A nanostructured aluminum oxide-based microfluidic device for enhancing immunoassay’s fluorescence and detection sensitivity

Microfluidic devices integrated with plasmonic nanostructures for sensitive fluorescent immunoassays

The robust identification and quantification of various biomarkers is of utmost significance in clinical diagnostics and precision medicine. Fluorescent immunoassays are widely used and considered as a gold standard for biomarker detection due to their high specificity and accuracy. However, current commercial immunoassay tests suffer from limited detection sensitivity and complicated, labor-intensive operation procedures, making them impractical for point-of-care diagnosis, particularly in resource-limited regions. Recently, microfluidic immunoassay devices integrated with plasmonic nanostructures have emerged as a powerful tool for sensitive detection of biomarkers, addressing specific issues, such as integration schemes, easy operation, multiplexed detection, and sensitivity enhancement. In this paper, we provide a discussion on the recent advances in the plasmonic nanostructures integrated with microfluidic devices for fluorescent immunoassays. We shed light on the nanofabrication strategies and various fluidic designs for rapid, sensitive, and highly efficient sensing of antigens. Finally, we share our perspectives on the potential directions of these integrated devices for practical applications.

Plasmonic Fluorescence Enhancement in Diagnostics for Clinical Tests at Point-of-Care: A Review of Recent Technologies

Fluorescence-based biosensors have widely been used in the life-sciences and biomedical applications due to their low limit of detection and a diverse selection of fluorophores that enable simultaneous measurements of multiple biomarkers. Recent research effort has been made to implement fluorescent biosensors into the exploding field of point-of-care testing (POCT), which uses cost-effective strategies for rapid and affordable diagnostic testing. However, fluorescence-based assays often suffer from their feeble signal at low analyte concentrations, which often requires sophisticated, costly, and bulky instrumentation to maintain high detection sensitivity. Metal- and metal oxide-based nanostructures offer a simple solution to increase the output signal from fluorescent biosensors due to the generation of high field enhancements close to a metal or metal oxide surface, which has been shown to improve the excitation rate, quantum yield, photostability, and radiation pattern of fluorophores. This article provides an overview of existing biosensors that employ various strategies for fluorescence enhancement via nanostructures and have demonstrated the potential for use as POCT. Biosensors using nanostructures such as planar substrates, freestanding nanoparticles, and metal-dielectric-metal nanocavities are discussed with an emphasis placed on technologies that have shown promise towards POCT applications without the need for centralized laboratories.

ZnO Nanowire-Based Early Detection of SARS-CoV-2 Antibody Responses in Asymptomatic Patients with COVID-19

A serological immunoassay based on enzyme-linked immunosorbent assay (ELISA) is a crucial tool for screening and identification of human SARS-CoV-2 seroconversion. Various immunoassays are developed to detect the spike 1 (S1) and nucleocapsid (NP) proteins of SARS-CoV-2; however, these serological tests have low sensitivity. Here, a novel microplate (MP) is developed on which a ZnO nanowire (NW) is fabricated by a modified hydrothermal synthesis method. This plate is coated with SARS-CoV-2 NP and used as a fluorescent immunoassay (FIA) to detect antibodies specific for SARS-CoV-2 NP. Compared with the bare MP, the ZnO-NW MP binds high levels (up to 5 µg mL^-1) of SARS-CoV-2 NP tagged to histidine without any surface treatment. A novel serological assay based on the ZnO-NW MP is more sensitive than a commercial immunoassay, enabling early detection (within <5 days of a reverse transcription polymerase chain reaction-confirmed COVID-19 infection) of anti-SARS-CoV-2 NP IgG antibodies in asymptomatic patients with COVID-19. This is the first assay to detect early antibody responses to SARS-CoV-2 in asymptomatic patients. Therefore, this serological assay will facilitate accurate diagnosis of COVID-19, as well as estimation of COVID-19 prevalence and incidence.

Microfluidic chip grafted with integrin tension sensors for evaluating the effects of flowing shear stress and ROCK inhibitor on platelets

Integrins are key players in platelet adhesion and aggregation. Integrin molecular tensions, the forces transmitted by integrin molecules, are regulated by both mechanical and biochemical cues, and the outside-in and inside-out signaling has been extensively studied. While the mechanical properties of platelets at static status have been studied by atomic force microscopy, traction force microscopy and tension sensors, the biomechanical properties of flowing platelets remain elusive. Herein, we report microfluidic chips grafted with integrin tension sensors for microfluidic-force mapping in platelets. Specifically, the process of integrin α_IIbβ₃ mediating tension transmission and platelet adhesion under low flow rates has been obtained, and the process of platelet clustering at post-stenotic regions has been demonstrated. We found that flowing shear force can postpone the integrin-mediated tension transmission and platelet adhesion. We further evaluated the effect of Y-27632, a ROCK inhibitor that has been proven to reduce integrin-mediated platelet adhesion, at a series of concentrations and demonstrated that microfluidic chips with integrin tension sensors are sensitive to the concentration-dependent effects of Y-27632. Given their low cost and scalable throughput, these chips are ideal technical platforms for biological studies of platelets at flowing status and for platelet inhibitor or potential antiplatelet drug screening.

Measurement of serum prostate cancer markers using a nanopore thin film based optofluidic chip

Currently used cancer marker for prostate adenocarcinoma (PC), serum prostate-specific antigen (PSA), greatly overestimates PC population. Patients with high PSA levels have to undergo unnecessary but physically painful and expensive procedure such as prostate biopsies repeatedly. The reliability of PC test can be greatly increased by finding a protein that is secreted selectively by malignant, but not normal, prostate cells. A recently discovered novel protein, referred as neuroendocrine marker (NEM), is secreted only by malignant prostate cells and released in blood circulation. Although NEM seems to be significantly more reliable based on the data obtained from a limited cohort, currently available NEM ELISA is not suitable for undertaking a large study. Therefore, the goal of the present study was to develop an alternative, label-free assay system that can reliably measure NEM and PSA in patient samples. Herein an optofluidic chip that can reliably detect PSA as well as NEM in patient samples has been developed. The optofluidic chip, which consists of arrayed nanopore-based sensors fabricated from anodic aluminum oxide (AAO) thin film, offers improved sensitivity upon the optimization of the concentration of the detector antibodies immobilized on the sensor surface. The results demonstrate that the chip is reliable, extremely sensitive and requires just 1 µl of patient serum (or even less) to measure PSA and NEM even in a non-cancer individual. Compared with the traditional ELISA for PSA, the nanopore-based sensor assay is 50-100 fold more sensitive, and offers many advantages such as elimination of labeled antigen, need for sophisticated equipment and highly trained individuals. These advantages, along with the low cost, should make the technology suitable for point-of-care application to screen elderly male populations for PC and to monitor the progress of patients undergoing PC treatment.

Opto-Microfluidic Immunosensors: From Colorimetric to Plasmonic

Optical detection has long been the most popular technique in immunosensing. Recent developments in the synthesis of luminescent probes and the fabrication of novel nanostructures enable more sensitive and efficient optical detection, which can be miniaturized and integrated with microfluidics to realize compact lab-on-a-chip immunosensors. These immunosensors are portable, economical and automated, but their sensitivity is not compromised. This review focuses on the incorporation and implementation of optical detection and microfluidics in immunosensors; it introduces the working principles of each optical detection technique and how it can be exploited in immunosensing. The recent progress in various opto-microfluidic immunosensor designs is described. Instead of being comprehensive to include all opto-microfluidic platforms, the report centers on the designs that are promising for point-of-care immunosensing diagnostics, in which ease of use, stability and cost-effective fabrication are emphasized.