New insights on surface‐enhanced Raman scattering based on controlled aggregation and spectroscopic studies, DFT calculations and symmetry analysis for 3,6‐bi‐2‐pyridyl‐1,2,4,5‐tetrazine adsorbed onto citrate‐stabilized gold nanoparticles

Abstract

A systematic study on the surface-enhanced Raman scattering (SERS) for 3,6-bi-2-pyridyl-1,2,4,5-tetrazine (bptz) adsorbed onto citrate-modified gold nanoparticles (cit-AuNps) was carried out based on electronic and vibrational spectroscopy and density functional methods. The citrate/bptz exchange was carefully controlled by the stepwise addition of bptz to the cit-AuNps, inducing flocculation and leading to the rise of a characteristic plasmon coupling band in the visible region. Such stepwise procedure led to a uniform decrease of the citrate SERS signals and to the rise of characteristic peaks of bptz, consistent with surface binding via the N heterocyclic atoms. In contrast, single addition of a large amount of bptz promoted complete aggregation of the nanoparticles, leading to a strong enhancement of the SERS signals. In this case, from the distinct Raman profiles involved, the formation of a new SERS environment became apparent, conjugating the influence of the local hot spots and charge-transfer (CT) effects. The most strongly enhanced vibrations belong to a 1 and b 2 representations, and were interpreted in terms of the electromagnetic and the CT mechanisms: the latter involving significant contribution of vibronic coupling in the system.

Surface‐enhanced Raman scattering of 4,4'‐diaminostilbene: Dependence of spectral features and resonant enhancement on surface coverage

“…Because an electron cloud movement is most significant during CT, the highest activation of considered normal vibration is achieved at these conditions. Similar stronger enhancement of some totally symmetric bending modes of aromatic system arising due to CT in conditions of hot spots was also revealed for 3,6‐bi‐2‐pyridyl‐1,2,4,5‐tetrazine adsorbed on gold nanoparticles …”

“…For instance, the changes of adsorbate/analyte spectral features can be caused by any of the following reasons: molecule reorientation, conformational transitions, protonation of functional group, coadsorption of ions, and so forth . In a number of recent studies, it was found that an aggregation of metal nanoparticles also can induce the changes of SERS profile . These observations deserve a particular attention due to their connection to another burning issue of SERS spectroscopy, namely, to the hot spots phenomenon.…”

Surface‐enhanced Raman scattering of N‐acetylneuraminic acid on silver nanoparticle surface

“…(A) and (B), respectively. The assignment of the main bands (Table S1) is made on the basis of previously reported Raman spectra of sodium citrate and related compounds . The vibrational modes are considerably broader and blue shifted in the SERS spectrum, suggesting a substantial interaction of citrate ion with metal surface.…”

The SERS effect in coordination chemistry

“…Another interesting case has been reported for the SERS dependence on the aggregation modes for the 3,6-bi-2-pyridyl-1,2,4,5-tetrazine (bptz) adsorbed onto citrate-modified gold nanoparticles (cit-AuNps) [229]. The bptz molecule is a linear Nheterocyclic ligand capable of interacting with the gold atoms by means of the aromatic nitrogen groups (scheme 15).…”