Raman, TEM, EELS, and Magnetic Studies of a Magnetically Reduced Graphene Oxide Nanohybrid following Exposure to Daphnia magna Biomarkers

Abstract

A ternary nanocomposite made of nanomaghemite, nanoanatase, and graphene oxide has been successfully synthesized using an inorganic coprecipitation approach, and it has been systematically investigated by X-ray diffraction, transmission electron microscopy, and different spectrocopic techniques (electron energy loss, µ-Raman, and ⁵⁷Fe Mössbauer) after interaction with an effluent containing Daphnia magna individuals. Specifically, the influence of the nanocomposite over the Daphnia magna carapace, administered in two doses (0.5 mg mL^-1 and 1 mg mL^-1), has been characterized using µ-Raman spectroscopy before and after laser burning protocols, producing information about the physicochemical interaction with the biomarker. The thermal stability of the nanocomposite was found to be equal to 500 °C, where the nanoanatase and the nanomaghemite phases have respectively conserved their structural identities. The magnetic properties of the nanomaghemite have also been kept unchanged even after the high-temperature experiments and exposure to Daphnia magna. In particular, the size, texture, and structural and morphological properties of the ternary nanocomposite have not shown any significant physicochemical modifications after magnetic decantation recuperation. A significant result is that the graphene oxide reduction was kept even after the ecotoxicological assays. These sets of observations are based on the fact that while the UV-Vis spectrum has confirmed the graphene oxide reduction with a localized peak at 260 nm, the 300-K and 15-K ⁵⁷Fe Mössbauer spectra have only revealed the presence of stoichiometric maghemite, i.e., the two well-defined static magnetic sextets often found in the bulk ferrimagnetic counterpart phase. The Mössbauer results have also agreed with the trivalent-like valence state of Fe ions, as also suggested by electron energy loss spectroscopy data. Thus, the ternary nanocomposite does not substantially affect the Daphnia magna, and it can be easily recovered using an ordinary magnetic decantation protocol due to the ferrimagnetic-like character of the nanomaghemite phase. Consequently, it shows remarkable physicochemical properties for further reuse, such as cleaning by polluted effluents, at least where Daphnia magna species are present.

Keywords: Daphnia magna biomarkers; lethal dose; nanohybrid recoverage; post-exposure characterization.

Figure 1

(a) µ-Raman spectra for glass substrate and D. magna immobilized on the glass substrate (DM1 sample, 0 mg mL⁻¹), showing the typical glass-substrate-related luminescence, (b) micrograph image of the D. magna carapace, and (c) gel electrophoresis for different concentrations of DNA from D. magna.

Figure 2

(a) Before- and after-burning µ-Raman spectra for the DM2 sample, 5× and (c) 50× are two distinct magnifications representing the DM2 sample before burning. The yellow box in (c) is a zoomed region of (b), where the 1, 2, and 3 index represents the exact illuminated position for the collected spectra given in (a).

Figure 3

(a) Before- and after-burning µ-Raman spectra for the DM3 sample, (b) micrograph representing the DM3 sample before burning. 1 is a measurement in a bright point, whilst 2 is a measurement in a dark/brownish place.

Figure 4

Before- and after-burning µ-Raman spectra for different spots in the ternary nanocomposite (a–d). (e) Note how the darker material becomes whitish after the burning due to the phase transition of the iron-oxide. (e) After-burning micrograph for the recovered ternary nanocomposite measured over a glass substrate. (b–d) 1, 2, 3 (before burning) correspond to Raman spectra taken by exciting bright, darker, middle (crack) spots, respectively in the picture (f).

Figure 5

(a) Rietveld refinement of the XRD diffractogram of the ternary nanocomposite using the TCH diffraction profile. The red points (I_obs) and the black lines (I_cal) respectively represent the observed experimental diffractograms and the calculated diffractograms, and the blue line is the residual lines. Miller indices with black and purple colors indicate the crystallographic γ-Fe₂O₃ and anatase TiO₂ phases, respectively, (b) TG curve, and (c) UV vis spectrum of the ternary nanocomposite.

Figure 6

(a,b) TEM images of the γ-Fe₂O₃ NPs, (c) the FFT of (b,d) high-resolution image, (e) the magnification of the white box in (d,f), FFT of (e,g) is the STEM image for faceted NPs and (h), and (i) the EELS images for Fe and O taken from (g). On the other hand, (j) is the STEM image for spherical NPs with their respective EELS mapping (k) and elemental identification for Fe (l) and O (m). (n) is the PSD for γ-Fe₂O₃ NPs considering both morphologies.

Figure 7

(a) TEM image for the nanocomposite, (b) magnification of the white box in (a) where the γ-Fe₂O₃ and TiO₂ NPs are observed (white arrows), (c) high-resolution magnification of (b) where the FFT was taken, see right top inset of (c), (d) FFT of Image (a), while (e) is the STEM image of (a,f–i), which are elemental EDS images for Fe, O, Ti, and mapping. (j) STEM image of another selected region from the sample with (k–n) representing EELS images for the Fe, Ti, O, and Fe+ Ti, where (o) is the total EELS mapping. (p) is the high-resolution image of the ternary nanocomposite, (q) the EELS image, and (r) is the PSD for the TiO₂ NPs.

Figure 8

(a) O-K edge and (b) Fe-L_2,3 edge in the EELS spectra of the spherical and faceted γ-Fe₂O₃ NPs identified in Figure 6g,j. (c) O-K edge and (d) Ti-L_2,3 edge in the EELS spectra of the porous and small TiO₂ NPs identified in Figure 7p,q.

Figure 9

(a) 300 K and (b) 15 K Mossbauer spectra for the ternary nanocomposite, (c) ZFC 300 K M(H) loop and (d) ZFC and FC 1T M(H) loop recorded at 5 K for the ternary nanocomposite. 1 T was the field used in the FC protocol when the sample was cooled down from 300 K to 5 K. Insets in (c) and (d) are zoomed regions of their respective M(H) loops.