Indirect Interactions between Raman Probes Encapsulated within Cucurbit[7]urils and Gold Nanorods to Enhance Long-term Stability and Signal

Abstract

a new host-guest chemistry and hybridization systems. [10][11][12][13] Cucurbit[n]urils (CB[n]) are macrocyclic host molecules composed of glycoluril repeat units bridged by methylene groups (Fig. 1). Compared with other host molecules, such as cyclodextrin and crown ethers, CB[n] molecules have unique properties that are contained within rigid, highly symmetric, and macrocyclic pumpkin-shaped structures. [14][15][16] Due to the hollow and cyclic structure of CB[n], target molecules position themselves inside the hydrophobic cavity of CB[n] via strong host-guest interactions. 15,16 Furthermore, CB[n] can attach itself onto gold surfaces through the interaction between the carbonyl groups of CB[n] and Au surface, which leads to the formation of uniform and stable sub-nanometer gaps at 0.9 nm, i.e., the height of the CB [n]. 17 This indicates that CB[n] induces a powerful hotspot between nanoparticles that is highly effective for SERS. 18 The encapsulation inside CB[n] may also contribute to the improved stability of the SERS structure and provide possible enhancements without direct interaction between metal nanoparticles and probe molecules. The carbonyl portals of CB[n], placed on the metal surface, allow indirect interactions due to a trapped state related to electromagnetic enhancements, which is also related to the SERS mechanism. 19 Despite recent studies on SERS that use CB[n], the interactions between CB[n] and various guest molecules to initiate effective SERS and the enhancement mechanisms behind SERS based on CB[n] and gold nanoparticles still lack fundamental understanding. Furthermore, the long-term stability and reproducibility of SERS signals for various Raman probes (e.g., R6G), which are encapsulated within CB[n] that is bound to gold nanoparticle surfaces, still require investigation to improve 2019 Surface-enhanced Raman scattering (SERS) is a powerful technique that enhances Raman signals by adsorbing probe molecules on rough metal surfaces. However, SERS is limited because target molecules must strongly interact with metal to enhance a stable Raman signal. In this study, to improve long-term SERS stability, we use cucurbit[7]urils (CB[7]) as bridge molecules and sample containers to probe Rhodamine 6G (R6G) molecules. We observed interactions between gold nanorods (AuNRs) and CB[7] via aggregate formation, which enhanced the Raman signal and improved long-term R6G probe stability by up to 20 days when encapsulated within CB[7] during SERS analysis. Fig. 1 A schematic displaying the repeating units and a representation of CB[n].

Emerging sensing platforms based on Cucurbit[n]uril functionalized gold nanoparticles and electrodes

“…Apart from the spherical Au NPs, Au NRs and a mixture of Au NR and Au NPs were also used to construct the CB[n] mediated Au nanoaggregates for enhanced SERS. 31,95 Although the CB[n] mediated nanoaggregates implement superior SERS enhancement and a highly sensitive detection of guest analytes, single-molecule SERS has been primarily limited to detecting and distinguishing different chemical species. Therefore, it is essential to expand the capabilities of SERS to investigate single-molecule chemical interactions.…”

“…Apart from the spherical Au NPs, Au NRs and a mixture of Au NR and Au NPs were also used to construct the CB[ n ] mediated Au nanoaggregates for enhanced SERS. 31,95…”

Cucurbiturils‐Mediated Noble Metal Nanoparticles for Applications in Sensing, SERS, Theranostics, and Catalysis

“…Large electromagnetic field enhancement can be achieved by end-to-end alignment of AuNRs with strong longitudinal dipole plasmons coupling. [81] CB[5]/CB [7] as rigid spacers could be used to construct precise and reproducible "hot spot" of 0.9 nm, generating stable 3D aggregates of AuNRs [82] or anisotropic AuNRs chains with the largest and most desirable field enhancements (Figure 8d). [40] Interestingly, CB [7] can capture the analyte and deliver it to the center of the "hot spot" constructed by CB[5], achieving the SERS-based sensitive detection at very low concentrations (<1.0 × 10 −7 m) in water.…”