Abstract
We report an ultra-sensitive and robust fluorescence sensor made by using a biconical taper with a waist diameter of 720 nm for both excitation and fluorescence collection. To enhance the stability of the fluorescence sensor, the biconical taper has been embedded in a 125 µm wide microchannel with a detection length of 2.5 cm. Investigated by measuring the fluorescence intensity of rhodamine 6G (R6G), the sensor shows a detection limit down to 100 pM, with excellent reversibility in a concentration range of 0–10 nM. The sensor has also been applied to quantum dot (QD)-labeled streptavidin measurements, yielding a detection sensitivity down to 10 pM for QDs. In addition, the small sample volume (ca. 500 nL), high sampling throughput, and seamless connection between the biconical taper and standard optical fibers offer a number of attractive advantages for chemical and biosensing applications.
Highlights
Optical microfibers and nanofibers (MNFs) have been emerging as a novel platform for exploring fiber-optic technology on the micro/nanoscale owing to their outstanding properties including low waveguiding losses, tight optical confinement, strong evanescent fields, and widely tailorable waveguideSensors 2015, 15 dispersion [1,2,3,4]
We systematically investigated the sensitivity and reproducibility of the sensor by measuring the fluorescence of rhodamine 6G (R6G), a dye widely used in fluorescence microscopy, flow cytometry, fluorescence correlation spectroscopy and enzyme-linked immunosorbent assay (ELISA)
Because the fluorescence collected by the biconical taper was guided in the fiber spectrometer through a long pass filter, the fluorescence collection efficiency of the sensing system will mainly depend on the coupling efficiency of fluorescence into the guided mode of the biconical taper (~20%), the transmission of the filter (~90%), and the light collection efficiency of the spectrometer (~75%)
Summary
Optical microfibers and nanofibers (MNFs) have been emerging as a novel platform for exploring fiber-optic technology on the micro/nanoscale owing to their outstanding properties including low waveguiding losses, tight optical confinement, strong evanescent fields, and widely tailorable waveguide. Unstretched fiber is completely embedded in the polydimethylsiloxane (PDMS), and only the taper waist used for sensing is surrounded by chemical or biological samples filling the microchannel, this configuration offers excellent stability and reproducibility, and makes effective use of the pronounced evanescent wave of the centimeter-long taper waist, resulting in an ultra-high sensitivity. Investigated by measuring the fluorescence intensity of rhodamine 6G (R6G) and quantum dot (QD)-labeled streptavidin, the sensor shows an ultra-high sensitivity owing to the strong evanescent field outside the taper waist for fluorescence excitation and the high efficiency for fluorescence collection. Compared to silica and polymer nanofiber sensors [14,32], which use an evanescent coupling method for optical waveguiding, biconical tapers can seamlessly connect with light source and detector through standard fibers, yielding a simple, robust, and sensitive sensing structure
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