D. Bhattacharjee

Preparation of polystyrene–clay nanocomposite by solution intercalation technique

Polymer–clay nanocomposites of commercial polystyrene (PS) and clay laponite were prepared via solution intercalation technique. Laponite was modified suitably with the well known cationic surfactant cetyltrimethyl ammonium bromide by ion-exchange reaction to render laponite miscible with hydrophobic PS. X-ray diffraction analysis in combination with scanning electron microscopy gives an idea of structural and morphological information of PS–laponite nanocomposite for different varying organo-laponite contents. Intercalation of PS chain occurs into the interlayer spacings of laponite for low organo-laponite concentration in the PS–O-laponite mixture. However, aggregation and agglomeration occur at higher clay concentration. The molecular bond vibrational profile of laponite as well as PS–laponite nanocomposite have been explored by Fourier transform infrared spectroscopy. Thermogravimetric analysis along with differential scanning calorimetry results reveal the enhancement of both thermal stability and glass transition temperature of PS due to the incorporation of clay platelets.

LANGMUIR–BLODGETT FILMS AND MOLECULAR ELECTRONICS

Molecular electronics is a new, exciting and interdisciplinary field of research. The main concern of the subject is to exploit the organic materials in electronic and optoelectronic devices. On the other hand, the Langmuir–Blodgett (LB) film deposition technique is one of the best among few methods used to manipulate materials at the molecular level. In this article, the LB film preparation technique is discussed briefly with an emphasis on its application towards molecular electronics.

Investigation of Fluorescence Resonance Energy Transfer between Fluorescein and Rhodamine 6G

Fluorescence Resonance Energy Transfer between two organic dyes Fluorescein and Rhodamine 6G was investigated in aqueous solution in presence and absence of synthetic clay laponite. Spectroscopic studies suggest that both the dyes were present mainly as monomer in solution. Fluorescence Resonance Energy Transfer occurred from Fluorescein to Rhodamine 6G in solutions. Energy transfer efficiency increases in presence of laponite and the maximum efficiency was 72.00% in aqueous laponite dispersion. Energy transfer efficiency was found to be pH sensitive. It has been demonstrated that with proper calibration it is possible to use the present system under investigation to sense pH over a wide range from 1.5 to 8.0.

Surface-Enhanced Raman Spectroscopic Study of Isomeric Formylthiophenes in Silver Colloid

Surface enhanced Raman (SER) spectra of 2- and 3-formylthiophenes (2FT and 3FT) in silver colloids are compared with their normal Raman spectra in bulk and in acetonitrile solution. Experimental results indicate adsorption of 2FT on metal surface through the sulfur and the oxygen atoms with the ring plane vertical to the metal surface, whereas the ring plane of the 3FT molecule is inclined to the surface. Moreover, the SER spectrum of 2FT molecule predicts the existence of syn-form of this molecule in the adsorbed state because of large enhancement of the carbonyl stretching mode. Concentration dependence of relative enhancement infers the monolayer formation on metal surface at a specific concentration, implying strong evidence of chemisorption of the adsorbates. The excitation profile of C identical withC symmetric stretching mode of 2FT suggests classical plasmon resonance as the main contributor to surface enhancement. Copyright 1997 Academic Press. Copyright 1997Academic Press

Adsorption kinetics of a fluorescent dye in a long chain fatty acid matrix.

This work reports the adsorption kinetics of a highly fluorescent laser dye rhodamine B (RhB) in a preformed stearic acid (SA) Langmuir monolayer. The reaction kinetics was studied by surface pressure-time (π-t) curve at constant area and in situ fluorescence imaging microscopy (FIM). Increase in surface pressure (at constant area) with time as well as increase in surface coverage of monolayer film at air-water interface provide direct evidence for the interaction. ATR-FTIR spectra also supported the interaction and consequent complexation in the complex films. UV-vis absorption and Fluorescence spectra of the complex Langmuir-Blodgett (LB) films confirm the presence of RhB molecules in the complex films transferred onto solid substrates. The outcome of this work clearly shows successful incorporation of RhB molecules into SA matrix without changing the photophysical characteristics of the dye, thus making the dye material as LB compatible.

Control of H-dimer formation of acridine orange using nano clay platelets

Acridine orange (AO) forms dimer even in aqueous solution. In layer-by-layer (LbL) film of AO dimeric sites predominate over monomeric sites. This communication reports the control of H-dimer of AO in LbL film by incorporating nano clay platelets. This was studied by using UV-Vis absorption spectroscopy. Atomic force microscopic (AFM) image of the LbL film was taken to confirm the presence of nano clay platelets in the LbL film.

Raman Spectral Study of Vibrational Relaxation of the CN Stretching Band of Acetonitrile and Benzonitrile

Raman spectroscopic studies on acetonitrile and benzonitrile as the liquids and in dilute solutions were used to investigate the vibrational relaxation processes which lead to broadening of the C≡N stretching vibrational band. Line widths of isotropic Raman spectra in a number of solvents revealed the quantitative dependence of vibrational relaxation rate on the hydrodynamic and dispersion forces.

Spectroscopic characteristics of 9-cyanoanthracene mixed with polymethyl methacrylate and stearic acid in Langmuir–Blodgett films

Abstract Multilayers of 9-cyanoanthracene (CNA) mixed with stearic acid (SA) as well as also polymethyl methacrylate (PMMA) at different mole fractions of CNA have been transferred onto quartz substrate to form stable Langmuir–Blodgett (LB) films. Surface pressure versus area per molecule isotherms confirms the formation of stable films. The absorption and fluorescence spectra of CNA–SA and CNA–PMMA mixed LB films were compared. In case of CNA–SA mixed LB films, even at very low mole fraction of CNA, aggregation was formed. While in case of CNA–PMMA mixed LB films, at very low mole fraction of CNA, no aggregation was formed. However with increasing mole fraction of CNA in PMMA formation of aggregate started which is indicated by the development of a new band in the absorption spectra. At higher concentration this band overlaps with the high energy band to form a broad band profile in the absorption spectra. Steady-state fluorescence spectra also confirm these phenomena.

Spectroscopic properties of non-amphiphilic 1-pyrene carboxaldehyde assembled in Langmuir-Blodgett films

Abstract 1-Pyrene carboxaldehyde (abbreviated as PyCHO), a non-amphiphilic derivative of pyrene has been successfully incorporated in Langmuir–Blodgett (LB) films, mixed with stearic acid (SA). Surface pressure ( π ) vs. area per molecule isotherms of PyCHO mixed with SA at different mole fractions reveal a plateau like region which indicates a phase transition corresponding to a reorientation of the PyCHO molecules at the air–water interface. Moreover, a positive deviation from the ideal behaviour of the area per molecule in different mole fraction in the mixed film at the air–water interface clearly reveals a repulsive interaction between the components resulting in the formation of aggregates of PyCHO molecules in the mixed films. Unlike pyrene molecules, formation of J-aggregates of PyCHO molecules in the mixed LB films has been confirmed by absorption spectroscopic studies. The fluorescence spectrum of the LB mixed film of PyCHO has been observed to lie between the solution and microcrystalline spectra and has a combination of both monomeric and excimeric species. A striking similarity between the LB film emission spectra and the emission spectra of PyCHO in ethanol–water binary mixture confirms the formation of low dimensional microcrystalline aggregates in the LB film.

Development of a DNA sensor using a molecular logic gate.

This communication reports the increase in fluorescence resonance energy transfer (FRET) efficiency between two laser dyes in the presence of deoxyribonucleic acid (DNA). Two types of molecular logic gates have been designed where DNA acts as input signal and fluorescence intensity of different bands are taken as output signal. Use of these logic gates as a DNA sensor has been demonstrated.