Highly selective Zn2+ near-infrared fluorescent probe and its application in biological imaging

Zn2+ plays a vital role in regulating various life processes, such as gene expression, cell signaling, and brain function. In this study, a near-infrared fluorescent probe AXS was synthesized to detect Zn2+ with good fluorescence specificity, high selectivity, and high sensitivity; the detection limit of Zn2+ was 6.924 × 10-11 M. The mechanism of Zn2+ recognition by the AXS probe was investigated by 1H nuclear magnetic resonance titrations, UV-visible spectroscopy, fluorescence spectroscopy, Fourier-transform infrared spectroscopy, and high-resolution mass spectrometry. Test paper experiments showed that the AXS probe could detect Zn2+ in real samples. In addition, quantitative and qualitative detection of Zn2+ in common foodstuffs was achieved. For portable Zn2+ detection, a smartphone detection platform was also developed based on the AXS probe. Importantly, the AXS probe showed good bioimaging capabilities in Caenorhabditis elegans and mice.

Unusual Reactivity and Metal Affinity of Water-Soluble Dipyrrins

Dipyrrins are a versatile class of organic ligands capable of fluorogenic complexation of metal ions. The primary goal of our study was to evaluate dipyrrins functionalized with ester and amide groups in 2,2'-positions in sensing applications. While developing the synthesis, we found that 3,3',4,4'-tetraalkyldipyrrins 2,2'-diesters as well as 2,2'-diamides can undergo facile addition of water at the meso-bridge, transforming into colorless meso-hydroxydipyrromethanes. Spectroscopic and computational investigation revealed that this transformation proceeds via dipyrrin cations, which exist in equilibrium with the hydroxydipyrromethanes. While trace amounts of acid favor conversion of dipyrrins to hydroxydipyrromethanes, excess acid shifts the equilibrium toward the cations. Similarly, the presence of Zn²⁺ facilitates elimination of water from hydroxydipyrromethanes with chromogenic regeneration of the dipyrrin system. In organic solutions in the presence of Zn²⁺, dipyrrin-2,2'-diesters exist as mixtures of mono-(LZnX) and bis-(L₂Zn) complexes. In L₂Zn, the dipyrrin ligands are oriented in a nonorthogonal fashion, causing strong exciton coupling. In aqueous solutions, dipyrrins bind Zn²⁺ in a 1:1 stoichiometry, forming mono-dipyrrinates (LZnX). Unexpectedly, dipyrrins with more electron-rich 2,2'-carboxamide groups revealed ∼20-fold lower affinity for Zn²⁺ than the corresponding 2,2'-diesters. Density Functional Theory (DFT) calculations with explicit inclusion of water reproduced the observed trends and allowed us to trace the low affinity of the dipyrrin-diamides to the stabilization of the corresponding free bases via hydrogen bonding with water molecules. Overall, our results reveal unusual trends in the reactivity of dipyrrins and provide clues for the design of dipyrrin-based sensors for biological applications.

A New Sensitive and Selective Off-On Fluorescent Zn2+ Chemosensor Based on 3,3',5,5'-Tetraphenylsubstituted Dipyrromethene

3,3',5,5'-Tetraphenyl-2,2'-dipyrromethene was described as a highly sensitive and selective Off-on fluorescent colorimetric chemosensor for Zn²⁺ based on the chelation-enhanced fluorescence (CHEF) effect. The reaction of dipyrromethene ligand with Zn²⁺ induces the formation of the [ZnL₂] complex, which exhibits the increasing fluorescence in 120 fold compared with ligand in the propanol-1/cyclohexane (1:30) binary mixture. The Zn²⁺ detection limit was 1.4 × 10^-7 М. The UV-Vis and fluorescence spectroscopic studies demonstrated that the dipyrromethene sensor was highly selective toward Zn²⁺ cations over other metal ions (Na⁺, Mg²⁺, Co²⁺, Ni²⁺, Fe³⁺, Cu²⁺, Mn²⁺, Cd²⁺ and Pb²⁺), excluding Hg²⁺.

Development of Ion Chemosensors Based on Porphyrin Analogues

Sensing of metal ions and anions is of great importance because of their widespread distribution in environmental systems and biological processes. Colorimetric and fluorescent chemosensors based on organic molecular species have been demonstrated to be effective for the detection of various ions and possess the significant advantages of low cost, high sensitivity, and convenient implementation. Of the available classes of organic molecules, porphyrin analogues possess inherently many advantageous features, making them suitable for the design of ion chemosensors, with the targeted sensing behavior achieved and easily modulated based on their following characteristics: (1) NH moieties properly disposed for binding of anions through cooperative hydrogen-bonding interactions; (2) multiple pyrrolic N atoms or other heteroatoms for selectively chelating metal ions; (3) variability of macrocycle size and peripheral substitution for modulation of ion selectivity and sensitivity; and (4) tunable near-infrared emission and good biocompatibility. In this Review, design strategies, sensing mechanisms, and sensing performance of ion chemosensors based on porphyrin analogues are described by use of extensive examples. Ion chemosensors based on normal porphyrins and linear oligopyrroles are also briefly described. This Review provides valuable information for researchers of related areas and thus may inspire the development of more practical and effective approaches for designing high-performance ion chemosensors based on porphyrin analogues and other relevant compounds.

Two Schiff base ligands for distinguishing ZnII/CdII sensing—effect of substituent on fluorescent sensing

Two Schiff base ligands (HL1, HL2) were synthesized as fluorescent sensors for Zn²⁺ or Cd²⁺. With an ethoxyl group, HL1 can distinguishingly sense Zn²⁺ and Cd²⁺, while HL2 exclusively responds to Zn²⁺.

Fluorescent and colorimetric ion probes based on conjugated oligopyrroles

In this tutorial review, the design strategies, syntheses, and ion sensing properties of conjugated oligopyrroles are described.

Regioselective dihalohydration reactions of propargylic alcohols: gold-catalyzed and noncatalyzed reactions

The regioselective conversion of propargylic alcohols into previously unreported α,α-diiodo-β-hydroxyketones was achieved by treatment with N-iodosuccinimide in the presence of a gold catalyst. The corresponding α,α-dichloro-β-hydroxyketones were obtained by treatment with trichloroisocyanuric acid in the absence of a catalyst. The latter reaction can be extended to other alkynols. These transformations can be used to prepare potentially useful halogenated building blocks. Preliminary mechanistic studies suggest that the reaction involves participation of the acetonitrile solvent in the formation of a 5-halo-1,3-oxazine intermediate.

α-Monoacylated and α,α'- and α,β'-diacylated dipyrrins as highly sensitive fluorescence "turn-on" Zn2+ probes

With the purpose of developing readily synthesized CHEF (chelation-enhanced fluorescence) type Zn(2+) probes with relatively simple molecular structures and excellent sensing behavior, p-anisoyl chloride was used for the acylation of 5-(pentafluorophenyl)dipyrromethane. Interestingly, the α,β'-diacylated product PS2 with a unique substitution mode was obtained in high yield in addition to the normal α-substituted mono- and diacylated products PS1 and PS3. Further oxidation of PS1-PS3 afforded dipyrrins S1-S3. Crystal structure and (1)H NMR measurements of S2 demonstrate the existence of a pure tautomer, which is consistent with DFT calculations. S1-S3 show highly Zn(2+) selective "turn-on" fluorescence based on a CHEF mechanism by the formation of 2:1 (probe:metal) Zn(2+) complexes. The emission colors can be easily tuned from green to red by changing the dipyrrin substitution modes. Furthermore, these probes demonstrate fast responses and wide applicable pH ranges. Among them, S2 shows the highest Zn(2+) sensitivity, with a detection limit of 4.4 × 10(-8) M.