1. IntroductionThough glucose biosensor has been already commercialized for decades, the basic research is still active [1-5]. Especially, amperometric biosensor based on glucose oxidase (GOD) is extremely popular due to its simplicity, reliability, low cost and very stable activity [6-9]. In this type, glucose level is indirectly monitored by measurement of current associated with the electrochemical detection of hydrogen peroxide produced during the enzymatic reaction in which glucose is oxidized by oxygen with the aid of GOD. The side product, hydrogen peroxide, is usually oxidized at conventional solid state electrodes, and unfortunately the kinetics is very sluggish, therefore, high over potentials are often required in order to get a sufficient sensitivities. This may give rise to serious interference from some electroactive species (i.e. ascorbate, urate, acetaminophen, etc.) which are usually present in real samples. To this end, various approaches have been proposed to circumvent or alleviate this issue, for example, coating the enzymes with a preoxidizing layer or a permselective membrane (e.g. Nafion) [10], or incorporation of redox mediators (e.g. porphyrin [11], phthalocyanine [12], Prussian blue [13], and peroxidase [14]) to diminish the over potential for the detection of hydrogen peroxide. Additionally, the oxidation of hydrogen peroxide can be promoted by some precious metal nanoparticles thanks to their large active surface area and favorable lattice planes compare to the bulks, such as Pt and Au [15,16]. Likewise, Pd nanoparticles are also frequently included into sensory configurations for this purpose and practically, they are generally dispersed with a conducting support to avoid their agglomerations and keep their activities. On this aspect, various carbon materials have been employed due to their good electrical conductivities, biocompatibilities and rich in functional groups used for anchoring Pd nanoparticles, for example, carbon nanotubes [17], graphite [18] and mesoporous carbon [19]. In this article, we exploit a new carbon material, reduced graphene oxide (RGO), as the support for Pd nanoparticles for this purpose.
Reduce graphene oxide (RGO), a flat monolayer of graphite packed into a two-dimensional (2D) honeycomb lattice, has become one of the most novel and exciting stars in material science. This inexpensive material has a unique ability to promote fast electron transfer kinetics for a wide range of electroactive species when it is employed as electrode substrate [20-24]. For example, Kang et al. [25] reported that the RGO electrode rich in hydroxyl and carboxylic groups showed promoted electrocatalytic activity towards the oxidation of paracetamol. Kim et al. [26] developed a sensitive sensor for the selective determination of dopamine without the interference of ascorbic acid based on a RGO modified electrode. Shan et al. [27] have used graphene as the electrode material for low-potential NADH detection and biosensing for ethanol. The RGO used in most of the works is generally obtained by the chemical reduction of graphene oxide in solution, and because of the van der Waals and π-π stacking interactions, the RGO sheets are especially easy to form irreversible agglomerates and even restack to form graphite, which may limit their applications. Herein, this issue is gotten over by incorporation of Pd nanoparticles between the RGO sheets and the agglomeration of Pd nanoparticles is circumvented at the same time by supported on RGO in that the Pd nanoparticles and RGO are achieved through a one-step reduction of PdCl2 and graphite oxide (GO) with hydrazine. During the reduction process, Pd nanoparticles are grown on the surface of RGO sheets and prevent their restacking. The obtained nanocomposite exhibits particularly electrocatalytic activities towards the oxidation of hydrogen peroxide. This feature encouraged us to develop a new amperometric glucose biosensor by combination with glucose oxidase.
2. Experimental2.1. ReagentsGlucose oxidase (GOD, 155.6 U/mg, from Aspergillusniger), palladium(II) chloride (PdCl2), graphite powder, ascorbic acid (AA), uric acid (UA) and acetaminophen (AP) were purchased from Sigma-Aldrich Co. (St. Louis, USA). β-D-Glucose was obtained from Beijing Chemical Reagent Company. Glucose stock solutions were stored at 4˚C and allowed to mutarotate for at least 24 h prior to use. All other chemicals were analytical grade and used as received. Doubly distilled water was used for the preparation of aqueous solutions throughout.
2.2. Preparation of Graphite OxideGraphite oxide (GO) was prepared according to a modified Hummer’s method [28]. Briefly, 0.3 g graphite was added into a mixture of 2.4 mL H2SO4 (98%), 0.5 g K2S2O8, and P2O5, and then the mixture was continuously stirred at 80˚C for 4.5 h. The resulted preoxidized product was filtrated and rinsed with copious distilled water, and dried at room temperature overnight. The preoxidized product was added to 12 mL H2SO4 (98%) in an ice-bath. And followed with addition of 1.5 g of KMnO4 gradually while maintaining vigorous agitation. The temperature of the mixture was maintained below 20˚C. The ice-bath was then removed and the temperature of the suspension was leveled up to 35˚C where it was maintained for 2 h. Then 25 mL H2O was added. After 0.5 h, additional 70 mL H2O, which was heated up early, was added to the solution, and 2 mL 30% H2O2 was slowly injected into the solution, then a bright yellow solution was obtained. After that, the mixture was filtrated and washed with water several times to remove the remaining metal ions and acid, then dried at 30˚C under vacuum overnight.
2.3. Synthesis of Pd/RGO NanocompositePd/RGO was prepared by the following procedures. In brief, 32.0 mg GO, 0.15 mL of 1.13 mM PdCl2, and 170 mL H2O were added into a flask, and vigorously stirred at room temperature for 0.5 h. Then the pH was adjusted to 13 by sodium hydroxide solution. Finally 2 mL 85% hydrazine solution was added into the flask. The solution was refluxed at 90˚C for 3 h, and additionally stirred at room temperature overnight. The resulted solution was filtrated and washed with copious distilled water, and then the filtrate was dried at room temperature overnight. RGO was identically prepared only except for the absence of PdCl2 in the beginning.
The particle sizes and morphologies of the Pd/RGO nanocomposite were characterized by a JEOL 2010 transmission electron microscope (TEM) at 200 kV. The X-ray diffraction (XRD) pattern was obtained with a Philips powder diffraction system (model PW 1830) using a Cu Kα source operating at 40 keV at a scan rate of 0.025∙s–1.
2.4. Preparation of GOD/Pd/RGO/GCEA glassy carbon electrode (GCE, dia: 3 mm) was polished with 0.05 mm alumina slurry on a piece of polishing cloth and then ultrasonically cleaned in ethanol and distilled water, successively. Pd/RG/GCE was made by casting 5 µL of the Pd/RGO suspension (1 mg/mL, dispersed in DMF with ultrasonic agitation) on the surface of the well-polished GCE. The solvent was allowed to dry under an infrared lamp for 0.5 h. Then the Pd/RGO/GCE was gradually cooled to room temperature. The RGO/ GCE was prepared in the same way.
GOD/Pd/RGO/GCE was prepared according to the following procedures. 0.5 mg GOD and 2.5 mg BSA were dissolved in 50 µL of deoxygenated phosphate buffer solution (0.05 M, pH 6.85), then 10 µL of glutaraldehyde (5%) was added and evenly shaken as soon as possible. After that 3 µL of the mixture was casted onto the surface of the Pd/RGO/GCE. Then the solvent was allowed to dry at room temperature. The GOD/RGO/GCE was prepared in the same way. Both the bioelectrodes were stored in phosphate buffer solution (0.05 M, pH 6.85) at 4˚C in a refrigerator when not in use.
2.5. ApparatusElectrochemical measurements were performed with a CHI660b electrochemical workstation (Chenhua Instruments Co., Shanghai, China). Three-electrode system with a platinum wire counter and an Ag/AgCl (3 M NaCl) reference electrode was employed in this study. The working electrode was one of the modified GCE described above. The counter, reference and working electrodes were inserted into a 10 mL beaker through holes in its Teflon cover. A magnetic Teflon stirrer provided the convective transport during the amperometric measurements. All the experiments were performed at room temperature (20˚C ± 1˚C).
3. Results and Discussion3.1. Characterization of the Pd/RGO NanocompositeThe X-ray diffraction (XRD) patterns of RGO (a) and Pd/RGO (b) are given in Figure 1. The diffraction peak at 23.50˚ and 43.00˚ are indexed to the (002) and (100) planes of RGO, while the diffraction peaks located at 2θ values of 38.59˚, 44.86˚, 66.85˚ and 79.67˚ are attributed to the (111), (200), (220) and (311) planes of Pd, respectively. These peaks indicate that Pd supported on RGO is present in the face-centered cubic (fcc) crystal lattice structure. We estimate the average particle size of the Pd nanoparticles according to the Scherrer equation [29]:
where L is the average particle size, λkαl is the X-ray wave length (0.154056 nm for Cu λkαl radiation), B2θ is the peak broadening, and θB is the angle corresponding to the peak maximum. The value of the volume averaged particle is about 4.09 nm calculated from the full width half maximum of the (220) peak.
Figure 2 shows the transmission electron microscopy (TEM) images of the RGO (A) and Pd/RGO (B). It can be clearly seen that the thickness of flakes is about 3 - 4 nm, which corresponds to about 10 reduced graphene
oxide layers, suggesting the electrochemical behavior of RGO prepared here is similar to that of single layer graphene, as demonstrated by Pumera et al. [30,31]. Yet, they are generally not especially flat, and their edges often exhibit folding or scrolling shapes. From Figure 2 (B), we can discern that Pd nanoparticles are clearly decorated in the RGO nanosheets. The particles display relatively uniform spherical shape with a nominal diameter of 3.5 - 4.5 nm, which is conformed to the datum obtained by the XRD analysis.
3.2. Electrochemical Performance of Pd/RGO/GCE towards Hydrogen PeroxideFigure 3 depicts the cyclic voltammograms for the oxidation of hydrogen peroxide recorded at Pd/RGO/GCE (A) and RGO/GCE (B) in phosphate buffer solution (0.05 M, pH 6.85) at a scan rate of 50 mV/s. As can be seen, the Pd/RGO/GCE shows much more favorable electrochemical activities towards the catalysis of H2O2 than does the RGO/GCE in terms of the onset potential and current response, suggesting the promotion of Pd nanoparticles towards the catalytic oxidation of H2O2.
Because of the significant electrocatalytic properties of Pd/RGO/GCE, the sensitive detection of hydrogen peroxide is reliable by amperometric method under low over potentials. Then the effect of applied potential on the
current response of the Pd/RGO/GCE (A) and RGO/GCE (B) to 1 mM H2O2 had been investigated in detail. The effect of applied potential on the response current was shown in Figure 4. In the case of Pd/RGO/GCE, the current response increases rapidly when the applied potential changes from 0.3 to 0.5 V but levels off when higher than 0.5 V, while the current response is gradually increased for RGO/GCE. Therefore, the operation potential of 0.5 V was selected for oxidation of H2O2 in the following experiments.
Figure 5 displays the typical amperometric responses (A) and the corresponding calibration plots (B) recorded at the RGO/GCE (a) and Pd/RGO/GCE (b) by increasing levels of 0.5 mM steps under the operation potential of 0.5 V. Apparently, As directly perceived through current staircases, the response of RGO/GCE was almost negligible while the response of Pd/RGO/GCE was dramatically increased after each addition of H2O2 indicating the effective detection of H2O2 catalyzed by Pd nanoparticles. To elaborate this point more in detail, we get the analytical data by resolving the calibration curves. The sensitivities are 3.78 and 0.14 µA/mM for the Pd/RGO/GCE and RGO/GCE, respectively. Correspondingly, the detection limit is lowered from 2.31 µM at the RGO/GCE to 1.00 µM at the Pd/RGO/GCE. The linear ranges are up to around 4.00 and 3.00 mM for the Pd/RGO/GCE and RGO/GCE, respectively.