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Reduced nicotinamide adenine dinucleotide (NADH)-detecting electrochemical sensors are attractive in monitoring and diagnosing various physiological disorders of NADH abnormalities. The NADH detection methods using conventional electrodes are challenging due to slow electron transfer and fouling effect. Interestingly, paper-based flexible and disposable electrodes (PE) are superior for sensing biomolecules through simple detection procedures with excellent sensitivity and selectivity. Herein, to construct a conducting polypeptide-modified paper electrode, initially, polytyrosine (PTyr) is synthesized from l-tyrosine N-carboxy anhydride through ring-opening polymerization, and PTyr is drop-coated on the PE. The PTyr-modified paper electrode (PMPE) demonstrated excellent electrochemical properties and facilitated the electrooxidation of NADH at a lower potential of 576 mV. The PMPE displayed a linear detection between 25 and 145 μM of NADH concentration, with a lower detection limit of 0.340 μM. Under ideal circumstances, the sensor developed displayed an excellent NADH detection capability without interference with the most common electroactive species, ascorbic acid. The PMPE facilitates good electrocatalytic activity toward NADH, which can also be employed as a substrate material for biofuel cells.

In this work, a self-circulation oxygen-hydrogen peroxide-oxygen (O 2 -H 2 O 2 -O 2 ) system with photogenerated electrons as fuel and highly active hemin monomers as operators was engineered for ultrasensitive cathode photoelectrochemical bioassay of microRNA-141 (miRNA-141) using a stacked sealed paper device. During the circulation, the photogenerated electrons from BiVO 4 /Cu 2 O photosensitive structures assembled on a reduced graphene oxide paper electrode first reduced the electron acceptors (dissolved O 2 ) to H 2 O 2 , which was then catalytically decomposed by hemin monomers to generate O 2 again. The regenerated O 2 continued to be reduced, which made O 2 and H 2 O 2 stuck in the infinite loop of O 2 -H 2 O 2 -O 2 accompanied by the fast consumption of photogenerated electrons, generating an amplified photocurrent signal. When a target existed, a duplex-specific nuclease-induced target recycling reaction with dual trigger DNA probes as the output was performed to initiate the assembly of bridge-like DNA nanostructures, which endowed the self-circulation system with dual destruction functions as follows. (i) Reduced fuel supply: the assembled DNA bridges acting as a negatively charged barrier prevented the photogenerated electrons from participating in the O 2 reduction to H 2 O 2 . (ii) Incapacitation of operators: DNA bridging induced the dimerization of hemin monomers linked on the DNA hairpins to catalytically inactive hemin dimers, leading to the abortive regeneration of O 2 . These destruction functions resulted in the circulation interruption and a remarkably decreased photocurrent signal. Thus, the developed cathode photoelectrochemical biosensing platform achieved ultrasensitive miRNA-141 detection with a linear range of 0.25 fM to 1 nM and a detection limit of 83 aM, and it also exhibited high accuracy, selectivity, and practicability.

In this work, a triggerable H 2 O 2 -cleavable fluid switch mediated paper-based biochip, being amenable to multiplexing and quantitative analysis with the dual-response output of visual screening and ratiometric electrochemistry, was developed for sensitive detection of target on-site. By properly implanting hydrophobic Ag-H 2 O 2 responsive material in specific zone to form a programmable fluid switch, the biochip could achieve different modes of blocking/connecting switching automatically. In order to improve the test performance, a ratiometric electrochemical signal readout was designed, which was enhanced by a secondary in situ growth method fabricating trepang-shaped Au modified paper working electrode. In virtue of hybridization chain reaction, classic competitive recognition interactions of aptamer and target, and ratiometric internally calibrated mechanisms, ultrasensitive detection of the target was realized. To acquire a more quantitative and straightforward naked eye visual screening, the hydrophobic Ag switch was triggered by stimulating instructions from H 2 O 2 , thus reconnecting the electrochemical and ratiometric units automatically and resulting in a "signal on" visual fluidic flow on the chemometer characterized by the accurate distance of color development as a detection motif. With MCF-7 and K562 cells as models, wider linear detection ranges from 150 to 1.0 × 10 7 and 220 to 7.0 × 10 6 cells mL -1 for MCF-7 and K562 cells, respectively, were achieved. Meanwhile, thanks to the paper fluid chemometer, an acceptable screening detection limit of 10 3 cells mL -1 was obtained in the quantitative colorimetric assays. The proposed paper-based biochips opened up new horizons for designing of integratable, easy-to-use, and precise point-of-care testing devices.

A novel paper electrode-based flexible pressure sensor modified with multiwalled carbon nanotubes was designed for point-of-care (POC) immunoassay of carcinoembryonic antigen (CEA) with digital multimeter readout. The portable POC testing device consisted of flexible pressure sensor equipped with a paper electrode and connected through syringe tubing to a single-break microplate. The immunoreaction was initially carried out on the microplate with a sandwich-type assay format using platinum nanozyme-labeled secondary antibody for the gas generation. Upon addition of hydrogen peroxide (H 2 O 2 ), platinum nanozyme (catalase-like mimic) reduced it into hydrogen oxide and oxygen (O 2 ). The overflowing oxygen gas increased the pressure of the multiwalled carbon nanotube-functionalized paper electrode in a homemade pressure-tight system, and the increased pressure could be readily monitored using the paper electrode-based flexible pressure-tight sensor with a digital multimeter readout. The detectable signal mainly derived from the resistance change of pressure sensor because of its deformation with the assistance of the as-generated gas, and the shift in the resistance could be allowed to detect the gas pressure even as low as 80 Pa. Under optimum conditions, pressure sensor-based immunoassay exhibited good resistance responses toward target CEA within a linear range of 0.5-60 ng/mL at a detection limit of 167 pg/mL. Moreover, our strategy provided acceptable reproducibility, precision, high specificity, and good accordance with the commercial CEA ELISA kit for detecting human serum specimens.

The development of flexible electrodes is of considerable current interest because of the increasing demand for modern electronics, portable medical products, and compact energy devices. We report a modular approach to fabricating high-performance flexible electrodes by structurally integrating 2D-assemblies of nanoparticles with freestanding graphene paper. We have shown that the 2D array of gold nanoparticles at oil-water interfaces can be transferred on freestanding graphene oxide paper, leading to a monolayer of densely packed gold nanoparticles of uniform sizes loaded on graphene oxide paper. One major finding is that the postassembly electrochemical reduction of graphene oxide paper restores the ordered structure and electron-transport properties of graphene, and gives rise to robust and biocompatible freestanding electrodes with outstanding electrocatalytic activities, which have been manifested by the sensitive and selective detection of two model analytes: glucose and hydrogen peroxide (H(2)O(2)) secreted by live cells. The modular nature of this approach coupled with recent progress in nanocrystal synthesis and surface engineering opens new possibilities to systematically study the dependence of catalytic performance on the structural parameters and chemical compositions of the nanocrystals., (© 2011 American Chemical Society)

The content of a bacterial consortium found on an electron beam (e-beam) Pt-deposited electrode in a mediator-less microbial fuel cell (MFC) using glucose and glutamate as fuel is reported in this paper. The e-beam Pt-deposited electrode and electrochemically active bacteria (EAB) consortium were developed to improve the mediator-less MFC performance. Denaturing gradient gel electrophoresis (DGGE), restriction fragment length polymorphism (RFLP), and 16S rRNA sequencing were used to identify the EAB consortia. Sequencing results showed that clone ASP-31 was predominant and was similar to Aeromonas hydrophila, an Fe(III)-reducing and EAB. The phylogenetic tree analysis disclosed the presence of gamma-proteobacteria groups such as Aeromonas genus, Enterobacter asburiae, and Klebsiella oxytoca. These results suggest that MFC performance of the e-beam Pt-deposited electrode with Aeromonas genus consortia dominated by A. hydrophila was higher than other MFCs within a short period. With the e-beam Pt-deposited electrode and Aeromonas genus consortia in the mediator-less MFC, it is possible to increase the efficiency of electron transfer between the bacteria and the electrode.