By diluting commercially available riboflavin (Peschke TE 0.25 %; Peschke, Huenenberg, Switzerland) in balanced salt solution (BSS; Serag-Wiessner, Naila, Germany), serial dilutions of 1, 5, 10, 50, 100, 1000, 10,000, 50,000, and 100,000 ng/mL were produced using calibrated research pipettes with disposable tips (Eppendorf, Hamburg, Germany). The solutions were protected from light and used within 2 hours of manufacture.