Hg(II) Ion Detection Using Thermally Reduced Graphene Oxide

Apr 12, 2012 - The lowest mercury(II) ion concentration detected by the sensor is 2.5 × 10–8 M. The drain current shows rapid response within less ...
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Hg(II) Ion Detection Using Thermally Reduced Graphene Oxide Decorated with Functionalized Gold Nanoparticles Kehung Chen, Ganhua Lu, Jingbo Chang, Shun Mao, Kehan Yu, Shumao Cui, and Junhong Chen* Department of Mechanical Engineering, University of Wisconsin-Milwaukee, 3200 North Cramer Street, Milwaukee, Wisconsin 53211, United States S Supporting Information *

ABSTRACT: Fast and accurate detection of aqueous contaminants is of significant importance as these contaminants raise serious risks for human health and the environment. Mercury and its compounds are highly toxic and can cause various illnesses; however, current mercury detectors suffer from several disadvantages, such as slow response, high cost, and lack of portability. Here, we report field-effect transistor (FET) sensors based on thermally reduced graphene oxide (rGO) with thioglycolic acid (TGA) functionalized gold nanoparticles (Au NPs) (or rGO/ TGA-AuNP hybrid structures) for detecting mercury(II) ions in aqueous solutions. The lowest mercury(II) ion concentration detected by the sensor is 2.5 × 10−8 M. The drain current shows rapid response within less than 10 s after the solution containing Hg2+ ions was added to the active area of the rGO/TGA-AuNP hybrid sensors. Our work suggests that rGO/TGA-AuNP hybrid structures are promising for low-cost, portable, real-time, heavy metal ion detectors.

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biomolecules with such advantages as small device size, low energy consumption, fast response, and a user-friendly analytical platform.8−11 Although graphene has huge potential for sensor applications, there are only a few reports on graphene sensors for online chemical detection in the aqueous environment.12,13 These field-effect transistor (FET) sensors used graphenebased materials as the conducting channel for detecting chemicals in water.12,13 The working principle is that the charge carrier concentration or mobility in graphene changes upon absorption of chemicals.9,14 To achieve sensing specificity, receptors for a target analyte are generally added onto the surface of graphene via covalent or noncovalent bonding. For instance, Zhang et al. showed noncovalent surface modification of graphene for mercury II (Hg2+) detection by taking advantage of the interactions between 1-octadecanethiol and graphene;12 however, the achieved detection limit for mercury II (Hg2+) was relatively low at 5 × 10−5 M. FETs based on reduced graphene oxide (rGO) have been recently demonstrated for real-time detection of various metal ions in solutions. The rGO was functionalized with metallothionein type II protein via the precoated pyrene linker molecules which react with protein on one end and attach to rGO on the other end through the strong π−π stacking covalent bonding.13 However, this approach requires a complex fabrication procedure to

ccess to clean water is one of the grand challenges for engineering. Mercury and its compounds are among major aqueous contaminants due to their high toxicity and risk to human health.1 Even a trace amount of mercury intake can lead to acute or chronic damage to the human body.1 Moreover, mercury and its derivatives also cause detrimental effects to the ecosystem. Therefore, it is important to develop methods to efficiently and effectively detect their presence in water systems, especially at innocuous levels.1−7 Traditionally, Hg ions are detected after chromatographic separation or cold vapor generation3 by inductively coupled plasma mass spectrometry,4 ion selective electrodes,5 atomic absorption spectrometry,6 and atomic fluorescence spectroscopy.7 However, these methods share several disadvantages such as low throughput, costly instrumentation, complicated sample preparation, the need for well-trained operators, and the possibility of introducing additional contamination. Therefore, it is highly desirable to develop a low-cost, portable, and userfriendly analytical platform for in-line analysis of mercury ion concentrations. Graphene is a two-dimensional (2D) sheet of sp2-bonded carbon atoms that are arranged in a hexagonal array. Recently, graphene has attracted significant attention in scientific research because of its unique and outstanding properties (i.e., high active surface area, extremely high surface-to-volume ratio, low Johnson noise, exceptional electrical, mechanical, thermal, optical, and chemical properties). Since graphene bears high sensitivity to electrical perturbations from the surrounding environment due to its ultrasmall thickness, it has been integrated into novel sensors for detecting gas molecules and © 2012 American Chemical Society

Received: January 4, 2012 Accepted: April 12, 2012 Published: April 12, 2012 4057

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anchor the metal ion binding molecule, a biological protein, to the surface of rGO. For example, in addition to the chemical reaction for forming the linker molecule to link the specific receptor molecule to rGO, another chemical reaction is needed to anchor the specific receptor molecule (metallothionein type II protein) to the linker molecule. Moreover, low temperature (4 °C) overnight incubation is necessary for the protein binding process. Because of the complex fabrication and labeling process, the biological characteristic for every individual protein may be a potential concern for the stability and repeatability of sensor performance. Besides chemical or biological modification of graphene or rGO, a hybrid platform composed of graphene and nanoparticles (NPs) can be another option to construct sensors because various NPs produced by either physical15,16 or chemical17−19 methods can react with target analytes to enhance sensitivity. Our group has demonstrated hybrid nanomaterial sensors based on NPs distributing on the surface of 1D (carbon nanotubes) or 2D (rGO) carbon nanomaterials for detecting various gases and biomolecules.20−24 NPs can be further modified to attain better sensitivity and selectivity, making the hybrid nanostructure even more promising as a versatile sensor platform.17,18 Au NPs are widely used for sensing applications because of the well-known Au NP functionalization chemistry.17,18 For example, Chang’s group exhibited successful modification of the surface of Au NPs by several thiol ligands (mercaptopropoinic acid, mercaptosuccinic acid, and homocystine) and used those modified Au NPs as fluorescent sensors for detecting the Hg2+ ion in aqueous solutions. However, the fluorescent sensors require detection time on the order of a few minutes and more than 4 h for preparation, which is relatively slow for real time detection.17 In this work, we report the fabrication and characterization of chemical sensors based on thermally reduced graphene oxide (rGO) sheets decorated with thioglycolic acid (TGA) functionalized Au NPs (thereafter denoted as rGO/TGAAuNP hybrid structures) to detect the Hg2+ ion through an FET characteristic change upon the introduction of the Hg2+ solution. The Hg2+ ion concentration range was investigated from 2.5 × 10−8 to 1.42 × 10−5 M. Sodium, calcium, zinc, cadmium, iron, copper, and lead ions were used to verify the selectivity of the rGO/TGA-AuNP hybrid sensor. The rapid, stable, and sensitive performance indicates the promising potential of rGO/TGA-AuNP hybrid sensor for Hg2+ ion detection in aqueous solutions.

Figure 1. (a) Schematic diagram of the rGO/TGA-AuNP hybrid sensor. TGA-modified Au NPs are anchored to the rGO sheet surface and function as a specific recognition group for immobilizing Hg(II) ion. (b) The fabrication process of the rGO/TGA-AuNP hybrid sensor with the assembly of Au NPs onto the TRGO shown in the dashed frame.

The sensor fabrication process is schematically shown in Figure 1b. The sensing device consisted of a 200 nm thermally formed SiO2 on Si substrates, where SiO2 layer acted as the gate dielectric and Si as a back gate. Interdigitated electrodes with both finger-width and interfinger spacing (source-drain separation) of about 1 μm were patterned using an e-beam lithography process followed by e-beam deposition of Cr/Au and lift-off. To place GO sheets between interdigitated electrodes, one droplet of the GO suspension was pipetted onto the electrodes and dried under room temperature. Thermal reduction of GO was carried out in a tube furnace (Lindberg Blue, TF55035A-1) by heating for 1 h at 300 °C in Ar flow (1 L/min) to remove residue solvents, reduce graphene oxide, and improve the contact between the rGO sheet and electrodes. After heating, samples were quickly cooled to room temperature within ∼5 min with the assistance of a blower. After the annealing process, rGO was found to be immobilized between interdigitated fingers even after several cycles of washing and drying, which was confirmed by SEM imaging. Au NPs were then assembled onto the surface of rGO sheets by a previously reported method, which combines electrospray with an electrostatic force directed assembly (ESFDA) technique.16,27 The Au NPs assembly time was around 2 h. To exclude solution-induced interference to the device, a standard e-beam lithography process was used to encapsulate the interdigitated electrode regions with 400 nm thick 4% polymethyl methacrylate (PMMA), leaving only the sensing region (rGO coated with Au NPs) accessible for the liquid solutions. Briefly, the PMMA solution was first spin-coated

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EXPERIMENTAL SECTION Graphite oxide was synthesized by the oxidative treatment of purified natural graphite (SP-1, Bay Carbon, MI) using a modified Hummers method.25 The graphite oxide was dissolved into water and centrifuged to remove possible agglomeration material. The graphite oxide was then fully exfoliated in water due to its strong hydrophilicity originated from the existence of oxygen functional groups. Individual graphene oxide (GO) sheets can be obtained from the stable suspension with the aid of ultrasonication.26 Au NPs (5 nm colloidal gold) were purchased from BB international. TGA was purchased from Sigma Aldrich. Mercury(II), sodium(I), calcium(II), zinc, cadmium(II), iron(III), copper(II), and lead(II) ion solutions were prepared by adding chloride salts in DI water. A schematic diagram of the FET sensor with Hg2+ ions bound to TGA functionalized Au NPs is shown in Figure 1a. 4058

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onto the device. E-beam lithography was then used to pattern the PMMA layer such that the PMMA covering sensing regions (between electrodes) could be removed, resulting in encapsulated electrodes and open sensing areas. After that, the device was submerged in 10 mM TGA solution for 24 h at room temperature to functionalize Au NPs. The sensor device was then rinsed with DI water for several times to remove extra TGA. Transport and electrical measurements were performed on rGO/TGA-AuNP hybrid sensors using a Keithley 4200 semiconductor characterization system. Three-terminal FET measurements were employed for device transport characteristics only, and all other electrical tests were operated by twoterminal measurement with a floating gate. Electrical conductance of the rGO/TGA-AuNP hybrid sensor was measured by fixing the drain voltage (Vds) and simultaneously recording the drain current (Ids) when the device was exposed to different concentrations of target ion solutions. All the sensing data was repeated by 3−4 sensors, and their similar sensing responses further confirmed sensor repeatability. A Hitachi S4800 field-emission scanning electron microscope (SEM) was used to characterize the morphology of rGO sheets at a 2 kV acceleration voltage.

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RESULTS AND DISCUSSION GO is normally electronically insulating because of the abundant existence of saturated sp3 bonds, the high density of electronegative oxygen atoms bonded to carbon, and other defects. The resistance of the GO at room temperature is on the order of tens of GΩ. The thermal annealing was used to partially reduce the GO sheets, allowing rGO to work as the conducting channel for the sensor device. The conductance of the rGO was mainly decided by annealing temperature, annealing time, annealing gaseous environment, and number of rGO layers in the sensor.8,20 Better reduction of GO sheets (with higher conductance) can be typically achieved at a higher annealing temperature. After reduction, the rGO device is ready for the Au NP assembly process. Figure 2a shows the SEM image of a single rGO sheet spanning across a pair of Au interdigitated electrodes. After the Au NP assembly, Au NPs were seen uniformly distributing on the surface of the rGO sheet without agglomeration (Figure 2b). The van der Waals binding between Au NPs and rGO is strong enough to retain Au NPs in place even after several cycles of washing and drying.23 The sensing signal from the hybrid structure of rGO decorated with recognition-group-functionalized Au NPs is based on the fact that the channel conductance changes sensitively due to either the electron donating or withdrawing effect of target ions. A specific recognition group (or a probe) is anchored to the rGO surface through Au NPs and further used to immobilize target ions. Because of the work function difference between Au NPs (5.1−5.47 eV)28 and reduced graphene oxide (4.2 eV),29 electrons may transfer between the rGO and the Au NPs and thereby change the drain current. The adsorption of target ions onto probes may lead to a carrier concentration change in rGO due to the effective electronic transfer between the rGO and Au NPs. The electrical detection of target agent that binds to probes is accomplished by measuring the change in the electrical characteristics of the device. The Au NPs were functionalized with TGA by immersing a device in 10 mM TGA solution. TGA (HS-CH-COOH) has

Figure 2. SEM images of a GO sheet (a) and an rGO sheet decorated with TGA-AuNPs (b) spanning across interdigitated electrodes.

both a thiol (-C-SH) group and a carboxylic acid (-COOH) group. The thiol group in TGA strongly interacts with the surface of Au NPs, facilitating anchoring TGA on Au NPs. On the other hand, the carboxylic acid in TGA acts as a linker to immobilize the Hg2+ ion because they can react to form RCOO-(Hg2+)-OOC-R chelates.18,30 Because of the strong bonding between gold and the thiol group, a self-assembled monolayer of TGA was formed on the gold surface, which was confirmed by X-ray photoelectron spectroscopy (XPS)31 and contact angle measurement.30 Functionalization of Au with TGA in a similar manner was also reported in a fluorescence sensor18 and a high electron mobility transistor sensor.30−32 Chen et al.18 functionalized the Au NPs with TGA by mixing TGA solution with Au NP-based solution. Ren’s group also performed a similar TGA functionalization on the Au gate surface in the high electron mobility transistor.30−32 The drain current (Ids) of the rGO/TGA-AuNP hybrid sensor as a function of the drain voltage (Vds) or the gate voltage (Vg) was measured as the sensor was exposed to water and 10−5 M Hg2+ ion solution, as shown in Figure 3a,b. The drain current increase for the rGO/TGA-AuNP hybrid sensor after exposure to the Hg2+ ion solution is due to the formation of R-COO-(Hg2+)-OOC-R chelates through reactions between Hg2+ ions and the carboxylic acid groups of the TGA molecules on the Au NPs. The coupling of Hg2+ ions with carboxylic acid groups can cause changes in the charge carrier concentration in rGO sheets. To counteract the accumulation of positive charges from Hg2+ ions, electrons may transfer from the rGO to the Au NPs, increasing the hole concentration in the rGO and thereby 4059

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Figure 4. Dynamic response (sensitivity versus time) of an rGO/ TGA-AuNP hybrid sensor for Hg2+ ion concentrations ranging from 2.5× 10−8 to 1.42 × 10−5 M (Vds = 0.01 V and Vg = 0).

testing and the sensitivity kept increasing with the addition of higher concentration Hg2+ ions. Three control experiments were performed to reveal the roles played by Au NPs and TGA probes in the hybrid sensing platform. The first control experiment was conducted using a bare rGO device without any Au NPs or TGA-functionalized Au NPs. The bare rGO device was insensitive to Hg2+ ions (Supporting Information, Figure S1). In the second control experiment, we fabricated an rGO device with the assembly of Au NPs, but without the TGA functionalization process. The rGO/AuNPs hybrid device was not responsive to the Hg2+ ions either (Supporting Information, Figure S2), implying that there was no obvious improvement in sensor sensitivity after the assembly of Au NPs. A third rGO device, which was processed with TGA modification but without the assembly of Au NPs, showed no sensitivity to the Hg 2+ ions (Supporting Information, Figure S3). This could be due to the lack of strong adhesion between TGA and rGO, which could lead to the removal of TGA from the rGO surface after washing with DI water. Therefore, these three control experiments suggest that the combination Au NPs and TGA modification of Au NPs is critical for rGO-based sensors to achieve good Hg2+ sensing performance as shown in Figure 4. To demonstrate the specificity of the rGO/TGA-AuNP hybrid sensor, we inspected its sensing behavior when it was exposed to solutions containing interference species such as Na+ and Ca2+ ions. As the chelating effect of thiolate compound favors heavy metal ions such as Hg2+, the interference of Na+ and Ca2+ ions was weak. The rGO/TGA-AuNP hybrid sensor indeed gave no obvious response upon the addition of Na+ and Ca2+ ions, as clearly shown in Figure 5. To further confirm the sensor specificity, sensor responses to a variety of common heavy metal ions, including Zn2+, Cd2+, and Fe3+, were investigated and presented in Figure 6. The Zn2+ and Cd2+ resulted in a very weak response. However, the device demonstrated some sensitivity to Fe3+, which may be attributed to the high affinity of Fe3+ with carboxylic acid groups and more net positive charges of Fe3+. The lower detection limit for Fe3+ was about 5 μM, which is much higher than that of Hg2+. To investigate the sensor sensitivity to ions that are chemically similar to Hg2+, Cu2+ and Pb2+ were examined and showed similar responses with Fe3+ (Supporting Information, Figures S4 and S5). The performance of our rGO/TGA-AuNP hybrid sensor platform is very encouraging for low-concentration Hg2+ detection and could be further enhanced. The improved detection limit, better selectivity, and extended applications of

Figure 3. Ids−Vds (a) and Ids−Vgs (b) characteristics of an rGO/TGAAuNP hybrid sensor exposed to water (black) and 10−5 M Hg2+ ion (red) solutions (at 0.01 V drain voltage).

increasing the drain current. Therefore, compared with water, exposure to Hg2+ ion solution increased the conductance of the rGO/TGA-AuNP hybrid sensor. As shown in Figure 3b, the Dirac point of the rGO/TGA-AuNP hybrid sensor shifted ∼+10 V because of the immobilization of the Hg2+ ions. The charge-transfer mechanism between the adsorbed analytes and carbon nanomaterials was also observed in another report.33 The gating effect was also reported as the possible sensing mechanism for a positively charged antigen binding event23 because the accumulation of positively charged target analyte can act as a positive potential gating and further reduce the electrical conductivity of the rGO. On the basis of the transport characteristic of the rGO/TGA-AuNP hybrid sensor, the transport through the rGO sheets is mainly dominated by positive charge carriers (holes) at a floating gate (Vgs = 0 V) condition. However, the electrical conductivity of rGO increased with the increase of the Hg2+ ion concentration, showing the gating effect was negligible for our sensor platform. Further studies are required for additional understanding of the sensing mechanisms. Figure 4 shows the dynamic response of an rGO/TGAAuNP hybrid sensor with Hg2+ ion concentrations ranging from 2.5 × 10−8 to 1.42 × 10−5 M. The drain current versus time was monitored and then the sensitivity (defined as the source-drain current change ratio or the ratio of the sensor conductance in Hg2+ solution to that in DI water) was obtained for different target Hg2+ ion concentrations. There was no noticeable change observed upon the addition of DI water, implying the specificity and stability of the device. The sensor showed a rapid response when solutions with varying Hg2+ concentrations were introduced to the device surface. The sensor responded within a few seconds for the Hg2+ ions to diffuse through the liquid drop on the top of the device, in marked contrast to minutes or even hours required for conventional fluorescence sensors. The binding sites on the Au NPs were not fully occupied by Hg2+ ions within a single 4060

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(Figure S2), and an rGO device with TGA modification (Figure S3) and dynamic response of an rGO/TGA-AuNP hybrid sensor for Cu2+ (Figure S4) and Pb2+ (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (414)229-2615. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The first three authors contributed equally. This work was supported by National Science Foundation through the Industry/University Cooperative Research Center on Water Equipment & Policy located at the University of WisconsinMilwaukee and Marquette University (Grant IIP-0968887) and a fundamental research grant (Grant IIP-1128158). The authors thank H. A. Owen for technical support with the SEM and L. E. Ocola, R. Diva, and D. Rosenmann for assistance in the electrode fabrication. The SEM imaging was conducted at the UWM Electron Microscope Laboratory. The e-beam lithography was performed at the Center for Nanoscale Materials of Argonne National Laboratory, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357.

Figure 5. An rGO/TGA-AuNP hybrid sensor showed no obvious response to Na+ and Ca2+ concentrations ranging from 2.5× 10−8 to 1.42 × 10−5 M.

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Figure 6. Dynamic response of an rGO/TGA-AuNP hybrid sensor for a variety of metal ions: Zn2+, Cd2+, and Fe3+.

the hybrid sensor may be achieved by several potential approaches. First, the areal density of the Au NPs and the TGA coverage can be furthered improved because more -COOH functional groups are beneficial for immobilizing target ions. Second, various other chemical structures can be produced via strong bonding between the thiol group and the Au. Third, the -COOH groups, available for binding TGA to the Au NP surface, can be employed for further chemical bonding to other functional groups. Furthermore, different NPs and chemical modification methods can be designed to form various chemical structures and sensors for other target analytes.

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CONCLUSIONS In summary, we have demonstrated a hybrid sensing system consisting of rGO and functionalized NPs for in situ electronic detection of ions in aqueous solutions. We fabricated an Hg2+ ion sensor using rGO sheets decorated with TGA-functionalized Au NPs. The as-fabricated hybrid sensors showed good Hg2+ ion sensing performance with a detection limit of 2.5 × 10−8 M and a response time as fast as a few seconds. The use of NPs provides more possibilities in surface modification of rGO for sensor applications. The encouraging results suggest the promise of using the rGO/NPs hybrid sensing platform for detection of various chemicals and bacteria in the water environment.

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ASSOCIATED CONTENT

S Supporting Information *

Five supplemental figures: dynamic response for Hg2+ solutions of an rGO sensor (Figure S1), an rGO-AuNP hybrid sensor 4061

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