Core@shell SPION@SiO 2 NPs were prepared using a reverse microemulsion method adapted from the literature. IGEPAL-co-520 (0.5 g, 1 mmol) was dispersed in cyclohexane (11 mL, 101 mmol) and sonicated for 10 min at ambient conditions. SPIONs in cyclohexane (1 mL, 5 mg/mL) were added to the stirred solution followed by the addition of ammonium hydroxide. The controlled addition of TEOS was performed using a fractionated drop-method; where a known volume of TEOS was added every 4 hours. A total of 3 additions were carried out per day and reacted overnight. For 6 and 8 additions, the 4 th (and 7 th ) addition were carried out 24 (and 48) hours after the first addition, and subsequent additions were carried out following the 4 hour interval. A schematic of the synthesis can be seen in Fig. , detailing how the the shell is formed within the reverse micelle. Once complete, SPION@SiO 2 NPs were washed in ethanol using centrifugation three times and redispersed in ethanol. During the study, the number of moles of TEOS and NH 4 OH, and the number of fac-torial drop-wise additions of TEOS were varied according to the design shown in Table , ESI.