Flow Alters the Interfacial Reactions of Upconversion Nanocrystals Probed by In Situ Sum Frequency Generation

Abstract Particle moving is a fundamental phenomenon of colloidal nanoparticles (NPs), but its influence on the interfacial interaction is far from been clarified. Herein, by mimicking the particle moving of upconverting fluoride NPs in a flowing cell, highly dependence of interfacial reactions on the particle moving of the colloidal NPs is clearly revealed by adopting combined in situ sum frequency generation and photoluminescence spectroscopies. It is found that particle moving can induce the rearrangement of the adsorbed interfacial water due to generating a more positively charged surface. More importantly, besides more prone to adsorb negatively charged carboxyl group of Rhodamine B via enhanced electrostatical interaction, particle moving at pH of 6.8 could reorient the xanthene ring of the dye parallel to the NP surface, via repelling the positively charged diethylamino groups. As a consequence, fluorescence resonance energy transfer efficiency from the upconverting NPs to Rhodamine B can be reversibly modulated by only tuning the flow. This work provides new insight into the interfacial interactions of the colloidal NPs, which are expected to guide their surface modification and practical applications.