Tyrosine has been introduced as one of the nutritionally essential amino acids and also one of the essential constituents of protein [1, 2], with a significant contribution to metabolism and development of animals and humans. Actually, it is a precursor of DOPA (dihydroxyphenylalanine), dopamine (DA), thyroxin and neurotransmitters in foods like fish, eggs, soy, bananas and milk [3,4]. The enhanced tyrosine contributes to Parkinson’s, mood disorder as well as depression and lower level of tyrosine may result in alkaptonuria and albinism [5,6]. Thus, effective and robust methods for tyrosine determination are of great importance. Over the past years, several conventional methods such as spectrophotometric , fluorescence , capillary electrophoresis (CE) , liquid chromatography coupled with mass spectrometry (LC-MS) , and gas chromatography coupled with mass spectrometry (GC–MS)  have been used to detect and quantify tyrosine in pharmaceutical formulations.
Although these techniques may show acceptable sensitivity and LODs, they frequently need laborious detection procedures and complicated pretreatment stages. In addition, they are rather complex, costly and it is not possible to employ them in on-site measurements. In comparison to these procedures, our new electro-chemical techniques are largely attracted in electroanalysis of various compounds because of merits like simplicity, inexpensiveness, higher sensitivity, and simplification for in-situ determination [12-19].
Miniaturizing the electrodes employed in voltammetric procedures is one of the options for reducing expenses and possible utilization of on-site analyses. Hence, screen-printed electrodes (SPEs) would be one of the alternatives to conventional electrodes due to their affordability, acceptable reproducibility, as well as lower level of toxicity [20,21]. Another proposed benefit has been the possible application of multiple conductive materials in its fabrication that may be modified with electro-catalytic compounds [22, 23].
The high-performance electrochemical sensing of detecting target molecules is significantly dependent on the electrode materials, so the modification of electrode is highly crucial. In addition, the chemically-modified electrodes (CMEs) have been highly considered in analytical chemistry according to electro-catalysis and design of novel of electro-chemical sensors and biosensors [24-27].
It is widely accepted that nanotechnology represents design, construction, and utilization of the nano-structures or nanomaterials as well as basic understanding of the correlation between physical features, or phenomena, and the size of the materials. Researchers have been attracted by nano-materials due to the emergence of specific optical, electrical, magnetic, and other features at this scale. Such features can be highly used in medicine, electronics, and other areas [28-30]. In recent years, nanomaterials have found a broad application in designing and developing the modified electrodes towards the potential application of medical diagnosis, environmental monitoring and food safety [31-34].
MXene has been introduced as one of the novel classes of two-dimensional nanomaterials first proposed in 2011  and has been considerably addressed in different researches [36,37]. The general formula of MXene is Mn+1XnTx where M represents the initial transition metal and X is C and or N. Moreover, T denotes the surface termination (H, O or F), and n=1, 2, or 3. Researchers have shown numerous benefits like larger surface areas, very good electrical conductivity, consistency with water and organic solvents as well as easier fabrication under room temperature, that have been related to the layered architecture, presence of metal in its components, and the presence of negative charge on the surface. Since it enjoys specific structural, chemical and electrical features, MXene has high potential in several utilizations like catalysts , super capacitors  and sensors . The most investigated form of MXene has been considered to be Ti3C2Tx that results from its simplified synthesis, higher mechanical strength, hydrophilicity and acceptable catalysis features .
Accordingly, the present work aimed to develop a sensitive, fast, portable, low cost electrochemical sensor for the determination of tyrosine.
Material and Methods
Chemicals and instrumentations
All materials in this study had analytical grade with no extra purification, which belonged to Sigma-Aldrich and Merck. A potentiostat/galvanostat AUTOLAB PGSTAT 302N (Metrohm, Switzerland) was utilized to carry out all experiments during the electrochemical processes, under monitoring of General Purpose Electrochemical System (GPES) software Version 4.9. The DropSens SPGE (DRP-110, Spain) was used for all electrochemical tests. The three-electrode cell contained a 4-mm graphite as the working electrode, a graphite as the auxiliary electrode, and a silver as pseudo-reference electrode. A Metrohm 713 pH-meter equipped with a glass electrode was utilized to measure the pH values of all solutions. In addition, orthophosphoric acid as well as the respective salts (KH2PO4, K2HPO4, and K3PO4) with a pH ranging between 2.0 and 9.0 was utilized to procure buffer solution.
Synthesis of Ti3C2 nano layers
In the first step, titanium powder, aluminum powder and graphite powder with molar ratios of 3, 1.1, and 1.9 were injected into the pellet mill over a period of 360 minutes at 600 rpm to synthesize the nano layers. MXene was prepared in powder form with Ti3AlC2 formula. Next, 60 ml of 40% purity hydrogen fluoride (HF) solution was added to 0.2 g of Ti3AlC2 powder and heated at 25 °C for 20 h. Afterwards, to prepare MXene with Ti3C2 formula, the prepared suspension was centrifuged and washed with deionized water, and the precipitate was dried at 55 °C .
Modification of SPE
The modification of SPE surface with Ti3C2 nano layers was accomplished in this way: 1 mg of Ti3C2 nano layers was suspended in 1 mL of the distilled water for forming the suspension, which was sonicated for 40 min for dispersing the nano-composite. Finally, a 4 μL aliquot of the suspension was pipetted over the surface of a SPE and drying was done at the ambient temperature.
Results and Discussion
Electrochemical behavior of tyrosine at the Ti3C2/SPE
The electrochemical determinations are significantly influenced by the solution pH. Hence, we conducted the tests to determine the pH effect on electrocatalytic behavior of tyrosine at Ti3C2/SPE. The differential pulse voltammetry (DPV) was employed to study the effect of electrolyte solution pH (0.1 M phosphate buffer solution (PBS)) under different values (2-9) in the presence of 200.0 μM of tyrosine at 50 mV/s on the Ti3C2/SPE. The oxidation peak current of tyrosine was maximum at pH 7.0, thereby selecting this value as the optimum pH in the droxidopa detection.
In the next step, we examined the potential application of Ti3C2/SPE for electro-oxidation and determination of tyrosine via CV. Figure 1 depicts the cyclic voltammetry (CV) response for 200.0 μM tyrosine oxidation on (a) Ti3C2/SPE, and (b) bare SPE in 0.1 M PBS of the pH=7.0 at the scan rate 50 mVs-1. Moreover, over-potential decreased and the peak current was enhanced for electroxidation of tyrosine at Ti3C2/SPE as compared to bare SPE that was caused by increasing the simplification of the electron transfer process.
Effect of scan rate on the results
Figure 2 shows the use of linear sweep voltammetry (LSV) to determine the scan rate influence on the tyrosine oxidation electrocatalytically on the Ti3C2/SPE. As seen in Figure 2, the peak current of oxidation increased by elevating the scan rate. The peak current (Ip) plot versus the scan rate square root (ν1/2) was linear ranging from 10 to 700 mV/s, which means diffusion process rather than surface-controlled process.
Figure 1: CV response of 200.0 μM tyrosine at (a) Ti3C2/SPE and (b) bare SPE in 0.1 M PBS of pH 7.0
Figure 2: LSV curves of 150.0 μM tyrosine in PBS (0.1 M, pH = 7.0) at different scan rates (10-700 mV/s) on Ti3C2/SPE (1-7 refers to 10, 30, 70, 100, 300, 500 and 700 mV s-1). Inset: Plot of scan rate square root versus tyrosine oxidation peak current
Chronoamperometric determinations of tyrosine on Ti3C2/SPE surface were done by adjusting the potential of working electrode at 0.75 V. The findings from various tyrosine contents in PBS (at pH value of 7.0) are depicted in Figure 3. The chronoamperometric measurement of electroactive moieties under the limited conditions of mass transfer was based on Cottrell equation as follows:
I = nFAD1/2Cbπ–1/2t–1/2
In this equation, D stands for the diffusion coefficient (cm2 /s) and Cb for the bulk concentration (mol/cm3). The I plot against t-1/2 was on the basis of empirical data (Figure 3A), with the optimal fits for various tyrosine contents. Then, the slopes from straight lines (Figure 3A) were drawn against tyrosine content (Figure 3B). At last, the slope from plot in Figure 3B and Cottrell equation were applied to calculate the mean D value, which was 5.3 ×10-5 cm2/s.
Figure 3: The chronoamperograms obtained on Ti3C2/SPE in PBS (0.1 M, pH = 7.0) at different tyrosine concentrations; Note: 1–4: 0.1, 0.5, 1.0, and 2.0 mM of tyrosine. Inset A) plot of I versus t-1/2 based on chronoamperograms (1-4). Inset B) slope plot of straight line versus tyrosine concentration
DPV analysis of tyrosine at Ti3C2/SPE
DPV can increase sensitivity and better features for analytical purposes. Therefore, the voltammetric sensor of Ti3C2/SPE towards tyrosine detection was investigated by DPV. Figure 4 shows the DPV curves of tyrosine with various concentrations in PBS (0.1 M, pH 7.0) solution (Step potential=0.01 V and pulse amplitude=0.025 V). Based on Figure 4, the anodic peak currents exhibit linear elevation with various droxidopa contents (0.5-700.0 μM). The LOD was estimated to be 0.15 μM. The LOD and linear range of tyrosine at Ti3C2/SPE electrode presented in this work was compared with the reported modified electrodes and are given in Table 1.
Figure 4: DPV response of tyrosine at Ti3C2/SPE in the concentration range 0.5 μM – 700.0 μM in PBS (0.1 M, pH = 7.0); 1-9 refers to 0.5, 5.0, 20.0, 40.0, 70.0, 100.0, 300.0, 500.0, and 700.0 µM; inset: The calibration curve of DPV peaks against concentration of tyrosine
One of the electro-chemical sensors for sensitive detection of tyrosine was constructed by modification of SPE with nanolayered Ti3C2. According to the analyses, the modified electrode demonstrated acceptable electrocatalytic activities and sensitivity. Compared with unmodified SPE, the Ti3C2/SPE decreased oxidation overpotential and increased current response of the tyrosine. Linear response from 0.5–700.0 μM was obtained based on tyrosine electrochemical oxidation through DPV. The detection limit of the method for tyrosine was 0.15 μM (S/N = 3).
The authors wish to thankful the Graduate University of Advanced Technology, Kerman, Iran for financial support of this research.
The authors wish to thankful the Graduate University of Advanced Technology, Kerman, Iran for financial support of this research
All authors contributed toward data analysis, drafting and revising the paper and agreed to responsible for all the aspects of this work.
Conflict of Interest
We have no conflicts of interest to disclose.
HOW TO CITE THIS ARTICLE
Hadi Beitollahia, Navid Arbabi, Nanolayered Ti3C2 Modified Screen Printed Electrode as High‐Performance Electrode for Electrochemical Detection of Tyrosine. Chem. Methodol., 2022, 6(4) 293-300