Shell-Isolated Nanoparticle-Enhanced Phosphorescence.

The emerging field of plasmonics has promoted applications of optical technology, especially in plasmon-enhanced spectroscopy (PES). However, in plasmon-enhanced fluorescence (PEF), "metal loss" could significantly quench the fluorescence during the process, which dramatically limits its applications in analysis and high-resolution imaging. In this report, silver core silica shell-isolated nanoparticles (Ag@SiO2 NPs or SHINs) with a tunable thickness of shell are used to investigate the interactions between NPs and emitters by constructing coupling and noncoupling modes. The plasmonic coupling mode between Ag@SiO2 NPs and Ag film reveals an exceeding integrating spectral intensity enhancement of 330 and about 124 times that of the radiative emission rate acceleration for shell-isolated nanoparticle enhanced phosphorescence (SHINEP). The experimental findings are supported by theoretical calculations using the finite-element method (FEM). Hence, the SHINEP may provide a novel approach for understanding the interaction of plasmon and phosphorescence, and it holds great potential in surface detection analysis and singlet-oxygen-based clinical therapy.

Plasmon-enhanced fluorescence spectroscopy.

Fluorescence spectroscopy with strong emitters is a remarkable tool with ultra-high sensitivity for detection and imaging down to the single-molecule level. Plasmon-enhanced fluorescence (PEF) not only offers enhanced emissions and decreased lifetimes, but also allows an expansion of the field of fluorescence by incorporating weak quantum emitters, avoiding photobleaching and providing the opportunity of imaging with resolutions significantly better than the diffraction limit. It also opens the window to a new class of photostable probes by combining metal nanostructures and quantum emitters. In particular, the shell-isolated nanostructure-enhanced fluorescence, an innovative new mode for plasmon-enhanced surface analysis, is included. These new developments are based on the coupling of the fluorophores in their excited states with localized surface plasmons in nanoparticles, where local field enhancement leads to improved brightness of molecular emission and higher detection sensitivity. Here, we review the recent progress in PEF with an emphasis on the mechanism of plasmon enhancement, substrate preparation, and some advanced applications, including an outlook on PEF with high time- and spatially resolved properties.

Co-Deposition of Gold Nanoparticles and Metalloporphyrin Using the Langmuir-Blodgett (LB) Technique for Surface-Enhanced Raman Scattering (SERS).

The synergistic effect produced by metallic nanoparticles when incorporated into different systems empowers a research field that is growing rapidly. In addition, organometallic materials are at the center of intensive research with diverse applications such as light-emitting devices, transistors, solar cells, and sensors. The Langmuir-Blodgett (LB) technique has proven to be suitable to address challenges inherent to organic devices, since the film properties can be tuned at the molecular level. Here we report a strategy to incorporate gold nanoparticles (AuNPs) into the LB film by co-deposition in order to achieve surface-enhanced Raman scattering (SERS) of the zinc(II)-protoporphyrin (IX) dimethyl ester (ZnPPIX-DME). Prior to the LB co-deposition, the properties of the Langmuir monolayer of ZnPPIX-DME at the air-water interface, containing AuNPs in the subphase, are studied through the surface-pressure versus mean molecular area (π-A) isotherms. The ZnPPIX-DME+AuNPs π-A isotherm presented a significant shift to higher molecular area, suggesting an interaction between both ZnPPIX-DME molecules and AuNPs. Those interactions are a key factor allowing the co-deposition of both AuNPs and ZnPPIX-DME molecules onto a solid substrate, thus forming the LB film. SERS of ZnPPIX-DME was successfully attained, ensuring the spatial distribution of the AuNPs. Higher enhancement factors were found at AuNP aggregates, as a result of the intense local electromagnetic field found in the metal nanoparticle aggregates. The main vibrational bands observed in the SERS spectra suggest a physical adsorption of the ZnPPIX-DME onto the surface of AuNPs. The latter is not only in agreement with the interactions pointed out by the π-A isotherms but also suggests that this interaction is kept upon LB film co-deposition.

Plasmon enhanced fluorescence with aggregated shell-isolated nanoparticles.

Shell-isolated nanoparticles (SHINs) nanostructures provide a versatile substrate where the localized surface plasmon resonances (LSPRs) are well-defined. For SHINEF, the silver (or gold) metal core is protected by the SiO2 coating, which is thicker than the critical distance for minimum quenching by the metal. In the present work, it is shown that an increase in the SHINEF enhancement factor may be achieved by inducing SHIN aggregation with electrolytes in solution. The proof of concept is demonstrated using NaCl as aggregating agent, although other inorganic salts will also aggregate SHIN nanoparticles. As much as a 10-fold enhancement in the SHINEF enhancement factor (EF) may be achieved by tuning the electrolyte concentrations in solution. The SHINEF experiments include the study of the aggregation effect controlling gold SHIN's surface concentration via spraying. Au-SHINs are sprayed onto layer-by-layer (LbL) and Langmuir-Blodgett (LB) films, and samples are fabricated using fluorophores with low and also high quantum yield.

Raman spectroscopy and scanning electron microscopy characterizations of fission track method datable zircon grains.

Spectroscopic and morphological studies, designed to improve our understanding of the physicochemical phenomena that occur during zircon crystallization, are presented. The zircon fission track method (ZFTM) is used routinely in various laboratories around the world; however, there are some methodological difficulties needing attention. Depending on the surface fission track density observed under an optical microscope, the zircon grain surfaces are classified as homogeneous, heterogeneous, hybrid, or anomalous. In this study, zircon grain surfaces are characterized using complementary techniques such as optical microscopy (OM), micro-Raman spectroscopy, and scanning electron microscopy (SEM), both before and after chemical etching. Our results suggest that anomalous grains have subfamilies and that etching anisotropy related to heterogeneous grains is due to different crystallographic faces within the same polished surface that cannot be observed under an optical microscope. The improved methodology was used to determine the zircon fission track ages of samples collected from the Bauru Group located in the north of Paraná Basin, Brazil. A total of 514 zircon grains were analyzed, consisting of 10% homogeneous, about 10% heterogeneous, about 20% hybrid, and 60% anomalous grains. These results show that the age distributions obtained for homogeneous, heterogeneous, and hybrid grains are both statistically and geologically compatible.

Organic acids and protein compounds causing the photoluminescence properties of natural rubber membranes and the quenching phenomena from Au nanoparticle incorporation.

Natural rubber membranes were fabricated using latex from Hevea brasiliensis trees (clone RRIM 600) by casting, and controlling the time and temperature of thermal treatment. Three temperatures were used: 65, 80 and 120 °C and the corresponding annealing times of 6, 8, 10 and 12 h. The centrifugation of the latex produces the constituent phases: solid rubber (F1), serum or protein components (F2) and bottom fraction (F3). The photoluminescence properties could be correlated with organic acid components of latex. Natural rubber membranes were used as the active substrate (reducing agent) for the incorporation of colloidal Au nanoparticles synthesized by in situ reduction at different times. The intensity of photoluminescence bands assigned to the natural rubber decreases with the increase in amount of nanoparticles present on the membrane surface. It can be assumed that Au nanoparticles may be formed by reduction of the Au cation reacting with functional groups that are directly related to photoluminescence properties. However, the quenching of fluorescence may be attributed to the formation of a large amount of metal nanostructures on the natural rubber surface.

Large-area plasmonic substrate of silver-coated iron oxide nanorod arrays for plasmon-enhanced spectroscopy.

One-dimensional iron oxide materials fabricated on conducting glass substrates and their unique properties make these nanostructures promising candidates for a wide range of applications. Herein, vertically oriented α-Fe2O3 nanorod arrays synthesized under hydrothermal conditions over a large area are described, as an active platform for surface-enhanced resonance Raman scattering (SERRS) and surface-enhanced fluorescence (SEF). From scanning electron microscopy images the formation of a homogeneous distribution of vertically oriented rods in a large area is confirmed. For activating the localized surface plasmon resonances, which are responsible for SERRS and SEF, a 6 nm layer of Ag is deposited onto the α-Fe2O3 nanorod arrays by physical vapor deposition to form Ag islands.

SERS mapping in Langmuir-Blodgett films and single-molecule detection.

Plasmon-enhanced spectroscopic techniques have expanded single-molecule detection (SMD) and are revolutionizing areas such as bio-imaging and single-cell manipulation. Surface-enhanced (resonance) Raman scattering (SERS or SERRS) combines high sensitivity with molecular-fingerprint information at the single-molecule level. Spectra originating from single-molecule SERS experiments are rare events, which occur only if a single molecule is located in a hot-spot zone. In this spot, the molecule is selectively exposed to a significant enhancement associated with a high, local electromagnetic field in the plasmonic substrate. Here, we report an SMD study with an electrostatic approach in which a Langmuir film of a phospholipid with anionic polar head groups (PO4(-)) was doped with cationic methylene blue (MB), creating a homogeneous, two-dimensional distribution of dyes in the monolayer. The number of dyes in the probed area of the Langmuir-Blodgett (LB) film coating the Ag nanostructures established a regime in which single-molecule events were observed, with the identification based on direct matching of the observed spectrum at each point of the mapping with a reference spectrum for the MB molecule. In addition, advanced fitting techniques were tested with the data obtained from micro-Raman mapping, thus achieving real-time processing to extract the MB single-molecule spectra.

Plasmon enhanced spectroscopy.

Surface enhanced spectroscopy encompasses a broad field of linear and nonlinear optical techniques that arose with the discovery of the surface-enhanced Raman scattering (SERS) effect. SERS enabled ultrasensitive and single molecule detection with molecular fingerprint specificity, opening the door for a large variety of chemical sensing applications. Basically, from the beginning it was realized that the necessary condition for SERS to be observed was the presence of a metallic nanostructure, and with this condition, the optical enhancement found a home in the field of plasmonics. Although plasmonic practitioners claim that SERS is "the most spectacular application of plasmonics", perhaps it is more appropriate to say that the spectacular development of plasmonics is due to SERS. Here is a brief recollection from surface enhanced spectroscopy to plasmon enhanced spectroscopy.

Biomimetic, mild chemical synthesis of CdTe-GSH quantum dots with improved biocompatibility.

Multiple applications of nanotechnology, especially those involving highly fluorescent nanoparticles (NPs) or quantum dots (QDs) have stimulated the research to develop simple, rapid and environmentally friendly protocols for synthesizing NPs exhibiting novel properties and increased biocompatibility. In this study, a simple protocol for the chemical synthesis of glutathione (GSH)-capped CdTe QDs (CdTe-GSH) resembling conditions found in biological systems is described. Using only CdCl(2), K(2)TeO(3) and GSH, highly fluorescent QDs were obtained under pH, temperature, buffer and oxygen conditions that allow microorganisms growth. These CdTe-GSH NPs displayed similar size, chemical composition, absorbance and fluorescence spectra and quantum yields as QDs synthesized using more complicated and expensive methods.CdTe QDs were not freely incorporated into eukaryotic cells thus favoring their biocompatibility and potential applications in biomedicine. In addition, NPs entry was facilitated by lipofectamine, resulting in intracellular fluorescence and a slight increase in cell death by necrosis. Toxicity of the as prepared CdTe QDs was lower than that observed with QDs produced by other chemical methods, probably as consequence of decreased levels of Cd(+2) and higher amounts of GSH. We present here the simplest, fast and economical method for CdTe QDs synthesis described to date. Also, this biomimetic protocol favors NPs biocompatibility and helps to establish the basis for the development of new, "greener" methods to synthesize cadmium-containing QDs.

Toward the optimization of an e-tongue system using information visualization: a case study with perylene tetracarboxylic derivative films in the sensing units.

The wide variety of molecular architectures used in sensors and biosensors and the large amount of data generated with some principles of detection have motivated the use of computational methods, such as information visualization techniques, not only to handle the data but also to optimize sensing performance. In this study, we combine projection techniques with micro-Raman scattering and atomic force microscopy (AFM) to address critical issues related to practical applications of electronic tongues (e-tongues) based on impedance spectroscopy. Experimentally, we used sensing units made with thin films of a perylene derivative (AzoPTCD acronym), coating Pt interdigitated electrodes, to detect CuCl(2) (Cu(2+)), methylene blue (MB), and saccharose in aqueous solutions, which were selected due to their distinct molecular sizes and ionic character in solution. The AzoPTCD films were deposited from monolayers to 120 nm via Langmuir-Blodgett (LB) and physical vapor deposition (PVD) techniques. Because the main aspects investigated were how the interdigitated electrodes are coated by thin films (architecture on e-tongue) and the film thickness, we decided to employ the same material for all sensing units. The capacitance data were projected into a 2D plot using the force scheme method, from which we could infer that at low analyte concentrations the electrical response of the units was determined by the film thickness. Concentrations at 10 µM or higher could be distinguished with thinner films--tens of nanometers at most--which could withstand the impedance measurements, and without causing significant changes in the Raman signal for the AzoPTCD film-forming molecules. The sensitivity to the analytes appears to be related to adsorption on the film surface, as inferred from Raman spectroscopy data using MB as analyte and from the multidimensional projections. The analysis of the results presented may serve as a new route to select materials and molecular architectures for novel sensors and biosensors, in addition to suggesting ways to unravel the mechanisms behind the high sensitivity obtained in various sensors.

Surface-enhanced spectra on D-gluconic acid coated silver nanoparticles.

Coated silver (Ag) colloids synthesized with D-glucose permit the observation of surface-enhanced fluorescence (SEF) and surface-enhanced resonance Raman scattering (SERRS) of the rhodamine B (RhB) molecule. The organic coating formed during the synthesis of the Ag nanostructures was identified by its surface-enhanced Raman scattering (SERS) spectrum as D-gluconic acid. The RhB molecule is used to exemplify the distance dependence of SEF and SERRS on the coated Ag nanostructures. The fluorescence enhancement factor for RhB on D-gluconic acid coated silver nanoparticles was determined experimentally and estimated using a simple model. Further support for the plasmon enhancement is obtained from the fact that the measured fluorescence lifetime of RhB on the silver coated with D-gluconic acid is shorter than that found on a glass surface. A very modest enhancement factor is obtained, as expected for very short distance between RhB and the metal surface. Given the very thin metal-fluorophore separation, estimated from the size of the D-gluconic acid, the energy transfer or fluorescence quenching is still efficient and the SEF enhancement is just overcoming the energy transfer. Therefore, both SEF and SERRS are observed. Notably, the aggregation of coated nanoparticles also increases the enhancement factor for SEF.

Plasmon-enhanced resonance Raman scattering and fluorescence in Langmuir-Blodgett monolayers.

Localized surface plasmon resonances in silver and gold nanostructures are engaged to enhance the inelastic Raman scattering and the fluorescence of a phopholipid containing a sulforhodamine 101 acid chloride dye known as Texas Red. The most efficient coupling and enhancement are attained when the excitation laser line is in resonance with both the chromophore and the plasmon absorption of the nanostructure, the case of surface-enhanced resonance Raman scattering, allowing single-molecule detection. The tagged phospholipid was incorporated into a single fatty acid Langmuir monolayer at varying concentrations and transferred onto an evaporated Ag nanoparticle film. Surface-enhanced fluorescence is achieved using shell-isolated silica-coated gold nanoparticles, an emission enhancement named SHINEF.

Chromic materials for responsive surface-enhanced resonance Raman scattering systems: a nanometric pH sensor.

The use of chromic materials for responsive surface-enhanced resonance Raman scattering (SERRS) based nanosensors is reported. The potential of nano-chromic SERRS is demonstrated with the use of the halochrome methyl yellow to fabricate an ultrasensitive pH optical sensor. Some of the challenges of the incorporation of chromic materials with metal nanostructures are addressed through the use of computational calculations and a comparison to measured SERRS and surface-enhanced Raman scattering (SERS) spectra is presented. A strong correlation between the measured SERRS and the medium's proton concentration is demonstrated for the pH range 2-6. The high sensitivity achieved by the use of resonance Raman conditions is shown through responsive SERRS measurements from only femtolitres of volume and with the concentration of the reporting molecules approaching the single molecule regime.

Synthesis of silver nanoparticles with controllable surface charge and their application to surface-enhanced Raman scattering.

The objective of the present work was to explore new methods of synthesis of silver nanocolloids using amino acids as reducing agents. The goal of the study was to fabricate nanostructures with controllable surface charge (zeta potential) that may allow optimizing the adsorption of target molecules for ultrasensitive analysis using surface-enhanced Raman scattering (SERS). The average SERS properties of these colloids are tested with two organic analytes and compared with those obtained by using the most commonly used citrate Ag sols.

The observation of high order overtones and combinations in the SERRS spectra of a perylene dye spin coated onto silver island films.

The intensity of high harmonics and the background observed in the SERRS spectra of the bis(4-chlorbenzylimido)perylene (CLPTCD), spin-coated onto silver nanostructures are discussed. The SERRS spectra of an Ag island film spin coated with 50 microL of a 8.7 x 10(-7) M of CLPTCD in CH2Cl2 excited at 514.5 nm show signals from the first to the third overtones and combinations of fundamental vibrational modes observed between 1200 and 1600 cm(-1). SERRS spectra were obtained where 2nd overtones were seen with relative intensity equal to that of the 1st overtones and in other SERRS spectra the 3rd overtones were more intense than the 2nd overtones. These varying intensity patterns are seen at submonolayer coverage where there is a breakdown of statistical average, and the spatial distribution of enhancement factors becomes evident. The apparent correlation between the background intensity profile and the enhancement of the overtones and combinations is also discussed.

Fabrication, structural characterization, and applications of langmuir and langmuir-blodgett films of a poly(azo)urethane.

The synthesis of a poly(azo)urethane by fixing CO(2) in bis-epoxide followed by a polymerization reaction with an azodiamine is presented. Since isocyanate is not used in the process, it is termed "clean method" and the polymers obtained are named "NIPUs" (non-isocyanate polyurethanes). Langmuir films were formed at the air-water interface and were characterized by surface pressure vs mean molecular area per mer unit (Pi-A) isotherms. The Langmuir monolayers were further studied by running stability tests and cycles of compression/expansion (possible hysteresis) and by varying the compression speed of the monolayer formation, the subphase temperature, and the solvents used to prepare the spreading polymer solutions. The Langmuir-Blodgett (LB) technique was used to fabricate ultrathin films of a particular polymer (PAzoU). It is possible to grow homogeneous LB films of up to 15 layers as monitored using UV-vis absorption spectroscopy. Higher number of layers can be deposited when PAzoU is mixed with stearic acid, producing mixed LB films. Fourier transform infrared (FTIR) absorption spectroscopy and Raman scattering showed that the materials do not interact chemically in the mixed LB films. The atomic force microscopy (AFM) and micro-Raman technique (optical microscopy coupled to Raman spectrograph) revealed that mixed LB films present a phase separation distinguishable at micrometer or nanometer scale. Finally, mixed and neat LB films were successfully characterized using impedance spectroscopy at different temperatures, a property that may lead to future application as temperature sensors. Principal component analysis (PCA) was used to correlate the data.

Single molecule analysis by surfaced-enhanced Raman scattering.

Our main objective in this tutorial review is to provide insight into some of the questions surrounding single molecule detection (SMD) using surface-enhanced Raman scattering (SERS) and surface-enhanced resonance Raman scattering (SERRS). Discovered thirty years ago, SERS is now a powerful analytical tool, strongly tied to plasmonics, a field that encompasses and profits from the optical enhancement found in nanostructures that support localized plasmon excitations. The spectrum of the single molecule carries the quantum fingerprints of the system modulated by the molecule-nanostructure interactions and the electronic resonances that may result under laser excitation. This information is embedded in vibrational band parameters. The dynamics and the molecular environment will affect the bandwidth of the observed Raman bands. In addition, the localized surface plasmon resonances (LSPR) empower the nanostructure with a number of optical properties that will also leave their mark on the observed inelastic scattering process. Therefore, controlling size, shape and the formation of the aggregation state (or fractality) of certain metallic nanostructures becomes a main task for experimental SERS/SERRS. This molecule-nanostructure coupling may, inevitably, lead to spectral fluctuations, increase photobleaching or photochemistry. An attempt is made here to guide the interpretation of this wealth of information when approaching the single molecule regime.