A facile UV-Vis spectrophotometric method for quantitative determination of Al³⁺ ion based on chromophoric Alizarin Red S (ARS) was carried out. ARS reagent was used as an organic chelator for Al³⁺ ion to form ARS–Al(III) complex. The yellow-coloured ARS changed to reddish-orange hue after reaction with Al³⁺ ion and formed ARS–Al(III) complex at optimum pH 4. ARS reagent yielded linear concentration range over 0.1–1.0 ppm Al³⁺ ion and a fast response time of ∼35 s. The reproducibility study for ARS–Al(III) complex was found to give satisfactory relative standard deviation (RSD) values of 2.4–3.1%. The ARS reagent demonstrated good photostability over an 8-hour experimental period. An attempt has been conducted to physically immobilize ARS reagent onto newly synthesized photocurable poly(n-butyl acrylate) [poly(nBA)] microspheres, followed by reflectometric transduction of ion recognition event. The micro-sized poly(nBA) spheres has been confirmed by scanning electron microscopy (SEM). The colour change of the immobilized ARS, i.e. before and after reaction with Al³⁺ ion was found to be similar to the ARS solution analysis at pH 4. The acrylic-based optical microsensor showed potential to accommodate both quantitative and semiquantitative in situ evaluation of Al³⁺ ion concentration in environmental water and biological fluids.