Morphology-Engineered Gold Nanostars for Activatable NIR-II Imaging of Intracellular miR-21 with Deep Tissue Capability.

Near-infrared II (NIR-II, 1000-1700 nm) fluorescence imaging enables deep-tissue analysis, but conventional "always-on" probes suffer from high background that compromises the signal-to-background ratio for low-abundance targets. Activatable probe strategies could mitigate this issue, yet most existing designs rely on fluorescence resonance energy transfer with acceptors confined to the visible region, hampering efficient signal quenching in the NIR-II window. Here, we present a generalizable strategy based on morphology-engineered nanometal surface energy transfer (NSET) to construct a NIR-II core-satellite nanoprobe for intracellular miR-21 detection. By systematically tuning the branch aspect ratio of gold nanostars (AuNSs), their localized surface plasmon resonance (LSPR) absorption can be red-shifted to achieve precise spectral overlap with the NIR-II emission of Ag2S quantum dots. Guided by DNA self-assembly, an AuNS@Ag2S core-satellite structure with controllable interparticle spacing is established, enabling an initial fluorescence quenching efficiency of up to 88%. Upon target recognition, miR-21-triggered strand displacement reaction induces the dissociation of Ag2S quantum dots from the AuNS surface, resulting in a pronounced NIR-II fluorescence "turn-on" response. The nanoprobe exhibits exceptional performance, including a low detection limit of 75.8 pM, robust linearity, single-base mismatch discrimination capability, and enables visualization of differential miR-21 expression in living cells. Notably, high-contrast NIR-II signals are retained at penetration depths of up to 15 mm in tissue-mimicking experiments. This work establishes an NSET-based analytical platform through precise spectral matching and rational structural engineering, providing a robust strategy for high signal-to-background biosensing and in situ analysis of low-abundance miR-21 in deep tissues.