Semiconductor quantum dots are nanoparticles with optical and electronic properties that can be integrated into society to improve the standard of living. The nanometric dimensions of quantum dots, which are smaller than 10,000 times the width of a hair, result in unique photophysical properties including high brightness in the visible range of the electromagnetic spectrum, high photostability, narrow emission peaks, and size dependent emission tunablity. As the size of the quantum dot increases, a longer wavelength light is emitted with a visible emission color of orange or red. As their size decreases, a shorter wavelength light is emitted with a visible emission color of blue or green, though these sizes only vary between 2 and 6 nanometers in diameter. The unique photophysical properties of luminescent semiconductor quantum dots enable their use in biological imaging, labeling and detection, and as optical elements in consumer electronics, light emitting displays, and photovoltaic devices. One of the major drawbacks of quantum dot technology, which hindered the incorporation of luminescent quantum dots into many consumer products is their chemical composition which includes cadmium, a toxic heavy metal. There is a significant concern that wide scale use of luminescent quantum dots could have adverse impacts on human health and the environment.
This summer, Hyo Park, an undergraduate student from Chaminade University in Honolulu, HI is working in the Rosenzweig laboratory at UMBC on a research project aiming to understand the parameters that control the chemical stability of luminescent semiconductor quantum dots under different environmental conditions, as they interact with living organisms. The Rosenzweig laboratory is a part of the NSF Center for Sustainable Nanotechnology (CSN), which aims to understand the interactions between engineered nanomaterials and model membranes, and living organisms in order to redesign and produce nanomaterials, which maintain high functionality while having a minimal impact on the environment and human health. Hyo Park is working with Melissa Muth, a CSN graduate student in the Rosenzweig laboratory who is pursuing her Ph.D in toxicology. Melissa’s research project is to understand and minimize the long-term biological impacts of nanomaterials on aquatic organism development prior to mass production of luminescent nanomaterials and their introduction to the market.
REU student Hyo Park (center), graduate student Melissa Muth (left) and faculty mentor Prof. Zeev Rosenzweig (right) are analyzing fluorescence microscopy images of zebrafish embryos that are exposed to semiconductor quantum dots during development (a fluorescence image of a zebrafish embryo is see on the computer monitor).
Hyo already completed a set of chemical stability measurements of luminescent quantum dots of CdSe/ZnS in aqueous buffer solutions. These core-shell luminescent quantum dots are modified with a unique molecular ligand, which is produced in the Rosenzweig laboratory to increase the chemical stability of quantum dots in biological solutions. The ligand is prepared by appending a polyethylene glycol chain to a derivative of dihydrolipoic acid (DHLA). Figure 1 describes the fluorescence intensity of 75nM describes the fluorescence intensity of 75nM CdSe/ZnS quantum dots that are capped with a commonly used ligand mercaptoundecanoic acid (MUA) (a) and with the DHLA PEG-CH3O ligand prepared in the Rosenzweig laboratory (b) in water, 10mM of phosphate, and 20 mM phosphate buffer (a); and in water, 10 mM phosphate buffer, and Phosphate Buffer Saline (PBS) solution, which contains 10mM phosphate and 135mM of sodium chloride (b). The luminescent quantum dots were excited at 450 nm, and their peak fluorescence intensity at 570 nm was monitored for seven days to determine the rate of degradation or level of chemical stability. The loss of fluorescence could be the result of quantum dots aggregation and precipitation, and quantum dots dissolution. UV/VIS spectra of the quantum dots solutions were also measured to monitor changes in the UV/VIS absorption profile during the week-long experiment. The figure clearly shows that the fluorescence intensity of quantum dots that are capped with the DHLA-PEG ligand are stable in all aqueous buffers. This is in contrast to quantum dots that are capped with mono-thiolated ligands like mercapto-undecanoic acid, which has been frequently used in biological experiments with quantum dots. The significant improvement in chemical stability will enable chronic exposure experiments of living organisms to luminescent quantum dots, which were not accessible to the community until the development of the DHLA-PEG ligand capping ligand technology.
Figure 1 – Chemical stability measurements of CdSe/ZnS quantum dots that are capped with mercaptoundecanoic acid (MUA) (a); and with a ligand prepared in our laboratory DHLA-PED-CH3O (b) in aqueous buffers of increasing ionic strength. Quantum dots that are capped with the new DHLA-PEG-CH3O ligand exhibit significantly higher chemical stability in aqueous buffers.