We present an optical sensor for solute concentration in solutions by using aluminum-doped zinc oxide (AZO) coatings on silica single-mode/coreless/single-mode and single-mode/multimode/single-mode fiber structures. The fiber sensor is based on the detection of the second lossy-mode resonance (LMR) order of the AZO coated fiber structures which covers the near-infrared C and L wavelength bands. AZO thin films with Zn:Al atomic concentration proportions of 92%:8% were deposited on the fiber structures by radio frequency magnetron sputtering technique. Wavelength displacement of the second order LMR notch as a function of concentration variation on isopropyl-alcohol/glycerin liquid mixtures where measured by a simple optical transmission setup, detected in the 1.55 μm wavelength range. In glycerin concentration percentages (c%) variations from 5% to 30%, maximum sensitivity of 1.42 nm/c% was obtained for coreless-fiber-based structures.