Esben Skovsen

Role of Solvent, pH, and Molecular Size in Excited-State Deactivation of Key Eumelanin Building Blocks : Implications for Melanin Pigment Photostability

Ultrafast time-resolved fluorescence spectroscopy has been used to investigate the excited-state dynamics of the basic eumelanin building block 5,6-dihydroxyindole-2-carboxylic acid (DHICA), its acetylated, methylated, and carboxylic ester derivatives, and two oligomers, a dimer and a trimer in the O-acetylated forms. The results show that (1) excited-state decays are faster for the trimer relative to the monomer; (2) for parent DHICA, excited-state lifetimes are much shorter in aqueous acidic medium (380 ps) as compared to organic solvent (acetonitrile, 2.6 ns); and (3) variation of fluorescence spectra and excited-state dynamics can be understood as a result of excited-state intramolecular proton transfer (ESIPT). The dependence on the DHICA oligomer size of the excited-state deactivation and its ESIPT mechanism provides important insight into the photostability and the photoprotective function of eumelanin. Mechanistic analogies with the corresponding processes in DNA and other biomolecules are recognized.

Two-Photon Singlet Oxygen Microscopy: The Challenges of Working with Single Cells

Abstract A microscope is described in which singlet molecular oxygen, O2(a1Δg), is produced in a femtoliter focal volume via a nonlinear two-photon photosensitized process, and the 1270 nm phosphorescence from this population of O2(a1Δg) is detected in a photon counting experiment. Although two-photon excitation of a sensitizer is less efficient than excitation by a one-photon process, nonlinear excitation has several distinct advantages with respect to the spatial resolution accessible. Pertinent aspects of this two-photon O2(a1Δg) microscope were characterized using bulk solutions of photosensitizers. These data were compared to those obtained from a single biological cell upon linear one-photon excitation of a sensitizer incorporated in the cell. On the basis of the results obtained, we outline the challenges of using nonlinear optical techniques to create O2(a1Δg) at the single cell level and to then optically detect the O2(a1Δg) thus produced in a time-resolved experiment.

Flash Photolysis of Cutinase: Identification and Decay Kinetics of Transient Intermediates Formed upon UV Excitation of Aromatic Residues

Aromatic amino acids play an important role in ultraviolet (UV)-induced photochemical reactions in proteins. In this work, we aim at gaining insight into the photochemical reactions induced by near-UV light excitation of aromatic residues that lead to breakage of disulfide bridges in our model enzyme, Fusarium solani pisi cutinase, a lipolytic enzyme. With this purpose, we acquired transient absorption data of cutinase, with supplemental experimental data on tryptophan (Trp) and lysozyme as reference molecules. We here report formation kinetics and lifetimes of transient chemical species created upon UV excitation of aromatic residues in proteins. Two proteins, lysozyme and cutinase, as well as the free amino acid Trp, were studied under acidic, neutral, and alkaline conditions. The shortest-lived species is assigned to solvated electrons (lifetimes of a few microseconds to nanoseconds), whereas the longer-lived species are assigned to aromatic neutral and ionic radicals, Trp triplet states, and radical ionic disulphide bridges. The pH-dependent lifetimes of each species are reported. Solvated electrons ejected from the side chain of free Trp residues and aromatic residues in proteins were observed 12 ns after excitation, reaching a maximum yield after approximately 40 ns. It is interesting to note that the formation kinetics of solvated electrons is not pH-dependent and is similar in the different samples. On the other hand, a clear increase of the solvated electron lifetime is observed with increasing pH. This observation is correlated with H3O+ being an electron scavenger. Prolonged UV illumination of cutinase leads to a larger concentration of solvated electrons and to greater absorption at 410 nm (assigned to disulphide electron adduct RSSR *-), with concomitant faster decay kinetics and near disappearance of the Trp* radical peak at 330 nm, indicating possible additional formation of TyrO* formed upon reaction of Trp* with Tyr residues. Prolonged UV illumination of cutinase also leads to a larger concentration of free thiol groups, known to originate from the dissociation of RSSR *-. Additional mechanisms that may lead to the near disappearance of Trp(*) are discussed. Our study provides insight into one key UV-light-induced reaction in cutinase, i.e., light-induced disruption of disulphide bridges mediated by the excitation of aromatic residues. Knowledge about the nature of the formed species and their lifetimes is important for the understanding of UV-induced reactions in humans that lead to light-induced diseases, e.g., skin cancer and cataract formation.

Photodissociation of laser aligned iodobenzene: Towards selective photoexcitation

Iodobenzene molecules, having their symmetry axis aligned to the polarization direction of a strong, linearly polarized nanosecond laser pulse, are photodissociated into phenyl and iodine radicals with a 1.5 ps long laser pulse at 266 nm. The yield of I photoproducts, detected by resonant multiphoton ionization, is enhanced up to a factor of 2.7 when the dissociation laser is polarized parallel instead of perpendicular to the alignment laser polarization. In particular, the high-velocity distribution of I products, corresponding to excitation of an (n,σ*) repulsive surface, is enhanced by a factor of 3.3, when comparing parallel and perpendicular polarizations, whereas the low-velocity distribution of I products, corresponding to excitation of predissociative (π,π*) surfaces is only enhanced by a factor of 2.2. The difference is explained by the different directions of the transition dipole moment for the two transitions. We discuss the perspectives for selective photoexcitation using three dimensionally ...

Quantum state tomography of dissociating molecules.

Using tomographic reconstruction we determine the complete internuclear quantum state, represented by the Wigner function, of a dissociating I2 molecule based on femtosecond time resolved position and momentum distributions of the atomic fragments. The experimental data are recorded by timed ionization of the photofragments with an intense 20 fs laser pulse. Our reconstruction method, which relies on Jayness maximum entropy principle, will also be applicable to time resolved position or momentum data obtained with other experimental techniques.

Pore size dependence of diffuse light scattering from anodized aluminum solar cell backside reflectors

The development of backside reflectors (BSRs) is crucial for the efficiency of future low cost thin-film silicon solar cells. In this work, the scattering efficiency of bare aluminum BSRs with different pore sizes and ordering of surface microstructures are investigated. The BSRs were fabricated by utilizing the process of self-ordering anodic oxidation on aluminum foils resulting in regions with an approximately hexagonally periodic surface microstructure. It was found that the total and diffuse light scattering reflectance spectra showed opposite tendencies when increasing the pore size of the microstructures. When the pore size was increased to 700 nm, more than 68% of the incident light with wavelengths from 250 nm to 800 nm was reflected by scattering. For a similar geometry, except that it had less ordering, this number was increased to around 80%. This large fraction of reflected light observed in the form of scattering is promising for the use of the considered geometries as BSRs in thin-film silicon solar cells.

Photonic immobilization of high-density protein arrays using Fourier optics

The technique of UV‐light‐assisted immobilization of disulfide containing proteins has been combined with the Fourier‐transforming properties of lenses as well as with a simple millimeter scale feature size spatial mask. The result is a new simple and inexpensive way of creating high‐density protein arrays with feature sizes down to a few hundred nanometers, which represents an improvement of tenfold over existing commercially available high‐density protein arraying methods.

Plasmonic black gold based broadband polarizers for ultra-short laser pulses

It has recently been demonstrated that adiabatic nanofocusing of light by gap-plasmon modes in ultra-sharp convex metal grooves can turn metallic surfaces with high reflectivity in the visible and near-infrared into excellent absorbers of light polarized perpendicular to the groove direction. Here we demonstrate that this effect can be used to design broadband linear polarizers, operating in reflection and inducing negligible dispersive stretching of ultra-short (5–10 fs) laser pulses.

Local excitation of surface plasmon polaritons by second-harmonic generation in crystalline organic nanofibers

Coherent local excitation of surface plasmon polaritons (SPPs) by second-harmonic generation (SHG) in aligned crystalline organic functionalized para-phenylene nanofibers deposited on a thin silver film is demonstrated. The excited SPPs are characterized using angle-resolved leakage radiation spectroscopy in the excitation wavelength range of 850-1325 nm and compared to simulations based on a Green’s function area integral equation method. Both experimental and theoretical results show that the SPP excitation efficiency increases with decreasing wavelength in this wavelength range. This is explained both as a consequence of approaching the peak of the fibers nonlinear response at the wavelength 772 nm, and as a consequence of better coupling to SPPs due to their stronger confinement.

Immobilization of biomolecules onto surfaces according to ultraviolet light diffraction patterns.

We developed a method for immobilization of biomolecules onto thiol functionalized surfaces according to UV diffraction patterns. UV light-assisted molecular immobilization proceeds through the formation of free, reactive thiol groups that can bind covalently to thiol reactive surfaces. We demonstrate that, by shaping the pattern of the UV light used to induce molecular immobilization, one can control the pattern of immobilized molecules onto the surface. Using a single-aperture spatial mask, combined with the Fourier transforming property of a focusing lens, we show that submicrometer (0.7 μm) resolved patterns of immobilized prostate-specific antigen biomolecules can be created. If a dual-aperture spatial mask is used, the results differ from the expected Fourier transform pattern of the mask. It appears as a superposition of two diffraction patterns produced by the two apertures, with a fine structured interference pattern superimposed.