Carbon disulfide, a potentially therapeutic small molecule, is generated via oxidative cleavage of 1,1-dithiooxalate (DTO) photosensitized by CdSe quantum dots (QDs). Irradiation of DTO-QD conjugates leads to λ(irr) independent photooxidation with a quantum yield of ~4% in aerated pH 9 buffer solution that drops sharply in deaerated solution. Excess DTO is similarly decomposed, indicating labile exchange at the QD surfaces and a photocatalytic cycle. Analogous photoreaction occurs with the O-tert-butyl ester (t)BuDTO in nonaqueous media. We propose that oxidation is initiated by hole transfer from photoexcited QD to surface DTO and that these substrates are a promising class of photocleavable ligands for modifying QD surface coordination

Bernt, Christopher M

Burks, Peter T

DeMartino, Anthony W

Ford, Peter C

Levy, Elizabeth S

Pierri, Agustin E

Zigler, David F

eScholarship - University of California

UC Santa BarbaraUC Santa Barbara Previously Published WorksTitlePhotocatalytic Carbon Disulfide Production via Charge Transfer Quenching of Quantum DotsPermalinkhttps://escholarship.org/uc/item/2rt61047JournalJournal of the American Chemical Society, 136(6)ISSN0002-7863AuthorsBernt, Christopher MBurks, Peter TDeMartino, Anthony Wet al.Publication Date2014-02-12DOI10.1021/ja4083599 Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaPhotocatalytic Carbon Disulfide Production via Charge TransferQuenching of Quantum DotsChristopher M. Bernt,† Peter T. Burks,† Anthony W. DeMartino, Agustin E. Pierri, Elizabeth S. Levy,David F. Zigler, and Peter C. Ford*Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States*S Supporting InformationABSTRACT: Carbon disulfide, a potentially therapeuticsmall molecule, is generated via oxidative cleavage of 1,1-dithiooxalate (DTO) photosensitized by CdSe quantumdots (QDs). Irradiation of DTO−QD conjugates leads toλirr independent photooxidation with a quantum yield of∼4% in aerated pH 9 buffer solution that drops sharply indeaerated solution. Excess DTO is similarly decomposed,indicating labile exchange at the QD surfaces and aphotocatalytic cycle. Analogous photoreaction occurs withthe O-tert-butyl ester tBuDTO in nonaqueous media. Wepropose that oxidation is initiated by hole transfer fromphotoexcited QD to surface DTO and that thesesubstrates are a promising class of photocleavable ligandsfor modifying QD surface coordination.Semiconductor quantum dots (QDs) exhibit size andcomposition dependent optical properties, strong absorp-tion cross sections, high photoluminescence (PL) quantumyields, and customizable solubility through surface ligandexchange.1 These properties position QDs as attractivesensitizers for photodynamic therapy,2 photoactivated drugdelivery,3 solar energy conversion,4 and photocatalysis.5 Herewe describe the photocatalytic cleavage of 1,1-dithiooxalate(DTO) to CS2 and CO2 mediated by CdSe QDs. This processserves to “uncage” carbon disulfide (CS2), a potentiallytherapeutic agent. Similar reactions are observed with theDTO ester tBuDTO, and light activated cleavage of suchligands suggests strategies for the controlled modification ofquantum dot surfaces.Fragmentary evidence points toward possible therapeuticroles for carbon disulfide.6 For example, CS2 may react withbiological amines to form dithiocarbamates, known inhibitors7of nuclear factor κB (NF-κB),8 a crucial mediator ininflammation induced tumor growth and progression.9 Indeed,dithiocarbamates have been proposed as anticancer agents.10Physiological responses to CS2 or dithiocarbamates include cellgrowth, apoptosis, and neurotransmission, which are alsofunctions of the small molecule bioregulators NO, CO, andH2S.11 Like each of these, CS2 is considered toxic at higherconcentrations, although epidemiological data are contra-dictory.12 Thus, the benefit of balancing the therapeutic andtoxic effects of CS2 make the prospect of controlled, targeteddelivery an attractive goal.In these contexts, we are probing DTO−QD conjugates asphotochemical CS2 precursors. These were readily prepared byexchanging DTO dianions for the myristate ligands (n-C13H27CO2−) originally terminating the QD surface (seeSupporting Information (SI) for procedures). Ligand exchangeis evidenced by the solubility shift from organic to aqueousmedia as dianionic DTO replaces the hydrophobic myristate.Purified conjugates show a very strong UV absorption band(∼335 nm) nearly the same as free DTO (λmax ∼335 nm, εmax =1.5 × 104 M−1 cm−1 in aq. solution) and a QD exciton bandred-shifted by as much as 40 nm (Figure 1, SI Table S-1).Analogous red shifts in exciton absorptions have been seenwhen dithiocarbamates were exchanged onto semiconductorQD surfaces,13 and this effect was attributed to relaxation ofexciton confinement in the QD−ligand conjugate owing toalignment of interfacial orbital energies. Lastly, the strong QDPL was completely quenched. Aqueous solutions of the DTO−QD conjugates are stable in the dark for at least 6 h at 37 °C(Figure S-2) and indefinitely stable when stored in arefrigerator.PL quenching in thiolate terminated QDs has been attributedto hole transfer from the valence band to the surface boundReceived: August 12, 2013Published: October 23, 2013Figure 1. Absorption spectrum of purified DTO−QD550 conjugates inpH 9 buffer (50 mM sodium borate) solution (solid line) and that ofQD550 nanoparticles in toluene prior to ligand exchange (dashed line).Inset: Expanded view of the exciton peak shift. (QD subscript refers toexciton λmax before ligand exchange.)14Communicationpubs.acs.org/JACS© 2013 American Chemical Society 2192 dx.doi.org/10.1021/ja4083599 | J. Am. Chem. Soc. 2014, 136, 2192−2195ligands.15 Such a mechanism suggested that CdSe QDs couldmediate DTO cleavage (eq 1).− ⎯ →⎯⎯⎯⎯⎯ +−S C CO CS CO2 22oxidanthv, QD2 2 (1)In order to test this hypothesis, aerated pH 9 aqueoussolutions of the purified DTO−QD550 conjugate14 wereirradiated with 365 nm light. This led to a systematic decreasein the 335 nm absorption band indicating that the surface-coordinated DTO was undergoing photodecomposition. Thequantum yield for the disappearance of this band (Φdis) was0.029 ± 0.008 (see SI, Sec. I and J for details). Notably, thisabsorbance decrease was accompanied by recovery of the QDPL (Figure 2). At higher photochemical conversion, the QDcores precipitated, owing to loss of the water-solvating surfaceligands. Photolysis (365 nm) of a comparable solution of DTOalone (∼70 μM) showed minimal photoinduced bleaching(Figure S-4) and a much smaller Φdis (0.004 ± 0.001).The difference between the DTO−QD conjugates and DTOitself is even more dramatic at longer irradiation wavelengths(λirr). When buffered solutions of DTO−QD550 were excitedwith light emitting diodes centered at 479, 498, or 530 nm(Figure S-5), bleaching of the 335 nm band occurred withcomparable efficiency (Φdis = 0.045 ± 0.005, 0.032 ± 0.003 and0.039 ± 0.005 for these respective λirr) to that seen for λirr =365 nm. Again, the PL of the QD cores was restored (Figure S-6). In contrast, solutions of free DTO were unaffected owing totheir optical transparency at these longer λirr, so there is littlequestion that the QDs sensitize the photochemical decom-position of the surface bound DTO.Similar sensitization of photodecomposition was observedwith other DTO−QD conjugates, although there were somedifferences in the quantum efficiencies. Respective Φdis valuesof 0.016 ± 0.003, 0.033 ± 0.003, and 0.035 ± 0.001 weremeasured for 498 nm irradiation of DTO−QD510, DTO−QD550, and DTO−QD580 (Figure 3); however, a larger data setis needed before considering a possible systematic trend.When the photoreactions were carried out under deaeratedconditions, Φdis values dropped precipitously. For example, 498nm irradiation of DTO−QD550 solutions deaerated byextensive bubbling with argon gave Φdis = 0.002 ± 0.001.However, an atmosphere of pure O2 did not increasephotoactivity (Φdis = 0.033 ± 0.001) significantly above thatseen in aerobic media. Thus, while the oxidant O2 appearsnecessary for efficient net photodecomposition of coordinatedDTO, this does not appear to be rate-limiting under normalconditions.If these spectral changes do indeed reflect DTO photo-decomposition, then other ligands in solution shouldcoordinate to the resulting open surface sites. Thus, the QDsshould be photocatalysts for oxidative decomposition of freeDTO in solution.This reactivity was demonstrated using anaerated pH 9 solution of DTO−QD550 to which a 2-fold excessof the K+ salt of DTO (115 μM) had been added. Photolysis at498 nm led to a rapid decrease in the characteristic DTOabsorbance at 335 nm (Figure S-7), and a Φdis = 0.052 ± 0.006was measured. Furthermore, the QDs remained in solution andshowed little PL until the DTO was nearly consumed accordingto λmax 335 nm. Thus, the system is indeed photocatalytic.Additionally, this confirms that the decomposition of DTOfrees the QD surface from dithiolcarboxylate coordination andsuggests a strategy for syntheses of new quantum dotconjugates via incorporation of other ligands.Although it has been speculated16 that photooxidativedecomposition of DTO would occur according to eq 1, CS2has not previously been detected as a photoproduct. To addressthis issue, we exhaustively photolyzed (λirr ≥ 479 nm) aeratedpH 9 solutions of DTO−QD550 in sealed, septa-capped vials.GC-MS analysis of headspace gases by Weck Laboratories (Cityof Industry, CA) showed that photolyzed samples gavesignificantly larger amounts of CS2 than “dark” controls,17thereby confirming, qualitatively, formation of CS2 as aphotoproduct.In order to evaluate the CS2 production quantitatively, wedeveloped a colorimetric assay based on the rapid reaction ofCS2 with amines to form dithiocarbamates.18 The latter havestrong UV absorptions. This procedure clearly confirmedphotochemical CS2 formation, although the amount detectedwas systematically about 50% that predicted by assuming eq 1to be the only reaction leading to photobleaching of the DTOpeak at 335 nm (SI, Sec. K for details). No CS2 formation wasapparent without photolysis (see Figure S-9).The other product predicted is CO2. This was determined byGC analysis of the headspace before and after acidifying thephotoproduct solution to ∼pH 1, since some of the CO2 wouldbe trapped as bicarbonate in the pH 9 buffered solutions.Consistent with the colorimetric analysis for CS2, the CO2generated by photolysis of DTO−QD550 was about half thatpredicted by eq 1 for the analysis before acidification and about70% after acidification (SI, Sec. M).Figure 2. Spectral changes for pH 9 solution of DTO−QD550 upon365 nm excitation over the course of 120 s at 20 s intervals. Inset:Photograph showing PL return as photolysis proceeds. (See video inthe SI.)Figure 3. (Left) Φdis values for the photolysis of DTO−QD550conjugates at different λirr (UV = 365, blue = 479, cyan = 498, andgreen = 530 nm). (Center) Φdis values for λirr 498 nm of DTO−QD510, DTO−QD550, and DTO−QD580. (Right) Φdis values for 498nm photolysis of DTO−QD550 under Ar, air, and O2. All in pH 9buffer solution.Journal of the American Chemical Society Communicationdx.doi.org/10.1021/ja4083599 | J. Am. Chem. Soc. 2014, 136, 2192−21952193These products and the requirement for O2 as a coreactantsuggest that photolysis of surface coordinated DTO reversiblyleads to transient species that are trapped by the externaloxidant (Scheme 1). Alternatively, it might be argued thatphotoinduced two-electron transfer from DTO to the QDoccurs either simultaneously or sequentially and that “charged”QDs are no longer photoactive until “discharged” by reactingwith O2 (SI, Scheme S-1). The formation of the DTO radicalssuggested by Scheme 1 offers a possible explanation for thenonstoichiometric formation of CS2 and CO2, given that suchspecies might dimerize to give a disulfide linkage.19 Regardlessof the actual mechanism, the catalytic behavior shows that theQD sensitized photooxidation and cleavage of DTO facilitatesremoval and replacement of the surface ligands. Ongoingstudies will attempt to differentiate the mechanistic possibilities.While DTO functionalization provides aqueous solubility,QD conjugates of tBuDTO remain soluble in organic media.Given that CO2 formation must contribute to the driving forcefor eq 1, analogous oxidative activation of an O-ester such astBuDTO may release a radical, e.g., eq 2.− ⎯ →⎯⎯⎯⎯⎯ + +− •S C CO R CS CO R2 2 oxidanthv, QD2 2 (2)Organic soluble conjugates were prepared as described in theSI by exchanging tBuDTO for the myristate surface ligands ofQD510. The spectrum oftBuDTO−QD510 displayed the strongabsorption band at 350 nm characteristic of the DTOchromophore and an exciton peak at 523 nm red-shiftedfrom that of the core QD exciton peak (Figure S-8). In additionthe tBuDTO conjugate was not luminescent, as seen for theQD conjugates with DTO. Photolysis of tBuDTO−QD510 withλirr 498 nm in aerated solution led to bleaching of the 350 nmDTO band, but Φdis values were small, 0.007 in toluene and0.0024 in chloroform. Exhaustive photolysis led to PL recoveryand a shift of the exciton peak to its original maximum (FigureS-8), while the QDs remained soluble. The colorimetricanalytical method described above demonstrated that CS2 isclearly formed during this photoreaction, but the yields areeven lower (∼12%) than with analogous DTO conjugates (SI,Sec. M).In summary, we have described the preparation of photo-sensitive conjugates by displacing the native surface ligands onCdSe QDs with 1,1-dithiooxalate and with O-tert-butyl-dithiooxalate. The resulting DTO−QD conjugates are water-soluble while the R-DTO conjugates are organic-soluble. Forboth, the QD exciton bands are shifted to the red and PL isquenched. Photolysis in aerated solutions leads to thephotocatalytic oxidative decomposition of these surface ligandswith modest quantum yields. By using a colorimetric assay, itwas shown in both cases that CS2 is generated. This representsthe first demonstrated photochemical release of this potentiallytherapeutic small molecule.We propose that the photooxidation mechanism involvesQD excitation-induced hole transfer to surface bound DTO.The efficiency is highly dependent upon the presence ofanother oxidant, O2, which serves as the ultimate electronacceptor. The reaction not only decomposes the multipleDTOs bound to the QD surface, but the facile ligand exchangeleads to a catalytic cycle for photooxidation of excess free DTOin solution. In addition, this offers a potential pathway for thesystematic removal or replacement of QD surface ligands andfor the photochemical syntheses of new QD−ligand conjugates.O-Ester-DTO−QD conjugates undergo analogous photo-decomposition. We propose that such a photocleavable ligandmay allow the controlled release of organic radicals from QDsurfaces. Ongoing studies are directed toward a betterquantification of the products as well as expanding thephotochemistry to a library of other esters in order to probethe utility of these as photocleavable surface anchors.■ ASSOCIATED CONTENT*S Supporting InformationExperimental details regarding the synthesis and character-ization of DTO−QD and tBuDTO−QD conjugates, photo-chemical procedures and photoproduct analyses, a videoillustrating the photoreaction, and additional data. This materialis available free of charge via the Internet at http://pubs.acs.org.■ AUTHOR INFORMATIONCorresponding Authorford@chem.ucsb.eduAuthor Contributions†These authors contributed equally.NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis work was funded by the National Science Foundation(NSF-CHE-1058794). P.T.B. thanks ConvEne-IGERT (NSF-DGE 0801627), C.M.B. thanks the UCSB Graduate Division,A.E.P. and A.W.D. thank PIRE-ECCI (NSF-OISE-0968399),and E.S.L. thanks IRES-ECCI (NSF-OISE-1065581) forfellowships. We thank Weck Laboratories for GC-MS measure-ments .■ REFERENCES(1) (a) Choi, C. L.; Alivisatos, A. P. Annu. Rev. Phys. Chem. 2010, 61,369. (b) Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H.Nat. Mater. 2005, 4, 435.(2) Samia, A. C. S.; Chen, X.; Burda, C. J. Am. Chem. Soc. 2003, 125,15736.(3) (a) Burks, P. T.; Ford, P. C. Dalton Trans. 2012, 41, 13030.(b) Sortino, S. J. Mater. Chem. 2012, 22, 301.(4) Robel, I. n.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am.Chem. Soc. 2006, 128, 2385.(5) (a) Ruberu, T. P. A.; Nelson, N. C.; Slowing, I. I.; Vela, J. J. Phys.Chem. Lett. 2012, 3, 2798. (b) Zhao, J.; Holmes, M. A.; Osterloh, F. E.ACS Nano 2013, 7, 4316.(6) (a) Huang, X.; Zhou, Y.; Ma, J.; Wang, N.; Zhang, Z.; Ji, J.; Ding,Q.; Chen, G. Environ. Toxicol. Pharm. 2012, 34, 679. 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Photocatalytic Carbon Disulfide Production via Charge Transfer Quenching of Quantum Dots

Photocatalytic carbon disulfide production via charge transfer quenching of quantum dots.

Photocatalytic carbon disulfide production via charge transfer quenching of quantum dots.

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