TABLE 1.

Comparison between nanoparticles for tracking mesenchymal stem cells (MSCs).

Example	Types of nanoparticles (NPs)	Preparation	Imaging modality	Types of MSCs	Advantages	Disadvantages	Main results
Lu et al., 2007; Andreas et al., 2012	Superparamagnetic iron oxide (SPIO) NPs	Easy	T2-weighted magnetic resonance imaging (MRI)	Human MSCs	Obvious contrast	Low cell-labeling efficiency for MSCs; necessary to functionalize SPIO NPs	No apparent influences on viability of MSCs
Babic et al., 2008; Kim et al., 2011; Szpak et al., 2013; Lewandowska-Łańcucka et al., 2014; Park et al., 2014; Liao et al., 2016; Rawat et al., 2019; Gu et al., 2018; Lee et al., 2020	SPIO NPs	Complex	T2-weighted MRI	Human MSCs; mouse MSCs	Obvious contrast; enhancement of cellular internalization	Induce precipitation of NPs; perturb the cell membrane	No apparent influences on viability of MSCs
Zhang et al., 2010	SPIO NPs	Easy	Proton (1H) MRI images	Human MSCs	High sensitivity of cell detection	Difficult to accurately quantify cell population	No apparent influences on viability of MSCs
Sehl et al., 2019	SPIO NPs	Complex	Proton (1H) MRI images	Mouse MSCs	High sensitivity of cell detection; quantify the persistence of transplanted MSCs		No apparent influences on viability of MSCs
Sherry et al., 1988; Ratzinger et al., 2010	Gd-based NPs	Easy	T1-weighted	Human MSCs	Distinguish some similar low signal	Low cellular uptake for MSCs	No apparent influences on viability of MSCs
Kim et al., 2015b; Santelli et al., 2018	Gd-based NPs	Easy	T1-weighted MRI	Human MSCs	Distinguish some similar low signal; long-term tracking		No apparent influences on viability of MSCs
Huang et al., 2020	Au-based NPs	Easy	Photoacoustic imaging and CT imaging	Mouse MSCs	Detected in deep tissue at a high resolution; directly labeled; excellent biocompatibility		No apparent influences on viability of MSCs
Chen G. et al., 2015; Li et al., 2016	QD-based NPs	Complex	Fluorescence imaging	Human MSCs	Longer lifetime than traditional fluorescence dye; photochemical stability	Unsatisfied cytotoxicity and stochastic blinking; the contradiction of sensitivity and definition	No apparent influences on viability of MSCs
Idris et al., 2009; Ang et al., 2011; Zhao et al., 2013; Chen X. et al., 2015; Ma et al., 2016	UC-based NPs	Complex	UC luminescence imaging	Human MSCs	Detected in deeper tissue; more stable and higher definition	Poor biocompatibility; low uptake for cellular	No apparent influences on viability of MSCs
Huang et al., 2005; Chen and Jokerst, 2020; Yao et al., 2020	Silicon-based NPs	Complex	Fluorescence imaging	Human MSCs	Excellent biocompatibility and chemical inertness; easily modified by bioconjugation; good cellular uptake		No apparent influences on viability of MSCs
Chen et al., 2019	Silicon-based NPs	Complex	Photoacoustic imaging	Human MSCs	Good biocompatible and cells tracking capacity; long time		No apparent influences on viability of MSCs
Gao et al., 2016	Other-based NPs	Complex	Aggregation-induced emission imaging	Mouse bone marrow-derived MSCs	Possessing long-term tracking and strong anti-photobleaching ability		No apparent influences on viability of MSCs
Yin et al., 2018	Other-based NPs	Complex	Photoacoustic imaging	Human MSCs	High signal-to-noise; deeper tissue imaging		No apparent influences on viability of MSCs