Portable self-powered electrochemical aptasensing platform for ratiometric detection of mycotoxins based on multichannel photofuel cell

Self-powered electrochemical sensors based on photofuel cells have attracted considerable research interest because their unique advantage of not requiring an external electric source, but their application in portable and multiplexed targets assay is limited by the inherent mechanism. In this work, a portable self-powered sensor constructed with multichannel photofuel cells was developed for the ratiometric detection of mycotoxins, namely ochratoxin A (OTA) and patulin (PAT). The spatially resolved CdS/Bi2S3-modified photoanodes and a shared Prussian Blue cathode were integrated on an etched indium-tin oxide slide to fabricate the multichannel photofuel cell. The aptamers of OTA and PAT were covalently bonded to individual photoanode regions to build sensitive interfaces, and the specific recognition of analytes impaired the output performance of constructed PFC. Accordingly, ratiometric sensing of OTA and PAT was achieved by utilizing the output performance of a control PFC as a reference signal. This approach effectively eliminates the impact of light intensity on the accuracy of the detection. Under the optimal conditions, the proposed sensing chip exhibited linear ranges of 2.0-1000 nM and 5.0-500 nM for OTA and PAT, respectively. The detection limits (3 S/N) were determined to be 0.25 nM for OTA and 0.27 nM for PAT. The developed ratiometric sensing method demonstrated good selectivity and stability in the simultaneous detection of OTA and PAT. It was successfully utilized for the analysis of OTA and PAT real samples. This work provides a new perspective for construction of portable and ratiometric self-powered sensing platform.

Nanozyme-assisted ratiometric photoelectrochemical aptasensor over Cu2O nanocubes mediated photocurrent-polarity-switching based on S-scheme FeCdS@FeIn2S4 heterostructure

The controllable modulation of the response mode is highly attractive for the construction of photoelectrochemical (PEC) sensors with improved sensitivity and anti-interference ability in complex real samples matrix. Here, we present a charming proof-of-concept ratiometric PEC aptasensor of enrofloxacin (ENR) analysis via the controllable signal transduction. Different with the traditional sensing mechanism, this ratiometric PEC aptasensor integrates the anodic PEC signal induced by PtCuCo nanozyme-catalyzed precipitation reaction and the polarity-switching cathodic PEC response mediated by Cu2O nanocubes on S-scheme FeCdS@FeIn2S4 heterostructure. Taking advantages of the photocurrent-polarity-switching signal response model and the superior performance of the photoactive substrate material, the proposed ratiometric PEC aptasensor displays a good detection linear range for ENR analysis from 0.01 pg mL-1 to 10 ng mL-1, with a detection limit of 3.3 fg mL-1. This study provides a general platform for detecting interested trace analytes in real samples and expands the diversity of sensing strategy design.

Nickel Single-Atom Catalyst-Mediated Efficient Redox Cycle Enables Self-Checking Photoelectrochemical Biosensing with Dual Photocurrent Readouts

Developing a self-checking photoelectrochemical biosensor with dual photocurrent signals could efficiently eliminate false-positive or false-negative signals. Herein, a novel biosensor with dual photocurrent responses was established for the detection of acetylcholinesterase activity. To achieve photocurrent polarity-switchable behavior, the iodide/tri-iodide redox couple was innovatively introduced to simultaneously consume the photoexcited electrons and holes, which circumvents the inconvenience caused by the addition of different hole- and electron-trapping agents in the electrolyte. Importantly, benefiting from the high catalytic activity, the enhanced photoelectric responsivity can be realized after decorating the counter electrode with nickel single-atom catalysts, which promotes a more efficient iodide/tri-iodide redox reaction under low applied voltages. It is envisioned that the proposed photocurrent polarity switching system offers new routes to sensitive and reliable biosensing.

Chitosan-based molecularly imprinted photoelectric sensor with ZnO/Bi2O3/Bi2S3 sensing layer for thiamethoxam determination

A molecularly imprinted photoelectrochemical sensor with high sensitivity and stable structure was constructed and applied to detect thiamethoxam pesticide. ZnO/Bi₂O₃/Bi₂S₃ heterojunction photoelectric material was formed on the fluorine-doped tin oxide (FTO) electrode by seed layer growth, drip coating, and in situ ion exchange. A chitosan-imprinted polymer membrane was prepared using chitosan as the functional monomer, glutaraldehyde as the cross-linking agent, and thiamethoxam as the template molecule. The photoelectric material was characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive x-ray spectroscopy analyses. The electron transfer mechanism of Z-type heterojunction was verified by ultraviolet-visible curve and Mott-Schottky curve. When thiamethoxam was re-adsorbed on the imprinted membrane, the current recorded at 0 V (vs. Ag/AgCl) was reduced because the thiamethoxam molecule blocked the electron transfer. The molecularly imprinted sensor exhibited a linear relationship to thiamethoxam concentration in the range from 7.0 × 10^-13 mol/L to 7.0 × 10^-10 mol/L and the detection limit was 3.32 × 10^-13 mol/L, which is lower than the values reported by other detection methods. Most pesticides, such as propoxur and isoprocarband carbaryl, do not interfere with the determination. The sensor also showed good practicability and suitability for the determination of trace thiamethoxam in environmental water and soil leaching solutions, with a recovery of 99.6-102.1% (RSD < 3.74%). A novel molecularly imprinted photoelectrochemical (MI-PEC) sensor with high sensitivity and selectivity for the determination of thiamethoxam (TMX) was developed. A Z-type heterojunction ZnO/Bi₂O₃/Bi₂S₃ photoelectric material was synthesized for the first time. The MI-PEC sensor was prepared with ZnO/Bi₂O₃/Bi₂S₃ as the sensitive material and MI membrane as the recognition element. The sensor exhibits an extremely sensitive response to thiamethoxam with a detection limit of 3.32 × 10^-13 mol/L due to the excellent photoelectrochemical properties of ZnO/Bi₂O₃/Bi₂S₃.

Addressable Label-Free Photoelectric Sensor Array with Self-Calibration for Detection of Neuron Specific Enolase

An addressable label-free photoelectric immunosensor array was designed for detection of neuron specific enolase (NSE) based on TiO₂/CdS as substrate materials. In this work, the hydrothermal synthesized TiO₂ nanorod film is evenly grown on the surface of the fluorine-doped tin oxide (FTO), and then CdS with a narrow band gap is added for sensitization through successive ionic layer adsorption reactions. The obtained TiO₂/CdS composite materials with matched energy band structures promote the rapid electron transfer and effectively reduce the recombination of electron hole pairs, which greatly enhance the visible light absorption and increased photocurrent intensity. In order to construct a suitable sensor array, the sensitized FTO electrode is divided into multiple regions of equal size by insulating stickers, and then the addressable and continuous detection of multiple samples can be achieved. Because multiple detection regions are prepared and tested under the same conditions, the difference effectively reduces, and the sensor can realize self-calibration and obtain more accurate results. Under optimal conditions, this sensor array can detect NSE in the linear range of 0.01-100 ng mL^-1 with a detection limit of 2.49 pg mL^-1 (S/N = 3). The sensor array has good selectivity, stability, and reproducibility, making it a viable approach for real sample detection.

A ratiometric photoelectrochemical microsensor based on a small-molecule organic semiconductor for reliable in vivo analysis

Photoelectrochemical (PEC) sensing has been developing quickly in recent years, while its in vivo application is still in the infancy. The complexity of biological environments poses a high challenge to the specificity and reliability of PEC sensing. We herein proposed the concept of small-molecule organic semiconductor (SMOS)-based ratiometric PEC sensing making use of the structural flexibility as well as readily tunable energy band of SMOS. Xanthene skeleton-based CyOH was prepared as a photoactive molecule, and its absorption band and corresponding PEC output can be modulated by an intramolecular charge transfer process. As such, the target mediated shift of absorption offered the opportunity to construct a ratiometric PEC sensor. A proof-of-concept probe CyOThiols was synthesized and assembled on a Ti wire electrode (TiWE) to prepare a highly selective microsensor for thiols. Under two monochromatic laser excitation (808 nm and 750 nm), CyOThiols/TiWE offered a ratiometric signal (j₈₀₈/j₇₅₀), which exhibited pronounced capacity to offset the disturbance of environmental factors, guaranteeing its reliability for application in vivo. The ratiometric PEC sensor achieved the observation of bio-thiol release induced by cytotoxic edema and fluctuations of thiols in drug-induced epilepsy in living rat brains.

The first small-molecule organic semiconductor-based ratiometric photoelectrochemical sensor was proposed, which exhibited pronounced selectivity and capacity to offset environmental disturbance, guaranteeing its reliability for in vivo analysis.

Microfluidic Ratiometric Photoelectrochemical Biosensor Using a Magnetic Field on a Photochromic Composite Platform: A Proof-of-Concept Study for Magnetic-Photoelectrochemical Bioanalysis

Integrating a microfluidic sensor with a ratiometric photoelectrochemical (PEC) strategy to build a bioanalysis device for actual sample testing is often limited to large-volume space-resolution equipment and wavelength-dependent or potential-dependent paired photoactive materials. This work reports a microfluidic ratiometric magnetic-photoelectrochemical (M-PEC) biosensor on the photochromic composite platform to solve the above problems. In particular, as a proof-of-concept study, the platform Bi₂WO_6-x/amorphous BiOCl nanosheets/Bi₂S₃ (p-BWO-s) mediated by photochromic color centers and the magnetic photoactive secondary antibody marker ZnFe₂O₄@Ag₂O are integrated on the microfluidic biosensor. By enhancement of the photochromic color centers, p-BWO-s outputs a considerable photocurrent signal. Meanwhile, the photoactivity of the secondary antibody marker can be changed with a magnetic field; thus, different photocurrent signals can be obtained to realize ratiometric detection. The quenching photocurrent signal without the magnetic field and the difference photocurrent signal under the magnetic field are quantitatively related to the target concentration, which unfolds a novel general strategy for bioanalysis. Different from traditional ratiometric PEC biosensors, this work characterizes the first ratiometric PEC biosensor based on an external magnetic field. Generally speaking, combined with different biorecognition cases, this scheme with good expansibility brings a unique new perspective.

A dual-signal self-checking photoelectrochemical immunosensor based on the sole composite of MIL-101(Cr) and CdSe quantum dots for the detection of α-fetoprotein

Designing a photoelectrochemical (PEC) immunosensor that can produce dual photocurrent signals which can refer to each other is a great importance but a big challenge. In this manuscript, a novel dual photocurrent signals immunosensor was constructed for the detection of α-fetoprotein (AFP). Unlike the usual method of using two composite materials to provide cathode and anode photocurrent respectively, this work applies only one compound of MIL-101 (Cr) and CdSe quantum dots (QDs). Thereinto, we found that the photocurrent polarity of MIL-101(Cr) would switch by adjusting applied voltage. And then CdSe QDs was introduced by simple ultrasound mixing to boost the dual photocurrent signals. Furthermore, in the composite of M&C, the electron transfer path between MIL-101(Cr) and CdSe QDs may switch between "Z-type" and "Ⅱ-type" by adjusting voltage. Benefiting by the dual signals, the proposed sensor can not only perform sensitively quantitative detection of α-fetoprotein (AFP), but also can intuitively estimate the accuracy and reliability of the test result by determining whether the corresponding relationship of "cathode photocurrent-analyte concentration-anode photocurrent" is established. The linear ranges of the sensing electrodes as cathode and anode are the same, both from 0.1 to 300 ng mL^-1. The limit of detection (LOD) is 0.082 ng mL^-1 (S/N = 3) when it used as an anode, and the LOD is 0.054 ng mL^-1 (S/N = 3) when it served as cathode. Furthermore, this sensor showed acceptable stability, reproducibility, specificity, and feasibility of detecting AFP in human serum, which has broad development prospects in the early clinical diagnosis.

A critical review on the use of potentiometric based biosensors for biomarkers detection

Potentiometric-based biosensors have the potential to advance the detection of several biological compounds and help in early diagnosis of various diseases. They belong to the portable analytical class of biosensors for monitoring biomarkers in the human body. They contain ion-sensitive membranes sensors can be used to determine potassium, sodium, and chloride ions activity while being used as a biomarker to gauge human health. The potentiometric based ion-sensitive membrane systems can be coupled with various techniques to create a sensitive tool for the fast and early detection of cancer biomarkers and other critical biological compounds. This paper discusses the application of potentiometric-based biosensors and classifies them into four major categories: photoelectrochemical potentiometric biomarkers, potentiometric biosensors amplified with molecular imprinted polymer systems, wearable potentiometric biomarkers and light-addressable potentiometric biosensors. This review demonstrated the development of several innovative biosensor-based techniques that could potentially provide reliable tools to test biomarkers. Some challenges however remain, but these can be removed by coupling techniques to maximize the testing sensitivity.

A novel SWCNT-amplified "signal-on" electrochemical aptasensor for the determination of trace level of bisphenol A in human serum and lake water

A novel "signal-on" electrochemical aptasensor was developed for ultrasensitive and specific detection of BPA, using single-walled carbon nanotubes (SWCNT) as the electro-catalytic probe for further signal amplification. The multi-walled carbon nanotubes (MWCNT), amino-functionalized magnetite, and gold nanoparticles (NH₂-Fe₃O₄/Au NPs) were applied first to modify the glassy carbon electrode (GCE) surface and to form a nanomaterial film with satisfactory conductive properties, stability, and biocompatibility. The BPA aptamer was then loaded onto the sensing platform by hybridization with complementary DNA (CDNA). In the presence of BPA it combines with the aptamer and the BPA-aptamer conjugate was released from the electrode;subsequently the added SWCNT and CDNA assembled quickly. Thus, the dual-amplification of the "signal-on" electrochemical aptasensor takes effect. The [Fe (CN)₆]^3-/4- redox probe signal (∆I) detected by DPV (differential pulse voltammetry) is proportional to the negative logarithm of BPA concentration between 10^-19 M and 10^-14 M. The detection limit is 0.08 aM. Importantly, the proposed biosensor represents a successful application for determination of BPA in human serum and lake water. Schematic representation of SWCNT-amplified "signal-on" electrochemical aptasensor for the detection of trace level of bisphenol A in human serum and lake water.

A self-calibrating electrochemical aptasensing platform: Correcting external interference errors for the reliable and stable detection of avian influenza viruses

Conventional electrochemical biosensing systems rely on a single output signal, which limits their certain practical application, specifically from the viewpoint of external interference factors causing electrochemical signal errors. This study reports a self-calibrating dual-electrode based electrochemical aptasensor for the reliable and independent detection of avian influenza viruses (AIVs), which are the primary cause of highly contagious respiratory diseases, under external interference factors. Both electrodes were fabricated using tungsten rods surface-modified with a 3D nanostructured porous silica film (3DNRE). Subsequently, methylene blue (MB) was loaded as a redox-active material into the pores and capped with corresponding aptamer. One electrode was capped with an anti-AIV nucleoprotein (NP) aptamer (Apt_AIV-MB@3DNRE) allowing target-specific binding, resulting in changes in electrochemical signal upon diffusional release of the loaded redox molecules. The other electrode was capped with a control aptamer (Apt_con-MB@3DNRE), serving as a reference to correct false responses generated by nonspecific aptamer detachment and MB release under environmental changes in pH and ion strength and presence of nontarget molecules from cell lysis debris. In the dual-electrode platform, Apt_con-MB@3DNRE provides a corrected baseline for the fluctuating original output signals from Apt_AIV-MB@3DNRE. Consequently, this dual-electrode platform exhibits excellent output-signal stability (relative standard deviation, RSD: 5.86%) compared to a conventional single-electrode platform (RSD: 30.13%) at equivalent concentrations of AIV NP samples under different reaction buffer conditions. Moreover, no further purification and washing steps were required, indicating that the strategy may represent a universal and reliable platform for the electrochemical aptamer-based detection of various biomolecules.

A potentiometric resolved photoelectrochemical system based on CdS nanowires and SnNb2O6 nanosheets: a case application for dual biomarker analysis

A potentiometric-resolved PEC immunosensor was developed for dual-target detection.

Detection signal amplification strategies at nanomaterial-based photoelectrochemical biosensors

This review focusses on unique material modification and signal amplification strategies reported in developing photoelectrochemical biosensors with utmost sensitivity and selectivity.

Photoelectric current as a highly sensitive readout for potentiometric sensors

The photocurrent at a working electrode coated with a ZnSe/r-GO composite can be modulated by a polymeric membrane ion-selective electrode that works as a reference electrode.

Donor/Acceptor-Induced Ratiometric Photoelectrochemical Paper Analytical Device with a Hollow Double-Hydrophilic-Walls Channel for microRNA Quantification

Integrating ratiometric photoelectrochemical (PEC) techniques with paper microfluidics to construct a ratiometric PEC paper analytical device for practical application is often restricted by the grave dependence of ratiometric assay on photoactive materials and low mass-transfer rates of the paper channel. Herein, a universal donor/acceptor-induced ratiometric PEC paper analytical device with a hollow double-hydrophilic-walls channel (HDHC) was fabricated for high-performance microRNA-141 (miRNA-141) quantification. Concretely, a photoanode and photocathode were integrated on the paper-based sensing platform in which the photocathode served as a biosensing site for the pursuit of higher selectivity. For formulation of a cascading signal amplification strategy, a unique duplex-specific nuclease-induced target recycling reaction was engineered for the output of a double amount of all useful DNA linkers instead of conventional output of only one available DNA product, which could guarantee the output of abundant DNA linkers with the initiation of a cascade of hybridization chain reaction on both the trunk and branch in the presence of miRNA-141. Then the formed dendriform polymeric DNA duplex structures were further decorated with glucose oxidase (GOx)-mimicking gold nanoparticles by the electrostatic interaction to form a branchy gold tree (BGT). Profiting from the perfect GOx-mimicking activity of BGT and high mass-transfer rates of HDHC, the cathodic photocurrent from Ag₂S/Cu₂O hybrid structure was in a "signal off" state while the anodic photocurrent from graphene quantum dots (GQDs) and Ag₂Se QDs cosensitized ZnO nanosheets was in a "signal on" state because BGT-catalyzed glucose oxidation reaction evoked the consumption of dissolved O₂ as an electron acceptor and the generation of H₂O₂ as an electron donor. With calculation of the ratio of two photocurrent intensities, the quantitative detection of miRNA-141 was achieved with high sensitivity, accuracy, and reliability.

Affinity-Based Detection of Biomolecules Using Photo-Electrochemical Readout

Detection and quantification of biologically-relevant analytes using handheld platforms are important for point-of-care diagnostics, real-time health monitoring, and treatment monitoring. Among the various signal transduction methods used in portable biosensors, photoelectrochemcial (PEC) readout has emerged as a promising approach due to its low limit-of-detection and high sensitivity. For this readout method to be applicable to analyzing native samples, performance requirements beyond sensitivity such as specificity, stability, and ease of operation are critical. These performance requirements are governed by the properties of the photoactive materials and signal transduction mechanisms that are used in PEC biosensing. In this review, we categorize PEC biosensors into five areas based on their signal transduction strategy: (a) introduction of photoactive species, (b) generation of electron/hole donors, (c) use of steric hinderance, (d) in situ induction of light, and (e) resonance energy transfer. We discuss the combination of strengths and weaknesses that these signal transduction systems and their material building blocks offer by reviewing the recent progress in this area. Developing the appropriate PEC biosensor starts with defining the application case followed by choosing the materials and signal transduction strategies that meet the application-based specifications.

Novel 3D Printed Device for Dual-Signaling Ratiometric Photoelectrochemical Readout of Biomarker Using λ-Exonuclease-Assisted Recycling Amplification

A ratiometric photoelectrochemical (PEC) sensing strategy was proposed for monitoring of carcinoembryonic antigen (CEA) based on a homemade 3D printing device with dual-working photoelectrodes (PE₁ and PE₂), coupling λ-exonuclease (λ-Exo)-assisted recycling amplification with CdS quantum dots. Gold nanoparticles-functionalized ZnO nanorods were utilized as PEC substrate for generating initial photocurrent and immobilizing DNA probe. Upon incubation of target with DNA trigger/CEA aptamer-modified magnetic bead (tri/apt-MB), DNA trigger dissociated from magnetic bead and then hybridized with capture probe (cp) on PE₁ or opened hairpin probe (hp) on PE₂ to form double-stranded DNA (dsDNA). The exonuclease could recognize and cleave two newly generated dsDNA, leading to the release of trigger. The free trigger strand continued to hybridize with the remaining cp/hp, which were cleaved by λ-Exo, and then trigger was released again and restarted next recycle with the λ-Exo. After digestion of λ-Exo, the number of capture probes on PE₁ was reduced, and many short DNA fragments were produced on PE₂, thereby resulting in the decreasing CdS QDs on PE₁ and the increasing CdS QDs on PE₂. As a result, it was observed that the ratio value of photocurrents (PE₁/PE₂) significantly decreased with the increasing CEA. Under optimum conditions, the sensing method showed a good linear relationship toward CEA within the dynamic range of 0.02-10 ng mL^-1 and a detection limit of 6.0 pg mL^-1. Moreover, the ratiometric PEC sensor exhibited good reproducibility, satisfying stability, and remarkable anti-interference performance, which suggests its promising application prospect to detect target CEA.

Oxygen vacancy enhanced photoelectrochemical performance of Bi2MoO6/B, N co-doped graphene for fabricating lincomycin aptasensor

Oxygen defect-engineered is an important strategy to improve the photoelectric activity of materials. Herein, a facile one-pot solvothermal method was utilized to synthesize visible light-responsive photoactive Bi₂MoO₆ nanoparticles anchored boron and nitrogen co-doped graphene (BNG) nanosheets nanocomposites with oxygen vacancy. The incorporation of BNG nanosheets increased the oxygen vacancies amounts on Bi₂MoO₆ remarkably, and the presences of oxygen vacancies can be beneficial to broaden the absorption range. The absorption edge of Bi₂MoO₆/BNG was widened from 500 nm to 550 nm compared to Bi₂MoO₆, and the charge transfer was accelerated to improve the photoactive of Bi₂MoO₆/BNG. Under visible light illumination, the photoelectrochemical (PEC) response of the as-prepared Bi₂MoO₆/BNG was 11.6-fold, 6.7-fold, 3.1-fold and 2.4-fold higher than that of pristine Bi₂MoO₆, Bi₂MoO₆/graphene, Bi₂MoO₆/nitrogen doped graphene and Bi₂MoO₆/boron doped graphene. Using Bi₂MoO₆/BNG nanocomposites with the superior PEC performance as photoactive materials in combination with specifically recognized lincomycin (LIN) aptamer, a highly efficient PEC aptasensor was successfully constructed for sensitive analysis of LIN. Under optimal conditions, the proposed PEC aptasensor exhibited excellent analytical performance for LIN with a wide linear response of 1 × 10^-11 to 1 × 10^-6 mol L^-1 along with a low detection limit of 3.7 × 10^-12 mol L^-1 (defined as S/N = 3). The as-prepared Bi₂MoO₆/BNG nanocomposites exhibit excellent visible light response and PEC performance, indicating its potential applications in PEC biosensor.

Enhanced photoelectrochemical sensing based on novel synthesized Bi2S3@Bi2O3 nanosheet heterostructure for ultrasensitive determination of L-cysteine

The design of a low-cost and highly efficient photoactive heterojunction material for sensing is still a challenging issue. On the basis of the formation of sheet-like Bi₂O₃ via coating Bi₂S₃, a novel Bi₂O₃@Bi₂S₃ heterostructure is controllably synthesized via a facile water bath approach. The prepared Bi₂O₃@Bi₂S₃ nanosheets show a superior photoelectrochemical (PEC) performance for the detection of L-cysteine (L-Cys), and the photocurrent signal is three and four times higher than those of Bi₂S₃ and Bi₂O₃ under visible irradiation, respectively. Also, the heterostructure presents an outstanding linear range for the detection of L-Cys: 0.1-10,000 μM. In addition, the mechanism of improved PEC response of Bi₂O₃@Bi₂S₃ nanosheets is investigated according to the estimated energy band positions. Thus, the integration of the novel heterostructure and the photoelectrochemical technique demonstrates a rapid photocurrent response, showing a great effect on the performance of the sensing system and a new PEC method for highly selective and sensitive chemical detection. Graphical abstract.

Dual-Channel Photoelectrochemical Ratiometric Aptasensor with up-Converting Nanocrystals Using Spatial-Resolved Technique on Homemade 3D Printed Device

A near-infrared light-activated ratiometric photoelectrochemical aptasensor was fabricated for detection of carcinoembryonic antigen (CEA) coupling with upconversion nanoparticles (UCNPs)-semiconductor nanocrystals-based spatial-resolved technique on a homemade 3D printing device in which a self-regulating integrated electrode was designed for dual signal readout. The as-prepared NaYF₄:Yb,Er UCNPs@CdTe nanocrystals were initially assembled on two adjacent photoelectrodes, then CEA aptamer 1 (A₁) and capture DNA (CA) were modified onto two working photoelectrodes (WP₁ and WP₂) through covalent binding, respectively, and then gold nanoparticle-labeled CEA aptamer 2 (Au NP-A₂) was immobilized on the surface of functional WP₂ for the formation of double-stranded DNA. Upon target CEA introduction, the various concentrations of CEA were captured on the WP₁, whereas the binding of the CEA with Au NP-A₂ could be released from the WP₂ thanks to the highly affinity of CEA toward A₂. The dual signal readout with the "signal-off" of WP₁ and "signal-on" of WP₂ were employed for the spatial-resolved PEC (SR-PEC) strategy to detect CEA as an analytical model. Combining NaYF₄:Yb,Er UCNPs@CdTe nanocrystals with spatial-resolved model on 3D printing device, the PEC ratiometric aptasensor based on steric hindrance effect and exciton-plasmon interactions (EPI) exhibited a linear range from 10.0 pg mL^-1 to 5.0 ng mL^-1 with a limit of detection of 4.8 pg mL^-1 under 980 nm illumination. The SR-PEC ratiometric strategy showed acceptable stability and reproducibility with a superior anti-interference ability. This approach can provide the guidance for the design of ratiometric, multiplexed, and point-of-care biosensors.

A Sunlight Powered Portable Photoelectrochemical Biosensor Based on a Potentiometric Resolve Ratiometric Principle

As a new analysis tool, photoelectrochemical (PEC) biosensors have been widely studied in recent years. However, common PEC biosensors usually require a highly stable light source to excite the electrical signal and an electrochemical workstation to collect and process the signal data, which limited the development of portable PEC devices. Herein, we propose the design of a sunlight powered portable PEC biosensor that uses sunlight as the light source. The sunlight intensity changes over time and weather and results in varied background PEC currents. To eliminate the interference caused by unstable excitation light, the potentiometric resolve ratiometric principle was introduced. Coupled with a miniature electrochemical workstation and a laptop, a sensitive and portable PEC sensing platform was successfully developed. The detection may be achieved under the irradiation of sunlight and will no longer need an extra light source. In a proof of concept experiment, this platform was successfully applied in aflatoxin B1 analysis, which was promising in the development of portable biosensors.

A sensitive Potentiometric resolved ratiometric Photoelectrochemical aptasensor for Escherichia coli detection fabricated with non-metallic nanomaterials

In this work, a sensitive potentiometric resolved ratiometric photoelectrochemical aptasensor for Escherichia coli (E. coli) detection was successfully fabricated with non-metallic nanomaterials. To avoid the use of precious metals or heavy metals, three-dimensional graphene hydrogel-loaded carbon quantum dots (C-dots/3DGH) and graphene-like carbon nitride (g-C₃N₄) with excellent PEC activity and matched potential were prepared. These two materials were modified onto two adjacent areas on the ITO electrode. By applying different bias voltage, the cathodic current generated by C-dots/3DGH and the anodic current generated by g-C₃N₄ can be clearly distinguished and would not interfere with one another. Then E. coli aptamer was modified onto the surface of C-dots/3DGH. In the presence of targets, the binding of E. coli with aptamer lead to the steric hindrance greatly increased and the cathodic current decreased significantly. Meanwhile, the anodic current generated by g-C₃N₄ was not influenced and it can serve as a stable reference to evaluate the environmental factors. Therefore, the concentration of E. coli can be quantified by the ratio of cathodic current to anodic current, which can effectively eliminate these analyte-independent factors and provide a more precise analysis. In addition, this ratiometric PEC biosensor also showed a good sensitivity and a wide linear range (2.9 cfu/mL to 2.9 × 10⁶ cfu/mL).

Photovoltage-triggered electrochromic tablet for visualized photoelectrochemical sensing

In this work, we designed an integrated photoelectrochemical (PEC) analytical system with photovoltage-regulated electrochromic tablet for visualized sensing. The electrochromic tablet consisted of electron-injector (EI) part and colorimetric system, which was functionalized with dye-sensitized titanium dioxide and nickel oxide nanosheet film as the recognition element and signal readout, respectively. Under irradiation of home-built white light, the EI part provided sufficient and stable photovoltage and injected the photoexcited electrons to nickel oxide nanosheet through the conductive inner side of indium tin oxide slide. Thus, the different color states of nickel oxide nanosheet were observed, resulting in a detectable signal by naked eyes for visual analysis. More importantly, the color change was identified to be dependent on the photovoltages generated from EI part. Using copper ions as a model analyte, the electrochromic tablet was successfully applied to visually detect copper ions based on its quenching effect on photovoltage due to the formation of exciton trapping. As a result, the copper ions could be simultaneously determined over a wide range through photovoltage record, average RGB value, and naked eyes. Furthermore, the device exhibited considerable stability and could be reused through simple washing steps. The smart tablet PEC system provides a new avenue to design practical PEC sensor in point-of-care diagnosis.

Formation of Homogeneous Epinephrine-Melanin Solutions to Fabricate Electrodes for Enhanced Photoelectrochemical Biosensing

The development of a simple but effective surface modification method is very important for the construction of biosensing interfaces. In this work, a postsynthetic water-soluble epinephrine-melanin (EPM) prepared from the self-polymerization of epinephrine has been demonstrated as an alternative of the widely used in situ formed polydopamine (PDA) for the surface coating of TiO₂ nanoparticles and the construction of a photoelectrochemical (PEC) biosensing interface. In contrast to the formation of insoluble aggregates in solution for dopamine, a homogeneous solution was obtained for epinephrine after the self-polymerization. The use of EPM as a postsynthetic material enables the surface coating of TiO₂ with the simple drop-casting method. Compared with the widely used dip-coating method for in situ PDA modification, the developed drop-casting method based on the use of water-soluble postsynthetic EPM saves more time, avoids the waste of bulk solution, and undoubtedly decreases the batch-to-batch inconsistencies. The simple coating of commercially available TiO₂ nanoparticles with EPM greatly enhances the PEC performance due to the charge transfer property of EPM. The application of EPM in the construction of the PEC biosensing interface was demonstrated by the immobilization of a model biorecognition element (prostate specific antigen (PSA) antibody) onto EPM modified indium tin oxide (ITO) photoanode. Sensitive detection of PSA with high selectivity and stability was obtained on the basis of the biological recognition ability of PSA antibody. This work may renew the use of postsynthetic melanin-like biopolymers in other fields.

Spatial-Resolved Photoelectrochemical Biosensing Array Based on a CdS@g-C3N4 Heterojunction: A Universal Immunosensing Platform for Accurate Detection

The detection of biomarkers with high sensitivity and accuracy in real biosamples remains challenging. Herein, a universal spatial-resolved photoelectrochemical (PEC) ratiometry for biodetection of prostate-specific antigen (PSA) as model biomarker was designed for the first time based on a dual-electrode array modified by CdS@g-C₃N₄ heterojunction coupled with CuS quantum dots (QDs) as signal amplification tags. Specifically, a new kind of photoactive material, the CdS@g-C₃N₄ p-n heterojunction with high photoelectric conversion efficiency and good chemical stability, was synthesized and immobilized on two spatial-resolved electrodes (WE1 and WE2). After immobilizing gold nanoparticles and capturing PSA antibodies on the electrodes, WE1 incubated with various concentrations of PSA was taken as a working electrode, whereas WE2 with a fixed concentration of PSA was used as an internal reference electrode. Next, signal antibodies of PSA-labeled CuS QDs as PEC signal quenchers were immobilized on the electrodes to form a sandwich-type immunocomplex. With the aid of a multiplexed disjunctor, the PEC responses of the dual electrodes were recorded, and the PSA was quantified via the ratio values of photocurrent intensities from WE1 and WE2. Combining the fine PEC performance of the CdS@g-C₃N₄ heterojunction with the superior quenching effect of CuS QDs in the spatial-resolved platform, the ratiometric system exhibits a linear range from 1.0 × 10^-11 to 5.0 × 10^-8 g mL^-1 with a limit of detection of 4.0 pg mL^-1. The results demonstrated herein may provide a new pattern for biomarker detection with high accuracy and good specificity as well as satisfactory applicability in real biosamples.

Label-Free Photoelectrochemical "Off-On" Platform Coupled with G-Wire-Enhanced Strategy for Highly Sensitive MicroRNA Sensing in Cancer Cells

MicroRNA (miRNAs) quantification, especially at low abundance, is vital for disease diagnosis, prognosis, and therapy. Herein we develop a distinctive label-free "off-on" configuration for photoelectrochemical (PEC) sensing platform fabrication, coupled with DNA four-way junction (4J) architecture as well as G-wire superstructure for signal amplification. In addition, ultrathin copper phosphate nanosheets (CuPi NSs) coating Au nanoparticles (Au-CuPi NSs) serve as a highly efficient photocathode substrate. To improve the sensitivity, and avoid the false positive signals, the quencher, gold nanoparticles (GNPs), is utilized to switch off the PEC signal because of the commendable surface plasmon resonance (SPR) absorption. Subsequently, ingenious DNA 4J architecture is applied to export proportional c-myc regions for target quantification. Assisted with the G-wire superstructure formation, the enhancer 5,10,15,20-tetra(4-sulfophenyl)-21H,23H-porphyrin (TSPP) is coupled on the substrate to switch on the PEC signal, thus realizing the miRNA assay with persuasive accuracy, high sensitivity, and low detection limit. In addition, we execute the miRNA detection in prostate carcinoma cell line 22Rv1, and acquire desirable quantitative capability. Remarkably, the prepared PEC sensing platform not only realizes the highly efficient miRNAs quantification, but also uncovers a marvelous horizon for sensing platform fabrication.

Design of a Dual Channel Self-Reference Photoelectrochemical Biosensor

Photoelectrochemical (PEC) biosensors are usually based on the single photocurrent change caused by biorecognition events between analytes and probes. However, the photocurrent may be influenced by other factors besides target analytes and bring a false result. To improve the accuracy and reliability of PEC detection, here we proposed the design of a dual channel self-reference PEC biosensors. CdTe and CdTe-graphene oxide (GO) were chosen as the two PEC active material and modified onto two adjacent areas on the ITO electrode. Then they were functionalized with Aflatoxin B1 (AFB1) aptamer through covalent binding or physical adsorption, respectively. The cathodic current from CdTe-GO and anodic current from CdTe can be well distinguished by adjusting the bias voltage. With the simultaneous application of "signal on" and "signal off" model, dual concentration information may be obtained in one detection and serve as a reference for each other. By comparing these two results, this sensor can clearly distinguish whether the signal change was caused by AFB1 or other interference factors. Compared to traditional PEC biosensors, this design can provide a better accuracy and reliability, which is promising in the future development of PEC detection.

Quantum material accompanied nonenzymatic cascade amplification for ultrasensitive photoelectrochemical DNA sensing

A novel cascade photoelectrochemical (PEC) signal amplification sensing strategy was designed and applied in target DNA detection by introducing quantum dots (QD) as the accompanying tag.