Plasmonic TiO2@Au NPs//CdS QDs photocurrent-direction switching system for ultrasensitive and selective photoelectrochemical biosensing with cathodic background signal

Split aptamer-based sandwich-type ratiometric biosensor for dual-modal photoelectrochemical and electrochemical detection of 17β-estradiol

BACKGROUND

As one of the most potent environmental estrogens, 17β-estradiol (E2), which can be enriched into organisms through the food chain and cause harmful biological effects in humans, has been frequently detected in the water environment of the world. High performance liquid chromatography (HPLC) and gas chromatograohy-mass spectrometry (GC/MS) have been widely used for quantification of E2. Despite excellent accuracy, tedious pretreatment and expensive instruments result in their limited application. It is clear that there is an urgent need to establish simple, sensitive and accurate methods for the determination of E2.

RESULTS

A split aptamer-based sandwich-type ratiometric biosensor based on split aptamer was developed by coupling photoelectrochemical and electrochemical assays for E2 detection. For analysis, the two fragments of split aptamer recognized E2 by forming sandwich structure, which triggered hybridization chain reaction (HCR) to produce double-stranded DNA (dsDNA) with CdTe quantum dots (QDs) labeled hairpin DNA. The resultant dsDNA can further absorb methylene blue (MB) to sensitize CdTe QDs for an enlarged photocurrent (IPEC) and output a redox current of IMB, and both of them acted as response signals for detection; [Fe(CN)6]3-/4- probe produced redox current of I[Fe(CN)6]3-/4- as reference signal. Using IMB/I[Fe(CN)6]3-/4- and IPEC/I[Fe(CN)6]3-/4- as yardsticks, the developed split aptamer-based sandwich-type ratiometric biosensor provides two linear ranges of 0.1-5000 pg mL-1 for IMB/I[Fe(CN)6]3-/4- and 0.1-10000 pg mL-1 for IPEC/I[Fe(CN)6]3-/4- with detection limits of 0.06 pg mL-1 and 0.02 pg mL-1, respectively.

SIGNIFICANCE

These results of the biosensor are benefiting from the coupling of photoelectrochemical (PEC) and electrochemical (EC) assays as well as the unique cooperative recognition mechanism of split aptamer. This method not only enabled the biosensor to be successfully applied to the determination of E2 in lake water, but also broadens the prospects for the realization of sensitive and accurate detection of E2.

A Direct-Contact Photocurrent-Direction-Switching Biosensing Platform Based on In Situ Formation of CN QDs/TiO2 Nanodiscs and Double-Supported 3D DNA Walking Amplification

Herein, a direct-contact photocurrent-direction-switching photoelectrochemical (PEC) biosensing platform for the ultrasensitive and selective detection of soluble CD146 (sCD146) is reported for the first time via in situ formation of carbon nitride quantum dots (CN QDs)/titanium dioxide (TiO2 ) nanodiscs with the double-supported 3D DNA walking amplification. In this platform, metal organic frameworks (MOFs)-derived porous TiO2 nanodiscs exhibit excellent anodic photocurrent, whereas a single-stranded auxiliary DNA (ssDNA) as biogate is absorbed onto the TiO2 nanodiscs to block active sites. Subsequently, with the help of intermediate DNAs from target sCD146-induced double-supported 3D DNA walking signal amplification, the ssDNA can leave away from the surface of TiO2 nanodiscs due to the specific hybridization with intermediate DNAs. Afterward, the successful direct contact of CN QDs on TiO2 nanodiscs by porosity and electrostatic adsorption, leads to the effective photocurrent-direction switching from anodic to cathodic photocurrent. Based on direct-contact photocurrent-direction-switching CN QDs/TiO2 nanodiscs system and double-supported 3D DNA walking signal amplification, sCD146 is detected sensitively with a wide linear range (10 fg mL-1 to 5 ng mL-1 ) and a low limit of detection (2.1 fg mL-1 ). Also, the environmentally friendly and direct-contact photocurrent-direction-switching PEC biosensor has an application prospect for cancer biomarker detection.

Distance-Regulated Photoelectrochemical Sensor "Signal-On" and "Signal-Off" Transitions for the Multiplexed Detection of Viruses Exposed in the Aquatic Environment

Photochemical (PEC) sensors were severely limited for multiplex detection applications due to the cross interference between multiplex signals at the single recognition interface. In this work, a distance-regulated PEC sensor was developed for multiplex detection by using an i-Motif sequence with conformational transformation activity as the signal transduction unit. Through dynamic regulation of the spatial distance between the end site of the functional sequence and the electrode material, the photogenerated electrons on the surface of the sensor were directionally transferred. Thus, a PEC sensor with "signal-on" and "signal-off" dual signal output modes was developed for simultaneous detection of multitarget molecules. Combining isothermal nucleic acid amplification, the PEC sensor constructed in this work was successfully applied to the detection of two virus (Norovirus and Rotavirus) nucleic acid sequences. Under the optimal condition, this bioassay protocol exhibits a linear range of 0.01-100 nM for both viruses with detection limits of 0.72 and 0.53 pM, respectively. In this study, a stimulus-mediated distance regulation strategy successfully addressed the transduction of multiplex detection signals at the single recognition interface of the PEC sensor. It is expected that the technical barriers to multiplex detection of PEC sensors will be overcome and the application of PEC sensing technology will be expanded in the field of environmental analysis.

Ultrathin MXene nanosheet-based TiO2/CdS heterostructure as a photoelectrochemical sensor for detection of CEA in human serum samples

To develop highly accurate and ultrasensitive strategies is of great importance for the clinical measurement, in particular, the detection of cancer biomarkers. Herein, we synthesized an ultrasensitive TiO₂/MXene/CdS QDs (TiO₂/MX/CdS) heterostructure as a photoelectrochemical immunosensor, which favors energy levels matching and fast electron transfer from CdS to TiO₂ in the help of ultrathin MXene nanosheet. Dramatic photocurrent quenching can be observed upon incubation of the TiO₂/MX/CdS electrode by Cu²⁺ solution from 96-well microplate, which caused by the formation of CuS and subsequent Cu_xS (x = 1, 2), reducing the absorption of light and boosting the electron-hole recombination upon irradiation. As a result, the as-prepared biosensor demonstrates a linearly increased photocurrent quenching percentage (Q%) value with CEA concentration ranging from 1 fg/mL to 10 ng/mL, as well as a low detection limit of 0.24 fg/mL. Benefit from its excellent stability, high selectivity and good reproducibility of as-prepared PEC immunosensor, we believe that this proposed strategy might provide new opportunities for clinical diagnosis of CEA and other tumor markers.

2D carbon materials based photoelectrochemical biosensors for detection of cancer antigens

Cancer is a leading cause of death globally and early diagnosis is of paramount importance for identifying appropriate treatment pathways to improve cancer patient survival. However, conventional methods for cancer detection such as biopsy, CT scan, magnetic resonance imaging, endoscopy, X-ray and ultrasound are limited and not efficient for early cancer detection. Advancements in molecular technology have enabled the identification of various cancer biomarkers for diagnosis and prognosis of the deadly disease. The detection of these biomarkers can be done by biosensors. Biosensors are less time consuming compared to conventional methods and has the potential to detect cancer at an earlier stage. Compared to conventional biosensors, photoelectrochemical (PEC) biosensors have improved selectivity and sensitivity and is a suitable tool for detecting cancer agents. Recently, 2D carbon materials have gained interest as a PEC sensing platform due to their high surface area and ease of surface modifications for improved electrical transfer and attachment of biorecognition elements. This review will focus on the development of 2D carbon nanomaterials as electrode platform in PEC biosensors for the detection of cancer biomarkers. The working principles, biorecognition strategies and key parameters that influence the performance of the biosensors will be critically discussed. In addition, the potential application of PEC biosensor in clinical settings will also be explored, providing insights into the future perspective and challenges of exploiting PEC biosensors for cancer diagnosis.

Carbon dots dominated photoelectric surface in titanium dioxide nanotube/nitrogen-doped carbon dot/gold nanocomposites for improved photoelectrochemical water splitting

The dynamic behavior of electron-hole pairs at the interface of the nanocomposites is important for photoelectrochemical catalysis, but it is difficult to characterize. Here we construct a ternary titanium dioxide/nitrogen-doped carbon dot/gold (TiO₂/NCD/Au) complex as the model catalyst to investigate the kinetic indexes at their interfaces. Under irradiation (200 mW cm^-2), the photocurrent density of TiO₂/NCD/Au is 10.26 mA cm^-2, which is higher than those of TiO₂/Au (4.34 mA cm^-2), TiO₂/NCD (7.55 mA cm^-2) and TiO₂ (3.34 mA cm^-2). The evolved oxygen of TiO₂/NCD/Au reaches 125.8 μmol after 5000 s test. The energy bands of complexes are very similar to that of the unmodified TiO₂ catalyst due to the low content modification of NCDs and Au. In addition, the transient photovoltage (TPV) tests with a series of control samples show differences about the carriers' separation and transfer process, which verify that Au can increase the separation quantity of electron-hole pairs while NCDs play a more important role on the increase of the separation quantity and separation rate simultaneously. This work quantifies the function of each component in a composite catalyst and deepens the understanding of the catalyst interface design.

Electrochemistry/Photoelectrochemistry-Based Immunosensing and Aptasensing of Carcinoembryonic Antigen

Recently, electrochemistry- and photoelectrochemistry-based biosensors have been regarded as powerful tools for trace monitoring of carcinoembryonic antigen (CEA) due to the fact of their intrinsic advantages (e.g., high sensitivity, excellent selectivity, small background, and low cost), which play an important role in early cancer screening and diagnosis and benefit people's increasing demands for medical and health services. Thus, this mini-review will introduce the current trends in electrochemical and photoelectrochemical biosensors for CEA assay and classify them into two main categories according to the interactions between target and biorecognition elements: immunosensors and aptasensors. Some recent illustrative examples are summarized for interested readers, accompanied by simple descriptions of the related signaling strategies, advanced materials, and detection modes. Finally, the development prospects and challenges of future electrochemical and photoelectrochemical biosensors are considered.

A sandwich-type photoelectrochemical aptasensor using Au/BiVO4 and CdS quantum dots for carcinoembryonic antigen assay

A novel sandwich-type photoelectrochemical (PEC) aptasensor for the carcinoembryonic antigen (CEA) assay was fabricated using the CEA aptamer, Au/BiVO₄ and CdS quantum dots (CdS QDs). In virtue of the localized surface plasmon resonance effect of Au nanoparticles, Au/BiVO₄ showed an effective utilization of visible light and excellent photoactivity, and was employed as the photoanode. After CdS QDs were conjugated to Au/BiVO₄ through the sandwich structure based on the hybridization of the CEA aptamer with two partially complementary single-stranded DNA molecules, the photocurrents were further enhanced by a resonance energy transfer between CdS QDs and Au nanoparticles. Meanwhile, the consumption of the photo-induced holes by ascorbic acid could also retard the combination of the electron-hole pairs and cause an increase of the photocurrents. However, the specific recognition of CEA by the CEA aptamer could destroy the sandwich structure and remarkably weaken the photocurrent response. Thus, the quantitative detection of CEA was connected with the decrease of the photocurrent. Benefitting from the above methods for signal enhancement, the PEC aptasensor showed a wide sensing range of 0.0001-10 ng mL^-1 and a low detection limit of 0.047 pg mL^-1 for CEA detection. The specificity, stability and recoveries of the PEC aptasensor were also excellent. Therefore, the construction of the present PEC aptasensor provides a universal and practical method for sensing other substances.

Recent Progress in DNA Hybridization Chain Reaction Strategies for Amplified Biosensing

With the continuous development of DNA nanotechnology, various spatial DNA structures and assembly techniques emerge. Hybridization chain reaction (HCR) is a typical example with exciting features and bright prospects in biosensing, which has been intensively investigated in the past decade. In this Spotlight on Applications, we summarize the assembly principles of conventional HCR and some novel forms of linear/nonlinear HCR. With advantages like great assembly kinetics, facile operation, and an enzyme-free and isothermal reaction, these strategies can be integrated with most mainstream reporters (e.g., fluorescence, electrochemistry, and colorimetry) for the ultrasensitive detection of abundant targets. Particularly, we select several representative studies to better illustrate the novel ideas and performances of HCR strategies. Theoretical and practical utilities are confirmed for a range of biosensing applications. In the end, a deep discussion is provided about the challenges and future tasks of this field.

A photoelectrochemical sensor based on Z-Scheme TiO2@Au@CdS and molecularly imprinted polymer for uric acid detection

A novel photoelectrochemical (PEC) sensor based on "Z-scheme" TiO₂@Au@CdS and molecularly imprinted polymer (MIP) was developed for the non-invasive detection of uric acid (UA). The "Z-scheme" material, consisting of an electron-transfer system (Au) and two isolated photochemical systems (CdS, TiO₂), was synthesized by chemical deposition method and it worked as a substrate for electro-polymerization of MIP. Due to the high photoelectric conversion efficiency provided by TiO₂@Au@CdS and specific imprinting effect afforded by MIP, the sensor displayed desirable sensing performance with the merits of sensitivity, selectivity, repeatability, and stability. The linear range for UA detection is from 1 nM to 9 μM with the detection limit of 0.3 nM (S/N = 3). Moreover, the assay was successfully utilized to measure UA in human tears and offered a reliable result. The incorporation of MIP and "Z-scheme" material into a PEC sensor system is expected to provide a promising strategy for detecting other small molecules.