A quantum dot-electrode system was developed which allows the sensitive detection of NADH (nicotinamide adenine dinucleotide). The colloidal semiconductive CdSe/ZnS nanocrystals (quantum dots) are attached to gold by chemisorption via a dithiol compound. The current signal can be triggered by illumination of the quantum-dot-modified electrode surface. Because of photoexcitation, electron-hole pairs are generated in the quantum dots, which can be detected as anodic or cathodic photocurrent. The immobilization of the nanocrystals is verified by amperometric photocurrent and quartz crystal microbalance (QCM) measurements. This study shows that CdSe/ZnS quantum dot-modified electrodes allow concentration dependent NADH detection in the range of 20 microM to 2 mM already at rather low potentials (around 0 V vs. Ag/AgCl, 1 M KCl). Therefore such electrodes can be used in combination with NADH-producing enzyme reactions for the light-triggered analysis of the respective substrates of the biocatalyst. It can be shown that glucose detection is feasible with such an electrode system and photocurrent measurements.
A light-addressable gold electrode modified with CdS and FePt or with CdS@FePt nanoparticles via an interfacial dithiol linker layer is presented. XPS measurements reveal that trans-stilbenedithiol provides high-quality self-assembled monolayers compared to benzenedithiol and biphenyldithiol, in case they are formed at elevated temperatures. The CdS nanoparticles in good electrical contact with the electrode allow for current generation under illumination and appropriate polarization. FePt nanoparticles serve as catalytic sites for the reduction of hydrogen peroxide to water. Advantageously, both properties can be combined by the use of hybrid nanoparticles fixed on the electrode by means of the optimized stilbenedithiol layer. This allows a light-controlled analysis of different hydrogen peroxide concentrations.
Gold electrodes with switchable conductance are created by coating the gold surface with different colloidal quantum dots. For the quantum dot immobilization, a dithiol compound was used. By polarizing the electrode and applying a light pointer, local photocurrents were generated. The performance of this setup was characterized for a variety of different nanoparticle materials regarding drift and signal-to-noise ratio. We varied the following parameters: quantum dot materials and immobilization protocol. The results indicate that the performance of the sensor strongly depends on how the quantum dots are bound to the gold electrode. The best results were obtained by inclusion of an additional polyelectrolyte film, which had been fabricated using layer-by-layer assembly.
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