environmental science nano

EnvironmentalScienceNano

Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30

.

TUTORIAL REVIEW View Article OnlineView Journal

This journal is © The Royal Society of Chemistry 2015

a IBM Microelectronics, Systems and Technology Group, Hopewell Jct., NY, USAb Arizona State University, School of Sustainable Engineering & The Built

Environment, PO Box 873005, Tempe, AZ 85287-3005, USA.

E-mail: [email protected]; Tel: +480 965 2885c University of Arizona, Tucson, AZ, USAdUniversity of Texas at Dallas, Richardson, TX, USAeNorth Carolina A&T State University, Greensboro, North Carolina, USAf Johns Hopkins University, Baltimore, MD, USAgClarkson University, Potsdam, NY, USA

Nano impact

The manuscript represents the efforts of an academic-industry consortium aiming to characterize the physical-chemical and biclass of engineered nanomaterials (CMP slurries). Results include intra-laboratory comparisons of measurements, multiple inparticle sizes and multiple, complimentary assays to assess human cell and bacterial response to nanomaterials. The resureported measurements, and put into a life cycle perspective that aims to understand both exposure and hazards. The conclusiopose relatively low risk based upon our current understanding, but that biological effect differences between fumed and counresolved when considering the available physical chemical data.

h Intel Corporation, Santa Clara, CA, USAi Colorado School of Mines, Golden, CO, USAj University of Washington, Seattle, WA, USA

† Electronic supplementary information (information includes some analytical methodsfate during on-site industrial wastewaterc5en00046g‡ Current address: Department of Civil, EnvironmFlorida Atlantic University, Boca Raton, Florida 334

Cite this: DOI: 10.1039/c5en00046g

Received 14th March 2015,Accepted 13th May 2015

DOI: 10.1039/c5en00046g

rsc.li/es-nano

Physical, chemical, and in vitro toxicologicalcharacterization of nanoparticles in chemicalmechanical planarization suspensions used in thesemiconductor industry: towards environmentalhealth and safety assessments†

David Speed,a Paul Westerhoff,*b Reyes Sierra-Alvarez,c Rockford Draper,d

Paul Pantano,d Shyam Aravamudhan,e Kai Loon Chen,f Kiril Hristovski,b

Pierre Herckes,b Xiangyu Bi,b Yu Yang,b Chao Zeng,c Lila Otero-Gonzalez,c

Carole Mikoryak,d Blake A. Wilson,d Karshak Kosaraju,e Mubin Tarannum,e

Steven Crawford,e Peng Yi,‡f Xitong Liu,f S. V. Babu,g Mansour Moinpour,h

James Ranville,i Manuel Montano,i Charlie Corredor,j Jonathan Posnerj andFarhang Shadmanc

This tutorial review focuses on aqueous slurries of dispersed engineered nanoparticles (ENPs) used in

chemical mechanical planarization (CMP) for polishing wafers during manufacturing of semiconductors. A

research consortium was assembled to procure and conduct physical, chemical, and in vitro toxicity

characterization of four ENPs used in CMP. Based on input from experts in semiconductor manufacturing,

slurries containing fumed silica (f-SiO2), colloidal silica (c-SiO2), ceria (CeO2), and alumina ĲAl2O3) were

selected and subsequently obtained from a commercial CMP vendor to represent realistic ENPs in

simplified CMP fluids absent of proprietary stabilizers, oxidants, or other chemical additives that are known

to be toxic. ENPs were stable in suspension for months, had highly positive or negative zeta potentials at

their slurry working pH, and had mean diameters measured by dynamic light scattering (DLS) of 46 ± 1 nm

for c-SiO2, 148 ± 5 nm for f-SiO2, 132 ± 1 nm for CeO2, and 129 ± 2 nm for Al2O3, all of which were larger

than the sub 100 nm diameter primary particle sizes measured by electron microscopy. The concentration

of ENPs in all four slurries that caused 50% inhibition (IC-50) was greater than 1 mg mL−1 based on in vitro

assays using bioluminescence of the bacterium Aliivibrio fischeri and the proliferation, viability, and integrity

of human cells (adenocarcinomic human alveolar basal epithelial cell line A549). The general practice in

Environ. Sci.: Nano

ological attributes of a majordependent measurements oflts are compared with othern is that CMP nanoparticleslloidal silica continue to be

ESI) available: Supplementaryand in-depth discussion of ENPtreatment. See DOI: 10.1039/

ental and Geomatics Engineering,31, USA.

http://crossmark.crossref.org/dialog/?doi=10.1039/c5en00046g&domain=pdf&date_stamp=2015-05-23

http://dx.doi.org/10.1039/c5en00046g

http://pubs.rsc.org/en/journals/journal/EN

Environ. Sci.: Nano

Environmental Science: NanoTutorial review

Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30

. View Article Online

the CMP industry is to dilute the slurry waste stream so actual abrasive concentrations are typically orders

of magnitude smaller than 1 mg mL−1, which is lower than IC-50 levels. In contrast to recent reports, we

observed similar toxicological characteristics between c-SiO2 and f-SiO2, and the materials exhibited similar

X-ray diffraction (XRD) spectra but different morphology observed using electron microscopy. The ENPs

and CMP slurries used in this study have been made available to a number of other research groups, and it

is the intention of the consortium for this paper to provide a basis for characterizing and understanding

human and environmental exposures for this important class of industrial ENPs.

1 Introduction

Some industrial processes use large quantities of engineerednanoparticles (ENPs) in ways that do not directly end up inconsumer products but which nonetheless require appropri-ate handling and environmental controls to assure that theydo not pose workplace and/or environmental risks. The provi-sion of effective and safe handling methods, and the provi-sion of effective waste treatment and disposal methodsrequires knowledge of how ENPs behave in air and aqueousmatrices, as well as the availability of well informed humanand ecotoxicity threshold values.4,5 The ability to establishthese factors is in turn predicated on the availability of vali-dated analytical methods for the quantification of ENP in rel-evant air, water and solid media. Efforts to evaluate ENPbehavior in environmental systems face difficult metrologychallenges.6,7 In particular, there is a need for validatedmethodologies that can discriminate between dissolved andnano-sized particulates, measure particle number and sizedistributions, and differentiate between ENPs and naturallyoccurring nanoparticles.

Alumina ĲAl2O3), ceria (CeO2), and silica (SiO2) representan important class of ENPs.1 One principal use of Al2O3,CeO2, and amorphous SiO2 nanoparticles is chemicalmechanical planarization (CMP) where particles in the formof abrasive slurries are used to polish wafers when fabricat-ing integrated circuits in the semiconductor industry.17,18 Inthis application, the ENPs are used to manufacture the prod-uct (semiconductor chips), but are not incorporated into theproduct. CMP nanoparticles constituted nearly 60% of thetotal $1 billion worldwide market for nano-powders in2005.19,20

Although Al2O3, CeO2, and amorphous SiO2 ENP are gen-erally believed to be relatively innocuous, toxicity evaluationsthat have been conducted on these materials, vary dependingon the particular aspects of the particles being assessed (e.g.,particle size, physiochemical properties, and dispersion state)as well as the particular toxicity assessment method beingutilized (e.g., dose, cell lines, exposure protocols, and assayend points).2,3 Likewise, there is some uncertainty regardingthe behavior and fate of these ENP in wastewater treatmentprocesses. For instance, the removal of CeO2, SiO2, and Al2O3

nanopartcles (NPs) during biological wastewater treatmenthas received some research attention,8–14 but less informa-tion exists on ENP removal in the type of physical-chemicaltreatment processes that are often employed as on-site indus-trial wastewater treatment. The available literature indicates

that Al2O3 and CeO2, but not necessarily SiO2, are typicallyremoved well by conventional biological wastewater treat-ment processes. ENPs removed from the wastewater accumu-late in biosolids and precipitated sludges, where their fateand ultimate stability are important considerations whendetermining the environmental effects of these ENPs. More-over, naturally occurring Al2O3 and SiO2 NPs are believed tobe common in natural water systems, where their fate inter-twined with a variety of geological and biological pro-cesses.15,16 An understanding of the background concentra-tions and geochemical processes that govern the occurrenceand behavior of these naturally occurring NPs, and how biotarespond to them, is needed to provide context for inter-preting the impacts of ENP wastewater discharges and deter-mining appropriate discharge levels. In light of these infor-mation gaps, we have pursued a multi-universitycollaborative research effort, the initial stages of which havecentered on developing and validating basic characterizationmethods for Al2O3, CeO2, and amorphous SiO2.

This tutorial review aims to characterize ENPs in modelCMP suspensions, to understand the ramifications of ENPproperties on potential exposure and toxicity to environmen-tal systems and humans, and to discuss research needs asso-ciated with the next generation of semiconductormanufacturing. Four model CMP fluids, each being the sim-plest formulation to generate a stable suspension of ENPs,were obtained from a major commercial slurry manufacturer.These slurries were thoroughly characterized via physical-chemical means, including intra-laboratory comparisons ofnano-measurements. Large volumes of these model CMP sus-pensions were procured and have been made available to sev-eral investigators that study workplace exposure, human andecosystem toxicity, industrial treatment efficiencies, sewerdischarge and wastewater treatment removal, fate and trans-port, and fundamental aspects of the nano-bio interface.In vitro assays were conducted using different cell lines tocompare the relative toxicity of ENPs used in CMP. Finally, adiscussion identifies potential scenarios for workplace expo-sure and flux of ENPs from CMP processes into the sewersystem.

1.1 Nanoparticle use in semiconductor production

Al2O3, CeO2, and amorphous SiO2 ENPs are used in a varietyof applications beyond CMP.1 For example, Al2O3 is used inthe production of tires, paper, catalyst, polymers, and per-sonal care products; CeO2 is used in fuel additives, catalyst,



Environmental Science: Nano Tutorial review

Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30


and biomedical applications; and SiO2 is used in the manu-facture of tires, paper, paints, coatings, catalyst, cement, poly-mers, and personal care products. Annual production volumeof Al2O3, CeO2, and SiO2 has been estimated and presentedin a number of recent publications but varies widely, as indi-cated in Table 1.

CMP fluids remove materials by a combination of chemi-cal and mechanical (or abrasive) actions to achieve highlysmooth and planar material surfaces. CMP, which can beused to planarize a variety of materials including dielectrics,semiconductors, metals, polymers, and composites, is one ofthe most important semiconductor processes for achievingthe performance goals of modern microprocessor and mem-ory chips.21,22 Fig. 1 shows a typical CMP operation schemewhere a robotic arm loads a semiconductor wafer onto arotating plate, a quantity of CMP slurry is dispensed, and apad is engaged in a polishing action that uses the slurry toplanarize the wafer surface. The polishing step is followed byadditional automated rinse and pad cleaning steps. Follow-ing the CMP operation, the height of the wafer surface is typi-cally uniform to within ±1 nm, or less. Because even thesmallest scratch or surface imperfection can ruin the geome-try of the integrated circuit being fabricated into a wafer,CMP slurries are typically crafted with highly controlled parti-cle size distributions, along with additives, as describedbelow.

The inorganic oxide abrasive particles are an importantconstituent for CMP slurries, with the three most commonlyused abrasive particles being Al2O3, amorphous SiO2, andCeO2.

17,23 Depending on the particular application, particlesize in CMP slurries can vary from 20 to 200 nm, and trendsare toward CMP particles <100 nm in size to achieve highlypolished surfaces.23,24 In a given slurry formulation, theseparticles usually have a uniform shape and size.


Table 1 Estimates of the annual production (metric tons per year) of Al2O3, C

NP material production Holden et al. (2014)73 Keller et a

Al2O3 >200 000 18 500–35CeO2 <10 000 7500–10SiO2 >2 400 000 82 500–95

Fig. 1 The CMP operation scheme.

Amorphous SiO2 can be distinguished as fumed silica(f-SiO2) and colloidal SiO2 (c-SiO2) based on the synthesismethods. f-SiO2 is formed in a pyrogenic process by oxidizingchlorosilane (SiCl4) at high temperature.25 c-SiO2 is formed inliquid phase by precipitating a Si precursor (e.g., Na2SiO3).

26 Awidely referenced method of synthesizing c-SiO2 that usesa tetraalkyl silicate as the Si precursor was presented byStöber.27 CeO2 NPs used in CMP slurries have a crystallinestructure, thus often yielding sharp edges, corners, andapexes.28 Al2O3 NPs used in CMP slurries can be α-Al2O3,ß-Al2O3, δ-Al2O3, or fumed Al2O3.

29 Al2O3 is softer than SiO2 orCeO2 and sometimes can be coated with a hard surface such asSiO2 to enhance semi-conductor polishing.23

In addition to the metal oxide NPs, CMP slurries may alsocontain a number of additives, such as pH adjustmentagents, oxidizers, dispersants, complexing agents, surfac-tants, biocides, and corrosion inhibitors, as summarized inTable 2. For instance, one additive may serve to aid in theselective dissolution and solubilization of a material that ispresent at the wafer surface, and a second additive may serveto protect or inhibit the removal of a different material thatis present on the exposed wafer surface during the CMP pro-cess. Likewise, surface active agents may be added to influ-ence particle surface charge and help maintain a stable parti-cle dispersion.

In a manufacturing line, a combination of CMP slurries isfed to a fleet of CMP tools. In each tool, the wafers undergothe CMP process and are rinsed with deionized (DI) water,and further chemicals may be added to clean and/or recondi-tion the wafer polishing pads. A typical wafer production stepmight involve applying between 0.2 and 0.8 L of CMP slurry,1 to 2 L of rinse water, and another 5 or more liters of padcleaner and rinse water. The total quantity of wastewater gen-erated per wafer undergoing CMP polishing may be on the

Environ. Sci.: Nano

eO2, and SiO2

l. (2014)1 Eur. Comm (2012)74 Piccinno et al. (2012)75

000 200 000 55–5500000 10 000 5.5–550000 1 500 000 55–55 000


Table 2 Typical CMP slurry additives

Component Function Examples

Abrasiveparticles

Polish surface Al2O3, CeO2, amorphousSiO2

pH adjust Adjust and buffer pH HCl, KOH, HNO3,NH4OH, H3PO4, TMAH,NH4OH, buffers

Complexingagents

Solubilize dissolved metals Amino acids (glycine,etc.), carboxylic acids(citric acid, etc.)

Oxidizers Promote metal removal viaoxidative dissolution

H2O2, Ferric nitrate,KIO4, KMnO4, etc.

Corrosioninhibitors

Selectivity against removalof certain surfaces,corrosion inhibition

Benzotriazole (BTA),3-amino-triazole

Surfaceactiveorganics

Maintain metal oxideparticles in a dispersed state

Polyacrylic acid,polyethylene glycolpolymer, cetyl trimethylammonium bromide,polyethylene cetyl ether

High MWpolymers

Flocculant and/or coatabrasives to “cushion” theirabrasiveness

High MW polyethyleneoxide

Biocides Prevent biological growth Hydrogen peroxide andothers


Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30


order of 10 or more liters. The effluent wastewater containsthe original slurries, associated rinse waters, and dissolvedand particulate material that is removed from the wafer dur-ing the CMP operation. The thickness of material removedfrom the wafer surface may vary from a few nanometers to100 or more nanometers. If, for instance, a 100 nm blanketlayer of Cu is removed from a 300 mm diameter wafer sur-face, 64 mg Cu/wafer would be added to the wastewater.

Detailed characterization of the compositional change thata CMP slurry undergoes throughout a given CMP process hasnot been reported in the literature. However, there arereports that the particle size distribution in CMP waste is typ-ically broader than the particle size distribution of the virginslurry,30,31 which is probably due to the release of small

Environ. Sci.: Nano

Table 3 Summary of key characteristics for the model CMP slurry compositio

Name c-SiO2

Manufacturer reported- Material Colloidal SiO2

- Composition 3% SiO2

- Additive <1% acetic acid- pH 2.5–4.5- Particle size (nm) 50–60Primary metal concentration 27 g Si L−1

Dissolved organic carbon (DOC; mg L−1) 320.5 ± 0.5Other additives 801.9 ± 1.3 mg L−1 acetic acidDiameter by SEM (nm) 37 ± 7Diameter by TEM (nm) 36 ± 9Mean diameter by DLS (nm) 46 ± 0.2(Polydispersity index) (0.08)Diameter by NTA (nm) 61 ± 0.9Single particle ICP-MS (nm) NDZeta potential at slurry pH (mV) −21a BDL = Below detection limit. b particles tended to coalesce, and primary

particles from the wafer surface, the formation of aggregates,or both.

2 Analytical & experimental methods2.1 CMP fluid procurement

Through an industry-university collaboration supported bythe Semiconductor Research Corporation (SRC) and the SRCEngineering Research Center for Environmentally BenignSemiconductor Manufacturing, our consortium was able towork with a CMP slurry provider (Cabot Inc.) to design andprocure large volumes of four fairly simple, industrially rele-vant, and well characterized CMP slurries. Because the slur-ries were custom synthesized, there were no intellectual prop-erty challenges to overcome nor any proprietary chemicaladditives. Table 3 summarizes the physiochemical propertiesof the four CMP slurries, including information provided bythe manufacturer (shown in the top row). Acidic c-SiO2 wasprepared in acetic acid while f-SiO2 was in a basic solution ofKOH. The CeO2 slurry was provided without any additives.According to supplier, the cerium oxide is made from a hightemperature (>300 °C) calcination process followed by mill-ing/re-sizing, and based upon our industrial authors themajority of commercially available ceria slurries used in thesilica wafer processing industry use calcined ceria. The Al2O3

slurry was provided in dilute nitric acid. CMP NPs were dis-persed using a common industry method involving a high-energy dispersion machine.32

2.2 Particle sizing and zeta potential analysis

Particle sizing was conducted using Brookhaven ZetaPALS orBl-200SM and Malvern ZS ZEN3600 instruments, differentlaser wavelengths (659, 488, 633 nm), and different scatteringangles (90°, 90°, 173°). Refractive indexes were 1.765 forAl2O3, 2.200 for CeO2, and 1.542 for SiO2. The electrophoreticmobilities (EPMs) of the slurries were measured using eitherthe Malvern Zetasizer Nano ZS ZEN3600 or the Brookhaven


n

f-SiO2 CeO2 Al2O3

Fumed SiO2 CeO2 Al2O3

5% SiO2 1% CeO2 3% Al2O3

<1% KOH none <1% nitric acid10 3–4 4.5–5.0120–140 60–100 80–10050 g Si L−1 9.6 g Ce L−1 29 g Al L−1

4.84 ± 0.03 1.90 ± 0.03 6.77 ± 0.18— — 134.7 ± 0.8 mg NO3

− L−1 BDLa for nitrite38 ± 14 43 ± 16 85 ± 21NDb 39 ± 19 38 ± 16148 ± 5.1 132 ± 0.1 129 ± 1.6(0.11) (0.16) (0.11)144 ± 1.8 79 ± 1.3 119 ± 1.1144 ± 26 60 ± 28 66 ± 23−50 43 55

particle size could not be determined.



Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30


ZetaPALS Analyzer. The EPMs of the slurries were thenconverted into zeta (ζ) potentials using the Smoluchowskiequation.33 Slurries were prepared for both measurementsusing either DI water or 10 mM ionic strength solutions(adjusted with either NaCl or NaHCO3), and the EPM mea-surements were conducted over a broad range of pH condi-tions (3–11).

Nanoparticle Tracking Analysis (NTA) was performedusing a NanoSight LM10 instrument (NanoSight Ltd.,Amesbury, United Kingdom) equipped with a 405 nm (blue)laser source, a temperature-controlled chamber, and a scien-tific CMOS camera (Hamamatsu). A video (30 s) of each sam-ple was collected and analyzed using NTA 2.3 Build 011 soft-ware (NanoSight Ltd.). The concentration (particles per mL)was calculated as the average of three replicates.

Single particle ICP-MS (spICP-MS) is an emerging nano-analysis to size and quantify NPs in liquid matrices.34–36 AnICP-MS instrument (Perkin-Elmer NexION 300q) was placedin time-resolved analysis mode in which the signal wasrecorded every dwell time (integration time of one reading bythe detector) of 10 ms. Thus, the detection of a particle gavea pulse signal. The sample flow rate was 0.6–0.7 mL min−1.Si,29 140Ce, and27Al were used as the analyte isotopes forSiO2, CeO2, and Al2O3 NPs.

2.3 Chemical digestion and analysis of CMP nanoparticles

All NPs were digested using a microwave-assisted system anda suitable digestion mixture. Tetramethylammonium hydrox-ide (4 mL of TMAH, 25%) was used to digest SiO2 NPs sam-ples (11 mL). For CeO2 and Al2O3, 2 mL HF (50%), 2 mL HCl(30%, J. T. Baker), and 6 mL HNO3 (70%) were added to thesample, and the total volume was adjusted to 15–20 mL. Themicrowave was operated as follows: ramping to 150 °C in 15min; next ramping to 180 °C in 15 min; and holding at 180°C for 30 min. Metals were analyzed using an ICP-MS(Thermo X series II ICP-MS).

2.4 Separation of nanoparticles from dissolved ions

Two methods were employed and compared to separate NPsfrom dissolved ions. First, a centrifugal ultrafiltration device(Millipore, Darmstadt, Germany), which combines an ultrafil-tration (UF, 30 kDa nominal molecular weight limit) mem-brane and a centrifuge tube, was adopted as a tool to sepa-rate NPs and the ionic species (liquid phase). Samples in thecentrifugal UF devices (10 mL) were centrifuged at 5000 × gfor 30 min. To demonstrate the effectiveness of the centrifu-gal UF device to separate NPs and dissolved species, a com-mercial SiO2 nanoparticle (PM1040, Nissan Chemical, Hous-ton) and an ionic SiO2 standard solution (HACH, Loveland)were used. Three samples containing: 1) 1000 μg L−1 (as SiO2)ionic standard and 1000 μg L−1 (as SiO2) NPs; 2) 1000 μg L−1

NPs; and 3) 1000 μg L−1 ionic standard were tested in tripli-cate. The recoveries of filtrate and concentrate for all caseswere ≥94%. In the second method, slurries were centrifugedin a two-step procedure to remove NPs and provide


supernatants for analysis. Slurry aliquots (1.5 mL) werecentrifuged at 20 000 × g for 60 min, and 1.2 mL of the super-natant was collected. Subsequently, the supernatant wascentrifuged at 100 000 × g for 60 min.

2.5 Anions and dissolved organic carbon

Acetate was monitored using an Agilent 7890A gas chroma-tography system (Agilent Technologies, Santa Clara, CA, USA)fitted with a Restek Stabilwax-DA column (30 m × 0.35 mm,ID 0.25 μm and a flame ionization detector. Nitrate was ana-lyzed by suppressed conductivity – ion chromatography usinga Dionex IC-3000 system (Sunnyvale, CA, USA) fitted with aDionex IonPac AS11 analytical column (4 mm × 250 mm) andan AG11 guard column (4 mm × 40 mm). The flow rate was 1mL min−1, and run time was 10 min per sample. An isocraticmobile phase containing 30 mM KOH was employed. Thedissolved organic carbon (DOC) was determined using aShimadzu TOC-500A total organic carbon analyzer (ShimadzuScientific Instruments, Columbia, MD, USA).

2.6 Solid state characterization

Scanning electron microscopy systems equipped with anenergy dispersive X-ray microanalysis system (SEM/EDX)(FEG ESEM Philips XL30 with EDAX system) and high-resolution transmission electron microscopy systems (HR-TEM) coupled with energy dispersive X-ray spectroscopy(EDX) (Philips CM200 FEG HR-TEM/STEM) were used. X-raydiffraction was performed using an Agilent-Gemini X-Ray dif-fractometer with a molybdenum source in a Bragg–Brentanoarrangement. All slurries were dried to a constant mass at125 °C prior to analysis. The Fourier transform infraredspectroscopy (FTIR) analysis was performed in an attenuatedtotal reflectance (ATR) spectrophotometer (Varian 600 FT-IR)in the range of 400–4000 cm−1 at a resolution of 1 cm−1.

2.7 CMPs catalytic activity analysis

The catalytic activity of CMPs (c-SiO2 and f-SiO2) using ourColorimetric Assay to Detect Engineered nanoparticles(CADE) technique described elsewhere in detail.37 Briefly,CADE employs a dye, methylene blue (MB), and a reducingagent, sodium borohydride (BH4), to colorimetrically assessthe catalytic activity of nanoparticles in an aqueous media(see ESI† for more information).

2.8. In vitro assays

In vitro assays were conducted using marine bacteriumAliivibrio fischeri (MicroTox® Bioasssay) and adenocarcinomichuman alveolar basal epithelial cells (A549 cell viability,ATCC® CCL-185™). The dye 3-Ĳ4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay kit (Sigma Aldrich)was used to quantitatively evaluate the cell viability of A549cells after exposure to the CMP slurries, and the Lactate dehy-drogenase (LDH) kit (Sigma Aldrich) was used to evaluate themembrane integrity of A549 cells. Proliferation of A549 cells

Environ. Sci.: Nano



Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30


was measured by two methods: determination of cell num-bers by staining nucleic acids with crystal violet dye (CV), ordirect counting of cell numbers with a Coulter counter.Details of these methods are provided in ESI.†

3 Results3.1 Chemical composition of nanoparticles and CMP slurries

The “model slurries” employed in the initial phase of thiswork represent the simplest possible stable dispersion of par-ticles in water. As such, they contrast with the complexity ofcommercial CMP slurries that are formulated with a widevariety of ingredients, including a number of chemicals thatare known to be toxic on their own and surface active andredox active chemicals (Table 2) that are intended to influ-ence particle behavior. Bulk primary metal concentrations inthe as-received slurries ranged from 9.6 to 50 g L−1 andagreed with the manufacturer reported data of 1 to 5%(Table 3). Digested slurries were analyzed for additional ele-ments (Fig. 2) to quantify the presence of impurities, espe-cially elements potentially toxic to cells. Each slurrycontained different ratios of trace elements relative to the pri-mary CMP NP. Fig. 2 presents the concentrations of elementsin the slurry that were detected at concentrations above labo-ratory blanks. Calcium and zinc were detected as impuritiesin all the samples at levels of 10 to 100 mg L−1, which isroughly 1000 times lower than the primary metal elements(Si, Ce, or Al) that were present in the slurry at 9.6 to 50 gL−1. The SiO2-based slurries contained aluminum at 1 to 20

Environ. Sci.: Nano

Fig. 2 Concentrations of elements other than the primary metal (Si, Cconcentration analyzed after a full sample digestion by ICP-MS; shadeddigestion blank control sample. A significant concentration difference betwtence of the corresponding element in the slurry sample.

mg L−1 and titanium, iron, and small amounts of either gold,magnesium, and/or copper at concentrations below 5 mg L−1.The high concentration of potassium in the f-SiO2 slurry isassociated with addition of KOH (Table 1). The Al2O3 slurrycontained less than 1 mg L−1 iron, magnesium, titanium, zir-conium, copper, and chromium. The CeO2 slurry containedthe largest number of detectable elements, including haf-nium, palladium, silver, and gold, which may have been co-occurring elements residual from the mining and separationprocess. Likewise, impurities in the feedstocks for SiO2 andAl2O3 probably cause impurities in the slurries and couldpossibly be used as unique markers for tracing the fate ofCMP NPs in the environment as has been attempted forother types of NPs (e.g., TiO2 NPs from sunscreens into riv-ers38). Analysis of metals was performed at two differentuniversities, and comparable results were obtained.

In order to differentiate elements associated with the CMPparticles from elements dissolved in solution, supernatantsafter high-speed centrifugation or permeates of 30 kDAultrafilters were analyzed (Fig. 3). Both preparation methodsyielded very similar concentrations, thus validating their usein laboratories that may only have access to one method(Fig. 3). In general, the same elements detected in thedigested, as-received slurry were also detectable in the NP-free solutions. In all cases, the concentrations of all detect-able elements in the aqueous aliquots of the slurries were farbelow levels detected in the as-received digested slurry, andmost trace elements were below 1000 μg L−1. This suggeststhat the NPs partially dissolved and released ionic forms ofthese elements. Whereas levels of zinc were high in some


e or Al) in the four CMP slurries. Open bars represent the elementbars represent the detectable concentration of these elements in theeen the slurry sample and blank control sample demonstrates the exis-


Fig. 3 Concentration of elements other than the major metals (Si, Ce or Al) in the liquid phase of four CMP slurries. Open bars represent liquidsample prepared by centrifugal ultrafiltration; solid bar represent liquid sample prepared by high speed centrifugation.


Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30


CMP slurry NPs (Fig. 2), they were much lower in free solu-tion (Fig. 3). The as-received slurries were diluted many foldprior to toxicity testing (discussed in section 3.5), and thepredicted effect of dissolved zinc was below levels of concernfor toxicity. Elevated levels of potassium in the f-SiO2 slurryand supernatant were notable but expected because the slurrywas adjusted to basic pH with KOH (Table 3).

Three of the slurries had low levels of dissolved organiccarbon (1.9 to 6.7 mg L−1), but the c-SiO2 dispersion hadmuch higher DOC (320 mg L−1) because high levels of aceticacid were present (~800 mg L−1) (Table 3). Nitrate wasdetected (135 mg NO3

− L−1) in the Al2O3 slurry that containednitric acid. Both the acetic acid and nitrate were associatedwith pH control agents added to adjust pH to levels where theNPs should be stable in suspension (discussed in section 3.4).

Fig. 4 X-ray diffraction pattern of SiO2, CeO2, and Al2O3 slurries afterdrying.

3.2 Solid-state analysis of nanoparticles

XRD spectra of the NPs in the CMP slurries were obtained tocharacterize their crystalline nature and purity (Fig. 4). Thetwo SiO2 samples gave similar spectra, showing a broad haloin XRD pattern, clearly indicating an amorphous SiO2 struc-ture. The CeO2 slurry shows strong peaks at (111), (200), and(220) for CeO2 that are consistent with literature.39 The Al2O3

slurry shows a strong peak at (111) and weaker peaks at (311)and (400), as observed elsewhere.40

Fig. 5 shows FTIR analysis of the slurries. Broad stretchingaround 3000–3500 cm−1 is attributed to OH stretch from


water, and the peak around 1650 cm−1 is attributed to CCstretching and indicates the presence of organic contami-nants in the slurries. Colloidal and fumed SiO2 showed aband around 1120 cm−1 corresponding to asymmetric

Environ. Sci.: Nano


Fig. 5 FT-IR spectra of c-SiO2 (A), f-SiO2 (B), CeO2 (C), and Al2O3 (D) slurries.


Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30


stretching vibration of Si–O–Si band41 in which the bridgingoxygen atom moves parallel to the Si–Si lines in the oppositedirection to their Si neighbors and a second band around~470 cm−1 corresponding to Si–O rocking vibration where theoxygen atom moves perpendicular to the Si–O–Si plane. FTIRspectra for the other two slurries show Ce–O and Al–Ostretching in the region of 500–750 cm−1.39,40

Differentiating forms of silica is important to the semicon-ductor industry which uses both fumed and colloidal silicafor CMP operations. Fumed silica has been used since CMPprocesses were first developed in the 1980s and provides acomparatively inexpensive and rapid means of planarizingoxide surfaces. However, it is stable only at alkaline pH andgenerally provides a lower quality surface than colloidal sil-ica, which became available in the 1990s. Fumed silica parti-cles are normally multi-fractal, irregularly shaped with sharpedges and surfaces and is produced via high temperaturecombustion of SiCl4 with oxygen, whereas colloidal silica,made via a sol–gel process using either water glass (Na or Ksilicates) or tetramethoxysilane (TMOS) and tetraethoxysilane(TEOS), is available in the form of uniform spherical particlesover a wide range of pH and size distributions and is gener-ally used where a highly smooth surface is required. Despitevery different uses by industry, it can be difficult to differenti-ate f-SiO2 from c-SiO2 using common solid-state characteriza-tion methods. However, literature incorporating nuclear mag-netic resonance analysis suggests that the surface hydroxylconcentration and formation of bi-nuclear surface complexeswith metals in solution reacting with the SiO2 surfaces (i.e.,proximity of surface group) are perhaps more important thanthe morphological structure.42,43

3.3 Shape and size characterization

Table 3 summarizes CMP NP sizing information and showssizing of primary particles via electron microscopy differs

Environ. Sci.: Nano

from measurements of the NPs in dispersions where aggre-gates are present.

Fig. 6 shows imaging and sizing results using electronmicroscopy. The c-SiO2 NPs are nearly spherical, comparedwith more angular and rectangular shaped CeO2 NPs. Thef-SiO2 NPs appeared fused together to an extent not apparentwith SiO2. The angular shape and non-spherical morphologyof three of the four NPs was initially unexpected becauseCMP NPs are routinely described as nearly sphericalpolishing agents. However, the non-spherical nature of someCMP NPs influences their ability to scratch surfaces beingpolished.44 The Al2O3 NPs appeared to be aggregates ofsmaller primary particles having a broad range of diameters.Using both TEM and SEM images, the particle size distribu-tions of the shortest dimension of the NPs were made (Fig. 6and Table 3). The primary particles in the two SiO2 slurrieswere similar (30 to 40 nm),and were also similar to the CeO2

NPs. The broader range of primary particle sizes for the Al2O3

resulted in a larger mean diameter and larger size distribu-tion than the other NPs. Differences between SEM and TEManalysis was low (Table 3), except for the Al2O3 NPs, whichmay have been associated with the modest number of pri-mary particles counted given the large observed distributionin diameters.

The particle size distribution of slurries diluted approxi-mately 10 : 1 with DI water was also analyzed by DLS in sixseparate laboratories, resulting in the following mean diame-ters: 46 ± 10 nm for c-SiO2, 137 ± 10 nm for CeO2, 141 ± 28nm for Al2O3, and 158 ± 16 nm for f-SiO2. Statistically, thereis little difference between the three larger NPs, but in allcases the order of mean sizes was consistent with c-SiO2 hav-ing the smallest diameter and Al2O3, CeO2, and f-SiO2 thelargest and relatively similar diameter. Differences in abso-lute diameters may be attributed to six different operatorsand three instrument models that used different laser wave-lengths (659, 488, 633 nm) and different scattering angles



Fig. 6 TEM images and TEM based particle size distributions for CMP nanoparticles. The size distribution histogram for colloidal silica, ceria, oralumina is obtained by sizing >50 particles under the corresponding TEM images. Fumed silica particles were not sized because of their coalescedstate.


Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30


(90°, 90°, 173°). The hydrodynamic diameter of the CMP NPswas larger compared to the size determined for the primaryparticles by electron microscopy (Table 3). c-SiO2 was anexception, and the particle size determined with these twotechniques was relatively similar. In contrast, the DLS sizingof the f-SiO2 resulted in an average size nearly 2–3 fold higherthan determined for the primary NPs, reflective of the aggre-gated nature of the primary NPs into a dendritic morphology,and the hydrodynamic size of the CeO2 NPs was approxi-mately two-fold higher compared to the primary particle sizedetermined by electron microscopy (Table 3, Fig. 6).

The NTA trends in NP size from smallest to largerst wereconsistent with DLS measurements, with exception of CeO2,which was sized smaller by NTA (79 nm) than by DLS (132nm). NTA analysis also determines particle number concen-trations, which were (# particles/ × 1012 per mL): 4.7 ± 0.2 forc-SiO2, 13.0 ± 0.3 for f-SiO2, 3.9 ± 0.2 for CeO2, and 22.0 ±1.1 for Al2O3.

An additional sizing method, spICP-MS, was alsoemployed to evaluate the particle size distribution in theCMP slurries (Fig. 7). In this method, the cloud of ions gener-ated from the ablation of a single particle is detected as apulse above the background by utilizing short dwell times.Calculated mean diameters from spICP-MS analysis were 144 ±26, 60 ± 28, and 66 ± 23 nm for f-SiO2, CeO2, and Al2O3,respectively. Limitations brought on by molecular interferingions (e.g., dinitrogen) hindered the sizing of the SiO2 NPsbelow 100 nm and biased the diameter toward a larger meansize than actually present in the sample. The average particlesizes determined for f-SiO2 using spICP-MS, DLS, and NTAare very similar (Table 3). The size of c-SiO2 was below cur-rent spICP-MS detection limits for silica, indicating it has


smaller diameter than f-SiO2, which is consistent with DLS,NTA, and SEM/TEM. Advances in micro-second dwell timeICP-MS technology and analysis may be capable of improvingsize resolution for c-SiO2 or other NPs with high backgroundnoise or poor detection resolution.45,46 Mean diameters forAl2O3 and CeO2 by spICP-MS were similar to each other andbetween electron microscopy methods and DLS or NTAresults. The size distributions of NPs based upon spICP-MSlose some resolution relative to background below ~25 nmfor Al2O3 and CeO2, which biases the mean to slightly largersizes. The size ranges near the peak of the Gaussian distribu-tions (Fig. 7) are in closer agreement with SEM/TEM results.

Few studies compare size measurements across as manytechniques on the same number of different, well-dispersedNPs as present in these CMP slurries. Fig. 8 compares meandiameters reported by the manufacturer to those measuredby the various techniques reported herein. Within any singleevaluation technique, the size trends from smallest to largestare generally consistent, but significant differences in abso-lute size vary dramatically. This points to both the bias andassumptions of each technique (e.g., hydrated radius, min-eral structure, density). Whereas DLS and NTA detect thehydrodynamic size, spICP-MS and SEM/TEM are notimpacted by the hydrated nature of NPs and thus hydrody-namic state partially accounts for observed differences inmean diameters reported in Table 3 and shown in Fig. 8.

3.4 Stability of nanoparticles in different matrices

Surface charge is a critical factor influencing the stability(i.e., aggregation potential) of NP dispersions. Zeta potentialmeasurements of the CMP slurries (Table 3), diluted with

Environ. Sci.: Nano


Fig. 7 Size distributions of Al2O3 (A), CeO2, (B), and f-SiO2 (C) CMPslurries by single particle ICP-MS.

Fig. 8 Comparison of the average particle size values determined inthis study for the various CMP slurries using different techniques withvalues reported by the slurry manufacturer.

Fig. 9 Dynamic light scattering (bars) and zeta potential analysis(squares) of CMP slurries (ambient slurry pH). Polydispersity index (Pdi)values are shown for DLS data.


Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30


ultrapure water to concentrations suitable for zeta potentialanalysis, resulted in highly positively charged NPs (>+40 mV)for CeO2 and Al2O3 or very negatively charged (<−20 mV) forthe two SiO2 NPs. Zeta potentials this far from zero indicatevery stable NP suspensions. The manufacturer claims thatthe NPs in the CMP slurries would remain stable for at leasttwo years if stored in the dark at room temperature. Fig. 9shows both the zeta potentials and DLS measurementsobtained at the same time. Separate measurements performed

Environ. Sci.: Nano

six months later showed no discernible differences in zetapotential or DLS.

Fig. 10 shows the electrophoretic mobilities (EPMs) of fourCMP NPs as a function of pH. At pH higher than 2, the c-SiO2 and f-SiO2 were negatively charged, and the magnitudeof surface charge generally increased with increasing pH. Thef-SiO2 was almost neutral at pH 2, which is consistent withthe reported pH of zero point of charge (pHZPC) of 2.0 forSiO2.

47 The pHZPC of c-SiO2 was lower than 2. CeO2 and Al2O3

colloids were both positively charged at pH lower than 7.0,and their surface charges reversed when pH was elevated to11. By extrapolation, the pHZPC for CeO2 and Al2O3 colloidswere determined to be approximately 8 and 10, respectively,which are consistent with the reported pHZPC for CeO2 (8.1)

48

and for Al2O3 (8.2–10).49

3.5 Surface reactivity of silica nanomaterials

The CADE uses the reduction rate of methylene blue by boro-hydride, which depends directly on the catalytic activity ofnanoparticles in CMP. Results in ESI† showed a statisticaldifferences at the 95% confidence interval between the



Fig. 10 EPMs of four CMP nanoparticles in 1 mM NaCl solutionsprepared at different pH conditions. The error bars represent thestandard deviation of triplicates.


Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30


catalytic reactivity in a control from the catalytic activityinduced by f- or c-SiO2 present at 100 ppm. The f-SiO2 nano-particles were also more catalytically active than the c-SiO2

nanoparticles from CMP slurries. At fixed CMP mass concen-tration, the surface charge of nanoparticles may have aninfluence on the catalytic reactivity of CMPs. We believe nega-tively charged particles, c-SiO2 and f-SiO2, with a surfacecharge of −21 and −50 mV, may electrostatically repel BH4

molecules to the surface of the particle, which then inhibitthe reduction of MB, resulting in high β values. According toAzad et al. when BH4 absorbs to the surface of nanoparticles,it creates a negatively charged layer that attracts cationicorganic dyes, such as CADE.50 This electrostatic attraction orrepulsion between particle surfaces and the reducing agentsincrease or decrease the reduction rate of MB.

3.6. In vitro toxicity

The potential toxicity of model CMP slurries to bacteria A.fischeri was assessed using the Microtox® assay. Microtox®assay is a highly sensitive test that is widely used to monitorthe toxicity of effluents and evaluate the toxic effects of chem-ical compounds.51 The results of the test have been shown tocorrelate well with toxicity values for fish, crustaceans, and


Table 4 The effects of slurries on the proliferation, viability, or membrane inseveral conditions as noted

Assay

IC-50 (mg mL−1)

c-SiO2

Bioluminescence of A. fischeri NDa

Proliferation of A549 cells 3.8 ± 1.3Viability of A549 cells 1.2 ± 0.2Integrity of A549 cells 4.6 ± 0.2

a 37.6% inhibition at the highest concentration tested (1.3 mg mL−1). b

inhibition at the highest concentration tested (0.7 mg mL−1). d Only 28.4%than 10% inhibition at highest concentration tested (2 mg mL−1). f 0% ihighest concentration tested (0.52 mg mL−1). h No effect at highest concen

algae for a wide range of organic and inorganic chemicals.The results in Table 4 indicate that the CMP NPs were not oronly mildly inhibitory to the metabolic activity of A. fischeri athigh concentrations ranging from 0.7 to 1.3 mg mL−1,depending on the assay. No effect was observed when cellswere exposed to f-SiO2 and CeO2 NPs. This observation issimilar to the CADE analysis shows that f-SiO2 posses highercatalytic reactivity than c-SiO2, where surface redox reactivityin nanoparticles is a key emerging property related to poten-tial cellular toxicity. Exposure to a concentration of 1.3 mgmL−1 of the c-SiO2 and Al2O3 suspensions resulted in 37.6and 28.4% inhibition, respectively.

For the eukaryote toxicity tests with A549 cells, the IC-50values for proliferation and plasma membrane integrity werein the range 1 to 4 mg mL−1 for both c-SiO2 and f-SiO2. Theviability tests using MTT resulted in IC-50 values for bothforms of SiO2 in the range of 1 to 2 mg mL−1. The CeO2 andAl2O3 had negligible effect in any of the A549 cell assays atthe highest tested slurry concentrations. In both prokaryoticand eukaryotic cell assays, the CMP NPs were unstable andaggregated when in biological medium. This was especiallypronounced for CeO2 and Al2O3.

With the prokaryote A. fischeri, none of the CMP metaloxides NPs resulted in as much as a 50% decrease in biolu-minescence in the Microtox® assay after a 30 min exposure.Thus, CMP NPs do not appear to be very toxic to A. fischeri.However, there was a statistically significant reduction in bio-luminescence from bacteria exposed to c-SiO2 and Al2O3 at1.3 mg mL−1 for 30 min (37.6 and 28.4% inhibition, respec-tively). It is therefore likely that there could be more signifi-cant adverse effects at higher doses and longer exposuretimes. Literature results confirm the low toxicity of silica,ceria, and alumina NPs towards A. fischeri. No appreciableeffects were observed in Microtox® assays supplemented withnominal concentrations of CeO2 and Al2O3 up to 0.1 mgmL−1.52,53 Similarly, amorphous SiO2 NPs of different diame-ters (50 and 100 nm) were not toxic to A. fischeri at a concen-tration as high as 1 mg mL−1.

There are many studies in the literature on the toxicity ofSiO2 NPs towards various cultured mammalian cells with awide range of toxicity values reported. Factors that influencesilica NP toxicity include cell type, differences in the physical

Environ. Sci.: Nano

tegrity of model organisms. IC-50 levels were not determined (ND) under

f-SiO2 CeO2 Al2O3

NDb NDc NDd

3.6 ± 0.2 NDe ND f

1.5 ± 0.2 NDg NDh

3.1 ± 0.2 NDg NDh

No effect at highest concentration tested (1.1 mg mL−1). c Only 4.3%inhibition at the highest concentration tested (1.3 mg mL−1). e Less

nhibition at highest concentration tested (6 mg mL−1). g No effect attration tested (2.0 mg mL−1).



Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30


and surface properties of the NPs, and the type of toxicityassay.54–57 The IC-50 values reported here for the variousassays with A549 cells (in the range of 1 to 4 mg mL−1 for a24 hour exposure) are consistent with data in the literature.For example, Lin et al. (2006) observed at most a 17% reduc-tion in viability after exposing A549 cells to 15 nm SiO2 NPsfor 24 hours, and at 72 h exposure the viability was reducedby about half.58 Yu et al. (2011) reported no effect of c-SiO2

colloidal silica on A549 cells in a 24 h exposure up to thehighest concentration tested of 0.5 mg mL−1.55 Yu et al.(2011) also reported that a murine macrophage cell line(RAW 264.7) responded to c-SiO2 with an IC-50 of ~200 μgmL−1, emphasizing that different cell types may respond dif-ferently to SiO2. Using the MTT assay and an impedance-based assay, Otero-Gonzalez et al. (2012) found IC-50 valuesin the range of 0.17–0.23 mg mL−1 with human bronchial epi-thelial cells (16HBE14o-) exposed to amorphous SiO2 for 48h.59 Zhang et al. (2012) reported that f-SiO2 prepared by ahigh temperature process was significantly more toxic than c-SiO2 prepared by a low temperature process.56 They attrib-uted the result to different surface chemistries generated bythe different synthesis methods. Nevertheless, we observedlittle difference in the toxicity of colloidal and fumed SiO2 ina variety of assays with A549 cells.

The biological effects of CeO2 have been enigmaticbecause of reports that it is both an oxidant capable of gener-ating reactive oxygen species (ROS) and an anti-oxidant capa-ble of protecting cells from oxidants by consuming ROS.60

The different oxidation properties are attributed to the pres-ence of both CeĲIII) and CeĲIV) in NPs. Interpretation of the lit-erature has been confusing because the same type of NPcould seemingly be oxidizing or anti-oxidizing, toxic or non-toxic. Recent work from the Baer group has brought insightto the problem.61 CeO2 is usually made by one of threemethods: high temperature heating (>300 °C), heated in asolvent (<100 °C), or prepared at room temperature. Karakotiet al. (2012) grouped biological responses to CeO2 in the liter-ature according to synthesis method and noticed that most(but not all) of the CeO2 made by the high temperature orhigh temperature in solvent methods were either pro-oxidative or had both oxidative and anti-oxidative propertiesas reported in the various assays used in papers.61 CeO2

made at room temperature was, with one exception, anti-oxi-dative. This analysis is another example demonstrating thatthe method of nanoparticle synthesis can have a large influ-ence on its properties and biological effects. However, theCMP CeO2 NPs we examined had no measureable toxicity inour A549 assays up to the highest concentrations tested.

As with many nanomaterials, there is a diversity of litera-ture and opinions on whether Al2O3 NPs are toxic. For exam-ple, nano-Al2O3 at 100 to 1000 mg L−1 was toxic to culturedhuman brain microvascular endothelial cells and alsoreduced tight junctions in brain endothelial cells in cerebralvasculature after infusion into rats.62 Al2O3 NPs were at leastmildly toxic to osteoblast-like UMR 106 cells using assays ofmitochondrial and lysosome function,63 and they were

Environ. Sci.: Nano

cytotoxic and genotoxic with CHO-K1 cells.64 Otero-Gonzalezet al. (2012) observed that exposing human bronchial epithe-lial cells (16HBE14o-) to 1 mg mL−1 of nano-Al2O3 (<50 nm)for 48 h resulted in 50% inhibition in the MTT assay.59 Inthe same study, the IC-50 value determined for nano-Al2O3

using an impedance-based real-time cell analyzer was 0.3 mgmL−1. In recent work, Al2O3 NPs were reported toxic to plantcells in culture and toxic to fresh water algae.65,66 In contrast,Al2O3 NPs had no measurable toxicity with mouse L929 cellsand normal human fibroblasts.67 Moreover, even at high con-centrations, nano-Al2O3 did not affect the phagocytic activityof rat alveolar macrophages.68 Our results with Al2O3 CMPslurry failed to find any toxic response with A549 cells inthree different types of assays.

Toxicity data developed herein for f-SiO2 and c-SiO2, Al2O3,and CeO2 were integrated with human and other organisms.These data are summarized along with corresponding IC-50and the half maximal effective concentration (EC-50) datareported in the literature in Fig. 11 and 12. The IC/EC-50 con-centrations for silica are higher than for alumina, which inturn is higher than for ceria. This is good because it is inreverse order of their prevalence and use in most semicon-ductor fabrication facilities (fabs). Silica, which is used inmost abundance, generally has the highest IC/EC-50 valuesand is therefore least toxic. It is evident that for a given mate-rial type, there is considerable variation among the IC/EC-50data, depending upon the particular test type, cell line or testorganism, test duration, and test endpoint.

3.7. Impact of findings on semiconductor industry

Another key aspect of this work has been a collaborativeeffort between universities and the semiconductor industryto determine the conditions and concentration ranges neces-sary for the ENP analytical methods. Using a materials bal-ance from one fab and drawing from reported concentrationdata in the literature (see ESI†), SiO2 concentrations in theeffluent wastewater that comes directly from CMP operations,but prior to treatment, might typically be on the order of1000 mg L−1, alumina concentrations on the order of 10 to100 mg L−1, and cerium concentrations on the order of 1 mgL−1 or less. Cerium is less prevalently used than either silicaor alumina in CMP operations, and none of the referencedliterature reports listed cerium concentration in wastewater.According to materials usage records at one fab, silica, alu-mina, and ceria may be used in proportions of roughly 90 : 9 :1. However, slurry formulations are both proprietary anddynamic.

There are also significant differences between fabs in themanner that CMP wastewater is routed through the fab andtreated. Some, like the fab described in ESI,† employ aphysical-chemical wastewater treatment system for the com-posite CMP water, followed by dilution and equalizationwith other on-site wastewater flows before treatment by anon-site biological wastewater system. For this particular fab,the waste streams that represent potential gateways for



Fig. 11 Ecotoxicity findings determined in bioassays with freshwater crustaceans (Cladocera) (A) and algae (B) for three major classes of metal-based nanoparticles used in CMP slurries. Note: D. similis = Daphnia similis; D. magna = Daphnia magna; D. pulex = Daphnia pulex; C. dubia =Cerodaphnia dubia; P. subcapitata = Pseudokirchneriella subcapitata. Legends: Data for CeO2 (■), Al2O3 (□), and SiO2 ( ).


Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30


releasing Al2O3, CeO2, and SiO2 ENPs to the environment arethe solids concentrate produced by the CMP wastewatertreatment process and/or discharges from municipal waste-water treatment plants that receive sewer discharges fromthe fab. In this fab, the on-site industrial CMP wastewatertreatment process produces a filter cake with a 52% watercontent and measured SiO2 and Al2O3 concentrations of 77and 8 wt%, respectively. Although this particular filter cakewas recycled for the production of cement, it demonstratesthe importance of evaluating the fate and long term stabilityof the solids concentrate waste streams from on-site CMPwastewater treatment processes as the ENP composition ofwaste sludges may range from less than 1 wt% to greaterthan 75 wt%. The treated effluent from municipal biologicalwastewater treatments is typically discharged to surfacewaters, and the waste sewage sludges or biosolds disposed asland soil amendments (~60%), landfills (~20%), or inciner-ated with ash being landfilled (~20%).69 SEM analysis ofENPs at the influent and effluent of on-site chemical waste-water treatment processes at a fab (see SI) indicate the pres-ence of SiO2 ENPs. While both locations have ENPs


approximately 70 nm in size, the effluent SiO2 NPs appear tohave slightly different surface morphologies. Overall, theresults and analytical methods herein with the four CMPslurries can be applied to effluent streams in fabs to deter-mine ENP behavior and fate. Ideally, speciated and size frac-tionated ENP concentration data are available for the influ-ent, effluent, and waste biosolids, such that a materialsbalance account can be made across wastewater treatmentfacilities.

Analytical method development is relevant not only forassessing the fate of ENPs, but also for determining theirimpact on biological processes (industrial on-site or off-sitemunicipal facilities). For instance, Zheng et al. (2012)reported 35% inhibition of N removal efficiency at 50 mg L−1

of 80 nm SiO2.70 Others observed a 37% inhibition of O2

uptake rate for 50 mg L−1 of 50 nm SiO2.71 Details for a par-

ticular fab with on-site chemical and biological treatment areprovided in ESI.† The mass balance indicates 2 mg L−1 ofSiO2 influent to the on-site treatment facility and 0.2 mg L−1

in the effluent from the biological wastewater treatment step.Thus, the biological treatment is important in reducing ENP

Environ. Sci.: Nano


Fig. 12 Human cytotoxicity findings for three major classes of metal-based nanoparticles used in CMP slurries. Legends: Data for CeO2 (■), Al2O3

(□), and SiO2 ( ). The asterisk (*) indicates data obtained in this study.76–85


Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30


levels. If a fab doesn't pre-treat waste streams in the fab orhave extensive dilution with other on-site wastewater flows,its potential ENP influent concentrations to the biologicaltreatment process could be several tens of mg L−1 or greater,which may inhibit the performance of the biological wastewa-ter treatment process.

4 Conclusions

A principal objective of this work was to develop a commonset of ENP samples that could be shared between differentlaboratories and used to develop validated analytical methodsfor characterizing CMP slurries and their associated wastestreams. The ENPs in the “model slurries” are representativeof those used in commercial CMP slurries, but they lack theadditives that are commonly employed in commercial slur-ries and thus are intended as only a first step in analyticalmethod development. Moreover, the four test slurries aremodels of the raw unused slurries prior to contact withwafers in CMP operations, and so likewise these model slur-ries serve as only a first step towards our ultimate goal ofusing validated methods to characterize the behavior and fateof alumina, ceria, and silica ENPs in real fab wastewaters andeffluent discharges. Towards these goals we have developedmetal oxide digestion methods that are appropriate for

Environ. Sci.: Nano

determining f-SiO2 and c-SiO2, Al2O3, and CeO2 concentra-tions. We have demonstrated two alternative methods, a cen-trifuge and an ultrafiltration method, for distinguishingbetween dissolved and ENP concentrations. We have demon-strated the applicability of four different particle size distri-bution methods and highlighted their relative differences.

Cytotoxicity using prokaryotic and eukaryotic toxicityassays showed a low inhibitory potential of the four ENPs inthe CMP slurries. The concentrations of ENPs in all four slur-ries causing 50% inhibition (IC-50) were greater than 1 mgmL−1 based upon in vitro assays using bioluminescence ofthe bacterium Aliivibrio fischeri and proliferation and viabilityor integrity of human cells (adenocarcinomic human alveolarbasal epithelial cell line A549). Based on these IC-50 values,none of these model slurry dispersions showed acute toxicityin assays performed. In contrast with some previous reports,f-SiO2 was not significantly more toxic than c-SiO2 in theCMP slurries, despite having different sizes and morphol-ogies but similar characterization by FTIR and XRD. Addi-tional characterization techniques that probe surface reactiv-ity or number and proximity of surface hydroxyl groups areneeded to improve our understanding of discrepancies in theliterature. Otherwise, the levels of toxicity of the ENPstowards human cells or model aquatic organisms were simi-lar to literature reports and suggest monitoring at mg L−1




Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30


levels would be adequate to meet IC-50 levels. IC-50 values(>1000 mg L−1) are much higher than ENP concentrationsexpected in semiconductor effluents,. It should be noted thatthe general practice in the CMP industry is to dilute theslurry waste stream so actual abrasive concentrations are typi-cally orders of magnitude smaller than 1 mg mL−1, which islower than IC-50 levels.

Among the most interesting observations was the abilityof the CMP slurry manufacturer to produce 1 to >5 wt%ENPs that have remained dispersed in solution for manymonths (i.e., stable; no aggregation). The special-order slur-ries did not contain organic surfactants, and we demon-strated through comprehensive analysis of the solution thatthere were no added stabilizers other than pH adjustment.The slurry manufacturers were able to disperse the ENPsusing mechanical, sound, or other non-chemical means andthen maintain a very highly negative (less than −20 mV for c-and f-SiO2) or very highly positive (greater than +40 mV forAl2O3 and CeO2) zeta potential through pH adjustment.

The size, morphology, and composition of the ENPs in theCMP slurries differed. Size measurements by TEM, SEM, andspICP-MS agreed well and were smaller than measurementsby DLS and NTA, which accounted for the hydrodynamicinfluence of the nanoparticles. There was excellent agreementamong multiple laboratories performing DLS measurementson the well-dispersed ENPs. f-SiO2 ENPs contained small pri-mary particles agglomerated together into dendritic struc-tures whereas the c-SiO2 ENPs were present as small and usu-ally singular (non-agglomerated) particles, indicating that thesynthesis method impacts the morphology more than struc-tural properties measured by FTIR, XRD, or XPS. CeO2 ENPswere cubic shaped and generally not-agglomerated whereasAl2O3 nanoparticles contained a wide range of primary parti-cle sizes and were agglomerated together. Elemental analysisof the ENPs (Fig. 2) revealed the presence of trace constitu-ents that, while representing low weight percentages of thenanoparticles, might influence their reactivity and or abilityto be traced in the environment. Such elemental data has notbeen reported for other nanoparticles, and the presence ofsome metals may be related to the purity of silica, ceria, oralumina used in bulk by the CMP slurry manufacturer, com-pared against high grade purity levels typically used in labo-ratory studies that synthesize nanoparticles for specificresearch applications. Unrelated observations, yet similarconclusions, have been reported when yttrium and othertrace metals were reported in carbon nanotubes.72 Additionalresearch is needed to understand the implications of differ-ences in stock reagent purity on nanoparticle properties asproduction of ENPs scales up.

Acknowledgements

This work was supported by the Semiconductor Research Cor-poration (SRC) Engineering Research Center for Environmen-tally Benign Semiconductor Manufacturing and the Universityof Arizona Water Sustainability Program. Gratitude is extended


to Cabot Inc. for preparing the CMP slurries used in thisstudy, which are now available through Farhang Shadman(Univ of Arizona) for other research groups to apply in EHSor other nano-related studies. This work was partially fundedby the USEPA (grant number RD83558001).

References

1 A. A. Keller and A. Lazareva, Predicted releases of engineered
nanomaterials:From global to regional to local, Environ. Sci.Technol. Lett., 2014, 1(1), 65–70.
2 A. Kroll, C. Dierker, C. Rommel, D. Hahn, W. Wohlleben, C.
Schulze-Isfort, C. Goebbert, M. Voetz, F. Hardinghaus and J.Schnekenburger, Cytotoxicity screening of 23 engineerednanomaterials using a test matrix of ten cell lines and threedifferent assays, Part. Fibre Toxicol., 2011, 8.
3 A. D. Maynard, D. B. Warheit and M. A. Philbert, The New
Toxicology of Sophisticated Materials: Nanotoxicology andBeyond, Toxicol. Sci., 2011, 120, S109–S129.
4 A. E. Nel, E. Nasser, H. Godwin, D. Avery, T. Bahadori, L.
Bergeson, E. Beryt, J. C. Bonner, D. Boverhof, J. Carter, V.Castranova, J. R. DeShazo, S. M. Hussain, A. B. Kane, F.Klaessig, E. Kuempel, M. Lafranconi, R. Landsiedel, T.Malloy, M. B. Miller, J. Morris, K. Moss, G. Oberdorster, K.Pinkerton, R. C. Pleus, J. A. Shatkin, R. Thomas, T.Tolaymat, A. Wang and J. Wong, A Multi-Stakeholder Per-spective on the Use of Alternative Test Strategies for Nano-material Safety Assessment, ACS Nano, 2013, 7(8),6422–6433.
5 G. Oberdorster, Safety assessment for nanotechnology and
nanomedicine: concepts of nanotoxicology, J. Intern. Med.,2010, 267(1), 89–105.
6 R. D. Handy, G. Cornelis, T. Fernandes, O. Tsyusko, A.
Decho, T. Sabo-Attwood, C. Metcalfe, J. A. Steevens, S. J.Klaine, A. A. Koelmans and N. Horne, Ecotoxicity testmethods for engineered nanomaterials: Practical experiencesand recommendations from the bench, Environ. Toxicol.Chem., 2012, 31(1), 15–31.
7 F. von der Kammer, P. L. Ferguson, P. A. Holden, A. Masion,
K. R. Rogers, S. J. Klaine, A. A. Koelmans, N. Horne andJ. M. Unrine, Analysis of engineered nanomaterials incomplex matrices (environment and biota): Generalconsiderations and conceptual case studies, Environ. Toxicol.Chem., 2012, 31(1), 32–49.
8 H. P. Jarvie, H. Al-Obaidi, S. M. King, M. J. Bowes, M. J.
Lawrence, A. F. Drake, M. A. Green and P. J. Dobson, Fate ofSilica Nanoparticles in Simulated Primary WastewaterTreatment, Environ. Sci. Technol., 2009, 43(22), 8622–8628.
9 L. K. Limbach, R. Bereiter, E. Mueller, R. Krebs, R. Gaelli
and W. J. Stark, Removal of oxide nanoparticles in a modelwastewater treatment plant: Influence of agglomeration andsurfactants on clearing efficiency, Environ. Sci. Technol.,2008, 42(15), 5828–5833.
10 F. Gomez-Rivera, J. A. Field, D. Brown and R. Sierra-Alvarez,
Fate of cerium dioxide (CeO2) nanoparticles in municipal
Environ. Sci.: Nano



Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30


wastewater during activated sludge treatment, Bioresour.Technol., 2012, 108, 300–304.

11 M. A. Kiser, P. Westerhoff, T. Benn, Y. Wang, J. Perez-Rivera
and K. Hristovski, Titanium Nanomaterial Removal andRelease from Wastewater Treatment Plants, Environ. Sci.Technol., 2009, 43(17), 6757–6763.
12 P. Westerhoff, et al., Occurrence and Removal of Titanium
at Full Scale Wastewater Treatment Plants: Implications forTiO2 Nanomaterials, J. Environ. Monit., 2011, 13(5),1195–1203.
13 L. E. Barton, M. Auffan, M. Bertrand, M. Barakat, C.
Santaella, A. Masion, D. Borschneck, L. Olivi, N. Roche,M. R. Wiesner and J.-Y. Bottero, Transformation of Pristineand Citrate-Functionalized CeO2 Nanoparticles in aLaboratory-Scale Activated Sludge Reactor, Environ. Sci.Technol., 2014, 48(13), 7289–7296.
14 J. Rottman, F. Shadman and R. Sierra-Alvarez, Interactions
of inorganic oxide nanoparticles with sewage biosolids,Water Sci. Technol., 2012, 66(9), 1821–1827.
15 M. R. Wiesner, G. V. Lowry, E. Casman, P. M. Bertsch, C. W.
Matson, R. T. Di Giulio, J. Liu and M. F. Hochella Jr.,Meditations on the Ubiquity and Mutability of Nano-SizedMaterials in the Environment, ACS Nano, 2011, 5(11),8466–8470.
16 C. F. Conrad, G. A. Icopini, H. Yasuhara, J. Z. Bandstra, S. L.
Brantley and P. J. Heaney, Modeling the kinetics of silicananocolloid formation and precipitation in geologicallyrelevant aqueous solutions, Geochim. Cosmochim. Acta,2007, 71(3), 531–542.
17 M. Krishnan, J. W. Nalaskowski and L. M. Cook, Chemical
mechanical planarization: slurry chemistry, materials, andmechanisms, Chem. Rev., 2010, 110(1), 178–204.
18 P. B. Zantye, A. Kumar and A. K. Sikder, Chemical
mechanical planarization for microelectronics applications,Mater. Sci. Eng., R, 2004, 45(3–6), 89–220.
19 R. K. Singh and R. Bajaj, Advances in chemical-mechanical
planarization, MRS Bull., 2002, 27(10), 743–751.
20 X. Feng, D. C. Sayle, Z. L. Wang, M. S. Paras, B. Santora,
A. C. Sutorik, T. X. Sayle, Y. Yang, Y. Ding and X. Wang,Converting ceria polyhedral nanoparticles into single-crystalnanospheres, Science, 2006, 312(5779), 1504–1508.
21 K. H. Brown, D. A. Grose, R. C. Lange, T. H. Ning and P. A.
Totta, Advancing the state of the art in high-performancelogicandarraytechnology, IBMJ.Res.Dev.,1992,36(5),821–828.
22 M. Krishnan, J. W. Nalaskowski and L. M. Cook, Chemical
mechanical planarization: slurry chemistry, materials, andmechanisms, Chem. Rev., 2009, 110(1), 178–204.
23 P. B. Zantye, A. Kumar and A. Sikder, Chemical mechanical
planarization for microelectronics applications, Mater. Sci.Eng., R, 2004, 45(3), 89–220.
24 Y. Liu, K. Zhang, F. Wang and W. Di, Investigation on the
final polishing slurry and technique of silicon substrate inULSI, Microelectron. Eng., 2003, 66(1), 438–444.
25 R. K. Willardson, E. R. Weber, S. M. H. Li and R. M. Miller,
Chemical mechanical polishing in silicon processing, Academicpress:, 1999, vol. 63.
Environ. Sci.: Nano

26 R. K. Iler, The chemistry of silica: solubility, polymerization,
colloid and surface properties, and biochemistry, 1979.
27 W. Stöber, A. Fink and E. Bohn, Controlled growth of
monodisperse silica spheres in the micron size range,J. Colloid Interface Sci., 1968, 26(1), 62–69.
28 Z. L. Wang and X. Feng, Polyhedral shapes of CeO2
nanoparticles, J. Phys. Chem. B, 2003, 107(49), 13563–13566.
29 D. J. Schroeder, K. J. Moeggenborg, H. Chou, J. P.
Chamberlain, J. D. Hawkins and P. Carter, CMPmethod utilizingamphiphilic nonionic surfactants, In Google Patents, 2005.
30 C. M. Coetsier, F. Testa, E. Carretier, M. Ennahali, B.
Laborie, C. Mouton-arnaud, O. Fluchere and P. Moulin,Static dissolution rate of tungsten film versus chemicaladjustments of a reused slurry for chemical mechanicalpolishing, Appl. Surf. Sci., 2011, 257(14), 6163–6170.
31 F. Testa, C. Coetsier, E. Carretier, M. Ennahali, B. Laborie,
C. Serafino, F. Bulgarelli and P. Moulin, Retreatment ofsilicon slurry by membrane processes, J. Hazard. Mater.,2011, 192(2), 440–450.
32 X. L. Song, N. Jiang, Y. K. Li, D. Y. Xu and G. Z. Qiu,
Synthesis of CeO2-coated SiO2 nanoparticle and dispersionstability of its suspension, Mater. Chem. Phys., 2008, 110(1),128–135.
33 M. Elimelech, J. Gregory, X. Jia and R. A. Williams, Particle
Deposition and Aggregation - Measurement, Modelling and Sim-ulation, In Butterworth-Heinemann, Oxford: England, 1995.
34 C. Degueldre, P. Y. Favarger and S. Wold, Gold colloid
analysis by inductively coupled plasma-mass spectrometry ina single particle mode, Anal. Chim. Acta, 2006, 555(2), 263–268.
35 H. E. Pace, N. J. Rogers, C. Jarolimek, V. A. Coleman, C. P.
Higgins and J. F. Ranville, Determining Transport Efficiencyfor the Purpose of Counting and Sizing Nanoparticles viaSingle Particle Inductively Coupled Plasma MassSpectrometry, Anal. Chem., 2011, 83(24), 9361–9369.
36 H. E. Pace, N. J. Rogers, C. Jarolimek, V. A. Coleman, E. P.
Gray, C. P. Higgins and J. F. Ranville, Single particleinductively coupled plasma-mass spectrometry: a perfor-mance evaluation and method comparison in the determina-tion of nanoparticle size, Environ. Sci. Technol., 2012, 46(22),12272–12280.
37 C. Corredor, M. Borysiak, J. Wolfer, P. Westerhoff and J.
Posner, Colorimetric Detection of Catalytic Reactivity ofNanoparticles in Complex Matrices, Environ. Sci. Technol.,2015, 49(6), 3611–3618.
38 A. P. Gondikas, F. von der Kammer, R. B. Reed, S. Wagner,
J. F. Ranville and T. Hofmann, Release of TiO2Nanoparticles from Sunscreens into Surface Waters: A One-Year Survey at the Old Danube Recreational Lake, Environ.Sci. Technol., 2014, 48(10), 5415–5422.
39 H. Gu and M. D. Soucek, Preparation and characterization of
monodisperse cerium oxide nanoparticles in hydrocarbonsolvents, Chem. Mater., 2007, 19(5), 1103–1110.
40 K. M. Parida, A. C. Pradhan, J. Das and N. Sahu, Synthesis
and characterization of nano-sized porous gamma-aluminaby control precipitation method, Mater. Chem. Phys.,2009, 113(1), 244–248.



Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30


41 P.-Y. Hsu, J.-J. Lin, B.-W. Lai, Y.-L. Wu, C.-F. Yang and S.-S.
Lin, FTIR Characterizations of the Gamma-Ray-Irradiated Sil-ica Nanoparticles/γ-APTES Nanocomposite with UVAnnealing, in Intelligent Technologies and Engineering Sys-tems, ed. J. Juang and Y.-C. Huang, Springer New York, 2013,vol. 234, pp. 893–899.
42 R. Mackay, H. Zhang, Q. Wu and Y. Z. Li, NMR investigation
of concentrated alumina and silica slurries, Colloids Surf., A,2004, 250(1–3), 343–348.
43 W. H. Kuan and C. Y. Hu, Chemical evidences for the
optimal coagulant dosage and pH adjustment of silicaremoval from chemical mechanical polishing (CMP)wastewater, Colloids Surf., A, 2009, 342(1–3), 1–7.
44 E. Matijevic and S. V. Babu, Colloid aspects of chemical-
mechanical planarization, J. Colloid Interface Sci.,2008, 320(1), 219–237.
45 M. D. Montano, H. R. Badiei, S. Bazargan and J. F. Ranville,
Improvements in the detection and characterization ofengineered nanoparticles using spICP-MS with microseconddwell times, Environ. Sci.: Nano, 2014, 1(4), 338–346.
46 X. Y. Bi, S. Lee, J. F. Ranville, P. Sattigeri, A. Spanias,
P. Herckes and P. Westerhoff, Quantitative resolution ofnanoparticle sizes using single particle inductivelycoupled plasma mass spectrometry with the K-meansclustering algorithm, J. Anal. At. Spectrom., 2014, 29(9),1630–1639.
47 W. Stumm, Chemistry of the Solid-Water Interface, New York,
Wiley, 1992, pp. 269–286.
48 L. A. Defaria and S. Trasatti, THE POINT OF ZERO CHARGE
OF CEO2, J. Colloid Interface Sci., 1994, 167(2), 352–357.
49 M. Kosmulski, The pH-dependent surface charging and the
points of zero charge, J. Colloid Interface Sci., 2002, 253(1),77–87.
50 U. P. Azad, V. Ganesan and M. Pal, Catalytic reduction of
organic dyes at gold nanoparticles impregnated silicamaterials: influence of functional groups and surfactants,J. Nanopart. Res., 2011, 13(9), 3951–3959.
51 B. Johnson, Microtox acute toxicity test, in Freshwater
Toxicity Investigations, Part 2, ed. C. Blaise and J. F. FerardSpringer: Dordrecht, The Netherlands, 2005, pp. 69–105.
52 R. Doshi, W. Braida, C. Christodoulatos, M. Wazne and G.
O'Connor, Nano-aluminum: Transport through sandcolumns and environmental effects on plants and soilcommunities, Environ. Res., 2008, 106(3), 296–303.
53 I. Velzeboer, A. J. Hendriks, A. M. J. Ragas and D. Van de
Meent, Aquatic ecotoxicity tests of some nanomaterials,Environ. Toxicol. Chem., 2008, 27(9), 1942–1947.
54 F. Schrurs and D. Lison, Focusing the research effort, Nat.
Nanotechnol., 2012, 7(9), 546–548.
55 T. Yu, A. Malugin and H. Ghandehari, Impact of Silica
Nanoparticle Design on Cellular Toxicity and HemolyticActivity, ACS Nano, 2011, 5(7), 5717–5728.
56 H. Zhang, D. R. Dunphy, X. Jiang, H. Meng, B. Sun, D. Tarn,
M. Xue, X. Wang, S. Lin, Z. Ji, R. Li, F. L. Garcia, J. Yang,M. L. Kirk, T. Xia, J. I. Zink, A. Nel and C. J. Brinker,Processing Pathway Dependence of Amorphous Silica

Nanoparticle Toxicity: Colloidal vs Pyrolytic, J. Am. Chem.Soc., 2012, 134(38), 15790–15804.

57 A. M. Schrand, M. F. Rahman, S. M. Hussain, J. J. Schlager,
D. A. Smith and S. F. Ali, Metal-based nanoparticles andtheir toxicity assessment, Wiley Interdiscip. Rev.: Nanomed.Nanobiotechnol., 2010, 2(5), 544–568.
58 W. Lin, Y.-W. Huang, X.-D. Zhou and Y. Ma, in vitro toxicity
of silica nanoparticles in human lung cancer cells, Toxicol.Appl. Pharmacol., 2006, 217(3), 252–259.
59 L. Otero-Gonzalez, R. Sierra-Alvarez, S. Boitano and J. A.
Field, Application and Validation of an Impedance-BasedReal Time Cell Analyzer to Measure the Toxicity of Nano-particles Impacting Human Bronchial Epithelial Cells, Envi-ron. Sci. Technol., 2012, 46(18), 10271–10278.
60 A. Karakoti, S. Singh, J. M. Dowding, S. Seal and W. T. Self,
Redox-active radical scavenging nanomaterials, Chem. Soc.Rev., 2010, 39(11), 4422–4432.
61 A. S. Karakoti, P. Munusamy, K. Hostetler, V. Kodali, S.
Kuchibhatla, G. Orr, J. G. Pounds, J. G. Teeguarden, B. D.Thrall and D. R. Baer, Preparation and characterizationchallenges to understanding environmental and biologicalimpacts of ceria nanoparticles, Surf. Interface Anal.,2012, 44(8), 882–889.
62 L. Chen, R. A. Yokel, B. Hennig and M. Toborek,
Manufactured Aluminum Oxide Nanoparticles DecreaseExpression of Tight Junction Proteins in Brain Vasculature,J. Neuroimmune Pharmacol., 2008, 3(4), 286–295.
63 A. L. Di Virgilio and M. Reigosa, Fernandez Lorenzo de
Mele, M., Response of UMR 106 cells exposed to titaniumoxide and aluminum oxide nanoparticles, J. Biomed. Mater.Res., Part A, 2010, 92A(1), 80–86.
64 A. L. Di Virgilio, M. Reigosa and P. M. Arnal, Fernandez
Lorenzo de Mele, M., Comparative study of the cytotoxic andgenotoxic effects of titanium oxide and aluminium oxidenanoparticles in Chinese hamster ovary (CHO-K1) cells,J. Hazard. Mater., 2010, 177(1–3), 711–718.
65 S. Pakrashi, S. Dalai, T. C. Prathna, S. Trivedi, R. Myneni,
A. M. Raichur, N. Chandrasekaran and A. Mukherjee,Cytotoxicity of aluminium oxide nanoparticles towards freshwater algal isolate at low exposure concentrations, Aquat.Toxicol., 2013, 132, 34–45.
66 Z. Poborilova, R. Opatrilova and P. Babula, Toxicity of
aluminium oxide nanoparticles demonstrated using a BY-2plant cell suspension culture model, Environ. Exp. Bot.,2013, 91, 1–11.
67 E. Radziun, J. D. Wilczynska, I. Ksiazek, K. Nowak, E. L.
Anuszewska, A. Kunicki, A. Olszyna and T. Zabkowski,Assessment of the cytotoxicity of aluminium oxidenanoparticles on selected mammalian cells, Toxicol. in vitro,2011, 25(8), 1694–1700.
68 A. J. Wagner, C. A. Bleckmann, R. C. Murdock, A. M. Schrand,
J. J. Schlager and S. M. Hussain, Cellular interaction ofdifferent forms of aluminum nanoparticles in rat alveolarmacrophages, J. Phys. Chem. B, 2007, 111(25), 7353–7359.
69 P. Westerhoff, S. Lee, Y. Yang, G. Gordon, K. Hristovski,
R. U. Halden and P. Herckes, Examination of opportunities
Environ. Sci.: Nano



Publ

ishe

d on

14

May

201

5. D

ownl

oade

d on

31/

05/2

015

21:5

1:30


for metal prospecting in municipal biosolids by analysis ofsamples from full scale wastewater treatment plants,Environ. Sci. Technol., 2015, DOI: 10.1021/es505329q.

70 X. Zheng, Y. Su and Y. Chen, Acute and Chronic Responses
of Activated Sludge Viability and Performance to SilicaNanoparticles, Environ. Sci. Technol., 2012, 46(13),7182–7188.
71 M. Sibag, B. G. Choi, C. Suh, K. H. Lee, J. W. Lee, S. K.
Maeng and J. Cho, Inhibition of total oxygen uptake by silicananoparticles in activated sludge, J. Hazard. Mater.,2015, 283, 841–846.
72 R. B. Reed, D. G. Goodwin, K. L. Marsh, S. S. Capracotta,
C. P. Higgins, D. H. Fairbrother and J. F. Ranville, Detectionof single walled carbon nanotubes by monitoring embeddedmetals, Environ. Sci.: Processes Impacts, 2013, 15(1), 204–213.
73 P. A. Holden, F. Klaessig, R. F. Turco, J. H. Priester, C. M.
Rico, H. Avila-Arias, M. Mortimer, K. Pacpaco and J. L.Gardea-Torresdey, Evaluation of Exposure ConcentrationsUsed in Assessing Manufactured NanomaterialEnvironmental Hazards: Are They Relevant?, Environ. Sci.Technol., 2014, 48(18), 10541–10551.
74 Commission, E., Eur. Commission., Commission Staff Working
Paper. Types and uses of nanomaterials, including safetyaspects, Accompanying the Communication from theCommission to the European Parliament, the Council and theEuropean Economic and Social Committee on the SecondRegulatory Review on Nanomaterials; SWD(2012) 288 final;European Commission: Brussels, 3.10.2012, 2012, p. 111,http://ec.europa.eu/nanotechnology/pdf/second_regulatory_review_on_nanomaterials_-_staff_working_paper_accompanying_com (2012)_572.pdf, (Aug. 15, 2014), 2012.
75 F. Piccinno, F. Gottschalk, S. Seeger and B. Nowack,
Industrial production quantities and uses of ten engineerednanomaterials in Europe and the world, J. Nanopart. Res.,2012, 14, 1109.
76 O. Bondarenko, A. Ivask, A. Kaekinen, V. Aruoja, I. Blinova,
K. Juganson, K. Kasemets, K. Kuennis-Beres, I. Kurvet, M.Mortimer, M. Sihtmaee and A. Kahru, Biological effects of
Environ. Sci.: Nano

nanoparticles of silver, gold, TiO2 and nanoporous silica toselected invertebrate species and bacteria: FP7 projectNanoValid, Toxicol. Lett., 2013, 221, S100–S100.

77 M. P. Casado, A. Macken and H. J. Byrne, Ecotoxicological
assessment of silica and polystyrene nanoparticles assessedby a multitrophic test battery, Environ. Int., 2013, 51, 97–105.
78 L. Clement, A. Zenerino, C. Hurel, S. Amigoni, E. T. de
Givenchy, F. Guittard and N. Marmier, Toxicity assessmentof silica nanoparticles, functionalised silica nanoparticles,and HASE-grafted silica nanoparticles, Sci. Total Environ.,2013, 450, 120–128.
79 L. De Marzi, A. Monaco, J. De Lapuente, D. Ramos, M.
Borras, M. Di Gioacchino, S. Santucci and A. Poma,Cytotoxicity and genotoxicity of ceria nanoparticles ondifferent cell lines in vitro, Int. J. Mol. Sci., 2013, 14(2),3065–3077.
80 J. Ji, Z. Long and D. Lin, Toxicity of oxide nanoparticles to
the green algae Chlorella sp, Chem. Eng. J., 2011, 170(2–3),525–530.
81 S. Lanone, F. Rogerieux, J. Geys, A. Dupont, E. Maillot-
Marechal, J. Boczkowski, G. Lacroix and P. Hoet,Comparative toxicity of 24 manufactured nanoparticles inhuman alveolar epithelial and macrophage cell lines, Part.Fibre Toxicol., 2009, 6(14), 1–12.
82 W. Lin, Y.-W. Huang, X.-D. Zhou and Y. Ma, Toxicity of
cerium oxide nanoparticles in human lung cancer cells, Int.J. Toxicol., 2006, 25(6), 451–457.
83 E.-J. Park, J. Choi, Y.-K. Park and K. Park, Oxidative stress
induced by cerium oxide nanoparticles in cultured BEAS-2Bcells, Toxicology, 2008, 245(1–2), 90–100.
84 J. Sun, S. Wang, D. Zhao, F. H. Hun, L. Weng and H. Liu,
Cytotoxicity, permeability, and inflammation of metal oxidenanoparticles in human cardiac microvascular endothelialcells. Cytotoxicity, permeability, and inflammation of metaloxide nanoparticles, Cell Biol. Toxicol., 2011, 27(5), 333–342.
85 S. Yang, R. Ye, B. Han, C. Wei and X. Yang, Ecotoxicological
effect of nano-silicon dioxide particles on Daphnia magna,Integr. Ferroelectr., 2014, 154(1), 64–72.

http://ec.europa.eu/nanotechnology/pdf/second_regulatory_review_on_nanomaterials_-_staff_working_paper_accompanying_com (2012)_572.pdf




environmental science nano

Documents