We have demonstrated a novel strategy for the synthesis of two different types of mesoporous silica nanoparticles (MSNPs) i.e. rambutan-like MSNPs (R-MSNPs) and hollow MSNPs (H-MSNPs) using a surfactant-free strategy at room temperature via an alkali-assisted stirring process. An amine-based cationic polymer (polyethylenimine; PEI) with various molecular weights (2-750 kD) were simple added in to a solution of silica nanoparticle (SNP). After magnetic stirring for various time duration (6-48h) in basic condition, obtained the desired structures. The particle sizes were uniform with easily tunable surface structures by varying the loading volume ratio of Si/PEI, reaction time, etc. The structural changes of the MSNPs were systematically studied using electron microscopic techniques, XPS, Raman and IR spectroscopy. Both types of MSNPs have a high specific surface area (up to 1025 m2/g) with large pore volumes. As the synthesized MSNPs are highly porous materials that contain internal organic/inorganic networks structure. Therefore, we investigated for the remediation applications of environmental pollutants and bulky dye molecules. For example, obtained materials exhibited promising performances for the removal of heavy metal with high adsorption capacity (qe), e.g. for Pb = 343.4 mg/g; Hg = 285.8 mg/g and As =75.0 mg/g, methyl orange dye absorption rate >95%. In addition, we have demonstrated for removals of toxic microcystin-YR even at low concentration (100 ìg/L). Also, high loading capacity of gold-NPs offered excellent catalytic activities for the reduction of nitrophenol to useful aminophenol. Keywords: Polyelectrolytes, mesoporous silica, environmental remediation