Nickerl et al. also showed that electrostatic attraction plays a vital role in the selective adsorption of dyes on an imidazolic linker-based cationic MOF, [Cu(Imid)(H2O)]+[114]. The studied dyes were fluorescein (FR), methyl red (MR), Nile blue (NB), and Nile red (NR); the first two dyes have carboxylic functional groups. NR and NB are structurally very similar, but the latter one is ionic in character. It was observed that [Cu(Imid)(H2O)]+ readily adsorbs NB, but cannot adsorb NR. This selective adsorption is because the anionic part of NB undergoes electrostatic interactions with the cationic [Cu(Imid)(H2O)]+, giving good adsorption, whereas no interaction occurs between non-ionic NR and cationic [Cu(Imid)(H2O)]+. Both MR and FR undergo deprotonation to produce the corresponding anions, and can be adsorbed on [Cu(Imid)(H2O)]+. However, MR and FR cannot be desorbed through exposure to excess solvent, whereas NB can be desorbed (see Fig. 2). This is because the corresponding anion (of MR or FR) is trapped inside the channels by electrostatic interactions or can replace the terminal water molecules coordinated to the copper paddle-wheels, resulting in the formation of a neutral framework with the composition [Cu(Imid)(dye)]. In this case, the dye becomes an integral part of the framework and cannot be desorbed