Cd2+-Specific Fluorescence Response of Methoxy-Substituted N,N-Bis(2-quinolylmethyl)-2-methoxyaniline Derivatives

The N3O1 tetradentate ligand, TriMeOBQMOA (N,N-bis(5,6,7-trimethoxy-2-quinolylmethyl)-2-methoxyaniline), was developed as a Cd2+-specific fluorescent sensor. The structure of TriMeOBQMOA is half of TriMeOBAPTQ (N,N,N',N'-tetrakis(5,6,7-trimethoxy-2-quinolylmethyl)-1,2-bis(2-aminophenoxy)ethane), which is a tetrakisquinoline derivative of the well-known calcium chelator BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid). The fluorescent Cd2+ selectivity of TriMeOBAPTQ (IZn/ICd = 5.3% in the presence of 3 equiv of metal ions in MeOH-HEPES buffer (9:1)) comes from the formation of fluorescent dinuclear cadmium (M2L) and nonfluorescent OH-bridged dizinc ((μ-OH)M2L) complexes. TriMeOBQMOA also exhibits excellent Cd2+ specificity in fluorescence enhancement (IZn/ICd = 2.3% in the presence of 5 equiv of metal ions in DMF-HEPES buffer (1:1, HEPES 50 mM, KCl 0.1 M, pH = 7.5)) via substantial formation of a highly fluorescent bis(μ-chloro)dinuclear cadmium complex ([Cd2(μ-Cl)2L2]2+), which is in equilibrium with the mononuclear Cd2+ complex ([CdLCl]+), and extremely poor stability of the TriMeOBQMOA-Zn2+ complex. The all-nitrogen derivatives of BQMOA and BAPTQ, namely, N,N-BQDMPHEN (N,N-bis(2-quinolylmethyl)-N',N'-dimethyl-1,2-phenylenediamine) and BPDTQ (N,N,N',N'-tetrakis(2-quinolylmethyl)-2,2'-(N,N'-dimethylethylenediamino)dianiline), respectively, and their methoxy-substituted derivatives were also prepared, and the fluorescent metal ion sensing properties are discussed.

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.

A phenazine-imidazole based ratiometric fluorescent probe for Cd2+ ions and its application in in vivo imaging

A new phenazine-imidazole based Schiff base (PIS) fluorescent probe has been developed for the ratiometric detection of Cd²⁺ ions in aqueous media at physiological pH. PIS upon binding with Cd²⁺ ions shows red shifted fluorescence and thereby, permits ratiometric detection of Cd(II) ions. A detection limit down to 2.10 × 10^-8 M was determined for Cd²⁺ quantitation. Also, the accompanying apparent fluorescence color change (from yellow to orange red) is noticeable to the naked eye under a UV-lamp. The sensing mechanism could be attributed to the 1 : 1 PIS-Cd complexation, followed by extension of the conjugation due to better planarity and modulation of the charge transfer efficiency in the probe. This was complemented by solvatochromism and density functional theory calculations. Furthermore, PIS was used to detect Cd²⁺ in Oxya chinensis cells, zebrafish larvae and live tissues of Arabidopsis thaliana under a fluorescence microscope, showing great potential in analyzing living biosystems.

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.

N,N-bis(2-quinolinylmethyl)benzylamine

N,N-bis(2-quinolinylmethyl)benzylamine (1) was synthesized under basic conditions from a pseudo-three-component reaction between benzylamine and two molecules of 2-(quinolinylmethyl)chloride, resulting in the formation of two N–C bonds in a single step. Compound (1) crystallizes in the triclinic system of the P-1 space group. The unit cell comprises a dimer of 1, in which the monomers are linked by two complementary hydrogen bonds between N1 and H′1–C′1 of another molecule. The dimers form chains along the a-axis through intermolecular interactions between the N′2 acceptor atoms and C″17 donors from molecules in the nearest neighboring dimer. These interactions form extended sheets of the dimers of 1, along the ab plane. The quinolinylmeth-2-yl groups of 1 lie in almost orthogonal planes and their N1/2(q) donor atoms are away from the apical amino N3 atom.

Aza- and Mixed Thia/Aza-Macrocyclic Receptors with Quinoline-Bearing Pendant Arms for Optical Discrimination of Zinc(II) or Cadmium(II) Ions

The synthesis and coordination properties of two fluorescent chemosensors, featuring [9]aneN₃ (1,4,7-triazacyclononane; L1) and [12]aneNS₃ (1-aza-4,7,10-trithiacyclododecane; L2) as receptor units, and a quinoline pendant arm with an amide group as a functional group spacer are described. The optical responses of L1 and L2 in the presence of several metal ions were analysed in MeCN/H₂ O (1 : 4 v/v) solutions. A selective chelation enhancement of fluorescence (CHEF) effect was observed in the presence of Zn²⁺ in the case of L1, and in the presence of Cd²⁺ in the case of L2, following the formation of a 1 : 1 and a 1 : 2 metal/ligand complex, respectively, which was also confirmed by potentiometric measurements. ¹ H and ¹³ C NMR measurements in CD₃ CN/CDCl₃ in combination with molecular mechanics calculations show that for both complexes of L1 and L2 with Zn²⁺ and Cd²⁺ , respectively, the coordination of the carbonyl group from the pendant arm could be the origin of the observed optical selectivity.

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.

Quinoline- and isoquinoline-derived ligand design on TQEN (N,N,N′,N′-tetrakis(2-quinolylmethyl)ethylenediamine) platform for fluorescent sensing of specific metal ions and phosphate species

Utilizing the unique metal-binding and fluorescent properties of methoxy-substituted (iso)quinolines, varieties of fluorescent probes were developed from TQEN (N,N,N′,N′-tetrakis(2-quinolylmethyl)ethylenediamine) structure.

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.

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).

Fabrication of an "ion-imprinting" dual-emission quantum dot nanohybrid for selective fluorescence turn-on and ratiometric detection of cadmium ions

In this work, we have fabricated a new dual-emission quantum dot (QD) nanohybrid for fluorescence ratiometric determination of cadmium ions (Cd²⁺) in water samples, where the "turn-on" model and "ion-imprinting" technique were incorporated simultaneously. The nanohybrid probe was composed of green-emitting CdSe QDs covalently linked onto the surface of silica nanoparticles embedded with red-emitting CdTe QDs. The chemical etching of ethylene diamine tetraacetic acid (EDTA) at the surface produced specific Cd²⁺ recognition sites and quenched the green fluorescence of outer CdSe QDs. Upon exposure to different amounts of Cd²⁺, the green fluorescence was gradually restored, whereas the inner red fluorescence remained constant. As a consequence, an obviously distinguishable fluorescence color variation (from red to green) of the probe solution was observed. Under the optimized conditions, the developed ratiometric sensor displayed a linear response range from 0.1 to 9 μM with a detection limit of 25 nM (S/N = 3) for Cd²⁺, which could offer an alternative sensing approach for the highly sensitive and selective detection of heavy metal ions.

A non-symmetric pillar[5]arene based on triazole-linked 8-oxyquinolines as a sequential sensor for thorium(iv) followed by fluoride ions

A novel non-symmetric pillar[5]arene bearing triazole-linked 8-oxyquinolines at one rim was synthesized and demonstrated as a sequential fluorescence sensor for thorium(iv) followed by fluoride ions with high sensitivity and selectivity.