G. J. Blanchard

Micelle-Induced Versatile Sensing Behavior of Bispyrene-Based Fluorescent Molecular Sensor for Picric Acid and PYX Explosives

The effect of surfactant micelles on the photophysical properties of a cationic bispyrene fluorophore, Py-diIM-Py, was systemically examined. The results from series of measurements including UV-vis absorption, steady-state fluorescence emission, quantum yield, fluorescence lifetime, and time-resolved emission spectra reveal that the cationic fluorophore is only encapsulated by the anionic sodium dodecyl sulfate (SDS) surfactant micelles and not incorporated in the cationic dodecyltrimethylammonium bromide (DTAB) and neutral Triton X-100 (TX100) surfactant micelles. This different fluorophore location in the micellar solutions significantly influences its sensing behavior to various explosives. Fluorescence quenching studies reveal that the simple variation of micellar systems leads to significant changes in the sensitivity and selectivity of the fluorescent sensor to explosives. The sensor exhibits an on-off response to multiple explosives with the highest sensitivity to picric acid (PA) in the anionic SDS micelles. In the cationic DTAB micelles, it displays the highest on-off responses to PYX. Both the sensitivity and selectivity to PYX in the cationic micelles are enhanced compared with that to PA in the anionic micelles. However, the poor encapsulation in the neutral surfactant TX100 micelles leads to fluorescence instability of the fluorophore and fails to function as a sensor system. Time-resolved fluorescence decays in the presence of explosives reveal that the quenching mechanism of two micellar sensor systems to explosives is static in nature. The present work demonstrates that the electrostatic interaction between the cationic fluorophore and differently charged micelles plays a determinative role in adjusting its distribution in micellar solutions, which further influences the sensing behavior of the obtained micellar sensor systems.

Picosecond spectroscopic measurement of a solvent dependent change of rotational diffusion rotor shape

The rotational diffusion behavior of cresyl violet is observed to be solvent dependent, producing an induced orientational anisotropy function with single exponential decays in both ethylene glycol at 26 °C and 1‐dodecanol at 37 °C, but a two‐component decay in 1‐dodecanol at 26 °C. It is clear from this data that cresyl violet is experiencing significant changes in its local solvation environment. Without knowledge of the orientation of the transition moment within the molecule, however, the interpretation of these results is ambiguous. A simple calculation is presented which allows for the estimation of the intramolecular orientation of the transition moment, thereby removing ambiguity from the interpretation. The observed behavior is shown to be consistent with cresyl violet reorienting as an oblate rotor in ethylene glycol and 1‐dodecanol at 37 °C, and as a prolate rotor in 1‐dodecanol at 26 °C.

Photopolymerized polypyrrole microvessels.

We report on the preparation of water-filled polymer microvessels through the photopolymerization of pyrrole in a water/chloroform emulsion. The resulting structures were characterized by complementary spectroscopic and microscopic techniques, including Raman spectroscopy, XPS, SEM, and TEM. The encapsulation of fluorescent, magnetic, and ionic species within the microvessels has been demonstrated. Confocal microscopy and fluorescence anisotropy measurements revealed that the encapsulated chromophore (Rhodamine 6G) resides within voids in the capsules; however, strong interaction of the dye with polypyrrole results in a measurable decrease in its rotational dynamics. Microvessels loaded with ferrofluid exhibit magnetic properties, and their structures can be directed with an external magnetic field. TEM measurements allowed imaging of individual nanoparticles entrapped within the vessels. The application of Cu(2+)-loaded microvessels as a transducer layer in all-solid-state ion-selective electrodes was also demonstrated.

Effects of ethanol on the organization of phosphocholine lipid bilayers

We have investigated the consequences of the addition of ethanol to aqueous solutions containing 100 nm diameter phosphocholine unilamellar vesicles. We have studied the effect of ethanol addition on both gel phase and fluid phase phospholipid bilayers of 1,2-dimyristoyl-sn-phosphatidylcholine (DMPC), using time-resolved fluorescence measurements of perylene incorporated into the vesicles. We observe an increase in the perylene rotational diffusion time constants for ethanol concentrations of ca. 0.6 M in both the gel phase (289 K) and the fluid phase (303 K), indicating a change in the bilayer interacyl chain spacing and/or organization. While the change in rotational diffusion behavior of perylene is seen for both phospholipid phases, the details of the change in chromophore dynamics are not the same for the two phases, likely due to the differing extents of disorder in the phospholipid acyl chain region at the two temperatures. These data provide insight into the effects of ethanol on the local environment of the probe in both gel phase and fluid phase lipid bilayers.

The role of multiple electronic states in the dissipative nergy dynamics of coumarin 153

Abstract We have examined the transient spectral relaxation properties of coumarin 153 in three polar solvents using ultrafast spontaneous and stimulate emission spectroscopies. The time evolution of the coumarin 153 spontaneous emission spectrum exhibits an excitation energy-dependence, demonstrating that the spectral dynamics of this molecule are determined promarily by intramolecular population relaxation processes. We have used ultrafast stimulated emission spectroscopy to determine the presence of at least two electronic state within the spontatneous emission envelope. The excitation energy dependence of the stumulated emission spectra, in conjunction with reorientation time measurements, show that the excited electronic states possess substantially different charge distributions and thus interact differently with the surrounding solvent medium. Semi-empirical molecular orbital calculations of the electronic states of coumarin 153 predict the presence of two closely spaced singlet states as well as several triplet states, consitent with our experimental data. Our data and calculations demonstrate collectively that coumarins are, in general, not ideal molecular probes of solvationn dynamics because of their complex electronic structure and intramolecular energy dissipation characteristics.

Franck–Condon enhancement of χ(3) in a conjugated polymer under double resonance conditions

Stimulated inverse Raman spectroscopy is used to investigate exciton–phonon coupling in the polydiacetylene PTS. The Raman resonances used the v=0 and one v=1 singlet exciton as intermediate states, and the consequent Stark shiftings and splittings of the excitons are measured. We use these optical Stark effects to determine the coupling between the ground state phonons and the v=0 exciton. This coupling is as strong for the combination modes as for the fundamental modes. The coupling between several combinations modes and the v=1 exciton is 4 to 10 times greater than it is for the v=0 exciton, in agreement with calculations of the overlap integrals for each transition.

Formation of air-stable supported lipid monolayers and bilayers

We have devised a means of depositing planar, air-stable supported lipid adlayers on modified Au substrates. Using the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), we form planar supported adlayer structures by vesicle fusion. Lipid bilayer formation proceeds on a hydroxythiol-terminated Au surface. Phospholipid monolayers form on hydroxythiol-terminated gold surfaces that have been treated with POCl3 and ZrOCl2(aq) prior to lipid deposition, providing an interface that interacts strongly with the DMPC phosphocholine headgroup. We use FTIR, cyclic voltammetry, optical ellipsometry, and water contact angle measurements to confirm the presence of lipid bilayers or monolayers on the modified Au substrates. For the zirconated surface, we observe the conversion of an initial partial lipid bilayer to a lipid monolayer, over a ca. 20 min time period, on the basis on ellipsometric thickness and contact angle data. 31P NMR measurements show the complexation of the phospholipid headgroup to a Zr-phosphate surface.

The picosecond spectroscopy of a polydiacetylene in the small signal limit: Detection and characterization of a new long-lived state

Abstract High-resolution, dual-wavelength picosecond pump-probe spectroscopy is used to study thin single crystals (⩽200 A thick) of the polydiacetylene PTS in the small signal limit (one pump photon per 120 unit cells). Excitation near the 2.0 eV exciton reveals a new slow (τ=130 ps at 300 K) spectral component in addition to the previously observed fast component. The slow response exhibits a clear build-up at low temperature, demonstrating that a finite creation time is associated with the new long-lived state. We also measure a nonuniform spectral dependence of the rapid excitonic bleaching signal.

Photoinduced reactivity of doxorubicin: catalysis and degradation.

Doxorubicin exhibits unusual photoreactivity in aqueous solutions. Our data show that there are two distinct photoreactive pathways for doxorubicin. One is a two-step process that leads to the formation of 3-methoxysalicylic acid, a stable degradation product. The other pathway is a photoreduction of doxorubicin to form the corresponding dihydroquinone, which undergoes spontaneous oxidation mediated by dissolved oxygen to recover doxorubicin with the formation of hydrogen peroxide. Our data account for the known nonlinear dependence of doxorubicin fluorescence intensity on concentration.

Ultrafast stimulated emission spectroscopy of perylene in dilute solution: Measurement of ground state vibrational population relaxation

We report ultrafast stimulated emission measurements of perylene in a series of polar and nonpolar solvents. In all solvents the perylene stimulated emission spectra evolve in time. We observe individual features corresponding to distinct vibronic resonances in the stimulated emission spectra. The intensities of these features increase subsequent to excitation and persist for hundreds of picoseconds. The fast build‐up seen at short delay times is related directly to the vibrational population relaxation time, T1 , of the ground vibrational state that is the lower energy state of the stimulated transition. The measured T1 times for perylene vary with both ground vibrational state and solvent. The slow decay rates for these data, the sum of the stimulated and spontaneous decay rates for the particular transition, depend critically on the particular transition that is resonant with the probe laser electric field.