A Rhodamine−Cyclen Conjugate as a Highly Sensitive and Selective Fluorescent Chemosensor for Hg(II)

Opposing roles of organic salts on mini-protein structure

We investigated the effects of 1-ethyl-3-methylimidazolium chloride ([EMIM][Cl]) and choline chloride ([Chol][Cl]) on the local environment and conformational landscapes of Trp-cage and Trpzip4 mini-proteins using experimental and computational approaches. Fluorescence experiments and computational simulations revealed distinct behaviors of the mini-proteins in the presence of these organic salts. [EMIM][Cl] showed a strong interaction with Trp-cage, leading to fluorescence quenching and destabilization of its native structural interactions. Conversely, [Chol][Cl] had a negligible impact on Trp-cage fluorescence at low concentrations but increased it at high concentrations, indicating a stabilizing role. Computational simulations elucidated that [EMIM][Cl] disrupted the hydrophobic core packing and decreased proline-aromatic residue contacts in Trp-cage, resulting in a more exposed environment for Trp residues. In contrast, [Chol][Cl] subtly influenced the hydrophobic core packing, creating a hydrophobic environment near the tryptophan residues. Circular dichroism experiments revealed that [Chol][Cl] stabilized the secondary structure of both mini-proteins, although computational simulations did not show significant changes in secondary content at the explored concentrations. The simulations also demonstrated a more rugged free energy landscape for Trp-cage and Trpzip4 in [EMIM][Cl], suggesting destabilization of the tertiary structure for Trp-cage and secondary structure for Trpzip4. Similar fluorescence trends were observed for Trpzip4, with [EMIM][Cl] quenching fluorescence and exhibiting stronger interaction, while [Chol][Cl] increased the fluorescence at high concentrations. These findings highlight the interplay between [EMIM][Cl] and [Chol][Cl] with the mini-proteins and provide a detailed molecular-level understanding of how these organic salts impact the nearby surroundings and structural variations. Understanding such interactions is valuable for diverse applications, from biochemistry to materials science.

Viscosity-sensitive and AIE-active bimodal fluorescent probe for the selective detection of OCl- and Cu2+: a dual sensing approach via DFT and biological studies using green gram seeds

A novel dual-mode viscosity-sensitive and AIE-active fluorescent chemosensor based on the naphthalene coupled pyrene (NCP) moiety was designed and synthesized for the selective detection of OCl- and Cu2+. In non-viscous media, NCP exhibited weak fluorescence; however, with an increase in viscosity using various proportions of glycerol, the fluorescence intensity was enhanced to 461 nm with a 6-fold increase in fluorescence quantum yields, which could be utilized for the quantitative determination of viscosity. Interestingly, NCP exhibited novel AIE characteristics in terms of size and growth in H2O-CH3CN mixtures with high water contents and different volume percentage of water, which was investigated using fluorescence, DLS study and SEM analysis. Interestingly, this probe can also be effectively employed as a dual-mode fluorescent probe for light up fluorescent detection of OCl- and Cu2+ at different emission wavelengths of 439 nm and 457 nm via chemodosimetric and chelation pathways, respectively. The fast-sensing ability of NCP towards OCl- was shown by a low detection limit of 0.546 μM and the binding affinity of NCP with Cu2+ was proved by a low detection limit of 3.97 μM and a high binding constant of 1.66 × 103 M-1. The sensing mechanism of NCP towards OCl- and Cu2+ was verified by UV-vis spectroscopy, fluorescence analysis, 1H-NMR analysis, mass spectroscopy, DFT study and Job plot analysis. For practical applications, the binding of NCP with OCl- and Cu2+ was determined using a dipstick method and a cell imaging study in a physiological medium using green gram seeds.

A turn-on fluorescent probe with high selectivity for Hg2+ and its applications in living cells

A novel fluorescent probe L with a rhodamine B lactam structure modified with 3-methyl-2-thiophenecarboxaldehyde has been prepared based on the thiophilicity of Hg²⁺. The probe L exhibits a unique response with an “off–on-type” mode towards Hg²⁺ among other biologically relevant metal cations. The limit of detection (LOD) for probe L is 1.5 ppb. In addition, in the presence of Hg²⁺, the probe L shows a colorimetric response from colorless to pink. The recognition behavior of probe L towards Hg²⁺ has been investigated by ¹H NMR titration experiments, Job's plot, and MS and IR analyses. As a result, the ligation between the probe and Hg²⁺ leads to the scission of the spirolactam bond of free L and the restoration of its conjugated structure, which can give rise to the fluorescence enhancement of the probe L. Besides, it also can be used as a sensitivity probe in living cells for Hg²⁺ sensing, which can meet various needs in genetic and environmental samples.

A novel colorimetric fluorescent probe with low detection limits has been designed and synthesized, which shows an unique response for Hg²⁺ in living cells.

Dual-mode chemosensor for the fluorescence detection of zinc and hypochlorite on a fluorescein backbone and its cell-imaging applications

Fluorescein coupled with 3-(aminomethyl)-4,6-dimethylpyridin-2(1H)-one (FAD) was synthesized for the selective recognition of Zn²⁺ over other interfering metal ions in acetonitrile/aqueous buffer (1 : 1). Interestingly, there was a significant fluorescence enhancement of FAD in association with Zn²⁺ at 426 nm by strong chelation-induced fluorescence enhancement (CHEF) without interrupting the cyclic spirolactam ring. A binding stoichiometric ratio of 1 : 2 for the ligand FAD with metal Zn²⁺ was proven by a Jobs plot. However, the cyclic spirolactam ring was opened by hypochlorite (OCl^-) as well as oxidative cleavage of the imine bond, which resulted in the emission enhancement of the wavelength at 520 nm. The binding constant and detection limit of FAD towards Zn²⁺ were determined to be 1 × 10⁴ M^-1 and 1.79 μM, respectively, and the detection limit for OCl^- was determined as 2.24 μM. We introduced here a dual-mode chemosensor FAD having both the reactive functionalities for the simultaneous detection of Zn²⁺ and OCl^- by employing a metal coordination (Zn²⁺) and analytes (OCl^-) induced chemodosimetric approach, respectively. Furthermore, for the practical application, we studied the fluorescence imaging inside HeLa cells by using FAD, which demonstrated it can be very useful as a selective and sensitive fluorescent probe for zinc.

A Pyrene-Rhodamine FRET couple as a chemosensor for selective detection of picric acid

Selective detection of nitroaromatic compounds such as Picric acid (PA), those being explosive materials and hazardous pollutants of environmental and biological concern is highly desirable. With the operational advantages of the chemosensing approach, a pyrene-rhodamine-B couple (1) was explored in this investigation as a ratiometric molecular probe for selective and sensitive detection of picric acid. The bi-fluorophoric probe displayed absorption and fluorescence enhancements along with colourless to reddish-brown colour transition as signaling responses in the selective presence of PA among all the nitro aromatic analyte investigated. The signaling module relies on PA- mediated modulation of various operational photo-physical processes such as (a) inhibition of photo-induced electron transfer (PET) operative from amino-donor to excited pyrene (b) a conformational translation through spiro-ring opening of rhodamine-B segment, and (c) initiation of Fluorescence Resonance Energy Transfer (FRET) between excited pyrene donor and ring-opened rhodamine acceptor. The ratio of fluorescence from both fluorophores (pyrene and Rhodamine) as output channel displayed sensitive signaling performance (LOD = 13.8 nM) in the detection of PA. The investigation that inferred to the PA-induced selectivity in signalling, higher binding affinity (log K_a≈11), a faster response time, and reversibility in signalling with a counter analyte and an operational pH range established the probe's efficacy as a chemosensor for PA detection.

A novel Schiff base macrocycle based on 1,1'-binaphthyl for fluorescence recognition

A novel chiral polyimine macrocycle C-1 was designed and synthesized by the self-condensation of the dialdehyde of the chiral dinaphtho[2,1-d:1',2'-f][1,3]dioxepine derivative and o-phenylenediamine by Schiff base formation, and the corresponding polyamine macrocycle C-1H was obtained by the reduction of the polyimine macrocycle. The UV-vis and fluorescence spectral studies indicated that both C-1 and C-1H form the complex with metal ions in a 1:2 ratio. The fluorescence behaviour of C-1 upon the addition of Zn²⁺ or Cd²⁺ showed a 'turn-on' response accompanied by fluorescence enhancement at 510 nm six times for Cd²⁺ and 13 times for Zn²⁺ . In contrast, C-1H revealed a 'turn-off' response upon the addition of Co²⁺ , Ni²⁺ , and Cu²⁺ .

In-vitro and in-vivo monitoring of gold(III) ions from intermediate metabolite of sodium aurothiomalate through water-soluble ruthenium (II) complex-based luminescent probe

Real-time monitoring of drug metabolism in vivo is of great significance to drug development and toxicology research. The purpose of this study is to establish a rapid and visual in vivo detection method for the detection of an intermediate metabolite of the gold (I) drug. Gold (I) drugs such as sodium aurothiomalate (AuTM) have anti-inflammatory effects in the treatment of rheumatoid arthritis. Gold(III) ions (Au³⁺) are the intermediate metabolite of gold medicine, and they are also the leading factor of side effects in the treatment of patients. However, the rapid reduction of Au³⁺ to Au⁺ by thiol proteins in organisms limits the in-depth study of metabolism of gold drugs in vivo. Here we describe a luminescence Au³⁺ probe (RA) based on ruthenium (II) complex for detecting Au³⁺ in vitro and in vivo. RA with large Stokes shift, good water solubility and biocompatibility was successfully applied to detect Au³⁺ in living cells and vivo by luminescence imaging, and to trap the fluctuation of Au³⁺ level produced by gold (I) medicine. More importantly, the luminescent probe was used to the detection of the intermediate metabolites of gold (I) drugs for the first time. Overall, this work offers a new detection tool/method for a deeper study of gold (I) drugs metabolite.

Novel integrated sensing system of calixarene and rhodamine molecules for selective colorimetric and fluorometric detection of Hg2+ ions in living cells

Three novel and facile calixarene derivatives (5, 6 and 7), which were appended with four rhodamine units at the upper rim of calixarene skeleton, were firstly prepared and evaluated for selective detection of metal ions in solution. Receptors (5) and (7) indicated immediate turn on fluorescence output toward Hg²⁺ ions over other most competitive metal ions with the ultralow detection limits, indicating their high efficiency and reliability. The binding response to Hg²⁺ ions in solution was also observed through a chromogenic change (from colorless to pale pink). Furthermore, in vitro and bio-imaging studies with MCF-7 or MIA PaCa-2 cell lines were also performed to investigate the use of receptors in biological systems in order to monitor of mercury ions. Results showed that new receptors (5) or (7) were cell permeable and suitable for real-time imaging of Hg²⁺ in living cells (MCF-7) or (MIA PaCa-2) without any damage to healthy cell lines (HEK 293).

New Pyridine-Bridged Ferrocene-Rhodamine Receptor for the Multifeature Detection of Hg2+ in Water and Living Cells

A challenge in the design of optical and redox-active receptors is how to combine a specific recognition center with an efficient responsive system to facilely achieve multifeature detection in biological and environmental analyses. Herein, a novel ferrocene-rhodamine receptor conjugated with a pyridine bridge was designed and synthesized. This receptor can sensitively sense Hg²⁺ in aqueous media via chromogenic, fluorogenic, and electrochemical multisignal outputs with a low detection limit and fast response time. Moreover, it can be qualified as a fluorescent probe for effectively monitoring Hg²⁺ in living cells. A plausible recognition mode was proposed and rationalized with theoretical calculations.

Mechanochemical synthesis of fluorescein-based receptor for CN- ion detection in aqueous solution and cigarette smoke residue

A facile green method for the mechanochemical synthesis of Schiff base phenylhydrazono-N-methylene fluorescein (PHMF) with 95% yields has been established. The synthesized receptor assists in the naked-eye detection of CN^- ions in organic and aqueous media, and F^- ions in acetonitrile over a series of anions with a color transfer from colorless to pink. A redshift of 160 nm of PHMF-CN^- complex in the absorbance spectrum and a turn-on response in the fluorescence spectrum were observed, respectively, at λmax 345 to 515 and 519 nm. A strong interaction of PHMF with CN^- and F^- ions forming a 1:3 binding stoichiometry has been noted in this study. In an aqueous medium for CN^- ion, the lower limit of detection (LOD) is defined as 9.204 nM, which is quite better in terms of sensitivity. In addition, PHMF's paper-strip sensor for rapid real-time CN^- ion sensing was found to be sufficiently sensitive to successfully detect CN^- ion in water and a solid state, resulting in a portable device for detecting cyanide ions. In acetonitrile, the receptor's ability to detect CN^- ion in cigarette smoke residue was also satisfactorily achieved. Graphical Abstract.

Synthesis and Antibacterial Activity of Rhodanine-Based Azo Dyes and Their Use as Spectrophotometric Chemosensor for Fe3+ Ions

This research includes the design and synthesis of new derivatives for rhodanine azo compounds (4a–c) containing a naphthalene ring. Physiochemical properties of the synthesized compounds were determined by their melting points, FTIR, 1H-NMR, 13C-NMR, and elemental analysis spectroscopic techniques. The biological activities of the newly prepared azo rhodanine compounds were evaluated against some pathogenic bacteria using three different bacterial species including (Escherichia coli., Pseudomonas aeruginosa, Staphylococcus aureus) and compared with amoxicillin as a reference drug. The results showed that our compounds have moderate-to-good vital activity against the mentioned pathogenic bacteria. The selectivity and sensitivity of the newly prepared rhodanine azo compounds with transition metals Co2+, Cu2+, Zn2+, Ni2+, and Fe3+ were studied using UV–vis and fluorescence spectroscopy techniques. Among the synthesized azos, azo 4c showed affinity toward Fe3+ ions with an association constant of 4.63 × 108 M−1. Furthermore, this azo showed high sensitivity toward Fe3+ ions with detection limits of 5.14 µM. The molar ratio and Benesi–Hildebrand methods confirmed the formation of complexes between azo 4c and Fe3+ with 1:2 binding stoichiometry. Therefore, azo 4c showed excellent potential for developing efficient Fe3+ chemosensors.

Effect of n-alkyl substitution on Cu(ii)-selective chemosensing of rhodamine B derivatives

Rhodamine B hydrazide-based molecular probes (1-10) were synthesized by derivatization with n-alkyl chains of different lengths at the hydrazide amino end. These probes exhibited selective absorption (A∼557) and fluorescence (I∼580) 'off-on' signal transduction along with a colourless → magenta colour transition in the presence of Cu(ii) ions among all the competitive metal ions investigated. The effective coordination of these probes to Cu(ii) ions under the investigated environment forming [Cu·L]2+ (L = 1-5) and [Cu·L2]2+ (L = 6-10) complexes led to their spiro-ring opening, which in turn was expressed through signatory spectral peaks of ring-opened rhodamine. All these probes exhibited Cu(ii) selectivity in signalling despite structural modifications to the core receptor unit through variation of the nature of the alkyl substituents. However, the sensitivity of the signalling and kinetics of the spiro-ring opening varied and could be correlated with the number of carbon atoms present in the n-alkyl substituents. Structural elucidation with X-ray diffraction and X-ray photoemission spectroscopic analyses provided further insight into the structure-function correlation in their Cu(ii) complexes. These probes with Cu(ii) coordination showed selectivity in signalling, high complexation affinity (log Ka = 4.8-8.8), high sensitivity (LOD = 4.1-80 nM), fast response time (rate = 0.0017-0.0159 s-1) and reversibility with counter anions, which ascertained their potential utility as chemosensors for Cu(ii) ion detection.

Chromogenic and fluorogenic “off–on–off” chemosensor for selective and sensitive detection of aluminum (Al3+) and bifluoride (HF2−) ions in solution and in living Hep G2 cells: synthesis, experimental and theoretical studies

A simple pyridine-dicarbohydrazide based colorimetric and fluorometric chemosensor L was designed and synthesized for Al³⁺ ion sensing in organo aqueous solution.

A colorimetric and “off–on” fluorescent Pd2+ chemosensor based on a rhodamine-ampyrone conjugate: synthesis, experimental and theoretical studies along with in vitro applications

We successfully designed and developed a rhodamine based “turn-on” chemosensor L for the detection of Pd²⁺ ions down to 1.19 × 10⁻⁵ M (11.9 μM).

Fluorescent peptide dH3w: A sensor for environmental monitoring of mercury (II)

Heavy metals are hazardous environmental contaminants, often highly toxic even at extremely low concentrations. Monitoring their presence in environmental samples is an important but complex task that has attracted the attention of many research groups. We have previously developed a fluorescent peptidyl sensor, dH3w, for monitoring Zn2+ in living cells. This probe, designed on the base on the internal repeats of the human histidine rich glycoprotein, shows a turn on response to Zn2+ and a turn off response to Cu2+. Other heavy metals (Mn2+, Fe2+, Ni2+, Co2+, Pb2+ and Cd2+) do not interfere with the detection of Zn2+ and Cu2+. Here we report that dH3w has an affinity for Hg2+ considerably higher than that for Zn2+ or Cu2+, therefore the strong fluorescence of the Zn2+/dH3w complex is quenched when it is exposed to aqueous solutions of Hg2+, allowing the detection of sub-micromolar levels of Hg2+. Fluorescence of the Zn2+/dH3w complex is also quenched by Cu2+ whereas other heavy metals (Mn2+, Fe2+, Ni2+, Co2+, Cd2+, Pb2+, Sn2+ and Cr3+) have no effect. The high affinity and selectivity suggest that dH3w and the Zn2+/dH3w complex are suited as fluorescent sensor for the detection of Hg2+ and Cu2+ in environmental as well as biological samples.

A dual-responsive colorimetric and fluorescent chemosensor based on diketopyrrolopyrrole derivative for naked-eye detection of Fe3+ and its practical application

A novel dual-responsive colorimetric and fluorescent chemosensor L based on diketopyrrolopyrrole derivative for Fe³⁺ detection was designed and synthesized. In presence of Fe³⁺, sensor L displayed strong colorimetric response as amaranth to rose pink and significant fluorescence enhancement and chromogenic change, which served as a naked-eye indicator by an obvious color change from purple to red. The binding constant for L-Fe³⁺ complex was found as 2.4×10⁴ with the lower detection limit of 14.3nM. The sensing mechanism was investigated in detail by fluorescence measurements, IR and ¹H NMR spectra. Sensor L for Fe³⁺ detection also exhibited high anti-interference performance, good reversibility, wide pH response range and instantaneous response time. Furthermore, the sensor L has been used to quantify Fe³⁺ ions in practical water samples with good recovery.

A pyrene-benzimidazole composed effective fluoride sensor: potential mimicking of a Boolean logic gate

A highly selective fluoride sensor based on a pyrene benzimidazole unit was developed and studied for recyclable memory function.

Terpyridine derivatives as “turn-on” fluorescence chemosensors for the selective and sensitive detection of Zn2+ ions in solution and in live cells

A terpyridine based compound L1 was designed and synthesized as an “off–on” chemosensor for the detection of Zn²⁺.

A Unique Sensitive and Highly Selective Fluorescent Naphthodiaza-Crown Macrocyclic Ligand Chemosensor for Hg2+ in Water

The noticeable enhancement in fluorescence emission of O₂N₂-donor naphthodiaza-crown macrocyclic ligand (L) in the presence of Hg²⁺ was observed in which the fluorescence quantum yield of free ligand L as well as L/Hg²⁺ complex were found to be as 0.29 and 0.49, respectively. The observed ultra-low limit of detection (LOD) for Hg²⁺ by L was determined as low as 1.0 × 10^-11 M in water. A 1:1 stoichiometry was also established for L/Hg²⁺ together with a binding constant K _BH = 66,543 by employing fluorescence spectrophotometry. The competition experiments on L/Hg²⁺ demonstrated highly selective detection of Hg²⁺ in the presence of the library cations. A two path mechanism for detection of metal ion in terms of coordination of metal ion to L and/or the formation of counter ion was proposed by using of ¹H NMR and fluorescence spectroscopy. Graphical Abstract pH dependence mechanism of interaction between Hg²⁺ and macrocyclic ligand L.

Spectroscopic Investigation of the Interaction of the Anticancer Drug Mitoxantrone with Sodium Taurodeoxycholate (NaTDC) and Sodium Taurocholate (NaTC) Bile Salts

The focus of the present work was to investigate the interaction of the anticancer drug mitoxantrone with two bile salts, sodium taurodeoxycholate (NaTDC) and sodium taurocholate (NaTC). Ultraviolet-visible (UV-Vis) absorption and electron paramagnetic resonance (EPR) spectroscopy were used to quantify the interaction and to obtain information on the location of mitoxantrone in bile salt micelles. The presence of submicellar concentrations of both bile salts induces mitoxantrone aggregation and the extent of drug aggregation in NaTDC is higher than in NaTC. For micellar bile salts concentrations, mitoxantrone monomers are entrapped in the micellar core. Binding constants, micelle/water partition coefficients and the corresponding thermodynamic parameters for binding and partitioning processes were estimated using the changes in monomer absorbance in the presence of bile salts. Binding interaction of mitoxantrone is stronger for NaTDC than NaTC micelles, whereas partitioning efficiency is higher for NaTC micelles for all investigated temperatures. Thermodynamic parameters indicate that both binding and partitioning processes are spontaneous and entropy controlled. The spectral behavior and thermodynamic parameters indicate distinct types of mitoxantrone interaction with NaTDC and NaTC micelles supported by the differences in nature and structure of bile salts micelles.

Physicochemical aspects of the energetics of binding of sulphanilic acid with bovine serum albumin

The thermodynamic study of the binding of sulphanilic acid with model transport protein bovine serum albumin is a promising approach in the area of synthesizing new sulfa drugs with improved therapeutic effect. Thus, such binding studies play an important role in the rational drug design process. The binding between sulphanilic acid and bovine serum albumin has been studied using calorimetry, light scattering in combination with spectroscopic and microscopic techniques. The calorimetric data reveals the presence of two sequential nature of binding sites where the first binding site has stronger affinity (~10(4)M(-1)) and second binding site has weaker affinity (~10(3)M(-1)). However, the spectroscopic (absorption and fluorescence) results suggest the presence of single low affinity binding site (~10(3)M(-1)) on protein. The contribution of polar and non-polar interactions to the binding process has been explored in the presence of various additives. It is found that sulphanilic acid binds with high affinity at Sudlow site II and with low affinity at Sudlow site I of protein. Light scattering and circular dichroism measurements have been used to study the effect on the molecular topology and conformation of protein, respectively. Thus these studies provide important insights into the binding of sulphanilic acid with bovine serum albumin both quantitatively and qualitatively.

Functionalized calix[4]arene as a colorimetric dual sensor for Cu(ii) and cysteine in aqueous media: experimental and computational study

The sensors developed detect Cu²⁺and the metal complex recognizes cysteine, detectable by the naked eye, and DFT calculations corroborate the experimental results.