Elisa S. Orth

The use of gum Arabic as "Green" stabilizer of poly(aniline) nanocomposites: a comprehensive study of spectroscopic, morphological and electrochemical properties.

Herein we show the synthesis and characterization of water dispersible composites formed by poly(aniline) and the natural polymer gum Arabic (GA), used as stabilizer. The materials were synthesized via a rapid and straightforward method and were fully characterized by different techniques such as UV-Vis, Raman, FTIR, TEM, SEM and cyclic voltammetry. TEM and SEM images revealed that the proportion of stabilizer highly influences the growth mechanism of the nanostructures. It was found spherical particles, elongated structures and large agglomerates at the lower, intermediate and at the higher GA amount, respectively. Accordingly to fluorescence spectra, different hydrophobic structures are formed depending on the GA amount in aqueous solutions, possibly acting as hosting sites for the PANI growth. In order to further study the PANI polymerization in the presence of GA, kinetics experiments were performed and showed that nucleation is the limiting step for the composite growth and a model is proposed. Spectroscopic experiments showed that the presence of GA affects the PANI conformation, avoiding the formation of phenazine structures which highly impairs the electroactivity of PANI. The material integrity is achieved by strong hydrogen bond interactions between PANI and GA as evidenced by the study of specific NH bands in FTIR and Raman analyses. The intensity of the hydrogen bonds decreased upon higher amounts of GA, probably due to steric impediment around the NH sites. Cyclic voltammograms showed a good electroactivity behavior of the modified electrodes presenting distinguishable diffusional processes through the adsorbed composites. By this way, we have thoroughly investigated the formation and properties of new conducting polymer composite materials. Taken into account the low toxicity of GA and the excellent dispersity in water, the materials can successfully be applied in bioelectrochemical applications or as green corrosion inhibitors.

pKa determination of graphene-like materials: Validating chemical functionalization.

We report a novel pKa determination for different graphene-like samples: graphene oxide (GO), reduced GO (rGO), graphene nanoribbons (GNR), oxidized GNR (GONR), thiol- and imidazole-functionalized GO (GOSH and GOIMZ, respectively) and thiol-functionalized GONR (GONRSH). Using the specialized computational program BEST7 for treating titration curves, pKas for different functional groups were discriminated (confirmed by infrared spectra) and their composition quantified. Overall, three equilibria were distinguished, two relative to carboxylic acids exhibiting different acidic degrees (pKa1∼4.0 and pKa2∼6.0) and one relative to alcohols (pKa4∼10.0). Upon functionalization on carboxylate sites, thiol (pKa(GOSH/GONRSH)=6.7) and imidazole (pKa(GOIMZ)=6.6) moieties were discerned, followed by a decrease of their carboxylate percentage (compared to the precursors), thus allowing determining the degree of functionalization (48% and 36% of thiol content for GOSH and GONRSH respectively, and 29% of imidazole for GOIMZ). The proposed method is innovative and simpler when compared to the traditional tools usually employed to quantify chemical functionalization.

Tailoring multifunctional graphene-based thin films: from nanocatalysts to SERS substrates

Anchoring different functional groups on graphene may modulate and broaden its application, providing free groups to act for example as catalytic sites or for metallic nanoparticles (NPs) passivation. In this work, two novel approaches, based on the liquid/liquid (L/L) interfacial method, were used to obtain thiolated graphene oxide (GO) thin films rationally modified with cysteamine, through stable amide bonds, with different degrees of functionalization. In addition, nanocomposites of the functionalized film with silver nanoparticles (Ag-NPs) were obtained by the direct heterogeneous reaction of Ag+ cations with the films. The functionalized films were applied as nanocatalysts for organophosphate degradation, as SERS substrates and finally as a SERS-based qualitative sensor for nitrophenols. The thin film nanocomposites presented high performance in the detection of 4-aminothiophenol with enhancement factors up to 108. Moreover, the thiolated thin films were effective catalysts for degrading organophosphates such as the toxic pesticide Paraoxon, which presented impressive catalytic activity (106-fold). Elegantly, we managed to detect by Raman spectroscopy the degradation product adsorbed on the nanocomposite catalyst, hence achieving a SERS sensor for nitrophenol. Thus, we report for the first time the synthesis of functionalized graphene-based thin films through the interfacial route and their nanocomposites with Ag-NPs, along with their multiple applications. In fact, tailoring such multifunctionalities is particularly interesting for the development of sensors and detoxifying agents to monitor and eradicate abusive uses of toxic organophosphorus substances.

Degrading Pesticides with Waste Product: Imidazole-Functionalized Rice Husk Catalyst for Organophosphate Detoxification

Rice husk (RH) is one the largest agricultural waste products worldwide, and rice is one of the crops that use the most pesticides. Among these, organophosphates have been of increasing concern due to their high toxicity. Herein, we report the functionalization of RH with imidazole groups (RHIMZ) to obtain sustainable catalysts from waste for organophosphate degradation. The waste-derived catalyst showed prominent catalytic activity in dephosphorylation reactions with the model substrate diethyl 2,4-dinitrophenyl phosphate (DENDPP), over 10-fold, compared to the spontaneous reaction (lifetime = 1 month). Finally, RHIMZ was also effective in degrading the pesticide Paraoxon (spontaneous lifetime = 1 million years), degrading 60% in 20 days, giving a 10-fold enhancement. Overall, the proposed approach is environmentally friendly for reusing the waste for a noble cause, i.e., degrading toxic pesticides, which is promising for designing sensors and detoxification processes.

Air stable black phosphorous in polyaniline-based nanocomposite

The greatest challenge regarding black phosphorus (BP) comes as a result of its fast degradation when exposed to ambient conditions, which has overshadowed its applications. Herein, we report a simple and efficient route towards overcoming BP deterioration by preparing a nanocomposite with the conducting polymer polyaniline (PANI). The liquid/liquid interfacial method was employed to produce transparent, freestanding and transferable thin film of BP covered by PANI, with high stability under ambient atmosphere, up to 60 days. Otherwise, the uncapped exfoliated neat BP degraded in solely 3 days under the same conditions. Characterization data show that PANI covers efficiently the BP flakes, indicating favorable interactions between the components. The results presented here can be considered a breakthrough for employing BP as thin film in different technological applications, considering the properties of BP itself or taking advantage of synergistically combining the properties of both components.

The Importance of Methyl Positioning and Tautomeric Equilibria for Imidazole Nucleophilicity

Imidazole (IMZ) rings catalyze many biological dephosphorylation processes. The methyl positioning effect on IMZs reactivity has long intrigued scientists and its full understanding comprises a promising tool for designing highly efficient IMZ-based catalysts. We evaluated all monosubstituted methylimidazoles (xMEI) in the reaction with diethyl 2,4-dinitrophenyl phosphate by kinetics studies, NMR analysis and DFT calculations. All xMEI showed remarkable rate enhancements, up to 1.9×105 fold, compared with spontaneous hydrolysis. Unexpectedly, the electron-donating methyl group acts to decrease the reactivity of the xMEI compared to IMZ, except for 4(5)methylimidazole, (4(5)MEI). This behavior was attributed to both electronic and steric effects. Moreover, reaction intermediates were monitored by NMR and surprisingly, the reactivity of the two different 4(5)MEI tautomers was distinguished.

Detoxification of organophosphates using imidazole-coated Ag, Au and AgAu nanoparticles

Organophosphate (OP) detoxification is a worldwide problem due to the high stability of P–O bonds. Here, we designed several imidazole-coated nanocatalysts targeted towards the cleavage of OP and thus their detoxification. Specifically, Ag, Au and bimetallic AgAu nanoparticles (NPs) supported on SiO2 were functionalized with methimazole (MTZ), which comprises thiol and imidazole groups, foreseeing enabling freely available imidazole groups. Raman analyses indicate that MTZ interacts preferably via its sulfur atom over Au NPs and via the nitrogen of the imidazole ring over Ag NPs. The MTZ-derived nanocatalysts were effective towards OP cleavage. For instance, rate enhancements of 108 fold were obtained for the toxic pesticide Paraoxon, compared to the uncatalyzed reaction. Interestingly, Au-derived nanocatalysts were significantly more effective since the imidazole group is free to react with the OP, which is not possible when Ag–N interactions take place.

Nanocatalysts for hydrogen production from borohydride hydrolysis: graphene-derived thin films with Ag- and Ni-based nanoparticles

Generation of hydrogen is one of the greatest challenges nowadays, especially chemically obtaining from the simple, straightforward borohydride hydrolysis reaction. Herein we are interested in this reaction for producing hydrogen using various nanocatalysts which are derived from graphene (reduced graphene oxide, rGO) and combined with metal-based nanoparticles (Ag, Ni and Ni(OH)2). Overall, four promising catalysts were obtained using the liquid–liquid interfacial route in order to obtain highly homogeneous thin films. We highlight the novelty in employing thin films for the generation of hydrogen gas, in which the reaction kinetics was thoroughly elucidated, obeying first-order profiles. In fact, very high catalytic activity was observed using low amounts of nanocatalysts (∼0.1–0.8 mg), with the maximum rate of hydrogen production reaching nearly 33 × 103 mL−1 min−1 g−1. rGO/Ni nanocomposites are reported here for the first time, which also showed the most prominent activity. The isolated components alone were not efficient, evidencing the synergism in the nanocomposites produced. Finally, the nanocatalysts were recycled for up to 10 consecutive cycles, without losing activity. Various characterization techniques were performed during the reactions and after the reuse and confirmed that the typical nature of the composites remains intact.

Puzzling reaction of imidazole with methyl parathion: P=S vs P=O mechanistic shift dilemma in organophosphates

Organophosphates (OPs) constitute many toxic agrochemicals and warfare and can undergo a wide spectrum of mechanisms, some which are fairly unexplored. In this sense, concise mechanistic elucidation stands out as a strategic tool for achieving efficient detoxification and for monitoring processes. Particularly intriguing is the effect of substituting the oxygen atom of the phosphoryl moiety (P=O) in OPs with a sulfur atom to give the thio-derived OPs (i.e., OTPs, P=S). In general, imidazole (IMZ) reacts very efficiently with OPs by targeting the phosphorus atom, although herein we evidence a thio-driven shift with OTPs: IMZ undergoes unusual nucleophilic attack at the aliphatic carbon atom of methyl parathion. Alkylation of IMZ under mild conditions (aqueous weakly basic medium) is also novel and should be applicable to other novel IMZ-based architectures, and thereby, it can be a great ally for organic synthesis. Overall, a broader understanding of the mechanistic trend involved in such highly toxic agents is provided.

Nonenzymatic electrochemical sensor based on imidazole-functionalized graphene oxide for progesterone detection

The modification of electrode surfaces has been the target of study for many researchers in order to improve the analytical performance of electrochemical sensors. Herein, the use of an imidazole-functionalized graphene oxide (GO-IMZ) as an artificial enzymatic active site for voltammetric determination of progesterone (P4) is described for the first time. The morphology and electrochemical performance of electrode modified with GO-IMZ were characterized by scanning electron microscopy and cyclic voltammetry, respectively. Under optimized conditions, the proposed sensor showed a synergistic effect of the GO sheets and the imidazole groups anchored on its backbone, which promoted a significant enhancement on electrochemical reduction of P4. Figures of merits such as linear dynamic response for P4 concentration ranging from 0.22 to 14.0 μmol L-1, limit of detection of 68 nmol L-1 and limit of quantification and 210 nmol L-1 were found. In addition, presented a higher sensitivity, 426 nA L µmol-1, when compared to the unmodified electrode. Overall, the proposed device showed to be a promising platform for a simple, rapid, and direct analysis of progesterone.