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Glucose sensor based on porous Ni by using a graphene bottom layer combined with a Ni middle layer
Carbon 149 (2019) 609e617
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Glucose sensor based on porous Ni by using a graphene bottom layer combined with a Ni middle layer Zhaodi Ren*, Haochao Mao, Haowen Luo, Yuanan Liu Beijing Key Laboratory of Work Safety Intelligent Monitoring, School of Electronic Engineering, Beijing University of Posts and Telecommunications, Beijing, 100876, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 December 2018 Received in revised form 7 April 2019 Accepted 19 April 2019 Available online 26 April 2019
Porous nickel (pNi) was prepared by a hydrogen bubble template method by using a graphene bottom layer combined with a Ni middle layer on Cu foil substrate. The crystal structure and the pore size of the pNi were characterized by XRD and SEM, respectively. The TEM, SAED and XPS exhibited that the pNi and graphene as well as the Ni(OH)2 with high activity were formed on the surface. The glucose sensing performance in alkaline environments was estimated by cyclic voltammetry and chronoamperometry. The pNi as-prepared exhibits a high sensitivity of 6161mA/mM1cm2 within a linear range of 0.0005 e1.0 mM, which is much better than that of pNi on Cu foil. The large quantities of active site supplied by the pNi with pore size as low as 1e15 mm as well as the high electron transfer ability between electrode and active reaction center induced by the graphene play important roles in obtaining the pNi asprepared with high sensitivity and a broad linear range. It provides a new idea for the development of non-enzymatic glucose sensor especially based on porous Ni with graphene bottom layer and Ni middle layer. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Diabetes mellitus is one of the most serious chronic diseases worldwide, which causes millions of people dying every year even though children due to the complication of stroke, heart attack, and renal failure [1]. Diagnosis and prognosis of both the hypoglycemia and hyperglycemia state of blood glucose level are important for the prevention and treatment of diabetes mellitus. Glucose sensor based on glucose oxidase is widely used in the real medical environment to estimate blood glucose level due to its good selectivity and high sensitivity [2e4]. However, it is limited due to the inherent instability of glucose oxidase to temperature, oxygen, pH and humidity [5,6]. Scientists have focused on the development of non-enzymatic glucose sensors, in which the materials can response to glucose without enzyme. Among the materials that being used in non-enzymatic glucose sensors, the transition metals and their oxides are widely investigated due to their high sensitivity and low cost [7e9]. Ni-based materials have the higher sensitivity than others metals due to the high catalytic activity of NiOOH produced by Ni(OH)2 and thus received great attentions
* Corresponding author. E-mail address: [email protected] (Z. Ren). https://doi.org/10.1016/j.carbon.2019.04.073 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
[10]. Porous structures of Ni-based materials have high surface areas due to the existence of the pores and thus expected to supply more active sites [11,12]. Ni foam with pore size larger than 100 mm has been used as sensitive material in glucose sensor [13]. However, due to the large pore size and low resistivity of Ni itself, carbon has been deposited on the surface of the Ni foam to increase the electron transfer ability for using in glucose sensor [14]. The Ni foam is thus mainly used as the electrode skeleton to deposit others materials other than electrode directly. The Ni thus was covered by others materials and the activity of Ni(OH)2 therefore cannot be fully used [15e17]. Hydrogen bubble template method is an efficient way to form porous Ni material with pore size lower than 1 mm [18e20]. Different kinds of substrate such as SPEC [11], Ni foil [18] and Cu foil [21] have been used to form porous Ni by hydrogen bubble template method and the porous Ni is used as electrode directly for non-enzymatic glucose sensors. Thus, the activity of Ni(OH)2 is possible to be fully exhibited. However, the electron transfer ability of porous Ni is still weak due to the porous structure. Graphene has high carrier mobility, large specific surface area, a wide electrochemical potential window and an excellent biocompatibility and have been widely used to increase the electron transfer rates between the electrode and the active reaction center [22e26], in which graphene provide actions of anchoring,
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separating and conducting while the catalytic action is provided by the metal oxide since the sensitivity of graphene itself is much lower than that of Ni [27e29]. In this paper, porous Ni was formed on exfoliated graphene (EG) bottom layer by hydrogen bubble template method on Cu foil substrate. Different than references [30e32], in which graphene is normally used either independently, or on the surface of the active materials, or as the electrode skeleton to deposit Ni nanoparticles, the graphene in this work was used as the bottom layer for deposition of the porous Ni to form a hybrid material of Ni and graphene. Besides that of the graphene bottom layer, the Ni middle layer was also designed to be used for not only obtaining smaller pore size and a thinner pore wall under the action of the nodular interlocking effect but also the lower contact resistance since the contact resistance between Ni and graphene is lower than that of Cu foil and graphene in theory [33]. The work in this paper provides a new idea for obtaining a glucose sensor with high sensitivity and a broad linear range induced by the porous Ni electrode through using a graphene bottom layer and Ni middle layer.
2 mol/L NH4Cl solution. For comparison, the Cu/pNi, Cu/EG/pNi, and Cu/Ni/pNi were also formed with the same conditions. The thicknesses of the Ni middle layer and the pNi are about 10 mm and 15e20 mm respectively, which are measured by Dektak XT profilometer. All the samples were rinsed with deionized water before being used for measurements. 2.3. Characterization of the materials and measurements of the electrochemical glucose sensing property
Cu foil, absolute ethanol, NiCl2$6H2O, NiSO4$6H2O, HCl, boric acid, ammonium chloride (NH4Cl), NaOH, ammonium sulfate [(NH4)2 SO4], fructose and galactose are all purchased from Sinopharm Chemical Reagent Co., Ltd (China).Uric acid (UA), L-abcorbic acid (AA), Citric acid (CA), D-glucose and KCl were obtained from sigma company (USA). All reagents were directly used for experiments without further purification.
The surface morphology and the pore size of the porous Ni were characterized by FESEM (Hitachi S-4800). The graphene and the hybrid material of Ni-EG/pNi were characterized by TEM and HRTEM (JEOL JEM 2010F), in which both EDS and SAED were used to characterize the element and the crystal structure of the samples. The graphene and the as-prepared pNi were firstly immersed in the ethanol solution, dispersed with the ultrasonic vibration method, and then were dropped on Cu grid using capillary for TEM observation. The graphene obtained in (NH4)2 SO4 solution was vacuum filtrated to form graphene paper for characterization by Raman (LabRAM Aramis, 532 nm). The Raman was also performed directly on the Cu/Ni-EG layer. The crystal structure of the plating films were characterized by XRD technique (Cu Ka, Bruker D8 Advance). The surface of the Cu/Ni-EG/pNi was also analyzed by XPS (PHI Quantera SXM, ULVAC-PHI, Al target, Japan). All the electrochemical measurements of CeV, amperometric response and EIS were performed with CHI660E electrochemical workstation in 0.1 mol/L NaOH solution using the reference electrode of Ag/ AgCl and counter electrode of platinum disk. The selectivity of the sample were performed with anti-interference substances of 0.1 mM UA, AA,CA, KCl, fructose and galactose.
2.2. Synthesis of Cu/Ni-EG/porous Ni (Cu/Ni-EG/pNi) electrode
3. Results and discussion
Copper foil was used as the substrate. It was washed with ethanol and 1 mol/L HCl to clean and remove the oxide on the surface of the Cu foil. A Ni middle layer was formed on the Cu foil with a current density of 1 A/dm2, a plating time of 1 h, a stirring rate of 100 r/min at 50 C realized by water bath through a plating method to form Cu/Ni layer. The graphene is electrochemical exfoliated on the Cu/Ni layer at a voltage of 10 V for 30 min in 0.1 mol/L (NH4)2SO4 solution using graphite paper as the cathode to form Cu/Ni-EG layer. After formation of the graphene bottom layer, porous Ni was deposited on the surface of the Cu/Ni-EG layer to obtain the Cu/Ni-EG/pNi layer. The porous Ni was obtained by a hydrogen bubble template method with a current density of 2A/ cm2 and the deposition time of 40s in 0.2 mol/L NiCl2$6H2O and
3.1. Materials preparation and characterization
2. Experimental section 2.1. Reagents and materials
The preparation process of the Cu/Ni-EG/pNi by hydrogen bubble template method is shown in Fig. 1. Porous Ni is formed on the surface of the exfoliated graphene on Ni middle layer with Cu foil substrate. Fig. 2aec shows the morphology of the Cu foil, Cu/Ni and Cu/NiEG respectively while Fig. 2deg shows the morphology of the porous Ni formed on the former layers. The pore size distribution of the Cu/Ni-EG/pNi is narrower and the surface morphology shows more uniform. As shown in Fig. 2deg, the Cu/Ni-EG/pNi has the lowest pore size of 1e15 mm while the Cu/Ni/pNi and Cu/EG/pNi have pore size of 5e20 mm and 3e30 mm respectively, which is
Fig. 1. Formation procedure of Cu/Ni-EG/pNi. (A colour version of this figure can be viewed online.)
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Fig. 2. SEM image of (a) Cu foil, (b)Cu/Ni, (c) Cu/Ni-EG, (d) Cu/pNi, (e) Cu/EG/pNi, (f) Cu/Ni/pNi, and (g) Cu/Ni-EG/pNi.
lower than that of Cu/pNi with pore size of 22e30 mm. It shows that the pore size is decreased by introducing of the graphene bottom layer and the Ni middle layer. In the early stage of hydrogen bubble accumulation, the Ni middle layer with rougher surface than that of Cu foil can inhibit the aggregation of H2 and thus produce more dense bubbles, resulting in the lower pore size, as exhibited in Fig. 2aeb, d and f. Besides that, graphene distributed on the surface of the Ni middle layer can not only favor the HER reaction but also can prevent the aggregation of H2 on the metal surface and thus decrease the pore size of porous Ni as shown in Fig. 2c, e and g. The adhesion strength between the porous Ni and the Ni middle layer can also be increased by an interlocking effect, which will further favor the formation of the pores [34].
The TEM of the graphene and the pNi as-prepared is shown in Fig. 3. The morphology of the graphene is exhibited clearly in Fig. 3a and b. The electron diffraction embedded in Fig. 3b shows the hexagonal structure of the graphene. The graphene formed is mainly 5e10 layers although there is monolayer graphene and the HRTEM of the graphene is given in Fig. 3c, in which graphene with 5 layers is shown as exhibited by the edge. The TEM image of the hybrid material of Ni-EG/pNi is shown in Fig. 3d. The electron diffraction in Fig. 3e shows the existence of both graphene and Ni, exhibiting the formation of the hybrid materials of graphene and Ni by using a graphene bottom layer. The EDS shows the relative contents of Ni, C and O in the hybrid material, which is exhibited in Fig. 3f. The EDS map scanning results in the indicated area in Fig. 3d
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Fig. 3. (a) (b)TEM images of EG. The inset in (b) shows the electron diffraction result of EG. (c) HRTEM of EG. (d)TEM of Ni-EG/pNi. The electron diffraction result and EDS of Ni-EG/ pNi in(d) is shown in (e) and (f) respectively while the EDS map scanning results in the area indicated by the red rectangular in (d) show in (g)e(i). (A colour version of this figure can be viewed online.)
are further shown in Fig. 3 g-i. It further confirms the existence of Ni, oxides of Ni and graphene in the hybrid material. The Raman spectrum in Fig. 4a shows the graphene obtained from the exfoliated solution. It exhibits two prominent characteristic peaks at 1591 and 2733 cm1, in corresponding to the G and 2D bands of graphene. The intensity ratio of the 2D and G peaks shows that the graphene is mostly in the 5e10 layers and belongs to the multilayer graphene [35], which is in accordance with that of the
HRTEM results in Fig. 3. The characteristic peaks appearing at 1349 cm1 indicate that the graphene has more defects. The intensity of the D, G and 2D peaks decrease greatly in Fig. 4b compared with that of Fig. 4a, which is induced by the interaction between the graphene and the Ni after graphene was exfoliated onto the Ni middle layer. As shown in Fig. 5, the XRD patterns of the porous Ni with and without the graphene bottom layer are exhibited. The peaks located
Fig. 4. Raman spectrum of the graphene measured with sample of (a) graphene film obtained by vacuum filtration method and (b) exfoliated graphene (EG) on Ni middle layer. (A colour version of this figure can be viewed online.)
Fig. 5. XRD patterns of the porous Ni. (A colour version of this figure can be viewed online.)
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at 2q of 44.55 , 51.9 , 76.6 are attributed to the (111), (200) and (220) crystal planes of Ni (JCPDS No.04e0850). The other two weak diffraction peaks located at 2q of 50.4 and 74.2 can be assigned to (200) and (220) crystal planes of Cu (JCPDS No. 85e1326), which come from the Cu foil substrate. The peak belongs to graphene is not occurred. It is probably drowned by the strong peak intensity of Ni crystal since the thickness of the porous Ni and Ni middle layer is within 10e20 mm while the graphene is only within nm range. The peak intensity of the Ni crystal with graphene bottom layer is stronger than that without graphene as exhibited in Fig. 5. It shows that the crystallization of the porous Ni is favored by the graphene bottom layer. The XPS analysis on the surface of the porous Ni with sample of Cu/Ni-EG/pNi is shown in Fig. 6. It can be clearly seen that there are oxides of Ni on the surface of the sample as exhibited in Fig. 6a. The existence of the graphene on the surface is confirmed by the peaks located at 284.6eV, 286.9eV, 288.3eV and 289.6eV, in correspondence to the bonds of sp2 CeC, CeO, C]O and OeC]O, respectively, as exhibited in Fig. 6b. The formation of the Ni(OH)2 with high activity on the surface besides that of Ni is exhibited in Fig. 6c.
3.2. Electrochemical measurement The CV curves and the EIS spectra of the Cu/Ni-EG/pNi as prepared are shown in Fig. 7. For comparison, the curves of the Cu/pNi,
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Cu/Ni/pNi and Cu/EG/pNi are also given. It can be seen in Fig. 7a that there is a pair of redox peaks appear at 0.3 V and 0.5 V, corresponding to the redox reaction of Ni(OH)2 and NiO(OH) in occurring of NaOH environment. The anodic peak represents the conversion of Ni2þ to Ni3þ, and the higher anodic peak current means that more Ni3þ participates in the catalytic oxidation of glucose. In the presence of 1 mM glucose, the anodic peak current of samples increases compared with that of without glucose, showing the good response to the glucose. The response current of the samples with EG and Ni middle layer is higher than that of without them while it is the highest for Cu/Ni-EG/pNi. As discussed above, the graphene and Ni middle layer can decrease the pore size of the porous Ni, which results in the higher surface area and thus high response current of Cu/EG/pNi and Cu/Ni/pNi than that of Cu/pNi. The Cu/NiGr/pNi has the lowest pore size and thus is probably to exhibit the highest response current. Fig. 7b shows the CVs of Cu/Ni-EG/pNi with different glucose concentrations. It exhibits that a remarkable increase of the current with occurring of glucose and the anodic peak moves to the positive potential direction while the reduction peak almost stable with increases of the glucose concentration. The shifts of the anodic peak potential are caused by the absorption of glucose and the oxidized intermediates on the active sites [36]. It can be attributed to the diffusion controlled process at the electrode surface [37]. As the scanning rate increases, the peak current increases and the anodic
Fig. 6. XPS results of the Cu/Ni-EG/pNi. (A colour version of this figure can be viewed online.)
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Fig. 7. CVs of Cu/Ni-EG/pNi. (a) Without and with 1 mM glucose (50 mV/s), (b) (c) effects of glucose concentrations (50 mV/s) and scanning rates (1 mM glucose), (d) Plots of peak currents versus the square root of scanning rate obtained from (c). (e) Multicycle voltamograms (1 mM glucose, 50 mV/s). (f) EIS spectra measured with frequency of 0.1 Hz-1MHz. All the measurements are performed in 0.1 M NaOH at 0.55 V and the measurements of the samples of Cu/pNi, Cu/EG/pNi and Cu/Ni/pNi as-prepared are also given for comparison in (a) and (f). (A colour version of this figure can be viewed online.)
peak potential moves in the positive direction while reduction peak potential moves in the negative direction as shown in Fig. 7c. This indicates that the redox reaction occurring on the Cu/Ni-EG/pNi electrode is fast and reversible. The movement of the anodic peak indicates a kinetic limitation of the reaction occurring at the anode. It is a diffusion controlled process as the anodic peak current and the square root of the sweep velocity meets the linear relationship as shown in Fig. 7d, which is in correspondence of the results that
Fig. 7b as obtained above [38]. Fig. 7e shows the CV measured 20 times. Peak currents increase gradually with continuous scanning. It indicates the formation of the Ni(OH)2 on the surface of porous Ni. The reaction that happened is shown as follows [11]: Ni(OH)2 þ OH / NiO(OH) þ H2O þ e
(1)
NiO(OH) þ glucose / Ni(OH)2 þ glucolacton
(2)
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Fig. 8. Amperometric response of Cu/Ni-EG/pNi (a) with different potentials with a dropwise addition of 0.1 mM glucose, (b) measured at 0.55 V. The insert shows the current response of the electrodes with low glucose concentration and (c) Relationship between the amperometric response and the glucose concentrations derived from (b), (d) Selectivity of the Cu/Ni-EG/pNi measured with 1 mM glucose, 0.1 mM UA,AA,CA,fructose, KCl, galactose and 0.5 mM glucose. All the measurements are performed in 0.1 M NaOH and the measurements of the samples of Cu/pNi, Cu/EG/pNi and Cu/Ni/pNi as-prepared are also given for comparison in (b) and (c). (A colour version of this figure can be viewed online.)
To further discuss the mechanism of the samples for glucose detect, EIS spectra is shown in Fig. 7f. It can be seen that the lower electrochemical impedance is obtained by introducing of the graphene and the Ni middle layer. The lower electrochemical impedance mainly caused by the lower contact resistance, the lower pore size of pNi, the high crystalline contents of the Ni and the high electron transfer ability of graphene induced by the introducing of the graphene bottom layer and the Ni middle layer. The amperometric response of the samples is shown in Fig. 8. The current response of Cu/Ni-EG/pNi in 0.55 V is the best compared with that of the others neighboring measurement potential as shown in Fig. 8a. The amperometric response curve is thus measured at 0.55 V and it can be seen from the curve that the response time is within 2s as shown in Fig. 8b. The linear fit curve is shown in Fig. 8c. The sensitivity, linear range and detection limit have been shown in Table 1. The detection limit is calculated by the 3S/N by using the error bar. It can be seen from Fig. 8b and c as well as Table 1 that the sensitivity of Cu/Ni-EG/pNi is 6161mA/ mM1cm2 within a linear range of 0.0005e1.0 mM, which is better than that of others samples. It exhibits that the sensitivity and the linear range is improved greatly by the introducing of the graphene bottom layer combined with the Ni middle layer. Fig. 8d shows the selectivity of the Cu/Ni-EG/pNi. Besides glucose, the electrochemical response of the sample to interferences of UA, AA, CA, fructose, KCl and galactose are measured. Considering the concentration of glucose in the human body is about 30 times than
the interferential compounds, both 1 mM/L and 0.5 mM/L glucose as well as 0.1 mmol/L interferences have been used. As shown in Fig. 8d, the responses come from the glucose are much higher than that of the others interferences, showing a good selectivity of the Cu/Ni-EG/pNi electrode in using as the glucose sensor. Table 2 shows the glucose sensing properties of Cu/Ni-EG/pNi obtained in this work in comparison to that of the previously reported Ni-based non-enzymatic glucose sensors [10,11,14,28,39,40]. It can be seen that the glucose sensor obtained in this work is better than that of the former reported work as a whole. For measuring the reproducibility of Cu/Ni-EG/pNi, three samples have been prepared and tested at þ0.55 V with 0.1 mM glucose for the observation of the amperometric responses. The calculated relative standard deviation (RSD) for these samples is 3.29%, indicating the good reproducibility of the Cu/Ni-EG/pNi. The measurement for the current response of the Cu/Ni-EG/pNi is executed each day and it is lasted for 10 days to show the long-term stability of Cu/Ni-EG/pNi as shown in Fig. 9. The Cu/Ni-EG/pNi to the glucose response almost stable after 8 days and it retains more than 92% of the initial sensitivity, suggesting that the long-term stability is good. The practicability of the sensor was further investigated by using human blood serum with three different glucose concentrations. The results of our measurement in comparing with that of the results from the hospital is shown in Table 3. 100 mL of the human blood serum samples were added into 20.0 mL of 0.1 M NaOH
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Table 1 The glucose sensing properties of the samples in this paper. Sample
Sensitivity (mA/mM1cm2)
Linear range (mM)
Detection Limit (mM)
R2
Cu Cu/pNi Cu/EG/pNi Cu/Ni/pNi Cu/Ni-EG/pNi
2739 5286 5341 5577 6161
0.0005e0.75 0.0005e0.5 0.0005e0.75 0.0005e0.75 0.0005e1.0
0.21 0.40 0.97 0.49 0.46
0.9971 0.9929 0.9953 0.9961 0.9969
Table 2 Comparison of the glucose sensing property between this work and the references previous reported in Ni-based non-enzymatic glucose sensors. Sample
Potential
Sensitivity (mAmM1cm2)
Linear range (mM)
Detection Limit (mM)
Ref
PI/CNTeNi(OH)2 Porous Ni/SPCE Ni@C/Ni foam Ni(OH)2@3DPN Ni(OH)2/Ni foam Ni nanofoam Cu/Ni-EG/pNi
0.6 V SCE 0.5 V Ag/AgCl 0.54 V Ag/AgCl 0.46 V Ag/AgCl 0.6 V Ag/AgCl 0.55 V SCE 0.55 V Ag/AgCl
2071.5 2900 32794.4 2761.6 2617.4 2370 6161
0.001e0.8 0.0005e4 0.15e1.475 Up to 2.1 0.0025e1.05 0.005e0.7 0.0005e1.0
0.36 0.07 0.05 0.46 2.5 5 0.46
[10] [11] [14] [28] [39] [40] This work
mM1cm2 within a linear range of 0.0005e1.0 mM with a low detection limit of 0.46 mM and the selectivity to quantities interferences of AA, UA, CA, KCl, fructose and galactose is fine. The Cu/ Ni-EG/pNi electrodes as-prepared thus is very promising to be used as non-enzymatic glucose sensor with excellent properties. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (No. 2016RC18, 2016RCGD15). References
Fig. 9. Normalized sensitivity of the Cu/Ni-EG/pNi to glucose analysis by amperometric measurements for 10 consecutive days.
Table 3 Measurement of glucose in human blood serum samples. Sample
Commercial method in hospital (mM/L)
Measured (mM/L)
RSD (%,n ¼ 3)
1 2 3
5.0 7.8 14.8
4.7 ± 0.06 7.91 ± 0.1 14.36 ± 0.04
5.2 3.47 2.9
solution with the measurement voltage of þ0.55 V. It shows that the sensor can be used for practical medical environment based on its favorable accuracy. 4. Conclusion Porous Ni is prepared on Cu foil substrate with a graphene bottom layer combined with a Ni middle layer. The pore size of the porous Ni as-prepared is lower than that of without graphene and Ni middle layer. Besides the smaller pore Size, the high crystalline contents of the porous Ni, the low contact resistance between Ni and graphene than that of Cu and graphene, the high electron transfer ability of graphene play important roles in obtaining the glucose sensor with high property. The sensitivity is 6161mA/
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Carbon journal homepage: www.elsevier.com/locate/carbon
Glucose sensor based on porous Ni by using a graphene bottom layer combined with a Ni middle layer Zhaodi Ren*, Haochao Mao, Haowen Luo, Yuanan Liu Beijing Key Laboratory of Work Safety Intelligent Monitoring, School of Electronic Engineering, Beijing University of Posts and Telecommunications, Beijing, 100876, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 December 2018 Received in revised form 7 April 2019 Accepted 19 April 2019 Available online 26 April 2019
Porous nickel (pNi) was prepared by a hydrogen bubble template method by using a graphene bottom layer combined with a Ni middle layer on Cu foil substrate. The crystal structure and the pore size of the pNi were characterized by XRD and SEM, respectively. The TEM, SAED and XPS exhibited that the pNi and graphene as well as the Ni(OH)2 with high activity were formed on the surface. The glucose sensing performance in alkaline environments was estimated by cyclic voltammetry and chronoamperometry. The pNi as-prepared exhibits a high sensitivity of 6161mA/mM1cm2 within a linear range of 0.0005 e1.0 mM, which is much better than that of pNi on Cu foil. The large quantities of active site supplied by the pNi with pore size as low as 1e15 mm as well as the high electron transfer ability between electrode and active reaction center induced by the graphene play important roles in obtaining the pNi asprepared with high sensitivity and a broad linear range. It provides a new idea for the development of non-enzymatic glucose sensor especially based on porous Ni with graphene bottom layer and Ni middle layer. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Diabetes mellitus is one of the most serious chronic diseases worldwide, which causes millions of people dying every year even though children due to the complication of stroke, heart attack, and renal failure [1]. Diagnosis and prognosis of both the hypoglycemia and hyperglycemia state of blood glucose level are important for the prevention and treatment of diabetes mellitus. Glucose sensor based on glucose oxidase is widely used in the real medical environment to estimate blood glucose level due to its good selectivity and high sensitivity [2e4]. However, it is limited due to the inherent instability of glucose oxidase to temperature, oxygen, pH and humidity [5,6]. Scientists have focused on the development of non-enzymatic glucose sensors, in which the materials can response to glucose without enzyme. Among the materials that being used in non-enzymatic glucose sensors, the transition metals and their oxides are widely investigated due to their high sensitivity and low cost [7e9]. Ni-based materials have the higher sensitivity than others metals due to the high catalytic activity of NiOOH produced by Ni(OH)2 and thus received great attentions
* Corresponding author. E-mail address: [email protected] (Z. Ren). https://doi.org/10.1016/j.carbon.2019.04.073 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
[10]. Porous structures of Ni-based materials have high surface areas due to the existence of the pores and thus expected to supply more active sites [11,12]. Ni foam with pore size larger than 100 mm has been used as sensitive material in glucose sensor [13]. However, due to the large pore size and low resistivity of Ni itself, carbon has been deposited on the surface of the Ni foam to increase the electron transfer ability for using in glucose sensor [14]. The Ni foam is thus mainly used as the electrode skeleton to deposit others materials other than electrode directly. The Ni thus was covered by others materials and the activity of Ni(OH)2 therefore cannot be fully used [15e17]. Hydrogen bubble template method is an efficient way to form porous Ni material with pore size lower than 1 mm [18e20]. Different kinds of substrate such as SPEC [11], Ni foil [18] and Cu foil [21] have been used to form porous Ni by hydrogen bubble template method and the porous Ni is used as electrode directly for non-enzymatic glucose sensors. Thus, the activity of Ni(OH)2 is possible to be fully exhibited. However, the electron transfer ability of porous Ni is still weak due to the porous structure. Graphene has high carrier mobility, large specific surface area, a wide electrochemical potential window and an excellent biocompatibility and have been widely used to increase the electron transfer rates between the electrode and the active reaction center [22e26], in which graphene provide actions of anchoring,
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separating and conducting while the catalytic action is provided by the metal oxide since the sensitivity of graphene itself is much lower than that of Ni [27e29]. In this paper, porous Ni was formed on exfoliated graphene (EG) bottom layer by hydrogen bubble template method on Cu foil substrate. Different than references [30e32], in which graphene is normally used either independently, or on the surface of the active materials, or as the electrode skeleton to deposit Ni nanoparticles, the graphene in this work was used as the bottom layer for deposition of the porous Ni to form a hybrid material of Ni and graphene. Besides that of the graphene bottom layer, the Ni middle layer was also designed to be used for not only obtaining smaller pore size and a thinner pore wall under the action of the nodular interlocking effect but also the lower contact resistance since the contact resistance between Ni and graphene is lower than that of Cu foil and graphene in theory [33]. The work in this paper provides a new idea for obtaining a glucose sensor with high sensitivity and a broad linear range induced by the porous Ni electrode through using a graphene bottom layer and Ni middle layer.
2 mol/L NH4Cl solution. For comparison, the Cu/pNi, Cu/EG/pNi, and Cu/Ni/pNi were also formed with the same conditions. The thicknesses of the Ni middle layer and the pNi are about 10 mm and 15e20 mm respectively, which are measured by Dektak XT profilometer. All the samples were rinsed with deionized water before being used for measurements. 2.3. Characterization of the materials and measurements of the electrochemical glucose sensing property
Cu foil, absolute ethanol, NiCl2$6H2O, NiSO4$6H2O, HCl, boric acid, ammonium chloride (NH4Cl), NaOH, ammonium sulfate [(NH4)2 SO4], fructose and galactose are all purchased from Sinopharm Chemical Reagent Co., Ltd (China).Uric acid (UA), L-abcorbic acid (AA), Citric acid (CA), D-glucose and KCl were obtained from sigma company (USA). All reagents were directly used for experiments without further purification.
The surface morphology and the pore size of the porous Ni were characterized by FESEM (Hitachi S-4800). The graphene and the hybrid material of Ni-EG/pNi were characterized by TEM and HRTEM (JEOL JEM 2010F), in which both EDS and SAED were used to characterize the element and the crystal structure of the samples. The graphene and the as-prepared pNi were firstly immersed in the ethanol solution, dispersed with the ultrasonic vibration method, and then were dropped on Cu grid using capillary for TEM observation. The graphene obtained in (NH4)2 SO4 solution was vacuum filtrated to form graphene paper for characterization by Raman (LabRAM Aramis, 532 nm). The Raman was also performed directly on the Cu/Ni-EG layer. The crystal structure of the plating films were characterized by XRD technique (Cu Ka, Bruker D8 Advance). The surface of the Cu/Ni-EG/pNi was also analyzed by XPS (PHI Quantera SXM, ULVAC-PHI, Al target, Japan). All the electrochemical measurements of CeV, amperometric response and EIS were performed with CHI660E electrochemical workstation in 0.1 mol/L NaOH solution using the reference electrode of Ag/ AgCl and counter electrode of platinum disk. The selectivity of the sample were performed with anti-interference substances of 0.1 mM UA, AA,CA, KCl, fructose and galactose.
2.2. Synthesis of Cu/Ni-EG/porous Ni (Cu/Ni-EG/pNi) electrode
3. Results and discussion
Copper foil was used as the substrate. It was washed with ethanol and 1 mol/L HCl to clean and remove the oxide on the surface of the Cu foil. A Ni middle layer was formed on the Cu foil with a current density of 1 A/dm2, a plating time of 1 h, a stirring rate of 100 r/min at 50 C realized by water bath through a plating method to form Cu/Ni layer. The graphene is electrochemical exfoliated on the Cu/Ni layer at a voltage of 10 V for 30 min in 0.1 mol/L (NH4)2SO4 solution using graphite paper as the cathode to form Cu/Ni-EG layer. After formation of the graphene bottom layer, porous Ni was deposited on the surface of the Cu/Ni-EG layer to obtain the Cu/Ni-EG/pNi layer. The porous Ni was obtained by a hydrogen bubble template method with a current density of 2A/ cm2 and the deposition time of 40s in 0.2 mol/L NiCl2$6H2O and
3.1. Materials preparation and characterization
2. Experimental section 2.1. Reagents and materials
The preparation process of the Cu/Ni-EG/pNi by hydrogen bubble template method is shown in Fig. 1. Porous Ni is formed on the surface of the exfoliated graphene on Ni middle layer with Cu foil substrate. Fig. 2aec shows the morphology of the Cu foil, Cu/Ni and Cu/NiEG respectively while Fig. 2deg shows the morphology of the porous Ni formed on the former layers. The pore size distribution of the Cu/Ni-EG/pNi is narrower and the surface morphology shows more uniform. As shown in Fig. 2deg, the Cu/Ni-EG/pNi has the lowest pore size of 1e15 mm while the Cu/Ni/pNi and Cu/EG/pNi have pore size of 5e20 mm and 3e30 mm respectively, which is
Fig. 1. Formation procedure of Cu/Ni-EG/pNi. (A colour version of this figure can be viewed online.)
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Fig. 2. SEM image of (a) Cu foil, (b)Cu/Ni, (c) Cu/Ni-EG, (d) Cu/pNi, (e) Cu/EG/pNi, (f) Cu/Ni/pNi, and (g) Cu/Ni-EG/pNi.
lower than that of Cu/pNi with pore size of 22e30 mm. It shows that the pore size is decreased by introducing of the graphene bottom layer and the Ni middle layer. In the early stage of hydrogen bubble accumulation, the Ni middle layer with rougher surface than that of Cu foil can inhibit the aggregation of H2 and thus produce more dense bubbles, resulting in the lower pore size, as exhibited in Fig. 2aeb, d and f. Besides that, graphene distributed on the surface of the Ni middle layer can not only favor the HER reaction but also can prevent the aggregation of H2 on the metal surface and thus decrease the pore size of porous Ni as shown in Fig. 2c, e and g. The adhesion strength between the porous Ni and the Ni middle layer can also be increased by an interlocking effect, which will further favor the formation of the pores [34].
The TEM of the graphene and the pNi as-prepared is shown in Fig. 3. The morphology of the graphene is exhibited clearly in Fig. 3a and b. The electron diffraction embedded in Fig. 3b shows the hexagonal structure of the graphene. The graphene formed is mainly 5e10 layers although there is monolayer graphene and the HRTEM of the graphene is given in Fig. 3c, in which graphene with 5 layers is shown as exhibited by the edge. The TEM image of the hybrid material of Ni-EG/pNi is shown in Fig. 3d. The electron diffraction in Fig. 3e shows the existence of both graphene and Ni, exhibiting the formation of the hybrid materials of graphene and Ni by using a graphene bottom layer. The EDS shows the relative contents of Ni, C and O in the hybrid material, which is exhibited in Fig. 3f. The EDS map scanning results in the indicated area in Fig. 3d
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Fig. 3. (a) (b)TEM images of EG. The inset in (b) shows the electron diffraction result of EG. (c) HRTEM of EG. (d)TEM of Ni-EG/pNi. The electron diffraction result and EDS of Ni-EG/ pNi in(d) is shown in (e) and (f) respectively while the EDS map scanning results in the area indicated by the red rectangular in (d) show in (g)e(i). (A colour version of this figure can be viewed online.)
are further shown in Fig. 3 g-i. It further confirms the existence of Ni, oxides of Ni and graphene in the hybrid material. The Raman spectrum in Fig. 4a shows the graphene obtained from the exfoliated solution. It exhibits two prominent characteristic peaks at 1591 and 2733 cm1, in corresponding to the G and 2D bands of graphene. The intensity ratio of the 2D and G peaks shows that the graphene is mostly in the 5e10 layers and belongs to the multilayer graphene [35], which is in accordance with that of the
HRTEM results in Fig. 3. The characteristic peaks appearing at 1349 cm1 indicate that the graphene has more defects. The intensity of the D, G and 2D peaks decrease greatly in Fig. 4b compared with that of Fig. 4a, which is induced by the interaction between the graphene and the Ni after graphene was exfoliated onto the Ni middle layer. As shown in Fig. 5, the XRD patterns of the porous Ni with and without the graphene bottom layer are exhibited. The peaks located
Fig. 4. Raman spectrum of the graphene measured with sample of (a) graphene film obtained by vacuum filtration method and (b) exfoliated graphene (EG) on Ni middle layer. (A colour version of this figure can be viewed online.)
Fig. 5. XRD patterns of the porous Ni. (A colour version of this figure can be viewed online.)
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at 2q of 44.55 , 51.9 , 76.6 are attributed to the (111), (200) and (220) crystal planes of Ni (JCPDS No.04e0850). The other two weak diffraction peaks located at 2q of 50.4 and 74.2 can be assigned to (200) and (220) crystal planes of Cu (JCPDS No. 85e1326), which come from the Cu foil substrate. The peak belongs to graphene is not occurred. It is probably drowned by the strong peak intensity of Ni crystal since the thickness of the porous Ni and Ni middle layer is within 10e20 mm while the graphene is only within nm range. The peak intensity of the Ni crystal with graphene bottom layer is stronger than that without graphene as exhibited in Fig. 5. It shows that the crystallization of the porous Ni is favored by the graphene bottom layer. The XPS analysis on the surface of the porous Ni with sample of Cu/Ni-EG/pNi is shown in Fig. 6. It can be clearly seen that there are oxides of Ni on the surface of the sample as exhibited in Fig. 6a. The existence of the graphene on the surface is confirmed by the peaks located at 284.6eV, 286.9eV, 288.3eV and 289.6eV, in correspondence to the bonds of sp2 CeC, CeO, C]O and OeC]O, respectively, as exhibited in Fig. 6b. The formation of the Ni(OH)2 with high activity on the surface besides that of Ni is exhibited in Fig. 6c.
3.2. Electrochemical measurement The CV curves and the EIS spectra of the Cu/Ni-EG/pNi as prepared are shown in Fig. 7. For comparison, the curves of the Cu/pNi,
613
Cu/Ni/pNi and Cu/EG/pNi are also given. It can be seen in Fig. 7a that there is a pair of redox peaks appear at 0.3 V and 0.5 V, corresponding to the redox reaction of Ni(OH)2 and NiO(OH) in occurring of NaOH environment. The anodic peak represents the conversion of Ni2þ to Ni3þ, and the higher anodic peak current means that more Ni3þ participates in the catalytic oxidation of glucose. In the presence of 1 mM glucose, the anodic peak current of samples increases compared with that of without glucose, showing the good response to the glucose. The response current of the samples with EG and Ni middle layer is higher than that of without them while it is the highest for Cu/Ni-EG/pNi. As discussed above, the graphene and Ni middle layer can decrease the pore size of the porous Ni, which results in the higher surface area and thus high response current of Cu/EG/pNi and Cu/Ni/pNi than that of Cu/pNi. The Cu/NiGr/pNi has the lowest pore size and thus is probably to exhibit the highest response current. Fig. 7b shows the CVs of Cu/Ni-EG/pNi with different glucose concentrations. It exhibits that a remarkable increase of the current with occurring of glucose and the anodic peak moves to the positive potential direction while the reduction peak almost stable with increases of the glucose concentration. The shifts of the anodic peak potential are caused by the absorption of glucose and the oxidized intermediates on the active sites [36]. It can be attributed to the diffusion controlled process at the electrode surface [37]. As the scanning rate increases, the peak current increases and the anodic
Fig. 6. XPS results of the Cu/Ni-EG/pNi. (A colour version of this figure can be viewed online.)
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Fig. 7. CVs of Cu/Ni-EG/pNi. (a) Without and with 1 mM glucose (50 mV/s), (b) (c) effects of glucose concentrations (50 mV/s) and scanning rates (1 mM glucose), (d) Plots of peak currents versus the square root of scanning rate obtained from (c). (e) Multicycle voltamograms (1 mM glucose, 50 mV/s). (f) EIS spectra measured with frequency of 0.1 Hz-1MHz. All the measurements are performed in 0.1 M NaOH at 0.55 V and the measurements of the samples of Cu/pNi, Cu/EG/pNi and Cu/Ni/pNi as-prepared are also given for comparison in (a) and (f). (A colour version of this figure can be viewed online.)
peak potential moves in the positive direction while reduction peak potential moves in the negative direction as shown in Fig. 7c. This indicates that the redox reaction occurring on the Cu/Ni-EG/pNi electrode is fast and reversible. The movement of the anodic peak indicates a kinetic limitation of the reaction occurring at the anode. It is a diffusion controlled process as the anodic peak current and the square root of the sweep velocity meets the linear relationship as shown in Fig. 7d, which is in correspondence of the results that
Fig. 7b as obtained above [38]. Fig. 7e shows the CV measured 20 times. Peak currents increase gradually with continuous scanning. It indicates the formation of the Ni(OH)2 on the surface of porous Ni. The reaction that happened is shown as follows [11]: Ni(OH)2 þ OH / NiO(OH) þ H2O þ e
(1)
NiO(OH) þ glucose / Ni(OH)2 þ glucolacton
(2)
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Fig. 8. Amperometric response of Cu/Ni-EG/pNi (a) with different potentials with a dropwise addition of 0.1 mM glucose, (b) measured at 0.55 V. The insert shows the current response of the electrodes with low glucose concentration and (c) Relationship between the amperometric response and the glucose concentrations derived from (b), (d) Selectivity of the Cu/Ni-EG/pNi measured with 1 mM glucose, 0.1 mM UA,AA,CA,fructose, KCl, galactose and 0.5 mM glucose. All the measurements are performed in 0.1 M NaOH and the measurements of the samples of Cu/pNi, Cu/EG/pNi and Cu/Ni/pNi as-prepared are also given for comparison in (b) and (c). (A colour version of this figure can be viewed online.)
To further discuss the mechanism of the samples for glucose detect, EIS spectra is shown in Fig. 7f. It can be seen that the lower electrochemical impedance is obtained by introducing of the graphene and the Ni middle layer. The lower electrochemical impedance mainly caused by the lower contact resistance, the lower pore size of pNi, the high crystalline contents of the Ni and the high electron transfer ability of graphene induced by the introducing of the graphene bottom layer and the Ni middle layer. The amperometric response of the samples is shown in Fig. 8. The current response of Cu/Ni-EG/pNi in 0.55 V is the best compared with that of the others neighboring measurement potential as shown in Fig. 8a. The amperometric response curve is thus measured at 0.55 V and it can be seen from the curve that the response time is within 2s as shown in Fig. 8b. The linear fit curve is shown in Fig. 8c. The sensitivity, linear range and detection limit have been shown in Table 1. The detection limit is calculated by the 3S/N by using the error bar. It can be seen from Fig. 8b and c as well as Table 1 that the sensitivity of Cu/Ni-EG/pNi is 6161mA/ mM1cm2 within a linear range of 0.0005e1.0 mM, which is better than that of others samples. It exhibits that the sensitivity and the linear range is improved greatly by the introducing of the graphene bottom layer combined with the Ni middle layer. Fig. 8d shows the selectivity of the Cu/Ni-EG/pNi. Besides glucose, the electrochemical response of the sample to interferences of UA, AA, CA, fructose, KCl and galactose are measured. Considering the concentration of glucose in the human body is about 30 times than
the interferential compounds, both 1 mM/L and 0.5 mM/L glucose as well as 0.1 mmol/L interferences have been used. As shown in Fig. 8d, the responses come from the glucose are much higher than that of the others interferences, showing a good selectivity of the Cu/Ni-EG/pNi electrode in using as the glucose sensor. Table 2 shows the glucose sensing properties of Cu/Ni-EG/pNi obtained in this work in comparison to that of the previously reported Ni-based non-enzymatic glucose sensors [10,11,14,28,39,40]. It can be seen that the glucose sensor obtained in this work is better than that of the former reported work as a whole. For measuring the reproducibility of Cu/Ni-EG/pNi, three samples have been prepared and tested at þ0.55 V with 0.1 mM glucose for the observation of the amperometric responses. The calculated relative standard deviation (RSD) for these samples is 3.29%, indicating the good reproducibility of the Cu/Ni-EG/pNi. The measurement for the current response of the Cu/Ni-EG/pNi is executed each day and it is lasted for 10 days to show the long-term stability of Cu/Ni-EG/pNi as shown in Fig. 9. The Cu/Ni-EG/pNi to the glucose response almost stable after 8 days and it retains more than 92% of the initial sensitivity, suggesting that the long-term stability is good. The practicability of the sensor was further investigated by using human blood serum with three different glucose concentrations. The results of our measurement in comparing with that of the results from the hospital is shown in Table 3. 100 mL of the human blood serum samples were added into 20.0 mL of 0.1 M NaOH
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Table 1 The glucose sensing properties of the samples in this paper. Sample
Sensitivity (mA/mM1cm2)
Linear range (mM)
Detection Limit (mM)
R2
Cu Cu/pNi Cu/EG/pNi Cu/Ni/pNi Cu/Ni-EG/pNi
2739 5286 5341 5577 6161
0.0005e0.75 0.0005e0.5 0.0005e0.75 0.0005e0.75 0.0005e1.0
0.21 0.40 0.97 0.49 0.46
0.9971 0.9929 0.9953 0.9961 0.9969
Table 2 Comparison of the glucose sensing property between this work and the references previous reported in Ni-based non-enzymatic glucose sensors. Sample
Potential
Sensitivity (mAmM1cm2)
Linear range (mM)
Detection Limit (mM)
Ref
PI/CNTeNi(OH)2 Porous Ni/SPCE Ni@C/Ni foam Ni(OH)2@3DPN Ni(OH)2/Ni foam Ni nanofoam Cu/Ni-EG/pNi
0.6 V SCE 0.5 V Ag/AgCl 0.54 V Ag/AgCl 0.46 V Ag/AgCl 0.6 V Ag/AgCl 0.55 V SCE 0.55 V Ag/AgCl
2071.5 2900 32794.4 2761.6 2617.4 2370 6161
0.001e0.8 0.0005e4 0.15e1.475 Up to 2.1 0.0025e1.05 0.005e0.7 0.0005e1.0
0.36 0.07 0.05 0.46 2.5 5 0.46
[10] [11] [14] [28] [39] [40] This work
mM1cm2 within a linear range of 0.0005e1.0 mM with a low detection limit of 0.46 mM and the selectivity to quantities interferences of AA, UA, CA, KCl, fructose and galactose is fine. The Cu/ Ni-EG/pNi electrodes as-prepared thus is very promising to be used as non-enzymatic glucose sensor with excellent properties. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (No. 2016RC18, 2016RCGD15). References
Fig. 9. Normalized sensitivity of the Cu/Ni-EG/pNi to glucose analysis by amperometric measurements for 10 consecutive days.
Table 3 Measurement of glucose in human blood serum samples. Sample
Commercial method in hospital (mM/L)
Measured (mM/L)
RSD (%,n ¼ 3)
1 2 3
5.0 7.8 14.8
4.7 ± 0.06 7.91 ± 0.1 14.36 ± 0.04
5.2 3.47 2.9
solution with the measurement voltage of þ0.55 V. It shows that the sensor can be used for practical medical environment based on its favorable accuracy. 4. Conclusion Porous Ni is prepared on Cu foil substrate with a graphene bottom layer combined with a Ni middle layer. The pore size of the porous Ni as-prepared is lower than that of without graphene and Ni middle layer. Besides the smaller pore Size, the high crystalline contents of the porous Ni, the low contact resistance between Ni and graphene than that of Cu and graphene, the high electron transfer ability of graphene play important roles in obtaining the glucose sensor with high property. The sensitivity is 6161mA/
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