Light addressable gold electrodes
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,...
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|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.