Evidence for the controlled cycling of Nd 3+ ions between the bulk and surface of the aqueous solution is shown in Fig. . Figures and show the temporal evolution of the surface density of Nd during three consecutive cycles of X-ray exposure. Each cycle consists of two steps: X-rays-on and X-rays-off. X-rays-on refers to continuous exposure of the sample to X-rays. During this exposure the element-specific surface density was measured by synchrotron X-ray fluorescence near total reflection (XFNTR, see Section S1 and Fig. in the SI). The X-ray fluorescence was generated by Xrays incident on the surface at an angle that is slightly above the angle for total reflection. These Xrays illuminate the surface region and transmit into the bulk aqueous phase with a penetration depth (normal to the surface) of 2.8 µm. XFNTR measurements last 5 seconds each and occur continuously during the X-rays-on exposure. X-rays-off refers to a greatly reduced exposure of the sample for which a 5-second XFNTR measurement occurs once every 5 minutes but without X-ray exposure between measurements. These experimental results were obtained from samples of a composition and pH for which DTPA is expected to chelate nearly 100% of the Nd 3+ ions in solution in the absence of other molecules, such as the DHDP monolayer, or effects such as X-ray exposure (Fig. ). Figures and show that the surface density of Nd increases during the X-rays-on step and decreases during the X-rays-off step of each cycle.