Evaluation of two-dimensional transition-metal carbides and carbonitrides (MXenes) for SERS substrates

This initial report demonstrates the family of two-dimensional transition-metal carbides and carbonitrides (MXenes) as promising plasmonic materials for surface-enhanced Raman spectroscopy (SERS), allowing sensitive detection of analyte molecules. Their surface plasmon resonances, metallic characters, and rich surface chemistries enable both SERS electromagnetic and chemical enhancements. We have synthesized seven MXenes—Nb2C, Mo2C, Ti2C, V2C, Ti3C2, Mo2TiC2, and Ti3CN, of which only Ti3C2 and Nb2C have been explored as pristine substrates for SERS applications. All explored MXenes could detect the 10–7 M concentrations of Rhodamine 6G dye. Ti3C2 and Ti2C show the highest enhancement and were compared to the commercially available substrates, where Ti3C2 outperformed gold nanoparticles. Additionally, the best-performing MXene was tested with a variety of dyes, suggesting the versatility of MXene sensing. Due to high charge carrier densities and tunable surface chemistries, MXenes show potential for SERS sensors that are inexpensive, easy to fabricate and optimize for specific applications. Surface-enhanced Raman spectroscopy is a powerful, non-destructive analytical tool that provides high selectivity and sensitivity and could be used for single-molecule characterization. It allows for fast detection of environmental pollutants and toxins, as well as biosensing and environmental monitoring. However, the prevalence of the application is limited by the cost and stability of commercially available substrates that employ noble metal nanostructures (Ag, Au, Pt). Recent research activities have focused on discovering new enhancing substrates: the Ti3C2 MXene has been gaining interest as a SERS substrate due to its surface plasmon resonance, metallic behavior, and surface terminations, achieving the record-setting enhancement factors. In contrast, this study elucidates the capability of MXenes beyond Ti3C2 to perform as SERS substrates. We showed that Nb2C, Mo2C, Ti2C, V2C, Ti3C2, Mo2TiC2, and Ti3CN MXenes could detect low concentrations of dye molecules. Furthermore, compared to the commercial gold nanoparticle substrate, Ti3C2 showed advantageous performance. There are approximately 50 MXenes that have been synthesized to date. Their high charge carrier densities and tunable surface chemistries open the avenues of SERS detection of various analytes, with the perspective of replacing noble metals with materials made of earth-abundant elements. This would make SERS more accessible for biomedical, environmental, and forensic applications.