*2.1. Synthesis and Chemical Functionalization of Nanostars*

Gold nanostars were prepared based on the procedure described by Hao et al. [38] and further optimized by Van de Broek et al. [39]. In brief, 2 mg bis(psulfonatophenyl) phenylphosphine dihydrate dipotassium (BSPP; Strem Chemicals, Newburyport, MA, USA) and 100 μL H2O2 (30% *v*/*v*, Air Products, Vilvoorde, Belgium) were added to 50 mL of a 6.8 <sup>×</sup> 10−<sup>3</sup> M aqueous sodium citrate solution (Acros Organics, Geel, Belgium). In a next step, 100 μL of 0.075 M HAuCl4 (Acros Organics) was added slowly under constant stirring at room temperature. By using an Atlas Syringe Pump (Syrris, Ruisbroek, Belgium) a slower addition rate of 12.5 μL/min was used in comparison to previously published articles in order to achieve the desired shape and size [36]. The 50 mL AuNP suspension was centrifuged at 4500 rpm for 1 h and the pellet was re-suspended in 10 mL of water. The star-shape of the AuNPs was stabilized using a disulfide molecule, according to Lin et al. [40]. Hereby, 1 mL of an 1.2 mM disulfide (S-(CH2)11-(O-CH2-CH2)6-O-CH2-CO-NH-(CH2)2-maleimide)2 (Prochimia, Sopot, Poland) solution was added to 10 mL of the AuNP suspension mixed with 100 μL 0.5 M NaOH (Merck, Overijse, Belgium). After 90 min of shaking, the mixture was centrifuged at 4000 rpm for 60 min and re-suspended in water resulting in an optical density of ~1 at their maximum plasmon band. These nanostars were characterized in water and cell culture medium using UV-Vis absorption spectroscopy (Shimadzu UV-1601PC, Brussels, Belgium), dynamic light scattering (DLS; Malvern Nanosizer, WR, United Kingdom) and transmission electron microscopy (TEM; Tecnai F30, FEI company, Eindhoven, The Netherlands). A terminal maleimide group was chosen for future functionalization with anti/nanobodies and targeting of specific cell types as described [22].
