Dual-Mechanism Enhanced Miniature Optical Fiber SPR Bio-Probe Based on Nanodiamonds

Optical-fiber-based surface plasmon resonance (SPR) biosensors, featuring label-free, high integration, and small size, have aroused great interest in recent years. However, the reported sensors always presented with a length of several millimeters even longer, and faced a huge task in sensitivity improvement. In this study, a dual-mechanism enhanced miniature optical fiber SPR bio-probe based on nanodiamonds (NDs) is proposed and demonstrated. The probe is composed of a multimode fiber terminated by a short section of no-core fiber, whose surface is deposited with a gold film. Then, carboxylated NDs are exploited to improve the probe sensitivity simultaneously from two aspects, namely plasmonic and biological aspects. On the one hand, the plasmonic sensing field is enhanced by chemically-modifying NDs on the gold surface, which generates 46.5% improvement in bulk refractive index sensitivity. On the other hand, NDs are chemically-bonded with second antibodies to further improve the biological signal using sandwich method. As a result, the sensitivity of probe to the immunodetection of biological protein (mouse immunoglobulin G in this work) is significantly improved by more than an order of magnitude. Besides, test results prove a good specificity and the capability of working in serum environment for the probe. Our work first demonstrates the NDs application as a gain medium in the second signal amplification of SPR biosensing. Meanwhile, the obtained miniature and high sensitivity bio-probe is very suitable for the case requiring a tiny probe size or sample volume.