Simultaneous Photoelectrochemical Immunoassay of Dual Cardiac Markers Using Specific Enzyme Tags: A Proof of Principle for Multiplexed Bioanalysis

HOF-derived Step-Scheme FJU-200@CdSe heterojunction: A photoelectrochemical sensing platform for sensitive detection of EGFR

Here, a photoelectrochemical (PEC) immunosensor based on the FJU-200@CdSe heterostructure was developed for epidermal growth factor receptor (EGFR) detection. This is the first application of FJU-200 in PEC. After modification using CdSe quantum dots (QDs), FJU-200 and CdSe QDs formed an S-scheme heterostructure due to the interleaved energy band structure and the difference in Fermi energy (Ef) levels, which generated an efficient and stable PEC signal. When EGFR bound specifically to the antibody, a large spatial site resistance was generated, which hindered the electron transfer at the interface and the PEC signal was quenched. The proposed PEC sensing platform exhibited excellent detection performance for EGFR, with a good linear relationship with the photocurrent change value (ΔI) in the detection range of 10 fg/mL-100 ng/mL, and the detection limit was as low as 1.08 fg/mL. This work illustrates the potential electron transfer pathway between FJU-200 and CdSe QDs and creatively applies to the construction of PEC immunosensors, providing a new option for the detection of EGFR as well as other substances to be tested.

Magnetically Controlled and Addressable Photoelectrochemical Sensor Array with Self-Calibration for the Label-Free Detection of Amyloid β-Proteins

A label-free addressable photoelectric immunosensor array was designed for the detection of amyloid β-proteins based on magnetic separation and self-calibration strategies. In this paper, Na2Ti6O13 with a flower-like morphology was prepared by the hydrothermal method; after continuously combining Fe3O4 and CdS, it was endowed with magnetism and better photoelectric activity. Subsequently, a series of reactions occurred in the solution, and the magnetic separation method was used to enrich the target. On the other hand, the ITO glass was separated into eight sites (2 × 4) using magnets, and a light shield was utilized to prevent light exposure, resulting in addressable and continuous detection. After the uniform preparation of magnetic photoelectric materials and precise control of testing conditions, the relative errors among different sites have been effectively reduced. Moreover, incorporating a self-calibration strategy has allowed the sensor array to achieve greater accuracy. The proposed photoelectrochemical biosensor exhibits a good relationship with amyloid β-protein ranging from 0.01 to 100 ng mL-1 with a limit of detection of 1.1 pg mL-1 and exhibits excellent specificity, reproducibility, and stability.

Heating promoted super sensitive electrochemical detection of p53 gene based on alkaline phosphatase and nicking endonuclease Nt.BstNBI-assisted target recycling amplification strategy at heated gold disk electrode

An ultrasensitive electrochemical biosensor for detecting p53 gene was fabricated based on heated gold disk electrode coupling with endonuclease Nt.BstNBI-assisted target recycle amplification and alkaline phosphatase (ALP)-based electrocatalytic signal amplification. For biosensor assembling, biotinylated ssDNA capture probes were first immobilized on heated Au disk electrode (HAuDE), then combined with streptavidin-alkaline phosphatase (SA-ALP) by biotin-SA interaction. ALP could catalyze the hydrolysis of ascorbic acid 2-phosphate (AAP) to produce ascorbic acid (AA). While AA could induce the redox cycling to generate electrocatalytic oxidation current in the presence of ferrocene methanol (FcM). When capture probes hybridized with p53, Nt.BstNBI would recognize and cleave the duplexes and p53 was released for recycling. Meanwhile, the biotin group dropt from the electrode surface and subsequently SA-ALP could not adhere to the electrode. The signal difference before and after cleavage was proportional to the p53 gene concentration. Furthermore, with electrode temperature elevated, the Nt.BstNBI and ALP activities could be increased, greatly improving the sensitivity and efficiency for p53 detection. A detection limit of 9.5 × 10^-17 M could be obtained (S/N = 3) with an electrode temperature of 40 °C, ca. four magnitudes lower than that at 25 °C.

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.

Improved Sensitivity and Selectivity of Glutathione Detection through Target-Driven Electron Donor Generation in Photoelectrochemical Electrodes

Glutathione (GSH) plays a vital role in many physiological processes, and its abnormal levels have been found to be associated with several diseases. In contrast to traditional methods using electron donor-containing electrolytes for photoelectrochemical (PEC) sensing, in this study, a target-driven electron donor generation in a PEC electrode was developed to detect GSH. Using well-aligned TiO2 nanotube arrays (TNTs) as the PEC substrate, mesoporous MIL-125(Ti) was grown in the TNTs through an in situ solvothermal method and subsequent two-step annealing treatment. The accommodation capacity of mesoporous MIL-125(Ti) allows a well loading of cystine and Pt nanoclusters (NCs). Taking advantage of the specific cleavage ability of disulfide bonds by GSH, cystine was converted to cysteine, which served as the electron donor for the PEC process. Benefiting from the confinement effect of mesoporous MIL-125(Ti), cysteine was effectively oxidized to cysteine sulfinic acid by the photogenerated holes. Importantly, the highly active Pt NCs decorated in the mesopores not only improved the charge transfer but also accelerated the above oxidation reaction. The synergistic effect of these factors enabled the efficient separation of the photogenerated electron-hole pairs, which induced a significant photocurrent increase and in turn led to the high-sensitivity detection of GSH. Consequently, the proposed PEC biosensor exhibited excellent performance in the detection of GSH in serum specimens. The target-driven electron donor generation designed in this study might open a new route for developing sensitive and selective PEC biosensors with application in complex biological environments.

Progress in Electrochemical Immunosensors with Alkaline Phosphatase as the Signal Label

Electrochemical immunosensors have shown great potential in clinical diagnosis, food safety, environmental protection, and other fields. The feasible and innovative combination of enzyme catalysis and other signal-amplified elements has yielded exciting progress in the development of electrochemical immunosensors. Alkaline phosphatase (ALP) is one of the most popularly used enzyme reporters in bioassays. It has been widely utilized to design electrochemical immunosensors owing to its significant advantages (e.g., high catalytic activity, high turnover number, and excellent substrate specificity). In this work, we summarized the achievements of electrochemical immunosensors with ALP as the signal reporter. We mainly focused on detection principles and signal amplification strategies and briefly discussed the challenges regarding how to further improve the performance of ALP-based immunoassays.

Photoelectrochemical Determination of Cardiac Troponin I as a Biomarker of Myocardial Infarction Using a Bi2S3 Film Electrodeposited on a BiVO4-Coated Fluorine-Doped Tin Oxide Electrode

A sensitive and selective label-free photoelectrochemical (PEC) immunosensor was designed for the detection of cardiac troponin I (cTnI). The platform was based on a fluorine-doped tin oxide (FTO)-coated glass photoelectrode modified with bismuth vanadate (BiVO₄) and sensitized by an electrodeposited bismuth sulfide (Bi₂S₃) film. The PEC response of the Bi₂S₃/BiVO₄/FTO platform for the ascorbic acid (AA) donor molecule was approximately 1.6-fold higher than the response observed in the absence of Bi₂S₃. The cTnI antibodies (anti-cTnI) were immobilized on the Bi₂S₃/BiVO₄/FTO platform surface to produce the anti-cTnI/Bi₂S₃/BiVO₄/FTO immunosensor, which was incubated in cTnI solution to inhibit the AA photocurrent. The photocurrent obtained by the proposed immunosensor presented a linear relationship with the logarithm of the cTnI concentration, ranging from 1 pg mL^-1 to 1000 ng mL^-1. The immunosensor was successfully employed in artificial blood plasma samples for the detection of cTnI, with recovery values ranging from 98.0% to 98.5%.

Tailoring of the photocatalytic activity of CeO2 nanoparticles by the presence of plasmonic Ag nanoparticles

The present study investigates basic features of a photoelectrochemical system based on CeO₂ nanoparticles fixed on gold electrodes. Since photocurrent generation is limited to the absorption range of the CeO₂ in the UV range, the combination with metal nanoparticles has been studied. It can be shown that the combination of silver nanoparticles with the CeO₂ can shift the excitation range into the visible light wavelength range. Here a close contact between both components has been found to be essential and thus, hybrid CeO₂@Ag nanoparticles have been prepared and analyzed. We have collected arguments that electron transfer occurs between both compositional elements of the hybrid nanoparticles.The photocurrent generation can be rationalized on the basis of an energy diagram underlying the necessity of surface plasmon excitation in the metal nanoparticles, which is also supported by wavelength-dependent photocurrent measurements. However, electrochemical reactions seem to occur at the CeO₂ surface and consequently, the catalytic properties of this material can be exploited as exemplified with the photoelectrochemical reduction of hydrogen peroxide. It can be further demonstrated that the layer-by layer technique can be exploited to create a multilayer system on top of a gold electrode which allows the adjustment of the sensitivity of the photoelectrochemical system. Thus, with a 5-layer electrode with hybrid CeO₂@Ag nanoparticles submicromolar hydrogen peroxide concentrations can be detected.

Light-Addressable Electrochemical Sensors toward Spatially Resolved Biosensing and Imaging Applications

The light-addressable electrochemical sensor (LAES) is a recently emerged bioanalysis technique combining electrochemistry with the photoelectric effect in a semiconductor. In an LAES, a semiconductor substrate is illuminated locally to generate charge carriers in a well-defined area, thereby confining the electrochemical process to a target site. Benefiting from the unique light addressability, an LAES can not only detect multiple analytes in parallel within a single sensor plate but also act as a bio(chemical) imaging sensor to visualize the two-dimensional distribution of specific analytes. An LAES usually has three working modes: a potentiometric mode using light-addressable potentiometric sensors (LAPS) and an impedance mode using scanning photoinduced impedance microscopy (SPIM), while an amperometric mode refers to light-addressable electrochemistry (LAE) and photoelectrochemical (PEC) sensing. In this review, we describe the detection principles of each mode of LAESs and the concept of light addressability. In addition, we highlight the recent progress and advance of LAESs in spatial resolution, sensor system design, multiplexed detection, and bio(chemical) imaging applications. An outlook on current research challenges and future prospects is also presented.

The TDs/aptamer cTnI biosensors based on HCR and Au/Ti3C2-MXene amplification for screening serious patient in COVID-19 pandemic

The accurate assay of cardiac troponin I (cTnI) is very important for acute myocardial infarction (AMI), it also can be employed as an effective index for screening serious patients in COVID-19 pandemic before fatal heart injury to reduce the mortality. A ratiometric sensing strategy was proposed based on electrochemiluminescent (ECL) signal of doxorubicin (Dox)-luminol or the electrochemical (EC) signal of methylene blue (MB) vs. referable EC signal of Dox. The bio-recognitive Tro4-aptamer ensures the high specificity of the sensor by affinity binding to catch cTnI, and the tetrahedral DNA (TDs) on Au/Ti₃C₂-MXene built an excellent sensing matrix. An in situ hybrid chain reaction (HCR) amplification greatly improved the sensitivity. The ratiometric sensing responses ECL_Dox-luminol/Current_Dox or Current_MB/Current_Dox linearly regressed to cTnI concentration in the range of 0.1 fM-1 pM or 0.1 fM-500 fM with the limit of detection (LOD) as 0.04 fM or 0.1 fM, respectively. Served as the reference signal, Current_Dox reflected the variation of sensor, it is very effective to ensure the accuracy of detection to obviate the false results. The proposed biosensors show good specificity, sensitivity, reproducibility and stability, have been applied to determine cTnI in real samples with satisfactory results. They are worth looking forward to be used for screening serious patient of COVID-19 to reduce the mortality, especially in mobile cabin hospital.

Tailoring the Photoelectrochemical Activity of Hexametaphosphate-Capped CdS Quantum Dots by Ca2+-Triggered Surface Charge Regulation: A New Signaling Strategy for Sensitive Immunoassay

The development of efficient signaling strategies is highly important for photoelectrochemical (PEC) immunoassay. We report here a new and efficient strategy for sensitive PEC immunoassay by tailoring the electrostatic interaction between the photoactive material and the electron donor. The photoelectric conversion of hexametaphosphate (HMP)-capped CdS quantum dots (QDs) in Na₂SO₃ solution is significantly boosted after Ca²⁺ incubation. The negative surface charges on CdS@HMP QDs decrease because of the complexation reaction between HMP and Ca²⁺, and the electrostatic repulsion between CdS@HMP QDs and electron donor (SO₃^2-) becomes weak accordingly, leading to an improved electron-hole separation efficiency. Inspired by the PEC response of CdS@HMP QDs to Ca²⁺, a novel "signal-on" PEC immunoassay platform is established by employing CaCO₃ nanoparticles as labels. By regulating the surface charge of CdS@HMP QDs with in situ-generated Ca²⁺ from CaCO₃ labels, sensitive detection of the carcinoembryonic antigen (CEA) is achieved. The linear detection range is 0.005-50 ng mL^-1 and the detection limit is 1 pg mL^-1 for CEA detection. Our work not only provides a facile route to tailor the photoelectric conversion but also lays the foundation for sensitive PEC immunoassay by simply regulating the surface charge of photoactive materials.

Proton-Regulated Catalytic Activity of Nanozymes for Dual-Modal Bioassay of Urease Activity

Benefiting from the merits of high stability and superior activity, nanozymes are recognized as promising alternatives to natural enzymes. Despite the great leaps in the field of therapy and colorimetric sensing, the development of highly sensitive nanozyme-involved photoelectrochemical (PEC) biosensors is still in its infancy. Specifically, the investigation of multifunctional nanozymes facilitating different catalytic reactions remains largely unexplored due to the difficulty in synergistically amplifying the PEC signals. In this work, mesoporous trimetallic AuPtPd nanospheres were synthesized with both efficient oxidase and peroxidase-like activities, which can synergistically catalyze the oxidation of 4-chloro-1-naphthol to produce benzo-4-chlorohexadienone precipitation on the surface of photoactive materials, and thus lead to the decreased photocurrent as well as increased charge-transfer resistance. Inspired by the proton-dependent catalytic activity of nanozymes, a self-regulated dual-modal PEC and electrochemical bioassay of urease activity was innovatively established by in situ regulating the activity of AuPtPd nanozymes through urease-mediated proton-consuming enzymatic reactions, which can remarkably improve the accuracy of the assay. Meanwhile, the determination of urease activity in spiked human saliva samples was successfully realized, indicating the reliability of the biosensor and its application prospects in clinical diagnosis.

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.

Double ion-exchange reaction-based photoelectrochemical immunoassay for sensitive detection of prostate-specific antigen

This work developed a double ion-exchange reaction-based photoelectrochemical (PEC) immunoassay with the split-type detection mode for sensitive detection of prostate-specific antigen (PSA, used as a model). The nanocomposite of cadmium sulfide and nickel sulfide (CdS@NiS nanocomposite), as the photoactive material, was rapidly synthesized by two-step hydrothermal treatment. In the presence of target PSA, the cupric oxide nanoparticle (CuO NP) labeled detection antibody was introduced into the detection system by sandwich immunoreaction and the copper (Cu²⁺) ions was released from CuO nanoparticles by acid to participate in double ion-exchange reaction. The double ion-exchange reaction on the photoelectric sensing interface between Cu²⁺ and CdS@NiS nanocomposites formed the weak photoactive material Cu_xS (x = 1, 2) to reduce the photocurrent. Under optimal conditions, the double ion-exchange reaction-based PEC immunoassay exhibited good photocurrent responses toward target PSA within the dynamic working range from 0.01 ng mL^-1 to 50 ng mL^-1 at a low limit of detection (LOD) of 2.9 pg mL^-1. Besides, our work could achieve good reproducibility and high specificity under the split-type detection mode. Compared with human PSA ELISA kit, the accuracy obtained by our strategy was satisfactory. Importantly, this Cu²⁺-activated double ion-exchange reaction-based PEC immunoassay provides a promising platform for the detection of biomarkers.

Biomarkers detection with magnetoresistance-based sensors

Biosensing platforms for detecting and quantifying biomarkers have played an important role in the past decade. Among them, platforms based on magnetoresistance (MR) sensing technology are attractive. The resistance value of the material changes with the externally applied magnetic field is the core mechanism of MR sensing technology. A typical MR-based sensor has the characteristics of cost-effective, simple operation, high compactness, and high sensitivity. Moreover, using magnetic nanoparticles (MNPs) as labels, MR-based sensors have the ability to overcome the high background noise of complex samples, so they are particularly suitable for point-of-care testing (POCT). However, the problem still exists. How to obtain high-throughput, that is, multiple detections of biomarkers in MR-based sensors, thereby improving detection efficiency and reducing the burden on patients is an important issue in future work. This paper reviews three MR-based detection technologies for the detection of biomarkers, i.e., anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunneling magnetoresistance (TMR). Based on these three common technologies, different typical applications that include biomedical diagnosis, food safety, and environmental monitoring are presented. Furthermore, the existing MR-based detection method is better expanded to make it more in line with present detection needs by combining different advanced technologies including microfluidics, Microelectromechanical systems (MEMS), and Immunochromatographic test strips (ICTS). And then, a brief discussion of current challenges and perspectives of MR-based sensors are pointed out.

Plasmonic Enhanced Gold Nanoclusters-Based Photoelectrochemical Biosensor for Sensitive Alkaline Phosphatase Activity Analysis

Low-toxicity gold nanoclusters-decorated Ag@SiO₂ (Au NCs-Ag@SiO₂) nanocomposites modified plasmonic photoelectrodes were first fabricated to improve the photoelectric properties of Au NCs and practical application in biological detection. Through adjusting distance between Au NCs and plasmonic silver nanoparticles (Ag NPs), the photocurrent intensity of Au NCs enhanced by 3.8 times attributed to strong competition between enhancement functions of hot electron transfer, local electric field, light scattering effects, and quenching functions of nonradiative energy transfer. Further comparison between experimental results and theoretical simulations were conducted to gain a deeper understanding toward the photoelectric enhancement mechanism. Moreover, Au NCs-Ag@SiO₂ nanocomposites was successfully applied to the construction of photoelectrochemical (PEC) biosensors for sensitively detecting alkaline phosphatase activity. This proposed PEC biosensor showed a wide linear range from 0.04 to 400 U·L^-1, and a low detection limit of 0.022 U·L^-1.

A label-free G-quadruplex-based fluorescence assay for sensitive detection of alkaline phosphatase with the assistance of Cu2

The level of alkaline phosphate (ALP) is a significant biomarker index in organism. In this work, a label-free and sensitive G-quadruplex fluorescence assay for monitoring ALP activity has been developed with the assistance of Cu²⁺ based on the competitive binding effect between pyrophosphate (PPi) and G-quadruplex-N-methylmesoporphyrin (G4/NMM) complex to Cu²⁺. In the sensing assay, the G4/NMM complex is employed as a signal indicator, while the Cu²⁺ as a quencher and the PPi as recovery agent as well as the hydrolytic substance for ALP. In details, the fluorescence of the G4/NMM complex was efficiently quenched by introducing Cu²⁺ due to the proximal carboxylate groups of NMM coordinating with the Cu²⁺ as well as the unfolding of G-quadruplex by Cu²⁺, while the higher affinity between PPi and Cu²⁺ could lead to the fluorescence recovery. However, in the presence of ALP, the PPi was hydrolyzed to phosphate ions (Pi) which cannot integrate with Cu²⁺, resulting in the fluorescence quenching once again. Thus, a simple and facile way to inspect ALP has been exploited. The proposed assay shows a good linear relationship in the range from 0.5 to 100 U/L with the detection limit of 0.3 U/L. Moreover, the fabricated method is succeeded in detecting ALP in human serum samples, indicating the potential as a profitable candidate in biological and biomedical application.

o-Phenylenediamine/gold nanocluster-based nanoplatform for ratiometric fluorescence detection of alkaline phosphatase activity

This study demonstrates a novel and convenient ratiometric fluorescent method for the detection of alkaline phosphatase (ALP) activity. Amino-functionalized mesoporous silica nanoparticle-gold nanoclusters (MSN-AuNCs) nanocomposites were integrated with o-phenylenediamine (OPD) to form a ratiometric fluorescence nanoplatform. The presence of ALP induced the generation of quinoxaline (QX) derivative which called 3-(dihydroxyethyl)furo[3,4-b]quinoxaline-1-one (DFQ) with strong fluorescence emission at 450 nm, while the orange-red fluorescence of MSN-AuNCs at 580 nm was slightly quenched. Meanwhile, an obvious fluorescence color change from orange-red to purple and finally to blue can be observed by naked eyes with the increasing of ALP concentration. Therefore, employing the fluorescence emission of DFQ at 450 nm as the reporter signal and the fluorescence emission of MSN-AuNCs at 580 nm as a reference signal, a sensitive ratiometric detection method for ALP was developed. Quantitative detection of ALP activity in the linear range from 0.2 to 80 U/L with a detection limit of 0.1 U/L can be realized in this way, which endows the assay with high sensitivity enough for practical detection of ALP in human serum samples.

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.

A near-infrared light-controlled, ultrasensitive one-step photoelectrochemical detection of dual cell apoptosis indicators in living cancer cells

The proposed near-infrared (NIR) light-controlled, one-step photoelectrochemical (PEC) strategy could simultaneously detect cell apoptosis indicators, phosphatidylserine (Pho) and sodium potassium adenosine triphosphatase (Sat), on living cancer cells.

Carboxyl Esterase-Like Activity of DT-Diaphorase and Its Use for Signal Amplification

Carboxyl esterases show limited use as catalytic labels in bioassays because of slow enzymatic reaction. We report that DT-diaphorase from Bacillus stearothermophilus (DT-D, EC 1.6.99.-) shows high carboxyl esterase-like activity in the presence of reduced β-nicotinamide adenine dinucleotide (NADH) and may be used as a better catalytic label than carboxyl esterases. DT-D is a redox enzyme and can participate in signal-amplifying redox cycling. Thus, an electrochemical immunosensor using a DT-D label allows for triple signal amplification based on (i) hydrolysis of a carboxyl ester, (ii) electrochemical-chemical (EC) redox cycling involving an electrode, a hydrolysis product, and NADH, and (iii) electrochemical-enzymatic (EN) redox cycling involving an electrode, a hydrolysis product, DT-D, and NADH. Ester hydrolysis by DT-D is confirmed via spectrophotometric measurement of a chromogenic substrate (4-nitrophenyl acetate) and ¹H NMR spectra. Among two phenyl acetates and four naphthyl acetates considered, 4-aminonaphthalene-1-yl acetate (4-NH₂-NAc) is chosen as the best acetyl ester substrate because 4-NH₂-NAc is stable, its hydrolysis is slow in the absence of DT-D, its hydrolysis is very fast in the presence of DT-D, and EC and EN redox cycling involving the hydrolysis product (4-amino-1-naphthol) is rapid. However, hydrolysis of 4-NH₂-NAc by esterase from porcine liver (EC 3.1.1.1.) is very slow. When DT-D is applied to sandwich-type detection of thyroid-stimulating hormone in artificial serum, the detection limit is ∼2 pg/mL, indicating that the developed immunosensor is highly sensitive because of triple signal amplification. DT-D may be used as a catalytic label in sensitive and stable bioassays instead of common alkaline phosphatase and horseradish peroxidase.

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.

2D-porphrinic covalent organic framework-based aptasensor with enhanced photoelectrochemical response for the detection of C-reactive protein

In this study, a novel photoelectrochemical (PEC) aptasensor based on two-dimensional (2D) porphyrinic covalent organic frameworks (p-COFs) for the label-free detection of C-reactive protein (CRP) is presented. The obtained p-COFs possess high conductivity and an improved stability due to strong and rigid covalent linkages. The introduction of p-COFs hinder the recombination of electrons and holes, decreasing their band gap (E_g), thereby which improved the photocurrent conversion efficiency. Compared with pure porphyrin, p-COFs exhibited enhanced photocurrent intensity. An amplified photocurrent conversion efficiency and enhanced photocurrent results from H₂O₂, which act as active molecules and electron donors. As an unprecedented application of COFs in PEC bioanalysis, the detection of CRP with a PEC aptasensor is presented. The assembly of a CRP aptamer on the surface of Ag nanoparticles hinders the electron transfer, resulting in the decrease of the photocurrent response. This PEC aptasensor exhibits good analytical performances such as a rapid response, high stability, wide linear range and excellent selectivity, making COFs promising candidates for PEC bioanalysis.

A universal photoelectrochemical biosensor for dual microRNA detection based on two CdTe nanocomposites

A universal photoelectrochemical biosensor for dual microRNA detection has been successfully developed.

A sensitive photoelectrochemical methyltransferase activity assay based on a novel “Z-scheme” CdSe QD/afGQD heterojunction and multiple signal amplification strategies

Based on a “Z-scheme” CdSe QD/afGQD heterojunction and multiple signal amplification strategies, a sensitive photoelectrochemical methyltransferase activity assay was carried out.

Multiplexed electrochemical immunoassay for two immunoglobulin proteins based on Cd and Cu nanocrystals

Herein, a simple and feasible electrochemical immunosensing method for simultaneous voltammetric detection of two immunoglobulin proteins, human IgG (HIgG) and rabbit IgG (RIgG), was developed using two distinguishable signal-generation tags on the same electrode. The immunosensor was prepared by immobilizing two Fab antibody fragments on a gold electrode. After this, Cu and Cd nanocrystals, as nanotags, were synthesized and functionalized with identical detection antibodies. Transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) were employed to characterize the Cu and Cd nanocrystals. The covalently modified electrode with the Fab antibody fragments through the Au-thiolate bond (to dispel the non-specific adsorption) was investigated via scanning electron microscopy (SEM). After the sandwiched immunoreaction, the antibody-modified nanocrystals were captured on the immunosensor, which could be interrogated in pH 3.5 HCl using square-wave anodic stripping voltammetry. Experimental results also indicated that the multiplexed immunoassay enabled the simultaneous detection of HIgG and RIgG in a single run with the similar linear range from 0.01 to 10 ng mL^-1, and the limits of detection (LODs) towards two analytes could be as low as 3.4 pg mL^-1 (at 3σ). Acceptable assay results on precision, reproducibility, specificity, and method accuracy were also acquired. Importantly, the newly designed strategy avoided cross-talk and enzymatic introduction as compared to conventional electrochemical immunoassays, thus exhibiting a promising potential in clinical applications.

High performance immunochromatographic assay for simultaneous quantitative detection of multiplex cardiac markers based on magnetic nanobeads

The detection of cardiac markers is critical to the diagnosis of acute myocardial infarction, and immunochromatographic assays are a common tool for point-of-care analysis. Methods: We report a multiplexed lateral flow test strip for simultaneous quantitative detection of cardiac troponin I (cTnI), creatine kinase isoenzyme MB (CKMB), and myoglobin (Myo). Hydrophilic, monodisperse, stable, and carboxyl-modified (COOH-) magnetic nanobeads (MNBs) were used to construct immunomagnetic probes specific to the three cardiac markers. The detection area of the sandwich-style complexes contained three test lines for cTnI, CKMB, and Myo. The magnetic signal intensity of the detection area in the nitrocellulose membrane was measured via a magnetic immunochromatography reader developed in house. Results: To optimize the assay, a modified working buffer was also investigated to improve the detection sensitivity, decrease the background noise, and shorten the detection time. The MNB-based immunochromatography test (MICT) strip offers a wide linear dynamic detection range, rapid detection, high sensitivity, and specificity. The limit of detection was 0.0089 ng/mL for cTnI, 0.063 ng/mL for CKMB, and 0.05 ng/mL for Myo with minimal cross-reactivity. There were 110 clinical human serum samples that were used to evaluate this platform with high correlation. Conclusion: MICT shows great potential as a supplemental method for in vitro diagnostics in the laboratory or in other point-of-care testing (POCT) applications.

Point-of-Care Compatibility of Ultra-Sensitive Detection Techniques for the Cardiac Biomarker Troponin I-Challenges and Potential Value

Cardiac biomarkers are frequently measured to provide guidance on the well-being of a patient in relation to cardiac health with many assays having been developed and widely utilised in clinical assessment. Effectively treating and managing cardiovascular disease (CVD) relies on swiftly responding to signs of cardiac symptoms, thus providing a basis for enhanced patient management and an overall better health outcome. Ultra-sensitive cardiac biomarker detection techniques play a pivotal role in improving the diagnostic capacity of an assay and thus enabling a better-informed decision. However, currently, the typical approach taken within healthcare depends on centralised laboratories performing analysis of cardiac biomarkers, thus restricting the roll-out of rapid diagnostics. Point-of-care testing (POCT) involves conducting the diagnostic test in the presence of the patient, with a short turnaround time, requiring small sample volumes without compromising the sensitivity of the assay. This technology is ideal for combatting CVD, thus the formulation of ultra-sensitive assays and the design of biosensors will be critically evaluated, focusing on the feasibility of these techniques for point-of-care (POC) integration. Moreover, there are several key factors, which in combination, contribute to the development of ultra-sensitive techniques, namely the incorporation of nanomaterials for sensitivity enhancement and manipulation of labelling methods. This review will explore the latest developments in cardiac biomarker detection, primarily focusing on the detection of cardiac troponin I (cTnI). Highly sensitive detection of cTnI is of paramount importance regarding the rapid rule-in/rule-out of acute myocardial infarction (AMI). Thus the challenges encountered during cTnI measurements are outlined in detail to assist in demonstrating the drawbacks of current commercial assays and the obstructions to standardisation. Furthermore, the added benefits of introducing multi-biomarker panels are reviewed, several key biomarkers are evaluated and the analytical benefits provided by multimarkers-based methods are highlighted.

Advances in the integration of quantum dots with various nanomaterials for biomedical and environmental applications

Quantum dots (QDs) are semiconductor nanocrystals with distinct characteristics of high brightness, large Stokes shift and broad absorption spectra, large molar extinction coefficients, high quantum yield, good photostability and long fluorescence lifetime. The QDs have replaced the conventional fluorophores with wide applications in immunoassays, microarrays, fluorescence imaging, targeted drug delivery and therapy. The integration of QDs with various nanomaterials such as noble metal nanoparticles, carbon allotropes, upconversion nanoparticles (UCNPs), metal oxides and metal-organic frameworks (MOFs) brings new opportunities and possibilities in nanoscience and nanotechnology. In this review, we summarize the recent advances in the integration of QDs with various nanomaterials for biomedical and environmental applications including sensing, bioimaging, theranostics and cancer therapy. We highlight the involved interactions such as fluorescence resonance energy transfer (FRET), plasmon enhanced fluorescence (PEF), and nanometal surface energy transfer (NSET) as well as the synergistic effect resulting from the integration of QDs with nanomaterials. In addition, we discuss the sensing and imaging mechanisms of different strategies and give new insight into the challenges and future direction as well.

Recent progress on the sensitive detection of cardiovascular disease markers by electrochemical-based biosensors

Cardiovascular disease is the most reason for deaths in all over the world. Hence, biomarkers of cardiovascular diseases are very crucial for diagnosis and management process. Biomarker detection demand is opened the important way in biosensor development field. Rapid, cheap, portable, precise, selective and sensitive biomarker sensing devices are needed at this point to detect and predict disease. A cardiac biomarker can be orderable as C-reactive protein, troponin I or T, myoglobin, tumor necrosis factor alpha, interleukin-6, interleukin-1, lipoprotein-associated phospholipase, low-density lipoprotein and myeloperoxidase. They are used for prediction of cardiovascular diseases. There are many methods for early diagnosis of cardiovascular diseases, but these have long time process and expensive devices. In recent studies, different biosensors have been developed to remove the problems in this field. Electrochemical devices and developed biosensors have many superiorities than others such as low cost, mobile, reliable, repeatable, need a little amount of solution. In this review, recent studies were presented as details for cardiovascular disease biomarkers detection using electrochemical methods.