Abstract
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
Highlights
In recent years, the research and development of optical chemical sensor technology has been increasing due to their low cost, low power consumption and good long-term stability [1]
We present the pH induced fluorescence lifetime (FL) response characteristic of two commercially available acridine derived fluorophores, 9-acridinemethanamine (9-AMA) and acridine-9carboxaldehyde (9-ACA), see Fig. 1, which we show are sensitive in the lower pH ranges from pH 2–6
The increase in FL shows that the fraction of protonated 9-AMA increases sharply with a decrease in pH from 5 to 2, indicating a pKa of around 3.5
Summary
The research and development of optical chemical sensor technology has been increasing due to their low cost, low power consumption and good long-term stability [1]. Such characteristics make optical sensors interesting for applications in oceanography [2], aquaculture [3] and biomedicine [4] amongst other applications. Optical pH sensor technologies in particular are attracting a lot of interest because of the widespread importance of pH
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