Recent Advances in Porous Hydrogen‐Bonded Organic Frameworks for Luminescent and Photoelectric Sensing

ABSTRACT

Hydrogen‐bonded organic frameworks (HOFs), self‐assembled from organic building blocks via hydrogen bonding and other weak interactions such as π‐π interactions, represent a distinct class of crystalline porous materials. Through rational framework design and functionalization, exceptional intrinsic luminescent and photoelectric properties can be integrated into these structures. Coupled with their unique advantages of solution processability, facile recyclability, and biocompatibility, HOFs emerge as an ideal platform for developing next‐generation high‐performance sensors. This review systematically summarizes recent advancements in HOF‐based materials for luminescent and photoelectric sensing. We elucidate the underlying signal transduction mechanisms, highlighting how specific intermolecular interactions induce detectable changes in optical or electrical signals. The discussion of applications ranges from the detection of hazardous gases, organic pollutants, and chemical warfare agents to the monitoring of antibiotics and biomarkers, as well as temperature sensing and chiral recognition. Finally, we critically assess the current landscape, identifying key challenges to guide the development of HOF‐based luminescent and photoelectric sensors.

This review summarizes advancements in HOF‐based materials for luminescent and photoelectric sensing. The underlying signal transduction mechanisms are elucidated, highlighting how intermolecular interactions induce detectable changes in optical or electrical signals. The applications ranges from detection of hazardous gases, organic pollutants, and chemical warfare agents to the monitoring of antibiotics and biomarkers, as well as temperature sensing and chiral recognition.

1. Introduction

Crystalline porous materials (CPMs), particularly metal‐organic frameworks (MOFs) [1, 2, 3] and covalent organic frameworks (COFs) [4, 5, 6], have achieved significant success in constructing high‐sensitivity sensors owing to their exceptional specific surface areas and tunable pore environments [7, 8, 9]. MOFs utilize the coordination between metal nodes and organic ligands to provide abundant luminescent sites [10, 11], while COFs offer superior chemical stability through their robust covalent skeletons [12, 13]. However, the rapid evolution of the Internet of Things and wearable electronics has imposed new requirements for next‐generation sensing devices, specifically regarding miniaturization, flexibility, and sustainability. In this context, the solution processability and interface compatibility of materials have become as critical as their intrinsic porosity. It is a challenge for most MOFs and COFs, which typically exist as highly cross‐linked insoluble powders, hindering the use of low‐cost liquid‐phase techniques for device fabrication. Furthermore, the development of green, metal‐free, and recyclable porous materials is increasing to avoid potential heavy metal toxicity in biological applications and ensure environmental sustainability.

Hydrogen‐bonded organic frameworks (HOFs), as a class of CPMs self‐assembled from organic molecules through hydrogen bonding and other noncovalent interactions, offer a novel approach to overcoming the aforementioned limitations [14, 15]. Compared with their structurally rigid coordinatively or covalently bonded counterparts, MOFs and COFs, HOFs fill a specific blank for soluble and recyclable crystalline porous materials. With the reversibility of hydrogen bonds, HOFs can be prepared, repaired, and regenerated through a simple dissolution‐recrystallization process [16, 17, 18]. This feature allows them to be processed into various sensing interfaces and enables material reuse through simple treatment after device performance attenuation [19, 20, 21].

Although in early studies, the relatively weak nature of hydrogen‐bonding interactions was often regarded as a source of structural instability, many HOFs were prone to framework collapse upon solvent removal [22, 23, 24]. Around 2010, the emergence of HOFs with permanent porosity marked a pivotal turning point in their developmental process [25, 26]. Recent research has demonstrated that the introduction of large π‐conjugated building blocks such as pyrene and porphyrins, shape‐fitted π‐π stacking, and multiple hydrogen‐bonding motifs have markedly improved their permanent porosity and framework stability [27, 28, 29]. With the continuous development of these strategies, HOFs reported in recent years have achieved significant improvements in pore stability and framework rigidity. With a controlled catenation strategy in HOFs by precisely manipulating the interpenetration of multiple hydrogen bonds in a point‐contact manner, researchers have successfully developed robust supramolecular HOF RP‐H101, which achieves record‐breaking gravimetric surface areas compared with all HOFs reported so far, of 3526 m²g⁻¹ [30]. These strategies not only proved that HOFs can rival or surpass other crystalline porous materials in terms of porosity, which is crucial for analyte enrichment, but also demonstrated that the weak interactions, such as π‐π stacking, can yield highly robust structures by precise engineering.

More significantly, these structural design strategies for HOFs not only ensure framework stability but also directly enhance sensing performance. On the one hand, the close π‐π stacking of conjugated organic monomers facilitates intermolecular orbital overlap and restricts nonradiative decay, thereby imparting intrinsic photoelectric conductivity and strong luminescence. On the other hand, the resultant stable porous architecture serves as a versatile host for the precise encapsulation of functional guests, such as lanthanide ions [31], metal nanoparticles [32], or enzymes [33] to introduce additional signal transduction channels. This capability to synergize intrinsic backbone activity with extrinsic guest functionality endows HOFs with a broad spectrum of tunable active sites, rendering them an ideal platform for constructing high‐performance luminescent and photoelectric sensors [34, 35].

Despite these promising attributes, the landscape of HOF‐based sensing is evolving rapidly. Recent advancements have transitioned from static powder‐based detection to flexible devices, in vivo monitoring, and smart analysis systems assisted by machine learning. Given the remarkable advancements of HOF‐based luminescent and photoelectric sensing research, particularly involving device integration and intelligent recognition, a systematic overview summarizing these cutting‐edge applications and strategies is urgently needed. This review focuses on the latest research progress of HOFs especially from 2020 to 2026, in the fields of luminescent and photoelectric sensing [36]. At the application level, the two approaches are complementary. Luminescent sensing is suitable for visual detection, while photoelectric sensing facilitates electronic device integration. Their combined review paves the way for the construction of multimodal intelligent sensing platforms. First, we focus on the luminescent and photoelectric sensing mechanisms, including Förster Resonance Energy Transfer (FRET), photoinduced electron transfer (PET), and so on. Furthermore, we classify and summarize the application instances of HOFs in detecting hazardous gases, organic pollutants, biomarkers, as well as temperature sensing and chiral recognition, while offering a prospect on the challenges facing this field, such as the further improvement of conductivity, long‐term stability, and future development directions.

2. Sensing Mechanism

The sensing capabilities of HOFs fundamentally stem from the dynamic interactions between the host framework and guest analytes, which modulate the electronic behavior of the excited states of HOFs. Due to the tunable organic building blocks and flexible hydrogen‐bonded networks, HOFs offer a versatile platform where guest molecules can precisely intervene in energy dissipation and charge transport pathways. We classify these signal transduction mechanisms into two distinct categories based on the readout modality. The first mode involves luminescent sensing, where analytes influence the radiative transition processes, leading to changes in emission intensity or wavelength (photon‐to‐photon). The second mode is photoelectric sensing, where analytes govern the generation, separation, and transport of photo‐induced charge carriers, resulting in detectable electrical signals (photon‐to‐electron). In this section, we systematically elucidate the underlying physical and chemical principles of these two transduction mechanisms to provide a theoretical basis for HOF‐based sensing applications [37, 38].

2.1. Luminescent Sensing Mechanism

HOFs have emerged as a versatile platform for luminescent sensing, fundamentally distinguished by their unique construction from optically active organic subunits. They are primarily self‐assembled from large π‐conjugated organic building blocks such as pyrene, porphyrin, or tetraphenylethylene which serve as intrinsic fluorophores or functionalizable chromophores. This feature endows HOFs with inherent, tunable luminescent properties directly derived from their constituent monomers. In the aspect of structure, these emissive characteristics are further potentiated by the porous architecture of HOFs. The directional hydrogen bonding creates ordered channels and specific recognition sites that function as “molecular traps”, effectively concentrating analytes via the nanoconfinement effect. This integration of intrinsic luminescence with a porous matrix facilitates intimate host–guest interactions, where the binding of an analyte modulates the framework's electronic state. Such interactions can trigger distinct optical responses through mechanisms including Förster Resonance Energy Transfer (FRET) or photoinduced electron transfer (PET), and so on, thereby converting molecular recognition events into readable luminescent signals. The superior sensitivity and selectivity make HOFs an ideal platform for luminescent sensing. The key mechanisms are detailed below.

2.1.1. Förster Resonance Energy Transfer (FRET)

Förster resonance energy transfer (FRET) is a nonradiative energy transfer process wherein dipole‐dipole coupling between an excited donor and a ground‐state acceptor chromophore enables energy transfer from the excited donor to the acceptor. This results in a decrease in the donor's emission intensity and excited‐state lifetime, while simultaneously enhancing the acceptor's emission intensity and excited‐state lifetime, thus eliminating the need for electron exchange [39]. This resonant process relies on energy conversion between the two species, necessitating spectral overlap between the donor's emission spectrum and the acceptor's ultraviolet‐visible absorption spectrum.

In HOF‐based luminescent sensing, the rigid hydrogen‐bonded framework offers the ordered porous structure which can confine analyte molecules within the nanoscopic channels [40]. This nanoconfinement effect significantly shortens the distance between the luminescent HOF ligands as donors and the encapsulated analytes as acceptors, thereby ensuring they remain within the effective Förster radius (<10 nm) to facilitate highly efficient energy transfer. Furthermore, the specific host–guest interactions within the HOF pores can orient the guest molecules to maximize dipole alignment, enhancing the sensitivity of the response. The FRET process always results in luminescence quenching/enhancement of HOFs or induces luminescence emission from the analyte [41, 42].

A representative application of this strategy was demonstrated by Xu et al. [43]. They reported a luminescent Eu³⁺‐functionalized HOF hydrogel film (designated as ‘1’) for the detection of two quinolone antibiotics ofloxacin (OF) and flumequine (FQ). In this system, the HOF is constructed from a room‐temperature phosphorescent (RTP) ligand, Melamine‐isophthalic acid (MA‐IPA), which serves as the energy donor crosslinked by Eu³⁺ to form a flexible sensor. The incorporation of Eu³⁺ acts as both a secondary luminescent center and a crosslinking agent to integrate the framework into a sodium alginate network. Firstly, the emission spectrum of MA‐IPA highly overlaps with the absorption spectrum of the guest OF molecule, satisfying the conditions for FRET occurrence (Figure 1a). Upon UV excitation, the energy from the HOF is nonradiatively transferred to adjacent OF molecules trapped in the framework via dipole‐dipole interactions, directly quenching the blue emission at 358 nm of MA‐IPA. Subsequently, the proximity induced by the HOF network allows the excited MA‐IPA* to further associate with ground‐state OF molecules, forming a novel, lower‐energy excited‐state complex—the MA‐IPA‐OF exciton complex. This complex serves as an efficient green luminescent center, exhibiting an emission peak at 496 nm (Figure 1b). Ultimately, the material's optical output manifests as a ratiometric‐based change characterized by blue light quenching and green light enhancement, resulting in a macroscopically observable color transition from blue to green. This enables selective and visual sensing of OF with high sensitivity (Figure 1c).

FIGURE 1.

(a) Schematic diagram illustrating the FRET‐based luminescence response mechanism of MA‐IPA in 1 toward OF. (b) Luminescence photographs of 1 after removal from OF solutions with increasing concentration from 10⁻⁹ to 10⁻³ M, showing visual color transition from blue to green. (c) Detailed mechanism schematic depicting the energy transfer pathway from MA‐IPA to OF, resulting in the FRET‐mediated ratiometric luminescence change described in the text [43]. Reproduced from ref. [43] Copyright (2021), with permission from WILEY‐VCH.

2.1.2. Photo‐Induced Electron Transfer (PET)

Photo‐induced electron transfer (PET) constitutes an excited‐state charge transfer process governed by the redox potentials of the host framework and guest analytes. Unlike FRET, which relies on dipole‐dipole coupling, PET involves the physical transfer of an excited‐state electron between a donor and a ground‐state acceptor. This process is thermodynamically favorable when the lowest unoccupied molecular orbital (LUMO) energy level of the donor is higher than that of the acceptor, enabling charge separation and subsequent quenching of the donor emission [44, 45].

HOFs offer distinct advantages for PET‐based sensing. The ordered nature of the HOF ensures precise, fixed spatial alignment between donor and acceptor molecules. The moderate hydrogen bonding strength permits the formation of close‐packed π‐π stacking, establishing continuous and ordered pathways for charge carrier transport that significantly enhance orbital overlap and electron coupling efficiency. Furthermore, the all‐organic composition of HOFs eliminates potential interference from metallic nodes, thereby yielding a cleaner and purer photophysical environment for charge transport. Concurrently, through judicious molecular design, the energy levels of structural units (such as HOMO/LUMO) can be finely tuned to ensure that the excited‐state energy level of the donor exceeds that of the LUMO of the acceptor. In a typical luminescence quenching sensing process, the HOF functions as the electron donor. When the analyte as an electron acceptor is encapsulated within the pores, its low‐lying LUMO accepts a photoelectron from the excited HOF, effectively suppressing the radiative recombination of the exciton and leading to reduced luminescence intensity of the HOF, which is the fundamental basis for detecting analytes [46]. The lower the LUMO energy level of the analyte, the more favorable it is for photoelectron transfer, consequently enhancing quenching efficiency [47].

The effectiveness of HOF‐based PET sensing is exemplified by Zhu et al. [48] They reported a Tb³⁺‐functionalized self‐assembled HOF composite based on melamine (ME) and phthalic acid (PA). Originally, in the absence of analytes, the excitation energy is transferred from the ME ligand to PA via thermally activated delayed luminescence (TADF)‐assisted energy transfer and then ultimately delivered to Tb³⁺ center, resulting in strong green luminescence. However, in the presence of tryptophan (Try) or its metabolite 5‐hydroxyindole‐3‐acetic acid (5‐HIAA), this pathway is disrupted. The LUMO levels of these analytes are lower than those of the ME unit in HOF. Consequently, a rapid PET process occurs from the excited ME to the analyte. Electrons spontaneously transfer from the excited‐state LUMO of ME to the LUMO of analytes, which essentially ‘hijacks’ the excited‐state energy, completely suppresses the pre‐set TADF process and highly efficient luminescence pathway (Figure 2a). The macroscopic result is specific quenching of Tb³⁺ green luminescence emission, while the intrinsic blue luminescence (approximately 380 nm) of the uncoupled ME ligand remains (Figure 2b). This results in the unexhausted energy from the ME excited state being released as intrinsic. This achieves ratio‐type luminescent signal output and a visual colourimetric response shifting from green to blue.

FIGURE 2.

(a) Dependence of the emission intensity of 544 and 380 nm peaks on Try concentrations. (b) Calibration curves of Tb@1 toward Try in the concentration range of 5×10⁻⁶–5×10⁻⁴ M [48]. Reproduced from ref. [48] Copyright (2023), with permission from WILEY‐VCH. (c) Energy transfer among the H3BTC ligand, DPA, and Tb³⁺ in Lumi‐HOF@Tb‐DPA. (d) Excitation and emission spectra of Lumi‐HOF@Tb‐DPA; inset shows a corresponding luminescence image by 254 nm UV irradiation [49]. Reproduced from ref. [49] Copyright (2022), with permission from American Chemical Society.

2.1.3. Inner Filter Effect (IFE)

The inner filter effect (IFE) refers to radiative attenuation of luminescence intensity caused by the presence of an absorbing substance within the detection system. Different from FRET or PET, which involve nonradiative excited‐state interactions, IFE does not involve direct energy transfer or electron transfer between the fluorophore and the absorber. IFE arises from the overlap between the absorption spectrum of the absorber and the excitation and/or emission spectrum of the fluorophore, leading to the competitive absorption of the excitation photons or reabsorption of the emitted light [50]. This effect results in a reduction of the observed luminescence intensity [51]. In the relevant research, IFE is regarded as a spectroscopic artifact that needs to be corrected [52]. While in the field of luminescent sensing by HOFs, IFE can be utilized as a sensitive transduction mechanism to achieve ultra‐high sensitivity and selective trace detection. The porous architecture of HOFs is particularly advantageous for this mode, which can effectively concentrate target analytes as absorbers within its channels. Through the absorption of excitation or emission light by target molecules, HOFs exhibit significant luminescence quenching, significantly enhancing the local optical density and thereby amplifying the signal response to trace‐level concentration changes. Furthermore, the dynamic response characteristics of IFE permit the development of intelligent optical switches and stimulus‐responsive materials using HOFs. This integrated design simplifies sensor architecture, enabling the fusion of recognition and signal transduction without complex labelling.

The research of Li et al. perfectly illustrated this concept [49]. They utilized multiple ligands, including melamine (MA), cyanuric acid (CA), luminol, and 1,3,5‐benzenetricarboxylic acid (H₃BTC) to build HOFs via hydrogen‐bonded self‐assembly. This system exhibits a dual‐emissive feature. The ligands including H₃BTC and others, sensitize Tb³⁺ via the “antenna effect” to produce green luminescence, while the confinement of luminol within the rigid framework enhances its quantum yield. The sensing mechanism for α‐glucosidase relies on the enzymatic hydrolysis of the substrate p‐nitrophenyl‐α‐D‐glucopyranoside (pNGP) to the p‐nitrophenol (pNP). The optical response is modulated by the spectral shift of the reaction product. Specifically, the absorption peak of the generated pNP shifts to 400 nm, eliminating the spectral overlap with the excitation wavelength that existed with the substrate pNGP. With the removal of the shielding effect, the internally protected Tb³⁺ luminescent center within the HOF regains full excitation, triggering a “turn‐on” response, with the recovered luminescence intensity proportional to enzyme activity (Figure 2c). Concurrently, the aromatic pore surfaces of HOF selectively adsorb the generated pNP, facilitating intermolecular interactions between pNP and luminol within the framework to generate a new luminescence peak at 466 nm. The combination of the IFE‐modulated Tb³⁺ emission at 546 nm and the guest‐induced luminole emission at 466 nm results in a ratiometric‐based self‐calibrating signal output to ensure high detection accuracy (Figure 2d).

2.1.4. Ligand‐to‐Metal Charge Transfer (LMCT)

Ligand‐to‐metal charge transfer refers to an efficient nonradiative relaxation process that absorbs light energy, electrons are excited from the molecular orbitals of the ligand, which are typically electron‐rich ligands and transferred to vacant orbitals of the metal center. This mechanism is often exploited to construct sensitive “turn‐off” sensors. The rigidly ordered channels of HOFs provide a unique microenvironment for LMCT‐based sensing. Especially in lanthanide‐functionized HOFs, luminescence is typically governed by the antenna effect, wherein organic ligands of the HOF framework capture excitation energy and sensitize the emissive lanthanide centers. Crucially, this process is highly susceptible to modulation by paramagnetic metal ions. When an appropriate metal center is introduced or generated in situ, a thermodynamically favorable LMCT pathway is established. Since the kinetics of this charge transfer are significantly faster than the energy transfer required for lanthanide sensitization, LMCT effectively suppresses the antenna effect, resulting in luminescence quenching [53, 54].

The LMCT mechanism is demonstrated by the research of Li et al. [31]. They successfully synthesized two bimetallic LnZn‐HOFs by reacting Schiff base ligands formed from the condensation of 3‐methoxysalicylaldehyde and 1,3‐diamino‐2,2‐dimethylpropane, with Ln(NO₃)₃ (Ln = Tb, Dy) and Zn(OAc)₂. When LnZn‐HOFs are dispersed in aqueous solution, the lanthanide ions (Ln (III)) situated at the periphery of the crystal structure undergo dissociation under the influence of water molecules [55, 56]. This process exposes the electron‐donating atoms, specifically nitrogen and oxygen on the organic ligands that were originally coordinated to Ln (III), thereby generating a multitude of vacant, highly reactive chelation sites. In the presence of target heavy metal ions such as Cu²⁺, these ions rapidly and selectively occupy the newly exposed sites due to their strong Lewis acidity and suitable coordination geometry, forming tightly bound ligand‐ Cu²⁺ complexes with the ligands. (Figure 3a). The coordinated Cu (II) ion, possessing vacant d orbitals, functions as a strong electron acceptor while the excited‐state ligand acts as an electron‐rich donor, facilitating highly efficient orbital coupling. As supported by DFT calculations, an electron from the HOMO of the excited‐state ligand is directly injected into the vacant d orbitals of Cu (II) via a thermodynamically favorable LMCT pathway (Figure 3b). Crucially, the nonradiative LMCT process operates at rates far exceeding energy transfer to the Tb (III) center by the antenna effect. Consequently, the vast majority of excited‐state energy is competitively dissipated via this electron transfer mechanism, leading to intense quenching of luminescence of the system. This quenching effect exhibits a strong correlation with Cu (II) concentration, thereby enabling ultra‐high‐sensitivity optical detection of trace amounts of Cu (II) (Figure 3c).

FIGURE 3.

(a) Jablonsky energy level diagrams comparing TbZn‐HOF and Tb‐HOF, illustrating the competitive energy dissipation pathways. (b) Comparison of HOMO and LUMO energy levels for the ligand, ZnL, Tb‐HOF, and TbZn‐HOF obtained via DFT calculations; the increase in the HOMO–LUMO energy gap of TbZn‐HOF facilitates energy transfer from the ligand to Tb³⁺. (c) Schematic of LMCT‐based fluorescence quenching sensing of Cu²⁺ by LnZn‐HOFs [31]. Reproduced from ref. [31] Copyright (2023), with permission from Elsevier.

2.1.5. Pore Sensing and Structural Transformation

Structurally, the luminescent sensing capability of HOF lies in the synergy between its dynamic porous structure and luminescent units. the structural flexibility. Fundamentally, HOFs are assembled from π‐conjugated organic building blocks such as tetraphenylethylene, carbazole, and triphenylamine, which function as intrinsic fluorophores. Within the initial framework, the rigid hydrogen‐bonded network imposes a restriction of intramolecular motion, effectively inhibiting internal rotations and vibrations. This confinement typically triggers aggregation‐induced emission (AIE) or significantly enhances intrinsic luminescence quantum yields. Upon exposure to analytes, the sensing mechanism operates through reversible structural transformations inherent to the hydrogen‐bonded network. The pore channels provide precise recognition where the dimensions and topology enable molecular sieving, while functionalized sites on the inner walls such as hydrogen bonding and π‐π interaction sites, can specifically capture targets. The host‐guest interaction directly perturbs the luminescent centers to trigger the initial luminescence response [57]. More notably, the inherent structural flexibility of HOFs allows reversible structural transitions induced by analytes. These conformational adaptations significantly modify the microenvironment surrounding luminescent groups, thereby amplifying subtle molecular recognition into detectable luminescent signals. Consequently, the synergistic integration of channel‐selective recognition and structural transition‐amplified signaling enables HOFs to achieve highly sensitive, selective, and reversible sensing.

In 2023, Chen reported HOF‐FJU‐206 constructed from the tripyridine‐carbazole molecule CPPY, where the carbazole unit serves as the luminescent center [58]. HOF‐FJU‐206 possesses narrow and flexible channels that facilitate the diffusion of organic acid vapors into the framework. H⁺ ions dissociated from acid molecules undergo controlled protonation at pre‐engineered pyridine sites. This process significantly alters the electronic structure of the luminescent carbazole center, inducing a red shift in luminescence emission from blue → yellow to achieve visual signal output. Crucially, the pore structure undergoes a slight and reversible expansion to accommodate the proton transport and binding. Different organic acids exhibit varying dissociation efficiencies and protonation degrees within the pores due to their distinct pK_a values, resulting in differing luminescence red‐shift magnitudes and response rates. Strong acids, however, cause irreversible sensor failure by disrupting the hydrogen‐bond network through excessive protonation, leading to pore collapse. Through controlled proton transport via the pore and reversible structural fine‐tuning of the framework, highly selective and reversible luminescent sensing for organic acids is achieved.

2.2. Photoelectric Sensing Mechanism

The preceding section highlights the radiative decay pathways utilized in luminescent sensing of HOFs. Furthermore, HOFs can also serve as superior platforms for photoelectric sensing. In particular, most current research focuses on photoelectrochemical (PEC) sensing, which harvests non‐radiative charge carriers owing to their structural flexibility and tailorable charge transport properties. Both Luminescent and photoelectric sensing rely on the interaction between the electronic structure of HOFs and photons, but exhibit distinct working mechanisms and application scenarios [59]. Luminescent sensing depends on the radiative properties of HOFs, achieving detection by analyzing changes in luminescence intensity, wavelength, or lifetime induced by the target analytes. Photoelectric sensing, conversely, focuses on the photovoltaic signals, such as photocurrent or photovoltage, generated by HOFs under illumination, enabling detection by measurement of electrical response. Specifically, the assembly of a large π‐conjugated framework via shape‐fitted π‐π stacking interactions creates efficient channels for through‐space orbital overlap. The structure arrangement facilitates long‐range charge delocalization and accelerates charge transport via the redox hopping processes, overcoming the low intrinsic conductivity typical of organic materials [60].

The photoelectric sensing of HOFs involves a sequence of photophysical events. Photon absorption generates tightly bound Frenkel excitons, which subsequently dissociate into free charge carriers including electrons and holes under a driving force typically provided by an internal electric field or interfacial band bending. These carriers then migrate to the electrode surface to participate in redox reactions, generating a measurable photocurrent. Efficient PEC sensing requires the maximization of nonradiative charge separation while suppressing radiative recombination. Therefore, through rational structural design such as modulating the degree of conjugation, constructing heterojunctions, introducing electron donor‐acceptor units, or optimizing stacking arrangements, it is critical to guide the distribution of excited‐state energy. This precise control not only enables sensitive PEC sensing but also builds the dual‐mode sensing platforms where the optical and electrical signals can be synergistically modulated [61].

2.2.1. Enhanced Photoactivity by Heterojunction Engineering

Despite the unique light‐harvesting capabilities of HOFs, their performance as pristine organic semiconductors is often constrained by high exciton binding energies and relatively low intrinsic conductivity, which severely limit their photoelectric conversion efficiency. To address this critical issue, heterojunction engineering is widely regarded as the most effective strategy to boost PEC sensing performance. With their tunable porous crystalline structure and high specific surface area, they can provide a uniformly dispersed carrier for inorganic semiconductors, including quantum dots (QDs), metal oxides or 2D materials, for example, g‐C₃N₄, possessing matched energy levels, based on band theory. Upon contact between HOFs and these counterparts, electron rearrangement occurs at the interface, driven by the difference in Fermi levels until equilibrium is reached. Through bandgap matching, an intrinsic electric field forms at the interface, efficiently separating photo‐generated carriers. This interfacial electric field acts as a driving force to propel the directional spatial separation of photogenerated electrons and holes, effectively enhancing photoconversion efficiency. Subsequently, by modifying the heterojunction surface with recognition elements such as antibodies or molecularly imprinted polymers, the formation of steric hindrance or insulating layers upon specific capture of target molecules significantly impedes interfacial electron transfer, leading to selective suppression of photocurrent. This synergistic mechanism first amplifies the substrate signal, then selectively suppresses it through specific binding, which ingeniously integrates optimized optoelectronic material properties with precise molecular recognition. It enables highly sensitive, selective, and quantitative detection of targets such as disease biomarkers and environmental pollutants.

The work of Hai et al. exemplifies this mechanism [62]. They combined the n‐type HOF FJU‐200 with n‐type CdSe QDs to construct a FJU‐200@CdSe step‐scheme (S‐scheme) heterojunction. Driven by the higher Fermi level of FJU‐200 compared to CdSe, electrons spontaneously diffuse across the interface to equate energy levels, establishing an internal electric field (IEF) pointing from FJU‐200 to CdSe. Under illumination, the IEF drives selective recombination of electrons in the CdSe conduction band with holes in the FJU‐200 valence band. Consequently, the strongly reducing electrons in the FJU‐200 conduction band are preserved for efficient export to the electrode, while holes in the CdSe valence band are retained, enabling efficient separation and rapid transport of photogenerated carriers. This generates a significantly enhanced and stable anodic photocurrent that serves as a robust signal baseline for high‐sensitivity PEC sensing (Figure 4a,b).

FIGURE 4.

(a) The photocurrent intensity of FJU‐200 under a bias voltage of 0–3 V. (b) photocurrent changes of immunosensor incubated step by step (curve a: FJU‐200/FTO, b: CdSe QDs/FTO, c: F&C/FTO, d: F&C/TGA/FTO, e: F&C/TGA/EDC/NHS/FTO, f: F&C/TGA/EDC/NHS/Ab/FTO, g: F&C/TGA/EDC/NHS/Ab/BSA/FTO, h: F&C/TGA/EDC/NHS/Ab/BSA/EGFR/FTO). [62] Reproduced from ref. [62] Copyright (2025), with permission from Elsevier. (c) The construction process of the PEC immunosensor for SCCA detection. [63] Reproduced from ref. [63] Copyright (2025), with permission from Elsevier. (d) Schematic presentation of the combination of FDU‐HOF‐2 and CO₂. [64]Reproduced from ref. [64] Copyright (2023), with permission from WILEY‐VCH.

2.2.2. Signal‐Off Strategy Induced by Steric Hindrance

While heterojunction engineering effectively establishes a robust baseline, the core task of PEC sensing lies in translating specific molecular recognition events into quantifiable electrical signal fluctuations. To address this, employing the steric hindrance effect based on the HOF flexible structure to construct signal‐off sensors has emerged as the most universal strategy. The abundant and easily modifiable active sites on HOF frameworks provide an ideal platform for the stable anchoring of diverse recognition elements, ranging from natural biomacromolecules (e.g., antibodies, aptamers) to biomimetic synthetic receptors. Upon the specific binding of target analytes to the recognition elements on the electrode surface, a dense and typically insulating organic barrier is constructed over the photoactive interface. This barrier not only significantly increases the interfacial electron transfer resistance but also physically obstructs the diffusion and permeation of electron donors from the electrolyte to the photoactive centers. As the concentration of the target increases, the interfacial steric hindrance intensifies, leading to a linear or logarithmic attenuation of the photocurrent signal, thereby achieving high‐sensitivity detection.

A recent study employs the HOF‐101/CdS heterojunction for the detection of squamous cell carcinoma antigen (SCCA) (Figure 4c). After establishing a highly photoactive interface based on a Z‐scheme heterojunction, researchers covalently immobilized specific SCCA antibodies via amide bonds using the abundant carboxyl groups on the HOF‐101 surface. Upon the capture of the target antigen SCCA, bulky antigen‐antibody immunocomplexes accumulated on the electrode surface, forming a dense, non‐conductive protein layer. This layer not only drastically increased the interfacial electron transfer resistance but also physically blocked the access of electron donors from the electrolyte to the photosensitive sites. Consequently, the photocurrent signal exhibited a highly sensitive linear quenching with increasing SCCA concentration. Benefiting from the high initial signal provided by the heterojunction and the effective modulation by steric hindrance, the sensor achieved a limit of detection as low as 0.6 pg/mL [63].

2.2.3. Host‐Guest Electronic Interactions

Distinct from the steric hindrance imposed by biological macromolecules, the detection of gases or small molecules leverages the intrinsic porosity and abundant active sites of HOFs to modulate the electronic state of the material via host‐guest chemical interactions. This mechanism involves a specific reaction between the target analyte and the HOF ligands within the framework, generating new species with altered energy levels. These species function as electron traps or recombination centers for photogenerated carriers, thereby triggering a sensitive PEC response.

A study on FDU‐HOF‐2 for CO₂ sensing elucidates this electronic modulation mechanism. [64] The pore channels of FDU‐HOF‐2 are decorated with diaminotriazine (DAT) moieties, which specifically capture CO₂ molecules to form carbamates in situ. The resulting carbamate derivatives introduce deep‐level states that capture photogenerated electrons, effectively impeding their transport to the electrode. Consequently, the photocurrent attenuates significantly with increasing CO₂ concentration. This mechanism, based on chemisorption‐induced electron trapping, enables high‐selectivity detection of small‐molecule gases without reliance on external recognition elements (Figure 4d).

3. Applications of HOF Luminescent Sensing and Photoelectric Sensing

3.1. Gas Sensing

HOFs represent an emerging class of crystalline porous materials exhibiting unique advantages in gas sensing applications. Their highly designable molecular architecture and abundant surface functional groups such as amino groups, carboxyl groups, and porphyrin rings, enable specific recognition for guest gas molecules, while their intrinsic porosity facilitates rapid gas diffusion [65, 66]. At the same time, their favorable crystallinity and structural stability provide a foundation for constructing reusable sensor devices. Leveraging these features, HOFs have been successfully developed into high‐performance gas sensing platforms.

Research initially focused on the intrinsic charge transport properties of HOFs. In 2019, Wang et al. [67] reported the first HOF‐based gas sensor, fabricating a porphyrin‐based HOF (UPC‐H4a) featuring ordered channels and abundant amino modifications. Leveraging the π‐π stacking channels for charge transport, the material functioned as an n‐type semiconductor that enabled high‐sensitivity room‐temperature detection and ultrafast response sensing of NO₂ through the specific electron capture interaction between amino groups within the channels and NO₂. Response and recovery times of 17.6 and 15.4 s respectively, with a detection limit below 40 ppb, pioneered a new direction for HOF sensing, demonstrating the feasibility of HOFs in electronic transduction.

In the domain of photoelectric gas sensing, the work by Wang et al. not only builds upon the PEC mechanism but also addresses a facile bottom‐up growth method to build HOF sheets with high yield. By synthesizing a porphyrin‐based HOF (FDU‐HOF‐2) that exhibits excellent solubility in specific solvents, the authors successfully combine the light‐harvesting capability of porphyrins with specific chemical recognition for highly selective photoelectric sensing. This work not only achieved high selectivity but, more importantly, demonstrated a versatile pathway for integrating crystalline HOFs into practical optoelectronic devices without complex in situ growth procedures [64].

Furthermore, Wang et al. [68] have successfully developed a hydrogen‐bonded organic framework (HOF‐6) based on tetra(4‐carboxyphenyl) porphyrin (Figure 5a). This material forms ammonia molecule recognition ‘pockets’ with confined spaces through its three‐dimensional interpenetrating structure. Ammonia molecules in exhaled breath are specifically captured within these pockets, forming multiple hydrogen‐bond interactions with nitrogen and oxygen atoms in the framework; The lone pair electrons within the ammonia molecules induce charge rearrangement and electron transfer within the hydrogen‐bonded organic framework, significantly altering the energy levels of its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), thereby narrowing the energy gap. This electronic structural change suppresses aggregation‐induced quenching within the material, triggering an intense ‘turn‐on’ luminescence response (Table 1).

FIGURE 5.

(a) Schematic diagram of the synthesis and structure of HOF‐6 [68]. Reproduced from ref. [68] Copyright (2025), with permission from Elsevier. (b) Energy level diagram indicating the sensing mechanism of 1 toward SO₂ [69]. Reproduced from ref. [69] Copyright (2021), with permission from Royal Society of Chemistry.

TABLE 1.

Comparison of detection performance between HOF‐based luminescent and photoelectric sensors.

Hydrogen‐bonded units	HOFs	Pore size (Å)	S BET (m2 g−1)	Analytes	Sensing modes	Detection limit (LOD)	Reference
H2TDPP	FDU‐HOF‐2	11.97 × 21.74	250	CO2	Photoelectric sensing	2.3 ppm	[40]
H2TDPP	UPC‐H4a	—	210.5	NO2	Photoelectric sensing (Resistor type)	< 40 ppb	[42]
ME, IPA	Tb@ME‐IPA	—	—	SO2	Luminescent sensing (Colourimetric and Ratiometric)	2.538 ppm	[44]
H4TCPP	HOF‐6	∼8.0 × 8.3 × 8.7	—	NH3	Luminescent sensing (turn‐on)	0.2 ppm	[43]
H4TBAPy, MA, TPA	Eu@PFC‐1@MA‐TPA	—	—	DCN	Luminescent sensing	0.3 µM	[51]
Tt, H2TPA	Eu@Tt‐TPA	6.78 × 2.88	—	MA	Luminescent sensing (Ratiometric)	0.87 ppm	[46]
H4TBAPy, PAH, AchE	AChE@PAH‐HOF	—	—	Fipronil	Luminescent sensing (Colourimetric)	1 ng mL−1	[50]
H3‐FLAC	MEP‐HOF	4.6	—	Nitrobenzene	Luminescent sensing (turn‐off)	61.8 ± 2.2 ppb	[48]
Triarylamidine bromide salts	BA‐C	7.4 × 4.6	—	Acetone	Luminescent sensing (emission peak shift)	66.74 ppm	[47]
H4TNAPy	AgNPs@HOF‐101	8.6	600	Mustard gas simulants	Photoelectric sensing	15.8 nmol L−1	[54]
H4TBAPy	MTV‐HOF‐101	～3.4	905‐1148	H2O, CEES	Pore sensing	—	[53]
Py‐4DAT	FDU‐HOF‐5	10.2	395.6	DCP	Luminescent sensing (dual mode)	31 ppb	[15]
MA, BTC	MA‐BTC HOF	—	—	DOX	Luminescent sensing (dual‐emission)	3.1 nmol L−1	[56]
ME, IPA	EuTb@ME‐IPA	—	—	CIP	Luminescent sensing (dual‐ emission)	0.037 µmol L−1	[57]
MA, PSA, TMA	Zn‐MOF/HOF heterojunction	Average 12.53	16.56	DOX	Luminescent sensing (dual‐ emission)	2.2 nmol L−1;	[14]
MA, CA, TMA, Luminol	Eu@L‐HOF	—	—	TA, KAE	Luminescent sensing	0.61 µM	[63]
MA, CA, TMA, Luminol, HBTc	Lumi‐HOF@Tb	—	—	α‐glucosidase	Luminescent Sensing (Ratiometric)	0.04 U L−1	[27]
PTCDA, DCD	PD‐HOF	—	—	α‐glucosidase	Photoelectric Sensing (Ratiometric)	0.0049 U mL−1	[64]
HOF‐TJ‐1, TA, ER	Eu@HOF‐TJ‐1@TA L‐skin (1)	—	—	Temperature	Luminescent Sensing (Ratiometric)	Sr max: 97.669% K−1 δT = 0.000952 K	[67]
H2BABDC	HOF‐50	5.9 × 6.6	171.5	Temperature (90–280 K)	Luminescent Sensing (Ratiometric)	Srmax: 4.52% K−1 δT = 0.48 K	[69]
MA, PDC	MA‐PDC	—	—	Temperature (303–403 K)	Luminescent Sensing (Ratiometric)	Sr max:1.03% K−1 δT = 0.059 K	[70]
H4TCBP	Eu@HOF‐TCBP	—	∼1900	Temperature (297–377K)	Luminescent Sensing (Ratiometric)	Sr max:5.787% K−1	[66]
TPE‐4PZ	8PZ	∼7.5–9.1 × 10.2–10.5	—	Temperature	Luminescent sensing	—	[68]
(R)‐BINOL derivatives	HOF‐2	∼4.8	237.6	1‐PEA	Chiral separation	ee value: 92%	[76]
H4TPE R/S‐DPEN	R/S‐iHOF‐19	—	—	Circularly polarized light (CPL)	Luminescent sensing (photochromic)	—	[77]
Zn2+, H2tpdc, Me2dpy	NKU‐777‐xD/xL	—	—	(1S,2S)/(1R,2R)‐DPEAN	Luminescent sensing (turn off)	KSV ratio: 2.07	[78]

HOFs are also being integrated into sophisticated optical devices. Nizamidin et al. [65] developed an optical waveguide sensor using a BIPA‐TC‐HOF film. By coupling the HOF with a LiNbO₃ film, they constructed a miniaturized photonic circuit. The interaction with acid or base gases modulates the light propagation loss within the waveguide, enabling an efficient ON/OFF optical switching response. This work marks a significant step from simple material synthesis to the fabrication of integrated photonic sensing components based on HOFs.

To address the challenge of sensing sensitivity in complex environments, researchers have introduced artificial intelligence (AI). Xu et al. [69] fabricated Tb³⁺‐functionalized ME‐IPA‐type HOF films (Tb@ME‐IPA/FTO) for highly selective detection of SO₂ and its derivatives. The analyte SO₂ forms N‐S non‐covalent interactions with amino groups in the HOF, inhibiting the LMCT process, and quenching the Tb³⁺ luminescence. Crucially, by establishing a backpropagation neural network (BPNN) model based on the luminescence response data, the sensor achieved intelligent identification and precise concentration prediction of SO₂, demonstrating the potential of the combination of AI and HOFs for smart environment monitoring (Figure 5b).

3.2. Organic Pollutants

Gas sensing focuses on volatile molecules in the atmosphere; the detection of organic pollutants in condensed phases presents distinct challenges, primarily due to complex matrix interference and the need for high specificity. The organic pollutants pose a grave threat to human health, ecological environments, and food safety. Their efficient detection constitutes a fundamental requirement for environmental monitoring, public health protection, and industrial safety. HOFs, with their solution processability and tunable pore environments, offer a versatile platform to address these issues. By leveraging their inherent properties such as hydrogen bonding, π‐π stacking, and hydrophobic adsorption, to engage in specific interactions with target pollutants, HOFs can convert pollutant binding events into detectable optical or electrical signals [70, 71].

In terms of fundamental building blocks design, the early classic research commenced with the classical Eu³⁺ functionalization design of melamine/HOF (p‐benzenedicarboxylic acid) derivatives. Liu et al. [72] constructed a dual‐emission system by coordinating Eu³⁺ to the free carboxyl groups on the HOF framework. This hybrid material comprises the intrinsic blue emission from the HOF host (∼425 nm) and the characteristic red emission from the Eu³⁺ center (∼615 nm). The sensing mechanism relies on the hydrogen bonding interactions between the analyte methylamine and the organic ligand. This interaction effectively disrupts the antenna effect, blocking the energy transfer from the ligand to the Eu³⁺, thereby selectively quenching the red emission. This pronounced ratiometric luminescence response enables highly selective and visual detection of methylamine, establishing a foundational paradigm for dual‐emission ratiometric sensing based on lanthanide‐functionalized HOF.

Subsequent research achieved significant breakthroughs in the chemistry of HOF monomers. The dynamic nature of HOFs allows for unique sensing mechanisms based on structural transformations. In 2024, Wang et al. [73] innovatively employed a tautomeric organic salt, triarylmethanamine bromide, as a building block to construct anionic HOFs, the densely stacked BA‐N‐HOF and the loosely packed BA‐C‐HOF that undergo reversible structural transformations triggered by acetone vapor (Figure 6a). The guest‐induced structural switching synchronizes with the change in luminescent behavior of the material, shifting from green thermally activated delayed luminescence (TADF) and red room‐temperature phosphorescence (RTP) of BA‐N, to blue transient luminescence and green RTP of BA‐C. Thus, acetone detection transforms into dynamic, reversible control over multimodal luminescent colors and properties, achieving ultra‐high sensitivity and specificity.

FIGURE 6.

(a) Tautomerism of BA‐N and BA‐C and conformation of BA‐N and BA‐C in the single crystal structure obtained at room temperature [73]. Reproduced from ref. [73] Copyright (2024), with permission from WILEY‐VCH. (b) Schematic depicting the chlorpyrifos detection principle [76]. Reproduced from ref. [76] Copyright (2025), with permission from WILEY‐VCH. (c) Machine Learning‐Assisted Eu(III)‐Functionalized HOF‐on‐HOF Composite‐Based Sensor Platform for Precise and Visual Identification of Multiple Pesticides [77]. Reproduced from ref. [77] Copyright (2024), with permission from American Chemical Society.

An alternative approach explores more complex topological structures. Zhao et al. [74] designed a unique homophotoluminescent compound (MEP‐HOF) based on C3‐symmetric rigid macrocyclic FLAC. The structural feature involves the supramolecular assembly of multiple macrocycles via C–H···π interactions, which further assemble into a three‐dimensional network with a sevenfold‐twisted topology. Through efficient PET between the electron‐rich triphenylamine cavity and electron‐deficient nitrobenzene, the material achieved highly sensitive and selective detection of trace nitrobenzene in water by significant fluorescence quenching change, highlighting the strategy of precise regulation of host‐guest electronic interactions through molecular and supramolecular design [75].

At the application integration level, research exhibits two major trends, including functional composites and system intelligence. In 2025, Li et al. [76] pioneered an alternative approach by co‐assembling acetylcholinesterase (AChE) with polyallylamine hydrochloride (PAH) into a HOF (AChE@PAH‐HOF) via electrostatic interactions and hydrogen bonding. This encapsulation effectively shields the enzyme and enhances its activity. The sensing mechanism reverted to the classical enzyme inhibition‐colorimetric approach, where organophosphorus pesticides such as chlorpyrifos inhibit AChE activity, thereby diminishing the color reaction between the substrate hydrolysis product thiocholine (TCh) and DTNB. By embedding AChE@PAH‐HOF within a glycerol‐sodium alginate hydrogel exhibiting high water retention and attaching it to plant surfaces, researchers successfully constructed a plant‐wearable sensor capable of long‐term, in situ monitoring of pesticide degradation dynamics (Figure 6b). This represents a leap from traditional ex vivo detection to continuous in vivo in situ monitoring.

The intelligent development in this field is exemplified by the research of Hu and Yan [77]. They advanced beyond a simple single‐component system to construct a hierarchical Eu(III)‐functionalized HOF‐on‐HOF composites Eu@PFC‐1@MA‐TPA, endowing the material with dual HOF components alongside Eu³⁺ luminescence. The distinct advantage of this architecture is the ability to generate distinctly different fluorescence response patterns for two distinct pesticides DCN and TBZ. DCN selectively quenches the 530 nm emission of the HOF framework (Type A), while TBZ simultaneously quenches both the framework and the Eu³⁺ center at 616 nm emissions (Type B). The researchers did not stop at pattern recognition via principal component analysis (PCA) (Figure 6c). Instead, they constructed these differentiated fluorescence images into datasets and trained them using a dense convolutional neural network (DenseNet), ultimately establishing an intelligent artificial vision platform integrating sensing materials with AI algorithms. This platform can automatically analyze fluorescence images within one second, akin to the human visual system, simultaneously outputting both pesticide type and concentration information with over 98% accuracy. This breakthrough marks a transition for HOF sensing from passive signal transducers to an intelligent new era capable of intelligent signal interpretation.

3.3. Chemical Warfare Agents

Chemical warfare agents (CWAs) such as nerve agents, for example, Sarin, VX and blister agents, for example, mustard gas, are a class of highly toxic chemicals designed for military applications that can rapidly cause death or severe injury through inhalation or skin contact. Consequently, the development of sensitive detection methods for CWAs is critical for early warning systems, personnel protection, timely medical intervention, and emergency response. The detection of CWAs demands not only ultra‐high sensitivity but also rigorous selectivity against complex environmental interferents. HOFs are uniquely positioned to address these challenges. Their flexible and lightweight organic backbones allow for the engineering of specific chemical recognition sites, for example, halogen bonds, Ag─Cl interactions that precisely target CWA functional groups. Furthermore, the intrinsic porosity and optical and electrical activity of HOFs facilitate the rapid transduction of these specific recognitions into readable signals, fulfilling the critical requirement for real‐time early warning systems.

Establishing structural stability is the foundational step for practical CWA sensing applications. Addressing the stability challenge, in 2020, Ma et al. [78] reported the hyperstable mesoporous HOF‐102 based on a ‘shape‐matching’ π–π stacking strategy. By the extension of geometrically matched pyrene‐based monomer conjugated arms to effectively avoid staggered stacking, the researchers expand the pore size to 3 nm while maintaining excellent thermochemical stability. This work not only demonstrated that HOFs can possess permanent mesoporous properties comparable to MOFs, but also, through the preparation of HOF‐102 fiber composites, first showcased their practicality in the photocatalytic degradation of mustard gas simulants. Although primarily focused on degradation, this work laid the critical material foundation, demonstrating that HOFs possess the requisite durability for CWAs sensing. Sensing specificity is often compromised by the environment, particularly humidity. In 2023, Gao et al. [79] pioneered a multivariate (MTV) strategy to engineer the pore structures of HOFs (Figure 7a). By co‐assembling pyrene monomers bearing electronically diverse substituents including ─H, ─CH₃, ─NH₂, and ─F, they achieved continuous, tunable hydrophilicity or hydrophobicity on pore surfaces. Although this study primarily focused on the selective capture of the simulant CEES over water, the ability to integrate electron‐donating and withdrawing groups into the photoactive pyrene backbone provides a versatile tool for modulating the optoelectronic properties of HOFs for sensing.

FIGURE 7.

(a) Schematic diagram of the structure of MTV‐HOF‐101 and CEES adsorption of HOF‐101‐R and MTV‐HOF‐101 [79]. Reproduced from ref. [79] Copyright (2023), with permission from WILEY‐VCH. (b) Schematic illustration of the fabrication process of AgNPs/HOFs and AgNPs@HOFs nanocomposites [80]. Reproduced from ref. [80] Copyright (2022), with permission from WILEY‐VCH. (c) Schematic illustration of the ICT mechanism for the fluorescence variation of FDU‐HOF‐5 induced by DCP [33]. Reproduced from ref. [33] Copyright (2025), with permission from WILEY‐VCH.

Building upon these structural and environmental regulation strategies, recent advances have finally realized the potential of HOFs as active sensing platforms. In 2022, Wang et al. [80] encapsulated Ag nanoparticles (AgNPs) within the channels of HOF‐101, constructing the AgNPs@HOF composite. The design combined size exclusion effects with specific Ag─Cl interactions, achieving synergistic selective detection for multiple mustard gas simulants with a detection limit as low as 15.8 nmol L⁻¹. This work marks the evolution of HOFs from simple adsorption materials to sensing platforms integrating physical and chemical which significantly enhances selectivity (Figure 7b).

Most recently, relevant research has advanced toward emulating the efficient, multifunctional synergistic mechanisms of biological enzymes. In 2025, Liu et al. were inspired by the active pockets of lipase enzymes. Inspired by the active pockets of lipase enzymes. Liu et al. [33] designed a biomimetic HOF (FDU‐HOF‐5) based on diamine‐triazine‐functionalized pyrene monomers. This material incorporated the principle of multiple weak interaction recognition. This endows the efficient capture of the nerve agent simulant DCP, rapid dual‐mode fluorescence and colorimetric detection with a response time <5 s, detection limit of 31 ppb, and humidity‐driven self‐degradation (Figure 7c). This represents a leap for HOFs in CWA sensing toward an integrated and intelligent response system.

Over the past five years, HOFs in CWA defence have progressed from static, hyperstable adsorbents toward dynamic, biomimetic sensing systems. Future research must confront challenges, including testing against authentic warfare agents, broad‐spectrum switchable recognition capabilities, and large‐scale flexible device integration. Leveraging AI to predict HOF structures and properties, alongside developing long‐lasting, stable, and renewable smart materials, will be pivotal in advancing these compounds from laboratory research toward practical combat protection equipment.

3.4. Antibiotics

The accumulation of antibiotic residues in food chains and the environment poses a significant public health risk, primarily by accelerating the proliferation of bacterial resistance. Necessarily, the development of rapid, sensitive on‐site detection technologies is critical. Luminescent and PEC sensing of HOF supports multi‐modal visualization and intelligent recognition detection, significantly enhancing the accuracy, reliability and practicality of detection, which provides a robust, innovative solution for food safety monitoring and environmental pollution early warning systems [81].

Li et al. have consecutively published two studies on intelligent sensing based on integrating HOFs with advanced chemometric analysis to achieve high‐precision sensing. First, a one‐pot synthesis was employed to form a Zn─MOF/HOF heterojunction between a ternary HOF monomer composed of melamine, 3‐pyridinesulfonic acid, and trimellitic acid, and Zn─MOF (Figure 8a). This sensor achieves dual‐emission fluorescence responses for four tetracycline antibiotics, including doxycycline, tetracycline, oxytetracycline, and chlortetracycline. By integrating adsorption enrichment of this material with detection recognition and machine learning algorithms such as convolutional neural networks (CNN), the system achieves 100% classification accuracy, eventually enabling a smartphone‐based visual detection platform [32]. Subsequently, an innovative HOF‐to‐MOF strategy was proposed by the same group, converting a melamine‐benzotriazole (MA‐BTC) HOF monomer into a Zn‐MBTC MOF with a nanocoral morphology [82]. Following functionalization with Ru(bpy)₃ ²⁺, the resulting dual‐emission luminescent sensor was integrated with machine learning algorithms, again achieving 100% accurate identification and classification of the aforementioned four antibiotics (Figure 8b). These works highlight the synergy between structural design and AI in resolving complex analyte mixtures.

FIGURE 8.

(a) Schematic illustration of multifunctional Zn‐MOF/HOF preparation procedure, capture [32]. Reproduced from ref. [32] Copyright (2025), with permission from Elsevier. (b) Detection and identification of TCs based on Ru@Zn‐MBTC [82]. Reproduced from ref. [82] Copyright (2025), with permission from Elsevier. (c) Schematic illustration of the preparation process of the sensor EuTb@ME‐IPA and EuTb@ME‐IPA film [83]. Reproduced from ref. [83] Copyright (2025), with permission from Elsevier.

Except for combining with machine learning, functionalization with lanthanide ions is also an effective way for enhancing sensitivity and medium adaptability. Furthermore, Meng et al. [83] developed a ratiometric fluorescence sensor by anchoring Eu³⁺/Tb³⁺onto melamine‐isophthalic acid (ME‐IPA) type HOF monomers. Leveraging the antenna effect, this system overcame solvent‐dependent quenching limitations, enabling flexible and rapid detection of norfloxacin (NOR) and ciprofloxacin (CIP) across ethanol, aqueous solution, and solid‐state films (Figure 8c).

These studies illustrate a significant evolution in HOF‐based antibiotic sensing, from exploring the intrinsic luminescence properties of HOFs to utilizing them in constructing complex sensing systems such as dual‐mode, bipolar electrodes for detecting single or dual‐target antibiotics, and further integrating intelligent data processing. These approaches span from fundamental property exploration to advanced intelligent applications, achieving multi‐target precise identification and multi‐medium universal detection, and demonstrating the innovation of HOFs in the field of antibiotic sensing.

3.5. Biosensing

Biosensing technology is essential for real‐time, highly sensitive detection in fields including disease diagnosis, environmental monitoring, and biosecurity [84]. HOFs, owing to their biocompatibility, structural tunability and functional designability, demonstrate significant application potential across diverse biomedical and sensing domains [85, 86, 87].

In the field of disease treatment, taking photodynamic therapy as an example, in 2022, Zhang et al. [88] selected tetracarboxyphenylporphyrin (TCPP) as the main component of the HOF and encapsulated the two‐photon dye DSM to construct a near‐infrared‐II (NIR‐II) excited photosensitizing HOF. Utilizing the ordered channels of the HOF effectively mitigated dye self‐quenching, while efficiently facilitating singlet oxygen generation through FRET between TCPP and DSM (Figure 9a). This architecture enabled the first realization of NIR‐II‐excited HOF‐based photo‐oxidative therapy for Alzheimer's disease, resolving critical challenges of deep tissue penetration and photobiological safety [89].

FIGURE 9.

(a) Schematic representation of the synthesis of the NIR‐II hydrogen‐bonded organic frameworks [88]. Reproduced from ref. [88] Copyright (2021), with permission from WILEY‐VCH. (b) Eu@L‐HOF dual‐mode optical signal for polyphenol detection [91]. Reproduced from ref. [91] Copyright (2025), with permission from Elsevier.

The biosensing of HOFs has expanded from in vivo therapy to high‐sensitivity in vitro detection, with the signal output evolving from single optical signals to multi‐modal synergistic responses [90]. For instance, in 2025, Chen et al. [91] engineered the first dual‐modal HOF probe Eu@L‐HOF by doping Eu³⁺ into a luminol‐functionalized matrix assembled from melamine (MA), cyanuric acid (CA), and trimellitic acid (TMA). This system enabled the detection of polyphenols through two concurrent mechanisms. The IFE quenched Eu³⁺ luminescence, while their antioxidant properties suppress luminol‐H₂O₂ chemiluminescence, enabling dual‐signal verification and highly sensitive detection of target compounds (Figure 9b). Similarly, Li et al. [49] previously proposed a multi‐ratio fluorescence‐chemiluminescence probe based on Lumi‐HOF@Tb for detecting α‐glucosidase activity and screening its inhibitors. The sensing mechanism relies on α‐glucosidase catalyzing the hydrolysis of substrate pNGP into p‐nitrophenol (pNP) and α‐glucose. pNP restores Tb³⁺ luminescence at 546 nm via an IFE and forms a luminol‐pNP complex emitting fluorescence at 466 nm [92].

In summary, these studies highlight the versatility of HOFs in biosensing, extending from optical signaling to photoelectrochemical sensing [93, 94]. When combined with immunorecognition or heterojunction designs, HOFs are establishing a new paradigm for the ultrasensitive detection of a broad spectrum of targets including allergens, bacteria, tumor markers, and other targets, ranging from small‐molecule biomarkers to intact microbial cells.

3.6. Temperature Sensing

Temperature sensing is indispensable in scientific research, healthcare, industrial applications, and environmental monitoring. HOFs can achieve highly sensitive, self‐calibrating ratiometric temperature detection by rationally regulating hydrogen‐bond networks and introducing lanthanide ions to construct dual‐emission systems. These systems offer visible colour‐change responses, wide temperature adaptability, and easy integration into flexible applications.

Over the past five years, research on HOF‐based ratiometric temperature sensors has evolved from functional integration to low‐cost simplification in both monomer design and sensing mechanisms. In 2020, Feng et al. [95] employed electrophoretic deposition to fabricate Eu³⁺‐functionalized HOF films based on the polycarboxylate ligand H₄TCBP. By harnessing energy transfer from the ligand to Eu³⁺, they achieved ratiometric temperature sensing with a sensitivity of 5.787% K⁻¹, enabling highly sensitive luminescence‐based temperature sensing. Subsequently, Xu and Yan drew inspiration from shark skin, innovatively encapsulating HOF‐TJ‐1 and Eu³⁺ with the phase‐change material tetradecanoic acid (TA) to construct a bionic skin capable of dual‐mode temperature and underwater acoustic sensing (Figure 10a) [96]. Their strategy of regulating energy transfer via TA phase change achieved an ultra‐high sensitivity of 97.669% K⁻¹ and an exceptionally low temperature uncertainty (0.000952 K), demonstrating the potential of HOFs in dual‐mode acoustic and thermal sensing.

FIGURE 10.

(a) Schematic representation indicating that a sharkskin‐inspired Eu@HOF‐TJ‐1@TA L‐skin (1) as a temperature‐acoustic bifunctional sensor can be employed for underwater temperature and sound sensing [96]. Reproduced from ref. [96] Copyright (2023), with permission from WILEY‐VCH. (b) the 3D framework of synthesized HOF‐50. (c) Schematic representation for the preparation of HOF/polymer membrane and anchoring lanthanide and a schematic diagram depicting the starting organic building blocks [98]. Reproduced from ref. [98] Copyright (2024), with permission from WILEY‐VCH.

Beyond incorporating rare‐earth metal ions, constructing purely organic host‐guest systems through the local dynamic behavior of organic segments within the HOF framework represents another frontier strategy for achieving programmable temperature sensing. Huang et al. [97] designed a flexible high‐performance polyamide (HOF) 8PZ using flexible ethyl ester chains and tetraphenylethylene (TPE) derivatives as building blocks. Upon external stimuli such as solvent and temperature changes, the ethyl ester chains can locally rearrange while preserving the overall 8PZ framework structure, effectively modulating pore size. Leveraging this unique local flexibility, 8PZ achieves adaptive accommodation of 3‐alkylthiophene guests with varying alkyl chain lengths, forming high‐quality host‐guest eutectic 8PZ‐SCX complexes whilst maintaining stable luminescent properties. Upon heating to a specific temperature, guest molecules escape from the eutectic, causing the HOF to transition from an open structure to a thermodynamically more stable compact structure. Consequently, the luminescent colour abruptly shifts from blue to yellow‐green, enabling visual temperature alarms.

To further enhance the stability of the sensing material under extreme conditions, Liu et al. [98] designed an allyl‐containing polymerizable HOF‐50 (Figure 10b). Employing an orthotopic copolymerization strategy, they covalently embedded it into a polymer matrix to fabricate PolyHOF films with tunable mechanical properties. Even at extremely low Ln³⁺ content (0.19 wt%), while maintaining outstanding temperature sensing performance across a pH range of 0–14, highlighting the advantages of covalent hybridization in stability and cost control (Figure 10c) [56]. In 2025, Dong et al. [99] pursued further material simplification by constructing layered HOF MA‐PDC using inexpensive bidentate ligands 2,5‐pyridinedicarboxylic acid (PDC) and melamine (MA). Post‐modification introduced Tb³⁺/Eu³⁺ ions leverage temperature‐enhanced Tb³⁺→Eu³⁺ energy transfer and the Mott‐Seitz model, they achieved ratiometric‐based temperature measurement across a broad range (303–443 K) with low cost (1.03% K⁻¹), offering a novel pathway for developing low‐cost, high‐performance colourimetric thermometers.

3.7. Chiral Recognition and Sensing

Chiral separation and recognition are critical requirements in the pharmaceutical and chemical industries, as pharmaceuticals and pesticides exhibit markedly different biological activities [100]. The core of chiral signal transduction lies in converting enantioselective recognition into optical signals. In chiral hydrogen‐bonded organic frameworks, chiral cavities selectively enrich specific enantiomers through steric matching and multiple hydrogen bonding, thereby triggering two signal transduction mechanisms [101]. One is excited‐state energy competition, where guest absorption of excitation energy leads to selective fluorescence quenching. The other is circularly polarized luminescence (CPL) emission [102]. The underlying physical principle of CPL generation is that the helical arrangement of chromophores in the chiral framework induces level splitting via exciton coupling after excitation, while the coupling between electric and magnetic dipole transitions results in unequal emission probabilities of left‐ and right‐handed circularly polarized light, with the discrepancy quantified by the dissymmetry factor g _lum [103]. Together, these two mechanisms enable efficient transduction from molecular recognition to chiroptical signals [104].

The feasibility of constructing stable porous frameworks with distinct chiral microenvironments was first demonstrated by Li et al. in 2014 [105]. By the combination of the hiral induction capabilities of 1,1’‐binaphthol (BINOL) framework—with the strong hydrogen‐bonding unit 2,4‐diaminotriazine (DAT) (Figure 11a), the authors successfully synthesized the first enantiomeric HOF, HOF‐2, exhibiting permanent micropores. The resulting helical three‐dimensional channels induced by chiral BINOL units, which engaged in precisely stereoselective hydrogen bonding interactions with guest molecules, achieved an enantiomeric excess (ee) separation of up to 92% for secondary alcohols. This work established the fundamental strategy of combining a chiral cavity with a strong hydrogen‐bond network for constructing high‐performance chiral separation materials.

FIGURE 11.

(a) Organic Building Block to Build HOF‐2 [105]. Reproduced from ref. [105]. Copyright (2014), with permission from American Chemical Society. (b) Chromophore tetrakis(4‐sulfophenyl) ethylene (H₄TPE), chiral source building blocks R/S‐DPEN and R/S‐iHOF‐19 constructs, and supramolecular structure [106]. Reproduced from ref. [106]. Copyright (2025), with permission from WILEY‐VCH. (c) The synthesis of racemic products of NKU‐777‐P/M and enantiomerically enriched NKU‐777‐xD/xL. (d) The coordination‐chain‐based hydrogen‐bonded framework (left) and two types of weak interactions (right). Atom code: Zn, light purple; S, yellow; N, blue; C, grey; O, red [107]. Reproduced from ref. [107]. Copyright (2025), with permission from Elsevier.

The foundational separation studies validated the recognition specificity of HOFs; translating this capability into readable signals is essential for sensing applications. ’Recent research has expanded beyond static separation to pursue intelligent responsive materials capable of signal transduction. The 2025 report by Zhou et al. on the ionic chiral HOF R/S‐iHOF‐19 marked a significant progress in this field [106]. Instead of the conventional neutral ligand strategy, they employed oppositely charged building blocks, tetra(4‐sulfonatophenyl) ethylene and chiral 1,2‐diphenylethylenediamine, to complete self‐assembly via the synergistic interaction of electrostatic forces and hydrogen bonds (Figure 11b). This approach achieves dual gating effects modulated by temperature and solvent molecules. At low temperatures of −20°C, the continuous hydrogen‐bond network formed by solvent molecules serves as an efficient electron transport channel, facilitating electron transport and photochromic phenomena. The removal of the solvent or elevation of temperature can close this channel. Concurrently, the helical framework constructed by chiral amines successfully transfers chirality to the luminescent moiety, yielding a pronounced circularly polarized luminescence (CPL) signal whose asymmetry factor intensifies with decreasing temperature. The integration of light response, temperature response, and inherent chiral optical properties positions this material as a candidate for highly secure and multi‐key information encryption materials.

Zhao et al. [107] reported a novel strategy for constructing enantiomerically enriched chiral materials from achiral building blocks by regulating spontaneous resolution via a recyclable chiral inducer. Employing D‐ or L‐mandelic acid (D/L‐MA) as the inducer, the researchers synthesized a series of hydrogen‐bonded framework (HBF) materials, NKU‐777‐xD/xL (where x = 0.1–0.7 denotes the molar amount of inducer), via a one‐pot synthesis. Mechanistic studies revealed that the key to the chiral induction process lies in the inducer forming a chiral microenvironment in solution. Through non‐covalent interactions, this microenvironment guides the enantioselective crystallization of Zn²⁺, 2,5‐thienedicarboxylic acid (H₂tpdc), and 5,5’‐dimethyl‐2,2’‐bipyridine (Me₂dpy) from achiral precursors. ‐dimethyl‐2,2’‐bipyridine (Me₂dpy) for enantioselective crystallization. This yields P‐type or M‐type (NKU‐777‐P/M) crystals with enantiomeric excess, whilst the inducer can be recovered and reused for at least four cycles (Figure 11c).

In sensing applications, this material exhibits outstanding enantioselective sensing performance toward the important chiral reagents (1S,2S)‐ and (1R,2R)‐1,2‐diphenyltetrahydrodiamine (S/R‐DPEAN). The ultraviolet absorption spectrum of the analyte S/R‐DPEAN significantly overlaps with the excitation spectrum of NKU‐777 within the 200–400 nm wavelength range, leading to competitive absorption of excitation light energy. Following absorption, the analyte dissipates energy via non‐radiative transitions, thereby quenching the framework luminescence (Figure 11d).

Overall, these endeavours of HOFs for chiral recognition and sensing have not only illustrated the profound integration of materials chemistry, supramolecular chemistry and chiral science, but also paved the way for the practical applications of HOFs in cutting‐edge fields such as drug purification, enantiomer analysis, advanced optical anti‐counterfeiting and information encryption.

4. Summary and Outlook

This review systematically summarizes the recent advancements in porous HOFs for luminescent and photoelectric sensing. From diverse signal transduction mechanisms to specific sensing applications for analytes, HOFs have demonstrated unique potential in the field of sensing. Compared to traditional inorganic zeolites or MOFs, the most distinguishing features of HOFs are their solution processability and intrinsic recyclability. The assembly driven by weak interactions including hydrogen bonding and π‐π stacking and so on, not only facilitates low‐cost preparation and structural regeneration of HOFs, but also facilitates the integration of HOF‐based sensors into devices.

In terms of monomer design, the field has progressed from utilizing classical ligands to incorporating functionalized macrocycles and dynamic tautomers, and further to the construction of hierarchical composites. Regarding sensing mechanisms, strategies have diversified from fundamental energy transfer and PET to structure‐induced ratiometric luminescence, recently expanding into biomimetic enzymatic inhibition. At the systems level, progress has advanced from single‐analyte detection in solution to in vivo monitoring, integrating with AI to achieve multi‐target and high‐throughput intelligent recognition. Collectively, these developments have propelled HOFs from static sensing materials into a comprehensive, dynamic, intelligent, and analytical platform integrating chemistry, materials science, biology, and information technology. chemistry, materials science, and information technology.

However, it must be acknowledged that despite the remarkable achievements in HOF‐based sensing materials, compared to other crystalline porous materials, such as MOFs and COFs, HOFs are still in an early stage of development regarding both structural design and luminescent and photoelectric sensing applications. To bridge the gap between fundamental research and practical deployment, future research must confront several critical challenges.

• (1) Current HOF‐based luminescent sensors often rely on encapsulated lanthanide ions to achieve high quantum yields or ratiometric responses. However, this dependence compromises the intrinsic advantages of HOFs as pure organic materials, introducing potential toxicity and cost concerns. Future efforts should focus on engineering metal‐free frameworks by incorporating highly luminescent organic tectons or constructing donor‐acceptor charge‐transfer networks directly within the backbone.

• (2) While strategies such as π‐π stacking and charge‐assisted hydrogen bonding have improved the stability of HOFs, many frameworks remain susceptible to hydrolysis or structural degradation under harsh acidic/basic conditions or high humidity. Future molecular design must move beyond simple assembly to engineering hydrophobic shielding layers or interpenetrated networks that kinetically protect the hydrogen‐bonded nodes, to ensure long‐term structural stability in complex environments.

• (3) While HOFs show outstanding performance in luminescent sensing, their application in PEC sensing is often limited by poor intrinsic conductivity. Most HOFs function as insulators, impeding efficient charge carrier transport. To advance photoelectric sensing, research must target the design of π‐conjugated frameworks with continuous orbital overlap. Strategies such as establishing long‐range π‐π stacking columns or introducing doping agents to form charge‐transfer complexes are essential to enhance sensitivity in HOF‐based PEC sensors.

Conflicts of Interest

The authors declare no conflicts of interest.

Biographies

Ziyun Chai is a master's student in the School of Chemical Engineering and Technology at Hebei University of Technology. Her current research focuses on the design and synthesis of multifunctional luminescent materials.

Chun Yang received his PhD degree from Nankai University in 2010 and served as a visiting scholar at Florida Institute of Technology from 2015 to 2016. He is currently an Associate Professor and Master's Supervisor at the School of Chemical Engineering Technology, Hebei University of Technology. His research focuses on the design, synthesis, and application of metastable photoacid and stimulus‐responsive functional coordination compounds and hybrid luminescent multifunctional materials.

Tian‐Meng Guo obtained her Bachelor's degree from Sichuan University in 2016 and her PhD degree from Nankai University in 2023. She is currently a Lecturer and Master's Supervisor at School of Chemical Engineering and Technology, Hebei University of Technology. Her research focuses on molecular‐based piezoelectric ferroelectric materials and hybrid luminescent multifunctional materials.

Zhiqiang Li received his BS at Inner Mongolia University in 2009 and his PhD at Nankai University under the guidance of Prof. Yu Liu in 2014. From 2017 to 2019, he worked as a Postdoctoral Research Fellow under the supervision of Prof. Banglin Chen at the University of Texas at San Antonio. Currently, he is a Full Professor of School of Chemical Engineering and Technology, Hebei University of Technology. His current research interest is the self‐assembly of lanthanide‐containing hybrid luminescent materials and functionalized HOFs.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.