Recent advances in photoelectrochemical platforms based on porous materials for environmental pollutant detection

Human health and ecology are seriously threatened by harmful environmental contaminants. It is essential to develop efficient and simple methods for their detection. Environmental pollutants can be detected using photoelectrochemical (PEC) detection technologies. The key ingredient in the PEC sensing system is the photoactive material. Due to the unique characteristics, such as a large surface area, enhanced exposure of active sites, and effective mass capture and diffusion, porous materials have been regarded as ideal sensing materials for the construction of PEC sensors. Extensive efforts have been devoted to the development and modification of PEC sensors based on porous materials. However, a review of the relationship between detection performance and the structure of porous materials is still lacking. In this work, we present an overview of PEC sensors based on porous materials. A number of typical porous materials are introduced separately, and their applications in PEC detection of different types of environmental pollutants are also discussed. More importantly, special attention has been paid to how the porous material's structure affects aspects like sensitivity, selectivity, and detection limits of the associated PEC sensor. In addition, future research perspectives in the area of PEC sensors based on porous materials are presented.


Introduction
Over the past few decades, global population growth and industrial development have resulted in serious environmental pollution. 1,2Substantial amounts of harmful chemical compounds, such as heavy metal ions, antibiotics, pesticides, and phenolics, have been discharged into the environment.The self-purication capacity of natural water bodies cannot cope with these increasing environmental pollutants, posing a severe threat to both the ecological environment and public health. 3,4hese contaminants can be identied using traditional

Shiben Liu
Shiben Liu received a PhD degree from the Ocean University of China in 2022 before he joined the School of Chemistry and Chemical Engineering at Shandong University as a postdoc.His scientic interests are devoted to the fabrication of multifunctional porous materials for photoelectrochemical applications.
6][7][8][9] However, the aforementioned detection techniques are limited by the large experimental instruments, specic operating conditions, and high training costs. 10,11herefore, it is crucial to develop efficient techniques for the sensitive identication of these environmental contaminants.
Recently, photoelectrochemical (PEC) detection methods, as a branch of electrochemical (EC) detection techniques, have gained much attention in the eld of tracing environmental pollutants, [12][13][14][15] which can be seen from the number of publications since 2010 (Fig. 1).7][18] Since light and photocurrent are employed as the excitation source and identication signal, respectively, a PEC sensor handles relatively lower background noise and higher sensitivity than those of conventional electrochemical detection methods due to the difference in the energy form of the excitation source and the converted electrical signal.Moreover, the PEC sensor is not directly in contact with the measured object, resulting in less inuence from the measurement condition. 19][22][23] In a typical PEC sensing system, the incoming light is regarded as the excitation source, and the electrical signal is used as the detection signal. 18,24,25When exposed to light, the electrode containing the photoactive material is excited, resulting in the generation of photogenerated electron-hole pairs, which are subsequently transferred to the electrode surface.Cathodic photocurrent results from the electrons on the condition band (CB) being trapped by electron acceptors (A); otherwise, the holes on the valence band (VB) transfer to the surface of photoactive materials, where they react with the electron donors (D) to form anodic photocurrent (Fig. 2A). 17ccording to the different generation processes of the photocurrents at the interface between the electrode and electrolyte, the PEC detection sensors can be classied into two categories: (1) the analytes serve as electron donors or acceptors to directly react with photoactive materials; 26,27 (2) the electrodes are rst modied by recognition elements, aer which the target concentration is determined through indirect physicochemical interactions between the targets and the recognition elements (Fig. 2B).For the commonly used recognition elements used in the PEC detection system, they can be divided into three categories: (1) aptamers refer to single-stranded synthetic nucleic acid molecules (DNA or RNA); [28][29][30] (2) antibodies, which are mainly proteins, can be used as recognition elements to bind with target antigens such as proteins, toxins, and pathogens; 31 (3) enzymes are biological catalysts that can bind to and act upon specic substrates. 32s can be seen from the inset above, whether the recognition element is contained or is free on photoelectrochemical sensing platforms, photoelectrochemical detection performance of a fabricated electrode is mainly determined by three factors, including light absorption capacity, photogenerated charge carrier separation and transportation within photoactive materials, and the transfer of surface charge carrier to the detection species. 33To achieve excellent detection activity with

Bin Cai
Bin Cai obtained his PhD from Technische Universitat Dresden under Prof. Alexander Eychmuller in 2017.He then continued his career as a postdoc at Massachusetts Institute of Technology with Profs.Yuriy Roman and Yang Shao-Horn, and Pacic Northwest National Laboratory with Drs Chun-Long Chen and James De Yoreo.In 2020, he joined the School of Chemistry and Chemical Engineering as a professor at Shandong University and his current research interest focuses on nanocrystals, electrochemistry and analytical chemistry.
PEC sensors, researchers have focused on exploiting various sensing devices and detection modes, and the design engineering of photoactive materials.Much effort has been directed towards reducing costs and simplifying the device fabrication for versatile and portable PEC sensing platforms.5][36] Another area of research involves exploring different detection patterns, such as split-type detection, self-power detection, visual detection, and highthroughput detection.Despite extensive efforts in designing detection devices and modifying detection modes, the primary focus in constructing PEC platforms has been on investigating photoelectrodes. 19,37To date, signicant effort has been devoted to the development of photoactive materials as electrodes for PEC sensors.These materials include metal oxides, metal organic frameworks, graphene and carbon nitride. 38,39][42] Research interests in the applications of porous material-based PEC sensors were rapidly increasing, particularly in the eld of determining environmental pollutants. 43Due to their unique structure, porous materials are preferred over bulk materials for photoelectrochemical determination of environmental contaminants.As illustrated in Fig. 3, the porous material has a structural inuence on the three factors governing the PEC detection activity of the sensor.
(1) Light absorption capability: porous materials with small pores enable the incident light to penetrate their pore walls and scatter intensely inside the pore channels, thereby greatly improving the light harvesting ability of the porous materials. 44urther, the feasible modications of the band structure of the porous materials by altering the pore diameter also promote light absorption, particularly in the visible light region.
(2) Photogenerated charge carrier separation and transport efficiency: the photoelectric conversion efficiency of a photoelectrode depends on the separation and transport of photogenerated charge pairs within the photoactive material.In the case of bulk materials, photogenerated charge pair separation normally occurs in the space charge layer and the adjacent layers, leading to easy recombination.However, for porous materials, the charge separation and migration paths can be shortened due to their unique porous structures, 45 enabling efficient photogenerated charge transfer within porous materials.
(3) Surface charge carrier transfer rate: porous materials possess a large specic surface area, which is advantageous for mass transfer during the photoelectrochemical detection.In addition, the exposed surface areas with abundant active sites provide sufficient functional groups conducive to interaction with recognition elements or direct trapping of the probe species.Hence, porous materials exhibit rapid surface charge carrier transfer rates and reaction kinetics.
Thus, photoelectrochemical sensing platforms based on porous materials have remarkable detection capabilities against environmental contaminants.This can be attributed to several factors, including enhanced light absorption, high photogenerated charge carrier separation and transport efficiency, and rapid surface charge carrier transfer rates during photoelectrochemical detection.As a result, these platforms offer a wide linear range, low detection limits, high sensitivity, excellent selectivity, and outstanding stability.
7][48][49][50] However, these studies focused on strategies for fabricating PEC detection systems and identifying the multiple contaminants, neglecting to discuss the effect of the presence of porous materials on the detection activity of PEC sensors.For instance, in 2020, Shu et al. provided a comprehensive summary of current research in photoelectrochemical sensing, with a particular emphasis on material design and engineering to regulate photoelectrochemical sensing performance.They also discussed photoelectrochemical sensing devices and detection modes. 33In another review by Li and coworkers, the central theme was a comparative analysis of electrochemical detection and photoelectrochemical detection, specically addressing the question of which analytical method is more effective for tracing environmental pollutants. 11urther, in 2021, Yan et al. provided a concise overview of the fundamental and research progress of functional materials (such as metals, metal oxides, inorganic 2D materials, and carbon nanomaterials) in electrochemical and photoelectrochemical technologies for monitoring environmental pollutants. 51Despite these efforts, few reviews have focused on the relationship between the structure of porous materials and the detection activity of associated PEC sensors.So, in this review, we seek to thoroughly investigate the recent advancements in porous materials for environmental contaminant detection.Indeed, porous materials encompass a wide range of different types.Here, we specically focus on photoactive porous materials, including but not limited to metal oxides, metal-organic frameworks (MOFs), covalent organic frameworks (COFs), graphitic carbon nitride (g-C 3 N 4 ), and MXene (Table 1).We extensively discuss the structural effects of porous materials on the performance of PEC sensors.Finally, we propose the main challenges and future prospects of porous materials in the realm of PEC detection sensors.However, due to the space limitations of this review and the widespread use of porous materials in the eld of PEC detection sensors, many intriguing and signicant studies may not have been addressed.We apologize for any inadvertent omissions and appreciate the valuable contributions of all researchers in this eld.

Metal oxides
Metal oxides play a crucial role in the eld of photoelectrochemical sensing. 52,53Indeed, there is a large body of research focusing on porous metal oxides for PEC sensing, which is difficult to categorize in a simple way, so here we only present some representative porous metal oxides, in particular semiconductor metal oxides (Table 2).5][56][57][58][59] In addition, PEC detection sensors have been constructed using ZnO, SnO 2 , WO 3 , and Fe 2 O 3 (ref.60-65)  (Table 3).For instance, in a study by Tavella and co-workers, a porous TiO 2 array modied titanium electrode was constructed for PEC sensing of dopamine. 66The as-prepared electrode performed well for dopamine and has a wide response range (200∼1500 mM) and a low detection limit (20 mM).In addition to nanoarrays, nanotubes also have a large number of pores.For instance, Fan et al. prepared a BiOI nanoowers/TiO 2 nanotubes (BiOI/TiO 2 ) composite through a hydrothermal method (Fig. 4A and B). 67Due to the presence of porous nanotube structure, the BiOI NFs/TiO 2 NTs electrode provided more spaces for anchoring the aptamer, and thus the PEC platform demonstrated a signicant activity for atrazine determination with a low detection limit of 0.5 pM (Fig. 4C and D).
Similarly, Wu et al. developed a PEC sensor for chlorpyrifos (CPF) detection based on bismuth sulde nanoparticles decorating TiO 2 nanotubes with oxygen vacancies (Bi 2 S 3 /V o -TNTAs), which realized a rapid photocurrent response to CPF within a linear range of 0.07∼3.0mM. 68n addition to the porous TiO 2 nanomaterials, other photoactive porous metal oxides (ZnO, SnO 2 , WO 3 , and Fe 2 O 3 ) have also exhibited excellent photoelectrochemical detection activity towards environmental pollutants.In a study by Wu and coworkers, CdS nanocrystals were decorated on porous onedimensional (1D) ZnO nanorods through a pulsed electrodeposition technique. 69The resulting CdS/ZnO hybrid photoelectrode was employed for PEC detection of the heavy metal ion, Cu 2+ .The porous ZnO nanorods provided an enlarged surface area for the dispersion of nano-sized CdS nanocrystals, thereby enhancing the photogenerated electron transfer via the "1D electron highway".The CdS/ZnO composite electrode demonstrated an ultrasensitive LOD of approximately 3 nM within a wide linear range of 0.01∼1000 mM.Meanwhile, Velmurugan et al. prepared a WO 3 /CuMnO 2 p-n heterojunction composite and applied it for "signal-on" PEC sensing of nitrofurazone. 70Porous WO 3 nano-tiles were synthesized via a hydrothermal method and coupled with CuMnO 2 nanoparticles by an evaporative impregnation method.The porous WO 3 provided abundant surface sites for interaction with CuMnO 2 nanoparticles.Further, a p-n heterojunction was formed between the p-type CuMnO 2 and the n-type WO 3 , which promoted photogenerated electron transfer due to the presence of a built-in electric eld. 71As a result, the PEC nitrofurazone sensing performance of the WO 3 /CuMnO 2 composite electrode demonstrated a wider detection range of 0.015∼32 mM with a lower LOD of 1. 19   a MIP: molecularly imprinted polymer; NTAs: nanotubes; NRs: nanorods; V o -TNTAs: TiO 2 nanotube arrays with oxygen vacancies; TiNTs: TiO 2 nanotube arrays; GQDs: graphene quantum dots; P(33DT-co-3TPCA): poly(3,3 0 -dithiophene-co-3-thiophenecarboxylic acid); [BMIM]Cl: 1-butyl-3methylimidazolium chloride.demonstrated a LOD of around 0.3 mM in the range from 0.01 mM to 25 mM.Hence, PEC sensors based on porous metal oxides hold great potential for tracing environmental contaminants.

Metal-organic frameworks
Metal-organic frameworks (MOFs), which are potential porous materials composed of transition metal ions and organic ligands, have attracted much attention in the area of environmental pollutants determination. 86,879][90][91] The Zeolitic Imidazolate Framework (ZIF), Materials Institute Lavoisier frameworks (MILs), Universitetet i Oslo (UiO), copper(II) benzene-1,3,5-tricarboxylate (Cu-BTC), and porphyrin-based MOFs (PCN) series MOFs have been extensively investigated for PEC sensing applications against hazardous pollutants (Table 4). 92.2.1.ZIF.The ZIF family of MOFs has been widely employed in the fabrication of PEC detection sensors.Among them, zeolitic imidazolate framework-8 (ZIF-8), a typical member of MOF materials, is composed of zinc ions and 2methyl-imidazole ligands.ZIF-8 possesses excellent stability in aqueous conditions.93,94 More importantly, ZIF-8 exhibits porous structures and has the ability to generate abundant reactive species under light irradiation.These unique properties make it suitable for constructing PEC detection platforms for environmental pollutant determination.For instance, Chen et al. prepared a ZIF-8/ZnIn 2 S 4 (ZIF-8@ZIS) photoelectrode for PEC detecting tetracycline (Fig. 5A).95 In this detection system, the porous ZIF-8, with plenty of active Zn(II) sites, could interact rapidly and specically with the tetracycline, resulting in quenched photocurrent due to the swi transfer of photoelectrons.The lowest detection limit calculated for the ZIF-8@ZIS PEC sensor was ca.0.1 pM (Fig. 5B and C).In addition, it exhibited exceptional speediness, high stability, and strong selectivity during PEC monitoring of tetracycline.
2.2.2.MIL.Similar to the ZIF series MOFs, the MIL series MOFs have also been utilized in PEC detection.Recently, NH 2 -MIL-125(Ti), a titanium-based metal organic framework (Ti-MOF) derived from the Ti metal ions and NH 2 -BDC ligands, has exhibited excellent visible light absorption ability and stable redox reaction properties. 96In 2021, Yang et al. developed a molecularly imprinted NH 2 -MIL-125(Ti)/TiO 2 compositebased photoelectrochemical sensor for oxytetracycline detection. 97The NH 2 -MIL-125(Ti)/TiO 2 hybrid composite was synthesized via a simple solvothermal method, and then a molecularly imprinted polymer (MIP) was modied as a recognition element.The photocurrent of the MIP@NH 2 -MIL-125(Ti)/TiO 2 -modied electrode was signicantly enhanced (0.8 mA) compared to that of the MIP@TiO 2 (0.1 mA), owing to the improved visible light absorption capacity and wellmatched band levels of the two components.As a result, the MIP@NH 2 -MIL-125(Ti)/TiO 2 -based photoelectrochemical detection sensor demonstrated a wide linear detection range from 0.1 nM to 10 mM, with a LOD of 60 pM.
2.2.3.UiO.UiO series MOFs have also demonstrated excellent photoelectrochemical detection activities against environmental contaminants.For instance, UiO-66 is composed of the Zr 6 O 4 (OH) 4 cluster and terephthalic acid.In 2021, Feng et al. synthesized [Ru(bpy) 3 ] 2+ @Ce-UiO-66/Mn:Bi 2 S 3 composites and utilized them to construct aptamer-based PEC sensors for ooxacin (OFL) detection. 98Ce-UiO-66 MOFs were obtained by doping UiO-66 with Ce-elements.The cycling of Zr 4+ -Zr 3+ and Ce 4+ -Ce 3+ in porous Ce-UiO-66 enhanced charge separation efficiency.Additionally, the [Ru(bpy) 3 ] 2+ broadened the range of light absorption, and Mn:Bi 2 S 3 acted as a photosensitizer, improving the separation efficiency of photogenerated charge pair.The photocurrent response of [Ru(bpy) 3 ] 2+ @Ce-UiO-66/ Mn:Bi 2 S 3 composite was improved in comparison to Mn:Bi 2 S 3 .Thus, the composite-modied photoelectrodes showed excellent photoelectrochemical detection activity (concentration range: 0.1∼100 nM, LOD: 6 pM).Similarly, in a study by Wu and co-workers, a photoelectrochemical sensor based on Au NPs loaded on porous g-C 3 N 4 nanosheets and hexagonal NH 2 -UiO-66 composite (g-C 3 N 4 /Au/NH 2 -UiO-66) was constructed for the detection of D-penicillamine. 99The presence of the Z-scheme heterojunction in g-C 3 N 4 /Au/NH 2 -UiO-66 promoted the photogenerated electron transfer, and the strong binding between Au NPs and D-penicillamine enhanced the selectivity and sensitivity of the sensor.The composite-modied electrode exhibited a maximum photocurrent about 10 times larger than that of g-C 3 N 4 .Thus, the proposed g-C 3 N 4 /Au/NH 2 -UiO-66-based photoelectrode demonstrated a low detection limit of 0.0046 mM in a wide linear range from 10 nM to 400 mM.2.2.4.Cu-BTC.In addition to the aforementioned MIL, ZIF, and UiO series MOFs, several other MOFs, such as Cu-BTC (HKUST-1), have been employed in the fabrication of PEC sensing platforms.As for Cu-BTC, it consists of copper ions and benzene-1,3,5-tricarboxylate ligands.In 2019, Cao et al. constructed a PEC sensor for glyphosate assays utilizing a porous Cu-BTC/g-C 3 N 4 composite. 100Porous Cu-BTC offered the benet of trapping glyphosate molecules and signicantly enhanced the photoelectric conversion efficiency of g-C 3 N 4 nanosheets.In this detection system, the formation of the Cu-glyphosate complex increased the steric hindrance to electron transfer between the composite and the electrode surface, leading to a reduction of photocurrent.Thus, a correlation between photocurrent and glyphosate concentration was established over a wide range from 1 pM to 1 nM.

PCN.
PCN is a type of MOF comprising zirconium (Zr) clusters and porphyrin ligands.Its remarkable stability and photoelectrochemical properties make it highly promising for the fabrication of PEC platforms.For instance, Dong and coworkers constructed a photoelectrochemical sensor for the identication of kanamycin sulfate based on a PCN-222@g-C 3 N 4 composite. 101Through a physical microwave mixing process, g-C 3 N 4 with a broad bandgap was effectively coupled with porous, narrow-band gap PCN-222.The composite-modied photoelectrode demonstrated a low detection limit of 0.127 nM with a wide linear range from 1 nM to 1000 nM.Similar work has also been conducted employing the CdS QDs/ PCN-224 composite. 102Sub-3 nm CdS quantum dots were uniformly distributed in the porous PCN-224.Compared to pure PCN-224 and CdS QDs, the photocurrent response of the composite-based electrode was improved.With regard to the detection of doxorubicin hydrochloride and gentamicin sulfate, the LODs were as low as 3.57 nM and 0.158 nM, respectively.
2.2.6.Ln-MOFs.Some other Ln-MOFs have also been employed in the fabrication of PEC sensing platforms.Eu-MOFs, as a representative class of Ln-MOFs, possess excellent lightharvesting properties.For instance, Gao et al. prepared a CdS nanoparticle/europium metal-organic framework (CdS/Eu-MOF) composite and utilized it for PEC monitoring of ampicillin. 110he porous structure of the Eu-MOF enhanced the light absorption capability of the composite while effectively inhibiting the recombination of photogenerated charge pairs.The CdS/Eu-MOFbased PEC sensor achieved a low detection limit of around 0.093 nM, along with great selectivity, outstanding repeatability, and desired stability.In summary, porous metal-organic frameworks have demonstrated remarkable potential for photoelectrochemical sensing of environmental toxic species.
2.2.7.MOF derivatives.Moreover, MOF derivatives have shown potential for identifying toxic species in the environment 116,120,123 (Table 5).For instance, Zhang et al. prepared a hollow CoS x @CdS polyhedron composite for photoelectrochemical Hg 2+ assays.The composites were obtained by decorating CdS nanoparticles on the surface of CoS x through a simple solvothermal method, using a zeolitic imidazolate framework-67 (ZIF-67) as the sacricial template and cobalt precursor 103 (Fig. 5D).The porous CoS x polyhedron component in the composite facilitates the transfer of photogenerated electrons during the PEC detection process (Fig. 5E).The photocurrent of the CdS/CoS x composite-modied electrode was about 50 nA, 2.5 times higher than that of CdS.Photoluminescence spectra (PL) and electrochemical impedance spectroscopy (EIS) measurements also conrmed its enhanced photogenerated charge pair separation ability.Compared to the CdS or CoS x modied electrodes, the CdS/CoS x modied electrode exhibited an increasing photocurrent in the presence of analyte-Hg 2+ due to the in situ formation of a Z-scheme CoS x @CdS/HgS via a selective ionexchange reaction.A strong linear relationship was established between the photocurrent and log(Hg 2+ ) concentrations in the range of 0.005 nM to 1000 nM, with a LOD of 0.002 nM (Fig. 5F).
Derivatives of the MIL family of MOFs have also demonstrated excellent PEC detection activities against environmental toxic species.In a study by Zhang and co-workers, COOHfunctionalized TiO 2 (TiO 2 -C) was achieved via one-step calcination of MIL-125(Ti). 114Due to the large specic surface area and abundant functional groups of TiO 2 -C, it demonstrated superior photochemical, electrochemical, and PEC detection performance compared to MIL-125(Ti).Furthermore, it was also useful for graing molecularly imprinted polymers (MIPs).The MIPs@TiO 2 -C, with a large number of binding sites, provides precise electron transfer channels, resulting in improved sensitivity and selectivity for antibiotics such as ooxacin.Under optimal conditions, the prepared sensor has a low detection limit (2.9 pg mL −1 ) and a wide linear concentration range (0.01 ng mL −1 ∼3 mg mL −1 ).
MIL-68(In) is also an ideal template for obtaining porous photoactive materials.For instance, Yan et al. synthesized an In 2 O 3 -In 2 S 3 -Ti 3 C 2 MXene composite on the base of MIL-68(In)derived In 2 O 3 hollow tubular 122 and then used it to construct a dual-mode (photoelectrochemical and photofuel cell) self-powered apta-sensing platform for detecting microcystin-LR (MC-LR).Porous In 2 O 3 hollow tubulars with a large specic surface area provide abundant active sites, while the wellmatched energy levels of In 2 O 3 and In 2 S 3 and the Ti 3 C 2 MXene quantum dots acting as electron transfer mediators both accelerate the separation of photogenerated charge carriers.This sensing platform revealed excellent PEC detection activity in the range from 0.5 pM to 400 nM, with a LOD of 0.169 pM.In another research, Feng et al. used MIL-68(In) as the precursor for fabricating homogeneous In 2 O 3 nanoparticles through high temperature calcination in an air atmosphere. 120he formation of a heterojunction between the porous In 2 O 3 and g-C 3 N 4 facilitated the separation and transfer of photogenerated charge pairs.The introduction of gold nanoparticles (Au NPs) with the localized surface plasmon resonance (LSPR) effect also improved visible light absorption and photoelectron transfer.The photocurrent of the Au NPs/In 2 O 3 @g-C 3 N 4 composite reached 1.75 mA, much larger than that of pure g-C 3 N 4 .The Au NPs/In 2 O 3 @g-C 3 N 4 was successfully applied to fabricate a label-free photoelectrochemical apta-sensing platform for tetracycline detection, which yielded a wide linear range from 0.01 nM to 500 nM with a LOD of 3.3 pM.Porous composites derived from Cu-BTC or Zn-BTC have also demonstrated great potential for PEC detection.In a study by Cao et al., Cu-BTC MOF (BTC: benzene-1,3,5-tricarboxylic acid) was calcinated to achieve porous hierarchical CuO. 115The CuO material has a large specic surface area, which is favorable for the capture of the target species, malathion.The facile PEC detection sensor achieved a LOD of 0.086 nM in the range of 0.1 nM to 104 nM.

Covalent organic frameworks
Like metal-organic frameworks, covalent organic frameworks (COFs) are a new class of porous materials that have attracted much interest for constructing the PEC analytic platforms. 128,129enerally, COFs are composed of covalently bonded light elements, such as C, N, H, O, B, and Si.The porous nature of COFs favors the trapping of probe species such as heavy metal ions.1][132] So far, several COFs have been employed in the eld of PEC detection, such as D-TA COF, 133,134 TAPP-COF, 124 TTPA-COF, 135 F-COF, 125 pbqy-COF, 136 and PFA-130. 137However, the studies on PEC sensors based on covalent organic frameworks for tracing environmental pollutants are still in their early stages, as summarized in Table 6.In a study by Zhao and co-workers, a porphyrin-based covalent organic framework thin lm (TAPP-COF) was synthesized via a liquid-liquid interfacial method and used as a photocathode material for photoelectrochemical "on-off-on" sensing of Pb 2+ . 124Due to the unique charge channels of porous COFs and the excellent photoelectric properties of porphyrin, 138 the TAPP-COF-based PEC sensor displayed an improved "signal-on" photocathodic current response.The CdSe@SiO 2 quantum dots, as a quenching agent, were introduced via a hybridization chain reaction to achieve a "signal off" photocurrent response, and then the PEC response switched to a "signal on" state aer the addition of the detected species, Pb 2+ (Fig. 6A).TAPP-COF is a p-type material that is benecial to photogenerated electron-hole pair transfer. 139Hence, the as-prepared PEC detection sensor displayed a wide linear range from 0.05 nM to 100 nM with a LOD of 0.012 nM (Fig. 6C).][142][143][144][145] Alternatively, the combination of COFs with other materials could enhance their performance in PEC detection platforms by providing more reactive sites for the determination of target environmental contaminants.In a study by Wang et al., a heterostructure was formed between TiO 2 NTAs and F-COFs through a simple hydrothermal method (Fig. 6D and E) and then used for constructing a PEC detection platform for dopamine. 125The introduction of F-COFs enhanced the visible light absorption and photogenerated electron-hole pairs separation efficiency.Meanwhile, the porous F-COFs processed a large specic surface area, and p-p interactions could be formed between aromatics of dopamine, 146 which contributed to their ultra-sensitive detection activity.The resulting PEC sensor demonstrated excellent photocurrent response stability.More importantly, a LOD of 0.032 mM was obtained in an extended linear range (Fig. 6F).Heterostructure COF/MOF hybrids have been fabricated as multifunctional materials for sensing applications. 127,147In 2021, Zhang et al. prepared a CoPc-PT-COF@Cu-MOF-based PEC-EC dual-mode biosensor for Cr(III) quantication. 126The 2D CoPc-PT-COF was in situ grown on a Cu-MOF by covalent binding between the carboxyl group in the Cu-MOF and the amino group in the CoPc-PT-COF.The DNA strands could be easily anchored on the composite via strong interaction owing to its large specic surface area and high porosity.Furthermore, due to the specic recognition between DNA and Cr(III), the composite-based biosensor could be used to trace Cr(III).The photocurrent of composite (419 nA) was about 14.5 times greater than that of Cu-MOF, indicating that the heterojunction between Cu-MOF and CoPc-PT-COF enhanced the photoelectric conversion efficiency.So, the obtained composite-based PEC biosensor displayed a LOD of 14.5 fM within the Cr(III) concentration range of 0.1 pM to 100 nM.Overall, porous COFs have signicant potential for PEC sensing applications against environmental contaminants.

Graphitic carbon nitride
9][150][151] In recent years, g-C 3 N 4 has been utilized in the area of PEC detection of environmental contaminants 120,152-156 (Table 7).However, for bulk g-C 3 N 4 , the PEC detection performance is poor due to its relatively low photoelectric conversion efficiency.Activating pristine g-C 3 N 4 with various treatments to achieve a porous structure is indeed an excellent way to speed up the separation of photogenerated charge pairs.Specically, the methods employed to treat bulk g-C 3 N 4 include chemical exfoliation and thermal oxidation. 149,157,158or the chemical exfoliation process, the morphology and porosity of the porous carbon nitride are determined by the choice of etchant and the reaction conditions. 159Common exfoliation agents include K 2 Cr 2 O 7 + H 2 SO 4 , KMnO 4 + H 2 SO 4 , and HNO 3 + H 2 SO 4 . 160However, these chemical agents pose environmental and safety concerns due to their hazardous nature.Thermal oxidation is another method for producing porous carbon nitride.This process disrupts the hydrogen bonds in carbon nitride framework, leading to the formation of porous carbon nitride nanosheets with a large specic surface area and a thin sheet structure. 161However, those above two aforementioned approaches can harm the environment.Further, it requires high-quality bulk g-C 3 N 4 , and the process is complicated and time-consuming.
For instance, in a study by Peng and co-workers, phosphorus-doped porous carbon nitride nanosheets (PCN-S) were synthesized using element doping and thermal oxidation method. 162The PCN-S demonstrated an ultrathin nanosheet structure, a large specic surface area, and numerous surface pores (Fig. 7D).Moreover, the Au NPs with the LSPR effect were in situ decorated on the porous surface to yield an Au/PCN-S composite via a photo-reduction method.The composite was used to construct a self-powered PEC apta-sensor for the oxytetracycline (OTC) assay.Under visible light irradiation, electrons were excited to the CB of PCN-S and subsequently transferred to the Au NPs.They then reacted with dissolvable oxygen in the aqueous solution to produce superoxide oxygen species, which react with analyte-OTC (Fig. 7F).Owing to its strong visible light absorption ability and enhanced photogenerated charge pair separation efficiency, the Au/PCN-S composite-based PEC sensor demonstrated excellent performance in terms of a wide linear detection range (0.5∼200 nM), a low detection limit (0.34 nM) (Fig. 7E).
Different from traditional methods, alkaline hydrothermal treatment is a new effective approach to achieving porous carbon nitride nanosheets. 187,188In 2020, Liang et al. prepared a porous carbon nitride via an alkaline hydrothermal method. 163he hydroxide effectively exfoliated the 2D framework of the pristine bulk graphitic carbon nitride to obtain a porous structure with a large number of functional groups on its surface.The TEM image of activated g-C 3 N 4 (A-CN) clearly revealed the presence of porous structures on its surface (Fig. 7A).Moreover, the specic surface area (SSA) of A-CN was about 10 times larger than that of bulk g-C 3 N 4 (Fig. 7B), further conrming the existence of porous structures in A-CN.In Fig. 7C, the photocurrent response of A-CN was much higher than that of bulk g-C 3 N 4 due to the porous structure and the large number of functional groups on the surface, which promoted photogenerated charge separation and transfer.Thus, the A-CN-based PEC sensor displayed an outstanding performance for Cu 2+ detection with a low detection limit of 0.31 mM in the range from 0.2 mM to 50 mM.
Aiming to overcome the inevitable restacking and agglomeration of the bulk g-C 3 N 4 nanosheets, the fabrication of threedimensional (3D) porous g-C 3 N 4 is another simple method.It not only prevents the agglomeration of nanosheets but also improves the utilization of irradiation through multiple reections in its open framework. 189,190For instance, Zhang et al. developed a dual-photoelectrode for detecting chloramphenicol, employing Au NPs doping porous three-dimensional g-C 3 N 4 and porous N-doped Cu 2 O/C used as the photoanode and photocathode, respectively. 173The small-scale Au NPs were uniformly distributed across the 3D CN nanosheets.The photocurrent of Au-doped 3D CN was much higher than that of bulk g-C 3 N 4 , and it also had the smallest electron transfer resistance.The porous 3D g-C 3 N 4 improved both the visible light absorption ability and photoelectric conversion efficiency.Additionally, the presence of Au NPs with the LSPR effect provided extra energy for photogenerated charge carrier generation with the aid of hot electron transfer.As a result, the prepared apta-sensor exhibited an ultra-low detection limit (0.1 pM) and a wide linear detection range (0.5 pM∼300 nM).

MXene
2][193][194] Porous MXenes have demonstrated outstanding chemical reactivity and hydrophilicity owing to their superior electrical conductivity and significant number of functional groups like -F and -OH, enabling them to form heterojunctions with semiconductors. 195Additionally, exposed connector metal sites, such as titanium on MXenes, present a higher redox reactivity compared to conventional carbon materials, positioning MXene nanolayers as ideal optoelectronic platforms.Recent reports on MXenebased PEC sensors for environmental pollutant detection are summarized in Table 8.These reports illustrate that MXene can be combined with a variety of materials, including graphene oxide, graphitic carbon nitride, metal oxide, metal halide, and metal sulde, to construct composite hybrid photoelectrochemical detection platforms.Graphene oxide and graphitic carbon nitride can be effectively coupled with the porous MXenes due to the strong p-p interaction between them. 196,197For instance, in a study by Yuan and co-workers, the g-C 3 N 4 /Ti 3 C 2 MXene composite was synthesized and proposed as a PEC sensing material for cipro-oxacin detection. 196In Fig. 8A, porous Ti 3 C 2 MXene was observed on the surface of g-C 3 N 4 , suggesting a close connection between the two components facilitated by electrostatic self-assembly.Transient photocurrent measurements (Fig. 8B) revealed a photocurrent density of 1.36 mA cm −2 for the g-C 3 N 4 / Ti 3 C 2 MXene, nearly twice that of g-C 3 N 4 .Meanwhile, the EIS of g-C 3 N 4 /Ti 3 C 2 MXene has a smaller arc size compared to that of pure g-C 3 N 4 .These ndings suggested that the introduction of Ti 3 C 2 MXene promotes photogenerated charge transfer.The fabricated PEC sensor demonstrated an ultra-low detection limit of 0.13 nM over a broad linear range from 0.4 nM to 1000 nM.
These results imply that the tight interaction between Ti 3 C 2 MXene and g-C 3 N 4 facilitates the smooth transfer of photogenerated electrons from g-C 3 N 4 to Ti 3 C 2 MXene, whose Fermi level (0.58 V vs. NHE) is much more positive than the conduction band energy of g-C 3 N 4 (Fig. 8C).
Similarly, another typical carbon material, graphene oxide, has also been coupled with porous Ti 3 C 2 MXene to construct smart PEC sensing platforms.In 2023, Zhang et al. reported a "signal-on" PEC sensor for sulde detection based on in situ growth of AgI on a 3D porous Ti 3 C 2 MXene/graphene aerogel (AgI/ Ti 3 C 2 MXene/GO). 198The porous Ti 3 C 2 MXene/GO was synthesized through the solvothermal method, followed by in situ decoration of AgI NPs onto the Ti 3 C 2 MXene/GO.Fig. 8D depicts the typical three-dimensional interconnected skeleton structure of aerogel.The porous structure of Ti 3 C 2 MXene/GO aerogel with large specic surface area favored the anchoring of AgI NPs.Upon the addition of detection analyte-sulde (S 2− ), Ag 2 S formed on the surface of AgI/Ti 3 C 2 MXene/GO, resulting in an enhanced photocurrent response due to the newly created Ag 2 S/AgI heterojunction (Fig. 8F).As a result, the associated PEC sensor revealed an outstanding S 2− detection activity, including a wide linear range (5 nM∼200 mM) and an ultra-low detection limit (1.54 nM).
Interestingly, MXene can serve as a co-catalyst to enhance the separation efficiency of photogenerated electron-hole pair. 204,205,212,213Recently, porous MXenes have been combined with various semiconductors (e.g., TiO 2 , BiVO 4 , MoS 2 , and BiOI) to fabricate composite-based PEC platforms for environmental contaminant assays.For example, Ling and co-workers reported a PEC sensing platform based on a BiVO 4 /Ti 3 C 2 MXene composite for oxytetracycline (OTC) detection. 205Within the composite, the small porous MXene nanosheets acted as a cocatalyst to promote photo-generated charge pair separation.This was conrmed by the enhanced photocurrent responses, weaker PL intensity, and smaller arc resistance values of BiVO 4 / Ti 3 C 2 MXene.The introduction of MXene created a Schottky barrier between BiVO 4 and MXene, effectively suppressing electron reux.During the PEC detection process, photogenerated electrons in the CB of BiVO 4 were transferred to MXene via the Schottky junction and then injected into a conductive substrate (in this case, ITO) to generate photocurrent.Simultaneously, holes in the VB of BiVO 4 contributed to the current generation by oxidizing OTC molecules on the Review RSC Advances composite surface.Thus, the improved photocurrent resulted in a wide detection range (0.1∼100 nM), an ultra-low detection limit (0.03 nM), and excellent sensitivity for the corresponding PEC sensing platform.Schottky heterojunctions have also been observed in other Ti 3 C 2 MXene-containing composites.For instance, Ye et al. reported the utilization of a ower-like BiOI/2D Ti 3 C 2 MXene (BiOI/Ti 3 C 2 ) heterostructure composite as a photocathode for Lcysteine (L-Cys) PEC biosensing. 203The Schottky interaction between the two components enhanced charge transfer, resulting in a superior cathodic photocurrent signal.Similarly, a Schottky junction was also found in CdS nanoparticles and Ti 3 C 2 MXene in other studies. 201The CdS/Ti 3 C 2 heterostructurebased PEC sensing platform showed a linear response for Cu 2+ ranging from 0.1 nM to 10 mM with a LOD of 0.05 nM, attributed to signicantly improved charge carrier transfer at the CdS/ Ti 3 C 2 interface.In another report, Du et al. employed a wetchemical method to fabricate AgBr/Ti 3 C 2 Schottky heterojunction composites for self-powered PEC sensing of chlorpyrifos (CPF). 207The synergistic effect of the Schottky heterojunction between AgBr and Ti 3 C 2 MXene, combined with metal-ligand charge transfer (MLCT), promoted photogenerated charge carrier separation and transfer.The PEC sensor demonstrated excellent performance in CPF monitoring, with a linear detection range of 0.001∼1 ng L −1 and a LOD of 0.33 pg L −1 .Recently, porous MBenes, which are respective transition metal borides, [214][215][216][217][218] have been applied in the areas of energy storage and electrocatalysis.Until now, there have been few reports on the application of MBenes to photoelectrochemical sensors.However, we believe that MBenes have great potential in the fabrication of PEC detection platforms.

Perspectives
In summary, over the past decade, metal oxides, MOFs, COFs, graphitic carbon nitride, and MXene have made signicant advances in the construction of PEC analytical platforms for environmental contaminants.Due to their porous structure, large specic surface area and abundant active sites, PEC sensors based on photoactive porous material exhibit superior optoelectronic response.(1) Among the photoactive metal oxides, TiO 2 nanotubes or nanoarrays with unique porous channels can efficiently transmit photogenerated charge pairs.PEC sensors based on TiO 2 nanotubes or nanoarrays can be applied to detect heavy metal ions and organic pollutants, such as Cr(VI) and atrazine.(2) MOFs and COFs both exhibit excellent photoelectrochemical detection performance against organic pollutants.Moreover, derivatives of MOFs have demonstrated robust PEC detection activities against environmental toxic species.(3) Bulk g-C 3 N 4 can be treated with a variety of methods to obtain a porous structure, including chemical exfoliation, thermal oxidation, and alkaline hydrothermal treatment.Meanwhile, the construction of 3D frameworks is another approach to obtain porous structures.A number of research works have demonstrated that porous graphitic carbon nitride-based PEC sensors exhibit outstanding detection activity in terms of ultra-low detection limit and wide linear detection range.(4) Porous MXenes characterized by high redox reactivity, superior electrical conductivity, and rich functional groups, can effectively be coupled with other materials to construct composite hybrid PEC platforms for pollutant determination.Therefore, porous materials have great potential for the fabrication of PEC sensors.

Heavy metal ions
0][221] Copper (Cu), iron (Fe), manganese (Mn), cobalt (Co), and zinc (Zn) are required for biological functions at relatively low concentrations but become toxic in excessive amounts.Low amounts of lead (Pb), mercury (Hg), chromium (Cr), cadmium (Cd), and arsenic (As) are nevertheless dangerous. 222,223The World Health Organization (WHO) has established stringent restrictions regarding the concentration of heavy metal ions in drinking water.In recent years, many efforts have been dedicated to developing porous materialbased PEC sensors for monitoring heavy metal ions. 46,167,224,225n this section, we present a selection of relevant works on PEC detection of heavy metal ions.
3.1.1.Cr(VI).Hexavalent chromium (Cr(VI)) has been iden-tied as one of the most toxic heavy metal ions, posing a signicant threat to biological and ecological systems. 226hus, it is critical for detecting Cr(VI) in water bodies.Several studies have been conducted to improve the PEC detection performance for Cr(VI). 227For instance, Qiao et al. constructed a NiCo-LDHs-modied TiO 2 NTAs/Ti photoelectrode using anodization and electrodeposition techniques. 55The proposed PEC sensor demonstrated a linear relationship between photocurrents and Cr(VI) concentrations ranging from 10 mM to 400 mM, with a LOD of 3.2 mM.Moreover, the sensor displayed no discernible reactions even when subjected to various interfering ions at high concentrations (e.g., Cr 3+ , Sn 4+ , Cd 2+ , Pb 2+ , Mn 2+ , Na + , and Fe 2+ ), indicating its strong selectivity for Cr(VI).More importantly, the practical application of the developed PEC sensor was examined by detecting Cr(VI) in river water, demonstrating its excellent potential in real-world scenarios.Additionally, porous graphitic carbon nitride has been employed for the construction of PEC platform for Cr(VI) detection.For instance, Fang et al. designed a formate anionincorporated graphitic carbon nitride (F-g-C 3 N 4 )-based PEC sensor for determining trace amounts of Cr(VI). 165The porous Fg-C 3 N 4 demonstrated enhanced photogenerated charge separation efficiency compared with bulk g-C 3 N 4 .Furthermore, when combined with the molecularly imprinted polymers (MIPs), F-g-C 3 N 4 served as a sensing platform for Cr(VI) determination.The resulting sensor exhibited remarkable sensitive, demonstrating a linear range from 0.01 mg L −1 to 100 mg L −1 , and achieving a LOD of approximately 0.006 mg L −1 .
The construction of heterojunctions was another strategy to improve the PEC detection activity for Cr(VI).For instance, in 2021, Cheng et al. devised a quick and highly sensitive PEC sensor for determining Cr(VI) in water samples. 166The photoactive electrode consisted of a p-n BiOI/CN heterojunction composite.The CN nanosheets were effectively coupled to the BiOI with a needle-like petal structure.The photocurrent density of BiOI/CN was signicantly higher than that of pure CN and BiOI, suggesting that the introduction of porous CN enhanced the visible light absorption.Moreover, the BiOI/CN processed the smallest charge transfer resistance due to its unique structure, leading to increased active area and electric conductivity of the photoelectrode.Furthermore, the PEC detection performance of the composite-based electrode for Cr(VI) was studied at various Cr(VI) concentrations.It displayed a wide linear detection range of 0.5∼190 mM with a LOD of 0.1 mM.Under visible light irradiation, the electrons in the VB of CN and BiOI were excited to their CB.Due to the Fermi level of the p-type BiOI being close to the VB and that of the n-type CN being close to the CB, electrons in the CB of the CN tended to diffuse into the BiOI, while holes in the VB were transferred from the BiOI to the CN.The electrons in CB of CN could react with Cr(VI) to convert it to harmless Cr(III).As a result, an interfacial electric eld was formed between BiOI and CN, facilitating the separation of photogenerated charge pairs.
3.1.2.Hg 2+ .Another harmful heavy metal contaminant commonly found in water bodies is Hg 2+ .It can cause damage to the brain, kidneys, and central nervous system.EPA has dened a maximum allowable limit of 10 nM of inorganic mercury in drinking water. 228,229Therefore, it is critical to detect it accurately.Porous MXene-based composites have been employed to construct PEC platforms for monitoring Hg 2+ in water bodies.For instance, Xiao et al. prepared BiOBi x I 1−x /Ti 3 C 2 MXene Schottky heterojunction nanocomposites using an electrostatic selfassembly method. 202Porous Ti 3 C 2 MXene possesses a high specic surface area, excellent electrical conductivity, and an abundance of surface chemical groups.In addition, the Schottky heterojunction of Ti 3 C 2 MXene and BiOBi x I 1−x signicantly enhances the separation rate of photogenerated charge pairs, resulting in a higher photocurrent signal compared to BiOBi x I 1−x alone.During the detection of Hg 2+ , an ultra-low detection limit of around 42.1 pM was obtained, ranging from 0.1 nM to 1000 nM.Moreover, this sensor has been successfully employed to determine Hg 2+ in environmental samples.Similarly, Jiang et al. used a BiVO 4 /Ti 3 C 2 MXene composite to construct a visible light-driven PEC sensor for probing Hg 2+ . 200A single Ti 3 C 2 MXene layer was coated on the BiVO 4 lm.Electrochemical impendence spectroscopy measurements revealed a smaller arc diameter for the interfacial charge transfer resistance of the composite photoelectrode compared to pure BiVO 4 .Further, the photocurrent signal of the composite-based electrode was higher than that of BiVO 4 (Fig. 9B).Meanwhile, the photocurrents of BiVO 4 and BiVO 4 /Ti 3 C 2 MXene-based electrodes increased upon the addition of glutathione (GSH) (Fig. 9A).The porous MXene, a metallike material with high conductivity, was coupled with BiVO 4 to form a Schottky junction, facilitating charge-pair separation.Thus, the performance of the PEC analysis for Hg 2+ demonstrated a detection limit of 1.0 pM within a range of 1 pM to 2 nM.
3.1.3.Cu 2+ .Cu 2+ is required by all living organisms.However, excessive amounts of copper can cause hepatic or renal damage, as well as gastrointestinal disturbances.Therefore, it is critical to detect Cu 2+ concentration levels in environmental and biological samples. 230,231Porous material-based PEC sensors for monitoring Cu 2+ have been investigated by several researchers. 43,73,163For instance, in a study by Xu and coworkers, a porous graphene-analogue carbon nitride (GA-C 3 N 4 ) was derived from the graphitic C 3 N 4 . 164GA-C 3 N 4 is an ideal candidate for a PEC sensor to determine Cu 2+ due to its thin layer structure and high specic surface area.The fabricated sensor demonstrated a linear photocurrent response with varying Cu 2+ concentrations over a wide range of 0.4∼7.6 mM.
3.1.4.Pb 2+ .Even at low concentrations, lead ions (Pb 2+ ) represent a major threat to both species and human health.4][235] For example, in a recent study by Yu and coworkers, a label-free and ultrasensitive PEC biosensor was developed for the determination of Pb 2+ . 112The NH 2 -MIL-125(Ti) was synthesized via a hydrothermal method and subsequently doped with Cu.The Cu 2+ -doped-titanium-based metal-organic framework (Cu 2+ /NH 2 -MIL-125(Ti)) was calcinated to create a porous Cu 2 O-CuO-TiO 2 heterojunction composite.In Fig. 9C, the composite exhibited a disk-like morphology, and the presence of Cu 2 O and CuO was conrmed by HR-TEM measurements.The nitrogen adsorption-desorption isotherm of the composite was shown in Fig. 9D, revealing a specic surface area (SSA) of up to 75.1 m 2 g −1 and an average pore size of 18.4 nm.Consequently, the porous structure and high SSA were benecial for enhancing light absorption and facilitating the photogenerated charge transport during PEC detection.In Fig. 9E, the composite-based sensor exhibited a broad detection range of 10 fM to 1 mM and a low detection limit (6.8 fM).
3.1.5.Perspectives.In summary, signicant efforts have been devoted to the fabrication of photoelectrochemical sensing platforms based on photoactive porous materials for heavy metal ion assays.These platforms exhibited improved sensing activity, such as excellent sensitivity, an ultra-low detection limit (pM), and a wide linear range.The advantage of porous materials in the detection of heavy metal ions has been proven by the reports cited above: (1) porous material with a large surface area and abundant functional groups facilitates interaction with recognition elements or heavy metal ions; (2) fast photogenerated charge carrier separation and transfer rate within the photoactive porous materials.However, some challenges still remain with the porous material-based PEC detection sensors for heavy metal ion assays and need to be resolved: (1) some toxic heavy metal ions, such as arsenic (As 5+ ), have not yet been monitored by porous materials-based PEC sensors; (2) despite the great potential of metal-organic frameworks for PEC detection of heavy metal ions, few works have been reported; and (3) the stability, repeatability, multiple detection, and antiinterference of heavy metal ion PEC sensors still require enhancement.

Organic pollutants
Organic pollutants found in wastewater encompass a wide range of compounds, including phenolics, antibiotics, pesticides, and toxins.These substances are extremely toxic and pose serious risks to human health.Thus, developing environmentally friendly methods for accurately determining these contaminants has become a focus of worldwide. 236,237.2.1.Phenolics.Phenols and their derivatives are known for their recalcitrance and acute toxicity.Under EPA regulations, the permissible limit for phenol in surface water is no more than 1 ppb.High concentrations of phenolic compounds can have adverse effects on human health, such as salivation and anorexia. 238Hence, increasing attention has been focused on detecting these phenolic species. 176Porous carbon nitride is an excellent choice for developing PEC platforms for phenolic assays.For instance, Chen et al. produced a sensitive PEC sensor for p-nitrotoluene (p-NT) detection based on a copper cluster-modied porous carbon nitride nanosheet composite (CNNS-Cu). 170Under light irradiation, the Cu 2+ cluster captures the electrons on the CB and converts them to Cu + .Meanwhile, the electrons excited from the VB of the CNNS are occupied by Cu 2+ , which enhances the separation and migration of photogenerated charge pairs.Upon the introduction of p-NT, Cu 2+ facilitates p-NT reduction via Cu 2+ /Cu + redox reduction.As a result, this PEC sensor exhibits a wide linear detection range (0.1∼100 mM) and a low detection limit of 0.13 mM.In addition to graphitic carbon nitride with a porous structure, TiO 2 nanotubes with unique pore channels have been employed in PEC applications for phenols determination.For example, Wang et al. developed a novel PEC sensor for bisphenol A detection by combining a TiO 2 nanotubes/CdS heterostructure with inorganic framework molecular technology in PEC sensor analysis. 78The porous pore structures of TiO 2 nanotubes favor photogenerated charge transfer, and the formation of CdS/TiO 2 heterojunctions enhances the visible light absorption and improves photoinduced charge pair separation efficiency.Moreover, the inorganic framework molecular imprinting method creates plenty of recognition sites for target analytes (BPA).Hence, the fabricated sensor demonstrated excellent PEC sensing performance (detection range: 1∼100 pM, low limit detection: 0.5 pM).
3.2.2.Antibiotics.Antibiotics are widely used in human healthcare, aquaculture, and crop growth.However, the misuse and discharge of antibiotics into the environment have raised signicant concerns. 2394][245][246] Recently, Liu et al. reported a simple PEC aptasensor for tracing sulfadimethoxine (SDM) based on a zinc phthalocyanine/graphitic carbon nitride composite (ZnPc/ CN). 174By modifying porous graphitic carbon nitride nanosheets with visible/near-infrared light-responsive ZnPc, ZnPc/ CN nanocomposites were created.The porous CN nanosheets not only provided abundant sites for binding ZnPc, but also facilitated transport of photogenerated charges.The assembled PEC apta-sensor exhibited a linear correlation with SDM concentration in the range of 0.1∼300 nM, with a LOD of 0.03 nM.Further, it demonstrated excellent selectivity towards SDM even in the presence of familiar interferences (e.g., oxytetracycline, kanamycin, bisphenol A, and diclofenac).Similarly, using the same sensing strategy, 175 a facile PEC aptasensor based on a p-type BiFeO 3 /n-type porous ultrathin graphitic carbon nitride (BiFeO 3 /utg-C 3 N 4 ) was constructed (Fig. 10A).The BiFeO 3 /utg-C 3 N 4 composite was obtained by a simple electrostatic interaction method.The TEM image conrmed the coupling of BiFeO 3 NPs to the porous graphitic carbon nitride surface.In Fig. 10B, the composite-based electrode exhibited a signicantly higher photocurrent intensity than pure BiFeO 3 and utg-C 3 N 4 , indicating the suppression of photogenerated charge pair recombination due to the presence of p-n heterojunction in the composite.The EIS analysis further conrmed the efficient charge separation and transfer in the BiFeO 3 /utg-C 3 N 4 composite-based electrodes, as they exhibited the smallest charge transfer resistance among all the electrodes.The fabricated PEC sensor was applied to ampicillin detection.As displayed in Fig. 10C, the apta-sensor achieved a relatively low LOD of 0.33 pM within a wide range from 1 pM to 1 mM.
Further, the porous graphitic carbon nitride can be combined with metal oxides derived from metal-organic frameworks to construct heterojunction composites.In 2023, Jiang et al. synthesized a MOF-derived ZnO nanopolyhedra/g-C 3 N 4 composite via calcination of ZIF-8 and melamine. 117The g-C 3 N 4 nanosheets were covered on the ZnO surface (Fig. 10D).In Fig. 10E, the remarkable photocurrent of the ZnO/g-C 3 N 4 composite-based electrodes compared to pure ZnO was attributed to the presence of heterojunctions that facilitate the separation of photogenerated electron-hole pairs.A plausible electron transfer mechanism was proposed in Fig. 10F.Due to the suitable band energy levels of ZnO and g-C 3 N 4 , a type-II heterojunction is formed at the contact interface of the two components.As a result, the corresponding PEC apta-sensor demonstrated an ultra-low LOD (1.49 pM) in a wide linear range spanning from 5 pM to 200 nM.Furthermore, in addition to aptamer-based PEC sensors, recognition element-free sensors have been employed for the detection of antibiotics.For instance, Yan et al. presented a non-recognition element PEC sensor using a porous nitrogen-decient graphitic carbon nitride nanosheet (ND-g-CN) for ciprooxacin (CIP) detection. 169otably, the presence of nitrogen vacancies serves as traps, effectively suppressing charge recombination, while the porous sheet structure promotes the separation and transfer of photogenerated charge carriers.Thus, the PEC sensor realized an ultra-sensitive determination of CIP (LOD: 20 ng L −1 ).
3.2.3.Pesticides.Pesticides, even in low concentrations, can have a signicant impact on the ecosystem and human health due to their high toxicity and resistance to decomposition. 247Therefore, extensive efforts have been made to trace these pesticides in the environment. 46,115,248,249In 2020, Qin et al. synthesized CdS nanocrystal-functionalized porous ultrathin MnO 2 nanosheet composites (CdS/MnO 2 ). 77They established a linear relationship between the concentration of organophosphorus pesticides (OPs) and photocurrent by utilizing enzymatic etching of MnO 2 nanosheets and enzyme inhibition by OPs.The constructed PEC sensor had the merits of excellent stability, a wide linear range (0.05∼10 ng mL −1 ), and outstanding sensitivity.
Different from the enzyme-based biosensor, a self-powered PEC apta-sensor was constructed for diazinon (DZN) detection based on porous graphitic carbon nitride (CN). 180In this PEC apta-sensor, sulfur-doped h-BN (S-BN) was combined with the porous graphitic carbon nitride (CN), and Au nanoparticles with localized surface plasmon resonance (LSPR) effect acted as bridges to create a Z-scheme heterojunction, thereby enhancing the separation efficiency of photogenerated charge pairs.The proposed PEC sensor exhibited the benets of a low detection limit (6.8 pM), a broad linear detection range (0.01∼10 000 nM), and excellent selectivity for DZN detection.
3.2.4.Toxins.Toxins are harmful compounds produced by living cells or organisms, including microcystin-LR (MC-LR), ochratoxin A (OTA), and aatoxin B1 (AFB1). 250Therefore, the development of simple and sensitive methods for detecting such toxins is critical.Recently, Li et al. constructed a photoelectrochemical sensor for MC-LR detection, employing a bismuth/two-dimensional graphitic carbon nitride/ deoxyribonucleic acid (BiVO 4 /2D-C 3 N 4 /DNA) aptamer system. 186The porous graphitic carbon nitride nanosheets were modied onto the surface of the BiVO 4 lm photoelectrode.The coupling of 2D-C 3 N 4 with BiVO 4 formed a type-II heterojunction, enhancing the transfer capacity of photogenerated electron-hole via an internal electric eld effect.Additionally, 2D-C 3 N 4 effectively absorbed the DNA aptamer probe owing to its large area of p-p bonds.Thus, the sensor achieved a LOD of 0.04 pg L −1 within the range of 0.5 pg L −1 ∼10 mg L −1 .Apart from the conventional type-II heterojunction, the porous g-C 3 N 4 has been combined with various semiconductors and electron transfer mediators (e.g., Ag, Au, and Pt) to construct Z-scheme heterojunction composites.For instance, Tang et al. developed a self-powered photoelectrochemical apta-sensor for MC-LR determination based on a Z-scheme CoO/Au/g-C 3 N 4 heterojunction composite. 185In this composite, both CoO and Au nanoparticles were well anchored to the surface of the porous g-C 3 N 4 .The well-matched energy bands between CoO and g-C 3 N 4 , along with the presence of Au NPs as electron transfer mediators, signicantly improved the separation efficiency of photogenerated charge pairs.Thus, the detection limit for this sensor was lowered to 0.01 pM over a wide range from 0.1 pM to 10 nM.
3.2.5.Perspectives.Overall, PEC sensors based on photoactive porous materials hold signicant promise for the sensitive and selective detection of organic environmental pollutants.Metal oxides (such as TiO 2 ), metal-organic frameworks, covalent organic frameworks, graphitic carbon nitride, and MXene all exhibit huge potential for detecting various types of toxic organic contaminants, including phenolics, antibiotics, pesticides, toxins, and dyes.These materials demonstrate robust photoelectrochemical detection performance with high sensitivity and excellent selectivity.Nevertheless, there are still some challenges in this eld: (1) most of reported PEC sensors for organic environmental pollutants assays still rely on the target as the electron donor during the detection procedures, which results in relatively low interference in complex samples.Therefore, there is a need for the development of novel photoactive porous materials that can directly interact with organic analytes.(2) The anti-fouling properties of photoactive porous materials need improvement due to the adsorption of organic molecules and their derivatives on the surface of these materials.

Non-metal ion pollutants
Apart from heavy metal ions and organic pollutants, there is growing concern regarding non-metal ion pollutants, including nitrites, suldes, and cyanides. 251Recently, several works have focused on the PEC detection of these non-metal ion pollutants in the environment. 252,253In 2017, Su et al. described a novel PEC sensor for sulde detection using Cu 2 O-decorated TiO 2 nanotubes. 84The p-n heterojunction in the porous Cu 2 O-TiO 2 heterostructure effectively suppressed the recombination of photogenerated charge pairs, leading to enhanced photocurrent response.The detection of sulde by this sensor displayed a wide linear range from 1 mM to 300 mM, and the LOD was 0.6 mM.Furthermore, it has been successfully employed for accurate sulde monitoring in tap and lake water, achieving outstanding recoveries ranging from 99.2% to 103%.
In addition to the detection of toxic suldes in water bodies, the PEC sensing platform has also been utilized for tracing nitrides.For instance, Luo et al. constructed a porous threedimensional (3D) network of SnO 2 nanobers on an ITO substrate using an electrospinning technique, followed by the electrodeposition of gold nanoparticles (Au NPs). 85The presence of porous SnO 2 nanobers with a 3D network structure was benecial for the photogenerated charge pair separation and transfer (as shown in Fig. 10G).Moreover, Au NPs with the LSPR effect suppressed recombination of electron-hole pairs.By utilizing the photosensitizer-Ru(bpy) 3 2− , the photocurrent response was further increased.As illustrated in Fig. 10I, under the 473 nm light irradiation, the SnO 2 could not be excited, whereas Au NPs and Ru(bpy) 3 2− were both excited.Ru 2+ was excited into unstable Ru 2+ *, and the electrons were transferred to the Au or SnO 2 nanobers.These electrons were then transferred into the conduction band of SnO 2 with the aid of Au NPs in the surface plasma state.The Ru 2+ ions were converted to Ru 3+ ions, which then reacted with NO 2 − to form NO 3 − and Ru 2+ ions.As a result, a linear relationship between photocurrent and NO 2 − concentration was established.Under optimized test conditions, the fabricated sensor demonstrated a wide linear range from 1 to 10 000 nM with a LOD of 0.48 nM (Fig. 10H).

Conclusion and outlook
Photoelectrochemical detection has emerged as a promising analytical method for tracing environmental pollutants due to its distinct advantages, including cost-effectiveness, rapid response, minimal background noise, and high sensitivity.Crucially, photoactive materials, which signicantly inuence the monitoring activity of PEC sensors, are key components of these sensing systems.In particular, porous materials have garnered considerable interest in the design and fabrication of photoelectrochemical sensing platforms due to their unique properties, such as a high surface area, tunable pore sizes, and an abundance of functional groups.In this review, we summarize recent advances in photoactive porous materialbased photoelectrochemical sensors and their applications in monitoring environmental contaminants in water bodies based on a substantial body of research.We introduce and categorize typical porous materials into ve groups: metal oxides, metalorganic frameworks, covalent organic frameworks, graphitic carbon nitride, and MXene.Additionally, we separately discuss their applications in detecting heavy metal ions, organic pollutants, and non-metal ion pollutants.Most importantly, the structural effects of porous materials on the photoelectrochemical detection performance (sensitivity, detection limits, selectivity, and stability) of associated sensors have been discussed in detail in several representative works.Based on the comparison of numerous references, the structural effects of porous materials on PEC detection activity can be summarized into the following critical points: (1) enhancement of the light absorption ability through the multiple reection of light in pores, improving the light harvesting capability; (2) acceleration of the photoinduced electron-hole pair separation and transfer through the unique porous structures with considerable small pores, shortening the separation and migration route and suppressing charge pair recombination; (3) improvement of the surface charge transfer rate, porous materials with the high surface area provide abundant active sites for chemical reactions and facilitate the detection of species capture.From the aforementioned advantages of porous materials for the construction of photoelectrochemical sensing platforms, we strongly conrm the remarkable potential of porous materials for environmental pollutant detection.
Despite excellent advances and rapid progress in the eld of photoelectrochemical sensors based on porous materials for environmental toxic species assays, some fundamental issues and emerging challenges remain: (1) Even though much effort has been devoted to the application of porous materials in PEC sensors for environmental hazard detection, only a small portion of the vast family of porous materials has been used to fabricate PEC sensing platforms.Meanwhile, some porous materials are only preliminary to PEC sensing applications for highly toxic species, such as photoactive COFs.Therefore, it still has huge potential for exploitation.Furthermore, relatively few porous materials have been used for heavy metal ion assays, only metal oxides and graphitic carbon nitride.Some toxic heavy metal ions, such as arsenic, have not been detected by porous material-based PEC sensors.
(2) Most studies of porous materials focus on the energy level structure and neglect the unique porous structures, e.g., pore diameter and surface area.In addition, the effect of the porous structure on the photoinduced charge transfer and separation efficiency needs to be explored more.More importantly, although the photoelectric coordination of the detection mechanism based on the porous materials has been explained in several works, they only prefer to discuss the photocurrent signal generation mechanism in terms of the classical semiconductor theory and the consideration of the structural effects on the detection mechanism are somewhat insufficient.More attention should be paid to it for future studies, as porous structures favor trapping of probe species and an abundance of functional groups provides more active sites for the occurrence of relevant redox reactions.Moreover, the stability of PEC sensors based on porous materials requires more attention due to the fragile nature of porous structures and multiple assembly processes.
(3) The application of porous materials to PEC sensing is primarily at the laboratory stage.For practical applications, it still suffers from several shortcomings, such as relatively low reliability and reproducibility.Moreover, real samples may contain multiple environmental hazards, thus requiring multiplexing sensing capabilities for PEC sensors based on porous materials.In the eld of environmental sample detection, integrating PEC detection methods with exible electrodes, such as conductive polymers and carbon paper, is highly recommended.Flexible electrodes support the miniaturization and development of wearable and smart devices.PEC detection strategies should be combined with intelligent technologies to enable real-time and in situ detection, while still emphasizing the stability, convenience, and operability of integrated detection devices.Additionally, there are currently few commercial PEC sensing devices on the market, which hinders the development of PEC sensing methods.Therefore, future research should focus more on the fabrication of PEC-related equipment.With the help of smart technologies and the potential for commercial benets, the current limitations of PEC sensing can be resolved.

Fig. 1
Fig. 1 Overview of the research contributions in photoelectrochemical sensors since 2010.

Fig. 2 (
Fig. 2 (A) Anodic and cathodic photocurrent generation mechanisms of photoactive material-based electrodes.(B) Different types of PEC sensors with or without the recognition elements.

Fig. 3
Fig. 3 Structural effects of photoactive porous materials on the performance of photoelectrochemical detection of environmental pollutants.

Fig. 5 (
Fig. 5 (A) The preparation of a ZIF-8@ZIS-based PEC sensor for tetracycline.(B) TEM image of ZIF-8@ZIS.(C) The corresponding calibration curve for the photocurrent signal responses toward various concentrations of tetracycline.Adopted from ref. 95.Copyright 2022, with permission of Elsevier.(D) The synthetic process of hollow CoS x @CdS composites and the band structures of CoS x @CdS/HgS composites and charge separation under visible light irradiation.(E) TEM image of CoS x @CdS composites.(F) Photocurrent responses of the CoS x @CdS-modified electrodes in the presence of Hg 2+ of different concentrations.Adopted from ref. 103.Copyright 2020, with permission of Elsevier.

mM 127 a
Fig. 6 (A) Illustration of the TAPP-COF-based PEC sensor for tracing Pb 2+ .(B) The flexible photograph of TAPP-COF thin films.(C) Photocurrent response of the photoelectrochemical detection sensor to different concentrations of Pb 2+ .Adopted from ref. 124.Copyright 2021, with permission of American Chemical Society.(D) The construction of the F-COF/TiO 2 NTA platform for PEC sensing for dopamine.(E) SEM image of F-COF/TiO 2 NTA.(F) Photocurrent response of F-COF/TiO 2 NTA to different concentrations of dopamine.Adopted from ref. 125.Copyright 2021, with permission of American Chemical Society.

Fig. 7 (
Fig. 7 (A) TEM image of A-CN.(B) Nitrogen adsorption-desorption isotherm curves for bulk CN and A-CN.(C) Photocurrent responses of bulk CN and A-CN.Adopted from ref. 163.Copyright 2020, with permission of Elsevier.(D) TEM images of PCN-S.(E) Photocurrent responses of the Au/PCN-S at various oxytetracycline concentrations.(F) The photocurrent generation mechanism of Au/PCN-S under visible light irradiation.Adopted from ref. 162.Copyright 2018, with permission of Elsevier.

Table 1
Typical photoactive porous materials for the photoelectrochemical monitoring of environmental contaminants 63yers on the Bi 2 WO 6 nanoakes.63Theinuence of the a-Fe 2 O 3 layer thickness on the PEC detection activity of tetracycline was thoroughly investigated.Aer introducing porous a-Fe 2 O 3 layers, the photocurrent of the hybrid composite-based photoelectrode increased (4.3 mA cm −2 ) compared to that of pristine Bi 2 WO 6 (1.2 mA cm −2 ).Under optimized conditions, the photoelectrode

Table 3
Metal oxide-based photoelectrochemical detection sensors for tracing environmental contaminants a

Table 6
Covalent organic framework-based photoelectrochemical monitoring of environmental pollutants a