Plasmon enhancement and surface wave quenching for phase ratiometry in coextraction-based fluorosensors

In this study, a novel fluorescence ratiometric sensing scheme is presented. Gold nanoparticles (GNPs) were lipophilized with dodecanethiol and incorporated in the ether phase of a coextraction based ammonium sensor. The novel concept of this sensing scheme is that the analyte response involves a movement of the fluorophore toward lipophilized GNPs in an organic phase. Properly arranged, the system benefits from surface plasmon enhancement as well as increased lossy surface wave quenching as the fluorophore moves toward the GNPs. Likely constructive scattering effects are also present due to some build-up of nanoparticle clusters. The sensor configuration significantly reduced the noise level, increased the ratiometric signal, and improved the limit of detection approximately 3 orders of magnitude as compared with the optical sensor without the nanoparticles and ion-selective ammonium electrodes. Qualitative and quantitative detection limits were 1.7 and 5.7 nM, respectively. The sensing scheme is versatile and can be utilized for many other ions.