2.5.1. Silicon analysis with ICP-OES

Inductively Coupled Plasma Optical Emission spectroscopy (ICP-OES) measurements were performed using a Thermo Scientific iCap 6000. The silicon (Si) content was determined from the spectral intensities for Si at the wavelengths 2124, 2516 and 2881 Å. The Si concentration at a given time point represents the average of three repeat measurements performed on three independent repeat sample solutions. The percentage of Si dissolution is calculated by dividing the measured Si concentration (in mg/L or ppm) by the initial theoretical mass of Si. The initial theoretical mass of Si is calculated from the weighed mass of the particles added to the solution multiplied by the ratio of the molecular weights of Si (28.08 g/mol) and silica (SiO₂; 80.08 g/mol). The time at which 50% Si dissolution occurs, denoted as the T_50%, was interpolated from the linear region of the plot of the percentage of Si versus time graph by interpolating the curve in the linear lower time point region (0–8 h). It is acknowledged that the calculation of Si dissolved assumes all the sample mass consists of SiO₂ and neglects any weight from surface groups, such as silanols. It is known from the literature that the degree of condensation does not have to be dramatic to result in 10–20% differences in the effective Si-loadings in the experiments (Braun et al., 2016). However, the production process of the MSP used in this study is designed to completely re-hydroxylate silica surface, as demonstrated in our previous works (Valetti et al., 2017; Angiolini et al., 2018). It is important to note that the Si dissolution of unloaded MSP were performed on filtered samples using 25 mm, 0.2 µm polyethersulfone (PES), whilst for LYS-MSP-w it was necessary to centrifuge the samples as described above to avoid obstruction of the filter with the sample and lose of measurable Si. The analysis was performed by triplicate.