In shell-isolated nanoparticle (NP)-enhanced Raman spectroscopy (SHINERS), traditional metal oxide-based shells have inferior chemical inertness, they require strict preparation conditions, and lack specific groups, which lead to their poor selectivity toward target molecules. In this study, ultrathin and compact gold (Au)@polydopamine (PDA) SHINERS NPs were successfully fabricated by a simple self-polymerization technique. High wrapping tendency of PDA, a multifunctional biopolymer, favored the fabrication process. Au@PDA NPs exhibited a typical shell-isolated effect, i.e., Au@PDA NPs with a thick shell (more than 2.3 nm) showed a lower SERS activity, while those with an ultrathin (1.3 nm) shell exhibited higher SERS activity compared to uncoated Au NPs. The Au@PDA SHINERS substrate shows high performance in terms of sensitivity, uniformity, and stability. The relative standard deviations (RSDs) of SERS intensities from ten positions on the same substrate were less than 4%. Their Raman intensities dropped by only 15% over two months. More importantly, the Au@PDA (1.3 nm) SHINERS substrate exhibited high SERS activity for label-free and quantitative detection of benzotriazole (BTA), an important corrosion inhibitor, through utilizing a presumed π-π stacking interaction. A broad linear range from 10-4 to 10-8 M was achieved with a low detection limit (LOD) of 1 nM (0.119 μg L-1). The LOD was not only significantly lower than the maximum allowable level (20 μg L-1) of the Australian government water guide, but also lower than that of some modern methods such as fluorescence, liquid chromatography, and gas chromatography coupled with mass spectrometry. Furthermore, the substrate showed excellent discrimination against other compounds with a single aromatic ring. It is expected that the Au@PDA SHINERS substrate will offer great potential for analysis application in a complicated environmental system.