Braun Group People
Asfand Waqar
Former Graduate Student in Materials Science and Engineering
B.S. in Metallurgy and Materials Engineering: GIK Institute of Engineering Sciences and Technology, Pakistan (2007)
Assembly of Colloidal Crystals Using Click Chemistry
In last decade or so, self assembly of complex systems has gained enormous importance because of their possible use in designing new nano-structured devices. Self assembly is a phenomenon widely observed in biological systems, for up to certain levels, nearly all complex biological systems have a tendency to self assemble. Colloidal crystals represent an ideal case for the study of self assembly in complex systems.
Several surface modifications have been studied to assist the assembly of colloids into well assembled colloidal crystals [1]. Our group has successfully demonstrated use of DNA and DNAzymes to obtain colloidal assembly in two dimensions [2]. DNA hybridization was used for sequence specific reversible self-assembly of well-ordered 2-D arrays. Two essential indicators of DNA-hybridization mediated assembly were also confirmed: thermal reversibility, as the arrays melted at 50o C, and sequence specificity, with less than 1% nonspecific binding confirmed using fluorescent polystyrene colloids.
Fig. 1. Confocal micrographs of 25µm pitch arrays of 100nm colloids (a) imaged in buffer and (b) after drying for 15 minutes in air. [2]
Since the introduction of the click chemistry by K. Barry Sharpless et. al. in 2001 [3], this idea borrowed from nature has been widely cited in various fields, from organic synthesis, medicinal chemistry, to polymer and materials sciences. Click chemistry’s applications in bioconjugate and peptide chemistry are growing exponentially with over 1000 papers published to date [4, 5]. Success of click chemistry has led to its rapid application in many other fields, so we are investigating its possible use in assisted colloidal assembly.
From the library of click chemistry, I have selected Cu-Catalyzed Azide-Alkyne Cycloaddition because of its high specificity, high yield and reactivity in aqueous conditions. The idea is to functionalize the surface of the substrate with either Azide or Alkyne and then use particles functionalized with the alternative functionality to click with it to form a monolayer, followed by formation of another layer by click chemistry using particles functionalized with the same functionality as the substrate. Use of this same process and Cu-catalyzed Azide-Alkyne Cycloaddition step by step can further be used to form a colloidal crystal. Below is the general schematic of the current methodology being followed.
Fig. 2. General schematic of current protocol
I am using fluorescent cored 0.5 micron polystyrene particles. I functionalize them with azide and use an alkyne functionalized glass substrate.
Fig. 3. Fluorescent micrograph of azide functionalized particles clicked on alkyne functionalized substrate patterned with 20 microns wide OTS self assembled monolayer.
Currently, my research involves using vertical deposition to form the required monolayer, followed by the click reaction.
[1] E. C. Nelson and P. V. Braun: Enhancing Colloids Through the Surface, Science, 318, 924-925 (2007).
[2] M. Shyr, D. Wernette, P. Wiltzius, Y. Lu, P. V. Braun: DNA and DNAzyme-mediated 2-D Colloidal Assembly, Journal of the American Chemical Society, 130, 8234-8240 (2008).
[3] H. C. Kolb, M. G. Finn, K. B. Sharpless: Click Chemistry: Diverse Chemical Functionality from a Few Good Reactions, Angew. Chem. Int. Ed., 40, 2004-2021 (2001).
[4] Richard A. Evans: The Rise of Azide-Alkyne 1,3-Dipolar ‘Click’ Cycloaddition and its Applications to Polymer Science and Surface Modification.
[5] Erik Van der Eycken, K. B. Sharpless: Editorial, QSAR Comb. Sci., 26, 2007, No. 11-12, 1115.