Nanoparticle Optics Part 1

1
Nanoparticle Optics Part 1

• Gold and Silver Nanoparticles
• Group 1 Luke, Matt, and Jeff

2
Theory

• The color of the sol arises from a combination of
  absorption and scattering of light and depends on
  particle size. More specifically it is due to a
  resonance of the free electrons in the metal
  particle. The lights electromagnetic field
  causes them to slosh back and forth (plasmon
  oscillations).
• At a characteristic frequency which depends of
  the size and the metal, the sloshing is the most
  intense. This is the frequency where plasmon
  oscillations are excited. The plasmon resonance
  is easily seen in the extinction spectrum of the
  sol.
• The particles experience the constant buffeting
  of Brownian motion which also helps to keep them
  in suspension.

3
Objective

• Learn about scattering and absorption in gold and
  silver nanoparticles.
• Visually observe how particle size effects
  scattering.
• Learn to use a Tyndall beam.
• Observe sub-diffraction limit nanoparticles in
  the optical microscope.
• Become familiar with optical and electron
  microscopy.

4
Procedure Gold Nanoparticles

• Bring to a boil 50 mL of 2.510-4 M chloroauric
  acid solution
• Add 0.16 mL of 34 mM sodium citrate solution to
  the boiling solution while stirring
• After a minute will be faint blue and then
  darkening over 5 min to a brilliant red
• Repeat procedure with 0.30 mL and 1.0 mL of
  sodium citrate to produce three samples of
  different sized gold nanoparticles.
• Fill cuvette with sample and place in
  spectrophotometer. Record absorbance of each
  sample from 200 nm to 800 nm.

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Procedure Optical Microscopy

• Use micropipette to place 20 mL of sample on a
  microscope slide.
• Place cover slip over sample.
• Place slide under 20X objective lens for total
  magnification of about 200X.
• Use coarse and fine focus knobs to find
  nanoparticles. Use the plane of cover slip and
  slide as a reference.
• Observe stationary nanoparticles on cover slip
  and slide. Nanoparticles in solution are
  randomly moving because of Brownian motion.

6
Procedure Electron Microscopy

• Using a micropipette, put a few drops of sample
  on light side of carbon grid.
• Wick away excess with filter paper.
• Let sample dry for several hours.
• View sample in TEM and take photos for later
  analysis.

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Results Visual Inspection

• Solution changed from blue to red within five
  minutes as expected.
• Sample with 0.16 mL sodium citrate turned a
  duller red than the other two samples.

8
Results Absorption Spectra

• Fit of Mie Plot data to Spectrophotometer data
  resulted in gold nanoparticle radii of 14.8 nm,
  15.6 nm, and 35 nm (diameter of 29.6 nm, 31.2 nm,
  and 70 nm).
• Shoulder on 0.3 mL sample is because of larger
  particles also in the solution.

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Results TEM
0.16 mL, 100 nm scale
0.16 mL, 100 nm scale
0.3 mL, 20 nm scale
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Results TEM
1 mL, diffraction
0.3 mL, 100 nm scale
0.3 mL, 20 nm scale
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Analysis TEM

• Particles in each picture were measured with a
  ruler to get a size distribution.
• Data agrees well with estimations from absorption
  spectra

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Analysis TEM Diffraction

• Measured radii of diffraction rings to determine
  lattice constant of gold.

Measured Radius (cm) Miller Index Plane Spacing (nm) Lattice Constant (nm)





13
Questions

• Approximate Size r 14.8 nm, 15.6 nm, and 35 nm
• Atoms per nanoparticle
• For r 14.8 nm

• For r 15.6 nm 9.4 106 atoms/nanoparticle
• For r 35 nm 106 106 atoms/nanoparticle

14
Questions

• Fraction of atoms on surface
• For r 14.8 nm

• For r 15.6 nm 0.0033
• For r 35 nm 0.0015

15
Questions

• Number of nanoparticles per mL
• For r 14.8 nm

• For r 15.6 nm 1.6 109 nanoparticles/mL
• For r 35 nm 0.14 109 nanoparticles/mL

16
Questions

• Nanoparticle surface area per mL
• For r 14.8 nm

• For r 15.6 nm 0.049 cm2/mL
• For r 35 nm 0.022 cm2/mL