Stir-controlled differential binding of folic acid in hollow silver nanocubes and the PERS enhancement in pre- and post-saturation regimes

The chemical identity of the biomolecules on nanostructured substrates is not only defined by plasmon-enhanced Raman spectroscopy (PERS) but also by the nature of the plasmon resonance that is facilitated by the substrate. In this case, we show that the rate of the stirring in the course of the synthesis regulates the sharpness of the corners of hollow silver nanocubes (HAgNCs) that determines the excitation of either the dipolar or both the dipolar and quadrupolar surface plasmon resonances with direct and molecularly specific implications on the PERS of the adsorbed folic acid (FA) capping agent of the hollow silver nanocube. Hollow silver nanocubes were made in unstirred (HAgNCs-Zero, 0 rpm) and stirring (HAgNCs-150, 150 rpm) conditions. The transmission electron microscopy proves that both samples produce hollow cubic nanoparticles with similar edge lengths (around 78–79 nm) and void diameters, and the edge of critical morphological distinction is the much sharper corners in HAgNCs-Zero. Second-derivative analysis of UV–Vis extinction spectroscopy shows that HAgNCs-Zero only supports dipolar plasmon modes. Meanwhile, HAgNCs-150, alone, has an extra quadrupolar resonance at 430 nm, the higher-order plasmon mode, which has not been reported in hollow silver nanocubes. FTIR spectroscopy of the binding geometries on the surface is fundamentally different: in the case of folic acid-binding to HAgNCs-Zero, the pteridine ring nitrogen/oxygen atoms coordinate the folic acid to the HAgNCs-Zero, and the peripheral NH groups are not bound, in contrast to HAgNCs-150 where the primary amine -NH₂ groups directly coordinate to the silver surface, collapsing the NH spectrophotometric band into a single broad featureless band. Correlatively, PERS measurements reveal that the symmetric and asymmetric NH folic acid vibrations of folic acid at 3200 and 3400 cm−1 are present only in the spectrum of HAgNCs-Zero, the first-ever NH folic acid modes to be observed in any SERS or PERS of folic acid on any substrate. This suppression in HAgNCs-150 is caused by a combination of two effects; direct -NH₂ surface coordination (chemical quenching) and the spatially localized near-field topology of the rounded-corner substrate. Measurement of concentration- dependence of PERS and zeta potential also shows that there are specific pre-saturation and post-saturation adsorption regimes. These results provide a direct mechanistic pathway between the synthesis environment to corner geometry, plasmon mode hierarchy, molecular binding orientation, and selective vibrational enhancement to provide new understanding on the rational design of morphology-controlled PERS substrate for bioanalytical applications.