Lecture 4 Flashcards
What is a monolayer?
A monolayer is a single layer of identical objects on a surface
What are the reactions of gold?
Gold has always been touted for its inertness to acids and bases. Though some molecules react with gold at room temperature, forming a uniform monolayer on its surface (as shown in figure 3.1 in the notes). These molecules all share the functional group of thoil -SH
Note that for gold, sulfur can only react with the most superficial layer of atoms, and the formation of such an Au2S layer is thus invisible to the human eye. while for silver it forms a dark coating of Ag2S.
What is the nature of the reaction and the bond formed during the thoil group and gold interaction?
As of yet, the reaction of the gold surface with the thiols is still surprisingly a topic of much debate, it is believed though to occur via the elimination of hydrogen from the thiol, which becomes a thiolate, while gold gets oxidized to Au^+
The nature of the bond formed between the gold surface and thiolate is still up for debate as well, but what we care about is the fact that the bond is strong enough to keep the R group that is attached to the thiol anchored on the gold surface at room temp and atmospheric pressure. Given the gold surface uniformity and the relative weakness of the bond we can think of the molecules anchored as rafts; their lateral diffusion at room temp is non-negligible.
What is the Gedanken experiment?
It is an experiment that gives a macroscopic example of how the thiolated molecules react with the golds surface:
Imagine your molecules on the gold surface to be beads on a tray. When you add enough amounts of beads to the tray and not shake it you will form an irregular, non-periodic, and amorphous array similar to that of 2D glass. This situation corresponds to thiols and on the gold’s surface at absolute zero temperature.
Temperature shakes molecules a little with an intensity that increases with degrees. if you start shaking the tray, emulating temperature, you will start to see the beads from hexagonally ordered clusters eventually growing in the area to occupy most of the tray; the beads would have crystallised. the same happens with the thiolated molecules on gold.
This thought experiment does not account for many complications within the real problem, For example how the beads only interact by colliding with one another while the thiolates attract each other via VdW forces and interact with eh underlying gold surface via the thiolate group
What is so unique about gold?
The uniqueness of gold is that distances between its atoms on the surface are placed in a way to achieve the minimum distance at which alkanethiols can pack.
These arrays of molecules on gold are called self-assembled monolayers, or SAMs.
What is the proper definition of self-assembled monolayers or SAMs?
They are ordered 2D arrays of molecules
Whta is the difference between SAM and grafted molecules on the silica surface? (question also includes future lectures)
The main difference is that SAM can completely cover the surface of gold, thoroughly changing its properties. This is because the bond between thiols and gold is liable enough to allow for efficient lateral diffusion of molecules; the reaction of silanes with silanols instead leads to strong covalent Si-O-Si bonds, which do not allow for much diffusion.
What does the structure of SAMs look like?
Alot of what is mentioned below is made to reference Figure 3.1:
The R in Figure 3.1 represents the alkyl chains, maximizing van der waals interactions by minimizing bulkiness and maximizing length.
From the top view, it is shown how the lattice of alkanethiol molecules is commensurate with the gold surface. In the front view is shown how the alkanethiols naturally lean at a certain angle with respect to the surface (this angle depends on the crystallographic facet on the metal)
Binary SAMs can also be produced when different alkanethiols are eco-assembled on the surface of gold; under specific conditions, you can observe the formation of binary-ordered arrays in which the arrangments of both species are periodic.
In Figure 3.1, we also see green and blue dots at the end of the alky tails, representing different terminal groups, which serve to remind you that SAMs can add organic functionality to the gold surface.
In the Row underneath we are shown some of SAM defects. On the left we show how gold can present grain boundaries and how the same would follow the change in orientation of the gold lattice underneath; in the middle, we show two different kinds of grain boundaries with SAM lattice. on the top, there is a top-down view of a grain boundary arrangement of the SAM molecules on the au surface, while below is shown how changes in the orientation of the alkanethiols can also constitute a grain boundary. on the right is shown how the gold surface can be terraced and how their end can influence the packing of the SAM
How can terminal groups on the end of the alkyl tail add organic functionality to the gold surface?
We can do multiple things for example have a polar group like OH on the end to allow for wetting or attach out an amine for the conjugation of biomolecules, or even carboxylic acids to make the surface negatively charged (upon deprotonation at pH 7 in water)
What is the influnce of defects in SAMs?
The influence of defects is that the properties of the block are modified. For example, the alkanethiols right next to a grain boundary or to a terrace are less tightly bound to the gold than when they are in the middle of a SAM lattice since the have fewer neighbouring molecules to interact with via VdW forces. This means that those molecules are more prone to be removed or exchanged than averge; this, in turn, means that under certain mild conditions, you might be able to selectively remove or exchange them while leaving the rest untouched
What is the ligand exchange?
It is a strategy that relies on the concept where alkanethiol molecules that are loosely bound to the surface can be removed (most often by raising the temp: in the case of thiols on gold, 80C in solution is usually enough) or exchanged.
If you put a SAM-protected gold surface in a solution containing another alkanethiol, equilibrium is established between the thiols on the surface and the thiols in the solution (shown in the notes).
When the amount of the other thiols is larger than the thiols attached to the gold surface the equilibrium will mostly be driven to the right side, representing an almost complete exchange of the initial thiol with the new thiol.
Sometimes the exchange is very slow (kinetically hampered) and it must be accelerated by an increase in the temperature of the voltage biasing the gold with negative charges and thereby weakening the gold-thiolate bond.
This strategy is often used to change or fine-tune the properties of surfaces and will play a major role in making nanocrystals soluble in water.
What happens when a gold particle is exposed to an electromagnetic wave (light)?
When a gold particle is exposed to an electromagnetic wave, the electrical field associated with it moves the free electrons of gold to one side of the particle building up the region of negative charge and leaving a positive charge on the opposite side, thus creating a dipole, which will oscillate at a certain frequency.
This oscillation of electron cloud is called the PLASMON; the natural frequency at which the electrons can oscillate in a specific particle is called the PLASMON RESONANCE FREQUENCY and if the incoming electromagnetic wave has the same frequency important effects happen.
You can image the free electron cloud as a spring, the more you stretch the more you will strengthen the opposing force of the generated dipole; such dipole represents the separation of positive and negative charges which spotaneously, due to the coulomb attraction will try to come back together. So, just like a spring, a free electron cloud can be stretched, but the more you stretch it, the more it will oppose resistance.
Such systems are called oscillators, this specific one is called a damp oscillator since as its oscillation starts it will wane down due to damping effects.
The general oscillation is shown in Figure 3.2 in the notes.
What is the formal definition of plasmons, resonance frequency, and plasmon resonance frequency?
- Plasmon: a coherent oscillation of electrons on the surface of a conductor
- Resonance frequency: The natural frequency of an object, which can build up to a large vibration amplitude; at this frequency, an oscillator absorbs much more than at other frequencies.
- Plasmon resonance frequency: The frequency of oscillation of a free electron cloud that when met by electromagnetic radiation of the same frequency absorbs or scatters the light significantly.
What happens when you hit the resonance frequency of gold nanoparticles?
When we heat the resonance frequency of the free electron cloud in our gold particles. What will happen (as shown in Figure 3.2) the electromagnetic wave is not allowed into the material but will is instead absorbed or scattered away very efficiently. (Linking back to lec 1: The origin of the purple-red colour of colloidal dispersion of gold nanocrystals comes from this specific absorption/scattering phenomenon)
It is important here to stress that the efficiency of this absorption/scattering (CALLED THE CROSS-SECTION) is the largest for this kind of phenomenon than for any other, which makes metal nanocrystals extremely detectable even at low concentrations. For example, a very low concentration of gold nanocrystals will, in fact, absorb enough visible light to be detected by the eye as a change in the colour of the solution they are dissolved in.
What are the important aspects of plasmons?
Alot of what is mentioned here makes reference to Figure 3.2.
There are several important aspects of plasmons:
First of all the plasmon resonance frequency depends on the size of the particles. For spherical gold nanocrystals, the main plasmon frequency can be tuned from 510 to 540 nm with size.
Shape also has a very important effect, as shown in Figure 3.2. In the case of oblate particles or nanorods, the system will have two separate plasmon frequencies depending on which axis the electrons are oscillating along; the resonance frequency on the short axis will be similar to one of the spherical nanocrystals, but the additional plasmon resonance appearing due to the long axis will be at a much lower frequency or larger wavelengths. Also, the frequency of this additional resonae can ve widely changed by altering the length of the gold nanorods.
What does the century-old Mie theory tell us?
The century-old Mie theory tells us which are the parameters determining the plasmon resonance frequency:
The formula shown in Figure 3.2 gives the absorbance as a function of two terms with different physical origins: the first term on the left represents scattering which depends on the dielectric constant of the medium Em, on the cube of the particle’s radius R, and on the inverse wavelength of the incoming radiation; the second term from the left represents the absorption due the plasmon resonance which depends on the real and imaginary part of the dielectric constant od the particle and on the dielectric constant f the medium.
The real part of the dielectric constant E’ is related to the refractive index and thus to the ability of the material to slow down light. THe imaginary part of the dielectric constant E’’, has to do instead with the damping losses. which come from the absorption of the light travelling through the material.
The denominator of the second term would approach zero if E’ would approach the value of -2Em. At that condition, as the denominator approaches zero, the second term would increase dramatically, generating the condition of resonance which is represented by the peak in plasmon absorbance.
How are gold nanocrystals synthesized?
You start from a water-soluble Au^3+ salt, usually HAuCl4, which is then reduced to metallic Au^0 by a reducing agent like citric acid, amines borohydrides (NaBH4), or even phosphorus, as described in Faraday’s original synthesis of colourful gold colloids.
The particles that are created need to be stabilised by a ligand, often a thiol, or the reducing agent itself in the case of citric acid.
The common strategy for colloid synthesis is to control the nucleation and growth of particles produced during a standard precipitation reaction: by reducing the surface energy of the growth particles by ligand-capping you alleviate their tendency to aggregation, and stabilize their size at the nanoscale.
VERY IMPORTANT LOOK AT DIGITAL NOTES FPR MORE INFOOOOO!!!!
what is the galvanic replacement reaction?
It is a reaction done by exposing a metal to a salt solution of the other metal; if the reduction/oxidation potentials for the two elements are correct, the salt will get reduced while the metal will get oxidised, leading to an EXCHANGE!!
This is illustrated in Figure 3.3
What is the difference between applying “trivial” reactions to a bulk material and a nanoscale material?
The main difference lies in the diffusion length of atoms or ions!
Diffusion length is the measure of how far a particle travels by diffusion before it is absorbed or otherwise consumed; for ions, this is usually of the order of nanometers.
What happens when you place a silver nanotube into a solution of HAuCl4?
The silver will start dissolving at a rhythm of three silver atoms for every gold atom (since Au is at a 3+ charge and Ag will have at most an oxidation state of 1+). An obvious consequence is that, at the end of the reaction, you will have three times less solid metal than you started with
At the onset of the reaction, there will be the formation of mixed Au/Ag alloy on the surface of the nanocubes. At some point the surface will develop a hole (presumably due to the presence of a defect ) through a process called “pitting”. The silver will be “eaten out” of the nanocubes from inside this hole and the reduced gold will end up thickening the Au/Ag alloy shell l. Once all Ag has been consumed, the pore will close and the shell will start the process of dealloying (removal of one element from an alloy of two or more elements). During this process the shell is losing atoms and forms pores, becoming a sort of holey golden nano-box.
All of this is shown in Figure 3.3 together with the representative scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
Note that you can also make holey nanotubes from full silver nanowires with the same principle!
What is so special about the reaction between Ag and HAuCl4 and how can we use that to our advantage?
The reaction is very similar to the crystallographic plane on which it operates. In the case of silver nanocubes (card before), the surfaces were all planes with miller indices (100) of FCC structure of solid silver. So if, by a chemical process, we start to selectively dissolve the corners of the cubes, (111) planes will start to be exposed as shown in the bottom diagram of Figure 3.3. With this starting material, the galvanic replacement proceeds in the same manner, but the pitting behaviour occurs selectively on the exposed (111) planes. Such holes are too large to be closed at the end of the hollowing process, and this leads to holey nanoboxes where the holes are exclusively where the (111) facets were.
Such very well-defined holey nano boxes have been redubbed as nanocages, and they show some remarkable optical properties as their plasmon resonance frequency can be widely and precisely tuned across the near-infrared, which is a very important frequency range for biological purposes as well will soon see. Their hollow structure also allows us to dream about the possibilities of using them as drug delivery vehicles or as catalysts!
What is Plasmon coupling?
Plasmon resonance has been applied in a multitude of devices, but what is discussed here is related to their coupling, which occurs during the self-assembly of gold nanocrystals, where neighbouring nanocrystals can influence each other.
The observation that leads to this methodology is that the plasmon resonance shifts to the red if gold nanocrystals get close together. This change can be tracked visually as the colloids change in colour from red to blue. So, If one had a system to direct gold colloids to aggregate upon exposure to an analyte, a very nice colourimetric sensor could be developed.
This has been developed by using the molecular recognition capabilities of DNA to detect oligonucleotides!
How is plasmon coupling used to detect oligonucleotides?
This explanation makes alot of reference to Figure 3.4:
The gold nanocrystals are functionalized via ligand exchange with a small number of thiol-terminated oligonucleotides (Shown in Figure 3.4 with a black backbone) which are complementary to the different parts of the analyte (The analyte oligonucleotide is shown in figure 3.4 with a red backbone) and will be defined as probes.
The gold nanocrystals are dispersed in solution as stable colloids since the probes are not complementary to each other. Once the analyte is introduced into the system, near-instantaneous aggregation of nanocrystals ensues, leading to a visible shift in the colour of the dispersion. The binding is based on hydrogen bonding (and ofc Watson crick base pairs) and hence it is reversible with temp; higher temperatures than 70-80 C melt the H-bonds, allowing for redispersion of the aggregates.
What is the bigger picture behind the plasmon coupling?
So the big idea of this is that the analyte does not need to be an oligonucleotide, as long as it can be selectively bound by at least two probes which are otherwise not influencing each other, you can develop a gold-nanocrystal-based detection scheme for it!!! (basically molecular recognition)
Note that this sensing methodology uses so little gold (which is an expensive regent) that is it one of the cheapest platforms for biomolecule detection and it is touted as being one of the solutions for low-resource settings like the developing world
Why does the colour shift occur when gold probes are brought near one another?
Alot of what is mentioned below is referenced in Figure 3.4:
When the gold probes are isolated they will only have one single plasmon resonance frequency r1 which is based on the formation of a dipole across the particle. since the dipole has a well-determined length (the diameter of the particles). the resonant frequency will also be well determined as shown in the first absorption spectrum in Figure 3.4. The plasmon frequency falls at 520nm which corresponds to a strong red colour.
If the two nanocrystals form a bound couple the dipole can be aligned in two different directions: in the first case both nanocrystals within the couple have a dipole r1 analogous to the one in the isolated particle case thus leading to resonance at 520 nm; on the second case, the diple is spread across the two nanocrystals, thus having a different length and thus different resonance frequency. This second resonance mode r2 would contribute to the absorption with a peak at lower frequencies than the r1 resonance, as shown in the second absorption spec in Figure 3.4.
In the final material, both resonances are allowed and they both contribute to the absorption; the total absorption spectrum is thus the sum of the r1 and r2 contributions and thus is red-shifted compared to the isolated particle case leading to the blue colloidal dispersion.
How are gold nanocubes and nanorods made? (Gl because it is long and we explained every structure that was mentioned)
The main idea here is to explain how 2D defects can be used in metallic crystals, like gold, to create unique shapes with novel properties. Much of the explanation refers to Figure 3.5.
In bulk, gold typically forms an FCC (face-centered cubic) lattice structure, as shown in the left model of Figure 3.5. By replicating this unit cell in all directions, you get a cube, which models a gold nanocube. The cube’s facets are indexed as (100) planes.
To grow a cubic nanocrystal, you need to stabilize these (100) planes by lowering their surface energy, which depends on the roughness and the number of dangling bonds. Chemists use ligands (molecules that bond to surface atoms) to saturate the dangling bonds and stabilize the surface, ensuring the (100) facet remains during growth.
If the ligand isn’t selective, you may end up with other facets, like (111), forming shapes like a cuboctahedron, which is a mix of a cube and an octahedron. The octahedron is excessively delimited by (111) planes like the tetrahedron which is obtained by slicing an FCC lattice along (111) planes. The difference between the two, which affects their occurrence as nanostructures, is that the surface-to-volume ratio is a lot higher for tetrahedra than for octahedra, making octahedra more stable and more likely to be created.
Tetrahedra can increase stability by joining along their facets to form decahedra (ten-sided solids). This fivefold symmetry involves defects called twinning planes.
Finally, decahedra can be transformed into nanorods by stabilizing the (100) planes more than the (111) planes, forcing growth to occur along the (111) facets, forming nanorods, as shown in the SEM micrograph in Figure 3.5.
What is twinning?
A planar defect; the atomic lattice, upon crossing the twinning plane, is rotated by a certain angle which depends on the lattice and on the crystallographic direction of the twinning plane!
Why does is such a complecated mechanism required to form gold nanoparticles?
okay, let’s say you take the approach of making nanorods by taking a nanocube and then trying to grow it along one of the facets. The problem you would find is that all facets in the cube are identical in terms of surface energy and atomic structure as they are all (100). The same is valid if you start from a tetrahedron or an octahedron where all faces are identical (111) planes. Thus all acets would grow at the same time with the same rate thus yielding just a larger version of the same cube or octahedron.
Even in the case of a cuboctahedron. selectively growing the particle along the (111) or the (100) planes would only lead to an eight-armed nanocrystal, which would be called an octopod.
Thus we must use a tetrahedron to make a decahedron to make a nanorod!
How can we get around the diffuclity of making nanorods?
A way to defeat the symmetry problem could be by using a template that has already the desired form, for example, a cylindrical micelle. If you could find a way to grow gold within a cylindrical micelle you could obtain a nanorod!
How can we use gold nanoparticles to treat cancerous cells/tissues?
Much of the explanation refers to Figure 3.6:
Okay first it is important to understand that the plasmon energy for gold nanoparticles can be approximately depicted as a two-level system where the dipole oscillations have an energy ground state and an excited level corresponds to the plasmon resonance frequency. So, if light with energy corresponding to the plasmon frequency impinges on the gold nanocrystals, it will enter a resonance state, and absorb the light. For a very brief time, the plasmons will stay in this excited state before relaxing to the ground state. This will then release heat.
Using this concept, we were able to develop a therapy for tumours called photo-thermal therapy, based on the phenomenon of hyperthermia, by which cells are destroyed by excessive heat which causes their cell wall to burst.
Why are nanocrystals favoured to be used photo-thermal therapy
Much of the explanation refers to Figure 3.6:
Because the heat generated to kill the cancerous cells is related to the intensity of the impinging radiation and the absorption cross-section of the particles, which we already know the absorption cross-section of gold nanoparticles is several orders of magnitude higher than for average molecules
Another reason is the biological window which is the wavelengths at which light is readily able to penetrate living tissues:
As seen in Figure 3.6, in the second row of diagrams, the tissue absorption drastically decreases in the red/near-infrared region. In the graph on the right, we show how the light intensity would fade within the tissues as a function of depth, for two different wavelengths, one outside a biological window (500nm) and the other within the biological window (1000nm). You can see how the 1000nm light can penetrate much deeper into the body owing to the lower absorbance/scattering of the tissues. To take advantage of this we know that gold nanoparticle’s absorption spectrum can be made to fall within the biological window in the NIR, making them suitable for probing and targeting deep tissues
What is STM
It is a microscopy that imagines the surface of gold nanoparticles via an AFM tip which oscillates on the surface measuring its stiffness based on the frequency of electrons that interact with it!
AFM tip for shaving: start with a monolayer then take the tip to shave some of the monolayer to form a hole to get direct contact with the gold which you can etch to make a bigger hole!!
Do more research about this waleed please a quick Google search :)
This is depicted in notes shown in the notes
Rest of the PowerPoint
Okay there is alot in the notes gllll don’t hate me I’m so sorry
