Spectroscopie ultraviolet-visible

La spectroscopie ultraviolet-visible ou spectrométrie ultraviolet-visible est une technique de spectroscopie mettant en jeu les photons dont les longueurs d'onde sont dans le domaine de l'ultraviolet ( - ), du visible ( - ) ou du proche infrarouge ( - ). Soumis à un rayonnement dans cette gamme de longueurs d'onde, les molécules, les ions ou les complexes sont susceptibles de subir une ou plusieurs transitions électroniques. Cette spectroscopie fait partie des méthodes de spectroscopie électronique. Les substrats analysés sont le plus souvent en solution, mais peuvent également être en phase gazeuse et plus rarement à l'état solide. Le spectre électronique est la fonction qui relie l'intensité lumineuse absorbée par l'échantillon analysé en fonction de la longueur d'onde. Le spectre est le plus souvent présenté comme une fonction de l'absorbance en fonction de la longueur d'onde. Il peut aussi être présenté comme le coefficient d'extinction molaire en fonction de la longueur d'onde.

Identification of Isomeric Lipids by UV Spectroscopy of Noncovalent Complexes with Aromatic Molecules

The tremendous structural and isomeric diversity of lipids enables a wide range of their functions in nature but makes the identification of these biomolecules challenging. We distinguish and quantify isomeric lipids using cold ion UV fragmentation spectroscopy of their noncovalent complexes with aromatic amino acids and dipeptides. On the basis of structural simulations, specific isomer-sensitive aromatic "sensors" have been preselected for lipids of each studied class. Tyrosine appeared to be a good "sensor" to distinguish steroids and prostaglandins, which are rich in functional groups, while diphenylalanine is a better choice for sensing largely hydrophobic phospholipids. With this sensor, the relative concentrations of two isomeric glycerophospholipids mixed in solution have been determined with 3.3% accuracy, which should degrade only to 3.7% for a 14 s express measurement.

AMER CHEMICAL SOC2021

Chemosensors Based on Metal-Dye Complexes for Bioanalytes

Friederike Gundula Marie Zaubitzer

Multicomponent chemosensors were prepared by mixing the rhodium(III) complex [(η5-C5Me5)RhCl 2]2, nickel(II) chloride and/or copper(II) chloride with commercially available dyes in buffered aqueous solution. The sensors were used to detect various biomolecules, which interact with the components of the sensing ensemble and alter the composition, and hence the color of the sample. The sensors were analyzed by UV/Vis spectroscopy. A mixture of [(η5-C5Me5)RhCl 2]2, three dyes and pyrophosphate was employed to determine in retrospect the time at which the target molecules ADP and ATP were added to the sensing ensemble. An artificial neural network was used to correlate absorbance data with analyte addition times. Dynamic combinatorial libraries of metal-dye complexes prepared from NiCl2, CuCl2 and three dyes were used to distinguish between mixtures of the peptide hormones angiotensin I and angiotensin II. The discriminative power of the sensor was increased by analyzing samples in a time-resolved fashion. Nine primary amines were identified in a sensing ensemble, where amines were first combined with pyridine-2-aldehyde in the presence of NiCl2 or CuCl2, in order to obtain metal-stabilized imine mixtures. A colorimetric output was generated by adding a dye. In an approach to sense conventional sugars in an indicator displacement assay, sugars were transformed in situ into sugar imines prior to the sensing experiment.

EPFL2010

Electrochemical Studies of Water Splitting in Organic Media

Jonnathan Cesar Hidalgo Acosta

The development of new routes towards the generation of environmentally friendly solar fuels, such as molecular hydrogen (H2) for use in fuel cells, is a grand challenge facing the scientific community. In this regard, water splitting represents one of the most promising, yet challenging, options. In nature, water splitting occurs under primarily non-aqueous conditions. This provides a blue print for the development of efficient systems for water oxidation (WOR) and hydrogen evolution (HER), profiting from the changes structure-reactivity in those complex environments. Herein, we investigate these two processes, ocurring in mainly non-aqueous environment. Another aspect addressed in this work, is the inmobilization of catalitycally active nanoparticles towards WOR, at the surface of an electrode. In first instance, a layer-by-layer methodology was used to prepare thin films consisting of bilayers of negatively charged citrate-stabilized IrOx nanoparticles (NPs) and the positively charged poly(diallyldimethylammonium chloride) (PDDA) polymer. The IrOx films obtained were amorphous, with the NPs therein being well dispersed and retaining their as-synthesized shape and sizes. UV/vis spectroscopic and electrochemical studies confirmed that electrochemically addressable surface coverage of IrOx NPs increased linearly with the number of bilayers. Moreover, the electrodes obtained exhibit an âidealâ electrochemical response, which resembles the highly reversible waves observed in hydrous iridium oxide films (HIROFs). Taking one step further, the concept of layer-by-layer inkjet printing was implemented for the fabrication of pH sensing electrodes based on IrOx. Those electrodes exhibited good performance, with a linear and near-Nernstian pH response. Water oxidation catalyzed by IrOx NPs in water/acetonitrile mixtures using [RuIII(bpy)3]3+ as oxidant was studied as a function of the water content, the acidity of the reaction media and the catalyst concentration. Under acidic conditions and at high water contents (80 % (v/v)) the reaction is slow, but its rate increases as the water content decreases, reaching a maximum at approximately equimolar proportions (â 25 % H2O (v/v)). At high water fractions, water is present in highly hydrogen-bonded arrangements and is less reactive. As the water content decreases, water-clustering gives rise to the formation of water-rich micro-domains, which are considerably more reactive towards oxygen production. Further analysis via electrochemical measurements at PDDA-IrOX modified electrodes, demonstrated that those changes in reactivity are correlated with decrease in the overpotential for WOR. Based on a straightforward thermodynamic analysis, those changes in reactivity were associated with favorable kinetics rather than a lowering in the thermodynamic barriers. Finally, the light-driven HER catalyzed by Pt in acidified acetonitrile solutions and using tetrathiafulvalene (TTF) as both, sensitizer and electron donor, was studied. Kinetic studies indicated that the reaction starts by strong and fast adsorption of TTF-H+ on the catalyst surface, followed by slow photoreduction of protons. On the other hand, the addition of water to the mixture of reaction leads to evolution of O2 by water oxidation, inhibiting HER. In both cases, no decomposition of the electron donor/acceptor was observed, indicating that both reactions are reversible and therefore the donor/acceptor potentially recyclable.

EPFL2016

Electrochemical Studies of Water Splitting in Organic Media

Jonnathan Cesar Hidalgo Acosta

EPFL2016