We present a new configuration for coupling fluorescence microscopy and voltammetry using self-induced redox cycling for ultrasensitive electrochemical measurements. An array of nanopores, each supporting a recessed disk electrode separated by 100 nm in depth from a planar multiscale bipolar top electrode, was fabricated using multilayer deposition, nanosphere lithography, and reactive-ion etching. Self-induced redox cycling was induced on the disk electrode producing ∼30× current amplification, which was independently confirmed by measuring induced electrogenerated chemiluminescence from Ru(bpy) 3 2/3+ /tri- n -propylamine on the floating bipolar electrode. In this design, redox cycling occurs between the recessed disk and the top planar portion of a macroscopic thin film bipolar electrode in each nanopore. Electron transfer also occurs on a remote (mm-distance) portion of the planar bipolar electrode to maintain electroneutrality. This couples the electrochemical reactions of the target redox pair in the nanopore array with a reporter, such as a potential-switchable fluorescent indicator, in the cell at the distal end of the bipolar electrode. Oxidation or reduction of reversible analytes on the disk electrodes were accompanied by reduction or oxidation, respectively, on the nanopore portion of the bipolar electrode and then monitored by the accompanying oxidation of dihydroresorufin or reduction of resorufin at the remote end of the bipolar electrode, respectively. In both cases, changes in fluorescence intensity were triggered by the reaction of the target couple on the disk electrode, while recovery was largely governed by diffusion of the fluorescent indicator. Reduction of 1 nM of Ru(NH 3 ) 6 3+ on the nanoelectrode array was detected by monitoring the fluorescence intensity of resorufin, demonstrating high sensitivity fluorescence-mediated electrochemical sensing coupled to self-induced redox cycling. more »« less
Han, Donghoon; Crouch, Garrison M.; Fu, Kaiyu; Zaino III, Lawrence P.; Bohn, Paul W(
, Chemical Science)
null
(Ed.)
The ability of zero-mode waveguides (ZMW) to guide light into subwavelength-diameter nanoapertures has been exploited for studying electron transfer dynamics in zeptoliter-volume nanopores under single-molecule occupancy conditions. In this work, we report the spectroelectrochemical detection of individual molecules of the redox-active, fluorogenic molecule flavin mononucleotide (FMN) freely diffusing in solution. Our approach is based on an array of nanopore-confined recessed dual ring electrodes, wherein repeated reduction and oxidation of a single molecule at two closely spaced annular working electrodes yields amplified electrochemical signals. We have articulated these structures with an optically transparent bottom, so that the nanopores are bifunctional, exhibiting both nanophotonic and nanoelectrochemical behaviors allowing the coupling between electron transfer and fluorescence dynamics to be studied under redox cycling conditions. We also investigated the electric field intensity in electrochemical ZMWs (E-ZMW) through finite-element simulations, and the amplification of fluorescence by redox cycling agrees well with predictions based on optical confinement effects inside the E-ZMW. Proof-of-principle experiments are conducted showing that electrochemical and fluorescence signals may be correlated to reveal single molecule fluctuations in the array population. Cross-correlation of single molecule fluctuations in amperometric response and single photon emission provides unequivocal evidence of single molecule sensitivity.
Electrochemical reactions at nanoscale structures possess unique characteristics, e.g. fast mass transport, high signal-to-noise ratio at low concentration, and insignificant ohmic losses even at low electrolyte concentrations. These properties motivate the fabrication of high density, laterally ordered arrays of nanopores, embedding vertically stacked metal–insulator–metal electrode structures and exhibiting precisely controlled pore size and interpore spacing for use in redox cycling. These nanoscale recessed ring-disk electrode (RRDE) arrays exhibit current amplification factors, AF RC , as large as 55-fold with Ru(NH 3 ) 6 2/3+ , indicative of capture efficiencies at the top and bottom electrodes, Φ t,b , exceeding 99%. Finite element simulations performed to investigate the concentration distribution of redox species and to assess operating characteristics are in excellent agreement with experiment. AF RC increases as the pore diameter, at constant pore spacing, increases in the range 200–500 nm and as the pore spacing, at constant pore diameter, decreases in the range 1000–460 nm. Optimized nanoscale RRDE arrays exhibit a linear current response with concentration ranging from 0.1 μM to 10 mM and a small capacitive current with scan rate up to 100 V s −1 . At the lowest concentrations, the average pore occupancy is 〈 n 〉 ∼ 0.13 molecule establishing productive electrochemical signals at occupancies at and below the single molecule level in these nanoscale RRDE arrays.
Interfacial oxidation–reduction reactions have important applications in corrosion and catalysis, but traditional electrochemical cell methods cannot be used to study these reactions in non-conducting environments such as non-aqueous solvents. We demonstrate that the molecule resazurin that can be reduced to highly-fluorescent resorufin is compatible for sensing in non-aqueous solvents. We characterize the spectral properties of the dyes in ethanol, dimethylformamide (DMF), acetone, and dimethyl sulfoxide (DMSO), showing a ∼10× increase in intensity for “turned-on” resorufin compared to resazurin in all four solvents. We then apply resazurin to sense corrosive reduction reactions at iron surfaces. Increases in fluorescence intensity due to resazurin reduction to resorufin are observed in ethanol, acetone, and DMSO, while DMF had no turn-on. Our work shows that fluorescent dyes have considerable potential to be used to understand redox reactions in non-aqueous solvents, but care must be taken to understand the interplay between the dye, the solvent, and the reactions occurring.
In this paper, we highlight the uniqueness of nanoporous film‐coated electrodes as electrochemical sensing platforms. Specifically, we focus on discussing electrodes coated with insulator‐based monolithic films comprising vertically‐oriented, rigid cylindrical nanopores of uniform diameters (2–200 nm). The electrode coating results in the formation of an array of recessed nanodisk electrodes, and thus we named them recessed nanodisk‐array electrodes (RNEs). We first summarize the properties of nanoporous films commonly used for RNE fabrication, including nanoporous anodic alumina membranes, track‐etched polymer membranes, block copolymer‐derived nanoporous films, and mesoporous silica films. Subsequently, we discuss representative works that take advantage of the uniform size/shape of the nanopores for enhancing electrochemical detection selectivity and sensitivity. RNE‐based sensors measure faradaic currents from redox‐active analytes or exogenously‐added electroactive species that penetrate through the nanopores, or those from redox‐active moieties tethered to the surface of the nanopores or underlying electrodes. The enhanced detection selectivity of these sensors is attributed to the preferential partitioning of analytes into the nanopores or steric/electrostatic exclusion of interfering species. In particular, the uniform sizes/shapes of RNE nanopores play key roles in their higher molecular sieving selectivity and also in the better control of the detection selectivity based on electrostatic/chemical interactions. The detection sensitivity of RNE‐based sensors can be improved by tailoring the chemical environments of the nanopores for analyte preconcentration or for steric/electrostatic manipulation of the dynamics of redox‐tagged binding moieties. These unique characteristics of RNEs, in addition to the mitigation of electrode fouling by the nanoporous films, will enable the development of pretreatment‐free electrochemical sensors for complex matrix solutions.
Biomass is abundant, inexpensive and renewable, therefore, it is highly expected to play a significant role in our future energy and chemical landscapes. The US DOE has identified furanic compounds (furfural and 5-(hydroxymethyl)furfural (HMF)) as the top platform building-block chemicals that can be readily derived from biomass sources [1]. Electrocatalytic conversion of these furanic compounds is an emerging route for the sustainable production of valuable chemicals [2, 3].
In my presentation, I will first present our recent mechanistic study of electrocatlytic hydrogenation (ECH) of furfural through a combination of voltammetry, preparative electrolysis, thiol-electrode modifications, and kinetic isotope studies [4]. It is demonstrated that two distinct mechanisms are operable on metallic Cu electrodes in acidic electrolytes: (i) electrocatalytic hydrogenation (ECH) and (ii) direct electroreduction. The contributions of each mechanism to the observed product distribution are clarified by evaluating the requirement for direct chemical interactions with the electrode surface and the role of adsorbed hydrogen. Further analysis reveals that hydrogenation and hydrogenolysis products are generated by parallel ECH pathways. Understanding the underlying mechanisms enables the manipulation of furfural reduction by rationally tuning the electrode potential, electrolyte pH, and furfural concentration to promote selective formation of important bio-based polymer precursors and fuels
We further studied the mechanisms on the Pb electrode, based on the potential regulated ECH and ER products. Isotopic incorporation studies and electrokinetics have confirmed ECH process to alcohol and alkyl product followed different pathways: alcohol was generated from Langmuir Hinshelwood step through surface-bound furfural and hydrogen, which is sensitive to surface structures. In contrast, alkyl product was formed through an Eley–Rideal step via surface-bound furfural and the inner-sphere protons. By modifying the electrode/electrolyte interface, we have elucidated H2O and H3O+ matters in forming alcohol and alkyl products, respectively. Dynamic oscillation studies and electron paramagnetic resonance (EPR) finally confirmed that the alcohol and dimer products shared the same intermediate. The dimer was formed through the intermediate desorption from the surface to form radicals and the self-coupling of two radicals at the bulk electrolyte.
Next, I will present electrocatalytic conversion of HMF to two biobased monomers in an H-type electrochemical cell [5]. HMF reduction (hydrogenation) to 2,5-bis(hydroxymethyl)furan (BHMF) was achieved under mild electrolyte conditions and ambient temperature using a Ag/C cathode. Meanwhile, HMF oxidation to 2,5-furandicarboxylic acid (FDCA) with ~100% efficiency was facilitated under the same conditions by a homogeneous nitroxyl radical redox mediator. We recently developed a three-electrode flow cell with an oxide-derived Ag (OD-Ag) cathode and catbon felt anode for paring elecatalytic oxidation and reduction of HMF [6]. The flow cell has a remarkably low cell voltage: from ~7.5 V to ~2.0 V by transferring the electrolysis from the H-type cell to the flow cell. This represents a more than fourfold increase in the energy efficiency at the 10 mA operation. A combined faradaic efficiency of 163% was obtained to BHMF and FDCA. Alternatively, the anodic hydrogen oxidation reaction on platinum further reduced the cell voltage to only ~0.85 V at 10 mA operation. These paired processes have shown potential for integrating renewable electricity and carbon for distributed chemical manufacturing in the future.
Ma, Chaoxiong, Zaino III, Lawrence P., and Bohn, Paul W. Self-induced redox cycling coupled luminescence on nanopore recessed disk-multiscale bipolar electrodes. Retrieved from https://par.nsf.gov/biblio/10053506. Chemical Science 6.5 Web. doi:10.1039/c5sc00433k.
Ma, Chaoxiong, Zaino III, Lawrence P., & Bohn, Paul W. Self-induced redox cycling coupled luminescence on nanopore recessed disk-multiscale bipolar electrodes. Chemical Science, 6 (5). Retrieved from https://par.nsf.gov/biblio/10053506. https://doi.org/10.1039/c5sc00433k
Ma, Chaoxiong, Zaino III, Lawrence P., and Bohn, Paul W.
"Self-induced redox cycling coupled luminescence on nanopore recessed disk-multiscale bipolar electrodes". Chemical Science 6 (5). Country unknown/Code not available. https://doi.org/10.1039/c5sc00433k.https://par.nsf.gov/biblio/10053506.
@article{osti_10053506,
place = {Country unknown/Code not available},
title = {Self-induced redox cycling coupled luminescence on nanopore recessed disk-multiscale bipolar electrodes},
url = {https://par.nsf.gov/biblio/10053506},
DOI = {10.1039/c5sc00433k},
abstractNote = {We present a new configuration for coupling fluorescence microscopy and voltammetry using self-induced redox cycling for ultrasensitive electrochemical measurements. An array of nanopores, each supporting a recessed disk electrode separated by 100 nm in depth from a planar multiscale bipolar top electrode, was fabricated using multilayer deposition, nanosphere lithography, and reactive-ion etching. Self-induced redox cycling was induced on the disk electrode producing ∼30× current amplification, which was independently confirmed by measuring induced electrogenerated chemiluminescence from Ru(bpy) 3 2/3+ /tri- n -propylamine on the floating bipolar electrode. In this design, redox cycling occurs between the recessed disk and the top planar portion of a macroscopic thin film bipolar electrode in each nanopore. Electron transfer also occurs on a remote (mm-distance) portion of the planar bipolar electrode to maintain electroneutrality. This couples the electrochemical reactions of the target redox pair in the nanopore array with a reporter, such as a potential-switchable fluorescent indicator, in the cell at the distal end of the bipolar electrode. Oxidation or reduction of reversible analytes on the disk electrodes were accompanied by reduction or oxidation, respectively, on the nanopore portion of the bipolar electrode and then monitored by the accompanying oxidation of dihydroresorufin or reduction of resorufin at the remote end of the bipolar electrode, respectively. In both cases, changes in fluorescence intensity were triggered by the reaction of the target couple on the disk electrode, while recovery was largely governed by diffusion of the fluorescent indicator. Reduction of 1 nM of Ru(NH 3 ) 6 3+ on the nanoelectrode array was detected by monitoring the fluorescence intensity of resorufin, demonstrating high sensitivity fluorescence-mediated electrochemical sensing coupled to self-induced redox cycling.},
journal = {Chemical Science},
volume = {6},
number = {5},
author = {Ma, Chaoxiong and Zaino III, Lawrence P. and Bohn, Paul W.},
}
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