Signal Amplification Mechanisms in Photoelectrochemical Biosensors： From Photoelectric Conversion to Signal Output

Sitian Long; Haibing Zhu; Yuchen Du; Yadong Xue; Juan Li; Zhanjun Yang

doi:10.7536/PC20260101

Progress in Chemistry >

2026 , Vol. 38 >Issue 3: 532 - 560

DOI: https://doi.org/10.7536/PC20260101

Review

Signal Amplification Mechanisms in Photoelectrochemical Biosensors： From Photoelectric Conversion to Signal Output

Sitian Long ² ,
Haibing Zhu ² ,
Yuchen Du ² ,
Yadong Xue ^,¹^,^* ,
Juan Li ^,²^,^* ,
Zhanjun Yang ^,²^,^*

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¹ Central Laboratory， Affiliated Jinhua Hospital， School of Medicine， Zhejiang University， Jinhua 321000， China
² School of Chemistry and Materials， Yangzhou University， Yangzhou 225002， China

^* xueyadong@zju.edu.cn （Yadong Xue）；

lijuan@yzu.edu.cn （Juan Li）；

zjyang@yzu.edu.cn （Zhanjun Yang）

^† These authors contributed equally to this work

Received date: 2026-01-04

Revised date: 2026-01-29

Online published: 2026-03-18

Supported by

National Natural Science Foundation of China(22474124)

National Natural Science Foundation of China(21475116)

National Natural Science Foundation of China(21575125)

Project for Yangzhou City and Yangzhou University corporation(YZ2023204)

Open Research Fund of State Key Laboratory of Analytical Chemistry for Life Science(SKLACLS2405)

Postgraduate Research & Practice Innovation Program of Jiangsu Province(KYCX25_3936)

Jiangsu Provincial Key Laboratory of Green & Functional Materials Environmental Chemistry

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Abstract

Photoelectrochemical （PEC） biosensors， as an emerging analytical platform， offer significant advantages， including low background signals， high sensitivity， and operational simplicity， due to the inherent separation of the excitation source and the detection signal. The core of achieving high performance in PEC biosensors lies in the development of efficient signal amplification strategies. This review systematically summarizes recent research progress on signal amplification mechanisms in PEC biosensors. Photoelectric †conversion constitutes the basis of PEC sensing， primarily involving three essential processes： light harvesting， charge carrier separation， and interfacial reaction. Based on this， the prevailing signal amplification mechanisms are reviewed from the core processes of photoelectric conversion to the design of signal output. Simultaneously， the design principles and characteristics of these mechanisms are delved. Finally， this review examines the challenges of PEC sensing technologies and explores future trends. This review aims to provide theoretical guidance for the rational design of high-performance PEC biosensors and to promote their further development in applications of analysis.

Contents

1 Introduction

2 Signal amplification mechanisms in PEC sensors

2.1 Modulating light absorption and photogenerated charge carriers separation

2.2 Modulating interfacial redox reactions

2.3 Modulating the output signal

3 Challenges and perspectives

Key words： photoelectrochemical biosensing; signal amplification; photogenerated carrier separation engineering; interfacial redox reactions; signal-output design

Cite this article

Sitian Long , Haibing Zhu , Yuchen Du , Yadong Xue , Juan Li , Zhanjun Yang . Signal Amplification Mechanisms in Photoelectrochemical Biosensors： From Photoelectric Conversion to Signal Output[J]. Progress in Chemistry, 2026 , 38(3) : 532 -560 . DOI: 10.7536/PC20260101

1 Introduction

Photoelectrochemical （PEC） sensing is an innovative analytical platform that integrates photoexcitation and electrochemical detection. It features operational simplicity， minimal background signals， high selectivity， and high sensitivity^[1-3]. PEC sensing inherits the benefits of electrochemical sensing while potentially offering superior sensitivity and anti-interference ability due to the physical separation of the excitation （light） and detection signals （current）. Compared with optical methods such as colorimetry， resonance light scattering， fluorescence， PEC sensing is generally more cost-effective， amenable to miniaturization， and capable of real-time monitoring^[4-5]. It is widely applicable in diverse fields including disease diagnosis， environmental pollutant detection， and food safety monitoring^[6-8].

The PEC sensing process is based on the photoelectric effect^[9-10]. Upon absorption of photons with sufficient energy， electrons （e^-） in the valence band （VB） of a semiconductor are excited to the conduction band （CB）， leaving behind positively charged holes （h⁺） in the VB， thereby forming e^--h⁺ pairs. The separated e^--h⁺ migrate to the substrate electrode surface and the electrode material/electrolyte interface， respectively， where interfacial redox reactions occur， facilitating energy transfer and generating a photocurrent. Fig.1 schematically illustrates the generation of anodic and cathodic photocurrent. A built-in electric field （BIEF） exists at the semiconductor/solution interface. For an n-type semiconductor， whose Fermi level （E_F） is close to the conduction band edge （E_CB）， alignment with the solution's redox potential results in an upwardly bent depletion layer^[11]. The BIEF is directed from the bulk semiconductor toward the electrolyte. Driven by this field， photogenerated holes accumulate at the electrode surface and participate in oxidation reactions. The resulting electron flow in the external circuit gives rise to an anodic photocurrent. Conversely， for a p-type semiconductor， whose E_F is near the valence band edge （E_VB）， the depletion layer bends downward， and the electric field direction is from the solution toward the bulk. Photogenerated electrons are driven to the surface， undergo reduction reactions， and result in a cathodic photocurrent.

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图1 （a）阳极光电流和（b）阴极光电流生成示意图

Fig.1 Schematic diagram of （a） anodic photocurrent and （b） cathodic photocurrent generation

In practical applications， the performance of PEC sensors is often limited by factors including low photoelectric conversion efficiency of the photoactive materials， insufficient light absorption， and weak recognition signals from trace analytes^[12-13]. To improve PEC sensor performance， research has focused on two main directions. One is the rational design of photoactive materials to enhance light harvesting and facilitate charge carrier transport and separation through strategies like bandgap engineering^[14-16] and morphology control^[17-18]. The other involves the development of signal amplification strategies^[19-22]. Several reviews have summarized approaches for signal amplification of PEC sensors. For example， Yang et al.^[23] reviewed the modification of photoactive materials and various signal amplification strategies. Xu et al.^[24] summarized the design of photoactive materials and signal amplification strategies in pathogen-related PEC sensors. Recently， Bao et al.^[25] reviewed various signal amplification strategies for PEC sensors in the analysis of carcinoembryonic antigen （CEA）. Existing reviews often catalog amplification strategies as discrete techniques， overlooking their shared mechanism. This review moves beyond by clarifying the key regulatory role of the BIEF in photogenerated charge dynamics， thereby constructing a unified mechanistic framework.

This review aims to summarize the signal amplification mechanisms in PEC sensors. Initially， the operational mechanisms of different signal amplification strategies are explored by analyzing various stages of the PEC process， including light harvesting， separation and transfer of photogenerated charge carriers， interfacial redox reactions， and electrical signal generation. Then the remaining challenges in the development of PEC sensors are discussed， and perspectives on future research directions are provided.

2 Signal amplification mechanisms in PEC sensors

To maximize the sensing performance of PEC biosensors， it is crucial to systematically optimize every key step within the sensing process. A typical PEC sensing process commences with light harvesting， where photoactive materials absorb photons to generate e^--h⁺ pairs. This is followed by the charge separation and transfer stage， during which the photogenerated carriers are efficiently separated and transported to the material surface. In the ensuing interfacial chemical reaction step， these charges participate in redox reactions with electron donors or acceptors in the solution. Finally， this reaction is transduced into a measurable signal output， such as a change in the intensity or direction of the photocurrent. These four stages are intricately linked， and an efficiency loss in any single step will directly constrain the final sensitivity， selectivity， and stability of the sensor. Consequently， the core of modern PEC signal amplification strategies revolves around the precise intervention and enhancement of these four physico-chemical stages. Specifically， photogenerated carrier separation engineering， through strategies such as bandgap engineering， morphology control， defect introduction， and heterojunction construction， aims to synergistically improve the light-harvesting capability and charge separation efficiency. By modulating interfacial reaction kinetics， for instance， through the in-situ generation of electron donors/acceptors or the introduction of steric hindrance effects， the efficiency and specificity of interfacial chemical reactions can be significantly enhanced. Furthermore， the integration of DNA amplification techniques or the design of photocurrent polarity switching modes can fundamentally amplify or convert the final output signal， thereby overcoming performance bottlenecks. This chapter will follow this intrinsic logic to systematically elucidate advanced signal amplification mechanisms aimed at optimizing the aforementioned steps， providing a comprehensive theoretical foundation and design guidelines for constructing next-generation， ultra-sensitive PEC biosensors.

2.1 Modulating light absorption and photogenerated charge carriers separation

Photoactive materials serve as the core functional elements in PEC sensing and are responsible for the conversion of light energy into electrical signals. Their performance directly dictates the sensitivity and selectivity of the sensor. Photoactive materials encompass organic semiconductors^[26-28]， inorganic semiconductors^[29-31]， and emerging materials， such as metal-organic frameworks （MOFs）， MXenes， and covalent organic frameworks （COFs）^[32-34]. According to band theory， a semiconductor is characterized by a bandgap situated between that of a conductor and an insulator. Upon excitation by external energy sources like light or heat， electrons in the VB can be excited to the CB， generating charge carriers. However， intrinsic semiconductors often lack the requisite properties to optimally fulfill every step in the PEC sensing process. Consequently， it is essential to engineer semiconductor materials to enhance their capabilities in light harvesting as well as in the separation and transport of photogenerated charge carriers， thereby improving the overall photoelectric conversion efficiency.

2.1.1 Surface sensitization

The light absorption capacity of a semiconductor is determined by its bandgap energy （E_g）， which is a consequence of its unique electronic band structure. Some commonly used semiconductors in PEC sensing， such as TiO₂^[35] and ZnO^[36]， are only responsive to ultraviolet （UV） light. To address this limitation， broadening the absorption range of semiconductor materials across the solar spectrum is an effective strategy for enhancing light harvesting. Surface sensitization represents a viable method to overcome the constraint of wide bandgaps in photoactive materials. This approach extends the spectral absorption into the visible （Vis） or even near-infrared （NIR） regions， thereby improving the overall photoelectric conversion efficiency^[37-39]. Common sensitizers include quantum dots （QDs）， organic dyes， and noble metal nanoparticles （NPs）.

2.1.1.1 QDs

Enhancing the light absorption capacity of photoactive materials necessitates broadening their absorption spectrum from the UV to the Vis region. QDs， typically composed of II-VI （e.g.， HgCdTe， HgSe， and HgTe）， III-V （e.g.， InP， InAs， and InSb）， or IV-VI （e.g.， PbS， PbSe， and PbTe）， are zero-dimensional （0D） nanomaterials with small size^[40-41]. They possess relatively narrow bandgaps， enabling them to serve as effective absorption centers for Vis light. The in-situ introduction of QDs onto photoactive materials is a common strategy to widen the light absorption range. Furthermore， the introduced QDs often exhibit matched energy levels with the substrate material， facilitating more efficient generation e^--h⁺ pairs. For instance， Song et al.^[42] electrochemically deposited ZnS QDs as photosensitizers onto an ITO/TiO₂ NPs substrate， as shown in Fig.2a. The modification with ZnS QDs extended the absorption range of excitation light. Simultaneously， the QD-sensitized heterojunction interface absorbed photon energy， thereby promoting the formation of e^--h⁺ pairs and resulting in a fourfold enhancement in photocurrent intensity （Fig. 2b）. QDs exhibit large Stokes shifts， indicating a significant difference exists between the excitation and emission wavelengths. This property is beneficial in PEC analysis as it effectively reduces background signal interference^[43]. Consequently， probe molecules labeled with QDs are widely employed in PEC biosensing^[44-45]. Biomolecules provide abundant attachment sites for QDs， thereby improving detection sensitivity. Sun et al.^[46] developed a PEC biosensing platform based on a CdTe QDs-sensitized WO₃/SnS₂ Type-II/Z-scheme tandem heterostructure for the detection of circulating tumor cells （CTCs）. CdTe QDs conjugated with aptamers were immobilized on the electrode via specific hybridization， forming the Type-II/Z-scheme tandem heterostructure and enhancing the PEC performance. Detection relied on the specific recognition and capture of MCF-7 cells by the aptamer-conjugated QDs （Apt-QDs）， followed by their detachment from the electrode surface， leading to a decrease in the photocurrent response. This “signal-on-off” PEC sensing strategy， involving QD-labeled probes， not only amplifies the photocurrent signal but also enables the specific recognition of target molecules， thereby significantly improving detection specificity^[47-48].

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图2 （a）ITO/TiO₂ NPs/ZnS QDs/胆碱氧化酶（ChOx）/磷脂酶D（PLD）电极的制备过程示意图^[42]；（b）裸ITO（a）、ITO/TiO₂ NPs（b）、ITO/TiO₂ NPs/ZnS QDs（c）、ITO/TiO₂ NPs/ZnS QDs/ChOx（d）和ITO/TiO₂ NPs/ZnS QDs/ChOx/PLD（e）的光电流响应曲线，PEC测试在含有1 mmol/L PC在PBS（10 mmol/L）中进行，偏置电压为0.4 V；ITO/TiO₂ NPs/ZnS QDs/ChOx/PLD（f）在不含1 mmol/L PC在PBS（10 mmol/L）中、偏置电压0.4 V条件下的光电流响应曲线^[42]；（c）[（C₆）₂Ir（dcbpy）]⁺敏化NiO光电阴极用于前列腺特异性抗原（PSA）检测的组装过程示意图^[52]；（d）基于Ag/ZIF-8复合材料检测DA的PEC传感机制示意图，以及Ag/ZIF-8电极在有光和无光条件下的DPV曲线^[58]

Fig.2 （a） Fabrication process of the ITO/TiO₂ NPs/ZnS QDs/choline oxidase （ChOx）/phospholipase D （PLD） electrode^[42]. Copyright 2025， Elsevier. （b） Photocurrent responses of bare ITO （a）， ITO/TiO₂ NPs （b）， ITO/TiO₂ NPs/ZnS QDs （c）， ITO/TiO₂ NPs/ZnS QDs/ChOx （d）， and ITO/TiO₂ NPs/ZnS QDs/ChOx/PLD （e）， PEC measurements were carried out in PBS （10 mmol/L） with 1 mmol/L PC under 0.4 V bias voltage. Photocurrent responses of ITO/TiO₂ NPs/ ZnS QDs/ChOx/PLD （f）， in absence of 1 mmol/L PC in PBS （10 mmol/L） under 0.4 V bias voltage^[42]. Copyright 2025， Elsevier. （c） Schematic diagram of the assembly process about [（C₆）₂Ir（dcbpy）]⁺-sensitized NiO photocathode for prostate-specific antigen （PSA） detection^[52]. Copyright 2022， Elsevier. （d） Schematic diagram of the PEC sensing mechanism for DA detection using the Ag/ZIF-8 composite. And DPV curves of the Ag/ZIF-8 electrode in the presence and absence of light^[58]. Copyright 2025， Elsevier

2.1.1.2 Dyes

In recent years， dyes have garnered significant attention as sensitizing materials in dye-sensitized solar cells^[49]. Owing to their broad absorption bands and efficient photoelectric conversion properties that significantly enhance light absorption and PEC performance， dye sensitization is also widely employed in constructing highly sensitive， Vis-light-driven sensors^[50-51]. In PEC sensing， transition metal complexes are utilized as photosensitizers. These dyes feature a central transition metal ion surrounded by organic ligands. The ligands are designed to absorb light at specific wavelengths and transfer the energy to the metal ion， initiating a metal-to-ligand charge transfer （MLCT） process. During MLCT， an electron is transferred to a ligand. Since the ligand is typically anchored to the semiconductor surface， this electron can be readily injected into the semiconductor CB， thereby achieving charge separation. Furthermore， the MLCT excited state in transition metal complexes often exhibits a relatively long lifetime. For example， as depicted in Fig.2c， Wang et al.^[52] reported the use of a cyclometalated iridium（III） complex to sensitize a NiO photocathode system， marking its first application in a cathodic PEC immunoassay. Beyond transition metal complexes， organic dyes also exhibit excellent photoresponse under Vis light. Organic dyes possess extensive conjugated π-bond systems. Upon light absorption， charge is transferred from the donor moiety to the acceptor moiety through the π-conjugated bridge， which facilitates high charge transfer efficiency. Gong et al.^[53] developed a sandwich-type PEC immunosensor based on a ZnO/poly（5-formylindole）（P5FIn）/anthocyanin heterostructure for the sensitive and background-free detection of the tumor biomarker CYFRA21-1. The introduction of the natural dye anthocyanin as a sensitizer enhanced the absorption and utilization of Vis light by the composite material， further improving the PEC conversion efficiency beyond that of the heterojunction alone.

2.1.1.3 Noble metal NPs

Doping semiconductors with noble metal NPs represents an effective strategy for enhancing their photoelectric conversion efficiency^[54]. This technique involves loading noble metal NPs （Au， Ag） onto the semiconductor surface to leverage their unique surface plasmon resonance （SPR） effect， which significantly boosts PEC performance^[55-57]. At the SPR frequency， an intensely localized electromagnetic field is generated around the NPs， producing “hot electrons” with energies higher than the E_F of the noble metal. These hot electrons can then be injected into the CB of the semiconductor， enabling it to absorb photons with energies lower than its bandgap， thereby extending its photoresponse range. Furthermore， larger NPs exhibit strong light-scattering capabilities， which enhance the light-harvesting efficiency of the semiconductor. The contact between the noble metal and the semiconductor forms a Schottky barrier， which acts as an efficient electron trap， thereby improving the separation efficiency of photogenerated charge carriers. Yan et al.^[58] developed a PEC biosensor based on an Ag/ZIF-8 composite， which demonstrated significantly enhanced performance for dopamine （DA） detection， as illustrated in Fig.2d. The synergistic effect between the localized surface plasmon resonance （LSPR） of Ag NPs and the strong interfacial coupling with ZIF-8 substantially increased the photocurrent response. Additionally， noble metal NPs can serve as catalytic centers for redox reactions， modulating the photocurrent signal by catalyzing the target analyte during sensing^[59-60]. They can also be conjugated with biomolecules； for instance， Au NPs can functionalize biomolecules via Au-S bonds， improving the selectivity of the biosensor^[61]. Therefore， noble metal NPs are considered a key technology for enabling the next generation of ultra-sensitive and miniaturized sensors.

2.1.2 Morphology engineering

The morphology of a photoactive material dictates its function. Through meticulous morphological design， the key processes such as light absorption， charge separation， and surface reaction can be significantly improved， and these steps collectively determine the overall PEC performance. Classified by spatial dimensionality， nanomaterials can be categorized as 0D （e.g.， QDs， nanoclusters （NCs）， and nanospheres）^[62]， one-dimensional （1D， e.g.， nanowires， nanofibers （NFs）， and nanotubes （NTs））^[63]， two-dimensional （2D， e.g.， nanosheets）^[64]， and three-dimensional （3D， e.g.， hollow structures， urchin-like， and flower-like structures）. From 0D to 3D， photoactive materials with different morphologies each possess distinct advantages and disadvantages for PEC sensing. 0D NPs represent the most fundamental and common morphology， such as TiO₂， ZnO， and CdS. Their large specific surface area provides abundant active sites. However， the numerous interfaces between particles create transport pathways in which photogenerated carriers must traverse many boundaries to reach the substrate， significantly increasing the probability of charge recombination. 1D nanomaterials provide a direct， unidirectional pathway for electron transport. Electrons can travel rapidly along the axial direction to the substrate， although the increase in specific surface area is somewhat limited （Fig.3a）^[65-66]. 2D nanosheets are ultra-thin sheet-like structures （Fig.3b）. Their extremely high specific surface area offers a large platform for reactions. The minimal thickness shortens the transport path for photogenerated carriers， allowing them to rapidly reach the surface and participate in reactions （Fig.3c）. Furthermore， the edges of 2D nanosheets are often rich in active sites， conferring high catalytic activity. Consequently， 2D nanosheets are widely used in sensing platforms requiring numerous surfaceactive sites^[67-68]. 3D hierarchical structures are complex architectures self-assembled from lower-dimensional building blocks （e.g.， nanowires， nanospheres）. They combine a high specific surface area with efficient electron transport capabilities^[69-70]. For example， Yang et al.^[71] designed a highly sensitive Z-scheme PEC biosensing platform using Cu₂O-sensitized hollow covalent organic framework microspheres （Fig.3d）. In such structures， the hollow nanoarchitecture promotes multiple light utilization through internal refraction and reflection， enhancing light-harvesting efficiency （Fig.3e）. Simultaneously， it provides abundant reaction sites and shortens charge transport distances （Fig.3f）， markedly amplifying the photocurrent of the sensing platform （Fig.3g）. Engineering different morphologies can improve PEC sensing performance in various ways. For instance， the direct conductive paths in 1D arrays and 3D structures facilitate more efficient electron delivery to the electrode， enhancing charge separation efficiency and accelerating response speed. The large specific surface area provided by 2D nanosheets and 3D structures allows for the immobilization of a greater number of biorecognition molecules， improving sensing sensitivity. Through the rational design of material morphology， researchers can precisely control the behavior of photons and electrons， thereby greatly optimizing PEC performance.

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图3 （a）用于检测Ca²⁺ PEC传感的改性赤铁矿纳米棒示意图^[65]；（b）原始ZnO NRs的SEM俯视图像^[68]；（c）原始ZnO、ZnO/V₂O₅及V₂O₅的能级排列示意图^[68]；（d）基于空心结构HCOF-OMe构建的用于微囊藻毒素-LR（MC-LR）检测的PEC生物传感平台示意图^[71]；（e）实心COF-OMe的TEM图像^[71]；（f，g）不同修饰电极的Nyquist图及拟合等效电路图（f）与光电流响应曲线（g）：（a）ITO、（b）HCOF-OMe/ITO、（c）GA/HCOF-OMe/ITO和（d）Cu₂O/HCOF-OMe/ITO^[71]

Fig.3 （a） Schematic diagram of the modified hematite NRs for the PEC sensing of Ca²⁺^[65]. Copyright 2022， American Chemical Society. （b） Top-view SEM image of the pristine ZnO NRs^[68]. Copyright 2026， Elsevier. （c） Energy level alignment of the pristine ZnO， ZnO/V₂O₅， and V₂O₅^[68]. Copyright 2026， Elsevier. （d） Schematic illustration of the proposed PEC biosensing platform for microcystin-LR （MC-LR） assay based on hollow structured HCOF-OMe^[71]. Copyright 2025， Elsevier. （e） TEM image of the solid COF-OMe^[71]. Copyright 2025， Elsevier. （f， g） Nyquist plots with fitted equivalent circuit diagram （f） and photocurrent responses （g） of different electrodes：（a） ITO，（b） HCOF-OMe/ITO，（c） GA/ HCOF-OMe/ITO， and （d） Cu₂O/HCOF-OMe/ITO^[71]. Copyright 2025， Elsevier

2.1.3 Defect engineering

A perfect crystal is not necessarily the material with the best performance. Defect engineering， the deliberate and controlled introduction， modulation， or elimination of point defects （vacancy defects， interstitial atoms， and substitutional atoms） in photoactive materials， enables precise tuning of their electronic structure， thereby dramatically enhancing their PEC properties. It has been demonstrated that defects can introduce defect energy levels within the semiconductor bandgap. Electrons can be excited first transition from the VB to these intragap states and then promoted to the CB， thereby extending the photoresponse range from the UV to the Vis light region. For instance， Kim et al.^[72] prepared defective amorphous TiO_x films and demonstrated that their light absorption， photogenerated charge separation， and electron transfer efficiencies were significantly improved compared to TiO₂. Moreover， these properties could be modulated by adjusting the defect concentration （Fig.4a）. Appropriately introduced defects can act as electron traps， capturing photogenerated electrons and suppressing e^--h⁺ recombination. Furthermore， defects can induce lattice distortion， forming a BIEF that aids in charge separation. Yang et al.^[73] reported on the influence of copper vacancies （V_Cu） on the PEC performance of CuO derived from a copper-based metal-organic gel （Cu-MOG） precursor， finding that a higher concentration of V_Cu enhanced the PEC activity of the CuO photocathode by promoting charge separation and transfer.

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图4 （a）ALD-TiO_x薄膜的（i）紫外-可见吸收光谱及（内插）Tauc图、（ii）光捕获效率曲线、（iii）电荷分离效率及（iv）电荷转移效率^[72]；（b）Vo-ZnO/ZnS/FTO电极的制备流程及葡萄糖传感器构建示意图^[76]；（c）ZnO与Vo-ZnO/ZnS的光电响应曲线^[76]

Fig.4 （a）（i） UV-vis absorbance spectra and （inset） Tauc plot，（ii） light-harvesting efficiency curves，（iii） charge separation efficiencies， and （iv） charge transfer efficiencies of ALD-TiO_x thin films^[72]. Copyright 2023， American Chemical Society. （b） Simple schematic of the preparation process of the Vo-ZnO/ZnS/FTO electrodes and glucose sensor preparation^[76]. Copyright 2023， The Royal Society of Chemistry. （c） Photoelectric responses of the ZnO and Vo-ZnO/ZnS^[76]. Copyright 2023， The Royal Society of Chemistry

Among vacancy defects， oxygen vacancies （OVs） are the most common and extensively studied type^[74-75]. They are typically introduced via simple and low-cost methods such as thermal treatment under specific atmospheres （H₂， Ar） or plasma treatment， which can easily extend the light absorption range into the Vis region. Xu et al.^[76] fabricated a heterojunction photoanode composed of ZnO/ZnS NCs rich in surface OVs （Vo-ZnO/ZnS）（Fig.4b）. This electrode exhibited high electron transfer efficiency under Vis light irradiation， yielding a photocurrent density three times higher than that of pure ZnO （Fig.4c）. Elemental doping represents another major strategy for introducing defects， enabling more precise control over the material’s conductivity type and E_F. Zeng et al.^[77] synthesized novel tungsten-doped In₂S₃ nanostructures with excellent PEC performance via a dissolution-regrowth process. Density functional theory （DFT） calculations confirmed that the W⁴⁺ dopants introduced impurity levels and defects， which improved the separation of photogenerated carriers and consequently enhanced the PEC performance. The key to defect engineering lies in the precise control of the defect type， concentration， and distribution， as an excessive density of defects can conversely act as recombination centers， leading to performance degradation.

2.1.4 Heterojunctions

The formation of heterojunctions is an effective strategy for promoting the separation of photogenerated charge carriers. The underlying mechanism involves the intimate contact of two or more semiconductor materials with matched band structures at their interface， which creates a BIEF driven by their intrinsic energy level difference. This field drives the directional migration of photogenerated electrons and holes in opposite directions， thereby suppressing their recombination and markedly enhancing PEC performance. Depending on the band alignment and charge transfer pathways， heterojunctions are primarily categorized into Type-II heterojunctions， Z-scheme heterojunctions， and p-n heterojunctions， each with distinct amplification mechanisms. Therefore， this section discusses the band structures， charge separation pathways， and recent advancements of these heterojunctions， from the perspective of electron transfer mechanisms.

2.1.4.1 Type-II heterojunctions

A Type-II heterojunction consists of two semiconductors with a staggered band alignment， where both the CB and VB edge energies of one material are lower than those of the other. The Type-II band alignment generates a BIEF at the interface when two materials are in contact， directed from the wide-bandgap material toward the narrow-bandgap material. This field provides a directional driving force for photogenerated electrons and holes， serving as the fundamental mechanism that enables their spatial separation and thereby enhances the PEC signal. Under illumination， photogenerated electrons spontaneously transfer from the higher CB to the lower CB， while photogenerated holes migrate from the lower VB to the higher VB. This process achieves spatial separation of electrons and holes， leading to their accumulation on different semiconductor materials. Consequently， the charge carrier lifetime is substantially prolonged， and their probability of participating in interfacial redox reactions is significantly increased， leading to a markedly enhanced and stable photocurrent. Feng et al.^[78] utilized Hg²⁺-integrated ZnS nanocubes （Hg²⁺@ZnS NCs） for the quantitative analysis of hydrogen sulfide （H₂S）， as shown in Fig.5a. Hg²⁺ probe ions were doped into the porous structure of the ZnS NCs via an adsorption reaction， which modulated the surface states/defects and band structure to suppress surface recombination. This resulted in a slightly stronger PEC response compared to pristine ZnS. Upon the addition of H₂S， a Type-II HgS@ZnS heterojunction was formed in situ. Photogenerated electrons transferred from the CB of HgS to that of the Hg²⁺@ZnS composite， while holes moved from the VB of Hg²⁺@ZnS to the VB of HgS （Fig.5a）. This mechanism further enhanced the spatial separation and efficient transfer of charge carriers. The combination of heterojunction engineering and morphological control synergistically improved the overall light utilization and reaction efficiency of the heterostructure （Fig.5b）. Liao et al.^[79] developed a Vis-light-driven PEC aptasensor based on 3D electrode MoS₂@TiO₂ nanofiber membrane （MoS₂@TiO₂ NFM）. Compared to pure TiO₂ NFM， the Type-II MoS₂@TiO₂ heterojunction exhibited an extended absorption range into the NIR region and demonstrated an approximately 9.7-fold enhancement in photocurrent intensity. Although Type-II heterojunctions effectively separate photogenerated charges， they often lead to a reduction in redox potential power. Constructing multi-component heterojunctions is an effective strategy to overcome this limitation. For instance， Yin et al.^[80] designed a Bi₂O₂S/Bi₄O₅Br₂/CeO₂ ternary Type-II/Z-scheme tandem heterostructure. This design leveraged a synergistic tandem mechanism， simultaneously harnessing the efficient charge separation of the Type-II structure and the strong redox capability of the Z-scheme structure， thereby providing a stronger signal for the sensor.

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图5 （a）Hg²⁺@ZnS的制备过程、添加S^2-或H₂S后原位形成HgS@ZnS样品的示意图；复合前ZnS与HgS的能带结构排列，以及复合后在模拟太阳光照射下形成的具有Type-II异质结构的HgS@ZnS相应的电荷转移过程，以及基于Hg²⁺@ZnS的PEC传感器在模拟太阳光照射下对痕量S^2-或H₂S进行“信号增强”型分析的传感机理示意图^[78]；（b）（i）ZnS、Hg²⁺@ZnS、HgS@ZnS和HgS样品的光电流响应曲线，（ii）Hg²⁺@ZnS在750 s光开关循环测试下的光电流稳定性，（iii）所得样品的电化学阻抗谱，（iv）ZnS和HgS@ZnS样品的稳态光致发光（PL）光谱及时间分辨PL衰减曲线^[78]

Fig.5 （a） Schematic illustration of the preparation procedure of Hg²⁺@ZnS and in-situ formed HgS@ZnS samples after the addition of S^2- or H₂S， and the band structure alignments of ZnS and HgS before combination and the corresponding charge transferring of HgS@ZnS with Type-II heterostructure formed after the combination under simulated sunlight irradiation， and the sensing mechanism of Hg²⁺@ZnS-based PEC sensor for the “signal-on” analysis of trace S^2- or H₂S under simulated sunlight irradiation^[78]. Copyright 2025， Elsevier. （b）（i） The photocurrent responses of ZnS， Hg²⁺@ZnS， HgS@ZnS and HgS samples，（ii） the photocurrent stability tests of Hg²⁺@ZnS under on/off light cycle test for 750 s. （iii） the electrochemical impedance spectra of so-obtained samples. （iv） the steady-state photoluminescence （PL） spectra and time-resolved PL decay of ZnS and HgS@ZnS samples^[78]. Copyright 2025， Elsevier

2.1.4.2 Z-scheme heterojunctions

Z-scheme heterojunctions resemble Type-II heterojunctions in their staggered band alignment but differ fundamentally in their electron transfer mechanisms. A Z-Scheme heterojunction typically consists of two semiconductors （PS I and PS II） bridged by an electron mediator. Two interfacial BIEFs are established at the contact between PS I and the mediator， PS II and the mediator. These fields direct the flow of photogenerated electrons and holes toward the central mediator， promoting their efficient recombination. This process indirectly enhances the longevity and concentration of the highly reactive holes and electrons retained within the respective active components， thereby providing a stronger driving force for subsequent high-energy redox reactions and leading to significant signal amplification. The charge separation pathway in a Z-scheme heterojunction follows a “Z”-shaped pattern. There are two primary types： liquid-phase and all-solid-state Z-scheme heterojunctions. The liquid-phase Z-scheme involves two semiconductors and a pair of liquid-phase redox mediators. However， the introduction of a liquid medium can lead to issues such as photocorrosion and competitive light absorption. Consequently， all-solid-state Z-scheme heterojunctions have been developed. In an all-solid-state Z-scheme heterojunction， photogenerated electrons from the CB of PS I recombine directionally with photogenerated holes in the VB of PS II via a nano-conductor （e.g.， a noble metal or carbon material）. This process leaves highly oxidative holes on the VB of PS I and highly reductive electrons on the CB of PS II. This configuration achieves efficient charge separation while simultaneously maximizing the preservation of strong redox capabilities within the material system， thereby enhancing detection sensitivity and broadening the application scope. Fig.6a illustrates a Ti₃C₂ MXene@TiO₂/Co_2.7Ni_0.3O₄ Z-scheme heterojunction with excellent conductivity and efficient electron transport properties. Their study indicated that the CB potential of Co_2.7Ni_0.3O₄ （0.66 eV） matched well with the VB potential of TiO₂ （-0.4 eV）， facilitating electron transfer. Concurrently， photogenerated electrons in the CB of TiO₂ recombined with photogenerated holes in the VB of Co_2.7Ni_0.3O₄ via the Ti₃C₂ MXene acting as an electron mediator. This process effectively suppressed the recombination of photogenerated electrons and holes within Co_2.7Ni_0.3O₄ and Ti₃C₂， respectively^[81]. In recent research， ternary dual Z-scheme heterojunctions have attracted significant attention^[82-84]. For example， as presented in Fig.6b， Ji et al.^[85] fabricated a Bi/Bi₂WO₆/g-C₃N₄ ternary dual Z-scheme heterojunction using a controlled in-situ reduction method with NaBH₄. The Bi NPs formed dual Z-scheme heterojunctions with both Bi₂WO₆ and g-C₃N₄， which promoted charge separation. This effect synergized with the SPR of Bi NPs， resulting in a significantly enhanced photoelectric conversion capability. The system exhibited a photocurrent response of 0.7 μA， which was 3.4 to 10.4 times higher than that of the individual component materials （Fig.6c）.

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图6 （a）PEC传感器检测H₂O₂的作用机理示意图^[81]；（b）毒死蜱（CPF）检测中可能的Z型电子转移机理示意图^[85]；（c）（i）不同修饰电极的光电流响应曲线及（ii）Nyquist图：Bi/Bi₂WO₆/g-C₃N₄/ITO、aptamer/Bi/Bi₂WO₆/g-C₃N₄/ITO和CPF/aptamer/Bi/Bi₂WO₆/g-C₃N₄/ITO；（iii）g-C₃N₄/ITO、Bi₂WO₆/ITO、Bi₂WO₆/g-C₃N₄/ITO和Bi/Bi₂WO₆/g-C₃N₄/ITO的光电流响应对比及（iv）电化学阻抗谱（EIS）图^[85]

Fig.6 （a） Detection mechanism of H₂O₂ at the PEC sensor^[81]. Copyright 2025， Elsevier. （b） Chlorpyrifos （CPF） detection of a possible Z-type electron transfer mechanism^[85]. Copyright 2026， Elsevier. （c）（i） Photocurrent responses and （ii） Nyquist plots of different modified electrodes： Bi/Bi₂WO₆/g-C₃N₄/ITO， aptamer/Bi/Bi₂WO₆/g-C₃N₄/ITO， and CPF/aptamer/Bi/Bi₂WO₆/g-C₃N₄/ITO. （iii） Comparative photocurrent responses and （iv） electrochemical impedance spectroscopy （EIS） spectra of g-C₃N₄/ITO， Bi₂WO₆/ITO， Bi₂WO₆/g-C₃N₄/ITO， and Bi/Bi₂WO₆/g-C₃N₄/ITO^[85]. Copyright 2026， Elsevier

2.1.4.3 p-n heterojunctions

A p-n heterojunction is formed by the contact between a p-type （hole-majority） semiconductor and an n-type （electron-majority） semiconductor. This interface creates a BIEF directed from the n-region to the p-region and a corresponding space charge region. Under illumination， this strong BIEF provides a powerful driving force for charge separation， propelling photogenerated electrons towards the n-region and holes towards the p-region. For example， as can be seen in Fig.7a， Tian et al.^[86] prepared a MoS₂/TiO₂ NTs p-n heterojunction nanocomposite via a hydrothermal method. Characterization techniques including PL and EIS confirmed that the formed heterojunction effectively enhanced the separation and transport efficiency of photogenerated charge carriers under Vis light （Fig.7b）. This improvement is conducive to increasing photoelectric conversion efficiency and sensing sensitivity. In such systems， if the separated holes flow from the p-region to the solution and an n-type semiconductor serves as the working electrode， an anodic photocurrent is predominantly observed. Conversely， if the separated electrons flow from the n-region to the solution using a p-type semiconductor as the working electrode， a cathodic photocurrent is generated. Leveraging this property， an innovative signal output mode has been developed where the sensor’s output signal reverses from one polarity （e.g.， anodic current） to the other （e.g.， cathodic current）. This strategy can fundamentally eliminate background current interference caused by factors like the sample matrix or instrumental fluctuations， offering an extremely high signal-to-noise ratio and superior anti-interference capability^[87-88]. Lin et al.^[89] synthesized a novel bifunctional Sb₂S₃/CdIn₂S₄ （SS/CIS） p-n heterojunction via a hydrothermal method for the sensitive PEC detection and detoxification of hexavalent chromium （Cr（VI））. The SS/CIS heterojunction exhibited an electron lifetime of 22.93 milliseconds and an incident photon-to-current conversion efficiency （IPCE） of 59.75%， significantly higher than those of pristine SS and CIS. A decrease in the anodic photocurrent was observed at low Cr（VI） concentrations （0.1~1 mmol/L）. At higher concentrations （2~8 mmol/L）， the reduction of Cr（VI） to Cr（III） induced a photocurrent polarity switch， simultaneously achieving the detoxification process.

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图7 （a）MoS₂/TiO₂ NTs p-n异质结可能的电子转移机理示意图；（b）MoS₂/TiO₂ NTs（1∶20）与TiO₂ NTs的（i）EIS图谱及（ii）PL光谱^[86]

Fig.7 （a） A possible MoS₂/TiO₂ NTs p-n heterojunction electron transfer mechanism. （b） EIS plots （i）， and PL spectrum （ii） of the MoS₂/TiO₂ NTs 1∶20 and TiO₂ NTs^[86]. Copyright 2025， American Chemical Society

2.1.5 Energy transfer effects

Energy transfer （ET） has been established as an effective signal amplification strategy^[90-91]. Based on the mechanism of Förster resonance ET （FRET）， when the emission spectrum of a donor overlaps with the absorption spectrum of an acceptor and their distance is less than 10 nm， energy from the donor can be transferred to the acceptor via dipole-dipole interactions^[92]. In PEC sensing， this ET mechanism can modulate the generation or recombination efficiency of photogenerated charge carriers， enabling the amplification of photocurrent signals in “signal-on” or “signal-off” modes. Small-molecule probes （SMPs） are widely employed as energy acceptors in the field of ET^[93]. Kong et al.^[94] demonstrated that an SMP acting as an energy acceptor can effectively quench the photocurrent in a PEC sensing process and constructed a PEC sensing platform based on this ET principle （Fig.8a）. They utilized CdTe QDs as both the photoactive material and the energy donor， and a rhodamine-based SMP （named XY-K） designed for SO₂ as the energy acceptor. In the absence of SO₂， XY-K induced e^--h⁺ recombination via ET， effectively attenuating the photocurrent of the CdTe QDs. However， a specific chemical reaction between SO₂ and XY-K inhibited the ET process， leading to the recovery of the photocurrent signal and enabling the sensitive detection of SO₂. Furthermore， differing from traditional solution or interfacial systems， intranetwork ET， where both energy donor and acceptor are anchored within the lattice of a MOF， has been developed as a highly efficient ET strategy^[95-96]. Kong et al.^[97] designed an intranetwork ET （IRET） strategy and synthesized a novel dual-photosensitized MOF （dpMOF）（Fig.8b）. The photosensitizers meso-tetra（4-carboxyphenyl）porphine （Por） and pyridine-functionalized boron dipyrromethene （BDP） served as the energy acceptor and donor， respectively， within the MOF framework after BDP ligand absorption under 460~550 nm light irradiation. Both experimental validation and DFT calculations confirmed the IRET process from the BDP ligand to the Por ligand within the MOF skeleton. This process promoted the separation of e^--h⁺ pairs， generated photoelectrons from the Por ligand， followed by electron transfer from the HOMO-1/-2 of the Por ligand to the LUMO of the BDP ligand. The achieved IRET efficiency was as high as 88.3%， and the resulting photocurrent was 2.6 to 6.5 times greater than that of single photosensitized MOFs. This research provides new insights for achieving high photon utilization in photoactive materials and expands the application of MOFs in photoelectric conversion.

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图8 （a）基于ET调制的PEC微传感器用于SO₂检测及体内分析的示意图，采用光寻址微电极（：XY-K的识别过程；：体内靶标识别；：体外信号检测）^[94]；（b）在存在DA和溶解氧的条件下，具有框架内IRET机制的dpMOF的PEC过程示意图^[97]

Fig.8 （a） The schematic illustration of the ET modulated PEC microsensor for the detection of SO₂ and in vivo assays using the light addressable microelectrode （*： recognition process of XY-K； **： in vivo target recognizing； ***： in vitro signal detecting）^[94]. Copyright 2024， Elsevier. （b） A schematic illustration of the PEC process of dpMOF with IRET mechanism in the presence of DA and dissolved O₂^[97]. Copyright 2025， American Chemical Society

2.2 Modulating interfacial redox reactions

The preceding sections have discussed strategies for enhancing PEC performance by modifying the intrinsic structure or adjusting the band structure of photoactive materials. Beyond improving light absorption and the separation/transfer of photogenerated charge carriers through material engineering， the fundamental PEC process critically relies on redox reactions at the semiconductor/electrolyte interface. Therefore， it is essential to discuss the impact of designing half-reactions on the performance of PEC sensing. In these half-reactions， electron donors or acceptors in the solution scavenge photogenerated holes or electrons at the semiconductor surface， thereby accelerating charge carrier separation and generating a sustained and stable photocurrent response. Consequently， modulating interfacial half-reactions represents an effective pathway for enhancing the PEC response. Current primary signal amplification strategies in PEC sensors involve either regulating the generation/consumption of electron donors/acceptors at the interface or introducing steric hindrance on the semiconductor surface to impede the half-reactions， thereby altering the current signal.

2.2.1 In-situ generation of electron donors/acceptors

A stable photocurrent signal in PEC processes relies on the efficient separation and subsequent quenching of photogenerated electrons and holes. This strategy does not alter the photoactive material itself but instead utilizes target-triggered biochemical reactions to continuously generate or consume electron donors or acceptors participating in the PEC reaction in situ at the electrode surface. This dramatically alters the interfacial reaction kinetics， leading to significant amplification of the photocurrent signal. Traditional methods involve pre-adding sacrificial agents to the solution， but their constant concentration limits the extent of PEC signal amplification^[98]. Currently， the most widely used strategy for introducing sacrificial agents involves enzymatic reactions. Catalysts like enzymes are immobilized at the interface via biorecognition events； in the presence of the target analyte， they catalyze the in-situ generation or consumption of these key species， enabling precise and efficient regulation of the interfacial reaction^[99]. Additionally， physical encapsulation （e.g.， within liposomes） allows for the target analyte to trigger the release of electron donors or acceptors from nanocarriers， effectively concentrating the sacrificial agents at the interface^[100-101].

The enzymatic generation of electron donors/acceptors to construct “signal-on” sensors is one of the most common and effective signal amplification pathways. An oxidase is immobilized on the electrode surface via a biorecognition element （e.g.， antibody， aptamer）. Upon capture of the target molecule， the enzyme catalyzes the in-situ， continuous， and efficient generation of an electron donor/acceptor from a substrate at the interface. The resulting half-reaction consumes a large number of photogenerated holes or electrons. For instance， as shown in Fig.9a， Zhang et al.^[102] developed an immunoassay platform with dual signal amplification based on a OVs-Bi₂₊_xWO₆ photoactive material. A sandwich immunoassay on a 96-well plate was used to prepare a detection antibody labeled with a secondary antibody/Au nanoparticle/glucose oxidase （GOx） conjugate. Specific recognition between the secondary antibody （Ab₂） and phosphorylated histone H₂AX （γH₂AX） introduced GOx， which catalyzed the rapid formation of the electron acceptor H₂O₂ from glucose， thereby enhancing the photocurrent response. Fig.9b shows a dual-mode self-powered PEC and colorimetric platform based on enzymatic catalysis and redox cycling amplification， using a Bi₂S₃/Bi₂O₃/ITO photoanode and a NiO/ITO cathode^[103]. When the target analyte is present， it participates in an immunoreaction with an antibody labeled with adenosine triphosphate （ATP）. The ATP catalyzes the conversion of ascorbic acid 2-phosphate （AAP） to the signaling reporter ascorbic acid （AA）. AA then reacts with photogenerated holes， effectively separating the photogenerated e^--h⁺ pairs. Furthermore， the oxidized AA participates in a colorimetric reaction and is continuously regenerated during the tris（2-carboxyethyl） phosphine-mediated chemical redox cycling process， effectively turning on both the photoelectrical and colorimetric signals. This dual-mode detection shows promising application prospects for the reliable detection of biomarkers.

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图9 （a）用于γH₂AX检测的分裂型PEC免疫分析示意图^[102]；（b）夹心型免疫反应及集成了自供能PEC与比色免疫分析的化学氧化还原循环生物传感器构建示意图^[103]；（c）用于CIP检测的PEC适配体传感器构建及可能的电子转移机理示意图^[104]；（d）Ag₂S QDs与Bi₂S₃的合成及Ag₂S/Bi₂S₃/Au-ALP PEC传感电极的制备流程示意图^[105]；（e）（i）ITO/Ag₂S/Au-ALP、（ii）ITO/Bi₂S₃/Au-ALP和（iii）ITO/Ag₂S/Bi₂S₃/Au-ALP在（a）不含APP及（b）含0.05 mol/L AAP的0.01 mol/L Tris-HCl缓冲液（pH=8.2）中的光电流响应；（iv）ITO/Ag₂S/Bi₂S₃/Au-ALP在（a）含0.05 mol/L AAP的0.01 mol/L Tris-HCl缓冲液（pH=8.2）及（b）在含0.05 mol/L AAP和50 nmol/L NaF的0.01 mol/L Tris-HCl缓冲液（pH=8.2）中的光电流响应^[105]

Fig.9 （a） Schematic illustration of the split-type PEC immunoassay for γH₂AX detection^[102]. Copyright 2023， Elsevier. （b） Schematic illustration on the fabrication of the sandwich-type immunoreaction and the designed chemical redox cycling biosensor， which integrated self-powered PEC and colorimetric immunoassay^[103]. Copyright 2025， American Chemical Society. （c） Fabrication of PEC aptasensors for CIP detection， and the possible mechanism of electron transfer^[104]. Copyright 2025， Elsevier. （d） Synthesis of Ag₂S QDs and Bi₂S₃ and the fabrication of the Ag₂S/Bi₂S₃/Au-ALP PEC sensing electrode^[105]. Copyright 2024， American Chemical Society. （e） Photocurrent responses of （i） ITO/Ag₂S/Au-ALP，（ii） ITO/Bi₂S₃/Au-ALP， and （iii） ITO/Ag₂S/Bi₂S₃/Au-ALP in （a） 0.01 mol/L Tris-HCl （pH 8.2） and （b） 0.01 mol/L Tris-HCl （pH 8.2） containing 0.05 mol/L AAP；（iv） photocurrent responses of ITO/Ag₂S/Bi₂S₃/Au-ALP in （a） 0.01 mol/L Tris-HCl （pH 8.2） containing 0.05 mol/L AAP and （b） 0.01 mol/L Tris-HCl （pH 8.2） containing 0.05 mol/L AAP in the presence of 50 nmol/L NaF^[105]. Copyright 2024， American Chemical Society

Effective detection of target molecules can also be achieved by constructing “signal-off” sensors that inhibit the generation of electron donors/acceptors. This can be accomplished by introducing enzyme inhibitors or causing the release of the enzyme from the electrode surface， preventing the generation of the species necessary to sustain the photocurrent and thus disrupting the original PEC reaction equilibrium， leading to photocurrent quenching. As illustrated in Fig.9c， Liu et al.^[104] innovatively employed a competitive binding mechanism to inhibit the in-situ generation of an electron donor， developing an aptasensing platform for the detection of ciprofloxacin （CIP）. The target molecule CIP preferentially binds specifically to the aptamer， releasing an alkaline phosphatase （ALP）-labeled aptamer （ALP-aptamer）. This prevents ALP from effectively catalyzing the hydrolysis of AAP to generate AA， resulting in a significant decrease in the photocurrent response and enabling sensitive detection of CIP. As shown in Fig.9d， Zhao et al.^[105] developed a PEC sensing platform based on an Ag₂S/Bi₂S₃ composite for the detection of ALP and its inhibitor， fluoride ions （F^-）. The Ag₂S/Bi₂S₃ composite exhibited a good photocurrent response. ALP was introduced via Au-S bonds using an Au-ALP conjugate. ALP catalyzes the hydrolysis of AAP to produce the electron donor AA， amplifying the photocurrent. F^- inhibit the catalytic activity of ALP， leading to a significant reduction in the photocurrent （Fig.9e）.

2.2.2 Steric hindrance

The steric hindrance effect is a direct and efficient physical modulation mechanism. It involves the creation of a physical barrier on the surface of the photoactive material， either by the target analyte itself or by a target-triggered event， which impedes the mass transfer of electron acceptors/donors to the electrode surface. This regulates the interfacial redox reactions， leading to a significant change in the signal. Consequently， the steric hindrance effect is commonly employed to construct “signal-off” sensors. For large-sized target analytes， the recognition elements can be densely immobilized on the electrode surface. When the target molecule is captured， its physical presence itself acts as a direct steric hindrance^[106-108]. Bo et al.^[109] constructed a novel sandwich-type PEC bioimmunosensing technique for the sensitive detection of CEA， using a 2D Z-scheme ZnIn₂S₄/g-C₃N₄ heterojunction as the photosensitive material and BiVO₄ as a photocurrent quencher （Fig.10a）. The Z-scheme heterojunction with well-aligned bandgaps provided a strong initial photocurrent， which was further amplified by introducing an electron donor. In the presence of CEA， a sandwich immunocomplex formed， loading BiVO₄-Ab₂ onto the material surface. This created steric hindrance， and BiVO₄ also competed with the heterojunction material for the electron donor， resulting in photocurrent quenching and enabling CEA detection. Under optimized conditions， this sensor demonstrated a wide detection range （0.0001~100 ng/mL） for CEA with a detection limit as low as 0.03 pg/mL. For target molecules whose size is insufficient to generate significant hindrance directly， a signal amplification process can be triggered by the target to form an insulating barrier on the electrode surface， thereby quenching the photocurrent^[110-111]. DNA self-assembled nano-structures serve as a typical example. As illustrated in Fig.10b， Hong et al.^[112] developed a novel， ultra-sensitive PEC sensing platform using loop-mediated isothermal amplification （LAMP） technology integrated with PEC detection （LAMP-PEC）. CdIn₂S₄ provided a stable photocurrent. The NH₂-modified LAMP products， obtained from the amplification of porcine epidemic diarrhea virus （PEDV） genes， were immobilized on the electrode surface via a Schiff base reaction. The amplified DNA assembled on the electrode surface increased steric hindrance and hindered electron transfer from the electrode to the electron acceptor in the solution， thereby reducing the photocurrent. This approach enhanced the accuracy of the quantitative analysis based on steric hindrance.

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图10 （a）用于CEA检测的免疫传感器示意图^[109]；（b）基于LAMP的PEC传感平台检测PEDV的示意图^[112]

Fig.10 （a） Schematic of the immunosensor intended for CEA detection^[109]. Copyright 2024， Elsevier. （b） Schematic illustration of the PEC sensing platform based on LAMP for detecting PEDV^[112]. Copyright 2023， Elsevier

2.3 Modulating the output signal

In the pursuit of high-performance PEC sensors， effective signal amplification can be achieved by engineering photoactive materials and modulating interfacial redox reactions， as previously discussed. These two strategies enhance photoelectric conversion capability and optimize reaction kinetics， respectively. However， a pivotal alternative pathway to ultra-high sensitivity lies in the precise manipulation of the final output signal itself. This section focuses on this concept， highlighting three strategies that do not rely solely on optimizing the intrinsic properties of materials or interfacial reaction efficiency. Instead， they employ ingenious sensing designs to convert， amplify， or even transduce biorecognition events into an output signal form that is more easily detectable and possesses greater resistance to interference. The core principle behind these strategies is post-signal processing. They leverage the programmability and catalytic power of functional biomolecules （e.g.， DNA， enzymes） or exploit the inherent properties of semiconductor materials to reconfigure the initial， weak photocurrent signal. Whether it involves using DNA amplification techniques to transform a single recognition event into the accumulation of numerous signal tags， employing enzyme-catalyzed reactions to construct a physical barrier at the interface that drastically alters signal intensity， or building a p-n junction to cause a fundamental reversal of photocurrent direction， the ultimate goal is the same： to amplify or convert minute， difficult-to-distinguish signal changes into a pronounced， distinctive detection signal that cannot be overlooked.

By actively modulating the output signal， sensors can overcome the limitations of traditional intensity-modulation modes， enabling exceptionally high sensitivity and specificity in complex biological samples and demonstrating significant application potential. The following sections will elaborate on three representative strategies： DNA amplification， enzyme-catalyzed precipitation， and photocurrent polarity switching.

2.3.1 DNA amplification

DNA amplification techniques have gained rapid prominence in photoelectrochemistry primarily because DNA itself possesses an inherent capacity for signal amplification and regulation. The specific base sequences and conformational changes of DNA， such as the amplification of a short strand into a long chain or the replication of multiple functional segments， represent minute alterations that can be strategically utilized to introduce photosensitive units or catalytic sites onto the photoelectrode. These modifications are subsequently transduced into macroscopically measurable changes in photocurrent. Based on whether enzymes are involved， DNA amplification strategies within PEC systems can be broadly categorized into two pathways. One pathway is driven by entropy or free energy gradients， encompassing enzyme-free cascades such as the hybridization chain reaction （HCR） and catalytic hairpin assembly （CHA）. The other pathway operates under the direction of enzymes， including polymerases， nickases， or exonucleases， enabling isothermal exponential amplification. Techniques like rolling circle amplification （RCA）， DNA walkers， and CRISPR-coupled amplification fall into this category， capable of converting minimal differences in target concentration into significant， quantifiable differences in photocurrent magnitude within a short timeframe. The common objective of both pathways is to establish a clear and predictable amplification curve correlating the number of “photoelectric conversion elements” at the electrode interface with the concentration of the target biomarker. This section will primarily elucidate the specific operational mechanisms of different DNA amplification techniques.

2.3.1.1 HCR

The HCR is an enzyme-free， isothermal DNA self-assembly technique. It is initiated by a single-stranded DNA （ssDNA） initiator that triggers the alternate hybridization of two metastable hairpin DNAs （H1 and H2）， ultimately forming a long double-stranded DNA （dsDNA） polymer comprising hundreds of repeating units. This method offers advantages such as operational simplicity， low background signal， and high compatibility， making it widely applicable in biosensing analysis^[113-115]. For example， as illustrated in the Fig.11a， when the target analyte PSA binds to its aptamer， a trigger DNA （T-DNA） is released. This T-DNA initiates the HCR process， causing it to alternately hybridize with two biotin-labeled hairpin DNAs （B-H1 and B-H2）. Subsequently， through the biotin-streptavidin interaction， streptavidin-conjugated ALP is assembled onto the surface of the hybridization chain， triggering an enzyme-catalyzed precipitation reaction^[116]. An organic photoelectrochemical transistor （OPECT） biosensor designed based on this mechanism achieved highly sensitive detection with a limit of detection of 10 fg/mL （S/N = 3）. In conventional HCR performed in homogeneous solution， the trigger strand and hairpin DNAs hybridize via diffusion， which often leads to issues such as long amplification times and low hybridization efficiency. To address these limitations， localized HCR strategies incorporating spatial confinement effects have been developed^[117-120]. This novel strategy demonstrates benefits including a high signal-to-noise ratio and a rapid response.

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图11 （a）HCR增强的生物催化沉淀（BCP）门控OPECT适配体传感器示意图，以及基于PSA触发的HCR及随后的ALP驱动的BCP反应原理（结合磁分离辅助），所构建的OPECT适配体传感器及其基于BCP门控效应的工作原理示意图^[116]；（b）FEN1检测的PEC分析示意图^[124]；（c）H₂O₂在CuBi₂O₄纳米多面体上的自配位用于表面陷阱态修复及增强电荷分离的示意图，以及基于四面体DNA辅助链置换扩增反应的FEN1检测流程示意图^[129]；（d）传感器构建流程示意图^[136]；（e）基于CRISPR/Cas12a G4/hemin DNAzyme级联催化信号放大的PEC适配体传感器用于ATZ检测的示意图^[144]；（f）靶标激活的CRISPR/Cas13a反式切割三螺旋分子结构示意图，以及基于CRISPR/Cas13a编程的Cu NCs与Z型T-COF/Ag₂S异质结的circRNA检测PEC生物传感器构建示意图^[145]

Fig.11 （a） Schematic of the HCR-enhanced biocatalytic precipitation （BCP）-gated OPECT aptasensor， and principle of PSA-dependent HCR and subsequent ALP-enabled BCP reaction with the assistance of magnetic separation. Construction of the proposed OPECT aptasensor and its working principle based on the gating effect of BCP^[116]. Copyright 2023， American Chemical Society. （b） Diagram of PEC assay for FEN1 detection^[124]. Copyright 2024， Elsevier. （c） Schematic illustration of the self-coordination of H₂O₂ onto CuBi₂O₄ nanopolyhedra for surface trap state remediation and reinforced charge carrier separation， and the bioassay processes for probing FEN1 with the THD-assisted SDA reaction^[129]. Copyright 2023， The Royal Society of Chemistry. （d） Diagram of the process of building a sensor^[136]. Copyright 2025， Elsevier. （e） Schematic of PEC aptamer sensor based on signal amplification by cascade catalysis of CRISPR/Cas12a and G4/hemin DNAzyme for ATZ detection^[144]. Copyright 2025， Springer Nature. （f） Diagram of target-activated CRISPR/Cas13a trans-cleavage triple-helix molecular， and construction diagram of pec biosensor for circRNA detection CRISPR/Cas13a-programmed Cu NCs and Z-scheme T-COF/Ag₂S^[145]. Copyright 2025， American Chemical Society

2.3.1.2 RCA

RCA is an isothermal enzymatic reaction driven by DNA polymerase， which uses a circular DNA as a template to continuously generate ultra-long ssDNA consisting of thousands to tens of thousands of tandem repeats. A typical RCA amplification system comprises the following components： a circular DNA template， a short DNA primer complementary to the template， DNA polymerase， and deoxyribonucleoside triphosphates （dNTPs）^[121]. Within the framework of PEC sensing， the core value of RCA lies in its ability to directly convert “molecular counting” into “interfacial charge density”， thereby conferring an order-of-magnitude improvement in sensor sensitivity. The DNA nanowire generated from the circular template can exponentially enrich substrates or signal probes^[122-123]. For instance， DA can enhance photocurrent output by modulating surface defects on Bi₂O₂S nanosheets. As displayed in Fig.11b， Hu et al.^[124] employed an RCA reaction to regulate the release of a DA aptamer， thereby enhancing the detection performance of a PEC sensing platform for Flap endonuclease 1 （FEN1） leveraging this sensing mechanism. In this design， when FEN1 is present， it recognizes and cleaves the 5’ flap of a dumbbell probe. The exposed 5’ phosphate group ligates with the 3’ hydroxyl terminus to form a closed dumbbell probe （CDP）. A primer （H1） hybridizes with the CDP. This hybridization event opens the CDP and creates a primer-template structure suitable for elongation. In the presence of Phi29 DNA polymerase and dNTPs， the RCA reaction is initiated， producing a long DNA strand complementary to the DA aptamer sequence. This long RCA product then hybridizes with the DA aptamer. Subsequently， Exonuclease III （Exo III） digests the aptamer strand， releasing DA molecules. The released DA modulates the surface defects of the Bi₂O₂S nanosheets， leading to an enhanced anodic photocurrent output. However， circular templates require precise design and ligase-mediated cyclization， leading to complex preparation and high cost. Coupled with the time-consuming nature of enzyme-catalyzed isothermal amplification， this poses a significant challenge for rapid， simple， and low-cost practical applications^[125-126].

2.3.1.3 Strand displacement amplification （SDA）

SDA is an isothermal DNA amplification technique that employs two enzymes （a DNA polymerase and a nicking enzyme） working cooperatively. It establishes a cyclical reaction of “nicking， extension， and displacement”， enabling the exponential replication of a target DNA sequence. Integrating SDA with various sensing strategies facilitates highly sensitive detection in PEC sensors^[127-128]. Gu et al.^[129] discovered that the surface trap states of CuBi₂O₄ polyhedra can act as charge carrier recombination centers， suppressing the charge separation rate and leading to a reduced cathodic photocurrent output. They found that H₂O₂ could spontaneously interact with the surface-uncoordinated Cu（II） sites， remedying these defective trap states. Leveraging this innovative sensing strategy of surface self-coordination-mediated trap state repair for cathodic PEC signal transduction， they introduced SDA （Fig.11c）. When the target analyte FEN1 is present， it initiates a continuous SDA reaction involving a G-rich tetrahedral DNA nanostructure （THD3）. FEN1 specifically recognizes and cleaves its target site， releasing a ssDNA. This ssDNA then hybridizes with an auxiliary probe （THD2） to form a dsDNA structure. This dsDNA subsequently hybridizes with the G-rich THD3， triggering another round of strand displacement and releasing the initial ssDNA， thereby establishing the SDA cycle. Crucially， this SDA process successfully prevents the formation of a G-quadruplex （G4）/hemin DNAzyme. The absence of this DNAzyme inhibits the catalytic reduction of H₂O₂， which in turn allows H₂O₂ to remediate the surface trap states of CuBi₂O₄. This remediation results in a significant enhancement of the PEC signal. This strategy demonstrated a wide linear range from 1.0 fmol/L to 100.0 pmol/L and achieved a remarkably low detection limit of 0.3 fmol/L （S/N = 3）. However， SDA involving one or more probes suffers from relatively low specificity. Compared to other amplification strategies， SDA requires the cooperative action of both a DNA polymerase and a nicking enzyme， significantly increasing the detection cost. Furthermore， achieving high amplification efficiency is time-consuming， which adds to the complexity and energy consumption of portable or point-of-care testing （POCT）^[130-131].

2.3.1.4 DNA walker

The DNA walker represents an emerging strategy that introduces DNA molecular machines into PEC sensing for signal amplification. Its essence lies in constructing an editable molecular track on the electrode surface. Activated by the recognition of a target aptamer， the DNA walker undergoes continuous mechanical movement along this predefined track， driven by an external energy source （typically chemical energy）^[132-133]. DNA walkers are usually powered by enzymes （e.g.， endonucleases， polymerases） or strand displacement reactions^[134-135]. For instance， as illustrated in Fig.11d， Wang et al.^[136]， leveraging the excellent photoelectric conversion properties of an SnS₂/In₂O₃ heterostructure and utilizing Fe₂O₃ as a PEC signal quencher， designed an “ON-OFF” mode PEC biosensing platform for miRNA detection， which employed an enzyme-assisted DNA walker for signal amplification. The strong photoelectric conversion capability of the SnS₂/In₂O₃ heterostructure resulted in an initial photocurrent in the “ON” state. The DNA walker was then activated on the electrode surface in the presence of the target miRNA and Exo III， leading to an amplification process. This process generated the signal quencher DNA1-Fe₂O₃-DNA2， which caused a significant decrease in the photocurrent， switching it to the “OFF” state. This “ON-OFF” mode enabled the sensitive detection of miRNA-let-7a within a range of 100 amol/L to 100 nmol/L， demonstrating a reliable linear correlation and achieving an ultra-low detection limit of 30.11 amol/L （S/N = 3）. Furthermore， combining DNA walkers with the HCR enables enzyme-free amplification， offering advantages such as operational simplicity and high versatility. For example， Miao et al.^[137] constructed a sensitive PEC biosensing platform for the detection of miRNA-486-5p based on an MXene/BiVO₄/Bi₂S₃ composite material and a cascade signal amplification technique. The integration of a DNA walker with HCR not only realized enzyme-free amplification but also achieved a satisfactory photocurrent response. Recently， researchers have further enhanced sensitivity by coupling DNA walkers with 3D magnetic bead tracks， self-powered enzyme fuel cells， or CRISPR-Cas12a cascades， achieving detection limits ranging from femtomolar to attomolar levels for various biomarkers such as microRNA， the mecA gene， and MC-LR^[138-140]. However， the walking behavior of DNA walkers on electrode surfaces is influenced by surface chemistry， DNA strand density， and orientation， leading to unpredictable and poorly reproducible amplification factors. Consequently， the actual detection sensitivity may fall far short of theoretical values predicted by ideal models.

2.3.1.5 CRISPR/Cas systems

CRISPR/Cas systems are widely utilized in sensing applications due to their exceptional specificity and high cleavage efficiency^[141-143]. The system structurally consists of two main components： clustered regularly interspaced short palindromic repeats （CRISPR） and CRISPR-associated （Cas） proteins， such as Cas12a， Cas13a， and Cas14a. Among these， Cas12a， upon RNA-guided recognition of target dsDNA/ssDNA， activates its non-specific single-stranded DNase （ssDNase） activity. This trans-cleavage characteristic enables it to indiscriminately cleave non-target ssDNA. For instance， as illustrated in Fig.11e， Chen et al.^[144] developed a sensitive PEC sensing platform for atrazine （ATZ） detection by amplifying the signal using CRISPR/Cas12a. This was based on a sensing strategy where a G4/hemin DNAzyme catalyzes the oxidation reaction between H₂O₂ and DA， leading to the formation of polydopamine （PDA） deposits on the electrode surface. This process consumes the electron donor DA， resulting in a decreased photocurrent. ATZ can hybridize with the ATZ aptamer （Apt） in an Apt/cDNA duplex， releasing an activator strand （cDNA）. This released cDNA activates the CRISPR/Cas12a system and triggers the cleavage of the G4 structure， causing the degradation of the G4/hemin DNAzyme immobilized on the electrode surface. This reduction in the amount of G4/hemin DNAzyme on the electrode decreases PDA production and DA consumption， leading to the recovery of the photocurrent signal. The photocurrent change showed a linear relationship with the logarithm of the ATZ concentration over a range from 1.00 × 10^-12 to 1.00 × 10^-5 mol/L， achieving a detection limit of 3.47 × 10^-13 mol/L. In contrast， Cas13a functions as an RNA-guided ribonuclease. In the presence of target ssRNA， it exhibits non-specific trans-cleavage activity， degrading non-target RNA sequences present in the system. For example， Fig.11f illustrates a PEC biosensor for highly sensitive circRNA detection was designed based on a CRISPR/Cas13a-programmed system utilizing copper NCs （Cu NCs） and a Z-scheme T-COF/Ag₂S heterostructure^[145]. The T-COF/Ag₂S photoactive material with the Z-scheme heterostructure generated a strong initial photocurrent. When CRISPR/Cas13a precisely targeted the circRNA， it nonspecifically cleaved a triplex molecular beacon， releasing DNA fragments. These released fragments then initiated an HCR with a DNA hairpin probe （HP）， forming AT-rich dsDNA. Subsequently incubated Cu²⁺ formed Cu NCs in situ on this dsDNA via A-Cu-T bonds. The formed Cu NCs effectively quenched the photocurrent， likely through an ET mechanism. This developed PEC biosensor for circRNA determination demonstrated a low detection limit of 0.5 fmol/L. Furthermore， the reusability of the DNA-modified magnetic beads （MB-DNA） helped reduce the detection cost. However， in practical PEC sensing applications， Cas proteins face challenges due to their large molecular size， high costs associated with synthesis， purification， and storage， as well as poor stability.

2.3.2 Enzyme-catalyzed precipitation

Enzyme-catalyzed precipitation is a signal amplification method that modulates the current signal by altering the electrode surface kinetics. It involves an enzymatic reaction that generates an insoluble product， forming a precipitate layer on the electrode surface. This layer impedes electron transfer or the output of the optical signal， leading to a reduction in photocurrent intensity. Commonly used enzymes include horseradish peroxidase （HRP） and ALP^[146-148]. In PEC sensors， these enzymes are typically conjugated with biorecognition elements （e.g.， antibodies）. The specific recognition of the target analyte triggers the enzymatic reaction. For example， as illustrated in Fig.12a， Jiang et al.^[149] reported a PEC immunosensing platform for the photocurrent measurement of thyroglobulin （TG）. The photoanode was prepared by immobilizing a capture anti-TG antibody on an AuNPs-decorated BiVO₄ surface. An HRP-conjugated detection antibody was used， where the target TG was sandwiched between the capture and detection antibodies. The conjugated HRP catalyzed the oxidation of 4-chloro-1-naphthol （4-CN） in the presence of H₂O₂ to generate an insoluble precipitate， resulting in a decreased photocurrent. This detection method， where the immunoassay and photocurrent measurement occur on the same photoanode， offers the advantage of a rapid response. However， byproducts from the enzymatic reaction might directly interfere with the PEC process， potentially affecting detection accuracy. An alternative， split-type PEC detection method has been developed， where the enzymatic reaction and the PEC reaction are physically separated （Fig.12b）^[150]. The products generated by the enzymatic reaction are transferred to the surface of the photoelectrode via diffusion or other means， subsequently influencing its PEC performance. This split-type PEC detection model maintains ultra-high sensitivity while exhibiting excellent anti-interference capabilities （Fig.12c）.

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图12 （a）基于纳米金功能化BiVO₄（AuNPsBiVO₄）光阳极结合酶促生物催化沉淀（EBCP）的TG PEC免疫分析示意图^[149]；（b）基于Cu₂O修饰的Cu₂O/FTO电极结合EBCP反应与数字万用表读出的分裂型PEC免疫分析用于检测心肌肌钙蛋白I（cTnI）的示意图（4-CD：苯并-4-氯己二烯酮；SDS：十二烷基硫酸钠）^[150]；（c）（i）基于Cu₂O/FTO PEC免疫分析的光电流峰值强度随cTnI浓度（0.01~10 ng/mL）变化的校准曲线；（ii）基于Cu₂O/FTO PEC免疫分析的特异性及（iii）储存稳定性（特异性和稳定性测试中使用10 ng/mL的cTnI）^[150]；（d）用于大肠杆菌O157：H7检测的近红外响应型光电流极性可切换PEC免疫分析示意图^[163]；（e）用于ALP测定的光电流极性切换型PEC分析示意图^[164]

Fig.12 （a） Schematic illustration of PEC immunoassay for TG on nanogold-functionalized BiVO₄ （AuNPsBiVO₄） photoanode coupling with enzymatic biocatalytic precipitation （EBCP）^[149]. Copyright 2023， Elsevier. （b） Schematic illustration of a split-type PEC immunoassay toward the detection of cardiac troponin I （cTnI） on a Cu₂O modified Cu₂O/FTO electrode by coupling with an EBCP reaction with a digital multimeter readout （4-CD： benzo-4-chlorohexadienone； SDS： sodium dodecyl sulfate）^[150]. Copyright 2023， Royal Society of Chemistry. （c）（i） Calibration plots corresponding to the photocurrent peak intensity of the Cu₂O/FTO-based PEC immunoassay as a function of cTnI concentration （0.01~10 ng/mL）；（ii） the specificity and （iii） the storage stability of the Cu₂O/FTO-based PEC immunoassay （10 ng/mL of cTnI for specificity and stability）^[150]. Copyright 2023， Royal Society of Chemistry. （d） Schematic illustration of an NIR-responsive photocurrent-polarity-switchable PEC immunoassay for E. coli O157：H7^[163]. Copyright 2023， American Chemical Society. （e） Schematic diagram of photocurrent-polarity switching PEC for ALP determination^[164]. Copyright 2023， American Chemical Society

However， natural enzymes suffer from inherent limitations such as poor stability， difficult storage， and complex preparation^[151]. To overcome these drawbacks， nanozymes， nanomaterials with enzyme-like activities， have been developed as substitutes. Nanozymes offer advantages including high stability， tunable activity， and good environmental tolerance， and are widely integrated with various analytical techniques in detection applications^[152-154].

In recent years， researchers have developed numerous photoactive materials endowed with enzymatic catalytic activity， including noble metals^[155-156]， metal oxides^[157-158]， and organic frameworks^[159]. By integrating these enzyme-mimicking photoactive materials with the original substrate to form composites， the photoelectric conversion capability is enhanced， leading to a more pronounced change in the photocurrent signal and achieving further signal amplification. For instance， a PEC immunosensor based on the coupling of CdS semiconductors with PdPt nanozymes was developed for the highly sensitive detection of CEA^[160]. Compared to pristine CdS， the LSPR effect of the CdS/PdPt composite significantly enhanced the PEC performance of the system. However， the spatial distribution and morphology of enzymatic insoluble precipitates on electrodes are difficult to control， impairing reproducibility. Moreover， their strong adsorption increases the cost per test by preventing electrode reuse.

2.3.3 Photocurrent polarity switching

Photocurrent polarity switching represents a signal amplification and anti-interference strategy. Traditional photocurrent signal output modes include “signal-on” and “signal-off” types， which are susceptible to interference from redox species， leading to false-positive or false-negative results. In contrast， photocurrent polarity reversal， which involves a change in photocurrent direction upon target recognition， effectively improves anti-interference capability. The key to this strategy lies in the design of the switching mechanism. In traditional PEC sensors， modulating the applied potential can alter the flow direction of photogenerated charge carriers， thereby changing the photocurrent polarity^[161-162]. With the advancement of signal amplification strategies， more controllable and intelligent approaches based on material design， interface control， and target-induced reactions have been developed.

Introducing inorganic semiconductors or organic small molecules onto photoactive materials is an effective strategy for achieving photocurrent polarity reversal. For instance， as shown in Fig.12d， Chen et al.^[163] constructed a PEC immunoassay platform based on NIR-responsive AgBiS₂ nanocrystals and Cu₂O nanocube composites for the detection of E. coli O157：H7. n-type AgBiS₂， immobilized with a monoclonal antibody （MAb1） on a paper-based working electrode （PWE）， bound to the E. coli O157：H7 target via an immunocomplex reaction. Simultaneously， an immunoprobe （Cu₂O/PAb2） was introduced onto the PWE surface， forming a p/n-type composite responsive to infrared light. The formation of this composite altered the charge transfer pathway of photogenerated carriers， inducing photocurrent polarity reversal. Furthermore， the introduction of more probes significantly amplified the cathodic photocurrent. Due to the spatial separation of charge carriers and the polarity switch， false positive/negative photocurrent signals were eliminated. The platform demonstrated a wide detection range （25 to 5 × 10⁷ CFU/mL） and a remarkably low detection limit （8 CFU/mL）. However， challenges regarding stable contact between materials may exist in this approach. Alternatively， photocurrent switching can be achieved by modulating the band structure of the photoactive material or the interface properties. For example， Fan et al.^[164] developed a PEC sensing platform with excellent performance by modulating photocurrent polarity switching in a BiOBr/CuMOF heterojunction through the introduction of OVs and enzyme-catalyzed generation of an electron donor （Fig.12e）. The BiOBr/CuMOF heterojunction initially exhibited a large cathodic photocurrent due to its Z-scheme structure. In the presence of the target analyte ALP， it catalyzed the decomposition of AAP to produce the electron donor AA. Under the combined action of light and the reductant AA， the Bi³⁺-O bonds in BiOBr were disrupted， generating OVs. This led to a change in the band structure of BiOBr. Concurrently， the altered redox conditions modified the transport path of photogenerated charge carriers， achieving photocurrent switching. The constructed low-background interference PEC sensing platform achieved a detection limit as low as 0.0017 U/L. While changing the interfacial redox conditions by introducing/consuming electron donors/acceptors improves the sensitivity of traditional polarity switching strategies， the instability of these species in solution might generate additional signals. Jiang et al.^[165] found that L-cysteine （L-cys） not only acted as a polarity switcher by altering the energy level positions through covalent bonding with Cu and Bi， thereby reversing the photocurrent， but also provided a relatively stable electron donor to efficiently consume photogenerated holes， ensuring high accuracy and sensitivity. Simultaneously， using acriflavine as a signal amplifier significantly accelerated the transfer of photogenerated electrons and amplified the photocurrent signal. This approach overcame the stringent requirements of signal suppression before reversal and amplification after reversal in traditional polarity reversal systems， further improving detection sensitivity. Furthermore， polarity switching can be realized by forming polymeric switch factors on the photoactive material surface or based on anion exchange reactions， among other methods^[166-168]. In summary， the photocurrent polarity switching strategy， by providing two distinct signal polarities， significantly enhances the anti-interference ability and reliability of PEC sensors， holding great potential in PEC sensing applications.

3 Challenges and perspectives

This review systematically demonstrates that the core of PEC signal amplification lies in the collaborative regulation of multiple processes in photoelectric conversion， including light harvesting， charge separation， interfacial reactions， and signal output. Different signal amplification strategies have their own distinct emphases and offer complementary advantages： Enhancing light harvesting （e.g.， surface sensitization） can directly increase the initial photon utilization efficiency， thereby enabling rapid signal amplification. Optimizing charge separation and transport （e.g.， heterojunction construction） focuses on regulating internal charge carrier dynamics， laying the foundation for high stability and gain. Modulating interfacial reaction processes （e.g.， electron donors/acceptors or steric hindrance） directly influences interfacial charge transfer efficiency， which determines the baseline performance of signal transduction. Building on this， regulating signal output （e.g.， DNA amplification and catalytic precipitation） enables highly specific bio-recognition and secondary amplification， thereby significantly improving sensing sensitivity and selectivity.

Although these amplification mechanisms have significantly enhanced the performance of PEC sensors， their further development and practical application face several challenges. Firstly， most research remains at the stage of laboratory validation of strategy effectiveness， lacking systematic analysis of the synergistic or antagonistic interactions between different amplification mechanisms. Secondly， many strategies relying on sophisticated DNA nanotechnology or multi-step enzymatic cascades are susceptible to interference in complex biological matrices. Finally， exploration of in practical applications remains insufficient. These challenges hinder the transition from high-performance proof-of-concept to reliable practical application.

Future developments in PEC sensing will focus on：（i） developing in situ characterization techniques and multi-scale simulations to rationally design sensing interfaces；（ii） addressing practical application challenges by enhancing sensor stability and anti-interference capability in complex biological media， and promoting integration with microfluidics and flexible electronics to enable point-of-care and wearable detection platforms；（iii） exploring the integration of novel physical effects， such as ferroelectricity， piezoelectricity， and plasmonic hot electrons， with PEC processes to break through the performance limits of existing materials and mechanisms；（iv） accelerating the translation of PEC sensors toward real-world deployment， including cost-effective materials， scalable and reproducible fabrication， long-term operational stability， and compatibility with existing analytical workflows；（v） at the methodological level， integrating artificial intelligence with high-throughput computing to achieve the efficient and rational screening of photoactive materials， the prediction of their performance， and the design of optimized sensing interfaces， thereby accelerating the discovery and application of novel materials and mechanisms.

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1 Introduction

图1 （a）阳极光电流和（b）阴极光电流生成示意图

2 Signal amplification mechanisms in PEC sensors

2.1 Modulating light absorption and photogenerated charge carriers separation

2.1.1 Surface sensitization

2.1.1.1 QDs

2.1.1.2 Dyes

2.1.1.3 Noble metal NPs

2.1.2 Morphology engineering

2.1.3 Defect engineering

2.1.4 Heterojunctions

2.1.4.1 Type-II heterojunctions

2.1.4.2 Z-scheme heterojunctions

2.1.4.3 p-n heterojunctions

图7 （a）MoS2/TiO2 NTs p-n异质结可能的电子转移机理示意图；（b）MoS2/TiO2 NTs（1∶20）与TiO2 NTs的（i）EIS图谱及（ii）PL光谱[86]

2.1.5 Energy transfer effects

2.2 Modulating interfacial redox reactions

2.2.1 In-situ generation of electron donors/acceptors

2.2.2 Steric hindrance

图10 （a）用于CEA检测的免疫传感器示意图[109]；（b）基于LAMP的PEC传感平台检测PEDV的示意图[112]

2.3 Modulating the output signal

2.3.1 DNA amplification

2.3.1.1 HCR

2.3.1.2 RCA

2.3.1.3 Strand displacement amplification （SDA）

2.3.1.4 DNA walker

2.3.1.5 CRISPR/Cas systems

2.3.2 Enzyme-catalyzed precipitation

2.3.3 Photocurrent polarity switching

3 Challenges and perspectives

References

图7 （a）MoS₂/TiO₂ NTs p-n异质结可能的电子转移机理示意图；（b）MoS₂/TiO₂ NTs（1∶20）与TiO₂ NTs的（i）EIS图谱及（ii）PL光谱^[86]

图10 （a）用于CEA检测的免疫传感器示意图^[109]；（b）基于LAMP的PEC传感平台检测PEDV的示意图^[112]