No abstract

Anthraimidazoledione-based charge transfer dyes have been designed for multimodal detection of a pathogenic biomarker, dipicolinic acid (DPA), at physiological pH. A change in visible color from yellow to red was observed along with an appearance of red luminescence when the probe-Eu3+ complex was exposed to DPA. Conversely, with the probe-Cu2+ complex, the solution color turns into orange in the presence of DPA with blue-colored fluorescence. Thus, the present sensory system can achieve naked-eye detection of DPA, which is very rare for metal complex-based probes. Mechanistic investigations revealed that variations in the metal ion center influence the nature of the DPA interaction, which subsequently dictates the output optical signal. DPA forms a ternary complex with the probe-Eu3+ conjugate, while in the case of Cu2+, it dissociates the preformed probe-metal ion conjugate. Interestingly, at a given concentration of DPA, the probe with the 2,2′-(phenylazanediyl)­diacetic acid group (1) at the receptor end shows a more prominent change with DPA as compared to the probe with the 2,2′-((2-(carboxymethoxy)­phenyl)­azanediyl)­diacetic acid (2) functional group. Subsequently, the system is involved in the screening of DPA in complex real-life samples, such as human blood serum, urine, natural water and soil samples, etc. In addition, the present assay can be employed for quantitative evaluation of Bacillus subtilis spores, and as low as 2.2 × 104 spores/mL was detected. Further, to extend the practical implication, low-cost paper-based devices are developed as an eco-friendly alternative for on-location detection of DPA.

Environmental pollution induced by toxic metal ions and harmful chemicals mainly from industrial waste poses a significant threat which urges for their rapid detection before release into the ecosystem above the permissible level. Optical sensors are inexpensive, simple, yet efficient in sensing such toxicants. Herein, we show that structurally simple π-conjugated pyridine-end p-phenylenevinylene oligomers can selectively sense toxic metal ions and anions in solution, supramolecular gels, as well as in solid support. Interaction of Hg2+ at nanomolar concentrations with the linear pyridine-ends via two-site coordination was clearly seen from “naked eye” color changes and fluorometric investigations. The sensitivity as well as selectivity of the oligomers toward Hg2+ was found to be greatly affected by the extent of aromatic conjugation and pK a of the end-pyridine functionalities. Interestingly, one of the oligomers (3) containing an isoniazid moiety renders visual color changes with both Hg2+ and CN– ions through two different binding sites involving two nonidentical sensing pathways which enable this probe for the detection of multiple analytes at the same time. Moreover, these ions (Hg2+/CN–) showed remarkable tuning of the supramolecular assembly (molecular gels) of 3, depicting reversible sol–gel transformation on complementary addition of Hg2+/CN– in proper stoichiometry which could be useful in scavenging toxic ions from industrial wastes. In addition, the low-cost, reusable paper discs coated with the probe molecules show rapid, onsite detection of toxic ions even from the contaminated water samples. Therefore, this highly efficient multimodal sensing of toxic ions by easy-to-synthesize molecular probes could inspire the design of new sensors with varying chromophores for the color-tunable sensing of toxic ions.

An easy to synthesize amphiphilic dye is developed whose sensing behavior in surfactant assemblies can be modulated through surface-charge, micropolarity, and local pH, etc. Thus, the micelle-bound probe shows remarkable ion-dependent bathochromic shifts in the charge-transfer band, enabling simultaneous ratiometric detection of four different metal ions, such as Cu2+, Ni2+, Hg2+, and Zn2+, at parts per billion (ppb) level in water. This is indeed a striking observation since naked-eye sensing of multiple metal ions at the mesoscopic interface is not known to date. Moreover, the probe even shows distinct color response against copper ion in different oxidation states, which is also unheard of. Further, the in situ formed metal complexes can be employed for naked-eye screening of three different amino acids, such as histidine, cysteine, and aspartic acid, in aqueous medium. Probes of this class, which are capable of multiplexing, offer new ways of efficiently screening multiple analytes in complex, real-life samples (e.g., wastewater management, analysis of pharmaceutical drugs, etc.). Further, low-cost reusable dye-coated paper discs were also developed as an ecofriendly method for on-site sensing of metal ions.

A two-component charge transfer (CT) hydrogel has been derived from supramolecular heteroassembly of a pyrene amino acid conjugate (PyHisOH, donor) with a 4-Chloro-7-nitrobenzofurazan (NBD-Ox, acceptor) derivative in the aqueous medium....