L7 Quantum Dots Flashcards
Re Quiz 6 W9
- Briefly describe how quantum dots work, in context of emitting light of certain frequencies.
(2 marks)
Model answer: Electrons in the quantum dot absorb the incoming photon and are subsequently excited, resulting in the ‘jumping’ of the electron from the valence band to the conduction band. This band gap is predominantly dependent on the QD size and material composition. The electrons then return to the valence band, leading to the emission of a photon with its frequency corresponding to the energy level of the band gap (E = hf)
Keywords: photon, emission/absorption, excitation, band gap, valence band, conduction band, wavelength/frequency
Also remember: it’s the ELECTRONS of the QD that move between the different bands. Not the QD.
- Name ONE advantageous property of quantum dots that render them favourable for various applications, including biomedical applications.
(1 mark)
- Tunable optical properties, better optical properties than most organic dyes
- Photostability
- Would also accept ‘biocompatibility’, but it is the ‘least’ biocompatible amongst all the nanoparticle materials that we have discussed prior to this point
‘Biologically stable’, is a vague answer; it can mean two things, one positive one negative:
- Positive – the QD will not dissolve/degrade in a biological environment, thus letting you image as much as you can
-
Negative – the QD will stay in a living system for so long that it would start inducing chronic toxicity.
- (do not use this during the exam; may not be accepted in the context of ‘advantageous properties’)
Regarding “compatible with various hosts”:
- very vague statement on its own, and you’ll need the additional materials qualifier (also in the lecture notes, SiO2 etc) to have this statement make any sense
- The band gap of quantum dots can be tuned, by changing which of the following about the quantum dots?
(Choose TWO correct answers – 0.5 mark for each correct answer, total 1 mark)
(A) Size of the quantum dots
(B) Material composition of the quantum dots
(C) Colour of the quantum dots
(D) Method employed to fabricate quantum dots
(A) Size of the quantum dots
(B) Material composition of the quantum dots
Incorrect:
(C) Colour of the quantum dots
(D) Method employed to fabricate quantum dots
- Which of the following stoichiometric formulae does NOT represent the formulation of a semiconductor-based quantum dot?
(A) CdSe (cadmium selenide)
(B) PbSe (lead selenide)
(C) GaAs (gallium arsenide)
(D) GeBr (germanium bromide)
(D) GeBr (germanium bromide)
(NB pending confirmation of this answer, since germanium is indeed a semiconductor)
Answer based off attached image (from L7 slide 2)
- (a) Name ONE top-down method employed to fabricate quantum dots. (1 mark)
(b) Briefly describe (or define) the method you answered in part (a) in context of quantum dot fabrication. (1 mark)
- My answers:*
a) Lithography - Pattern-based
b) Mask-pattern defines QD distribution and size to be etched onto a preformed substrate
Young’s wording, (b):
Lithography: etching of the material with predetermined size and shape (often by using a mask or other mechanism) using high energy beams such as electron beams.
- Other*:
a) epitaxy
b) crystal growth of QD on another crystalline substrate (by firing the elements onto the substrate using a beam) whose crystallographic orientation enables the growth of QD on its surface to progress in a particular manner and direction.
- Which of the following reasons best describes why surfactants should have optimal binding capacity to quantum dots during synthesis?
(A) Suboptimal binding capacities of surfactants can lead to either quantum dot aggregation, or prevent quantum dot growth.
(B) Suboptimal binding capacities render quantum dots insoluble in aqueous solutions.
(C) Suboptimal binding capacities lead to poor biocompatibility.
(D) Suboptimal binding capacities can lead to gradual disintegration of quantum dots.
(A) Suboptimal binding capacities of surfactants can lead to either quantum dot aggregation, or prevent quantum dot growth.
- Which of the following statements best describes the hot injection method to synthesize quantum dots through colloidal synthesis?
(A) Aqueous solutions of the precursors of the quantum dot material are injected into an aqueous medium at high temperatures (~300ºC), after which the quantum dots spontaneously precipitate.
(B) Precursors of the quantum dot material dissolved in appropriate solvents are injected into a hot coordinating solvent resulting in supersaturation and subsequently very fast nucleation. This is followed by growth of the nuclei at lower temperature.
(C) Molecular beams of precursor material are expelled through a jet at very high temperatures onto a substrate in a controlled manner, to form quantum dots on the substrate.
(D) Precursors of the quantum dot material dissolved in organic solvents are injected into a hot aqueous solution. The differences in polarity of the two solvents force the precursors to nucleate and subsequently grow.
(B) Precursors of the quantum dot material dissolved in appropriate solvents are injected into a hot coordinating solvent resulting in supersaturation and subsequently very fast nucleation. This is followed by growth of the nuclei at lower temperature.
- Name TWO biomedical applications for fluorescent nanoparticles such as quantum dots.
(2 marks)
(NOTE: need more specific applications other than ‘in vitro imaging’ and ‘in vivo imaging’)
My answers:
- Drug Delivery
- DNA region identification in cancer cells
Notes:
- See attached images. Any TWO of these terms (or any more advanced ones) would have been enough.
- “Diagnosis”, “treatment”, “bioimaging”, – can’t accept these single word answers… need more specificity
- Even ‘Imaging cancer cells’, ‘targeted imaging cancer’, ‘bioimaging cancer cells’ would have sufficed, though I would have preferred proper names of the experiments/applications
- List TWO advantages of using quantum dots over organic molecular dye tags for in vitro and in vivo imaging applications.
(2 marks)
- Protein coated QDs very stable (> 2 yrs)
- Narrow spectral width (FWHM ~25nm)
- Broad excitation spectrum
- High quantum yields (40-50%)
- High absorption cross-section
- Low photobleaching
- Compared to rhodamine 6G:
- 20x brighter
- 100-200x more stable
- List TWO ways in which biomolecules can be tagged to functionalize quantum dots for biomedical applications.
(2 marks)
- Bifunctional linkage
- Hydrophobic attraction
- Silanization
- Electrostatic attraction
- Nanobeads
(see image)
(‘Covalent bonding’ is too general.)
- Briefly describe in your own words the technique “Fluorescent resonance energy transfer” (FRET).
(2 marks)
Keywords: distance-dependent interactions, donor/acceptor proteins/fluorescent molecules/quantum dots
Model answer: FRET is a technique where it relies on the process where a donor molecule (such as proteins) can absorb energy from a photon, and then subsequently transfer this energy to a neighbouring acceptor molecule that would ideally subsequently fluoresce, provided that the donor and acceptor molecule are sufficiently close for the energy transfer to succeed.
- List TWO different types of quantum dots (imaging probes) whose primary constituent is carbon. (2 marks)
Carbon nanotubes, graphene nanoparticles/nanosheets, carbon quantum dots, nanodiamonds, fullerene.
(note - all these materials below have their own characteristic properties and structure.)
- Name ONE advantage of carbon quantum dots (i.e. quantum dots whose primary constituent is carbon) over semiconductor-based quantum dots.
(1 mark)
Biocompatible
- the desired answer is ‘better biocompatibility’ (or along those lines).
- ‘water solubility’ is acceptable
Regarding:
- ‘unique optical properties’ - too vague
- ‘multiphoton/two-photon’, can be done with semiconductor based QDs as well (do not use in exam)
