Water-stable perovskite-silica nanocomposites for encoded microbeads construction and multiplexed detection

All-inorganic lead halide perovskite nanocrystals exhibiting bright luminescence have great potential as fluorescence elements for optical encoding. However, their limited stability in water hinders the application in biosensing. In this study, novel optical encoded microbeads based on CsPbX3 (X = Cl, Br) nanocrystals are developed and applied in bead-based suspension arrays for the first time. Through the in-situ crystallization of CsPbX3 nanocrystals within mesoporous silica nano-templates (MSNs), accompanied by mesopores collapse after sintering, CsPbX3@MSNs (X3M) nanocomposites with uniform morphology and stable fluorescence intensity in aqueous solutions for up to 50 days are obtained. By assembling X3M with microspheres to form a host-guest structure, an optical encoding microbead (MX3M) library is established by varying the X3M ratio, halide composition, and the size of host microspheres, which can be easily decoded under multi-channel flow cytometer. As a result, MX3M exhibits outstanding capacity for specific target capture and negligible nonspecific absorption performance in the multiplex nucleic acid detection of respiratory viruses, with a low limit of detection (10 copies/rxn). This result highlights the tremendous potential of MX3M encoded microbeads constructed based on CsPbX3 nanocrystals for multiplexed bioassays.

Recent advances in quantum dot-based fluorescence-linked immunosorbent assays

Fluorescence immunoassays have been given considerable attention among the quantitative detection methods in the clinical medicine and food safety testing fields. In particular, semiconductor quantum dots (QDs) have become ideal fluorescent probes for highly sensitive and multiplexed detection due to their unique photophysical properties, and the QD fluorescence-linked immunosorbent assay (FLISA) with high sensitivity, high accuracy, and high throughput has been greatly developed recently. In this manuscript, the advantages of applying QDs to FLISA platforms and some strategies for their application to in vitro diagnostics and food safety are discussed. Given the rapid development of this field, we classify these strategies based on the combination of QD types and detection targets, including traditional QDs or QD micro/nano-spheres-FLISA, and multiple FLISA platforms. In addition, some new sensors based on the QD-FLISA are introduced; this is one of the hot spots in this field. The current focus and future direction of QD-FLISA are also discussed, which provides important guidance for the further development of FLISA.

Preparation of Fluorescence-Encoded Microspheres Based on Hydrophobic Conjugated Polymer-Dye Combination and the Immunoassay

Fluorescent microspheres are greatly demanded in many applications based on high-throughput suspension array technology. To realize the multiplexed assay, microspheres should be encoded to identify the interaction between analytes and spheres. This study advanced a strategy for preparing fluorescence-encoded microspheres, employing two hydrophobic fluorophores, poly(p-phenyleneethylene) (PPE), and Nile Red (NR), as well as the monodisperse amino-modified porous substrate polymeric spheres, poly(glycidyl methacrylate) microspheres (APGMA). Loading the fluorophores sequentially onto the substrate spheres via adsorption by immersing the spheres in the dipping solution of fluorophores resulted in the APGMA-PPE-NR spheres. By varying the concentration and combination of fluorophores in the solution, an array of 64-code APGMA-PPE-NR spheres was obtained and could be easily individually decoded via flow cytometry. A 2D dot plot from the flow cytometry of a set of mixed spheres with four different codes could also be differentiated, coincident with the overlaid plots of the spheres' corresponding codes but measured individually. These spheres were found to have good stability against washing, photobleaching, and thermal treatment. In addition, a sandwich immunoassay for the detection of goat IgG was performed, and the capability of the encoded spheres to be used in suspension array technology was demonstrated.

Fluorescent microbeads for point-of-care testing: a review

Microbead-based point-of-care testing (POCT) has demonstrated great promise in translating detection modalities from bench-side to bed-side. This is due to the ease of visualization, high surface area-to-volume ratio of beads for efficient target binding, and efficient encoding capability for simultaneous detection of multiple analytes. This review (with 112 references) summarizes the progress made in the field of fluorescent microbead-based POCT. Following an introduction into the field, a first large section sums up techniques and materials for preparing microbeads, typically of dye-labelled particles, various kinds of quantum dots and upconversion materials. Further subsections cover the encapsulation of nanoparticles into microbeads, decoration of nanoparticles on microbeads, and in situ embedding of nanoparticles during microbead synthesis. A next large section summarizes microbead-based fluorometric POCT, with subsections on detection of nucleic acids, proteins, circulating tumor cells and bacteria. A further section covers emerging POCT based on the use of smartphones or flexible microchips. The last section gives conclusions and an outlook on current challenges and possible solutions. Aside from giving an overview on the state of the art, we expect this article to boost the further development of POCT technology. Graphical Abstract Schematic presentation of the fabrication of microbeads, the detection targets of interest including bacteria, circulating tumor cells (CTCs), protein and nucleic acid, and the emerging point-of-care testing (POCT) platform. The colored wheels of the bus represent the fluorescent materials embedded in (red color) or decorated on the surface of microbeads (green color).

Polymers mediate a one-pot route for functionalized quantum dot barcodes with a large encoding capacity

With the increasing demands for high-throughput multiplexed bioassays, quantum dot (QD)-encoded microbeads as biocarriers for various bioreactions have attracted considerable attention. However, three key requirements for these biocarriers are still longstanding issues: a stable fluorescence intensity, a large encoding capacity and abundant surface functional groups. Here, a novel one-pot strategy is developed, generating functionalized QD-encoded microspheres with a strong fluorescence intensity and optical stability. With poly(styrene-co-maleic anhydride) (PSMA) molecules as mediators, the encapsulation of QDs and carboxylation of the bead surface are integrated together, greatly improving the preparation efficiency and guaranteeing their potential application in biodetection. Moreover, the mechanism for preparing QD-doped beads is further proposed, which helps to precisely manipulate the preparation process and accurately encode the beads. Through this approach, a single- and dual-color barcode library of QD-encoded microspheres has been successfully established, which demonstrates their great potential in suspension arrays.

On-demand one-step synthesis of small-sized fluorescent–magnetic bifunctional microparticles on a droplet-splitting chip

Generation of small-sized multifunctional microparticles: multifunctional microparticles were easily produced based on droplet splitting and photopolymerization in a single step.

Layer-by-layer fabrication of fluorescent microspheres using micelles as a spacer: simultaneously realizing fluorescence enhancement and introduction of bioconjugation sites

Fluorescent microspheres fabricated using a conjugated polymer and micelles are demonstrated to have strong emission and are effective for bioconjugation.

Dual-spectra encoded suspension array using reversed-phase microemulsion UV curing and electrostatic self-assembling

The rapid growth of demand for high-throughput multiplexed biochips from modern biotechnology has led to growing interest in suspension array based on multi-channel encoded microbeads. We prepare dual-spectra encoded PEGDA microbeads (DSEPM) by reversed-phase microemulsion UV curing method and layer-by-layer electrostatic self-assembly method. Excitation of the synthesized DSEPM results in two spectra, including fluorescence spectra from quantum dots and laser induced breakdown spectra from nanoparticles with specific elements. With further surface modification and bio-probes grafting, we use DSEPM to carry a series of detection experiments of biomolecules. The adsorption experiment to two types of anti-IgG in mixture sample has demonstrated the availability of DSEPM in multiplexing. Then, the contrast experiment has verified the specificity of DSEPM in detection. Finally, we carry out the concentration gradient experiment and obtain the response curve to show the performance of DSEPM in quantitative analysis. The results indicate our method provide an effective way to improve multiplexed biochips with more coding capacity, accuracy and stability.

The rapid growth of demand for high-throughput multiplexed biochips from modern biotechnology has led to growing interest in suspension array based on multi-channel encoded microbeads.

Microfluidic synthesis of QD-encoded PEGDA microspheres for suspension assay

A simple microfluidic device is designed to generate monodispersed QD-encoded PEGDA microbeads. PEGDA/PDA composite microspheres are prepared to easily couple protein on their surface. A sandwich immunoassay of rabbit IgG is performed to indicate that PDA on the bead surface facilitates efficient attachment of biomacromolecules.

Suspension arrays based on nanoparticle-encoded microspheres for high-throughput multiplexed detection

Spectrometrically or optically encoded microsphere based suspension array technology (SAT) is applicable to the high-throughput, simultaneous detection of multiple analytes within a small, single sample volume. Thanks to the rapid development of nanotechnology, tremendous progress has been made in the multiplexed detecting capability, sensitivity, and photostability of suspension arrays. In this review, we first focus on the current stock of nanoparticle-based barcodes as well as the manufacturing technologies required for their production. We then move on to discuss all existing barcode-based bioanalysis patterns, including the various labels used in suspension arrays, label-free platforms, signal amplification methods, and fluorescence resonance energy transfer (FRET)-based platforms. We then introduce automatic platforms for suspension arrays that use superparamagnetic nanoparticle-based microspheres. Finally, we summarize the current challenges and their proposed solutions, which are centered on improving encoding capacities, alternative probe possibilities, nonspecificity suppression, directional immobilization, and "point of care" platforms. Throughout this review, we aim to provide a comprehensive guide for the design of suspension arrays, with the goal of improving their performance in areas such as multiplexing capacity, throughput, sensitivity, and cost effectiveness. We hope that our summary on the state-of-the-art development of these arrays, our commentary on future challenges, and some proposed avenues for further advances will help drive the development of suspension array technology and its related fields.

Enhancement of performance in porous bead-based microchip sensors: Effects of chip geometry on bio-agent capture

Measuring low concentrations of clinically-important biomarkers using porous bead-based lab-on-a-chip (LOC) platforms is critical for the successful implementation of point-of-care (POC) devices. One way to meet this objective is to optimize the geometry of the bead holder, referred to here as a micro-container. In this work, two geometric micro-containers were explored, the inverted pyramid frustum (PF) and the inverted clipped pyramid frustum (CPF). Finite element models of this bead array assay system were developed to optimize the micro-container and bead geometries for increased pressure, to increase analyte capture in porous bead-based fluorescence immunoassays. Custom micro-milled micro-container structures containing an inverted CPF geometry resulted in a 28% reduction in flow-through regions from traditional anisotropically-etched pyramidal geometry derived from Si-111 termination layers. This novel "reduced flow-through" design resulted in a 33% increase in analyte penetration into the bead and twofold increase in fluorescence signal intensity as demonstrated with C-Reactive Protein (CRP) antigen, an important biomarker of inflammation. A consequent twofold decrease in the limit of detection (LOD) and the limit of quantification (LOQ) of a proof-of-concept assay for the free isoform of Prostate-Specific Antigen (free PSA), an important biomarker for prostate cancer detection, is also presented. Furthermore, a 53% decrease in the bead diameter is shown to result in a 160% increase in pressure and 2.5-fold increase in signal, as estimated by COMSOL models and confirmed experimentally by epi-fluorescence microscopy. Such optimizations of the bead micro-container and bead geometries have the potential to significantly reduce the LODs and reagent costs for spatially programmed bead-based assay systems of this type.

Microfluidic generation of uniform quantum dot-encoded microbeads by gelation of alginate

A facile method was reported to generate monodispersed QD encoded alginate microbeads by employing a simple microfluidic device using an internal gelation approach. The application of the as-prepared microbeads for a suspension assay was demonstrated.

Realization of fluorescence color tuning for poly(p-phenylenevinylene) coated microspheres via a heterogeneous catalytic thermal elimination process

Fluorescent microspheres with clear core–shell structures and various emission colors were successfully prepared via a catalytic elimination process.

Preparation of multi-shell structured fluorescent composite nanoparticles for ultrasensitive human procalcitonin detection

In this paper, we reported the preparation of carboxyl functionalized quantum dots (QDs)-embedded silica nanoparticles by combining layer-by-layer (LbL) self-assembly technique and a multi-layer protection method.

Self-healing encapsulation strategy for preparing highly stable, functionalized quantum-dot barcodes

Quantum dot (QD) barcodes are becoming an urgent requirement for researchers and clinicians to obtain high-density information in multiplexed suspension (bead-based) assay. However, how to improve the stability of quantum dot barcodes is a longstanding issue. Here, we present a new self-healing encapsulation strategy to generate functionalized uniform quantum dots barcodes with high physical and chemical stability. This efficient and facile strategy could make porous polymer microspheres self-heal to encapsulate QDs via the thermal motion and interaction of the molecular chains. Consequently, the new strategy solved especially the QDs leakage problem and improved the chemical stability under different pH physiological conditions as well as the longtime storage stability. In the meantime, the encoding capacity and the spatial distribution uniformity of quantum dots could be also improved. Furthermore, immunofluorescence assays for alpha fetoprotein (AFP) detections indicated that carboxyl groups on the surface of QD-encoded microspheres could facilitate efficient attachment of biomacromolecules.

Incorporating fluorescent dyes into monodisperse melamine–formaldehyde resin microspheres via an organic sol–gel process: a pre-polymer doping strategy

An organic sol–gel process is developed to incorporate various organic fluorescent dyes into monodisperse melamine–formaldehyde (MF) resin microspheres. The dye incorporating mechanism is investigated and fluorescence-encoded microsphere arrays are prepared.

NIR-emitting quantum dot-encoded microbeads through membrane emulsification for multiplexed immunoassays

NIR-emitting CdSeTe/CdS/ZnS core/shell/shell QD-encoded microbeads are combined with common flow cytometry with one laser for multiplexed detection of hepatitis B virus (HBV). A facile one-pot synthetic route is developed to prepare CdSeTe/CdS/ZnS core/shell/shell QDs with high photoluminescence quantum yield and excellent stability in liquid paraffin, and a Shirasu porous glass (SPG) membrane emulsification technique is applied to incorporate the QDs into polystyrene-maleic anhydride (PSMA) microbeads to obtain highly fluorescent QD-encoded microbeads. The relatively wide NIR photoluminescence full width half maximum of the CdSeTe/CdS/ZnS QDs is used to develop a 'single wavelength' encoding method to obtain different optical codes by changing the wavelengh and emission intensity of the QDs incorporated into the microbeads. Moreover, a detection platform combining NIR-emitting CdSeTe/CdS/ZnS QD-encoded microbeads and Beckman Coulter FC 500 flow cytometry with one laser of 488 nm is successfully used to conduct a 2-plex hybridization assay for hepatitis B surface antigen (HBsAg), hepatitis B e antigen (HBeAg), and a 3-plex hybridization assay for hepatitis B surface antibody (HBsAb), hepatitis B e antibody (HBeAb), and hepatitis B core antibody (HBcAb), which suggests the promising application of NIR QD-encoded microbeads for multiplex immunoassays.

Sandwich-type immunosensors and immunoassays exploiting nanostructure labels: A review

Methods based on sandwich-type immunosensors and immunoassays have been developed for detection of multivalent antigens/analytes with more than one eptiope due to the use of two matched antibodies. High-affinity antibodies and appropriate labels are usually employed for the amplification of detectable signal. Recent research has looked to develop innovative and powerful novel nanoparticle labels, controlling and tailoring their properties in a very predictable manner to meet the requirements of specific applications. This articles reviews recent advances, exploiting nanoparticle labels, in the sandwich-type immunosensors and immunoassays. Routine approaches involve noble metal nanoparticles, carbon nanomaterials, semiconductor nanoparticles, metal oxide nanostructures, and hybrid nanostructures. The enormous signal enhancement associated with the use of nanoparticle labels and with the formation of nanoparticle-antibody-antigen assemblies provides the basis for sensitive detection of disease-related proteins or biomolecules. Techniques commonly rely on the use of biofunctionalized nanoparticles, inorganic-biological hybrid nanoparticles, and signal tag-doped nanoparticles. Rather than being exhaustive, this review focuses on selected examples to illustrate novel concepts and promising applications. Approaches described include the biofunctionalized nanoparticles, inorganic-biological hybrid nanoparticles, and signal tage-doped nanoparticles. Further, promising application in electrochemical, mass-sensitive, optical and multianalyte detection are discussed in detail.

Efficient incorporation of quantum dots into porous microspheres through a solvent-evaporation approach

Quantum dot (QD)-encoded microspheres play an important role in suspension arrays by acting as supports for various reactions between biomolecules. With regard to QD-encoded microspheres utilized in suspension arrays, three key requirements are controllable size, abundant surface functional groups, and especially excellent fluorescence properties. In this paper, narrowly dispersed poly(styrene-co-divinylbenzene-co-methylacrylic acid) (PSDM) microspheres with specific size, surface carboxyl groups, and porous structures were synthesized by seeded copolymerization. In order to improve the incorporation efficiency of QDs within microspheres, we developed a swelling-evaporation approach in which the swelling process was combined with gradual evaporation of the solvent and thus gradual concentration of QDs in the dispersion solution. This approach was demonstrated to be an efficient method for improving the fluorescence intensity of resultant microspheres compared with the use of swelling alone. Moreover, the porous structure was shown to aid the penetration of QDs into the interiors of the microspheres. Through this approach, microspheres encoded with either single or multiple wavelength-emitting QDs were fabricated effectively. The suspension immunoassays were then founded based on the QD-encoded microspheres, by coating mouse antihuman chorionic gonadotropin as the probe for goat antimouse IgG detection. The positive results determined by Luminex 100 and the low cytotoxicity of the QD-encoded microspheres demonstrated their great potential in suspension arrays.

Fluorescent-magnetic dual-encoded nanospheres: a promising tool for fast-simultaneous-addressable high-throughput analysis

Bead-based optical encoding or magnetic encoding techniques are promising in high-throughput multiplexed detection and separation of numerous species under complicated conditions. Therefore, a self-assembly strategy implemented in an organic solvent is put forward to fabricate fluorescent-magnetic dual-encoded nanospheres. Briefly, hydrophobic trioctylphosphine oxide-capped CdSe/ZnS quantum dots (QDs) and oleic acid-capped nano-γ-Fe2O3 magnetic particles are directly, selectively and controllably assembled on branched poly(ethylene imine)-coated nanospheres without any pretreatment, which is crucial to keep the high quantum yield of QDs and good dispersibility of γ-Fe2O3. Owing to the tunability of coating amounts of QDs and γ-Fe2O3 as well as controllable fluorescent emissions of deposited-QDs, dual-encoded nanospheres with different photoluminescent emissions and gradient magnetic susceptibility are constructed. Using this improved layer-by-layer self-assembly approach, deposition of hydrophobic nanoparticles onto hydrophilic carriers in organic media can be easily realized; meanwhile, fluorescent-magnetic dual-functional nanospheres can be further equipped with readable optical and magnetic addresses. The resultant fluorescent-magnetic dual-encoded nanospheres possess both the unique optical properties of QDs and the superparamagnetic properties of γ-Fe2O3, exhibiting good monodispersibility, huge encoding capacity and nanoscale particle size. Compared with the encoded microbeads reported by others, the nanometre scale of the dual-encoded nanospheres gives them minimum steric hindrance and higher flexibility.