MoS₂-quantum dot triggered reactive oxygen species generation and depletion: responsible for enhanced chemiluminescence

Activated MoS₂-QDs exhibit a promising capability for the generation of reactive oxygen species.

Abstract

Reactive oxygen species (ROS) generation is of intense interest because of its crucial role in many fields. Here we demonstrate that MoS₂-QDs exhibit a promising capability for the generation of reactive oxygen species, which leads to enhanced chemiluminescence. We discovered that the unique performance is due to hydroxyl radical activation increasing the active catalytic sites on molybdenum sulphide quantum dots (MoS₂-QDs). The reactive oxygen species, such as hydroxyl radicals (˙OH), superoxide radicals (˙O₂^–) and singlet oxygen (¹O₂) have been efficiently generated from H₂O₂ solution in alkaline conditions. In particular, the maximum ˙OH yield was enhanced significantly (9.18 times) compared to the Fe(ii)/H₂O₂ Fenton system under neutral conditions. These findings not only enrich our understanding of the fascinating performance of MoS₂ QDs, but also provide a new pathway for ROS generation in all kinds of pH environment.

Introduction

Reactive oxygen species (ROS) is a collective term for oxygen free radicals and molecules including superoxide radicals (˙O₂^–), hydroxyl radicals (˙OH) and singlet oxygen (¹O₂), which possess high reactivity compared to molecular O₂. Generation of abundant ROS is of immense interest in environmental and biological sciences.1 To date, the ROS are usually generated using a photocatalytic process via light-activated photosensitizers or a chemical process via iron-mediated Fenton reaction.2 However, the photocatalytic process suffers from undesirable bio-damage by ultra-violet (UV) radiation and low reactive oxygen species production. This has restricted the role of photo-catalysis for biomedical applications.3 The Fenton reaction can generate adequate ROS, however, this requires acidic conditions (pH = 3–4). Therefore, alternative reactions and conditions for ROS generation need to be pursued.

Nanoscale molybdenum sulphides (MoS₂) have gained widespread application in photo-responsive, energy storage and biosensor devices.4 Besides, MoS₂ has been used as an excellent electro-catalyst for the hydrogen evolution reaction (HER).5 However, the performance of molybdenum sulphide-quantum dots (MoS₂-QDs) toward chemiluminescence (CL) based on ROS generation has not been explored.

Here, we demonstrate the excellent capability of MoS₂-QDs to generate ROS from hydrogen peroxide in alkaline solution, which gives rise to CL emission (Fig. 1A). The hydroxyl radicals (˙OH) from intrinsic reactions such as H₂O₂ in alkaline medium and the Fe(ii)/H₂O₂ Fenton system activate MoS₂-QDs and generate active catalytic sites on their surface. These active sites then facilitate the conversion of H₂O₂ to generate a sufficient amount of ROS, which leads to a strong CL-emission.

Fig. 1. MoS₂-QDs promote the generation of reactive oxygen species and induce chemiluminescence with H₂O₂ in alkaline conditions. (A) Schematic illustration of ROS generation by MoS₂-QDs. (B) The CL spectrum of injection order with (a) injection of MoS₂-QDs into a mixture of H₂O₂ + NaOH, (b) injection of NaOH into a mixture of H₂O₂ + MoS₂-QDs and (c) injection of H₂O₂ into a mixture of MoS₂-QDs + NaOH. (C) The dose-dependence of H₂O₂ concentration on kinetic decay time and CL intensity by injection of MoS₂-QDs into a H₂O₂–NaOH solution. (D) The CL kinetic curve at H₂O₂ concentrations of (a) 0.01, (b) 0.025, (c) 0.05, and (d) 0.1 M.

Results and discussion

The MoS₂-QDs were prepared by solvothermal treatment in N,N-dimethylformamide (DMF), as proposed by Wang et al.6 The MoS₂-QDs exhibited an average size of ∼3.7 nm (Fig. S1, ESI†). The as-prepared product (0.9 mg ml^–1) was dispersed in water and a transparent pale-yellow solution was obtained, which exhibited a strong fluorescence emission over a wide range of excitation wavelengths (Fig. S2, ESI†).

The chemiluminescence (CL) was produced by MoS₂-QDs and H₂O₂. A stronger CL was obtained with injection of the MoS₂-QD suspension into a pre-mixed H₂O₂/NaOH solution, whereas those with the addition of H₂O₂ to NaOH/MoS₂-QDs or with the addition of NaOH to H₂O₂/MoS₂-QDs, exhibited relatively weaker CL-emission (Fig. 1B). We found that the CL was not only correlated to the injection order, but also to the NaOH concentration. The CL intensity was found to increase with increasing concentration of NaOH and no CL emission was found under acidic or neutral conditions. This was attributed to the remarkable decomposition of H₂O₂ in alkaline medium7 (80% and 15% at pH = 13 and 10.5 respectively) (Fig. S3, ESI†). The results suggested that the decomposition of H₂O₂ plays an important role in the enhanced CL-emission with the addition of MoS₂-QDs. The hydroxyl radical (˙OH) generation from H₂O₂ in the presence of base is given in eqn (1) and (2). Perhydroxyl ions (HO₂^–) were formed through the decomposition of H₂O₂ in base; then the HO₂^– reacted with undissociated H₂O₂ molecules to produce hydroperoxide radicals (HO₂˙) and ˙OH.8

H₂O₂ + OH^– → H₂O + HO₂^– 1

HO₂^– + H₂O₂ → HO₂˙ + ˙OH + OH^– 2

Ouyang et al. have reported that intrinsic defects can be generated to activate and increase the number of catalytic sites on MoS₂.9 The ˙OH is a strong oxidizing agent, which has a redox potential of 2.8 V,10 therefore, the production of ˙OH leads to radical attack on the MoS₂-QDs and generates defects. This increases the effective catalytic sites of the MoS₂-QDs and hence facilitates the further decomposition of H₂O₂. The occurrence of defects was evidenced by the short lifetime of about 5.2 ns of the MoS₂-QDs (Fig. S4, ESI†). To clarify the role of ˙OH in the strong CL-emission of the MoS₂-QD/H₂O₂ system, experiments with H₂O₂ at different concentrations were performed. We found that the CL-emission intensity of the MoS₂-QD/H₂O₂ system is dependent on the concentration of H₂O₂ (Fig. 1C). With increasing [H₂O₂] from 0.01 M to 0.1 M, the intensity of CL increased progressively. The CL kinetic decay time was also found to be dependent on the H₂O₂ concentration. A flash CL-emission was observed at high H₂O₂ concentration (0.1 M), whereas a relatively slower decay was observed at low concentration (Fig. 1D).

Electron Spin Resonance (ESR) spectroscopy was performed to directly examine the generation of ˙OH under different H₂O₂ conditions. A larger yield of ˙OH was observed as [H₂O₂] was increased (Fig. 2A(a) and B(a)). This provides clear evidence that activation of the MoS₂-QDs is highly dependent on ˙OH formation. No obvious ESR signals for DMPO–OH were observed without MoS₂-QDs, while the ˙OH was significantly enhanced in the presence of activated MoS₂-QDs at [H₂O₂] = 0.01 M in NaOH (0.1 M) (Fig. 2A(b)). In addition, we found the signal for DMPO–OH and DMPO–˙O₂^– (ref. 11) in the presence of MoS₂-QDs is stronger than that in the absence of MoS₂-QDs at H₂O₂ = 0.1 M in NaOH (0.1 M) (Fig. 2B(b)). These results suggest the dependence of ˙OH generation on H₂O₂ concentration and the ability of the MoS₂-QDs for ROS generation, which leads to the strong CL-emission of the MoS₂-QD/H₂O₂ system.

Fig. 2. Evaluation of the enhancement of ˙OH, ˙O₂^– and ¹O₂ production by MoS₂-QDs in the H₂O₂–NaOH system. (A) ESR spectrum of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) with ˙OH obtained from 0.01 M H₂O₂ in 0.1 M NaOH (a), and the same as the above conditions but with MoS₂-QDs (b). (B) ESR spectrum of DMPO with ˙OH or ˙O₂^– obtained from 0.1 M H₂O₂ in 0.1 M NaOH (a), and the same as the above conditions but with MoS₂-QDs (b). (C) The absorbance of NBT in the H₂O₂–NaOH system with the addition of MoS₂-QDs. (D) ESR spectrum of TEMP with ¹O₂ production from H₂O₂–NaOH (black) and MoS₂-QDs–H₂O₂–NaOH (red).

Moreover, the UV-vis spectra (Fig. 2C) with nitrotetrazolium blue chloride (NBT) as a superoxide radical (˙O₂^–) probe12 and ESR spectra (Fig. 2D) with 2,2,6,6-tetramethyl-4-piperidine (TEMP) as a singlet oxygen (¹O₂) probe13 also showed a significant increase in ˙O₂^– and ¹O₂ production in the presence of MoS₂-QDs. The generation pathways of ˙O₂^– and ¹O₂via radical reactions are given in eqn (3)–(7).14,15 We hypothesized that the active sites induced by ˙OH were responsible for the generation of ˙OH, ˙O₂^–, and ¹O₂ by facilitating the conversion of H₂O₂ into more ROS.

˙OH + H₂O₂ → HO₂˙ + H₂O 3

HO₂˙ + OH^– → ˙O₂^– + H₂O 4

5

˙OH + HO₂˙ → H₂O + O₂ + ¹O₂ 6

˙OH + ˙O₂^– → OH^– + ¹O₂ 7

In order to further evaluate the dependence of MoS₂-QD activation and CL-emission on the generation and concentration of ˙OH, a classic ˙OH generation Fenton system was investigated. We found that, MoS₂-QDs strongly enhanced the CL-emission of the Fe²⁺/H₂O₂ system. It was striking that equivalent CL-emission was produced under both neutral and acidic conditions (Fig. 3A). Besides, ˙OH generation was significantly accelerated and compared to the Fe²⁺/H₂O₂ system, a 9.18 times greater yield of ˙OH was observed (Fig. 3B). These results confirmed the hypothesis of ROS generation through ˙OH-activated MoS₂-QDs.

Fig. 3. Enhancement of chemiluminescence and hydroxyl radical (˙OH) production by MoS₂-QDs in the Fe²⁺–H₂O₂ system (A) the chemiluminescence of Fe²⁺–H₂O₂ (blank); and the CL produced by the addition of MoS₂-QDs to the Fe²⁺–H₂O₂ system under acidic (pH = 3.7) and neutral conditions. (B) The hydroxyl radicals generated from Fe²⁺–H₂O₂ (blank) and from the addition of MoS₂-QDs with a concentrations of 0.09 mg ml^–1 and 0.45 mg ml^–1. The inset in panel B is the ESR spectrum of DMPO with ˙OH generated from Fe²⁺–H₂O₂ (black) and with the addition of MoS₂-QDs with concentrations of 0.09 mg ml^–1 (red) and 0.45 mg ml^–1 (blue).

To further verify the generation of ˙OH, the Fenton-oxidation of tetramethylbenzidine (TMB) was introduced, which is known to be oxidized by ˙OH.16 In the absence of MoS₂-QDs, TMB was oxidized slowly and reached a maximum absorbance intensity (λ_max = 652 nm) after 60 s. Nevertheless, in presence of MoS₂-QDs, a rapid strong absorbance at 652 nm was observed (Fig. S5, ESI†), indicating a rapid formation of the bluish oxidized TMB because of ˙OH being generated in large amounts. This confirmed that MoS₂-QD driven Fenton reaction can effectively produce ˙OH.

The generation of ˙OH can effectively degrade organic contaminants.17 Therefore, phenol, pyrocatechol, rhodamine B (RhB) and methylene blue (MB) were used to find the degradation efficiency and fate of ˙OH in the Fenton (Fe²⁺/H₂O₂) and MoS₂-QD supported Fenton system. Phenolic compound concentrations were measured by using 4-aminoantipyrine for the colorimetric determination.18 As expected, a significant decrease of phenol and pyrocatechol (Fig. S6, ESI†) was observed when phenol or pyrocatechol were incubated with the MoS₂-QD supported Fe²⁺/H₂O₂ system as compared to the traditional Fe²⁺/H₂O₂ Fenton system, indicating a higher degradation efficiency for phenolic compounds with the Fe²⁺/H₂O₂–MoS₂-QD system. In addition, the degradation of RhB and MB has been detected by the UV-visible method. In the presence of MoS₂-QDs, MoS₂-QDs were found to significantly inhibit both RhB and MB degradation efficiency (Fig. 4). Moreover, the presence of MoS₂-QDs showed a more negative influence on MB degradation, inhibiting the efficiency of MB degradation by 72%, as well as that of RhB degradation efficiency by 8%. This difference towards RhB and MB degradation is probably attributable to the lower reactivity of MB compared to RhB in the Fe²⁺/H₂O₂–MoS₂-QD system, because of significant structural differences between the two different organic fractions (RhB and MB). It has been found previously that ˙OH could be consumed by injection of holes into semiconductor nano-materials.19 The selective degradation of phenolic compounds as compared to RhB and MB is probably due to the higher reaction rate of ˙OH with phenolic compounds (k ≈ 10¹⁰ M^–1 s^–1),20 which makes phenolic compounds more competitive for reacting with ˙OH rather than MoS₂-QDs in the degradation process. In contrast, ˙OH reacts with MoS₂-QDs preferentially rather than the aromatic heterocyclic compounds, such as RhB and MB, due to the one order of magnitude lower rate (k ≈ 10⁹ M^–1 s^–1).21 These results strongly indicated that a rapid reaction of MoS₂-QDs with ˙OH (and ˙O₂^–) forming hole-injection ((MoS₂)˙⁺ QDs) and electron-injection ((MoS₂)˙^– QDs) species (eqn (8) and (9)) occurred.

MoS₂-QDs + ˙OH → (MoS₂)˙⁺ QDs + OH^– 8

MoS₂-QDs + ˙O₂^– → (MoS₂) ˙^– QDs + O₂ 9

Fig. 4. The influence of MoS₂-QDs on organic contaminant degradation in the Fe²⁺–H₂O₂ Fenton system. (A) The absorbance of rhodamine B at 554 nm in the Fe²⁺–H₂O₂ and Fe²⁺–H₂O₂ + MoS₂-QD systems; (B) the absorbance of methylene blue at 626 nm in the Fe²⁺–H₂O₂ and Fe²⁺–H₂O₂ + MoS₂-QD systems. The solution conditions were 0.01 M H₂O₂, 0.45 mg ml^–1 MoS₂-QDs, 1 mM Fe²⁺, 5 μg ml^–1 RhB and 5 μg ml^–1 MB.

The annihilation of (MoS₂)˙⁺ and (MoS₂)˙^– resulted in energy release in the form of CL emission,22 which has been clarified through a CL-emission spectrum. The CL emission spectrum was centered at 490 nm and 550 nm, which may contain overlap of ¹O₂ and MoS₂ emission23 (Fig. S7, ESI†). To verify that this emission was from the recombination of (MoS₂)˙⁺ and (MoS₂)˙^– and not from the overlap of ¹O₂ and MoS₂-QD emission, the CL-emission was recorded in D₂O as a solvent instead of H₂O, because ¹O₂ possesses a 13 times longer lifetime in D₂O than in H₂O.24 Generally, if the CL emission originated from ¹O₂, one would expect alteration of the kinetic reaction in D₂O. However, no obvious change was observed, indicating no contribution of the ¹O₂ to the CL-emission due to the annihilation of electron–hole combination of (MoS₂)˙⁺ QDs and (MoS₂)˙^– QDs (Fig. S8, ESI†).

Conclusions

For the first time, we have demonstrated the excellent capability of MoS₂-QDs toward ROS generation. We explored that the synergistic effect of enhanced ROS production and ROS depletion were the main factors leading to the CL-emission of MoS₂-QDs with H₂O₂ in alkaline medium; and with the Fenton reagent under neutral and acidic conditions. These findings present a new pathway for ROS generation over the whole pH-range. Thus, MoS₂-QDs have potential applications for degradation of organic pollutants and chemo-dynamic therapy.

Conflicts of interest

There are no conflicts to declare.