TDDFT study on the photophysical properties of coumarinyl chalcones and sensing mechanism of a derived fluorescent probe for hydrogen sulfide

A Simple Fluorescent Chemo-Dosimeter for Sensitive Detection of Cyanide and Hydrogen Sulfide Ions: Spectroscopic and TD-DFT Studies

The lethal potency and pervasive existence of both cyanide and hydrogen sulphide in various processes mandates their detection. In this context, a simple chemo-dosimeter (AK6) containing 2,2'-bisthiophene fluorophore moiety has been synthesized and spectroscopically characterized. The selectivity of AK6 toward CN‒ and HS‒ ions was fortified via fluorescence enhancement, which is attributed to the nucleophilic addition of CN‒ and HS‒ ions to the olefinic C-atom of the probe, as manifested by 1H and 13C NMR, and mass spectral experiments. The AK6 could sense CN‒ and HS‒ ions with a limit of detection (LOD) values of 0.42 and 13 µM, respectively, with high selectivity without interference from other common anions and cations. The pH studies revealed that AK6 is suitable for detecting CN- and HS in physiological and environmental conditions (pH range 4-10). The Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) calculations substantiated the proposed mechanism, which alters the intramolecular charge transfer (ICT) transition in the probe to impart fluorescence enhancement that can be seen visually. Besides, AK6 could be utilized to assess CN‒ and HS‒ ions in actual water samples.

Dithiophene chemosensor for ultrasensitive intracellular detection of Al3+: Design, DFT analysis, and ESIPT-PET mechanisms

Metal ions play essential roles in living cells, yet their biological functions, which depend on intracellular concentrations, are not fully understood. Therefore, there is a critical need for efficient and sensitive methods to monitor metal ion levels in biological systems. Herein, we report the development of a fluorescent probe, 2-hydroxy-1-naphthaldehyde-(dithiophen-2-yl)ethanediamine (NS), for the precise and sensitive detection of intracellular Al3+ at concentrations as low as 3.92 × 10-8 M. The probe features a bifunctional thienyl ethanol ligand, consisting of two thiophene rings and a hydroxyl group, which forms stable coordination with Al3+. This interaction modifies the electron allocation within the ligand, suppressing the excited-state intramolecular proton transfer (ESIPT) mechanism and significantly increasing fluorescence intensity. Notably, in the presence of Al3+, compared to other ions, the fluorescence intensity of NS at 452 nm increases by 77-fold, with an exceptional sensitivity and selectivity for Al3+. Furthermore, the hydroxyl group enhances the probe's solubility and stability in aqueous solutions, making it highly effective for intracellular detection of Al3+ in prostate cancer RM-1 cells. The response mechanism is further investigated through 1H NMR and DFT studies, revealing the contributions of ESIPT, photoinduced electron transfer (PET), and CN isomerization to the probe's fluorescence behavior. This work provides a promising and advanced tool for ionobiology, opening new avenues for research into metal ion-related biological processes.

Design and optimization of encapsulated sensor materials with diverse binding sites for efficient cyanide ion detection

Developing colorimetric and fluorimetric sensors with a new design strategy incorporates the same electron donor and acceptor units by changing the binding site by expecting different mechanisms. The sensors YS and RS have the D-π-A concept, having phenothiazine as an electron donor and benzothiazole as an electron acceptor for sensing cyanide ions in various spectral techniques. Both the sensors showed an efficient color change with cyanide ion in day light and UV light, which was confirmed by UV-vis and Fluorescence spectral analysis. The mechanism of sensing cyanide ion by the sensor YS via hydrogen bond formation followed by deprotonation and RS via nucleophilic addition reaction was confirmed with a 1H NMR, FT-IR and HRMS spectral studies. The detection limit was found to be 1.36 μM and 0.78 μM by UV-vis, 0.13 nM and 0.39 nM by fluorescence technique are for sensors YS and RS, which are significantly lower than the WHO criterion of 1.9 μM for cyanide ions in water used for drinking. Furthermore, the real-world application showed that the sensors could quantitatively identify the quantity of cyanide ion present in different types of water samples. Besides, the fabricated test strips make the sensors easy to utilize for detecting CN- in the field without the need for complicated devices. Also, the developed sensor-encapsulated Polysulfone (PSF) capsule kit effectively senses cyanide ion in water.

ICT / PET Modulated Chromogenic/Fluorogenic Detection of Cyanide Ions: A Spectroscopic and Theoretical Study

The prevalence use of cyanide and its pernicious risks to human health accentuates the prominence of cyanide detection. Concerning this, we have developed a probe VDP4 by coupling a donor (8-hydroxyquinoline-2-carbaldehyde) and an acceptor (2-(1H-benzimidazole-2-yl)acetonitrile) by Knoevenagel condensation reaction for the exclusive detection of cyanide in an aqueous solution. The VDP4 responded to CN- by changing its color to yellow and switching on its fluorescence following contact with cyanide. 1H-NMR, 13C-NMR, LC-MS, FT-IR, and DFT studies provide an attestation for the signaling mechanism for cyanide detection is the deprotonation of the -OH group combined with nucleophilic addition of cyanide at the electron-deficient vinylic carbon atom of VDP4. A very low colourimetric (86 nM) and fluorometric (44 nM) detection limit highlighted its practicability, and Job studies ratified the 1:1 binding interaction. The DFT/TDDFT studies uncovered that colour changes of VDP4 with CN- were caused by ICT variations, and PET modulations were responsible for fluorescent alterations. The spatial ICT was a key factor for the deep yellow coloration of the VDP4 in the presence of CN-. The energy level diagram of frontier molecular orbitals unveiled that Photoinduced electron transfer from the benzimidazole part to the 8-hydroxy quinoline part was liable for the frail emission of VDP4 which occluded in the presence of CN- thereby turning on the fluorescence of the fluorophore. The real-time analysis results advocated that VDP4 was the optimum choice for the qualitative and quantitative estimation of CN- in food samples.

Spectroscopic and Theoretical Studies on the Selective Detection of Cyanide Ions by a Turn-On Fluorescent Chemo-Dosimeter and its Application in Living Cell Imaging

A new chemo-dosimeter AK4 containing quinoline fluorophore has rationally been designed, synthesised and characterized using 1H and 13C NMR and mass spectral techniques. The probe senses explicitly CN- ion through a dramatic enhancement in fluorescence over other commonly coexistent anions in H2O:DMSO (9:1 v/v) medium over a broad pH range (4-10). 1H NMR titration revealed the deprotonation followed by nucleophilic addition reaction of CN-, which was supported by 13C NMR and mass spectral examinations. The Job's continuous variation method indicated the formation of a 1:1 adduct between AK4 and CN- with a binding constant of 1.62 × 104 M-1. A limit of detection (LOD) towards CN- of 0.69 µM has been determined, which is much lower than the World Health Organization (WHO) recommended limit of CN- in drinking water (1.9 µM). The changes in the optical properties of AK4 upon reaction with CN- were delineated using Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) calculations. Moreover, fluorescence microscopic studies established that AK4 could be an effective probe for imaging intracellular CN- in HeLa cells.

Comprehensive investigations of four ratiometric fluorescent chemosensors based on 4-(1H-imidazol-2-yl)benzaldehyde skeleton for malononitrile detection

Malononitrile is a very important chemical material and has wide application fields in production of medicines, pesticides, and extraction of gold. However, its nonnegligible hypertoxicity inspired researchers to develop more efficient analysis techniques to sensitively and selectively detect malononitrile. Nopinone derivatives initiated by our research group have been developed as a class of organic fluorescent chemosensors for identifying multiple analytes in recent years. Different heterocyclic compounds based on nopinone were designed and synthesized to be applied in the fields of environmental analysis, food detection and bioimaging. Nevertheless, the comparison research on the optical properties of fluorescent compounds containing the nopinyl matrix with other structural analogs including alkyl, cyclohexyl and phenyl groups was deficient. Herein, four 4-(1H-imidazol-2-yl)benzaldehyde-based ratiometric fluorescent chemosensors based on o-dimethyl cyclohexyl, phenyl and nopinyl units for recognizing malononitrile were designed and developed, and their differences in the optical properties and detection performances were investigated by using spectral analysis combined with theoretical calculations. Moreover, the nopinone-based 4-(1H-imidazol-2-yl)benzaldehyde fluorescent chemosensor NMZQ was successfully applied in the dual channel fluorescence bioimaging of malononitrile in living HeLa cells and zebrafish, which attributed to its outstanding spectral property and detection performance.

A ferrocene-based chemo-dosimeter for colorimetric and electrochemical detection of cyanide and its estimation in cassava flour

A simple chemo-dosimeter VDP2 bearing a ferrocene moiety was designed, synthesized, and characterized, and exhibited both chromogenic and electrochemical responses selectively for CN- in H2O-DMSO (9 : 1, v/v) medium. The probe VDP2 showed an instantaneous color change from colorless to yellow with CN- that can readily be observed visually. The deprotonation of the benzimidazole -NH, followed by nucleophilic addition of CN- to the olefinic C-atom, as evidenced by 1H and 13C NMR titration experiments, caused the colorimetric and electrochemical responses. The mass spectral study, CV, FTIR and Mulliken charges computed well supported the proposed mechanism. The electrochemical limit of detection was calculated to be 72 nM. The results of DFT and TD-DFT calculations suggested that the colorless nature of the probe VDP2 is due to weak intramolecular charge transfer (ICT) transition and the yellow color of the VDP2+CN adduct is due to through-space ICT transition. Above all, the probe could be an ideal candidate for monitoring cyanide in water samples and cassava flour with practical significance. A simple and convenient colorimetric method was developed to determine cyanide content in cassava flour.

A Chalcone-based Fluorescence Probe for H2S Detecting Utilizing ESIPT Coupled ICT Mechanism

The accurate and effective identification of hydrogen sulfide holds great significance for environmental monitoring. Azide-binding fluorescent probes are powerful tools for hydrogen sulfide detection. We combined the 2'-Hydroxychalcone scaffold with azide moiety to construct probe Chal-N3, the electron-withdrawing azide moiety was utilized to block the ESIPT process of 2'-Hydroxychalcone and quenches the fluorescence. The fluorescent probe was triggered with the addition of hydrogen sulfide, accompanied by great fluorescence intensity enhancement with a large Stokes shift. With excellent fluorescence properties including high sensitivity, specificity selectivity, and wider pH range tolerance, the probe was successfully applied to natural water samples.

TDDFT study on a fluorescent probe for distinguishing analogous thiols based on smiles rearrangement

The complete excited-state sensing mechanism of a fluorescent probe capable of distinguishing cysteine/homocysteine and glutathione from analogous biological thiols has been investigated. Using a TDDFT method, the nature of the fluorescence differences in the detection of thiols by the probe has been explained at the molecular level. Calculation results imply that the probe undergoes photoinduced electron transfer (PET) from the fluorophore to the nitrobenzooxadiazole (NBD)-based acceptor in the excited state. In the presence of a thiol, the NBD moiety is cleaved and the red fluorescence emission of the fluorophore is enhanced through inhibition of the PET process. The sulfur-substituted NBD-thiol product is predisposed to undergo excited-state torsion, leading to fluorescence quenching. However, for cysteine and homocysteine, their appropriate distances lead to Smiles rearrangements with relatively low activation energies (26.60 kJ/mol and 42.94 kJ/mol, respectively) and the emission of a distinct green fluorescence at ambient temperature. It has been theoretically confirmed that the distance between two reactive sites, such as sulfhydryl and amino moieties, can be used to distinguish different thiols, thus providing rational support for the control of fluorescence activity and the design of probe molecules.

Spectroscopic and TD-DFT studies on the chromo-fluorogenic detection of cyanide ions in organic and aquo-organic media

A new naked-eye chromogenic and fluorogenic probe KS5 has been developed for the detection of CN- ions in neat DMSO and H2O:DMSO (1:1 v/v) media. The probe KS5 exhibited selectivity towards CN- and F- ions in organic and high selectivity towards CN- ions in aquo-organic media resulting in a colour change from brown to colourless and a turn-on fluorescence response. The probe could able to detect CN- ions via a deprotonation process, which was conceived by consecutive addition of hydroxide and hydrogen ions and confirmed using 1H NMR studies. The limit of detection (LOD) of KS5 towards CN- ions were in the range of 0.07-0.62 µM in both these solvent systems. Suppression of intra-molecular charge transfer (ICT) transition and photoinduced electron transfer (PET) process of KS5 by the added CN- ions are responsible for the chromogenic and fluorogenic changes observed, respectively. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) calculations strongly supported the proposed mechanism along with the optical properties of the probe before and after the addition of CN- ions. To prove the practical applicability, KS5 was successfully utilized to detect CN- ions in cassava powder and bitter almonds as well as to determine CN- ions in various real water samples.

Fluorescent Mechanism of a Highly Selective Probe for Copper(II) Detection: A Theoretical Study

A highly selective probe for copper(II) detection based on the dansyl group was theoretically studied by means of (time-dependent) density functional theory. The calculated results indicated that the oscillator strength of the fluorescent process for the probe molecule is considerably large, but the counterpart of its copper(II) complex is nearly zero; therefore, the predicted radiative rate k_r of the probe is several orders of magnitude larger than that of its complex; however, the predicted internal conversion rate k_ic of both the probe and its complex is of the same order of magnitude. In addition, the simulated intersystem crossing rate k_isc of the complex is much greater than that of the probe due to the effect of heavy atom from the copper atom in the complex. Based on the above information, the calculated fluorescence quantum yield of the probe is 0.16% and that of the complex becomes 10^-6%, which implies that the first excited state of the probe is bright state and that of the complex is dark state. For the complex, the hole-electron pair analysis indicates that the process of S₀ → S₁ belongs to metal-to-ligand charge transfer; its density-of-state diagram visually illustrates that the highest occupied molecular orbital (HOMO) contains the ingredient of the s orbital from the copper atom, which decreases the frontier orbital energy level and the overlap integral of HOMO and LUMO.

TDDFT study on the simultaneous sensing mechanism for peroxynitrite and glutathione by a bifunctional fluorescent probe

Using time-dependent density functional theory (TDDFT) method, the response mechanism of a reported bifunctional fluorescent probe for simultaneous recognition of peroxynitrite and glutathione (Chem. Commun. 2018, 54, 11336) was theoretically studied. Calculated vertical excitation energies based on the ground-state and excited-state geometries were consistent with the corresponding experimental ultraviolet-visible and fluorescence spectra. In the ground state, electron delocalization in the probe was limited because its geometry was restrained by steric hindrance. Frontier molecular orbital analysis has shown that the probe should undergo photoinduced electron transfer (PET) from the benzothiazole moiety to the maleimide moiety after excitation. The nonplanar structure together with PET led to fluorescence quenching of the probe. The probe could be dealkylated by peroxynitrite anion. The resulting intramolecular hydrogen bond increasesd the planarity of the molecule, while also gave rise to an excited-state proton-transfer process. Moreover, the addition reaction between the probe and glutathione inhibited the PET process. These two analytes together contributed to the fluorescence enhancement of the final product. This theoretical sensing mechanism for peroxynitrite and glutathione may potentially be important for the design and enhancement of novel probes.

A highly selective red-emitting fluorescent probe and its micro-nano-assembly for imaging endogenous peroxynitrite (ONOO-) in living cells

Endogenous peroxynitrite plays a very important role in the regulation of life activities. However, validated tools for ONOO^- tests are currently insufficient. We designed a fluorescent probe TPA-F-NO₂ with a low fluorescence background in water based on the D-π-A structure for the imaging of endogenous ONOO^- in living cells. TPA-F-NO₂ can realize the naked eye detection of ONOO^- due to the obvious color change. TPA-F-NO₂ has the advantages of large stokes shift, high signal-to-noise ratio, high selectivity and sensitivity. The quantitative detection can be achieved in the range of 0-14 μM ONOO^-. Due to its solvatochromic characteristics, TPA-F-NO₂ has the potential to be used in OLEDs and other fields. In addition, 4-methylumbelliferone has a wide range of anticancer effects as an inhibitor of hyaluronic acid. We prepared TPA-MU-NPs by assembling TPA-F-NO₂ and 4-methylumbelliferone. It also endows TPA-MU-NPs with ONOO^- imaging function and anti-proliferation effect on breast cancer cells and other cells. This 'probe-drug' assembly strategy provides ideas for the design and optimization of dual-functional probes.

Development and application of a novel β-diketone difluoroboron-derivatized fluorescent probe for sensitively detecting H2S

Hydrogen sulfide{Wang, 2018 #4}{Wang, 2018 #4}{Zhong, 2020 #9} (H₂S) is a poisonous and harmful gas molecule. Certain concentrations of H₂S{Liu, 2021 #8} can irritate the eyes, respiratory system, and central nervous system of human beings. Therefore, it was an urgent need for highly selective, anti-interference, and sensitive detection technology for hydrogen sulfide. Herein, a novel "turn-on" fluorescent probe 1-(2-(6,6-dimethylbicyclo[3.1.1]heptyl-2-ene-2-yl))-9-(4-(dimethylaminophenyl))non-1,6,8-triene-3,5-dione boron difluoride complex (MCBF) was designed and synthesized for detecting H₂S sensitively. MCBF displayed a remarkable fluorescence enhancement response to H₂S with a large Stokes shift of 220 nm. The sensitive detection of MCBF towards H₂S owned good selectivity, fast response time (6 min), excellent photostability, and low detection limit (0.44 μM). The sensing mechanism of MCBF towards H₂S was well confirmed by HRMS analysis, ¹H NMR titration, and density functional theory (DFT) calculations. What's more, probe MCBF was successfully applied to detect the contained H₂S in red wine, which showed the potential practicability of MCBF in real samples analysis.

A highly selective and sensitive ratiometric fluorescent probe for quantitative detection of Al(III) in different natural matrices

Highly selective and sensitive assay of Al(III) using ratiometric fluorescence enhancement is reported in an aqueous solution. The probe (named as RS5) exhibits a red-shift of 54 nm upon binding with Al(III) ion. The significant enhancement response of RS5 at 481 nm is attributed to the formation of a 1:1 complex between the probe and Al(III), wherein RS5 acts as a tridentate NNN-donor ligand. The complexation process is ascertained by 1H, 13C and 27Al NMR and HR-MS spectral techniques. The binding constant of the complex is determined to be 1.3x105 M-1. The ratiometric change in fluorescence upon complexation with Al(III) is ascribed to increase in intramolecular charge transfer (ICT) transition along with chelation enhanced fluorescence (CHEF) processes. The probe can be applied for monitoring Al(III) in a pH range of 6 - 8. The limit of detection (LOD) of RS5 for the examination of Al(III) is found to be 0.3 µM. With an aim to understand the sensing behaviour of RS5, the optical properties of the probe and its Al(III) complex are investigated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The probe is successfully employed for the determination of Al(III), with very high recovery percentages, in natural matrices like deep well water, tap water, drinking water, pond water, river water, bovine serum albumin (BSA) solution and blood serum.

Ligand-based design of chalcone analogues and thermodynamic analysis of their mechanism of free radical scavenge

Overproduction of free radicals in the body may result in oxidative stress, which plays an active role in the development of various health disorders. Consequently, the development of efficient free radical scavengers and evaluation of their antioxidant properties is a research area of interest. In the present research, computational quantum chemical approach based on the density functional theory (DFT) method was employed to elucidate the free radical scavenge of chalcone derivatives via thermodynamic studies. New set of chalcone antioxidants were designed. Their reactivity towards hydroperoxyl (HOO·) and methyl peroxyl (CH₃OO·) radicals were investigated through systematic study of their mechanism of free radical scavenge. Various reaction enthalpies and Gibbs free energy that characterize the various steps in these mechanisms were computed in the gas phase and aqueous solution, in order to identify the main channels of reaction. Results in the gas phase indicate that hydrogen atom transfer (HAT) and sequential proton loss electron transfer (SPLET) mechanisms represent the most plausible reaction pathways, while single electron transfer followed by proton transfer (SET-PT) mechanism was thermodynamically unfeasible. However, these mechanisms were thermodynamically favoured in aqueous solution. Also, these chalcone derivatives were observed to be more effective in scavenging HOO· than CH₃OO· radicals in both phases. Based on the exergonicity of the obtained results, the molecule MCHM 17 ((E)-1-(3-bromo-5-hydroxyphenyl)-3-(2,5-dihydroxyphenyl)prop-2-en-1-one) at the 5-OH site was found to exhibit the greatest potential to scavenge HOO· and CH₃OO· radicals in both phases. This research is a gateway to the efficient exploitation of these compounds in pharmacy and food chemistry.