The use of surfactant-filled mesoporous silica as an immobilising medium for a fluorescence lifetime pH indicator, providing long-term calibration stability

Fluorescence intensity and fluorescence lifetime measurements of various carbon dots as a function of pH

Measurement and monitoring of pH are essential in both the industry and academia. It is therefore important to continue developing novel, low-cost pH sensors that provide increased accuracy over long periods of time. Particularly promising are sensors based on materials that show pH-dependent fluorescence intensity (FI) and lifetime (FL). Carbon dots (CDs) are emerging as promising candidates because of their low cost, ease of manufacturing, low toxicity, and negligible photobleaching. However, little has been done to quantify the FI and FL values of CDs. Here we report the characterisation of the pH-dependent FI and FL of four novel solvothermal synthesised CDs. The fifth CD is used as a reference sample and was synthesised following a published synthesis. The precursors for the CDs include disperse blue 1 dye, phloroglucinol, m-phenylenediamine (m-PD), N, and N-dimethylformamide (DMF). The average diameter size of the CDs ranges from 1.5 to 15 nm. An excitation wavelength of 452 nm with a bandwidth of 45 nm was used to quantify the fluorescence in the pH range 5-9. Three CDs show a decreasing trend in FI with pH, while two CDs show an increasing trend. None of the CDs shows strong FL dependence. The FL changes around 0.5 ± 0.2 ns across the tested pH range. We suggest that the differences in the fluorescence trends can be attributed to the precursors chosen for synthesising the CDs.

9-Acridinemethanamine and Acridine-9-Carboxaldehyde as Potential Fluorescence Lifetime pH Indicators

A significant challenge concerning the development of fluorescence lifetime (FL) based pH sensors is the paucity of fluorophores with sufficiently large FL variation with pH. Acridine is amongst the indicators with highest fluoresce lifetime responses to pH, with a change in lifetime of about 13 ns within a pH range of 5-8. Here we examine the two acridine derivatives, 9-acridinemethanamine (9-AMA) and acridine-9-carbaldehyde (9-ACA) in terms of their FL pH sensitivity and pH sensing range. Both indicators are characterized when dissolved in buffer solutions, as well as when immobilized in support materials. 9-AMA has a change in FL of 11 ns between pH 2-5, both when dissolved in solution and when immobilized in surfactant-filled mesoporous silica. The FL of 9-ACA is not sensitive to pH when dissolved in buffer solutions; however, when covalently bound to amine-modified silica, its FL changes 15 ns between pH 3-6. 9-AMA and 9-ACA represent promising FL in the pH range of pH 2-6, and could potentially form the basis of new FL pH sensors. Graphical Abstract.