Luminescent Lanthanide Research Articles

Front Cover: Luminescent Lanthanide Complexes for Effective Oxygen‐Sensing and Singlet Oxygen Generation (ChemPlusChem 6/2023)

The front cover picture shows triplet-oxygen sensing and singlet-oxygen generation using lanthanide complexes. The lanthanides consist of elements of the f-block in period six in the periodic table. Organic ligands play crucial roles in lanthanide complexes as light-harvesting antennas for lanthanide luminescence and singlet-oxygen generation. More information can be found in the Review by Y. Kitagawa, Y. Hasegawa, and co-workers.

Upconversion Luminescence from Sol-Gel-Derived Erbium- and Ytterbium-Doped BaTiO3 Film Structures and the Target Form

Sol-gel technology has attracted attention in the fabrication of diverse luminescent materials and thin film structures, with forms that range from powders to microcavities. The optical properties of sol-gel-derived structures depend on the sol composition, deposition, and heat treatment conditions, as well as on the film thicknesses and other factors. Investigations on the upconversion luminescence of lanthanides in film structures and materials are also ongoing. In this study, we synthesized three different types of materials and film structures using the same sol, which corresponded to a Ba0.76Er0.04Yb0.20TiO3 xerogel, as follows: (a) the target form, which used the explosive compaction method for sol-gel-derived powder; (b) single-layer spin-on xerogel films annealed at 450 and 800 °C; and (c) microcavities with an undoped SiO2/BaTiO3 Bragg reflector surrounding a Ba0.76Er0.04Yb0.20TiO3 active layer. The BaTiO3:(Er,Yb)/SiO2 microcavity exhibited an enhancement of the upconversion luminescence when compared to the BaTiO3:(Er,Yb) double-layer film fabricated directly on a crystalline silicon substrate. The reflection spectra of the BaTiO3:(Er, Yb)/SiO2 microcavity annealed at 800 °C demonstrated a deviation of the maxima of the reflection within 15% for temperature measurements ranging from 26 to 120 °C. From the analyses of the transmission and reflection spectra, the optical band gap for the indirect optical transition in the single layer of the BaTiO3:(Er,Yb) spin-on film annealed at 450 °C was estimated to be 3.82 eV, while that for the film annealed at 800 °C was approximately 3.87 eV. The optical properties, upconversion luminescence, and potential applications of the BaTiO3:(Er,Yb) sol-gel-derived materials and structures are discussed in this paper.

Lanthanide and Ladder-Structured Polysilsesquioxane Composites for Transparent Color Conversion Layers

Ladder-type polysilsesquioxanes (LPSQs) containing phenyl as a high refractive index unit and cyclic epoxy as a curable unit were found to be excellent candidates for a transparent color conversion layer for displays due to being miscible with organic solvents and amenable to transparent film formation. Therefore, the LPSQs were combined with luminescent lanthanide metals, europium Eu(III), and terbium Tb(III), to fabricate transparent films with various emission colors, including red, orange, yellow, and green. The high luminescence and transmittance properties of the LPSQs-lanthanide composite films after thermal curing were attributed to chelating properties of hydroxyl and polyether side chains of LPSQs to lanthanide ions, as well as a light sensitizing effect of phenyl side chains of the LPSQs. Furthermore, Fourier-transform infrared (FT-IR) and X-ray photoelectron spectroscopy and nanoindentation tests indicated that the addition of the nanoparticles to the LPSQs moderately enhanced the epoxy conversion rate and substantially improved the wear resistance, including hardness, adhesion, and insusceptibility to atmospheric corrosion in a saline environment. Thus, the achieved LPGSG-lanthanide hybrid organic-inorganic material could effectively serve as a color conversion layer for displays.

Rapid handheld time-resolved circularly polarised luminescence photography camera for life and material sciences

Circularly polarised luminescence (CPL) is gaining a rapidly increasing following and finding new applications in both life and material sciences. Spurred by recent instrumental advancements, the development of CPL active chiral emitters is going through a renaissance, especially the design and synthesis of CPL active luminescent lanthanide complexes owing to their unique and robust photophysical properties. They possess superior circularly polarised brightness (CPB) and can encode vital chiral molecular fingerprints in their long-lived emission spectrum. However, their application as embedded CPL emitters in intelligent security inks has not yet been fully exploited. This major bottleneck is purely hardware related: there is currently no suitable compact CPL instrumentation available, and handheld CPL photography remains an uncharted territory. Here we present a solution: an all solid-state small footprint CPL camera with no moving parts to facilitate ad hoc time-resolved enantioselective differential chiral contrast (EDCC) based one-shot CPL photography (CPLP).

Butterfly-Inspired Tri-State Photonic Crystal Composite Film for Multilevel Information Encryption and Anti-Counterfeiting.

Counterfeiting is a worldwide issue and has long troubled legitimate businesses, while nowadays anti-counterfeiting materials and technology are still insufficient to combat the escalating counterfeit behaviors. Inspired by hindwing structure of Troides magellanus, a new kind of anti-counterfeiting material taking advantage of both physical and chemical structures to display multiple optical states is prepared. The chemical units (luminescent lanthanide) are blended with physical units (monodispersed colloidal particles) and mediating molecules, which are then assembled into a photonic crystal structure at room temperature in less than 10s through a new assembly technique called molecule-mediated shear-induced assembly technique (MSAT). The as-prepared photonic crystal films feature three unique optical states, each displaying structural, fluorescent, and phosphorescent color under different lighting conditions, which integrates colors from both physical and chemical origins. Furthermore, by incorporating different luminescent materials into different parts of the photonic crystal pattern, a high-level information encryption system is designed to be capable of carrying three distinct types of information. Thanks to this powerful tool of MSAT, it is now possible to assemble different-sized, even irregular non-spherical units with monodispersed spherical units into high-quality photonic crystal films, which provides easy access to incorporating new features into photonic crystal systems.

Facile synthesis, novel structure, multicolor luminescence, and potential applications of lanthanide ions doped bismuth titanate phosphors

A variety of lanthanide ions doped bismuth titanate (Bi4Ti3O12) luminescent materials with eminent down-conversion (DC) and up-conversion (UC) luminescence performance have been fabricated via a facile sol-gel approach. The XRD, XPS, and EDX elemental mapping results confirm the phase structure of orthorhombic Bi4Ti3O12 (BTO), and the lanthanide activator ions occupy the Bi3+ lattice sites in the BTO crystal. Under UV or NIR excitation, the Eu3+, Yb3+/Ln3+ (Ln = Er, Tm, and Ho) doped Bi4Ti3O12 samples exhibit characteristic red, green, blue, and green emissions. The luminescent mechanisms of the BTO:Eu3+ and BTO:Yb3+/Ln3+ samples are discussed based on the energy level diagrams. The doping concentrations of Eu3+, Yb3+, Er3+, Tm3+, Ho3+ ions and annealing temperature and time are optimized, whose optimal values are determined to be 14, 8, 1, 0.4, 1 mol% and 800 oC, 4 h. The as-obtained LED devices fabricated by Bi4Ti3O12:Eu3+ and Yb3+/Ln3+ phosphors exhibit dazzling multicolor visible light emissions from different Ln3+ ions. The results indicate that the as-obtained Ln3+ doped BTO phosphors may be potentially utilized in LED devices and solid-state lighting. Furthermore, the Eu3+ and Er3+ co-doped BTO samples exhibit different DC and UC luminescence spectral profiles when excited at various UV, visible, or NIR wavelengths, revealing their eminent feasibility and great potential in anti-counterfeiting applications.

Plasmon-enhanced upconversion luminescence on horizontally aligned gold nanorod arrays with self-contained spacer

The growing interest in converting infrared light to visible light in many photovoltaic fields requires the development of upconversion systems with high luminescence quantum yields and stability without blinking. However, the weak and narrow absorption of rare earth ions has greatly restricted the luminescent efficiency of rare earth dopants and hampered their translation from experimental research to real world applications. In this work, horizontally aligned gold nanorod arrays with self-contained spacer and obvious polarization dependent plasmonic properties are prepared on the substrate of silicon wafers by a self-assembly method of droplet evaporation, and polarization-dependent enhancement on the upconversion luminescence of rare earth ions doped nanoparticles (NaYF4: Yb3+, Er3+) are investigated. The results show that the luminescence intensity of the nanoparticles is greatly enhanced by a factor of about 140, when the horizontally aligned gold nanorod arrays are available as a substrate. The effect of self-contained spacer of the arrays to prevent quenching of the luminescence is investigated with the introduction of an Al2O3 layer as an isolate and the adjustment of its thickness. It is found that the self-contained spacer can effectively block the non-radiative energy transfer without the need of additional spacer. In addition, the horizontally aligned gold nanorod arrays also exhibit excellent optical polarization anisotropy. As the polarization angle of the incident light gradually increases, the intensity of the fluorescent radiation decreases, which is interpreted and analyzed by simulating the near field distribution. This strategy not only provides a powerful solution to build high-efficiency surface enhanced fluorescence systems, but also suggests a new pathway to realize the polarization-selective excitation of upconversion luminescence.

Fabrication of the Antibiotic Sensor by the Multifunctional Stable Adjustable Luminescent Lanthanide Metal–Organic Frameworks

In recent years, the irrational use of antibiotics has become very widespread. It is necessary to regulate this phenomenon through antibiotic detection. In this work, a series of isomorphic Ln-MOFs (Ln = Tb3+ and Eu3+) were synthesized from 1,3,5-tri(4-carboxyphenyl)benzene (H3L) and Ln3+ by the solvothermal method for the first time. A series of 1-EuxTb1-x with different luminescence were doped by changing the molar ratio of Tb3+ and Eu3+. Ln3+ forms a 4-connected 2D network structure through self-assembly with fully deprotonated L3-. It shows good chemical stability in water, and its luminescence is not affected by aqueous solutions with different pH values. 1-Eu demonstrates rapid and sensitive detection capabilities for MDZ and TET with good recyclability and low detection limits (10-5). In order to increase the practicability of 1-Eu, two portable sensors have been prepared, in which the fluorescent film (Film@1-Eu) has a detection limit of 10-4, and the sensitivity is only less than 10% of the titration results. A portable fluorescent test paper can reach the detection limit of 14.7 ppm. This study provides a new idea for the application of stable multifunctional materials in the field of fluorescence sensing.

Detecting phosphate using lysine-sensitized terbium coordination polymer nanoparticles as ratiometric luminescence probes.

Probes for detecting phosphate ions (Pi) are required for environmental monitoring and to protect human health. Here, novel ratiometric luminescent lanthanide coordination polymer nanoparticles (CPNs) were successfully prepared and used to selectively and sensitively detect Pi. The nanoparticles were prepared from adenosine monophosphate (AMP) and Tb3+, and lysine (Lys) was used as a sensitizer (through the antenna effect) to switch on Tb3+ luminescence at 488 and 544nm while Lys luminescence at 375nm was quenched because of energy transfer from Lys to Tb3+. The complex involved is here labeled AMP-Tb/Lys. Pi destroyed the AMP-Tb/Lys CPNs and therefore decreased the AMP-Tb/Lys luminescence intensity at 544nm and increased the luminescence intensity at 375nm at an excitation wavelength of 290nm, meaning ratiometric luminescence detection was possible. The ratio between the luminescence intensities at 544 and 375nm (I544/I375) was strongly associated with the Pi concentration between 0.1 and 6.0μM, and the detection limit was 0.08μM. The dual-emission reverse-change ratio luminescence sensing method can exclude environmental effects, so the proposed assay was found to be very selective. The method was successfully used to detect Pi in real water samples, and acceptable recoveries were found, suggesting that the method could be used in practice to detect Pi in water samples.

Structural metamorphosis and photophysical properties of thermostable nano- and microcrystalline lanthanide polymer with flexible coordination chains

ABSTRACT Luminescent lanthanide coordination polymer crystals (LCPCs) represent an area of growing interest in materials chemistry owing to their unique and tailorable functional properties. The LCPCs provide a high level of structural tunability, including size- and morphology-dependent properties; therefore, they are promising materials for next-generation phosphors in a wide range of applications such as light emitting diodes. Here, by controlling the morphology of thermostable europium coordination polymer crystals, [Eu(hfa)3(dpbp)]n, hfa: hexafluoroacetylacetonate and dpbp:4,4’-bis(diphenyl phosphoryl) biphenyl), we realized a novel red phosphor with narrow linewidth emission (FWHM = 7.8 nm). The obtained luminescent LCPCs with unique structures were characterized by X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), dynamic light scattering (DLS) and thermogravimetric analysis. Among, them, size tunable crystalline polymer spheres were found to have high internal quantum efficiency (ex., IQE = 79%) and highly thermostability (>300°C), and to exhibit dispersibility in PMMA media. The obtained results on the structural tunability of these materials can be used for the development of synthesis techniques for nanoscale materials based on crystalline lanthanide-based coordination phosphors.

Boosting the optical properties of polylactic acid/ lanthanide-based metal-organic framework composites

Luminescent lanthanide metal-organic frameworks (LnMOF) are gathering much interest in the scientific community due to their exceptional optical properties, which have the potential to revolutionize several application fields, including anti-counterfeiting. The traceability of the original products is an ever-growing demand for producers and end-users to verify the authenticity of produced goods. Following this newfound need, we propose an anti-counterfeiting method based on a quick-response (QR) code, with a very specific verification procedure, using a luminescent LnMOF-based composite. The embedded code will only react to certain stimulation, hence it can only be unlocked using the sequence that is programmed to respond to. For that purpose, we explored the incorporation of fresh LnMOFs, with europium (EuMOF) and terbium (TbMOF), on a biodegradable polymer matrix of polylactic acid (PLA). A homogeneous mixture of PLA/LnMOF was produced via a two-step process including solvent casting followed by thermal mixing, after achieving the optimized concentration for each filler. The optical characterization of the fresh composites showed an excellent response to selective excitation, with well-defined intraionic lines for TbMOF @ 542 nm (5D4→7F5) and for EuMOF @ 615 nm (5D0→7F2). PLA/Eu, TbMOF composites were also explored to create an additional level of encryption by tuning the emitted color with the appropriate excitation stimuli. Hence, these new luminescent composite materials can be used as traceable optical tags for a wide variety of polymeric products, capable to respond to a very specific excitation wavelength and act as a level three luminescent security marker, whose decoding demands the assessment of more than one factor, with near-impossible replication. Additionally, this technology has a high potential for scalability since the applied processing techniques used for the production of the optically active QR code are commonly employed in the polymer industry. Nevertheless, the analysis of the aged effects clearly indicates the need for a definition of new strategies to enhance the efficiency of the sensitization process of the Ln3+ intraionic emission.

Plasmon-enhanced luminescence of rare-earth ions by gold and silver nanoparticles in PMMA

In this work plasmon resonances in gold and silver nanoparticles to enhance luminescent properties of PMMA doped with different rare-earth ions densities are investigated. For this purpose, a rate equation model based on the Förster theory is used. This model describes the luminescent dynamics between donor and acceptor rare earth ions and considers the electric field changes due to metallic nanoparticles. The MNPBEM Matlab toolbox is used to calculate the electric field in our system, including the electric field of the excitation and the electric field induced by the nanoparticle. The results show that rare earth luminescence can be increased in the proximity of spherical gold and silver nanoparticles due to the change in the optical field and the transition rates. Accordingly, the rate equation model proposed here explains the increase in luminescent emission when the dopants interact with the metal nanoparticles. Additionally, the calculations allow to find the optimal conditions of nanoparticle size and excitation wavelength to achieve the maximum amplification of optical field and, therefore, the maximum amplification of the emission of rare earth ions.

Exploring the impact of structure-sensitive factors on luminescence and absorption of trivalent rare earth embedded in disordered crystal structures of Gd3Ga5O12–Gd3Al5O12 and Lu2SiO5–Gd2SiO5 solid solution crystals

High resolution spectra of optical absorption and luminescence were recorded at temperatures below 6 K for Pr3+, Sm3+, Er3+, and Yb3+ ions embedded in Gd3Ga5O12–Gd3Al5O12 and Lu2SiO5–Gd2SiO5 solid solution crystals fabricated by the Czochralski method. Detailed analysis of acquired spectral bands related to transitions between the lowest energy components (0–0) lines of initial and terminal multiplets provided quantitative information regarding bandwidths, peak transition energies and number of band components. Combination of these data with structural peculiarities of the hosts made it possible to propose the interpretation of optical interaction in systems under study.

A multicenter lanthanide coordination polymer for ratiometric pesticide monitoring

Due to the increasing environmental and public health issues, efficient recognition and detection approaches for pesticides has attracted concerns worldwide. Herein, luminescent lanthanide coordination polymers (Ln-CPs) with tunable emissive centers are synthesized by simultaneously doping controlled amounts of Eu3+, Tb3+ and Gd3+ into the same framework. An optimized version of the multicenter Ln-CP (Eu0.048Tb0.052Gd0.9-CPBA) is prepared for 2D fingerprint recognizing pesticides. Additionally, the special “turn-on” response of ligand-based emission and “turn-off” response of Eu3+/Tb3+-based emission toward glyphosate (GPS) endow Eu0.048Tb0.052Gd0.9-CPBA with highly selective ratiometric sensing capability. The rapid, sensitive, selective and recyclable sensor for GPS provides a facile and adjustable synthetic approach for the development of multicenter ratiometric sensors. The real-time monitoring platform not only improves the sensitivity and selectivity but also holds great environmental significance.

Impact of chiral ligands on photophysical and electro-optical properties of β-diketonate europium complexes in circularly polarized OLEDs.

Luminescent lanthanide complexes exhibiting chiroptical properties are attracting attention for their application in chiral optoelectronics and photonics, thanks to their unique optical properties, allied to intraconfigurational f-f transitions, which are generally electric-dipole-forbidden and can be magnetic dipole-allowed, which in an appropriate environment can lead to high dissymmetry factors and strong luminescence, in the presence of an antenna ligand. However, because luminescence and chiroptical activity are governed by different selection rules, their successful application in commonly used technologies is still an expectation. Recently, we showed that europium complexes bearing β-diketonates acting as luminescence sensitizers, and chiral bis(oxazolinyl) pyridine derivatives as the chirality inducer, reasonably perform in circularly polarized (CP) organic light-emitting devices (OLEDs). Indeed, europium β-diketonate complexes are an interesting molecular starting point, given their strong luminescence and their established use in conventional (i.e., nonpolarized) OLEDs. In this context, it is interesting to investigate in detail the impact of the ancillary chiral ligand on complex emission properties and the performances of corresponding CP-OLEDs. Here we show that, by incorporating the chiral compound as emitter in the architecture of solution processed electroluminescent devices, CP emission is retained, and the efficiency of the device is comparable to reference unpolarized OLED. The observed remarkable dissymmetry values strengthen the position of chiral lanthanide-OLEDs as CP-emitting devices.

Facile Assembly Strategy for Luminescent Lanthanide Nanoparticles with Antibacterial Activity Using Aggregation-Inducing Emission Polymers

Aggregation-induced emission (AIE) polymers that permit both outstanding biocompatibility and excellent bioimaging properties are a highly appealing means by which to meet the demands of new-generation biosensors. However, dozens of problems puzzling researchers include the poor water solubility, lack of self-assembling ability, and a luminescence quenching phenomenon cannot be solved thoroughly until now, which limits the further application of AIE polymers. Herein, we developed new polymers with terminal-functionalized 2,6-dipiclinic acid (DPA) groups. The biocompatible polymers with intrinsic AIE properties can form supramolecular aggregates. Furthermore, the polymers can coordinate with Eu3+ to contribute dynamic complexing networks to remain luminescent in water solution. More importantly, all the supramolecular polymers and luminescent lanthanide nanoparticles are demonstrated with satisfying antifouling and antibacterial properties, which provides a bright prospect for medical antifouling coatings and biosensors.

Fabrication of Eu3+-dipicolinic acid complex functionalized nanoSiO2 composites and their Langmuir-Blodgett films as visual fluorescence probe for tetracycline and oxytetracycline

Luminescent lanthanide complexes have attracted growing attention in the visual probe for detection of toxic and dangerous materials. Here, we report an interfacial self-assembly of Eu3+-dipicolinic acid complex functionalized nanoSiO2 composites (nanoSiO2PyTAEu) and their Langmuir-Blodgett (LB) films as well as the luminescent probes to tetracycline (TC) and oxytetracycline (OTC). Organic linkers and Eu3+ ions are anchored on the hydrophilic nanoSiO2 particles’ surface via the surface silanization, substitution and coordination reactions, so they exhibit high stability and well distribution in the solutions and LB films. Both the nanocomposites of nanoSiO2PyTAEu and their LB films show strong and enhanced red fluorescence emissions of Eu3+ ions when they are immersed in the aqueous TC solution, with the detection limit to 3.3 and 0.91 nmol/L, respectively. The present nanocomposites and LB films have the advantages of high sensitivity and stability.

Luminescent lanthanide metallogel as a sensor array to efficiently discriminate various saccharides

By loading 3-nitrophenylboronic acid (3-NPBA) which can react with saccharide, the fluorescence lanthanide metallogel can emit fluorescence signals at 398, 490 and 546 nm. Thus, a three-dimensional responsive fluorescence sensor array (H6L/Tb3+/3-NPBA) was prepared to recognize saccharides. Due to the typical pH responsiveness of the fluorescence behavior of the sensor array, it is suitable for H+ generation systems, such as the reaction of 3-NPBA with cis-diol of saccharides. The response of fluorescence signal intensity to 15 different saccharides was measured, and the qualitative discrimination of those saccharides was investigated by principal component analysis (PCA). The results showed that PCA elliptic clusters did not overlap, even though the concentration of the saccharides was as low as 3 mmol/L, and the discrimination rate of each saccharide reached 100 %. The “double blind” experiment indicated that the sensor system has good ability to discriminate unknown component samples. Furthermore, the discrimination rate of fructose/glucose mixtures with varied mole ratios also reached 100 %. The method of Euclidean distance confirmed that there was a good linear correlation between PCA elliptic clusters of fructose and its concentration, which could realize the quantitative analysis of fructose. The results of PCA indicated that the elliptic clusters of tea drinks with various sugars and commercially available sugar-containing drinks were loosely distributed, which further confirmed that the sensor system can effectively discriminate the saccharides under the interference of complex components.

Luminescent Lanthanide Complexes for Effective Oxygen-Sensing and Singlet Oxygen Generation.

Oxygen quantification using luminescence has attracted considerable attention in various fields, including environmental monitoring and clinical analysis. Among the reported luminophores, trivalent lanthanide complexes have displayed characteristic narrow emission bands with high brightness. This bright emission is based on photo-sensitized energy transfer via organic triplet states. The organic triplet states in lanthanide complexes effectively react with the triplet oxygen, enabling oxygen quantification by lanthanide luminescence. Some TbIII and EuIII complexes with slow deactivation processes have also formed the excited state equilibrium, thus resulting in the emission-lifetime based oxygen sensing property. The combination of TbIII /EuIII emission, EuIII /SmIII emission, EuIII /ligand phosphorescence, and ligand fluorescence/ligand phosphorescence provide the ratiometric oxygen-sensing properties. Moreover, the reaction generates singlet oxygen species which exhibit numerous applications in the photo-medical field. The ligands with large π-conjugated aromatic systems, such as porphyrin, phthalocyanine, and polyaromatic compounds, induces highly efficient oxygen generation. The combination of effective luminescence with singlet-oxygen generation by the lanthanide complexes render them suitable for photo-driven theranostics. This review summarizes the research progress of lanthanide complexes with efficient oxygen-sensing and singlet-oxygen generation properties.

Shape memory luminescent cellulose/chitosan hydrogel for high sensitive detection of formaldehyde

Hybrid hydrogels containing biomacromolecules have been widely used in sensors, fluorescent probes, and other fields due to their high biocompatibility and nontoxicity. In this paper, tough hydrogels with interconnected macro-pores have been fabricated by freeze-induced chemical cross-linking of microfibrillated cellulose (MFC) and organic modified chitosan (CS). In this hydrogel materials, three-dimensional networks were formed by abundant hydrogen bonds and chemical cross-linking. Luminescent lanthanide complexes were covalently bonded to the hydrogel networks through coordination of Eu3+ ions with 2, 3-pyridine dicarboxylic acid modified chitosan. The luminescence of hydrogel materials was further improved by the replacement of coordination water with 2-thiophenyltrifluoroacetone (TTA). The prepared hydrogels showed excellent shape memory properties both under water and in air. The stress of the hybrid hydrogel at 80 % strain can reach 159 kPa, which is much higher than that of the traditional microfibrillated cellulose-based hydrogels. The obtained luminescent hybrid hydrogels exhibited an excellent fluorescence detection effect on formaldehyde. The detection limit for formaldehyde is 45.7 ppb, which is much lower than the WHO standard (80 ppb for indoor air). The novel, facile preparing procedure may extend the potential applications of hybrid lanthanide luminescent hydrogel as fluorescence probes for pollution monitoring, especially for formaldehyde and other organic aldehydes.