Applied Materials Today
pH-regulated reversible photoluminescence and localized surface plasmon resonances arising from molybdenum oxide quantum dot
Introduction
Transition-metal oxides such as molybdenum oxide (MoOx) have unique physical and chemical properties [[1], [2], [3]]. MoOx nanomaterials with various allotropes and suboxide phases, such as MoO3, MoO2, Mo2O5 [2,[4], [5], [6]], exhibit excellent localized surface plasmon resonances (LSPR) due to outer-d valence electrons [7]. Because of surface plasmon bands in the near-infrared (700–1100 nm) [[8], [9], [10]], MoOx may be preferred over conventional noble metals [[11], [12], [13]] for switching or modulation applications [14,15]. Both theoretical and experimental results demonstrated that MoOx nanomaterials were good photocatalysts [3,7,9,[15], [16], [17]]. Additionally, they have been used for optical sensing of bovine serum albumin [14] and ascorbic acid [18] via LSPR tunability. The enhanced light-matter interactions by surface plasmons coupling light strongly to the metal surface can confine light in an smaller area compared with that calculated by the diffraction limit, and increase the local electromagnetic field intensity by many orders of magnitude [19]. In this case, high-sensitive detection is able to be achieved since small variations of the local environment can be evidently amplified when nanomaterials with LSPR properties is applied to optical and sensing devices [20]. Therefore, nanomaterials with tunable LSPR properties would be advantageous for optical and sensing devices.
Changes in free charge carrier concentrations could tune LSPR characteristics in two-dimensional semiconducting oxides [19,21,22]. The transformation of stoichiometric MoO3 to non-stoichiometric (MoO3−x) or hydrogen-doped MoO3 (HxMoO3) could produce a tunable LSPR, because of the introduction of aliovalent heteroatoms or lattice vacancies that provide free charge carriers [7,15]. However, generation of MoOx nanomaterials with sufficient free charge carriers was mainly implemented by heating under supercritical CO2 treatments or hydrogen reduction [7,14,16,23,24], requiring complex equipment and high-pressure or highly toxic reducing agents. Therefore, a more facile approach is desirable.
The quantum dots (QDs) have more active sites and higher charge carrier mobility than bulk nanomaterials, which may be a favorable candidate for tunable optical device [23,[25], [26], [27], [28]]. Although most of the reports about MoOx-related structure are nanosheet, nanowire, nanotube and nanobelt [4,23,24], MoOx QDs bearing excellent photoluminescent property have been followed with interest, which has some particular properties, such as quantum size effect, good stability, etc [29]. Photoluminescent probes based on MoOx QDs have been used for the detection of 2,4,6-trinitrotoluene [29], phosphate [30], and acetylcholine esterase and its inhibitors [31]. The photoluminescence of MoOx derives from impurities, Mo interstitials, or surface and structural defects, and varies with defect density or size [2]. Hence, MoOx QDs could be a dual-modal probe via simultaneous tunable LSPR and photoluminescent properties. A dual-modal sensor would open up possibilities of multimodal nanodevices in living and environmental systems.
Here, simultaneous photoluminescence and LSPR tuning in MoOx QDs is demonstrated. An easy one-pot protocol was adopted to prepare N-doped MoOx QDs. The introduction of N in the MoOx QD surfaces via ammonia (NH3) could be favorable for trapping oxygen molecules for forming MoOx QDs, and for providing free electrons to enable tunability. The formation of lattice oxygen vacancies in MoOx QDs is target concentration dependent, which could simultaneously regulate the fluorescence and LSPR in the visible and near-infrared regions. As a proof-of concept, a dual-modal sensor for the determination of extremely acidic pH was demonstrated. Hydrogen ions embedded in the MoOx QDs lattice could bond to oxygen atoms in the surface to generate water molecules. In addition, Mo6+ in MoOx QDs could be reduced to Mo5+ by electron transfer from surface N, transforming stoichiometric MoO3 to non-stoichiometric MoO3−x and increasing the size of the QDs with decreasing pH. The strong photoluminescence decreased with the decreasing pH value, while the 700 nm LSPR absorbance increased with the decreasing pH value. Thus, a simple dual fluorescence and LSPR sensor for extreme acidity was designed. The MoOx QDs responded to pH values in the range 1.0–5.0, which was suitable for many microorganisms. It was used for sensing extreme acidity in bacterial cells and exhibited good stability, selectivity, and reversibility. The dual-modal strategy, which utilizes the best optical properties of nanomaterials, provides a concrete basis for analogous nanomaterials and novel applications.
Section snippets
Synthesis of MoOx QDs
Na2MoO4·2H2O (0.25 g) and glutathione (GSH) (0.3 g) were dissolved in ultrapure water (68 mL) and ultrasonicated for five minutes. After the addition of 7 mL of NH3·H2O (25 %), the mixed solution was placed in a 100 mL polytetrafluoroethylene autoclave and heated at 200 °C for 48 h. When the reaction solution cooled to room temperature, MoOx QDs were obtained by centrifugation at 14,000 rpm. The pH was adjusted to 7.0 with HCl and the solution was then subjected to dialysis (MWCO: 1 KD) for
Synthesis and characterization of MoOx QDs
MoOx QDs were synthesized through a facile hydrothermal procedure, in which GSH was used as both a reducer and a stabilizer during the formation of the MoOx lattice. GSH is an endogenous antioxidant that can prevent reactive oxygen species (ROS)-induced cellular damage, thereby enhancing the biocompatibility of nanomaterials [32,33]. Ammonia was utilized to introduce an alkaline medium and N-atoms into the MoOx lattice via a high-temperature hydrothermal process. In addition, the N enabled
Conclusion
By controlling lattice vacancies, a facile strategy was reported that simultaneously tuned the visible photoluminescence and LSPR properties of MoOx QDs. Based on the mechanism of lattice vacancy tuning, a MoOx QDs-based dual-modal fluorescence and localized surface plasmon resonance assay was demonstrated. As a proof-of concept, the determination of extremely acidic pH was shown in solution and within bacteria. The dual-mode probe had significant advantages, including simplicity, label-free
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant no. 21707137), the Municipal Natural Science Foundation of Chongqing City (No. CSTC-2018jcyjAX0140), Chongqing Science and Technology Innovation in Social Livelihood of the People (No. CSTC-2017shmsA0159), Scientific and Technological Research Program of Chongqing Municipal Education Commission (Grant no. KJQN201801401), Research Funding Project of Yangtze Normal University (No.
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