<p><a>Molecular imaging revolutionized the way researchers and clinicians visualize and investigate complex biochemical phenomena, and it is beneficial </a><a>for disease diagnosis, drug design and therapy assessment</a>. Among a variety of different imaging techniques, the non-ionizing and non-invasive photoacoustic (PA) imaging is attracting increased attentions, owing to its high spatial and temporal resolutions with reasonable penetration depth in tissue. Parallel efforts have been the preparation of PA imaging agents which has high PA efficacy and can specifically label the targets at cellular or molecular level. Particularly, there is exponentially growing interest in imaging in the second near-infrared (NIR-II) window (1000–1700 nm), where offers reduced tissue background and improved penetration depth. However, study of PA imaging in the NIR-II window is incomplete, partly due to the lack of suitable materials. Therefore, in my dissertation work I studied NIR-II PA imaging through semiconducting polymer. </p> <p>Firstly, the performance of PA imaging in the NIR-II window is explored by using a semiconducting polymer nanoparticle (SPN) which has strong absorption in the NIR-II window. Compared with lipid, blood and water, such SPN shows outstanding PA contrast in the NIR-II window <i>in situ</i> and <i>in vivo</i>, and an imaging depth of more than 5 cm at 1064 nm excitation is achieved in chicken-breast tissue. These results suggest that SPN as a PA contrast in the NIR-II window opens new opportunities for biomedical imaging with improved imaging contrast and centimeter-deep imaging depth.</p> <p>Next, targeted PA imaging of prostate cancer is achieved by functionalizing a NIR-II absorbing SPN with prostate-specific membrane antigen (PSMA)-targeted ligands. Insights into the interaction of the imaging probes with the biological targets are obtained from single-cell to whole-organ by transient absorption (TA) microscopy and PA imaging. TA microscopy reveals the targeting efficiency, kinetics, and specificity of the functionalized SPN to PSMA-positive prostate cancer at cellular level. Meanwhile, the functionalized SPN demonstrates selective accumulation and retention in the PSMA-positive tumor after intravenous administration <i>in vivo</i>. Taken together, it is demonstrated that BTII-DUPA SPN is a promising targeted probe for prostate cancer diagnosis by PA imaging. </p> <p>Lastly, PA imaging in the NIR-II window is also achieved water-soluble semiconducting polymer, which is prepared by oxygen-doping. After doping, it shows broadband absorption in the entire NIR-II window, with great chemical stability, photostability and biocompatibility. Owing to its merit of broadband absorption, the imaging depth comparison among different NIR-II wavelengths is also achieved. Moreover, this doped semiconducting polymer is readily soluble in normal physiological pH by virtue of carboxyl groups on side chains and tends to aggregate at an acidic environment which results in a 7.6-fold PA enhancement at pH 5.0. Importantly, a 3.4±1.0-fold greater signal in tumor tissue than that in muscle is revealed <i>in vivo</i>. This study provides a more attainable yet effective platform to the field for achieving water-soluble NIR-II absorbing contrast agents for activatable PA imaging. </p>

Significance Cerebrospinal fluid (CSF) rhinorrhea (leakage of brain fluid from the nose) can be difficult to identify and currently requires invasive procedures, such as intrathecal fluorescein, which requires a lumbar drain placement. Fluorescein is also known to have rare but significant side effects including seizures and death. As the number of endonasal skull base cases increases, the number of CSF leaks has also increased for which an alternative diagnostic method would be highly advantageous to patients. Aim We aim to develop an instrument to identify CSF leaks based on water absorption in the shortwave infrared (SWIR) without the need of intrathecal contrast agents. This device needed to be adapted to the anatomy of the human nasal cavity while maintaining low weight and ergonomic characteristics of current surgical instruments. Approach Absorption spectra of CSF and artificial CSF were obtained to characterize the absorption peaks that could be targeted with SWIR light. Different illumination systems were tested and refined prior to adapting them into a portable endoscope for testing in 3D-printed models and cadavers for feasibility. Results We identified CSF to have an identical absorption profile as water. In our testing, a narrowband laser source at 1480 nm proved superior to using a broad 1450 nm LED. Using a SWIR enabling endoscope set up, we tested the ability to detect artificial CSF in a cadaver model. Conclusions An endoscopic system based on SWIR narrowband imaging can provide an alternative in the future to invasive methods of CSF leak detection.