Conversion of Carbon Dioxide into Chemical Vapor Deposited Graphene with Controllable Number of Layers via Hydrogen Plasma Pre-Treatment.

In this work, we report the conversion of carbon dioxide (CO2) gas into graphene on copper foil by using a thermal chemical vapor deposition (CVD) method assisted by hydrogen (H2) plasma pre-treatment. The synthesized graphene has been characterized by Raman spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results show the controllable number of layers (two to six layers) of high-quality graphene by adjusting H2 plasma pre-treatment powers (100-400 W). The number of layers is reduced with increasing H2 plasma pre-treatment powers due to the direct modification of metal catalyst surfaces. Bilayer graphene can be well grown with H2 plasma pre-treatment powers of 400 W while few-layer graphene has been successfully formed under H2 plasma pre-treatment powers ranging from 100 to 300 W. The formation mechanism is highlighted.

An alternative ready-to-use electrochemical immunosensor for point-of-care COVID-19 diagnosis using graphene screen-printed electrodes coupled with a 3D-printed portable potentiostat.

A severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a cause of worldwide Coronavirus 2019 (COVID-19) disease pandemic. It is thus important to develop ultra-sensitive, rapid and easy-to-use methods for the identification of COVID-19 infected patients. Herein, an alternative electrochemical immunosensor based on poly(pyrrolepropionic acid) (pPPA) modified graphene screen-printed electrode (GSPE) was proposed for rapid COVID-19 detection. The method was based on a competitive enzyme immunoassay process utilizing horseradish peroxidase (HRP)-conjugated SARS-CoV-2 as a reporter binding molecule to compete binding with antibody against the SARS-CoV-2 receptor binding domain (SARS-CoV-2 RBD) protein. This strategy enhanced the current signal via the enzymatic reaction of HRP-conjugated SARS-CoV-2 RBD antibody on the electrode surface. The modification, immobilization, blocking, and detection processes were optimized and evaluated by amperometry. The quantitative analysis of SARS-CoV-2 was conducted based on competitive enzyme immunoassay with amperometric detection using a 3D-printed portable potentiostat for point-of-care COVID-19 diagnosis. The current measurements at -0.2 V yielded a calibration curve with a linear range of 0.01-1500 ng mL-1 (r2 = 0.983), a low detection limit of 2 pg mL-1 and a low quantification limit of 10 pg mL-1. In addition, the analyzed results of practical samples using the developed method were successfully verified with ELISA and RT-PCR. Therefore, the proposed portable electrochemical immunosensor is highly sensitive, rapid, and reliable. Thus, it is an alternative ready-to-use sensor for COVID-19 point-of-care diagnosis.

Alpha-MnO2 nanofibers/nitrogen and sulfur-co-doped reduced graphene oxide for 4.5 V quasi-solid state supercapacitors using ionic liquid-based polymer electrolyte.

α-MnO2 nanofibers combined with nitrogen and sulfur co-doped reduced graphene oxide (α-MnO2/N&S-rGO) were prepared through simple hydrothermal and ball milling processes. Structural characterization results by X-ray diffraction, X-ray photoemission spectroscopy, electron microscopy and Raman spectroscopy demonstrated that α-MnO2 nanofibers with the average diameter of ~40 nm were well dispersed on N&S-rGO nanoflakes. The synthesized material was incorporated into supercapacitor (SC) electrodes and assembled with the quasi-solid-state electrolyte comprising N,N-Diethyl-N-methyl-N-(2-methoxy-ethyl)ammonium bis (trifluoromethyl-sulfonyl)amide [DEME][TFSA]/polyvinylidene fluoride-hexafluoropropylene (PVDF-co-HFP) to produce coin-cell SCs. Electrochemical performances of SCs were measured by cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy. From the electrochemical data, SC using α-MnO2/N&S-rGO exhibited a good specific capacitance of 165F g-1 at 0.25 A g-1 with a wide potential window of 0-4.5 V, corresponding to a high energy density of 110 Wh kg-1 and a power density of 550 W kg-1. In addition, it exhibited good electrochemical stability with a capacitance retention of 75% after 10,000 cycles at 1 A g-1 and a low self-discharge loss. The attained energy-storage performances indicated that the α-MnO2/N&S-rGO composite could be highly promising for high-performance ionic liquid-based quasi solid-state supercapacitors.

On-chip immunoassay for determination of urinary albumin.

An immunoassay performed on a portable microfluidic device was evaluated for the determination of urinary albumin. An increase in absorbance at 500 nm resulting from immunoagglutination was monitored directly on the poly(dimethylsiloxane) (PDMS) microchip using a portable miniature fibre-optic spectrometer. A calibration curve was linear up to 10 mg L(-1) (r(2) = 0.993), with a detection limit of 0.81 mg L(-1) (S/N = 3). The proposed system showed good precision, with relative standard deviations (RSDs) of 5.1%, when evaluated with 10 mg L(-1) albumin (n = 10). Determination of urinary albumin with the proposed system gave results highly similar to those determined by the conventional spectrophotometric method using immunoturbidimetric detection (r(2) = 0.995; n = 15).

Detection of shrimp Taura syndrome virus by loop-mediated isothermal amplification using a designed portable multi-channel turbidimeter.

In this study, a portable turbidimetric end-point detection method was devised and tested for the detection of Taura syndrome virus (TSV) using spectroscopic measurement of a loop-mediated isothermal amplification (LAMP) by-product: magnesium pyrophosphate (Mg(2)P(2)O(7)). The device incorporated a heating block that maintained an optimal temperature of 63°C for the duration of the RT-LAMP reaction. Turbidity of the RT-LAMP by-product was measured when light from a light-emitting diode (LED) passed through the tube to reach a light dependent resistance (LDR) detector. Results revealed that turbidity measurement of the RT-LAMP reactions using this device provided the same detection sensitivity as the agarose gel electrophoresis detection of RT-LAMP and nested RT-PCR (IQ2000™) products. Cross reactions with other shrimp viruses were not found, indicating that the RT-LAMP-turbidity measurement was highly specific to TSV. The combination of 10 min for rapid RNA preparation with 30 min for RT-LAMP amplification followed by turbidity measurement resulted in a total assay time of less than 1h compared to 4-8h for the nested RT-PCR method. RT-LAMP plus turbidity measurement constitutes a platform for the development of more rapid and user-friendly detection of TSV in the field.

AAO-CNTs electrode on microfluidic flow injection system for rapid iodide sensing.

In this work, carbon nanotubes (CNTs) nanoarrays in anodized aluminum oxide (AAO-CNTs) nanopore is integrated on a microfluidic flow injection system for in-channel electrochemical detection of iodide. The device was fabricated from PDMS (polydimethylsiloxane) microchannel bonded on glass substrates that contains three-electrode electrochemical system, including AAO-CNTs as a working electrode, silver as a reference electrode and platinum as an auxiliary electrode. Aluminum, stainless steel catalyst, silver and platinum layers were sputtered on the glass substrate through shadow masks. Aluminum layer was then anodized by two-step anodization process to form nanopore template. CNTs were then grown in AAO template by thermal chemical vapor deposition. The amperometric detection of iodide was performed in 500-µm-wide and 100-µm-deep microchannels on the microfluidic chip. The influences of flow rate, injection volume and detection potential on the current response were optimized. From experimental results, AAO-CNTs electrode on chip offers higher sensitivity and wider dynamic range than CNTs electrode with no AAO template.

DNA hybridization enhancement using piezoelectric microagitation through a liquid coupling medium.

In conventional DNA microarray hybridization, delivery of target cDNAs to surface-bounded probes depends solely on diffusion, which is notoriously slow, and thus typically requires 6-20 h to complete. In this study, piezoelectric microagitation through a liquid coupling medium is employed to enhance DNA hybridization efficiency and the results are compared with the standard static hybridization method. DNA hybridization was performed in a sealed aluminium chamber containing DNA microarray glass chip, coupling medium and piezoelectric transducers. 3×SSC (Saline Sodium Citrate) was used as a coupling medium to prevent overheating of the piezoelectric transducers and to effectively transmit ultrasonic wave to the glass chip. Flow visualization using fluidic dye and velocimetry (PTV) technique was applied to observe fluid transport in the hybridization chamber. It was revealed that the dye solution was homogeneously distributed within 10 min under dynamic agitation while it took over 1 h to reach the same level of homogeneity in static condition. Plasmodium falciparum DNA microarrays and total RNA extracted from parasite cells were used as a model for DNA microarray experiments. It was found that the required hybridization time may be substantially reduced from 16 h to 4 h by the use of dynamic hybridization scheme. With the same hybridization time of 16 h, dynamic hybridization resulted in higher fluorescent signals of ∼33% and ∼24% compared to static hybridization in Cy3 and Cy5 channels, respectively. Additionally, good/effective spots, some of which were not formed by static method, were enhanced and distributed more uniformly over the microarray. Therefore, the developed dynamic hybridization with integrated piezoelectric microagitation platform is highly promising for DNA analysis in molecular biology and medical applications.

Flow injection based microfluidic device with carbon nanotube electrode for rapid salbutamol detection.

A microfabicated flow injection device has been developed for in-channel electrochemical detection (ECD) of a beta-agonist, namely salbutamol. The microfluidic system consists of PDMS (polydimethylsiloxane) microchannel and electrochemical electrodes formed on glass substrate. The carbon nanotube (CNT) on gold layer as working electrode, silver as reference electrode and platinum as auxiliary electrode were deposited on a glass substrate. Silver, platinum, gold and stainless steel catalyst layers were coated by DC-sputtering. CNTs were then grown on the glass substance by thermal chemical vapor deposition (CVD) with gravity effect and water-assisted etching. 100-microm-deep and 500-microm-wide PDMS microchannels fabricated by SU-8 molding and casting were then bonded on glass substrate by oxygen plasma treatment. Flow injection and ECD of salbutamol was performed with the amperometric detection mode for in-channel detection of salbutamol. The influences of flow rate, injection volume, and detection potential on the response of current signal were optimized. Analytical characteristics, such as sensitivity, repeatability and dynamic range have been evaluated. Fast and highly sensitive detection of salbutamol have been achieved. Thus, the proposed combination of the efficient CNT electrode and miniaturized lab-on-a-chip is a powerful platform for beta-agonists detection.