Robust silica-coated quantum dot-molecular beacon for highly sensitive DNA detection

We synthesized and characterized small yet highly robust silica-coated quantum dots (QDs) and then used them to develop highly sensitive molecular beacon (MB) for DNA detection. As compared to the previously reported methods, our silica coating approach enabled simple and rapid synthesis of silica-coated QDs in large quantities and high concentrations with a well-controlled silica layer. The QDs such made were stable and had a high quantum yield in a wide range of pH values (1-14) and high salt concentrations (up to 2 M). They were less than 10 nm in diameter, much smaller than current silica-coated QDs, thus allowing for efficient energy transfer. The MB sensor based on these silica-coated QDs was capable of rapidly detecting the target DNA at 0.1 nM concentration within 15 min. It could also differentiate the target DNA from the single base mismatched DNA. The QD-MB developed in this work can be used for highly sensitive and selective detection of DNA and other biomolecules in homogeneous solution and inside a cell, as well as in harsh environment.

Compact quantum dot probes for rapid and sensitive DNA detection using highly efficient fluorescence resonant energy transfer

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.

Versatile opto-fluidic ring resonator lasers with ultra-low threshold

We develop a versatile integrated opto-fluidic ring resonator (OFRR) dye laser that can be operated regardless of the refractive index (RI) of the liquid. The OFRR is a micro-sized glass capillary with a wall thickness of a few micrometers. When the liquid in the core has an RI lower than that of the capillary wall (n=1.45), the capillary circular cross-section forms the ring resonator and supports the whispering gallery modes (WGMs) that interact evanescently with the gain medium in the core. When the core RI is higher than that of the wall, the WGMs exist at the core/wall interface. In both cases, the WGMs can have extremely high Q-factor (>109), providing excellent optical feedback for low-threshold lasing. In this paper, we analyze the OFRR laser for various core RI's and then we demonstrate the R6G laser when the dye is in ethanol (n=1.36), chloroform (n=1.445), and quinoline (n=1.626). The lasing threshold of 25 nJ/mm(2) is achieved, two to three orders of magnitude lower than the previous work in microfluidic lasers. We further show that the laser emission can be efficiently out-coupled via an optical waveguide in touch with the OFRR for both high and low RI liquid core, allowing for easy guiding and delivery of the laser light.

Opto-fluidic ring resonator lasers based on highly efficient resonant energy transfer

We demonstrate an opto-fluidic ring resonator dye laser using highly efficient energy transfer. The active lasing material consists of a donor and acceptor mixture and flows in a fused silica capillary whose circular cross section forms a ring resonator and supports the whispering gallery modes (WGMs) of high Q-factors (>107). The excited states are created in the donor and transferred to the acceptor through the fluorescence resonant energy transfer (FRET), whose emission is coupled into the WGM. Due to the high energy transfer efficiency and high Q-factors, the acceptor exhibits a lasing threshold as low as 0.3 muJ/mm2. We further analyze the energy transfer mechanisms and find that non-radiative Förster transfer is the dominant effect to support the acceptor lasing. FRET lasers using cascade energy transfer and using quantum dots (QDs) as the donor are also presented. Our study will not only lead to development of novel microfluidic lasers with low lasing thresholds and excitation/emission flexibility, but also open an avenue for future laser intra-cavity bio/chemical sensing.

Interaction of fluorescent molecular rotors with blood plasma proteins

Many disease states have associated blood viscosity changes. Molecular rotors, fluorescent molecules with viscosity sensitive quantum yields, have recently been investigated as a new method for biofluid viscosity measurement. Current viscometer measurements are complicated by proteins adhering to surfaces and forming air-surface layers. It is unknown at this time what effects proteins may have on biofluid viscosity measurements using molecular rotors. To answer this question, binding affinities to blood plasma proteins were investigated by equilibrium dialysis for four hydrophilic molecular rotors. Aqueous solutions of 9-[(2-cyano-2-hydroxy-carbonyl)vinyl]julolidine (CCVJ) and three derivatives were prepared and dialyzed against solutions of bovine source albumin, fibrinogen and immunoglobulin G approximating normal physiologic concentrations and fresh-frozen human plasma. After equilibration, dye concentration on each side of the dialysis membrane was assessed by spectrophotometry. The relative binding affinity of the four dyes to the proteins and to the plasma was compared. Affinity of all dyes was highest for albumin. The bound dye fraction showed little change in relation to protein concentration in the physiological concentration range. Diol, the most hydrophilic molecular rotor tested showed the lowest affinity for albumin. This study indicates that hydrophilic molecular rotors are well-suited for biofluid viscosity measurement.