Analytical Nanosphere Sensors Using Quantum Dot−Enzyme Conjugates for Urea and Creatinine

An enzyme-linked analytical nanosphere sensor (ANSor) is described, responding to enzyme−substrate turnover in the vicinity of a quantum dot (QD) due to coimmobilized enzyme and pH sensitive ligand. QD capping by mercapto-alkanoic acids were rejected as a pH sensitive ligand, but with the use of a layer-by-layer assembly on mercaptopropionic capped QDs and an intermediate poly(allylamine hydrochloride) layer, anthraquinone sulfonate (calcium red, CaR) was introduced to modify the pKa in the immobilized system > 8. QD−CaR absorption shows spectral overlap with QD530 emission at all pHs and gives a complex pH dependent fluorescence resonance energy transfer (FRET) efficiency, due to excited state proton transfer (λex = 540 nm; λem = 585 nm). In contrast QD615−CaR with spectral overlap between the QD and CaR gave a strong and reproducible pH response. QD−urease and QD−creatinine deiminase conjugates could be linked with pH changes produced by enzyme degradation of urea and creatinine, respectively. Close cou...