Fluorescent and chromogenic organic probes to detect group 10 metal ions: design strategies and sensing applications

Group 10 metals including Ni, Pd and Pt have been extensively applied in various essential aspects of human social life, material science, industrial manufactures, medicines and biology. The ionic forms of these metals are involved in several biologically important processes due to their strong binding capability towards different biomolecules. However, the mishandling or overuse of such metals has been linked to serious contamination of our ecological system, more specifically in soil and water bodies with acute consequences. Therefore, the detection of group 10 metal ions in biological as well as environmental samples is of huge significance from the human health point of view. Related to this, considerable efforts are underway to develop adequately efficient and facile methods to achieve their selective detection. Optical sensing of metal ions has gained increasing attention of researchers, particularly in the environmental and biological settings. Innovatively designed optical probes (fluorescent or colorimetric) are usually comprised of three basic components: an explicitly tailored receptor unit, a signalling unit and a clearly defined reporter unit. This review deals with the recent progress in the design and fabrication of fluorescent or colorimetric organic sensors for the detection of group 10 metal ions (Ni(II), Pd(II) and Pt(II)), with attention to the general aspects for design of such sensors.

Simple Coumarin-Based Fluorescent Probe for Recognition of Pd(II) and Its Live Cell Imaging

A simple coumarin hydrazine Schiff base bearing a thioether recognition fragment (compound CBBS) has been rationally designed and easily prepared. CBBS exhibited an excellent selectivity for Pd(II) and a low detection limit of 65 nM (S/N = 3). The fluorescence emission intensities of CBBS at 495 nm were linear to Pd(II) concentrations in a wide range from 0 to 80 μM. Moreover, CBBS has been well used in fluorescence imaging of Pd(II) in living A549 cells. CBBS as a simple coordination-based fluorescent probe will inspire the researchers to develop a polymer for selective detection and adsorption of Pd(II).

A quinoline-based fluorescent chemosensor for palladium ion (Pd2+)-selective detection in aqueous solution

Quinoline-based fluorescent chemosensors have been extensively developed for various metal cations, but it was still rare for Pd²⁺-selective detection. In this work, a novel quinoline-benzimidazole conjugate containing one carboxylic acid group (QBM) was designed, and the QBM displayed highly selective fluorescence quenching response towards Pd²⁺ over other metal cations in aqueous solution. The fluorescence titration revealed a good linear relationship between the fluorescence intensity and the Pd²⁺ concentration in the range of 0.5-10 μmol L^-1, with the detection limit of 0.26 μmol L^-1 (S/N = 3). Fluorescence detection of Pd²⁺ in practical water sample was also successfully achieved.

Ratiometric fluorescent determination of palladium based on the C–N bond cleavage of allyl quaternary ammonium

Allylpyridinium conjugated phenanthroimidazole is encapsulated into CTAC micelles, which can ratiometrically detect Pd⁰ with a LOD of 11.5 nM.

A fluorescent probe based on novel fused four ring quinoxalinamine for palladium detection and bio-imaging

An “off–on” fluorescent probe was designed to detect palladium of all the typical oxidation states (0, +2, +4) without additional additives.

Sequential Detection of Palladium and Chromium Oxyanion by a Fluorescein Based Chemosensor in Mixed Aqueous Media

A new highly selective chemosensor, based on fluorescein-allyloxy benzene conjugate 1, was developed for the sequential detection of palladium and chromium oxyanions in a mixed aqueous media, and was studied by UV-visible and fluorescence spectroscopy. The sensing of palladium ions produces a chemodosimetric and ratiometric change in the emission band of 1 from 450 to 525 nm, followed by the sensing of chromate ions by 2 that quenches the emission band at 525 nm in a buffered H2O: DMF solution (9:1, pH = 7.4). The rate constants of palladium and chromate ions were found to be 8.6 × 105 M−1, 2.1 × 105 M−1, and 5.4 × 104 M−1 respectively. The chemosensor 1 has a palladium detection limit of 49 ppb while the sequential detection limit of chromate ions (CrO42− and Cr2O72−) were 127 and 259 ppb. The ratiometric change in the emission is produced due to the deallylation of 1 by palladium to produce 2 that restores the ESIPT (excited state intramolecular proton transfer) of the phenolic ring and enhances the electron transfer (ET) phenomenon from the phenolic group to fluorescein. The sequential binding of chromate ions to 2 inhibits the ESIPT and causes chelation enhanced quenching (CHEQ) of the fluorescence.

Iodine-Promoted Domino Oxidative Cyclization for the One-Pot Synthesis of Novel Fused Four-Ring Quinoxaline Fluorophores by sp3 C-H Functionalization

A method for the synthesis of novel fused four-ring quinoxaline skeleton has been described by an I₂ promoted sp³ C-H functionalization between 1,2,3,3-tetramethyl-3H-indolium iodides and 1,2-diamines. This transformation proceeds smoothly under metal- and peroxide-free conditions through a sequential iodination, oxidation, annulation and rearrangement. Moreover, 8,9-dichloro-5,12,12-trimethyl-2-(trifluoromethyl)-5,12-dihydroquinolino[2,3-b]quinoxaline showed good photophysical properties and was used in live cell imaging, indicating the potential value of this skeleton as a fluorophore in probes.

Simple fabrication of a carbaldehyde based fluorescent “turn-on” probe for the selective and sole detection of Pd2+: application as test strips

A simple fluorescent “turn-on” probe (DHMC) has been designed for selective and sole detection of Pd²⁺.