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John M. Khosrofian

Bio: John M. Khosrofian is an academic researcher from New York University. The author has contributed to research in topics: Flash photolysis & Pyrene. The author has an hindex of 4, co-authored 5 publications receiving 426 citations.

Papers
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Journal ArticleDOI
TL;DR: 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, and diameters can be measured to an accuracy of 0.05%.
Abstract: 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.

366 citations

Journal ArticleDOI
TL;DR: 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 intercalated sites.
Abstract: 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

63 citations

Book ChapterDOI
TL;DR: In this paper, various ratios of Eu and UO2 acetates were prepared by dissolving Eu in acetic acid and removing acetic acids in vacuum, and fluorescence intensities at 616 nm were measured under excitation at wavelength of 429 nm.
Abstract: 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.

21 citations

Journal ArticleDOI
TL;DR: Using flash photolysis techniques, the triplet excited states of benzo(a)pyrene, pyrene, benz( a)anthracene and other aromatic hydrocarbons have been detected in complexes of bovine (and human) serum albumin dissolved in aqueous solutions at room temperature.

9 citations

Journal ArticleDOI
TL;DR: In this article, the optical Kerr susceptibility of liquid carbon diselenide was measured and found to be (74 ± 11) × 10 − 14 csu with pump and probe wavelengths of 619 and 579 nm, respectively.

3 citations


Cited by
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Journal ArticleDOI
TL;DR: It appears that physical binding of polycyclic aromatic hydrocarbons consists of intercalation into these sites, and increasing salt concentration increases the duplex content of denatured DNA.
Abstract: 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.

1,808 citations

Journal ArticleDOI
TL;DR: A review of the use of the technique of solute fluorescence quenching to study the structure and dynamics of proteins and a number of factors are discussed that must be considered in analyzing such data.

1,644 citations

Journal ArticleDOI
TL;DR: In this article, femtosecond stimulated Raman microscopy (FSRM) is introduced, which is a non-linear nonlinear Raman imaging method that uses femtocond white light pulses and intense picosecond pulses derived from a femto-cond laser/amplifier system.
Abstract: 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.

293 citations

Journal ArticleDOI
TL;DR: In this article, optically pumped GeSn edge-emitting lasers were grown on Si substrates and 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.
Abstract: 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

187 citations

Journal ArticleDOI
TL;DR: This work reveals that the family of halide perovskite colloidal nanocrystals transcend these constraints with highly efficient five-photon-excited upconversion fluorescence—unprecedented for semiconductor nanocry crystals.
Abstract: 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.

170 citations