Question 12

Question 1

q12a)

It was observed that UV irradiation of spirothiopyran in an aqueous solution of gold nanoparticles (AuNPs) gives AuNP aggregates. Scheme 1 shows (a) the photoisomerization of spirothiopyran and (b) the mechanism for light‐induced aggregation of AuNPs.

Using the force balance approach, explain the principle of this assembly experiment.

Answer 1

aqueous solution of gold: citrate-route: negative charged gold NP

Before SP (spirothiopyran) on gold: no agglomeration bc electrostatic repulsion (negative charge)
→ SP screens the charges of the citrate when under UV light (has positive charge) → no electrostatic repulsion => agglomeration

attraction force: always van der Waals

upon UV irradiation attractive forces overcome electrostatic repulsion → agglomeration

Question 2

q12b)

How would you characterize the size and the shape of the aggregates?

Answer 2

• SEM: Visualizes the surface morphology and size of the aggregates
• TEM: Provides detailed images of the size and shape of individual nanoparticles and aggregates
• UV-Vis: If small enough: size of colloid changes surface plasmon resonance → different colors absorbed (the bigger the NP (>100nm) the less defined the peak)

Question 3

q12c)

You would like to incorporate CdSe quantum dots into mesoporous silica particles to make them luminescent. As starting material, you have a powder of mesoporous silica and a dispersion of CdSe quantum dots synthesized by the hot injection method. Explain/sketch briefly the different experimental steps considering i) the solvents used, and ii) the surface chemistry and interaction between the two materials.

Explain how you would modify the surfaces and explain which interaction you would like to establish between the silica and the quantum dots.

Answer 3

Mesoporous: 2-50 nm
mesoporous silica, not functionalized: hydrophilic, OH groups at the end
CdSe QD by hot injection (in organic solvent, 2-6nm): hydrophobic bc organic surfactants for stabilization

Steps:
- Change surfactant of CdSe to PEG → make it hydrophilic (solvents: polar solvents to facilitate ligand exchange e.g. ethanol)
- Put QD and silica particles in a little bit non-polar solvent (e.g. Dioxane: when too much hydrophobic: precipitation possible, if too much hydrophilic: no arrangement), mix well (QD should go into silica due to hydrogen bonds: silica: has the OH and QD the O with the free electrons)
- evaporation of solvent with elevated temperature
- wash to remove excess QD (by centrifugation, silica particles heavier than QD)

Critical thinking:
- don’t know if Hydrogen bonds are “strong” enough to fill all the pores → maybe use charges?
- Way with charges: modify CdSe with amine-terminated silane (positively charged) and use high pH for negative charge on Si → mix together in polar solvent (e.g. water)