Sol-, Gel-, and Paper-Based Detection of Picric Acid at Femtogram Level by a Short Peptide Gelator

Owing to the worldwide threats from the terrorist activities, easy and convenient sensing of explosives is extremely important. Picric acid (PA) is a well-known explosive that is commonly used in military operations. Such uses pollute the environment, resulting in a menace to human health. Development of a sensor for PA at a femtogram scale detection is a challenging task. In this work, we demonstrate a small peptide-based gelator capable of sensing PA selectively in solution-, gel-, and a gel-coated paper-based system. In solution, detection of PA is achieved through quenching of the monomeric emission of the gelator, while in the gel- or paper-based system, the sensing mechanism relies entirely on the decrease in excimer emission in the presence of PA. In the solution and gel state, the detection limits were found to be 115.24 ppt and 22.91 ppb, respectively. The gel, when coated on paper strips, could detect PA at femtogram scale (11.45 × 10–15 g cm–2) with a detection limit of 0.229 ppt. Notably, in spite of a very low vapor pressure of PA, the newly developed paper strip is able to sense PA vapor with a detection limit between ppt and ppb. DFT calculation revealed that energetically favorable complex formation between the pyrene of the gelator and PA is behind the quenching. The self-assembly of the peptide and its photophysical properties were used to create this simple, convenient, and economic method of PA detection on surfaces in a contact mode, and the paper-strip-based method holds promise for efficient practical uses.