Cadmium-Containing Quantum Dots Used in Electronic Displays: Implications for Toxicity and Environmental Transformations

Abstract

Cadmium-containing quantum dot (QD) nanoparticles are integrated into electronic displays because of their ability to efficiently convert colors. There are conflicting accounts as to whether these particles present a hazard to the environment, as they have been studied either as (1) embedded QDs in display screen films or (2) as model QDs with small, hydrophilic ligands. Both approaches have limitations that we addressed by synthesizing QDs featuring the core–shell structure and the thick polymer coating present in commercial devices to probe the dissolution of QDs in response to two environmental factors (pH and dissolved oxygen) over 1 day and 6 months. Results show that QDs were chemically stable at circumneutral pH (0% Cd dissolution after 6 months), but low pH initiated rapid dissolution under both aerobic and anaerobic conditions (up to 100% Cd dissolution after 6 months). In addition to the presence of a capping polymer, the QD shell structure led to more chemically stable nanoparticles compared to nonshelled QDs, as the presence of ZnS shells decreased Cd dissolution by 75%. The dense aggregation of QDs into structures of ∼100 nm diameter over time was observed as well, which could lead to decreased bioavailability. To test this, we used liver cells to compare the toxicity of pristine QDs to those subjected to acid dissolution. Our results reveal that low-pH-exposed QDs separated from dissolved ions are less toxic than pristine QDs (half maximal inhibitory concentration, IC50, of 290 and 150 mg/L, respectively) and suggest a key role of dissolved ions and capping polymers for QD toxicity. These findings highlight the use of a commercially relevant nanoparticle structure to demonstrate fate and toxicity.

Hazard Profiling of Commercially Relevant Quantum Dot Components Revealed Synergistic Interactions between Heavy Metals and Polymers

“…The QD model used in this study was CdSe/ZnS_P&E, as described in our prior study . Individual components of CdSe/ZnS_P&E were also synthesized, namely, CdSe cores, ZnS nanoparticles, and polymer P&E, to assess the effects of the core, shell, and surface coating (Figure A).…”

“…The QDs used in this study are as follows: (1) cadmium selenide core with cadmium sulfide and zinc sulfide shell with octadecylamine ligand (CdSe/ZnS), (2) cadmium selenide core with myristic acid ligand (CdSe), and (3) zinc sulfide QD with oleylamine ligand (ZnS). Detailed QD synthesis as well as the procedure to synthesize and integrate the polymer coating can be found in the SI and a previous publication . Briefly, CdSe QDs were synthesized by injecting Se into a hot solution of Cd complexed with myristic acid in octadecene .…”

“…The cells were then imaged by LSM 710 confocal laser scanning microscopy (Carl Zeiss). Based on the emission wavelength (590 nm) identified in the previous study, the red channel was used to capture images and measure the fluorescence intensity of the QDs …”

“…In our previous study, we synthesized such a model for commercially relevant QD based on a published patent (US 9,199,842 B2) and our analysis of the various metal ratios in QD-containing TV screen films . The QD designed for television screens in the patent contained a CdSe core covered by a graded CdS and ZnS shell and a cross-linked polymer of polyethyleneimine and 1,2 epoxy-3-phenoxypropane (P&E). , In this context, the polymer may be an independent source of toxicity as previous reports have revealed its role in facilitating cellular uptake through endocytosis and acidification of endosomes. , In our previous study, we found that the LC 50 (lethal concentration that caused 50% death of the exposed HepG2 cells) of pristine QDs was on par with a simple mixture of its labile components, which is a combination of Cd 2+ , Zn 2+ ions, and the polymer.…”

Interactions of semiconductor Cd-based quantum dots and Cd2+ with gut bacteria isolated from wild Salmo trutta fry

“…Balde et al (2017) stated that only 20% of electronic waste is properly recycled and a significant number of manufactured displays have QDs cores containing Cd. Additionally, Bechu et al (2021) noted that the release of QDs into the environment occurs through leakages and discharges from engineered landfills and unregulated waste dumps into the soil and groundwater, as well as from unregulated recycling in unsafe conditions. It can be stated that as the demand for QDs and production volumes further increase, their release into the environment will only increase.…”