Dear Ayesha Urooj,

Given that chemical and physical structural changes in light absorbing entities under observation may either cause the absorption maxima of observed spectral absorption bands to move up or down the visible spectrum, or alter their spectral bandwidth, I reason as follows.

The term SPR {Spectral Plasmon Resonance) refers to the mode of vibrational displacement of free (un-bonded) electrons either within a given molecule’s chemical structure, or within a more loosely bonded molecular aggregation.

Three rules now follow: (1) the larger the free path of the electron within the structure the lower the oscillation frequency and therefore, the longer the mean wavelength of light absorption maximum. (2) If more than one resonating entity (each with a distinct frequency) is present peak broadening is observed. (3) Any given resonance mode may easily be inhibited or even quashed by introducing additional resonation modes.

I suggest firstly that in terms of colour and at the low molecular count associated with nano-particles, they behave as ‘super molecules’. Thus, in the initial change from yellow to orange small two and three molecule aggregations are generated which exhibit slightly longer electron resonance path. As aggregation proceeds the subsequent quenching and peak broadening indicate that there is a rapid rise in both quantity and variety of contributing vibrational modes and frequencies.