A fiber gas sensor including a core fiber comprising at least one Bragg grating region, a fiber cladding in contact with the core fiber along an entire length of the core fiber, and a sensing matrix structure disposed upon the outer surface of the fiber cladding along a portion of the length of the fiber cladding and surrounding the fiber Bragg grating region. The sensing matrix structure comprising a bonding layer disposed on the outer surface of the fiber cladding layer, a nano-structured trampoline matrix layer disposed on the outer surface of the bonding layer and a capping layer disposed on the outer surface of the matrix layer. The thermally modulated response amplitude of the fiber gas sensor is found to linearly depend upon the gas molecular weight, and can be directly used to determine heat specific capacity ratio of Cp/Cv.