The Morphological Evolution of Self-Assembled Silver Nanoparticles under Photoirradiation and Their SERS Performance

In this study, we conducted a one-step photochemical synthesis to produce silver nanoparticles
(AgNPs) with irregular morphology. The synthesis process involved the photoconversion of
Ag nanoseeds into self-assembled Ag nanostructures of various morphologies using a high-pressure
sodium lamp with a wavelength of 589 nm, corresponding to an energy of 2.1 eV. During the synthesis,
the color of the colloidal Ag nanoseeds gradually changed as the irradiation time increased,
transitioning from yellow to brown, juniper green, basil green, ocean green, aegean blue, and finally
to true blue. We characterized the morphological evolution of the resulting AgNPs, as well as their
optical properties and aggregation behavior, using transmission electron microscopy, UV-vis spectroscopy,
and dynamic light scattering. Furthermore, we evaluated the impact of the self-assembled
morphology of the AgNPs on their surface-enhanced Raman scattering efficiency, using R6G as
the target analyte. The results revealed that the colloidal AgNPs synthesized under a visible light
irradiation time of 1 h consisted of circular nanoplates, hexagonal nanoplates, trapezoid nanoplates,
and triangular nanoplates. These colloidal AgNPs exhibited excellent SERS activity when used as an
SERS-active substrate in the form of an aqueous solution, enabling the detection of low concentrations
of R6G down to 10^-12 M.