Effect of synthesis methods, biocompatibility and photoluminescence of scheelite type sodium lanthanide double tungstates

The scheelite type sodium lanthanide double tungstates NaRE(WO₄)₂ (RE = Sm³⁺, Ho³⁺ and Pr³⁺) were synthesized by three different synthesis methods, namely solid-state reaction, hydrothermal method and solvothermal method using ethylene glycol. The synthesized samples were characterized by X-ray diffraction (XRD), scanning electron microscopy, UV-DRS and photoluminescence spectroscopy. The phase pure sample preparations with the Scheelite-like crystal structure in I4₁/a space group were established by XRD. Among the three synthesis methods, the broader XRD peaks were noticed for the solvothermal samples and resulting in crystallite sizes of ~ 11-28 nm. The SEM micrographs supported the XRD results and the agglomeration of particles with similar morphology was confirmed for all synthesis methods. The smallest particle size was obtained in the solvothermal method with average particle size distributions of ~ 100-140 nm. Indeed, the synthesis methods and particle size played a crucial role in the photoluminescence (PL) emission spectra. The PL emission intensity was very low for as-synthesized solvothermal samples where the particle size was considerably smaller compared to the other two methods. However, the calcination of solvothermal samples at different temperatures has improved their PL emission such that the emission intensity gradually increased with increasing the calcination temperature. Simultaneously, the SEM confirmed that the particle size remained the same till the calcination temperature of 600 °C. The microbial cell viability and cytotoxicity experiments were performed on the solvothermal samples using E. coli, S. aureus and HeLa cells, respectively. The biological studies demonstrated the biocompatibility of synthesized samples. The biocompatibility and smaller particle size in the solvothermal method could be useful in developing improved phosphors for cell imaging and theranostic applications.