Polyethylenimine-carbon nanotubes composite as an ...

Talanta 160 (2016) 607–613

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Polyethylenimine-carbon nanotubes composite as an electrochemical sensing platform for silver nanoparticles Shuo Duan, Rui Yue, Yuming Huang n The Key Laboratory of Luminescence and Real-Time Analytical Chemistry, Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China

art ic l e i nf o

a b s t r a c t

Article history: Received 4 June 2016 Received in revised form 28 July 2016 Accepted 2 August 2016 Available online 2 August 2016

For the first time, polyethylenimine (PEI) and carbon nanotubes (CNT) composites (PEI-CNTs) were employed for investigation of electrochemical response of silver nanoparticles (AgNPs) in this work. The PEI-CNTs were easily prepared by sonicating the mixture of PEI and CNTs solution and characterized by SEM, Raman and electrochemical impedance spectroscopy (EIS). The PEI-CNTs/GCE enhanced the electrochemical response for three AgNPs with different capping agents, including citrate-AgNPs (Cit-AgNPs), humic acid-AgNPs (HA-AgNPs) and gum acacia-AgNPs (GA-AgNPs), by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronocoulometry (CC). On these findings, a new electrochemical method based on LSV was developed for the detection of AgNPs. Under the optimum conditions, the proposed method showed a good linear range from 5 to 200 ng/L, and the limits of detection (LODs) were 4.77 ng/L for Cit-AgNPs, 2.48 ng/L for HA-AgNPs and 1.01 ng/L for GA-AgNPs, respectively. The potential of using the PEI-CNTs modified GCE for determining AgNPs in water samples has been successfully demonstrated. & 2016 Elsevier B.V. All rights reserved.

Keywords: Silver nanoparticles Carbon nanotube Polyethylenimine Cyclic voltammetry Linear sweep voltammetry Chronocoulometry

1. Introduction Due to the unique optical, electronic, antibacterial and catalytic properties, silver nanoparticles (AgNPs) have been widely used in abroad range of fields, such as medical industry, water treatment, catalytic and analytical areas [1–7]. As one of the most common nanotechnology-based consumer products, AgNPs has triggered the concern over their potential risks to both environmental and human health [8–10]. Previous studies showed that the potential toxicity of AgNPs cannot be underestimated in their transformation progress in natural environment, including aggregation, sedimentation, adsorption, dissolution and reduction [6,11]. In addition, AgNPs could be formed naturally and exist in the natural environment [12]. For these critical reasons, there is an imperative to develop analytical methods for AgNPs detection [13]. Various analytical techniques have been developed for detection and characterization of AgNPs, including ICP-MS [14,15], UV– Vis spectra [16] and electrochemical methods [17,18]. Among these techniques, electrochemical methods have attracted a great attention due to their advantages of quick response, inexpensive instrument and low energy consumption, and various NPs including AuNPs [19,20], MoNPs [21], PtNPs [22], Cd-based QDs [23], fullerene [24] and AgNPs [25–31] have been measured or n

Corresponding author. E-mail address: [email protected] (Y. Huang).

http://dx.doi.org/10.1016/j.talanta.2016.08.011 0039-9140/& 2016 Elsevier B.V. All rights reserved.

investigated by electrochemical methods [32]. In particular, since anodic particle coulometry (APC) was firstly reported for the electrochemical detection of AgNPs by Compton's group [32], a series of electrochemical methods [25–29] and related coupling techniques [30–32] have been utilized in the detection of AgNPs. For instance, Cheng et al. fabricated a disposable poly (L-cysteine) modified electrode for the detection and quantification of commercial AgNPs [26]. Also, the commercial gold recordable CDs could be fabricated as a disposable gold electrode for AgNPs detection [27]. Cepriá et al. proposed a quick and simple method for the detection of AgNPs using screen printed electrodes [28]. In another work, Goda et al. reported an electrochemical method for simultaneous detection of AgNPs and MoNPs at a commercial glassy carbon electrode [29]. On the other hand, previous work has demonstrated that the capping agents could affect the catalytic properties of the resulting metal NPs [33]. However, the investigation on the effect of capping agents on the electrochemical response of AgNPs is rare. To our knowledge, there is only one paper dealing with such an effect reported by Compton's group, who investigated the influence of capping agents on the oxidation of silver nanoparticles by using anodic particle coulometry on an unmodified GCE [34]. Herein, we reported the electrochemical measurement of AgNPs by PEI-CNTs modified GCE for the first time. PEI-CNTs composite was used because there was a possible synergic effect between CNTs (electron-transfer promotion and electrochemical signal enhancement) and PEI (dispersion for CNTs and enrichment for AgNPs). In

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S. Duan et al. / Talanta 160 (2016) 607–613

addition, PEI is an important type of nitrogen-rich polymers and contains primary and secondary amine groups [35], which possess good metal chelation properties. Thus, it is anticipated that AgNPs may achieve satisfactory responses at the PEI-CNTs composite modified electrode. To this end, the electrochemical response of the Cit-AgNPs (widely used AgNPs in a lab), GA-AgNPs (a kind of commercial engineering AgNPs) and HA-AgNPs (a kind of natural AgNPs [12]) were investigated at PEI-CNTs/GCE by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronocoulometry (CC). The effect of capping agent on electrochemical behaviors of three AgNPs was also discussed. Under the optimization conditions, the linear relationships and detection limits of Cit-AgNPs, HA-AgNPs and GA-AgNPs were obtained. The potential of using the PEI-CNTs modified GCE for determining AgNPs in water samples has been successfully demonstrated.

2.4. Preparation of PEI-CNTs modified electrodes Prior to modification, the GCE (Φ ¼ 3 mm) was polished carefully on a polishing cloth with 0.05 mm alumina powder and rinsed with ultrapure water. Then the GCE was treated by sonication in ethanol and ultrapure water prior to use. For preparation of PEI-CNTs modified electrode, 1 mg PEI-CNTs was dispersed in 1 mL ultrapure water by ultrasonic to form PEI-CNTs dispersion (1 mg/mL). Then an aliquot of 2 μL of PEI-CNTs dispersion was droplet coating on the GCE surface and then the solvent was evaporated at room temperature to obtain the PEI-CNTs/GCE. For comparison, PEI/GCE and CNTs/GCE were prepared by a similar process, namely, 2 μL PEI solution (1 mg/mL) or 2 μL CNTs dispersion (1 mg/mL) was droplet coating on the GCE surface and then dried in air, respectively. 2.5. Electrochemical measurements

2. Experimental 2.1. Reagents and instruments Single-walled carbon nanotubes (SWCNTs) (with diameter of 1–2 nm and length of 30 mm) were purchased from XFNANO Materials Tech Co., Ltd. PEI (with average MW of 25000), humic acid and gum acacia were purchased from Aladdin. Trisodium citrate and other chemicals were purchased from Sinopharm-Group chemical reagent corporation. They were used without further purification. All the solutions were prepared and diluted by ultrapure water. Electrochemical measurements were carried on a CHI660E electrochemical workstation (Chenhua, Shanghai) or an IM6ex electrochemical workstation (Zahner, Germany). The zeta potential determination was carried on a Zetasizer Nano-ZS90 instrument (Malvern, UK). Scanning electron microscopy (SEM) images were taken on Hitachi S-4800 field emission scanning electron microscope (Hitachi, Japan). Transmission electron microscopy (TEM) images were obtained on Phlips Tecnai G2 20 electron microscopy (FEI, USA). 2.2. Preparation of AgNPs with different capping agents Cit-AgNPs, HA-AgNPs and GA-AgNPs were synthesized following the procedures reported [36–38] with modifications. For synthesis of Cit-AgNPs, 320 mg trisodium citrate was added to a flask containing 800 mL ultrapure water with vigorous stirring for about 10 min, and then 10 mL AgNO3 aqueous solutions (0.02 M) were added dropwise with constant stirring. Finally, 10 mL NaBH4 aqueous solutions (8 mg/mL) were quickly added with vigorous stirring for 12 h. For synthesis of HA-AgNPs or GA-AgNPs, 0.5 mL AgNO3 aqueous solutions (0.02 M) were added dropwise into 50 mL HA or GA aqueous solutions (6 mg/L) with the 5 min stirring. Then 2 mL NaBH4 aqueous solutions (2 mg/mL) were quickly added with vigorous stirring for another 30 min. Three AgNPs were diluted with ultrapure water to achieve standard solution with different concentrations. TEM images showed that three AgNPs exhibited similar particle sizes and morphologies (Fig. S1). 2.3. Preparation of PEI-CNTs composite PEI-CNTs composite was prepared based on previous works by ultrasonic method [39,40] with a minor modification. Briefly, 100 mg CNTs and 10 mg PEI were mixed and sonicated in 200 mL ultrapure water for 1 h. Then the mixed solution was transferred into a flask and stirred overnight. After completion of the reaction, the resulting suspension was centrifuged and the obtained product was washed by ethanol and ultrapure water in turn.

The employed three-electrode system consisted of a modified glass carbon electrode (GCE) as the working electrode, a platinum wire as the counter electrode, and an Ag/AgCl (3.0 M KCl) electrode as the reference electrode. All the electrochemical measurements were performed at room temperature. Electrochemical tests, including cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronocoulometry (CC), were performed on a CHI660E electrochemical workstation. The above measurements were performed in pH 5.6 PBS solutions containing a certain amount of analytes. Before LSV or CV measurement, electrode was immersed in electrolyte for a certain preconcentration time. Electrochemical impedance spectroscopy (EIS) was carried out by an IM6ex electrochemical workstation. EIS measurement was performed in 0.1 M KCl solution containing 1.0 10 3 M [Fe (CN)6]3 /[Fe(CN)6]4 .

3. Results and discussion 3.1. Characterization of PEI-CNTs composite The morphologies of the CNTs and PEI-CNTs were characterized by SEM. Fig. 1A and B show the SEM images of CNTs before and after PEI treatment. It can be seen that CNTs exhibit uniform tubular structure. After PEI treatment, CNTs were wrapped by PEI and the obtained PEI-CNTs exhibit thicker and smooth tube wall, suggesting successful synthesis of PEI-CNTs composite. Fig. S2 displays the Raman spectra of raw CNTs and PEI-CNTs composite. Two characteristic peaks were recorded for the CNTs and PEI-CNTs. The band in 1300–1340 cm 1 is common in disordered sp2 carbon material and has been considered as the D-band. The band in 1580–1600 cm 1 attributes to the G-band and is corresponding to well-ordered graphite. The calculated ID/IG value for CNTs and PEI-CNTs are 0.93 and 0.75, respectively. This demonstrates that the wrapping of PEI on CNTs reduces the number of defects on the surface of CNTs, resulting in a decrease of disordered sp2 carbon material [39,40]. From the above result, it is clear that CNTs have been wrapped well by PEI, which was consistent with the previous reports [39,40]. In addition, PEI wrapping increased the hydrophilic property of CNTs surface due to the presence of amino group in PEI, thus the formed PEI-CNTs composite could be well dispersed in aqueous solution. Hence, PEI was not only the wrapping reagent but also the dispersant of CNTs. 3.2. Electrochemical characterization of electrodes EIS was a powerful tool to study the electrode interface properties and could provide information about the electrode surface.


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Fig. 1. SEM images of CNTs before (A) and after PEI treatment (B).

Fig. 2. (A) The EIS Nyquist plots of [Fe(CN)6]3 /[Fe(CN)6]4 at different electrodes. (B) The CVs of [Fe(CN)6]3 /[Fe(CN)6]4 at different electrodes.

EIS results (Nyquist plots,Z′′ vs. Z′) for different electrodes in a solution containing of 1.0 mM [Fe(CN)6]3 /[Fe(CN)6]4 and 0.1 M KCl with the frequencies swept from 105 to 0.1 Hz were given in Fig. 2A. After PEI and CNTs treatment, the EIS changed obviously. The EIS displayed semicircle in the high frequency region and straight line in the low frequency region. The diameters of the semicircle were corresponding to the charge transfer resistance of the electrodes. The charge transfer resistance (Rct) values of GCE, PEI/GCE, CNTs/GCE and PEI-CNTs/GCE were estimated as 58.53 Ω, 608.9 Ω, 264.1 Ω and 386.9 Ω, respectively. The Rct value of PEICNTs/GCE was lower than that of PEI/GCE and higher than that of CNTs/GCE. This is because CNTs are well known excellent electroconductive materials [41] and frequently used to improve the electroconductivity of non-electroactive materials. In contrary, PEI is a nonconductive material and has been demonstrated to cause significant changes in the electrical conductance of carbon materials [42,43]. Thus, the modification of PEI increased the resistance while the modification of CNTs promoted the charge transfer. This was confirmed by CV results for different electrodes in the same electrolyte in Fig. 2B. For all the electrodes, the peak potential difference (ΔEp) of [Fe(CN)6]3 /[Fe(CN)6]4 was lower than 80 mV. At PEI-CNTs/GCE, the ΔEp value was 70 mV. The low ΔEp indicated all the electrodes showed passable electrochemical responses and reversibility. In addition, the electrochemical redox of [Fe(CN)6]3 /[Fe(CN)6]/4 at PEI-CNTs/GCE exhibited higher responses than those at GCE and PEI/GCE. This indicated that CNTs could promote the charge transfer effectively. Compared with CNTs/GCE, the lower peak currents at PEI-CNTs/GCE indicated that

PEI enlarged the internal resistance of electrode slightly. 3.3. Electrochemical properties of AgNPs at PEI-CNTs/GCE In order to investigate the electrochemical properties of AgNPs at PEI-CNTs/GCE, CV was performed in PBS (pH 5.6) at the potential range between 0.4 V and þ0.6 V. The resulting voltammograms are shown in Fig. 3A to C. As can be seen, compared to that at GCE, CNTs/GCE and PEI/GCE, significant oxidation signals were observed for Cit-AgNPs, HA-AgNPs and GA-AgNPs at PEICNTs/GCE at the peak potentials of 313 mV, 353 mV and 378 mV (versus a Ag/AgCl reference electrode), which were ascribed for the oxidation of Ag nanoparticles to silver(I) ions [26,28]. It is noted that a positive shift in oxidation potential (from 313 mV to 353 mV and to 378 mV) was observed for Cit-AgNPs, HA-AgNPs and GA-AgNPs. Previous studies indicated that both diameter of metal NPs [44] and the coverage of metal NPs on an electrode affected the oxidation potential [20]. However, the later one is a more influential factor than the size of metal NPs on the oxidation potential [20]. The absorption amount of AgNPs in electrochemical reaction could be measured by CC and was used as surface coverage in this work. Fig. S3 shows the CC results of three AgNPs at PEI-CNTs/GCE. The calculated adsorption amount Γ followed the order ΓCit o ΓHA o ΓGA (See Supporting information). The surface coverage followed the same order as Γ, leading to the oxidation potential order of ECit oEHA o EGA. The result was consistent with previous report [20]. The above discussion indicated that the capping agents could affect the adsorption ability of AgNPs onto

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Fig. 3. (A)–(C) The CVs of Cit-AgNPs, HA-AgNPs and GA-AgNPs at GCE, CNTs/GCE, PEI/GCE and PEI-CNTs/GCE. (D) The LSVs of Cit-AgNPs, HA-AgNPs and GA-AgNPs.

electrode surface, leading to different surface coverage and electrochemical behaviors. Above result indicated that PEI-CNTs composite enhanced the electrochemical oxidation of AgNPs effectively. Thus, the PEI-CNTs/GCE was employed as a platform for AgNPs investigation in this work. Fig. 3D shows the LSVs of CitAgNPs, HA-AgNPs and GA-AgNPs at PEI-CNTs/GCE. With the different capping agents, AgNPs exhibited discrepant electrochemical properties on electrode surface. Compared with Cit-AgNPs and HA-AgNPs, GA-AgNPs exhibited better responses and higher oxidation peak current (Ipa), which might be attributed to the discrepant capping agents. Fig. S4 displays the reaction charge values of three AgNPs at different preconcentration time ranging from 0.5 h to 8 h. The higher reaction charge of GA-AgNPs indicated that GA-AgNPs could be captured and adsorbed onto electrode surface more easily than Cit-AgNPs and HA-AgNPs. Similar to previous works, the reaction charge increased linearly with the preconcentration time approximately [26,45]. In our work, 3 h was selected as the optimized preconcentration time for further work. 3.4. The effects of scan rate In order to get better electrochemical response, the electrochemical tests with different scanning rates were investigated by LSV. Fig. 4A to C show the LSVs of Cit-AgNPs, HA-AgNPs and GAAgNPs at PEI-CNTs/GCE with scanning rates from 10 mV/s to 400 mV/s, respectively. As seen, the oxidation peak current increased linearly with increase in scan rate. Fig. 4D shows the values of the oxidation peak currents of three AgNPs at different scan

rates. For three AgNPs, the oxidation peak currents and scan rates exhibited good linear relationship. The linear regression equation towards Cit-AgNPs, HA-AgNPs and GA-AgNPs were Ipa (μA) ¼ 0.8488 þ0.01716 v (mV/s), r2 ¼ 0.9963; Ipa (μA) ¼0.701þ 0.02765 v (mV/s), r2 ¼0.9969 and Ipa (μA)¼ 1.423 þ0.05375 v (mV/s), r2 ¼ 0.9960, respectively. The results indicate that the electro-oxidative reaction of three AgNPs on electrode surface is an adsorption-controlled process [46]. The adsorption rate of AgNPs from electrolyte to electrode surface would be the critical factor controlling electrochemical response. In our work, the 100 mV/s was chosen as the optimized scan rate. The apparent heterogeneous electron transfer rate constant (ks) could be calculated by the Laviron equation [46]:

Ep = E 0 + (RT /αnF )ln(RTks/αnF ) − (RT /αnF )lnv

(1)

where R is the gas constant (R¼8.314 J/(K mol)); n is the number of electrons transferred (n ¼1); F is the Faraday constant (F¼96,500 C/mol); T is the Kelvin temperature; v is the scan rate; α is the electron transfer coefficient. E0 is the standard potential and could be obtained from the Y-intercept of Ep v graph (Fig. S5). From the intercept and slope of the Ep lnv graph (Fig. S6), the ks values for three AgNPs were calculated as 0.01148 s 1 for CitAgNPs, 0.01015 s 1 for HA-AgNPs, 0.008268 s 1 for GA-AgNPs, respectively. Fig. S7 shows the relationships of lnks with Ep and Ip for three AgNPs at 100 mV/s. The lower ks indicated that more NPs were oxidized in electrochemical reaction, resulting in higher peak current and more positive peak potential.


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Fig. 4. The LSVs of Cit-AgNPs (A), HA-AgNPs (B) and GA-AgNPs (C) at different scan rates. The linear relationship between scan rate and oxidation peak current for three AgNPs (D).

3.5. Effect of pH Effect of pH on the electrochemical oxidation of AgNPs was investigated by LSVs in 0.1 M PBS electrolyte. Fig. 5B shows the oxidation peak current values of AgNPs at different pHs. As the pH increased, the electrochemical responses decreased obviously. The highest peak current of AgNPs at PEI-CNTs/GCE was achieved at

pH 5.6. Fig. 5B displays zeta potential values of PEI-CNTs and AgNPs at different pHs. At acidic condition, more electropositive zeta potential values of PEI-CNTs were observed. At pH 5.6, the difference of zeta potential between PEI-CNTs and AgNPs reached maximum. Hence, low pH could promote the electrostatic adsorption for AgNPs, and pH 5.6 was chosen as the optimized pH in our work. For HA-AgNPs and GA-AgNPs, similar relationship

Fig. 5. (A) The oxidation peak current values of Cit-AgNPs, HA-AgNPs and GA-AgNPs at different electrolyte pH. (B) Zeta potential values of PEI-CNTs, Cit-AgNPs, HA-AgNPs and GA-AgNPs at different pH.

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between pH and Ipa was observed. Above result suggested that capping agents affected the electrostatic charge of particle surface and their electrochemical oxidation.

The interference of chloride ion was efficiently masked by ClO4 (Fig. S8). 3.7. Analytical performance and real sample applications

3.6. Stability and anti-reference ability of PEI-CNTs/GCE Under the above optimized conditions, the PEI-CNTs/GCE was used to determine 100 ng/L Cit-AgNPs for 10 times by LSVs repetitively. The relative standard deviation (RSD) was 1.4%. Also, the reproducibility among six PEI-CNTs modified GCE electrodes prepared at different time is satisfactory (RSD among inter-batch is 3.0%). These features suggest good stability of the PEI-CNTs/GCE for sensing of AgNPs. To verify the selectivity and anti-interference ability of PEI-CNTs/GCE, the interference of some common ions was evaluated. It was found that 0.1 M of Na þ , K þ and PO43 , 1 mM of Ca2 þ , Mg2 þ and NH4 þ , 0.1 mM of SO42 and NO3 , 100 μg/L of Cu2 þ , Pb2 þ and Cd2 þ did not interfere with the determination of 200 ng/L AgNPs. Three electroactive interferents, including ascorbic acid (AA), urine acid (UA) and dopamine (DA), were added in measurement system successively. The result was shown in Fig. S8. When 1 μL of 5 10 3 M AA, UA and DA were added, the current had no obvious change, indicating that AA, UA and DA had no interference for the response of AgNPs. However, addition of Cl increased the peak current of AgNPs significantly, indicating that chloride ion was the main interferent in AgNPs determination. The phenomenon was consistent with a previous report about AgNPs electrochemical investigation in seawater [17].

Based on the above investigation, PEI-CNTs/GCE could be used for AgNPs sensing. Under the optimal conditions, LSVs responses of different concentrations of Cit-AgNPs, HA-AgNPs and GA-AgNPs were performed and the results were shown in Fig. 6A to C, respectively. The good linear concentration ranges of 5–200 ng/L for 3 AgNPs were obtained (Fig. 6D). The linear regression equation towards Cit-AgNPs, HA-AgNPs and GA-AgNPs were Ipa (μA) ¼ 0.0109 cþ 2.931 (r2 ¼0.9945, n ¼12); Ipa (μA) ¼0.0225c þ3.035 (r2 ¼0.9886, n ¼ 12); Ipa (μA)¼0.1049cþ 3.6333 (r2 ¼0.9916, n¼ 12), respectively. The LODs for Cit-AgNPs, HA-AgNPs and GA-AgNPs were 4.77 ng/L, 2.48 ng/L and 1.01 ng/L, respectively. Table 1 shows the comparison of analytical performances of the proposed method with those previously reported methods [25–31,45,47,48]. As can be seen, the PEI-CNTs composite gave lower LOD for AgNPs than some of previous works [28,29,31,45]. To examine the applicability of the proposed method, the recovery tests for three AgNPs were performed in lake water samples. At first, water samples were adjusted pH to 5.6 by dilute nitric acid. Then, 100 μL of the sample was transferred to a 10 mL electrolytic cell containing 0.1 M PBS solution (pH 5.6). 10.0 μL of 1 M NaClO4 aqueous solution was also added to the electrolytic cell to mask the chloride ion [17]. Using standard addition method,

Fig. 6. The LSVs of different concentrations of Cit-AgNPs (A), HA-AgNPs (B) and GA-AgNPs (C) at PEI-CNTs/GCE. (D) The linear relationship between concentration and oxidation peak current for three AgNPs.


Table 1 Comparison of analytical performances of the proposed method with those previously reported methods for electrochemical detection of AgNPs. Method

Working electrode

LOD (normalized, g/L)

Refs

N/A 3.56 10 10

[25] [26]

10

2.37 10 5 10 7 3.31 10 7 N/A 1.4 10 4

[27] [28] [29] [30] [31]

1 10 4 N/A N/A 4.77 10 9 for CitAgNPs; 2.48 10 9 for HAAgNPs; 1.01 10 9 for GAAgNPs

[45] [47] [48] This work

613

Foundation of China (No. 21277111).

Appendix A. Supplementary material oxidation Macro-GCE oxidation L-cysteine modified screen printed electrode oxidation Commercial gold compact disks oxidation Screen printed electrode oxidation GCE oxidation Pt disc electrode oxidation Poly(amic) acid filter membrane electrode oxidation GC microelectrode oxidation DMSA modified gold electrode oxidation Carbon fiber microelectrode oxidation PEI-CNTs modified GCE

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.talanta.2016.08. 011.

References [1] [2] [3] [4] [5] [6] [7] [8] [9]

Table 2 Results of recovery tests for 3 AgNPs added in lake water sample.

[10] [11]

AgNPs

Spiked (ng/L)

Found (ng/L)

Recovery (%)

Cit-AgNPs

0 10 30

– 8.95 7 0.1 26.5 7 0.3

– 89.5 88.4

[12] [13] [14]

HA-AgNPs

0 10 30

– 9.617 0.2 28.5 7 0.3

– 96.1 95.1

[15]

0 10 30

– 9.47 70.0 28.3 7 0.1

– 94.7 94.3

[17] [18] [19] [20]

GA-AgNPs

[16]

[21]

the recovery of Cit-AgNPs, HA-AgNPs and GA-AgNPs in lake water samples were determined by proposed PEI-CNTs/GCE. The results were presented in Table 2. The recoveries ranged from 88.4% to 96.1%, demonstrating that the proposed method was potentially applicable for the determination of AgNPs in environmental water samples.

[22] [23]

[24] [25] [26]

4. Conclusion In summary, PEI-CNTs composite exhibited outstanding electroanalytical performance for AgNPs. Due to the adsorption affinity towards AgNPs, PEI-CNTs/GCE could effectively promote the electrochemical response of AgNPs. At PEI-CNTs/GCE, Cit-AgNPs, HAAgNPs and GA-AgNPs all achieved significant oxidation peaks. The electro-oxidative reaction of three AgNPs on electrode surface is an adsorption-controlled process. The adsorption rate of AgNPs from electrolyte to electrode surface is a critical factor controlling electrochemical response. The capping agents of AgNPs affected the electrostatic charge of particle surface, resulting in difference in their electrochemical oxidation. The PEI-CNTs/GCE promises good stability and anti-reference ability. The success of this work offers a new approach to simple detection of AgNPs and provides a new strategy to design an electrochemical sensing platform for other nanoparticles.

Acknowledgments This work was supported by the National Natural Science

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