Overview of Gas-Generating-Reaction-Based Immunoassays

Point-of-care (POC) immunoassays have become convincing alternatives to traditional immunosensing methods for the sensitive and real-time detection of targets. Immunoassays based on gas-generating reactions were recently developed and have been used in various fields due to their advantages, such as rapid measurement, direct reading, simple operation, and low cost. Enzymes or nanoparticles modified with antibodies can effectively catalyze gas-generating reactions and convert immunorecognition events into gas pressure signals, which can be easily recorded by multifunctional portable devices. This article summarizes the advances in gas-generating-reaction-based immunoassays, according to different types of signal output systems, including distance-based readout, pressure differential, visualized detection, and thermal measurement. The review mainly focuses on the role of photothermal materials and the working principle of immunoassays. In addition, the challenges and prospects for the future development of gas-generating-reaction-based immunoassays are briefly discussed.

Designing and prototyping a novel biosensor based on a volumetric bar-chart chip for urea detection

A volumetric bar-chart chip (V-chip) is a microfluidic device based on distance-based quantitative measurement that visualizes analyte concentration without the need for apparatus or data processing. This typically utilizes special receptors and catalysis parts that generate oxygen, so ink can be moved inside the channels, and enables instant visual quantitation of the analyte. However, the low stability of some macromolecules, the use of expensive catalysts, and difficulty in controlling the process cause inaccurate readings, and therefore, limit further development and the use of these systems. In this article, we introduced a novel approach that eliminates the use of catalysts in V-chips and provides an efficient and simple path in the design of biosensors. The product of the enzymatic reaction of urease with urea is bicarbonate, which turns into CO2 gas in an acidic environment. Therefore, the amount of gas produced is proportional to the amount of urea in the sample, and it can be quantitatively measured by visual detection from the amount of ink movement caused by CO2 gas pressure. This biosensor has a linear response range of 0 to 1000 μg ml-1 and a detection limit of 3.6 μg ml-1 in raw milk. The recovery of urea in raw milk at 100 and 400 μg ml-1 concentrations was 96.5% and 98.9%, respectively. This volumetric chip shows potential for determining urea levels in real samples without requiring additional equipment. The combination of the sensitivity and specificity of enzymatic reactions, inherent gas-generating reactions, and the processability of microchips discussed in this paper can be the basis for the comprehensive development of volumetric chips, which can create a new path for the development of efficient and cheap biosensors.

Enhanced portable detection for Sars-CoV-2 utilizing DNA tetrahedron-tethered aptamers and a pressure meter

Tethering oligonucleotide aptamers to a DNA tetrahedron structure can enhance the recognition of SARS-CoV-2 spike protein to effectively overcome challenges with its detection in current diagnostic assays. Building on this framework, we have developed a unique portable detection method for COVID-19 that provides exceptional sensitivity and selectivity via pressure meter readout. This innovative assay streamlines the detection process, providing a rapid, sensitive, cost-effective, and user-friendly diagnostic tool. This point-of-care test exhibits high sensitivity and specificity, achieving an impressive detection limit of 0.1 pg mL-1 for the spike protein. The effectiveness of this method was validated using pseudoviruses and oropharyngeal swab samples, and its utility for environmental monitoring is demonstrated by testing sewage samples. With a wide linear range and strong potential for clinical or home application, our assay represents a major innovation in point-of-care diagnostics and provides a vital contribution to the current toolkit for controlling the impacts of COVID-19.

The application of nanoparticles in point-of-care testing (POCT) immunoassays

The Covid-19 pandemic has led to greater recognition of the importance of the fast and timely detection of pathogens. Recent advances in point-of-care testing (POCT) technology have shown promising results for rapid diagnosis. Immunoassays are among the most extensive POCT assays, in which specific labels are used to indicate and amplify the immune signal. Nanoparticles (NPs) are above the rest because of their versatile properties. Much work has been devoted to NPs to find more efficient immunoassays. Herein, we comprehensively describe NP-based immunoassays with a focus on particle species and their specific applications. This review describes immunoassays along with key concepts surrounding their preparation and bioconjugation to show their defining role in immunosensors. The specific mechanisms, microfluidic immunoassays, electrochemical immunoassays (ELCAs), immunochromatographic assays (ICAs), enzyme-linked immunosorbent assays (ELISA), and microarrays are covered herein. For each mechanism, a working explanation of the appropriate background theory and formalism is articulated before examining the biosensing and related point-of-care (POC) utility. Given their maturity, some specific applications using different nanomaterials are discussed in more detail. Finally, we outline future challenges and perspectives to give a brief guideline for the development of appropriate platforms.

Diagnosis of disease relevant nucleic acid biomarkers with off-the-shelf devices

Changes in the level of nucleic acids in blood may be correlated with some clinical disorders like cancer, stroke, trauma and autoimmune diseases, and thus, nucleic acids can serve as potential biomarkers for pathological processes. The requirement of technical equipment and operator expertise in effective information readout of modern molecular diagnostic technologies significantly restricted application outside clinical laboratories. The ability to detect nucleic acid biomarkers with off-the-shelf devices, which have the advantages of portability, simplicity, low cost and short response time, is critical to provide a prompt clinical result in circumstances where the laboratory instruments are not available. This review throws light on the current strategies and challenges for nucleic acid diagnosis with commercial portable devices, indicating the future prospect of portable diagnostic devices and making a great difference in improving the healthcare and disease surveillance in resource-limited areas.

NIR II Light-Response Au Nanoframes: Amplification of a Pressure- and Temperature-Sensing Strategy for Portable Detection and Photothermal Therapy of Cancer Cells

Quantitative detection of cancer cells using portable devices is promising for the development of simple, fast, and point-of-care cancer diagnostic techniques. However, how to further amplify the detection signal to improve the sensitivity and accuracy of detecting cancer cells by portable devices remains a challenge. To solve the problem, we, for the first time, synthesized folic-acid-conjugated Au nanoframes (FA-Au NFs) with amplification of pressure and temperature signals for highly sensitive and accurate detection of cancer cells by portable pressure meters and thermometers. The resulting Au NFs exhibit excellent near-infrared (NIR) photothermal performance and catalase activity, which can promote the decomposition of NH₄HCO₃ and H₂O₂ to generate corresponding gases (CO₂, NH₃, and O₂), thereby synergistically amplifying pressure signals in a closed reaction vessel. At the same time, Au NFs with excellent peroxidase-like activity can catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to produce TMB oxide (oxTMB) with a strong photothermal effect, thereby cooperating with Au NFs to amplify the photothermal signal. In the presence of cancer cells with overexpressing folate receptors (FRs), the molecular recognition signals between FA and FR can be converted into amplified pressure and temperature signals, which can be easily read by portable pressure meters and thermometers, respectively. The detection limits for cancer cells using pressure meters and thermometers are 6 and 5 cells/mL, respectively, which are better than other reported methods. Moreover, such Au NFs can improve tumor hypoxia by catalyzing the decomposition of H₂O₂ to produce O₂ and perform photothermal therapy of cancer. Together, our work provides new insight into the application of Au NFs to develop a dual-signal sensing platform with amplification of pressure and temperature signals for portable and ultrasensitive detection of cancer cells as well as personalized cancer therapy.

Pressure/colorimetric dual-readout immunochromatographic test strip for point-of-care testing of aflatoxin B1

Immunochromatographic test strip (ITS) for point-of-care testing (POCT) has attracted prominent attention due to the advantages including rapid response, low cost and good portability. Here, we developed a sensitive ITS for detecting aflatoxin B₁ (AFB₁) by using dendritic platinum nanoparticles (DPNs) as novel pressure/colorimetric dual-readout probes. DPNs-labeled antibody of AFB₁ were used as the signal tracer of the immunochromatographic process. After 10-min competitive immunoreaction, black color appeared on the test line of ITS due to the accumulation of DPNs, which was observed visually as a colorimetric readout for qualitation purpose. Furthermore, DPNs with peroxidase-like activity caused decomposition of hydrogen peroxide aqueous solution to produce pressure change signal in vials, which was detected by a hand-held pressure meter for quantitation purpose. With the pressure readout mode, the detection range was 0.05-10 ng mL^-1, and the detection limit was 0.03 ng mL^-1 (S/N = 3) for AFB₁. The proposed ITS was successfully utilized for detecting AFB₁ in herbal medicine samples, and the acceptable recoveries of 93.77-114.09% indicated the reliability for real sample detection. It provides a new avenue for POCT with great application potential in various area including drug and food quality control, pollutants monitoring as well as medical diagnosis.

A distance-based capillary biosensor using wettability alteration

Distance-based detection methods with a quantitative readout are of great significance to point-of-care testing (POCT), are low-cost and user-friendly, and can be integrated into portable analytical devices. Here, we submit a visual quantitative distance-based sensor by capillary force alteration in a capillary tube. This sensor converts the wettability alteration caused by the target molecules into a capillary rise height signal. Moreover, the sensor profits from isothermal amplification technology, achieving the detection of miRNAs with high sensitivity and specificity by visually reading the height of the water in the capillary tube. The proposed biosensor shows great potential in routine clinical diagnosis as well as POCT in resource-limited settings.

Electrochemical Detection and Point-of-Care Testing for Circulating Tumor Cells: Current Techniques and Future Potentials

Circulating tumor cells (CTCs) are tumor cells that escaped from the primary tumor or the metastasis into the blood and they play a major role in the initiation of metastasis and tumor recurrence. Thus, it is widely accepted that CTC is the main target of liquid biopsy. In the past few decades, the separation of CTC based on the electrochemical method has attracted widespread attention due to its convenience, rapidness, low cost, high sensitivity, and no need for complex instruments and equipment. At present, CTC detection is not widely used in the clinic due to various reasons. Point-of-care CTC detection provides us with a possibility, which is sensitive, fast, cheap, and easy to operate. More importantly, the testing instrument is small and portable, and the testing does not require specialized laboratories and specialized clinical examiners. In this review, we summarized the latest developments in the electrochemical-based CTC detection and point-of-care CTC detection, and discussed the challenges and possible trends.

A sensitive biosensor for determination of pathogenic bacteria using aldehyde dehydrogenase signaling system

Aldehyde dehydrogenase (ALDH) was first developed as an enzymatic signaling system of a biosensor for sensitive point-of-care detection of pathogenic bacteria. ALDH and specific aptamers to Salmonella typhimurium (S. typhimurium), as organic components, were embedded in organic-inorganic nanocomposites as a biosensor signal label, integrating the functions of signal amplification and target recognition. The biosensing mechanism is based on the fact that ALDH can catalyze rapid oxidation of acetaldehyde into acetic acid, resulting in pH change with portable pH meter readout. The altered pH exhibited a linear relationship with the logarithm of S. typhimurium from 10² to 10⁸ CFU/mL and detection limit of 46 CFU/mL. Thus, the proposed biosensor has potential application in the diagnosis of pathogenic bacteria.

Point-of-care assay for drunken driving with Pd@Pt core-shell nanoparticles-decorated ploy(vinyl alcohol) aerogel assisted by portable pressure meter

Alcohol abuse causes health problems and security accidents. A reliable and sensitive detection system for alcohol has been an instinctive demand in law enforcement and forensic. More efforts are demanded in developing new sensing strategy preferably with portable and non-invasive traits for the pushforward of point-of-care (POC) device popularization.

Methods: We developed a POC diagnosis system for alcohol assay with the aid of alcohol oxidase (AOX) pre-joining in the system as well as Pd@Pt core-shell nanoparticles (abbreviated to Pd@Pt) that were decorated on ploy(vinyl alcohol) aerogel with amphiphilicity. Biological samples like saliva and whole blood can be absorbed by the aerogel in a quick process, in which the analyte would go through a transformation from alcohol, H₂O₂, to a final production of O₂, causing an analyte dose-dependent signal change in the commercial portable pressure meter. The cascade reactions are readily catalyzed by AOX and Pd@Pt, of which the latter one possesses excellent peroxidase-like activity.

Results: Our design has smartness embodied in the aerogel circumvents the interference from methanol which is more ready to be catalyzed by AOX. Under the optimal conditions, the limit of detection for alcohol was 0.50 mM in saliva, and is able to distinguish the driving under the influence (DUI) (1.74 mM in saliva) and driving while impaired (DWI) (6.95 mM in saliva) in the national standard of China.

Conclusion: Our proof-of-concept study provides the possibility for the establishment of POC device for alcohol and other target detection, not only owing to the sensing qualification but also thanks to the architecture of such sensor that has great flexibility by replacing the AOX with glucose oxidase (GOX), thenceforth realizing the accurate detection of glucose in 0.5% whole blood sample. With the advantages of easy accessibility and anti-interference ability, our sensor exhibits great potential for quantitative diagnostics in biological system.

A Sensitive, Point-of-Care Detection of Small Molecules Based on a Portable Barometer: Aflatoxins In Agricultural Products

Sensitive and point-of-care detection of small toxic molecules plays a key role in food safety. Aflatoxin, a typical small toxic molecule, can cause serious healthcare and economic issues, thereby promoting the development of sensitive and point-of-care detection. Although ELISA is one of the official detection methods, it cannot fill the gap between sensitivity and point-of-care application because it requires a large-scale microplate reader. To employ portable readers in food safety, Pt-catalysis has attracted increasing attention due to its portability and reliability. In this study, we developed a sensitive point-of-care aflatoxin detection (POCAD) method via a portable handheld barometer. We synthesized and characterized Au@PtNPs and Au@PtNPs conjugated with a second antibody (Au@PtNPs-IgG). A competitive immunoassay was established based on the homemade monoclonal antibody against aflatoxins. Au@PtNPs-IgG was used to catalyze the production of O2 from H2O2 in a sealed vessel. The pressure of O2 was then recorded by a handheld barometer. The aflatoxin concentration was inversely proportional to the pressure recorded via the barometer reading. After optimization, a limit of detection of 0.03 ng/mL and a linear range from 0.09 to 16.0 ng/mL were achieved. Recovery was recorded as 83.1%-112.0% along with satisfactory results regarding inner- and inter-assay precision (relative standard deviation, RSD < 6.4%). Little cross-reaction was observed. Additionally, the POCAD was validated by high-performance liquid chromatography (HPLC) by using peanut and corn samples. The portable POCAD exhibits strong potential for applications in the on-site detection of small toxic molecules to ensure food safety.

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.

Saw-Toothed Microstructure-Based Flexible Pressure Sensor as the Signal Readout for Point-of-Care Immunoassay

A saw-toothed microstructure-based flexible pressure sensor was designed as the signal readout for point-of-care (POC) immunoassay of carcinoembryonic antigen (CEA). In this portable POC bioassay, the sandwich-type immunoreaction was first carried out to capture the target on a microplate, which simultaneously introduced the platinum nanoparticles (PtNPs). Upon adding hydrogen peroxide (H₂O₂), the pressure as the bridge between the molecular recognition event and detectable signal was increasing rapidly, resulting from the decomposition of H₂O₂ accelerated by PtNPs. Meanwhile, a flexible pressure sensor was fabricated with high sensitivity; the ability of the pressure response was dramatically improved by adopting the saw-toothed microstructure. By coupling with the pressure sensor, the pressure change could be monitored in real time to achieve the portable detection of CEA. Under the optimum conditions, the proposed pressure-based bioassay presented good sensing performance within 0.1-40 ng/mL at a detection limit of 87 pg/mL. The reproducibility, precision, and accuracy provided by the method were also studied and satisfied.

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.

Quantitative assessment of disease markers using the naked eye: point-of-care testing with gas generation-based biosensor immunochromatographic strips

BACKGROUND

Immunochromatographic strips (ICSs) are a practical tool commonly used in point-of-care testing (POCT) applications. However, ICSs that are currently available have low sensitivity and require expensive equipment for quantitative analysis. These limitations prohibit their extensive use in areas where medical resources are scarce.

METHODS

We developed a novel POCT platform by integrating a gas generation biosensor with Au@Pt Core/Shell nanoparticle (Au@PtNPs)-based ICSs (G-ICSs). The resulting G-ICSs enabled the convenient and quantitative assessment of a target protein using the naked eye, without the need for auxiliary equipment or complicated computation. To assess this platform, C-reactive protein (CRP), a biomarker commonly used for the diagnosis of acute, infectious diseases was chosen as a proof-of-concept test.

RESULTS

The linear detection range (LDR) of the G-ICSs for CRP was 0.05-6.25 μg/L with a limit of detection (LOD) of 0.041 μg/L. The G-ICSs had higher sensitivity and wider LDR when compared with commonly used AuNPs and fluorescent-based ICSs. When compared with results from a chemiluminescent immunoassay, G-ICS concordance rates for CRP detection in serum samples ranged from 93.72 to 110.99%.

CONCLUSIONS

These results demonstrated that G-ICSs have wide applicability in family diagnosis and community medical institutions, especially in areas with poor medical resources.

Immunoassay for pathogenic bacteria using platinum nanoparticles and a hand-held hydrogen detector as transducer. Application to the detection of Escherichia coli O157:H7

An innovative approach is presented for portable and sensitive detection of pathogenic bacteria. A novel synthetic hybrid nanocomposite encapsulating platinum nanoparticles, as a highly efficient catalyst, catalyzes the hydrolysis of the ammonia-borane complex to generate hydrogen gas. The nanocomposites are used as a label for immunoassays. A portable hand-held hydrogen detector combined with nanocomposite-induced signal conversion was applied for point-of-care testing of pathogenic bacteria. A hand-held hydrogen detector was used as the transducer. Escherichia coli O157:H7 (E. coli O157: H7), as detection target, formed a sandwich structure with magnetic beads and hybrid nanocomposites. Magnetic beads were used for separation of the sandwich structure, and hybrid nanocomposites as catalysts to catalyze the generation of hydrogen from ammonia-borane. The generated hydrogen was detected by a hydrogen detector using an electrochemical method. E. coli O157:H7 has a detection limit of 10 CFU·mL^-1. The immunosensor made the hand-held hydrogen detector a point-of-care meter to be used outdoors for the detection and quantification of targets beyond hydrogen. Graphical abstract Schematic presentation of one-pot synthetic peptide-Cu₃(PO₄)₂ hybrid nanocomposites embedded PtNPs (PPNs), encapsulating many Pt particles. The PPNs acts as an ideal immunoprobe for hand-held H₂ detector signal readouts, by transforming pathogenic bacteria recognition events into H₂ signals.

Lateral flow assay ruler for quantitative and rapid point-of-care testing

Lateral flow assay (LFA) is a well-established platform for point-of-care (POC) testing due to its low cost and user friendliness. Conventional LFAs provide qualitative or semi-quantitative results and require dedicated instruments for quantitative detection. Here, we developed an "LFA ruler" for quantitative and rapid readout of LFA results, using a 3D printed strip cassette and a simple, inexpensive microfluidic chip. Platinum nanoparticles are used as signal amplification reporters, which catalyze the generation of oxygen to push ink advancement in the microfluidic channel. The concentration of the target is linearly correlated with the ink advancement distance. The entire assay can be completed within 30 minutes without external instruments and complicated operations. We demonstrated quantitative prostate specific antigen testing using the LFA ruler, with a limit of detection of 0.54 ng mL^-1, linear range of 0-12 ng mL^-1, and high correlation with a clinical gold standard assay. The LFA ruler achieves low cost, quantitative, sensitive and rapid detection, which has great potential in POC testing and can be extended to quantify other disease biomarkers.

Staining Traditional Colloidal Gold Test Strips with Pt Nanoshell Enables Quantitative Point-of-Care Testing with Simple and Portable Pressure Meter Readout

Traditional immunochromatographic test strips based on colloidal gold are effective devices for portable and low-cost point-of-care (POC) testing. Nevertheless, they still suffer from the limitation of qualitative or semiquantitative tests via naked-eye detection. Replacement of gold with other signal entities, such as magnetic particles or fluorescent particles, requires professional instrumentation to obtain quantitative results. A pressure-based assay with platinum nanoparticles (PtNPs) can provide quantitative results using a portable pressure meter but is also hampered by the long-term instability of PtNPs. Consequently, we developed a Pt-staining method based on test strips to create platinum nanoshells on the surface of colloidal gold. This method not only preserves the original advantages of colloidal gold with easy synthesis and decoration but also introduces PtNPs with excellent catalytic activity as signal labels to achieve sensitive quantitative detection. Myoglobin was tested as a model target, and the limit of detection was 5.47 ng/mL in 20% diluted serum samples, which satisfies the requirements for clinical monitoring of acute myocardial infarction. In addition, the two most common colloidal gold strips available in the marketplace were applied to demonstrate the compatibility of Pt-staining. Taking advantage of low cost, user-friendliness, compatibility, simplicity, and stability, colloidal gold test strips with Pt-staining are expected to satisfy the need for quantitative POC testing of biomarkers, especially in resource-limited regions.

Multiplexed Instrument-Free Bar-Chart SpinChip Integrated with Nanoparticle-Mediated Magnetic Aptasensors for Visual Quantitative Detection of Multiple Pathogens

A portable multiplexed bar-chart SpinChip (MB-SpinChip) integrated with nanoparticle-mediated magnetic aptasensors was developed for visual quantitative instrument-free detection of multiple pathogens. This versatile multiplexed SpinChip combines aptamer-specific recognition and nanoparticle-catalyzed pressure amplification to achieve a sample-to-answer output for sensitive point-of-care testing (POCT). This is the first report of pathogen detection using a volumetric bar-chart chip, and it is also the first bar-chart chip using a "spinning" mechanism to achieve multiplexed bar-chart detection. Additionally, the introduction of the spin unit not only enabled convenient sample introduction from one inlet to multiple separate channels in the multiplexed detection, but also elegantly solved the pressure cross-interference problem in the multiplexed volumetric bar-chart chip. This user-friendly MB-SpinChip allows visual quantitative detection of multiple pathogens simultaneously with high sensitivity but without utilizing any specialized instruments. Using this MB-SpinChip, three major foodborne pathogens including Salmonella enterica, Escherichia coli, and Listeria monocytogenes were specifically quantified in apple juice with limits of detection of about 10 CFU/mL. This MB-SpinChip with a bar-chart-based visual quantitative readout has great potential for the rapid simultaneous detection of various pathogens at the point of care and wide applications in food safety, environmental surveillance, and infectious disease diagnosis.