Three-Dimensional Ni2P Nanoarray: An Efficient Catalyst Electrode for

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Three-Dimensional Ni2P Nanoarray: An Efficient Catalyst Electrode for Sensitive and Selective Nonenzymatic Glucose Sensing with High Specificity Tao Chen,† Danni Liu,† Wenbo Lu,† Kunyang Wang,‡ Gu Du,‡ Abdullah M. Asiri,§ and Xuping Sun*,† †

College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China Chengdu Institute of Geology and Mineral Resources, Chengdu 610081, Sichuan, China § Chemistry Department, King Abdulaziz University, Jeddah 21589, Saudi Arabia ‡

S Supporting Information *

ABSTRACT: It is highly attractive to construct a natural enzyme-free electrode for sensitive and selective detection of glucose. In this Letter, we report that a Ni2P nanoarray on conductive carbon cloth (Ni2P NA/CC) behaves as an efficient three-dimensional catalyst electrode for glucose electrooxidation under alkaline conditions. Electrochemical measurements demonstrate that the Ni2P NA/CC, when used as a nonenzymatic glucose sensor, offers superior analytical performances with a short response time of 5 s, a wide detection range of 1 μM to 3 mM, a low detection limit of 0.18 μM (S/N = 3), a response sensitivity of 7792 μA mM−1 cm−2, and satisfactory selectivity, specificity, and reproducibility. Moreover, it can also be used for glucose detection in human blood serum, promising its application toward determination of glucose in real samples.

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(Ni2P NA/CC) as a high-performance catalyst electrode for glucose oxidation in alkaline media. Direct growth of Ni2P nanoarray ensures strong adhesion and good electronic contact between catalyst and current collector and the nanoarray configuration allows for easy diffusion of electrolytes and exposure of more active sites. As a nonenzymatic glucose sensor, Ni2P NA/CC is superior in sensing performances with a short response time of 5 s, a wide detection range of 1 μM to 3 mM, a low detection limit of 0.18 μM (S/N = 3), and a response sensitivity of 7792 μA mM−1 cm−2, with satisfactory selectivity and reproducibility. Furthermore, it also performs efficiently for glucose detection in human blood serum. Figure 1A presents the X-ray diffraction (XRD) pattern of the phosphided product scratched down from CC. As observed, it displays several diffraction peaks at 30.4°, 31.7°, 35.3°, 40.7°, 44.6°, 47.3°, 54.1° and 54.9°, which can be indexed as (110), (101), (200), (111), (201), (210), (300), and (211) planes of the Ni2P phase (JCPDS No. 65-1989), respectively. The scanning electron microscopy (SEM) images of Ni2P NA/CC indicate that the entire surface of CC is completely covered by nanosheets array, as shown in Figure 1B,C. Energy dispersive X-ray (EDX) elemental mapping analysis (Figure 1D) confirms the uniform distribution of both Ni and P elements throughout the whole nanoarray. The EDX spectrum (Figure S1) suggests a nearly 2:1 atomic ratio of Ni to P. The high-resolution

s an important necessity for the human body, glucose generally exists in blood to provide energy for normal activities. An excess amount of glucose in the blood however could lead to diabetes mellitus, one of the leading diseases causing death and disability in the world.1 Currently, diabetes mellitus affects about 29.1 million individuals in the United States and 347 million worldwide.2,3 In 1962, Clark et al. developed the first enzymatic electrochemical glucose biosensor.4 Despite high sensitivity and selectivity, such sensor suffers from high cost of enzyme, difficult enzyme immobilization process, and degradation of enzymatic activity.5 Although the high catalytic activity for glucose electrooxidation of noble metals and their metal alloys enables its use for nonenzymatic glucose sensing, the high cost of such catalysts limits the widespread applications.6−9 Therefore, it is highly attractive to develop earth-abundant nanostructures for electrocatalytic glucose sensing.10,11 Ni is an interesting transition metal with catalytic power toward glucose electrooxidation and its oxides and hydroxides have received intensive research attention in recent years.12−21 Transition metal phosphides (TMPs) are interstitial alloys with superior electrical conductivity,22 which offers a great benefit to electrochemical performance of catalysts. While recent studies have demonstrated that Ni phosphide nanostructures are efficient electrocatalysts for water splitting,23−28 their application toward electrochemical glucose detection has not been explored before. In this Letter, we describe the first use of three-dimensional (3D) Ni2P nanoarray supported on conductive carbon cloth © XXXX American Chemical Society

Received: June 7, 2016 Accepted: August 4, 2016

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DOI: 10.1021/acs.analchem.6b02216 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry

Figure 2. (A) CVs of blank CC (a, b) and Ni2P NA/CC (c, d) in 0.1 M NaOH (pH = 13) with the absence (a, c) and presence (b, d) of 1 mM glucose (scan rate, 50 mV s−1). (B) CVs for Ni2P NA/CC in 1 mM glucose at scan rates from 20 to 100 mV s−1 and (C) the corresponding plots of current density vs the square root of scan rate. (D) Plot of anodic peak current density vs pH for Ni2P NA/CC electrode with the presence of 1 mM glucose at 0.5 V (scan rate, 50 mV s−1).

absence and presence of 1 mM glucose at a scan rate of 50 mV s−1 within the potential range of 0 to 0.8 V. Blank CC gives no redox peak in the absence of glucose (curve 1). In contrast, Ni2P NA/CC shows a pair of redox peaks at about 0.52 and 0.30 V (curve 2), which could be assigned to the Ni(II)/(III).37 Previous work reported that anodic scanning leads to NiOx/ Ni(OH)x on Ni2P surface in basic media.27,28 It is reasonable to conclude that the above peaks arise from the surface Faradaic reaction between NiOx/Ni(OH)x and NiOOH.38,39 The introduction of glucose causes a negligible oxidation current density change for blank CC (curve 3) while a notable increase in anodic peak current density occurs for Ni2P NA/CC with the presence of 1 mM glucose (curve 4). These observations suggest Ni2P NA/CC is efficient for electrooxidation of glucose. This enhancement in anodic current density is attributed to the electrocatalytic oxidation of glucose with the participation of NiOOH,12,17,40 which can be further explained as follows. Ni(II) is electrochemically oxidized to Ni(III) in NaOH, and at the same time, glucose is oxidized to gluconic acid by Ni(III) which further deoxidized to Ni(II). Cyclic voltammograms (CVs) for Ni2P NA/CC electrode at different scan rates were also acquired in 0.1 M NaOH containing 1 mM glucose. It is shown that both anodic and cathodic peak current densities increase with the scan rates in the range of 20 to 100 mV s−1 (Figure 2B). The good linear relationship between peak current densities and the square root of scan rates implies a diffusioncontrolled process of glucose oxidation on the Ni2P NA/CC electrode,41 as shown in Figure 2C. We further examined the electrochemical responses of Ni2P NA/CC toward 1 mM glucose at different pH in the range of 10−14 (Figure 2D). As observed, the anodic peak current density increases with increased pH and a linear curve is only obtained at pH ranging from 10 to 13. The sharp increase in current density at pH 14 can be attributed to oxygen evolution reaction under such strong alkaline conditions. Given that the high concentration of 1 M NaOH is also not eco-friendly, we thus chose 13 as the optimal pH in our present study.

Figure 1. (A) XRD pattern for Ni2P. (B, C) SEM images for Ni2P NA/CC. (D) SEM and EDX elemental mapping images of Ni and P for Ni2P NA/CC. (E) TEM image of one single Ni2P nanosheet. (F) HRTEM image taken from the Ni2P. XPS spectra of Ni2P in the (G) Ni 2p and (H) P 2p regions.

transmission electron microscopy (HRTEM) image taken from one nanosheet (Figure 1E) presents well-resolved lattice fringes with an interplanar distance of 0.225 nm corresponding to the (111) planes of Ni2P, as shown in Figure 1F. Figure 1G,H offers the X-ray photoelectron spectroscopy (XPS) spectra for Ni2P in the Ni 2p and P 2p regions. As shown in Figure 1G, the Ni 2p3/2 region exhibits two main peaks at 852.7 and 856.4 eV, along with one satellite peak at 861.5 eV,29,30 and the Ni 2p1/2 spectrum shows two peaks at 870.1 and 874.3 eV along with one satellite peak at 880.4 eV.31 The peaks at 852.7 and 870.1 eV can be assigned to Ni in Ni2P28,31 while those at 856.4 and 874.3 eV are characteristic of oxidized Ni species.29,32 The P 2p region (Figure 1H) shows two peaks at 128.7 and 129.5 eV assigned to the binding energies (BEs) of P 2p3/2 and P 2p1/2 in Ni2P, respectively,31,33,34 and another peak at 132.7 eV can be attribute to oxidized phosphate species formed on Ni2P surface due to air contact.35 In addition, the Ni 2p3/2 BE of 852.7 eV is positively shifted compared to that of Ni metal, suggesting the Ni in Ni2P has a partial positive charge.29,36 The P 2p3/2 BE of 128.7 eV is negatively shifted compared to element P (130.0 eV), indicating P in Ni2P has a negative charge.32,33 All these results strongly support the successful growth of Ni2P nanoarray on CC. Ni2P NA/CC was directly utilized working electrode for electrochemical tests, with platinum wire as the counter electrode and a saturated calomel electrode as the reference electrode. Figure 2A exhibits the electrochemical responses of Ni2P NA/CC and blank CC in 0.1 M NaOH (pH 13) in the B

DOI: 10.1021/acs.analchem.6b02216 Anal. Chem. XXXX, XXX, XXX−XXX

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after multiple injections of glucose (Figure 3C,D) may be ascribed to small variation of local pH, faster glucose consumption than its diffusion, or intermediates adsorption on the active sites.42 The calibration curve for our glucose sensor is shown in Figure 3E, which gives a linear dependence on glucose concentration in the range of 1 μM to 3 mM (R2 = 0.998). The detection limit and sensitivity were calculated as 0.18 μM (S/N = 3) and 7792 μA mM−1 cm−2, respectively. These values compare favorably to the behaviors of most reported Ni-based electrochemical glucose sensors (Table 1). Eliminating the interference responses is one of the major challenges in nonenzymatic glucose analysis. Fructose (Fru) and lactose (Lac) have similar electroactive behavior to glucose.43 Ascorbic acid (AA), uric acid (UA), and dopamine (DA) coexist with glucose in human blood serum.15 We thus evaluated the anti-interference performance of Ni2P NA/CC toward these interfering species. Figure 4A shows the

Above results suggest that our Ni2P NA/CC offers us as a 3D catalyst electrode for nonenzymatic glucose sensing. To this end, we examined the electrochemical behaviors of Ni2P NA/ CC toward different concentrations of glucose. As observed, the current density for oxidation peak increases with increased glucose concentration in range between 0 and 8 mM (Figure 3A). The corresponding calibration curve suggests that the

Figure 4. (A) Amperometric responses of Ni2P NA/CC to the successive addition of 1 mM glucose, 5 mM Lac, 5 mM AA, 5 mM Fru, 5 mM Gal, 5 mM UA, 5 mM DA, 5 mM Rib and 1 mM glucose at 0.5 V in 0.1 M NaOH. (B) CVs of Ni2P NA/CC in 0.1 M NaOH with the absence (a) and presence of ribose (b), fructose (c), galactose (d), and glucose (e) at a scan rate of 50 mV s−1 (concentration for all monosaccharide is 4 mM).

Figure 3. (A) CVs for Ni2P NA/CC in 0.1 M NaOH (pH = 13) with the presence of varied glucose concentrations: 0, 1, 2, 3, 4, 5, 6, 7, and 8 mM from inner to outer (scan rate, 50 mV s−1) and (B) the corresponding calibration curve. (C, D) Amperometric responses and (E) the corresponding calibration curve of Ni2P NA/CC electrode to successive additions of glucose at 0.5 V in 0.1 M NaOH.

amperometric responses of the Ni2P NA/CC electrode at 0.50 V in 0.1 M NaOH with successive addition of 1 mM glucose, 5 mM Fru, 5 mM Lac, 5 mM AA, 5 mM DA, 5 mM UA, and 1 mM glucose. It can be clearly seen that Ni2P NA/ CC has remarkable response for glucose but negligible response for interfering species and the current density increases again with another addition of glucose. These results demonstrate the superior selectivity of Ni2P NA/CC toward glucose. CVs for Ni2P NA/CC electrode were also acquired in 0.1 M NaOH containing 4 mM monosaccharide such as ribose, fructose, and galactose. As observed, Ni2P NA/CC only shows a poor change in oxidation current density toward ribose, fructose, or

anodic peak current density increases linearly with the rise of glucose concentrations in the whole concentration range (Figure 3B). By holding the potential at +0.5 V, we measured the amperometric responses of Ni2P NA/CC to the successive addition of glucose in 0.1 M NaOH. This electrode exhibits a fast amperometric response toward glucose and can achieve steady state current density within 5 s. The slight baseline drift

Table 1. Comparison of Sensing Performances of Ni2P NA/CC with Other Ni-Based Nonenzymatic Electrochemical Glucose Sensors catalysts

sensitivity (μA mM−1 cm−2)

linear range (mM)

detection limit (μM)

ref

NiO nanowalls HAC/NiO nanocomposite 600-NiO/SiC NiO-MWCNT/CPE NiO nanoflake arrays c-Ni(OH)2 HR PI/CNT-Ni(OH)2 nanospheres rGO/Ni(OH)2 Ni(OH)2/3DGF Ni(OH)2@oPPyNW Ni2P NA/CC

2300 1721.5 2037 6527 8500 1569 2071.5 11400 2650 1049 7792

0.0002−1 0.005−4.793 0.004−7.5 0.001−14 0.01−0.8 0.002−3.8 0.001−0.8 0.02−30 0.001−1.17 0.001−3.86 0.001−3.0

0.2 0.055 0.32 19 1.2 0.6 0.36 15 0.34 0.3 0.18

12 13 14 15 16 17 18 19 20 21 this work

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galactose while a notable increase occurs when glucose with the same concentration is introduced (Figure 4B), implying the high specificity of Ni2P NA/CC electrode toward glucose. We also tested the long-term stability of Ni2P NA/CC for practical application consideration. Ni2P NA/CC was stored at 4 °C when not in use to evaluate the stability and its current response to 1 mM glucose in 0.1 M NaOH was tested every 3 days. This sensor only shows 17.5% loss in current density after 30 days (Figure S2A). SEM observations indicate that this catalyst electrode preserves its nanoarray morphology and structural integration after repeated usage (Figure S2B). The reproducibility of the Ni2P NA/CC was also investigated by measuring the response to 1 mM glucose at 10 Ni2P NA/CC electrodes. The relative standard deviation (RSD) of anode peak current densities is only 4.7%, demonstrating a good reproducibility. To verify its feasibility for routine analysis, the Ni2P NA/CC was applied to detect glucose in human blood serum, peach juice, and human blood. Figure 5A shows that the oxidation

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b02216.

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Experimental section, EDX spectrum, stability test, SEM images, CVs, and calibration curves (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

T.C. and D.L. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21575137).

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REFERENCES

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Figure 5. (A) CV curves of Ni2P NA/CC in human blood serum sample (10%) in 0.1 M NaOH with the presence of varied glucose concentrations: 0, 1, 2, 3, 4, 5, 6, 7, and 8 mM from inner to outer (scan rate, 50 mV s−1). (B) The corresponding calibration curve at 0.52 V.

peak current densities increase with successive addition of glucose into the diluted blood serum sample (10%). The corresponding calibration curve (Figure 5B) indicates that the peak current densities increase linearly with increased glucose concentrations ranging from 1 to 8 mM. Because the level of glucose in the physiological condition is between 4 and 7 mM,42 our Ni2P NA/CC sensor is promising for nonenzymatic glucose monitoring for clinical samples at alkaline pH. Figure S3 exhibits that the oxidation peak current densities increase with successive addition of glucose into the diluted peach juice (10%) and human blood (10%). These observations indicate that Ni2P NA/CC electrode could be utilized for practical sample testing. In summary, Ni2P nanoarray has been proven as an efficient catalyst electrode for glucose electrooxidation under alkaline conditions. As a nonenzymatic glucose sensor, this electrode exhibits superior sensing performances with a linear range of 1 μM to 3 mM, a detection limit of 0.18 μM, and high selectivity, specificity, and reproducibility. Our present study not only provides an attractive low-cost easily made electrode material for high-efficiency glucose detection but would open new opportunity to explore using electronically conductive 3D TMPs nanoarray as electrochemical sensors for analytical applications. D

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DOI: 10.1021/acs.analchem.6b02216 Anal. Chem. XXXX, XXX, XXX−XXX