New natural organic–inorganic pH indicators: Synthesis and characterization of pro-ecological hybrid pigments based on anthraquinone dyes and mineral supports

Abstract

We present a strategy that leads to the formation of high-performance hybrid pigments made from natural anthraquinone chromophores (alizarin, purpurin) and mineral supports (palygorskite (PAL), sepiolite (SEP), and hydroxyapatite (HAP)). Thanks to the mineral filler form features and the synergy between the organic and inorganic components, the as-prepared hybrid pigments showed excellent thermal and chemical stability. The natural pigments exhibited strong pH-sensing activity, changing their color upon exposure to HCl and/or NH3 vapors. The PAL-based pigments were found to be the most effective pH indicators, showing pronounced color changes after 10 min of exposure to HCl (ΔE for PAL/A was 7.4). ToF-SIMS, 13C NMR and XPS techniques revealed interactions between the organic dyes and metal ions present in the minerals. XPS spectroscopy was also used to examine the alterations in the structure of the hybrid colorants following exposure to acid vapors. The proposed pH sensors were successfully applied to elastomer and thermoplastic elastomer matrices. The purpurin-based hybrid pigments exhibited significantly reduced migration from the polymer (about six times lower concentration in ethanol after 14 days). The results of this study pave a new way for the design, development, and application of advanced hybrid colorants with pH-sensing activity and improved stability.