Fluorescent materials for short-wave infrared imaging

Abstract

Our understanding of the fundamental processes that drive biology and medicine is, in large part, based on our ability to visualize biological structures and monitor their transformations over time. Fluorescence imaging is one of the most transformative technologies of modern biomedical imaging as it provides a low cost, high sensitivity method for real-time molecular imaging in vivo. As the scattering and absorption of light through biological tissue impose significant restrictions on imaging penetration depth, acquisition speed, and spatial resolution, the development of novel optical imaging technologies has increasingly shifted toward the use of light of longer wavelengths. Fluorescence imaging in the shortwave infrared (SWIR, 1000 - 2000 nm) spectral region mitigates the negative effects of light attenuation and benefits from a general lack of tissue autofluorescence.

As a result, SWIR imaging promises higher contrast, sensitivity, and penetration depths compared to conventional visible and near-infrared (NIR) fluorescence imaging. However, the lack of versatile and functional SWIR emitters has prevented the general adoption of SWIR imaging both in academic and clinical settings. Here, we will present progress toward the synthesis of a new generation of SWIR-emissive materials and discuss their use in enabling biomedical imaging applications. In the first part of this thesis, we will examine the synthesis of SWIR-emissive indium arsenide (InAs) quantum dots (QDs). To address existing challenges in the synthesis of these semiconductor nanocrystals, we will investigate the processes that govern nanoparticle formation and growth.

Combining experimental and theoretical methods, we demonstrate that the synthesis of large nanocrystals is hindered by slow growth rates for large particles, as well as the formation and persistence of small cluster intermediates throughout nanocrystal growth. Based on these insights, we design a novel, rational synthesis for large InAs QDs with high brightness across the SWIR spectral region. Second, we will discuss the use of InAs-based QDs in functional SWIR imaging applications in pre-clinical settings. We will present three QD surface functionalizations that enable the non-invasive real-time imaging of hemorrhagic stroke, the quantification of metabolic activity in genetically-engineered animals, and the measurement of hemodynamics in the brain vasculature of mice. In addition, we will present preliminary results for the synthesis of SWIR-emissive QD probes for the molecular targeting of biological entities and for advanced particle tracking applications.

Using a QD-based broadband SWIR emitter, we will further investigate the eæect of SWIR imaging wavelength on image contrast and tissue penetration depth. While it was previously assumed that reduced scattering of light at longer wavelengths is the primary cause for increased image contrast, our results indicate that for imaging scenarios with strong fluorescent background signals, image contrast and penetration depth correlate closely with the absorptive properties of biological tissue. As a result, deliberate selection of imaging wavelengths at which biological tissue is highly absorptive can help to overcome contrast-limited imaging scenarios. In the last part of this thesis, we will take a closer look at SWIR emitters with the potential for translation into clinical settings.

We will demonstrate that the FDA-approved NIR dye indocyanine green (ICG) exhibits an unexpectedly high SWIR brightness that arises from a large absorption cross-section and a vibronic shoulder in its fluorescence spectrum that extends well into the SWIR spectral region. We expand on this finding by showing that ICG outperforms commercial SWIR dyes during in vivo imaging, and additionally by demonstrating a variety of high-contrast and high-speed imaging applications in small animals. These results suggest that ICG enables the direct translation of SWIR imaging into the clinic. In summary, this thesis will paint a comprehensive picture of the current state of SWIR-emissive materials, present the synthesis of novel versatile SWIR probes, and show their application in unprecedented functional SWIR imaging applications.