Development of an Inner Filter Effects-Based Upconversion

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Development of an Inner Filter Effects-based Upconversion NanoparticlesCurcumin Nanosystem for the Sensitive Sensing of Fluoride Ion Yan Liu, Qin Ouyang, Huanhuan Li, Zhengzhu Zhang, and Quansheng Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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ACS Applied Materials & Interfaces

Development of an Inner Filter Effects-based Upconversion Nanoparticles-Curcumin Nanosystem for the Sensitive Sensing of Fluoride Ion Yan Liu a, Qin Ouyang a, Huanhuan Li a, Zhengzhu Zhang b, Quansheng Chen a,b,∗ a

School of Food and Biological Engineering, Jiangsu University, Zhenjiang 212013, China

b

State Key Laboratory of Tea Plant Biology and Utilization, Anhui Agricultural University, Hefei

210036, China

Keywords: fluoride ion, upconversion fluorescence, curcumin, inner filter effects, sensor.

∗ Corresponding author. Tel.: +86-511-88790318. Fax: +86-511-88780201. E-mail: [email protected] (Q.S.Chen)

ACS Paragon Plus Environment


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Abstract This paper developed a novel ratiometric fluorescence based sensor for the detection of fluoride ion. Yb3+, Er3+ and Tm3+ co-doped NaYF4 upconversion nanoparticles (UCNPs), which can emit fluorescence at 546 nm, 657 nm, 758 nm and 812 nm under the 980 nm single wavelength excitation, was synthesized, amino-modified and applied as the fluorescent signal indicator. The natural chemical curcumin served as specific recognition element and mixed with UCNPs to make a nanosystem. In this nanosystem, the absorption peak of curcumin shows bathochromic shift when F- was added, causing an upconversion fluorescence quenching at 546 nm and 657 nm through inner filter effects (IFE), while the upconversion emission at 758 nm and 812 nm remained unchanged. Thus, the fluorescence ratio I546/I758 was inversely proportional to F- concentration. Meanwhile, the large absorption bathochromic shift also lead to a color change, based on the colorimetric analysis of F- by the naked eye. Under the optimized conditions, the developed UCNPs-curcumin mixed system achieved the colorimetric and ratiometric fluorescence sensing towards F- in the linear range of 25-200 µM and 5–200 µM, with the detection limits as low as 25 µM (ca. 0.48 ppm) and 5 µM (ca. 0.10 ppm), respectively. The developed nanosystem also has high selectivity and anti-jamming ability. Furthermore, this method showed promising practical applications in spiked real samples (ex., tap water and milk) with recoveries of 79.58% to 134.02% and RSD values in the range of 0.94% to 22.11%, which confirmed its great potential in harmful substance detection.


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Introduction Fluoride ion, as one of the most common anions, is useful for dental care and treatment of osteoporosis in small amounts, but it may lead to a serious health hazard in the event of its excessive intake, resulting in conditions such as mottling and destruction of teeth, crippling skeletal fluorosis and nephrolithiasis in severe case1-3. Considering that above dual nature of fluoride on human body were dependent on its concentrations, makes it quantitative detection crucial and relevant. Generally, fluoride ion content is determined by ion chromatography4, ion-selective electrodes5, and

19

F-NMR spectroscopy6. While these methods can provide the

accurate results, they have many limitations that should be addressed, such as requires expensive facilities, tedious operation and high skilled persons with its technical know-how. In recent times, , considering the manifold advantages of high sensitivity, easy operation and high speed, fluorescent detection techniques have been recognized as promising and powerful tool for quantitative detection. Some fluoride sensors have been developed, such as multifunctional Schiff base fluorescent sensor7, silyl capped hydroxylpyrenealdehyde based colorimetric and fluorescence dual-readout sensor8 and calix[4]arene-based colorimetric and fluorescence dual-readout sensors9. However, all of the above sensors use traditional fluorescence probes as sensitive material and signal element, which exhibit fluorescence with a Stokes shift - they emit short-wavelength light (higher energy photons) under excitation with long-wavelength light (lower energy photons). Under this scheme, it is easy to generate a low signal-to-background ratio caused by unwanted autofluorescence interference as well as by strong light scattering from the detection system. Therefore, it is a challenge to develop a novel sensor to satisfy sensitive detection of fluoride ion with high signal-to-noise ratio. Rare-earth doped upconversion nanoparticles (UCNPs), which can convert near-infrared light to visible light, strongly reduces autofluorescence and light scattering10, 11. With these outstanding features, UCNPs have become a hotspot in a wide range of bioimaging12-14, therapies15-17 and detection applications18-20. However, most application mechanisms were based on the fluorescence quenching through



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fluorescence resonance energy transfer (FRET), which has high requirements for the distance between the absorbers and UCNPs11,

21-24

. Alternatively, fluorescence

quenching based on IFE can also achieve the same result but it does not need to meet such strict requirements25. Through IFE, which can be caused only by the overlapping between the absorption band of the absorber and the excitation and/or emission bands of the fluorophores in the detection system, many optical sensors have been fabricated for trace element detection with good results achieved

26-30

. Among them, the most

currently used receptors are usually synthesized through painstaking chemical procedures and always hazardous to humans. So it is essential to find eco-friendly natural materials as receptors to establish the IFE based sensor. To meet the above challenges, a high-efficiency upconversion fluoresence sensor based on the IFE of natural curcumin on the fluorescence of UCNPs was developed for the detection of fluoride ion. Yb3+, Er3+and Tm3+ co-doped NaYF4 nanoparticles with desired emission wavelength were synthesized and mixed with natural curcumin to make a mixed nanosystem for colorimetric and fluorescent detection of fluoride ions. The proposed strategy is depicted in Scheme 1. The assay principle is attributed to the remarkable fluorescence quenching of UCNPs by curcumin through IFE and the formation of curcumin-F- complex. In the presence of F-, a bathochromic shift of the maximum UV absorption peak of curcumin occurs and leads to the IFE-quenched fluorescence of UCNPs. Therefore, absorption and fluorescence spectroscopy change with the concentration of F-. On the basis of these changes of the detection nanosystem, a sensitive sensor for F- was developed. [Here for Scheme 1] RESULTS AND DISCUSSION Characteristics of UCNPs The morphology, structure distribution and changes in surface groups of the UCNPs were observed via TEM, XRD and FTIR, and the results are shown in Figure 1. The TEM in Figure 1A shows that the synthesized UCNPs have globular shape and homogeneous size distribution in the average particle diameter of 55 nm. The XRD patterns were applied to study the chemical composition and crystal type of the


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UCNPs. As presented in Figure 1B, the XRD pattern of the prepared UCNPs is consistent with the standard values for JCPDS no. 49-1896, suggesting that the synthesized UCNPs were highly crystalline. FTIR was used to characterize the modification groups on the surface of UCNPs. The broad absorption bands at 3400 cm−1 was corresponded to the stretching vibration of hydroxide radicals (–OH) of oleic acid on the surface of UCNPs. Meanwhile, the absorptions at 2926 cm−1 and 2853 cm−1; and the peaks at 1557 cm−1 and 1418 cm−1 were related to the asymmetric and symmetric stretching vibrations of the methylene group (–CH2–) and the carboxylic

group

(–COOH)

respectively

(curve

a

in

Figure

1C).

After

amino-modification for UCNPs, three characteristic peaks at 3410, 1625 and 1068 cm−1 (curve b in Figure 1C) appeared: the first one was attributed to the stretching vibration of a hydroxyl group from silanol groups (–Si–OH); the second one corresponded to the stretching and bending vibration of amine groups (–NH2); the last one was linked to the stretching vibration of Si–O. [Here for Figure 1] Detection principle of upconversion fluorescence mixed nanosystem UCNPs-curcumin mixed nanosystem is implemented based on upconversion fluorescence quenching induced by a combination of curcumin and F-. The absorption and the emission spectra of individual constituents of the proposed mixed nanosystem is shown in Figure 2. With the addition of F-, the ultraviolet absorption band of curcumin at 417 nm decreased, meanwhile a new absorption peak at 560 nm appeared and continued to increase, leading to a spectral overlap between the upconversion fluorescence emission peak of UCNPs and the ultraviolet absorption band of curcumin. This resulted in a fluorescence quenching of UCNPs at 546 nm and 657 nm through IFE. Fluorescence quenching at 546 nm is the most significant, so it is chosen as the signal peak. Furthermore, to control the potential interferences caused by the probe concentration, environmental conditions or other factors in lower range, this paper used upconversion fluorescence at 758 nm which remains almost the same as the internal standard. Therefore, ratiometric upconversion fluorescence at 546 nm to that at 758 nm (I546/I758) was used as the index to detect the fluoride ion content.



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[Here for Figure 2] To confirm the fluorescence quenching mechanism, UV–vis absorption spectroscopy, fluorescence emission spectra and zeta potentials were measured. As shown in UV–vis absorption spectroscopy of curcumin solution before and after adding UCNPs (Figure 3A), the absorption bands position and intensity of curcumin had almost no change after adding UCNPs, which verified that no complex was formed. This proved there was no chemical bonding between UCNPs and curcumin. Also, the zeta potential was applied to measure the surface charges of UCNPs and curcumin. As shown in Figure 3B, zeta potentials of the dispersed UCNPs and curcumin were +48.3 mV and +11.4 mV, respectively. The result shows that both UCNPs and curcumin were positively charged, indicating there was no electrostatic attraction between them. In consideration of the above two points, the study ascertained that the distance between the UCNPs and curcumin was larger than 10 nm, and thus proved that the fluorescence quenching mechanism was based on IFE. Furthermore, the upconversion fluorescence spectra of UCNPs was unchanged after adding the curcumin while there was a distinct decrease at 546 nm and 657 nm with continuous addition of fluoride (Figure 3C). Therefore, it was confirmed that the observed fluorescence quenching resulted from IFE between curcumin-F- complex and UCNPs. [Here for Figure 3] Optimization of detection conditions In order to obtain high detection sensitivity, the factors such as the solvent, pH value and mixing ratio of UCNPs and curcumin were investigated for optimizing the detection conditions. A detailed absorption and fluorescence spectra study of UCNPs-curcumin in 8 different solvent (water, dichloromethane, N,N-dimethylformamide, methanol, acetonitrile, ethanol, acetone, dimethylsulfoxide) has been conducted in our research group prior this formal detection, from which the supporing information (Figure S1, S2) were provided. After choosing acetonitrile as the solvent in this study, an attempt of the mixed aqueous-organic solvent system and the effect of pH value were studied


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by dissolving UCNPs-curcumin into water-acetonitrile (v:v) in different ratio (v:v =9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9, 0:10) and water-acetonitrile (v:v=1:1) in different pH (pH=2, 3, 4, 5, 6, 7, 8, 9, 10, 11) respectively. Through analyzing the related information in absorption and fluorescence spectra which can be seen in supporing information (Figure S3, S4) in details, reveals the pure acetonitrile was selected as the optimized solvent for all experiments, which both ensured an excellcent detection effect and avoided the effect of pH. Additionally, the mixing ratio of UCNPs and curcumin were investigated by comparing the effect of different concentration of curcumin on fluorescence in mixed system at a fixed fluoride concentration. As shown in Figure 4, the fluorescence quenching efficiency increased at first and decreased later with the increasing concentration of curcumin. After the concentration increased to 8×10-5 M, the IFE between curcumin-F- complex and UCNPs reached the best result. Meanwhile, a distinct color change could be observed as in insert of Figure 4. It could be seen that the nanosystem had no significant color change of curcumin at low concentrations and had inhibitory effect on color change of curcumin at high concentrations. Consequently, 8×10-5 M curcumin mixed with 1 mg/mL UCNPs in equal volume were selected as the optimum mixing ratio. [Here for Figure 4] Sensitivity assay for FThe colorimetric and fluorescent responses of the optimal UCNPs-curcumin mixed nanosystem to fluoride ions at different concentrations were recorded to analyze its sensitivity. As shown in Figure 5A，the intensity of UV absorption peak at 417 nm decreased gradually while a new UV absorbance peak at 560 nm was found clearly to increase with the increasing concentration of F-. These two changes were attributed to the involvement of –OH groups and the intra-molecular charge transfer transition between the curcumin-F- complex respectively31. A linear response of the mixed nanosystem between the absorption and the F- can be obtained at F- concentration range of 5-200 µM. It can be seen that within the range of 5-25 µM, the linear relationship (red standard curve in Figure 5B) between the concentration of F- and



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absorbance at 560 nm was not obvious, so the colorimetric method cannot easily achieve the detection when the F- is below 25 µM. Meanwhile, it is produced almost no changes in colors by visually observing the mixed nanosystem solutions in bottle when the F- fell below 25 µM (inset of Figure 5B). For ratiometric fluorescence response of the developed mixed nanosystem, as shown in Figure 5C, there was a decrease in the upconversion fluorescence intensity at 546 nm and 657 nm but no change at 758 nm and 812 nm, confirming that the efficiency of IFE between UCNPs and curcumin-F- complex. As discussed above, I546/I758 was chosen as the output signal and its variation shown in Figure 5D. There was a significant inverse correlation between I546/I758 and the concentration of F- from 5 µM to 200 µM expressed by the linear regression equation y=-0.0175x + 5.8014 (R2=0.996). The detection limit (LOD) of the colorimetric and fluorescence methods were 25 µM (ca. 0.48 ppm) and 5 µM (ca. 0.10 ppm) respectively, the former was obtained from actual tests while the latter was obtained by the equation LOD =3S0/S (3 is the factor at the 99% confidence level, S0 is the standard deviation of 10 blank measurements, and S is the slope of the calibration curve).The LOD of ratiometric upconversion fluorescence spectroscopy is an order of magnitude lower than that of colorimetric detection which is approximately consistent with that of the curcumin in sensing F- 32. This is mainly ascribed to the fact that changes in the absorbance of the curcumin can translate into exponential changes in the fluorescence intensity of the UCNPs through IFE33. In addition, the high signal-to-background ratio and improved sensitivities due to their excellent optical properties and good chemical stability of UCNPs can be cited as another reason. [Here for Figure 5] In the summary and contrastive analysis, a comparison between the developed sensors and other reported methods for F- detection was obtained as captured in Table S1, and it was confirmed that the developed sensors could achieve a more sensitive detection than other reported methods34-38. In addition, the continuous noticeable color changes from yellow to light purple to royal purple observed with the increase in Fconcentration (inset of Figure 5B), may offer a rough visual estimation.


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Selectivity assay Selectivity and anti-jamming ability are important factors for an excellent sensor. In this work, the optical UCNPs-curcumin mixed nanosystem was extended to the detection of other anions. Figure 6A shows that fluorescence intensity at 546 nm and 657 nm decreased only in the presence of F-, while there was no significant change with addition of other anions, indicating that only F- could interact with UCNPs-curcumin mixed nanosystem. Simultaneously, the mixed nanosystem showed distinct color change from yellow to violet for F- in low concentration, whereas the remaining anions did not show any color change in higher concentration (inset of Figure 6A). Moreover, competition experiments were carried out by adding F- to solutions of UCNPs-curcumin in the presence of other anions. It was obvious that whether in the absence or presence of other anions, significant decreases of I546/I758 could be obtained for UCNPs-curcumin mixed nanosystem upon addition of F(Figure 6B). The results indicate that the sensing of F- by UCNPs-curcumin mixed nanosystem was hardly affected by these commonly coexistent anions. Therefore, UCNPs-curcumin mixed nanosystem can be used as a highly selective ratiometric upconversion fluorescence and colorimetric dual-readout sensor for F-. [Here for Figure 6] Among the common cations, ferrous ions can be complexed with curcumin molecules and will certainly affect the results of nanosystems in vivo assays39. As seen in Figure S5A, the UV–visible spectra of curcumin-UCNPs solution was carried out with addition of Fe2+ ions. Upon the addition of Fe2+, it showed gradual decrease in the absorption peak at 417 and the appearance of a new absorption peak at 485 nm with isosbestic point at 454 nm due to the charge transfer (CT) transition between curcumin and Fe2+ ions40. Meanwhile, the new absorption peak at 485 nm with bathochromic shift of 68 nm occur due to the formation of curcumin-Fe2+ complex. Binding of Fe2+ with curcumin can been seen in Figure S6a. It is clear that phenolic – OH functional groups are involved in the binding process, which is same in the reaction mechanism of curcumin with F-. During the detection of F-, phenolic –OH groups of curcumin in the developed system can form intermolecular hydrogen bonds



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with F-, resulting in curcumin-F- complexation (Figure S6b). Therefore，the presence of Fe2+ can compete with F- in binding with curcumin, leading to the reduction in the amount of captured curcumin and thus affects the F- detection results of nanosystems in practice. Later, upconversion fluorescence study of curcumin-UCNPs mixed system was carried out to investigate the influence of Fe2+ on F- detection. It can be seen from Figure S5B, Fe2+did not produce any fluorescence intensity change at 546 nm and 657 nm, which were the center position of signal peaks in the F- detection. This may be attributed to the fact that, the developed detection system is based on the inner filter effect (IFE) and therefore requires the overlaps between the absorption band of the absorbent and the excitation or emission bands or with the both chromospheres to some extent. In F- detection, the absorption peak at 417 nm bathochromic shifted to 560 nm, leading to a spectral overlap between the upconversion fluorescence emission peak of UCNPs and the ultraviolet absorption band of curcumin, which consequently resulted in a fluorescence quenching of UCNPs at 546 nm and 657 nm through IFE. While the absorption peak at 417 nm showed a bathochromic shift to 485 nm, which cannot overlap the upconversion fluorescence emission peak, resulting in no fluorescence quenching by IFE. In summary, the presence of Fe2+ will only affect the complexation of curcumin and F-, and has no influence on the absorption and upconversion fluorescence spectra. Thus, its influence can be eliminated by improving the concentration of curcumin. To test the idea, competition experiments were carried out by adding F- in the absence or presence of Fe2+ after increasing the proportion of curcumin. An obvious fluorescence quenching occurred, which can be seen from Figure S5B (also can be seen in Figure 6A), indicating the feasibility of the method. Therefore, Fe2+ can complex with curcumin, but this effect can be avoided in vivo assays by using developed curcumin-UCNPs solution. Application in real samples On further evaluation of the feasibility and repeatability of the UCNPs-curcumin mixed nanosystem in actual test, tap water and milk samples, spiked with different concentrations of F-, were used as real samples. Intra-assay variations were


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determined with five replicates in the same developed systems which were composed of the same batch of UCNPs on a single day. Inter-assay variations were determined by a single test in different developed systems which were composed of different batch of UCNPs on five different days. The recoveries of F- are shown in Table 1, within the range of 79.58% to 134.02%, with the RSD values in the range of 0.94% to 22.11%, indicating the relevant prospect of the developed system. [Here for Table 1] CONCLUSION In summary, it can confidently be said that a novel upconversion fluorescence nanosensor for trace amounts of F- based on the IFE between UCNPs and curcumin-Fcomplex has been successfully developed. The fluorescence quenching efficiency and the absorption of UCNPs-curcumin mixed nanosystem are all quantitatively correlated to the concentration of F–, achieving a ratiometric fluorescence and colorimetric dual-readout detection for F- with high selectivity, rapid response, and high sensitivity (LOD=0.10 ppm). Moreover, the developed sensor has been successfully applied to detecting F- in tap water and milk, demonstrating its great potential for wide application in the detection of harmful substances. Experimental section Materials All starting materials were obtained from commercial suppliers and used as received. Rare-earth chloride hexahydrate: Gadolinium(III) chloride hexahydrate (GdCl3·6H2O, 99.99 %), yttrium (III) chloride anhydrous (YCl3·6H2O, 99.9%), ytterbium (III) chloride anhydrous (YbCl3·6H2O, 99.9%), holmium (III) chloride anhydrous (HoCl3·6H2O, 99.9%), as well as 1-octadecene (ODE, 90 %), oleic acid (OA, 90 %), were purchased from Sigma-Aldrich Company (Shanghai, China). Curcumin, acetonitrile and all the other chemical reagents with analytical grade were purchased from Sinopharm Chemical Reagent Company, Ltd. (Shanghai, China) and used directly without further purification. Synthesis and modification of UCNPs Oleic acid-capped upconversion nanoparticles were developed on the basis of our



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group's previous research11. First, 0.2 M rare-earth trichlorides were dissolved in 12 mL methanol and added into a 250 mL flask containing 18 mL oleic acid and 42 mL 1-octadecene. Secondly, after heating the above solution to 160 ºC for 30 min under argon atmosphere and cooled down to 50 ºC, 10 mL methanol solution which involves NH4F (0.16 g) and NaOH (0.1 g) was introduced into the flask under vigorous stirring conditions. Next, the mixture was heated up to 70 ºC to make the methanol volatilize and be removed. The content of the flask was further heated to 300 ºC under argon atmosphere for 1 h and cooled down to room temperature subsequently. Finally, the products were separated from the solution by centrifugation at 8000 rotations per minute for 8 minutes, washed three times with ethanol-cyclohexane (v: v = 2: 1) mixture, and dried overnight in a vacuum oven at 60 ºC. To acquire the excellent dispersal of UCNPs, amino-modification for them were completed by using a typical Stöber-based method with minor modifications. First, 20 mg of the prepared UCNPs was dispersed in 60 mL ethanol by ultrasonic dispersion for 30 min. Secondly, 2.5 mL of ammonia (25%) and 20 mL of distilled water were added into the solution under heating at 70 ºC and vigorous stirring for 15 min. Afterwards, 200 µL tetraethoxysilane (TEOS) was added dropwise into the flask and the reaction was maintained for another 6 h under vigorous stirring. Then, 200 µL 3-Aminopropyltriethoxysilane (APTES) was added dropwise. After further stiffing for 3-4 h, the solvent was removed by centrifuging at 8000 rotations per minute for 8 minutes, and the precipitate was washed three times with ethanol. Finally, amino-functionalized silica-capped UCNPs were obtained after drying in vacuum oven at 60 ºC overnight. Characterization Transmission electron microscopy (TEM) images were captured by the Tecnai 12 transmission electron microscopy (Philips, Holland) using 120 kV acceleration voltage. Powder X-ray diffraction (XRD) spectra were performed on a Siemens D5005 instrument (Bruker AXS, Ltd., Germany) in a 2θ range from 10° to 70° with a 7°/min step scan mode. The reference data was taken from the JCPDS database (Joint Committee on Powder Diffraction Standards). Ultraviolet-visible (UV-vis) absorption


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spectra were collected using an 8454 UV-vis spectrophotometer (Agilent Technologies Inc., U.S.A.). Fourier transform infrared spectra (FT-IR) at the wave numbers range of 4000 -500 cm-1 were recorded by the Nicolet Nexus 470 Fourier transform infrared spectrophotometer (Thermo Electron Co., U.S.A.) with the KBr method. The surface charges were measured using Malvern zetasizer Nano ZS90 (Malvern Instruments Ltd., U.K.). Spectroscopy assays of fluoride ions using UCNPs-curcumin mixed nanosystem Fluoride ions in the forms of tetrabutylammonium (TBA) salts were prepared using acetonitrile as solvent. Curcumin solutions at optimal concentration were added into 3 mL glass bottle containing equal amounts of 1 mg/mL UCNPs, and mixed evenly by ultrasonic dispersion. Then, F- solution was added to the content of the bottle to give a series of samples with different concentrations (cF- = 0, 5, 10, 15, 20, 25, 50, 75, 100, 125, 150, 175, 200 µM). The bottle was subsequently shaken for a few seconds to enable adequate reaction between its content to occur. Finally, UV-vis spectra and upconversion fluorescence spectra were performed at room temperature. Preparation and detection of real samples Water was collected directly from the tap in the laboratory and centrifuged at 12000 rpm for 20 min. After filtering through a 0.22 µm membrane, different concentrations of F- were spiked into the filtrates to evaluate the performance of developed method. Milk samples were purchased from the local supermarkets and 10 mL of it was mixed with 2 mL of chloroform and 2 mL trichloroacetic acid. Subsequently, the milk samples were defatted and de-proteinized by centrifugation at 4 ºC for 10 min at 12000 rpm. After that, 1 M NaOH was added dropwise to the supernatant to adjust the pH to 7.4. Finally, the supernatant fluid was filtered through a 0.22 µm membrane and spiked with F- to test the samples in the same way. Selectivity assay To verify the selectivity of the developed methods, the following procedures were done in this study: first, various anions such as H2PO4−, AcO−, HSO4−, ClO4−, NO3−, OH−, Cl−, Br−, and I− in the form of tetrabutylammonium salts, were dissolved in acetonitrile with a final concentration higher than 100 times that of F− concentration



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(150 µM of F− and 15 mM of H2PO4−, AcO−, HSO4−, ClO4−, NO3−, OH−, Cl−, Br−, and I−). Secondly, the solution of optimal UCNPs-curcumin in acetonitrile was prepared. Then after, the above anions solutions were added into the optimal UCNPs-curcumin mixed nanosystem to study the response. Besides, same amounts of F- were added into the optimal UCNPs-curcumin mixed nanosystem containing other anions to perform a competition experiment. ASSOCIATED CONTENT Supporting Information Optimization of the reaction solvents, Optimization of the pH for the detection system, Effect of Fe2+ on the Hg2+ detection, and comparison between the developed sensors and other reported methods for the determination of F-. ACKNOWLEDGEMENTS This work has been financially supported by the National Natural Science Foundation of China (31471646), the Natural Science Foundation of Jiangsu Province (Youth) (BK20150502), the China Postdoctoral Science Foundation (2015M571698 and 2016T90429), and the Advanced Talents Science Foundation of Jiangsu University (15JDG064). REFERENCES (1) Ali, R.; Razi, S. S.; Shahid, M.; Srivastava, P.; Misra, A. Off–On–Off Fluorescence Behavior of an Intramolecular Charge Transfer Probe Toward Anions and CO2. Spectrochim. Acta, Part A 2016, 168, 21-28. (2) Irigoyen-Camacho, M. E.; García Pérez, A.; Mejía González, A.; Huizar Alvarez, R. Nutritional Status and Dental Fluorosis Among Schoolchildren in Communities with Different Drinking Water Fluoride Concentrations in a Central Region in Mexico. Sci. Total Environ. 2016, 541, 512-519. (3) Chuah, C. J.; Lye, H. R.; Ziegler, A. D.; Wood, S. H.; Kongpun, C.; Rajchagool, S. Fluoride: A Naturally-Occurring Health Hazard in Drinking-Water Resources of Northern Thailand. Sci. Total Environ. 2016, 545–546, 266-279. (4) Hattab, F. N. Analytical Methods for the Determination of Various Forms of Fluoride in Toothpastes. J. Dent. 1989, 17, 77-83. (5) De Marco, R.; Clarke, G.; Pejcic, B. Ion-Selective Electrode Potentiometry in Environmental Analysis. Electroanalysis 2007, 19, 1987-2001. (6) Gerken, M.; Boatz, J. A.; Kornath, A.; Haiges, R.; Schneider, S.; Schroer, T.; Christe, K. O. The 19F NMR shifts are not a Measure for the Nakedness of the Fluoride Anion. J. Fluorine Chem. 2002, 116 ,


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Table 1 Recoveries and relative standard deviation (RSD) values for fluoride ions in real samples by developed method. c (µM)

Tap water

5 10 15 20 25 50 75 100 125 150 175 200

intra-assay recovery (%) RSD (%) 79. 58 15.12 86.31 11.34 74.18 14.63 93.85 10.07 105.98 8.34 83.15 7. 25 97.07 9.42 101.38 4.61 134.02 3.2 112.31 14.5 116.76 10.84 102.19 0.94

93.55 1.97 10 96.81 21.20 15 108.3 13.54 20 96.03 7.48 25 107.76 4.56 50 111.4 16.71 Milk 75 97.6 20.08 100 109.4 12.54 125 101.7 8.04 150 80.34 2.03 175 108.5 9.79 200 85.78 2.87 c denotes the spiked concentration in real samples. RSD denotes relative standard deviation. 5

inter-assay recovery(%) RSD(%) 88.61 17.53 121.35 11.21 94.83 16.49 120.26 11.19 106.24 6.87 102.32 8.53 105.88 21.30 90. 13 17.79 102.77 16.74 117.32 11.43 95.52 21.63 119.93 15.23 89.43 101.91 111.41 109.83 104.28 99.46 96.83 109.12 119.92 99.91 100.44 82.46


20.38 21.04 22.11 13.84 9.98 16.05 7.67 16.51 8.59 2.18 10.39 6.8

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For Table of Contents Only



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Scheme 1. Schematic illustration of UCNPs-curcumin nanosystem response to F-. Changes in ultraviolet absorbance and upconversion emission are also shown in their corresponding spectra and photos. 113x66mm (300 x 300 DPI)


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Figure 1. (A) TEM images of oleic acid-capped UCNPs, (B) XRD pattern of oleic acid-capped UCNPs, (C) FTIR spectra of oleic acid-capped UCNPs (curve a) and amino-modified silica coated UCNPs (curve b). 309x80mm (300 x 300 DPI)



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Figure 2. UV-vis spectra of the curcumin in the absence (red dashed line) and presence of F- (red solid line), and upconversion fluorescence spectrum of UCNPs (black dashed line) and UCNPs-curcumin mixed system with addition of F- under excitation with 980 nm laser. 113x72mm (300 x 300 DPI)


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Figure 3. (A) UV-vis spectra of the curcumin and UCNPs-curcumin. (B) Zeta potential distribution of amino modified UCNPs (a) and curcumin (b). (C) Upconversion fluorescence spectra of UCNPs UCNPs-curcumin and UCNPs-curcumin-F-. 185x94mm (300 x 300 DPI)



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Figure 4. Upconversion fluorescence quenching efficiency of the (1 mg/mL) UCNPs added with 200 µM Fafter addition of various concentrations of curcumin under 980 nm laser excitation. Inset: Visible change of UCNPs-curcumin nanosystem with addition of various concentration of curcumin at a fixed fluoride concentration. 151x95mm (300 x 300 DPI)


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Figure 5. (A) Absorption spectra of 1mg/mL UCNPs with 8×10-5 M curcumin after addition of different concentrations of F- (5-200 µM). (B) Absorption intensity of UCNPs-curcumin versus different concentrations of F-（0, 5, 10, 15, 20, 25, 50, 75, 100, 125, 150, 175 and 200 µM）and the linear fits of the two regions at concentrations below and above 25 µM. Inset: Change in color of UCNPs-curcumin nanosystem with addition of different concentrations of F-. (C) Upconversion fluorescence spectra of 1mg/mL UCNPs with 8×10-5 M curcumin after addition of different concentrations of F- (5-200 µM). (D) Upconversion fluorescence intensity ratio (I550/I758) of UCNPs-curcumin versus different concentrations of F-（0, 5, 10, 15, 20, 25, 50, 75, 100, 125, 150, 175 and 200 µM）and the linear fits. 246x176mm (300 x 300 DPI)



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Figure 6. (A) Absorption spectra of optimized UCNPs-curcumin nanosystem in the presence of various representative anions (H2PO4−, Fe2+, AcO−, HSO4−, NO3−, ClO4−, OH-, CN-, I−, Br−, Cl−, and F-). Inset: Change in color of UCNPs-curcumin nanosystem with addition of various representative anions. (B) Upconversion fluorescence intensity ratio I546/I785 of optimized UCNPs-curcumin nanosystem in the presence of 150 µM F-, 1.5 mM other anions (as shown in bar forms). Solid columns represent the subsequent addition of 150 µM F- to the above solution. 75x110mm (300 x 300 DPI)


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Development of an Inner Filter Effects-Based Upconversion

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