Selection Of Excitation Wavelength Research Articles

Probing the structural and spectroscopic characteristics of Ag2O-modified Li2O–CaO–B2O3 glasses doped with Nd2O3

Currently, there is growing interest in Ag2O doping in oxide glasses through optimal selection of excitation wavelengths to achieve efficient luminescence via either up or down-conversion processes. In this present work, we report the influence of Ag nanoparticles/aggregates on the structural and spectroscopic characteristics of melt-quenched Ag2O - co-doped (19.5-x)Li2O+70B2O3+10CaO+0.5Nd2O3+xAg2O glasses. Systematic studies were conducted using XRD, Raman spectroscopy, HR-TEM, MAS-NMR, UV–Visible–NIR, Photoluminescence, and decay kinetics techniques. The XRD analysis conclusively indicates the absence of crystalline peaks. However, HR-TEM analysis showed a reduction in the size of AgNPs from 14 nm to 11 nm with an increase of Ag2O concentration from 0.25 mol% to 0.5 mol%, respectively. The RAMAN and MAS-NMR (11B and 7Li) revealed the modification in the local structure induced by the Ag2O as revealed by the predominant borate structural units, and silver (Ag) acted as a modifier in the glass matrix. The absorption spectra of all samples exhibit a characteristic peak of Nd3+ ions. Interestingly, the LCBNd0.5Ag0.5 glass sample exhibits the SPR peak at 416 nm, indicating that the lower limit of Ag2O in LCBNd0.5 is <0.5 mol%. Luminescence studies at different excitation wavelengths (350, 525, and 584 nm) reveal the predominant emission at 877 nm and 1058 nm, whose emission intensity increased with Ag2O concentration. Samples with Ag2O-doped LCBNd glasses exhibit identical lifetime values, indicating the absence of luminescence-quenching and the presence of Ag aggregates leading to the SPR effect. Based on our findings, the LCBNd0.5Ag0.5 sample demonstrated superior performance to other samples, with the highest emission under an excitation of 350 nm.

The application of laser‑induced fluorescence in oil spill detection.

Over the past two decades, oil spills have been one of the most serious ecological disasters, causing massive damage to the aquatic and terrestrial ecosystems as well as the socio-economy. In view of this situation, several methods have been developed and utilized to analyze oil samples. Among these methods, laser-induced fluorescence (LIF) technology has been widely used in oil spill detection due to its classification method, which is based on the fluorescence characteristics of chemical material in oil. This review systematically summarized the LIF technology from the perspective of excitation wavelength selection and the application of traditional and novel machine learning algorithms to fluorescence spectrum processing, both of which are critical for qualitative and quantitative analysis of oil spills. It can be seen that an appropriate excitation wavelength is indispensable for spectral discrimination due to different kinds of polycyclic aromatic hydrocarbons' (PAHs) compounds in petroleum products. By summarizing some articles related to LIF technology, we discuss the influence of the excitation wavelength on the accuracy of the oil spill detection model and proposed several suggestions on the selection of excitation wavelength. In addition, we introduced some traditional and novel machine learning (ML) algorithms and discussed the strengths and weaknesses of these algorithms and their applicable scenarios. With an appropriate excitation wavelength and data processing algorithm, it is believed that laser-induced fluorescence technology will become an efficient technique for real-time detection and analysis of oil spills.

Colour-tunable and warm white light emitting thermally stable Dy3+/Sm3+ co-activated tungstate-tellurite glasses for photonic applications

Transparent single Dy3+, Sm3+, and Dy3+/Sm3+ co-activated tungstate-tellurite (TWKZBi) glass matrices were synthesized via melt-quenching process. The optical features of the prepared glass matrices were examined with the aid of the absorption spectra. Under various excitation wavelengths (366, 388 and 454 nm), Dy3+ doped TWKZBi (TWKZBiDy) glasses revealed a prevailing emission peak ⁓575 nm related to the yellow band (4F9/2 → 6H13/2) transition, while Sm3+ doped TWKZBi (TWKZBiSm) glass matrices reveal strong intense emission peak at ∼600 nm associated with the orange-red band (4G5/2 → 6H7/2) transition. The emission intensity progressively surges with an increment in Sm3+ ion content (up to 1.5 mol%), confirming the ET mechanism between Dy3+ and Sm3+ ions. Application of Dexter's ET formulation with the Reisfeld's approximation revealed that the ET mechanism involves a non-radiative dipole-dipole (d-d) interaction between Dy3+ and Sm3+ ions. The average lifetime (τavg) values for the 4F9/2 state of Dy3+ ions in the TWKZBiDySm glass sample was measured and found that these values decreased with increasing Sm3+ ion content. In addition, the ET efficiencies (ηDy→Sm) of the TWKZBiDySm glass matrices were evaluated using the lifetime curves. The color coordinates (x, y) and the correlated color temperature (CCT) values of the co-activated glass samples were estimated. Based on the chromaticity coordinates and CCT values, it was found that the desired color of the TWKZBiDySm glass samples can be modulated from warm white light to orange-red light via precisely adjusting the concentration of Sm3+ ions and at selected excitation wavelengths. Furthermore, the emission intensity observed at 373 K was 83.83 % compared to the emission intensity at ambient temperature, indicating the excellent thermal stability of the TWKZBiDySm glasses prepared. The above results confirm that the prepared Dy3+/Sm3+ co-activated TWKZBi glass matrix can be a promising candidate for white light and other photonic devices.

Factor analysis of the time series of SERS spectra reveals water arrangement and surface plasmon changes in Ag nanoparticle systems

The enhancement of Raman signals of molecules localized in the vicinity of plasmonic nanoparticles, known as surface-enhanced Raman scattering (SERS) effect, is strongly influenced by the selected excitation wavelength. The optimal excitation wavelength in SERS measurements is given by the position of the surface plasmon extinction (SPE) band of the studied system. Even a small change of the SPE band intensity, position and/or shape during the measurement may influence the SERS signal significantly. In this work, we prepared several systems of Ag nanoparticles, which were used for the demonstration how the information about SPE changes can be obtained by multivariate statistical analysis (factor analysis; FA) from SERS spectral sets, and employed in more precise and more comprehensive interpretation of the results. In non-aggregated Ag colloidal systems measured at the excitation wavelength of 445 nm, SPE band changes could be monitored by the analysis of water stretching vibration together with the vibrations in the fingerprint region. The FA of the water stretching band region was shown to provide unique information on both arrangement and disarrangement of water molecules in the vicinity of Ag NPs during the time evolution of these SERS active systems. In addition, the FA of the fingerprint region helped to monitor a rapid metalation of meso-tetrakis(N-methyl-4-pyridyl)porphine in etched SERS systems with Ag+ ions released from the NPs surface. In aggregated Ag colloidal systems measured at the excitation wavelength of 785 nm, the FA of SERS spectral sets enabled us to reveal the contribution of the 2nd electromagnetic enhancement to the overall SERS signal. The reliability of our conclusions was verified by comparing the results obtained from FA of SERS spectral sets with the data obtained from the parallel SPE measurements of the studied systems.

Wavelength and solvent controlled energy and charge transfer in donor-acceptor substituted platinum acetylide complexes

• Photophysics of a Donor-Acceptor substituted Pt(II) Complex are studied. • Complex displays wavelength and solvent tuned photophysics. • Ultrafast spectroscopy reveals the existence of ultrafast energy and charge transfer. An electron donor-acceptor platinum complex trans -Pt(PBu 3 ) 2 (C≡C-T 4 )(C≡C-TBT) (Pt(T 4 )(TBT)) incorporating (3,3'''-dihexyl-2,2':5',2'':5'',2'''-quaterthiophene, T 4 ) as donor and (4,7-bis(2-thienyl)-2,1,3-benzothiadiazole, TBT) as acceptor has been synthesized and fully characterized. Model complexes trans -Pt(PBu 3 ) 2 (C≡C-T 4 ) 2 and trans -Pt(PBu 3 ) 2 (C≡CTBT) 2 featuring only donor or acceptor moieties were synthesized for reference. Steady state absorption, fluorescence, and ultrafast visible-near infrared transient absorption spectroscopy were applied to evaluate intracomplex energy and charge transfer in different solvents and by using selective excitation wavelengths corresponding to T 4 or TBT units. The fluorescence spectrum of Pt(T 4 )(TBT) is red-shifted with increasing solvent polarity, consistent with the existence of a charge transfer excited state. Detailed solvent and excitation wavelength dependent ultrafast transient absorption experiments were carried out on all of the complexes. For Pt(T 4 )(TBT) in non-polar solution excitation of T 4 leads to ultrafast (∼200 fs) T 4 to TBT energy transfer. However, in more polar solvents, T 4 excitation gives rise to an ultrafast T 4 to TBT electron transfer affording a “ligand to ligand” charge transfer excited state. The results provide insight into the existence of donor-acceptor interactions between weakly coupled π-conjugated units that are separated by a metal center.

Generation of Pure Green Up-Conversion Luminescence in Er3+ Doped and Yb3+-Er3+ Co-Doped YVO4 Nanomaterials under 785 and 975 nm Excitation.

Materials that generate pure, single-color emission are desirable in the development and manufacturing of modern optoelectronic devices. This work shows the possibility of generating pure, green up-conversion luminescence upon the excitation of Er3+-doped nanomaterials with a 785 nm NIR laser. The up-converting inorganic nanoluminophores YVO4: Er3+ and YVO4: Yb3+ and Er3+ were obtained using a hydrothermal method and subsequent calcination. The synthesized vanadate nanomaterials had a tetragonal structure and crystallized in the form of nearly spherical nanoparticles. Up-conversion emission spectra of the nanomaterials were measured using laser light sources with λex = 785 and 975 nm. Importantly, under the influence of the mentioned laser irradiation, the as-prepared samples exhibited bright green up-conversion luminescence that was visible to the naked eye. Depending on the dopant ions used and the selected excitation wavelengths, two (green) or three (green and red) bands originating from erbium ions appeared in the emission spectra. In this way, by changing the UC mechanisms, pure green luminescence of the material can be obtained. The proposed strategy, in combination with various single-doped UC nanomaterials activated with Er3+, might be beneficial for modern optoelectronics, such as light-emitting diodes with a rich color gamut for back-light display applications.

White light emission and energy transfer in Pr3+/Er3+ co-doped InF3-based glass

Pr3+/Er3+ co-doped glasses based on InF3 were synthesized and then studied for white light emission. The emission spectra consist of several bands corresponding to characteristic 4f-4f electronic transitions of Pr3+ and Er3+ ions. The relative intensities of emission bands depend on the selective excitation wavelengths. The energy transfer process from Pr3+ to Er3+ ions was confirmed by the excitation and emission spectra measurements. From emission spectra, the CIE chromaticity coordinates, the correlated color temperature (CCT) and the color purity (CP) were determined. The results indicate that Pr3+/Er3+ co-doped glasses based on InF3 are excellent fluoride candidate to achieve warm white light emission.

Dual-emitter polymer carbon dots with spectral selection towards nanomolar detection of iron and aluminum ions

We report on the spectral selection of excitation wavelength towards selective detection of aluminum and iron ions using dual emission polymer carbon dots (PCDs). PCDs were prepared from glucose and dilute sulfuric acid using one-pot solvothermal method. The PCDs emit blue light at 480 nm when excited at 340 nm, while emit red light at 590 nm when excited with 400 nm. Spectral selection (selection of excited state) showed sensitivity enhancement for detection of some metal ions. The PCDs showed fluorescence enhancement when combined with Al3+ ions with hypsochromic shift centered at 470 nm when excited at 440 nm. While the PCDs selectively quenched via addition of Fe3+ ions, when excited at 400 nm. The wavelength selection of the same carbon dots increases signal to noise ration. The PCDs showed thermo-sensing behavior from 0 °C to 90 °C with reasonably good reversibility. The PCDs acted as fluorescent probes for multicolor (green and yellow) imaging of MCF-7 cells while not inducing cell death, which indicates that the PCDs are biocompatible and nontoxic to the cells. Therefore, the PCDs can be used as probes for cell-imaging applications in vitro and in vivo. The PCDs proved to be a multi-purpose polymer carbon nanomaterial that can used for pharmaceutical analysis, bio-imaging and thermo-sensing while providing high accuracy, selectivity and a limit of detection in the nano range.

Enhanced luminescence, electric-field and actinic-light modulation of emission in nematic-CdSeS gradient nanocrystal composites by polymer confinement

We demonstrate that dispersing a small amount of CdSeS gradient quantum dots (QDs) in a nematic liquid crystal and geometrically confining the nanocomposite in a polymer network leads to their ordered linear assembly, as observed through electron microscopy. Such an orientational order not only enhances the base photoluminescence (PL), but brings in significant anisotropy in the emission magnitude, a feature barely seen for the system without the polymer network. A high frequency (∼kHz) AC field adds the attractive dimension of on-demand and spatially addressable PL modulation. Using a photoisomerizable azobenzene derivative and a carefully selected excitation wavelength, it is shown that an actinic-light driven modulation of PL is also possible, which is furtherenhanced by the presence of the polymer network.

Excitation and activator concentration induced color tuning and white light generation from Bi3+ sensitized Y2O3:Eu3+: Energy transfer studies

Hydrothermally synthesized Bi3+ sensitized Y2O3:Eu3+ phosphors were characterized using X-ray diffraction, Fourier transform infrared spectroscopy and transmission electron microscopy. The substitution of Bi3+ in the different symmetry sites of Y3+ lattice in Y2O3 i.e., C2 and S6 induces two different excitation (1S0 → 3P1) and emission (3P1 → 1S0) bands of Bi3+. These two excitation bands of Bi3+ efficiently transfer the energy to the excited states of Eu3+ in Y2O3:Eu3+. The occurrence of energy transfer from Bi3+ to Eu3+ ions is carefully investigated using the steady state and decay lifetime measurements. The energy transfer efficiency reaches up to 93% and 99% for S6 and C2 related excitations, respectively. The energy transfer occurs mainly through dipole-dipole interactions. The appropriate selection of excitation wavelength and careful choose of Eu3+ concentration in Y2O3 can provide the multicolor emission as well as white light emission. The maximum quantum yield of 24% was observed from Bi3+ (1 at%) sensitized Y2O3:Eu3+ (2 at%).

Fluoroindate glasses co-doped with Pr3+/Er3+ for near-infrared luminescence applications

Fluoroindate glasses co-doped with Pr3+/Er3+ ions were synthesized and their near-infrared luminescence properties have been examined under selective excitation wavelengths. For the Pr3+/Er3+ co-doped glass samples several radiative and nonradiative relaxation channels and their mechanisms are proposed under direct excitation of Pr3+ and/or Er3+. The energy transfer processes between Pr3+ and Er3+ ions in fluoroindate glasses were identified. In particular, broadband near-infrared luminescence (FWHM = 278 nm) associated to the 1G4 → 3H5 (Pr3+), 1D2 → 1G4 (Pr3+) and 4I13/2 → 4I15/2 (Er3+) transitions of rare earth ions in fluoroindate glass is successfully observed under direct excitation at 483 nm. Near-infrared luminescence spectra and their decays for glass samples co-doped with Pr3+/Er3+ are compared to the experimental results obtained for fluoroindate glasses singly doped with rare earth ions.

Vibration–rotation interactions in H2, HD and D2 : centrifugal distortion factors and the derivatives of polarisability invariants

ABSTRACT We report the correction factors for centrifugal distortion in Raman intensities for pure rotation (O0- and S0-branch) and vibration–rotation (O1- and S1-branch) transitions in the ground electronic state of H2, HD and . These factors are presented for 52 selected excitation wavelengths and for the initial rotational states, . This data is useful in applications of intensity calibration of spectrometers and the spectroscopy of flames. The classical treatment of centrifugal distortion involved the expansion of polarisability anisotropy (γ) over the internuclear distance, while assuming the diatomic molecule behaves as a harmonic oscillator. Here, this approximation of polarisability invariants as a Taylor series expansion is tested, revealing that truncation up to the second-order derivatives of mean polarisability () and polarisability anisotropy (γ) gives faithful representations, yielding accurate expectation values with error , for the ground rovibrational state and for the fundamental transition.

Measurement of 3-photon excitation and emission spectra and verification of Kasha’s rule for selected fluorescent proteins excited at the 1700-nm window

Fluorescent proteins are widely used to visualize structures and dynamics in various biological samples. Multiphoton microscopy is especially suitable for imaging structures expressing fluorescent proteins with subcellular resolution. 3-photon microscopy (3PM) excited at the 1700-nm window has proven to be promising for deep-tissue (such as brain) imaging expressing red fluorescent proteins. However, the 3-photon excitation and emission spectra of fluorescent proteins suitable at this window remain largely unknown, hampering protein selection and detection optimization. Here we demonstrate detailed measurement of 3-photon excitation and emission spectra for selected fluorescent proteins, suitable for 3-photon excitation at the 1700-nm window. The measured 3-photon excitation spectra will provide guidelines for protein and excitation wavelength selection. The measured 3-photon emission spectra and comparison with the 1-photon emission spectra, on one hand proves that the fundamental Kasha's rule is still valid for 3-photon fluorescence in these fluorescent proteins, on the other hand will be helpful for efficient fluorescence signal detection.

Visualizing astrocytes in the deep mouse brain in vivo.

Astrocytes play a key role in the central nervous system. However, methods of visualizing astrocytes in the deep brain in vivo have been lacking. 3-photon fluorescence imaging of astrocytes labeled by sulforhodamine 101 (SR101) is demonstrated in deep mouse brain in vivo. Excitation wavelength selection was guided by wavelength-dependent 3-photon action cross section (ησ 3 ) measurement of SR101. 3-photon fluorescence imaging of the SR101-labeled vasculature enabled an imaging depth of 1340-μm into the mouse brain. This justifies the deep imaging capability of the technique and indicates that the imaging depth is not determined by the signal-to-background ratio limit encountered in 2-photon fluorescence imaging. Visualization of astrocytes 910 μm below the surface of the mouse brain in vivo is demonstrated, 30% deeper than that using 2-photon fluorescence microscopy. Through quantitative comparison of the signal difference between the SR101-labeled blood vessels and astrocytes, the challenges of visualizing astrocytes below the white matter is further elucidated.

A novel LED-based 2D-fluorescence spectroscopy system for in-line bioprocess monitoring of Chinese hamster ovary cell cultivations-Part II.

This study was performed in order to evaluate a new LED-based 2D-fluorescence spectrometer for in-line bioprocess monitoring of Chinese hamster ovary (CHO) cell culture processes. The new spectrometer used selected excitation wavelengths of 280, 365, and 455nm to collect spectral data from six 10-L fed-batch processes. The technique provides data on various fluorescent compounds from the cultivation medium as well as from cell metabolism. In addition, scattered light offers information about the cultivation status. Multivariate data analysis tools were applied to analyze the large data sets of the collected fluorescence spectra. First, principal component analysis was used to accomplish an overview of all spectral data from all six CHO cultivations. Partial least square regression models were developed to correlate 2D-fluorescence spectral data with selected critical process variables as offline reference values. A separate independent fed-batch process was used for model validation and prediction. An almost continuous in-line bioprocess monitoring was realized because 2D-fluorescence spectra were collected every 10 min during the whole cultivation. The new 2D-fluorescence device demonstrates the significant potential for accurate prediction of the total cell count, viable cell count, and the cell viability. The results strongly indicated that the technique is particularly capable to distinguish between different cell statuses inside the bioreactor. In addition, spectral data provided information about the lactate metabolism shift and cellular respiration during the cultivation process. Overall, the 2D-fluorescence device is a highly sensitive tool for process analytical technology applications in mammalian cell cultures.

Energy transfer studies and neutral to warm white light generation in Dy3+-Sm3+ co-doped bismuth phosphate glasses for lighting applications

A series of bismuth phosphate (BiP) glasses activated with 0.5 mol% of Dy3+ and different concentrations (0.1, 0.3, 0.5, 1.0, 1.5 and 2.0 mol%) of Sm3+ ions were prepared by melt quenching technique. These are characterized through optical absorption, photoluminescence and decay measurements for spectroscopic investigations. The luminescence spectra have been studied under selected excitation wavelengths to examine the energy transfer mechanism in the present Dy3+-Sm3+ co-doped glass by varying Sm3+ ion concentration. The lifetimes of both 4F9/2 level of Dy3+ ion and 4G5/2 level of Sm3+ ion of co-doped glasses have obtained and found to be decreased with increase in Sm3+ ion concentration. The emission intensity increased with the increase of Sm3+ ion concentration (upto 1.5 mol%) which confirms the energy transfer process between Dy3+ to Sm3+ ions. The CIE chromaticity co-ordinates (x, y), correlated color temperature (CCT) and color purity (CP) were determined from the emission data. From the CIE, CCT and CP values, it can be concluded that there is a color hue from neutral to warm white light region due to Sm3+ ion concentration and selected excitation wavelength. Hence, the obtained results strongly suggest that the present Dy3+-Sm3+ co-doped bismuth phosphate glasses could be useful for lighting applications.

Ex and in vivo characterization of the wavelength‐dependent 3‐photon action cross‐sections of red fluorescent proteins covering the 1700‐nm window

Multiphoton action cross-sections are the prerequisite for excitation light selection. At the 1700-nm window suitable for deep-tissue imaging, wavelength-dependent 3-photon action cross-sections ησ3 for RFPs are unknown, preventing wavelength selection. Here we demonstrate: (1) ex vivo measurement of wavelength-dependent ησ3 for purified RFPs; (2) a multiphoton imaging guided measurement system for in vivo measurement; and (3) in vivo measurement of wavelength-dependent ησ3 in RFP labeled cells. These fundamental results will provide guidelines for excitation wavelength selection for 3-photon fluorescence imaging of RFPs at the 1700-nm window, and augment the existing database of multiphoton action cross-sections for fluorophores.

Excitation wavelength selection for quantitative analysis of carotenoids in tomatoes using Raman spectroscopy

The difference in Raman spectra for different excitation wavelengths (532 nm, 785 nm, and 1064 nm) was investigated to identify an appropriate wavelength for the quantitative analysis of carotenoids in tomatoes. For the 532 nm-excited Raman spectra, the intensity of the peak assigned to the carotenoid has no correlation with carotenoid concentration, and the peak shift reflects carotenoid composition changing from lycopene to β-carotene and lutein. Thus, 532 nm-excited Raman spectra are useful for the qualitative analysis of carotenoids. For the 785 nm- and 1064 nm-excited Raman spectra, the peak intensity of the carotenoid showed good correlation with carotenoid concentration; thus, regression models for carotenoid concentration were developed using these Raman spectra and partial least squares regression. A regression model designed using the 785 nm-excited Raman spectra showed a better result than the 532 nm- and 1064 nm-excited Raman spectra. Therefore, it can be concluded that 785 nm is the most suitable excitation wavelength for the quantitative analysis of carotenoid concentration in tomatoes.

Modality switching between therapy and imaging based on the excitation wavelength dependence of dual-function agents in folic acid-conjugated graphene oxides.

Owing to its near infrared (NIR) absorption, graphene oxide (GO) is promising for both photothermal (PT) therapy and multiphoton (MP) imaging. Novel therapy/imaging modality switching is proposed here based on the selected excitation wavelength of femtosecond (FS) laser. GO-based destruction of cancer cells is demonstrated when the laser power of 800-nm-wavelength FS laser is increased above 7 mW. However, GO-based imaging is mainly monitored without damaging the sample when using 1200-nm wavelength FS laser in the same laser power range. Folic acid (FA) conjugated graphene oxide (FA-GO) was synthesized for selective cancer cell targeting. Dual-function FA-GO-based cancer cell targeting agents were experimentally optimized to enable therapy/imaging modality switching.

Ratiometric Fluorescence Sensing and Real-Time Detection of Water in Organic Solvents with One-Pot Synthesis of Ru@MIL-101(Al)-NH2.

Ratiometric fluorescence detection attracts much attention because of its decreased environmental influence and easy-to-differentiate color and intensity change. Herein, a guest-encapsulation metal-organic framework (MOF), Ru@MIL-NH2, is prepared with 2-aminoterephthalic acid, AlCl3, and Ru(bpy)32+ by a simple one-pot method for ratiometric fluorescence sensing of water in organic solvents. The rational selection of the excitation wavelength provides dual emission at 465 and 615 nm from Ru@MIL-NH2 under a single excitation of 300 nm. High sensitivity, low detection limit (0.02% v/v), wide response range (0-100%), and fast response (less than 1 min) are obtained for ratiometric fluorescence sensing of water under single excitation with Ru@MIL-NH2 as the probe. Moreover, the result of water content is independent of the concentration of Ru@MIL-NH2 as the merit of ratiometric fluorescence detection. The response mechanism reveals that the protonation of the nitrogen atom of the MIL-NH2, the π-conjugation system, and the stable fluorescence of Ru(bpy)32+ achieve the ratiometric fluorescence. The analysis of real spirit samples confirms the proposed method. A test strip is prepared with Ru@MIL-NH2 for convenient use. We believe that such turn-on ratiometric host-guest MOFs and the rational selection of excitation wavelength will offer guidance for ratiometric fluorescence detection with wide applications.