UM co-develops NIR-II quantum yield measurement to advance medical imaging

Xu Changhuo, research assistant professor in the Faculty of Health Sciences (FHS) at the University of Macau (UM), in collaboration with Lam Wing Yip, research associate professor in the Department of Chemistry at the Hong Kong University of Science and Technology (HKUST), and Tang Ben Zhong, dean of the School of Science and Engineering at the Chinese University of Hong Kong, Shenzhen, has made a significant breakthrough in the field of second near-infrared (NIR-II) materials. They have developed a new organic fluorophore, TPE-BBT, which sets a reliable standard for measuring photoluminescence quantum yield (PLQY) in the NIR-II window. This advancement promises to unify the evaluation criteria for NIR-II materials, paving the way for more accurate and consistent research and offering new tools for bioimaging and cancer diagnostics. The research has been published in the prestigious journal Small.

NIR-II materials have extensive applications in deep-tissue bioimaging, organic light-emitting diodes, and communication technologies. A key factor determining the performance of these materials is the PLQY, which is the ratio of the number of emitted photons to the number of absorbed photons. Traditional integrating sphere technology for determining absolute PLQY is challenged by the low signal-to-noise ratio and significant measurement error caused by the low PLQY values of most NIR-II materials (often less than 1%). The most common approach is to use the relative method based on reference standards such as IR-26, which can accurately measure PLQY even when the PLQY value is less than 1%. However, the PLQY values of IR-26 are not clearly defined, and are reported to range from 0.05% to 0.50%, leading to unreliable results.

To address these challenges, the research team introduced TPE-BBT, an aggregation-induced emission (AIE) luminogen, which exhibits high PLQY in low-polarity solvents. Its molecular structure, comprising a benzobisthiadiazole acceptor and a tetraphenylethylene donor, enables strong emission in the NIR-II window. The PLQY of TPE-BBT is highly dependent on solvent polarity, with toluene solution achieving an impressive PLQY of over 6%—one of the highest values reported for NIR-II organic small-molecule dyes. In tetrahydrofuran, TPE-BBT exhibited a moderate PLQY of 3.94%, which can be used as a standard for relative NIR-II PLQY measurements.

Using TPE-BBT as a reference, the research team recalibrated the PLQY values of two widely used NIR-II standards, IR-26 and IR-1061. The results showed that IR-26 has a PLQY of 0.0284% in 1,2-dichloroethane, while IR-1061 has a PLQY of 0.182% in dichloromethane. These calibrated values allowed for more accurate relative PLQY measurements, with results closely matching the absolute values. This breakthrough establishes a more consistent framework for future NIR-II PLQY measurements, reducing discrepancies between absolute and relative methods and enabling more reliable comparisons across studies. By providing a more accurate and standardised way to measure PLQY, the research team’s work is expected to accelerate the development of NIR-II materials for applications in bioimaging, optoelectronics, and beyond.

Xu, Lam, and Tang are the co-corresponding authors of this study. PhD graduate Shen Hanchen and PhD student Zhu Xinyan from the Department of Chemistry at HKUST are the co-first authors. Zhang Jianyu, a postdoctoral fellow at HKUST, also contributed to the study. The research was funded by the Science and Technology Development Fund of the Macao SAR (File No.: 0049/2023/ITP2). The full text of the research article is available at https://onlinelibrary.wiley.com/doi/10.1002/smll.202411866.

Schematic illustration of the absolute and relative PLQY measurements of NIR-II materials

Chemical structure of TPE-BBT and its photophysical properties in different states

Xu Changhuo