Fig. S3.

Nonspecific protein detection on graphene sensors. Response of an ethanolamine (ETA)-modified graphene sensor vs. time during addition of 100 nM prostate-specific antigen (PSA) in pH6 phosphate buffer (PB) (green arrow) and subsequent addition of pure PB (purple arrow). The red, green, and blue traces correspond to PB concentrations of 10, 50, and 100 mM, respectively. The signal amplitudes were 7.5, 1.5, and 0.4 mV for 10 mM (red), 50 mM (green), and 100 mM (blue) PB conditions, respectively. The 50 mM trace is also shown in Fig. 3A as a control. ETA was coupled to PYCOOH-modified graphene surface (see Materials and Methods for detailed procedures) to yield a hydroxyl-terminated graphene surface. The ETA-modified sensors show a much smaller PSA response vs. PB concentration compared with PEG/ETA-modified sensors, consistent with the concept that the permeable PEG layer increases the effective Debye screening length.