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pH-regulated reversible photoluminescence and localized surface plasmon resonances arising from molybdenum oxide quantum dot
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pH-regulated reversible photoluminescence and localized surface plasmon resonances arising from molybdenum oxide quantum dot Haiyan Cao a,∗ , Xue Hu b , Wenbing Shi a , Siqi Li b , Yuming Huang b,∗ a
The Key Laboratory of Chongqing Inorganic Special Functional Materials, College of Chemistry and Chemical Engineering, Yangtze Normal University, Chongqing, 408100, China The Key Laboratory of Luminescence and Real-Time Analytical Chemistry, Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, China b
a r t i c l e
i n f o
Article history: Received 23 August 2019 Received in revised form 20 November 2019 Accepted 22 November 2019 Keywords: Molybdenum oxide Localized surface plasmon resonances Photoluminescence Dual-modal sensor pH sensor
a b s t r a c t Localized surface plasmon resonances (LSPR) and photoluminescence are important for applications of transition-metal oxides. However, simultaneous control of these optical properties is challenging. Here, a facile strategy is presented that simultaneously tunes photoluminescence and visible LSPR of molybdenum oxide quantum dots (MoOx QDs) by controlling lattice vacancies. Specifically, N-doped MoOx QDs were prepared with a one-pot protocol. The introduction of N in MoOx QDs surfaces via ammonia (NH3 ) not only trapped oxygen molecules in the process of forming MoOx QDs, but also provided enough free electrons to enable tunable optical properties. Thus, a MoOx QD-based dual-modal fluorescence and LSPR assay was demonstrated via lattice vacancy concentration tuning. Upon introduction of H+ or OH− , pHreversible tunability of the fluorescence and plasmonic resonance was observed. The dual-mode probe was used to detect extreme acidity in bacterial cells. Overall, tunable LSPR and photoluminescence within one nanostructure via pH-regulation should enable multi-modal signal-outputs for sensing platforms and photoelectric nanodevices. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Transition-metal oxides such as molybdenum oxide (MoOx ) have unique physical and chemical properties [1–3]. MoOx nanomaterials with various allotropes and suboxide phases, such as MoO3 , MoO2 , Mo2 O5 [2,4–6], 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) [8–10], MoOx may be preferred over conventional noble metals [11–13] for switching or modulation applications [14,15]. Both theoretical and experimental results demonstrated that MoOx nanomaterials were good photocatalysts [3,7,9,15–17]. 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
∗ Corresponding authors. E-mail addresses: [email protected], [email protected] (H. Cao), [email protected] (Y. Huang).
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 nonstoichiometric (MoO3−x ) or hydrogen-doped MoO3 (Hx MoO3 ) 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 [23,25–28]. 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 quan-
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tum size effect, good stability, etc [29]. Photoluminescent probes based on MoOx QDs have been used for the detection of 2,4,6trinitrotoluene [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 dualmodal 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. 2. Experimental 2.1. Synthesis of MoOx QDs Na2 MoO4 ·2H2 O (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 ·H2 O (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 72 h to remove residual unreacted components. 2.2. Spectral measurements In the pH titration via fluorescence (FL) spectra, a 0.1 mL of MoOx QDs stock solution and 4.9 mL of HAc-NaAc buffer solutions (0.1 M) at various pH values were added successively into a 5 mL colorimetric tube. FL spectra were acquired with 305 nm excitation. For pH titration via UV–vis-near-infrared-(NIR) absorption, 2 mL of MoOx QDs stock solution and 2 mL of HAc-NaAc buffer solutions (0.1 M) at various pHs were added successively into a 5 mL colorimetric tube. After 30 min, UV–vis-NIR absorption spectra were acquired. To investigate the reversibility of MoOx QDs, the pH solution was adjusted between 1.5 and 5.0 by using HCl or NaOH solutions. Interference from other ions or biomolecules (K+ , Na+ , Ca2+ , Mg2+ , Zn2+ , Ba2+ , Cd2+ , Pb2+ , Mn2+ , Co2+ , Cr3+ , Cu2+ , Ag+ , Fe2+ , Fe3+ , Al3+ ;
arginine (Arg), glycine (Gly), threonine (Thr), leucine (Leu), alanine (Ala), histidine (His), cysteine (Cys), glucose (Glc), tyrosine (Tyr), serine (Ser), methionine (Met), asparagine (Asn), glutamate (Glu)) with the FL intensity was examined at pH 2.5 and 5, respectively.
3. Results and discussion 3.1. 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 hightemperature hydrothermal process. In addition, the N enabled trapping of oxygen molecules in the process of forming MoOx QDs [34]. Dynamic light scattering (DLS) measurements indicated that the non-aggregated MoOx QDs were about 3.0 nm in diameter (Fig. S1A). Transmission electron microscopy (TEM) images (Fig. 1A) revealed monodispersity (2.8 ± 0.6 nm, n = 90, Fig. S1B). Lattice fringes in high-resolution TEM (HRTEM) images of MoOx QDs indicated a lattice distance of 0.26 nm (Fig. 1B), which was the (011) plane of MoO3 [16]. The zeta potential of the MoOx QDs was −32 mV (Fig. S1C), verifying colloidal stability in aqueous media. To investigate the structural evolution of the MoOx QDs, X-ray powder diffraction patterns (XRD) were acquired (Fig. S1D). Two main reflection peaks of MoO3 at 25.8◦ and 58.9◦ were observed and attributed to the (210) and (424) planes of MoO3 of standard JCPDS data card (no. 21-0569). The result indicated that the MoOx QDs had a crystal structure. Fourier-transform infrared (FTIR) spectra (Fig. S2A) exhibited oxygen functionalities at 1655 cm−1 and 1385 cm−1 from the deformation of C O in the carboxyl group and N H bending [35,36], a broad peak at 3391 cm−1 from O H vibrations [37] and N H stretching [36], respectively. A peak at 2978 cm−1 was assigned to C H stretching [38]. Peaks at 617 cm−1 and 879 cm−1 were attributed to MoO3 fingerprint characteristics [39]. A peak at 1047 cm−1 was attributed to the N in-plane bending vibration consistent with those of N-doped graphene dots (GQDs) [35]. These indicated the conjugated hydroxyl and amino-group on nano-material surface, which enabled tunable optical properties. Raman spectroscopy was used to identify vibrational and structural characteristics [40] of the MoOx QDs (Fig. 1C). Peaks at 941 cm−1 and 992 cm−1 were attributed to O Mo O and Mo6+ O stretching modes, while peaks at 454 cm−1 , 638 cm−1 , and 669 cm−1 corresponded to the Mo3 O stretching mode [23,41,42]. Peaks at 826 cm−1 and 885 cm−1 were assigned to the Mo O Mo stretching mode [41]. A band at 304 cm−1 was assigned to O Mo O wagging modes. The results demonstrated that the Raman characteristic of MoOx QDs is mainly consistent with that of MoO3 in previously reported results [23,40,43]. X-ray photoelectron spectroscopy (XPS) was used to verify the in-depth chemistry of the MoOx QDs. High-resolution XPS spectra in Fig. 1D show two peaks at 235.6 eV and 232.5 eV that were attributed to the binding energies of Mo6+ 3d3/2 and the Mo6+ 3d5/2 , respectively, which indicated that the Mo was in a high oxidation state [23]. Apart from Mo6+ , there is no lower oxidation states of Mo species, that is, Mo5+ (BE at 231.4 and 234.5 eV) and Mo4+ (bind energy (BE) at 229.7 and 232.7 eV) ions. As shown in Fig. S2B, the only O 1s peak was located at 531.2 eV, which was perfectly symmetrical and attributed to oxygen bonded to metal [17]. The N 1s level XPS spectra (Fig. S2C) exhibited four group peaks attributed to Mo-N, pyridinic N, pyrrolic N, and quaternary N at
Please cite this article as: H. Cao, X. Hu, W. Shi et al., pH-regulated reversible photoluminescence and localized surface plasmon resonances arising from molybdenum oxide quantum dot, Appl. Mater. Today, https://doi.org/10.1016/j.apmt.2019.100516
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Fig. 1. (A) HRTEM image of MoOx QDs. (B) The magnified HRTEM image of a MoOx QDs. (C) Raman spectrum of MoOx QDs. (D) The Mo 3d XPS spectral region of MoOx QDs.
397.5 eV, 399.2 eV, 400.1 eV, and 401.2 eV, respectively, which was in agreement with other N-doped nanomaterials [44,45].
3.2. Optical properties of MoOx QDs A absorption peak was around 302 nm, attributed to the excitonic features of the Mo O band in the MoO6 6− octahedron (Fig. 2A), which was consistent with previous report [46]. There were no absorption peaks in the visible or near-infrared regions. The MoOx QDs solution was faint yellow under sunlight and emitted blue-green FL upon irradiation by a 302 nm lamp (see Fig. 2A, inset). From the three-dimensional FL spectrum shown in Fig. 2B, the most intense peak was at 414 nm with 305 nm excitation. Luminescence properties of dilute MoOx QDs soultions at different excitation wavelengths were investigated (Fig. S2D). With 270–350 nm excitation, the FL spectra had a 405 nm emission maximum, and the maximum fluorescence was observed at 305 nm excitation. Because there was no change in the emission maximum with excitation wavelength, the luminescence derived from excited states and not scattering effects. The quantum yield of MoOx QDs was 10.6 %, which was calculated with quinine sulfate as standard. The MoOx QDs stability was investigated by FL measurements in a solution with different concentrations of NaCl or H2 O2 . From Fig.
S3A and B, there were no changes in FL intensity, demonstrating that MoOx QDs could tolerate high ionic strengths and oxidation in 3 M NaCl solution or 200 M H2 O2 . Hence, they could be used for sensing in high-ionic-strength environments. In Fig. S3C, the FL intensity of MoOx QDs was almost unchanged after 60 min of continuous irradiation at pH 5.0 and 2.0, respectively, indicating good photostability. A slight change in FL intensity could be found after storage for six months at 4 ◦ C (Fig. S3D). Thus, the MoOx QDs have anti-oxidation and long-term storage stabilities, and tolerance for high salt concentrations.
3.3. Simultaneous tuning of MoOx QD photoluminescence and LSPR The tuning mechanism involves pH regulation of the oxygen vacancies in the MoOx QDs surface, as depicted in Scheme 1. The oxygen atoms of the MoOx QDs bond to hydrogen ions embedded in the lattice to generate water molecules. Some Mo6+ ions were reduced to Mo5+ by electron transfer from N in the surface, which induced transformation of stoichiometric MoO3 to nonstoichiometric MoO3−x [15,47]. This was verified by XPS, as shown in Fig. S4 and Table S1 in Supporting Information. For each doublet, the 3d5/2 and 3d3/2 peaks had separation energies close to 3.1 eV,
Please cite this article as: H. Cao, X. Hu, W. Shi et al., pH-regulated reversible photoluminescence and localized surface plasmon resonances arising from molybdenum oxide quantum dot, Appl. Mater. Today, https://doi.org/10.1016/j.apmt.2019.100516
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Fig. 2. (A) UV absorption spectrum of MoOx QDs. The inset shows photographs of the MoOx QDs under visible light (left) and 365 nm light (right). (B) Contour map of the three-dimensional FL spectra of MoOx QDs. The arrow represents the position of the FL peak (excitation = 305 nm, emission = 414 nm).
Scheme 1. Schematic representation of the fabricated dual-mode optical probe of pH based on MoOx QDs.
which was characteristic of Mo compounds. The ratio of the peak intensities was about 1.5, which was characteristic of the 3d orbital [48]. As noted above, only Mo6+ ions (232.5 eV and 235.6 eV) [49] were observed for MoOx QDs at pH 10.0. When the pH was adjusted to 2.0, both Mo6+ and Mo5+ were present in the MoOx QDs at 53.8 % and 46.2 % proportions, respectively, from the XPS data. Thus, the average Mo oxidation state was 5.54, suggesting that the mixedvalence state may have resulted from oxygen vacancies. In acidic media, a MoOx QDs solution is blue; whereas, as noted above, it is light yellow in a neutral or alkaline medium (Fig. S5). Thus, the addition of H+ to a MoOx QDs solution could introduce lattice vacancies that contribute to a tunable carrier concentration, resulting in plasmonic absorption. TEM images of MoOx QDs at pH values of 10.0, 5.0, and 2.0 are shown in Fig. S6. The MoOx QDs dimensions increased with decreasing pH, in good agreement with the DLS data in Fig. S6D. The crystal lattice of Mo4 O11 with a (6¯ 01) plane distance of 0.36 nm [10] was revealed in the lattice fringes of MoOx QDs at pH 2.0 (Fig. S7), demonstrating the presence of partly Mo5+ derived from the reduced Mo6+ and presence of oxygen vacancies. Due to the quantum size effect, MoOx QDs photoluminescence would be regulated by pH, thus achieving tunable LSPR and photoluminescence within the same nanostructure. 3.4. Sensing studies Nanomaterials were synthesized with different volumes of concentrated ammonium hydroxide (0, 5, 7, 10 mL), corresponding to QDs-0, QDs-5, QDs-7, QDs-10, respectively, and their FL responses were investigated in the pH range 2.0–10.0. As shown in Fig. S8A, the FL intensities of nanomaterials made without ammonium
hydroxide had changed little. However, when the volume of ammonium hydroxide was equal to or greater than 5 mL, the FL intensity increased, with a maximum at pH 10.0, while it decreased approximately three-fold at pH 2.0. Furthermore, when the QDs were synthesized with NaOH instead of NH3 in the pH range 2.0–5.0, no pH-derived FL variance was observed (Fig. S8B). Thus, the addition of NH3 formed sufficient N-atoms in the MoOx QDs surface to generate free electrons and enhance the tunability [14]. Accordingly, the MoOx QDs were sensitive from pH 2.0 to 10.0, especially pH 1.0–5.0, and were suitable for studying pH variations in surrounding environments of microorganisms. Time-resolved emission measurements were performed to gain insight into the excited state relaxation of MoOx QDs at different pH values (Fig. S9 and Table S2). The average lifetimes were 6.09 ns for pH 10.0, 6.33 ns for pH 5.0, and 6.57 ns for pH 2.0, respectively. These minor differences in lifetimes as a function of pH indicated that there was no significant charge transfer according to the previous literatures [50,51]. The size effect in the TEM images (Fig. S6) might be the main reason for the tunable photoluminescence. The FL intensity of the MoOx QDs was linear over the pH range of 1.0–5.0 (Fig. 3), with a fit y = 365.40 + 162.02 x (r2 = 0.9982, n = 21). It is noted that the fluorescent peaks exhibited slightly red-shift with the decreasing pH value, probably due to the size effect. From TEM in Fig. S6, the size increased when the pH was decreasing. The fluorescent peak of nanomaterials can be tuned by the size, and the climbing size might lead to red-shift of wavelength [52]. The RSD was 0.37 % for pH 4.2 (n = 13). The MoOx QDs thus exhibited a high sensitivity to H+ . In addition, a comparison with other nanomaterials based on the fluorescence sensing system for extreme pH was made. As shown in Table S3, this work involved simpler available materials and displayed relatively broader response range.
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Fig. 3. (A) FL of MoOx QDs as a function of pH (1.0–7.0). (B) Plots of FL intensity at 414 nm vs. pH.
Fig. 4. (A) UV–vis-NIR absorption spectra of 280.6 mg L−1 MoOx QDs as a function of pH (1.5–4.0). (B) UV–vis-NIR absorbance of 280.6 mg L−1 MoOx QDs at 700 nm vs. pH.
Meanwhile, the LSPR absorption intensity of MoOx QDs increase with decreasing pH values, and the corresponding absorption peaks exhibits gradually red-shifted from 700 nm to 816 nm (Fig. 4A). This was probably because of the decreased free carrier concentration induced by Mo5+ and the size distribution [15,53]. In addition, because the LSPR peaks were closely related to the aspect ratio of the plasmon material [53], the LSPR energy shifted to lower energies when the size of the MoOx QDs increased. This was consistent with that of the longitudinal mode of metal and ReO3 nanoparticles as a function of aspect ratio [54]. The relationship between resonance and pH is shown in Fig. 4B. The RSD was 1.12 % for pH 3.0 (n = 11). Compared with other techniques for detecting extremely acid pH, MoOx QDs possessed unique dual signal readout and exhibited better reliability and sensitivity.
3.5. Reversibility and selectivity of MoOx QDs as probe for acidic pH Reversibility of the probe was studied in the range of pH 1.5–5.0 for FL and pH 1.5–3.5 for LSPR, respectively. The probe still worked well after five cycles, indicating that it was highly reversible for sensing pH (Fig. S10). To examine MoOx QDs anti-interference ability and selectivity in a complex environment, the following ions or biomolecules were separately added to a MoOx QDs solution at pH 2.5 and 5.0: 100 mM Na+ and K+ , 1 mM Ca2+ , Cd2+ , Mn2+ and Cr3+ , 0.5 mM Mg2+ , Zn2+ , Ba2+ , Pb2+ , and Co2+ , 0.1 mM Cu2+ and Al3+ , 0.01
mM Fe3+ , Fe2+ and Ag+ , 1 mM Arg, Gly, Thr, Leu, Ala, His, Cys, Glc, Tyr, Ser, Met, Asn, Glu. As shown in Fig. 5, very small changes in FL intensity were observed, indicating that the MoOx QDs were highly selective to pH against metal ions and biomolecules.
3.6. Cytotoxicity and extremely acidic pH detection in bacteria The cytotoxicity of MoOx QDs to E. coli cells was examined for 24 h at different QD concentrations. In Fig. 6A, the number of surviving colonies on agar plates changed little with increasing MoOx QDs concentration. The numbers of viable bacteria were estimated by calculating the exact number of colonies in each sample. The results in Fig. 6B indicated that almost 100 % of the E. coli cells were alive in the presence of 561.2 mg L−1 MoOx QDs. The main reason was that GSH was on the MoOx QDs surfaces; it could prevent cellular damage induced by ROS [32,33]. Thus, MoOx QDs exhibited very low cytotoxicity and could be used to detect pH in bacteria. The E. coli cells were then used to demonstrate the reliability of MoOx QDs for sensing pH values within the microorganisms. First, the E. coli cells were incubated at pH 5.0, 3.5, and 2.5. The E. coli cells still formed colonies on the agar plates (Fig. S11). The cells were then incubated for 2 h, 6 h, and 24 h, respectively, with MoOx QDs (280.6 mg L−1 ) at the different pHs. After centrifugation to remove extra probes outside the cells, FL and LSPR spectra of the periplasmic region of the bacteria were acquired. As shown in Figs. 6C
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Fig. 5. Fluorescence intensity at 414 nm of the probe at pH 2.5 and 5 in the presence of (A) metal ions: 100 mM Na+ and K+ , 1 mM Ca2+ , Cd2+ , Mn2+ and Cr3+ , 0.5 mM Mg2+ , Zn2+ , Ba2+ , Pb2+ , and Co2+ , 0.1 mM Cu2+ and Al3+ , 0.01 mM Fe3+ , Fe2+ and Ag+ ; and (B) biomolecules: 1 mM Arg, Gly, Thr, Leu, Ala, His, Cys, Glc, Tyr, Ser, Met, Asn and Glu.
Fig. 6. (A) Biocompatibility of the pH indicator for different concentrations of MoOx QDs. a: 0.00 mg L−1 ; b: 11.22 mg L−1 ; c: 22.45 mg L−1 ; d: 56.12 mg L−1 ; e: 112.24 mg L−1 ; f: 561.20 mg L−1 . (B) Quantitative evaluation of the effect of MoOx QDs concentration on the survival rate of E. coli by counting the colonies grown on agar plates. (C) FL spectra of the probe inside E. coli cells at pH 2.5, pH 3.5, and pH 5.0; also plotted is the background fluorescence of the bacterium. (D) Quantitative analysis of FL intensity in E. coli cells vs. pH for different incubation times.
and S12, significant enhancement in FL emission was observed with increasing pH, while significant enhancement of the LSPR was observed with decreasing pH. Thus, the dual-modal sensor probe could be used for sensing extremely acid pH in microorganisms. In addition, the quantitative analysis shown in Fig. 6D indicates that the FL intensity significantly decreasing with pH. The same results were observed after incubation for longer times (over 6 h), indicating that if the incubation time was too long, it may affect the detection of the acidity in the bacteria.
4. 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
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advantages, including simplicity, label-free detection, excellent reversibility, high selectivity, and accuracy. This work may have potential for MoOx QD multimodal sensors in environmental and biological applications. 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. 2017KYQD23), Young Scientist Research Fund of Yangtze Normal University (2017QNRC20). We thank Alan Burns, PhD, from the Liwen Bianji, Edanz Group China (www.liwenbianji. cn/ac), for editing the English text of a draft of this manuscript. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apmt.2019. 100516. References [1] Z. Chen, D. Cummins, B.N. Reinecke, E. Clark, M.K. Sunkara, T.F. Jaramillo, Core–shell MoO3 –MoS2 nanowires for hydrogen evolution: a functional design for electrocatalytic materials, Nano Lett. 11 (2011) 4168–4175. [2] I. Navas, R. Vinodkumar, V.P. Mahadevan Pillai, Self-assembly and photoluminescence of molybdenum oxide nanoparticles, Appl. Phys. A 103 (2011) 373. [3] C. Hefeng, K. Takashi, M. Kohsuke, Y. Hiromi, Surfactant-free nonaqueous synthesis of plasmonic molybdenum oxide nanosheets with enhanced catalytic activity for hydrogen generation from ammonia borane under visible light, Angew. Chem. Int. Ed. 53 (2014) 2910–2914. [4] X. Tan, L. Wang, C. Cheng, X. Yan, B. Shen, J. Zhang, Plasmonic MoO3-x @MoO3 nanosheets for highly sensitive SERS detection through nanoshell-isolated electromagnetic enhancement, Chem. Commun. 52 (2016) 2893–2896. [5] C. Wang, J. Jiang, Y. Ruan, X. Ao, K. Ostrikov, W. Zhang, J. Lu, Y.Y. Li, Construction of MoO2 quantum dot–graphene and MoS2 nanoparticle–graphene nanoarchitectures toward ultrahigh lithium storage capability, ACS Appl. Mater. Interfaces 9 (2017) 28441. [6] M. Maaza, B.D. Ngom, S. Khamlich, J.B.K. Kana, P. Sibuyi, D. Hamidi, S. Ekambaram, Valency control in MoO3−␦ nanoparticles generated by pulsed laser liquid solid interaction, J. Nanopart. Res. 14 (2012) 714. [7] H.F. Cheng, M.C. Wen, X.C. Ma, Y. Kuwahara, K. Mori, Y. Dai, B.B. Huang, H. Yamashita, Hydrogen doped metal oxide semiconductors with exceptional and tunable localized surface plasmon resonances, J. Am. Chem. Soc. 138 (2016) 9316–9324. [8] H. Qingquan, H. Shi, Z. Jing, W. Xun, MoO(3-x)-based hybrids with tunable localized surface plasmon resonances: chemical oxidation driving transformation from ultrathin nanosheets to nanotubes, Chemistry 18 (2012) 15283–15287. [9] H.F. Cheng, T. Kamegawa, K. Mori, H. Yamashita, Surfactant-free nonaqueous synthesis of plasmonic molybdenum oxide nanosheets with enhanced catalytic activity for hydrogen generation from ammonia borane under visible light, Angew. Chem. Int. Ed. 53 (2014) 2910–2914. [10] M.M. Alsaif, K. Latham, M.R. Field, D.D. Yao, N.V. Medhekar, G.A. Beane, R.B. Kaner, S.P. Russo, J.Z. Ou, K. Kalantar-Zadeh, Tunable plasmon resonances in two-dimensional molybdenum oxide nanoflakes, Adv. Mater. 26 (2014) 3931–3937. [11] K.L. Kelly, E. Coronado, L.Z. Lin, G.C. Schatz, The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment, Cheminform 34 (2003) 668–677. [12] H. Chen, L. Shao, Q. Li, J. Wang, Gold nanorods and their plasmonic properties, Chem. Soc. Rev. 42 (2013) 2679–2724.
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pH-regulated reversible photoluminescence and localized surface plasmon resonances arising from molybdenum oxide quantum dot Haiyan Cao a,∗ , Xue Hu b , Wenbing Shi a , Siqi Li b , Yuming Huang b,∗ a
The Key Laboratory of Chongqing Inorganic Special Functional Materials, College of Chemistry and Chemical Engineering, Yangtze Normal University, Chongqing, 408100, China The Key Laboratory of Luminescence and Real-Time Analytical Chemistry, Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, China b
a r t i c l e
i n f o
Article history: Received 23 August 2019 Received in revised form 20 November 2019 Accepted 22 November 2019 Keywords: Molybdenum oxide Localized surface plasmon resonances Photoluminescence Dual-modal sensor pH sensor
a b s t r a c t Localized surface plasmon resonances (LSPR) and photoluminescence are important for applications of transition-metal oxides. However, simultaneous control of these optical properties is challenging. Here, a facile strategy is presented that simultaneously tunes photoluminescence and visible LSPR of molybdenum oxide quantum dots (MoOx QDs) by controlling lattice vacancies. Specifically, N-doped MoOx QDs were prepared with a one-pot protocol. The introduction of N in MoOx QDs surfaces via ammonia (NH3 ) not only trapped oxygen molecules in the process of forming MoOx QDs, but also provided enough free electrons to enable tunable optical properties. Thus, a MoOx QD-based dual-modal fluorescence and LSPR assay was demonstrated via lattice vacancy concentration tuning. Upon introduction of H+ or OH− , pHreversible tunability of the fluorescence and plasmonic resonance was observed. The dual-mode probe was used to detect extreme acidity in bacterial cells. Overall, tunable LSPR and photoluminescence within one nanostructure via pH-regulation should enable multi-modal signal-outputs for sensing platforms and photoelectric nanodevices. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Transition-metal oxides such as molybdenum oxide (MoOx ) have unique physical and chemical properties [1–3]. MoOx nanomaterials with various allotropes and suboxide phases, such as MoO3 , MoO2 , Mo2 O5 [2,4–6], 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) [8–10], MoOx may be preferred over conventional noble metals [11–13] for switching or modulation applications [14,15]. Both theoretical and experimental results demonstrated that MoOx nanomaterials were good photocatalysts [3,7,9,15–17]. 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
∗ Corresponding authors. E-mail addresses: [email protected], [email protected] (H. Cao), [email protected] (Y. Huang).
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 nonstoichiometric (MoO3−x ) or hydrogen-doped MoO3 (Hx MoO3 ) 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 [23,25–28]. 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 quan-
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tum size effect, good stability, etc [29]. Photoluminescent probes based on MoOx QDs have been used for the detection of 2,4,6trinitrotoluene [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 dualmodal 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. 2. Experimental 2.1. Synthesis of MoOx QDs Na2 MoO4 ·2H2 O (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 ·H2 O (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 72 h to remove residual unreacted components. 2.2. Spectral measurements In the pH titration via fluorescence (FL) spectra, a 0.1 mL of MoOx QDs stock solution and 4.9 mL of HAc-NaAc buffer solutions (0.1 M) at various pH values were added successively into a 5 mL colorimetric tube. FL spectra were acquired with 305 nm excitation. For pH titration via UV–vis-near-infrared-(NIR) absorption, 2 mL of MoOx QDs stock solution and 2 mL of HAc-NaAc buffer solutions (0.1 M) at various pHs were added successively into a 5 mL colorimetric tube. After 30 min, UV–vis-NIR absorption spectra were acquired. To investigate the reversibility of MoOx QDs, the pH solution was adjusted between 1.5 and 5.0 by using HCl or NaOH solutions. Interference from other ions or biomolecules (K+ , Na+ , Ca2+ , Mg2+ , Zn2+ , Ba2+ , Cd2+ , Pb2+ , Mn2+ , Co2+ , Cr3+ , Cu2+ , Ag+ , Fe2+ , Fe3+ , Al3+ ;
arginine (Arg), glycine (Gly), threonine (Thr), leucine (Leu), alanine (Ala), histidine (His), cysteine (Cys), glucose (Glc), tyrosine (Tyr), serine (Ser), methionine (Met), asparagine (Asn), glutamate (Glu)) with the FL intensity was examined at pH 2.5 and 5, respectively.
3. Results and discussion 3.1. 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 hightemperature hydrothermal process. In addition, the N enabled trapping of oxygen molecules in the process of forming MoOx QDs [34]. Dynamic light scattering (DLS) measurements indicated that the non-aggregated MoOx QDs were about 3.0 nm in diameter (Fig. S1A). Transmission electron microscopy (TEM) images (Fig. 1A) revealed monodispersity (2.8 ± 0.6 nm, n = 90, Fig. S1B). Lattice fringes in high-resolution TEM (HRTEM) images of MoOx QDs indicated a lattice distance of 0.26 nm (Fig. 1B), which was the (011) plane of MoO3 [16]. The zeta potential of the MoOx QDs was −32 mV (Fig. S1C), verifying colloidal stability in aqueous media. To investigate the structural evolution of the MoOx QDs, X-ray powder diffraction patterns (XRD) were acquired (Fig. S1D). Two main reflection peaks of MoO3 at 25.8◦ and 58.9◦ were observed and attributed to the (210) and (424) planes of MoO3 of standard JCPDS data card (no. 21-0569). The result indicated that the MoOx QDs had a crystal structure. Fourier-transform infrared (FTIR) spectra (Fig. S2A) exhibited oxygen functionalities at 1655 cm−1 and 1385 cm−1 from the deformation of C O in the carboxyl group and N H bending [35,36], a broad peak at 3391 cm−1 from O H vibrations [37] and N H stretching [36], respectively. A peak at 2978 cm−1 was assigned to C H stretching [38]. Peaks at 617 cm−1 and 879 cm−1 were attributed to MoO3 fingerprint characteristics [39]. A peak at 1047 cm−1 was attributed to the N in-plane bending vibration consistent with those of N-doped graphene dots (GQDs) [35]. These indicated the conjugated hydroxyl and amino-group on nano-material surface, which enabled tunable optical properties. Raman spectroscopy was used to identify vibrational and structural characteristics [40] of the MoOx QDs (Fig. 1C). Peaks at 941 cm−1 and 992 cm−1 were attributed to O Mo O and Mo6+ O stretching modes, while peaks at 454 cm−1 , 638 cm−1 , and 669 cm−1 corresponded to the Mo3 O stretching mode [23,41,42]. Peaks at 826 cm−1 and 885 cm−1 were assigned to the Mo O Mo stretching mode [41]. A band at 304 cm−1 was assigned to O Mo O wagging modes. The results demonstrated that the Raman characteristic of MoOx QDs is mainly consistent with that of MoO3 in previously reported results [23,40,43]. X-ray photoelectron spectroscopy (XPS) was used to verify the in-depth chemistry of the MoOx QDs. High-resolution XPS spectra in Fig. 1D show two peaks at 235.6 eV and 232.5 eV that were attributed to the binding energies of Mo6+ 3d3/2 and the Mo6+ 3d5/2 , respectively, which indicated that the Mo was in a high oxidation state [23]. Apart from Mo6+ , there is no lower oxidation states of Mo species, that is, Mo5+ (BE at 231.4 and 234.5 eV) and Mo4+ (bind energy (BE) at 229.7 and 232.7 eV) ions. As shown in Fig. S2B, the only O 1s peak was located at 531.2 eV, which was perfectly symmetrical and attributed to oxygen bonded to metal [17]. The N 1s level XPS spectra (Fig. S2C) exhibited four group peaks attributed to Mo-N, pyridinic N, pyrrolic N, and quaternary N at
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Fig. 1. (A) HRTEM image of MoOx QDs. (B) The magnified HRTEM image of a MoOx QDs. (C) Raman spectrum of MoOx QDs. (D) The Mo 3d XPS spectral region of MoOx QDs.
397.5 eV, 399.2 eV, 400.1 eV, and 401.2 eV, respectively, which was in agreement with other N-doped nanomaterials [44,45].
3.2. Optical properties of MoOx QDs A absorption peak was around 302 nm, attributed to the excitonic features of the Mo O band in the MoO6 6− octahedron (Fig. 2A), which was consistent with previous report [46]. There were no absorption peaks in the visible or near-infrared regions. The MoOx QDs solution was faint yellow under sunlight and emitted blue-green FL upon irradiation by a 302 nm lamp (see Fig. 2A, inset). From the three-dimensional FL spectrum shown in Fig. 2B, the most intense peak was at 414 nm with 305 nm excitation. Luminescence properties of dilute MoOx QDs soultions at different excitation wavelengths were investigated (Fig. S2D). With 270–350 nm excitation, the FL spectra had a 405 nm emission maximum, and the maximum fluorescence was observed at 305 nm excitation. Because there was no change in the emission maximum with excitation wavelength, the luminescence derived from excited states and not scattering effects. The quantum yield of MoOx QDs was 10.6 %, which was calculated with quinine sulfate as standard. The MoOx QDs stability was investigated by FL measurements in a solution with different concentrations of NaCl or H2 O2 . From Fig.
S3A and B, there were no changes in FL intensity, demonstrating that MoOx QDs could tolerate high ionic strengths and oxidation in 3 M NaCl solution or 200 M H2 O2 . Hence, they could be used for sensing in high-ionic-strength environments. In Fig. S3C, the FL intensity of MoOx QDs was almost unchanged after 60 min of continuous irradiation at pH 5.0 and 2.0, respectively, indicating good photostability. A slight change in FL intensity could be found after storage for six months at 4 ◦ C (Fig. S3D). Thus, the MoOx QDs have anti-oxidation and long-term storage stabilities, and tolerance for high salt concentrations.
3.3. Simultaneous tuning of MoOx QD photoluminescence and LSPR The tuning mechanism involves pH regulation of the oxygen vacancies in the MoOx QDs surface, as depicted in Scheme 1. The oxygen atoms of the MoOx QDs bond to hydrogen ions embedded in the lattice to generate water molecules. Some Mo6+ ions were reduced to Mo5+ by electron transfer from N in the surface, which induced transformation of stoichiometric MoO3 to nonstoichiometric MoO3−x [15,47]. This was verified by XPS, as shown in Fig. S4 and Table S1 in Supporting Information. For each doublet, the 3d5/2 and 3d3/2 peaks had separation energies close to 3.1 eV,
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Fig. 2. (A) UV absorption spectrum of MoOx QDs. The inset shows photographs of the MoOx QDs under visible light (left) and 365 nm light (right). (B) Contour map of the three-dimensional FL spectra of MoOx QDs. The arrow represents the position of the FL peak (excitation = 305 nm, emission = 414 nm).
Scheme 1. Schematic representation of the fabricated dual-mode optical probe of pH based on MoOx QDs.
which was characteristic of Mo compounds. The ratio of the peak intensities was about 1.5, which was characteristic of the 3d orbital [48]. As noted above, only Mo6+ ions (232.5 eV and 235.6 eV) [49] were observed for MoOx QDs at pH 10.0. When the pH was adjusted to 2.0, both Mo6+ and Mo5+ were present in the MoOx QDs at 53.8 % and 46.2 % proportions, respectively, from the XPS data. Thus, the average Mo oxidation state was 5.54, suggesting that the mixedvalence state may have resulted from oxygen vacancies. In acidic media, a MoOx QDs solution is blue; whereas, as noted above, it is light yellow in a neutral or alkaline medium (Fig. S5). Thus, the addition of H+ to a MoOx QDs solution could introduce lattice vacancies that contribute to a tunable carrier concentration, resulting in plasmonic absorption. TEM images of MoOx QDs at pH values of 10.0, 5.0, and 2.0 are shown in Fig. S6. The MoOx QDs dimensions increased with decreasing pH, in good agreement with the DLS data in Fig. S6D. The crystal lattice of Mo4 O11 with a (6¯ 01) plane distance of 0.36 nm [10] was revealed in the lattice fringes of MoOx QDs at pH 2.0 (Fig. S7), demonstrating the presence of partly Mo5+ derived from the reduced Mo6+ and presence of oxygen vacancies. Due to the quantum size effect, MoOx QDs photoluminescence would be regulated by pH, thus achieving tunable LSPR and photoluminescence within the same nanostructure. 3.4. Sensing studies Nanomaterials were synthesized with different volumes of concentrated ammonium hydroxide (0, 5, 7, 10 mL), corresponding to QDs-0, QDs-5, QDs-7, QDs-10, respectively, and their FL responses were investigated in the pH range 2.0–10.0. As shown in Fig. S8A, the FL intensities of nanomaterials made without ammonium
hydroxide had changed little. However, when the volume of ammonium hydroxide was equal to or greater than 5 mL, the FL intensity increased, with a maximum at pH 10.0, while it decreased approximately three-fold at pH 2.0. Furthermore, when the QDs were synthesized with NaOH instead of NH3 in the pH range 2.0–5.0, no pH-derived FL variance was observed (Fig. S8B). Thus, the addition of NH3 formed sufficient N-atoms in the MoOx QDs surface to generate free electrons and enhance the tunability [14]. Accordingly, the MoOx QDs were sensitive from pH 2.0 to 10.0, especially pH 1.0–5.0, and were suitable for studying pH variations in surrounding environments of microorganisms. Time-resolved emission measurements were performed to gain insight into the excited state relaxation of MoOx QDs at different pH values (Fig. S9 and Table S2). The average lifetimes were 6.09 ns for pH 10.0, 6.33 ns for pH 5.0, and 6.57 ns for pH 2.0, respectively. These minor differences in lifetimes as a function of pH indicated that there was no significant charge transfer according to the previous literatures [50,51]. The size effect in the TEM images (Fig. S6) might be the main reason for the tunable photoluminescence. The FL intensity of the MoOx QDs was linear over the pH range of 1.0–5.0 (Fig. 3), with a fit y = 365.40 + 162.02 x (r2 = 0.9982, n = 21). It is noted that the fluorescent peaks exhibited slightly red-shift with the decreasing pH value, probably due to the size effect. From TEM in Fig. S6, the size increased when the pH was decreasing. The fluorescent peak of nanomaterials can be tuned by the size, and the climbing size might lead to red-shift of wavelength [52]. The RSD was 0.37 % for pH 4.2 (n = 13). The MoOx QDs thus exhibited a high sensitivity to H+ . In addition, a comparison with other nanomaterials based on the fluorescence sensing system for extreme pH was made. As shown in Table S3, this work involved simpler available materials and displayed relatively broader response range.
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Fig. 3. (A) FL of MoOx QDs as a function of pH (1.0–7.0). (B) Plots of FL intensity at 414 nm vs. pH.
Fig. 4. (A) UV–vis-NIR absorption spectra of 280.6 mg L−1 MoOx QDs as a function of pH (1.5–4.0). (B) UV–vis-NIR absorbance of 280.6 mg L−1 MoOx QDs at 700 nm vs. pH.
Meanwhile, the LSPR absorption intensity of MoOx QDs increase with decreasing pH values, and the corresponding absorption peaks exhibits gradually red-shifted from 700 nm to 816 nm (Fig. 4A). This was probably because of the decreased free carrier concentration induced by Mo5+ and the size distribution [15,53]. In addition, because the LSPR peaks were closely related to the aspect ratio of the plasmon material [53], the LSPR energy shifted to lower energies when the size of the MoOx QDs increased. This was consistent with that of the longitudinal mode of metal and ReO3 nanoparticles as a function of aspect ratio [54]. The relationship between resonance and pH is shown in Fig. 4B. The RSD was 1.12 % for pH 3.0 (n = 11). Compared with other techniques for detecting extremely acid pH, MoOx QDs possessed unique dual signal readout and exhibited better reliability and sensitivity.
3.5. Reversibility and selectivity of MoOx QDs as probe for acidic pH Reversibility of the probe was studied in the range of pH 1.5–5.0 for FL and pH 1.5–3.5 for LSPR, respectively. The probe still worked well after five cycles, indicating that it was highly reversible for sensing pH (Fig. S10). To examine MoOx QDs anti-interference ability and selectivity in a complex environment, the following ions or biomolecules were separately added to a MoOx QDs solution at pH 2.5 and 5.0: 100 mM Na+ and K+ , 1 mM Ca2+ , Cd2+ , Mn2+ and Cr3+ , 0.5 mM Mg2+ , Zn2+ , Ba2+ , Pb2+ , and Co2+ , 0.1 mM Cu2+ and Al3+ , 0.01
mM Fe3+ , Fe2+ and Ag+ , 1 mM Arg, Gly, Thr, Leu, Ala, His, Cys, Glc, Tyr, Ser, Met, Asn, Glu. As shown in Fig. 5, very small changes in FL intensity were observed, indicating that the MoOx QDs were highly selective to pH against metal ions and biomolecules.
3.6. Cytotoxicity and extremely acidic pH detection in bacteria The cytotoxicity of MoOx QDs to E. coli cells was examined for 24 h at different QD concentrations. In Fig. 6A, the number of surviving colonies on agar plates changed little with increasing MoOx QDs concentration. The numbers of viable bacteria were estimated by calculating the exact number of colonies in each sample. The results in Fig. 6B indicated that almost 100 % of the E. coli cells were alive in the presence of 561.2 mg L−1 MoOx QDs. The main reason was that GSH was on the MoOx QDs surfaces; it could prevent cellular damage induced by ROS [32,33]. Thus, MoOx QDs exhibited very low cytotoxicity and could be used to detect pH in bacteria. The E. coli cells were then used to demonstrate the reliability of MoOx QDs for sensing pH values within the microorganisms. First, the E. coli cells were incubated at pH 5.0, 3.5, and 2.5. The E. coli cells still formed colonies on the agar plates (Fig. S11). The cells were then incubated for 2 h, 6 h, and 24 h, respectively, with MoOx QDs (280.6 mg L−1 ) at the different pHs. After centrifugation to remove extra probes outside the cells, FL and LSPR spectra of the periplasmic region of the bacteria were acquired. As shown in Figs. 6C
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Fig. 5. Fluorescence intensity at 414 nm of the probe at pH 2.5 and 5 in the presence of (A) metal ions: 100 mM Na+ and K+ , 1 mM Ca2+ , Cd2+ , Mn2+ and Cr3+ , 0.5 mM Mg2+ , Zn2+ , Ba2+ , Pb2+ , and Co2+ , 0.1 mM Cu2+ and Al3+ , 0.01 mM Fe3+ , Fe2+ and Ag+ ; and (B) biomolecules: 1 mM Arg, Gly, Thr, Leu, Ala, His, Cys, Glc, Tyr, Ser, Met, Asn and Glu.
Fig. 6. (A) Biocompatibility of the pH indicator for different concentrations of MoOx QDs. a: 0.00 mg L−1 ; b: 11.22 mg L−1 ; c: 22.45 mg L−1 ; d: 56.12 mg L−1 ; e: 112.24 mg L−1 ; f: 561.20 mg L−1 . (B) Quantitative evaluation of the effect of MoOx QDs concentration on the survival rate of E. coli by counting the colonies grown on agar plates. (C) FL spectra of the probe inside E. coli cells at pH 2.5, pH 3.5, and pH 5.0; also plotted is the background fluorescence of the bacterium. (D) Quantitative analysis of FL intensity in E. coli cells vs. pH for different incubation times.
and S12, significant enhancement in FL emission was observed with increasing pH, while significant enhancement of the LSPR was observed with decreasing pH. Thus, the dual-modal sensor probe could be used for sensing extremely acid pH in microorganisms. In addition, the quantitative analysis shown in Fig. 6D indicates that the FL intensity significantly decreasing with pH. The same results were observed after incubation for longer times (over 6 h), indicating that if the incubation time was too long, it may affect the detection of the acidity in the bacteria.
4. 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
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advantages, including simplicity, label-free detection, excellent reversibility, high selectivity, and accuracy. This work may have potential for MoOx QD multimodal sensors in environmental and biological applications. 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. 2017KYQD23), Young Scientist Research Fund of Yangtze Normal University (2017QNRC20). We thank Alan Burns, PhD, from the Liwen Bianji, Edanz Group China (www.liwenbianji. cn/ac), for editing the English text of a draft of this manuscript. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apmt.2019. 100516. References [1] Z. Chen, D. Cummins, B.N. Reinecke, E. Clark, M.K. Sunkara, T.F. Jaramillo, Core–shell MoO3 –MoS2 nanowires for hydrogen evolution: a functional design for electrocatalytic materials, Nano Lett. 11 (2011) 4168–4175. [2] I. Navas, R. Vinodkumar, V.P. Mahadevan Pillai, Self-assembly and photoluminescence of molybdenum oxide nanoparticles, Appl. Phys. A 103 (2011) 373. [3] C. Hefeng, K. Takashi, M. Kohsuke, Y. Hiromi, Surfactant-free nonaqueous synthesis of plasmonic molybdenum oxide nanosheets with enhanced catalytic activity for hydrogen generation from ammonia borane under visible light, Angew. Chem. Int. Ed. 53 (2014) 2910–2914. [4] X. Tan, L. Wang, C. Cheng, X. Yan, B. Shen, J. Zhang, Plasmonic MoO3-x @MoO3 nanosheets for highly sensitive SERS detection through nanoshell-isolated electromagnetic enhancement, Chem. Commun. 52 (2016) 2893–2896. [5] C. Wang, J. Jiang, Y. Ruan, X. Ao, K. Ostrikov, W. Zhang, J. Lu, Y.Y. Li, Construction of MoO2 quantum dot–graphene and MoS2 nanoparticle–graphene nanoarchitectures toward ultrahigh lithium storage capability, ACS Appl. Mater. Interfaces 9 (2017) 28441. [6] M. Maaza, B.D. Ngom, S. Khamlich, J.B.K. Kana, P. Sibuyi, D. Hamidi, S. Ekambaram, Valency control in MoO3−␦ nanoparticles generated by pulsed laser liquid solid interaction, J. Nanopart. Res. 14 (2012) 714. [7] H.F. Cheng, M.C. Wen, X.C. Ma, Y. Kuwahara, K. Mori, Y. Dai, B.B. Huang, H. Yamashita, Hydrogen doped metal oxide semiconductors with exceptional and tunable localized surface plasmon resonances, J. Am. Chem. Soc. 138 (2016) 9316–9324. [8] H. Qingquan, H. Shi, Z. Jing, W. Xun, MoO(3-x)-based hybrids with tunable localized surface plasmon resonances: chemical oxidation driving transformation from ultrathin nanosheets to nanotubes, Chemistry 18 (2012) 15283–15287. [9] H.F. Cheng, T. Kamegawa, K. Mori, H. Yamashita, Surfactant-free nonaqueous synthesis of plasmonic molybdenum oxide nanosheets with enhanced catalytic activity for hydrogen generation from ammonia borane under visible light, Angew. Chem. Int. Ed. 53 (2014) 2910–2914. [10] M.M. Alsaif, K. Latham, M.R. Field, D.D. Yao, N.V. Medhekar, G.A. Beane, R.B. Kaner, S.P. Russo, J.Z. Ou, K. Kalantar-Zadeh, Tunable plasmon resonances in two-dimensional molybdenum oxide nanoflakes, Adv. Mater. 26 (2014) 3931–3937. [11] K.L. Kelly, E. Coronado, L.Z. Lin, G.C. Schatz, The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment, Cheminform 34 (2003) 668–677. [12] H. Chen, L. Shao, Q. Li, J. Wang, Gold nanorods and their plasmonic properties, Chem. Soc. Rev. 42 (2013) 2679–2724.
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