Lecture 7

Question 1

What is Hematite?

Answer 1

Fe2O3; rust

Question 2

What are the effects of the ligand shell on the properties of colloidal nanocrystals? (introduction)

Answer 2

This is just an introduction because I’m not sure how to represent this info in a flashcard.

Figure 6.2 shows the equilibrium that exists in the solution when a ligand-capped iron oxide nanocrystal is exposed to a different ligand. As we can see from the figure, both ligands rely on carboxylate groups to attach to the surface iron cations. The bond between the carboxylate and the iron is coordinative and ionic, with a negative charge delocalized on the carboxylate anion and a positive charge associated with the surface iron(III) centre.

Question 3

What are the head groups and the briefing groups?

Answer 3

The head group is the group that is present at the end of a ligand and that interacts with the solvent

The bridging group is the middle part of a ligand, usually a long-chain alkyl group, which connects the head group with the functional group that is anchored to a surface

Both components of a ligand can have very important functions which are summarized in Figure 3.1 and more detail about them will be represented below

Question 4

What is the solvent-particle effect?

Answer 4

This is the best-known effect of ligands which can be tuned by head and riding groups. The effect represents the colloidal stability of the particles in apolar solvents like toluene or hexane, the stabilization of colloids mostly happens via steric stabilisation. Where the alkyl chains, which in our case would constitute the bridging group, have a high solubility in apolar solutions and thus enjoy being solvated by them allowing the alkyl chains to have the maximum freedom they prefer.

Particle-solvent interaction also affects the biodistribution of colloids, as this depends on the colloidal stability and the surface charge distribution (more about this in Flashcard 11)

Note that alkyl chains are limited to apolar organic solvents only, if water or another polar solvent is used the alkyl chains will collapse on themselves, trying to reduce their contact with the “bad” solvent and maximize instead their interaction with each other. In this case, the collapsed ligand shell would be ineffective at preventing aggregation, and instead would work as a glue between particles; by aggregating, the ligands could decrease their exposure to water, and thus be in a more favourable environment.

Question 5

What happens when two ligan-capped particles get close enough to each other?

Answer 5

Their ligand shells start interpenetrating, a repulsive force ensues to obtain two effects:

1) to restore the entropy of the ligand shells.

2) to minimize the local concentration of ligands and thus increase favourable solvation of the ligands by the solvent.

Question 6

How can we use solvents to functionalize ligands?

Answer 6

By using a specific nature of solvent we can control the ability of ligands to behave as either a glue ot a repellent between particles.

For example, to remove precursors we can slowly introduce a bad solvent into the system and the nanocrystals are made to aggregate while the excess precursor remains in solution. In this way, by centrifugation and removal of the supernatant, you can remove the excess precursors from the nanocrystal aggregates, which are then resolubilized by exposure to a good solvent

Question 7

What is magnetite?

Answer 7

Fe3O4

Question 8

What can we do to achieve ligand stabilization in water?

Answer 8

POE poly(ethylene oxide) bridging groups can be used. A more common technique is to use head groups that are charged in water at the pH of interest.

In this case, as well, the nanocrystals can be precipitated by addition of a bad solvent.

Question 9

what are the most commonly used charged head groups?

Answer 9

ammonium groups for a positive charge, and carboxylic acid for a negative charge.

It is important to note that in oxide colloids the use of carboxylic acid as a head group, will cause the ligand to work effectively as a bidentate ligand, connecting particles at each end, and aggregating the nanocrystals together. One must therefore be very careful in choosing a head group which cannot bind to the surface of the other nanocrystal, unless you want to obtain a crosslinked network for nanocrystals.

Question 10

What does this in vivo and in vitro mean?

Answer 10

In vivo means in an actual living organism; by contrast in vitro is under conditions that replicate a living organism but are actually artificial and contained in apparatus

Question 11

What is the issue of using nanocrystals in vivo?

Answer 11

Just as you have an equilibrium between ligand A on the surface of the nanocrystals and ligand B in the solvent (shown in figure 6.1), you will have an eq between ligand A on the nanocrystal and ligand A in the solvent. This means that the ligands try to maintain a certain ratio of concentration between the solvent and the surface of nanocrystals, a ratio determined by the strength of the bond between the surface and the ligand. Therefore if you add pure solvent to a solution of nanocrystals, ligands will detach from the particle’s surface in order to maintain the eq concentration of liands in the solution.

this effect makes dilute colloidal solutions of nanocrystals very unstable, as ligands detach from the nanocrystal surface there will be an increasing probability that they will aggregate irreversibly.

When doing work in vivo, you hope to use very small concentrations of nanocrystals inorder for them not to be recognised by the body and then be excreted through the kidneys leaving no trace. If the nanocrystals aggregate and form large particles they will be immediately recognized by the body as an alien, and this be sequestered in the liver where the organism will try to metabolize them

(This is in relation to flashcard 4)

Question 12

What is ligand-ligand interactions?

Answer 12

It is another function of ligands that can be manipulated by changing the ligand’s bridging and head group.

They have the following aspects (which are discussed in the upcoming Flashcards):

1. Crosslinking
2. Phase segregation
3. Bunching
4. Permeability

Question 13

How can crosslinking happen via ligand-ligand interactions?

Answer 13

We previously mentioned the possibility of crosslinking the nanocrystals together by using a head group able to bind to the surface of the nanocrystals. It is possible to obtain such crosslinking also by matching head groups so that they will react and condense together.

Question 14

How can we control phase aggregation via ligand-ligand interactions?

Answer 14

We mentioned how exposure to polar solvents can lead nanocrystals bearing ligands with alkyl chains to aggregate. We also discussed how we can crosslink ligands by matching the head groups.

Therefore by controlling these two effects, we will have a situation where the nanocrystals aren’t sure whether to aggregate or not, leading to a reversible aggregation mechanism which usually brings about order. Very large crystals or nanocrystals have been grown in such a manner

Question 15

What is bunching?

Answer 15

Another phenomenon that can be observed with alkyl-chain ligands is their bunching on the surface due to their relatively strong van der Waals attraction. We have seen how alkanethiols from ordered arrays on gold, representing an example of a self-assembled monolayer (SAM). Well, alkyl chain ligands will attempt to do the same on gold nanocrystals, but since the surface of the nanocrystals is faceted they will only be able to form small bunching of ligands on each facet.

Such bunching is deleterious to the colloidal stability of the nanocrystals as they leave areas of the surface not adequately protected from interaction with the solvent.

This is one reason why unsaturated alkyl chains are often used as ligands for nanocrystal synthesis. The double bond forces the alky chains to assume a crooked conformation which prevents them from packing efficiently, reducing the van der Waals interaction, and thus preventing bunching.

Question 16

How can ligand-ligand interactions lead to phase-segregated domains?

Answer 16

We have seen how it is possible to form binary-ordered SAMs by using two different alkanethiols with different alkanethiols with different head groups. The same happens on the faceted surface of nanocrystals. It was found recently that this specific phase segregation of ligands on the surface of the nanocrystals strongly affects their solubility and their ability to penetrate cell membranes.

Question 17

What is permeability used in ligand-ligand interactions

Answer 17

First let’s set the general definition of permeability:

the ability of a substance to allow another substance to pass through it.

so when we create covalent bonds between the ligands to create a network of molecules we play with the permeability of the network to create a network that on a surface would be very hard to remove

(check this I’m not too sure)

Question 18

How can particle-particle interaction be affected by the ligands?

Answer 18

The pIt has been found that different ligands bring about different nanocrystal surface charges. This has obvious consequences on their self-assembly, since with different charges you can change the nanocrystals mutual attraction from repulsive to attractive, or vice versa

Question 19

How can particle-biomembrane interaction be effected by the ligands?

Answer 19

The Final interaction that can be affected by ligands is the particle-biomembrane interaction, where surface charge and its distribution play a key role in the particle-surface interaction.

We can induce the sticking of nanocrystals on a SAM-covered gold surface by using ligands that interact strongly with that SAM. Often, the van der Waals interactions between the interdigitating ligands and alken thiols are not enough to make gold surfaces sticky!

Question 20

What is magnetism?

Answer 20

Magnetism is a force that is still not fully understood but known to be generated by rotatry motion of charges

Question 21

What is magnetism in solids caused by?

Answer 21

Magnetism in solids is caused by the dynamics of the electrons. As known since Faraday, magnetism is generated by the rotary motion of charges. If electrons circulate in a round circuit they produce a magnetic field. If a magnetic field is applied to a stream of electrons they will beond in a circle.

Question 22

What is spin?

Answer 22

Electrons around atoms have magnetic properties - spins - whih are attributed to their spinning on their axis. Orbitals have magnetic properties which are attributed to electrons circulating in them. All these magnetic fields and spins can couple together in very sophisticated ways, generating spin waves (magnons) and other sorts of weird phenomena typical of strongly coupled systems.

Among such phenomena is the formation of Wiess domains, which are shown in Figure 6.2.

This is depicted in the lecture slides which are displayed in digital notes

Question 23

What are Weiss domains?

Answer 23

Wiess domains occur when the magnetic dipoles in a material interact and align together within certain domains; this happens below a certain temperature known as the curie temperature.

Question 24

What are the domains in different materials?

Answer 24

In ferromagnetic materials (like iron) all dipoles align in the same direction; in ferrimagnetic materials (like magnetite; Fe3O4) the dipoles are in two groups pointing in opposite directions, but one group is stronger than the other leading to non-zero magnetization; in antiferromagnetic materials (hematite; Fe2O3) the diples are in two groups that have equal strength in opposite directions, but if a field is applied in one direction, one of the groups of dipoles can become stronger than the other.

This is depicted in the lecture slides which are displayed in digital notes

Question 25

What happens to Weiss domains as you apply a magnetic field? (This is basically a description of the hysteresis loop in terms of Weiss domains)

Answer 25

Alot of what is mentioned here is in reference to Figure 6.2) First, an ordeer (Ferro, ferri, or aniferro-) magnetic bulk material at zero applied field will not show a magnetic moment M, since the Weiss domains are magnetically randomly oriented, and thus cancel each other out (Point 1). In the presence of a magnetic field H (point 2), the randomness is lifted, as various Weiss domains try to align with the applied field. The material starts to display a net magnetic moment which will saturate at M=Ms, the saturation magnetization, when all the domains are aligned with the applied field. Once the applied magnetic field H is removed (Point 3), the Weiss domains will try to return to a random state. They will encounter though a certain "friction" between each other that will prevent them from doing so. The result is that the material will maintain a certain remanent field Mr. In order to recover a zero-sum magnetization the magnetic material must be exposed to an opposing coercive field of strenght equal to Hc (point 4) or be heated above Tc

Question 26

How can you make a hysteresis loop? (there will be some clash with flashcard 25 sorry)

Answer 26

Alot of what is mentioned here is in reference to Figure 6.2): You start from a demagnetized material at zero fields 1; you can increase the applied field till saturation, 2 then you bring back the field to zero, 3; you invert the direction of the field till you bring the magnetization in the material to zero, 4; you increase the intensity of the inverse applied field till saturation; and then you bring the applied field in the original orientation and increase it til saturation, 2. This hysteresis cycle should generally form a closed loop and the area enclosed within it is a measure of the energy dissipated in the process in the form of heat.

Question 27

What does the shape of the hysteresis loop tell us?

Answer 27

The shape of the cycle will tell us if the material is a ferromagnetic, a paramagnetic or a superparamagnetic (defined in Flashcard 28) if it has a disordered surface if it has a core-shell structure and so forth.

Question 28

How does size affect the magnetic properties of a ferromagnetic material?

Answer 28

In Figure 6.2 is shown that with decreasing the size, Mr and Hc will decrease due to the decreased amounts of Wiess-domains boundaries (called 'walls') causing friction. Losing that friction makes the material less hysteretic (more reversible). By the time the nanocrystal has a size below the size of a single Wiess domain, there will be no walls left. The whole nanocrystal will be a single Weiss domain. All the magnetic dipoles within the same nanocrystal are parallel and there will be no remanent magnetization or coercive field (basically superparamagnetism but we will define it formally in flash card 28)

Question 29

What is superparamagnetism?

Answer 29

Superparamagnetism is a magnetic ordering phase in which the material is constituted by a single Wiess domain; the whole material can thus orient its magnetic dipole with the applied magnetic field and this leads to the absence of remanent magnetic field and to very large maximum magnetizations!

Question 30

How can the magnetization of nanocrystals ever give zero-sum if all its magnetic dipoles are already perfectly aligned?

Answer 30

The point here is that the energy required to change the orientation of the dipoles in a nanocrystal of that size is ver, as there are no domain walls. Such energy is less than the thermal energy that the material will absorb from the environment. The dipole orientation will thus oscillate rapidly in time, leading to a zero-sum at zero-field. An array of nanocrystals - a solution of them - will have nanocrystals aligned randomly with respect to each other, also leading to a zero-sum. in the case of an applied field, the dipoles will still oscillate but will preferentially point towards the direction of the applied field, thus leading to a non-zero magnetization. Again this type of magnetisation is called superparamagnetic. This is depicted in the lecture slides which are displayed in digital notes

Question 31

What are ferrofluids?

Answer 31

They are concentrated colloidal suspensions of superparamagnetic particles. Droplets of these black liquids show fantastic shapes when exposed to a magnetic field, as can be seen in Figure 6.3. The Image shows a liquid surface which is modulated by a magnetic field. What you see is the compromise between the magnetic force exerted in the particles and the capillary force of the fluid which tries to minimize the liquid-air interface. The result is a form of self-assembly that gives a nearly periodic structure, whose period and amplitude of the oscillations depend on the strength of the applied field the period decreases and the amp increases with increasing field) Besides being able to make fancy structures, some of these liquids are able to change their viscosity upon exposure to magnetic fields. Under a magnetic field, the dipoles formed on each nanocrystal will align with each other forming microfiber of nanocrystals. Such fibres can strongly constrain the flow of the liquid, increasing its viscosity on a macroscopic scale. The important aspect is that this change can be rapid. This is depicted in the lecture slides which are displayed in digital notes

Question 32

What is the Kirkendall effect?

Answer 32

It is the effect that occurs when you place against each other two solids, A and B, that can react to produce a third material, C, in the middle, and you heat, you will observe that the interfaces A-C and C-B will move at different rates. Note that in the case of nanoscale structure, you can observe a kind of Kirkendall effect which is not observed in the bulk phases. The reason for this is that the mean free path of diffusion is comparable in size with the size of the nanocrystals, thus altering all diffusion-limited processes like solid-state reactions. This is depicted in the lecture slides which are displayed in digital notes

Question 33

What happens when you expose a metallic nanocrystal to oxygen or to a chalcogen like sulfur or selenium (Figure 6.4)?

Answer 33

Let's assume the thermodynamic conditions are such that the reaction between the two is spontaneous. the anion deposits on the surface of the iron. To complete the reaction, one element has to diffuse into the other in order to form a metal oxide lattice. The metal has a larget mobility and a more favourable diffusion coefficient than the oxygen. Thus, instead of the oxygen migrating into the metal lattice, it is the metal which moves outwards to surround the oxygen. If the iron cations move outward they must leave vacancies (see Figure 6.4). Such vacancies are highly energetic and will tend to coalesce together in order to minimize the dangling bonds they create. Such coalescence leads to the formation of a large void at the centre of the nanocrystal. A hollow nanocrystal! Note that in this case oxygen is much larger than the Fe so it will diffuse slower The resulting capsules are not single-crystalline but polycrystalline, which actually turns out to be a very good thing. The boundaries between the grains in the hallow shell provide nice diffusion channels between the hallow and the exterior. The interior surface and cavity also is then chemically accessible and thus can be functionalized and used for things like catalysis, drug delivery, encoded tags, or chemical sensors

Question 34

How does nature minimize the consumption of enegry?

Answer 34

If you look closely at the microscopy images of the nanocrystals taken at different stages of the reaction (Figure 6.4) you will see that the voids initially coalesce, forming an empty layer which separates the iron oxide shell from the unreacted core. The two parts are connected together by thin 'Threads' of atoms which serve as conduits for the ion diffusion. This is very smart. In fact, while it is more expensive in terms of surface energy, it saves a lot more energy by letting diffuse only the minimum amount of ions; in the model of a permanent central hole, all unreacted atoms in the nanocrystal would have had to be shuffled in the process.

Question 35

How do you make self-assembly of nanocrystals into ordered superlattices?

Answer 35

The principle here is similar to the one we saw for opals: we start from highly stable colloidal dispersions of highly monodisperse colloids; the substrate is placed in dispersion, which is allowed to evaporate; the colloids will self-assemble in superlattices, such as the ones shown in Figure 6.5

Question 35

Where does self-assembly occur + a more detailed mechanism?

Answer 35

It takes place in the meniscus region, where the particles are forced by the enhanced flux of liquid to pack against the growing interface. The small volume of the meniscus enforces a preordering of the nanocrystals before they hit the growing interface; the small distance between the nanocrystals in the meniscus region strengthens their mutual repulsive interactions and thus by trying to keep apart from each other, they will preorder in configuration in which the nearest -neighbour distances are maximized

Question 36

What is the difference between this self-assembly and the self-assembly of silica colloids?

Answer 36

The remarkable difference between this self-assembly and the self-assembly of silica colloids is the relative ease with which you can produce binary lattices with nanocrystals such as the ones shown in the bottom right corner of Figure 6.5. Such binary crystals mimic the symmetries of binary atomic NaCl structure or the CaF2 structure and can have radically different properties from their unary counterparts. Researchers are pursuing binary superlattices formed by two different classes of non-crystals: magnetic and luminescent or magnetic and plasmonic All these properties can be mixed by Design in binary superlattices with the added advantage that the properties of the complicit of the composite will be exceedingly homogeneous due to the order of the structure much in the same way crystal properties are more homogeneous than the properties of glasses.

Question 37

What are the applications of magnetic nanoparticles?

Answer 37

Magnetic nanoparticles can be used in theranostics which is a mix of therapy and diagonostic applications: i) Sensing and actuation (shown in digital notes) ii) Magnetohyperthermia (shown in digital notes) iii) Magnetic resonance imaging (MRI) (shown in Flashcard 39)

Question 38

What is MRI and how are nanoparticles used in its application?

Answer 38

MRI is an imaging technique which is very good resolution and sensitivity to soft tissues as a signal come from water molecules. The principle behind MRI is a relatively simple. You have a very strong magnetic field, which orientates the nuclei of the water hydrogen atoms. nuclei, just like electrons, spin on their axes and this gives them a nuclear magnetic moment which can align with the applied magnetic field .The intensity of that moment is fairly small and this is why you need a strong magnetic Field. Once the nuclear are aligned a really wave is emitted which collapses temporarily the alignment of the nuclei. The contrast from the MRI comes from the differences in the relaxation time of the nuclei. Just as any excited States, the collapsed nuclei have a certain lifetime before theiy reorient with the applied magnetic field. Such lifetimes are profoundly affected by the local environment of the proton. MRI contrast agents which is the application of the nanoparticles more specifically iron oxide enhance the water protein relaxation rates does enhancing this contrast in the MRI image. As you can see from figure 6.6 eficiency of a contrast agent depends on many factors, the most important being the magnetic-field gradient it produces, the water exchange rate in its vacinity, and the tumbling rate (how fast the agent wobbles in space). Note that one advatahe of iron oxide in this application is that it is metanolized easliy by the organism, thus preventing long-term accumulation.