Enzyme conjugation and biosensing with quantum dots: A photoluminescence study

In this paper we investigate the enzyme conjugation of CdSe/ZnS core/shell quantum dots (QDs) by means of their photoluminescent properties, as well as the influence of the enzymatic reaction on these properties. The QDs were first transferred from toluene to water by encapsulation in poly(ethylene glycol)-phospholipid micelles. The water solubilized QDs were bioconjugated with the enzymes glucose oxidase (GOX) and horseradish peroxidase (HRP). For covalent coupling the crosslinkers 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and hydroxysulfosuccinimide (sulfo-NHS) were used. Photoluminescence (PL) measurements showed that EDC had a large effect on the PL intensity. Also, the PL intensity increased in PBS buffer as compared to water. The enzyme conjugation caused a noticeable shift in the PL peak wavelength, but had a minor effect on the PL intensity. Sensing experiments confirmed the well-known quenching of PL by H₂O₂. Similarly, the PL intensity was sensitive to glucose in the mM range in the presence of GOX. This can be understood since the conversion of glucose by GOX yields H₂O₂ as a by-product. Measurements of the quantum yield (QY) showed increased values for QD solutions treated with EDC. The QY decreased with increasing glucose concentration in the presence of GOX. The change in QY was accompanied by a change in pH, suggesting a correlation between pH and QY.