Ratiometric Fluorescent Detection of Phosphate in Aqueous Solution

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Ratiometric Fluorescent Detection of Phosphate in Aqueous Solution Based on Near Infrared Fluorescent Silver Nanoclusters/Metal-Organic Shell Composite Dai Cong, Cheng-Xiong Yang, and Xiu-Ping Yan Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b03086 • Publication Date (Web): 22 Oct 2015 Downloaded from http://pubs.acs.org on October 22, 2015

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Analytical Chemistry

Ratiometric Fluorescent Detection of Phosphate in Aqueous Solution Based on Near Infrared Fluorescent Silver Nanoclusters/Metal‐Organic Shell Composite Cong Dai, Cheng-Xiong Yang, and Xiu-Ping Yan* College of Chemistry, Research Center for Analytical Sciences, State Key Laboratory of Medicinal Chemical Biology (Nankai University), Tianjin Key Laboratory of Molecular Recognition and Biosensing, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, 94 Weijin Road, Tianjin 300071, China *Fax: +86-22-23506075 E-mail: [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT Synthesis of near infrared (NIR) fluorescent AgNCs with high quantum yield and stability is challenging, but important for sensing and bioimaging application. Here, we report the fabrication of AgNCs/metal-organic shell composite via the deposition of metal-organic (zincnitrogen) coordination shell around AgNCs for ratiometric detection of phosphate. The composite exhibits NIR emission at 720 nm with 30 nm red-shift in comparison to bare AgNCs, and a weak emission at 510 nm from the shell. The absolute quantum yield of NIR fluorescence of the composite is 15% owing to FRET from the shell to the AgNCs core under the excitation at 430 nm. Besides, the composite is stable due to the protection of the shell. Based on the composite, a novel ratiometric fluorescence probe for the detection of phosphate in aqueous solution with good sensitivity and selectivity was developed. The limit of detection (3s) is 0.06 μM, and the relative standard deviation for 10 replicate detections of 10 μM phosphate was 0.6%. The recoveries of spiked phosphate in water, human urine and serum samples ranged from 94.1% to 103.4%.


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INTRODUCTION Silver nanoclusters (AgNCs) have great potential for sensing and bioimaging owing to their good fluorescence properties, low toxicity and small size in comparison with traditional quantum dots.1-7 However, the high tendency of AgNCs to irreversible aggregation due to their high surface energy limits their wide applications. To solve this problem, great efforts have been made to synthesize stable AgNCs using a variety of ligands, such as short thiols,8,9 oligonucleotides,10-17 peptides,18-20 and polymers.21,22 Recently, short thiol modified polymer capped AgNCs23-25 and self-assemble thiolated AgNCs26 have also been fabricated to address this issue. Short thiols stabilized AgNCs exhibit near-infrared (NIR) fluorescence while polymers stabilized AgNCs own good stability, oligonucleotides capped and self-assemble thiolated AgNCs have high quantum yield. But, none of them simultaneously possesses NIR fluorescence, high quantum yield and good stability. Therefore, it is highly desirable to prepare NIR fluorescent AgNCs with high quantum yield and good stability for sensing and bioimaging. Herein, we report the fabrication of NIR fluorescent AgNCs/metal-organic shell composite with high quantum yield and good stability for ratiometric detection of phosphate in aqueous solution. Phosphate is not only very significant for aquatic ecosystems, but also plays important roles in biological systems. An understanding of phosphate levels in water and biological fluids can provide useful information about eutrophication, several diseases and many other problems.27,28 Thus, it is vital to monitor phosphate in water and biological fluids. However, traditional molecular fluorescence probes with poor water solubility are usually applied in homogenous assay systems.29 To enhance the efficiency of fluorescent sensing, various fluorescent sensors have been developed.29-34 In particular, a turn-on coordination nanoparticle fluorescent probe for phosphate based on a single fluorescence change from rhodamine B and the



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affinity of zinc ion and phosphate was reported recently.33 However, the sensing signal originating from single fluorescence of these fluorescent sensors may easily be affected by excitation intensity, emission collection efficiency and probe concentration.35 In contrast, ratiometric methods based on the ratio of dual fluorescence intensities can alleviate most of these problems, and possess excellent ability of quantitative analysis.35 In this work, we show that the prepared AgNCs/metal-organic shell composite is promising as a fluorescence probe for ratiometric detection of phosphate in aqueous solution with good sensitivity and selectivity. EXPERIMENTAL SECTION Chemicals and Materials. All the chemicals are at least of analytical grade. Ultrapure water (Hangzhou Wahaha Group Co. Ltd., Hangzhou, China) was used throughout. Silver nitrate, lipoic acid, glutathione (GSH), zinc nitrate hexahydrate, and sodium borohydride were obtained from Aladdin (Shanghai, China). Imidazole-2-carboxaldehyde (ICA) was purchased from Heowns (Tianjin, China). Sodium hydrogen carbonate, sodium sulfate, sodium chloride, sodium acetate, calcium chloride, magnesium sulfate, aluminum nitrate, cadmium chloride and potassium dihydrogen phosphate were purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). Potassium chloride, chromium nitrate, manganese chloride, ferric chloride, ferrous chloride, cobalt nitrate, lead nitrate, and copper nitrate were obtained from Aladdin (Shanghai, China). Sodium nitrate was purchased from Alfa Aesar (China). Instrumentation and Characterization. The content of nitrogen was determined on a vario EL CUBE element analyzer (Elementar, GER), and the content of silver and zinc was measured on an X series inductively coupled plasma mass spectrometer (ICP-MS) (Thermo Elemental, UK). The high resolution transmission electron microscopy (HRTEM) image and the energy dispersive X-ray (EDX) analysis were obtained on a Tecnai G2 F20 transmission electron


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microscope (FEI, USA) with an accelerating voltage of 200 kV. Fourier transform infrared (FTIR) spectra in KBr were recorded on a Nicolet 6700 spectrometer (Thermo Fisher Scientific, USA). X-ray photoelectron spectrometer (XPS) spectra was investigated on an Axis Ultra DLD spectrometer fitted with a monochromated Al Kα X-ray source (hν = 1486.6 eV), hybrid (magnetic/electrostatic) optics, and a multichannel plate and delay line detector (Kratos Analytical Ltd., UK). The hydrodynamic size and zeta potential were measured on a Nano-ZS Zetasizer (Malvern, UK). All fluorescence measurements were carried out on an F-4500 spectrofluorometer (Hitachi, Japan). The fluorescence quantum yield was determined on an FLS920 spectrometer with an integration sphere attachment under excitation at 430 nm (Edinburgh, UK). The absorption spectra were recorded on a UV-3600 UV-vis-NIR spectrophotometer (Shimadzu, Japan). The long-term colloidal stability was evaluated on a ZF-8 ultraviolet analyzer (Jiapeng technology Co. Ltd., Shanghai, China) under the irradiation at 365 nm. Synthesis of ICA-modified GSH (ICA-GSH). GSH (1.2 mmol) was dissolved in 60 mL ultrapure water, and pH was adjusted to 9 with NaOH solution (2 M). Then, ICA (1.2 mmol) was added to GSH solution, and the mixture was stirred overnight to obtain ICA-GSH solution. Synthesis of ICA Functionalized AgNCs (ICA-AgNCs). ICA-GSH (10 mL), lipoic acid (0.35 mmol) and NaOH (0.6 mL, 1 M) were added to 55 mL ultrapure water under ultrasonication until lipoic acid was dissolved completely. AgNO3 solution (1.4 mL, 25 mM) was then added to the mixture under vigorous stirring, and NaBH4 solution (3 mL, 50 mM) was introduced 5 min later. The color of the mixture was changed to orange red after stirring for 2 h. The resulting ICA-AgNCs was collected without purification and stored at 4 oC for further use.



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Synthesis of AgNCs/Metal-Organic Shell Composite. ICA-AgNCs (24 mL) was mixed with 20 mL methanol, and then ICA methanol solution (7.2 mL, 100 mM) was added under stirring. Subsequently, Zn(NO3)2 (0.9 mL, 100 mM) was mixed, and kept stirring for 10 min. The composite was collected by centrifugation (10000 rpm, 10 min) and washed with methanol and ultrapure water sequentially, then redispersed in ultrapure water at 4 oC. Ratiometric Detection of Phosphate. Tap water and mineral water samples were collected locally. Human urine and serum samples were obtained from Solarbio (Beijing, China). Human serum samples were filtrated with Amicon Ultra-4 centrifugal Filter Units (30 kDa) by centrifugation at 5000 rpm for 15 min and were diluted 100 times. The supernatant of human urine samples was collected after coagulation for 12 h at 4 oC, and was diluted 1000 times. After all samples were adjusted to pH 6, 1 mL of sample solution was added to a clean quartz cell (1 cm × 1 cm), and 10 μL of the composite containing 6 mM Ag was mixed with a slight shaking. The fluorescence spectra were recorded after 5-min incubation. RESULTS AND DISCUSSION Synthesis and Characterization of AgNCs/Metal-Organic Shell Composite. The fabrication of AgNCs/metal-organic shell composite and the mechanism for ratiometric detection of phosphate is illustrated in Scheme 1. To obtain the composite, ICA-GSH was firstly synthesized via a Schiff-base reaction (Figure S1, Supporting Information). Then, ICA-AgNCs was formed by reduction with NaBH4 after the incubation of Ag+, ICA-GSH and lipoic acid. ICA was functionalized on AgNCs through the coordination of the thiol in ICA-GSH and AgNCs to facilitate the formation of the Zn-ICA coordination shell via a subsequent coordination of ICA to Zn2+. Thus, AgNCs/metal-organic shell composite was prepared.


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Scheme 1. Schematic illustration for the fabrication of AgNCs/metal-organic shell composite and the mechanism for ratiometric detection of phosphate. The morphology and composition of the as-prepared composite was characterized by FTIR, HRTEM, EDX and XPS. The characteristic FTIR peak of C=N at 1600 cm-1 shows the successful preparation of ICA-GSH, and the presence of ICA-GSH in ICA-AgNCs and the composite (Figure 1A). The characteristic band of N-Zn coordination bond stretching vibration at 540 cm-1 in the composite verifies the Zn-ICA coordination shell (Figure 1A).36



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Figure 1. (A) FTIR spectra of ICA-GSH (black), ICA-AgNCs (red) and the composite (blue). (B) HRTEM image of the composite. Inset: the lattice fringes of ICA-AgNCs in the composite. (C) EDX line scan across the composite along the white line, and there is one ICA-AgNCs with high contrast in the red circle. (D) EDX element analysis of the composite. The diameter of ICA-AgNCs in the composite is about 3 nm (Figure 1B), with a lattice spacing of 0.2 nm corresponding to the (111) lattice plane, which is the same as that of bare ICAAgNCs (Figure S2, Supporting Information). The moiety with lower contrast around ICAAgNCs in Figure 1B is the Zn-ICA shell, which is also confirmed by EDX line scan across the composite (Figure 1C). The EDX measurement shows the presence of N and Zn in the composite along the whole white line, but Ag only in the red circle. The EDX element analysis also reveals the presence of Ag, S, Zn and N in the composite (Figure 1D). The hydrodynamic diameter of the composite is 170 nm.


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The XPS peaks of Ag 3d5/2 and 3d3/2 are at 367.6 eV and 373.4 eV, respectively (Figure S3A, Supporting Information). The peak of S 2p at 161.2 eV owes to the S-Ag bond in the composite (Figure S3B, Supporting Information).24 In addition, both the Zn 2p doublet in the range of 1021.4-1044.5 eV and the N 1s symmetric peak at 399.1 eV (Figure S3C and S3D, Supporting Information), corresponding to typical zinc-nitrogen coordination compounds,37,38 also demonstrate the formation of Zn-ICA coordination shell. Moreover, the abundance of Ag, N and Zn is 9.29%, 13.07% and 23.65%, respectively. The above results confirm the successful fabrication of the AgNCs/metal-organic shell composite. Fluorescence Properties of AgNCs/Metal-Organic Shell Composite. The as-prepared composite gives the same absorption peaks at 430 nm and 510 nm as ICA-AgNCs (Figure 2A and 2B), and exhibits a strong NIR emission at 720 nm with a 30 nm red-shift in comparison to the NIR emission of bare ICA-AgNCs (Figure 2A). The significant NIR emission red-shift is caused by reabsorption, due to a certain degree of inevitable ICA-AgNCs aggregation in the formation of Zn-ICA coordination shell.39-42 In addition to NIR emission, the composite and ICA-AgNCs also give a weak emission at 510 nm from ICA (Figure 2A Inset). The absolute quantum yield (QY) of the NIR fluorescence of the composite is 15%, while that of ICA-AgNCs is 8%. Under the excitation at 430 nm, the emission of ICA at 510 nm has a high degree of overlap with the absorption of ICA-AgNCs (Figure 2B), leading to FRET from ICA to the AgNCs core. Because of the much greater amount of ICA in the composite, FRET enhancement of the NIR fluorescence of the composite is much greater than that of ICA-AgNCs. So, the QY of the composite is about double that of ICA-AgNCs, and even 7.5 times that of typical dihydrolipoic acid capped AgNCs (2%).8 The contribution of the FRET from ICA to the enhancement of the QY of the composite was further confirmed by measuring the QY of the



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composites with various contents of ICA (Table S1, Supporting Information). The results show that the QY of the as-prepared composite increased with the content of ICA, verifying the FRET process from ICA to AgNCs.

Figure 2. (A) UV-vis absorption spectra of the composite (black), and fluorescence spectra of ICA-AgNCs (blue) and the composite (red) with the same concentration of Ag under the excitation at 430 nm. Inset: the weak emission at 510 nm. (B) UV-vis absorption spectra of ICAAgNCs (black), and fluorescence spectra of ICA (red) under the excitation at 430 nm. (C) Longterm colloidal stability of the composite and ICA-AgNCs at different pHs. Besides the high QY, the composite also offered good long-term colloidal stability. With the protection of the coordination shell, the composite is stable in a pH range of 5-9 and a NaCl concentration range of 0-1 M for at least 24 h (Figure 2C; Figure S4, Supporting Information). The hydrodynamic diameter and the zeta potential of the composite are almost constant in these


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different solutions, showing no change of the composite and good stability (Figure S5 and S6, Supporting Information). In contrast, ICA-AgNCs exhibit poor stability for the weak protection of short thiols (Figure 2C; Figure S4, Supporting Information).5-7 The above results indicate the high quantum yield and good stability of the composite with NIR emission at 720 nm. Ratiometric Detection of Phosphate. The dual emissions of the composite and the high affinity between phosphate and zinc offer the potential of the composite as a fluorescent probe for the ratiometric detection of phosphate. As shown above, the prepared composite gave the strong NIR fluorescence at 720 nm from the AgNCs and the weak fluorescence at 510 nm from the ICA due to the FRET process from the ICA to the AgNCs. Considering the high affinity between phosphate and Zn2+ ion, the Zn-ICA shell would be destroyed by phosphate due to its competition for Zn2+ with ICA. To address this point, the hydrodynamic diameters of the composite in various concentrations of phosphate solution were measured (Figure S7, Supporting Information). The obvious decrease of the hydrodynamic diameter of the composite due to increased concentrations of phosphate confirms that the extent of the Zn-ICA shell destruction depends on the concentration of phosphate. The binding of phosphate with the Zn2+ from the composite led to the departure of the ICA from the composite to the solution. In turn, the FRET process from the shell ICA to the AgNCs core was inhibited to some extent. As a result, the NIR fluorescence at 720 nm from the AgNCs decreased while the fluorescence at 510 nm from ICA increased, depending on the concentration of phosphate (Figure 3A). Thus, the intensity ratio of the fluorescence at 510 nm to that at 720 nm (I510/I720) increased with the concentration of phosphate (Figure 3B), allowing ratiometric detection of phosphate.



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The effect of pH on the I510/I720 of the probe in 10 μM phosphate aqueous solution was tested (Figure 3C). The I510/I720 decreased as pH increased from 6 to 9 because of the increased affinity of deprotonated ICA to zinc at higher pH.43 As a control, the I510/I720 in ultrapure water was almost constant in the same pH range (Figure 3C). Thus, the highest net ratio of I510/I720 was obtained at pH 6.

Figure 3. (A) Effect of the concentration of phosphate on the fluorescence spectra of the probe. Inset: the emission spectra at 510 nm. (B) Plot of I510/I720 of the probe as a function of the concentration of phosphate. (C) Effect of pH on I510/I720 of the probe in ultrapure water as a control (black) and 10 μM phosphate aqueous solution (red). (D) Normalized I510/I720 in the presence of various ions in aqueous solution. Note: 1, 80 μM phosphate; 2, 80 μM phosphate + 1 mM HCO-3 ; 3, 80 μM phosphate + 1 mM NO-3 ; 4, 80 μM phosphate + 500 μM SO2-4 ; 5, 80 μM phosphate + 200 μM CH3COO-; 6, 80 μM phosphate + 1 mM Cl-; 7, 80 μM phosphate + 500 μM Zn2+; 8, 80 μM phosphate + 500 μM Cd2+; 9, 80 μM phosphate + 1 mM K+; 10, 80 μM phosphate + 500 μM Ca2+; 11, 80 μM phosphate + 200 μM Mn2+; 12, 80 μM phosphate + 200


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μM Mg2+; 13, 80 μM phosphate + 100 μM Al3+; 14, 80 μM phosphate + 20 μM Pb2+; 15, 80 μM phosphate + 20 μM Co2+; 16, 80 μM phosphate + 20 μM Cr3+; 17, 80 μM phosphate + 20 μM Fe3+; 18, 80 μM phosphate + 20 μM Fe2+; 19, 80 μM phosphate + 20 μM Cu2+. We also examined the change of I510/I720 of the probe with the concentration of aqueous phosphate at pH 6. The I510/I720 linearly increased with the concentration of phosphate ([phosphate]) from 1 μM to 100 μM with a calibration function of I510/I720 = 0.0013 [phosphate] + 0.0153 ([phosphate] in μM, R2 = 0.9954) (Figure 3B). The relative standard deviation for 10 replicate detections of 10 μM phosphate was 0.6%, showing the high precision for phosphate detection. The limit of detection (3s) is 0.06 μM, which is comparable to or better than those obtained by other fluorescence probes for phosphate (Table S2, Supporting Information). To evaluate the selectivity of the probe for ratiometric detection of phosphate, the effect of co-existing ions on the I510/I720 of the probe in the presence of 80 μM phosphate was tested (Figure 3D). For comparison, the I510/I720 in the presence of other ions was then normalized to that in the presence of only 80 μM phosphate. The tolerant concentration of HCO-3 , NO-3 , SO2-4 , CH3COO-, Cl-, Zn2+, Cd2+, K+, Ca2+, Mn2+, Mg2+, Al3+, Pb2+, Co2+, Cr3+, Fe3+, Fe2+, and Cu2+ was up to 1000, 1000, 500, 200, 1000, 500, 500, 1000, 500, 200, 200, 100, 20, 20, 20, 20, 20, and 20 μM, respectively. Although the tolerant concentration of the last six ions is 20 μM, it is still higher than the content of those ions in water.44 To show the practical utility of the probe, we applied it to the detection of phosphate in water, human urine and serum samples. The recoveries of spiked phosphate in these samples ranged from 94.1% to 103.4% (Table 1), indicating no significant interferences encountered for the determination of phosphate in these samples. The concentrations of phosphate in the water, human urine and serum samples obtained by our proposed method are in good agreement with



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those measured by a phosphomolybdenum blue spectrophotometric method (Table 1). The above results show that the proposed method has great potential for quantitative detection of phosphate in water and biological samples with good sensitivity and selectivity. Table 1. Analytical Results (Mean ± s, n=3) for the Determination of Phosphate in Water, Human Urine and Serum Samples

samples

concentration of phosphate in original samples this work

phosphomolybdenum blue spectrophotometric method

recovery (%)a

tap water

0.52 ± 0.09 μM

0.65 ± 0.08 μM

100.3 ± 1.1

mineral water 1

2.48 ± 0.22 μM

2.77 ± 0.15 μM

100.3 ± 2.2

mineral water 2

2.42 ± 0.30 μM

2.72 ± 0.07 μM

100.5 ± 1.6

mineral water 3

9.36 ± 0.17 μM

9.13 ± 0.04 μM

99.8 ± 0.5

mineral water 4

1.35 ± 0.43 μM

1.39 ± 0.04 μM

100.2 ± 1.0

human urine 1

34.86 ± 1.72 mM

33.64 ± 0.04 mM

94.1 ± 2.8

human urine 2

48.37 ± 1.95 mM

48.00 ± 0.04 mM

100.0 ± 2.0

human serum 1

1.35 ± 0.02 mM

1.35 ± 0.01 mM

103.4 ± 1.6

human serum 2

1.56 ± 0.11 mM

1.44 ± 0.01 mM

101.0 ± 2.7

a

the spiked phosphate of original water, human urine and serum samples is 5 μM, 20 mM and 2

mM, respectively.

CONCLUSIONS In summary, we have prepared AgNCs/metal-organic shell composite via the formation of metalorganic shell around AgNCs. Owing to the coordination shell, the as-prepared composite possesses NIR emission at 720 nm, high quantum yield and good stability. Furthermore, we have


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also shown that the composite is promising as a fluorescent probe for sensitive and selective ratiometric detection of phosphate in aqueous solution. ASSOCIATED CONTENT Supporting Information Additional figures and tables on the synthesis and characterization of the composite, and comparison of the proposed method with other methods. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Fax: +86-22-23506075 E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grants 21435001, 21275079) and Open Funds of the State Key Laboratory of Electroanalytical Chemistry (SKLEAC201406).



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For TOC only


Ratiometric Fluorescent Detection of Phosphate in Aqueous Solution

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