Fluorescent nanocrystal-encoded microbeads for multiplexed cancer imaging and diagnosis

State-of-the-art developments in carbon quantum dots (CQDs): Photo-catalysis, bio-imaging, and bio-sensing applications

Carbon quantum dots (CQDs), the intensifying nanostructured form of carbon material, have exhibited incredible impetus in several research fields such as bio-imaging, bio-sensing, drug delivery systems, optoelectronics, photovoltaics, and photocatalysis, thanks to their exceptional properties. The CQDs show extensive photonic and electronic properties, as well as their light-collecting, tunable photoluminescence, remarkable up-converted photoluminescence, and photo-induced transfer of electrons were widely studied. These properties have great advantages in a variety of visible-light-induced catalytic applications for the purpose of fully utilizing the energy from the solar spectrum. The major purpose of this review is to validate current improvements in the fabrication of CQDs, characteristics, and visible-light-induced catalytic applications, with a focus on CQDs multiple functions in photo-redox processes. We also examine the problems and future directions of CQD-based nanostructured materials in this growing research field, with an eye toward establishing a decisive role for CQDs in photocatalysis, bio-imaging, and bio-sensing applications that are enormously effective and stable over time. In the end, a look forward to future developments is presented, with a view to overcoming challenges and encouraging further research into this promising field.

Trends of Bead Counting-Based Technologies Toward the Detection of Disease-Related Biomarkers

Nowadays, the biomolecular assay platforms built-up based on bead counting technologies have emerged to be powerful tools for the sensitive and high-throughput detection of disease biomarkers. In this mini-review, we classified the bead counting technologies into statistical counting platforms and digital counting platforms. The design principles, the readout strategies, as well as the pros and cons of these platforms are introduced in detail. Finally, we point out that the digital bead counting technologies will lead the future trend for the absolute quantification of critical biomarkers, and the integration of new signal amplification approaches and routine optical/clinical instruments may provide new opportunities in building-up easily accessible digital assay platforms.

Spectral-Time Multiplexing in FRET Complexes of AgInS2/ZnS Quantum Dot and Organic Dyes

Nowadays, multiplex analysis is very popular, since it allows to detect a large number of biomarkers simultaneously. Traditional multiplex analysis is usually based on changes of photoluminescence (PL) intensity and/or PL band spectral positions in the presence of analytes. Using PL lifetime as an additional parameter might increase the efficiency of multiplex methods. Quantum dots (QDs) can be used as luminescent markers for multiplex analysis. Ternary in-based QDs are a great alternative to the traditional Cd-based one. Ternary QDs possess all advantages of traditional QDs, including tunable photoluminescence in visible range. At the same time ternary QDs do not have Cd-toxicity, and moreover they possess long spectral dependent lifetimes. This allows the use of ternary QDs as a donor for time-resolved multiplex sensing based on Förster resonance energy transfer (FRET). In the present work, we implemented FRET from AgInS₂/ZnS ternary QDs to cyanine dyes absorbing in different spectral regions of QD luminescence with different lifetimes. As the result, FRET-induced luminescence of dyes differed not only in wavelengths but also in lifetimes of luminescence, which can be used for time-resolved multiplex analysis in biology and medicine.

Tempo-spectral multiplexing in flow cytometry with lifetime detection using QD-encoded polymer beads

Semiconductor quantum dots (QDs) embedded into polymer microbeads are known to be very attractive emitters for spectral multiplexing and colour encoding. Their luminescence lifetimes or decay kinetics have been, however, rarely exploited as encoding parameter, although they cover time ranges which are not easily accessible with other luminophores. We demonstrate here the potential of QDs made from II/VI semiconductors with luminescence lifetimes of several 10 ns to expand the lifetime range of organic encoding luminophores in multiplexing applications using time-resolved flow cytometry (LT-FCM). For this purpose, two different types of QD-loaded beads were prepared and characterized by photoluminescence measurements on the ensemble level and by single-particle confocal laser scanning microscopy. Subsequently, these lifetime-encoded microbeads were combined with dye-encoded microparticles in systematic studies to demonstrate the potential of these QDs to increase the number of lifetime codes for lifetime multiplexing and combined multiplexing in the time and colour domain (tempo-spectral multiplexing). These studies were done with a recently developed novel luminescence lifetime flow cytometer (LT-FCM setup) operating in the time-domain, that presents an alternative to reports on phase-sensitive lifetime detection in flow cytometry.

Integration of Flow Cytometry and Single Cell Sequencing

Integrating cytometric analysis of cells, mitochondria, and other polynucleotide-containing biological particles with high-throughput single particle sequencing would provide an ultimate bioanalytical tool, simultaneously assessing phenotype, functionality, genome, and transcriptome of each particle in a large population. Here, we describe how such integration could be performed by adapting existing, well-established technologies.

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.

Bioimaging Tools Based on Polyelectrolyte Microcapsules Encoded with Fluorescent Semiconductor Nanoparticles: Design and Characterization of the Fluorescent Properties

Fluorescent imaging is a widely used technique for detecting and monitoring the distribution, interaction, and transformation processes at molecular, cellular, and tissue level in modern diagnostic and other biomedical applications. Unique photophysical properties of fluorescent semiconductor nanocrystals "quantum dots" (QDs) make them advanced fluorophores for fluorescent labeling of biomolecules or optical encoding of microparticles to be used as bioimaging and theranostic agents in targeted delivery, visualization, diagnostics, and imaging. This paper reports on the results of development of an improved approach to the optical encoding of polyelectrolyte microcapsules with stable, covered with the multifunctional polyethyleneglycol derivatives water-soluble QDs, as well as characterization of the optical properties, morphological and structural properties of the encoded microcapsules. The embedding of QDs into the polymer microcapsule membrane through layer-by-layer deposition on a preliminarily formed polymeric polyelectrolyte shell makes it possible to obtain bright fluorescent particles with an adapted charge and size distribution that are distinctly discernible by flow cytometry as individual homogeneous populations. The fluorescent microcapsules developed can be used in further designing bioimaging and theranostic agents sensitive to various external stimuli along with photoexcitation.

Preparation and characterization of multimodal hybrid organic and inorganic nanocrystals of camptothecin and gold

We demonstrate a novel inorganic-organic crystalline nanoconstruct, where gold atoms were imbedded in the crystal lattices as defects of camptothecin nanocrystals, suggesting its potential use as simultaneous agents for cancer therapy and bioimaging. The incorporation of gold, a potential computed tomography (CT) contrast agent, in the nanocrystals of camptothecin was detected by transmission electron microscope (TEM) and further quantified by energy dispersive X-ray spectrometry (EDS) and inductively coupled plasma-optical emission spectrometers (ICP-OES). Due to gold's high attenuation coefficient, only a relatively small amount needs to be present in order to create a good noise-to-contrast ratio in CT imaging. The imbedded gold atoms and clusters are expected to share the same biological fate as the camptothecin nanocrystals, reaching and accumulating in tumor site due to the enhanced permeation and retention (EPR) effect.

Early diagnosis of disease using microbead array technology: A review

Early diagnosis of diseases (before they become advanced and incurable) is essential to reduce morbidity and mortality rates. With the advent of novel technologies in clinical laboratory diagnosis, microbead-based arrays have come to be recognized as an efficient approach, that demonstrates useful advantages over traditional assay methods for multiple disease-related biomarkers. Multiplexed microbead assays provide a robust, rapid, specific, and cost-effective approach for high-throughput and simultaneous screening of many different targets. Biomolecular binding interactions occur after applying a biological sample (such as blood plasma, saliva, cerebrospinal fluid etc.) containing the target analyte(s) to a set of microbeads with different ligand-specificities that have been coded in planar or suspension arrays. The ligand-receptor binding activity is tracked by optical signals generated by means of flow cytometry analysis in the case of suspension arrays, or by image processing devices in the case of planar arrays. In this review paper, we discuss diagnosis of cancer, neurological and infectious diseases by using optically-encoded microbead-based arrays (both multiplexed and single-analyte assays) as a reliable tool for detection and quantification of various analytes.

Luminescence lifetime encoding in time-domain flow cytometry

Time-resolved flow cytometry represents an alternative to commonly applied spectral or intensity multiplexing in bioanalytics. At present, the vast majority of the reports on this topic focuses on phase-domain techniques and specific applications. In this report, we present a flow cytometry platform with time-resolved detection based on a compact setup and straightforward time-domain measurements utilizing lifetime-encoded beads with lifetimes in the nanosecond range. We provide general assessment of time-domain flow cytometry and discuss the concept of this platform to address achievable resolution limits, data analysis, and requirements on suitable encoding dyes. Experimental data are complemented by numerical calculations on photon count numbers and impact of noise and measurement time on the obtained lifetime values.

Multiplexed microarrays based on optically encoded microbeads

In recent years, there has been growing interest in optically-encoded or tagged functionalized microbeads as a solid support platform to capture proteins or nucleotides which may serve as biomarkers of various diseases. Multiplexing technologies (suspension array or planar array) based on optically encoded microspheres have made possible the observation of relatively minor changes in biomarkers related to specific diseases. The ability to identify these changes at an early stage may allow the diagnosis of serious diseases (e.g. cancer) at a time-point when curative treatment may still be possible. As the overall accuracy of current diagnostic methods for some diseases is often disappointing, multiplexed assays based on optically encoded microbeads could play an important role to detect biomarkers of diseases in a non-invasive and accurate manner. However, detection systems based on functionalized encoded microbeads are still an emerging technology, and more research needs to be done in the future. This review paper is a preliminary attempt to summarize the state-of-the-art concerning diagnostic microbeads; including microsphere composition, synthesis, encoding technology, detection systems, and applications.

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.

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.

Quantum-dot-based suspension microarray for multiplex detection of lung cancer markers: preclinical validation and comparison with the Luminex xMAP® system

A novel suspension multiplex immunoassay for the simultaneous specific detection of lung cancer markers in bronchoalveolar lavage fluid (BALF) clinical samples based on fluorescent microspheres having different size and spectrally encoded with quantum dots (QDEM) was developed. The designed suspension immunoassay was validated for the quantitative detection of three lung cancer markers in BALF samples from 42 lung cancer patients and 10 control subjects. Tumor markers were detected through simultaneous formation of specific immune complexes consisting of a capture molecule, the target antigen, and biotinylated recognition molecule on the surface of the different QDEM in a mixture. The immune complexes were visualized by fluorescently labeled streptavidin and simultaneously analyzed using a flow cytometer. Preclinical validation of the immunoassay was performed and results were compared with those obtained using an alternative 3-plex immunoassay based on Luminex xMAP^® technology, developed on classical organic fluorophores. The comparison showed that the QDEM and xMAP^® assays yielded almost identical results, with clear discrimination between control and clinical samples. Thus, developed QDEM technology can become a good alternative to xMAP^® assays permitting analysis of multiple protein biomarkers using conventional flow cytometers.

Engineering of Optically Encoded Microbeads with FRET-Free Spatially Separated Quantum-Dot Layers for Multiplexed Assays

Quantum dot (QD) encoded microbeads are emerging for multiplexed analysis of biological markers. The quantitative encoding of microbeads prepared with different concentrations of QDs of different colors suffers from resonance energy transfer from the QDs fluorescing at shorter wavelengths to the QDs fluorescing at longer wavelengths. Here, we used the layer-by-layer deposition technique to spatially separate QDs of different colors with several polymer layers so that the distance between them would be larger than the Förster energy transfer radius. We performed fluorescence lifetime measurements to investigate and determine the conditions excluding significant resonance energy transfer between QDs within QD-encoded microbeads. Additionally, the number of QDs adsorbed onto microbeads was systematically established and multilayer structures of the QD-encoded microbead shells were characterized by scanning probe nanotomography. Finally, we prepared eight populations of FRET-free microbeads encoded with QDs of three colors at two intensity levels and demonstrated that all the optical codes are excitable at a single wavelength and may be clearly identified in three channels of a flow cytometer. The developed approach for engineering QD-encoded microbeads that are free from optical artefacts related to inter-QD resonance energy transfer paves the way to quantitative QD-based multiplexed assays.

Application of semiconductor quantum dots in bioimaging and biosensing

In this review we present new concepts and recent progress in the application of semiconductor quantum dots (QD) as labels in two important areas of biology, bioimaging and biosensing.

Fluorescent/magnetic micro/nano-spheres based on quantum dots and/or magnetic nanoparticles: preparation, properties, and their applications in cancer studies

The study of cancer is of great significance to human survival and development, due to the fact that cancer has become one of the greatest threats to human health. In recent years, the rapid progress of nanoscience and nanotechnology has brought new and bright opportunities to this field. In particular, the applications of quantum dots (QDs) and magnetic nanoparticles (MNPs) have greatly promoted early diagnosis and effective therapy of cancer. In this review, we focus on fluorescent/magnetic micro/nano-spheres based on QDs and/or MNPs (we may call them "nanoparticle-sphere (NP-sphere) composites") from their preparation to their bio-application in cancer research. Firstly, we outline and compare the main four kinds of methods for fabricating NP-sphere composites, including their design principles, operation processes, and characteristics (merits and limitations). The NP-sphere composites successfully inherit the unique fluorescence or magnetic properties of QDs or MNPs. Moreover, compared with the nanoparticles (NPs) alone, the NP-sphere composites show superior properties, which are also discussed in this review. Then, we summarize their recent applications in cancer research from three aspects, that is: separation and enrichment of target tumor cells or biomarkers; cancer diagnosis mainly through medical imaging or tumor biomarker detection; and cancer therapy via targeted drug delivery systems. Finally, we provide some perspectives on the future challenges and development trends of the NP-sphere composites.

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.

Quantum dot-based lab-on-a-bead system for multiplexed detection of free and total prostate-specific antigens in clinical human serum samples

An immunodiagnostic lab-on-a-bead suspension microarray based on microbeads encoded with quantum dots (QDs) has been developed and preclinically validated for multiplexed quantitative detection of prostate cancer markers in human serum samples. The sensitivity and specificity of the microarray are similar to those of "gold-standard" single-analyte ELISA. Moreover, the array has an improved immunoassay capacity, ensures quantitative detection of multiple cancer biomarkers and may be operational in a considerably wider dynamic range of concentrations. The array is characterized by reduced time and cost of analysis and is compatible with classical flow cytometers. Proof-of-concept preclinical tests ensured simultaneous quantitative determination of free and total prostate-specific antigens in human serum, with clear discrimination between the control and clinical samples. The proposed approach is flexible and paves the way to development of a wide variety of immunodiagnostic assays for multiplexed early diagnosis of various diseases.

FROM THE CLINICAL EDITOR

Early diagnosis of cancer can result in better prognosis for patients. Thus, the use of specific tumor markers is widely employed in clinical practice. Traditional screening methods only employ single markers. The authors here developed a microarray system based on microbeads encoded with quantum dots (QDs), which can be used for multiplexed quantitative detection. The validated results on patient samples should lead to the development of a wider variety of assays for other diseases.

Nanoparticle based fluorescence resonance energy transfer (FRET) for biosensing applications

Nanoparticle based FRET assays have higher energy transfer efficiency and better performance compared with traditional organic fluorophore based FRET assays.

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.

Highly sensitive single domain antibody-quantum dot conjugates for detection of HER2 biomarker in lung and breast cancer cells

Despite the widespread availability of immunohistochemical and other methodologies for screening and early detection of lung and breast cancer biomarkers, diagnosis of the early stage of cancers can be difficult and prone to error. The identification and validation of early biomarkers specific to lung and breast cancers, which would permit the development of more sensitive methods for detection of early disease onset, is urgently needed. In this paper, ultra-small and bright nanoprobes based on quantum dots (QDs) conjugated to single domain anti-HER2 (human epidermal growth factor receptor 2) antibodies (sdAbs) were applied for immunolabeling of breast and lung cancer cell lines, and their performance was compared to that of anti-HER2 monoclonal antibodies conjugated to conventional organic dyes Alexa Fluor 488 and Alexa Fluor 568. The sdAbs-QD conjugates achieved superior staining in a panel of lung cancer cell lines with differential HER2 expression. This shows their outstanding potential for the development of more sensitive assays for early detection of cancer biomarkers.

Fluorescent cross-linked polystyrene perylenebisimide/oligo(p-phenylenevinylene) microbeads with controlled particle size, tunable colors, and high solid state emission

A series fo cross-linked fluorescent polystyrene (PS) microbeads with narrow size distribution and intense solid state emission was developed. Fluorophores based on perylene bisimide (PBI) and oligo(p-phenylenevinylene) (OPV) designed as acrylic cross-linkers were introduced into the polymerization recipe in a two-stage dispersion polymerization, carried out in ethanol in the presence of poly(vinylpyrrolidone) (PVP) as stabilizer. The structural design permitted introduction of up to 10(-5) moles of the fluorophores into the polymerization medium without fouling of the dispersion. The particle size measured using dynamic light scattering (DLS) indicated that they were nearly monodisperse with size in the range 2-3 μm depending on the amount of fluorophore incorporated. Fluorescence microscope images of ethanol dispersion of the sample exhibited intense orange red emission for PS-PBI-X series and green emission for PS-OPV-X series. A PS incorporated with both OPVX and PBIX exhibited dual emission upon exciting at the OPV wavelength of 350 nm and PBI wavelength of 490 nm, respectively. The low incorporation of fluorophore resulted in almost complete absence of aggregation induced reduction in fluorescence as well as red-shifted aggregate emission. The solid state emission quantum yield measured using integrating-sphere setup indicated a very high quantum yield of ϕpowder = 0.71 for PS-OPV-X and ϕpowder = 0.25 for PS-PBI-X series. The cross-linked PS microbeads incorporating both OPV and PBI chromophores had a ϕpowder = 0.33 for PBI emission and ϕpowder = 0.20 for OPV emission. This strategy of introducing fluorophore as cross-linkers into the PS backbone is very versatile and amenable to simultaneous addition of different suitably designed fluorophores emitting at different wavelengths.

A microfluidic concentration-gradient droplet array generator for the production of multi-color nanoparticles

A microfluidic concentration-gradient droplet array generator (CDrAG) with parallel multi-channels and multi-layers was developed with 64 outlet channels producing 33 droplet gradient concentrations. A droplet production rate of 5 × 10(4) min(-1) was obtained, and the RSD value of droplet diameters in 64 groups is 5.5% (n = 64). Using the concentration gradient droplet array as parallel microreactors, 33 Au/Ag ratio nanoparticles were synthesized. The absorption spectra of the Au/Ag nanoparticles shifted from the spectrum of pure gold to one of pure silver. This demonstrates the CDrAG platform's promising potential to produce specific nanoparticle barcodes for high-throughput screening in chemistry, biology and a broad range of life science applications.

Quantum dots: heralding a brighter future for clinical diagnostics

Quantum dots (QDs) are semiconductor nanocrystals that possess unique optical properties including broad-range excitation, size-tunable narrow emission spectra and high photostability, giving them considerable value in various biomedical applications. The size and composition of QDs can be varied to obtain the desired emission properties and make them amenable to simultaneous detection of multiple targets. Furthermore, numerous surface functionalizations can be used to adapt QDs to the needed application. The successful use of QDs has been reported in the areas of in vitro diagnostics and imaging. There is also potential for multimodal applications for simultaneous imaging. Toxicity issues are still a prime concern with regards to in vivo applications on account of the toxic constituents of QDs.

Protein analytical assays for diagnosing, monitoring, and choosing treatment for cancer patients

Cancer treatment is often hindered by inadequate methods for diagnosing the disease or insufficient predictive capacity regarding therapeutic efficacy. Targeted cancer treatments, including Bcr-Abl and EGFR kinase inhibitors, have increased survival for some cancer patients but are ineffective in other patients. In addition, many patients who initially respond to targeted inhibitor therapy develop resistance during the course of treatment. Molecular analysis of cancer cells has emerged as a means to tailor treatment to particular patients. While DNA analysis can provide important diagnostic information, protein analysis is particularly valuable because proteins are more direct mediators of normal and diseased cellular processes. In this review article, we discuss current and emerging protein assays for improving cancer treatment, including trends toward assay miniaturization and measurement of protein activity.

Layered double hydroxides as carriers for quantum dots@silica nanospheres

Quantum dot-hydrotalcite layered nanoplatforms were successfully prepared following a one-pot synthesis. The process is very fast and a priori delamination of hydrotalcite is not a prerequisite for the intercalation of quantum dots. The novel materials were extensively characterized by X-ray diffraction, thermogravimetry, infrared spectroscopy, transmission electron microscopy, true color fluorescence microscopy, photoluminescence, and nitrogen adsorption. The quantum dot-hydrotalcite nanomaterials display extremely high stability in mimicking physiological media such as saline serum (pH 5.5) and PBS (pH 7.2). Yet, quantum dot release from the solid structure is noted. In order to prevent the leaking of quantum dots we have developed a novel strategy which consists of using tailor made double layered hydrotalcites as protecting shells for quantum dots embedded into silica nanospheres without changing either the materials or the optical properties.

Quantum dot-containing polymer particles with thermosensitive fluorescence

Composite polymer particles consisting of a solid poly(acrolein-co-styrene) core and a poly(N-vinylcaprolactam) (PVCL) polymer shell doped with CdSe/ZnS semiconductor quantum dots (QDs) were fabricated. The temperature response of the composite particles was observed as a decrease in their hydrodynamic diameter upon heating above the lower critical solution temperature of the thermosensitive PVCL polymer. Embedding QDs in the PVCL shell yields particles whose fluorescence is sensitive to temperature changes. This sensitivity was determined by the dependence of the QD fluorescence intensity on the distances between them in the PVCL shell, which reversibly change as a result of the temperature-driven conformational changes in the polymer. The QD-containing thermosensitive particles were assembled with protein molecules in such a way that they retained their thermosensitive properties, including the completely reversible temperature dependence of their fluorescence response. The composite particles developed can be used as local temperature sensors, as carriers for biomolecules, as well as in biosensing and various bioassays employing optical detection schemes.

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.

On-demand preparation of quantum dot-encoded microparticles using a droplet microfluidic system

Optical barcoding technology based on quantum dot (QD)-encoded microparticles has attracted increasing attention in high-throughput multiplexed biological assays, which is realized by embedding different-sized QDs into polymeric matrixes at precisely controlled ratios. Considering the advantage of droplet-based microfluidics, producing monodisperse particles with precise control over the size, shape and composition, we present a proof-of-concept approach for on-demand preparation of QD-encoded microparticles based on this versatile new strategy. Combining a flow-focusing microchannel with a double T-junction in a microfluidic chip, biocompatible QD-doped microparticles were constructed by shearing sodium alginate solution into microdroplets and on-chip gelating these droplets into a hydrogel matrix to encapsulate CdSe/ZnS QDs. Size-controllable QD-doped hydrogel microparticles were produced under the optimum flow conditions, and their fluorescent properties were investigated. A novel multiplex optical encoding strategy was realized by loading different sized QDs into a single droplet (and thus a hydrogel microparticle) with different concentrations, which was triggered by tuning the flow rates of the sodium alginate solutions entrapped with different-colored QDs. A series of QD-encoded microparticles were controllably, and continuously, produced in a single step with the present approach. Their application in a model immunoassay demonstrated the potential practicability of QD-encoded hydrogel microparticles in multiplexed biomolecular detection. This simple and robust strategy should be further improved and practically used in making barcode microparticles with various polymer matrixes.

Nanodevices in diagnostics

The real-time, personalized and highly sensitive early-stage diagnosis of disease remains an important challenge in modern medicine. With the ability to interact with matter at the nanoscale, the development of nanotechnology architectures and materials could potentially extend subcellular and molecular detection beyond the limits of conventional diagnostic modalities. At the very least, nanotechnology should be able to dramatically accelerate biomarker discovery, as well as facilitate disease monitoring, especially of maladies presenting a high degree of molecular and compositional heterogeneity. This article gives an overview of several of the most promising nanodevices and nanomaterials along with their applications in clinical practice. Significant work to adapt nanoscale materials and devices to clinical applications involving large interdisciplinary collaborations is already underway with the potential for nanotechnology to become an important enabling diagnostic technology.

Quantum dots in cell biology

Quantum dots are semiconductor nanocrystals that have broad excitation spectra, narrow emission spectra, tunable emission peaks, long fluorescence lifetimes, negligible photobleaching, and ability to be conjugated to proteins, making them excellent probes for bioimaging applications. Here the author reviews the advantages and disadvantages of using quantum dots in bioimaging applications, such as single-particle tracking and fluorescence resonance energy transfer, to study receptor-mediated transport.

Application of multiple response optimization design to quantum dot-encoded microsphere bioconjugates hybridization assay

The optimization of DNA hybridization for genotyping assays is a complex experimental problem that depends on multiple factors such as assay formats, fluorescent probes, target sequence, experimental conditions, and data analysis. Quantum dot-doped particle bioconjugates have been previously described as fluorescent probes to identify single nucleotide polymorphisms even though this advanced fluorescent material has shown structural instability in aqueous environments. To achieve the optimization of DNA hybridization to quantum dot-doped particle bioconjugates in suspension while maximizing the stability of the probe materials, a nonsequential optimization approach was evaluated. The design of experiment with response surface methodology and multiple optimization response was used to maximize the recovery of fluorescent probe at the end of the assay simultaneously with the optimization of target-probe binding. Hybridization efficiency was evaluated by the attachment of fluorescent oligonucleotides to the fluorescent probe through continuous flow cytometry detection. Optimal conditions were predicted with the model and tested for the identification of single nucleotide polymorphisms. The design of experiment has been shown to significantly improve biochemistry and biotechnology optimization processes. Here we demonstrate the potential of this statistical approach to facilitate the optimization of experimental protocol that involves material science and molecular biology.

Detection of Staphylococcus aureus carrying the gene for toxic shock syndrome toxin 1 by quantum-dot-probe complexes

In this study, a high-sensitive and high-specific method to detect the toxic shock syndrome toxin-1 (TSST-1)-producing Staphylococcus aureus was developed based on quantum dot (QD) and oligonucleotide probe complexes. S. aureus carrying tst gene which is responsible for the production of TSST-1 were detected based on fluorescence resonance energy transfer (FRET) occurring between CdSe/ZnS QD donors and black hole quencher (BHQ) acceptors. QD-DNA probe was prepared by conjugating the carboxyl-modified QD and the amino-modified DNA with the EDC. Photoluminescence (PL) quenching was achieved through FRET after the addition of BHQ-DNA which was attached to tst gene probe by match sequence hybridization. The PL recovery was detected in the presence of target DNA by BHQ-DNA detached from QD-DNA probe because of the different affinities. In contrast, mismatch oligonucleotides and DNAs of other bacteria did not contribute to fluorescence intensity recovery, which exhibits the higher selectivity of the biosensor. The experimental results showed clearly that the intensity of recovered QD PL is linear to the concentration of target DNA within the range of 0.2-1.2 μM and the detection limit was 0.2 μM.

RGB colour coding of Y-shaped DNA for simultaneous tri-analyte solid phase hybridization detection

We present a new concept for tri-analyte DNA detection based on the idea of a Y-shaped capture probe which, after tri-target and fluorescently labeled reporter probe binding, becomes colour-coded to generate images in an RGB colour scheme. Hence, the RGB value of the resulting secondary pseudo-colour presented by the hybridized Y-DNA can be related to the ratio of the primary pseudo-colours present in its make-up, and thus to the ratio of the three target concentrations. As a proof of concept we detect sequences from the genes of the pathogenic bacterial strains Escherichia coli O157:H7, Vibrio cholera and Salmonella enteric in a semi-quantitative manner across the range 20-167 nM. The assay was relatively quick, with a time from hybridization to completed data interpretation of approximately 4 h.