Advancing thermoplasmonic sensing: gold nanobipyramids for enhanced light-to-heat conversion†

Thermoplasmonic detection is a newly emerging application in the rapidly growing and promising field of thermoplasmonics. Accordingly, herein, an in-depth evaluation of thermoplasmonic performances of gold nanobipyramids (AuBPs) dispersed in colloidal solutions and immobilized on a filter paper substrate was provided, which revealed their ability for efficient and sensitive thermoplasmonic detection of simple and complex molecules. Concretely, AuBPs in aqueous solution with optical responses in and out of resonance with the 808 nm laser line were synthesized and their intrinsic light-to-heat conversion performances were assessed, revealing photothermal efficiencies (η) up to 74%. Subsequently, colloidal AuBPs were functionalized with 4-mercaptobenzoic acid (4-MBA), which is a simple and small molecule. Consequently, η decreased by up to 4%. Furthermore, their immobilization on Whatman no. 1 filter paper through immersion resulted in the preservation of their optical properties and intrinsic thermoplasmonic activity. Thermoplasmonic detection capabilities of the plasmonic paper were tested using 4-MBA and thiol-polyethylene glycol-amine (a thermo-sensitive complex polymer). Following the functionalization of the plasmonic paper, its photothermal activity significantly decreased, causing an increase in the cooling time constant; thus, both 4-MBA and thiolated PEG were detected via thermoplasmonic detection. Moreover, a LOD of 0.19 nM and a LOQ of 0.58 nM were determined for 4-MBA, proving the high biosensing efficiency of the plasmonic paper. Hence, these results contribute to the consolidation of the versatile thermoplasmonic detection of both simple and complex interactions, being a stepping stone in the development of simple and efficient thermoplasmonic nanosensors.