Specific Adsorption of Magnetic Fe₃O₄@hydroxyapatite Nanocomposites to a Hybridized Duplex for Immobilization and Label-Free Electrochemical Biosensing of MicroRNA

The unique discriminating ability of hydroxyapatite (HAP) to single- and double-stranded nucleic acid implies its potential as a biosensing material for nucleic acid analysis. Herein, a novel immobilization- and label-free electrochemical strategy has been developed for miRNA let-7a detection based on the specific adsorption of magnetic Fe 3 O 4 @hydroxyapatite (Fe 3 O 4 @HAP) to a duplex structure. Typically, the highly efficient homogeneous reaction between probe DNA (pDNA) and target miRNA let-7a produces the nucleic acid duplex, which is specifically adsorbed on the surface of Fe 3 O 4 @HAP. Thereafter, the electroactive intercalator Nile blue (NB) is introduced as the label-free signal molecule and inserts between the stacked base pairs of the pDNA–miRNA duplex. Thus, the Fe 3 O 4 @HAP bearing pDNA–miRNA and numerous intercalated NBs are adsorbed by the magnetic glassy carbon electrode, giving rise to an intense sensing signal without the need of nuclease-assisted cyclic amplification. With this advantage, the sensing system shows a wide linear range from 1.0 fM to 100 nM and a low detection limit of 0.051 fM for miRNA let-7a analysis. What is more, the synergy of the base-pairing-driven hybridization, the specific intercalation of NB to the duplex structure, and the selective adsorption of Fe 3 O 4 @HAP contribute to a high sensing specificity of the biosensing strategy. The sensing method provides a state-of-the-art strategy for the sensitive and facile bioanalysis and clinical diagnosis of miRNA.