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Graded‐Bandgap Quantum‐ Dot‐Modified Nanotubes: A Sensitive Biosensor for Enhanced Detection of DNA Hybridization

Biosensors containing quantum dots (QDs) as fluorescent markers have been employed for immunoassays, gene expression, genomic analysis, and fluorescence imaging because QDs have superior properties to traditional organic fluorophores, such as higher quantum yield and much sharper emission spectra. Recently, the use of QDs as fluorescence resonance energy transfer (FRET) donors to detect DNA, RNA, or proteins was reported, where an amplification of the detected fluorescent signal was achieved by exploiting FRET. One particularly promising strategy in this context involves funneling energy from strongly light-harvesting species towards a reaction center, because energy is always transferred from a species with a larger bandgap to a species with a smaller bandgap. In this way, a cascaded-energy-transfer structure based on QD assembly has been constructed with a high photoluminescence (PL) intensity originating from the center layer. The use of QDs as biomarkers to functionalize nanotubes (NTs) has been also reported, as a new and active field of research. It is known that the preparation of functionalized NTs, for example, inside an ordered porous alumina membrane, can yield hybrid systems containing a large number of aligned and functionalized channels for biological applications. For example, NTs functionalized with QDs have found promising applications as drug carriers, biomarkers, and biosensors. Hence, taking advantage of the superior properties of QDs described above, we describe in this Communication a novel approach to designing functionalized NTs with a cascaded-energy-transfer architecture by incorporating graded-bandgap QDs into ordered porous alumina membranes (Scheme 1) and their use as highly sensitive biosensor component for the detection of DNA hybridization through energy transfer. The incorporation of QDs into the NTs was performed by the well-known approach of layer-by-layer (LBL) deposition. Mercaptoundecanoic acid (MPA) ligand coated ZnCdSe alloy QDs with luminescence maxima at k= 561 nm (QD) (d = 6.6 nm), 594 nm (QD) (d = 6.1 nm), and 614 nm (QD) (d = 5.5 nm) were used. Their PL emission and UV absorption spectra of aqueous suspensions are shown in Figure 1A and Figure S1 (Supporting Information), respectively. Several advantages of ZnCdSe alloy QDs compared with other types of QDs, for example, core/shell QDs, are summarized here: 1) There are very few surface defects around the luminescent particles owing to the high crystallinity. 2) They have large particle sizes, hardened lattice structures, and decreased interdiffusion. 3) ZnCdSe alloy QDs can produce atomically abrupt jumps in the chemical potential, which can further localize free exciton states in the alloy crystallites. Therefore, the fluorescence stability of ZnCdSe alloy QDs can be greatly improved. As a matrix forming the walls of the NTs we used globular N,N-disubstituted hydrazine phosphorus-containing dendrimers of the fourth generation having 96 terminal groups with either cationic [G4(NH Et2Cl )96] (G4 ) or anionic [G4(CHCOONa)96] (G4 ) character. The two dendrimers possess a controlled shape and well-defined external and internal surfaces. The multilayer structures consisting of the two dendritic polyelectrolytes show a lower degree of interpenetration than multilayer systems composed of common linear polyelectrolytes. Therefore, they are ideal components for constructing compartmentalized NT walls, in which different functional species, such as water-soluble QDs with different size or composition in the case of the alloy particles used in this experiment, can be deposited with high spatial precision. For LBL deposition in an ordered porous alumina membrane, the pore walls were first coated with the positively charged 3-aminopropyl dimethylethoxysilane (3-APS), further allowing the deposition of a negatively charged denC O M M U N IC A IO N