Self-Recognition between Two Almost Identical Macroions during Their Assembly: The Effects of pH and Temperature

Abstract

Two Keplerate-type macroions, [Mo VI 72 Fe III 30 O 252 - (CH 3 COO) 12 {Mo 2 O 7 (H 2 O)} 2 {H 2 Mo 2 O 8 (H 2 O)}(H 2 O) 91 ]·ca.150H 2 O= {Mo 72 Fe 30 } and [{Na(H 2 O) 12 }⊂{Mo VI 72 Cr III 30 O 252 (CH 3 COO) 19 - (H 2 O) 94 }]·ca.120H 2 O={Mo 72 Cr 30 }, with identical size and shape but different charge density, can self-assemble into spherical "blackberry"-like structures in aqueous solution by means of electrostatic interactions. These two macroanions can self-recognize each other and self-assemble into two separate types of homogeneous blackberries in their mixed dilute aqueous solution, in which they carry -7 and -5 net charges, respectively. Either adjusting the solution pH or raising temperature is expected to make the self-recognition more difficult, by making the charge densities of the two clusters closer, or by decreasing the activation energy barrier for the blackberry formation, respectively. Amazingly, the self-recognition behavior remains, as confirmed by dynamic and static light scattering, TEM, and energy dispersive spectroscopy techniques. The results prove that the self-recognition behavior of the macroions due to the long-range electrostatic interaction is universal and can be achieved when only minimum differences exist between two types of macroanions. Universal self-recognition among macroions: Two almost identical POM-based macroions can self-assemble into "blackberry"-like structures and self-recognize each other in solution. The self-recognition behavior remains when the clusters have high (high pH) and low (low pH) charge density on their surface, as well as at elevated temperatures.