Abstract: Disclosed are methods for determining the presence or absence of a target nucleic acid (e.g. DNA) sequence in a sample nucleic acid, the method comprising: (a) exposing the sample to a detection agent comprising a colloid metal surface associated with a SER(R)S active species (SAS) such as an azo dye and with a target binding species (TBS) which may be PNA which is complementary to the target, and (b) observing the sample agent mixture using SER(R)S to detect any surface enhancement of the label wherein the binding of the TBS to the target sequence causes surface enhancement SAS.