Upconverting nanoparticles
Hacking complex photophysical networks for optical applications
Energy transfer network in Er/Tm-doped upconverting nanoparticles (J. Phys. Chem. B 2012)
In doped nanoparticles and semiconductor quantum dots, excited states can follow multiple competing pathways (e.g., resonant energy transfer or Auger recombination). The non-linear and interconnected nature of these interactions ultimately lead to complex photophysical networks whose behavior can be counterintuitive. To understand how the optical properties of nanoparticles are governed by these photophysical networks, we use our high-throughput combinatorial nanoscience workflows to systematically investigate libraries of nanomaterials with varying composition and structure. This understanding allows us to discover new photophysical phenomena and hack energy transfer networks to produce emergent non-linear behavior.
Upconverting nanoparticles
A major focus of our research is on lanthanide-doped upconverting nanoparticles (UCNPs), whose complex energy transfer networks enable these materials to emit light with higher energy than the incident light. The ability to excite UCNPs with NIR light with little scattering, photodamage, or autofluorescence allows us to image these materials through >1 mm of tissue. The ability to deliver visible light deep inside tissue enables applications such as the optogenetic stimulation of neurons (ACS Nano 2016), photodynamic therapy (ACS Nano 2014), and optically controlled release of therapeutics (Chemical Science 2018). Molecular Foundry Users have also utilized lanthanide-doped materials as nanothermometers (Nanoscale 2016, Nature Comm. 2018) and cathodoluminescent probes (Nature Nano. 2019). Understanding the complex energy transfer (ET) networks that govern upconversion have allowed us to controllably manipulate these networks in order to overcome major challenges with upconverting nanoparticles. In close collaboration with the groups of Bruce Cohen (Foundry) and Jim Schuck (Columbia), we have developed dye conjugation to counter low absorption, alloyed UCNPs for low luminescence efficiency, and new sensitizing dopants to counter the limited range of excitation wavelengths). We also develop novel applications for UCNPs and have leveraged the chemically interchangeable nature of lanthanide ions to develop a high-throughput workflow for the modeling, synthesis, and screening of lanthanide-doped upconverting nanoparticles for new functionality.
Top: (a) basic mechanism for upconversion. (b) combinatorial screening of triply doped UCNPs (Nano Letters 2012).
Bottom: Lanthanide-doped NaYF4 upconverting nanoparticles.
Energy-looping nanoparticles
Seeking to expand the small number of wavelengths used to excite UCNPs, we used in silico combinatorial screening to identify Tm3+:NaYF4 nanoparticles that can upconvert light efficiently to 800 nm when excited at 1064 nm, which we confirmed using high-throughput synthesis with our robot, WANDA. This combination of wavelengths is particularly useful for biological applications because 800 and 1064 nm fall respectively within the 1st and 2nd NIR windows for biological imaging. Our computational package revealed that these particles generate upconverted light through a unique, avalanche-like mechanism called “energy looping,” which amplifies excited state populations nonlinearly through repeated cycles of excited state absorption and cross-relaxation (an ET process inverse to upconversion).
Upconverting microlasers
The amplification of excited state populations in energy looping nanoparticles (ELNPs) make them desirable gain media for the amplification of photons. To demonstrate this concept, we have fabricated whispering gallery microresonators by assembling ELNPs at the surface of polystyrene microspheres. These structures host sharp whispering gallery modes (Q > 2000) as evident in light emitted from these cavities and to demonstrate upconverted lasing at multiple wavelengths (475 and 800 nm) Collaborating with Schuck group (Columbia), we demonstrated that these “looping” lasers exhibited the lowest reported lasing threshold for any upconverting nanoparticle laser. They are stable for over 6 h of continuous pumping and over a year of repeated use. Their low threshold and high stability allowed us to excite these resonators with continuous wave excitation at room temperature Record low thresholds and high quality factors can be achieved with high fidelity by controlling assembly of the ELNPs onto cavity surfaces. These microlasers operate and can be used to measure temperature even in complex biological media such as serum and through tissue-mimicking phantoms. We further reduce the dimensions and thresholds of upconverting nanoparticle lasers by coupling them to plasmonic arrays. These results suggest that upconverting microlasers, which are smaller than red blood cells, may be applied to in vivo sensing and imaging through tissue.
Avalanching nanoparticles for sub-diffraction imaging
By increasing the Tm3+ doping and growing thick, high quality passiviting shells, we can access an "extreme" version of energy looping known as photon avalanching (PA). We have reported the first example of PA at room temperature in single nanostructures. Avalanching nanoparticles (ANPs) can be pumped by continuous-wave lasers and exhibit all of the defining features of PA, including clear excitation power thresholds, exceptionally long rise time at threshold, and a dominant excited-state absorption that is >10,000 times larger than ground-state absorption. Since ANP emission scales nonlinearly with the 26th power of pump intensity, we can use them in photon-avalanche single-beam superresolution imaging (PASSI). The Schuck group has achieved sub-70 nm spatial resolution, using only simple scanning confocal microscopy on ANPs synthesized in our group. The low PA threshold and exceptional photostability of ANPs also suggest their utility in applications including optical and environmental sensing.
