2.1. Materials

All chemicals were purchased from Sigma Aldrich (Milan, Italy). Caffeine in anhydrous powder form ReagentPlus Grade and PCL preformed in flakes with an average molecular weight of 14,000 g/mol were used. Acetone with purity ≥ 99.5% in compliance with European Pharmacopedia standards and acetonitrile of an HPLC grade ≥ 99.9% were employed as solvents. Phosphate buffer solution was prepared with the following chemicals: 8 mg/mL sodium chloride anhydrous ≥ 99%, 0.2 mg/mL potassium chloride ACS grade ≥ 99.5%, 1.44 mg/mL sodium phosphate dibasic dehydrate ≥ 99%, and 0.24 mg/mL potassium dihydrogen phosphate ACS reagent ≥ 99%. Ultrapure water was produced by means of a Milli-Q RG system by Millipore R (Billerica, MA, USA).