Eu3+ doped ZnAl layered double hydroxides as calibrationless, fluorescent sensors for carbonate

The photoluminescence properties (PL) of Eu3+ hosted in the hydroxide layers of layered double hydroxides (LDHs) enables calibrationless quantification of anions in the interlayers. The concept is demonstrated during the nitrate-to-carbonate ion exchange in Zn2+/Al3+/Eu3+ LDHs and can be implemented as a remote optical sensor to detect intrusion of anions such as Cl− or CO32−.

For the synthesis, a ten-millilitres aqueous solution (solution A) containing Zn(NO3)2, Al(NO3)3 and Eu(NO3)3 at concentrations of 0.66, 0.314 and 0.016 mol.L -1 , respectively, was added dropwise (10 mL.h -1 ) into 200 mL of Milli-Q water (solution B).During the dosing, the pH of solution B was kept in the range 7.8-8.2by the addition of a 1 mol.L -1 NaOH solution by the automatic titrator Metrohm 785 DMP.After the end of the synthesis, the resulting slurry was aged statically for two days at 60 °C to optimize crystallization and subsequently centrifuged, rinsed with distilled water and dried in a drying oven at 60 °C for 3 days.The final solid was ground with mortar and pestle to produce a fine powder.For the synthesis of LDH-CO3 2-, the same procedure was repeated, but solution B comprised an aqueous solution of Na2CO3 0.5 mol.L -1 .

Anion exchange:
To exchange the nitrate originally intercalated in the LDHs, 300 mg of pristine LDH-NO3 were dispersed by continuous stirring in 5 mL of a Na2CO3 aqueous solutions at concentrations of 20, 40, 60, 80, 100 and 200 mmol.L -1 (samples LDH-A, A = 20, 40, 60, 80, 100 and 200).The suspensions were equilibrated for 24 h and then centrifuged and washed with deionized water up to a final dilution of more than 100 times.
After the final centrifugation step the product was dried at 60 °C for 3 days.
Electronic Supplementary Material (ESI) for Chemical Communications.This journal is © The Royal Society of Chemistry 2023 Characterization: Powder X-ray diffraction (PXRD) was performed in a Bruker D8 (Massachusetts, EUA) diffractometer (CuKα, 1.5418 Å).For Carbon and Nitrogen (CHN) elemental analysis, a Perkin Elmer (Massachusetts, EUA) 2400 series ii elemental analyzer was used.For Zn, Al and Eu quantification, Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) was performed with a Spectro Arcos (SPECTRO Analytical Instruments GmbH, Germany) optical spectrometer with radial view.The ex situ photoluminescence measurements were performed in a FS5 Spectrofluorometer (Edinburgh Instruments) spectrofluorometer with a 450 W Xenon lamp equipped with singular monochromators both at the excitation and the emission optics.All spectra shown were corrected for the intensity profile of the Xenon lamp and detection response.X-ray absorption spectroscopy (XAS) experiments were performed at the XAFS2 1 beamline of the Brazilian Synchrotron Light Laboratory (LNLS, Campinas, Brazil).X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were collected at the Eu L III -edge (6.9769 keV).A double-crystal Si(111) monochromator was used.The powder samples were pressed into pellets and the absorption spectra were measured in the fluorescence mode.
In situ photoluminescence measurements: For the in situ measurements of the luminescence properties of LDHs during anion exchange reactions, 300 mg of pristine LDH-NO3 were suspended in 5 mL of a Na2CO3 aqueous solutions at concentrations of 20, 100 and 200 mmol.L -1 .The LDHs used in the different suspensions originated from the same batch.To follow the luminescence properties of LDHs in situ during the anion exchange reaction, a setup comprising an Ocean Optics USB2000 portable fluorimeter equipped with an optical fibre to monitor the emission of the sample was assembled.A commercial 10 W UV LED lamp (Figure S2) was used as excitation source and was place on top of the suspension.EXFAS data analysis: For the EXAFS data analysis, an amplitude reduction factor S0 2 = 0.95 ± 0.11 has been obtained from ab-initio calculations 3 using the FEFF8.4code 4 for the model compound Eu2O3 (cubic phase 5 , coordination number = 6, see Ref. 6 ).Chemical transferability has been assumed and this amplitude factor was used in the analysis of the LDH samples.The Demeter platform 7 has been used to model and fit the EXAFS data.Crystallographic information from layered Europium hydroxides 8 , assumed to be crystal analogs for the local environment of Eu 3+ in the LDHs, were used as input for the calculations implemented along with IFEFFIT.The coordination of Eu 3+ in LDH-NO3 and LDH-CO3 as derived from the fitting of the Eu 3+ L III -edge EXAFS data (Figure S4) is summarized on Table S3.

Figure S4 .
Figure S4.Eu L III -edge EXAFS spectra (left) and phase-corrected Fourier transform (right) of LDH-NO3 and LDH-CO3.The data (circles), fits (solid lines) and residuals (faint lines) are shown with matching colors for each sample.
* The NO3 -stoichiometry in LDH-100 is lower than 0.002 and has not been shown.

Table S2 .
Luminescence decay lifetime of the (Eu 3+ ) 5 D0→ 7 F1 transition in the LDH samples after direct excitation of Eu 3+ at 394 nm.Spectrum of the UV lamp used as excitation source for the in situ PL measurements.
Figure S3.PXRD pattern of LDH-CO3 2-and comparison to a reference structure (CCDC 1402449) of a ZnAl LDH intercalated with carbonate as shown on Ref. 2 .
Detailed inspection of the Fourier transformed EXAFS spectra for each coordination shell observed in the spectra revealed no evidence of high-Z backscatterers in the near coordination environment of Eu 3+ , excluding the presence of Eu segregation in Eu-rich side phases in both samples.In LDH-NO3, Europium is surrounded by 7 Oxygen atoms at 2.42 Å, 1 Oxygen atom at 3.03 Å, 4 Oxygen atom at 3.57 Å and 6 Zn atoms at 4.00 Å.The 6-fold Zn coordination shell observed at 4.0 Å is in full accordance with Eu 3+ being built into the hydroxide layers of the LDH structure.A similar coordination structure is observed for Europium in LDH-CO3, Eu 3+ being surrounded by 7 Oxygen atoms at 2.32 Å, 1 Oxygen atom at 2.84 Å (this one shorter than that in LDH-NO3), 5(2) Oxygen atom at 3.63 Å and 6 Zn atoms at 3.93 Å.Also for LDH-CO3 the proximity to the Zn atoms in the hydroxide layers indisputably shows Eu 3+ to be replacing Al 3+ in the hydroxide layers.

Table S3 .
Fit parameters from the Eu L III -edge EXAFS data*.