Abstract
The full potential of triplet fusion photon upconversion (TF-UC) of providing high-energy photons locally with low-energy excitation is limited in biomedicine and life sciences by its oxygen sensitivity. This hampers the applicability of TF-UC systems in sensors, imaging, optogenetics and drug release. Despite the advances in improving the oxygen tolerability of TF-UC systems, the evaluation of oxygen tolerability is based on comparing the performance at completely deoxygenated (0% oxygen) and ambient (20–21%) conditions, leaving the physiological oxygen levels (0.3–13.5%) neglected. This oversight is not deliberate and is only the result of the lack of simple and predictable methods to obtain and maintain these physiological oxygen levels in an optical setup. Herein, we demonstrate the use of microfluidic chips made of oxygen depleting materials to study the oxygen tolerability of four different micellar nanocarriers made of FDA-approved materials with various oxygen scavenging capabilities by screening their TF-UC performance over physiological oxygen levels. All nanocarriers were capable of efficient TF-UC even in ambient conditions. However, utilizing oxygen scavengers in the oil phase of the nanocarrier improves the oxygen tolerability considerably. For example, at the mean tumour oxygen level (1.4%), nanocarriers made of surfactants and oil phase both capable of oxygen scavenging retained remarkably 80% of their TF-UC emission. This microfluidic concept enables faster, simpler and more realistic evaluation of, not only TF-UC, but any micro or nanoscale oxygen-sensitive system and facilitates their development and implementation in biomedical and life science applications.
Highlights
The issue of molecular oxygen is intrinsic to triplet fusion photon upconversion (TF-UC), since it is based on excited triplet states that are quenched by molecular oxygen to yield singlet oxygen
We have formulated four micellar nanocarriers with varying oxygen scavenging capabilities made of FDA-approved surfactants and oils for green-to-violet triplet fusion upconversion and studied their upconversion performance under a range of oxygen levels
Each formulation was capable of upconversion even in ambient conditions, utilizing an oxygen scavenger as the oil phase improves the oxygen tolerability considerably
Summary
The issue of molecular oxygen is intrinsic to TF-UC, since it is based on excited triplet states that are quenched by molecular oxygen to yield singlet oxygen. The generation of singlet oxygen competes with not one but two steps of TF-UC, illustrated, reducing the quantum yield of upconversion (QY) and increasing the excitation power density threshold (Ith) of efficient UC. Singlet oxygen is a powerful oxidant that can damage the dye molecules of the UC system or its immediate surroundings. This naturally leads to degradation of the UC system[12] and toxicity to cells and tissue through the photodynamic effect that can be either beneficial in the case of photodynamic therapy or harmful in the case of, for example, bioimaging.[13] the effects of molecular oxygen, when unmitigated, can lead to no upconversion and even loss of the UC system.
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