Characterization of the structural, optical, photocatalytic and in vitro and in vivo anti-inflammatory properties of Mn2+ doped Zn2GeO4 nanorods

V. Y. Suzuki a, L. H. C. Amorin a, N. M. Lima b, E. G. Machado c, P. E. Carvalho d, S. B. R. Castro d, C. C. Souza Alves c, A. P. Carli c, Maximo Siu Li e, Elson Longo f and Felipe A. La Porta *a
aFederal Technological University of Paraná, Nanotechnology and Computational Chemistry Laboratory, Avenida dos Pioneiros 3131, 86036-370, Londrina, PR, Brazil. E-mail:
bFederal University of Juiz de Fora, Chemistry Department, CEP 36036-900 Juiz de Fora-MG, Brazil
cFederal University of the Valleys of Jequitinhonha and Mucuri, CEP 39803-371, Teófilo Otoni, MG, Brazil
dFederal University of Juiz de Fora, Department of Pharmacy, CEP 35010-180, Governador Valadares, MG, Brazil
eUniversidade de São Paulo, Instituto de Física de São Carlos, CEP 13560-970, São Carlos, SP, Brazil
fINCTMN-UFSCar, Universidade Federal de São Carlos, P.O. Box 676, 13565-905 São Carlos, SP, Brazil

Received 2nd March 2019 , Accepted 14th May 2019

First published on 14th May 2019

The effect of Mn incorporation on the structural, optical, and photocatalytic properties of Zn2GeO4 (ZGO) host matrices was investigated. Moreover, the ability to modulate in vitro inflammatory mediators and in vivo acute inflammation of Mn-doped ZGO (ZGO:Mn) was evaluated for the first time. We used a one-pot hydrothermal approach for preparing shape-controlled ZGO nanorods with 0, 1, 2, and 4% Mn2+ (ZGO:Mn). Both the ZGO and ZGO:Mn nanorods showed a rhombohedral structure and excellent optical properties with photoluminescence (PL) emission in the visible region at 543 nm attributed to the electronic transitions (4T16A1) among the 3d5 orbitals of the incorporated Mn2+ ions in [MnO4] clusters. The nanorods were found to be efficient photocatalysts for the ultraviolet degradation of methylene blue in the presence of H2O2. The ZGO:Mn nanorod catalyst with 2% Mn showed the best methylene blue degradation (complete degradation within 10 min). The surface acidity of the ZGO:Mn nanorods also contributed to their photocatalytic ability. Meanwhile, the ZGO:Mn nanorods significantly reduced the tumor necrosis factor and nitric oxide content. Hence, the 4%-doped nanorods were found to be highly effective in the reduction of carrageenan-induced paw edema. The acute inflammation (on macrophages) results revealed that the nanorods with 4% Mn2+ exhibited promising anti-inflammatory properties.


Nanocrystalline zinc germanete (Zn2GeO4 (ZGO)), which is an n-type oxide semiconductor with a direct bandgap (≈4.4 eV) and large exciton binding energy, crystallizes in a phenakite structure and possesses unique properties for various technological applications.1–5 ZGO shows excellent chemical and thermal stability, and hence finds applications in photocatalysis, electrochemical sensors, lithium-ion battery (LIB) components, electroluminescent devices, and toxic heavy metal ion removal.6–14 ZGO nano/microstructures are grown via various synthesis routes such as solid-state reactions, hydroxylation of alkoxides, steam growth techniques, and a variety of aqueous processes.15–17 Among these methods, the hydrothermal method is considered to be the most efficient approach for scalable preparation of various low-temperature metastable ternary germanate oxides.18–20 This approach yields highly crystalline products without any additional treatment.21 Hence, microwave-assisted hydrothermal synthesis has attracted immense attention for preparing of a diversity of crystals with well-defined shapes and sizes while reducing the synthesis time.17,21–25 In addition, the synthesis conditions such as the supersaturation, temperature, solvent, and precursors significantly affect the crystal growth kinetics, structure, and morphology of the crystals prepared using this method.26,27

Owing to its excellent physical and chemical properties, ZGO is considered to be an excellent host material for various activator ions to generate multicolor emission28–30 and for photocatalytic degradation of pollutants and reduction of CO2.22,31–34 For instance, Co2+-doped ZGO is widely used in lithium-ion batteries (LIBs).11 Eu3+ doping affects the morphology of ZGO crystals, making them suitable for the preparation of white light emitting devices.29 Since Mn2+ ions emit intense green light (530 nm) upon ultraviolet (UV) irradiation, they are widely used for doping ZGO.35–37 In addition, Mn2+-doped ZGO (referred to as ZGO:Mn henceforth) shows long wavelength photoluminescence (PL) when co-doped with Cr3+ and Pr3+ and is used as a near-infrared down-conversion material when co-doped with Yb3+.17,38,39 Hence, the incorporation of such activator ions can tailor the properties of ZGO.

Although nanostructures have been widely used in various fields including health and medicine, they exhibit pronounced toxicity, and their long-term toxic effects on humans are unknown.40–43 Zinc- and germanium-based nanoparticles have already been shown to modulate inflammatory responses.44–46 However, to the best of our knowledge, the ability of ZGO:Mn crystals to modulate in vitro inflammatory mediators and in vivo acute inflammation has not been investigated.

Therefore, in this study, we investigated the effects of the structural defects, optical and photocatalytic properties, and cytotoxicity of the ZGO:Mn nanostructures (with 0, 1, 2, and 4% Mn doping) prepared using the microwave-assisted hydrothermal method on their ability to modulate inflammatory responses.

Results and discussion

From the X-ray diffraction (XRD) pattern shown in Fig. 1a, it can be observed that ZGO showed a high degree of crystallinity. The peaks could be indexed to Joint Committee on Powder Diffraction Standards (JCPDS) 11-687 with lattice constants of a = b = 14.231 Å, c = 9.53 Å, α = β = 90°, and γ = 120°. It showed a rhombohedral crystalline structure belonging to space group R[3 with combining macron] (no. 148). It consisted of tetrahedral germanium [GeO4] and zinc [ZnO4] clusters bound to four oxygen atoms. In the ZGO lattice, [GeO4] acted as the network former, while [ZnO4] acted as the network modifier. Since the ionic radii (Ir) of components Mn2+ and zinc are similar, when Mn2+ is used as a dopant in ZGO, it is more likely to replace zinc (Ir = 0.60 pm) than germanium (Ir = 0.39 pm). The Ir were chosen taking into account the coordination number, which in this case for all discussed samples follows tetrahedral coordination of atoms.47,48 Therefore, [MnO4] clusters were also observed in the doped samples. These clusters are shown in Fig. 1f. The texture coefficient (TC) of these crystals was calculated using the XRD results to determine their degree of orientation. The TC values were calculated using eqn (1),
image file: c9tc01189g-t1.tif(1)
where I(hkl)i is the intensity of the (hkl)i plane, I0(hkl)i is the intensity of the (hkl)i plane according to JCPDS 11-687, and N is the number of total peaks analyzed. Fig. 1b shows the TC values of the crystals. The dotted red line represents the division of the presence or absence of preferential growth directions. At TC > 1, the growth occurred preferentially in the direction indicated by the corresponding Miller indices. Hence, it was found that the doping content significantly affected the crystal growth in the (113), (006), and (306) directions, suggesting that the tetrahedral [MnO4] clusters showed strong interactions in these directions because of the environmental growth conditions. With an increase in the doping content, the crystal growth occurred preferentially along a direction parallel to the (113) and (306) directions. This preferential growth is related to the degree of agglomeration of particles (which is very common in polycrystalline structures), i.e., being able to be in a non-random network mainly on the abovementioned crystal planes.49Fig. 1c–e show the planes ((220), (410), and (113), respectively) corresponding to the rhombohedral structure of ZGO.

image file: c9tc01189g-f1.tif
Fig. 1 (a) XRD patterns and (b) TC analysis for the indexed peaks of the ZGO:Mn samples. Representation of the (c) (220), (d) (410), and (e) (113) directions and (f) polyhedral clusters of the rhombohedral ZGO structure.

In order to confirm these findings, we carried out the Rietveld refinement analysis of the samples. The structural parameters obtained using the Rietveld refinement method are shown in Fig. 2a–d. These parameters (Rwp, Rp, Rexp, and GoF) were consistent with the parameters obtained from the XRD patterns (Table 1). Hence, the ZGO:Mn samples showed a certain degree of structural disorder. These structural defects can be attributed to the short-, medium-, and long-range structural distortions of the ([O–Mn–O]/[O–Ge–O]/[O–Zn–O]) angles and bonds of ZGO, resulting in most likely an increase in the number of oxygen vacancies. These defects altered the properties of the ZGO lattice. The ZGO lattice parameters obtained in this study were slightly different from those reported previously (Table 1). This can be attributed to the various processing methods and orientation, size, and structural disorder of the ZGO crystals used in this study. The samples showed slightly different mean crystallite sizes, as calculated using the Scherrer method.49 The crystallite sizes of the samples with 0, 1, 2, and 4% Mn2+ were found to be 4.179, 3.792, 3.309, and 3.259 Å, respectively. This decrease in the crystallite size with an increase in the Mn doping content was confirmed by the transmission electron microscopy (TEM) results (Fig. 2e and f). The images showed that the nanorods had soft edges and uniform morphology. Furthermore, the degree of agglomeration decreased, which is important for the physical phenomena that depend on the contact interface.

image file: c9tc01189g-f2.tif
Fig. 2 (a–d) Rietveld refinement analysis of the ZGO:Mn samples. TEM images of the ZGO:Mn samples with (e) 0% and (f) 4% Mn2+.
Table 1 Lattice parameters obtained for Rietveld refinements
Sample a (Å) c (Å) Volume (Å)
0% 14.218 9.533 1668.977
1% 14.217 9.537 1669.417
2% 14.254 9.561 1682.366
4% 14.263 9.556 1683.541
ICSD no. 11-687 14.231 9.530 1686.930

The optical bandgap energies of the ZGO:Mn nanorods with different Mn doping contents (0, 1, 2 and 4%) were determined from the peaks of their first-derivative DRS/UV-vis curves. Fig. 3 shows the optical bandgap energies of the nanorods. ZGO showed a bandgap energy of 3.34 eV. The bandgap energy increased from 3.45 to 3.38 eV with an increase in the doping content from 1 to 2%. However, a further increase in the doping content to 4% resulted in a significant decrease in the bandgap energy to 2.95 eV. These variations can be attributed to the nucleation–dissolution–recrystallization mechanism of the primary building particles or cluster subunits of the ZGO:Mn samples. Such mechanisms are strongly driven by the chemistry of the surface, which in turn contributes to phase stability under microwave-assisted hydrothermal conditions.50,51 In addition, this variation in the optical band gap energy of the samples suggests the generation of intermediate states in the forbidden electronic states of ZGO:Mn because of the deformation of the ZGO crystal lattice. These variations altered the properties of the ZGO crystal lattice.52,53

image file: c9tc01189g-f3.tif
Fig. 3 First-derivative curves of the DRS spectra.

Correspondingly, the optical properties of the ZGO:Mn samples were investigated by obtaining their PL spectra. Fig. 4a–d show the multi-peak fitting of the PL spectra. In the case of the 0, 1, and 2%-doped samples, five peaks (PL sub-bands) were used to deconvolve their spectra. On the other hand, only three peaks were used for the deconvolution in the case of the 4%-doped sample because of its narrow peaks. Each component contributed to the broadband profile of the samples. This can be attributed to the presence of defect states in the forbidden gap.54–57 Hence, it can be stated that the PL emission of the ZGO:Mn samples in the visible region was related to the structural defects of the [GeO4], [ZnO4] and [MnO4] clusters. The crystal structure disorder of the samples (short-, medium-, and long-range) affected their charge distributions. Thus, electronically excited states cause broad-band PL emission at room temperature.54–57 The PL profile of the ZGO host lattice changed with the substitution of Zn2+ ions by Mn2+ ions. The 4%-doped sample showed a significantly narrow bandwidth (Fig. 4e). From the inset of Fig. 4e, it can be observed that the luminescence intensity increased with an increase in the doping content. The full width at half maximum of the 4%-doped sample was about 58% lower than that of the undoped sample. The maximum PL emission of the samples varied from 522.7 (0%) to 543.0 nm (4%). This red shift in the PL emission was caused by the generation of a sub-bandgap between the valence and conduction bands.

image file: c9tc01189g-f4.tif
Fig. 4 PL spectra at room temperature of the ZGO:Mn samples. (a–d) Deconvolved into five and three bands and (e) with (inset without) normalization of the ZGO lattice with different concentrations of Mn2+.

As is well-known, Mn2+-doped ZGO shows green luminescence. This can be attributed to the electronic transitions (4T16A1) between the 3d5 orbitals of Mn2+ ions in the [MnO4] clusters.10,17,48 The undoped ZGO sample also showed green luminescence. This can be attributed to the incorporation of some intrinsic defects (e.g. oxygen vacancies (VO), Zn interstitials (Zni), Zn vacancies (VZn), or Ge vacancies (VGe)) during its processing.10,48,58 However, the visible-region PL band profile of ZGO is attributed to the structural distortions of [GeO4] clusters, especially VO.4,14

In addition, the surface acidity of the samples was investigated by carrying out their titration with HCl (0.01 M) to determine the concentration of acid groups on their surface. Table 2 lists the H+ concentration of the samples. The 2% ZGO:Mn sample showed the highest acidity (consistent with the photocatalysis results discussed later). The surface acidity of the samples contributed to their photocatalytic activity for methylene blue (MB) degradation. Fig. 5 shows that the 2% ZGO:Mn catalyst showed excellent MB degradation efficiency. Theoretically, this effect of increasing the acidity could promote a better interaction between the catalyst and the pollutant with unpaired electrons, allowing a more effective absorption of H2O and H2O2. Thus, there is likely a greater formation of OH* radicals, responsible for promoting the degradation of the MB.59,60

Table 2 Results of the surface acidity of ZGO doped with different ratios of Mn2+
Sample HCl 0.01 M (mL) H+/mcatalyst (10−3 mmol g−1)
0% 17.53 1.493 ± 0.098
1% 16.20 1.760 ± 0.231
2% 15.90 1.820 ± 0.198
4% 16.50 1.700 ± 0.159

The significant reduction in the maximum MB absorption peaks of the catalysts (Fig. 5a–d) shows that the photoactivated catalysts degraded almost all the dye in the solution within 10 min. On the other hand, the photodegradation process required 30 min when the degradation was carried out only in the presence of H2O2 without the catalysts. The −ln(C/C0) vs. degradation time plots of the catalysts were obtained to calculate the k value of the degradation reaction (using the slope) (Fig. 5b and c). Our results show a better activity for the photocatalytic test in the presence of H2O2.

image file: c9tc01189g-f5.tif
Fig. 5 (a) Degradation percentage of the MB dye and −ln(C/C0) vs. degradation time plot (b) with and (c) without H2O2.

image file: c9tc01189g-f6.tif
Fig. 6 Inflammatory modulation. Evaluation of the (A) cytotoxicity, (B) nitric oxide production and (C) TNF-α, (D) IL-6 and (E) IL-1β release of the J774A.1 macrophages stimulated or not with LPS + IFN-γ and treated with ZGO or ZGO:Mn (1, 2 or 4%) nanoparticles at concentrations 100, 10 and 1 μg mL−1. Cell = J774A.1 not treated. *p < 0.05 versus cell. Data are representative of three independent experiments.

Table 3 lists the degradation reaction rates of the catalysts. The kinetic (k) constant for the reaction without the catalyst was 0.04709 min−1. It was found that the presence of ZGO nanocrystals (doped and undoped) accelerated the degradation of MB significantly. In this case, the 2% ZGO:Mn sample showed the best photocatalytic performance. Meanwhile, for the photocatalysis performed without H2O2, in particular, the most satisfactory result was for the 1% ZGO:Mn. Also, the correlation coefficient values (R2) were consistent with the proposed pseudo-first order kinetics model.

Table 3 Summary of the pseudo-first-order kinetics for the photodegradation of MB dye using ZGO:Mn as a catalyst
Sample k (min−1) R 2
No catalyst 0.04709 ± 0.00415 0.92129
0% 0.14708 ± 0.01514 0.95935
1% 0.13168 ± 0.01398 0.95686
2% 0.15759 ± 0.02027 0.93791
4% 0.13033 ± 0.01479 0.94989

In order to investigate the interactions of H2O, O2, and H2O2 with the nanorod surface, we used the complex cluster notation (based on the Kröger–Vink notation61). These reactions are shown in the equations given below (M = Zn, Ge, and Mn). Mn appeared only in the doped ZGO samples. Structural defects (at short-, medium- and long-range) display an important role in the adsorption process by forming radical groups such as H2O* and OH*.26 Since ZGO is an n-type semiconductor, photoexcitation occurs on its surface, which is responsible for the generation of hydroxyl radicals. These radicals trigger the decomposition of organic molecules such as dyes in solutions.62

[MO4]× + [MO4]× → [MO4]′ + [MO4(2)
[MO4]˙ + H2O → [MO4]× + OH* + H˙(3)
[MO4]′ + O2 → [MO4]× + O2(4)
O2′ + H˙ → O2H*(5)
[MO4]′ + H2O2 → [MO4]× + OH* + OH(6)
H2O2 + O2′ → OH* + OH + O2(7)

Eqn (2) shows the formation of electron–hole pairs by the charge transfer between the clusters. The left part represents the ordered crystal, while the right corresponds to the disordered one. The lattice defects introduced in the crystal during the synthesis induce the order–disorder effect, which makes this charge transfer possible. This behavior is characteristic of crystals with native defects. The distortions between the angles and bonds of the [MnO4], [GeO4], and [ZnO4] clusters confirm the presence of native defects in the ZGO:Mn nanorod crystals. These defects are governed by the electron–hole separation and dipole moment induction, and contribute significantly to the photocatalytic activity of a complex material.14

Eqn (3), (4) and (6) show the reaction between the surface of the nanocrystals and the reagents dissolved in the medium: H2O, O2, and H2O2. The interaction of these species contributed to the electron transfer, which accelerated the subsequent reactions. In the next step, radicals such as OH* (in eqn (3), (6) and (7)) and O2H* (in eqn (5)) were formed. These radicals have a high oxidation potential and hence promote the photocatalytic reaction. The photocatalysis results revealed that the 2% ZGO:Mn sample showed the highest concentration of surface defects.

It has been reported that zinc- and germanium-based nanoparticles can increase the release of inflammatory mediators (at low concentrations) in diverse cell lines46,63–65 and can even reduce the TNF and NO concentrations. This can be attributed to the different patterns of these nanoparticles.66 In this case, the nanoparticle treatment reduced the viability of the cells at ZGO (100 and 10 μg mL−1) and 2% ZGO:Mn (100 μg mL−1) samples. Their NO, IL-6, IL-1β and TNF-α reduction were not considered because of their cytotoxicity (Fig. 6a–c). 1% ZGO:Mn showed a NO reduction of around 50% at all the concentrations. 4% ZGO:Mn showed the highest NO reduction (more than 86% (100 μg mL−1)) without cytotoxicity (Fig. 6(b, c and e)). The doped compounds showed a significant reduction in TNF-α production (80% reduction by 10 μg mL−1 of 1 and 2% ZGO:Mn) (Fig. 6c). ZGO showed no reduction in the TNF-α production, probably because of its different reaction pathway and the absence of interacting structures in it.67 The presence of Mn can reduce the cytotoxicity of ZGO, probably because of the interactions of the doped nanoparticles.67

image file: c9tc01189g-f7.tif
Fig. 7 Acute inflammation. Female BALB/c mice (n = 5 per group) were treated intraperitoneally (100 μL) with PBS, ZGO:Mn 4% (1 mg kg−1) or dexamethasone (0.5 mg kg−1), 30 minutes before the induction of the carrageenan-induced paw edema. Paw edema (Δ paw thickness (mm) = [footpad thickness of carrageenan (mm)] − [footpad thickness of PBS (mm)]) was monitored until 4 hours. Each point represents the arithmetic mean ± SEM. *p < 0.05 versus untreated group (PBS). Data are representative of three independent experiments.

Hence, 4% ZGO:Mn showed the reduction of carrageenan-induced paw edema after 3 h of induction and showed the same efficiency as that shown by dexamethasone (Fig. 7). This sample was chosen for use in this assay because of its good TNF-α, IL-6, IL-1β and NO inhibition without cytotoxicity. This acute inflammation model is widely used to evaluate the anti-inflammatory activity of different compounds.68,69 In this direction, we should mention that the direct use of Zn-containing nanoparticles (e.g., with well-defined morphologies and sizes), have a well-known inhibitory effect on the proliferation and adhesion of macrophages. Because of this, they could be widely used to prevent or still control many anti-inflammatory processes of interest. Since the release of Zn ions from the nanoparticles is a slow and long-term process,70 in particular, this factor may be beneficial allows to a bone integration, including secretion of tumor necrosis factor alpha by macrophages could regulate neighboring cells, as well as, control the healing process. A more fundamental understanding of these factors and their critical role in the modulation of anti-inflammatory activity, in principle, they can decisively influence the proliferation, viability and migration of macrophages to retard the immune response as well.70 Therefore, our results suggest that the Mn incorporation and Vo have a positive effect on the modulation of the inflammatory response of the prepared ZGO:Mn nanorods. In this way, studies on the association of inflammation with cancer are quite attractive for the research of new therapeutic targets and biologically active compounds.

image file: c9tc01189g-f8.tif
Fig. 8 Scheme for the synthesis procedure of the ZGO:Mn materials.


We investigated the effect of Mn2+ incorporation on the structural, optical, and photocatalytic properties; cytotoxicity; and modulation of the inflammatory response of ZGO:Mn nanorods prepared using the microwave-assisted hydrothermal method. The results were correlated with the structural defects of the nanorods, which contributed to their outstanding physical and chemical properties. The PL emission center of the nanorods shifted from 522.7 to 543.0 nm with an increase in the doping content from 0 to 4%, respectively. Mn doping also resulted in a significant increase in the luminescence intensity and a significant narrowing of the PL band profile (58%). Narrower PL bands impart a purer color for application in red, green and blue light emitting (RGB)-based optoelectronics devices. In addition, the photocatalysis results showed that the ZGO:Mn catalysts could efficiently degrade MB under UV-light. The sample with 2% doping showed the highest photocatalytic degradation efficiency owing to its highest surface acidity. Moreover, the ZGO:Mn nanorods could modulate the inflammatory mediators in macrophages without reducing the viability of this cell line. The sample with 4% doping was found to be potent for in vitro modulation and could reduce acute inflammation as effectively as dexamethasone.


Material and synthesis

Firstly, two aqueous solutions of 40 ml each were prepared, one with zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 98%, Sigma Aldrich) and another one with germanium dioxide (GeO2, 99.99%, Alfa Aesar). After mixing these two solutions with constant stirring, manganese acetate tetrahydrate is added (Mn(CHCOO)2·4H2O, 99.999%, Alfa Aesar). Four syntheses were performed: one with only Zn(NO3)2·6H2O and GeO2 in a molar ratio of 2[thin space (1/6-em)]:[thin space (1/6-em)]1 and the others replacing 1%, 2% and 4% in moles of Zn(NO3)2·6H2O by Mn(CHCOO)2·4H2O, respectively. After preparing the mixture of all reagents, ammonium hydroxide (NH4OH) is added to adjust the pH to 8. After constant stirring for 10 minutes, the mixture is placed in a Teflon reactor and put in a microwave. All hydrothermal synthesis was performed at 140 °C for 10 minutes. Then, the material is washed several times with distilled water and alcohol until neutralized. Finally, the material is dried in an oven at 80 °C for about 24 hours to obtain a white powder. The samples will be denominated, respectively, by their doping percentage content. The synthesis scheme is shown in Fig. 8.


All materials obtained were characterized by powder X-ray diffraction (XRD) in the 2θ range from 5 to 80° at 0.02° min−1, using a Rigaku-Max/2500 PC with a Cu Kα X-ray source (λ = 1.5406 Å). Phase analysis by the Rietveld method71 was carried out using the General Structure Analysis System (GSAS) software.72 The PL spectra were collected at room temperature with a Thermal Jarrel-Ash Monospec monochromator and a Hamamatsu R955 photomultiplier with a lock-in setup. The 350.7 nm (2.57 eV) excitation wavelength of a krypton ion laser (Coherent Innova 200 K) was used with the output of the laser kept at 500 mW and 15 mW arriving at the sample after modulation and filtering through a prism. Diffuse-reflectance spectroscopy in the ultraviolet-visible region (DRS/UV-vis) of ZGO and ZGO:Mn was performed using a Cary 5G spectrophotometer. Transmission electron microscopy (TEM) analysis was performed using a JEOL JEM 2100F microscope operating at 200 kV.

Photocatalysis test

For the MB degradation test, 40 mg of the catalyst was used in a mixture of the dye solution (10 mg L−1) with hydrogen peroxide in the ratio 9[thin space (1/6-em)]:[thin space (1/6-em)]1 (volumetric ratio). After mixing all the components for carrying out the photocatalysis under constant stirring, the mixture was left in the dark for 10 minutes to reach the adsorption–desorption equilibrium. Then, with photoreactor UVC lamps at 254 nm (15W, G15T8/OF, OSRAM) attached, 1.00 mL aliquots were collected at times of 0, 1, 2, 3, 5, 7 and 10 minutes. Finally, the aliquots were diluted in 100 mL of distilled water and spectroscopy was performed in the visible ultraviolet (UV-vis) region. The photocatalytic degradation process could be well-fitted by pseudo-first-order kinetics (eqn (8)). Therefore, the pseudo-first-order rate (k) was used to compare the photodegradation efficiency of the catalysts:
−ln(C/C0) = kt(8)
where C is the MB concentration at the analyzed photocatalysis time, C0 is the concentration of MB in the photocatalysis reaction, k is the reaction kinetic rate and t is the degradation time.71

Determination of surface acidity

To determine the acidity of the surface of the synthesized material, suspensions with about 50.0 mg of catalyst were prepared in 25 mL of sodium hydroxide solution (NaOH (0.01 M)). The mixture was stirred for 300 minutes at room temperature to obtain the title compound. Then, as a pH indicator, phenol red (0.02%) was used for identification of the turning point around pH 7, the point of neutralization. The used titrant was hydrochloric acid (0.01 M HCl). These experiments were performed in triplicate.

Cell culture assay

The J774A.1 macrophage lineage was maintained in culture bottles containing RPMI-1640 medium (Lonza) supplemented with 1% non-essential amino acids, 100 μg mL−1 streptomycin and penicillin, and 5% fetal bovine serum (Sigma) in a humid atmosphere of approximately 5% CO2 at 37 °C. After reaching confluence the bottles were scraped and the cells plated in 96-well plates at a concentration of 2 × 105 cells mL−1.

For the cell viability assay, the J774A.1 lineage was incubated in a humidified atmosphere of approximately 5% CO2 at 37 °C in the presence or not of the ZGO or ZGO:Mn (1%, 2% or 4%) nanorods at concentrations of 100, 10 and 1 μg mL−1, respectively, for 48 hours. For the nitric oxide (NO), IL-6, IL-1β and TNF-α dosage, the macrophages were incubated in a humidified atmosphere of approximately 5% CO2, kept at 37 °C in the presence or not of the nanoparticles at concentrations of 100, 10 and 1 μg mL−1 for one hour and subsequently stimulated with LPS (10 μg mL−1) and IFN-γ (9 ng mL−1) at 10% of the culture volume. After 6 hours (TNF-α) or 48 hours (NO) of culture, the supernatants were collected in this case for further analysis.

Cell viability assay

Cell viability was measured using the standard MTT [3-(4,5-dimethylthiazol-2yl)-2,5-diphenyltetrazolium bromide] assay on a culture of unstimulated cells. Throughout 48 hours of culture, for each experiment the supernatant was discarded. Then, the cells were immediately incubated with about 100 μL of supplemented RPMI and 10 μL of MTT (5 mg mL−1) in a humidified atmosphere of approximately 5% CO2 kept at 37 °C for a period of 4 hours. Next, the supernatant was discarded from the wells without any change in the precipitate. The formed formazan crystals were then dissolved by the brief addition of 100 μL of DMSO into each well. Complete solubilization was done by gentle agitation of the plates. Optical density in this experiment was measured at a wavelength of 560 nm (EZ Read 2000, Biochrom). Hence, the cellular viability was then calculated using the formula (X1/X2) × 100, considering X1 the mean OD of treated cells and X2 the mean OD of untreated cells.

Nitric oxide dosage

The 48 hour supernatants from the stimulated cultures also were analyzed for quantification of nitrites by the Griess method. For this assay, aliquots of supernatants were plated with equal volumes of 1% sulfanilamide and 0.1% N-(1-naphthyl)ethylenediamine. Hence, the resulting nitrite production was quantified by comparison to a standard curve with different NaNO2 concentrations. In this case, the optical density was then measured at a standard wavelength of 540 nm (EZ Read 2000, Biochrom).

Cytokines dosage

The three-hour culture supernatants were measured using commercially available ELISA kits. BD OptEIA kits (BD Bioscience) were used for IL-6, IL-1β and TNF-α dosage, in accordance with the manufacturer's instructions.


For these experiments, we employ female BALB/c mice about 4–6 weeks old (n = 5 per group), which were obtained from the animal care facilities of the Federal University of Minas Gerais (UFMG) – Belo Horizonte-MG, Brazil – and maintained in microisolator cages. It is worth emphasizing that all procedures adopted followed the principles of the Brazilian code for the use of animals in the laboratory and, consequently, were pre-approved by the committee on the use of laboratory animals at the Federal University of Vales do Jequitinhonha e Mucuri (UFVJM) – Teófilo Otoni-MG, Brazil – (Protocol no. 001/2018). Hence, the mice were monitored for clinical signs of toxicity in this case after the realized treatments.

Induction of acute inflammatory response by carrageenan-induced paw edema

Initially the mice (n = 5 per group) were weighed (20.16 ± 0.2 g) and their right and left paws were measured with a pachymeter (0 h). Dexamethasone was essentially used as a positive control treatment due to its anti-inflammatory activity. Thus, 30 minutes before induction of edema in the paw, PBS, dexamethasone (0.5 mg kg−1) or ZGO:Mn 4% (1 mg kg−1) was administered intraperitoneally (100 μL). Carrageenan (2.5%) was dissolved in PBS, and 20 μL injected into the left footpad, and 20 μL of PBS into the right footpad of all groups. At regular intervals in the period of 1–4 h, soon after the injection application of carrageenan, both paws (left and right) were measured. The resulting magnitude of the carrageenan-induced paw edema in this case was determined as follows: [paw edema (mm)] = [footpad thickness of carrageenan (mm)] − [footpad thickness of PBS (mm)].73

Statistical analysis

Herein, the obtained results can be conveniently analyzed by means of at least three independent experiments and are presented as the mean ± SEM. All data were analyzed using ANOVA or Mann–Whitney tests (GraphPad Prism 5.00), when appropriate, and the differences were considered statistically significant at p < 0.05.

Author contributions

F. A. L. and E. L. conceived the project; V. Y. S. and L. H. C. A. performed the synthesis, characterization and data analysis under the direction of F. A. L.; and M. S. L. carried out the optical analysis. N. M. Lima, E. G. Machado, P. E. Carvalho, S. B. R. Castro, C. C. Souza Alves and A. P. Carli carried out the anti-inflammatory response experiments for the as-prepared samples. All of the authors participated and contributed to the writing of the manuscript and discussion of the results.

Conflicts of interest

There are no conflicts to declare.


The authors gratefully acknowledge the support from the Brazillian agencies CNPq (203012/2017-8, 159387/2015-9, 460184/2014-8), CAPES, Fundação Araucária, FAPESP (2013/07296-2) and FAPEMIG (MPR 00262-16).

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