Embedded Au Nanoparticles-Based Ratiometric Electrochemical

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Embedded Au Nanoparticles-Based Ratiometric Electrochemical Sensing Strategy for Sensitive and Reliable Detection of Copper Ions Xinxing Wang, Guangmao Liu, Youxiao Qi, Yue Yuan, Jian Gao, Xiliang Luo, and Tao Yang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b02945 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 21, 2019

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Analytical Chemistry

Embedded Au Nanoparticles-Based Ratiometric Electrochemical Sensing Strategy for Sensitive and Reliable Detection of Copper Ions Xinxing Wang,† Guangmao Liu,† Youxiao Qi,§ Yue Yuan,† Jian Gao,† Xiliang Luo,*,† and Tao Yang*,‡ †

Key Laboratory of Optic-electric Sensing and Analytical Chemistry for Life Science, MOE; Shandong Key Laboratory of Biochemical Analysis; Key Laboratory of Analytical Chemistry for Life Science in Universities of Shandong; Key Laboratory of Eco-chemical Engineering; College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China. ‡ School of Chemical Engineering and Technology, Sun Yat-Sen University, Zhuhai, 519082, China. § College of Environment and Safety Engineering, Qingdao University of Science and Technology, Qingdao 266042, China. ABSTRACT: Ratiometric method allows the measurement of ratio changes between two signals, which can reduce the detection signal fluctuations caused by distinct background conditions, and greatly improve the reproducibility and reliability of detection. However, in contrast with the emerging dual excitation or dual emission dyes applied in ratiometric luminescence measurement, only a few internal reference probes have been exploited for the ratiometric electrochemical detection. In this paper, a gold nanoparticles@carbonized resin nanospheres composite with thermally reduced graphene oxide as scaffold (AuNPs@CRSTrGNO) has been fabricated, and the AuNPs embedded in the CRS was first used as an internal reference probe for the ratiometric electrochemical detection. The detachment and aggregation of AuNPs is suppressed by embedding in the CRS, so its redox signal is very stable, which provides feasibility for ratiometric detection. Moreover, the embedment of AuNPs, carbonization of resin spheres and hybridization with TrGNO all have played positive roles in improving the charge transfer rate, which leads to excellent electrochemical performance of the composite. Based on these characteristics of the AuNPs@CRS-TrGNO, a new ratiometric electrochemical detection platform was constructed, and copper ions (Cu2+) in simulated seawater was successfully detected. This ratiometric method has the advantages of simple design and convenient operation, and obviously improves the reproducibility and reliability of the electrochemical sensor.

Copper is one of the essential trace elements for human beings. It has important roles in various physiological processes, for instance, participating in hematopoiesis and iron metabolism. However, high level of copper can lead to serious health issues, like gastrointestinal disturbance, kidney or liver damage, and various neurological diseases.1,2 Copper ions (Cu2+), a widespread environmental pollutant, can migrate and accumulate in living organisms and result in the body’s copper content over standard,3 so convenient and reliable methods for Cu2+ determination are much-needed for effective risk assessment. Up to now, several instrumental analysis methods for Cu2+ determination have been exploited, such as atomic absorption or emission spectrometry, inductively coupled plasma mass spectroscopy, fluorescence chemical sensing, and electrochemical sensing.4-6 Among them, the electrochemical sensing method received special attention because of its simple measurement system, easy operation, low cost and high sensitivity, which shown great potential in real-time and onsite detection.7 In addition, the progress of modern nano and functional material technology provides a much broader space for the development of electrochemical sensing.8,9 However, the practical application of electrochemical sensors is greatly limited due to their poor reproducibility and reliability, which stem from hard-to-avoid variations in characteristics of electrodes (i.e., morphology and area), density and orientation

of probes, nontarget-induced reagent degradation/dissociation, as well as complex detection environments.10 To solve this problem, the ratiometric strategy originally used in fluorescence and electrochemiluminescence analysis11,12 has been introduced into electrochemical detection in the last few years.13,14 The ratiometric method measures the changes of the ratio between two electrochemical signals (a analyte-dependent signal and a reference signal), which provides more precise measurement through built-in correction to normalize electrode and environmental variables, and improves the reliability of the electrochemical detection method.10,15-17 Unfortunately, compared with the emerging dual excitation or dual emission dyes applied in luminescence ratiometric method, only a few reference signal molecules (internal reference probes) have been exploited for the ratiometric electrochemical detection. At present, it is mainly confined to a few classical electrochemical probes, like ferrocene (Fc) and methylene blue (MB). Moreover, small molecules (e.g. MB) are difficult to be directly immobilized on the electrode surfaces with high stability, they need to be labeled on DNA chains to develop DNA-assisted ratiometric sensing, which greatly increases the complexity of operation and limits the wide application of electrochemical ratiometric detection.18 For this reason, there is an urgent need to develop diversified internal reference probes that are easier and extensive to use in electrochemical ratiometric detection.

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Gold nanoparticles (AuNPs) are one of the most commonly used nanomaterials in the field of sensing, because of their large surface area, versatile surface chemistry, unique size or shape-dependent properties, and high conductivity, stability, catalytic activity as well as good biocompatibility.19 Beyond these characteristics, AuNPs also exhibits redox activity20,21 which was seldom discussed. But there are precedents for the use of AuNPs redox signals as the indicator in electrochemical detection. For instance, Ozsoz et al. reported an electrochemical genosensor for determining of Factor V Leiden mutation by using AuNPs oxidation signal as measurement signal.22 Kerman et al. also described an electrochemical-based scheme for DNA hybridization detection based on the change of AuNPs oxidation signal.23 These studies indicate that AuNPs have potential applications in providing reference signals as an internal reference probe. In this paper, a gold nanoparticles@carbonized resin nanospheres composite with thermally reduced graphene oxide as scaffold (AuNPs@CRS-TrGNO) has been fabricated, and the AuNPs embedded in the CRS was first used as an internal reference probe for the electrochemical ratiometric detection of Cu2+ in simulated seawater. The fully embedded AuNPs was straightforward synthesized in-situ in resin spheres (RS) by matrix-assisted reduction, which effectively avoids the detachment and aggregation, and greatly improves the stability of the electrode as well as the self-redox signal.24,25 Moreover, the embedment of AuNPs, carbonization of the RS and hybridization with TrGNO all could improve the electrochemical properties of the material. These characteristics of the AuNPs@CRS-TrGNO make it possible to construct ratiometric electrochemical sensing platform for Cu2+ detection with high sensitivity and reliability. The properties and electrochemical performance of the AuNPs@CRS-TrGNO-based electrochemical sensor were investigated and elaborated in detail. EXPERIMENTAL SECTION Apparatus and reagents. The electrochemical testing was performed on an electrochemical workstation (CHI 660E, Shanghai Chenhua, China) with a three-electrode cell. The working, reference and auxiliary electrodes were glassy carbon electrode (GCE) or modified GCE, saturated calomel electrode (SCE) and platinum electrode, respectively. Morphologies of the resulting materials were characterized by a transmission electron microscopy (TEM, JEM-2100, JEOL, Japan). Ultrasonic treatment was performed by an ultrasonic cleaner (KQ-500B, Kunshan Ultrasound Instrument, China) and an ultrasonic homogenizer (JY92-IIN, Ningbo Scientz, China). pH values of the working solutions were measured by a digital acidimeter (pHS-25, Shanghai Leici, China). 3-aminophenol, ethanol and chloroauric acid (HAuCl4) were obtained from the Sinopharm Chemical Reagent Co., Ltd. (China) and used as received. Ammonium hydroxide and formaldehyde solution were analytical grade and purchased from local suppliers. The TrGNO was obtained from Shanghai Second Polytechnic University. CuCl2, Pb(NO3)2, Cd(NO3)2·4H2O, Zn(NO3)2·6H2O and CoCl2 (Adamas Reagent Co., Ltd., China) were dissolved in 1% (v/v) nitric acid to prepare the stock solutions of heavy metal ions. Ultrapure water (≥18 MΩ cm-1) for the preparation of aqueous solutions was obtained from the Millipore system. The seawater sample was provided by Yellow Sea Fisheries Research Institute,

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Chinese Academy of Fishery Sciences. Before analysis, 0.22 μm filter membrane was used for filtration treatment. Preparation of the AuNPs@CRS. The AuNPs@CRS was prepared according to MacLachlan’s method with minor modifications.24 The detail is as follows: firstly, a mixture of water (24 mL) and ethanol (9.6 mL) was prepared, and 3aminophenol (0.3640 g, 3.34 mmol) was added and dissolved in it. And then 220 μL of ammonium hydroxide was injected into the above mixture, and magnetically stirred for 10 min at 30°C. Afterward, 250 μL of formaldehyde solution was injected into the reaction mixture, and stirred for 4 h at 30°C to yield the monodisperse RS. The obtained RS was purified with distilled water by repeated sonication (10 min) and centrifugation (4500 rpm, 40 min) cycles for 10 times, and dried at 60°C under vacuum. The dried RS (15 mg) was dispersed in an aqueous solution of HAuCl4 (30 mL, 1.0 mM) under magnetic stirring at room temperature for 2 h. The acquired reddish brown suspension was centrifuged and washed by ultrapure water for 3 times, and dried at 60°C under vacuum, resulting in the gold nanoparticles@resin nanospheres (AuNPs@RS). Finally, the AuNPs@CRS was obtained by pyrolysis of the AuNPs@RS under N2 atmosphere. The pyrolysis performed by heating from room temperature to 100°C (2°C/min), staying at 100°C for 2 h, then heating to 600°C (2°C/min). The sample was kept at 600°C for 6 h, then cooled to room temperature under continuous N2 flow. Preparation of the AuNPs@CRS-TrGNO. The procedure for preparation of the AuNPs@CRS-TrGNO composite solutions is as follows: certain amounts of TrGNO were dispersed in ultrapure water and processed by ultrasonication for 2 h, resulting suspensions of TrGNO with concentrations of 0.5, 1.0, and 2.0 g L-1, respectively. Then, certain amounts of AuNPs@CRS were added to the above TrGNO suspensions, respectively, with a same concentration of 1.0 g L-1. The mixed solutions were treated by ultrasonic homogenizer for 3 min, and the AuNPs@CRS-TrGNO composite solutions were obtained. According to the ratio between AuNPs@CRS and TrGNO, the obtained AuNPs@CRS-TrGNO composites were named as AuNPs@CRS-TrGNO(1:0.5), AuNPs@CRSTrGNO(1:1) and AuNPs@CRS-TrGNO(1:2), respectively. Unless otherwise stated, the AuNPs@CRS-TrGNO refers to AuNPs@CRS-TrGNO(1:1). Preparation of the modified electrodes. 10 μL of suspensions of the as-prepared materials were dripped onto fresh GCE surfaces and allowed to dry naturally in air, respectively. Then, 5 μL of 1% nafion ethanol solution was coated on these electrodes, and dried naturally to obtain the modified electrodes. The concentrations of the RS, AuNPs@RS and AuNPs@CRS suspensions are 1.0 g L-1, and that of the AuNPs@CRS-TrGNO is as described above. The corresponding modified electrodes were denoted as RS/GCE, AuNPs@RS/GCE, AuNPs@CRS/GCE and AuNPs@CRSTrGNO/GCE, respectively. Preparation of the Au/CRS-TrGNO/GCE. In order to compare the signal stability with the AuNPs@CRSTrGNO/GCE, Au/CRS-TrGNO/GCE was prepared by electrodeposition of Au on the CRS-TrGNO modified electrode. CRS was obtained by direct pyrolysis of RS according to the above operation. The preparation process of the CRS-TrGNO suspension was the same as that of the AuNPs@CRS-TrGNO suspension, except that AuNPs@CRS


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Analytical Chemistry was replaced by CRS. Then, 10 μL of the CRS-TrGNO suspension was modified onto fresh GCE surface by the above-mentioned dropping method. The obtained electrode was denoted as the CRS-TrGNO/GCE. Electrodeposition of Au on the CRS-TrGNO/GCE was proceeded as follows: the CRS-TrGNO/GCE was immersed in an aqueous solution of 5.0 mM HAuCl4, and electrodeposited by potentiostatic electrodeposition method under an atmosphere of N2. The parameters for the deposition process were: potential, -0.2 V; deposition time, 400 s. Afterward, 5 μL of 1% nafion ethanol solution was coated on the electrode, and dried naturally to obtain the Au/CRS-TrGNO/GCE. Electrochemical measurements. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used for electrochemical characterization of the modified electrodes. The electrolyte was a solution of 0.1 M KCl containing 1.0 mM [Fe(CN)6]3−/4−. Except for special declarations, the scan rate in CV test is 50 mV s-1. The parameters of EIS tests were: applied potential, 0.2 V; amplitude, 0.005 V; high frequency, 105 Hz; low frequency, 0.1 Hz. Differential pulse voltammetry (DPV) was adopted for Cu2+ determination. The pulse width, pulse period and pulse amplitude were 0.05 s, 0.5 s and 0.05 V, respectively. RESULTS AND DISCUSSION Morphology characterization. The TEM images of the RS (A), AuNPs@RS (B), AuNPs@CRS (C) and AuNPs@CRSTrGNO (D) are shown in Figure 1. It can be seen that the RS presents spherical morphology, and the diameter is around 550 nm. When HAuCl4 met with the purified RS, AuNPs were formed inside the spheres and the AuNPs@RS was obtained. This is because the RS is porous, and contains abundant hydroxymethyl groups which are reducing agents for Au3+. Under the action of hydroxymethyl groups, the Au3+ penetrated into the RS were reduced to AuNPs without external reducing agent. The AuNPs@CRS exhibits similar morphology with that of the AuNPs@RS, indicating that pyrolysis has no obvious effect on morphology of the RS and AuNPs. After hybridization with TrGNO, the AuNPs@CRS were dispersed on TrGNO nanosheets or embedded in layers, ensuring good contact between the two. Electrochemical study of different electrodes. Figure 2 shows CV and EIS plots of the AuNPs@CRS-TrGNO/GCE in a solution of 0.1 M KCl containing 1.0 mM [Fe(CN)6]3-/4-. For the sake of better understanding the function of each component and pyrolysis, the electrochemical behaviors of the other related electrodes, namely bare GCE, RS/GCE, AuNPs@RS/GCE, and AuNPs@CRS/GCE were also studied. As shown in Figure 2A, bare GCE exhibits a couple of welldefined [Fe(CN)6]3-/4- redox peaks. By contrast, the response currents of [Fe(CN)6]3-/4- decreases dramatically on the RS/GCE, indicating poor conductivity of the RS. Compared with the RS/GCE, the electrochemical response of [Fe(CN)6]3/4- on the AuNPs@RS/GCE increases obviously, due to the good electrochemical activity and high conductivity of AuNPs. At the AuNPs@CRS/GCE, the electrochemical response of the redox couple is further increased, suggesting that the carbon material obtained by pyrolysis (namely, CRS) has better electrical conductivity than the RS. In addition, it can be seen that the CV curve of the AuNPs@CRS/GCE has a large rectangular area, which indicates that the composite modified electrode has a large electrical double-layer capacitance. This can be attributed to the unique porous carbon structure and

high conductivity of the CRS.26 This phenomenon is in line with expectations and demonstrates that the RS has been successfully carbonized. AuNPs@CRS-TrGNO/GCE exhibits higher response signals of [Fe(CN)6]3-/4- and larger rectangular area of the CV curve than the AuNPs@CRS/GCE. These both should be attributed to the introduction of TrGNO, which has large specific surface area, high electrical conductivity and large capacitance.27

Figure 1. TEM images of the (A) RS, (B) AuNPs@RS, (C) AuNPs@CRS and (D) AuNPs@CRS-TrGNO. The EIS result is exhibited in Figure 2B. The Nyquist impedance plots of the above electrodes are composed of semicircle arcs and straight lines at high-frequency and lowfrequency regions, respetively. The semicircle arc corresponds to charge transfer resistance (Rct) between the electrode surface and solution. As can be seen, the Nyquist plot of the RS/GCE shows an obviously enlarged semicircular arc compared with that of bare GCE, suggesting that the modification of RS hinders charge transfer due to its poor conductivity. After the step by step improvement, the semicircle associated with Rct at the AuNPs@RS/GCE, AuNPs@CRS/GCE and AuNPs@CRS-TrGNO/GCE decrease gradually. This indicates that these strategies, namely, AuNPs embedment, RS carbonization and TrGNO hybridization, all have played positive roles in accelerating charge transfer. On the AuNPs@CRS-TrGNO/GCE, the semicircular arc is almost invisible, indicating the superior electrical conductivity of the AuNPs@CRS-TrGNO composite. Except for changes in highfrequency region, the differences in low-frequency region are also observed. As shown, the AuNPs@CRS/GCE and AuNPs@CRS-TrGNO/GCE exhibit lines with higher slope in low-frequency region compared with the other electrodes. Especially for the AuNPs@CRS-TrGNO/GCE, the line deviates significantly from the classical Warburg diffusion line with ideal 45° slant, which indicates low diffusion resistance and high capacitance performance of the modified electrode.28 The results given here are agree with the above CV results, and demonstrate that the AuNPs@CRSTrGNO/GCE has lower charge transfer resistance and higher diffusion rate of ions than the other four electrodes. These factors are beneficial to improve the electrochemical sensing performance of the AuNPs@CRS-TrGNO/GCE.



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the AuNPs@CRS-TrGNO/GCE, which caused larger effective Au surface area.25 These results confirm that the couple of redox peaks is assigned to AuNPs surface oxidation and reduction.

Figure 3. CV plot of bare GCE and the modified electrodes in a solution of 0.6 M NaCl.

Figure 2. (A) CV plot and (B) Nyquist plot of bare GCE and the modified electrodes in a solution of 0.1 M KCl containing 1.0 mM [Fe(CN)6]3−/4−. The scan rate in CV test is 5 mV s-1. CV plot obtained from the above electrodes in a solution of 0.6 M NaCl (simulated seawater) is shown in Figure 3. As can be seen, the CV curves of the bare GCE and RS/GCE exhibit no significant redox peaks. While on the AuNPs@RS/GCE, a couple of redox peak with an oxidation peak at about 0.8 V and a reduction peak at about 0.3 V is observed. The couple of redox peak is also observed on the AuNPs@CRS/GCE and AuNPs@CRS-TrGNO/GCE. In comparison with the AuNPs@RS/GCE, the redox peaks at the AuNPs@CRS/GCE and AuNPs@CRS-TrGNO/GCE show higher peak current and finer peak shape. For the AuNPs@CRS-TrGNO/GCE, the intensity of the redox peaks is much increased and the peak potential difference (ΔEp) decreases significantly, indicating that modification of electrodes with AuNPs@CRS-TrGNO caused easier and faster charge transfer at the electrode surface. We speculate that these peaks are associated with the redox activity of AuNPs. According to the literature,21 thiol protected AuNPs are redox species with multivalent and the observable redox peaks are affected by the dispersity of AuNPs and experimental conditions. Yancey et al. have observed the characteristic Au surface oxidation and reduction peaks on the Au DENs modified GCE in 0.5 M H2SO4, which are similar to our experimental result.29 Kato et al. investigated the electrochemical properties of the Au-embedded UBM carbon film electrodes, and also observed oxidation and reduction peaks assigned to Au surface oxidation and reduction.25 To further demonstrate, the CV plot of the bulk Au electrode was also tested in 0.6 M NaCl. As indicated in Figure S1, the bulk Au electrode shows similar electrochemical behavior with that of the AuNPs@CRS-TrGNO/GCE, except that the peak current intensity is much higher. This maybe because that Au content of the bulk Au electrode is much higher than that of

Figure 4. CV curves of 10 consecutive cycles on the (A) AuNPs@CRS-TrGNO/GCE and (B) Au/CRS-TrGNO/GCE in a solution of 0.6 M NaCl. For evaluating the stability of the electrode signal, the CV scanning on the AuNPs@CRS-TrGNO/GCE for 10 consecutive cycles was conducted in a solution of 0.6 M NaCl. Meanwhile, the CV performance of the Au/CRS-TrGNO/GCE, which was prepared by electrodeposition of Au on the CRSTrGNO modified electrode, was also investigated for comparison. As shown in Figure 4A, the current response of AuNPs surface oxidation and reduction remains almost unchanged on the AuNPs@CRS-TrGNO/GCE after 10 cycles of scanning. By contrast, the change of the current response on the Au/CRS-TrGNO/GCE is obvious, as demonstrated in


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Analytical Chemistry Figure 4B. This maybe because that the electrodeposited Au particles of the Au/CRS-TrGNO/GCE are located on the outer surface, which are easily detached from the surface of the electrode or aggregation with each other. While for the AuNPs@CRS-TrGNO/GCE, the AuNPs are embedded in the porous structure of CRS, so the detachment as well as aggregation is suppressed. This special embedded structure ensures the stability of the electrode signal.24,25

Figure 5. (A) CV plot and (B) DPV plot obtained from the AuNPs@CRS-TrGNO/GCE in a solution of 0.6 M NaCl containing different concentrations of Cu2+. Feasibility investigation for the ratiometric detection of Cu2+. The stable electrochemical signal of the AuNPs@CRSTrGNO/GCE makes it possible to develop electrode signal as internal reference for ratiometric detection. As a proof of concept, the Cu2+ in simulated seawater was selected as the target and detected. To study the feasibility of the ratiometric strategy in Cu2+ detection, a set of preresearch experiments was performed. Figure 5A shows the CV plot of the AuNPs@CRS-TrGNO/GCE in solutions of 0.6 M NaCl containing 25 mg L-1 and 250 mg L-1 Cu2+, respectively. As can be seen, the CV curves both exhibit three couples of redox peaks. The couple of peaks denoted as A1/C1 is related to AuNPs redox, which remains nearly unchanged with the variation of Cu2+ concentration. The other two pairs of peaks correspond to the redox couples of Cu2+/Cu+ (A2/C2) and Cu+/Cu0 (A3/C3), respectively.30 Their peak currents both increase with increasing of Cu2+ concentration. The above result demonstrates the feasibility for adopting the electrode signal, specifically, the AuNPs redox signal, as the internal reference for ratiometric detection of Cu2+. However, the sensitivity of CV is low because of the large background current. DPV technology with small background current can be used for sensitive detection. The DPV curves of the AuNPs@CRS-TrGNO/GCE in solutions of 0.6 M NaCl

containing low concentration of Cu2+ (50 μg L-1 and 500 μg L-1) is exhibited in Figure 5B. With increasing of Cu2+ concentration, the peak current of AuNPs keeps almost unchanged, and the Cu2+-related peak current increases, which is in agreement with the result of CV. It is noteworthy that the current intensity of the peak C3 is very weak at low concentration, far below that of the peak C2, which is consistent with the reported result.30 Therefore, the peak C2 centered at 0.15 V was adopted as the detection signal for sensitive detection of Cu2+.

Figure 6. (A) DPV plot obtained from AuNPs@CRSTrGNO/GCEs with different composition ratios in a solution of 0.6 M NaCl containing 250 μg L-1 Cu2+. (B) DPV plot obtained from the AuNPs@CRS-TrGNO/GCE in a solution of 0.6 M NaCl containing 250 μg L-1 Cu2+ with varied pH. Optimization of material composition ratio and detection condition. The composition ratio of the AuNPs@CRS and TrGNO in composites as well as pH value of the working solution were optimized to achieve the best performance of the electrochemical sensing platform. Figure 6A shows the DPV plot of AuNPs@CRS-TrGNO/GCEs with different compositions in a solution of 0.6 M NaCl containing 250 μg L-1 Cu2+. The order of the peak current of Cu2+ on different electrodes was found to be AuNPs@CRSTrGNO/GCE(1:1) > AuNPs@CRS-TrGNO/GCE(1:2) > AuNPs@CRS-TrGNO/GCE(1:0.5). This may be because high concentration of TrGNO (2.0 g L-1) is difficult to disperse effectively, which lead to poor combination of AuNPs@CRS with TrGNO in AuNPs@CRS-TrGNO(1:2). While low concentration of TrGNO (0.5 g L-1) has limited effect on improving of the charge transfer rate, and the advantage of large specific surface area for TrGNO can not be fully exploited in AuNPs@CRS-TrGNO(1:0.5). Based on the above result, AuNPs@CRS-TrGNO/GCE(1:1) was adopted for the electrochemical sensing.



pH value of the working solution also has a significant effect on electrochemical response of the sensing platform. As indicated in Figure 6B, the peak current of AuNPs and Cu2+ both varies with the change of pH, but the variation between the two is inconsistent. The current intensity of Cu2+ reaches its maximum at pH 5, while that of AuNPs is the highest at pH 6. Considering that Cu2+ is the target of detection, so 0.6 M NaCl (pH 5) was adopted for subsequent detection.

Table 1. Different Electrochemical Platforms for Cu2+ determination

Based on this sensing platform, the non-ratiometric detection method by using ICu2+ as measurement signal was compared with the above ratiometric detection method. The calibration curve of ICu2+ vs CCu2+ is shown in Figure S2. In contrast to the ratiometric method, the non-ratiometric method exhibited relatively large standard deviations, which are mainly ascribed to the variations of parallel electrodes (i.e., area and morphology), and complex detection environments. The result demonstrates that the ratiometric method we used can reduce the background effects by introduction a built-in correction, and improve reproducibility and reliability of the electrochemical method.

Figure 7. (A) DPV response of the ratiometric sensing platform for various concentrations of Cu2+ (2.5, 5.0, 10, 15, 25, 50, 100, 250, 500 μg L-1) in a solution of 0.6 M NaCl (pH 5). (B) Calibration curve of ICu2+/IAuNPs vs CCu2+. Results of three parallel experiments. Determination of Cu2+ by using the AuNPs@CRSTrGNO/GCE. The sensing performance of the as-prepared ratiometric electrochemical platform was evaluated under the optimized condition. Figure 7A shows the DPV current response of various concentrations of Cu2+ on the AuNPs@CRS-TrGNO/GCE. The peak current of Cu2+ gradually increases and that of AuNPs is nearly unchanged with increasing of Cu2+ concentration. The ratio of peak current between Cu2+ and AuNPs (ICu2+/IAuNPs) was found to be linear relative to Cu2+ concentration (CCu2+) in ranges of 2.5 to 25 μg L-1 (sensitivity of 0.0313 (μg L-1)-1) and 25 to 500 μg L-1 (sensitivity of 0.0100 (μg L-1)-1), as shown in Figure 7B. The limit of detection (LOD) is estimated to be 0.9 μg L-1 (S/N = 3), which is comparable to or even lower than the reported electrochemical platforms for Cu2+ determination in aqueous solution (Table 1).

Figure 8. Specificity of the ratiometric electrochemical sensor toward different interference ions (Pb2+, Cd2+, Zn2+, and Co2+) and mixture of them. The concentration of each interference ion was 500 μg L-1, 10 times more than that of Cu2+ (50 μg L-1). Selectivity, repeatability, reproducibility and stability study. The selectivity of the present ratiometric electrochemical sensor was evaluated. Several major heavy metal ion pollutants, namely Pb2+, Cd2+, Zn2+, and Co2+, were selected as interference ions. The concentration of the interference ions is 500 μg L-1, which is 10-fold in excess as compared to that of Cu2+ (50 μg L-1). As indicated in Figure 8 and Figure S3, the addition of Pb2+, Zn2+, Co2+, and Cd2+ did not result in an obvious change of the ICu2+/IAuNPs value, indicating good selectivity of the sensing platform toward Cu2+ detection.


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Analytical Chemistry The repeatability of the ratiometric electrochemical sensor was also evaluated by detecting 50 μg L-1 Cu2+ at the same electrode for ten measurements and relative standard deviation (RSD) of the ICu2+/IAuNPs was 3.8%. The reproducibility was studied by intra-assay. The RSD of the ICu2+/IAuNPs obtained at five parallel electrodes was calculated to be 3.4%, which is much lower than that of the ICu2+ (13.2%). The result indicates that the ratiometric electrochemical sensor shows good repeatability, and the reproducibility is much better than that obtained by using just ICu2+ as the measurement signal, which is also confirmed by the result of calibration curve. To explore its stability, the sensing platform was stored at 4°C when it is not used, and fifteen measurements were conducted in five days. The result showed that the DPV response could retain 92% of its original value after fifteen measurements, which implies that the sensing platform exhibited good stability. The repeatability, reproducibility and stability test results are shown in Figure S4. Recovery Test. Standard addition method was adopted to investigate the potential application value of this electrochemical sensor for Cu2+ determination in real seawater. The seawater sample was obtained from the pacific ocean, and Cu2+ is not detectable in it. For recovery rate testing, certain amounts of Cu2+ were added to the original sample. DPV plot obtained from the AuNPs@CRS-TrGNO/GCE in seawater samples with different concentrations of Cu2+ is demonstrated in Figure S5, and the experimental results are sorted in Table 2. As is shown in the table, the recovery values were 94.0%– 102.0% and RSDs varied from 4.3% to 5.3%. The result is satisfactory, which suggests that the prepared ratiometric electrochemical sensor has great potential in practical Cu2+ detection in the high salinity of seawater. Table 2. Determination of Artificially Added Cu2+ in seawater samples

CONCLUSION In this study, a novel and simple ratiometric electrochemical detection method was proposed by using AuNPs as internal reference probe. The AuNPs are embedded in the CRS, which effectively inhibits the detachment and aggregation of them and ensures the stability of their electrochemical signal. Moreover, the introduction of electrochemically active AuNPs, carbonization of RS and hybridization with TrGNO all have played positive roles in accelerating charge transfer, which leads to excellent electrochemical performance of the AuNPs@CRS-TrGNO. Based on the AuNPs@CRS-TrGNO modified electrode, a sensitive detection of Cu2+ in simulated seawater has been realized with a LOD of 0.9 μg L-1, and the reproducibility has been greatly improved in contrast with that of using single ICu2+ as measurement signal. The result of standard addition experiment indicates that the prepared ratiometric electrochemical sensor has great potential in practical Cu2+ detection in high salinity of seawater. ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. CV plot of the bulk Au electrode in a solution of 0.6 M NaCl; Calibration curve of ICu2+ vs CCu2+; DPV plot of the AuNPs@CRS-TrGNO/GCE in the presence of interferences; Repeatability, reproducibility and stability test results and DPV plot obtained from the AuNPs@CRS-TrGNO/GCE in seawater samples (PDF)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. * E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The work is supported by the National Natural Science Foundation of China (Grant 21804076, 21675092), the Taishan Scholar Program of Shandong Province of China (Grant ts20110829), the Natural Science Foundation of Shandong Province (Grant ZR2017BB040), the Applied Basic Research Program of Qingdao (Grant 17-1-1-65-jch) and the Open Fund of Key Laboratory of Optic-electric Sensing and Analytical Chemistry for Life Science, MOE, Qingdao University of Science and Technology (Grant SATM201708).

REFERENCES (1) Huang, P. J. J.; Liu, J. W. An Ultrasensitive Light-up Cu2+ Biosensor Using a New DNAzyme Cleaving a PhosphorothioateModified Substrate. Anal. Chem. 2016, 88, 3341−3347. (2) Wang, Y. L.; Su, Z. H.; Wang, L. M.; Dong, J. B.; Xue, J. J.; Yu, J.; Wang, Y.; Hua, X. D.; Wang, M. H.; Zhang, C. Z.; Liu, F. Q. SERS Assay for Copper(II) Ions Based on Dual Hot-Spot Model Coupling with MarR Protein: New Cu2+-Specific Biorecognition Element. Anal. Chem. 2017, 89, 6392−6398. (3) Liu, Q.; Liao, Y. B.; Shou, L. Concentration and Potential Health Risk of Heavy Metals in Seafoods Collected from Sanmen Bay and Its Adjacent Areas, China. Mar. Pollut. Bull. 2018, 131, 356−364. (4) Xiong, W.; Zhou, L.; Liu, S. T. Development of Gold-Doped Carbon Foams as a Sensitive Electrochemical Sensor for Simultaneous Determination of Pb (II) and Cu (II). Chem. Eng. J. 2016, 284, 650–656. (5) Deng, H. M.; Huang, L. J.; Chai, Y. Q.; Yuan, R.; Yuan, Y. L. Ultrasensitive Photoelectrochemical Detection of Multiple Metal Ions Based on Wavelength-Resolved Dual-Signal Output Triggered by Click Reaction. Anal. Chem. 2019, 91, 2861−2868. (6) Lin, Y. Q.; Wang, C.; Li, L. B.; Wang, H.; Liu, K. Y.; Wang, K. Q.; Li, B. Tunable Fluorescent Silica-Coated Carbon Dots: A Synergistic Effect for Enhancing the Fluorescence Sensing of Extracellular Cu2+ in Rat Brain. ACS Appl. Mater. Interfaces 2015, 7, 27262−27270. (7) Zhan, F. P.; Liao, X. L.; Gao, F.; Qiu, W. W.; Wang, Q. X. Electroactive Crown Ester-Cu2+ Complex with In-Situ Modification at Molecular Beacon Probe Serving as a Facile Electrochemical DNA Biosensor for the Detection of CaMV 35s. Biosens. Bioelectron. 2017, 92, 589−595. (8) Lu, L. L.; Zhou, L.; Chen, J.; Yan, F.; Liu, J. Y.; Dong, X. P.; Xi, F. N.; Chen, P. Nanochannel-Confined Graphene Quantum Dots for Ultrasensitive Electrochemical Analysis of Complex Samples. ACS Nano 2018, 12, 12673−12681. (9) Jin, J. C.; Wu, J.; Yang, G. P.; Wu Y. L.; Wang, Y. Y. A Microporous Anionic Metal–Organic Framework for a Highly Selective and Sensitive Electrochemical Sensor of Cu2+ Ions. Chem. Commun. 2016, 52, 8475−8478.



(10) Du, Y.; Lim, B. J.; Li, B. L.; Jiang, Y. S.; Sessler, J. L.; Ellington, A. D. Reagentless, Ratiometric Electrochemical DNA Sensors with Improved Robustness and Reproducibility. Anal. Chem. 2014, 86, 8010−8016. (11) Kawanishi, Y.; Kikuchi, K.; Takakusa, H.; Mizukami, S.; Urano, Y.; Higuchi, T.; Nagano, T. Design and Synthesis of Intramolecular Resonance-Energy Transfer Probes for Use in Ratiometric Measurements in Aqueous Solution. Angew. Chem. Int. Ed. 2000, 39, 3438−3440. (12) Zhang, H. R.; Xu, J. J.; Chen, H. Y. Electrochemiluminescence Ratiometry: A New Approach to DNA Biosensing. Anal. Chem. 2013, 85, 5321−5325. (13) Chai, X. L.; Zhou, X. G.; Zhu, A. W.; Zhang, L. M.; Qin, Y.; Shi, G. Y.; Tian, Y. A Two-Channel Ratiometric Electrochemical Biosensor for In Vivo Monitoring of Copper Ions in a Rat Brain Using Gold Truncated Octahedral Microcages. Angew. Chem. Int. Ed. 2013, 52, 8129–8133. (14) Luo, Y. P.; Zhang, L. M.; Liu, W.; Yu, Y. Y.; Tian, Y. A Single Biosensor for Evaluating the Levels of Copper Ion and LCysteine in a Live Rat Brain with Alzheimer’s Disease. Angew. Chem. Int. Ed. 2015, 54, 14053–14056. (15) Zhang, L. M.; Tian, Y. Designing Recognition Molecules and Tailoring Functional Surfaces for In Vivo Monitoring of Small Molecules in the Brain. Acc. Chem. Res. 2018, 51, 688−696. (16) Cui, L.; Lu, M. F.; Li, Y.; Tang, B.; Zhang, C. Y. A Reusable Ratiometric Electrochemical Biosensor on the Basis of the Binding of Methylene Blue to DNA with Alternating AT Base Sequence for Sensitive Detection of Adenosine. Biosens. Bioelectron. 2018, 102, 87–93. (17) Zhu, C. X.; Liu, M. Y.; Li, X. Y.; Zhang, X. H.; Chen, J. H. A New Electrochemical Aptasensor for Sensitive Assay of a Protein Based on the Dual-Signaling Electrochemical Ratiometric Method and DNA Walker Strategy. Chem. Commun. 2018, 54, 10359–10362. (18) Tang, Z. X.; Ma, Z. F. Ratiometric Ultrasensitive Electrochemical Immunosensor Based on Redox Substrate and Immunoprobe. Sci. Rep. 2016, 6, 35440. (19) Saha, K.; Agasti, S. S.; Kim, C.; Li, X. N.; Rotello, V. M. Gold Nanoparticles in Chemical and Biological Sensing. Chem. Rev. 2012, 112, 2739–2779. (20) Antonello, S.; Holm, A. H.; Instuli, E.; Maran, F. Molecular Electron-Transfer Properties of Au38 Clusters. J. Am. Chem. Soc. 2007, 129, 9836–9837. (21) Quinn, B. M., Liljeroth, P.; Ruiz, V.; Laaksonen, T.; Kontturi, K. Electrochemical Resolution of 15 Oxidation States for Monolayer Protected Gold Nanoparticles. J. Am. Chem. Soc. 2003, 125, 6644– 6645. (22) Ozsoz, M.; Erdem, A.; Kerman, K.; Ozkan, D.; Tugrul, B.; Topcuoglu, N.; Ekren, H.; Taylan, M. Electrochemical Genosensor Based on Colloidal Gold Nanoparticles for the Detection of Factor V Leiden Mutation Using Disposable Pencil Graphite Electrodes. Anal. Chem. 2003, 75, 2181–2187. (23) Kerman, K.; Morita, Y.; Takamura, Y.; Ozsoz, M.; Tamiya, E. Modification of Escherichia Coli Single-Stranded DNA Binding Protein with Gold Nanoparticles for Electrochemical Detection of DNA Hybridization. Anal. Chim. Acta 2004, 510, 169–174. (24) Khan, M. K.; MacLachlan, M. J. Polymer and Carbon Spheres with an Embedded Shell of Plasmonic Gold Nanoparticles. ACS Macro Lett. 2015, 4, 1351−1355. (25) Kato, D.; Kamata, T.; Kato, D.; Yanagisawa, H.; Niwa, O. Au Nanoparticle-Embedded Carbon Films for Electrochemical As3+ Detection with High Sensitivity and Stability. Anal. Chem. 2016, 88, 2944−2951. (26) Yao, L.; Wu, Q.; Zhang, P. X.; Zhang, J. M.; Wang, D. R.; Li, Y. L.; Ren, X. Z.; Mi, H. W.; Deng, L. B.; Zheng, Z. J. Scalable 2D Hierarchical Porous Carbon Nanosheets for Flexible Supercapacitors with Ultrahigh Energy Density. Adv. Mater. 2018, 30, 1706054. (27) Wang, H.; Feng, H. B.; Li, J. H. Graphene and Graphene-like Layered Transition Metal Dichalcogenides in Energy Conversion and Storage. Small 2014, 10, 2165−2181. (28) Xu, X. L.; Shi, W. H.; Li, P.; Ye, S. F.; Ye, C. Z.; Ye, H. J.; Lu, T. M.; Zheng, A. A.; Zhu, J. X.; Xu, L. X.; Zhong, M. Q.; Cao, X.

Page 8 of 9

H. Facile Fabrication of Three-Dimensional Graphene and Metal−Organic Framework Composites and Their Derivatives for Flexible All-Solid-State Supercapacitors. Chem. Mater. 2017, 29, 6058−6065. (29) Yancey, D. F.; Carino, E. V.; Crooks, R. M. Electrochemical Synthesis and Electrocatalytic Properties of Au@Pt DendrimerEncapsulated Nanoparticles. J. Am. Chem. Soc. 2010, 132, 10988– 10989. (30) Nie, M.; Neodo, S.; Wharton, J. A.; Cranny, A.; Harris, N. R.; Wood, R. J. K.; Stokes, K. R. Electrochemical Detection of Cupric Ions with Boron-Doped Diamond Electrode for Marine Corrosion Monitoring. Electrochim. Acta 2016, 202, 345–356. (31) Zhao, G. T.; Liang, R. N.; Wang, F. F.; Ding, J. W.; Qin, W. An All-Solid-State Potentiometric Microelectrode for Detection of Copper in Coastal Sediment Pore Water. Sensor. Actuat. B-Chem. 2019, 279, 369–373. (32) Lu, M. X.; Deng, Y. J.; Luo, Y.; Lv, J. P.; Li, T. B.; Xu, J.; Chen, S. W.; Wang, J. Y. Graphene Aerogel-Metal-Organic Framework-Based Electrochemical Method for Simultaneous Detection of Multiple Heavy-Metal Ions. Anal. Chem. 2019, 91,888– 895. (33) Cinti, S.; Mazzaracchio, V.; Öztürk, G.; Moscone, D.; Arduini, F. A Lab-on-a-Tip Approach to Make Electroanalysis User-Friendly and Decentralized: Detection of Copper Ions in River Water. Anal. Chim. Acta 2018, 1029, 1–7. (34) Cheng, B. W.; Zhou, L.; Lu, L. L.; Liu, J. Y.; Dong, X. P.; Xi, F. N.; Chen, P. Simultaneous Label-Free and Pretreatment-Free Detection of Heavy Metal Ions in Complex Samples Using Electrodes Decorated with Vertically Ordered Silica Nanochannels. Sensor. Actuat. B-Chem. 2018, 259, 364–371. (35) Gu, B.; Ye, M. N.; Nie, L. N.; Fang, Y.; Wang, Z. L.; Zhang, X.; Zhang, H.; Zhou, Y.; Zhang, Q. C. Organic-Dye-Modified Upconversion Nanoparticle as a Multichannel Probe To Detect Cu2+ in Living Cells. ACS Appl. Mater. Interfaces 2018, 10, 1028−1032. (36) Jiang, Y.; Chen, X. F.; Lan, L. T.; Pan, Y.; Zhu, G. X.; Miao, P. Gly–Gly–His Tripeptide- and Silver Nanoparticle-Assisted Electrochemical Evaluation of Copper(II) Ions in Aqueous Environment. New J. Chem. 2018, 42, 14733−14737. (37) Yu, Y. Y.; Yu, C.; Yin, T. X.; Ou, S. S.; Sun, X. Y.; Wen, X. R.; Zhang, L.; Tang, D. Q.; Yin, X. X. Functionalized Poly (Ionic Liquid) as the Support to Construct Aratiometric Electrochemical Biosensor for the Selective Determination of Copper Ions in AD Rats. Biosens. Bioelectron. 2017, 87, 278–284. (38) Veerakumar, P.; Veeramani, V.; Chen, S. M.; Madhu, R.; Liu, S. B. Palladium Nanoparticle Incorporated Porous Activated Carbon: Electrochemical Detection of Toxic Metal Ions. ACS Appl. Mater. Interfaces 2016, 8, 1319−1326.


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