In Situ Synthesis of Reduced Graphene Oxide and Gold Nanocomposites for Nanoelectronics and Biosensing
© Dong et al. 2010
Received: 26 July 2010
Accepted: 14 September 2010
Published: 6 October 2010
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© Dong et al. 2010
Received: 26 July 2010
Accepted: 14 September 2010
Published: 6 October 2010
In this study, an in situ chemical synthesis approach has been developed to prepare graphene–Au nanocomposites from chemically reduced graphene oxide (rGO) in aqueous media. UV–Vis absorption, atomic force microscopy, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy were used to demonstrate the successful attachment of Au nanoparticles to graphene sheets. Configured as field-effect transistors (FETs), the as-synthesized single-layered rGO-Au nanocomposites exhibit higher hole mobility and conductance when compared to the rGO sheets, promising its applications in nanoelectronics. Furthermore, we demonstrate that the rGO-Au FETs are able to label-freely detect DNA hybridization with high sensitivity, indicating its potentials in nanoelectronic biosensing.
Graphene, a single-layer of carbon atoms densely compacted into a two-dimensional honeycomb crystal lattice, has attracted tremendous attention in recent years, because of its unique electronic, optical, thermal, and mechanical properties [1–9]. It provides a variety of novel applications such as field-effect transistors (FETs) [10, 11], ultrasensitive sensors [12, 13], transparent electrodes , and novel nanocomposites [15, 16]. Graphene is particularly advantageous in biosensing due to its large detection area, high charge mobility, low noise, and biocompatibility .
The common fabrication methods of graphene sheets include mechanical exfoliation from graphite , chemical exfoliation (chemical oxidation of graphite and subsequent reduction of the exfoliated graphite oxide sheets) , and epitaxial growth on the surface of silicon carbide crystals or metal substrates [20, 21]. Among these methods, chemical exfoliation is particularly advantageous for low-cost, large-scale, high-yield preparation of graphene sheets. However, the electronic properties of the graphene sheets obtained by this method are not good enough for nanoelectronics. In order to enhance the electrical properties of the chemically reduced GO (rGO) sheets, many approaches have been investigated, such as changing the oxidation methods , reduction with different reduction agents or reduction conditions [23, 24], and modification with metal nanoparticles .
In this study, we report a simple method for in situ synthesis of rGO-Au nanocomposites. With the assistance of sodium dodecyl sulfate (SDS), the resultant rGO-Au nanocomposites can form stable dispersion in water. Au nanoparticles formed uniformly on the rGO surface. The as-prepared rGO-Au hybrids show significantly higher hole mobility than Au-free rGO sheets. Furthermore, we demonstrate that that field-effect transistors (FETs) based on rGO-Au nanocomposites were able to detect DNA hybridization with high sensitivity, indicating its promising potentials in printable nanoelectronics and bio-nanoelectronics .
Nature graphite flakes were obtained from NGS, Germany. Also, 98% H2SO4, 30% H2O2, potassium permanganate (KMnO4), and other reagents were of analytical grade and used as received. Distilled water (18 MΩ cm) from a Millipore system was used in all studies. The sequences of the DNA strands used in the experiments are as follows: probe DNA: 5'-AGG-TCG-CCG-CCC-SH-3', target DNA: 3'-TCC-AGC-GGC-GGG-5', single-base mismatched DNA: 3'-TCC-AGC-GGC-GTG-5'
GO was synthesised by a modified Hummers methods . Three grams of graphite was added to 12 ml H2SO4 and kept at 80°C for 4.5 h. After sonication at room temperature, the solution was filtered using 200-nm porous filter and obtained the pre-oxidized graphite powder. To exfoliate the pre-oxidized graphite powder into monolayer graphene sheets, 2 g powder and 15 g KMnO4 were added into 120 ml H2SO4 with stirring and an ice-water bath was adopted to ensure the temperature remain below 10°C. The mixture was stirred for 2 h under ice-water bath. About 250 ml distilled water and 20 ml H2O2 (30%) were added to dilute the solution at room temperature. The suspending solution was precipitated for 12 h and the upper supernatant was collected and centrifuged. Successively, the GO powders were washed with 10% HCl and distilled water three times. The obtained GO was dispersed in distilled water to get a stable brown solution.
Prior to the deposition of Au nanoparticles, the GO sheets were chemically reduced assisted by microwave irradiation (MWI). In a typical experiment of chemical reduction of GO, 3 μl of hydrazine and 0.1 g SDS were added to 20 ml of GO aqueous solution (0.5 mg/ml). After rigorous shaking, the mixture was heated with microwave irradiation (900 W, 2.40 GHz) for 30 s and obtained a stable rGO sheets dispersions. The reduced GO sheets were centrifuged and washed three times with distilled water to remove the hydrate residue, and dissolved into the SDS aqueous solution again. Au nanoparticles were deposited on rGO sheets by a chemical reduction of HAuCl4 in aqueous solution. In a typical procedure, 0.1 ml aqueous of 0.05 M HAuCl4 was added into 5 ml rGO SDS aqueous dispersion in a 10-ml bottle under rigorous stirring for 1 h. After the reduction reaction, the stable graphene–Au dispersion was obtained. The UV–Vis absorption curves of the resulting dispersions were measured using a UV–Vis spectrometer. The morphology of the rGO-Au nanocomposites was observed by transmission electron microscopy (TEM), tapping mode atomic force microscope (AFM), and scanning electron microscopy (SEM). Raman spectra were obtained with a WITec CRM200 confocal Raman microscopy system (laser wavelength 488 nm and laser spot size about 0.5 mm); the Si peak at 520 cm-1 was used as a reference for wavenumber calibration.
The rGO or rGO-Au sheets were deposited by dip-coating method onto Si substrate with 300 nm thermal oxide on top first. To fabricate bottom-gated graphene FETs, selected monolayer rGO or rGO-Au sheet was located under a microscope, and then covered by a copper grid hard mask. The gold source and drain electrodes were subsequently deposited onto the sheets by thermal evaporation. The channel length between source and drain electrodes is 20 μm, and the thickness of gold electrode is about 50 nm. The electrical measurements were performed in ambient condition using a Keithley semiconductor parameter analyzer (model 4200-SCS). The effective carrier mobility was calculated using μ = (L/WCoxVd)(ΔId/ΔVg) , where L and W are the channel length and width, Cox the gate oxide capacitance, Vd the source–drain voltage, Id the source–drain current, and Vg the gate voltage. The linear regime of the transfer characteristics was used to obtain the slope ΔId/ΔVg.
It has been reported that the single-strand DNA with –SH end group can react with Au nanoparticles and can be used to realize DNA sensing . To immobilize probe DNAs, the rGO-Au devices were immersed in 1 μM probe DNA solution for a period of 16 h. The samples were washed with PBS buffer to remove the weakly bound DNA, dried and characterized its electrical properties. Then, ten microliter target or mismatch DNA solution (200 nM) was pipetted onto the probe DNA decorated devices to hybridize 1 h, washed, dried, and characterized its electrical properties.
As shown in Figure 1d, the UV–Vis spectrum of the rGO dispersion in SDS solution (black line) possesses one absorption peak at ~271 nm, which is similar to rGO in water . Also, there is a weak absorption peak at ~306 nm due to incomplete reduction of GO. In contrast, the adsorption spectrum of rGO-Au nanocomposite dispersion (red line) peaks at ~570 nm, indicating that the Au nanoparticles firmly attach to the surface of the rGO sheets. The reaction mechanism for Au nanoparticles formed (reduced) on the rGO sheets may be attributable to the difference of chemical potential between rGO and gold. This reaction mechanism has been proposed to explain the decoration of carbon nanotubes with metal nanoparticles .
In summary, we have demonstrated a simple approach to prepare stably dispersed rGO-Au sheets in aqueous media. Compared with the rGO sheet, the rGO-Au sheet FETs exhibit much higher hole mobility. In addition, the Au nanoparticles provide readily functionalization sites for conjugating biomolecular probes on the rGO-Au nanocomposites. We further showed that rGO-Au-based FETs are able to detect DNA hybridization with sequence specificity.
We acknowledge the financial support from the NSF of China (50902071, 61076067), the 973 Program (China, 2009CB930601), the NSF of the Education Committee of Jiangsu Province (TJ207035), the Science Foundation of Nanjing University of Posts and Telecommunications (NY208058), and A-Star SERC grant (No. 072 101 0020) of Singapore.
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