A novel "turn off-on" sensor for detection of interaction between DNA and acrylamide (ACR) was developed. In this method, L-cysteine capped Mn-doped ZnS quantum dots (QDs) were used as room temperature phosphorescent probes. In the ``turn-off'' mode, ACR was absorbed onto the surface of QDs via electrostatic interaction, which caused a quenching effect of room-temperature phosphorescence signal by photoinduced electron-transfer mechanism. ACR was removed from the QDs' surface with the addition of DNA. Thus, the phosphorescence emission of QDs was recovered and the system was turned to the ``turn-on'' mode. The quenching mechanism of QDs by ACR was collisional (dynamic) and the quenching constant, binding constant, and binding site number were calculated as 3.2 x 10$^{4\, }$ M$^{-1}$, 2.04 x 10$^{4\, }$ M$^{-1}$, and 1.2, respectively. An absorption spectrometric method was also used to evaluate ACR-DNA interaction and the binding constant (K) was found as 2.4 x 10$^{5\, }$ M$^{-1}$. The developed biosensor is simple, is free of interferences coming from autofluorescence and scattering light, and does not need any derivatization step or sample pretreatment.