and J.L. affinity molecule, which can bind tightly to a broad range of focuses on from small metallic ions to whole cells1,2,3. Compared with standard antibodies, nucleic acid centered aptamer probes have a number of advantages such as avoidance of using animals’ productions, better thermostability, and more flexibility in developing various types of electrochemical4,5, fluorescence6,7, chemiluminescence8, or colorimetric9,10sensing techniques for a broad spectrum of focuses on. Among the methods, unmodified platinum nanoparticle (AuNP) centered colorimetric aptasensors have been given prominent attention largely because of the simplicity, high level of sensitivity, and a potential for high-throughput analysis11,12,13. In these assays, AuNPs are used as an extremely sensitive colorimetric indication due to an extraordinarily high extinction coefficient and strong distance-dependent optical properties14,15. Random coil single-strand DNA (ssDNA) aptamers are postulated to be adsorbed onto the surface of AuNPs through coordination between Au and N atoms in DNA bases16,17,18,19,20. Then, the AuNPs are stabilized by ssDNA aptamers against aggregation upon salt addition and the perfect solution is remains wine-colored. However, in the presence of focuses on, the aptamers become folded by binding to the focuses on and becoming desorbed from the surface of the AuNPs, resulting in subsequent aggregation of AuNPs and the solution’s color change from reddish to purple-blue. In fact, the folded aptamers or double-strand DNAs are not very easily adsorbed onto the AuNPs mainly due to the higher rigidity in the constructions with the hidden status of the positively charged bases situated inside the double-stranded. Even though the simple colorimetric aptasensor has been developed for numerous target molecules16,17,21,22,23,24,25, most of the target aptamers have short sequences such as potassium ions 21 mer16, thrombin 29 mer17, ochratoxin A 36 mer21, ampicillin 19 mer22, and sulfadimethoxine 22 mer23. Most of long-sequence aptamers showed poor overall performance in detection level of sensitivity as we tested aptamers such as 17-estradiol 76 mer26, chloramphenicol 80 mer27, and oxytetracycline 76 mer28. A lower affinity to focuses on may be one reason as Kim et al. has significantly improved Oleanolic Acid (Caryophyllin) the level of sensitivity by truncating the 76 mer oxytetracycline aptamer to shortened 8 mer ssDNAs with an enhanced affinity28. There may be some other reasons since the mechanism of DNA adsorption onto the surface of AuNPs is still not well understood29,30. One possible reason is definitely that short ssDNAs are more easily adsorbed onto the surface of AuNPs than long ssDNAs, which have more folded structures. That means more long aptamers are needed to stabilize AuNPs against the same concentration of salt than short aptamers. As we know, excessive aptamers mean less level of sensitivity in the aspect of competitive connection dynamics among aptamers, AuNPs, and focuses on23. Another possible reason is that once the aptamers are adsorbed onto the surface of AuNPs, the long aptamers bind more tightly Oleanolic Acid (Caryophyllin) than short aptamers as they have more bases binding to the surface of AuNPs. A higher aptamer affinity to AuNPs shows more focuses on are needed to detach aptamers from the surface of AuNPs, which also means lesser level of sensitivity31. Since many long-sequence aptamers have been selected by SELEX and it is difficult to find a short core sequence with a higher affinity, to find Oleanolic Acid (Caryophyllin) a fresh way is critical to develop a highly sensitive unmodified AuNP centered colorimetric aptasensor with long-sequence aptamers. 17-estradiol, as an endocrine disrupting chemical (EDC), has the very Oleanolic Acid (Caryophyllin) best estrogenic activity and has been found regularly in natural water sources and in wastewater effluents within the range of 0.2 to 3 Oleanolic Acid (Caryophyllin) 3 mg/L32. 17-estradiol entering into the organism from outside interferes with normal physiological processes and creates many deleterious effects33. The detection of such chemicals, therefore, is necessary for Rabbit Polyclonal to OR2T2 protecting general public and environmental health. To address this problem, for the first time, we focused on 17-estradiol with its 76 mer aptamer split into two short pieces.