Highly Sensitive Detection of Ammonia Based on Protonated Cytosine-containing Colorimetric and Fluorometric Polydiacetylene

Abstract

Owing to color change (typically blue-to-red) in response to chemical and physical stimuli, polydiacetylenes (PDAs) have been extensively used in colorimetric sensors. Among target molecules, gaseous ammonia has received a special attention due to the disease (kidney) and food spoilage-related properties. Conventional PDA-based ammonia sensors have been based on the acid-base reactions and thus, carboxylic acid-containing PDAs have been almost exclusively utilized. The carboxylic acid-based strategies, however, have proven to be ineffective for the detection of ammonia in high sensitivity since the sensor systems have displayed approximately 300-1000 ppm level of detection limit. In this study, we have developed a new PDA-based colorimetric ammonia sensor. A cytosine-containing diacetylene (DA), PDC, was found to form a hemiprotonated structure in the presence of acid. UV irradiation (254 nm) to the solid form of the hemiprotonated DA afforded a blue-colored PDA. Very interestingly, the cytosine-derived PDA displayed a blue-to-red color change as well as fluorescence turn-on feature upon exposure to ammonia with detection limit of ca. 200 ppm. It is believed that the abstraction of proton by ammonia causes partial distortion of the conjugated ene-yne backbone of the PDA, resulting in the decrease of effective conjugation length of the polymer. More interestingly, addition of sulfuric acid to the cytosine-containing diacetylene PDC afforded a more sensitive ammonia sensor when the protonated PDC was transformed to PDA. Accordingly, ca 10 ppm level of ammonia was detected by using this sensor system. The increased sensitivity, compare to the hemiprotonated form, is believed to be the more facile abstraction of the proton. The strategy developed in this thesis study should open a new avenue for the design of PDA-based ammonia sensors.