Electrochemical determination of L-Tryptophan based on a multiwall carbon nanotube/Mg–Al layered double hydroxide modified carbon paste electrode as a sensor

• Combination of two modifiers shows excellent electro-catalytic activity towards tryptophan.
• The sensor combines properties of both CNT and LDH.
• No interference from other biomolecules in the determination of nanomolar Trp.
• Successful application of the sensor in the estimation of tryptophan in milk.
• Excellent lower detection limit, higher efficiency, stability and reproducibility.

A biocompatible modified carbon paste electrode (CPE) for the selective detection of L-Tryptophan (Trp) at physiological pH in the presence of Ascorbic acid (AA), Dopamine (DA) and Uric acid (UA) has been constructed. The surface of CPE was modified with a combination of two modifiers such as the multi-walled carbon nanotubes and Mg–Al–CO3 layered double hydroxide by drop cast method. The surface morphology of the electrodes was studied using FE-SEM images. The Nyquist plots reveal the least charge transfer resistance for the modified electrode. The electrochemical behaviour of the modified electrode was investigated using the cyclic voltammetry method. A very little amount of the modifiers was enough to bring about an excellent sensitivity and electrocatalytic activity towards the oxidation of Trp. Trp exhibits an irreversible electrochemical charge transfer. The quantification of Trp was done by hydrodynamic linear sweep voltammetry using the rotated modified electrode. Two wide linear response ranges were observed for Trp between 3–90 μM and 90–1000 μM with a lower detection limit of 6.8 × 10−9 M (S/N = 3). A large peak separation between Trp and the interfering molecules, good sensitivity and stability enable the modified electrode to be amenable for use in the analysis of Trp individually and in the simultaneous presence of AA, DA and UA at physiological pH. The practical application was successfully demonstrated by estimating Trp in spiked milk and the blood serum. The modified electrode has all the characteristics to be used as a potential sensor for the analysis of Trp in real samples.

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