Sensitive Naked Eye Detection of Hydrogen Sulfide and Nitric Oxide by Aza-BODIPY Dyes in Aqueous Medium Nagappanpillai Adarsh, † Megha S. Krishnan, † and Danaboyina Ramaiah* ,†,‡ † Photosciences and Photonics, Chemical Sciences and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Trivandrum-695 019, Kerala, India ‡ CSIR-North East Institute of Science and Technology (CSIR-NEIST), Jorhat-785 006, Assam, India * S Supporting Information ABSTRACT: With an objective to develop optical probes for biologically important anions and neutral molecules, we synthesized three novel NIR absorbing aza-BODIPY derivatives, 3a-3c, and have systematically tuned their photophysical properties by changing the peripheral substitution. A profound red-shift was observed in the absorption and fluorescence spectra of the aza-BODIPY dyes with the change in substitution from azido (3a) to amino (3b) to dimethylamino (3c) groups. Theoretical calculations of 3a- 3c showed a consistent decrease in bandgap, which supports the observed spectral changes. The study of their interactions with various analytes revealed that the azido-aza-BODIPY 3a selectively interacts with hydrogen sulfide (H 2 S) when compared to other molecules. Uniquely, the detection of H 2 S can be visualized through a change in color from bright blue to purple with a detection limit of 0.5 ppm. The sensitivity of the probe was observed to be ∼20-fold higher than the allowed exposure limits of H 2 S as defined by EPA (10 ppm). The aza-BODIPY derivative 3b, on the other hand, exhibited selective interactions with nitrite ions (NO 2 - ) and nitric oxide (NO) in aqueous medium through a visible color change from blue to green with a sensitivity of 20 and 0.15 ppb, respectively. In contrast, the dimethylamino-aza-BODIPY derivative, 3c, showed negligible affinity for the anions and neutral molecules tested. By tuning the photophysical properties through the judicious functionalization, the aza-BODIPY dyes thus synthesized can be utilized for the sensitive on-site detection and analysis of H 2 S, NO 2 - , and NO in the aqueous medium. I n recent years, there has been increased interest in the synthesis and tuning of BODIPY dyes due to their potential as optical and electronic materials. 1-5 The photophysical properties such as absorption and fluorescence properties of these dyes can be modulated through extending conjugation, or the use of fused pyrrole rings, or by substituting with electron withdrawing or donating groups in the core and peripheral positions. 6-12 Alternatively, O’Shea, Akkaya, and our group have achieved the synthesis of 8-aza-boradiazaindacenes (aza- BODIPYs), 13-20 which exhibit around 100 nm bathochromic shifted absorption in the region 600-800 nm, when compared to BODIPYs. 21-25 These aza-BODIPY dyes which exhibit strong absorption in the NIR region, and high triplet yields offer a tremendous potential as PDT agents. The potential of aza-BODIPY dyes as the probe for metal cations, anions, and pH are recently reported in the literature. 16,19 However, the recognition of neutral molecules by the aza-BODIPY dyes is yet to be exploited. Among the neutral species, hydrogen sulfide (H 2 S) and nitric oxide (NO) are recognized as important gasotransmitters in many physiological events and the development of probes for their detection has gained much attraction. 26-33 The higher levels of H 2 S can induce shock, convulsions, and coma. H 2 S exposure prevents cellular respiration due to its complexation with cytochromes, which ultimately leads to death. 34 Depend- ing upon the physiological system, it has been shown that high levels of NO can either be toxic or promote cell survival. In biological systems, NO is considerably less stable (half-life <30 s) and is rapidly oxidized to nitrate and nitrite. Because of its short half-life, direct measurement of NO is difficult; therefore, monitoring of nitrate and nitrite has been employed as a useful indicator of NO production. 35-39 Because of the high reactivity and adverse effects of these species, a rapid detection strategy is very essential. The most common techniques used for the detection of hydrogen sulfide and nitric oxide include electrochemical, gas chromatography, fluorescence, and colori- metric methods. 40-48 Of these techniques, the colorimetric methods are quite simple and convenient for the on-site visual analysis of these biologically important analytes. Herein, we report the synthesis of three novel aza-BODIPY derivatives, 3a-3c having azido, amino, and dimethylamino functional groups at the peripheral phenyl rings and investigation of their photophysical properties and interactions with various analytes. Uniquely, of these three derivatives, the Received: July 30, 2014 Accepted: August 25, 2014 Article pubs.acs.org/ac © XXXX American Chemical Society A dx.doi.org/10.1021/ac502849d | Anal. Chem. XXXX, XXX, XXX-XXX