*2.2. Chemicals*

Polystyrene (PS) NPs were synthesised in IPREM, Institut des Sciences Analytiques et de Physico-Chimie pour l'Environnement et les Matériaux, Pau, France, avoiding any additives, especially surfactants, bactericides (e.g., sodium azide), and the trace metals usually present in commercial standards [55,56]. The synthesis and characteristics of soapfree polystyrene models have been detailed elsewhere, e.g., in [55]. Spherical PS NPs were used (diameter of 420 ± 20 nm determined by scanning electron microscopy) with a surface functionalized by carboxylic groups, a low polydispersity (PDI of 0.009), and the zeta potential of −46 mV at a pH value of 7.

Enrofloxacin powder (purity ≥ 98%) was acquired from Sigma-Aldrich (St. Louis, MO, USA). The stock solution was prepared daily to get 100 μg ml−<sup>1</sup> by dissolving the weighted amount in milliQ water in an ultrasonic bath. The stock solution of enrofloxacin was prepared without using organic solvents or buffer solutions to avoid changing the water parameters.
