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Synergistic Photogeneration of Reactive Oxygen Species by Dissolved Organic Matter and C60 in Aqueous Phase Junfeng Niu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es505089e • Publication Date (Web): 23 Dec 2014 Downloaded from http://pubs.acs.org on December 26, 2014

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Synergistic Photogeneration of Reactive Oxygen Species by Dissolved Organic Matter and C60 in Aqueous Phase

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

Environmental Science & Technology es-2014-05089e.R1 Article 20-Dec-2014 Li, Yang; Beijing Normal University, School of Environment Niu, Junfeng; Beijing Normal University, School of Environment Shang, Enxiang; Beijing Normal University, School of Environment Crittenden, John; Georgia Institute of Technology, Brook Byers Institute for Sustainable Systems

ACS Paragon Plus Environment

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Synergistic Photogeneration of Reactive Oxygen

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Species by Dissolved Organic Matter and C60 in

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Aqueous Phase Yang Li1, Junfeng Niu1∗, Enxiang Shang1, and John Charles Crittenden1,2

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Beijing Normal University, Beijing 100875, People’s Republic of China

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State Key Laboratory of Water Environment Simulation, School of Environment,

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School of Civil and Environmental Engineering and the Brook Byers Institute for

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Sustainable Systems, Georgia Institute of Technology, Atlanta, GA 30332, United

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States

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Corresponding author: e-mail: [email protected], phone: +86-10-5880 7612, fax: +86-10-5880 7612. 1



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ABSTRACT: We investigated the photogeneration of reactive oxygen species (ROS)

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by C60 under UV irradiation, when humic acid (HA) or fulvic acid (FA) are present.

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When C60 and dissolved organic matter (DOM) were present as a mixture, singlet

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oxygen (1O2) generation concentrations were 1.2~1.5 times higher than the sum of 1O2

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concentrations that were produced when C60 and DOM were present in water by

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themselves. When C60 and HA were present as a mixture, superoxide radical (O2•−)

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were 2.2~2.6 times larger than when C60 and HA were present in water by themselves.

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A synergistic ROS photogeneration mechanism involved in energy and electron

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transfer between DOM and C60 was proposed. Enhanced 1O2 generation in the

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mixtures was partly due to 3DOM* energy transfer to O2. However it was mostly due

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to 3DOM* energy transfer to C60 producing 3C60*. 3C60* has a prolonged lifetime (> 4

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μs) in the mixture and provides sufficient time for energy transfer to O2 which

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produces 1O2. The enhanced O2•− generation for HA/C60 mixture was because 3C60*

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mediated electron transfer from photoionized HA to O2. This study demonstrates the

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importance of considering DOM when investigating ROS production by C60.

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INTRODUCTION

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C60 fullerenes possess unique photochemical reactivity because they have strong

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absorbance in the ultraviolet-visual spectrum and have conjugated double bonds.1, 2

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When irradiated with light, the individual C60 molecule is excited to singlet state

42

(1C60*) followed by conversion to triplet state (3C60*). The process has a high

43

quantum yield (approximately 100%) for light that has photonic energy greater than

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2.3 eV.2-4 Subsequently, the excess energy in 3C60* is efficiently transferred to O2

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resulting in singlet oxygen (1O2) generation via a type II energy-transfer pathway.4, 5

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In addition, C60 is an excellent electron acceptor and can accept up to six electrons per

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molecule due to its delocalized π-electrons within the spherical carbon framework,

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small reorganization energy, and low reduction potential.6, 7 In the presence of an

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electron donor (ED), 3C60* can mediate electron transfer from ED to O2 and produce

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superoxide radical (O2•−) through a type I electron-transfer pathway.7-9 Concerns over

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the potential threat of C60 to human health and ecosystems have stimulated intense

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study on their photoreactivity in the natural aqueous environment.

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C60* is a key intermediate for both energy and electron transfer and its decay

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rate and lifetime governs C60’s photoreactivity in water.5 Once released into water, C60

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forms aggregates in which the lifetime of 3C60* is less than 100 ns due to their

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quenching by surrounding ground-state C60 (self-quenching) or another

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(triplet-triplet annihilation).5 This short lifetime of 3C60* does not provide sufficient

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time for energy and electron transfer and this leads to loss of intrinsic molecular

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photoreactivity of C60 in water.5 C60 will not flocculate (and remain in solution as

60

stable single particles) when surfactants are adsorbed to C60 surface, such as dissolved

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organic matter (DOM) that impose electrosteric repulsion (i.e., combination of

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electrostatic and steric) between particles.10-12 The presence of individual particles in

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3

C60*


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solution significantly prolongs the lifetime of 3C60*, and facilitates both energy and

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electron transfer to O2 and increases their photochemical activity.5 What remains

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unclear is how DOM influences the decay rate and lifetime of 3C60*, and the

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photochemical activity of C60 in aqueous phase and our study focuses on this.

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DOM can absorb photons in the 300-500 nm range of the solar spectrum because

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they contain conjugated unsaturated bonds and free electron pairs on heteroatoms.13-15

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After light absorbance, DOM can form short-lived reactive species such as excited

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triplet states (3DOM*), hydrated electrons (e-), O2•−, and 1O2.16,

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released into natural waters, they can absorb DOM on C60 surfaces due to their high

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surface-to-volume ratios and DOM surfactant properties.18 Also, the electrons

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produced from DOM photoionization could become trapped on C60 surfaces.18-20

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DOM photoionization raises several important questions that we address in the study.

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Can a charged DOM transfer an electron to C60 and can this increase O2•− generation

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for DOM/C60 mixtures? Can DOM photosensitize C60 and enhance

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photogeneration in mixtures of C60 and DOM? Accordingly, we investigated the

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electron and energy transfer between C60 and DOM and how their interactions

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facilitated production of 1O2 and O2•−. This facilitated production is crucial for

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assessing the toxicity of C60 upon their release into natural waters.

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Once C60 were

1

O2

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Humic acid (HA) and fulvic acid (FA) are main components of DOM in surface

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waters.13-15 Their distinct physicochemical properties (e.g., chemical composition,

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molecular structure, surface charge, solubility, and molecular weight) result in

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different photochemical reactivity.21, 22 HA has a higher aromatic content and a lower

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photosensitizing ability to generate 1O2 than that of FA. Because the quantum yields

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for

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aromaticity.19, 21 In addition, HA contains more phenolic functional groups than those

1

O2 generation have been shown to be inversely proportional to DOM

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of FA, which are primary chromophores responsible for DOM photoionization and

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charge-transfer reactions.22 Therefore, it is particularly important to compare the

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energy transfer and electron donation capacity of HA and FA to C60 and their impact

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on 1O2 and O2•− photogeneration in DOM/C60 mixtures.

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The objective of this work was to compare the effect of different DOM fractions

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(HA and FA) on the photoreactivity of C60. We investigated the production kinetics

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and concentrations of 1O2 and O2•− in C60 aqueous suspension, DOM solutions, or

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their mixtures under UV irradiation for a wavelength of 365 nm (UV-365), which is

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the primary wavelength of UV irradiation that reaches the earth’s surface.23 The decay

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kinetics and lifetime of 3C60* in C60 aqueous suspension with or without DOM was

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also investigated. The generation mechanism of 1O2 and O2•− in DOM/C60 mixtures

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was elucidated by the energy and electron transfer between C60 and DOM. Overall,

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this work provides insight into the photoreactivity and transformation of C60 in natural

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aqueous environment under UV light irradiation.

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MATERIALS AND METHODS

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Preparation of DOM Solutions. Chemicals used in this study are provided in

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Section S1 of Supporting Information (SI). DOM stock solutions were prepared

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following published method.24 1 g/L HA stock solution was prepared by dissolving

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HA powder in DI water, adjusting pH to 8.0 with 0.01 mol/L NaOH solution. The

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solution was then filtered through a 0.45 μm pore size membrane filter (Millipore,

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Billerica, MA, USA) to remove undissolved HA. 1 g/L FA stock solution was

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prepared by dissolving FA powder in DI water. The concentrations of DOM stock

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solution were confirmed by a total organic carbon (TOC) analyzer (HACH IL 550,

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Loveland, CO, USA). The standard redox potentials of HA or FA were measured by

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the titration method reported by Struyk et al.25

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Preparation of C60 Stock Suspension and Particle Characterization. The

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aqueous C60 stock suspension was prepared by direct sonication method.26, 27 Section

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S2 of SI provides detail about the preparation and characterization method of C60

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stock suspension. The concentration of C60 stock suspension was 10.0 mg/L and was

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measured using a TOC analyzer (pH = 5.7). The hydrodynamic sizes, particle size

118

distributions (PSDs), and zeta potentials of C60 aqueous suspensions (5 mg/L) in the

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absence or in the presence of DOM (10, 20, or 50 mg/L) were characterized by

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dynamic light scattering (DLS) on a Zetasizer Nano ZS instrument (Malvern,

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Worcestershire, UK).

122

ESR detection of 1O2 and O2•−. Electron spin resonance spectrometry (ESR;

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Bruker ESP-300E, Karlsruhe, Germany) was employed to determine ROS that was

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photogenerated in C60 aqueous suspensions, DOM solutions, and their mixtures. The

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setting of ESR spectrometer was as follows: (1) center field of 3480.00 G; (2)

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microwave power of 10 mW; (3) modulation frequency of 100.00 kHz; (4)

127

modulation amplitude of 2.071 G; (5) sweep width of 100.00 G; and (6) sweep time

128

of 41.943 sec.

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(1)

1

O2 Detection. Production of

1

O2 was monitored using TEMP as

130

spin-trapping agent. Experimental solution of 5 mg/L C60 aqueous suspension was

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prepared by mixing 6 μL TEMP (4 M), 150 μL C60 stock solutions (10 mg/L), and 144

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μL DI water. The mixture containing 5 mg/L C60 and 10 mg/L DOM was prepared by

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mixing 6 μL TEMP (4 M), 150 μL C60 stock solution (10 mg/L), 3 μL DOM (1000

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mg/L), and 141 μL DI water. The reaction solutions were placed into the cylindrical

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quartz cell and irradiated by a 4-W compact ultraviolet lamp (UV-365, UVP, San

136

Gabriel, CA, USA). The UV lamp has an output spectrum ranging from 315 to 400

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nm with peak intensity at a wavelength of 365 nm as measured by a

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spectrophotometer (Spectropro-500, Acton, MA, USA). The light intensity in the

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reaction solution was 1.0 × 10-6 Einstein·L-1·s-1 as measured by a UVX radiometer

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(Model UVX-25, UVP, San Gabriel, CA, USA). After 30 min, the quartz cell was

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quickly subjected to ESR measurement. When 1O2 was produced, TEMP was

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oxidized by 1O2 to 4-oxo-2, 2, 6, 6-tetramethyl-1-piperdinyloxy radical (TEMPO).

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Accordingly, the ESR signals for TEMPO were used to measure 1O2 formation.

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(2) O2•− Detection. DMPO was used as spin-trapping agent for O2•− detection.

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The reaction solution of 5 mg/L C60 was prepared by mixing 150 μL C60 dispersed in

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DMSO (10 mg/L), 20 μL DMPO (0.5 M), and 130 μL DMSO. The mixture of 5 mg/L

147

C60 and 10 mg/L DOM was prepared by mixing 20 μL DMPO, 150 μL C60 dispersed

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in DMSO (10 mg/L), 3 μL DOM (1000 mg/L), and 127 μL DMSO. The

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photochemical reaction was performed following the same procedure as described for

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1

O2. ESR signals for DMPO-O2•− adducts were used to measure O2•− formation.

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Detection of 1O2 and O2•− Concentrations. The molecular probe assays were

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conducted to confirm the photogeneration of ROS and measure their concentrations.

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One hundred mL of aqueous suspension containing 5 mg/L C60, DOM with different

154

concentrations (10, 20, or 50 mg/L), or their mixture was placed into a beaker and

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irradiated by the same UV lamp. No buffer solutions or salts were added to the

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reaction solutions to prevent the possibility of colloidal instability for C60 during the

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photochemical experiments. The reaction temperature was maintained at (20 ± 2)oC

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by a recirculating water bath and the reaction solutions were stirred by a magnetic

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stirrer (RO 10; Staufen, Germany). Furfuryl alcohol (FFA, 0.85 mM) and XTT (200

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µM) were used as molecular probes for 1O2 and O2•−, respectively. XTT reduction by

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O2•− results in the formation of orange-colored XTT-formazan, which is water-soluble

162

and has an adsorption peak at 470 nm. The UV-vis spectrophotometer (Beckman, DU

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163

7700, Brea, CA, USA) was used to measure the concentrations of XTT-formazan,

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which can be used to quantify the concentration of O2•−. Our previous studies have

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demonstrated that XTT-formazan was stable under UV-365 light irradiation.28-31 After

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UV illumination for several elapsed times, the reaction solutions were sampled,

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prepared, and analyzed according to the reported method.29-32 The average molar

168

concentration of 1O2 and O2•− was calculated following the previous methods.29, 30

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Laser Flash Photolysis. The decay kinetics of 3C60* in C60 aqueous suspensions

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or DOM/C60 mixtures were traced by a nanosecond laser flash photolysis instrument

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(LP920, Edinburgh, England). Prior to the experiment, 3 mL of solution that contains

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5 mg/L of C60 with or without 10 mg/L of DOM was placed in a rectangular quartz

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cell, purged with ultrapure nitrogen gas for at least 30 min, and sealed from the

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atmosphere to inhibit energy transfer from 3C60* to O2. A 355 nm laser pulse (10 mJ,

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pulse width of 7 nm) was generated from a Quanta Ray Nd: YAG laser system

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(Continuum, Santa Clara, CA, USA) and was used as an excitation source. A 500 W

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xenon lamp was used as a monitoring light. Instantaneous formation of 3C60* after

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laser pulse and its decay kinetics was obtained from the time resolved data using

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absorbance at 740 nm.3, 5 The lifetime of 3C60* was determined by the least squares fit

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of a single exponential equation with offset, performed with Origin 7.5.

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RESULTS AND DISCUSSION

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Particle Size Distribution. The intensity-averaged particle size distributions

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(PSDs) of C60 dispersed in water after addition of different concentrations of HA or

184

FA are presented in Figure 1. The average hydrodynamic radius of C60 with 10.0, 20.0,

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and 50.0 mg/L HA were 70.3, 61.5, and 36.6 nm, respectively, which were much

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smaller than the hydrodynamic radius of C60 without HA (108.1 nm). Similarly, the

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particle sizes of C60 decreased with increasing FA concentrations, with the average

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hydrodynamic radius of C60; i.e., 72.8, 67.0, and 43.5 nm for FA concentrations of

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10.0, 20.0, and 50.0 mg/L, respectively. Even though DOM reduced the aggregation

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degree of C60, they did not prevent their aggregation. Our experimental results are

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consistent with previous reports that DOM decreased the particle size and increased

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colloidal stability of C60 with increasing DOM concentrations.20, 33 Also, HA has been

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shown to be more effective than FA in suppressing C60 aggregation.20,

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mass-averaged PSDs of C60 in the presence of different concentrations of HA or FA

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are provided in Figure S1 of SI.

33

The

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The zeta potential measurements (Table S1 of SI) indicated that the negative

197

potential of C60 increased slightly when DOM was added. The changes in particle

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sizes of C60 did not correlate well to their changes in zeta potentials, indicating that

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the electrostatic interaction was not the primary mechanism for particle stability. The

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decreased particle size and increased C60 stability in the presence of DOM was due to

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the steric force they impart as discussed below.10-12, 34 The different structural and

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conformational characteristics between HA and FA account for their distinct

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stabilization capacity for C60 aqueous suspensions.10, 34 HA has less polarity, longer

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chain hydrophobic moieties, and higher molecular weight than FA,35 and this causes

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stronger sorption of HA onto C60 surface via hydrophobic and π-π interactions

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providing more steric stability. Also HA have a thicker organic coating on C60 surface

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preventing C60 getting closer to form bigger aggregates.10, 34

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(a)

(b)

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Figure 1. Intensity-averaged PSD diagrams of C60 aqueous suspensions without or with different concentrations of (a) HA or (b) FA. 9



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ESR Detection of 1O2. As shown in Figure 2(a)-(d), three characteristic peaks of

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TEMPO spin adducts could be detected by ESR for DOM solutions with or without

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C60 when irradiated by UV light. The intensities of the TEMPO spin adducts increased

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as the irradiation time was increased. In contrast, no TEMPO signals were detected in

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C60 aqueous suspension (Figure 2(e)), indicating that C60 aggregates lost their

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photochemical reactivity with respect to 1O2 production. These are consistent with

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previous studies that 1O2 could not be detected in C60 aggregates. These studies

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hypothesize that the close proximity of the C60 molecules in an aggregate result in

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rapid decay of 3C60* and its dramatically shortened lifetime.5,

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aggregates exposed less C60 molecules to photons and O2,37, 38 potentially reducing

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active surface areas for 1O2 production.

36

In addition,

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The TEMPO signals have been detected in DOM solutions, which is consistent

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with previous studies that both HA and FA could photosensitize 1O2 generation under

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UV irradiation.39, 40 The excited triplets of phenyl ketone or aromatic quinone in the

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DOM molecule are primarily responsible for their photosensitization effects.41,

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Moreover, the TEMPO signals in FA solution are stronger than those in HA solution

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with or without C60. The higher photosensitization capacity of FA than HA is

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primarily due to the presence of more chromophores in the lower-molecular-weight

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fractions of DOM.17,

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mixtures are stronger than those in solutions that contained only DOM or C60. The

234

reasons for the enhanced 1O2 generation in DOM/C60 mixtures will be discussed in the

235

following “Proposed Reaction Framework” Section. No TEMPO signals have been

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detected in C60 aqueous suspension, DOM solutions, or their mixtures in the dark

237

(Figure S2 of SI).

39

42

It is also noteworthy that TEMPO signals in DOM/C60

238

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239 240 241 242

243 244 245

Figure 2. ESR spectra recorded at ambient temperature for TEMP adduct with 1O2 in the C60 aqueous suspension, DOM, or their mixtures (UV irradiation at 365 nm, light intensity of 1.0 × 10-6 Einstein·L-1·s-1, C60 of 5 mg/L, and DOM of 10 mg/L).

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ESR Detection of O2•−. As shown in Figure 3, six characteristic peaks of the

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DMPO-O2•− spin adducts could be detected using ESR under UV irradiation in HA

248

solution or HA/C60 mixture. Similarly, the strengths of DMPO-O2•− spin adducts were

249

enhanced with prolonged UV irradiation time (Figure 3(a) and (c)). C60 aqueous

250

suspension did not induce O2•− generation primarily due to lack of ED and short

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lifetime of 3C60*.8, 9, 43 DMPO-O2•− signals were observed in HA solution primarily

252

due to electron transfer from HA photoionization to O2.44 The phenolic moieties in

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HA molecules were responsible for their photoionization and charge-transfer

254

transitions.19, 45 DMPO-O2•− signals in HA/C60 mixture were stronger than those in

255

HA solution, indicating that HA could serve as ED for C60 and subsequently enhanced

256

O2•− production in the mixtures. Unlike HA solution, no DMPO-O2•− adducts were

257

detected in FA solution with or without C60, demonstrating that FA could not undergo

258

photoionization and then inject electrons to O2 or C60. A positive correlation has been

259

found between the charge transfer efficiency of DOM and their aromaticity.19, 45 The

260

lower electron donation capacity of FA than that of HA was probably due to the lower

261

aromaticity of FA (0.28) as compared to that of HA (0.42).46 No measurable amount

262

of O2•− was detected in C60 aqueous suspension, DOM solutions, or their mixtures in

263

the dark (Figure S2 of SI). It is worth mention that DMSO was used as solvent to 11



264

detect O2•− due to its spontaneous dismutation in water.47 The potential effect of

265

DMSO on particle size distribution and O2•− generation by C60 with or without DOM

266

was discussed in Section S6 of SI.

267 268 269 270 271 272 273 274

Figure 3. ESR spectra recorded at ambient temperature for DMPO adduct with O2•− in the C60 aqueous suspension, DOM, or their mixtures (UV irradiation at 365 nm, light intensity of 1.0 × 10-6 Einstein·L-1·s-1, C60 of 5 mg/L, and DOM of 10 mg/L).

275

Generation Kinetics of 1O2 and O2•−. Molecular probe assays were conducted

276

to measure the photogenerated concentrations of 1O2 and O2•− in C60 aqueous

277

suspensions, DOM solutions, or their mixtures under UV-365 irradiation. Basically,

278

the ROS production profile that was measured by the molecular probe assays was

279

similar to that detected using ESR. Previous studies have shown that FFA photolysis

280

under UV-365 irradiation was negligible.29-31 As shown in Figure 4(a), (b), we

281

observed that both HA and FA solutions result in FFA photodegradation, and the

282

degradation rate increases with DOM concentration. At the same mass concentration,

283

the photodegradation rate of FFA by FA is higher than that by HA. This is consistent

284

with our ESR results and previous studies that showed FA produced more 1O2 than

285

HA.16, 19 Aqueous C60 suspensions alone did not degrade FFA, which indicates that

286

1

287

few sites for UV absorption and O2 exposure.5 When DOM was added to C60 aqueous

288

suspensions, the photodegradation rate of FFA was significantly enhanced and the

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FFA photodegradation rate increased with DOM concentration. A comparison

290

between Figure 4 (a) and (b) shows the FFA photodegradation rates in FA/C60

O2 was not generated by C60. This was primarily due to short lifetime of 3C60* and

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mixtures were much faster than those in HA/C60 mixtures at the same DOM mass

292

concentrations. This indicated that the photosensitization capacity of FA for 1O2

293

generation in DOM/C60 mixture was higher than that of HA because most of

294

chromophores (such as phenyl ketone and aromatic quinone) were located in the

295

low-molecular-weight fractions of DOM.16,

296

mixtures did not induce significant degradation of FFA (data not shown).

19

In the dark, C60, DOM, or their

297

Figure 4(c)-(f) shows the changes in the absorption spectrum of C60 aqueous

298

suspension, HA solutions, or their mixtures during a 48-h exposure to UV-365

299

irradiation. The absorption peak at λ = 470 nm indicates the production of O2•−. As

300

shown in Figure 4(c), no absorption peaks are detected for C60 aqueous suspension

301

under UV irradiation. This is consistent with our ESR results which have shown that

302

C60 aggregates do not produce O2•−, because of a lack of ED and short lifetime of

303

3

304

increased with increasing HA concentrations. Previous studies have also detected O2•−

305

in HA solution under simulated sunlight or UV irradiation.17, 44 Our work has shown

306

that C60 alone did not produce O2•−, and their addition to HA solutions significantly

307

enhanced O2•− production rates, and the production rate increased with HA

308

concentration. This indicated that C60 could photocatalytically promote O2•−

309

generation by HA. In marked contrast, no measurable amount of O2•− was detected in

310

FA solution with or without C60 (Figure S4 of SI). These results indicated that the

311

electron donation efficiency of HA was higher than that of FA.

C60*.8, 9, 43 HA solution generated O2•− under UV irradiation and the production rate

312

Two factors can influence the electron transfer capacity of DOM, i.e.,

313

aromaticity and phenolic group content.19, 45 Lower aromaticity and phenolic group

314

content result in lower electron transfer efficiency of DOM.19, 45 Previous studies have

315

demonstrated that the aromaticity of FA (0.28) is approximately 1.5-fold lower than

13



316

the aromaticity of HA (0.42).26, 27 Furthermore, the phenolic moieties are responsible

317

for charge transfer in FA (1.5 meq/g), which is approximately two-fold lower than that

318

in HA (2.8 meq/g).46 In the dark, no absorption peaks were detected for a C60 solution,

319

DOM solutions, or their mixtures (Figure S5). Many previous studies have

320

demonstrated that the degradation of XTT by UV-365 light was negligible. 29-31

321 322 323 324 325 326 327

(c)

(d)

10 mg/L HA

(e)

(f)

50 mg/L HA

328 329 330 331 332 333 334 335

Figure 4. 1O2 generation kinetics indicated by the degradation of 0.85 mM FFA (a-b) and O2•− generation kinetics (c-f) in C60 aqueous suspension, DOM solutions, or their mixtures as shown by the reduction of 200 μM XTT (UV irradiation at 365 nm, light intensity of 1.0 × 10-6 Einstein·L-1·s-1, and C60 of 5 mg/L).

336

ROS Concentration Photogenerated by C60, DOM Solution, or their

337

Mixtures. Table 1 summarizes the average molar concentrations of 1O2 and O2•−

338

photogenerated in C60 aqueous suspension, DOM solutions, or their mixtures. Only

339

C60 dispersed in water did not produce measurable amount of ROS, whereas DOM

340

solutions with or without C60 generated at least one type of ROS. The average molar

341

concentrations of total ROS (the sum of the concentrations of 1O2 and O2•−) generated

342

in DOM/C60 mixtures were 1.3~1.8 times higher than the sum of their individual 14


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343

production concentrations, indicating that DOM/C60 mixtures generated ROS in a

344

synergistic way. This was primarily because the decreased C60 aggregation by DOM

345

increased the specific surface area for light adsorption and O2 exposure. It was

346

observed that: (1) The photogenerated 1O2 concentrations in decreasing order

347

followed this trend: FA/C60 mixture > FA > HA/C60 mixture > HA at the same DOM

348

concentration and C60 concentration of 5 mg/L. The 1O2 production concentrations for

349

FA solutions were approximately 1.6-fold, 1.9-fold, and 2.3-fold more than the

350

concentrations generated in HA solutions at 10.0, 20.0, and 50.0 mg/L, respectively.

351

After addition of 5.0 mg/L C60, 1O2 production concentrations in FA solutions were

352

approximately 2.0 times higher than those generated in HA solutions at all DOM

353

concentrations. (2) No measurable amount of O2•− was detected in C60 aqueous

354

suspension, FA solution, or their mixture. The addition of C60 into HA solution

355

significantly enhanced O2•− generation concentrations, which were increased by up to

356

1.8 fold as compared with those generated in HA solution alone.

357 358

Table 1. Average molar concentrations of ROS generated by C60, DOM solutions, or their mixtures (UV-365 irradiation and light intensity of 1.0 × 10-6 Einstein·L-1·s-1). C60 (mg/L) 0

HA

0

FA

5.0

5.0

5.0 5.0 359 360

DOM (mg/L)

HA

FA

HA FA

1

O2 (μM)

O2•− (μM)

Total (μM)

10.0 20.0 50.0 10.0

49.8 ± 2.1 63.0 ± 2.5 72.8 ± 2.8 79.2 ± 0.3

18.9 ± 1.8 36.8 ± 2.0 58.2 ± 2.2 N.D.

68.7 ± 3.9 99.8 ± 4.5 131.0 ± 5.0 79.2 ± 0.3

20.0

121.6 ± 7.2

N.D.

121.6 ± 7.2

50.0 10.0

165.1 ± 8.2 60.8 ± 2.3

N.D. 49.0 ± 2.1

165.1 ± 8.2 109.8 ± 4.4

20.0

91.1 ± 4.1

83.8 ± 3.8

174.9 ± 7.9

50.0 10.0

108.7 ± 6.6 114.8 ± 6.8

127.9 ± 7.4 N.D.

236.6 ± 14.0 114.8 ± 6.8

20.0

163.4 ± 8.6

N.D.

163.4 ± 8.6

50.0 0 0

215.9 ± 13.2 N.D. N.D.

N.D. N.D. N.D.

215.9 ± 13.2 0 0

N.D. indicates that ROS were not detected or were not statistically significant. Decay Kinetics and Lifetime of 3C60*. 3C60* is the key transient intermediate 15



Page 16 of 24

361

for both energy transfer (i.e., 1O2 generation) and electron injection (i.e., O2•−

362

generation) process in C60 aqueous suspensions after light irradiation.1,

363

quantitatively analyze the lifetime of 3C60*, the decay kinetics of 3C60* in the C60

364

aqueous suspension with or without DOM was fitted by the following single

365

exponential function:

2

It = I0 exp (-t/τ0) + C

366

To

(1)

367

where It is the fluorescence intensity of 3C60* after a UV irradiation time of t (μs), I0 is

368

the initial fluorescence intensity of 3C60*, τ0 is the lifetime of 3C60* (μs), and C is a

369

constant. The experimental data have been fitted by Eq 1, and the model fits are

370

shown by red lines in Figure 5. The correlation coefficients (R2), fitted values of I0, τ0,

371

and C are shown in Table S2 of SI. Apparently, the addition of HA or FA into C60

372

aqueous suspension significantly prolonged the lifetime of 3C60*.

373

As shown in Table S2, the lifetime of 3C60* in the C60 aqueous suspension is 0.09

374

± 0.01 μs. This is consistent with previous studies that showed 3C60* in the C60

375

aqueous suspension decayed within 100 ns.9, 10 As demonstrated by our DLS results

376

and previous studies,37, 38 C60 dispersed in water was comprised of closely packed C60

377

molecules. The short lifetime of 3C60* in C60 aggregates was primarily due to

378

quenching by surrounding ground-state C60 (self-quenching) or another

379

(triplet-triplet annihilation) on the aggregate surface.3, 5 Therefore, the pathway for

380

energy and electron transfer to O2 is fundamentally blocked as 3C60* becomes

381

short-lived and less available,3, 5 which could explain why no ROS was detected in C60

382

aqueous suspenion.

3

C60*

383

In contrast, when HA or FA was added into C60 aqueous suspensions, the

384

liftetime of 3C60* extended to 4.63 ± 0.05 or 4.59 ± 0.04 μs, respectively, which was

385

approximately fifty-fold longer than that in C60 aqueous suspension (Table S2 of SI).

16


Page 17 of 24


386

This was primarily because DOM decreased the aggregate size and enhanced the

387

colloidal stability of C60 aggregates. As the size of aggregates decreased, more C60

388

molecules on aggregate surface were exposed to light forming more 3C60*.37, 38 These

389

lead to less ground state C60 and subsequently decreased their quenching for 3C60*.

390

Meanwhile, the enhanced colloidal stability decreased the possibility of triplet-triplet

391

annihilation among different C60 aggregates. Therefore, the lifetime of 3C60* in the

392

mixture is sufficiently long for 3C60* to mediate both energy transfer and charge

393

injection to O2. No absorbance has been observed at 740 nm in HA or FA solution.

394

Hotze et al.48 have also demonstrated that aggregate size and structure strongly

395

affected the fraction of 3C60* within an aggregate and influenced ROS generation in

396

C60 aqueous suspension.

397 398 399 400 401 402 403

Figure 5. Absorption-time profiles of 3C60* recorded at 740 nm in C60 aqueous suspensions with or without HA/FA at different laser excitation time in N2-saturated condition (C60 of 5 mg/L, HA of 10 mg/L, and FA of 10 mg/L).

404

Proposed Reaction Framework. We propose a ROS photogeneration

405

mechanism for DOM/C60 mixtures. As shown in Figure 6, the synergistic

406

photogeneration of 1O2 in the DOM/C60 mixutres could be attributed to the energy

407

transfer between them. The 365 nm UV light can induce the generation of 3DOM*

408

because the incident photon energy (3.4 eV) is higher than the energy of 3DOM*

409

(180~250 kJ/mol, 1.9 to 2.6 eV).45, 49 As far as the photophysics is concerned, 3DOM*

410

has a much higher energy level than that of 3C60* (157~176 kJ/mol, 1.63~1.83 eV) or

411

1

O2 (94.3 kJ/mol, 0.98 eV).19, 50, 51 The fractions of light absorbed by C60 or DOM in

17



412

their mixtures are calculated in Section S10 of SI. DOM absorbs much more light

413

than C60 in C60/DOM mixtures. Therefore 3DOM* can transfer energy to O2 but more

414

importantly 3DOM* can transfer energy to C60 and create 3C60*. And the prolonged

415

lifetime of 3C60* provides more opportunity for the energy transfer to O2 and

416

generates more 1O2. It is important to note that the creation of smaller C60 aggregates

417

from the adsorption and stabilization caused by DOM increases the lifetime of 3C60*

418

because larger aggregates contain several 3C60* and they can annihilate each other

419

before energy transfer to O2.

420

As shown in Figure 6, the synergistic photogeneration of O2•− in HA/C60 mixture

421

is primarily attributed to the electron donation from photoionized HA to electrophile,

422

C60. With respect to thermodynamics, the standard oxidation potential of HA was

423

+0.797 V, demonstrating that HA can reduce 3C60* which has a standard reduction

424

potential of E0(3C60*/C60•−) +1.1 V.3, 5, 36 The ground state C60 has a standard redox

425

potential of E0(C60/C60•−) -0.2 V,3, 5, 36 indicating that the electron transfer from HA is

426

more favorable to 3C60* (1.897 V) than the ground state of C60 (0.597 V), resulting in

427

C60•− formation. Subsequently, C60•− transfers electrons to O2 and produces O2•−.

428

Moreover, C60 promotes electron depletion from HA during photoionization and

429

photocatalytically promotes O2•− generation by HA. The standard oxidation potential

430

of FA is +0.507 V, which indicates that FA could reduce 3C60*. However, our

431

experimental results of fluorescence quenching of DOM by C60 showed that the

432

electron donation capacity of FA was lower than that of HA (Section S11 of SI). In

433

addition, the quantum yield for e- production by FA (7.9 × 10-6)52 was two orders of

434

magnitude lower than that by HA (1.7 × 10-3) under UV irradiation.53 Therefore, it is

435

hard for FA to loose an electron and serve as electron donors for C60. This explains

436

why no measurable amount of O2•− was detected in FA/C60 mixture.

18


Page 18 of 24

Page 19 of 24


437

C60 is electron deficient and a good electron acceptor (can reversibly accept up to

438

six electrons). Thus the electron transfer from C60 to DOM is not likely to occur in

439

principle. Kong et al.19 have also demonstrated that DOM can act as electron donors

440

and transfer electron to C60 in their mixtures, and the reverse, did not take place. The

441

lower energy level of 3C60* than that of 3DOM* makes it impossible for energy

442

transfer from C60 to DOM. Therefore, C60 cannot transfer electron and energy to DOM

443

under light illumination. In conclusion, the electron and energy transfer from DOM to

444

C60 contributes to the synergistic ROS generation in the mixed system.

445 446 447 448 449 450 451 452 453

Figure 6. The reaction schema of the synergistic generation of 1O2 and O2•− by the mixture of C60 and DOM in aqueous phase under UV-365 irradiation.

454

Our work demonstrated that the energy and electron transfer between DOM and

455

C60 resulted in the synergistic generation of 1O2 and O2•− for DOM/C60 mixtures when

456

exposed with 365 nm UV light. This strongly suggests that DOM and sunlight may

457

play critical roles in the fate and photochemical activity of C60 in the natural aqueous

458

environment. After C60 are released into surface waters where DOM is ubiquitous, the

459

enhanced ROS generation concentrations of C60 could increase their possibility to

460

pose hazardous to aquatic organisms under natural solar irradiation. Thus the risk

461

assessment of C60 should consider both their inherent toxicity and their possible

462

interactions with coexisting DOM in natural waters. 19



463

ASSOCIATED CONTENT

464

Supporting Information Available

465

Chemicals used in this study, preparation and characterization method of C60,

466

mass-averaged PSDs of C60 with or without DOM, zeta potential of C60, O2•−

467

production kinetics in FA solutions with or without C60, correlation coefficient for

468

3

469

Supporting Information. This material is available free of charge via the Internet at

470

http://pubs.acs.org.

471

ACKNOWLEDGMENTS

C60* decay kinetics, and fluorescence quenching of DOM by C60 are available in the

472

This study was financially supported by the Fund for Innovative Research Group

473

of the National Natural Science Foundation of China (Grant No. 51421065), the

474

National Natural Science Foundation of China (Nos. 51378065 and 21407010), and

475

China Postdoctoral Science Foundation (No. 224234).

476

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477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494

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Synergistic Photogeneration of Reactive Oxygen Species by

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