We have investigated the physical binding of pyrene and benzo[a]pyrene derivatives to denatured DNA. These compounds exhibit a red shift in their absorbance spectra of 9 nm when bound to denatured calf thymus DNA, compared to a shift of 10 nm when binding occurs to native DNA. Fluorescence from the hydrocarbons is severely quenched when bound to both native and denatured DNA. Increasing sodium ion concentration decreases binding of neutral polycyclic aromatic hydrocarbons to native DNA and increases binding to denatured DNA. The direct relationship between binding to denatured DNA and salt concentration appears to be a general property of neutral polycyclic aromatic hydrocarbons. Absorption measurements at 260 nm were used to determine the duplex content of denatured DNA. When calculated on the basis of duplex binding sites, equilibrium constants for binding of 7,8,9,10-tetrahydroxy-7,8,9,10-tetrahydro-benzo[a]pyrene to denatured DNA are an order of magnitude larger than for binding to native DNA. The effect of salt on the binding constant was used to calculate the sodium ion release per bound ligand, which was 0.36 for both native and denatured DNA. Increasing salt concentration increases the duplex content of denatured DNA, and it appears that physical binding of polycyclic aromatic hydrocarbons consists of intercalation into these sites.

Polycyclic aromatic hydrocarbons physically intercalate into duplex regions of denatured DNA

Fluorescence quenching studies with proteins.

A novel type of non-linear Raman microscopy, femtosecond stimulated Raman microscopy (FSRM), is introduced. It employs femtosecond white light pulses and intense picosecond pulses which are derived from a femtosecond laser/amplifier system. The pulses are coupled into a microscope set-up and induce a stimulated Raman process at the focus. The Raman interaction spectrally modulates the white light. These modulations are read-out in multi-channel fashion and allow recording of a complete Raman spectrum of the focal region. By raster-scanning the sample, complete Raman images can be obtained. Raman images of polystyrene beads in water demonstrate the feasibility of the approach.

Femtosecond stimulated Raman microscopy

This paper reports the demonstration of optically pumped GeSn edge-emitting lasers grown on Si substrates The whole device structures were grown by an industry standard chemical vapor deposition reactor using the low cost commercially available precursors SnCl4 and GeH4 in a single run epitaxy process Temperature-dependent characteristics of laser-output versus pumping-laser-input showed lasing operation up to 110 K The 10 K lasing threshold and wavelength were measured as 68 kW/cm2 and 2476 nm, respectively Lasing characteristic temperature (T0) was extracted as 65 K

An optically pumped 2.5 μm GeSn laser on Si operating at 110 K

Multiphoton absorption processes enable many technologically important applications, such as in vivo imaging, photodynamic therapy and optical limiting, and so on. Specifically, higher-order nonlinear absorption such as five-photon absorption offers significant advantages of greater spatial confinement, increased penetration depth, reduced autofluorescence, enhanced sensitivity and improved resolution over lower orders in bioimaging. Organic chromophores and conventional semiconductor nanocrystals are leaders in two-/three-photon absorption applications, but face considerable challenges from their small five-photon action cross-sections. Herein, we reveal that the family of halide perovskite colloidal nanocrystals transcend these constraints with highly efficient five-photon-excited upconversion fluorescence-unprecedented for semiconductor nanocrystals. Amazingly, their multidimensional type I (both conduction and valence band edges of core lie within bandgap of shell) core-shell (three-dimensional methylammonium lead bromide/two-dimensional octylammonium lead bromide) perovskite nanocrystals exhibit five-photon action cross-sections that are at least 9 orders larger than state-of-the-art specially designed organic molecules. Importantly, this family of halide perovskite nanocrystals may enable fresh approaches for next-generation multiphoton imaging applications.

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Giant five-photon absorption from multidimensional core-shell halide perovskite colloidal nanocrystals.

An inversion algorithm, involving a linear least-squares method, is developed to analyze the intensity data obtained by application of the traveling knife-edge method. Through use of simulated intensity data, diameters can be measured to an accuracy of 0.05%. The method also yields good results when it is applied to He–Ne laser beam data.

Measurement of a Gaussian laser beam diameter through the direct inversion of knife-edge data

Properties of benzo(a)pyrene (BaP) and benzo(e)pyrene (BeP) in physical complexes with native calf thymus DNA dissolved in aqueous solutions at 24 +- 1/sup 0/C were studied by means of fluorescence quantum yield, single photon counting fluorescence decay, and triplet flash photolysis techniques. It is proposed that most of the fluorescence originates from BaP and BeP bound to GC-GC intercalation sites, while the nonfluorescent molecules are bound at AT containing intercalation sites. This conclusion is supported by the relative quenching efficiencies of the fluorescence of BaP by mononucleosides dissolved in aqueous ethanol solutions: 2'-deoxythymidine is more effective (by a factor of greater than 10) as a quencher than cytidine, 2'-deoxyguanosine and -adenosine. The strong quenching of the fluorescence in DNA is partially accompanied by the production of triplet excited states. The triplets are not as susceptible to quenching in DNA as the singlets and the lifetimes of BaP and BeP triplets in DNA are 35 and 155 ms, respectively. The triplet lifetimes are sensitive to the conformation of the DNA in solution, to the ionic strength, and to the oxygen concentration. Employing oxygen quenching techniques, the triplets can be used as probes to determine the accessibility of the polycyclic aromatic moleculesmore » in macromolecular complexes. Oxygen quenching constants for triplets of BaP, BeP, benz(a)anthracene, 7,12-dimethylbenz(a)anthracene, and acridine orange are in the range of 1-2 x 10/sup 8/ l. mole/sup -1/ s/sup -1/, about 10 to 20 times smaller than in fluid solution; this decrease is attributed to intercalation.« less

Properties of benzopyrene--DNA complexes investigated by fluorescence and triplet flash photolysis techniques

Various ratios of Eu and UO2 acetates were prepared by dissolving Eu and UO2 acetates in acetic acid and removing acetic acid in vacuum. The fluorescence intensities at 616 nm of these mixed acetates were measured under excitation at wavelength of 429 nm. Strong energy transfer from UO 2 2+ to Eu3+ was observed. This result indicated that these ions were in very close contact in this system. Various concentrations of Eu3+ and UO 2 2+ salts of polymers containing carboxyl ligands were prepared and fluorescence properties of these polymers were investigated. The polymers investigated were poly (methacrylic acid) (PMA), and copolymers of styrene-acrylic acid (PSAA) and methyl methacrylate-methacrylic acid (PMM/MA). When Eu3+ and UO 2 2+ were introduced to the polymers of PSAA and PMM/MA, very effective energy transfer from UO 2 2+ to Eu3+ was observed because of the proximity of these metal ions in ionic aggregates of these ionomers. However, the PMA-UO2 and Eu salts did not show energy transfer even when a large excess of UO2+ 2 was added to the polymer. The PMA salts are reported to have no ion aggregate structure; it is concluded that Eu3+ and UO 2 2+ ions were uniformly dispersed throughout the polymer system so that the distances between these ions were far beyond the energy transfer range.

John M. Khosrofian

Papers

Measurement of a Gaussian laser beam diameter through the direct inversion of knife-edge data

Properties of benzopyrene--DNA complexes investigated by fluorescence and triplet flash photolysis techniques

Rare Earth Metal Containing Polymers: Energy Transfer from Uranyl to Europium Ions in Ionomers

Triplet excited states of polycyclic aromatic compounds as probes of their microenvironment in serum albumin complexes.

Optical Kerr effect in carbon diselenide