Starch-mediated shape-selective synthesis of Au nanoparticles with tunable longitudinal plasmon resonance

We report the synthesis of Au nanoparticles, with tunable longitudinal plasmon band and shape selectivity, mediated by starch in the presence of ultrasonic waves. The synthesis was carried out by reduction of HAuCl4, at various concentrations, using H2O2 as the reducing agent. When the reactions were carried out in the absence of ultrasonic waves, there was no occurrence of the longitudinal resonance band, while the transverse plasmon resonance band shifted toward a higher wavelength. Transmission electron microscopic measurements revealed an increase in particle sizes with increasing higher initial HAuCl4 concentration. On the other hand, in the presence of ultrasonic waves, as the initial concentration of HAuCl4 was increased, while the transverse plasmon resonance band remained the same, the longitudinal plasmon resonance band increasingly shifted toward a higher wavelength. Transmission electron microscopic measurements revealed the change in shape from spherical to triangular to hexagonal particles with increasing initial HAuCl4 concentration. We also report that the starch-stabilized nanoparticles could be precipitated from the solution by a starch digesting enzyme which also binds with the particles resulting in its precipitation.