What color are Gold Nanoparticles?

this is a vial of gold nanoparticles and this is another vial of gold nanoparticles but clearly they have different colors this one's kind of a purply violet and this one's more of a ruby red now you might think that the difference here is simply due to concentration the purple vial has a lot more particles in it so it's darker but if we dilute down the purple vial you'll see that the color actually doesn't change it stays a violet color albeit more transparent as you kind of expect so the difference here is not due to just particle concentration there's something else going on here that makes these two different vials of gold nanoparticles look different even though it's the same metal this doesn't just apply to gold either here we have a vial of silver nanoparticles and it's kind of like a dark brownish color and here is another vial which is more yellow and transparent and this is also silver nanoparticles these nanoparticles are so small that they start to interact with light in kind of funny and unintuitive ways and the mechanism that's responsible for this is known as plasmon resonance but we'll talk about that in a little bit but first i want to talk about how to synthesize these nanoparticles because it's actually pretty easy the technique i use goes by a lot of different names but it's most commonly referred to as laser ablation in liquid and it's basically what it sounds like you have a laser and you have some type of liquid that you choose and your substrate of choice and you ablate the substrate in the liquid and you get nanoparticles and it's really that simple although there is some nuance into what parameters you choose and what solutions you use the substrate is typically a metal although it doesn't have to be it can be any material that ablates under your laser of choice the liquid is most often distilled or deionized water although organic solvents like ethanol or methanol or acetone are frequently used for different effects and the laser is typically a near infrared laser like an nd yag or in my case a fiber laser and it has to be a pulsed laser because you need really high peak energy to actually ablate the material in the liquid i let my samples run for about five minutes each which seemed to be a good amount for you know this quantity of nanoparticles but this is something that you can scale up very easily by either using bigger lasers or more lasers in parallel or like continuous flow systems rather than small confined vials like this so it's a technique that has a lot of flexibility and has some distinct advantages that we'll talk about in a bit the reason this works is actually very similar to the glass etching that we did in a prior video using the laser induced plasma except it's a little bit in reverse so in that video we used the plasma to etch and melt the surface of the glass and the glass substrate was what we were after here it's kind of the opposite we're using the plasma and etching the surface but we don't really care about the surface we want all the products that come off of the ablation etching process essentially the metal absorbs the laser pulse and ablates a pocket of superheated gas and plasma into the liquid and because the liquid is incompressible it doesn't really want to expand out of the way and you're left with this pretty tightly packed little bubble where the laser pulse continues to interact with because my laser pulses are on the order of a hundred nanoseconds pretty slow in this scheme of things there's actually a very long period where the pulse actually stops interacting with the surface of the metal and instead starts heating up the plasma that's now on the way so this kind of continues to increase the pressure and the temperature inside this little bubble until the pulse stops at which point the liquid compresses the bubble back down the whole thing implodes on itself this rapid implosion flings molten metal and plasma into the liquid where it's immediately quenched by the drastically cooler liquid everywhere else and it's this fast quenching process that actually generates the nanoparticles now there are dozens of different ways to generate nanoparticles like the literature is filled with different ways to make it so the question is why is this preferable to those other techniques well for one it's simple you don't need anything other than your metal your liquid and your laser and so the setup is pretty easy to get up and running quickly it's clean meaning that the only thing that's in your vial is the liquid which is usually pure distilled water and the nanoparticles particle size and to some degree shape is adjustable based on the different parameters in the setup so like laser power or frequency or the type of solution you're using so for example this vial of purple nanoparticles i showed at the beginning this was generated with a relatively high power pulse the large pulse ablates bigger fragments of metal and so when those quench you tend to get larger nanoparticles whereas this vial was done at a lower power so you get smaller ablation products and then thus smaller nanoparticles there's other things you can tweak too like the constituents of the solution so for example this vial is red but you'll notice it's slightly different than the other red and that's because this one contains a very small amount of sodium chloride and sodium chloride in this particular application adds a little bit of electrostatic repulsion between all the nanoparticles which helps keep them in suspension and not agglomerate as much so you get a slightly different color between the two so there's different things like that you can kind of tweak and tune to get the properties you desire the solvent of choice also plays a big role so a lot of the samples that i did for example this titanium were done in organic solvent i used isopropyl alcohol but ethanol or acetone are pretty common in the literature and the reason for that is to help minimize the amount of oxidation so if you were to ablate titanium in water you often will get titanium dioxide because the water when it's split in this plasma process will liberate a bunch of oxygens that can then react with your titanium so you have to be cognizant of which solvents you use and what your end product is because you're essentially making a really tiny little reactor vessel each time there's a plasma pulse but in some cases that's a desired property so a lot of times you'll use just pure water to get different oxidation effects in different metals i even generated this cute little cubic carbon crystal so carbon doesn't like to crystallize into a cube form that's not a natural configuration for it but by adding a very tiny amount of salt to the solution the carbon can preferentially crystallize into this cubic form which it wouldn't normally do i tried this on a bunch of different metals everything i could get my hands on gold silver titanium copper two different types of graphite nickel the only one that didn't work for me was platinum which apparently just doesn't etch under my laser's wavelength okay so with the fabrication background out of the way let's take a look at that gold and silver again and try to understand what's going on with the colors so with the gold we have the purple and the red and with the silver we have kind of the brown and the yellow the only difference between the two different samples of each is the nanoparticle size so the nanoparticles in the purple vial are larger than those in the red vial and similarly the ones in the brown are larger than the ones in the yellow so the color effect that we're seeing is purely due to the size of the nanoparticles so this effect is caused by a phenomenon known as plasma and resonance and it's a big topic so we're not going to go into all the details here this is kind of cliff notes version and honestly i don't really understand enough of it to go super deep but this is my understanding of how plasmon resonance works and why it generates the colors that it does in a bulk metal like just a chunk of aluminum there's a cloud of delocalized electrons all just sort of flying around and this is often known as a free electron gas and these electrons are independent but they influence each other because no electron can share the same quantum state as any other electron so they're all kind of pushing and shoving on each other sort of like molecules in a gas and because these are electrons they're affected by electromagnetic fields and light is just an oscillating electromagnetic field which means that all the electrons in a metal are affected by any light that's incident on surface so all these electrons that are flying around in a cloud on the surface of the metal will absorb the electromagnetic field all move together as kind of a wave and then bounce backwards and re-emit a photon back out and that's essentially why metals are shiny and reflective because all these electrons are working together in a coordinated motion and then re-emit the photon now my understanding of it which hopefully isn't too wrong is that this coordinated motion of the electrons is known as a plasmon so it's not like a thing you can't go find a plasmon particle but it describes accurately the behavior of these electrons in this situation it's sort of like how a wave in an ocean isn't a thing you can't pick up a wave and put it down somewhere else the wave is just describing the general behavior of the water molecules kind of at that position but you can still talk about a wave as if it were an object right it has a height and a position and a direction and so it's helpful to think about a wave as a distinct entity even though it's really just describing the collective behavior of a bunch of smaller components and that's essentially what a plasmon is but applied to the free electrons in a metal whew okay still with me so far all right we're almost there so that's plasmons and the thing we care about is plasmon resonance or when the electrons in this cloud start to resonate in a particular way and this happens in nanoparticles because of their size so if you think about a nanoparticle that's say 50 nanometers that's considerably smaller than even the smallest wavelength of visible light to 300 nanometers and that starts to do funny things to the electrons on the surface of that nanoparticle now they're constrained to this very small surface which is entirely inside of a light wave's amplitude and in the situation where the wavelength of light exactly matches the resonant frequency of those electrons traveling across the surface you get plasmon resonance where the electrons are resonating with the light wave and that causes that particular wavelength of light to be reflected back the direction it came and so that's why you get a bright ruby red with certain sized gold nanoparticles because the red wavelength happens to resonate with these size nanoparticles and is reflected back to the viewer while the blue greens and yellows are all absorbed and that's also why you get a purple color because now the purple wavelength is being reflected while everything else is absorbed so the wavelength that you see is specifically tied to the size because of the electrons resonating with that particular wavelength of light these nanoparticles also can exhibit another neat property due to different scattering effects both me scattering and rally scattering now i don't know a whole lot about either of those so i don't want to speak too much on it but it is a neat effect to see and so i want to briefly mention it so my understanding is that knee scattering is an effect caused by spherical particles that are smaller but not significantly smaller than the wavelength of light and this causes a sort of diffraction effect that's the inverse of a pinhole so if you have a pinhole light kind of diffracts outwards in this case light kind of diffracts around the particle and this is what gives clouds their white appearance you have water molecules suspended in the atmosphere and they're pretty small so they scatter light and it gives them kind of an opaque whitish color and so we can actually see this by shining a light either through or against the nanoparticles in transmission you'll see the same colors we've been seeing so far where the gold is purple and red and the silver is kind of a brownish and a yellowish but if we flip the flashlight around and look at it from the front so that we're looking at light that's being reflected off of the nanoparticles you'll see that the purple vial becomes more of an oranges color and the yellow vial takes on a more opaque greenish color there's actually a famous example of this known as the lycurgus cup i think i'm pronouncing that correctly i stumbled on this while doing research on this topic it's a very decorative glass cup from the roman period and the neat thing about is if you shine a flashlight through the cup it takes on a very rich ruby red color but if you shine a light on the front so you're seeing reflected light it takes on this greenish opaque white color and when scientists have taken a closer look at the compositions of the glass they've found that it contains small nanoparticles of gold and silver alloy so they're pretty sure that the optical properties of this cup are due somehow to the combination of the gold and the silver that's inside of the glass itself which is pretty cool now it should be noted that plasmon resonance is mostly associated with the noble metals i believe it applies to all metals but due to their electron configurations it's not nearly as impressive as with gold and silver so you don't see these really remarkable color changes like you do and i can back that up i've tried different combinations of copper or nickel or titanium and you don't really see any difference in the coloration they kind of just look like the metal but diluted into solution now i don't have any specific plans for these nanoparticles yet i just kind of wanted to try out the technique and see if it worked and you know it's always good to have a new tool in your toolbox if anyone knows what i could use some of these nanoparticles for definitely let me know leave a comment down below i'm always interested in trying to use the stuff for the tools that i have to work on new projects and it seems like this is a cool way to generate nanoparticles now i just need to actually use those nanoparticles for something if you enjoyed today's video and aren't subscribed yet please consider subscribing it helps me out and it helps you not miss any of my future videos and otherwise i think that's all i got i hope you enjoyed this look at the weird and kind of strange world of nanoparticles and i'll see you next time thanks for watching

Channel: Breaking Taps

Views: 116,128

Keywords: silver nanoparticles, silver nanoparticles synthesis, gold nanoparticles, nanoparticles, laser ablation, laser ablation in liquid, colloid, copper nanoparticles, plasmon, plasmon resonance, surface plasmon, atomic force microscopy, silver nanoparticles solution, silver nanoparticles and colloidal silver, gold nanoparticles surface plasmon resonance, nanoparticles explained, nanoparticles synthesis, nanoparticles properties, laser ablation in liquids

Id: m_ScOUNAdlM

Length: 14min 11sec (851 seconds)

Published: Fri Jul 02 2021