Computational Insights into Sensing Mechanism for Al3+ in a New Acylhydrazone Fluorescent Probe Based on Excited-State Intramolecular Proton Transfer (ESIPT) and Twisted Intramolecular Charge Transfer (TICT)

The work explored the fluorescent properties of probe N'-(2, 4-dihydroxy-benzylidene)pyridine-3-carbohydrazide (HL) and its sensing mechanism for the Al³⁺ ion in detail. HL has two competing deactivation processes: ESIPT and TICT. Upon light-excitation, only one proton can transfer, and the SPT1 structure is generated. The SPT1 form is highly emissive, which is inconsistent with the colorless emission observed in the experiment. Then a nonemissive TICT state was obtained by rotating the C-N single bond. The energy barrier of the TICT process is lower than that of the ESIPT process, which indicates that probe HL will decay to the TICT state and quench the fluorescence. When Al³⁺ is recognized by probe HL, strong coordinate bonds are formed between HL and Al³⁺, and then the TICT state is prohibited, and the fluorescence of HL is turned on. Al³⁺ as a coordinated ion can effectively remove the TICT state but cannot influence the photoinduced electron transfer (PET) process of HL.

Stimuli-Modulated Metal Oxidation States in Photochromic MOFs

Tuning metal oxidation states in metal-organic framework (MOF) nodes by switching between two discrete linker photoisomers via an external stimulus was probed for the first time. On the examples of three novel photochromic copper-based frameworks, we demonstrated the capability of switching between +2 and +1 oxidation states, on demand. In addition to crystallographic methods used for material characterization, the role of the photochromic moieties for tuning the oxidation state was probed via conductivity measurements, cyclic voltammetry, and electron paramagnetic resonance, X-ray photoelectron, and diffuse reflectance spectroscopies. We confirmed the reversible photoswitching activity including photoisomerization rate determination of spiropyran- and diarylethene-containing linkers in extended frameworks, resulting in changes in metal oxidation states as a function of alternating excitation wavelengths. To elucidate the switching process between two states, the photoisomerization quantum yield of photochromic MOFs was determined for the first time. Overall, the introduced noninvasive concept of metal oxidation state modulation on the examples of stimuli-responsive MOFs foreshadows a new pathway for alternation of material properties toward targeted applications.

Metal-Organic Frameworks Encapsulating Carbon Dots Enable Fast Speciation of Mono- and Divalent Copper

Copper is an essential element to play significant roles in human health associated to the strong redox properties of Cu(I) and Cu(II). The concurrent monitoring of copper species in biological matrixes is highly desired. Herein, a dual-channel fluorescence nanoprobe was designed for the speciation of mono- and divalent copper by conjugating carbon dots (CDs) with Eu-based metal-organic frameworks (Eu-MOFs). The obtained Eu-MOFs@CD nanoprobe exhibits fluorescence at λ_ex/λ_em = 380/454 nm from CDs and λ_ex/λ_em = 275/615 nm from Eu-MOFs. Bathocuproine disulfonate (BCS) specifically chelates Cu⁺ to produce a BCS-Cu⁺ adduct with absorption at 480 nm, which quenches the fluorescence of CDs at 454 nm due to the inner filter effect. On the other hand, Cu²⁺ quenches the fluorescence of Eu-MOFs due to the replacement of Eu³⁺ by Cu²⁺. Thus, Eu-MOFs@CDs enable extremely fast detection of Cu⁺ and Cu²⁺ within 1 min. Furthermore, the nanoprobe is demonstrated by monitoring the variation of Cu⁺ and Cu²⁺ in the degradation process of copper nanoparticles and Cu-based MOFs.

Ag(i) and Cu(i) cyclic-triimidazole coordination polymers: revealing different deactivation channels for multiple room temperature phosphorescences

The photophysics of isostructural Ag(i) and Cu(i) 1D and 3D coordination polymers based on cyclic triimidazole reveals excitation wavelength-dependent multiple emissions with different radiative paths according to the coordinated metal.

Simultaneous detection and speciation of mono- and di-valent copper ions with a dual-channel fluorescent nanoprobe

Redox cyclings between mono-/di-valent copper oxidation states occur in electron transfer reactions that underlie their biological functions. We report herein a dual channel fluorescent nanoprobe for the detection of mono- and di-valent copper ions. The probe, BSA-CDs@RBH/BCS, is designed by decorating carbon dots (CDs) on BSA encapsulated rhodamine hydrazide (RBH) and conjugating with bathocuproine disulfonate (BCS). Cu²⁺ catalyzes the hydrolysis of RBH, and the formed rhodamine B (RhB) shows emission at λ_ex/λ_em = 360/575 nm which ensures Cu²⁺ detection. BSA reduces Cu²⁺ to Cu⁺ and the BCS-Cu⁺ chelate shows emission at λ_ex/λ_em 360/450 nm which ensures Cu⁺ assay. Thus, the dual-channel fluorescence enables speciation of Cu²⁺ and Cu⁺.

Biologically active Co (II), Cu (II), Zn (II) centered water soluble novel isoniazid grafted O-carboxymethyl chitosan Schiff base ligand metal complexes: Synthesis, spectral characterisation and DNA nuclease activity

In this study, the new N, N, O tridentate donor water soluble isoniazid based biopolymer Schiff base ligand and their Co (II), Cu (II), Zn (II) metal complexes were prepared. The compounds were designed for potential biological application such as antibacterial, antifungal, anti-inflammatory, total antioxidant, antidiabetic and DNA binding studies. The synthesized compounds were illuminated in different light sources of various spectra were used to explore the functional groups of Biopolymer derivatives. Thermal degradation, thermal stability and percentage of mass loss for the prepared compounds were investigated through thermo gravimetric and differential thermal (TGA-DTA) analyses. Crystalline structure of synthesized biopolymer derivatives were explored by X-ray diffraction (XRD) studies, the crystallinity of chitosan is gradually decreased after the Schiff base and complex formation. Surface morphology and structures of the prepared compounds were examined using SEM analysis. The magnetic moment and magnetism of the metal complexes were studied using Vibrating-sample magnetometer (VSM). Antidiabetic studies of Biopolymer Schiff base and metal complexes were carried out by α-amylose inhibitory method. DNA nuclease activities of synthesized metal complexes were investigated by Ultra-Violet (UV) and viscometry methods. The Cu (II) complexes showed better DNA binding results than Co (II) and Zn (II) complexes.

Bilirubin Quantification in Human Blood Serum by Deoxygenation Reaction Switch-Triggered Fluorescent Probe

A coumarin-based fluorescent compound, bilirubin fluorescent probe N-oxide (BFPNox), was successfully designed and synthesized for highly selective and sensitive detection of free bilirubin with short response time. The fluorescence "turn-on" response of the probe is based on the in situ generated Fe²⁺-mediated deoxygenation reaction of N-oxide from the diethylarylamine group of the probe, where the group attached to the coumarin π-conjugated system is responsible for the fluorescence quenching state of the probe, BFPNox. Here, the reaction of the added Fe³⁺ ions with bilirubin produces Fe²⁺ ions in situ in aqueous buffer. Fluorescence enhancement of BFPNox was achieved by more than 12-fold when a double equivalent of bilirubin solution was added in reaction buffer at pH 7.2 (50 mM HEPES, 5% DMSO) at 25 °C under excitation at 400 nm. It detected free bilirubin as low as 76 nM in an aqueous system without any interference of metal ions, anions, and other important biomolecules with a linear concentration range of 0-10 μM (R² = 0.991). The probe was also employed in the estimation of free bilirubin in human serum specimens to verify the efficacy of this probe. With these, it is revealed that this probe is a good candidate to be used as a powerful diagnostic tool for the assessment of free bilirubin with significant accuracy and reliability.

Sensing mechanism of a fluorescent probe for thiophenols: Invalidity of excited-state intramolecular proton transfer mechanism

Simple and effective detection of thiophenols has attracted great attention. A fluorescent probe 1 with high selectivity and sensitivity is designed and synthesized based on the excited-state intramolecular proton transfer (ESIPT) in experiment. However, we conclude that the ESIPT process fails to happen actually based on the calculation results. In the present work, the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods are employed to investigate the real sensing mechanism. The calculated absorption and emission spectra agree well with the experimental results. By comparing the energy of enol and keto configurations and the constructed potential energy surfaces (PESs) in the ground (S₀) and excited (S₁) states of 3-(benzo[d]thiazol-2-yl)-10-butyl-10H-phenothiazin-2-ol (dye 2), the ESIPT process is confirmed impossible because of the relatively high keto form energy and potential energy barrier. Besides, the transition state of dye 2 is optimized to offer the accurate potential energy barrier. The results of calculated frontier molecular orbitals (FMOs) and spectra indicate that it is the photoinduced electron transfer (PET) process that results in the fluorescence quenching of probe 1. After adding thiophenols, the thiolysis of 2,4-dinitrophenyl ether bond is triggered and dye 2, which emits strong fluorescence because of the absence of PET process, is obtained. Consequently, our study has demonstrated that probe 1 can act as a fluorescent probe to detect thiophenols through the off-on fluorescence variation based on the PET mechanism but not the ESIPT process.

Study on synthesis and fluorescence property of rhodamine-naphthalene conjugate

In this study, a novel ligand (HL) consisting of 2-methyl quinoline-4-carboxylic acid, rhodamine and naphthalene moiety, was designed and synthesized, it could be developed a ratiometric fluorescent sensor for selective detection of Al³⁺ via fluorescence resonance energy transfer (FRET) from naphthalimide moiety to rhodamine moiety. The addition of Al³⁺ trigger the significant fluorescence enhancement of HL at 550 nm at the expense of the fluorescent emission of HL centered at 524 nm.

Sensing mechanism of a ratiometric near-infrared fluorescent chemosensor for cysteine hydropersulfide: Intramolecular charge transfer

Previous studies have shown that the cysteine hydropersulfide (Cys-SSH) as the sulfur donor is crucial to sulfur-containing cofactors synthesis. Recently, a selective and sensitive near-infrared ratiometric fluorescent chemosensor Cy-DiSe has been designed and synthesized to detect Cys-SSH spontaneously. Herein, by means of the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) approaches, the sensing mechanism has been thoroughly explored. According to our calculations, the experimental data have been reproduced. The results indicate the intramolecular charge transfer (ICT) is the reason for changes in fluorescence wavelengths. Compared with the chemosensor Cy-DiSe, the larger energy gap of Cy induced by ICT mechanism leads to the blue-shift of the absorption and emission spectra, which guarantees that Cy-DiSe can become a ratiometric fluorescent chemosensor to detect Cys-SSH.

Novel rhodamine-based colorimetric and fluorescent sensor for the dual-channel detection of Cu2+ and Co2+/trivalent metal ions and its AIRE activities

A rhodamine hydrazone 1 bearing coumarin moiety was designed and prepared. Compound 1 exhibited high selectivity toward Co²⁺ and trivalent metal ions with fluorescence enhancement in CH₃OH solution. However, 1 selectively responded to Al³⁺ in nearly pure H₂O media and was further applied to monitor Al³⁺ in live cells. Moreover, 1 could also act as a colorimetric probe toward Cu²⁺ in either CH₃OH or H₂O solution. In addition, sensor 1 displayed aggregation-induced ratiometric emission (AIRE) activities in mixed H₂O/CH₃OH solution.

Ratiometric fluorescent probe based on pyrrole-modified rhodamine 6G hydrazone for the imaging of Cu2+ in lysosomes

A novel rhodamine-based Schiff base derivative was obtained via the simple condensation of substituted formyl-1H-pyrrole and rhodamine 6G hydrazone. Fluorescence resonance energy transfer enabled the subsequent use of the derivative as a naked-eye colorimetric and ratiometric fluorescent sensor for Cu²⁺ in semi-aqueous solution, and the existence of the morpholine group enabled the further application of the sensor in imaging Cu²⁺ in the lysosomes of HeLa cells.

A multi-functional chemosensor for highly selective ratiometric fluorescent detection of silver(I) ion and dual turn-on fluorescent and colorimetric detection of sulfide

A multi-functional chemosensor 1 as silver and sulfide detector was synthesized by the combination of octopamine and 4-dimethylaminocinnamaldehyde. Sensor 1 exhibited a ratiometric fluorescence emission for Ag+ from blue to sky. The binding mode of 1 and Ag+ turned out to be a 1 : 1 ratio as determined using Job plot and electrospray ionization (ESI) mass spectral analyses. The sensing mechanism of 1 with silver ion was unravelled by 1H NMR titrations and theoretical calculations. Sensor 1 also discerned sulfide by enhancing fluorescence intensity and changing colour from yellow to colourless in aqueous solution. The sensing properties of 1 toward S2- were investigated by using ESI-mass analysis, Job plot and 1H NMR titrations. Moreover, 1 could be used as a detector for sulfide in a wide pH range.

A New Lysosome-Targetable Turn-On Fluorogenic Probe for Carbon Monoxide Imaging in Living Cells

A lysosome-targetable fluorogenic probe, LysoFP-NO₂, was designed and synthesized based on a naphthalimide fluorophore that can detect selectively carbon monoxide (CO) in HEPES buffer (pH 7.4, 37 °C) through the transformation of the nitro group into an amino-functionalized system in the presence of CO. LysoFP-NO₂ triggered a "turn-on" fluorescence response to CO with a simultaneous increase of fluorescence intensity by more than 75 times. The response is selective over a variety of relevant reactive nitrogen, oxygen, and sulfur species. Also, the probe is an efficient candidate for monitoring changes in intracellular CO in living cells (MCF7), and the fluorescence signals specifically localize in the lysosome compartment.

A naphthalimide-based fluorescence ‘‘turn-on’’ chemosensor for highly selective detection of carbon monoxide: imaging applications in living cells

A naphthalimide-based fluorescence chemosensor, COFP, was designed and synthesized for the detection of carbon monoxide (CO) in HEPES buffer (pH 7.4, 37 °C).

A simple hydrazone as a multianalyte (Cu2+, Al3+, Zn2+) sensor at different pH values and the resultant Al3+ complex as a sensor for F−

A new colorimetric and fluorescence molecular chemosensor based on triazole hydrazone can be used as a multi-probe for selective detection of Al³⁺, Zn²⁺, and Cu²⁺ by monitoring changes in the absorption and fluorescence spectral patterns. Results show that Al³⁺ and Zn²⁺ ions can induce remarkable fluorescence enhancement at pH 6.0 and pH 10.0, respectively, while the addition of Cu²⁺ ions leads to a significant UV-visible absorption enhancement in the visible range at pH 6.0. In addition, the resultant Al³⁺ complex could act as an ‘on–off’ fluorescence sensor for F⁻. The fluorescence sensor was also used to monitor intracellular Al³⁺, Zn²⁺, and F⁻ in Hela cells.

A fluorescent probe for Al³⁺ and Zn²⁺ was synthesised, and the resultant Al³⁺-complex was used for the detection of F⁻.

New Six-Membered pH-Insensitive Rhodamine Spirocycle in Selective Sensing of Cu2+ through C-C Bond Cleavage and Its Application in Cell Imaging

A new rhodamine-based chemosensor 1 with a six-membered spirocyclic ring has been synthesized, which exhibits excellent pH stability and shows selective "turn-on" fluorescent detection of Cu²⁺ ions over a series of other metal ions including Cu⁺ ions. The expansion of spirocycle improves the stability and selectivity of the chemosensors in sensing of metal ions. Till today only few rhodamine structures R1-R5 with thiourea-, hydrazine amide-, or pyrrole-decorated six-membered spirocyclic rings are known that exhibit metal-ion sensing via C-N bond cleavage of the spiro ring. In this context, rhodamine compound that responds to the metal ion through C-C bond cleavage of the six-membered spiro ring is completely unknown. The present example is a first-time report that demonstrates selective sensing of Cu²⁺ ions through C-C bond cleavage over the conventional existing systems in the literature. The chemosensor 1 is cell permeable and can detect Cu²⁺ in live cells using confocal microscopy in the biologically relevant pH range with high photostability.

Photoluminescence properties of TADF-emitting three-coordinate silver(i) halide complexes with diphosphine ligands: a comparison study with copper(i) complexes

A series of three- and four-coordinate silver(i) halide complexes exhibiting efficient blue thermally activated delayed fluorescence have been prepared.

A new turn-on benzimidazole-based greenish-yellow fluorescent sensor for Zn2+ions at biological pH applicable in cell imaging

A newly designed and structurally characterized non-cytotoxic benzimidazole containing quinazoline derivative (HL) acts as a ‘turn-on’ greenish-yellow fluorescent sensor selective for Zn²⁺ions at as low as 39.91 nM in 5 mM HEPES buffer (DMSO/water: 1/5, v/v) at biological pH.

Development of a cell permeable red-shifted CHEF-based chemosensor for Al(3+) ion by controlling PET

A structurally modified quinazoline derivative (L) acts as highly selective chemosensor for Al(3+) ions in DMSO-H2O (1:9, v/v) over the other competitive metal ions. L shows a red shifted fluorescence after the addition of Al(3+) ions and later the further fluorescence enhancement is due to chelation enhanced fluorescence (CHEF) through inhibition of photoinduced electron transfer (PET). This probe (L) detects Al(3+) ions as low as 9nM in DMSO-H2O (1:9, v/v) at biological pH. The non-cytotoxic probe (L) can efficiently detect the intercellular distribution of Al(3+) ions in living cells under a fluorescence microscope to exhibit its sensible applications in the biological systems.

A fluorescent probe for the selective detection of creatinine in aqueous buffer applicable to human blood serum

A naphthalimide-based fluorescence light-up probe detects creatinine selectively in PBS buffer of pH 7.2 at 37 °C. It exhibits a ‘turn-on’ response to creatinine over a variety of biologically relevant ions and species and can also detect creatinine in human blood serum.

On the outside looking in: redefining the role of analytical chemistry in the biosciences

Analytical chemistry has much to offer to an improved understanding of biological systems.

Selective and sensitive turn-on chemosensor for Al(iii) ions applicable in living organisms: nanomolar detection in aqueous medium

A new crystallographically characterized non-cytotoxic CHEF based highly sensitive Al(iii) ion selective chemosensor (L) is useful to detect the Al(iii) ions' distribution in A549 cell lines and also able to sense Al(iii) ions in tea extract.

A new fluorogenic probe for the selective detection of carbon monoxide in aqueous medium based on Pd(0) mediated reaction

A coumarin-based fluorogenic probe, PCO-1, detects carbon monoxide selectively in buffer at pH 8.0 through the intramolecular cyclization–elimination pathway based on Pd(0) mediated reaction.