Carbon Based Enzymeless Hydrogen Peroxide Sensor and Further Applications
Suazo Dávila, Dámaris
Cabrera, Carlos R. (Consejero)
Hydrogen peroxide is a very well known molecule and because of that it is extremely attractive in many fields, such as pharmaceutical, textile, sensors, medicine and cosmetics. The research presented in this dissertation focuses on the development of two different carbon-based sensors for hydrogen peroxide: vertically aligned carbon nanofibers (VACNF) and reduced graphene oxide modified with silver (r-GO/Ag). In addition, both sensors were used for further applications; VACNFs were used for covalent immobilization of cholesterol oxidase and graphene oxide for electrochemical preparation of graphene quantum dots (GQDs). Chapter 2 shows the experimental details for the preparation of the VACNFNE and GO, as well as the steps for the modification of graphene oxide with silver and the electrochemical experiment to produce GQDs. Chapter 3 shows the characterization of the VACNFNEs. The VACNFNEs are presented as Surface-I and Surface-II for the hydrogen peroxide sensor and for the immobilization of the cholesterol oxidase, respectively. The VACNF have heights on the order of 2-3 microns, and diameters that vary from 50 to 100 nm. The VACNFs were grown as individual, vertical, freestanding structures using plasma-enhanced chemical vapor deposition. The VACNFNEs were used for two purposes: as hydrogen sensor and for cholesterol oxidase immobilization. Therefore, they are presented as Surface-I and Surface-II, respectively. Chapter 4 shows Surface-I and Surface-II as the hydrogen peroxide sensor and the immobilization matrix for cholesterol oxidase, respectively. The VACNFNE enzymeless H2O2 sensor showed stability and reproducibility in five consecutive calibration curves with different hydrogen peroxide concentrations over a period of 3 days. The detection limit was 66 μM. The sensitivity for hydrogen peroxide electrochemical detection was 0.0906 mA cm-2 mM-1. The sensor was also used for the measurement of hydrogen peroxide as the by-product of the reaction of cholesterol with cholesterol oxidase as a biosensor application. The sensor exhibits linear behavior in the range of 50 μM to 1 mM cholesterol concentrations. For cholesterol oxidase immobilization characterization, cyclic voltammograms of the VACNFNE prior to ChOx modification produced quasi-reversible redox peaks characteristic of the Fe(CN)6redox couple, with a decrease in current after the modification with ChOx. X-ray photoelectron spectroscopy was used to study ChOx immobilization. An enzyme apparent activity study was done to ensure the presence and activity of the enzyme after immobilization. Chapter 5 shows the characterization, modification and application of rGOX/Ag composite for a hydrogen peroxide sensor. Different techniques were used to characterize the electrochemically prepared rGO/Ag such as FT-IR spectroscopy, UV–vis absorption spectroscopy, Raman spectroscopy, XRD, XPS, ICP and SEM. A 55% concentration of Ag nanoparticles was estimated in r- GO/Ag from ICP experiment and dendritic structures were observed on the SEM. The amperometric performance of the r-GO/Ag toward H2O2 reduction in N2- purged solution was studied, I (mA)= -37.63 mAmM-1 x -0.2042 (where x is the concentration of H2O2 in mM) with a detection limit of 3.2 μM based on 3σ/slope. The selectivity of the sensor toward H2O2 was studied with an amperometric response of H2O2 with the addition of different common interferences such as ascorbic acid, glucose, and urea. No response was measured when these interferences were added in solution. Chapter 6 shows the preparation of the GQDs using graphene oxide as the precursor. A slurry solution of previously prepared 10 mg graphene oxide in 6 mL 0.1 M PBS pH 7.5 was used and a cyclic potential between +3.0 V and -3.0 V vs. Ag/AgCl was applied at 400 rpm and scan rate of 500 mV/s for a total of 2,200 potential cycles. HRTEM of the nanomaterial presented particles of 3.4 nm and the measured fluorescence emission in water evidenced the presence of graphene quantum dots.