Upconversion nanoparticles as intracellular pH messengers
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RESEARCH PAPER
Upconversion nanoparticles as intracellular pH messengers Evaline S. Tsai 1 & Fadwa Joud 2 & Lisa M. Wiesholler 3 & Thomas Hirsch 3 & Elizabeth A. H. Hall 1 Received: 30 December 2019 / Revised: 7 June 2020 / Accepted: 11 June 2020 # The Author(s) 2020
Abstract Upconversion nanoparticles (UCNPs) should be particularly well suited for measurement inside cells because they can be imaged down to submicrometer dimensions in near real time using fluorescence microscopy, and they overcome problems, such as photobleaching, autofluorescence, and deep tissue penetration, that are commonly encountered in cellular imaging applications. In this study, the performance of an UCNP modified with a pH-sensitive dye (pHAb) is studied. The dye (emission wavelength 580 nm) was attached in a polyethylene imine (PEI) coating on the UCNP and excited via the 540-nm UCNP emission under 980-nm excitation. The UC resonance energy transfer efficiencies at different pHs ranged from 25 to 30% and a Förster distance of 2.56 nm was predicted from these results. Human neuroblastoma SH-SY5Y cells, equilibrated with nigericin H+/K+ ionophore to equalize the intra- and extracellular pH‚ showed uptake of the UCNP-pHAb conjugate particles and, taking the ratio of the intensity collected from the pHAb emission channel (565–630 nm) to that from the UCNP red emission channel (640–680 nm), produced a sigmoidal pH response curve with an apparent pKa for the UCNP-pHAb of ~ 5.1. The UCNP-pHAb were shown to colocalize with LysoBrite dye, a lysosome marker. Drug inhibitors such as chlorpromazine (CPZ) and nystatin (NYS) that interfere with clathrin-mediated endocytosis and caveolae-mediated endocytosis, respectively, were investigated to elucidate the mechanism of nanoparticle uptake into the cell. This preliminary study suggests that pH indicator–modified UCNPs such as UCNP-pHAb can report pH in SH-SY5Y cells and that the incorporation of the nanoparticles into the cell occurs via clathrinmediated endocytosis. Keywords Upconversionnanoparticles . Nanosensors . pHmeasurements . Intracellular . Fluorescencemicroscopy . Endocytosis
Introduction Dyes that have a particular affinity for the nucleus, membrane or some specific organelle in the cell can provide exquisite spatial or structural information about the cell, but it is a further challenge to obtain spatiotemporal information about Published in the topical collection featuring Female Role Models in Analytical Chemistry. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00216-020-02768-5) contains supplementary material, which is available to authorized users. * Elizabeth A. H. Hall [email protected] 1
Department of Chemical Engineering and Biotechnology, University of Cambridge, Philippa Fawcett Dr., Cambridge CB3 0AS, UK
2
Cancer Research UK Cambridge Institute, University of Cambridge, LiKa Shing Centre, Robinson Way, Cambridge CB2 0RE, UK
3
Institute of Analytical Chemistry, University of Regensburg, Universitätsstr. 31, 93053 R
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