Question 6

Question 1

q6a)

Your task is to synthesize complex colloidal particles as shown in figure (a) that can be assembled into triangles as shown in figure (b) and then switched into other triangles as shown in figure (c)

The yellow particles are gold nanoparticles of 5‐10 nm.

How would you synthesize the building blocks shown in figure (a)?
Which composition would you choose for the blue and the black part of the spheres?

Answer 1

Principle: half hydrophobic, half hydrophilic → with different solvents rotation → blue: hydrophobic, black: positively charged, so that hydrophilic gold particles can attach to it (negatively charged & hydrophilic from citrate route) => (a): neutral solvent (dioxane) (b): polar solvent (e.g. water) (c) non-polar solvent (e.g. hexane)

Janus particles (half half):
* blue: one side hydrophobic and not charged
* * Carbon black
* black: other side hydrophilic and positively charged (no pH dependance) → use EtO3Si-3CNH2 (always charged, no pH dependance, called amine terminated Silane) as surfactant (makes a positive charge)
* * SiO2 with the surfactant
* How to fabricate them:
* * make the Silica spheres (Stöber process) and attach the surfactant (not in water because else surfactant would react with itself).
* * Masking: put the stöber particles on a substrate (soft polymer) = immobilization, by tuning softness of substrate (=temp) one can control the penetration depth (other: mechanical compressing or swelling)
* * Deposition of carbon black (with spin coating or dip coating as sputtering is not possible bc it consists of particle), add PVA (surfactant) for no agglomeration
* * detach: sonicate or dissolve substrate

Gold nanoparticle:
- Citrate route → gives hydrophilic negatively charged particles. solvent: dioxan (6 ring with 2O and 4C) bc its in between polar and apolar.

Critical thinking:
- Trizma as surfactant for black part: has positive charge
- Surfactant can react with water and make sol-gel process → but we think it won’t react as it has already attached to the surface

Question 2

q6b)

How would you assemble the building blocks in (a) into the triangular assemblies shown in (b)? Describe briefly the experimental steps.

Answer 2

Change the polarity of the solvent (solvent exchange): polar solvent (hydrophilic) → hydrophobic part of janus particle come close together, as they don’t like the solvent.

To make triangular assembly: use a flat 2D arrangement (limit the height of the solvent) to prevent assembly of more than 3 particles. Or use sedimentation and remove all others (just take the triangles)

Question 3

q6c)

c) What and where are the attractive and repulsive forces within such a structure?

Explain

Answer 3

Repulsive forces: hydrophobic interactions, don’t stick together bc steric repulsion of PVA

Attractive forces: hydrophilic interactions, van der Waals (in general), gold sticks to silica bc electrostatic attraction (pos & neg charged)

Question 4

q6d)

Could you think of a trigger that changes the triangular arrangement in such a way that the gold nanoparticles attract each other as shown in (c)?
You don’t have to explain all the experimental steps.
Just describe, how the attractive and repulsive forces change and how you could do that.

Answer 4

Trigger could be a solvent exchange from a polar to a non-polar solvent (e.g. hexane, hydrophobic)

Repulsive: hydrophilic interactions, electrostatic repulsion between the gold

Attractive: hydrophobic interactions, hydrogen bonds from NH+ on gold and water, gold sticks to silica bc electrostatic attraction (pos & neg charged)