A microstructure-based chemical sensor that can be interrogated by a remote observer. The device acts as an electromagnetic wave filter in the optical region of the spectrum, filtering one or more wavelength bands where the band spectral notch location shifts in response to the accumulation of material on the surface of the microstructure sensor. The apparatus has a substrate having a surface relief structure containing dielectric bodies with one or more physical dimensions smaller than the wavelength of the filtered electromagnetic waves, such structures repeated in an array covering at least a portion of the surface of the substrate. A retro-reflecting structure allows interrogation of the sensor over a wide field of view. The device is particularly useful as a water monitoring device in hard to reach locations, and as a chemical warfare or explosives detector that can be read from a safe distance.