Method for Determining Average Iron Content of Ferritin by Measuring its Optical Dispersion

We report a method where the refractive index increments of an iron storage protein, ferritin, and apoferritin (ferritin minus iron) were measured over the wavelength range of 450-678 nm to determine the average iron content of the protein. The protein used in this study had ∼3375 iron atoms per molecule. The measurement of optical dispersion over the broad wavelength range was enabled by the use of mesoporous leaky waveguides (LWs) made of chitosan. We present a facile approach for fabricating mesoporous chitosan waveguides for improving the measurement sensitivity of macromolecules such as ferritin. Mesoporous materials allow macromolecules to diffuse into the waveguide, maximizing their interaction with the optical mode and thus increasing sensitivity by a factor of ∼9 in comparison to nonporous waveguides. The sensitivity was further improved and selectivity toward ferritin was achieved by the incorporation of antibodies in the waveguide. The method presented in this work is a significant advance over the state of the art method, the enzyme linked immunosorbent assay (ELISA) used in clinics, because it allows determining the average content of ferritin in a single step. The average iron content of ferritin is an important marker for conditions such as injury, inflammation, and infection. Thus, the approach presented here of measuring optical dispersion to determine the average iron content of ferritin has a significant potential to improve the point of care analysis of the protein for disease diagnosis and screening.