Fig. 3. (A) Schematic illustrations of constructing a homochiral environment by π–π stacking for sensing DOPA enantiomers. XPS signals of (B) P 2p and (C) N 1s before and after l-Trp was anchored onto PA/TiO₂ NTs. (D) CD spectra of TiO₂ NTs, PA/TiO₂ NTs, and l-Trp/PA/TiO₂ NTs. SERS spectra of PB for sensing different concentrations of (E) l-DOPA and (F) d-DOPA on l-Trp/PA/TiO₂ NTs, and (G) the corresponding Raman intensities at 2158 cm⁻¹.