www.hsph.harvard.edu/nano13 November 2013 slide 1

Safer by design, transparent, UV-absorbing ZnO

nanorods with minimal genotoxicity

Georgios A. Sotiriou, Christa Watson, Kim M. Murdaugh, Alison Elder1 and

Philip Demokritou

Center for Nanotechnology and Nanotoxicology, Department of Environmental Health, School of Public Health, Harvard

University, 665 Huntington Avenue, 02115 Boston, MA

1University of Rochester Medical Center, 601 Elmwood Ave, 14642 Rochester, NY

www.hsph.harvard.edu/nano13 November 2013 slide 2

ZnO nanoparticles

• Inorganic semiconductor[1,2]

◦ Band-gap Eg = 3.3 eV (white color)

◦ UV-absorbing

• Transparent in visible wavelength

• Applications

◦ Cosmetics

◦ Sunscreens

◦ Filler in polymers

• Exposure to humans: inevitable

[2] King, Liang, Carney, Hakim, Li, Weimer, Adv. Funct. Mater. 18, 607 (2008).[1] Janotti, Van de Walle, Rep. Prog. Phys. 72, 126501 (2009).

[3]

[3] Hikov, Rittermeier, Luedeman, Herrmann, Muhler, Fischer, J. Mater. Chem. 18, 3325 (2008).

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Toxicological implications

• Many in-vitro tox studies linked

ZnO NPs to cytotoxicity[1]

• Primary cytox mechanism:

◦ Zn2+ ion release

◦ Direct nanoparticle contact/ ROS

generation

• ZnO NPs exhibit high DNA

damage potential[2]

◦ Doses below cytotoxic level

evaluated using the CometChip technology. Expanded view illustrates the morphology of the

comet structure induced from 4hr exposure of zinc oxide ENP in TK-6 revealing significant

DNA damage. C.) Positive control cells treated with H2O2 (100μM)  fo r  2 0 minutes. D.)

Traditional comet assay of TK-6  cells  treated  with  ZnO  (2 0μg/ ml)  fo r  4h

r

s  fo r  c omparison  to  

CometChip quantitative assessments.

Figure 3 Quantitative DNA damage assessments of TK-6 cells exposed to ENPs for 4hrs

and H9T3 cells for 24hrs. A.) TK-6 cells were seeded at a density of 1x106 cells/well and

[2] Watson, Ge, Cohen, Pyrgiotakis, Engelward, Demokritou, in revision (2013).

[1]

[2]

[1] George, Xia, Rallo, Zhao, Ji, Lin, Wang, Zhang, France, Schoenfeld, Damoiseaux, Liu, Lin, Bradley, Cohen, Nel, ACS Nano 5, 1805 (2011).

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• The likely success or failure of NT industry depends on

nano-EHS matters.

• While nano-EHS research is progressing, research on

“safer by design” approaches is lacking behind

• Elements of a “Safer by design” approach:

◦ Reduce Toxicological footprint

◦ Maintain functional properties of ENMs

◦ Scalability

Safer by Design approaches

for ENMs

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Safer-by-design approaches for ZnO NPs

• Altering the tox profile◦ Zn2+ ion release

◦ Direct nanoparticle contact

• Example: Fe-doping of ZnO[1]

◦ Decreases Zn2+ ion release

◦ Lower cytotoxicity

• Pitfall: Fe-doping changes the optoelectronic properties[2]

◦ Color changes from white to brown–undesired for many applications

• Safer by design approach◦ Engineer safer ZnO nanoparticles while

maintaining their optoelectronic properties

◦ Scalability of method for industry

[1]

increasing Fe-content at%

[1]

[2]

0 %

10 %

[1] George, Pokhrel, Xia, Gilbert, Ji, Schowalter, Rosenauer, Damoisaeux, Bradley, Madler, Nel, ACS Nano 4, 15 (2010).[2] Aydin, El-sadek, Zheng, Yahia, Yakuphanoglu, Optic Laser Technol. 48, 447 (2013).

www.hsph.harvard.edu/nano13 November 2013 slide 6

A Safer Formulation Concept for

flame generated ENMs

Develop a concept to coat in flight flame generated

ENMs with a nanothin layer of SiO2

Features: Scalability, no chemical by products and

impurities, high volume production

(1) Gass et al.,Sus. Chem & Eng., 2013,(2) Xia et al., ACS Nano 2011, 5, 1223 – 1235 (3) Napierska et al., Particle and Fibre Toxicology 2010, 7,39(4) Teleki et al., Chem. Mater. 2009, 21, 2094–2100(5) Sotiriou et al., Adv. Funct. Mater. 2010, 20, 4250–4257

Scalability?

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Strategy: In flight SiO2 coating on ZnO nanoparticles

Core nanoparticle

synthesis

SiO2 coating

formation

Particle collection

Si-precursor vapor injection

[2] Gass, Cohen, Pyrgiotakis, Sotiriou, Pratsinis, Demokritou, ACS Sustainable Chem. Eng. 1, 843 (2013).[1] Teleki, Heine, Krumeich, Akhtar Pratsinis, Langmuir 24, 12553 (2008).

[1,2]

SiO2-coating

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Crystallinity

• Highly crystalline core

• Identical XRD

patterns

◦ SiO2 shell does not

influence core

crystallinity[1]

◦ No “free” SiO2[2,3]

[3] Gass, Cohen, Pyrgiotakis, Sotiriou, Pratsinis, Demokritou, ACS Sustainable Chem. Eng. 1, 843 (2013).[2] Sotiriou, Schneider, Pratsinis, J. Phys. Chem. C 116, 4493 (2012).[1] Teleki, Heine, Krumeich, Akhtar Pratsinis, Langmuir 24, 12553 (2008).

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Hermetic nature of SiO2 coating

coat ing ef f iciency = 95%

Zn 3s Zn 3p

Si 2sSi 2s

X-ray photoelectron spectroscopy

(XPS)

Photocatalytic activity

(MB degradation under UV, l = 254 nm)

• Zn-related XPS peaks diminish

• High coating efficiency

◦ Inelastic mean free path of Zn free electrons may penetrate through SiO2

• Pure ZnO is a photocatalyst◦ Degrade organic dyes under UV light

• Coated ZnO show no MB degradation◦ Hermetic coating

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Optoelectronic properties?

Eg

UV-vis transmission of aqueous

suspensions (100 mg/mL)

Diffuse-reflectance UV-vis

(powder form)

• SiO2-coated ZnO nanorods are more

transparent

• Both uncoated and SiO2-coated block

UV (< 400 nm)

• Identical optoelectronic properties◦ Eg = 3.3 eV

◦ In agreement with XRD

• No sensory changes

• May be used in cosmetics

Sotiriou et al., in submission, 2013.

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Genotoxicity - DNA damage

Nano Cometchip assay[1,2] MTT cytotoxicity assay

**p-value < 0.01, ***p-value < 0.001

• Uncoated ZnO nanorods induce DNA

damage

• SiO2-coated ZnO nanorods show a

protective DNA damage effect

• No significant cytotoxicity is observed for

all doses[1]

• DNA damage may be present at no- or

low-cytotoxicity doses

TK-6 human lymphoblastoid cells

1 Watson et al , in review, 2013, 2 Sotiriou et al., in submission, 2013.

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Conclusions

• Synthesis of hermetically SiO2-coated ZnOnanorods inflight

• Hermetic SiO2 coating◦ No photocatalytic activity

• SiO2 presence does not influence the core crystallinity◦ Maintain the desired visible transparency and UV

absorption

• SiO2-coated ZnO nanorods exhibit reduced DNA damage

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Acknowledgements

• NSF (#1235806)

• NIEHS (ES-0000002)

• SNSF (#145392)

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APPENDIX

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Zeta-potential

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TGA

Dm = 0.9 wt%