Three-Dimensional CdS@Carbon Fiber Networks: Innovative Synthesis and Application as a General Platform for Photoelectrochemical Bioanalysis

A flexible electrochemical sensor for bisphenol A detection based on photoinitiated molecular imprinting on CdS functionalized carbon felt

Long-term and excessive exposure to bisphenol A (BPA) has an extremely detrimental effect on human health and ecological system. Hence, there is an urgent need to develop a sensitive and selective sensor for precisely monitoring BPA levels. In this work, a flexible and tailor-made electrochemical sensor for BPA has been fabricated based on in situ photopolymerization of molecular imprinting on cadmium sulfide (CdS) modified carbon felt (MIP@CdS-CF). It is worth nothing that CdS acts as a photocatalyst to enhance the capacity of photopolymerization, accordingly upgrading imprinting efficiency. Meanwhile, carbon felt (CF) exhibits attractive merits in term of superior electrical conductivity, enlarged electrochemical active areas and unique flexibility. Consequently, the novel MIP@CdS-CF electrochemical sensor shows superior sensitivity, high selectivity, extraordinary reproducibility and stability to detect BPA. The detection limit is 1.5 nM, which is lower than those of previously reported electrochemical sensors for the detection of BPA. More importantly, this newly developed electrochemical sensor can be utilized for detecting BPA in plastic bottles with satisfactory results.

A dual-model immobilization-free photoelectrochemical/visual colorimetric bioanalysis based on microemulsion self-assemblies mediated multifunctional signal amplification strategy

Herein, a novel silver ion-loaded gold microemulsion assemblies (Au/Ag+ MAs) mediated multifunctional signal amplification strategy was proposed to construct a sensitive immobilization-free photoelectrochemical (PEC)/colorimetric biosensor for carcinoembryonic antigen (CEA) detection. Through the sandwiched reaction among CEA, the CEA aptamer (DNA1) loaded on the Au nanoparticles (NPs) functionalized iron oxide (Fe3O4) nanospheres and another CEA aptamer (DNA2) immobilized on Au/Ag+ MAs, a complex is formed and acquired by magnetic separation. Then, Au/Ag+ MAs of the complex are disassembled into Au NPs and Ag+ ions driven by an acetone response, and the obtained demulsification solution is transferred to the cadmium sulfide/cadmium telluride (CdS/CdTe) photoactive composites modified electrode. Based on the multiple inhibition functions (blocking effect of oleylamine; energy transfer effect of Au NPs; and electron snatching effect of Ag+), the photocurrent of the electrode decreases obviously, resulting in the ultrasensitive detection of CEA (a detection limit of 16 fg mL-1). Interestingly, the ion-exchange reactions between CdS/CdTe composites and Ag+ ions generate silver sulfide/silver telluride (Ag2S/Ag2Te) composites, and a color change of composites can be distinguished directly, leading to a quick visual detection of CEA. Compared with the traditional single-modal assay for CEA, such dual-modal PEC/colorimetric assay is a more accurate and reliable due to different mechanisms and independent signal conversion. This work will offer a new perspective for the applications of various self-assemblies in PEC bioanalysis.

Liposome-Mediated In Situ Formation of Type-I Heterojunction for Amplified Photoelectrochemical Immunoassay

Exploiting innovative sensing mechanisms and their rational implementation for selective and sensitive detection has recently become one of the mainstream research directions of photoelectrochemical (PEC) bioanalysis. In contrast to existing conventional strategies, this study presents a new liposome-mediated method via in situ combining ZnInS nanosheets (ZIS NSs) with SnS₂ to form a ZIS NSs/SnS₂ type-I heterojunction on fluorine-doped tin oxide (FTO) electrodes for highly sensitive PEC immunoassays. Specifically, alkaline phosphatase (ALP)-encapsulated liposomes were confined within 96-well plates by sandwich immunorecognition and subsequently subjected to lysis treatment. Enzymatically produced H₂S by the released ALP was then directed to react with Sn(IV) to engender the ZIS NSs/SnS₂ type-I heterojunction on the FTO/ZIS NSs-Sn(IV) electrode, resulting in a change in the photogenerated electron-hole transfer path of the photoelectrode and reduction in current signaling. Exemplified by heart-type fatty acid binding protein (h-FABP) as a target, the constructed PEC sensor showed good stability and selectivity in a biosensing system. Under optimal conditions, the as-prepared sensing platform displayed high sensitivity for h-FABP with a dynamic linear response range of 0.1-1000 pg/mL and a lower detection limit of 55 fg/mL. This research presents the liposome-mediated PEC immunoassay based on in situ type-I heterojunction establishment, providing a new protocol for analyzing various targets of interest.

Recent Advances of Nanostructured Materials for Photoelectrochemical Bioanalysis

Nowadays, the emerging photoelectrochemical (PEC) bioanalysis has drawn intensive interest due to its numerous merits. As one of its core elements, functional nanostructured materials play a crucial role during the construction of PEC biosensors, which can not only be employed as transducers but also act as signal probes. Although both chemical composition and morphology control of nanostructured materials contribute to the excellent analytical performance of PEC bioassay, surveys addressing nanostructures with different dimensionality have rarely been reported. In this review, according to classification based on dimensionality, zero-dimensional, one-dimensional, two-dimensional, and three-dimensional nanostructures used in PEC bioanalysis are evaluated, with an emphasis on the effect of morphology on the detection performances. Furthermore, using the illustration of recent works, related novel PEC biosensing patterns with promising applications are also discussed. Finally, the current challenges and some future perspectives in this field are addressed based on our opinions.

Bi2S3@MoS2 Nanoflowers on Cellulose Fibers Combined with Octahedral CeO2 for Dual-Mode Microfluidic Paper-Based MiRNA-141 Sensors

An effective dual-mode microfluidic paper-based analysis device (μPAD) was proposed via Bi₂S₃@MoS₂ nanoflowers combined with octahedral CeO₂ for ultrasensitive miRNA-141 bioassay. To obtain the amplified electrochemical signal, Bi₂S₃@MoS₂ nanoflowers were first in situ grown onto the surface of cellulose fibers to promote the reduction of H₂O₂. The prism-anchored octahedral CeO₂ nanoparticles with a great catalytic function on the reduction of H₂O₂ were linked up to the functionalized cellulose fibers through the hybridization chain reaction to further enhance the electrochemical signal. By means of the catalysis effect of Bi₂S₃@MoS₂ nanoflowers and octahedral CeO₂ nanoparticles, the obtained signal was amplified, thereby achieving ultrasensitive electrochemical detection of the target. With the help of duplex specific nuclease, the octahedral CeO₂ could be released from the electrochemical detection area and flow to the color channel through capillary action, which could initiate the oxidation reaction of 3,3',5,5'-tetramethylbenzidine in the existence of H₂O₂ to generate a blue visual band, avoiding the error of distinguishing color depth caused by the naked eye and thus improving the accuracy of the visual method. Under the optimal conditions, satisfactory prediction and accurate detection performance were achieved in the range of 10 fM-1 nM and 0.5 fM-1 nM, respectively, by measuring the length of the blue product and the electrochemical signal intensity. The electrochemical/visual detection limits of the proposed μPAD for miRNA-141 were as low as 0.12 and 2.65 fM (S/N = 3). This work provides great potential for the construction of low-cost and high-performance dual-mode biosensors for the detection of biomarkers.

A signal-on photoelectrochemical aptasensor for chloramphenicol assay based on 3D self-supporting AgI/Ag/BiOI Z-scheme heterojunction arrays

Severe challenges are still remained for development of highly sensitive, selective and stable photoelectrochemical (PEC) sensing technology, albeit with its broad application for chloramphenicol (CAP) detection. Herein, a novel "signal-on" PEC aptasensor was fabricated based on a 3D self-supporting Z-scheme AgI/Ag/BiOI heterojunction arrays subtly integrated with in-situ formed biocatalytic precipitation (BCP) for highly sensitive and selective determination of CAP. Impressively, the HRP modified CAP aptamer (HRP-CAP aptamer) was released from the electrode by its strong affinity to the introduced CAP, and gradually terminated the BCP reaction, in turn recovering the photocurrent. By virtues of the 3D self-supporting AgI/Ag/BiOI Z-scheme heterojunction arrays and BCP signal amplification strategy, the resultant PEC sensor exhibited a wide linear range of 2-250 nM with a limit of detection (LOD) as low as 0.226 nM (S/N = 3). This work opens a new avenue for design of PEC aptasensing strategy and exhibits the marvelous potential in bioanalysis of environmental samples.

Photo-electrochemical detection of dopamine in human urine and calf serum based on MIL-101 (Cr)/carbon black

A new photo-electrochemical sensor based on MIL-101(Cr) MOF/carbon black (CB) is fabricated and characterized. By using differential pulse voltammetry, dopamine (DA) can be effectively detected using a photo-electrochemical MIL-101(Cr)/CB sensor under visible light. The CB acts as the electron bridge to combine with the large specific surface area and photo-catalytic feature of MOF, which contribute to the improvements of sensitivity of DA detection. The concentration of the catalyst, pH value, accumulation potential, and accumulation time were also optimized. Furthermore, the electrochemical performances of MIL-101(Cr)/CB sensor was investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scan rate, electrochemically active surface area (ECSA), and amperometric responses. A detection limit of 0.38 nM (LOD = 3 s_b/S, s_b = 0.028) and a working range of 1 nM to 2.22 μM has been achieved. The MIL-101(Cr)/CB sensor exhibits excellent reproducibility, stability, and selectivity and also has satisfactory recovery rate for the analysis of real samples including calf serum and human urine. Graphical abstract.

Ultrasensitive Photoelectrochemical Detection of MicroRNA on Paper by Combining a Cascade Nanozyme-Engineered Biocatalytic Precipitation Reaction and Target-Triggerable DNA Motor

Developing efficient strategies for sensitive detection of microRNAs, the noncoding bioactive molecules and well-established biomarkers, has aroused great interests due to its great potential values in genetic and pathological analyses. Herein, a highly selective and disposable paper-based photoelectrochemical (PEC) sensor was rationally designed for sensing microRNA based on simple self-assembly of a target-triggerable DNA motor and nanozyme-catalyzed multistage biocatalytic precipitation reaction. Specifically, a brand-new type II heterojunction of TiO₂-CeO₂ nanotubes decorated with carbon fiber paper (CFP) was first prepared, which gave an enhanced photoreactive surface and realized fast electron transport and extraction, markedly accelerating photoelectric conversion efficiency of the sensor. For achieving target detection, cascade nanozyme centers of the CeO₂ and Au nanoparticles modified by cyclodextrin were drafted, greatly decreasing the photocurrent intensity and achieving an ultralow background signal. With target introduction, the DNA motor was activated and automatically moved along the predesigned route driven by an endonuclease cleavage reaction, resulting in more substrate probe digestion and nanozyme release from CFP. Consequently, the repressive inner enhancement mechanism was gradually renewed with constant advancement of the enzymatic reaction and walker probe walking progressively, eventually allowing multiple enzymatic factor output in each target import. As a proof-of-concept application, the developed PEC sensor successfully performed detection of miRNA-141, showing a low detection limit of 0.6 fM, and was further applied to real sample bioassays with satisfying results. This work proposes promising strategies to boost the catalytic cascade DNA-motor adhibition in biological samples analysis and also exhibits potential capability in detection of other targets.

Ultrasensitive Paper-Based Photoelectrochemical Sensing Platform Enabled by the Polar Charge Carriers-Created Electric Field

Efficient separation of electron-hole pairs is vitally crucial to enhancing the analytical performance of paper-based photoelectrochemical (PEC) bioanalysis. Herein, a simple but effective strategy is developed to modulate the effective separation of photogenerated electrons and holes via introducing a polar charge carriers-created (PCC) electric field induced by a classical perovskite ferroelectric BaTiO₃ (BTO). By inserting it between the n-type WO₃ nanoflakes and p-type Cu₂O (WO₃ nanoflakes/BTO/Cu₂O), the photoelectrode is endowed with a renewable PCC electric field, as a sustaining driving force, to guarantee the realization of directional separation of charge carrier (DSCC) strategy in PEC bioanalysis. The enduring PCC electric field can attract the electrons of Cu₂O and holes of WO₃, respectively, thereby regulating the directional migration of charge carriers and achieving an enhanced PEC photocurrent for the ultrasensitive quantification based on the highly efficient separation of electron-hole pairs. Consequently, with respect to WO₃ nanoflakes/Cu₂O and WO₃ nanoflakes photoelectrode, the polarized WO₃ nanoflakes/BTO/Cu₂O photoelectrode exhibits 1.7 and 10.9 times higher photocurrent density, respectively. Benefiting from this, the prominent photocurrent density is obtained which is extremely beneficial for enhancing the sensitivity of PEC bioanalysis. Ultimately, the ultrasensitive detection of model prostate specific antigen (PSA) is realized and presents a linear range of 0.1 pg/mL-50 ng/mL with the detection limitation of 0.036 pg/mL. This work provides the basis for understanding the role of the polarized electric field induced by ferroelectric in tuning the charge separation as well as insights on strategies for constructing high-performance paper-based PEC bioanalysis.

Wavelength distinguishable signal quenching and enhancing toward photoactive material 3,4,9,10-perylenetetracarboxylic dianhydride for simultaneous assay of dual metal ions

Photoelectrochemical (PEC) assay with low background, simple instrumentation and high sensitivity has deemed as one of the most potential strategies to simultaneous multi-component detection. How to distinguish photocurrent changes caused by various targets on a single sensing platform thus becomes the key issue to be resolved. Herein, we innovatively proposed a multiplex PEC biosensor based on wavelength distinguishable signal quenching and enhancing toward photoactive material 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) for simultaneous assay of dual metal ions. Briefly, S1 and S2 ssDNA containing sensitizer methylene blue and quencher ferrocene (termed as MB-S1 and Fc-S2), respectively, were first generated through target Pb²⁺ and Mg²⁺-induced DNAzyme-assisted target recycling, which thereafter were modified on PTCDA sensing platform specifically via host-guest recognition with β-cyclodextrin (β-CD). Interestingly, the sensitizer MB could enhance photocurrent of PTCDA under the excitation wavelength of 623 nm and 590 nm, respectively, while the quencher Fc just quencher the photocurrent of PTCDA under the excitation wavelength of 590 nm, thereby achieving wavelength distinguishable signal quenching and enhancing toward photoactive material PTCDA for simultaneous assay of dual metal ions. As a result, the conceived biosensor for Mg²⁺ and Pb²⁺ detection realized high sensitivity with detection limit of 0.3 pM and 0.3 nM, respectively. The proposed strategy not only for the first time achieved the discrimination of varied PEC signal caused by two targets with usage of sole photoelectric material, but also realized the simultaneous multiplex assay on a single sensing platform, providing a new way for constructing effective and sensitive PEC biosensor for multi-component detection.

Three-Dimensional TiO2@Cu2O@Nickel Foam Electrodes: Design, Characterization, and Validation of O2-Independent Photocathodic Enzymatic Bioanalysis

This work reports the innovative design and application of a three-dimensional (3D) TiO₂@Cu₂O@nickel foam electrode synergized with enzyme catalysis toward the proof-of-concept study for oxygen-independent photocathodic enzymatic detection. Specifically, a 3D-nanostructured photoelectrode has great potential in the semiconductor-based photoelectrochemical (PEC) biological analysis. On the other hand, using various photocathodes, cathodic PEC bioanalysis, especially the photocathodic enzymatic detection, represents an attractive frontier in the field. Different from state-of-the-art photocathodic enzymatic studies that are oxygen-dependent, herein, we present the ingenious design, characterization, and implementation of 3D TiO₂@Cu₂O@nickel foam photocathodes for the first oxygen-independent example. In such a configuration, the Cu₂O acted as the visible-light absorber, while the TiO₂ shell would simultaneously function as a protective layer for Cu₂O and as a desirable substrate for the immobilization of enzyme biomolecules. Especially, because of the proper band positions, the as-designed photocathode exhibited unique O₂-independent PEC property. Exemplified by glucose oxidases, the as-developed sensor exhibited positive response to glucose with good performance. Because various oxidases could be integrated with the system, this protocol could serve as a universal O₂-independent platform for many other targets. This work is also anticipated to catalyze more studies in the advanced 3D photoelectrodes toward innovative enzymatic applications.