magnetite.41 Lattice parameters were varied with background, peak shape and zero error parameters refined. Dynamic light scattering (DLS) and zeta potential (ZP). light scattering (DLS) and zeta potential (ZP). Measurements were carried out on a Malvern Zetasizer Nano ZSP instrument. ZP measurements were carried out in 0.01 M NaCl solution. Thermogravimetric analysis (TGA). analysis (TGA). Measurements were carried out on a TA min-1. Instruments SDT Q600 TGA machine using a constant air flow of 100 mL min-1 Samples were heated up to 120 °C at a heating rate of 20 °C and kept at 120 °C for 20 minutes to remove any water, then ramped up to 1000 °C at a heating rate of 10 min-1. °C Ultraviolet-visible spectroscopy (UV-Vis). spectroscopy (UV-Vis). Colloidal stability measurements were carried out on a Thermo Scientific 2000c nanodrop UV-Vis spectrophotometer in a cuvette with a path length of 1 cm at the relevant temperature. Magnetic measurements. measurements. Magnetic isotherm data were collected at 300 K in DC scan mode (scan length 30 mm, scan time 4 seconds, 2 scans per measurement) using a MPMS3 SQUID (superconducting quantum interface device) magnetometer (Quantum Design). Freeze-dried samples (0.5-1.5 mg) were packed in a size 4 gelatin capsule and fixed in a plastic drinking straw. Freeze dryer. dryer. Samples for TGA and SQUID measurements were freeze dried using a Labconco freezone 4.5 freeze-dryer with a condenser temperature of –50 °C and a shelf temperature of 20 °C.

## Materials

Fluorescein isothiocyanate diethylaminoethyl (FITC-DEAE)-dextran (Mw 40,000 Da), Iron (III) chloride hexahydrate FeCl 6(H O) (ACS reagent, 97%), Iron (II) chloride 3. 2 tetrahydrate FeCl 4(H O) (reagent plus®, 98%), ammonium hydroxide solution (28- 2. 2 30 %), Corning® Spin-X® (100k), sodium azide, Sephadex® G-100 beads were all purchased from Sigma Aldrich. Spectra/Pore Biotech cellulose ester (CE) dialysis membranes (MWCO: 100,000 Da) and Millex GP syringe filters with a polyethersulfone (PES) 0.22 µm membranes were purchased from Fisher Scientific. Deionised water from a Milli-Q system (resistivity 15 MΩcm at 25 °C) was used for all experiments.

Methods SPION synthesis and purification. Three samples were prepared using the generic SPION synthesis and purification. Three samples were prepared using the generic method below which employed different amounts of FITC-DEAE dextran polymer whilst all other reaction parameters were kept constant. For the three samples, 0.2, 0.05 or 0.0125 g of FITC-DEAE polymer (Mw 40,000 Da) in 10 mL water was added 10-4 to 0.03 g (1.1 x mol) ferric chloride hexahydrate (FeCl .6H O) and 0.015 g (7.5 x 3 2 10-5 mol) ferrous chloride tetrahydrate (FeCl .4H O) in 15 mL water in a two necked 2 2 flask fitted to a non-magnetic stirrer through an airtight connection. The flask was purged with nitrogen for 30 minutes on ice and, whilst stirring at 200 rpm, 1 mL ammonium hydroxide (28-30 %) was added dropwise over a period of 120 seconds. The mixture was then heated to 80°C over a period of 15 minutes and held at 80°C for 1 hour. After cooling to room temperature, the reaction mixture was dialysed repeatedly using 100 kDa membranes until the particle solution reached pH 7 for 48 hours. The particles were then passed through G-100 Sephadex® beads and spin- concentrated 3 times using 15 mL water containing 0.01 wt % sodium azide. When spin-concentrating, 1 mL of solution was left in the filter compartment between each cycle to avoid drying up the SPIONs. All particles were passed through a sterile 0.22 µm polyethersulfone syringe filter prior to cell labelling.