Ultrabroadband and Highly Sensitive Short-Wave Infrared Molecular Fingerprinting via Acoustic MXene Plasmons

Surface-enhanced infrared absorption (SEIRA) spectroscopy has emerged as a powerful technique, amplifying inherently weak molecular vibration signatures to enable ultrasensitive detection of molecular structure and dynamics. However, conventional SEIRA platforms based on noble metals or 2D materials are fundamentally constrained by their narrow spectral bandwidths and limited access to high-frequency vibrational modes. Here, we demonstrate an ultrabroadband SEIRA approach that overcomes these limitations by activating acoustic plasmon modes in two-dimensional Ti₃C₂T_x MXene. These acoustic plasmons provide deep subwavelength confinement, compressing wavelengths by more than two orders of magnitude relative to free space in the short-wave infrared (SWIR), and sustaining an unprecedented spectral bandwidth of approximately 5000 cm^{- 1}. Using this platform, we achieve simultaneous detection of distinct vibrational fingerprints-from high-frequency CH₃ combination bands near 4700 cm^-1 to low-frequency out-of-plane bending modes around 700 cm^-1-in ultrathin analytes such as 8 nm PMMA and 10 nm graphene oxide films, with up to an order-of-magnitude sensitivity enhancement. These results establish acoustic plasmon modes in MXene as a transformative foundation for ultrabroadband, high-sensitivity deep-infrared spectroscopy, paving the way for next-generation molecular sensing technologies.