Tailoring the pH response range of fluorescent-based pH sensing phases by sol–gel surfactants co-immobilization

Abstract

New fluorescence optical sensing phases for pH measurements have been developed, based on the use of 2′,7′-dibromo-5′-(hydroxymercuri)fluorescein (mercurochrome) as the fluorescent pH indicator. Fluorescence emission of mercurochrome changes reversibly with the pH in a relatively wide range of pH. The pH sensing material has been prepared by trapping this fluorescent dye in a rigid inorganic matrix prepared following the sol–gel technology. The resulting sensing phase showed a strong pH dependence ( λ ex = 528 nm, λ em = 549 nm) over 6 pH units, with reversible fluorescent changes. The co-immobilization of different surfactants and mercurochrome in the sol–gel matrix has been evaluated for tuning the apparent p K a of the luminescent pH indicator and thus the sensing response. In this vein, a wide variety of surfactants of different nature (cationic, anionic, zwitterionic and non-ionic) were tested. Incorporation of surfactants induces large p K a shifts of the sensing phase depending on the nature of the co-immobilized surfactant. Using three of such mercurochrome pH sensing phases, each of them covering a different range of pH, pH-reversible sensing between pH 1–9, with response times in the order of about 1 min, could be accomplished. The long-term stability of the sensing materials was established over at least eight months, when stored in the absence of light. Additionally, transmission electron microscopy (TEM) analysis of the sol–gel sensing phases to investigate the effect of surfactants immobilization on the microscopic sol–gel structure has also been carried out. The applicability of the synthesized sensing phases has been tested for pH analysis in several mineral and tap water samples. The results obtained showed good agreement with the corresponding pH values provided by a commercial glass electrode.