how the properties of nanoparticles can vary from other chemical compounds by comparing the colour of gold nanoparticle complexes in solution. Also to learn how changing the composition of nanoparticles can affect their spectoral properties.

1. Synthesis and Characterisation of Gold­Silver Nanoparticles.
by Awad Albalwi
Aim:
To prepare gold and gold/silver alloy nanoparticles and to see how the properties of
nanoparticles can vary from other chemical compounds by comparing the colour of
gold nanoparticle complexes in solution. Also to learn how changing the composition
of nanoparticles can affect their spectoral properties.
Method:
Refer to practical notes.
Preparation of Gold Nanoparticles
1ml of gold solution was pipetted into a 10ml volumetric flask, made up with water
and 2.5ml was transferred into a conical flask with 90ml of added water. This was
brought to the boil and 5ml of sodium citrate 1% w/w. The solutions were left to boil
for 30 minutes, then cooled to room temperature and transferred into a 100ml
volumetric flask and made up to the mark with distilled water.
Preparation of gold/silver nanoparticles and gold nanoparticles.
Basically the same as above, except for exchanging some gold with silver. Using the
0.0025 M stock gold solution and a 0.0025 M silver nitrate solution, adding the
amounts as described in the laboratory manual. Colour changes were observed and
recorded.
Characterisation of Nanoparticles
UV­vis spectra of gold, silver and gold­silver nanoparticles
A UV­vis absorbance spectra was taken for each of the six solutions, measured
between 300 and 700nm using a 1 cm cuvette and water as a reference.
Particle size analysis of gold and gold/silver nanoparticles
Due to time constraints and the length of time required to do the particle size analysis

2. via the Malvern Zetasizer, the samples were given to the demonstrator who did the
measurement of particle size for us and sent the results via email.
Application of gold nanoparticles as sensors
~5ml of gold nanoparticle solution was placed into 3 test tubes. 1 M NaCl was added
to the first drop wise and observations were noted. It was then repeated using a
saturated NaCl solution. The third test tube was used as a control, using water for a
comparison.
Results:
Colour observations:
Solution: gold: silver ratio Colour changes Final colour
100% gold Blue to purple, pink and red Red
4:1 Pink, salmon, blood orange Blood orange
7:3 Pink, salmon, light orange,
bright orange
Orange
3:2 Pink, yellow, goldy orange Goldy orange
3:7 Wheat to cloudy
yellowy­grey
Yellow/grey
100% silver Light yellow then cloudy
yellow
Cloudy yellow

3.
Gold Nanoparticles as a sensor observations:
­When 1M NaCl was added to the filtered nano particle solution it required many
drops, close to the volume of 5ml, the same volume as the nanoparticle solution for it
to go close to clear. It was easily visible to see its colour changes from the initial red
solution then a lighter pink, transparent purple and finally a very faint almost clear
transparent blue.
­When repeated with a saturated NaCl solution the gold nanoparticle solution went
through its various colour changes to almost clear with only a few drops, much less
than required with the 1M NaCl solution.
Visible spectra
x(Au) Λ max (nm) Molar extinction coefficient
(M­1 cm­1)
1 522.5 3833.9
0.8 500 4669.9
0.7 486.5 4916.0
0.6 478 5246.1
0.3 450 1564.5
0 420 8238.24

4. Particle size analysis
x(Au) Av. Particle size
1 46.38
0.8 89.76
0.7 62.63
0.6 67.08
0.3 122
0 58.8
Basically it can be said that these results do not reflect literature or an ideal
distribution, looking at another group member Nicole’s results, it appears that the
average particle size was decreasing. The individual results shown don’t show any
systematic conformity.
Discussion
It appeared that with the addition of more silver solution or nanoparticles, the colour
of the gold/silver alloy solutions were getting lighter, ranging in a series of colours
that ranged from a red of the gold complex to a cloudy yellow of the silver
nanoparticle solution. As described the complexes also appeared cloudier with
varying amounts of silver added. This indicates that the size of the nanoparticles in
the complexes were getting larger and with a large enough concentration of silver
appeared to aggregate.
In the λmax verses mole fraction of gold graph​​it estimates that the maximum
absorption of nanoparticles of pure silver would be 420 nm, this does not cohere with
literature as the theoretical value lies around 400nm, 420nm is much too high.
Whereas the value for gold obtained was 522.5nm which was almost the exact
theoretical value of ~520nm, generally around 520­525nm.
Physicists predicted that nanoparticles in diameter 1­10 nm would display
electronic structures, reflecting the nanoparticle band structure (Daniel M, 2003).​ ​The
advantage of gold colloids is that they can be prepared with a much narrower particle
size distribution than silver colloids. Colloid gold has an absorbance of approximately

5. 520nm which is caused by the oscillation of the electron gas (Mulvaney P et al, 1992).
Absorbance in this wavelength region is also influenced by inter band transitions.
1. Aggregation of gold nanoparticles to form larger particles is prevented by the
addition of sodium citrate when the solutions started to boil. It acts as a reducing
agent, which is then absorbed onto the surface of each gold nanoparticle. This
introduces a surface charge that repels the particles from one another and prevents
them from aggregation called electrostatic repulsion. As this occurs the solution
turns to a red colour, indicating maximum absorption of light at ~520nm, the
green region (McFarland et al, 2004).
2. As seen in the maximum wavelength of absorption verses the mole fraction of
gold data, the curve is blue shifted with the increasing amount of silver added i.e.
there is a linear relationship towards a blue shift as the mole fraction of gold is
decreased. The results show that the predicted wavelength of silver is 420 nm.
This is quite higher than the theoretical maximum absorption of ~400nm. This is
evidential that pure silver nanoparticles cannot be made from this method of
conversion from gold nanoparticles through a series of alloy stages.
Silver and gold particles are miscible in all proportions due to there almost
identical lattice constants of 0.408 and 0.409nm (Link et al, 1999). There lattice
do not mix with the addition of silver to gold nanoparticles. The unit cell size of
Ag­Au changes theoretically by less than 1% over an entire range between 100%
Au to 100% Ag. Therefore electron diffraction pattern obtained from individual
colloid particles does not allow one to differentiate between the two possibilities
(Mulvaney P et al, 1993)
This experiment therefore also shows that silver nanoparticles in this size
range could not be obtained via the reduction with sodium citrate, the cloudiness
of the solution indicates larger particles. Although it is possible to make silver
nanoparticles, a method is described by Mulvaney P. et al, 1993, Electrochemistry
of Multilayer colloids. Silver sols can be prepared by y­irradiation of deaerated
solutions containing 50 or 100 µM​ ​AgCl04, 0.1 M 2­propanol, 0.01 M acetone,
and 0.1 mM sodium polyphosphate, (NaPOs) as stabilizer (Mulvaney P et al,
1993). This produces nanoparticles with maximum absorption around 380nm,
quite close to its theoretical value.

6. 3. From the results shown we can predict that we have the foundation of bimetallic
gold­silver alloys and not just a mixture of gold and silver because the band
structure of the varying ratio mixtures had different UV band structures than the
pure gold and silver nanoparticles. Evidence is confirmed with the stacked uv­vis
spectra and also the zetasizer analysis, as the stacking pattern does not remain
constant among the varying alloys in either of the data sets, evidently in the
gold/silver 3:7 complex. This suggests that individual particles of gold or silver
are absent in the alloy nanoparticles (Mulvaney et al, 1993).
The alloy formation can be concluded by the UV­vis absorption spectrum,
which shows only one absorbance bands. If there were to be a mixture of gold and
silver nanoparticles two bands would have been visible (Link et al, 1999).
4. The Malvern Zetasizer measures the average size of the particles in the solution
via dynamic light scattering (Ralph S, 2008). ​Dynamic Light Scattering
measures Brownian motion​ ​and relates this to the size of the particles.
Brownian motion principle suggests that the movement of
particles is due to the random collision with the molecules of the
liquid that surrounds the particle (Xu R, 2002).​ The size of the
particles are measured by illuminating the particles with a laser over a number of
intervals over ~30 minutes and analyses the fluctuation intensities in the scattered
light over the time, which is related to and used to calculate particle size.
​A Zetasizer has six main components, a laser, cell, detector, attenuator, a
correlator and a computer. ​A laser is used to provide a light source to
illuminate the sample particles within a cell, some light is scattered
by the particles within the sample. A detector is used to measure
the intensity of the scattered light, theoretically as light bouncing of
the particles should scatter in all directions, therefore the detector
could be placed anywhere around the cell but is typically place at
either 173° or 90°.. The intensity of scattered light is measured
from the detector, although it must be within a specific range,
therefore the zetasizer is also fitted with an attenuator which
reduces the intensity of light emitted from the laser and also
reduces the scattering​. ​Finally the scattering intensity signal is passed on to a

7. correlator, which compares the scattering intensity at successive time intervals to
determine the rate the intensity is varying ​(Xu R, 2002)​. The computer then
analyses data and determines the size of the particles in solution.
5. When 1M NaCl, an electrolyte was added to the 100% filtered gold nanoparticle
solution, colour changes were observed varying from the initial red solution, a
lighter pink, transparent purple and finally a very faint almost clear transparent
blue. This occurred as the high concentration of ions has a screening effect that
screens repulsive electrostatic forces between the nanoparticles (McFarland et al,
2004). This eliminates the repulsive forces between the gold nanoparticles and
they begin to aggregate. Identified by the colour changes observed upon addition
of 1M NaCl it forms and almost clear solution similar to its starting point.
It should also be noted that in another experiment with a saturated NaCl
solution that the gold nanoparticles went to an almost clear solution with much
fewer drops of the saturated solution. This occurred because the saturated solution
was a much stronger electrolyte with a higher concentration of free ions in
solution and was hence more affective screening the repulsive forces. The ​positive
charges of the electrolyte bind to the negative charges on the surfaces of the
nanoparticles​​created by the sodium citrate solution (McFarland et al, 2004).
Conclusion​:
In this experiment gold and gold/silver alloy nanoparticles were prepared and the
properties of varying chemical compounds and nanoparticles were observed by
comparing the colour changes of the gold and gold-silver nanoparticle complexes in
solution. A UV-vis specta analysis was also conducted to determine and identify the
different spectoral properties of the various nanoparticles. A particle size analysis was
also conducted with the used of an instrument called zetasizer.

8. References:
● Daniel MC and Astruc D, 2004, ​Gold Nanoparticles: Assembly,
Supramolecular Chemistry, Quantum­Size­Related Properties, and
Applications toward Biology, Catalysis, and Nanotechnology​, ​Chem. Rev.
2004, ​104, ​293­346
● Link S, Wang ZL and El­Sayad MA, 1999, ​Alloy Formation of Gold­Silver
Nanoparticles and the Dependence of the Plasmon Absorption on Their
Composition, ​Journal of Physical Chemistry, vol 103, 3529­3533
● McFarland AD et al, 2004, ​Colour my Nanoworld, ​Journal of Chemical
Education, vol 81
● Mulvaney P, Giersig M and Henglein A, 1992, ​Surface Chemistry of Colloidal
Gold: Deposition of Lead and Accompanying Optical Effects, ​Journal of
Physical Chemistry, vol 96, 10419­10424
● Mulvaney P, Giersig M and Henglein A, 1993, ​Electrochemistry of Multilayer
Colloids: Preparation and Absorption Spectrum of Gold­Coated Silver
Particles​, Journal of Physical Chemistry, vol 97, 7061­7064
● Ralph S, 2008, experiment 6 practical notes
● Xu R, 2004, Particle Characterisation: Light Scattering Methods, Chapter 14