Rapid naked-eye colorimetric detection of gaseous alkaline analytes using rhodamine B hydrazone-coated silica strips

A novel chemosensor based on rhodamine B hydrazone was developed. The chemical structure of the new rhodamine B hydrazone was verified using different spectroscopic techniques. The surface morphology of the rhodamine B hydrazone was determined by scanning electron microscope (SEM), which revealed self-assembly in the form of layered aggregations. A thin layer chromatography (TLC) test strip impregnated with rhodamine B hydrazone was fabricated to function as a qualitative optical tool for the rapid detection of alkaline vapors (1 min), including ammonia, triethylamine, piperidine, pyridine, and hydrazine monohydrate. The colorimetric sensing of alkaline vapors was achieved through color changes of the rhodamine B hydrazone-coated silica strips from pink to colorless or faded pink upon exposure to the vapors of the alkaline analytes at 0.1 µl. The colorimetric changes can be tracked by the naked-eye or by using a smartphone camera. Reversible ring-opening/ring-closure of the spirolactam rings of the probe was confirmed by UV/Vis and 1H NMR analyses. The colorimetric detection of alkaline vapors occurred through deprotonation of the ring-opened amide form of the bis-hydrazone material supported on the TLC silica strip. The proposed detection method offers several advantages, including facile fabrication, simple operation without the need for a special device or skilled operators, easy handling, and real-time sensing of alkaline vapors.