Upconversion luminescence nanoprobe based on luminescence resonance energy transfer from NaYF₄:Yb, Tm to Ag nanodisks

Abstract

In this work, we reported a luminescence probe based on luminescence resonance energy transfer (LRET), in which the positively charged upconversion nanoparticles (UCNPs) were used as the energy donors and the negatively charged Ag nanodisks (Ag NPs) as the energy acceptors. They bound together through electrostatic interaction, and luminescence quenching was observed due to the LRET from UCNPs to Ag NPs. However, the luminescence intensities recovered with the addition of cysteine (Cys). This was due to the formation of S–Ag bonds, which caused the inhibition of the LRET process. Under the optimized conditions, the relationship between the luminescence intensity and the concentration of cysteine were linear in the range from 50 to 2000 μM. The proposed LRET system also showed high selectivity to Cys and would also be used to detect other biothiols. Thus, the as-prepared probe possesses potential applications in the detection of biothiols.

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1. Introduction

In recent years, luminescence resonance energy transfer (LRET) has been used as a powerful tool widely applied in detection,¹ biosensing,² bioimaging³ and immunoassay.⁴ LRET is an non-radiative process where an excited state donor transfers its energy to a proximal ground state acceptor.⁵ The spectral overlap and the distance between the donor and the acceptor are the two major factors that determine the LRET efficiency.⁶ To date, a series of nanoparticles have been used in a LRET system including semiconductor quantum dots (QDs),⁷ graphene quantum dots (GO),⁸ carbon dots,⁹ upconversion nanoparticles (UCNPs)¹⁰ and so on.

Among them, UCNPs have received great attention because of the excellent optical properties in recent years.^11–13 Under 980 near-infrared (NIR) laser diode excitation, UCNPs have high signal-to-noise ratio, narrow emission peaks, good chemical and thermal stability, and low toxicity.^14,15 Owing to their unsurpassed properties, UCNPs are generally used as energy donors to form a nanoprobe. As far as now, organic dyes and inorganic materials have been successfully employed as energy acceptors, such as carbon nanoparticles,¹⁶ rose bengal photosensitizer,⁵ Fe³⁺,¹⁷ europium complex¹⁸ and so on. Nevertheless, organic molecules as energy acceptors always fall short on energy-transfer efficiency. Among the inorganic materials, noble-metals nanoparticles were employed as good quenching effect acceptors due to the strong and specific absorption which was overlapped with the emissions of the UCNPs. Therefore, UCNPs bound with noble-metals nanoparticles have been used as donor–acceptor pairs to build LRET systems, which has increased researchers' attention. Wu and coworkers presented a system in which two color UCNPs were employed as the donors, and controlled gold nanoparticles as the acceptors for the simultaneous detection of Pb²⁺ and Hg²⁺. They found that the relative luminescence intensities increased as the metal ion concentrations were increased, allowing for the quantification of Pb²⁺ and Hg²⁺.¹⁹ Yan et al. reported a LRET probe for detecting biothiols, which was constructed by decorating UCNPs with dithiol-stabilized Ag nanoparticles.²⁰

However, to the best of our knowledge, less studies focused on developing UCNPs-Ag nanoparticles as donor–acceptor for detection are reported. Compared with Au nanoparticles, Ag nanoparticles are more electrical and thermal conductivity, making it an ideal component for electrical interconnection. Besides, Ag nanoparticles are economical for their excellent qualities in terms of plasmonic ability, available nanostructures, and material cost.²¹

In this work, we have developed a nanoprobe composed of UCNPs and Ag NPs, in which UCNPs and Ag NPs were respectively as the energy donors and acceptors. Polyetherimide (PEI) stabilized NaYF₄:Yb, Tm UCNPs were synthesized through hydrothermal method. The blue emission peak of UCNPs located at the 478 nm was the strongest among all peaks, which was good overlapped with the absorption spectrum of L-ascorbic acid stabilized Ag NPs. Thus, the emission at 478 nm was quenched due to the LRET from UCNPs to Ag NPs. The detection of Cys based on the LRET system was achieved by the recovery of the luminescence intensities (emission at 478 nm). Therefore, the nanoprobe shows the great potential for biothiols detection in biological and analytical fields.

2. Experimental

2.1 Materials and instrumentations

Yttrium nitrate, ytterbium nitrate, thulium nitrate and polyethyleneimine (PEI) were purchased from Xiya Chemical Industry Co Ltd Shandong. AgNO₃ was purchased from Guangdong Guanghua Sci-Tech Co., Ltd. L-ascorbic acid (ASC) was purchased from Tianjin Tianli Sci-Tech Co.,Ltd. All reagents were of analytical grade and used without further purification. And double distilled water was used throughout the experiments.

2.2 Synthesis of PEI modified NaYF₄:Yb, Tm nanoparticles

NaYF₄:Yb, Tm nanoparticles were synthesized by one-step hydrothermal process. Briefly, a total of 2 mmol of RE(NO₃)₃ (Y/Yb/Tm = 0.78 : 0.2 : 0.02, mole-to-mole ratio) was mixed in deionized water. Then aqueous NaNO₃ (5 mL, 0.4 M), 25 mL diethylene glycol (DEG), aqueous solution of PEI (10 mL, 10 wt%) were added one by one. After magnetic stirring about 30 min, 6 mmol NaBF₄ was added in the solution for another 1 h. Then the mixture was transferred into a 100 mL Teflon autoclave and heated at 180 °C for 5 h. As the Teflon autoclave was cooled down naturally, the products were precipitated and washed with ethanol and water several times. Finally, the obtained UCNPs were redispersed into PBS solutions (pH = 6.864) for the next process.

2.3 Synthesis of ASC modified Ag nanodisks

The silver nanodisks were synthesized according to the Xia's method.²² Firstly, 33 mL of an aqueous solution containing 0.11 mM AgNO₃ and 2.05 mM trisodium citrate was prepared. Under stirring, aqueous NaBH₄ (fisher, 0.6 mL, 5 mM) was added dropwise. After stirring 10 min, the seed solution was stored at 4 °C for 9 hours. Secondly, 100 mL double distilled water was mixed with aqueous AgNO₃ (2.5 mL, 5 mM), aqueous PVP (7.5 mL, 0.7 mM), aqueous sodium citrate (7.5 mL, 30 mM), and 2 mL of seed solution, followed by slow dropping into aqueous L-ascorbic acid (62.5 mL, 1 mM). After about 30 min, purple silver nanodisks solution was obtained.

2.4 Detection of Cys in aqueous solutions

Various amount of Ag NPs (0.068 mM) were added to the same UCNPs solution (10 mL, 0.015 mM), which were then incubated for 30 min at room temperature, the emission spectra were recorded. Afterwards, 20 mL of different concentrations of Cys solutions were added and reacted for another 30 min, the luminescence spectra were recorded as well.

2.5 Characterizations

Transmission electron microscopy (TEM) images were taken with the Hitachi H-800 electron microscope at an acceleration voltage of 200 kV with a CCD camera. Scanning electron microscopy (SEM) images were taken with the JEOL JSM-6390A system. The crystalline phases of UCNPs were characterized by a Shimadzu XRD-6000 powder diffractometer. The upconversion emission spectra of UCNPs were recorded using an F-7000 luminescence spectrometer (Hitachi, Japan) equipped with a 980 nm laser diode as the excitation source. The Shimadzu UV-3600 UV/Vis/NIR spectrophotometer was used to obtain the UV-visible absorption spectra. The FT-IR spectra were recorded by BRUKER VERTEX70 FT-IR Spectrometer in a range of 4000–500 cm⁻¹ using KBr pellets.

3. Results and discussion

3.1 Morphology and structure of NaYF₄:Yb,Tm UCNPs and Ag NPs

The NaYF₄:Yb, Tm UCNPs in this study were synthesized via hydrothermal method, and the experimental results revealed that the high quality UCNPs were obtained. The morphology and structure of NaYF₄:Yb, Tm UCNPs were characterized by SEM and TEM firstly. As we can see from Fig. 1a–c, UCNPs were quasi-spherical, narrow-distribution and had a smooth surface. And it could be observed that the average diameter of the as-prepared UCNPs was approximately 58 nm (Fig. 1b), and the typical high-resolution TEM (HR-TEM) exhibited legible lattice distances, corresponding to the (111) lattice planes of cubic phase NaYF₄:Yb, Tm (JCPDs no. 06-0342). In addition, the crystal structures and the phase purity of the as-prepared NaYF₄:Yb, Tm UCNPs were identified by XRD. Fig. 1d shows the XRD patterns of UCNPs agreed well with the values of α-NaYF₄ (JCPDs: 06-0342) and no other diffraction peaks of impurities had been detected, confirming the HR-TEM results. These results indicated that the as-prepared UCNPs had good crystalline.

Fig. 1 (a) SEM image, (b) size distribution histogram, (c) TEM and HR-TEM images, (d) XRD patterns of PEI–NaYF₄:Yb, Tm UCNPs.

Fig. 2a and b showed the typical TEM and HR-TEM images of the Ag nanodisks. It was clear that the diameter was approximately 10 nm, and lattice fringes with a spacing of 0.25 nm, corresponding to the (111) lattice plane of the Ag nanodisks, which could be further confirmed by the XRD patterns of Ag NPs (Fig. 2c).

Fig. 2 (a) TEM image, (b) HR-TEM image, (c) XRD patterns of Ag NPs.

The surface modification of the UCNPs and Ag NPs were critical for the further construction of UCNPs–Ag NPs LRET nanoprobe. In this study, the UCNPs were coated with PEI, which made the UCNPs positively charged. Meanwhile, negatively charged Ag NPs were obtained by coating with L-ascorbic acid. Thus, the UCNPs and Ag NPs could be assembled by electrostatic interaction. The presence of PEI and L-ascorbic acid on the surface of UCNPs and Ag NPs were verified by FT-IR spectroscopy, respectively (Fig. 3).

Fig. 3 FT-IR spectra of (a) PEI–NaYF₄:Yb, Tm UCNPs, (b) L-ascorbic acid–Ag NPs.

As shown in Fig. 3a, the absorption peaks of methylene asymmetric and symmetric C–H stretching (3128 and 2985 cm⁻¹), bending and stretching vibrations of the N–H bands (3436 and 1635 cm⁻¹), methylene scissoring (1400 cm⁻¹), and the stretching vibrations of the C–N band (1083 cm⁻¹) were observed, proving the successful coating of PEI on the surface of UCNPs. In Fig. 3b, the absorption peaks of stretching vibration of C–H band (3157 cm⁻¹), methylene scissoring (1400 cm⁻¹), and the stretching vibrations of the C–N band (1290 cm⁻¹), meanwhile, the stretching vibration of O–H band (3450 cm⁻¹) and C=O (1660 cm⁻¹), indicating that PVP and ASC were coated on the surface of Ag NPs.

3.2 The optical properties of UCNPs and Ag NPs

In order to construct the effective LRET systems, the absorption spectrum of the Ag NPs (acceptors) should overlap with the luminescence spectrum of the UCNPs (donors). Thus, we had investigated varies of factors that influenced the optical properties of Ag NPs and UCNPs.

We firstly optimized the absorption spectrum of Ag NPs. The Ag nanoparticles could be synthesized by using a series of approaches, and the UV absorption spectra of Ag nanoparticles could be tuned by using different reducing agents as shown in Fig. 4. The results indicated that the absorption peaks of Ag nanoparticles were around at 400–420 nm with trisodium citrate (TSC) and NaBH₄ as reductants, which was not overlapped with the emission peaks (478 nm) of UCNPs. However, in the presence of L-ascorbic acid, three characteristic peaks of Ag nanodisks were observed in the UV absorbance spectrum, the sharp peak at around 330 nm could be attributed to the out-of-plane quadrupole plasmon resonance. The intense peak at around 500 nm could be assigned to in-plane dipole resonance, and the shoulder peak at around 420 nm could be attributed to the in-plane quadrupole resonance.²³ Thus, the emission spectrum of UCNPs could overlap with the absorption spectrum of Ag NPs, leading to the effective LRET process. Therefore, we prepared the Ag NPs by using the L-ascorbic acid as reductant.

Fig. 4 UV absorption spectra of Ag NPs using different reducing agents.

Secondly, we investigated the four factors that influenced the Ag NPs' absorption peaks (Fig. 5), such as the role of seed solution, poly(vinyl pyrrolidone) (PVP), trisodium citrate (TSC), and L-ascorbic acid (ASC). The seed solution possibly assisted to the formation of plate-like structure. When the volume of seed solution (0.11 mM) varied from 0 to 2 mL, a sharp peak around 510 nm appeared along with a weak shoulder at around 420 nm (Fig. 5a), suggesting the formation of Ag nanodisks.

Fig. 5 UV absorption spectra of Ag NPs prepared by controlling different (a) volumes of seed solution, molar ratios of Ag to (b) PVP, (c) TSC, and (d) ASC.

PVP has been widely used to improve the stability of silver nanoparticles, assisting the synthesis of silver nanodisks.^24–26 As can be seen in Fig. 5b, the UV absorption spectra of Ag NPs indicated that the PVP in solution were favorable for stabilizing Ag NPs and preventing aggregation.

Serving as a shape-directing agent and stabilizer, citrate has been widely used in the preparation of noble-metals nanoparticles, particularly Au and Ag.^27,28 It could be selectively and preferentially bind to (111) facets, which may be the basis of the faster two-dimensional growth at the edges.²⁹ In other words, (100) facets growed faster than (111) facets that led to the formation of Ag nanodisks (Fig. 5c).

The silver ions in solution were reduced by the ASC to become silver atoms. With the addition of ASC, the reactions produced silver nanodisks which displayed three characteristic peaks in the spectra (Fig. 5d). Besides as stabilizer, the other role of ASC was capping agent, which could stabilize the Ag NPs with carboxy group.

Thirdly, by controlling the molar ratio of Tm³⁺ and Yb³⁺ ions, the intensity ratio of different emissions could be modulated (Fig. 6). It was found in Fig. 6 that under 980 nm laser diode excitation, the products showed four upconversion emission peaks at 362, 478, 645, 800 nm, which could be assigned to ¹D₂ → ¹H₆, ¹G₄ → ³H₆, ¹G₄ → ³H₄, ³F₄ → ³H₆ transitions of Tm³⁺, respectively.³⁰ Notably, the blue upconversion emission peaks at 478 nm was dominatly strong, which was overlapped with absorption spectrum of Ag NPs.

Fig. 6 Luminescence emission spectra of NaYF₄:Yb, Tm UCNPs at different molar ratio (a) Tm³⁺ and (b) Yb³⁺ (all the emission intensities were normalized at the maximum of 478 nm emission intensity).

As shown in Fig. 6a, the luminescence intensities continuous decreased when the Tm³⁺ content increased from 0.2% to 0.8%, which was due to the increase of the probability of collision between Tm³⁺ increased, resulting in the luminescence quenching phenomenon and the decrease of luminescence intensities.³¹

The energy level scheme of Yb³⁺ is quite simple, and thus shows a certain tolerance to a high doping concentration. Before reaching the optimal doping concentration, the luminescence intensities of the UCNPs continued to increase with the increasing Yb³⁺ content, which could greatly increase the absorption probability of NIR photons (Fig. 6b). However, too high concentration would result in cross-relaxations between Yb³⁺–Yb³⁺ pairs, which led to the quenching of luminescence.³¹

3.3 Upconversion LRET process from UCNPs to Ag NPs

As Ag NPs were stabilized by L-ascorbic acid, which would form an negatively charged layer on the surface of Ag NPs and prevent nanoparticles from aggregating. On the contrary, the NaYF₄:Yb, Tm UCNPs capped by PEI were positively charged. Thus, the electrostatic interaction made it possible to connect Ag NPs with PEI–UCNPs, and reduced the distance between them, which would be the critical factor for LRET. The EDS analysis and TEM results had proved that the LRET nanoprobe composed of NaYF₄:Yb, Tm and Ag nanodisks (Fig. 7b and c). As we can see from Fig. 7a, Ag NPs showed a strong absorption band around 500 nm, which perfectly overlapped with the upconversion emission peaks of NaYF₄:Yb, Tm UCNPs located at 478 nm. Therefore, the LRET process between UCNPs and Ag NPs could occur, in other words, the luminescence of UCNPs could be quenched by Ag NPs.

Fig. 7 (a) Normalized upconversion luminescence spectrum of the NaYF₄:Yb, Tm UCNPs and UV absorption spectrum of Ag NPs, (b) EDS analysis, (c) TEM image and (d) HR-TEM image of UCNPs–Ag NPs LRET nanoprobe.

As we can see from Fig. 8a, the luminescence intensity of UCNPs had no changes in the absence of Ag NPs. However, the luminescence intensities gradually decreased with the increasing amount of Ag NPs from 340 μmol to 408 μmol, and the maximum quenching was observed at the 408 μmol, which closed to 88% (Fig. 8b). All the results provided evidence that LRET between NaYF₄:Yb, Tm to Ag NPs had been established successfully.

Fig. 8 (a) Luminescence emission spectra of UCNPs along with the increasing amount of Ag NPs (a–i: 0, 34, 68, 136, 204, 272, 34, 408 μmol), (b) relative luminescence intensities of UCNPs in the presence of varying quantity of Ag NPs (F/F₀, in which F and F₀ were the emission intensity of UCNPs at the 478 nm in the absence and presence of Ag NPs, respectively).

3.4 The detection of biothiol based on the LRET system

To test the performance of the UCNPs–Ag NPs LRET nanoprobe, cysteine (Cys) was chose as a model target in this work. Cys is an abundant biothiol in biological systems, it plays a vital role in reversible redox reactions inside cells, which have numerous biological functions in metabolism and detoxification, therefore, a sensitive and specific method for detecting Cys is of important significance.^32–35 As shown in Fig. 9a, the luminescence intensities of UCNPs–Ag NPs LRET nanoprobe were gradually increased with the increasing of concentrations of Cys. Cys could bind with Ag NPs and cause the aggregation of the Ag NPs, so the distance between the UCNPs and Ag NPs was too large, which will inhibit the LRET process (Scheme 1). As a result, upconversion emissions were recovered. The relative luminescence intensities of the UCNPs–Ag NPs LRET nanoprobe (F/F₀, in which F and F₀ were the emission intensities of UCNPs–Ag NPs LRET nanoprobe in the absence and presence of Cys, respectively) in varying concentrations of Cys were shown in Fig. 9b.

Fig. 9 (a) Luminescence spectra of UCNPs–Ag NPs LRET nanoprobe in the presence of different concentrations of Cys (a–i: 0, 50, 100, 200, 600, 1000, 1200, 1600, 2000 μM), (b) standard curve of the luminescence enhancement efficiency obtained for 50–2000 μM.

Scheme 1 Schematic representation of UCNPs–Ag NPs LRET nanoprobe for the detection of Cys.

In order to assess the selectivity of the LRET nanoprobe for biothiols, the potential interfering substances including BSA, L-aspartic acid, L-serine, L-lysine, glycine, L-arabinose, glucose, Na⁺, K⁺ were investigated at the same experimental conditions. Fig. 10 showed the results of interfering substances on the UCNPs' luminescence intensities at 478 nm, indicating that no obvious change was observed in the presence of these substances. These results demonstrated that the LRET system is of high selectivity towards biothiols.

Fig. 10 Relative luminescence intensities for the UCNPs–Ag NPs LRET nanoprobe in the presence of different substances (the concentration of each substances was 1000 μM).

4. Conclusions

In conclusion, we have developed a LRET-based nanoprobe, which was selective and sensitive for the detection of biothiols. The LRET nanoprobe was the assembly of Ag NPs and NaYF₄:Yb, Tm UCNPs through electrostatic interaction. In this system, PEI-coated NaYF₄:Yb, Tm UCNPs with a strong emission at 478 nm were used as the energy donors and Ag NPs as the energy acceptors. The LRET took place due to the spectral overlap between the absorbance spectrum of the Ag NPs and the emission spectrum of the UCNPs and their proximity, thus leading to the luminescence quenching of UCNPs. With the addition of Cys, LRET process was inhibited and then the luminescence intensities of UCNPs recovered. Moreover, the luminescence recovery was found to show a good linear to its concentration and could be used to detect Cys. Therefore, the as-prepared UCNPs–Ag NPs LRET nanoprobe in this study has the potential applications in the detection of biothiols.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NO. 51372201, 21476183, and 21306150), the Specialized Research Fund for the Doctoral Program of Higher Education of China (NO. 20136101110009), the Shaanxi Provincial Research Foundation for Basic Research of China (NO. 2015JM5159).

Notes and references

1. W. W. Hu, Q. S. Chen, H. H. Li, Q. Ouyang and J. W. Zhao, Biosens. Bioelectron., 2016, 80, 398–404.
2. J. Zhang, B. Li, L. M. Zhang and H. Jiang, Chem. Commun., 2012, 48, 4860–4862.
3. Y. Liu, M. Chen, T. Y. Cao, Y. Sun, C. Y. Li, Q. Liu, T. S. Yang, L. M. Yao, W. Feng and F. Y. Li, J. Am. Chem. Soc., 2013, 135, 9869–9876.
4. Y. H. Wang, P. Shen, C. Y. Li and Y. Y. Wang, Anal. Chem., 2012, 84, 1466–1473.
5. Y. Wang, K. Liu, X. M. Liu, K. i. Dohnalová, T. Gregorkiewicz, X. G. Kong, M. C. G. Aalders, W. J. Buma and H. Zhang, J. Phys. Chem. Lett., 2011, 2, 2083–2088.
6. J. Zhou, Q. Liu, W. Feng, Y. Sun and F. Y. Li, Chem. Rev., 2015, 115, 395–465.
7. Y. S. Xia, L. Song and C. Q. Zhu, Anal. Chem., 2011, 83, 1401–1407.
8. L. T. Kong, J. Wang, G. C. Zheng and J. H. Liu, Chem. Commun., 2011, 47, 10389–10391.
9. J. Tang, B. Kong, H. Wu, M. Xu, Y. C. Wang, Y. L. Wang, D. Y. Zhao and G. F. Zheng, Adv. Mater., 2013, 25, 6569–6574.
10. Y. Zhang, L. Q. Wu, Y. R. Tang, Y. Y. Su and Y. Lv, Anal. Methods, 2014, 6, 9073–9077.
11. Y. H. Hu, X. H. Liang, Y. B. Wang, E. Z. Liu, X. Y. Hu and J. Fan, Ceram. Int., 2015, 41, 14545–14553.
12. L. Mattsson, K. D. Wegner, N. Hildebrandt and T. Soukka, RSC Adv., 2015, 5, 13270–13277.
13. X. H. Liang, Y. H. Hu, X. Ma, Y. B. Wang, J. Fan and X. Y. Hu, Mater. Lett., 2014, 129, 107–110.
14. S. L. Wang, X. B. Wang, X. X. Chen, X. Z. Cao, J. Cao, X. F. Xiong and W. B. Zeng, RSC Adv., 2016, 6, 46317–46324.
15. H. S. Qian, H. C. Guo, P. C. Ho, R. Mahendran and Y. Zhang, Small, 2009, 5(20), 2285–2290.
16. C. M. Yu, X. Z. Li, F. Zeng, F. Y. Zheng and S. Z. Wu, Chem. Commun., 2013, 49, 403–405.
17. Y. J. Ding, H. Zhu, X. X. Zhang, J. J. Zhu and C. Burda, Chem. Commun., 2013, 49, 7797–7799.
18. S. Lahtinen, Q. Wang and T. Soukka, Anal. Chem., 2016, 88, 653–658.
19. S. J. Wu, N. Duan, Z. Shi, C. C. Fang and Z. P. Wang, Talanta, 2014, 128, 327–336.
20. Y. Xiao, L. Y. Zeng, T. Xia, Z. J. Wu and Z. H. Liu, Angew. Chem., Int. Ed., 2015, 54, 5323–5327.
21. M. Rycenga, C. M. Cobley, J. Zeng, W. Li, C. H. Moran, Q. Zhang, D. Qin and Y. Xia, Chem. Rev., 2011, 111, 3669–3712.
22. J. Zeng, S. Roberts and Y. N. Xia, Chem.–Eur. J., 2010, 16, 12559–12563.
23. B. Tang, S. P. Xu, X. L. Hou, J. L. Li, L. Sun, W. Q. Xu and X. G. Wang, ACS Appl. Mater. Interfaces, 2013, 5, 646–653.
24. Y. G. Sun, B. Mayers and Y. N. Xia, Nano Lett., 2003, 3(5), 675–679.
25. Q. Zhang, J. P. Ge, T. Pham, J. Goebl, Y. X. Hu, Z. D. Lu and Y. D. Yin, Angew. Chem., 2009, 121, 3568–3571.
26. Q. Zhang, N. Li, J. Goebl, Z. D. Lu and Y. D. Yin, J. Am. Chem. Soc., 2011, 133, 18931–18939.
27. D. M. Ledwith, A. M. Whelan and J. M. Kelly, J. Mater. Chem., 2007, 17, 2459–2464.
28. J. E. Millstone, G. S. Métraux and C. A. Mirkin, Adv. Funct. Mater., 2006, 16, 1209–1214.
29. D. Aherne, D. M. Ledwith, M. Gara and J. M. Kelly, Adv. Funct. Mater., 2008, 18, 2005–2016.
30. W. F. Shan, R. X. Li, J. Feng, Y. W. Chen and D. C. Guo, Mater. Chem. Phys., 2015, 162, 617–627.
31. H. Dong, L. D. Sun and C. H. Yan, Chem. Soc. Rev., 2015, 44, 1608–1634.
32. X. Yuan, Y. Q. Tay, X. Y. Dou, Z. T. Luo, D. T. Leong and J. P. Xie, Anal. Chem., 2013, 85, 1913–1919.
33. R. O. Ball, G. Courtney-Martin and P. B. Pencharz, J. Nutr., 2006, 136, 1682S–1693S.
34. E. Weerapana, C. Wang, G. M. Simon, F. Richter, S. Khare, M. B. Dillon, D. A. Bachovchin, K. Mowen, D. Baker and B. F. Cravatt, Nature, 2010, 468, 790–795.
35. K. G. Reddie and K. S. Carroll, Curr. Opin. Chem. Biol., 2008, 12, 746–754.

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