Supramolecular Assembly of Poly(propyleneimine) Dendrimers Driven By Simple Monovalent Counterions

Abstract

The self-assembly of semiglobular, positively charged poly(propyleneimine) (PPI) dendrimers with small monovalent counterions (e.g., Cl(-)) in water/acetone mixtures was investigated. We showed that PPI dendrimers can assemble into hollow, spherical, single-layered blackberry-type structures mediated by the presence of monovalent counterions. The effects on the assembly of changing the solvent polarity and adjusting the pH were further investigated to confirm the presence of electrostatic interactions and hydrogen bonding as the driving forces. Results showed that PPI dendrimers form stable, hollow spheres in 5-20% v/v acetone/water and that the size of the spheres decreases monotonically as the solvent polarity and/or the charge on the dendrimers (i.e., lower solution pH) increases. This is the first example to show that small monovalent counterions can trigger attraction among PPI dendrimers (or broadly defined polyelectrolytes) that is strong enough to bring them together to form large, stable supramolecular assemblies, which indicates that these organic macroions have similar solution behavior to more-well-defined inorganic molecular macroions.

Elucidating the Origin of the Attractive Force among Hydrophilic Macroions

“… ( a ) Examples of different kinds of macroions that are able to form blackberries, including inorganic metal-oxide molecular clusters (1, 2) 13 , metal-organic nanocages (3) 32 , functionalized fullerenes (4) 33 , cyclodextrins (5) 34 , dendrimers (6) 35 . ( b ) A typical blackberry structure self-assembled from metal-oxide molecular clusters (a2), which is a highly ordered monolayer hollow sphere.…”

Tuning of Polyoxopalladate Macroanionic Hydration Shell via Countercation Interaction

“…Tremendous efforts have been made to understand the ion-pairs and hydrations hells in solution, [2] especially in biological systems, [3] because they are critical for ion transport, biochemical hydrolysis and protein stability.H owever,u nderstanding these biomacromolecules in vitro is difficult, because they are fragile and unstable once extracted from the biological environment. [4] Studying structurally well-defined, simple macromolecules with identical size and tunable charge density would be an alternative way.P olyoxometalates [5] (POMs) are discrete, anionic polyatomic ions with sizes in the nanometer regime and excellent modelsw ith similars olution behavior as biomacromolecules in the wayt hey interact with small countercations in solution.P OMs, alongw ith many othert ypes of macroions, such as metal-organic nanocages, [6] polyhedralo ligomeric silsesquioxane( POSS), [7] functionalized C 60 [8] and dendrimers, [9] can self-assemble into single-layered, hollow,s pherical "blackberry"-type structures by counterion-mediated attraction, [10] with a mechanism similar to virus capsid formation, demonstrating impressive self-recognition capabilityd ue to delicate longrange electrostatic interaction, [11] etc. Such hydrophilic macroions possess well-definedh ydration shells,w hich couldb e studiedt or evealt he interaction between hydration shells and counterions and the type of ion-pairs.…”

Unique Symmetry-Breaking Phenomenon during the Self-assembly of Macroions Elucidated by Simulation

“…Recent studies indicate that the like-charged macroions can strongly attract with each other when carrying moderate amount of charges, leading to the reversible formation of thermodynamically stable, hollow, spherical, and monolayered “blackberry”-type structures in polar solvents 4 – 7 . Vairous macroions (1-6-nm size) are found to do so, such as inorganic metal-oxide molecular clusters 8 – 13 , polyhedral oligomeric silsesquioxane 14 , functionalized fullerenes 15 , dendrimers 16 , 17 , metal-organic nanocages 18 – 21 , bio-macromolecules and small nanoparticles 22 , 23 (Fig. 1 ).…”

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Figure 1 Coarse-graining of various macroions that form blackberry structures. ( a ) Examples of different kinds of macroions, including inorganic metal-oxide molecular clusters (1, 2) 8 – 13 , metal-organic nanocages (3) 18 – 21 , functionalized fullerenes (4) 15 , cyclodextrins (5) 14 and dendrimers (6) 17 . ( b ) A typical blackberry structure self-assembled from metal-oxide molecular clusters (a1), which is a monolayer hollow sphere.

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