Ζ potential evidences silanol heterogeneity induced by metal contaminants at the quartz surface: Implications in...

Among the physico-chemical features responsible for the so-called "variability of quartz hazard", a key role has been assigned to the silica surface charge, evaluated by means of ζ potential measurement. The ζ potential of silica describes the protonation state of silanols which, in turn, determine interactions with cell membranes. To gain a molecular understanding of the role of silanols in silica pathogenicity, we conducted a systematic investigation of the variation of the ζ potential as a function of pH (ζ plot titration curve) on a large set of respirable quartz particles with different levels of metal contaminants. The membranolytic activity of the particles on red blood cells, used as a readout of pathogenic activity, was assessed in parallel. Pure quartz surfaces showed sigmoid-shaped ζ plots suggesting the presence of silanol families with similar acidity, whereas contaminated dusts exhibited convex-shaped ζ plots, indicating a higher silanol heterogeneity on contaminated surfaces with respect to the pure ones. The quartz particles with a higher surface heterogeneity related to metal contamination showed a higher membranolytic activity. By removing structural defects and chemical heterogeneity, the ζ plot shifted towards the typical shape of pure quartz and the membranolytic activity was reduced. We conclude that the ζ plot is a useful readout to measure the acid-base behavior of quartz surfaces and to describe the chemical heterogeneity of quartz silanols. Surface heterogeneity, here induced by metal contamination, is proposed as the main cause of quartz membranolytic activity, further supporting the hypothesis that surface silanol disorganization determines silica pathogenicity.