Ecotoxicological Properties of Titanium Dioxide Nanomorphologies in Daphnia magna

Abstract

In this work, the structural, vibrational, morphological, and colloidal properties of commercial 15.1 nm TiO₂ nanoparticles (NPs) and nanowires (NWs, 5.6 thickness, 74.6 nm length) were studied with the purpose of determining their ecotoxicological properties. This was achieved by evaluating acute ecotoxicity experiments carried out in the environmental bioindicator Daphnia magna, where their 24-h lethal concentration (LC₅₀) and morphological changes were evaluated using a TiO₂ suspension (pH = 7) with point of zero charge at 6.5 for TiO₂ NPs (hydrodynamic diameter of 130 nm) and 5.3 for TiO₂ NWs (hydrodynamic diameter of 118 nm). Their LC₅₀ values were 157 and 166 mg L^-1 for TiO₂ NWs and TiO₂ NPs, respectively. The reproduction rate of D. magna after fifteen days of exposure to TiO₂ nanomorphologies was delayed (0 pups for TiO₂ NWs and 45 neonates for TiO₂ NPs) in comparison with the negative control (104 pups). From the morphological experiments, we may conclude that the harmful effects of TiO₂ NWs are more severe than those of 100% anatase TiO₂ NPs, likely associated with brookite (36.5 wt. %) and protonic trititanate (63.5 wt. %) presented in TiO₂ NWs according to Rietveld quantitative phase analysis. Specifically, significant change in the heart morphological parameter was observed. In addition, the structural and morphological properties of TiO₂ nanomorphologies were investigated using X-ray diffraction and electron microscopy techniques to confirm the physicochemical properties after the ecotoxicological experiments. The results reveal that no alteration in the chemical structure, size (16.5 nm for TiO2 NPs and 6.6 thickness and 79.2 nm length for NWs), and composition occurred. Hence, both TiO₂ samples can be stored and reused for future environmental purposes, e.g., water nanoremediation.

Keywords: aquatic biota; biomarkers; metal-oxides; nanoparticles; nanowires.

Figure 1

Rietveld refinement of X-ray diffractograms obtained for (A) TiO₂ NPs and (B) TiO₂ NWs. The black line corresponds to experimental data, red line to calculated diffractogram, and the blue line is the residual data between the experimental and calculated data. Green vertical lines indicate the Bragg’s positions and Miller indices which are given by vertical number in parentheses. Blue numbers between parentheses are assigned to brookite phase.

Figure 2

Fit to experimental µ-Raman spectrum for TiO₂ NWs sample. The laser power over the sample was of 8.3 mW. Lorentzian subcomponents were highlighted with different colors and main Raman modes are indicated on the top of each identified active optical mode. B indicates the brookite phase.

Figure 3

Zeta potential measurements as a function of pH for TiO₂ samples. (A) First measurement for TiO₂ NPs, (B) second measurement for TiO₂ NPs, (C) first measurement for TiO₂ NWs, (D) second measurement for TiO₂ NWs.

Figure 4

(a) TEM image of the TiO₂ NPs before the ecotoxicity experiment, (b) 10 nm zoomed TEM image, (c) normal distribution of the measurements of the TiO₂ NPs from before the ecotoxicity experiment, (d) TEM image of the TiO₂ NPs after the ecotoxicity experiment, (e) 5 nm zoomed TEM image, and (f) normal distribution of the TiO₂ NPs measurements after the ecotoxicity experiment.

Figure 5

(a) TEM image of the TiO₂ NWs before the ecotoxicity experiment, (b) TEM image of the TiO₂ NWs after the ecotoxicity experiment. (c) Log-normal distribution of the TiO₂ NWs length measurements before the experiment of ecotoxicity and (d) log-normal distribution of the measurements of the length of the TiO₂ NWs after the ecotoxicity experiment. (e) Lorentz distribution of the measurements of the diameter of the TiO₂ NWs before the ecotoxicity experiment and (f) normal distribution of the measurements of the diameter of the TiO₂ NWs after the ecotoxicity experiment.

Figure 6

(A) Mortality (%) for the TiO₂ NP sample vs. concentration and (B) probit of mortality vs. log10 concentration (right) fitted with a non-linear cubic function. Goodness of fit (${\mathit{R}}^{\mathbf{2}}$ = 1.0). (C) Mortality (%) for the TiO₂ NWs sample against concentration and (D) mortality probit vs. log10 concentration (right) fitted with a nonlinear parabolic function. ${\mathit{R}}^{\mathbf{2}}$ = 0.859.

Figure 7

Boxplot for all morphological parameters (tail, body, heart, antenna, and eye) measured after TiO₂ NPs exposure. Numbers above boxes are their p-values, with those written in bold numbers indicating significance (p-value < 0.05).

Figure 8

Box plot for all morphological parameters (tail, body, heart, antenna, and eye) measured after TiO₂ NWs exposure. Numbers above boxes are their p-values, with those written in bold numbers indicating significance (p-value < 0.05).

Figure 9

(A) D. magna individual from the negative control, (B) D. magna individual exposed to 37.5 mg ${\mathrm{L}}^{-1}$ of TiO₂ NPs, (C) D. magna individual exposed to 75 mg ${\mathrm{L}}^{-1}$ of TiO₂ NPs, (D) D. magna individual exposed to 150 mg ${\mathrm{L}}^{-1}$ of TiO₂ NPs, (E) D. magna individual exposed to 300 mg ${\mathrm{L}}^{-1}$ of TiO₂ NPs, and (F) D. magna individual exposed to 600 mg ${\mathrm{L}}^{-1}$ of TiO₂ NPs.

Figure 10

(A) D. magna individual from the negative control, (B) D. magna individual exposed to 50 mg ${\mathrm{L}}^{-1}$ TiO₂ NWs, (C) D. magna individual exposed to 100 mg ${\mathrm{L}}^{-1}$ TiO₂ NWs, (D) D. magna exposed to 200 mg ${\mathrm{L}}^{-1}$ TiO₂ NWs, (E) D. magna individual exposed to 400 mg ${\mathrm{L}}^{-1}$ TiO₂ NWs, and (F) D. magna exposed to 800 mg ${\mathrm{L}}^{-1}$ TiO₂ NWs.

Figure 11

SAED pattern for before (a) and after (b) TiO₂ NPs. SAED pattern for before (c) and after (d) TiO₂ NWs. The inset (bottom-right) in blue indicates the indexed rotational average pattern for the identified crystalline phases before and after ecotoxicological experiments.

Figure 12

(a) SEM image, (b) EDS mapping, (c,d) elemental (Ti,O) EDS images for before TiO₂ NPs, (e) SEM image, (f) EDS mapping, (g,h) elemental (Ti,O) EDS images for after TiO₂ NPs, (i) SEM image, (j) EDS mapping, (k,l) elemental (Ti,O) EDS images for before TiO₂ NWs, (m) SEM image, (n) EDS mapping, and (o,p) elemental (Ti,O) EDS images for after TiO₂ NWs.

Figure 13

(a) Ti-L_2,3 edge and (b) O-K edge in the EELS spectra for before and after TiO₂ NPs. (c) Ti-L_2,3 edge and (d) O-K edge in the EELS spectra for before and after TiO₂ NWs. The upper letters (A, B, C, and D) indicate the characteristic energy loss positions (eV).