Highly selective and sensitive near-infrared fluorescent sensors for cadmium in aqueous solution

A 1,8-Naphthalimide-based Tripodal Fluorescent Chemosensor to Selectively Detect Copper Ions

A 1,8-naphthalimide-based tripodal fluorescent ligand (L3) was synthesized through the copper (I) catalyzed Huisgen azide-alkyne cycloaddition reaction of 2-(2-azidoethyl)-6-morpholino-1 H-benzo[de]isoquinoline-1,3(2 H)-dione with triproparagylamine. Naphthalimide acts as the fluorophore while the triazole and amine nitrogens chelate the metal ion. L3 showed a selective fluorescence turn-off for Cu(II) over other metal ions in aqueous acetonitrile solution. A Job's plot, Benesi-Hildbrand plot and high-resolution mass spectrometry data confirm a 1:1 binding stoichiometry with a binding constant of 7.8 х105 M- 1 while addition of disodium EDTA demonstrates its reversibility. The structure and stability of the complex was supported by theoretical calculations. The limit of detection for Cu(II) was calculated to be 0.3 µM which is considerably lower than WHO recommended Cu(II) limit in drinking water.

Cellulose-based adsorbent using in mercury detection and removal from water via an efficient grafting strategy of fluorometric sensors by click reaction

Mercury pollution in waters attracts lots of attention due to its serious toxicity and high bioenrichment and many efforts have been devoted in the development of adsorbents for mercury detection and removal. Herein, a cellulose-based adsorbent Cell-TriA-HQ is functionalized with quinoline fluorophore by covalent immobilization through "Click reaction" with high yield. In addition to the admirable adsorptive performance, the prepared adsorbent exhibits excellent selectivity and sensitivity towards Hg (II) in water that the detection limit for Hg (II) is determined to be as low as 1.92 × 10-7 M. The sensitive fluorescence enhancement response is considered to be resulted from the inhibition of photo-induced electron transfer between triazole and quinoline groups and the reinforcement of structural rigidity. The easy manipulation along with excellent performance of adsorption capacity, detective ability and reusability for the multifunctional adsorbent makes it potential in mercury monitoring and removal from aqueous solutions in the field of water treatment.

Rapid Detection of Cd2+ Ions in the Aqueous Medium Using a Highly Sensitive and Selective Turn-On Fluorescent Chemosensor

Herein, a novel optical chemosensor, (CM1 = 2, 6-di((E)-benzylidene)-4-methylcyclohexan-1-one), was designed/synthesized and characterized by ¹H-NMR and FT-IR spectroscopy. The experimental observations indicated that CM1 is an efficient and selective chemosensor towards Cd²⁺, even in the presence of other metal ions, such as Mn²⁺, Cu²⁺, Co^2+, Ce³⁺, K⁺, Hg²⁺,^, and Zn²⁺ in the aqueous medium. The newly synthesized chemosensor, CM1, showed a significant change in the fluorescence emission spectrum upon coordination with Cd²⁺. The formation of the Cd²⁺ complex with CM1 was confirmed from the fluorometric response. The 1:2 combination of Cd²⁺ with CM1 was found optimum for the desired optical properties, which was confirmed through fluorescent titration, Job's plot, and DFT calculation. Moreover, CM1 showed high sensitivity towards Cd²⁺ with a very low detection limit (19.25 nM). Additionally, the CM1 was recovered and recycled by the addition of EDTA solution that combines with Cd²⁺ ion and, hence, frees up the chemosensor.

Aza-BODIPY-based Fluorescent and Colorimetric Sensors and Probes

Aza-boron-dipyrromethenes (Aza-BODIPYs) represent an important class of chromophores absorbing and emitting in the near-infrared (NIR) region. They have unique optical and electronic features and higher physiological and photo stability than other NIR dyes. Especially after the development of facile synthetic routes, Aza-BODIPYs have become indispensable fluors that can find various applications ranging from chemosensors, bioimaging, phototherapy, solar energy materials, photocatalysis, photon upconversion, lasers, and optoelectronics. Herein, we review Aza-BODIPY based fluorescent and colorimetric chemosensors. We show the potential and untapped toolbox of Aza-BODIPY based fluorescent and colorimetric chemosensors. Hence, we divide the fluorescent and colorimetric chemosensors and probes into five sections according to the target analytes. The first section begins with the chemosensors developed for pH. Next, we discuss Aza-BODIPY based ion sensors, including metal ions and anions. Finally, we present the chemosensors and probes concerning reactive oxygen (ROS) and nitrogen species (RNS) along with biologically relevant species in the last two sections. We believe that Aza-BODIPYs are still in their infancy, and they have a promising future for translation from the bench to real biomedical and materials science applications. After two decades of intensive research, it seems that there are many more to come in this already fertile field. Overall, we hope that future work will further expand the applications of Aza-BODIPY in many areas.

Cd2+-selective fluorescence response of TQEN (N,N,N′,N′-tetrakis(2-quinolylmethyl)ethylenediamine) derivatives bearing ether oxygen-binding sites

Introduction of a methoxymethyloxy (MOMO) group at one of the four quinoline rings of TQEN (N,N,N′,N′-tetrakis-(2-quinolylmethyl)ethylenediamine) affords Cd2+-selective fluorescence enhancement.

Strategy to Design a Flexible and Macromolecular Sensor to Bind Cd2+ Ions: A Complete Photophysical Analysis and Bio-Imaging Study

A novel triazole-bridged coumarin-benzimidazole-conjugated fluorescence sensor (4) has been developed for selective detection of Cd2+ over other competitive metal ions. The sensor exhibited quick "turn-on" responses upon interaction with a very low level of Cd2+ (14 nM). The photophysical changes in the complexation of Cd2+ with sensor 4 have been explained through the excited-state intramolecular proton transfer mechanism. The involvement of benzimidazole and triazole moieties in Cd2+ binding was confirmed by different spectroscopic techniques such as UV-vis, Fourier transform infrared, nuclear magnetic resonance, and ESI mass. The diameter of the circular shape of the sensor decreased upon complexation with Cd2+, which was confirmed by field-emission scanning electron microscopy. Furthermore, the quantum chemical (density functional theory) calculation supported the mechanism of interactions and the mode of binding of 4 toward Cd2+. The sensor was more effective for finding Cd2+ in two living cells, C6 (rat glial cell) and Hep G2 (human liver cell).

Rapid and visual detection of Cd2+ based on aza-BODIPY near infrared dye and its application in real and biological samples for environmental contamination screening

Cadmium highly toxic and hazardous, and it can adversely affect human health leading to serious disorders. Herein, a water-soluble near-infrared sensor based on aza-BODIPY (1) was developed for dual determination of Cd²⁺ in environmental and biological media. This sensor exhibited color change from colorless to green along with a fluorescence enhancement in the near-infrared (NIR) region via photoinduced electron transfer (PET) after complexation with Cd²⁺. Sensor 1 can be employed in aqueous media at physiological pH for quantitative monitoring. It shows rapid response with high sensitivity (detection limit of 2.8 ppb; linear correlation over [Cd²⁺] 1.33 - 6.67 µM) and selectivity over potentially interfering ions. NIR sensor 1 can be used to determine [Cd²⁺] in living cells and environmental samples.

Tuned Cd2+ Selectivity: Showcase of Electronic and Regio-Effect of π-Extended Di-2-Picolylamine-Substituted Quinoline-Based Tolans

π-Extended di-2-picolylamine (DPA)-substituted 8-hydroxyquinoline (8-HQ) tolans (2) were synthesized for testing electronic and regio-effects. The electron-poor CN-tolan (2b) showed clear selectivity for Cd2+ (>>Zn2+) over other metal ions via turn-on fluorescence, while the electron-rich MeO-tolan (2a) displayed no clear metal selectivity. Furthermore, considering that there was no significant energy difference between the Cd2+ complexes of 1 and 2b, the intended regio-effect (7- vs. 5-substituted effect) did not induce steric hindrance. Thus, the regio-effect is mainly electronic. Considering the above, 2a and 2b constitute a complete showcase in which electronic and regio-effects modulate the metal selectivity. The fluorescence titration of 2b (10 mM) with Cd2+ showed that the limit of detection (LOD) of the Cd2+-selective 2b was 158 nM in PBS (phosphate-buffered saline) (10 mM, pH 7.2) containing 50% MeOH.

Laurdan and Di-4-ANEPPDHQ Influence the Properties of Lipid Membranes: A Classical Molecular Dynamics and Fluorescence Study

Environmentally sensitive (ES) dyes have been used for many decades to study the lipid order of cell membranes, as different lipid phases play a crucial role in a wide variety of cell processes. Yet, the understanding of how ES dyes behave, interact, and affect membranes at the atomistic scale is lacking, partially due to the lack of molecular dynamics (MD) models of these dyes. Here, we present ground- and excited-state MD models of commonly used ES dyes, Laurdan and di-4-ANEPPDHQ, and use MD simulations to study the behavior of these dyes in a disordered and an ordered membrane. We also investigate the effect that these two dyes have on the hydration and lipid order of the membranes, where we see a significant effect on the hydration of lipids proximal to the dyes. These findings are combined with experimental fluorescence experiments of ordered and disordered vesicles and live HeLa cells stained by the aforementioned dyes, where the generalized polarization (GP) values were measured at different concentrations of the dyes. We observe a small but significant decrease of GP at higher Laurdan concentrations in vesicles, while the same effect is not observed in cell membranes. The opposite effect is observed with di-4-ANEPPDHQ where no significant change in GP is seen for vesicles but a very substantial and significant decrease is seen in cell membranes. Together, our results show the profound effect that ES dyes have on membranes, and the presented MD models will be important for further understanding of these effects.

Research progress of near-infrared fluorescence probes based on indole heptamethine cyanine dyes in vivo and in vitro

Near-infrared (NIR) fluorescence imaging is a noninvasive technique that provides numerous advantages for the real-time in vivo monitoring of biological information in living subjects without the use of ionizing radiation. Near-infrared fluorescent (NIRF) dyes are widely used as fluorescent imaging probes. These fluorescent dyes remarkably decrease the interference caused by the self-absorption of substances and autofluorescence, increase detection selectivity and sensitivity, and reduce damage to the human body. Thus, they are beneficial for bioassays. Indole heptamethine cyanine dyes are widely investigated in the field of near-infrared fluorescence imaging. They are mainly composed of indole heterocyclics, heptamethine chains, and N-substituent side chains. With indole heptamethine cyanine dyes as the parent, introducing reactive groups to the parent compounds or changing their structures can make fluorescent probes have different functions like labeling protein and tumor, detecting intracellular metal cations, which has become the hotspot in the field of fluorescence imaging of biological research. Therefore, this study reviewed the applications of indole heptamethine cyanine fluorescent probes to metal cation detection, pH, molecules, tumor imaging, and protein in vivo. The distribution, imaging results, and metabolism of the probes in vivo and in vitro were described. The biological application trends and existing problems of fluorescent probes were discussed.

Unusual absence of FRET in triazole bridged coumarin-hydroxyquinoline, an active sensor for Hg2+ detection

A triazole-bridged coumarin conjugated quinoline sensor has been 'click'-synthesized by Cu(i) catalyzed Huisgen cycloaddition, and it exhibited high selectivity for toxic Hg2+. Surprisingly, no evidence of energy transfer from the quinoline moiety to coumarin has been found, substantiated by time-resolved fluorescence study. The possible binding mode of this sensor to Hg2+ has been established via NMR study, steady-state and time-resolved fluorescence spectroscopy, which is further supported by TDDFT calculations. The sensor has been found to be cell membrane permeable and non-toxic, and hence is suitable for intracellular Hg2+ detection.

Differential Interaction of Metal Ions with Gold Nanoclusters and Application in Detection of Cobalt and Cadmium

Interest in biosensing platforms using protein fluorescent gold nanoclusters (FGNCs) has grown significantly in the past due to the unique optical properties they offer. This study investigates the interaction of metal ions with FGNCs, and the structural modifications brought about by the interaction resulting in fluorescence changes of the cluster and its successful application in the detection of two heavy metals, cobalt and cadmium. The binding of cobalt and cadmium to FGNCs synthesized from BSA significantly altered the secondary structure of the protein, causing a change in its hydrophobicity. It also resulted in a change in fluorescence properties of FGNCs by intersystem crossing (ICT) and fluorescence resonance energy transfer (FRET). Cobalt and cadmium could successfully be detected in the range of 5-165 ng/mL (R² = 0.95) and 20-1000 ng/ mL (R² = 0.91), respectively, with appreciable sensitivity. The principle was also applied for the detection of Vitamin B₁₂ in commercially available ampoules, validating the proposed method. Graphical Abstract Proposed detection method of cadmium and cobalt using FGNCs.

Multianalyte azo dye as an on-site assay kit for colorimetric detection of Hg2+ions and electrochemical sensing of Zn2+ ions

A new tailor-made colorimetric chemosensor, (E)-1-(benzo[d]thiazol-2-yl)-3-(pyridin-3-yldiazenyl)naphthalen-2-ol (1), containing benzothiazole and pyridine moieties connected through an azo (-N=N-) linkage has been designed and synthesized. The synthesized chemosensor displayed an eye-catching color change upon binding to acetate [AcO^-;colorless to russet] and mercury (II) [Hg²⁺;colorless to greenish blue] ions in 9:1 (v/v) aqueous CH₃CN (pH 7.0 HEPES buffer).The mechanism of interaction between the chemosensor and the Hg²⁺/AcO^- ions has been confirmed by ¹H NMR titration experiments. Moreover, the colorimetric chemosensor 1 displayed potential in-field applications as on-site assay kit and detection of Hg²⁺ ions in real water samples. Importantly chemosensor 1 gave selective electrochemical response towards Zn²⁺ ions, enabling simple azo-dye 1 as multichannel chemosensor for colorimetric detection of Hg²⁺ ions and electrochemical detection of Zn²⁺ ions.

Switching of Fluorescent Zn/Cd Selectivity in N,N,N',N'-Tetrakis(6-methoxy-2-quinolylmethyl)-1,2-diphenylethylenediamine by One Asymmetric Carbon Atom Inversion

A quinoline-based hexadentate ligand, (S,S)-N,N,N',N'-tetrakis(6-methoxy-2-quinolylmethyl)-1,2-diphenylethylenediamine ((S,S)-6-MeOTQPh₂EN), exhibits fluorescence enhancement at 498 nm upon addition of 1 equiv of Zn²⁺ (I_Zn/I₀ = 12, φ_Zn = 0.047) in aqueous DMF solution (DMF/H₂O = 2:1). Addition of 1 equiv of Cd²⁺ affords a much smaller fluorescence increase at the same wavelength (I_Cd/I₀ = 2.5, I_Cd/I_Zn = 21%). The trivalent metal ions such as Al³⁺, Cr³⁺, and Fe³⁺ also exhibit fluorescence enhancement at 395 nm (I_Al/I₀ = 22, I_Cr/I₀ = 6 and I_Fe3+/I₀ = 13). In contrast, meso-6-MeOTQPh₂EN exhibits a Cd²⁺-selective fluorescence increase at 405 nm in the presence of 1 equiv of metal ion (I_Cd/I₀ = 11.5, φ_Cd = 0.022), while Zn²⁺ induces a smaller fluorescent response under the same experimental conditions (I_Zn/I₀ = 3.3, I_Zn/I_Cd = 29%). In this case, the fluorescence intensities of meso-6-MeOTQPh₂EN in the presence of a large amount of Zn²⁺ and Cd²⁺ become similar. This diastereomer-dependent, fluorescent metal ion specificity is derived from the Zn²⁺-specific intramolecular excimer formation in (S,S)-6-MeOTQPh₂EN-Zn²⁺ complex and higher binding affinity of meso-6-MeOTQPh₂EN with Cd²⁺ in comparison to Zn²⁺. The more conformationally restricted diastereomeric pair, namely, cis- and trans-TQDACHs (cis- and trans-N,N,N',N'-tetrakis(2-quinolylmethyl)-1,2-diaminocyclohexanes), both exhibit Zn²⁺-specific fluorescence enhancement because of the high metal binding affinity and intramolecular excimer forming property derived from the rigid DACH backbone.

A water-soluble BODIPY based 'OFF/ON' fluorescent probe for the detection of Cd2+ ions with high selectivity and sensitivity

A water-soluble dilithium salt BODIPY derivative (LiBDP) with appended dicarboxylate pseudo-crown ether [NO4] coordinating sites has been designed, synthesized and characterized successfully for the selective and sensitive recognition of Cd2+ in aqueous media. The chemosensor exhibits a remarkable increase in fluorescence intensity as well as a distinct color change upon the addition of Cd2+ over other environmentally and biologically relevant metal ions in H2O. The fluorometric response of LiBDP is attributed to the metal chelation-enhanced fluorescence (MCHEF) effect which has been confirmed by a strong association constant of 2.57 ± 1.06 × 105 M-1 and Job's plot, indicating 1 : 1 binding stoichiometry between LiBDP and Cd2+. Frontier molecular orbital analysis (obtained from DFT studies) also illustrates the turn-on fluorescence of the probe by blocking photoinduced electron transfer (PET) after coordination to Cd2+. The probe can detect Cd2+ in a competitive environment up to a submicromolar level in a biologically significant pH range. The sensor is proved to be reversible and reusable by the alternative addition of Cd2+ followed by S2-. The OFF/ON/OFF sensing behavior is utilized to construct an INHIBIT molecular logic gate based on the two inputs of Cd2+ and S2- and a fluorescence intensity at 512 nm as an output. The test paper experiment demonstrates the practical utility of LiBDP to monitor Cd2+ in an aqueous sample. Finally, the sensing probe was utilized to monitor Cd2+ in living cells.

Methoxy-substituted tetrakisquinoline analogs of EGTA and BAPTA for fluorescence detection of Cd2+

A 5,6,7-trimethoxyquinoline-based octadentate ligand with a BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid) structure exhibits Cd²⁺-specific fluorescence enhancement.

Fluorescent probes guided by a new practical performance regulation strategy to monitor glutathione in living systems

A practical regulation strategy for the design of glutathione specific probes and their application in living systems.

Glutathione (GSH) plays an important role in the body's biochemical defense system, and the detection of GSH in a physiological system is an important tool for understanding redox homeostasis. Protection–deprotection strategies have proven to be the most reliable, among existing detection methods. However, the understanding of how various electronic and steric effects influence a probe's ability to recognize a substrate is still lacking. In this study, we have analyzed various substituent effects on a GSH probe template via theoretical calculations and constructed the performance regulation and control strategy for this kind of probe. We then developed a series of guided probes using eighteen different acrylic ester derivatives to mask the fluorescence of fluorescein. The optical performance differences between the guided probes strongly supported the applicability of our proposed guiding strategy. Moreover, the positively guided probes are excellent for imaging GSH distribution in living cells and mice.

Replacement of quinolines with isoquinolines affords target metal ion switching from Zn2+ to Cd2+ in the fluorescent sensor TQLN (N,N,N',N'-tetrakis(2-quinolylmethyl)-2,6-bis(aminomethyl)pyridine)

A quinoline-based heptadentate ligand, N,N,N',N'-tetrakis(2-quinolylmethyl)-2,6-bis(aminomethyl)pyridine (TQLN), exhibits a Zn²⁺-specific fluorescence increase at 428 nm, which is assigned to excimer emission (I_Zn/I₀ = 38, I_Cd/I_Zn = 24%, ϕ_Zn = 0.069). In contrast, the isoquinoline counterpart 1-isoTQLN exhibits a Cd²⁺-specific fluorescence increase at 365 nm attributable to monomer emission (I_Cd/I₀ = 83, I_Zn/I_Cd = 19%, ϕ_Cd = 0.015).

Green silver nanoparticles as a multifunctional sensor for toxic Cd(ii) ions

Silver nanoparticles (AgNPs) were synthesized using Allium sativum (AS) extract. The AgNP-AS was able to detect Cd(ii) ions with various techniques such as optical, fluorescence and electrochemical sensing. The limit of detection was found to be 0.277 μM. Silver nanoparticles were able to quantify Cd(ii) ions from environmental samples. The antibacterial activity of AgNP-AS was explored towards waterborne bacteria.

Intracellular detection of hazardous Cd2+ through a fluorescence imaging technique by using a nontoxic coumarin based sensor

A new coumarin based turn on fluorescent sensor (R1) was reported for the detection of highly hazardous Cd²⁺ with excellent selectivity and sensitivity without any interference of other metal ions.

A 4,5-quinolimide-based fluorescent sensor for the turn-on detection of Cd2+ with live-cell imaging

The first 4,5-quinolimide sensor, BNA, bearing the amide-DPA receptor, was reported for sensing Cd²⁺ with live-cell imaging.

A real time colorimetric ‘two in one’ kit for tracking ppb levels of uric acid and Hg2+ in live HeLa S3 cells and Hg2+ induced keto–enol tautomerism

A TBET based ‘turn-on’ fluorescent reagent with high Stokes shifts, exhibits selective and colorimetric detection of biologically important uric acid as well as toxic Hg2+.

Near-infrared cyanine-based sensor for Fe3+ with high sensitivity: its intracellular imaging application in colorectal cancer cells

A cyanine-based probe with N-(2-hydroxyethyl) amide arms was designed for Fe³⁺ with a remarkable colorimetric and fluorometric response. It was successfully applied to the imaging of Fe³⁺ ions in colorectal cancer cells.

Lead ion induced chemodosimeter approach of a tripodal hydroxyl-quinoline based phospho-ester through P–O bond cleavage

Pb²⁺ induced phosphoester hydrolysis followed by the formation of a penta coordinated chelate complex of lead.

A selective fluorescent probe for the detection of Cd2+ in different buffer solutions and water

A fluorescent probe NHQ, which exhibited excellent selectivity toward Cd²⁺ in different buffer solutions such as Tris-HCl buffer solution, HEPES buffer solution, and PBS buffer solution, and even in water, was developed.