Enzyme Mediated Increase in Methanol Production from Photoelectrochemical Cells and CO₂

While success has been shown in utilizing photocatalytic systems to reduce CO₂ in water, most of these studies have yielded formic acid as the major product with trace amounts of formaldehyde or methanol. One reason for this is the strong equilibrium of formaldehyde toward the hydrate methanediol. To increase methanol yields from CO₂, we show here the combined use of the biological catalyst alcohol dehydrogenase (ADH) from <i>Saccharomyces cerevisiae</i> with CO₂ reduction products obtained from photoelectrochemical cells (PEC). We first show that ADH can reduce very low micromolar amounts of formaldehyde in solution. Upon adding ADH to the PEC products, a rapid three- to four-fold gain in methanol production was observed, which we also attribute to the lack of back reaction by the enzyme. Lastly, because formaldehyde dehydrogenase (FalDH) showed very low reactivity with formate, the addition of FalDH and ADH to the PEC products demonstrated no difference in methanol yields as compared to ADH alone

Graphene Oxide/Nucleic-Acid-Stabilized Silver Nanoclusters: Functional Hybrid Materials for Optical Aptamer Sensing and Multiplexed Analysis of Pathogenic DNAs

Hybrid systems consisting of nucleic-acid-functionalized silver nanoclusters (AgNCs) and graphene oxide (GO) are used for the development of fluorescent DNA sensors and aptasensors, and for the multiplexed analysis of a series of genes of infectious pathogens. Two types of nucleic-acid-stabilized AgNCs are used: one type includes the red-emitting AgNCs (616 nm) and the second type is near-infrared-emitting AgNCs (775 nm). Whereas the nucleic-acid-stabilized AgNCs do not bind to GO, the conjugation of single-stranded nucleic acid to the DNA-stabilized AgNCs leads to the adsorption of the hybrid nanostructures to GO and to the fluorescence quenching of the AgNCs. By the conjugation of oligonucleotide sequences acting as probes for target genes, or as aptamer sequences, to the nucleic-acid-protected AgNCs, the desorption of the probe/nucleic-acid-stabilized AgNCs from GO through the formation of duplex DNA structures or aptamer–substrate complexes leads to the generation of fluorescence as a readout signal for the sensing events. The hybrid nanostructures are implemented for the analysis of hepatitis B virus gene (HBV), the immunodeficiency virus gene (HIV), and the syphilis (<i>Treponema pallidum</i>) gene. Multiplexed analysis of the genes is demonstrated. The nucleic-acid-AgNCs-modified GO is also applied to detect ATP or thrombin through the release of the respective AgNCs-labeled aptamer–substrate complexes from GO

Boranephosphonate DNA-Mediated Metallization of Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWNTs), when dispersed in DMSO with boranephosphonate DNA (bpDNA), were efficiently metalized with silver, gold, and palladium nanoparticles (NPs). This was possible by first adsorbing boranephosphonate DNA onto the surface of SWNTs and then bathing with silver, gold, and palladium metal salts, which form the corresponding nanoparticles by reduction of their respective ions without addition of any external reducing agent. Reduction of a redox dye, 2,6-dichlorophenolindophenol (DCPIP), by Pd nanoparticle conjugates (PdNP/bpDNA/SWNT) disclosed the efficient electron transfer properties of these metallized SWNTs. These PdNP/bpDNA/SWNT conjugates were also successfully used to catalyze Heck and Suzuki coupling reactions. Boranephosphonate DNA-mediated metallization of SWNTs therefore provides a new method for fabricating well-defined SWNT-based nanostructures. This discovery should reveal unexpected applications in various research areas ranging from nanoelectronic devices to nanoscale SWNT supported multimetallic catalysts having different compositions

Multiplexed Aptasensors and Amplified DNA Sensors Using Functionalized Graphene Oxide: Application for Logic Gate Operations

Graphene oxide (GO) is implemented as a functional matrix for developing fluorescent sensors for the amplified multiplexed detection of DNA, aptamer–substrate complexes, and for the integration of predesigned DNA constructs that activate logic gate operations. Fluorophore-labeled DNA strands acting as probes for two different DNA targets are adsorbed onto GO, leading to the quenching of the luminescence of the fluorophores. Desorption of the probes from the GO, through hybridization with the target DNAs, leads to the fluorescence of the respective label. By coupling exonuclease III, Exo III, to the system, the recycling of the target DNAs is demonstrated, and this leads to the amplified detection of the DNA targets (detection limit 5 × 10^–12 M). Similarly, adsorption of fluorophore-functionalized aptamers against thrombin or ATP onto the GO leads to the desorption of the aptamer–substrate complexes from GO and to the triggering of the luminescence corresponding to the respective fluorophore, thus, allowing the multiplexed analysis of the aptamer–substrate complexes. By designing functional fluorophore-labeled DNA constructs and their interaction with GO, in the presence (or absence) of nucleic acids, or two different substrates for aptamers, as inputs, the activation of the “OR” and “AND” logic gates is demonstrated

Biocatalytic Implant of Pt Nanoclusters into Glucose Oxidase: A Method to Electrically Wire the Enzyme and to Transform It from an Oxidase to a Hydrogenase

The enzyme glucose oxidase (GOx) reduces, in the presence of glucose and under anaerobic conditions, PtCl₆²⁻ to Pt nanoclusters (NCs) that are implanted in the protein. The assembly of the Pt NC/GOx hybrid on a dithiol monolayer yields an electrically contacted enzyme electrode, and the bioelectrocatalytic oxidation of glucose is activated (turnover rate ca. 2780 ± 70 s⁻¹). The Pt NC/GOx hybrid is also used, under anaerobic conditions, as a biocatalyst for H₂ evolution

Light-Driven Catalytic Upgrading of Butanol in a Biohybrid Photoelectrochemical System

This paper reports the design and preparation of a biohybrid photoelectrochemical cell (PEC) that can drive the tandem enzymatic oxidation and aldol condensation of <i>n</i>-butanol (BuOH) to C₈ 2-ethylhexenal (2-EH). In this work, BuOH was first oxidized to <i>n</i>-butyraldehyde (BA) by the alcohol oxidase enzyme (AOx), concurrently generating hydrogen peroxide (H₂O₂). To preserve enzyme activity and increase kinetics nearly 2-fold, the H₂O₂ was removed by oxidation at a bismuth vanadate (BiVO₄) photoanode. Organocatalyzed aldol condensation of C₄ BA to C₈ 2-EH improved the overall BuOH conversion to 6.2 ± 0.1% in a biased PEC after 16 h. A purely light-driven, unbiased PEC showed 3.1 ± 0.1% BuOH conversion, or ∼50% of that obtained from the biased system. Replacing AOx with the enzyme alcohol dehydrogenase (ADH), which requires the diffusional nicotinamide adenine dinucleotide cofactor (NAD⁺/NADH), resulted in only 0.2% BuOH conversion due to NAD⁺ dimerization at the photoanode. Lastly, the application of more positive biases with the biohybrid AOx PEC led to measurable production of H₂ at the cathode, but at the cost of lower BA and 2-EH yields due to both product overoxidation and decreased enzyme activity

Enzyme-Capped Relay-Functionalized Mesoporous Carbon Nanoparticles: Effective Bioelectrocatalytic Matrices for Sensing and Biofuel Cell Applications

The porous high surface area and conducting properties of mesoporous carbon nanoparticles, CNPs (<500 nm diameter of NPs, pore dimensions ∼6.3 nm), are implemented to design electrically contacted enzyme electrodes for biosensing and biofuel cell applications. The relay units ferrocene methanol, Fc-MeOH, methylene blue, MB⁺, and 2,2′-azino­bis(3-ethyl­benzo­thiazoline-6-sulfonic acid), ABTS^2–, are loaded in the pores of the mesoporous CNPs, and the pores are capped with glucose oxidase, GOx, horseradish peroxidase, HRP, or bilirubin oxidase, BOD, respectively. The resulting relay/enzyme-functionalized CNPs are immobilized on glassy carbon electrodes, and the relays encapsulated in the pores are sufficiently free to electrically contact the different enzymes with the bulk electrode supports. The Fc-MeOH/GOx CNP-functionalized electrode is implemented for the bio­electro­catalyzed sensing of glucose, and the MB⁺/HRP-modified CNPs are applied for the electrochemical sensing of H₂O₂. The ABTS^2–/BOD-modified CNPs provide an effective electrically contacted material for the bio­electro­catalyzed reduction of O₂ (<i>k</i>_cat = 94 electrons·s^–1). Integration of the Fc-MeOH/GOx CNP electrode and of the electrically wired ABTS^2–/BOD CNP electrode as anode and cathode, respectively, yields a biofuel cell revealing a power output of ∼95 μW·cm^–2

Electrochemical Switching of Photoelectrochemical Processes at CdS QDs and Photosystem I‑Modified Electrodes

Photoactive inorganic CdS quantum dots (QDs) or the native photosystem I (PSI) is immobilized onto a pyrroloquinoline quinone (PQQ) monolayer linked to Au electrodes to yield hybrid relay/QDs (or photosystem) assemblies. By the electrochemical biasing of the electrode potential, the relay units are retained in their oxidized PQQ or reduced PQQH₂ states. The oxidized or reduced states of the relay units dictate the direction of the photocurrent (anodic or cathodic). By the cyclic biasing of the electrode potential between the values <i>E </i>≥ −0.05 V and <i>E</i> ≤ −0.3 V <i>vs</i> Ag quasi-reference electrode (Ag QRE), retaining the relay units in the oxidized PQQ or reduced PQQH₂ states, the photocurrents are respectively switched between anodic and cathodic values. Different configurations of electrically switchable photoelectrochemical systems are described: (i) the PQQ/CdS QDs/(triethanolamine, TEOA) or PQQ/PSI/(ascorbic acid/dichlorophenolindophenol, DCPIP) systems, leading to anodic photocurrents; (ii) the PQQ/CdS QDs (or PSI)/(flavin adenine dinucleotide) systems, leading to cathodic photocurrents; (iii) the PQQ/CdS QDs (or PSI)/(O₂) switchable systems, leading to cyclic anodic/cathodic switching of the photocurrents

Multicatalytic, Light-Driven Upgrading of Butanol to 2‑Ethylhexenal and Hydrogen under Mild Aqueous Conditions

Microbes produce low-molecular-weight alcohols from sugar, but these metabolites are difficult to separate from water and possess relatively low heating values. A combination of photo-, organo-, and enzyme catalysis is shown here to convert C₄ butanol (BuOH) to C₈ 2-ethylhexenal (2-EH) using only solar energy to drive the process. First, alcohol dehydrogenase (ADH) catalyzed the oxidation of BuOH to butyraldehyde (BA), using NAD⁺ as a cofactor. To prevent back reaction, NAD⁺ was regenerated using a platinum-seeded cadmium sulfide (Pt@CdS) photocatalyst. An amine-based organocatalyst then upgraded BA to 2-EH under mild aqueous conditions rather than harsh basic conditions in order to preserve enzyme and photocatalyst stability. The process also simultaneously increased total BuOH conversion. Thus, three disparate types of catalysts synergistically generated C₈ products from C₄ alcohols under green chemistry conditions of neutral pH, low temperature, and pressure