Recent progress on noble-free substrates for surface-enhanced Raman spectroscopy analysis

Surface-enhanced Raman spectroscopy (SERS) has gained recognition as a potential vibrational spectroscopy technique due to its distinctive fingerprint characteristics, non-invasive nature, high sensitivity, and quick detection response. Following the initial observation of enhanced Raman signals from a pyridine molecule adsorbed on a rough Ag electrode in the 1970 s, a variety of substrates with enhanced capabilities have been explored for diverse applications. In recent times, a range of noble-free SERS substrates has been developed for broad analytical applications owing to their improved activity, inherent chemical stability, and high specificity for target molecules. This review comprehensively examines recent advances in noble-free SERS substrates, primarily focusing on main categories, fabrication strategies, basic enhancement mechanisms, corresponding SERS performance, and representative applications. Firstly, the review illustrates the fundamental working principles of SERS, including electromagnetic (EM) and chemical (CM) enhancements. Subsequently, a detailed overview of the general categories of noble-free SERS substrates is presented. These include carbon-based ma-terials, metal oxides, transition metal dichalcogenides, transition metal nitrides/carbides, polymers, metal--organic frameworks (MOFs), MXene, Si, heterojunctions, and other noble-free SERS substrates. Next, a brief view of their typical applications in bio-analysis, bioimaging, and small molecule analysis is outlined. In the end, this review addresses the challenges and discusses perspectives, including improvement in SERS enhancement, the unification of enhancement mechanisms, and the expansion of applications. The challenges and potential avenues for future research and development are thoroughly discussed and proposed.