PCR is an important biosensing method since it can amplify DNA by many orders of magnitude. However, for food safety and environmental monitoring, the target molecule is usually not DNA. Here we demonstrate a detection method that integrates competitive assay format, magnetic bead separation, and isothermal DNA amplification for sensitive detection of tetracycline (TET) and other molecules. While TET is an antibiotic rather than oligonucleotide, our method allows the signal of TET detection to be amplified like conventional PCR. In addition, since we use isothermal amplification instead of conventional PCR, the temperature of the sample solution only needs to be maintained at a fixed temperature during the amplification process. We have successfully used the method to detect 10 pM TET. In addition, to improve the sensitivity of detection further, besides using EvaGreen dyes to quantify the amplified DNA, we have also tested another DNA quantification method that uses graphene oxide and a fluorescent DNA probe to increase the signal to background ratio of fluorescence detection.