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.

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.

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.

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.

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.

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.

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.

Vibrational and surface-enhanced Raman spectra of 1,6-diphenyl-1,3,5-hexatriene.

The vibrational spectra and surface-enhanced Raman scattering (SERS) of 1,6-diphenyl-1,3,5-hexatriene (DPH) are discussed. The fundamental vibrational frequencies, overtones, and combinations observed in the infrared and Raman spectra of DPH are reported. The interpretation of the observed vibrational spectra was supported by a complete geometry optimization, followed by vibrational frequency and intensity computations for the cis- and trans- isomers of the DPH using density functional theory at the B3LYP/6-31G(d,p) level of theory. Because the molecule is photo-chemically active on Ag metal surfaces, the best SERS results for silver islands were obtained at low temperature and low energy density of the exciting laser line. DPH SERS on Au films was obtained at room temperature.

Role of nanoparticle surface charge in surface-enhanced Raman scattering.

In this work, the role of nanoparticle surface charge in surface-enhanced Raman scattering (SERS) is examined for the common case of measurements made in colloidal solutions of Ag and Au. Average SERS intensities obtained for several analytes (salicylic acid, pyridine, and 2-naphthalenethiol) on Ag and Au colloids are correlated with the pH and zeta potential (zeta) values of the nanoparticle solutions from which they were recorded. The consequence of the electrostatic interaction between the analyte and the metallic nanoparticle is stressed. The zeta potentials of three commonly used colloidal solutions are reported as a function of pH, and a discussion is given on how these influence SERS intensity. Also examined is the importance of nanoparticle aggregation (and colloidal solution collapse) in determining SERS intensities, and how this varies with the pH of the solution. The results show that SERS enhancement is highest at zeta potential values where the colloidal nanoparticle solutions are most stable and where the electrostatic repulsion between the particles and the analyte molecules is minimized. These results suggest some important criteria for consideration in all SERS measurements and also provide important insights into the problem of predicting SERS activities for different molecular systems.

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.

Photoinduced phenomena in layer-by-layer films of poly(allylamine hydrochloride) and Brilliant Yellow azodye.

The nanoscale interactions between adjacent layers of layer-by-layer (LBL) films from poly(allylamine hydrochloride) (PAH) and azodye Brilliant Yellow (BY) have been investigated, with the films employed for optical storage and the formation of surface-relief gratings. Using Fourier transform infrared spectroscopy, we identified interactions involving SO3- groups from BY and NH+ groups from PAH. These electrostatic interactions were responsible for the slow kinetics of writing in the optical storage experiments, due to a tendency to hinder photoisomerization and the subsequent reorientation of the azochromophores. The photoinduced birefringence did not saturate after one hour of exposure to the writing laser, whereas in azopolymer films, saturation is normally reached within a few minutes. On the other hand, the presence of such interactions prevented thermal relaxation of the chromophores after the writing laser was switched off, leading to a very stable written pattern. Moreover, the nanoscale interactions promoted mass transport for photoinscription of surface-relief gratings on PAH/BY LBL films, with the azochromophores being able to drag the inert PAH chains when undergoing the trans-cis-trans photoisomerization cycles. A low level of chromophore degradation was involved in the SRG photoinscription, which was confirmed with micro-Raman and fluorescence spectroscopies.

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.

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.

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.

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.