Theoretical elucidation of the origin of surface‐enhanced Raman spectra of PCB52 adsorbed on silver substrates

Abstract

To better understand experimentally observed surface-enhanced Raman Scattering (SERS) of polychlorinated biphenyls (PCBs) adsorbed on nanoscaled silver substrates, a systematic theoretical study was performed by carrying out density functional theory and time-dependent density functional theory calculations. 2,2′,5,5′-tetrachlorobiphenyl (PCB52) was chosen as a model molecule of PCBs, and Ag n (n = 2, 4, 6, and 10) clusters were used to mimic active sites of substrates. Calculated normal Raman spectra of PCB52-Ag n (n = 2, 4, 6, and 10) complexes are analogical in profile to that of isolated PCB52 with only slightly enhanced intensity. In contrast, the corresponding SERS spectra calculated at adopted incident light are strongly enhanced, and the calculated enhancement factors are 10 4~1 0 5 . Thus, the experimentally observed SERS phenomenon of PCBs supported on Ag substrates should correspond to the SERS spectra rather than the normal Raman spectra. The dominant enhancement in Raman intensities origins from the charge transfer resonance enhancement between the molecule and clusters.

Rapid detection of 2,2′,4,4′‐tetrabromodiphenyl ether (BDE‐47) using a portable Au‐colloid SERS sensor

“…Indeed, the DFT results show that the Mulliken charge transferred from BDE‐47 to Au ranged from 0.30 to 0.60 electrons in different Au n ( n = 0–10) clusters (Table S1). The same CT direction was also reported when PCB‐52 adsorbed on Ag clusters . The CT accompanied different Au–Br bond lengths ranging 2.6 to 3.1 Å (Table S1).…”

“…For BDE‐47, our DFT calculation indicated that the stable complexes were formed when Br atoms of BDE‐47 were closely coordinated with Au clusters as evidenced by their 2.6–3.1‐Å atomic distances (Table S2). Similarly, close association of Ag–Cl atoms was observed in the PCB‐52/Ag n ( n = 0–10) system, and the calculated Ag–Cl distances were in the range 2.9–3.8 Å . The SERS peaks of pyrene were resolved at 405, 592, 1135, and 1234 cm −1 , corresponding to the Raman shifts of pyrene standard .…”

The SERS effect in coordination chemistry

“…The higher polarizability of the Au 20 -enol system, in relation to the free molecule, would also facilitate the occurrence of a non-resonant chemical mechanism (CHEM). However, in spite of the several theoretical attempts to explain the interactions between the ligand and the metallic clusters [188,[206][207][208]226,227], their contribution to the transition polarizability moment remains yet poorly understood.…”

“…However, this mechanism is not yet completely understood, representing a current theoretical [181,201,[206][207][208][209][210][211][212] and experimental challenge [198,199,213]. It requires the understanding of the nature of the complexes formed at the nanoparticle-molecule interface, including their geometry, bond strengths and electronic transitions [214].…”

Uma abordagem fundamental sobre nanopartículas de ouro

“…24,25,153 [188][189][190] Quanticamente, a intensidade da radiação espalhada é dependente do momento de transição de polarizabilidade, entre um estado inicial e final, definidos pelas suas respectivas funções de onda, que serão representadas simplificadamente por e f v f R f e e i v i R i , onde "e" é a parte eletrônica, "v" a vibracional e "R" a rotacional. O efeito químico não ressonante vem sendo estudado teoricamente por métodos de primeiros princípios, como DFT/TD-DFT, [41][42][43]47,48,197,198 CH2rock + C 6 =C 5 C 2 O 15 C 8 C 6 + O 9 C 5 =C 6 435 O 13 C 8 C 6 ip + O 9 C 5 =C 6 ip + CH2rock(vw) O 13 C 1 C 2 + O 9 C 5 -C 2 380 375 CH2rock + C 6 =C 5 C 2 (w)+ C 5 =C 6 C 8 (vw) ν = estiramento,  = deformação de ânculo , ip = no plano, op = for a do plano, iph = em fase, oph = fora de fase, sym = simétrico, as = assímétrico Para a cit-AuNP a interação entre as moléculas de citrato e os átomos superficiais das partículas de ouro é predominantemente eletrostática e como foi mostrado as transições eletrônicas no sistema Au20-citrato apresentam maior caráter de transições intracluster do que de transferência de carga. Neste caso o espectro SERS é observado apenas em condição de agregação, na qual a geração de hot-spots é induzida e comprovada pelo aparecimento da banda de acoplamento plasmônico na região acima de 700 nm, Figura 69.…”