Recent Progress in Surface-Enhanced Fluorescence Using Gold Nanorods

Abstract

Plasmonic nanoparticles have been intensely used in research because they possess powerful optical properties. Gold nanorods (Au NRs), in particular, feature the interesting ability to absorb and scatter light in the near-infrared region through their longitudinal localized surface plasmon resonance. This property is particularly interesting in biology because these wavelengths are associated with maximum tissue penetration. The interplay between plasmonic nanoparticles and fluorophores has also triggered exciting research. Specifically, illuminating the Au NRs produces an enhanced electromagnetic field in the vicinity of the particles. This allows nearby fluorophores to feature enhanced optical properties through a phenomenon known as surfaceenhanced fluorescence. This effect, however, must be counterbalanced by potential quenching that can happen from the Au NRs to the fluorophore. Systems have been developed whereby a spacer layer insulates the Au NR from the fluorophore. Multiple studies have explored different materials to use as the spacer layer such as silica or polyelectrolyte multilayers. Many of these studies have also investigated the optimal spacer thickness for achieving maximum enhancement. This review summarizes and provides an outlook on important developments in this field that have been reported in the last 10 years.

Enhancement of Vibrant Up-Conversion in (Ca,Sr)F2:Er,Yb,Al Phosphors via Energy Transfer, Localized Interstitial Distortion, and Au Nanostructures

“…Up-conversion phosphors with low excitation power are attractive for applications in bioimaging, displays, sensors, therapeutics, photovoltaics, etc. − CaF 2 , an alkaline earth fluoride, is acknowledged as an efficient host due to its excellent stability and lower phonon energy, preventing nonradiative decay. − Furthermore, Er 3+ -Yb 3+ -doped CaF 2 phosphors, with a cubic fluorite structure in the Fd 3̅ m space group, are considered a compatible up-conversion luminescence material compared to cubic NaYF 4 :Er 3+ ,Yb 3+ phosphors in previous reports. − The presence of interstitial ion in optical materials is crucial as it significantly decreases the local symmetry of rare-earth ion, leading to increasing homogeneity in the host lattice and effectively enhancing up-conversion intensity. − This occurs because interstitial ions reduce the phonon energy, thereby reducing the nonradiative relaxation rate. LSPR effect, caused by collective oscillation of free electrons, especially in Au metal nanoparticles, further enriches the emission intensity of up-conversion phosphors. − The shape and size of the Au nanoparticles play a crucial role in the LSPR effect as their irradiations are specific to certain wavelengths. − In this study, the photoluminescent emission of up-conversion CaF 2 :Er phosphors was dramatically enhanced to approximately over 1100-fold under excitation with a 980 nm laser diode, showing the synergistic effects of energy transfer, interstitial, distortion, and LSPR.…”

“…Up-conversion luminescent materials have the capability to absorb lower-energy infrared photons and subsequently emit high-energy visible photons. , Distinct rare-earth ions, such as Er 3+ , Ho 3+ , or Tm 3+ , serve as efficient activators, generating multiphotons in host lattices under 980 nm excitation. Additionally, Yb 3+ ions, acting as sensitizers, can be co-doped to enhance up-conversion luminescence through energy transfer to the activators. − The integration of localized surface plasmon resonance (LSPR) from metal nanostructures, which absorb and scatter light with the up-conversion phosphors, further improves luminescence intensity. − In a previous report, the Er 3+ -Yb 3+ -doped NaYF 4 up-conversion phosphor, considered one of the most efficient luminescent materials, exhibited over 1000-fold luminescence enhancement when incorporated with metal–insulator–metal nanostructures . Up-conversion phosphors with low excitation power are attractive for applications in bioimaging, displays, sensors, therapeutics, photovoltaics, etc. − CaF 2 , an alkaline earth fluoride, is acknowledged as an efficient host due to its excellent stability and lower phonon energy, preventing nonradiative decay. − Furthermore, Er 3+ -Yb 3+ -doped CaF 2 phosphors, with a cubic fluorite structure in the Fd 3̅ m space group, are considered a compatible up-conversion luminescence material compared to cubic NaYF 4 :Er 3+ ,Yb 3+ phosphors in previous reports. − The presence of interstitial ion in optical materials is crucial as it significantly decreases the local symmetry of rare-earth ion, leading to increasing homogeneity in the host lattice and effectively enhancing up-conversion intensity. − This occurs because interstitial ions reduce the phonon energy, thereby reducing the nonradiative relaxation rate.…”

Multiscale modeling of surface enhanced fluorescence

“…In this context, reliable computational approaches capable of accurately describing how the NP structure affects SEF signals are needed, especially those models featuring a fully atomistic, realistic description of the nanosystem. 18,22,[25][26][27][28][29][30] In this work, the rst fully atomistic multiscale approach to SEF is developed. The model can be used to describe realistic nanostructures with atomistic details, substantially overcoming common issues related to the huge computational cost of full QM treatments of the system, which only allows the description of systems (molecule + nanostructure) composed of a few atoms.…”