Kinetic Rotating Droplet Electrochemistry: A Simple and Versatile Method for Reaction Progress Kinetic Analysis in Microliter Volumes

Here, we demonstrate a new generic, affordable, simple, versatile, sensitive, and easy-to-implement electrochemical kinetic method for monitoring, in real time, the progress of a chemical or biological reaction in a microdrop of a few tens of microliters, with a kinetic time resolution of ca. 1 s. The methodology is based on a fast injection and mixing of a reactant solution (1–10 μL) in a reaction droplet (15–50 μL) rapidly rotated over the surface of a nonmoving working electrode and on the recording of the ensuing transient faradaic current associated with the transformation of one of the components. Rapid rotation of the droplet was ensured mechanically by a rotating rod brought in contact atop the droplet. This simple setup makes it possible to mix reactants efficiently and rotate the droplet at a high spin rate, hence generating a well-defined hydrodynamic steady-state convection layer at the underlying stationary electrode. The features afforded by this new kinetic method were investigated for three different reaction schemes: (i) the chemical oxidative deprotection of a boronic ester by H₂O₂, (ii) a biomolecular binding recognition between a small target and an aptamer, and (iii) the inhibition of the redox-mediated catalytic cycle of horseradish peroxidase (HRP) by its substrate H₂O₂. For the small target/aptamer binding reaction, the kinetic and thermodynamic parameters were recovered from rational analysis of the kinetic plots, whereas for the HRP catalytic/inhibition reaction, the experimental amperometric kinetic plots were reproduced from numerical simulations. From the best fits of simulations to the experimental data, the kinetics rate constants primarily associated with the inactivation/reactivation pathways of the enzyme were retrieved. The ability to perform kinetics in microliter-size samples makes this methodology particularly attractive for reactions involving low-abundance or expensive reagents

Simple and Highly Enantioselective Electrochemical Aptamer-Based Binding Assay for Trace Detection of Chiral Compounds

A new electrochemical methodology is reported for monitoring in homogeneous solution the enantiospecific binding of a small chiral analyte to an aptamer. The principle relies on the difference of diffusion rates between the targeted molecule and the aptamer/target complex, and thus on the ability to more easily electrochemically detect the former over the latter in a homogeneous solution. This electrochemical detection strategy is significant because, in contrast to the common laborious and time-consuming heterogeneous binding approaches, it is based on a simple and fast homogeneous binding assay which does not call for an aptamer conformational change upon ligand binding. The methodology is here exemplified with the specific chiral recognition of trace amounts of l- or d-tyrosinamide by a 49-mer d- or l-deoxyribooligonucleotide receptor. Detection as low as 0.1% of the minor enantiomer in a nonracemic mixture can be achieved in a very short analysis time (<1 min). The assay finally combines numerous attractive features including simplicity, rapidity, low cost, flexibility, low volume samples (few microliters), and homogeneous format

Redox Controls over the Stability of U(IV) in Floodplains of the Upper Colorado River Basin

Aquifers in the Upper Colorado River Basin (UCRB) exhibit persistent uranium (U) groundwater contamination plumes originating from former ore processing operations. Previous observations at Rifle, Colorado, have shown that fine grained, sulfidic, organic-enriched sediments accumulate U in its reduced form, U­(IV), which is less mobile than oxidized U­(VI). These reduced sediment bodies can subsequently act as secondary sources, releasing U back to the aquifer. There is a need to understand if U­(IV) accumulation in reduced sediments is a common process at contaminated sites basin-wide, to constrain accumulated U­(IV) speciation, and to define the biogeochemical factors controlling its reactivity. We have investigated U­(IV) accumulation in organic-enriched reduced sediments at three UCRB floodplains. Noncrystalline U­(IV) is the dominant form of accumulated U, but crystalline U­(IV) comprises up to ca. 30% of total U at some locations. Differing susceptibilities of these species to oxidative remobilization can explain this variability. Particle size, organic carbon content, and pore saturation, control the exposure of U­(IV) to oxidants, moderating its oxidative release. Further, our data suggest that U­(IV) can be mobilized under deeply reducing conditions, which may contribute to maintenance and seasonal variability of U in groundwater plumes in the UCRB

Physico-Chemical Heterogeneity of Organic-Rich Sediments in the Rifle Aquifer, CO: Impact on Uranium Biogeochemistry

The Rifle alluvial aquifer along the Colorado River in west central Colorado contains fine-grained, diffusion-limited sediment lenses that are substantially enriched in organic carbon and sulfides, as well as uranium, from previous milling operations. These naturally reduced zones (NRZs) coincide spatially with a persistent uranium groundwater plume. There is concern that uranium release from NRZs is contributing to plume persistence or will do so in the future. To better define the physical extent, heterogeneity and biogeochemistry of these NRZs, we investigated sediment cores from five neighboring wells. The main NRZ body exhibited uranium concentrations up to 100 mg/kg U as U­(IV) and contains ca. 286 g of U in total. Uranium accumulated only in areas where organic carbon and reduced sulfur (as iron sulfides) were present, emphasizing the importance of sulfate-reducing conditions to uranium retention and the essential role of organic matter. NRZs further exhibited centimeter-scale variations in both redox status and particle size. Mackinawite, greigite, pyrite and sulfate coexist in the sediments, indicating that dynamic redox cycling occurs within NRZs and that their internal portions can be seasonally oxidized. We show that oxidative U­(VI) release to the aquifer has the potential to sustain a groundwater contaminant plume for centuries. NRZs, known to exist in other uranium-contaminated aquifers, may be regionally important to uranium persistence

Reducing Conditions Influence U(IV) Accumulation in Sediments during <i>In Situ</i> Bioremediation

This study presents field experiments conducted in a contaminated aquifer in Rifle, CO, to determine the speciation and accumulation of uranium in sediments during in situ bioreduction. We applied synchrotron-based X-ray spectroscopy and imaging techniques as well as aqueous chemistry measurements to identify changes in U speciation in water and sediment in the first days follwing electron donor amendment. Limited changes in U solid speciation were observed throughout the duration of this study, and non-crystalline U(IV) was identified in all samples obtained. However, U accumulation rates strongly increased during in situ bioreduction, when the dominant microbial regime transitioned from iron- to sulfate-reducing conditions. Results suggest that uranium is enzymatically reduced during Fe reduction, as expected. Mineral grain coatings newly formed during sulfate reduction act as reduction hotspots, where numerous reductants can act as electron donors [Fe(II), S(II), and microbial extracellular polymeric substances] that bind and reduce U. The results have implications for identifying how changes in the dominant reducing mechanism, such as Fe versus sulfate reduction, affect trace metal speciation and accumulation. The outcomes from this study provide additional insights into uranium accumulation mechanisms in sediments that could be useful for the refinement of quantitative models describing redox processes and contaminant dynamics in floodplain aquifers