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. 2020 Mar 5;23(3):100962. doi: 10.1016/j.isci.2020.100962

A Universal Strategy to Construct Lanthanide-Doped Nanoparticles-Based Activable NIR-II Luminescence Probe for Bioimaging

Zhen Li 1, Junjie Wu 1, Qirong Wang 1, Tao Liang 2, Juan Ge 1, Peipei Wang 2, Zhihong Liu 1,2,3,∗
PMCID: PMC7090340  PMID: 32200096

Summary

Lanthanide-doped nanoparticles (LnNPs) have gained increasing attention recently for bioimaging in the second near-infrared window (NIR-II, 1,000–1,700 nm) because of their excellent photophysical properties, but the construction of LnNPs-based activable probe responding to specific targets remains a challenge. Herein, we proposed an uncomplicated and universal strategy to fabricate LnNPs-based NIR-II probes by target-triggered dye-sensitization process. The dye acts as both the recognition motif of the target and a potential antenna for LnNPs, which can be activated by the target to sensitize the NIR-II luminescence of LnNPs. A proof-of-concept probe for glutathione (GSH) was constructed to validate this approach. It was able to track the fluctuation of GSH level in liver and lymphatic drainage and provide clear images with high contrast and resolution in vivo. This strategy can be generalized to construct NIR-II probes for various analytes by simply changing the recognition motif of the dye, greatly promoting the application of LnNPs.

Subject Areas: Medical Imaging, Nanoparticles, Radiation Physics

Graphical Abstract

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Highlights

  • •

    A universal strategy was proposed to construct LnNPs-based activable NIR-II probe

  • •

    The strategy circumvents the difficulty in designing activable probe in NIR-II region

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    This probe can be applied to monitor the subtle fluctuation of GSH level in vivo

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    It shows excellent performance in bioimaging with deep penetration and high contrast

Medical Imaging; Nanoparticles; Radiation Physics

Introduction

Fluorescence imaging with the features of real-time tracking, high temporal-spatial resolution, and non-invasiveness has been extensively studied and holds great promise for monitoring biological substances and processes in vivo (He et al., 2018, Owens et al., 2016, Miao et al., 2017, Kim et al., 2018, Cheng et al., 2017). Especially, near-infrared fluorescence imaging in the second biological window (NIR-II), from 1,000 to 1700 nm, has evoked considerable attention for bioimaging because of its unparalleled spatial resolution (approximately 10 μm) and tissue penetration (centimeter level), benefiting from the negligible autofluorescence as well as the reduced photon absorption and scattering by biosamples (Tang et al., 2019, Yang et al., 2018, Sun et al., 2019a, Sun et al., 2019b, Alifu et al., 2018, Sheng et al., 2018, Antaris et al., 2016, Deng et al., 2019, Lei et al., 2019). Lanthanide-doped nanoparticles (LnNPs) as a new kind of promising NIR-II luminescent materials with tunable emission, sharp f-f emission peaks, high chemical stability and photostability, long luminescence lifetime, and low toxicity have received increasing interest in fields of chemical and biomedical science (Song et al., 2019b, Liu et al., 2018a, Wang et al., 2018, Ximendes et al., 2016, Ximendes et al., 2017, Zhang et al., 2019a, Fan et al., 2018; Fan and Zhang, 2019, Zhao et al., 2018). However, in most of the previous reports, LnNPs always acted as a fluorescent tag in in vivo imaging, outputting “always-on” luminescence signals (He et al., 2019, Li et al., 2019, Dai et al., 2017, Zhao et al., 2019, Xue et al., 2018), which does not allow for the recognition of specific targets or events. This restriction is derived from the lack of flexible design strategies to construct activable LnNPs-based NIR-II probes, which require an on-site modulation of lanthanide emission by analytes. Because of the structural and spectral characteristics of lanthanide ions and LnNPs, it is a challenge to modulate such long-wavelength emissions via common fabricating principles for optical probes, such as photoinduced electron transfer or resonance energy transfer.

Researchers have recently made a number of efforts to deal with this problem. For example, Chen et al. designed a kind of NaCeF4:Yb,Er nanocrystal with photoluminescence emission at 1,525 nm originating from the 4I13/2→4I15/2 transition of Er3+ because of the cross-relaxation between Er3+ and Ce3+ (Lei et al., 2018, Zhang et al., 2019b). After the oxidization of Ce3+ to Ce4+ by H2O2, the emission at 1,525 nm was quenched, enabling the detection of H2O2. But the application of this kind of probe is restricted since it can be utilized only for monitoring H2O2 or H2O2-participated processes, and the expansibility of the design is somewhat limited. Besides, Zhang et al. proposed a relatively more flexible strategy, named absorption competition-induced emission (ACIE), to build LnNPs-based NIR-II probes, in which the target was detected through regulating the filtration effect of excitation light by dyes (Wang et al., 2017, Wang et al., 2019). The occurrence of efficient filtration effect normally requires a high concentration of dye, which may add difficulties in the fabrication of probe for in vivo usage.

Herein, we propose a universal strategy for fabricating LnNPs-based activable NIR-II luminescence probe through target-regulated dye sensitization of Ln3+ emission. As well recognized, the faint absorptivity of lanthanide ions leads to a rather low photoluminescence efficiency of LnNPs. Through the introduction of an organic dye with strong NIR absorption as an antenna, it is possible to increase the utilizing efficiency of excitation light and enhance the luminescence of LnNPs (Zou et al., 2012, Wu et al., 2016, Liu et al., 2018b, Zhang et al., 2019c). In this sensitization process, the dyes harvest energy from external photons and subsequently transfer their excited-state energy to the ions in LnNPs (such as Nd3+ or Yb3+) via a resonance energy transfer process (Xu et al., 2017a, Liang et al., 2018, Song et al., 2019a, Shao et al., 2016, Wei et al., 2016). A prerequisite of such sensitization is the spectral match between the emission of the dye and the absorption of Ln3+. We thus consider modulating the NIR-II luminescence of LnNPs through handling the emission of a dye. In this way, the sensitization process can be readily regulated by a target specifically reacting with the dye, so as to construct an activable LnNPs-based NIR-II probe. To verify our design strategy, a proof-of-concept probe was built for detection of glutathione (GSH), which is crucial in maintaining the intracellular redox homeostasis (Lu, 2009, Estrela et al., 2006). As shown in Scheme 1, a non-emissive dye, HC-Ni, was designed as both the recognition motif for GSH and a potential antenna, which cannot sensitize the NIR-II luminescence of LnNPs in its present form. After reacting with GSH, HC-Ni transferred into the emissive form, HC-SG, triggering the sensitization of NIR-II luminescence. Hence GSH can be quantitatively detected since the enhancement of NIR-II luminescence is correlated with GSH concentration. We have demonstrated excellent biocompatibility and superior performance of the proposed probe in bioimaging in vivo. More significantly, this strategy provides a new approach to the construction of LnNPs-based activable NIR-II probe and can be generalized to various targets by simply changing the recognition unit of the dye.

Scheme 1.

Scheme 1

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Schematic Illustration of the LnNPs-Based Activable NIR-II Luminescence Probe

The design strategy of probe is based on a target-triggered dye-sensitization process.

Results

Photophysical Property of HC-Ni and Its Response to GSH

HC-Ni was designed as the potential antenna with a heptamethine cyanine as the fluorophore and an m-dinitrophenol motif as the recognition unit. Owing to the large spectral overlap between the emission spectrum of heptamethine cyanine and the absorption spectrum of Nd3+, the excited-state energy of heptamethine cyanine can be effectively transferred to LnNPs through a non-radiative energy transfer process (Chen et al., 2015, Xu et al., 2017b). Introduction of the m-dinitrophenol into heptamethine cyanine quenches its fluorescence to generate the non-emissive HC-Ni due to the photoinduced electron transfer (PET) process. As shown in Figure S1A, HC-Ni displayed an intense absorption peak at 794 nm with a molar absorption coefficient of 2.27×105 mol−1⋅L⋅cm−1 (Figure S1B). At the wavelength of excitation light (808 nm), the molar absorption coefficient of HC-Ni was as high as 1.59×105 mol−1⋅L⋅cm−1 (Figure S1C), which was at least five orders of magnitude higher than that of Nd3+ ions (0.77 mol−1⋅L⋅cm−1, Figure S1D). Such a high molar absorption coefficient suggested the possibility of HC-Ni to act as the potential antenna. After reacting with GSH, the photoluminescence intensity of the reaction product HC-SG at 819 nm obviously increased in accordance with the concentration of GSH, owing to the substitution of m-dinitrophenol by GSH and the inhibition of PET process (Figures S2A and S2B). As shown in Figure 1, the emission peak of HC-SG matched well with the absorption spectrum of Nd3+, fulfilling the prerequisite of non-radiative energy transfer from HC-SG to LnNPs. Meanwhile, the absorbance of HC-SG at 794 nm only changed slightly (Figures S2C and S2D). Therefore, with the intense absorption and strong emission at appropriate wavelengths, the reaction product HC-SG can function as an efficient antenna to sensitize the NIR-II luminescence of LnNPs. To achieve the assembly between HC-Ni and LnNPs, two long hydrophobic alkyl chains were introduced into HC-Ni to combine with LnNPs through hydrophobic interaction (vide infra).

Figure 1.

Figure 1

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Spectral Properties of the Antenna

Emission of HC-SG overlaps well with the absorption of Nd3+, but HC-Ni is non-emissive. And HC-SG shows an intense absorption band around 800 nm matching with the excitation light.

Fabrication and Characterization of HC-Ni-LnNPs

To establish the energy-transfer chain of dye→Nd3+→Yb3+→Er3+ (Figure S3), we designed LnNPs with a core-shell structure of NaYF4:20%Yb,2%Er@NaYF4:30%Nd. The transmission electron microcopy (TEM) images illustrated the obtained LnNPs were spherical with uniform size (Figures 2A and 2B). According to the size histograms, the diameter of LnNPs was 36.8 nm with a core of 33.6 nm and a shell thickness of 1.6 nm (Figures 2C and 2D). The crystal phase of core and core-shell structured LnNPs was characterized by X-ray diffraction (XRD) patterns (Figure 2E), which were both consistent with the hexagonal-phase of NaYF4 (JCPDS No. 16-0334). Then, HC-Ni and oleic acid (OA)-coated LnNPs were assembled through hydrophobic interaction and coated with an amphiphilic molecule DSPE-PEG to obtain the water-soluble nanoprobe HC-Ni-LnNPs. TEM image and XRD analysis indicated that the modification by HC-Ni did not induce significant changes in shape, size, and crystallinity of LnNPs (Figures S4A and S4B). The successful construction of HC-Ni-LnNPs was confirmed by Fourier transform infrared (FTIR) spectra and UV-vis spectra. As shown in Figure 2F, the typical peak at 1,630 cm−1 in the FTIR spectra of HC-Ni-LnNPs were assigned to C=O of the amide group stretching vibrations in DSPE-PEG, confirming its existence on the surface of HC-Ni-LnNPs. Meanwhile, the absorption around 800 nm of HC-Ni appeared on the spectrum of HC-Ni-LnNPs (Figure 2G). In addition, because of the positive charge of HC-Ni, the ζ potential of DSPE-PEG-coated LnNPs shifted from −1.8 mV to +30 mV after HC-Ni loading (Figure S5). All the above results confirmed that the HC-Ni-LnNPs nanoprobe was successfully constructed.

Figure 2.

Figure 2

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Characterization of LnNPs and HC-Ni-LnNPs

(A–D) TEM images and size distribution of core (A and C) and core-shell (B and D) structured LnNPs. Scale bar: 100 nm.

(E) XRD patterns of core and core-shell structured LnNPs.

(F) FTIR spectra of DSPE-PEG, OA-LnNPs, and HC-Ni-LnNPs.

(G) The UV-vis spectra of the LnNPs before and after loading with HC-Ni.

Sensing of GSH in Solution

Then, we investigated the sensing ability of HC-Ni-LnNPs for GSH in buffer solution. First, we optimized the structure of LnNPs including the amount of Nd3+ in shell and the thickness of shell, which decided the sensitization efficiency. In the energy transfer process, Nd3+ ions absorb the excited-state energy of dyes and subsequently transfer their energy to Yb3+ ions. Thus, a high concentration of Nd3+ will favor this process owing to the enhanced ability of energy harvesting and rate of energy transfer from Nd3+ to Yb3+. However, the detrimental cross-relaxation process between Nd3+ ions (4F3/2+4I9/2→24I15/2) may arise with elevating the Nd3+ concentration, which will invalidate the energy harvested by Nd3+ (Chen et al., 2015). Therefore, for purpose of optimization, we prepared a series of core-shell structured LnNPs, NaYF4:20%Yb,2%Er@NaYF4:x%Nd (x = 10, 20, 30, 40, 50 and 60, respectively), with core diameter of 33.9 nm and shell thickness of about 2 nm (Figure S6). As shown in Figure 3A, at the Nd3+ concentration of 30%, a balance between the two effects can be obtained, achieving the maximal luminescence enhancement factor (F/F0, in which F and F0 are the luminescence intensity at 1,530 nm with and without GSH, respectively, termed as LEF below). The corresponding NIR-II luminescence spectra are shown in Figure S7. Furthermore, the shell thickness of LnNPs can significantly affect the performance of the probe. As known, since the non-radiative energy transfer process is highly dependent on the distance of energy donor-acceptor pair, a thinner shell is beneficial for the effective energy transfer from dye to Nd3+ and Nd3+ to Yb3+. However, a thinner shell layer will cause more serious surface quenching of luminescence. With the fixed size of the core, we synthesized four groups of core-shell structured LnNPs with the shell thickness of 1.9, 3.1, 3.9, and 5.6 nm, respectively (Figure S8). As shown in Figure 3B, the best response performance was achieved with the shell thickness of 3.1 nm (the corresponding NIR-II luminescence spectra are shown in Figure S9).

Figure 3.

Figure 3

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Sensing of GSH in Solution

(A–C) Luminescence enhancement factor (LEF), F/F0, at 1,530 nm of HC-Ni-LnNPs responding to GSH with (A) different Nd3+ doping concentrations of LnNPs; (B) various shell thickness of LnNPs; (C) different loading capacity of HC-Ni.

(D) NIR-II luminescence spectra of HC-Ni-LnNPs responding to GSH under the optimal conditions.

(E and F) (E) Dependence of F/F0 at 1,530 nm of HC-Ni-LnNPs on the GSH concentration and (F) the linear relationship between F/F0 and logarithm of GSH concentration. Data are represented as mean ± SD (n = 3).

With the optimal structure of LnNPs, we investigated the influence of the loading capacity of HC-Ni assembled on the surface of LnNPs on the response performance. According to the molar absorption coefficient of HC-Ni and the absorbance of HC-Ni-LnNPs at 794 nm, the loading capacities of HC-Ni for the five batches of probes prepared were calculated as 0.78, 1.4, 3.16, 7.6, and 8.36 nmol/mg LnNPs, respectively (Figure S10). As shown in Figure 3C, the LEF increased with enhancing the amount of HC-Ni loaded on the surface of nanoprobe. Notably, the LEF reached the maximum at a loading capacity of 8.36 nmol/mg, which was the maximal amount of HC-Ni that could be loaded (the corresponding NIR-II luminescence spectra are shown in Figure S11). In addition, the luminescence of HC-Ni-LnNPs after reacting with GSH as a function of time was detected, which showed that the LEF gradually increased and reached the plateau after 50 min (Figure S12). With the above optimized experimental conditions, we recorded the NIR-II luminescence signal of HC-Ni-LnNPs after introducing various concentrations of GSH. As shown in Figure 3D, the NIR-II luminescence was enhanced stepwise with increasing the concentration of GSH. More importantly, the LEF at 1,530 nm was dependent on the concentration of GSH in the range of 0.03–9 mM (Figure 3E), implying the feasibility of quantitatively detecting GSH. As shown in Figure 3F, the LEF exhibited a good linear relationship with the logarithm of [GSH] and the detection limit was calculated to be 1.9 μM according to three times the standard deviation of blank signal (n = 7). In contrast, no significant luminescence response was measured after introduction of other species, such as metal ions, amino acid, reactive oxygen species (ROS), and other biothiols, suggesting the excellent specificity of HC-Ni-LnNPs (Figure S13). The energy transfer efficiency from HC-SG to Nd3+ was studied by comparing the lifetime of HC-SG assembled on the surface of nanoparticles with or without doping Nd3+. As shown in Figure S14, the lifetime of HC-SG shortened from 0.92 to 0.73 ns after doping Nd3+, by which an energy transfer efficiency of 20% was calculated.

Biosafety of the HC-Ni-LnNPs Probe

Before being used in bioimaging of GSH in vivo, the stability and biocompatibility of HC-Ni-LnNPs were first assessed. As exhibited in Figure S15, HC-Ni-LnNPs emitted stable NIR-II luminescence signals in HEPES buffer, DMEM medium, and fetal bovine serum (FBS) during 2 h incubation at 37°C. This luminescence signal also changed very slightly under different pH value from 5.7 to 8.2 (Figure S16). Furthermore, incubation of the product, HC-SG-LnNPs, at 37°C for 2 h in the above-mentioned three kinds of media did not cause obvious variation in the luminescence signal either (Figure S17). These results demonstrated that both the probe and the reaction product possessed adequate stability and were suitable for long-term observation in bioimaging. Subsequently, the cytotoxicity of HC-Ni-LnNPs was evaluated by the cell counting kit-8 (CCK-8) assay. As shown in Figure S18, the cell viability maintained >90% after incubation with various concentrations of HC-Ni-LnNPs in the range of 0–0.6 mg/mL, illustrating the very low cytotoxicity of HC-Ni-LnNPs. This result was also in agreement with the co-staining by calcein-AM, which could stain living cells with a green fluorescence, and propidium iodide (PI), which could stain dead cells with a red fluorescence. As shown in Figure S19, strong green fluorescence and slight red fluorescence were observed with each concentration of HC-Ni-LnNPs-treated cells. Furthermore, different amounts of HC-Ni-LnNPs (0, 3, and 4.5 mg/100 g body weight) were intravenously (i.v.) injected into the tail of healthy mice to investigate the in vivo biosafety of this nanoprobe. After 7 days, the blood routine and blood biochemical parameters of the mice injected with HC-Ni-LnNPs or physiological saline showed no significant difference (Figure S20). The hematoxylin and eosin (H&E) staining results of the major organs including heart, liver, spleen, lung, and kidney further confirmed that HC-Ni-LnNPs did not induce obvious damage to these organs (Figure S21). All the above results revealed the excellent biocompatibility of HC-Ni-LnNPs, paving the way to apply for in vivo bioimaging.

In Vivo Imaging of GSH Level in Mice

With the superior response performance and biocompatibility of HC-Ni-LnNPs verified, we then examined the bioimaging capability of this NIR-II luminescence probe. First, we investigated the wavelength dependence of imaging by a tissue phantom study covered with different thicknesses of lard, which was detected by an InGaAs camera equipped with different long-pass (LP) filters (1,000, 1,100, 1,200, and 1,300 nm). Owing to the reduced scattering coefficient of NIR-II luminescence, the cuvette profiles were discernible in all images with the depth range from 0 to 4.5 mm (Figure S22A). Furthermore, the signal-to-background ratio (SBR) improved with increasing the wavelength because of the inverse wavelength dependence of scattering coefficient (Figure S22B). Therefore, the 1,300-nm LP filter was utilized in subsequent bioimaging experiments. Next, we applied this nanoprobe to track the intrinsic GSH level in the body. After i.v. injection of HC-Ni-LnNPs, the NIR-II luminescence signals of mice were recorded at different time points. As shown in Figure S23, a clear blood-vessel profile can be observed 1 min post injection (p.i.). This NIR-II luminescence signal gradually strengthened, reached a maximum at 7 min of p.i., and then decreased stepwise. Because of the lower GSH concentration in blood than in liver, the intensity of NIR-II luminescence signal in blood vessel was much weaker than that in the liver. After 20 min of p.i., the NIR-II luminescence signal was detectable mainly in the liver. We further investigated the metabolic time of HC-Ni-LnNPs in the liver. As shown in Figure S24, the NIR-II luminescence signal reached a maximum at about 3–6 h and then gradually decreased until 32 h. Taken together, the above results have shown the promising capacity of HC-Ni-LnNPs to image GSH in vivo.

Thereafter, we estimated the performance of the nanoprobe to sense GSH variations in vivo. As an initial trial, N-methylmaleimide (NMM), a thiol-blocking reagent, was utilized to modulate the GSH level in mice liver. As shown in Figure S25, the control group of mice i.v. injected with HC-Ni-LnNPs only exhibited the brightest NIR-II luminescence. The luminescence intensities of the liver region in other four groups, which were pretreated with NMM (0.5, 1, 1.5, and 2 mg/100 g body weight) for 30 min before injection of HC-Ni-LnNPs, were obviously lower than that of the control group, and the intensity decreased corresponding to the concentration of NMM administrated. Furthermore, we investigated the ability of HC-Ni-LnNPs to monitor the subtle fluctuation of GSH level in two physiological processes. In one mice model, lipopolysaccharide (LPS) was intraperitoneally (i.p.) injected into mice to induce acute inflammation, decreasing the ratio of GSH/GSSG in liver because of the activation of macrophages and neutrophils to produce ROS (Kundu et al., 2009, Yamada et al., 2006). Before i.v. injection of HC-Ni-LnNPs, the mice were pretreated with different dosages of LPS from 0 to 0.75 mg/100 g body weight for 24 h. With increasing dosage of LPS, the NIR-II luminescence signal in liver weakened gradually (Figure S26) because of the reduction of GSH content. We also recorded the luminescence signals of these four batches of samples at different time points after injecting the nanoprobe. We found that the change tendency of luminescence intensity versus the dosage of LPS was consistent at each time point (Figures S26 and S27), indicating the applicability of HC-Ni-LnNPs for long-term bioimaging in vivo. Another mice model was the drug overdose-induced liver injury. Acetaminophen (APAP), a painkiller that may lead to grievous damage to liver and consumption of GSH with excessive amount (Saito et al., 2010, Mitchell et al., 1985, Yin et al., 2014), was i.v. injected into the mice body to cause hepatotoxicity. As shown in Figure 4A, the normal mice injected with HC-Ni-LnNPs exhibited strong NIR-II luminescence because of the high concentration of GSH in normal liver cells. For the three groups with hepatotoxicity, mice were i.v. injected with APAP at 10, 20, and 30 mg/100 g body weight, respectively, 0.5 h before loading with HC-Ni-LnNPs. As expected, the NIR-II luminescence signals of these samples gradually decreased with the increased degree of hepatotoxicity (Figures 4B–4D and 4G). As revealed in previous studies, α-lipoic acid (α-LA) was able to suppress the APAP-induced hepatotoxicity, recovering GSH level in liver cells (Abdel-Zaher et al., 2008, Xiao et al., 2015, Liang et al., 2016). Therefore, other two batches of mice were pretreated with α-LA for 1 h before injecting APAP to investigate the protective effect of α-LA with regulation of GSH level in the liver. As shown in Figures 4E and 4F, the NIR-II luminescence signals were brighter than that of the group injected with APAP at the same amount, and the intensity was enhanced corresponding to the increase of α-LA dosage. To rule out the possible effect of APAP or LPS on the NIR-II luminescence intensity of LnNPs, four sets of control experiment were conducted, where LnNPs or HC-Ni-LnNPs were incubated with APAP or LPS. As shown in Figure S28, the NIR-II luminescence of both LnNPs and HC-Ni-LnNPs kept almost unchanged. In addition, to preclude the potential contribution of ROS caused by liver failure to the luminescence change of HC-Ni-LnNPs, another control experiment was performed. The NIR-II luminescence intensity of HC-SG-LnNPs incubated with ROS was detected, which exhibited no significant change (Figure S29). All the above results have illustrated the satisfying ability of HC-Ni-LnNPs to track the fluctuation of GSH in specific processes.

Figure 4.

Figure 4

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In Vivo NIR-II Luminescence Imaging of GSH in Liver Injury Model

(A–D) In vivo NIR-II luminescence imaging of mice injected with APAP (A: 0, B: 10, C: 20, D: 30 mg/100 g body weight) 0.5 h before loading with HC-Ni-LnNPs.

(E and F) Mice injected with α-LA (E: 1; f: 2 mg/100 g body weight) 1 h before the treatment with APAP (30 mg/100 g body weight). (G) Luminescence intensities of images (A)–(F). Data are represented as mean ± SD (n = 3).

In Vivo Imaging of GSH in Lymphatic Drainage

The reduced scattering coefficient and near-zero autofluorescence endow NIR-II luminescence imaging with high spatial resolution. To demonstrate the capability of HC-Ni-LnNPs for high-resolution imaging in vivo, it was then applied to detect the GSH level in lymphatic drainage in the hindlimb of mice, which is composed of two connective lymph vessels and two lymph nodes and appropriate for estimating imaging resolution. The mice were intradermally (i.d.) injected with LPS (0, 0.125, 0.25, and 0.5 mg/100 g body weight) to cause lymphatic inflammation and reduce GSH level, followed by i.d. injection of HC-Ni-LnNPs 6 h later. As shown in Figure 5, the lymph structure can be observed clearly. The NIR-II luminescence intensity was the highest in the normal group and obviously weakened in the lymphatic inflammation groups. In addition, the luminescence intensities decreased in accordance with the dosage of LPS injected, revealing the different levels of LPS-induced lymphatic inflammation. These results supported the fact that the LnNPs-based NIR-II luminescence probe possessed high-resolution in bioimaging.

Figure 5.

Figure 5

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In Vivo NIR-II Imaging of Lymphatic Drainage in Lymphatic Inflammation Model

(A–D) Mice were i.d. injected with LPS (A: 0, b: 0.125, C: 0.25, D: 0.5 mg/100 g body weight) for 6 h before the injection of HC-Ni-LnNPs.

(E) Luminescence intensities of images (A)–(D). Data are represented as mean ± SD (n = 3).

Discussion

In summary, we proposed an uncomplicated and straightforward strategy to construct LnNPs-based activable NIR-II luminescence probe based on a target-triggered dye-sensitization process. Through regulating the emission of the dye by a specific analyte, the NIR-II luminescence of LnNPs can be readily modulated in response to the analyte. A proof-of-concept probe for GSH, namely, HC-Ni-LnNPs, was fabricated, which showed good biocompatibility and excellent response performance toward the target both in vitro and in vivo. HC-Ni-LnNPs were successfully applied in tracking the fluctuation of GSH level in the liver induced by drugs and imaging GSH with high resolution in lymphatic drainage. Our approach can be extended to construct probes for various targets by simply altering the recognition unit of the dye. We thus expect that this strategy will facilitate the design of LnNPs-based activable NIR-II luminescence probes and promote their applications in the future.

Limitations of the Study

We have proposed a universal strategy to construct LnNPs-based activable NIR-II luminescence probe through a target-triggered dye-sensitization process, by which a probe for GSH detection was fabricated. Probes for other targets have not been confirmed and are worth investigating in the future.

Methods

All methods can be found in the accompanying Transparent Methods supplemental file.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (No. 21625503, 21807028).

Author Contributions

The manuscript was written through the contributions of all the authors. All authors have given approval to the final version of the manuscript. Z. Li, J.W., Q.W., T.L., J.G., and P.W. performed the experiments. Z. Li and Z. Liu analyzed the data, wrote the manuscript, and designed the experimental approach.

Declaration of Interests

The authors declare no competing interest.

Published: March 27, 2020

Footnotes

Supplemental Information can be found online at https://doi.org/10.1016/j.isci.2020.100962.

Supplemental Information

Document S1. Transparent Methods and Figures S1–S33
mmc1.pdf (3.5MB, pdf)

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Supplementary Materials

Document S1. Transparent Methods and Figures S1–S33
mmc1.pdf (3.5MB, pdf)

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