Application of Optical Hydrogels in Environmental Sensing

The ever‐increasing complexity of environmental pollutants urgently warrants the development of new detection technologies. Sensors based on the optical properties of hydrogels enabling fast and easy in situ detection are attracting increasing attention. In this paper, the data from 138 papers about different optical hydrogels (OHs) are extracted for statistical analysis. The detection performance and potential of various types of OHs in different environmental pollutant detection scenarios were evaluated and compared to those obtained using the standard detection method. Based on this analysis, the target recognition and sensing mechanisms of two main types of OHs are reviewed and discussed: photonic crystal hydrogels (PCHs) and fluorescent hydrogels (FHs). For PCHs, the environmental stimulus response, target receptors, inverse opal structures, and molecular imprinting techniques related to PCHs are reviewed and summarized. Furthermore, the different types of fluorophores (i.e., compound probes, biomacromolecules, quantum dots, and luminescent microbes) of FHs are discussed. Finally, the potential academic research directions to address the challenges of applying and developing OHs in environmental sensing are proposed, including the fusion of various OHs, introduction of the latest technologies in various fields to the construction of OHs, and development of multifunctional sensor arrays.

The ever-increasing complexity of environmental pollutants urgently warrants the development of new detection technologies.Sensors based on the optical properties of hydrogels enabling fast and easy in situ detection are attracting increasing attention.In this paper, the data from 138 papers about different optical hydrogels (OHs) are extracted for statistical analysis.The detection performance and potential of various types of OHs in different environmental pollutant detection scenarios were evaluated and compared to those obtained using the standard detection method.Based on this analysis, the target recognition and sensing mechanisms of two main types of OHs are reviewed and discussed: photonic crystal hydrogels (PCHs) and fluorescent hydrogels (FHs).For PCHs, the environmental stimulus response, target receptors, inverse opal structures, and molecular imprinting techniques related to PCHs are reviewed and summarized.Furthermore, the different types of fluorophores (i.e., compound probes, biomacromolecules, quantum dots, and luminescent microbes) of FHs are discussed.Finally, the potential academic research directions to address the challenges of applying and developing OHs in environmental sensing are proposed, including the fusion of various OHs, introduction of the latest technologies in various fields to the construction of OHs, and development of multifunctional sensor arrays.
researchers have seen the potential of OHs and applied them to pollutant detection. [6,22]Because hydrogels contain a high amount of moisture, most OHs are well-suited for quickly and inexpensively detecting water pollutants on-site.However, researchers have also developed some OHs that can detect pollutants in complex soil environments or gaseous pollutants that are more challenging to detect.Due to the intricate connection between the environment, microorganisms, and pollutants, researchers have also developed OHs that can detect environmental microorganisms.Even so, OH sensor application studies still concentrate on biological and medical testing. [23,24]Therefore, there is currently a lack of reviews and perspective studies on the applications of OHs in environmental sensing, particularly on the feasibility and design strategies for implementing OHs in the practical detection of environmental pollutants.
Because OH research is far less active in environmental sensing than in the medical field, it is unclear how to determine the types of OHs that are most suitable when targeting specific pollutants.To address this, we first chose 138 papers related to OHs that are representative and performed a statistical analysis.The types of pollutants that could be detected by different OHs, the detection sensitivity, and the relationship between different OHs are discussed in (Chapter 2).Based on these 138 papers, Chapter 3 and 4 further review photonic crystal hydrogels (PCHs) and fluorescent hydrogels (FHs), two main types of OHs, respectively.They also detail the types of target pollutants (including environmental microorganisms) that they can detect and their sensing mechanisms.In the next section, we briefly supplemented our review with discussion of other types of OHs used for environmental sensing.Based on this analysis, we then discussed the relationship between different OHs, and the application potential of various types of OHs in the field of pollutant detection.Some future research directions for intelligent, high-sensitivity, and rapid OH-based pollutant detection technologies are proposed.Table 1 lists all the abbreviations used in this review.Table 2 lists the beneficial properties of typical monomers for OH construction, asnd the review structure is shown in Figure 2.

Concept and Performance Analysis of Different Optical Sensors
The most significant advantages of OHs are portability, rapidity, and low detection cost.However, for any detection technique, the detection sensitivity and variety of detectable objects are the most critical metrics.Therefore, we identified 138 papers that serve as representations of the detection of various environmental pollutants using OHs.These OHs were divided into photonic crystal hydrogels (PCHs) and fluorescent hydrogels (FHs) for further classification and statistics.

The Concept of Photonic Crystal Hydrogels
Light can generate diffraction or surface plasmon resonance (SPR) at the interface of different media.The regular arrangement of different media can form a photonic band gap and make photons exhibit periodic changes in light intensity or wavelength. [25,26]The photonic structures (gratings) of PCHs comprise regular arrangements between hydrogel matrixes and different block copolymers or polyelectrolyte layers.Their specificity is achieved by embedding receptors [27,28] or building specific response structures.When the trigger conditions are met, PCHs have two response mechanisms: 1 PCHs made from functional hydrogels respond to physical (e.g., temperature, osmotic pressure, and magnetic field) or chemical (e.g., pH and salinity) stimuli.The stimuli cause changes in hydrogel volume and gratings.Thus, PCHs enable the detection of tested objects in the detection environment via wavelength monitoring, SPR, and even apparent color changes (Figure 3a). [29,30] Analytical targets bind to specific receptors on modified PCHs, transforming the electrostatic forces in the hydrogel grids.This transformation causes the expansion or contraction of the PCHs, changing the spacing of their internal gratings.The analytical targets can be accurately detected by monitoring wavelength variations, SPR, and even apparent color changes (Figure 3b). [31,32]modified PCHs can detect some environmental indicators, such as temperature, humidity, and pH.Incorporating specific chemical or biomolecular receptors in PCHs enables the detection of almost all common substances.Moreover, constructing certain unique structures or introducing new technologies can further improve the detection accuracy of PCHs.PCHs have promising practical applications in disease diagnostics, biopharmaceuticals, pathogen screening, toxicity monitoring, and food security. [27]Recently, they have been gradually introduced to environmental sensing.

Concept of Fluorescent Hydrogel
Fluorescence is a typical photoluminescence phenomenon.When fluorophores absorb high-energy light, their electrons enter the excited state from the ground state, then release energy in the form of light emissions. [33,34]Fluorescence-based detection technologies have the advantages of high sensitivity, high selectivity, and convenience. [35]In addition, hydrogels have porous junctions, excellent biocompatibility, and highly controllable physicochemical properties.Introducing fluorophores into hydrogel matrices produces fluorescent hydrogels (FHs), which combine the advantages of both materials.FHs have broad application potential in bioimaging, medical diagnosis, pollutant detection, and gene technologies. [35,36]When analytical targets contact FHs, the fluorescent modules respond through five main pathways: [37,38] photoinduced electron transfer (PET), [39] intramolecular charge transfer (ICT), [40] excited-state intramolecular proton transfer (ESIPT), [41] fluorescence resonance energy transfer (FRET), [42] and excimer/ exciplex interaction (Table 3). [43]

Detection Performance Analysis
Firstly, we conducted a statistical analysis on the types of pollutants detected by both types of OHs.The result is shown in Figure 4a.In the selected papers, OHs can detect heavy metals, anions, antibiotics, pesticides, small organics, gas molecules, environmental parameters, and microorganisms in the environment.Additionally, some other report show that OHs can detect much more targets such as anesthetics in fish, [44] metabolites of microorganisms in complex media, [45] melamine in milk, [46] and catecholamines in serum. [47]These examples indicate that OHs have broad potential for detecting increasingly complex contaminants in the environment.
To further explore the sensitivities of different OHs in different kinds of contaminant detection, we extracted the LODs of OHs for different targets from 138 papers and compared them with the reported LODs of standard detection methods (Table 4).We calculated the normalized sensitivities as Q = L o /L s , where, L o represents the LOD for a certain pollutant using a certain OH, L s represents the LOD for this pollutant with the standard method from China, the USA, Australia, the UK, or the International Organization for Standardization.Q < 1 indicates more sensitivity than the standard method, and vice versa.We processed the Q values with the TRIMMEAN (percent = 22%) function to reduce the impacts of outliers.We transformed normalized data into a natural logarithmic function form and plotted the results as a heatmap (Figure 4b).
Overall, the PCHs had higher normalized sensitivities for pollutant detection than FHs because all the PCHs are nanoscale grating structures that can reflect any tiny change caused by the targets.Additionally, the target receptors can be directly grafted on the gratings by the hydrogel backbones.PCHs are both target recognition and signal expression units, without the need for "bridges" for signal transmission.However, in FHs, the fluorescence and target recognition units usually exist separately, and the signal needs to be converted and transmitted via one or more steps.Therefore, the density, uniformity, coordination of fluorophores and acceptors, and complexity of signal transmission may all affect the sensitivity of FHs.
PCHs detected more types of metal ions than FHs did, and compound RBPCHs (C-RBPCHs) and MIPCHs showed extremely high detection sensitivities for metal ions.These disparities come from the abundance of metal chelators, the fact that even an extremely small amount of chelation causes obvious changes in hydrogel properties, and the advantages of molecular (ionic) imprinting technologies.Since metal ions can directly quench the fluorescence of some fluorophores, FHs for metal ion detection are easy to construct.However, these FHs lack specificity, and their detection sensitivities are relatively low.Therefore, they are mainly used as the indicator of heavy metals adsorbent materials.Compared to cations (metal ions), FHs can detect even more anions.As anions are electron-deficient groups, they can directly induce electron transfer and affect the electronic arrangement of fluorophores.These phenomena affect the fluorescence properties, particularly those of fluorophores.Therefore, CFHs also have the highest sensitivity for anion detection.
Considering their potential hazards, complex organic compounds, such as antibiotics, pesticides, and poisons, require high-precision linear quantitative detection.Therefore, researchers prefer to use FHs to detect them.However at present, FHs are not as sensitive as IOPCHs, MIPCHs, and biomolecule RBPCHs (B-RBPCHs) for these complex organic compounds.As for small organic molecules, they directly react with the functionalized hydrogel matrixes of PCHs, affecting their diffraction wavelengths; this is a simple detection strategy.However, FHs (and particularly the emerging GQDFHs) had far stronger detection sensitivities for small organic compounds than PCHs.The extremely rich organic groups of GQDs play an essential role in the detection of small molecular organics by GQDFHs.At last, Gas molecules can replace the medium in the pores of hydrogel matrixes, directly changing the refractive index of PCHs; thus, the vast majority of OHs that can detect gas molecules are PCHs.However, the detection sensitivities of OHs to gas molecules need further improvement.
In microbiological detection, FHs were not used as the main body of detection but assisted other detection methods, such as RT-LAMP and PCR.This special mechanism allows FHs to detect more kinds of microorganisms than PCHs, [48][49][50][51] and BAFHs also have high sensitivities for microbiological detection.Furthermore, molecular imprinting technologies can create highly specific cavities using microorganisms as templates; thus, MIPCHs also exhibit high sensitivities for microorganisms.Because there were no LODs reported, we plotted the detection frequency in the heatmap for environmental parametersdetecting OHs instead.Owing to the direct environmental stimuli-responsiveness of hydrogels, DRPCHs are undoubtedly the most suitable OHs for environmental pollutant detection.
Next, we will discuss the detection mechanisms of each type of OH for various targets, as well as the pros and cons of different hydrogel matrices used in OHs.The subsequent discussion is primarily based on the 138 selected papers, but to provide a more comprehensive discussion, there are also additional papers included as supplementary cases.

Directly Responsive Photonic Crystals Hydrogels
Hydrogels that are responsive to one or more environmental stimuli can act as matrixes to build photonic crystal structures directly.The formed sensors are called directly responsive photonic crystal hydrogels (DRPCHs), and are usually be used to detect humidity, pH, and temperature in the environment.Acrylamide glycol hydrogel and hydrogel synthesized from acrylamide (AAm) and acrylic acid (AAc) have high hygroscopicity; they shrink and swell at low and high humidity levels, respectively (Figure 5a).When constructed into regular photonic crystal structures, the changes in refractive index and apparent color induced by transformations in the hydrogel lattice constant can be used to detect environmental humidity (Figure 5b). [52,53]Additionally, PCHs synthesized from 2-hydroxyethyl methacrylate (HEMA) and AAc respond to pH.Monitoring their swelling rate enables the real-time detection of ambient pH [54] (Figure 5c).PCHs constructed from the temperatureresponsive polymer N-isopropylacrylamide (NIPAm) can also detect temperature changes. [55]RPCHs, which are responsive to multiple environmental stimuli, are a notable research topic.They are formed by aggregating functional monomers with different stimuli-responsive properties.For instance, DRPCHs constructed from temperature-responsive N,N 0 -methylene acrylamide (BisAA) monomers and pH-responsive N,N,N,N-tetraethyl ethylenediamine (TEMED) monomers can simultaneously detect pH and temperature. [56]In DRPCHs formed from temperature-responsive NIPAm monomers and glycidyl methacrylate (GAM) via a double crosslinking system, the primary amine groups of NIPAm and epoxy groups of GAM form a stable network structure.Besides temperature, these DRPCHs can detect alcohols via the destruction of the amineepoxy bonds of the hydrogel by alcohols (Figure 5d,e). [57]nterestingly, some researchers have proposed a modular DRPCH synthetic method that enables polymerization of functional monomers around nanomagnetic particles (Fe 3 O 4 @polyvinylpyrrolidone) using a H-bond-guided template.These particles then aggregate under a magnetic field to form submicron photonic crystal chains (Figure 6a). [58]his modular preparation approach is universal, so these DRPCHs can aggregate monomers with various functions to respond to various All data came from the most relevant documents on the Web of Science Core Collection in the past 10 years, and the extracted contents include the title, keywords, abstract, and references of papers.The nodes represent keywords, the size of nodes indicates the frequency of the keyword in the journal, and the connection between the nodes represents the interconnection between the keywords.The keywords with the highest frequency show in the form of tags.The smaller the serial number, the higher the frequency, and #0 is the search keyword.
environmental stimuli, and their submicron structure enables them to monitor the microenvironment.Although only pH-responsive chains have been produced through this method, the modularity and submicron structure aspects are particularly promising.
Some researchers have reported that besides environmental parameters, DRPCHs can potentially detect some microorganisms.For example, DRPCHs constructed with pH-sensitive hydrogels (HEMA-AAm polymer) shrink and expand depending on the pH, enabling the detection of acidic products from bacterial glucose metabolism (Figure 6b). [59]Furthermore, gelatin-based DRPCHs can be decomposed by gelatinase-producing bacteria, such as Pseudomonas aeruginosa.The swelling and decrease in crosslinking degree of DRPCHs under decomposed gelatin enable bacterial detection. [60]Because carbohydrate and protein metabolism are widespread in microorganisms, DRPCHs for bacterial detection have no selectivity, along with low detection sensitivity.
Overall, DRPCHs can hardly be used for the specific detection of pollutants.However, the stimuli-responsive capabilities of DRPCHs are the basis of most other types of PCHs and a critical bridge between object recognition and optical response in PCHs.The receptor-bound photonic crystals hydrogels (RBPCHs) can be obtained by introducing compounds to the hydrogel matrices that specifically recognize pollutants.Molecules contacting these sensors bind to the targets, affecting the hydrogel structure and thereby changing the wavelength of incident light and plasma.

Photonic Crystals Hydrogels with Compounds Receptors
Because many compounds used to make RBPCHs have metal ion-chelating properties, these hydrogels have been used for environmental heavy metal detection.For example, 3arylamidopropyl-trimethylammonium chloride (ATAC) is a complexant for Cr 6+ with hydrogel-forming properties. [61,62]Although aggregating ATAC with BisAA yields RBPCHs with good swelling properties, the formed complexes between Cr 6+ and the tertiary amine groups of ATCT can affect the density and osmotic pressure of the RBPCHs.Cr 6+ can thus be selectively detected by Bragg diffraction [61] or SPR (Figure 7a). [62]As pH-sensitive anionic hydrogels, ATAC-based RBPCHs require a continually maintained neutral test environment (pH = 7).
The bidentate chelator 8-hydroxyquinoline (8-HQ) possessing two metal coordination groups (O covalent bond and N coordination bond) can chelate divalent and trivalent metal ions such as Zn 2+ , Cd 2+ , Mg 2+ , Li 2+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Al 3+ , and Fe 3+ .Each metal ion chelated to 8-HQ has specific O-metal and N-metal bond strengths (Figure 7b).Specific bonding between metal ions and coordination groups can change the grating structure in 8-HQ-based RBPCHs and modify their wavelength.Therefore, 8-HQ-based RBPCHs can identify and quantify different metal ions based on the change of wavelength and interference images. [63]This complexation occurs even at extremely low metal ion concentrations, and the formed complexes  7 of 26 will continue to chelate other 8-HQ molecules; [64] thus, RBPCHs containing 8-HQ are extremely sensitive to metal ions.However, 8-HQ cannot form a hydrogel alone.They require the use of hydrogel matrixes with excellent swelling properties to construct RBPCHs, such as poly(vinyl alcohol) (PVA), AAc, and polystyrene (PS). [63,65]owever, similarly to ATCT-based RBPCHs, 8-HQ-based RBPCHs rely on changes in osmotic pressure to detect targets.Therefore, the assay needs to be performed in a solution with certain osmotic pressure.Similar to ATAC and 8-HQ-based RBPCHs, the RBPCHs made from thiourea [66] or dithiothreitol [67] can specifically recognize Cd 2+ and Hg 2+ , respectively.
Instead of integrating ligand compounds directly in PCHs, grafting the target-interacting groups on hydrogel monomers can be a better strategy.For instance, RBPCHs containing NIPAm with crown ether groups as the side chains (CE-NIPAm) have a unique network structure.Their side chains can chelate Pb 2+ [68,69] and Be 2+ , [70] and the main chains swell under the effect of electrostatic repulsion between the charged complexes and NIPAm backbone (Figure 7c). [71]Besides Fluorophores (F) and acceptors (A) are linked by a chemical rigid bridge (B) to form an F-B-A system.Normally, excitation light causes electron transfer from A to F and inhibits fluorescence.When the target interacts with A, electron transfer is inhibited or suppressed, allowing the fluorescence of F recovers [39] Target Acceptors hydrogel swelling, metal-crown ether group polymers enhance the sensitivity of CE-NIPAM-based RBPCHs by affecting the osmotic pressure, as previously described.Therefore, the detection limit of CE-NIPAM for Pb 2+ was as low as 10 À9 M. The osmotic pressure of the hydrogel depends on the difference in ion concentration inside and outside the hydrogel; thus, this type of RBPCH needs to be kept in a solution with specific ionic concentration to achieve detection.

Chemical
Besides metal ions, RBPCHs can detect certain small molecules.The amide groups of AAm undergo a methylolation reaction with aldehydes in Na 2 CO 3 solution, changing the chemical structure of the hydrogel.Thus, the RBPCHs constructed from AAm hydrogel can detect aldehydes via the apparent color changes caused by structural changes. [72]hen the RBPCHs are built from amine-rich hydrogel monomers, the amino groups from ionic liquids contact CO 2 gas and swell the hydrogel matrix.CO 2 gas can then be detected by monitoring the refractive index changes of the RBPCHs. [73]

Photonic Crystals Hydrogels Combined with Biomacromolecule Receptors
It is difficult for RBPCHs with simple compound receptors to detect complex compounds or organisms.For these applications, enzymes, functional proteins, nucleotides, and DNA fragments are better receptor choices.A study with penicillin as the tested target [74] directly introduced penicillinases as acceptors in AAm&AAc hydrogels.Penicillinases then hydrolyzed penicillin G to produce penicilloic acid.Electrostatic repulsion between the carboxylic acid groups of the hydrogel matrixes decreased with increasing pH.Periodic shrinkage of the RBPCHs under the change in osmotic pressure afforded penicillin G detection at very low concentrations (Figure 8a).Likewise, butyrylcholinesterase (BuChE) could decompose the sarin reagent (a highly toxic nerve agent) and generate hydrofluoric acid.When BuChE was immobilized into the AAm hydrogel matrix owing to an amide bond condensation reagent, the resulting RBPCHs could detect sarin with great sensitivity. [75]or functional proteins, RBPCHs can exploit the well-known antibody-antigen recognition relation to detect microorganisms.Taking the rotavirus as an example, staphylococcal protein A can bind to immunoglobulins at non-antigen-binding sites.Staphylococcal protein A could be immobilized on a polyacrylamide (PAM) hydrogel with nanopores using silanol condensing agents.Subsequently, monoclonal rotavirus antibodies could be immobilized in the nanopores of this RBPCH to capture rotavirus.When the virus filled the nanopores, the refractive index of the RBPCH changed, enabling the detection of rotavirus (Figure 8b). [76]Additionally, Escherichia coli could be indirectly detected by immobilizing polyclonal E. coli antibodies in RBPCHs using an immobilized biotin-streptavidin system.As a signal amplification system, each streptavidin molecule could bind four biotinylated antibodies. [77]High-density loading of antibodies in hydrogels endows RBPCHs with high sensitivity for bacterial detection.Besides antibodies, RBPCHs directly constructed by pure lectin concanavalin A could detect Candida albicans owing to the polymer effect between the glycoprotein mannan on the surface of C. albicans cells and concanavalin A. [78] Glycated protein-functionalized RBPCHs can immobilize glycosyl groups on amino acid groups via nonenzymatic reactions.Hence, RBPCHs can detect Gram-negative bacteria by combining glycated proteins with lipopolysaccharides (D-glucosamine disaccharide, D-glucosamine, etc.) on the surface of bacteria (Figure 8c). [79]oreover, to improve detection accuracy, some researchers have constructed ternary complex systems in RBPCHs. [80,81]For example, RBPCHs based on the tryptophan (Trp)-Zn(II)-ciprofloxacin (CIP) ternary complex [80] recognize and detect CIP owing to Zn(II)-CIP complex formation, which changes the osmotic pressure of RBPCHs.The amino and carboxyl groups of Trp were connected on the hydrogel backbone, and the exposed indolyl groups bound to Zn(II), forming a local positively charged region.When CIP was introduced as the target, Trp amplified the signal by exacerbating the osmotic pressure change (counterion effect; Figure 8d), affording a CIP detection limit of 10 À10 M. In contrast, in the tetracycline-Cu(II)-glycine (Gly) ternary complex system, the RBPCHs prepared by immobilizing tetracycline in the PAM matrix could detect Gly with high sensitivity. [81]n summary, almost any biological or chemical reaction that can directly or indirectly cause structural changes in hydrogels can be used to build RBPCHs.The substances involved in the reaction can be regarded as detection targets and their receptors.Therefore, RBPCHs have a wide scope of development in pollutant detection.Any substance may become a receptor for a pollutant as long as a chain of "detection Energy Environ.Mater.2024, 7, e12646 target-receptor-hydrogel structural change" can exist.This also means that RBPCHs could theoretically be constructed for the selective detection of any pollutant and even organisms.

Construction of Photonic Crystals Hydrogels by Inverse Opal Structures
PCHs come in many different structures, but the inverse opal structure is the most widely used (Table 5).PCHs with the inverse opal structure are called inverse opal-structured photonic crystal hydrogels (IOPCHs).IOPCHs use regularly arranged nanoparticles as templates, and the voids are filled with high-refractive-index hydrogel matrixes.Removing the templates yields a symmetric and periodic structure with high porosity (Figure 9a). [82]When the refractive index of the medium filling the inverse opal structure reaches a certain value, complete photonic band gaps (PBGs) appear.Analytical targets can change the PBGs, which can be detected by monitoring diffraction wavelength.Therefore, the most significant advantage of IOPCHs is that the PBGs can be easily controlled by adjusting the size and spacing of the templates.
In the preparation process, the most significant components of IOPCHs are templates, functional monomers, crosslinkers, and binding sites.PS and its functionalized particles are commonly used as templates (Table 5).Because PS can form highly ordered particle arrays via self-  assembly at the gas-liquid interface, the array has a huge surface area and good crystal quality.More importantly, the size of PS (5 nm-5 mm) and space between the spheres can be easily tuned by plasmonic resonance; [83] thus, the PBGs of PS-based IOPCHs are also easy to control.Besides, most IOPCHs use methacrylic acid (MAA), AAm, AAc, and their derivates as functional monomers.Because these monomers can act as H-bond donors and acceptors and show good adaptability to ionic interactions, they are called "universal" functional monomers. [84]s the crosslinking agent for functional monomers, BisAA enables hydrogels to undergo significant volume change under physical or chemical stimuli, which is key to the Bragg diffraction response during detection. [85]ditionally, some IOPCHs without any receptor can also selectively detect targets.Replacing the filling medium of IOPCHs with tested targets with different refractive indexes transforms their PBGs.For example, based on the differences in hydrogel swelling rates in different solvents, a PAM-based IOPCH encapsulating TiO 2 rapidly quantified different solvents using the PBG change. [86]When the filling medium is air, the IOPCHs can detect solvents and gaseous molecules.Gaseous molecules such as H 2 O 2, [87] toluene vapor, [88] acetone vapor, and ethanol vapor [89] can be rapidly adsorbed and diffused in the special porous structure of IOPCHs, and the PBGs of the IOPCHs migrate directionally when the targets replace the media (Figure 9b).Particularly, highly polar anions (e.g., SCN À ) can form strongly hydrated ions.Hence, Figure 5.The response modes of directly responsive photonic crystals hydrogels to environmental parameters.a) The matrices of directly responsive photonic crystals hydrogels shrinkage and expansion due to changes in environmental parameters (humidity). [52]Effects of apparent colors and reflectance spectrums of directly responsive photonic crystals hydrogels caused by changes in environmental parameters b) humidity [53] and c) pH values [54] .Variations of the multiatomic-responsive optical hydrogel in resonance spectrum at d) different ethanol concentrations and e) temperatures. [57]gure 6.Mechanisms of two typical direct-response photonic crystal hydrogels.a) Synthetic mechanism and characterization of submicron photonic crystals via the H-bond-guided template method, taking the synthesis of pH-responsive directly responsive photonic crystal hydrogels as an example. [58]b) Illustration of the fabrication and responsive mechanism of directly responsive photonic crystal hydrogels for bacterial diagnosis and disinfection. [59]ergy Environ.Mater.2024, 7, e12646 using water as a medium in IOPCHs inhibits H-bond formation between the hydrophilic group of IOPCHs and water, causing expansion of the material and enabling SCN À detection. [90]nverse opal structures can easily form dense nanophotonic crystals and synchronously modify various specific receptors by self-assembly.The ample nanopores allow high receptor density and target capture capability.Therefore, the detection sensitivity of IOPCHs is usually higher than that of PCHs with other structures.

Application of Molecular Imprinting Technique in Photonic Crystals Hydrogels
Molecular imprinting-based PCHs (MIPCHs) can detect targets without corresponding receptors with high recognition accuracy.The targets serve as templates during the assembly of the functional monomers via covalent/noncovalent interactions.Removal of the imprinted molecules leaves a "blot," conferring MIPCHs with the ability to selectively detect the corresponding objects (Figure 10a). [91,92]These imprinted molecules can be metal ions, [93,94] compounds, [95][96][97][98] and even organisms. [79]Ions cannot usually be direct template molecules for MIPCHs and imprinting them into the hydrogel grid requires the assistance of certain complexes.For instance, in 5 0 -O-acryloyl-2 0 ,3 0 -Oisopropylidene guanosine (APG)-modified MIPCHs, Sr 2+ induces the formation of planar G-quartets by APG units.After removing the Sr 2+ templates, the cavities remaining in the hydrogel network provide accurate binding sites for Sr 2+ .Upon reexposure to Sr 2+ , APG-modified MIPCHs shrink due to the weakened electrostatic repulsion between the four O atoms in the relaxed G-quadruplex (Figure 10b). [93]Similarly, pentaethylenehexamine (PEHA)can coordinate with Pb(II) to form an imprint.MIPCHs formed from secondarily crosslinked Pb(II)-treated pentaethylenehexamine and a PAM hydrogel detected Pb(II) with high sensitivity. [94]Besides, with MIPCHs, some compounds can directly act as imprinted molecules.For example, tetracyclines used as a template molecule increased the hydrogel crosslinking degree of MIPCHs formed from AAm [95] or AAc. [97]When the imprints were secondarily bonded with tetracyclines, the MIPCHs could detect tetracyclines via changes in the hydrogel crosslinking degree (Figure 10c).
Notably, almost all MIPCHs have adopted the inverse opal structure because it allows molecular imprints can be densely and evenly imprinted in this 3D-ordered-nanochannel structure.Imprinted molecules branded with specific receptors can greatly improve the detection sensitivity of RBPCHs.In a previous report, [79] PCHs containing glycated proteins imprinted with E. coli exhibited 2.67 times in E. coli detection sensitivity, compared with the PCHs without imprinting.
All these techniques for PCHs are not independent: combining them affords better performance.In the face of increasingly complex environmental pollutants, PCHs combining multiple detection approaches may be one of the most promising development directions of OH-based environmental pollutant detection technologies.

Hydrogels Loaded with Compound Fluorophores
In compound fluorophore hydrogels (CFHs), the turn-on system comprising compound fluorophores and hydrogel matrixes is the most typical and is based on the PET principle.For instance, rhodamine fluorophore can be grafted onto oxidized agarose via a carboxyl-amine reaction, [99] and the modified oxidized agarose can be oxidatively polymerized directly with AAm from CFHs. [100]At that moment, the Figure 8.The operating principles of some special photonic crystal hydrogels.a) Recognition and response strategies of photonic crystal hydrogels to penicillin G; [74] b) Working principle of a label-free virus sensor based on an inverse opal 3D Photonic crystal hydrogels; [76] c) Construction of photonic crystal hydrogels that can recognize E. coli by glycated proteins; [79] d) The construction mechanism of ternary complexes-based photonic crystal hydrogel sensors, and how it captures ciprofloxacin compounds. [80]ergy Environ.Mater.2024, 7, e12646 formed spirocyclic lactam structure in the CFHs quenches the fluorescence.After binding to the targets (Pb 2+ and Al 3+ ), the CFHs convert to the ring-opened amide structure and generate pink (Pb 2+ ) or yellow (Al 3+ ) fluorescence (Figure 11a,b).Besides their use as a fluorescence "switch," hydrogel matrixes can condense fluorophores and amplify the detection signal by shrinkage, [99] yielding detection sensitivities of 10 À7 M and 1.5 × 10 À6 M for Pb 2+ and Al 3+ , respectively.Likewise, 3D porous frameworks with abundant H-bond donors and acceptors, constructed using sodium alginate (SA), can combine with 9anthraaldehyde via the supramolecular Cu coordination polymer Cuatda and form CFHs. The differences in electron energy levels between 9-anthraaldehyde/Cu-atda and different antibiotics (flumequine and nitrofurans) causes electron transfer between them.Thus, multiple antibiotics can be detected via fluorescence quenching or enhancement. [101]reover, the fluorescence of boron-dipyrromethene (BODIPY) is quenched when its pyridine groups are complexed with the Co core of cobaloxime.HS À can replace the pyridine group to generate HScobaloxime, restoring isolated BODIPY fluorescence (Figure 11c).Besides, polyurethane can capture and ionize H 2 S gas into HS À .Thus, CFHs comprising BODIPY, cobaloxime, and polyurethane enable the detection of gaseous H 2 S. [102] Meanwhile, when BODIPY was introduced to the symmetrical thiophene aldehyde groups and grafted on chitosan, the formed CFHs enabled Cu 2+ detection, as Cu 2+ quenched the fluorescence of the modified BODIPY (Figure 11d). [103]nother major type of CFHs are those based on the ICT principle.The electron distribution changes within BODIPY are influenced by pH changes and cause a red/blue shift of fluorescence (ICT principle).Therefore, BODIPY-based CFHs can be used to detect pH [104,105] or NH 3 . [106]Typically, morin and the Al 3+ of hydrotalcite (MR-HT) form  11e). Hence, CFHs comprising polyurethane and MR-HT can detect HPO 2À 4 and H 2 PO À 4 with high sensitivity.Additionally, polyurethane prevents fluorophore leakage, and its polarity increases the sensitivity of MR-HT to PO 3À 4 ions. [107]oreover, some CFHs detect pollutants through the ESIPT principle.For example, N- (3-(benzo[d] thiazol-2-yl)-4-(tertbutyldiphenylsilyloxy)phenyl)acetamide (BTBPA) can be isomerized to N- (3-(benzo[d]thiazol-2-yl)-4hydroxyphenyl)acetamide (BTHPA) with a longer fluorescence wavelength by cleaving the Si-O bond using F À (Figure 11f).Thus, a BTBPA-immobilized poly(N-vinyl-2pyrrolidone) hydrogel detected F À , and the strong adsorption and diffusion of F À by poly (N-vinyl-2-pyrrolidone) afforded a detection time of only 15 s. [108]Based on the same mechanism, 2-hydroxy-1-naphthaldehyde was introduced into the amino side chain of chitosan and aggregated into a CFH.This CFH achieved Cd 2+ detection as the direct bonding of Cd 2+ to the hydroxyl and imine groups of chitosan caused the tautomerism of fluorophores and quenched the fluorescence (Figure 11g). [109]luorescent pyrenes with chiral aromatic amino acid amides can bind oligo oxyethylene chains through gallic acid anchors to form CFHs without hydrogel matrixes.ClO À detection was achieved as acyl aroyl hydrazine units of the CFHs were selectively oxidized, changing the fluorescence of pyrene (Figure 11h).Changing the anchors and groups attached to the pyrenes would allow the detection of other oxidizable substances.However, this hydrolysis caused a sol reaction, which was detrimental to the stability of the CFHs. [110]Interestingly, 8-HQ, described in Section 2, is weakly fluorescent, and complexes between 8-HQ and metal ions can enhance the fluorescence by blocking the ESIPT channel. [111]Therefore, PCHs with 8-HQ as an acceptor could detect metal ions more sensitively with the assistance of a fluorescence enhancement system (Figure 11i).Different metal ions could also be identified owing to their ESIPT-blocking ability. [65]inally, some CFHs achieved pollutant detection by quenching the fluorescence of unstable complexation to ions.Therefore, some ions can be detected by repeatedly breaking and recovering unstable complexes.For example, salicylaldehyde (SD) and thiosemicarbazide (TB) can form a p-π conjugated system, with their respective unsaturated conjugated chains connected to electron-poor and electron-rich groups.The SD-TB complex fluoresces via electron transfer at specific excitation wavelengths.CFHs constructed on this basis can detect Cu 2+ via fluorescence quenching owing to the transfer of Cu 2+ electrons to the fluorophore.More importantly, Cu 2+ captured by the CFHs could be extracted under the action of chelating Figure 9. a) Preparation process of inverse opal photonic crystal hydrogels.SEM images 1, [86] 2, [87] and 3 [88] from different literatures; b) Construction of photonic crystal hydrogels that can identify gas targets based on anti-opal structures. [89]gure 10.Constructive process of molecular imprinting based photonic crystals hydrogels a) Five main types of molecular imprinting: (i) noncovalent, (ii) electrostatic/ionic, (iii) covalent, (iv) semicovalent, and (v) metal center coordination; [92] b) The planar G-quartets formed in photonic crystals hydrogels using Sr 2+ as templates; [93] c) Using tetracycline as the template molecules, specific imprints are engraved in photonic crystal hydrogels with anti-opal structures. [95]ergy Environ.Mater.2024, 7, e12646 agents (ethylenediaminetetraacetic acid) to restore fluorescence (Figure 11j).Therefore, the Cu-complexed SD-TB hydrogel could also detect compounds with metal-chelating properties. [112]Based on the same "on-off-on" mechanism, CFHs loaded with R19S fluorophores were used for the repeated detection of Hg 2+ (Figure 11k). [113]In these CFHs, along with specific functions, the hydrogel matrixes played the role of dispersing and immobilizing fluorophores and preventing contamination of the sensors.

Hydrogels Loaded with Biomacromolecules
Highly editable biomacromolecule assisted fluorophore hydrogels (BAFHs) were created to exploit the fact that embedded proteins and DNA can maintain their native structures and functions owing to the high-water content and excellent biocompatibility of hydrogels.In BAFHs, biomacromolecules usually cannot emit fluorescence directly.However, as the bridge between targets and fluorophores, they play the role of capturing targets and controlling fluorescence.This mechanism is similar to that of RBPCHs, but BAFHs are usually more scalable and editable and their sensitivity and specificity are higher.
BAFHs embedded with functionalized DNA are the most common.The fluorophore SYBR Green I emits weak fluorescence when linked to single-stranded DNA (ssDNA).Meanwhile, its fluorescence dramatically increases when it binds to double-stranded DNA (dsDNA), and the intensity is related to the amount of dsDNA (FRET principle).Hg 2+ can then promote the transformation of ssDNA to dsDNA with a hairpin structure by inducing base pairing between thymine and thymine (Figure 12a).[116] Hydrogel matrixes prevent leakage of the DNA-fluorophore system, and their strong adsorption of Hg 2+ affords highly sensitive detection.
Similarly, Cu 2+ can bind to thymine, but thymine-complexed Cu 2+ can be reduced by a reducing agent (such as ascorbic acid) into Cu 0 , which has a particle size close to single Cu atom.Cu 0 then forms Cu nanoclusters (NCs) that emit red fluorescence (FRET principle).Based on this, polythymidine DNA (PT-DNA) immobilized on agar can emit fluorescence and portably detect Cu 2+ without additional fluorophores. [117]Moreover, when introducing both Hg 2+ and Cu 2+ into BAFHs, PT-DNA tended to combine with Hg 2+ first.If a compound with stronger affinity for Hg 2+ entered the BAFHs, Cu 2+ would combine with PT-DNA, form Cu NCs, and emit fluorescence. [118]Hence, these BAFHs can be used to detect some metal chelators.Moreover, more complex PT-DNA-based BAFHs have also been reported.Tyrosinase had an enzyme-controlled quenching effect on the fluorescence of Cu NCs, and organophosphorus compounds could inhibit tyrosinase and recover fluorescence.Therefore, PT-DNA-based BAFHs combined with tyrosinase enabled pesticide detection. [119]Based on similar mechanisms, higher-cost DNA-templated fluorescent Au NCs [120] and Ag NCs [121] could also detect different pollutants.
Owing to the inherent advantages of DNA technologies for microbial detection, DNAbased BAFHs were mostly used to detect environmental pathogenic microorganisms.For viruses, one end of the target virus (e.g., HIV, HCV, SARS, or H5N1) sequences (ssDNA) was immobilized on hydrogel matrixes by amino groups, and another end was modified with a fluorophore.The modified ssDNA hybridized with fully complementary fluorescence-quenching strands to form dsDNA.When the target virus entered the BAFHs, they combined with the quenching strands, restoring fluorescence (Figure 13a). [122,123]Although reverse transcription loop-mediated isothermal amplification (RT-LAMP) is simple, scalable, widely applicable, and does not require complex microbial equipment, [124] the complexity of environmental samples often affects the detection of viruses.When RT-LAMP-based reagents were crosslinked with the microporous track-etched polycarbonate membranes through polyethylene glycol (PEG) monomers, impurities in the sample were filtered out or adsorbed by the special polycarbonate membrane. [48]This technique enabled accurate on-site detection of viruses (SARS-CoV-2).In the above two examples, the hydrogel matrixes used embedded ssDNA in a highly dispersed state to enhance nucleic acid hybridization rates.Their dense microporous structure and rich groups filtered out some impurities in the samples and enhanced the adsorbability to viruses.Therefore, the sensors exhibit excellent antifouling performance and detection efficiency. [49,50]or bacterial detection, fluorescence probes and specific bacterial (E.coli) recognition ssDNA strands were immobilized on arrayed 3D hydrogel chips.When the chips were placed in bacterial lysis fluid, the polymerase chain reaction (PCR) was started directly without the DNA template extraction procedure, and the resulting fluorescence signal was immediately detectable. [51]Therefore, the required inspection time was significantly reduced.Interestingly, a similar mechanism was used for detecting some biological toxins.gure 13.The operating principles of some special fluorescent hydrogels, which use DNA as the acceptor.a) In the microgel particles, the target viral DNA hybridizes to quencher in the tail of singlestranded DNA and results in fluorescence recovery; [122] b) Ochratoxin A binds to aptamers to initiate rolling circle amplification and induce fluorescence of Cy3dUTP. [125]ergy Environ.Mater.2024, 7, e12646 Aptamers can bind to ochratoxin A, releasing the primers and enabling ochratoxin A detection.The separated primers combined with the padlock probe to form a circular template and started the rolling cycle amplification reaction under enzymatic action.The rolling cycle amplification products wrapped fluorophores to form DNA hydrogels, and the polymerization process made the dispersed fluorophores continue to aggregate (Figure 13b).There was a positive correlation between fluorescence intensity and ochratoxin A concentration. [125] Unlike DNA-based BAFHs, BAFHs containing biological enzymes can sensitively detect pollutants without complicated biomimetic procedures.Typically, BAFHs containing acetylcholinesterase (AChE) enable the detection of residual pesticides (paraoxon, [126] carbaryl, [127] and organophosphorus [128] ) and insecticides (dichlorvos [129] ) in the environment via various mechanisms: 1) BAFHs with immobilized AChE and quantum dots (QDs) could detect paraoxon because p-nitrophenol, produced from the reaction between paraoxon and AChE, quenched the fluorescence of QDs; [126] 2) H 2 O 2 produced by BAFHs containing AChE and choline oxidase quenched the fluorescence of QDs, and the combination of dichlorvos with AChE prevented H 2 O 2 production, enabling dichlorvos detection; [129] 3) AChE produced thiocholine via hydrolysis, quenching the fluorescence of QDs by complexing with Ag 2+ or triggering the fluorescence of upconversion nanoparticles by hindering dopamine polymerization. [127,128]In all these systems, pesticides inhibit AChE, which affects fluorescence.Similarly, dichlorvos can be detected by BAFHs equipped with a Cu 2+ -AChE-carbon dot (CD) system. [130]Besides, BAFHs containing tyrosinase-QD conjugates can also emit bright orange fluorescence.Quinone intermediates generated by the catalytic oxidation of tyrosinase to phenol and its derivatives can act as electron acceptors and quench fluorescence (FRET principle). [131- 133]Hence, these BAFHs detected phenol and its derivatives with high sensitivity.Besides enzymes, peptide sequence-based BAFHs can also detect pollutants.When polypeptides modified with fluorophores were immobilized on dispersed tetraphenylethylene, organophosphorus could covalently bind the serine of peptides, forcing their aggregation and the gelation of tetraphenylethylene.The resulting fluorophore aggregation increased fluorescence intensity. [134]This was a special mechanism as the hydrogel no longer acted as a matrix, but the polymerization process of hydrogel monomers was used to gather free fluorophores and enhance fluorescence instead.

Quantum Dots Loaded in the Hydrogels
QDs are colloidal semiconductor nanocrystals measuring between 1 and 10 nm.Their fluorescence is generated by electron-hole recombination upon irradiation with excitation light. [135]QDs have bright fluorescence, photostability, excellent multiplexing ability, and their emission wavelength can be controlled by modulating crystal size and material properties. [136,137]QD fluorescent hydrogels (QDFHs), made by combining hydrogel matrixes with CDs, have been used for environmental pollutant detection in recent years.

Carbon Dots
CDs have stable fluorescence without additional excitation conditions; thus, analytical targets can be detected only if they quench or enhance fluorescence.Besides, CDs contain abundant carboxyl groups, affording them good water solubility and compatibility with various organic substances, polymers, inorganic substances, and even biomolecules (Figure 14a). [138]Compared with expensive precious metal or heavy metal-based QDs, which are potential environmental and biological hazards, CDs have low cost, abundant sources, low toxicity, and good biocompatibility. [138,139]The applications of CDs in environmental pollutant detection have been reported the most and demonstrate great potential.
The most notable property of CDs is that their fluorescence can be directly and significantly quenched by metal ions. [140]Therefore, metal ions can be quantitatively detected owing to their direct fluorescence quenching of CDs.This property is most commonly used by FHs with CDs (CDFHs).Cellulose nanofibrils (CNFs) are an ideal hydrogel matrixes material [141][142][143][144][145][146][147] of CDs.They possess nanoscale size, strong stiffness, high specific surface area, and special 3D network structures. [148,149]The porous structure of CNFs endows CDFHs with many CD and metal ion-binding sites.Besides, they have great fluorescence quantum yield, density, and adsorption capacity for heavy metal ions (Figure 14b).Hence, CNF-based CDFHs not only have excellent detection abilities for heavy metals but can also adsorb heavy metal pollution. [142,144,150]More importantly, all C-rich materials are theoretically sources for CDs and CNFs.CNF hydrogel precursors and CDs can be generated simultaneously from biosourced materials via one-pot hydrothermal methods. [151,152]This method is cheap and nontoxic.
Notably, hydrogel matrixes prepared from single natural CNFs have various drawbacks, such as poor flexibility and environmental adaptability.To overcome this, CDs-CFNs were crosslinked with other hydrogel matrixes, including AAm&AAc, [141,142] chitosan, [144] polyethyleneimine (PEI), [145] and epichlorohydrin. [146]In these materials, the abundant amino groups provided by chitosan can couple with sulfonic acid.Chitosan-based CDFHs combined with molecular imprinting technology enable the detection of perfluorooctanesulfonic acid (PFOS). [153]Furthermore, PEI can be used as a N-rich surface passivator to modify CDs (N doping) and improve the fluorescence quantum yield and performance. [145,154]In CDs treated with PEI (PEI-CD), ClO À oxidized the phenol groups of PEI-CD into benzoquinone groups and quenched fluorescence via the ICT principle (Figure 14c).Hence, this system allowed specific ClO À detection. [155]Besides PIE and its derivatives, some compounds, such as urea [143,156] and ethanediamine, [144,157] could also be used as N dopants.
CDFHs were also used to detect antibiotics such as tetracycline [158][159][160] and rifampicin. [161]All of these studies have shown that the absorption spectra of antibiotics and the excitation spectrum of CDs have enough overlap to cause fluorescence quenching.Some researchers attributed this to the inner filter effect (IFE) between antibiotics and CDs, [158,160] while others attributed it to the FRET effect caused by the combination of antibiotics and surface functional groups of CDs. [159,161]CDFHs also show potential for bacterial detection.When amphiphilic CDs were assembled into the hydrogel matrixes constructed using 6-O-acylated fatty acid esters, the esterase secreted by bacteria cleaved the ester bonds, making the hydrogel network collapse.This collapse caused CD aggregation, which quenched fluorescence and enabled bacterial detection (Figure 14d). [162]Finally, CDFHs can be used to measure pH.The surface functional groups (amino, amide, carbonyl, and carboxyl groups) of CDs undergo reversible protonation and deprotonation based on pH.When the valence bands of CDs are filled or depleted, the fluorescence intensity and emission peaks change (Figure 14e). [152,163]Because CDs exhibit stable fluorescence, the tested targets only affect the fluorescence intensity and hardly change the fluorescence properties.Therefore, CDFHs can Energy Environ.Mater.2024, 7, e12646 recognize multiple targets simultaneously, but they usually lack specificity for targets without the help of additional recognition modalities, particularly for detecting metal ions.

Graphene Quantum Dots
As a special kind of CDs, GQDs have a layered structure and good crystallinity, and can therefore provide higher quantum yields than CDs and exhibit excellent trapping properties for photons in the shortwavelength region. [164,165]Additionally, as giant polycyclic aromatic hydrocarbon molecules, GQDs have complex chemical groups, functional groups, defects, and dopants (Figure 15a). [166]These unique properties can also be controlled by changing the size of graphene, disrupting the integrity of the π-electron system, and adjusting the chemical or layered structure. [167]Therefore, GQD-based FHs (GQDFHs) provide significant flexibility for environmental sensing applications.
For example, GQDs were crosslinked with reduced graphene oxide to form porous 3D GQDFHs via multilayer H-bonding.Reduced graphene oxide adsorbed U 6+ via strong complexation, while coordination interactions between the O and N groups of GQDs and U 6+ quenched the fluorescence.These two phenomena enabled sensitive U 6+ detection sensitively. [168]Similarly, glyceryl methacrylate-functionalized GQDs were crosslinked with polythioctic acid, BisAA, and gelatin to form GQDFHs.These GQDFHs adsorbed Cd 2+ and Pb 2+ via coordination with C-S bonds and chelation with thiol groups and enabled the detection of these metal ions via IFE-based chelation-enhanced fluorescence or chelation-quenched fluorescence (Figure 15b). [169]esides metal ions, the PN junction synthesized from Ni 2+ and histidine-functionalized GQDs to form GQDFHs with PVA could oxidize 3,3 0 ,5,5 0 -tetramethylbenzidine and quench the fluorescence.Lambda-cyhalothrin blocked this process by inhibiting the activity of the PN junction, enabling detection. [170]Besides, graphitic carbonitride QDs encapsulated into 3D GQDFHs have much higher fluorescence intensity than ordinary GQDs.With the assistance of biological aptamers (specificity dsDNA), these 3D GADFHs enabled the selective and sensitive detection of kanamycin [171] and even E. coli. [172]These examples show that the complex organic structure of GQDs endows GQDFHs with strong scalability and different expansion possibilities.As one of the newest nanomaterials, GQDFHs are one of the most promising pollutant detection sensors, even though few reports on their application in environmental pollutant detection exist.

Other QDs
Besides the above two QDFHs, SiQDs have fluorescence properties similar to those of CDs as well as low toxicity and good biocompatibility. [173,174]Based on the same fluorescence suppression mechanism, QDFHs containing SiQDs have been used to detect metal ions (such as Cr 6+ , Fe 3+ , or Cu 2+ ). [175,176]Additionally, the QDs in the aforementioned Ag + -AChE-QD system for detecting organophosphorus and  [155] d) Fluorescence of the enzyme-embedded carbon dot fluorescent hydrogels is quenched in by induction of bacteria. [162]e) The changes of photoluminescence spectate of two carbon dot fluorescent hydrogels with pH detection capability at different pH. [152]ergy Environ.Mater.2024, 7, e12646 paraoxon were SiQDs. [126,128]Meanwhile, SiQDs synthesized chemically using Si solution possess hydrogel properties and can form QDFHs even without the aid of hydrogel matrixes.More importantly, SiO 2 hydrogels can efficiently be turned into aerogels through various drying methods; hus, SiQD-based fluorescent aerogels are conceivable.This could potentially enable air pollutant detection, which most OHs cannot do, but there have been no reports on this so far.
Cd-based QDs (CdQDs) were the earliest reported QDs, and many types of CdQD-based FHs have been developed to detect various pollutants.For example, NO 3 À anions can bind to positively charged PEI-CdS QDs and be detected via the resulting fluorescence quenching during electron transfer (ESIPT principle). [177,178]Meanwhile, cations (H + and Fe 3+ ) could be captured by the negatively charged carboxyl groups of SA hydrogels and detected by CdS immobilized in the hydrogels. [179] QDFH with CdTe QDs in a QD-AChE system could detect organophosphorus without the coordination of metal ions, unlike other AChE-based FHs (Section 3.2).[180] Besides, CdSe/ZnS cooperated with specific dsDNA to realize virus detection, [123] and carboxylated Cdbased QDs were also used to detect phenols via a tyrosinase-QD system.[131][132][133] The commercialization of various CdQDs enabled the easy preparation and even mass production of CdQD-based FHs for different detection targets.However, the potential environmental and health hazards of heavy metal-based QDs have been worrisome, even though the hydrogel matrixes have prevented the leakage of heavy metal ions to a certain extent.

Living Luminescent Microorganisms Immobilized in Hydrogels
The unique sensor formed by immobilizing natural or artificial living luminescent microorganisms in hydrogel matrixes arecalled bioluminescence hydrogels (BLHs).They are a kind of special OH, as the hydrogel does not directly contribute to the detection process.Instead, it creates a suitable living environment for microorganisms and amplifies the adsorption of BLHs to the target.BLHs are mainly used for toxicity detection.Toxicity involves biochemical processes that are complex, hard to elucidate clearly, and difficult to simulate in vitro.It also involves the uniform expression of complex mixtures in polluted environments.Currently designed OHs based on chemical and physicochemical processes can detect most environmental pollutants, but all the previously described strategies for OH design were unable to evaluate pollutant toxicity.However, the implantation of living luminescent microorganisms can allow OHs to detect toxicity.The stable grid structure, mechanical strength, and high hydrogel biocompatibility allow living cells to be confined within the hydrogel grids and maintain their original activities. [21,181]Furthermore, the porous structure of hydrogels is an excellent environment for cell attachment and proliferation.The rich functional groups facilitate the transport of metabolites and nutrients in and out of the capsules. [182,183]More importantly, it is also feasible to add nutrients in hydrogel matrixes to maintain the long-term activity of microorganisms because of the similarities between hydrogels and water.Finally, the antifouling and adsorption properties of hydrogels described above make BLHs more resistant to environmental disturbances and more sensitive to targets than unimmobilized living luminescent microorganisms.All of these unique properties enable the construction of toxicity sensors.
Living luminescent microorganisms include natural luminescent bacteria and genetically recombinant bacteria.Natural luminescent bacteria include Vibrio fischeri, Vibrio harveyi, Pseudomonas fluorescens, and Pseudomonas leiognathid.Toxic substances can affect their cellular metabolic states and quickly suppress their luminescence intensity, [184] making them useful for evaluating the acute toxicity of samples.For example, V. fischeri immobilized in a PEG diacrylate hydrogel to form test paper enabled the quick assessment of water toxicity. [185]Pseudomonas leiognathid immobilized on agar, carrageenan, and SA was used to test heavy metal and pesticide-associated toxicity in water systems.These studies showed that SA-based BLHs performed best. [186]Besides, as a natural anionic polymer, SA has high biocompatibility, and its crosslinking process is mild and nontoxic.Therefore, using SA to encapsulate cells provides the highest benefits for BLHs. [187,188]or genetically recombinant bacteria, E. coli [189,190] and yeast [191,192] are the two most common host cells.They were implanted with lowtranscript plasmids that contained a transcribed fusion gene of the green fluorescent protein and an antibiotic resistance gene.The region between two adjacent open reading frames on the plasmids was amplified and embedded with different promoters for stress-related genes (Figure 16a). [193,194]When the stress conditions of the promoters were reached, the corresponding gene channels opened to drive the expression of the fluorescent protein gene, and the bacteria thus emitted specific fluorescence.In toxicity assays, these stress genes were associated with oxidative stress, protein stress, detoxification, electron transport, and DNA damage. [189,190]However, the possible escape of genetically recombinant bacteria during testing and diffusion of their unnatural  [169] Energy Environ.Mater.2024, 7, e12646 genes in natural microbial systems, especially antibiotic resistance genes, is worrisome. [195,196]Immobilization of genetically recombinant bacteria on hydrogel matrixes can effectively prevent the leakage of bionts and their unnatural genes.For example, a directionally designed genetically recombined E. coli embedded in SA hydrogels enabled the quantification of 2,4,6-trinitrotoloune via the toxicity response (Figure 16b). [197]nterestingly, under the action of the gas-liquid interface and organic matter adsorption effect of SA, BLHs embedded with E. coli could detect formaldehyde, cigarette smoke, acetone, and other toxins in the gas phase. [198,199]To further prevent the leakage of genetically recombinant bacteria and their unnatural genes, researchers constructed a hard BisAA shell outside the E. coli-embedded SA hydrogel core (Figure 16c).This process did not compromise the sensitivity of the sensor to heavy metals in real water. [200]Additionally, E. coli, embedded in PEG diacrylate (with better mechanical strength than SA) [201] or agar (without chemical crosslinking) [202,203] enabled the detection of environmental toxicity in water.Agar-embedded E. coli-based BLHs were used for continuous monitoring and early warning of water toxicity.However, the leakage of genetically recombinant bacteria that were not restricted by the crosslinked chemical network could easily cause new environmental concerns.
Presently, there are few reports on BLHs, but their special detection properties could not be achieved by other OH sensors.Solving biological leakage, stability, and long-term storage problems will make BLHs particularly attractive for environmental pollutant detection.

Others
Light diffraction-based PCHs and fluorescencebased FHs have accounted for almost all the existing studies using OHs for environmental sensing.There are few reports about OHs that are neither PCHs nor FHs.
In some studies, hydrogels with specific stimulus responsiveness were wrapped at the end of the fiber to form the interference structure.When the stimulation conditions were reached, the expansion or contraction of the hydrogel changed the wavelength of light in the fiber, [204][205][206][207] enabling target detection (Figure 17a).For example, ambient humidity could be measured by the interference structure of optical fibers wrapped with a strongly hygroscopic hydroxypropyl methylcellulose hydrogel. [204]dditionally, fibers coated with DNA aptamermodified [205] or glucose recognition agentdoped [206,207] hydrogels enabled the detection of more complex targets, such as K + and glucose.In another method, hydrogel films with specific stimuli-responsiveness covered the surface of a metal sensor with a glass substrate to  [197] c) Hydrogel microspheres immobilized with recombinant E. coli are used to detect heavy metals in water, each microsphere is wrapped in a tough semi-permeable hydrogel shell to prevent leakage of engineered bacteria. [200]gure 17.Three other types of optical hydrogel sensors.a) Fiber-based optical hydrogel sensors; [206] b) Waveguide-based optical hydrogel sensors; [208] c) Fresnel lens-based optical hydrogel sensor. [211]ergy Environ.Mater.2024, 7, e12646 make an optical waveguide structure.When the stimulation conditions were reached, the changes in hydrogel film volume affected the refractive index of the incident laser, ultimately changing the waveguide spectra [208][209][210] (Figure 17b).Likewise, when the hydrogel film was pH-sensitive, the optical waveguide sensor could detect pH, [208] and when a streptavidin system was added to the film, the sensor could detect bacteria. [209]Sensors with waveguide-structured hydrogels could also detect biomolecules with more complex designs.For instance, a terpolymer hydrogel layer doped with estradiol monoclonal antibody was covered on a laser prism with gold film.When contacting estradiol molecules, the refractive index changes were detected through plasmon resonance-excited optical waveguide modes. [210]Particularly, some researchers made Fresnel lenses from pH-sensitive hydrogels using the replica mold method.Changes in focal length and focusing efficiency caused by changes in hydrogel volume enabled pH detection (Figure 17c). [211]verall, the basic detection mechanism of these sensors is similar to that of PCHs.In both fiber-based and optical waveguide-based OHs, the hydrogel matrixes are just the auxiliary structures of various optical devices (fiber optics, laser transmitters, signal receivers, precious metal resonance films, prisms, glass baseboards, etc.).These hydrogel-assisted optical sensors may have advantages for developing wearable flexible detection devices.However, optical waveguide or optical fiber detection equipment may have insufficient reliability and sensitivity for pollutant detection.They are far less convenient, more expensive, and less efficient than PCHs.Compared with PCHs and FHs, most other OHs have no advantages or applicability in environmental sensing.

Cross-Relationships of Different OHs
FHs and PCHs are two different development directions of OHs.They are not in competition but complement each other.In the future, PCHs need to be faster and more sensitive to identify more environmental pollutants through visual detection.FHs need to accurately quantify more environmental pollutants through more portable fluorescent devices.Figure 18 presents the relationships between the different kinds of OHs.Although some PCHs (DRPCHs) can detect certain pollutants without target recognition modules, they cannot achieve high sensitivities.Improving the sensitivity of PCHs for complex macromolecules and even organisms requires combining molecular imprinting technologies and biological macromolecules.It is also necessary to use inverse opal structures to improve the sensitivity of signal expression.Additionally, to improve the detection accuracy of PCHs, multiple PCHs can be combined.For example, two or more environmental stimuli-responsive hydrogels can be used to construct joint inverse opal structures [54,90] and compounds [93] or biomacromolecule [74,79] receptors can be added to molecular imprinting cavities.
This synergy can also be applied to FHs, for example, by combining QDs and compounds [107,177] or simultaneously doping CDs and other QDs to form a double fluorescence system. [130]In particular, all BAFHs require the assistance of QDs or fluorophores, as no biomacromolecule emits fluorescence.Furthermore, some recent reports fused PCHs and FHs into a system called fluorescent PCHs. [152,153]This may be an interesting new development direction, but more research is needed to determine whether the effects of the change in grating diffraction and SPR on fluorescence are beneficial.

Concluding Remarks
Although OHs are mostly used in biological and medical fields, they have extensive application and development scope in environmental pollutant detection.Research and development of OHs may make detection of environmental pollutants more convenient, inexpensive, and efficient.It is also one of the ways to achieve in situ detection and online monitoring of the environment.However, applying OHs in environmental sensing still presents significant challenges.Firstly, there are numerous types of OHs, and designing optimal OHs for environmental sensing requires researchers to possess a diverse range of interdisciplinary knowledge, including polymers, molecular biology, chemistry, physics, and more.Moreover, environmental pollutants are diverse and constantly emerging.Even if the properties of a newly emerging pollutant are fully understood, it remains challenging to identify an appropriate response process to construct effective OHs.Finally, unlike the clear and controllable testing environments in biological and medical fields, such as blood, urine, tissue fluid, and nutrient solution, environmental sensing often involves extremely intricate testing environments, making it challenging for OHs to precisely identify and detect target pollutants amidst the complexity.This demands that OHs possess robust anti-interference capabilities.
More than 60% of the examples of OHs were FHs, and only 30% were PCHs.The main advantage of FHs is the rapid quantitative onsite detection of pollutants as these sensitive fluorescence-based devices are small, portable, and inexpensive.Among them, QDFHs may become one of the most important research directions for OHs.QDFHs are one of the most promising classes of FHs because QDs exhibit bright fluorescence, photostability, excellent multiplexing ability, and controllable emission wavelength based on crystal size and material properties.More importantly, QDs are one of the best commercialized high-performance fluorophores. [212,213]affording the developed QDFHs high potential for inexpensive and large-scale production.However, owing to their potential environmental risks, heavy metal-based QDs are not suitable for environmental pollutant detection.By contrast, CDFHs may become mainstream in environmental pollutant detection owing to the wide sources and environmental friendliness of CDs. [138,139]otably, CDs can be simultaneously synthesized with hydrogel matrixes while preparing CDFHs, [151,152] which not only reduces the sensor fabrication cost and difficulty but also improves the stability and uniformity of fluorescence.[171][172] GQDFHs have shown extremely high detection sensitivities to pollutants, some even far surpassing those of traditional detection methods, and their properties are more controllable and scalable than those of ordinary CDs.Hence, GQDFHs may be one of the most promising directions in environmental pollutant detection, although their cost remains high.
Although there are fewer reports on PCHs than FHs, PCHs are easily turned into visual detection kits and can rapidly and directly identify pollutants through changes in apparent color.Recently, rapid target recognition kits based on visual detection technologies have attracted increasing attention as they do not require any detection equipment or professional operators.The most salient example is the widespread application of COVID-19 antigen kits worldwide during the pandemic.Hence, PCHs hold great promise for inexpensive, rapid batch in situ detection in emergency cases.
In order to advance the development of OHs in environmental sensing, environmental researchers need to continually search for new technologies that are suitable for constructing OHs based on the latest research results across all fields of study.For example, the detection sensitivity may be affected by the density, uniformity, coordination, and complex signal transmission path of FH's internal fluorescence and target recognition unit.Therefore, we think that applying supramolecular nanohydrogels, which have been widely studied in drug delivery systems, [214,215] to FHs could solve this problem.A single fluorescence unit, target recognition unit, and signal transmission and conversion unit are formed into a microscopic system and packaged in a nanohydrogel container.After multiple microscopic systems are encapsulated and form complete FHs, the density and uniformity of the fluorescence and acceptor units are guaranteed, and the microsystems do not interfere with each other.Moreover, when dealing with a complex detection environment, it is essential to utilize materials that possess exceptional anti-pollution or directional adsorption capabilities to make OHs accurately identify the targets.For instance, functionalized cellulose nanocrystals and TiO 2 nanotube can selectively adsorb some organic dye pollutants. [216,217]Incorporating certain specialized sample pretreatment techniques for the OHs is achievable.
Finally, almost all OHs can only be applied to detect one pollutant or several with similar properties.Therefore, an interesting OH-based sensor or detection kit development direction would be to integrate multiple OHs for different pollutant detection into a single sensor array to simultaneously detect multiple common or harmful environmental pollutants.
We hope that this review will spark new ideas for the further application and development of OHs in the field of environmental pollutant detection.

Dan
Li is a Professor at Department of Environmental Science and Engineering, Fudan University, China.She received her Ph.D. from Tsinghua University, China in 2010, in collaboration with the University of California, Irvine.She conducted postdoctoral research at Tsinghua University (2010-2011) and Massachusetts Institute of Technology (2011-2013), U.S.She is an advocate of interdisciplinary science, and her researches focus on environmental sensing, atmospheric pollution, and health sciences.She has published over 40 papers, and four papers have been highly cited by ESI.Dr. Li was awarded the Outstanding Youth Fund of the National Science Foundation of China (2021).

Figure 1 .
Figure 1.Research trend analysis of optical hydrogel detection technologies based on CiteSpace 6.1.All data came from the most relevant documents on the Web of Science Core Collection in the past 10 years, and the extracted contents include the title, keywords, abstract, and references of papers.The nodes represent keywords, the size of nodes indicates the frequency of the keyword in the journal, and the connection between the nodes represents the interconnection between the keywords.The keywords with the highest frequency show in the form of tags.The smaller the serial number, the higher the frequency, and #0 is the search keyword.

Figure 2 .
Figure 2. Chapter based mind map of the review.

Figure 3 .
Figure 3. Schematic diagrams of photonic crystal hydrogel sensors.a) Environmental stimuliresponsive photonic crystal hydrogel sensors.b) Photonic crystal hydrogel sensors combined with ions and receptors.
direct contact with acceptors, allowing orbital overlap.One end of the molecule is electron-rich and the other is electron-poor.Normally, this tendency is exacerbated by excitation light, generating numerous dipoles.When the target interacts with the acceptor, the excited state dipoles are repelled (blue shift) or attracted (red shift)[40] by excitation light puts the fluorophore in the excited state, inducing proton transfer between the proton donors and proton acceptors inside the molecule, forming tautomers with different fluorescence properties[41] fluorescence donor (FD) and fluorescence acceptor (FA).Interaction with the target induces energy transfer from FD to FA via nonradiative dipole-dipole coupling under the action of excitation light, inducing fluorescence of FA[42] excited-state fluorophore forms a complex with the same ground-state fluorophore (excimer) or different groundstate fluorophore (exciplex).When the excimer/exciplex interacts with the target, double fluorescence of the excited-state fluorophore and excimer/exciplex can be observed

Figure 4 .
Figure 4. Statistical analysis of optical hydrogel research cases.a) Sample numbers for different pollutants detected by fluorescent hydrogels and photonic crystal hydrogels (numbers on stacking columns represent identifiable target species).Number of samples: 114.b) Detection sensitivities of different types of optical hydrogels for different contaminants (Note: 1.0 indicates that the sensitivity of this type of optical hydrogel is the same as that of the standard methods).

Figure 7 .
Figure 7. Recognition of heavy metals by compound receptors in photonic crystal hydrogels.a) Molecular structure of CP-ATAC and chelation of Cr 6+ in CP-ATAC.b) Molecular structure of 8-HQ and chelation of different valence metals.c) Introduction of crown ether groups, molecular structure of CE-PNIPAM, and chelation of Pb 2+ and Be 2+ in CE-PNIPAM.

Figure 12 .
Figure 12.DNA-based fluorescent hydrogels that can detect heavy metals.a) Under the guidance of Hg 2+ , single-stranded DNA is folded into double-stranded DNA with a hairpin structure and further enhances the fluorescence of SYBR Green I; b) The response principles of DNA-based fluorescent hydrogels to different metal ions.[115]

Figure 14 .
Figure 14.a) Structure of fluorescent carbon dots; b) Fluorescent carbon dots immobilized by cellulose nanofibers; c) The fluorescence quenching of polyethyleneimine-doped carbon dot fluorescent hydrogels by ClO À ;[155] d) Fluorescence of the enzyme-embedded carbon dot fluorescent hydrogels is quenched in by induction of bacteria.[162]e) The changes of photoluminescence spectate of two carbon dot fluorescent hydrogels with pH detection capability at different pH.[152]

Figure 16 .
Figure 16.Environmental sensing by the luminescent microorganisms embedded hydrogels.a) The plasmids of Genetic recombination fluorescent microorganisms (GFPG: green fluorescent protein gene, ARG: antibiotics resistance gene, ORF: open reading frame); b) Luminescent response of strains wrapped in hydrogel to 2,4-dinitotoluene in soil,[197] c) Hydrogel microspheres immobilized with recombinant E. coli are used to detect heavy metals in water, each microsphere is wrapped in a tough semi-permeable hydrogel shell to prevent leakage of engineered bacteria.[200]

Figure 18 .
Figure 18.Relations diagram of different types of optical hydrogels.Note: rings with apertures indicate that the type can fluoresce.

Table 2 .
Properties and chemical construction of several typical hydrogel monomers used to construct different optical hydrogels.

Table 3 .
Introduction to the principles of fluorescence emission and quenching.

Table 4 .
Limit of detection (LOD) of different pollutants in their corresponding reference test standards.

Table 4 .
Continued Illustrate: When multiple detection standards exist for a given substance, the one with the lowest detection limit is chosen.Energy Environ.Mater.2024, 7, e12646

Table 5 .
Inverse opal-structured photonic crystal hydrogels for detecting different kinds of chemical molecules.