A few months ago, I joined Prof. Jim Schuck’s research group here at Columbia University and aim to study upconverting nanoparticles. If you have read my research portfolio sections, you will see that this is a new area for me, as my past research is mostly 2-dimensional materials (graphene) studies. Why did I make this switch? First of all, 2D materials research has gained a lot of traffic since the Nobel Physics Prize in 2010, and it is getting too saturated. Almost everyone in materials science wants to combine their research with 2D materials, there even are many scientific journals dedicated specifically for 2D materials. Despite all the passion and funding going into 2D research, and almost 15 years since the first exfoliated monolayer graphene in 2004, there really has not been any commercially promising applications coming out of it. Because just as there are many desirable qualities of these materials, there are also many limitations. I decided to stop pursuing something that is too hyped and not very promising, even though Columbia is probably the best place for 2D research.
Upconverting nanoparticles (UCNPs) on the other hand, has a timeline similar to that of graphene’s (I stole this slide from my advisor). However, instead of blowing up immediately, they saw a steady increase in attention over the years, and also the brightness of the particles increased by many orders of magnitude over a very short period of time. This means that with relatively small amount of research yield such significant result, there must still be a lot of room for improvement. And this is what appeals to me, that I get to be a part of the early research effort, and this could be a very rewarding experience. Moreover, UCNPs has already been used for in vivo and in vitro bioimaging studies, so we should be able to see them being commercialized for real world applications a lot earlier than graphene.
Upon the recent switching of research topic, I found myself having to learn a new field from scratch, and that I had to explain this to people outside of my field (which is everyone because my advisor is the only one studying UCNPs at Columbia, and probably one of a few on the east coast). I decided to write this blog to help me and everyone else better understand this research topic. I will try to explain everything in layman terms, but if I am not doing a good job, please feel free to leave a comment and I will try to explain better.
First of all, I will introduce the composition of the particle. These particles can be synthesized from a few nanometers to a few hundred nanometers. 
Note that one nanometer is 10 to the -9 power of a meter, and roughly the size a 2~3 atoms. Here I show a toy model of such particle, each “ball” of the ensemble is one atom. The black and grey ones compose of the host matrix, meaning that they are the basic building blocks, like the wood or concrete used to build houses. However the choice is not arbitrary, the specific combination of NaYF4 is chosen because it has just the right distance between the atoms to allow Lanthanide ions to be added and bonded to the rest of the structure. The green balls are the sensitizers, their goal is to take in energy. When we shine a light onto the ensemble, the energy from the light waves are absorbed by the green balls. Subsequently, that energy is transferred to the red balls, which are the emitters. Upon receiving the energy from the sensitizer, the red balls gets really energized, and they have to give off that energy somehow, and that’s when they will emit a photon, but note that this light is much more energetic than the input light.
I realized that it might be clearer to explain this energy jumping with a diagram. Instead of getting intimidated thinking in terms of energy levels, we can think of it as hopping on and off ladders. In the middle column is where the ladder of our sensitizer sits. when we provide it with an external force, the particle is able to hop on the ladder. Once it is on the top, it can hop onto another ladder. In this example, the energy can be transferred to Er3+ or Tm3+ ions, which would give off light of different wavelengths. We can see that there are various arrows coming off of the top of the Yb3+ ladder. It can hop onto any other allowed ladders steps of either Er3+ or Ym3+. And when it arrives onto a neighboring ladder, it cannot stay there for too long, it will fall to the bottom of that ladder. During that process, a photon will be emitted.
In most UCNP systems, the incoming light will be near infrared (NIR), and the emitted light will be in the visible light spectrum, hence we can use it in a biological system and clearly see any irregularities within. There are many advantages to use UCNPs as the preferred method for bioimaging: no photo-blinking, that is, the particles will not turn itself on or off randomly; no photo-bleaching, the particle does not just die randomly; the signal does not interfere with cellular auto-florescence; most importantly, it does not compose of toxic components compared to quantum dots. As it is still a relatively new field, there are still lots of interesting properties to be explored. My project in the lab is to study single molecule imaging with UCNPs. it could be revolutionary because it will be able to retain high resolution images locally, and reveal much more detailed information as compared to imaging with a whole ensemble of such particles. Please get in touch if you would like to learn more, or if you are interested in a collaboration!
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