Gold nanostructure-related non-plasmon resonance absorption band as a fingerprint of ortho-alkyl substituted phenolic compounds

•An absorption band at 400 nm is proposed for AuNP-based colorimetric assays.•o-Alkyl substituted phenolic compounds generate this selective absorption band.•Metastable gold nanoclusters are responsible of the band at 400 nm.•The approach improves the selectivity of AuNP-based colorimetric assay for phenolics.