Metal-enhanced Luminescence Research Articles

Engineering the luminescence and photothermal properties of metal complex Ruphen functionalized with silver nanoparticles for infrared human detection

A luminescence quenching enhanced infrared (IR) detector has been developed to achieve highly sensitive recognition of human body by adding metallic luminescence quenchers. Spherical Ag NPs were mixed with Ru(phen)32+ complex to obtain temperature-sensitive Ag/Ruphen/PVDF films. The relationships among luminescence output, temperature rise and silver addition were investigated and a satisfactory combination of enhanced luminescence factor and temperature change was found with an addition of 5 mol% of 71 nm Ag NPs. It is found that the metal-enhanced luminescence output is insignificant compared to its thermal ingredient since silver provides additional possible electron transfer channels, promoting non-radiative transitions and weakening luminescence. Besides, the Ag NPs induced additional temperature rise bringing further reduction of luminescence. The photothermal effect of Ag NPs can attain a higher temperature change under a certain IR radiation without an observed degradation of luminescence output. A coupled optical-thermal model was established to explain the schematic of the constructed infrared thermal detector. The non-invasive detections of moving and stationary human by the detector were successfully demonstrated. The work tends to provide an engineered optical-thermal model in luminophores and construct highly sensitive luminescence-based infrared thermal detectors.

Optical Binding-Driven Micropatterning and Photosculpting with Silver Nanorods.

Controlling the nano- and micropatterning of metal structures is an important requirement for various technological applications in photonics and biosensing. This work presents a method for controllably creating silver micropatterns by laser-induced photosculpting. Photosculpting is driven by plasmonic interactions between pulsed laser radiation and silver nanorods (AgNRs) in aqueous suspension; this process leads to optical binding forces transporting the AgNRs in the surroundings, while electronic thermalization results in photooxidation, melting, and ripening of the AgNRs into well-defined 3D structures. This work call these structures Airy castles due to their structural similarity with a diffraction-limited Airy disk. The photosculpted Airy castles contain emissive Ag nanoclusters, allowing for the visualization and examination of the aggregation process using luminescence microscopy. This work comprehensively examines the factors that define the photosculpting process, namely, the concentration and shape of the AgNRs, as well as the energy, power, and repetition rate of the laser. Finally, this work investigates the potential applications by measuring the metal-enhanced luminescence of a europium-based luminophore using Airy castles.

Nanoparticle core size and spacer coating thickness-dependence on metal-enhanced luminescence in optical oxygen sensors

Metal-enhanced luminescence (MEL) originated from near field interactions of luminescence with the surface plasmon resonance (SPR) of nearby metallic nanoparticles (NPs) is an effective strategy to increase luminescence detection sensitivity in oxygen sensors. Once the excitation light induces SPR, the generated enhanced local electromagnetic filed will result in an enhanced excitation efficiency and an increased radiative decay rates of luminescence in close proximity. Meanwhile, the nonradioactive energy transfer from the dyes to the metal NPs, leading to emission quenching, can also be affected by their separation. The extent of the intensity enhancement depends critically on the particle size, shape and the separation distance between the dye and the metal surface. Here, we prepared core-shell Ag@SiO2 with different core sizes (35 nm, 58 nm and 95 nm) and shell thickness (5–25 nm) to investigate the size and separation dependence on the emission enhancement in oxygen sensors at 0–21% oxygen concentration. Intensity enhancement factors of 4–9 were observed with a silver core size of 95 nm and silica shell thickness of 5 nm at 0–21% O2. In addition, the intensity enhancement factor increases with increasing core size and decreasing shell thickness in the Ag@SiO2-based oxygen sensors. Using Ag@SiO2 NPs result in brighter emission throughout the 0–21% oxygen concentration. Our fundamental understanding of MEP in the oxygen sensors provides us the opportunity to design and control efficient luminescence enhancement in oxygen and other sensors. .

Factors affecting the metal-enhanced luminescence of lanthanide complexes by Ag@SiO2 nanoparticles

Metal-enhanced luminescence (MEL) via Ag@SiO2 nanoparticles provides a promising strategy for the superior luminescence of lanthanide complexes. However, because of the delicate structure of the composites of Ag@SiO2 and complexes, it remains a great challenge to achieve the ideal MEL effect. Herein, five types of Ag@SiO2 nanoparticles with distinct core sizes (100 and 46 nm) and varied silica shell thickness (3, 24, 32, 52 and 55 nm) were prepared. Four kinds of lanthanide complexes RE(BA)3·H2O and RE(pyca)3·2H2O (RE = Eu and Tb, BA = benzoate, pyca = 2-pyrazine carboxylate) were synthesized. A series of REL3-Ag@SiO2 (L = BA−, pyca−) composite nanoparticles were prepared through the interaction of the Ag@SiO2 nanoparticles and the complexes. The adsorption of the complexes on the surface of Ag@SiO2 nanoparticles was confirmed by transmission electron microscope and energy dispersive X-ray spectroscopy. Luminescence property investigation showed that the factors affecting the MEL effect included the excitation and emission wavelength of lanthanide complexes, the kinds of lanthanide ions and organic ligands, as well as the core size and shell thickness of Ag@SiO2. Among these factors, the excitation wavelength of lanthanide complexes and the SiO2 shell thickness were found to play decisive roles. Finally, 25.92 times enhancement in luminescent intensity and 7.4 times increase in luminescence quantum yield can be achieved on Tb(BA)3·H2O by Ag@SiO2 with core size of 46 nm and shell thickness of 24 nm.

Effect of the Composition of Lanthanide Complexes on Their Luminescence Enhancement by Ag@SiO2 Core-Shell Nanoparticles

Metal-enhanced luminescence of lanthanide complexes by noble metal nanoparticles has attracted much attention because of its high efficiency in improving the luminescent properties of lanthanide ions. Herein, nine kinds of europium and terbium complexes—RE(TPTZ)(ampca)3·3H2O, RE(TPTZ)(BA)3·3H2O, RE(phen)(ampca)3·3H2O, RE(phen)(PTA)1.5·3H2O (RE = Eu, Tb) and Eu(phen)(BA)3·3H2O (TPTZ = 2,4,6-tri(2-pyridyl)-s-triazine, ampca = 3-aminopyrazine-2-carboxylic acid, BA = benzoic acid, phen = 1,10-phenanthroline, PTA = phthalic acid)—have been synthesized. Meanwhile, seven kinds of core-shell Ag@SiO2 nanoparticles of two different core sizes (80–100 nm and 40–60 nm) and varied shell thicknesses (5, 12, 20, 30 and 40 nm) have been prepared. The combination of these nine types of lanthanide complexes and seven kinds of Ag@SiO2 nanoparticles provides an opportunity for a thorough investigation of the metal-enhanced luminescence effect. Luminescence spectra analysis showed that the luminescence enhancement factor not only depends on the size of the Ag@SiO2 nanoparticles, but also strongly relates to the composition of the lanthanide complexes. Terbium complexes typically possess higher enhancement factors than their corresponding europium complexes with the same ligands, which may result from better spectral overlap between the emission bands of Tb complexes and surface plasmon resonance (SPR) absorption bands of Ag@SiO2. For the complexes with the same lanthanide ion but varied ligands, the complexes with high enhancement factors are typically those with excitation wavelengths located nearby the SPR absorption bands of Ag@SiO2 nanoparticles. These findings suggest a combinatorial chemistry strategy is necessary to obtain an optimal metal-enhanced luminescence effect for lanthanide complexes.

Metal-enhanced luminescence of silicon quantum dots: effects of nanoparticles and molecular electron donors and acceptors on the photofading kinetics

Alkyl-capped silicon quantum dots (SiQDs) show enhanced luminescence when drop cast as films on glass slides in mixtures with Ag or Au nanoparticles or the electron donor ferrocene (Fc). Metal enhancement of quantum dot photoluminescence (PL) is known to arise from a combination of the intense near-field associated with the surface plasmon of the metal on the rate of absorption and the decrease in the lifetime of the excited state. Here we present evidence that an additional factor is also involved: electron transfer from the metal to the quantum dot. Under CW irradiation with an argon ion laser at 488 nm, SiQDs undergo a reversible photofading of the PL as the particles photoionize. A steady-state condition is established by the competition between photoionization and electron–hole recombination. The fading of the initial PL I0 to the steady-state value can be modelled by a simple first order decay with a lognormal distribution of rates, which reflects the heterogeneity of the sample. In the presence of Ag and Au nanoparticles, the modal rate constants of photofading increase by factors of up to 4-fold and the ratio decreases by factors up to 5-fold; this is consistent with an increase in the rate of electron–hole recombination facilitated by the metal nanoparticles acting as sources of electrons. Further support for this interpretation comes from the enhancement in PL observed in photofading experiments with films of SiQDs mixed with Fc; this compound is a well-known one-electron donor, but shows no plasmon band which complicates the estimation of PL enhancement with Ag NPs.

Metal-enhanced luminescence in colloidal solutions of CdSe and metal nanoparticles: investigation of density dependence and optical band overlap.

The photoluminescence (PL) of semiconductor nanoparticles (SNP) is strongly modified when the semiconductor is in the proximity of a metal surface or a metal nanoparticle (MNP). The effect may be due to two different phenomena which are (a) (Förster) resonant energy transfer ((F)RET) between the semiconductor and the metal and (b) the enhanced electric field around metallic structures that arises from surface plasmon oscillations. Here we present experimental evidence for enhancement and quenching of the PL of dilute SNP colloidal solutions depending on the amount of admixed MNP and the position of the MNP plasmon band with respect to the excitation wavelength and the optical bands in the SNP. The average distance between an MNP and its next neighbor MNP is varied between ∼0.1 and 2 μm by varying the MNP concentration, whereas that between MNP and SNP as well as between SNP and SNP is kept at about 0.1 μm. A model function based on the rate equations of the system is developed that yields a satisfactory description of the measured data by considering solely FRET between the particle species. The derived function is an extension of the Stern-Volmer equation, as it not only accounts for the energy transfer from the fluorescent SNPs to the MNPs, but also for the transfer of excitation from MNPs to SNPs and between MNPs. This theory provides a deeper insight into the mechanisms of metal-enhanced fluorescence and fluorescence quenching phenomena.

A Nano-Silver Enhancement Effect on the Luminescence of a Ligand–Eu3+ Complex via a SiO2 Spacer

Luminescent rare earth complex (REC) nanocomposites, Eu(TTA)3Phen attached onto Ag@SiO2 nanoshells, were fabricated by facile wet chemistry and self-assembly techniques. Transmission electron microscopy, and fourier transform infrared and UV–Vis spectroscopy were used to investigate the step-by-step fabrication. The luminescence of REC was significantly enhanced using a silver core (size: 45 nm) surrounded by a 20-nm thick silica shell. Thicker or thinner silica shells afforded tuning of the metal-enhanced luminescence. The thiophene-TTA-containing REC fluorophore was able to etch the silver core, resulting in hollow silica shells, consequently displaying no luminescence enhancing capabilities. The etching efficiency was proportional to the concentration of Eu(TTA)3Phen, and decreased with increasing shell thickness.

Luminescent Quantum Clusters of Gold in Bulk by Albumin‐Induced Core Etching of Nanoparticles: Metal Ion Sensing, Metal‐Enhanced Luminescence, and Biolabeling

The synthesis of a luminescent quantum cluster (QC) of gold with a quantum yield of approximately 4 % is reported. It was synthesized in gram quantities by the core etching of mercaptosuccinic acid protected gold nanoparticles by bovine serum albumin (BSA), abbreviated as Au(QC)@BSA. The cluster was characterized and a core of Au(38) was assigned tentatively from mass spectrometric analysis. Luminescence of the QC is exploited as a "turn-off" sensor for Cu(2+) ions and a "turn-on" sensor for glutathione detection. Metal-enhanced luminescence (MEL) of this QC in the presence of silver nanoparticles is demonstrated and a ninefold maximum enhancement is seen. This is the first report of the observation of MEL from QCs. Folic acid conjugated Au(QC)@BSA was found to be internalized to a significant extent by oral carcinoma KB cells through folic acid mediated endocytosis. The inherent luminescence of the internalized Au(QC)@BSA was used in cell imaging.