Novel methods allowing for the simple optical detection and electrochemical detection of doubled-straded oligonucleotides are disclosed. The methods are rapid, selective and versatile. Advantageously, they do not require any chemical reaction on the probes or on the analytes since they are based on different electrostatic interactions between cationic poly(3-alkoxy-4-methylthiophene) derivatives and single-stranded or doubled-stranded (hybridized) oligonucleotides.