A Microfluidic Platform with Nanoparticle-Based Metal-Enhanced Fluorescence for pH Mapping Acidified Aqueous Solutions b
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A Microfluidic Platform with Nanoparticle-Based Metal-Enhanced Fluorescence for pH Mapping Acidified Aqueous Solutions by CO2 Microbubbles Jérémie Asselin,1,2 Mazeyar Parvinzadeh Gashti,1 Denis Boudreau,1,2 Jesse Greener1* 1
Département de Chimie, Université Laval, Québec (QC), Canada G1V 0A6 Centre d’optique, photonique et laser (COPL), Québec (QC), Canada G1V 0A6 Corresponding email: [email protected]
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ABSTRACT A method for creating pH maps in microchannels containing liquid/liquid and liquid/gas phases comprised of water and CO2 is demonstrated. It is based on a glass slide coated with nanoparticles exhibiting metal-enhanced fluorescence. The time resolution is better than 500 ms, limited by the fluidic control. This work opens the way for new in situ measurements of physical dissolution and chemical reactions in dynamic CO2 gas-liquid systems. INTRODUCTION The effect of rising CO2 emissions on climate and ocean acidification has led to extensive research on its capture and long-term sequestration.[1,2] Approaches include stable storage and chemical conversion into usable byproducts.[3,4] High surface area of suspended microbubbles can aide these approaches by increasing CO2 mass-transfer from gas to the liquid. Pickering stabilized gas microbubbles produced using nano-catalytic materials have even been demonstrated for enhanced reaction efficiency.[5] Microfluidics offers a unique tool to study physical and chemical gas/liquid interactions. Advantages include: superior heat and mass transfer, control over interfacial surface area via formation of precisely sized bubbles at high frequencies, control of reaction variables and suitability for well-timed multi-step reactions.[6-8] Typically, such studies are conducted by optical characterization of bubble sizes, from which dissolution rates and secondary chemical reactions in the liquid phase can be inferred.[9-11] However, drawbacks include pressure drop along the microchannel, and complications to masstransfer, due to surfactant accumulation at the gas/liquid interface, flow rate-dependent mixing between gas and liquid plugs and channel geometry. Meanwhile, monitoring CO2 reactions in liquid-only microchannels require direct chemical measurements. Direct, spatially-resolved measurements of dissolved CO2 concentrations, especially related to their effect on water acidification, would open the way for in situ studies of reaction kinetics, concentration gradients. In situ characterization in microchannels have been conducted by miniaturized electrodes for single- or multi-point pH measurements.[12-14] However, challenges related to interfacing, local flow dynamics and probe density limit their use for chemical imaging. The pH-dependent swelling of hydrogels has also been used for microfluidic sensing applications, though the effects of shear and low spatial resolution are limitations.[15] In general, immobilized sensors for should maximize analytical figures of merit: sensitivity, linearity, response time, limited photobleaching and resistance to physic
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