Monday, 14 August 4:00pm – 5:00pm

This seminar will be delivered in Chemistry Lecture Theatre 4 and Online (Zoom) Please email chemistry.researchsupport@sydney.edu.au for zoom link and password.

Talk 1: Methods to modulate the fluorescence of naphthalimides

Speaker: Hazel Chan

Abstract: Naphthalimides are attractive small-molecule fluorophores for use in designing chemosensors for biological and cellular applications owing to its high photostability, high quantum yields, and large Stokes’ shifts. They are highly tunable intermolecular charge transfer (ICT) probes, with applications in metal sensing, enzyme sensing, and sensors for toxic chemicals. Recently, their synthetic versatility has been increasingly explored, by inputting recognition sites for the desired analyte, organelle-targeting groups, and moieties for fluorescence tuning, onto the naphthalimide scaffold.

Photoinduced electron transfer (PET) is a phenomenon that often results in the quenching of fluorescence. PET quenching occurs when a nearby lone pair electron can relax into the highest occupied molecular orbital (HOMO) of a fluorophore, thus blocking the pathway for the excited electron in the lowest unoccupied molecular orbital (LUMO) from relaxing back to the HOMO, preventing fluorescence. In fluorescent sensors, PET quenching is often incorporated into a design, such that interaction with the desired analyte will inhibit PET quenching, leading to fluorescence. A series of pH-responsive naphthalimide-based sensors have been designed to understand how changes in structure will affect PET quenching.

Förster resonance energy transfer (FRET) occurs when energy is transferred between two fluorophores known as a “FRET pair”. The FRET pair consists of a donor and an acceptor joined by a linker. When there is maximum FRET efficiency, the donor is excited and transfers all its energy to the acceptor, which fluoresces, while emission from the donor is not observed. Coumarin-naphthalimide FRET pairs with different linkers have been synthesised to understand how modifying the bridge between the two fluorophores will affect FRET efficiency, which can help illuminate ways of designing ratiometric probes.

Talk 2: Fluorescence sensing approaches to pH in biological context

Speaker: Nian Kee Tan

Abstract: Small molecule fluorescent sensors are excellent tools to probe cellular environments with temporal and spatial resolution, playing a large role in our understanding of health and disease. However, despite the advances in fluorescence imaging, the current state of art for fluorescent probes are far from ideal and needs to keep up with new discoveries in health and disease.

Our interest in probing cellular pH environment comes from it being an important parameter with significant roles in homeostasis, proteolysis, ATP production and apoptosis. Abnormal regulation of pH inflict stress to cells and is a potential driver towards diseased state in cells. For example, cancer cells often exhibit abnormal pH values between acidic pH 4-5, which decreases drug efficacy and conditions them better against cancer therapy and metastasis. Using fluorescent pH probes, the relationship between the acidification of pH environment and the corresponding disease pathology can be investigated. However, the shortfall to most organic fluorescent probe is its limited sensing range of ±1 pK_a, restricting its use in studying biological processes.

To circumvent this, the first half of this seminar details the development of a fluorescent pH sensor for biological imaging with extended sensing range. This was achieved by grafting novel organic fluorophore with carbon quantum dot. The final probe responds to pH environment through modulating its emissive intensity and was able to distinguish in the biological relevant range of pH 4-7, an extension to ±1.5 pK_a of conventional pH probes.

The second half of this talk will discuss the development of arrays for distinguishing pH using a combination of organic pH sensors and multivariate statistical analysis. Two approaches to accurately discriminate and predict pH will be showcased, to which lays the groundwork for multi-probe fluorescent analysis.