Upconversion Photoluminescence and Materials Design Concepts for those with a Sweet Tooth

– Before you start reading: Get your mind set by grabbing a cup of coffee or tea and your favorite cookie or chocolate. –

Are you fascinated by how materials design can tune materials properties? If so, have you ever wondered how this can potentially relate to cookies, chocolates and even covid-19 distancing rules? Well, here we go… (I hope you have your tea or coffee and cookie or chocolate ready; if not: this is a good moment to fix this)

You may (or may not) have heard about the concept of upconversion photoluminescence.1 Simply said, that is the capability of a material (and more recently molecules2) to emit light of higher energy (for instance UV or visible light) under illumination (excitation) with light of lower energy, typically in the near-infrared (e.g., 808 nm or 980 nm). While this may sound counter-intuitive – how to get more out of less? – this process is not violating fundamental physics, but is based on the step-wise excitation of an electron from its ground state into a higher energy level using more than one low-energy photons, followed by the emission of one photon of higher energy. In a simple way, we can imagine it as shown in the following scheme or check out the great animation here.

Why should we care about so-called upconverters?

Indeed, it is this use of near-infrared light for excitation that has triggered the attention of materials scientists, chemists and physicists. Such light can penetrate deeper through skin and into biological tissue than UV or visible light that is typically used for optical bioimaging in medicine and biomedical research. The human body is not transparent to UV or visible light (then why do we use sun’blocks’? Answer is present at the end of this paragraph). While this is nice in daily life, it is barrier for medical diagnostics or light-based therapies (such as photodynamic therapy or optogenetics). Hence, research suggested to use near-infrared light that can penetrate deeper into biological tissue and also does less harm to the biological tissue than UV light (UV light gives you a sunburn, infrared light warms you).

Thus, the general idea is to use upconverting nanoparticles – UCNPs – for application such as light-based therapy or optogenetics.3-4 Conventionally, UV-Vis light is needed for these treatments. But UV-Vis light cannot penetrate into biological tissue. Yet, when UCNPs are brought inside the body (diseased tissue for therapy or brain for optogenetics), they are excited through the tissue using near-infrared light, and UV-Vis light is subsequently emitted very locally from these nanoparticles. This emitted light can ultimately be used for optogenetics or photodynamic therapy.

In addition, wouldn’t it be convenient if the same optical probe is not only emitting UV-Vis light for therapy but also emitting light of longer wavelength (a classical downshifting) falling into the near-infrared transparency window for imaging through tissue? Well, indeed, there are some types of materials able to do so, including lanthanide-based nanoparticles (more on those below), modern organic dyes, carbon-based nanomaterials and quantum dots.5

Disclaimer: The idea of UCNPs and near-infrared emitting nanomaterials for biomedicine sounds great, but it’s still at its infancy with a lot of research needed to actually penetrate deep into the tissue and make the processes more efficient, not to forget about the many difficulties that come with nanomedicine in general. Maybe, you can help scale up the technology someday!

This brings us to the question: How to make Upconverting and Near-infrared Emitting Nanoparticles?

One of the key players in the design of these materials are the elements that form the f-block in the periodical table; those that are typically drawn under the table (to easily be ignored?); the group of LANTHANIDES! Lanthanides are known for their outstanding optical (and magnetic) properties. Among them, for instance Erbium, Thulium, Holmium, Ytterbium, and Neodymium. These are a great choice when aiming for upconversion and near-infrared emission given their rich energy level schemes that match well with near-infrared excitation and upconversion or near-infrared emission (the energy levels are summarized in the so-called Dieke Diagram6).7

However, the optical transitions that take place during the photoluminescence process happen within the f-shell, they are so-called f-f transitions. Quantum mechanics (i.e., the Laporte Selection Rule) forbid this. Yet, luckily, forbidden does not mean that it cannot happen – just think of a child not allowed to eat that extra cookie: there still is a probability that the kid will find a way to get to that extra cookie, maybe by playing some tricks… (By the way, maybe you want to take the opportunity to get an extra cookie as well?). Materials chemists do the same when designing luminescent lanthanide-based nanoparticles.8 Here, the trick is to partially overcome the selection rules by doping lanthanide ions (Ln3+) into a suitable inorganic crystalline host material (the materials community would call this a “ceramic”).

We can imagine this like baking our favorite cookies: we need cookie dough and chocolate chips, and nuts, and salty caramel, and raisins… you name it. It all depends on your sweet tooth. But you want to make sure that the ratio between chocolate chips, nuts, … and the cookie dough is a balanced one. You also want to make sure that all cookies have the same amount of nuts, raisins, … who wants to get the one cookie made of dough only? Or just a huge junk of chocolate at one side of the cookie? Also, while we may pick our favorite cookie based on the added flavors, if the dough isn’t good, the whole cookie isn’t worth it.

Now, you get this: replace nuts, raisins, chocolate chips etc by “lanthanide ions” (for instance, Er3+ and Yb3+, or Tm3+ and Yb3+, or add a bit of Nd3+ or Ho3+) and cookie dough by “host material” [state-of-the-art materials are for instance, alkali metal lanthanide (or rare earth to be more precise) fluorides like NaGdF4 or LiYF4]. Hence, when synthesizing lanthanide-based nanoparticles (let’s call them Ln-NPs from now on), it is important to use the right amount of Ln-dopants in the best host material (for optimized emission intensity) and to chose the type of Ln-ions to tune the emission color (for instance, Er3+ gives green and red, Tm3+ gives blue upconversion9).

The Core-Shell Concept

Over the past years, researchers learned that the photoluminescence from Ln-NPs is rather sensitive towards the environment in which you place them. More precisely, when dispersed in solvents like water as needed for biomedical applications, the water molecules can quench the luminescence, resulting in loss of the luminescence. In addition, small nanoparticles tend not to be perfect at their surface, but to be prone to surface defects in the crystal lattice. These defects are also known to induce loss of luminescence. How to overcome this? The core-shell concept is the answer.

You can imagine this like an onion: a nanoparticle built by several layers. But if you are like me preferring chocolate with your coffee over onion rings, let’s stick to the world of chocolate. So, let’s imagine a delicate chocolate truffle. Soft, delicious – but also easy to be smashed. The problem can be overcome by adding a shell of more ridged and protective chocolate. Similar with Ln-NPs: growing a shell of the same material as the host material around the Ln-doped nanoparticle (but no dopants) will boost the photoluminescence as healing surface defects and adding a barrier layer between the emitting Ln-ions in the core and the quenching solvent molecules outside.

Let’s dream a bit more about our finest chocolate fantasies… Not only a chocolate filled with soft truffle or fruity jelly, but one that has several layers combining various flavours and textures in each layer. Sounds mouth-watering? Not only in the world of chocolate, but also when designing Ln-NPs with tuned emission colours. Of course, we could add a variety of Ln-ions each with its own fingerprint-like emission profile all together into that doped core (like mixing nuts and raisings and chocolate all in one cookie). But it turns out that this approach risks to be an overkill. More scientifically speaking, we risk energy transfer between the Ln-ions ultimately not adding on their optical performance (i.e., emitting light of various wavelength from UV via visible into in the near-infrared), but resulting in loss of photoluminescence through energy migration and cross-relaxation processes. Instead, let’s create a nanoparticle where specific Ln-ions are doped into different layers of a core/multi-shell Ln-NPs, like a chocolate with a nut in the center, embedded in soft and smooth truffle, and covered in an outer shell of chocolate or cocoa; each layer having its own distinct feature. This has been proven to result in multi-color optical probes as is now a commonly applied strategy to control and boost photoluminescence from Ln-NPs!7, 10-12

Bonus Material:
Last but not least, in the words of today’s world of Covid-19: Lanthanide ions need to maintain physical distance! If they are getting too close to each other or if they form large clusters, there is a high risk of energy transfer ultimately leading to loss of brightness or even complete quenching of their luminescence. Similar to us humans, while it is hard, we need to keep our distance and add an extra layer of protection in form of masks or face shields to keep ourselves and others safe. The same is true for lanthanide ions doped into nanoparticles at all times.

Having said this, I hope you and your loved ones are staying safe and sound. I also hope you enjoyed this read and learned a bit about the world of lanthanide-based nanoparticles while having a cup of coffee or tea and your favorite cookie or chocolate. And who knows: maybe UCNPs will even help to fight this or any future pandemic!?13-14 That’s just one of the reasons why Ln-NP research is going on.

(1) Auzel, F. Upconversion and anti-Stokes processes with f and d ions in solids. Chem. Rev. 2004, 104 (1), 139.
(2) Nonat, A.; Chan, C. F.; Liu, T.; Platas-Iglesias, C.; Liu, Z. Y.; Wong, W. T.; Wong, W. K.; Wong, K. L.; Charbonniere, L. J. Room temperature molecular up conversion in solution. Nat. Commun. 2016, 7, 8.
(3) Hemmer, E.; Acosta-Mora, P.; Mendez-Ramos, J.; Fischer, S. Optical nanoprobes for biomedical applications: Shining a light on upconverting and near-infrared emitting nanoparticles for imaging, thermal sensing, and photodynamic therapy. J. Mater. Chem. B 2017, 5 (23), 4365.
(4) Wu, Y.; Ang, M. J. Y.; Sun, M.; Huang, B.; Liu, X. Expanding the toolbox for lanthanide-doped upconversion nanocrystals. J. Phys. D: Appl. Phys. 2019, 52 (38), 383002.
(5) Hemmer, E.; Benayas, A.; Legare, F.; Vetrone, F. Exploiting the biological windows: Current perspectives on fluorescent bioprobes emitting above 1000 nm. Nanoscale Horiz. 2016, 1, 168.
(6) Withnall, R.; Silver, J., Physics of light emission from rare-earth doped phosphors. In Handbook of Visual Display Technology, Chen, J.; Cranton, W.; Fihn, M., Eds. Springer Berlin Heidelberg: Berlin, Heidelberg, 2012; pp 1019-1028.
(7) Skripka, A.; Marin, R.; Benayas, A.; Canton, P.; Hemmer, E.; Vetrone, F. Covering the optical spectrum through collective rare-earth doping of NaGdF4 nanoparticles: 806 and 980 nm excitation routes. Phys. Chem. Chem. Phys. 2017, 19 (19), 11825.
(8) Heer, S.; Kömpe, K.; Güdel, H.-U.; Haase, M. Highly efficient multicolour upconversion emission in transparent colloids of lanthanide-doped NaYF4 nanocrystals. Advanced Materials 2004, 16 (23-24), 2102.
(9) Wang, F.; Liu, X. G. Upconversion multicolor fine-tuning: Visible to near-infrared emission from lanthanide-doped NaYF4 nanoparticles. J. Am. Chem. Soc. 2008, 130 (17), 5642.
(10) Wang, F.; Deng, R.; Wang, J.; Wang, Q.; Han, Y.; Zhu, H.; Chen, X.; Liu, X. Tuning upconversion through energy migration in core-shell nanoparticles. Nat. Mater. 2011, 10 (12), 968.
(11) Quintanilla, M.; Ren, F.; Ma, D.; Vetrone, F. Light management in upconverting nanoparticles: Ultrasmall core/shell architectures to tune the emission color. ACS Photonics 2014, 1 (8), 662.
(12) Vetrone, F.; Naccache, R.; Mahalingam, V.; Morgan, C. G.; Capobianco, J. A. The active-core/active-shell approach: A strategy to enhance the upconversion luminescence in lanthanide-doped nanoparticles. Adv. Funct. Mater. 2009, 19 (18), 2924.
(13) Weiss, C.; Carriere, M.; Fusco, L.; Capua, I.; Regla-Nava, J. A.; Pasquali, M.; Scott, J. A.; Vitale, F.; Unal, M. A.; Mattevi, C.; Bedognetti, D.; Merkoçi, A.; Tasciotti, E.; Yilmazer, A.; Gogotsi, Y.; Stellacci, F.; Delogu, L. G. Toward nanotechnology-enabled approaches against the COVID-19 pandemic. ACS Nano 2020, 14 (6), 6383.
(14) Medhi, R.; Srinoi, P.; Ngo, N.; Tran, H.-V.; Lee, T. R. Nanoparticle-based strategies to combat COVID-19. ACS Appl. Nano Mater. 2020, 3 (9), 8557.

Rishabh Kundu whole-heartedly acknowledges, thanks and applauds Dr. Eva Hemmer (ehemmer@uottawa.ca), an Associate Professor at the University of Ottawa, Canada, for answering his invitation for contribution and sparing her precious time to draft up this article. She leads a young and enthusiastic team working hard on the development of tomorrow’s multifunctional nanomaterials and is an incredibly kind, smart, and passionate individual. RK wishes her all the best for her future endeavours.

Disclaimer: This article has been drafted by Dr. Eva Hemmer and moderated by Rishabh Kundu. The content of this article is the sole responsibility of Dr. Hemmer.

Published by Dr. Eva Hemmer

Eva is food lover, hobby photographer, and travel enthusiast. She is also a professor at the University of Ottawa, Department of Chemistry and Biomolecular Sciences, Canada, and passionate about materials chemistry and nanophotonics, particularly when lanthanides are involved. To find out more about her team's work check out the groups webpage at www.hemmerlab.com/.

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