Thermochromic Strategy Driven by Asymmetric Heat Distribution of Chiral Nanogold for the Time-Resolved Immunosensing

Photothermal immunosensor is widely used in point-of-care testing (POCT) due to its high sensitivity, repeatability, and ease of operation. In this work, a colorimetric photothermal immunosensor was constructed based on chiral Au nanohelix cubes (NHCs), realizing the visible readout of a photothermal signal through the time-resolved change of color. Specially, based on the experiment and simulation, the chiral Au NHCs exhibited enhanced photothermal performance due to their unique chiral structure, which not only broadened the range of absorbing light through cross-section of chiral structure to enhance localized surface plasmon resonance but also created an asymmetric heat distribution to enhance fluid convection of water by weakening light propagation and increasing light reflection on the surface of chiral structure. As a result, the chiral Au NHCs showed a photothermal conversion efficiency of 77.1%, an instantaneous heating rate of 18.1 °C/min, and a final temperature of 85.2 °C, better than achiral Au NHCs and Au nanocubes (NCs) under the irradiation of 808 nm. When incorporating thermochromic pigments, the photothermal signal can be visualized through a distinct color transition from gray-green to blue-violet, where the time of color transition was based on the amount of chiral Au NHCs. Herein, by using chiral Au NHCs to immobilize secondary antibody, the photothermal immunosensor, combining with a homemade instrument, was realized to recognize different concentrations of antipyrine from 0.01 to 100 μg/L based on the relationship between the target concentration and the time of color change. By converting the photothermal signal into visible signals, it provides an effective visualization method for the photothermal immunosensor.