Diameter-optimized high-order waveguide nanorods for fluorescence enhancement applied in ultrasensitive bioassays.

Development of fluorescence enhancement (FE) platforms based on ZnO nanorods (NRs) has sparked considerable interest, thanks to their well-demonstrated potential in chemical and biological detection. Among the multiple factors determining the FE performance, high-order waveguide modes are specifically promising in boosting the sensitivity and realizing selective detection. However, quantitative experimental studies on the influence of the NR diameter, substrate, and surrounding medium, on the waveguide-based FE properties remain lacking. In this work, we have designed and fabricated a FE platform based on patterned and well-defined arrays of vertical, hexagonal prism ZnO NRs with six distinct diameters. Both direct experimental evidence and theoretical simulations demonstrate that high-order waveguide modes play a crucial role in FE, and are strongly dependent on the NR diameter, substrate, and surrounding medium. Using the optimized FE platform, a significant limit of detection (LOD) of 10-16 mol L-1 for Rhodamine-6G probe detection is achieved. Especially, a LOD as low as 10-14 g mL-1 is demonstrated for a prototype biomarker of carcinoembryonic antigen, which is improved by one order compared with the best LOD ever reported using fluorescence-based detection. This work provides an efficient path to design waveguiding NRs-based biochips for ultrasensitive and highly-selective biosensing.