Single-molecule immunoassay and DNA diagnosis

Advances in Optical Single-Molecule Detection: En Route to Supersensitive Bioaffinity Assays

The ability to detect low concentrations of analytes and in particular low-abundance biomarkers is of fundamental importance, e.g., for early-stage disease diagnosis. The prospect of reaching the ultimate limit of detection has driven the development of single-molecule bioaffinity assays. While many review articles have highlighted the potentials of single-molecule technologies for analytical and diagnostic applications, these technologies are not as widespread in real-world applications as one should expect. This Review provides a theoretical background on single-molecule-or better digital-assays to critically assess their potential compared to traditional analog assays. Selected examples from the literature include bioaffinity assays for the detection of biomolecules such as proteins, nucleic acids, and viruses. The structure of the Review highlights the versatility of optical single-molecule labeling techniques, including enzymatic amplification, molecular labels, and innovative nanomaterials.

Separation and determination of alpha-synuclein monomeric and oligomeric species using two electrophoretic approaches

Parkinson's disease (PD) is a frequent degenerative disorder that is diagnosed based on clinical symptoms. When the first symptoms appear, more than 70% of the dopaminergic cells are already lost. Therefore, it is of utmost importance to have reliable biomarkers to diagnose much earlier PD. In this context, alpha-synuclein (aSyn) is a protein of high interest because of its tendency to form oligomers and amyloid fibrils. The oligomeric forms seem to play a critical pathological role in PD. To date, most of studies aiming at detecting and quantifying aSyn oligomers were performed by immunoassays, mainly by ELISA using specific antibodies. In this study a capillary gel electrophoresis (CGE) coupled with fluorescence detection method was developed to detect and quantify the oligomeric forms of aSyn formed in vitro. All the results obtained were supported by SDS-PAGE analysis, a widely used and well-known technique but exhibiting a main drawback since it is not an automated technique. The repeatability and the intermediate precision of the method were evaluated, as well as the stability of the labeled and non-labeled aSyn samples. After careful screening and optimization of various labeling reagents, 4-fluoro-7-nitrobenzofurazan (NBD-F) was selected and used to establish a calibration curve with monomeric fluorescently-labeled aSyn. Finally, the method was used to study the effect of doxycycline on the oligomerization process. Altogether, our results show that CGE is a very promising automated technique to analyze aSyn monomers, as well as small oligomers.

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.

Counting Low-Copy Number Proteins in a Single Cell

We have designed a microfluidic device in which we can manipulate, lyse, label, separate, and quantify the protein contents of a single cell using single-molecule fluorescence counting. Generic labeling of proteins is achieved through fluorescent-antibody binding. The use of cylindrical optics enables high-efficiency (approximately 60%) counting of molecules in micrometer-sized channels. We used this microfluidic device to quantify beta2 adrenergic receptors expressed in insect cells (SF9). We also analyzed phycobiliprotein contents in individual cyanobacterial cells (Synechococcus sp. PCC 7942) and observed marked differences in the levels of specific complexes in cell populations that were grown under nitrogen-depleted conditions.

Enhanced fluorescence of Cy5-labeled DNA tethered to silver island films: fluorescence images and time-resolved studies using single-molecule spectroscopy

Methods that increase the total emission per fluorophore would provide increased sensitivity and a wider dynamic range for chemical analysis, medical diagnostics, and in vivo molecular imaging. The use of fluorophore-metal interactions has the potential to dramatically increase the detectability of single fluorophores for bioanalytical monitoring. The fabrication and single-molecule analysis of fluorophore-labeled DNA molecules tethered to silver island films are described in this article. The single-molecule spectroscopic method reveals some insightful information on the behaviors of single molecules, rather than an ensemble of molecules. Analysis of fluorescence images, intensity profiles, total emitted photons, and lifetime distributions reveals some of sample heterogeneities. Investigations of time-dependent emission characteristics of single molecules indicate that the total number of emitted photons on the silvered surface is more than 10 times greater than on free labeled DNA molecules on a glass substrate. In addition, time-correlated single-photon counting results reveal the reduced lifetimes of single molecules tethered to silver island films.

Simultaneous measurements of the flow velocities in a microchannel by wide/evanescent field illuminations with particle/single molecules

A laser-induced fluorescence imaging method was developed to simultaneously measure flow velocities in the middle and near wall of a channel with particles or single molecules, by selectively switching from the wide field excitation mode to the evanescent wave excitation mode. Fluorescent microbeads with a diameter of 175 nm were used to calibrate the system, and the collisions of microbeads with channel walls were directly observed. The 175 nm microbeads velocities in the main flow and at 275 nm from the bottom of the channel were measured. The measured velocities of particles or single molecules in two positions in a microchannel were consistent with the calculated value based on Poiseuille flow theory when the diameter of a microbead was considered. The errors caused by Brownian diffusion in our measurement were negligible compared to the flow velocity. Single lambda DNA molecules were then used as a flowing tracer to measure the velocities. The velocity can be obtained at a distance of 309.0 +/- 82.6 nm away from bottom surface of the channel. The technique may be potentially useful for studying molecular transportation both in the center and at the bottom of the channel, and interactions between molecules and microchannel surfaces. It is especially important that the technique can be permitted to measure both velocities in the same experiment to eliminate possible experimental inconsistencies.

Comparative quantification of nucleic acids using single-molecule detection and molecular beacons

This paper reports a highly sensitive homogenous method for comparative quantification of nucleic acids based on single-molecule detection (SMD) and molecular beacons (MBs). Two different color MBs were used to perform a separation-free comparative hybridization assay for simultaneous quantification of both target and control strands. A fluorescent burst, emitted from a single hybrid when it passes through a minuscule laser-focused region, is detected with high signal-to-noise ratio (SNR) by using single-molecule fluorescence spectroscopy. Targets are quantified via counting of discrete fluorescent bursts. The high SNR achieved in both detection channels overcame the complications of fluorescent variability usually observed in dual-color ensemble measurements. In comparison with the conventional ensemble methods, this method improved the detection limit by 3 orders of magnitude and reduced the probe consumption by 6 orders of magnitude, facilitating a highly sensitive approach for comparative quantification of nucleic acids and offering great promise for genomic quantification without amplification.

Capillary electrophoresis-based immunoassay

Capillary electrophoresis-based immunoassay (CEIA) is a developing analytical technique with a number of advantages over conventional immunoassay, such as reduced sample consumption, simpler procedure, easy simultaneous determination of multiple analytes, and short analysis time. However, there are still a number of technical issues that researchers on CEIA have to solve before the assay can be more widely used. These issues include method to improve the concentration sensitivity of the assay, requirement for robust separation strategy for different analytes, and method to increase the throughput of the assay. The approaches to solve these issues are reviewed. Several studies have been devoted to develop general separation strategies for CEIA, and to enhance the sensitivity of detection. The recent development of microchip-based CEIA is encouraging and is likely to address more drawbacks of CEIA, particularly on the throughput issue.