Here, a modular approach is reported to introduce a specific function into single-chain polymeric nanoparticles (SCPNs). Hereto, an amphiphilic polymer with pendant benzene-1,3,5-tricarboxamide (BTA) units is mixed with a "free" BTA that contains a functional group, either a fluorescent naphthalimide or a catalytically active L-proline. Taking advantage of hydrophobic interactions and self-recognition properties of the BTA units, the "free" BTAs are captured into the interior of the SCPN in water as evidenced by fluorescence studies. To illustrate that function can be readily introduced using a modular approach, L-proline-based BTAs are incorporated to procure a catalytically active SCPN in water. The aldol reaction between p-nitroben-zaldehyde and cyclohexanone shows good conversions at low catalyst loadings and substrate concentrations, and high stereoselectivities are obtained (de = 91% and ee = 98%).