Luminescent and absorptive metal-coated emulsions for micro-velocimetry
Fluorescent latex beads have been widely used as tracers in microfluidics for the last decades. They have the advantages to be density matched with water and to be easily localizable using fluorescence microscopy. We have recently synthesized silver-coated oil droplets that are both luminescent and absorptive, by first coating the oil interface with a polydopamine layer and then depositing a silver layer by a redox process. They have a mean diameter of 6 μm and their density has been matched to the density of water by adjusting the thickness of the metallic layer. In this work we used these particles as tracers to measure the velocity profile of an aqueous solution in a PDMS microchannel with a rectangular cross-section. This allowed us to confirm the predictions of the Stokes equation with results comparable to those of common polystyrene particles.
Photoresponsive Core-Shell Emulsion Droplets
Polydopamine (PDA), a multifunctional biomaterial with strong adhesion and coating properties, exhibits melanin-like opto-electronic properties but is virtually devoid of intrinsic fluorescence. Herein we disclose the first PDA-based system that can develop fluorescence without chemical manipulation.
Polymerization of dopamine in the presence of oil microdroplets resulted in the formation of smooth PDA-coated core-shell droplets, which developed a bright and persistent fluorescence upon UVA illumination. Fluorescence induction depends on excitation wavelength, nature of the oil, oxidation and buffer conditions, suggesting photooxidation of PDA components within the oil matrix which shields developing fluorophores from water quenching. Peroxyl radical scavengers enhance fluorescence whereas hydrogen peroxide induces quenching. The new system holds potential interest for imaging and sensing applications.
We have developped a simple fabrication process of fluorescent silver-coated liquid particles. The synthesis involves the encapsulation of quasi-monodisperse micronic soybean oil-in-water emulsion droplets in a polydopamine shell followed by an electroless silver plating of the surface. Due to the presence of the thin silver layer, the droplets exhibit a broad range fluorescence that shows no photobleaching upon illumination. The method is a proof of principle that multimodal colloidal microparticles can be produced via a high-yield method from sustainable, off-the-shelf products with mild synthesis routes in aqueous conditions.
Reference : G.M. Nocera, K. Ben M’Barek, D.G. Bazzoli, G. Fraux, M. Bontems-Van Heijenoort, J. Chokki, S. Georgeault, Y. Chen, and J. Fattaccioli. Fluorescent microparticles fabrication through chemical coating of O/W emulsion droplets with a thin metallic film. RSC Advances 2014, 4 (23), 11564 – 11568 – [pdf][doi]
This fabrication method led to a patent :
J. Fattaccioli. Méthode de dépôt de de polymère sur des particules liquides. Patent Number : PCT/FR2014/050461 (2013). [url]