Enzyme-Encapsulated Liposome-Linked Immunosorbent Assay Enabling Sensitive Personal Glucose Meter Readout for Portable Detection of Disease Biomarkers

New sensing methods using commercially available products: Based on PGM and PTS

In recent years, detection technology has made remarkable progress in the field of food safety, in vitro diagnosis, and environment monitoring under the impetus of trace substances detection requirements. However, in sharp contrast to the rapid development of detection technology, its marketization process is relatively lagging behind. One possible approach is to integrate novel sensing strategies with mature commercial devices, such as personal glucose meters (PGMs) and pregnancy test strips (PTS) to speed up their marketization process. In this review, we systematically summarized design principle, evolution, and application progress for the integration of novel sensing strategies with commercial devices PGMs and PTS. Meanwhile, key factors and difficulties for the integration novel sensing strategies with commercial devices were emphasized. More importantly, the future of prospects and remaining challenges were discussed.

Microvolumetric determination of thrombomodulin based on competitive immunoreaction using a portable glucometer

A new method of reducing the amount of reagent and sample for determination of thrombomodulin (TM) was developed based on competitive immunoreaction using a portable glucometer (PGM). Two types of nanocomposites, TM protein-modified magnetic nanoparticles (MNPs-TM) and TM antibody-/glucose oxidase-modified gold nanoparticles (Ab-GNPs-GOx), were prepared. Their binding product, MNPs-TM-Ab-GNPs-GOx, in the microvolumetric solution was used to catalyze the oxidation of glucose, leading to a decline of the glucose content. The TM-involved competitive immunoreaction had a negative effect on the generation of MNPs-/GNPs-based nanocomposites and inhibited the catalytic oxidation of glucose. The glucose content difference in the microvolumetric solution, which was revealed by a PGM, was in proportion to the logarithm of the TM concentration from 25 ng mL-1 to 2.5 μg mL-1. The limit of detection was 5.7 ng mL-1. Microvolumetric solution and a PGM were used in the measurement, which overcame some deficiencies of classical methods in chemo/biosensing, for example, special instrument, complicated measurement procedure, and high cost.

Specific surface modification of liposomes for gut targeting of food bioactive agents

Liposomes have become a research hotspot in recent years as food delivery systems with attractive properties, including the bilayer structure assembled like the cell membrane, reducing the side-effect and improving environmental stability of cargos, controlling release, extending duration of functional ingredients, and high biodegradable and biocompatible abilities in the body. However, the conventional liposomes lack stability during storage and are weak in targeted absorption in the gastrointestinal track. At present, surface modification has been approved to be an effective platform to shield these barricades and help liposomes deliver the agents safely and effectively to the ideal site. In this review, the gastrointestinal stability of conventional liposomes, cargo release models from liposomes, and the biological fate of the core materials after release were emphasized. Then, the strategies in both physical and chemical perspectives to improve the stability and utilization of liposomes in the gastrointestinal tract, and the emerging approaches for improving gut targeting by specifically modified liposomes and the intestinal receptors relative to liposomes/cargos absorption were highlighted. Last but not the least, the safety, challenges, and opportunities for the improvement of liposomal bioavailability were also discussed to inspire new applications of liposomes as oral carriers.

A liposome-based aptasensor integrated with competitive reaction enabling portable and electrochemical detection of Aβ oligomer

Aggregation of β-amyloid (Aβ) were considered as a typical pathological feature of Alzheimer's disease (AD). Extensive studies have verified that soluble Aβ oligomers (AβO) were more toxic to neurons than plaques. Herein, in this work, a glucose entrapped liposome-based portable aptasensor was fabricated for recognizing and interacting with AβO by specific aptamer on liposome (G-Lip-Apt). Then, a single strand DNA, designed to be partially complementary to AβO aptamer, was modified on amino-functionalized Fe₃O₄@SiO₂ to obtain a magnetic nanocomposite (Fe₃O₄@SiO₂/NH₂-DNA). In the presence of AβO, the specific recognition between AβO and its aptamer on G-Lip-Apt made AβO bounded with G-Lip-Apt. With subsequent introduction of Fe₃O₄@SiO₂/NH₂-DNA, the unreacted G-Lip-Apt was further linked with Fe₃O₄@SiO₂/NH₂-DNA by double stranded complementary pairing interaction. Along with the addition of TritonX-100 into the formed G-Lip-Apt/Fe₃O₄@SiO₂/NH₂-DNA complex, the encapsulated glucose was released from liposome and then measured by a personal glucose meter (PGM). Good linear correlation was acquired over concentration of 5.0-1000 nM and the limit of detection (LOD) was calculated to be 2.27 nM for AβO. The developed portable electrochemical strategy integrated magnetic separation, competitive reaction and point of care test (POCT) to achieve high sensitivity, selectivity and accuracy, therefore enabled it successfully applied to the analysis of AβO in the hippocampus and cortex of APP/PS1 transgenic AD mice.

Personal Glucose Meters Coupled with Signal Amplification Technologies for Quantitative Detection of Non-Glucose Targets: Recent Progress and Challenges in Food Safety Hazards Analysis

Ensuring food safety is paramount worldwide. Developing effective detection methods to ensure food safety can be challenging owing to trace hazards, long detection time, and resource-poor sites, in addition to the matrix effects of food. Personal glucose meter (PGM), a classic point-of-care testing device, possesses unique application advantages, demonstrating promise in food safety. Currently, many studies have used PGM-based biosensors and signal amplification technologies to achieve sensitive and specific detection of food hazards. Signal amplification technologies have the potential to greatly improve the analytical performance and integration of PGMs with biosensors, which is crucial for solving the challenges associated with the use of PGMs for food safety analysis. This review introduces the basic detection principle of a PGM-based sensing strategy, which consists of three key factors: target recognition, signal transduction, and signal output. Representative studies of existing PGM-based sensing strategies combined with various signal amplification technologies (nanomaterial-loaded multienzyme labeling, nucleic acid reaction, DNAzyme catalysis, responsive nanomaterial encapsulation, and others) in the field of food safety detection are reviewed. Future perspectives and potential opportunities and challenges associated with PGMs in the field of food safety are discussed. Despite the need for complex sample preparation and the lack of standardization in the field, using PGMs in combination with signal amplification technology shows promise as a rapid and cost-effective method for food safety hazard analysis.

Nanotechnology‐based electrochemical biosensors for monitoring breast cancer biomarkers

Breast cancer is categorized as the most widespread cancer type among women globally. On-time diagnosis can decrease the mortality rate by making the right decision in the therapy procedure. These features lead to a reduction in medication time and socioeconomic burden. The current review article provides a comprehensive assessment for breast cancer diagnosis using nanomaterials and related technologies. Growing use of the nano/biotechnology domain in terms of electrochemical nanobiosensor designing was discussed in detail. In this regard, recent advances in nanomaterial applied for amplified biosensing methodologies were assessed for breast cancer diagnosis by focusing on the advantages and disadvantages of these approaches. We also monitored designing methods, advantages, and the necessity of suitable (nano) materials from a statistical standpoint. The main objective of this review is to classify the applicable biosensors based on breast cancer biomarkers. With numerous nano-sized platforms published for breast cancer diagnosis, this review tried to collect the most suitable methodologies for detecting biomarkers and certain breast cancer cell types.

A Simple and Universal Nucleic Acid Assay Platform Based on Personal Glucose Meter Using SARS-CoV-2 N Gene as the Model

A simple, selective, and quantitative platform for point-of-care diagnostic of COVID-19 is urgently needed as a complement in areas where resources are currently relatively scarce. To meet the needs of early diagnosis and intervention, a proof-of-concept demonstration of a universal personal glucose meter-based nucleic acid assay platform (PGM-NAAP) is presented, which converts to SARS-CoV-2 detection from glucose detection. By using magnetic bead separation together with the hand-held PGM for quantitative readout, PGM-NAAP achieves the 98 pM limit of detection for a sequence related to SARS-CoV-2. The ability to discriminate target nucleic acid from genomic DNA, the satisfactory spike recoveries of saliva and serum samples, as well as the good stability all together suggest the potential of the PGM-NAAP for the screening and diagnosis of suspected patients during the outbreaks of COVID-19 in resource-limited settings without sophisticated instruments. On the basis of these findings, PGM-NAAP can be expected to provide an accurate and convenient path for diagnosis of disease-associated nucleic acid.

A polypyrrole-mediated photothermal biosensor with a temperature and pressure dual readout for the detection of protein biomarkers

A novel photothermal biosensor with a temperature and pressure dual readout was developed for CRP detection. The in situ synthesized polypyrrole exhibits photothermal effect under NIR light to increase temperature and pressure for portable readout.

Dual-recognition colorimetric sensing of thrombin based on surface-imprinted aptamer-Fe3O4

Thrombin plays an essential role in blood coagulation and some physiological and pathological processes. The convenient, rapid, sensitive, and specific detection of thrombin is of great significance in clinical research and diagnosis. Herein, surface molecularly imprinted polymer (MIP) was modified on aptamer-functionalized Fe3O4 nanoparticles (MIP-aptamer-Fe3O4 NP) for thrombin colorimetric assay by taking advantage of the peroxidase-like activity of Fe3O4 NP. With the adsorption of thrombin into imprinted cavities, the exposed surface area of Fe3O4 NP decreased, causing a decrease in its peroxidase-like activity toward 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H2O2. On the other hand, the reductive amino acids on the thrombin surface also impeded the oxidation of TMB. Both phenomena caused the light blue color of the sensing solution. Thus, a specifically sensitive colorimetric approach for the visual detection of thrombin was proposed with a linear range and limit of detection of 108.1 pmol L-1-2.7 × 10-5 mol L-1 and 27.8 pmol L-1, respectively. Moreover, due to the double recognition elements of MIP and aptamer, the prepared MIP-aptamer-Fe3O4 NP showed higher selectivity to thrombin than that based on only one recognition element. It is worth noting that no special property (e.g. electrochemical or fluorescence activity) of the template was required in this work. Thus, more template molecules can be easily, selectively, and sensitively detected based on the proposed MIP-aptamer-mimic enzyme colorimetric sensing strategy.

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.

Recent progress of personal glucose meters integrated methods in food safety hazards detection

Development of personal glucose meters (PGMs) for blood glucose monitoring and management by the diabetic patients has been a long history since its first invention in 1968 and commercial application in 1975. The main reasons for its wide acceptance and popularity can be attributed mainly to the easy operation, test-to-result model, low cost, and small volume of sample required for blood glucose concentration test. During past decades, advances in analytical techniques have repurposed the use of PGMs into a general point-of-care testing platform for a variety of non-glucose targets, especially the food hazards. In this review, we summarized the recent published research using PGMs to detect the food safety hazards of mycotoxins, illegal additives, pathogen bacteria, and pesticide and veterinary drug residues detection with PGMs. The progress on PGM-based detection achieved in food safety have been carefully compared and analyzed. Furthermore, the current bottlenecks and challenges for practical applications of PGM for hazards detection in food safety have also been proposed.

"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.

Portable detection of Staphylococcus aureus using personal glucose meter based on hybridization chain reaction strategy

Staphylococcus aureus is one of the most important food-borne bacterial pathogens and causes numerous illnesses. In this work, we report a sensitive and highly selective magnetic-aptamer biosensor based on a personal glucose meter (PGM) platform for the detection of Staphylococcus aureus. The aptamer for Staphylococcus aureus was immobilized on the magnetic bead by hybridization with the capture probe P. In the presence of Staphylococcus aureus, the aptamer was dissociated from the magnetic bead. Then the capture probe was exposed and could be hybridized with a biotinylated probe to trigger the DNA hybridization chain reaction (HCR), thus achieving the signal amplification. The concentration of streptavidin-labeled invertase can be read by PGM, thus can lead to the portable quantitative detection of Staphylococcus aureus. After optimization of various conditions, 5 μM probe P, the MB-P reaction time for 36 h, the competition time for 60 min, 0.5 μM H1 & H2, 0.5 M sucrose and the sucrose invertase catalytic reaction time for 50 min was chosen to achieve the better sensor performance. Under the optimal conditions, the fabricated sensor offers high sensitivity with the limit of detection about 2 CFU/mL. This sensitive PGM based sensor could successfully evaluate the Staphylococcus aureus concentration in real food samples, and the results are consistent with those obtained by using plate counting methods. Moreover, the PGM sensor can greatly reduce the required time compared to the plate counting methods. The fabricated sensor supplies an ideal solution for rapid portable detection of bacterial pathogens and holds its potential use in the quality control for agriculture and food enterprises, entry-exit inspection and quality testing for food.

Dual cascade isothermal amplification reaction based glucometer sensors for point-of-care diagnostics of cancer-related microRNAs

The practical use of a point-of-care (POC) device is of particular interest in performing liquid biopsies related to cancer. Herein, taking advantage of the practical convenience of a commercially available personal glucose meter (PGM), we report a convenient, low-cost and sensitive detection strategy for circulating microRNA-155 (miRNA155) in human serum. First, miRNA155 in serum triggers the catalyzed hairpin assembly (CHA) reaction, and then the CHA product is specifically captured by the peptide nucleic acid (PNA) probes attached to the surface of a 96-well plate, which in turn triggers the hybridization chain reaction (HCR), resulting in the local enrichment of invertase. Next, introduction of a substrate (sucrose) for the invertase results in the generation of glucose, which can be detected by a PGM. In this sensor, neutrally charged PNA (12 nt) is more likely to hybridize with the CHA products than with the negatively charged DNA in kinetics, which improves the detection sensitivity and specificity. Due to the synergistic isothermal amplification reaction between CHA and HCR, the sensor is able to achieve a broad dynamic range (from 1 fM to 10 nM) with a detection limit down to 0.36 fM (3 orders of magnitude lower than that without HCR) and is capable of distinguishing single-base mismatched sequences. Thus the convenient, sensitive, robust and low-cost PGM sensor makes on-site nucleic acids detection possible, suggesting its great application prospect as a promising POC device in cancer diagnostics.

DNAzyme Amplified Aptasensing Platform for Ochratoxin A Detection Using a Personal Glucose Meter

Aptamer-based sensors have emerged as a major platform for detecting small-molecular targets, because aptamers can be selected to bind these small molecules with higher affinity and selectivity than other receptors such as antibodies. However, portable, accurate, sensitive, and affordable detection of these targets remains a challenge. In this work, we developed an aptasensing platform incorporating magnetic beads and a DNAzyme for signal amplification, resulting in high sensitivity. The biosensing platform was constructed by conjugating a biotin-labeled aptamer probe of small-molecular targets such as toxins and a biotin-labeled substrate strand on magnetic beads, and the DNAzyme strand hybridized with the aptamer probe to block the substrate cleavage activity. The specific binding of the small-molecular target by the aptamer probe can replace the DNAzyme strand and then induce the hybridization between the DNAzyme strand and substrate strand, and the iterative signal amplification reaction of hydrolysis and cleavage of the substrate chain occurs in the presence of a metal ion cofactor. Using invertase to label the substrate strand, the detection of small molecules of the toxin is successfully transformed into the measurement of glucose, and the sensitive analysis of small molecules such as toxins can be realized by using the household portable glucose meter as a readout. This platform is shown to detect ochratoxin, a common toxin in food, with a linear detection range of 5 orders of magnitude, a low detection limit of 0.88 pg/mL, and good selectivity. The platform is easy to operate and can be used as a potential choice for quantitative analysis of small molecules, at home or under point-of-care settings. Moreover, by changing and designing the aptamer probe and the arm of DNAzyme strand, it can be used for the analysis of other analytes.

A flexible assay strategy for non-glucose targets based on sulfhydryl-terminated liposomes combined with personal glucometer

The personal glucose meter (PGM) is one of the most successful point-of-care (POC) testing devices. It is simple, robust and inexpensive, but cannot be easily adapted to analytes other than glucose. We report a novel chemical conjugation-based assay strategy, using rational design of chemically-derivatized glucose-encapsulating liposomes, to repurpose a PGM, taking an important mycotoxin patulin as the model analyte. Sulfhydryl (-SH) was proposed for the first time as a specific functional group for efficient recognition of patulin. Multifunctional sulfhydryl-terminated glucose-encapsulating liposomes (G-LIP-SH) were synthesized in a simple, single step, which efficiently captured patulin by covalent bonding, and interacted strongly with NH₂-Au@Fe₃O₄ nanoparticles. Magnetic removal of nanoparticles efficiently and selectively separated patulin-derivatized from un-derivatized G-LIP-SH, permitting the latter to be lysed and the released glucose measured by PGM. The PGM signal was inversely proportional to the patulin concentration, over the range of 0.1-50 ng mL^-1 (R² = 0.995) with a detection limit of 0.05 ng mL^-1 (S/N = 3). This approach overcame interference from endogenous glucose, other mycotoxins and metal ions, allowing the analysis of a wide range of sample matrices and showed high specificity, acceptable reproducibility, good accuracy and optimal applicability. Other derivatization chemistries will enable this approach to be adapted to analytes with a wide range of chemical structures, to facilitate development of rapid, portable, user-friendly and cost-effective assays applicable to diverse analytes and sample matrices.

A rapid and highly sensitive biomarker detection platform based on a temperature-responsive liposome-linked immunosorbent assay

The enzyme-linked immunosorbent assay (ELISA) is widely used in various fields to detect specific biomarkers. However, ELISA tests have limited detection sensitivity (≥ 1 pM), which is insufficiently sensitive for the detection of small amounts of biomarkers in the early stages of disease or infection. Herein, a method for the rapid and highly sensitive detection of specific antigens, using temperature-responsive liposomes (TLip) containing a squaraine dye that exhibits fluorescence at the phase transition temperature of the liposomes, was developed. A proof-of-concept study using biotinylated TLip and a streptavidin-immobilized microwell plate showed that the TLip bound to the plate via specific molecular recognition could be distinguished from unbound TLip within 1 min because of the difference in the heating time required for the fluorescence emission of TLip. This system could be used to detect prostate specific antigen (PSA) based on a sandwich immunosorbent assay using detection and capture antibodies, in which the limit of detection was as low as 27.6 ag/mL in a 100-μL PSA solution, 0.97 aM in terms of molar concentration. The present temperature-responsive liposome-linked immunosorbent assay provides an advanced platform for the rapid and highly sensitive detection of biomarkers for use in diagnosis and biological inspections.

Harmful effects of high amounts of glucose on the immune system: An updated review

Release and storage of energy can be regulated by the metabolic parameter dependent on the central nervous system. Macrophages are one of the most professional antigen-presenting cells that are formed by the accumulation of dead or damaged cells or in response to the infection, which has the main function of phagocytosis, secretion of cytokines, and presenting antigen to T cells. A proper immune response is needed for the production of effector cytokines along with comprehensive and rapid cell proliferation and growth. Activation of the immune system and immune cells is needed to increase glucose metabolism. When the immune system responds to pathogens, chemokines inform immune cells such as macrophages and T cells to travel to the infected area. Although glucose is vital for the proper function of immune cells and their proliferation, a high amount of glucose may lead to impaired function of the immune system and pathological conditions. However, a suitable amount of glucose is indispensable for the immune system, but its elevated amount leads to excessive proinflammatory cytokines production. In this study, we focused on the master regulatory role of glucose on the immune system.

Magnetic bead-enzyme assemble for triple-parameter telomerase detection at single-cell level

In this work, we developed a triple-parameter strategy for the detection of telomerase activity from cancer cells and urine samples. This strategy was developed based on magnetic bead-enzyme hybrids combined with fluorescence analysis, colorimetric assay, or adenosine triphosphate (ATP) meter as readout. The application of magnetic bead-enzyme hybrids has the advantages of magnetic separation and signal amplification. These detection methods can be used individually or in combination to achieve the optimal sensing performance and make the results more convincing. Among them, the ATP meter with portable size had easy operation and low cost, and this response strategy provided a higher sensitivity at the single-cell level. The designed strategy was suitable as naked-eye sensor and point-of-care testing (POCT) for rapid assaying of telomerase activity. Graphical abstract Magnetic bead-enzyme assemble for triple-parameter telomerase detection.

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.

Translating in vitro diagnostics from centralized laboratories to point-of-care locations using commercially-available handheld meters

There is a growing demand for high-performance point-of-care (POC) diagnostic technologies where in vitro diagnostics (IVD) is fundamental for prevention, identification, and treatment of many diseases. Over the past decade, a shift of IVDs from the centralized laboratories to POC settings is emerging. In this review, we summarize recent progress in translating IVDs from centralized labs to POC settings using commercially available handheld meters. After introducing typical workflows for IVDs and highlight innovative technologies in this area, we discuss advantages of using commercially available handheld meters for translating IVDs from centralized labs to POC settings. We then provide comprehensive coverage of different signal transduction strategies to repurpose the commercially-available handheld meters, including personal glucose meter, pH meter, thermometer and pressure meter, for detecting a wide range of targets by integrating biochemical assays with the meters for POC testing. Finally, we identify remaining challenges and offer future outlook in this area.

Electrochemiluminescence Biosensor for Hyaluronidase Based on the Ru(bpy)32+ Doped SiO2 Nanoparticles Embedded in the Hydrogel Fabricated by Hyaluronic Acid and Polyethylenimine

Hyaluronidase (HAase), a specific enzyme of hyaluronic acid (HA), has been reported as a potential tumor biomarker in recent years. Hence, developing some simple, rapid, and sensitive methods for HAase assay is necessary. In this work, a simple and sensitive biosensor constructed by a reliable controlled release system and a mature electrochemiluminescence (ECL) analytical technique has been devised for the quantification of HAase with high efficiency and selectivity. Tris (2,2'-bipyridyl) ruthenium(II) chloride hexahydrate doped SiO₂ nanoparticles (Ru@SiO₂ NPs), as ECL signal probes, were trapped in the hydrogel fabricated by HA and polyethylenimine evenly and steadily. When HAase existed, the hydrogel was decomposed by HAase, and the Ru@SiO₂ NPs escaped from the hydrogel into the supernate. The ECL signal produced from the supernate can be detected and used to characterize HAase concentration. The result showed a good linear relationship between ECL intensity, and HAase concentration ranged from 2 to 60 U/mL and the limit of detection was 2 U/mL. The developed controlled release ECL biosensor has been used for HAase quantification in urine samples with satisfactory results.

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.

Tethering functionality to lipid interfaces by a fast, simple and controllable post synthesis method

HYPOTHESIS

Liposomes require careful control of the surface design to ensure colloidal stability in complex matrices and target-specific binding to desired receptor units. Ideally, surface functionalization should be smart and controllable in terms of composition which is seldomly achieved by conventional methods. Therefore, a new strategy (insertion method) was developed and compared to the standard method (modification post-synthesis) using the model receptor biotin.

EXPERIMENTS

Dipalmitoylphosphatidylethanolamine-biotin (DPPE-biotin) was used in both procedures, lipopeptide-biotin and cholesterol-biotin were tested additionally for insertion into the intact lipid bilayer. The insertion method was optimized regarding incubation time, temperature and vesicle stability. The biotinylated vesicles of both functionalization methods were characterized with respect to their size, ζ-potential and binding functionality.

FINDINGS

Standard incorporation resulted in large variations in insertion-efficiency, high batch-to-batch differences, and an incorporation limit of 4 mol%. Best results were obtained through effortless insertion of the lipopeptide-biotin at room temperature. The concentration-controlled functionalization of liposomes (up to 10 mol%) could easily be monitored by the ζ-potential, resulted in reliable, quantitative binding to streptavidin and did not affect the analytical properties of the nanomaterial. This offers the possibility for a general modification strategy for lipid-based nanomaterials ideal for assay optimizations or multi-analyte detection.

Catalase-linked immunosorbent pressure assay for portable quantitative analysis

In this study, catalase-linked immunosorbent pressure assay with a gas-generation reaction was established for quantitative detection of disease biomarker C-reactive protein (CRP) by a portable pressuremeter. The pressure-based detection system recognizes, transduces, and amplifies the target signal to a convenient target-correlated pressure signal reading in a closed chamber. Biotin molecules were modified on the surface of catalase in order to incorporate catalase into the pressure immunoassay by the streptavidin-biotin interaction. To improve the assay performance, the modification ratios of biotin molecules to catalase, and the concentrations of capture and detection antibodies were further optimized. The catalase-linked immunosorbent pressure assay allows portable and quantitation analysis of CRP with a limit of detection of 1.8 nM, which can satisfy the clinical needs for determining the risk of cardiovascular disease. The catalase-linked immunosorbent pressure assay also shows superior specificity and good accuracy. Compared to the previously reported assay catalyzed by PtNP nanozyme, catalase is not easily deactivated during storage and operation. With the merits of enzymatic efficiency, biocompatibility, low non-specific adsorption and facile modification, catalase can be reasonably used for reproducible, stable, simple quantitative detection of disease markers using a portable pressure-based assay in resource-limited settings.

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.

A three-dimensional DNA walking machine for the ultrasensitive dual-modal detection of miRNA using a fluorometer and personal glucose meter

Three-dimensional (3D) DNA walking machines inspired by natural molecular machines have attracted significant attention due to their high walking efficiency and signal amplification capability. Herein, we report a 3D DNA walking machine for the dual-modal detection of miRNA using a fluorometer and personal glucose meter (PGM). The 3D DNA walking machine on magnetic beads (MBs) was coated with the BHQ-H1-FAM hairpin structures (DNA tracks), activated by target miRNA-21 (walking strand) and propelled by a strand displacement reaction. During these processes, the fluorescence of FAM on H1 was turned on (first signal), and the invertase on H2 was introduced into the surface of the MBs. After being separated by an external magnetic field, the invertase hydrolyzed sucrose into glucose to generate a second signal, which was quantified by the PGM. The developed 3D DNA walking machine showed high sensitivity and good specificity, and the detection limits of 98 pM and 60 pM were obtained for the fluorescence-based assay and PGM-based assay, respectively. Compared with the single-modal detection, the developed DNA walking machine can achieve a unique double signal readout and more reliable sensitive performance. In addition, the proposed 3D DNA walking machine was successfully applied to detect miRNA in real biological samples. The 3D DNA walking machine with dual-modal detection has potential applications in disease diagnostics and clinical applications and can satisfy different testing requirements both in the laboratory and field.

Portable detection of colorectal cancer SW620 cells by using a personal glucose meter

It is of great value to develop general, low-cost and even household methods for colorectal cancer detection. Here, a portable detection strategy based on a personal glucose meter (PGM) was designed for meeting this purpose. In this strategy, the anti-EpCAM coated magnet beads (MBs) were used as capture probes for enriching cancer cells and the aptamer modified and invertase loaded graphene oxides (GO) were used as report probes for producing glucose signal. This method is sensitive with detection limit as low as 560 cells, and demonstrates excellent detection specificity. Meanwhile, we succeeded in the specific detection of target cells in 20% human serum samples, indicating this method has great prospect in clinical diagnosis. Moreover, this method presents favourable universality for detecting different colorectal cancer cells by just using different recognition aptamers. Importantly, this method can be implemented for the target cell detection at room temperature without any expensive and large-scale instruments but a portable PGM. Therefore, this portable detection method possesses great potential in point-of-care detection of colorectal cancer cells.

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.

Bioinspired DNA-Inorganic Hybrid Nanoflowers Combined with a Personal Glucose Meter for Onsite Detection of miRNA

Biomineralization is an important process in nature, by which living organisms participate in producing organic/inorganic hybrid materials and the resultant materials show sophisticated structures and excellent physical and chemical properties. Inspired by biomineralization, DNA-Cu₃(PO₄)₂ hybrid nanoflowers (HNFs) were prepared, which exhibited high stability, a high surface-to-volume ratio, and good DNA encapsulation ability. A facile thread platform for microRNA (miRNA) detection was fabricated by employing DNA-Cu₃(PO₄)₂ HNFs as captors, and the signal could be easily read out by a personal glucose meter. The fabricated biosensor could detect miRNA-21 quantitatively and a detection limit of 0.41 nM was achieved. Furthermore, miRNA in A549 cell lysate could also be detected without pretreatment. In this work, we achieved a fast, simple, low-cost method based on the bioinspired DNA-inorganic HNFs for the specific and sensitive detection of miRNA in both aqueous solution and biological samples, indicating its great promise in biomedical and clinical applications.

A sensitive and low background fluorescent sensing strategy based on g-C3N4-MnO2 sandwich nanocomposite and liposome amplification for ricin detection

Ricin is an extremely potent ribosome-inactivating protein and serves as a likely food biocontaminant or biological weapon. Thus, simple, sensitive and accurate analytical assays capable of detecting ricin are urgently needed to be established. Herein, we present a novel method for ricin B-chain (RTB) detection by using two materials: (a) a highly efficient hybrid probe that was formed by linking a glucose oxidase (GOD)-encapsulated liposome (GOD-L) to magnetic beads (MBs) through hybridization between an aptamer and a blocker and (b) a new low-background g-C3N4-MnO2 sandwich nanocomposite that exhibits fluorescence resonance energy transfer (FRET) between the g-C3N4 nanosheet and MnO2. In the presence of RTB, the strong binding between RTB and the aptamer can release the blocker-linked liposome from the surface of the MBs. After magnetic separation, the decomposed liposome can release GOD to catalyze the oxidation of glucose, generating a certain amount of H2O2. Then, H2O2 can reduce MnO2 of the g-C3N4-MnO2 nanocomposite to Mn2+, which leads to the elimination of FRET. Thus, the fluorescence of the g-C3N4 nanosheet will be turned on. Because of the excellent signal amplification ability of liposome and the characteristic highly sensitive response of the g-C3N4-MnO2 nanocomposite toward H2O2, RTB could be detected sensitively based on the significantly enhanced fluorescent intensity. The linear range of detection was from 0.25 μg mL-1 to 50 μg mL-1 and the limit of detection (LOD) was 190 ng mL-1. Moreover, the proposed assay was successfully applied in the detection of the entire ricin toxin content in a castor seed.

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.

Portable Aptasensor of Aflatoxin B1 in Bread Based on a Personal Glucose Meter and DNA Walking Machine

Despite some recent developments on the portable on-site sensor of Aflatoxin B1 (AFB1), the complex and expensive preparation of recognition elements have still limited their wide applications. In this paper, using the fast, low-cost, and stable recognition of aptamer DNA-AFB1, a portable aptasensor was constructed for the on-site detection of AFB1 in food matrixes, with the readout of personal glucose meter (PGM) and DNA walking machine for signal probe separation. In such an assay protocol, the target could trigger the DNA walker to autonomously move on the electrode surface, propelled by unidirectional Pb²⁺-specific DNAzyme digestion, which could amplify the signal and separate the signal probe as well for further quantification by the PGM. Under optimized conditions, the increase of PGM signal was relative with the concentration of AFB1 ranging from 0.02 to 10 nM and the low limit of detection (LOD) was 10 pM (S/N = 3). With the features of portability, and cheapness, the presented user-friendly method could be extended to various other analytes for wide point-of-care applications.

A novel method for sensitive, low-cost and portable detection of hepatitis B surface antigen using a personal glucose meter

Hepatitis B virus (HBV) infection is the major public health problem leading cause of death worldwide. The most important diagnostic marker for this infection is hepatitis B surface antigen (HBsAg). In this study, a novel, inexpensive, portable and sensitive ELISA method was designed and investigated for diagnosis of HBsAg based on the functionalized Fe₃O₄ and Al₂O₃ nanoparticles, with the strategy for detecting the concentration of glucose using a cheap and accessible personal glucose meter (PGM). The ELISA system was constructed using hepatitis B antibody against HBsAg immobilized on streptavidin coated magnetic iron oxide particles (S-Fe₃O₄) as the capture antibody (Ab₁). In addition, another hepatitis B antibody against different epitope of HBsAg (Ab₂) and glucoamylase both were immobilized on Al₂O₃ nanoparticles. After formation of the sandwich immune complex between Ab₁ and Ab₂ immobilized on S-Fe₃O₄ and Al₂O₃ NPs, respectively, through HBsAg, starch was converted into glucose using glucoamylase. Then, the glucose concentration was measured using PGM. The concentration of HBsAg was calculated based on the linear relation between the concentrations of HBsAg and glucose. Under optimal conditions, this assay showed detection limit values of 0.3 to 0.4 ng ml^-1 for "ay" and "ad" subtypes of HBsAg, respectively. The results indicate that the designed assay is comparable to the commercial kits in terms of sensitivity, on-site, specificity, cost, simplicity, portability and reproducibility. The presented method can be used in disadvantaged areas of the world and blood transfusion centers. To the best of our knowledge, this is the first report of using PGMs for HBSAg detection.

Liposomes and lipid bilayers in biosensors

Biosensors for the rapid, specific, and sensitive detection of analytes play a vital role in healthcare, drug discovery, food safety, and environmental monitoring. Although a number of sensing concepts and devices have been developed, many longstanding challenges to obtain inexpensive, easy-to-use, and reliable sensor platforms remain largely unmet. Nanomaterials offer exciting possibilities for enhancing the assay sensitivity and for lowering the detection limits down to single-molecule resolution. In this review, we present an overview of liposomes and lipid bilayers in biosensing applications. Lipid assemblies in the form of spherical liposomes or two-dimensional planar membranes have been widely used in the design of biosensing assays; in particular, we highlight a number of recent promising developments of biosensors based on liposomes in suspension, liposome arrays, and lipid bilayers arrays. Assay sensitivity and specificity are discussed, advantages and drawbacks are reviewed, and possible further developments are outlined.

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

Liposomes are an excellent candidate component for biosensors to transduce and amplify detection signals due to their outstanding ability in encapsulating signal marker compounds. However, the use of liposomes for photoelectrochemical (PEC) signal transduction has not yet been achieved due the lack of appropriate sensing strategy. Herein, we report on a novel liposomes-amplified PEC immunoassay (LAPIA) method for sensitive HIV-p24 antigen (p24) detection based on a split-type strategy. Initially, liposomes were encapsulated with alkaline phosphatase (ALP) in their hydrophilic chamber and conjugated with secondary antibody on the surface to form the ALP-encapsulated liposomes (ALP-Ls) based PEC signal label. Sandwiched immunoassay based on the ALP-Ls label was then carried out in microwell plate. Upon addition of tween 20, the ALP molecules were released and catalyzed the hydrolysis of ascorbic acid 2-phosphate (AA-p) to produce ascorbic acid (AA). The latter then donated electron to the graphene/g-C₃N₄ nanohybrids based photoelectrode, arousing an increased photocurrent signal. The separation of immunoreaction step and PEC signal excitation (i.e. split-type) not only enabled the realization of liposomes based amplification strategy, but also could eliminate the PEC-caused biomolecules damage. The developed PEC method possessed a wide calibration range from 1.0pgmL^-1 to 50ngmL^-1 and a low detection limit of 0.63pgmL^-1. Its practicability was demonstrated by assaying human serum samples. Moreover, the universality of the liposomes-amplified PEC sensing strategy was also demonstrated by developing it into a sensitive microRNA detection method.