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.