One of the primary factors that induce Alzheimer's disease (AD) is the deposition of beta-amyloid (Aβ). The Aβ molecules can self-assemble to form neurotoxic aggregates with various morphologies, such as dimers, oligomers, protofibrils and fibrils. For this aspect, we demonstrated that the amyloid fibrillation can be inhibited by quenching the nucleation and elongation processes with a low concentration of water dispersed N-acetyl-l-cysteine capped quantum dots (NAC-QDs). Based on the concentration dependence of NAC-QDs on the seeded fibril growth, there is a remarkable inhibition effect when the NAC-QDs concentration is increased by 100-fold from 10-9 to 10-7 m. The NAC-QDs concentration required to show inhibition effect is much lower than that of the amyloid peptide concentration (50 μm). The step-like change suggests that the inhibition effect of NAC-QDs displays a threshold response. The inhibition is likely due to the intermolecular attractive interactions such as the hydrogen bonding between NAC-QDs and amyloid fibrils resulting in the blockage of the active elongation sites on the fibrils. © 2009 Elsevier Ltd. All rights reserved.