MoS2 nanosheets based label-free colorimetric aptasensor for Escherichia coli O157: H7 detection

The aggregation behavior of large-scale two-dimensional molybdenum disulfide (MoS2) nanosheets prepared by sonication-assisted exfoliation in environmental friendly 45% ethanol/water mixture under different conditions (e.g., salt concentration, cation valence and nanosheets size) had been examined to make them more appropriate for biological sensing application. Adjustment of sonication time and centrifuging speed could be utilized to select MoS2 nanosheets with various concentrations and sizes. Higher concentrations of salt and cations with higher valence were more effective to aggregate MoS2 nanosheets and settle them than their counterparts, which could be attributed to the perturbation of attractive van der Waals forces and repulsive Coulombic interactions. Moreover, the ethanol in mixture was capable of assisting MoS2 nanosheets to resist this aggregation initiated by salt but it would disturb the stability of aptamer functionalized MoS2 nanosheets in salt solution. The interplay of Escherichia coli O157: H7 (E. coli O157:H7) aptamer functionalized MoS2 nanosheets and salt was then inspired to develop a high sensitive detection method of E. coli O157:H7. The introduction of target bacteria induced MoS2 nanosheets to be exposed to cations, and promoted the aggregation and sedimentation behaviour of MoS2 nanosheets. The change in extinction of MoS2 nanosheets had a linear relationship with E. coli O157:H7 concentration in a range of 500-5000 CFU/mL with a detection limit of 116 CFU/mL. This proposed colorimetric sensor expands the application scope of MoS2 nanosheets and shows its promising potential as an alternative tool for bacteria detection in food and water quality monitoring.