Quantum dot-based sensitive detection of disease specific exosome in serum

Universal Ti3C2 MXenes Based Self-Standard Ratiometric Fluorescence Resonance Energy Transfer Platform for Highly Sensitive Detection of Exosomes

Exosomes, as novel noninvasive biomarkers for disease prediction and diagnosis, have shown fascinating prospects in monitoring cancer-linked public health issues. Herein, a unique Cy3 labeled CD63 aptamer (Cy3-CD63 aptamer)/Ti₃C₂ MXenes nanocomplex was constructed as a self-standard ratiometric fluorescence resonance energy transfer (FRET) nanoprobe for quantitative detection of exosomes. The Cy3-CD63 aptamer can be selectively adsorbed onto the Ti₃C₂ MXene nanosheets by hydrogen bond and metal chelate interaction between the aptamer and MXenes, and the fluorescence signal from Cy3-CD63 aptamer was quenched quickly owing to the FRET between the Cy3 and MXenes. The fluorescence of Cy3 greatly recovered after the addition of the exosomes which can specifically combine with the aptamer and release from the surface of Ti₃C₂ MXenes due to the high affinity between the aptamer and CD63 protein on exosome surface. Meanwhile, the self-fluorescence signal of MXenes in the whole process showed little change, which can be used as a standard reference. Based on the self-standard turn-on FRET biosensing platform the detection limit of exosome was determined as 1.4 × 10³ particles mL^-1, which was over 1000× lower than that of conventional ELISA method. This fluorescence sensor can also be used for the identification of multiple biomarkers on the exosome surface and different kinds of exosomes, combining with the fluorescent confocal scanning microscope image. The proposed strategy not only provides a universal nanoplatform for exosomes, but also can be extensively expanded to multiple biomarkers detection, which may promise the prospect of MXenes as robust candidates in biological fields.

Cadmium-Free Quantum Dots as Fluorescent Labels for Exosomes

Quantum dots are attractive alternatives to organic fluorophores for the purposes of fluorescent labeling and the detection of biomarkers. They can also be made to specifically target a protein of interest by conjugating biomolecules, such as antibodies. However, the majority of the fluorescent labeling using quantum dots is done using toxic materials such as cadmium or lead due to the well-established synthetic processes for these quantum dots. Here, we demonstrate the use of indium phosphide quantum dots with a zinc sulfide shell for the purposes of labeling and the detection of exosomes derived from the THP-1 cell line (monocyte cell line). Exosomes are nano-sized vesicles that have the potential to be used as biomarkers due to their involvement in complex cell processes. However, the lack of standardized methodology around the detection and analysis of exosomes has made it difficult to detect these membrane-containing vesicles. We targeted a protein that is known to exist on the surface of the exosomes (CD63) using a CD63 antibody. The antibody was conjugated to the quantum dots that were first made water-soluble using a ligand-exchange method. The conjugation was done using carbodiimide coupling, and was confirmed using a range of different methods such as dynamic light scattering, surface plasmon resonance, fluorescent microscopy, and Fourier transform infrared spectroscopy. The conjugation of the quantum dot antibody to the exosomes was further confirmed using similar methods. This demonstrates the potential for the use of a non-toxic conjugate to target nano-sized biomarkers that could be further used for the detection of different diseases.

Single-Exosome-Counting Immunoassays for Cancer Diagnostics

Exosomes shed by tumor cells have been recognized as promising biomarkers for cancer diagnostics due to their unique composition and functions. Quantification of low concentrations of specific exosomes present in very small volumes of clinical samples may be used for noninvasive cancer diagnosis and prognosis. We developed an immunosorbent assay for digital qualification of target exosomes using droplet microfluidics. The exosomes were immobilized on magnetic microbeads through sandwich ELISA complexes tagged with an enzymatic reporter that produces a fluorescent signal. The constructed beads were further isolated and encapsulated into a sufficient number of droplets to ensure only a single bead was encapsulated in a droplet. Our droplet-based single-exosome-counting enzyme-linked immunoassay (droplet digital ExoELISA) approach enables absolute counting of cancer-specific exosomes to achieve unprecedented accuracy. We were able to achieve a limit of detection (LOD) down to 10 enzyme-labeled exosome complexes per microliter (∼10^-17 M). We demonstrated the application of the droplet digital ExoELISA platform in quantitative detection of exosomes in plasma samples directly from breast cancer patients. We believe our approach may have the potential for early diagnosis of cancer and accelerate the discovery of cancer exosomal biomarkers for clinical diagnosis.

Label-free detection of hypoxia-induced extracellular vesicle secretion from MCF-7 cells

Nanoscale extracellular vesicles (EVs) including exosomes (50-150 nm membrane particles) have emerged as promising cancer biomarkers due to the carried genetic information about the parental cells. However the sensitive detection of these vesicles remains a challenge. Here we present a label-free electrochemical sensor to measure the EVs secretion levels of hypoxic and normoxic MCF-7 cells. The sensor design includes two consecutive steps; i) Au electrode surface functionalization for anti-CD81 Antibody and ii) EVs capture. The label-free detection of EVs was done via Differential Pulse Voltammetry (DPV) and Electrochemical Impedance Spectroscopy (EIS). The working linear range for the sensor was 102-109 EVs/ml with an LOD 77 EVs/mL and 379 EVs/ml for EIS and DPV based detection. A blood-abundant protein, RhD was used for the selectivity test. In order to assess the performance of the biosensor, the level of EVs secretion by the human breast cancer MCF-7 cell line was compared with enzyme-linked immunosorbent assays (ELISA) and Nanoparticle Tracking Analysis (NTA). Designed label-free electrochemical sensors utilized for quantification of EVs secretion enhancement due to CoCl2-induced hypoxia and 1.23 fold increase with respect to normoxic conditions was found.

Quantification of Exosome Based on a Copper-Mediated Signal Amplification Strategy

Exosomes, a class of small extracellular vesicles, play important roles in various physiological and pathological processes by serving as vehicles for transferring and delivering membrane and cytosolic molecules between cells. Since exosomes widely exist in various body fluids and carry molecular information on their originating cells, they are being regarded as potential noninvasive biomarkers. Nevertheless, the development of convenient and quantitative exosome analysis methods is still technically challenging. Here, we present a low-cost assay for direct capture and rapid detection of exosomes based on a copper-mediated signal amplification strategy. The assay involves three steps. First, bulk nanovesicles are magnetically captured by cholesterol-modified magnetic beads (MB) via hydrophobic interaction between cholesterol moieties and lipid membranes. Second, bead-binding nanovesicles of exosomes with a specific membrane protein are anchored with aptamer-modified copper oxide nanoparticles (CuO NPs) to form sandwich complexes (MB-exosome-CuO NP). Third, the resultant sandwich complexes are dissolved by acidolysis to turn CuO NP into copper(II) ions (Cu²⁺), which can be reduced to fluorescent copper nanoparticles (CuNPs) by sodium ascorbate in the presence of poly(thymine). The fluorescence emission of CuNPs increases with the increase of Cu²⁺ concentration, which is directly proportional to the concentration of exosomes. Our method allows quantitative analysis of exosomes in the range of 7.5 × 10⁴ to 1.5 × 10⁷ particles/μL with a detection of limit of 4.8 × 10⁴ particles/μL in biological sample. The total working time is about 2 h. The assay has the potential to be a simple and cost-effective method for routine exosome analysis in biological samples.

Screening and multiple detection of cancer exosomes using an SERS-based method

As a kind of most important cancer biomarker, exosomes are getting more frequently investigated in cancer diagnosis. In this study, we proposed an SERS-based method for the screening and simultaneous multiple detection of exosomes using magnetic substrates and SERS probes. Specifically, the capturing substrates are achieved using gold shell magnetic nanobeads modified by aptamers, which can capture most kinds of exosomes by recognizing the generic surface protein CD63. Moreover, the SERS probes are made of gold nanoparticles decorated with a Raman reporter and a specific aptamer for targeting exosomes. Further, for the simultaneous detection of multiple kinds of exosomes, three kinds of SERS probes were designed using different SERS reporters. While detecting specific kinds of exosomes, the capturing substrates were mixed with these three kinds of SERS probes. When the target exosome is present, an apta-immunocomplex can be formed among the target exosomes, the substrate, and the corresponding kind of SERS probes, and the other non-specific SERS probes remain in the suspension. Hence, an SERS signal with a decreased intensity will be detected in the supernatant, indicating the presence of the target exosomes. Finally, this detection method has also been successfully employed for the detection of exosomes in real blood samples; this proves that the proposed SERS-based method is a promising tool for clinical cancer screening based on exosomes.

Biophysical properties of extracellular vesicles in diagnostics

Extracellular vesicles (EVs) are cell-derived nanoparticles, involved in cell-to-cell communication, in both normal and pathological processes. Originating by the outward budding of the plasma membrane or released by exocytosis, they are natural cargoes for lipids, carbohydrates, proteins and nucleic acids. EV-based diagnostics promises unique advantages compared with conventional strategies involving whole body fluid analysis, including the reduction of biofluids complexity and more specific and sensitive detection of low abundance biomacromolecules. Besides EV cargoes, new breakthrough technologies are addressing EV 'colloidal properties' - including particle content, size and membrane mechanical properties - directly experienced by researchers to be critical factors in biomarkers discovery. This article focuses on the progresses in EV biophysical properties characterization as diagnostic tools for different pathological conditions.

Biological Functions and Current Advances in Isolation and Detection Strategies for Exosome Nanovesicles

Exosomes are nanoscale (≈30-150 nm) extracellular vesicles of endocytic origin that are shed by most types of cells and circulate in bodily fluids. Exosomes carry a specific composition of proteins, lipids, RNA, and DNA and can work as cargo to transfer this information to recipient cells. Recent studies on exosomes have shown that they play an important role in various biological processes, such as intercellular signaling, coagulation, inflammation, and cellular homeostasis. These functional roles are attributed to their ability to transfer RNA, proteins, enzymes, and lipids, thereby affecting the physiological and pathological conditions in various diseases, including cancer and neurodegenerative, infectious, and autoimmune diseases (e.g., cancer initiation, progression, and metastasis). Due to these unique characteristics, exosomes are considered promising biomarkers for the diagnosis and prognosis of various diseases via noninvasive or minimally invasive procedures. Over the last decade, a plethora of methodologies have been developed for analyzing disease-specific exosomes using optical and nonoptical tools. Here, the major biological functions, significance, and potential role of exosomes as biomarkers and therapeutics are discussed. Furthermore, an overview of the most commonly used techniques for exosome analysis, highlighting the major technical challenges and limitations of existing techniques, is presented.