Gold Nanoparticles/Nanographene-Based 3D Sensors Integrated in Mini-Platforms for Thiamine Detection

Vitamins are essential for sustaining daily activities and perform crucial roles in metabolism, such as preventing vascular events and delaying the development of diabetic nephropathy. The ultrasensitive assessment of thiamine in foods is required for food quality evaluation. A mini-platform utilizing two 3D sensors based on nanographene and gold nanoparticles paste modified with protoporphyrin IX and protoporphyrin IX cobalt chloride is proposed for the detection of thiamine in blueberry syrup, multivitamin tablets, water, and a biological sample (urine). Differential pulse voltammetry was utilized for the characterization and validation of the suggested sensors. The sensor modified with protoporphyrin IX has a detection limit of 3.0 × 10−13 mol L−1 and a quantification limit of 1.0 × 10−12 mol L−1, whereas the sensor modified with protoporphyrin IX cobalt chloride has detection and quantification limits of 3.0 × 10−12 and 1.0 × 10−11 mol L−1, respectively. High recoveries (values greater than 95.00%) and low RSD (%) values (less than 5.00%) are recorded for both 3D sensors when used for the determination of thiamine in blueberry syrup, multivitamin tablets, water, and urine, demonstrating the 3D sensors’ and suggested method’s high reliability.


Introduction
Thiamine, also known as vitamin B1, is part of the vitamin B complex and plays a role in the development of the brain and neurons [1]. The human body stores at least 30 g worth of vitamin B1 at any given time [2]. Although it is a water-soluble vitamin, it cannot be stored in the human body because it travels with water and is excreted from the body through urine; therefore, it must be ingested regularly [3]. The deficiency of thiamine can lead to beriberi, which can have detrimental effects on the neuron system. Consuming foods rich in thiamine from any source or supplement may serve as a means of protection against the condition. As vitamin B1 plays such an important role in the human body, researchers developed methods to determine whether or not foods or particular supplements contain it [4]. Methods such as high-performance liquid chromatography [1] and fluorescence [5] are just a few of the methods that have been developed. These techniques are superior in that they have a high accuracy and coefficient of determination, in addition to a low detection limit. Nevertheless, these techniques come with a few drawbacks, such as timeconsuming sample preparation, the need for expensive high-tech equipment, and a high price tag. These issues could be circumvented by employing an electrochemical technique, specifically voltammetry. These techniques have a number of benefits, some of which are listed below: sensitivity, the ability to generate data that can be interpreted even at

Methods and Equipment
A mini potentiostat, widely recognized as the EmStat Pico, was connected to a laptop running PSTrace software version 5.9 (PalmSens BV, Houten, The Netherlands) for the purpose of acquiring data in order to carry out electrochemical measurements such as CV, DPV, and electrochemical impedance spectroscopy (EIS). A conventional three-electrode system was implemented by using PIX/AuNPsnGr and PIXCoCl/AuNPsnGr as working electrodes, an Ag/AgCl wire (1 mol L −1 KCl) as a reference electrode, and a Pt wire as a counter electrode. A Mettler Toledo pH meter was employed in order to make the necessary pH adjustments.
Every measurement was performed with the instrument at ambient temperature. A qualitative analysis of the materials that were studied was carried out with the assistance of scanning electron microscopy (SEM) (Inspect S), manufactured by FEI Company Netherlands. All of the samples were analyzed using the ETD detector in high-vacuum mode with a high voltage (HV) of 25 kV and a spot value of 2 at 1600 times the magnification. This was performed so that the picture resolution could be obtained.

Development of the PIX/AuNPsnGr and PIX/AuNPsnGr 3D Sensors' Designs
In order to put together the mini-platform containing the PIX/AuNPsnGr and PIX/ AuNPsnGr electrochemical sensors (Scheme 1), 100 mg of nanographene powder, 10 µL of AuNPs, a suitable quantity of paraffin oil, and 100 µL of PIX and PIXCoCl were physically mixed together in order to obtain homogenous pastes. Following the placement of the pastes into the plastic tubes, a silver wire was used to establish the electrical contact. Before performing each analysis, the electrodes were rinsed with deionized water to remove any residue. Polishing on aluminum foil until a smooth surface was attained allowed the PIX/AuNPsnGr and PIX/AuNPsnGr surfaces to be refreshed. Whenever they were not in use, the 3D sensors were kept at a temperature between 2 and 8 • C.
(Milwaukee, WI, USA). Fluka supplied the paraffin oil (d420, 0.86 g cm −1 ) (Buchs, Sweden). Phosphate buffer solution (PBS, 0.1 mol L −1 ) was made by combining monosodium phosphate and disodium phosphate in a solution-making process. To achieve the desired range of pH values (2.0, 3.0, 4.0, 5.0, 6.0, 7.0, and 8.0), the pH of the buffer solution was modified by adding varying amounts of solutions with a concentration of 0.1 mol L −1 of either NaOH or HCl.
For the preparation of thiamine solutions, PBS (pH 2.0) was utilized. The stock solution (10 −2 mol L −1 ) was prepared only with PBS (pH 2.0) and had a concentration of 10 −2 mol L −1 . The remaining solutions (10 −3 mol L −1 −10 −12 mol L −1 ) were prepared using PBS and 0.1 mol L −1 NaNO3 as supporting electrolytes. When not in use, all solutions were stored in a dark, dry place, at room temperature.

Methods and Equipment
A mini potentiostat, widely recognized as the EmStat Pico, was connected to a laptop running PSTrace software version 5.9 (PalmSens BV, Houten, The Netherlands) for the purpose of acquiring data in order to carry out electrochemical measurements such as CV, DPV, and electrochemical impedance spectroscopy (EIS). A conventional three-electrode system was implemented by using PIX/AuNPsnGr and PIXCoCl/AuNPsnGr as working electrodes, an Ag/AgCl wire (1 mol L −1 KCl) as a reference electrode, and a Pt wire as a counter electrode. A Mettler Toledo pH meter was employed in order to make the necessary pH adjustments.
Every measurement was performed with the instrument at ambient temperature. A qualitative analysis of the materials that were studied was carried out with the assistance of scanning electron microscopy (SEM) (Inspect S), manufactured by FEI Company Netherlands. All of the samples were analyzed using the ETD detector in highvacuum mode with a high voltage (HV) of 25 kV and a spot value of 2 at 1600 times the magnification. This was performed so that the picture resolution could be obtained.

Development of the PIX/AuNPsnGr and PIX/AuNPsnGr 3D Sensors' Designs
In order to put together the mini-platform containing the PIX/AuNPsnGr and PIX/AuNPsnGr electrochemical sensors (Scheme 1), 100 mg of nanographene powder, 10 µL of AuNPs, a suitable quantity of paraffin oil, and 100 µL of PIX and PIXCoCl were physically mixed together in order to obtain homogenous pastes. Following the placement of the pastes into the plastic tubes, a silver wire was used to establish the electrical contact. Before performing each analysis, the electrodes were rinsed with deionized water to remove any residue. Polishing on aluminum foil until a smooth surface was attained allowed the PIX/AuNPsnGr and PIX/AuNPsnGr surfaces to be refreshed. Whenever they were not in use, the 3D sensors were kept at a temperature between 2 and 8 °C.

Procedure: Differential Pulse Voltammetry
Each DPV measurement was carried out within a potential domain that varied from −1.00 V to 1.00 V, a step potential that was 25.0 mV s −1 , and a modulation amplitude that was 100 mV. The calibration curve was established by graphing thiamine concentrations ranging from 10 −3 mol L −1 to 10 −10 mol L −1 against their respective peak heights with a correlation coefficient higher than 0.991. The DPV peaks were given a baseline adjustment.

Samples
The created electrochemical sensors were utilized to determine the thiamine concentration in blueberry syrup, multivitamin tablets, water, and a healthy subject's biological sample (urine). After diluting the samples in PBS with a pH of 2.0 using a volume-tovolume ratio of 1:1, a range of thiamine concentrations were added to the samples.
The following were performed in order to prepare the samples: after 1 g of blueberry syrup was measured using an analytical balance, the sample was diluted with 8 mL of PBS with a pH of 2.0 and 1 mL of 0.1 mol L −1 of NaNO 3 . Before analyzing the water, urine, and multivitamin tablet samples, 1 mL of each sample was diluted with 8 mL of PBS with a pH of 2.0 and 1 mL of 0.1 mol L −1 of NaNO 3 .

Morphological Characterization of the Pastes
For the purpose of determining the morphology of the material, SEM microscopy was performed. Figure 1 illustrates the surface morphology of AuNPSnGr (Figure 1a), PIX/AuNPsnGr (Figure 1b), and PIXCoCl/AuNPsnGr (Figure 1c) pastes. Figure 1a shows the surface morphology of the paste, which consists of numerous layers of irregularly shaped flakes of varying sizes. It is possible to see that the particles in the pastes that are modified with PIX and PIXCoCl, shown in Figure 1b,c, have an irregular shape and are agglomerated in asymmetric formations.
Each DPV measurement was carried out within a potential domain that varied from −1.00 V to 1.00 V, a step potential that was 25.0 mV s −1 , and a modulation amplitude that was 100 mV. The calibration curve was established by graphing thiamine concentrations ranging from 10 −3 mol L −1 to 10 −10 mol L −1 against their respective peak heights with a correlation coefficient higher than 0.991. The DPV peaks were given a baseline adjustment.

Samples
The created electrochemical sensors were utilized to determine the thiamine concentration in blueberry syrup, multivitamin tablets, water, and a healthy subject's biological sample (urine). After diluting the samples in PBS with a pH of 2.0 using a volume-to-volume ratio of 1:1, a range of thiamine concentrations were added to the samples.
The following were performed in order to prepare the samples: after 1 g of blueberry syrup was measured using an analytical balance, the sample was diluted with 8 mL of PBS with a pH of 2.0 and 1 mL of 0.1 mol L −1 of NaNO3. Before analyzing the water, urine, and multivitamin tablet samples, 1 mL of each sample was diluted with 8 mL of PBS with a pH of 2.0 and 1 mL of 0.1 mol L −1 of NaNO3.

Morphological Characterization of the Pastes
For the purpose of determining the morphology of the material, SEM microscopy was performed. Figure 1 illustrates the surface morphology of AuNPSnGr (Figure 1a), PIX/AuNPsnGr (Figure 1b), and PIXCoCl/AuNPsnGr (Figure 1c) pastes. Figure 1a shows the surface morphology of the paste, which consists of numerous layers of irregularly shaped flakes of varying sizes. It is possible to see that the particles in the pastes that are modified with PIX and PIXCoCl, shown in Figure 1b,c, have an irregular shape and are agglomerated in asymmetric formations.

Electrochemical Characterization of the 3D Sensors
Electrochemical characterization was performed using two types of methods (CV and EIS) on bare (AuNPsnGr) and modified (PIX/AuNPsnGr and PIX/CoClAuNPsnGr) electrodes. The CV method was applied in order to assess the electrochemical response of the PIX/AuNPsnGr and PIXCoCl/AuNPsnGr. The CVs displayed in Figure

Electrochemical Characterization of the 3D Sensors
Electrochemical characterization was performed using two types of methods (CV and EIS) on bare (AuNPsnGr) and modified (PIX/AuNPsnGr and PIX/CoClAuNPsnGr) electrodes. The CV method was applied in order to assess the electrochemical response throughout a potential range of −0.6 to 1.0 V, at a scan rate of 0.1 V s −1 , using AuNPsnGr, PIX/AuNPsnGr, and PIXCoCl/AuNPsnGr as working electrodes. When compared to the unmodified sensor, the modified 3D sensors exhibit a higher conductivity. According to these findings, the modifications with both PIX and PIXCoCl result in an improvement in the electrochemical response. where Ip is the peak current (µA), n is the number of transferred electrons (n = 1, in this example), A is the electrode active surface area (cm 2 ), C0 is the concentration of K3[Fe(CN)6] (mol cm −3 ), DR is the diffusion coefficient (7.60 × 10 −6 cm 2 s −1 ), and υ is the scan rate (V s −1 ). The fact that the anodic and cathodic peaks, Ipa (purple dots) and Ipc (pink dots), respectively, exhibit a linear dependence on the square root of the scan rate (Figures 3 and  4) while the scan rate varies from 0.010 to 0.100 V s −1 indicates that the redox process is controlled by diffusion. Figures 3a and 4a illustrate the pattern that emerges as the scan rate and current intensity both continue to increase, whereas Figures 3b and 4b depict the linear dependences of the two peaks, Ipa vs. υ 1/2 and Ipc vs. υ 1/2 , respectively. The sensor based on PIXCoCl/AuNPsnGr exhibits the highest active area (7.23 × 10 −4 cm 2 ) when compared to the other modified sensor, PIX/AuNPsnGr (5.58 × 10 −4 cm 2 ), and when compared to the sensor AuNPsnGr that has not been modified (4.53 × 10 −4 cm 2 ). In order to conduct an analysis of the interfaces of the bare and modified electrodes using the EIS method, the frequency range for the analysis was between 10 5 and 10 −1 Hz. The experiments were carried out in a solution containing 5.0 × 10 −3 mol L −1 of K 3 [Fe(CN) 6 ] and 0.1 mol L −1 of KCl. Figure 2b displays the Nyquist plots obtained for AuNPsnGr (colored red), PIX/AuNPsnGr (colored dark), and PIXCoCl/AuNPsnGr (colored green). According to the Nyquist plot, AuNPsnGr exhibits a large, well-defined semicircle at low frequencies, which matches to a high electrical resistance (R ct = 1899.0). This is due to the high electrical resistance of the material. Following the modification of AuNPsnGr with PIX and PIXCoCl, a semicircle with a reduced diameter (R ct = 572.9) and a semicircle with a more noticeably reduced diameter (R ct = 299.3), respectively, are recorded. This Sensors 2023, 23, 344 6 of 14 modification is successful. As both the diameter of the semicircle and the R ct value are reduced, the electron transfer rate at PIX/AuNPsnGr and PIX/CoClAuNPsnGr surfaces is accelerated. The results of the EIS correspond adequately with the CV measurements.
The electrochemical behavior of AuNPsnGr, PIX/AuNPsnGr, and PIXCoCl/AuNPsnGr was further investigated utilizing the DPV method in pH 2.0 PBS containing 0.1 mol L −1 NaNO 3 as a supporting electrolyte and 10 −9 mol L −1 thiamine. In Figure 2c, the oxidation results of PIX/AuNPsnGr and PIXCoCl/AuNPsnGr for thiamine are greater than those of AuNPsnGr.
Calculating the electroactive surface area permitted the study of the electrocatalytic activity of the sensors, which was performed by applying the Randles-Sevick equation [20,21] to quasi-reversible redox processes controlled by diffusion. As demonstrated below, the peak current intensity on both the anodic and cathodic peaks is directly proportional to the square root of the scan rate: where Ip is the peak current (µA), n is the number of transferred electrons (n = 1, in this example), A is the electrode active surface area (cm 2 ), C 0 is the concentration of K 3 [Fe(CN) 6 ] (mol cm −3 ), D R is the diffusion coefficient (7.60 × 10 −6 cm 2 s −1 ), and υ is the scan rate (V s −1 ). The fact that the anodic and cathodic peaks, Ip a (purple dots) and Ip c (pink dots), respectively, exhibit a linear dependence on the square root of the scan rate (Figures 3 and 4) while the scan rate varies from 0.010 to 0.100 V s −1 indicates that the redox process is controlled by diffusion. Figures 3a and 4a illustrate the pattern that emerges as the scan rate and current intensity both continue to increase, whereas Figures 3b and 4b depict the linear dependences of the two peaks, Ip a vs. υ 1/2 and Ip c vs. υ 1/2 , respectively. The sensor based on PIXCoCl/AuNPsnGr exhibits the highest active area (7.23 × 10 −4 cm 2 ) when compared to the other modified sensor, PIX/AuNPsnGr (5.58 × 10 −4 cm 2 ), and when compared to the sensor AuNPsnGr that has not been modified (4.53 × 10 −4 cm 2 ).

The Optimization of the pH Values and the Supporting Electrolyte
To achieve the best possible results, research was conducted to determine how pH and supporting electrolytes influenced the oxidation of thiamine. Analyses were performed on solutions of PBS with varying pH values, ranging from 1.7 to 8.0, and comprising 10 −4 mol L −1 of thiamine as part of the study on the influence of pH. The maximum oxidation peak of thiamine is obtained in an acidic medium, as shown in Figures 5a and 6a (pH 2.0). As a result of this, pH 2.0 PBS was utilized for all of the subsequent measurements.
To determine the effect of the supporting electrolyte on the electrooxidation of thiamine, different electrolyte solutions (0.1 mol L −1 of NaCl, KCl, NaNO3, and KNO3)

The Optimization of the pH Values and the Supporting Electrolyte
To achieve the best possible results, research was conducted to determine how pH and supporting electrolytes influenced the oxidation of thiamine. Analyses were performed on solutions of PBS with varying pH values, ranging from 1.7 to 8.0, and comprising 10 −4 mol L −1 of thiamine as part of the study on the influence of pH. The maximum oxidation peak of thiamine is obtained in an acidic medium, as shown in Figures 5a and 6a (pH 2.0). As a result of this, pH 2.0 PBS was utilized for all of the subsequent measurements. were added to a solution of pH 2.0 PBS containing 10 −4 mol L −1 of thiamine. When using 0.1 mol L −1 NaNO3, the peak current is the highest; hence, this is the best supporting electrolyte. This can be seen in Figure 5b and 6b. As a result of this, all subsequent thiamine working solutions were prepared using 0.1 mol L −1 of NaNO3 as a supporting electrolyte.

Response Characteristics of the 3D Sensors in Differential Pulse Voltammetry Mode
The response characteristics of the presented 3D electrochemical sensors were identified using differential pulse voltammetry (at the optimal pH level of 2.0), and they can be found in Table 1. Due to the optimal working conditions and electrocatalytic capacity of the porphyrins utilized in their design, the proposed 3D sensors are able to achieve a broad concentration range, high sensitivities, and low quantification and detection limits. Figure 7b shows the calibration graph for thiamine, along with the peaks To determine the effect of the supporting electrolyte on the electrooxidation of thiamine, different electrolyte solutions (0.1 mol L −1 of NaCl, KCl, NaNO 3 , and KNO 3 ) were added to a solution of pH 2.0 PBS containing 10 −4 mol L −1 of thiamine. When using 0.1 mol L −1 NaNO 3 , the peak current is the highest; hence, this is the best supporting electrolyte. This can be seen in Figures 5b and 6b. As a result of this, all subsequent thiamine working solutions were prepared using 0.1 mol L −1 of NaNO 3 as a supporting electrolyte.

Response Characteristics of the 3D Sensors in Differential Pulse Voltammetry Mode
The response characteristics of the presented 3D electrochemical sensors were identified using differential pulse voltammetry (at the optimal pH level of 2.0), and they can be found in Table 1. Due to the optimal working conditions and electrocatalytic capacity of the porphyrins utilized in their design, the proposed 3D sensors are able to achieve a broad concentration range, high sensitivities, and low quantification and detection limits. Figure 7b shows the calibration graph for thiamine, along with the peaks that are produced during the calibration of the PIX/AuNPsnGr sensor. With a correlation coefficient of 0.9916, the linear concentration range is from 1.0 × 10 −12 mol L −1 to 1.0 × 10 −5 mol L −1 . Both the limits of detection (LOD) and the limits of quantification (LOQ) are determined to be 3.0 × 10 −13 mol L −1 and 1.0 × 10 −12 mol L −1 , respectively The following are the LOD and LOQ values: LOD = 3 s/m and LOQ = 10 s/m; where s is the standard deviation of the peak current (10 measurements) of the blank and m is the slope of the calibration curve. The PIX/AuNPsnGr sensor has a sensitivity of 1.49 × 10 8 A mol L −1 .

Studies of the Interference That Occurs with the 3D Electrochemical Sensors
Vitamin B12, ascorbic acid, maltodextrin, fructose, glucose, FeSO 4 , CH 3 COONa, and NH 4 Cl were subjected to a series of tests in an effort to identify whether or not they interfered with the detection of thiamine. The molecules frequently identified with thiamine in blueberry syrup, multivitamin tablets, water, and urine samples were chosen as possible interfering substances. The tolerance limit was defined as the highest interference concentration that caused a change in current intensity in terms of relative error (5% acceptance level), bias (%), and signal change (%). All measurements were conducted with pH 2.0 thiamine solutions that were buffered with PBS at a concentration of 1.0 × 10 −9 mol L −1 and 0.1 mol L −1 NaNO 3 . Experiments demonstrate that the employment of the PIX/AuNPsnGr sensor has no influence on the detection of thiamine, despite the addition of 10-fold CH 3 COONa, 25-fold NH 4 Cl, 50-fold FeSO 4 , fructose, and 100-fold glucose, maltodextrin, ascorbic acid, and vitamin B12 ( Table 2). This demonstrates that the suggested sensor has a good selectivity for thiamine determination. Table 2. The influence of possible interfering species on the detection of 1.0 × 10 −9 thiamine (n = 6) using the PIX/AuNPsnGr sensor. Experiments reveal that the addition of 25-fold NH 4 Cl, 50-fold FeSO 4 , 100-fold ascorbic acid, fructose, glucose, CH 3 COONa, maltodextrin, and vitamin B12 has no influence on the detection of thiamine using the PIXCoCl/AuNPsnGr sensor (Table 3). This demonstrates that the suggested sensor has a good selectivity for thiamine determination. Table 3. The influence of possible interfering species on the detection of 1.0 × 10 −9 thiamine (n = 6) using the PIXCoCl/AuNPsnGr sensor.

Reproducibility, Repeatability, and Stability
The repeatability, reproducibility, and stability of the designed 3D sensors (PIX/ AuNPsnGr and PIXCoCl/AuNPsnGr) were studied by DPV using a solution of thiamine (1.0 × 10 −9 mol L −1 ) in PBS pH 2.0 and 0.1 mol L −1 NaNO 3 , under the ideal experimental conditions (Figure 9). Using three identically made new sensors of each type, the reproducibility was studied. It is determined that the relative standard deviation (RSD%) for the PIX/AuNPsnGr sensor is 0.15% (n = 3), whereas the PIXCoCl/AuNPsnGr sensor has an RSD% of 0.16% (n = 3). Sensors 2023, 23, x FOR PEER REVIEW 12 of 16 Repeatability within a single day is determined to be 4.31% (n = 10) for the PIX/AuNPsnGr sensor and 3.21% (n = 10) for the PIXCoCl/AuNPsnGr sensor. Ten days were used to test the sensors' stability ( Figure 10). During the whole stability study, the modified electrodes were stored at 2-8 °C. After 10 days, for the PIX/AuNPsnGr sensor, the current intensity of thiamine (1.0 × 10 −9 mol L −1 ) in PBS pH 2.0 and 0.1 mol L −1 NaNO3 decreases to 91.38% of the initial value from the first day of analysis; and for the PIXCoCl/AuNPsnGr sensor, the peak current also decreases to 92.82% of the initial value from the first day of analysis. The number of sensors used to determine reproducibility is n = 3. In contrast, n = 10 denotes the number of determinations in the case of repeatability and stability.

Determination of Thiamine in Food, Water and Biological Samples
Prior to their widespread use in the monitoring of food quality and control of food security, the suggested 3D sensors must be validated. Standard addition was used to test the 3D sensors' capability to reliably assess thiamine in blueberry syrup, multivitamin tablets, water, and urine samples. This method was utilized to present the precision of 3D sensors. As shown in Tables 4 and 5, the reproducibility of the thiamine analysis in blueberry syrup, multivitamin tablets, water, and urine samples is quite high.
After inserting the 3D sensors into the samples, the peak current was measured. The aforementioned calibration equation was used to calculate the thiamine concentrations. Recovery, RSD, and bias (%) results are displayed in Tables 4 and 5. Repeatability within a single day is determined to be 4.31% (n = 10) for the PIX/ AuNPsnGr sensor and 3.21% (n = 10) for the PIXCoCl/AuNPsnGr sensor. Ten days were used to test the sensors' stability ( Figure 10). During the whole stability study, the modified electrodes were stored at 2-8 • C. After 10 days, for the PIX/AuNPsnGr sensor, the current intensity of thiamine (1.0 × 10 −9 mol L −1 ) in PBS pH 2.0 and 0.1 mol L −1 NaNO 3 decreases to 91.38% of the initial value from the first day of analysis; and for the PIXCoCl/AuNPsnGr sensor, the peak current also decreases to 92.82% of the initial value from the first day of analysis. The number of sensors used to determine reproducibility is n = 3. In contrast, n = 10 denotes the number of determinations in the case of repeatability and stability. Repeatability within a single day is determined to be 4.31% (n = 10) for the PIX/AuNPsnGr sensor and 3.21% (n = 10) for the PIXCoCl/AuNPsnGr sensor. Ten days were used to test the sensors' stability ( Figure 10). During the whole stability study, the modified electrodes were stored at 2-8 °C. After 10 days, for the PIX/AuNPsnGr sensor, the current intensity of thiamine (1.0 × 10 −9 mol L −1 ) in PBS pH 2.0 and 0.1 mol L −1 NaNO3 decreases to 91.38% of the initial value from the first day of analysis; and for the PIXCoCl/AuNPsnGr sensor, the peak current also decreases to 92.82% of the initial value from the first day of analysis. The number of sensors used to determine reproducibility is n = 3. In contrast, n = 10 denotes the number of determinations in the case of repeatability and stability.

Determination of Thiamine in Food, Water and Biological Samples
Prior to their widespread use in the monitoring of food quality and control of food security, the suggested 3D sensors must be validated. Standard addition was used to test the 3D sensors' capability to reliably assess thiamine in blueberry syrup, multivitamin tablets, water, and urine samples. This method was utilized to present the precision of 3D sensors. As shown in Tables 4 and 5, the reproducibility of the thiamine analysis in blueberry syrup, multivitamin tablets, water, and urine samples is quite high.
After inserting the 3D sensors into the samples, the peak current was measured. The aforementioned calibration equation was used to calculate the thiamine concentrations. Recovery, RSD, and bias (%) results are displayed in Tables 4 and 5.

Determination of Thiamine in Food, Water and Biological Samples
Prior to their widespread use in the monitoring of food quality and control of food security, the suggested 3D sensors must be validated. Standard addition was used to test the 3D sensors' capability to reliably assess thiamine in blueberry syrup, multivitamin tablets, water, and urine samples. This method was utilized to present the precision of 3D sensors. As shown in Tables 4 and 5, the reproducibility of the thiamine analysis in blueberry syrup, multivitamin tablets, water, and urine samples is quite high. After inserting the 3D sensors into the samples, the peak current was measured. The aforementioned calibration equation was used to calculate the thiamine concentrations. Recovery, RSD, and bias (%) results are displayed in Tables 4 and 5.
The employment of the PIX/AuNPsnGr sensor results in the outcomes of excellent recovery values, as shown in Table 4. The recoveries for both the water and food samples are above 95%. In addition, the urine sample has a recovery value greater than 100%. RSD values for the samples range from 0.07 to 3.04. In the instance of the PIXCoCl/AuNPsnGr sensor, Table 5 reveals that the recoveries in water, food, and urine samples are greater than 95%, with RSD values ranging between 0.19 and 2.01. Consequently, based on the fact that Tables 4 and 5 contain very good values, it can be stated that both sensors, PIX/AuNPsnGr and PIXCoCl/AuNPsnGr, demonstrate that they are able to provide better outcomes for sensitivity and selectivity of the assay for thiamine in food, water, and biological sample. Table 6 shows that compared with other electrochemical methods proposed in the literature for the assay of thiamine, the 3D sensors present wider linear concentration ranges, as well as lower limits of detection. Table 6. Comparison of the new developed method of analysis of thiamine (using 3D sensors) with other methods presented in the literature.

Conclusions
The technological application of nanographenes will undoubtedly pave the way for the development of progressively more sensitive electrochemical sensors. Nanographenes are fascinating new instruments that, perhaps, have the potential to enhance food safety and quality control monitoring. In the perspective of such, the present study offers a miniplatform comprising two 3D electrochemical sensors that were developed, characterized, tested, and validated for the assessment of thiamine in blueberry syrup, multivitamin tablets, water, and a biological sample. Based on AuNPs and nGr paste, these 3D sensors were modified with PIX and PIXCoCl. Both sensors display extraordinarily high levels of stability, selectivity, sensitivity, and reproducibility in their respective studies. The suggested 3D sensors have the advantage of being able to be employed in the analysis of food, pharmaceutical, water, and biological samples in relation to the detection of the amount of thiamine contained by those samples.

Institutional Review Board Statement:
The study was conducted in accordance with the Declaration of Helsinki, and approved by Ethics Committee approval number 75/2015 awarded by the University of Medicine and Pharmacy "Carol Davila" from Bucharest.