This paper presents a novel biosensing approach that combines the high sensitivity of whispering gallery modes (WGM) with a metallic nanoparticle-based assay. The authors developed a computational model based on generalized Mie theory to explain the enhanced sensitivity of protein detection through plasmon enhancement. They quantitatively analyzed the binding of bovine serum albumin (BSA) to gold nanoparticles (Au NPs) by measuring frequency shifts in high-Q WGM resonance frequencies. The results were fit to an adsorption isotherm, which agreed with theoretical predictions of a two-component adsorption model. The method demonstrated picomolar sensitivity levels, decoupling the WGM transducer from the NP recognition element. The study also showed that the hybrid photonic-plasmonic modes significantly enhanced the signal-to-noise ratio due to plasmon coupling. The authors concluded that WGM modes can be effectively used to quantitatively probe the amount of BSA protein adsorbed to plasmonic Au NPs, marking the first time such a combination of high-sensitivity WGM biosensing with a NP-based assay has been achieved for optical detection.This paper presents a novel biosensing approach that combines the high sensitivity of whispering gallery modes (WGM) with a metallic nanoparticle-based assay. The authors developed a computational model based on generalized Mie theory to explain the enhanced sensitivity of protein detection through plasmon enhancement. They quantitatively analyzed the binding of bovine serum albumin (BSA) to gold nanoparticles (Au NPs) by measuring frequency shifts in high-Q WGM resonance frequencies. The results were fit to an adsorption isotherm, which agreed with theoretical predictions of a two-component adsorption model. The method demonstrated picomolar sensitivity levels, decoupling the WGM transducer from the NP recognition element. The study also showed that the hybrid photonic-plasmonic modes significantly enhanced the signal-to-noise ratio due to plasmon coupling. The authors concluded that WGM modes can be effectively used to quantitatively probe the amount of BSA protein adsorbed to plasmonic Au NPs, marking the first time such a combination of high-sensitivity WGM biosensing with a NP-based assay has been achieved for optical detection.