Correspondence/Rebuttal pubs.acs.org/est
Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Comment on The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics
I
n this recent article,1 the authors argue that charge transfer (CT) interactions cannot be important to the optical properties of humic substances (HS) and dissolved organic matter because perturbations of solvent temperature, viscosity and polarity do not affect the absorbance and fluorescence spectral line shapes. However, this conclusion is predicated on three very important assumptions: (1) the electron donors (D) and acceptors (A) are freely diffusing in solution; (2) the D and A pairs are in dynamic equilibrium with the DA complex; (3) the D and A pairs and the DA complex are completely solvent accessible. Based on these three assumptions, the authors rightly exclude the simple models shown in their Figure 1. However, all three of these assumptions are questionable for HS based on existing information. First, work by Dreyer et al.2 has demonstrated for closely related materials (polydopamine) that D and A pairs will form higher molecular weight aggregates that are stabilized through a combination of charge transfer, πstacking and hydrogen bonding interactions (see also, ref 3). Second, numerous studies have provided strong evidence that HS are organized in microheterogenous domains,4−8 in which outer charged groups encompass an inner core,7,8 lowering or limiting solvent accessibility. As pointed out in a recent paper,9 our working structural model is similar to that of Dreyer et al.,2 wherein the D and A pairs are in contact, helping to stabilize the formation of an ensemble of higher molecular weight static aggregates (not in dynamic exchange), with outer charged groups surrounding the individual aggregates,7,8 improving water solubility while further stabilizing the contact DA complexes within the interior, potentially both thermodynamically and kinetically. Because changes in solvent polarity and viscosity did not largely effect the spectral dependence of either the absorption or emission properties of these HS, a key question is whether these solvent perturbations were sufficient to disrupt any CT interactions within these HS. The authors argue that they would expect this to be the case, based on the ability of an organic solvent, dimethylformamide (DMF), to disrupt πstacking in eumelanin (ref 48 in ref 1),10 another material closely related to polydopamine. However, this argument is specious, as the eumelanin was first treated with a concentrated ammonia solution, sonicated for 20 min, stirred at 90 °C for several hours and then rotoevaporated at 80 °C in water/DMF before final addition to DMF. Even then, this treatment only disrupted the higher order structure and not the primary structure of the eumelanin.10 Although we would not anticipate that the HS would be stabilized to the same degree as polydopamine or eumelain due to the longer-range order in these large uncharged supramolecular aggregates, the authors provide no experimental evidence that these solvents (or temperature over the range tested) would in fact disrupt this type of structure within the HS. The inability of tetrahydrofuran to solubilize the higher molecular weight soil HS (PPHA and ESHA), although not a proof, is consistent with the idea that they do not. © XXXX American Chemical Society
Although the authors employ these data to argue that an electronic interaction model, more specifically a CT model,11,12 is incorrect, and to promote the idea that a superposition of noninteracting chromophores can account for the visible absorption and emission properties of the HS, they ignore numerous lines of evidence that are either inconsistent or incompatible with this idea. As pointed out in Sharpless and Blough,12 these include but are not limited to the following: (1) continuously red-shifting emission maxima with increasing excitation wavelength (SI in ref 1), combined with monotonically decreasing apparent fluorescence quantum yields (Figures 2,3 and SI in ref 1) and fluorescence lifetimes13 - note that this applies to the soil HS as well, but the dependence is shifted to lower energies (Figure 2 in ref 1); (2) the broad-band loss of absorption across the visible combined with enhanced blueshifted fluorescence emission following removal of putative electron acceptors (ketone/aldehydes) by borohydride reduction;9,14−18 (3) the broad-band increase in absorption across the visible with increasing pH, particularly over the pKa range associated with putative phenol electron donors;18−21 (4) the lowering or elimination of this pH-induced absorption enhancement in the visible following reduction with borohydride to remove the electron acceptors.18 As noted previously,12 if one wishes to develop a comprehensive photophysical model of HS and chromophoric dissolved organic matter, then one should consider all of the data, not just a self-selected portion.
Neil V. Blough*,† Rossana Del Vecchio§
■
† Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States § Earth System Science Interdisciplinary Center, University of Maryland, College Park, Maryland 20770, United States
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Neil V. Blough: 0000-0002-7938-4961 Rossana Del Vecchio: 0000-0001-5087-7028 Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) McKay, G.; Korak, J. A.; Erickson, P. R.; Latch, D. E.; McNeill, K.; Rosario-Ortiz, F. L. The case against charge transfer interactions in dissolved organic matter photophysics. Environ. Sci. Technol. 2018, 52, 406−414. (2) Dreyer, D. R.; Miller, D. J.; Freeman, B. D.; Paul, D. R.; Bielawski, C. W. Elucidating the structure of poly(dopamine). Langmuir 2012, 28 (15), 6428−6435.
A
DOI: 10.1021/acs.est.8b01189 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
(3) Liu, Y. L.; Ai, K. L.; Lu, L. H. Polydopamine and its derivative materials: Synthesis and promising applications in energy, environmental, and biomedical fields. Chem. Rev. 2014, 114 (9), 5057−5115. (4) Latch, D. E.; McNeill, K. Microheterogeneity of singlet oxygen distributions in irradiated humic acid solutions. Science 2006, 311 (5768), 1743−1747.4. (5) Grandbois, M.; Latch, D. E.; McKneill, K. Microheterogeneous concentrations of singlet oxygen in natural organic matter isolate solutions. Environ. Sci. Technol. 2008, 42, 9184−9190. (6) Chu, C.; Lundeen, R. A.; Remucal, C. K.; Sander, M.; McNeill, K. Enhanced indirect photochemical transformations of histidine and histamine through association with chromophoric dissolved organic matter. Environ. Sci. Technol. 2015, 49, 5511−5519. (7) Blough, N. V. Electron-paramagnetic resonance measurements of photochemical radical production in humic substances.1. Effects of O2 and charge on radical scavenging by nitroxides. Environ. Sci. Technol. 1988, 22 (1), 77−82. (8) Green, S. A.; Morel, F. M. M.; Blough, N. V. Investigation of the electrostatic properties of humic substances by fluorescence quenching. Environ. Sci. Technol. 1992, 26 (2), 294−302. (9) Del Vecchio, R.; Schendorf, T. M.; Blough, N. V. Contribution of quinones and ketones/aldehydes to the optical properties of humic substances (HS) and chromophoric dissolved organic matter (CDOM). Environ. Sci. Technol. 2017, 51, 13624−13632. (10) Watt, A. A. R.; Bothma, J. P.; Meredith, P. The supramolecular structure of melanin. Soft Matter 2009, 5 (19), 3754−3757. (11) Del Vecchio, R.; Blough, N. V. On the origin of the optical properties of humic substances. Environ. Sci. Technol. 2004, 38 (14), 3885−3891. (12) Sharpless, C. M.; Blough, N. V. The importance of charge transfer interactions in determining chromophoric dissolved organic matter (CDOM) optical and photochemical properties. Environmental Science-Processes & Impacts 2014, 16 (4), 654−671. (13) Boyle, E. S.; Guerriero, N.; Thiallet, A.; Del Vecchio, R.; Blough, N. V. Optical properties of humic substances and CDOM: Relation to structure. Environ. Sci. Technol. 2009, 43 (7), 2262−2268. (14) Ma, J. H.; Del Vecchio, R.; Golanoski, K. S.; Boyle, E. S.; Blough, N. V. Optical properties of humic substances and CDOM: Effects of borohydride reduction. Environ. Sci. Technol. 2010, 44 (14), 5395−5402. (15) Sharpless, C. M. Lifetimes of triplet dissolved natural organic matter (DOM) and the effect of NaBH4 reduction on singlet oxygen quantum yields: Implications for DOM photophysics. Environ. Sci. Technol. 2012, 46 (8), 4466−4473. (16) Phillips, S. M.; Smith, G. D. Light absorption by charge transfer complexes in brown carbon aerosols. Environ. Sci. Technol. Lett. 2014, 1 (10), 382−386. (17) Schendorf, T. M.; Del Vecchio, R.; Koech, K.; Blough, N. V. A standard protocol for NaBH4 reduction of CDOM and HS. Limnol. Oceanogr.: Methods 2016, 14 (6), 414−423. (18) Schendorf, T. M. Effect of Borohydride Reduction and pH on the Optical Properties of Humic Substances; MS, University of Maryland, UMD Theses and Dissertations, 2014. (19) Dryer, D. J.; Korshin, G. V.; Fabbricino, M. In situ examination of the protonation behavior of fulvic acids using differential absorbance spectroscopy. Environ. Sci. Technol. 2008, 42 (17), 6644−6649. (20) Yan, M.; Korshin, G. V.; Claret, F.; Croué, J.-P.; Fabbricino, M.; Gallard, H.; Schäfer, T.; Benedetti, M. F. Effects of charging on the chromophores of dissolved organic matter from the Rio Negro Basin. Water Res. 2014, 59, 154−164. (21) Phillips, S. M.; Bellcross, A. D.; Smith, G. D. Light absorption by brown carbon in the southeastern United States is pH-dependent. Environ. Sci. Technol. 2017, 51 (12), 6782−6790.
B
DOI: 10.1021/acs.est.8b01189 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
