Fig. 1

Characterization and pH-dependent colloidal stability of bPAuNC. a Colloidal size of bPAuNC repeatedly measured by DLS. b AFM images of bPAuNC (left) and the magnified microscopic image (right). Scale bar is 50 nm. c Photographs of bPAuNC containing various concentrations (3.91, 7.81, 15.63, 31.25, 62.5, 125, 250 mg mL⁻¹) of albumin. d Fluorescence emission spectra of bPAuNC depending on albumin concentration (1X = 125 mg albumin mL⁻¹). e Linear relationship between the normalized fluorescence intensity of bPAuNC (F/F0) and the concentration of albumin. f Photographs of bPAuNC solution (125 mg albumin mL⁻¹) depicting colloidal stability agglomeration state of bPAuNC under different pH values by addition of HCl (0.1 M) or NaOH (0.1 M) to the solution. The pH ranges from 4.041 to 12.064. g Fluorescence decay of the bPAuNC affected by the pH value of the reaction solution. h The separated supernatant or re-suspended bPAuNC solution according to its change in solubility at pH 5 as analyzed by fluorescence spectra. Inset of h: Photographs of the separated supernatant (left) and resuspended bPAuNC solution (right) at pH 5