Hydroxylated graphene quantum dots (GQDs) (1 mg/mL, purity >80%) were purchased from ACS Material. Lyophilized human beta-amyloid (1–42) (Aβ42 monomer, DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVGGVVIA; HPLC purity≥95%48 (link)) was purchased from AnaSpec. Oligomer A11 polyclonal antibody was obtained from Invitrogen. Donkey anti-rabbit Alex 594 secondary antibody and phalloidin-iFluor 488 were purchased from Abcam. OxiSelect intracellular ROS detection kit was obtained from Cell Biolabs. All sample solutions were prepared in Milli-Q water and the solvents used were of analytical grade.