In this work, we manifest the fabrication of a simple, portable, and benign solid-state sensor for the quantification of toxic Hg²⁺ ions through ocular colorimetric sensing. The sensor fabrication is modulated through sol-gel process using two different block-polymer surfactants (PEO and F108) for the formation of mesoporous silica monolithic designs that are immobilized with tailor-made probe molecules. The crack-free porous silica monoliths are structurally engineered to form, (i) highly ordered honey-combed 3D cubic, and (ii) disoriented worm-like mesopore structures that facilitates homogeneous probe anchoring through constrained spatial orientations for the selective detection of ultra-trace Hg²⁺ ions. The monolithic sensor materials are characterized by FE-SEM, HR-TEM, p-XRD, SAED, EDAX, XPS, FT-IR, TGA, and N₂ isotherm analysis. For ensuring unambiguous ion-sensing, analytical parameters such as solution pH, temperature, kinetics, probe concentration, sensor quantity, linear signal response, matrix tolerance, the limit of detection (L_D), and quantification (L_Q) are optimized. The L_D and L_Q values for probe anchored PEO-MSM based sensor are 0.61 & 2.05 ppb and for probe anchored F108-MSM based sensor are 0.22 & 0.72 ppb, respectively, in the corresponding linear response range of 0−100 ppb and 0−50 ppb of Hg²⁺, which has been validated by real-time analysis of natural water samples.