Engineering multiphoton-active gold and carbon nanoparticles for precision biophotonics

Engineering light-responsive nanoparticles (NPs) with tailored nonlinear optical properties is opening new opportunities in precision biophotonics. This article highlights gold nanoparticles (AuNPs) and carbon dots (CDs) as complementary nanoscale platforms for multiphoton bioimaging, photodynamic therapy, sensing, and theranostic applications. Key methods for probing nonlinear optical behaviour, including Z-scan, two-photon excited fluorescence spectroscopy, and hyper-Rayleigh scattering, are first outlined. The discussion focuses on AuNPs, covering bioinspired synthesis, biofriendly post-processing, and representative functions in antimicrobial photodynamic inactivation, plasmonic catalysis, sensing, and nonlinear optical readout. Special attention is also given to lyotropic liquid-crystalline systems (LLCs), particularly DNA-based mesophases and myelin figures, as biomimetic environments that enable NP organisation, polarisation-sensitive studies, and photothermal control of local order of LLCs. CDs are presented more concisely as an earth-abundant complementary platform whose multiphoton responses can be tuned through internal structure and surface chemistry, with emerging relevance in biomolecular environments and soft-matter imaging. Overall, this review connects materials design with optical function and biological performance, and points to future opportunities in deeper-window operation, hybrid nanostructures, and translational biophotonics.