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Ionic liquid functionalized graphene oxide-Au nanoparticles assembly for fabrication of electrochemical 2,4-dichlorophenol sensor
Accepted Manuscript Title: Ionic Liquid Functionalized Graphene Oxide-Au Nanoparticles Assembly for Fabrication of Electrochemical 2,4-Dichlorophenol Sensor Authors: Tianrong Zhan, Zhengwei Tan, Xia Tian, Wanguo Hou PII: DOI: Reference:
S0925-4005(17)30334-9 http://dx.doi.org/doi:10.1016/j.snb.2017.02.107 SNB 21838
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
15-10-2016 15-1-2017 17-2-2017
Please cite this article as: Tianrong Zhan, Zhengwei Tan, Xia Tian, Wanguo Hou, Ionic Liquid Functionalized Graphene Oxide-Au Nanoparticles Assembly for Fabrication of Electrochemical 2,4-Dichlorophenol Sensor, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2017.02.107 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ionic Liquid Functionalized Graphene Oxide-Au Nanoparticles Assembly for Fabrication of Electrochemical 2,4-Dichlorophenol Sensor
Tianrong Zhana*, Zhengwei Tana, Xia Tiana, Wanguo Houb**
a Key
Laboratory of Sensor Analysis of Tumor Marker (Ministry of Education), State Key
Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China b Key
Laboratory of Colloid & Interface Chemistry (Ministry of Education), Shandong University,
Jinan 250100, China Corresponding author, Tel & Fax: +86-532-84023927 E-mail address: [email protected] (T. Zhan) and [email protected] (W. Hou).
Ipa/A
Graphic abstract
Epa/ V
Highlights: ► The 2,4-Dichlorophenol (2,4-DCP) sensor was fabricated based on IL-GO-AuNPs modified glass carbon electrode (IL-GO-AuNPs/GCE).
► IL-GO-AuNPs/GCE effectively enhanced the electron transfer and electrocatalytic capacity toward the oxidation of 2,4-DCP.
► Differential pulse voltammetry detecting 2,4-DCP displayed a wide linear range between 0.01 to 5 μM with a low detection limit of 3 nM (S/N =3).
ABSTRACT Self-assembly of graphene oxide (GO) and Au nanoparticles (AuNPs) was covalently modified by ionic liquid (IL) containing amino groups to prepare the ternary nanocomposite of IL-GO-AuNPs. UV, IR spectra, X-ray power diffraction and scanning electron microscope confirmed the successful hybridization of these three components, leading to the larger surface area and better dispersity. Then a sensitive 2,4-Dichlorophenol (2,4-DCP) sensor was established based on IL-GO-AuNPs modified glassy carbon electrode. The IL-GO-AuNPs composite gave rise to a desirable access to the electron transfer and electrocatalytic capacity toward the oxidation of 2,4-DCP. Differential pulse voltammetry was employed for the quantitative analysis of 2,4-DCP. The proposed sensor showed a wide linear range from 0.01 to 5 μM with a low detection limit of 3 nM (S/N =3). The developed sensor was successfully used for the determination of 2,4-DCP in water samples with satisfactory recoveries and stability. The excellent electrocatalytic performance could be attributed to the high conductivity and more active sites of IL-GO-AuNPs.
Keywords: ionic liquid functionalization; graphene oxide; Au nanoparticles; 2,4-Dichlorophenol sensor; differential pulse voltammetry
1. Introduction 2,4-Dichlorophenol (2,4-DCP), a representative member of chlorphenols family, is widely used as intermediate in the production of herbicides, fungicides, pharmaceuticals and insecticides due to its broad spectrum antimicrobial properties [1]. However, European Union and the US Environment Protection Agency have listed 2,4-DCP as a class of priority control pollutants because of its high
toxicity, poor biodegradation, potentially mutagenic and carcinogenic effects on living organisms albeit at a very low concentration [2]. Gao’s group has detected 2,4-DCP in more than half of China’s surface water samples (51.3%, 1.1–19960.0 ng L−1) from over 600 sites in the seven major watersheds and three drainage areas [3].The 2,4-DCP concentrations in some water area are beyond its maximum admissible concentration in drinking water (500 ng L−1) [4]. For the sake of public health and environmental safety, it is essential to develop a highly sensitive, selective, and reliable method for 2,4-DCP determination. Recently, several analytical methods such as HPLC [5], GC-MS [6], chemiluminescence [7], capillary electrophoresis [8], and UV spectrophotometry [9] have been established for determination of 2,4-DCP. By comparison with expensive instrument, complex manipulation and inaccessibility of in situ analysis of above methods, electrochemical sensors have attracted tremendous attention because of their low-cost, real-time detection, instrument simplicity, simple operation and high sensitivity [1,4]. Various kinds of modified electrodes for 2,4-DCP detection have been successfully constructed by using different materials including imprinted microgel [10], metal-organic frameworks (MOFs) [11], carbon dots composite [4], ZnSe QDs [12] and enzyme [13]. Whereas, it is difficult to evenly immobilize these nano-modifiers on electrode surface because of their either propensity to agglomerate, or easy denaturation and complicated fabrication process. Au nanoparticles (AuNPs) present many promising physicochemical properties, such as good conductivity, high surface area and low toxicity, which afford them distinct superiorities in the fabrication of electrochemical sensors. Nevertheless, the tendency to aggregate makes AuNPs to be used in combination with other nanomaterials for the improved sensing performance. For example, the electrochemical sensors based on HS-β-cyclodextrin/AuNPs, HS-β-cyclodextrin/AuNPs/MWCNTs and AuNPs/MoS2 composites have been developed for phenol sensitive detection [14-16]. Graphene (GR) is a lightweight 2D atom-thick and honeycomb carbon material composing of all-sp2-hybridized carbon atoms. GR has especially drawn much attention in electrochemical sensing fields owing to its low mass density, high surface areas (2630 m2/g), and high electrical conductivity [17,18]. Chemical reduction of exfoliated graphene oxide (GO) are generally employed for the low-cost and large-scale production of GR sheets [19,20]. However, the van der waals force and π-π interaction can inevitably result in irreversible agglomeration of GR sheets during reduction process, which therefore compromises the full performance of this carbon material [21]. Currently, covalent [22-25] functionalizations of GO are widely used to obtain the partially reduced GO (rGO) with enhanced dispersibility [26]. Ionic liquid (IL) is a kind of organic room temperature molten salts completely comprising of cations and anions. Due to high conductivity, wide potential window and innate liquid state, IL has been widely used as modifier or binder of modified electrode for the better sensing performance. Grafting of IL can also promote the dispersity of GO or rGO based materials
owing to its special solubility [27,28]. More recently, IL has been used for covalent modification of GO or rGO sheets to improve their electrochemical properties. Studies have demonstrated an enhanced ability to store energy due to modification of GO surface with IL [29,30]. IL grafted GO has also been coated on the solid-phase microextraction fiber for extraction and enrichment of polycyclic aromatic hydrocarbons in food wrap due to the modified surface properties and the increased active sites [31]. Covalently bonding IL to GO may not only shield the π-π stacking interaction between the GO (or rGO) nanosheets, but also partially reduce GO to rGO for the better conductivity [26]. Although the covalent attachment of IL on GO or rGO is widely used in energy and separation, the synthesis of IL modified GO-AuNPs and their application in electrochemical sensor have not been reported so far. In this article, the positively charged cetyltrimethylammonium bromide (CTAB)-AuNPs solution was firstly assembled on the negatively charged GO sheets through electrostatic attraction. Then IL containing amino group, 1-aminopropyl-3-methylimidzaolium tetrafluoroborate (APMIMBF4), was covalently bound on GO surface by reaction with oxygen containing group. In the resultant hybrid, IL could effectively inhibit nanoparticles from aggregating and form a molecular film enhancing the direct electron transfer. Functionalization and hybridization endowed the composite with more active sites to facilitate the entrapment of analyte. Therefore, this IL functionalized GO-AuNPs composite (IL-GO-AuNPs) was employed to fabricate the modified electrode for investigation of behaviors and detection of 2,4-DCP. The sensing performance of IL-GO-AuNPs was also compared with the GO-AuNPs counterpart, demonstrating the construction of an acceptable 2,4-DCP sensor.
2. Experimental 2.1 Reagents and apparatus 2,4-DCP, HAuCl4·4H2O (99.9%) and trisodium citrate (99.9%) and CTAB were purchased from Aladdin Chemistry Co. (Shanghai). Graphite powder (average particle size 30 μm) and IL (APMIMBF4) were repectively obtained from Colloid Chemical Co. (Shanghai) and Lanzhou Greenchem. ILS, LICP, CAS). Other chemicals were analytical grade and directly used as received. Electrochemical measurements were carried out on CHI660D electrochemical workstation (CHI, China). Three electrode system including a modified glass carbon electrode (GCE, Φ = 3 mm) as working electrode, a platinum wire as auxiliary electrode and a saturated calomel electrode (SCE) as reference electrode was used. X-ray power diffraction (XRD) patterns were collected using a Rigaku DLMAX 2550V diffractometer (Japan) with Cu Kα radiation (λ = 0.154178 nm, graphite monochromator, 28 kV and 20 mA) from 10° to 80° with a scanning speed of 10° min−1. Morphology of solid sample was evaluated by a JSM-6610 scanning electron microscope (SEM, Japan). UV absorption spectroscopy was recorded on A TU-1901 spectrophotometer (Beijing Puxi,
China). 2.2 Synthesis of IL-GO-AuNPs Synthesis of AuNPs. AuNPs was synthesized according to the previous procedure [32]. Briefly, 1.5 ml of 1% sodium citrate solution (w%) was slowly added to 100 mL of the 0.01% HAuCl4 aqueous solution (w%) under stirring. Then 0.3 mL of 0.075% sodium borohydride solution was added to reduce Au3+ to Au0. The continuous stirring gave rise to purple AuNPs solution. Finally, 10 mL of 10 mM CTAB solution was added to the above AuNPs solution for a well dispersed AuNPs solution. Synthesis of IL-GO-AuNPs.
Firstly, GO was prepared from nature graphite powder by a modified
Hummer’s technique [33]. The resultant GO powder was ultrasonically dispersed in ultrapure water with concentration of 1 mg/mL. The same volume of GO aqueous solution (50 mL) was slowly added to the well-dispersed AuNPs suspension. The mixed solution was stirred for 8 h at 160 rpm. The brown GO-AuNPs was obtained by 10 min centrifugation at 5000 rpm. The GO-AuNPs hybrid was re-dispersed in 50 mL of ultrapure water. Then 25 mg IL (APMIMBF4) and 25 mg KOH were added to the mixture. After refluxing for 2 h, IL-GO-AuNPs composite was afforded after centrifugation and successive washing with ultrapure water and absolute alcohol. 2.3 Preparation of IL-GO-AuNPs/GCE Before modification, the GCE was polished with 0.05 μm alumina slurry to a mirror-like surface and thoroughly cleaned with ultrapure water. For preparation of IL-GO-AuNPs composite suspension, 5 mg of hybrid was dispersed in 5 mL ultrapure water. After ultrasonic reaction for 2 h, the IL-GO-AuNPs suspension was obtained with concentration as 1 mg/mL. Then, 8 μL IL-GO-AuNPs suspension was coated on the clean GCE surface and dried at room temperature. The corresponding modified electrode was denoted as IL-GO-AuNPs/GCE. Similarly, IL-GO/GCE and GO-AuNPs/GCE were also constructed through similar procedure for comparison.
3. Results and discussion 3.1 Morphology and structural characteristics Fig. 1 gives the SEM images of GO, GO-AuNPs and IL-GO-AuNPs. GO shows the large sheets with some fine crumples on its surface and scrolled edges (Fig. 1a). This large sheet may be formed through van der Waals interactions among the oxygenous groups [34]. After self-assembling between AuNPs and GO, AuNPs were dispersedly attached on GO substrate through electrostatic interaction. Form the image of GO-AuNPs, it could be seen that some AuNPs were encapsulated by the soft and thin GO slice, which resulted in more distinct wrinkles on GO surface (Fig. 1b). As for
IL-GO-AuNPs (Fig. 1c), the covalent functionalization of IL provided a layer of molecular membrance on composite. Although this process partly flattened the crumples on GO sheets, the bound IL produced the rougher surface. It was seemed that IL also promoted the dispersity of AuNPs by an amount of charges. Therefore, it was believed that the designed IL-GO-AuNPs could exert excellent sensing performance due to the increased effective surface area, conductivity and more active sites. The synthesized products were also characterized by UV-vis spectrum. As shown in Fig.2A, AuNPs (curve a) presented its typical peaks at 550 nm. GO (curve b) showed the classic π-π* plasmon peak at 235 nm [35]. As for GO-AuNPs (curve c), the adsorption peak of AuNPs unchanged with a weakened intensity. However, the peak of GO shifted to ca. 210 nm because of the strong interaction between GO and AuNPs. After IL functionalization, by comparison with GO-AuNPs, IL-GO-AuNPs displayed a negligible broad at ca. 270 nm (indexed by arrow), indicating the small partial reduction of GO. These results demonstrated the successful preparation of IL-GO-AuNPs. The XRD patterns of GO-AuNPs and IL-GO-AuNPs were shown in Fig. 2B. GO-AuNPs (curve a) showed a diffraction (002) peak of GO at 11.2° and typical reflections of AuNPs including the (111), (200), (220), and (311) diffraction peaks at 38.04º, 44.62º, 63.26º and 78.92º, respectively (JCPDS 04-0784). The coexistence of GO and AuNPs signals revealed the successful self-assembling of two species. However, as regards IL-GO-AuNPs (curve b), a new (002) diffraction peak of rGO at 23.16 º was also observed except for the typical reflections of GO and AuNPs. This new phase suggested that partial oxygen functional groups had been removed [36] due to the covalent modification of IL. These results manifested that functionalization of IL toward GO-AuNPs was realized. The FT-IR technique was also employed to evaluate the covalent functionalization of GO-AuNPs with IL containing amino group as shown in Fig. 3. It could be seen that GO-AuNPs (curve a) displayed the peaks of aromatic C-H stretching vibrations at 703, 830 and 983 cm-1, C-O and C-O-C vibrations in epoxy at 1120 and 1405 cm-1. The C=C and C=O stretching bands were also distinctly observed at 1637 and 1856 cm-1, respectively. Additionally, C-H and -NH2 characteristic absorptions appeared at 2940 and 3455 cm-1, respectively [37]. After functionalization with IL, the intensity of peaks related to the C-O, C-O-C and C=O stretching bands was obviously decreased. Particularly, the aromatic C-H and C=O stretching vibrations respectively at 983 cm-1 and 1856 cm-1 were almost disappeared after IL functionalization. The results confirmed that the amino group in IL reacted with oxygen-containing groups in GO sheets. Therefore, it was suggested that IL molecules were successfully attached onto the GO nanosheets surface via amide reaction.
3.2 Electrochemical characterization of electrodes The modified electrodes were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in 5 mM [Fe(CN)6]3−/4− solution containing 0.1 M KCl. As could be seen from CVs results of Fig. 4A, GO/GCE (curve a) showed the smallest voltammetric response because of the intrinsic non-conductive nature of GO. The background and peak current of IL-GO/GCE (curve c), GO-AuNPs/GCE (curve d) were obviously larger than those for bare GCE (curve b). It was suggested that IL-GO or GO-AuNPs composite films improved the effective active area of electrodes and high conductive IL or AuNPs components accelerated the electron transfer rate. As for IL-GO-AuNPs/GCE (curve e), the voltammetric response is further higher than those for IL-GO/GCE and GO-AuNPs/GCE. It was attributed to the fact that the covalent introduction of IL not only enhanced the surface area and active sites, but also improved the conductivity and dispersity of the ternary composite. Nyquist plots of same modified electrodes as in Fig. 4A were depicted in Fig. 4B. The impedance spectroscopy contains a linear section indexing the diffusion-limited process, and the semicircle section signifying the charge transfer-limited process (Rct) [34,38]. GO/GCE (curve a) displayed the largest impedance owing to the poor conductivity of GO. For the bare GCE (curve b), a well-defined semicircle with Rct of 790 Ω was found, revealing the distinct interface resistance against electron transfer. When IL-GO was modified on the GCE surface (curve c), its impedance was greatly decreased owing to the improved electron transfer efficiency from GO and conductive IL. As for GO-AuNPs/GCE (curve d), it could be observed a much smaller semicircle than IL-GO/GCE, which was ascribed to the electric conductor of AuNPs. By comparison, IL-GO-AuNPs/GCE (curve e) exhibited almost a straight line with a very small Rct value. Hence, it could be suggested that the synergistic effects of the components in IL-GO-AuNPs entrapped more Fe(CN)63−/4− on the basing electrode surface and facilitated the electron communication. 3.3 CV behaviors of 2,4-DCP on modified electrodes The CV behaviors of 0.1 mM 2,4-DCP in 0.1 M pH=8.0 PBS on different electrodes were investigated. As shown in Fig. 5, the GO/GCE (curve a) presented the lowest oxidative signal, manifesting the resistance of GO film against electrocatalytic oxidation toward 2,4-DCP. As for the bare GCE (curve b), a slightly enhanced oxidative peak was observed at 0.57 V. After coating IL-GO (curve c), the oxidative peak current dramatically increased, manifesting that IL-GO film could promote the electrochemical oxidization toward 2,4-DCP. GO-AuNPs/GCE (curve d) displayed the stronger oxidative responses than those at IL-GO/GCE. It was supposed that the stronger conductivity of AuNPs than IL highly promoted the electron transportation from 2,4-DCP to GCE. However, IL-GO-AuNPs/GCE (curve e) gave rise to the biggest oxidative peak among five electrodes. The excellent electrocatalytic performance of IL-GO-AuNPs could be resulted from the
following aspects. i) Electrostatic assembling and covalent binding of IL avoided the aggregation of building blocks and enhanced the dispersity of IL-GO-AuNPs. ii) The covalent modification of IL toward GO-AuNPs highly enlarged the surface area and further increased the conductivity and active sites of the IL-GO-AuNPs composite. iii) The π-π and hydrophobic interactions between 2,4-DCP with GO (part rGO) and IL also played important role on the prominent electrocatalytic capability [39]. 3.4 Effects of pH The effects of PBS with different pH on CV responses of 2,4-DCP were investigated. Fig. 6A shows the CVs of 2,4-DCP at IL-GO-AuNPs/GCE in 0.1 M PBS with pH ranged from 4 to 10. In the pH range from 4-10, the oxidative peak current of 2,4-DCP reached the biggest value at pH of 8.0 (Fig. 6B, curve a). Thus pH 8 of PBS was chosen as the supporting electrolyte for the expectable sensitivity. The optimal response at pH about 8.0 could be due to the existing form of 2,4-DCP with a pKa value of 7.98 [10]. On the other hand, the oxidation peak potential proportionally shifted to the negative direction as pH increased (Fig. 5B, curve b). The linear equation of the peak potential (Epa) vs. pH was obtained as Epa (V) = −0.0528 pH + 1.0384 (R = 0.9993). A slope of 52.8 mV/pH is approximately close to the theoretical Nerstian value of −59 mV/pH, implying that the equal number of electron transfer and protons electro-oxidation process of 2,4-DCP on IL-GO-AuNPs/GCE [27,40]. 3.5 Effects of scan rate The influences of different scan rate on IL-GO-AuNPs/GCE were also carried out for the further understanding of electrocatalytic mechanism of 2,4-DCP. As illustrated in Fig. 7A, it exhibited a linear relationship between the oxidation peak currents (Ipa) of 2,4-DCP and the square root of scan rate from 20 to 300 mV/s (Inset in Fig. 7A). The regressing equation could be expressed as Ipa (μA) = 0.24 v1/2 (mV/s) － 0.3045 (R = 0.993), indexing a typical diffusion-controlled process of 2,4-DCP at IL-GO-AuNPs/GCE [41]. It was also seen that the oxidative peak potential (Epa) increased against lnv with a regression equation of Epa (V) = 0.025 ln v + 0.5069 (R = 0.999) (Fig. 7B). Based on the Laviron’s theory, Epa could be presented by the Laviron equation [42] as following: Epa= E0 + (RT/αnF) ln(RTk0/αnF) + (RT/αnF) ln ν
(1)
For which α is transfer coefficient (assumed as 0.5 for the totally irreversible electrode process), k0 is standard rate constant, n is electron transfer number, ν is scan rate, E0 is formal redox potential, R is the gas constant (R = 8.314), T is the absolute temperature (T = 298), and F is the Faraday constant (F = 96485). According to the slope (equal to RT/αnF) of linear equation, the electron transfer number (n) was thus calculated as about 2. Therefore, the oxidation process of
2,4-DCP on IL-GO-AuNPs/GCE is a two-electron and two-proton process [4]. The possible electrochemical process of 2,4-DCP on the modified electrode might be depicted as shown in Scheme 1 [11]. 3.6 Effects of IL-GO-AuNPs content, accumulation time and potential For better sensitivity, effects of the loading of IL-GO-AuNPs, accumulation time and accumulation potential were also carefully optimized. For the content of IL-GO-AuNPs suspension (Fig. 8A), the peak current of 2,4-DCP reached the maximum at 1.0 mg mL-1 in the concentration range from 0.5 to 3.0 mg mL-1. So 1.0 mg mL−1 of IL-GO-AuNPs colloidal suspension was chosen to avoid too much modifier blocking the electron transfer. As for the accumulation time (Fig. 8B), the oxidation current peak response continuously increased before the accumulation time 120 s, then slightly decreased and leveled off after that moment. It could be supposed that the binding of 2,4-DCP and IL-GO-AuNPs had reached equilibrium. So the 120 s accumulation time was used in experiment. In the accumulation potential diagram (Fig. 8C), the oxidation peak showed the maximum at −0.25 V in the potential window between −0.1 to −0.4 V. Therefore, the accumulation potential was set at −0.20 V in further experiments. 3.7 Chronocoulumetry Chronocoulometry was employed to comparatively study the electrochemically effective surface areas of bare GCE, GO-AuNPs/GCE and IL-GO-AuNPs/GCE in 0.1 mM K3[Fe(CN)6] solution containing 1.0 M KCl (Fig. 8A) by using the following Anson equation [43]: 1 1
𝑄 (𝑡) =
2𝑛𝐹𝐴𝑐𝐷 2 𝑡 2 1
𝜋2
+ 𝑄𝑑𝑙 + 𝑄𝑎𝑑𝑠
(2)
wherein n is the number of electron transfer, A is the effective surface area of working electrode, c is the concentration of substrate, D is the standard diffusion coefficient of K3[Fe(CN)6] (7.6×10−6 cm2 s−1 at 25 ℃), Qdl is double layer charge which could be eliminated by background subtraction, Qads is Faradic charge. It could be seen from inset in Fig. 9A, the plots of Q against t1/2 all exhibited the good linear relationship for the oxidation reaction of K3[Fe(CN)6] on three working electrodes. According to the slope of equation (2), the values of A were respectively estimated as 0.064, 0.091 and 0.202 cm2 for GCE, IL-GO/GCE and IL-GO-AuNPs/GCE. The obviously increased electrode effective surface area arise from the high conductivity, large surface area and more active sites of IL-GO-AuNPs composite, ensuring the excellent adsorption capacity and detection sensitivity for 2,4-DCP. The diffusion coefficient D and Qads of 2,4-DCP at IL-GO-AuNPs/GCE were also evaluated by chronocoulometry in PBS (pH 8) containing 0.0 and 0.1 mM 2,4-DCP (Fig. 9B). After subtracting background, the plots of Q against t1/2 also displayed the linear relationship with a slope of 8.05× 10−6 C s1/2 and an intercept (Qads) of 8.77 × 10−7 C. Basing on the equation (2), D was
calculated to be 3.35 × 10−6 cm2 s−1. Considering Qads = nFAГs, so the adsorption capacity Гs of 2,4-DCP at IL-GO-AuNPs/GCE was obtained as 2.25 × 10−11 mol cm−2. The standard heterogeneous rate constant (ks) could be calculated by the following equation [44]: ks=2.415 exp(−0.02F/RT)D1/2(Ep−Ep/2)−1/2v1/2
(2)
where Ep − Ep/2 = 75 mV, D = 3.35 × 10−6 cm2 s−1, v = 100 mV/s and T=298 K. Thereby the ks value was calculated to be 2.34 × 10−3 cm s−1, meaning a fast electron transfer process. 3.8 Differential pulse voltammetric determinations Given that the higher sensitivity than CV, differential pulse voltammetry (DPV) was used to determine the electrochemical response of 2,4-DCP with different concentrations at IL-GO-AuNPs/GCE. As presented in Fig. 10, the oxidation peak currents linearly increased with 2,4-DCP concentration in a large range from 1 × 10−8 to 5× 10−6 M. The linear equation was expressed as Ipa (μA) = −1.50c (μM) − 0.2155 (R=0.997). The detection limit was calculated as 3 nM (S/N = 3). The results showed that the performance of the proposed sensor was better than many previous literatures as illustrated in Table 1. The low detection limit and wide linear range were mainly ascribed to the high conductivity, large surface area and more active sites. 3.9 Reproducibility, stability and interferences Five IL-GO-AuNPs/GCE electrodes were prepared by the same method and the 3 μmol/L of 2,4-DCP solution was detected by DPV. The relative standard deviation (RSD) was obtained as 4.7%, demonstrating the desirable reproducibility of proposed sensor. Under the optimized conditions, the IL-GO-AuNPs/GCE was applied to determine 3 μmol/L 2,4-DCP seven times by CVs. Prior to each measurement, the modified electrode was thoroughly rinsed with water, and then regenerated by five cycles CV scanning at 100 mV/s in a blank PBS solution to remove any residue. The RSD of the results is 4.82% for 2,4-DCP. On the other hand, the modified electrode was also used to detect 3 μmol/L of 2,4-DCP solution after three weeks storage at 4 ℃ in refrigerator. The current peak signals were reduced by 5.78%, implying a better stability. Considering that there are some interfering substances, the effects of possible interference was also investigated for 3 μmol/L 2,4-DCP solution by DPV. As outlined in Table 2, 100-fold content of Ag+, Al3+, Ca2+, Cu2+, Fe3+, Mg2+ and Mn2+ gave rise to insignificant influences on 2,4-DCP determination with less than 5% anodic currents changes. Moreover, 60-fold concentration of phenolic compounds such as phenol, hydroquinone, hydroxyphenol, pyrocatechol, bisphenol A and phloroglucinol induced less than 8% peak currents variations toward 2,4-DCP. The results demonstrated that the novel 2,4-DCP sensor based on IL-GO-AuNPs/GCE possessed the excellent anti-interfering capability. 3.10 Preliminary analysis of water samples
The developed sensor was used to analyze 2,4-DCP in two types of water samples for the evaluation of the performance. Prior to determination, the fresh River and Tape water samples were filtered through a millipore membrane (0.45 μm) to eliminate suspended particles. By adding PBS, the pH values of all water samples were adjusted to 8.0. Known concentrations of 2,4-DCP were added the water samples and determination by the proposed DPV method. The recoveries were obtained in range from 96% to 109% with the average RSD of less than 5.6%, suggesting the excellent reliability and promising applicability.
4. Conclusion In this work, a novel and sensitive 2,4-DCP electrochemical sensor was developed based on the IL-GO-AuNPs nanocomposite, which was synthesized through functionalization of IL toward the self-assembly of GO and AuNPs. The comprehensive effects of high conductivity, large surface area, more active sites and better dispersity greatly improved the electrocatalytic performance of IL-GO-AuNPs toward the oxidation of 2,4-DCP. Compared to other reported methods, the proposed sensor showed better sensitivity, stability, selectivity and reproducibility. Therefore, this work provided a new type of sensor for the detection of 2,4-DCP. The proposed sensor showed a promising potential for analysis of water samples.
Acknowledgements This study was supported by the National Natural Science Foundation of China (21173135, 21403121, 21573133, 21275085), the Natural Science Foundation of Shandong Province, China (ZR2014JL013 and ZR2013BQ013), the open foundation from the Key Laboratory of Marine Bioactive Substance and Modern Analysis Technology, SOA (MBSMAT-2016-02, MBSMAT-2015-04, MBSMAT-2014-02 and MBSMAT-2013-01.).
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Biographies Tianrong Zhan is an associated professor in College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology. He received his Ph.D. degree from Institute of Oceanology, Chinese Academy of Science in 2004. He worked in the School of Pharmaceutical sciences and the School of chemistry and chemical engineering of Shandong University as a postdoctoral research fellow from 2004 to 2006, and 2009 to 2011, respectively. Now his research concentrates on colloid and interface sciences and electrochemical sensor.
Zhengwei Tan is a master candidate in Qingdao University of Science and Technology. He majors in the modified electrode based on layered materials. Xia Tian is a master candidate in Qingdao University of Science and Technology. He majors in preparation and properties of layered materials and modified electrode. Wanguo Hou received his MS and PhD degree from the Chemistry Department of Shandong University in 1986 and 1989, respectively. He is presently a professor in the School of chemistry and chemical engineering, Shandong University. His main research interests are in colloid and interface chemistry, micro(nano)materials, dispersion system, and oilfield chemistry.
b
Au(200)
b
c a 200
300
a 400
500
600
700
800
20
40
60
2 (degree)
Wavelength/nm
Fig. 2. (A) UV-vis absorption spectra of Au (a), GO (b), GO-AuNPs (c) and IL-GO-AuNPs (d). (B) XRD
Transmittance
patterns of GO-AuNPs (a) and IL-GO-AuNPs (b).
b
a
3600
Au(311)
rGO(002)
B
Au(220)
550 nm
GO(002)
d
Intensity (a.u.)
Absorbance
A
Au(111)
Fig. 1. The SEM images of GO (a), GO-AuNPs (b) and IL-GO-AuNPs (c).
3000
2400
1800
1200
600
Wavelength (cm-1) Fig. 3. FTIR spectra of GO-AuNPs (a) and IL-GO-AuNPs (b).
80
10
A
d
B 800
c
d
a
b
a
-Z''/ohm
Ipa/10-5A
5 0
600
e
400
-5 200
-10 0 0.6
0.4
0.2
0.0
-0.2
0
Epa/V
500
1000
1500
2000
Z'/ohm
Fig. 4. (A) CVs and (B) Nyquist plots of GO/GCE (a), GCE (b), IL-GO/GCE (c), GO-AuNPs/GCE (d) and IL-GO-AuNPs/GCE (e) in 5 mM Fe(CN)63-/4- (1:1) solution containing 0.1 M KCl. (Scan rate: 100 mV/s).
0
a Ipa/10-5A
b
-1
c d
-2
e 1.0
0.8
0.6
0.4
0.2
Epa/V Fig. 5. CVs of GO/GCE (a), GCE (b), IL-GO/GCE (c), GO-AuNPs/GCE (d) and IL-GO-AuNPs/GCE (e) of 0.1mM chlorophenol in 0.1 M PBS (pH=8.0). Scan rate: 100 mV s-1. Accumulation time: 100 s. Accumulation potential: -0.25 V.
B
0.7
-1.8
0.6
-1.5
Epa/V
Ipa/10-5A
-0.8
-2.1
b
0.8
-1.6
Ipa/10-5A
A 0.0
a a g:4-10
-1.2
0.5
-2.4 0.90
0.75
0.60
0.45
0.30
4
Epa/V
5
6
7
8
9
10
pH
Fig. 6. (A) CVs of 0.1 mM 2,4-DCP at IL-GO-AuNPs/GCE in 0.1 M PBS with different pH values (from a to g: 4, 5, 6, 7, 8, 9 and 10). (B) Effects of pH value on the peak current (a) and potential (b) with scan rate at 100 mV/s.
0
A
0.66
B
Epa/ V
0.63
-2 4
-3
Ipa/10-5A
Ipa/10-5A
-1
-4
0.60
3 2 1 6
9
12 -1
15
18
(v/mv s )1/2
-5 1.0
0.9
0.8
0.7
0.6
0.5
0.4
Epa/V
0.3
0.57 3.0
3.5
4.0
4.5
5.0
5.5
lnV
Fig. 7. (A) CVs of 0.1 mM 2,4-DCP at IL-GO-AuNPs/GCE with different scan rates. Curves a-i are obtained at 20, 40，60, 80, 100, 150, 200, 250 and 300 mV/s, respectively. Inset is the linear relationship between the peak current and the square root of the scan rate. (B) The relationship between Epa and ln v.
Fig. 8. Effects of IL-GO-AuNPs content (A), accumulation time (B) and accumulation potential (C) on the oxidation peak current of 0.1mM 2,4-DCP in 0.1 M pH 8.0 PBS.
-6
-5
A
-5
c
B
-4 -4
b
-3
a -2
-6
-5 -4
-2
Q/C
Q/C
Q/C
-3
-5
Q/C
-1 0
-3
-4 -3
-1
-2
-2
-1
1
0
0.1
0.2
0.3
0.4
0.5
t1/2/s1/2
2 0.00
0.05
0.10
0.15
0.20
0.1
0
0.25
0.3
0.4
0.5
t1/2/s1/2 0.00
t/s
0.2
0.05
0.10
t/s
0.15
0.20
0.25
Fig. 9. (A) Plot of Q-t curves of GCE (a), GO-AuNPs/GCE (b) and IL-GO-AuNPs/GCE (c) in 0.1 mM K3[Fe(CN)6] containing 1 M KCl. Inset: plot of Q–t1/2 curve on GCE (a), GO-AuNPs/GCE (b) and IL-GO-AuNPs/GCE (c). (B) Plot of Q–t curve of IL-GO-AuNPs/GCE in 0.1 M PBS (pH 8.0) containing 0.1 mM 2,4-DCP after background subtracted. Inset: plot of Q–t1/2 curve on IL-GO-AuNPs/GCE.
0
a -8
-4
l
-6
Ipa/A
Ipa/A
-2
-4
-6
-2 0 0
-8 0.0
1
2
3
C/mol L-1
0.2
4
0.4
5
0.6
0.8
1.0
Epa/ V Fig. 10. DPV curves of 2,4-DCP at IL-GO-AuNPs/GCE in 0.1 M pH 8.0 PBS containing different concentrations of 2,4-DCP (a–k: 0.01, 0.05, 0.1, 0.2, 0.5, 0.8, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0 μM). Insert: liner calibration curve. Amplitude: 0.05 V; pulse width: 0.2 s; pulse period: 0.5 s.
Scheme 1. The oxidation reaction mechanism of 2,4-DCP on IL-GO-AuNPs/GCE.
Table 1. Comparison of proposed sensor for determination of 2,4-DCP with others. Modified electrode
Analytical technique
Linear range (μM)
LOD (nM)
Ref.
Nafion/PSS-GN-CTAB/GCE
LSV
0.01-2
2
[1]
CS/CDs-CTAB/GCE
DPV
0.04-8
10
[4]
MIP/GCE
DPV
5-100
1600
[10]
Cu3(BTC)2/CPE
DPV
0.04-1
9
[11]
HRP/MWNT/GCE
Amperometry
1.0-100
380
[13]
SWCNT/PEDOT/GCE
Amperometry
0.4-120
228
[45]
Tyrosinase/MWCNT/GCE
Amperometry
2.0-100
660
[46]
Mb-AG/GCE
Amperometry
12.5-208
206
[47]
Nafion/MWCNT/GCE
Amperometry
0.1-100
37
[48]
IL-GO-AuNPs/GCE
DPV
0.01-5
3
this work
LOD: limit of detection, LSV: linear sweep voltammetry, CPE: carbon paste electrode, MIP: molecule imprinted polymers, MWCNT: multi-wall carbon nanotube, SWCNT: single-wall carbon nanotube, Mb: myoglobin.
Table 2. Influences of other interfering species on 3 μM 2,4-DCP Interferents
C (μM)
Ipa change (%)
Ag+
300
-4.8
Al3+
300
+3.2
Ca2+
300
-2.7
Cu2+
300
-2.5
Fe3+
300
-3.8
Mg2+
300
-1.6
Mn2+
300
-1.8
Phenol
180
+4.3
Hydroquinone
180
+5.4
Hydroxyphenol
180
+4.9
Pyrocatechol
180
+7.2
Bisphenol A
180
+7.4
Phloroglucinol
180
+6.8
Table 3. Recoveries of 2,4-DCP from spiked water samples (n = 5). Sample a
River water
Tape water
a
Added (nM)
Found (nM) b
Recovery (%)
RSD (%) c
10
9.75
97
4.3
20
20.8
104
5.2
40
43.6
109
3.7
10
10.2
102
5.6
20
21.3
106
3.6
40
38.4
96
4.6
River water samples were collected fromLicun River (Qingdao) and Tap water was taken from our laboratory. b
Mean of five measurements; c Relative standard deviation for n = 5.
S0925-4005(17)30334-9 http://dx.doi.org/doi:10.1016/j.snb.2017.02.107 SNB 21838
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
15-10-2016 15-1-2017 17-2-2017
Please cite this article as: Tianrong Zhan, Zhengwei Tan, Xia Tian, Wanguo Hou, Ionic Liquid Functionalized Graphene Oxide-Au Nanoparticles Assembly for Fabrication of Electrochemical 2,4-Dichlorophenol Sensor, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2017.02.107 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ionic Liquid Functionalized Graphene Oxide-Au Nanoparticles Assembly for Fabrication of Electrochemical 2,4-Dichlorophenol Sensor
Tianrong Zhana*, Zhengwei Tana, Xia Tiana, Wanguo Houb**
a Key
Laboratory of Sensor Analysis of Tumor Marker (Ministry of Education), State Key
Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China b Key
Laboratory of Colloid & Interface Chemistry (Ministry of Education), Shandong University,
Jinan 250100, China Corresponding author, Tel & Fax: +86-532-84023927 E-mail address: [email protected] (T. Zhan) and [email protected] (W. Hou).
Ipa/A
Graphic abstract
Epa/ V
Highlights: ► The 2,4-Dichlorophenol (2,4-DCP) sensor was fabricated based on IL-GO-AuNPs modified glass carbon electrode (IL-GO-AuNPs/GCE).
► IL-GO-AuNPs/GCE effectively enhanced the electron transfer and electrocatalytic capacity toward the oxidation of 2,4-DCP.
► Differential pulse voltammetry detecting 2,4-DCP displayed a wide linear range between 0.01 to 5 μM with a low detection limit of 3 nM (S/N =3).
ABSTRACT Self-assembly of graphene oxide (GO) and Au nanoparticles (AuNPs) was covalently modified by ionic liquid (IL) containing amino groups to prepare the ternary nanocomposite of IL-GO-AuNPs. UV, IR spectra, X-ray power diffraction and scanning electron microscope confirmed the successful hybridization of these three components, leading to the larger surface area and better dispersity. Then a sensitive 2,4-Dichlorophenol (2,4-DCP) sensor was established based on IL-GO-AuNPs modified glassy carbon electrode. The IL-GO-AuNPs composite gave rise to a desirable access to the electron transfer and electrocatalytic capacity toward the oxidation of 2,4-DCP. Differential pulse voltammetry was employed for the quantitative analysis of 2,4-DCP. The proposed sensor showed a wide linear range from 0.01 to 5 μM with a low detection limit of 3 nM (S/N =3). The developed sensor was successfully used for the determination of 2,4-DCP in water samples with satisfactory recoveries and stability. The excellent electrocatalytic performance could be attributed to the high conductivity and more active sites of IL-GO-AuNPs.
Keywords: ionic liquid functionalization; graphene oxide; Au nanoparticles; 2,4-Dichlorophenol sensor; differential pulse voltammetry
1. Introduction 2,4-Dichlorophenol (2,4-DCP), a representative member of chlorphenols family, is widely used as intermediate in the production of herbicides, fungicides, pharmaceuticals and insecticides due to its broad spectrum antimicrobial properties [1]. However, European Union and the US Environment Protection Agency have listed 2,4-DCP as a class of priority control pollutants because of its high
toxicity, poor biodegradation, potentially mutagenic and carcinogenic effects on living organisms albeit at a very low concentration [2]. Gao’s group has detected 2,4-DCP in more than half of China’s surface water samples (51.3%, 1.1–19960.0 ng L−1) from over 600 sites in the seven major watersheds and three drainage areas [3].The 2,4-DCP concentrations in some water area are beyond its maximum admissible concentration in drinking water (500 ng L−1) [4]. For the sake of public health and environmental safety, it is essential to develop a highly sensitive, selective, and reliable method for 2,4-DCP determination. Recently, several analytical methods such as HPLC [5], GC-MS [6], chemiluminescence [7], capillary electrophoresis [8], and UV spectrophotometry [9] have been established for determination of 2,4-DCP. By comparison with expensive instrument, complex manipulation and inaccessibility of in situ analysis of above methods, electrochemical sensors have attracted tremendous attention because of their low-cost, real-time detection, instrument simplicity, simple operation and high sensitivity [1,4]. Various kinds of modified electrodes for 2,4-DCP detection have been successfully constructed by using different materials including imprinted microgel [10], metal-organic frameworks (MOFs) [11], carbon dots composite [4], ZnSe QDs [12] and enzyme [13]. Whereas, it is difficult to evenly immobilize these nano-modifiers on electrode surface because of their either propensity to agglomerate, or easy denaturation and complicated fabrication process. Au nanoparticles (AuNPs) present many promising physicochemical properties, such as good conductivity, high surface area and low toxicity, which afford them distinct superiorities in the fabrication of electrochemical sensors. Nevertheless, the tendency to aggregate makes AuNPs to be used in combination with other nanomaterials for the improved sensing performance. For example, the electrochemical sensors based on HS-β-cyclodextrin/AuNPs, HS-β-cyclodextrin/AuNPs/MWCNTs and AuNPs/MoS2 composites have been developed for phenol sensitive detection [14-16]. Graphene (GR) is a lightweight 2D atom-thick and honeycomb carbon material composing of all-sp2-hybridized carbon atoms. GR has especially drawn much attention in electrochemical sensing fields owing to its low mass density, high surface areas (2630 m2/g), and high electrical conductivity [17,18]. Chemical reduction of exfoliated graphene oxide (GO) are generally employed for the low-cost and large-scale production of GR sheets [19,20]. However, the van der waals force and π-π interaction can inevitably result in irreversible agglomeration of GR sheets during reduction process, which therefore compromises the full performance of this carbon material [21]. Currently, covalent [22-25] functionalizations of GO are widely used to obtain the partially reduced GO (rGO) with enhanced dispersibility [26]. Ionic liquid (IL) is a kind of organic room temperature molten salts completely comprising of cations and anions. Due to high conductivity, wide potential window and innate liquid state, IL has been widely used as modifier or binder of modified electrode for the better sensing performance. Grafting of IL can also promote the dispersity of GO or rGO based materials
owing to its special solubility [27,28]. More recently, IL has been used for covalent modification of GO or rGO sheets to improve their electrochemical properties. Studies have demonstrated an enhanced ability to store energy due to modification of GO surface with IL [29,30]. IL grafted GO has also been coated on the solid-phase microextraction fiber for extraction and enrichment of polycyclic aromatic hydrocarbons in food wrap due to the modified surface properties and the increased active sites [31]. Covalently bonding IL to GO may not only shield the π-π stacking interaction between the GO (or rGO) nanosheets, but also partially reduce GO to rGO for the better conductivity [26]. Although the covalent attachment of IL on GO or rGO is widely used in energy and separation, the synthesis of IL modified GO-AuNPs and their application in electrochemical sensor have not been reported so far. In this article, the positively charged cetyltrimethylammonium bromide (CTAB)-AuNPs solution was firstly assembled on the negatively charged GO sheets through electrostatic attraction. Then IL containing amino group, 1-aminopropyl-3-methylimidzaolium tetrafluoroborate (APMIMBF4), was covalently bound on GO surface by reaction with oxygen containing group. In the resultant hybrid, IL could effectively inhibit nanoparticles from aggregating and form a molecular film enhancing the direct electron transfer. Functionalization and hybridization endowed the composite with more active sites to facilitate the entrapment of analyte. Therefore, this IL functionalized GO-AuNPs composite (IL-GO-AuNPs) was employed to fabricate the modified electrode for investigation of behaviors and detection of 2,4-DCP. The sensing performance of IL-GO-AuNPs was also compared with the GO-AuNPs counterpart, demonstrating the construction of an acceptable 2,4-DCP sensor.
2. Experimental 2.1 Reagents and apparatus 2,4-DCP, HAuCl4·4H2O (99.9%) and trisodium citrate (99.9%) and CTAB were purchased from Aladdin Chemistry Co. (Shanghai). Graphite powder (average particle size 30 μm) and IL (APMIMBF4) were repectively obtained from Colloid Chemical Co. (Shanghai) and Lanzhou Greenchem. ILS, LICP, CAS). Other chemicals were analytical grade and directly used as received. Electrochemical measurements were carried out on CHI660D electrochemical workstation (CHI, China). Three electrode system including a modified glass carbon electrode (GCE, Φ = 3 mm) as working electrode, a platinum wire as auxiliary electrode and a saturated calomel electrode (SCE) as reference electrode was used. X-ray power diffraction (XRD) patterns were collected using a Rigaku DLMAX 2550V diffractometer (Japan) with Cu Kα radiation (λ = 0.154178 nm, graphite monochromator, 28 kV and 20 mA) from 10° to 80° with a scanning speed of 10° min−1. Morphology of solid sample was evaluated by a JSM-6610 scanning electron microscope (SEM, Japan). UV absorption spectroscopy was recorded on A TU-1901 spectrophotometer (Beijing Puxi,
China). 2.2 Synthesis of IL-GO-AuNPs Synthesis of AuNPs. AuNPs was synthesized according to the previous procedure [32]. Briefly, 1.5 ml of 1% sodium citrate solution (w%) was slowly added to 100 mL of the 0.01% HAuCl4 aqueous solution (w%) under stirring. Then 0.3 mL of 0.075% sodium borohydride solution was added to reduce Au3+ to Au0. The continuous stirring gave rise to purple AuNPs solution. Finally, 10 mL of 10 mM CTAB solution was added to the above AuNPs solution for a well dispersed AuNPs solution. Synthesis of IL-GO-AuNPs.
Firstly, GO was prepared from nature graphite powder by a modified
Hummer’s technique [33]. The resultant GO powder was ultrasonically dispersed in ultrapure water with concentration of 1 mg/mL. The same volume of GO aqueous solution (50 mL) was slowly added to the well-dispersed AuNPs suspension. The mixed solution was stirred for 8 h at 160 rpm. The brown GO-AuNPs was obtained by 10 min centrifugation at 5000 rpm. The GO-AuNPs hybrid was re-dispersed in 50 mL of ultrapure water. Then 25 mg IL (APMIMBF4) and 25 mg KOH were added to the mixture. After refluxing for 2 h, IL-GO-AuNPs composite was afforded after centrifugation and successive washing with ultrapure water and absolute alcohol. 2.3 Preparation of IL-GO-AuNPs/GCE Before modification, the GCE was polished with 0.05 μm alumina slurry to a mirror-like surface and thoroughly cleaned with ultrapure water. For preparation of IL-GO-AuNPs composite suspension, 5 mg of hybrid was dispersed in 5 mL ultrapure water. After ultrasonic reaction for 2 h, the IL-GO-AuNPs suspension was obtained with concentration as 1 mg/mL. Then, 8 μL IL-GO-AuNPs suspension was coated on the clean GCE surface and dried at room temperature. The corresponding modified electrode was denoted as IL-GO-AuNPs/GCE. Similarly, IL-GO/GCE and GO-AuNPs/GCE were also constructed through similar procedure for comparison.
3. Results and discussion 3.1 Morphology and structural characteristics Fig. 1 gives the SEM images of GO, GO-AuNPs and IL-GO-AuNPs. GO shows the large sheets with some fine crumples on its surface and scrolled edges (Fig. 1a). This large sheet may be formed through van der Waals interactions among the oxygenous groups [34]. After self-assembling between AuNPs and GO, AuNPs were dispersedly attached on GO substrate through electrostatic interaction. Form the image of GO-AuNPs, it could be seen that some AuNPs were encapsulated by the soft and thin GO slice, which resulted in more distinct wrinkles on GO surface (Fig. 1b). As for
IL-GO-AuNPs (Fig. 1c), the covalent functionalization of IL provided a layer of molecular membrance on composite. Although this process partly flattened the crumples on GO sheets, the bound IL produced the rougher surface. It was seemed that IL also promoted the dispersity of AuNPs by an amount of charges. Therefore, it was believed that the designed IL-GO-AuNPs could exert excellent sensing performance due to the increased effective surface area, conductivity and more active sites. The synthesized products were also characterized by UV-vis spectrum. As shown in Fig.2A, AuNPs (curve a) presented its typical peaks at 550 nm. GO (curve b) showed the classic π-π* plasmon peak at 235 nm [35]. As for GO-AuNPs (curve c), the adsorption peak of AuNPs unchanged with a weakened intensity. However, the peak of GO shifted to ca. 210 nm because of the strong interaction between GO and AuNPs. After IL functionalization, by comparison with GO-AuNPs, IL-GO-AuNPs displayed a negligible broad at ca. 270 nm (indexed by arrow), indicating the small partial reduction of GO. These results demonstrated the successful preparation of IL-GO-AuNPs. The XRD patterns of GO-AuNPs and IL-GO-AuNPs were shown in Fig. 2B. GO-AuNPs (curve a) showed a diffraction (002) peak of GO at 11.2° and typical reflections of AuNPs including the (111), (200), (220), and (311) diffraction peaks at 38.04º, 44.62º, 63.26º and 78.92º, respectively (JCPDS 04-0784). The coexistence of GO and AuNPs signals revealed the successful self-assembling of two species. However, as regards IL-GO-AuNPs (curve b), a new (002) diffraction peak of rGO at 23.16 º was also observed except for the typical reflections of GO and AuNPs. This new phase suggested that partial oxygen functional groups had been removed [36] due to the covalent modification of IL. These results manifested that functionalization of IL toward GO-AuNPs was realized. The FT-IR technique was also employed to evaluate the covalent functionalization of GO-AuNPs with IL containing amino group as shown in Fig. 3. It could be seen that GO-AuNPs (curve a) displayed the peaks of aromatic C-H stretching vibrations at 703, 830 and 983 cm-1, C-O and C-O-C vibrations in epoxy at 1120 and 1405 cm-1. The C=C and C=O stretching bands were also distinctly observed at 1637 and 1856 cm-1, respectively. Additionally, C-H and -NH2 characteristic absorptions appeared at 2940 and 3455 cm-1, respectively [37]. After functionalization with IL, the intensity of peaks related to the C-O, C-O-C and C=O stretching bands was obviously decreased. Particularly, the aromatic C-H and C=O stretching vibrations respectively at 983 cm-1 and 1856 cm-1 were almost disappeared after IL functionalization. The results confirmed that the amino group in IL reacted with oxygen-containing groups in GO sheets. Therefore, it was suggested that IL molecules were successfully attached onto the GO nanosheets surface via amide reaction.
3.2 Electrochemical characterization of electrodes The modified electrodes were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in 5 mM [Fe(CN)6]3−/4− solution containing 0.1 M KCl. As could be seen from CVs results of Fig. 4A, GO/GCE (curve a) showed the smallest voltammetric response because of the intrinsic non-conductive nature of GO. The background and peak current of IL-GO/GCE (curve c), GO-AuNPs/GCE (curve d) were obviously larger than those for bare GCE (curve b). It was suggested that IL-GO or GO-AuNPs composite films improved the effective active area of electrodes and high conductive IL or AuNPs components accelerated the electron transfer rate. As for IL-GO-AuNPs/GCE (curve e), the voltammetric response is further higher than those for IL-GO/GCE and GO-AuNPs/GCE. It was attributed to the fact that the covalent introduction of IL not only enhanced the surface area and active sites, but also improved the conductivity and dispersity of the ternary composite. Nyquist plots of same modified electrodes as in Fig. 4A were depicted in Fig. 4B. The impedance spectroscopy contains a linear section indexing the diffusion-limited process, and the semicircle section signifying the charge transfer-limited process (Rct) [34,38]. GO/GCE (curve a) displayed the largest impedance owing to the poor conductivity of GO. For the bare GCE (curve b), a well-defined semicircle with Rct of 790 Ω was found, revealing the distinct interface resistance against electron transfer. When IL-GO was modified on the GCE surface (curve c), its impedance was greatly decreased owing to the improved electron transfer efficiency from GO and conductive IL. As for GO-AuNPs/GCE (curve d), it could be observed a much smaller semicircle than IL-GO/GCE, which was ascribed to the electric conductor of AuNPs. By comparison, IL-GO-AuNPs/GCE (curve e) exhibited almost a straight line with a very small Rct value. Hence, it could be suggested that the synergistic effects of the components in IL-GO-AuNPs entrapped more Fe(CN)63−/4− on the basing electrode surface and facilitated the electron communication. 3.3 CV behaviors of 2,4-DCP on modified electrodes The CV behaviors of 0.1 mM 2,4-DCP in 0.1 M pH=8.0 PBS on different electrodes were investigated. As shown in Fig. 5, the GO/GCE (curve a) presented the lowest oxidative signal, manifesting the resistance of GO film against electrocatalytic oxidation toward 2,4-DCP. As for the bare GCE (curve b), a slightly enhanced oxidative peak was observed at 0.57 V. After coating IL-GO (curve c), the oxidative peak current dramatically increased, manifesting that IL-GO film could promote the electrochemical oxidization toward 2,4-DCP. GO-AuNPs/GCE (curve d) displayed the stronger oxidative responses than those at IL-GO/GCE. It was supposed that the stronger conductivity of AuNPs than IL highly promoted the electron transportation from 2,4-DCP to GCE. However, IL-GO-AuNPs/GCE (curve e) gave rise to the biggest oxidative peak among five electrodes. The excellent electrocatalytic performance of IL-GO-AuNPs could be resulted from the
following aspects. i) Electrostatic assembling and covalent binding of IL avoided the aggregation of building blocks and enhanced the dispersity of IL-GO-AuNPs. ii) The covalent modification of IL toward GO-AuNPs highly enlarged the surface area and further increased the conductivity and active sites of the IL-GO-AuNPs composite. iii) The π-π and hydrophobic interactions between 2,4-DCP with GO (part rGO) and IL also played important role on the prominent electrocatalytic capability [39]. 3.4 Effects of pH The effects of PBS with different pH on CV responses of 2,4-DCP were investigated. Fig. 6A shows the CVs of 2,4-DCP at IL-GO-AuNPs/GCE in 0.1 M PBS with pH ranged from 4 to 10. In the pH range from 4-10, the oxidative peak current of 2,4-DCP reached the biggest value at pH of 8.0 (Fig. 6B, curve a). Thus pH 8 of PBS was chosen as the supporting electrolyte for the expectable sensitivity. The optimal response at pH about 8.0 could be due to the existing form of 2,4-DCP with a pKa value of 7.98 [10]. On the other hand, the oxidation peak potential proportionally shifted to the negative direction as pH increased (Fig. 5B, curve b). The linear equation of the peak potential (Epa) vs. pH was obtained as Epa (V) = −0.0528 pH + 1.0384 (R = 0.9993). A slope of 52.8 mV/pH is approximately close to the theoretical Nerstian value of −59 mV/pH, implying that the equal number of electron transfer and protons electro-oxidation process of 2,4-DCP on IL-GO-AuNPs/GCE [27,40]. 3.5 Effects of scan rate The influences of different scan rate on IL-GO-AuNPs/GCE were also carried out for the further understanding of electrocatalytic mechanism of 2,4-DCP. As illustrated in Fig. 7A, it exhibited a linear relationship between the oxidation peak currents (Ipa) of 2,4-DCP and the square root of scan rate from 20 to 300 mV/s (Inset in Fig. 7A). The regressing equation could be expressed as Ipa (μA) = 0.24 v1/2 (mV/s) － 0.3045 (R = 0.993), indexing a typical diffusion-controlled process of 2,4-DCP at IL-GO-AuNPs/GCE [41]. It was also seen that the oxidative peak potential (Epa) increased against lnv with a regression equation of Epa (V) = 0.025 ln v + 0.5069 (R = 0.999) (Fig. 7B). Based on the Laviron’s theory, Epa could be presented by the Laviron equation [42] as following: Epa= E0 + (RT/αnF) ln(RTk0/αnF) + (RT/αnF) ln ν
(1)
For which α is transfer coefficient (assumed as 0.5 for the totally irreversible electrode process), k0 is standard rate constant, n is electron transfer number, ν is scan rate, E0 is formal redox potential, R is the gas constant (R = 8.314), T is the absolute temperature (T = 298), and F is the Faraday constant (F = 96485). According to the slope (equal to RT/αnF) of linear equation, the electron transfer number (n) was thus calculated as about 2. Therefore, the oxidation process of
2,4-DCP on IL-GO-AuNPs/GCE is a two-electron and two-proton process [4]. The possible electrochemical process of 2,4-DCP on the modified electrode might be depicted as shown in Scheme 1 [11]. 3.6 Effects of IL-GO-AuNPs content, accumulation time and potential For better sensitivity, effects of the loading of IL-GO-AuNPs, accumulation time and accumulation potential were also carefully optimized. For the content of IL-GO-AuNPs suspension (Fig. 8A), the peak current of 2,4-DCP reached the maximum at 1.0 mg mL-1 in the concentration range from 0.5 to 3.0 mg mL-1. So 1.0 mg mL−1 of IL-GO-AuNPs colloidal suspension was chosen to avoid too much modifier blocking the electron transfer. As for the accumulation time (Fig. 8B), the oxidation current peak response continuously increased before the accumulation time 120 s, then slightly decreased and leveled off after that moment. It could be supposed that the binding of 2,4-DCP and IL-GO-AuNPs had reached equilibrium. So the 120 s accumulation time was used in experiment. In the accumulation potential diagram (Fig. 8C), the oxidation peak showed the maximum at −0.25 V in the potential window between −0.1 to −0.4 V. Therefore, the accumulation potential was set at −0.20 V in further experiments. 3.7 Chronocoulumetry Chronocoulometry was employed to comparatively study the electrochemically effective surface areas of bare GCE, GO-AuNPs/GCE and IL-GO-AuNPs/GCE in 0.1 mM K3[Fe(CN)6] solution containing 1.0 M KCl (Fig. 8A) by using the following Anson equation [43]: 1 1
𝑄 (𝑡) =
2𝑛𝐹𝐴𝑐𝐷 2 𝑡 2 1
𝜋2
+ 𝑄𝑑𝑙 + 𝑄𝑎𝑑𝑠
(2)
wherein n is the number of electron transfer, A is the effective surface area of working electrode, c is the concentration of substrate, D is the standard diffusion coefficient of K3[Fe(CN)6] (7.6×10−6 cm2 s−1 at 25 ℃), Qdl is double layer charge which could be eliminated by background subtraction, Qads is Faradic charge. It could be seen from inset in Fig. 9A, the plots of Q against t1/2 all exhibited the good linear relationship for the oxidation reaction of K3[Fe(CN)6] on three working electrodes. According to the slope of equation (2), the values of A were respectively estimated as 0.064, 0.091 and 0.202 cm2 for GCE, IL-GO/GCE and IL-GO-AuNPs/GCE. The obviously increased electrode effective surface area arise from the high conductivity, large surface area and more active sites of IL-GO-AuNPs composite, ensuring the excellent adsorption capacity and detection sensitivity for 2,4-DCP. The diffusion coefficient D and Qads of 2,4-DCP at IL-GO-AuNPs/GCE were also evaluated by chronocoulometry in PBS (pH 8) containing 0.0 and 0.1 mM 2,4-DCP (Fig. 9B). After subtracting background, the plots of Q against t1/2 also displayed the linear relationship with a slope of 8.05× 10−6 C s1/2 and an intercept (Qads) of 8.77 × 10−7 C. Basing on the equation (2), D was
calculated to be 3.35 × 10−6 cm2 s−1. Considering Qads = nFAГs, so the adsorption capacity Гs of 2,4-DCP at IL-GO-AuNPs/GCE was obtained as 2.25 × 10−11 mol cm−2. The standard heterogeneous rate constant (ks) could be calculated by the following equation [44]: ks=2.415 exp(−0.02F/RT)D1/2(Ep−Ep/2)−1/2v1/2
(2)
where Ep − Ep/2 = 75 mV, D = 3.35 × 10−6 cm2 s−1, v = 100 mV/s and T=298 K. Thereby the ks value was calculated to be 2.34 × 10−3 cm s−1, meaning a fast electron transfer process. 3.8 Differential pulse voltammetric determinations Given that the higher sensitivity than CV, differential pulse voltammetry (DPV) was used to determine the electrochemical response of 2,4-DCP with different concentrations at IL-GO-AuNPs/GCE. As presented in Fig. 10, the oxidation peak currents linearly increased with 2,4-DCP concentration in a large range from 1 × 10−8 to 5× 10−6 M. The linear equation was expressed as Ipa (μA) = −1.50c (μM) − 0.2155 (R=0.997). The detection limit was calculated as 3 nM (S/N = 3). The results showed that the performance of the proposed sensor was better than many previous literatures as illustrated in Table 1. The low detection limit and wide linear range were mainly ascribed to the high conductivity, large surface area and more active sites. 3.9 Reproducibility, stability and interferences Five IL-GO-AuNPs/GCE electrodes were prepared by the same method and the 3 μmol/L of 2,4-DCP solution was detected by DPV. The relative standard deviation (RSD) was obtained as 4.7%, demonstrating the desirable reproducibility of proposed sensor. Under the optimized conditions, the IL-GO-AuNPs/GCE was applied to determine 3 μmol/L 2,4-DCP seven times by CVs. Prior to each measurement, the modified electrode was thoroughly rinsed with water, and then regenerated by five cycles CV scanning at 100 mV/s in a blank PBS solution to remove any residue. The RSD of the results is 4.82% for 2,4-DCP. On the other hand, the modified electrode was also used to detect 3 μmol/L of 2,4-DCP solution after three weeks storage at 4 ℃ in refrigerator. The current peak signals were reduced by 5.78%, implying a better stability. Considering that there are some interfering substances, the effects of possible interference was also investigated for 3 μmol/L 2,4-DCP solution by DPV. As outlined in Table 2, 100-fold content of Ag+, Al3+, Ca2+, Cu2+, Fe3+, Mg2+ and Mn2+ gave rise to insignificant influences on 2,4-DCP determination with less than 5% anodic currents changes. Moreover, 60-fold concentration of phenolic compounds such as phenol, hydroquinone, hydroxyphenol, pyrocatechol, bisphenol A and phloroglucinol induced less than 8% peak currents variations toward 2,4-DCP. The results demonstrated that the novel 2,4-DCP sensor based on IL-GO-AuNPs/GCE possessed the excellent anti-interfering capability. 3.10 Preliminary analysis of water samples
The developed sensor was used to analyze 2,4-DCP in two types of water samples for the evaluation of the performance. Prior to determination, the fresh River and Tape water samples were filtered through a millipore membrane (0.45 μm) to eliminate suspended particles. By adding PBS, the pH values of all water samples were adjusted to 8.0. Known concentrations of 2,4-DCP were added the water samples and determination by the proposed DPV method. The recoveries were obtained in range from 96% to 109% with the average RSD of less than 5.6%, suggesting the excellent reliability and promising applicability.
4. Conclusion In this work, a novel and sensitive 2,4-DCP electrochemical sensor was developed based on the IL-GO-AuNPs nanocomposite, which was synthesized through functionalization of IL toward the self-assembly of GO and AuNPs. The comprehensive effects of high conductivity, large surface area, more active sites and better dispersity greatly improved the electrocatalytic performance of IL-GO-AuNPs toward the oxidation of 2,4-DCP. Compared to other reported methods, the proposed sensor showed better sensitivity, stability, selectivity and reproducibility. Therefore, this work provided a new type of sensor for the detection of 2,4-DCP. The proposed sensor showed a promising potential for analysis of water samples.
Acknowledgements This study was supported by the National Natural Science Foundation of China (21173135, 21403121, 21573133, 21275085), the Natural Science Foundation of Shandong Province, China (ZR2014JL013 and ZR2013BQ013), the open foundation from the Key Laboratory of Marine Bioactive Substance and Modern Analysis Technology, SOA (MBSMAT-2016-02, MBSMAT-2015-04, MBSMAT-2014-02 and MBSMAT-2013-01.).
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Biographies Tianrong Zhan is an associated professor in College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology. He received his Ph.D. degree from Institute of Oceanology, Chinese Academy of Science in 2004. He worked in the School of Pharmaceutical sciences and the School of chemistry and chemical engineering of Shandong University as a postdoctoral research fellow from 2004 to 2006, and 2009 to 2011, respectively. Now his research concentrates on colloid and interface sciences and electrochemical sensor.
Zhengwei Tan is a master candidate in Qingdao University of Science and Technology. He majors in the modified electrode based on layered materials. Xia Tian is a master candidate in Qingdao University of Science and Technology. He majors in preparation and properties of layered materials and modified electrode. Wanguo Hou received his MS and PhD degree from the Chemistry Department of Shandong University in 1986 and 1989, respectively. He is presently a professor in the School of chemistry and chemical engineering, Shandong University. His main research interests are in colloid and interface chemistry, micro(nano)materials, dispersion system, and oilfield chemistry.
b
Au(200)
b
c a 200
300
a 400
500
600
700
800
20
40
60
2 (degree)
Wavelength/nm
Fig. 2. (A) UV-vis absorption spectra of Au (a), GO (b), GO-AuNPs (c) and IL-GO-AuNPs (d). (B) XRD
Transmittance
patterns of GO-AuNPs (a) and IL-GO-AuNPs (b).
b
a
3600
Au(311)
rGO(002)
B
Au(220)
550 nm
GO(002)
d
Intensity (a.u.)
Absorbance
A
Au(111)
Fig. 1. The SEM images of GO (a), GO-AuNPs (b) and IL-GO-AuNPs (c).
3000
2400
1800
1200
600
Wavelength (cm-1) Fig. 3. FTIR spectra of GO-AuNPs (a) and IL-GO-AuNPs (b).
80
10
A
d
B 800
c
d
a
b
a
-Z''/ohm
Ipa/10-5A
5 0
600
e
400
-5 200
-10 0 0.6
0.4
0.2
0.0
-0.2
0
Epa/V
500
1000
1500
2000
Z'/ohm
Fig. 4. (A) CVs and (B) Nyquist plots of GO/GCE (a), GCE (b), IL-GO/GCE (c), GO-AuNPs/GCE (d) and IL-GO-AuNPs/GCE (e) in 5 mM Fe(CN)63-/4- (1:1) solution containing 0.1 M KCl. (Scan rate: 100 mV/s).
0
a Ipa/10-5A
b
-1
c d
-2
e 1.0
0.8
0.6
0.4
0.2
Epa/V Fig. 5. CVs of GO/GCE (a), GCE (b), IL-GO/GCE (c), GO-AuNPs/GCE (d) and IL-GO-AuNPs/GCE (e) of 0.1mM chlorophenol in 0.1 M PBS (pH=8.0). Scan rate: 100 mV s-1. Accumulation time: 100 s. Accumulation potential: -0.25 V.
B
0.7
-1.8
0.6
-1.5
Epa/V
Ipa/10-5A
-0.8
-2.1
b
0.8
-1.6
Ipa/10-5A
A 0.0
a a g:4-10
-1.2
0.5
-2.4 0.90
0.75
0.60
0.45
0.30
4
Epa/V
5
6
7
8
9
10
pH
Fig. 6. (A) CVs of 0.1 mM 2,4-DCP at IL-GO-AuNPs/GCE in 0.1 M PBS with different pH values (from a to g: 4, 5, 6, 7, 8, 9 and 10). (B) Effects of pH value on the peak current (a) and potential (b) with scan rate at 100 mV/s.
0
A
0.66
B
Epa/ V
0.63
-2 4
-3
Ipa/10-5A
Ipa/10-5A
-1
-4
0.60
3 2 1 6
9
12 -1
15
18
(v/mv s )1/2
-5 1.0
0.9
0.8
0.7
0.6
0.5
0.4
Epa/V
0.3
0.57 3.0
3.5
4.0
4.5
5.0
5.5
lnV
Fig. 7. (A) CVs of 0.1 mM 2,4-DCP at IL-GO-AuNPs/GCE with different scan rates. Curves a-i are obtained at 20, 40，60, 80, 100, 150, 200, 250 and 300 mV/s, respectively. Inset is the linear relationship between the peak current and the square root of the scan rate. (B) The relationship between Epa and ln v.
Fig. 8. Effects of IL-GO-AuNPs content (A), accumulation time (B) and accumulation potential (C) on the oxidation peak current of 0.1mM 2,4-DCP in 0.1 M pH 8.0 PBS.
-6
-5
A
-5
c
B
-4 -4
b
-3
a -2
-6
-5 -4
-2
Q/C
Q/C
Q/C
-3
-5
Q/C
-1 0
-3
-4 -3
-1
-2
-2
-1
1
0
0.1
0.2
0.3
0.4
0.5
t1/2/s1/2
2 0.00
0.05
0.10
0.15
0.20
0.1
0
0.25
0.3
0.4
0.5
t1/2/s1/2 0.00
t/s
0.2
0.05
0.10
t/s
0.15
0.20
0.25
Fig. 9. (A) Plot of Q-t curves of GCE (a), GO-AuNPs/GCE (b) and IL-GO-AuNPs/GCE (c) in 0.1 mM K3[Fe(CN)6] containing 1 M KCl. Inset: plot of Q–t1/2 curve on GCE (a), GO-AuNPs/GCE (b) and IL-GO-AuNPs/GCE (c). (B) Plot of Q–t curve of IL-GO-AuNPs/GCE in 0.1 M PBS (pH 8.0) containing 0.1 mM 2,4-DCP after background subtracted. Inset: plot of Q–t1/2 curve on IL-GO-AuNPs/GCE.
0
a -8
-4
l
-6
Ipa/A
Ipa/A
-2
-4
-6
-2 0 0
-8 0.0
1
2
3
C/mol L-1
0.2
4
0.4
5
0.6
0.8
1.0
Epa/ V Fig. 10. DPV curves of 2,4-DCP at IL-GO-AuNPs/GCE in 0.1 M pH 8.0 PBS containing different concentrations of 2,4-DCP (a–k: 0.01, 0.05, 0.1, 0.2, 0.5, 0.8, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0 μM). Insert: liner calibration curve. Amplitude: 0.05 V; pulse width: 0.2 s; pulse period: 0.5 s.
Scheme 1. The oxidation reaction mechanism of 2,4-DCP on IL-GO-AuNPs/GCE.
Table 1. Comparison of proposed sensor for determination of 2,4-DCP with others. Modified electrode
Analytical technique
Linear range (μM)
LOD (nM)
Ref.
Nafion/PSS-GN-CTAB/GCE
LSV
0.01-2
2
[1]
CS/CDs-CTAB/GCE
DPV
0.04-8
10
[4]
MIP/GCE
DPV
5-100
1600
[10]
Cu3(BTC)2/CPE
DPV
0.04-1
9
[11]
HRP/MWNT/GCE
Amperometry
1.0-100
380
[13]
SWCNT/PEDOT/GCE
Amperometry
0.4-120
228
[45]
Tyrosinase/MWCNT/GCE
Amperometry
2.0-100
660
[46]
Mb-AG/GCE
Amperometry
12.5-208
206
[47]
Nafion/MWCNT/GCE
Amperometry
0.1-100
37
[48]
IL-GO-AuNPs/GCE
DPV
0.01-5
3
this work
LOD: limit of detection, LSV: linear sweep voltammetry, CPE: carbon paste electrode, MIP: molecule imprinted polymers, MWCNT: multi-wall carbon nanotube, SWCNT: single-wall carbon nanotube, Mb: myoglobin.
Table 2. Influences of other interfering species on 3 μM 2,4-DCP Interferents
C (μM)
Ipa change (%)
Ag+
300
-4.8
Al3+
300
+3.2
Ca2+
300
-2.7
Cu2+
300
-2.5
Fe3+
300
-3.8
Mg2+
300
-1.6
Mn2+
300
-1.8
Phenol
180
+4.3
Hydroquinone
180
+5.4
Hydroxyphenol
180
+4.9
Pyrocatechol
180
+7.2
Bisphenol A
180
+7.4
Phloroglucinol
180
+6.8
Table 3. Recoveries of 2,4-DCP from spiked water samples (n = 5). Sample a
River water
Tape water
a
Added (nM)
Found (nM) b
Recovery (%)
RSD (%) c
10
9.75
97
4.3
20
20.8
104
5.2
40
43.6
109
3.7
10
10.2
102
5.6
20
21.3
106
3.6
40
38.4
96
4.6
River water samples were collected fromLicun River (Qingdao) and Tap water was taken from our laboratory. b
Mean of five measurements; c Relative standard deviation for n = 5.