Designing Paper-Based Immunoassays for Biomedical Applications

Point-of-care diagnostics approaches for detection of lung cancer-associated circulating miRNAs

Circulating cell-free miRNAs (ccf-miRs) have gained significant interest as biomarkers for lung cancer (LC) diagnosis. However, the clinical application of ccf-miRs is mainly limited by time, cost, and expertise-related problems of existing detection strategies. Recently, the development of different point-of-care (POC) approaches offers useful on-site platforms, because these technologies have important features such as portability, rapid turnaround time, minimal sample requirement, and cost-effectiveness. In this review, we discuss different POC approaches for detecting ccf-miRs and highlight the utility of incorporating nanomaterials for enhanced biorecognition and signal transduction, further improving their diagnostic applicability in LC settings.

Protein-based lateral flow assays for COVID-19 detection

To combat the enduring and dangerous spread of COVID-19, many innovations to rapid diagnostics have been developed based on proteinprotein interactions of the SARS-CoV-2 spike and nucleocapsid proteins to increase testing accessibility. These antigen tests have most prominently been developed using the lateral flow assay (LFA) test platform which has the benefit of administration at point-of-care, delivering quick results, lower cost, and does not require skilled personnel. However, they have gained criticism for an inferior sensitivity. In the last year, much attention has been given to creating a rapid LFA test for detection of COVID-19 antigens that can address its high limit of detection while retaining the advantages of rapid antibodyantigen interaction. In this review, a summary of these proteinprotein interactions as well as the challenges, benefits, and recent improvements to protein based LFA for detection of COVID-19 are discussed.

Microfluidic Point-of-Care Testing: Commercial Landscape and Future Directions

Point-of-care testing (POCT) allows physicians to detect and diagnose diseases at or near the patient site, faster than conventional lab-based testing. The importance of POCT is considerably amplified in the trying times of the COVID-19 pandemic. Numerous point-of-care tests and diagnostic devices are available in the market including, but not limited to, glucose monitoring, pregnancy and infertility testing, infectious disease testing, cholesterol testing and cardiac markers. Integrating microfluidics in POCT allows fluid manipulation and detection in a singular device with minimal sample requirements. This review presents an overview of two technologies - (a.) Lateral Flow Assay (LFA) and (b.) Nucleic Acid Amplification - upon which a large chunk of microfluidic POCT diagnostics is based, some of their applications, and commercially available products. Apart from this, we also delve into other microfluidic-based diagnostics that currently dominate the in-vitro diagnostic (IVD) market, current testing landscape for COVID-19 and prospects of microfluidics in next generation diagnostics.

A New Paper-Based Microfluidic Device for Improved Detection of Nitrate in Water

In this paper, we report a simple and inexpensive paper-based microfluidic device for detecting nitrate in water. This device incorporates two recent developments in paper-based technology suitable for nitrate detection and has an optimized microfluidic design. The first technical advancement employed is an innovative fibrous composite material made up of cotton fibers and zinc microparticles that can be incorporated in paper-based devices and results in better nitrate reduction. The second is a detection zone with an immobilized reagent that allows the passage of a larger sample volume. Different acids were tested—citric and phosphoric acids gave better results than hydrochloric acid since this acid evaporates completely without leaving any residue behind on paper. Different microfluidic designs that utilize various fluid control technologies were investigated and a design with a folding detection zone was chosen and optimized to improve the uniformity of the signal produced. The optimized design allowed the device to achieve a limit of detection and quantification of 0.53 ppm and 1.18 ppm, respectively, for nitrate in water. This accounted for more than a 40% improvement on what has been previously realized for the detection of nitrate in water using paper-based technology.

Higher Sensitivity Provided by the Combination of Two Lateral Flow Immunoassay Tests for the Detection of COVID-19 Immunoglobulins

SARS-Cov-2 was identified in Wuhan, China in December 2019. The World Health Organization (WHO) declared it a pandemic in March of 2020. COVID-19 has now been reported on every continent. In the United States, the total number of confirmed reported cases of COVID-19 has exceeded 1.8 million with the total death exceeding 100,000 people. The most common investigational diagnostics of this disease are RT-PCR and serology testing. The objective of this work was to validate two commercial kits for the detection of IgM and IgG using lateral flow immunoassay tests and to study the effect of the combination of both serology kits for better detection of immunoglobulins. A total of 195 patients presenting with respiratory symptoms suggestive of infection with SARS-Cov-2 were subject to serology and molecular testing. Two lateral flow immunochromatographic assay kits were used: the Healgen Scientific for SARS-CoV-2 IgM/IgG and the Raybiotech for SARS-CoV-2 IgM/IgG. Sensitivity and specificity of each kit alone and in combination were determined and compared. The limit of detection, inter and intra test variations, as well interfering substances and cross reactivity were also studied for both kits. The results show sensitivities for IgM detection varying between 58.9 and 66.2% for the kits alone and 87.7% of the combination of both kits. IgG detection was not significantly affected by this combination. Both kits manifested high specificities (99.2–100%). Both kits showed high clinical performance in terms of cross reactivity and interfering substances. Our results suggest using combinatory testing for the serology of COVID-19 after a full evaluation study, assessing all the parameters affecting their clinical performance before deciding on this combination.

Surface-Enhanced Raman Scattering-Based Lateral-Flow Immunoassay

Lateral flow immunoassays (LFIAs) have been developed and used in a wide range of applications, in point-of-care disease diagnoses, environmental safety, and food control. However, in its classical version, it has low sensitivity and can only perform semiquantitative detection, based on colorimetric signals. Over the past decade, surface-enhanced Raman scattering (SERS) tags have been developed in order to decrease the detection limit and enable the quantitative analysis of analytes. Of note, these tags needed new readout systems and signal processing algorithms, while the LFIA design remained unchanged. This review highlights SERS strategies of signal enhancement for LFIAs. The types of labels used, the possible gain in sensitivity from their use, methods of reading and processing the signal, and the prospects for use are discussed.

Detection of Bacterial and Viral Pathogens Using Photonic Point-of-Care Devices

Infectious diseases caused by bacteria and viruses are highly contagious and can easily be transmitted via air, water, body fluids, etc. Throughout human civilization, there have been several pandemic outbreaks, such as the Plague, Spanish Flu, Swine-Flu, and, recently, COVID-19, amongst many others. Early diagnosis not only increases the chance of quick recovery but also helps prevent the spread of infections. Conventional diagnostic techniques can provide reliable results but have several drawbacks, including costly devices, lengthy wait time, and requirement of trained professionals to operate the devices, making them inaccessible in low-resource settings. Thus, a significant effort has been directed towards point-of-care (POC) devices that enable rapid diagnosis of bacterial and viral infections. A majority of the POC devices are based on plasmonics and/or microfluidics-based platforms integrated with mobile readers and imaging systems. These techniques have been shown to provide rapid, sensitive detection of pathogens. The advantages of POC devices include low-cost, rapid results, and portability, which enables on-site testing anywhere across the globe. Here we aim to review the recent advances in novel POC technologies in detecting bacteria and viruses that led to a breakthrough in the modern healthcare industry.

Nanomaterial-mediated paper-based biosensors for colorimetric pathogen detection

The pervasive spread of infectious diseases and pandemics, such as the 2019 coronavirus disease (COVID-19), are becoming increasingly serious and urgent threats to human health. Preventing the spread of such diseases prioritizes the development of sensing devices that can rapidly, selectively, and reliably detect pathogens at minimal cost. Paper-based analytical devices (PADs) are promising tools that satisfy those criteria. Numerous paper-based biosensors have been established that rival conventional pathogen detection methods. Among them, colorimetric strategies are promising since results can be interpreted by eye, and are simple to operate, which is advantageous for point-of-care testing (POCT). Particularly, the application of nanomaterials on paper-based biosensors has become important as these materials are capable of converting signals from pathogens through unique mechanisms to yield an amplified colorimetric readout. To highlight the research progress on using nanomaterials in colorimetric paper-based biosensor for pathogen detection, we discuss the sensing mechanisms of how they work, structural and analytical characteristics of the devices, and representative recent applications. Current challenges and future directions of using PADs and nanomaterial-mediated strategies are also discussed.

A panel of anti-influenza virus nucleoprotein antibodies selected from phage-displayed synthetic antibody libraries with rapid diagnostic capability to distinguish diverse influenza virus subtypes

Immunoassays based on sandwich immuno-complexes of capture and detection antibodies simultaneously binding to the target analytes have been powerful technologies in molecular analyses. Recent developments in single molecule detection technologies enable the detection limit of the sandwich immunoassays approaching femtomolar (10^-15 M), driving the needs of developing sensitive and specific antibodies for ever-increasingly broad applications in detecting and quantifying biomarkers. The key components underlying the sandwich immunoassays are antibody-based affinity reagents, for which the conventional sources are mono- or poly-clonal antibodies from immunized animals. The downsides of the animal-based antibodies as affinity reagents arise from the requirement of months of development timespan and limited choices of antibody candidates due to immunodominance of humoral immune responses in animals. Hence, developing animal antibodies capable of distinguishing highly related antigens could be challenging. To overcome the limitation imposed by the animal immune systems, we developed an in vitro methodology based on phage-displayed synthetic antibody libraries for diverse antibodies as affinity reagents against closely related influenza virus nucleoprotein (NP) subtypes, aiming to differentiating avian influenza virus (H5N1) from seasonal influenza viruses (H1N1 and H3N2), for which the NPs are closely related by 90-94% in terms of pairwise amino acid sequence identity. We applied the methodology to attain, within four weeks, a panel of IgGs with distinguishable specificities against a group of representative NPs with pairwise amino acid sequence identities up to more than 90%, and the antibodies derived from the antibody libraries without further affinity refinement had comparable affinity of mouse antibodies to the NPs with the detection limit less than 1 nM of viral NP from lysed virus with sandwich ELISA. The panel of IgGs were capable of rapidly distinguishing infections due to virulent avian influenza virus from infections of seasonal flu, in responding to a probable emergency scenario where avian influenza virus would be transmissible among humans overlapping with the seasonal influenza infections. The results indicate that the in vitro antibody development methodology enables developing diagnostic antibodies that would not otherwise be available from animal-based antibody technologies.

The Immunoprobe Aggregation State is Central to Dipstick Immunoassay Performance

As new infectious disease outbreaks become more likely, it is important to be able to develop and deploy appropriate testing in time. Paper-based immunoassays are rapid, cheap, and easy to produce at scale and relatively user friendly but often suffer from low selectivity and sensitivity. Understanding the molecular mechanisms of paper immunoassays may help improve and hasten development and therefore production and market availability. Here, we study how the behavior of nanoparticle-antibody immunoprobes in paper dipstick immunoassays is impacted by synthesis strategy and surface chemistry architecture. We conjugate gold nanoparticles to polyclonal anti-immunoglobulin G (IgG) and anti-zika NS1 antibodies by electrostatic adsorption and N-hydroxysuccinimide (NHS) and hydrazide (Hz) chemistries. The immunoprobes were used in paper immunoassays and the effective affinity for the antigen was quantified from the test line intensities, as well as the distribution of the immunoprobes throughout the strips. The results show that nanoparticle colloidal stability, both post synthesis and during antigen binding, is a key factor and affects immunoassay results and performance, often through reduction or loss of signal.

Optimization of paper-based nanoparticle immunoassays for direct detection of the bacterial pathogen V. parahaemolyticus in oyster hemolymph

The detection of foodborne pathogens is critical for disease control and infection prevention, especially in seafood consumed raw or undercooked. Paper-based diagnostic tools are promising for rapid fieldable detection and provide a readout by eye due to the use of gold nanoparticle immunoprobes. Here we study different strategies to overcome these challenges in a real biological matrix, oyster hemolymph, for the detection of the pathogenic bacteria Vibrio parahaemolyticus (Vp). Nanoparticle surface chemistry, nitrocellulose speed and blocking, running steps, and antibody concentrations on the NP and nitrocellulose were all studied. Their effect on paper immunoassay signal intensity was quantified to determine optimal conditions, which enabled the detection of Vp directly from hemolymph below pathogenic concentrations.

Highly Sensitive Chemiluminescence-Based Lateral Flow Immunoassay for Cardiac Troponin I Detection in Human Serum

Lateral flow assays (LFAs) have become the most common biosensing platforms for point-of-care testing due to their compliance with the ASSURED (affordable, sensitive, specific, user-friendly, rapid/robust, equipment-free, and deliverable to end-users) guidelines stipulated by the World Health Organization. However, the limited analytical sensitivity and low quantitative capability of conventional LFAs, which use gold nanoparticles (AuNPs) for colorimetric labeling, have prevented high-performance testing. Here, we report the development of a highly sensitive chemiluminescence (CL)-based LFA involving AuNPs conjugated with aldehyde-activated peroxidase and antibody molecules-i.e., AuNP-(ald)HRP-Ab-as a new conjugation scheme for high-performance testing in LFAs. When paired with the CL-based signal readout modality, the AuNP-(ald)HRP-Ab conjugate resulted in 110-fold enhanced sensitivity over the colorimetric response of a typical AuNP-Ab conjugate. To evaluate the performance of the CL-based LFA, we tested it with human cardiac troponin I (cTnI; a standard cardiac biomarker used to diagnose myocardial infarction) in standard and clinical serum samples. Testing the standard samples revealed a detection limit of 5.6 pg·mL^-1 and acceptably reliable precision (with a coefficient of variation of 2.3%-8.4%), according to clinical guidelines. Moreover, testing the clinical samples revealed a high correlation (r = 0.97) with standard biochemical analyzers, demonstrating the potential clinical utility of the CL-based LFA for high-performance cTnI testing.

Oligonucleotide-templated lateral flow assays for amplification-free sensing of circulating microRNAs

Herein we demonstrate the first example of oligonucleotide-templated reaction (OTR) performed on paper, using lateral flow to capture and concentrate specific nucleic acid biomarkers on a test line. Quantitative analysis, using a low-cost benchtop fluorescence reader showed very high specificity down to the single nucleotide level and proved sensitive enough for amplification-free, on-chip, detection of endogenous concentrations of miR-150-5p, a recently identified predictive blood biomarker for preterm birth.