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A cantilever sensor integrated with an on-tip micro spherical reflecting mirror (MSRM) exhibits a larger optical beam displacement than a conventional one, i.e. the system sensitivity/responsivity is enhanced. In this paper, it is employed as a concentration gauge that only requires 0.5 muL of solution. The MSRM-integrated cantilever is first set above a sodium chloride-water droplet carried by a glass substrate, and then the droplet is moved up gradually. Once the cantilever is touched by the droplet, it is pulled and bent down as the droplet is reshaped. The cantilever deformation amount is related to the surface tension of the solution, which increases with the mass percentage of sodium chloride. Therefore, by measuring the bending amount of the cantilever, the concentration of the NaCl-water solution can be determined. We also perform the experiments on the alcohol-water mixture, whose surface tension, conversely, reduces as the alcohol percentage increases.