We developed highly sensitive and specific nanosensors based on quantum dots (QDs) and DNAzyme for multiplexed detection of heavy metal ions in liquid. The QDs were coated with a thin silica layer for increased stability and higher quantum yield while maintaining a relatively small size for highly efficient energy transfer. The QD-DNAzyme nanosensors were constructed by conjugating quencher-labeled DNAzymes onto the surface of carboxyl-silanized QDs. In the presence of metal ions, the emission is restored due to the cleavage of DNAzymes. The detection could be completed within 25 min with a single laser excitation source. The detection limit of 0.2 and 0.5 nM was experimentally achieved for Pb(2+) and Cu(2+), respectively, which is a 50- and 70-fold improvement over the recent results obtained with dye molecules. Multiplexed detection was also demonstrated using two different colors of QDs, showing negligible cross-talk between the Pb(2+) detection and Cu(2+) detection.
Optofluidic dye lasers hold great promise for adaptive photonic devices, compact and wavelength-tunable light sources, and micro total analysis systems. To date, however, nearly all those lasers are directly excited by tuning the pump laser into the gain medium absorption band. Here we demonstrate bioinspired optofluidic dye lasers excited by FRET, in which the donor-acceptor distance, ratio, and spatial configuration can be precisely controlled by DNA scaffolds. The characteristics of the FRET lasers such as spectrum, threshold, and energy conversion efficiency are reported. Through DNA scaffolds, nearly 100% energy transfer can be maintained regardless of the donor and acceptor concentration. As a result, efficient FRET lasing is achieved at an unusually low acceptor concentration of micromolar, over 1,000 times lower than that in conventional optofluidic dye lasers. The lasing threshold is on the order of μJ∕mm 2 . Various DNA scaffold FRET lasers are demonstrated to illustrate vast possibilities in optofluidic laser designs. Our work opens a door to many researches and applications such as intracavity bio/chemical sensing, biocontrolled photonic devices, and biophysics. O ptofluidic lasers are an emerging technology that combines the advantages of compactness and easy liquid manipulation of microfluidics, and dynamic wavelength tunability and broad spectral coverage of dye lasers (1-3). Optical feedback in those optofluidic lasers has been achieved using high-Q ring resonators [e.g., microdroplets (4, 5), microspheres (6), microcylinders (7), microcapillaries (8, 9), and microfiber knots (10)], Fabry-Pérot cavities (11, 12), and distributed feedback gratings (3, 13). In nearly all those lasers, the gain medium is directly excited by tuning the pump laser into the dye absorption band, which requires that the pump laser wavelength match the particular dye absorption. An alternative excitation scheme is through energy transfer, in which dye mixtures, composed of the donor and the acceptor, are used. Donors are directly excited and subsequently transfer energy to acceptors for lasing. The energy transfer significantly extends the laser emission wavelength range without the need to change the pump wavelength. Moreover, dye lasers based on energy transfer have a much higher pump efficiency and lower lasing threshold than the corresponding single dye lasers due to the low donor absorption loss at the acceptor lasing wavelength (14, 15).Generally, there are two transfer mechanisms between the donor and the acceptor in an optical cavity: nonradiative FRET (14-17), in which the transfer is mediated by short-ranged resonant dipole-dipole interaction, and cavity-assisted radiative transfer (18)(19)(20), in which the emission from the donor is first coupled into the cavity, which stores photons for an extended amount of time before they are reabsorbed by the acceptor. The FRET efficiency between a donor and acceptor pair is R 0 6 ∕ðR 0 6 þ r 6 Þ, where R 0 and r are the Förster distance and the donor-acceptor distance, r...
We developed a simple method for quickly synthesizing compact quantum dot (QD)-DNA probes for sensitive DNA detection using fluorescence resonant energy transfer (FRET). The density of DNA probes on the QD surface was controlled to avoid steric hindrance and to promote rapid hybridization with target DNA molecules. The radius of the final QDs was only around 3 nm after applying the functional coating, enabling highly efficient energy transfer. It was demonstrated that nearly 70% transfer efficiency could be achieved with only a few DNA molecules on each QD and that the FRET-based DNA detection could be carried out within 10 min with a sub-nM detection limit. Theoretical analysis was also performed to confirm our results.
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