A Portable Immunosensor with Differential Pressure Gauges Readout for Alpha Fetoprotein Detection

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.

A flexible optical gas pressure sensor as the signal readout for point-of-care immunoassay

Inspired by the concept of pneumatic micro/nanoscale surface morphing, an optical flexible gas pressure immunosensor constructed with an optical Ag/PDMS BGPS and a SiO2/Pt immunocomplex induced gas-generated reaction element for the sensitive detection of AFP was proposed.

High Sensitivity Label-Free Quantitative Method for Detecting Tumor Biomarkers in Human Serum by Optical Microfiber Couplers

Label-free optical fiber immunosensors have attracted widespread attention in recent decades due to their high sensitivity. However, nonspecific adsorption in serum has remained a critical bottleneck in existing label-free fiber optic biosensors, which hinders their widespread use in diagnostics. In addition, individual differences in clinical human serum (HS) negatively impact biosensing results. In this work, the modified serum preadsorption strategy was applied to reduce nonspecific adsorption by forming a saturated antifouling interface on an optical microfiber coupler (OMC). Furthermore, to reduce the effect of the differences between individual HS samples, we proposed a new method where Sigma HS was used as a wavelength shift reference due to being close to clinical serum compared to other serums. Sigma HS was used first to reduce the differences in immune sensors before performing a clinical sample test in which quantitative detection was achieved based on the independent calibration of several sensors with wide dynamic ranges via dissociation processes. The individual differences in 25% HS were corrected by 30% Sigma HS. As a proof of concept, the label-free OMC immune sensor demonstrates good sensitivity and specificity for the detection of carcinoembryonic antigen (CEA) in 25% Sigma HS at different concentrations. The detection limit of CEA reached as low as 34.6 fg/mL (0.475 fM). Additionally, label-free quantitative detection of CEA using this OMC immune sensor was verified experimentally according to the calibration line, and the results agree well with clinical examination detection. To our knowledge, it is the first study to employ an OMC immune sensor in point-of-care label-free quantitative detection for clinical HS.

Multifunctional nanoplatform for dual-mode sensitive detection of pathogenic bacteria and the real-time bacteria inactivation

Bacterial infection is a growing public health concern and causes a huge medical and financial burden. It is of significance to efficiently construct multifunctional platforms for bacterial point-of-care testing (POCT) and elimination. Herein, near-infrared (NIR) light-responded vancomycin-doped prussian blue nanoparticles (PB-VANNPs) with high efficient photothermal conversion was synthesized for binding, dual-mode portable detection, and elimination of bacteria. The PB-VANNPs can bind to the surface of Gram-positive bacteria such as Staphylococcus aureus (S. aureus), forming complex of PB-VANNPs/S. aureus. After being centrifugated, the suspension solution of PB-VANNPs can stimulate perfluorohexane (PFH) to rapidly release oxygen (O2) under NIR irradiation. Thus, the bacteria can be sensitively detected with portable pressure meter as signal reader, reporting a limit of detection (LOD) of 1.0 CFU mL-1. On the other side, the sediment of PB-VANNPs/S. aureus can be detected via thermal camera, reporting a LOD of 1.0 CFU mL-1. Interestingly, the bacteria can be effectively inactivated with the local temperature elevation during temperature-based detection. The antibacterial efficiency reaches as high as 99.8%. The developed multifunctional nanoplatform not only provides a straightforward "mix-then-test" way for portable detection of bacteria with high sensitivity, also realizes high efficiency elimination of bacteria simultaneously. The developed strategy was further applied for promoting wound healing of bacteria-infected mice.

A cost-effective and rapid quantitative test of polymyxin B sulphate with a simple and portable pressure meter readout

Any signals, if their intensities have a simple functional relationship with analyte concentration, can be applied for analytical purposes. Among them, pressure measurement as a signalling technique has been extensively utilized to develop portable and quantitative bioanalysis, with the advantages of highly sensitive detection of a variety of biomedical targets. In this contribution, it was found that polymyxin B sulphate (PMB) could significantly inhibit the catalytic ability of platinum nanoparticles (PtNPs) prepared in different protocols in the H2O2 breakdown reaction. By employing sodium citrate as the reducing agent to prepare the platinum nanoparticles (C-PtNPs) as an example and monitoring the pressure signal changes of the C-PtNP catalyzed H2O2 breakdown against the reaction time, a sensitive, cost-effective, rapid and reliable analysis method for PMB is established on the basis of the pressure signal readout. The proposed method has a detection limit of 28.6 nM and can selectively detect PMB in both POLY-MxB powder injection and human urine samples, demonstrating its potential in bioanalysis, which would be significant to address biological, clinical and medicinal requirements.

Emission Wavelength Switchable Carbon Dots Combined with Biomimetic Inorganic Nanozymes for a Two-Photon Fluorescence Immunoassay

In this work, o-phenylenediamine is utilized as a precursor to synthesize the fluorescent emission wavelength switchable carbon dots (o-CDs). Our investigation reveals that ferrous ions (Fe²⁺) can effectively induce fluorescence quenching of o-CDs by chelation and aggregation. After the addition of hydrogen peroxide (H₂O₂), the fluorescence of o-CDs recovers and the fluorescent color changes from yellow to green. As far as we know, o-CDs are the first reported CDs with switchable fluorescence emission wavelength. In order to fabricate an enzyme-free immunosensor, an amino-functionalized dendritic mesoporous silica nanoparticle (DMSN)-gold nanoparticle (Au NP) nanostructure was fabricated as a glucose oxidase mimetic nanoenzyme by in situ coating of the Au NPs on the surface of the DMSNs. Then, the functionalized DMSN-Au NPs were modified on the detection antibody and hydrolyzed with glucose to produce H₂O₂. This immune induced recognition strategy combines with the o-CDs+Fe²⁺ signal generation system to achieve specific and sensitive detection of the target. The replacement of glucose oxidase by DMSN-Au NPs not only reduces the cost but also provides significantly amplified signals due to DMSNs haing a high specific surface area. We show the detection of carcinoembryonic antigen (CEA) as an example target to evaluate the analytical figure of merits of the proposed strategy. Under the optimal conditions, two-photon-based o-CDs displayed excellent performances for CEA and the limit of detection as low as 74.5 pg/mL with a linear range from 0.1 to 80 ng/mL. The proposed fluorescent immunosensor provides an optional and potential scheme for low cost, high sensitivity, and versatile discovery of clinical biomarkers.

A novel and facile immunosensor based on a barometer: Application for rapid analysis of Escherichia coli in waters

A facile immunosensor was constructed based on a barometer indicator using a double-antibody- sandwich method for rapid and sensitive detection of Escherichia coli (E. coli) from water samples. At the present study, Anti- E. coli DH-5 polyclonal antibody was modified through enriching carboxylated magnetic beads and catalase functionalized gold nanoparticles (AuNPs)- loaded nanospheres. The functionalized AuNPs-loaded nanospheres exhibited an excellent catalysis towards decomposition of hydrogen peroxide (H₂O₂), generating a large volume of oxygen (O₂) into waters, increasing the pressure inside the glass vial, which in turn raised the water level in barometer. Under the optimized experimental conditions, the proposed method showed wide linear ranges (10²- 10⁷ cfu mL^-1), good accuracy and precision (recoveries, 86.7- 107%; CV, 3.2- 8.1%) with a limit of detection (LOD, S/N = 3) and the limit of quantification (LOQ, S/N = 10) were 80 cfu mL^-1and 267 cfu mL^-1, respectively. Furthermore, the fabricated portable immunosensor device showed some distinct features in low cost and visibility, suggesting great potential for rapid and on-site analysis of this bacteria from waters in less developed areas of developing countries.

Recent Advances of Fluid Manipulation Technologies in Microfluidic Paper-Based Analytical Devices (μPADs) toward Multi-Step Assays

Microfluidic paper-based analytical devices (μPADs) have been suggested as alternatives for developing countries with suboptimal medical conditions because of their low diagnostic cost, high portability, and disposable characteristics. Recently, paper-based diagnostic devices enabling multi-step assays have been drawing attention, as they allow complicated tests, such as enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR), which were previously only conducted in the laboratory, to be performed on-site. In addition, user convenience and price of paper-based diagnostic devices are other competitive points over other point-of-care testing (POCT) devices, which are more critical in developing countries. Fluid manipulation technologies in paper play a key role in realizing multi-step assays via μPADs, and the expansion of biochemical applications will provide developing countries with more medical benefits. Therefore, we herein aimed to investigate recent fluid manipulation technologies utilized in paper-based devices and to introduce various approaches adopting several principles to control fluids on papers. Fluid manipulation technologies are classified into passive and active methods. While passive valves are structurally simple and easy to fabricate, they are difficult to control in terms of flow at a specific spatiotemporal condition. On the contrary, active valves are more complicated and mostly require external systems, but they provide much freedom of fluid manipulation and programmable operation. Both technologies have been revolutionized in the way to compensate for their limitations, and their advances will lead to improved performance of μPADs, increasing the level of healthcare around the world.

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.

Lateral flow assay with pressure meter readout for rapid point-of-care detection of disease-associated protein

Paper-based assays such as lateral flow assays are good candidates for portable diagnostics owing to their user-friendly format and low cost. In terms of analytical detection, lateral flow assays usually require dedicated instruments to obtain quantitative results. Here we demonstrate a lateral flow assay with handheld pressure meter readout for the rapid detection of disease-related protein with high sensitivity and selectivity. Based on the pressure change produced by the catalytic reaction of Pt nanoparticles related to the concentration of the target, a quantitative reaction platform was established. During the lateral flow assay, the Pt nanoparticles are aggregated in the test line to form a gray band by biomolecular recognition and finally convert the recognition signal into highly sensitive pressure readout for quantitative analysis. Without sophisticated instrumentation and complicated operations, the whole detection process can be completed within 20 minutes. The limit of detection for myoglobin (2.9 ng mL^-1 in diluted serum samples) meets the requirements of clinical monitoring. With the advantages of low cost, ease of operation, high sensitivity and selectivity, the method represents a versatile platform for point-of-care testing of disease biomarkers.

Gas-generating reactions for point-of-care testing

Gas generation-based measurement is an attractive alternative approach for POC (Point-of-care) testing, which relies on the amount of generated gas to detect the corresponding target concentrations.

A Simple and Convenient Aptasensor for Protein Using an Electronic Balance as a Readout

The electronic balance, one of the most common pieces of equipment in the laboratory, is normally used to directly measure the weight of a target with high accuracy. However, little attention has been paid to the extension of its applications. In this study, an electronic balance was used as a readout to develop a novel aptasensor for protein quantification for the first time. Thrombin was selected as a model target, and its two aptamers recognizing different sites of the protein were used (one aptamer was immobilized on the surface of magnetic microparticles and the other aptamer was functionalized with platinum nanoparticles). The two aptamers were specifically bound with the thrombin to form a sandwich structure; thus, the platinum nanoparticles were linked to the magnetic microparticles, and they were separated by a magnet easily. The captured platinum nanoparticles effectively catalyzed the decomposition of H₂O₂, generating a large volume of O₂ to discharge a certain amount of water in a drainage device, because the pressure in the vial is higher than that outside of the vial. The weight of water was accurately measured by an electronic balance. The weight of water increased with the increasing of the thrombin concentration in the range of 0 to 100 nM with a detection limit of 2.8 nM. This is the first time the use of an electronic balance as a signal readout for biomolecule quantitation in bioassay has been reported.