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

The ultimate objective of this research work is to design a sensitive and selective electrochemical sensor for the 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.


Introduction
Ascorbic acid (AA) is a vital biomolecule that is present in a variety of naturally occurring sources, including fruits and vegetables, and that 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 such as 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. sensing applications. Hence, in this work, we developed and examined Yb 2 O 3 .CuO@rGO nanocomposite (NC) as the active sensing material for AA detection.
Inspired by previous works available in the literature, we synthesized Yb 2 O 3 -doped CuO nanoparticles to improve stability, sensitivity, and selectivity, and then used a simple sonication technique to synthesize the Yb 2 O 3 .CuO@rGO NC sensing material. This study presents a simple method for preparing an electrochemical AA sensor using Yb 2 O 3 .CuO@rGO NC that offered improved selectivity and sensitivity. As far as we are aware, this will be the maiden article utilizing Yb 2 O 3 .CuO@rGO NC to develop an enzyme-less AA electrochemical sensor.

Materials
All necessary chemicals, including ascorbic acid, copper (II) nitrate, ytterbium (III) nitrate, sodium hydroxide, reduced graphene oxide, NaH 2 PO 4 , Na 2 HPO 4 , 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 Yb 2 O 3 .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, λα 1 = 154.060 pm, λα 2 = 154.439 pm 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 Yb 2 O 3 .CuO@rGO. EDS (JEOL) was used to investigate the elemental composition of the Yb 2 O 3 .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.

Synthesis of CuO, Yb 2 O 3 , Yb 2 O 3 .CuO, and Yb 2 O 3 .CuO@rGO Nanocomposite
To synthesize the CuO, Yb 2 O 3 , Yb 2 O 3 .CuO, and Yb 2 O 3 .CuO@rGO nanocomposites, the following process was followed: firstly, equimolar Cu(NO 3 ) 2 and Yb(NO 3 ) 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 h. Afterwards, the ensuing dark precipitate was cleaned with distilled water and ethanol to remove contaminants and the resulting black precipitate was dried at 80 • C. This as-grown Yb 2 O 3 .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: Precursors, Yb 3+ and Cu 2+ 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 Cu 2+ and OH − exceeds the K sp value. Similarly, Yb(OH) 3 was also produced. Cu 2+ ions easily incorporate themselves into the Yb 2 O 3 lattice because of the similar ionic radii. On heating, hydroxides decompose to produce respective oxides. Similarly, CuO and Yb 2 O 3 NPs were also synthesized.
To synthesize the Yb 2 O 3 .CuO@rGO nanocomposite, 0.5 g of Yb 2 O 3 .CuO NPs and 0.025 g of reduced graphene oxide (rGO) were mixed followed by 40 min of sonication in 80 mL distilled water. This resulting mixture was then filtered and had 12 h of drying in an oven at 70 • C.

Glassy Carbon Electrode Modification Using Yb 2 O 3 .CuO@rGO Nanocomposite
Glassy carbon electrodes (GCEs) (diameter = 3 mm; BAS Inc., Sumida-Ku, Japan) were cleaned using a 1 µm diamond past, followed by a 0.05 µm alumina slurry using the commercially available polishing pads. Next, the GCE was fabricated utilizing Yb 2 O 3 .CuO@rGO nanocomposite using a Nafion solution. During the fabrication process, 4.0 mg of Yb 2 O 3 .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 min. Such a fabricated GCE was labeled as the Yb 2 O 3 .CuO@rGO/GCE. Control experiments were also conducted, in which CuO/GCE, Yb 2 O 3 /GCE, rGO/GCE, and Yb 2 O 3 .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 Yb 2 O 3 .CuO@rGO/GCE, Ag/AgCl, and a platinum spiral were served as the working, reference, and counter electrodes, respectively.

Characterization of Yb 2 O 3 .CuO@rGO Nanocomposite
The elemental compositions and structure of Yb 2 O 3 .CuO@rGO were examined using XPS. It is evident from the XPS analysis shown in Figure 1a-e that Yb 2 O 3 .CuO@rGO nanocomposite is composed of Yb, Cu, O, and C atoms only. The Yb4d 5/2 spectrum has three clearly defined peaks appearing at energies of 187.2, 188.4, and 189.1, which are compatible with Yb4d ( Figure 1b) [50]. In the deconvoluted Cu2p spectrum in Figure 1c, there are two peaks at 937.1 and 956.8 eV that may be related to Cu2p 3/2 and Cu2p 1/2 , respectively [51]. In between these two peaks, there are some satellite peaks that appeared that are also consistent with the literature [52]. Figure 1d shows two peaks from the finescan O1s spectra that are associated with the Yb-O and Cu-O bonds, respectively, at 533.3 and 535.2 eV [13]. Three peaks are shown in the fine-scan C1s spectrum in Figure 1e at energies of 284.6, 287.2, and 289.1 eV. The 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 [55]. The Yb 2 O 3 .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 Yb 2 O 3 .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 Yb 2 O 3 , while bands at 328 and 850 cm −1 were connected to CuO [57]. The characteristic carbon bands at 1344 and 1676 cm −1 are related to the D and G bands of rGO [58].  [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]. The characteristic carbon bands at 1344 and 1676 cm− 1 are related to the 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   [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]. The characteristic carbon bands at 1344 and 1676 cm− 1 are related to the 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 tive 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 was provided by the TEM images in Figure 3f-i that show a collection of spherical Yb2O3 and elongated CuO NPs dispersed on sheet-like structures of rGO. Figure 3j presents an HR-TEM image of the Yb2O3.CuO@rGO nanocomposite and Figure 3k displays the selected area electron diffraction (SAED) patterns, which unequivocally reveal that the composite is polycrystalline.

Electrochemical Study of Yb2O3.CuO@rGO/GCE Assembly
We evaluated the electro-chemical activity of the modified electrodes through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).

Electrochemical Study of Yb 2 O 3 .CuO@rGO/GCE Assembly
We evaluated the electro-chemical activity of the 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 µM AA at +0.52 V; however, the CuO/GCE and Yb 2 O 3 .CuO/GCE showed enhanced CV outputs at +0.41 V and 0.28 V, respectively. For the Yb 2 O 3 /GCE and rGO/GCE electrodes, no CV response was detected. A significantly improved CV result at a low potential of +0.25 V was obtained from the Yb 2 O 3 .CuO@rGO/GCE. This demonstrates that this Yb 2 O 3 .CuO@rGO/GCE assembly possessed the greatest electrocatalytic performance during AA determination in comparison to other the electrodes shown in Figure 4a. Therefore, we designated the Yb 2 O 3 .CuO@rGO/GCE assembly as an AA sensor in this investigation. Additionally, a definite CV peak was produced for the Yb 2 O 3 .CuO@rGO/GCE sensor with 40 µM AA while, in the absence of AA, no CV response was seen (Figure 4b), further emphasizing the effective electro-chemical properties of Yb 2 O 3 .CuO@rGO/GCE as an AA sensor. Figure 4c displays EIS Nyquist plots of bare GCE, CuO/GCE, Yb 2 O 3 /GCE, Yb 2 O 3 .CuO/GCE, and Yb 2 O 3 .CuO@rGO/GCE, and a relevant equivalent circuit is presented in the inset. The Yb 2 O 3 .CuO@rGO/GCE electrode was found to have the shortest semicircle diameter, which indicates that its charge transfer resistance (Rct = 9.2 kΩ) value is lower than that of other electrodes including bare GCE (75.2 kΩ), CuO/GCE (35.2 kΩ), Yb 2 O 3 /GCE (94.7 kΩ), and Yb 2 O 3 .CuO/GCE (22.9 kΩ), which were acquired through fitting utilizing the EIS Spectrum Analyzer Software. The smallest semicircular diameter of the Yb 2 O 3 .CuO@rGO/GCE electrode suggests that the fabrication process lowered its Rct value. We therefore draw the conclusion that the Yb 2 O 3 .CuO@rGO/GCE electrode provided improved electron transfer performance than the other modified electrodes shown in Figure 4a. was detected. A significantly improved CV result at a 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 in comparison to other the 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 µM 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 a relevant equivalent circuit is presented in the inset. The Yb2O3.CuO@rGO/GCE electrode was found to have the shortest semicircle diameter, which indicates that its charge transfer resistance (Rct = 9.2 kΩ) 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 the other modified electrodes shown in Figure 4a.  We investigated the impact of a pH between 6.0 and 8.0 with 40 µM AA to better understand the electrochemical AA oxidation. Figure 5a,b shows that, for a pH of 6.0 to 7.0, the anodic peak current (I pa ) value steadily increased and, for a pH 7.0 to 8.0, a declining trend was seen. The extreme I pa was seen at a pH~7.0, as shown in Figure 5b. As a result, pH 7.0 was set as the standard for the remaining tests in this paper. Figure 5c displayed a straight-line plot for anodic peak potential (E pa ) vs. pH having a regression Equation (1): understand the electrochemical AA oxidation. Figure 5a,b shows that, for a pH of 6.0 to 7.0, the anodic peak current (Ipa) value steadily increased and, for a pH 7.0 to 8.0, a declining trend was seen. The extreme Ipa was seen at a pH ~ 7.0, as shown in Figure 5b. As a result, pH 7.0 was set as the standard for the remaining tests in this paper. Figure 5c displayed a straight-line plot for anodic peak potential (Epa) vs. pH having a regression Equation (1):  Figure 5c 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 (v) analysis in Figure 6a shows the CVs of 40 µM AA acquired using different scan rates (20-200 mVs −1 ) using a Yb2O3.CuO@rGO/GCE sensor. The Ipa value in Figure  6a was rising as v increased, although the Epa value only marginally changed in a positive way. The nonlinear change in Ipa vs. v in Figure 6b suggested that AA oxidation is not a surface-controlled process [59] while, in Figure 6c, a linear Ipa vs. v 1/2 curve was seen, validating a diffusion-controlled process [60] using Equation (2) Figure 5c 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 (v) analysis in Figure 6a shows the CVs of 40 µM AA acquired using different scan rates (20-200 mVs −1 ) using a Yb 2 O 3 .CuO@rGO/GCE sensor. The I pa value in Figure 6a was rising as v increased, although the E pa value only marginally changed in a positive way. The nonlinear change in I pa vs. v in Figure 6b suggested that AA oxidation is not a surface-controlled process [59] while, in Figure 6c, a linear I pa vs. v 1/2 curve was seen, validating a diffusion-controlled process [60] using Equation (2) below.
Additionally, in Figure 6d, a straight line from E pa vs. log(v) plot was seen using Equation  Figure 6a exhibited that for v > 70 mVs −1 , the value of [E pa − E pc ]/2 remained essentially unchanged. Hence, at 100 mVs −1 scan rate, the [E pa − E pc ]/2 value assume to be 90.5/nα mV [61], consequently, it was determined that there were 2.29 ≈ 2 transferred electrons (n α ). Therefore, it is established that AA oxidation at the Yb 2 O 3 .CuO@rGO/GCE surface was a two-electron-transfer system. Overall, the scan rate and pH investigations determined that AA oxidation at the Yb 2 O 3 .CuO@rGO/GCE surface is a combined twoelectrons and two-protons reaction, which is consistent with the literature [13]. Biosensors 2023, 13, x 9 of 17  [61], 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, the scan rate and pH investigations determined that AA oxidation at the Yb2O3.CuO@rGO/GCE surface is a combined two-electrons and two-protons reaction, which is consistent with the literature [13].

Sensor Parameters Determination
We used amperometry for evaluating the sensor performance of the Yb2O3.CuO@rGO/GCE sensor. An 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 the amperometric responses achieved from the Yb2O3.CuO@rGO/GCE sensor for AA additions. Here, the current response in each AA addition increased to around 95% of its maximum current in just 4 s. Figure 7b shows a linear segment of calibration plot for 0.5-1744 µM AA using the Equation (4).

Sensor Parameters Determination
We used amperometry for evaluating the sensor performance of the Yb 2 O 3 .CuO@rGO/ GCE sensor. An 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 the amperometric responses achieved from the Yb 2 O 3 .CuO@rGO/GCE sensor for AA additions. Here, the current response in each AA addition increased to around 95% of its maximum current in just 4 s. Figure 7b shows a linear segment of calibration plot for 0.5-1744 µM AA using the Equation (4).
As a result, the Yb 2 O 3 .CuO@rGO/GCE sensor's linear detection range (LDR) was determined to be 0.5-1571 µM. Additionally, the Yb 2 O 3 .CuO@rGO/GCE sensor's estimated sensitivity value was found to be 0.4341 µAµM −1 cm −2 and limit of detection (LOD) and limit of quantification (LOQ) were determined to be~0.062 µM (S/N = 3) and 0.1887 µM, respectively. The sensitivity was calculated using the equation, sensitivity = S/A eff [62], where A eff stands for the surface area of the modified electrode (0.0493 cm 2 ), as provided in the electronic Supplementary Materials of Figure S1 [19,63,64]. The equations were used to calculate LOD and LOQ are LOD = 3.3(S b /S) and LOQ = 10(S b /S), respectively [65]; here, S b (0.000403) stands for relative standard deviation (RSD) related to five blank responses, and S stands for calibration curve's slope. limit of quantification (LOQ) were determined to be ~0.062 µM (S/N = 3) and 0.1887 µM, respectively. The sensitivity was calculated using the equation, sensitivity = S/Aeff [62], where Aeff stands for the surface area of the modified electrode (0.0493 cm 2 ), as provided in the electronic Supplementary Materials of Figure S1 [19,63,64]. The equations were used to calculate LOD and LOQ are LOD = 3.3(Sb/S) and LOQ = 10(Sb/S), respectively [65]; here, Sb (0.000403) stands for relative standard deviation (RSD) related to five blank responses, and S stands for calibration curve's slope. The 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. The 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 a three-fold Ipa from the Yb2O3.CuO@rGO/GCE compared to a bare GCE during AA oxidation.

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), Clions, and NO3 -ions. Here, 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, the 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 the repeatability study shown in Figure  8b. Five runs with a 4.2% RSD and with nearly similar CV responses showed good repeatability. Figure 8c showed the reproducibility study of the 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. The electrocatalytic performance is dependent on two variables: (i) increase in I pa and (ii) decreased E pa . Hence, attempts have been made to improve the electrocatalytic activity of GCEs by fabricating them using Yb 2 O 3 .CuO@rGO NC. The achieved results showed that the Yb 2 O 3 .CuO@rGO/GCE sensor successfully satisfied both of the aforementioned requirements. Figure 4a showed a substantial negative shift of E pa and a significant increase in I pa from the Yb 2 O 3 .CuO@rGO/GCE sensor compared to other electrodes used in this study. We achieved about a three-fold I pa from the Yb 2 O 3 .CuO@rGO/GCE compared to a bare GCE during AA oxidation.

Selectivity, Repeatability, Reproducibility, and Stability
To test the Yb 2 O 3 .CuO@rGO/GCE sensor's selectivity, we used common interfering chemicals such as uric acid (UA), glucose (Glc), citric acid (CA), dopamine (DA), Clions, and NO 3 ions. Here, 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 Yb 2 O 3 .CuO@rGO/GCE assembly during the AA detection. Furthermore, the various sensor characteristics of Yb 2 O 3 .CuO@rGO/GCE were also investigated using CV with 40 M AA. A freshly fabricated Yb 2 O 3 .CuO@rGO/GCE assembly was employed to measure 40 M AA for the repeatability study shown in Figure 8b. Five runs with a 4.2% RSD and with nearly similar CV responses showed good repeatability. Figure 8c showed the reproducibility study of the Yb 2 O 3 .CuO@rGO/GCE assembly that used five newly modified Yb 2 O 3 .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 Yb 2 O 3 .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 I pa value in CVs was retained at approximately 81% of its initial value after being stored for 20 days at ambient conditions and that the Yb 2 O 3 .CuO@rGO/GCE sensor surface remained undamaged.
Biosensors 2023, 13, x 11 of 17 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 that the Yb2O3.CuO@rGO/GCE sensor surface remained undamaged. When the AA molecule touches the Yb2O3.CuO@rGO surface, an electro-oxidation reaction occurs. AA molecules release electrons to the conduction-band of the Yb2O3.CuO@rGO nanocomposite that ultimately enhance the conductivity of the Yb2O3.CuO@rGO/GCE sensor and, hence, an enhanced CV response can be obtained. In comparison to other AA sensors, the Yb2O3.CuO@rGO/GCE sensor demonstrated a greater sensitivity for AA detection (Table 1) [13,17,39,[65][66][67][68][69][70][71][72][73].    Considering the experimental findings stated above, we may say that AA oxidation at the Yb 2 O 3 .CuO@rGO NC is a combined two-electrons and two-protons transfer reaction and, in this AA oxidation, the Yb 2 O 3 .CuO@rGO NC is exceedingly active. The Yb 2 O 3 .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 Yb 2 O 3 .CuO@rGO/GCE sensor. 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. Scheme 1. Schematic representation for Yb2O3.CuO@rGO/GCE−based ascorbic acid sensor.

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. Firstly, we measured the 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 the 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, the calculated AA amount was 98.1% of the manufacturer's specification, confirming that the newlydeveloped Yb2O3.CuO@rGO/GCE sensor is appropriately validated.

Analyses of Real Samples: AA Detection from Blood Serum and Vitamin C Tablet
The developed suggested Yb 2 O 3 .CuO@rGO/GCE sensor's efficacy was tested by measuring AA in blood serums and vitamin C tablets utilizing the standard addition method. Firstly, we measured the Yb 2 O 3 .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 the real sample investigations. These results indicate that, with approximately 100% quantitative recovery, this novel Yb 2 O 3 .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, the calculated AA amount was 98.1% of the manufacturer's specification, confirming that the newly-developed Yb 2 O 3 .CuO@rGO/GCE sensor is appropriately validated.

Conclusions
Herein, we successfully synthesized and characterized the Yb 2 O 3 .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 a minimal interference effect, a 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 the Yb 2 O 3 .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 a highly efficient AA sensor in the future.

Informed Consent Statement:
The verbal informed consent of the participants was obtained.
Data Availability Statement: Data will be available upon request.