Haiying Gu

Quantum Dot (QD)-Modified Carbon Tape Electrodes for Reproducible Electrochemiluminescence (ECL) Emission on a Paper-Based Platform

Stable and sensitive electrochemiluminescence (ECL) detection relies on successful immobilization of quantum dots (QDs) on working electrodes. Herein, we report a new technique to apply double-sided carbon adhesive tape as the working electrode to improve the stability and reproducibility of QD-based ECL emission. CdS QD-modified electrodes were prepared by dropping and drying CdS QD suspension on the carbon adhesive tape supported by indium tin oxide (ITO) glass. The ECL detection was performed with the prepared electrode on a paper-based platform. We tested our system using H(2)O(2) of various concentrations and demonstrated that consistent ECL emission could be obtained. We attribute stable and reproducible ECL emission to the robust attachment of CdS QDs on the carbon adhesive tape. The proposed method could be used to quantify the concentration of dopamine from 1 μM to 10 mM based on the quenching effect of dopamine on ECL emission of CdS QD system using H(2)O(2) as the coreactant. Our approach addressed the problem in the integration of stable QD-based ECL detection with portable paper-based analytical devices. The similar design offers great potential for low-cost electrochemical and ECL analytical instruments.

Nanostructured biosensors built with layer-by-layer electrostatic assembly of hemoglobin and Fe3O4@Pt nanoparticles

This paper reports layer-by-layer (LBL) films fabricated with hemoglobin and core-shell nanoparticles with Fe(3)O(4) as the core covered by Pt (Fe(3)O(4)@Pt) and their applications in biosensing. The characterization of {Hb/Fe(3)O(4)@Pt}(n) LBL films at different layers revealed that the formation of films is step-by-step and uniform. Meanwhile, at glassy carbon electrodes modified with {Hb/Fe(3)O(4)@Pt}(n) film at different layers there was a pair of well-defined and nearly reversible peaks in cyclic voltammetry (CV). CV results indicated that the electroactivity of the structure with four bilayers was the best. The {Hb/Fe(3)O(4)@Pt}(4) film modified electrode could be used to detect H(2)O(2) and nitrite with the linear range from 0.125 μM to 0.16 mM for H(2)O(2) and 1.5 μM to 0.12 mM for nitrite as well as the detection limits of 0.03 μM for H(2)O(2) and 0.29 μM for nitrite (S/N=3). The biosensors also exhibited good reproducibility, high selectivity, and long-term stability. Our investigation showed that the strategy taking advantages of Fe(3)O(4)@Pt and LBL assembly is ideal for direct electrochemistry of redox proteins as well as the sensitive and stable mediator-free biosensors.

Paper-based electroanalytical devices for in situ determination of salicylic acid in living tomato leaves

Detection of phytohormones in situ has gained significant attention due to their critical roles in regulating developmental processes and signaling for defenses in plants at low concentration. As one type of plant hormones, salicylic acid has recently been found to be one of pivotal signal molecules for physiological behaviors of plants. Here we report the application of paper-based electroanalytical devices for sensitively in situ detection of salicylic acid in tomato leaves with the sample volume of several microliters. Specifically, disposable working electrodes were fabricated by coating carbon tape with the mixture of multiwall carbon nanotubes and nafion. We observed that the treatment of the modified carbon tape electrodes with oxygen plasma could significantly improve electrochemical responses of salicylic acid. The tomato leaves had a punched hole of 1.5mm diameter to release salicylic acid with minor influence on continuous growth of tomatoes. By incorporating the tomato leaf with the paper-based analytical device, we were able to perform in situ determination of salicylic acid based on its electrocatalytic oxidation. Our experimental results demonstrated that the amounts of salicylic acid differed statistically in normal, phytoene desaturase (PDS) gene silent and diseased (infected by Botrytis cinerea) tomato leaves. By quantifying salicylic acid at the level of several nanograms in situ, the simple paper-based electroanalytical devices could potentially facilitate the study of defense mechanism of plants under biotic and abiotic stresses. This study might also provide a sensitive method with spatiotemporal resolution for mapping of chemicals released from living organisms.

Using nanostructured conductive carbon tape modified with bismuth as the disposable working electrode for stripping analysis in paper-based analytical devices

Low cost disposable working electrodes are specifically desired for practical applications of electrochemical detection considering maturity of electrochemical stations and data collection protocols. In this paper double-sided conductive adhesive carbon tape with nanostructure was applied to fabricate disposable working electrodes. Being supported by indium tin oxide glass, the prepared carbon tape electrodes were coated with bismuth film for stripping analysis of heavy metal ions. By integrating the bismuth modified electrodes with paper-based analytical devices, we were able to differentiate Zn, Cd and Pb ions with the sample volume of around 15 μL. After the optimization of parameters, including modification of bismuth film and the area of the electrodes, etc., Pb ions could be measured in the linear range from 10 to 500 μg/L with the detection limit of 2 μg/L. Our experimental results revealed that the disposable modified electrodes could be used to quantify migrated lead from toys with the results agreed well with that using atomic absorption spectrometry. Although bismuth modification and stripping analysis could be influenced by the low conductivity of the carbon tape, the low cost disposable carbon tape electrodes take the advantages of large-scaled produced double-sided carbon tape, including its reproducible nanostructure and scaled-up fabrication process. In addition, the preparation of disposable electrodes avoids time-consuming pretreatment and experienced operation. This study implied that the carbon tape might be an alternative candidate for practical applications of electrochemical detection.

Layer by layer assembled films between hemoglobin and multiwall carbon nanotubes for pH-switchable biosensing.

Although pH-switchable behaviors have been reported based on multilayer films modified electrodes, their pH-switchable biosensing is still difficult due to the existence of the electroactive mediator. In this study, we report the pH-dependable determination of hydrogen peroxide (H2O2) based on a four-bilayer film fabricated through layer by layer assembly between hemoglobin (Hb) and multiwall carbon nanotubes (MWCNTs). We observed that response of electroactive probe Fe(CN)6(3-) at the multilayer films was very sensitive and reversible to pH values of phosphate buffer solutions phosphate buffer solution with cyclic voltammetry. The reduction peak height of Fe(CN)6(3-) at the multilayer film could reach ∼221μA at pH 3.0 while 0μA at pH 9.0. The linear range for the detection of H2O2 at pH 3.0 was from 12.5μM to 10.4mM, which was much wider than that at pH 9.0. Our results demonstrated that the detection of H2O2 with the proposed modified electrode is dependent on pH values of phosphate buffer solution. Moreover, the component of multilayer films has impacts on the performance of biosensors with pH-switchable behaviors.

Simultaneous Electrochemical Determination of Indole-3-acetic Acid and Salicylic Acid in Pea Roots using a Multiwalled Carbon Nanotube Modified Electrode

Indole-3-acetic acid and salicylic acid are essential phytohormones with profound effects on growth and development as well as stress responses of plants. Accumulating evidence has suggested that these compounds mediate the same biological process by collaboration or antagonistic actions. Simultaneous determination of indole-3-acetic acid and salicylic acid may significantly improve the understanding on mechanisms of their interaction. In this study, the simultaneous determination of these analytes was electrochemically performed using a multiwalled carbon nanotubes-chitosan modified glassy carbon electrode. Based on differential pulse voltammetry, indole-3-acetic acid and salicylic acid were determined from 0.67 to 48.82 micromolar with detection limits of 0.1 micromolar. In addition, no interferences were observed from other molecules. This method was employed for the simultaneous determination of the analytes in pea root extracts. The approach may be extended for additional study of the interaction of indole-3-acetic acid and salicylic acid in plants.

Paper-based cell impedance sensor and its application for cytotoxic evaluation.

Disposable analytical devices for developing sensitive and label-free monitoring of cancerous cells would be attractive for cancer research. Here, paper-based electroanalytical devices based on impedance spectrometry were applied for the study of K562 cells and the toxic effect of anticancer drugs. The proposed device integrating gold nanorods modified ITO electrodes could provide a biocompatible surface for immobilization of living cells maintaining their bioactivity. The impedance results exhibited good correlation to the logarithmic value of cell numbers ranging from 7.5 × 10(2) to 3.9 × 10(6) cells mL(-1) with a detection limit of 500 cells mL(-1). Furthermore, this strategy was used to evaluate the cytotoxic effects of arsenic trioxide and cyclophosphamide. Results obtained by the impedimetric method correlate well with the conventional cell viability assay. Cells exposed to drugs exhibited a prominent reduction of impedance data, showing an inverse dose-dependent relationship. This simple, cost-effective and portable paper-based electrochemical analytical device could provide a new impedance platform for applications in monitoring cell behavior, pharmacological studies and toxicological analyses.

Layer-by-layer assembly of hemoglobin-coated microspheres for enhancing the oxygen carrying capacity

Hemoglobin-coated microspheres with one layer and five layers were fabricated by layer-by-layer assembly. The electrochemical methods were used to study their capacity for carrying oxygen. The results provided direct evidences that microspheres with five-layer hemoglobin could store and release more oxygen than those with one layer. This study also provided an alternative method for evaluating the oxygen carrying capacity of hemoglobin microspheres.

Paper-based analytical devices for electrochemical study of the breathing process of red blood cells.

Herein we utilized the filter paper to physically trap red blood cells (RBC) to observe the breathing process of red blood cells based on the permeability of the filter paper. By integrating double-sided conductive carbon tape as the working electrodes, the device could be applied to monitor electrochemical responses of RBC for up to hundreds of minutes. The differential pulse voltammetry (DPV) peak currents increased under oxygen while decreased under nitrogen, indicating that RBC could take in and release oxygen. Further studies demonstrated that the RBC suspension could more effectively take in oxygen than the solution of hemoglobin and the supernatant of RBC, suggesting the natural advantage of RBC on oxygen transportation. This study implied that simple paper-based analytical devices might be effectively applied in the study of gas-participating reactions and biochemical detections.

Ultrafast glucose-responsive, high loading capacity erythrocyte to self-regulate the release of insulin

Insulin (INS) delivery system that can mimic normal insulin secretion to maintain the blood glucose level (BGL) in the normal range is an ideal treatment for diabetes. However, most of the existing closed-loop INS delivery systems respond slowly to the changes in BGL, resulting in a time lag between the abnormal BGL and the release of INS, which is not suitable for practical application. In this study, glucose oxidase (GOx)-modified erythrocytes are used as INS carriers (GOx-INS-ER) that can rapidly self-regulate the release of INS upon the changes in BGL. In this system, glucose can be broken down into gluconic acid and hydrogen peroxide by GOx-INS-ER, and the latter will rupture the erythrocyte membrane to release INS within minutes. A pulsatile release of INS can be achieved upon the changes in the glucose concentration. This GOx-INS-ER enables diabetic rats to overcome hyperglycemia within 1 h, and a single injection of this GOx-INS-ER into the STZ-induced diabetic rats can maintain the BGL in the normal range up to 9 days. STATEMENT OF SIGNIFICANCE Diabetes mellitus has been a major public health threatener with global prevalence. Although, glucose-responsive carriers that can release insulin (INS) in a closed loop have been explored greatly in recent years, their sluggish glucose-responsive property and low INS-loading content greatly restrict their practical application [ACS Nano, 2013, 7, 4194]. In this work, we reported INS-loaded erythrocytes featuring ultrafast glucose-responsive property and high INS loading content, which could release INS in a closed loop. These GOX-INS-ERs could respond to the changes in glucose level within several minutes and self-regulate the release of INS for a long time. Single injection of GOX-INS-ER can overcome hyperglycemia in diabetic mice within 1 h and maintain the baseline level of glucose up to 9 days. We think our method may provide a robust way to potentiate diabetes treatment.