Detection of small numbers of cancer cells is one of the significant challenges in oncology, which prevents detecting a tumor in its early stages or onset of metastatic spreading of disease in cancer patients. Developing nanoparticles for molecular specific imaging, and therapy is a promising strategy to address this challenge. Many inorganic nanoparticles exhibit a bright signal for enhanced imaging contrast and exhibit unique size- dependent optical properties. Organic nanoparticles require to load dyes to serve as imaging agents and are able to degrade inside the host body to be excreted without long- term bodily accumulation. Photoacoustic imaging is a novel non-invasive imaging modality that can be used for highly sensitive detection of near-infrared (NIR) absorbing nanoparticles in tissue. NIR optical window is ideal for medical imaging due to the low tissue scattering and absorption, which in turn, provides longer tissue penetration depth. Therefore, nanoparticles with NIR absorption have great potential for photoacoustic imaging for highly sensitive cancer detection. In addition, nanoparticles can render molecular-specific toward cancer cells of interest via attachment of antibodies that target cancer biomarkers. During my doctoral study, I used ultra-small gold nanoparticles (AuNP) and organic nanoparticles loaded with indocyanine green (ICG) (Lipo-ICG and ICGJ@PEI Ps) that are conjugated with antibodies (anti-EGFR) in conjunction with photoacoustic imaging (PAI) for highly sensitive and specific cancer detection. Ultra-small AuNPs of 5 nm in diameter have two important advantages: 1) improved delivery in tissue due to their small size; and 2) an efficient renal clearance after administration. Renal clearance is a prerequisite for the approval of nanoparticles in clinics. As an alternative

approach to inorganic AuNPs, organic nanoparticles such as liposomes and polymersomes were loaded with s (ICGJs), which displays a narrow peak in the NIR window, ideal for PA imaging. This approach provides the following advantages: 1) biodegradability to achieve renal clearance without long-term bodily accumulation, and 2) improved PAI sensitivity and molecular specificity. The main idea of my approach is based on the targeting of the epidermal growth factor receptor (EGFR). Individual 5 nm AuNPs do not exhibit NIR absorbance that is required for sensitive PAI. Attachment of anti-EGFR antibodies to 5 nm AuNPs renders their molecular specificity and receptor-mediated accumulation inside EGFR-expressing cancer cells; nanoparticle aggregation occurs in endosomal compartments after the accumulation, which leads to a dramatic increase of their absorption in the NIR spectrum. For organic nanoparticles, retention of ICGJs inside cancer cells after EGFR receptor-mediated accumulation can enable molecular PAI. Therefore, labeled cancer cells can be detected by the PAI with NIR excitation. In order to fully understand the contrast enhancement mechanism of ultra-small AuNPs, I carried out a series of experiments to study the endocytosis and aggregation of targeted 5 nm AuNPs using two-photon microscopy. Then, the feasibility of sensitive PAI was evaluated with the targeted 5 nm AuNPs at the cellular level. Liposomes and polymersomes loaded with ICGJs were tested as a new design of organic PA contrast agents for cancer detection. Future research directions can explore theranostic applications, including 1) radiosensitization by ultra-small AuNPs, and 2) longitudinal monitoring of molecular therapy using PAI with targeted organic nanoparticles loaded with ICG J aggregates. Future research directions can explore theranostic applications including: 1) radiosensitization by ultra-small AuNPs, and 2) longitudinal monitoring of molecular therapy using PAI with targeted organic nanoparticles loaded with ICG J aggregates.