Hello, my name is Angela Hess and I’m here
at the Ginger Lab at the University of Washington and I’ll be synthesizing silver nanoprisms
today. Silver nanoprisms are very useful because
they have a very specific plasmon resonance, and the plasmon resonance is basically adjusted
by the size of the prisms and it is visible through the color of the colloid.
First I will be synthesizing silver nanospheres and these will be converted to the larger
nanoprisms by putting them under LED of a specific color, and based on the color of
the LED I can get different colors of the resulting colloid.
Silver nanoprisms are actually small silver triangles approximately 2-30 nanometers on
the side length and the fact that they are triangles that it’s actually very special
because tips of the prisms experience a strong field enhancement and the absorption spectrum
is very sensitive to binding particles or the media of the prisms.
We are now setting up to do the first step of silver nanoprism synthesis.
So each synthesis you prepare the stock solutions fresh and put them on ice before using. All
the vials are cleaned with acetone and the sodium borohydride solution is purged with
nitrogen before actually adding the borohydride salt.
We use a 250 mL, three necked round bottom flask and put it in an ice bath with a stir
bar. We add 95 mLs of purified water to the flask.
And then we start purging the flask with nitrogen. Now we add 2 mL of our silver nitrate stock
solution, and 1 mL of the stock sodium citrate. After an about an hour of purging we add 1
mL of the sodium borohydride to the silver and sodium citrate solution.
The solution should turn a very pale yellow. This is a second set up and this has only
been purged for about half an hour and as we add the sodium borohydride the color does
not change to pale yellow and instead it changes to a slightly grayish yellow and the hint
of gray indicates that it has not been purged sufficiently and the rest of the reaction
will likely not go as desired. After adding the borohydride we stop purging
the solution. At around five minutes we add 100 microliters
of sodium borohydride. We repeat this step at 7, 9 and 11 minutes.
Now we mix together the two stock solutions for BSPP and sodium borohydride
We take 1 mL of the BSPP and 500 microliters of sodium borohydride and put them together
on ice. At fifteen minutes we add 150 microliters
of this mix to our solution and we repeat this step every thirty seconds until all of
the mix solution is gone. So after adding the last of the BSPP and sodium
borohydride we let the solution age for about fifteen hours and we stopper the solution.
We wrap it in tinfoil so that it will be completely in the dark for this aging.
We also replace the ice so that it will be cold for the duration of the aging.
After aging for 15 hours the absorbance of the sample should be at about 393-394 nanometers
and that will indicate that it’s been sufficiently aged.
After this we’re going to remove from the foil and the ice bath and adjust the pH.
The starting pH will be around 9.5 and the ending ph should be 11.0
We are going to use .2 molar sodium hydroxide and add drop wise to elevate the pH to 11.0
After adjusting the pH we’re going to pour the solution into small vials that have been
cleaned. And it is important to use plastic or Teflon caps because if it is a metal lined
cap the metal will react with the solution. After the pH conversion I’ll take the seed
solution and put it under an LED and wrap it in tinfoil to redirect as much of the light
as possible onto the solution. And put directly under the LED, and run the LED at about its
maximum current. And cover with a black cloth.
The conversion process from silver nanospheres to prisms will depends on the light that is
being used but for most wavelengths it they will take approximately 12-24 hours
After 24 hours under the LED the conversion process is complete.
The shift in color is extremely obvious and the absorption spectrum should also display
a decrease in the original silver nanosphere peak and the development of a new peak for
the silver prisms. And I can achieve based on a color of the
LED different colors of prisms.