Highly sensitive detection of DNA hybridization is demonstrated for the functionalized nanotubes (NTs) described here. With a cascaded-energy-transfer architecture produced by depositing, layer-by-layer, three types of ZnxCd1-xSe alloy quantum dots (QDs)-that is, graded-bandgap QDs-into ordered porous alumina membranes (see figure), the NTs provide the ability to detect small but significant changes in the signal during biological detection.