On-Chip Electrochemical Sensing with an Enhanced Detecting Signal Due to Concentration Polarization-Based Analyte Preconcentration

Here, we integrated two key technologies within a microfluidic system, an electrokinetic preconcentration of analytes by ion Concentration Polarization (CP) and local electrochemical sensors to detect the analytes, which can synergistically act to significantly enhance the detection signal. This synergistic combination, offering both decoupled and coupled operation modes for continuous monitoring, was validated by the intensified fluorescent intensities of CP-preconcentrated analytes and the associated enhanced electrochemical response using differential pulse voltammetry and chronoamperometry. The system performance was evaluated by varying the location of the active electrochemical sensor, target analyte concentrations, and electrolyte concentration using fluorescein molecules as the model analyte and Homovanillic acid (HVA) as the target bioanalyte within both phosphate-buffered saline (PBS) and artificial sweat solution. The combination of on-chip electrochemical sensing with CP-based preconcentration renders this generic approach adaptable to various analytes. This advanced system shows remarkable promise for enhancing biosensing detection in practical applications while bridging the gap between fundamental research and practical implementation.

A Kinetic Approach to Oxygen Radical Absorbance Capacity (ORAC): Restoring Order to the Antioxidant Activity of Hydroxycinnamic Acids and Fruit Juices

This study introduces a kinetic model that significantly improves the interpretation of the oxygen radical absorbance capacity (ORAC) assay. Our model accurately simulates and fits the bleaching kinetics of fluorescein in the presence of various antioxidants, achieving high correlation values (R2 > 0.99) with the experimental data. The fit to the experimental data is achieved by optimizing two rate constants, k5 and k6. The k5 value reflects the reactivity of antioxidants toward scavenging peroxyl radicals, whereas k6 measures the ability of antioxidants to regenerate oxidized fluorescein. These parameters (1) allow the detailed classification of cinnamic acids based on their structure-activity relationships, (2) provide insights into the interaction of alkoxyl radicals with fluorescein, and (3) account for the regeneration of fluorescein radicals by antioxidants. The application of the model to different antioxidants and fruit extracts reveals significant deviations from the results of traditional ORAC tests based on the area under the curve (AUC) approach. For example, lemon juice, rich in 'fast' antioxidants such as ascorbic acid, shows a high k5 value, in contrast to its low AUC values. This finding underscores the limitations of the AUC approach and highlights the advantages of our kinetic model in understanding antioxidative dynamics in food systems. This study presents a comprehensive, quantitative, mechanism-oriented approach to assessing antioxidant reactivity, demonstrating a significant improvement in ORAC assay applications.

Polymerase incorporation of fluorescein or rhodamine modified 2'-deoxyuridine-5'-triphosphates into double-stranded DNA for direct electrochemical detection

The 2'-deoxyuridine-5'-triphosphates modified with fluorescein (dUTP-Fl) or rhodamine (dUTP-Rh) were tested as bearers of electroactive labels and as proper substrates for polymerases used in polymerase chain reaction (PCR) and isothermal recombinase polymerase amplification (RPA) with the aim of electrochemical detection of double-stranded DNA (dsDNA) amplification products. For this purpose, electrochemical behavior of free fluorescein and rhodamine as well as the modified nucleotides, dUTP-Fl and dUTP-Rh, was studied by cyclic (CV) and square wave (SWV) voltammetry on carbon screen printed electrodes. Both free fluorescein and dUTP-Fl underwent a two-step oxidation at the peak potentials (Ep) of 0.6-0.7 V and 0.8-0.9 V (phosphate buffer, pH 7.4). The reduction peaks of fluorescein and dUTP-Fl were registered between -0.9 V and -1 V, but they did not depend on concentration. The free rhodamine and dUTP-Rh have demonstrated the well-defined oxidation peaks at 0.8-0.9 V. In addition, the distinct reduction peaks at Ep between -0.8 V and -0.9 V were registered for both rhodamine and dUTP-Rh. The dUTP-Fl and dUTP-Rh were further tested as substrates to incorporate an electroactive label into 210 or 206 base pair long dsDNA amplicons generated either by PCR or RPA. Among two dUTP derivatives tested, dUTP-Fl revealed significantly better compatibility with PCR and RPA, producing the full-size amplicons at 50-90% substitution of dTTP in the reaction mixture. In the PCR, the best compromise between amplicon output and labeling was achieved at the dUTP-Fl : dTTP and dUTP-Rh : dTTP molar ratios of 70% : 30% and 20% : 80% in the PCR mixture, respectively, allowing the direct electrochemical detection of amplicons at micromolar concentrations. Alongside with fluorescence DNA assays, the fluorescein and rhodamine modified dUTP appear as promising electroactive labels to develop direct electrochemical DNA assays for detecting PCR and RPA products.

Visible-Light-Mediated Radical Arylations Using a Fluorescein-Derived Diazonium Salt: Reactions Proceeding via an Intramolecular Forth and Back Electron Transfer

Functionalizations of arenes and alkenes via additive-free radical reactions using highly photosensitive, fluorescein-derived diazonium salts are described. The particular properties of the diazonium salts enable unique Meerwein-type carbohydroxylations of non-activated alkenes, which can be rationalized by a reaction mechanism involving forth and back electron transfer from and to the xanthene subunit of the fluorescein moiety.

Electrical Stimuli-Responsive Decomposition of Layer-by-Layer Films Composed of Polycations and TEMPO-Modified Poly(acrylic acid)

We previously reported that layer-by-layer (LbL) film prepared by a combination of 2,2,6,6-tetramethylpiperidinyl N-oxyl (TEMPO)-modified polyacrylic acid (PAA) and polyethyleneimine (PEI) were decomposed by application of an electric potential. However, there have been no reports yet for other polycationic species. In this study, LbL films were prepared by combining various polycationics (PEI, poly(allylamine hydrochloride) (PAH), poly(diallydimethylammonium chloride) (PDDA), and polyamidoamine (PAMAM) dendrimer) and TEMPO-PAA, and the decomposition of the thin films was evaluated using cyclic voltammetry (CV) and constant potential using an electrochemical quartz crystal microbalance (eQCM). When a potential was applied to an electrode coated on an LbL thin film of polycations and TEMPO-PAA, an oxidation potential peak (Ep^a) was obtained around +0.6 V vs. Ag/AgCl in CV measurements. EQCM measurements showed the decomposition of the LbL films at voltages near the Ep^a of the TEMPO residues. Decomposition rate was 82% for the (PEI/TEMPO-PAA)₅ film, 52% for the (PAH/TEMPO-PAA)₅ film, and 49% for the (PDDA/TEMPO-PAA)₅ film. It is considered that the oxoammonium ion has a positive charge, and the LbL films were decomposed due to electrostatic repulsion with the polycations (PEI, PAH, and PDDA). These LbL films may lead to applications in drug release by electrical stimulation. On the other hand, the CV of the (PAMAM/TEMPO-PAA)₅ film did not decompose. It is possible that the decomposition of the thin film is not promoted, probably because the amount of TEMPO-PAA absorbed is small.

Spatially isolated redox processes enabled by ambipolar charge transport in multi-walled carbon nanotube mats

This work demonstrates a simple dual-well device which enables spatially isolated solutions to undergo complementary redox reactions. The device functions by the ambipolar transport of charge carriers between two spatially isolated poly(dimethylsiloxane) (PDMS) microwells through an underlying multi-walled carbon nanotube (MWCNT) mat. This MWCNT mat enables charge carriers, produced from the decomposition of an analyte in one solution, to drive a redox reaction in a spatially isolated second colorimetric read-out solution via a potential difference between the wells. As proof-of-concept a visible colorimetric read-out was shown using an enzyme, cytochrome c (reduced in 16 h), and the visualizing reagent 3,3',5,5'-tetramethylbenzidine (TMB) (oxidized in 2.5 h) for the detection of dithionite and hydrogen peroxide, respectively, without any external energy input. We discuss the origin of this phenomenon and highlight the ability of MWCNTs to accept and transport both electrons and holes efficiently between spatially isolated solutions giving rise to a highly versatile sensor suitable for use in simple, low-cost point-of-care diagnostics.

An effective electrochemical sensing platform for fluoride ions based on fluorescein isothiocyanate–MWCNT composite

The intrinsic redox properties of fluorescein isothiocyanate, rather than its fluorescence, were employed as a transducing signal to detect fluoride ions in aqueous media.