This work presents the design and implementation of a reusable-tapered optical fiber sensor (RFS). We used the RFS together with a white light source to analyze the metastability of gold nanoparticles. With the RFS we observed that reaction limited aggregation occurred when modifying the concentration of metastable gold nanoparticles (MAuNPs). This was detected by a red-shift and a broadening of the absorption band. The transition from a metastable to a stable state, regardless of the concentration of the MAuNPs, was achieved by the addition of glucose. The changes in the refractive index of the solution due to the presence of glucose and the stability of the MAuNPs were successfully detected by the RFS. Diffusion limited aggregation of the MAuNPs was rapidly triggered after the addition of insulin. Finally, to gain a better understanding of our results we used Raman spectroscopy together with a hollow core photonic crystal optical fiber (HC-PCF) and studied the chemical interactions between the MAuNPs and glucose and between the MAuNPs and insulin.