The problem of functionalizing and coating nanoparticles with surfactants dispersed in a colloid is a prevalent case in nanoscience and related studies. It is known that surfactants could easily cause a shift in the absorption peak in metallic nanoparticles (NPs). Here, a precise theoretical model is presented to simulate the ultraviolet-visible (UV-vis) absorption spectrum of a colloid containing gold nanoparticles (Au NPs) in the presence of different surfactants. Based on the Lorentz–Drude model, this model is able to justify the fact that surfactants with a higher refractive index lead to movement of the absorption peak toward longer wavelengths (red shift). Also, relative concentrations of agents in a solvent can be analyzed using this model. The presented descriptive model illustrates gold-based biosensors with a physical point of view that leads to an increase in their efficiency. Several experimental cases are considered and are examined to calculate and compare the refractive index of the surfactants. In accordance with the results, it is found that this model is compatible with a wide range of molecular sizes, and here, the model is applied for a typical size range of micromolecules such as citrate ions to macromolecules such as polyethylene glycol (PEG) as a polyether. The suggested method revealed that it is appropriate for different surfactants with various chemical structures and refractive indexes. Utilization of approximations in this theoretical model is limited, thus, a method with the least deviation from real measurements has been introduced. The applicability of this model can be extended to practical purposes, including optical bio-sensors and detectors of organic and biological moieties such as viruses and antibodies.