(559e) Micro-RNA Profiling for Oral Cancer Screening By a Nanocone Optical Fiber Array

Abnormal experssion of MicroRNAs has been connected to cancer, diabetes, cardiovascular and other chronic diseases. Screening platforms based on miRNA profiling are hence promising early prescreening tests that can significantly improve the survival rate of the more deadly diseases. However, short life time (< 1 hour) and low experssion level of miRNAs pose chanllenges for accurate microRNA profiling, particularly for future transformative home-use personalized medical devices that cannot rely on expensive instrumentation and extensive manual manipulation. Real-time PCR can quantify low number of miRNAs, but can only detect one target at a time, making it impossible to profile a panel of target miRNAs. DNA microarray can detect thousands of targets but only at higher concentrations and takes tens of hours due to diffusion limitation to surface functionalized probes. Both require reverse transciption with expensive instrumentation. Rapid and sensitive multi-target detection and quantification would eliminate reverse-transcription and allow direct miRNA profiling from the sample.

We report a novel conical optical fiber platform, with low-cost LED light sources and miniature cameras in a turn-key instrument, that can profile a large panel of microRNAs down to fentoMolar within 15 minutes, thus allowing direct profiling. The platform relies on unique transport and optical physics at nanoscale geometric singularities.  Molecular diffusion to a conic tip with a nanoscale diffusion length and a correspondingly large concentration gradient is much faster than to a planar microarray. Focused optical and plasmonic fields at cone tip also amplifies the fluorescence intensity by 3 orders compared to flat SPR. We recently showed theoretically that plasmonic excitation at a metal cone is broadbanded, encompassing the entire visible spectrum rather than the monochromatic plasmonic frequencey of a gold nanoparticle (Wang, Plouraboue and Chang, Optics Exp, 21, 6609(2013)). A generic multi-spectral platform for SERS, FRET, transmission/absorption, second-harmonic CARS, near-field, interference biosensing can hence be designed based on such unique optical phenomena at geometric singularities.  A first prototype with plasmonically enhanced FRET hairpin probes for miRNA profiling is reported here.

Conical array is fabricated by wet etching commercial imaging fiber bundles, with thousands of small fiber cores with optimum spacing for coupling with bulk light. The cone tip has a radius of curvature below 10 nm and can be flattened for near-field applications. A thin gold layer (5 nm) is thermally evaporated onto the array to sustain plasmonics. Hairpin probes with fluorophore tags quenched by the gold film are functionalized to the gold surface. Plasmonically enhanced fluorescent reporting of hybridization occurs when the target molecule breaks the hairpin. Detection of multiple targets in a panel can be achieved by addressing each fiber core optically and using photochemistry to selectively attach different probes. Detailed spectral measurements show broadbanded plasmonic excitation of 3 fluorophores per cone to minimize interference. Intensity tracking in time show a signature constant rapid absorption flux to a conic tip, followed by a slow t^-1/2 diffusive flux to the substrate. The enhanced tip fluorescence allows detection of the approximately 100 miRNA at the tip in the first region within 15 minutes. Profiling data for miRNA of oral cancer cells from a heterogeneous sample will be shown.