A simple and environmentally‐friendly approach was developed to synthesize 2D CuO nanosheets using electrochemical deposition. The formed 2D CuO nanosheets (NSs) exhibit numerous advantageous properties such as no toxicity, high electrical conductivity, large active surface area, and a p‐type semiconducting nature with a band gap of 1.2eV. A sensitive electrochemical sensor was constructed for the amperometric detection of glucose to take advantage of these characteristics. The fabricated sensor displayed an excellent sensitivity of 2710μAmM−1cm−2along with a wide linear range of 0.001–1.0mM and a lower limit of detection of 0.8μM (S/N=3). Additionally, the modified electrode possesses high selectivity and good stability. The outstanding electrocatalytic performance of the electrode is attributed to a large active surface area, unique structural morphology, and the high conductivity of the 2D CuO nanosheets.
]]>Glucose is one of the major sources of energy for humans and is a fundamental part of blood serum [1]. The standard level of body glucose in a healthy person ranges from 3.9-6.1 mM, while levels higher than 6.1 mM are indicative of diabetes mellitus [2]. Diabetes is caused by an increase in blood glucose concentration (hyperglycemia) that originates from a lack of insulin secretion that decreases glucose metabolism [3][4]. For the clinical diagnosis and management of diabetes, controlling glucose levels is essential [5]. For this reason, the fast, accurate, sensitive, and selective detection of blood glucose is important for the diagnosis and management of diabetes.
Electrochemical sensors are commonly classified as enzymatic and non-enzymatic sensors. Due to their high sensitivity and selectivity, enzymatic sensors have received much research attention. However, enzymes suffer from several drawbacks including the instability of enzymes, sensitivity associated with environmental factors, relatively high cost, the need for complex immobilization procedures and electron mediators, non-recyclability for subsequent analysis due to deposition of serum proteins and interferent species in blood serum [2,10]. To overcome these shortcomings, non-enzymatic electrochemical sensors are a promising alternative for the detection and measurement of glucose [9][10][11].
In the last decade, noble metals in the form of Ag nanoparticles (NPs) [12][13], Au NPs [14][15][16], Pt NPs [17][18][19], Pd NPs [20][21], and noble metal alloys [22][23][24] have been employed for the fabrication of glucose sensors because they have high conductivity and excellent catalytic activity. Nonetheless, these materials based on noble metals possess several disadvantages including poisoning of Au and Pt by the absorption of intermediates and chloride ions, which impedes electron transfer and ultimately reduces the sensitivity and selectivity of the sensors. These sensors also possess low working stability and are relatively expensive [2][3]4,25]. For these reasons, there is great interest in developing non-enzymatic glucose sensors based on earth-abundant and low-cost transition metal oxides.
Currently, several low-cost transition metal oxides such as Fe 2 O 3 [26][27], Mn x O x [6,28], MoO 2 [29], CdO [30], NiO [1][2][31][32], ZnO [33][34], and CuO [25,35] have been studied for use in non-enzymatic glucose sensors. Among all of these metal oxides, CuO has been widely explored as a catalytic material for different applications including semiconductors, supercapacitors, batteries, solar energy conversion, gas sensing devices, catalysis, biosensors, and transistors because it is a p-type semiconductor with a band gap of 1.2 eV, and it exhibits a low overpotential for electron exchange, excellent electrocatalytic activity, high active surface area, good chemical stability, and biocompatibility. Additionally, CuO is nontoxic, inexpensive, and abundant [25,[35][36].
The particle size and shape of materials significantly affect their electrocatalytic activity [25]. CuO nanomaterials of various shapes have been designed and studied for glucose sensing applications including nanospheres [48], nanorods [49], nanowires [39][40], nanocubes [41], nanosheets [42][43], nanofibers [44], nanoflowers [25,45], and octahedra [46][47]. In addition, the fabrication of electrodes for non-enzymatic sensors by a drop casting process through the help of costly binder polymer solutions possesses some disadvantages. First, polymer binder solutions may block the electroactive sites of the catalytic materials, and accordingly decrease the catalytic activity of the materials. Second, catalytic materials can desorb off of the electrode during the electrochemical reactions. Third, the binder solutions significantly decrease the electrical conductivity of the electrode. To overcome these issues, sensor electrodes designed by direct deposition of catalytic materials on the electrode surface with well-defined morphological shape are of great interest for the nonenzymatic detection of biomolecules [48].
In this work, we report non-enzymatic electrochemical sensors based on 2D CuO nanosheets electrochemically deposited on Pt foil for the detection of glucose. The synthesized materials were characterized by FE-SEM, HR-TEM, XRD, and cyclic voltammetry techniques. The fabricated 2D CuONSs@Pt foil sensors exhibit high sensitivity, excellent selectivity, and stability with good reproducibility due to the morphology of the 2D nanostructures and large active surface area of the CuO NSs. The effects of different synthetic parameters on the morphology and catalytic activity were also investigated. Generally, it was found that the size, morphology and electrocatalytic performance strongly depend upon the concentration of Cu 2+ ions, deposition time, deposition potential, and solution pH.
Cupric nitrate trihydrate [Cu(NO 3 ) 2 .3H 2 O] (SD-Fine chem India), sodium hydroxide (SD-Fine chem India), H 2 O 2 , ethanol, ascorbic acid, dopamine, uric acid, KCl, NaCl, and all other chemicals were of analytical grade and used without additional purification. All of the solutions and reagents were prepared using ultra-pure water.
Electrochemical experiments were conducted using a Potentiostat-galvanostat DY2300 series Potentiostat electrochemical workstation (Austin, TX, USA) with a conventional three-electrode system. The CuO@Pt foil (3×3 mm 2 ) was used as the working electrode, a glassy carbon electrode (GCE, 3 mm) was used as the auxiliary electrode, and a saturated calomel electrode (SCE) was used as the reference electrode, respectively. All potentials in this manuscript are reported versus SCE. Materials characterization was performed using an HR-transmission electron microscope (HR-TEM, JEOL, JEM 2100F) operated at 10 kV and a field emission gun scanning electron microscope (FEG-SEM, JSM-7600F) operated at 10 kV. The X-ray diffraction (XRD) analysis was performed with a PANalytical, Netherlands (k α Cu -1.54184 A o ) instrument.
Commercially purchased Pt foil (3×3 mm 2 ) was thoroughly polished with a slurry of 0.05 µm alumina powder and water. The Pt was then successively sonicated in 0.1 M HCl, deionized water, and absolute ethanol for 5 min each. A precursor Cu(NO 3 ) 2 solution (10 mM) was placed in two 100 cm 3 of beakers. 0.1 M NaOH was slowly added to one beaker with constant stirring, and 0.1 M HNO 3 was slowly added to the other beaker to form Cu(OH) 2 and Cu(NO 3 ) 2 , respectively. The precursor solutions were then vigorously stirred for an additional 15 min to avoid agglomeration and ensure uniform mixing. Cleaned Pt foil was then immersed in each of the two beakers and connected to the -ve terminal (cathode) of a DC power supply. At the same time, a stainless steel rod (2 mm) was connected to the +ve terminal (anode) of the power supply. Both the electrodes were kept at a distance of approximately 2 cm. A constant potential of 1.5 V was applied to deposit CuO on the Pt electrode. The CuO-modified electrode was then rinsed with deionized water and dried in oven at 150 o C. The electrode was then used for electrochemical studies. An overview of the electrodeposition process of the 2D CuO NSs and their electrochemical sensing application for glucose is displayed in Scheme 1.
The simultaneous electrochemical exfoliation and deposition of CuO nanostructures on Pt foil is a straight forward approach to design modified electrodes for glucose sensing. The morphology of the synthesized nanostructure was examined by FE-SEM and FE-TEM scanning microscopic techniques. When synthesized under alkaline conditions (0.1 M NaOH), large agglomerates of CuO nanoparticles were observed (Fig 1A). However, when CuO was synthesized in acid (0.1 M HNO 3 ), sheet structures were formed instead of agglomerates (Fig. 1B). A high-resolution TEM image indicates that a large amount of 2D CuO sheets are uniformly spread over the surface of the Pt electrode and that the nanosheets are about 10-15 nm in size (Fig. 1C).
X-ray diffraction (XRD) was used to further interrogate the structure of the synthesized nanomaterials (Fig. 1D). Sharp and intense diffraction peaks observed at 2θ = 32. where, D, K, λ, 𝛽, and θ are the crystal size (nm), Scherer constant, X-ray wavelength (nm), half width full maxima (radians), and angle of diffraction (degree), respectively. From this equation, the crystal size was calculated to be 14.73 nm, which matches well with the 10-15 nm particles observed in the HR-TEM image.
Scheme1.Synthesis of 2DCuO NSs and application for electrooxidation of glucose.
The electrochemically active surface area (EASA) of Pt electrode modified with CuO NSs was determined by measuring cyclic voltammograms (CVs) in 20 mM K 3 [Fe(CN) 6 ] and 0.1 M KCl at a scan rate of 50 mVs -1 . CVs for bare Pt and Pt modified with CuO nanosheets are shown in Fig. 2. The active surface area was calculated using the Randles-Sevcik equation given below [51]. I p = 2.69×10 5 × n 3/2 ×A×D 1/2 ×v 1/2 ×C (n=1, D=0.76 ×10 -5 cm 2 s -1 , v=0.05 Vs -1 )............(eq. 2)
where, I p , n, A, D, v, and C are the peak current (µA), number of electrons transferred in the redox reaction, electrode area (cm 2 ), diffusion coefficient (cm 2 s -1 ), scan rate (mVs -1 ), and concentration of the analyte (mM), respectively. The active surface areas of the bare and CuO-modified-electrodes were found to be 0.2755 cm 2 and 0.4768 cm 2 , respectively. In contrast, the geometric surface area of the bare Pt foil electrode is only 0.09 cm 2 . These results demonstrate that the active surface area of the Pt foil electrode after electrochemical deposition of CuO nanosheets is greatly increased, which is advantageous to accelerate the glucose oxidation reaction.
Fig. 3 illustrates cyclic voltammograms of bare Pt foil and 2D CuONSs@Pt foil in the presence (Fig. 3A) and absence (Fig. 3B) of 2 mM glucose in 0.1 M NaOH as a supporting electrolyte solution at a potential range of -0.2 V to 0.8 V at a scan rate of 50 mVs -1 . Notably, bare Pt foil exhibits no redox peaks in the absence of glucose (Fig. 3B, black line). However, a well-defined peak is seen at 0.48 V for the 2D CuONSs@Pt foil electrode (Fig. 3B,red line). This peak is attributed to the reduction of Cu(III) to Cu(II). The corresponding oxidation of Cu(II) to Cu(III) does not result in a peak because it is masked due to the simultaneous O 2 evolution reaction from water splitting [47]. Upon addition of 2 mM glucose to the electrolyte, oxidation peaks are observed at 0.42 V for the 2D CuONSs@Pt foil electrode (acidic medium) with increased oxidation current as compared to bare Pt and the Pt modified with CuNPs (alkaline medium, Fig. 3A).This difference in the current density of the oxidation peaks is due to the difference in morphology of the CuO nanomaterial. The agglomerated particles when the CuO is synthesized in base result in a decreased surface area of the catalytic material. In contrast, when the CuO is synthesized in acid, 2D nanosheets are formed, which increase the electrochemical active surface area and correspondingly increase the rate of the glucose oxidation reaction. The redox mechanism of the 2D CuO nanosheets and glucose is displayed in the following equations. In the first step, Cu(OH) 2 is oxidized to CuOOH. In the second step, CuOOH oxidizes glucoses, regenerating Cu(OH) 2 in the process (36).
Cyclic voltammograms of the 2D CuONSs@Pt foil electrode were performed at various scan rates ranging from 2 mVs -1 to 100 mVs -1 in 0.1 M NaOH solution (Fig. 3C). The oxidation current gradually increases with increasing sweep rate as expected from the Randles-Sevcik equation. The oxidation current follows a good linear fit (R = 0.9986) with I pc (µA) = 1166.12 + 994.12v (mVs -1 ) 1/2 (Fig. 3D). The linear relation between oxidation current and square root of scan rate indicates that the electro-oxidation reaction of glucose follows a diffusion-controlled process (1).
To optimize the performance of the glucose sensor, several different experimental parameters were varied. First, the effect of Cu 2+ concentration, ranging from 2 mM to 30 mM, used to synthesize the CuO NSs was studied. The CVs of glucose oxidation with Pt electrodes modified with CuO synthesized at various Cu 2+ concentrations are shown in Fig. 4A, and the figure inset illustrates the relationship between the peak oxidation current and the Cu 2+ ion concentration. At a Cu 2+ concentration of 10 mM, the CuO consists of welldeveloped 2D nanosheets, which enhance the rate of electron transfer, resulting in greater anodic current. When a higher concentration of Cu 2+ is used to synthesize the CuO, agglomeration of CuO occurs during deposition. This agglomeration decreases the number of active sites, which decreases the reaction rate (Fig. S1). Therefore, a Cu 2+ concentration of 10 mM was selected as an optimized concentration for further investigation in this manuscript.
The deposition time also impacts the morphology of the CuO nanostructures. For this reason, CuO was deposited using the optimized 10 mM Cu 2+ concentration at various times, and the current responses of the glucose oxidation reaction were measured (Fig. 4B). Initially, the oxidation current of glucose was significantly improved when increasing the deposition time from 1 to 2 mins due to rapid electron transfer and catalytic activity of 2D CuO nanosheets. However, the anodic current decreases as the deposition time increase from 2 to 3 mins because longer deposition time increases agglomeration (Fig. S2), which inhibits the electron transfer rate and decreases the electrochemically active surface area of the catalyst. Thus, 2 mins was chosen as optimized time for deposition of the CuO nanosheets.
The influence of applied potential ranging 0.5 V to 2.5 V on the deposition during the synthesis of the CuO nanosheets was examined using the optimized 10 mM Cu 2+ concentration for 2 mins of deposition time in acidic medium. The corresponding CVs of the modified electrodes are shown in Fig. 4C. The inset of Fig. 4C demonstrates the relationship between the oxidation current and deposition potential. The current signal of the glucose oxidation reaction rises appreciably as the deposition potential increases from 0.5 V to 1.5 V. At potentials more positive than 1.5 V, however, the current response gradually decreases. This trend is attributed to the formation of good morphology at 1.5 V, but at high potential there might be overlapping of CuO material (Fig. S3) and evolution of H 2 or O 2 , which interferes in the deposition of CuO nanomaterials and decreases active binding sites and ultimately affects the reaction rate. Therefore, 1.5 V was selected as the optimal potential for the deposition of CuO nanosheets.
The effect of concentration on the glucose oxidation reaction was studied in 0.1 M NaOH solution. Fig. 4D displays voltammograms in electrolytes containing various glucose concentrations. The oxidation current linearly increases with increasing concentration of glucose. Fig. 5A shows the linear plot of the anodic current response versus concentrations of glucose. The 2D CuONSs@Pt exhibits good linearity from 0.001 mM to 1.0 mM and a high sensitivity of 2710 µA.mM -1 .cm -2 which follows the equation I pa (µA) = 3010.64 + 1292.11x (mM) (R = 0.998). The detection limit was found to be 0.8 µM.
The electrochemical performance of the sensors compared with previously reported non-enzymatic glucose sensors are shown in Table 1. The electrochemical sensor developed in this work possesses a very high sensitivity, broad linear range, and good limit of detection compared to other works. These excellent metrics are attributed to the large active surface area as well as the 2D morphology of the synthesized nanomaterial.
Anti-interference properties of non-enzymatic glucose sensors are of great importance because other species such as ascorbic acid (AA), uric acid (UA), dopamine (DA), hydrogen peroxide (H 2 O 2 ), and Na + are also present in blood serum and might interfere in the amperometric detection of glucose. Thus, the amperometric response of 0.5 mM glucose was measured with successive additions of the possibly interfering species AA, UA, DA, H 2 O 2 , and Na + at a concentration of 0.5 mM each in 0.1M NaOH. With all of these species, there is no significant change in the oxidation current of glucose (Fig. 5B). Consequently, the fabricated 2D CuONSs@Pt foil sensor exhibit excellent anti-interference ability due to the strong electrostatic repulsion force between negatively-charged CuO catalyst at the electrode surface and predominantly negatively-charged interfering species [35].
In addition, the reproducibility and stability properties of glucose sensors are also essential for practical applications. The reproducibility of the 2D CuONSs@Pt foil sensor was determined by measuring the current response of the five modified electrodes through same procedure at room temperature for 2 mM glucose solution (Fig 5C). The calculated value of relative standard deviation (RSD) for the oxidation current was 2.57%. To evaluate long-term stability of the sensor, the modified electrode was stored at room temperature for three weeks, and the current response measured was similar before and after storage. Specifically, the anodic current signal retained 96% of its original current (Fig. 5D). The above results demonstrate that the modified 2D CuONSs@Pt foil sensor possesses excellent reproducibility and stability.
In this manuscript, Pt electrodes modified with 2D CuO nanosheets were fabricated by a fast, simple, inexpensive, and stabilizer-free galvanostatic method that induced the direct electrooxidation of Cu(NO 3 ) 2 on Pt foil. Moreover, numerous parameters including Cu 2+ ion concentration, deposition potential, deposition time, and pH of the precursor solution were investigated to understand how they affect the growth and morphology of the CuO nanosheets. The materials were characterized by FE-SEM, HR-TEM, XRD, and CV. The materials were then used to fabricate non-enzymatic glucose sensors that exhibit fast response times, high sensitivity, excellent selectivity, and good stability under a wide linear concentration range. These results are ascribed to the 2D nanostructure, which possess large electrochemical surface areas. Taken together, these findings demonstrate that 2D CuO nanosheets are a promising electrocatalytic material for the practical non-enzymatic detection of glucose.