Quinoline based chromogenic and fluorescence chemosensor for pH: Effect of isomer

Active Hydrogen Free, Z-Isomer Selective Isatin Derived "Turn on" Fluorescent Dual Anions Sensor

An efficient and anions fluorescence "on-off" sensor of 1-(prop-2-yn-1-yl)-3-(quinolin-3-ylimino)indolin-2-one (PQI) has been developed for the selective sensing of dual anions of F- and NO3- ions in aqueous medium. Active hydrogen and Lewis acidic binding sites free, Z- isomer of isatin based π-conjugated quinoline exhibited excellent sensing activity against F- and NO3- ions in UV light. The fluorescence turns on the process accomplished via the PET "on-off" mechanism. The interaction between probe molecule and anions is thought to be a non-covalent interaction of the low electron density covalently bonded N-methylene moiety of propargyl isatin (-N-CH2-) of probe molecule with F- ion and the terminal acidic proton of propargyl group of isatin (-C≡C-H) with NO3- ions. The modes of anions binding with PQI and plausible mechanisms are proposed by 1H and 13C NMR titrations. The selectivity of anions sensing may be offered by the bucked structure of the Z-isomer. The calculated association constant values for PQI and F- and NO3- are ions 2.5 × 104 M-1 and 2.2 × 103 M-1, respectively, indicating strong binding interaction between the PQI and anions. The association nature of anions and probes was analyzed by a Jobs plot and the finding indicates both F- and NO3- ions are in 1:1 complexation with PQI. The limit of detection (LOD) of the probe with F- and NO3- ions is calculated and is to be 6.91 × 10-7 M and 9.93 × 10-7 M, respectively. The proposed PQI fluorophore possesses a low limit of detection (LOD) for both F- and NO3- ions which is within the WHO prescribed detection limit.

Schiff base compounds as fluorimetric pH sensor: a review

With the recent progress of biological and environmental research, detection of pH values has become one of the most indispensable requirements. To determine the pH values of a certain medium, organic Schiff base compounds and their derivatives have been observed to play pivotal roles because of their smooth synthetic roots, easily tuneable structural architecture, non-destructive signals of emission, visually differentiable colour generation and capability of real sample analysis. Therefore with the revolutionary upgradation of wavelength radiometric techniques, the construction of molecular structures which can exhibit dual emission and absorption characteristics and which can be regulated by the change in pH values, has been a stimulating challenge. Generally a pH sensor molecule has a chromophoric or fluorophoric portion. Normally heteroatoms attached to these chromophore units either get protonated or deprotonated in acidic or basic media which gives rise to changes in absorption and emission properties of the molecule.

Recent Advancements in Colorimetric and Fluorescent pH Chemosensors: From Design Principles to Applications

Due to the immense biological significance of pH in diverse living systems, the design, synthesis, and development of pH chemosensors for pH monitoring has been a very active research field in recent times. In this review, we summarize the designing strategies, sensing mechanisms, biological and environmental applications of fluorogenic and chromogenic pH chemosensors of the last three years (2018-2020). We categorized these pH probes into seven types based on their applications, including 1) Cancer cell discriminating pH probes; 2) Lysosome targetable pH probes; 3) Mitochondria targetable pH probes; 4) Golgi body targetable pH probes; 5) Endoplasmic reticulum targetable pH probes; 6) pH probes used in nonspecific cell imaging; and 7) pH probes without cell imaging. All these different categories exhibit diverse applications of pH probes in biological and environmental fields.

Synthesis, Crystal Structures, Fluorescence and Quantum Chemical Investigations of some Multi-Substituted Quinoline Derivatives

Starting from eugenol (4-allyl-2-methoxyphenol) three new quinoline derivatives, namely 5-bromo-7-(carboxymethoxy)-6-hydroxy-1-methylquinolin-1-ium-3-sulfonate (Q2, C₁₂H₁₀BrNO₇S), 5-amino-7-(carboxymethoxy)-6-hydroxyquinolin-1-ium-3-sulfonate (Q4, C₁₁H₁₀N₂O₇S) and 7-(carboxymethoxy)-5,6-dihydroxylquinolin-1-ium-3-sulfonate (Q6, C₁₁H₉NO₈), have been synthesized and crystallised as dihydrate. The best planes through the quinoline ring and the carboxymethoxy substituent is 6.60 (14), 7.28 (6) and 4.73 (7)° for Q2, Q4 and Q6, respectively. The crystal packing of Q2 is characterised by O-H…O, π …π and Br …pyridine interactions. The two water molecules bridge three sulphate groups. Infinite chains of Q4 running in the direction [021] are formed by O/N-H …O hydrogen bonds at both ends of the molecule. Parallel chains interact by O/N-H…O hydrogen bonds and π…π and C=O…phenyl stacking. The -NH₂ substituent bridges two sulphate groups, while the two water molecules bridge the other functional groups. The packing of Q6 consists of sheets of molecules interaction through O/N-H…O hydrogen bonds while the two water molecules bridge all function groups present. Parallel sheets interact through π…π and C=O…pyridine stacking. An aqueous solution of Q2 and its precursor 7-(carboxymethoxy)-6-hydroxyquinolin-1-ium-3-sulfonate (Q) exhibits fluorescence which is pH dependent. The fluorescence intensity of a 10 μM solution of Q containing Zn²⁺ reaches its maximum for a [Zn²⁺]:[Q] ratio of 1:1. The fluorescence properties of Q, Q2, Q4 and Q6 were further investigated by DFT calculation methods.

4-Methyl-2,6-diformylphenol based biocompatible chemosensors for pH: discrimination between normal cells and cancer cells

Two compounds, namely, 2-hydroxy-5-methyl-3-((pyridin-2-ylimino)methyl)benzaldehyde (HM-2py-B) and 2-hydroxy-5-methyl-3-((pyridin-3-ylimino)methyl)benzaldehyde (HM-3py-B), have been explored as fluorescent chemosensors for pH. HM-2py-B and HM-3py-B were synthesized by single step condensation reaction between 4-methyl-2,6-diformylphenol and the appropriate aminopyridine. These compounds have been characterized by elemental analysis, FT-IR, ¹H NMR, ¹³C NMR, ESI mass spectrometry, and absorption and fluorescence spectroscopy. Their structures have been confirmed by single crystal X-ray diffraction analysis. Both of the compounds show low emission at 530 nm at low pH. Fluorescence intensity increases with the increase in pH. With the alteration in pH of the medium from 4.0 to 10.0, the fluorescence intensity at 530 nm enhances by 66 and 195 fold for HM-2py-B and HM-3py-B, respectively. pK_a values of HM-2py-B and HM-3py-B have been determined to be 7.15 and 6.57, respectively. Fluorescence increase occurs mainly due to deprotonation of the phenolic OH group. Several cations and anions could not induce significant change in fluorescence behavior for both of the probes. The quantum yield and life-time enhance significantly when the pH of the medium is changed from 5.0 to 9.0. Naked eye identification of different pH environments is possible by using these compounds. Some theoretical calculations have been carried out to support experimentally obtained spectral transitions. As cancer cell has a pH in the range of 5.5–7.0 in comparison to normal cell pH of 7.4, these probes have been used effectively to discriminate between normal cells and cancer cells.

Two 4-methyl-2,6-diformylphenol based compounds with pyridylamine have been established as chemosensors for pH. The probes are able to differentiate between normal cells and cancer cells.