Vimal K. Bhardwaj

Highly selective turn-on fluorescent sensor for nanomolar detection of biologically important Zn2+ based on isonicotinohydrazide derivative: application in cellular imaging.

A new Zn(2+) selective chemosensor (3) was synthesized by condensation of commercially available substituted salicylaldehyde and isonicotinohydrazide, and characterized by single crystal X-ray crystallography. Receptor 3 with Zn(2+) exhibited a highly selective and pronounced enhancement in the fluorescence emission among different cations by forming a 2:1 complex. The receptor can detect Zn(2+) up to nanomolar level (6.75 nM) with good tolerance of other metal ions and can be used for in vitro cellular imaging.

Comparative DNA binding abilities and phosphatase-like activities of mono-, di-, and trinuclear Ni(II) complexes: the influence of ligand denticity, metal-metal distance, and coordinating solvent/anion on kinetics studies.

Six novel Ni(II) complexes, namely, [Ni2(HL(1))(OAc)2] (1), [Ni3L(1)2]·H2O·2CH3CN (2), [Ni2(L(2))(L(3))(CH3CN)] (3), [Ni2(L(2))2(H2O)2] (4), [Ni2(L(2))2(DMF)2]2·2H2O (5), and [Ni(HL(2))2]·H2O (6), were synthesized by reacting nitrophenol-based tripodal (H3L(1)) and dipodal (H2L(2)) Schiff base ligands with Ni(II) metal salts at ambient conditions. All the complexes were fully characterized with different spectroscopic techniques such as elemental analyses, IR, UV-vis spectroscopy, and electrospray ionization mass spectrometry. The solid-state structures of 2, 3, 5, and 6 were determined using single-crystal X-ray crystallography. The compounds 1, 3, 4, and 5 are dinuclear complexes where the two Ni(II) centers have octahedral geometry with bridging phenoxo groups. Compound 2 is a trinuclear complex with two different types of Ni(II) centers. In compound 3 one of the Ni(II) centers has a coordinated acetonitrile molecule, whereas in compound 4, a water molecule has occupied one coordination site of each Ni(II) center. In complex 5, the coordinated water of complex 4 was displaced by the dimethylformamide (DMF) during its crystallization. Complex 6 is mononuclear with two amine-bis(phenolate) ligands in scissorlike fashion around the Ni(II) metal center. The single crystals of 1 and 4 could not be obtained; however, from the spectroscopic data and physicochemical properties (electronic and redox properties) it was assumed that the structures of these complexes are quite similar to other analogues. DNA binding abilities and phosphatase-like activities of all characterized complexes were also investigated. The ligand denticity, coordinated anions/solvents (such as acetate, acetonitrile, water, and DMF), and cooperative action of two metal centers play a significant role in the phosphate ester bond cleavage of 2-hydroxypropyl-p-nitropenylphosphate by transesterification mechanism. Complex 3 exhibits highest activity among complexes 1-6 with 3.86 × 10(5) times greater rate enhancement than uncatalyzed reaction.

Fluorescent organic nanoparticles (FONs) of rhodamine-appended dipodal derivative: highly sensitive fluorescent sensor for the detection of Hg2+ in aqueous media

A rhodamine-based receptor consisting of a dipodal framework and a pseudo-cavity compatible with cation binding has exhibited fluorescence recognition of Hg2+ with a detection limit of 0.1 nm in a buffered aqueous medium. The Hg2+ coordination in the receptor pseudocavity resulted in the enhancement of fluorescence intensity due to spirolactam ring-opening.

New tripodal and dipodal colorimetric sensors for anions based on tris/bis-urea/thiourea moieties

Seven neutral tripodal and dipodal receptors having mesitylene/triethylbenzene as core moiety, urea/thiourea as binding groups and p-nitrobenzene as signalling unit have been reported. The receptors act as selective colorimetric, naked eye sensors for small and spherical F− ion with some interference from tetrahedral ions. Thiourea derivatives form stable 1:1 H-bonded complexes with F− anion and to some extent with anions, whereas for urea derivatives, the recognition is simply based on acid–base reaction between ureidic protons and basic F− ions and is a completely reversible phenomenon. The pre-organisation of thiourea derivatives coupled with their high-intrinsic acidity is supposed to help them in the formation of strong H-bonded complexes with F− anion. The urea-based receptors do not respond at lower concentrations of the anion, but at higher concentrations, they undergo completely reversible deprotonation concomitant with a colorimetric change, with the production of stable [HF2]− anion.

Imine-linked chemosensors for the detection of Zn2+ in biological samples

A chemosensor 1 with a long hydrocarbon chain and polar end group is synthesized by the simple condensation reaction of a long chain amine with salicylaldehyde. A long chain hydrocarbon with a polar end group is used because of its solubility in an aqueous surfactant solution, which ensures that it can be used in a neutral water medium. The rationale for choosing an aryl aldehyde with –OH functionality is based upon the fact that a chelate ring consisting of an –OH group and an sp2 nitrogen donor is always better for the selective recognition of Zn2+. The sensor shows selective binding to Zn2+ in 1% Triton-X-100 solution. Binding of Zn2+ by sensor 3 leads to an approximately 300% enhancement in the fluorescence intensity of the sensor, due to the combined effects of excited state intramolecular proton transfer (ESIPT) and the inhibition of the photo-induced electron transfer (PET) process by the –OH group. The fluorescence emission profiles of sensor 1 show some changes in the low and high pH ranges, however the sensor remains stable in the pH range 4–9, which makes it appropriate for use in biological fluids.

Ratiometric fluorescent probe for biothiol in aqueous medium with fluorescent organic nanoparticles

A dipodal rhodamine-based mercury complex have been designed and synthesized, for the selective detection of 3-mercaptopropionic acid (MPA). To avoid the poor solubility of rhodamine-based ligand in pure water, the Hg(2+) complex of fluorescent organic nanoparticles (FONs) of ligand have been developed using reprecipitation method and the formation of 1:1 complex has been confirmed with various spectroscopic techniques. The resultant chemosensor can detect MPA in a concentration range of 60 nM-1 μM (in buffered aqueous medium) with detection limit of 60 nM.

Synthesis and optical characterization of ZnO nanoparticles capped with 2-aminothiols

ZnO nanoparticles were synthesized by precipitation method. To reduce the agglomeration among small ZnO nanoparticles, an efficient surface modification method was proposed using 2-aminothiols as a capping agent. The effect of capping reagent is investigated on optoelectronics properties of ZnO. The capping of ZnO with 2-aminothiol leads to the shift in fluorescence intensity and also effected the UV–vis spectra of ZnO. The strategy exposed new dimensions to fine tune the fluorescence signatures of the ZnO.

Influence of surface modification by 2-aminothiophenol on optoelectronics properties of ZnO nanoparticles

In this study, a precipitation method was used to synthesise ZnO nanoparticles using suitable precursors. An efficient surface modification method was proposed in order to reduce the agglomeration among synthesised small sized ZnO nanoparticles using 2-aminothiophenol as a capping agent. This article briefly investigated the effects of capping agent like 2-aminothiophenol on the optoelectronic properties of ZnO nanoparticles. The modified effectivity of 2-aminothiophenol has been examined on the nanosized ZnO nanoparticle for fluorescence and UV–visible (UV–vis) studies. The mechanism was studied for ZnO nanoparticles light emitting capability under different conditions. By facilitating the capping of ZnO with 2-aminothiophenol, fluorescence emission of the surface defects vanishes and ultraviolet (UV) emission increases. Surface capping by 2-aminothiophenol effectively covers most of the surface defects of ZnO and results in quenching of the visible region. The UV–vis absorption spectra of modified ZnO nanoparticles has been influenced by modified ZnO nanoparticles as a result of surface modification; where marked blue shift in absorption edge results. By surface modification of ZnO nanoparticles, change in optoelectronics properties has opened the new scope and possibilities to explore and fine tune the optical character of the modified ZnO for various optoelectronics applications such as UV laser.

Catalyst-Controlled Structural Divergence: Selective Intramolecular 7-endo-dig and 6-exo-dig Post-Ugi Cyclization for the Synthesis of Benzoxazepinones and Benzoxazinones

Metal catalyzed post-Ugi cyclization of bis-amides is reported in this study. Exposure of bis-amides to Pd(II) catalyst triggered the formation of seven-membered benzoxazepinones. This investigation established that changing the catalyst to a Echavarrens gold(I) turned off cyclization to seven member ring and turned on 6-exo-dig annulations to afford family of six-membered benzoxazinones. To support the proposed mechanisms, quantum chemical based density functional theory calculations have been performed and validated. This novel method obtained molecular complexity up to four modular inputs and divergence of two different skeletons. 2D NMR spectroscopic techniques and single crystal X-ray diffraction established the proposed structures.

Enhanced Activity of Trinuclear Zn(II) Complex towards Phosphate Ester Bond Cleavage by Introducing Three Metal Cooperativity

Three Zn(II) complexes, namely, [Zn3(L1)2]·4CH3CN (1), [Zn3(L1)2(H2O)4]·H2O·2DMF (2) and [Zn2(L2)2·(H2O)2·2H+]·2ClO4− (3) were synthesized by reacting nitrophenol-based tripodal and dipodal Schiff-base ligands with Zn(II) salts. The complexes were characterized using different techniques such as elemental analyses, ESI-MS, IR and NMR. The solid-state structures of all the complexes were determined. Complex 1 has a rigid trinuclear structure with an octahedral environment around each metal ion. The coordination environment of the rigid trinuclear complex 1 was purposely disturbed by changing the reaction conditions to obtain the flexible trinuclear complex 2. The dinuclear complex 3, containing coordinated water molecules, was also synthesized. DNA binding abilities and phosphatase-like activities of all the complexes were investigated. The catalytic activity of the rigid trinuclear complex 1 was compared with the cup shaped trinuclear complex 2 and closed structure dinuclear complex 3. The different structural factors (such as flexibility of the complex, number of metal centers, labile sites at the metal centre and coordination environment around the metal center) collectively modulate the phosphatase-like activity toward the transesterification of 2-hydroxypropyl-p-nitropenylphosphate (HPNP). Complex 2 exhibits the highest catalytic activity among all the complexes due to its flexible cup-shaped structure, three metal centers and coordinated water molecules.