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[Display omitted] • Homogeneous, finely tuned gold nanorods were obtained by combining various synthesis protocols. • A biosensor exploiting plasmonic coupling as the sensing mechanism was developed. • Plasmon coupling was detected through strait forward pixel extraction. • The biosensor was used to accurately detect domoic acid with cost-efficient equipment. An immunoassay was developed that utilized plasmonic coupling between immobilised gold nanorods and colloid gold nanospheres to detect the marine toxin domoic acid (DA). The aspect ratio of the nanorods was optimised and the effects of variation in acidity, silver to gold ratio, cetyltrimethylammonium bromide (CTAB) concentration and seed addition in the growth solution on the yield, size variance and LSPR peak position was investigated. Excellent nanorods (size variation < 15%; aspect ratio 3.5–5; yield 0.26–0.35 nM mL−1) were obtained for the LSPR range 785–867 nm using strong acidic conditions (12 µl HCl (37%)), silver to gold ratio of 1:5, 0.05–0.1 M CTAB and 20–30 µl seed addition to 10 mL of growth solution. One set of nanorods (54.9 X 15.7 nm; LSPR 785 nm) were immobilised onto a silica support and bio-functionalised with DA hapten. Colloid nanospheres (15 nm; LSPR 519 nm) were bio-functionalised with an anti-domoic-acid monoclonal antibody. The functionalised nanoparticles were used to detect DA by plasmon coupling by quantifying the average LSPR shift of individual plasmon couples with hyperspectral imaging or quantifying the pixels count caused by the particle aggregation visible under darkfield microscopy. The first method led to a LSPR blue-shift of ~55 nm caused by the immunoreaction. The second, simpler method, enabled very clear qualitative detection (p < 0.0005) of domoic acid when 10 µM domoic acid was added. Both methods show potential though the novelty and simplicity of the second platform allowing rapid (~30 min) detection with high-throughput possibilities using a simple set-up is of most interest. [ABSTRACT FROM AUTHOR]