Ultra-sensitive biosensor for K-ras gene detection using enzyme capped gold nanoparticles conjugates for signal amplification.

In this study, an ultra-sensitive hairpin DNA-based electrochemical DNA biosensor for K-ras gene detection is described. Gold nanoparticles (Au-NPs) and horseradish peroxidase (HRP)-streptavidin capped Au-NPs (HAS) conjugates are used for signal amplification. Initially, hairpin DNA dually labeled with thiol at its 5' end and with biotin at its 3' end is immobilized on the surface of Au-NPs modified electrode, and the hairpin DNA is in a "closed" state; hence, the HAS conjugates are shielded from being approached by the biotin due to steric hindrance. However, in the presence of target DNA, the target DNA hybridizes with the loop structure of hairpin DNA and causes conformational change, resulting in biotin forced away from the electrode surface, thereby becoming accessible for the HAS conjugates. Thus, the HAS conjugates are linked to the electrode surface via the specific interaction between biotin and streptavidin. Electrochemical detection was performed in phosphate-buffered saline (PBS) containing tetramethylbenzidine (TMB) and H2O2. Under optimal conditions, the peak current differences (ΔI) are linear with the target DNA in the range from 0.1 fM to 1 nM with a detection limit of 0.035 fM. Furthermore, this biosensor also demonstrates its excellent specificity for single-base mismatched DNA.