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We report on a novel very high Q-factor dual-band Terahertz perfect absorber composed of a metasurface located on top of a flexible polyimide spacer deposited on a silver ground layer. The metasurface is a 2D-array of plasmonic nanoantennas with the shape of two concentric square rings and a cylinder positioned at their centre. By performing numerical simulations, we studied the polarisation insensitive electromagnetic response of the absorber for incident angles varying from 0° up to±30°. The two resonant modes centred at f 1= 1. 80 THz and f 2= 2. 26 THz have Q-factors Q (f 1)= 120 and Q (f 2)= 94 and absorption coefficients A (f 1)= 99. 8% and A (f 2)= 99. 6%. Moreover, we investigated how the resonant mode frequencies change with the refractive index and thickness of transparent analytes adsorbed on the metasurface. In terms of the Refractive Index Units (RIU), we obtained sensitivities equal to 187.5 …