The subject invention provides methods, assays, and products for detecting small molecules in a sample, in particular, in both clinical and field settings. The method for detecting a small-molecule target, preferably, a synthetic cathinone in a sample comprises contacting the sample with an aptamer-based sensor selective for the small-molecule target, and detecting the small-molecule target in the sample. Specifically, the method utilizes an aptamer-based sensor comprising a dye binding to a three-way junction binding domain of an aptamer. Binding of small-molecule target to the aptamer displaces the dye, generating a spectroscopic signal that can be used for detection of the small-molecule target and quantitative measurement of the target concentration.