Question 10

Question 1

q10a)

Negatively charged Fe3O4@SiO2 particles can effectively assemble in a hydrogel matrix into one‐dimensional chains with internal periodicity when exposed to an external magnetic field and display a fast, fully reversible, and tunable photonic response to the changes in the external field. The key lies in the use of agarose (a polysaccharide, see Figure). The steric hindrance and the hydrogen bonding from the agarose network effectively limit the migration of the Fe3O4@SiO2 particles and their chain‐like assemblies.

What do you think is the role of the agarose?
Briefly explain. List three properties of the agarose gel, which you think are crucial for the functioning of such a magnetically responsive photonic film.

Answer 1

Agarose hydrogel: sol-gel reaction and forms a dense 3D network (double helices of agarose chains stabilized by hydrogen bonds)
Fe2O3@SiO2 particles: Fe2O3 colloids in cluster stabilized with silica coating (silica: very rigid, repulsion of clusters, mechanical stability)

Properties of agarose gel:
- Provides gel/stability: uniform distribution of particles and structural stability
- hydrogen bonds with the silica → more stability and hindered movements
- Hydrophilicity: agarose gel is hydrophilic and silica as well → stabilization
- no magnetic response
- transparent

Question 2

q10b)

What determines the color?
Explain briefly the underlying physical principle of the color formation.
Based on your explanation, how can the color systematically be changed?

Answer 2

Color determination: distance between clusters determines color via Bragg diffraction
→ (physical principle: constructive & destructive interference, light scattered by periodic structures, Braggs law: nλ=2dsinθ)

change color: change magnetic field to change the distance between the colloids (stronger magnetic field
→ colloids closer together → blue color)

Question 3

q10c)

What do you think is a suitable size range of the Fe3O4@SiO2 particles to efficiently display the desired color?
Why?

Answer 3

< 30 nm particles to be superparamagnetic assembled into >100 nm colloids (half of wavelength of light to make Bragg diffraction and display visible colors)

→ superparamagnetic for no hysteresis in the response of the magnetic field

Question 4

q10d)

Considering the force balance approach: What are the attractive and repulsive forces in such a system?

Answer 4

Attractive:
- magnetic attraction when applied H-field → particles align
- hydrogen bonds between helices to stabilize gel

Repulsive:
- electrostatic repulsion of negative charged Fe2O3@SiO2 particles prevent agglomeration
- steric hindrance of gel → physical barrier that limit the movement of the clusters