Method to detect and/or quantify miRNA, RNA or DNA molecules of interest in at least one isolated biological sample, through an electrochemical sensor based on the use of magnetic beads (magnetic beads-based electrochemical sensor), comprising the following sequential steps: a. Obtaining a RNA/DNA, DNA/DNA or RNA/RNA hybrid by carrying out a homogenous hybridization in solution of a miRNA, RNA or DNA molecule of interest present in the isolated biological sample to a complementary RNA or DNA sequence labeled with a compound A, to provide the RNA/DNA, DNA/DNA or RNA/RNA hybrid; b. Capturing the product of the hybridization in solution of step a) by adding to said solution a suspension of magnetic particles, preferably superparamagnetic particles (MBs), previously bound to an antibody or a fragment thereof capable of recognizing RNA-DNA heteroduplexes or DNA-DNA or RNA-RNA duplexes through a protein G coupled to the surface of the magnetic particles, wherein said product of the hybridization is captured onto the said magnetic particles; c. Adding an element capable of being detected and/or quantified by a magnetic beads-based electrochemical sensor, wherein said element binds to compound A, thereby generating a complex capable of being detected and/or quantified by a magnetic beads-based electrochemical sensor; d. Magnetic capturing the product of step c) comprising the magnetic particles onto an electrochemical support; and e. detecting and/or quantifying a electrochemical signal produced by the complex of step d), thereby detecting and/or quantifying the product of hybridization of step a) and thereby detecting and/or quantifying the miRNA, RNA or DNA molecules of interest present in the at least one biological sample; wherein the antibody fragment is selected from the group consisting of "Fab" fragments, "F(ab')2" fragments, "Fv" fragments, single chain Fv fragments or "scFv", "Diabodies" and "bispecific antibodies" (Bab); wherein "compound A" is any compound capable of giving an electrochemical signal after binding with another compound; wherein "electrochemical support" is any rigid or flexible support which can function as a working electrode in an electrochemical transduction; and wherein an element capable of being detected and/or quantified by a magnetic beads-based electrochemical biosensor is an element capable of producing an electrochemical reaction by itself or after reaction with other molecules. a. Obtaining a RNA/DNA, DNA/DNA or RNA/RNA hybrid by carrying out a homogenous hybridization in solution of a miRNA, RNA or DNA molecule of interest present in the isolated biological sample to a complementary RNA or DNA sequence labeled with a compound A, to provide the RNA/DNA, DNA/DNA or RNA/RNA hybrid; b. Capturing the product of the hybridization in solution of step a) by adding to said solution a suspension of magnetic particles, preferably superparamagnetic particles (MBs), previously bound to an antibody or a fragment thereof capable of recognizing RNA-DNA heteroduplexes or DNA-DNA or RNA-RNA duplexes through a protein G coupled to the surface of the magnetic particles, wherein said product of the hybridization is captured onto the said magnetic particles; c. Adding an element capable of being detected and/or quantified by a magnetic beads-based electrochemical sensor, wherein said element binds to compound A, thereby generating a complex capable of being detected and/or quantified by a magnetic beads-based electrochemical sensor; d. Magnetic capturing the product of step c) comprising the magnetic particles onto an electrochemical support; and e. detecting and/or quantifying a electrochemical signal produced by the complex of step d), thereby detecting and/or quantifying the product of hybridization of step a) and thereby detecting and/or quantifying the miRNA, RNA or DNA molecules of interest present in the at least one biological sample;