Discovery of a Peptide Nucleic Acid (PNA) aptamer for cardiac troponin I: Substituting DNA with neutral PNA maintains picomolar affinity and improves performances for electronic sensing with graphene field-effect transistors (gFET)

DNA-based aptamers are widely employed as bioreceptors, owing to their tuneable affinity and specificity towards their targets. The use of peptide nucleic acids (PNAs) has instead proven challenging for this purpose, due to the absence of selection methods for the independent discovery of suitable receptors and due to the difficult mimicry of established DNA-based ones. Despite that PNAs exceed homologous DNA or RNA in terms of complementary base pairing, they can fail to reproduce alternative modes of binding because of their different structural features. The remarkable stability and charge distribution of PNAs could be beneficial to produce sensing bioreceptors, especially in the development of electronic devices such as field-effect transistor-based (FET) biosensors. We hereby report for the first time a high-affinity PNA aptamer for cardiac Troponin I (cTnI), a biomarker of acute myocardial infarction, able to interact with this specific protein in the picomolar range. The PNA aptamer was immobilized onto a graphene-based FET (gFET) transducer, and its ability in the direct detection of cTnI was compared with that of a DNA-based one of the same sequence. Similar dissociation constants were recorded for both receptors in 0.01 × PBS, as well as comparable detection limits of 6.0 ± 1.0 pg mL-1 (PNA aptamer) and 3.3 ± 0.7 pg mL-1 (DNA aptamer). Apart from the non-trivial demonstration that a PNA can behave as an aptamer, the tested receptor proved to be more consistent upon working in more complex biological matrices.