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In this article, we demonstrate a method for inducing reversible crystal-to-crystal transitions in binary mixtures of soft colloidal particles. Through a controlled decrease of salinity and increasingly dominating electrostatic interactions, a single sample is shown to reversibly organize into entropic crystals, electrostatic attraction-dominated crystals, or aggregated gels, which we quantify using microscopy and image analysis. We furthermore analyze crystalline structures with bond order analysis to discern between two crystal phases. We observe the different phases using a sample holder geometry that allows both in situ salinity control and imaging through confocal laser scanning microscopy and apply a synthesis method producing particles with high resolvability in microscopy with control over particle size. The particle softness provides for an enhanced crystallization speed, while altering the re-entrant melting behavior as compared to hard sphere systems. This work thus provides several tools for use in the reproducible manufacture and analysis of binary colloidal crystals.

Colloid-probe-AFM has been widely used to measure the forces of interaction between two surfaces such as particles of different size and functionality or a flat surface and a particle in aqueous environments. However, the applicability of colloid-probe AFM to detect different surface chemistries on chemical or protein patterned surfaces has not been demonstrated. We show that the technique can be used to distinguish the regions of different surface chemistries or biomolecules located within a patterned surface in buffer solution. Our previously developed method, based on evaporation induced self-assembly, was used to generate binary colloidal crystal (bCC) layers of different chemical and/or protein patterns. Particles of different size and functionality, with or without adsorbed protein(s), were used to decorate solid supports with the bCCs. The results from the force measurements on the patterned surfaces revealed that the probe experiences both strong or weak repulsive interactions and attractive interactions depending on the net surface charge present on the particles within the bCC patterns. Measurements on bCC patterns where one particle is coated with a single protein demonstrate that the technique can also be used to probe surface diffusion of adsorbed proteins. Therefore, we are able to detect whether a protein remains adsorbed or diffuses to a region on the bCC containing no proteins. Furthermore, the applicability of this technique was extended to detect the presence of two different biomolecules (i.e., lysozyme and bovine serum albumin adsorbed on both the large and small particles) within the protein patterned bCC surface by their different interaction forces. The study demonstrates that colloid-probe AFM can be used to discriminate between the surface properties of binary colloid crystal patterned surfaces where the binary patterns are comprised of particles of either different surface chemistry or particles with different adsorbed proteins. © The Royal Society of Chemistry 2017. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Material from this article can be used in other publications provided that the correct acknowledgement is given with the reproduced material and it is not used for commercial purposes.