Mechanically robust cuprorivaite coated supramarbles

Abstract

Liquid marbles (LMs) have received an increasing amount of attention in sensing applications in various fields including biomedical and water quality. Generally, the interior fluid is the sensing element, but in the present work the coating is desired as the sensing element. A 2-D nanoparticulate pigment, cuprorivaite (CV, Egyptian Blue), has found application in sensing recently due to its intense near-infrared (NIR) emission at ∼900 nm. There is little in terms of a systematic report of the conditions for engineering a uniform coating of 2-D CV particles on liquid phase surfaces, and we believe that such a study is essential to understanding how to utilize this emerging photoprobe in sensing. LMs may be an excellent multiscale platform to prepare uniform coatings when the particles have an irregular nanoscale shape (which is common for 2-D nanoparticles commercially available). That irregular shape makes manufacture of a uniform coating on a flat surface problematic, one which LMs are proposed to address. Conformance through van der Waals layering can lead to better control of particle layering. To overcome the typical LM breakage issue we also introduced a durable, non-sensing core. CV was exfoliated and then surface modified with trichlorovinylsilane to become hydrophobic. A drop of POLYCRYLIC topcoat or Gorilla Epoxy Glue was placed into the hydrophobic CV powder and rolled to engineer the coating around the marble. SEM micrographs revealed large (1–250 μm) crystallite features, which did not produce a uniform fluorescent signal initially. The hydrophobic CV powder was then sieved at 212 μm, 75 μm, 45 μm, 38 μm and ∼1.5 μm diameters, and supramarbles (SMs) were prepared with these powders. Supramarbles are suggested as having the sensing element assembled in the coating and solid cores. Here we report the reaction conditions, fabrication method, SEM micrographs, and NIR fluorescent emission of such SMs aimed at sensing applications, as well as a discussion on how surface modification and particle size influenced the CV's fluorescent signal. Fluorescence spectroscopy data revealed uniform CV coatings on the marbles. No trend was found for particle size dependence of the emission wavelength. The fluorescent intensities appeared more uniform in the cases of marbles coated with 75 μm, 45 μm, and ∼1.5 μm CV powders, though the root mean squares of the fluorescent intensities were lower. The present work offers fundamental insights into understanding how to implement CV as a sensing coating with respect to surface modification and particle size.