(A) SERS spectra of SS solutions at increasing concentrations of urea (0.01%, 0.1%, 0.2%, 0.4%, 0.8%, 1.5% (w/v)), acquired at three fixed pH values (black lines pH = 4.8, red lines pH = 6.6, blue lines pH = 9.0). The experiments were repeated three times on different AgNWs spots, the reported spectra represent the average of the three repetitions and were stacked for clarity. (B) Calibration curve obtained integrating the spectral raw data of the urea vibrational band comprised in the range 950–1040 cm⁻¹ and peaked at 1002 cm⁻¹ for each concentration value in three different experiments and for three pH values. Experimental data (scatter points) follow a sigmoidal model (coloured lines). The inset shows a log–log plot of the data, fitted by a linear function (y = 4.10 + 0.77x, r ² = 0.98).