| dc.description | 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. | |
