Quartz-crystal microbalance (QCM)

AFFINIX Q4 was used as a 27 MHz QCM instrument (Initium Co. Ltd, Tokyo, Japan: http://www.initium2000.com) having four 500 μL cells equipped with a QCM plate (8.7 mm diameter quartz plate; 4.9 mm² gold electrode) at the bottom of each cell, a stirring bar and a temperature control system. The instrument was calibrated with a change frequency of 1 Hz in response to a mass change of 0.62 ng/cm² on the electrode (i.e., corresponding to 1 Hz frequency decrease per 30 pg mass increase).

The αAC nanofiber was immobilized on the sensor chip using an immobilization kit coupling for AFFINIX (Initium Co. Ltd., Tokyo, Japan). The amino group of the peptide nanofiber was covalently linked to the carboxylic acid terminus of self-assembled monolayers prepared on the sensor chip by (3-dimethylaminopropyl)-3-ethylcarbodiimide (EDC)-N-hydroxysuccinimide (NHS) activation. The amount of immobilized neutral αAC nanofiber on the sensor chip was 155 ng/cm². The stock solution Aβ (1–42) was diluted into buffer (50 mM phosphate pH 7.5, 100 mM NaCl) in the reaction vessel at 37°C with stirring. Binding of Aβ (1–42) to the αAC nanofiber was monitored by measuring the frequency change (ΔF, Hz) obtained according to the manufacture's protocol using the Sauerbrey equation:

where F₀ is the fundamental frequency of the QCM (27 x 10⁶ Hz), Δm is the mass change (g), A is the electrode area (4.9 mm²), ρ_q is the density of quartz (2.65 g cm^-3), and μ _q is the shear modulus of quartz (2.95 x 10¹¹ dyn cm^-2).