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Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX-XXX

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Nanoparticle Synthesis, Characterization, and Ecotoxicity: A Research-Based Set of Laboratory Experiments for a General Chemistry Course Zoe N. Amaris,† Daniel N. Freitas,† Karen Mac,† Kyle T. Gerner,† Catherine Nameth,‡ and Korin E. Wheeler*,† †

Department of Chemistry & Biochemistry, Santa Clara University, 500 El Camino Real, Santa Clara, California 95053, United States University of California Center for Environmental Implications of Nanotechnology (UC-CEIN), University of California, Los Angeles, 570 Westwood Plaza, Los Angeles, California 90095, United States

‡

S Supporting Information *

ABSTRACT: A series of laboratory experiments were developed to introduce first-year chemistry students to nanoscience through a green chemistry approach. Students made and characterized the stability of silver nanoparticles using two different methods: UV-visible spectroscopy and dynamic light scattering. They then assessed the ecotoxicity of silver nanoparticles in Escherichia coli sublethal growth curve assays. Bacterial growth was monitored via optical density and carbon dioxide measurements. Finally, students designed and implemented their own nanomaterial characterization and ecotoxicity experiments based upon the insights gained in previous tests. The experiments translated current research in sustainable nanomaterials to an undergraduate population while introducing them to research methodology and experiences early in their undergraduate career. Moreover, the experiments emphasized the interdisciplinary nature of the sciences to provide real-world applications and connect concepts learned within general chemistry lecture and lab to other classes commonly taken by first-year STEM students, including biology, materials science, and environmental sciences. KEYWORDS: First-Year Undergraduate/General, Environmental Chemistry, Interdisciplinary/Multidisciplinary, Collaborative/Cooperative Learning, Inquiry-Based/Discovery Learning, Colloids, Green Chemistry, Materials Science, Nanotechnology

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INTRODUCTION Green chemistry incorporates the ideas of sustainable design into chemistry and chemical engineering to reduce waste, conserve energy, and reduce hazards for new substances. Implementation of principles of green chemistry requires an iterative process to thoughtfully design materials, test for environmental and health impacts, and then redesign according to new findings. With the dramatic advancement of nanoscience over the past few decades, interdisciplinary groups of researchers are collaborating to apply the principles of green chemistry from the earliest stages of development for new nanomaterials.1−5 With a strong foundation in such research, nanoscientists will be able to design new products with predictable and beneficial impacts on the environment and human health. Given the exciting potential of nanotechnology,6 a library of educational experiments have highlighted the synthesis and unique properties of nanomaterials.7−14 Of those, several nanoscience-focused laboratories integrate principles of green chemistry to nanomaterial synthesis.8,12,15,16 Indeed, the introduction of nanomaterials into the undergraduate curricu© XXXX American Chemical Society and Division of Chemical Education, Inc.

lum is increasing; this increase is, in part, a response to the American Chemical Society’s recent initiatives to increase materials science in the curriculum.17 Additionally, the National Nanotechnology Initiative (NNI) has highlighted the need for skilled technicians, which adds incentive to train students in nanoscience and expose them to the many potential applications of nanomaterials.18 Herein, a research-focused laboratory experiment is described that introduces general chemistry undergraduate students to nanomaterial synthesis, characterization, and ecotoxicity testing. With a focus on developing authentic research experiences for undergraduates in a laboratory classroom setting, the experimental approach is based upon a recent study evaluating the toxicity of silver nanomaterials to environmentally relevant bacteria.19 The advantages of integrating discovery-based research into undergraduate curricula are well-documented to promote student learning and Received: June 5, 2017 Revised: October 18, 2017

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DOI: 10.1021/acs.jchemed.7b00369 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

diversity.20−23 Inquiry-based approaches are of growing interest when introducing nanoscience to an undergraduate lab.9,11,16,24−27 Yet, few reports include ecotoxicity of nanomaterials.16,27,28 Inclusion of an ecotoxicity component in experimental design easily connects the laboratory experiments to real-world applications and emphasizes the increasingly interdisciplinary nature of modern scientific research. With interdisciplinary work, we aim to allow undergraduate students to build connections across their introductory STEM courses in chemistry, material science, environmental science, and biology. Connections across disciplines are reinforced through emphasis on the many roles of nanotechnology in student’s modern lives, as reinforced by readings outside of lab and a final paper. One of the complications in research aimed at assessing the ecotoxicity of nanomaterials is that the chemical and physical properties of nanomaterials are dependent on a wide range of variables.2−5 Not only is the chemical composition of the nanomaterial essential to reactivity, but also researchers must also consider, for example, nanomaterial size, shape, surface coating, and aggregation state. Because many students participate in laboratory courses, this is an ideal venue to compare the properties and ecotoxicity of a diverse library of nanomaterials. Characterization and ecotoxicity experiments are both technically and conceptually accessible to undergraduates, while their relevance to human health and the environment engages students and enables them to make connections to their everyday lives.

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Figure 1. Three week module is diagramed. Week 1 included NP synthesis and characterization of NP stability in water. These skills were transferred to week 2, where nanomaterials were characterized in bacterial media and ecotoxicity tests were performed. In week 3, students designed their own experiments using the skill sets from the previous weeks.

Table 1. Three Weeks of Experiments with Details by Week Week

Synthesize silver nanoparticles (AgNPs) Compare the size of AgNPs using 2 methods: dynamic light scattering and UV−vis spectroscopy Evaluate AgNP stability in diverse solutions

2 Ecocharacterization and toxicity

Evaluate AgNP aggregation in bacterial media Assess AgNP dissolution in bacterial media Monitor the effects of nonlethal concentrations of AgNPs on short-term bacterial growth using 2 methods: cell density and CO2 production Evaluate AgNP impacts on overnight bacterial growth

3 Student-designed research projects

Design and propose experiments Peer review and critically evaluate experimental proposals Revise procedure based upon reviewer comments Perform proposed lab and evaluate results

Final project

Position paper arguing for or against use of nanoparticles in a consumer product.

EXPERIMENTAL SUMMARY

Overview and Timeline

The procedures were performed in an undergraduate general chemistry course during the second quarter of a three-quarter course sequence. Three laboratory periods (four hours each) were allocated for students to complete their experiments, followed by a short position paper to analyze and communicate their experimental outcomes. Students worked in groups of three to complete the lab. Prior to lab, students completed a one-page prelab worksheet with questions that connected experimental concepts to lecture topics, reinforced the principles of new techniques, and introduced new concepts in nanoscience. Near the end of each lab, roughly 20 minutes (min) were dedicated to discussion of results and connection of concepts from the lab to the course and/or everyday life. Data analysis and conclusions were communicated in postlab worksheets turned in at the end of the lab period. Over the three weeks of experiments, students collaborated to synthesize, characterize, and evaluate the ecotoxicity of silver nanoparticles. Silver nanoparticles were chosen because of their readily accessible synthesis, size-sensitive electronic spectra, and sensitivity in ecotoxicity due to their antimicrobial properties. Students also explored the use of silver nanomaterials in many of the consumer products they already use, including select clothing, food storage containers, and cosmetics. An outline of the weekly experiments is diagrammed in Figure 1 with details in Table 1. In the first week, each student group synthesized silver nanoparticles, compared two different methods of assessing nanoparticle size, and evaluated particle stability with diverse solutes. In the second week, the students characterized the size and dissolution of silver nanoparticles in bacterial media while monitoring their effects on bacterial growth using several different methods. In the third week, each group designed and implemented their own experiments to

List of Experiments

1 Nanomaterial synthesis and characterization

expand their understanding of silver nanoparticle behavior and ecotoxicity. A final position paper reinforced the principles of green chemistry and connected their lab work to the consumer products in their everyday lives. Preparation and Materials

The methods chosen for silver nanoparticle synthesis and characterization require no specialized chemicals, but the synthesis works best if the sodium borohydride and silver nitrate solutions are freshly prepared. Nanoparticle characterization was performed using both UV−Vis spectroscopy and dynamic light scattering (DLS). Although the use of spectrophotometers is common in general chemistry laboratories (e.g. Vernier UV−visible spectrophotometers designed for college and high school classrooms), DLS may not be as readily available. If DLS is unavailable, instructors may wish to only use UV−visible (UV−vis) characterization and provide sample DLS data (vida inf ra) to expose students to the technique. For characterization of nanoparticles in biological media and ecotoxicity testing, particles were purchased from nanoComposix to readily enable comparison of ecotoxicity across particle properties of the instructor’s choosing. For example, in the most recent iteration of the lab, students compared the effects of surface charge on ecotoxicity of 40 nm silver B




Evaluation of Nanoparticle Behavior and Stability

nanoparticles by using particles with two different charged coatings: (1) negatively charged citrate and (2) positively charged branched polyethylenimine (bPEI). The commercial particles are purified to ensure a monodisperse size and to remove residual reactants. Although it is not essential to use commercial particles, purchasing particles provided students with access to an array of particles with different properties and ensured that ecotoxicity experiments tested the effects of the particles, rather than that of residual reactants in solution. To save money or simplify experiments, the experimental procedures could be performed with the student-synthesized particles. Ecotoxicity tests were performed with ampicillin (amp) resistant Escherichia coli (E. coli), which can be purchased from ATCC. Protocols for E. coli preparation are included in instructor’s notes (Supporting Information). To ensure cells were ready for student use, the instructor or teaching assistant prepared cells during week 1 for student use in weeks 2 and 3. Students monitored cell growth using a spectrophotometer (SpectraMax i3x Multi-Mode Microplate Reader) and a carbon dioxide monitor (SBA-5 from PP Systems) during the lab in weeks 2 and 3. Because the 4 hour (h) period was too short to monitor a full bacterial growth cycle, students also set up a microwell plate with samples for ecotoxicity testing for a 24 h period.

In procedures modified from McFarland et al.,7 students monitored the optical properties of their synthesized silver nanoparticles using UV−vis upon addition of 36 and 360 μL of 1 M NaCl, KCl, or glucose. Because there were multiple UV− Vis instruments available to students in the lab, these particle stability experiments were streamlined to only include evaluation with UV−Vis (not DLS). The bathochromatic shift and increased PWHM of their absorbance spectra for NaCl and KCl indicate particle aggregation. The addition of glucose led to few spectral changes, indicating no change in particle stability upon addition of this polar solute. The students then added one drop of a stock solution of 3% polyvinylpyrrolidone (PVP) to the dilute solutions of their synthesized particles. After reaction with PVP, the same addition of NaCl, KCl, or glucose resulted in little to no change in the particle absorbance spectra. The PVP coating of the particles stabilized the particles and enabled them to resist aggregation, even upon addition of ionic salts to solution. Characterization of Nanoparticle Stability in Bacterial Growth Media

In the second week of lab, students applied their understanding of particle characterization and stability to characterize particles in biologically and environmentally relevant solutions. To assess particle aggregation in the cell growth media, students diluted 50 μL of the purchased particles to 3 mL with Davis minimal media (DMM), and then worked with a teaching assistant to obtain hydrodynamic radii of the particles with DLS. Sample DLS data is shown in Figure 2. Since the media

Nanoparticle Synthesis

The synthesis was performed following a previously published method.28 Stock solutions were prepared beforehand by the instructor or teaching assistant. Students added 1 mL each of 7.5 mM silver nitrate and 7.5 mM sodium citrate stock solutions to a glass vial with 13 mL of bioling deionized water with stirring. After a couple of minutes, they then added an additional 1 mL of 10 mM sodium borohydride stock solution. Upon addition of the reducing agent, the solution immediately turned yellow upon formation of nanoparticles. After 10 min of additional heat with stirring, the students allowed the solution to cool and characterize their silver nanoparticles. This method was chosen because it is robust and adheres to many green chemistry principles, including the use of inherently safer nontoxic water-soluble reactants reacting within water, a safer solvent than the organic alternatives. In addition, because the reaction ran nearly to completion, the synthesis is also atom economical. Characterization of Synthesized Nanoparticles

Students characterized the size of their nanoparticles using two methods: UV−vis spectroscopy and DLS. The students diluted 50 μL of their synthesized particles to 3 mL in a quartz cuvette and took an absorbance spectrum from 280 to 800 nm. The maximum wavelength of absorption and calculated peak width at half the absorption maximum (PWHM) was compared to a table of known optical properties for an array of particle sizes to estimate the size range for their synthesized particles. Students also prepared samples for DLS using 50 μL of their synthesized particles diluted to 2 mL with filtered, deionized water. They then signed up to use the DLS to obtain hydrodynamic radii for their particles with a teaching assistant. The mean hydrodynamic diameter for student synthesized particles was 30.0 ± 4.8 nm with a bimodal distribution including a population at 5−10 nm and 45−60 nm.

Figure 2. Sample student DLS sizing data on nanoparticles in water () and DMM (---). Hydrodynamic diameter was measured for 40 nm AgNPs with citrate (a) and BPEI coatings (b). C




shield the surface charge of the particles, the particles aggregate in media. Rigorous mixing of the particles into the media tends to better distribute them into solution and reduces aggregation closer to literature values29 (nearer an average hydrodynamic diameter of 200 nm). Because silver toxicity is primarily linked to oxidative dissolution of silver into silver ions, students evaluated silver nanoparticle dissolution in the growth media as well. To assess dissolution, students added 50 μL of 0.18 M silver nitrate or stock particles to DMM. After a 1 h reaction time, the samples were centrifuged for 5 min at 15,000 rpm. The particles pelleted out of solution, but the dissolved silver ions remained in the DMM supernatant. The students removed the supernatant and added one drop of 0.25 M potassium chromate. The formation of red precipitate after addition of potassium chromate was indicative of the formation of silver chromate from dissolved silver in solution. Ecotoxicity Assay

Following procedures modified from Priester et al.,19 student teams compared the effects of sublethal concentrations of silver particles and dissolved silver ions on E. coli growth. To a test tube with 1.5 mL stock solutions of E. coli (optical density, OD (600 nm) = 0.2), students added 750 μL of stock silver nitrate or purchased silver nanoparticles (1 mg/mL NPs) and 750 μL DMM for a final OD = 0.1. The teams then monitored E. coli growth over the course of the 4 h lab period by taking OD (600 nm) and CO2 measurements every 30−45 min. Figure 3a,b includes sample student data over the 4 h period. For longer growth curves, each student team set up an additional growth assay on a microwell plate shared by the class. Each group prepared samples in triplicate to assess reproducibility. Cell growth was monitored by OD (600 nm) every 20 min for 24 h. Student data collected from the microplate assay was shared with the class by a teaching assistant immediately after collection via the course Web site. Sample growth curves are shown in Figure 3.

Figure 3. Sample student growth curve data as monitored by optical density, OD (600 nm) (a) and concentration of carbon dioxide (b). Sample student data is also provided for the longer 24 h growth curve as monitored by OD (600 nm) (c). For clarity, only growth curve data for 40 nm AgNPs with citrate coating is shown here.

Student-Designed Projects

Students were tasked with designing and proposing three new experiments for the last week to reinforce student understanding of the scientific process. To expose students to a range of motivations for scientific exploration, students were required to write a research proposal under each of three levels of risk tiers: (1, repeat) repeat an experiment to hone a new skill or correct previous error, (2, logical next step) evaluate a direct concept or phenomena closely aligned with previous experiments, and (3, leap ahead) explore entirely new, often complex phenomena using the available toolset. The instructor and teaching assistant reviewed and chose the experiment that each team performed in week 3 based upon availability of materials, safety, and likelihood of success (e.g., success in learning outcomes or experimental outcomes).

microcentrifuge tubes that were in contact with the bacterial culture should be placed in biohazardous bags and autoclaved. All nanoparticle solutions, including reagents and exposed E. coli, should be collected and disposed in accordance with hazardous waste procedures.

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DISCUSSION The procedures were performed three times in the laboratory portion of the second quarter of an undergraduate general chemistry honors course. A majority of students were first-year undergraduate STEM majors, including chemistry, biology, bioengineering, and physics majors. The students enrolled in the course with recommendation from their previous general chemistry instructor and were generally self-selecting for a more rigorous course experience. Enrollment was 15−20 students supervised by an instructor and one or two undergraduate student teaching assistants. During the most recent laboratory iteration, the stability and ecotoxicity of 40 nm silver particles with two types of coatings were compared in week 2, including (1) negatively charged citrate and (2) positively charged branched polyethylenimine. To provide instructor flexibility, the handouts for week 2 can be readily modified for other particles of interest; we recommend

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HAZARDS Sodium borohydride is flammable. Silver nitrate may be harmful if swallowed or inhaled, and cause irritation to skin, eyes, and respiratory tract. Sodium citrate may cause irritation to skin, eyes, and respiratory tract. Although the K12 strain of E. coli is nonpathogenic, the students should observe all standard laboratory precautions. This includes wearing gloves and goggles at all times during the experiment, in addition to the optional use of lab coats/aprons. Biohazardous waste should be treated with bleach; pipet tips, cuvettes, and D




Table 2. Student Learning Goals and Objectives Number A A.1 A.2 A.3 A.4 A.5 A.6 B B.1 B.2 B.3 B.4 B.5 B.6 B.7 C C.1 C.2 C.3 C.4 C.5 C.6 C.7 C.8 C.9 C.10

Learning Goals (in Bold) Followed by Abbreviated Statements of Learning Objectivesa Goal: Reinforce student learning in concepts taught in general chemistry lecture. Define the nanoscale and highlight the unique properties of nanomaterial at the nanoscale using technically correct terminology Provide the correct half reactions necessary for formation of silver nanoparticles Summarize the strengths and weaknesses of both techniques Describe the interactions between particles in solution using the language of intermolecular forces Perform conversion calculations to predict the ppm of CO2 cells will consume Apply solubility rules to properly predict the formation of insoluble silver compounds Goal: Establish student confidence in using new experimental approaches. After reading about and watching a video on micropipette use, students will be able to identify the parts of and properly use a micropipette Synthesize silver nanoparticles Use a UV−vis spectrometer and interpret the data to assess the size of silver nanoparticles Interpret dynamic light scattering (DLS) data to assess the size of silver nanoparticles Perform experiments with E.coli and outline key steps to minimize contamination of their samples Interpret data monitoring cell growth with OD600 and CO2 measurements Compare the utility of two methods used to answer an experimental question Goal: Teach students skills and concepts that connect STEM disciplines. Relate nanotechnology to their everyday lives Connect the fields of chemistry with materials science, the environment, and biology Construct a peer review and build awareness of the peer review process Demonstrate a capacity to constructively respond to failure Demonstrate the ability to find and read the primarily scientific literature in chemistry Connect chemical concepts to real-world experiences Develop a testable hypothesis Design an experiment with proper controls Assess the quality of their own data, even in the absence of known experimental values Suggest revisions of their approaches to improve their data quality

Figure 4. Bar graph of student achievement within learning objectives. The codes on the y-axis correspond to the learning objectives outlined in Table 2. Details on the evaluation of each benchmark are provided in Supporting Information.

Results from the microwell plate growth experiments rely upon new skill sets for many students. Instructors observed that student success in the ecotoxicity experiments depends upon two key skills: (a) use of micropipettes and (b) sterile technique. Earlier laboratories in the general chemistry honors series include micropipettes and provide additional opportunities for students to master this key skill set. To minimize the impact of contamination in the E. coli samples, it was essential to use amp resistant E. coli. Importantly, silver nanoparticles showed little change in hydrodynamic radius or zeta potential upon addition of amp in concentrations used here. In addition, amp did not impact silver nanoparticle dissolution as evaluated with ICP-MS (data not shown). In the third week of experiments, student groups performed their designed experiments to learn about new aspects of sustainable nanomaterials. Sample titles from student-designed research projects included the following: (1) Ecotoxicity of different sized AgNPs (2) Ecotoxicity and properties of AgNPs vs AuNPs (3) Ecotoxicity and properties of citrate and cysteine coated AgNPs (4) AgNP properties and ecotoxicity in orange juice (5) Ecotoxicity of AgNPs in clothing detergent In rare cases where students were struggling with micropipetting or sterile technique, the instructor encouraged the student group to repeat the experiments from week 2 to ensure additional opportunity to hone their skills. For each class, most student groups were encouraged to do their “logical next step” experiments, as these were straightforward to design, tended to include the appropriate controls, and were likely to yield insightful results. Examples of “logical step” experiments include sample titles 1, 2, and 3 (above). Knowing that students were often eager to perform their most ambitious projects, the best-designed projects from the “leap ahead” categories were chosen for one or two groups. Although the projects from the latter category were least likely to provide

a

For clarity, student learning objectives are abbreviated. Each begins with a statement such as “After completion of a task in the module, students will be able to...”. Complete learning objectives are included in Supporting Information.

including discussion of particle choice in pre- and postlab lecture discussions. Although the stability and ecotoxicity of similar particles were previously reported by Priester et al.,19 the results for these specific particles have not been reported and outcomes were unknown prior to this laboratory experiment. Dynamic light scattering sizing, silver dissolution, and 4 h cell growth experiments gave reproducible results across each student group. Yet, as is common when working with cells (especially in smaller volumes), not all samples in the 24 h microwell plate growths gave reproducible results. Despite this challenge, these experiments captured student imagination and excitement. Moreover, the disparity in results was used as a catalyst for class discussions on how to evaluate the best data set in scientific research when “the right answer” is unknown. E




students demonstrated that they had not yet built strengths that spanned all three disciplines.

discerning results, they were essential fodder for in class discussion about the scientific method and research approaches. Examples of “leap ahead” experiments include sample titles 4 and 5 (above). In larger laboratory classes with multiple sections, we recommend streamlining this exploratory third week of student-designed experiments, since managing many student projects with multiple sections may become onerous. To encourage the students to connect their experimental results to their modern lives, the final assignment for the series of experiments was to write a brief position paper on the use of a specific nanomaterial within a consumer product. In this report, students used their experimental data to justify or refute the use of silver nanomaterials in a variety of consumer products. Some chose to support the use of silver nanomaterials as antimicrobials in food storage containers to reduce illness, while others refuted the use of silver nanomaterials in socks because of environmental concerns. Students were evaluated on their ability to properly communicate the conclusions from their research and use scientific references to justify their argument. For many students, this assignment served as an important capstone to tie together the detailed experimental analyses, new concepts on nanotechnology, environmental impacts, and relevance to their daily lives. Student worksheets and notebooks confirm that this set of activities and experiments enables students to achieve the three main learning goals of the module, including (A) reinforcing general chemistry concepts, (B) building confidence in approaching new experimental methods, and (C) forming a skill set in the foundational approaches in STEM. These goals and the underlying student learning objectives are provided in Table 2. The first learning objective states, “the student should be able to define the nanoscale and highlight the unique properties of nanomaterials using technically correct terminology” [A.1]. To assess student progress to this objective, we employed the first question on the student prelab worksheets, student notebooks, and their final papers. Students who met the benchmark toward this objective correctly answered the prelab assignment question asking students to (a) provide two physical and chemical differences between nano- and macroscale, (b) describe the importance of surface-area-to-volume ratios in nanomaterials, and (c) state at least one nanomaterial property that affects color (aside from size). After watching the video on nanomaterials, all of the students met this benchmark. Exceeding the benchmark in this area meant applying the principles of nanochemistry to describe laboratory observations in their notebook or use nanochemistry to accurately argue their points in the final paper. Fifty percent of the students achieved this higher benchmark for demonstrating an understanding of nanoscale phenomena. The full analysis of students meeting and achieving each learning objective is provided in Figure 4. Over 90% of the students met or exceeded the benchmarks for 18 of the 23 learning objectives. Under two of the three goals, students found 1−2 learning objectives challenging to achieve. This includes applications of redox chemistry to silver nanoparticle formation [A.2], which relies on general chemistry concepts the students were introduced to the quarter before and had not yet reviewed in lecture. The other learning objective that seemed to prove most challenging was to connect the fields of materials science, the environment, and biology [C.2]. Although many students clearly made broad connections between the fields in their final paper, the benchmark was written to assess their ability to tackle the more conceptually challenging pre- and postlab questions; here,

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SUMMARY A three week series of laboratory experiments were developed to engage general chemistry undergraduate students in the synthesis, characterization, and ecotoxicity evaluation of silver nanoparticles. This research-focused series was designed to enable students to apply their general chemistry knowledge to gain an introduction to nanoscience. Simultaneously, students were exposed to modern techniques while connecting their foundational knowledge in chemistry to interdisciplinary concepts in biology, environmental studies, and materials science. Moreover, students were introduced to essential research approaches, including experimental design and analysis of new experimental outcomes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00369.

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All student handouts (PDF, DOCX) All prep notes and instructor manuals (PDF, DOCX)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Korin E. Wheeler: 0000-0002-4711-5062 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge University of California Center for the Environmental Implications of Nanotechnology Co-PIs, Professors Hilary Godwin (UCLA) and Patricia Holden (UCSB), for their support in designing the ecotoxicity experiments. We’d also like to thank Professor Jillian Blatti (Pasadena City College) for thoughtful discussion, the students in Santa Clara University’s general chemistry honors course (CHEM 12H from 2015−2017), and course instructors Lindsay Sperling and Stephen Reaney (SCU) for assistance and feedback on the pilot runs of these experiments. This work was supported by the National Science Foundation (NSF DBI1266377).

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