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We present a systematic study of the effect of higher-multipolar order plasmon modes on the spectral response and plasmonic coupling of silver nanoparticle dimers at nanojunction separation and introduce a coupling mechanism. The most prominent plasmonic band within the extinction spectra of coupled resonators is the dipolar coupling band. A detailed calculation of the plasmonic coupling between equivalent particles suggests that the coupling is not limited to the overlap between the main bands of individual particles but can also be affected by the contribution of the higher-order modes in the multipolar region. This requires an appropriate description of the mechanism that goes beyond the general coupling phenomenon introduced as the plasmonic ruler equation in 2007. In the present work, we found that the plasmonic coupling of nearby Ag nanocubes does not only depend on the plasmonic properties of the main band. The results suggest the decay length of the higher-order plasmon mode is more sensitive to changes in the magnitude of the interparticle axis and is a function of the gap size. For cubic particles, the contribution of the higher-order modes becomes significant due to the high density of oscillating dipoles localized on the corners. This gives rise to changes in the decay length of the plasmonic ruler equation. For spherical particles, as the size of the particle increases (i.e., ≥80 nm), the number of dipoles increases, which results in higher dipole–multipole interactions. This exhibits a strong impact on the plasmonic coupling, even at long separation distances (20 nm). 

We report for the first time the usage of plasmonically enhanced Raman spectroscopy (PERS) to directly monitor the dynamics of pharmacologically generated hemeoxygenase-1 (HO-1) by evaluating the kinetics of formation of carbon monoxide (CO), one of the metabolites of HO-1 activation, in live cells during cisplatin treatment. Being an endogenous signaling molecule, CO plays an important role in cancer regression. Many aspects of HO-1's and CO's functions in biology are still unclear largely due to the lack of technological tools for the real-time monitoring of their dynamics in live cells and tissues. In this study, we found that, together with nuclear region-targeted gold nanocubes (AuNCs), cisplatin treatment can dramatically trigger the activation of HO-1 and thereby the rate and production of CO in mammalian cells in a dose-dependent manner. Though quantitative molecular data revealed that a lower concentration of cisplatin up-regulates HO-1 expression in cancer cells, PERS data suggest that it poorly facilitates the activation of HO-1 and thereby the production of CO. However, at a higher dose, cisplatin along with AuNCs could significantly enhance the activation of HO-1 in cancer cells, which could be probed in real-time by monitoring the CO generation by using PERS. Under the same conditions, the rate of formation of CO in healthy cells was relatively higher in comparison to the cancer cells. Additionally, molecular data revealed that AuNCs have the potential to suppress the up-regulation of HO-1 in cancer cells during cisplatin treatment at a lower concentration. As up-regulation of HO-1 has a significant role in cell adaptation to oxidative stress in cancer cells, the ability of AuNCs in suppressing the HO-1 overexpression will have a remarkable impact in the development of nanoformulations for combination cancer therapy. This exploratory study demonstrates the unique possibilities of PERS in the real-time monitoring of endogenously generated CO and thereby the dynamics of HO-1 in live cells, which could expedite our understanding of the signaling action of CO and HO-1 in cancer progression.