Optical fiber biosensor based on localized surface plasmon resonance in gold nanoparticles

We developed a label-free, highly sensitive, real time, and micrometer-sized optical fiber biosensor without using any attenuated total reflection (ATR) optics. The sensor is based on the localized surface plasmon resonance (LPR) in gold nanoparticles. The gold nanoparitlces show a large absorption band at around 520 nm due to LPR, which is quite sensitive to the environment around the nanoparticles. Thus change of the refractive index of the ambient medium around the nanoparticles or overcoating a thin dielectric layer on the particles results in a red-shift of the absorption band and change of the absorption intensity. This paper reports a novel optical fiber biosensor that is constructed at the endface of the optical fiber. The fiber probe is consisting of gold nanoparticles that plays as a transducer, where ligand is coated. The return light intensity from the fiber probe is changed by the adsorption of the biomolecules that has affinity to the ligand. The sensitivity was evaluated to be 10 pg/mm² (2x10^-5 RIU) with an LED light, which is compatible with the conventional surface plasmon resonance biosensors that use the ATR optics.