The use of polymer brushes has been attracting attention in recent years due to its ability to reduce non-specific adsorption of proteins on a surface. At the same time, noble metal nanoparticles has been widely exploited for chemical and biomolecular sensing applications, particularly via localized surface plasmon resonance and surface enhanced raman spectroscopy. While many conventional plasmonic, nanoparticle solution-based detection methods offer direct observation with high sensitivity and selectivity, they have several limitations. For example, it is difficult to discern color hue or intensity changes in colored samples or complex mixtures such as blood and serum. Nanoparticle in solution are also prone to aggregation over time, reducing shelf life. This work explores the use of polymer brushes as a matrix for the incorporation of nanoparticles. Sensing interfaces were facilely fabricated by loading unmodified gold nanoparticles (AuNPs) and gold nanorods (AuNRs) into poly(oligo(ethylene glycol) methacrylate) (POEGMA) brushes grown on glass. Depending on their size, the AuNPs would either infiltrate into the brush or adsorb on the brush/solution interface. The attainment of a three-dimensional (3D) assembly of non-infiltrating AuNP was also achieved via a novel process described as in-stacking. The AuNP-POEGMA nanocomposite structure with infiltrated AuNPs was used for lead detection based on lead ion-induced release of thiosulfate-functionalized AuNPs. The decrease in colorimetric intensity over increasing lead concentrations could easily be observed iv via unaided eyes. We obtained a limit-of-detection as low as 25 pM and a dynamic range as wide as 4 orders of magnitude. Lead detection in human serum gave recoveries of 91.0-109.9%, testimony of the effective role of POEGMA as a nonfouling polymer matrix. The AuNP-POEGMA nanostructure with adsorbed AuNPs were employed for the detection of proteins. We found that large plasmonic peak shifts of ~50 nm were actuated upon in situ functionalization as well as during target detection. They were used for thrombin detection based on aggregation and deaggregation induced upon aptamerization and protein sensing respectively. We obtained a limit-of-detection as low as 0.64 pM. Finally, AuNR-POEGMA nanocomposite structures was conveniently achieved by initially studying the influence of ionic strength and surfactant concentration on the assembly of AuNRs on a polystyrene-sulfonate (PSS)-modified substrate. The same principle was eventually applied to obtain tunable assemblies of AuNRs incorporated within the POEGMA.