Amino acids are derived by the human body through the breakdown of proteins that enters in the form of diet in foods. Among 22 standard amino acids, L-Tyrosine (Tyr) and L-Tryptophan (Trp) play important roles in the biochemical, nutritional and clinical significance in mammalian systems. Tyrosine (4- hydroxyphenylalanine) is a non-essential amino acid because, under normal conditions, the body synthesizes sufficient quantities from phenylalanine. It acts as a precursor for catecholamine neurotransmitters and hormones such as L-dopa, dopamine, norepinephrine and epinephrine, melanin and thyroid hormones. Tryptophan (2S-2-amino-3-(1H-indol-3-yl) propanoic acid) is one of the essential amino acids which is commonly synthesized in plants and microorganism from shikimic acid. Trp acts as precursor for the indolamine neurotransmitters such as serotonin, melatonin and vitamin B6 niacin. Improper metabolism of both the aromatic amino acids leads to depression, Parkinson’s disease, sister chromatid exchange, hallucinations, schizophrenia, drowsiness and appetite suppression [1]. Therefore, the development of a simple and sensitive tool for the determination of amino acids in food products and pharmaceuticals is necessary to have great importance in life science research.

Here, we report a simple method for the electrochemical determination of Tyr and Trp based on ultraviolet (UV) irradiated tungsten trioxide (WO₃) nanoparticles modified glassy carbon electrode (GCE) without using surface mediators or promotors. Irradiation with appropriate sources such as gamma rays, swift heavy ion and low energy ion beam have been used as an efficient tool to improve its physic-chemical properties [2-5]. In addition to these fairly sophisticated tools, UV light irradiation can be considered as an alternative method due to its advantageous features like room temperature processes, creating defects in a controlled manner, easy to operate and long service life. Li et al., reported that the exposure of WO₃ photoanodes to UV irradiation leads to improvements in photoelectrochemical activity towards the oxygen evolution reaction [6].

In the current work, WO₃ nanoparticles were synthesized via UV irradiation with variant exposure time (2, 4, 6 and 8 hr) and characterized by powder XRD, FESEM, Raman, UV-Vis absorption and photoluminescence spectra. Optimally irradiated (4 hr) WO₃ NPs exhibited reduced crystallite size with fragmented spherical particles and increased band gap energy due to induced oxygen defects and vacancies. Cyclic Voltammetry (CV) studies of UV-4 hr WO₃/GCE showed an excellent electrocatalytic activity towards the oxidation of Tyr and Trp in phosphate buffer solution (PBS, pH 6.0) in the presence of several potentially interfering compounds. Differential pulse voltammetry (DPV) studies of the modified GCE exhibited linear response over a wide concentration range of 0.1 to 1100 µM (Tyr) and 0.1 to 1000 µM (Trp) with the lowest detection limit of 41 nM and 55 nM respectively for Tyr and Trp.

Under optimum experimental conditions, influence of various interfering compounds was investigated by DPV method. A series of foreign species were added into 0.1 M PBS (pH 6.0) containing 2 µM Tyr and 5 µM Trp and the tolerance limit was found to be approximately ±2% relative error in the determination of Tyr and Trp. It was noted that 500-fold excess of ascorbic acid, uric acid, cysteine, Na⁺ and K⁺ ions and 50-fold excess of dopamine, serotonin, melatonin, epinephrine and norepinephrine had no significant effect on the response of the two analytes. These results indicate that the proposed electrode can act as a highly selective sensor for recognition of Tyr and Trp in aqueous solutions. The fabricated sensor was successfully applied to determine Tyr and Trp concentration in milk, curd and egg samples with satisfactory recoveries.

Keywords: WO₃ nanoparticles; Ultraviolet irradiation; Electrochemical sensor; Tyrosine; Tryptophan; Dairy products.

References

[1] A. B. Hughes, WILEY Publishers, 1 (2009) 1-734. ISBN: 978-3-527-32096-7.

[2] A.C. Anithaa, N. Lavanya, K. Asokan, C. Sekar, Electrochim. Acta 16 (2015) 294-302.

[3] A.C. Anithaa, K. Asokan, C. Sekar, Sens. Actuators B Chem. 238 (2017) 667-675.

[4] A.C. Anithaa, K. Asokan, C. Sekar, Electrochim. Acta 237 (2017) 44-53.

[5] A.C. Anithaa, K. Asokan, C. Sekar, Sens. Actuators B Chem. 247 (2017) 814-822.

[6] T. Li, J. He, B. Pena, C. P. Berlinguette, ACS Appl. Mater. Interfaces 8 (2016) 25010-25013.