An Engineering Approach Investigating the Uptake and Phytotoxicity of One Type of Engineered Nanoparticle (CdSe/ZnS Quantum Dots) by Solanum Lycopersicum
Authors: Salverson Lynsey A. M. , Saniei Nader , Mendelson Mel , Lum Michelle 
Published: January 2014

The novel and extraordinary physiochemical properties of engineered nanoparticles (ENPs) is certain, yet, at the same time, their unique characteristics raise growing concerns regarding potentially adverse effects on biological and ecological systems. It is becoming increasingly evident, that before the full potential of nanotechnology can be realized, standardized characterization of ENPs behavior, fate, and their effects in the ecosystem are essential, to ensure the safe manufacturing and use of ENP products. Otherwise, the promise of such extraordinary advancements may find itself limited to applications such as electronics, and sporting equipments, industries in which it currently resides. The current toxicity profile of engineered nanomaterials is not only preliminary, but highly variable amongst researchers. Emphasizing the great need to develop a highly organized, efficient, and precise approach to assess the hazardous potential ENPs may pose, and address the safety concerns surrounding and limiting nanotechnology. In response to such concerns, the present study took an engineering approach, in an otherwise traditionally viewed discipline, to assess the potential impact of engineered nanoparticles on tomato (Solanum lycopersicum) seedlings, by implementing a full factorial design of experiment (FDOE) in an effort to identify what factors, and their interactions, have a significant (p ≤ 0.05) effect on root and shoot elongation, and if any observed effects are a result of particle uptake, evaluated via fluorescence microscopy imaging. Therefore, the goal of our study was to design and implement an efficient, effective, and precise method to assess the effect of one type of ENP, water-soluble CdSe/ZnS quantum dots, using Solanum lycopersicum as our model organism, one of 10 species recommended by the Unites States Environmental Protection Agency (US EPA) for use in phytotoxicity studies, via a methodology we believe novel to nanotechnology. By implementing factorial experimental design methodologies, not only are we efficiently identifying the factors that affect phytotoxicity, we are providing, for the first time to our knowledge, the first scientific data to report the significant interaction effects between the factors responsible for ENP toxicity.

Water soluble (MUA) CdSe/ZnS quantum dots used in our study had a negative influence on root and shoot lengths of tomato seeds exposed for 3 and 6 days. The observed influence depended on (MUA) CdSe/ZnS concentration and QD exposure time. The importance of the factor effects were examined via analysis of variance (ANOVA), t-tests, confidence intervals, and normal plot statistical analyses. The findings concluded that factors B, C, and the BC-interaction (CdSe/ZnS: Exposure time, concentration, and exposure time–concentration interaction) significantly (p ≤ 0.05) affected root and shoot lengths of tomato seedlings. Thus, factors A, AB, AC, and ABC (CdSe/ZnS QD: Size, size–concentration, size-exposure time, and size-concentration-exposure time interactions) were not found to have a significant effect on root and shoot lengths of tomato seedlings, and ultimately eliminated from our model. After analyzing the interaction plots, it became evident that low percentages of root reduction are obtained at low concentration levels for short lengths of time; thus, to obtain the least amount of phytotoxic effects one would set factors B (concentration) and C (exposure time) to their low levels, 125 mg/L for 3 days, respectively. Alternatively, high percentages of root reduction are obtained at high concentration levels for long lengths of time; thus, to obtain the greatest phytotoxic effect one would set factors B and C to their high levels, 1000 mg/L for 6 days, respectively. This indicates that as exposure time increases, root reduction increases; thus, phytotoxicity increases. Since our study attempted to realize which factors minimize phytotoxicity effects of one type of ENP, these findings suggest that to minimize phytotoxicity effects (i.e. maximize root length or minimize percent of root reduction) of (MUA) CdSe/ZnS QDs on tomato seeds, set factors B and C (QD concentration and exposure time) to their low levels; that is, expose tomato seeds to 125 mg/L of QD solution for a maximum of 3 days. These settings will yield the least amount of root reduction (5.15%) and; thus, phytotoxicity effects will be minimized.

With regard to tomato roots ability to uptake MUA QDs, our results contribute to the literature by reporting uptake possible. Although we did see particles inside the root, it was sporadic and difficult to quantify. As to whether it was intracellular (within the cell) or intercellular (in the spaces between the cells, i.e., outside the cells) we could not conclude with certainty, although we suspect the QDs were intercellular. Thus, we highly recommend future experiments involving cross sections and more in-depth microscopy imaging. Additionally, although the results of our experiment failed to support that particle size (t = 2.13; d.f. = 1; p = 0.065) or the particle size-exposure time interaction (t = 2.17; d.f. = 1; p = 0.062) had a significant effect on root and shoot lengths of tomato seedlings, due to the small p-value associated with both test statistics, it is our belief that particle size and the particle size-exposure time interaction may, in fact, be a real effect; thus, further investigation is recommended.