| 1. |
- Biswas, Abhijit, et al.
(författare)
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Control of the size and shape of TiO2 nanoparticles in restricted media
- 2013
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 24:19
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Tidskriftsartikel (refereegranskat)abstract
- Template-capped TiO2 nanostructures have been synthesized. In certain template conditions, TiO2 hexagons are found to form. These hexagonal structures can be effectively sensitized by fluorescein dye without any change in the protonation state of the dye. Bare TiO2 nanoparticles are not so useful for sensitization with dyes like fluorescein as they alter the dye protonation state. The novelty of this work is twofold-the hitherto elusive hexagonal phase of TiO2 nanoparticles has been stabilized and the synthesis of TiO2 in the rutile phase has been achieved under mild conditions.
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| 2. |
- Corani, Alice, et al.
(författare)
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Bottom-Up Approach to Eumelanin Photoprotection: Emission Dynamics in Parallel Sets of Water-Soluble 5,6-Dihydroxyindole-Based Model Systems.
- 2012
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 116:44, s. 13151-13158
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Tidskriftsartikel (refereegranskat)abstract
- The molecular mechanisms by which the black eumelanin biopolymers exert their photoprotective action on human skin and eyes are still poorly understood, owing to critical insolubility and structural heterogeneity issues hindering direct investigation of excitation and emission behavior. Recently, we set up strategies to obtain water-soluble 5,6-dihydroxyindole (DHI)-based polymers as useful models for disentangling intrinsic photophysical properties of eumelanin components from aggregation and scattering effects. Herein, we report the absorption properties and ultrafast emission dynamics of two separate sets of DHI-based monomer-dimer-polymer systems which were made water-soluble by means of poly(vinyl alcohol) or by galactosyl-thio substitution. Data showed that dimerization and polymerization of DHI result in long-lived excited states with profoundly altered properties relative to the monomer and that glycosylation of DHI imparts monomer-like behavior to oligomers and polymers, due to steric effects hindering planar conformations and efficient interunit electron communication. The potential of S-glycation as an effective tool to probe and control emission characteristics of eumelanin-like polymers is disclosed.
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| 3. |
- Corani, Alice, et al.
(författare)
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Excited-State Proton-Transfer Processes of DHICA Resolved: From Sub-Picoseconds to Nanoseconds
- 2013
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 4:9, s. 1383-1388
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Tidskriftsartikel (refereegranskat)abstract
- Excited-state proton transfer has been hypothesized as a mechanism for UV energy dissipation in eumelanin skin pigments. By using time resolved fluorescence spectroscopy, we show that the previously proposed, but unresolved, excited-state intramolecular proton transfer (ESIPT) of the eumelanin building block 5,6-dihydroxyindole-2-carboxylic acid (DHICA) occurs with a time constant of 300 fs in aqueous solution but completely stops in methanol. The previously disputed excited-state proton transfer involving the 5- or 6-OH groups of the DHICA anion is now found to occur from the 6-OH group to aqueous solvent with a rate constant of 4.0 x 10(8) s(-1).
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| 4. |
- Corani, Alice
(författare)
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Photochemistry of Eumelanin Precursors Role of Excited State Proton Transfer for UV Photoprotection
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Melanin is an epidermal pigment commonly known to give darker skin coloration under sun exposure. It is also present in the hair, eyes, inner ear and brain. The first function of epidermis melanin is believed to be photoprotective against harmful ultraviolet (UV) light, but the recent increase of skin cancer correlated to an increase of sun exposure questions the properties of melanin. Its presence in different body parts suggests that its function is not solely protective against UV-light. Melanin in epidermis is divided in two categories eumelanin responsible of the dark coloration and pheomelanin, which does not have great influence on the skin shade, but gives the red coloration of hair. The amount of skin cancer has been observed to be greater in patients presenting a fair type of skin. The mechanism after melanin UV absorption is poorly understood. Two main problems arise in the study of melanin photochemistry. First the pigment is believed to be an oligomer assembly of different sizes, resulting in a broad heterogeneity of a studied sample, which makes the distinction of active species difficult. On the other hand, this is probably a key property of melanin, to ensure a photoprotective barrier against especially UV-light. The second main difficulty in the study of melanin is the solubility. The larger the pigment the less soluble in aqueous solution. An additional issue in the study of melanin is the reproducibility of the sample. The work presented here focuses on eumelanin and its interaction with UV-light. With help of fluorescence steady state and time-resolved methods we have investigated eumelanin photochemistry. We present here a model of the energy dissipation mechanism of the pigment after UV absorption. Our method is based first on the study of synthetic samples, which allows us to have control over the heterogeneity and thus identify the function of each molecule involved in the whole melanin structure. Secondly, we have performed a bottom up approach, starting with the study of monomer constituents up to the polymer. Moreover, we have developed a method to solubilize the polymer, which does not interfere with the photodynamics of the molecules. We demonstrate that the main dissipation channel of eumelanin after UV absorption in aqueous solution is controlled by Excited State Proton Transfer (ESPT). The surrounding solvent is essential to have a rapid and efficient UV dissipation on the order of hundreds of femtoseconds. We show that the melanin precursor DHICA, in its polymeric form, is much more efficient than the DHI precursor in the dissipation mechanism. Our approach brings new insight to the eumelanin photochemistry and shows that one of the eumelanin components has great photoprotection properties against UV-light, while the other one present longer excited state lifetimes that leave more time to the molecule to produce radicals and reactive species, possibly responsible of melanoma formation. We hope to have brought a better understanding to the property of the black polymer and opened a way to deepen the study of melanin and its interaction with UV-light.
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| 5. |
- Corani, Alice, et al.
(författare)
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Superior photoprotective motifs and mechanisms in eumelanins uncovered.
- 2014
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 136:33, s. 11626-11635
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Tidskriftsartikel (refereegranskat)abstract
- Human pigmentation is a complex phenomenon commonly believed to serve a photoprotective function through the generation and strategic localization of black insoluble eumelanin biopolymers in sun exposed areas of the body. Despite compelling biomedical relevance to skin cancer and melanoma, eumelanin photoprotection is still an enigma: What makes this pigment so efficient in dissipating the excess energy brought by harmful UV-light as heat? Why has Nature selected 5,6-dihydroxyindole-2-carboxylic acid (DHICA) as the major building block of the pigment instead of the decarboxylated derivative (DHI)? By using pico- and femtosecond fluorescence spectroscopy we demonstrate herein that the excited state deactivation in DHICA oligomers is 3 orders of magnitude faster compared to DHI oligomers. This drastic effect is attributed to their specific structural patterns enabling multiple pathways of intra- and interunit proton transfer. The discovery that DHICA-based scaffolds specifically confer uniquely robust photoprotective properties to natural eumelanins settles a fundamental gap in the biology of human pigmentation and opens the doorway to attractive advances and applications.
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| 6. |
- Corani, Alice, et al.
(författare)
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Ultrafast Dynamics of Hole Injection and Recombination in Organometal Halide Perovskite Using Nickel Oxide as p-Type Contact Electrode.
- 2016
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 7, s. 1096-1101
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Tidskriftsartikel (refereegranskat)abstract
- There is a mounting effort to use nickel oxide (NiO) as p-type selective electrode for organometal halide perovskite-based solar cells. Recently, an overall power conversion efficiency using this hole acceptor has reached 18%. However, ultrafast spectroscopic investigations on the mechanism of charge injection as well as recombination dynamics have yet to be studied and understood. Using time-resolved terahertz spectroscopy, we show that hole transfer is complete on the subpicosecond time scale, driven by the favorable band alignment between the valence bands of perovskite and NiO nanoparticles (NiO(np)). Recombination time between holes injected into NiO(np) and mobile electrons in the perovskite material is shown to be hundreds of picoseconds to a few nanoseconds. Because of the low conductivity of NiO(np), holes are pinned at the interface, and it is electrons that determine the recombination rate. This recombination competes with charge collection and therefore must be minimized. Doping NiO to promote higher mobility of holes is desirable in order to prevent back recombination.
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| 7. |
- Huijser, Annemarie, et al.
(författare)
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Photophysics of indole-2-carboxylic acid in an aqueous environment studied by fluorescence spectroscopy in combination with ab initio calculations.
- 2012
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Ingår i: Physical chemistry chemical physics : PCCP. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 14:6, s. 2078-2086
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Tidskriftsartikel (refereegranskat)abstract
- The photo-physics and -chemistry of indoles are known to be highly complex and strongly dependent on their precise molecular structure and environment. Combination of spectroscopic analysis with quantum chemical calculations should be a powerful tool to unravel precise excited state deactivation mechanisms. At the same time, combined studies are seldom and likely far from trivial. In this work we explore the feasibility of combining spectroscopic and quantum-chemical data into one consistent model. The molecule of choice is indole-2-carboxylic acid (ICA) in aqueous media. Excited state dynamics are determined by time-resolved fluorescence experiments, while excited state reaction pathways of ICA-H(2)O clusters are explored by ab initio calculations.
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| 8. |
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| 9. |
- Nogueira, Juan J., et al.
(författare)
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Sequential Proton-Coupled Electron Transfer Mediates Excited-State Deactivation of a Eumelanin Building Block
- 2017
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 8:5, s. 1004-1008
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Tidskriftsartikel (refereegranskat)abstract
- Skin photoprotection is commonly believed to rely on the photochemistry of 5,6-dihydroxyindole (DHI)- and 5,6-dihydroxyindole-2-carboxylic acid (DHICA)-based eumelanin building blocks. Attempts to elucidate the underlying excited-state relaxation mechanisms have been partly unsuccessful due to the marked instability to oxidation. We report a study of the excited-state deactivation of DHI using steady-state and time-resolved fluorescence accompanied by high-level quantum-chemistry calculations including solvent effects. Spectroscopic data show that deactivation of the lowest excited state of DHI in aqueous buffer proceeds on the 100 ps time scale and is 20 times faster than in methanol. Quantum-chemical calculations reveal that the excited-state decay mechanism is a sequential proton-coupled electron transfer, which involves the initial formation of a solvated electron from DHI, followed by the transfer of a proton to the solvent. This unexpected finding would prompt a revision of current notions about eumelanin photophysics and photobiology.
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