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Probing photosensitization by functionalized carbon nanotubes

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Chia-Ying Chen*,†,⊥ , Richard G. Zepp*,§

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†

National Research Council Associate, National Exposure Research Laboratory, Ecosystems Research Division, United States Environmental Protection Agency, Athens, Georgia 30605, United States §

National Exposure Research Laboratory, Ecosystems Research Division, United States Environmental Protection Agency, Athens, Georgia 30605, United States ⊥Department

of Environmental Engineering, National Chung Hsing University, Taichung City 402,

Taiwan

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*Corresponding author phone: (706) 355-8249 (C-Y C); (706) 355-8117 (R.G.Z). E-mail: [email protected] (C-Y C); [email protected] (R.G.Z)

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Abstract

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Carbon nanotubes (CNTs) photosensitize the production of reactive oxygen species that may damage

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organisms by biomembrane oxidation or mediate environmental transformations of CNTs.

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Photosensitization by derivatized carbon nanotubes from various synthetic methods, and thus with

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different intrinsic characteristics(e.g., diameter and electronic properties), has been investigated under

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environmentally-relevant aquatic conditions. We used the CNT-sensitized photoisomerization of sorbic

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acid ((2E,4E)-hexa-2,4-dienoic acid) and singlet oxygen formation to quantify the triplet states (3CNT*)

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formed upon irradiation of selected single-walled carbon nanotubes (SWCNTs) and multiwalled carbon

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nanotubes (MWCNTs). The CNTs used in our studies were derivatized by carboxyl groups to facilitate

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their dispersion in water. Results indicate that high-defect-density (thus well-stabilized), small-

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diameter, and semiconducting-rich CNTs have higher-measured excited triplet state formation and

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therefore singlet oxygen (1O2) yield. Derivatized SWCNTs were significantly more photoreactive than

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derivatized MWCNTs. Moreover, addition of sodium chloride resulted in increased aggregation and

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small increases in 1O2 production of CNTs. The most photoreactive CNTs exhibited comparable

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photoreactivity (in terms of 3CNT* formation and 1O2 yield) to reference natural organic matter (NOM)

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under sunlight irradiation with the same mass-based concentration. Selected reference NOM could 1 ACS Paragon Plus Environment

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therefore be useful in evaluating environmental photoreactivity or intended antibacterial applications of

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CNTs.

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Introduction Carbon nanotubes (CNTs), which are widely used in industry, have been studied extensively.

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Single-walled carbon nanotubes (SWCNTs) have attracted attention due to their well-defined properties

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and small diameters, while multiwalled carbon nanotubes (MWCNTs) have lower production costs and

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are available in large quantities. Due to pristine strong bundling of CNTs in aqueous solutions,

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dispersants, solvents or surface functional groups are often added to increase transport and dispersion.

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Unlike pristine CNTs, however, covalently functionalized CNTs readily disperse in aqueous solution,

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are likely to be more mobile in aquatic environments, and have more interactions with aquatic

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organisms. Chemical functionalization also enhances chemical-tailoring surface properties of CNTs for

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various applications.1, 2 Covalently carboxylated CNTs are one of the most easily prepared derivatives

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used to achieve dispersed suspensions. Carboxylated derivatives also can be readily modified by

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techniques such as esterification to produce CNTs with a variety of useful properties.3, 4

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Photosensitization is a major photochemical process that affects transformation and toxicity of any

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material in the environment. Photogenerated reactive oxygen species (ROS) can mediate

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transformations of nanomaterials themselves, as well as other compounds or microorganisms. ROS

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(including singlet oxygen (1O2) in natural environment) may act as oxidizing agents in aqueous media or

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biological systems.5 Sunlight-induced 1O2 production by nanomaterials has been observed, including

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aqueous Buckminster fullerene clusters (aqu/nC60),6 fullerol,7-10 engineered metal oxides,11 SWCNTs,12

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and MWCNTs.13 In aquatic systems, photoexcited functionalized SWCNTs undergo efficient energy

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transfer to molecular oxygen to produce 1O2, as well as electron transfer, to form superoxide radical

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anions (O2-) and hydroxyl radical (·OH) under sunlight or UVA irradiation.12, 14 Functionalized

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MWCNTs were reported to generate 1O2 and ·OH, but no detectable O2- during UVA irradiation.13

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Direct comparison or investigation of underlying mechanisms in well-defined conditions of CNTs from

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different sources (e.g., SWCNTs vs. MWCNTs) have not been considered previously. Indeed, the

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diversity of emerging nanomaterials from various synthesis methods and treatments often leads to 3 ACS Paragon Plus Environment

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differences in reported reactivity. In particular, characteristic variations of CNTs including tube

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diameters, chiral angles, and metal residuals make it difficult to consider them a uniform group of

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substances, or even to characterize them by a typical set of physicochemical parameters.15 Unveiling

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the underlying mechanisms of photoreaction dynamics after exposure to solar radiation among different

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CNTs at various aquatic conditions is thus challenging, yet crucial to assessing their environmental

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impacts. Relationships between physicochemical properties of CNTs and their environmental fate,

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including photoreactivity, are still not well understood, however.

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Aquatic chemistry such as changes in colloidal stability and ionic strength may affect reactivity of

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nanomaterials, subsequently altering their properties and fate.16 The colloidal stability of CNTs has been

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shown to correlate strongly with intrinsic characteristics of the CNTs (e.g., chirality,17 and surface

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functional groups18) or surrounding media (e.g., dispersant,19 pH, and ionic strength20). The aggregation

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status of nanomaterials has previously been reported as a critical governing factor of reactivity,

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including photosensitization. For example, aggregation of metal oxide nanoparticles was reported to

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reduce their photocatalytic generation of free hydroxyl radicals.21 The higher local concentration of

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cages within aqu/nC60 aggregates was proposed as leading to triplet-triplet annihilation, self-quenching

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reactions, and limited mass transfer of oxygen. These processes significantly accelerate the decay of

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excited triplet state C60 thus limiting sensitized photoproduction of singlet oxygen.22, 23 Moreover, it

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was suggested that the structure of the aggregates, rather than aggregation itself, affects

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photosensitization properties.7

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In this study, we sought to answer if and how intrinsic characters (e.g., diameter and electronic

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properties), solution chemistry (e.g., ionic strength) and colloidal stability of carboxylated CNTs interact

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to affect their photoreactivity under sunlight irradiation. These interactions have not been considered

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previously. This study sheds light on mechanisms of aquatic photoreactivity of CNTs under sunlight

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exposure which is important to evaluating exposure to CNTs in the environment, as well as potential

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antibacterial applications of CNTs. Three representative, commercially available carboxylated CNTs 4 ACS Paragon Plus Environment

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were selected, one of which was further oxidized/purified in-house before study. Attachment efficiency

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and dynamic light scattering were used to study colloidal stability CNTs, while sorbic acid and furfuryl

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alcohol (FFA) were applied as probes of photoproduced triplet state and ROS, respectively. This work

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experimentally proves that interactions of CNTs with environmental constituents affect their

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photoreactivity in the aquatic environment by altering their photoinduced triplet excited state formation

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and ROS generation.

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Materials and Methods

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Materials and Preparation of Aqueous Stock Dispersions of CNTs. CNTs used in this study are

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listed in Table S1; two carboxylated CNTs were purchased from NanoLab, Inc. (D1.5L1-5-COOH and

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PD15L1-5-COOH, Waltham, MA). According to NanoLab, Inc., pristine SWCNTs and MWCNTs

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produced by chemical vapor deposition were refluxed in concentrated sulfuric/nitric acid, resulting in

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CNTs hereafter referred to as NLSWCNT and NLMWCNT, respectively. P3SWCNT is a carboxylated

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SWCNT from Carbon Solutions, Inc. (Riverside, CA), made by an electric arc discharge technique.

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These three tubes were used without further modification. HOMWCNT was highly oxidized using

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NLMWCNT as starting material by a similar method described by Smith et al.24 and Peng et al.25

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Briefly, NLMWCNT was refluxed in concentrated acid mixture (sulfuric acid: nitric acid = 3:1, v/v) at

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80oC for 8 h, while stirring. The oxidation products were repeatedly filtered through a 0.45 µm

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membrane and resuspended in water until the filtrate reached pH 5-6, then dried overnight at 100oC.

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Aqueous dispersions of CNTs were prepared by probe-sonicating (Misonix, Inc., Model XL2020;

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550 W; 20kHz, Farmingdale, NY) a mixture of 5 mg CNTs with 50 mL of pure water in an ice-water

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bath until an energy of ~ 23400 J was delivered. After ultrasonication, CNT suspensions were

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centrifuged (Eppendorf model 5424) at 10000 g for 30 min, followed by collecting 70-80% of the

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supernatant. Stability and consistency between batches of CNTs suspensions was checked by

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monitoring their UV-visible and near infrared (NIR) absorption spectra and averaged hydrodynamic

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radius with dynamic light scattering (DLS). Resulting stock suspensions were stored in the dark at 4oC 5 ACS Paragon Plus Environment

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and were stable during use, except for NLMWCNT. NLMWCNT stock suspensions were freshly made

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each month since agglomerated tubes were found at around one month. Note that the majority of the

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results presented were collected in phosphate buffer at pH = 7 ± 0.2.

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Natural organic matter solutions (200 mg /L) including Suwannee River natural organic matter

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(SRNOM, International Humic Substances Society) and Aldrich humic acid (AHA, Sigma Aldrich)

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were mixed in pure water, adjusting pH to ∼10 by NaOH to facilitate dissolution, stirred overnight, then

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passed through 0.45 µm membrane filters. Dissolved organic carbon of the filtrate of SRNOM and AHA

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was determined with a Shimadzu carbon analyzer (TOC-VCPH, Columbia, MD). Chemicals of the

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highest purity available from Sigma Aldrich (St. Louis, MO) were not purified further, and all aqueous

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solutions were prepared in water purified with a Barnstead Smart2Pure© System (≥18.2 MΩ·cm).

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Irradiation. There are two major irradiation data sets in this study. One is derived from detailed

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experiments carried out with one CNT and one source of DOM under monochromatic irradiation at 366

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nm. Photoreaction kinetics were compared between samples optically matched to same initial

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absorption coefficient (0.58 cm-1, NLSWCNT: 8 mg-C/L and SRNOM: 75 mg-C/L) (Figure S1). The

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other data set involved experiments conducted with a wider array of CNT and NOM samples

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concurrently exposed to broad-band irradiation by simulated sunlight. Samples with similar mass

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concentrations were used (CNTs: 8 mg-C/L; SRNOM: 10 mg-C/L and AHA: 6 mg-C/L).

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Solar irradiations were performed in an Atlas SunTest CPS/CPS+ solar simulator (Atlas Materials

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Testing Technology, Chicago, IL) equipped with a 1kW xenon arc lamp. Irradiance of the simulator in

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the UV spectral region was very similar to mid-summer, midday natural sunlight at 33.95oN, 83.33oW

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(Athens, GA) (Figure S2). Reactions were carried out in 8 mL Pyrex tubes containing 5 mL solutions.

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Throughout irradiation, tubes were maintained at 25oC in a NESLAB recirculating water bath. Spectral

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irradiance at the surface of the tubes was measured using an Optronic Laboratories OL756

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Spectroradiometer (spectrum in Figure S2); incident irradiance at the tube surface, summed from 290 to 6 ACS Paragon Plus Environment

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700 nm, was 0.065 W/cm2. Additionally, monochromatic irradiation at 366 nm was conducted in a

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rotating turntable merry-go-round reactor (MGRR), in the center of which was a Hanovia medium

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pressure Hg vapor lamp (450 W). A borosilicate glass sleeve in the immersion well of the lamp blocked

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light of wavelengths < 300 nm. A combination of Corning 0-52 and 7-37 filters on a box surrounding

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the lamp isolated 366 nm light. Using ferrioxalate actinometers in Pyrex tubes, the light intensity was

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measured as 1.19 ± 0.12 × 10-6 mol photon L-1 s-1 at 366 nm.

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Characterization and ROS Production Measurement of CNT suspensions. The absorption

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spectra of CNTs were scanned by a Perkin-Elmer Lambda 35 UV-visible-NIR absorption

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spectrophotometer equipped with a 1 cm quartz cuvette. Additionally, a Perkin Elmer Lambda 900

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spectrophotometer equipped with an integrating sphere attachment (150 mm in i.d. and with BaSO4

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inside coating) was used to evaluate light scattering effects. Minimal differences were found in CNT

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absorbance spectra collected with or without the integrating sphere attachment (Figure S1).

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Hydrodynamic radius (Rh), electrophoretic mobilities (EPMs) and zeta potential of CNTs were

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measured by a ZetaSizer Nano ZS (Malvern Instrument, Worcestershire, U.K.) with a monochromatic

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coherent 633 nm He-Ne laser, over NaCl concentrations ranging from 1 to 100 mM. CNT suspensions

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were also characterized by a Renishaw inVia Reflex Raman Microscope System (Renishaw, Hoffman

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Estates, IL) at λ = 514, 632, and 785 nm. Photoproduction of 1O2 was monitored via loss of furfuryl

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alcohol (FFA) from which the rates and pseudo-steady-state concentrations of 1O2 were determined

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(Text S1 and Figure S4 in Support Information). The FFA concentration (0.2 mM) used was

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sufficiently low that the photochemical loss of furfuryl alcohol obeyed first order kinetics, as expected

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when the steady state concentration of singlet oxygen was not significantly repressed.26 During

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experiments, samples were periodically removed from the light source and filtered through 0.2-µm

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membrane filters prior to high performance liquid chromatography (HPLC) analysis, using a C18

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column with acetonitrile/water (30:70) as the mobile phase.

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Triplet Formation Measurement. The formation rate of excited triplet states of CNTs and NOM

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under simulated sunlight or monochromatic 366 nm irradiation was probed by following the

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photosensitized isomerization of sorbic acid ((2E,4E)-hexa-2,4-dienoic acid (t,t-HDA)) (100, 500, 750,

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1000 µM in this study) with a modified version of a recently-developed method that determines triplet

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kinetics of organic compounds.27-29 During simulated sunlight exposures, a glass filter blocked

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irradiance lower than 316 nm to retard direct photoisomerization of t,t-HDA (Figure S6). With the filter

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in place, direct photoisomerization of t,t-HDA was determined by experiments in purified water (no

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added photosensitizer); the minimal concentrations of isomers produced by this process were subtracted

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from final concentrations observed in other studies with added photosensitizer. After energy transfer

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from the photoexcited triplet states of the CNTs, NOM and benzophenone, excited t,t-HDA isomerizes

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to four HDA isomers ((2Z,4E)-hexa-2,4-dienoic acid (c,t-HDA), (2Z,4Z)-hexa-2,4-dienoic acid (c,c-

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HDA), (2E,4E)-hexa-2,4-dienoic acid (t,t-HDA), and (2E,4Z)-hexa-2,4-dienoic acid (t,c-HDA))

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including reformation of t,t-HDA itself. Formation of c,t-HDA, c,c-HDA, and t,c-HDA were directly

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determined, while the reformation rate of t,t-HDA was calculated for each photosensitizer, based on t,t-

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HDA relative ratio to c,t-HDA derived by a multiple linear regression. Triplet formation rates (FT) under

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366 nm or sunlight irradiation were then calculated using the sum of c,t-HDA, c,c-HDA, and t,c-HDA

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formation rates, plus the t,t-HDA reformation rate (detailed in Text S2 in Supporting Information).

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Effects of changing ionic strength on NLSWCNT triplet energy transfer to HDA were quantified as

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relative ratios of the triplet loss rate constant (k’s) and steady-state triplet concentration ([Tss]) to those at

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buffer-only conditions. HDA isomers were measured by HPLC using a C18 column with 15%

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acetonitrile and 85% acetate buffer (30 mM, pH = 4.75), detected at UV 254 nm. More details and a set

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of representative HPLC chromatograms showing full resolution of four isomers (Figure S7) are

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presented in Supporting Information.

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Aggregation Kinetics. Early-stage aggregation of four CNTs as a function of ionic strength was

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monitored using dynamic light scattering (TRDLS) to determine the time-dependent increase of Rh,30 8 ACS Paragon Plus Environment

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Initial particle size (Rh0) was measured immediately after mixing CNTs (approximately 1 mg/L) at pH =

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7.0 ± 0.2. Aggregation was initiated by adding the phosphate buffer and a series of NaCl electrolyte

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solutions at final concentrations varying from 1-1000 mM. Hydrodynamic radius measurements were

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started promptly after momentary vortexing of the cuvette containing 1 mL final suspension. The

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autocorrelation function was analyzed every 15 s within 60 min, until the Rh value exceeded 1.5 Rh0.19

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The initial aggregation rate constant (ka) correlated to increases in Rh and initial particle concentration,

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N0:19, 24, 30, 31

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ka =

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To quantify and compare aggregation kinetics of different samples, the aggregation attachment

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efficiencies α (also known as inverse stability ratio, 1/W) is defined as the ka at varied electrolyte

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concentrations, normalized by ka under diffusion-limited (fast/favorable) conditions:19, 24, 30, 31

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α=

1  dR h (t )    N 0  dt  t → 0

k 1 = a = W k a , fast

1 N0 1 N 0, fast

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 dR h (t )     dt  t → 0  dR h (t )     dt  t → 0, fast

Results and Discussion Surface Characterization and Electrokinetic Properties of CNTs. Figure 1(a) shows

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electrophoretic mobilities (EPMs) of four CNTs, determined as a function of NaCl concentrations at pH

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7. Over the entire range of concentrations, the CNTs are negatively charged, becoming less negative as

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electrolyte concentrations increase due to electrical double-layer compression.32 The hydrophilic nature

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of carboxylated CNTs is consistent with their high negative surface charge at low salt concentrations.

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NLMWCNT and HOMWCNTs are less negatively charged compared to the other two SWCNTs,

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especially in the high NaCl concentration region, which is consistent with faster aggregation (i.e., easier

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screening by Na+) with increasing electrolytes (see Aggregation Studies). Despite the fact that DLS is

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not ideal for determining non-spherical particles, the intensity’s averaged hydrodynamic radius (Rh) 9 ACS Paragon Plus Environment

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could be used to obtain a general index of the size population of CNT suspensions.24, 25 NLSWCNT has

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the smallest Rh (∼60 nm), followed by P3SWCNT, HOMWCNT and NLMWCNT, as shown in Table

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S1. High polydispersity indices (PDIs) of all four tubes reflect the high size heterogeneity of CNT

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suspensions. Raman spectroscopy is one of the most sensitive tools for characterizing CNTs. Radial

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breathing mode (RBM) is a bond stretching out of the plane phonon mode whose frequency (between

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120 and 350 cm-1) is inversely proportional to the tube diameter of SWCNTs: ωRBM=A/d+B; A=234 cm-

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1

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stretching vibration of the C-C bonds within the pristine graphitic surface of SWCNTs, while the D

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band peak is associated with surface structural disorder arising from any surface defect sites that

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interrupt the sp2 carbon structure (i.e., disorder-induced band). Hence, the ratio of intensities of the D

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and G bands (ID/IG) is commonly used to evaluate the extent of defects. RBM peaks (at 633 nm laser

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excitation) of NLSWCNT (164.6 cm-1 and 219.3 cm-1) and P3SWCNT (164.6 cm-1) were used to

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estimate diameters. The estimates were 1.51 nm and 1.12 nm for NLSWCNT and 1.51 nm for

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P3SWCNT (Figure 1(b)). Note that RBM signals of NLSWCNT are weaker than that of P3SWCNT,

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which may be attributed to a higher breakdown of van Hove singularities NLSWCMTs due to

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introducing irregular distribution of sp3-sites from covalent functional groups.34 The resulting

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suppression of RBM signals of NLSWCNT is in line with a much higher D over G (ID/IG) ratio than that

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of P3SWCNT, showing a higher degree of functionalization NLSWCNT that we will address later.

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Additionally, RBM at 514 nm laser excitation probes mostly the v1-to-c1 transition of metallic CNTs

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with smaller diameter.34, 35 RBM peaks at 514 nm laser excitation of P3SWCNT (Figure S8 (a)) suggest

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the presence of small-diameter metallic nanotubes, while no RBM peaks were detected for NLSWCNT

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at 514 nm laser excitation (Figure S8 (b)). The intensity of the RBM feature of NLMWCNT and

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HOMWCNT is weak, consistent with the RBM signal from large-diameter tubes often being hardly

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visible.36 The fine structure in the RBM regime of HOMWCNT was suppressed due to further

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functionalization.34 The ID/IG of P3SWCNT is 0.09; an increase to 1.03 of NLSWCNT indicates greater

and B=10 cm-1 or A=248 cm-1 and B=0 cm-1.33 The G band peak is associated with tangential, in-plane

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destruction of sp2 carbon bonds by insertion of functional groups or side wall defects. P3SWCNT,

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shows a slightly more negative EPM value than NLSWCNT, however, which is likely attributable in

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part to its higher suspension pH, since a strong dependence of EPM on CNT solution pH was reported

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previously.37 A pronounced increase in D-band intensity of HOMWCNT, on the other hand, confirmed

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successful further functionalization of NLMWCNT (Figure S8 (c)).

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Aggregation Studies. Colloidal stability profiles of NLMWCNT, HOMWCNT, P3SWCNT and

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NLSWCNT are shown in Figure 1(c). The CNT suspensions were diluted to around 1 mg/L to slow

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aggregation rates so that early-stage aggregation kinetics could be accurately measured by time-resolved

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DLS. Representative aggregation profiles of NLSWCNT are presented in Figure S9. For all CNTs,

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distinct reaction-limited and diffusion-limited aggregation regimes were observed, indicating that

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Derjaguin-Landau-Verwey-Overbeek (DLVO) theory controls their aggregation kinetics,38 which is

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consistent with previous studies of MWCNTs24, 25, 37 and SWCNTs in sodium dodecylsulfate (SDS)19

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and humic acid39 aqueous solutions. In reaction-limited regimes, the aggregation attachment efficiencies

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α progressively increased with NaCl concentration; this is attributed to more Na+ ions screening the

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surface of negatively charged CNTs, reducing the energy barrier and resulting in increased aggregation

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tendency. When the NaCl concentration has no effect on α, it reaches the diffusion-limited regime

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where completely suppressed electrostatic repulsion and lack of an energy barrier leads to favorable

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aggregation. The critical coagulation concentration (CCC) obtained from the intersection of extrapolated

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lines through two regimes is the minimum concentration of electrolyte needed to induce diffusion-

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limited aggregation and, thus, often used as a representative metric of the colloidal stability of

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nanoparticle suspensions.18, 25, 30, 40 The CCCs of NLMWCNT, HOMWCNT, P3SWCNT, and

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NLSWCNT were determined to be 60, 148, 212, and 262 mM NaCl, respectively (Figure 1(c)). Further

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oxidation of NLMWCNT significantly enhanced the density of defects and, therefore, their dispersal

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ability, leading to a much more stable dispersion (HOMWCNT). This is consistent with the observation

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found,25 as well as a linear correlation at pH = 4 to 8 between surface total oxygen concentration of

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oxidized MWCNTs and CCC18 in which the carboxylic acid group was most closely correlated with

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CCC.

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Triplet State Formation of CNTs in Sunlight. Rapid singlet-to-triplet intersystem crossings (e.g.,

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S1→T1, τisc of (6,5) chirality-enriched SWCNTs ~ 20 ps), coupled with relatively longer triplet lifetime

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(τt of (6,5) SWCNTs ~ 15 µs),41 suggested to us that triplet states may be responsible for most

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photosensitization processes of CNTs. Photosensitization by CNTs was first probed by quantifying

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their triplet state formation via energy transfer to photoisomerize dienes; HDA was the diene in this

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study. This technique was previously used to evaluate excited triplet states of natural organic matter.27,

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42

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t,t-HDA. Not all excited triplet states of CNTs are capable of transferring energy to HDA: energy

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transfer occurs rapidly only when triplet energies of the CNT excited states are equal to or greater than

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the excited state energy of the diene.27, 42, 43 Because the excited state energy of oxygen in its singlet

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state is generally lower than those of dienes, a wider range of CNT triplets may sensitize singlet oxygen

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formation. For example, the fraction of NOM excited triplets capable of producing singlet oxygen by

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energy transfer to molecular oxygen is significantly larger than the fraction capable of sensitizing diene

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photoisomerization.42 This approach is utilized as a probe for 3CNT* capable of sensitizing

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photoisomerization of t,t-HDA -- that is, 3CNT* with energy higher than approximately 2.1 eV. In the

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air-equilibrated solutions used in this work, 3CNT* were expected to be scavenged by both oxygen and

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t,t-HDA, and the triplet formation rate calculated would therefore be lower than in de-aerated solutions.

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We conducted initial kinetic studies of c,t-HDA formation on monochromatic irradiation (366 nm) of

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t,t-HDA (100 µM) (Figure 2(a)). In these monochromatic experiments solutions containing SRNOM or

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SWCNTs as photosensitizers were optically matched to the same absorbance at 366 nm (Figure S1);

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photosensitized c,t-HDA formation rates decreased in the following order: SRNOM > NLSWCNT >

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P3SWCNT > NLMWCNT (Figure 2(a)). P3SWCNT and NLMWCNT showed low to no c,t-HDA

Before irradiation, the presence of isomers in the solutions was negligible, except for unisomerized

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formation (≤ rate of direct photoisomerization) under monochromatic light. Additional detailed studies

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at 366 nm were conducted with SRNOM and NLSWCNT as photosensitizers under both air-saturated

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and nitrogen-purged conditions (Table 1). The triplet quantum yields for SRNOM were ten- and four-

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times higher than those of NLSWCNT under air-saturated and nitrogen-purged conditions, respectively,

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while the 1O2 quantum yield was three-fold higher. These results indicate that only part of the 3CNT*

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has sufficient triplet energy to excite both oxygen and t,t-HDA. Indeed, under air-saturated conditions,

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the observed ratio of triplet quantum yield to 1O2 quantum yield of SRNOM was about 0.5 which is

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close to the previously reported ratio,42 while a lower ratio was observed for NLSWCNT. This further

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indicated that under 366 nm irradiation energy transfer from CNT triplets to diene is comparatively less

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efficient than from NOM triplets. The singlet oxygen quantum yield (1.6 ± 0.8 × 10-2) determined for

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SRNOM agrees with the previously reported value (1.7 × 10-2).42 It should be noted that the

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concentration of SRNOM (75 mg-C/L) used in the 366 nm studies was much higher than that of

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NLSWCNT (8 mg-C/L) to reach the same absorbance value.

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Triplet formation rates were measured under sunlight irradiation for comparison to the 366 nm

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results. Under similar mass concentrations, the triplet formation rate of NLSWCNT

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(1.7 ± 0.2 × 10-8 M s-1) was comparable to, or slightly higher than, SRNOM (1.5 ± 0.2 × 10-8 M s-1) and

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AHA (1.5 ± 0.1 × 10-8 M s-1) (Figure 2(b), Table 2). This might be attributed to higher absorbance of

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NLSWCNT in the visible spectral region where solar irradiance is highest, compared to the rapid

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absorption decline of SRNOM and AHA in the visible region. The higher sunlight absorption rates of

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CNTs in the visible region result in some CNTs (e.g., NLSWCNT) having comparable triplet formation

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rates to NOM, especially under sunlight exposure. Among CNTs tested, NLSWCNT had the highest

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triplet formation rate which was approximately 19-fold and 131-fold higher than rates observed for

301

P3SWCNT (9.0 ± 3.0 × 10-10 M s-1) and HOMWCNT (1.3 ± 0.5 × 10-10 M s-1), respectively. Minimum

302

triplet formation (less than direct photoisomerization of t,t-HDA) was detected for NLMWCNT using

303

this approach. The band gap of semiconducting CNTs is proportional to the reciprocal of CNTs radius,44 13 ACS Paragon Plus Environment

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and CNTs with larger band gaps exhibited shorter wavelength emission (i.e., higher energy emitted

305

photons).45 NLSWCNT with its smaller diameter and thus larger band gap likely has excited triplet

306

states with higher energy, resulting in an increased likelihood of photoisomerization of t,t-HDA and the

307

highest 3CNT* formation rate. The band gap of semiconducting SWCNTs is ∼0.1 to 2 eV for most

308

tubes.46, 47 Semiconducting MWCNTs are semi-metallic (like graphite) due to the reduced band gap for

309

large tubes and electron-hole pairing for multiwall coupling.47 Light absorption generates an excited

310

state which initiates reactions with surrounding compounds such as oxygen and sorbate. Although the

311

energy provided by 366 nm photons (3.4 eV) or sunlight is sufficient to photoexcite all four CNTs from

312

the ground state, our results indicate that light absorption by MWCNTs with a smaller band gap leads to

313

fewer excited triplets with energy sufficient for energy transfer to oxygen (0.97 eV) and especially to

314

sorbate (∼2.1 eV). It has been reported that semiconducting SWCNTs have a longer-lived electronic

315

excited state than metallic SWCNTs.48 Strong RBM peaks under 514 nm Raman resonance of

316

P3SWCNT (Figure S8 (a)) suggest the presence of metallic tubes that could shorten their photoexcited

317

state lifetime. It should be noted that triplet state formation rate measured here corresponds to triplets

318

with enough energy to isomerize sorbate. The triplet state formation rate may be underestimated for

319

CNTs, including P3SWCNT, NLMWCNT, and HOMWCNT, because lower triplet formation rates

320

detected by this approach could mean less efficient energy transfer from 3CNT. Defects on the tube

321

walls were implicated as sites where trapped excitation energy participates in formation of excited

322

triplet states on the CNTs, leading to enhanced energy transfer to O2 producing 1O2.49, 50 This possible

323

involvement of defects is supported by our observation of enhanced triplet reactivity of HOMWCNT

324

compared to NLMWCNT (Fig. 2 (b)). As we will discuss later in the section on ionic strength effects

325

on photoreactivity of CNTs, aggregation doesn’t seem to affect the participation of 3CNT* in type II

326

sensitization.

327

To learn more about the triplet state energy of the CNTs used in this work, we determined the

328

dependence of the composition of the HDA isomers on photosensitizer triplet energy after isomerization 14 ACS Paragon Plus Environment

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to photostationary mixtures (Figure 2(c)) whose composition was sensitive to triplet energy.42 For

330

example, the ratios of the percent of c,t-HDA to c,c-HDA or c,t-HDA to t,c-HDA were significantly

331

lower with the high energy photosensitizer, benzophenone (triplet energy 289 kJ/mol),49 than with the

332

lower energy SRNOM ( ≤ 250 kJ/mol).42 The ratios observed with the CNTs as photosensitizers were

333

significantly higher than with the SRNOM, indicating their triplets were lower in energy than 250

334

kJ/mol; these results further suggest that the triplet energy of P3SWCNT was lower than NLSWCNT.

335

More detailed studies with a broader range of triplet sensitizers are required to confirm and further

336

clarify these results. It also should be noted that the SRNOM sample here was isolated by reverse-

337

osmosis and may contain a more diverse compound class than NOM isolates used in the prior study.42

338

Singlet Oxygen Production of CNTs under Sunlight. The production of 1O2 was measured by

339

monitoring the loss of furfuryl alcohol (Figure 3) in aqueous suspensions of CNTs. Blank tests

340

performed in the absence of CNTs indicated a negligible photolysis of FFA ( < 2%) after the first 10 h

341

solar irradiation, and 11% photolysis after prolonged (95 h) sunlight exposure. No FFA loss was

342

observed in the dark control samples within the experiment’s duration (data not shown). 1O2 steady state

343

concentration ([1O2]ss) in solutions of NLMWCNT and HOMWCNT under sunlight were calculated

344

after correcting for FFA direct photolysis during 95 h sunlight exposure. 1O2 production in CNTs

345

suspensions was further confirmed by azide ion (N3-) quenching (Figure S4). The rates of FFA loss

346

were suppressed by about 60% with addition of 1 mM azide. This quenching effect agrees well with

347

estimates based on the reported second order quenching rate constant of 1O2 by azide ion (kd (N3-) = 5 ×

348

108 M-1s-1).50 [1O2]ss of both NLSWCNT and P3SWCNT were higher than that of SRNOM at similar

349

concentrations under sunlight exposure (Table 2). Lower photoproduction of 1O2 by two MWCNTs, on

350

the other hand, was due primarily to the aforementioned inefficient energy transfer from 3CNT* to

351

oxygen, coupled with low 3CNT* production yield. Electronic band structure is presumably responsible

352

for distinctive photochemical reactivity of different CNTs. 1O2 has been suggested as forming through

353

energy transfer from 3CNTs* to dissolved oxygen.12 Also, applying femtosecond-to-microsecond time 15 ACS Paragon Plus Environment

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domain pump-probe transient absorption spectroscopy, 3O2 quenching of (6,5) chirality-enriched

355

SWCNTs excited states was observed in air-saturated solutions41 which suggests energy transfer from

356

excited CNTs to molecular oxygen. Although oxygen has been shown to be capable of quenching both

357

excited singlet and triplet states of sensitizers, such as polycyclic aromatic hydrocarbon,51 1O2 formation

358

through direct energy transfer from excited singlet states of CNTs is likely to be kinetically limited

359

because of rapid singlet-to-triplet intersystem crossing41 and the very short lifetimes of CNT singlet

360

states.52

361

Ionic Strength Effects. The ratios of 3CNT* formation rate (FT), 3CNT* loss rate constant (k’s)

362

and 3CNT* steady state concentration [T]ss of NLSWCNT, as well as ratios of [1O2]ss of NLSWCNT,

363

P3SWCNT, and HOMWCNT under varied ionic strength (IS) conditions normalized to buffer-only

364

conditions at pH 7, are shown in Figure 4. Three pathways are assumed for the triplet decay and any, or

365

all, could contribute to the observed loss rate constant: (1) intrinsic decay, (2) triplet-triplet annihilation,

366

and (3) ground-state quenching. The triplet kinetics of NLSWCNT are not significantly affected by

367

ionic strength at NaCl concentrations lower than 150 mM. At NaCl = 250 mM; the triplet formation rate

368

and loss rate constant are slightly higher, and triplet steady state concentration is lower. The [1O2]ss of

369

NLSWCNT and P3SWCNT solutions increased about 30% and 8%, respectively, at the highest tested

370

IS (250 mM) which is consistent with the higher triplet formation rate observed. Interestingly, all CNTs

371

were partially or fully aggregated at IS above 80 mM (Figure S10) at a CNT concentration ~ 8 mg-C/L.

372

Photosensitized production of 1O2 by SWCNTs was slightly enhanced in high IS suspensions where

373

extensive aggregation was observed, in contrast to the suppression of 1O2 production in aggregated C60

374

suspensions.16 One possible explanation for the positive effect of increasing IS in the SWCNT systems

375

is that more available metals released from CNTs at high IS promote inter-system crossing of CNTs,

376

then lead to higher singlet-to-triplet intersystem crossing efficiency and 1O2 production. Desferal, a

377

metal chelator, was added to both SWCNT suspensions at buffer-only and 250 mM NaCl conditions to

378

evaluate potential impact of available metal ions on photoreactivity. Addition of desferal to reduce 16 ACS Paragon Plus Environment

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available metals in the SWCNTs suspensions, however, did not significantly suppress 1O2 production,

380

excluding the possibility of significant involvement of free metals in energy transfer from 3CNT* to

381

oxygen. A controlled experiment using Rose Bengal, an efficient 1O2 photosensitizer, showed desferal

382

(50 µM) had no significant impact on 1O2 production (Figure S5). Moreover, it’s unlikely that

383

aggregation resulting from changing ionic strength significantly affects the energy of the excited states

384

involved with energy transfer to oxygen and other energy acceptors. Aggregates of carboxylated CNTs

385

likely resemble fullerol (hydroxylated fullerene) aggregates which have an amorphous structure with

386

hydrophilic functional groups on the surface, and are much less closely packed than aqu/nC60 aggregates

387

where the closely packed crystalline structure facilitates triplet-triplet annihilation and sharply reduced

388

energy transfer efficiency.7, 22, 23, 53 CNTs have surface defects in the form of pentagon and heptagon

389

irregularities at their carbon scaffold as well as incomplete carbon rings at the end termini.37 Covalent

390

functionalization of the sidewall of CNTs introduces a higher density of defects which have been shown

391

to trap excited state energy and, as a result, to slightly enhance formation of triplet states that can

392

transfer energy to singlet oxygen.54 As a consequence, no difference to slightly higher 1O2 production

393

was observed with increasing ionic strength.

394

Environmental Significance. Four acid-treated CNTs synthesized by various methods that led to

395

varied carboxyl functional groups, and thus different surface characteristics and dispersion states in

396

aqueous solutions, were systematically investigated for photoreactivity under sunlight. Our work

397

suggests that high-defect-density (thus well-stabilized), small-diameter, semiconducting-rich SWCNTs

398

have higher measured triplet excited state formation and subsequent 1O2 production. Surface

399

functionality promotes excited energy trapping and, subsequently, photoreactivity. Small-diameter,

400

semiconducting-rich CNTs likely would form triplet excited states with higher energy and longer

401

lifetime, as well as higher 1O2 production rates. Potential reasons for lower triplet photosensitization

402

efficiencies by two MWCNTs could be low 3CNT* production yield, and/or less efficient energy

403

transfer from 3CNT. t,t-HDA was used to probe formation rates of 3CNT* with triplet state energy 17 ACS Paragon Plus Environment

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greater than 2.1 eV. These higher energy 3CNT*s may be significant in affecting photochemical

405

interaction/degradation of certain compounds susceptible to NOM-sensitized photoreactions,55 or in

406

impacting aquatic organisms through photosensitized production of singlet oxygen. Although quantum

407

efficiencies for triplet production are generally lower for CNTs than NOM, the higher sunlight

408

absorption rates of CNTs in the visible region result in some CNTs (e.g., NLSWCNT) having

409

comparable triplet and 1O2 formation rates to NOM, especially under sunlight exposure. Selected

410

reference NOM therefore could be useful in evaluating environmental photoreactivity or intended

411

antibacterial applications of CNTs.

412

Acknowledgments

413

This paper has been reviewed in accordance with the U.S. Environmental Protection Agency’s

414

(U.S. EPA) peer and administrative review policies and approved for publication. Mention of trade

415

names or commercial products does not constitute an endorsement or recommendation for use by the

416

U.S. EPA. Financial support provided by National Research Council (C-Y C) is acknowledged. We

417

thank Dr. John Washington and Tom Jenkins for helping with absorbance measurements using the UV-

418

visible spectrometer equipped with an integrating sphere attachment, Dr. Jack Jones for help with N2

419

glove box, and Ernest Walton and Jeffrey Hendel of Science and Ecosystem Support Division, USEPA -

420

Region 4 for technical assistance with metals measurements.

421

Supporting Information Available Additional information is available free of charge via the Internet at http://pubs.acs.org.

422 423

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Table 1. 1O2 quantum yield, triplet formation rate (FT), and triplet quantum yield of SRNOM and NLSWCNT under monochromatic irradiation at 366 nm (initial absorption coefficient was 0.58 cm-1, NLSWCNT: 8 mg-C/L; SRNOM: 75 mg-C/L) under air-saturated and N2-purged condition. Errors represent one standard deviation from duplicate or triplicate measurements. FT (Ms-1)a O2 quantum yield N2 purged air-saturated -2 -9 SRNOM 1.6 (±0.8)×10 7.5 (±0.1)×10 1.3 (±0.1)×10-8 -3 -10 NLSWCNT 5.2 (±0.4)×10 6.1 (±0.2)×10 3.1 (±0.1)×10-9 a probed by photosensitized isomerization of t,t-HDA 1

576 577 578 579 580 581

582 583 584

Triplet quantum yielda N2 purged air-saturated -3 8.8 (±0.1)×10 1.5 (±0.1)×10-2 -4 7.3 (±0.2)×10 3.7 (±0.1)×10-3

Table 2. Comparison of triplet formation rate (FT) and [1O2]ss of SRNOM, AHA and CNTs under sunlight (CNTs: 8 mg-C/L; SRNOM: 10 mg-C/L and AHA: 6 mg-C/L). Errors represent one standard deviation from duplicate or triplicate measurements. FT (Ms-1)a [1O2]ss (M) -8 SRNOM 1.5 (±0.2)×10 18.6 (±0.6)×10-14 -8 AHA 1.5 (±0.1)×10 41.0 (±1.3)×10-14 -8 NLSWCNT 1.7 (±0.2)×10 46.7 (±0.7)×10-14 P3SWCNT 9.0 (±3.0)×10-10 20.2 (±0.3)×10-14 -10 HOMWCNT 1.3 (±0.5)×10 1.3 (±0.1)×10-14 b NLMWCNT -2.5 (±0.1)×10-15 a probed by photosensitized isomerization of t,t-HDA b ≤ rate of direct photoisomerization of t,t-HDA

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585 586

587 588 589 590 591 592 593

Figure 1. Characterization of CNTs of (a) Electrophoretic mobility (EPM) as a function of NaCl concentration at pH 7. Error bars represent one standard deviation, (b) Raman spectrum (633 nm laser excitation) of P3SWCNT and NLSWCNT, and (c) aggregation attachment efficiencies α (inverse stability ratios, 1/W) as a function of NaCl concentration at pH 6.9 (each stability profile was obtained at CNT concentration of 1 mg/L).

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594 595

596 597 598 599 600 601 602 603 604

Figure 2. HDA isomer formation of SRNOM, AHA, NLSWCNT, P3SWCNT, HOMWCNT, and NLMWCNT in 100 µM t,t-HDA at pH 7: (a) cis, trans-HDA formation as a function of exposure time to monochromatic irradiation at 366 nm (the initial absorption coefficient was 0.58 cm-1 at 366 nm, CNTs: 8 mg-C/L; SRNOM: 75 mg-C/L); (b) cis, trans-HDA formation as a function of exposure time to sunlight (CNTs: 8 mg-C/L; SRNOM: 10 mg-C/L and AHA: 7 mg-C/L); (c) fraction yield of four HDA isomers under sunlight exposure at 3 and 6 hr.

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605 606

607 608 609 610 611 612

Figure 3. Loss of the singlet oxygen acceptor, furfuryl alcohol (FFA), under simulated sunlight exposure at pH = 7, 25.0○C of FFA control, NLMWCNT, HOMWCNT, P3SWCNT, and NLSWCNT. Error bars represent one standard deviation.

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613 614

615 616 617 618 619 620

Figure 4. (a) Normalized 3CNT* formation rates (FT), loss rate constants (k’s), and steady state concentrations ([T]ss) of NLSWCNT, and (b) normalized [1O2]ss NLSWCNT, P3SWCNT, and HOMWCNT at various ionic strengths, to those at buffer-only conditions at pH = 7.

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