Ratiometric fluorescent sensor based on a truncated specific aptamer by MGO-SELEX screening for streptomycin detection

Streptomycin (STR) is an antibiotic fungicide that is easily enriched in the environment and threatens human health. In this paper, a ratiometric fluorescence aptasensor with silver nanoclusters-skimmed milk powder@zeolitic imidazolate framework-8 (AgNCs-SMP@ZIF-8) fluorescent material as the response signal and carbon quantum dots (CQDs) functionalized with aptamer as the reference signal was prepared. Magnetic graphene oxide selection technology was used to screen high-affinity STR-specific aptamer. The truncation strategy for removing non-binding site bases was based on molecular simulation docking, and it was found that the stem-loop structure formed by the truncated aptamer 52 (Apt52) was used for STR-specific recognition and had a higher STR affinity (Kd = 14.52 nM) compared to the initial 79-nucleotide aptamer. AgNCs was then covered in a protein coating and encapsulated in ZIF-8 to obtain AgNCs-SMP@ZIF-8 with enhanced red fluorescence intensity. The added Cu2+ rapidly penetrated into ZIF-8 to induce red fluorescence quenching, while the CQDs modified by Apt52 maintained green fluorescence. Hence, the constructed ratiometric fluorescent sensor with Apt52 as the recognizing element can detect STR within 90 s with a detection limit of 0.98 nM. The ratiometric probe proposed has considerable potential for STR residue detection in milk.