General Bioluminescence Resonance Energy Transfer Homogeneous Immunoassay for Small Molecules Based on Quantum Dots

Nanocrystals for Deep-Tissue In Vivo Luminescence Imaging in the Near-Infrared Region

In vivo imaging technologies have emerged as a powerful tool for both fundamental research and clinical practice. In particular, luminescence imaging in the tissue-transparent near-infrared (NIR, 700-1700 nm) region offers tremendous potential for visualizing biological architectures and pathophysiological events in living subjects with deep tissue penetration and high imaging contrast owing to the reduced light-tissue interactions of absorption, scattering, and autofluorescence. The distinctive quantum effects of nanocrystals have been harnessed to achieve exceptional photophysical properties, establishing them as a promising category of luminescent probes. In this comprehensive review, the interactions between light and biological tissues, as well as the advantages of NIR light for in vivo luminescence imaging, are initially elaborated. Subsequently, we focus on achieving deep tissue penetration and improved imaging contrast by optimizing the performance of nanocrystal fluorophores. The ingenious design strategies of NIR nanocrystal probes are discussed, along with their respective biomedical applications in versatile in vivo luminescence imaging modalities. Finally, thought-provoking reflections on the challenges and prospects for future clinical translation of nanocrystal-based in vivo luminescence imaging in the NIR region are wisely provided.

Design of an Antigen-Triggered Nanobody-Based Fluorescence Probe for PET Immunoassay to Detect Quinalphos in Food Samples

Photoinduced electron-transfer (PET) immunoassay based on a fluorescence site-specifically labeled nanobody, also called mini Quenchbody (Q-body), exhibits extraordinary sensitivity and saves much time in the homogeneous noncompetitive mode and is therefore regarded as a valuable method. However, limited by the efficiency of both quenching and dequenching of the fluorescence signal before and after antigen binding associated with the PET principle, not all original nanobodies can be used as candidates for mini Q-bodies. Herein, with the anti-quinalphos nanobody 11A (Nb-11A) as the model, we, for the first time, adopt a strategy by combining X-ray structural analysis with site-directed mutagenesis to design and produce a mutant Nb-R29W, and then successfully generate a mini Q-body by labeling with ATTO520 fluorescein. Based on this, a novel PET immunoassay is established, which exhibits a limit of detection of 0.007 μg/mL with a detection time of only 15 min, 25-fold improved sensitivity, and faster by 5-fold compared to the competitive immunoassay. Meanwhile, the recovery test of vegetable samples and validation by the standard ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) both demonstrated that the established PET immunoassay is a novel, sensitive, and accurate detection method for quinalphos. Ultimately, the findings of this work will provide valuable insights into the development of triggered PET fluorescence probes by using existing antibody resources.

Highly Sensitive and Selective Detection of Inorganic Phosphates in the Water Environment by Biosensors Based on Bioluminescence Resonance Energy Transfer

The accurate detection of phosphate in water is very important to prevent water eutrophication and ensure the health of water quality. However, traditional phosphomolybdenum blue spectrophotometry is not sensitive, is time-consuming, and demands large amounts of chemical reagents. Therefore, highly sensitive, rapid, and environmentally friendly Pi detection methods are urgently needed. Here, we developed a bioluminescence resonance energy transfer (BRET)-based biosensor, which can detect Pi in water quickly, highly sensitively, and highly selectively. The NanoLuc and the Venus fluorescent protein were selected as the bioluminescence donor and energy acceptor, respectively. The best-performing BRET sensor variant, VenusΔC10-PΔC12-ΔN4Nluc, was identified by Pi-specific binding protein (PiBP) screening and systematic truncation. Single-factor experiments optimized the key parameters affecting the detection performance of the sensor. Under the optimal detection conditions, the detection limit of this method was 1.3 μg·L^-1, the detection range was 3.3-434 μg·L^-1, and it had excellent selectivity, repeatability, and stability. This low-cost and environment-friendly BRET sensor showed a good application prospect in real water quality detection.

Recent progress in homogeneous immunosensors based on fluorescence or bioluminescence using antibody engineering

Homogeneous immunosensors integrate the advantages of both biosensors and immunoassays; they include speed, high sensitivity, and accuracy. They have been developed rapidly in the past few years and offer a cost-effective alternative technology with rapidity, sensitivity, and user-friendliness, which has been applied in a wide variety of applications. This review introduces the current directions of immunosensor development, focusing on fluorescent and bioluminescent immunosensors and highlighting the advantages, improvements, and key approaches to overcome the limitations of each.

AIEgens: Next Generation Signaling Source for Immunoassays?

Luminogens with aggregation-induced emission (AIEgens) properties have numerous broad applications in fields of chemical and biological analyses due to their exceptional photostability, excellent signal reliability, high quantum yield, and large Stokes' shift. In particular, AIEgens also bring new blood for immunoassay. Since publication of the first 2004 paper, AIEgens-based immunoassays have received significant attention because of their high sensitivity, specificity, accuracy, and reliability. However, until now, there have been no comprehensive literature reviews focused on the evolving field of AIEgens-based immunoassays. Thus, we have extensively reviewed AIEgens-based immunoassays from their basic working principles to specific applications. We focus on several fundamental elements of AIEgens-based immunoassays, including the typical structures of AIEgens, emission mechanism of AIEgens probes, function of AIEgens in immunoassays, and platform of AIEgens-based immunoassays. Then, the representative applications of AIEgens-based immunoassays in food safety, medical diagnostics, and environmental monitoring are explored. Thus, proposals on how to further improve the AIEgens-based immunoassay performance are also discussed, as well as future challenges and perspectives, aiming to provide brief and valid guidelines for choosing suitable AIEgens-based immunoassays according to specific application requirements.

New Approach to Generate Ratiometric Signals on Immunochromatographic Strips for Small Molecules

The self-calibration capability of ratiometric signals has been widely considered to enhance the accuracy, sensitivity, and anti-interference ability of immunoassays. Exploring a new approach to generate ratiometric signals can provide more options for various requirements. Herein, we integrated the negative-readout competitive and positive-readout noncompetitive immunoassays into a single assay by employing different color tracers, labeled peptidomimetic and anti-immunocomplex peptides, to create a new unconstrained ratiometric signal approach. Using an immunochromatographic strip (ICS) and a fungicide benzothiostrobin as the analytical platform and analyte, respectively, we showed that this approach can be extensively applied to fluorescence and colorimetry readouts, which have also been proven for strong anti-interference ability to an external light environment. Moreover, the enormous intuitional color changes of ratiometric fluorescent and colorimetric ICSs (RFICS and RCICS) enabled the formation of the color reference cards (like the pH paper) for visual judgment. After adaptation with a portable smartphone, the quantitative detection limits for RFICS and RCICS were 0.17 and 0.44 ng mL^-1, respectively. In addition, the ICSs showed good accuracy for the detection of benzothiostrobin in spiked samples.

Recent Advances in Engineered Noble Metal Nanomaterials as a Surface-Enhanced Raman Scattering Active Platform for Cancer Diagnostics

Recently, noble metal nanomaterials have been extensively studied in the fields of biosensing, environmental catalysis, and cancer diagnosis and treatment, due to their excellent electrical conductivity, high surface area, and individual physical and optical properties. Early research on the surface-enhanced Raman scattering (SERS) effect was focused on the cognition of the SERS phenomenon and enhancing its sensitivity for single-molecule detection. With the development of nanomaterials and nanotechnology, the advances and applications based on SERS substrates have been accelerated. Among them, noble metal nanomaterials are mainly used as SERS-active substrates to enhance SERS signals owing to their compelling surface plasmon resonance (SPR) properties. This review provides recent advances, perspectives, and challenges in SERS assays based on engineered noble metal nanomaterials for early cancer diagnosis.

In Vivo Assessment of Protein-Protein Interactions Using BRET Assay

Proteins play an important part in almost all life activities and across all organisms. Proteins occasionally act on their own but rather fulfill most of their biological tasks by cooperating with other proteins or ligand molecules. The bioluminescence resonance energy transfer (BRET) assay serves to measure dynamic events such as protein-protein or protein-ligand interactions in vitro or in-vivo. With several inherent attributes such as rapid and fairly sensitive ratio-metric measurements, assessment of interactions irrespective of protein location within the cellular compartment, cost-effectiveness consenting to high-throughput screening compatibility, makes BRET a popular genetic reporter-based assay system for protein-protein interaction (PPI) studies. Based on the Förster principle, BRET allows to judge if the proximity has been achieved between the interacting partners. In recent years, the BRET application has emerged as a significantly versatile assay format by using multiple detection devices such as a plate reader or in-vivo optical imaging platform, or even a bioluminescence microscope has expanded its scope for advancing PPI studies. Beyond the scope of quantitative measurement of PPIs, molecular optical imaging applications based on BRET assay have expanded the scope for screening pharmacological compounds by unifying live cell and in-vivo animal-/plant-based experiments using the same platform technology. In this chapter, we have given intricate methodological details for performing in-vitro and in-vivo BRET experiments, primarily by using donor/acceptor reporter protein combinations.

Recombinant antibodies and their use for food immunoanalysis

Antibodies are widely employed as biorecognition elements for the detection of a plethora of compounds including food and environmental contaminants, biomarkers, or illicit drugs. They are also applied in therapeutics for the treatment of several disorders. Recent recommendations from the EU on animal protection and the replacement of animal-derived antibodies by non-animal-derived ones have raised a great controversy in the scientific community. The application of recombinant antibodies is expected to achieve a high growth rate in the years to come thanks to their versatility and beneficial characteristics in comparison to monoclonal and polyclonal antibodies, such as stability in harsh conditions, small size, relatively low production costs, and batch-to-batch reproducibility. This review describes the characteristics, advantages, and disadvantages of recombinant antibodies including antigen-binding fragments (Fab), single-chain fragment variable (scFv), and single-domain antibodies (V_HH) and their application in food analysis with especial emphasis on the analysis of biotoxins, antibiotics, pesticides, and foodborne pathogens. Although the wide application of recombinant antibodies has been hampered by a number of challenges, this review demonstrates their potential for the sensitive, selective, and rapid detection of food contaminants.

Biosensors and bioassays for determination of matrix metalloproteinases: state of the art and recent advances

Matrix metalloproteinases (MMPs) are closely associated with various physiological and pathological processes, and have been regarded as potential biomarkers for severe diseases including cancer. Accurate determination of MMPs would advance our understanding of their roles in disease progression, and is of great significance for disease diagnosis, treatment and prognosis. In this review, we present a comprehensive overview of the developed bioassays/biosensors for detection of MMPs, and highlight the recent advancement in nanomaterial-based immunoassays for MMP abundance measurements and nanomaterial-based biosensors for MMP activity determination. Enzyme-linked immunosorbent assay (ELISA)-based immunoassays provide information about total levels of MMPs with high specificity and sensitivity, while target-based biosensors measure the amounts of active MMPs, and allow imaging of MMP activities in vivo. For multiplex and high-throughput analysis of MMPs, microfluidics and microarray-based assays are described. Additionally, we put forward the existing challenges and future prospects from our perspective.

Nanostructures for Biosensing, with a Brief Overview on Cancer Detection, IoT, and the Role of Machine Learning in Smart Biosensors

Biosensors are essential tools which have been traditionally used to monitor environmental pollution and detect the presence of toxic elements and biohazardous bacteria or virus in organic matter and biomolecules for clinical diagnostics. In the last couple of decades, the scientific community has witnessed their widespread application in the fields of military, health care, industrial process control, environmental monitoring, food-quality control, and microbiology. Biosensor technology has greatly evolved from in vitro studies based on the biosensing ability of organic beings to the highly sophisticated world of nanofabrication-enabled miniaturized biosensors. The incorporation of nanotechnology in the vast field of biosensing has led to the development of novel sensors and sensing mechanisms, as well as an increase in the sensitivity and performance of the existing biosensors. Additionally, the nanoscale dimension further assists the development of sensors for rapid and simple detection in vivo as well as the ability to probe single biomolecules and obtain critical information for their detection and analysis. However, the major drawbacks of this include, but are not limited to, potential toxicities associated with the unavoidable release of nanoparticles into the environment, miniaturization-induced unreliability, lack of automation, and difficulty of integrating the nanostructured-based biosensors, as well as unreliable transduction signals from these devices. Although the field of biosensors is vast, we intend to explore various nanotechnology-enabled biosensors as part of this review article and provide a brief description of their fundamental working principles and potential applications. The article aims to provide the reader a holistic overview of different nanostructures which have been used for biosensing purposes along with some specific applications in the field of cancer detection and the Internet of things (IoT), as well as a brief overview of machine-learning-based biosensing.

An ultrasensitive, homogeneous fluorescence quenching immunoassay integrating separation and detection of aflatoxin M1 based on magnetic graphene composites

A homogeneous fluorescence quenching immunoassay is described for simultaneous separation and detection of aflatoxin M₁ (AFM₁) in milk. The novel assay relies on monoclonal antibody (mAb) functionalized Fe₃O₄ decorated reduced-graphene oxide (rGO-Fe₃O₄-mAb) as both capture probe and energy acceptor, combined with tetramethylrhodamine cadaverine-labeled aflatoxin B₁ (AFB₁-TRCA) as the energy donor. In the assay, AFB₁-TRCA binds to rGO-Fe₃O₄-mAb in the absence of AFM₁, quenching the fluorescence of TRCA by resonance energy transfer. Significantly, the immunoassay integrates sample preparation and detection into a single step, by using magnetic graphene composites to avoid washing and centrifugation steps, and the assay can be completed within 10 min. Under optimized conditions, the visual and quantitative detection limits of the assay for AFM₁ were 50 and 3.8 ng L^-1, respectively, which were significantly lower than those obtained by fluorescence polarization immunoassay using the same immunoreagents. Owing to its operation and highly sensitivity, the proposed assay provides a powerful tool for the detection of AFM₁.

Antibody engineering-driven controllable chemiluminescence resonance energy transfer for immunoassay with tunable dynamic range

The donor-acceptor distance is a critical factor for the occurrence of chemiluminescent resonance energy transfer (CRET). We herein evaluate the donor-acceptor distance and transfer efficiency of CRET immunoassays of a series of donors which contain different sized antibody fragments, intact monoclonal antibody (IgG), antigen binding fragment (Fab), and single chain fragment antibody (scFv). Core/multishell quantum dots were used as the acceptor in three CRET systems. IgG is the maximum antibody fragment, leading to the longest donor-acceptor distance and the lowest transfer efficiency. Donors with Fab and scFv show significantly decreased distance and increased transfer efficiency. These results suggest an inverse correlation between donor size and transfer efficiency and can be used to provide guidance for the construction of controllable CRET. By combining the controllable CRET with immunoassay, we further develop a tunable sulfamethazine analytical system. Three different sized donors based CRET immunoassay show a markedly different sensitivity and dynamic range. Such adjustable detection provides greater flexibility for contaminant detection in different foodstuffs with different residue limits. This work not only illustrates the effect of donor-acceptor distance on regulating the energy transfer efficiency of CRET system, but also provides a guideline for the construction of a tunable immunoassay.

Multiplexed homogeneous digital immunoassay based on single-particle motion analysis

Homogeneous digital immunoassay is a powerful analytical method for highly sensitive protein biomarker detection with a simple protocol. However, it has not been multiplexed yet. In this study, we developed a multiplexed homogeneous digital immunoassay based on single-particle motion analysis (digital homogeneous non-enzyme-linked immunosorbent assay, digital Ho-Non ELISA). In this assay, multiple target antigen molecules react with the optical subpopulation of magnetic nanobeads labeled with fluorescent dyes and capture antigen-specific antibodies. Then, these beads are magnetically pulled into femtoliter-sized reactors. The surface of these reactors is modified with multiple detection antibodies specific to each antigen by molecular tethers. Each antigen on the particles reacts with the detection antibodies anchored to the surface of the reactors. Magnetic force enhances the efficiency of bead encapsulation in the reactors, and subsequent physical compartmentalization of beads enhances the binding efficiency of the antigen-antibody reaction. The tethered beads show characteristic Brownian motion distinct from free diffusion or non-specific binding of the antigen-free beads. The color of the beads is attributed to target-identification, and the number of tethered beads is attributed to the concentration of the specific target. We measured two biomarkers (PSA and IL6) as model targets by multiplexed digital Ho-Non ELISA. Our method showed higher sensitivity compared to previous digital Ho-Non ELISA and could detect multiple targets simultaneously with the same performance as in single-plex detection. This new strategy has the potential to open a new avenue for next-generation multiplexed immunoassays in in vitro diagnostics.

Bioluminescence-Based Energy Transfer Using Semiconductor Quantum Dots as Acceptors

Bioluminescence resonance energy transfer (BRET) is the non-radiative transfer of energy from a bioluminescent protein donor to a fluorophore acceptor. It shares all the formalism of Förster resonance energy transfer (FRET) but differs in one key aspect: that the excited donor here is produced by biochemical means and not by an external illumination. Often the choice of BRET source is the bioluminescent protein Renilla luciferase, which catalyzes the oxidation of a substrate, typically coelenterazine, producing an oxidized product in its electronic excited state that, in turn, couples with a proximal fluorophore resulting in a fluorescence emission from the acceptor. The acceptors pertinent to this discussion are semiconductor quantum dots (QDs), which offer some unrivalled photophysical properties. Amongst other advantages, the QD's large Stokes shift is particularly advantageous as it allows easy and accurate deconstruction of acceptor signal, which is difficult to attain using organic dyes or fluorescent proteins. QD-BRET systems are gaining popularity in non-invasive bioimaging and as probes for biosensing as they don't require external optical illumination, which dramatically improves the signal-to-noise ratio by avoiding background auto-fluorescence. Despite the additional advantages such systems offer, there are challenges lying ahead that need to be addressed before they are utilized for translational types of research.

A Visual FRET Immunofluorescent Biosensor for Ratiometric Parathyroid Hormone (1-84) Antigen Point-of-Care Detection

Excessive secretion of PTH leads to disturbance of calcium and phosphorus metabolism in the body, which promotes bone, kidney, digestive system and nervous system diseases. Due to the short half-life of PTH, it becomes a difficult issue for PTH detection in the clinical diagnosis field. We explored a competitive immunofluorescent sensing mode based on FRET of two-color CdTe QDs for ratiometric PTH 1-84 antigen detection. The FRET effect and ratiometric fluorescence between the two-color CdTe QDs motivated accurate quantification of PTH 1-84 antigen concentration from 0.01 ng mL^-1 to 0.08 ng mL^-1 with a limit of detection of 3 pg mL^-1. More importantly, under UV irradiation, samples with different concentrations of PTH 1-84 antigen achieved fluorescence visualization, which provides huge possibility for the practical application of PTH 1-84 antigen point-of-care detection.

Bioluminescence Resonance Energy Transfer (BRET) Coupled Near-Infrared Imaging of Apoptotic Cells

Detection of apoptotic cells is crucial for understanding the mechanism of diseases and for therapy development. So far, visible-emitting fluorescent probes such as FITC-labeled Annexin V has been widely used for the detection of apoptotic cells. However, such probes cannot be applied to noninvasive imaging in the near-infrared (NIR) region. Compared with visible light, NIR light is highly permeable in turbid biological samples and tissues. In addition, NIR optical imaging has several advantages such as lower autofluorescence and scattering from biological samples, leading to clearer images with high signal to background ratios. Here, we describe the synthesis and application of bioluminescence resonance energy transfer (BRET)-coupled quantum dots (QDs) for the NIR optical imaging of apoptotic cells.

Rapidly detecting antibiotics with magnetic nanoparticle coated CdTe quantum dots

A reusable magnetic-quantum dot material (MNP–SiO₂–QD) with good magnetic properties and high fluorescence retention was successfully fabricated from linked magnetic nanoparticles and quantum dots. The resulting material can qualitatively and quantitatively detect four kinds of antibiotics and maintain high recovery rates.

A reusable magnetic-quantum dot material with good magnetic property and high fluorescence retention can qualitatively and quantitatively detect four kinds of antibiotics.

Wash- and Amplification-Free Digital Immunoassay Based on Single-Particle Motion Analysis

Digital enzyme-linked immunosorbent assay (ELISA) is a powerful analytical method for highly sensitive protein biomarker detection. The current protocol of digital ELISA requires multiple washing steps and signal amplification using an enzyme, which could be the potential drawback in in vitro diagnosis. In this study, we propose a digital immunoassay method, which we call "Digital HoNon-ELISA" (digital homogeneous non-enzymatic immunosorbent assay) for highly sensitive detection without washing and signal amplification. Target antigen molecules react with antibody-coated magnetic nanoparticles, which are then magnetically pulled into femtoliter-sized reactors. The antigens on the particles are captured by antibodies anchored on the bottom surface of the reactor via molecular tethers. Magnetic force enhances the efficiency of particle encapsulation in the reactors. Subsequent physical compartmentalization of the particles enhances the binding efficiency of antigen-carrying particles to the antibodies. The tethered particles show characteristic Brownian motion within a limited space by the molecular tethering, which is distinct from free diffusion or nonspecific binding of antigen-free particles. The number of tethered particles directly correlates with the concentration of the target antigen. Digital HoNon-ELISA was used with a prostate-specific antigen to achieve a detection of 0.093 pg/mL, which is over 9.0-fold the sensitivity of commercialized highly sensitive ELISA (0.84 pg/mL) and comparable to digital ELISA (0.055 pg/mL). This digital immunoassay strategy has sensitivity similar to digital ELISA with simplicity similar to homogeneous assay. Such similarity allows for potential application in rapid and simple digital diagnostic tests without the need for washing and enzymatic amplification.

Fluorescent "Turn off-on" Small-Molecule-Monitoring Nanoplatform Based on Dendrimer-like Peptides as Competitors

Peptides isolated from phage display libraries are powerful reagents for small-molecule immunoassay; however, their application as phage-borne peptides is significantly limited by the biological nature of the phage. Here, we present the use of lysine scaffold to prepare a series of different valence peptides to serve as replacements for phage-borne peptides. Benzothiostrobin was selected as a model analyte, the cyclic benzothiostrobin-peptidomimetic in the form of monomer, dendrimer-like dimer, and tetramer were designed and synthesized. Compared with the monomer, the affinity of dendrimer-like dimer and tetramer increased 1.87 and 13.6 times, respectively, as determined by isothermal titration calorimetry (ITC). A novel inner filter effect immunoassay (IFE-IA) with positive readout was developed for benzothiostrobin detection utilizing the peptidomimetics attached to upconversion nanoparticles (UCNPs) as energy donor and monoclonal antibody (mAb)-labeled urchin-like gold nanoflowers (AuNFs) as energy absorber, respectively. The sensitivity of the assay based on dendrimer-like tetramer was approximately 6 and 3 times higher than monomer and dendrimer-like dimer, respectively. After optimization, 50% saturation of the signal (SC₅₀) and detection range (SC₁₀ to SC₉₀) of the IFE-IA based on dendrimer-like tetramer were 11.81 ng mL^-1 and 2.04-106.17 ng mL^-1, respectively. The IFE-IA also shows good accuracy for the detection of benzothiostrobin in authentic samples.

Biosensors for epigenetic biomarkers detection: A review

Epigenetic inheritance is a heritable change in gene function independent of alterations in nucleotide sequence. It regulates the normal cellular activities of the organisms by affecting gene expression and transcription, and its abnormal expression may lead to the developmental disorder, senile dementia, and carcinogenesis progression. Thus, epigenetic inheritance is recognized as an important biomarker, and the accurate quantification of epigenetic inheritance is crucial to clinical diagnosis, drug development and cancer treatment. Noncoding RNA, DNA methylation and histone modification are the most common epigenetic biomarkers. The conventional biosensors (e.g., northern blotting, radiometric, mass spectrometry and immunosorbent biosensors) for epigenetic biomarkers assay usually suffer from hazardous radiation, complicated manipulation, and time-consuming procedures. To facilitate the practical applications, some new biosensors including colorimetric, luminescent, Raman scattering spectroscopy, electrochemical and fluorescent biosensors have been developed for the detection of epigenetic biomarkers with simplicity, rapidity, high throughput and high sensitivity. In this review, we summarize the recent advances in epigenetic biomarkers assay. We classify the biosensors into the direct amplification-free and the nucleotide amplification-assisted ones, and describe the principles of various biosensors, and further compare their performance for epigenetic biomarkers detection. Moreover, we discuss the emerging trends and challenges in the future development of epigenetic biomarkers biosensors.

Integration of Nanomaterials and Bioluminescence Resonance Energy Transfer Techniques for Sensing Biomolecules

Bioluminescence resonance energy transfer (BRET) techniques offer a high degree of sensitivity, reliability and ease of use for their application to sensing biomolecules. BRET is a distance dependent, non-radiative energy transfer, which uses a bioluminescent protein to excite an acceptor through the resonance energy transfer. A BRET sensor can quickly detect the change of a target biomolecule quantitatively without an external electromagnetic field, e.g., UV light, which normally can damage tissue. Having been developed quite recently, this technique has evolved rapidly. Here, different bioluminescent proteins have been reviewed. In addition to a multitude of bioluminescent proteins, this manuscript focuses on the recent development of BRET sensors by utilizing quantum dots. The special size-dependent properties of quantum dots have made the BRET sensing technique attractive for the real-time monitoring of the changes of target molecules and bioimaging in vivo. This review offers a look into the basis of the technique, donor/acceptor pairs, experimental applications and prospects.

Bioluminescence Resonance Energy Transfer as a Method to Study Protein-Protein Interactions: Application to G Protein Coupled Receptor Biology

The bioluminescence resonance energy transfer (BRET) approach involves resonance energy transfer between a light-emitting enzyme and fluorescent acceptors. The major advantage of this technique over biochemical methods is that protein-protein interactions (PPI) can be monitored without disrupting the natural environment, frequently altered by detergents and membrane preparations. Thus, it is considered as one of the most versatile technique for studying molecular interactions in living cells at "physiological" expression levels. BRET analysis has been applied to study many transmembrane receptor classes including G-protein coupled receptors (GPCR). It is well established that these receptors may function as dimeric/oligomeric forms and interact with multiple effectors to transduce the signal. Therefore, they are considered as attractive targets to identify PPI modulators. In this review, we present an overview of the different BRET systems developed up to now and their relevance to identify inhibitors/modulators of protein⁻protein interaction. Then, we introduce the different classes of agents that have been recently developed to target PPI, and provide some examples illustrating the use of BRET-based assays to identify and characterize innovative PPI modulators in the field of GPCRs biology. Finally, we discuss the main advantages and the limits of BRET approach to characterize PPI modulators.

A Class-Selective Immunoassay for Sulfonamides Residue Detection in Milk Using a Superior Polyclonal Antibody with Broad Specificity and Highly Uniform Affinity

The development of multianalyte immunoassays with an emphasis on food safety has attracted increasing interest, due to its high target throughput, short detection time, reduced sample consumption, and low overall cost. In this study, a superior polyclonal antibody (pAb) against sulfonamides (SAs) was raised by using a bioconjugate of bovine serum albumin with a rationally designed hapten 4-[(4-aminophenyl) sulfonyl-amino]-2-methoxybenzoic acid (SA10-X). The results showed that the pAb could recognize 19 SAs with 50% inhibition (IC₅₀) below 100 µg L^-1 and a recognition profile for SAs containing, either a five-atom ring or a six-atom ring, with highly uniform affinity. A three-dimensional quantitative structure-activity relationship analysis indicated that the electrostatic features of SAs play a considerably important role, during recognition with pAb than stereochemical effects. Skimmed milk samples were directly diluted five times before analysis. After optimization, the limit of detection for sulfamonomethoxine, sulfamethoxazole, sulfaquinoxaline, sulfadimethoxine, and sulfamethazine were 1.00, 1.25, 2.95, 3.35, and 6.10 µg L^-1, respectively. The average recoveries for these 5 SAs were 72.0⁻107.5% with coefficients of variation less than 14.1%. The established method, based on pAb, with broad specificity and uniform affinity, offered a simple, sensitive, and high-throughput screening tool for the detection of multi-SAs in milk samples.

Small molecule-protein interactions in branch migration thermodynamics: modelling and application in the homogeneous detection of proteins and small molecules

We have disclosed the unique inhibition effect of small molecule-protein interactions toward the DNA branch migration process and constructed a complete thermodynamic model for it. The disclosed effect was further coupled with the steric hindrance effect to establish a homogeneous assay for proteins and small molecules with ultra-high inhibition factors and sensitivity.

Asynchrony of spectral blue-shifts of quantum dot based digital homogeneous immunoassay

We present a femtomolar digital homogeneous immunoassay for the detection of cancer biomarkers.

Development of quantum dot-based biosensors: principles and applications

We review the recent advances in quantum dot-based biosensors and focus on quantum dot-based fluorescent, bioluminescent, chemiluminescent, and photoelectrochemical biosensors.

Bioluminescence Resonance Energy Transfer (BRET)-coupled Annexin V-functionalized Quantum Dots for Near-Infrared Optical Detection of Apoptotic Cells

Deregulation in apoptosis induces numerous diseases such as cancer, cardiovascular, and neurodegenerative diseases. Detection of apoptotic cells is crucial for understanding the mechanism of these diseases and for therapy development. Although optical imaging using visible-emitting fluorescent probes, such as FITC-labeled annexin V, is widely used for the detection of apoptotic cells, there are very limited probes that can be used in the near-infrared region (NIR) over 700 nm. Compared with visible light, NIR light is highly permeable in turbid biological samples and tissues. In addition, optical imaging in the NIR region shows low autofluorescence from biological samples, leading to clearer images with high signal to background ratios. Here, we report the synthesis of bioluminescence resonance energy transfer (BRET)-coupled annexin V-functionalized quantum dots (QDs) and their application to NIR optical detection of apoptotic cells.

Quantum dots: from fluorescence to chemiluminescence, bioluminescence, electrochemiluminescence, and electrochemistry

During the past decade, nanotechnology has become one of the major forces driving basic and applied research. As a novel class of inorganic fluorochromes, research into quantum dots (QDs) has become one of the fastest growing fields of nanotechnology today. QDs are made of a semiconductor material with tunable physical dimensions as well as unique optoelectronic properties, and have attracted multidisciplinary research efforts to further their potential bioanalytical applications. Recently, numerous optical properties of QDs, such as narrow emission band peaks, broad absorption spectra, intense signals, and remarkable resistance to photobleaching, have made them biocompatible and sensitive for biological assays. In this review, we give an overview of these exciting materials and describe their potential, especially in biomolecules analysis, including fluorescence detection, chemiluminescence detection, bioluminescence detection, electrochemiluminescence detection, and electrochemical detection. Finally, conclusions are made, including highlighting some critical challenges remaining and a perspective of how this field can be expected to develop in the future.

Nanotechnology-Enhanced No-Wash Biosensors for in Vitro Diagnostics of Cancer

In vitro biosensors have been an integral component for early diagnosis of cancer in the clinic. Among them, no-wash biosensors, which only depend on the simple mixing of the signal generating probes and the sample solution without additional washing and separation steps, have been found to be particularly attractive. The outstanding advantages of facile, convenient, and rapid response of no-wash biosensors are especially suitable for point-of-care testing (POCT). One fast-growing field of no-wash biosensor design involves the usage of nanomaterials as signal amplification carriers or direct signal generating elements. The analytical capacity of no-wash biosensors with respect to sensitivity or limit of detection, specificity, stability, and multiplexing detection capacity is largely improved because of their large surface area, excellent optical, electrical, catalytic, and magnetic properties. This review provides a comprehensive overview of various nanomaterial-enhanced no-wash biosensing technologies and focuses on the analysis of the underlying mechanism of these technologies applied for the early detection of cancer biomarkers ranging from small molecules to proteins, and even whole cancerous cells. Representative examples are selected to demonstrate the proof-of-concept with promising applications for in vitro diagnostics of cancer. Finally, a brief discussion of common unresolved issues and a perspective outlook on the field are provided.

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.

"Turn-off" fluorescent sensor for highly sensitive and specific simultaneous recognition of 29 famous green teas based on quantum dots combined with chemometrics

Fluorescent "turn-off" sensors based on water-soluble quantum dots (QDs) have drawn increasing attention owing to their unique properties such as high fluorescence quantum yields, chemical stability and low toxicity. In this work, a novel method based on the fluorescence "turn-off" model with water-soluble CdTe QDs as the fluorescent probes for differentiation of 29 different famous green teas is established. The fluorescence of the QDs can be quenched in different degrees in light of positions and intensities of the fluorescent peaks for the green teas. Subsequently, with aid of classic partial least square discriminant analysis (PLSDA), all the green teas can be discriminated with high sensitivity, specificity and a satisfactory recognition rate of 100% for training set and 98.3% for prediction set, respectively. Especially, the "turn-off" fluorescence PLSDA model based on second-order derivatives (2nd der) with reduced least complexity (LVs = 3) was the most effective one for modeling. Most importantly, we further demonstrated the established "turn-off" fluorescent sensor mode has several significant advantages and appealing properties over the conventional fluorescent method for large-class-number classification (LCNC) of green teas. This work is, to the best of our knowledge, the first report on the rapid and effective identification of so many kinds of famous green teas based on the "turn-off" model of QDs combined with chemometrics, which also implies other potential applications on complex LCNC classification system with weak fluorescence or even without fluorescence to achieve higher detective response and specificity.