A major international effort is underway to identify and manage the potential risks of nanotechnology to human health and the environment.1-4
Most research in this area focuses on primary nanomaterials, whose behavior is directly relevant to occupational risks in nanomanufacturing facilities.5
Consumer and environmental risks, in contrast, are most often associated with nano-enabled products
, which are typically composites in which primary nanomaterials are bound on substrates or embedded in liquid or solid matrices.6
A particularly common product formulation is a polymeric matrix with embedded nanomaterials that enhance the optical, mechanical, or electronic properties or impart wholly new function to the base polymer.
It is widely understood that the consumer and environmental risks associated with nano-enabled consumer products may differ from those of the primary particles on which most current research is based. Embedded nanoparticles (NPs) typically have lower exposure potential than free particles,7
but exposure can occur in principle if there is damage to or degradation of the polymer matrix during use, misuse, or disposal.8,9
The exposure can be to liberated particles, chemical components of degraded particles, or particle-containing polymer wear debris.
In one example, Wohlleben et al.9
reported that no free nanomaterials were liberated by sanding or weathering of polymeric and cementitious nanocomposites. While the wear debris contained nanofillers, some of which appeared on the debris particle surfaces, there was no apparent toxicological effect of the nanofiller component of the composite debris. It is a challenge to study nanocomposite risks in any general way, because each commercial product is a unique formulation, and both the degradation processes and the resulting debris depend on the specific manner in which the product is stressed during use or disposal. Nevertheless, more information and data are needed on the important topic of nanocomposite risks, and in light of the challenges cited above, a practical way forward is in the consideration of commercial product case studies
One emerging nano-enabled consumer product is the Quantum Light™ Optic
of QD Vision Inc.
(Lexington, MA). The optic is an acrylate polymer with embedded quantum dots (QDs) used as a faceplate on light-emitting diode (LED) lamps to shift the output spectrum to more closely resemble incandescent lamps, which have higher consumer acceptance. LED lamps are more efficient than incandescent lamps and their widespread acceptance would lead to significant energy savings and environmental protection. For illustration, today’s typical luminous efficiencies of LED and incandescent lamps are 113 Lm/W and 17 Lm/W respectively,10
and complete substitution in the world market would lead to approximately 1,000 TWh/yr energy savings and 200 million tones reduction in CO2
The use of quantum dots for spectral correction saves an additional 25 - 40% of energy relative to LEDs with broadband downconversion material such as rare-earth phosphors. QD-enabled LED lighting products with 250 Lm/W are within reach in the coming decade. Quantum dots absorb blue light and re-emit the energy through fluorescence downconversion at longer wavelengths in a very narrow band (~30 nm full-width at half of maximum (FWHM)) of color, while conventional phosphors have a typical FWHM of 90 - 120 nm, resulting in significant deep red and infrared emission. Realizing the energy and environmental benefits of QD-enhanced LED technologies will require careful consideration of their potential risk and net benefits as nanoproducts.
Quantum dots in the form of suspended primary particles have been the subject of numerous environment and health-oriented studies, reviewed by Hardman12
and Pelley et al..13
QDs have been reported to be benign or toxic depending on the specific material, biological system, dose, and length of test.12,13
Adverse responses have been attributed to associated cadmium ions, NP surfaces, or a combination of the two.14,15
Many studies have considered the effects of QD formulation, in the sense that toxicity is reported to be sensitive to the presence and chemical nature of stabilizing shells, ligands, and other conjugated moieties.12,13
Essentially all of the studies reviewed12,13
focus on primary QDs rather than composites.
A subset of the literature deals with the issue of QD stability. Mahendra et al.16
report bacterial toxicity of quantum dots due to cadmium or selenite ion, but only after a pre-weathering step under acidic or basic conditions to destabilize the dots and cause ion release. Navarro et al.17
provide evidence for degradation of quantum dots in soil and study both particle and ion mobility in soil columns. King-Heiden et al.14
report little QD degradation during zebrafish exposure, and a toxicity mechanism that involves both particles and associated ions. Hardman12
notes that adverse effects are more typically seen in longer term studies. This may reflect the time required for the relevant toxicity pathways, or the increased toxicity of QDs over time due to slow degradation and release of toxic cadmium and selenium species. A number of studies report toxicity mitigation by inclusion of a ZnS shell13
, which may reflect stabilization of the CdSe core to ion release or passivation of surface reactions. These degradation studies also focus on primary quantum dots rather than composite materials. Relevant to polymer composites is a significant literature on (pure) polymer degradation, and there have been several published life cycle analyses on polymer/nanoclay composites.18,19
To our knowledge, there have been no studies of the environmental or health implications of quantum dot composite materials.
The quantum dot product chosen for this case study is not a structural material that is likely to be drilled, sawed or otherwise modified by consumers, so the main issue in environmental risk is end-of-life behavior. Common environmental fates include landfilling, incineration, improper disposal and product recycling. In this study we focus on landfilling, where the optics will be exposed to fluid phases and may leach heavy metals, liberated QDs or particle-containing polymeric debris. Cadmium, selenium, and zinc are known environmental toxicants. If present in soluble form in the leachate, they represent chemical pollutants whose risks may be assessed by conventional methods without the need for new information on unique properties or behaviors that emerge at the nanoscale. If NPs are liberated, however, they may have environmental mobilities, bioaccumulation propensities, and toxicities that are different from the soluble forms of the constituent elements. The presence of NPs in leachate does not necessarily imply an elevated risk, but does imply uncertainty that warrants special consideration and possibly new regulation. A key question for risk assessment and regulation of nanocomposites, therefore, is whether or not disposal leads to NP liberation to the natural environment.20
The present article examines the end-of-life behavior of the Quantum Light™ Optic as a case study in nanocomposite environmental risks. The study focuses on the extent and forms of Cd release using a prototype optic in commercial size and form and a range of fluid simulants. A particular interest is to determine whether this material liberates NPs following shredding and long-term fluid exposure, which would imply a nanotechnology-specific environmental issue, or whether the product liberates soluble cadmium and is more usefully regarded as a conventional chemical product that is a potential source of leachate fluids containing small amounts of soluble cadmium.