Method for selective detection of l-tryptophan using formaldehyde as medium

A method for a selective detection of L-tryptophan (L-Trp) using formaldehyde as a medium, comprising the following steps:

• (1) a preparation of a copper sulfide nanosheets-chitosan/acidified functionalized multi-wall carbon nanotubes (CuS NS-CS/F-MWCNTs) composite material:
• a) preparing surface carboxylated F-MWCNTs by a blending acidification method;
• b) preparing CuS NS;
• c) dissolving the surface carboxylated F-MWCNTs prepared in step a) in anhydrous ethanol to form an F-MWCNT solution with a concentration of 0.8-1.2 mg/mL, mixing the CuS NS prepared in step b) with a CS solution to obtain a first mixture, wherein the CS solution has a mass percentage of 0.8-1.2%, and a mass/volume ratio of the CuS NS and the CS solution is (4.5-5.5) mg:(0.05-0.15) mL, and then dissolving the first mixture in anhydrous ethanol to prepare a CuS NS-CS solution with a CuS concentration of 4.5-5.5 mg/mL;
• (2) a preparation of a composite film-modified electrode CuS NS-CS/F-MWCNTs/GCE:
• polishing a surface of a glassy carbon electrode to obtain a polished surface of the glassy carbon electrode, performing an ultrasonic cleaning and a first air drying on the polished surface of the glassy carbon electrode, and performing a first ultrasonic dispersion on the F-MWCNT solution to obtain an F-MWCNT dispersed solution, then coating the F-MWCNT dispersed solution on the polished surface of the glassy carbon electrode dropwise to obtain a first processed electrode and performing a second air drying on the first processed electrode, to obtain an F-MWCNT-modified glassy carbon electrode;
• performing a second ultrasonic dispersion on the CuS NS-CS solution to obtain a CuS NS-CS homogeneous solution, then coating the CuS NS-CS homogeneous solution on the F-MWCNT-modified glassy carbon electrode to obtain a second processed electrode and performing a third air drying on the second processed electrode, to obtain the composite film-modified electrode CuS NS-CS/F-MWCNTs/GCE;
• (3) using the composite film-modified electrode CuS NS-CS/F-MWCNTs/GCE as a working electrode, an Ag/AgCl electrode as a reference electrode and a platinum wire electrode as a counter electrode to form a three electrode system;
• detecting an electrochemical signal of the L-Trp in a formaldehyde-containing buffer solution by differential pulse voltammetry (DPV), wherein an oxidation peak potential of the L-Trp is 0.82 V;
• and testing the L-Trp with different concentrations, drawing a working standard curve, then detecting the L-Trp in a sample to be tested by a standard addition method, wherein a concentration of the formaldehyde in the formaldehyde-containing buffer solution is 0.02-0.2 M.