Adapting fluorescence resonance energy transfer with quantum dot donors for solid-phase hybridization assays in microtiter plate format

Development of three-dimensional semi-solid hydrogel matrices for ratiometric fluorescence sensing of Amyloid β peptide and imaging in SH-SY5 cells: Improvement of point of care diagnosis of Alzheimer's disease biomarker

Alzheimer's is a neurodegenerative disease with high morbidity and mortality in the elderly, so, detection of its biomarker for definite diagnosis of Alzheimer's in the early stage of disease is a challenge. Amyloid beta peptide (Aβ) chosen as an Alzheimer's biomarker. Here, we developed novel, semi-solid, three-dimensional hydrogel matrices for ratiometric fluorescence detection of Aβ. This assay's great performance stems from the employment of a hybrid conjugate composed of Rhodamine B (RB), Carbon dots (CDs), and an Aβ probe entrapped in Polyvinyl alcohol (PVA), and then detection of fluorescence resonance energy transfer (FRET) that occurs in the presence of AuNP/target-Aβ, as a result of hybridization. The RB-CDs' fluorescence (at 582 nm and 675 nm under 430 nm excitation) is quenched in the presence of AuNPs, while the ratio of fluorescence (I₅₈₂/I₆₇₅) is increased by the addition of Aβ target, and shows a linear relationship in the range of 75 pM-250 nM, with a detection limit of 0.5 pM. Furthermore, the assay possesses strong selectivity for Aβ compared to other proteins, and different quantities of a human serum sample successfully analyzed with excellent sensitivity, satisfactory precision, and reliability. Due to distribution of Aβ in SH-SY5 human neuroblastoma cells, extending this UV-Vis-NIR full-range responsive CDs bio-probe to imaging of Aβ in cells. In both fixed and living SH-SY5 cells, the nanoprobe delivers a clear signal to the Aβ target. Because of its high sensitivity, selectivity, biocompatibility and affordability, this nanoprobe is a good option for early Alzheimer's disease diagnosis.

Recent advances in nanomaterial-assisted detection coupled with capillary and microchip electrophoresis

Nanomaterials have drawn much attention because of their unique properties enabling them to play important roles in various applications in different areas. This review covers literature data in the Web of Science from January 2017 to August 2020, focusing on the applications of nanomaterials (nanoparticles, quantum dots, nanotubes, and graphene) in CE and MCE to achieve enhanced sensitivity of several detection techniques: fluorescence, colorimetry, amperometry, and chemiluminescence /electrochemiluminescence. For the articles surveyed, the types of nanomaterials used, detection mechanisms, analytical performance, and applications are presented and discussed.

Capillary electrophoresis, a method for the determination of nucleic acid ligands covalently attached to quantum dots representing a donor of Förster resonance energy transfer

The synthesis and determination of the structure of a Förster resonance energy transfer probe intended for the detection of specific nucleic acid sequences are described here. The probe is based on the hybridization of oligonucleotide modified quantum dots with a fluorescently labeled nucleic acid sample resulting in changes of the fluorescence emission due to the energy transfer effect. The stoichiometry distribution of oligonucleotides conjugated to quantum dots was determined by capillary electrophoresis separation. The results indicate that one to four molecules of oligonucleotide are conjugated to the surface of a single nanoparticle. This conclusion is confirmed by the course of the dependence of Förster resonance energy transfer efficiency on the concentration of fluorescently labeled complementary single-stranded nucleic acid, showing saturation. While the energy transfer efficiency of the probe hybridized with complementary nucleic acid strands was 30%, negligible efficiency was observed with a noncomplementary strand.

Inorganic Nanoparticles as Donors in Resonance Energy Transfer for Solid-Phase Bioassays and Biosensors

Bioassays for the rapid detection and quantification of specific nucleic acids, proteins, and peptides are fundamental tools in many clinical settings. Traditional optical emission methods have focused on the use of molecular dyes as labels to track selective binding interactions and as probes that are sensitive to environmental changes. Such dyes can offer good detection limits based on brightness but typically have broad emission bands and suffer from time-dependent photobleaching. Inorganic nanoparticles such as quantum dots and upconversion nanoparticles are photostable over prolonged exposure to excitation radiation and tend to offer narrow emission bands, providing a greater opportunity for multiwavelength multiplexing. Importantly, in contrast to molecular dyes, nanoparticles offer substantial surface area and can serve as platforms to carry a large number of conjugated molecules. The surface chemistry of inorganic nanoparticles offers both challenges and opportunities for the control of solubility and functionality for selective molecular interactions by the assembly of coatings through coordination chemistry. This report reviews advances in the compositional design and methods of conjugation of inorganic quantum dots and upconversion nanoparticles and the assembly of combinations of nanoparticles to achieve energy exchange. Our interest is the exploration of configurations where the modified nanoparticles can be immobilized to solid substrates for the development of bioassays and biosensors that operate by resonance energy transfer (RET).

A Dual-Color Quantum Dots Encoded Frit-Based Immunoassay for Visual Detection of Aflatoxin M1 and Pirlimycin Residues in Milk

Mycotoxins and antibacterial agents are the main chemical hazards that lead to several health problems. Nowadays, multiplex immunoassay is a primary goal throughout the world. Here, aflatoxin M₁ and pirlimycin were selected as models, and a novel dual colorimetric encoded frit-based immunoassay was developed for simultaneously screening of aflatoxin M₁ and pirlimycin residues in milk. This multiplex frit-based immunoassay combined two monoclonal antibodies to extend the spectrum of analytes and to enable detection of two classes of analytes in a single test. The cutoff values were 0.02 μg/kg for aflatoxin M₁ and 0.5 μg/kg for pirlimycin, which satisfied the requirement to measure the maximum residue levels. The novel colorimetric frit-based immunoassay has the advantage of high throughput, short analysis time, reduced overall cost per assay, and can be used as a rapid screening technique for simultaneously detecting aflatoxin M₁ and pirlimycin residues in milk.

Self-Assembled Gold Nanoparticle-Fluorescent Protein Conjugates as Platforms for Sensing Thiolate Compounds via Modulation of Energy Transfer Quenching

The ability of Au and other metal nanostructures to strongly quench the fluorescence of proximal fluorophores (dyes and fluorescent proteins) has made AuNP conjugates attractive for use as platforms for sensor development based on energy transfer interactions. In this study, we first characterize the energy transfer quenching of mCherry fluorescent proteins immobilized on AuNPs via metal-histidine coordination, where parameters such as NP size and number of attached proteins are varied. Using steady-state and time-resolved fluorescence measurements, we recorded very high mCherry quenching, with efficiency reaching ∼95-97%, independent of the NP size or number of bound fluorophores (i.e., conjugate valence). We further exploited these findings to develop a solution phase sensing platform targeting thiolate compounds. Energy transfer (ET) was employed as a transduction mechanism to monitor the competitive displacement of mCherry from the Au surface upon the introduction of varying amounts of thiolates with different size and coordination numbers. Our results show that the competitive displacement of mCherry depends on the thiolate concentration, time of reaction, and type of thiol derivatives used. Further analysis of the PL recovery data provides a measure for the equilibrium dissociation constant (Kd-1) for these compounds. These findings combined indicate that the AuNP-fluorescent protein conjugates may offer a potentially useful platform for thiol sensing both in solution and in cell cultures.

A SPR biosensor based on signal amplification using antibody-QD conjugates for quantitative determination of multiple tumor markers

The detection of tumor markers is very important in early cancer diagnosis; however, tumor markers are usually present at very low concentrations, especially in the early stages of tumor development. Surface plasmon resonance (SPR) is widely used to detect biomolecular interactions; it has inherent advantages of being high-throughput, real-time, and label-free technique. However, its sensitivity needs essential improvement for practical applications. In this study, we developed a signal amplification strategy using antibody-quantum dot (QD) conjugates for the sensitive and quantitative detection of α-fetoprotein (AFP), carcinoembryonic antigen (CEA) and cytokeratin fragment 21-1 (CYFRA 21-1) in clinical samples. The use of a dual signal amplification strategy using AuNP-antibody and antibody-QD conjugates increased the signal amplification by 50-folds. The constructed SPR biosensor showed a detection limit as low as 0.1 ng/mL for AFP, CEA, and CYFRA 21-1. Moreover, the results obtained using this SPR biosensor were consistent with those obtained using the electrochemiluminescence method. Thus, the constructed SPR biosensor provides a highly sensitive and specific approach for the detection of tumor markers. This SPR biosensor can be expected to be readily applied for the detection of other tumor markers and can offer a potentially powerful solution for tumor screening.

Surface labeling of enveloped virus with polymeric imidazole ligand-capped quantum dots via the metabolic incorporation of phospholipids into host cells

We report a general method for the preparation of quantum dot-labeled viruses through a strain-promoted azide-alkyne cycloaddition (SPAAC) reaction. The quantum dot sample was functionalized with methacrylate-based polymeric imidazole ligands (MA-PILs) bearing dibenzocyclooctyne groups. Enveloped measles virus was labeled with azide groups through the metabolic incorporation of a choline analogue into the host cell membrane, and then linked with the modified QDs. The virus retained its infectious ability against host cells after the modification with MA-PIL capped QDs.

Ratiometric fluorescence transduction by hybridization after isothermal amplification for determination of zeptomole quantities of oligonucleotide biomarkers with a paper-based platform and camera-based detection

Paper is a promising platform for the development of decentralized diagnostic assays owing to the low cost and ease of use of paper-based analytical devices (PADs). It can be challenging to detect on PADs very low concentrations of nucleic acid biomarkers of lengths as used in clinical assays. Herein we report the use of thermophilic helicase-dependent amplification (tHDA) in combination with a paper-based platform for fluorescence detection of probe-target hybridization. Paper substrates were patterned using wax printing. The cellulosic fibers were chemically derivatized with imidazole groups for the assembly of the transduction interface that consisted of immobilized quantum dot (QD)-probe oligonucleotide conjugates. Green-emitting QDs (gQDs) served as donors with Cy3 as the acceptor dye in a fluorescence resonance energy transfer (FRET)-based transduction method. After probe-target hybridization, a further hybridization event with a reporter sequence brought the Cy3 acceptor dye in close proximity to the surface of immobilized gQDs, triggering a FRET sensitized emission that served as an analytical signal. Ratiometric detection was evaluated using both an epifluorescence microscope and a low-cost iPad camera as detectors. Addition of the tHDA method for target amplification to produce sequences of ∼100 base length allowed for the detection of zmol quantities of nucleic acid targets using the two detection platforms. The ratiometric QD-FRET transduction method not only offered improved assay precision, but also lowered the limit of detection of the assay when compared with the non-ratiometric QD-FRET transduction method. The selectivity of the hybridization assays was demonstrated by the detection of single nucleotide polymorphism.

Synthesis of random terpolymers bearing multidentate imidazole units and their use in functionalization of cadmium sulfide nanowires

This work reports on a new synthesis method for random ternary copolymers that are shown to tether a molecular dye payload to cadmium sulfide nanowires in aqueous solution.

Camera-based ratiometric fluorescence transduction of nucleic acid hybridization with reagentless signal amplification on a paper-based platform using immobilized quantum dots as donors

Paper-based diagnostic assays are gaining increasing popularity for their potential application in resource-limited settings and for point-of-care screening. Achievement of high sensitivity with precision and accuracy can be challenging when using paper substrates. Herein, we implement the red-green-blue color palette of a digital camera for quantitative ratiometric transduction of nucleic acid hybridization on a paper-based platform using immobilized quantum dots (QDs) as donors in fluorescence resonance energy transfer (FRET). A nonenzymatic and reagentless means of signal enhancement for QD-FRET assays on paper substrates is based on the use of dry paper substrates for data acquisition. This approach offered at least a 10-fold higher assay sensitivity and at least a 10-fold lower limit of detection (LOD) as compared to hydrated paper substrates. The surface of paper was modified with imidazole groups to assemble a transduction interface that consisted of immobilized QD-probe oligonucleotide conjugates. Green-emitting QDs (gQDs) served as donors with Cy3 as an acceptor. A hybridization event that brought the Cy3 acceptor dye in close proximity to the surface of immobilized gQDs was responsible for a FRET-sensitized emission from the acceptor dye, which served as an analytical signal. A hand-held UV lamp was used as an excitation source and ratiometric analysis using an iPad camera was possible by a relative intensity analysis of the red (Cy3 photoluminescence (PL)) and green (gQD PL) color channels of the digital camera. For digital imaging using an iPad camera, the LOD of the assay in a sandwich format was 450 fmol with a dynamic range spanning 2 orders of magnitude, while an epifluorescence microscope detection platform offered a LOD of 30 fmol and a dynamic range spanning 3 orders of magnitude. The selectivity of the hybridization assay was demonstrated by detection of a single nucleotide polymorphism at a contrast ratio of 60:1. This work provides an important framework for the integration of QD-FRET methods with digital imaging for a ratiometric transduction of nucleic acid hybridization on a paper-based platform.

Solid-phase covalent immobilization of upconverting nanoparticles for biosensing by luminescence resonance energy transfer

Monodisperse water-soluble upconverting nanoparticles (UCNPs) were immobilized onto modified glass substrates for development of biosensing surfaces that operated using luminescence resonance energy transfer (LRET). Amine modified UCNPs were prepared from oleic acid capped UCNPs by ligand exchange using o-phosphorylethanolamine (PEA). PEA-UCNPs were covalently immobilized on aldehyde functionalized coverslips. Environmental scanning electron microscopy (ESEM) images indicated a homogeneous distribution of UCNPs on surfaces with a high immobilization density of approximately 1.3 × 10(11) UCNP cm(-2). This is the first account of covalent immobilization of UCNPs for bioassay and biosensor development where the density is on par with the high immobilization densities reported for other types of nanoparticles. The functionality and stability of the immobilized NPs were demonstrated by examining an LRET-based bioassay. The well-known sandwich assay for the detection of thrombin was selected as a model in which UCNPs were used as donors and quantum dots (QDs) as acceptors. The closely packed UCNPs on the glass surface showed a 2.5-fold enhancement in assay sensitivity compared to less-densely packed surfaces. In addition, a 1.5-fold enhancement in energy transfer efficiency was shown for solid-phase compared to solution-phase LRET.

Solid-phase supports for the in situ assembly of quantum dot-FRET hybridization assays in channel microfluidics

Semiconductor quantum dots (QDs) have long served as integral components in signal transduction modalities such as Förster resonance energy transfer (FRET). The majority of bioanalytical methods using QDs for FRET-based techniques simply monitor binding-induced conformational changes. In more recent work, QDs have been incorporated into solid-phase support systems, such as microfluidic chips, to serve as physical platforms in the development of functional biosensors and bioprobes. Herein, we describe a simple strategy for the transduction of nucleic acid hybridization that combines a novel design method based on FRET with an electrokinetically controlled microfluidic technology, and that offers further potential for amelioration of sample-handling issues and for simplification of dynamic stringency control.

Multidentate zwitterionic ligands provide compact and highly biocompatible quantum dots

Hydrophilic functional semiconductor nanocrystals that are also compact provide greatly promising platforms for use in bioinspired applications and are thus highly needed. To address this, we designed a set of metal coordinating ligands where we combined two lipoic acid groups, bis(LA)-ZW, (as a multicoordinating anchor) with a zwitterion group for water compatibility. We further combined this ligand design with a new photoligation strategy, which relies on optical means instead of chemical reduction of the lipoic acid, to promote the transfer of CdSe-ZnS QDs to buffer media. In particular, we found that the QDs photoligated with this zwitterion-terminated bis(lipoic) acid exhibit great colloidal stability over a wide range of pHs, to an excess of electrolytes, and in the presence of growth media and reducing agents, in addition to preserving their optical and spectroscopic properties. These QDs are also stable at nanomolar concentrations and under ambient conditions (room temperature and white light exposure), a very promising property for fluorescent labeling in biology. In addition, the compact ligands permitted metal-histidine self-assembly between QDs photoligated with bis(LA)-ZW and two different His-tagged proteins, maltose binding protein and fluorescent mCherry protein. The remarkable stability of QDs capped with these multicoordinating and compact ligands over a broad range of conditions and at very small concentrations, combined with the compatibility with metal-histidine conjugation, could be very useful for a variety of applications, ranging from protein tracking and ligand-receptor binding to intracellular sensing using energy transfer interactions.

Paper-based solid-phase multiplexed nucleic acid hybridization assay with tunable dynamic range using immobilized quantum dots as donors in fluorescence resonance energy transfer

A multiplexed solid-phase nucleic acid hybridization assay on a paper-based platform is presented using multicolor immobilized quantum dots (QDs) as donors in fluorescence resonance energy transfer (FRET). The surface of paper was modified with imidazole groups to immobilize two types of QD-probe oligonucleotide conjugates that were assembled in solution. Green-emitting QDs (gQDs) and red-emitting QDs (rQDs) served as donors with Cy3 and Alexa Fluor 647 (A647) acceptors. The gQD/Cy3 FRET pair served as an internal standard, while the rQD/A647 FRET pair served as a detection channel, combining the control and analytical test zones in one physical location. Hybridization of dye-labeled oligonucleotide targets provided the proximity for FRET sensitized emission from the acceptor dyes, which served as an analytical signal. Hybridization assays in the multicolor format provided a limit of detection of 90 fmol and an upper limit of dynamic range of 3.5 pmol. The use of an array of detection zones was designed to provide improved analytical figures of merit compared to that which could be achieved on one type of array design in terms of relative concentration of multicolor QDs. The hybridization assays showed excellent resistance to nonspecific adsorption of oligonucleotides. Selectivity of the two-plex hybridization assay was demonstrated by single nucleotide polymorphism (SNP) detection at a contrast ratio of 50:1. Additionally, it is shown that the use of preformed QD-probe oligonucleotide conjugates and consideration of the relative number density of the two types of QD-probe conjugates in the two-color assay format is advantageous to maximize assay sensitivity and the upper limit of dynamic range.