A droplet-based fluorescence polarization immunoassay (dFPIA) platform for rapid and quantitative analysis of biomarkers

Vision-Based Performance Analysis of an Active Microfluidic Droplet Generation System Using Droplet Images

This paper discusses an active droplet generation system, and the presented droplet generator successfully performs droplet generation using two fluid phases: continuous phase fluid and dispersed phase fluid. The performance of an active droplet generation system is analysed based on the droplet morphology using vision sensing and digital image processing. The proposed system in the study includes a droplet generator, camera module with image pre-processing and identification algorithm, and controller and control algorithm with a workstation computer. The overall system is able to control, sense, and analyse the generation of droplets. The main controller consists of a microcontroller, motor controller, voltage regulator, and power supply. Among the morphological features of droplets, the diameter is extracted from the images to observe the system performance. The MATLAB-based image processing algorithm consists of image acquisition, image enhancement, droplet identification, feature extraction, and analysis. RGB band filtering, thresholding, and opening are used in image pre-processing. After the image enhancement, droplet identification is performed by tracing the boundary of the droplets. The average droplet diameter varied from ~3.05 mm to ~4.04 mm in the experiments, and the average droplet diameter decrement presented a relationship of a second-order polynomial with the droplet generation time.

An amplified fluorescence polarization assay for sensitive sensing of organophosphorus pesticides via MnO2 nanosheets

It is highly desirable to develop a simple, efficient and sensitive strategy for organophosphorus pesticides (OPs) in both environment pollution and human health. Herein, a novel amplified fluorescence polarization (FP) biosensor was established for highly sensitive detection of OPs using MnO₂ nanosheets as the signal enhancer. In this system, OPs can suppress the activity of acetylcholinesterase (AChE) efficiently, blocking the hydrolysis reaction of acetylthiocholine (ATCh) to generate thiocholine (TCh) by AChE. TCh can lead the decomposition of MnO₂ nanosheets to manganese ions. So, without the influence of TCh, MnO₂ nanosheets can maintain its original shape and form a stable complex with FAM-DNA, which greatly enhanced the FP signal. This method can tremendously improve the sensitivity of FP with a detection limit of 0.01 ng/mL for diazinon. In addition, it was also applicable to determine other four OPs and investigate the level of diazinon in real water samples. Consequently, the proposed approach provides a new promising platform for detection of OPs and is expected to be used in application of environmental monitoring.

A near-infrared-II fluorescence anisotropy strategy for separation-free detection of adenosine triphosphate in complex media

Fluorescence anisotropy (FA) has been widely applied for detecting and monitoring special targets in life sciences. However, matrix autofluorescence restricted its further application in complex biological samples. Herein, we report a near-infrared-II (NIR-II) FA strategy for detecting adenosine triphosphate (ATP) in human serum samples and breast cancer cell lysate, which employed NIR-II fluorescent Ag₂Se quantum dots (QDs) as tags to reduce matrix autofluorescence effect and applied graphene oxide (GO) to enhance fluorescence anisotropy signals. In the presence of ATP, the recognition between NIR-II Ag₂Se QDs labeled aptamer (QD-pDNA) and ATP led to the release of QD-pDNA from GO, resulting in the obvious decrease of FA values. ATP could be quantitatively detected in concentrations ranged from 3 nM to 2500 nM, with a detection limit down to 1.01 nM. This study showed that the developed NIR-II FA strategy could be applied for detecting targets in complex biological samples and had great potential for monitoring interactions between biomolecules in biomedical research.

Multiplexed Luminescence Oxygen Channeling Immunoassay Based on Dual-Functional Barcodes with a Host-Guest Structure: A Facile and Robust Suspension Array Platform

The development of a powerful immunoassay platform with capacities of both simplicity and high multiplexing is promising for disease diagnosis. To meet this urgent need, for the first time, a multiplexed luminescent oxygen channeling immunoassay (multi-LOCI) platform by implementation of LOCI with suspension array technology is reported. As the microcarrier of the platform, a unique dual-functional barcode with a host-guest structure composed of a quantum dot host bead (QDH) and LOCI acceptor beads (ABs) is designed, in which QDH provides function of high coding capacity while ABs facilitate the LOCI function. The analytes bridge QDH@ABs and LOCI donor beads (DBs) into a close proximity, forming a QDH@ABs-DBs "host-guest-satellite" superstructure that generates both barcode signal from QDH and LOCI signal induced by singlet oxygen channeling between ABs and DBs. Through imaging-based decoding, different barcodes are automatically distinguished and colocalized with LOCI signals. Importantly, the assay achieves simultaneous detection of multiple analytes within one reaction, simply by following a "mix-and-measure" protocol without the need for tedious washing steps. Furthermore, the multi-LOCI platform is validated for real sample measurements. With the advantages of robustness, simplicity, and high multiplexing, the platform holds great potential for the development of point-of-care diagnostics.

Analysis of deoxyribonuclease activity by conjugation-free fluorescence polarisation in sub-nanolitre droplets

We report the analysis of deoxyribonuclease (DNase) activity by conjugation-free fluorescence polarisation in a droplet-based microfluidic chip. DNase is a DNA cleaving enzyme and its activity is important in the maintenance of normal cellular functions. Alterations in DNase activity have been implicated as the cause of various cancers and autoimmune diseases. To date, various methods for the analysis of DNase activity have been reported. However, they are not cost effective due to the requirement of large sample volumes and the need for the conjugation of fluorescent dyes. In this study, we have used ethidium bromide (EtBr), a DNA intercalating reagent, as a fluorescent reporter without any prior conjugation or modification of DNA. Degradation of DNA by DNase 1 was monitored at a steady state by making changes in the fluorescence polarisation of EtBr in droplets with a volume of 330 picolitre at a 40 hertz frequency under visible light. Using this technique, we successfully determined the half-maximal inhibitory concentration (IC₅₀) of ethylenediaminetetraacetic acid (EDTA) for the inhibition of DNase 1 activity to be 1.56 ± 0.91 mM.

Machine Learning Approach to Enhance the Performance of MNP-Labeled Lateral Flow Immunoassay

The use of magnetic nanoparticle (MNP)-labeled immunochromatography test strips (ICTSs) is very important for point-of-care testing (POCT). However, common diagnostic methods cannot accurately analyze the weak magnetic signal from ICTSs, limiting the applications of POCT. In this study, an ultrasensitive multiplex biosensor was designed to overcome the limitations of capturing and normalization of the weak magnetic signal from MNPs on ICTSs. A machine learning model for sandwich assays was constructed and used to classify weakly positive and negative samples, which significantly enhanced the specificity and sensitivity. The potential clinical application was evaluated by detecting 50 human chorionic gonadotropin (HCG) samples and 59 myocardial infarction serum samples. The quantitative range for HCG was 1-1000 mIU mL^-1 and the ideal detection limit was 0.014 mIU mL^-1, which was well below the clinical threshold. Quantitative detection results of multiplex cardiac markers showed good linear correlations with standard values. The proposed multiplex assay can be readily adapted for identifying other biomolecules and also be used in other applications such as environmental monitoring, food analysis, and national security.

Streptavidin-triggered signal amplified fluorescence polarization for analysis of DNA-protein interactions

Fluorescence polarization (FP) is a sensitive, robust, and homogeneous assay format, able to probe a diversity of biological molecules and their interactions. Herein, we describe a new FP strategy based on the use of streptavidin as a signal amplifier. Such signal amplified fluorescence polarization (SAFP) was used to monitor the binding affinity of human angiogenin and a single-stranded DNA aptamer. Streptavidin was bound to a biotinylated single-stranded DNA aptamer and the interaction between this complex and Alexa Fluor 488 labelled human angiogenin was measured. A dissociation constant of 135.3 ± 32.9 nM and a limit of detection of 6.3 nM were successfully extracted only when the FP signal was increased (without binding hindrance) via streptavidin. Moreover, the demonstrated approach was specific to target molecules without any non-specific binding. The streptavidin-triggered SAFP method unlike amplification strategies that utilize nanomaterials (such as graphene oxides, carbon nanotubes, and metal nanoparticles) is not compromised by fluorescence quenching, and it is able to operate within nanomolar concentration regimes. Furthermore, unlike the other FP signal amplification strategies that use dual binding DNA probes, the presented method is simple to implement with signal amplification only requiring the binding of streptavidin with biotinylated DNA. This method could be expanded to analyze molecular interactions and it may be a useful tool for FP measurement by reducing the concentration of rare and expensive protein samples.

Cellphone-Enabled Microwell-Based Microbead Aggregation Assay for Portable Biomarker Detection

Quantitative biomarker detection methods featured with rapidity, high accuracy, and label-free are demonstrated for the development of point-of-care (POC) technologies or "beside" diagnostics. Microbead aggregation via protein-specific linkage provides an effective approach for selective capture of biomarkers from the samples, and can directly readout the presence and amount of the targets. However, sensors or microfluidic analyzers that can accurately quantify the microbead aggregation are scared. In this work, we demonstrate a microwell-based microbeads analyzing system, by which online manipulations of microbeads including trapping, arraying, and rotations can be realized, providing a series of microfluidic approaches to layout the aggregated microbeads for further convenient characterizations. Prostate specific antigen is detected using the proposed system, demonstrating the limit of detection as low as 0.125 ng/mL (3.67 pM). A two-step reaction kinetics model is proposed for the first time to explain the dynamic process of microbeads aggregation. The developed microbeads aggregation analysis system has the advantages of label-free detection, high throughput, and low cost, showing great potential for portable biomarker detection.

A compact fluorescence polarization analyzer with high-transmittance liquid crystal layer

Fluorescence polarization (FP) offers easy operation and rapid processing, making it implementable in molecular interaction analysis. Previously we have developed a unique FP measurement system using a liquid crystal (LC) layer and an image sensor. The system is based on a principle of synchronized detection between the switching rate of the LC layer and the sampling rate of the CCD. The FP system realized simultaneous multiple sample detection; however, the measurement precision was lower than that of the conventional FP apparatus. The main drawbacks were low light transmittance of the LC layer and insufficient synchronization between the LC layer and CCD. In this paper, we developed a new FP analyzer based on LC-CCD synchronization detection. By using a newly designed LC with high transmittance and improving synchronization, the performance of the system has been dramatically improved. Additionally, we reduced the cost by using an inexpensive CCD and an LED as the excitation source. Simultaneous FP immunoassay of multiple samples of prostaglandin E2 was performed. The error rate of the FP system is reduced from 16.9% to 3.9%, as comparable to the commercial conventional FP system.

An Integrated Microfluidic Processor for DNA-Encoded Combinatorial Library Functional Screening

DNA-encoded synthesis is rekindling interest in combinatorial compound libraries for drug discovery and in technology for automated and quantitative library screening. Here, we disclose a microfluidic circuit that enables functional screens of DNA-encoded compound beads. The device carries out library bead distribution into picoliter-scale assay reagent droplets, photochemical cleavage of compound from the bead, assay incubation, laser-induced fluorescence-based assay detection, and fluorescence-activated droplet sorting to isolate hits. DNA-encoded compound beads (10-μm diameter) displaying a photocleavable positive control inhibitor pepstatin A were mixed (1920 beads, 729 encoding sequences) with negative control beads (58 000 beads, 1728 encoding sequences) and screened for cathepsin D inhibition using a biochemical enzyme activity assay. The circuit sorted 1518 hit droplets for collection following 18 min incubation over a 240 min analysis. Visual inspection of a subset of droplets (1188 droplets) yielded a 24% false discovery rate (1166 pepstatin A beads; 366 negative control beads). Using template barcoding strategies, it was possible to count hit collection beads (1863) using next-generation sequencing data. Bead-specific barcodes enabled replicate counting, and the false discovery rate was reduced to 2.6% by only considering hit-encoding sequences that were observed on >2 beads. This work represents a complete distributable small molecule discovery platform, from microfluidic miniaturized automation to ultrahigh-throughput hit deconvolution by sequencing.

Analysis of ribonuclease activity in sub-nanoliter droplets by label-free fluorescence measurements

We report the results of a label-free analysis of ribonuclease activity using droplet-based microfluidics.

Fluorescence polarization measurement system using a liquid crystal layer and an image sensor

The detection system which enables simultaneous fluorescence polarization (FP) measurement of multiple samples was proposed and proven by a proof-of-concept experiment on the viscosity dependence of FP of fluorescein sample in water-ethylene glycol solution and another experiment on the FP immunoassay of prostaglandin E2 sample. The measurement principle of FP is based on the synchronization between the orientation of the liquid crystal molecules and the sampling frequency of a CCD. This report is the first description of the simultaneous FP measurement of multiple samples. This system has a great potential for equipment miniaturization and price reduction as well as providing simultaneous FP measurement of multiple samples.