Tapanendu Kundu

Novel U-bent fiber optic probe for localized surface plasmon resonance based biosensor.

The aim of this study is to develop an optical absorbance based biosensor suitable for wide scale use in resource-poor locales. A sensor for sensitive measurement of refractive index (RI) with the help of optical absorbance properties of gold nanoparticles (GNP) coupled to an efficient optical transducer in the form of a U-bent fiber optic probe is described. A U-bent probe was fabricated by a simple procedure. The absorbance due to the localized surface plasmon resonance (LSPR) of fiber-bound GNP was found to be linear to refractive index changes between 1.33 and 1.35. A U-bent probe of 200 microm diameter with a bend radius of 0.75 mm gave rise to a sensitivity of 35 DeltaA/RIU at 540 nm. The resolution of the sensor probe was 3.8x10(-5) RIU. Label-free biosensing was demonstrated using these probes with the help of IgG-anti IgG as bioreceptor-analyte pair.

Evanescent wave absorbance based fiber optic biosensor for label-free detection of E. coli at 280 nm wavelength

A novel label-free technique for the detection of pathogens based on evanescent wave absorbance (EWA) changes at 280 nm from a U-bent optical fiber sensor is demonstrated. Bending a decladded fiber into a U-shaped structure enhances the penetration depth of evanescent waves and hence sensitivity of the probe. We show that the enhanced EWA response from such U-bent probes, caused by the inherent optical absorbance properties of bacterial cells or biomolecules specifically bound to the sensor surface, can be exploited for the detection of pathogens. A portable optical set-up with a UV light emitting diode, a spectrometer and U-bent fiber optic probe of 200 μm core diameter, 0.75 mm bend radius and effective probe length of 1cm demonstrated an ability to detect less than 1000 cfu/ml.

Effects of Metal Substitution on Third-order Optical Non-linearity of Porphyrin Macrocycle

The effects of axial ligands in Sn(IV)-substituted 5,10,15,20-tetrakis [4-(carboethoxymethyleneoxy)phenyl]porphyrin and divalent transition metal substitution in 5,10,15,20-tetraphenylporphyrin on the second molecular hyperpolarizability (γ) at 802 nm were studied using the Z-scan technique. A significant increase in γ for divalent metal ions is observed with decreasing d-shell occupancy. In the case of the tetravalent metal-substituted porphyrin a dramatic enhancement of γ is seen when a strongly electronegative axial ligand such as I2 is attached to the metal ion. A plausible explanation for the observed trend of γ has been sought in terms of metal-ligand interaction.

Study of localized surface-plasmon-resonance-based optical fiber sensor

A U-bent fiber-optic sensor based on the localized surface plasmon resonance (LSPR) of spherical silver nanoparticles has been studied. The redshift of the absorption maximum of the bound silver nanoparticles was observed due to the increment of nanoparticle density on the surface of the fiber. On the other hand, the blueshift was observed when the refractive index of the environment surrounding the nanoparticle was increased. These observations were analyzed in terms of a single nanoparticle theoretical framework. The departure from the spherical symmetry of the nanoparticle is attributed to the plasmonic coupling effect between the randomly distributed nanoparticles on the surface of the fiber core. This phenomenon can be cleverly exploited to develop different kinds of optical fiber sensors.

Origin of methyl torsional barrier in 1-methyl-2-(1H)-pyridone.

The laser induced fluorescence excitation and single vibronic excitation dispersed fluorescence spectra have been studied for supersonic jet cooled 1-methyl-2(1h)-pyridone. The methyl torsional bands and some low frequency vibrational transitions were assigned for both ground and excited states. The torsional parameters V(3)=244 cm(-1) and V(6)=15 cm(-1) for the ground state and V(3)=164 cm(-1) and V(6)=40 cm(-1) for the excited state were obtained. To get the insight into the methyl torsional barrier, ab initio calculations were performed and compared with the experimental results. Origin of potential barrier was traced by partitioning the barrier energy into changes in bond-antibond interaction, structural, and steric energies accompanying methyl rotation using natural bond orbital analysis. The role of local interactions in ascertaining the barrier potential reveals that its nature cannot be understood without considering the molecular flexing. The hyperconjugation between CHsigma(*) and ring pi(*) observed in lowest unoccupied molecular orbital (LUMO) stabilizes the methyl group conformer that undergoes a 60 degrees rotation in the excited state with respect to that of the ground state, and it is the change in LUMO that plays important role in the excited state barrier formation.

Origin of methyl torsional potential barrier — An overview

This paper presents the evolution of views on methyl internal rotation potential barrier. Various mechanisms proposed for the origin of torsional barrier in ethane have been reviewed. Inadequacy of one dimensional description of internal rotation has been highlighted in small methyl conjugated molecules in the light of its multidimensional nature. The effect of skeletal flexing on the picture of barrier formation by dissecting the barrier energy into potential type, virial type and symmetry type is described. The role of π and σ electrons at different stages of molecular flexing is discussed. The analysis identifies the dominant contributions to barrier origin as π-bonding changes during rigid rotation and σ-bonding changes resulting from bond lengthening during methyl group rotation. The contribution of lone pair electrons in determining the preferred structure of the methyl group in imine compounds such as 1-methyl 2-(1H)-pyridinimine is presented.

Origin of methyl torsional barrier in 1-methyl-2(1H)-pyridinimine and 3-methyl-2(1H)-pyridone: II. Ground state

To get the insight into the electronic structure-methyl torsion correlation in nitrogen heterocyclic molecules, a comparative study on torsion of the methyl group in 1-methyl-2(1H)pyridone (1MPY), 1-methyl-2(1H)pyridinimine (1MPI), and 3-methyl-2(1H)pyridone (3MPY) was carried out using ab initio calculations. To understand the barrier forming mechanism in the ground state and its consequence on the molecular structure, the ground state torsional potential has been investigated by partitioning the barrier energy using the natural bond orbital (NBO) theoretical framework. The NBO analysis reveals that the delocalization energy is the barrier forming term whereas the Lewis energy is always antibarrier for all these molecules. To get further insight into the effect of local electronic structure on the methyl torsional barrier, the individual bond-antibond interactions and structural energy contributions have been investigated. It was found that when the bond order difference between the vicinal bonds does not change appreciably during the course of methyl rotation, the local electronic interactions with the methyl group do not play any decisive role in barrier formation as observed in the case of 1MPY and 1MPI. In these cases, it is the skeletal relaxation during methyl rotation that plays an important role in determining the barrier. On the other hand, if the bond order change is appreciable as is the case for 3MPY, the local interactions alone suffice to describe the origin of the torsional barrier of the methyl group.

Synthesis, mixed valence aspects and non-linear optical properties of the triruthenium complexes [{(bpy) 2 Ru II } 3 (L)] 3+ and [{(phen) 2 Ru II } 3 (L)] 3+ (bpy = 2,2-bipyridine, phen = 1,10-phenanthroline and L 3- = 1,3,5-triazine-2,4,6-trithiol)

The triruthenium complexes [{(bpy)2RuII}3L]3+ [1]3+ and [{(phen)2RuII}3L]3+ [2]3+ have been synthesized via the reactions of [RuII(bpy)2(EtOH)2]2+ and [RuII(phen)2(EtOH)2]2+ with the trisodium salt of 1,3,5-triazine-2,4,6 trithiol (Na3L) respectively. In CH3CN, the complexes [1]3+ and [2]3+exhibit three reversible one-electron redox processes corresponding to successive Ru(II)/Ru(III) couples. The 190-250 mV separation in potential between the successive Ru(II)/Ru(III) couples is indicative of moderate intermetallic electronic coupling in the mixed valence states. The bipyridine and phenanthroline based reductions are observed at -1.58, -1.86 V and -1.77, -2.01, -2.43 V versus SCE respectively. The spectroelectrochemical study on the bipyridine derivative [1]n+ (n = 3-6) in acetonitrile medium at 243 κ shows a broad and relatively weak intervalence charge-transfer transition (IVCT) near 1900 nm for both the mixed valence states RuIIRuIIRuIII [1]4+ and RuIIRuIIIRuIII [1]5+, characteristic of class II behaviour. The calculated coupling constant (Vab), 560 cm-1 is also supportive of class II mixed-valence states. The electrochemically generated one-electron oxidised species [1]4+ or [2]4+ exhibits an EPR spectrum characteristic of low-spin RuIII ion in a distorted octahedral environment (g1 = 2.246, g2 = 1.993 for [1]4+ and g1 = 2.469, g2 = 2.191 for [2]4+). The complexes are moderately strongly luminescent at 77 κ . Both the complexes have also shown third order non-linear optical properties with γ = -4.5 × 10-29 esu for [1]3+ and -5.09 × 10-29 esu for [2]3+.

Non-resonant third-order optical non-linearity of porphyrinderivatives

Non-resonant second molecular hyperpolarisabilities (γ) of tetraphenylporphyrin (H 2 tpp) and five substituted derivatives are measured by the Z-scan technique at 784 nm; the relationship between the various structures and their corresponding molecular hyperpolarisabilities is explained on the basis of electronic interaction between the external substituents and the porphyrin core.

Polymer-Based MEMS Photodetector With Spectral Response in UV-Vis-NIR and Mid-IR Region

Integrating various components on the same chip is highly sought after for various optoelectronic applications. In an attempt to provide an on-chip photodetection, a MEMS-based photodetector device with a wide spectral response is presented. The design merges the photoconductive and pyroelectric properties of nanomorphology-controlled polyvinyl alcohol as a photoactive layer. The fabrication technology is low cost with a single-layer deposition of photoactive polymer on a MEMS low thermal mass platform designed to improve the heat loss to the substrate. This fabricated device with a metal-semiconductor-metal structure shows Schottky diode behavior. The photoresponse of this device was observed from UV to mid-IR region with minimum light detection capability of 30 nW in UV, 120 nW for visible light, and 100 μW for IR light. The effect of nanomorphology and the thickness of the photoactive layer were studied to optimize the responsivity in the different waveband regions. Typically, at zero bias, under 405-nm illumination with light intensity of 170 μW/cm2, the photodetector exhibited responsivity of 0.53 A/W. The wavelength response of this detector was found to be similar with standard detectors of the UV visible as well as mid-IR region (6.3-10.6 μm). The proposed on-chip MEMS-based photodetection module with the broad-spectrum detection capability and lower power consumption is useful for lab-on-chip-based technologies for a wide range of optical/spectroscopic applications.