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Rapid and selective detection of trace Cu2+ by accumulation- reaction-based Raman spectroscopy
Sensors & Actuators: B. Chemical 283 (2019) 278–283
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Rapid and selective detection of trace Cu2+ by accumulation- reaction-based Raman spectroscopy ⁎
T
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Ying Liu, Yiping Wu, Xiaoyu Guo , Ying Wen, Haifeng Yang
The Education Ministry Key Lab of Resource Chemistry, Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Municipal Education Committee Key Laboratory of Molecular Imaging Probes and Sensors and Department of Chemistry, Shanghai Normal University, Shanghai, 200234, China
A R T I C LE I N FO
A B S T R A C T
Keywords: L-cysteine Inositol hexaphosphate Silver nanoparticles Raman Trace Cu2+
Developing detection method for trace copper ions (Cu2+) has attracted great attention since Cu2+ ions would threaten to human health, including cellular toxicity, liver damage, and neuro degenerative diseases. In this work, adding L-cysteine (L-Cys) into inositol hexaphosphate (IP6) modified Ag nanoparticles (designated as LCys/IP6@Ag) induces the certain aggregation of nanoparticles, resulting in increasng the surface enhanced Raman scattering (SERS) signal of rhodamine 6 G (R6 G). The presence of Cu2+ ions could be adsorbed by IP6 and the concentrated Cu2+ ions could further selectively oxide Cys molecules under certain pH value, which disperses the IP6@Ag again and decreases the SERS signal of R6 G. The linear range of determination of Cu2+ ions is from 10−5 to 10-1° M as well as a low detection concentration is of 10 pM. Furthermore, the L-Cys/ IP6@Ag-R6 G-based SERS sensor has been successfully applied to detect trace Cu2+ in river water.
1. Introduction Nowadays, environmental pollution from toxic metals has become a serious problem because of the long-term harm to human being. Therefore, detection of toxic metals is highly desired as an important issue both in environmental monitoring and clinical research. [1,2] Copper as an essential trace element plays a key role in various biological processes. [3,4] However, through food chains, the accumulation of copper ions to relatively high concentration in both human and animal would result in toxic effect to living organisms [5] including cellular toxicity, liver damage, neurodegenerative diseases (e.g., Wilson’s diseases and Alzheimer’s disease) and so on. [6] The safety limit of copper content in drinking water set by the U.S. Environmental Protection Agency (EPA) is 1.3 ppm (20 mM).[7] The average concentration of Cu2+ in sea water and in fresh water are 0.25 ppb(∼4 nM) and 3 ppb (∼47 nM), respectively. [8] Thus, the measurement of copper ions (Cu2+) in environmental matrix and biological fluids becomes increasingly important and is highly demanded. Routine detection methods for trace Cu2+ include atomic absorption spectrometry (AAS),[9] inductively coupled plasma-mass spectrometry (ICP-MS),[10] electrochemical methods,[11] chemical sensors. [12]and fluorescent analysis.[13] Normally, the above techniques need expensive instruments, complex pre-processing, pre-concentration, or on-lab testing. Therefore, it calls for development of a simple, sensitive
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and fast assay to meet the requirement of on-site and rapid test. Surface-enhanced Raman scattering (SERS) spectroscopy with rich information of fingerprints has many merits of facile sample pretreatment, noninvasive detection, and high sensitivity, [14–17] which is regarded as a promising technique in the field of environmental monitoring. For instance, Sougata et al. established a Cu2+ SERS method by taking advantage of 2-mercaptobenzimidazole responsive exclusively towards Cu2+ ions but the sensitivity is not good enough to meet the requirement of EPA.[18] Li et al. synthesized 3,5-dimethoxy-4-(6 -azobenzotria-zolyl) phenol modified silver nanoparticles as SERS reporter and then was further functionalized with L-cysteine (L-Cys) for SERS detection of Cu2+ ions. [19] However, this synthesis method is time-consuming of 18 h. Ndokoye et al. used gold nanostars coated with L-Cys to realize SERS detection of Cu2+ but its sensitivity should be further improved. [20] What’s more, nanostars had poor stability and they were easily transformed into spherical and metamorphic. [21] It needs more effort to explore the new strategies for elevating SERS sensitivity in case of the detection of trace Cu2+ (Table 1). In this work, we obtained a stable SERS substrate by preparing inositol hexaphosphate (IP6) protected Ag nanoparticles (Ag NPs), designated as IP6@Ag. And then, IP6@Ag was modified with L-cysteine and Rhodamine 6 G(R6 G), denoted as L-Cys/IP6@Ag-R6 G. Interestingly, L-Cys/IP6@Ag-R6 G trending to aggregation could have the strong SERS signal of R6 G. In the presence of trace Cu2+, IP6
Corresponding authors. E-mail addresses: [email protected] (X. Guo), [email protected] (H. Yang).
https://doi.org/10.1016/j.snb.2018.12.043 Received 8 June 2018; Received in revised form 2 December 2018; Accepted 9 December 2018 Available online 10 December 2018 0925-4005/ © 2018 Published by Elsevier B.V.
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Table 1 Determination of Cu2+ in river water by Inductive Coupled Plasma Emission Spectrometer and L-Cys/IP6@Ag-R6 G-based SERS methods (n = 5). Samples
riverwater
Spiked amount
SERS amount
SERS Recovery%
RSD%
Spiked amount
ICP amount
ICP Recovery%
(nM) 10 1 0.1
8.87 0.97 0.113
88.7 97.2 113.4
7.2 9.5 10.3
(μM) 10 1 0.1
(μM) 9.77 1.05 0.115
97.7 105.4 114.9
possessing six phosphates could accumulate Cu2+ ions via strong interaction. Next, L-Cys molecules in this detection system happened a selective oxidation reaction by Cu2+ ions and it resulted in dispersing IP6@Ag to extent and decreasing the SERS signal of R6 G. Consequently, L-Cys/IP6@Ag- R6 G system contributed a synergetic effect to increase analytic sensitivity of Cu2+ ions and limit of detection (LOD) could be down to 10 pM, which meeting the requirement of EPA.
phosphate and 20 mmol/L sodium dihydrogen phosphate (solvent A) and methyl alcohol：acetonitrile 10:90 (solvent B), which was pumped at a flow rate of 1.0 mL/min. The ratio of solvent A to solvent B was 67:33. The wavelength of the detector was 360 nm and the injection volume was 20 μL.
2. Experimental section
L-Cys/IP6@Ag was pre-mixed with target sample at 50 °C for 15 min under keeping stirring. The Raman and SERS detections were carried out by using Dilor's SuperLabram (II) confocal microscopic laser Raman apparatus with semiconductor-cooled CCD detector (1024 × 256 pixels). The light source was helium-neon ion laser with excitation wavelength of 633 nm and the acquirement time for each spectrum was set at 8 s with 3 accumulations.
2.4. SERS detection
2.1. Materials and instruments Silver nitrate (AgNO3), trisodium citrate, sodium of inositol hexaphosphate (IP6), phytic acid (PA), rhodamine 6 G(R6 G), L-cysteine (LCys), copper chloride (CuCl2), calcium chloride (CaCl2), cobalt nitrate (Co(NO3)2), manganese nitrate (Mn(NO3)2), zinc nitrate (Zn(NO3)2), sodium chloride (NaCl), ferric chloride (FeCl3), ferrous chloride (FeCl2), lead nitrate (Pb(NO3)2), nickel nitrate (Ni(NO3)2), and mercury chloride (HgCl2) were purchased from sigma. All chemicals were analytical grade and used without further purification. Solutions were prepared in high-purity water from a Millipore purification system (18.2 MΩ cm).
3. Results and discussion 3.1. Characterization of L-Cys/IP6@Ag and its detection mechanism The preparation process of L-Cys/IP6@Ag- R6 G-based SERS probe and the detection mechanism for Cu2+ are cartooned in Scheme 1. First, IP6@Ag functionalized with L-cysteine could bring to certain aggregation of the nanoparticles via –COOH binding, which enables to improve SERS signal of R6 G. Next, when adding trace Cu2+ ions, Cu2+ ions could be accumalted by both of IP6 and Cys and then it happens a oxidation reaction of L-Cys by Cu2+ ions. It results in despersing the Ag NPs again and decreasing SERS signa of R6 G. The above inferring mechanism could ascertained by TEM observation. As shown in Fig. 1b, L-Cys anchoring onto Ag NPs surface via S − Ag bonding and free COO− and NH3+ groups of L-Cys through coordination bonding23 drag nanoparticles closer.[24] By DLS measurement results given in Fig. 1d, the average size of IP6@Ag NPs without L-Cys is 49 nm and IP6@Ag NPs with L-Cys, the average size of Ag NPs becomes 57 nm. The latter mgiht produce more hot spots for promoting SERS signal. Clearly, as seen in Fig. 1c, the bound Ag NPs, which are broken by adding Cu2+, would suppress the SERS signal and correspondingly, the average size of LCys/ IP6@Ag-Cu2+ alters into 52 nm (Fig. 1d). Fig. S1 demonstrates the reasonable uniformity of as-prepared Ag nanoparticles. In Fig. 2A of UV–vis spectra, surface plasmon band of L-Cys/ IP6@Ag exhibits a slight red shift from 404 to 406 nm in comparison with IP6@Ag. It should be due to increase of the local dielectric constant after introduction of L-Cys. By adding excess amount of Cu2+ (10−5 M) into L-Cys modified Ag nanoparticles in volume ratio of 1:1, the surface plasmon band of Ag NPs is observed at 404 nm with low intensity, meaning the redispersion of nanoparticles. It should be mentioned that the slight shifts might be due to the AgNPs protected by IP6. FT-IR experiment was carried out to validate the successful modification of L-Cys at the surface of IP6@Ag. In Fig. 2B(a) the spectral bands at 1590 and 1400 cm−1 correspond to the asymmetric and symmetric stretching of C = O (COOH), respectively. A band at 1540 cm −1 is due to NeH bending. The broad bands of N−H and OH− stretching are observed in the range of 3000 − 3500 cm−1. A weak band near 2550 cm−1 virtually confirms the presence of S−H group in the L-Cys molecule. [25,26] In Fig. 2B(b) recorded from L-Cys/IP6@Ag,
2.2. Synthesis of Ag NPs Ag nanoparticles were synthesized according to the previous method [22]. In detail, 0.255 g AgNO3 dissolved in 150 mL Milli-Q water, and boiled to 90 °C. Then, 5 mL of 10−3 M IP6 solution was added under keeping boiling. After 15 min, we added 3 mL of 1% trisodium citrate solution. After boiling for 6 h, the Ag nanoparticles protected with IP6 were obtained. Ag NPs were mixed with L-Cys solution (10−6 M) and R6 G solution -5 (10 M) in volume ratio of 1:1 and then the mixture was heated with water bath under 40 °C for 1.5 h under stirring. It should be mentioned the final pH should be adjusted to 7.0 by using raw PA soulution. [23]After centrifuged and washed with water, L-Cys/IP6@Ag probe was stored in a refrigerator at 4 °C. 2.3. Characterization of composition The surface plasmon resonance spectra of the prepared Ag NPs, LCys/IP6@Ag and mixture of L-Cys/IP6@Ag and Cu2+ ions were measured with a UV-7504 UV–vis spectrophotometer (Shanghai XinMao Instrument Co..Ltd.). The morphology of the nanoparticles were measured with a JEOL JEM-2000 FX transmission electron microscopy (TEM) operating at 200 kV and a JEOL JEM-2100 high-resolution transmission electron microscope (HR-TEM) instrument. For TEM experiments, the samples were directly dropped onto the carbon coated Cu grids. FTIR experiment was conducted by using Nicolet iS5 (Thermo Fisher, USA). ICP measurement was performed on Vista MPX inductive coupled plasma emission spectrometer. DLS experiment was employed by Zetasizer Nano ZS90. We conducted a verification experiment using HPLC method (a Thermo U3000 HPLC system). Chromatographic separation was achieved on a Spherisorb CNW Athena C18-Wp (4.6 mm × 150 mm, 5 um) S/N:R4780008 at 35 °C by a thermostat. The binary mobile phase contained both 20 mmol/L sodium hydrogen 279
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Scheme 1. Preparation process of Cys modified IP6@Ag NPs with R6 G and possible detection mechanism.
interaction of −COO– and Ag presents a stretching band of −COO– at 1740 cm −1 and the cleavage of S−H results in the invisibility of the corresponding band at 2550 cm−1, which hints that L-Cys molecules acts as a bridging reagent in connection of the Ag NPs.
We have done a control experiment under nitrogen atmosphere. Briefly, after the sample solution was degassed by nitrogen for 20 min to remove oxygen, the reaction of Cu2+ and cysteine was observed and as presented in Fig S2. The similar results with and without oxygen were acquired, showing no obvious interference from the oxygen on the above reaction. As consequence, in this work, the mechanism by Cu2+induced oxidation of L-Cys could be confirmed. In order to further prove that copper ions and cysteine have been oxidized, we conducted a verification experiment using HPLC method. Briefly, the solution of LCys /IP6@Ag and Cu2+/ L-Cys /IP6@Ag were centrifuged, and then the supernatant was tested using HPLC to examine the content of L-Cys. The results shown in Fig. S3, and we found that after the copper ions reacted with L-Cys/IP6@Ag substrate, cysteine was not detected in the
3.2. Quantitative detection performance In Fig. 3a, clearly, with increasing concentration of Cu2+, SERS intensity of R6 G is decreased. The cysteine oxidation-reduction reaction equation is given as follows: Cu2+ + cysteine = cystine + Cu+ (1) The standard potential of Cu2+ + e− = Cu+ is 150 mV, while the standard potential for oxidation of L-cysteine to cystine is 340 mV [27].
Fig. 1. TEM images of (A)Ag nanoparticles protected with phytate (B) Cys modified Ag NPs (C) Cu2+ mixed with L-Cys/IP6@Ag substrate.(D) average particle size of Ag NPs, L-Cys/IP6@Ag or L-Cys/IP6@Ag-Cu2+ estimated by DLS method. 280
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Fig. 2. (A)UV–vis spectra of AgNPs, L-Cys/IP6@Ag and L-Cys/ IP6@Ag-Cu2+. (B)FTIR spectra of powder Cys (a) and L-Cys/IP6@Ag (b).
Ca2+, Co2+, K+, Na+, Fe2+, Fe3+, Ni2+, Pb2+, Mn2+, Zn2+, and Hg2+ were examined. According to the statistic profiles in Fig. 3d, this L-Cys/ IP6@Ag-R6 G-based SERS method could selectively detect trace Cu2+. Based on previous research [28], L-Cys can be selectively oxidize by cupric ions. Inter-molecular copper complexes are the easiest to form and their enthalpy will be higher. Additionally, in the case of intramolecular reactions, the thiol moieties are correctly coordinate Cu2+ ions than other ions. However, the as-prepared L-Cys/IP6@Ag substrate is slightly
supernatant, which proved that copper ions oxidized cysteine rather than the dissociation of L-Cys from IP6@Ag by coordination between Cu and L-Cys. The lowest detectable concentration for Cu2+ in deionized water could be down to 10 pM (S/N = 3). In Fig. 3c, a linear relationship plotted between the intensities of indicative peak at 1509 cm−1 and the logarithm of concentrations of Cu2+ is ranging from 1 × 10-6 to 1 × 10−1° M. The error bars were obtained from at least 5 times measurements. In order to test the selectivity of this method, several possible co-existing ions involving
Fig. 3. (A) SERS spectra of R6 G (a) L-Cys/IP6@Ag, (b) L-Cys/ IP6@Ag with Cu2+ (c) Ag NPs (B) SERS spectra of L-Cys/IP6@Ag react with different concentrations of Cu2+ after adjust pH to 7.0. Spectrum from a to h:0 M,10-5 M, 10-6 M,10-7 M, 10-8 M, 10-9 M, 10-10 M, and 10-11 M. (C) The calibration curve plotted by the SERS intensities at 1509 cm-1 and the varying concentrations of Cu2+ with R2 = 0.9907 (D) the SERS selectivity of detecting Cu2+ by using L-Cys/IP6@Ag and the concentrations for other metal ions are set at 10-4 M. 281
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Table 2 Comparisons of analytical performances of various detection methods for Cu2+. Method
Linear range
Limit of detection
Detection time
Stability
Reference
SERS Fluorescence SERS Electrodeposition Colorimetry MC-LR SERS
8.5-40μM 1-10μM 10−5-10-8M 50-300ppm 0.2-0.6M 0-120nM 10−6-10-10M
10μM 2.7 × 10−8M 10−8M 50ppm 0.04μM 2.8nM 10pM
1.5h — 140s — 5min 1h 15min
— — 24.4% — — 1.1% 10.15%
[20] [31] [30] [28] [32] [33] This work
Appendix A. Supplementary data
alkaline due to using inositol hexaphosphate and it results in poor selectivity of SERS detection as shown in Fig. S4a. Therefore, HCl and phytic acid were repectively employed to adjust the pH. Clearly, in Fig. S4b, using phytic acid to control pH of SERS substrate is much better than HCl. The possible reason is that Ag NPs protected by phytate ions mixed with phytic acid might introduce the buffer effect with high torelence to acid and basic media. Additionally, HCl resulted in the serious agglogation of Ag NPs. In order to study the reproducibility of this method, the SERS spectra of R6 G were recorded by using 10 batches L-Cys/IP6@Ag substrates, prepared by controlling the same conditions. As shown in Fig. S5, the relative standard deviation (RSD) is estimated about 10.15%, suggesting that the reasonable preparation reproducibility and the quick SERS method meets the requirement for real application. For evaluating the method reliability, trace Cu2+ ions spiked river water samples were detected by inductive coupled plasma emission spectrometer (ICP) and the proposed L-Cys/IP6@Ag-R6 G-based SERS assay. It should be stated, for SERS measurements, the mimic samples (10−2–10−4 M) were respectively diluted several times with ultrapure water to meet the concentration linear dynamic range. As a consequence, the L-Cys/IP6@Ag-R6 G-based SERS method could determine trace Cu2+ in river water down to 10 pM with the recoveries from 88.7% to 113.4%, indicating the high sensitivity of the method with satisfactory robustness. The SERS spectra of the determination of Cu2+ in river water could be found in Fig. S6. For highlighting the merit of the L-Cys/IP6@Ag-R6 G-based SERS method, the some determination assays of Cu2+ in literature are listed in Table 2 [29–33]. Clearly, the detection time, Stability, sensitivity and linear range of the present method with a lower limit of detection are comparable or at the better position with respect to other methods.
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4. Conclusion In this work, we proposed a new strategy that modification of Ag nanoparticles (Ag NPs) by L-cysteine and IP6. The L-Cys/IP6@Ag probe could synergistically elevate SERS sensitivity for realizing detection of trace Cu2+, when Rhodamine 6 G was used as Raman probe. In addition, the SERS detection selectivity was due to the specific reaction between L-Cys and Cu2+ ions. L-Cys/IP6@Ag-R6 G-based SERS protocol has been applied to quickly determine trace Cu2+ in river water with high sensitivity down to 10 pM without sample pretreatment.
Acknowledgements This work is supported by the National Natural Science Foundation of China (21475088), Yangfan Program of Shanghai Science and Technology Committee (15YF1409000), project from Shanghai science and Technol-ogy Committee (15520502900), International Joint Laboratory on Resource Chemistry (IJLRC) and State Key Laboratory of Environmental Chemistry and Ecotoxicology.
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Rapid and selective detection of trace Cu2+ by accumulation- reaction-based Raman spectroscopy ⁎
T
⁎
Ying Liu, Yiping Wu, Xiaoyu Guo , Ying Wen, Haifeng Yang
The Education Ministry Key Lab of Resource Chemistry, Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Municipal Education Committee Key Laboratory of Molecular Imaging Probes and Sensors and Department of Chemistry, Shanghai Normal University, Shanghai, 200234, China
A R T I C LE I N FO
A B S T R A C T
Keywords: L-cysteine Inositol hexaphosphate Silver nanoparticles Raman Trace Cu2+
Developing detection method for trace copper ions (Cu2+) has attracted great attention since Cu2+ ions would threaten to human health, including cellular toxicity, liver damage, and neuro degenerative diseases. In this work, adding L-cysteine (L-Cys) into inositol hexaphosphate (IP6) modified Ag nanoparticles (designated as LCys/IP6@Ag) induces the certain aggregation of nanoparticles, resulting in increasng the surface enhanced Raman scattering (SERS) signal of rhodamine 6 G (R6 G). The presence of Cu2+ ions could be adsorbed by IP6 and the concentrated Cu2+ ions could further selectively oxide Cys molecules under certain pH value, which disperses the IP6@Ag again and decreases the SERS signal of R6 G. The linear range of determination of Cu2+ ions is from 10−5 to 10-1° M as well as a low detection concentration is of 10 pM. Furthermore, the L-Cys/ IP6@Ag-R6 G-based SERS sensor has been successfully applied to detect trace Cu2+ in river water.
1. Introduction Nowadays, environmental pollution from toxic metals has become a serious problem because of the long-term harm to human being. Therefore, detection of toxic metals is highly desired as an important issue both in environmental monitoring and clinical research. [1,2] Copper as an essential trace element plays a key role in various biological processes. [3,4] However, through food chains, the accumulation of copper ions to relatively high concentration in both human and animal would result in toxic effect to living organisms [5] including cellular toxicity, liver damage, neurodegenerative diseases (e.g., Wilson’s diseases and Alzheimer’s disease) and so on. [6] The safety limit of copper content in drinking water set by the U.S. Environmental Protection Agency (EPA) is 1.3 ppm (20 mM).[7] The average concentration of Cu2+ in sea water and in fresh water are 0.25 ppb(∼4 nM) and 3 ppb (∼47 nM), respectively. [8] Thus, the measurement of copper ions (Cu2+) in environmental matrix and biological fluids becomes increasingly important and is highly demanded. Routine detection methods for trace Cu2+ include atomic absorption spectrometry (AAS),[9] inductively coupled plasma-mass spectrometry (ICP-MS),[10] electrochemical methods,[11] chemical sensors. [12]and fluorescent analysis.[13] Normally, the above techniques need expensive instruments, complex pre-processing, pre-concentration, or on-lab testing. Therefore, it calls for development of a simple, sensitive
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and fast assay to meet the requirement of on-site and rapid test. Surface-enhanced Raman scattering (SERS) spectroscopy with rich information of fingerprints has many merits of facile sample pretreatment, noninvasive detection, and high sensitivity, [14–17] which is regarded as a promising technique in the field of environmental monitoring. For instance, Sougata et al. established a Cu2+ SERS method by taking advantage of 2-mercaptobenzimidazole responsive exclusively towards Cu2+ ions but the sensitivity is not good enough to meet the requirement of EPA.[18] Li et al. synthesized 3,5-dimethoxy-4-(6 -azobenzotria-zolyl) phenol modified silver nanoparticles as SERS reporter and then was further functionalized with L-cysteine (L-Cys) for SERS detection of Cu2+ ions. [19] However, this synthesis method is time-consuming of 18 h. Ndokoye et al. used gold nanostars coated with L-Cys to realize SERS detection of Cu2+ but its sensitivity should be further improved. [20] What’s more, nanostars had poor stability and they were easily transformed into spherical and metamorphic. [21] It needs more effort to explore the new strategies for elevating SERS sensitivity in case of the detection of trace Cu2+ (Table 1). In this work, we obtained a stable SERS substrate by preparing inositol hexaphosphate (IP6) protected Ag nanoparticles (Ag NPs), designated as IP6@Ag. And then, IP6@Ag was modified with L-cysteine and Rhodamine 6 G(R6 G), denoted as L-Cys/IP6@Ag-R6 G. Interestingly, L-Cys/IP6@Ag-R6 G trending to aggregation could have the strong SERS signal of R6 G. In the presence of trace Cu2+, IP6
Corresponding authors. E-mail addresses: [email protected] (X. Guo), [email protected] (H. Yang).
https://doi.org/10.1016/j.snb.2018.12.043 Received 8 June 2018; Received in revised form 2 December 2018; Accepted 9 December 2018 Available online 10 December 2018 0925-4005/ © 2018 Published by Elsevier B.V.
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Table 1 Determination of Cu2+ in river water by Inductive Coupled Plasma Emission Spectrometer and L-Cys/IP6@Ag-R6 G-based SERS methods (n = 5). Samples
riverwater
Spiked amount
SERS amount
SERS Recovery%
RSD%
Spiked amount
ICP amount
ICP Recovery%
(nM) 10 1 0.1
8.87 0.97 0.113
88.7 97.2 113.4
7.2 9.5 10.3
(μM) 10 1 0.1
(μM) 9.77 1.05 0.115
97.7 105.4 114.9
possessing six phosphates could accumulate Cu2+ ions via strong interaction. Next, L-Cys molecules in this detection system happened a selective oxidation reaction by Cu2+ ions and it resulted in dispersing IP6@Ag to extent and decreasing the SERS signal of R6 G. Consequently, L-Cys/IP6@Ag- R6 G system contributed a synergetic effect to increase analytic sensitivity of Cu2+ ions and limit of detection (LOD) could be down to 10 pM, which meeting the requirement of EPA.
phosphate and 20 mmol/L sodium dihydrogen phosphate (solvent A) and methyl alcohol：acetonitrile 10:90 (solvent B), which was pumped at a flow rate of 1.0 mL/min. The ratio of solvent A to solvent B was 67:33. The wavelength of the detector was 360 nm and the injection volume was 20 μL.
2. Experimental section
L-Cys/IP6@Ag was pre-mixed with target sample at 50 °C for 15 min under keeping stirring. The Raman and SERS detections were carried out by using Dilor's SuperLabram (II) confocal microscopic laser Raman apparatus with semiconductor-cooled CCD detector (1024 × 256 pixels). The light source was helium-neon ion laser with excitation wavelength of 633 nm and the acquirement time for each spectrum was set at 8 s with 3 accumulations.
2.4. SERS detection
2.1. Materials and instruments Silver nitrate (AgNO3), trisodium citrate, sodium of inositol hexaphosphate (IP6), phytic acid (PA), rhodamine 6 G(R6 G), L-cysteine (LCys), copper chloride (CuCl2), calcium chloride (CaCl2), cobalt nitrate (Co(NO3)2), manganese nitrate (Mn(NO3)2), zinc nitrate (Zn(NO3)2), sodium chloride (NaCl), ferric chloride (FeCl3), ferrous chloride (FeCl2), lead nitrate (Pb(NO3)2), nickel nitrate (Ni(NO3)2), and mercury chloride (HgCl2) were purchased from sigma. All chemicals were analytical grade and used without further purification. Solutions were prepared in high-purity water from a Millipore purification system (18.2 MΩ cm).
3. Results and discussion 3.1. Characterization of L-Cys/IP6@Ag and its detection mechanism The preparation process of L-Cys/IP6@Ag- R6 G-based SERS probe and the detection mechanism for Cu2+ are cartooned in Scheme 1. First, IP6@Ag functionalized with L-cysteine could bring to certain aggregation of the nanoparticles via –COOH binding, which enables to improve SERS signal of R6 G. Next, when adding trace Cu2+ ions, Cu2+ ions could be accumalted by both of IP6 and Cys and then it happens a oxidation reaction of L-Cys by Cu2+ ions. It results in despersing the Ag NPs again and decreasing SERS signa of R6 G. The above inferring mechanism could ascertained by TEM observation. As shown in Fig. 1b, L-Cys anchoring onto Ag NPs surface via S − Ag bonding and free COO− and NH3+ groups of L-Cys through coordination bonding23 drag nanoparticles closer.[24] By DLS measurement results given in Fig. 1d, the average size of IP6@Ag NPs without L-Cys is 49 nm and IP6@Ag NPs with L-Cys, the average size of Ag NPs becomes 57 nm. The latter mgiht produce more hot spots for promoting SERS signal. Clearly, as seen in Fig. 1c, the bound Ag NPs, which are broken by adding Cu2+, would suppress the SERS signal and correspondingly, the average size of LCys/ IP6@Ag-Cu2+ alters into 52 nm (Fig. 1d). Fig. S1 demonstrates the reasonable uniformity of as-prepared Ag nanoparticles. In Fig. 2A of UV–vis spectra, surface plasmon band of L-Cys/ IP6@Ag exhibits a slight red shift from 404 to 406 nm in comparison with IP6@Ag. It should be due to increase of the local dielectric constant after introduction of L-Cys. By adding excess amount of Cu2+ (10−5 M) into L-Cys modified Ag nanoparticles in volume ratio of 1:1, the surface plasmon band of Ag NPs is observed at 404 nm with low intensity, meaning the redispersion of nanoparticles. It should be mentioned that the slight shifts might be due to the AgNPs protected by IP6. FT-IR experiment was carried out to validate the successful modification of L-Cys at the surface of IP6@Ag. In Fig. 2B(a) the spectral bands at 1590 and 1400 cm−1 correspond to the asymmetric and symmetric stretching of C = O (COOH), respectively. A band at 1540 cm −1 is due to NeH bending. The broad bands of N−H and OH− stretching are observed in the range of 3000 − 3500 cm−1. A weak band near 2550 cm−1 virtually confirms the presence of S−H group in the L-Cys molecule. [25,26] In Fig. 2B(b) recorded from L-Cys/IP6@Ag,
2.2. Synthesis of Ag NPs Ag nanoparticles were synthesized according to the previous method [22]. In detail, 0.255 g AgNO3 dissolved in 150 mL Milli-Q water, and boiled to 90 °C. Then, 5 mL of 10−3 M IP6 solution was added under keeping boiling. After 15 min, we added 3 mL of 1% trisodium citrate solution. After boiling for 6 h, the Ag nanoparticles protected with IP6 were obtained. Ag NPs were mixed with L-Cys solution (10−6 M) and R6 G solution -5 (10 M) in volume ratio of 1:1 and then the mixture was heated with water bath under 40 °C for 1.5 h under stirring. It should be mentioned the final pH should be adjusted to 7.0 by using raw PA soulution. [23]After centrifuged and washed with water, L-Cys/IP6@Ag probe was stored in a refrigerator at 4 °C. 2.3. Characterization of composition The surface plasmon resonance spectra of the prepared Ag NPs, LCys/IP6@Ag and mixture of L-Cys/IP6@Ag and Cu2+ ions were measured with a UV-7504 UV–vis spectrophotometer (Shanghai XinMao Instrument Co..Ltd.). The morphology of the nanoparticles were measured with a JEOL JEM-2000 FX transmission electron microscopy (TEM) operating at 200 kV and a JEOL JEM-2100 high-resolution transmission electron microscope (HR-TEM) instrument. For TEM experiments, the samples were directly dropped onto the carbon coated Cu grids. FTIR experiment was conducted by using Nicolet iS5 (Thermo Fisher, USA). ICP measurement was performed on Vista MPX inductive coupled plasma emission spectrometer. DLS experiment was employed by Zetasizer Nano ZS90. We conducted a verification experiment using HPLC method (a Thermo U3000 HPLC system). Chromatographic separation was achieved on a Spherisorb CNW Athena C18-Wp (4.6 mm × 150 mm, 5 um) S/N:R4780008 at 35 °C by a thermostat. The binary mobile phase contained both 20 mmol/L sodium hydrogen 279
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Scheme 1. Preparation process of Cys modified IP6@Ag NPs with R6 G and possible detection mechanism.
interaction of −COO– and Ag presents a stretching band of −COO– at 1740 cm −1 and the cleavage of S−H results in the invisibility of the corresponding band at 2550 cm−1, which hints that L-Cys molecules acts as a bridging reagent in connection of the Ag NPs.
We have done a control experiment under nitrogen atmosphere. Briefly, after the sample solution was degassed by nitrogen for 20 min to remove oxygen, the reaction of Cu2+ and cysteine was observed and as presented in Fig S2. The similar results with and without oxygen were acquired, showing no obvious interference from the oxygen on the above reaction. As consequence, in this work, the mechanism by Cu2+induced oxidation of L-Cys could be confirmed. In order to further prove that copper ions and cysteine have been oxidized, we conducted a verification experiment using HPLC method. Briefly, the solution of LCys /IP6@Ag and Cu2+/ L-Cys /IP6@Ag were centrifuged, and then the supernatant was tested using HPLC to examine the content of L-Cys. The results shown in Fig. S3, and we found that after the copper ions reacted with L-Cys/IP6@Ag substrate, cysteine was not detected in the
3.2. Quantitative detection performance In Fig. 3a, clearly, with increasing concentration of Cu2+, SERS intensity of R6 G is decreased. The cysteine oxidation-reduction reaction equation is given as follows: Cu2+ + cysteine = cystine + Cu+ (1) The standard potential of Cu2+ + e− = Cu+ is 150 mV, while the standard potential for oxidation of L-cysteine to cystine is 340 mV [27].
Fig. 1. TEM images of (A)Ag nanoparticles protected with phytate (B) Cys modified Ag NPs (C) Cu2+ mixed with L-Cys/IP6@Ag substrate.(D) average particle size of Ag NPs, L-Cys/IP6@Ag or L-Cys/IP6@Ag-Cu2+ estimated by DLS method. 280
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Fig. 2. (A)UV–vis spectra of AgNPs, L-Cys/IP6@Ag and L-Cys/ IP6@Ag-Cu2+. (B)FTIR spectra of powder Cys (a) and L-Cys/IP6@Ag (b).
Ca2+, Co2+, K+, Na+, Fe2+, Fe3+, Ni2+, Pb2+, Mn2+, Zn2+, and Hg2+ were examined. According to the statistic profiles in Fig. 3d, this L-Cys/ IP6@Ag-R6 G-based SERS method could selectively detect trace Cu2+. Based on previous research [28], L-Cys can be selectively oxidize by cupric ions. Inter-molecular copper complexes are the easiest to form and their enthalpy will be higher. Additionally, in the case of intramolecular reactions, the thiol moieties are correctly coordinate Cu2+ ions than other ions. However, the as-prepared L-Cys/IP6@Ag substrate is slightly
supernatant, which proved that copper ions oxidized cysteine rather than the dissociation of L-Cys from IP6@Ag by coordination between Cu and L-Cys. The lowest detectable concentration for Cu2+ in deionized water could be down to 10 pM (S/N = 3). In Fig. 3c, a linear relationship plotted between the intensities of indicative peak at 1509 cm−1 and the logarithm of concentrations of Cu2+ is ranging from 1 × 10-6 to 1 × 10−1° M. The error bars were obtained from at least 5 times measurements. In order to test the selectivity of this method, several possible co-existing ions involving
Fig. 3. (A) SERS spectra of R6 G (a) L-Cys/IP6@Ag, (b) L-Cys/ IP6@Ag with Cu2+ (c) Ag NPs (B) SERS spectra of L-Cys/IP6@Ag react with different concentrations of Cu2+ after adjust pH to 7.0. Spectrum from a to h:0 M,10-5 M, 10-6 M,10-7 M, 10-8 M, 10-9 M, 10-10 M, and 10-11 M. (C) The calibration curve plotted by the SERS intensities at 1509 cm-1 and the varying concentrations of Cu2+ with R2 = 0.9907 (D) the SERS selectivity of detecting Cu2+ by using L-Cys/IP6@Ag and the concentrations for other metal ions are set at 10-4 M. 281
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Table 2 Comparisons of analytical performances of various detection methods for Cu2+. Method
Linear range
Limit of detection
Detection time
Stability
Reference
SERS Fluorescence SERS Electrodeposition Colorimetry MC-LR SERS
8.5-40μM 1-10μM 10−5-10-8M 50-300ppm 0.2-0.6M 0-120nM 10−6-10-10M
10μM 2.7 × 10−8M 10−8M 50ppm 0.04μM 2.8nM 10pM
1.5h — 140s — 5min 1h 15min
— — 24.4% — — 1.1% 10.15%
[20] [31] [30] [28] [32] [33] This work
Appendix A. Supplementary data
alkaline due to using inositol hexaphosphate and it results in poor selectivity of SERS detection as shown in Fig. S4a. Therefore, HCl and phytic acid were repectively employed to adjust the pH. Clearly, in Fig. S4b, using phytic acid to control pH of SERS substrate is much better than HCl. The possible reason is that Ag NPs protected by phytate ions mixed with phytic acid might introduce the buffer effect with high torelence to acid and basic media. Additionally, HCl resulted in the serious agglogation of Ag NPs. In order to study the reproducibility of this method, the SERS spectra of R6 G were recorded by using 10 batches L-Cys/IP6@Ag substrates, prepared by controlling the same conditions. As shown in Fig. S5, the relative standard deviation (RSD) is estimated about 10.15%, suggesting that the reasonable preparation reproducibility and the quick SERS method meets the requirement for real application. For evaluating the method reliability, trace Cu2+ ions spiked river water samples were detected by inductive coupled plasma emission spectrometer (ICP) and the proposed L-Cys/IP6@Ag-R6 G-based SERS assay. It should be stated, for SERS measurements, the mimic samples (10−2–10−4 M) were respectively diluted several times with ultrapure water to meet the concentration linear dynamic range. As a consequence, the L-Cys/IP6@Ag-R6 G-based SERS method could determine trace Cu2+ in river water down to 10 pM with the recoveries from 88.7% to 113.4%, indicating the high sensitivity of the method with satisfactory robustness. The SERS spectra of the determination of Cu2+ in river water could be found in Fig. S6. For highlighting the merit of the L-Cys/IP6@Ag-R6 G-based SERS method, the some determination assays of Cu2+ in literature are listed in Table 2 [29–33]. Clearly, the detection time, Stability, sensitivity and linear range of the present method with a lower limit of detection are comparable or at the better position with respect to other methods.
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4. Conclusion In this work, we proposed a new strategy that modification of Ag nanoparticles (Ag NPs) by L-cysteine and IP6. The L-Cys/IP6@Ag probe could synergistically elevate SERS sensitivity for realizing detection of trace Cu2+, when Rhodamine 6 G was used as Raman probe. In addition, the SERS detection selectivity was due to the specific reaction between L-Cys and Cu2+ ions. L-Cys/IP6@Ag-R6 G-based SERS protocol has been applied to quickly determine trace Cu2+ in river water with high sensitivity down to 10 pM without sample pretreatment.
Acknowledgements This work is supported by the National Natural Science Foundation of China (21475088), Yangfan Program of Shanghai Science and Technology Committee (15YF1409000), project from Shanghai science and Technol-ogy Committee (15520502900), International Joint Laboratory on Resource Chemistry (IJLRC) and State Key Laboratory of Environmental Chemistry and Ecotoxicology.
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