The integration of lab-on-a-chip technologies with single-molecule detection techniques may enable new applications in analytical chemistry, biotechnology, and medicine. We describe a method based on the reciprocity theorem of electromagnetic theory to determine and optimize the detection efficiency of photons emitted by single quantum emitters through truncated dielectric waveguides of arbitrary shape positioned in their proximity. We demonstrate experimentally that detection of single quantum emitters via such waveguides is possible, confirming the predicted behavior of the detection efficiency. Our findings blaze the trail towards efficient lensless single-emitter detection compatible with large-scale optofluidic integration.

The applicability of single-molecule fluorescence assays in liquids is limited by diffusion to concentrations in the low picomolar range. Here, we demonstrate quantitative single-molecule detection at attomolar concentrations within 1 min by excitation and detection of fluorescence through a single-mode optical fiber in presence of turbulent flow. The combination of high detectability and short measurement times promises applications in ultrasensitive assays, sensors, and point-of-care medical diagnostics.

National Center of Competence for Research in Nanoscale Science (NCCR), Institute of Physics, Klingelbergstr. 82, CH-4056 Basel, Switzerland, Nano-Optics & Biophotonics Group, Physikalisches Institut, Wilhelm-Conrad-Röntgen-Center for Complex Material Systems (RCCM), Experimentelle Physik 5, Am Hubland, 97074 Würzburg, Germany, Institut Jacques Monod, 15 rue Hélène Brion, 75013 Paris, France, Centre for Research on Adaptive Nanostructures and Nanodevices, College Green, Dublin 2, Ireland, Klingelbergstrasse 82, CH-4056 Basel, Switzerland, Klingelbergstrasse 50-70, CH-4056 Basel, Switzerland, and Mittlere Strasse 91, CH-4031 Basel, Switzerland

We describe a method to detect and count transient burstlike signals in the presence of a significant stationary noise. To discriminate a transient signal from the background noise, an optimum threshold is determined using an iterative algorithm that yields the probability distribution of the background noise. Knowledge of the probability distribution of the noise then allows the determination of the number of transient events with a quantifiable error (wrong-positives). We apply the method, which does not rely on the choice of free parameters, to the detection and counting of transient single-molecule fluorescence events in the presence of a strong background noise. The method will be of importance in various ultra sensing applications.

The purpose of our studies is to better understand the morphology and functioning of the arteries and their changes in pathogenesis. The most frequently used imaging techniques are intravascular ultrasound, magnetic resonance imaging, and optical coherence tomography. These methods do not image cell-level structural details and only provide biomechanical properties indirectly. We present a new protocol for imaging the endothelial surface and measuring elastic properties of vascular tissue by scanning force microscopy. Full-thickness sections of native pig coronary arteries were prepared. In addition, cultured human umbilical vein endothelial cells were studied as an in vitro model system and for comparison. We encountered a variety of difficulties mostly due to the softness of vascular tissue which required significant adaptations of standard equipment: (i) a new specimen holder designed to stably immobilize the coronary arteries; (ii) a phase-contrast microscope incorporated for assessing the status of the cultured endothelial cells and positioning the scanning force microscope (SFM) tip at a site of interest; and (iii) a continuous exchange of the culture medium at 37 degrees C to assure viability of the cells in the SFM over extended times. We were thus able to investigate both fresh arterial tissue and living endothelial cells in a near-physiological environment. We present initial SFM images of vascular tissue at a spatial resolution similar to scanning electron microscopy, but which also provide a closer view of the bona fide structure of native tissue. Novel morphological features such as distinct granular particles were observed. Moreover, we report initial measurements of vascular tissue surface stiffness, obtained by indentation-type SFM.