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SB.Nano > Part 1 > Flashcards

Part 1 Flashcards

1
Q

What does TEM stand for

A

Transmission electron microscopy

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2
Q

How does TEM work?

A

A beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes. Due to the adsorption of the electrons in the material (due to the thickness and composition of material) TEM contrast is achieved and an image can be created.

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3
Q

What does HRTEM stand for?

A

High resolution transmission electron microscopy

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4
Q

How does HRTEM work?

A

Allows the imaging of the crystallographic structure of a sample at an atomic scale. At higher magnifications, contrast arises from the interference in the imagining plane of the electron wave itself.

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5
Q

What is TEM and HRTEM used for?

A

Imaging of material morphology, size, size distribution and drug delivery application. HRTEM can be used specifically to determine crystal structure, interface between core/shell structure of NPs

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6
Q

What is UV-Vis?

A

Refers to ultraviolet-visible spectroscopy

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7
Q

What affects the colour of chemicals?

A

The absorption or reflectance in the visible range directly affects the perceived colour of the chemicals involved.

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8
Q

What is the mechanism of absorption spectroscopy?

A

Upon striking the sample, photons that match the energy gap of the molecules present are absorbed in order to excite the molecule. Other photons transmit unaffected. . An absorption spectrum can be obtained.

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9
Q

What is fluorescence?

A

Fluorescence is the emission of light by a substance that has absorbed light (re-emission). It is a form of luminescence.

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10
Q

What is a typical difference between emitted light and absorbed radiation?

A

Emitted light typically has a longer wavelength and therefore lower energy.

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11
Q

Give the equation for the band gap, knowing the wavelength

A

E = 1240/gamma

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12
Q

How do you approximate band gap?

A

from an absorption spectrum (absorbance vs wavelength), you take the onset of absorption from the low energy side (high wavelength). See diagram

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13
Q

What are the limitations of TEM

A
  • HRTEM may be affected by sample thickness and defocus, which can make results difficult to interpret.
  • 1 single image is insufficient to infer the complete structure
  • High energy used may destroy or damage the structure, or change its properties.
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14
Q

What does SEM stand for?

A

Scanning Electron Microscopy.

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15
Q

How does SEM work?

A

Produces images of a sample by scanning it with a focussed beam of electrons.
Electrons interact with atoms in the sample, producing various signals that can be detected.

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16
Q

What information does SEM tell you?

A

Gives information about the sample’s surface topography and composition.

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17
Q

What is the role of electrons in SEM?

A

The electrons are scattered and collected by detectors. Electrons that are back scattered from the specimen are collected and form an image.

Electrons do three things:

  • pass right through the sample without colliding with any of the sample atoms (atom mostly empty space).
  • can collide with specimen electrons, creating showers of secondary electrons.
  • Can collide with the nucleus of sample atoms, creating backscattered electrons.
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18
Q

Give advantages for SEM

A

Since electrons do not need to penetrate the specimen, this can be bulky and opaque.

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19
Q

Give details about secondary electrons

A

Incident beam is highly energised electrons. When they collide with sample atom electrons, they will knock them out of their orbitals. This is a secondary electron with very weak energy. A single impinging, high energy electron can cause a shower of thousands of low energy secondary electrons.

If these secondary electrons are close enough to the surface they can be collected to form an image.

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20
Q

Give details about backscattered electrons in SEM

A

when the incident beam collides with the nucleus of the sample atom, it bounces back and retain their high energy.
A sample with a higher density will create more of them so the image can typically discern differences in sample concentration.

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21
Q

Give details about X-ray emission in SEM

A

Upon emission of secondary electrons, this leaves a “hole” in the shell. To stabilise the atoms, electrons from outer shells will drop into the inner shells. These outer orbitals have a higher energy and therefore X-ray emission occurs.

The X-rays emitted are characteristic in energy and wavelength depending on the sample atom. Therefore, can determine chemical composition.

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22
Q

What is super-resolution microscopy?

A

Due to the diffraction of light, the resolution of conventional light microscopy is limited to approx. 200 nm.

Super-resolution allows images to be taken with a higher resolution that the diffraction limit.

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23
Q

How does a super-resolution microscope work

A

Insert answer

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24
Q

Give the generic term for microscopes that can scan sample surfaces with an extremely sharp probe to observe their 3-D structure.

A

Scanning Probe Microscopy (SPM)

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25
Q

What is STM?

A

Scanning tunnel microscopy. Uses the basics of quantum tunnelling. When the conducting tip is brought very near the surface to be examined, a voltage difference applied between the two can allow electrons to tunnel through the vacuum between them

The “tunnelling current” is a function of tip position, applied voltage, and local density of states.

26
Q

Give an advantage for STM

A

It doesn’t necessarily require ultra high vacuum, but because of the quantum tunnelling effect it can be used also in air, water, and various other gases and liquids. A wide range of temperatures can be utilised too.

27
Q

Define the 2 modes of STM

A

Constant current: tip is vertically adjusted to maintain a constant current. As the current is proportional to the local density of states, the tip follows a contour. A topographic image is generated by the vertical position of the tip.

Constant height mode: the current is a function of lateral position and represents the surface image. Only really applicable for atomically flat surfaces. An advantage is that it can be utilised at high scanning frequencies.

28
Q

What is the mechanism behind AFM?

A

Atomic force microscopy (AFM) used to “feel” across the surface. Uses a cantilever with a sharp tip. When the tip is brought into close proximity of the sample surface, forces between the tip and the sample lead to a deflection of the cantilever, according to Hooke’s law. Deflection is measured by a laser spot, reflected from the surface of the cantilever.

29
Q

What is the operation of the cantilever in AFM?

A

As the cantilever is displaced, via its interaction with the surface, so too will the reflection of the laser beam be displaced on the surface of the photodiode.

30
Q

What are the uses for X-ray crystallography?

A
  • Find crystal structure
  • Measure the size, shape and internal stress of small crystalline regions.
  • Measure the average spacings between layers or rows of atoms.
  • Determine the orientation of a single crystal or grain
31
Q

Give the formula for particle diameter from X-ray crystallography (Debye-Scherrer)

A

D = 0.9gamma/(beta cos(theta))

32
Q

What are the uses for DLS?

A

Dynamic light scattering (DLS) used to determine the size distribution profile of small particles in suspension or polymers in solution.

33
Q

How does DLS work?

A

Temporal fluctuations are usually analysed by means of the intensity or photon autocorrelation function. Faster dynamics due to smaller particles lead to faster decor relation of scatter intensity trace.

34
Q

Give the equation for hydrodynamic radius

A

d (H) = kT/ (3pi n D)

35
Q

What is the zeta potential?

A

describes the electrokinetic potential in colloidal dispersions.

It is the electric potential in the interfacial double layer at the location of the slipping plane, relative to a point in the bulk fluid away from the interface.

i.e. it is the potential difference between the dispersion medium and stationary layer of fluid attached to the dispersed particle.

36
Q

What creates a surface trap?

A

Surface traps are due to imperfections (surface defects) and dangling bonds. The formation of solid surface results in the breakdown of periodic crystal lattice, that is, the chemical bonding between atoms are disconnected and remains as electron lone pairs or vacant sites.

37
Q

What is typical evidence of surface trap emission?

A

A broad absorbance peak is witnessed at low energy, which is related to surface state emission. They typically possess poor photoluminescence energy.

38
Q

How can nanoparticles be stabilised?

A

By steric repulsion (hindering nanoparticles from being close to each other stabilise with long chains) and electrostatic repulsion (same surface charge).

39
Q

How are surface traps created?

A

Due to insufficient passivation with organic ligands, QDs are easily exposed to the environment and result in the undesired non-radiative carrier relaxation process or chemical reaction. These sites are called surface traps.

40
Q

How can you remove a surface trap?

A

Surface traps can be partially removed by surface coating with organic molecules (ligands)

41
Q

What is X-ray photoelectron spectroscopy (XPS) used for?

A

It can measure elemental composition, chemical state and electronic state of the elements within the material. It can be used to study the nonmaterial surface coating coverage (e.g. how much surface area is covered by surfactant).

42
Q

What is the mechanism of XPS?

A

XPS spectra are obtained by irradiating a material with a beam of X-rays whilst simultaneously measuring the kinetic energy and no. of electrons that escape from the surface (top 10 nm of material)

43
Q

Give the energy equation for XPS

A

E_binding = E_photon - (E_kinetic)

44
Q

What factors influence surface passivation?

A
  • Binding energy between ligands and the surface of nano materials.
  • Charge transfer between nano materials and ligands.
45
Q

What are the limitations of surface passivation by organic ligands?

A
  • it is difficult to simultaneously passivate both anionic and cationic surface traps.
  • steric hindrance between bulky organic ligands results in incomplete surface coverage and unpassivated dangling bonds.
46
Q

What is an advantage of surface passivation?

A

The ability to significantly improve the fluorescent efficiency.
They can also greatly improve the materials biocompatibility.

47
Q

Why is the control of passivation shell thickness paramount?

A

If the shell is too thin, the passivation of the core NC is inefficient, resulting in reduced photo stability.
If the shell is too thick, the optical properties can generally deteriorate as a consequence of stain induced by the lattice mismatch, and by the generation of defect states.

48
Q

What is SPR?

A

Surface Plasmon Resonance (SPR) is the collective oscillation of electrons in a solid or liquid stimulated by incident light.

49
Q

What condition must exist for SPR?

A

Resonance condition is established when the frequency of light photons matches the natural frequency of surface electrons oscillation against the restoring force of the positive nuclei.

50
Q

What is the photo thermal effect (SPR)?

A

When gold and silver nanoparticles absorb laser radiation of specific wavelength, it triggers the electrons in the metal nanoparticles to oscillate with the frequency of the electromagnetic field and generate heat. This is SPR

51
Q

What are the advantages of SPR?

A

Photothermal conversion allows for very rapid, localised heating with high selectivity.

52
Q

How do you use hyperthermia for cancer therapy?

A

Gold NPs are adsorbed onto the surface of cell membranes or uptake by cells. The temperature inside the cells is raised by illumination of the gold particles. A temperature increase of only a few degrees is sufficient to kill cells.

53
Q

What is the seed-mediated method of creating gold nanorods?

A

Two steps: production of seed particles and seed growth into rod.

54
Q

What is the growth mechanism of gold nano rods?

A

“Zipping” fashion formation of gold nano rods.
Step 1: nucleation, growth, development of facets
Step 2: Preferential surfactant binding to specific crystal faces. (The positively charged CTAB bilayer stabilises the nano rod).

55
Q

What makes gold nano rods useful for cancer treatment?

A

Gold nano rods can absorb energy from near infrared light and convert the energy to heat, which can be used to kill cancer cells.

56
Q

What are dendrimers?

A

Highly branched, star-shaped macromolecules with nm-scale dimensions. defined by three components: central core, interior dendritic structure and an exterior surface with functional surface groups.

57
Q

What is a use for dendrimers?

A

Used to coat the surface of CdSe quantum dots and reduce surface oxidation.

58
Q

Why are CdSe cytotoxic and what can be done to reduce this?

A

Surface oxidation leads to the release of cadmium ions. This problem can be alleviated by coating the surface with an inorganic layer such as ZnS, or by proteins such as BSA (bovine serum albumin).

59
Q

What are the 4 classes of biomolecule conjugation?

A
  • ligand-like bonding to the surface of the inorganic particle core, commonly by chemisorption
  • electrostatic adsorption of positively charged biomolecules to negatively charge NPs or vice versa.
  • Covalent bonding by exploiting functional groups.
  • non-covalent, affinity-based receptor-ligand systems.
60
Q

How can nanomedicine be used for targeted delivery?

A

Targeted delivery can be achieved by coating nano materials with surface ligands that have a strong affinity with cancer cell septic receptors (e.g. anti-Her-2 antibody, which can specifically bind to the Her-2 receptor on breast cancer cells).

61
Q

What is the advantage of using nanoscale drug delivery systems?

A

Targeted delivery. It is possible to minimise the uptake of anticancer agent by normal cells, thus minimising the side effects of therapy.

SB.Nano (8 decks)

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