Synthesizing Ag+: MgS, Ag+: Nb2S5, Sm3+: Y2S3, Sm3+:Er2S3, and Sm3+:ZrS2 Compound Nanoparticles for Multicolor Fluorescence Imaging of Biotissues.

Development of the fluorophores whose fluorescence bands can be flexibly selected is of great interest for biotissue imaging. Compounds of Ag+:MgS, Ag+:Nb2S5, Sm3+:Y2S3, Sm3+:Er2S3, and Sm3+:ZrS2 were obtained through new chemical synthesis. They were characterized by X-ray photoelectron spectroscopy, X-ray diffraction spectroscopy, and transmission electron microscopy. They revealed polychromatic-photoluminescence spectra when excited by 280, 380, 480, 580, 680, and 785 nm light. Especially, near-infrared emission ranging from 800-1100 nm was found upon 785 nm light excitation. A band model was proposed to explain transitions responsible for the observed components of emission. Their broad fluorescence spectra cover from the ultraviolet to near-infrared spectral range. Their ability of emitting wide-range fluorescence was utilized for multicolor fluorescence imaging of biotissues, as demonstrated by pig-kidney tissue samples.

Introduction

The development of fluorophores whose fluorescence bands can be flexibly selected is important for practical biotissue imaging and clinic/surgery procedures. In the field of biotissue imaging, conventional fluorophores are associated with very narrow spectral window, which have been developed to only show ultraviolet (UV), blue, green, yellow, red, or near-infrared (NIR) fluorescence. However, it should be mentioned that there is complex biofluid existing in biotissues. As a result, the optical absorption band of the biotissues can be associated with a very wide optical band ranging from UV to NIR. Therefore, more than one type of the fluorophores are generally required in order to get multicolor fluorescence imaging of the biotissues. New materials need to be identified or developed for readily available and effective point-of-use fluorophores which can emit multicolor fluorescence upon excitation.

Several groups have reported progress in the fabrication of fluorescence imaging contrast agents with polychromatic luminescence,[1−5] which includes silver- or samarium-based compounds.

Silver is an attractive metal for optical investigation when considering the position of its d-electrons compared to gold.[6−12] Silver-based optical materials have been attracting significant attention from many chemists, biologists, and pharmacists during the last few decades.[6−11] This phenomenon is the result of the fact that these materials show particularly interesting visible and NIR emissions. Several examples are discussed as follows; Zheng and Dickson. made water-soluble dendrimer: Ag nanodots, which show green or red emission upon blue light excitation.[6] Lu et al. reported the synthesis of three silver coordination polymers by coordinating three pyridine carboxylic hydrazide (4-pyridine carboxylic hydrazide, 3-pyridine carboxylic hydrazide, and 2-carboxylic hydrazide) with silver nanoclusters, which present green emission upon the excitation of UV light.[7] Ashenfelter et al. synthesized molecular Ag-glutathione compounds of Ag32(SG)19, Ag15(SG)11, and Ag11(SG)7, which emit red light upon the excitation of blue light.[8] Xie et al. made silver-organic gels (NH4)9[Ag9(mba)9], which undergo the aggregation-induced emission of red light and fluorescence-to-phosphorescence switching.[9] Khan et al. reported the synthesis of Ag:CdSe, which results in an emission that is tunable from 609 to 880 nm, with a Stokes shift up to 1 eV and a fluorescence quantum efficiency exceeding 50%.[10] This material is highly fluorescent and its large Stokes shift can provide a strongly suppressed emission reabsorption. He et al. synthesized three kinds of silver phosphinate in three-dimensional frameworks with argentophilic interactions, which reveal a green emission upon blue light excitation.[11] Shu et al. used Pb ions to modify Ag2S quantum dots for tuning its emission in the second near-infrared window.[12]

Samarium-based compounds have been another class of interesting fluorophores because of their orange/red light emission,[13−18] which is just between the edge of the visible light and the beginning of the NIR light. There are several reports showing that Sm-based compounds can emit fluorescence other than orange/red light.[19−21]

These studies presented will not only enrich our understanding on silver/samarium-based compounds but also provide new opportunities to better engineer them into new luminescent materials with real applications.

Then, a question stands. How can we tailor the luminescence of the silver/samarium-based compounds to show different colors, such as UV-, blue-, yellow-, red-, or NIR light?

It is said that spectroscopic chemistry behind the luminescence can be associated with ligands to metal charge transfer.[7,8] Therefore, either the modification of the ligands or the metal ions can bring new spectral regions.

It is reported that introducing a heterofunctional ligand, namely, the presence of a second metal ion,[22,23] opens up one possibility for tailoring the luminescence of compounds.

Based on this strategy, it can be speculated that if the second metal ion is introduced into the silver compounds, the heterofunctional ligand may create a donor to be excited. Subsequently, the photoluminescence band may be expanded.[22,23]

We plan to use metal ion-modified Ag/Sm-based compounds in order to obtain a polychrome emission. The theory for this is that the introduction of another metal ion can bring some defects inside the materials, which build up the extra band. The interaction between these extra bands and the conduction band or valence band can lead to the formation of many energy levels. These energy levels introduce multiple transitions, resulting in a multicolor fluorescence.

Using the idea, in this work, we aimed to combine silver/samarium compounds with another metal ion. We developed a method for making compounds of Ag+:MgS, Ag+:Nb2S5, Sm3+:Y2S3, Sm3+:Er2S3, and Sm3+:ZrS2. They present the polychromatic luminescence ranging from the UV to NIR region when different light excitation is applied.

X-ray photoelectron spectroscopy (XPS), X-ray diffraction spectroscopy (XRD), and transmission electron microscopy (TEM) were used to characterize these synthesized nanoparticles.

Biotissue imaging experiments were performed via UV light (359–371 nm), blue light (450–490 nm), green light (540–552 nm), and NIR light (center wavelength = 785 nm) excitation with pig-kidney tissue samples. It experimentally demonstrates that these nanoparticles could be utilized for UV–vis–NIR ultrawide-range fluorescence imaging of the biotissue. Taking advantage of these exclusive optical properties, these compound nanoparticles were envisioned to be employed as a kind of multicolor fluorophore for biotissue imaging.

Results and Discussion

XRD, TEM, and XPS Characterization

The XRD data of the samples were analyzed by the software packages of Mercury and GSAS. Structure analysis and refinement were performed, leading to the indexing of all the peaks in XRD profiles (see Figures S1–S6 in Supporting Information).The particle size and morphology of the synthesized samples were determined by TEM. Figure a–f presents the representative TEM images of Ag+:MgS, Ag+:Nb2S5, Ag+:WS3, Sm3+:Y2S3, Sm3+:Er2S3, and Sm3+:ZrS2.

Figure 1

TEM images of the samples: (a) Ag+:MgS; (b) Ag+:Nb2S5; (d) Sm3+:Y2S3; (e) Sm3+:Er2S3; and (f) Sm3+:ZrS2. (c) Amplified image of the rectangular region in (b).

Ag+:MgS, Ag+:Nb2S5, Sm3+:Y2S3, Sm3+:Er2S3, and Sm3+:ZrS2 nanoparticles show an irregular shape. In particular, Ag+:Nb2S5 nanoparticles form clusters. As shown in the low-resolution TEM image (Figure b), these particles look like a big spot. However, in the high-resolution TEM image (Figure c), we can see that they are clusters gathered by tiny nanoparticles.

After counting the size of the nanoparticles, the average size of Ag+:MgS, Ag+:Nb2S5, Sm3+:Y2S3, Sm3+:Er2S3, and Sm3+:ZrS2 is found to be 22, 6.5, 47.4, 4.2, and 164.4 nm, respectively. The size distribution of these nanoparticles is shown in Figures S6–S10.

XPS was employed to detect the nature of Ag 3d, Mg 1s, Nb 3d, S 2p, Sm 3d, Y 3d, Er 4d, and Zr 3d species at the surface of these compounds. The molar ratio of Ag+ with respect to MgS and Nb2S5 was determined by XPS survey to be 0.6 and 0.8, respectively. Similarly, the molar ratio of Sm3+ with respect to Y2S3, Er2S3, and ZrS2 was determined by XPS survey to be 0.1, 1.8, and 0.5, respectively.

Figure a–c shows the XPS scanning spectra of Ag+:MgS. Ag 3d spectra (see Figure a) reveal peaks at 367.2, 372.9, and 373.8 eV, which are attributed to Ag–S bonding, Ag–S–Mg bonding, and satellite peaks.[24−27] Analysis of Mg 1s spectra (see Figure b) reveals peaks at 1299.4 and 1306.2 eV, which are related to the oxidation of Mg.[25] Simulation of S 2p spectra (Figure c) reveals peaks at 162.9, 163.1, and 163.7 eV, which are assigned to S–Ag bonding, S–Mg–Ag bonding, and S–Ag–S bonding.[26,27] The XPS analysis of Ag+:Nb2S5, Sm3+:Y2S3, Sm3+:Er2S3, and Sm3+:ZrS2 is shown in the Supporting Information (See Figures S11–S14).

Figure 2

XPS scanning spectra of Ag+:MgS: (a) Ag 3d; (b) Mg 1s; and (c) S 2p.

Optical Spectra Study

Fluorescence and optical absorption spectra were used to investigate the optical properties of these compounds. Figure a–f presents fluorescence spectra of Ag+:MgS generated by various light sources. The fluorescence spectra of g1+:MgS generated by 280 nm light are shown in Figure a, which present two peaks at 311 and 529 nm. The fluorescence spectra with 380 nm light excitation show a major peak at 533 nm and a minor peak at 713 nm (see Figure b). The fluorescence spectra with 480 nm light excitation (see Figure c) show two peaks at 590 and 716 nm. The fluorescence spectra with 580 nm light excitation (see Figure d) show two weak peaks at 654.6 and 715.4 nm. The fluorescence spectra with 680 nm light excitation (see Figure e) show a peak with weak intensity around 738 nm. The fluorescence spectra with 785 nm light excitation (see Figure f) reveal a peak at 807.5 nm. The optical absorption spectrum of Ag+:MgS reveals several peaks at 741, 833, 876, and 974 nm (see Figure ).

Figure 3

Fluorescence spectra of Ag+:MgS generated by different light sources; (a) generated by 280 nm light; (b) generated by 380 nm light; (c) generated by 480 nm light; (d) generated by 580 nm light; (e) generated by 680 nm light; and (f) generated by 785 nm light.

Figure 4

Optical absorption of Ag+:MgS.

The other samples, including Ag+:Nb2S5, Sm3+:Y2S3, Sm3+:Er2S3, and Sm3+:ZrS2, show similar multicolor fluorescence (see Figures S17–S22 in Supporting Information for detailed analysis). All these results indicate that our samples can emit fluorescence of different colors when various light sources are applied.

It is interesting to point out that MgO nanoparticles show a multicolor fluorescence, which can be tuned from blue/violet to green and then yellow by using various excitation light.[28,29] It can be achieved by introducing different amounts of acetic acid to replace oleic acid in the reaction solvent. It is believed that acid etching can bring defects in the samples for making a multicolor fluorescence. Given that Ag+:MgS and MgO share the same element of Mg and a similar optical emission, this makes us speculate that the defects in our samples may cause this unusual emission of multicolor fluorescence. Our samples show a near-infrared emission, which has rarely been reported in previous works about multicolor fluorescence in Mg-based compounds.[28,29]

Optical Fluorescence Imaging of Biotissues

The biotissue imaging ability of the Ag+:MgS, Ag+:Nb2S5, Sm3+:Y2S3, Sm3+:Er2S3, and Sm3+:ZrS2 compound nanoparticles was tested by using the pig-kidney samples. As shown in Figures and 6, S23–S25 (see Supporting Information), millimeter and micrometer features can be seen on the tissue surface. When the tissue is shined by UV-light, a blue/green morphology can be shown (see Figures a, 6a, S23a, S24a, S25a). When the tissue was irradiated by blue light, a green morphology can be visioned (see Figures b, 6b, S23b, S24b, S25b). When the tissue was shined by green light, a yellow morphology can be observed (see Figures c, 6c, S23c, S24c, S25c). When the tissue was irradiated by 785 nm laser, a valley-like feature can be captured (see Figures d, 6d, S23d, S24d, S25d). It can be noted that some blood vessels are clearly seen (check Figure S24a). Moreover, some fat granules were identified in NIR fluorescence imaging (see Figures d, 6d, S23d, S24d, S25d). These nanoparticles present a multicolor fluorescence in normal tissues. A possible problem might arise: how the multicolor fluorescence will change when these nanoparticles are applied to a tumor? Studying this brings a potential opportunity to discover materials that target the tumor, which is useful in nanomedicine.[30−37]

Figure 5

Pig-kidney tissue imaging generated by Ag+:MgS nanoparticles generated by various light sources; (a) generated by UV light; (b) generated by blue light; (c) generated by green light; and (d) generated by 785 nm light.

Figure 6

Pig-kidney tissue imaging generated by Ag+:Nb2S5 nanoparticles generated by various light sources; (a) generated by UV light; (b) generated by blue light; (c) generated by green light; and (d) generated by 785 nm light.

In order to get more details about the fluorescence images of the tissue, we plotted intensity contour, as shown in Figures S26–S30, associated with tissue fluorescence images, as shown in Figures and 6, S23, S24 and S25 (see the Supporting Information). After we compared the contour images with the fluorescence spectra in Figures , S15, S17, S19 and S21, we could find out that the existing biofluid in the tissue shows interesting impacts on the fluorescence generated by the nanoparticles. As presented in Figure S26c, these yellow areas show a strong intensity around 100–120 A.U. However, the same areas present very weak fluorescence intensity in Figures S26a and S26b. This means that the blue/green fluorescence emitted is weaker than the yellow/red fluorescence on the tissue surface. However, when we checked the fluorescence spectra of Ag+:MgS, as shown in Figure , we could find out that blue or green fluorescence intensities generated by the UV light or blue light is stronger compared to the yellow/red fluorescence generated by the green light. This indicates that the biofluid in tissues impacts the fluorescence of the nanoparticles. It may depend on the nature of the biofluid such as PH level, viscosity, refractive index, and coordinating ability of the biofluid and so forth.[38] The biofluid may enhance or weaken the fluorescence of the nanoparticles. The fluorescence presented depends on the combinational impacts of the biofluid and the nanoparticles. Similar impacts can be spotted in Figures S27–S30, where those areas showing a strong red fluorescence present weak blue or green fluorescence, which does not match the profiles of the fluorescence spectra, as indicated in Figures S15, S17, S19 and S21. It should be noted that a very high contrast can be spotted in the near-infrared fluorescence images (see Figures d, 6d, S23d, s24d and S25d). A detailed analysis of the fluorescence intensity contour, as shown in Figures S26d, S27d, S28d, S29d and S30d, can lead to the conclusion that it is because of the existence of very clear and sharp contour boundaries in the NIR fluorescence images. On the contrary, the blue or green fluorescence images get vague contour boundaries showing indistinct sections.

Our Method of Synthesizing

Our study has provided a useful method for making nanoparticles that show a wide spectral range fluorescence. The detailed synthesis is shown in the Experimental Section. In the following, we outline the synthesis shown in Figure . It starts from preparing the solution that contains silver or samarium ions and other metal ions. An organic acid solution that provides sulfur was prepared for introducing a ligand. Then, the metal ion solution was mixed with the organic acid solution. The obtained solution was heated with a certain temperature to get crystalline. High-speed centrifuging was applied to get the final product of the nanoparticles.

Figure 7

Outline of our synthesis method.

Possible Mechanism of Optical Emission

One may consider that the nature of the fluorescence can be explained by considering the energy transferring from molecular groups to specific metal ions and then the decays of electrons from high energy levels to low energy levels.[39−42] However, these metal ions contained in our samples (e.g. Ag+, Mg2+, S2–, Nb5+, Y3+, and Zr4+) do not seem optically active given that they are typically not showing optical energy levels.

One considerable reason about the nature of the luminescence may be associated with the existence of defects. Most commonly, the green luminescence has been attributed to the recombination of an electron with an ionized vacancy. Our studies suggested that those uncommon luminescence bands arise from the phosphorescent decay of an excited state of the vacancy. The orange/red luminescence on the other hand has been attributed to metal ions impurities, which are Mg2+, Nb5+, Y3+, Er3+, and Zr4+ in the samples. It has also been theoretically proposed that the internal decay of antisite defects could give rise to the red luminescence.[39−42]

Now, we continue to discuss with much more insights into the origin of the luminescence along with the surface nature of these defects. Excitations of core level electrons lead to thermalization. This procedure happens very fast and can be completed through filling of the high energy core-hole by electrons on the atom on which the core-hole is localized. Moreover, when we consider the case of the 1s electron excitation, it can be found that the core-hole initially formed may be directly populated by electrons decaying from the p orbitals of the valence band. Therefore, the electron is excited into an unoccupied p symmetry orbital, which subsequently decay into the s-like conduction band. Also, we have to consider the case of the Ag or Sm excitation. Here, the electron directly populates the conduction band. The core-hole is filled by the atomic-like Ag or Sm d electrons essentially, whose vacancy can subsequently decay via the interaction with a valence band electron.[39−43]

Figure a,b presents our proposed band model depicting transitions responsible for the observed components of the emission. The blue luminescence can be attributed to the electron transition between the high energy levels created by shallow donor defects and the valance band. The green emission can be due to the transition between the conduction band and the high energy levels created by shallow acceptor defects. The red emission may be corresponding to the electron transition between the high energy levels created by the shallow donor defects and the high energy levels created by the shallow acceptor defects. The NIR emission can be assigned to the transition between the low energy levels of shallow donor defects and the high energy levels of shallow acceptor defects.

Figure 8

Proposed band model depicting transitions responsible for the observed components of the emission in material systems; (a) Ag+:MgS and Ag+:Nb2S5 and (b) Sm3+:Y2S3, Sm3+:Er2S3, and Sm3+:ZrS2.

Envisioning of Our Work

It is worth mentioning that our samples present interesting NIR fluorescence ranging from 800 to 1100 nm, which is the so-called NIR-I region for deep tissue imaging.[51] However, we have to optimize the sample prepared in order to obtain a strong NIR fluorescence intensity for taking their unique advantage in achievable penetration depth. This can be done by adjusting the amount of Mg2+, Nb5+, Y3+, Er3+, and Zr4+ ion treatment, which can change the amount of donors and acceptors and ultimately, the configuration of energy levels.

Fluorescence imaging of biological tissues is important for clinical applications. It helps doctors to acquire critical information of patients. Based on the imaging results, the doctors can decide the following treatment for the diseases. As a matter of fact, our study has demonstrated the synthesis of a new kind of optical material and their promising application in biotissue fluorescence imaging. Their wide spectral range and multicolor photoluminescence is useful, given that the various fluorescence bands generated can be selected for specific clinical imaging of various tissues and other biomedical fields.

Here, the Ag+ and Sm3+ ions can be considered as impurities of the whole material system. This inspires us that we might utilize other metal ions as impurities, and the selection of other metal ions might bring special optical properties because of the special transition between energy levels. There are a wide variety of metal ions existing in the earth. It is our future endeavor to develop those metal ion-modified materials for creating fluorescence for imaging purposes.

It is generally believed that the width of the emission peak is dependent on the level of sample purity.[26] Therefore, one of our future investigations can be focused on the study about the impact of the impurity level for the peak profile of the fluorescence. It is also generally believed that the nanoparticle size might impact the photoluminescence because of the quantum confinement effect.[44] This might be another direction of our future study, where the dimension-varied nanoparticles were made and studied for the enhancement or suppression of the fluorescence. We may also study fluorescence versus temperature for figuring out the thermal stability of the fluorescence for practical application of our samples.

During the past tens of years, solid-state fluorophores have presented tremendous applicability in lasers, lighting, displays, lighting, optical telecommunications, and various other fields.[44−50] Because of the incredible development of technology, demand in these fields are not sufficiently met and new challenges for traditional phosphors are presented.[28−33] Now, in the field of solid-state lighting, the efficiency of luminescent materials has already exceeded that of the incandescent lamps.[44−50] Indeed, it is expected to achieve a higher luminescence efficiency to replace the conventional fluorescent lamps.[44−50] Therefore, the luminescence efficiency may not become the focus of our investigation. In our current study, the spectra under different excitation wavelengths increasing from 280 to 785 nm are shown. When using shorter excitation wavelengths (see Figures a, S15a, S17a, S19a and 21a), the samples show white light emission. As the excitation wavelength moves toward the longer wavelength, the emission light changes to blue, green, yellow and red, and NIR. Our model shows that the possible introduction of Mg2+, Nb5+, Y3+, Er3+, and Zr4+ ions for the composite nanoparticles and using different excitation wavelengths will result in light emissions with different colors. This type of tunable visible/NIR luminescence has shown potential multiple functions with applications as a light source for compact display and light systems.[44−50]

Conclusions

We have reported the synthesis of Ag+:MgS, Ag+:Nb2S5, Sm3+:Y2S3, Sm3+:Er2S3, and Sm3+:ZrS2 compound nanoparticles. XRD, XPS, and TEM were utilized to characterize these nanoparticles. They exhibited polychromatic fluorescence in the board range of UV–vis–NIR when excited by various light sources. When excited by 280 nm light, they emitted white light. When excited by 785 nm, they revealed fluorescence ranging around 800–1100 nm, which is the so-called NIR-I region for deep tissue imaging. A band model was proposed to describe the electron transitions responsible for the observed components of emission, which happen between energy levels created by shallow donor/acceptor defects. Their ability for multicolor fluorescence imaging of the biotissues was examined by using pig-kidney samples. In future applications, these novel silver- or samarium-based compounds can be expected to play a vital role in wide spectral range biotissue fluorescence imaging. Future works can be focused on animal study, toxicity, biocompatibility, and nanoparticle morphology versus fluorescence properties.

Experimental Section

Preparing of Organic Solutions

All the chemicals are purchased from Alfa Aesar. Compound 1 was prepared by mixing d-(+)-glucose (5 g), 2, 5-dihydroxyterephthalic acid (0.5 g), fumaric acid (5 g), terephtalimidazole (5 g), 2-methylimidazole (2 g), and oxalic acid dehydrate (5 g). Solution 1 was prepared by mixing diethyleneglycol (40 mL), oleic acid (40 mL), triethylamine (40 mL), dimethyl sulfoxide (20 mL), methacrylic anhydride (10 mL), and N,N-dimethylformamide (300 mL). Solution 2 was prepared by mixing l-cysteine (2 g) with deionized water (50 mL). Compound 1 was dissolved in solution 1 to get a yellow solution, which was mixed with solution 2. Then, the acquired solution was stirred for 5 h to acquire solution 3.

Preparing of Ag+:MgS

Magnesium chloride hexahydrate (0.1 g), silver nitrate (0.4 g), and sodium hydroxide (0.016 g) were dissolved in deionized water (10 mL) to get solution 4. Solution 4 and solution 3 (40 mL) were mixed and heated at 120 °C for 2 h. The acquired solution was ultrasonically processed for 9 h and then centrifuged with a speed of 14,000 rpm for 0.5 h to get the nanoparticles of Ag+:MgS.

Preparing of Ag+:Nb2S5

Niobium(V)chloride (9.32 g) was added to sulfuric acid (40 mL) to get solution 5. Then, deionized water (150 mL) was added into solution 5 drop by drop to get solution 6. Silver nitrate (0.4 g) and sodium hydroxide (0.016 g) was dissolved in deionized water (10 mL) to get solution 7. Solution 3(40 mL), solution 6 (200 μL), and solution 7 were mixed and heated at 120 °C for 2 h. The acquired solution was ultrasonically processed for 9 h and then centrifuged with a speed of 14,000 rpm for 0.5 h to get the nanoparticles of Ag+:Nb2S5.

Preparing of Sm3+:Y2S3

Yttrium nitrate hexahydrate (0.1 g), sodium hydroxide (0.016 g), and samarium(III) nitrate hexahydrate (0.4 g) were dissolved in deionized water (5 mL) to get solution 10; solution 10 was dissolved in solution 3 (40 mL) to get solution 11. Solution 11 was heated at a temperature of 180 °C for 2 h to get dark red solution 12. Solution 12 was centrifuged with a speed of 10,000 rpm for 0.5 h. The top layer of solution 12 was put into one centrifuge tube and then centrifuged with a speed of 14,000 rpm for 0.5 h to get the final nanoparticles of Sm3+:Y2S3.

Preparing of Sm3+:Er2S3

Erbium(III) nitrate pentahydrate (0.1 g), sodium hydroxide (0.016 g), and samarium(III) nitrate hexahydrate (0.4 g) were dissolved in deionized water (5 mL) to obtain solution 13, which was dissolved in solution 3 (40 mL) to get solution 14. Solution 14 was heated at a temperature of 180 °C for 2 h to get dark yellow solution 15. Solution 15 was centrifuged with a speed of 10,000 rpm for 0.5 h. The top layer of solution 15 was put into one centrifuge tube and then centrifuged with a speed of 14,000 rpm for 0.5 h to get the final nanoparticles Sm3+:Er2S3.

Preparing of Sm3+:ZrS2

Zirconium tetrachloride (0.1 g), sodium hydroxide (0.016 g), and samarium(III) nitrate hexahydrate (0.4 g) were dissolved in deionized water (5 mL) to get solution 16. Solution 16 was dissolved in solution 3 (40 mL) to get solution 17. Solution 17 was heated at a temperature of 180 °C for 2 h to get dark red solution 18. Solution 18 was centrifuged with a speed of 10,000 rpm for 0.5 h. After that, the top layer of solution 18 was centrifuged with a speed of 14,000 rpm to get the final nanoparticles of Sm3+:ZrS2.

Instruments

A K-Alpha XPS instrument (ThermoScientific, Inc., Waltham, Massachusetts, MA, USA) was employed to collect the XPS data. Powder XRD analysis was performed on a Panalytical Empyrean XRD instrument. TEM characterization was conducted through a JEOL1400 TEM (120 kV). The optical absorption data were collected through a Spark spectrometer. A Zeiss Lumar fluorescence microscope with UV-light (359–371 nm), blue-light (450–490 nm), and green-light (540–552 nm) excitation was used to acquire fluorescence images of the samples in the UV–vis range. NIR fluorescence spectra analysis was analyzed through an optical spectrometer (Henggong Instrument Inc.) combined with a 785 nm laser diode and an NIR long-pass optical filter. An NIR fluorescence imaging system was built by using a 785 nm laser diode, an NIR long-pass optical filter, an NIR camera, and a platform that can be moved in the X-, Y-, and Z-directions. A pig kidney bought from a local shop was cut into many pieces for the biotissue imaging. The dimension of every piece was set to around 3 × 4 × 3 mm3. One drop of the synthesized solution that contained the nanoparticles was coated onto the surface of the pig-kidney pieces. Then, after waiting for around 3–5 min, the pig-kidney samples were taken for the fluorescence imaging experiments.