Photocatalytic Carbon Disulfide Production via Charge Transfer Quenching of Quantum Dots

Abstract

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.S emiconductor quantum dots (QDs) exhibit size and composition dependent optical properties, strong absorption cross sections, high photoluminescence (PL) quantum yields, and customizable solubility through surface ligand exchange. 1 These properties position QDs as attractive sensitizers for photodynamic therapy, 2 photoactivated drug delivery, 3 solar energy conversion, 4 and photocatalysis. 5 Here we describe the photocatalytic cleavage of 1,1-dithiooxalate (DTO) to CS 2 and CO 2 mediated by CdSe QDs. This process serves to "uncage" carbon disulfide (CS 2 ), a potentially therapeutic agent. Similar reactions are observed with the DTO ester t BuDTO, and light activated cleavage of such ligands suggests strategies for the controlled modification of quantum dot surfaces.Fragmentary evidence points toward possible therapeutic roles for carbon disulfide. 6 For example, CS 2 may react with biological amines to form dithiocarbamates, known inhibitors 7 of nuclear factor κB (NF-κB), 8 a crucial mediator in inflammation induced tumor growth and progression. 9 Indeed, dithiocarbamates have been proposed as anticancer agents. 10 Physiological responses to CS 2 or dithiocarbamates include cell growth, apoptosis, and neurotransmission, which are also functions of the small molecule bioregulators NO, CO, and H 2 S. 11 Like each of these, CS 2 is considered toxic at higher concentrations, although epidemiological data are contradictory. 12 Thus, the benefit of balancing the therapeutic and toxic effects of CS 2 make the prospect of controlled, targeted delivery an attractive goal.In these contexts, we are probing DTO−QD conjugates as photochemical CS 2 precursors. These were readily prepared by exchanging DTO dianions for the myristate ligands (n-C 13 H 27 CO 2 − ) originally terminating the QD surface (see Supporting Information (SI) for procedures). Ligand exchange is evidenced by the solubility shift from organic to aqueous media as dianionic DTO replaces the hydrophobic myristate. Purified conjugates show a very strong UV absorption band (∼335 nm) nearly the same as free DTO (λ max ∼335 nm, ε max = 1.5 × 10 4 M −1 cm −1 in aq. solution) and a QD exciton band red-shifted by as much as 40 nm (Figure 1, SI Table S-1).Analogous red shifts in exciton absorptions have been seen when...

Biological Thiols and Carbon Disulfide: The Formation and Decay of Trithiocarbonates under Physiologically Relevant Conditions

“…In this context, we describe the kinetics of CS 2 dissociation from several prepared TTC – salts in aqueous solution. The latter studies complement earlier investigations of CS 2 generation by photosensitized oxidation of 1,1-dithiooxalate 7 and by the thermal decay of dithiocarbamate anions. 8 The CS 2 release rates from the latter precursors vary considerably, thereby providing a wide range of activities for physiological experiments.…”

Uncaging carbon disulfide. Delivery platforms for potential pharmacological applications: a mechanistic approach

“…This procedure was similar to that described by Schwack and Nyanzi 52 and to that used to detect CS 2 released by the photolysis of CdSe quantum dots surface decorated with 1,1-dithiooxalate. 25 In the present case, the CS 2 was detected by determining absorption spectral changes upon reaction with alkaline ethanol to form sodium ethylxanthate ( λ max 303 nm, ε = 1.74 × 10 4 M –1 cm –1 ). 53 The DTC substrates were allowed to degrade completely (as monitored from their characteristic absorption spectra), then flowing medical grade air was used to transfer the volatile CS 2 from the substrate solution via PEEK or Teflon tubing to a stirred cuvette containing 3.0 mL of ∼10 mM NaOH in 190 proof ethanol.…”

“…Advances in understanding the biological signalling and therapeutic roles of the known SMBs have depended in part upon the accessibility of molecular species that provide controlled release under physiological conditions. 19 – 24 In complement to earlier studies of CS 2 release by photosensitized decomposition of 1,1-dithiooxalate, 25 the present study is an investigation of thermally activated CS 2 donors with the goal of providing a toolbox of compounds with well-defined release rates that can be used to evaluate potential signalling and/or therapeutic properties of CS 2 . In this context, we note the recent reports by Powell et al 26 and by Steiger et al 27 – 29 of donors for carbonyl sulfide (OCS), which serves as a precursor for the release of H 2 S, a known SMB in human physiology.…”

Mechanistic Insights into the Interface‐Directed Transformation of Thiols into Disulfides and Molecular Hydrogen by Visible‐Light Irradiation of Quantum Dots

“…[5] Most recently, Ford and co-workers found that 1,1-dithiooxalate bound to the surfaces of CdSe QDs can be converted into carbon disulfide under aerobic conditions through oxidative cleavage. [6] Herein, we report that visible-light irradiation of CdSe QDs leads to virtually quantitative coupling of a variety of thiols to give disulfides and molecular hydrogen without the need for sacrificial reagents or external oxidants and with relatively high turnover numbers (TONs). The mechanistic insights presented offer a blueprint for how these QDs may be used in other applications.Coupling reactions are useful in the synthesis of new molecules, as well as for various medical, biological, materials, and nanotechnological applications.…”