Quantum Dot-Fluorescence-Based Biosensing - Quantum Dots for DNA Biosensing - page 32

instrumental design, increasing detection speed, and lowering cost. QD emission
bands can be as narrow as 20 nm in the visible range, enabling distinct signals to
be detected simultaneously with very little cross talk [ 18 ]. In 2001, Nie's group
achieved multicolor optical coding for biological assays by embedding different-
sized QDs into polymeric microbeads at precisely controlled ratios. They designed
a model DNA hybridization system using oligonucleotide probes and triple-color-
encoded beads and the coding signals could identify different DNA sequences [ 19 ].
In 2005, Zhang et al. developed a sandwich type DNA nanosensor based on
single quantum dot. They chose CdSe/ZnS core-shell nanocrystals as donors and
Cy5 as acceptors. As shown in Fig. 3.7 , reporter probe was labeled with Cy5, and
capture probe was modified with biotin to conjugate with streptavidin function-
alized QDs. When a target DNA was present in solution, it was sandwiched by
the two probes. Several sandwiched hybrids were then captured by a single QD
through biotin-streptavidin binding, resulting in a local concentration of targets in
a nanoscale domain. The occurrence of FRET enabled the detection of low con-
centrations of DNA in a separation-free format. The functions of QD are not only
as a FRET energy donor but also a target concentrator to amplify the target signal
Fig. 3.7 Schematic of single QD-based DNA nanosensors. a Conceptual scheme showing
the formation of a nanosensor assembly in the presence of targets. b Fluorescence emission
from Cy5 on illumination on QD caused by FRET between Cy5 acceptors and a QD donor in
a nanosensor assembly. c Experimental setup. Reproduced with permission from Ref. [ 21 ].
Copyright 2005, Macmillan Publishers Ltd