1：Experimental Design and Method Selection:

The study designed a composite electrode by mixing carbon nanotubes (CNTs) and TiO2 nanoparticles (NPs) on a glassy carbon electrode to combine high electrochemical activity with photocatalytic self-cleaning properties. Electrochemical methods such as cyclic voltammetry (CV), amperometry, and differential pulse voltammetry (DPV) were employed to evaluate sensor performance.

2：Sample Selection and Data Sources:

Multi-walled CNTs (purity >95%, diameter 40-60 nm) from Shenzhen Nanotech Port Inc., TiO2 NPs (P25 powder, ~25 nm diameter, 80wt% anatase, 20wt% rutile) from Evonik, and analytes 5-hydroxytryptamine (5-HT) and dopamine (DA) from Sigma-Aldrich were used. Solutions were prepared in phosphate buffer (PBS, pH

3：0). List of Experimental Equipment and Materials:

Equipment included a CHI660c potentiostat (CH Instruments Inc.), FE-SEM (FEI Nova 400 Nano), XPS (ESCALB MK-II), UV lamp (CHF-XM500W, Trusttech Inc.), and standard electrochemical cells with SCE reference and Pt auxiliary electrodes. Materials included CNTs, TiO2 NPs, SDS, Nafion, and chemicals for buffer preparation.

4：Experimental Procedures and Operational Workflow:

CNTs and CNTs/TiO2-NPs suspensions were prepared via ultrasonic agitation, coated on polished GC electrodes, stabilized with Nafion, and dried. Electrochemical measurements were conducted in a three-electrode system. Electrode fouling was induced by CV cycles in 5-HT solution, and refreshment was achieved by UV irradiation. Morphology and composition were characterized by SEM and XPS.

5：Data Analysis Methods:

Data from CV, amperometry, and DPV were analyzed for peak currents, linear ranges, detection limits (S/N=3), and reproducibility (RSD). Calibration curves were plotted, and statistical analysis was performed on triplicate experiments.