Liposome-amplified photoelectrochemical immunoassay for highly sensitive monitoring of disease biomarkers based on a split-type strategy

A liposome encapsulated methylene blue-mediated electrochemical and UV-visible dual mode split-type immunoassay for the detection of 17β-estradiol

Herein, we fabricated an electrochemical (EC) and UV-visible absorption (UV-vis) dual mode split-type immunoassay for the detection of 17β-estradiol (E2), which was mediated by liposome encapsulated methylene blue (MB@lip). MB molecule acted as the probe in the EC and UV-vis absorption dual mode detections, and its release was controlled by liposome. The competitive immune recognition was conducted between the E2 in the sample and E2 conjugated bovine serum protein (E2-BSA) adsorbed on the 96-wells plate in combining with E2 antibody labeled with MB@lip (E2-Ab/MB@lip). MB molecule could be released from the resulting immune composite of E2-BSA/E2-Ab/MB@lip in the presence of Triton X-100, and quantified by UV-vis and EC methods. The three-dimensional cross-linked reduced graphene oxide/Ti3C2 (3D-rGO/Ti3C2) aerogel was prepared through hydrothermal method, then complexed with the electroactive anthraquinone (AQ) and used as the electrode modified material. The AQ/3D-rGO/Ti3C2 composite had high surface area and provided abundant adsorption sites for MB, and the displacement/competitive behavior between AQ and MB could dexterously achieve the ratiometric EC detection of E2. In addition, the inherent blue color of MB allowed it to be analyzed by UV-vis absorption method. The proposed dual mode detection method exhibited broad linear ranges of 0.1 pg mL-1 to 50 ng mL-1 (by UV-vis) and 0.03 pg mL-1 to 50 ng mL-1 (by EC) for E2 detection, and the detection limits were 0.023 pg mL-1 (S/N = 3) and 8.0 fg mL-1 (S/N = 3), respectively. Moreover, the proposed immunoassay exhibited good practicability and was applied to monitor E2 in milk and serum successfully.

Combining Bioorthogonal Chemistry with Fluorescent Silica Nanoparticles for the Ultrasensitive Detection of the HIV-1 p24 Antigen

Early detection and viral concentration monitoring of human immunodeficiency virus in resource-poor settings are important to control disease spread and reduce mortality. Nucleic acid amplification tests are expensive for low-resource settings. Lateral flow antibody tests are not sensitive if testing is performed within 7-10 days, and these tests are not quantitative. We describe a signal enhancement technique based on fluorescent silica nanoparticles and bioorthogonal chemistries for the femtomolar detection of the HIV-1 p24 antigen. We developed a magnetic bead-based assay, wherein we used fluorescent-dye-encapsulated silica nanoparticles as reporters. The number of reporters was increased by using bioorthogonal chemistry to provide signal enhancement. The limit and range of detection of the sandwich immunoassay using alternating multiple layers for p24 in human serum were found to be 46 fg/mL (1.84 fM) and 46 fg/mL to 10 ng/mL, respectively. This simple assay was 217-fold higher in sensitivity compared to that of commercial enzyme-linked immunoassays (limit of detection of 10 pg/mL).

Ti3C2@UiO-TCPP Schottky junction photoelectrochemical sensor for detecting alkaline phosphatase through the steric hindrance effect of phosphopeptide

Alkaline phosphatase (ALP) is a major biomarker for clinical diagnosis, but detection methods of ALP are limited in sensitivity and selectivity. In this paper, a novel method for ALP determination is proposed. A photoelectrochemical (PEC) sensor was prepared by growing UiO-tetratopic tetrakis (4-carbox-yphenyl) porphyrin (TCPP) in situ between layered Ti3C2 through a one-pot hydrothermal method. The obtained Schottky heterojunction photoelectric material Ti3C2@UiO-TCPP not only has a large light absorption range but also greatly improves the efficiency of photogenerated electron hole separation and thereby enhances sensitivity for PEC detection. The phosphate group on the phosphorylated polypeptide was utilized to form a Zr-O-P bond with the zirconium ion on UiO-66, and then photocurrent decreases due to the steric hindrance effect of phosphorylated polypeptides, that is, the hindrance of electron transfer between the photoelectric material and a solution. The specific interaction between ALP and phosphorylated polypeptides shears the bond between phosphate and zirconium ion on UiO-66 in the peptides then weakens the hindrance effect and increases the photocurrent, thus realizing ALP detection. The linear range of ALP is 0.03-10,000 U·L-1, and the detection limit is 0.012 U·L-1. The method is highly sensitive and selective, and has been applied in detection of ALP in serum samples.

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.

Electrochemical Biosensors for Whole Blood Analysis: Recent Progress, Challenges, and Future Perspectives

Whole blood, as one of the most significant biological fluids, provides critical information for health management and disease monitoring. Over the past 10 years, advances in nanotechnology, microfluidics, and biomarker research have spurred the development of powerful miniaturized diagnostic systems for whole blood testing toward the goal of disease monitoring and treatment. Among the techniques employed for whole-blood diagnostics, electrochemical biosensors, as known to be rapid, sensitive, capable of miniaturization, reagentless and washing free, become a class of emerging technology to achieve the target detection specifically and directly in complex media, e.g., whole blood or even in the living body. Here we are aiming to provide a comprehensive review to summarize advances over the past decade in the development of electrochemical sensors for whole blood analysis. Further, we address the remaining challenges and opportunities to integrate electrochemical sensing platforms.

Ultrasensitive alkaline phosphatase activity assay based on controllable signal probe production coupled with the cathodic photoelectrochemical analysis

A highly sensitive and selective split-type perovskite-based photoelectrochemical (PEC) platform was developed for measuring alkaline phosphatase (ALP) activity in milk and serum samples. ALP in the test sample hydrolyzed 2-phosphate sesquimagnesium salt hydrate (AAPS) in a 96-microwell plate to produce ascorbic acid (AA), a PEC electron donor. The resulting AA, which could preferentially annihilate the photogenerated holes, indirectly reflects ALP activity. The PEC used a cetyltrimethylammonium bromide (CTAB)-functionalized CH₃NH₃PbI₃ (CTAB@CH₃NH₃PbI₃) film as the cathode to monitor the controlled AA production. Due to the excellent photoelectric characteristics of the CH₃NH₃PbI₃ perovskite and the split-type assay, excellent sensitivity and selectivity for ALP detection were obtained. Under the optimum experimental conditions, ALP activity with a limit of detection (LOD) of 2.6 × 10^-4 U/L in a linear dynamic range of 10^-3 ∼ 10² U/L was obtained. With its sensitive, rapid, and high-throughput detection capabilities, this split-type and label-free PEC platform has great potential for use in food and biomedical analysis.

PB@PDA nanocomposites as nanolabels and signal reporters for separate-type cathodic photoelectrochemical immunosensors in the detection of carcinoembryonic antigens

Photoelectrochemical (PEC) immunoassays exhibiting high sensitivity and decent operability have considerable potential in areas such as cancer diagnostics. In particular, cathodic PEC configurations can prevent interference from reductive substances, which can occur in biological samples; however, challenges remain in terms of sensitivity and operability. In this study, separate-type PEC immunoassays were developed for carcinoembryonic antigen (CEA) by combining microplate-based immune recognition and off-on cathodic PEC detection. Polydopamine (PDA)-coated Prussian blue (PB) nanoparticles (PB@PDA NPs) were used as signal tags to label the detection antibody. The PB NPs and PDA captured on the microplates both disassembled under strongly alkaline conditions to generate redox-active electron acceptors. The disassembled products were quantitatively transferred to PEC detection cells and synergistically enhanced the PEC current with microstructured BiOI, which operated as a cathodic semiconductor electrode. As proof of principle, carcinoembryonic antigen (CEA) was applied to elucidate the potential application of PEC immunoassay in clinical diagnosis, and the obtained linear range of the sensor was 0.001-100 ng mL^-1 with the detection limit of 54.9 fg mL^-1 (S/N = 3). The proposed separate-type off-on PEC strategy showed high sensitivity and decent operability for CEA detection, indicating its potential for the identification of other tumor markers.

Nano drug-delivery systems for management of AIDS: liposomes, dendrimers, gold and silver nanoparticles

AIDS causes increasing mortality every year. With advancements in nanomedicine, different nanomaterials (NMs) have been applied to treat AIDS and overcome its limitations. Among different NMs, nanoparticles (NPs) can act as nanocarriers due to their enhanced solubility, sustained release, targeting abilities and facilitation of drug-dose reductions. This review discusses recent advancements in therapeutics for AIDS/HIV using various NMs, mainly focused on three classifications: polymeric, liposomal and inorganic NMs. Polymeric dendrimers, polyethylenimine-NPs, poly(lactic-co-glycolic acid)-NPs, chitosan and the use of liposomal-based delivery systems and inorganic NPs, including gold and silver NPs, are explored. Recent advances, current challenges and future perspectives on the use of these NMs for better management of HIV/AIDS are also discussed.

Catalytic Decomposition of the Hole-Derived H2O2 by AgBiS2@Ag Nanozyme to Enhance the Photocurrent of Z-Scheme BiVO4/ZnIn2S4 Photoelectrode in Microfluidic Immunosensing Platform

A novel microfluidic photoelectrochemical (PEC) analytical device based on AgBiS₂@Ag nanozyme-mediated signal amplification was developed for ultrasensitive detection of cytokeratin 19 fragment 21-1 (CYFRA 21-1). First, a brand new Z-scheme BiVO₄/ZnIn₂S₄ (BZIS) photoactive material was utilized as a sensing matrix to supply a stable photocurrent. Under anodic bias, the photoexcited holes in BiVO₄ could oxidize water to produce hydrogen peroxide (H₂O₂), which markedly enhanced the separation efficiency of the electron-hole pairs. Besides, the Z-scheme heterojunction formed between BiVO₄ and ZnIn₂S₄ further accelerated the transport of the electron. Second, for improving the sensitivity of the PEC sensor, a new strategy of catalytic dissociation of the hole-derived H₂O₂ by AgBiS₂@Ag nanozyme was proposed to amplify the PEC signal. AgBiS₂@Ag composites, possessing an excellent peroxidase-mimicking feature, could efficiently catalyze the H₂O₂ to produce hydroxyl radicals (^•OH) and lead to the significant enhancement of the photocurrent. Third, automatic sample injection and detection were successfully realized by integrating the photoelectrode into microfluidic chips. Based on this advanced sensing strategy, the designed microfluidic PEC sensor displayed a wide linear range (0.1 pg/mL - 100 ng/mL) and a low detection limit of 35 fg/mL (S/N = 3), which could be efficiently applied to the ultrasensitive determination of CYFRA 21-1 in a human serum sample.

CuO Nanozymes as Multifunctional Signal Labels for Efficiently Quenching the Photocurrent of ZnO/Au/AgSbS2 Hybrids and Initiating a Strong Fluorescent Signal in a Dual-Mode Microfluidic Sensing Platform

A novel dual-mode microfluidic sensing platform based on CuO nanozymes as a photoelectrochemical (PEC)-fluorescent (FL) multifunctional signal label was developed for ultrasensitive neuron specific enolase (NSE) detection. Herein, ZnO/Au/AgSbS₂ hybrids, possessing excellent PEC properties, were first exploited as a sensing matrix to provide a stable photocurrent. The controlled synthesis of photoactive ZnO nanoflowers (NFs) was successfully conducted using a microfluidic reactor in the scale of seconds. Furthermore, the photocurrent of ZnO NFs decorated by Au and AgSbS₂ nanoparticles significantly improved, owing to the local surface plasma resonance effect of Au and matching band structure between ZnO and AgSbS₂. A strategy of catalytic oxidation ascorbic acid (AA) by CuO nanozymes was proposed to quench the PEC signals and initiate FL signals. CuO nanoparticles growing on conductive carbon spheres (CuO@CSs) as secondary antibodies' labels could efficiently catalyze the oxidation of AA to achieve a PEC "signal-off" state. Then, the produced dehydroascorbic acid reacting with o-phenylenediamine opportunely generated a strong FL signal. Importantly, wide linear ranges of 0.0001-150 ng/mL for the PEC technique and 0.001-150 ng/mL for the FL method with a low detection limit of 0.028 and 0.25 pg/mL, respectively, could guarantee the sensitive detection of NSE.

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

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

Ascorbic acid-mediated organic photoelectrochemical transistor sensing strategy for highly sensitive detection of heart-type fatty acid binding protein

Heart-type fatty acid binding protein (H-FABP) has been regarded as a promising biomarker for early diagnosis of acute myocardial infarction (AMI). Developing fast and reliable method for H-FABP detection is still highly desirable but challenging. Herein, an ascorbic acid (AA)-mediated organic photoelectrochemical transistor (OPECT) sensing strategy was reported for the detection of H-FABP in phosphate buffer saline (PBS) solution and human serum. A primary antibody/H-FABP/secondary antibody-Au NPs-alkaline phosphatase (ALP) sandwich immunorecognition structure was constructed. The modified ALP could catalytically convert ascorbic acid-2-phosphate to AA, which was then analyzed by OPECT. As a result, the AA-mediated OPECT sensing strategy realized highly sensitive detection of H-FABP with a detection limit of 3.23 × 10^-14 g/mL which is two orders of magnitude lower than that of PEC method. Under optimal experimental conditions, H-FABP concentration could be obtained in ∼90 min. Importantly, the analysis of H-FABP was resistant to the interference from immunoglobulin G, bovine serum albumin, cysteine, AA and human serum. The proposed AA-mediated OPECT sensing strategy provides a simple, fast, and accurate way for H-FABP detection in AMI suspected patients.

Ultrasensitive detection of vitamin E by signal conversion combined with core-satellite structure-based plasmon coupling effect

A smart signal conversion and amplification strategy based on silver–gold–silica core-satellite structure nanoparticles to sensitively SERS detect vitamin E.

A versatile and ultrasensitive molecularly imprinted electrochemiluminescence sensor with HRP-encapsulated liposome labeled by light-triggered click reaction for pesticide residues

A novel approach for trace detection of fipronil with a molecularly imprinted electrochemiluminescence sensor (MIECLS) is proposed. The sensitivity is significantly improved via signal amplification of the enzymatic reaction of horseradish peroxidase (HRP) released from encapsulated liposomes which linked onto the template molecules after rebinding. The molecularly imprinted polymer membrane was prepared through the electropolymerization of monomers with fipronil as a template. After the elution of the template molecules, the analyte fipronil was reabsorbed into the cavities. HRP-encapsulated liposomes were linked to the target molecules by light-triggered click reaction. The higher the concentration of the target was, the more HRP-encapsulated liposomes were present on the molecularly imprinted polymer (MIP) sensor. Then, HRP was liberated from liposomes, and the catalytic degradation of hydrogen peroxide (H₂O₂) by HRP occurs, which changed the electrochemiluminescence intensity of luminol significantly. The change of the ∆ECL intensity was linearly proportional to the logarithm of the fipronil concentration ranging from 1.00 × 10^-14 to 1.00 × 10^-9 mol/L, and the detection limit was 7.77 × 10^-16 mol/L. The recoveries obtained ranged from 95.7 to 105.8% with RSD < 5%. The sensitivity of the detection was significantly improved, and the analysis process was simplified in that the incubation step required in the conventional method was avoided. The sensor proposed provides a feasible platform for ultra-trace amount determination.

Fluorescence and visual immunoassay of HIV-1 p24 antigen in clinical samples via multiple selective recognitions of CdTe QDs

Human immunodeficiency virus (HIV) infection inflicts significant economic and social burdens on many countries worldwide. Given the substantial morbidity and mortality from HIV infection, there is an urgent need for accurate and early detection of the virus. In this study, immunofluorescence and visual techniques are described that detect the HIV-1 p24 antigen, which relied on selective recognition of Ag⁺/Ag nanoparticles (Ag NPs) and Cu²⁺/Cu⁺ using cadmium telluride quantum dots (CdTe QDs). After the sandwich immunoreactions were accomplished, the alkaline phosphatase (ALP) hydrolyzed L-ascorbic acid 2-phosphate (AAP) to form ascorbic acid (AA) that further reduces Ag⁺ and Cu²⁺ to Ag NPs and Cu⁺, respectively. This method was highly sensitive and selective and could detect as low as 1 pg/mL of p24 antigen by naked eyes and had a good linearity in the concentration range 1-100 pg/mL. When using Ag⁺ and Cu²⁺ as media, the limit of detection (LOD) of the new method was 0.3 pg/mL and 0.2 pg/mL, respectively. Compared with clinical electrochemiluminescence immunoassay (ECLIA) results and clinical data, this method demonstrated good consistency for the quantification of HIV-1 p24 antigen in 34 clinical serum samples. In addition, this method could accurately distinguish HIV from other viruses and infections such as hepatitis B virus, systemic lupus erythematosus, hepatitis C virus, Epstein-Barr virus, cytomegalovirus, lipemia, and hemolysis. Therefore, our dual-mode analysis method may provide additional solutions to identify clinical HIV infection. An immunofluorescence and visualization dual-mode strategy for the detection of p24 antigen was constructed based on immune recognition reaction and a phenomenon that cadmium telluride quantum dots (CdTe QDs) can selectively recognize Ag⁺/Ag nanoparticles (Ag NPs) and Cu²⁺/Cu⁺.

Recent advances in enzyme-enhanced immunosensors

Among the products for rapid detection in different fields, enzyme-based immunosensors have received considerable attention. Recently, great efforts have been devoted to enhancing the output signals of enzymes through different strategies that can significantly improve the sensitivity of enzyme-based immunosensors for the need of practical applications. In this manuscript, the significance of enzyme-based signal transduction patterns in immunoassay and the central role of enzymes in achieving precise control of reaction systems are systematically described. In view of the rapid development of this field, we classify these strategies based on the combination of immune recognition and enzyme amplification into three categories, namely enzyme-based enhancement strategies, combination of the catalytic amplification of enzymes with other signal amplification methods, and substrate-based enhancement strategies. The current focus and future direction of enzyme-based immunoassays are also discussed. This article is not exhaustive, but focuses on the latest advances in different signal generation methods based on enzyme-initiated catalytic reactions and their applications in the detection field, which could provide an accessible introduction of enzyme-based immunosensors for the community with a view to further improving its application efficiency.

Rolling Circle Amplification as an Efficient Analytical Tool for Rapid Detection of Contaminants in Aqueous Environments

Environmental contaminants are a global concern, and an effective strategy for remediation is to develop a rapid, on-site, and affordable monitoring method. However, this remains challenging, especially with regard to the detection of various contaminants in complex water environments. The application of molecular methods has recently attracted increasing attention; for example, rolling circle amplification (RCA) is an isothermal enzymatic process in which a short nucleic acid primer is amplified to form a long single-stranded nucleic acid using a circular template and special nucleic acid polymerases. Furthermore, this approach can be further engineered into a device for point-of-need monitoring of environmental pollutants. In this paper, we describe the fundamental principles of RCA and the advantages and disadvantages of RCA assays. Then, we discuss the recently developed RCA-based tools for environmental analysis to determine various targets, including heavy metals, organic small molecules, nucleic acids, peptides, proteins, and even microorganisms in aqueous environments. Finally, we summarize the challenges and outline strategies for the advancement of this technique for application in contaminant monitoring.

A photocurrent-polarity-switching biosensor for highly selective assay of mucin 1 based on target-induced hemin transfer from ZrO2 hollow spheres to G-quadruplex nanowires

Herein, a photocurrent polarity switching platform for highly selective assay of mucin 1 (MUC1) was developed based on target-induced hemin transfer from ZrO₂ hollow spheres (ZrO₂ HSs) to G-quadruplex nanowires (G wires). In this system, SiO₂ spheres were used as templates to synthesize the uniform and mesoporous ZrO₂ HSs. As nanocontainers, ZrO₂ HSs could load hemin in its cavity via pores. Then, the aptamers of MUC1, as bio-gates, blocked the pores of ZrO₂ HSs based on the specific binding of Zr⁴⁺ and the phosphate groups of aptamer. In the presence of MUC1, the aptamer could specifically recognize and bind with MUC1, and then leave away from the surface of ZrO₂ HSs, which resulted in the opening of the bio-gates and releasing of hemin. Assisted with the G wires formed on the Au NPs/In₂S₃/ITO, the released hemin was captured on the electrode through the formation of hemin/G-quadruplex structure, leading to the switch of the photocurrent polarity of the electrode from anodic photocurrent to cathodic photocurrent. The proposed photoelectrochemical biosensor showed outstanding performance for MUC1 assay with high selectivity, wide linear response range (1 fg mL^-1 -10 ng mL^-1) and lower detection limit (0.48 fg mL^-1). And the strategy could be easily extended to a triple-mode detection of MUC1 because the hemin/G-quadruplex structure was widely used in electrochemical and colorimetric methods as a hydrogen peroxide mimetic enzyme, which might provide wide applications in biological or clinical studies.

Liposome-assisted enzyme catalysis: toward signal amplification for sensitive split-type electrochemiluminescence immunoassay

Developing an efficient signal amplification strategy is very important to improve the sensitivity of bioanalysis. In this paper, a liposome-assisted enzyme catalysis signal amplification strategy was developed for electrochemiluminescence (ECL) immunoassay of prostate specific antigen (PSA) in a split-type mode. The sandwich immunoreaction occurred in a 96-well plate, and glucose oxidase (GOx) encapsulated and antibody-modified liposomes were used as labels. The ECL detection was carried out using a rGO-Au NP modified glassy carbon electrode (GCE). The large amount of generated H2O2, i.e. the coreactant of the luminol system, and the excellent catalytic behavior of rGO-Au NPs greatly boosted the ECL signal, resulting in the signal amplification. The developed ECL immunosensor for detecting PSA achieved a wider linear range from 1.0 × 10-13 to 1.0 × 10-8 g mL-1 and a detection limit of 1.7 × 10-14 g mL-1. The application of the proposed strategy was demonstrated by analyzing PSA in human serum samples with recoveries from 89.0% to 113.0%, and relative standard deviations (RSDs) were less than 6.6%. This work provides a new horizon to expand the application of liposomes for ECL bioanalysis.

A novel split-type photoelectrochemical immunosensor based on chemical redox cycling amplification for sensitive detection of cardiac troponin I

Photoelectrochemical (PEC) immunoassay is a burgeoning and promising bioanalytical method. However, the practical application of PEC still exist some challenges such as the inevitable damage of biomolecules caused by the PEC system and the unsatisfactory sensitivity for biomarkers with low abundance in real sample. To solve the problems, we integrated the cosensitized structure of Ag2S/ZnO nanocomposities as photoelectrode with photogenerated hole-induced chemical redox cycling amplification (CRCA) strategy to develop a split-type PEC immunosensor for cardiac troponin I (cTnI) with high sensitivity. Initially, the immunoreaction was carried out on the 96-well plates in which alkaline phosphatase (ALP) could catalyze ascorbic acid 2-phosphate (AAP) to generate the signal-reporting species ascorbic acid (AA). Subsequently, the AA participated and the tris (2-carboxyethyl) phosphine (TCEP) mediated chemical redox cycling reaction took place on the photoelectrode, thus leading to signal amplification. Under the optimized conditions, the immunosensor demonstrated a detection limit (LOD) of 3.0 × 10^-15 g mL^-1 with a detection range of 1.0 × 10^-14 g mL^-1 to 1.0 × 10^-9 g mL^-1 for cTnI. Impressively, the proposed method could determine the cTnI in human serum samples with high sensitivity and satisfactory accuracy. Considering the virtues of the photoelectrode and the chemical redox cycling strategy, the method would hold great potential for highly sensitive biosensing and bioanalysis.

"Dual Signal-On" Split-Type Aptasensor for TNF-α: Integrating MQDs/ZIF-8@ZnO NR Arrays with MB-Liposome-Mediated Signal Amplification

Ultrasensitive and accurate detection of biomarkers in serum is of great importance for disease diagnosis and treatment. So far, the commonly used single-mode signal suffers from certain instinct drawbacks that restrict assay performances. Herein, we report the proof-of-concept fabrication of a split-type photoelectrochemical (PEC) and electrochemical (EC) dual-modal aptasensor for ultrasensitively tracing tumor necrosis factor-α, a noteworthy biological biomarker with essential clinical importance. By smart integrating molybdenum disulfide QDs/zeolitic imidazolate framework-8@ZnO nanorod arrays with a methylene blue-liposome-mediated signal amplification strategy, "dual signal-on" detection is accomplished based on a sandwich reaction of the target with aptamer-anchored carboxyl magnetic beads and an aptamer-confined MB liposome. Linear ranges of 5 fg/mL-5 μg/mL (detection limit 1.46 fg/mL) for PEC and 10 fg/mL-0.5 μg/mL (detection limit 6.14 fg/mL) for EC are obtained, respectively. An independent signal transduction mechanism supports the accuracy improvement, and a separate biological process from a translator enables convenient fabrication, short-time consumption, wider linearity, as well as outstanding reproducibility and stability in practical application. This work presents a universal bioassay route with prospects in biomedical and related areas.

Nanozyme-Activated Synergistic Amplification for Ultrasensitive Photoelectrochemical Immunoassay

At present, enzyme-mediated signal amplification strategies have been widely applied in photoelectrochemical (PEC) biosensing systems, while the introduction of natural enzymes onto the surface of photoelectrodes inevitably obstructs the electron transfer due to their insulating properties as proteins, leading to severe damage to photocurrent. In this work, the PdPt bimetallic nanozymes with the efficient peroxidase-like activity were used as alternatives to natural enzymes and amplified PEC biosensing signals via their efficient enzymatic reaction and remarkable enhancement in photocurrent. As a result, photoactive CdS nanorods modified with PdPt bimetallic nanozymes showed a boosted PEC performance compared with the pristine CdS nanorods due to the localized surface plasmon resonance effect and Schottky junction. On the basis of the as-prepared CdS/PdPt photoelectrode, a sensitive split-type glucose oxidase-mediated PEC immunoassay for carcinoembryonic antigen (CEA) detection was successfully constructed. Along with the sandwich immunocomplexing, the subsequently produced hydrogen peroxide (H₂O₂) can oxidize 4-chloro-1-naphthol into insoluble precipitates to inhibit photocurrent and simultaneously trigger the bio-etching of CdS to further restrain photocurrent signals due to the excellent peroxidase-mimicking activity of PdPt nanozymes. Owing to the synergistic signal amplification fulfilled by PdPt nanozymes, an ultrasensitive immunoassay of CEA was realized with a wider linear range from 1 to 5000 pg/mL and a low detection limit of 0.21 pg/mL, opening a new avenue for building ultrasensitive PEC biosensors with nanozymes.

Poly(amino acid) Multilayers Modified Dendritic Mesoporous Silica Nanoparticles Achieve Effective Enzyme Stability for Ultrasensitive Immunoassay

Enzyme-linked immunosorbent assay (ELISA) is one of the most common techniques in biomedical detection; however, the poor sensitivity in early diagnosis for some diseases seriously limits its application. In this work, we developed an ultrasensitive ELISA system that is based on horseradish peroxidase (HRP)-loaded dendritic mesoporous silica nanoparticles (DMSN) modified with poly(amino acid) multilayers (defined as DSHP). A large amount of HRP adsorption was achieved in center-radial mesoporous channels of DMSN because of the high specific surface area and large pore size, leading to significant signal amplification. Additionally, DSHP could not only effectively maintain HRP activity for at least 10 days but also provide preferable protection for HRP activity even at high temperatures or a wide pH range. Moreover, the DSHP system exhibited admirable signal amplification performance with a limit of detection of 0.667 fM and a wide detectable range from 6.67 × 10^-4 to 6.67 × 10⁵ pM, whose sensitivity was 10⁴ times higher than that of the conventional ELISA. We believe that the DSHP will offer a new strategy for signal amplification of the ELISA system in clinical diagnosis.

Cationic liposome-triggered luminol chemiluminescence reaction and its applications

Liposomes are spherical phospholipid bilayer vesicles. In the present study, we found that cationic liposomes made by (2,3-dioleoyloxy-propyl)-trimethylammonium (DOTAP) could enhance the luminol-H₂O₂ chemiluminescence (CL) reaction. Mechanism studies showed that the positive charge on the surface of liposomes plays an important role in the CL process. We speculated that the cationic liposomes with quaternary ammonium groups on the surface may be capable of catalyzing the decomposition of H₂O₂ leading to the formation of oxygen-related free radicals including ˙OH, ¹O₂, and O₂˙^-. The luminol anions tend to move close to the surface of the cationic liposomes and then to be oxidized by the oxidizing radical species which may be around the surface of cationic liposomes forming excited-state 3-aminophthalate* (3-APA*). When the 3-APA* returns to the ground state, an enhanced CL is observed. In addition, the single-strand DNA (ssDNA) showed a significant inhibition effect on the proposed CL reaction. The CL intensity decreased linearly with an increasing amount of DNA from 0.05 to 2 pmol. We assumed that the binding of ssDNA with cationic liposomes would neutralize the positive charge on the surface of liposomes and inhibit the catalytic activity of DOTAP cationic liposomes. Based on the ssDNA-inhibited luminol-H₂O₂-cationic liposome CL reaction, simple label-free CL sensing platforms were developed for the detection of sequence-specific DNA related to the hepatitis B virus (HBV) gene and for the detection of ATP (as a model analyte) using an anti-ATP aptamer as the recognition element.

Boosting the biocatalytic precipitation with enzyme-loaded liposomes: Toward a general platform for amplified photoelectrochemical immunoassay

Liposome-assisted photoelectrochemical (PEC) bioanalysis represents one of the latest frontiers in the arena of PEC bioanalysis. This work reports a general enzyme-amplified liposomal PEC bioanalysis protocol via the use of enzyme-loaded liposomes to boost the biocatalytic precipitation (BCP) effect. In the representative system, the horseradish peroxidase (HRP)-loaded liposome (HRPLL) and the Au nanoclusters (NCs)/Au nanoparticles (NPs)/TiO₂ nanotubes (NTs) framework (AATF) were used as liposomal label and photoelectrode, respectively. In the detection, the sandwich immunocomplex reaction was accomplished in a 96-well plate to confine the HRPLL label, which was then lysed to release the HRP molecules to initiate the BCP process. Due to the amplified formation of HRP-induced BCP on the AATF scaffold, the photo-current response correlated closely with the immunorecognition process and the analyte could be detected very sensitively. This work features the first integration of enzyme-loaded liposomes and the BCP for sensitive PEC bioanalysis, which to our knowledge has not been reported. With the use of various other enzymes, this work could serve as a general basis for the PEC bioanalysis of numerous other target of interest.

Ti3C2 MXene quantum dot-encapsulated liposomes for photothermal immunoassays using a portable near-infrared imaging camera on a smartphone

Methods based on the photothermal effect (a common phenomenon in nature) have been widely applied in different fields; however, their application in bioanalysis has lagged behind. Herein, we designed a near-infrared (NIR) photothermal immunoassay for the qualitative or quantitative detection of prostate-specific antigen (PSA) using titanium carbide (Ti₃C₂) MXene quantum dot (QD)-encapsulated liposomes with high photothermal efficiency. This system involves a sandwich-type immunoreaction and photothermal measurements. Ti₃C₂ MXene QDs were utilized as innovative photothermal signal beacons and were encapsulated in liposomes for the labeling of the secondary antibody. The assay was carried out by coupling a low-cost microplate with a homemade 3D printed device. Under NIR-laser irradiation, the Ti₃C₂ MXene QDs converted the light energy into heat, and a shift in temperature corresponding with the analyte concentration was obtained on a handheld thermometer. Under optimal conditions, the Ti₃C₂ MXene QD-based photothermal immunoassay exhibited a dynamic linear range from 1.0 ng mL^-1 to 50 ng mL^-1 with a limit of detection of 0.4 ng mL^-1 for PSA detection. Also, we constructed portable equipment using a portable near-infrared imaging camera to collect visual thermal data for the semi-quantitative analysis of the target PSA within 3 min. The specificity, reproducibility and accuracy of the photothermal immunoassay were acceptable. Importantly, our strategy opens new opportunities for protein point-of-care (POC) testing and biosecurity diagnostics.

Multifunctional Peroxidase-Encapsulated Nanoliposomes: Bioetching-Induced Photoelectrometric and Colorimetric Immunoassay for Broad-Spectrum Detection of Ochratoxins

In this study, a versatile dual-modal readout immunoassay platform was achieved for sensitive and broad-spectrum detection of ochratoxins based on the photocurrent response of flexible CdS/ZnO nanorod arrays/reduced graphene oxide and the localized surface plasmon resonance (LSPR) peak shift of Au nanobipyramids (Au NBPs). By using nanoliposomes as the vehicle to carry the secondary antibody and encapsulate horseradish peroxidase (HRP), the photocurrent change and the peak shift can be greatly amplified. The reaction mechanism was investigated in detail, indicating that HRP can trigger enzymatic bioetching in the presence of H₂O₂. In the photoelectrochemical detection, the oxidized HRP can etch CdS on the photoelectrode, resulting in the photocurrent change, while in the colorimetric detection, HRP can oxidize H₂O₂ to produce hydroxyl radicals that can etch Au NBPs to form multiple color changes and LSPR shifts. Compared with the common single-modal immunoassay for ochratoxins, such dual-modal immunoassay is more precise and reliable, owing to the completely independent signal conversion and transmission mechanism. Therefore, we hope that this accurate, simple, and visualized strategy may create a new avenue and provide innovative inspiration for food analysis.

Disposable Sensors in Diagnostics, Food, and Environmental Monitoring

Disposable sensors are low-cost and easy-to-use sensing devices intended for short-term or rapid single-point measurements. The growing demand for fast, accessible, and reliable information in a vastly connected world makes disposable sensors increasingly important. The areas of application for such devices are numerous, ranging from pharmaceutical, agricultural, environmental, forensic, and food sciences to wearables and clinical diagnostics, especially in resource-limited settings. The capabilities of disposable sensors can extend beyond measuring traditional physical quantities (for example, temperature or pressure); they can provide critical chemical and biological information (chemo- and biosensors) that can be digitized and made available to users and centralized/decentralized facilities for data storage, remotely. These features could pave the way for new classes of low-cost systems for health, food, and environmental monitoring that can democratize sensing across the globe. Here, a brief insight into the materials and basics of sensors (methods of transduction, molecular recognition, and amplification) is provided followed by a comprehensive and critical overview of the disposable sensors currently used for medical diagnostics, food, and environmental analysis. Finally, views on how the field of disposable sensing devices will continue its evolution are discussed, including the future trends, challenges, and opportunities.

Nanoporous Semiconductor Electrode Captures the Quantum Dots: Toward Ultrasensitive Signal-On Liposomal Photoelectrochemical Immunoassay

Liposomal photoelectrochemical (PEC) bioanalysis has recently emerged and exhibited great potential in sensitive biomolecular detection. Exploration of the facile and efficient route for advanced liposomal PEC bioanalysis is highly appealing. In this work, we report the split-type liposomal PEC immunoassay system consisting of sandwich immunorecognition, CdS quantum dots (QDs)-loaded liposomes (QDLL), and the release and subsequent capture of the QDs by a separated TiO₂ nanotubes (NTs) electrode. The system elegantly operated upon the protein binding and lysis treatment of CdS QDLL labels within the 96-well plate, and then the CdS QDs-enabled sensitization of TiO₂ NTs electrode. Exemplified by cardiac markers troponin I (cTnI) as target, the proposed system achieved efficient activation of TiO₂ NTs electrode and thus the signal generation toward the split-type PEC immunoassay. This work features the first use of QDs for liposomal PEC bioanalysis and is expected to inspire more interests in the design and implementation of numerous QDs-involved liposomal PEC bioanalysis.

A 3D printing-based portable photoelectrochemical sensing device using a digital multimeter

An enzyme-free photoelectrochemical sensing method based on a 3D-printing device was developed for CEA detection coupling glucose-encapsulated liposomes with digital multimeter readout.

Cu2+-Modulated in situ growth of quantum dots for split-type photoelectrochemical immunoassay of prostate-specific antigen

A split-type photoelectrochemical immunosensor of PSA was developed using Cu²⁺-dependent catalytic oxidation for inhibiting the in situ growth of CdS QDs.

A target-responsive liposome activated by catalytic hairpin assembly enables highly sensitive detection of tuberculosis-related cytokine

Here, we propose a new fluorescence method to detect tuberculosis-related cytokine by using a target-responsive liposome activated by catalytic hairpin assembly. The method combines a DNA self-assembly based amplification process with a liposome-based signal amplification process, therefore offering a very high sensitivity.

Dual-Modal Split-Type Immunosensor for Sensitive Detection of Microcystin-LR: Enzyme-Induced Photoelectrochemistry and Colorimetry

Microcystins, the lethal cyanotoxins from Microcystis aeruginosa, can inhibit the activity of protein phosphatase and promote liver tumors. Herein, a dual-modal split-type immunosensor was constructed to detect microcystin-LR (MC-LR), based on the photocurrent change of CdS/ZnO hollow nanorod arrays (HNRs) and the blue shift of the surface plasmon resonance peak from Au nanobipyramids@Ag. By using mesoporous silica nanospheres as the carrier to immobilize secondary antibody and DNA primer, a hybridization chain reaction was adopted to capture alkaline phosphatase, while its catalytic reaction product, ascorbic acid, exhibited dual functions. The detailed mechanism was investigated, showing that ascorbic acid can not only act as the electron donor to capture the holes in CdS/ZnO-HNRs, leading to the increase photocurrent, but also as the reductant to form silver shells on Au nanobipyramids, generating multiply vivid color variations and blue shifts. Compared with the traditional photoelectrochemical immunosensor or colorimetric method for MC-LR, a more accurate and reliable result can be obtained, due to different mechanisms and independent signal transduction. Therefore, this work can not only propose a new dual-modal immunosensor for MC-LR detection but also provide innovative inspiration for constructing sensitive, accurate, and visual analysis for toxins.