Confocal microscopy demonstrates that Nterminal poly-histidine tagged rGFP (rGFP H ) strongly partitions within crystals previously washed with a NiCl 2 solution. Control experiments show that samples lacking either NTA functionalization, NiCl 2 or protein exhibit no fluorescence signal under identical imaging conditions. Additionally, no fluorescence signal was detected when performing the experiment using a protein lacking the His-tag (rGFP), fully demonstrating the specificity of the partitioning. Fig. shows that protein entrapment can be controllably reversed by incubating the crystals loaded with rGFP H with the chelating agent ethylenediaminetetraacetic acid (EDTA), which strips the nickel from the NTA. The protein can again be captured upon removal of EDTA and addition of new NiCl 2 . Diffraction patterns obtained from non-treated NTA C-star frameworks (Fig. ) and from those treated with Ni and loaded with rGFP H (Fig. ) show the same BCC symmetry and identical lattice parameter as the inert C-star design with the same arm length, demonstrating that neither including the NTA moiety nor trapping the protein affect the microstructure of the frame-works.