Fluorescent and colorimetric detection of pH by a rhodamine-based probe

Described here is a rhodamine-based derivative (1) that acts as a colorimetric and fluorescent off-on probe for the detection of pH. The design tactics for the probe is based on the change in structure between spirocyclic (non-fluorescent) and quinoid-type (fluorescent) forms of rhodamine dyes. The tetrahydrofuran/Tris(hydroxymethyl)aminomethane-HCl (4:6, v:v, pH 7.5) buffer solution of 1 is colorless and nonfluorescent, whereas ring-opening of the corresponding spirolactam to a conjugated quinoid form induced by H+ gives rise to strong absorption and fluorescence emission as well as a pink color. The fluorescence change of 1 is reversible and takes place mainly within the pH range from 1.1 to 4.0, which is valuable for the detection of strong acidic environment. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.