Palladium is used as a pivotal material in various industries that include organic synthesis processes, the manufacture of multilayer ceramic capacitors, fuel cells, and automobile catalytic converters. 1 In particular, palladium-catalyzed reactions are powerful synthetic tools for carbon-carbon bond formation, and these organic reactions have been widely applied to the production of medicinal substances. 2 However, residual palladium species in final products may remain even after rigorous

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... fication, which raises a great concern because palladium species may tightly bind to various materials which can lead to disruption of biological processes. 3 Common tools for the detection of palladium species are atomic absorption/emission spectrophotometry, ion-coupled plasma emission-mass spectrometry, and X-ray fluorescence spectroscopy. 4 Although these are capable tools for the extremely sensitive analysis of palladium species in various matrixes, these methods require laborious and complicated sample preparation steps and expensive instruments. Thus, many sensing probes have been developed that offer operational simplicity, low cost, and high sensitivity which leads to longer reaction times from the added chemical transformation. 5 Most sensing probes for Pd 2+ are chemodosimeters that are based on palladium-induced reactions. 6 Chemodosimeters are used to detect an analyte through significant chemical transformations involving both irreversible breaking and formation of covalent bonds induced by analytes. 7 These chemical transformations lead to an observable signal that has an accumulative effect, which is directly related to the concentration of the analyte. Also, well designed chemical transformations of these chemodosimeters provide highly selective and sensitive detection of Pd 2+ . 6,7 Although these chemodosimeters can detect Pd 2+ with high sensitivity and selectivity over various metal ions, many chemodosimeters require the initial conversion of Pd 2+ to Pd 0 using a reducing agent, which lengthens the reaction time because it utilizes a further chemical transformation, or they exhibit low selectivity over Pd 0 . These problems can be addressed by selective complexation between a probe and Pd 2+ , but such examples are few. 8 Therefore, it is desirable to develop a probe that can immediately detect Pd 2+ without needing a sample pretreatment procedure. Pd 2+ can promote a moderately regioselective cleavage of cytochrome c, myoglobin, three albumins, and several other proteins by anchoring to methionine residues in the peptide and hydrolyzing proximal peptide bonds. 9 These results imply that the 2-(methylthio)ethyl amide group would be a good ligand for palladium. With this information, we expected that fluorogenic dyes with a 2-(methylthio)ethyl amide moiety could be used as a chemosensor for Pd 2+ . These molecules were synthesized by the coupling of pyrenecarboxylic acid and 2-(methylthio)ethylamine as shown in Scheme 1. Initially, the fluorescence emission spectra of N-(2-(methylthio)ethyl)pyrene-1-carboxamide (1) and N-(2-(methylthio)ethyl)-2-(pyren-1-yl)acetamide (2) were measured in the presence of various concentrations of Pd 2+ . The addition of Pd 2+ induced a decrease in the fluorescence of 1 and 2, and as shown in Figure 1 , the observed fluorescence intensities were nearly inversely proportional to the Pd 2+ concentrations. The fluorescence change of 1 was more significant than for 2 because Pd 2+ acts as a quencher and the quenching efficiency is inversely proportional to the distance between the fluorescent moiety and the quencher. 10 From these titration results, the detection limits of 1 and 2 for Pd 2+ were estimated to be 1.3 µM and 1.6 µM, respectively (see Supplementary information). Although both probes are turn-off type chemosensors and such decreased emission is less desirable for analytical purposes, the probes were able to detect only Pd 2+ without the need of a reducing agent, and they did not require long reaction times unlike previously reported chemodosimeter type probes for Pd 2+ . The fluorescence change of 1 in the presence of various metal ions was measured to evaluate the selectivity of 1 for these metal ions. Fluorescence spectra of solutions of 1 (2 µM) were recorded after the addition of 2.5 equivalents of each metal ion, spectra shown in Figure 2 , because the a These authors contributed equally to this work. Scheme 1. Synthetic route to probes 1 and 2.