Sensitive and Selective Detection of Silver(I) Ion in Aqueous Solution

Mar 24, 2011 - Sensitive and Selective Detection of Silver(I) Ion in Aqueous Solution Using Carbon Nanoparticles as a Cheap, Effective Fluorescent Sen...
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LETTER pubs.acs.org/Langmuir

Sensitive and Selective Detection of Silver(I) Ion in Aqueous Solution Using Carbon Nanoparticles as a Cheap, Effective Fluorescent Sensing Platform Hailong Li,†,‡ Junfeng Zhai,† and Xuping Sun*,† †

State Key Lab of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China ‡ Graduate School of the Chinese Academy of Sciences, Beijing 100039, China

bS Supporting Information ABSTRACT: In this Letter, we demonstrate the first use of carbon nanoparticles (CNPs) obtained from carbon soot by lighting a candle as a cheap, effective fluorescent sensing platform for Agþ detection with a detection limit as low as 500 pM and high selectivity. We further demonstrate its practical application to detect Agþ in a real sample.

iven the threat posed by the high toxicity of Agþ to aquatic organisms, it is of particular importance to monitor the content levels in aquatic ecosystems. Owing to the broad employment in industries, such as electronics, photography, mirrors, and pharmacy, a large amount of silver is released to the environment annually from industrial wastes and emissions especially to the sludge waste and even to surface waters.1,2 Several methods for the detection of silver ions at trace quantity levels in various samples have been developed, including atomic absorption spectroscopy,35 inductively coupled plasma-mass spectroscopy,6,7 and ion-selective electrodes (ISEs).8,9 Much effort has also been made to develop a fluorescence chemosensor as an alternative method, which provides new prospects of improvement in sensitivity, selectivity, simple instrumental implementation, and easy operation.1013 However, there are only a few successful examples of molecular probes for Agþ detection.14,15 Interactions of Agþ ions with nucleic acids have attracted considerable interest and have been extensively studied.1218 Agþ is capable of selectively coordinating cytosine (C) bases to form strong and stable CAgþC complexes, laying the foundation for oligonucleotide (OND)-based silver(I) ion assay development.1523 For example, Ono et al. have reported a specific C-rich OND probe labeled with a fluorophore and a quencher at each terminus for Agþ detection.16 The presence of Agþ induces the OND to fold into a hairpin structure, which brings both termini into close proximity and results in significant fluorescence quenching via intramolecular fluorescence resonance energy transfer (FRET). Although such a method is sensitive and selective, it still has some drawbacks in that it

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r 2011 American Chemical Society

requires labeling at both ends of the OND probe with specific dyes that suffer from low overall yield and are not cost-effective.24 More recently, We25,26 and other researchers21,27 have demonstrated the use of single-labeled fluorescent OND for Agþ detection with the use of deoxyguanosine base or nanostructure including nano-C60, graphene oxide (GO), and single-walled carbon nanotubes (SWCNTs) as a quencher. However, SWCNT or graphite used for GO preparation must be purchased from some sources. On the other hand, both SWCNT treatment and GO preparation by the Hummers method28 are timeconsuming and labor-intensive. In this Letter, we demonstrate the first use of carbon nanoparticles (CNPs) obtained from candle soot by lighting a candle as a cheap, effective fluorescent sensing platform for highly sensitive and selective detection of Agþ with a detection limit as low as 500 pM, which is much lower than that on SWCNTs21 and GO.27 Furthermore, we demonstrate its practical application to detect Agþ in a real sample. The general concept used in this approach is described as below: in the absence of Agþ, fluorescently labeled single-stranded DNA (ssDNA) probe adsorbs on CNP via ππ stacking interactions between DNA bases and CNP,29,30 leading to substantial fluorescence quenching; however, in the presence of Agþ, CAgþC coordination induces the probe to form a hairpin structure, which does not adsorb on CNP and thus retains the dye fluorescence. Scheme 1 illustrates the CNP-based fluorescent Agþ detection. Received: January 5, 2011 Revised: March 13, 2011 Published: March 24, 2011 4305

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Scheme 1. Scheme (not to scale) Illustrating the CNP-Based Fluorescent Agþ Detection and the Conformational Change of Agþ-Specific C-Rich OND (PAg)

All chemically synthesized oligonucleotides were purchased from Shanghai Sangon Biotechnology Co. Ltd. (Shanghai, China). DNA concentration was estimated by measuring the absorbance at 260 nm. All the other chemicals were purchased from Aladin Ltd. (Shanghai, China) and used as received without further purification. Metal ion solutions were prepared from corresponding nitrate salts. The water used throughout all experiments was purified through a Millipore system. CNPs were prepared as follows: In brief, 3 mg of carbon soot obtained by lighting a candle using a well-established method31 was suspended in 12 mL of a water/ethanol mixture (1:1) with the help of ultrasonication. After that, the black solution was centrifuged at 3000 rpm for 2 min to separate out large carbon soot particle. The supernatant was collected and then subjected to centrifugation at 6000 rpm for 2 min. The black precipitate was collected and redispersed in 12 mL of a water/ethanol mixture (1:1) for further characterization and sensing application. We repeated the separation and collected the desired CNPs; both of water and ethanol can be removed by lyophilization. According to the mass of the final residue, the concentration of the previously prepared suspension was about 0.15 mg/mL. For scanning electron microscopy (SEM) characterization, 20 μL of the suspension was placed on an indium tin oxide (ITO) glass slide and air-dried at room temperature. Scanning electron microscopy (SEM) measurements were made on a XL30 ESEM FEG scanning electron microscope at an accelerating voltage of 20 kV. The sample for transmission electron microscopy (TEM) measurements was prepared by placing a dilution of colloidal solution on a carbon-coated copper grid and drying at room temperature. Transmission electron microscopy (TEM) measurements were made on a HITACHI H-8100 EM instrument (Hitachi, Tokyo, Japan). Fluorescent emission spectra were recorded on a RF-5301PC spectrofluorometer (Shimadzu, Japan). Zeta potential measurements were performed on a Nano-ZS Zetasizer ZEN3600 (Malvern Instruments Ltd., U.K.). The total volume of each sample for fluorescence measurement was 400 μL in 10 mM 3-(N-morpholino)propanesulfonic acid (MOPS) buffer containing 50 mM NaNO3 (pH 7.0). The volume of CNPs used for each measurement was 35 μL if not specified, and the fluorescence quenching measurement was done after 15 min incubation in buffer. Oligonucleotide sequences are listed as follows. PAg (ROX dye-labeled ssDNA probe for Agþ): 50 -ROX-CCT CCC TCC TTT TCC ACC CAC C-30 . Too small CNPs (below 10 nm) produce strong photoluminescence emission interfering with detection32 and too large CNPs tend to precipitate; therefore, suitable CNPs should be

Figure 1. (a) Low and (b) high magnification SEM images; (c, d) TEM images of the resulting products thus obtained.

selected. A separation process was involved in our present study to collect CNPs of desired size (as mentioned above). Figure 1a and b shows the low and high magnification SEM images of the resulting products, respectively, indicating that they consist of a large amount of nanoparticles. The corresponding TEM images reveal that they are nearly spherical in the diameter range of 2540 nm, as shown in Figure 1c and d. It is of great importance to note that the obtained CNPs have a zeta potential of 7.19 mV and can be well-dispersed in water/ethanol mixture. However, it is impossible to obtain reliable overall information about the CNPs existing in solution by using SEM and TEM techniques, because the evaporation of the solvent during the sample preparation may lead to secondary aggregations of the CNPs on the substrate used.29 Figure 2 shows the fluorescence emission spectra of ROXlabeled ssDNA probe for Agþ and PAg, under different conditions. In the absence of CNPs, PAg exhibits strong fluorescence emission due to the presence of the ROX dye (curve a). However, the presence of CNPs results in about 84% fluorescence quenching after 15 min incubation in MOPS buffer (curve c), indicating that CNP can adsorb ssDNA and quench the fluorescent dye very effectively. In the presence of Agþ, the fluorescence intensity is 4-fold higher than that without Agþ (curve d). It should be pointed out that the fluorescence of free PAg was slightly influenced by Agþ in the absence of CNPs (curve b). We also investigated the influence of ethanol on the fluorescence intensity of PAg with or without Agþ (Supporting Information Figure S1); it can be clearly seen that there is not any effect from ethanol used in our experiments. It should be noted that although CNPs tend to precipitate in mixed water/ethanol after a long time, they can be well-dispersed by shaking and kept stable during our measurements. When CNPs were added for fluorescence quenching, the final mixture was almost clear and no precipitate was observed after several hours. For the sensitivity study, different concentrations of Agþ in the range of 0400 nM were investigated. Figure 3 shows the 4306

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Figure 2. Fluorescence spectra of (a) PAg, (b) PAg þ Agþ, (c) PAg þ CNPs, and (d) PAg þ CNPs þ Agþ. Excitation was at 580 nm. All measurements were done in 10 mM MOPS buffer containing 50 mM NaNO3 (pH 7.0). ([PAg] = 50 nM; [Agþ] = 200 nM.)

Figure 4. Histograms of FF0 value with error bar (standard deviation from the mean, n = 3), where F0 and F are the fluorescence intensity of PAg þ CNPs in the absence and presence of different metal ions, respectively. ([Agþ] = 200 nM; (A) [other metal ion] = 10 μM; (B) [other metal ion] = 50 μM.) Excitation was at 580 nm, and the emission intensity was monitored at 601 nm. All measurements were done in 10 mM MOPS buffer containing 50 mM NaNO3 (pH 7.0). ([PAg] = 50 nM.)

Figure 3. Fluorescence spectra of PAg þ CNPs in the presence of different Agþ concentrations (from bottom to top: 0, 0.5, 1, 2, 4, 8, 10, 40, 80, 100, 150, 200, 300, and 400 nM). Inset: F/F0  1 value plotted against the concentration of Agþ with error bar (standard deviation from the mean, n = 3), where F0 and F are the fluorescence intensity of PAgCNP without and with Agþ, respectively. Excitation was at 580 nm, and the emission intensity was monitored at 601 nm. All measurements were done in 10 mM MOPS buffer containing 50 mM NaNO3 (pH 7.0). ([PAg] = 50 nM.)

fluorescence intensity of PAg-CNP in the presence of different Agþ concentrations in MOPS buffer. It is obvious that the fluorescence intensity of the mixture is sensitive to Agþ and increases with the increase of Agþ concentration. The inset in Figure 3 shows the value of F/F0  1 plotted against the concentration of Agþ, where F0 and F are the fluorescence intensity of PAg-CNP without and with Agþ, respectively. No further increase of the fluorescence intensity was observed when a higher Agþ concentration (>400 nM) was used, indicating that the interaction between PAg and Agþ reaches a balance. It should be noted that although CNP, CNT, and GO are made by carbon and the interactions between such nanocarbon and ssDNA are ππ stacking interactions, the detection limit on CNP is estimated to be 500 pM (practical measurement indicated in Figure 3), which is much lower than that on SWCNTs21 and

GO.27 We speculate the reason for the much higher sensitivity of CNP compared to the CNTs and GO could be due to its spherical morphology and unsmoothed surface, but the exact reason is not completely understood at the present time. The selectivity of the present detection system for Agþ was further evaluated. To do this, a variety of environmentally relevant metal ions were examined, including Agþ, Ca2þ, Cd2þ, Co2þ, Cu2þ, Fe2þ, Fe3þ, Mg2þ, Mn2þ, Ni2þ, Pb2þ, Zn2þ, and Hg2þ. Figure 4 shows the difference in fluorescence intensity between the blank and solutions containing Agþ (200 nM) or other metal ions ((A) 10 μM; (B) 50 μM). It is interesting to find that Mg2þ ions only lead to small difference of fluorescence intensity, but other metal ions lead to decrease of fluorescence intensity, including Ca2þ, Cd2þ, Co2þ, Cu2þ, Fe2þ, Fe3þ, Mn2þ, Ni2þ, Pb2þ, Zn2þ, and Hg2þ. This could be attributed to that these metal ions themselves can quench dye fluorescence to one degree or another (Supporting Information Figure S2). All these observations indicate that the present sensing system exhibits very high selectivity for Agþ. In addition to evaluating the Agþ sensor in an artificial system using pure buffer, the performance of the present method for real sample analysis was also investigated. Lake water samples were obtained from the South Lake of Changchun, Jilin province, China. Figure 5 compares the response of this sensing platform to blank lake water (curve a) and lake water spiked with 100 nM 4307

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Figure 5. Fluorescence spectra of (a) PAg þ CNPs and (b) PAg þ CNPs þ 100 nM Agþ in lake water. Inset: corresponding histograms with error bar (standard deviation from the mean, n = 3). Excitation was at 580 nm, and the emission intensity was monitored at 601 nm. ([PAg] = 50 nM.)

Agþ (curve b). It was found that this CNP-based probe can withstand the interference existing in lake water. The fluorescence response to 100 nM Agþ is about 9-fold higher than that to blank sample, which is much lower than the toxicity level of Agþ in drinking water (460 nM) defined by U.S. Environmental Protection Agency (EPA).2 In summary, for the first time, we demonstrate the successful use of CNPs as a cheap, effective fluorescent sensing platform for silver ion detection in pure buffer and real samples. Compared to previously reported SWCNT and GO sensing platforms,21,27 the present CNP-based platform has the following two obvious advantages: (1) CNPs are easier to produce and more costeffective than SWCNTs and GO; (2) it exhibits a much lower detection limit of 500 pM. We believe that this sensing platform will find great application in real environmental analysis.

’ ASSOCIATED CONTENT

bS

Supporting Information. Fluorescence spectra of PAg, PAg þ 17.5 μL EtOH, PAg þ Agþ, and PAg þ Agþ þ 17.5 μL EtOH; fluorescence intensity histograms of PAg in the presence of different metal ions. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*Telephone/Fax: 0086-431-85262065. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by National Basic Research Program of China (No. 2011CB935800). ’ REFERENCES (1) Barriada, J. L.; Tappin, A. D.; Evans, E. H.; Achterberg, E. P. TrAC, Trends Anal. Chem. 2007, 26, 809. (2) Ratte, H. T. Environ. Toxicol. Chem. 1999, 18, 89. (3) Chakrapani, G.; Mahanta, P. L.; Murty, D. S. R.; Gomathy, B. Talanta 2001, 53, 1139. (4) Pu, Q.; Sun, Q.; Hu, Z.; Su, Z. Analyst 1998, 123, 239. 4308

dx.doi.org/10.1021/la200052t |Langmuir 2011, 27, 4305–4308