Guillermo Orellana

Influence of Surface Hydroxylation on 3-Aminopropyltriethoxysilane Growth Mode during Chemical Functionalization of GaN Surfaces: An Angle-Resolved X-ray Photoelectron Spectroscopy Study

A comparative study of the chemical functionalization of undoped, n- and p-type GaN layers grown on sapphire substrates by metal-organic chemical vapor deposition was carried out. Both types of samples were chemically functionalized with 3-aminopropyltriethoxysilane (APTES) using a well-established silane-based approach for functionalizing hydroxylated surfaces. The untreated surfaces as well as those modified by hydroxylation and APTES deposition were analyzed using angle-resolved X-ray photoelectron spectroscopy. Strong differences were found between the APTES growth modes on n- and p-GaN surfaces that can be associated with the number of available hydroxyl groups on the GaN surface of each sample. Depending on the density of surface hydroxyl groups, different mechanisms of APTES attachment to the GaN surface take place in such a way that the APTES growth mode changes from a monolayer to a multilayer growth mode when the number of surface hydroxyl groups is decreased. Specifically, a monolayer growth mode with a surface coverage of approximately 78% was found on p-GaN, whereas the formation of a dense film, approximately 3 monolayers thick, was observed on n-GaN.

PHOTOINDUCED ELECTRON TRANSFER QUENCHING OF EXCITED Ru(II) POLYPYRIDYLS BOUND TO DNA: THE ROLE OF THE NUCLEIC ACID DOUBLE HELIX

In the presence of double helical polynucleotides (sodium poly(dA‐dT)‐poly(dA‐dT) or calf thymus DNA), the efficiency of oxidative or reductive electron transfer between photoexcited ruthenium(II) chelates Ru(tap)2(hat)2+ or Ru(phen)⅔+ (where tap =1,4,5,8‐tetraazaphenanthrene, hat = 1,4,5,8,9,12‐hexaazatriphenylene, and phen = 1,10‐phenanthroline) and appropriate cationic quenchers (ethidium, Ru(NH3)3/6+, methyl viologen, or M(phen)3/3+, where M = Co, Rh, Cr) increases1–2 orders of magnitude compared to the efficiency of the same quenching in microhomogeneous aqueous medium (kq= 0.3‐1.8 × 109 Ms−1). The enhancement is more pronounced when the binding constant of the quencher (103 < Kh < 106M−1) is large. Similar trends are found when the biopolymers are replaced by sodium poly(styrenesulfonate) (PSS). The accelerated electron transfer process is proposed to be due mainly to the effect of accumulation of the reagents in the electrostatic field of the polymer; if corrections for this effect are introduced (e.g. ratioing [quencher]/[polynucleotide]), the reaction rate becomes essentially independent of the polymer concentration. Based upon a model for electron transfer reaction of the complexes within a small cylindrical interface around the DNA helix, calculations of the bimolecular electron transfer rate constants are computed to be 10′ times smaller when the reactants are bound to the double‐stranded polynucleotides and decreased mobility of the cationic species is apparent. The effect is less pronounced if a simpler polyelectrolyte (PSS) is employed. Emission lifetimes of the Ru(II) polypyridyls bound to the DNA (0.32–2μs, double exponential decays) are discussed as well.

Effect of the template and functional monomer on the textural properties of molecularly imprinted polymers.

Molecularly imprinted polymers (MIPs) for zearalenone analysis have been synthesized using the template mimics cyclododecyl 2,4-dihydroxybenzoate (CDHB), resorcinol and resorcylic acid. The MIPs are photochemically prepared from 2-(diethylamino)ethyl methacrylate (2-DAEM), 4-vinylpyridine (VIPY), 2-hydroxyethyl methacrylate (HEMA) or 1-allylpiperazine (1-ALPP) as the functional monomers, trimethylolpropane trimethacrylate (TRIM) as cross-linker, azobis(isobutyronitrile) as initiator and acetonitrile as porogen. Non-imprinted polymers have been also synthesized for reference purposes. The textural properties of the novel polymers (BET areas, pore volumes and pore size distributions) have been determined from nitrogen adsorption-desorption isotherms. These parameters have shown to be strongly dependent on the presence of the template and the monomer nature. Scanning electron microscopy and solvent uptake experiments support these findings. Microporosity contributes less than 7% to the total pore volume for all the polymers prepared. Interestingly, a 3.5 nm pore opening is observed for all the polymers and additional pore apertures in the 20-40 nm region for VIPY-, HEMA- and 2-DAEM-based MIPs whereas a much wider opening size distribution has been measured for the 1-ALPP-based MIP. Molecular modeling and, particularly, (1)H NMR experiments demonstrate the strong (2:1) complex formed between 1-ALPP and the diphenolic CDHB (K(11)=4.7 x 10(4)M(-1) and K(12) = 2.6 x 10(2)M(-1) in acetonitrile) that make the corresponding MIP the most suitable for zearalenone recognition in real samples.

Luminescent core-shell imprinted nanoparticles engineered for targeted Förster resonance energy transfer-based sensing.

Red-luminescent 200 nm silica nanoparticles have been designed and prepared as a versatile platform for developing FRET (Förster resonance energy transfer) biomimetic assays. Ru(phen)₃²⁺ dye molecules embedded off-center in the silica core provide the long-lived donor emission, and a near-infrared labeled analyte serves as fluorescent acceptor (the measured R₀ of this D-A pair is 4.3 nm). A thin surface-grafted molecularly imprinted polymer (MIP) shell intervenes as selective enrofloxacin-binding element. These nanoparticles have been tested for photochemical detection of enrofloxacin by using a competitive scheme that can be readily performed in MeCN-HEPES (pH 7.5) 7:3 (v/v) mixtures and allows for the antibiotic detection in the μM range (LOD = 2 μM) without optimization of the assay. Given the well-known difficulties of coupling the target-binding-to-MIP and the transducing events, the novel photochemical approach tuned up here will be valuable in future developments of MIP-based assays and optosensors that capitalize also on the advantages of nanomaterials for (bio)analysis.

A Ruthenium Probe for Cell Viability Measurement Using Flow Cytometry, Confocal Microscopy and Time‐resolved Luminescence ¶

The capability of the new luminescent probe (dibenzo[h,j]dipyrido[3,2‐a:2′,3′‐c]phenazine)bis(2,2′‐bipyridine)ruthenium(II) dication, (RB2Z), to discriminate live and dead cells has been tested on rat hepatocytes and mouse lymphocytes. RB2Z‐stained cells were analyzed using flow cytometry, fluorescence (confocal) microscopy and time‐resolved luminescence measurements. The established viability probes propidium iodide (PI) and SYTOX® green (SG) were used as controls. The intense luminescence of RB2Z at 606 nm is localized in the nucleus of nonviable cells. Viability measurements by flow cytometry and fluorescence microscopy using RB2Z as dead‐cell marker yield the same results as PI and SG. The luminescence lifetime of RB2Z also displays sensitivity to cell viability (0.45 and 0.82 μs in presence of fully viable and dead cells, respectively). This ruthenium complex is photostable under laser sources and its 200 nm Stokes shift facilitates multicolor labeling experiments in flow cytometry and fluorescence microscopy. Unlike the currently available probes, the long‐lived excited state of RB2Z also allows assays based on luminescence decay measurements.

Immuno-Like Assays and Biomimetic Microchips

Biomimetic assays with molecularly imprinted polymers (MIPs) are bound to be an alternative to the traditional immuno-analytical methods based on antibodies. This is due to the unique combination of advantages displayed by the artificial materials including the absence of animal inoculation and sacrifice, unnecessary hapten conjugation to a carrier protein for stimulated production, the possibility of manufacturing MIPs against toxic substances, excellent physicochemical stability, reusability, ease of storage, and recognition in organic media. If the selectivity and affinity of MIPs are increased, many more immuno-like assays will be developed using radioactive, enzymatic, colorimetric, fluorescent, chemiluminescent, or electrochemical interrogation methods. This chapter provides a comprehensive comparison between the bio- and biomimetic entities and their usage.

In vitro antiamyloidogenic properties of 1,4-naphthoquinones.

The aim of this study is to find out whether several 1,4-naphthoquinones (1,4-NQ) can interact with the amyloidogenic pathway of the amyloid precursor protein processing, particularly targeting at β-secretase (BACE), as well as at β-amyloid peptide (Aβ) aggregation and disaggregating preformed Aβ fibrils. Compounds bearing hydroxyl groups at the quinoid (2) or benzenoid rings (5, 6) as well as some 2- and 3-aryl derivatives (11-15) showed BACE inhibitory activity, without effect on amyloid aggregation or disaggregation. The halogenated compounds 8 and 10 were selective for the inhibition of amyloid aggregation. On the other hand, 1,4-naphthoquinone (1), 6-hydroxy-1,4-naphthoquinone (4) and 2-(3,4-dichlorophenyl)-1,4-naphthoquinone (26) did not show any BACE inhibitory activity but were active on amyloid aggregation and disaggregation preformed Aβ fibrils. Juglone (5-hydroxy-1,4-naphthoquinone (3), and 3-(p-hydroxyphenyl)-5-methoxy-1,4-napththoquinone (19) were active on all the three targets. Therefore, we suggest that 1,4-NQ derivatives, specially 3 and 19, should be explored as possible drug candidates or lead compounds for the development of drugs to prevent amyloid aggregation and neurotoxicity in Alzheimers disease.

Interaction of Sulfonated Ruthenium(II) Polypyridine Complexes with Surfactants Probed by Luminescence Spectroscopy

Novel anionic [RuL2L′]2− complexes, where L stands for (1,10-phenanthroline-4,7-diyl)bis(benzenesulfonate) (pbbs; 3a) or (2,2′-bipyridine)-4,4′-disulfonate (bpds; 3b), and L′ is N-(1,10-phenanthrolin-5-yl)tetradecanamide (pta; 2a) or N-(1,10-phenanthrolin-5-yl)acetamide (paa; 2b), were synthesized, and their interaction with the prototypical surfactants sodium dodecylsulfate (SDS), cetyl trimethyl ammonium bromide (CTAB), and Triton X-100 (TX-100) was investigated by electronic absorption, luminescence spectroscopy, emission-lifetime determinations, and O2-quenching measurements. [Ru(pbbs)2(pta)]2− (5a) displayed cooperative self-aggregation in aqueous medium at concentrations above 1.3 μM; the observed association was enhanced in the presence of either β-cyclodextrin or NaCl. This amphiphilic RuII compound showed the strongest interaction with all the detergents tested: nucleation of surfactant molecules around the luminescent probe was observed below their respective critical micellar concentrations. As much as a 12-fold increase of the emission intensity and a 3-fold rise in the lifetime were measured for 5a bound to TX-100 micelles; the other complexes showed smaller variations. The O2-quenching rate constants decreased up to 1/8 of their original value in H2O (e.g., for [Ru(bpds)2(pta)]2− (6a) bound to CTAB micelles). Luminescence-lifetime experiments in H2O/D2O allowed the determination of the metal-complex fraction exposed to solvent after binding to surfactant micelles. For instance, such exposure was as low as 25% for pta complexes⋅CTAB aggregates. The different behaviors observed were rationalized in terms of the RuII complex structure, the electrostatic/hydrophobic interactions, and the probe environment.

Enhanced performance of a fibre-optic luminescence CO2 sensor using carbonic anhydrase

Abstract Incorporation of carbonic anhydrase into the indicator layer placed at the distal end of an optical fibre has yielded a fast-responding luminescence sensor for carbon dioxide in the gas phase. The measuring principles rests on a CO 2 -modulated proton transfer from the inner phosphate buffer molecules to photoexcited tris[2-(2-pyrazinyl)thiazole]ruthenium(II) cation, immobilized onto an anionic dextra gel. The best performance of the optical device id discussed in terms of internal buffeer pH, enzymatic activity and temperature of operation. Optimization of such parameters yields values of 8.5, 4500 U ml −1 and 20–25 °C, respectively. Under these conditions, the fibre-optic sensor is able to respond in 40 s ( t 90 ) to a 0–100% CO 2 concentration change, and in 2 min ( t 90 ) to a reverse 100-0% CO 2 dose variation, compared to 2 and 6 min, respectively, for the non-enzymatic device.

Pulmonary surfactant layers accelerate O2 diffusion through the air-water interface

During respiration, it is accepted that oxygen diffuses passively from the lung alveolar spaces through the respiratory epithelium until reaching the pulmonary capillaries, where blood is oxygenated. It is also widely assumed that pulmonary surfactant, a lipid-protein complex secreted into alveolar spaces, has a main surface active function, essential to stabilize the air-liquid interface, reducing in this way the work of breathing. The results of the present work show that capillary water layers containing enough density of pulmonary surfactant membranes transport oxygen much faster than a pure water phase or a water layer saturated with purely lipidic membranes. Membranes reconstituted from whole pulmonary surfactant organic extract, containing all the lipids plus the hydrophobic surfactant proteins, permit also very rapid oxygen diffusion, substantially faster than achieved by membranes prepared from the surfactant lipid fraction depleted of proteins. A model is proposed suggesting that protein-promoted membrane networks formed by pulmonary surfactant might have important properties to facilitate oxygenation through the thin water layer covering the respiratory surface.