Publications

Abstract Herein, we present a simple microelectrode preparation methodology consisting in coating a platinum wire or a carbon fiber with a thin insulating Parylene C film (e.g. 1-10 mu m), to produce SECM probes with a small and constant probe RG (i.e. ratio between the radius of the insulating sheath and the radius of the active electrode area). After exposition of a fresh active electrode area by blade cutting, a disc shaped electrode is obtained thanks to a protective hot mounting wax layer that avoids Parylene C coating deformation and is easily removed with acetone. Stiffness and straightness of the probe can be tuned by modifying the Parylene C coating thickness and the length of the carbon fiber or platinum wire. This simple electrode preparation method is highly reproducible (c.a. > 90%). The prepared Parylene C coated microelectrodes were characterized by optical microscopy, cyclic voltammetry, scanning electrochemical microscopy (SECM) approach curves and finally applied to SECM imaging of Pt band structures in contact-less and contact mode.

Abstract A novel optical fiber sensor is presented for measuring dissolved CO2 for water quality monitoring applications, where the optical signal is based either on refractive index changes or on color change. The sensing chemistry is based on the acid-basic equilibrium of 4-nitrophenol, that is converted into the anionic form by addition quaternary ammonium hydroxide. The CO2 sensitive layer was characterized and tested by using simple absorbance/reflectance measurement setups where the sensor was connected to a fiber optic CCD spectrometer. A prototype simulating a real shallow raceway aquaculture system was developed and its hydraulic behavior characterized. A commercially available partial-pressure-NDIR sensor was used as a reference for dissolved CO2 tests with the new optical fiber sensor under development. Preliminary tests allowed verifying the suitability of the new optical sensor for accurately tracking the dissolved carbon dioxide concentration in a suitable operation range. Direct comparison of the new sensor and the reference sensor system allowed to demonstrate the suitability of the new technology but also to identify some fragilities there are presently being addressed.

Abstract Gold nanoparticle (Au NP) mirrors, which exhibit both high reflectance and electrical conductance, were self-assembled at a [heptane + 1,2-dichloroethane]/water liquid/liquid interface. The highest reflectance, as observed experimentally and confirmed by finite difference time domain calculations, occurred for Au NP films consisting of 60 nm diameter NPs and approximate monolayer surface coverage. Scanning electrochemical microscopy approach curves over the interfacial metallic NP films revealed a transition from an insulating to a conducting electrical material on reaching a surface coverage at least equivalent to the formation of a single monolayer. Reflectance and conductance transitions were Interpreted as critical junctures corresponding to a surface coverage that exceeded the percolation threshold of the Au NP films at the [heptane + 1,2-dichloroethane]/water interface.

Abstract The decomposition reaction of dimethyl-1,2-dioxetanone in dichloromethane was studied by using a DFT approach. The low efficiency of triplet and singlet excited-state formation was rationalised. A charge-transfer process was demonstrated to be involved in the chemiluminescence process. Present and previous results allow us to define an interstate crossing-induced chemiexcitation (ICIC) mechanism for the chemiluminescence of dioxetanones. Charge transfer is needed to reach a transition state, in the vicinity of which direct population of excited states is possible. The chemiexcitation process is then governed by singlet/triplet intersystem crossings. Structural modifications then modify the rate of these crossings and the singlet ground and excited-state interaction, thereby modulating the efficiency of this process and the spin of the resulting products.

Abstract Macromolecules, such as polyethylene glycol (PEG), have been frequently used in the preparation of xerogels, mainly with the purpose of tuning the meso- or macroporosity. However, PEG has never been applied in the context of the preparation of molecularly imprinted xerogels for small molecules. Thus, we decided to conduct a computational and experimental study of the incorporation of PEG into formerly studied sol-gel mixtures for the preparation of damascenone-imprinted xerogels. Computationally, two types of pregelification models were studied, one representing the initial mixture (SI3/SIPA:S:3 models) and the other representing the same mixtures after considerable solvent loss (SI3/SIPA:40:1 models). The latter ones were particularly prolific in providing clear effects of the PEG. In the SI3:40:1 model (containing SI3 units of Si3O3(OH)(6) mimicking the final xerogels backbone), a prohibitive instead of a promoting effect of PEG on the template-SI3 association was observed. PEG was found to interpose the SI3 aggregates, turning them smaller and more disperse. In agreement with that, a much higher porosity and surface area were found for the corresponding xerogel prepared with PEG, while no appreciable improvement of the imprinting efficiency could be observed. In the SIPA:40:1 model (containing both SI3 and SIPA units; SIPA, Si3O3(OH)(5)C3H6NHC6H5, representing the introduction of the organic functional group into the xerogel network), the interactions related to the network structuring were not significantly affected. This was due to the fact that the SIPA units themselves had a dispersive effect on the silica network; the PEG molecules were "pushed" into the aqueous/methanolic continuum, and their presence was somewhat redundant. Accordingly, both prepared SIPA-xerogels (with PEG or not) exhibited higher porosity compared to SI3-xerogels. Although the simulation results were not conclusive about the effect of PEG on the template-functional group association, experimentally it was clear that the imprinting effect was not improved with PEG.

Abstract Firefly oxyluciferin presents a pH-sensitive fluorescence in aqueous solutions. Its fluorescence spectra are composed of two green peaks at different pH values, despite the enolate anion being the only emitter. A computational approach was used to further elucidate the photoprotolytic cycle of oxyluciferin and investigate its pH sensitivity. It was found that oxyluciferin forms - stacking complexes both in the ground and excited states, at basic and acidic/neutral pH. However, at different pH values, these complexes adopt a different conformation, which explains the lower energy of the emission at acidic/neutral pH, in comparison with the emission at basic pH.

Abstract Herein we describe the synthesis of water-soluble platinum nanodendrites in dimethylformamide (DMF), in the presence of polyethyleneimine (PEI) as a stabilizing agent. The average size of the dendrites is in the range of 20-25 nm while their porosity can be tuned by modifying the concentration of the metal precursor. Electron tomography revealed different crystalline orientations of nanocrystallites in the nanodendrites and allowed a better understanding of their peculiar branching and porosity. The high surface area of the dendrites (up to 22 m(2) g(-1)) was confirmed by BET measurements, while X-ray diffraction confirmed the abundance of high-index facets in the face-centered-cubic crystal structure of Pt. The prepared nanodendrites exhibit excellent performance in the electrocatalytic oxidation of ethanol in alkaline solution. Sensing, selectivity, cycleability and great tolerance toward poisoning were demonstrated by cyclic voltammetry measurements.

Abstract To gain deeper insight into the structure of the electrode-ionic liquid interface differential capacitance-potential curves have been acquired through electrochemical impedance measurements at the mercury electrode. Understanding the role of the nature and size of the ionic species in the ionic liquid, namely the influence of different lengths of alkyl side chains of the imidazolium cation on the structure of the double layer, is the aim of the work being carried out. Differential capacitance-potential curves were obtained at the mercury electrode in contact with various mixtures of room-temperature ionic liquids of varying cation chain length (1-alkyl-3-methylimidazolium, [CnMIM](+) where n = 2, 6 and 12) with the same anion (bis(trifluoromethylsulfonyl)imide, [Tf2N](-)). For comparison purposes we have also carried out electrochemical measurements of the pure ionic liquids under the same conditions. Systematic variations in the electrocapillary drop-time curves and capacitance were found with increasing amounts of the cation with longer alkyl chains in the mixture. The electrocapillary curve results may be interpreted as indicating the preferential adsorption of the longer alkyl group on the mercury surface.

Abstract The use of agar-based biomaterials for the development of emerging areas, such as tissue engineering or 'smart materials' production has recently gained great interest. Understanding how these gel-forming polysaccharides self-organise in aqueous media and how these associations can be tuned to meet the specific needs of each application is thus of great relevance. As an extension of previous pioneering research concerning the application of the microwave-assisted extraction (MAE) technique in the recovery of native (NA) and alkali-modified (AA) agars, this article focuses on the different molecular assemblies assumed by these novel NA and AA when using different MAE routes. The molecular architectures in dilute (5, 10, 50 and 100 mg mL(-1)) and concentrated (1.5% (w/w)) aqueous media were imaged by AFM and cryoSEM, respectively. Relevant structural and physicochemical properties were investigated to support the microscopic data. Different extraction routes led to polysaccharides with unique properties, which in turn resulted in different molecular assemblies. Even at 5 mu g mL(-1), AFM images included individual fibers, cyclic segments, aggregates and local networks. At higher polymer concentrations, the structures further aggregated forming multilayer polymeric networks for AA. The more compact and denser 3D networks of AA, imaged by cryoSEM, and their higher resistance to large deformations matched the 2D-shapes observed by AFM. Depending on the nature of the AA chains, homogeneous or heterogeneous growth of assemblies was seen during network formation. The obtained results support well the view of double helix formation followed by intensive double helix association proposed for agar gelation.