# Enhancement of the Luminescence Properties of Eu (III) Containing Paramagnetic Saponite Clays
## 2 Materials and Methods
### Analytical Methods  
The cationic exchange capacity (C.E.C.) parameter of Na-GdEuSAP clay was determined by the Ultraviolet-Visible (UV-Vis) method reported here: 0.300 g of clay were exchanged with 10 mL of a 0.02 M solution of hexamminecobalt(III) chloride ([Co (NH\({}_{3}\))\({}_{6}\)]\({}^{3+}\)at RT for 60 h. After separation by centrifugation (5000 rpm for 5 min, two times), the solution was analyzed by UV-Vis spectroscopy. UV-Visible spectra were recorded at RT in the range 300-600 nm with a resolution of 1 nm, using a double-beam Perkin Elmer Lambda 900 Spectrophotometer (Perkin Elmer, Waltham, MA, USA). The absorbance of the band at 475 nm (\({}^{1}\)A\({}_{1g}\) \(\rightarrow\)\({}^{1}\)T\({}_{1g}\)), relative to a \(d\)-\(d\) spin-allowed Laporte-forbidden transition of Co\({}^{3+}\), was evaluated to quantify the amount of free Co\({}^{3+}\) ions in solution, thereby determining the amount exchanged in the procedure and thus the C.E.C. of the saponite sample.  
The elemental analyses were performed on a Thermo Fisher Scientific X5 Series Inductively Coupled Plasma Mass Spectrometer (ICP-MS; Waltham, MA, USA). Prior to the analyses, the solids were mineralized by treatment with a mixture of nitric acid (HNO\({}_{3}\), 5 mL) and hydrofluoric acid (HF, 5 mL) at 100 \({}^{\circ}\)C for 8 h. The samples were opportunely diluted in nitric acid aqueous solution (1 wt%) before the analysis.  
X-ray powder (XRPD) diffractograms were collected on unoriented ground powders with a ThermoARL XTRA-048 Powder Diffractometer (Thermo Fisher Scientific, Waltham, MA, USA) with a Cu-K\({}_{\alpha 1}\) (\(\lambda\) = 1.54062 A) monochromatic radiation. Diffractograms were recorded at room temperature (RT) in the 2\({}^{\circ}\)-65\({}^{\circ}\)\(2\theta\) range with a step size of 0.02\({}^{\circ}\) and a rate of 1.0\({}^{\circ}\)/min. The X-ray profiles at low angles (2-15\({}^{\circ}\)\(2\theta\)) were collected with narrower slits and rate of 0.25\({}^{\circ}\)/min.  
Thermogravimetric analyses (TGA) were carried out with a Setaram SETSYS Evolution Thermobalance. The data were collected in the range of 50 to 900 \({}^{\circ}\)C, with a scan rate of 10 \({}^{\circ}\)C/min and under oxygen (O\({}_{2}\)) flow (100 mL/min).  
Photoexcitation and photoluminescence (PL) emission spectra were recorded on a Horiba Jobin-Yvon Model IBH FL-322 Fluorolog 3 Spectrometer equipped with a 450 W xenon arc lamp, double grating excitation and emission monochromators (2.1 nm-mm\({}^{-1}\) dispersion; 1200 grooves per mm) and a Hamamatsu Model R928 photomultiplier tube. Time-resolved measurements were performed by using the time-correlated single-photon counting (TCSPC) option. A 295 nm nanoLED laser was used to excite the samples. Signals were collected using an IBH DataStation Hub photon counting module. Data analysis was performed using the commercially available DAS6 software (HORIBA Jobin Yvon IBH).  
The water proton longitudinal relaxation rates (\(R_{1}\)) were measured by using a variable field relaxometer equipped with an HTS-110 3T Metrology Cryogen-free Superconducting Magnet (Mede, Italy), operating in the overall range of proton Larmor frequencies of 20-120 MHz (0.47-3.00 T). The measurements were performed using the standard inversion recovery sequence (20 experiments, 2 scans) with a typical 90\({}^{\circ}\) pulse width of 3.5 us and the reproducibility of the data was within \(\pm\)0.5%. The temperature was controlled with a Stelar VTC-91 heater airflow equipped with a copper-constant thermocouple (uncertainty of \(\pm\)0.1 \({}^{\circ}\)C). Additional points in the 0.01-10 MHz frequency range were collected on a Fast-Field Cycling (FFC) Stelar SmarTracer Relaxometer. Na-GdEuSAP, Phen-GdEuSAP_1 and Phen-GdEuSAP_2 solids (10 mg) were dispersed in 1.5 mL of ultrapure water, in the presence of xanthan gum (0.1 wt.%) to improve particle dispersion. Before measurements, the suspensions were sonicated for 30 min. The pH of the suspensions was _ca. 7.0_.