Question 3

Question 1

q3a)

Why are aqueous gold nanoparticle dispersions red?

Answer 1

Gold dispersion has red color bc plasmons absorb green & blue light (plasmon frequency determined by shape and size of particles)

→ Gold aggregation (when NaCl added): are blue bc particle is bigger → red-shift of absorbing light
→ Gold foil: golden color bc particles are very close together

Question 2

q3b)

How would you change the color of such a gold nanoparticle dispersion without changing the particle size and shape?

Answer 2

Change the distance between the particles by:
- change ionic strength: adding salt if there are surface charges (citrate-route)
- pH adjustment for OH-groups (citrate-route and adjust surfactant for thiol-route)
- introduce or remove steric hindrance
- introduce groups with charges which bond to surface
- add metal ions which coordinate via surfactant
- change the solvent that alter the local refractive index

Question 3

q3c)

Could you imagine an application, in which such a color change would be useful?

Answer 3

Colorimetric sensing with gold nanoparticles:
- add surfactant to gold particle which bonds to the specific metal → clusters are formed → color turns from red to blue (should be reversible but difficult)
- detect DNA: two different batches of gold particles (which go to beginning and end of DNA), cluster form when specific DNA is there, reversible with temperature (DNA detaches)

Application as biosensors (SERS effect):
Gold nanoparticle (with raman reporter molecule) with antibody on surface goes to e.g. tumor and enhances its raman signal

Question 4

q3d)

d) Discuss the ways in which you could avoid the fowling of Ag NPs on the walls of a flow reactor

Answer 4

Fowling on walls happens with continuous flow and conventional heating → nonuniform reaction
- Microwave heating and not conventional heating (heats up regularly and conventional heating from the outside towards inside)
- use droplet flow (droplets never touch the wall)
- use two immiscible fluids (only nucleation in fluid in the middle)
- apply magnetic field for more turbulent flow
- put in seeds for more homogeneous nucleation

Question 5

q3e)

Describe the ways to obtain a glass‐ceramic, i.e. crystalline nanoparticles embedded in amorphous silica

Answer 5

MxOy:SiO2 glass-ceramic: SiO2 glass with MxOy (ceramic) phases
1. Have MxOy doped SiO2 xerogel (homogeneous & porous)
2. Increase temperature → softening
3. increase temperature even more → M switches places with Si to form a more stable phase (nucleation and growth due to higher ionic mobility) & densification
4. MxOy nucleates → biphasic dense glass-ceramic

Number of size of nanocrystals can be tailored by dopant concentration, densification temperature & densification atmosphere (less O2 → more vacancies → easier & faster hopping)
With more dopants (e.g. Sn) → color changes as nanocrystals have different BG

Tuning refractive index with light: with laser deposit energy in nanocrystals → partially dissolve → less nanocrystals => refractive index decreases

or could add preformed crystalline nanoparticles in a silica melt but have a lot of limitations

Question 6

q3f)

What is oriented attachment?

Answer 6

With oriented attachment one can control the direction of attachment. It‘s a spontaneous self-organization of adjacent particles so that they share a common crystallographic orientation (entropy driven → release of surface attached molecules which increases entropy)

Example: with directional forces (oriented attachment): oriented attachment of surface-functionalized TiO2 electrostatically stabilized by Trizma

Question 7

q3g)

Explain in your own words the difference between the hot‐injection method and the heating‐up method.
What is common for both methods such that they often result in the formation of monodisperse nanoparticles?

Answer 7

Hot injection mode:
1. injection of a cold precursor (organometallic of metal organic compounds/metal salts) solution to a hot surfactant (polar head group and long alkyl chain) or surfactant/solvent mix
2. nucleation and growth
3. Surfactant controls size and surface properties
4. precipitation by addition of a “nonsolvent”

precursor reacts into „monomer“ and this monomer forms the particle Heating-up method:
mix all precursors together, heat up

Essential difference between the two processes: the way in which the high supersaturation level is achieved.
- Hot-injection: rapid injection of precursors,
- Heating-up method: at low temp you have a high energy barrier for homogeneous nucleation → no nanocrystal formation). As the temp is increased the supersaturation goes up until high enough for nucleation.

Common for both:
- Size Focusing: The smaller particles grow faster than the larger ones. Bc its diffusion controlled growth (growth rate follows 1/r) → after some time every particle the same size.
- Diffusion controlled: surface energy low, supersaturation high