Fluorescence Single-Molecule Counting Assays for High-Sensitivity Detection of Cytokines and Chemokines

A highly selective and sensitive salamo-salen-salamo hybrid fluorometic chemosensor for identification of Zn2+ and the continuous recognition of phosphate anions

A salamo-salen-salamo hybrid fluorescent chemical sensor (H₄L) was synthesized and characterized. It exhibits high selectivity and sensitivity to Zn²⁺ in physiological pH range. Meanwhile, its zinc(II) complex (L-Zn²⁺) continuously responses phosphate anions in DMF/H₂O (v/v, 9:1) solution. Moreover, the identification processes are explored using characterization methods such as UV-absorption spectra, IR spectra and ESI-MS spectrum. In addition, the coordination mechanism of H₂PO₄^- and Zn²⁺ were successfully exploited to make the chemical sensor reproducible. In short, the sensors H₄L and L-Zn²⁺ will be promising detection devices for Zn²⁺ and phosphate anions.

Ultrasensitive protein detection: a case for microfluidic magnetic bead-based assays

We review the use of magnetic micro- and nanoparticles ('magnetic beads') in microfluidic systems for ultrasensitive protein detection. During recent years magnetic beads have been used frequently in immunoassays, either as mobile substrates on which the target antigen is captured, as detection labels, or simultaneously as substrates and labels. The major part of the reviewed work has as application the detection of antibodies or disease biomarkers in serum or of biotoxins from food samples. Several of the most sensitive assays allow protein detection down to fg mL(-1) concentrations. We benchmark the performance of these microfluidic magnetic bead-based assays with the most promising earlier work and with alternative solutions.

Design and development of a field-deployable single-molecule detector (SMD) for the analysis of molecular markers

Single-molecule detection (SMD) has demonstrated some attractive benefits for many types of biomolecular analyses including enhanced processing speed by eliminating processing steps, elimination of ensemble averaging and single-molecule sensitivity. However, it's wide spread use has been hampered by the complex instrumentation required for its implementation when using fluorescence as the readout modality. We report herein a simple and compact fluorescence single-molecule instrument that is straightforward to operate and consisted of fiber optics directly coupled to a microfluidic device. The integrated fiber optics served as waveguides to deliver the laser excitation light to the sample and collecting the resulting emission, simplifying the optical requirements associated with traditional SMD instruments by eliminating the need for optical alignment and simplification of the optical train. Additionally, the use of a vertical cavity surface emitting laser and a single photon avalanche diode serving as the excitation source and photon transducer, respectively, as well as a field programmable gate array (FPGA) integrated into the processing electronics assisted in reducing the instrument footprint. This small footprint SMD platform was tested using fluorescent microspheres and single AlexaFluor 660 molecules to determine the optimal operating parameters and system performance. As a demonstration of the utility of this instrument for biomolecular analyses, molecular beacons (MBs) were designed to probe bacterial cells for the gene encoding Gram-positive species. The ability to monitor biomarkers using this simple and portable instrument will have a number of important applications, such as strain-specific detection of pathogenic bacteria or the molecular diagnosis of diseases requiring rapid turn-around-times directly at the point-of-use.

A microfluidic chip-compatible bioassay based on single-molecule detection with high sensitivity and multiplexing

Many applications in pharmaceutical development, clinical diagnostics, and biological research demand rapid detection of multiple analytes (multiplexed detection) in a minimal volume. This need has led to the development of several novel array-based sensors. The most successful of these so far have been suspension arrays based on polystyrene beads. However, the 5 microm beads used for these assays are incompatible with most microfluidic chip technologies, mostly due to clogging problems. The challenge, then, is to design a detection particle that has high information content (for multiplexed detection), is compatible with miniaturization, and can be manufactured easily at low cost. DNA is a solid molecular wire that is easily produced and manipulated, which makes it a useful material for nanoparticles. DNA molecules are very information-rich, readily deformable, and easily propagated. We exploit these attributes in a suspension array sensor built from specialized recombinant DNA, Digital DNA, that carries both specific analyte-recognition units, and a geometrically encoded identification pattern. Here we show that this sensor combines high multiplexing with high sensitivity, is biocompatible, and has sufficiently small particle size to be used within microfluidic chips that are only 1 microm deep. We expect this technology will be the foundation of a broadly applicable technique to identify and quantitate proteins, nucleic acids, viruses, and toxins simultaneously in a minimal volume.

Single-molecule immunosorbent assay as a tool for human immunodeficiency virus-1 antigen detection

Ultrasensitive detection and quantification of viral antigen with a novel single-molecule immunosorbent assay (SMISA) was achieved. Antigen from human immunodeficiency virus type 1 (HIV-1), the major etiological agent of acquired immune deficiency syndrome, served as the screening target in this study. The target molecule was sandwiched between a polyclonal capture antibody and a monoclonal detector antibody. The capture antibody was covalently immobilized on (3-glycidoxypropyl) trimethoxy silane-modified glass slides. The detector antibody was conjugated with fluorescent Alexa Fluor 532 labeled secondary antibody prior to being used as a probe for the antigen. Imaging was performed with a total internal reflection fluorescence single-molecule detection system. This technique is demonstrated for detecting HIV-1 p24 antigen down to 0.1 pg/mL with a dynamic range of over four orders of magnitude. A Langmuir isotherm fits the molecule count dependence on the target concentration. The target antigen was further tested in 20% human serum, and the results showed that neither sensitivity nor dynamic range was affected by the biological matrix. SMISA is therefore a promising approach for the early diagnosis of viral induced diseases.

Single molecule studies of multiple-fluorophore labeled antibodies. Effect of homo-FRET on the number of photons available before photobleaching

Advancements in single molecule detection (SMD) continue to unfold powerful ways to study the behavior of individual and complex molecular systems in real time. SMD enables the characterization of complex molecular interactions and reveals basic physical phenomena underlying chemical and biological processes. We present here a systematic study of the quenching efficiency of Förster-type energy-transfer (FRET) for multiple fluorophores immobilized on a single antibody. We simultaneously monitor the fluorescence intensity, fluorescence lifetime, and the number of available photons before photobleaching as a function of the number of identical emitters bound to a single IgG antibody. The detailed studies of FRET between individual fluorophores reveal complex through-space interactions. In general, even for two or three fluorophores immobilized on a single protein, homo-FRET interactions lead to an overall non-linear intensity increase and shortening of fluorescence lifetime. Over-labeling of protein in solution (ensemble) results in the loss of fluorescence signal due to the self-quenching of fluorophores making it useless for assays applications. However, in the single molecule regime, over-labeling may bring significant benefits in regards to the number of available photons and the overall survival time. Our investigation reveals possibilities to significantly increase the observation time for a single macromolecule allowing studies of macromolecular interactions that are not obscured by ensemble averaging. Extending the observation time will be crucial for developing immunoassays based on single-antibody.