Article pubs.acs.org/est
Functionalized Fullerenes in Water: A Closer Look Samuel D. Snow,†,⊥ Ki Chul Kim,‡,⊥ Kyle J. Moor,§ Seung Soon Jang,*,‡ and Jae-Hong Kim*,†,§ †
School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States § Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06510, United States ‡
S Supporting Information *
ABSTRACT: The excellent photophysical properties of C60 fullerenes have spurred much research on their application to aqueous systems for biological and environmental applications. Spontaneous aggregation of C60 in water and the consequent diminution of photoactivity present a significant challenge to aqueous applications. The mechanisms driving the reduction of photoactivity in fullerene aggregates and the effects of functionalization on these processes, however, are not well understood. Here, we take a closer look at the molecular phenomena of functionalized fullerene interactions in water utilizing simulation and experimental tools. Molecular dynamic simulations were performed to investigate time-evolved molecular interactions in systems containing fullerenes with water, oxygen, and/or neighboring fullerene molecules, complimented by physical and chemical characterizations of the fullerenes pre- and postaggregation. Aggregates with widely different photoactivities exhibit similar fullerene−water interactions as well as surface and aggregation characteristics. Photoactive fullerene aggregates had weaker fullerene−fullerene and fullerene−O2 interactions, suggesting the importance of molecular interactions in the sensitization route.
■
INTRODUCTION The unique photochemical properties of (C60) fullerenes and their derivatives have been a topic of great interest to researchers for use in organic photovoltaics,1,2 as efficient singlet oxygen (1O2) sensitizers,3 and for understanding their potential toxicology in aqueous environments.4,5 In aqueous or biological media, the fullerene−fullerene and fullerene−H2O interactions drive the aggregation of C60 into colloidal nanosized clusters, often termed nC60.6,7 In these cases where 1 O2 photosensitization is generally the outcome of interest, nC60 was found to lose most of its photoactivity, whereas aggregates/agglomerates of hydrophilic functionalized C60 may or may not have reduced photoactivity, depending on the functional characteristics.8,9 Although the dynamics of the formation of pristine and functionalized C60 aggregates in water are still not fully understood, several reports have demonstrated the importance of chemical transformation and surface oxidation upon aggregation on fullerene photochemistry.6,10−12 The reduction of photosensitization capacity during aggregation has been attributed to fullerene−fullerene quenching pathways associated with triplet−triplet and triple−singlet annihilation due to the crystalline nature of the aggregates combined with reduced surface area available for sensitization.13−18 Our past work with amino-functionalized fullerenes provided evidence for aggregate morphology and crystallinity determining the photochemistry of fullerene aggregates.16 In addition, Hotze et al. established a theoretical framework for understanding particle photochemistry based on the size and © 2015 American Chemical Society
crystallinity of the aggregates, where smaller and less crystalline particles are predicted to be more photoactive.6,10−12 Several reports have documented enhanced photochemistry of nC60 after, or due to, transformation or oxidation of nC60. Intense UVC treatment and ozonation of nC60 aggregates both resulted in highly oxidized, photoactive aggregates.11,12,19 Further, aggregate size, morphology, and photochemistry were related in detail by Chae et al., finding that surface oxidation groups allowed smaller, and therefore more photoactive, particles to be more stable and photoactive than larger particles that were less oxidized.10 The specific effects of functionalization on aggregate formation photochemistry have been probed by two recent studies that have both shown that photoactivity may depend on the type of functionalization on the C60 cage.8,9 Studies on the effects of functionalization on C60 aggregates have produced intriguing insights, questioning the standing theories of aggregate photochemistry, or lack thereof. Our previous work examined three series of fullerene derivatives with a particularly interesting contrast between the fulleropyrrolidine (A) and the positively charged fulleropyrrolidinium ion (B) series (see Figure S1 of the Supporting Information for a molecular diagram. The following nomenclature will be used to be consistent with our previous work: A1 Received: Revised: Accepted: Published: 2147
September 27, 2014 January 14, 2015 January 16, 2015 January 29, 2015 DOI: 10.1021/es504735h Environ. Sci. Technol. 2015, 49, 2147−2155
Article
Environmental Science & Technology
Batches of aggregates were dried at 105 °C for 24 h for XPS and FTIR analyses. Physical and Chemical Characterizations. High resolution transmission electron microscopic (HR-TEM) images were collected using a Hitachi 7700 TEM. Samples were prepared on carbon-coated copper grids with square mesh; drops of aggregate solution were added and dried onto the TEM grids prior to imaging. A Thermo K-Alpha XPS was used for XPS analysis of ca. 10 mg of dried aggregate (105 °C for 24 h) or dry fullerene powder. Peaks of C 1s scans were fitted using the Thermo Advantage software with Gaussian− Lorentzian computations with a fixed fwhm among all peaks identified. A Thermo Scientific Nicolet 6700 Fourier-transform infrared (FTIR) spectrometer with attenuated total reflectance (ATR) attachment (ZnSe crystal plate) was used to further assess the degree to which the aggregate formation methods functionalized the aggregates. Aggregates were dried and directly placed on the ZnSe crystal for FTIR analysis. A Thermo Almega XR Raman analysis system was used to probe the nature of water’s interaction with the fullerenes on aqueous samples of fullerene aggregates. Raman spectra were collected at varying concentrations and subsequently fitted with Gaussian−Lorentzian peaks. The peak fitting calculations provided variable results, depending on initial peak location guesses; 10 fitting attempts were therefore performed and averaged for each of the three characteristic peaks at each concentration and sample tested. Laser Flash Photolysis. Nanosecond laser flash photolysis (LFP) was used to trace the decay kinetics of fullerene derivatives’ triplet excited states in aqueous media. The experimental solutions consisted of ca. 5 μM fullerene derivatives buffered at pH 7 with 1 mM phosphate buffer. To inhibit energy transfer from triplet excited fullerene to 3O2, solutions (4 mL) were purged with argon gas for at least 30 min prior and throughout the experiments. A 355 nm laser pulse (50 mJ, pulse width = 6 ns) from a Quanta Ray Nd:YAG laser system was used as an excitation source and a xenon lamp was used as a probe source. Formation of the fullerene derivative’s triplet excited state was measured at 650 nm, the local maximum absorption for the triplet excited state of the B derivatives. Molecular Dynamics Simulations. Simulated fullerenes were assumed to be pristine and free of oxidation, as would be the case in the initial step of aggregation and in the aggregate cores. This assumption is justified by prior EDS and MALDIMS observations9 and concurrent XPS and ATR-FTIR observations of fullerene aggregates. The fullerene−water systems were initially prepared by adding one or two fullerenes and 1000 water molecules into a cubic cell having a size of 30 × 30 × 30 Å (Figure S2, Supporting Information). All the atoms in the systems were subsequently geometrically optimized using newly developed force field parameters (see below) in the Cerius2 software package.21 The cell size and shape were fixed during the geometry optimization. Then, MD simulations were performed through the LAMMPS code combined by reliable atomic charges and force field.22 Nose-Hoover thermo- and baro-stats were used to control the temperature and pressure in the NVT (298 K) and NPT ensembles (298 K and 1 atm). The quantum mechanical (QM) density functional theory (DFT) calculation was performed to obtain reliable atomic charges using B3LYP functional and 6-31G** basis set in the Jaguar software package.23 Charge-based Coulombic interactions were
corresponds to monofunctionalized A, A2 is bis-, and A3 is tris-, with the same scheme for B.). It was observed that A aggregates were not measurably photoactive, whereas B aggregates were efficient 1O2 sensitizers.9,20 Surprisingly, the size and crystallinity of the A and B aggregates failed to account for the discrepancy in photoactivity. The only observed difference between photoactive and nonphotoactive aggregates was the presence of cationic functional groups on the C60 cage. These reports, along with others examining the effects of size and functionalization,8,10 demonstrated that the photoactivity of fullerene aggregates do not necessarily depend on aggregate crystallinity or size. The significant disparity of photoactivity between aggregates of different types of fullerene derivatives highlights the lack of understanding about fundamental aggregate photochemistry. Here we present complementary lines of empirical and theoretical evidence as a step toward a deeper understanding of fullerene to fullerene, fullerene to O2, and fullerene to H2O interactions driving the occurrence or lack of photochemistry in aqueous fullerene aggregates. Continuing from our previous work on functionalized fullerenes,9,20 the A and B series are further characterized by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy for chemical changes upon aggregation. Molecular dynamics (MD) simulations are introduced to provide insights on fundamental molecular interactions that state of the art instrumentation cannot provide: simulations predict the different interactions of two series of fullerene derivatives with neighboring fullerenes, solvating H2O molecules, and ambient O2 molecules. Raman spectroscopy and high-resolution transmission electron microscopy (HR-TEM) provide evidence of water structure and aggregate morphology. These theoretical and empirical techniques combined provide a thorough examination of the dynamics of functionalized fullerene interactions in aqueous systems, and the implications of the observations are discussed in the context of the documented photochemical and antimicrobial properties of the fullerenes. The work presented herein is expected to provide significant advances in the understanding of the photochemistry of fullerenes and related materials in aqueous systems.
■
EXPERIMENTAL SECTION Fullerene Materials and Aggregate Preparation. Deionized/distilled water (resistivity > 18.2 MΩ) from a Millipore ultrapure system (Millipore Co.) was used to prepare aggregate suspensions. All fullerene materials were used as received without further purification. A and B derivatives were obtained from The Chemistry Research Solution LCC, Bristol PA. The purity of A and B were guaranteed to be above 95% by the manufacturer, with impurities consisting of the same derivative with more or less addends (e.g., B2 contains trace amounts of B1 and B3). Aqueous aggregates of the fullerene materials were prepared via sonication. Briefly, 9 mg of C60 or derivatives was added to 10 mL of toluene, and the solution was sonicated in a bath sonicator (Fisher Scientific FS30) for 6 h in a sealed jar at 60 °C. Ultrapure water (90 mL) was then added, and the mixture was further sonicated for 24 h. B derivatives were added directly to water, due to their insolubility in toluene. Finally, the mixtures were unsealed and sonicated for an additional 24 h, allowing the toluene to evaporate. All fullerenes underwent a total of 56 h of sonication. Aggregate solutions were then filtered with 0.45 μm nitrocellulose filters. 2148
DOI: 10.1021/es504735h Environ. Sci. Technol. 2015, 49, 2147−2155
Article
Environmental Science & Technology simulated using the particle−particle particle−mesh (PPPM) solver. Force Field Parameterization. In general, the MD simulations in many studies have been performed on the basis of the bonded and nonbonded interactions defined by the well-known DREIDING force field parameters.24 However, the pre-equipped force field parameters do not always accurately describe the nonbonded dispersion interactions in simulation systems, prompting many efforts to develop or improve the force field parameters.25−27 Here, these adaptations are employed to reliably describe the potential energy surfaces between a fullerene and a water molecule or between a fullerene and an oxygen molecule. Detailed methods of force field parametrization are provided in Text S1 (Supporting Information). Figure S3 (Supporting Information) shows the comparison between the DFT-based potential energy curves and the fitted force field based potential energy curves. For all four cases in the figure, the potential energy surfaces generated using the fitted force field are in a great agreement with those generated from the DFT method. The fitted force field parameters are listed in Table S1 (Supporting Information).
■
RESULTS AND DISCUSSION Chemical Characterization. XPS analyses on C60, A3, and B3 (as representatives of each series of derivatives) shown in Figure S4, Supporting Information, (C 1s) and Figure 1 (O 1s) suggest the occurrence of significant surface oxidation during aqueous aggregation. The C 1s peak at 284.5 eV is attributed to CC bonds of the fullerene cage.28 Peaks shifted 1−2 eV higher in energy represent CO bonds, whereas peaks in the 288 to 291 eV range are assigned as dioxygenated bonds (e.g., CO or OCO).29−31 The broad signal spanning about 4 eV, which begins 6 eV higher than the CC peak corresponds to the π−π* shakeup from the significant aromaticity in the fullerenes.28,32 The percentages of total carbon oxidized by one or two oxygen bonds are listed in Table S2 (Supporting Information) alongside the total elemental compositions. In general, the quantities of oxidation observed for the dry versus aggregated fullerenes are consistent with reports in the literature.19,29−31,33 The significant presence of oxygen functionalization suggests that the sonication method employed herein chemically alters the fullerenes as they aggregate, introducing oxygen moieties which are likely part of the reason for the negative charge of nonionic fullerene aggregates. It is worth noting that other methods of aggregate preparation have yielded much lower oxygen functionalization, particularly for cases prepared with stirring only or in dark conditions.33,34 The O 1s scan (Figure 1) also suggests an increase in signal in the binding energy (BE) range of 1−2 eV above the peak of the pristine fullerenes, although individual peaks were not resolved, due to the complicated nature of O 1s peaks and the many overlapping signals. The peak in the pristine fullerenes is centered at ca. 532 eV, which aligns well with organic CO bonds, and the increase in signal in the 533−534 eV range is indicative of organic CO bonds,32 supporting the analysis of the C 1s peaks. This increase and shifting in BE for the O 1s peak has been reported previously for nC60, particularly for cases of nC60 formation under light and oxic conditions.34 Although the presence of CO and CO functional groups have been evidenced for the aggregated fullerenes, contamination from adsorbed O2 or CO2 may have contributed to the peaks, which would help explain the O 1s signals for the pristine cases. Additionally, the oxidation of the pristine
Figure 1. O 1s XPS peaks for (a) C60, (b) A3, and (c) B3.
fullerenes may also be due to the reaction of fullerite powder with ambient ozone, as recently suggested by Murdianti et al.35 It is important to note the ratio of O to C in the aggregate scans. Although a high percentage of carbon atoms experience shifts in BE for their C 1s electrons, it is not likely that a full 26% of carbon is oxidized in the nB3 case. It is doubtful even that the apparent 16% oxygen observed in nB3 all comes from functionalized carbon, because the fullerenes retain a significant portion of their photochemical character that arises from their highly π-conjugated chromosphere. Residual adsorbed, or likely trapped within the aggregates during the aggregate formation process, O2, CO2, and H2O are likely responsible for the large O content observed. ATR-FTIR spectroscopy analysis (Figure 2) confirms the oxidation of the fullerene aggregates observed with XPS. The FTIR peaks generally match well with reports of nC60 in the literature. Pristine C60 exhibits expected peaks at 1430 and 1180 c m − 1 , co n s ist e n t w it h kn o w n C C v ibr at i o n al peaks.29,30,34,36,37 Upon aggregation, several new peaks are present for nC60, including a broad band centered at 1000 cm−1 that is indicative of a mixture of CO bonds,29,30,37 a peak at 1710 cm−1 indicative of carbonyl (CO) bonds, a minor peak centered at 2900 cm−1, indicative of CH bonds that likely 2149
DOI: 10.1021/es504735h Environ. Sci. Technol. 2015, 49, 2147−2155
Article
Environmental Science & Technology
possibility of adsorbed or trapped H2O molecules on or within the aggregates. Unlike previous reports suggesting that increased aggregate oxidation leads to increased photoactivity,6,8,11,12 the nB2 and nB3 fullerenes here are shown to be less oxidized than the nonphotoactive nC60 and A aggregates. Therefore, it can be inferred that the nature of aggregate formation and consequential surface oxygenation is not primarily responsible for the discrepancy in photoactivities between the B derivatives and all others compared. While it is known that intense UV irradiation or ozonation effectively oxidize nC60, resulting in more hydrophilic suspensions that are photoactive in water,12,19 the present results suggest that mechanisms other than surface oxygen content and hydrophilicity drive the photoactivity (or lack thereof) in the derivatives studied herein. Examination of Particle Morphology. High magnification HR-TEM imaging of nC60 and A1 in Figure 3 yielded a clear view of crystal lattice fringes, consistent with the crystallinity observed with energy dispersive X-ray spectroscopy (EDS) observed previously.9 Counting the number of lattice fringes found within the 20 nm scale bar yielded 24 crystal lattice lines (or one line every 0.83 nm) for both nC60 and A1, consistent with previous reports of nC60.30,38 Assuming a 0.35 nm radius of the C60 cage, the fullerenes are calculated to be 0.16 nm apart. Crystal lattice fringes were not found in any of the remaining derivatives, despite the fact that EDS showed crystallinity for A2 and A3.9 The lack of observable lattice lines may be due to the presence of functional groups causing a less uniform crystal structure or a blurring of the interstitial spaces between lattice lines. With progressively more functional
Figure 2. ATR-FTIR spectra for C60, A3, and B3 and their aggregates.
formed when O groups were added to CC bonds,30,37 and a broad signal between 3200 and 3600, consistent with OH stretching from water or alcohol groups.29,30,37 The nB3 sample had very little 2900 cm−1 signal compared with B3, indicating an unexpected decrease in CH vibrations, perhaps due to oxygenation of the methyl groups. These changes in FTIR spectra appear to be consistent across the various functionalized fullerenes. Each case shows an increase in and broadening of signal around the 1000 and 1710 cm−1 regions. The significant increase of signal in the 3200 to 3600 cm−1 range, which has been observed in other studies on nC60,30,34 also points to the
Figure 3. TEM images of (a, b) nC60 under 60k× and 400k× magnification; images of (c, d) nA1 under 100k× and 400k× magnification; images of (e, f) nA2 under 50k× and 300k× magnification; images of (g, h) nA3 under 50k× and 300k× magnification; images of (i, j) nB2 under 50k× and 100k× magnification; images of (k, l) nB3 under 50k× and 200k× magnification. 2150
DOI: 10.1021/es504735h Environ. Sci. Technol. 2015, 49, 2147−2155
Article
Environmental Science & Technology
together once the fullerenes are near each other. These observed interaction dynamics may be responsible for more fractured or jagged morphology with effective binding leading to rigid particle edges. The B series, however, exhibits significantly different and weaker interactions than C60 or the A series. For each B derivative, the distances never became closer than ca. 1.3 nm and generally varied between 1.3 and 2 nm apart, indicating weaker interactions. Electrostatic repulsion is possible; for example, the B2 molecules traveled up to 2.5 nm apart from 0.3 ns and on, resulting in no interaction. B3 interactions are observed to remain at a distance of approximately 1.4 nm apart during the simulation. The longer distances between B fullerenes may be important in preventing the self-quenching pathway that drastically reduces fullerene aggregates ability to photosensitize 1O2 in aqueous media. The LFP analyses suggest that both B2 and B3 aggregates exhibited long-lived excited triplet states in aqueous media, with lifetimes of ca. 28 and 34 μs, respectively (Figure S5, Supporting Information). Conversely, a triplet excited state was not observable for A derivatives in the nanosecond range, which is likely a result of fast deactivation (picoseconds) of the excited triplet through increased self-quenching or triplet−triplet annihilation pathways. Furthermore, less dense particles may also allow for 1O2 sensitized within the aggregate to more efficiently diffuse out of the particle.38 The weaker interactions are consistent with the observed photochemistry and particle morphology; B aggregates (excluding B1, which did not form stable aggregates and therefore did not stay suspended in solution long enough to perform photochemical experiments) were found to be significantly photoactive9,20 and are now observed to be less dense, have less defined particles, and have long-lived triplet lifetimes in water. Dense aggregate structures have also been shown to limit the diffusion of dissolved oxygen into and 1O2 out and away from the aggregate to the bulk phase, limiting their effective 1O2 production.38 The difference in “adhesion” characteristics between fullerenes is the most notable finding predicted from the MD simulations on the fullerene−fullerene interactions and is understood based on the expected electrostatic repulsion of the B derivatives and the dominant van der Waals interactions of the A derivatives. Fullerene−Water Interactions. The effects of fullerene aggregates on their surrounding water solvation structures were probed using Raman spectroscopy, and the results are shown in Figure 5 for the O−H symmetric bending mode (see Figure S6, Supporting Information for the spectra for DI water). The characteristic Raman signal from liquid water is known to shift upon addition of solutes39,40 and has recently been employed to observe more structured hydration environments with increasing nC60 concentrations.41 The stretching modes (Figure S7 and S8, Supporting Information) did not appear to exhibit any clear trend, given the error bars, which represent the standard deviation of the 10 fitting attempts. The symmetric bending, however, appears to experience a slight blue shift with increasing aggregate concentrations, consistent with the previous report on nC60.41 This blue shift is likely due to an increase in the order of the solvating water molecules around the fullerenes. All fullerenes appear to shift the bending mode to a higher energy, indicating that they are all affecting the localized water structures, whereas differences between fullerenes were not clearly noticeable. MD simulations also suggested that the fullerenes do not appear to be significantly different in the way they interact with
groups, the A and B aggregates both appear to have less discrete particle morphology. A3 retains distinct particle boundaries, whereas B2 and B3 have progressively more ambiguous particle edges, as observed consistently throughout TEM analysis. B3 lacked clearly defined particles; even at lower magnifications, particles were difficult to image fully in focus, suggesting that the B3 particles are more amorphous than others. Prior work on analyzing the dependence of fullerene aggregate photoactivity on particle morphology has suggested that higher fractal dimensions (i.e. higher particle surface area within a given area) have greater ROS production efficiencies.6,10 Although the fractal dimension analysis is not performed here, it is noteworthy that the observations based on the images presented seem to be in agreement with the theory of higher photoactivity with higher fractal dimensions. Although the drying and vacuuming processes involved in TEM imaging significantly alter the actual state of the aggregates as compared to their state in solution, these analyses provide some evidence of the importance of fullerene−fullerene interactions within the aggregates. The dichotomy of photoactivities between A and B, however, is not fully explained without further investigations. Simulated Fullerene−Fullerene Interactions. Timeevolved MD simulation trajectories of two fullerenes initially placed 1.3 nm apart are shown in Figure 4 in terms of distances
Figure 4. Fullerene−fullerene interaction distances in a system of two fullerenes, or fullerene derivatives of the same functionalization, hydrated by 1000 water molecules for C60, A, and B.
between their center of mass (COM). The C60, A1, and A2 cases show rapid attraction, sticking at a distance of 1 nm, or 0.3 nm from edge to edge of the fullerene cages, with no subsequent movement away from each other. This result is in good agreement with 0.16 nm spacing measured in the TEM imaging, considering that additional forces will likely to induce more compact packing in a complex crystal environment. The simulated interaction of A2 fullerenes also predicts close attraction, even though lattice fringes were not found during TEM observation. The A2 particles were observed, however, to have very clearly defined particle boundaries with similar surface morphology as nC60 or A1 aggregates. The A3 fullerenes do not immediately attract and bind together, but did so after 0.7 ns of the simulation. The A3 interaction appears to be less probable, perhaps dependent on the physical alignment of functional groups, but strong enough to stick 2151
DOI: 10.1021/es504735h Environ. Sci. Technol. 2015, 49, 2147−2155
Article
Environmental Science & Technology
molecules in the first water layer. A corresponding increase in density in the boundary between the first and second hydration shells with increasing functionalization provides further evidence. The results of mean-squared displacement (MSD) simulation, indicative of how fullerenes affect the mobility of the neighboring water molecules, are shown in Figure S9 (Supporting Information). Hydrophilic functionality appears to cause a slight decrease in water mobility around the fullerenes; A and B derivatives yield MSDs between 100 and 120 Å2 after 60 ps, whereas C60’s MSD was 125 Å2 after 60 ps. This decreased mobility likely indicates a shift in the nature of the hydration of water around the fullerene cage; water molecules would be more stable next to hydrophilic functional groups compared with the hydrophobic cage of C60, resulting in reduced movement. However, the differences between different types of functionalized fullerenes were not noticeable; these simulation data combined with the Raman observations suggest that the photoactivity of fullerene derivatives is not dependent on the fullerene−water interactions. Fullerene−Oxygen Interactions. Figure 7 shows the interactions of O2 with various fullerenes, predicted by MD simulations. Pristine C60 (Figure 7a) showed strong interactions between 0.2 and 0.4 ns, for a total of 201 ps of interaction time, with interaction energies ranging between −0.5 and −1.4 kcal/mol and the COM distance of 0.6 to 0.8 nm. Slightly less O2−fullerene interactions for the A series were observed (Figures 7(b)-7(d)), and much less for B series (Figures 7(e)-7(g)). On average, the A derivatives had interaction times of 103 ps during the 1 ns simulation, while the B derivatives had an average of 30 ps of interaction time. The interaction energies were generally in the range of −0.5 to −1.8 kcal/mol, with the notable exception of B2 which only reached an interaction energy of −0.18 kcal/mol. These varying fullerene−O2 interactions are interesting to note since it is
Figure 5. Raman peak locations of the second order O−H symmetric bending (2v2) for varying concentrations of fullerene aggregates.
water molecules immediately surrounding them. Figure 6 shows radial distribution functions (RDFs) of water molecules around the fullerene center of mass, predicted from the MD simulations. The simulations predict a first hydration shell at approximately 6.5 Å and a second at around 9.5 Å from the COM for all the fullerenes. These values match the reported radii for hydration shells around C60 very well.42−45 A slight reduction in the density of water molecules at the first hydration shell is noted in the derivatives as compared with C60, likely due to the presence of the functional groups occupying space that would otherwise have water molecules. This observation is consistent within each series of derivative: with each functional group added there is a lower density of water
Figure 6. RDFs of water molecules from the COM of (a) C60, (b) C60 compared with A3 and B3, (c) the A series, and (d) the B series. ρg(r) indicates the density of water molecules at a given radius r. 2152
DOI: 10.1021/es504735h Environ. Sci. Technol. 2015, 49, 2147−2155
Article
Environmental Science & Technology
solvent polarity47,49−53 and decreasing sensitizer oxidation potentials.47,49−54 The relatively low oxidation potential of C60 in water (1.76 V vs SCE),55 therefore, may limit the sensitization of 1O2 through the pCT channel. In addition, strong interactions between O2 and C60 could prevent or limit diffusion of 1O2 back into the bulk solution; studies have observed 1O2 trapped within the aggregates or at higher concentrations at the surface of the nC60.8,38 A study of sensitizing 1O2 in the gas phase using fullerene coated surfaces showed that 1O2 rapidly adsorbed onto the fullerene surfaces without significantly escaping back to the gas phase.56 It has also been shown that a ground state fullerene could quench 1O2 via back electron transfer (BET) reactions in a zinc porphyrin− fullerene system.17,57 Alternatively, if an O2 molecule remains at the surface of a fullerene too long, the BET process may proceed and quench the excitation before the C60−O2 exciplex dissociates. As an example of typical exciplex lifetimes, charge transfer exciplexes for 9-cyanophenanthrene and electron donors were reported in the nanosecond time frame.58 A similar study for fullerene−O2 exciplex lifetimes in various solvents may be warranted to fully elucidate the potential for BET quenching. In contrast to the cases of C60 and the A fullerenes, the photochemistry of the B series likely involves a weakly interacting, nonbinding nCT complex wherein free rotation of O2 molecule in the solvation shell is required for facile internal conversion. Note that 1O2 sensitization involving nCT complex is not limited by the sensitizer’s oxidation potential. B functionality is expected to have an innately higher oxidation potential than A, given the cationic charge(s) of B, which would increase 1O2 production for B compared to A series even if both form pCT complexes. Combined with weaker fullerene− fullerene interactions for the B series and consequently less triplet−triplet quenching during photosensitization process, the weaker fullerene−O2 interaction may explain significant aqueous photoactivity observed with B series fullerene aggregates. Overall, the suite of analytical and computational observations made in this study shed light on mechanism how the fullerene preserves or loses aqueous photoactivity depending on their surface functionalization. Fullerene−fullerene and fullerene−O2 interactions are potentially determining factors, while other factors such as aggregate size, water solvation, and surface oxidation appear to be less relevant.
Figure 7. Interactions between (a) C60, (b) A1, (c) A2, (d) A3, (e) B1, (f) B2, and (g) B3 and O2 over 1 ns; the black line represents distance between the COMs for C60 and O2 and the red represents interaction energy between C60 and the O2 molecule.
■
ASSOCIATED CONTENT
■
AUTHOR INFORMATION
* Supporting Information S
Additional figures of molecular diagrams of A and B fullerenes, depiction of the C60-1000 water molecule modeled system, comparison between QM-based and force-field fitted potential energy curves, C 1s scans with fitted peaks, nanosecond decay traces of triplet excited A and B fullerene derivatives, characteristic Raman signal for pure water with fitted peaks, fullerene concentration-dependent peak locations for the firstorder stretching modes of water’s Raman signal, MSD plots of C60, A, and B, and tables containing the fitted force field parameters and elemental compositions of fullerene species from XPS scans. This material is available free of charge via the Internet at http://pubs.acs.org.
known that, during the photosensitization process, a sensitizer and 3O2 form an exciplex where energy is transferred through either a partial charge transfer (pCT) or a non charge transfer (nCT) pathway, depending on the binding interactions of the sensitizer with the O2.46,47 The difference in the O2 interactions between C60, A and B fullerenes correlates well with the photoactivities reported previously9,20 and may indicate the involvement of two different types of 1O2 sensitization pathways, as discussed below. The relatively strong interactions between sensitizers (C60 and A fullerenes) and O2 suggest a partial charge transfer (pCT) as the mechanism for 1O2 generation.46−48 Note that pCT can proceed only when the sensitizer’s oxidation potential and triplet energy state allow for the free energy for complete electron transfer (CET), ΔGCET, < −30 kJmol−1. Past studies suggest that sensitizers with strong sensitizer-O2 interactions experience decreasing 1O2 production efficiency with increasing
Corresponding Authors
*S. S. Jang. E-mail:
[email protected]. *J.-H. Kim. E-mail:
[email protected]. 2153
DOI: 10.1021/es504735h Environ. Sci. Technol. 2015, 49, 2147−2155
Article
Environmental Science & Technology Author Contributions
C60 derivatives in aqueous systems. Environ. Sci. Technol. 2009, 43 (17), 6604−6610. (17) Ebbesen, T. W.; Tanigaki, K.; Kuroshima, S. Excited-state properties of C60. Chem. Phys. Lett. 1991, 181 (6), 501−504. (18) Sternlicht, H.; Nieman, G. C.; Robinson, G. W. Triplettriplet annihilation and delayed fluorescence in molecular aggregates. J. Chem. Phys. 1963, 38 (6), 1326. (19) Lee, J.; Cho, M.; Fortner, J. D.; Hughes, J. B.; Kim, J. H. Transformation of aggregate C60 in the aqueous phase by UV irradiation. Environ. Sci. Technol. 2009, 43 (13), 4878−4883. (20) Snow, S. D.; Park, K.; Kim, J.-H. Cationic fullerene aggregates with unprecedented virus photoinactivation efficiencies in water. Environ. Sci. Technol. Lett. 2014, 1 (6), 290−294. (21) CERIUS2 Modeling Environment, Release 4.0; Accelrys Inc., San Diego, CA, 1999. (22) Plimpton, S. Fast parallel algorithms for short-range moleculardynamics. J. Comput. Phys. 1995, 117 (1), 1−19. (23) Jaguar, version 7.5; Schrödinger, LLC: New York, NY, 2008. (24) Mayo, S. L.; Olafson, B. D.; Goddard, W. A. DREIDING - A generic force-field for molecular simulations. J. Phys. Chem. 1990, 94 (26), 8897−8909. (25) Kaminski, G. A.; Stern, H. A.; Berne, B. J.; Friesner, R. A. Development of an accurate and robust polarizable molecular mechanics force field from ab initio quantum chemistry. J. Phys. Chem. A 2004, 108 (4), 621−627. (26) Getman, R. B.; Miller, J. H.; Wang, K.; Snurr, R. Q. Metal alkoxide functionalization in metal-organic frameworks for enhanced ambient-temperature hydrogen storage. J. Phys. Chem. C 2011, 115 (5), 2066−2075. (27) Yu, D.; Ghosh, P.; Snurr, R. Q. Hierarchical modeling of ammonia adsorption in functionalized metal-organic frameworks. Dalton Trans. 2012, 41 (14), 3962−3973. (28) Onoe, J.; Nakao, A.; Takeuchi, K. XPS study of a photopolymerized C60 film. Phys. Rev. 1997, 55 (15), 10051−10056. (29) Hwang, Y. S.; Li, Q. Characterizing photochemical transformation of aqueous nC60 under environmentally relevant conditions. Environ. Sci. Technol. 2010, 44 (8), 3008−3013. (30) Labille, J.; Masion, A.; Ziarelli, F.; Rose, J.; Brant, J.; Villieras, F.; Pelletier, M.; Borschneck, D.; Wiesner, M. R.; Bottero, J. Y. Hydration and dispersion of C60 in aqueous systems: The nature of waterfullerene interactions. Langmuir 2009, 25 (19), 11232−11235. (31) Fortner, J. D.; Kim, D. I.; Boyd, A. M.; Falkner, J. C.; Moran, S.; Colvin, V. L.; Hughes, J. B.; Kim, J. H. Reaction of water-stable C60 aggregates with ozone. Environ. Sci. Technol. 2007, 41 (21), 7497− 7502. (32) Langley, L. A.; Villanueva, D. E.; Fairbrother, D. H. Quantification of surface oxides on carbonaceous materials. Chem. Mater. 2006, 18 (1), 169−178. (33) Chen, K. L.; Elimelech, M. Relating colloidal stability of fullerene C60 nanoparticles to nanoparticle charge and electrokinetic properties. Environ. Sci. Technol. 2009, 43 (19), 7270−7276. (34) Hüffer, T.; Kah, M.; Hofmann, T.; Schmidt, T. C. How redox conditions and irradiation affect sorption of PAHs by dispersed fullerenes nC60. Environ. Sci. Technol. 2012, 47 (13), 6935−6942. (35) Murdianti, B. S.; Damron, J. T.; Hilburn, M. E.; Maples, R. D.; Hikkaduwa Koralege, R. S.; Kuriyavar, S. I.; Ausman, K. D. C60 oxide as a key component of aqueous C60 colloidal suspensions. Environ. Sci. Technol. 2012, 46 (14), 7446−7453. (36) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Solid C60 - A new form of carbon. Nature 1990, 347 (6291), 354−358. (37) Prilutski, Y. I.; Durov, S. S.; Yashchuk, V. N.; Ogul’chansky, T. Y.; Pogorelov, V. E.; Astashkin, Y. A.; Buzaneva, E. V.; Kirghisov, Y. D.; Andrievsky, G. V.; Scharff, P. Theoretical predictions and experimental studies of self-organized C60 nanoparticles in water solution and on the support. Eur. Phys. J. D 1999, 9 (1−4), 341−343. (38) Wang, J.; Leng, J.; Yang, H.; Sha, G.; Zhang, C. Long-lifetime and asymmetric singlet oxygen photoluminescence from aqueous fullerene suspensions. Langmuir 2013, 29 (29), 9051−6.
⊥
These authors contributed equally.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was partly supported by the National Science Foundation (Grant # 1235916 for S. Snow and 1439048 for K. Moor). S. S. Jang also acknowledges that this research used resources of the Keeneland Computing Facility at the Georgia Institute of Technology, supported by the National Science Foundation under Contract OCI-0910735. We thank Dr. Prashant Kamat and Dr. Kevin Stamplecoskie at University of Notre Dame Radiation Laboratory for their assistance with LFP experiments.
■
REFERENCES
(1) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Plastic Solar Cells. Adv. Funct. Mater. 2001, 11 (1), 15−26. (2) Mayer, A. C.; Scully, S. R.; Hardin, B. E.; Rowell, M. W.; McGehee, M. D. Polymer-based solar cells. Mater. Today 2007, 10 (11), 28−33. (3) Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.; Diederich, F. N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. Photophysical properties of C60. J. Phys. Chem. 1991, 95 (1), 11−12. (4) Ray, P. C.; Yu, H.; Fu, P. P. Toxicity and environmental risks of nanomaterials: Challenges and future needs. J. Environ. Sci. Health, Part C: Environ. Carcinog. Ecotoxicol. Rev. 2009, 27 (1), 1−35. (5) Wiesner, M. R.; Lowry, G. V.; Alvarez, P.; Dionysiou, D.; Biswas, P. Assessing the risks of manufactured nanomaterials. Environ. Sci. Technol. 2006, 40 (14), 4336−4345. (6) Hotze, E. M.; Bottero, J. Y.; Wiesner, M. R. Theoretical framework for nanoparticle reactivity as a function of aggregation state. Langmuir 2010, 26 (13), 11170−11175. (7) Jafvert, C. T.; Kulkarni, P. P. Buckminsterfullerene’s (C60) octanol-water partition coefficient (Kow) and aqueous solubility. Environ. Sci. Technol. 2008, 42 (16), 5945−5950. (8) Badireddy, A. R.; Budarz, J. F.; Chellam, S.; Wiesner, M. R. Bacteriophage inactivation by UV-A illuminated fullerenes: Role of nanoparticle-virus association and biological targets. Environ. Sci. Technol. 2012, 46 (11), 5963−5970. (9) Snow, S. D.; Lee, J.; Kim, J. H. Photochemical and photophysical properties of sequentially functionalized fullerenes in the aqueous phase. Environ. Sci. Technol. 2012, 46 (24), 13227−13234. (10) Chae, S. R.; Badireddy, A. R.; Budarz, J. F.; Lin, S. H.; Xiao, Y.; Therezien, M.; Wiesner, M. R. Heterogeneities in fullerene nanoparticle aggregates affecting reactivity, bioactivity, and transport. ACS Nano 2010, 4 (9), 5011−5018. (11) Cho, M.; Fortner, J. D.; Hughes, J. B.; Kim, J. H. Escherichia coli inactivation by water-soluble, ozonated C60 derivative: Kinetics and Mechanisms. Environ. Sci. Technol. 2009, 43 (19), 7410−7415. (12) Cho, M.; Snow, S. D.; Hughes, J. B.; Kim, J. H. Escherichia coli inactivation by UVC-Irradiated C60: Kinetics and mechanisms. Environ. Sci. Technol. 2011, 45 (22), 9627−33. (13) Duncan, L. K.; Jinschek, J. R.; Vikesland, P. J. C60 colloid formation in aqueous systems: Effects of preparation method on size, structure, and surface, charge. Environ. Sci. Technol. 2008, 42 (1), 173− 178. (14) Fortner, J. D.; Lyon, D. Y.; Sayes, C. M.; Boyd, A. M.; Falkner, J. C.; Hotze, E. M.; Alemany, L. B.; Tao, Y. J.; Guo, W.; Ausman, K. D.; Colvin, V. L.; Hughes, J. B. C60 in water: Nanocrystal formation and microbial response. Environ. Sci. Technol. 2005, 39 (11), 4307−4316. (15) Lee, J.; Kim, J. H. Effect of encapsulating agents on dispersion status and photochemical reactivity of C60 in the aqueous phase. Environ. Sci. Technol. 2008, 42 (5), 1552−1557. (16) Lee, J.; Mackeyev, Y.; Cho, M.; Li, D.; Kim, J. H.; Wilson, L. J.; Alvarez, P. J. J. Photochemical and antimicrobial properties of novel 2154
DOI: 10.1021/es504735h Environ. Sci. Technol. 2015, 49, 2147−2155
Article
Environmental Science & Technology (39) Rice, S. Conjectures on the structure of amorphous solid and liquid water. In Top. Curr. Chem.; Springer: Berlin/Heidelberg, 1975; Vol. 60, pp 109−200. (40) Deak, J. C.; Rhea, S. T.; Iwaki, L. K.; Dlott, D. D. Vibrational energy relaxation and spectral diffusion in water and deuterated water. J. Phys. Chem. A 2000, 104 (21), 4866−4875. (41) Amer, M. S.; Wang, W. H. Effect of fullerene nanospheres on water evaporation kinetics and first-order thermal transitions. J. Phys. Chem. C 2012, 116 (14), 8216−8222. (42) Choudhury, N. A molecular dynamics simulation study of buckyballs in water: Atomistic versus coarse-grained models of C60. J. Chem. Phys. 2006, 125 (3), 34502. (43) Choudhury, N. Dynamics of water in solvation shells and intersolute regions of C60: A molecular dynamics simulation study. J. Phys. Chem. C 2007, 111 (6), 2565−2572. (44) Hotta, T.; Kimura, A.; Sasai, M. Fluctuating hydration structure around nanometer-size hydrophobic solutes. I. Caging and drying around C60 and C60H60 spheres. J. Phys. Chem. B 2005, 109 (39), 18600−18608. (45) Weiss, D. R.; Raschke, T. M.; Levitt, M. How hydrophobic buckminsterfullerene affects surrounding water structure. J. Phys. Chem. B 2008, 112 (10), 2981−90. (46) Schmidt, R. Photosensitized generation of singlet oxygen. Photochem. Photobiol. 2006, 82 (5), 1161−1177. (47) Schweitzer, C.; Mehrdad, Z.; Noll, A.; Grabner, E. W.; Schmidt, R. Mechanism of photosensitized generation of singlet oxygen during oxygen quenching of triplet states and the general dependence of the rate constants and efficiencies of O2(1Σg+), O2(1Δg), and O2(3Σg−) formation on sensitizer triplet state energy and oxidation potential. J. Phys. Chem. A 2003, 107 (13), 2192−2198. (48) Mehrdad, Z.; Noll, A.; Grabner, E. W.; Schmidt, R. Sensitization of singlet oxygen via encounter complexes and via exciplexes of π π* triplet excited sensitizers and oxygen. Photochem. Photobiol. Sci. 2002, 1 (4), 263−269. (49) Mcgarvey, D. J.; Szekeres, P. G.; Wilkinson, F. The efficiency of singlet oxygen generation by substituted naphthalenes in benzene Evidence for the participation of charge-transfer interactions. Chem. Phys. Lett. 1992, 199 (3−4), 314−319. (50) Wilkinson, F.; AbdelShafi, A. A. Mechanism of quenching of triplet states by oxygen: Biphenyl derivatives in acetonitrile. J. Phys. Chem. A 1997, 101 (30), 5509−5516. (51) Abdel-Shafi, A. A.; Wilkinson, F. Charge transfer effects on the efficiency of singlet oxygen production following oxygen quenching of excited singlet and triplet states of aromatic hydrocarbons in acetonitrile. J. Phys. Chem. A 2000, 104 (24), 5747−5757. (52) Abdel-Shafi, A. A.; Wilkinson, F. Electronic to vibrational energy conversion and charge transfer contributions during quenching by molecular oxygen of electronically excited triplet states. Phys. Chem. Chem. Phys. 2002, 4 (2), 248−254. (53) Schmidt, R. The effect of solvent polarity on the balance between charge transfer and non-charge transfer pathways in the sensitization of singlet oxygen by π−π triplet states. J. Phys. Chem. A 2006, 110 (18), 5990−5997. (54) Wilkinson, F.; Mcgarvey, D. J.; Olea, A. F. Factors governing the efficiency of singlet oxygen production during oxygen quenching of singlet and triplet-states of anthracene-derivatives in cyclohexane solution. J. Am. Chem. Soc. 1993, 115 (25), 12144−12151. (55) Dubois, D.; Kadish, K. M.; Flanagan, S.; Wilson, L. J. Electrochemical detection of fulleronium and highly reduced fulleride (C605−) ions in solution. J. Am. Chem. Soc. 1991, 113 (20), 7773− 7774. (56) Danilov, O. B.; Belousova, I. M.; Mak, A. A.; Belousov, V. P.; Grenishin, A. S.; Kiselev, V. M.; Kris’ko, A. V.; Ponomarev, A. N.; Sosnov, E. N. Generation of singlet oxygen with the use of optically excited fullerenes and fullerene-like nanoparticles. Opt. Spectrosc. 2003, 95 (6), 833−842. (57) Fukuzumi, S.; Imahori, H.; Yamada, H.; El-Khouly, M. E.; Fujitsuka, M.; Ito, O.; Guldi, D. M. Catalytic effects of dioxygen on
intramolecular electron transfer in radical ion pairs of zinc porphyrinlinked fullerenes. J. Am. Chem. Soc. 2001, 123 (11), 2571−2575. (58) Dolotova, E.; Dogadkin, D.; Soboleva, I.; Kuzmin, M.; Nicolet, O.; Vauthey, E. Lifetimes of partial charge transfer exciplexes of 9cyanophenanthrene and 9-cyanoanthracene. Chem. Phys. Lett. 2003, 380 (5−6), 729−735.
2155
DOI: 10.1021/es504735h Environ. Sci. Technol. 2015, 49, 2147−2155
