Photocurrent Measurements at Quantum Dot Electrodes for Bioanalytical Applications

Quantum dots (QDs) allow the generation of electron-hole pairs upon illumination. When these particles are fixed to an electrode a photocurrent can be generated. This allows their use as a light-switchable layer on the surface. The QDs can not only exchange electrons with the electrode, but can also show reactions with donor or acceptor compounds in solution. This provides access to the construction of signal chains starting from an analyte molecule to be detected.

The direction and magnitude of the photocurrent depend on several factors such as electrode polarization, solution pH and composition. These defined dependencies have been evaluated with respect to the combination of QD-electrodes with enzyme reactions for sensorial purpose [1,2].

Thus, it has been found that the cofactor of many dehydrogenase-based reactions NADH can be oxidized at the illuminated QDs without applying high potentials. This provides the basis for a coupling strategy with the enzyme glucose dehydrogenase for the analysis of glucose concentrations in solution by photocurrent measurements.

CdSe/ZnS-QD-modified electrodes can also be used to follow enzymatic reactions in solution based on their capability to reduce molecular oxygen while being excited [3]. In order to develop a photoelectrochemical biosensor, glucose oxidase (GOD) is immobilized on the QD-electrode. One immobilization strategy applies the layer-by-layer-technique of GOD and a polyelectrolyte (polyallylamine hydrochloride). Photocurrent measurements of such a sensor show a clear concentration dependent behavior for millimolar glucose concentrations. Sensitivity can be influenced by the enzyme concentration and the number of layers deposited. The principle of combining QD electrodes with a layered protein architecture and light-triggered read-out can also be transferred to other enzymes such sarcosine oxidase and sarcosine detection. The sensing properties of quantum dot electrodes can be significantly influenced by additional nanoparticles giving access to the detection of other substances, but also by immobilizing multiple layers of the QDs.

In another direction of research it can be demonstrated that direct electron transfer from excited quantum dots can be achieved with the redox protein cytochrome c provided the QD surface is properly modified. This allows the detection of the protein in different concentrations and redox states, but also of interaction partners which show a defined reaction with the protein. Examples are the cyt c reduction by superoxide radicals resulting in an anodic photocurrent and the cyt c oxidation by nitrate reductase in the presence of nitrate resulting in a cathodic photocurrent .

[1] Ch. Stoll, S. Kudera, W.J. Parak, F. Lisdat, SMALL 2 (6) (2006) 741-743).

[2] K. Schubert, W. Khalid, Z. Yue, W. Parak, F. Lisdat, Langmuir 26 (2) (2010) 1395-1400

[3] J. Tanne, D. Schäfer, W. Khalid, W. J. Parak, F. Lisdat, Analytical Chemistry 83 (20) (2011) 7778–7785