When evaluating the stability of LDH in RPMI containing 1% FBS, Cu-40 and Ag-35 were found to cause decreased LDH activity over time (Figure 2). Since TiO2-25 does not inactivate LDH, a reasonable explanation for the decreasing part of the curve in Figure 3 is that a significant portion of the LDH activity could not be measured due to adsorption of LDH molecules onto the TiO2-25 particle surface. The similar LDH inactivation pattern shared by Cu-40 NPs and Cu2+ (in form of CuSO4) (Figure 4) at an equimolar concentration seems to suggest that LDH inactivation by Cu NPs was predominantly through a dissolution process that gradually releases copper ions. Though we did not studied the exact form of these ions, Cu+ ions could be the directly released form and they were probably quickly oxidized into Cu2+, the more stable form of copper ions. There are some differences between the curves of the copper NP and Cu2+ at time zero and 1 hr time point and these could have been caused by a time delay for Cu NPs to release copper ions. Note that one-phase exponential decay model used for fitting the curves in Figure 4