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In this study, a colorimetric sensing assay of isoniazid based on excellent oxidase-like activity of heparin sodium stabilized platinum nanoparticles (HS-PtNPs) has been demonstrated. The newly prepared HS-PtNPs exhibit a great dispersion with an average size distribution of 4.8 ± 0.6 nm, and maintain more than 90% catalytic activity under strong acid and alkali or long-term storage conditions, indicating a robust nanomaterial with attractive potential. The HS-PtNPs show distinct oxidase-like activity with an ultrahigh affinity (Km = 0.01012 mM) for 3, 3', 5, 5'-tetramethylbenzidine (TMB). More significantly, we found that the pyridine ring of isoniazid has a strong reductive hydrazyl substitution, which can compete with TMB for the catalytic site of HS-PtNPs resulting in a colorless solution. Accordingly, a colorimetric sensing of isoniazid was fabricated. A linear relationship for isoniazid was achieved in 2.5 × 10 -6 to 2.5 × 10 -4  M (R 2  = 0.998) with a low limit of detection 1.7 × 10 -6  M (S/N = 3). Recovery experiments in drug tablets show that the standard recovery rates were 95%-103%. The quantitative detection data for isoniazid in drug tablets calculated respectively from the standard method and this method exhibited a high correlation coefficient (a slope of 0.9995), suggesting that high accuracy in isoniazid detection., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Microfluidic paper-based analytical devices (μPADs) for detection of hydrogen peroxide and glucose have been developed. The analytical performance of colorimetric detection using the conventional starch-iodine color reaction has been significantly improved by using gelatin as the surface modifier which retains the enzyme activity in the dry filter paper strip, improves antioxidability, as well as decreases the strong background signal. Under optimal conditions, the color intensities show a good linear relationship with glucose concentration ranging from 0.5 to 5 mM and hydrogen peroxide concentration from 0.5 to 6 mM, with the detection limit of 0.05 mM and 0.1 mM, respectively. In addition, the accuracy of colorimetric sensor has been successfully assessed in detecting glucose from real human serum samples and recovery value ranges from 95.7% to 97%, which are approaching to the glucose oxidase endpoint. The new colorimetric assay exhibits high sensitivity, good selectivity, acceptable stability and reproducibility. The present approach is promising for monitoring glucose for point of care diagnostic applications, especially in regions with resource-limited settings., (Copyright © 2019 Elsevier B.V. All rights reserved.)

We demonstrated a sensitive method for detection of glutathione (GSH) based on LSPR scattering spectrum using gold core-satellite nanostructure linked by T-Hg 2+ -T base pair. The core-satellite assembly caused coupling between plasmonic nanoparticles, which inducing distinct change of LSPR peak wavelength. As the interaction between Hg 2+ and GSH, the core-satellite nanostructure would be disassembled, which accompanied with spectral blue-shift of the scattering spectrum. By using this method, GSH could be quantitatively detected, and the detection limits can reach to 0.1 µM., (Copyright © 2018 Elsevier B.V. All rights reserved.)

For study of the photocatalytic reaction kinetics in a confined microsystem, a photocatalysis microreactor integrated on a microfluidic device has been fabricated using an on-line UV/vis detector. The performance of the photocatalysis microreactor is evaluated by the photocatalytic degradation of Rhodamine B chosen as model target by using commercial titanium dioxide (Degussa P25, TiO 2 ) nanoparticles as a photocatalyst. Results show that the photocatalytic reaction occurs via the Langmuir-Hinshelwood mechanism and the photocatalysis kinetics in the confined microsystem (r = 0.359 min -1 ) is about 10 times larger than that in macrosystem (r = 0.033 min -1 ). In addition, the photocatalysis activity of the immobilized TiO 2 nanoparticles in the microreactor exhibits good stability under flowing conditions. The present microchip device offers an interesting platform for screening of photocatalysts and exploration of photocatalysis mechanisms and kinetics., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Self-assembled monolayer (SAM) has been extensively applied as ideal interface layer for construction of biosensors. Its chain length and end functional groups determine the physical and chemical properties of the modified surfaces, which will affect the performance of constructed biosensors. Herein, we studied the influence of chain length of n-alkanethiols SAMs on the immunoreaction kinetics employing attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). Antibody (rabbit immunoglobulin) is assembled on carboxyl terminated SAMs of n-alkanethiols with different chain lengths (n = 3, 6, 11, 16). The whole fabrication steps of the immunoassay can be monitored in situ by the ATR-SEIRAS. From the time-dependent SEIRA spectra, the interfacial immunoreaction kinetics between the immobilized antibody and antigen (goat anti-rabbit immunoglobulin) can be evaluated. We found that the immunoreaction became faster with increasing the chain length of SAMs. This chain length dependent kinetics might be attributed to different orientations of the assembled antibody caused by different packing densities of SAMs. The present research offers a sensing platform to evaluate immunoassay kinetics and provides fundamentals for construction of immunoassay with high performance., (Copyright © 2017 Elsevier B.V. All rights reserved.)

A novel microfluidic method for measuring liquid interfacial tension using monodisperse microbubbles generated in situ has been proposed. Instead of bulky gas supply used in traditional microfluidic devices, microbubbles are efficiently generated via water electrolysis in the devices. Since the bubble formation frequency is related to the interfacial tension of liquids used, thus, precisely measuring the interfacial tension of liquids in microfluidics can be achieved. In addition, it is found that during the microbubble formation, the electrochemical potential fluctuates regularly at controlled electrolysis current, and the fluctuating period depends on the microbubble generation rate. Therefore, the change in electrochemical potential can be directly used to monitor the bubble formation process, which avoids the use of an external optical detection system. As demonstration, the interfacial tension of isopentanol solutions with different concentrations was measured, and the results show good agreement with the ones obtained using the maximum bubble pressure method, confirming the accuracy of the present method. The proposed strategy offers a simple, low cost and accurate solution to measure the liquid interfacial tension confined in microfluidic channels. The present platform is easily constructed and facilely manipulated in common laboratories, which is expected to be widely used in microfluidic-based research and application fields., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Numerical simulation can provide valuable insights for complex microfluidic phenomena coupling mixing and diffusion processes. Herein, a novel finite element model (FEM) has been established to extract chemical reaction kinetics in a microfluidic flow injection analysis (micro-FIA) system using high throughput sample introduction. To reduce the computation burden, the finite element mesh generation is performed with different scales based on the different geometric sizes of micro-FIA. In order to study the contribution of chemical reaction kinetics under non-equilibrium condition, a pseudo-first-order chemical kinetics equation is adopted in the numerical simulations. The effect of reactants diffusion on reaction products is evaluated, and the results demonstrate that the Taylor dispersion plays a determining role in the micro-FIA system. In addition, the effects of flow velocity and injection volume on the reaction product are also simulated. The simulated results agree well with the ones from experiments. Although gravity driven flow is used to the numerical model in the present study, the FEM model also can be applied into the systems with other driving forces such as pressure. Therefore, the established FEM model will facilitate the understanding of reaction mechanism in micro-FIA systems and help us to optimize the manifold of micro-FIA systems., (Copyright © 2015 Elsevier B.V. All rights reserved.)

Determination of protein concentration in mild environments is of great significance in the clinic diagnose and bioassay. Herein, a simple, fast and sensitive method for protein quantitative determination in neutral solution (pH 7.0) is developed. This assay is based on competition adsorption of the sample protein and fluorescently labeled dog serum albumin (FITC-DSA) onto gold nanoparticles (AuNPs). As the competitor FITC-DSA molecules are added into the mixture solution of sample protein conjugated AuNPs, they will compete for active sites of AuNPs, resulting in decrease in fluorescence intensity due to the quenching effect of AuNPs via fluorescence resonance energy transfer (FRET). Thus, quantitative determination of sample protein concentration can be achieved. Under the optimum conditions, the decrease in fluorescence intensity of the solution is related to the concentration of sample protein and a low detection limit of 0.01 µg/mL BSA can be achieved in 5 min. For the validation of our strategy in practical applications, the total protein content in human serum was determined using the as-proposed method. The result is in well agreement with that of measured by other conventional methods, suggesting a simple, accurate, and mild approach for protein detection in bioassay., (Copyright © 2015 Elsevier B.V. All rights reserved.)

A facile electrochemical immunosensor based on graphene-three dimensional nanostructure gold nanocomposites (G-3D Au) using simple and rapid one-step electrochemical co-reduction technique was developed for sensitive detection of topoisomerase. The resultant G-3D Au nanocomposites were characterized by scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy, and then were used as a substrate for construction of the "sandwich-type" immunosensor. Amperometric current-time curve was employed to monitor the immunoreaction on the protein modified electrode. The proposed method could respond to topoisomerase with a linear calibration range from 0.5 ng mL(-1) to 50 ng mL(-1) with a detection limit of 10 pg mL(-1). This new biosensor exhibited a fast amperometric response, high sensitivity and selectivity, and was successfully used in determining the topoisomerase which was added in human serum with a relative standard deviation (n=5)<5%. The immunosensor served as a significant step toward the practical application of the immunosensor in clinical diagnosis and prognosis monitor., (Copyright © 2014 Elsevier B.V. All rights reserved.)

As a peroxidase mimic, cupric oxide nanoparticles were found to enhance the chemiluminescence (CL) of luminol-H(2)O(2) system up to 400 folds. The CL spectra and radical scavengers were conducted to investigate the possible CL enhancement mechanism. It was suggested that the enhanced CL could be attributed to the peroxidase-like activity of CuO nanoparticles, which effectively catalyzed the decomposition of hydrogen peroxide into hydroxyl radicals. The effects of the reactant concentrations and some organic compounds were also investigated. The proposed method could be used as a sensitive detection tool for hydrogen peroxide and glucose., (Copyright © 2012 Elsevier B.V. All rights reserved.)

This paper presents a simple and cost-effective UV-ablation technique for fabrication of size-tunable nanofluidics devices via photochemical decomposition reaction. UV-irradiation through a PET photomask results in continuous decomposition of poly(carbonate) (PC), forming nanochannel and carboxyl groups on the surface of the etched PC. This photochemical decomposition process occurs at molecular scale, therefore, the depth of nanochannels can be controlled at nanometer level. The etching rate is estimated to be ca. 0.015 nms(-1). To demonstrate the potential application of the present UV-ablation technique, a nanochannel was fabricated and integrated with microchannels to form a micro/nanofluidics chip for protein concentration. Using this device, about 10(3)-10(5) fold protein concentration can be achieved within 10 min. The present approach offers a simple and practical solution to fabricate nanofluidics devices at low-cost, and the resulting device could provide ideal platforms for μTAS towards various applications in biology and chemistry., (Copyright © 2011 Elsevier B.V. All rights reserved.)

Unit operations for complicated biochemical analysis cannot usually be integrated into one substrate. A possible solution to solve this problem is to integrate multi-unit operations into two or more substrates. In this case, transporting droplets from one substrate to another is essential. In this work, a new method to transport droplets from a hydrophobic glass substrate to a piezoelectric substrate is proposed. An interdigitated transducer (IDT) and reflectors were fabricated on an optic grade 128°YX-cut lithium niobate (LiNbO(3)) substrate, and its working surface between the IDT and a reflector was modified to be hydrophobic. Droplets to be transported were first pipetted onto a glass substrate. Adjust the glass substrate so that the droplets could contact the working surface of the piezoelectric substrate, and then was moved down. These droplets could be successfully transported from the glass surface to the piezoelectric substrate because of their "adhesion work" difference. By using this mechanism, water and red dye droplets were successfully transported from glass substrate to piezoelectric substrate. As an application, droplets mixing process was demonstrated in the piezoelectric substrate by using surface acoustic wave after they have been transported from the glass substrate., (Copyright © 2011 Elsevier B.V. All rights reserved.)

The electrochemistry of a macrocyclic metal complex Fe(notpH(3)) [notpH(6)=1,4,7-triazacyclononane-1,4,7-triyl-tris(methylene-phosphonic acid)] reveals that the protonation/deprotonation of the non-coordinated P-OH groups in Fe(notpH(3)) affects its formal potential value (E(0)') considerably. Plotting E(0)' as function of solution pH gives a straight line with a slope of -585 mV pH(-1) in the pH range of 3.4-4.0, which is about ten times larger than the theoretical value of -58 mV pH(-1) for a reversible proton-coupled single-electron transfer at 20 °C. A sensitive pH responsive electrochemical switch sensor is thus developed based on Fe(notpH(3)) which shows an "on/off" switching at pH ∼ 4.0., (Copyright © 2010 Elsevier B.V. All rights reserved.)

We have successfully constructed a novel gold film with open interconnected macroporous walls of nanoparticles by combining the hydrogen bubble dynamic template synthesis with galvanic replacement reaction. After modified by a self-assembled monolayer (SAM) of 11-mercaptoundecanoic acid (MUA), the three-dimensionally (3D) interconnected macroporous Au film has been used as a biocompatible substrate for the immobilization of cytochrome c. The morphology, structure and electrochemical features of the modified and unmodified macroporous Au films were characterized by field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray (EDX), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results reveal that the resultant films had a large electroactive surface area for high protein loading, enhanced electron transfer of cytochrome c, retained electrochemical activity, good stability and repeatability. And the excellent electrochemical behaviors could be attributed to the hierarchical structure of the macroporous Au film constructed by nanoparticles., (Copyright (c) 2010 Elsevier B.V. All rights reserved.)

Nickel hexacyanoferrate film modified gold electrode was prepared by a simple chemical deposition procedure from a fresh prepared solution containing ferricyanide, Ni(2+), and sodium nitrate. The resultant films have solo composition and are significantly stable as compared to the electrochemically deposited NiHCF films. For different concentrations of Na(+) in the solution, the formal potential values of NiHCF shift according to the Nernstian behavior with a slope of 48 mV in the range of 10(-4) to 1.0M. The NiHCF film was also used for the electrocatalytic oxidation of ascorbic acid. The anodic peak current observed in cyclic voltammetry increased with the ascorbic acid concentration. At a fixed potential under hydrodynamic conditions, the calibration plot was linear over the ascorbic acid concentration range 0.1-12 mM.

The effect of surface microstructures of microchannels on the separation efficiency in microchip electrophoresis has been studied. In order to understand the influence of the surface microstructures on the microchip electrophoresis performance, a thermostatic system was designed and fabricated for precisely control of the chip temperature since the surrounding temperature causes a significant effect on the separation efficiency. The periodically ordered microstructures (dams) of different sizes were fabricated on the microchannels by a simple printing process using different gray levels of the vector graphic software. And the influence of dam sizes in microchannels on the separation efficiency was evaluated by using the number of theoretical plates (N) and the relative resolution (Rs'). Analyzing the variations of mixing elements caused by the periodically ordered dams in the separate channel and elution time, we found that with small dam size fabricated at low gray level of ca. 30%, the resolution of dopamine and epinephrine increased slightly, while it decreased obviously with larger dam sizes especially fabricated at gray level of 50%.

The direct electrochemistry of cytochrome c (cyt c) on a gold electrode modified with 3-mercaptopropylphosphonic acid [HS-(CH(2))(3)-PO(3)H(2), MPPA] self-assembled monolayers (SAMs) was for the first time investigated. Electrochemical measurements and surface-enhanced infrared absorption spectroscopic reveal that the adsorption kinetics of cyt c on the MPPA-SAMs is very fast (saturation adsorption is completed within 5s) and the immobilized cyt c molecules retain their native secondary protein structure. The nature of interaction between cyt c and -PO(3)H(2) groups is mainly the electrostatic interaction. The direct electrochemistry of the immobilized cyt c on the -PO(3)H(2) terminated SAMs with short chain is nearly reversible. Its formal potential (E(0)'=18+/-3 mV vs. SCE) is very close to that of cyt c in an aqueous solution (E(0)'=18-22 mV vs. SCE). In addition, the electron transfer rate of cyt c immobilized on -PO(3)H(2) terminated SAMs is relatively slow as compared to -SO(3)H and -COOH terminated SAMs, indicating excess negative charge density on the SAMs surface will decrease the electron transfer rate of cyt c.

The electrochemical behavior of isorhamnetin (ISO) at a glassy carbon electrode was studied in a phosphate buffer solution (PBS) of pH 4.0 by cyclic voltammetry (CV) and differential pulse voltammetric method (DPV). A well-defined redox wave of ISO involving one electrons and one proton appeared. The electrode reaction is a reactant weak adsorption-controlled process with a charge transfer coefficient (alpha) of 0.586. Based on the understanding of the electrochemical process of ISO at the glassy carbon electrode, analysis of ISO can be realized. Under optimal conditions, the oxidation peak current showed linear dependence on the concentration of ISO in the range of 1.0x10(-8) to 4.0x10(-7)M and 1.0x10(-6) to 1.0x10(-5)M. The detection limit is 5.0x10(-9)M. This method has been successfully applied to the detection of ISO in tablets.

Separation and determination of dopamine and epinephrine with end-channel electrochemical (EC) detection integrated on a native printed microchip capillary electrophoresis (CE) system was investigated. Factors influencing the separation and detection were investigated and optimized. Results show manipulating EOF, which can be easily achieved by adjusting buffer pH, is a simple and effective way to achieve the baseline separation of dopamine and epinephrine in native polymeric microchips. Without surface modification of microchannel, printed microchips with advantages of low cost and easy preparation can achieve high performance like other microfluidic devices.

Generally speaking, measurement of hydroxylated radical products of salicylic acid requires a fussy separation process. In this study, we describe a simple method to electrochemically detect hydroxyl radicals (*OH) using 4-hydroxybenzoic acid (4-HBA) as the *OH trap. The *OH is generated by the Fenton reaction from iron (II) sulfate and hydrogen peroxide in a phosphate buffer solution. Experimental results show that our method can detect the OH with high sensitivity without any separation process. The differential pulse voltammetric responses show a linear dependence on the concentration of *OH in a range of 2.0x10(-6) and 1.0x10(-3)M with a determination limit down to 5.0x10(-7)M. As a demonstration, the kinetics of the Fenton reaction was mapped by measuring the reaction product of hydroxyl radical trapped by 4-HBA. The result is in good agreement with that reported previously. All the results show that the present approach could provide a simple, inexpensive and promising method for biomedicine and iatrology.

A novel assay for the voltammetric detection of 18-bases DNA sequences relating to Chronic Myelogenous Leukemia (CML, Type b3a2) using methylene blue (MB) as the hybridization indicator was reported. DNA was covalently attached onto a glassy carbon electrode (GCE) through amines of the DNA bases using N-hydroxysulfosuccinimide (NHS) and N-(3-dimethylamion)propyl-N'-ethyl carbodiimidehydrochloride (EDC). The covalently immobilized single-stranded DNA (ssDNA) could selectively hybridize with its complementary DNA (cDNA) in solution to form double-stranded DNA (dsDNA) on the surface. A significant increase of the peak current for methylene blue upon the hybridization of immobilized ssDNA with cDNA in the solution was observed. This peak current change was used to monitor the recognition of CML DNA sequence. This electrochemical approach is sequence specific as indicated by the control experiments in which no peak current change was observed if a non-complementary DNA sequence was used. Factors, such as DNA target concentration and hybridization conditions determining the sensitivity of the electrochemical assay were investigated. Under optimal conditions, this sensor has a good calibration range between 1.25x10(-7) and 6.75x10(-7)M, with CML DNA sequence detection limit of 5.9x10(-8)M.

An amperometric biosensor for sensitive and selective detection of glucose has been constructed by using highly dispersed Pt nanoparticles supported on carbon nanotubes (Pt-MWCNTs) as sensing interface. The Pt-MWCNTs were synthesized by using the two-step pyrolysis method. This composite shows good electrocatalytic activity towards the oxidation of glucose in alkaline and thus can be used to selectively detect glucose. We found that detection potential and Nafion amount covered on the Pt-MWCNTs modified glassy carbon electrode had considerable influence on the selectivity for amperometric detection of glucose. Under optimal detection conditions (detection potential of 0.0V versus SCE and 10muL 1.5% Nafion), selective detection of glucose in the glucose concentration range of 1.0-26.5mM (correlation coefficient, >0.999) can be performed. The results demonstrate that the Pt-MWCNTs composite is promising for the fabrication of nonenzymatic glucose sensors.

Rapid separation and determination of acetaminophen and its hydrolysate with end-channel electrochemical (EC) detection integrated on a plastified poly(ethylene terephthalate) (PET)-toner microchip capillary electrophoresis (CE) system was investigated. In this separation and detection system, a Pt ultramicroelectrode integrated on a three-dimensional adjustor was used as working electrode. Factors influencing the separation and detection were investigated and optimized. Results show that acetaminophen and p-aminophenol can be well separated within 84s with R.S.D.<1% for migration time and R.S.D.<3.6% for detection current for both analytes. Detection limits for both analytes are determined to be 5.0muM (S/N=3). This method has been successfully applied to the detection of trace p-aminophenol in paracetamol tablets. The results demonstrate that the PET-toner microchips can obtain better performance than PDMS microfluidic devices but at much lower cost.