Ultrasensitive IgG quantification using DNA nano-pyramids (2014)

In addition to being a repository of genetic information, DNA is a bio-polymer that can be formed into various nanostructures. This profound ability to engineer various moieties has expanded its role from data storage to a structural biomaterial for sensing applications. In this study, we anchored DNA nano-pyramids (DPs) to gold electrodes for the electrochemical sensing of immunoglobulin G (IgG), an important antibody produced in response to infection. The pyramidal DNA structure not only avoids entanglement with neighboring probes through the use of spatially separating pendant probes but also reduces the local overcrowding effect with the overall enhanced packing of targets. The results from electrochemical impedance spectroscopy measurements also show that DP layer has better conductivity, with the hollow structure further facilitating electron transfer and increasing the sensitivity of electrochemical detection. We are able to selectively detect IgG in the presence of other proteins in an analyte solution. The limit of detection was 2.8 pg ml−1. Our ferrocene-labeled sandwich immunoassay works at 37 °C under a neutral pH environment. It also produces stable and reproducible signals even after storage for 1 week at 4 °C, further demonstrating the potential of this sensing system for clinical applications. Early diagnosis of central nervous system diseases like including brain infections and inflammatory multiple sclerosis, hinges on the detection of immunoglobulin G antibodies (IgG) at ultralow concentrations. However, the immobilization of the recognition moieties, anti-IgG on most biosensor surfaces often suffers from overcrowding thus limiting sensitivity. Now, a team led by David Tai Leong from the National University of Singapore has created ultrasmall DNA pyramid arrays that selectively sense IgG at the part-per-trillion level. To achieve such sensitivity, DNA pyramids were bound on gold electrodes through their three triangular vertices and simultaneously anchored anti-IgG molecules on their top vertices. The rigid nanostructures served to maximize the accessibility of the analytes to the recognition probes and facilitated the formation of stable anti-IgG and IgG complexes thus achieving a highly specific and sensitive immunosensing result. This DNA nanostructure-based biosensor is expected to play a crucial role in clinical settings. Four oligonucleotide strands are hybridized to form the 3D DNA nano-pyramid. Three thiol groups-terminated vertex can immobilize the pyramid firmly onto the surface of gold electrode, while the remaining non-thiolated vertex at the top with carboxyl group allows the covalent binding of anti-IgG antibody. Through traditional sandwich immunoreaction, the electroactive tag, ferrocene (FeC) generates electrochemical signals used to detect the analyte IgG. The pyramidal structure with higher rigidity encourages more uniform surface assembly and less steric effect, resulting in lower background interference. The pyramid's hollow structure further contributes to efficient electron transfer and makes this immunoassay system achieve an ultrasensitive detection limit.