We have developed a scalable fabrication process for a sensor array for DNA based on graphene field effect transistor (GFETs). Sensitivity of nucleic acid detection is essentially governed by the binding affinity of the target oligonucleotide to the complementary probe. Therefore, it is challenging to detect relatively short oligonucleotide sequence ~20 mer at a low concentration sub-fM. We developed a signal amplification approach based on the self-assembly of DNA for the detection of a 21-mer target DNA at sub-fM concentration. GFETs were functionalized with hairpin prob DNA with stem-loop secondary structure that can be specifically opened by the target DNA, triggering the self-assembly reactions which amplify the electrical sensing signal as monitored by the Dirac voltage shifts of the GFET. The self-assembly amplification approach has time-dependent limit of detection, allowing the detection of a 21-mer DNA oligonucleotide at 100 fM in 1 hour and at 100aM in 24 hours. The hairpin probe DNA exhibits high specificity against non-complementary target DNA with one base mismatch at the end, which is challenging for single-stranded probe due to the lack of stem-loop structure of the hairpin probe. Our approach paves the way for multiplexed and label-free nucleic acid testing with high selectivity for clinical disease diagnosis.