Reduction of Nitrate in Agricultural Soils by Bio-electrokinetics

High nitrate concentration in surface soils is a serious concern for the agricultural industry throughout the world. Nitrate reduction can be achieved by chemical or biological processes; however, these processes are difficult to achieve in-situ because of the low permeability of clays. This study evaluates bio-electrokinetic processes for nitrate treatment in low-permeability soils. The concept is based on using iron electrodes to generate an electric field and a reducing environment in the soil to facilitate nitrate reduction by existing bacteria. Experiments were conducted using starch as an additive for microbial activity in the anolyte. Three sets of experiments were conducted under 0.5, 1.0, and 2.0 V cm^?1 voltage gradient, and using starch as an anolyte for enhancing existing microbial activity in the soil. Initial nitrate concentration in the soil (agricultural soil collected from Jinju, Korea) was between 782 and 800 mg kg^?1. Removal of 100% nitrate was achieved in the soil under 0.5 and 1.0 V cm^?1 due to the combined effect of biological and iron reduction. A control experiment with iron electrodes, but without starch, was not as effective. Ammonium was also effectively removed by the combined action of starch and iron under 0.5 and 1.0 V cm^?1. The role of starch with iron on the nitrate and ammonium removal process is evaluated along with the role of transport by electro-osmosis and electro-migration. The bacterial action on denitrification and nitrification is assessed and the relationship between pH and the efficiency of nitrate reduction in the bio-EK systems is evaluated.     

Carbon nanotubes promote Cr(VI) reduction by alginate-immobilized Shewanella oneidensis MR-1

Bioreduction of hexavalent chromium (Cr(VI)) into trivalent one (Cr(III)) based on microbial immobilization techniques has been recognized as a promising way to remove Cr contaminants from wastewater. However, such a bioreduction process is inefficient due to limited electron transfer through the immobilization matrix. In this study, a modified immobilization process was proposed by impregnating carbon nanotubes (CNTs) into Ca-alginate beads, which were then used to immobilize Shewanella oneidensis MR-1 for enhanced Cr(VI) reduction. Compared with the free cells and the beads without CNTs, the AL/CNT/cell beads showed up to 4 times higher reduction rates, mainly attributed to an enhanced electron transfer by the CNTs. In addition, the dose of CNTs greatly improved the stability of beads, suggesting a high feasibility of the AL/CNT/cell beads for repeated use. The optimized CNT concentration, temperature and pH for Cr(VI) reduction by the AL/CNT/cell beads were 0.5%, 30 °C and 6.0–7.0, respectively.     

Unimodal size scaling of phytoplankton growth and the size dependence of nutrient uptake and use

Phytoplankton size structure is key for the ecology and biogeochemistry of pelagic ecosystems, but the relationship between cell size and maximum growth rate (μ_max) is not yet well understood. We used cultures of 22 species of marine phytoplankton from five phyla, ranging from 0.1 to 10⁶ μm³ in cell volume (V_cell), to determine experimentally the size dependence of growth, metabolic rate, elemental stoichiometry and nutrient uptake. We show that both μ_max and carbon‐specific photosynthesis peak at intermediate cell sizes. Maximum nitrogen uptake rate (V_maxN) scales isometrically with V_cell, whereas nitrogen minimum quota scales as V_cell^0.84. Large cells thus possess high ability to take up nitrogen, relative to their requirements, and large storage capacity, but their growth is limited by the conversion of nutrients into biomass. Small species show similar volume‐specific V_maxN compared to their larger counterparts, but have higher nitrogen requirements. We suggest that the unimodal size scaling of phytoplankton growth arises from taxon‐independent, size‐related constraints in nutrient uptake, requirement and assimilation.     

Fabrication of Carbon Nanotube High-Frequency Nanoelectronic Biosensor for Sensing in High Ionic Strength Solutions

The unique electronic properties and high surface-to-volume ratios of single-walled carbon nanotubes (SWNT) and semiconductor nanowires (NW) ^1-4 make them good candidates for high sensitivity biosensors. When a charged molecule binds to such a sensor surface, it alters the carrier density⁵ in the sensor, resulting in changes in its DC conductance. However, in an ionic solution a charged surface also attracts counter-ions from the solution, forming an electrical double layer (EDL). This EDL effectively screens off the charge, and in physiologically relevant conditions ~100 millimolar (mM), the characteristic charge screening length (Debye length) is less than a nanometer (nm). Thus, in high ionic strength solutions, charge based (DC) detection is fundamentally impeded^6-8.We overcome charge screening effects by detecting molecular dipoles rather than charges at high frequency, by operating carbon nanotube field effect transistors as high frequency mixers^9-11. At high frequencies, the AC drive force can no longer overcome the solution drag and the ions in solution do not have sufficient time to form the EDL. Further, frequency mixing technique allows us to operate at frequencies high enough to overcome ionic screening, and yet detect the sensing signals at lower frequencies^11-12. Also, the high transconductance of SWNT transistors provides an internal gain for the sensing signal, which obviates the need for external signal amplifier.Here, we describe the protocol to (a) fabricate SWNT transistors, (b) functionalize biomolecules to the nanotube¹³, (c) design and stamp a poly-dimethylsiloxane (PDMS) micro-fluidic chamber¹⁴ onto the device, and (d) carry out high frequency sensing in different ionic strength solutions¹¹.     

Localization and Relative Quantification of Carbon Nanotubes in Cells with Multispectral Imaging Flow Cytometry

Carbon-based nanomaterials, like carbon nanotubes (CNTs), belong to this type of nanoparticles which are very difficult to discriminate from carbon-rich cell structures and de facto there is still no quantitative method to assess their distribution at cell and tissue levels. What we propose here is an innovative method allowing the detection and quantification of CNTs in cells using a multispectral imaging flow cytometer (ImageStream, Amnis). This newly developed device integrates both a high-throughput of cells and high resolution imaging, providing thus images for each cell directly in flow and therefore statistically relevant image analysis. Each cell image is acquired on bright-field (BF), dark-field (DF), and fluorescent channels, giving access respectively to the level and the distribution of light absorption, light scattered and fluorescence for each cell. The analysis consists then in a pixel-by-pixel comparison of each image, of the 7,000-10,000 cells acquired for each condition of the experiment. Localization and quantification of CNTs is made possible thanks to some particular intrinsic properties of CNTs: strong light absorbance and scattering; indeed CNTs appear as strongly absorbed dark spots on BF and bright spots on DF with a precise colocalization.This methodology could have a considerable impact on studies about interactions between nanomaterials and cells given that this protocol is applicable for a large range of nanomaterials, insofar as they are capable of absorbing (and/or scattering) strongly enough the light.     

Fused Heteroaromatic Organic Compounds for High‐Power Electrodes of Rechargeable Lithium Batteries

Organic redox compounds are emerging electrode materials for rechargeable lithium batteries. However, their electrically insulating nature plagues efficient charge transport within the electroactive bulk. Alternative to the popular solution of elaborating nanocomposite materials, herein we report on a molecular‐level engineering strategy towards high‐power organic electrode materials with multi‐electron reactions. Systematic comparisons of anthraquinone analogues incorporating fused heteroaromatic structures as cathode materials in rechargeable lithium batteries reveal that the judicious incorporation of heteroaromatics improves the cell performance in terms of specific gravimetric capacity, working potential, rate capability, and cyclability. Combination studies with morphological observation, electrochemical impedance characterization, and theoretical modeling provide insight into the advantage of heteroaromatic building blocks. In particular, benzofuro[5,6‐b]furan‐4,8‐dione ( BFFD ) bearing furan moeities shows a reversible capacity of 181 mAh g^?1 when charged/discharged at 100C, corresponding to a power density of 29.8 kW kg^?1. These results have pointed to a general design route of high‐rate organic electrode materials by rational functionalization of redox compounds with appropriate heteroaromatic units as versatile structural tools.     

Cu3P Binary Phosphide: Synthesis via a Wet Mechanochemical Method and Electrochemical Behavior as Negative Electrode Material for Lithium‐Ion Batteries

Mechanochemical synthesis of Cu₃P in the presence of n‐dodecane results in a material with a secondary particle size distribution of 10 μm, secondary particles which consist of homogeneously agglomerated 20 nm primary particles. The electrochemical performance of Cu₃P with lithium is influenced by the reaction depth, in other words by the lower potential cut‐off. During the electrochemical reaction, the displacement of copper by lithium from the Cu₃P structure until the formation of Li₃P and Cu deteriorates the capacity retention. Improved performance was obtained when the charge potential was limited to 0.50 V (vs. Li/Li⁺) and the formation of the Li_xCu_3‐xP phase (0 ≤ × ≤ 2). In this case, when the potential is limited to 0.5 V, the capacity is stable for more than 50 cycles. Acceptable electrochemical performances in Li‐ion cells within the voltage range 0.50–2.0 V (vs. Li/Li⁺) were shown when Cu₃P was used as an anode and Li_1.2(Ni_0.13Mn_0.54Co_0.13)O₂ and LiNi_0.5Mn_1.5O₄ as positive electrode materials.     

Si‐Encapsulating Hollow Carbon Electrodes via Electroless Etching for Lithium‐Ion Batteries

Remarkable improvements in the electrochemical performance of Si materials for Li‐ion batteries have been recently achieved, but the inherent volume change of Si still induces electrode expansion and external cell deformation. Here, the void structure in Si‐encapsulating hollow carbons is optimized in order to minimize the volume expansion of Si‐based anodes and improve electrochemical performance. When compared to chemical etching, the hollow structure is achieved via electroless etching is more advanced due to the improved electrical contact between carbon and Si. Despite the very thick electrodes (30 ～ 40 μm), this results in better cycle and rate performances including little capacity fading over 50 cycles and 1100 mA h g^?1 at 2C rate. Also, an in situ dilatometer technique is used to perform a comprehensive study of electrode thickness change, and Si‐encapsulating hollow carbon mitigates the volume change of electrodes by adoption of void space, resulting in a small volume increase of 18% after full lithiation corresponding with a reversible capacity of about 2000 mA h g^?1.     

Random CNT network and regioregular poly(3-hexylthiophen) FETs for pH sensing applications: A comparison

Background

Nowadays, there is a tremendous need for cheap disposable sensing devices for medical applications. Materials such as Carbon Nanotubes (CNTs) and regioregular P3HT are proven to offer a huge potential as cost-effective and solution processable semiconductors for (bio)sensing applications.

Methods

CNT-based field-effect transistors (CNT-FETs) as well as regioregular P3HT-based ones (P3HT-FETs) are fabricated and operated in the so-called electrolyte-gated configuration. The active layer of the P3HT-FETs consists of a spin-coated regioregular P3HT layer, which serves on one hand as the active sensing element and on the other hand as passivation layer for the transistor's metal contacts. The active layer of the nanotube transistors consists of a randomly distributed single walled CNT-network (> 90% semiconducting tubes) deposited from a CNT-ink solution by spin-coating.

Results

We compare both devices concerning their stability in aqueous environment and their response when exposed to buffers with different pH. We found that even if P3HT shows lower stability its pH sensitivity is reproducible even after long-term measurements.

Conclusion

CNT-FETs and P3HT-FETs offer different advantages and drawbacks concerning their stability in solution and the ease of fabrication. A discussion of their different sensing mechanisms as well as sensitivity is given here.

General Significance

This work reports on fast and cost-effective production of solution processable thin-film transistors based on carbon nanotubes and regioregular P3HT and demonstrates their suitability as reliable pH sensors. This article is part of a Special Issue entitled Organic Bioelectronics — Novel Applications in Biomedicine.