Engineered IrO2@NiO CoreâShell Nanowires for Sensitive Non

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Engineered IrO2@NiO core-shell nanowires for sensitive nonenzymatic detection of trace glucose in saliva Junjun Wang, Lin Xu, Yang Lu, Kuang Sheng, Wei Liu, Cong Chen, Yang Li, Biao Dong, and Hongwei Song Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b03558 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 27, 2016

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Analytical Chemistry

Engineered IrO2@NiO core-shell nanowires for sensitive nonenzymatic detection of trace glucose in saliva Junjun Wang,a Lin Xu ,a* Yang Lu,b Kuang Sheng,a Wei Liu,a Cong Chen, a Yang Li,a Biao Dong, a

a

and Hongwei Songa State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and

Engineering, Jilin University, Changchun, 130012, People’s Republic of China. b

The Second Hospital of Jilin University, Jilin University, Changchun, 130041, People’s

Republic of China.

* To whom correspondence should be addressed. Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Hierarchical core-shell IrO2@NiO nanowires (NWs) have been designed through two simple steps, which combined electrospinning of IrO2 conductive core and chemical bath deposition (CBD) growth of ultra-continuous NiO nanoflakes. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectrometry (EDX) mapping and X-ray photoelectron spectroscopy (XPS) were employed to characterize the morphologies and structures of the as-prepared samples and the results were also carefully compared with that of pure NiO nanoflowers and IrO2 NWs. Electrochemical studies indicate that the as-prepared core-shell IrO2@NiO NWs exhibited excellent nonenzymatic detection ability to glucose. At 0.35 V, it offered a sensitivity of 1439.4 µAmM-1cm-2 (one order higher than pure NiO) with a wider linear range from 0.5 µM to 2.5 mM, an low detection limit of 0.31 µM (signal-to-noise ratio=3), and moreover, good resolution in low glucose concentration, reproducibility, and long-term performance stability. Owing to the high sensitivity and performance, application of the proposed sensor in monitoring saliva glucose was also demonstrated; the results indicated that the sensor can effectively distinguish the diabetes from the healthy people and even the varying degrees of diabetic. KEYWORDS core-shell, nanowires, nonenzymatic biosensor, traces glucose detection, electrospinning


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1. INTRODUCTION Diabetes, as one of the most common and serious threat to human health, is a chronic disease due to metabolic disturbances. In order to monitor the illness condition and the therapeutic effect, frequent measurement of blood glucose levels is a routine method in daily control of diabetes, while the recurring pain and the potential risk of cross infection during blood collection bring so much psychological stress and additional pain to the patient. Thus, the requirement for noninvasive, simple and responsive glucose monitoring has been strongly emphasized and highly expected. 1-4 Saliva is a good alternative candidate for noninvasive determination of glucose, due to its easy collection by individuals with modest instruction, little discomfort of the tests, and most importantly, saliva has good correlation with blood concentrations of numerous analytes.4-6 However, the concentration of glucose in saliva (30 to 80 µM) is much lower than the detection range of blood glucose in normal seniors (3.6–7.5 mM for healthy people and 1.1–20.8 mM for diabetic patients).7 Hence, it is pertinent to explore and develop reliable sensors with high sensitivity and low detection limit to achieve convincing clinical measurements from saliva. In different technology for glucose detection, electrochemical nonenzymatic biosensors have drawn so much attention recently, owing to their long stability, rapid response, flexibility, and low expense compared to the enzyme based one. 8, 9 Due to the absence of target enzyme, an efficient and effective sensitive electrocatalyst is essential for the construction of excellent nonenzymatic biosensors, as it will great effect the processes of direct substrate oxidization with minimum resistance, electrochemical reactions occurring at electrolyte/electrode interfaces, and electron conduction in electrodes.8-10 Accordingly, it should have improved conductivity in order to assist the electron transfer and a large surface area so as to contact with more target analytes, as well as electrocatalytic activity, stability, and good biocompatibility. Metal oxide based


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nanomaterial can meet well with these stringent requirements.10-13 Amoung these, NiO is a transition metal oxide with intriguing electronic, optical, electrochemical, and electrocatalytic properties, which has demonstrated great potentials in the applications of nonenzymatic electrochemical sensors.14-18 However, the selectivity and the signal-to-noise ratio (SNR), especially to low concentration glucose, of NiO based and even the metal oxide based biosensors still need to be improved, which is hindered by the relatively large resistance instinct of metal oxide.10 As is known, the glucose response of nonenzymatic biosensor is essentially an electrochemical redox process including electrons transfer process. Hence the introduction of the noble metal catalyst to accelerate the corresponding reaction and improve the conductivity of metal oxide based biosensors is very helpful.19-22 However, the noble metal catalyst is not so stable in nanoscale which is easy to be poisoned and lead to a poor long stability. Recently, IrO2, which has metal-like conductivity (resistance of ~50 µΩcm-2 in bulk) is coming into view as a good substitutes for noble metals,23-25 moreover, its especially superior chemical and thermal stability also make it much easier to keep the sensitivity and control the structure. Up to now, to the best of our knowledge, there are limited instance to use IrO2/metal oxide based nanocomposite materials to actually and accurately measure the glucose concentration in human saliva from clinical practice view. Herein, we designed IrO2@NiO core-shell NWs and carefully studied their enhanced biosensing properties for low concentration glucose detection. Electrospinning is used to fabricate the IrO2 NWs core, which is a very powerful and simple method to obtain ultra-long and high surface to volume ratio NWs. Generally, this method needs high temperature annealing to obtain the finial inorganic product. Compared to the most materials with good conductivity


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with poor thermal stability, such as carbon, Au, Ag, and Cu, IrO2 which has metal-like conductivity and also shows very good thermal stability will be a very good candidate. Thus, in this study, the IrO2 NWs were fabricated through one step electrospinning, which provided a larger surface area for NiO loading, plus a good electron transport path. Ultra-continuous NiO nanoflakes were decorated on the surface of the IrO2 NWs by a CBD procedure, which have large surface active area and act as specific glucose recognizer. We observed that IrO2@NiO core-shell NWs biosensor demonstrated excellent sensing permanence toward dilute glucose. Moreover, clinical tests proved that the as-prepared IrO2@NiO core-shell NWs biosensor not only could separate healthy people from diabetes patient according to glucose concentration in saliva, but also could further distinguish the concentration in varying degrees of diabetic clearly. 2. EXPERIMENTAL SECTION Synthesis of IrO2@NiO core-shell NWs. All chemicals used were analytical grade and utilized without further purification. First, IrO2 NWs were obtained through a simple singlespinneret electrospinning process. In a typical process, 43.3 mg of IrCl3 was added into 1 mL N,N-dimethylformamide

(DMF)

solution

and

stirred

a

few

minutes,

then

0.2

g

polyvinylpyrrolidone (PVP; Mw = 1 300 000) was added into the above mixed solution to make the weight ratio of the inorganic salt to PVP (IrCl3/PVP) is 0.22, and continue stirred until obtained a viscous and clear solution. The precursor nanofibers were fabricated by electrospinning with an applied steady voltage of 15 kV and a collection distance of 15 cm between the spinneret tip and the collector. After being dried for 12 h at room temperature, the electrospun fibers were annealed in a tube furnace with a heating rate of 1 °C/min from room temperature to 900 °C for 3 h, and then the furnace self-cooled down to room temperature. The final products were IrO2 NWs.


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Then the as prepared IrO2 NWs was used as the scaffold for growth of ultra-continued NiO nanoflake through simple CBD method with some improvements.26 The solution for CBD process was first prepared by mixing 1.25 mL of 1 M NiSO4.6H2O and 1 mL of 0.25 M K2S2O8. Then all the IrO2 NWs as obtained in the electrospinning were added into the mixture solution and ultrasonic dispersed for 5 min. After that, 1 mL of aqueous ammonia (25-28%) were added into the above solution and let stand for 8 min at 20 °C. Then, the samples were washed with distilled water for three times by centrifugation, and annealed at 350 °C for 1.5 h. Finally, IrO2@NiO core-shell NWs were obtained. For comparison, the pure NiO nanoflakes were also prepared in the same condition as the above process without IrO2 NWs. Characterization The morphology of the products was inspected using JEOL JSM-500F field SEM (Japan) with accelerating voltage of 15 kV and gold sputtering on the surface of the samples. TEM and high-resolution TEM (HRTEM) images were recorded on JEM-2010 transmission electron microscope under a working voltage of 200 Kv equipped with EDX spectrometer. The microstructure of the products were characterized by an XRD (Rigaku D/maxRA power diffractometer using Cu KR radiation (λ=1.541 78 Å), and the corresponding lattice constants were calculated by MDI Jade 5.0 software. XPS was characterized on an ESCAlab250 Analytical XPL spectrometer with a monochromatic Al KR source, the binding energies were referred to the C 1s peak at 284.7 eV of the surface adventitious carbon, and the fitted peaks in XPS spectra were deconvoluted using the XPS Peak 4.1 software. Preparation of IrO2@NiO, IrO2, and NiO modified Electrodes. First, the glassy carbon electrode (GCE, diameter of 3 mm) was polished by using 1 mm and 0.05 mm alumina slurries. Then the electrode was washed with nitric acid (0.2 M), acetone, ethanol, deionized water and dried at room temperature. For surface modification, 5 mg IrO2@NiO was mixed with 1 mL


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ethanol and sonicated for 1 h. After that, 5 µL IrO2@NiO suspension was dropped onto the surface of GCE. In order to entrap the sample, 5 µL Nafion solution (0.5 wt % in ethanol) were further covered on the GCE and dried in air. Note that the as-prepared electrode (denoted as IrO2@NiO/GCE) needed to be wetted before use. We also made the pure IrO2 and NiO modified GCEs (denoted as IrO2/GCE and NiO/GCE, respectively) as comparison in a similar way. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and current−time (I −t) measurements were recorded on a model CHI660D electrochemical analyzer (ChenHua Instruments Co. Ltd., Shanghai, China). All the electrochemical measurements were conducted using a three-electrode electrochemical cell, in which the as modified GCEs were used as working electrode, Ag/AgCl electrode was used as the reference electrode, and a platinum wire was used as the counter electrode. 3. RESULTS AND DISCUSSION Morphological and Structural Characteristics. The morphologies and nanostructures of as prepared samples are first illuminated. Figure 1a shows the SEM images of IrO2 scaffold. As can be seen, uniform, long, and continuous IrO2 NWs with an average diameter of ~70 nm are formed. When the SEM image is further enlarged (inset of Figure 1a), porous surface of IrO2 NWs can be clearly observed due to the removal of PVP and formation of IrO2 nanoparticals during the annealing process. Then, CBD procedure was introduced. When there is no IrO2 scaffold, NiO nanoflakes intend to self-assemble to a flower-like structure (Figure 1b and its inset). When IrO2 NWs exist, NiO nanoflakes will attach on IrO2 scaffold and continuous grow. The average diameter of IrO2@NiO core-shell NWs is ~220 nm, which is obviously larger than that of pure IrO2 NWs. Furthermore, Figure 1d shows the enlarged view of IrO2@NiO core-shell NWs, uniform and dense appearance of surface NiO nanoflakes can be further seen.


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Figure 1. SEM images of (a) the IrO2 nanofibers and (b) the NiO nanoflowers, the insets in panels (a) and (b) are their corresponding enlargement. (c) The SEM image of core-shell IrO2@NiO NWs, and (d) is the enlargement of (c). The heteroarchitecture of the IrO2@NiO core-shell NWs can be further confirmed from TEM images. Compared to the pure NiO nanoflowers (Figure 2a), the IrO2@NiO core-shell architecture can be clearly seen (Figure 2b and 2c) and the thick and continues structure of ultracontinues NiO nanoflakes can be further confirmed. HRTEM images are taken from the square areas in Figure 2c, distinct lattice fringes with an interplanar spacing of 0.317 nm (Figure 2d) and 0.207 nm (Figure 2e) can be observed, matching well with the (002) plane of IrO2 and the (111) plane of NiO, respectively. Further, as shown in EDX mapping images (Figure. 2f-i), the distribution of O element is match well with that of STEM image of IrO2@NiO core-shell NWs in Figure 2f. For the distribution of Ir element, it shows an obvious and homogeneous NWs structure, while the distribution of Ni element is much more unregularly and shows a wider distribution area without inside NWs trunk. This further proved that the IrO2@NiO core-shell NWs were successfully synthesized.


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Figure 2. TEM images of (a) NiO nanoflowers and (b) core-shell IrO2@NiO NWs, (c) is the enlargment of (b). (d) and (e) are the HRTEM images taken from the panel (c). (f) is the STEM images of IrO2@NiO NWs.(g)-(i) are the corresponding EDX mapping of O, Ir, and Ni elements.

(a) IrO2@NiO

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NiO

IrO2

20

30

40 50 60 70 80 2 Theta (degree) Figure 3. (a) XRD patterns of NiO nanoflowers, IrO2 NWs, and IrO2@NiO NWs.


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The composition and crystal structure were characterized by XRD patterns, as shown in Figure 3. The XRD pattern of pure IrO2 NWs and NiO nanoflowers can be well assigned to the facecentered cubic crystalline NiO (JCPDS 47-1049) and IrO2 (JCPDS 15-0870). In addition, the XRD patterns taken from the IrO2@NiO core-shell NWs can be indexed to a mixture of cubic NiO and IrO2 with separate phases, and no any trace of other phases could be detected. (a) Intensity (a. u.)

IrO2@NiO NiO

IrO2

0

400 800 Binding energy (eV)

Intensity (a. u.)

(b)

IrO2@NiO

NiO IrO2

1200

528

532 Binding energy (eV)

536

(d)

IrO2@NiO NWs

Intensity (a. u.)

(c)

Intensity (a. u.)

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IrO2@NiO

NiO

IrO2 NWs

58

60

62 64 66 68 Binding energy (eV)

70 850

860 870 880 Binding energy (eV)

890

Figure 4. (a) survey, (b) O 1s, (c) Ni 2p, and (d) Ir 4f XPS spectra of IrO2@NiO NWs, NiO nanoflowers, and/or IrO2 NWs. To further demonstrate the valence chemistry and binding energy of the constituent elements present in the as-prepared IrO2@NiO core-shell NWs, XPS spectrum was studied and compared with that of pure IrO2 NWs and NiO nanoflowers. As shown in Figure 4a, all the peaks in full XPS spectra can be ascribed to O, C, Ir or/and Ni elements, which is consistent with the above XRD and TEM results. The O 1s XPS spectra are enlarged in Figure 4b. Accordingly, The XPS spectra of O1s core level electrons measured from pure IrO2 display three peaks and NiO show


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two peaks. The binding energies of 529.3 in IrO2 and 531.2 in NiO can be assigned to the lattice oxygen in crystalline IrO2 and NiO, respectively.27, 28 The other peaks has been proposed for the detect sites within the nanocrystal,29 absorbed oxygen,30 or hydroxide spicies.31 In addition, Figure 4c and 4d show the Ir 4f and Ni 2p high-resolution XPS spectra of the as-prepared IrO2@NiO core-shell NWs compared with pure Ir NWs or NiO nanoflowers. As can be seen in Fig. 4c, the Ir 4f signal of IrO2 NWs shows two different binding states from iridium of dioxide, identified as 4f7/2 and 4f5/2 at 60.7 and 63.5 eV, respectively.32 For the Ir 4f signal of IrO2@NiO core-shell NWs, it shifts to higher binding energy obviously, 61.5 of 4f7/2 and 64.5 of 4f5/2, respectively. What’s interesting, the opposite shift of binding energy can be observed in Ni case. As shown in Figure 4d, the Ni 2p signals in both samples could be deconvoluted into five peaks. In pure NiO nanoflower, the binding energies at 854.8, 856.1, and 861.2 eV are attributed to the Ni 2p3/2 peaks, and the 873.1 and 879.5 eV peaks can be assigned to Ni 2p1/2.28 Compared to the pure NiO, the binding energies of Ni element in IrO2@NiO core-shell NWs shift obviously to the lower side (about 0.9 eV). Generally, the shift of the binding energies in XPS spectra can be explained by the different electronegativity of the corresponding metal ions or the electron transfer due to the strong interaction.33, 34 In our case, the electronegativity of Ni2+ ion and Ir4+ ion are about 1.91 and 2.2, respectively.35,

36

The Ir4+ ions with larger electronegativity can

withdraw the electrons from Ni2+ ions and this will lead to the electrons decrease in the Ni2+ ions but increase for the Ir4+ ions due to the screening effect. As consequence, the Ir 4f peaks shifted toward the higher binding energy, while the Ni 2p peaks shifted to the lower binding energy. On the other hand, because the excellent conductivity of IrO2, when the IrO2 and NiO nanoparticles attach together, electron transfer could occur from IrO2 to p-type NiO nanoflakes until the system attains equilibration, it is indicated a modification of the electron density on NiO due to


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the interactive coupling between IrO2 and NiO. It worth mentioning that the decrease of NiO binding energy can lead to the increased surface activity of NiO nanocrystals in IrO2@NiO coreshell NWs which is very helpful to the following glucose detection. Nonenzymatic Glucose Electrochemistry Behavior. IrO2@NiO core-shell NWs, in which NiO nanoflakes have been proved to be very thick and highly continuous growth on the surface of IrO2 NWs with good electron transport ability, is expected to provide a significantly unhindered interface for glucose molecules under an electrochemical environment. EIS can give the information about the electron transport ability of as prepared nanomaterials from the view of impedance changes of the electrode surface. Figure 5a shows the Nyquist plots of IrO2@NiO/GCE compared with NiO/GCE and IrO2/GCE in the presence of redox probe [Fe(CN)6]3-/4-. The semicircular diameter of EIS that reflects the interfacial electron transfer resistance (Rct) can be used to describe the capability of electron transfer on the different electrode. Here, a simple equivalent circuit used to model the impedance data in the presence of redox couples and also to further calculate the Rct value of each curve, which is shown in inset of Figure 5a. In this circuit, Rs, Cdl and Rct represent solution resistance, the double layer capacitance, and the charge transfer resistance, respectively. As can be clearly observed, the EIS curve of IrO2/GCE is almost a straight line with Rct =0.24 Ω, indicating the very good electron transport ability of IrO2 NWs, and IrO2@NiO/GCE only shows a little semicircular diameter (Rct =0.35 KΩ) compared to that of NiO/GCE (Rct =2.55 KΩ), demonstrating the conductivity of core-shell IrO2@NiO NWs has been greatly improved after introducing the IrO2 core. The preliminary information on the electrochemical kinetics of IrO2@NiO/GCE was derived from CV curves in 0.1 M NaOH solution with the scan rate of 100 mV/s. Figure 5b presents the CVs of IrO2@NiO/GCE without (trace a) and with (trace a’) of 5 mM glucose. Note that the


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amount of modified IrO2@NiO nanowires on the electrode surface is optimized and the changing trend of CVs peak current of the electrodes modified by different concentration of IrO2@NiO NWs (5 µL in ethanol) to the presentation of 5 mM glucose in NaOH solution is shown in Figure S1. As can be seen, the current intensity sharply increases with the increased concentration of IrO2@NiO NWs from 3 to 5 mg/mL and this can be attributed to the effectively increased the amount of sensitive material that can reactive to the glucose. When further increased the concentration, it just leads to continuous decrease of the current intensity, this can be explained by the overloading of the sensitive material which hindered the electron transport. For comparison, CVs of NiO/GCE (trace b and b’) and IrO2/GCE (trace c and c’) under similar conditions are also depicted in Figure S2. Note that the IrO2/GCE showed almost no response to glucose, indicating that NiO is the active sensitive material to glucose. The corresponding nonenzymatic response process to glucose is as depicted in Figure 5c. When in blank NaOH solution, one pair of well-defined redox peaks can be obtained, especially in IrO2@NiO/GCE from 0.15-0.4 V (trace a), this can be attributed to the Ni(II)/Ni(III) redox couple in the NaOH solution under the special potential region.37-39 The reaction can be formulated as: NiO + OH- - e- → NiO(OH). Note that OH- ions should be involvement to generate Ni(III), which is crucial for the oxidation of glucose to occur. After the injection of glucose, Ni(III) ion obtains an electron and acts as an electron delivery system, electrons are transferred from glucose to the electrode which leads to the increasing of the peak current, 39-41 the reaction process is as follows: 2NiO(OH) + glucose → 2NiO + gluconolactone + H2O.


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Figure 5. (a) EIS of CVs of IrO2@NiO/GCE, IrO2 /GCE, and NiO/GCE. (b) The CVs of IrO2@NiO/GCE without (trace a) and with (trace a’) of 5 mM glucose. (c) The schematic diagram of nonenzymatic response process of core-shell IrO2@NiO NWs to glucose. (d) The comparison of the CVs of IrO2@NiO/GCE, NiO/GCE, and IrO2/GCE with existing of 5 mM glucose. In this process, the NiO/GCE shows an oxidation peak around 0.42 V vs. Ag/AgCl while the IrO2@NiO/GCE displays an oxidation peak centred at 0.35 V vs. Ag/AgCl. This obvious decrease in the anodic over potential indicates a strong catalytic function of the IrO2@NiO coreshell NWs in the direct oxidation of glucose. Furthermore, the CVs of IrO2@NiO/GCE, NiO/GCE, and IrO2/GCE with existing of 5 mM glucose were also compared in Figure 5d.


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Compared to NiO/GCE, the current intensity IrO2@NiO/GCE is 4 times higher, which clearly shows strong catalytic activity of IrO2@NiO core-shell NWs. The shifts in this overpotential and the increased current may be due to a kinetic effect by an increase in the electroactive surface area which is consistent with the XPS result and the faster rate of electron transfer from glucose to the electrode due to the introduction of IrO2 with good electrical conductivity. These results in Figure 2a and b demonstrate the core-shell IrO2@NiO NWs have higher electron transfer ability and electrochemical activity. Furthermore, the CVs as function of scan rates varying from 50-250 mVs-1 were recorded and as shown in. As the scan rate increases, the corresponding oxidation and reduction peak currents increase along with a gradual potential shift of the oxidation peak towards positive direction and reduction peak towards negative direction implying the quasi-reversibility of the system. It can be found in the inset of Figure S3, both the anodic (Ipa) and cathodic (Ipc) peak currents are linearly proportional to the scan rate over the studied range (50–250 mVs−1) with the correlation coefficient (R2) of 0.9900 and 0.9970, respectively, suggesting that the redox kinetics controlled by surface electrochemical process, this is consisted with the previous study which indicated that the NiO biosensor would be under surface controlled when the electrode reaction was at the lower scan rate. 37 To determine the response efficiency and sensitivity of as prepared electrode for glucose detection, amperometric measurements were further studied. As the intensity of Ipa at +0.35 V vs. Ag/AgCl increases significantly with the increase of glucose concentration, an applied potential of +0.35 V vs. Ag/AgCl (peak potential of trace a’ in Figure 5a) was applied to conduct nonenzymatic amperometric detection of glucose in 0.1 M NaOH solution. The characteristic i–t curves of IrO2@NiO/GCE as well as NiO/GCE with regular injection of different concentrations


15


of glucose are presented in Figure 6a. As can be clearly seen, relative to NiO/GCE, the IrO2@NiO/GCE shows more rapid and sensitive response to the change of glucose concentration, the average response time of IrO2@NiO/GCE was determined to be ~ 3.3 s for the electrodes to reach 95% of steady-state current, which is 2 times shorter than that of NiO/GCE (~ 6.5 s), this can be ascribed to the good electrocatalytic property and lager effective surface area of NiO after introduced of IrO2 core. Moreover, the electrochemical response of IrO2@NiO/GCE is much higher than that of NiO/GCE, indicating a faster and higher electron exchange between glucose and NiO in IrO2@NiO core-shell NWs. 300

800 µΜ

400

200

IrO 2@NiO/GCE NiO/GCE

200

400 µΜ

R2 =0.9983 R2=0.9967

0

100

18 (b)

(a)

0

3 6 Glucose/mM 200 µΜ 5 µΜ 20 µΜ

0 300

9

IrO2@NiO/GCE

NiO/GCE 600 900 Time/s

Current/µA

Current/µA

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12

R 2=0.9980

6

R 2=0.9968

0

0

40 80 Glucose/µM

120

Figure 6. (a) The i-t curves of IrO2@NiO/GCE compared with NiO/GCE response to different concentration of glucose in 0.1 M NaOH at 0.35 V, the inset is the corresponding calibration curves of (a), (b) the linear range of the electrodes from 5-120 µM due to the much lower glucose concentration in saliva. The corresponding calibration curves for the electrodes under the optimized experimental conditions are shown in inset of Figure 6a. The IrO2@NiO/GCE displays an excellent linear response to glucose in the concentration range from 0-2.5 mM (R2 = 0.9983), while the NiO/GCE shows a linear range of 0-2 mM (R2 = 0.9956). The sensitivity of IrO2@NiO/GCE was


16

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estimated to be 1439.4 µAmM-1cm-2 and it increases almost one magnitude compared to that of NiO/GCE (114.9 µAmM-1cm-2), and this sensitivity also better than the other typical nonenzymatic core-shell material based glucose sensors as compared in Table S1.40-42 Besides, according to the intercept of the regression plot and standard deviation, the detection limit of the IrO2@NiO/GCE and NiO/GCE were determined to be 0.3 and 0.5 µM, under a signal/noise ratio of 3. Because the glucose concentration is in a much lower range in saliva, the linear range of the electrodes from 5-120 µM was further enlarged in Figure 6b. It can be found that the as prepared IrO2@NiO/GCE exhibits a good resolution to the trace concentration of glucose. Table 1 shows a comparison of the sensitivity, linear range, detection limit, and applied potential of several typical non-enzymatic nickel based glucose sensors as well as IrO2@NiO/GCE sensor in our study. As compared, IrO2@NiO/GCE sensor exhibits satisfactory integrative performances with very high sensitivity and low applied potential. Table 1. A comparison of the sensitivity, linear range, detection limit, and applied potential of several typical non-enzymatic nickel based glucose sensors as well as IrO2@NiO/GCE sensor in this work. Sensing materials NiO/CeO2 hybrid nanoflake arrays Ni foam Self-assembled nano-NiTiO3/NiO NiO/Pt NPs/ERGOa Ni loaded carbon nanofiber paste Ni–Co alloy on silica nanoparticles Ni-coordinated vertically aligned CNTsb Ni(OH)2 nanoboxes Flower-like Ni7S6 Nanostructured α-Ni(OH)2 IrO2@NiO core-shell NWs a ERGO:reduced graphene oxide b CNTs:carbon nanotubes

Sensitivity /µAmM-1cm-2 154.4 --1454 and 52.86 668.2 420.4 536.62 910.7 487.3 271.8 466 1439.4

Linear range /mM 0.001−2.9 0.05–7.35 0.0001–0.0188 and 0.04–2.07 0.05–5.66 0.002–2.5 0.001−5 0.05−1.0 0.005−5 0.005−3.7 0.01−0.75 0.005−2.5

Detection limited/µM 1.0 2.2 0.06

Applied potencial /V vs. Ag/AgCl 0.6 0.45 0.55

15 43 44

0.2 1 0.39 30 0.07 0.15 2.5 0.3

0.6 0.6 0.5 0.5 0.58 0.45 0.44 0.35

45 46 47 48 49 50 51 This work


Ref

17


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Since the avoidance of endogenous interfering species is a challenge in nonenzymatic glucose detection because a few of the structurally similar organic substances, such as dopamine, ascorbic acid, uric acid and other carbohydrate compounds, are also simultaneously oxidized along with glucose at the electrode surface and, hence, give interfering electrochemical signals. Figure 7a shows the anti-interference property to some typical interference in human fluid of asprepared IrO2@NiO/GCE. Considering the concentration of glucose is at least 30-50 times higher than that of interfering species in body fluid, such as the physiological level of glucose in normal serum is 3–8 mM and the levels of these interfering species are no more than 0.1 mM and its environment is much complexes than that of in saliva,48 the interference experiment was carried out by successive injection of 100 µM glucose and 10 µM other interfering species in NaOH solution. As is shown, well-defined glucose responses could be obtained, while the current responses of interfering species have little effect compared to glucose. In addition, the response of other sugars, such as fructose, mannose, lactose, galactose, maltose, and sucrose compared to that of glucose are also evaluated as shown in Figure 7b. The current responses of the corresponding interfering species are also very low in the presence of 100 µM glucose, demonstrating the physiological level of saccharides will not affect the detection of glucose. Note that although compared to the current responses of the other interfering species, the addition of 10 µM DA, AA, maltose and mannose also induces the increasing the current of IrO2@NiO/GCE, the intensity increased is much smaller than that of glucose. For AA and DA, this may due to the increased conductivity of coreshell IrO2@NiO NWs make the IrO2@NiO/GCE can work under a low potential, in which range


18

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the reaction can be happed easier for AA and DA.49,

52

For maltose and mannose, the little

increase of current can be explained that maltose and mannose may change to glucose in a some condition and then increased the current response.53, 54 Overall, it can be concluded that small amount of DA, AA, UA, NADH, Mg2+, Ca2+, and other carbohydrate compounds have unneglectable influence on the glucose response. In other words, the IrO2@NiO/GCE shows high selectivity for low concentration glucose detection.

(a)

Ca Mg2+

2+

60

(b)

NADH

120

fructose

DA

40

Current/µA

160

Current/µA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AA

80

UA 40

Glucose

lactose

maltose

mannose galactose

sucrose

20

100 µM glucose 0

0 200

400 Time/sec

600

0

120

240

360

Time/sec

Figure 7. (a) The amperometric response of the IrO2@NiO/GCE with successive additions of different interfering species (UA, AA, DA, NADH, Mg2+, and Ca2+) and (b) with sequential addition of 10 mM various interfering sugars (fructose, mannose, lactose, galactose, maltose, and sucrose) after initial addition of 100 mM glucose. The stability of IrO2@NiO/GCE was also evaluated. The current response decayed by 7.4% after 2 months storage, demonstrating a long-term storage stability of the electrode. The high stability of IrO2@NiO/GCE in glucose electro-oxidation resulted from the high physical and chemical stability of as fabricated IrO2@NiO core-shell and network-like structure formed on GCE surface, which prevented the nickel-based materials from conglomeration.


19


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The Practical Test of IrO2@NiO/GCEs towards Clinical Samples. To investigate the nonenzymatic glucose sensing ability in real human saliva condition, the i − t curves of IrO2@NiO/GCE by addition of healthy human saliva containing different concentration of glucose were carried out. As can be seen in Figure 8a, the current response is quite steady and fast with each addition of saliva in the studied range. Under the same fabrication condition, three electrodes were prepared and the corresponding calibration curve is shown in Figure 8b. The maximum RSD of 2.8% for the steady state current were obtained, indicating a satisfied reproducibility of this procedure in practical saliva measurement. In the studied range (0-180 µM glucose), the sensor displays a good linear dependence in saliva with a correlation coefficient of 0.9953, a sensitivity of 1478.2 µAmM−1cm−2. Furthermore, we also test i-t curves by addition of saliva samples coming from three different healthy human. In Figure 8c, it can be found that the responses are very similar, and the sensor also exhibits a good linear range with a correlation coefficient of 0.9945, a sensitivity of 1539.0 µAmM−1cm−2. The maximum RSD of the steadystate current response for three individually saliva is 3.0%. It is worth mentioning that the IrO2@NiO/GCE can detect as low as 5 µM glucose in saliva samples, which is significantly lower or comparable with the literature reported nanostructured nickel based electrodes in lab conditions. 48, 55Note that the sensitivity in saliva is a little higher than the sensitivity in PBS and this can be attributed to the influence brought by the other interferences. Above results essentially suggests very high reproducibility of the IrO2@NiO/GCE sensing performance, which is crucial for the most precise estimation of glucose in real physiological conditions, especially for the very low concentration glucose detection.


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25

(a)

Current in Saliva

(b)

20

5

5 µΜ glucose

0

0 200

400 600 Time/s

20 µΜ glucose 10 µΜ glucose

10 5

20

S3 S2 S1

(d)

15 R 2 =0.9945

10

5 µΜ glucose

5

0

0

200

180 80 120 40 Glucose Concentration/µM

Current/µA

15

0

800

25 (c) 20

R2=0.9953

10

10 µΜ glucose

10 5

15

20 µΜ glucose

15

Current/µA

Current/µA

20

Current/µA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400 600 Time/s

800

0

40 80 120 160 Glucose concentration/µ M

Figure 8. I−t curves of IrO2@NiO/GCE by addition of healthy human saliva containing different concentration of glucose at 0.35 V. (a) The three responses to the saliva sample taken from one volunteer and (b) is the corresponding calibration curve. (c) The responses to the saliva samples taken from three volunteers and (d) is the corresponding calibration curve. In order to further explore the IrO2@NiO/GCE for practical applications, the sensor was applied to determine the glucose in human saliva in 30 diabetics and 10 healthy people, each clinical samples were maeasured by 3 different IrO2@NiO/GCEs. 500 µL of serum sample was added to 20 mL NaOH solution, and the current responses were recorded at +0.35 V. As can be seen in Figure 9, the response current of 10 healthy people is very close which are between 1-5.6


21


µA, while the corresponding current intensity of diabetics is obviously higher (>20.4 µA), what’s more, the as prepared IrO2@NiO/GCE can clearly distinguish blood glucose levels (after meals for 2 h) from range of 7-8 or >8 (clinical blood glucose data) through testing the glucose concentration in saliva. The results demonstrated here reveal the potential clinical applications of the IrO2@NiO/GCE for determination of glucose in biological fluids, especially in saliva.

270

Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

180

90

0 Health

7-8

>8

Figure 9. The responses of 3 different IrO2@NiO/GCEs to human saliva samples took from 30 diabetics and 10 healthy people. 4. CONCLUSION In this study, we demonstrate a facile two-step synthesis route of core-shell IrO2@NiO NWs (electrospinning of IrO2 NWs scaffold and in situ CBD synthesis of ultra-continuous NiO nanoflake shell). The NiO nanoflakes have been proved to be very thick and highly continuous growth on the surface of IrO2 NWs which with good electron transport ability. The as-prepared IrO2@NiO NWs as well as pure NiO nanoflowers and IrO2 NWs have been successfully used to construct enzyme-free biosensor based on GCE. Due to the introduction of IrO2, IrO2@NiO NWs shows much better sensing performance to glucose, such as a sensitivity of 1439.4 µAmM-


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1

cm-2, a wider linear range of 0.5 µM-2.5 mM, an low detection limit of 0.31 µM (signal-to-noise

ratio=3), good resolution in low glucose concentration, reproducibility, and long-term performance stability. What’s important, the as-prepared IrO2@NiO NWs sensor can clearly distinguish the diabetes from the healthy people and even the varying degrees of diabetic in clinic. Accordingly, as-prepared IrO2@NiO NWs bio sensor could be an extremely promising candidate applicable for low concentration glucose detection in complex environment.

ASSOCIATED CONTENT Supporting Information. Supporting Information is available free of charge on the ACS Publications website. The CVs peak current of the electrodes modified by different concentration of IrO2@NiO NWs (5 µL in ethanol) to the presentation of 5 mM glucose in NaOH solution. The CVs of IrO2@NiO/GCE as function of scan rates varying from 50-250 mVs-1. The CVs of IrO2@NiO/GCE as function of scan rates varying from 50-250 mVs-1. A comparison of the sensitivity, linear range, and detection limit of several typical non-enzymatic core-shell material based glucose sensors as well as IrO2@NiO/GCE sensor in this work.

AUTHOR INFORMATION Corresponding Author *L. Xu. E-Mail: [email protected]


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Present Addresses State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun, 130012, People’s Republic of China. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes Any additional relevant notes should be placed here.

ACKNOWLEDGMENT This work was supported by NSFC (Grant no. 81301289, 11374127, 11304118, and 21403084). The

Jilin

Province

Natural

Science

Foundation

of

China

(No.

20140101171JC,

20150520090JH).

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Table of Contents Graphic


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