Direct binding procedure for host 5: The salt form of 5 was dissolved in H 2 O to give a 0.1 mM solution, to which was added 1 v% of 1 M H 2 SO 4 , resulting in a pH 2 solution. The metal oxide substrate was then immersed in this solution for 1 h, after which it was placed in pure H 2 O (15 ml, 5 min) and MeOH (10 ml, 5 min). The samples were then dried under a stream of N 2 .