Tailoring optical tweezers for quantum dot based fluorescence sensing

Abstract

Optical trapping is a venerable topic and has been an invaluable and pioneering tool in biological research, but it has been mostly unexplored by the analytical science community. This study lays the groundwork for a novel analytical technique by combining holographic optical tweezers and fluorescence spectroscopy of quantum dots (QDs).

QDs are artificial semiconducting nanoparticles with fascinating optical properties, making them ideal for fluorescence sensing applications. In this study, QD fluorescent probes fit for optical trapping were created, which included the synthesis of L-cysteine capped CdSe/ZnS core/shell QDs and the coupling thereof to micro-sized polymer beads through EDC/NHS chemistry. The synthesis process was optimized in order to gain control over the QD size, fluorescence emission, adhesion, agglomeration formation and purity of the sample. Several techniques including fluorescence spectroscopy, transmission electron microscopy (TEM), Fourier transform infrared (FTIR) and absorbance spectroscopy were used to characterize the structural and optical properties of the QDs and QD fluorescent probes. To showcase the sensing ability of these QD probes, they were used to detect atrazine, a harmful herbicide, at environmentally relevant surface water concentrations.

Optical trapping or tweezing describes the manipulation of nano- to micro-sized particles through momentum transfer from tightly focused light. In this study, the advantages of employing vectorial light in optical traps were explored, especially for the control of fluorescent particles. A holographic optical tweezer (HOT) that leverages on vectorially structured light with integrated fluorescence detection was built. The setup included a diode laser with wavelength λ = 532 nm, a spatial light modulator (SLM), an interferometer to combine the beams, an inverted microscope for trapping and imaging, and a photon counting fluorescence detection stage. This advanced setup allowed for the delivery of tuneable forms of light from purely scalar to purely vector beams resulting in tailored intensity gradient landscapes of the trapping beam. Trap stiffness calibration was performed with 2 μm polymer beads and in-situ fluorescence measurements of an optically trapped QD probe was achieved. This study focused specifically on propagation invariant vector flat-top beams which showed potential to reduce photobleaching of QDs in a single wavelength optical trap. The aim of the research presented in this dissertation is to advance the nascent field of chemistry applications in optics.