<p>Lead is a highly toxic and persistent heavy metal that poses serious risks to human health. The detection of lead ions in water is therefore essential not only for balancing economic and environmental priorities, improving public services, and ensuring agricultural safety, but also for preventing lead poisoning, promoting health equity, and safeguarding international trade in the context of global health. Conventional detection methods are often limited by expensive instrumentation and complex procedures, whereas surface-enhanced Raman scattering (SERS) has emerged as a promising alternative due to its high sensitivity and operational simplicity. In this study, we developed an ultrasensitive SERS-based method for the Pb²⁺ detection using L-cysteine-functionalized bismuth nanoparticles as probes. L-cysteine binds Pb²⁺ through its −COOH and −NH₂ groups, inducing nanoparticle aggregations and generating Raman hotspots that enhance the signal of 4-aminothiophenol (4-ATP). Additionally, an electrodeposited bismuth substrate further amplifies the SERS response. This method achieves a detection limit as low as 0.005 nmol·L⁻¹ (1.04 · 10⁻³ µg·L⁻¹), demonstrating 2–5 orders of magnitude greater sensitivity compared to conventional lead ion detection techniques.</p>