Luminescence enhancement of CaF₂:Nd³⁺ nanoparticles in the second near-infrared window for in vivo imaging through Y³⁺ doping†

Zhen-feng Yu ^ab, Jun-peng Shi ^a, Jin-lei Li ^ab, Peng-hui Li ^ab and Hong-wu Zhang *^a
aKey Lab of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy of Sciences, 1799 Jimei Road, Xiamen, 361021, China
^bCollege of Resources and Environment, University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China. E-mail: hwzhang@iue.ac.cn

First published on 23rd January 2018

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1. Introduction

Near infrared (NIR) luminescent nanoparticles have been widely considered for bioimaging owing to their outstanding properties, such as high contrast, deep tissue penetration, independent fluorescence spectra and low toxicity.^1–5 Up until now, a series of upconversion nanoparticles, namely, III–IV quantum dots (QDs) have been developed and applied to in vivo and in vitro imaging. These NIR nanoparticles have shown great potential for in vivo and in vitro diagnosis, photodynamic therapy, traceable drug carrier and so on, which thus has attracted much attention in the medical field. However, there were still many shortcomings and restrictions for practical application of these NIR nanoparticles. Among the challenges, low luminescence efficiency of upconversion nanoparticles and low tissue penetration of III–IV quantum dots have limited their further medical application.

To solve these problems, novel NIR nanoparticles that can emit highly efficient and deeply penetrating NIR light in the second biological window (1000–1400 nm, NIR-II) excited by NIR light in the first biological window (650–1000 nm, NIR-I) have attracted increasing attention. Some NIR-II nanoparticles have been widely applied to in vivo imaging. Dai et al. first utilized carbon nanotubes to realize NIR-II luminescent in vivo imaging, which showed excellent bioimaging perforce.⁶ Wang et al. have developed high efficiency NIR-II Ag₂S nanoprobes to realize in vivo anatomical imaging and early stage tumor diagnosis.⁷ Zou and Dai have found that the pDA-PEG polymer can be regarded as an NIR-II fluorophore with high efficiency.⁸ Subsequently, a series of NIR-II probes, such as PbSe, PbS, Ag₂Se, QDs, and Cu₃BiS₃ nanorods, have been developed as in vivo imaging nanoprobes.^9–12 However, until now, there are relatively few NIR-II nanoparticles reported as novel imaging agents, which hinders their further application for in vivo imaging. Nd³⁺ is well-suited for NIR-II in vivo imaging because Nd³⁺ ions can strongly absorb NIR-I light at 748 and 808 nm and then release strong NIR-II emissions centered at 1056 and 1328 nm. The absorption is attributed to a transition from the ground state ⁴I_9/2 to the excited state (⁴F_7/2, ²S_3/2) and (⁴F_5/2, ²H_9/2) and the emission corresponds to the excited states of ⁴F_3/2 to ⁴I_11/2 and ⁴I_13/2.¹³ It is well known that Nd³⁺ doped alkaline earth fluoride (CaF₂, SrF₂) crystals have been widely developed as efficient 1064 nm laser materials because of their excellent properties, such as high transmittance in the far UV to mid IR range, low refractive index, high chemical resistance and high laser damage threshold.¹⁴ Villa et al. have utilized NIR-II emission of SrF₂:Nd³⁺ NPs to realize in vivo imaging with high sensitivity under the irradiation of an 808 nm laser.¹⁵ However, the easy aggregation of Nd³⁺ in alkaline earth fluoride resulted in severe quenching of Nd³⁺ luminescence, which inevitably lowered the in vivo imaging sensitivity.¹⁶ Thus, enhancing the dispersion of Nd³⁺ is important for the enhancement of NIR-II emission efficiency of Nd³⁺ doped alkaline earth fluoride nanoparticles.

In this study (as shown in Scheme 1), we prepared water-soluble NIR-II luminescent CaF₂:Nd³⁺ NPs using a facile hydrothermal method.¹⁷ In order to enhance the NIR-II luminescence efficiency, Y³⁺ was introduced into CaF₂:Nd³⁺ NPs to break the structure of Nd³⁺–Nd³⁺ clusters and form Nd³⁺–Y³⁺ clusters that isolate Nd³⁺ from each other, which can increase the number of free Nd³⁺ and then enhance the luminescence intensity of CaF₂:Y³⁺,Nd³⁺ NPs. The effect of co-doped Y³⁺ concentration on the crystal structure and the NIR-II emission of Nd³⁺ has been investigated in detail to determine the optimum NIR-II luminescent nanoparticles.¹⁸ In addition, under irradiation from an 808 nm laser, the Y³⁺, Nd³⁺ co-doped CaF₂ showed excellent NIR-II emission with luminescence efficiency reaching 9.30%. Furthermore, the CaF₂:Y³⁺,Nd³⁺ NPs possessed excellent tissue penetration ability due to their strong NIR-II emission. All of these results indicated that CaF₂:Y³⁺,Nd³⁺ NPs can be regarded as potential in vivo imaging probes for biological imaging.

2. Experimental

2.1 Materials

2.2 Characterization of CaF₂:Y³⁺,Nd³⁺ nanoparticles

To investigate the crystal structure of the as-obtained powder, X-ray diffraction (XRD) was performed in the range of 20° ≤ 2θ ≤ 80° using a Panalytical X’pert PRO. The diffraction patterns were acquired using Cu Kα radiation (λ = 1.5405 Å) and the scan rate was 6.0° min⁻¹. The instrument was operated with a 40 kV tube voltage and a 40 mA tube current. The morphology of the CaF₂:Y³⁺,Nd³⁺ NPs was measured with a transmission electron microscope (TEM, JEOL, JEM-2100, Japan). The accelerating voltage of the instrument was 100 kV. The hydrodynamic size was acquired using a Zetasizer Nano-ZS (Malvern, UK). The NIR-II luminescence was recorded using an Edinburgh FLS920 instrument with a 450 W xenon lamp or an 808 nm NIR diode laser (0–1 W power). All luminescent spectra were recorded with 1 nm resolution and the excitation spectrum was measured with 2 nm resolution. The date acquisition and analysis were handled by F-900 software of Edinburgh Analytical Instruments, UK. For the quantum yield (QY) measurements in powder form, a barium sulfate coated integrating sphere was used with a mounted spectrofluorometer sample chamber on the side, opposite to the excitation source. The diffuse fluorescence spectra from the nanoparticles and the laser profile were recorded with an Edinburgh FLS980 instrument exciting the sample with an 808 nm NIR diode laser (0–1 W power).

2.3 Toxicity assay

The typical MTT assay was used to evaluate the toxicity of the sample. First, the A549 cells were placed into a 96-well culture plate (Corning, USA) with 100 μL culture medium and then transferred to an incubator at 37 °C in 5% CO₂ atmosphere for 48 h. Following this, instead of primary culture medium, 100 μL culture with various concentrations of Ca_0.68Y_0.30Nd_0.02F_2.32 NPs (0, 100, 200, 300, 400, 500, 600, 700, 800 and 900 mg L⁻¹) were incubated with A549 cell in each well. Incubating for 12 h at 37 °C in 5% CO₂, 20 μL of the prepared MTT dye solution (0.05 mg mL⁻¹) was added to each well and cultured for 4 h under the same conditions. For this, the previous medium was removed carefully and formazan crystals were solubilized in 0.1 mL DMSO. Finally, to acquire the data, we measured the plate by a microplate reader at 574 nm (Molecular Devices, USA).

2.4 In vivo imaging

All animal experiments were performed in accordance with the guidelines of the National Regulation of China for Care and Use of Laboratory Animals.

The in vivo imaging was performed on a Du888 camera (400–1200 nm, Andor company) and the results were analyzed using the Andor Solis Imaging System. For all of the in vivo experiments, the concentrations of Ca_0.68Y_0.30Nd_0.02F_2.32 NPs and Ca_0.98F_2.02:Nd_0.02³⁺ NPs were 1 mg mL⁻¹ dispersed in phosphate buffered saline (PBS). First, 80 μL of two solutions were injected into the same mouse via subcutaneous injection and the excitation intensity of the 808 nm laser was maintained at 483 mW cm⁻². The exposure time was 3 s. In addition, a 125 μL Ca_0.68Y_0.30Nd_0.02F_2.32 NPs solution was injected into the mouse through the tail vein and the laser power was maintained at 483 mW cm⁻² for 30 s.

3. Results and discussion

3.1 Characterization of CaF₂:Y³⁺,Nd³⁺ NPs

In order to investigate the structure of the samples, their XRD patterns were measured as shown in Fig. 1. Most of the diffraction peaks of all the samples can be indexed to the standard reference pattern for CaF₂ cubic phase with the fluorite-type structure (space group Fm3m). When the content of Y³⁺ was more than 10%, the (200) diffraction peak appeared and its intensity clearly enhanced with an increase in the Y³⁺ content. Previous studies have shown that the appearance of the (200) diffraction peak is a signature of the RE incorporation into the CaF₂ host. This peak has been found herein in the XRD patterns of CaF₂:Yb³⁺ NPs and CaF₂:Er³⁺NPs.¹⁹ Thus, all of these results indicate that the incorporated Y³⁺ can dissolve in CaF₂ by occupying the Ca²⁺ sites to form a CaF₂–YF₃ solid solution. The previous studies have showed that CaF₂:Nd³⁺ NPs were not efficient NIR phosphors since the aggregation of Nd³⁺ in CaF₂ resulted in severe quenching of Nd³⁺ luminescence. Due to the similar ionic radii and valence of Y³⁺ (89 pm) and Nd³⁺ (99.5 pm), Nd³⁺ more easily enters the Y³⁺ sites than the Ca²⁺ sites. The excellent dispersion of Y³⁺ in CaF₂ would significantly decrease the aggregation of Nd³⁺ in CaF₂, which induces greater luminescence enhancement of Nd³⁺.

From the TEM images of the samples (Fig. 2(a) and Fig. S1, ESI†), it can be noted that the addition of Y³⁺ has little effect on the morphology and particle size of CaF₂, which all show a regular pavement-like shape with particle sizes of 10–15 nm. The HRTEM images show that the distance between the lattice fringes of Ca_0.98F_2.02:Nd_0.02³⁺ NPs is 2.81 Å (Fig. S1, ESI†), corresponding to the (111) plane of bulk crystalline CaF₂. The addition of Y³⁺ can induce the broadening of the interplanar spacing of the (111) plane, which can approach 3.32 Å when the content of Y³⁺ reaches 0.30 (Fig. 2(b)). This result indicates that the Y³⁺ readily enters into the lattice of CaF₂ and forms an excellent dispersed state. Furthermore, the elemental mapping (Fig. S2, ESI†) confirmed that Y³⁺ ions dispersed uniformly throughout the CaF₂. Since the emitting center of Nd³⁺ was prone to occupy the Y³⁺ sites in CaF₂, Nd³⁺ NPs would also be effectively distributed in Y³⁺-doped CaF₂ and would not easily aggregate, suggesting that the NIR luminescence of Nd³⁺ would be enhanced greatly through the co-doping of Y³⁺. The SAED pattern (Fig. 2(c)) demonstrates excellent crystallinity of Ca_0.68Y_0.30Nd_0.02F_2.32 NPs, which is consistent with the XRD results. Simultaneously, the as-prepared Ca_0.68Y_0.30Nd_0.02F_2.32 NPs show excellent water-solubility and can be easily dispersed into PBS solution to form a transparent sol (Fig. 2(e)) with a hydrated radius located at about 100 nm (Fig. 2(d)). Furthermore, the Ca_0.68Y_0.30Nd_0.02F_2.32 NPs solution emits strong NIR light excited by an 808 nm laser (Fig. 2(f)). All these results indicate that the as-prepared CaF₂:Y³⁺,Nd³⁺ NPs are good candidates for biological imaging.

3.2 NIR luminescent properties of CaF₂:Y³⁺,Nd³⁺ NPs

Fig. 3(a) shows the excitation spectra of CaF₂:Y³⁺,Nd³⁺ NPs under the NIR-II emission wavelength of 1064 nm. There are several strong NIR-I excitation peaks located at 730 nm, 808 nm, and 865 nm, which can be ascribed to ⁴I_9/2 → ⁴F_7/2, ⁴I_9/2 → ⁴F_5/2 and ⁴I_9/2 → ⁴F_3/2 transitions, respectively.²⁰ Simultaneously, under the excitation of 808 nm, CaF₂:Y³⁺,Nd³⁺ NPs showed strong NIR light emissions located at 989, 1056 and 1328 nm (Fig. 3(b)), corresponding to the ⁴F_3/2 → ⁴I_9/2, ⁴F_3/2 → ⁴I_11/2 and ⁴F_3/2 → ⁴I_13/2 intra-4f electronic transitions of Nd³⁺, respectively.^21–24 Since the excitation and emission wavelengths all belong to the biological first (650–1000 nm) and second (1000–1400 nm) windows, CaF₂:Y³⁺, Nd³⁺ NPs can be utilized to realize the deep tissue in vivo imaging. Furthermore, the addition of Y³⁺ had a visible effect on the emission spectra of the samples. Without the co-doped Y³⁺ (Fig. S3, ESI†), CaF₂:Nd³⁺ NPs showed the strongest emission peak at 989 nm (⁴F_3/2 → ⁴I_9/2). Following the addition of Y³⁺, the ⁴F_3/2 → ⁴I_11/2 transition of Nd³⁺ located at 1058 nm was the most intense (Fig. 3(b)). These results further confirmed that the emitting center Nd³⁺ would preferentially occupy the Y³⁺ sites over Ca²⁺ sites in CaF₂:Y³⁺, Nd³⁺ NPs. Since the NIR-II light possessed higher penetration depth than NIR-I light, CaF₂:Y³⁺, Nd³⁺ NPs were more suitable for in vivo imaging. Furthermore, from Fig. 3(c), it can be observed that the increase in Y³⁺ doping concentration can enhance the NIR emission intensity of the samples. When the Y³⁺ doping concentration reached 0.30, the luminescent intensity of Ca_0.68Y_0.30Nd_0.02F_2.32 NPs was about 8 times that of CaF₂:Nd³⁺ NPs. As reported in previous studies, the NIR-II emission of Nd³⁺ was easily quenched because of clustering of Nd³⁺ which started at even very small dopant concentrations.^16,25 Due to a cross-relaxation processes between neighbouring Nd³⁺, which quench the ⁴F_3/2 emitting level; this quenching greatly decreased the Nd³⁺ emission.²⁶ When Y³⁺ was co-doped into the CaF₂:Nd³⁺ NPs, Nd³⁺ also would enter into Y³⁺ sites and distribute well in the Y³⁺-doped CaF₂. In addition, the Nd³⁺–Nd³⁺ cluster will be broken and the amount of free Nd³⁺ can increase, which induces the large increase of NIR luminescent intensity. All of these results show that the CaF₂:Y³⁺, Nd³⁺ NPs were excellent candidates for biological imaging.

In order to enhance the in vivo imaging efficiency, usually high-energy NIR lasers are utilized to realize in vivo imaging. Thus, the NIR-II luminescent efficiency of CaF₂:Y³⁺,Nd³⁺ NPs excited by an 808 nm laser was investigated, as shown in Fig. 4. Similarly, the NIR luminescent intensity increased following the increase of Y³⁺ concentration under the excitation of an 808 nm NIR laser (500 mW cm⁻²). When the Y³⁺ concentration reached 0.30, the luminescence intensity of CaF₂:Y³⁺,Nd³⁺ NPs was about 65 times that of CaF₂:Nd³⁺ NPs (Fig. 4(a)). Simultaneously, under the irradiation of an 808 nm laser, the ⁴F_3/2 → ⁴I_11/2 transition of Nd³⁺ located at 1058 nm was the strongest and the emission peaks below 1000 nm is hardly observed, indicating that CaF₂:Y³⁺,Nd³⁺ NPs were good NIR-II luminescent probes. Moreover, the increased laser power clearly induced the enhancement of NIR-II luminescence intensity of Nd³⁺ (Fig. 4(b)). When the laser power reached 483 mW cm⁻², the NIR-II luminescence intensity of Ca_0.68Y_0.30Nd_0.02F_2.32 NPs was the strongest. Furthermore, the quantum yield of Ca_0.68Y_0.30Nd_0.02F_2.32 NPs reached 9.30% under the excitation of an 808 nm laser with 483 mW cm⁻² power (Fig. 4(c)), which was about 3 times higher than that of CaF₂:Nd³⁺ (3.10%). It is well known that in order to ensure bio-safety, the laser power applied in bioimaging must be lower than 600 mW cm⁻². Thus, the high efficiency NIR-II emissions of CaF₂:Y³⁺,Nd³⁺ NPs shows that they are suitable for in vivo imaging.

3.3 NIR-II luminescent in vivo imaging of Ca_0.68Y_0.30Nd_0.02F_2.32 NPs

The MTT assays were used to evaluate the toxicity of Ca_0.68Y_0.30Nd_0.02F_2.32 NPs as shown in Fig. 5. It can be observed that after incubation with Ca_0.68Y_0.30Nd_0.02F_2.32 NPs, the cell viability did not decrease, but gradually increased following the increase of the sample concentration. When the incubation concentration of Ca_0.68Y_0.30Nd_0.02F_2.32 NPs reached 900 mg L⁻¹, the cell viability increased to 150%. This phenomenon may be induced by the existence of abundant calcium citrate on the surface of the nanoparticles,²⁷ which provides nourishment that promotes cell growth. These toxicity results show that Ca_0.68Y_0.30Nd_0.02F_2.32 NPs possess excellent biocompatibility and can thus be used for in vivo imaging.

In order to investigate the effect of co-coped Y³⁺ on the in vivo imaging performance of the samples, two 80 μL Ca_0.98F_2.02:Nd³⁺_0.02 NPs and Ca_0.68Y_0.30Nd_0.02F_2.32 NPs solutions (1 mg mL⁻¹) were injected into the mouse by subcutaneous injection, and labeled as ROI₁ and ROI₂. Under the irradiation exposure of an 808 nm laser (483 mW cm⁻²), the in vivo imaging intensity of Ca_0.68Y_0.30Nd_0.02F_2.32 NPs was about 2.38 times stronger than that of Ca_0.98F_2.02:Nd_0.02³⁺ NPs (Fig. 6(a)), which is consistent with the luminescence results. This result confirms that the co-doped Y³⁺ can enhance the in vivo imaging efficiency. In order to investigate the in vivo imaging penetration ability of Ca_0.68Y_0.30Nd_0.02F_2.32 NPs, a 125 μL Ca_0.68Y_0.30Nd_0.02F_2.32 NPs solution (1 mg mL⁻¹) was injected into the mouse tail via intravenous injection (Fig. 6(b)). Then, the mouse was totally exposed to the 808 nm laser illumination(483 mW cm⁻²) and the in vivo imaging were recorded using an Andor 888 EM CCD camera. It can be observed that there were strong imaging signals of the liver because most of the Ca_0.68Y_0.30Nd_0.02F_2.32 NPs were accumulated in the liver via the RES system.²⁸ This suggests that Ca_0.68Y_0.30Nd_0.02F_2.32 NPs possess excellent tissue penetration ability due to their strong NIR-II emission. All these results indicate that Ca_0.68Y_0.30Nd_0.02F_2.32 NPs can be regarded as potential candidates for in vivo imaging probes used in biological imaging.

4. Conclusions

In this study, we prepared approximately 10–15 nm-sized NIR-II luminescent CaF₂:Nd³⁺ NPs using a facile hydrothermal method. In order to enhance the NIR-II luminescence efficiency, the Y³⁺ ions were introduced into CaF₂:Nd³⁺ NPs to break the Nd³⁺–Nd³⁺ clusters. The XRD results showed that when co-doped Y³⁺ concentration was more than 10%, the (200) diffraction peak appeared and its intensity enhanced significantly following the increase in co-doped Y³⁺ content, which indicates the formation of CaF₂–YF₃ solid solution. Furthermore, the broadening of the interplanar spacing of the (111) plane also confirms this result. The luminescence intensity data showed that under the excitation of an 808 nm light, CaF₂:Y³⁺,Nd³⁺ NPs show strong NIR light emissions located at 989, 1056 and 1328 nm (Fig. 3(b)), corresponding to the ⁴F_3/2 → ⁴I_9/2, ⁴F_3/2 → ⁴I_11/2 and ⁴F_3/2 → ⁴I_13/2 intra-4f electronic transitions of Nd³⁺, respectively. Furthermore, when the Y³⁺ doped concentration reached 0.30, the luminescence intensity of CaF₂:Y³⁺,Nd³⁺ NPs was about 65 times that of CaF₂:Nd³⁺ NPs. Moreover, the quantum yield of Ca_0.68Y_0.30Nd_0.02F_2.32 NPs approached 9.30% under the excitation of the 808 nm laser at 483 mW cm⁻², which was about 3 times higher than that of CaF₂:Nd³⁺ NPs (3.10%). The MTT assay results indicate that the Ca_0.68Y_0.30Nd_0.02F_2.32 NPs possess excellent biocompatibility. Furthermore, under the irradiation exposure of an 808 nm laser (483 mW cm⁻²), the in vivo imaging intensity of Ca_0.68Y_0.30Nd_0.02F_2.32 NPs was about 2.38 times stronger than that of Ca_0.98F_2.02: Nd³⁺_0.02 NPs. In addition, the signals of liver imaging can also be clearly observed, suggesting that Ca_0.68Y_0.30Nd_0.02F_2.32 NPs possessed excellent tissue penetration due to their strong NIR-II emission. Therefore, the CaF₂:Y³⁺,Nd³⁺ NPs are very attractive infrared nanomaterials for deep tissue, high resolution in vivo imaging in the second biological window.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work has been financially supported by Bureau of International Cooperation, the Chinese Academy of Sciences (132C35KYSB20160021), the National Natural Science Foundation of China (21507129), the Program of International Cooperation of Xiamen (3502Z20161248), the National Natural Science Foundation of China (61705228), and the Shuang Bai Plan of Fujian Province.

References

1. T. Troy, D. Jekic-Mcmullen, L. Sambucetti and B. Rice, Mol. Imaging, 2004, 3, 9 CrossRef CAS PubMed .
2. J. C. Zhou, Z. L. Yang, W. Dong, R. J. Tang, L. D. Sun and C. H. Yan, Biomaterials, 2011, 32, 9059–9067 CrossRef CAS PubMed .
3. B. Fadeel and A. E. Garciabennett, Adv. Drug Delivery Rev., 2010, 62, 362 CrossRef CAS PubMed .
4. A. Gautam and F. C. J. M. V. Veggel, J. Mater. Chem. B, 2013, 1, 5186–5200 RSC .
5. F. Boschi, S. L. Meo, P. L. Rossi, R. Calandrino, A. Sbarbati and A. E. Spinelli, J. Biomed. Opt., 2011, 16, 126011 CrossRef PubMed .
6. N. W. Kam, M. O'Connell, J. A. Wisdom and H. Dai, Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 11600 CrossRef CAS PubMed .
7. G. Hong, J. T. Robinson, Y. Zhang, S. Diao, A. L. Antaris, Q. Wang and H. Dai, Angew. Chem., 2012, 124, 9818–9821 CrossRef PubMed .
8. G. Hong, Y. Zou, A. L. Antaris, S. Diao, D. Wu, K. Cheng, X. Zhang, C. Chen, B. Liu and Y. He, Nat. Commun., 2014, 5, 4206 CAS .
9. U. Aeberhard, R. Vaxenburg, E. Lifshitz and S. Tomić, Phys. Chem. Chem. Phys., 2012, 14, 16223–16228 RSC .
10. J. Cao, H. Zhu, D. Deng, B. Xue, L. Tang, D. Mahounga, Z. Qian and Y. Gu, J. Biomed. Mater. Res., Part A, 2012, 100, 958–968 CrossRef PubMed .
11. B. Dong, C. Li, G. Chen, Y. Zhang, Y. Zhang, M. Deng and Q. Wang, Chem. Mater., 2013, 25, 2503–2509 CrossRef CAS .
12. J. Du, X. Zheng, Y. Yong, J. Yu, X. Dong, C. Zhang, R. Zhou, B. Li, L. Yan and C. Chen, Nanoscale, 2017, 9, 8229–8239 RSC .
13. J. B. Gruber, G. W. Burdick, N. T. Woodward, V. Dierolf, S. Chandra and D. K. Sardar, J. Appl. Phys., 2011, 110, 043109 CrossRef .
14. V. Petit, J. L. Doualan, P. Camy, V. Ménard and R. Moncorgé, Appl. Phys. B: Lasers Opt., 2004, 78, 681–684 CrossRef CAS .
15. I. Villa, A. Vedda, I. X. Cantarelli, M. Pedroni, F. Piccinelli, M. Bettinelli, A. Speghini, M. Quintanilla, F. Vetrone and U. Rocha, Nano Res., 2015, 8, 649–665 CrossRef CAS .
16. J. A. Caird, L. K. Smith, L. L. Chase, N. D. Nielsen, S. A. Payne and W. F. Krupke, J. Opt. Soc. Am. B, 1991, 8, 726–740 CrossRef .
17. M. Pedroni, F. Piccinelli, T. Passuello, S. Polizzi, J. Ueda, P. Harogonzález, L. M. Maestro, D. Jaque, J. Garcíasolé and M. Bettinelli, Cryst. Growth Des., 2013, 13, 4906–4913 CAS .
18. Z. P. Qin, G. Q. Xie, J. Ma, W. Y. Ge, P. Yuan, L. J. Qian, L. B. Su, D. P. Jiang, F. K. Ma and Q. Zhang, Opt. Lett., 2014, 39, 1737 CrossRef CAS PubMed .
19. A. Bensalah, M. Mortier, G. Patriarche, P. Gredin and D. Vivien, J. Solid State Chem., 2006, 179, 2636–2644 CrossRef CAS .
20. Y. F. Wang, G. Y. Liu, L. D. Sun, J. W. Xiao, J. C. Zhou and C. H. Yan, ACS Nano, 2013, 7, 7200 CrossRef CAS PubMed .
21. B. Henderson and G. F. Imbusch, Optical spectroscopy of inorganic solids, Clarendon Press, 1989 Search PubMed .
22. R. Balda, J. Fernandez, A. Mendioroz, J. L. Adams and B. Boulard, J. Phys.: Condens. Matter, 1999, 6, 913–924 CrossRef .
23. E. O. Serqueira, N. O. Dantas, A. F. G. Monte and M. J. V. Bell, J. Non-Cryst. Solids, 2006, 352, 3628–3632 CrossRef CAS .
24. E. D. L. Rosa-Cruz, G. A. Kumar, L. A. Diaz-Torres, A. Martínez and O. Barbosa-García, Opt. Mater., 2001, 18, 321–329 Search PubMed .
25. Y. K. Voron'Ko, V. V. Osiko, A. M. Prokhorov and I. A. Shcherbakov, J. Exp. Theor. Phys., 1971, 33, 37–55 Search PubMed .
26. H. G. Danielmeyer, M. Blätte and P. Balmer, Appl. Phys., 1973, 1, 269–274 CAS .
27. W. Zhang, W. Wang, Q. Y. Chen, Z. Q. Lin, S. W. Cheng, D. Q. Kou, X. Z. Ying, Y. Shen, X. J. Cheng and P. F. Nie, Asian Pac. J. Trop. Med., 2012, 5, 310–314 CrossRef CAS PubMed .
28. R. Kumar, I. Roy, T. Y. Ohulchanskky, L. A. Vathy, E. J. Bergey, M. Sajjad and P. N. Prasad, ACS Nano, 2010, 4, 699 CrossRef CAS PubMed .

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7tb03052e

This journal is © The Royal Society of Chemistry 2018