Display omitted •A simple sensitive photothermal LFIA platform was demonstrated.•Nanoengineering the GNS morphology enables to achieve LSPR in the NIR-II region.•Our optimized GNS (GNS-3) shows excellent photothermal efficiency.•A portable device was developed for the PPh-LFIA assay.•GNS-3 demonstrates remarkable photothermal sensitivity in detecting cTnI. In recent years, photothermal lateral flow immunoassay (LFIA) has gained widespread use as a point-of-care testing (POCT) technique because of its cost-effective portable instrumentation and easy-to-handle procedures. However, developing a highly sensitive photothermal LFIA platform with high photothermal efficiency remains challenging. In this study, we developed a portable smartphone-based plasmonics-enhanced photothermal LFIA (PPh-LFIA) platform to improve the photothermal LFIA sensitivity by engineering plasmonic-active gold nanostars (GNS) having large-sized sharp branches. We investigated the photothermal effect using various GNS engineered to have different sizes and spike lengths of GNSs (GNS-1, GNS-2, GNS-3, and GNS-4) in order to exhibit optimal plasmonics properties. The results showed that the GNS-3 having a broad plasmon band with a plasmon maximum band at 995 nm exhibited the strongest photothermal efficiency (80 %). As a proof-of demonstration, we have employed GNS-3 for the detection of the cardiac biomarker, troponin I (cTnI), used as the model system. The results show high sensitivity with a detection limit (LOD) of 5.5 pg/mL, which was around 1000 times more sensitive than traditional colorimetric LFIA based on gold nanospheres (GNSP). To verify the applicability of the PPh-LFIA platform, cTnI was spiked into human blood serum samples, enabling the detection of cTnI with an LOD of 6.7 pg/mL. Overall, this study offers insights for nanoengineering and fine-tuning the GNS morphology to increase the photothermal efficiency, consequently improving the PPh-LFIA detection sensitivity, which makes it suitable for use as a sensitive and portable POCT tool.