Upconversion nanoparticles: a versatile solution to multiscale biological imaging

Abstract

Lanthanide-doped photon upconverting nanomaterials are emerging as a new class of imaging contrast agents, providing numerous unprecedented possibilities in the realm of biomedical imaging. Because of their ability to convert long-wavelength near-infrared excitation radiation into shorter-wavelength emissions, these nanomaterials are able to produce assets of low imaging background, large anti-Stokes shift, as well as high optical penetration depth of light for deep tissue optical imaging or light-activated drug release and therapy. The aim of this review is to line up some issues associated with conventional fluorescent probes, and to address the recent advances of upconverting nanoparticles (UCNPs) as a solution to multiscale biological imaging applications.

Scheme 1. Overview of the Present Tutorial Review

Figure 1

Schematic representation of the excitation/emission and interatomic energy transfer profiles of UCNPs.

Figure 2

(a) Confocal upconverted luminescent image of individual UCNPs. (b) Live-cell imaging of UCNPs in NIH 3T3 murine fibroblasts, showing virtually zero autofluorescence background. (c,d) Zoom-in time trace and histogram of emission intensity, showing no on/off behavior-nonblinking. (Reprinted with permission from ref (20). Copyright 2009 Highwire press PNAS.)

Figure 3

(left) TEM microgragh of 4.5 nm ultrasmall UCNP; (right) time trace showing no blinking. (Reprinted with permission from ref (24). Copyright 2012 American Chemical Society.)

Figure 4

Compiled luminescent spectrum and photos showing corresponding colloidal solution of series of Ln³⁺-doped nanoparticles. (Reprinted with permission from refs (27) and (28). Copyright 2014 American Chemical Society and 2012 Royal Society of Chemistry Publishing.)

Figure 5

(top) left: the tissue depth of NIR and visible light. right: (a) UCPL bright-field image of a cuvette filled with a suspension of the core/shell nanoparticles, (b) bright-field image of a cuvette covered with pork tissue with a quarter coin stood aside showing its thickness, (c) merged UCPL/bright-field image of the cuvette covered with pork tissue, and (d) bright-field image of the pork tissue (side view). The inset in (c) shows the spectra obtained from the circled areas. (bottom) Polyethylenimine-coated NIR_in-NIR_out R-(NaYbF4:0.5%Tm₃t)/CaF₂ core/shell nanoparticles for imaging a synthetic periosteal mesh implanted around a rat femur. (a) UCNPs were loaded on a 7-mm-wide sulfated polymer mesh and wrapped around the mid shaft of a rat femur. Scale bar: 500 μm. (b) Bright-field image of the rat hind leg after closing muscle/skin by suture (left) and PL image (right) of the deeply embedded UCNP-stained synthetic mesh wrapped around the rat femur. Scale bar: 2 cm. (Reprinted with permission from ref (34). Copyright 2012 American Chemical Society.)

Figure 6

Four-model imaging of the focused tumor from the tumor-bearing nude mouse 1 h after intravenous injection of NaLuF₄:Yb,Tm@NaGdF₄(¹⁵³Sm): (a) In vivo UCL-image, (b) X-ray CT image, (c) SPECT image, (d) MR image of tumor. (e) UCL confocal image of the paraffin section of tumor tissue. (f) Schematic illustration of tumor angiogenesis imaging using aLuF₄:Yb,Tm@NaGdF₄(¹⁵³Sm) as the probe. (Reprinted with permission from ref (36). Copyright 2013 American Chemical Society.)

Figure 7

Ex vivo fluorescent images of glioblastoma-bearing brain in 1 h after the intravenous injection with ANG/PEG-UCNPs, PEG-UCNPs (excitation, 980 nm; emission, 800 nm), and 5-ALA (excitation, 470 nm; emission, 650 nm). All imaging experiments were performed under the same condition. H&E-staining of the tumor tissues from glioblastoma-bearing mice brain was used to demonstrate the existence of glionblastoma. Scale bar: 100 μm. (Reprinted with permission from ref (38). Copyright 2014 American Chemical Society.)

Figure 8

(a) Upconversion process of Nd³⁺ → Yb³⁺ → Er³⁺(Tm³⁺) tridopants system with 800 nm excitation. (b) Spectra profiles of tissue optical window. The extinction coefficient of water at 800 nm is about 20 timers lower than that at 980 nm (Hb: hemoglobin; HbO₂: oxyhemoglobin). (c) In vivo heating effect induced by laser irradiation with 808 and 980 nm. (Reprinted with permission from refs (39) and (40). Copyright 2013 John Wiley and Sons and 2013 American Chemical Society.)

Figure 9

(top) NIR photoactivation process of caged fluorescein on UCNPs. (bottom) Photoactivation of cF-UCNPs in live HeLa cells by 975 nm laser confocal microscope scanning. (Reprinted with permission from ref (45). Copyright 2013 John Wiley and Sons.)

Figure 10

(a) Representative gross photos of a mouse showing tumors (highlighted by dashed white circles) at 14 d after treatment with the conditions described for groups 1–4. Scale bars, 10 mm. (b) Tumor volumes in the four treatment groups at 6, 8, 10, 12, and 14 d after treatment to determine the effectiveness of the treatment in terms of tumor cell growth inhibition. (c) TUNEL staining of tissue sections from the treatment groups at 24 h after treatment to determine the effectiveness of the treatment in terms of tumor cell death by apoptosis. DAPI counterstaining indicates the nuclear region, and upconversion fluorescence imaging indicates the position of the injected UCNP-labeled cell (400× magnification). Scale bar, 20 μm. (d) The apoptotic index charted as the percentage of TUNEL-positive apoptotic nuclei divided by the total number of nuclei visualized by counterstaining with DAPI obtained from counts of randomly chosen microscopic fields. (Reprinted with permission from ref (49). Copyright 2012 Nature Publishing Group.)