Copper nanoparticles/polyaniline/molybdenum disulfide composite as a nonenzymatic electrochemical glucose sensor

Abstract

A Cu@Pani/MoS₂ nanocomposite was successfully synthesized via combined in-situ oxidative polymerization and hydrothermal reaction and applied to an electrochemical nonenzymatic glucose sensor. The morphology of the prepared Cu@Pani/MoS₂ nanocomposite was characterized using FE-SEM and Cs-STEM, and electrochemical analysis was performed using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronoamperometry techniques. Electrostatic interaction between Cu@Pani and MoS₂ greatly enhanced the charge dispersion, electrical conductivity, and stability, resulting in excellent electrochemical performance. The Cu@Pani/MoS₂ was used as an electrocatalyst to detect glucose in an alkaline medium. The proposed glucose sensor exhibited a sensitivity, detection limit, and wide linear range of 69.82 μAmM⁻¹cm⁻², 1.78 μM, and 0.1–11 mM, respectively. The stability and selectivity of the Cu@Pani/MoS₂ composite for glucose compared to that of the potential interfering species, as well as its ability to determine the glucose concentration in diluted human serum samples at a high recovery percentage, demonstrated its viability as a nonenzymatic glucose sensor.

Graphical abstract

Highlights

• •Cu@Pani/MoS₂ nanocomposite was prepared via in-situ oxidative polymerization followed by hydrothermal method.
• •Cu@Pani/MoS₂ nanocomposite exhibits flower like nanosheet morphology.
• •The π-π interaction between MoS₂ and Cu@Pani greatly enhance the sensitivity and selectivity of glucose sensor.
• •The Cu@Pani/MoS₂ composite reveal high sensitivity of 69.82 μAmM⁻¹cm⁻² and detection limit of 1.78 μM.

1. Introduction

Inadequate insulin secretion by pancreatic beta cells is a characteristic of diabetes mellitus, which is a metabolic disorder that can cause serious health issues and even death [[1], [2], [3]]. “According to the World Health Organization diabetes is a leading cause of blindness, kidney failure, heart attacks, strokes, and lower limb amputation” [4]. These associated health risks have led to an emerging interest in the advancement of straightforward early diagnosis and treatment techniques for diabetes mellitus [5]. To this end, researchers have gained considerable interest in electrochemical glucose sensors, because of their cost effectiveness, high sensitivity, high stability, and ease of handling during experiments [[6], [7], [8], [9]], Due to these advantages, highly-sensitive glucose sensors are used in diverse fields, such as medical diagnostics [10], agro-food technology [11], and pharmaceuticals [12]. Enzyme-based electrochemical sensors immobilize enzyme glucose oxidase (GO) on an electrode surface and exhibit high selectivity; however, they have shortcomings such as poor stability in pH, and temperature variation, high production cost, short lifetime of the enzyme, and low electrochemical signals attributed to the low energy transfer. In addition, the adsorption of protein molecules form the blood on the enzyme surface further reduces the utility of enzymatic sensors [13,14]. To overcome these problems, electrochemical non-enzymatic glucose sensors with characteristic features such as effective selectivity, high sensitivity, low detection limit, and better stability have been developed [[15], [16], [17]]. Nanomaterials composed of metals such as copper (Cu), iron (Fe), gold (Au), silver (Ag), manganese (Mn), nickel (Ni) and zinc (Zn) have been examined for their applicability in non-enzymatic glucose sensors. Among these, metal oxides and metal sulfides have been used owing to their large surface areas and high surface energies, which substantially increase electrocatalytic activity during the electrooxidation of glucose [14], [[18], [19], [20], [21], [22], [23]]. Copper nanoparticles (CuNPs) have gained considerable interest in electrochemical research because of their low cost, better electrocatalytic activity, biocompatibility, and stability [14,24]. To this end, numerous studies have focused on developing electrochemical nonenzymatic glucose sensors for fabricating nanomaterials with high porosity, large surface area, low detection limit, and excellent sensitivity [25,26]. Inorganic/organic support-catalyst based composite nanoparticles may be a suitable electrocatalyst material, because of their improved ion dispersion, enhanced electron transfer, and better stability between the electrode surface and electrolyte [[27], [28], [29], [30], [31]]. Conducting polymers have been used as an electrocatalyst support and metal nanoparticles stabilizing agent because of their intriguing redox properties, high stability, and ease of synthesis [[32], [33], [34]].

Polyaniline (Pani) conducting polymers have attracted research interest because of their redox properties. The surface area of a conducting polymer can be increased via synthesis in the presence of metal nanoparticles [[35], [36], [37]]. The synergistic properties of metal nanoparticles dispersed onto a polymer composite exhibits enhanced electrochemical properties [27,38]. Owing to its large surface area, and distinct physicochemical characteristics, molybdenum disulfide (MoS₂), has been used in nanoelectronics, lithium-ion batteries [39], catalysis [40], supercapacitors [41,42], and sensors [43,44]. However, MoS₂ also has disadvantages that inhibit its wider application, e.g., restacking, intrinsically low conductivity, and tendency to agglomerate. These issues can be resolved by doping metal nanoparticles with the conducting polymer and carbon based materials [1,45,46]. Novel nanocomposites based on conducting polymers, metal nanoparticles, and MoS₂ featuring a unique structures and properties could significantly improve electrochemical nonenzymatic glucose sensors.

In previously studies, Yu et al. demonstrated that a NiCo₂O₄@PANI core-shell nanocomposite is relevant for sensitive glucose determination [47]; Yang et al. reported CuNPs deposited multi-walled carbon nanotube arrays for amperometric non-enzymatic glucose sensor [48]; and Belgherbi et al. used copper-polyaniline composite films for enzyme-free glucose sensors [6]. Further, Zheng et al. reported an electrochemical glucose sensor based on a copper nanoparticles/polyaniline/graphene composite [14]. Esmaeeli et al. reported a copper-oxide decorated polyaniline nanofiber applicable to non-enzymatic glucose sensors [49]. Moreover, Pratap et al. applied copper nanoparticles-supported mesoporous polyaniline (Cu/Meso-PANI) to nonenzymatic glucose sensor [50]. Chen et al. synthesized Cu-doped polyaniline through the H₂O₂-promoted oxidative polymerization of aniline [51]. To the best of the authors’ knowledge, this is the first study that presents a synthesis procedure combining in-situ oxidative polymerization, and a hydrothermal process for synthesizing a Cu@Pani/MoS₂ nanocomposite. CuNPs were doped onto polyaniline during in-situ polymerization, nucleation, and growth during the hydrothermal reaction. The as-prepared Cu@Pani/MoS₂ composite was loaded on a glassy carbon electrode, and it exhibited the excellent electrocatalytic performance, indicating its effectiveness for nonenzymatic glucose sensors.

2. Materials and methods

2.1. Chemicals and synthesis

Hydrochloric acid (Samchun), aniline (Alfa Aesar), copper sulfate anhydrous (Daejung), hydrogen peroxide (Samchun), trisodium citrate dihydrate (Shova chemicals), sodium molybdate dihydrate (Samchun), thiourea (Junsei) and human serum (Sigma- Aldrich H4522) were purchased and used directly without additional purification. Ultrapure double distilled water (Millipore, USA) was used for solution preparation throughout the experimental process.

2.2. Synthesis of Cu@Pani and Cu@Pani/MoS₂

Cu@Pani was prepared using a slight modification to the method reported by Chen et al. [51], through the in-situ oxidative polymerization of aniline along with a copper salt. 50 mM aniline, 10 mM copper sulfate anhydrous, 34 mM trisodium citrate dihydrate (NaCt), and 50 mM hydrogen peroxide (H₂O₂) were added, to a 70 mL 1 M HCl solution, and the mixture was stirred for 24 h at 400 rpm to synthesize Cu@Pani. Similarly, for Cu@Pani/MoS₂, 30 mM sodium molybdate and 100 mM thiourea were also added by following the above-mentioned procedure for Cu@Pani. After stirring 30 min, the solution was kept in a Teflon-lined hydrothermal vessel at 180 °C for 24 h. The obtained Cu@Pani and CuPani/MoS₂ composites were separated using a centrifuge, washed in distilled water, and ethanol, and then dried at 70 °C for 12 h.

2.3. Electrode preparation

The following procedures were followed to prepare Cu@Pani and CuPani/MoS₂ modified electrodes: Before loading the electrode material, a glassy carbon (GC) electrode, which has a diameter of 5 mm (A = 0.1963 cm²), was polished with 0.05, 0.3 and 1.0 μm alumina slurry, washed with water and ethanol, and allowed to dry at room temperature. 3 mg of both Cu@Pani and CuPani/MoS₂ were dispersed separately in 200 μL isopropanol and 5 μL 5 % Nafion solution to prepare the ink solution. The GC electrode was modified by drop-casting 20 μL of the dispersed solution onto the electrode surface and allowed it to dry at room temperature.

2.4. Characterization

Field emission scanning electron microscopy (Ultrahigh FE‐SEM: SU8220 HITACHI, Japan) coupled with energy dispersive X‐ray spectroscopy (EDX) and Cs‐corrected scanning transmission electron microscopy (Cs‐STEM; JEOL/JEM‐ARM200F) were used to characterize the surface morphologies of the Cu@Pani and CuPani/MoS₂ nanocomposites. The Fourier‐transform infrared spectroscopy (FTIR, PerkinElmer, Spectrum GX, USA) KBr plate method was used to record the FT-IR spectra in the transmission mode. The XRD patterns of Cu@Pani and Cu@Pani/MoS₂ composites were examined by using an X‐ray powder diffractometer (XRD, Rigaku Co., Japan) with monochromatic Cu Kα radiation (λ = 1.54056 Å) at a Bragg angle (2θ) range of 5‐70°. X‐ray photoelectron spectroscopy (XPS) was performed using a Nexsa XPS Surface Analysis System (Thermo Fisher Scientific, UK) outfitted with a monochromatic Al Kα (1486.6 eV) X‐ray light source.

2.5. Electrochemical measurements

All electrochemical measurements including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronoamperometry (i-t) were performed using a ZIVE SP2 potentiostat (Won ATech-ZIVE LAB). A GC electrode, Pt wire, and calomel electrode Hg₂Cl₂ were used as the working, counter, and reference electrodes, respectively. CV was performed with 0.1 M NaOH solution as the working electrolyte at various scan rates (5-100 mVs⁻¹) and potential ranges between 0 and 0.8 V. Between 0.1 Hz and 100 KHz, EIS was conducted using a 1:1 mixture of 5 mM Fe (CN)₆^3−/4- and 0.1 M KCl solution as the supporting electrolyte The Chronoamperometry was performed at + 0.5 V while stirring at 500 rpm.

3. Results and discussion

The processes used for Cu@Pani and Cu@Pani/MoS₂ synthesis is illustrated in Fig. 1. The Cu@Pani/MoS₂ composite was prepared through in-situ polymerization and hydrothermal reaction using hydrogen peroxide (H₂O₂) as a green oxidant followed by a hydrothermal reaction. During, the in-situ polymerization, H₂O₂ acts as the green oxidant that generates a green waste product, i.e., water. Further, CuNPs are doped into polyaniline, resulting Cu@Pani composite during polymerization. The oxidation potential of Cu (+0.34 V) is not sufficiently high to oxidize aniline [51], and therefore, we added H₂O₂ to the copper and aniline solution to promote the oxidation of aniline to polyaniline, which undergoes in-situ polymerization to form black precipitates of the copper polyaniline composite (i.e., Cu@Pani). Sodium molybdate and thiourea were reduced and sulfurized during a hydrothermal reaction to prepare Cu@Pani/MoS₂ [51,52]. Simultaneously, after the addition of Nact and H₂O₂, the copper salt is reduced to produce CuNPs. Meanwhile, H₂O₂ acts as a oxidizing agent that initiates polymerization [53], and NaCt act as a reducing and stabilizing agent such that the nucleation and growth processes and polymerization occur in parallel [38,[54], [55], [56]]. The CuNPs were doped onto polyaniline during in-situ polymerization, and MoS₂ nanosheets were grown on the surface of Cu@Pani during the hydrothermal reaction. The covalent linkage between polyaniline (N-atoms) and MoS₂ occurs via metal (dπ) - nitrogen pπ bonding and bonding between nitrogen and sulfur atom. The linkage of MoS₂ nanosheet and conjugated polyaniline occurs through π-π interaction between MoS₂ nanosheet and conjugated polyaniline [57,58], as displayed in Fig. 1.

Fig. 1.

Scheme for the synthesis of Cu@Pani and Cu@Pani/MoS₂ nanocomposites.

The Cu@Pani and Cu@Pani/MoS₂ FTIR spectra are shown in Fig. 2(a). The peaks located at 3352 and 3050 cm ⁻¹ are attributed to the N–H stretching vibration. The FT-IR absorption peaks at 1599, 1496, and 1290 cm ⁻¹ are attributed to the N = Q = N stretching of the quinine ring, C C bonding in the conjugated polyaniline, and C–N bond vibration of aromatic benzene ring, respectively [51,59]. [60]. Similarly, the peak at 692 cm ⁻¹ corresponds to the copper metal polyaniline linkage through the amino group of aniline [61]. The increased transmittance of Cu@Pani/MoS₂ composite as compared to Cu@Pani confirmed the proper conjugation, and the weak band at 588 cm ⁻¹ attributed to Mo–S bond vibration in Cu@Pani/MoS₂ [37,62]. The Cu@Pani and Cu@Pani/MoS₂ XRD spectra are shown in Fig. 2(b). The XRD patterns for Cu@Pani/MoS₂ at 14.1^°, 27.75^°, 31.73^°, 32.83^°, 36.95^°, 46.32^°, 52.67^°, 54.98^°, and 59.42^° corresponds to (002), (004), (100), (101), (102), (006), (105), (106), and (008) crystal planes, respectively (JCPDS: 03-065-1951) [63]. Similarly, Cu@Pani diffraction at 16.08^° can be attributed to the (011) crystal plane, which is parallel to the long chain polymer of aniline [59,63,64]. The deviation of the peak position from the usual peak around the 2θ angle 10.37^° and the emergence of new peaks at 2θ angles 18.22^°, 29.34^°, and 48.07^°, can be attributed to the scattered orientation of the parallel aniline chain as well as copper, trisodium citrate dihydrate and MoS₂, respectively, as indicated by asterisk (*). The characteristic peaks of Cu@Pani disappeared in the final Cu@Pani/MoS₂ composite, indicating the successful in-situ polymerization of aniline [65].

Fig. 2.

(a) FT-IR spectra (b) XRD spectra of Cu@Pani and Cu@Pani/MoS₂, respectively.

The Cu@Pani and Cu@Pani/MoS₂ composite FESEM and CS-TEM images were recorded to study the surface morphology, and they are presented in Fig. 3. As shown in the FE-SEM image of Cu@Pani, nanoflowers composed of Cu nanoparticles doped polyaniline showed a ball-like structure. The surface morphology of Cu@Pani/MoS₂ revealed that MoS₂ nanosheet were grew on the surface of Cu@Pani, as represented in Fig. 3 (a, b) [51,66]. The enlarged image of Cu@Pani/MoS₂ in Fig. 3(c) shows a flower-like MoS₂ nanosheet structure, which confirmed the formation of MoS₂ nanosheets [67,68]. Cs-STEM imagery revealed a nanosheet structure with agglomerated MoS₂, as shown in Fig. 3(e). Elemental color mapping revealed that Mo, Cu, N, S and C were homogeneously distributed over the composite 3(d). The EDX spectrum in Fig. 3(f) showed that the composite was composed of 28.49 % carbon, 24.22 % molybdenum, 25.08 % sulfur, 16.41 % copper, and 4.92 % nitrogen [69,70]. This information is presented in Fig. S3(c).

Fig. 3.

(a) FE-SEM of Cu@Pani and (b) Cu@Pani/MoS₂, (c) enlarged FE-SEM image of Cu@Pani/MoS₂, (d) EDX color mapping of Cu@Pani/MoS₂, (e) Cs-STEM of Cu@Pani/MoS_2, (f) EDX spectrum of Cu@Pani/MoS, and (g) a high-resolution TEM of Cu@Pani/MoS₂ with SEAD pattern.

The chemical composition of the nanocomposite material was studied by using XPS, and the results are shown in Fig. 4(a). The broad scan XPS spectrum revealed the presence of copper (Cu), molybdenum (Mo), nitrogen(N), sulfur (S) and carbon (C) and oxygen (O) elements [3]. After deconvolution, peaks at 932.2 and 952.2 eV binding energies corresponds to Cu 2P_3/2 and Cu 2P_1/2 respectively, and the small peaks at 944 eV and 940 eV represent satellite peaks of copper, which indicate the paramagnetic behavior, of the Cu²⁺ oxidation state. The binding energy difference of 19.6 eV between the 2P_3/2 and 2P_1/2 states of Cu resembles the reported 20 eV binding energy difference, as shown in Fig. 4(b) [51]. [71,72]. The carbon peaks displayed in Fig. 4(c) fit at 284.6, 285.5 and 286.9 eV binding energies assigned for the C–C, C–N and C $=$ O species [3,[73], [74], [75]]. Binding energies at 161.3, 162.3 and 168.9 eV shown in Fig. 4(d) are attributed to S 2P_1/2, S 2P_3/2 and oxidized sulfur, respectively [43,76,77]. The XPS spectrum shown in Fig. 4(e) with binding energies at 394.4, 399.6 and 412.7 eV are attributed to the nitride like –N, pyrrolic-N, and Mo 3P_3/2 respectively [[78], [79], [80]]. Binding energy peaks at 225.8, 228.5, 331.6, 232.6 and 235.7 eV are responsible for S 2s, Mo⁴⁺ 3d_5/2, Mo⁶⁺ 3d_5/2, Mo⁴⁺ 3d_3/2, and Mo⁶⁺ 3d_3/2, respectively, as represented in Fig. 4 (f), [81,82].

Fig. 4.

(a) XPS spectrum of Cu@Pani/MoS₂, (b–f) Cu 2P, C 1s, S 2P, N 1s + Mo 3P, and Mo 3d in Cu@Pani/MoS₂.

3.1. Electrochemical study

The electrochemical properties of the Cu@Pani, and Cu@Pani/MoS₂ electrodes were analyzed using 0.1 M NaOH solution as working electrolyte before fabricating a modified electrode for glucose sensing. As shown in Fig. 5(a), using the 0.1 M NaOH electrolyte cyclic voltammogram of Cu@Pani, Cu@Pani/MoS₂ and Cu@Pani/MoS₂ in the presence of 2 mM glucose were measured using a three-electrode system [50]. The Cu@Pani electrode exhibited small current response, whereas the current response of the Cu@Pani/MoS₂ electrode was higher than that of the Cu@Pani electrode. In the presence of 2 mM glucose, the Cu@Pani/MoS₂ electrode demonstrated a significantly increased current response, which can be attributed to glucose oxidation [14]. The comparatively lower current response of the Cu@Pani can be attributed to the selection of H₂O₂ for polymerization instead of ammonium persulfate (APS), and simultaneous use of oxidizing agent (H₂O₂) and reducing agent (NaCt), which leads to the incomplete oxidation of aniline to polyaniline, is also represented in Fig. 5(a) [83,84]. The Cu@Pani/MoS₂ CV spectrum exhibits a current response increment compared to that of Cu@Pani, which can be attributed to the two - dimensional layered structure of MoS₂ providing a large surface area for electron dispersion at the electrode surface. This greatly increases the electrocatalytic performance of the material [85].

Fig. 5.

(a) Cyclic voltammograms (CVs) of Cu@Pani and Cu@Pani/MoS₂ and Cu@Pani/MoS₂ in the presence of glucose in 0.1 M NaOH at 50 mVs⁻¹ (b) Nyquist plot of Cu@Pani and Cu@Pani/MoS₂ in 5 mM [Fe(CN)₆]^3-/4- redox electrolyte and 0.1 M KCl solution, (c) CV of Cu@Pani/MoS₂ with different glucose concentrations in the 0.1 M NaOH solution, and (d) Plot of oxidation peak current vs the concentration of glucose solution.

An electrolyte containing a mixture of 5 mM [Fe (CN)₆]^3−/4−redox electrolyte and 0.1 M KCl as the supporting electrolyte, was employed to conduct EIS analysis for the further electrochemical study of the Cu@Pani, and Cu@Pani/MoS₂ electrodes, as revealed in Fig. 5(b). The EIS analysis revealed a semicircular region diameter showing charge transfer resistance (Rct); shifting of the inclined line in the low-frequency region indicated an increased charge transfer kinetics, and diffusion control process respectively for the Cu@Pani/MoS₂ electrode as compared to that for Cu@Pani electrode [86]. The inset image on Fig. 5(b) shows Randels equivalent circuit, which describes the electrolyte solution resistance (R_s), R_ct, double-layer capacitance (Q₁), and Warburg impedance (Q₂), where the electrolyte resistance R_s, is fitted in series combination, and double layer capacitance (Q₁), R_ct, and impedance of the faradaic reaction are arranged in parallel combination [[87], [88], [89]].

Fig. 5(c) shows the CV of Cu@Pani and Cu@Pani/MoS₂ when exposed to 0, 1, 2, 3, 4, and 5 mM glucose solutions. Fig. 5(d) displays a plot of glucose concentration (mM) and anodic current (Ip). A linear regression line with Ip (μA) = 27.3x + 76.5 (R² = 0.9804), reveals that a diffusion-controlled reaction occurred on the Cu@Pani/MoS₂ electrode surface [90,91]. MoS₂ nano structure improves the electrocatalytic activity because of its high surface area, electron transfer abilities and rapid diffusion of the electrons and ions on the electrode surface, providing active sites for the electrocatalytic reactions during electrochemical glucose oxidation [92]. The electro-oxidation of glucose in the Cu@Pani/MoS₂ electrode involves the following reaction mechanisms [93].

Cu + 2OH⁻ → CuO + H₂O + 2e⁻ (1)

CuO + OH⁻ → CuOOH + e⁻ (2)

Cu³⁺ + Glucose → Gluconolactone + Cu²⁺ (3)

In an alkaline solution, glucose undergoes deprotonation which further initiates the isomerization, thereby forming an enediol structure for further oxidation. In the presence of Cu³⁺, it yields gluconolactone, which produces gluconic acid via hydrolysis [14,94]. The increased oxidation current response after the glucose addition can be attributed to the oxidation of Cu²⁺ to Cu³⁺, which greatly enhances the electrocatalytic activity and electron transfer process at the surface of the electrode material [95,96].

3.2. Chronoamperometry analysis

The results of the chronoamperometry analysis of the Cu@Pani/MoS₂ electrode with glucose are presented in Fig. 6(a). The gradual addition of different glucose concentrations to the Cu@Pani/MoS₂ electrode led to a gradual increase in the current response, which demonstrate the applicability of the Cu@Pani/MoS₂ electrode for measuring different glucose concentration. The plot of the observed oxidation current against the glucose concentration, follows a linear regression line with a co-relation coefficient of R² = 0.9950, as illustrated in Fig. 6(b). The linear fitting sensitivity of the electrochemical sensor was calculated and was found to be 69.82 μAmM⁻¹cm⁻². The calculated limit of detection (LOD) was found to be 1.78 μM, which was obtained using the formula LOD = 3.3 σ/m, (where m = slope and σ = standard deviation) [3]. The glucose sensor demonstrated a linear detection range of 100 μM to 11 mM. The obtained results were compared with those of Cu-, polyaniline-, and MoS₂-based electrode materials available in the literatures, as detailed in Table 1.

Fig. 6.

(a) Chronoamperometric i-t curves of the Cu@Pani/MoS₂ electrode at potential +0.5 V (vs. Hg₂Cl₂) at different glucose concentrations (100 μM–11 mM) in 0.1 M NaOH, (b) Plot of current vs. concentration Cu@Pani/MoS₂, (c) Chronoamperometric response of Cu@Pani/MoS₂ in 1 mM glucose and 0.1 mM various interfering species in 0.1 M NaOH, (d) Stability of Cu@Pani/MoS₂ in 2.0 mM glucose solution tested over ten days.

Table 1.

Comparison of polyaniline, Cu, and MoS₂ based electrodes for the glucose sensor.

Electrode material	Sensitivity μAmM−1cm−2	Detection limit	Linear detection range	Ref.
CuO-PANI/FTO	1359	0.24 μM	0.28–4.6 mM	[49]
CuO/NiO/PANI/GCE	–	2 μM	0.02–2.5 mM	[97]
PANI-CuNi	1030	0.2 μM	Up to 5.6 mM	[33]
Cu-MWCNTs	1096	1.0 μM	Up to 7.5 mM	[98]
CuOG-SPCE	2367	34.3 nM	0.122 μM- 0.5 mM	[99]
NiNPs-MWCNTs/Copper	3822	0.7 μM	2 μM-10 mM	[100]
Ni–MoS2 hybrid	1824	0.31 μM	0–4 mM	[101]
NiCo2O4@PANI	4550	4.7350 μM	0.3833 mM	[47]
Cu–PANI/ITO	4140	5 μM	0.02–1 mM	[6]
CuO/MoS2	1055	0.017 μM	35–800 μM	[102]
Cu/PANI	61.6	9.36 μM	0.02–10 mM	[50]
Cu–MoS2	1055	–	0–4 mM	[103]
CuNPs/PANI/graphene	150	0.27 μM	0.001–3.7 mM	[14]
Cu@Pani/MoS2	69.82	1.78 μM	0.1–11 mM	Our work

3.3. Selectivity, stability, and real sample analysis of the sensor

We further analyzed the selectivity of the Cu@Pani/MoS₂ electrode by adding 1 mM same volume of glucose solution and different interfering biomolecules such as, maelic acid (MA), potassium chloride (KCl), ascorbic acid (AA), urea (UR), uric acid (UA), and sodium chloride (NaCl) (0.1 mM) [3,104], [105]. Fig. 6(c) shows the selectivity of the Cu@Pani/MoS₂-based electrochemical non-enzymatic glucose sensor. Our results indicate that there is no noticeable current for interfering biomolecules; but increased current response for glucose, both before and after the addition of interfering biomolecules [106]. The relatively high current response for glucose compared to that of the interfering species, demonstrates the better selectivity of the glucose sensor [49].

Successive chronoamperometric measurements are performed over ten days using 2 mM glucose solution to confirm the stability of the electrode material; the corresponding results are compared in Fig. 6 (d). The results revealed that the sensitivity of the Cu@Pani/MoS₂ based electrochemical non-enzymatic glucose sensor decreases by 7.25 % and it retains a stability of 92.75 % as depicted in Fig. 6 (d). These results confirm the satisfactory stability of the Cu@Pani/MoS₂-based sensor. For practical application glucose concentration in human serum (H4522, Sigma - Aldrich, 4.89 mM) sample was measured using the Cu@Pani/MoS₂-based electrode [3]. For the glucose concentration measurement 20 μL of the serum sample was diluted by adding 30 mL 0.1 M NaOH solution, and the current response was measured using chronoamperometry at 0.5 V potential [33]. Subsequently, different glucose solution of known concentrations (500 μM, 1000 μM, and 2000 μM) were added to the diluted serum solution and current response were recorded using the standard addition method. The corresponding results are displayed in Table 2. Recovery percentages of 94.28 %, 94.92 % and 96.21 % were obtained by adding 500 μM, 1000 μM, and 2000 μM glucose, respectively [95]. Our results demonstrate the viability of the Cu@Pani/MoS₂ electrode for electrochemical non-enzymatic glucose sensing of biological samples in the presence of interfering species [[107], [108], [109]].

Table 2.

Recovery test in human blood serum sample.

Samples	Added glucose concentration (μM)	Observed glucose concentration (μM)	Recovery (%)	RSD (%) (n = 3)
1	500	471	94.28	2.15
2	1000	949	94.92	2.08
3	2000	1924	96.21	1.16

4. Conclusions

A conclusion, combination of in-situ oxidative polymerization and a hydrothermal process was employed to synthesize Cu@Pani/MoS₂ nanocomposite. The structural morphology of the nanocomposite was studied using different analytical tools, and the fabricated nanocomposite was used as a non-enzymatic electrochemical glucose sensor. Cu@Pani/MoS₂ composite-based sensor revealed a sensitivity of 69.82 μAmM⁻¹cm⁻², detection limit of 1.78 μM, and linear range of 100 μM to 11 mM with 92.75 % stability. The Cu@Pani/MoS₂ nanocomposite exhibited effective sensitivity, good selectivity, stability, and a short electrochemical response time, which indicates that the Cu@Pani/MoS₂ composite electrode is viable for electrochemical non-enzymatic glucose sensor. Our work provides a new method for the design of composite electrode materials for electrochemical applications such as supercapacitor, electrochemical sensor, batteries etc.

Data availability statement

Data will be made available on request.

CRediT authorship contribution statement

Krishna Prasad Sharma: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing - original draft, Writing - review & editing. Miyeon Shin: Data curation, Formal analysis, Project administration, Resources, Software, Writing – review & editing. Kyong Kim: Data curation, Project administration, Resources, Software. Kyungmin Woo: Data curation, Formal analysis, Investigation, Resources, Software. Ganesh Prasad Awasthi: Data curation, Formal analysis, Methodology, Validation. Changho Yu: Formal analysis, Funding acquisition, Project administration, Resources, Supervision, Validation, Writing – review & editing.

Declaration of competing interest

There is no conflict of interest to disclose for this research paper. The human serum samples were not taken form human patients. We have used commercially available human serum (H4522, Sigma- Aldrich) for the real sample analysis.

Appendix A. Supplementary data

The following is the Supplementary data to this article: