Graphite Carbon Nitride

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A novel aptasensor based on graphene/graphite carbon nitride nanocomposites for cadmium detection with high selectivity and sensitivity Xiaofang Wang, Wenyu Gao, Wei Yan, Pei Li, Haihan Zou, Zhengxuan Wei, Weijun Guan, Yuehui Ma, Songmei Wu, Yu Yu, and Kejian Ding ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00380 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 23, 2018

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ACS Applied Nano Materials

A Novel Aptasensor Based on Graphene/Graphite Carbon Nitride Nanocomposites for Cadmium Detection with High Selectivity and Sensitivity †

‡

Xiaofang Wang,†,a Wenyu Gao,†,a Wei Yan,† Pei Li,† Haihan Zou, Zhengxuan Wei,† Weijun Guan, Yuehui Ma, ‡

†

School of Science, Beijing Jiaotong University, Beijing 100044, P R China

‡

a

Songmei Wu,† Yu Yu,†,* Kejian Ding,†,*

Institute of Animal Sciences, Chinese Academy of Agricultural Sciences, Beijing 100083, P R China

These authors have equal contribution to this work.

ABSTRACT: Aptamers as new detection modes have strong affinity and specificity for targets. A novel sensor was developed by constructing a composite system of specific aptamers and reduced graphene oxide (rGO)/graphitic carbon nitride (g-C3N4) (GCN) for diagnosing cadmium cation. Attributed to the incorporation of rGO and aptamers with designed terminal groups, as well as the delicately bonding of aptamers with g-C3N4, this electrochemical biosensor exhibited good sensitivity, specificity, reproducibility, and stability for Cd2+ detection. The linear calibration curves range from 1nM to 1µM and from 1µM to 1mM，and the limitation of detection (LOD) was calculated to be 0.337 nM. The practical application of the proposed method was also verified in the real sample. KEYWORDS: graphite carbon nitride, reduced graphene oxide, aptamer sensor; cadmium; DPASV

INTRODUCTION

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As a worldwide environmental issue, heavy metal pollution has aroused global attention because it does potentially harmful to human health at low concentrations.1 Heavy metals not only are disabled to biodegrade but also accumulate in different body organs. Cadmium ion is detrimental to human and other living beings, as the excess content of cadmium damages the nervous system, alimentary system and hemopoietic system. It is pivotal to cause mutagenesis, carcinogenesis and teratogenesis.2 There are numerous methods to monitor and determine heavy metals, but these techniques inherent limitations which rely on complicated instruments and professional technical personnels have restricted their practical applications. In order to overcome these shortcomings, various electrochemical methods are extensively investigated and applied, since they are easily performed, highly sensitive and low cost. Proposed as a potential material with 2D monoatomic-thick sheets of carbon atoms arranged in the honeycomb pattern, graphene has intriguing features such as high chemical thermal stability, excellent conductivity and vast surface. Reduced graphene oxide (rGO) originated from graphene oxide (GO) by chemical reduction always possesses these analogous advantages, as well as designed surface property.3 So it is vastly employed in the construction of electrochemical sensor. Beyond that, graphite carbon nitride (g-C3N4) is another important material consisting of graphitic-like tri-striazine units, whose layers combined together with the help of weak van der Waals force.4-6 The planar configuration of g-C3N4 may provide an ideal substrate for rGO to sit on due to its dangling bonds.7-9 The composing of graphene and g-C3N4 (denoted as GCN) may endow the composite with not only their respective advantages, but also some new characters. DNA or RNA sequences selected in vitro have the ability to bind to certain molecular targets such as proteins and metal ions. In electrochemical sensing, some small DNA (RNA) with low molecule weight and only the pivotal short sequences, called as aptamers, are more widely


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regarded as attractive and applicative recognition elements, due to their better selectivity, affinity and electroconductivity compared to the antibody with larger biological structure. In addition, it is easy to manipulate and synthesize aptamers, accelerating the application of aptamers in the development of electrochemical sensors. Involved in the metal coordination and electrostatic effects and so on, aptamers targeting on metal ions always have a large-scale, binding-induced conformational change after catching targets. Therefore based on aptamers towards heavy-metal ions, a proper sensor could be enabled.10 Herein, we reported an elaborate aptamers grafted GCN aptasensor, and by differential anodic stripping voltammetry (DPASV), it exhibited high sensitivity, large range and low detection limitation to Cd2+. As illustrated in scheme 1, the substrated GCN composite was prepared by hydrothermal treatment of GO and g-C3N4. Since g-C3N4 was synthesized by thermal polymerization of urea, plentiful amino functional groups existed in g-C3N4 as incomplete product even after hydrothermal treatment. Accompanied with the carboxyl/hydroxyl groups in rGO, these composites are able to chemically bond to the aptamers with carboxyls or aminos which could specifically bind to Cd2+. After self-assembling on the accomplished glassy carbon electrode, GCN was grafted with aptamers under the catalysis of EDC-HCl, to construct highefficiency and performance APT-GCN functional biosensors. DPASV method was used to monitor the variation of Cd2+ concentration in solution. Consequently, besides rGO were affiliated to improve the electroconductivity, the aptamers with good affinity for cadmium ions also enabled this sensor to preferentially adsorb and capture cadmium both at low and high concentrations. As a result, the fabricated sensor exhibited large detection range, high sensitivity and low detect limit. Furthermore, according to the design of two aptamers with different terminal groups which bonded to g-C3N4 and rGO respectively, it confirmed that the delicately


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incorporation of the aptamer and g-C3N4 was one of the key points to obtain the optimal sensing performance with largest detecting range and smallest detect limit. RESULTS AND DISCUSSION Characterization of the Reduced Graphene/Graphitic Carbon Nitride Compounds. After hydrothermal composing, GCNs showed flake-like aggregates (Fig. S1A-D), similar to the rGO and g-C3N4 raw materials (Fig. S1E, F). As the increase of g-C3N4’ weight ratio, the thick and crimped texture increased. However in all the GCN composites, attributed to their strong Π-Π stacking interaction and chemical bonding between functional groups,11-12 it could be observed that the films of the two components quite closely contacted, which was beneficial to the fluent electron transfer in the composites. The morphology of GCN after grafted with aptamers (Fig. 1A and B) displayed no obvious difference from GCN in morphology. Under TEM observation of APT2-GCN(1:1) (Fig. 1B), it could clearly verify the co-existence of layered-structured rGO and g-C3N4 and their tight touch. As shown in Fig. 1B, the light-color areas pointed by black arrow were assigned to rGO, while the dark-color regions directed by white arrows were g-C3N4 with more layers. Although the DNA chain shape molecule was too small to be observed, we carried elements distribution scanning on the region of Fig. 1B to confirm its riveting in GCN. It found that C element spread over all areas (Fig. 1C), because it was the main element of both rGO and g-C3N4. Contrastively, N displayed in the corresponding areas of g-C3N4, indicating that little N was doped into rGO during the hydrothermal process. As the major element of the phosphate unit in DNA, the distribution of P could be regarded equivalent to the distribution of aptamers. The P mapping gave strong signal in the g-C3N4 sections, and trifling intensity in rGO sections. It demonstrated that substantially all the DNA aptamers were grafted on g-C3N4. It was reasonable, as APT2 was terminated with carboxyls, and could be bonded with amino functional


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groups in g-C3N4. The slight deposition of APT2 on rGO was resulted from the inevitable physical absorption. The composing of GO and g-C3N4 and the reduction of GO into rGO during hydrothermal reaction were also certified by X-ray diffraction (XRD) patterns measurement. XRD patterns of rGO, GO, g-C3N4, GCN(1:1) were shown in Fig. 1D, displaying the peaks positions of rGO and g-C3N4 at 25.9° and 27.3°, corresponding to interlayer distances are 0.344 and 0.327 nm. It was obvious that the peak of GCN(1:1) was at 26.9°, associating with an interlayer distance was 0.331 nm, between those of rGO and g-C3N4. Besides, the characteristic peak of GO at 10.2° disappeared in GCN(1:1), verifying the adequate reduction of GO raw material. Infrared spectroscopy (IR) was used to examine the change in chemical constitutions of GCN during the composing. Fig. 1E showed that the strong adsorption at 1650 and 1720 (C=O and O-C=O), 3000-3700 cm-1 (C-OH) by carboxyl/hydroxyl in GO got quite weak in both rGO and GCN(1:1), proving again the reduction of GO in composites. Meanwhile, the sharp peak at ~809 cm-1 in gC3N4, which was the symbol of the out-of-plane bending vibration characteristics of heptazine rings, was also considerably conspicuous in GCN(1:1). The wide and intensive bands 1200-1720 cm-1 by the stretching vibration modes of heptazine of g-C3N4 were also explicitly exhibited in GCN(1:1). The composing of rGO and g-C3N4 not only maintained their respective chemical property and functions, but also contributed to the build of a spatial structure with more contact areas with electrolyte solution. As a spacer, g-C3N4 can tremendously inhibit the restack of rGO sheets by attaching on its surface. N2 adsorption-desorption was used to investigate the specific surface areas of GCNs. Except GCN(1:10), almost all of GCN produced much larger BET surface areas than individual rGO and g-C3N4 (Table.S1; Figure S2). For example, GCN(1:1) suggested the surface area of 297.1 m2 g-1 (about twice over rGO and fourfold to g-C3N4) and a


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pore volume of 0.874 cm3 g-1. Its pore size distribution calculated by Barrett–Joyner–Halenda (BJH) method is centred at 19.1 nm. The larger surface areas and pore volume, as well as appropriate pore size endowed this GCN composite with more monitoring sites and mass transfer passway for metal ion-containing solution. Sensor Construction and Characterization. The assembly process was characterized by cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS) showing in Fig. S3. In the Nyquist plots, the high-frequency responses relate to the electron transfer-limited process and the linear portion associates with the diffusion process at low frequencies. So both the redox peak current and fitting value of charge transfer resistance (Rct) in the [Fe(CN)6]3-/4electrolyte could reflect the change on the surface of GCE, further confirming the successful construction of aptasensor. The redox peak current was 2.50 mA on bare GCE and there was a small semicircle domain with the Rct of 20.0 Ω, indicating a very fast charge-transfer process of [Fe(CN)6]3-/4- on the surface of bare GCE. After assembly of GCN(1:1), the redox peak current declined to 0.67 mA and Rct increased to 267.3 Ω, which was resulted from the partly repelling [Fe(CN)6]3-/4- by the electronegative GCN(1:1) and its obstructing charge transfer in and on the surface of electrode. A further decline in redox peak current (0.18 mA) and rise in Rct (513.2 Ω) was caused by the subsequent immobilization of APT2 on GCN(1:1). This fabricated aptasensor was then used to conduct DPASV measurements to investigate its electrochemical sensing for Cd2+. Analytical Performance for Detection of Cd2+. As shown in Fig. 2, the stripping peak responses APT2-GCN(1:1) showed two different relations with the concentrations of target metal. The sectionalized fitting discipline was concerned with the different reaction mechanisms. The combination of cadmium ions and aptamers leads to a change in conformation of the


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aptamers, resulting in the electrode surface capacitance changeable, and the transformation in capacitance caused the slope of the curve to alter. In the low concentration range (1 - 1000 nM), the stripping peak areas exhibited the logarithmic relationship versus Cd2+ concentrations due to the chemical coordination of Cd2+ with aptamers. This logarithmic relationship between current response and the concentration of detected heavy metal ions was vastly revealed in the previously reported aptamer/DNA involved current-response type aptasensor.13-15 When the reaction of cadmium ions and aptamers was saturated, the stripping peak areas satisfied the linear relationship versus Cd2+ concentrations in high concentration range (1 - 1000 µM). The reason was that abundant Cd2+ could bind directly with the functional groups g-C3N4 or GO, as well as intercalate into the holes or interlayers of the g-C3N4.16-18 The correlation equations for 1 - 1000 nM and 1 - 1000 µM were defined as A (V·µA) = 0.083 lgc(nM) + 0.289 (R2 = 0.996, Fig. 2B) and A (V·µA) = 9.017 c(mM) + 0.862 (R2=0.999, Fig. 2D) respectively. The detection limits estimated by the former equation and standard deviation of the baseline was 0.337 nM. Compared with other reported Cd2+ electrochemical sensing materials (i.e. Fe3O4/rGO nanoparticles,19 polycyclodextrin,20 bismuth film,21 cupferron&ß-naphthol/MWCNTs,22 Table S5), this aptamers-assistant sensors showed excellent sensitivity, larger range, and lower detection limit. As mentioned before, the introduction of rGO always played an important role in electrochemical measurement, as it could greatly improve the electroconductivity of electrode materials. The similar monitoring on the preparation of aptasensor electrode in the absence of rGO pointed out, both g-C3N4 and APT2-g-C3N4 had significantly larger Rct compared to their rGO-existing analogues (Fig. S4B), indicating the worse electron transfer. The negative influence in electrochemistry by poor conductivity was reflected at the DPASV responses to


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Cd2+ (Fig. S4C), in which APT2-g-C3N4 provided the much weaker sensitivity, smaller detection range and higher limit (Table S2). However, the proportion of rGO in the substrated composite should be controlled. In spite that the excellent electroconductivity brought by rGO was the necessity and promotion for the involved electrochemical reactions of metal ions in the DPASV conditions, g-C3N4 was responsible for the deposition and stripping of cadmium species. The abundant N element relevant binding sites and mesopores in g-C3N4 allowed the metal ions of small size to coordinate and intercalate into the holes. We investigated the detecting performance towards Cd2+ of the four APT2-GCN samples with different weight ratios of rGO/g-C3N4. Although Rct of GCNs got increasingly smaller, along with the augment of rGO component in composites (Fig. S5), the DPASV response didn’t present a continuous enlargement. Among these APT2-GCN samples, APT2-GCN(1:1) produced the most distinguished response in a large range (100 µM to 1 mM, Fig. 3A). APT2-GCN(1:5) and APT2-GCN(1:10) have the poorer property resulting from their inferior electrical conductivity, while APT2-GCN(5:1) performed worst as little g-C3N4 provided holes to interact with Cd2+ in composite. The vital role of g-C3N4 could be further demonstrated, the sensing performance of controlled aptasensor without g-C3N4 (APT1-rGO) was revealed a dramatically attenuation (Fig. S6, Table S2). Besides that the content of rGO and g-C3N4, aptamers was another pivotal issue in this highperformance biosensor. The aptamers employed in this work by selection in vitro were bound to specifically target Cd2+ and cannot do response towards other common objects (i.e. Mg2+, Co2+, Fe2+, Zn2+ et al). Hence the combination of aptamers with GCN was immensely benefited to the efficient and specific capture and collection of Cd2+. As expected, the GCN(1:1) sensor exhibited smaller responses than APT2-GCN(1:1) toward both 10 nM and 10 µM Cd2+ (Fig. S7). The detection range of aptamers-free GCN(1:1) biosensor diminished with the minimum of 10 nM


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and maximum of 400 µM, accompanied by the recession in sensitivities (Fig. S8, Table S2). Similar to rGO, excessive introduction of aptamers conversely damaged the sensing property. The immobilization of aptamers in this establishment of aptasensor was implemented by chemical bonding of its terminated carboxyls with the N-containing groups in g-C3N4 under the inducing of EDC-HCl catalyst. Nevertheless, these N-referred groups were also absorbing and intercalating sites towards Cd2+. The over-consumption of active sites, plugging the holes, and luxuriant physisorptions of aptamers on GCN under the excessively high concentrations during modifying aptamers into GCN was proved to weaken the DPASV responses. As shown in Fig. 3B, all of DPASV responses to 100 nM, 1 µM, and 10 µM Cd2+ increased along with the aggrandizement of adopted aptamers concentrations from 10 pM to 1 nM, whereas declined as aptamers concentrations continued to enlarge to 1000 nM. It is worth mentioning that the grafting of aptamers in aptasensor construction was delicately designed to combine with g-C3N4, as verified by element distribution mapping (Fig. 1C). This designated incorporation was key point to obtain the optimal sensing performance, and then the catalyst-induced bonding was the assurance of this specific uniting. Although a certain degree of non-specific adsorption might happen to aptamers, the resulting feeble adhesion cannot guarantee the stable cooperation during the detection. As revealed in Fig. S9 and Table S2, APT2/GCN(1:1) mixture didn’t behave as good as APT2-GCN(1:1). We also established another controlled aptasensor with the specific conjunction of amino-terminated aptamers and rGO also by EDC-HCl catalyst. This APT1-GCN(1:1) aptasensor gave better sensitivity to Cd2+ in low concentrations range, due to the exclusive interaction between metal ions and selected sequences in aptamers. However, as shown in Fig. S10 and Table S2, the sensitivity in high Cd2+


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concentrations range (5.850 V·µA mM-1), detection range (10 nM - 700 µM) and detection limitation (2.048 nM) couldn’t compete with APT2-GCN(1:1). Interferences, Repeatability and Stability of Sensor. Anti-interference ability was also an important performance index for heavy ion sensor. APT2-GCN(1:1) aptasensor showed highly selectivity for Cd2+ detection, as less than 1% change occurred when interfering ions (100 µΜ) including Mn2+, Fe2+, Hg2+, Zn2+, K+, Mg2+, Ca2+, Co2+ were added into 100 µΜ Cd2+ solutions (Table S3). The interferences did not remarkably affect the detection of Cd2+ because of not only the difference of stripping potentials but also the introduction of aptamers. Firstly, the different metals could be distinguished according to their respective stripping peak at different stripping potentials. However, the multitudinous cations in the homogeneous electrolyte may have a competitive adsorption on the surface of electrode. The introduction of aptamers had better affinity for cadmium ions, thus it was advantageous for the enrichment of cadmium ions on the electrode surface, decreasing the interference from other cations. In addition, the aptasensor also exhibited operation repeatability and stability. When exposed to air for one week, its daily testing responses to 100 nM and 10 µM Cd2+ gave relative standard deviations (RSD) of were 2.82 % and 9.63 %, and maintained the retention of over 80 % (Fig. S11). Determination of Cd2+ in Real Sample. Three kinds of real samples including tap water, lake water and industrial waste from paper mill were tested to evaluate the applicability of this aptasensor towards the heavy metal detection in real samples. The APT2-GCN aptasensor showed the high recoveries and small RSDs to the common pollution concentration range (Table S4) in the three series of real samples. CONCLUSIONS


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In summary, a delicate aptasensor was established by specific combining carboxyl-terminated aptamers with appropriately regulated rGO/g-C3N4 nanocomposite. The exclusive interaction with target metal ions originated from the selected sequences in aptamers and its cooperation with g-C3N4 expedited the aptasensor’s capture and collection of Cd2+, bringing in improved high sensitivity and selectivity, as well as large detection range and low limitation. This strategy would offer potential applications for monitoring of heavy metals and other pollutants in environment. ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website at DOI: Details of experimental, more TEM images of GCNs, fitting lined of DPASV responses of controlled sensors, the list of analytical performances of different sensors, results of interferences, repeatability and stability, real samples measurements. Corresponding Author *Email: [email protected], [email protected]. ACKNOWLEDGEMENT The authors gratefully acknowledge the support for this work from National Key R&D Program of China (No.2017YFC0805900), Fundamental Research Funds for the Central Universities (No.S13JB00200, S14RC00050, S15JB00310), the National Natural Science Foundation of China (No. 21503013, 31600771, 21373255).


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Figures and Figure Captions

Figure 1. (A) SEM and (B) TEM images of APT2-GCN(1:1), (C) element distribution mapping of C, N and P corresponding to (B); (D) XRD patterns and (E) IR spectra of rGO, GO, g-C3N4, GCN(1:1)

Figure 2. DPASV curves towards Cd2+ on APT2-GCN(1:1) in the 0.1 M HAc-NaAc buffer solution in the concentration range of (A) 1-1000 nM and (C) 1-1000 mM, and their corresponding fitting lines (B: 1-1000 nM; D: 1-1000 mM).


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Figure 3. (A) DPASV response of APT2-GCN(5:1)/GCE (black); APT2-GCN(1:1)/GCE (red); APT2-GCN(1:5)/GCE (blue) and APT2-GCN(1:10)/GCE (purple), Cd2+ concentration were 100 µM, 400 µM ,700 µM, 1000 µM. (B) The responses obtained on APT2-GCN(1:1)/GCE when activated in aptamer2 solution of different concentrations. The detected Cd2+ concentration were 100 nM, 1µM, 10 µM.

Scheme 1. Schematic illustration of the fabrication of APT-GCN aptasensor and its detection of cadmium cation.

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The importance of the aptamer in the sensor to detect Cd2+ and the significant enhancement of signal after adding aptamers


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Graphite Carbon Nitride

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