Degradation of quantum dots in aqueous environments

• Online Access: http://hdl.handle.net/2047/D20315043

QDs are mixed-metal nanocrystals with the smallest of particle sizes (2-10 nm). QDs application (in electronics, pharmaceutical industry, etc.) has been rising dramatically in the recent years. QDs readily leach heavy metal cations in water, potentially creating a co-occurrence of nanoparticulate and dissolved metal pollutants. It has been observed in the literature that QDs degradation in photic and aphotic environments are different. QDs degrade faster in light and produce reactive oxygen (ROS) species. These ROS are thought to be the culprit in QDs fast degradation. However, knowledge on QD's degradation kinetics, mechanism, and interfering parameters in photic and aphotic conditions lag behind their application and vast usage. This lag in QD studies partly comes from the difficulty in QD and dissolved metals separation and quantification. This work aimed to first develop a cheap and fast analytical method to separate and quantify QDs and their corresponding dissolved phase metals and then use it to describe QDs degradation kinetics, mechanism and influencing factors in photic and aphotic conditions. SEC-ICPMS method was developed and used to separate CdSe/ZnS and InP/Zn QDs, along with Au NPs from their corresponding dissolved phase metals. SEC-ICPMS lowers cost and time associated with NP characterization 90% and 50% respectively. Next, using the developed SEC-ICPMS method, QD's degradation in the dark was studied. A systematic approach was used to detect the driving force behind CdSe/ZnS QD's dissolution in aphotic aqueous environmental solutions. QDs dissolution in the presence of excess QD precursor molecules (MPA, Zn2+, Cd2+), natural solutes (O2, H2O2, natural organic matter), or a model organic metal complexing ligand (ethylenediaminetetraacetate) was studied. In most conditions, with the dissolved O2 present, the ZnS shell degraded fairly rapidly over ~1 week while some of the CdSe core remained up to 80 days. Additional MPA, Zn2+, and Cd2+ temporarily delayed dissolution, indicating a moderate role for capping agent detachment and mineral solubility. No dissolution of CdSe was observed when O2 was absent or when QDs formed aggregates at higher concentrations. Results show that the most important mechanism for QD degradation in the dark is oxidation and QDs can persist in dark and anoxic waters up to several months. This is important as QDs might end up in anoxic sediments and thus last for long periods of time. Next, QD's photodegradation was investigated. While H2O2 was detected in the solution, attempts to detect other reactive iii oxygen species (hydroxyl radicals, singlet oxygen, super oxide) were inconclusive. Then a systematic approach was used and effect of several environmentally relevant water chemistry parameters on QD degradation was investigated. NOM, nitrate, nitrite and ferrous/ferric ion increased QD's degradation. Basic pH decreased QDs degradation rate while other environmentally relevant water chemistry parameters like high ionic strength, phosphate, carbonate, and chloride ions had no apparent effects on QD's photodegradation. A new kinetic model for QDs photodegradation was also proposed and then fitted to the data using the Runge-Kutta method. Using the proposed kinetic model and by perfuming experiments on a wide range of commercially available QDs' it was observed that higher band gap values, smaller QD diameters, and low shell thickness are the most important structural factors in QDs rapid photodegradation.