[Emil]

Hi there

We have made an experiment with Colloidal gold, the instruction we followed can be found on Wikipedia, the link is:

http://en.wikipedia.org/wiki/Colloidal_gold

We used Turkevich's method.

We do however not know very much about this method, so some help would be most appreciated.

We mixed the Colloidal gold with both a 0.5 % solution of Sodium Citrate and a 0.05 % solution.

I do not know very much about what happens, during the reation but the product was red. We have also got a sprectrum, from which we should be able to determine the size of the particles we created? Furthermore one can proof that one has created Nanoparticles by shining light on the solution, could you please tell me something about that?

[Bakegaku]

It mentions the colour change in colloidal gold as particles fall below 100nm.  Basically when things get small, they act weird (especially when one gets into quantum mechanics).  I think the gold takes on a red colour because with fewer atoms around, the electron shells are drawn closer, giving them less energy, and causing them to give off a lower energy radiation.

[pantone159]

Actually, there is no quantum mechanics needed, only classical electromagnetism.  The calculations were first done in 1908, by Gustav Mie.

In this experiment, the gold is forming small colloidal spheres.  These spheres scatter light according to Maxwell's equations, just like small spheres of water (raindrops) scatter light (making rainbows, etc.)  I think that gold absorbs light, which means its refractive index is complex (has an imaginary part), while water is just 3/4 or something like that (and real, meaning no absorption).  Also, the sizes are different.

The mathematical problem is, however, identical for the two cases (gold sols, rainbows).  The Mie scattering solution applies to both.  (In fact, understanding the colors of gold sols was the original reason Mie had for pursuing this.)  Anyways, you can use this to calculate the spectrum and color for various sizes of gold spheres.  It is a fairly involved calculation, I doubt you are expected to duplicate it.  The results for gold spheres happen to be different colors for different sizes.  If I am reading the notation in my book right, for spheres diameter dia = 0.04 micron, you get red.  dia = 0.10 micron makes purple, dia = 0.14 micron makes blue.

I haven't done the calculation for gold sols, although I have done it for rainbows.

The book 'Light Scattering by Small Particles' by H. C. van de Hulst (a Dover book, so cheap) has all kinds of cool info about this kind of stuff.

[trueapprentice]

The Red Colour has to do with the size of the nanoparticles, basically, when the size of the particles become less than the wavelength of light (in the hundreds of nanometers), then it displays a 'surface plasmon resonance' effect.

Different sizes will absorb the wavelength of light differently, therefore giving them a spectrum which can be used to calculate their approx. particle size, but transmission electron microscopy is the usual technique to characterise the actual size of the particles properly.

the sodium citrate is used as a capping agent as well as the reducing agent, to reduce the gold (III) ion to gold atom clusters which are then capped by the citrate anion, hence the gold nanoparticles have a negatively charged shell around them in solution.

Thiols, can be used to cap gold nanoparticles too, from the 'self-assembly of Au-S'

[green-goblin]

Look up quantum dots.

You can use the partical in a box quantum mechanics equation to calculate the box size, (in this case the size of the nanopartical).