We demonstrate a new diffraction resonance mode in waveguide gold nanowire grating structures, which shows much higher sensitivity than the intrinsic waveguide resonance mode to the change in the environmental refractive index. This enables realization of a molecular concentration sensor. Measurements on ethanol/water mixture and glucose solutions in water with different concentrations indicate a sensitivity of this sensor as high as 400 nm per unit refractive index. However, the narrow-band optical response implies higher resolution than the displayed spectrum shift. This sensor device provides a simple and practical approach to perform on-site and contamination-free characterization of the concentrations of chemical and biological solutions.