Emergence of antibiotic bacterial resistance has caused serious clinical issues worldwide due to increasingly difficult treatment. Development of a specific approach for selective visualization of resistant bacteria will be highly significant for clinical investigations to promote timely diagnosis and treatment of bacterial infections. In this article, we present an effective method that not only is able to selectively recognize drug resistant AmpC β-lactamases enzyme but, more importantly, is able to interact with bacterial cell wall components, resulting in a desired localization effect on the bacterial surface. A unique and specific enzyme-responsive cephalosporin probe (DFD-1) has been developed for the selective recognition of resistance bacteria AmpC β-lactamase, by employing fluorescence resonance energy transfer with an "off-on" bioimaging. To achieve the desired localization, a lipid-azide conjugate (LA-12) was utilized to facilitate its penetration into the bacterial surface, followed by copper-free click chemistry. This enables the probe DFD-1 to be anchored onto the cell surface. In the presence of AmpC enzymes, the cephalosporin β-lactam ring on DFD-1 will be hydrolyzed, leading to the quencher release, thus generating fluorescence for real-time resistant bacterial screening. More importantly, the bulky dibenzocyclooctyne group in DFD-1 allowed selective recognition toward the AmpC bacterial enzyme instead of its counterpart ( e.g., TEM-1 β-lactamase). Both live cell imaging and cell cytometry assays showed the great selectivity of DFD-1 to drug resistant bacterial pathogens containing the AmpC enzyme with significant fluorescence enhancement (∼67-fold). This probe presented promising capability to selectively localize and screen for AmpC resistance bacteria, providing great promise for clinical microbiological applications.

Denitrogenative 6-endo-dig azide-yne cyclization of α-propargyloxy-β-haloalkylazides was enabled by gold catalysis, thus providing 2H-1,3-oxazines. This rare cyclization mode in gold-catalyzed reactions of azide-yne substrates was demonstrated to be facilitated and controlled by electronic and resonance effects of the alkyne substituents. Molecular transformations of the as-prepared 2H-1,3-oxazines were also investigated.

Cross-contamination during pharmaceutical drug manufacturing can result in expensive recalls. To counter that, companies spend significant time and resources to ensure equipment cleanliness, often relying on the compound solubility data in various solvents as the main indicator of cleaning success. The aim of this work is to provide an alternative way to analyze the fouling and cleaning of surfaces in pharmaceutical manufacturing processes by using the quartz crystal microbalance with dissipation (QCM-D) and Raman spectroscopy. In this study, we chose an active pharmaceutical ingredient (API), sitagliptin phosphate monohydrate (SIT), as the model drug compound and observed its adsorption and desorption on stainless steel (SS2343), borosilicate glass (glass), and polytetrafluoroethylene (PTFE) surfaces. SIT was selected as the model API since it is a product manufactured on a large scale and is part of the widely used dipeptidyl peptidase-IV inhibitor class of oral hypoglycemics used to treat type 2 diabetes mellitus, while the chosen surfaces mimic the wall materials of manufacturing equipment and components such as reactors, transfer lines, and valves. Both the QCM-D and Raman spectroscopy results show the highest physisorption on PTFE, followed by SS2343 and glass. Additionally, QCM-D revealed a harder removal of SIT from SS2343 compared to glass and PTFE. Raman analysis of the chemical interactions disclosed C−F and C�O bond interactions between SIT and the surfaces, and the lack of a peak shift suggested dipole−dipole interactions. Furthermore, contact angle measurements indicate that hydrophobic attraction contributed to SIT adhesion to the PTFE surface. Subsequently, SIT coverage upon deposition on a PTFE surface has a significantly smaller surface area than on SS2343 and glass due to surface hydrophobicity, hence resulting in a longer removal time. These results provide a practical use of QCM-D and Raman spectroscopy to enhance the understanding of fouling and improve the cleaning of complex small molecules on relevant surfaces during the pharmaceutical manufacturing process.

I would like to thank Assoc. Prof. Xing Bengang for his guidance during my M.Sc. research. I am thankful to Junxin and other group members for their support and assistance during my period of research. I would like to express my gratitude towards Prof. Chiba Shunsuke for his approval of his laboratory usage for the synthetic work, which greatly facilitated this research. I am also extremely thankful towards Ciputra, Derek, Sherman, Hiro, and others for their constant guidance and support from my undergraduate to graduate period of studies. I would like to thank Jin Yong for his contributions during his final year project, to the work presented in this thesis. I would like to express my appreciation to the staffs from the Division of Chemistry and Biological Chemistry (CBC), Ee Ling and Wen Wei for their assistance with instrument trainings/usages; Janice, Si Yun, and Yean Chin for their assistance in graduate studies matters. I would like to express my gratitude to Nanyang Technological University for the financial support during the original Ph.D. candidature, in the form of Nanyang Research Scholarship. I would also like to thank CBC for giving me the opportunity to attend the 2 nd International Biophotonics Conference, Singapore. Last but not least, I would like to thank my daddy and mummy for their support in my decisions regarding further studies. I am also extremely grateful to my friends, Gillian, Wei Ting, Wei Li, and others for their constant support and encouragement. ii