Engineering the physical properties and photocatalytic activities of a ß-ketoenamine COF using continuous flow synthesis.

Covalent Organic Frameworks (COF) having conjugated backbone are an interesting class of metal-free, visible light active, heterogeneous photocatalysts. Interestingly, synthesis of COF using continuous flow process has emerged as an efficient, alternative method when compared to the traditional batch process. Here, we demonstrate the possibility to engineer the physical properties and hence the adsorption and catalytic activities of a ß-ketoenamine COF by varying monomer flow rate and microreactor design during the continuous flow synthesis. Crystallinity of the COF increases on varying the monomer flow rate from 100 (S-100) to 500 (S-500) and up to 1000 µLmin-1 (S-1000), in an S-shaped microreactor, resulting in an enhanced surface area: 525, 722 and 1119 m2g-1 respectively. The photophysical properties of the COF are also found to vary significantly with the change in flow synthesis conditions. S-1000 is characterized by the highest adsorption of MB, due to its high surface area and accessible pores. On the other hand, S-500 shows the highest photocurrent, a low recombination of photogenerated charges and the lowest charge transfer resistance. Thus, S-500 is found to be the best photocatalyst for the removal of a model pollutant (methylene blue, MB). Further, enhanced photocatalytic removal of MB using S-500 could be achieved by performing the photocatalysis in continuous flow.

Visible-light active conducting polymer nanostructures with superior photocatalytic activity.

The development of visible-light responsive photocatalysts would permit more efficient use of solar energy, and thus would bring sustainable solutions to many environmental issues. Conductive polymers appear as a new class of very active photocatalysts under visible light. Among them poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most promising conjugated polymer with a wide range of applications. PEDOT nanostructures synthesized in soft templates via chemical oxidative polymerization demonstrate unprecedented photocatalytic activities for water treatment without the assistance of sacrificial reagents or noble metal co-catalysts and turn out to be better than TiO2 as benchmark catalyst. The PEDOT nanostructures exhibit a narrow band gap (E = 1.69 eV) and are characterized by excellent ability to absorb light in visible and near infrared region. The novel PEDOT-based photocatalysts are very stable with cycling and can be reused without appreciable loss of activity. Interestingly, hollow micrometric vesicular structures of PEDOT are not effective photocatalysts as compared to nanometric spindles suggesting size and shape dependent photocatalytic properties. The visible-light active photocatalytic properties of the polymer nanostructures present promising applications in solar light harvesting and broader fields.

Electron-induced growth mechanism of conducting polymers: a coupled experimental and computational investigation.

Pulse radiolysis was used to study the mechanism of HO(•)-induced polymerization of poly(3,4-ethylenedioxythiophene), PEDOT, in aqueous solution. A step-by-step mechanism has been found which involves a recurrent oxidation process by HO(•) hydroxyl radicals produced by water radiolysis. Furthermore, the cation radical, EDOT(•)(+), has been proposed as the promoter of the first step of polymerization. The determination of rate constants values and the attribution of transient and stable species were confirmed by molecular simulations and spectrokinetic analysis. Moreover, applying a series of electron pulses enabled in situ PEDOT polymerization. These polymers, which were characterized in solution or after deposition, form globular self-assembled structures with interesting conducting properties. Such a synthesis initiated for the first time by an electron accelerator gives us a glimpse of future promising industrial applications in the field of conducting polymers synthesis.

Conducting polymer nanostructures for photocatalysis under visible light.

Visible-light-responsive photocatalysts can directly harvest energy from solar light, offering a desirable way to solve energy and environment issues. Here, we show that one-dimensional poly(diphenylbutadiyne) nanostructures synthesized by photopolymerization using a soft templating approach have high photocatalytic activity under visible light without the assistance of sacrificial reagents or precious metal co-catalysts. These polymer nanostructures are very stable even after repeated cycling. Transmission electron microscopy and nanoscale infrared characterizations reveal that the morphology and structure of the polymer nanostructures remain unchanged after many photocatalytic cycles. These stable and cheap polymer nanofibres are easy to process and can be reused without appreciable loss of activity. Our findings may help the development of semiconducting-based polymers for applications in self-cleaning surfaces, hydrogen generation and photovoltaics.

Radiolytic method as a novel approach for the synthesis of nanostructured conducting polypyrrole.

In this study, a novel and extremely facile method for the synthesis of conducting polypyrrole (PPy) was achieved in aqueous solution. This radiolytic method is totally free of template and environmentally friendly compared with traditional chemical methods. According to ultraviolet-visible (UV-vis) spectroscopy and Fourier transform infrared (FTIR) spectroscopy analysis, pyrrole (Py) monomers were polymerized into PPy thanks to their oxidation by HO(•) radicals produced by the radiolysis of water when exposed to γ irradiation. The morphology of PPy was characterized by cryo-transmission electron microscopy (cryo-TEM) in aqueous solution and by scanning electron microscopy (SEM) after deposition. In an original way, high-resolution atomic force microscopy, coupled with infrared nanospectroscopy, was used to probe the local chemical composition of PPy nanostructures. The results demonstrated that spherical and chaplet-like PPy nanostructures were formed by γ-radiolysis. Thermogravimetric analysis (TGA) and electronic conductivity measurements showed that radiosynthesized PPy had good thermal stability and an electrical conductivity higher than that of chemically synthesized PPy.

Radiation-induced synthesis of nanostructured conjugated polymers in aqueous solution: fundamental effect of oxidizing species.

Synthesis of conjugated poly(3,4-ethylenedioxythiophene) (PEDOT) polymers is achieved through the radiolysis of N2O-saturated aqueous solutions of 3,4-ethylenedioxythiophene by using two different oxidizing species: HO(·) (hydroxyl) and N3(·) (azide) radicals. Both oxidative species lead to self-assembled polymers that are evidenced in solution by cryotransmission electron microscopy and UV/Vis absorption spectroscopy and, after centrifugation and deposition, by scanning electron microscopy and attenuated total reflectance FTIR techniques. Whereas HO(·) radicals lead to PEDOT-OH globular nanostructures with hydrophilic properties, N3(·) radicals enable the formation of amphiphilic PEDOT-N3 fibrillar nanostructures. These results, which highlight the differences in the intermolecular interaction behaviors of the two kinds of PEDOT polymers, are discussed in terms of polymerization mechanisms.

Influence of chemical and structural properties of functionalized polythiophene-based layers on electrochemical sensing of atrazine.

Sensitive layers based on conducting homopolymer [poly(3,4-ethylenedioxythiophene), denoted PEDOT] and copolymers [molecularly imprinted and non-imprinted poly(EDOT-co-3-thiophene acetic acid), denoted MICP and NICP, respectively] are electrosynthesized on gold substrates and used for the electrochemical detection of atrazine. These layers are characterized by cyclic voltammetry, ATR-FTIR spectroscopy, optical profilemetry, and AFM microscopy in order to study the effect of the chemical functionalities and of the structural properties of these conducting polymers on the physical chemistry of the interaction with atrazine targets and with the aim to improve the sensitivity of the recognition process. In particular, due to the presence in their backbones of preshaped functionalized cavities which keep the molecular memory of the targets, MICP layers show remarkable sensitivity, a low detection limit (10(-9) mol L(-1)), and a large linear range of detection (10(-8) to 10(-4) mol L(-1)), as demonstrated by square-wave voltammetry.

Influence of the chemical functionalities of a molecularly imprinted conducting polymer on its sensing properties: electrochemical measurements and semiempirical DFT calculations.

Starting from thiophene-based functional monomers (FM), namely, TMA, TAA, TMeOH, EDOT, and Th, bonded to atrazine (ATZ) target molecules into FM/ATZ prepolymerization dimers in acetonitrile solutions, differently functionalized molecularly imprinted conducting polymers (FM-MICP) are electrosynthesized and then washed and used as sensitive layers for ATZ recognition. Sensitivity of these layers toward ATZ, which is quantified by cyclic voltammetric measurements, decreases in the following order of functional monomers: TMA, TAA, TMeOH, EDOT, and Th. Absolute values of the FM-ATZ dimerization free energies are calculated with the help of DFT/PCM calculations and of an empirical correction of the entropy effects, using a modified Wertz formula. A strong correlation is found between FM-MICP sensitivity and the amount of FM/ATZ prepolymerization complexes.

Molecularly imprinted conducting polymer based electrochemical sensor for detection of atrazine.

An original electrochemical sensor based on molecularly imprinted conducting polymer (MICP) is developed, which enables the recognition of a small pesticide target molecule, atrazine. The conjugated MICP, poly(3,4-ethylenedioxythiophene-co-thiophene-acetic acid), has been electrochemically synthesized onto a platinum electrode following two steps: (i) polymerization of comonomers in the presence of atrazine, already associated to the acetic acid substituent through hydrogen bonding, and (ii) removal of atrazine from the resulting polymer, which leaves the acetic acid substituents open for association with atrazine. The obtained sensing MICP is highly specific towards newly added atrazine and the recognition can be quantitatively analyzed by the variation of the cyclic voltammogram of MICP. The developed sensor shows remarkable properties: selectivity towards triazinic family, large range of detection (10(-9) mol L(-1) to 1.5 x 10(-2) mol L(-1) in atrazine) and low detection threshold (10(-7) mol L(-1)).

Hydroperoxide characterisation as a signature of the micelle/monomer balance in radiation-induced peroxidation of arachidonate.

Archidonate peroxidation has been studied using HO* radicals radiolytically generated as initiators of this process. Irradiated aqueous solutions of arachidonate (between 0.01 and 25 mM at pH 10.5) have been characterised by means of conjugated dienes measurement (234 nm-absorption spectroscopy) and hydroperoxide detection (high-performance liquid chromatography coupled with a chemiluminescence detection). Radiation-induced peroxidation of arachidonate gives a different trend of peroxide products, depending on the degree of substrate interaction; endoperoxide and hydro-endoperoxide being favored at low concentrations (monomer/oligomer) and monohydroperoxide at high concentrations (micellar form). The experimental ratios G(Hydro2)/G(Hydro1) increase significantly only for arachidonate concentrations higher than 1 mM, i.e. in micellar medium. However, between 0.1 and 1?mM in arachidonate, G-values (for conjugated dienes, Hydro2 and Hydro1) remain nearly constant, meaning that the physical arrangement of the solution changes: Aggregation occurs. The experimental yields of conjugated dienes formation indicated that GDienes > GHO for [arachidonate]>2.5 mM, indicating that a chain propagation process had occurred. Radiolytic yields and structural identification (HPLC-MS analysis) of peroxidation products allowed us to propose a mechanism for the formation of both hydroperoxides.

Synthesis of nanostructured metal-organic films: surface X-ray radiolysis of silver ions using a langmuir monolayer as a template.

An application of the radiolysis method using an X-ray synchrotron beam is developed as a novel approach to the synthesis of metal-organic films with controlled shapes and thickness. We demonstrate that a Langmuir monolayer deposited onto a silver ion containing subphase, irradiated by an incident beam impinging below the critical angle for total reflection, induces the synthesis of a stable nanostructured silver-organic ultrathin film at the air-water interface. The X-ray scattering is also used to monitor in situ the structure of the silver layer during the synthesis process. The layer is observed by atomic force microscopy after its transfer onto a silicon substrate. One observes a film thickness of 4.6 nm, in good agreement with the X-ray penetration depth, about 4.5 nm. The silver structure is oriented by the initial organic film phase. This experiment demonstrates the considerable potential of this approach to produce various controlled metal-organic films with a surfactant self-assembly as a template.