Chinese Chemical Letters  2014, Vol.25 Issue (04):496-500   PDF    
Pd-based nanoporous metals for enzyme-free electrochemical glucose sensors
Chun-Lei Yanga, Xu-Hai Zhanga, Guo Lana, Lu-Yang Chenb, Ming-Wei Chenb, Yu-Qiao Zenga, Jian-Qing Jianga    
a Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, China;
b Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
Abstract: Nanoporous metals (NPMs) show potential applications as enzyme-free glucose sensors. There are few reports on nanoporous Pd in this area even though their cost is much lower than other NPMs. In this work, we report the formation of Pd-based NPM with improved catalytic activity towards the oxidation of glucose. By dealloying metallic glasses, Pd-based NPMs with bi-continuous networks were obtained. All the Pd-based NPMs show high electrochemical catalytic activity towards glucose oxidation. In this study, NPM with an open, three-dimensional, ligament-channel nanoporous structure resulted by dealloying metallic Pd30Cu40Ni10P20, producing a pore size of 11 nm and a ligament size of 7 nm as the best configuration towards the direct oxidation reaction of glucose.
Key words: Glucose sensors     Pd-based NPM     Dealloying     Electrochemical    
1. Introduction

An accurate and reliable method for the determination of glucose is the basis for the diagnostic response of several important medical sensing devices,such as blood glucose monitors. One of the most widely used techniques is based on the utilization of an enzyme for the conversion of the target analytes into electrochemically detectable products. However,the limited life and the stringent requirements for immobilization of the enzyme impose great restrictions on the further development of glucose sensors [1, 2]. Enzyme-free electrochemical glucose sensors are thus attracting considerable interest in the field of both industrial applications and theoretical studies. Recently,nanoporous metals (NPMs) with a bi-continuous network structure,and a tunable feature dimension are proved to be very active in some catalytic reactions [3, 4, 5, 6, 7]. For instance,the NPAu exhibits high catalytic activity and selectivity towards the oxidation of glucose,making it an ideal glucose sensing material [8, 9, 10, 11]. Recently,it was found that good catalytic performance in chemical-electrical oxidation can be maintained by using NPPd as the catalyst at a relatively lower cost [12, 13]. However,there are still some issues which need to be solved before the commercialization of NPPd as a glucose sensor. Firstly,the catalytic activity needs to be improved when compared with NPAu. Secondly,similar to other NPMs,NPPd can only maintain its catalytic activity for a few minutes due to the severe particle aggregation during catalyzation,and therefore, great efforts have been devoted to improve the catalytic performance of NPPd. A palladium coated nanoporous gold film was synthesized by Tavakkoliet al.[12] exhibiting both higher catalytic activity and enhanced stability towards the oxidation of glucose. Baiet al. [13] improved the catalytic performance by forming a new porous tubular Pd structure. Chen [14] reported the homogeneously modified Pd particles on FCNTs,which presented excellent catalytic activity,high resistance towards the poisoning of chloride ions,high selectivity and long-term stability towards the oxidation of glucose. In this study,we report the formation of a novel NPPd catalyst by dealloying multicomponent metallic glasses. 2. Experimental

The Pd30Cux Ni50-xP20(x= 20,30 and 40) alloys were prepared by melting the mixture of pure Pd (purity,99.9 wt%),Ni (purity, 99.95 wt%) and Cu (purity,99.95 wt%) elements with pre-alloyed Pd-P ingots in vacuum fused silica tubes,followed by B2O3flux treatment. From the mother alloy,glassy ribbon samples with a thickness of about 0.02 mm and a width of about 2 mm were fabricated by the single-roller melt-spinning technique. The amorphous structure of the as-prepared ribbons was confirmed by X-ray diffraction. The Pd-based multicomponent NPMs were fabricated by potentiostatically dealloying the Pd30Cux Ni50-xP20 metallic glasses in a mixed solution (0.8 mol/L H2SO4and 0.2 mol/L H3PO4),followed by repeatedly washing in triply distilled water. The dealloying process was carried out by using a classical threeelectrode setup (ZF-10) with a saturated calomel reference electrode (SCE) and a Pt counter electrode. Scanning electron microscopy (SEM) with an energy dispersive X-ray spectrometer (EDS) was used to observe the surface morphologies and detect the compositions of the NPMs. The electrochemical properties of the Pd-based NPMs for glucose electro-oxidation were evaluated by cyclic voltammetry (CV) measurements in a CHI-760D electrochemical workstation at r.t.,where the NPMs were modified onto the glassy carbon electrode with a diameter of 2 mm and used as the working electrode. A Pt foil was used as a counter electrode and a SCE was used as the reference electrode. All the aqueous solutions used in the CV test were prepared with triply distilled water and deoxygenated by bubbling high purity N2for half an hour. 3. Results and discussion 3.1. Surface morphology and nanostructure of Pd-based NPMs

Since Ni and Cu additions are beneficial for the catalytic activities and stability of nanostructured Pd,the Pd30Cux Ni50-xP20 metallic glasses are chosen as the dealloying precursors to prepare the Ni-Cu-containing multicomponent NPPd in this study. The potentiodynamic polarization test was performed on the Pd30Cux Ni50-xP20glassy ribbon in 0.8 mol/L H2SO4and 0.2 mol/L H3PO4solution with the results shown in Fig. 1(a). In the anodic region,all the Pd30Cux Ni50-xP20ribbons experienced a spontaneous passivation in the range of about 0-700 mV,followed by a remarkable current density rise at about 750 mV. It should be noticed that the potential corresponding to the current ‘‘apex nasi’’ increases with the Cu content in Pd30Cux Ni50-xP20,demonstrating that the Pd30Cux Ni50-xP20with higher Ni content has lower critical dealloying potential. This result may be attributed to the difference in potential required to dissolve the Ni and Cu components in their pure form (standard electrode potential: Ni/Ni2+=-0.25 V,Cu/Cu2+= 0.34 V). According to the theoretical computation [15] and practical experiments [16],the bulk critical dealloying potential of the precursor alloys decreases with the content of the component with lower standard electrode potential. With the aim of achieving NPMs with fine nanostructure [15],the Pd30Cux Ni50-xP20metallic glasses were dealloyed at 800 mV in this research (above all the critical dealloying potentials of the Pd30Cux Ni50-xP20 metallic glasses). As shown in Fig. 1(b),the dealloying rate increases with the increase of Ni amounts in the glassy ribbons.

Fig. 1. Potentiodynamic polarization (tafel) curves (a) and dealloying curves (b) of Pd30Cux Ni50-xP20(x= 40, 30 and 20) in 0.8 mol/L H2SO4and 0.2 mol/L H3PO4mixedsolutions (vs. SCE). Scan rate: 10 mV/s.

Fig. 2 shows the SEM images of the dealloyed samples. All the samples exhibit an open,three-dimensional,ligament-channel nanoporous structure on the surface,indicating that NPMs can be obtained fromdealloying the Pd30Cux Ni50-xP20metallic glasses with different Cu contents. The NPM dealloyed from Pd30Cu20Ni30P20 glassy ribbon leads to a course nanoporous structure,as is evidenced by the pore size ranging within 10-50 nm and the agglomerated ligaments with a width of 10-50 nm. The increase of the Cu content to 30 at% in the metallic glass results in a dramatic refined structure, where homogenous nanopores and metallic ligaments in a size of about 7-8 nmcan be observed. The refined structure with increasing Cu content in the metallic glass may be attributed to the decreasing dealloying overpotential (the difference between the dealloying potential and the critical dealloying potential). The lower overpotential leads to higher activation energy of the noble element for surface diffusion,and thus,improves the fine nanostructure in the NPM [17, 18]. The NPM dealloyed from the metallic glass with the highest Cu content of 40 at% exhibits an inhomogeneous microstructure. Besides the nanopores (about 11 nm) and metallic ligaments (about 7 nm),some flat areas corresponding to incomplete dealloying can been observed. The ‘‘island’’ like metallic glass areas are about 20-50 nm in diameter.

Fig. 2. SEM images and EDS spectra of dealloyed Pd30CuxNi50-xP20(x= 20, 30 and 40). (a) (d)x= 20, (b) (e)x= 30 and (c) (f)x= 40

The chemical compositions of the NPMs measured by EDS are shown in Fig. 2 and the corresponding data are summarized in Table 1. The results show that all the NPMs are mainly composed of Pd,together with a small amount of Cu,Ni and P,indicating Pd-based NPMs are obtained. The large dealloying overpotential (in the case of Pd30Cu20Ni30P20) leads to a large amounts of active element residues,i.e.,Cu: 5 at%,Ni: 9 at% and P: 5 at%. The other two NPMs have similar compositions,i.e.,Cu: 1 at%,Ni 1-2 at% and P: 1-2 at%.

Table 1
The chemical composition contents of the NPMs.
3.2. Electrocatalytic activities of Pd-based NPMs towards glucose

The electrocatalytic activities of the resultant NPMs towards glucose electro-oxidation were characterized by CV curves in 0.1 mol/L KOH alkaline aqueous solutions with,and without, 50 mmol/L of glucose (Fig. 3). In the KOH solutions without glucose,no current signals can be detected. On the contrary,when immersed in the solutions containing 50 mmol/L glucose,the Pdbased NPMs exhibit two peaks at about -0.4 V and around 0 V during the positive scan. The peak around -0.4 V is due to the OH - adsorption and the generation of intermediates [19]. The sharp peak around 0 V corresponds to the oxidation of glucose and the intermediates [19]. The current density at around 0 V of NPMs obtained from dealloying Pd30Cux Ni50-xP20(x= 20,30 and 40) are 4.51 mA cm-2 ,8.04 mA cm-2 and 6.04 mA cm-2 ,respectively,and increases clearly with the decreasing pore and ligament sizes of the NPMs. The NPM with a pore size of 7 nm and a ligament size of 8 nm (dealloyed from Pd30Cu30Ni20P20) shows the highest oxidation current,demonstrating that the Pd-based NPM with a smaller pore size are more chemically active towards the direct oxidation reaction of glucose. Such phenomenon can also be found in NPG [20]. Besides that,according to the density functional theory (DFT) based on a serial of cluster models,when adding Ni into pure Pd nanoparticles,the energy levels of HOMO and LUMO were uplifted and the energy gaps between the two orbitals were shortened, making it easier for Pd-based catalyst to pass electrons to achieve ORR. However,when too many Ni atoms replace Pd atoms in the cluster,the active positions of Pd on the surface of the catalyst will decrease,leading to the decreasing of catalytic activity. Thus,when the content of Pd was 97 at%,the catalytic activity of NPM was high,when the content of Pd decreased to 95 at% and even to 79 at%,the current density of NPMs decreased,demonstrating the decrease of catalytic activity.

Fig. 3. Cyclic voltammogram curves of dealloyed Pd30CuxNi50-xP20(x= 20, 30 and 40) in 0.1 mol/L KOH solution with and without 50 mmol/L glucose, (a)x= 20, (b)x= 30 and (c)x= 40. Scan rate: 50 mV/s

The electrochemical stabilities of these electrocatalysts were investigated by CV tests for 1000 potential cycles performed in KOH alkaline aqueous solutions with 50 mmol/L of glucose (Fig. 4). In the case of the NPM obtained from Pd30Cu40Ni10P20,no obvious change can be found in the potential and current of the oxidation peak after 100 cycles. After 1000 cycles,the current of the oxidation peak drops by only 29%. For the NPMs dealloyed from Pd30Cu30Ni20P20and Pd30Cu20Ni30P20,the oxidation peak current deceases continuously and decreases by about 42% and 38%, respectively,after 1000 cycles. The surface morphology of the NPMs after the 1000 cycles was observed by SEM as shown in Fig. 5. It can be seen that after 1000 cycles,the size of the nanopores and ligaments remain at the same scale (11 nm) in the NPM obtained from Pd30Cu40Ni10P20. On the contrary,other NPMs prepared by dealloying Pd30Cu30Ni20P20 and Pd30Cu20Ni30P20 exhibit courser morphology (NPM from Pd30Cu30Ni20P20: pores size 34 nm,ligaments 50 nm,NPM from Pd30Cu20Ni30P20increased to 100 nm and 150 nm). The structure stability is always associated with the surface diffusion of noble atoms. In the NPM dealloyed from Pd30Cu30Ni20P20,there are some homogenously disperse, undealloyed glassy islands. The diffusion of Pd through the islands is much slower due to the much lower surface energy than that of the NPM. Moreover,the glassy islands are also chemically stable in alkaline and weak acid conditions. Thus,they can work as the pin sites for the diffusion of the Pd during the oxidation of glucose. Moreover,the simultaneous addition of Ni,Cu and P onto the NPM structure,especially the Cu element,can also improve the stability of the NPM by forming a Pd-M (M = Ni,Cu and P) solid solution and lowering the diffusion rate of Pd in NPM. Therefore,high stability is obtained in the NPM dealloyed from the Pd30Cu30Ni20P20glass.

Fig. 4. Cyclic voltammogram curves of dealloyed Pd30CuxNi50-xP20(x= 40, 30 and 20) during 1000 potential cycle tests in 0.1 mol/L KOH solution with 50 mmol/L glucose, (a) x= 40, (b)x= 30 and (c)x= 20. Scan rate: 50 mV/s.

Fig. 5. SEM images of dealloyed Pd30CuxNi50-xP20(x= 20, 30 and 40) after 1000 potential cycle tests in 0.1 mol/L KOH solution containing 50 mmol/L glucose, (a)x= 20, (b)x= 30 and (c)x= 40
4. Conclusion

Pd-based NPMs can be obtained by dealloying metallic glass Pd30Cux Ni50-xP20(x= 20,30 and 40). When the content of Cu is 40 at%,NPM with an open,three-dimensional,ligament-channel nanoporous structure,a nearly homogeneous microstructure can be obtained. Pd-based NPM with a pore size of 11 nm and a ligament size of 7 nm is chemically active and electrochemical stability and thus,is the best configuration in the oxidation reaction of glucose in this study. Acknowledgments

This work is supported by the National Science Foundation of China (Nos. 51001026,21173041),the Project-sponsored by SRF for ROCS,SEM (No. 6812000013),the Project-sponsored by Nanjing for ROCS (No. 7912000011),Opening Project of Jiangsu Key Laboratory of Advanced Metallic Materials (No. AMM201101), and the Fundamental Research Funds for the Central Universities (Nos. 3212002205,3212003102).

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