lecture 2

Question 1

name the 4 most common techniques to visualize nanomaterials

Answer 1

1- TEM
2- cryo-EM
3- SEM
4- Atomic Force Microscopy

Question 2

name the 4 most common techniques to analyze nanomaterials

Answer 2

1- dynamic light scattering
2- zeta potential
3- UV spectroscopy / fluorescence

Question 3

what is the difference b/w transmission electron microscopy and light microscopes?

Answer 3

TEM operates on the same basic principles as light microscope but uses electrons instead of light

• the beam of electrons goes through lenses and through the samples

Question 4

what can you see with common light microscopes

Answer 4

cells- can distinguish between organelles

Question 5

what is the resolution of TEM

Answer 5

0.2 nm

Question 6

compare resolution of light vs. TEM

Answer 6

wavelength of electrons is much smaller than that of light, which is why they can resolve smaller things

• cannot see objects smaller than the wavelength (if wavelength is 500 nm, cannot see anything smaller than 500 nm)

Question 7

in TEM, electrons can behave as…

Answer 7

as a wave and as a particle

Question 8

TEM can be used to visualize what?

Answer 8

viruses
- everything inside the microscope is in high vacuum

Question 9

what is a specific capability in TEM?

Answer 9

EELS (electron energy loss spectroscopy) for chemical and compositional analysis

Question 10

in TEM, as electrons pass through a specimen, they interact with atoms of the solid…

Answer 10

1- many of the electrons pass through the thin sample without losing energy

2- a fraction will lose energy as they interact with the specimen (EELS), this leaves the sample in an excited state , the material can de-excite by giving up energy typically in the form of x-rays or Auger electrons (unique to each element)

3- electron diffraction

Question 11

what is the main difference between TEM and cryo-EM?

Answer 11

cryo-EM does not need high vacuum

• instead, has vitreous water environment, frozen water surrounds the electron beam

Question 12

define cryo-EM

Answer 12

technique applied on samples cooled to cryogenic temperatures and embedded in an environment of vitreous water

Question 13

advantage of cryo-EM

Answer 13

allows us to see virus/material in a more realistic environment

Question 14

describe SEM

Answer 14

very similar to TEM, also uses electrons instead of light
- but rather than a broad static beam used in TEM, SEM beam is focused to a fine point and scans line by line over the sample surface (so instead of penetrating sample, just scans sample surface- simple scan, not much info about composition)

• voltages much lower than in TEM b/c does not need to penetrate
• specimen doesn’t need to be thin,, greatly simplifying specimen preparation

Question 15

describe atomic force microscopy (afm)

Answer 15

entirely different microscope, just a scanner & computer
- AFM uses a cantilever with a very sharp tip to scan over a sample surface- detects changes in height of surface

• laser is reflected on photodiode and causing image to be produced
• can measure an attractive or repulsive charge- creates an image through the software

Question 16

AFM relies on the ___ between tip and surface, these impact the imaging

Answer 16

forces

Question 17

list the 3 scanning modes in AFM

Answer 17

1- contact mode (tip in continuous contact with sample, preferably used for hard samples like metal)

3- non contact mode (tip not in contact with sample, used for soft samples like viruses)

3- intermittent/tapping mode (oscillating cantilever, tip touches gently and frequently, often used for biological samples, imaging in air and liquid

Question 18

name a few examples of cantilevers

Answer 18

Si3N4 (silicon nitrade) , Si
- different spring constants (or stiffness)

typical cantilevers: 1 micrometer thick, 100’s of micrometers long

Question 19

how does AFM imaging differ from TEM?

Answer 19

AFM provides 3D profiles

Question 20

what technique can be used to determine the size of homogenous particles

Answer 20

DLS

Question 21

describe DLS (dynamic light scattering)

Answer 21

particles in suspension cause laser light to be scattered at different intensities depending on size

• particles that are smaller scatter less light & larger particles scatter more light
• different NP’s scatter incident light proportional to the 6th power of their radii and hence the particle size can be estimated using the Stokes-Einstein relationship

Question 22

name 2 limitations of DLS

Answer 22

• some samples are not homogenous
• larger particles will scatter more light and mask smaller particles

Question 23

name the advantages & disadvantages of DLS

Answer 23

advantages:
1- rapid analysis of samples
2- analysis can be done in function of solvent
3- can mimic physiological functions
disadvantages:
1- not ideal for heterogenous solutions
2- works better with spherical-type NP’s

Question 24

what is the purpose of zeta potential?

Answer 24

gives us an idea about the charge of nanomaterials, important to know b/c we want materials that are neutral (not toxic, charge can cause aggregation)

• important that measurements are being done in solutions that have low salt concentration and neutral pH

Question 25

how is ZP calculated?

Answer 25

electrophoretic mobility under an applied electric field - place nanomaterial in solution and apply electric field (positively charged materials go to anode, negatively charged go to cathode; machine can detect this mobility) - desirable ZP for medical applications = between -25 and +25

Question 26

zeta potential is related to electrophoretic mobility by the ___

Answer 26

Henry equation

Question 27

describe the technique using UV-visible spectroscopy

Answer 27

electromagnetic spectrum is 190-800 nm (molecules absorb light in this range) - DCBZ absorbs at 500 nm (not a nanomaterial, just a molecule) - direct relationship b/w absorbance and concentration- dark has higher concentration (more DCBZ molecules) - certain metals produce UV spectra when they cluster to form nanomaterials - if you see a pic, there is a nanoparticle, if no pic, no NP

Question 28

name a limitation and advantage of UV-visible spectroscopy

Answer 28

- UV-visible is cheaper & faster than TEM and DLS - limitations: 1- Abs in the UV-vis range is required 2- measurement is done in a liquid interface 3- spectra should be different from native components

Question 29

describe fluorescence spectroscopy

Answer 29

- involves using a beam of light, usually UV light, that excites the electrons in molecules of certain compounds and causes them to emit light, typically, but not necessarily, visible light - absorb light --> go back to ground state and emit light - common in nature- scorpions & jellyfish can emit fluorescence - there are certain nanomaterials that when disintegrated (not intact), give a different color than when they were intact- helps determine whether the NP has been formed or not

Question 30

describe nano-molding techniques

Answer 30

- PRINT enables the fabrication of nanomaterials with the precise control over the shape, size, composition, and surface functionality (helps synthesize nanomaterials into a certain size & shape) - a template in the nanoscale, pour a specific polymer onto the template --> polymer with nanocavities is peeled away --> pre-particle solution is distributed in the mold --> particles are removed from the mold by bringing the mold in contact with a harvesting film

Question 31

what is the ultimate goal of nano-molding techniques?

Answer 31

the challenge is during the synthesis process, mixture of sizes and shapes (not homogenous), so this template helps create homogenous NP's