Electrochemical Grafting of Graphene Nano Platelets with Aryl

Jan 25, 2016 - To vary interfacial properties, electrochemical grafting of graphene nano platelets (GNP) with 3,5-dichlorophenyl diazonium tetrafluoro...
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Electrochemical Grafting of Graphene Nano Platelets with Aryl Diazonium Salts Zhipeng Qiu,†,§ Jun Yu,†,§ Peng Yan,† Zhijie Wang,† Qijin Wan,*,† and Nianjun Yang*,†,‡ †

School of Chemistry and Environmental Engineering, Key Laboratory for Green Chemical Process of Ministry of Education, Hubei Key Lab of Novel Reactor & Green Chemical Technology, Wuhan Institute of Technology, Wuhan 430073, China ‡ Institute of Materials Engineering, University of Siegen, Siegen 57076, Germany ABSTRACT: To vary interfacial properties, electrochemical grafting of graphene nano platelets (GNP) with 3,5-dichlorophenyl diazonium tetrafluoroborate (aryl-Cl) and 4-nitrobenzene diazonium tetrafluoroborate (aryl-NO2) was realized in a potentiodynamic mode. The covalently bonded aryl layers on GNP were characterized using atomic force microscopy and X-ray photoelectron spectroscopy. Electrochemical conversion of aryl-NO2 into aryl-NH2 was conducted. The voltammetric and impedance behavior of negatively and positively charged redox probes (Fe(CN)63−/4− and Ru(NH3)62+/3+) on three kinds of aryl layers grafted on GNP reveal that their interfacial properties are determined by the charge states of redox probes and reactive terminal groups (-Cl, -NO2, -NH2) in aryl layers. On aryl-Cl and aryl-NH2 garted GNP, selective and sensitive monitoring of positively charged lead ions as well as negatively charged nitrite and sulfite ions was achieved, respectively. Such a grafting procedure is thus a perfect way to design and control interfacial properties of graphene. KEYWORDS: graphene nanoplatelets, aryl diazonium salts, electrochemical grafting, charge effect, inorganic ions

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INTRODUCTION

Electrochemical grafting using aryl diazonium salts, pioneered by Pinson and co-workers in 1992,11 will be the best method to functionalize graphene. Using such an approach, the type and amount of reactive terminal groups on the surface of graphene surface can be precisely controlled and altered,9,12−14 for example, through selecting the monomer of aryl diazonium salts, grafting mode (e.g., potentiostatic, potentiodynamic, galvanostatic), and grafting conditions (e.g., concentration of monomer of aryl diazonium salts, applied potentials or potential ranges, grafting times, solvent, etc.). In other words, the required interfacial properties of graphene are possible to be artificially designed via electrochemical grafting of aryl diazonium salts. Until now, electrochemical grafting of aryl diazonium salts on graphene modified screen-printed carbon electrodes15−17 monolayer graphene on Si,10,18 multilayer graphene on nickel19 have been reported. After further crosslinking or modification (e.g., EDC-NHS process) of these grafted aryl layers, several biosensors have been constructed, such as label-free voltammetric15 and impedimetric10 immunosensor. Surprisingly, until now, few studies are devoted to direct use of aryl layers grafted graphene for electrochemical sensing. Moreover, the effect of the types and amounts of reactive

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Graphene, a layer of two-dimensional sp -hybridized carbon, has become the most popular nanomaterial for chemists, physicists, biologists, material scientists, and engineers.1 Although it possesses extraordinary structural, mechanical, and electronic features, chemical and physical introduction of functional groups and molecules to the surface of graphene are of great significance for both fundamental studies and practical applications.2−10 In recent years covalent and noncovalent functionalization of graphene with aryl diazonium salts has been paid extensive attention.2−10 Using various aryl diazonium salts and the approaches such as click chemistry, electroless, photochemical illumination, electrochemical grafting, and other related methods, epitaxial graphene,6 graphene nanoribbons,7 chemically converted graphene,8 mechanically exfoliated graphene,9 and CVD graphene10 have been functionalized with different aryl layers. Depending on the amounts and types of reactive functional groups in aryl layers grafted on graphene, they have been employed for numerous different applications in the fields of electronic, chemical, biomedical, and related aspects.2−10 The features of such approaches are of easy preparation of modifer, flexible choice of reactive terminal groups, and strong aryl-surface covalent bonding.2−4 However, in some cases, precise control of such modification processes (e.g., the amounts or densities of reactive functional groups) is relatively difficult to be realized. Long reaction time up to several hours and heating process or the addition of surfactants were required.2−10 © XXXX American Chemical Society

Special Issue: Electrochemical Applications of Carbon Nanomaterials and Interfaces Received: November 29, 2015 Accepted: January 13, 2016

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DOI: 10.1021/acsami.5b11593 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION Electrochemical grafting of GNP with aryl diazonium salts was conducted by use of a potentiodynamic mode. Figure 1 shows

terminal groups on the performance of electrochemical and bioelectrochemical sensors based on aryl layers grafted graphene is missing in the literature. Therefore, in this contribution we report about electrochemical grafting of graphene nano platelet (GNP), a multilayer graphene, via reduction of aryl diazonium salts. Further direct utilization of aryl layers grafted GNP for electrochemical sensing of charged inorganic ions is shown. To vary the types and amounts of aryl layers on GNP, aryl diazonium salts with reactive terminal groups of -NO2 and -Cl were selected. Electrochemical conversion of aryl-NO2 into aryl-NH2 was conducted. Because of the introduction of reactive functional groups of -NH2, -NO2, and -Cl, the grafted aryl layers are promising to own different interfacial properties due to altered charge/polar states of these interfaces. These aryl layers grafted on GNP were characterized with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). To examine the effect of the type of the reactive terminal groups on the interfacial properties, voltammetric and impedance behavior of negatively and positively charged redox probes (e.g., Fe(CN)63−/4−, Ru(NH3)62+/3+) were investigated on these aryl layers grafted GNP. On the basis of these results, selective electrochemical sensing of negatively and positively charged inorganic ions, namely, lead, nitrite, and sulfite ions, were conducted on aryl-Cl and aryl-NH2 layers grafted GNP.

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Figure 1. Electrochemical grafting of GNP with 5 mM aryl-Cl (A) and aryl-NO2 (B) in 0.1 M TBABF4 solution. The scan rates were 50 mV s−1. (C) Electrochemical reduction of aryl-NO2 grafted on GNP in 0.1 M KCl containing ethanol/water (v/v = 1:9). The scan rate was 40 mV s−1.

cyclic voltammograms for electrochemical grafting of GNP with aryl-Cl (A) and aryl-NO2 (B). Irreversible reduction peaks centered around 100 ± 10 and 400 ± 20 mV are detected during the first cycle for aryl-Cl (A) and aryl-NO2 (B), respectively. The reduction potential of aryl-NO2 (B) on GNP is different from that reported on the glassy carbon electrode,8 conductive diamond,20,21 and silicon carbide,22 indicative of altered activities of aryl diazonium salts on these electrodes. As expected, these reduction currents and potentials varied as well with scan rates and the electrolytes used. For both aryl-Cl (A) and aryl-NO2 (B), the cathodic peaks disappear quickly in subsequent cycles, similar to those noticed on conductive diamond20,21 and silicon carbide.22 The decreased cathodic peak current in subsequent cycles and the disappearance of the cathodic peak potential indicate the formation of an aryl layer on GNP, where either the reduction is halted or the electron transfer through the formed film is kinetically slowed as a function of time.12,13 The densities of electrons exchanged during the grafting process in Figure 1A,B were calculated. It was enhanced with an increase of scanning cycles. For aryl-Cl, the density of electrons in the first cycle reached 55% of the total electron density of 4.4 × 1015 cm−2. For aryl-NO2, this density of electrons was 1.2 × 1016 cm−2 in the first cycle, which was 89% of the total electron density. This density of of electrons for aryl-NO2 obtained on GNP is almost 10 times higher than that on boron-doped single-crystalline diamond,20 indicative of higher reactivity of aryl diazonium salts on GNP than on diamond. It is believed that the mechanism of this covalent bonding of nitrophenyl molecules proceeds via reductive formation of aryl radicals that form covalent bonds to the carbon surface.12,13 Then the densities or the amounts of reactive terminal groups of -Cl and -NO2 on GNP are 4.4 × 1015 and 1.3 × 1016 cm−2, if assuming all aryl radicals react with GNP. The difference in their densities reveals the effect of the chemical nature (e.g., size, charges/polarity, etc.) of the terminal groups in the diazonium salts on their electrochemical grafting abilities to GNP. The reasons behind might include the altered interaction of diazonium salts with GNP, the changed assembly of the radicals from diazonium salts on GNP, and further their varied growth steps/mechanisms during electrochemical grafting. Further electrochemical reduction of aryl-NO2 grafted on GNP was investigated in a mixture of 0.1 M KCl and the

EXPERIMENTAL SECTION

Reagents and Solutions. GNP was obtained from Nanjing Xi Nano Mstar Technology Ltd. (Nanjing, China). 3,5-dichlorophenyl diazonium tetrafluoroborate (aryl-Cl; wt % ≥ 97%) and 4-nitrobenzene diazonium tetrafluoroborate (aryl-NO2; wt % ≥ 97%) were obtained from Aladdin Chemistry Co. Ltd. (Shanghai, China). Other chemicals were of analytical grade with highest purity and used as received. The chitosan solution (1%) was prepared by dissolving 0.5 g of chitosan in 50 mL of 0.1 M pH 5 acetate buffer solution. The preparation of 2.0 mg mL−1 GNP suspension was done by dispersing 10 mg of GNP in 5.0 mL of chitosan solution. Apparatus. Scanning electron microscope (SEM) images were obtained on a Quanta 200 microscope (FEI Company, Netherlands). XPS measurements were performed on a Quanta 200 microscope (FEI Company, Netherlands). A monochromated Al Kα beam was used as X-ray source. AFM images were recorded in a tapping mode on a Veeco Multimode Nanoscope V probe microscope (Shanghai, China). Amperometric and cyclic voltammetric experiments were performed on a CHI 760C electrochemical workstation (Shanghai Chenhua Apparatus Corporation, China) at room temperature. A conventional three-electrode system was applied with a modified glassy carbon electrode disk electrode (3.0 mm in diameter) as the working electrode, a platinum foil as the auxiliary electrode, and a saturated calomel electrode (SCE) as the reference electrode. The used modified electrodes included GNP coated glassy carbon electrode (GNP), an aryl-Cl layer grafted GNP electrode (GNP-Cl), an aryl-NO2 layer grafted GNP electrode (GNP-NO2), and an aryl-NH2 layer modified GNP electrode (GNP-NH2). Electrochemical Grating with Aryl Diazonium Salts. Prior to electrochemical grafting, the glassy carbon electrode was polished, cleaned, and then coated with 5.0 μL of 2.0 mg mL−1 GNP suspension via drop-casting. Electrochemical grating of GNP with aryl diazonium salts was conducted using a potentiodynamic mode at a scan rate of 50 mV s−1 in acetonitrile containing 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4) and 5 mM aryl-Cl or aryl-NO2. The scanned potential ranges were 0.4 to −0.5 V and 0.6 to −0.3 V for arylCl and aryl-NO2, respectively. For these grafting, only three cycles were applied. Electrochemical conversion of aryl-NO2 into aryl-NH2 was done in a mixture of 5 mL of water containing 10% ethanol and 1.0 M KCl. B

DOI: 10.1021/acsami.5b11593 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. SEM images of GNP before (A) and after grafted with aryl-Cl (B) and aryl-NO2 (C).

particles are detected, due to agglomerative polymer structures formed during the grafting process. The island sizes for an arylCl layer are bigger than those for an aryl-NO2 layer, probably due to different grafting mechanisms of aryl-Cl and aryl-NO2 on GNP (e.g., reaction speed, growth sites and mode, etc.). The measurements of lateral dimensions of these islands were failed, limited by the radius of curvature of the AFM tips and the roughness of GNP. With an attempt to measure the thickness of these aryl layers, AFM scratching experiments were tried.28 Owing to weak adhesion force of GNP to the glassy carbon electrode underneath, it was unfortunately not successful. To reach such a goal, we are currently trying to covalently bond micrometer-size (0.65 to 1.5 μm) GNP to the surface of atomically flat highly oriented pyrolytic graphite. Therefore, aryl-Cl and aryl-NO2 layers were grafted to GNP. XPS measurement is one of the most conventional methods to characterize an organic layer attached to a solid surface. High-resolution XPS spectra of aryl layers electrochemically grafted on GNP were then recorded. Figure 4 displays the XPS survey spectra of GNP after grafted with aryl-Cl and aryl-NO2. Well-defined XPS signals of Cl(2p) in Figure 4A and N(1s) in Figure 4B are seen. The peaks centered at 532.5 eV and at 283 to 285 eV correspond to XPS signals of O(1s) and C(1s). The atomic ratio of C/Cl in Figure 4A is ∼3:1, the same as that derived from the monomer, aryl-Cl. The atomic ratio of C/N in Figure 4B is ∼6:1, in agreement with that obtained from arylNO2. A detailed spectrum of Cl(2p) is shown as inset in Figure 4A. The typical spin−orbit splitting of the Cl(2p) line is clearly observed, namely, Cl(2p3/2) at 200.1 eV and Cl(2p1/2) at 201.6 eV. The intensity ratio of the peak Cl(2p3/2) to Cl(2p1/2) is 2:1, and the energy separation is 1.5 eV, the same as those reported.23 In Figure 4B, there is no N(1s) signal (∼403.8 eV) for -N2+, indicating full loss of diazonium moiety during such grafting process. A detailed spectrum of the N(1s) is shown as inset in Figure 4B, where two peaks at 400.0 and 406.0 eV are detected, the same as those reported.20,21 The N(1s) peak at 406 eV is related with -NO2 groups. The lower binding energy N(1s) peak at 399 eV is due to reduced nitrogen-containing functionalities.20,21 These together with electrochemical data and AFM results confirm covalent bonding of two different aryl layers to GNP. To get insight into interfacial properties and the effect of reactive terminal groups of aryl layers on the interfacial properties of GNP, the voltammetric and impedance behavior of these aryl layers grafted on GNP was investigated, in comparison with those on bare GNP. Negatively and positively charged redox probes, namely, Fe(CN)63−/4− and Ru(NH3)62+/3+, were utilized. Figure 5A shows the voltammograms of Fe(CN)63−/4− on GNP, GNP-Cl, GNP-NO2, and GNP-NH2. The oxidation peak current is almost equal to the reduction current on each

solution of ethanol/water (v/v = 1:9). As shown Figure 1C, a sharp irreversible reduction peak appears at −880 ± 20 mV, again different from that obtained on conductive diamond20,21 and silicon carbide.22 This cathodic wave is due to the electrochemical reduction of nitrophenyl (aryl-NO2) groups to amine- (aryl-NH2) or hydroxy aminophenyl (aryl-NHOH) groups. In the second and subsequent cycles, the reduction peak is drastically diminished, indicating that nearly all electroactive -NO2 terminal groups are reduced in the first scan. The anodic wave at −250 ± 10 mV appeares only after the first cycle, the same as that observed on conductive diamond,20,21 silicon carbide,22 and other electrodes.12,13 Different peak potentials for the reduction of aryl-NO2 on these electrodes are again the fingerprints of changed features of the electrodes used. The density of aryl-NO2 on GNP, evaluated from the integrated charge of the wave at −880 ± 20 mV, is 2.2 × 1015 cm−2. This density is only 17% of the value obtained in Figure 1A, indicating that most of aryl radicals did not react with GNP or attach to GNP, the same as those found at other metal and carbon electrodes.12,13,20−23 However, it is much higher than those on diamond20,21 and silicon carbide.22 The reasons behind are enhanced active surface area of GNP (in comparison to its geometric area) and improved electrochemical activity of graphene. Therefore, electrochemical grafting using aryl diazonium salts provides a simple and versatile method to functionalize graphene with different amounts or densities and types of reactive terminal groups. The topography of GNP before and after grafting aryl layers was investigated using SEM. As shown in Figure 2A, GNP coated on the glassy carbon electrode for grafting aryl layers shows similar features as those used previously.24−27 The GNP surface looks quite smooth and no special structures are seen. It has an average flake thickness of 6−8 nm, namely, a multilayer graphene. The lateral particle size is in the range from 0.65 to 1.5 μm. Similar features were confirmed in the tapping mode AFM images of a GNP electrode. After grafted with aryl-Cl (Figure 2B) or aryl-NO2 (Figure 2C), the smooth GNP surface is not visible. The SEM images become blurred, an indication of a film formation. The reduced conductivities of the measured samples result partially in such unclear images. Although the detailed nanostructures are not able to be checked, some particles or islands are clearly noticed. These results demonstrate obviously the formation of aryl layers on GNP. To gain the detailed nanostructures of aryl layers grafted on GNP, tapping-mode AFM was applied to characterize the morphology of a GNP electrode before and after grafting aryl layers. Figure 3 shows related AFM images of GNP before (A) and after grafted with aryl-Cl (B) and aryl-NO2 (C) in a selective area of 500 × 500 nm2. Before grafting, the surface of GNP is relatively smooth. After electrochemically grafted with aryl diazonium salts, it becomes rough. Many islands or C

DOI: 10.1021/acsami.5b11593 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. XPS survey spectra (a) of GNP grafted with aryl-Cl (A) and aryl-NO2 (B). (inset) Detailed spectra (b) for Cl(2p) and N(1s).

Figure 5. Cyclic voltammograms (A,B) and Nyquist plots (C,D) of 5.0 mM [Fe(CN)6]3−/4− (A,C) and [Ru(NH3)6]2+/3+ (B,D) in 0.1 M KCl on GNP (a), GNP-Cl (b), GNP-NO2 (c), and GNP-NH2 (d). The voltammograms were recorded at a scan rate of 50 mV s−1, and the Nyquist plots did at open circuit potentials.

are strongly suppressed, and the peak difference is much enlarged. The difference of peak potentials increases to 170 mV, in comparison to 100 mV on GNP. The redox current is only 60% of that on GNP. The decreases of redox currents on GNP-NO2 and GNP-Cl might be due to the repulsive or blockage effect of negatively charged species to negatively charged/polarized aryl-Cl interface. On GNP-NH2, the highest redox currents (200% higher than those on GNP-NO2) is obtained together with a peak difference of 100 mV. This indicates the highest activities of negatively charged redox probes on the positively charged/polarized aryl-NH2 interface. However, when the positively charged Ru(NH3)62+/3+ are used, the variation of both peak currents and peak potential (Figure 5B) is not so obvious as those shown in Figure 5A. The differences are due to altered electron transfer mechanisms of redox probes. For example, the redox probes of Fe(CN)63−/4− are surface-sensitive, while those of Ru(NH3)62+/3+ are not surface sensitive.22,23 In other words, the electron transfer for Fe(CN)63−/4− is extremely sensitive to the surface status of electrodes (e.g., cleanness, roughness, etc.) and modifiers (density, types, etc.) on the electrode surface. Namely, it occurs through an inner-sphere transfer mechanism. As the outersphere system, the redox probes of Ru(NH3)62+/3+ are not

Figure 3. AFM images of GNP before (A) and after grafted with arylCl (B) and aryl-NO2 (C). The images were recorded using tapping mode in air.

electrode. The magnitude of redox (oxidation or reduction) currents varies in the order of GNP-NH2 (Figure 5A-d) > GNP (Figure 5A-a) > GNP-Cl (Figure 5A-b) > GNP-NO2 (Figure 5A-c). The tendency for the difference of anodic peak potential from cathodic one is GNP-NH2 (Figure 5A-d) = GNP (Figure 5A-a) < GNP-Cl (Figure 5A-b) < GNP-NO2 (Figure 5A-c). For example, the redox currents on GNP-NO2 (Figure 5A-c) D

DOI: 10.1021/acsami.5b11593 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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carbon electrode. This is due to enhanced electrochemically active area and the involvement of active sites on graphene (or catalytic ability of graphene).29−33 Figure 6A shows the square wave voltammograms of accumulated lead at an enrichment potential of −1.2 V for

sensitive to those variations of the electrodes. Their electrontransfer processes are realized by electron tunneling from Ru(NH3)62+/3+ complexes through the modifiers.22,23 The magnitude of the oxidation currents on negatively charged interfaces (e.g., GNP-NO2, GNP-Cl) is generally a little higher than that on a positively charged interface (e.g., GNP-NH2). Note that the oxidation peak currents on all electrodes are smaller than the reduction peak currents, probably due to the involvement of oxygen during reduction processes. However, on the same electrode the redox currents of Fe(CN)63−/4− are higher than those of Ru(NH3)62+/3+, resulting from different electrochemical activities of redox probes on those electrodes. Figure 5C,D shows the Nyquist plots of Fe(CN)63−/4− and Ru(NH3)62+/3+ on above four electrodes, respectively. The electron-transfer resistance of redox reactions was calculated from the diameters of the semicircles in the Nyquist plots. For the spectra using Fe(CN)63−/4−, the electron-transfer resistance is ∼30 (Figure 5C-d), 60 (Figure 5C-a), 370 (Figure 5C-c), and 850 (Figure 5C-d) Ω for GNP-NH2, GNP, GNP-Cl, and GNP-NO2, respectively. The biggest electron-transfer resistance for GNP-NO2 is indicative of the slowest electrode kinetics, in agreement with the smallest magnitude of redox currents of Fe(CN)63−/4− in Figure 5A. This is due to a thick insulating aryl layer grafted on GNP, supported by those calculations shown in Figure 1. Moreover, the variation of these electron-transfer resistances is in line with the variation order of peak currents from the voltammetry, as expected for surfacesensitive redox probes. When redox couple of Ru(NH3)62+/3+ is applied, the electron transfer resistance is 20, 30, 70, and 170 Ω for GNP-NH2 (Figure 5D-d), GNP (Figure 5D-a), GNP-Cl (Figure 5D-b), and GNP-NO2 (Figure 5D-c), respectively. Although the order of these resistance is the same as that when redox couple of Fe(CN)63−/4− is used, these resistances are smaller than those obtained when Fe(CN)63−/4− redox probes are applied. This is because Fe(CN)63−/4− redox probes are surface-sensitive probes, while Ru(NH3)62+/3+ are not.23 However, as shown in Figure 5B, the variation of these resistance is smaller, leading to ignorable changes of redox currents and the difference of peak potentials, different from those shown in Figure 5A. However, as expected for a thick insulating aryl layer, GNP-NO2 shows the highest resistance. Therefore, in comparison with GNPNH2 and GNP-Cl, GNP-NO2 is not a suitable and promising electrode for electrochemical sensing applications. These results from voltammetry and impedance of Fe(CN)63−/4− and Ru(NH3)62+/3+ indicate that the aryl layers grafted on GNP are sensitive to its own reactive terminal groups (e.g., -Cl, -NO2, -NH2) and the charges of target analytes in solutions. Taking the redox currents, the charges of redox probes, polarized or charge states of reactive functional groups, and the electron-transfer resistances of the electrodes into account, one can conclude that positively charged/ polarized GNP-NH2 will be favorable for the electron-transfer processes of negatively charged inorganic ions and negatively charged/polarized GNP-Cl for positively charged ions. To further confirm the concepts discussed, the voltammetric behavior of three inorganic ions, namely, one positively charged lead ion and two negatively charged nitrite and sulfite ions, were investigated on negatively charged/polarized GNP-Cl and positively charged/polarized GNP-NH2. These results are compared with those obtained on GNP. Note that the introduction of GNP generally enlarged the oxidation currents of above three ions, in comparison to those got on a bare glassy

Figure 6. Voltammograms of 100 nM lead ions (A), 100 μM nitrite ions (B), and 500 μM sulfite ions (C) on GNP (a), GNP-NH2 (b), GNP-Cl (c). The solution was 0.1 M pH 6.0 acetate buffer (A), 0.1 M pH 6 phosphate buffer (B), and 0.1 M KNO3 (C). For the voltammograms in (A), an accumulation process was applied with an enrichment potential of −1.2 V vs SCE and an enrichment time of 120 s. The scan rate was 50 mV s−1.

120 s. The oxidation peak potential of accumulated lead is located at −0.55 V. The biggest oxidation peak current is seen on GNP-Cl, which is 1.4 and 2.1 times higher than that obtained on GNP and GNP-NH2, respectively. These facts support the idea that a higher activity of positively charged analysts can be realized on the negatively charged/polarized GNP-Cl interface. Figure 6B,C shows the cyclic voltammograms of nitrite and sulfite ions, respectively. For nitrite ions, the oxidation peak potential is at 0.91 V on both GNP and GNP-Cl. On GNPNH2, it shifts to 0.77 V, which is 0.14 V more negative that that on GNP and GNP-Cl. Moreover, the oxidation peak currents increase 20% and 40% in comparison to those obtained on GNP and GNP-Cl. For sulfite ions, the oxidation peak potential is ∼0.82 V on both GNP and GNP-Cl. It moves to 0.62 V when GNP-NH2 is employed. The smallest oxidation current is seen on negatively charged/polarized GNP-Cl. If GNP-NO2 is applied, this oxidation current reduced further. Subsequently a positively charged/polarized GNP-NH2 interface promotes the reactivity of negatively charged target species (e.g., nitrite and sulfite ions). Therefore, electrochemical sensing of charged analytes using aryl-layers grated on GNP can be designed as required. The sensing performance will be determined by the types/amounts of reactive functional groups in aryl layers grafted on GNP as well as the charges of target analytes. The performance of monitoring lead ions using GNP-Cl as well as nitrite and sulfite ions using GNP-NH2 were examined. For the detection of lead ions, the accumulation step was applied, and the accumulation conditions were optimized. The optimized conditions for lead accumulation from the solutions of lead ions were 0.1 M pH 6.0 acetate buffer, an enrichment potential of −1.2 V, and an enrichment time of 120 s. Figure 7A shows the square wave voltammograms of accumulated lead on GNP-Cl. The oxidation peak current (Ip) of accumulated lead was found to be linear with the concentration (c) of lead ions in the range from 1.0 to 30 nM. The linear regression equation is E

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GNP-Cl has a long lifetime up to months, even in air. While GNP-NH2 lost its activity soon in air but remained stable for weeks in a nitrogen-purged 0.1 M pH 6 phosphate buffer. The selectivity of determining of metal ions was tested on GNP-Cl. Figure 8 shows the voltammograms of accumulated

Figure 7. Electrochemical monitoring of lead ions on GNP-Cl in 0.1 M pH 6.0 acetate buffer (A), nitrite ions on GNP-NH2 in 0.1 M pH 6 phosphate buffer (B), and sulfite ions on GNP-NH2 in 0.1 M KNO3 (C). For the voltammograms in (A), the accumulation was done at a potential of −1.2 V for 120 s. The scan rate was 50 mV s−1. The concentrations of lead ions are 1.0 (a), 2.0 (b), 3.0 (c), 4.0 (d), 6.0 (e), 9 (f), and 12 (g) nM. For amperometric measurements in (B) and (C), the potentials applied were 0.75 and 0.60 V, respectively. The concentrations for nitrite ions in (B) were 0.1 (a), 0.15 (b), 8.0 (c), 20 (d), and 120 (e) μM. The concentrations for sulfite ions in (C) were 5 (a), 25 (b), 200 (c), 250 (d), and 500 (e) μM.

Figure 8. Square-wave voltammograms of accumulated lead, copper, and cadmium on GNP-Cl in 0.1 M pH 6 acetate buffer solution at a scan rate of 50 mV s−1. The concentrations of these ions are 20 (a), 200 (b), and 400 (c) nM. The accumulation was conducted at −1.2 V for 120 s.

Ip (nA) = 0.51 ± 0.1 c (nM) (R = 0.9996). The detection limit was 0.3 nM. The performance of GNP-Cl toward the detection of lead ions (e.g., linear concentration range, detection limit) is comparable to and in most cases even better than those reported using, for example, a gold electrode modified with graphene,34 cysteine-functionalized graphene oxide,35 waterdispersible magnetic chitosan/graphene oxide composites,36 and pellet-like reduced graphene oxide.37 For the detection of nitrite and sulfite ions, amperometry was applied. Figure 7B,C shows the related current−time curves on GNP-NH2 when nitrite and sulfite ions with different concentrations were continuously added, respectively. In both cases the oxidation currents reach to the steady-state within 3 s when a potential of 0.75 for nitrite ions and 0.6 V for sulfite ions was applied. For nitrite ions, a linear relationship between the steady-state current (I) and the concentration of nitrite (c) was found in the range from 0.1 μM to 1.2 mM with a linear regression equation of I (nA) = 7.5(±0.2) c (nM) (R = 0.9999). The detection limit was 33 nM. For sulfite ions, the linear regression equation is I (nA) = 2.1(±0.2) c (nM) (R = 0.9995) in the range from 5 μM to 1.0 mM, and the detection limit was 1.66 μM. Therefore, GNP-NH2 exhibits better performance not only for the detection of nitrite ions (e.g., a detection limit of 33 nM, higher than 15 nM on Fe2O3 nanoparticles decorated reduced graphene oxide nanosheets,38 but lower than 46 nM on graphene oxide supported gold nanoclusters,39 37 nM graphene oxide-silver nanocomposite,40 200 nM on gold nanoparticles-sulfonated graphene,41 250 nM on gold nanoparticle/graphene/chitosan,42 and 0.2 mM on polyaniline/graphene/ferrocenecarboxylic acid composite43) but also for the detection of sulfite.42,44 The reproducibility of measuring 5.0 nM lead ions with GNP-Cl as well as 1.5 μM nitrite ions and 25 μM sulfite ions with GNP-NH2 was checked via monitoring once with five freshly prepared electrodes or measuring five times with one electrode. The relative standard deviations of these measurements were 3.9%, 5.9%, and 9.8% for the detection of lead ions with GNP-Cl, nitrite, and sulfite ions with GNP-NH2, respectively.

lead, copper, and cadmium on GNP-Cl in 0.1 M pH 6 acetate buffer solution. The oxidation peak potentials are −0.73, −0.55, and −0.08 V for accumulated cadmium, lead, and copper, respectively. From the difference of these peak potentials, one can tell that GNP-Cl can be applied for selective detection of cadmium, lead, and copper ions. Moreover, when the concentrations of cadmium, lead, and copper ions were varied, the oxidation currents increase accordingly. For example, the oxidation peak currents of accumulated of cadmium, lead, and copper are 1.0, 10, and 0 μA, respectively, when their concentrations are 20 nM. They are 10, 90, and 0 μA, respectively, when the concentrations are 200 nM. Further increase the concentrations of these ions to 400 nm leads the enhancement of the oxidation peak currents to 60, 200, and 8 μA for of cadmium, lead, and copper, respectively. Note that the oxidation currents in these cases are higher than those when individual ions are present. This probably results from cocatalytic effects of accumulated metal and metal ions. Therefore, GNP-Cl is sensitive and selective toward the detection of cadmium, lead, and copper ions. The selective detection of nitrite and sulfite ions using GNP-NH2 is promising to be achieved as well because the oxidation current of nitrite ions on GNP-NH2 is almost 10 times higher than that of sulfite. Moreover, the difference or separation of their oxidation peak potentials is over ∼150 mV. All these facts point out aryl layers grafted on GNP are suitable and promising for electrochemical sensing applications with much improved selectivities and sensitivities, which is comparable with previously proposed using Bucky−Gel paper-based electrodes.45 In our laboratory, electrochemical grafting of other novel carbons such as carbon nanotubes,46 carbon fibers,47 and diamond48,14 with differently terminated diazonium salts are undergoing. These aryl layers grafted electrodes are expected to bring in different or even better performance for sensing these ions. For real sample determination, the standard addition experiments were conducted. The water sample was from Wuhan East Lake (Wuhan, China) and analyzed without any further pretreatment. GNP-Cl was applied for detecting lead F

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ACS Applied Materials & Interfaces Table 1. Analytical Results for Three Ions Using GNP Grafted with Diazonium Salts (n = 3) ions lead ions

nitrite ions

sulfite ions

sample, μM

added, μM

found, μM

recovery, %

RSD, %

20 × 10−3 20 × 10−3 20 × 10−3 10.0 10.0 10.0 100.0 100.0 100.0

5.0 × 10−3 10.0 × 10−3 15.0 × 10−3 1.5 3.0 4.5 25 50 75

23.8 × 10−3 29.8 × 10−3 36.0 × 10−3 11.0 12.5 14.8 119.8 154.0 166.7

95.2 99.3 102.8 95.6 96.1 102.1 95.8 102.7 95.3

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CONCLUSIONS Electrochemical grafting of graphene nanoplatelets with aryl diazonium salts have been conducted using a potentiodynamic mode. The covalently bonded aryl layers on graphene were characterized with atomic force microscope and X-ray photoelectron spectroscopy. Their interfacial properties have been investigated using differently charged redox probes. Through the introduction of different types and amounts of reactive terminal groups (-Cl, NO2, -NH2), the interfacial properties of these layers were altered. Depending on the types (charge states) of reactive terminal groups and target analytes, selective and sensitive detection of three inorganic ions (lead, nitrite, sulfite ions) have been realized. This study thus provides a guide to investigate fundamental properties and practical applications of aryl layers functionalized graphene. In summary, electrochemical grafting using diazonium salts is the best method to functionalize graphene and to introduce more artificially designed functionalities as needed. Future activity should focus on the control of the densities of reactive terminal groups of aryl layers and their effect on their interfacial properties as well as their electrochemical sensing applications. AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

Zhipeng Qiu and Jun Yu contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Project No. 21275113) and the Graduate Innovative Fund of Wuhan Institute of Technology (Project No. CX2014110).

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ions, and GNP-NH2 was used for nitrite ions and sulfite ions. The recoveries were in the range from 95.2% to 102.8%. The obtained relative standard deviations were 2.7%, 6.2%, and 6.2% for the detection of lead ions with GNP-Cl, nitrite, and sulfite ions with GNP-NH2, respectively. These results are listed in Table 1. Therefore, these electrodes are efficient and sensitive for the detection of these ions.

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