Nanobodies and nanocrystals: highly sensitive quantum dot-based homogeneous FRET immunoassay for serum-based EGFR detection

Molecular and functional insight into anti-EGFR nanobody: Theranostic implications for malignancies

Targeted therapy and imaging are the most popular techniques for the intervention and diagnosis of cancer. A potential therapeutic target for the treatment of cancer is the epidermal growth factor receptor (EGFR), primarily for glioblastoma, lung, and breast cancer. Over-production of ligand, transcriptional up-regulation due to autocrine/paracrine signalling, or point mutations at the genomic locus may contribute to the malfunction of EGFR in malignancies. This exploit makes use of EGFR, an established biomarker for cancer diagnostics and treatment. Despite considerable development in the last several decades in making EGFR inhibitors, they are still not free from limitations like toxicity and a short serum half-life. Nanobodies and antibodies share similar binding properties, but nanobodies have the additional advantage that they can bind to antigenic epitopes deep inside the target that conventional antibodies are unable to access. For targeted therapy, anti-EGFR nanobodies can be conjugated to various molecules such as drugs, peptides, toxins and photosensitizers. These nanobodies can be designed as novel immunoconjugates using the universal modular antibody-based platform technology (UniCAR). Furthermore, Anti-EGFR nanobodies can be expressed in neural stem cells and visualised by effective fluorescent and radioisotope labelling.

Homogeneous immunoassay utilizing fluorescence resonance energy transfer from quantum dots to tyramide dyes deposited on full immunocomplexes

There is an urgent need for homogeneous immunoassays that offer sufficient sensitivity for routine clinical practice. In this study, we have developed a highly sensitive, fluorescence resonance energy transfer (FRET)-based homogeneous immunoassay. Unlike previous FRET-based homogeneous immunoassays, where acceptors were attached to antibody molecules located far from the donor, we employed acceptors to label the entire sandwich-structured immunocomplex, including two antibodies and one antigen. As a result, the FRET signal was amplified by a factor of 10, owing to the reduced distance between the donor and acceptors. We validated our method by quantifying carcinoembryonic antigen (CEA) and α-fetoprotein (AFP) in PBS buffer and blank plasma. The limits of detection (LOD) for CEA and AFP in both PBS buffer and blank plasma were comparable, reaching sub-femtomolar levels. Furthermore, we successfully quantified CEA and AFP in three human plasma samples, thereby confirming the reliability of our method for clinical applications.

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.

Development of Electrochemical Immunosensors for HER-1 and HER-2 Analysis in Serum for Breast Cancer Patients

In this work, two human epidermal growth factor receptors, HER-1 and HER-2, were selected as biomarkers to enable the detection of breast cancer. Therefore, two biosensors were developed using gold sensor chips coupled with amperometric detection of the enzyme label horse radish peroxidase (HRP). The biosensors/immunosensors relied on indirect sandwich enzyme-linked immunosorbent assays with monoclonal antibodies (Ab) against HER-1 and HER-2 attached to the sensors to capture the biomarkers. Detection polyclonal antibodies followed by secondary anti-rabbit (for HER-1) and anti-goat (for HER-2) IgG antibody-HRP were then applied for signal generation. In buffer, the developed sensors showed limits of detections (LOD) of 1.06 ng mL^-1 and 0.95 ng mL^-1 and limits of quantification (LOQ) of 2.1 ng mL^-1 and 1.5 ng mL^-1 for HER-1 and HER-2, respectively. In 100% (undiluted) serum, LODs of 1.2 ng mL^-1 and 1.47 ng mL^-1 and LOQs of 1.5 ng mL^-1 and 2.1 ng mL^-1 were obtained for HER-1 and HER-2, respectively. Such limits of detections are within the serum clinical range for the two biomarkers. Furthermore, gold nanoparticles (AuNP) labelled with secondary anti-rabbit and anti-goat IgG antibody-HRP were then used to enhance the assay signal and increase the sensitivity. In buffers, LODs of 30 pg mL^-1 were seen for both sensors and LOQs of 98 pg mL^-1 and 35 pg mL^-1 were recorded for HER-1 and HER-2, respectively. For HER-2 the AuNPs biosensor was also tested in 100% serum obtaining a LOD of 50 pg mL^-1 and a LOQ of 80 pg mL^-1. The HER-2 AuNP electrochemical immunosensor showed high specificity with very low cross-reactivity to HER-1. These findings demonstrate that the two developed sensors can enable early detection as well as monitoring of disease progression with a beneficial impact on patient survival and clinical outcomes.

Detection of Alpha-Fetoprotein Using Aptamer-Based Sensors

Alpha-fetoprotein (AFP) is widely-known as the most commonly used protein biomarker for liver cancer diagnosis at the early stage. Therefore, developing the highly sensitive and reliable method of AFP detection is of essential demand for practical applications. Herein, two types of aptamer-based AFP detection methods, i.e., optical and electrochemical biosensors, are reviewed in detail. The optical biosensors include Raman spectroscopy, dual-polarization interferometry, resonance light-scattering, fluorescence, and chemiluminescence. The electrochemical biosensors include cyclic voltammetry, electrochemical impedance spectroscopy, and giant magnetic impedance. Looking into the future, methods for AFP detection that are high sensitivity, long-term stability, low cost, and operation convenience will continue to be developed.

Anti-EGF nanobodies enhance the antitumoral effect of osimertinib and overcome resistance in non-small cell lung cancer (NSCLC) cellular models

Osimertinib is a third-generation epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKI) that is effective against the EGFR T790M mutation in patients with advanced non-small-cell lung cancer (NSCLC). However, acquired resistance appears invariably due to several mechanisms. The strategy of using EGF-targeted nanobodies (Nbs) to block the initial step of the EGFR pathway constitutes a new research area. Nbs offer several advantages compared to traditional mAbs, such as their reduced size, increased stability, and tissue penetration, which provide key advantages for targeting soluble tumoral growth factors. In this study we investigated the efficacy of anti-EGF Nbs to reduce Osimertinib resistance. Two anti-EGF Nbs, generated in our laboratory, were shown to inhibit cell viability and colony formation in PC9 and PC9-derived osimertinib-resistant cell lines. The combination of these Nbs with osimertinib improved the antitumor efficacy of this EGFR-TKI in cell viability and colony formation experiments. In a mechanistic study of the EGFR pathway, the combination treatment dampened the activation of downstream proteins such as Akt and Erk1/2 MAP kinases. In addition, it increased cellular apoptosis and decreased the expression of Hes1, a cancer stem cell marker involved in metastasis and osimertinib resistance. We conclude that the addition of anti-EGF nanobodies enhances the antitumor properties of osimertinib, thus representing a potentially effective strategy for NSCLC patients.

Future of Digital Assays to Resolve Clinical Heterogeneity of Single Extracellular Vesicles

Extracellular vesicles (EVs) are complex lipid membrane vehicles with variable expressions of molecular cargo, composed of diverse subpopulations that participate in the intercellular signaling of biological responses in disease. EV-based liquid biopsies demonstrate invaluable clinical potential for overhauling current practices of disease management. Yet, EV heterogeneity is a major needle-in-a-haystack challenge to translate their use into clinical practice. In this review, existing digital assays will be discussed to analyze EVs at a single vesicle resolution, and future opportunities to optimize the throughput, multiplexing, and sensitivity of current digital EV assays will be highlighted. Furthermore, this review will outline the challenges and opportunities that impact the clinical translation of single EV technologies for disease diagnostics and treatment monitoring.

A Nanobody‐on‐Quantum Dot Displacement Assay for Rapid and Sensitive Quantification of the Epidermal Growth Factor Receptor (EGFR)

Biosensing approaches that combine small, engineered antibodies (nanobodies) with nanoparticles are often complicated. Here, we show that nanobodies with different C‐terminal tags can be efficiently attached to a range of the most widely used biocompatible semiconductor quantum dots (QDs). Direct implementation into simplified assay formats was demonstrated by designing a rapid and wash‐free mix‐and‐measure immunoassay for the epidermal growth factor receptor (EGFR). Terbium complex (Tb)‐labeled hexahistidine‐tagged nanobodies were specifically displaced from QD surfaces via EGFR‐nanobody binding, leading to an EGFR concentration‐dependent decrease of the Tb‐to‐QD Förster resonance energy transfer (FRET) signal. The detection limit of 80±20 pM (16±4 ng mL⁻¹) was 3‐fold lower than the clinical cut‐off concentration for soluble EGFR and up to 10‐fold lower compared to conventional sandwich FRET assays that required a pair of different nanobodies.

A Dendrimer-Based Time-Gated Concentric FRET Configuration for Multiplexed Sensing

Förster resonance energy transfer (FRET) is widely used for the development of biological probes and sensors. In this context, the norm for multiplexed detection is deployment of multiple probes, each a discrete donor-acceptor pair. Concentric FRET (cFRET) probes enable multiplexed sensing with a single vector but, to date, have only been developed around semiconductor quantum dots, which may limit the scope of biological applications for such probes. Here, we demonstrate that dendrimers labeled with a luminescent terbium complex (Tb) are a viable and advantageous alternative platform for cFRET probes. Polyamidoamine dendrimers were functionalized with Tb, biotin, NeutrAvidin, and three types of dye-labeled oligonucleotide probes to establish a network of competitive and sequential Tb-to-dye and dye-to-dye FRET pathways. These probes were characterized physically and photophysically, and a time-gated multiplexed assay for DNA targets was demonstrated. The time-gating offered by the Tb allowed the rejection of background autofluorescence from serum. More broadly, this dendrimer-based architecture shows that cFRET is a general concept and is an important step toward a new generation of probes for biological sensing.

An Electrochemical and Raman Scattering Dual Detection Biosensor for Rapid Screening and Biomolecular Profiling of Cancer Biomarkers

Detecting circulating biomarkers sensitively and quantitatively is paramount for cancer screening, diagnosis, and treatment selection. Particularly, screening of a panel of circulating protein biomarkers followed by mapping of individual biomarkers could assist better diagnosis and understanding of the cancer progression mechanisms. Herein, we present a miniaturized biosensing platform with dual readout schemes (electrochemical and Surface enhanced Raman scattering (SERS)) for rapid cancer screening and specific biomarker expressional profiling to support cancer management. Our approach utilizes a controlled nanomixing phenomena under alternative current electrohydrodynamic condition to improve the isolation of cancer-associated circulating proteins (i.e., Epidermal growth factor receptor (EGFR), BRAF, Programmed death-ligand 1 (PD-L1)) with antibody functionalized sensor surface for rapid and efficient isolation of the targets and subsequent labelling with SERS nanotags. The method employs Differential Pulse Voltammetry (DPV) for rapidly screening for the presence of the circulating proteins on biosensor surface irrespective of their type. Upon positive DPV detection, SERS is applied for sensitive read-out of individual biomarkers biomarker levels. In a proof-of-concept study, we demonstrate the dual detection biosensor for analysing circulating BRAF, EGFR and PDL-1 proteins and successfully screened both ensemble and individual biomarker expressional levels as low as 10 pg (1 ng/mL). Our findings clearly indicate the potential of the proposed method for cancer biomarker analysis which may drive the translation of this dual sensing concept in clinical settings.

Multiplexed Biosensing and Bioimaging Using Lanthanide-Based Time-Gated Förster Resonance Energy Transfer

The necessity to scrutinize more and more biological molecules and interactions both in solution and on the cellular level has led to an increasing demand for sensitive and specific multiplexed diagnostic analysis. Photoluminescence (PL) detection is ideally suited for multiplexed biosensing and bioimaging because it is rapid and sensitive and there is an almost unlimited choice of fluorophores that provide a large versatility of photophysical properties, including PL intensities, spectra, and lifetimes.The most frequently used technique to detect multiple parameters from a single sample is spectral (or color) multiplexing with different fluorophores, such as organic dyes, fluorescent proteins, quantum dots, or lanthanide nanoparticles and complexes. In conventional PL biosensing approaches, each fluorophore requires a distinct detection channel and excitation wavelength. This drawback can be overcome by Förster resonance energy transfer (FRET) from lanthanide donors to other fluorophore acceptors. The lanthanides' multiple and spectrally narrow emission bands over a broad spectral range can overlap with several different acceptors at once, thereby allowing FRET from one donor to multiple acceptors. The lanthanides' extremely long PL lifetimes provide two important features. First, time-gated (TG) detection allows for efficient suppression of background fluorescence from the biological environment or directly excited acceptors. Second, temporal multiplexing, for which the PL lifetimes are adjusted by the interaction with the FRET acceptor, can be used to determine specific biomolecules and/or their conformation via distinct PL decays. The high signal-to-background ratios, reproducible and precise ratiometric and homogeneous (washing-free) sensing formats, and higher-order multiplexing capabilities of lanthanide-based TG-FRET have resulted in significant advances in the analysis of biomolecular recognition. Applications range from fundamental analysis of biomolecular interactions and conformations to high-throughput and point-of-care in vitro diagnostics and DNA sequencing to advanced optical encoding, using both liquid and solid samples and in situ, in vitro, and in vivo detection with high sensitivity and selectivity.In this Account, we discuss recent advances in lanthanide-based TG-FRET for the development and application of advanced immunoassays, nucleic acid sensing, and fluorescence imaging. In addition to the different spectral and temporal multiplexing approaches, we highlight the importance of the careful design and combination of different biological, organic, and inorganic molecules and nanomaterials for an adjustable FRET donor-acceptor distance that determines the ultimate performance of the diagnostic assays and conformational sensors in their physiological environment. We conclude by sharing our vision on how progress in the development of new sensing concepts, material combinations, and instrumentation can further advance TG-FRET multiplexing and accelerate its translation into routine clinical practice and the investigation of challenging biological systems.

Advances in Chicken IgY-Based Immunoassays for the Detection of Chemical and Biological Hazards in Food Samples

As antibodies are the main biological binder for hazards in food samples, their performance directly determines the sensitivity, specificity, and reproducibility of the developed immunoassay. The overwhelmingly used mammalian-derived antibodies usually suffer from complicated preparation, high cost, frequent bleeding of animals, and sometimes low titer and affinity. Chicken yolk antibody (IgY) has recently attracted considerable attention in the bioanalytical field owing to its advantages in productivity, animal welfare, comparable affinity, and high specificity. However, a broad understanding of the application of IgY-based immunoassay for the detection of chemical and biological hazards in food samples remains limited. Here, we briefly summarized the diversity, structure, and production of IgY including polyclonal and monoclonal formats. Then, a comprehensive overview of the principles, designs, and applications of IgY-based immunoassays for these hazards was reviewed and discussed, including food-borne pathogens, food allergens, veterinary drugs, pesticides, toxins, endocrine disrupting chemicals, etc. Thus, the trend of IgY-based immunoassays is expected, and more IgY types, higher sensitivity, and diversification of recognition-to-signal manners are necessary in the future.

Electron transfer-triggered imaging of EGFR signaling activity

In vivo electron transfer processes are closely related to the activation of signaling pathways, and, thus, affect various life processes. Indeed, the signaling pathway activation of key molecules may be associated with certain diseases. For example, epidermal growth factor receptor (EGFR) activation is related to the occurrence and development of tumors. Hence, monitoring the activation of EGFR-related signaling pathways can help reveal the progression of tumor development. However, it is challenging for current detection methods to monitor the activation of specific signaling pathways in complex biochemical reactions. Here we designed a highly sensitive and specific nanoprobe that enables in vivo imaging of electronic transfer over a broad range of spatial and temporal scales. By using the ferrocene-DNA polymer “wire”, the electrons transferred in a biochemical reaction can flow to persistent luminescent nanoparticles and change their electron distribution, thereby altering the optical signal of the particles. This electron transfer-triggered imaging probe enables mapping the activation of EGFR-related signaling pathways in a temporally and spatially precise manner. By offering precise visualization of signaling activity, this approach may offer a general platform not only for understanding molecular mechanisms in various biological processes but also for promoting disease therapies and drug evaluation.

Here, the authors design a nanoprobe for in vivo imaging of electronic transfer, consisting of a ferrocene-DNA polymer to transfer electrons to luminescent nanoparticles, changing their optical signal. Using this probe, they map activation of EGFR signalling during tumour treatment.

Paper Title "Hu7CG2: A Novel Humanized Anti-Epidermal Growth Factor Receptor (EGFR) Biparatopic Nanobody"

Targeted therapy is an effective and appropriate approach with low side effects in cancer therapy compared with other treatment approaches. Epidermal growth factor receptor, EGFR, is a favorable biomarker as targeted therapy because it overexpresses in several cancers. Monoclonal antibodies are common agents for targeted therapy. Nanobody is the smallest format of monoclonal antibodies with unique properties that include hiding epitope targeting, high stability, low production cost, and ease of connection to other components. The main challenge in targeted therapy by monoclonal antibodies is their immunogenicity due to their non-human nature. In this study, we designed, constructed, and evaluated a novel humanized anti- EGFR biparatopic nanobody, hu7CG2. The hu7CG2 was designed by grafting the complementarity-determining regions of two camelid anti- EGFR nanobodies known as 7C12 and EG2 to a universal scaffold and then connected with a glycine-serine linker. The results of antigen-binding activity and cell viability assays showed that the hu7CG2 inhibited the growth of EGFR overexpression tumor cells. The data showed that hu7CG2 might be a useful tool in the targeting and treatment of tumor cells.

Nanobodies derived from Camelids represent versatile biomolecules for biomedical applications

Nanobodies are antigen binding variable domains of heavy-chain antibodies without light-chains, and these biomolecules occur naturally in the serum of Camelidae species. Nanobodies have a compact structure and low molecular weight when compared with antibodies, and are the smallest active antigen-binding fragments. Because of their remarkable stability and manipulable characteristics, nanobodies have been incorporated into biomaterials and used as molecular recognition and tracing agents, drug delivery systems, molecular imaging tools and disease therapeutics. This review summarizes recent progress in this field focusing on nanobodies as versatile biomolecules for biomedical applications.

Molecularly imprinted polyaniline for detection of horseradish peroxidase

A new facile and fast approach to the synthesis of polyaniline (PANi) molecularly imprinted polymers (MIPs) based on aniline oxidative chemical polymerization was proposed for protein recognition. For the first time, a surface imprinting strategy was implemented for the synthesis of PANi MIPs on the inner surface of soft glass polycapillaries (PC) with a large (2237) number of individual microcapillaries. Two different PANi layers-(i) PANi film and (ii) protein imprinted PANi nanowires-were synthesized sequentially. Uniform and highly stable PANi film was synthesized by oxidative polymerization at pH< 1. The synthesis of PANi MIPs on the PANi film pre-coated surface improved the reproducibility of PANi MIP formation. PANi MIP nanowires were synthesized at "mild" conditions (pH > 4.5) to preserve the protein template activity. The binding of horseradish peroxidase (HRP) molecules on the PANi MIP selective sites was confirmed by photometry (TMB chromogenic reaction), SEM images, and FTIR spectroscopy. The developed PANi MIPs enable HRP determination with a limit of detection (LOD) as low as 1.00 and 0.07 ng mL^-1 on the glass slips and PC, respectively. The PANi MIPs are characterized by high stability; they are reversible and selective to HRP. The proposed approach allows PANi MIPs to be obtained for proteins on different supports and to create new materials for separation and sensing. Graphical abstract.

Investigation of the Small Size of Nanobodies for a Sensitive Fluorescence Polarization Immunoassay for Small Molecules: 3-Phenoxybenzoic Acid, an Exposure Biomarker of Pyrethroid Insecticides as a Model

Limited reports on the use of nanobodies (Nbs) in fluorescence polarization immunoassay (FPIA) aroused us to explore if the small size of Nbs is a drawback for the development of sensitive FPIA to small molecular compounds, particularly since FPIA is a technology strongly dependent on molecular weight. In the present work, three different molecular weight Nbs against 3-phenoxybenzoic acid (3-PBA), an exposure biomarker of pyrethroid insecticides, including bare Nbs (15 kDa), Nbs-Avidin (Nbs-AV, 60 kDa), and Nbs-Alkaline phosphatase (Nbs-AP, 130 kDa) were specifically generated to cover distinct regions on the polarization and molecular weight relationship curve for a fluorescein tracer. In competitive FPIA, similar half-maximal inhibitory concentrations (IC₅₀) of 3-PBA of 16.4, 12.2, and 14.8 ng mL^-1 were obtained for Nbs, Nbs-AV, and Nbs-AP, respectively, indicating that the size of Nbs in the range tested had no significant effect on the sensitivity of the resulting competitive FPIA. An IC₅₀ of 20.2 ng mL^-1 for an anti-3-PBA polyconal antibody based FPIA further demonstrated the performance of Nbs, which was comparable to that of traditional antibodies in FPIA. Spike-recovery studies showed good and reproducible recovery of 3-PBA in urine samples, demonstrating the applicability of Nb-based FPIA. Overall, our results show that Nb-based FPIA achieves sensitivity levels of FPIA based on conventional antibodies and further indicate that Nb absolutely meets the sensitivity requirement of FPIA.

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.

Selective determination of acetone by carbon nanodots based on inner filter effect

Fluorescent sensing of acetone has been achieved taking advantage of the unique optical property of acetone. However, the complicated synthesis process of the fluorescent probes limited their application. Here, carbon nanodots (CNDs) derived from glucose were chosen as the sensing material for the first time, which could be obtained by a one-pot microwave-assisted synthesis within 2 min. The CND had ultraviolet excitation spectrum, resulting in an obvious overlap between its excitation band with the absorption band of acetone. The fluorescent quenching occurred via inner filter effect with fast response. The CNDs showed sensitive response to acetone with a detection limit of 0.09 vol%, which was comparable to most previously reported MOF probes. The results of our work indicated the feasibility of IFE-based sensing method, and demonstrated an efficient and convenient way for acetone detection.

In Vivo Biosensing Using Resonance Energy Transfer

Solution-phase and intracellular biosensing has substantially enhanced our understanding of molecular processes foundational to biology and pathology. Optical methods are favored because of the low cost of probes and instrumentation. While chromatographic methods are helpful, fluorescent biosensing further increases sensitivity and can be more effective in complex media. Resonance energy transfer (RET)-based sensors have been developed to use fluorescence, bioluminescence, or chemiluminescence (FRET, BRET, or CRET, respectively) as an energy donor, yielding changes in emission spectra, lifetime, or intensity in response to a molecular or environmental change. These methods hold great promise for expanding our understanding of molecular processes not just in solution and in vitro studies, but also in vivo, generating information about complex activities in a natural, organismal setting. In this review, we focus on dyes, fluorescent proteins, and nanoparticles used as energy transfer-based optical transducers in vivo in mice; there are examples of optical sensing using FRET, BRET, and in this mammalian model system. After a description of the energy transfer mechanisms and their contribution to in vivo imaging, we give a short perspective of RET-based in vivo sensors and the importance of imaging in the infrared for reduced tissue autofluorescence and improved sensitivity.

Sensing with photoluminescent semiconductor quantum dots

Fluorescent sensors benefit from high signal-to-noise and multiple measurement modalities, enabling a multitude of applications and flexibility of design. Semiconductor nanocrystal quantum dots (QDs) are excellent fluorophores for sensors because of their extraordinary optical properties. They have high thermal and photochemical stability compared to organic dyes or fluorescent proteins and are extremely bright due to their large molar cross-sections. In contrast to organic dyes, QD emission profiles are symmetric, with relatively narrow bandwidths. In addition, the size tunability of their emission color, which is a result of quantum confinement, make QDs exceptional emitters with high color purity from the ultra-violet to near infrared wavelength range. The role of QDs in sensors ranges from simple fluorescent tags, as used in immunoassays, to intrinsic sensors that utilize the inherent photophysical response of QDs to fluctuations in temperature, electric field, or ion concentration. In more complex configurations, QDs and biomolecular recognition moieties like antibodies are combined with a third component to modulate the optical signal via energy transfer. QDs can act as donors, acceptors, or both in energy transfer-based sensors using Förster resonance energy transfer (FRET), nanometal surface energy transfer (NSET), or charge or electron transfer. The changes in both spectral response and photoluminescent lifetimes have been successfully harnessed to produce sensitive sensors and multiplexed devices. While technical challenges related to biofunctionalization and the high cost of laboratory-grade fluorimeters have thus far prevented broad implementation of QD-based sensing in clinical or commercial settings, improvements in bioconjugation methods and detection schemes, including using simple consumer devices like cell phone cameras, are lowering the barrier to broad use of more sensitive QD-based devices.

Quantum Dot-Based FRET Immunoassay for HER2 Using Ultrasmall Affinity Proteins

Engineered scaffold affinity proteins are used in many biological applications with the aim of replacing natural antibodies. Although their very small sizes are beneficial for multivalent nanoparticle conjugation and efficient Förster resonance energy transfer (FRET), the application of engineered affinity proteins in such nanobiosensing formats has been largely neglected. Here, it is shown that very small (≈6.5 kDa) histidine-tagged albumin-binding domain-derived affinity proteins (ADAPTs) can efficiently self-assemble to zwitterionic ligand-coated quantum dots (QDs). These ADAPT-QD conjugates are significantly smaller than QD-conjugates based on IgG, Fab', or single-domain antibodies. Immediate applicability by the quantification of the human epidermal growth factor receptor 2 (HER2) in serum-containing samples using time-gated Tb-to-QD FRET detection on the clinical benchtop immunoassay analyzer KRYPTOR is demonstrated here. Limits of detection down to 40 × 10^-12 m (≈8 ng mL^-1 ) are in a relevant clinical concentration range and outperform previously tested assays with antibodies, antibody fragments, and nanobodies.

Highly sensitive detection of epidermal growth factor receptor in lung cancer cells by aptamer-based target-/probe-mediated cyclic signal amplification

We develop an antibody-free fluorescence method for the epidermal growth factor receptor (EGFR) assay using aptamer-based target-/probe-mediated cyclic signal amplification. The method is highly sensitive with a detection limit of 0.16 fM, and it can be applied to detect EGFR in lung cancer cells, holding great potential in clinical diagnosis.

Palladium Nanoparticles-Based Fluorescence Resonance Energy Transfer Aptasensor for Highly Sensitive Detection of Aflatoxin M₁ in Milk

A highly sensitive aptasensor for aflatoxin M₁ (AFM₁) detection was constructed based on fluorescence resonance energy transfer (FRET) between 5-carboxyfluorescein (FAM) and palladium nanoparticles (PdNPs). PdNPs (33 nm) were synthesized through a seed-mediated growth method and exhibited broad and strong absorption in the whole ultraviolet-visible (UV-Vis) range. The strong coordination interaction between nitrogen functional groups of the AFM₁ aptamer and PdNPs brought FAM and PdNPs in close proximity, which resulted in the fluorescence quenching of FAM to a maximum extent of 95%. The non-specific fluorescence quenching caused by PdNPs towards fluorescein was negligible. After the introduction of AFM₁ into the FAM-AFM₁ aptamer-PdNPs FRET system, the AFM₁ aptamer preferentially combined with AFM₁ accompanied by conformational change, which greatly weakened the coordination interaction between the AFM₁ aptamer and PdNPs. Thus, fluorescence recovery of FAM was observed and a linear relationship between the fluorescence recovery and the concentration of AFM₁ was obtained in the range of 5–150 pg/mL in aqueous buffer with the detection limit of 1.5 pg/mL. AFM₁ detection was also realized in milk samples with a linear detection range from 6 pg/mL to 150 pg/mL. The highly sensitive FRET aptasensor with simple configuration shows promising prospect in detecting a variety of food contaminants.

Single-Domain Antibodies As Versatile Affinity Reagents for Analytical and Diagnostic Applications

With just three CDRs in their variable domains, the antigen-binding site of camelid heavy-chain-only antibodies (HcAbs) has a more limited structural diversity than that of conventional antibodies. Even so, this does not seem to limit their specificity and high affinity as HcAbs against a broad range of structurally diverse antigens have been reported. The recombinant form of their variable domain [nanobody (Nb)] has outstanding properties that make Nbs, not just an alternative option to conventional antibodies, but in many cases, these properties allow them to reach analytical or diagnostic performances that cannot be accomplished with conventional antibodies. These attributes include comprehensive representation of the immune specificity in display libraries, easy adaptation to high-throughput screening, exceptional stability, minimal size, and versatility as affinity building block. Here, we critically reviewed each of these properties and highlight their relevance with regard to recent developments in different fields of immunosensing applications.

Graphene oxide and DNA aptamer based sub-nanomolar potassium detecting optical nanosensor

Quantum-dot (QD) based nanosensors are frequently used by researchers to detect small molecules, ions and different biomolecules. In this article, we present a sensor complex/system comprised of deoxyribonucleic acid (DNA) aptamer, gold nanoparticle and semiconductor QD, attached to a graphene oxide (GO) flake for detection of potassium. As reported herein, it is demonstrated that QD-aptamer-quencher nanosensor functions even when tethered to GO, opening the way to future applications where sensing can be accomplished simultaneously with other previously demonstrated applications of GO such as serving as a nanocarrier for drug delivery. Herein, it is demonstrated that the DNA based thrombin binding aptamer used in this study undergoes the conformational change needed for sensing even when the nanosensor complex is anchored to the GO. Analysis with the Hill equation indicates the interaction between aptamer and potassium follows sigmoidal Hill kinetics. It is found that the quenching efficiency of the optical sensor is linear with the logarithm of concentration from 1 pM to 100 nM and decreases for higher concentration due to unavailability of aptamer binding sites. Such a simple and sensitive optical aptasensor with minimum detection capability of 1.96 pM for potassium ion can also be employed in-vitro detection of different physiological ions, pathogens and disease detection methods.

Unravelling the Molecular Basis of High Affinity Nanobodies against HIV p24: In Vitro Functional, Structural, and in Silico Insights

Preventing the spread of infectious diseases remains an urgent priority worldwide, and this is driving the development of advanced nanotechnology to diagnose infections at the point of care. Herein, we report the creation of a library of novel nanobody capture ligands to detect p24, one of the earliest markers of HIV infection. We demonstrate that these nanobodies, one tenth the size of conventional antibodies, exhibit high sensitivity and broad specificity to global HIV-1 subtypes. Biophysical characterization indicates strong 690 pM binding constants and fast kinetic on-rates, 1 to 2 orders of magnitude better than monoclonal antibody comparators. A crystal structure of the lead nanobody and p24 was obtained and used alongside molecular dynamics simulations to elucidate the molecular basis of these enhanced performance characteristics. They indicate that binding occurs at C-terminal helices 10 and 11 of p24, a negatively charged region of p24 complemented by the positive surface of the nanobody binding interface involving CDR1, CDR2, and CDR3 loops. Our findings have broad implications on the design of novel antibodies and a wide range of advanced biomedical applications.

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.

Nanobody Based Immunoassay for Human Soluble Epoxide Hydrolase Detection Using Polymeric Horseradish Peroxidase (PolyHRP) for Signal Enhancement: The Rediscovery of PolyHRP?

Soluble epoxide hydrolase (sEH) is a potential pharmacological target for treating hypertension, vascular inflammation, cancer, pain, and multiple cardiovascular related diseases. A variable domain of the heavy chain antibody (termed single domain antibody (sdAb), nanobody, or VHH) possesses the advantages of small size, high stability, ease of genetic manipulation, and ability for continuous manufacture, making such nanobody a superior choice as an immunoreagent. In this work, we developed an ultrasensitive nanobody based immunoassay for human sEH detection using polymeric horseradish peroxidase (PolyHRP) for signal enhancement. Llama nanobodies against human sEH were used as the detection antibody in sandwich enzyme linked immunosorbent assays (ELISA) with polyclonal anti-sEH as the capture antibody. A conventional sandwich ELISA using a horseradish peroxidase (HRP) labeled anti-hemeagglutinin (HA) tag as the tracer showed a marginal sensitivity (0.0015 optical density (OD)·mL/ng) and limit of detection (LOD) of 3.02 ng/mL. However, the introduction of the PolyHRP as the tracer demonstrated a 141-fold increase in the sensitivity (0.21 OD·mL/ng) and 57-fold decrease in LOD (0.05 ng/mL). Systematic comparison of three different tracers in four ELISA formats demonstrated the overwhelming advantage of PolyHRP as a label for nanobody based immunoassay. This enhanced sEH immunoassay was further evaluated in terms of selectivity against other epoxide hydrolases and detection of the target protein in human tissue homogenate samples. Comparison with an enzyme activity based assay and a Western blot for sEH detection reveals good correlation with the immunoassay. This work demonstrates increased competiveness of nanobodies for practical sEH protein detection utilizing PolyHRP. It is worthwhile to rediscover the promising potential of PolyHRP in nanobody and other affinity based methods after its low-profile existence for decades.

Ultraefficient Cap-Exchange Protocol To Compact Biofunctional Quantum Dots for Sensitive Ratiometric Biosensing and Cell Imaging

An ultraefficient cap-exchange protocol (UCEP) that can convert hydrophobic quantum dots (QDs) into stable, biocompatible, and aggregation-free water-dispersed ones at a ligand:QD molar ratio (LQMR) as low as 500, some 20-200-fold less than most literature methods, has been developed. The UCEP works conveniently with air-stable lipoic acid (LA)-based ligands by exploiting tris(2-carboxylethyl phosphine)-based rapid in situ reduction. The resulting QDs are compact (hydrodynamic radius, R_h, < 4.5 nm) and bright (retaining > 90% of original fluorescence), resist nonspecific adsorption of proteins, and display good stability in biological buffers even with high salt content (e.g., 2 M NaCl). These advantageous properties make them well suited for cellular imaging and ratiometric biosensing applications. The QDs prepared by UCEP using dihydrolipoic acid (DHLA)-zwitterion ligand can be readily conjugated with octa-histidine (His₈)-tagged antibody mimetic proteins (known as Affimers). These QDs allow rapid, ratiometric detection of the Affimer target protein down to 10 pM via a QD-sensitized Förster resonance energy transfer (FRET) readout signal. Moreover, compact biotinylated QDs can be readily prepared by UCEP in a facile, one-step process. The resulting QDs have been further employed for ratiometric detection of protein, exemplified by neutravidin, down to 5 pM, as well as for fluorescence imaging of target cancer cells.