An improved frequency selective surface (FSS) comprises a periodically replicated unit cell, the unit cell including a material having a first electrical conductivity in the presence of an external condition, and a second electrical conductivity in the absence of an external condition, or in the presence of a modified external condition. For example, the material may be a chemoresistive material, having an electrical conductivity that changes in the presence of a chemical or biological analyte, i.e. having a first value of electrical conductivity in the presence of the analyte, and a second value of electrical conductivity in the absence of the analyte.