Sanchari Chowdhury

Silver-copper alloy nanoparticles for metal enhanced luminescence

Large metal enhanced luminescence was realized at the vicinity of easily fabricated Ag–Cu alloy nanoparticles upon tuning of their surface plasmon resonance spectra by changing only one experimental variable—the annealing temperature, for maximum spectral overlap with the emission and excitation spectra of the luminophores. We observed strong emission enhancement of luminophores (141.48±19.20 times for Alexa Fluor 488 and 23.91±12.37 times for Alexa Fluor 594) at the vicinity of these Ag–Cu nanoparticles, which is significantly larger than for pure Ag nanoparticles. We present theoretical calculations to provide insights into these experimental findings.

Measurement of Oxygen Permeability of Epoxy Polymers

This paper describes an experimental technique that refines and extends available methods to measure the oxygen permeability of polymers such as epoxies that cannot easily be prepared as thin films. A simple Fick’s law-based quasi-steady-state diffusion model was developed to extract the permeation coefficient from the experimental data. The validity of this technique was established by comparing results with published values. Tests were subsequently conducted to determine the oxygen permeation rates for five representative commercially available epoxy polymers. The results showed that the oxygen permeation rates for the different epoxies were comparable, making them all suitable for corrosion repair. This suggests that repair costs may be optimized by selecting the most cost-effective epoxy system. The applicability of the findings is discussed and an illustrative numerical example is presented.

Exciton–exciton annihilation as a probe of interchain interactions in PPV–oligomer aggregates

One measure of exciton mobility in an aggregate is the efficiency of exciton-exciton annihilation (EEA). Both exciton mobilities and EEA are enhanced for aggregate morphologies in which the distances between chromophores and their relative orientations are favorable for Förster energy transfer. Here this principle is applied to gauge the strength of interchain interactions in aggregates of two substituted PPV oligomers of 7 (OPPV7) and 13 (OPPV13) phenylene rings. These are models of the semiconducting conjugated polymer MEH-PPV. The aggregates were formed by adding a poor solvent (methanol or water) to the oligomers dissolved in a good solvent. Aggregates formed from the longer-chain oligomer and/or by addition of the more polar solvent showed the largest contribution of EEA in their emission decay dynamics. This was found to correlate with the degree to which the steady-state emission spectrum of the monomer is altered by aggregation. The wavelength dependence of the EEA signal was also shown to be useful in differentiating emission features due to monomeric and aggregated chains when their spectra overlap significantly.

Measurement of oxygen diffusivity and permeability in polymers using fluorescence microscopy.

A simple fluorescence microscopy technique is developed and presented to investigate heterogeneities in emission intensity and quenching responses of luminescence sensors and to measure diffusion and permeation coefficients of oxygen in polymers. Most luminescence oxygen sensors do not follow linearity of the Stern-Volmer (SV) equation due to heterogeneity of luminophore in the polymer matrix. To circumvent this limitation, inverted fluorescence microscopy is utilized in this work to investigate the SV response of the sensors at the micron scale. It was found that intensity is higher in regions where the luminophore is aggregated, but the response is poorer to oxygen concentration. In contrast, the nearly homogeneous regions exhibit linearity with high SV constants. In these diffusion experiments, oxygen concentration was measured by luminescence changes in regions with high SV constants and good linearity. Two diffusion experiments were performed-termed film-on-sensor and accumulation-in-volume techniques. A new Ficks law based quasi-steady-state diffusion model was developed and combined with the SV equation to obtain effective permeation coefficients for the accumulation-in-volume technique. Using these experimental techniques, oxygen diffusion properties in free-standing Teflon polymer films, cast silicon elastomers, and cast polydimethylsiloxane films containing different weight percentages of zeolite were determined with good precision.