Formation of Gold Nanoparticles inside a Hydrogel by Multiphoton Photoreduction for Plasmonic Sensing

Plasmonic sensing using metal nanoparticles has received considerable attention because of its high sensitivity for detection. In this study, gold nanoparticles (AuNPs) were formed near the focal point of femtosecond laser pulses inside a hydrogel containing gold chloride and glutamine for plasmonic sensing. The presence of glutamine suppressed the formation of AuNPs in the region outside of the laser focal point. The proposed method for the formation of AuNPs enables spatially selective fabrication of structures with optical properties attributed to localized surface plasmon resonance. Typical biomolecules present in sweat were detected using hydrogels with the fabricated structures. Among the biomolecules, the peaks of the absorbance spectra of the AuNPs inside hydrogels exhibited a blue shift only for urea. This result indicates that urea can be detected using hydrogels with fabricated structures. The method presented in this study facilitates spatially selective fabrication of biosensing structures inside the bulk of hydrogels.