NIST Special Publication 1200-5r1

Preparation of Nanoscale TiO2

Dispersions in an Environmental 

Matrix for Eco-Toxicological 

Assessment 

Version 1.2

J. S. Taurozzi

V. A. Hackley

M. R. Wiesner

http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.1200-5r1.pdf

This version supersedes

http://dx.doi.org/10.6028/NIST.SP.1200-5
NIST Special Publication 1200-5r1

NanoEHS Protocols

Preparation of Nanoscale TiO2

Dispersions in an Environmental 

Matrix for Eco-Toxicological 

Assessment

Version 1.2

J. S. Taurozzi

V. A. Hackley

Material Measurement Laboratory

Ceramics Division

M. R. Wiesner

Duke University

Durham, NC

http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.1200-5r1.pdf

This version supersedes

http://dx.doi.org/10.6028/NIST.SP.1200-5

June 2013

U.S. Department of Commerce 

Cameron F. Kerry, Acting Secretary

National Institute of Standards and Technology 

Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and Director
Certain commercial entities, equipment or materials may be identified in this

document in order to describe an experimental procedure or concept adequately.

Such identification is not intended to imply recommendation or endorsement by the

National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials or equipment are necessarily the best available for the purpose. 

National Institute of Standards and Technology Special Publication 1200-5r1

Natl. Inst. Stand. Technol. Spec. Publ. 1200-5, 14 pages (June 2013)

http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.1200-5r1.pdf

CODEN: NSPUE
iii

FOREWORD

This special publication is one in a series of protocols resulting from a collaborative research 

agreement between the National Institute of Standards and Technology (NIST) and Duke 

University’s Center for the Environmental Implications of Nanotechnology (CEINT). The 

original version of this protocol (Ver. 1.0) was first posted on the CEINT web site 

(http://ceint.duke.edu) and it, along with any other previous version, is superseded by this 

updated special publication version. Updates to this protocol may be released in the future. 

Visit http://nist.gov/mml/np-measurement-protocols.cfm to check for revisions of this protocol 

or new protocols in the series.

NIST and CEINT are interested in soliciting feedback on this method. We value user comments 

and suggestions to improve or further validate this protocol. Please send your name, email 

address, and comments/suggestions to nanoprotocols@nist.gov. We also encourage users to 

report citations to published work in which this protocol has been applied.
1

1. Introduction

Toxicity and fate assessment are key elements in the evaluation of the environmental, health and 

safety risks of engineered nanomaterials (ENMs). While significant effort and resources have 

been devoted to the toxicological evaluation of many ENMs, including nanoscale TiO2 (1-4), 

obtaining conclusive and reproducible results continues to be a challenge (5). This can be traced 

in part to the lack of standardized dispersion protocols and the inconsistent application of 

dispersion procedures in relevant biological and environmental matrices (6, 7). In order to 

address these issues, the National Institute of Standards and Technology (NIST) jointly with the 

Center for the Environmental Implications of Nanotechnology (CEINT) have developed a series 

of standardized and validated protocols for the dispersion of ENMs from a powdered material 

source for both human health and environmental testing applications. This protocol has been 

developed and validated using NIST Standard Reference Material (SRM) 1898a

. SRM 1898

consists of a widely studied and industrially relevant TiO2 nanomaterial with broad commercial 

penetration and a production history dating back several decades (3, 8-10).

While the procedures detailed in this series focus on the dispersion of SRM 1898 in specific 

aqueous media, it is believed that the adopted characterization, optimization and validation 

approaches can be more generally applied to the preparation of ENM dispersions in any relevant 

matrix. For this reason, and to allow for broader applicability, experimental details and 

discussions regarding the characterization, process optimization and validation steps adopted for 

the development of the dispersion method are detailed in a separate publication (11). The 

appendix contains a summary of validation results characterizing the dispersed state of the 

suspensions resulting from application of the protocol in both OECD and EPA standardized 

water using humic acid (HA) as the dispersing agent; however, the OECD medium was the 

principal focus for protocol development.

2. Principles and scope

This protocol is proposed for the preparation of dispersions for generic acute eco-toxicity 

applications; its use for chronic toxicity or other environmental studies, while potentially 

efficacious, is beyond the scope of the present work and should be validated by use of proper 

controls.

In this protocol, a TiO2 nanoparticle dispersion in a relevant environmental matrix is produced 

by following a series of steps applied to a TiO2 aqueous nanoparticle stock. Following the 

Organization for Economic Cooperation and Development (OECD) Guideline 202 for testing of 

chemicals (12), the environmental matrix used in this work is reconstituted from four stock 

solutions defined in ISO 6341 (13), yielding a calculated hardness of ≈ 170 mg/kg (ppm) as 

CaCO3.

 a SRM 1898 is available for purchase through the NIST SRM program office. Information, including the certificate 

of analysis, can be accessed at http://www.nist.gov/srm/.
2

In this protocol, HA is utilized as a stabilizing agent to disperse SRM 1898 in the selected test 

matrix. Natural organic matter – and HA in particular – has been demonstrated to function as a 

non-specific stabilizer in environmental matrices (14-16).

The method described herein, if applied correctly, yields 15 mL of a 100 μg/mL monomodal

nanoscale TiO2 dispersion in the selected test matrix, characterized by a mean particle diameter 

of ≈ 75 nm and pH values in the (7.0 to 7.7) range, without the need for pH adjustment steps. 

The tested TiO2 concentration was adopted from the limit test concentration recommended in 

OECD Guideline 202, while the pH of the dispersion was validated to fall within the Guideline’s 

recommended pH range. The dispersion retains its particle size distribution and pH at room 

temperature for up to 96 h, which is the maximum duration for acute toxicity assays (17). 

Dispersions prepared following this protocol should be stored in darkness or in amber glass vials, 

as TiO2 is photoactive.

3. Terminology

This protocol complies with definitions relevant to nanotechnology as set forth in the ASTM 

International standard E2456 (18) and is consistent with the draft standard ISO TS 80004-1 (19).

Additional guidance is derived from recommendations of the International Union of Pure and 

Applied Chemistry (20).

nanoparticle—sub-classification of ultrafine particle that is characterized by dimensions in the 

nanoscale (i.e., between approximately 1 nm and 100 nm) in at least two dimensions; also 

referred to as “nano-object” in ISO TS 80004-1 (19).

primary particle—the smallest discrete identifiable entity associated with a particle system; in 

this context, larger particle structures (e.g., aggregates and agglomerates) may be composed of

primary particles. 

aggregate—a discrete assemblage of primary particles strongly bonded together (i.e., fused, 

sintered, or metallically bonded).

Note—The adjective "primary", when used in conjunction with the term aggregate, is employed in the present 

context to indicate the smallest achievable dispersed particle entity.

agglomerate—assemblage of particles (including primary particles and/or smaller aggregates) 

held together by relatively weak forces (e.g., van der Waals, capillary, or electrostatic), that may 

break apart into smaller particles upon further processing.

Note—Although we define them as distinct entities, the terms aggregate and agglomerate have often been used 

interchangeably to denote particle assemblies.

dispersion—used in the present context to denote a liquid (aqueous) in which particles are 

homogeneously suspended, or the process of creating a suspension in which discrete particles are 

homogeneously distributed throughout a continuous fluid phase; implies the intention to break 

down agglomerates into their principal components (i.e., primary particles and/or aggregates). 

4. Reagents, materials and equipment
3

4.1 Reagents

4.1.1. 200 μg/mL stock TiO2 aqueous nanoparticle dispersion.

4.1.2. Type I biological grade de-ionized (DI) water (≥ 18 MΩ·cm resistivity); biological grade 

implies sterile and pyrogen-free water.

Note—Pyrogens (also known as endotoxins) are shed from the outer membrane of Gram-negative bacteria 

during cell division or lysis. These toxins are relatively heat-stable and are not destroyed under typical 

sterilizing conditions. As a result, pyrogens are ubiquitous and can interfere with the accuracy of toxicity assays. 

To depyrogenize glassware, bake at 250 °C for 2 h or at 200 °C overnight.

Note—Limulus Amoebocyte Lysate (LAL) reagent grade pyrogen-free water can be obtained from commercial 

vendors. 

Note—Sterility and absence of pyrogen contamination should be verified for all materials in contact with the 

dispersion. If using the LAL test for pyrogens, avoid using cellulose-based filters, as they can be a source of 

beta-glucan, which interferes with the LAL assay.

Note—If the dispersion is not intended for toxicological assessment, pyrogen-free conditions may not be 

necessary.

4.1.3. Humic acid (HA) powder (e.g., Suwanee River Humic Acid Standard II, International 

Humic Substances Society, MN, USA)

4.1.4. ISO Test Water 6341 aqueous stocks (in order of increasing ionic strength):

Stock A: 3 mmol potassium chloride: 0.23 g of potassium chloride, KCl, in 1 L of DI 

water 

Stock B: 31 mmol sodium bicarbonate: 2.59 g of sodium bicarbonate, NaHCO3, in 1 L of 

DI water

Stock C: 20 mmol magnesium sulfate heptahydrate: 4.93 g of magnesium sulfate

heptahydrate, MgSO4 • 7H2O, in 1 L of DI water 

Stock D: 80 mmol calcium chloride dihydrate: 11.76 g of calcium chloride dihydrate, 

CaCl2 • 2H2O, in 1 L of DI water

4.2. Materials

4.2.1. 20 mL and 30 mL sterilized and pyrogen-free glass vials 

4.2.2. aluminum or polystyrene weighing dishes

4.3.2. calibrated pipettes and sterile and pyrogen-free disposable tips covering a (0.020 to 5.000) 

mL range; adjustable volume pipettes are most convenient

4.3. Equipment

4.3.1. analytical balance with readability to 0.1 mg

For verification of expected outcome:

4.3.3. pH meter

4.3.4. Laser Diffraction Spectrometer (LDS), or

4.3.5. Dynamic Light Scattering (DLS) instrument
4
5

5. Preparation of TiO2 nanoparticle dispersions

Note—To avoid contamination, all glassware in contact with the media or suspensions should be meticulously 

cleaned, rinsed with ethanol, and dried prior to use. Glassware can be sterilized using an autoclave, by exposure 

to hot dry air (130 °C to 170 °C) for 2 h to 4 h in an oven, or by prolonged contact with alcohol. Avoid 

detergents if possible; if detergents are used, rinse with copious amounts of DI water prior to rinsing with 

ethanol and drying. Store and work in high-efficiency particulate air (HEPA) filtered clean bench if available; if 

not, containers should be capped or sealed with thermoplastic (e.g., Parafilm). 

Note—Use clean sterile pipette tips and sterile procedures.

5.1.1. Prepare 20 mL of 100 mg/L HA (aq.) solution, by adding 0.002 g of HA and 20 mL of DI 

water in a 20 mL glass vial. After adding both components, allow the covered solution to 

equilibrate for 48 h. The solution should have a pH of 4.0 ± 0.2 after equilibration. Proceed 

to the following steps after equilibration of the HA solution (i.e., 48 h after preparation of the 

solution).

5.1.2. Prepare 50 mL of 200 μg/mL TiO2 aqueous nanoparticle dispersion, by adding 0.01 g of 

SRM 1898 or equivalent into 50 mL of water, and following the sonication conditions 

prescribed in (21).

5.1.3. In a 30 mL amber glass vial, add 3 mL of the HA solution (5.1.1)

5.1.4. Add 7.5 mL of the TiO2 stock prepared in 5.1.2. into the vial with the HA solution (5.1.3)

5.1.5. Add 3.46 mL of water into the above mixture (5.1.4)

5.1.6. Add 0.26 mL of ISO stock solution A into the above mixture (5.1.5)

5.1.7. Add 0.26 mL of ISO stock solution B into the above mixture (5.1.6)

5.1.8. Add 0.26 mL of ISO stock solution C into the above mixture (5.1.7)

5.1.9. Add 0.26 mL of ISO stock solution D into the above mixture (5.1.8)

5.1.10. This procedure will yield a dispersion containing 100 μg/mL TiO2 and 20 mg/L HA in 

OECD compliant hard water with a hardness of ≈ 170 mg/L (ppm) as CaCO3.

Note—The above mentioned hardness value is calculated from the amount of Ca and Mg added with the starting 

stock solutions. 

5.1.11. If intended for toxicological assessment, the user is advised to conduct separate control 

tests for HA (20 mg/L) in the test medium (in the absence of TiO2).

6. Expected Outcome

Note—The particle size distribution (PSD) of the resulting dispersions was monitored for 96 h, corresponding 

to recommended acute toxicity assay timeframes ranging from 24 h to 96h (17). Although beyond the scope of 

this protocol, dispersions may remain stable for longer periods of time. This behavior has been validated 

without the presence of cells or other added components in the test media.

Note—Serial dilutions may cause agglomeration, this should be tested accordingly.

6.1.The resulting TiO2 dispersions should have a white but translucent appearance if prepared 

using SRM 1898 or commercial P25.
6

Note—If source powders other than SRM 1898 or P25are used, the appearance may vary depending on the final 

particle size, particle concentration, and other factors.

6.2.The particle size distribution (PSD) of the P25 dispersion should be monomodal, with the 

following volume-based mean particle diameter (Dm), D10 and D90 values:

b

If measured using LDS:

Dm ≈ (70 to 81) nm;

D10 ≈ (59 to 63) nm

D90 ≈ (79 to 102) nm

If measured using DLS:

Dm ≈ (120 to 127) nm;

D10 ≈ (68 to 86) nm

D90 ≈ (146 to 163) nm

The expected range for size parameters was calculated from three independent replicates 

obtained following the prescribed procedure. Refer to the Appendix for details on the 

calculation of the expected size parameter ranges, and illustrations of representative PSD 

profiles. Refer to (11) for details and discussions on PSD characterization and validation 

criteria.

The volume-based mean particle diameter, as well as the D10 and D90 valuesfor aqueous P25 

dispersions prepared following the protocol should be reported by the user to allow for 

comparison with the values specified herein.

6.3.The pH of dispersions after preparation should be ≈ 7.0. After (24 – 96) h, dispersions may

experience a slight increase in pH ranging from 0.3 to 0.7 units. The dispersions should 

remain well within the OECD recommended pH range of 6 to 9 (12) during the studied 

timeframe (24 to 96) h.

7. Abbreviations

DI de-ionized

DLS dynamic light scattering

ENM engineered nanomaterial

EPA U.S. Environmental Protection Agency

HA humic acid

HEPA high-efficiency particulate air

ISO International Organization for Standardization

IUPAC International Union of Pure and Applied Chemistry

 b D10 and D90 refer to characteristic percentile size values associated with the cumulative volume or mass less than 

10 % and 90 %, respectively, of the total volume or mass within the distribution. These parameters are routinely 

reported by LDS instruments. They may or may not be obtainable directly from commercial DLS instruments, 

depending on the manufacturer.
7

LAL Limulus Amoebocyte Lysate

LDS laser diffraction spectrometry

OECD Organization for Economic Cooperation and Development

PSD particle size distribution

SRM Standard Reference Material (a registered trademark of the National Institute of 

Standards & Technology)

8. Acknowledgements

We thank Michelle Twyla Blickley, NIST Materials Measurement Science Division, who

provided the ISO stock solutions used in the development of this protocol.

9. References

1. Landsiedel, R., L. Ma-Hock, A. Kroll, D. Hahn, J. Schnekenburger, K. Wiench, and W. 

Wohlleben, Testing metal-oxide nanomaterials for human safety. J. Adv. Mater., 2010. 

22(24): p. 2601-2627.

2. Pichat, P., A brief survey of the potential health risks of TiO2 particles and TiO2-

containing photocatalytic or non-photocatalytic materials. J. Adv. Oxid. Technol., 2010. 

13(3): p. 238-246.

3. Warheit, D.B., T.R. Webb, K.L. Reed, S. Frerichs, and C.M. Sayes, Pulmonary toxicity 

study in rats with three forms of ultrafine-TiO2 particles: Differential responses related to 

surface properties. Toxicology, 2007. 230(1): p. 90-104.

4. Wu, J.H., W. Liu, C.B. Xue, S.C. Zhou, F.L. Lan, L. Bi, H.B. Xu, X.L. Yang, and F.D. 

Zeng, Toxicity and penetration of TiO2 nanoparticles in hairless mice and porcine skin 

after subchronic dermal exposure. Toxicology Lett., 2009. 191(1): p. 1-8.

5. Titanium Dioxide (TiO2) IARC Monograph 93. International Agency for Research on 

Cancer, 2006.

6. Roebben, G., S.H. Ramirez-Garcia, V.A., M. Roesslein, F. Klaessig, V. Kestens, I. 

Lynch, C.M. Garner, A. Rawle, A. Elder, V. Colvin, W. Kreyling, H.F. Krug, Z. 

Lewicka, S. McNeil, A. Nel, A. Patri, P. Wick, M.R. Wiesner, T. Xia, G. Oberdörster, 

and K. Dawson, Interlaboratory reproducibility of size and surface charge measurements 

on nanoparticles prior to biological impact assessment. J. Nanopart. Res., 2011. 13(7): p. 

2675-2687.

7. Taurozzi, J.S., V.A. Hackley, and M.R. Wiesner, Ultrasonic dispersion of nanoparticles 

for environmental, health and safety assessment – Issues and recommendations.

Nanotoxicology, 2011. 5(4): p. 711-729.

8. Standard Reference Material 1898, National Institute of Standards and Technology, 

Gaithersburg, MD, http://www.nist.gov/srm/.

9. List of Manufactured Nanomaterials and List of Endpoints for Phase One of the OECD’s 

Testing Program, Series on the safety of manufactured nanomaterials, Number 6. 2008, 

Environmental Diractorate, Organization for Economic Co-operation and Development. 

Paris, France.
8

10. Porter, D., K. Sriram, M. Wolfarth, A. Jefferson, D. Schwegler-Berry, M. Andrew, and 

V. Castranova, A biocompatible medium for nanoparticle dispersion. Nanotoxicology, 

2008. 2(3): p. 144-154.

11. Taurozzi, J.S., V.A. Hackley, and M.R. Wiesner, A standardized approach for the 

dispersion of titanium dioxide nanoparticles in biological media. Nanotoxicology, Just 

Accepted, 2012. DOI:10.3109/17435390.2012.665506. 

12. OECD Guideline 202 for Testing of Chemicals, Daphnia sp., Acute Immobilisation Test. 

2004, Organization for Economic Cooperation and Development.

13. ISO 6341: Water quality - Determination of the inhibition of the mobility of Daphnia 

magna Straus (Cladocera, Crustacea) - Acute toxicity test, Third Edition. 1996, 

International Standards Organization, Geneva, Switzerland.

14. Thio, B.J.R., D.X. Zhou, and A.A. Keller, Influence of natural organic matter on the 

aggregation and deposition of titanium dioxide nanoparticles. J. Hazard. Mater., 2011. 

189(1-2): p. 556-563.

15. Keller, A.A., H.T. Wang, D.X. Zhou, H.S. Lenihan, G. Cherr, B.J. Cardinale, R. Miller, 

and Z.X. Ji, Stability and aggregation of metal oxide nanoparticles in natural aqueous 

matrices. Environ. Sci. Technol., 2010. 44(6): p. 1962-1967.

16. Gao, J., S. Youn, A. Hovsepyan, V.L. Llaneza, Y. Wang, G. Bitton, and J.C.J. Bonzongo, 

Dispersion and toxicity of selected manufactured nanomaterials in natural river water 

samples: Effects of water chemical composition. Environ. Sci. Technol., 2009. 43(9): p. 

3322-3328.

17. EPA-821-R-02-012: Methods for measuring the acute toxicity of effluents and receiving 

waters to freshwater and marine organisms, Fifth edition. 2002, U.S. Environmental 

Protection Agency.

18. ASTM E2456-06: Standard terminology relating to nanotechnology. 2006, ASTM 

International 

19. ISO TS 80004-1: Nanotechnologies - Vocabulary - Part 1: Core terms. 2007, 

International Standards Organization, Geneva, Switzerland.

20. Version 2009-09-07, Release 2.1.5: Compendium of chemical terminology. 2009, 

International Union of Pure and Applied Chemistry (IUPAC).

21. Taurozzi, J.S., V.A. Hackley, and M.R. Wiesner, Preparation of a nanoscale TiO2

aqueous dispersion for toxicological or environmental testing. Special Publication 1200-

3, National Institute of Standards and Technology, Gaithersburg, MD, 2012.
9

Appendix

A.1. Calculation of expected particle size parameters

The expected range for Dm, D10 and D90 values was obtained using the following equation:

range = �x − t. s

√n

to x +

t. s

√n

�

Where x and s are the average and standard deviation, respectively, of the measured size 

parameter from three independent replicates, t is the student test parameter for a 95% confidence 

interval and two degrees of freedom (t = 4.30), and n is the number of tested samples (n = 3).

A.2. Validation data for replicate sample preparations using OECD hard water

LDS and DLS were used as size characterization techniques to validate the protocol with respect 

to yielding stable monomodal dispersions in the nanoscale size range. These techniques were 

selected as they allowed for in situ measurements with minimal sample transformations, ensuring 

that the measured PSD profiles reflected the actual state of the “as produced” dispersions. 

Additionally, LDS measurements were confirmed previously for P25 using an x-ray disc 

centrifuge (see Ref.11) that utilizes an orthogonal measurement basis.

LDS measurements were performed using a Partica LA-950 V2 (Horiba Instruments Inc., Irvine 

CA, USA), equipped with an 87 detector, high-resolution silicon photodiode array (75 detectors 

for forward/low-angle light scattering and 12 detectors for high-angle and backscatter light 

scattering); operating with a 5 mW 650 nm red laser and a 3 mW 405 nm blue light emitting 

diode. Measurements were conducted by introducing the sample into a stirred 15 mL quartz cell. 

First, the system was blanked against a solution consisting of 12 mL of a HA blanking solution. 

The blanking solution was prepared by diluting 3 mL of stock HA solution into 14 mL of DI 

water, so as to yield a final HA concentration of approximately 20 mg/L. Then 1.5 mL of the test 

suspension was added to the cell containing the blanking solution, and the measurement was 

initiated. An appropriate blue line transmittance level was attained (between 70 % and 90 % 

transmittance) for all samples under these conditions. Volumetric PSDs were calculated by 

application of the Mie scattering model, with a particle refractive index of 2.5.

DLS measurements were performed using a Zetasizer Nano ZS (Malvern Instruments Inc., 

Westborough MA, USA) in backscatter configuration (θ = 173°) at a laser wavelength of 

633 nm. Samples were measured in 1.5 mL disposable cuvettes by dilution into DI water (20 μL 

of test suspension was diluted into 1 mL of DI water) to achieve an appropriate light scattering 

level. For each test sample, measurements were performed in triplicate, with the number and 

duration of sub-runs for each measurement determined automatically by the instrument software. 

A non-negatively constrained least squares inversion algorithm was used to generate the PSD. A 

regularization parameter of 0.01 was selected, with data parsed over 70 bins. For conversion 

from intensity-weighted to volumetric PSDs, the Mie scattering model was applied using a 

particle refractive index of 2.5.

For validation purposes, three independent replicate test suspensions were prepared following the 

prescribed protocol, and the PSD of each sample was measured using LDS and DLS shortly after 
10

preparation. Afterwards, LDS measurements were performed on each test suspension at 48 h, 

5 d, 7 d and 9 d following preparation. Figure A1 compares representative LSD and DLS PSD 

profiles obtained for the freshly prepared suspensions, while Tables A1 and A2 summarize the 

size values obtained for each replicate after the initial preparation as determined by LDS and 

DLS, respectively.

Figure A1. LDS (left) and DLS (right) volume-based PSD profiles of HA modified SRM 1898 

dispersions in the OECD synthetic hard water prepared following the procedure described in this 

protocol. The x-axis is shown on a log scale.

Table A1. LDS summary for freshly prepared triplicate 

samples in OECD hard water.

Replicate # Mean size D10 D90 pH

1 75.1 60.8 89.4 7.0

2 73.6 60.3 86.5 7.0

3 77.9 61.7 95.8 7.0

SD 2.2 0.7 4.8

95% CI 5.3 1.8 11.8

Average 75.6 60.9 90.6

Range up 81 63 102

Range down 70 59 79

Coverage factor k = 2.48 for a 95 % confidence interval.

0

5

10

15

20

25

30

35

40

10 100 1000 10000 100000

Volume (%)

Diameter (nm)

0

5

10

15

20

25

10 100 1000 10000 100000

Volume (%)

Diameter (nm)
11

Table A2. DLS summary for freshly prepared 

triplicate samples in OECD hard water.

Replicate #

Mean 

size D10 D90

1 122.6 74.3 154.0

2 123.3 81.4 151.5

3 125.1 76.6 158.0

SD 1.3 3.6 3.3

95% CI 3.3 9.0 8.2

Mean 123.7 77.4 154.5

Range up 127 86 163

Range down 120 68 146

Coverage factor k = 2.48 for a 95 % confidence interval.

The triplicate LDS results are presented in stacked format in Figure A2 for each test time 

following the initial preparation in the OECD hard water. Note the appearance of a micrometer 

scale component (agglomerates) after 7 d. The data clearly show that the suspensions are stable 

toward significant agglomeration for more than the typical 24 h to 96 h acute toxicity assay 

range. Even after 9 d, the nanoscale component is predominant while the agglomeration appears 

to proceed slowly, with little apparent difference between 7 d and 9 d.

Figure A2. LDS volume-based PSD profiles of HA modified SRM 1898 suspensions in OECD 

synthetic hard water prepared in triplicate following the procedure described in this protocol. The 

x-axis is presented on a log scale. Time after sample preparation is indicated on the right side. 

The mean pH of the suspensions is indicated on the left side. The appearance of a micrometer 

scale component (indicated by arrow) is apparent after 7 days, suggesting the onset of significant 

agglomeration.

pH 7.2

pH 7.4

pH 7.7

pH 7.3

pH 7.0

48 h

5 d

7 d

9 d

initial

Diameter (nm)

10 100 1000 10000
12

A.3. Tests using EPA synthetic hard freshwater 

The prescribed protocol was also tested using EPA standard synthetic hard water with a nominal 

hardness of 170 mg/L as CaCO3. Directions for preparing the EPA water can be found in Ref. 17 

(using the composition identified as “Hard” in Table 7 of that document).The PSD was

determined up to 5 d after initial preparation using independently prepared triplicate test 

suspensions. The LDS PSD profiles are presented in stacked format in Figure A3. Since this was 

a secondary test medium, only three time points were analyzed, but the results clearly show that 

the protocol yields a monomodal nanoscale suspension that is stable for at least 24 h and possibly 

up to 5 d. After 5 d the PSDs show the growth of microscale agglomerates, though the nanoscale 

component still predominates the volume or mass distribution of particles.

Figure A3. LDS volume-based PSD profiles of HA modified SRM 1898 suspensions in EPA

hard water prepared in triplicate following the protocol. The x-axis is presented on a log scale.

Time after sample preparation is indicated on the right side. The mean pH of the suspensions is 

indicated on the left side. The appearance of a micrometer scale component (indicated by arrow) 

is apparent after 5 days, suggesting the onset of significant agglomeration.

Table A3. LDS summary for freshly prepared 

triplicate samples in EPA hard water.

Replicate #

Mean 

size D10 D90

1 76.6 61.6 96.1

2 76.7 61.5 97.0

3 76.6 61.6 95.9

SD 0.1 0.1 0.6

95% CI 0.1 0.1 1.5

Mean 76.6 61.6 96.3

Range up 76.7 61.6 97.0

Range down 76.6 61.5 95.9

Coverage factor k = 2.48 for a 95 % confidence interval.