Matrix-tolerant dual-route detection of antioxidants and nitrite using gold nanorod cascade etching with localized surface plasmon resonance and photothermal readouts.

Colorimetric assays in complex food matrices are often affected by background color, turbidity and light scattering, which can mask target-induced absorbance changes and lead to false-negative results. Herein, a dual-route gold nanorod (AuNR) cascade-etching platform was developed for matrix-tolerant detection of reducing antioxidants or nitrite (NO₂⁻) through different reaction pathways. In the AuNR₁-TMB-AuNR₂ system, bilayer-structured AuNR₁ served as an oxidase-like catalyst for TMB oxidation, while AuNR₂ acted as a morphology-dependent localized surface plasmon resonance (LSPR) and photothermal transducer. Reducing antioxidants, represented by ascorbic acid (Vc), reduced oxTMB and inhibited the formation of etching-active TMB-derived species, thereby suppressing AuNR₂ etching. By contrast, NO₂⁻ participated in an oxTMB-involved reaction pathway and promoted AuNR₂ etching under acidic conditions. The two routes produced opposite signal trends. Increasing antioxidant concentration increased residual AuNR₂ absorption and photothermal heating while attenuating the LSPR blue shift. Increasing NO₂⁻ concentration decreased AuNR₂ absorption, enlarged the blue shift and reduced photothermal output. Using Vc as the reference antioxidant, the platform achieved LODs of 0.23-0.27 µM, while NO₂⁻ was detected with LODs of 0.15-0.28 µM. In milk powder matrices, direct oxTMB colorimetry was strongly affected by positive optical background, whereas AuNR₂ LSPR-shift and photothermal readouts provided reliable physical signals after simple dilution. This work offers a dual-route sensing strategy that enables selective antioxidant or nitrite detection in complex food samples via pathway-specific assay conditions.