Abstract
Human epidermal growth factor receptor 2 (HER2) is a key tumor marker for several common and deadly cancers. It is of great importance to develop efficient detection methods for its over-expression. In this work, an electrochemical impedance spectroscopy (EIS) method adjustable by anionic porphyrin for HER2 gene detection has been proposed, based on the impedance difference between multi-walled carbon nanotubes (MWCNTs) and DNA. The interesting finding herein is that with the addition of anionic porphyrin, i.e., meso-tetra(4-sulfophenyl)-porphyrin (TSPP), the impedance value obtained at a glass carbon electrode (GCE) modified with MWCNTs and a single stranded DNA (ssDNA), the probe DNA that might be assembled tightly onto MWCNTs through π-π stacking interaction, gets a slight decrease; however, the impedance value from a GCE modified with MWCNTs and a double stranded DNA (dsDNA), the hybrid of the probe DNA with a target DNA, which might be assembled loosely onto MWCNTs for the screening effect of phosphate backbones in dsDNA, gets an obvious decrease. The reason may be that on the one hand, being rich in negative sulfonate groups, TSPP will try to push DNA far away from CNTs surface due to its strong electrostatic repulsion towards DNA; on the other hand, rich in planar phenyl or pyrrole rings, TSPP will compete with DNA for the surface of CNTs since it can also be assembled onto CNTs through conjugative interactions. In this way, the “loosely assembled” dsDNA will be repelled by this anionic porphyrin and released off CNTs surface much more than the “tightly assembled” ssDNA, leading to a bigger difference in the impedance value between dsDNA and ssDNA. Thus, through the amplification effect of TSPP on the impedance difference, the perfectly matched target DNA could be easily determined by EIS without any label. Under the optimized experimental conditions, this electrochemical sensor shows an excellent linear response to target DNA in a concentration range of 2.0 × 10−11–2.0 × 10−6 M with a limit of detection (LOD) of 6.34 × 10−11 M (S/N = 3). This abnormally sensitive electrochemical sensing performance resulting from anionic porphyrin for DNA sequences specific to HER2 gene will offer considerable promise for tumor diagnosis and treatment.
Highlights
DNA biosensors are powerful tools for cancer diagnosis and detection [1] and have been extensively developed by different methods including fluorescence [2], luminescence [3], surface plasma resonance spectroscopy [4] and electrochemistry [5]
The proposed DNA detection strategy (Scheme 1) was tested for DNA fragments specific to Human epidermal growth factor receptor 2 (HER2) gene that is a tumor marker related to several kinds of common cancers
The limit of detection is found to be 6.34 × 10−11 M at a signal-to-noise ratio of 3. These results indicate that the proposed DNA sensor based on multi-walled carbon nanotubes (MWCNTs) and TSPP probably exhibits a high sensitivity for all previously ascribed reasons such as the high specific surface of carbon nanotubes (CNTs), the short distance between porphyrin and CNTs that increases electron transfer [19,23], and especially different attractions or repulsions occurring among CNTs, single stranded DNA (ssDNA)/double stranded DNA (dsDNA) and anionic porphyrin, which produce synergetic effects to amplify the impedance difference to be big enough for target DNA
Summary
DNA biosensors are powerful tools for cancer diagnosis and detection [1] and have been extensively developed by different methods including fluorescence [2], luminescence [3], surface plasma resonance spectroscopy [4] and electrochemistry [5]. It is more difficult for dsDNA to be assembled onto CNTs, because of the screening effect of its negatively charged phosphate backbones, which blocks the π-stacking interaction of the bases with CNTs [34,35,36] In this electrochemical platform of E1, the probe ssDNA can be tightly assembled onto the surface of MWCNTs and the resulting steric hindrance will block the electron transfer channel of the [Fe(CN)6 ]3−/4− , leading to an increase of the impedance value compared with that of the bare. This current work will surely offer a novel perspective for sensor fabrication via unconventional addition of anionic porphyrin to generate and amplify impedance disparity instead of utilizing optical and chemical sensing amplification, and it will enrich the research on the unusual composite system containing anionic porphyrin, DNA and CNTs which will combine the advantages of these three kinds of compounds, especially by using the exceptional sensing effect of anionic porphyrin, which will will open and pave many potential avenues towards for various applications in electrochemically biomedical fields
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