotic strains, L. fermentum and L. curvatus. Vibrational infrared spectroscopy, photoluminescence (PL) and mass spectrometry (LDI-TOF-MS) have been used to monitor the ZnO NCs surface chemistry and allowed for better description of bio-NCs organic coating composition (amino acids residues). The characterized ZnO formulations were then assessed for their photocatalytic properties against methylene blue (MB). Both types of bio-ZnO NCs exhibited good photocatalytic activity, however, the effect of CHEM_ZnO NCs was more potent than bio-ZnO NCs. Finally, the colloidal stability of the tested nanoparticles were investigated based on the zeta potential (ZP) and hydrodynamic diameter measurements in dependence of the nanocomposites concentration and investigation time. During the biosynthesis of nano-ZnO, the increment of pH from 5.7 to around 8 were observed which suggested possible contribution of zinc aquacomplexes and carboxyl-rich compounds resulted in conversion of zinc tetrahydroxy ion complex to ZnO NCs. Overall results in present study suggest that used accessible source such us probiotic strains, L. fermentum and L. curvatus, for extracellular bio-ZnO NCs synthesis are of high interest. What is important, no significant differences between organic deposit (e.g. metabolites) produced by tested strains were noticed-both of them allowed to form the nanoparticles with natural origin coating. In comparison to chemical ZnO NCs, those synthetized via microbiological route are promising material with further biological potential once have shown high stability during 7 days.

Figure 1

Physicochemical characterization of 1C, 4a and CHEM_ZnO NCs. XRD pattern (A); the FT-IR spectra (B) and the TGA/DTA pattern (C).

Figure 2

The electron micrographs for the 1C and 4a_ZnO NCs; SEM images in SE mode (A), SEM mapping (B), STEM images in SE and BF modes (C) and TEM images in BF, DF, SAED modes with EDX detector (D).

Figure 3

STEM images of all tested ZnO NCs recorded with SE, HAADF, BF and FFT detectors (A); the EDX spectra for CHEM, 1C and 4a_ZnO NCs (B).

Figure 4

The AFM 2D (A) and 3D (B) images of chemically synthesized CHEM and biologically synthesized: 1C and 4a_ZnO NCs; the size population distribution (C).

Figure 5

The LDI-TOF-MS spectra for core–shell signals of CHEM_ZnO NCs at 62.6 (A), 250 (B) and 500 (C) µg/mL concentration.

Figure 6

The LDI-TOF-MS spectra for core–shell signals of 1C_ZnO NCs at 62.6 (A), 250 (B) and 500 (C) µg/mL concentration.

Figure 7

The LDI-TOF-MS spectra for core–shell signals of 4a_ZnO NCs at 62.6 (A), 250 (B) and 500 (C) µg/mL concentration.

Figure 8

The photoluminescence spectra at two specific excitations regions (λ = 540 nm and λ = 420 nm) for CHEM_ZnO NCs (A), 1C_ZnO NCs (B) and 4a_ZnO NCs (C) at different concentration; organic deposit of tested NCs (D).

Figure 9

The UV–vis plot and efficiency of MB photocatalytic degradation (%) after CHEM_ZnO NCs (A), 1C_ZnO NCs (B) and 4a_ZnO NCs (C) treatment.

Figure 10

Study of the CHEM_ZnO NCs (A), 1C_ZnO NCs (B) and 4a_ZnO NCs (C) shown for hydrodynamic size (top) and zeta potential (bottom) for 7 days.