Hierarchical Self-Assembly of Cyclodextrin and Dimethylamino

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Hierarchical Self-Assembly of Cyclodextrin and DimethylaminoSubstituted Arylene-Ethynylene on N-doped Graphene for Synergistically Enhanced Electrochemical Sensing of Dihydroxybenzene Isomers Juanjuan Gao, Jingkun Fang, Xuehai Ju, Weiqing Zhu, Xuezhen Lin, Shupeng Zhang, Chuang Ma, and Haiou Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12463 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017

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ACS Applied Materials & Interfaces

Hierarchical

Self-Assembly

of

Cyclodextrin

and

Dimethylamino-Substituted

Arylene-Ethynylene on N-doped Graphene for Synergistically Enhanced Electrochemical Sensing of Dihydroxybenzene Isomers

Juanjuan Gao †, Jingkun Fang †, Xuehai Ju †, Weiqing Zhu†, Xuezhen Lin †, Shupeng Zhang *,†,ǁ, Chuang Ma †, Haiou Song *,‡

†

School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094,

PR China ‡

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing

University, Nanjing 210023, PR China ǁ

Nanjing University & Yancheng Academy of Environmental Protection Technology and Engineering,

Yancheng 210009, P.R. China

*

Corresponding authors.

Tel/Fax: +86 25 84315519 E-mail address: [email protected]

(S.P. Zhang)

[email protected]

(H.O. Song)

1

ACS Paragon Plus Environment


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ABSTRACT:

An electrochemically active sensing nanomaterial (denoted as CD-MPEA-NG) has been successfully constructed by an hierarchical self-assembly of cyclodextrin (CD) and N, N-dimethyl-4-(phenylethynyl)aniline (MPEA) on N-doped graphene (NG) in a low-temperature hydrothermal process. The unique nanostructure of the high-performance CD-MPEA-NG was confirmed by utilizing the Fourier transform infrared spectra, an X-ray diffractometer, a differential pulse voltammetry (DPV), etc. In particular, the method of density functional theory with dispersion energy (DFT-D) of wB97XD/LanL2DZ was employed to optimize and describe the face-to-face packing structure of heterodimers of NG and MPEA. The CD-MPEA-NG sensor exhibits highly sensitive performance towards dihydroxybenzene isomers, without relying on expensive noble metal or complicated preparation process. The experimental results demonstrate that, given the synergistic effect of NG and MPEA as a coupled sensing platform, CD as a supramolecular cavity can significantly enhance the electrochemical response. The detection limits (S/N = 3) for catechol (CT), resorcinol (RS) and hydroquinone (HQ) are 0.008, 0.018 and 0.011 µM by DPV, respectively. Besides, the CD-MPEA-NG sensor shows a superb anti-interference, reproducibility and stability, and satisfactory recovery aimed at detecting isomers in Nanjing River water. The encouraging performance as well as simplified preparation approach strongly support the CD-MPEA-NG sensor is a fascinating electrode to develop a seamless and sensitive electroanalytical technique.

KEYWORDS: N-doped graphene, dihydroxybenzene, sensor, cyclodextrin, arylene-ethynylene

2


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1. INTRODUCTION

Noncovalent interactions in nanomaterial architecture play a crucial role in fabricating nanostructure and controlling performance. Attractive π-π interaction as one of the backbones has been widely applied in material science so as to govern the properties and structures of solid nanomaterials.1,

2

architectures5,

based on π-π interaction, have been attracting great interest derived from the

6

The constructed host-guest complexes3,

4

and self-assembled supramolecular

requirement of understanding the detailed molecular behavior.7, 8 Graphene exhibits an unpredictable large aspect ratio, high-charge carrier mobility and excellent flexibility.9 It is irrefutable that pristine defect-free graphene possesses unique π-electron delocalization networks and intriguing 2D morphology. So, graphene is regarded to as an important substrate material for adsorbing guest atoms or molecules to achieve special applications, for example, in sensors, detectors, energy storage and other electronic devices.8,

10

However, the cost-efficient

synthesis of high-quality graphene by wet chemistry approaches has always presented a major challenge. Graphite oxide (GO), as a fascinating precursor, is easily accessible by oxidizing graphite with strong acids and oxidants.10 Subsequently, the exfoliation-reduction strategy of GO is very efficient for obtaining reduced graphene oxide (RGO) because of its inexpensive cost and mass production.11 During the chemical reduction process, a majority of the oxygenated defects can be eliminated and the sp2-conjugated nanostructure of GO can also be partially recovered. However, it should be noted that, in comparion to the pure graphene, the chemically-converted graphene (CCG) is still a nanostructure that is to some extent disordered.12 In addition, the deliberate introduction of heteroatom (N, P, B, and S) doping has a great technological importance thanks to the tailoring of CCG electronic band structure for its wide range of applications in the nanodevices.9, 13 Especially, N atoms, as the neighbors of C in the periodic table, have stronger electronegativity that can form the p-π conjugation between the N lone pair electrons 3



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and the GO π-system. The increased the n-type carrier density of CCG can significantly improve the electrocatalytic performance and durability.14-16 Now, N-doped graphene (NG) can be prepared by the chemical-vapor deposition,17, 18 nitrogen plasma,19, 20 thermal annealing,21, 22 etc. However, the doping process often require the complicated and extreme reaction conditions including higher temperature and unknown pressure,23-25 accompanied by the lower yield, the higher defect and a serious environmental pollution.9, 26 Therefore, it is necessary to develop an easy and environmental-friendly way for fabricating low-deficiency NG with the similar performance.27 Further functionalization in CCG, RGO and NG can alter the density distribution of electrons in response to the special chemical properties of graphene during the remediation process.28 Organic molecules with highly-delocalized conjugated π-system can be modified onto the surface of CCG by non-covalent interactions, which would inspire new chemical performance of functionalized CCG due to the rearrangement of electronic clouds.29 Recent studies have revealed that the π-π interactions between

aromatic

molecules

such

as

polycyclic

aromatic

hydrocarbons

(benzylamine,30

aminopyrene,31 aminotriazines,32 and phthalocyanine33) or conjugated polymers (e.g., polyaniline,34 polypyrrole35) and graphene derivatives. Arylene-ethynylenes have attracted extensive attention in fluorescent materials, optoelectrical materials, liquid crystal materials and dye-sensitized solar cells. Generally, para-dimethylamine-substituted arylene-ethynylenes are widely adopted in these fields. Among them, N, N-dimethyl-4-(phenylethynyl)aniline (MPEA) is a classical building block, which can be synthesized by Sonogashira coupling reactions. In MPEA, two aromatic delocalizing electronic structure and one carbon-carbon triple bond can generate a strong π-π conjugated effect, and the p orbitals of N atom can further form a p-π conjugated system with the multiple π-bonds in the whole molecule. The p-π effects of charge delocalization could cause the uniform distribution of electronic clouds in MPEA. Herein, the hierarchical MPEA functionalized NG sensing platform (MPEA-NG) was produced successfully by combing simultaneously a reduction and N-doping of GO, and supramolecular 4


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self-assembly without employing high temperature and pressure.36 Importantly, the oxygen-containing substitutions and nitrogen-vacancy centers can serve as “anchor points” to anisotropic assembly with environmentally friendly, water-soluble cyclodextrin (CD) through non-covalent interactions in order to increase the electrocatalytic sensing performance due to the host-guest recognition.37-39 Dihydroxybenzene has three isomers: catechol (CT), resorcinol (RS) and hydroquinone (HQ), respectively. They were widely used in the dyeing, antioxidant, pesticides, cosmetics and pharmaceutical industries. 40 But they are very poisonous, even in very low concentrations, to both human and the environment. Hence, the rapid and sensitive determination of CT, RS and HQ is a crucial topic for the accurate analysis. Trouble is that CT, RS and HQ often coexist due to their similar chemical structures, and difficulty in separation and detection. So, it is practically impossible that CT, RS and HQ can be detected utilizing conventional electrodes. Here, the distinguishable signal labels were achieved thanks to the three-layer nanostructure. Thus, the obtained electrochemical sensor (CD-MPEA-NG/GCE) presents a superior simultaneous trace determination of dihydroxybenzene isomers (Scheme 1).

Scheme 1

2. RESULTS AND DISCUSSION

Figure 1

The evaluation of electrochemical performance depends on the accurate nanostructure of testing materials scientifically. FT-IR spectroscopy can clearly reveal the surface chemistry of functional sensing materials, such as bonding nature and chemical structure, etc. Figure 1A shows the FT-IR patterns of GO, CDs, MPEA, NG, MPEA-NG, CD-NG and CD-MPEA-NG in order to confirm the 5



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interfacial interaction. For GO, the strong and broad absorption peak at around 3337 cm-1 can be ascribed to the O-H stretching vibration. The absorption peak at 1728 cm-1 is attributed to the carbonyl groups (C=O) in COOH, and the peak at 1624 cm-1 is due to O-H bending vibrations, epoxide groups or skeletal ring vibrations of graphitic domains.41 In addition, the band at 1048 cm-1 is owing to C-O in epoxy groups (COH/COC).42 After experiencing a chemical reduction and nitrogen doping in the presence of ammonium hydroxide, the FT-IR spectrum of NG exhibits the obvious decrease, even disappearances of peak intensities owing to C=O and O-H groups, indicate an efficient deoxidization. At the same time, the appearance of a new peak at 1580 cm-1 strongly supports the generation of C=N groups, demonstrating the formation of nitrogen-containing heterocyclic structures. In addition, there is a broad peak at about 1096 cm-1, possibly because of the generation of C-N bonds and the residual C-O groups.9 These observations indicate that nitrogen atoms can be doped into the graphene skeleton in the low-temperature hydrothermal reaction.41, 43 The FT-IR spectrum of MPEA shows the C≡C absorption peak (3430 and 2206 cm-1), C-H in CH3 units symmetrical stretching vibrations (2901 cm-1) and aromatic rings framework vibrations (1610, 1592 and 1521 cm-1, etc).12 Specifically, the peak at 845 cm-1 represents para-substitution of benzene ring in MPEA. The combination of MPEA and NG results in the peak changes. The peak at 3430 cm-1 owing to C≡C in MPEA is red-shifted to 3407 cm-1 in MPEA-NG, accompanying with the increase of intensity. Most importantly, the several peaks owing to aromatic skeleton ring vibrations in MPEA and the peak at 1580 cm-1 in NG are simultaneously blue-shifted and red-shifted to a single position at 1646 cm-1 in MPEA-NG. Additionally, there is no peak detected from 2280 to 2100 cm-1, demonstrating that excellent coupling of MPEA and NG due to the flowing of electron cloud based on π-π interactions. And the existence of typical peak at 844 cm-1 can further support the formation of π-π interactions between NG and MPEA. After functionalization with CD, several characteristic peaks at about 2900, 1416 and 1088 cm-1 were observed in CD-NG and CD-MPEA-NG. These are attributed from the -CH2- stretching, -CH- bending vibrations and C-O-C groups, respectively. On the other hand, the disappearance and appearance of peak at 844 cm-1 6


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in CD-NG and CD-MPEA-NG illustrate the effectiveness of MPEA modification to the NG. These results could confirm the success of those nanostructure constructions by using chemical functionalization. Thermal gravimetric analysis (TGA) can be utilized to assess the intensive interfacial interactions. As shown in Figure 1B, the functionalized graphene hybrids show the excellent thermal stabilities. GO shows about 14 wt% weight loss with an onset temperature of 200 °C owing to the evaporation of interlamellar water.11,

44

From 200 to 500 °C, a 28 wt% weight loss is due to the thermal

decomposition of hydroxyl and carboxyl groups. At temperatures higher than 500 °C, a 17 wt% weight loss was attributed to the loss of the more stable functional groups.12 In addition, NG posesses most excellent thermal stabilities, contributing to the doping of electron-rich N atoms into the graphene lattice and the reconstruction of the NG structure during the pyrolysis process.9 This results in additional π electrons-conjugation to the C network that increase the delocalization ability of the frame system.13 Single MPEA or CDs exhibits very poor thermal stability and start to decompose at about 160 and 278 °C, respectively. So, this cross temperature at 230 °C can be regarded as a key point to evaluate the thermal stability. The thermal stability of MPEA-NG is higher than that of NG before 230 °C, suggesting the formation of π-π interactions between MPEA and NG. In addition, the thermal stability of CD-NG is obviously higher than that of NG owing to the generation of hydrogen bonds between CDs and NG. Most importantly, after modification with MPEA and CD to NG, the resultant CD-MPEA-NG exhibits the highest thermal stability in contrast to the one of MPEA-NG and CD-NG. The enhanced performance strongly demonstrates that π-π interactions and hydrogen bonds can be effectively formed among CD, MPEA and NG. Interestingly, the thermal stability of NG is higher than that of other nanomaterials at temperatures higher than 230 °C. Incorporation of CD or MPEA into NG layers can result in effective exfoliation of NG nanoplates, which would promote their easier decomposition at higher temperature. Therefore, NG with the tight nanostructure shows the lower weight loss. 7



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Raman spectroscopy is a critical tool for characterizing disorder structures, defect density, and doping levels of the materials.37 In Figure 1C, all samples display the distorted graphitic carbon peaks at ∼1350 and ∼1580 cm−1 correspond to the first-order zone boundary phonon mode associated with the sp3 structural defect sites in the in-plane or edge of graphene (D band) and the radial C-C stretching mode of sp2 bonded carbon (G band), respectively.45 The weak D band reveals low density of defects and the D/G intensity ratio (ID/IG) reflects the disorder degree and the average size of the sp2 domains.11, 46 The apparent D band indicates that GO has multi-layered nanostructure and there are plenty of structural defects due to introduction of oxygen-containing functional groups during the preparation, which will favor the doping of foreign atoms. It was found that the ID/IG ratio in NG was 1.05, being slightly higher than 1.00 of GO. The increased relative intensity is probably due to the production of smaller nanocrystalline graphene domains, the loss of carbon atoms by the decomposition of oxygen-containing groups, as well as the incorporation of N heteroatoms.9, 14 The increased ID/IG ratio of CD-NG indicates more edge-plane defects owing to modification of CDs with sp3 defects. It is worth mentioning that the relative intensity of MPEA-NG is almost identical to that of NG, strongly demonstrating that MPEA have anchored onto the surface of NG based on π-π interactions to become monolitic. Especially, the dimethyl amine group in MPEA may be inserted into the defect hole of NG. The obtained nanostructure maintains the similar disorder to that of NG.11 The Raman spectrum of CD-MPEA-NG exhibits increased ID/IG intensity ratio relatively to the one of MPEA-NG due to incorporation of CDs. The synergistic combination of CD, MPEA and NG can effectively prevent the restacking of NG and significantly enhance the electrochemical performance.47 Electrochemical impedance spectroscopy (EIS) is considered as an important tool for investigating the interface properties of electrodes. The value of charge transfer resistance (Rct) can be estimated by the semicircle diameter of EIS at a higher frequency, and a linear portion at lower frequency is associated with the controlled diffusion process.37,

40

Figure 1D illustrates the results of the

impedance spectra on a bare GCE, GO/GCE, NG/GCE, MPEA-NG/GCE, CD-NG/GCE and 8


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CD-MPEA-NG/GCE, and the Rct values are listed in Table S1. An obviously decreased charge transfer impedance of NG indicates the increase of conductivity compared with GO. The substitution of electron-rich N into the C network can improve the formation of π electron conjugated system so as to increase conductivity and charge carrier density of graphene.13, 48 When MPEA-NG is modified onto the surface of GCE, the semicircle slightly increases compared to the NG electrode, indicating conductive MPEA almost can barely impact the conductivity. However, the Rct value of CD-NG is dramatically higher than that of GO/GCE and NG/GCE, demonstrating that the insulating CD decreases the electron transfer property. The rational introduction of hydrophobic MPEA or insulating CD makes an electrostatic repulsive force to Fe(CN)63−/4−, which should be the main reason of the Rct value increase.31 It is noteworthy that the MPEA with plane-like conjugated structure is more suitable than the cup-shaped CD, because the strong π-π conjugated structure could promote an electron transfer compared to the hydrogen bonding effect. Similarly, when the MPEA and CD are hierarchically functionalized onto the surface of NG, the Rct value of the obtained CD-MPEA-NG/GCE is very closer to that of CD-NG/GCE. The results prove the scientificity of the nanostructure design, which can enhance the host-guest recognition considerably without the decreasing the conductivity of the sensing electrode. The method of density functional theory with corrected dispersion energy (DFT-D) has been adopted to be appropriate for the hydrogen-bonded systems.49 The DFT-D method of wB97XD/LanL2DZ was employed to optimize the structures of heterodimers of NG and MPEA. To evaluate the accuracy of this scheme, we optimized the π-π face-to-face dimer of benzene to use it as a benchmark. The binding energies for the parallel-displaced benzene dimer are 9.37 and 9.00 kJ/mol at the CCSD(T)/aug-cc-pVDZ

50

and wB97XD/LanL2DZ levels, respectively. The binding energies

from these two methods are in good agreement with each other. So, the wB97XD/LanL2DZ method is adequate to describe face-to-face packing of benzene dimer, and is computationally economic one for a large system consisting of weak intermolecular forces. Four optimized MPEA-NG dimers were 9



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obtained with the use of Gaussian 09 program51. As it can be seen in Figure 1E, the binding energies are in the range of ‒130.61 to ‒147.86 kJ/mol (Table S2). Although the MPEA is on top of NG in all the initial configurations of dimers, in some cases, it moves outside the peripheral of NG. The dimers thus formed, turn out to be more stable than that of MPEA being on top of NG (Dimer 4)

Figure 2

The electrocatalytic activities of the bare GCE, GO/GCE, NG/GCE, MPEA-NG/GCE, CD-NG/GCE and CD-MPEA-NG/GCE were investigated utilizing differential pulse voltammetry (DPV) in 0.1 M citrate-phosphate buffer (pH 4.0) by comparing the electrochemical response of CT, RS and HQ. In Figure 2A-C, the obviously enhanced anodic peak currents of dihydroxybenzene isomers using the GO/GCE were obtained compared to that on the bare GCE. These increases are mainly attributed to the π–π interactions between the benzene ring of isomers and the graphene layer.52 In the case of NG/GCE, the catalytic activities towards three isomers were enhanced compared to that of the GO/GCE. That is due to the increased conductivity of the NG and the formation of hydrogen bonds between the hydroxyl in the target molecules and the nitrogen atoms within the NG layers.52 As seen, the oxidation peak currents towards CT, RS and HQ at MPEA-NG/GCE, were higher than that at NG/GCE, demonstrating the effective healing of the intrinsic defects in NG and the prevention of NG aggregation. Higher electron collection and transfer are the basis for enhancing performance of electrochemical sensors. Incorporation of MPEA can effectively reconstruct π-conjugated system of NG, promoting fast transportation of electrons. In addition, the target molecules could easily accumulate around the surface of the MPEA/NG layers owing to similar hydrophobic properties. Thus, the proton-donating groups of analytes facilitated the charge transport rate, strongly enhancing the corresponding peak currents at the NG-MPEA/GCE.37, 53 Furthermore, all the oxidation peak currents on the CD-NG/GCE were also increased for investigated analytes, which 10


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could be ascribed to the enrichment effect and host-guest recognition between CDs and the guests, in comparison to the currents at the MPEA-NG/GCE.54-56 Interestingly, the oxidation signals of three isomers on the surface of the CD-MPEA-NG/GCE exhibited the most significant enhancements in the peak current compared with other modified electrodes. The current densities of three compounds with the highest peak indicate that the ternary combination of CD/MPEA/NG exhibits the enhanced synergetic effect for target molecules. CD-MPEA-NG sensing nanomaterial integrates the high conductivity and large specific surface area of NG, defect-healed function of MPEA as well as the hydrophilicity and supramolecular host-guest recognition of CDs. Based on the above discussion, the unique three-layers architecture of the CD-MPEA-NG nanocomposite can be regarded to as an excellent sensing material for the determination of dihydroxybenzene isomers. The inner layer of NG is an efficient redox catalytic activity; the middle conjunction layer of MPEA can be used to improve the stability of the CDs and also to remedy the π-conjugated system of the NG surface; the outer layer of CDs can easily enrich targets so as to increase the regional concentration and accelerate the electronic transfer rate during the electro-catalysis reactions on the electrode surface. In

order to further investigate

the

electrocatalytic

property and

selectivity

of

the

CD-MPEA-NG/GCE, the compared DPV behaviors of the different modified electrodes in 0.1 M citrate-phosphate buffer (pH 4.0) contain CT, RS and HQ with the same concentration of 0.3 mM (Figure 2D). Only two broad oxidation peaks with small peak currents at the bare GCE were obtained (curve a), one is the unresolved peak at 0.456 V owing to the overlap of the oxidation peaks of HQ and CT. The other current peak at 0.764 V corresponds to the oxidation of RS. The low sensitivity and selectivity are possibly attributed from the GCE surface fouling, which is caused by the oxidation products of CT and HQ. Three well-separated oxidation peaks were observed at GO/GCE, NG/GCE, MPEA-NG/GCE, CD-NG/GCE and CD-MPEA-NG/GCE. The proton-donating groups of CT, RS and HQ can facilitate the charge transport rate and enhance the oxidation performance, which would gradually increase the corresponding peak currents and exhibited raising catalytic separation 11



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performance of the three coexisting three isomers.53, 57 The oxidation signals at CD-MPEA-NG/GCE exhibited the great improvements in the current peaks compared to that for the other modified electrodes. The well-defined oxidation peaks were observed at 0.224, 0.336 and 0.768 V for HQ, CT and RS, respectively. The high-sensitive performance is due to the superior antifouling property and synergetic effect of CD, MPEA and NG.58 Hence, the CD-MPEA-NG/GCE would be used for further investigation.

Figure 3

Optimization of detection conditions would significantly enhance the electrocatalytic performance of sensors for the simultaneous determination of three dihydroxybenzene isomers. The acidity of the detection medium can efficiently control the rate of mass transport to the electrode surface. The composition strongly affects the thermodynamics and kinetics of the charge transfer process.59 Therefore, the simultaneous current responses and oxidation potentials of CT, RS and HQ at CD-MPEA-NG/GCE were evaluated by DPV in 0.1 M citrate-phosphate buffer with the pH values varying from 3.0 to 7.0. It can be concluded from Figure 3A that the maximum oxidation peak currents of CT, RS and HQ increase with increasing the pH value until it reaches 4.0, and then decrease with further increase of the pH values. The pKa values of CT, RS and HQ are 9.4, 9.4 and 9.85, respectively. Three isomers are protic aromatic molecules that will readily be deprotonated and converted to anions at a higher pH value.60 Besides, the surface of N-containing functional groups in NG and electron-donating cavity of CDs at CD-MPEA-NG/GCE also become deprotonated and possess negative charges. Thus, electrostatic repulsion between the isomers and electrode might result in the decrease of adsorption capacity of the isomers on the electrode surface. So, the peak currents decrease with an increasing pH value. 40,

52, 57

The detection medium with pH of 4.0 should be

chosen as a more suitable environmental pH value. 12


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Additionally, the oxidation peak potentials of the three isomers shifted to negative direction with the pH value increasing, and these peak potentials decreases linearly with the increasing pH values (Figure 3B). The equations of linear regression and correlation coefficients for these relationships are described as follows: Ep (V) = 0.542 - 0.0572 pH (R = 0.9994) for CT, Ep (V) = 1.010 - 0.0620 pH (R = 0.9996) for RS, and Ep (V) = 0.451 - 0.0615 pH (R = 0.9994) for HQ, respectively. The three parallel lines indicate that the peak-to-peak separation between isomers is constant in the different pH solutions.61 Besides, these slopes were close to the expected Nernst theoretical value (- 59.1 mV per pH at 25 °C), implying that the entire electrochemical process at CD-MPEA-NG/GCE was proton-dependent. According to the following equation (1): dE p dpH

= −2.303

mRT nF

(1)

where m and n are the number of protons and electrons respectively. The value of m/n in this process was calculated to be 0.97 (CT), 1.05 (RS), 1.04 (HQ), which are so close to 1, implying that the electron transfer was accompanied by an equal number of protons in the electrode reaction.37, 59, 62 The expected results were consistent with electrochemical oxidation mechanisms shown in Figure S163, 64 involving two-electron and two-proton process.57, 63, 65, 66 Therefore, the selection of pH 4.0 is scientific. The accumulation step is an effective strategy of enhancing electrochemical sensitivity. Figure 3C and D illustrated the relationship between the peak current and the accumulation time for CT, RS and HQ (stirring speed = 400 rpm) at 0 V. The oxidation peak currents of three phenols increased with the increase in the deposition time and the maximum peak currents are obtained at 30 s for all isomers. The accumulation time longer than 30s did not increase the electrocatalytic response, even decreased it, indicating that the rapid surface saturation tended to reach a limited value. Furthermore, the effect of accumulation potential ranging from −0.2 to 0.2 V on dihydroxybenzene isomer peak currents was also studied. As depicted in Figure 3D, the highest oxidation peak current was observed at 0 V. These observations indicate that accumulation is an efficient tool to improve the determination sensitivity. 13



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Thus, to gain the lower detection limit and wider response range, an accumulation step was carried out at 0 V for 30 s was selected.

Figure 4

According to the above discussions, the CD-MPEA-NG/GCE exhibits fascinating electrochemical activity towards CT, RS and HQ due to the synergistic effect of enhanced conductivity and host-guest recognition.

37

The CD-MPEA-NG with the enhanced electrochemical properties and electrocatalytic

activities can be employed here to construct a sensor for simultaneously detecting dihydroxybenzene isomes. The linear range and limit of detection were demonstrated using DPV under the optimized conditions. The individual determination of CT, RS or HQ in their mixture solution was performed by increasing the concentration of the target analyte while keeping the concentration of the other two-analyte constants. The oxidation peak currents of CT, RS and HQ increase with the increase of their concentrations, respectively (Figure 4A, C and E). Interestingly, the almost constant peak currents of other two analytes indicated that the oxidation of CT, RS and HQ at CD-MPEA-NG/GCE takes place independently. 40, 54 Moreover, the oxidation peak currents of CT, RS and HQ (Ip, µA) were proportional to their concentrations (C, µM) on CD-MPEA-NG/GCE. A clearly defined oxidation current versus the concentration of CT has the good linearity in the range from 0.25 to 50 µM and from 50 to 1400 µM (Figure 4B). The linear regression equations are expressed as Ip, CT1 = 0.270 CCT + 3.764 (R = 0.9992), Ip, CT2 = 0.069 CCT + 12.468 (R = 0.9998). Similarly, Figure 4D revealed the anodic peak current increase linearly with increasing the concentration of RS from 0.5 to 14 µM and from 14 to 620 µM. Hence, the regression equation was Ip, RS1 = 0.202 CRS + 0.730 (R = 0.9954), Ip, RS2 = 0.026 CRS+ 3.079 (R = 0.9997). As shown in Figure 4F, the oxidation peak currents of HQ were also linear with the HQ concentration in the range of from 0.5 to 60 µM and from 60 to 1400 µM, the linear relation could be expressed by regression equation of Ip, HQ1 = 0.265 CHQ + 5.310 (R = 0.9992), 14


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Ip, HQ2 = 0.046 CHQ+ 19.245 (R = 0.9994). The detection limit of 0.008, 0.018 and 0.011 µM (S/N = 3) was obtained for CT, RS and HQ, respectively. Two linear ranges of CT, RS and HQ exist, from low to high concentrations for each compound. The reason may be attributed to the fact that the CD-MPEA-NG is a heterogeneous multi-layer enrichment process with limited surface of active sites. In the process of enrichment, the target molecules firstly dominate the high-energy active sites on the CD-MPEA-NG. While the utilization of high-energy active sites reach saturation, more analytes would be slowly enriched onto the other sites with low-energy activity. Therefore, the slopes of the linear fitting curves in the low concentration range are greater than the slopes of the curves in the higher concentration range. Moreover, with the enrichment reaction proceeding, the active site decreased gradually, and the enrichment gradually reached the dynamic equilibrium, that is, with the increase of the concentration, there is no linear relationship with the peak current.16, 67 It is notable that the outstanding baseline of DPV curves has a good coincidence and the reliable linear relationships between the peak current and the concentration of three isomers (R = 0.999). All observations prove that the CD-MPEA-NG/GCE has better stability for the sensing detection of the three isomers. It is worth noting that the detectable concentration ranges of CT, RS and HQ at CD-MPEA-NG/GCE are expanded mostly with lower detection limits, suggesting that CD-MPEA-NG/GCE had an excellent sensitivity to selective detection of three isomers. The comparison of the proposed CD-MPEA-NG with other sensing methods for CT, RS or HQ detection was listed in Table S3. 40 The selectivity and sensitivity of the CD-MPEA-NG/GCE towards dihydroxybenzene isomers are of significance to further reveal the electrochemical performance features. The major interferences come from the organic interfering species (e.g., glucose, ascorbic acid, bisphenol A, p-chlorophenol), which may lead to an overlapping with the existing peaks. The anti-interference experiment was performed by measuring the solution containing 5 µM of CT, RS and HQ in the presence of 10-fold concentration of organic interfering species using the CD-MPEA-NG/GCE by DPV. In addition, no 15



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significant interference was observed for the current response of three isomers, as shown in Figure S2. Moreover, the interferences of some inorganic species were also investigated at CD-MPEA-NG/GCE by i-t curve and the results were shown in Figure S3. The 100-fold concentration of KNO3, ZnSO4, NaAC, CaCl2, Cu(NO3)2 compounds have hardly any effect on determination of three isomers, exhibiting an excellent tolerance to interference of the modified electrode. 40 Stability is a key factor in the practical application for a successful sensor. The stability of the CD-MPEA-NG/GCE was evaluated by DPV. After the CD-MPEA-NG/GCE was stored at room temperature for over four weeks, the peak currents maintain 97.7%, 97.9% and 96.5% of its initial value to CT, RS and HQ, respectively, presenting a good long-term stability of the modified sensor. In order to investigate reproducibility, five modified electrodes fabricated independently in the same conditions were estimated by comparing with the peak currents of the same concentration of CT, RS and HQ, and satisfactory RSD values of 0.93%, 1.15% and 1.10% were obtained, respectively. The excellent reproducibility of the CD-MPEA-NG/GCE makes it attractive for the construction of electrochemical sensor. Additionally, under the optimized conditions, the RSD of the modified electrode in response to the same concentration of CT, RS and HQ for five measurements was 0.05%, 0.22% and 0.11%, respectively, indicating an acceptable repeatability for sensor preparation. The Nanjing river water was tested in order to investigate the possible applications of the method for determining CT, HQ and RS. The real samples were collected directly from the Nanjing river water. No dihydroxybenzenes could be detected in the water samples, which meant that aimed analytes contents were lower than the detection limits. Before measurement, the pH value and ionic strength of the water sample solutions were adjusted to match the citrate-phosphate buffer. The standard addition method was adopted for the determination of CT, RS and HQ by adding the known concentrations of target concentrations. The results are listed in Table 1. Each of the samples was measured for three times. The results obtained by DPV were in good agreement with the actual addition and the recovery rates were in the range of 100% to 103.9%, indicating potential for practical 16


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CT, RS and HQ detection. Furthermore, the recoveries of the proposed electrochemical method were in good accordance with the ranges obtained by the UV–vis spectrophotometry, indicating that the proposed method could be reliably utilized for practical application.

Table 1

3. CONCLUSIONS

In summary, we presented an electrochemical sensor by constructing hierarchically rational embedding of MPEA between cyclodextrin and N-doped graphene, whose electrocatalytic performance was enhanced significantly towards dihydroxybenzene isomers thanks to the synergistic effect. The strong face-to-face coupling between NG and MPEA can be described by DFT-D of wB97XD/LanL2DZ. The CD-MPEA-NG sensor can demonstrate fast and sensitive electrochemical response towards dihydroxybenzene isomers without employing expensive noble metal or complicated preparation process. Besides, the sensor also exhibits the highly sensitive determination towards trace dihydroxybenzene isomers, the obtained results are even better than many electrodes reported in the relevant literatures. All the advantageous observations showed that the proposed CD-MPEA-NG sensor is a simple, cost-effective and efficient electrode, which can promote the development of novel electrochemical sensing platform.

4. EXPERIMENTAL

Preparation of CD-MPEA-GN composite. CD-MPEA-GN was synthesized via two-step low-temperature hydrothermal process under the mild conditions, which is schematically illustrated in Scheme 1. In the first step, the MPEA molecule was prepared according to the method previously 17



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reported,68 and its structure is shown in Scheme 1. The detailed procedure can be found in the Supporting Information file. The GO as precursor was synthesized from graphite through a modified Hummers’ method,69, 70 and the electronic structure perturbation of nitrogen-vacancy was induced by using ammonia as intercalator. The simple and mild approach provides a means to remove oxygen species effectively with little contamination on the basal plane,71 and create a π-π interfacial interactions between NG and MPEA,10 which may allow electron transfer from the MPEA, MPEA-NG to an electron-accepting material placed nearby. Rochefort et al. revealed that the interaction between aminotriazine on graphene is partly driven by the specific attractive interaction of the –NR2 group with the underlying graphene surface, indicating that the substituent is a key factor in determining the interaction.8 In the second step, the electron communication between NG and β-CDs can be further strengthened via hydrogen bonding and sp2 and sp3 hybrid structures cooperative interactions, which contribute to accumulate CDs around the modified graphene layers. The details of the CD-MPEA-GN preparation were described as follows: 10 mg of obtained dry GO powers were ultrasonically dispersed into 5 mL of DMF solution for about 5 h, and then 500 µL of MPEA in DMF solution (0.01 mol/L) was added to the above GO dispersion. The mixed suspension was transferred to a reaction vial, and 500 µL of ammonium hydroxide (25-28 wt%) was quickly added into the suspension. After turning the vial upside down several times, the vial was kept in the oven quietly at 80 °C for 2 days to enable the complete assembly of MPEA into the surface of GO sheets and nitrogen doping. The prepared composite, named MPEA-NG, was washed by centrifuging for several times, and then dispersed in 10 mL of DMF. The concentration is 2mg/mL. After that, 1 mL of CD in DMF solution (320mg/mL) was added into the obtained suspension. After sonication for 5 h, the mixture was kept in the oven quietly at 80 °C for 2 days to enable the complete self-assembly of CD onto the surface of the MPEA-NG. Finally, the resulting product was washed with ethanol for three times, then dried at 70 °C under vacuum. For comparison purpose, the MPEA-NG, NG, CD-NG hybrids were fabricated using the similar method. The chemical structure of NG is shown in Figure S4. 18


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Fabrication of the sensor. Before its use, the glassy carbon electrode (GCE, 3 mm in diameter) was polished wiht 0.3 µm alumina slurry before use. After rinsed thoroughly in doubly-distilled water, the synthesized material (2.0 mg) was firstly dispersed in 2-mL solvent mixture of Nafion (5%) and water (V:V ratio = 1:9) by ultrasonication to enhance the tackiness and selectivity of the electrode. And then 5 µL of the suspension was cautiously dropped onto the surface of the GCE and dried at room temperature.53 The obtained electrode was noted as CD-MPEA-NG/GCE. For comparison, a similar procedure was used to prepare CD-NG/GCE and MPEA-NG/GCE. Finally, before measurement, the modified electrodes were activated before measurement by successive scans in citrate phosphate buffer solutions (pH = 4) until the steady voltammograms were achieved.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: More details of materials, experimental procedures, instrumentation, the mechanisms of electron generation, DPV, amperometric I-t, charge transfer resistance, total energies and binding energies and comparison of the proposed sensor for three isomers detection with other reported electrodes (PDF).

AUTHOR INFORMATION

Corresonding Authors *E-mail: [email protected] (S.P. Zhang) [email protected] (H.O. Song) 19



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Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

This work was supported by the Natural Science Foundation of China (51402151, 51408297, 51778281); the Natural Science Foundation of the Jiangsu province (BK20171342, BK20161493, BK20140780) and Major Science and Technology Program for Water Pollution Control and Treatment, PR China (No. 2014ZX07214-001); the QingLan Project, Jiangsu Province; and the Zijin Intelligent Program, the Nanjing University of Science and Technology; Fundamental Research Funds for the Central Universities (30917011309); Graduate research and innovation project of Jiangsu province (No.KYLX16-0470). The authors also acknowledge appreciation to Prof. Xin Wang and the Institute of Water Environmental Engineering, Jiangsu Industrial Technology Research Institute (Yancheng) for their helpful support.

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of

2-Aminophenol

and

4-Aminophenol

Based

on

Poly(

l

-Arginine)-β-Cyclodextrin/Carbon Nanotubes@Graphene Nanoribbons Modified Electrode. Biosens.

Bioelectron. 2016, 77, 353–358. (56) Zhu, G.; Yi, Y.; Sun, H.; Wang, K.; Han, Z.; Wu, X. Y. Cyclodextrin-Functionalized Hollow Carbon Nanospheres by Introducing Nanogold for Enhanced Electrochemical Sensing of o-Dihydroxybenzene and p-Dihydroxybenzene. J. Mater. Chem. B 2014, 3, 45–52. (57) Wang, X.; Wu, M.; Li, H.; Wang, Q.; He, P.; Fang, Y. Simultaneous Electrochemical 27



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Determination

of

Hydroquinone

and

Catechol

Based

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29



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Page 30 of 37

Captions

Scheme 1. Illustration of the procedure for fabricating CD-MPEA-NG. Figure 1. FT-IR spectra (A), TGA (B) and Raman spectra (C) of GO, CDs, MPEA, NG, MPEA-NG, CD-NG and CD-MPEA-NG hybrids; EIS characterization (D) of bare GCE (a), GO/GCE (b), NG/GCE (c), MPEA-NG/GCE (d), CD-NG/GCE (e) and CD-MPEA-NG/GCE (f) in 5.0 mM Fe(CN)63−/4− (1:1) containing 0.1 M KCl solution. (E) Optimized structures of MPEA/NG dimers at wB97XD/LanL2DZ level (Data in parenthesis are binding energy in kJ/mol)

Figure 2. DPVs obtained for the oxidation of 0.3 mM CT (A), RS (B), HQ (C), and CT, RS, HQ in the mixed solution with the same concentration (0.3 mM) (D) at bare GCE (a), GO/GCE (b), NG/GCE (c), MPEA-NG/GCE (d), CD-NG/GCE (e), CD-MPEA-NG/GCE (f) in 0.1 M pH 4.0 citrate-phosphate buffer. Pulse width: 0.05 s; amplitude: 0.05 V.

Figure 3. (A) Effect of detection medium pH on the anodic peak currents of 0.1 mM of CT, RS and HQ at CD-MPEA-NG/GCE in 0.1 M citrate-phosphate buffer; (B) The plots of anodic peak potential of these species versus pH values; (C) Effect of accumulation time on the oxidation peak currents of 0.2 mM CT, RS and HQ at CD-MPEA-NG/GCE in 0.1 M pH 4.0 citrate-phosphate buffer by DPV; (D) Effect of accumulation potential on the oxidation peak currents of 0.2 mM CT, RS and HQ at CD-MPEA-NG/GCE in 0.1 M pH 4.0 citrate-phosphate buffer by LSV;

Figure 4. DPV curves of CT (A), RS (C), HQ (E) with different concentrations in the mixture of 50 µM RS and 50 µM HQ, 50 µM CT and 50 µM HQ, 50 µM CT and 50 µM RS at CD-MPEA-NG/GCE in 0.1 M pH 4.0 citrate-phosphate buffer, respectively; The plots of Ip vs. concentrations for CT (B), RS (D) and HQ (F), respectively.

Table 1 Determination of 4-AP, 4-CP and 4-NP at various concentrations in river water.

30


Page 31 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1.

31



Figure 1.

(A)

(B) 100

(a) GO

1728 1624

Transmittance[a.u]

3337 2925

(b) CDs

80

1368

2206 1610 15211228 1592

(c) MPEA (d) NG

3309 2872 3407

Weight (wt %)

3430

1580

(e) MPEA-NG

845

1096 844

1646 1456 1093

2843

(f) CD-NG

1416

3500

2916

(g) CD-MPEA-NG

3000

2500

2000

1500

60 40

1236 1088 844

0

1000

100

200

-1

Wavenumber[cm ]

300

400

500

600

700

800

Temperature ( C ) O

(D)150

D

120

resistance / Rct

(C)

MPEA NG CD-NG

20

1077 1416

GO CDs MPEA-NG CD-MPEA-NG

O

230 C

1048 1029

2901

3226

G

ID/IG=1.07

CD-MPEA-NG

ID/IG=1.05

MPEA-NG

ID/IG=1.13

CD-NG

ID/IG=1.05

NG

ID/IG=1.00

Charge transfer

120

Z"/ohm

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

90

100

80

c d b

60

e f

40

20

0

GCE

GO

NG

MPEA-NG CD-NG CD-MPEA-NG

60

a

30

GO

0

750

1000

1250

1500

1750

2000

2250

0

50

-1

Raman Shift (cm )

100

Z'/ohm

32


150

200

Page 33 of 37

Figure 2.

(A)

f

60

(B)

CT

75

d c

Current/μ A

Current/μ A

45

30

b 15

0 0.0

a

e

45

b

30

a

15 0

0.2

0.4

0.6

0.0

(D)

HQ

f 30

Current/μA

20

c b

15 10

a

0.8

1.0

0.6

e

50

d 40

c

30 20

0 -0.2

1.2

CT+RS+HQ

f

a

b

10

5 0.6

0.4

70 60

e d

25

0.2

Potential/V vs. SCE

35

0 0.4

d

c

Potential/V vs. SCE

(C)

RS

f

e 60

Current/μ A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

Potential/V vs. SCE

0.2

0.4

0.6

0.8

Potential/V vs. SCE

33


1.0

1.2


Figure 3.

(B) pH 3 pH 4 pH 5 pH 6 pH 7

20

Current/μ A

15

10

5

0 -0.2

RS

0.8

Peak potential/V

(A)

0.6

0.4

CT HQ

0.2

0.0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

2

3

4

51

(C)

7

(D) 17.30

48 12

HQ

10

45 42

39

RS

8

CT

6

39

36 36

-10

0

10

20

30

40

50

60

17.20

30

20

25 20

RS

17.15

15 17.10

18 16 14 12

CT

10 10

-0.3

-0.2

-0.1

0.0

0.1

Accumulation potential / V

Accumulation time / s

22

HQ

33 -20

35 17.25

Current/µ A

45

33

6

14

48

42

5

Detection medium pH

Potential/V vs. SCE

Current/µA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 37

34


0.2

Page 35 of 37

Figure 4.

120 1400 μM 1100 μM 900 μM 700 μM 500 μM 300 μM 200 μM 150 μM 50 μM 35 μM 25 μM 15 μM 9 μM 7 μM 5 μM 3 μM 2 μM 1.4 μM 1 μM 0.7 μM 0.25 μM 0 μM

Current/μA

80 60 40 20

100

0.2

0.4

IP=12.468+0.069CCT R=0.9998

60 40

IP=3.764+0.270CCT

20

R=0.9992

0

0 0.0

(B)

80

IP/μ A

(A)

100

0.6

0

0.8

300

600

Potential/V vs. SCE

(C)

620 μM 450 μM 280 μM 150 μM 70 μM 14 μM 8 μM 5 μM 4 μM 3 μM 2 μM 1 μM 0.5 μM 0 μM

Current/μA

15 12 9 6 3

20

(D)

0.2

0.4

0.6

R=0.9997

10 5

IP=0.730+0.202CRS

0.8

1.0

1.2

R=0.9954 0

100

200

Potential/V vs. SCE

90

70 60 50 40 30 20 10

(F) 80

0.0

0.2

0.4

500

600

700

IP=19.245+0.046CHQ

R=0.9994

60 40 20

IP=5.310+0.265CHQ R=0.9992

0

0 -0.2

400

100

1400 μM 1000 μM 700 μM 500 μM 320 μM 160 μM 60 μM 45 μM 35 μM 25 μM 15 μM 10 μM 8 μM 6 μM 4 μM 2.5 μM 1 μM 0.5 μM 0 μM

(E)

300

CRS/µM

IP/μ A

80

1500

IP=3.079+0.026CRS

0

0.0

1200

15

0 -0.2

900

CCT/µM

IP/μ A

18

Current/µA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

0.8

1.0

0

1.2

300

Potential/V vs. SCE

600

900

CHQ/µM

35


1200

1500


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 37

Table 1 Samples

CT

RS

HQ

a

Spiked (µM)

DPV

UV–vis

Founda (µM)

Recovery (%)

Founda (µM)

Recovery (%)

0

-

-

-

-

10

10.15±0.07

101.5

10.16±0.15

101.6

70

70.11±0.65

100.2

70.05±0.30

100.1

0

-

-

-

-

10

10.39±0.29

103.9

10.24±0.25

102.4

70

70.08±0.61

100.1

70.39±0.25

100.6

0

-

-

-

-

10

10.02±0.09

100.2

10.56±0.23

105.6

70

70.02±0.07

100.0

70.50±0.13

100.7

Average of three measurements.

36


Page 37 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


343x160mm (300 x 300 DPI)


Hierarchical Self-Assembly of Cyclodextrin and Dimethylamino

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