72 W. T. Anderson et al. Accurate Particle Selection The choice of particle in a particle tracking experiment is important. Accurate analysis of passive, individual probe tracks in a biological material requires careful consideration of what probe to use, as inappropriate probe choice can mask results or lead to wrong conclusions. As noted earlier, several factors including probe size, charge, coating, fluorescence, and concentration each contribute to how a probe will interact with the sample and experimental setup of choice. While commercial probes are easily accessible to researchers who do not have the expertise, facilities, or time to create custom probes or modify existing probes, these probes are often not well characterized by their respective vendors. Consequently, the authors of this study aim to provide an in-depth characterization of 10+ purchasable probes to assist future researchers make informed decisions. Preliminary results with a focus on three quantum dots are presented here. Quantum dots (QDs) are semiconductor nanocrystals and are desirable for their small size and inherent, stable fluorescence. QDs are often coated with a polymer like polyethylene glycol (PEG) when used in biological applications. This addition can serve several purposes [19]: to protect cells from the toxic heavy metal core, prevent specific binding if the polymer is nonreactive/nonfunctionalized (necessary for passive particle tracking), or to hold reactive/functional groups for specific binding (necessary for active particle tracking). Fig. 2 Representative TEM images of commercially available quantum dots from Ocean NanoTech (a), Strem (b), and Thermo Fisher (c). Scale bars are 50 nm. Feret diameters (d) of Thermo Fisher and Strem QDs display narrow distributions while Ocean NanoTech QDs exhibit a wide distribution of sizes
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