Calibration Solution for Gold Nanoparticles

2% w/v methylcellulose (25 centipoise, Sigma Aldrich, Dorset, UK) was prepared as described in Griffiths et al.27 (link). Gold particles were prepared by citrate reduction according to Frens44 and stored at 4 °C. To make the calibration solution, freshly made colloid was mixed 1:1 with 2% methylcellulose. To remove citrate and sodium ions, the mix was centrifuged for 2 hours at 8000 × g at 4 °C and sedimented particles resuspended in fresh 1% methylcellulose. The calibration solution was stored at months at 4 °C. Nanodiscs were prepared as described35 (link), and liposomes according to Marius et al.41 (link). Standard plastic support films were made using 1% pioloform (Agar Scientific, Stansted, Essex, UK) and copper hexagonal 100-mesh or square 200 mesh grids applied to the film. Nanorods (10 nm thick) were obtained from Sigma Aldrich (Dorset, UK).