Morphology-Preserved Ag Alloying in Au Nanosheets Enables Tunable NIR-I/II Plasmonics for Enhanced Photothermal Conversion

Precise spectral modulation of plasmonic absorptions in the near-infrared (NIR-I and NIR-II) regions is crucial for matching laser wavelengths in practical applications, driving significant interest in sensing, photothermal therapy, and bioimaging. In this work, we show that silver (Ag) alloying in gold (Au) nanosheets provides tunable plasmonic absorption across both NIR-I (700–950 nm) and NIR-II (1000–1350 nm) spectral windows while preserving their intrinsic two-dimensional morphology. Through controlled variation of the Ag/Au ratio in a seed-mediated synthesis, we observe a continuous blue-shift of the in-plane dipole plasmon resonance from 1000 nm (NIR-II) to 700 nm (NIR-I). Theoretical simulations confirm that Ag incorporation tunes plasmonic responses by modulating both the free electron density and local dielectric environment. This morphology-preserved alloying approach establishes a versatile platform for developing optically switchable nanomaterials with promising applications in multiplexed biosensing and wavelength-selective photothermal therapy.