Water-soluble gold ultrathin nanorods with NIR-II longitudinal localized surface plasmon resonance and improved stability.

Oleylamine-coated gold ultrathin nanorods (AuUNR:OA), featuring an anisotropic morphology with a diameter of ∼1.8 nm, exhibit exclusively the longitudinal mode of localized surface plasmon resonance (LSPR). However, reinforcing their structural stability and tuning their LSPR wavelengths into the second biological window (NIR-II) have remained critical challenges. Here, we successfully regulate the LSPR of these AuUNRs within the NIR-II window via a controlled ligand-exchange reaction on AuUNR:OA with dihydrolipoic acid (LA(SH)2). The resulting series of LAS2-capped AuUNRs (AuUNR:S2LA), with aspect ratios (ARs) ranging from 3.3 to 5.9, exhibits LSPR wavelengths spanning from 1.09 to 1.44 μm. The alteration in the surface coordination environment imparts excellent water dispersibility owing to the presence of terminal carboxyl groups on the LAS2 shell. Furthermore, the strong, bidentate Au-S anchoring on the AuUNRs suppresses shape relaxation, leading to a significant improvement of both colloidal and morphological stability. The photothermal conversion efficiencies of these AuUNR:S2LA under 1.064 μm excitation were evaluated to be 0.37 to 0.72 depending on the ARs. The synergistic combination of enhanced stability, high hydrophilicity, and LSPR tunability within the NIR-II window renders AuUNR:S2LA a promising precursor for advanced biological applications.