Review of Some Interesting Surface Plasmon Resonance-enhanced Properties of Noble Metal Nanoparticles and Their Applications to Biosystems

Noble metal, especially gold (Au) and silver (Ag) nanoparticles exhibit unique and tunable optical properties on account of their surface plasmon resonance (SPR). In this review, we discuss the SPR-enhanced optical properties of noble metal nanoparticles, with an emphasis on the recent advances in the utility of these plasmonic properties in molecular-specific imaging and sensing, photo-diagnostics, and selective photothermal therapy. The strongly enhanced SPR scattering from Au nanoparticles makes them useful as bright optical tags for molecular-specific biological imaging and detection using simple dark-field optical microscopy. On the other hand, the SPR absorption of the nanoparticles has allowed their use in the selective laser photothermal therapy of cancer. We also discuss the sensitivity of the nanoparticle SPR frequency to the local medium dielectric constant, which has been successfully exploited for the optical sensing of chemical and biological analytes. Plasmon coupling between metal nanoparticle pairs is also discussed, which forms the basis for nanoparticle assembly-based biodiagnostics and the plasmon ruler for dynamic measurement of nanoscale distances in biological systems.     

Optimization of immunolabeled plasmonic nanoparticles for cell surface receptor analysis

Noble metal nanoparticles hold great potential as optical contrast agents due to a unique feature, known as the plasmon resonance, which produces enhanced scattering and absorption at specific frequencies. The plasmon resonance also provides a spectral tunability that is not often found in organic fluorophores or other labeling methods. The ability to functionalize these nanoparticles with antibodies has led to their development as contrast agents for molecular optical imaging. In this review article, we present methods for optimizing the spectral agility of these labels. We discuss synthesis of gold nanorods, a plasmonic nanoparticle in which the plasmonic resonance can be tuned during synthesis to provide imaging within the spectral window commonly utilized in biomedical applications. We describe recent advances in our group to functionalize gold and silver nanoparticles using distinct antibodies, including EGFR, HER-2 and IGF-1, selected for their relevance to tumor imaging. Finally, we present characterization of these nanoparticle labels to verify their spectral properties and molecular specificity.     

Solvent-Induced Morphological Changes of Polyhedral Silver Nanoparticles in Epoxy Resin

The use of just one material as reducing and capping agent during the synthesis of noble metal nanoparticles is of great interest for potential applications. This paper reports a simple method to prepare polyhedral silver nanoparticles at 80 °C using an epoxy resin (Araldite 506) as both reducing and capping agent. The formation of metal nanoparticles was investigated by Fourier-transform infrared spectroscopy, atomic force microscopy, transmission electronic microscopy, high-resolution transmission electronic microscopy and UV-vis spectroscopy. The proposed mechanism for the reduction of silver ions involves radicals as precursors of ketones and other products originated by thermo-oxidation of the resin. The nucleation of small spherical and polyhedral seeds (～7 nm) of polycrystalline silver and twin planes lead to big polyhedral silver nanoparticles of average size, 68 nm. On the other hand, the polyhedral silver nanoparticles dispersed in toluene changed to prolate-like particles, and their dispersion in dimethyl-sulphoxide and formamide originated elongated polyhedrons and concave nanostructures, respectively. These structural changes lead to unusual solvent-induced optical properties. For instance, the polyhedral nanoparticles dispersed in toluene red-shifted their surface plasmon resonance from 425 to 540 nm, in dimethyl-sulphoxide the spectrum exhibited a peak at 418 nm and a shoulder at 520 nm, and for the silver nanoparticles in formamide a broad band with maximum peak at 420 nm was observed. It is showed that the solvent/resin system works itself as structure-directing agent of silver nanoparticles. These results open the doors to achieve silver nanostructures highly sensitive to the dielectric environment, an ideal condition for applications in colorimetric sensors of molecules of biological or chemical interest.     

Ultrasensitive and selective detection of copper (II) and mercury (II) ions by dye-coded silver nanoparticle-based SERS probes

A simple and distinctive method for the ultrasensitive detection of Cu(2+) and Hg(2+) based on surface-enhanced Raman scattering (SERS) using cysteine-functionalized silver nanoparticles (AgNPs) attached with Raman-labeling molecules was developed. The glycine residue in a silver nanoparticle-bound cysteine can selectively bind with Cu(2+) and Hg(2+) and form a stable inner complex. Silver nanoparticles co-functionalized with cysteine and 3,5-Dimethoxy-4-(6'-azobenzotriazolyl)phenol (AgNP conjugates) can be used to detect Cu(2+) and Hg(2+) based on aggregation-induced SERS of the Raman tags. The addition of SCN(-) to the analyte can successfully mask Hg(2+) and allow for the selective detection of Cu(2+). This SERS-based assay showed an unprecedented limit of detection (LOD) of 10pM for Cu(2+) and 1pM for Hg(2+); these LODs are a few orders of magnitude more sensitive than the typical colorimetric approach based on the aggregation of noble nanoparticles. The analysis of real water samples diluted with pure water was performed and verified this conclusion. We envisage that this SERS-based assay may provide a general and simple approach for the detection of other metal ions of interest, which can be adopted from their corresponding colorimetric assays that have already been developed with significantly improved sensitivity and thus have wide-range applications in many areas.     

Enhanced Nonlinear Optical Response of Resonantly Coupled Silver Nanoparticle–Organic Dye Complexes

The influence of metal nanoparticles on linear and nonlinear optical properties of surrounding organic molecules has been widely investigated, whereas much less attention has been paid to the influence of molecules on properties of nanoparticles. Here, we employ transient absorption spectroscopy to address the nonlinear optical responses of the resonantly coupled silver nanoparticle–organic dye systems and demonstrate that silver nanoparticles covered with dye molecules show enhanced and spectrally different nonlinear extinction changes from pristine nanoparticles. We identify changes of the plasmon resonance band of nanoparticles induced by excitation of surrounding dye. We attribute these exciton–plasmon coupling effects to the excitation-induced refractive index modifications of the dye layer surrounding a nanoparticle and to the back-transfer of the oscillator strength borrowed by the dye from the nanoparticle.     

Optical Absorption in Overcoats of Nanoparticle Arrays on a Metallic Substrate

Surface plasma oscillations in metallic particles as well as in thin metallic films have been studied extensively in the past decades. New features regarding surface plasma excitations are, however, constantly discovered, leading, for example, to surface-enhanced Raman scattering studies and enhanced optical transmission though metal films with nanohole arrays. In the present work, the role of a metallic substrate is examined in two cases, one involving an overcoat of dielectric nanoparticles and the other an overcoat of metallic nanoparticles. Theoretical results are obtained by modeling the nanoparticles as forming a two-dimensional, hexagonal lattice of spheres. The scattered electromagnetic field is then calculated using a variant of the Green function method. Comparison with experimental results is made for nanoparticles of tungsten oxide and tin oxide deposited on either gold or silver substrates, giving qualitative agreement on the extra absorption observed when the dielectric nanoparticles are added to the metallic surfaces. Such absorption would be attributed to the mirror image effects between the particles and the substrate. On the other hand, calculations of the optical properties of silver or gold nanoparticle arrays on a gold or a silver substrate demonstrate very interesting features in the spectral region from 400 to 1,000 nm. Interactions between the nanoparticle arrays surface plasmons and their images in the metallic substrate would be responsible for the red shift observed in the absorption resonance. Moreover, effects of particle size and ambient index of refraction are studied, showing a great potential for applications in biosensing with structures consisting of metallic nanoparticle arrays on metallic substrates.     

Optically Stimulated Luminescence Under Plasmon Resonance Conditions Enhances X-Ray Detection

Plasmon enhancement of luminescence close to noble metal nanoparticles is a powerful tool for many optical purposes. Although the plasmon properties of noble metal nanoparticles found application in many different areas, no reports on their use to detect ionizing radiation exist. Here, we investigate the use of the localized surface plasmon resonance of noble metal nanoparticles, to obtain plasmon-enhanced optically stimulated luminescence (OSL). The OSL intensity depends on particle size: we observed enhanced OSL in samples containing silver nanoparticles and quenched OSL in samples bearing silver microparticles. The local field close to the nanoparticles surface under surface plasmon resonance condition increased the excitation rate of the X-ray-generated F centers, enhancing luminescence. Moreover, noble metal nanoparticles can also concentrate luminescent centers close to their surface, leading to a synergistic effect that facilitates detection and intensify OSL. These promising findings may give rise to a new class of ionizing radiation detectors. Figure

OSL intensity dependence on concentration of nanoparticles and microparticles of a NaCl/Ag composite. For nanoparticles, there is a sustained enhancement with mass content.     

Mycoendophytes as efficient synthesizers of bionanoparticles: nanoantimicrobials,mechanism, and cytotoxicity

Mycoendophytes are the fungi that occur inside the plant tissues without exerting any negative impact on the host plant. They are most frequently isolated endophytes from the leaf, stem, and root tissues of various plants. Among all fungi, the mycoendophytes as biosynthesizer of noble metal nanoparticles (NPs) are less known. However, some reports showing efficient synthesis of metal nanoparticles, mainly silver nanoparticles and its remarkable antimicrobial activity against bacterial and fungal pathogens of humans and plants. The nanoparticles synthesized from mycoendophytes present stability, polydispersity, and biocompatibility. These are non-toxic to humans and environment, can be gained in an easy and cost-effective manner, have wide applicability and could be explored as promising candidates for a variety of biomedical, pharmaceutical, and agricultural applications. Mycogenic silver nanoparticles have also demonstrated cytotoxic activity against cancer cell lines and may prove to be a promising anticancer agent. The present review focuses on the biological synthesis of metal nanoparticles from mycoendophytes and their application in medicine. In addition, different mechanisms of biosynthesis and activity of nanoparticles on microbial cells, as well as toxicity of these mycogenic metal nanoparticles, have also been discussed.     

Study of Broadband Tunable Properties of Surface Plasmon Resonances of Noble Metal Nanoparticles Using Mie Scattering Theory: Plasmonic Perovskite Interaction

We suggest semi-analytical approach to study the optical properties of noble metal nanoparticles and their interaction to the perovskite material (methyl ammonia lead halide: CH₃NH₃PbI₃). Metal nanoparticles embedded in perovskite matrix exhibits broadband surface plasmon resonances, and the tunability of these plasmonic resonances is highly sensitive to particle size. The calculation of optical cross section have been done using Mie scattering theory which is applicable to arbitrary size and spherical-shape metal nanoparticles. We have taken five different radii ranging from 15 to 100 nm to understand the plasmonic resonances and its spectral width in the wavelength range 300 to 800 nm. Out of these noble metal nanoparticles, silver have highest scattering efficiency nearly of the order of 18 for the case of 15 nm radii at resonance wavelength 613 nm. Our finding reveals a new concept to understand the applications of plasmonic resonances in order to enhance the photon absorption inside the thin film of perovskite.     

Refractive Index Sensitivity Analysis of Ag, Au, and Cu Nanoparticles

The localized surface plasmon resonance (LSPR) spectrum of noble metal nanoparticles is studied by quasi-static approximation. Taking the sensitivity of LSPR shape to the size and shape of nanoparticle along with surrounding refractive index, parameters like refractive index sensitivity and sensing figure of merit have been determined. In the present analysis from the sensing relevant parameters, it is concluded that Ag represents a better sensing behavior than Au and Cu over the entire visible to infrared regime of EM spectrum.     

Synthesis and Optical Properties of Highly Stabilized Peptide-Coated Silver Nanoparticles

The interaction between peptide and silver nanoparticle surfaces has been increasingly of interest for bionanotechnology applications. To fully understand how to control such interactions, we have studied the optical properties of peptide-modified silver nanoparticles. However, the impacts of peptide binding motif upon the surface characteristics and physicochemical properties of nanoparticles remain not yet fully understood. Here, we have prepared sodium citrate-stabilized silver nanoparticles and coated with peptide IVD (ID₃). These nanomaterials were characterized by UV-visible, transmission electron microscopy (TEM), and z-potential measurement. The results indicate that silver nanoparticles (AgNP)-peptide interface is generated using ID₃ peptide and suggested that the reactivity of peptide is governed by the conformation of the bound peptide on the nanoparticle surface. The peptide-nanoparticle interactions could potentially be used to make specific functionality into the peptide capped nanomaterials and antibacterial applications.     

Fluorescence modulation of 1,7‐bis(4‐hydroxy‐3‐methoxyphenyl)‐1,6‐heptadiene‐3,5‐dione by silver nanoparticles and its possible analytical application

The effects of silver nanoparticles on the photophysical properties of 1,7‐bis(4‐hydroxy‐3‐methoxyphenyl)‐1,6‐heptadiene‐3,5‐dione, popularly known as curcumin, have been investigated using optical absorption and fluorescence techniques. Although absorption spectroscopy suggests a ground‐state complex formation, fluorescence quenching data confirms a simultaneous static and dynamic quenching, inferring ground as well as excited‐state complex formation. The recovery of fluorescence quenching of the curcumin–silver nanoparticle complex in the presence of ascorbic acid or uric acid emphasizes a strong interaction between the silver nanoparticles and ascorbic acid/uric acid, suggesting that fluorescence recovery after the quenching of curcumin–silver nanoparticle complexes has potential for ascorbic acid or uric acid assay development. Copyright © 2011 John Wiley & Sons, Ltd.     

Multifunctional bionanocomposite films of poly(lactic acid), cellulose nanocrystals and silver nanoparticles

Nanocomposite films were prepared by the addition of cellulose nanocrystals (CNCs) eventually surfactant modified (s-CNC) and silver (Ag) nanoparticles in the polylactic acid (PLA) matrix using melt extrusion followed by a film formation process. Multifunctional composite materials were investigated in terms of morphological, mechanical, thermal and antibacterial response. The nanocomposite films maintained the transparency properties of the PLA matrix. Thermal analysis showed increased values of crystallinity in the nanocomposites, more evident in the s-CNC based formulations that had the highest tensile Young modulus. The presence of surfactant favoured the dispersion of cellulose nanocrystals in the polymer matrix and the nucleation effect was remarkably enhanced. Moreover, an antibacterial activity against Staphylococcus aureus and Escherichia coli cells was detected for ternary systems, suggesting that these novel nanocomposites may offer good perspectives for food packaging applications which require an antibacterial effect constant over time.     

Colorimetric and luminescent bifunctional Ru(II) complex-modified gold nano probe for sensing of DNA

A colorimetric and luminescent bifunctional Ru(II) complex-modified gold nano-probe for sensing ctDNA was developed. A new water-soluble Ru(II) complex containing an aromatic α-diimine ligand with an extended π system was found to be emissive at about 588nm in water and the emission intensity of the complex was enhanced about 3.4-fold upon addition of calf thymus DNA (ctDNA) in aqueous buffer with no color changes. The detection limit was 70nM with a 1.2% relative standard deviation (RSD, n=5). The Ru(II) complex chromophore was used to modify 8nm MHA carboxylate-functionalized gold nanoparticles to produce a colorimetric and luminescent bifunctional probe for sensing ctDNA in aqueous buffer at room temperature. The obvious change in visible color and enhancement of emission intensity of the functionalized gold-Ru(II) complex colloids upon addition of ctDNA was due to the electron or energy transfer between Ru(II) chromophore and the Au-NPs. The limit of detection was 1.0nM for ctDNA with a 4.5% RSD (n=5). With such a high sensitivity, the bifunctional Ru(II) complex-modified gold nano-probe will be potentially suitable for the DNA sensing in bioanalytical application.     

Determination of Fundamental Morphological Parameters of Supported Nanoparticle Ensembles

A new model to extract important morphological parameters of noble metal nanoparticle ensembles with a broad size and shape distribution is presented. The technique is based on a rigorous simulation of the inhomogeneously broadened extinction profiles of nanoparticle ensembles. As input data, only experimentally accessible parameters, such as the amount of deposited material, the nanoparticle number density, and the relative size distribution of the nanoparticles, are used. The model can be applied to oblate nanoparticles, which exhibit a strong correlation between their shape and size, e.g., to supported nanoparticles generated, for example, by deposition of atoms and subsequent nucleation or by gas phase deposition. Both methods are standard preparation techniques to generate well-defined nanoparticle ensembles under ultra high vacuum conditions. We apply our model to gold and silver nanoparticles on sapphire and TiO₂ supports and obtain a perfect agreement between the calculated and experimental data. More importantly, we could extract the functional dependence between the axial ratio and the radius of the nanoparticles within the ensemble and, therewith, the most probable axial ratio in the ensemble. In addition, the extinction spectrum of a nanoparticle ensemble irradiated with nanosecond pulsed laser light during growth has been successfully modeled. This demonstrates, that the model is able to describe shape changes of resonantly heated nanoparticles within the ensemble. By using the coverage as a free parameter, we could calculate from the extinction spectrum the average particle radius as well as the amount of desorbed atoms after irradiation with laser light. In summary, the model allows a fast, easy, but extensive morphological characterization of nanoparticle ensembles that exhibit a broad size and shape distribution.     

Preparation of aptamer-linked gold nanoparticle purple aggregates for colorimetric sensing of analytes

Aptamers are single-stranded DNA or RNA molecules that can bind target molecules with high affinity and specificity. The conformation of an aptamer usually changes upon binding to its target analyte, and this property has been used in a wide variety of sensing applications, including detection based on fluorescence intensity, polarization, energy transfer, electrochemistry or color change. Colorimetric sensors are particularly important because they minimize or eliminate the necessity of using expensive and complicated instruments. Among the many colorimetric sensing strategies, metallic nanoparticle-based detection is desirable because of the high extinction coefficients and strong distance-dependent optical properties of the nanoparticles. Here, we describe a protocol for the preparation of aptamer-linked gold nanoparticle purple aggregates that undergo fast disassembly into red dispersed nanoparticles upon binding of target analytes. This method has proved to be generally applicable for colorimetric sensing of a broad range of analytes. The time range for the entire protocol is approximately 5 d, including synthesis and functionalization of nanoparticles, preparation of nanoparticle aggregates and sensing.     

Photoconductivity of Silver Nanoparticle Ensembles on Quartz Glass (SiO2) Supports Assisted by Localized Surface Plasmon Excitations

We have studied the conductivity and photoconductivity in silver nanoparticle ensembles on quartz glass substrates. We observed a significant increase of the photoconductivity if the localized surface plasmon resonance in the metal nanoparticles was excited. A detailed analysis of the temperature dependence of the conductivity as well as dependences of the conductivity and photoconductivity on the amount of deposited metal led to the mechanism of the charge transfer in these structures. We found that the primary role in this mechanism is due to defects in the quartz glass structure which act as traps for electrons.     

Nanosilver—The burgeoning therapeutic molecule and its green synthesis

Nanotechnology is a field that is burgeoning day by day, making an impact in all spheres of human life. Biological methods of synthesis have paved way for the “greener synthesis” of nanoparticles and these have proven to be better methods due to slower kinetics, they offer better manipulation and control over crystal growth and their stabilization. This has motivated an upsurge in research on the synthesis routes that allow better control of shape and size for various nanotechnological applications. Nanosilver has developed as a potent antibacterial, antifungal, anti-viral and anti-inflammatory agent. The recent advancement in the field includes the enzymatic method of synthesis suggesting enzymes to be responsible for the nanoparticle formation. The biomedical applications of silver nanoparticle can be effective by the use of biologically synthesized nanoparticles which minimize the factors such as toxicity and cost and are found to be exceptionally stable. The targeting of cancer cells using silver nanoparticles has proven to be effective, but neither the exact mechanism of action nor the modes of activation of the downstream signaling molecules have been revealed yet. The review illustrates a probable signaling pathway and mechanism by which silver nanoparticles target the cancer cells. The current review also examines the historical background of nanoparticles, role of silver nanoparticles in various biomedical applications and also focusing on better methods of the synthesis of nanoparticles.     

Biogenic synthesis of silver nanoparticles using Brazilian propolis

Biological methods have been used to synthesize silver nanoparticles through materials such as bacteria, fungi, plants, and propolis due to their reducing properties, stabilizer role and environmentally friendly characteristic. Considering the antimicrobial activity of propolis as well as the broad-spectrum antibacterial effects of silver nanoparticles, this study aim to describe the use of Brazilian propolis to synthesize silver nanoparticles (AgNP-P) and investigate its antimicrobial activity. The synthesis was optimized by factorial design, choosing the best conditions for smaller size particles. AgNP-P demonstrated a maximum absorbance at 412 nm in ultraviolet-visible spectra, which indicated a spherical format and its formation. Dynamic light scattering demonstrated a hydrodynamic size of 109 nm and polydispersity index less than 0.3, showing a good size distribution and stability. After its purification via centrifugation, microscopy analysis corroborates the format and showed the presence of propolis around silver nanoparticle. X-ray diffraction peaks were attributed to the main planes of the metallic silver crystalline structure; meanwhile infrared spectroscopy demonstrated the main groups responsible for silver reduction, represented by ∼22% of AgNP-P indicates by thermal analysis. Our product revealed an important antimicrobial activity indicating a synergism between propolis and silver nanoparticles as expected and promising to be an effective antimicrobial product to be used in infections.     

Engineering Self‐Supported Noble Metal Foams Toward Electrocatalysis and Beyond

Noble metals, despite their expensiveness, display irreplaceable roles in widespread fields. To acquire novel physicochemical properties and boost the performance‐to‐price ratio for practical applications, one core direction is to engineer noble metals into nanostructured porous networks. Noble metal foams (NMFs), featuring self‐supported, 3D interconnected networks structured from noble‐metal‐based building blocks, have drawn tremendous attention in the last two decades. Inheriting structural traits of foams and physicochemical properties of noble metals, NMFs showcase a variety of interesting properties and impressive prospect in diverse fields, including electrocatalysis, heterogeneous catalysis, surface‐enhanced Raman scattering, sensing and actuation, etc. A number of NMFs have been created and versatile synthetic approaches have been developed. However, because of the innate limitation of specific methods and the insufficient understanding of formation mechanisms, flexible manipulation of compositions, structures, and corresponding properties of NMFs are still challenging. Thus, the correlations between composition/structure and properties are seldom established, retarding material design/optimization for specific applications. This review is devoted to a comprehensive introduction of NMFs ranging from synthesis to applications, with an emphasis on electrocatalysis. Challenges and opportunities are also included to guide possible research directions in this field and promote the interest of interdisciplinary scientists.