Gold nanoparticle-core spherical nucleic acids (AuNP core-SNAs), by virtue of the programmable nature of oligonucleotides, have yielded access to the innovative strategies for targeted biodiagnostics. DNA strands when bond on a nanoparticle scaffold can be programmed to achieve as powerful diagnostic tools for detection of specific oligonucleotide sequences with good selectivity. Here, we describe an assembly system consists of AuNP core-SNAs, an oligonucleotide assembler and a dye-labeled oligonucleotide (probe) for fluorometric detection of nucleic acid targets. The dye molecules emit fluorescence in solution upon excitation. The oligonucleotide assembler collects the dye-labeled oligonucleotide probe from the solution via its simultaneous interaction with SNAs and dye-labeled oligonucleotide probe. Upon the addition of oligonucleotide assembler, SNAs and dye-labeled oligonucleotide bind to different positions of the assembler molecules forming sandwich assemblies. This assembly formation is due to semi-complementarity of the assembler to recognition sequence of SNAs and dye-labeled oligonucleotide. The aggregated particles settle out of the solution upon centrifuging. Consequently, the fluorescence intensity of the solution decreases. However, in the presence of the nucleic acid target, the assembler preferably hybridizes the individual target molecules due to full complementarity between the assembler and the target and inhibits SNAs sandwich assembly. So the fluorescence signal will recover upon the concentration of nucleic acid target.
