Dokument

Summary:
The main objective carried out in this dissertation was to fabricate Light Amplified Potentiometric sensors (LAPS) based upon the semiconductor nanoparticles (quantum dots) instead of its bulk form. Quantum dots (QDs) were opted for this device fabrication because of their superior fluorescent, electric and catalytic properties. Also in comparison to their bulk counterparts they will make device small, light weighted and power consumption is much lower. QDs were immobilized on a Au substrate via 1,4 benzene dithiol (BDT) molecule. Initially a self-assembled monolayer (SAM) of BDT was established on Au substrate. Because of SAM, the conductivity of Au substrate decreased dramatically. Furthermore QDs were anchored with the help of BDT molecule on Au substrate. When QDs immobilized on Au substrate (QD/Au) via BDT molecule were irradiated with UV-visible light, electron-hole pairs were generated in QDs. The surface defect states in QDs trapped the excited electrons and long lived electron-hole pairs were formed. By the application of an appropriate bias potential on Au substrate the electrons could be supplied or extracted from the QDs via tunneling through BDT. Thus a cathodic or anodic current could be observed depending upon bias potential under illumination. However without light illumination the QD/Au electrode remained an insulator. To improve the device different modifications were made, including different substrates (Au evaporated on glass, Au evaporated on mica sheets and Au sputtered on SiO2/Si) and different dithiol molecules (capped and uncapped biphenyl 4,4 dithiol and capped and uncapped 4,4 dimercaptostilbenes) were tried. Also different QD immobilization techniques (normal incubation, spin coating, layer by layer assembly (LbL) of polyelectrolytes and heat immobilization) were employed. This device was able to detect electrochemically different analytes depending upon the QDs incorporated. For example CdS QDs were able to detect 4-Aminophenol, a product of an enzymatic reaction of Alkaline Phosphatase with p-Aminophenyl Phosphate. Subsequently this reaction was observed at CdS/Au electrode, by enzyme-substrate reaction within the electrolyte solution, and also by immobilizing the enzyme on top of QDs via LbL assembly of polyelectrolytes. With another kind of CdS-FePt dimer QDs, detection of hydrogen peroxide (H2O2) was demonstrated. Only at CdS/Au electrode there was no impact made by H2O2 but with the presence of Pt within QDs H2O2 was detected via reduction even at a bias potential of -100mV.

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