Differentiation of cell surface-bound and internalized AuNPs by tuning laser fluency (a) ICP-MS measurement of AuNP 1 levels in the cells after wash cycles showing essentially complete removal of surface-bound NPs. Paired sample t-test were performed, n=3; ***, P<0.01; **, P<0.05; n.s., P>0.05. (b) LDI-MS quantification of two AuNPs in cell lysate at different AuNP 2 incubation times. Note that AuNP 1 was first incubated for 60 min and then the cell monolayer was washed five times before incubation with AuNP 2. One-way ANOVA were performed on anount of AuNP 1, n=3, P<0.01, no significant difference between different time points was identified. (c) LDI-MS detection of AuNPs 1 and 2 from the intact cells. All error bars represent the standard deviation.