Tuning of the Spectroscopic Properties of Composite Nanoparticles by the Insertion of a Spacer Layer: Effect of Exciton−Plasmon Coupling

Abstract

Composite nanoparticles (NPs) having a double-shell structure, Au core, spacer layer (inner shell), and J-aggregate (JA) layer (outer shell) (Au/spacer/JA) have been synthesized. The spacer layer composed of N,N,N-trimethyl(11-mercaptoundecyl)ammonium chloride played an important role in promoting the J-aggregation of anionic cyanine dyes on the surface, as evidenced by the successful formation of the JA layers with four kinds of anionic cyanine dyes. It was found that the presence of a spacer layer causes a significant change in the line shape of the absorption spectrum, particularly near the J-band; there is the appearance of a peak type absorption for the composite NPs with the double-shell structure, while there is a dip type absorption for the ones without the spacer layer. The change from the peak type absorption to the dip type absorption in the Au/spacer/JA NPs occurs when the size of the Au core is varied from 5 to 15 nm. These observations would indicate that the strength of exciton-plasmon coupling between the Au core and the JA layer is enhanced with the increase in the core size or the decrease in the separation between the Au core and the JA shell. The photoluminescence arising from the JA can be detected for the composite NPs with the double-shell structure, showing that the quenching by the Au core is effectively suppressed by the spacer layer.

Plasmonic Fluorescent Nanocomposites of Cyanines Self-assembled upon Gold Nanoparticle Scaffolds

“…With 10 nm AuNPs-SA, the protein acts as a spacer layer protecting the fluorophores from quenching by AuNPs. 10,12,14,15 Hence, the LOD and LOQ values of 10 nm AuNPs-SA-nanocomposites were orders of magnitude smaller than 10 nm AuNPs-nanocomposites (Table 1). Our model (Fig.…”

“…Fluorescence quench in the vicinity of metal NPs is due to exciton coupling to LSPR. [5][6][7][8][9][10][11][12] However, spectral overlap between excitation or absorption of nanocomposites and LSPR of AuNPs enables fluorescence due to the AuNPs acting as local antennae, increasing the absorption. 19,20 Thus, 1 J-band absorption at 560 -600 nm (Fig.…”

“…[5][6][7][8][9][10][11][12] Metal enhanced fluorescence is also displayed by fluorophores located close to metal NPs. 26 Both processes are distance-dependent; quench dominates over short (≈5 nm to <10 nm) distance and enhancement is prominent at >10 nm distance separating fluorophores and AuNPs.…”

“…3 During a literature review of chromophore assemblies on metal nanostructures, we found several papers reporting no fluorescent J-aggregates being formed or the fluorescence from such aggregates was quenched or photobleached. [4][5][6][7][8][9][10][11][12][13] Others reported a need for spacer layers in order to distance the aggregates from the noble nanometal surface for avoiding quench; 10,12,14,15 sometimes, even with a spacer layer no J-aggregation was observed with certain cyanines. 10 There were a few reports of J-aggregates on AgNPs 13,16,17 but not on AuNPs.…”

Coherent Plasmon-Exciton Coupling in Silver Platelet-J-aggregate Nanocomposites

“…These hybrid systems often consist of a core-shell geometry in which the localized surface plasmon resonance (LSPR) of the metallic core couples with the exciton resonance exhibited by a J-aggregate dye or a quantum dot shell. [1][2][3][4][5][6][7][8][9][10] This architecture provides a means of studying plasmon-exciton interactions, which have resulted in unique optical phenomena such as induced transparency. 1,2,[5][6][11][12] The ability to control the morphology and dimensions of the individual layers at the nanoscale, and the subsequent control of optical properties, are ultimately what drive this field of research.…”