Lecture 6 Flashcards
What is silica made up of?
Silica is constituted of tetrahedral [SiO4]^-4 building blocks; these tetrahedra are rigid, but they can connect via the oxygen atoms (forming Si-O-Si bridges) at fairly flexible angles ranging from linear to tetrahedral. This flexibility is what makes silica an excellent glass former!
What is the main method to produce glass and how does that tie back to sillica?
The main method to produce glass is to quickly cool (qunech) the melt. If a materila is a good glass former, like silica, its atoms will not have time, during this rapid cooling, to assemble in the positions they would occupy in a crystals and will instead remain frozen in a metastable phase called glass.
A consequence of the excellent glass-forming characteristics of silica is that most of the silica you will see is in the amorphous (glassy) phase.
What is the reaction to form silanol groups on the surface of silica?
It is an equilibrium reaction that is depicted in Figure 2.1
Why are silanol important for handling silica surfaces?
They are so important because they affect the surface charge and reactivity, which, in turn, determine what chemistry you can do with silica
How can the concentration of silanol on the surface of silica be modified?
They can be modified in the following ways:
- it can be increased by exposure to oxygen plasma or air plasma, or by immersing the surface into a strong acid, possibly with oxidizing characteristics. Both methods break Si-O-Si bonds, increasing temporarily the concentration of the silanols above the equilibrium level. Within a few hours, if exposed to air, the surface will return to the equilibrium state.
- It can be decreased to about 1 per 10nm^s by heating the silica to about 800C; at such temperature the silanols will condense, releasing water, and forming new -Si-O-Si- bridges.
another point for temp is that when you increase the temp high enough water will evaporate driving the equilibrium to the right and decreasing silanols, and vice versa, if we have high humidity the concentration of water will increase causing the equilibrium to move to the left increasing the silanols!
what are the properties of Silanol at pH=7
Before I answer I want to make it clear that the pH of the environment strongly determines whether the silanol is protonated or deprotonated (thats why also acids increase the concentration of silanols!)
Okay in the case of pH 7 the silica surfaces are negatively charged, hence silanols are more DEprotonated than protonated. This makes silica generally hydrophilic (loves water) in normal conditions.
The hydration layer that forms will hardly evaporate at room temperature due to the strong electrostatic and hydrogen-bonding interaction it has with the surface: 300C is often necessary to temporarily remove such adsorbed water, and exposure to water vapor or liquid will reinstate immediately such hydration layer
How is the surface charges of Silanols identified?
The surface charges can be identified by the contact angle of water on the surface; the higher surface charge will correspond to smaller contact angles, or higher wettability, as the water will try to maximize its interaction with the surface.
What is the formal defination of wettability?
How easily a solid surface can be covered by a liquid; related to contact angle
How do silanols determine the surface reactivity?
Silanols can react very easily with chlorides (or alkoxides), as shown in Figure 2.1, and this can be used to covalently attach any molecule to the surface of silica. This allows us to be able to functionalize the surface of silica.
A nice aspect of this reaction is that it can be performed in solution (where the silica sample is immersed in a solution of the alkoxide or the chloride) or in the vapour phase (where the silica sample is placed into a low-pressure chamber containing the alkoxide or the chloride), whenever the precursor is sufficiently volatile.
Note that in Figure 2,1, R is an arbitrary group which can be an ALKY CHAIN, CARBOXYLIC ACID, AMINE for bioconjugation, OR ANYTHING ELSE, even Fullerences.
What interactions do hydroflouric acid (HF) and silia surfaces have?
Silica surfaces have a high susceptibility to hydrofluoric acid (HF) and resistance to all the other common acids like sulfuric acid and hydrochloric acid; Few other materials possess such selective reactivity.
This peculiarity makes it possible to selectively etch silica in the presence of many other materials or vice versa.
How does HF etching of silica happen?
The chemistry of HF etching of silica is shown in Figure 2.1:
HF attacks the Si-O bond by protonating the oxygen and coordinating the silicon atom with fluorine; this leads to the cleavage of the Si-O bond and the formation of a silanol and a Si-F bond, this process continues and, as HF is consumed, H2O and SiF4 (which is a gas at room temp and atmospheric pressure) are produced. The Si-F bond is very stable and the backwards reaction is thus strongly unfavourable.
Under aq condition conditions the F^- anion can also strongly coordinate SiF4 to yeild the stable hexaflourosilicate anion [SiF6]^-2
note that HF is extremely dangerous even though it is a mild acid.
When studying the size of silica, what discoveries are opened for us?
colliod and sol-gel chemsitry
What is the formal defination of colloids?
Colloids are generally defined as mixtures formed from a continuous phase and a dispersed phase whose colloidal stability (i.e., their tendency to remain homogeneously mixed) is determined by the surface energy and charge of both phases.
The term colloid in nanochemistry is most often used to indicate dispersions of solid particles in a liquid continuous phase.
What is polydispersity?
The amount of size inhomogeneity is often defined as the standard deviation of the size distribution (see Figure 2.2).
As a consequence, well defined and homogenous properties will only result if one can limit polydispersity.
(This is generally and can be applied to colloids as homogeneity increases as we limit the colloidal polydispersity)
What is the general sequecence for the production of Colliods?
A colloid is produced in solution by a sequence of controlled nucleation, growth, and precipitation reactions.
How is the partical size controlled.
The control of particle size is mostly exerted through the following three principles:
- By controlling the surface chemistry or charge of the particles, to prevent them from aggregating into a single lump.
- By controlling the initial supersaturation, which determines the number of nuclei that are formed at the beginning of the reaction: If the reaction proceeds to the same extent, having more nuclei will yield smaller particles as the SiO2 is distributed to a larger number of particles.
- By supplying such nuclei with enough regent for them to grow to the desired size.
How can we make silica colliods?
Silica colloids can be made in solution using the Stober process which is the main sol-gel method for producing silica collides
Firstly we will explain the chemistry behind sol-gel and then (in the next flash cars) define the Stober process:
Sol-Gel chemistry is based on hydrolysis and condensation reactions of suitable metal-organic precursors, such as alkoxides with formula M(OR)n where M is a metal with oxidation state n and R is an organic group (Often ethyl).
The sequence of hydrolysis and condensation reaction leads to the formation of M-O-M bonds (eventually leading to a network!) with concomitant release of water and alcohol ROH molecules, as shown in Figure 2.2. (dont get confused the metal is basically silica okay!!)
It is important to note that each step of a sol-gel reaction is differently sensitive to humidity, to the nature of the R group, and to Ph. The use of sol-gel chemistry often implies a study of these conditions and how to optimize them to get the reaction to proceed as desired!
What is the stober prrocess
TEOS is first dissolved into a water/ethanol mixture. TEOS reacts very slowly with water in the air at pH 7. The initiation of the reaction is thus performed by adding ammonia which, by increasing the pH, catalyses the process. Once ammonia is added the TEOS will begin to HYDROLYZE by the water, then subsequently CONDENSED by heat to form active nuclei silica centers.
after a few minutes of this mechanism, the dispersion is composed of three changing species: The nuclei that are being formed, the hydrolyzed TEOS which can add to the nuclei, and the not-yet-hydrolyzed TEOS, acting as a neutral bystander (as shown in Figure 2.2). The nuclei, due to the naturally high negative surface charge of silica, electrostatically repel each other maintaining their identity and minimizing aggregation.
With time, the hydrolyzed TEOS molecules add to the nuclei, while the non-hydrolyzed TEOS have time to hydrolyze. Given the amorphous nature of the product, the absence of crystalline facets and any other kind of atomic lattice anisotropy, there is no substantial difference between different spots on the surface of the colloids, so the growth happens on all sides at the same rate, isotropically, and thus yielding perfectly spherical particles.
By the end of the reaction, all of the TOES will have hydrolyzed and added to the particles; their final size will be determined by the nuclei initially formed, and by the amount of TEOS available after that. This principle is used in seeding
Along with Figure 2.2, there is a lecture slide shown to give a better picture.
THIS IS VERY IMPORTANT IT WILL COME IN THE EXAM
What is seeding
It is a process where small silica particles are synthesized separately via Stober synthesis and then used as seeds to grow larger particles.
The advantage of this approach is that you already know how many “nuclei” (seeds) you have in your reaction vessel. Since you can measure how much TEOS will react with them. It is worth mentioning that the seeds can be other nanoparticles like gold nanocrystals or iron oxide nanocrystals, and regrowth with TEOS creates core-shell Particles. Depicted in digital notes.
The sheath of silica can serve to protect the nanomaterial core and also facilitate the chemical functionalization of the silica surface.
What does the efficienct of the electrostatic repulsion between the silica nuclie depend on?
The efficiency of this electrostatic repulsion is depended on the ionic strenght of the continous phase; high ionic strenghts will cause the surface charges on the neighboring silica particles to be screened from each other, reducing the repulsion beyween particles and thus leading to more aggregation.
What is a standard test to see if dispersion in water is a colloid?
add salt to the water and see if it precipitates
What is a general annoyance when it comes to the synthesis of colliods and how can we deal with it?
A reccurring annoyance in the synthesis of silica colloids is the formation of aggregates, such as seen in Figure 2.2; such particles will disturb the self-assembly of said colloids since their size is much larger than average. To alleviate this problem these aggregates can be separated from the product solution by centrifugation, since they are larger and heavier so they will settle faster.
An alternative yet slower process than centrifugation is gravity driven sedimenation.
When do silica gels form pores?
You can understand the origin of this porosity if you relize that the condenstation reaction does not need to be completet for the partice to be a slod. A large number of silanols within the particle will not have had the change to condense with their neighbors thereby leaving empty spaces within the particle. The progressice reticulation of the silica matrix makes further reaction of silanols increasingly diffuclt due to thier diminshing freedon of movement. As the silanols are bound to an increasngly rigid matric they will find it increasingly difficult to move and find another silanol to react with. The properties of many materials will be affected by the degree of porosity, like density, mechanical strenght, dielectric constant, and so on.
How can we levitate the porosity formed in the silica gels?
In order to remove this porosity, so-gel oxides like amorphous silica are ‘calcined’, which means that they are heated to high temperatures in the air. This process leads to a shrinking of the solids as residual silanol groups further condense, according to the equation Si-OH +OH-Si —> Si-O-Si _H2O, causing the material to lose porosity, decrease its volume, and increase its density.
What are the problems/difficulties associated with calcination?
The shrinking caused by calcination is one of the main challenges of sol-gel chemistry applied to materials; it usually leads to internal stress resulting in the cracking of the oxide, especially when the material is anchored to a surface. This problem can be somewhat ameliorated by careful control of calcination temperature ramping and humidity.
In the case of most oxides, calcination also leads to crystallisation. Silica will instead remain amorphous and, if heated too much (over 100c) will melt. Thus it is important at this stage to use a temperature which is high enough to complete the condensation, but also not so high as to lead to sintering (explained in the next card) of the particles
What is sintering
Sintering is a process by which distinct and touching grains of material are heated at a temperature at which one starts to observe the diffusion of the atoms at the surface (Viscous flow), which eventually leads to the smoothening of the sharp edges and the fusion of the grains together. It is the process at the heart of the preparation of bricks for construction.
What is the formal definition of templating? (I feel it will help me think more logically)
Templating: Using an existing material of a certain shape as a mold to create a negative reproduction in a new material!
What is the first challenge of templating?
The first challenge is to find a plausible mould. Scientists have developed a wide array of them. In Figure 2.3, we have indicated a very popular one which is a nanochannel membrane. Such architectures exist in different materials and their channels like in the nanometer scale. In particular cases, the channels can be arranged into periodic patterns and can even be coerced to show periodic variations in their diameter length.
One of the reasons why there is so much research into developing such membranes is that they constitute a near-ideal template for the fabrication of 1D nanomaterials, as shown in Figure 2.3
What are the steps for templating
After we form our mold, the hydrolyzed TEOS molecules will react preferentially with the membrane, forming initially a film of silica on the pore walls. As the reaction proceeds, the pores are progressively filled with silica, till they are closed. Once this is accomplished, the template can be selectively dissolved (For example, in the case of aluminium oxide, a mild acid can be used, leaving the silica unscathed), leaving behind a replica of the membrane, as shown at eh bottom of Figure 2.3, consisting of silica nanowires.
What are the problems (and solutions) that arise during templating
- Problem: The precursor, For example, a sol-gel precursor, reacts on contact with the substrate, hydrolyzes in situ, and keeps on reacting with the precursor in solution leading to a fast clogging of all channels. Solution: reduce the kinetics of the sol-gel reaction and/or dilute the precursor
- Problem: The precursor does not fully infiltrate the template leaving a bubble behind which harms the homogeneity of the product architecture. Solution: Use ultrasound to drive out the bubble before beginning the solution-based infiltration.
- Problem: The precursor undergoes dramatic shrinking upon condensation leading to a fragmented and broken product. Solution: deposit the precursor by successive depositions, letting the precursor condense in between; each following deposition will cover the cracks left by the previous one, healing the product.
How versatile is the templating process?
Even though there are challenges in achieving good templating, the process of extremely versatile. For example, we could have stopped the infiltration depicted in Figure 2.3 when the channels were not completely filled with silica so that the removal of the template would have left us with an array of nanotubes.
The versatility of templating is not limited to shape control but extends to the choice of templates and precursors that are available. For example, any nanostructure can be used for templating, as long as the template can be dissolved (etched) and separated from the infiltrated material. Also, any liquid or gas-dispersible moiety with a size smaller than the channels of the template can be used as an infiltrating material.
What is single and double inversion?
Single inversion Creatin negative replica of a template (other word for templating)
Double inversion: using negative replicas of a template as a new template to recreate the shape of the original template (basically a positive not a negative replica)
What are micelles?
Micells are assemblies of amphiphilic molecules made up of two connected parts: One that loves water, often a small charged or polar chemical group, and one that hates water, often a long alkyl chain of methylenes terminated by a methyl group. Such molecules spontaneously form assemblies with defined shapes and sizes in order to find the best compromise between the needs of both parts of themselves. Cylindrical micelles are one of the possible assemblies; they are spontaneously created in order to maximize the contact of water with the hydrophilic heads of the molecules and minimize it with the hydrophobic tails (Figure 2.4). Such micelles can be spherical as well-form in water when the concentration of their amphiphilic molecules exceeds a so-called critical micelle concentration (CMC). Below this concentration are molecular solutions.
Real quickly to make sure that me and you are on the same page. Why are micelles important
because they serve as templates for silica allowing it to form porous (and many other) structures.
What are periodic mesoporous silicas (PMSs)?
A type of silica with regularly arranged pores in a size range of 2-50nm; they can have very high surface areas, as high as 1000m^2/g (bigger area than my studio Subhanallah) which makes them usefull in a wide range of fields such as catalysis, chromatography, sensing, batteries, fuel, and solar cells and any other function in which the interaction of a liquid, gas, or light with a solid surface is relevant.
Note: In Figure 2.4, a picture is taken by a scanning transmission electron microscope (STEM) of the resulting material, which clearly shows the pores and their HEXAGONAL ordering.
how are periodic mesoporous silicas made (VERY IMPORTANT)?
They are made by the co-assembly of micelles with silica. The mechanism is shown in Figure 2.4
what are the added technical advantages of PMSs
- the prores are interconnected
- the pore size and shape can be controlled by design
- The pore morphology can be controlled by design
- The pore orientation can be controlled by the design
- the choice of composition for the pore walls is very broad, as it is in principle just limited by sol-gel precursor availability.
What are the two methods to characterize PMSs?
- Adsorption isotherm
- Small-angle X-ray Scattering (SAXS)
How can we characterize PMSs using an adsorption isotherm? (first, describe the method of getting to the adsorption isotherm
The sample is placed in a vacuum chamber at the temperature of liquid nitrogen and then a gas is let inside the chamber; you can then monitor how much of that gas is adsorbed within the pores of the material. After having reached atmospheric pressure, the sample is pumped down again to monitor the reveres process of gas desorption from the pores. The adsorption-desorption curves are generally different and display a hysteresis cycle (which basically measures how much harder it is to desorb the gas than to adsorb it)
There are theories available that allow one to determine from such curves not only the surface area of the material, but also the average pore size, pore-size distribution, and even elasticity of the material.
How can we characterize PMSs using a Small-angle X-ray scattering (SAXS)? (first describe the method)
This method is used from materials that show some kind of meso- or nanoscale periodicity, where we monitor the amount of scattered X-rays, which monitors the amount of scattered X-rays, depending on the angle of (Usually between 0.1 to 10 (THIS IS WHY IT IS CALLED SMALL) deflection of the scattered X-ray beam. The SAXS pattern of a mesoporous material allows one to evaluate the periodicity of the channels, the distance pores, and sometimes even their shape.
This technique is based on the fact that electromagnetic radiation, like X-rays, is diffracted when it interacts with periodic structures. This behaviour is expressed by Bragg’s law;
n*λ = 2dsin(angle)
Quick note: Bragg’s law tells us that to examine a structure with a specific length scale, you should use a probe, like an electromagnetic wave, with a similar wavelength. While X-rays can, in theory, detect structures on the micrometre scale, the angles required for diffraction are often too small for typical lab X-ray equipment. Although synchrotron facilities can achieve this, it’s easier to use infrared light, which has wavelengths in the micrometre range, making it more suitable for optical studies.
How can we make the porous in PMSs denser?
Similar to what we did in the Stober process we use calcination (the same challenges are still felt).
How can we remove the micelles from PMSs
Via calcination, solvent extractions, ultraviolet photolysis, and plasma treatments.
What are opals?
Opals are ordered self-assemblies of nearly monodisperse silica microspheres allowing them to have angle-dependent colour.
Opals can be natural, such as opalescent gemstones, or they can be artificial, such as self-assembled Stober silica colloids into order arrays.
What is the method of making Opals?
the method follows a substrate (usually glass) immersed into an ethanolic dispersion of silica colloids, which is then allowed to evaporate.
The ensuing process is shown in Figure 2.4:
During evaporation, the meniscus is forced to move downwards and a few colloids will remain pinned between the substrate and the air-solvent interface; the colloids that are re-pinned will corrugate the air-solvent interface around them leading to an increased air-solvent surface area; increasing the air-solvent surface area leads to enhanced evaporation since there is more surface available to the molecules of the solvent to evaporate from. Enchancignt the evaporation leads to increased flux of solvent towards the region: the solvent ties to reduce the air-solvent interface because of its high surface energy. Since letting the eater evaporate from a wet array of colloids would only increase the area of such interface, more solvent is drawn from the bulk of the dispersion of compensate.
This enhanced flux of liquid will carry an increasing number of colloids, which will have to pack against the ones that are already pinned at the air-solvent interface, facilitated by the capillary force between the spheres. Such strong forces cause the colloids to order in the most efficient way, which is the FCC lattice.
At the end of evaporation one can observe a very nice order array of spheres deposited on the substrate as shown in the SEM micrograph in Figure 2.4
Note that controlling the evaporation rate and volume fraction of colloids in the dispersion will allow you to control the thickness of the film!
What are the different things that opal can be seen as?
- a 3D porous template
- an ordered macroporous material
- a scaffold for the growth of cells
- a scaffold for the study of biomolecule diffusion in confined systems
- a cellular scaffold for structural materials
- Photonic crystals
7.ect
What is so unique about opals
As we hinted while describing the SAXS technique, light interacts strongly with periodic structures that have a length scale commensurate with their wavelength. In the case of opals, this is evident by their colors which are diffracted from light impinging on them. This has a clear advantage over dyes and pigments as such ‘structural’ colours get increasingly bright with increasing light exposure, and they dont not fade with time. Such periodic structures are called ‘Photonic crystals” since they are able to diffract photons, similar to the way atomic crystals diffract electrons.
Such diffractions can be seen in the absorbance spectrum shown in Figure 2.4. The strong peak at 700nm comes from the diffraction of that wavelength. The presence of such a peak could also be predicted by Bragg’s law.
Important note (was on the slides): The same principle can be used by Morpho butterflies (shown in Figure 2.4), whose wings made of chitinous protein do not contain any dye or pigment, but whose exceedingly Brillant colours are due to the periodic structural framework composing their wings
What is the formal definition of Photonic crystals?
Periodic structures (Opals) that are able to interact with light by virtue of said periodicity
What is a stopgap?
A range of wavelengths where light is forbidden to propagate
Look at me photonic crystals…this is not you…dont let them anger you…I know the real you (What is the real potential of photonic crystals)
It is important to understand that photonic crystals are not just pretty coloured materials; they are able to control the flow of light, much like semiconductors control the flow of electrons. For example, in the absorbance spectrum (Figure 2.4), light which is absorbed at the edge will be slowed down while the light at 700mn will be bounced back. This is due to the light at 700nm having a wavelength corresponding to the centre of the stopgap causing the rate at which opal will emit photons to greatly diminish.
Note that photonic crystals modify the speed of light diffusion within themselves.
How can we play with photonic crystals function?
a photonic crystal’s optical performance greatly depends on the materials used and on the quality of the self-assembly.
This allows us to very easily change the optical properties of photonic crystals!
What are inverse opals?
Negative replicas of opals which often perform better optically
Why do we need systems for drug delivery? (This is a general question because everything we learned in all the lectures can be applied to drug delivery!)
We need drug delivery platforms as common drugs that we take every day do not maintain a constant concentration of the active molecule in the blood but rather show a concentration peak followed by a sudden decrease. This means that the drug concentration in the bloodstream is only optimal for a relatively short period of time.
In addition, drugs are generally administered systemwide (throughout the body) and they can thus elicit side effects in the tissue that were not meant to be exposed.
What are the applications of silica in Bionano?
- Drug delivery using PMSs
- Detecting DNA by creating a defect layer in photonic crystals causing a change in their optical abilities!
Both of these applications are shown in Figure 2.6 (read the caption for more insight)
