This journal is © The Royal Society of Chemistry 2014 Chem. Commun. Cite this: DOI: 10.1039/c4cc03891f A fast and selective probe for monitoring Pd 2+ in aqueous medium via the dual-optical readout†‡ Anup Kumar, Megha Chhatwal, Alok Kumar Singh,* Vikram Singh and Manoj Trivedi A ready-to-use coordinative probe has been outlined for the detection of Pd 2+ at parts-per-million (ppm) levels which potentially meets real- world-challenges through a simple synthetic route, water based-activity, fast response, by-eye visualization, regenerative-action, high selectivity and the dual-optical readout for precise quantification. Palladium, a heavy metal, has found enormous applications in pharmaceuticals and commercial materials such as fuel cells, dental crowns, medical instruments, jewellery etc., owing to its inertness, biocompatibility and versatility as a catalyst. 1 However, as a consequence, the rapid increase of palladium concentrations in the natural resources and the risk of bio-accumulation have raised great concern among the scientific community due to its deleterious effect on human health. 2 Thus, the threshold level of Pd in drugs has been limited to 5–10 ppm and its maximum dietary intake is restricted to o1.5–15 mg per person. 3 This demands ready-to-use protocols for palladium recognition enabled by low cost, label-free detection, on-spot monitoring and easily miniaturizable characteristics. The traditional methods employed for Pd 2+ sensing include expensive and time-consuming analytical- techniques such as atomic absorption spectroscopy (AAS), HPLC coupled with solid-phase micro-extraction, inductively coupled plasma atomic emission (ICP-AES/OES) and X-ray fluorescence. 4 As attractive alternatives, many excellent colorimetric and fluor- escent probes for determination of Pd 2+ have been reported. 5 However, some of these probes face serious drawbacks: (a) poor water-solubility; (b) vigorous reaction conditions and (c) extended response time. In view of these limitations, we introduce a hitch- free Ru(II)–bipyridyl complex (1) rich in photo-physical properties, which produces measurable changes in its characteristic MLCT band in the presence of an external stimulus (Pd 2+ ). Probe 1 exhibits fast and selective detection of ppm-concentrations of Pd 2+ in water with real-time monitoring/regeneration. Notably, the distinct ‘‘turn-on’’ readout in the chromogenic mode allows the measurement of the low-concentration of the analyte relative to the ‘‘dark’’ background and the associative ‘‘turn-off’’ readout in the fluorogenic mode reduces the probability of a false positive signal. The probe, 1 (Scheme 1), was prepared in good yield (62%) by adopting a straightforward synthetic pathway 6 and characterized by full battery of physico-chemical techniques (Fig. S1–S4, ESI ‡). It crystallizes with the C2/ c space group and the monoclinic point group. The UV-vis spectrum of 1 in water displays a sharp peak at l = 283 nm (e = 80 174 M À1 cm À1 ) associated with a broad peak at l = 428 nm (e = 11 646 M À1 cm À1 ) (Fig. S2, ESI‡) that can be assigned to p–p* transition of the ligand-centred (LC) band and the metal-to-ligand charge transfer (MLCT) band, respectively. 6 Interestingly, the absorption spectrum of 1 exhibits remarkable perturbation upon addition of Pd 2+ in water coupled with a visible colour change from orange to dark red. The MLCT band at l = 428 nm shows moderate hypsochromic (Dl = 16 nm) and hyperchromic (1.5 fold) shifts (i.e., ‘‘turn-on’’ coupled with ‘‘turn-left’’ output response) upon addition of Pd 2+ (50 ppm in water) to 1 in water after 3 min of response time. Additionally, appearance of a relatively new band at l = 565 nm (‘‘turn on’’ 1.3 fold) allows for potential label-free detection (Fig. 1a). Also, a hypochromic shift (1.8 fold) is observed for the LC band centered at l = 283 nm (Fig. S5, ESI‡). Moreover three isosbestic points were observed at l = 364, 432 and 503 nm indicating the formation of Scheme 1 (a) Chemical structure of 1; ORTEP representation of (b) 1 and (c) 1-Pd 2+ (thermal ellipsoids are drawn at 30% probability level). H-atoms and counter anions are omitted for clarity. Department of Chemistry, University of Delhi, New Delhi-110 007, India. E-mail: aloksinghchemistry@gmail.com † A tribute to late Dr Tarkeshwar Gupta. ‡ Electronic supplementary information (ESI) available: Experimental details and X-ray analysis data. CCDC 991470 (1) and 991331 (1-Pd 2+ ). For ESI and crystallo- graphic data in CIF or other electronic format see DOI: 10.1039/c4cc03891f Received 21st May 2014, Accepted 12th June 2014 DOI: 10.1039/c4cc03891f www.rsc.org/chemcomm ChemComm COMMUNICATION Published on 12 June 2014. Downloaded by University of Delhi on 05/07/2014 06:06:25. View Article Online View Journal