“We’ve known for a few years that it is possible to make triangular silver nanoplates by photochemical methods - that is, by exposing a colloidal solution of spherical nanoparticles to light,” Andrea Callegari told nanotechweb.org. “In the process of optimizing the preparation procedure we noticed a very interesting and unusual thing: the size and shape of the particles depended on the colour of the light used. Since this was highly unusual, we decided to investigate further.”
Callegari and colleagues experimented on an aqueous solution of spherical silver nanoparticles grown by borohydrate reduction of silver nitrate. To begin with, the researchers exposed all their samples to the same light source - a conventional fluorescent tube that emitted wavelengths in the 350-700 nm range. After about an hour the solution changed colour, from yellow to pale green. Then the team treated each sample with different light conditions for several hours by placing a coloured glass filter between the sample and the light source.
Samples positioned behind an orange filter predominantly grew triangular plates about 7 nm thick. A broad blue filter (which allowed through wavelengths of 354-589 nm) resulted in smaller triangular plates. In contrast, a narrow blue filter that transmitted wavelengths of 349-467 nm led to the formation of a large number of tetrahedral particles, some cubic and octahedral particles and just a few triangular plates.
Callegari believes that the light plays a “double role” in the nanoparticle growth process, affecting the photooxidation and coalescence of the particles. At the start of the process, scientists believe that excess hydroborate ions form a negatively charged layer on the nanoparticle surface and prevent the particles from aggregating. Photooxidation removes the borohydrate ions and allows the silver nanoparticles to come together.
“Coalescence, i.e. transforming these aggregates into a single, larger particle, is then stimulated by light exciting a collective motion of the particle electrons called ‘plasmon resonance’,” said Callegari. “The spectral region where this excitation occurs depends on the size and shape of the particles and aggregates, hence different colours of light can result in the growth of different shape/size particles.”
By altering the particles in this way, the scientists can control their spectral response across most of the visible region of the spectrum, and potentially well into the near-infrared. “The wavelength at which the particles’ scattering efficiency peaks can now be tuned by controlling the size and shape, which means that we can tune the optical properties of the particles to specific needs,” said Callegari. “Bio-markers for multicolour diagnostic labels, optical components such as filters, and optical probes are just a few of the most immediate potential applications. Some intriguing possibilities include using these particles as building blocks for complex nanodevices.”
The researchers reported their work in Nano Letters.