Ultrasensitive photoelectrochemical immunoassay for matrix metalloproteinase-2 detection based on CdS:Mn/CdTe cosensitized TiO2 nanotubes and signal amplification of SiO2@Ab2 conjugates

A disposable immunosensor for the detection of salivary MMP-8 as biomarker of periodontitis

This work describes the development of a novel voltammetric immunosensor for the detection of salivary MMP-8 at the point-of-care. The electrochemical platform was based on a graphene (GPH) screen-printed electrode (SPE) functionalized by gold-nanospheres (AuNSs) and antibodies against MMP-8 protein (anti-MMP-8). The functionalization with anti-MMP-8 was realized by using 11-mercaptoundecanoic acid (11-MUA), thanks to its ability to give strong sulfur bonds with its -SH end, and to cross-link the -NH2 groups of the antibody molecule with the other -COOH end, using the traditional EDC-NHS method. The voltammetric sensor showed good performances with a linear range of 2.5-300 ng mL-1, a LOD value of 1.0 ± 0.1 ng mL-1 and a sensitivity of 0.05 µA mL cm-2 ng-1. Moreover, the proposed immunosensor was tested in real saliva samples, showing comparable results to those obtained with the conventional ELISA method. The biosensor was single-use and cost-effective and required a small quantity of test medium and a short preparation time, representing a very attractive biosensor for MMP-8 detection in human saliva.

Highly efficient photoelectrochemical aptasensor based on CdS/CdTe QDs co-sensitized TiO2 nanoparticles designed for thrombin detection

A photoelectrochemical (PEC) sensor for the sensitive detection of thrombin (TB) was established. Co-sensitized combination of TiO2 nanoparticles combined with modified cadmium sulfide and cadmium telluride quantum dots (CdS/CdTe QDs) was utilized as a photoactive material. Successful growth of CdS/CdTe quantum dots on mesoporous TiO2 films occured by successive ion-layer adsorption and reaction. This interesting formation of co-sensitive structure is conducive to enhancing the photocurrent response by improving the use rate of light energy. Additionally, the step-level structure of CdS/CdTe QDs and TiO2 NPs shows a wide range of visible light absorption, facilitating the dissociation of excitons into free electrons and holes. Consequently, the photoelectric response of the PEC analysis platform is significantly enhanced. This constructed PEC aptasensor shows good detection of thrombin with a low detection limit (0.033 pM) and a wide linear range (0.0001-100 nM) in diluted actual human serum samples. In addition, this PEC aptasensor also has the characteristics of good stability and good reproducibility, which provides a novel insight for the quantitative measurement of other similar analytes.

Inverting the DNA tetrahedron: A novel strategy for sensitive and stable detection of aging-related enzyme MMP2

In the current study, a novel electrochemiluminescence biosensor based on the entropy-driven DNA tetrahedron for the detection of matrix metalloproteinase 2 (MMP2), an enzyme that regulates extracellular matrix remodeling and affects aging was reported. The biosensor utilizes an inverted DNA tetrahedron structure, which exposes three vertices to the solution, as molecular recognition units for capturing specific biomolecules. The biosensor also employs a ratiometric method and an entropy-driven reaction, which enhance the response rate and sensitivity of the detection. The biosensor can detect MMP2 with a detection limit of 55.2 fM, which is lower than that of conventional sensors. The biosensor also exhibits excellent stability and reproducibility, and can accurately measure MMP2 levels in complex samples, such as human serum. The paper demonstrates the feasibility and effectiveness of using the "inverted" DNA tetrahedron structure and the entropy-driven process to construct interfacial biosensors. The paper also discusses the potential applications of the biosensor in clinical diagnosis and anti-aging research, where MMP2 plays a crucial role in tissue damage and repair. The paper provides a valuable contribution to the field of biosensor development, and opens up new possibilities for using DNA nanotechnology for sensitive and reliable detection of various biomolecules.

Fabrication and nano-engineering of non-/noble metal-coupled plasmonic heterostructures for ultrasensitive photoelectrochemical immunoassays

To achieve reliable and ultrasensitive detection for disease markers in PEC bioanalysis, constructing and nano-engineering of ideal photoelectrodes and signal transduction strategies are of vital importance. Herein, a non-/noble metal coupled plasmonic nanostructure (TiO₂/r-STO/Au) was tactically designed with high-efficient PEC performance. Evidenced by the DFT and FDTD calculations, the reduced SrTiO₃ (r-STO) was found to support the localized surface plasmon resonance due to the sufficiently increased and delocalized local charge in r-STO. Under the synergistic coupling of plasmonic r-STO and AuNPs, the PEC performance of TiO₂/r-STO/Au was found remarkably promoted with reduced onset potential. This merit supported TiO₂/r-STO/Au as a self-powered immunoassay via a proposed oxygen-evolution-reaction mediated signal transduction strategy. With the increase of the target biomolecules (PSA), the catalytic active sites of TiO₂/r-STO/Au would be blocked and result in the decrease of the oxygen evaluation reaction. Under optimal conditions, the immunoassays exhibited an excellent detection performance with a LOD as low as 1.1 fg/mL. This work proposed a new type of plasmonic nanomaterial for ultrasensitive PEC bioanalysis.

Construction of a competitive electrochemical immunosensor based on sacrifice of Prussian blue and its ultrasensitive detection of alpha-fetoprotein

Effective signal amplification is a prerequisite for ultrasensitive detection by electrochemical immunosensors. For quantitative and ultrasensitive detection of alpha-fetoprotein (AFP), we designed a competitive electrochemical immunosensor and transferred the immunoreactivity from the electrode surface to the cuvette. AFP antigen was captured using AFP primary antibody (Ab₁) immobilized on magnetic nanobeads (MBs), and ZIF-8 nanomaterials attached to secondary antibody (Ab₂) were used as probes. MBs helped retain the sandwich structure in the test tube through incubation and washing steps. Then, an appropriately fixed excess of sodium ethylenediaminetetraacetic acid (EDTA) solution was added to the cuvettes, resulting in etching of Zn ions from ZIF-8 and formation of Zn-EDTA complexes. After magnetic separation, a certain amount of supernatant is added dropwise to the Prussian blue (PB)-modified electrode (GCE), and Fe ions (from PB) complex with the remaining EDTA in the supernatant, thus reducing the signal response value of PB. The higher the AFP concentration, the lower the amount of free EDTA in the supernatant, the less the destruction of PB, and therefore the higher the current. Under optimal conditions, the immunosensor achieved ultra-sensitive detection of AFP in the range of 10^-4 ng/mL-100 ng/mL with a limit of detection (LOD) as low as 0.032 pg/mL (S/N = 3). The excellent performance provides an important tool for the early screening and detection of AFP.

Sandwich photoelectrochemical biosensing of concanavalin A based on CdS/AuNPs/NiO Z-scheme heterojunction and lectin-sugar binding

A CdS/AuNPs/NiO Z-scheme heterojunction was prepared on a fluorine-doped tin oxide (FTO) electrode by hydrothermal synthesis of NiO on FTO, electrodeposition of AuNPs on NiO/FTO electrode and then cast-coating of CdS quantum dots. The CdS/AuNPs/NiO/FTO electrode gave a notably increased photocurrent versus NiO/FTO, CdS/FTO, AuNPs/NiO/FTO, CdS/AuNPs/FTO and CdS/NiO/FTO electrodes. The CdS/AuNPs/NiO/FTO electrode was further cast-coated with chitosan to immobilize d-mannose by Schiff base reaction, and concanavalin A (ConA) and then horseradish peroxidase (HRP) were captured on the electrode surface by lectin-sugar binding. 4-Chloro-1-naphthol (4-CN) was oxidized to form an insoluble precipitate catalyzed by HRP in the presence of H₂O₂, and the presence of precipitate on the photoelectrode inhibited the photocurrent in the presence of holes scavenger ascorbic acid. The relevant electrodes were characterized by electrochemistry, quartz crystal microbalance (QCM), UV-vis spectrophotometry, scanning electron microscopy/energy dispersive spectroscopy, and transmission electron microscopy. The QCM revealed that the collection efficiency (η) of the 4-CN-electrooxidation precipitate on the electrode can be as high as 91.8%. Under the optimal conditions, the decline of photocurrent responded linearly to the common logarithm of ConA concentration from 50 pM to 500 nM, with a limit of detection of 17 pM (S/N = 3). Satisfactory results were obtained in the detection of real soybean samples.

Novel electrogravimetric biosensors for the ultrasensitive detection of plasma matrix metalloproteinase-2 considered a potential tumor biomarker

In this study, we developed novel, simple gravimetric and voltammetric sensors for the ultrasensitive detection of active matrix metalloproteinase (MMP)-2 in plasma. The developed sensors are cost-effective, require a very less amount of reagents, and are time-saving. They detect MMP-2 based on antigen-antibody recognition and its ability to cleave glycine-leucine peptide bond. The three-dimensional bioplatform of the sensors consisted of a cationic polyethyleneimine (PEI) polymer that facilitated robust immobilization of the dipeptide labeled with anthraquinone (AQ), or antibody molecules in appropriate density, which was crucial for biosensing. Detection was performed using quartz crystal microbalance with dissipation and voltammetry. The results showed that the developed sensors were characterized by high stability, wide analytical range (2.0 pg mL^-1 to 5.0 μg mL^-1), and low detection limit (ca. 10 fg mL^-1). They also exhibited excellent efficiency in the determination of active MMP-2 in real samples, such as blood plasma. The developed sensors may hold great promise for the early diagnosis of cancers.

Proximity hybridization induced DNA assembly for label-free surface-enhanced Raman spectroscopic detection of carcinoembryonic antigen

In our research, label-free and surface-enhanced Raman dyes-free Raman spectroscopy which was used to detect carcinoembryonic antigen (CEA) according to poly adenine (Poly A)-regulated self-assembly methods was developed and studied. CEA induced partial hybridization of Ab-H2 and Ab-H1, and Ab-H1-CEA-Ab-H2 (a sandwich proximity CEA-DNA complex) was formed, which unfolded molecular beacon 1 (MB1) and modified the substrate. Subsequently, MB2-AuNPs were hybridized with MB1, and Ab-H1-CEA-Ab-H2 was released via toehold regulated displacements of DNA strands. Therefore, hybridization processes of MB2 and MB1 were induced and promoted by CEA-DNA complexes which worked as catalysts. The misplaced target then induced a next round of strand exchange, and the signals for determination of CEA were amplified by AuNPs absorbed on the substrate. It was indicated that the spectral characteristics of adenine at 736 cm^-1 were consistent with the SERS spectrum of DNA. Adenine acted as an internal marker for label-free SERS detection of CEA. Moreover, satisfactory stability and reproducibility were found. Meanwhile, the antibody could specifically recognize the corresponding antigen. Since adenine was dominant in SERS spectra, which was also proximal to Au surface, the sensitivity of the novel method was high without modifications. The analytical performance of this method in determining serum CEA was satisfactory.

A sensitive electrochemiluminescence biosensor for assay of cancer biomarker (MMP-2) based on NGQDs-Ru@SiO2 luminophore

A sensitive biosensor that can be used for the determination of matrix metalloproteinase 2 (MMP-2) was proposed. The biosensor was developed by using an excellent self-enhanced nanocomposites as an illuminant and a peptide as a recognition element. For the electrostatic attraction between Ru(bpy)₃²⁺ and nitrogen-doped graphene quantum dots (NGQDs), the self-enhanced electrochemiluminescence (ECL) nanocomposites of NGQDs-Ru(bpy)₃²⁺-doped silica nanoparticles (NGQDs-Ru@SiO₂) were synthesized through a simple sol-gel process. Then, a specific peptide (labeled sulfhydryl) was combined with the self-enhanced ECL nanocomposites (carboxyl in NGQDs) via acylation reaction to obtain the peptide-NGQDs-Ru@SiO₂ nanoprobe, which was fabricated onto the gold electrode surface via Au-S bond. The peptide of the ECL nanoprobe was exposed to cleavage in the presence of MMP-2, which caused the signal substance to move farther away from the electrode, leading to a decrease of the ECL signal. The proposed NGQDs-Ru@SiO₂-labeled peptide ECL biosensor displayed a lower detection limit of 6.5 pg mL^-1 than those of reported ECL methods. The proposed biosensor provided an outlook for future applications in other disease-associated biomarkers.

A visible and near-infrared light dual responsive "signal-off" and "signal-on" photoelectrochemical aptasensor for prostate-specific antigen

A visible and near-infrared light dual responsive "signal-off" and "signal-on" photoelectrochemical aptasensor was constructed for determining prostate-specific antigen (PSA) based on MoS₂ nanoflowers and gold nanobipyramids. The dual responsive photoelectrochemical aptasensor can provide accurate results for PSA determination. For the photoelectrochemical aptasensor fabrication, amino-group functionalized aptamers were immobilized on a MoS₂ nanoflowers modified glassy carbon electrode surface for the specific recognition, and thus to achieve a "signal-off" aptasensor for PSA under visible light illumination. Subsequently, gold nanobipyramids integrated with thiol-functional aptamer were introduced to the "signal-off" aptasensing interface after PSA recognition. Under excitation with near-infrared light at 808 nm, the photocurrent response can be amplified significantly due to the excellent conductivity and local surface plasmon resonance effect of gold nanobipyramids, thus to producing a "signal-on" model for determining PSA. Under the optimized conditions, the dual-responsive photoelectrochemical aptasensor shows a linear response to the logarithm of PSA concentration in the range of 0.005-100 ng/mL. The detection limits for PSA determination with a "signal-off" or a "signal-on" mode are 1.75 pg mL^-1 and 0.39 pg mL^-1, respectively. The dual-responsive photoelectrochemical aptasensor was also employed for determining PSA in clinical serum samples with satisfactory selectivity and excellent accuracy.

Sandwich-type photoelectrochemical immunosensor for procalcitonin detection based on Mn2+ doped CdS sensitized Bi2WO6 and signal amplification of NaYF4:Yb, Tm upconversion nanomaterial

In this paper, we constructed a sandwich-type photoelectrochemical (PEC) immunosensor for the quantitative detection of procalcitonin (PCT) based on the sensitization of Mn²⁺ doped CdS (CdS:Mn) nanocomposites to Bi₂WO₆ and the signal amplification effect of an upconversion material NaYF₄:Yb,Tm. Bi₂WO₆ was synthesized with a three-dimensional flowered structure. CdS:Mn reduced the recombination of photogenerated carriers, and significantly improved photocurrent response. Lanthanide-doped upconversion nanomaterials were used as the label of secondary antibody. NaYF₄:Yb,Tm have two functions, not only connected with the secondary antibody, but also can further amplify the photocurrent response. The proposed immunosensor for detecting PCT provided a desired linear range of 0.5 pg mL^-1-100 ng mL^-1 and a detection limit of 0.13 pg mL^-1 under optimal experimental conditions. Besides, the PEC immunosensor demonstrated good reproducibility, specificity and stability. The results of determination of PCT in real human serum samples were satisfactory. Thus, the immunosensor may be applied in the clinical diagnosis of PCT and other biomarkers.

Fluorescence-Based On-Resin Detection of Three Model Proteases

A new approach to on-resin detection of three model proteases (trypsin, chymotrypsin, and thrombin) has been developed, while at the same time already described methodology for simultaneous detection of two enzymes (trypsin and chymotrypsin) has been additionally generalized. Appropriate immobilized substrates, comprising specifically cleavable peptide sequences capped with fluorescent dyes, have been synthesized on Rink Amide PEGA resin or Amino PEGA resin modified with backbone amide linker (BAL). Resulting solid support-bound probes were then dispersed into Tris-HCl buffer solution (pH = 8.0) and subjected to enzymatic cleavage. Liberated fluorophores have been tracked by fluorescence measuring. The competitive activities of studied proteases towards the thrombin probe have been efficiently limited and controlled by employing a Bowman-Birk inhibitor into a system.

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.

Cyclometalated Iridium(III) Complex-Sensitized NiO-Based-Cathodic Photoelectrochemical Platform for DNA Methyltransferase Assay

This work reports an efficient [(C6)₂Ir(dppz)]⁺PF₆^- (C6 = coumarin 6 and dppz = dipyridophenazine)-sensitized NiO photocathode and its application in photoelectrochemical (PEC) bioanalysis field for the first time. This dye-sensitized NiO photocathode was found to exhibit a markedly enhanced cathodic photocurrent. A sensitive cathodic PEC platform was proposed integrating the as-prepared photocathode with enzyme-free cascaded amplification strategies of the catalytic hairpin assembly (CHA) and the hybridization chain reaction (HCR) for DNA methyltransferase (MTase) assay. A hairpin DNA(H_Dam) with specific recognition site of Dam MTase in its stem was designed. The site of H_Dam was methylated in the presence of Dam MTase and then cut by endonuclease DpnI. The released loop fragment, as an initiator, triggered the CHA circuit and the follow-up HCR circuit, resulting in long dsDNA concatemers on the ITO electrode. Numerous [(C6)₂Ir(dppz)]⁺PF₆^- were intercalated into dsDNA, and highly efficient signal amplification was realized. Benefiting from the superior iridium(III) complex-sensitized NiO photocathode and effective amplification strategy, a detection limit of 0.0028 U/mL for the determination of Dam MTase was achieved. Moreover, this work further demonstrated that these proposed tactics could be applied to screen Dam MTase activity inhibitors.

Interference-free photoelectrochemical immunoassays using carboxymethylated dextran-coated and gold-modified TiO2 nanotube arrays

An interference-free photoelectrochemical (PEC) immunoassay was developed for cardiac troponin I (cTnI) detection. Covalent linkage of cTnI antibody to carboxymethylated (CM-) dextran pre-immobilized onto a gold nanoparticles (AuNPs)-modified TiO₂ nanotube array (NTA) affords five consecutive analyte captures with surface regenerations in between. Changes in the photocurrents at this photoanode before and after cTnI captures can be well fitted with the Langmuir isotherm from 0.220 pM to 2.20 nM cTnI. Owing to the inherently high sensitivity of the PEC detection, the detection limit (2.20 pg/mL) is lower than the range attainable with the enzyme-linked immunosorbent assay (ELISA) (6.00-40.0 pg/mL). Furthermore, CM-dextran prevents species in complex biological matrices from nonspecifically adsorbing onto the sensor surface, a feature not attainable with uncoated semiconductor electrodes or those coated with non-hydrogel-based chemical modifiers. The excellent anti-fouling property of dextran hydrogel allowed us to validate the accuracy of our regenerable sensors through a comparison of PEC immunoassays of patient sera to those of ELISA.

Bioanalytical methods for circulating extracellular matrix-related proteins: new opportunities in cancer diagnosis

The role of the extracellular matrix (ECM) remodeling in tumorigenesis and metastasis is becoming increasingly clear. Cancer development requires that tumor cells recruit a tumor microenvironment permissive for further tumor growth. This is a dynamic process that takes place by a cross-talk between tumor cells and ECM. As a consequence, molecules derived from the ECM changes associated to cancer are released into the bloodstream, representing potential biomarkers of tumor development. This article highlights the importance of developing and improving bioanalytical methods for the detection of ECM remodeling-derived components, as a step forward to translate the basic knowledge about cancer progression into the clinical practice.

Electrochemical sensing of blood proteins for mild traumatic brain injury (mTBI) diagnostics and prognostics: towards a point-of-care application

Traumatic Brain Injury (TBI) being one of the principal causes of death and acquired disability in the world imposes a large burden on the global economy. Mild TBI (mTBI) is particularly challenging to assess due to the frequent lack of well-pronounced post-injury symptoms. However, if left untreated mTBI (especially when repetitive) can lead to serious long-term implications such as cognitive and neuropathological disorders. Computer tomography and magnetic resonance imaging commonly used for TBI diagnostics require well-trained personnel, are costly, difficult to adapt for on-site measurements and are not always reliable in identifying small brain lesions. Thus, there is an increasing demand for sensitive point-of-care (POC) testing tools in order to aid mTBI diagnostics and prediction of long-term effects. Biomarker quantification in body fluids is a promising basis for POC measurements, even though establishing a clinically relevant mTBI biomarker panel remains a challenge. Actually, a minimally invasive, rapid and reliable multianalyte detection device would allow the efficient determination of injury biomarker release kinetics and thus support the preclinical evaluation and clinical validation of a proposed biomarker panel for future decentralized in vitro diagnostics. In this respect electrochemical biosensors have recently attracted great attention and the present article provides a critical study on the electrochemical protocols suggested in the literature for detection of mTBI-relevant protein biomarkers. The authors give an overview of the analytical approaches for transduction element functionalization, review recent technological advances and highlight the key challenges remaining in view of an eventual integration of the proposed concepts into POC diagnostic solutions.

Highly Sensitive and Selective Photoelectrochemical Aptasensors for Cancer Biomarkers Based on MoS2/Au/GaN Photoelectrodes

An Au/GaN photoelectrode was prepared by sputtering 30 nm thick Au film on the surface of n-type gallium nitride (GaN). When the electrode contacts with multilayered molybdenum disulfide (MoS₂), photogenerated electrons and photogenerated holes transfer to MoS₂ because of the band gap matching of MoS₂ and GaN. The presence of Au promotes charge transfer and results in a greater recombination of electrons and holes; by this means, a more significant suppression of photocurrent can be detected. This characteristic has been coupled with the high selectivity of an aptamer and applied to develop a novel photoelectrochemical aptasensor for cancer biomarkers (alpha-fetoprotein (AFP) as a model). The aptamer of AFP was modified on the surface of the Au/GaN photoelectrode by Au-S bonds, which can bind to the target protein with high selectivity. Then, the transfer process of the charge carriers of GaN to MoS₂ can be blocked by the target protein so that the suppression of photocurrent is reduced. The difference of the photocurrent in the presence and absence of AFP (ΔI) showed a linear relationship with AFP concentration that ranged from 1.0-150 ng/mL (R² = 0.9995), and the detection limit was 0.3 ng/mL. The standard addition recovery rates ranged from 85.2 to 91.7%. The method possessed good sensitivity and high selectivity for AFP detection. The developed biosensor can be modified to detect other cancer biomarkers by simply replacing the aptamer used.

In Situ Protease Secretion Visualization and Metastatic Lymph Nodes Imaging via a Cell Membrane-Anchored Upconversion Nanoprobe

Matrix metalloproteinase (MMP) secretion is highly associated with tumor invasion and metastasis; therefore, monitoring MMP secretion is important for disease progression study and therapy choosing. Though working well for intracellular MMP imaging, the performance of current MMP detection probes is impaired in secretion monitoring due to the diffusion of MMP in an extracellular environment after secretion and low secreted amount. Here, we design a cell membrane-anchored ratiometric upconversion nanoprobe (UCNPs-Cy3/Pep-QSY7/Ab) for in situ MMP secretion visualization. Anti-EGFR is functionalized on the nanoprobe to provide specific recognition to tumor cells and guarantee fast response to MMP2 in the local place of secretion. MMP-responsive cleavage of Pep-QSY7 results in Cy3 luminescence recovery at 580 nm, which is ratioed over an internal standard of UCNP emission at 654 nm for MMP2 detection. The presented cell membrane-anchored ratiometric upconversion nanoprobe demonstrated that satisfactory results for in situ monitoring of MMP2 secretion from MDA-MB-231 cells and MCF-7 cells, as well as in vivo imaging of metastatic lymph nodes, would provide a universal platform for protease secretion study and contribute to tumor invasiveness assessment.

Characteristics of P-Type and N-Type Photoelectrochemical Biosensors: A Case Study for Esophageal Cancer Detection

P-type and N-type photoelectrochemical (PEC) biosensors were established in the laboratory to discuss the correlation between characteristic substances and photoactive material properties through the photogenerated charge carrier transport mechanism. Four types of human esophageal cancer cells (ECCs) were analyzed without requiring additional bias voltage. Photoelectrical characteristics were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), UV–vis reflectance spectroscopy, and photocurrent response analyses. Results showed that smaller photocurrent was measured in cases with advanced cancer stages. Glutathione (L-glutathione reduced, GSH) and Glutathione disulfide (GSSG) in cancer cells carry out redox reactions during carrier separation, which changes the photocurrent. The sensor can identify ECC stages with a certain level of photoelectrochemical response. The detection error can be optimized by adjusting the number of cells, and the detection time of about 5 min allowed repeated measurement.

Nanostructure-based photoelectrochemical sensing platforms for biomedical applications

As a newly developed and powerful analytical method, the use of photoelectrochemical (PEC) biosensors opens up new opportunities to provide wide applications in the early diagnosis of diseases, environmental monitoring and food safety detection. The properties of diverse photoactive materials are one of the essential factors, which can greatly impact the PEC performance. The continuous development of nanotechnology has injected new vitality into the field of PEC biosensors. In many studies, much effort on PEC sensing with semiconductor materials is highlighted. Thus, we propose a systematic introduction to the recent progress in nanostructure-based PEC biosensors to exploit more promising materials and advanced PEC technologies. This review briefly evaluates the several advanced photoactive nanomaterials in the PEC field with an emphasis on the charge separation and transfer mechanism over the past few years. In addition, we introduce the application and research progress of PEC sensors from the perspective of basic principles, and give a brief overview of the main advances in the versatile sensing pattern of nanostructure-based PEC platforms. This last section covers the aspects of future prospects and challenges in the nanostructure-based PEC analysis field.

Biosensors and bioassays for determination of matrix metalloproteinases: state of the art and recent advances

Matrix metalloproteinases (MMPs) are closely associated with various physiological and pathological processes, and have been regarded as potential biomarkers for severe diseases including cancer. Accurate determination of MMPs would advance our understanding of their roles in disease progression, and is of great significance for disease diagnosis, treatment and prognosis. In this review, we present a comprehensive overview of the developed bioassays/biosensors for detection of MMPs, and highlight the recent advancement in nanomaterial-based immunoassays for MMP abundance measurements and nanomaterial-based biosensors for MMP activity determination. Enzyme-linked immunosorbent assay (ELISA)-based immunoassays provide information about total levels of MMPs with high specificity and sensitivity, while target-based biosensors measure the amounts of active MMPs, and allow imaging of MMP activities in vivo. For multiplex and high-throughput analysis of MMPs, microfluidics and microarray-based assays are described. Additionally, we put forward the existing challenges and future prospects from our perspective.

Highly Sensitive and Selective Photoelectrochemical Aptasensor for Cancer Biomarker CA125 Based on AuNPs/GaN Schottky Junction

A gold nanoparticle (AuNPs)/gallium nitride (GaN) Schottky junction was fabricated by growing AuNPs in situ on the surface of GaN and then etched by H₂O₂ to appropriate diameter. The photogenerated electrons of GaN can be captured and transferred by the AuNPs to increase the migration efficiency, and the electron-hole pairs were separated, which results in the enhancement of the photoelectric performance of the system. The Fermi energy level of AuNPs and the charge transfer efficiency of the AuNPs/GaN can be adjusted by controlling the size of the AuNPs. Then the AuNPs/GaN Schottky photoelectrode had been applied to develop a novel photoelectrochemical (PEC) aptasensor for the epithelial ovarian cancer marker-CA125 detection. The DNA aptamer of CA125 was modified on the surface of the AuNPs via Au-S bonds. The aptamer can bind with the target with high selectivity, and the photoelectron transfer process of the system can be blocked by the protein, which results in the decrease of the photocurrent of the system. The ratio of photocurrent before and after incubation with CA125 (I₁/I₀) has a linear with the concentration of CA125 in the range of 1-100 U/mL with a detection limit of 0.3 U/mL. The standard addition recovery rates were between 86.01% and 90.09%. This method showed good sensitivity, selectivity, and reliability in detecting CA125 in serum.

CdS Quantum-Dots-Decorated V2O5 Nanosheets as Chemically Etchable Active Materials for Sensitive Photoelectrochemical Immunoassay of Carcinoembryonic Antigen

We report here CdS quantum-dots (QDs)-decorated V₂O₅ nanosheets as high-performance and chemically etchable photoelectric active materials for constructing a photoelectrochemical (PEC) immunoassay platform. CdS QDs-decorated V₂O₅ nanosheets as new photoelectric materials can show superior photocurrent to V₂O₅ nanosheets and CdS QDs under visible-light irradiation because of the promoted photogenerated electron-hole separation and the increased visible-light absorption. V₂O₅ nanosheets can be etched by ascorbic acid (AA) because of the reduction of V₂O₅ to V⁴⁺, and the photocurrent of CdS/V₂O₅-nanocomposite-modified indium tin oxide electrode decreases significantly after being etched by AA. Inspired by this phenomenon, a PEC immunoassay platform is constructed for carcinoembryonic antigen (CEA) detection by using CdS/V₂O₅ nanocomposite as the photoelectric material and AA-encapsulated liposome immunonanocapsules as labels. The linear detection range for detecting CEA is from 0.5 pg mL^-1 to 1 ng mL^-1, with a limit of detection of 0.1 pg mL^-1. The proposed method also shows good selectivity, excellent reproducibility, and satisfactory recovery in detection of CEA in human serum samples. We believe that this work will lay the foundation for the future development of V₂O₅-based materials for PEC analysis, and also provide a reasonable design and implementation for the development of PEC immunoassay.

Three-dimensional macroporous gold electrodes superior to conventional gold disk electrodes in the construction of an electrochemical immunobiosensor for Staphylococcus aureus detection

Herein, a three-dimensional macroporous gold (3DMG) electrode is demonstrated to be a better choice than a conventional gold disk electrode in the construction of an electrochemical immunobiosensor for Staphylococcus aureus (S. aureus) detection. The 3DMG electrode was prepared on a gold disk electrode by one-step electrodeposition using hydrogen bubbles as dynamic templates. The 3DMG electrode has a high electrochemically active surface area with pore sizes ranging from 20 to 50 μm, and these unique features are conducive to the immobilization of primary antibodies and the capture of S. aureus. Secondary antibodies (Ab2) and alkaline phosphatase (ALP) were immobilized on mesoporous silica nanospheres (MSNs), and the resulting ALP-MSNs-Ab2 composites were utilized as signal tags to construct a sandwich-type electrochemical immunobiosensor. S. aureus was measured based on alkaline phosphatase-catalyzed silver deposition and differential pulse voltammetric detection. The linear range is from 5 to 109 CFU mL-1, and the detection limit is 2 CFU mL-1 for S. aureus detection. Due to the signal amplification of the 3DMG electrode, the sensitivity of the immunobiosensor constructed on the 3DMG electrode is 9 times that of an immunobiosensor constructed on a gold disc electrode. The proposed biosensor was successfully applied for detecting S. aureus in milk samples.

Detection of Tetanus Antibody Applying a Cu-Zn-In-S/ZnS Quantum Dot-Based Lateral Flow Immunoassay

Lateral flow test strip (LFTS) enables rapid, portable, and low-cost point-of-care testing (POCT) diagnosis. Quantum dots (QDs), which are fluorescent semiconductor nanocrystals with distinctive and unique photophysical properties, have become promising candidates to serve as labels for LFTS with improved sensitivity. Here, by using QDs as a signal reporter, we report a fluorescent LFTS for detection of tetanus antibody. This LFTS possess a high sensitivity for tetanus antibody, with a detection limit of 0.001 IU/mL. This assay was also applied for detection of tetanus antibody in human serum. More importantly, these strips can retain their specificity and sensitivity for at least 4 months when they are stored at 4 °C.

Polydopamine-PEG-Folic Acid Conjugate Film Engineered TiO2 Nanotube Arrays for Photoelectrochemical Sensing of Folate Binding Protein

Serum-soluble folate binding protein (FBP) is an important tumor marker, and the development of a simple biosensing method is highly needed. In this work, a photoelectrochemical (PEC) biosensor for the detection of FBP was proposed based on the construction of an antifouling interface and the unique ligand-protein recognition. The PEC sensing platform was prepared by the biomimetic polydopamine (PDA) coating on TiO₂ nanotubes arrays (NTAs). A significant PEC enhancement effect was obtained due to the macroporous structures. Excellent antifouling performance was achieved by conjugation of amino-group-terminated 8-arm poly(ethylene glycol) (PEG). The incorporation of folic acid (FA) retains the antifouling property and shows recognition abilities toward FBP. The fabricated PEC biosensor shows good analytical performance. The combination of ligand-protein recognition and a PEC antifouling interface provides a good consideration for the development of FBP biosensors.

A photoelectrochemical immunosensor based on CdS/CdTe-cosensitized SnO2 as a platform for the ultrasensitive detection of amyloid β-protein

An ultrasensitive label-free photoelectrochemical immunosensor was developed to detect amyloid β-protein based on CdS/CdTe-cosensitized SnO₂ nanoflowers.

Ti3C2/BiVO4 Schottky junction as a signal indicator for ultrasensitive photoelectrochemical detection of VEGF165

A "signal-on" photoelectrochemical (PEC) biosensor was developed for ultrasensitive detection of vascular endothelial growth factor 165 (VEGF165) based on a Ti3C2/BiVO4 Schottky junction, which was a novel sensitized structure with excellent photovoltaic performance, prominent film-forming ability and stability. The generation of a built-in electric field by the Ti3C2/BiVO4 Schottky junction efficiently suppressed the combination of photogenerated electron-hole pairs and enhanced the photocurrent, providing a high initial photocurrent signal for the construction of a photoelectrochemical (PEC) biosensor.

Fluorescent polymer dots and graphene oxide based nanocomplexes for "off-on" detection of metalloproteinase-9

Numerous studies have demonstrated that cancer-related matrix metalloproteinase-9 (MMP-9) is an ideal biomarker for cancer diagnosis. However, most MMP-9 detection methods are expensive and time-consuming, and more convenient and specific MMP-9 detection methods are needed both clinically and in research. In the present study, peptide-linked polymer dots were assembled onto a graphene oxide surface to construct a graphene oxide-peptide-polymer dot (GO-Pep-Pdot) nanocomplex for sensitive, rapid, and accurate detection of MMP-9. In the absence of MMP-9, the nanocomplex was in an "off" state, whereas in the presence of MMP-9, the nanocomplex was turned "on", resulting in the emission of a fluorescence signal that is linearly correlated with the MMP-9 concentration. The limit of detection of the nanocomplex was 3.75 ng mL^-1, lower than most methods. This method was successfully verified by detecting MMP-9 in clinical serum samples of prostate cancer. The results suggest that this protease nanocomplex is generic and can be adopted to respond to other proteases by selecting specific peptides with suitable cleavage sites in clinics.

TiO2 nanotubes/reduced GO nanoparticles for sensitive detection of breast cancer cells and photothermal performance

In this study, we developed a simple and cost effective aptasensor based on TiO₂ nanotubes-reduced graphene oxide (TiO₂ nanotube-rGO) linked to MUC1 aptamers for ultrasensitive electrochemical detection of breast cancer cell (MCF-7). Moreover, the photothermal performance of nanohybrid TiO₂-rGO was investigated for cancer treatment. In this regard, after synthesize of TiO₂ nanotubes via anodization process, TiO₂ nanotubes-rGO hybrid was synthesized by UV assisted reduction of GO and subsequent TiO₂ nanotubes attachment to rGO sheets. The resultant hybrid could provide an excellent large surface area leading to improvement of suitable sites for MUC1 aptamer immobilization. Our results revealed that TiO₂-rGO aptasensor exhibited superior analytical performance for MCF-7 cell detection with the detection limit of 40 cells.ml^-1 within the detection range of 10³-10⁷ cells. ml^-1. In addition, the designed aptasensor was effectively applied to detect MUC1 marker in a real sample. Moreover, the TiO₂ nanotube-rGO hybrid nanoparticles revealed great photothermal performance exposed to NIR laser. It could be concluded that nanohybrid TiO₂-rGO would be a useful and beneficial platform for detection and treatment of breast cancer.

A novel "signal on" photoelectrochemical strategy based on dual functional hemin for microRNA assay

In this work, a novel "signal on" photoelectrochemistry (PEC) biosensor was constructed with dual-functional hemin as a signal quencher and electronic mediator for ultrasensitive target microRNA-141 assay with the assistance of T7 exonuclease (Exo)-initiated target amplification technology.

Simple "signal-on" photoelectrochemical aptasensor for ultrasensitive detecting AFB1 based on electrochemically reduced graphene oxide/poly(5-formylindole)/Au nanocomposites

A simple "signal-on" photoelectrochemical (PEC) aptasensor is constructed for Aflatoxin B1 (AFB1) detection based on electrochemically reduced graphene oxide/poly(5-formylindole)/Au (erGO/P5FIn/Au) nanocomposites. The nanocomposites are synthesized by simple electrochemical deposition method and show good photoelectrochemical performance. Poly(5-formylindole) (P5FIn) can generate electron-hole pairs under light irradiation, leading to the formation of robust cathode photocurrent. Au can be acted as signal amplifier due to the high conductivity. The erGO is used to immobilize AFB1 aptamer chain by π-π stacking interaction between the carbon six-membered ring in graphene and the C-N heterocyclic ring in nucleobases of ssDNA. After the insulating AFB1 aptamer chain is fixed to the electrode, the signal of PEC sensor is "OFF". In the process of AFB1 detection, the aptamer chain detaches from the surface of erGO, which results in "ON" of the sensor signal. Based on this design, this constructed PEC aptasensor shows a high sensitivity for AFB1 with a wide linear detection range (LDR) from 0.01 ng mL^-1 to 100 ng mL^-1. The limit of detection (LOD) is 0.002 ng mL^-1. This PEC sensor also exhibits good stability, selectivity, specificity, and satisfactory practical sample analysis ability. This work may provide a new promising PEC platform for AFB1 detection as well as some other small molecules analysis.

Facile Synthesis of Cu2O@TiO2-PtCu Nanocomposites as a Signal Amplification Strategy for the Insulin Detection

Novel ultrasensitive sandwich-type electrochemical immunosensor was proposed for the quantitative detection of insulin, a representative biomarker for diabetes. To this end, molybdenum disulfide nanosheet-loaded gold nanoparticles (MoS₂/Au NPs) were used as substrates to modify bare glassy carbon electrodes. MoS₂/Au NPs not only present superior biocompatible and large specific surface area to enhance the loading capacity of primary antibody (Ab₁) but also present good electrical conductivity to accelerate electron transfer rate. Moreover, the amino-functionalized cuprous oxide decorated with titanium dioxide octahedral composites (Cu₂O@TiO₂-NH₂) were prepared to load dendritic platinum-copper nanoparticles (PtCu NPs) to realize signal amplification strategy. The resultant nanocomposites (cuprous oxide decorated with titanium dioxide octahedral loaded dendritic platinum-copper nanoparticles) demonstrate uniform octahedral morphology and size, which effectively increases the catalytically active sites and specific surface area to load the secondary antibody (Ab₂), even increases conductivity. Most importantly, the resultant nanocomposites possess superior electrocatalytic activity for hydrogen peroxide (H₂O₂) reduction, which present the signal amplification strategy. Under the optimal conditions, the proposed immunosensor exhibited a linear relationship between logarithm of insulin antigen concentration and amperometric response within a broad range from 0.1 pg/mL to 100 ng/mL and a limit detection of 0.024 pg/mL. Meanwhile, the immunosensor was employed to detect insulin in human serum with satisfactory results. Furthermore, it also presents good reproducibility, selectivity, and stability, which exhibits broad application prospects in biometric analysis.

Improving Photovoltaic and Enzymatic Sensing Performance by Coupling a Core-Shell Au Nanorod@TiO2 Heterostructure with the Bioinspired l-DOPA Polymer

The photoelectrochemistry (PEC) performance of TiO₂ is somewhat limited by its wide band gap and low quantum efficiency, and the innovation of its composite materials provides a promising solution for an improved performance. Herein, a composite of a Au nanorod@TiO₂ core-shell nanostructure (AuNR@TiO₂) and a melanin-like l-DOPA polymer (PD) is designed and prepared, where the outer layer PD tethered by TiO₂-hydroxyl complexation and the AuNR core can intensify the long-wavelength light harvesting, and the AuNR@TiO₂ core-shell structure can strengthen the hot-electron transfer to TiO₂. The photocurrent of PD/AuNR@TiO₂ is 8.4-fold improved versus that of commercial TiO₂, and the maximum incident photon-to-electron conversion efficiency reaches 65% in the UV-visible-near-infrared region. In addition, the novel PD/AuNR@TiO₂ photocatalyst possesses the advantages of good biocompatibility and stability, which can act as a versatile PEC biosensing platform for providing a biocompatible environment and improving detection sensitivity. Herein, a PEC enzymatic biosensor of glucose is developed on the basis of the immobilization of dual enzyme [glucose oxidase (GOx) and horseradish peroxidase (HRP)] in PD and the signaling strategy of biocatalytic precipitation. In phosphate buffer containing glucose and 4-chloro-1-naphthol, the HRP-catalyzed oxidation of 4-chloro-1-naphthol by GOx-generated H₂O₂ can form a precipitate on the electrode, by which the decrement of photocurrent intensity is proportional to the common logarithm of glucose concentration. The linear detection range is from 0.05 μM to 10.0 mM glucose, with a limit of detection of 0.01 μM (S/N = 3). Glucose in some human serum samples is analyzed with satisfactory results.

Manganese doped CdS sensitized graphene/Cu2MoS4 composite for the photoelectrochemical immunoassay of cardiac troponin I

As a newly emerged photoactive material, Cu₂MoS₄ has motivated wide research interests in the field of photoelectrochemistry. Based on manganese doped CdS (CdS:Mn) sensitized graphene (G)/Cu₂MoS₄ composite, we developed a label-free photoelectrochemical (PEC) immunosensor for the detection of cardiac troponin I (cTnI). G as an excellent 2D conductive material, combined with Cu₂MoS₄ could improve its charge transfer efficiency. CdS:Mn nanoparticles (NPs) loaded on G/Cu₂MoS₄ further enlarged the light absorption range of Cu₂MoS₄ and restrained the electron-hole pairs recombination. Under optimal conditions, the proposed PEC immunosensor responded sensitively to cTnI with a low detection limit of 0.18 pg/mL and a wide linear range (0.005-1000 ng/mL). Moreover, as-fabricated immunosensor also exhibited high sensitivity, excellent selectivity and good stability. This work also was extended to real samples analysis and obtained satisfied results.

Monitoring matrix metalloproteases based on the selective interaction between an Ir(iii) solvent complex and a histidine-rich polypeptide

A luminescent biosensor was developed for MMP-9 assays based on the selective interaction between an Ir(iii) solvent complex and a histidine-rich peptide.

A peptide-based four-color fluorescent polydopamine nanoprobe for multiplexed sensing and imaging of proteases in living cells

A novel peptide-based four-color fluorescent polydopamine nanoprobe has been developed for multiplexed sensing and imaging of tumor-related proteases in living cells. This nanoprobe responds rapidly and selectively, enabling accurate differentiation between cancer cells and normal cells.