Multiplexed Bioanalysis Based on Light-sensitive Electrodes

Abstract

Introduction Quantum dots (QDs) have become attractive tools in bioanalysis because of their advantageous optical and electrochemical properties. This relates to applications exploiting fluorescence, electrochemiluminescence or photocurrent generation [1]. For the latter case QDs are immobilized on conductive electrodes. By light excitation charge carriers are formed inside the nanoparticles which can be used for photocurrent generation. Signal chains can be constructed when appropriate donor or acceptor components are present in solution. The special feature of such electrodes is that the response is triggered by illumination of the sensing electrode [1]. In this study, a light-addressable photoelectrochemical sensor was built by coupling QDs with oxidases to detect different enzyme substrates. Results and Conclusions At first CdSe/ZnS QDs have been immobilised via a dithiol compound on gold electrodes. Two different enzymes, glucose oxidase (GOD) and sarcosine oxidase (SOD) have been deposited on this QDs-electrode at spatially well resolved places. Here small spots have been achieved by a dropping method. A laser beam is moved over the spots to detect photocurrents. Since oxygen can act as electron acceptor at the illuminated QDs the photocurrent is correlated to the oxygen concentration in solution. By action of the enzymes in the presence of the corresponding substrate a competitive situation for oxygen is created in front of the QD layer and thus, the photocurrent decreases [2, 3]. Since this happens in a concentration dependent way, analysis of the enzyme substrate is feasible.Furthermore, due to the spatially separated immobilisation of the two enzymes, an independent read-out of the two enzyme reactions is possible. The results indicate good lateral resolution and selectivity of the multi-detection system without any crosstalk between the two sensing regions and also without any response at regions, which have not been modified with an enzyme [4].After that, the possibility of an imaging of the enzyme spot by photocurrent measurement has been studied. When the laser is moved over the spot of GOD with a constant speed, the size of the spot can be determined from the photocurrent - time curve (with a defined moving speed). The photocurrent correlates to different local enzyme activities. Consequently, an imaging of the enzyme activity distribution on the surface has also been achieved by this method.