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Low Overpotential Amperometric Sensor using Yb2O3.CuO@rGO Nanocomposite for Sensitive Detection of Ascorbic Acid in Real Samples

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25 April 2023

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26 April 2023

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Abstract
The ultimate objective of this research work is to design a sensitive and selective electrochemical sensor for efficient detection of ascorbic acid (AA), a vital antioxidant found in blood serum that may serve as a biomarker for oxidative stress. To achieve this, we utilized a novel Yb2O3.CuO@rGO nanocomposite (NC) as the active material to modify the glassy carbon working electrode (GCE). The structural properties and morphological characteristics of the Yb2O3.CuO@rGO NC were investigated using various techniques to ensure their suitability for the sensor. The resulting sensor electrode was able to detect a broad range of AA concentrations (0.5 - 1571 µM) in neutral phosphate buffer solution with a high sensitivity of 0.4341 µAµM-1cm-2 and a reasonable detection limit of 0.062 µM. The sensor's great sensitivity and selectivity allowed it to accurately determine the levels of AA in human blood serum and commercial vitamin C tablets. It demonstrated high levels of reproducibility, repeatability, and stability, making it a reliable and robust sensor for the measurement of AA at low overpotential. Overall, the Yb2O3.CuO@rGO/GCE sensor showed great potential in detecting AA from real samples.
Keywords: 
Subject: Chemistry and Materials Science  -   Analytical Chemistry

1. Introduction

Ascorbic acid (AA) is a vital biomolecule that is present in a variety of naturally occurring sources, including fruits and vegetables, and functions as a nutrient and antioxidant [1]. It is essential to numerous bodily metabolic activities including activating the immune system, aiding in wound healing, helping with the absorption of iron, and protecting against damage to bones and teeth [2]. Additionally, AA serves as a cofactor during the synthesis of collagen and carnitine [3]. In addition, AA has been demonstrated to provide protective effects against oxidative illnesses like heart disease, several cancers, AIDS, the common cold, etc. [4]. However, there is no AA produced by the human body and it can only be obtained through the consumption of foods and medicines [5]. AA is a crucial ingredient in dietary and pharmaceutical supplements [6]. Human blood serum typically contains between 28.5 and 85.2 µM of AA, and the amount of AA in blood serum can provide information about a person's general state of health [7]. Scurvy and anemia can result from an AA deficit in blood serum, while an excess of AA can lead to gastric irritation or diarrhea [8]. Therefore, it is crucial to have precise and efficient techniques for figuring out how much AA is present in foods, medications, and blood serum.
Direct titration [9], chromatography [10], spectrophotometry [11], and solid-phase spectrophotometry [12] are currently used methods for AA measurement. However, such methods are costly and require skilled personnel as well as challenging analytical measures for multi-sample preparation. Such techniques also require challenging analytical measures in multiple-sample preparations. To address these issues, researchers are working to develop efficient and cost-effective methods for real-time and in-situ AA determination. The advantages of electrochemical detection include quicker measurements, reduced sample size, reduced costs, and an absence of pre-concentration processes, making them handy, portable, and simple to use with miniaturized electrodes [13]. However, due to its irreversible nature and high over-potential requirements, the electrochemical AA oxidation at a bare electrode might have negative effects on selectivity, electrode fouling, and repeatability. Therefore, it is indispensable to fabricate an electrode surface that enables efficient AA detection with less over-potential. In recent years, to detect AA, researchers have proposed several sensors including electrodes modified with metals and metal oxides [14], alkylimidazolium salt [15], graphene derivatives [16], carbon nanotubes [14], and polymers [17].
Due to their potential technological uses and intriguing optical and structural characteristics, semiconducting doped nanostructured materials comprised of transition metal oxides have drawn a lot of attention. Because of their size, shape, and surface, these materials have distinct physical and chemical properties that make them relevant in a variety of study fields and applications for industry. In particular, metal oxide-based sensors have been explored for their diverse uses in areas such as the protection of the environment, chemical process management, personal security, healthcare, and military [18,19,20]. These sensors have several advantages, including their compact size, affordable price, less power consumption, straightforward processing, and good stability [21]. Previous researchers have investigated various types of metal oxides, such as CuO [22,23], MnO2 [24], NiO [25], Fe2O3 [26], and ZnO [27], as electron mediators for sensing applications. Additionally, doped metal oxides such as NiO.CoO nanocomposites [28], CdO.SnO2.V2O5 [29], CuO.In2O3 [30], CuO.Nd2O5 [31], CuO.NiO [32], CuO.ZnO [33] have been studied as efficient sensing materials with higher sensitivity, small detection limits, wide linear dynamic ranges, and quick response times. CuO, a p-type semiconductor, has shown particularly good performance as an electrocatalyst in sensing applications [34]. To improve the performance of CuO, researchers have also investigated using other semiconductor metal oxides such as In2O3 [30], Nd2O5 [31], NiO [32], TiO2 [35], SnO2 [36] in combination with CuO as bimetallic oxide pairs. Yb2O3 has also been explored for use in sensing applications [37,38,39,40].
A lot of research has been done on the applicability of carbonaceous nanomaterials for sensing applications, including reduced graphene oxide, activated carbon, mesoporous carbon, and carbon nanotubes. In particular, graphene, a sheet of sp2-bonded carbon atoms, with a particular surface area, low density, outstanding electrical conductivity, and great mechanical properties, has drawn a lot of interest [41,42]. Graphene-based 3D nanomaterials have also generated huge interest due to their high surface area, lower density, better electrical conductivity, and exceptional mechanical properties [43,44]. Composite materials made of metal oxides and graphene have recently been explored for their stability, long-term storage, and photo-catalytic capabilities [45,46]. Many graphene based nanomaterials have been studied in sensing applications [47,48,49]. However, rare earth oxide-transition metal oxide-reduced graphene nanocomposite has hardly been studied in sensing applications. Hence, in this work, we developed and examined Yb2O3.CuO@rGO nanocomposite as the active sensing material for AA detection.
Inspired by previous works available in the literature, we synthesized Yb2O3-doped CuO nanoparticles to improve stability, sensitivity, and selectivity, and then used a simple sonication technique to synthesize the Yb2O3.CuO@rGO nanocomposite sensing material. This study presents a simple method for preparing an electrochemical AA sensor using Yb2O3.CuO@rGO nanocomposite that offered improved selectivity and sensitivity. To the extent that we are aware, this will be the maiden article utilizing Yb2O3.CuO@rGO nanocomposite to develop an enzymeless AA electrochemical sensor.

2. Materials and Methods

2.1. Materials

All of the necessary chemicals, including ascorbic acid, copper (II) nitrate, ytterbium (III) nitrate, sodium hydroxide, reduced graphene oxide, NaH2PO4, Na2HPO4, citric acid, glucose, uric acid, dopamine, sodium chloride, and calcium nitrate, were purchased from Sigma Aldrich and utilized exactly as they were given. All solutions were made using double-distilled water. The XPS investigation of Yb2O3.CuO@rGO was performed using a MgKα spectrometer (JEOL, JPS 9200) in the subsequent circumstances: pass energy = 50 eV (wide-scan) and 30 eV (narrow-scan), Voltage = 10 kV, Current = 20 mA. A PANalytical X-ray diffractometer was used to acquire X-ray diffraction (XRD) spectra with Cu Kα1/2, λα1 = 154.060 p.m., λα2 = 154.439 p.m. radiation. A "Raman station 400 (Perkin Elmer)" spectrometer was used to acquire the Raman spectra. A FE-SEM (JEOL-6300F, 5 kV) was used to analyze the morphology and structural characteristics of Yb2O3.CuO@rGO. EDS (JEOL, Japan) was used to investigate the elemental composition of the Yb2O3.CuO@rGO. A JEOL JEM-2100F-UHR field emission apparatus fitted with a Gatan GIF 2001 energy filter and a 1 k-CCD camera was used to capture Transmission electron microscopy (TEM) micrographs at 200 kV. Electrochemical measurements were conducted using a Zahner Zennium potentiostat (German).

2.2. Synthesis of CuO, Yb2O3, Yb2O3.CuO, and Yb2O3.CuO@rGO nanocomposite

To synthesize the CuO, Yb2O3, Yb2O3.CuO, and Yb2O3.CuO@rGO nanocomposites, the following process was followed: First, equimolar Cu(NO3)2 and Yb(NO3)3 solutions were mixed in a beaker and stirred for half an hour at 70°C. This mixture was then combined with NaOH and stirred vigorously at 80°C for 8 hours. Afterwards the ensuing dark precipitate was cleaned with distilled water and ethanol to get rid of contaminants and the resulting black precipitate was dried at 80°C. This as-grown Yb2O3.CuO nanoparticle (NP) was then calcined by heating it for six hours at 500°C in a furnace. During this synthesis process, the following chemical reactions occurred:
Cu(NO3)2 + 2NaOH → Cu(OH)2 + 2NaCl
Yb(NO3)3 + 3NaOH → Yb(OH)3 + 3NaCl
Cu(OH)2 + 2Yb(OH)3 → Yb2O3.CuO + 4H2O
Precursors, Yb3+ and Cu2+ ions are soluble in NaOH solution, where NaOH keeps the pH constant during the reaction and continuously releases OH-. The development of the Cu(OH)2 nucleus starts when the ionic product of Cu2+ and OH- exceeds the Ksp value. Similarly, Yb(OH)3 was also produced. Cu2+ ions easily incorporate themselves into the Yb2O3 lattice because of the similar ionic radii. On heating, hydroxides decompose to produce respective oxides. Similarly CuO and Yb2O3 NPs were also synthesized.
To synthesize the Yb2O3.CuO@rGO nanocomposite, 0.5 g Yb2O3.CuO NPs and 0.025 g reduced graphene oxide (rGO) were mixed followed by 40 minutes of sonication in 80 ml distilled water. This resulting mixture was then filtered and had 12 hours of drying in an oven at 70°C.

2.3. GCE modification using Yb2O3.CuO@rGO nanocomposite

GCEs were first cleaned by using a 1 µm diamond and then a 0.05 µm alumina. Next, the GCE was fabricated utilizing Yb2O3.CuO@rGO nanocomposite using a Nafion solution. During the fabrication process, 4.0 mg of Yb2O3.CuO@rGO was uniformly mixed with 0.05 ml Nafion and 0.45 ml propan-2-ol, and then 2 µl of this suspension was carefully applied to a pre-cleaned GCE and dried at 60°C for 20 minutes. Such a fabricated GCE was labeled as the Yb2O3.CuO@rGO/GCE. Control experiments were also conducted, in which CuO/GCE, Yb2O3/GCE, rGO/GCE, and Yb2O3.CuO/GCE were fabricated using similar procedures. The electrochemical investigations of AA (0.5 – 1744 µM) were carried out in a typical three-electrode electrochemical cell at ambient conditions in 0.1 M PBS (pH 7.0), a Yb2O3.CuO@rGO/GCE, Ag/AgCl, and a platinum spiral were served as the working, reference, and counter electrodes, respectively.

3. Results and Discussion

3.1. Characterization of Yb2O3.CuO@rGO nanocomposite

Elemental compositions and structure of Yb2O3.CuO@rGO were examined using XPS. It is evident from the XPS analysis shown in Figure 1(a-e) that Yb2O3.CuO@rGO nanocomposite is composed of Yb, Cu, O, and C atoms only. The Yb4d5/2 spectrum has three clearly defined peaks appearing at energies of 187.2, 188.4, and 189.1, are compatible with Yb4d (Figure 1b) [50]. In the deconvoluted Cu2p spectrum in Figure 1c, two peaks at 937.1 and 956.8 eV that may be related to Cu2p3/2 and Cu2p1/2 respectively [51]. In between these two peaks there are some satellite peaks appeared that are also consistent with the literature [52]. Figure 1d shows two peaks from the fine scan O1s spectra that are associated to the Yb-O and Cu-O bonds, respectively, at 533.3 and 535.2 eV [13]. Three peaks are shown in fine scan C1s spectrum in Figure 1e at energies of 284.6, 287.2, and 289.1 eV. Peaks at 284.6 and 287.2 eV may be attributed to C-C and C-O-H bonds, respectively [53] and the remaining peak at 289.1 eV can be correlated to COOH [54].
XRD patterns in Figure 2a showed diffraction bands at 2θ = 20.80, 29.50, 34.30, 36.50, 40.60, 44.00, 47.50, 49.20, 51.00, 54.10, 57.10, 58.50, 60.00, and 61.50, which are related to the (211), (222), (400), (411), (332), (134), (125), (440), (443), (611), (145), (662), (136), and (444) planes for Yb2O3 NPs (JCPDS#65-3173), respectively [50]. The diffraction bands at 35.40, 38.60, 48.60, 58.20, 61.60, 66.30, and 68.10 can be related to (002), (111), (-202), (202), (-113), (-311), and (220) planes of CuO NPs ((JCPDS#45-0937), respectively [55]. The Yb2O3.CuO@rGO contains the rGO peak connected to carbon that is often appearing at 2θ = 24.30 which is correlated to (002) plane [56] but is not easily visible in Figure 2a due to low intensity. However, the presence of carbon in Yb2O3.CuO@rGO was established by XPS, EDS, SEM and TEM. Figure 2b shows the Raman spectra, where bands at 359.3, 718, and 1060 cm-1 can be related to Yb2O3; while bands at 328, and 850 cm-1 were connected to CuO [57]. Characteristic carbon bands at 1344 and 1676 cm-1 are related to D and G bands of rGO [58].
FESEM was employed to analyze the morphological and surface structure of CuO, Yb2O3, Yb2O3.CuO, and Yb2O3.CuO@rGO nanocomposite as presented in Figure 3a-d. The Yb2O3.CuO@rGO nanocomposite was found to be made up of Yb2O3.CuO composites that were randomly distributed over the graphene sheets. EDS was used to determine the Yb2O3.CuO@rGO nanocomposite's elemental composition (Figure 3e), and the results showed that the nanocomposite is exclusively made of Yb, Cu, O, and C with their respective weight percentages as 39.37%, 17.02%, 29.27%, and 14.34%. This elemental composition agrees with the findings of XPS and XRD. A more thorough morphology of CuO, Yb2O3, Yb2O3.CuO, and Yb2O3.CuO@rGO nanocomposite were provided by the TEM images in Figure 3(f–i) that are showing a collection of spherical Yb2O3 and elongated CuO NPs dispersed on sheet-like structures of rGO. Figure 3(j) presents an HR-TEM image of the Yb2O3.CuO@rGO nanocomposite and Figure 3(k) displays the SAED patterns, which unequivocally reveal that the composite is polycrystalline.

3.2. Ascorbic acid sensor development

3.2.1. Electrochemical study of Yb2O3.CuO@rGO/GCE assembly

We evaluated the electro-chemical activity of modified electrodes through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Figure 4a illustrates the feeble CV response from a bare GCE in the presence of 40 M AA at +0.52 V; however, the CuO/GCE and Yb2O3.CuO/GCE showed enhanced CV outputs at +0.41 V and 0.28 V, respectively. For Yb2O3/GCE and rGO/GCE electrodes, no CV response was detected. A significantly improved CV result at low potential of +0.25 V was obtained from the Yb2O3.CuO@rGO/GCE. This demonstrates that this Yb2O3.CuO@rGO/GCE assembly possessed the greatest electrocatalytic performance during AA determination than other electrodes shown in Figure 4a. Therefore, we designated the Yb2O3.CuO@rGO/GCE assembly as an AA sensor in this investigation. Additionally, a definite CV peak was produced for the Yb2O3.CuO@rGO/GCE sensor with 40 M AA, while in the absence of AA, no CV response was seen (Figure 4b), further emphasizing the effective electro-chemical properties of Yb2O3.CuO@rGO/GCE as an AA sensor. Figure 4c displays EIS Nyquist plots of bare GCE, CuO/GCE, Yb2O3/GCE, Yb2O3.CuO/GCE, and Yb2O3.CuO@rGO/GCE and relevant equivalent circuit is presented in inset. The Yb2O3.CuO@rGO/GCE electrode was found to have the shortest semicircle diameter, which indicates that its Rct (9.2 k) value is lower than that of other electrodes including bare GCE (75.2 kΩ), CuO/GCE (35.2 kΩ), Yb2O3/GCE (94.7 kΩ), and Yb2O3.CuO/GCE (22.9 kΩ), which were acquired through fitting utilizing the EIS Spectrum Analyzer Software. The smallest semicircular diameter of the Yb2O3.CuO@rGO/GCE electrode suggests that the fabrication process lowered its Rct value. We therefore draw the conclusion that the Yb2O3.CuO@rGO/GCE electrode provided improved electron transfer performance than other modified electrodes shown in Figure 4a.
We investigated the impact of pH between 6.0 and 8.0 with 40 µM AA to better understand the electrochemical AA oxidation. Figure 5(a-b) shows that for pH 6.0 to 7.0, the Ipa value steadily increased, and for pH 7.0 to 8.0, a declining trend was seen. The extreme Ipa was seen at pH ~ 7.0 as shown in Figure 5(b). As a result, pH 7.0 was set as the standard for the remaining tests in this paper. Figure 5(c) displayed a straight-line plot for Epa vs. pH having regression equation (i):
Epa(V) = 0.5614 - 0.0467pH (R2 = 0.9750)
Figure 5(c) showed that the gradient of -56 mV per pH unit over the selected pH range is extremely near to the predicted value of -59, demonstrating that the quantity of transported protons and electrons involved in this AA oxidation are equal [13,19].
Scan rate (ʋ) analysis in Figure 6(a) shows CVs of 40 µM AA acquired using different scan rates (20 - 200 mVs-1) using Yb2O3.CuO@rGO/GCE sensor. The Ipa value in Figure 6(a) was rising as ʋ increased, although the Epa value only marginally changed in a positive way. The nonlinear change of Ipa vs. ʋ in Figure 6(b) suggested that AA oxidation is not a surface-controlled process [59]. While in Figure 6(c), a linear Ipa vs. ʋ1/2 curve was seen, validating a diffusion-controlled process [60] using equation (ii) below.
Ipa(µA) = 190.3043 ʋ1/2 (V1/2s-1/2) - 9.5808 (R2=0.9978)
In addition, a second straight line plot for log(Ipa) vs. log(ʋ) was obtained in Figure 6(d) using equation (iii), which also supports the diffusion-controlled process [61].
log[Ipa (µA)] = 0.6382 log [ʋ (Vs-1)] + 2.3361 (R2=0.9941)
Also, in Figure 6(e), a straight line from Epa vs. log(ʋ) plot was seen using equation (iv).
Epa (V) = 0.0615 log[ʋ (Vs-1)] + 0.3385 (R2=0.9989)
Figure 6(a) exhibited that for ʋ > 70 mVs−1, the value of [Epa-Epc]/2 remained essentially unchanged. Hence, at 100 mVs−1 scan rate, the [Epa-Epc]/2 value assume to be 90.5/nα mV [62], consequently, it was determined that there were 2.29 ≈ 2 transferred electrons (nα). Therefore, it is established that AA oxidation at the Yb2O3.CuO@rGO/GCE surface was a two-electron-transfer system. Overall, scan rate and pH investigations determined that AA oxidation at Yb2O3.CuO@rGO/GCE surface is a combined two-electrons and two-protons, which is consistent with the literature [13].

3.2.2. Sensor parameters determination

We used amperometry for evaluating the sensor performance of the Yb2O3.CuO@rGO/GCE sensor. Amperometric response was acquired at +0.3 V after adding AA of varying concentrations (0.5- 1744 µM) at consecutive time intervals. Figure 7a displays amperometric responses achieved from Yb2O3.CuO@rGO/GCE sensor for AA additions. Herein, the current response in each AA addition increased to around 95% of its maximum current in just 4 seconds. Figure 7b shows a linear segment of calibration plot for 0.5-1744 µM AA using the equation (v).
I(μA) = 0.0214 [AA](µM) + 0.1527 (R2 = 0.9989)
As a result, the Yb2O3.CuO@rGO/GCE sensor's linear detection range (LDR) was determined to be 0.5 - 1571 μM. Additionally, the Yb2O3.CuO@rGO/GCE sensor’s estimated sensitivity value was found to be 0.4341 μAμM-1cm-2 and LOD and LOQ were determined to be ~ 0.062 μM (S/N = 3) and 0.1887 µM, respectively. Sensitivity was calculated using the equation, sensitivity = S/Aeff [61], where Aeff stands for the surface area of the modified electrode (0.0493 cm2) as stated in the electronic supplemental materials [63]. Equations were used to calculate LOD and LOQ are: LOD = 3.3(Sb/S) and LOQ = 10(Sb/S), respectively [64]; here, Sb (0.000403) stands for RSD related to five blank responses and S stands for calibration curve’s slope.
Electrocatalytic performance is dependent on two variables: (i) increase in Ipa and (ii) decreased Epa. Hence, attempts have been made to improve the electrocatalytic activity of GCEs by fabricating them using Yb2O3.CuO@rGO NC. Achieved results showed that the Yb2O3.CuO@rGO/GCE sensor successfully satisfied both of the aforementioned requirements. Figure 4a showed a substantial negative shift of Epa and a significant increase in Ipa from the Yb2O3.CuO@rGO/GCE sensor compared to other electrodes used in this study. We achieved about 3-fold Ipa from the Yb2O3.CuO@rGO/GCE compared to a bare GCE during AA oxidation.

3.2.3. Selectivity, repeatability, reproducibility, and stability

To test the Yb2O3.CuO@rGO/GCE sensor's selectivity, we used common interfering chemicals such as uric acid (UA), glucose (Glc), citric acid (CA), dopamine (DA), Cl- ions, and NO3- ions. Herein, 90 μM AA and an equal concentration of each interfering chemical were used to record the amperometric response (Figure 8a). While AA addition generated a significant amperometric response, no response was observed for the interfering chemicals. This confirms the selectivity of the Yb2O3.CuO@rGO/GCE assembly during the AA detection. Furthermore, various sensor characteristics of Yb2O3.CuO@rGO/GCE were also investigated using CV with 40 M AA. A freshly fabricated Yb2O3.CuO@rGO/GCE assembly was employed to measure 40 M AA for he repeatability study shown in Figure 8b. Five runs with a 4.2% RSD and nearly similar CV responses showed good repeatability. Figure 8c showed the reproducibility study of Yb2O3.CuO@rGO/GCE assembly that used five newly modified Yb2O3.CuO@rGO/GCE electrodes (E1-E5). The Ipa variations in CV responses revealed a 4.7% RSD, demonstrating remarkable reproducibility. In addition, we recorded CV responses every fourth day for a newly modified Yb2O3.CuO@rGO/GCE sensor to assess its stability while keeping it at room temperature. Figure 8d displays the stability investigation bar graph. It demonstrates that the Ipa value in CVs was retained at approximately 81% of its initial value after being stored for 20 days at ambient conditions and Yb2O3.CuO@rGO/GCE sensor surface remained undamaged.
When AA molecule touches the Yb2O3.CuO@rGO surface, an electro-oxidation reaction occurred. AA molecules release electrons to the conduction-band of Yb2O3.CuO@rGO nanocomposite that ultimately enhances the conductivity of Yb2O3.CuO@rGO/GCE sensor and hence an enhanced CV response was obtained. In comparison to other AA sensors, the Yb2O3.CuO@rGO/GCE sensor demonstrated greater sensitivity for AA detection (Table 1) [13,17,39,65-73].
Considering the experimental findings stated above, we may say that AA oxidation at the Yb2O3.CuO@rGO NC is a combined two-electrons and two-protons transfer reaction and in this AA oxidation the Yb2O3.CuO@rGO NC is exceedingly active. The Yb2O3.CuO@rGO/GCE sensor’s appropriateness in detecting AA can be attributed to the effective electrode-analyte interaction. Scheme 1 shows a concise model for electrochemical AA oxidation at this novel Yb2O3.CuO@rGO/GCE sensor.

3.3. Analyses of real samples: AA detection from blood serum and vitamin C tablet

The developed suggested Yb2O3.CuO@rGO/GCE sensor's efficacy was tested by measuring AA in blood serums and vitamin C tablets utilizing the standard addition method. First, we measured Yb2O3.CuO@rGO/GCE sensor's (i-t) response at +0.3 V in 10 ml PBS with 200 µl of undiluted blood serum (BS1) and then three repeated injections of 50 µl 0.01 M AA. Such processes were carried out three times under the same circumstances. Next, we performed the same standard addition procedure using the second blood serum (BS2). Furthermore, we used a dissolved Vitamin C 1000 tablet (Vit-C) from Dallah Pharma Factory, KSA as the real sample as in our previous report [13]. Finally, we repeated the whole standard addition process using 100 µl of Vitamin C and then three repeated injections of 100 µl 0.01 M AA. Table 2 summarizes the outcomes of real sample investigations. These results indicate that with approximately 100% quantitative recovery, this novel Yb2O3.CuO@rGO/GCE sensor can be utilized to precisely assess the presence of AA in real samples. Additionally, the measured level of AA in blood serums is within AA levels typically found in adults (28.5 - 85.2 µM) [7] and for the Vitamin C tablets, calculated AA amount was 98.1 % of the manufacturer’s specification, confirming that the newly-developed Yb2O3.CuO@rGO/GCE sensor is appropriately validated.

4. Conclusions

Herein, we successfully synthesized and characterized the Yb2O3.CuO@rGO nanocomposite. This nanocomposite material was then used to design a sensitive, selective, and reusable electrochemical AA sensor. This AA sensor was developed by a facile technique and is able to measure both high and low levels of AA because of its broad linear dynamic range and high sensitivity. Additionally, this AA sensor demonstrated minimal interference effect, fast response time, a reasonable limit of detection, excellent stability, reproducibility, and repeatability. These features make it a promising tool for detecting AA. To further validate Yb2O3.CuO@rGO/GCE sensor’s accuracy, it was tested utilizing blood serums and vitamin C tablets, and the results were consistent and encouraging. Overall, the method of sensor fabrication presented in this study offers a promising platform for developing highly efficient AA sensor in the future.

Acknowledgments

Authors would like to acknowledge the support of the Deputyship for Research and Innovation-Ministry of Education, Kingdom of Saudi Arabia for this research through a grant (NU/IFC/2/SERC/-/15) under the Institutional Funding Committee at Najran University, Kingdom of Saudi Arabia.

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Figure 1. (a) Survey XPS spectrum of Yb2O3.CuO@rGO NC, (b) Deconvoluted spectra of Yb4d, (c) Cu2p, (d) O1s, and (e) C1s of Yb2O3.CuO@rGO nanocomposite.
Figure 1. (a) Survey XPS spectrum of Yb2O3.CuO@rGO NC, (b) Deconvoluted spectra of Yb4d, (c) Cu2p, (d) O1s, and (e) C1s of Yb2O3.CuO@rGO nanocomposite.
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Figure 2. (a) XRD patterns and (b) Raman spectra of CuO, Yb2O3, and Yb2O3.CuO@rGO NC.
Figure 2. (a) XRD patterns and (b) Raman spectra of CuO, Yb2O3, and Yb2O3.CuO@rGO NC.
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Figure 3. FESEM image: (a) CuO, (b) Yb2O3, (c) Yb2O3.CuO, (d) Yb2O3.CuO@rGO, (e) EDS spectrum of Yb2O3.CuO@rGO; TEM micrograph from (f) CuO, (g) Yb2O3, (h) Yb2O3.CuO, (i) Yb2O3.CuO@rGO; (j) HR-TEM image, and (k) SAED patterns of Yb2O3.CuO@rGO nanocomposite.
Figure 3. FESEM image: (a) CuO, (b) Yb2O3, (c) Yb2O3.CuO, (d) Yb2O3.CuO@rGO, (e) EDS spectrum of Yb2O3.CuO@rGO; TEM micrograph from (f) CuO, (g) Yb2O3, (h) Yb2O3.CuO, (i) Yb2O3.CuO@rGO; (j) HR-TEM image, and (k) SAED patterns of Yb2O3.CuO@rGO nanocomposite.
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Figure 4. CVs recorded at scan rate 0.05 Vs-1 in 0.1 M PBS (pH 7.0) (a) CVs from bare GCE, CuO/GCE, Yb2O3/GCE, rGO/GCE , Yb2O3.CuO/GCE, Yb2O3.CuO@rGO/GCE with 40 µM AA, (b) CVs from the Yb2O3.CuO@rGO/GCE with 40 µM AA and without AA, and (c) EIS Nyquist plots acquired using various electrodes in 1.0 mM [Fe(CN)6]3-/4- in 0.1 M KCl at +0.50 V, at signal amplitude 10 mV and frequency ranging from 0.1 Hz to 100 KHz with a relevant equivalent circuit in the inset.
Figure 4. CVs recorded at scan rate 0.05 Vs-1 in 0.1 M PBS (pH 7.0) (a) CVs from bare GCE, CuO/GCE, Yb2O3/GCE, rGO/GCE , Yb2O3.CuO/GCE, Yb2O3.CuO@rGO/GCE with 40 µM AA, (b) CVs from the Yb2O3.CuO@rGO/GCE with 40 µM AA and without AA, and (c) EIS Nyquist plots acquired using various electrodes in 1.0 mM [Fe(CN)6]3-/4- in 0.1 M KCl at +0.50 V, at signal amplitude 10 mV and frequency ranging from 0.1 Hz to 100 KHz with a relevant equivalent circuit in the inset.
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Figure 5. (a) CVs recorded using 40 µM AA in 0.1 M PBS at varying pH (6.0 – 8.0) at 0.05 Vs-1 scan rate, (b) Ipa vs. pH, and (c) Epa vs. pH.
Figure 5. (a) CVs recorded using 40 µM AA in 0.1 M PBS at varying pH (6.0 – 8.0) at 0.05 Vs-1 scan rate, (b) Ipa vs. pH, and (c) Epa vs. pH.
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Figure 6. Investigation of scan rate effect of Yb2O3.CuO@rGO/GCE sensor: (a) CVs recorded at different scan rates (20 - 200 mVs-1) with 40 µM AA in 0.1 M PBS (b) Ipa vs. ʋ, (c) Ipa vs. √ v , (d) log(Ipa) vs log(ʋ), and (e) Epa vs. log(ʋ).
Figure 6. Investigation of scan rate effect of Yb2O3.CuO@rGO/GCE sensor: (a) CVs recorded at different scan rates (20 - 200 mVs-1) with 40 µM AA in 0.1 M PBS (b) Ipa vs. ʋ, (c) Ipa vs. √ v , (d) log(Ipa) vs log(ʋ), and (e) Epa vs. log(ʋ).
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Figure 7. (a) Yb2O3.CuO@rGO/GCE sensor's amperometric response for AA (0.5 - 1744 M) at +0.3 V potential, and (b) related calibration plot.
Figure 7. (a) Yb2O3.CuO@rGO/GCE sensor's amperometric response for AA (0.5 - 1744 M) at +0.3 V potential, and (b) related calibration plot.
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Figure 8. (a) Amperometric (i-t) response at +0.3 V from Yb2O3.CuO@rGO/GCE sensor upon successive additions of 90 µM of AA, UA, Glc, CA, DA, Cl-, NO3- and AA, (b) repeatability, (c) reproducibility, and (d) stability investigations.
Figure 8. (a) Amperometric (i-t) response at +0.3 V from Yb2O3.CuO@rGO/GCE sensor upon successive additions of 90 µM of AA, UA, Glc, CA, DA, Cl-, NO3- and AA, (b) repeatability, (c) reproducibility, and (d) stability investigations.
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Scheme 1. Schematic representation for Yb2O3.CuO@rGO/GCE-based ascorbic acid sensor.
Scheme 1. Schematic representation for Yb2O3.CuO@rGO/GCE-based ascorbic acid sensor.
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Table 1. Comparative ascorbic acid sensor performance employing various electrodes.
Table 1. Comparative ascorbic acid sensor performance employing various electrodes.
Electrode Technique LDR/
μM		 LOD/
µM		 Sensitivity/
μAμM-1cm-2 		Applied potential/V Ref.
PSi-MC/GCE Amp 0.5–2473 0.03 0.1982 +0.7 [13]
Poly(Py-oPD)/PGE SWV 1-1000 0.026 - - [17]
GO-IL/GCE Amp 10-4000 3.33 - +0.8 [39]
DMA/GCE Amp 25-1650 - 0.178 +0.35 [65]
PoPDoAP/GCE DPV 100-1000 36.4 0.0306 μAμM-1 - [66]
NFG/Ag/PANI Amp 10-11460 8.0 - +1.2 [67]
PG/GCE Amp 9.0-2314 6.45 0.0667 μAμM-1 -0.01 [68]
ZnO/GCE Amp 1-800 0.27 0.1156 μAμM-1 +0.36 [69]
ERGO/GCE DPV 500-2000 150 0.0054 μAμM-1 - [70]
PMES/RGO/GCE DPV 30-100 0.43 - - [71]
NPG Amp 10-1100 2.0 0.0021 μAμM-1 +0.3 [72]
GCE/Au@Pd-RGO DPV 0.01–100 0.002 - - [73]
Yb2O3.CuO@rGO/GCE Amp 0.5–1571 0.062 0.4341 +0.25 This work
PSi-MC = porous silicon-mesoporous carbon; Amp = Amperometry, DMA = N,N Dimethylaniline, GO-IL = Graphene oxide –Ionic liquid, PoPDoAP = poly(o-phenylenediamineco-o-aminophenol), NFG = nanoparticles grafted functionalized graphene, PG = pristine graphene, Poly(Py-oPD)/PGE = pencil graphite electrode modified with a molecularly imprinted copolymer of pyrrole and o-phenylenediamine, PMES = poly(2-(N -morpholine)ethane sulfonic acid), ERGO = electrochemically reduced graphene oxide, NPG = nanoporous gold.
Table 2. AA Detection from commercial vitamin C tablets and blood serums (BS1 & BS2) using the Yb2O3.CuO@rGO/GCE sensor.
Table 2. AA Detection from commercial vitamin C tablets and blood serums (BS1 & BS2) using the Yb2O3.CuO@rGO/GCE sensor.
Real samples Added std. AA (µM) Total AA measured (µM) AA measured in real samples (µM) Recovery (%) RSD (%)
(n = 3)
BS1 48.8 96.2 46.2 102.4 4.52
97.6 147.4 103.7
BS2 48.8 88.1 36.5 105.7 4.13
97.6 137.0 103.0
Vit-C 98.0 176.6 82.4 96.1 4.37
194.2 271.1 97.2
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