Reviewing the effect of aggregates in Rhodamine 6G aqueous solution on fluorescence quantum efficiency.

Rhodamines constitute a class of dyes extensively investigated and applied in various contexts, primarily attributed to their high luminescence quantum yield. This study delves into the impact of aggregation on the thermal and optical properties of Rhodamine 6G (R-6G) solutions in distilled water. Examined properties encompass thermal diffusivity (D), temperature coefficient of the refractive index (dn/dT), fluorescence quantum efficiency (η), and energy transfer (ET). These parameters were assessed through thermal lens (TL) and conventional absorption and emission spectroscopic techniques. The dimerization of R-6G solutions was revisited, revealing that an increase in R-6G concentration alters the features of absorption and emission spectra due to dimer formation, resulting in unexpected behavior of η. Consequently, we introduce a novel model for the fraction of absorbed energy converted into heat (φ), which accounts for emissions from both monomers and dimers. Employing this model, we investigate and discuss the concentration-dependent behaviors of η for monomers (ηm) and dimers (ηd). Notably, our findings demonstrate that ηm values necessitate ηd = 0.2, a relatively substantial value that cannot be disregarded. Additionally, applying the Förster theory for dipole-dipole electric ET, we calculate microparameters for ET between monomers (CDD) and monomer-dimer (CDA). Critical ranges for ET in each case are quantified. Microparameter analysis indicates that ET between monomer-monomer and monomer-dimer species of R-6G dissolved in distilled water holds significance, particularly in determining ηm. These results bear significance, especially in scenarios involving high dye concentrations. While applicable to R-6G in water, similar assessments in other media featuring aggregates are encouraged.

Influence of gold nanoholes and nanoslits arrays on Raman spectra and optical reflectance of graphene oxide.

We report the effect of gold nanostructured substrates, fabricated by interference lithography technique (IL), on the Raman spectra and optical reflectance of graphene oxide (GO) layers. For purposes of comparison two gold nanostructured substrates, nanoslits (AuNSs) and circular nanoholes (AuNHs) were compared with a non-nanostructured gold substrate. Effects induced by the gold nanostructured substrates are discussed in terms of the ID/IG ratio and the FWHM of the G band (FWHM(G)) as a function of the G band intensity (IG), showing that both ID/IG and FWHM(G) parameters are highly sensitive to the number of GO layers (nGO), which would allow to identify the number of GO layers in a reliable way. Optical reflectance spectra (R(λ)) reveal that plasmons are generated on the surface of nanostructured substrates by the incident radiation. Dips in R(λ) are ascribed as coupling by surface plasmon polaritons described by Bloch waves (BW-SPP). A peak in R(λ) is also observed and it is ascribed to visible radiation produced by Förster resonance energy transfer and Purcell effect. The relevance of these results lies in the possibility of designing colorimetric plasmonic sensors, based on few layers of GO with an excellent control of nGO and with potential in detection of molecules by fluorescent absorption.