The shell thickness was observed to be linearly dependent on the number of moles of TEOS. The number of moles of NH 4 OH similarly impacted the size. Additionally, an interaction between the number of moles of NH 4 OH and the number of fractionated additions was identified. It is important to note here, that the fractionated addition of TEOS alone did not have a significant effect (p-value = 0.9263), however it is included due to its presence in the interaction term. A larger mean shell thickness was achieved for higher number of moles and more additions, while the inverse produced smaller particles. These relationships are summarised in Equation 2 and the contour maps presented in Fig. . Overall, it is seen that increasing each of the terms increases the size of particles: increasing the number of moles of TEOS means more material present to form silica shells; increasing the number of moles of NH 4 OH increases the presence of aqueous domains, resulting in larger micelles as the number of moles increases; and increasing fractionated additions favours the growth of the silica shell around the SPION core. These observations correlate with literature descriptions of silica shell growth, however this study illustrates and quantifies the relationship of the interaction between NH 4 OH and fractionated addition of TEOS for the first time. By combining the number of moles of each parameter and alternating the fractionated additions of TEOS, exceptional control over shell thickness can be achieved through careful manipulation of experimental conditions.