Maryam Borghei

Disposable amperometric biosensor based on lactate oxidase immobilised on platinum nanoparticle-decorated carbon nanofiber and poly(diallyldimethylammonium chloride) films.

A novel biosensor for lactate has been developed, using screen-printed carbon electrodes (SPCE) and lactate oxidase (LOx). The active surface of the electrodes was modified using a dispersion of platinum nanoparticle decorated carbon nanofibers (PtNp-CNF) in poly(diallyldimethylammonium) chloride (PDDA) solution. In this way, sensitive, disposable, low cost and reliable hydrogen peroxide sensors were obtained. The immobilisation of LOx on top of these PtNp-CNF-PDDA/SPCEs resulted in amperometric biosensors with high operational stability. The sensitivity of the optimised lactate biosensor was 36.8 (mA/Mcm(2)) with a linear range of 25-1500 µM. The limit of detection was 11 µM (S/N=3). Reproducibility, selectivity and storage stability were also evaluated. Additionally, the stability of the biosensor was also predicted by a model based on thermal degradation. Finally, lactate in sweat and blood samples was determined in a sport test using LOx/PtNp-CNF-PDDA/SPCEs and commercial biosensors respectively. Based on these data, the validity of the sweat lactate for the determination of the lactate threshold is discussed.

Electrocatalysis of oxygen reduction on heteroatom-doped nanocarbons and transition metal–nitrogen–carbon catalysts for alkaline membrane fuel cells

Over the last decade, great progress has been made in the development of non-precious metal catalysts for the electrochemical oxygen reduction reaction (ORR). Among these, heteroatom-doped carbon nanomaterials and transition metal–nitrogen–carbon (M–N–C) catalysts are especially advantageous in an alkaline environment, showing high electrocatalytic activity for the ORR and good durability. Over the past few years, substantial achievements have also been made in improving the performance of anion exchange membrane fuel cells (AEMFCs) and the commercialisation of these devices has emerged as a viable option. This review article provides an outline to the most relevant studies of the ORR on heteroatom-doped nanocarbons and M–N–C type catalysts in alkaline media. In addition, an overview of the studies employing these materials as cathodes in AEMFCs is presented. A separate section is devoted to the results obtained with alkaline direct methanol and ethanol fuel cells. Further perspectives in the field of AEMFC research and development are also highlighted.

Graphitized carbon nanofiber-Pt nanoparticle hybrids as sensitive tool for preparation of screen printing biosensors. Detection of lactate in wines and ciders

This work describes the fabrication of a new lactate biosensor. The strategy is based on the use of a novel hybrid nanomaterial for amperometric biosensors i.e. platinum nanoparticles (PtNps) supported on graphitized carbon nanofibers (PtNps/GCNF) prepared by chemical reduction of the Pt precursor at GCNF surfaces. The biosensors were constructed by covalent immobilization of lactate oxidase (LOx) onto screen printed carbon electrodes (SPCEs) modified with PtNps (PtNps/GCNF-SPCEs) using polyethyleneimine (PEI) and glutaraldehyde (GA). Experimental variables concerning both the biosensor design and the detection process were investigated for an optimal analytical performance. Lactate biosensors show good reproducibility (RSD 4.9%, n=10) and sensitivity (41,302±546) μA/Mcm(2), with a good limit of detection (6.9μM). Covalent immobilization of the enzyme allows the reuse of the biosensor for several measurements, converting them in a cheap alternative to the solid electrodes. The long-term stability of the biosensors was also evaluated. 90% of the signal was kept after 3months of storage at room temperature (RT), while 95% was retained after 18months at -20°C. These results demonstrate that the method provides sensitive electrochemical lactate biosensors where the stability of the enzymatic activity can be preserved for a long period of time in adequate storage conditions.

Dissociation of oxygen on pristine and nitrogen-doped carbon nanotubes: a spin-polarized density functional study

Although nitrogen-doped nanocarbon systems have recently received intense attention, the mechanism for the observed highly efficient oxygen reduction is still under debate. To address this issue, we investigated the adsorption and dissociation of an oxygen molecule on three pristine or nitrogen-doped nanocarbon systems: graphene, single-walled and double-walled carbon nanotubes using density functional theory calculations. The adsorption and dissociation energies were determined for both pristine and N-doped single-walled carbon nanotubes of different diameters with graphitic-like N substitutions in order to see the effect of diameter on oxygen dissociation. It was found that the energy barrier for oxygen dissociation, chemisorption energy and reaction energy are a function of carbon nanotube diameter, but independent of the number of walls. We also investigated the energy barrier of oxygen dissociation on single-walled carbon nanotubes with different types of nitrogen doping (i.e. pyridinic and graphitic). It was observed that higher nitrogen concentrations greatly reduce the energy barrier for graphitic nitrogen. Our results contribute towards a better understanding of the reaction mechanism for nitrogen-doped carbon nanomaterials involving oxygen molecule dissociation in the first step.

High-Throughput Synthesis of Lignin Particles (∼30 nm to ∼2 μm) via Aerosol Flow Reactor: Size Fractionation and Utilization in Pickering Emulsions.

An aerosol flow reactor was used for the first time for high-throughput, high yield synthesis of spherical lignin particles with given inherent hydrophilicity, depending on the precursor biomolecule. In situ fractionation via Berner type impactor afforded populations with characteristic sizes ranging from ∼30 nm to 2 μm. The as-produced, dry lignin particles displayed excellent mechanical integrity, even after redispersion under high shear in either mineral oil or water. They were effective in the stabilization of oil-in-water (O/W) Pickering emulsions with tunable droplet size, depending on the dimension of the lignin particles used for emulsification. The emulsion stability correlated with particle concentration as well as the respective lignin type. For the O/W emulsions stabilized with the more hydrophilic lignin particles, negligible changes in phase separation via Ostwald ripening and coalescence were observed over a period of time of more than two months. Together with the fact that the lignin particle concentrations used in emulsification were as low as 0.1%, our results reveal a remarkable ability to endow emulsified systems with high colloidal stability. Overall, we offer a new, high-yield, scalable nanomanufacturing approach to producing dry spherical lignin particles with size control and high production capacity. A number of emerging applications for these organic particles can be envisioned and, as a proof-of-concept, we illustrate here surfactant-free emulsification.

Preparation Methods for Multi‐Walled Carbon Nanotube Supported Palladium Catalysts

Atomic layer deposition (ALD) in preparation of MWCNT‐supported Pd catalysts was compared to dry and wet impregnation. The prepared catalysts were characterized by various methods and cinnamaldehyde hydrogenation was used as a model reaction to test the activity of the prepared catalysts. The main factor affecting the properties of the catalyst was found to be the precursor medium used during catalyst preparation. ALD, with a gaseous precursor, led to the smallest metal particles with the highest dispersion, whereas impregnation methods using liquid precursors led to the largest particles with the lowest dispersion. The variation in the metal particle sizes of different catalysts was shown to affect the product distribution in the model reaction and the reaction mechanisms of the surface reactions.

Advanced Biomass-Derived Electrocatalysts for the Oxygen Reduction Reaction

Recent progress in advanced nanostructures synthesized from biomass resources for the oxygen reduction reaction (ORR) is reviewed. The ORR plays a significant role in the performance of numerous energy-conversion devices, including low-temperature hydrogen and alcohol fuel cells, microbial fuel cells, as well as metal-air batteries. The viability of such fuel cells is strongly related to the cost of the electrodes, especially the cathodic ORR electrocatalyst. Hence, inexpensive and abundant plant and animal biomass have become attractive options to obtain electrocatalysts upon conversion into active carbon. Bioresource selection and processing criteria are discussed in light of their influence on the physicochemical properties of the ORR nanostructures. The resulting electrocatalytic activity and durability are introduced and compared to those from conventional Pt/C-based electrocatalysts. These ORR catalysts are also active for oxygen or hydrogen evolution reactions.

Mesoporous carbon soft-templated from lignin nanofiber networks: microphase separation boosts supercapacitance in conductive electrodes

Flexible electrodes with supercapacitance were developed from highly mesoporous carbon fibers synthesized from lignin. Polyvinyl alcohol (PVA) facilitated the electrospinning of aqueous solutions of lignin and was used as a sacrificial polymer. Most importantly, PVA produced phase-separated domains for extreme surface area (>2000 m2 g−1) and mesoporous volume (0.7 cm3 g−1). An optimized sequential thermal treatment that initially included stabilization at 250 °C, allowed the formation of flexible, freestanding carbon networks upon PVA evolution to the gas phase and carbonization of the as-spun lignin-based fibers. Their main morphological and chemical characteristics were assessed by field emission scanning microscopy, transmission electron tomography reconstructions and Raman spectroscopy. The carbon fiber networks were used directly as electrodes with electrochemical double layer capacitance as determined by cyclic voltammetry and galvanostatic charge/discharge methods. Excellent electrochemical performance was demonstrated from the measured high rate capability and long-term cycling stability. The determined specific capacitance (∼205 F g−1 in 0.5 M Na2SO4 electrolyte) is one of the highest recorded for electrodes obtained from biopolymer precursors. Moreover, the electrical conductivity of the carbon fiber network (386 S m−1) was significantly higher, by two-orders of magnitude, than that obtained from the precursor (non-fibrous, powder) sample (2.47 S m−1). The remarkable performance of the synthesized electrodes is ascribed to the robust network morphology and mesoporosity obtained by soft-templating from the phase-separated sacrificial polymer. This is a demonstration of lignin valorization for novel application in advanced materials.

Filaments with Affinity Binding and Wet Strength Can Be Achieved by Spinning Bifunctional Cellulose Nanofibrils

We demonstrate benzophenone (BP) conjugation via amine-reactive esters onto oxidized cellulosic fibers that were used as precursors, after microfluidization, of photoactive cellulose nanofibrils (CNF). From these fibrils, cellulose I filaments were synthesized by hydrogel spinning in an antisolvent followed by fast biradical UV cross-linking. As a result, the wet BP-CNF filaments retained extensively the original dry strength (a remarkable ∼80% retention). Thus, the principal limitation of these emerging materials was overcome (the wet tensile strength is typically <0.5% of the value measured in dry conditions). Subsequently, antihuman hemoglobin (anti-Hb) antibodies were conjugated onto residual surface carboxyl groups, making the filaments bifunctional for their active groups and properties (wet strength and bioactivity). Optical (surface plasmon resonance) and electroacoustic (quartz crystal microgravimetry) measurements conducted with the bifunctional CNF indicated effective anti-Hb conjugation (2.4 mg m-2), endowing an excellent sensitivity toward Hb targets (1.7 ± 0.12 mg m-2) and negligible nonspecific binding. Thus, the anti-Hb biointerface was deployed on filaments that captured Hb efficiently from aqueous matrices (confocal laser microscopy of FITC-labeled antibodies). Significantly, the anti-Hb biointerface was suitable for regeneration, while its sensitivity and selectivity in affinity binding can be tailored by application of blocking copolymers. The developed bifunctional filaments based on nanocellulose offer great promise in detection and affinity binding built upon 1D systems, which can be engineered into other structures for rational use of material and space.

Retention of lysozyme activity by physical immobilization in nanocellulose aerogels and antibacterial effects

Aerogels prepared from aqueous dispersions of anionic and cationic cellulose nanofibrils (CNFs) were investigated as solid supports for enzymes and silver nanoparticles and to elicit a sustained antibacterial effect. The imparted stabilization in dry conditions was studied with aerogels that were cast after mixing the enzymes with CNFs followed by dehydration (freeze-drying). The activity of lysozyme immobilized in the given CNF system was analyzed upon storage in liquid and air media. In contrast with aqueous solutions of free, unbound enzyme, which lost activity after the first day, the enzyme immobilized physically in unmodified and cationic CNF presented better stability (activity for a longer time). However, the enzyme activity was reduced in the case of anionic CNF, which was prepared by TEMPO-mediated oxidation (TO-CNF). Both humidity and temperature reduced the stability of the enzyme immobilized in the respective CNF aerogel. The antibacterial activity of CNF aerogels carrying lysozyme was also tested against gram-negative and gram-positive bacteria. The results were compared with those obtained from CNF systems loaded with silver nanoparticles (AgNP) after in situ synthesis via UV reduction. Storage in cold or dry conditions preserved the activity and antibacterial performance of enzyme-loaded CNF aerogels. As expected, the lysozyme-containing aerogels showed lower inhibition than the AgNP-containing aerogel. In this latter case, the antibacterial activity depended on the concentration and size of the nanoparticles. Compared to unmodified CNF and TO-CNF, the aerogels prepared with cationic CNF, loaded with either lysozyme or AgNPs, showed remarkably better antibacterial activity. Similar experiments were conducted with horseradish peroxidase, which confirmed, to different degrees, the observations derived from the lysozyme systems. Overall, the results indicate that non-toxic and biodegradable CNF is a suitable support for bio-active materials and is effective in protecting and retaining enzymatic and antibacterial activities.