Nonenzymatic Glucose Sensor Using Bimetallic Catalysts
Rashmi Ghosh Xiao Li Matthew Z. Yates
Bimetallic glucose oxidation electrocatalysts were synthesized
by two electrochemical reduction reactions carried out in series onto
a titanium electrode. Nickel was deposited in the first synthesis
stage followed by either silver or copper in the second ...
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PubMed Central (PMC)
1. Introduction

Fast and reliable detection of glucose is of great scientific and technological importance both in healthcare and in industrial analytical applications. The measuring of glucose concentration is important in the food and biotechnology industries as well as in clinic diagnostics, where monitoring glucose levels plays a critical role in treating diabetic patients. The most common glucose sensors use an electric current response from an electrode covered with a glucose oxidation catalyst. When an appropriate electric potential is applied to the electrode, the catalytic oxidation of glucose results in a measurable current that can be correlated to the glucose concentration. Traditionally, the glucose oxidation catalyst is an immobilized enzyme, but there is growing interest in developing nonenzymatic sensors.1 Enzymatic glucose sensors suffer from drawbacks due to denaturization of enzymes, which can occur due to variations in temperature, pH, and humidity.2−4 Nonenzymatic sensors have been developed using a wide range of electrocatalysts.5−11

The main advantage of nonenzymatic sensors is their robust stability under a range of storage and operating conditions that would destroy enzymes. Depending on how they are constructed, nonenzymatic sensors have the potential to also be of much lower cost than enzymatic sensors. The detection of glucose using nonenzymatic metal-based sensors via direct oxidation has its own set of challenges, however. Since the most important application is in glucose monitoring for diabetes treatment, the nonenzymatic sensors should ideally work on biological samples, such as blood plasma or whole blood. In addition, the sensors should provide accurate measurements for patients whose blood glucose level falls outside the normal range of 4.4–6.6 mM.12 Blood sugar concentration for healthy people ranges between 4.0 and 5.4 mM in fasting and up to 7.8 mM postprandial. Prediabetes is when values are between 5.5 and 6.9 mM in fasting and between 7.8 and 11.0 mM postprandial. Blood sugar levels are >7.0 mM in fasting and >11.1 mM postprandial for people suffering from diabetes. Accuracy over a wide range of glucose concentration is required as well as performance in the presence of numerous other biochemical compounds found in blood.

Some of the earliest nonenzymatic sensors used precious metal-based electrocatalysts. Besides being expensive, they have limited sensitivity and selectivity, surface poisoning from adsorbed intermediates, and interference from chloride ions.7,13 There is ongoing development of transition metal-based glucose sensors, with particular focus on nickel- and cobalt-based catalysts, to overcome the limitations of precious metal catalysts.1 Transition-metal catalysts have been created that have good stability, low detection limit, fast response, high sensitivity, and low cost.14−17 The lower selectivity of metal-based sensors with respect to glucose oxidase enzyme-based sensors is found to be offset by engineering the morphology and surface of the electrode. Here, we report on the development of novel bimetallic catalysts synthesized directly on the surface of a titanium sensor electrode. By carrying out short electrochemical reduction reactions, metal nanoparticles can be uniformly deposited on the electrode surface. A second electrochemical reduction reaction can then be carried out that deposits nanoparticles of a different type of metal on top of the first. The bimetallic film that is formed is composed of two types of transition-metal nanoparticles deposited separately and not an alloy. Nickel was chosen for the first layer deposition because it is one of the most widely studied glucose oxidation catalysts.1,18 For the second layer, copper and silver were chosen for investigation because of their known performance in catalyzing glucose oxidation alone or when coupled with other metals.9,18−20 The fabricated bimetallic thin film electrodes have high surface area and higher catalytic active sites and were found to have excellent electrochemical properties for use in glucose sensing.

3. Results and Discussion

3.1. Morphology and Composition of Coatings

The morphology and size of the nanoparticles in the single transition metal-based film or bimetallic composite film were studied in detail using scanning electron microscopy (SEM). The three samples, namely, Ni, Cu@Ni, and Ag@Ni, were synthesized using the previously described electrolytic deposition process at a constant current density. In all the experiments, a 25 mm × 25 mm sized platinum plate was used as the anode and an 8 mm × 8 mm sized titanium plate was used as the cathode. All of the electrochemical deposition reactions were carried out in a two-electrode system. Crystalline coatings of Ni with nanosphere-shaped particles were fabricated at varying reaction times, including, 2, 4, 6, 8, and 12 min at a reaction temperature of 95 °C under constant current density of 62.5 mA/cm2. As seen in , the size and distribution of the Ni nanoparticles varied with the reaction time. At the highest reaction time of 12 min, larger Ni crystals were formed sometimes overlapping each other with an increasing tendency of agglomeration at irregular spots on the surface of the coatings. At the lowest reaction time of 2 min, smaller distinct Ni crystals were formed which were spherical in the shape. The electrochemically deposited Ni crystals on the Ti plate acted as the cathode for a second stage deposition of Ag or Cu.

Coatings of Ni nanocrystals deposited for shorter reaction times had void regions without Ni at some places on the surface of the electrode which reduced the effective surface area and catalytic activity toward glucose molecules. At higher reaction times, electrodeposited Ni nanocrystals tended to agglomerate with materials being formed in the bulk, which reduced the effective outer surface area. Hence, for all of the second-stage deposition reactions, Ni coatings electrochemically deposited for a reaction time of 8 min on a Ti plate were chosen as the working electrode. The Ni coating deposited using the 8 min reaction time from here on will be termed sample Ni. Sample Ni was used as a cathode in the second-stage deposition of Cu or Ag crystals. The average diameter of Ni crystals in both the single metal and bimetallic samples was approximately 75 nm as measured from the SEM images.

For the sample Cu@Ni as shown in , copper (Cu) nanoparticles electrodeposited on Ni crystals showed a tendency to agglomerate at the surface of Ni. At higher copper concentration, these agglomerates formed a single nanostructure, whereas at an optimized low Cu concentration distinct Cu nanocrystals were observed on the surface. The diameter of an individual Cu nanoparticle averaged 30 nm, whereas the size of these agglomerated Cu@Ni nanostructures varied from 100 to 250 nm. For the sample Ag@Ni as shown in , Ag nanoparticles were deposited uniformly and distinctly all over the Ni crystals. The diameter of the Ag nanoparticles averaged 20 nm. The compositions of the individual as well as composite metallic coatings were studied using electron dispersive X-ray spectroscopy (EDX). Elemental composition was measured at three different positions of the sample, and the data were averaged. The Ni concentration was 20 wt % averaged across all the samples, whereas the amount of Ag and Cu was found to be approximately 5 and 4 wt %, respectively. The average elemental composition along with standard deviation in measurement is shown in Table 1. Additionally based on the individual elemental ion mapping images (the Supporting Information file), the transition-metal ions were found to be uniformly distributed on the surface of the Ti substrate.

Table 1

sampleNi (wt %)Ag (wt %)Cu (wt %)Ni18.59 ± 1.50nilnilAg@Ni14.80 ± 3.494.78 ± 1.69nilCu@Ni23.31 ± 0.84nil3.56 ± 0.46

Six samples each of Ni, Cu@Ni, and Ag@Ni were synthesized under the same conditions used for the samples analyzed in Table 1 and then removed from the Ti surface using an ultrasonic bath for XRD analysis. Dry metallic powder was collected by briefly heating the ultrasonicated solution containing metal nanoparticles at 80 °C. Ultrasonication was used to effectively remove the coatings from the Ti substrate; however, the possibility of breaking the nanoparticles in further small fragments remained. As the nanoparticles were heated for drying, oxide peaks were recorded in the XRD spectra. The XRD spectra of all three samples are shown in . The diffraction patterns of Ni, Ag, CuO, and Cu matched the standard reference peaks with ICDD card numbers of 03-065-0380, 01-077-6577, 41-0254, and 01-071-4607, respectively. The crystallite size of the metal nanoparticles may be calculated using the Debye–Scherrer relation

1

where D is the mean size of the crystalline domain, K is the dimensionless shape factor, λ is the X-ray wavelength, β is the line broadening at half the maximum intensity [full width at half-maximum(fwhm)], and θ is the Bragg angle.

For the sample Ni as shown in a, strong Ni diffraction peaks were seen at 44.40, 51.74, and 76.15°. These three peaks represented Ni crystalline planes (111), (200), and (220), respectively. Calculating the Ni crystallite size using the Debye–Scherrer relation for the (111) peak gave a result of 28.8 nm which was much smaller than the nanoparticle size obtained from SEM images (75 nm). The difference in size is possibly due to nanoparticles being polycrystalline. For the sample Cu@Ni, the Ni peaks were seen at 44.46 and 51.80° as shown in c representing the crystalline planes (111) and (200), respectively. Strong diffraction peaks were also recorded at 38.10 and 64.95° due to the CuO crystalline planes of (111) and (022), respectively. Observation of CuO peaks instead of Cu is due to the heating of the samples at 80 °C. The (111) crystalline plane of Cu is located at 43.379° which is located near the (111) plane of Ni and the two peaks are probably convoluted, leading to the broadening of fwhm (β) for the peak located at 44.46°. Hence, the XRD spectrum peaks were studied as a qualitative measurement and not as a quantitative measurement of the crystallite size. Nanoparticle sizes were successfully measured from SEM images. In b for the sample Ag@Ni, the Ni peaks were observed at 44.42 and 51.81° representing the crystalline planes (111) and (200), respectively. Strong diffraction peaks were also recorded at 38.06, 64.82, and 77.85° representing the crystalline planes (111), (220), and (311), respectively, for the Ag nanoparticles. The (200) crystalline plane of Ag is located at 44.599° which is near to the (111) plane of Ni. Here also, there was a possibility that the two peaks were convoluted, leading to the broadening of fwhm (β) for the peak located at 44.42°.

The chemical states of the metal atoms near the surface were investigated in detail using X-ray photoelectron spectroscopy (XPS) (the Supporting Information file). The results confirmed the presence of metallic Ni in all three samples. Metallic copper was detected in the Cu@Ni sample, along with copper oxide. Metallic Ag was detected in the Ag@Ni sample. The XPS data confirm the XRD results.

3.2. Glucose Oxidation: Electrocatalytic and Electrokinetic Activity

The electrocatalytic activity of bare Ti, Ni, Cu@Ni, and Ag@Ni toward glucose oxidation was examined by CV in a 0.1 M NaOH aqueous solution at a scan rate of 10 mV/s. CV experiments were conducted between −0.6 and 0.7 V using glucose concentrations of 0, 1, and 3 mM. An additional CV was conducted for Ag@Ni between −0.6 and 1.1 V ( e) using glucose concentrations of 0, 1, and 3 mM to allow the completion of the anodic scan of the Ag@Ni electrocatalyst. As shown in a, bare Ti showed a small oxidation peak that was independent of the glucose concentration. Bare Ti was inert and acted as a control for all the reactions. All the three other samples showed oxidation peaks that increase with the glucose concentration, indicating catalytic activity due to glucose oxidation.

The transition metal-based nonenzymatic glucose sensor depends on the transition metal/oxide surface getting activated in the presence of hydroxide ions in a basic environment to act as a catalyst for glucose oxidation. The three transition metals concerned in our study are nickel (Ni), copper (Cu), and silver (Ag), forming glucose sensor materials, namely, Ni, Cu@Ni, and Ag@Ni. Under the alkaline conditions of our experiment, metallic Ni in the presence of hydroxide ions is expected to get transformed to Ni(OH)2 which further reacts with hydroxide ions to form nickel oxyhydroxide (NiOOH) at 0.54 V during the anodic scan.18,22 This NiOOH intermediate generated acts as an electrocatalyst for the oxidation of glucose to gluconolactone and then itself gets reduced back to Ni(OH)2 during the cathodic scan.23−25

2

3

In regards to the catalyst Cu@Ni, copper (Cu) at atmospheric conditions gets oxidized to copper oxide (CuO).26 Besides Ni transforming to Ni(OH)2, CuO reacts with the water to form Cu(OH)2 which then further changes to CuOOH forming a combined electrocatalyst of CuOOH/NiOOH. Glucose gets oxidized to gluconolactone in the presence of CuOOH/NiOOH as the electrocatalyst.27,28

4

5

6

In regard to the electrocatalyst Ag@Ni, glucose is oxidized in two steps in the presence of Ag-based electrocatalysts. As shown in e, during the anodic scan Ag nanoparticles in Ag@Ni (1 mM glucose curve) form Ag2O/AgOH in the presence of hydroxide ions during the first peak at 0.62 V and then Ag2O/AgOH changes to AgO during the second anodic peak at 0.80 V.29,30 Similarly, Ni(OH)2 forms the intermediate NiOOH. The AgO/NiOOH intermediate then acts as an electrocatalyst for the oxidation of glucose to gluconolactone which is further oxidized to gluconic acid. During the cathodic scan at peak locations of 0.34 and −0.05 V, Ag2O was regenerated from Ag and then Ag was again regenerated from Ag2O.31−34 The peak identifications are consistent with the mechanism of silver nanoparticle-catalyzed glucose oxidation described by Poletti Papi et al.29

7

8

9

10

11

12

In the presence of glucose, all three catalysts Ni, Cu@Ni, and Ag@Ni exhibited higher anodic redox peak current (Ipa) and the observed redox anodic peak potential (Epa) resembled their respective redox units of Ni3+, Ni3+/Cu3+, and Ni3+/Ag2+, validating the effective participation of the active catalytic centers of the electroredox couples toward oxidation of glucose. The increase in the anodic current due to an increase of glucose concentrations from 0 to 3 mM establishes the effective electrocatalytic activity of all the three catalysts Ni, Cu@Ni, and Ag@Ni.18,24,35

The high effective surface area of all the samples with 75, 30, and 20 nm sized nanocrystals (based on SEM images) was a major driving force behind the excellent electrocatalytic activity toward glucose oxidation. In this study, we used geometric area of the electrode for calculating the performance of the electrodes toward glucose oxidation. In future work, we measure the ECSA. Since, the electrodes were made of thin films of nanostructured transition metals, the ECSA is likely higher than the geometric surface area. In the case of bimetallic thin film deposition, due to the second set of nanoparticles deposited on the surface of Ni nanocrystals, the effective surface area is expected to increase and enhance the number of active catalytic sites. In a later discussion, it will be shown that the overall electrocatalytic performance of bimetallic catalysts exceeds that of Ni alone.

To understand the electrokinetics of the catalytic glucose oxidation system for all the three samples Ni, Cu@Ni, and Ag@Ni, a set of CV measurements were recorded with the scanning speed ranging from 10 to 100 mV/s in increments of 10 mV/s in the presence of 1 and 3 mM glucose (the Supporting Information). As expected, the redox peak current density increased with an increase in the scan rate. In addition, the peak-to-peak separation between oxidation and reduction peaks increased with the scan rate, and the ratio of oxidation/reduction real current was not unity. The CV results indicate that the reaction is partially irreversible or quasi-reversible.36,37 The CV data at various scan rates were converted to a Randles–Sevcik plot of peak current versus the square root of the scan rate as shown in for each of the three catalytic electrodes. For a surface electrochemical reaction that is diffusion controlled, it is expected to follow the Randles–Sevcik equation

13

where, D0 is the diffusion coefficient of the analyte molecules, A is the area of the electrode, C0* is the concentration of the analyte molecules that diffuses, α is the transfer coefficient, and ν is the potential sweep speed.38 The excellent linear fit of the data in is strong evidence that the electrocatalytic process is controlled by the diffusion of glucose molecules into the electrode/electrolyte interface.39−42

For all three catalysts Ni, Cu@Ni, and Ag@Ni with an increasing glucose concentration from 0, 1, to 3 mM, the anodic peak current was also found to increase. For our diffusion-controlled process, as more glucose molecules are present in the solution in a high glucose concentration, higher number of glucose molecules diffuses to the electrocatalyst surface requiring a higher number of redox units of Ni3+, Ni3+/Cu3+, and Ni3+/Ag2+ to act as electrocatalysts for glucose oxidation. From this increase in the anodic peak current with an increasing glucose concentration, we can conclude a successful formation of the redox units, like, Ni3+ from Ni2+ and participation of Ni3+ in the catalytic oxidation of glucose.43,44

3.3. Glucose Oxidation: Amperometric Measurement

For glucose concentration measurement, amperometry is typically used where the electrode is held at a fixed potential, and current response is correlated to the glucose concentration. This requires amperometric calibration of the electrodes against solutions of a known glucose concentration. A series of amperometric measurements were taken to calibrate each of the three types of electrodes (the Supporting Information). The applied fixed potential was +0.54, +0.62, and +0.62 V vs Ag/AgCl for Ni, Cu@Ni, and Ag@Ni. Before the addition of glucose to the NaOH solution to conduct amperometry experiments, the electrodes were stabilized in 0.1 M NaOH solution at the fixed activating potential for 10 min. A quasi-stationary current of 28.52 μA/cm2 (±0.51 μA/cm2), 79.77 μA/cm2 (±8.75 μA/cm2), and 275.76 μA/cm2 (±14.92 μA/cm2) was recorded for Ni, Cu@Ni, and Ag@Ni, respectively, at the end of the stabilization period at a zero glucose concentration. The quasi-stationary current was determined by averaging the current response in the last 30 s of the stabilization period. After stabilization and blank current measurement, the current response was measured in the same 60 mL of 0.1 M NaOH as a 120 mM glucose stock solution was added incrementally. After each addition of glucose, the solution was stirred for 40 s, followed by a 260 s stabilization period. The current response to a particular glucose concentration was obtained by averaging the current readings in the last 30 s of the stabilization period. shows the resulting calibration curve for each of the three types of electrodes with the glucose concentration in mM as the x axis and the current response in mA/cm2 (based on geometric surface area of the electrode) as the y axis.

The calibration curves in were used to calculate the limit of detection, sensitivity, and linear range of response. The linear response range was defined as the bracketed glucose concentration range for which a linear regression could accurately represent the glucose concentration versus the current response. Sensitivity was calculated as the slope of the linear regression. The detection limit was calculated using the formula: (3 × sd)/S (where, sd is the standard deviation of the blank signal and S is the slope of the calibration curve). Table 2 compares the figures of merit for all three samples Ni, Cu@Ni, and Ag@Ni. The samples Ni, Ag@Ni, and Cu@Ni exhibited a linear range of 0.2–1.8, 0.2–6.4, and 0.2–12.2 mM, respectively, with a high coefficient of determination of 0.983, 0.995, and 0.995, respectively. Cu@Ni showed the highest sensitivity of 420 μA/(mM cm2) and the lowest for Ni with a value of 110 μA/(mM cm2). The detection limits for Ni, Cu@Ni, and Ag@Ni were 14, 62.5, and 140 μM, respectively. The linear range of response as well as the sensitivity improved significantly for the bimetallic samples Cu@Ni and Ag@Ni in comparison to Ni. The sensitivity of Ag@Ni and Cu@Ni was 3× and 4× higher than that of Ni due to higher active catalytic sites in the bimetallic samples. The linear range of response for Ni was 0.2–1.8 mM of the glucose concentration. The upper limit of the linear response range increased from 1.8 to 6.4 and 12.2 mM for Ag@Ni and Cu@Ni, respectively. The linear response of the Cu@Ni electrode spans the entire normal glucose concentration range in human blood, and the upper limit of 12.2 mM is high enough to encompass warning levels indicating prediabetic and diabetic conditions.

Table 2

samplelinear range (mM)sensitivity detection limit (μM)Ni0.2–1.811014.0Ag@Ni0.2–6.4320139.9Cu@Ni0.2–12.242062.5

3.4. Comparison to Other Nonenzymatic Sensors

The literature review on nonenzymatic electrochemical glucose sensors by Hwang et al. shows that noble metal, nonprecious transition metal/metal oxides, and metal alloy/composite catalytic materials have been designed and developed for electrochemical glucose sensing.15 Noble metal-based electrodes show a high linear range of response toward glucose oxidation covering the human blood sugar levels of 2–8 mM; however, they showed poor sensitivity and were easily poisoned by interfering molecules besides being expensive. The nonprecious transition-metal counterpart had good sensitivity as well as anti-interference properties. However, the majority of the transition metal-based glucose sensing electrodes have a linear range of response that does not sufficiently cover human blood glucose concentrations. Furthermore, extremely high sensitivity in the case of some of the transition metal/metal oxide electrodes was due to high surface area substrates in the form of foam or porous substrates.15 In this work, a broad linear response of 0.2–12.2 mM was achieved with a sensitivity of 420 μA/(mM cm2) for the sample Cu@Ni. In one of the research articles, Anu Prathap et al. prepared CuO nanoparticles in the presence of tartaric acid/citric acid/amino acid and achieved a linear range of response 0.9–16.0 mM for glucose oxidation. However, during the amperometric experiment, the author modified a Pt electrode with the prepared CuO. There was no discussion if the underlying Pt electrode played a role in the high linear range recorded. Additionally, the author reported a sensitivity of only 9.02 μA/mM.45 Similarly, in another work, Subramanian et al. deposited rGO/Ni(OH)2 composites on Au electrodes to get a linear range of response of 15 μM–30 mM with a sensitivity of 11.4 mA mM–1 cm–2. However, here also there was no discussion if the underlying gold substrate played a role in the high value of figure of merits.46 In another work by Zhang et al., the author started with a Cu–Zr–Ag ingot and developed metallic glass ribbons by melt spinning followed by dealloying and other procedures, like anodizing, to finally form nanowires on nanoporous substrates. No detailed discussion was carried out about the dimension of the nanoporous substrate and how that might impact the result of a linear range of glucose detection of up to 15 mM with a sensitivity of 1310 μA/(mM cm2).47 A glucose oxidase-based enzymatic glucose sensor, Gox/Au–ZnO/GCE, prepared by Fang et al. recorded a linear range of response of 1–20 mM with a sensitivity of 1.409 μA/mM.48 Here also, in addition to enzyme, gold was used in the electrode. Jeong et al. prepared another enzymatic sensor, Gox/3D MoS2/graphene aerogel, with a linear range of glucose detection of 2–20 mM and a sensitivity of 3.36 μA/mM.49 In a recent review, Sehit and Altintas tabulated the performance of an enzyme-based glucose sensor with a widest linear range of glucose detection reported as 0–25 mM and a highest value of sensitivity noted as 289 μA/(mM cm2) for a different electrode material.50 Additionally, a very recent review of copper-based glucose sensors reported figures of merits from the literature reports of 520+ sensors.51 From this list, we find only eight studies that reported a higher upper limit of linear response range to the glucose concentration while maintaining a greater sensitivity than our best catalyst (Cu@Ni; linear response range: 0.2–12.2 mM and sensitivity: 420 μA/(mM cm2). Our work also has the added advantage of a simple low-cost preparation of bimetallic catalysts and sensors. There were no expensive materials involved in the catalyst synthesis, and the resulting catalyst-coated solid titanium plate is used directly as the sensing electrode without requiring other materials to facilitate electrode transfer during the glucose oxidation reaction.

3.5. Glucose Oxidation: Selectivity, Stability, and Reproducibility

One of the important parameters to be considered for fabricating a sensor material for the catalytic oxidation of glucose is its ability to eliminate the interfering responses generated by the species with similar electroactivity as that of the target analyte. In this work, for the Ni, Cu@Ni, and Ag@Ni samples, the selectivity of glucose was tested in the presence of the common interferents, including ascorbic acid (AA), uric acid (UA), dopamine (DA), lactose, maltose, fructose, and galactose. The experiments were carried out at fixed applied potentials of +0.54, +0.62, and +0.62 V for Ni, Cu@Ni, and Ag@Ni, respectively. The changes in the current response after the addition of the glucose and interferent solutions were studied. After each addition of glucose or an interferent, the solution was stirred for 40 s followed by 260 s of the stabilization period. The current response to a particular chemical addition was obtained by averaging the current response readings in the last 30 s of the stabilization period. After the initial stabilization of 10 min, 1 mM glucose was added to 60 mL of 0.1 M NaOH followed by 0.2 mM of each of the interferents every 300 s, and at the end, another 2 mM glucose was added. From a, it was seen that for Ni, the current response after the addition of 1 mM glucose was 0.37 mA and the current increased to only 0.40 mA after addition of all the seven interferents with each constituent’s concentration being 0.2 mM. After the addition 2 mM glucose at the end there was no significant increase in the current response for the sample Ni. The electrode material sample Ni surface was positioned and deactivated in the presence of all the interfering chemicals and did not respond to the glucose molecules added at the end of the reaction. In the case of Cu@Ni, ( c) the current was stabilized to 0.45 mA after addition of the initial 1 mM glucose and the current increased at an average rate of 10% after the addition of each interferent. Further work on improving the selectivity of Cu@Ni needs to be pursued. Here, the Cu@Ni material electrode was fully functional even after adding all the interferents, and the current increased to 1.93 mA after the addition of 2 mM glucose at the end. The selectivity toward catalytic glucose oxidation for the sample Ag@Ni was recorded as shown in b. The initial current after 1 mM of glucose addition was measured as 0.62 mA and the average current increment after the addition of each of the interferents was only 2%. Ag@Ni was fully stable and functional until the end of the selectivity test and recorded an increased current response of 1.44 mA at the end after the addition of 2 mM glucose. The selectivity performance of Ag@Ni was attributed to the fine uniform deposition of Ag nanoparticles on the surface of Ni nanoparticles and to the individual material properties of Ni and Ag.

The reproducibility of each of the electrode samples was tested by fabricating four replicates of each of Ni, Cu@Ni, and Ag@Ni and then measuring the anodic oxidation current at +0.54, +0.62, and +0.62 V, respectively, from the oxidation of 1 mM of glucose in 60 mL of 0.1 M NaOH solution. The mean anodic oxidation current was calculated, along with the standard deviation. The relative standard deviations for Ni, Cu@Ni, and Ag@Ni were 9.67, 6.47, and 5.12%, respectively.

The stability of Ni nanospheres electrodeposited on the Ti surface as well as the stability of bimetallic nanocrystalline Cu@Ni and Ag@Ni after repeated CV experiments in the presence of 1 mM glucose in 0.1 M NaOH was studied using ICP-MS analysis. The ICP-MS measurements were conducted for the Ni, Cu, and Ag ions that leached out into NaOH solution after the repeated 25 CV cycles at 50 mV/s between −0.6 and 1.1 V. Three replicates of each of the samples were tested along with a control experiment where CV was done with bare Ti without any material deposited on its surface. Low amount of metal ions leached into the solution as shown in the Table 3 validating the stability of the materials deposited on the surface of Ti even after long experimental runs.

Table 3

sampleNi conc. (ppb)Ag conc. (ppb)Cu conc. (ppb)control3.55 ± 0.101.15 ± 0.3613.34 ± 1.78Ni3.58 ± 0.301.68 ± 1.0915.02 ± 0.16Ag@Ni2.95 ± 0.1317.44 ± 4.5115.73 ± 1.04Cu@Ni3.46 ± 0.081.49 ± 0.8141.91 ± 4.45