Optima 3300

A W-precursor solution was prepared by adding tungstic ac id (H₂WO₄, ≥99%, Sigma Aldrich, Overijse, Belgium) and citric acid hydrate (CA, ≥99%, Sigma Aldrich, Overijse, Belgium) to a round bottom flask, dispersed with a small amount of water. The H₂WO₄ to CA ratio was 1:4. This yellow suspension was stirred and heated at 120 °C for 2 h under reflux conditions. Subsequently, the pH was raised to pH > 12 with ammonia (NH₃, 32%, Merck, Overijse, Belgium) and left to stir for 24 h. After cooling, a transparent, grey colored solution was obtained. The final pH was 8, with a concentration of 0.35 mol·L⁻¹, as was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Optima 3300, PerkinElmer, Zaventem, Belgium). Before ultrasonic spray deposition, the precursor was diluted and mixed with ethanol (10:9 water/ethanol volume ratio) to yield a 25 mM concentration [20 ,21 (link)].

The samples were UV-sterilized for 30 min, followed by autoclaving for 20 min at 121 °C before immersion in PBS. The bare and coated specimens were incubated in 10 ml of PBS at 37 ± 0.5 °C. The solution was collected and changed with a fresh one at specified intervals, i.e., 1, 3, 7, 14, and 30 days. Then, 100 µL of HNO₃ was added to the collected media, and the resultant media were kept in the freezer at − 32 °C. All of the experimental procedures were carried out under sterile conditions.
The surface morphology and elemental constituents of the SBF- and PBS-immersed specimens were studied by a FESEM (MIRA3 TESCAN, Czech Republic) equipped with EDS. The phase composition of the PBS-soaked samples was determined by XRD Cu-Kα radiation (Bruker D8 Advance, Germany), working at voltage and current of 40 kV and 40 mA, respectively. The spectra were collected in the 2Ө range from 10 to 60° with a step size of 0.04°. The concentration of released Ni²⁺ and Ca²⁺ ions in the PBS solution upon soaking at specified intervals was measured via ICP-MS (Elan DRC-e, Perkin Elmer, Shelton, USA) and ICP-OES (OPTIMA 3300 DV Perkin Elmer, Shelton, USA), respectively.

Three brine solutions were tested in this study: magnesium chloride (MgCl₂), calcium chloride (CaCl₂) and a solution sampled from the Dead Sea in Israel. MgCl₂ and CaCl₂ salts were purchased from Carlo Erba Reagents, Italy. MgCl₂ and CaCl₂ solutions were freshly prepared in deionized water at concentrations of 30% w/v and 35% w/v, respectively, with similar concentrations reported previously [2 ,3 ]. Samples of Dead Sea solution were collected in 3 L containers a few days prior to the experiments (Figure 1). Samples were analyzed for Na, K, Ca, Mg and Sr by ICP-AES (Optima 3300, Perkin Elmer, Seer Green, UK) and Br by ICP-MS (NexION 300D, Perkin Elmer, Seer Green, UK). Cl concentrations were calculated by subtracting Br concentrations, analyzed by ICP-MS, from the total Br and Cl concentrations obtained using AgNO3 titration. Titrations were performed using an 848 Titrino plus (Metrohm, Herisau, Switzerland). Chemical analysis was performed at the Geological Survey of Israel. The chemical composition of the Dead Sea salt solutions is presented in Table 1.