Photophysics of representative ketocyanine dyes: dependence on molecular structure

Spectral properties of a new fluorescent ketocyanine dye have been discussed. The energy of maximum absorption/fluorescence of the dye exhibits bathochromic shift with increasing polarity of the medium. Both dipolarity-polarisability and hydrogen bond donation interaction contribute to solvation of the dye. Study of fluorescence parameters points to existence of different emitting states of the dye for aprotic and protic solvents. While the emitting state is the (1)(π, π*) state for aprotic solvents, fluorescence supposedly take place from a different emitting state involving H-bond formation in the excited state in protic solvents. Fluorescence parameters of the dye have been compared with those for a structurally similar symmetric ketocyanine dye. The faster decay of the dye relative to its symmetric counterpart has been explained as due to an increase of nonradiative decay.     

Synthesis and structural characterisation of new Re(III) complexes using aldimines of α-amino acids as coligands

Complexes of the general formula [Re^III(L)Cl(PPh₃)₂] have been synthesised by reacting H₂L and [ReOCl₃(PPh₃)₂] in ethanol. Here H₂L represents imines of α-amino acids (glycine, l-alanine, l-valine, l-phenylalanine) derived from salicylaldehyde and naphthaldehyde. The crystal structure of one complex has been determined. The complexes are mononuclear, paramagnetic and display paramagnetic ¹H NMR in CDCl₃ solution. Their spectral and redox properties are scrutinised.     

Photoinduced intramolecular charge transfer reaction in (E)-3-(4-Methylamino-phenyl)-acrylic acid methyl ester: A fluorescence study in combination with TDDFT calculation

A donor-acceptor substituted aromatic system (E)-3-(4-Methylamino-phenyl)-acrylic acid methyl ester (MAPAME) has been synthesized, and its photophysical behavior obtained spectroscopically has been compared with the theoretical results. The observed dual fluorescence from MAPAME has been assigned to emission from locally excited and twisted intramolecular charge transfer states. The donor and acceptor angular dependency on the ground and excited states potential energy surfaces have been calculated both in vacuo and in acetonitrile solvent using time dependent density functional theory (TDDFT) and TDDFT polarized continuum model (TDDFT-PCM), respectively. Calculation predicts that a stabilized twisted excited state is responsible for red shifted charge transfer emission.     

Fluorescence Spectroscopic and Calorimetry Based Approaches to Characterize the Mode of Interaction of Small Molecules with DNA

Ethidium bromide displacement assay by fluorescence is frequently used as a diagnostic tool to identify the intercalation ability of DNA binding small molecules. Here we have demonstrated that the method has pitfalls. We have employed fluorescence, absorbance and label free technique such as isothermal titration calorimetry to probe the limitations. Ethidium bromide, a non-specific intercalator, netropsin, a (A-T) specific minor groove binder, and sanguinarine, a (G-C) specific intercalator, have been used in our experiments to study the association of a ligand with DNA in presence of a competing ligand. Here we have shown that netropsin quenches the fluorescence intensity of an equilibrium mixture of ethidium bromide - calf thymus DNA via displacement of ethidium bromide. Isothermal titration calorimetry results question the accepted interpretation of the observed decrease in fluorescence of bound ethidium bromide in terms of competitive binding of two ligands to DNA. Furthermore, isothermal titration calorimetry experiments and absorbance measurements indicate that the fluorescence change might be due to formation of ternary complex and not displacement of one ligand by another.     

Synthesis of a series of ethylene glycol modified water-soluble tetrameric TPE-amphiphiles with pyridinium polar heads: Towards applications as light-up bioprobes in protein and DNA assay,and wash-free imaging of bacteria

Available online Development of water soluble AIE-active “light-up” bioprobes for the detection of biomacromolecules has drawn huge research interests in recent past. In this study, a series of ethylene glycol modified water soluble tetrameric tetraphenylethylene amphiphiles with pyridinium polar heads (TPE-xEG-Py, x = 3, 4, 6 or 1a-c) have been synthesized by varying the ethylene glycol spacer. Their unique structure allows them to form vesicles and other nanoaggregates in aqueous solutions. These amphiphiles were successfully utilized for fluorimetric detection and quantitation of BSA and DNA based on the electrostatic interactions to trigger AIE-emission from the TPE moiety. The electrostatic interaction was also proved very effective in wash-free imaging of both Gram-negative (E. coli) and Gram-positive (S. aureus) bacteria with up to 92 folds increase in fluorescence response within bacterial concentration 0–12 × 10⁸ CFU mL^?1. The strategy is advantageous due to cost-effective and easy synthesis, high water solubility, and fast response.     

Hydrogen‐Bonding‐Regulated Supramolecular Nanostructures and Impact on Multivalent Binding

Herein we describe the H‐bonding‐regulated nanostructure, thermodynamics, and multivalent binding of two bolaamphiphiles NDI‐1 and NDI‐2 consisting of a hydrophobic naphthalene diimide connected to a hydrophilic wedge by a H‐bonding group and a glucose moiety on its two arms. NDI‐1 and NDI‐2 differ by the single H‐bonding group, namely, hydrazide or amide, which triggers the formation of vesicles and cylindrical micelles, respectively. Although the extended H‐bonding ensures stacking with head‐to‐head orientation and the formation of an array of the appended glucose moieties in both systems, the adaptive cylindrical structure exhibited superior multivalent binding with concanavalin A (ConA) to that of the vesicle. A control amphiphile lacking a H‐bonding group assembled with a random lateral orientation to produce spherical micelles without any notable multivalent binding.     

Atomically Precise Nanocluster Assemblies Encapsulating Plasmonic Gold Nanorods

The self‐assembled structures of atomically precise, ligand‐protected noble metal nanoclusters leading to encapsulation of plasmonic gold nanorods (GNRs) is presented. Unlike highly sophisticated DNA nanotechnology, this strategically simple hydrogen bonding‐directed self‐assembly of nanoclusters leads to octahedral nanocrystals encapsulating GNRs. Specifically, the p‐mercaptobenzoic acid (pMBA)‐protected atomically precise silver nanocluster, Na₄[Ag₄₄(pMBA)₃₀], and pMBA‐functionalized GNRs were used. High‐resolution transmission and scanning transmission electron tomographic reconstructions suggest that the geometry of the GNR surface is responsible for directing the assembly of silver nanoclusters via H‐bonding, leading to octahedral symmetry. The use of water‐dispersible gold nanoclusters, Au_≈250(pMBA)_n and Au₁₀₂(pMBA)₄₄, also formed layered shells encapsulating GNRs. Such cluster assemblies on colloidal particles are a new category of precision hybrids with diverse possibilities.     

A simple chemosensor for Hg2+ and Cu2+ that works as a molecular keypad lock

A new chemosensor which can detect Hg(2+) in water and Hg(2+)/Cu(2+) in acetonitrile and its application as a molecular keypad lock using Cu(2+) and F(-) as ionic inputs are demonstrated.     

Investigation of non-nucleophilic additives for the reduction of morphological anomalies in protein arrays

Uniform spot morphology is of critical importance in the fabrication and successful use of protein arrays, and solution additives are often needed to ensure good spot quality. Whereas hydroxyl-bearing molecules such as glycerol have found wide use, in our experience these reduce the efficiency of probe immobilization (particularly in the context of aldehyde-terminated surfaces). Here, we report a series of non-nucleophilic molecules that can be used as additives to improve spot homogeneity in protein arrays. Arrayed imaging reflectometry, a label-free optical biosensing technique, has been used along with spectroscopic ellipsometry to test the spot homogeneity, antibody immobilization efficiency, and activity of antihuman IgG arrays prepared with these non-nucleophilic additives on glutaraldehyde surfaces. It has been determined that 0.1% v/v 12-crown-4 performs optimally in MPBS buffer.     

Urea-based ruthenium(II)-polypyridyl complex as an optical sensor for anions: synthesis, characterization, and binding studies

A new ruthenium(II) complex [Ru(bpy)2(1-(6-nitro-[1,10]phenanthrolin-5-yl)-3-(4-nitrophenyl)-urea)] (bpy=2,2'-bipyridyl) was synthesized and characterized using standard analytical and spectroscopic techniques. Detailed absorption, emission, and 1H NMR spectral studies revealed that this receptor molecule acts as a sensor for F-, CH3COO-, and H2PO4- in acetonitrile solution. Binding of these anions caused an appreciable change in the color of the acetonitrile solution, which could be detected with the naked eye. At relatively lower concentration of anions, 1:1 H-bonded adduct was formed; however, at higher concentration, classical Br?nsted acid-base-type reaction prevailed. The relative binding affinity of different anions toward this receptor was evaluated and was rationalized with quantum chemical calculations. Narrowing of the gap between the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels on deprotonation of the receptor molecule caused a faster decay of the luminescence lifetime for the Rudpi-->Lpi*/bpypi*-based triplet excited state.