TOWARD A BETTER CLEANING AND SELECTIVITY IN ELECTROANALYSIS: TIO2-BASED SENSORS AND ENANTIOSELECTIVE MATERIALS
In the latest years the concern on environmental issues has grown dramatically. One of the major problems of environmental chemistry are emerging contaminants (ECs), which are not commonly monitored in the environment. They can cause undesirable effects on ecosystems and human health. This group of substances includes pharmaceutical and personal care products (PPCPs), endocrine disrupting chemicals, hormones, pesticides, plasticizers, flame retardants and so on. Some of these emerging contaminants have been added to the list of priority substances and corresponding individual environmental quality standards have been set. These substances include pharmaceuticals (macrolide antibiotics, estrogenic hormones, carbamazepine, diclofenac, ibuprofen), industrial chemicals (bisphenol A), metals (silver) and pesticides (triclosan, nicosulfuron, glyphosate, neonicotinoids, pyrethroids). Modern analytical techniques can easily detect emerging contaminants in the environment, but they are expensive and time consuming. In this context, electrochemical sensors could overcome these problems since they are easy to use, they can be portable, disposable and they can be adapted to every necessity.Thanks to their properties and characteristics, such as high active surface, increased surface/volume ratio, increased selectivity, nanomaterials are broadly used in a wide range of applications, like energy, catalysis and electroanalysis. One of the most valued characteristics for modern sensors are self-cleaning properties, i.e. the possibility to use the sensor more than once and to detect analytes in complex matrices, remotely without the need to clean and restore them for long time. Starting from the aforementioned points, the aim of this PhD thesis is the study of different materials to enhance the selectivity and the cleaning properties of electrochemical sensors. In particular, titanium dioxide was studied for its photocatalytic activity to give photo-renewable properties to electrochemical sensors using different supports (fluorine-doped tin oxide glass electrodes and gold disk microelectrodes) in the detection of different emerging contaminants. Its properties were also studied for the photoelectrochemical detection of ciprofloxacin, a fluoroquinolone antibiotic. Titanium dioxide was coupled with gold nanoparticles (AuNPs) to enhance its signal and with mesoporous silica to increase the selectivity of the devices. At first, the focus was on the study of the aging time of the titanium dioxide sol and how it influences the electrochemical and photoelectrochemical performances on the final structures. The starting device, an FTO glass, was covered with titanium dioxide and it was electrochemically characterised by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), atomic force microscopy (AFM) and transient photocurrent analysis. Once the study of the aging time of the sol was over, AuNPs were added to the device to form a heterojunction. This device was used for the detection of different emerging contaminants and the photocatalytic properties of TiO2 were used to photo-renew the surface of the sensor. Then this device was used for the photoelectrochemical detection of ciprofloxacin with different light sources. In particular, a low-power light source was used for this purpose, an UV LED. The determination of ciprofloxacin was also possible at low applied potential (+0.1 V vs SCE) and with good detection limits and good sensitivity. The analysis of possible interferences showed that the device was sensible to the presence of humic acids at high concentrations. For this reason, vertically aligned mesoporous silica was added to the device, in order to use its porosity to load gold nanoparticles and to use the charge- and size-exclusion properties of the material. The device was electrochemically characterised with CV, EIS, transmission electron microscopy (TEM) and transient photocurrent analysis. Then ciprofloxacin was detected with this device, obtaining good analytical parameters. Moreover, possible interferents were analysed and, in this case, even high concentrations of humic acids did not interfere with the determination of ciprofloxacin. The photo-renewable properties of titanium dioxide were also studied using a gold disk microelectrode in the presence of dopamine, a molecule known for its soiling problems. The titanium dioxide layer was deposited onto the microelectrode surface through a pseudo-calcination process and its behaviour was studied after the use of the sensor for the detection of dopamine. It has been seen that it is possible to restore the initial properties of the electrode even after its soiling. In last part of this PhD thesis, the enantioselective properties of oligo-BT2T4 enantiopure films and the electrocatalytic properties of multi-walled carbon nanotubes (MWCNTs) were studied together on the same device at the same time. At first the electrochemical properties of the materials were studied using a non-chiral probe in CV and EIS. Then the enantioselectivity of the oligo-BT2T4 enantiopure films was tested using a chiral probe in cyclic voltammetry. The surface of the device was also analysed using scanning electron microscopy (SEM). The presence of MWCNTs allows the detection of the chiral probe at lower potentials than would have happened only with the enantiopure film, thus having a catalytic effect. In conclusion, several materials are here investigated starting from their physico-chemical and electrochemical characterizations. Their application as electroanalytical sensors is explored, evidencing the presence of synergistic effects in the hybrid systems with respect to the single components, assuring enhanced properties and superior performances.
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