Electrochemical Control of pH in Nanoliter Volumes.

The electrochemical management of the proton concentration in miniaturized dimensions opens the way to control and parallelize multistep chemical reactions, but still it faces many challenges linked to the efficient proton generation and control of their diffusion. Here we present a device operated electrochemically that demonstrates the control of the pH in a cell of ∼140 nL. The device comprises a microfluidic reactor integrated with a pneumatic mechanism that allows the exchange of reagents and the isolation of protons to decrease the effect of their diffusion. We monitored the pH with a fluorescence marker and calculated the final value from the redox currents. We demonstrate a large pH amplitude control from neutral pH values beyond the fluorescence marker range at pH 5. On the basis of the calculations from the Faradaic currents, the minimum pH reached should undergo pH ∼ 0.9. The pH contrast between neutral and acid pH cells can be maintained during periods longer than 15 min with an appropriate design of a diffusion barrier.

Aerobic denitration of 2,4,6-trinitrotoluene in the presence of phenazine compounds and reduced pyridine nucleotides.

Phenazine-containing spent culture supernatants of Pseudomonas aeruginosa concentrated with a C18 solid-phase extraction cartridge initiate NAD(P)H-dependent denitration of 2,4,6-trinitrotoluene (TNT). In this study, TNT denitration was investigated under aerobic conditions using two phenazine secondary metabolites excreted by P. aeruginosa, pyocyanin (Py) and its precursor phenazine-1- carboxylic acid (PCA), and two chemically synthesized pyocyanin analogs, phenazine methosulfate (PMS+) and phenazine ethosulfate (PES+). The biomimetic Py/NAD(P)H/O2 system was characterized and found to extensively denitrate TNT in unbuffered aqueous solution with minor production of toxic amino aromatic derivatives. To a much lesser extent, TNT denitration was also observed with PMS+ and PES+ in the presence of NAD(P)H. No TNT denitration was detected with the biomimetic PCA/NAD(P)H/O2 system. Electron paramagnetic resonance (EPR) spectroscopy analysis of the biomimetic Py/NAD(P)H/O2 system revealed the generation of superoxide radical anions (O2 •−). In vitro TNT degradation experiments in the presence of specific inhibitors of reactive oxygen species suggest a nucleophilic attack of superoxide radical anion followed by TNT denitration through an as yet unknown mechanism. The results of this research confirm the high functional versatility of the redox-active metabolite pyocyanin and the susceptibility of aromatic compounds bearing electron withdrawing substituents, such as nitro groups, to superoxide-driven nucleophilic attack.

Copper-CNT interfacing with Cu-doped polydopamine in CNT carpet: copper nucleation and resistance decrease upon soft annealing.

In recent years, Cu-CNT composites have attracted much attention due to their remarkable properties, in comparison to pure copper, such as higher ampacity and a lower thermal coefficient of resistance. However, the fabrication of an efficient Cu-CNT composite is still challenging, mainly due to the high cuprophobicity of CNTs. To strengthen the chemical interactions between Cu and CNTs, we propose using a Cu-doped polydopamine coating as an interface between CNTs and metallic copper. This work reports on the nucleation of copper particles on the surface of MWCNTs that are coated with Cu-doped polydopamine after annealing in an inert atmosphere at 573, 673 and 773 K. We show, for the first time to the best of our knowledge, that the polydopamine coating oxidizes during annealing and efficiently reduces Cu ions into metallic Cu. Interestingly, the sheet resistance of coated CNT carpets can be reduced by 33 and 37.6% after annealing at 573 and 773 K, respectively. Furthermore, the sheet resistance decrease does not depend on the size of copper particles (diameter ranging from 13 to 27 nm) or their surface density (from 2.27 × 1010 to 5.7 × 108 particles per cm2). This sheet resistance drop is mainly attributed to the appearance of pyridinic nitrogen in the Cu-doped polydopamine structure after annealing at 673 K and above. Finally, we measure a negative temperature coefficient of resistance for all of the CNT carpets.

Mesoporous TiO2 anatase films for enhanced photocatalytic activity under UV and visible light.

Mesoporous TiO2 films with enhanced photocatalytic activity in both UV and visible wavelength ranges were developed through a non-conventional atomic layer deposition (ALD) process at room temperature. Deposition at such a low temperature promotes the accumulation of by-products in the amorphous TiO2 films, caused by the incomplete hydrolysis of the TiCl4 precursor. The additional thermal annealing induces the fast recrystallisation of amorphous films, as well as an in situ acidic treatment of TiO2. The interplay between the deposition parameters, such as purge time, the amount of structural defects introduced and the enhancement of the photocatalytic properties from different mesoporous films clearly shows that our easily upscalable non-conventional ALD process is of great industrial interest for environmental remediation and other photocatalytic applications, such as hydrogen production.

Influence of polymerisation on the reversibility of low-energy proton exchange reactions by Para-Aminothiolphenol.

The reversibility of redox processes is an important function for sensing and molecular electronic devices such as pH reporters or molecular switches. Here we report the electrochemical behaviour and redox reversibility of para-aminothiolphenol (PATP) after different polymerisation methods. We used electrochemical and photo-polymerisation in neutral buffers and plasma polymerisation in air to induce reversible redox states. The chemical stoichiometry and surface coverage of PATP in the polymerized layers were characterized by X-ray photoelectron spectroscopy (XPS), while cyclic voltammetry (CV) was used to measure the charge transfer, double layer capacitance and electrochemical rate of the layers during successive potential cycles. Our results show that the surface coverage of the redox active species is higher on electro-polymerised samples, however, after consecutive cycles all the methods converge to the same charge transfer, while the plasma polymerised samples achieve higher efficiency per molecule and UV polymerised samples have a higher electron transfer rate.

Colloidal lithography using silica particles: improved particle distribution and tunable wetting properties.

Colloidal lithography rests on the adhesion of colloids in a relatively ordered pattern on a charged surface owing to electrostatic interactions. However, due to capillary forces, the colloids tend to form aggregates during the drying process. These capillary forces are especially strong for large hydrophilic particles. In this paper, different experimental approaches are explored to limit the aggregation of large (500 nm) silica particles deposited on glass substrates that were previously treated with polyallylamine (PAH), a polycation. These approaches consist in the addition of smaller colloids between the large ones, or of a layer of macromolecules (PAH or albumin) on top of the deposit. Scanning electron microscopy observations show that the addition of PAH and even better of albumin on top of the adherent colloids efficiently limits the formation of aggregates. Interestingly, the water contact angle of the surface obtained after silica colloid deposition and albumin adsorption is very high (~95°), while very hydrophilic surfaces are obtained after calcination. This is discussed in light of the Wenzel and Cassie-Baxter models. In conclusion, the proposed method allows a nanoscale topographic pattern with tunable wettability to be created on large surface areas using a soft and inexpensive technique.

Hydroxyl radical release from dental resins: electron paramagnetic resonance evidence.

It is well known that polymeric free radicals remain trapped inside dental resins for a long time after photopolymerization. Moreover, although these high molecular mass compounds have very limited mobility, there is evidence to suggest that they disappear progressively over time. The purpose of this study was to provide new experimental data to help understand this phenomenon. To determine whether low molecular mass free radicals are released by dental composites stored in hydrophilic media, we used electron paramagnetic resonance spectroscopy to perform spin-trapping experiments on experimental and commercial samples stored in ethanol. Under these conditions, ethoxy radicals were produced. Further experiments demonstrated that (1) hydroxyl radicals were released from the methacrylated resin and (2) they reacted with ethanol molecules to produce "secondary" ethoxy free radicals. In addition to the well-known monomer toxicity of methacrylated resins, we may have identified a new source of concern for these biomaterials.

Kinetic study of free radicals trapped in dental resins stored in different environments.

In this work, we used electron paramagnetic resonance to follow the decrease kinetics of free radicals trapped in an experimental resin (ER) and in a commercial composite (Charisma (Ch)) stored under different conditions (in air at 25 and 37 degrees C; in argon, oxygen and water at 25 degrees C). During the first day, the decay was fast (0-24h-rate of decay of allylic radical: 1700-1000a.u. for Ch, 1700-1500a.u. for ER) and the storage conditions had no influence on the kinetics. This phase was ascribed to a post-polymerization phenomenon. From 1day to 1month, the rate of decay depended on the storage environment. In argon, free radicals were quite stable (1day to 1month-rate of decay of allylic radical: 1200-1000a.u. for Ch, 1400-1200a.u. for ER). For the other storage environments, in ER, the rate of decay was higher in water than in oxygen and in air (1day to 1month-rate of decay of allyl radical: 1400a.u. to 100, 500 and 800a.u., respectively). In Ch, free radicals faded quicker than in ER, as undetectable levels were reached before 1month, which attests to the influence of fillers on radical decrease kinetics. Heating experiments were also performed, and free radical concentrations decreased faster at higher temperatures, especially above the glass transition temperature. In conclusion, ambient oxygen is mainly involved in the termination process of free radicals. Therefore, conditions influencing oxygen diffusion have an impact on radical kinetics as well.