No abstract

A facile method for the preparation of silver nanoparticles (AgNPs) of various sizes and morphologies, including dodecahedra, nanorods, and nanoplates, has been discovered. By choosing the desired optical properties (absorption maximum) and irradiating spherical AgNP seeds with a selected light emitting diode, we achieve control over the size, morphology, and optical properties. The properties of AgNPs are intrinsically dependent on the size and shape of the particles, which can be readily controlled with the strategies reported here. Literature methods for the synthesis of anisotropic AgNPs require complex solutions containing seed nanoparticles with specific twinning defects, and a variety of specific stabilizing ligands direct the growth of the seeds but limit the versatility of the particles. In general, these thermal routes to anisotropic AgNPs give particles with high polydispersity, limiting their applications in single molecule spectroscopy and surface plasmon resonance spectroscopy. We describe a single photochemical method for preparation of AgNPs with predictable and controllable size and morphology that are produced from a single source of photochemically grown AgNP seeds stabilized only by sodium citrate. We also describe a common mechanism for the formation of the various types of AgNPs.

Aqueous, unprotected gold nanoparticles were prepared from HAuCl4 using a water-soluble benzoin (Irgacure-2959) as a photochemical source of strongly reducing ketyl radicals. This rapid method provides spatiotemporal control of nanoparticle generation, while light intensity can be used to control particle size. The particles are stable for months and do not require any of the conventional (S, N, or P) stabilizing ligands, although these can be readily incorporated if required.

The optimal size of spherical silver nanoparticles (AgNPs) for off-resonance surface-enhanced Raman scattering (SERS) was found to be ∼50 nm based on the equivalent Ag content in AgNP colloids. It is understood that the SERS intensity of adsorbates on the surface of metal nanoparticles is dependent on the size and shape of the particles of interest. Herein, we report a seeded growth mechanism for the formation of silver nanoparticles that allows superior control over the size of the resultant nanoparticles with relatively low polydispersity. The high degree of size control allows for a better understanding of the study of the effect of particle size on SERS intensity. The Raman study performed here employed a long-wavelength excitation (785 nm) so as to avoid photochemical degradation of adsorbed species and photochemical transformation under intense excitation. Under these experimental conditions, it was found that the optimal size of AgNPs for providing a maximum SERS intensity of adsorbed R6G is ∼50−60 nm, a result that is expected to extend to other adsorbates as well.

The photocatalytic hydroxylation of boronic acids with methylene blue as photosensitizer proceeds with high efficiency. Detailed time-resolved studies of the relevant rate constants provide a clear mechanistic understanding of excited-state processes and guided the selection of the photocatalyst and the optimization of experimental conditions.