Photodegradation of Ciprofloxacin-Zinc Complexes Produced at the Interface of ZnO and Cu-Doped ZnO Crystals

Ciprofloxacin hydrochloride (CIPRO) is considered an emerging pollutant in aquatic environments with the capacity to disseminate antibiotic resistance. Considering the pro-oxidant potential of ZnO and Cu-doped ZnO (Cu-ZnO) wurtzite crystals, the potential Ciprofloxacin photodegradation by these materials was investigated. CIPRO titration with ZnO and Cu-ZnO promoted the formation of zinc complexes and ~4% antibiotic adsorption. The carboxylic groups of CIPRO can complex Zn²⁺ by promoting the nanoetching of ZnO and Cu-ZnO crystallite surfaces. The alkaline interfaces provided by ZnO create a microenvironment favorable for Zn²⁺ chelation by CIPRO carboxylates. The photodegradation degree was similar for CIPRO and CIPRO-Zn under UV light, as revealed by UV-visible spectroscopy and FTIR. Therefore, the ZnO and Cu-ZnO crystals contributed to the formation of CIPRO-Zn rather than the photo-oxidative degradation of the antibiotic. Considering that CIPRO-Zn chelates disfavor bacterial selection for resistance, the treatment of CIPRO-contaminated effluents with ZnO and Cu-ZnO can facilitate desirable metal chelation without impairing photodegradation.

Keywords:
Ciprofloxacin; emergent pollutant; photodegradation; ZnO; Cu-ZnO nanoetching

Authorship

Aryane Tofanello

Universidade Federal do ABC, Centro de Ciências Naturais e Humanas, Av. dos Estados, 5001, 09210-580, Santo André, SP, Brasil.Universidade Federal do ABCBrasilSanto André, SP, BrasilUniversidade Federal do ABC, Centro de Ciências Naturais e Humanas, Av. dos Estados, 5001, 09210-580, Santo André, SP, Brasil.

Elisângela Belleti

Adrianne M. M. Brito

https://orcid.org/0000-0002-1187-6003

Iseli L. Nantes-Cardoso ^**e-mail: ilnantes@ufabc.edu.br

https://orcid.org/0000-0003-1434-3154

*e-mail: ilnantes@ufabc.edu.br

SCIMAGO INSTITUTIONS RANKINGS

Figures

Figures (8)

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Figure 1
Chemical structure of CIPRO molecule with its ionizable groups and indication of the respective pK_a values.

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Figure 2
Scanning electron microscope images of ZnO (A) and (B) and Cu-ZnO microcrystals (C) and (D).

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Figure 3
Structural characterization of ZnO and Cu-ZnO. A) X-ray diffraction of ZnO; B) XPS spectra of O1s, Zn2p of ZnO and C) XPS spectra of O1s, Zn2p of ZnO and Cu2p of Cu-ZnO.

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Figure 4
Changes in the CIPRO spectrum promoted by the interaction with ZnO and Cu-ZnO materials. A-B) and F-G) CIPRO photobleaching promoted by titration with ZnO and Cu-ZnO, respectively. The spectra obtained by titration with 0.012 to 0.06 mg/mL of ZnO and Cu-ZnO is shown in A and F, respectively, and B and G show the spectra obtained with 0.06 to 0.12 mg/ mL of ZnO and Cu-ZnO. C) and H) shows the corresponding differential spectra for ZnO and Cu-ZnO titration; D) and I) show the spectral changes at 260 and 285 nm and E) and J) the corresponding linearization by using logA_{260 nm}.

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Figure 5
Binding of CIPRO on ZnO crystal surface and complexation with Zn²⁺. A) Spectra of CIPRO titrated with ZnO before (black) and after (yellow) centrifugation at 14,000 rpm. B) Spectra of CIPRO before (black) after addition of 0.20 mg/mL of ZnCl₂ at pH 6.0 (yellow). C) Spectra of CIPRO aqueous solution (black) and in the supernatant of a colloidal suspension of ZnO after two centrifugations at 14,000 rpm (yellow).

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Figure 6
Photodegradation of CIPRO. A) Photobleaching of CIPRO-Zn complex obtained with ZnO and promoted by UV irradiation in two steps of 1 h as indicated by the arrow, B) Differential spectra obtained by subtracting CIPRO-Zn spectrum after 2 h of UV irradiation from the initial spectrum before irradiation (violet) and overlayed with the differential spectra of CIPRO-Zn minus CIPRO (blue). C) Photobleaching of CIPRO aqueous solution promoted by UV irradiation in two steps of 1 h as indicated by the arrow with the corresponding differential spectrum in the inset, D) Photobleaching of CIPRO-Zn complex obtained with Cu-ZnO and promoted by UV irradiation in two steps of 1 h as indicated by the arrow, E) Differential spectra obtained by subtracting CIPRO-Zn spectrum after 2 h of UV irradiation from the initial spectrum before irradiation (violet) and overlayed with the differential spectra of CIPRO-Zn minus CIPRO (blue).

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Figure 7
FTIR spectra of CIPRO exposed to UV irradiation under different conditions. A) FTIR spectra of CIPRO as powder (gray dotted line), paste (gray line), aqueous solution, and in aqueous solution after UV-irradiation (violet line); B) FTIR spectra of CIPRO aqueous solution (black line), as Zn complex before (gray line) and after 2 h of UV irradiation (violet line). The spectra panels are colored to a delimited spectral range associated to vibrations of the functional groups. In the molecular structure of CIPRO, the functional groups are indicated by the corresponding color.

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Figure 8
Formation of CIPRO-Zn complexes by nanoetching of ZnO surfaces. ZnO suspension promotes alkalinization of pure water to 7.9 that is above CIPRO pI (7.14). The unprotonated carboxylic group and the carbonyl group and pyridone carbonyl form a bidentate complex with Zn²⁺ that can coordinate with another CIPRO molecule forming a 2:1 CIPRO-Zn complex.

Figure 1 Chemical structure of CIPRO molecule with its ionizable groups and indication of the respective pK_a values.

Figure 2 Scanning electron microscope images of ZnO (A) and (B) and Cu-ZnO microcrystals (C) and (D).

Figure 3 Structural characterization of ZnO and Cu-ZnO. A) X-ray diffraction of ZnO; B) XPS spectra of O1s, Zn2p of ZnO and C) XPS spectra of O1s, Zn2p of ZnO and Cu2p of Cu-ZnO.

Figure 4 Changes in the CIPRO spectrum promoted by the interaction with ZnO and Cu-ZnO materials. A-B) and F-G) CIPRO photobleaching promoted by titration with ZnO and Cu-ZnO, respectively. The spectra obtained by titration with 0.012 to 0.06 mg/mL of ZnO and Cu-ZnO is shown in A and F, respectively, and B and G show the spectra obtained with 0.06 to 0.12 mg/ mL of ZnO and Cu-ZnO. C) and H) shows the corresponding differential spectra for ZnO and Cu-ZnO titration; D) and I) show the spectral changes at 260 and 285 nm and E) and J) the corresponding linearization by using logA_{260 nm}.

Figure 5 Binding of CIPRO on ZnO crystal surface and complexation with Zn²⁺. A) Spectra of CIPRO titrated with ZnO before (black) and after (yellow) centrifugation at 14,000 rpm. B) Spectra of CIPRO before (black) after addition of 0.20 mg/mL of ZnCl₂ at pH 6.0 (yellow). C) Spectra of CIPRO aqueous solution (black) and in the supernatant of a colloidal suspension of ZnO after two centrifugations at 14,000 rpm (yellow).

Figure 6 Photodegradation of CIPRO. A) Photobleaching of CIPRO-Zn complex obtained with ZnO and promoted by UV irradiation in two steps of 1 h as indicated by the arrow, B) Differential spectra obtained by subtracting CIPRO-Zn spectrum after 2 h of UV irradiation from the initial spectrum before irradiation (violet) and overlayed with the differential spectra of CIPRO-Zn minus CIPRO (blue). C) Photobleaching of CIPRO aqueous solution promoted by UV irradiation in two steps of 1 h as indicated by the arrow with the corresponding differential spectrum in the inset, D) Photobleaching of CIPRO-Zn complex obtained with Cu-ZnO and promoted by UV irradiation in two steps of 1 h as indicated by the arrow, E) Differential spectra obtained by subtracting CIPRO-Zn spectrum after 2 h of UV irradiation from the initial spectrum before irradiation (violet) and overlayed with the differential spectra of CIPRO-Zn minus CIPRO (blue).

Figure 7 FTIR spectra of CIPRO exposed to UV irradiation under different conditions. A) FTIR spectra of CIPRO as powder (gray dotted line), paste (gray line), aqueous solution, and in aqueous solution after UV-irradiation (violet line); B) FTIR spectra of CIPRO aqueous solution (black line), as Zn complex before (gray line) and after 2 h of UV irradiation (violet line). The spectra panels are colored to a delimited spectral range associated to vibrations of the functional groups. In the molecular structure of CIPRO, the functional groups are indicated by the corresponding color.

Figure 8 Formation of CIPRO-Zn complexes by nanoetching of ZnO surfaces. ZnO suspension promotes alkalinization of pure water to 7.9 that is above CIPRO pI (7.14). The unprotonated carboxylic group and the carbonyl group and pyridone carbonyl form a bidentate complex with Zn²⁺ that can coordinate with another CIPRO molecule forming a 2:1 CIPRO-Zn complex.

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Photodegradation of Ciprofloxacin-Zinc Complexes Produced at the Interface of ZnO and Cu-Doped ZnO Crystals

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[1] *e-mail: ilnantes@ufabc.edu.br