Highly Efficient Near-Infrared II Electrochemiluminescence from NaYbF 4 Core Mesoporous Silica Shell Nanoparticles

Open AccessCCS ChemistryRESEARCH ARTICLE5 Sep 2022Highly Efficient Near-Infrared II Electrochemiluminescence from NaYbF4 Core Mesoporous Silica Shell Nanoparticles Si-Yuan Ji, Jian-Bin Pan, Hao-Zhi Wang, Wei Zhao, Hong-Yuan Chen and Jing-Juan Xu Si-Yuan Ji State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023 , Jian-Bin Pan State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023 , Hao-Zhi Wang Department Key Laboratory of Advanced Ceramics and Machining Technology (Ministry of Education), School of Materials Science and Engineering, Tianjin University, Tianjin 300072 , Wei Zhao State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023 , Hong-Yuan Chen State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023 and Jing-Juan Xu *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023 https://doi.org/10.31635/ccschem.021.202101473 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Due to less interference in biological imaging, nanomaterials with second near-infrared (NIR-II) window (950–1700 nm) emission have received tremendous attention. However, no reports on NIR-II electrochemiluminescence (ECL) imaging exist because of the lack of high-efficiency NIR-II ECL luminophores. Herein, we designed and synthesized a NaYbF4@SiO2 core–shell nanoparticle for the first time, which exhibited a record-high ECL efficiency of 384% relative to the benchmark Ru(bpy)32+. The NaYbF4 core ensured a maximum emission peak at 983 nm; the synergistic effect of enrichment and catalysis provided by an external mesoporous shell significantly enhanced the ECL signal of particles and improved the stability. Moreover, the ECL generation mechanism was identified through density functional theory calculations. This work has broken through the wavelength bottleneck of NIR-ECL probes in the water-phase and can guide the design of subsequent NIR-II ECL emitters. Download figure Download PowerPoint Introduction The second near-infrared (NIR-II) window fluorescence emission between 950–1700 nm, with significant advantages of deeper penetration, higher spatiotemporal resolution, and better signal-to-background ratio, has received considerable attention in bioimaging and disease diagnosis.1–4 Meanwhile, this spectral domain is also very attractive for emerging electrochemiluminescence (ECL) biological applications, such as cell plasma membrane imaging and electric-driven antibacterials,5–7 because it can minimize the impact of photon scattering and autofluorescence. However, compared with the rapid development of NIR-II fluorescent materials in recent years,8,9 NIR-II ECL luminophores are still difficult to obtain because in addition to requiring energy level matching, the accessibility of their electrochemical reactions must also be considered. Therefore, there is an urgent need to develop highly efficient NIR-II ECL emitters. Currently, Ding and co-workers10,11 observed NIR-ECL (peaks at 900–950 nm) of gold nanoclusters (Au-NCs, Au25, and Au38) in organic solution, and elucidated the emission mechanism. Afterward, Wang and co-workers12 proposed a lipoic acid stabilized Au22-NC to achieve water-phase NIR-ECL emission. Thereby, Chen et al.13 and Zou et al.14 enhanced the ECL efficiency of water-dispersible Au-NCs through pre-oxidation and host–guest recognition methods; however, ligand-replacement caused the ECL wavelength of treated Au-NCs to blue shift (peaks at 800 and 532 nm) relative to the previous one. Most reported NIR luminophores, such as quantum dots,15 dyes, and organic molecules,16,17 have their ECL wavelengths concentrated in the NIR-I region (700–900 nm). Furthermore, the high biotoxicity of quantum dots and extremely poor water-solubility of dyes severely limit their applications in bioanalysis. In response to these deficiencies, it is critical to develop efficient NIR-II ECL luminophores with good biocompatibility and water dispersibility. NaYbF4 has been extensively researched as a host material of upconversion nanoparticles due to its sharp emission bandwidth, low toxicity, and superior photostability.18–20 Generally, Yb3+ ions only act as a sensitizer that absorbs NIR excitation light. Li's team21 recently proposed a creative undoped NaYbF4@CaF2 nanoparticle as an optical transducer, in which Yb3+ ions were used as absorption and emission centers, that could produce efficient emission at the NIR-II window. Currently, no research has been reported on the application and mechanism of NaYbF4 in electrogenerated luminescence. Herein, we first synthesized NaYbF4@SiO2 core–shell nanoparticles via the Stöber sol–gel method, which exhibited highly efficient NIR-II ECL in aqueous medium. Our design strategy involved choosing NaYbF4 as the core to ensure emission at the NIR-II window (983 nm) and wrapping a mesoporous shell outside the core to improve ECL intensity because the inert shell significantly reduced the surface quenching and the pore confinement effect enhanced the ECL signal. Moreover, the single excited state of Yb3+ ions avoided intersystem cross relaxation.22 Remarkably, the ECL efficiency of NaYbF4@SiO2 nanoparticles is 384% that of the Ru(bpy)32+-K2S2O8 system, which is highest among the reported water-phase NIR-ECL materials. Density functional theory (DFT) calculation demonstrated that the NaYbF4 anion radical exhibited better stability, and the ECL generation mechanism was revealed in detail. This work opens up a new avenue for the design of NIR-II ECL material, and has broad application prospects in ECL biosensing and imaging.23,24 Experimental Methods Reagents and apparatus All the chemicals were obtained from commercial suppliers and used as received without further purification. Phosphate-buffered saline (PBS, 0.1 M, pH 7.4) was prepared by mixing stock solutions of NaH2PO4 and Na2HPO4 that contained 0.1 M NaCl. Ultrapure fresh water was obtained from the Millipore water purification system (resistivity of 18.2 MΩ·cm−1 at 25 °C) and used for preparation of all aqueous solutions. The further details of other reagents and apparatus are available in the Supporting Information. Synthesis of NaYbF4 nanoparticles The NaYbF4 nanoparticles were synthesized according to the previously reported method with some modifications.21 Typically, 1 mmol YbCl3·6H2O was mixed with 6 mL oleic acid (OA) and 15 mL octadecene (ODE) in a 100 mL flask, and then the resulting mixture was heated to 160 °C for 60 min with magnetic stirring to form an optically transparent solution. Subsequently, the solution was cooled to room temperature under argon protection. During this time, 2.5 mmol NaOH and 4 mmol NH4F dissolved in 10 mL methanol were added dropwise to the flask and stirred until a transparent solution was formed. The temperature was then increased to 90 °C for degassing. Then the solution was heated to 300 °C and maintained for 45 min and then cooled naturally to room temperature. The nanoparticles were collected by centrifugation at 10,000 rpm for 8 min, and washed with a cyclohexane/ethanol mixture (v/v = 1∶3) three times. The prepared nanoparticles were finally re-dispersed in cyclohexane and stored at 4 °C. Synthesis of poly(acrylic acid)-modified NaYbF4 nanoparticles Poly(acrylic acid) (PAA)-modified NaYbF4 nanoparticles were prepared by the ligand exchange method at the liquid–liquid interface.25 OA-NaYbF4 nanoparticles (10 mg) were dispersed in dimethylformamide (DMF), and 50 mg PAA (MW = 3000) was injected into the solution under magnetic stirring. The resulting mixture was then thoroughly stirred overnight at 37 °C. Finally, nanoparticles were collected by centrifugation at 10,000 rpm for 10 min and rinsed with ultrapure water several times. The obtained nanoparticles were re-dispersed in Millipore water and stored at 4 °C. Synthesis of NaYbF4@SiO2 core–shell nanoparticles The NaYbF4@SiO2 core–shell nanoparticles were prepared following the Stöber sol–gel process.26 In short, 10 mg OA-NaYbF4 dissolved in cyclohexane was mixed with 120 mg cetyltrimethylammonium bromide (CTAB) and 20 mL Millipore water in a 100 mL flask. Cyclohexane was removed by rotary evaporation, resulting in a clear solution. Then the resulting solution was quickly added to a mixture of 5 mL ethanol, 40 mL Millipore water, and 300 μL NaOH solution (2 M). Subsequently, the mixture was heated to 80 °C for 15 min under magnetic stirring. During this process, 150 μL tetraethylorthosilicate (TEOS) was injected dropwise. The obtained nanoparticles were centrifuged and washed with isopropanol three times, and then transferred into 50 mL ethanol solution containing 300 mg NH4NO3, and maintained at 60 °C for 2 h to remove CTAB. Finally, mesoporous silica-coated NaYbF4 nanoparticles (NaYbF4@SiO2) were dispersed in Millipore water for further use. The calculation of particle concentration of NaYbF4 and NaYbF4@SiO2, as well as the electrochemical and ECL test process, are described in the Supporting Information. Computational method All calculations in this work were performed with the Vienna Ab Initio Software Package (VASP 5.3.5).27 The projected augmented wave (PAW) method was used to describe the electron–ion interaction.28,29 And the exchange-correlation term was described by Perdew–Burke–Ernzerhof (PBE) functional within the generalized gradient approximation to evaluate the electronic properties of NaYbF4.30 The cutoff energy for the plane-wave expansion in all the computations was set to 400 eV. The Brillouin zone of the surface unit cell was sampled by Monkhorst–Pack (MP) grids, with the k-point mesh for NaYbF4 bulk optimizations.31 And NaYbF4 bulk was determined by a 6 × 6 × 4 MP grid. The convergence criterion for the electronic self-consistent iteration and force were set to 10−7 eV and 0.01 eV/Å, respectively. Results and Discussion Morphology characterization of the NaYbF4 and NaYbF4@SiO2 core–shell nanoparticles Figure 1a shows the synthetic route for preparing mesoporous silica-coated NaYbF4 nanoparticles, and the modification procedure of PAA on hydrophobic NaYbF4 to achieve water-phase conversion is described in the Experimental Methods. The corresponding results were verified by Fourier transform infrared (FT-IR) spectroscopy ( Supporting Information Figure S1a). Here, PAA-NaYbF4 was used as a control to illustrate the importance of the mesoporous shell. The morphology and structure of the as-prepared nanoparticles were characterized by transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM (HAADF-STEM). As shown in Figure 1b and Supporting Information Figure S2, TEM images of NaYbF4 exhibited monodispersed nanoparticles of uniform spherical shape, and the particle size was not affected by ligand-exchange. After growing a SiO2 shell, according to the TEM image and size distribution histogram (Figures 1d and 1g), the particle size evolution could be clearly observed, that is, from a NaYbF4 core with an average diameter of 25.9 nm to a NaYbF4@SiO2 core–shell structure of 42.0 nm diameter. High-resolution TEM (HRTEM) images (Figure 1c and Supporting Information Figure S3a) revealed the highly crystalline nature, and lattice fringes spacing of 0.51 nm matched well with the (100) crystal plane of β-NaYbF4. Furthermore, the core–shell structure of NaYbF4@SiO2 was confirmed by HAADF-STEM (Figure 1e). The magnified view of an individual particle (insets in Figures 1d and 1e) displayed a well-defined mesoporous shell with average mesopore size of 2.5 nm, as further proved by a pore size distribution curve ( Supporting Information Figure S4b). The results of elemental mapping demonstrated that NaYbF4 was perfectly encapsulated in a mesoporous silica shell (Figure 1f); all expected elements were detected and matched well with the relative positions in the nanocomposite ( Supporting Information Figure S3). Figure 1 | (a) Schematic of synthetic process for NaYbF4@SiO2 core–shell nanoparticles. TEM images of (b) NaYbF4 and (d) NaYbF4@SiO2. (c) HRTEM image of a single NaYbF4 particle. (e) HAADF-STEM image of NaYbF4@SiO2; both core (bright) and shell (dark) were clearly visible. Inset: magnified view of individual particle showing the pore size. (f) HAADF-STEM-EDS element mapping images of Yb, F, Si, and O for NaYbF4@SiO2 (scale bars, 20 nm). (g) Size distributions diagram of NaYbF4 and NaYbF4@SiO2. EDS, energy-dispersive system. Download figure Download PowerPoint The crystal and geometric structural information of NaYbF4 was further uncovered. The X-ray diffraction (XRD) patterns of two nanoparticles ( Supporting Information Figure S5) have identical positions to hexagonal phase of β-NaYbF4 (JCPDS 061-0779). The peak at 2θ = 22° belonged to amorphous silica, suggesting the successful coating of silica. It was also proved via a distinct change of ζ-potential ( Supporting Information Figure S1b). Moreover, a N2 adsorption–desorption isothermal measurement was carried out to determine the surface area and mesoporous structure of NaYbF4@SiO2 ( Supporting Information Figure S4). The sample exhibited a type-IV curve, indicating the presence of mesopore, and the Brunauer–Emmett–Teller (BET) surface area was estimated to be 421.5 m2·g−1. Afterward, we performed the DFT calculation on the basis of a unit cell, and the deformation charge density structure of NaYbF4 is presented in Supporting Information Figure S6 and Table S1. Spectral characterization of the NaYbF4 and NaYbF4@SiO2 core–shell nanoparticles The optical properties of particles were systematically characterized, and the corresponding results are shown in Figure 2 and Supporting Information Figure S7. From the illustration of Figure 2a, the luminescence peaks of NaYbF4 and NaYbF4@SiO2 were at 980 and 983 nm, respectively, which were attributed to an efficient energy migration between Yb3+ ions. Notably, the luminescence intensity of NaYbF4@SiO2 was approximately 4.7 times higher than that of NaYbF4 under the same condition, which originated from the protection of the core nanocrystals by the inert shell. More precisely, the shell suppressed surface quenching and minimized nonradiative energy loss.32 For the NaYbF4 nanoparticle, since there was no external mesoporous shell protection, the excitation energy reached the surface quenching sites through energy migration, resulting in the loss of excitation energy, which would seriously decrease the lifetime and luminescence intensity. This was further supported by decay curves (Figure 2b) that showed a much-prolonged lifetime of NaYbF4@SiO2 (929.15 μs) in contrast to NaYbF4 (20.92 μs). And X-ray photoelectron spectroscopic (XPS) analysis results displayed that the prepared core–shell nanoparticles could be stored stably for a period of time ( Supporting Information Figure S8). On this foundation, the ECL behavior of particles immobilized on a glassy carbon electrode (GCE) was investigated, and several experimental conditions were optimized to achieve optimal ECL performance, including particles modification volume, K2S2O8 concentration, and solution pH ( Supporting Information Figure S9). A low ECL intensity of NaYbF4/GCE (1014 a.u., Figure 2c) was observed in PBS solution containing 100 mM K2S2O8. However, after encasing in mesoporous shell, the ECL intensity of NaYbF4@SiO2 (13788 a.u.) increased dramatically to 13.6 times that of NaYbF4. Herein, the highly efficient ECL emission from NaYbF4@SiO2 was ascribed to two factors: First, NaYbF4@SiO2 with large surface area and high porosity enriched a large amount of co-reactant K2S2O8 from the solution, and the relatively confined pore space served as a microreactor to accelerate electrochemical reduction of K2S2O8, thereby improving the ECL reaction efficiency.33,34 Second, the channel structure protected the metastable reaction intermediates from environmental quenching, thus synergistically enhancing the ECL signal. The contact angle photographs also indicated that NaYbF4@SiO2 exhibited the best wettability ( Supporting Information Figure S10), which was more conducive to the transportation of co-reactant molecules. To display the ECL performance of NaYbF4@SiO2 nanoparticles more intuitively, the most commonly used benchmark Ru(bpy)32+ was selected as a reference. The relative ECL efficiency (ΦECL) of NaYbF4@SiO2 was calculated to be 384% that of the cathodic Ru(bpy)32+-K2S2O8 system under the same experimental condition, which is a record-high ΦECL among the reported NIR-ECL luminophores (see Supporting Information Figure S11 and Tables S2 and S3 for more details). In addition, the stability of nanoparticles in aqueous solution was evaluated by continuous potential scanning (Figure 2d). The ECL intensity of the NaYbF4@SiO2-K2S2O8 system remained constant with a smaller relative standard deviation (RSD) of only 1.42%. The corresponding ECL spectra of particles were recorded under the optimal experimental conditions (Figure 2e). The ECL peaks centered at 981 nm (NaYbF4) and 983 nm (NaYbF4@SiO2), respectively, agreed with their luminescence peaks, indicating that the excited states of particles were caused by the bandgap transition.35 The observed λECL was at least 183 nm red-shifted compared with other water-phase NIR-ECL materials reported so far ( Supporting Information Table S3).36 Combined with the intensity curves in Figure 2e, we determined that the mesoporous shell only significantly enhanced the ECL signal without changing the emission wavelength. Figure 2 | (a) Luminescence spectra of NaYbF4 and NaYbF4@SiO2 nanoparticles. Inset: Normalized luminescence spectra of the above two particles. (Excited at 915 nm and collected by a 950 nm long-pass filter.) (b) Normalized decay curves of NaYbF4 and NaYbF4@SiO2, respectively. (c) ECL intensity-time curves of NaYbF4/GCE and NaYbF4@SiO2/GCE in 0.10 M PBS (pH 7.4) with 100 mM K2S2O8. Inset: Schematic of corresponding material/GCE. (d) ECL signals of NaYbF4/GCE and NaYbF4@SiO2/GCE under continuous potential scan between 0 and −2.5 V. (e) ECL spectra of NaYbF4@SiO2 (a, red), NaYbF4 (b, green) modified GCE and bare GCE (c, black) in PBS solution with 100 mM K2S2O8. Download figure Download PowerPoint Electrochemical and ECL performances of the NaYbF4 and NaYbF4@SiO2 core–shell nanoparticles To further clarify the ECL generation mechanism of the NaYbF4@SiO2-K2S2O8 system, correlative electrochemical and ECL tests were performed. Figure 3a illustrates a differential pulse voltammogram (DPV) of NaYbF4 in acetonitrile solution containing 0.1 M TBAPF6 as the supporting electrolyte. A well-defined quasi-reversible redox wave with a formal potential of −2.00 V versus saturated calomel electrode (SCE) was observed, corresponding to electrochemical reactions of the NaYbF4−/NaYbF40 redox couple. At positive potential, no conspicuous oxidation and reduction peaks were displayed, indicating that NaYbF4 could not be electrochemically oxidized to positively charged NaYbF4•+ by removing electrons from the highest occupied molecular orbital (HOMO). Furthermore, the relative stability of NaYbF4 with different electron numbers was determined through DFT calculation (Figure 3d). Compared with the original NaYbF4, the electron-rich system with one electron added had lower energy, indicating that the corresponding anionic radical (NaYbF4•−) exhibited better stability. Meanwhile, we noticed that the system energy of other electron numbers (such as NaYbF4•+ or NaYbF42−) rose sharply, which meant that they were extremely unstable and difficult to generate through redox reactions. Based on the above analysis, it can be concluded that NaYbF4 gained an electron to form NaYbF4•− radical. Figure 3 | (a) DPV of NaYbF4 in acetonitrile solution containing 0.1 M TBAPF6 with scan rate of 0.1 V s−1. Arrows showed potential scan directions. (b) CV and (c) corresponding ECL intensity-potential profiles of NaYbF4/GCE (i and iii), bare GCE (ii), NaYbF4@SiO2/GCE (iv) in 0.10 M PBS without (i) and with (ii, and 100 mM K2S2O8. and of CV of NaYbF4@SiO2/GCE and ECL curve of respectively. (d) relative energy and (e) density of states of NaYbF4 with different electron level is set at 0 eV. (f) Schematic of the proposed ECL mechanism of NaYbF4@SiO2-K2S2O8 system. Download figure Download PowerPoint Figures and and corresponding ECL intensity-potential curves of different in the presence of 100 mM K2S2O8 as The reduction peak of bare GCE at V was attributed to K2S2O8 reduction to generate the Moreover, NaYbF4/GCE in PBS solution displayed a reduction peak at indicating that NaYbF4 could be electrochemically reduced to NaYbF4•− by electrons into the molecular orbital This is with the results of DPV and DFT and an identical reduction peak was observed in the CV curve of NaYbF4@SiO2/GCE in Figure Electrochemical spectroscopy Supporting Information Figure exhibited that the of NaYbF4@SiO2/GCE was than that of bare GCE and significantly lower than that of NaYbF4/GCE the of the reduction of on NaYbF4@SiO2/GCE be far smaller than the of bare However, the was than the of bare and the reduction peak of K2S2O8 positively to V (Figure It was reported that the in the mesoporous channel the reaction energy of the reduction thereby the which was further confirmed by the results of electron spectra in Supporting Information Figure the density of states calculated by DFT revealed that NaYbF4 maintained the electron or loss process (Figure to space charge electrons were stored within the and the amount of charge on the surface electron density of states and the that the between electrons and the co-reactant anion be to ECL the electron-rich system with one electron added (NaYbF4•−) perfectly minimized the Meanwhile, cathodic ECL of both NaYbF4 and NaYbF4@SiO2 was observed with an and peak potential and respectively, which matched well with the reduction potential of NaYbF4 to NaYbF4•− of Figure More the ECL signal by NaYbF4@SiO2 relatively after the peak compared to NaYbF4 and a peak This proved that the mesoporous shell could the radical ions from quenching to the Based on the above an ECL mechanism of the NaYbF4@SiO2-K2S2O8 system was proposed in and described in Figure the potential to a anion and were via electrochemical of K2S2O8 and NaYbF4@SiO2, 1 and Subsequently, an electron reaction between and in the of the electrode to generate the excited state which would to the state through ECL emission 2 O 8 2 4 2 4 4 2 4 2 4 2 4 4 2 4 2 4 2 4 2 h for the first time a NIR-II ECL (NaYbF4@SiO2) in aqueous solution with a relative ECL efficiency as high as which is the highest among the NIR-ECL materials. The particle structure design the of NIR-ECL to the of emission and good Due to the NaYbF4 it perfectly the wavelength bottleneck and exhibited efficient ECL at 983 The synergistic effect of enrichment and catalysis provided by the mesoporous shell significantly enhanced the ECL signal of the particles and improved the stability. 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