Surface-enhanced fluorescence of fluorescein-labeled oligonucleotides capped on silver nanoparticles

Molecularly imprinted polymers-isolated AuNP-enhanced CdTe QD fluorescence sensor for selective and sensitive oxytetracycline detection in real water samples

A molecularly imprinted polymers (MIPs)-isolated AuNP-enhanced fluorescence sensor, AuNP@MIPs-CdTe QDs, was developed for highly sensitive and selective detection of oxytetracycline (OTC) in aqueous medium. The developed sensor combined the advantages of strong fluorescence signal of metal-enhanced fluorescence (MEF), high selectivity of MIPs, and stability of CdTe QDs. The MIPs shell with specific recognition served as an isolation layer to adjust the distance between AuNP and CdTe QDs to optimize the MEF system. The sensor demonstrated the detection limit as low as 5.22 nM (2.40 μg/L) for a concentration range of 0.1-3.0 μM OTC and good recovery rates of 96.0-103.0% in real water samples. In addition, high specificity recognition for OTC over its analogs was achieved with an imprinting factor of 6.10. Molecular dynamics (MD) simulation was utilized to simulate the polymerization process of MIPs and revealed H-bond formation as the mainly binding sites of APTES and OTC, and finite-difference time-domain (FDTD) analysis was employed to obtain the distribution of electromagnetic field (EM) for AuNP@MIPs-CdTe QDs. The experimental results combined with theoretical analyses not only provided a novel MIP-isolated MEF sensor with excellent detection performance for OTC but also established a theoretical basis for the development of a new generation of sensors.

Enhancement of Fluorescence Mediated by Silver Nanoparticles: Implications for Cell Imaging

In this study, we report the surface enhanced fluorescence (SEF) of a biologically important organic dye, fluorescein (FL), by silver nanoparticles (Ag NPs) in an aqueous medium and its implications for human cell imaging. The as-synthesized Ag NPs were characterized by dynamic light scattering (DLS), zeta potential, transmission electron microscopy (TEM), and UV-vis absorption spectroscopic studies. The interaction and aggregation of FL dye with Ag NPs and a cationic surfactant, namely, cetyltrimethylammonium bromide (CTAB), were explored by UV-vis absorption and steady-state and time-resolved fluorescence spectroscopic methods. The distance-dependent fluorescence enhancement of FL due to Ag NPs in the solution was also theoretically correlated by three-dimensional finite-difference time-domain (3D-FDTD) simulation. The plasmonic coupling between neighboring NPs facilitated the augmentation of the local electric field, thereby producing various "hotspots" that influence the overall fluorescence of the emitter. J-type aggregates of FL in the presence of the CTAB micelles and Ag NP mixed solution were confirmed by electronic spectroscopy. The density functional theoretical (DFT) study revealed the electronic energy levels associated with different forms of FL dye in the aqueous solution. Most interestingly, the Ag NP/FL mixed system used in fluorescence imaging of human lung fibroblast cells (WI 38 cell line) showed a significantly stronger green fluorescence signal compared to that of FL after an incubation period of only 3 h. This study confirms that the Ag NP mediated SEF phenomenon of the FL dye is also manifested in the intracellular medium of human cells giving a brighter and more intense fluorescence image. The cell viability test after exposure to the Ag NP/FL mixed system was confirmed by the MTT assay method. The proposed study may have an implication as an alternate approach for human cell imaging with higher resolution and more contrast.

Asymmetric parameter enhancement in the split-ring cavity array for virus-like particle sensing

Quantitative detection of virus-like particles under a low concentration is of vital importance for early infection diagnosis and water pollution analysis. In this paper, a novel virus detection method is proposed using indirect polarization parametric imaging method combined with a plasmonic split-ring nanocavity array coated with an Au film and a quantitative algorithm is implemented based on the extended Laplace operator. The attachment of viruses to the split-ring cavity breaks the structural symmetry, and such asymmetry can be enhanced by depositing a thin gold film on the sample, which allows an asymmetrical plasmon mode with a large shift of resonance peak generated under transverse polarization. Correspondingly, the far-field scattering state distribution encoded by the attached virus exhibits a specific asymmetric pattern that is highly correlated to the structural feature of the virus. By utilizing the parametric image sinδ to collect information on the spatial photon state distribution and far-field asymmetry with a sub-wavelength resolution, the appearance of viruses can be detected. To further reduce the background noise and enhance the asymmetric signals, an extended Laplace operator method which divides the detection area into topological units and then calculates the asymmetric parameter is applied, enabling easier determination of virus appearance. Experimental results show that the developed method can provide a detection limit as low as 56 vp/150µL on a large scale, which has great potential in early virus screening and other applications.

Enhanced Photoluminescence of R6G Dyes from Metal Decorated Silicon Nanowires Fabricated through Metal Assisted Chemical Etching

In this study, we developed active substrates consisting of Ag-decorated silicon nanowires on a Si substrate using a single-step Metal Assisted Chemical Etching (MACE) process, and evaluated their performance in the identification of low concentrations of Rhodamine 6G using surface-enhanced photoluminescence spectroscopy. Different structures with Ag-aggregates as well as Ag-dendrites were fabricated and studied depending on the etching parameters. Moreover, the addition of Au nanoparticles by simple drop-casting on the MACE-treated surfaces can enhance the photoluminescence significantly, and the structures have shown a Limit of Detection of Rhodamine 6G down to 10-12 M for the case of the Ag-dendrites enriched with Au nanoparticles.

Indium nanoparticle-based surface enhanced fluorescence from deep ultraviolet to near-infrared: A theoretical study

Herein, for the first time, we report a theoretical investigation on Indium nanoparticle-based surface enhanced fluorescence (SEF) from deep ultraviolet (UV) to near-infrared (NIR). In the beginning, the far- and near-field plasmonic properties of the Indium nanospheres of different sizes are studied to extract the wavelengths of lower and higher-order localized surface plasmon modes and the corresponding local electric field enhancement (EFE) values. Later, the dependence of the SEF enhancement with the separation between the fluorophore and nanoparticle (d), fluorescence, and excitation wavelengths is studied systematically. The role of the surrounding medium on plasmon mode wavelength and the SEF enhancement is also shown. Moreover, the effect of d and fluorescence wavelength on the average SEF enhancement is investigated. Finally, the variation in the plasmonic properties after thin dielectric coating on the surface of single Indium nanospheres is studied.

Enhanced Stability and Emission Properties of Perylene Dyes by Surface Tethering: Preparation of Fluorescent Ru Nanoparticle Suspensions by Alkyne Linker Chemistry

Spherical ruthenium nanoparticles (NPs) with a narrow size distribution were synthesised in ethanol by a facile low-temperature solvothermal process without the assistance of templates, structure-directing agents or post annealing/reduction treatments. Surface passivation with a fluorescent perylene dye (EP), and with silane ligands (ETMS), both initially bearing alkyne groups and subsequently forming vinylidene linkages, provided stable suspensions of the marginally soluble free EP. Quantitative analysis of the suspension gave an estimated EP surface coverage of 15 %, corresponding to an EP/ETMS mole ratio of ≈1:6. Photophysical evaluation of the bound and free dye revealed similar absorption bands and extinction coefficients and improved properties for the bound state, including enhanced fluorescence in the visible range for the bound dye, an extended absorption range into the near-UV providing strong emission in the visible, and significantly improved photostability. The physical basis of the enhanced photophysical properties, potential routes to further improvements and the implications for applications are discussed.

Fluorescent Noble Metal Nanoclusters Loaded Protein Hydrogel Exhibiting Anti-Biofouling and Self-Healing Properties for Electrochemiluminescence Biosensing Applications

Electrochemiluminescence (ECL) showed great potential in various analytical applications, especially in the sensing of biotargets, taking advantage of its high sensitivity, selectivity, ease of spatial and temporal control, and simplified optical setup. However, during the sensing of complex biological samples, ECL sensors often suffered severe interferences from unavoidable nonspecific-binding of biomacromolecules and physical damages of ECL sensing interfaces. Herein, a hydrogel based ECL biosensing system exhibiting excellent anti-biofouling and self-healing properties is developed. A protein hydrogel composed of bovine serum albumin (BSA) directed fluorescent Au/Ag alloy nanoclusters (Au/Ag NCs) is applied in building ECL sensing systems. The hydrogel matrix facilitates the immobilization of fluorescent Au/Ag NCs as excellent ECL probes, and the porous hydrophilic structure allows the free diffusion of small molecular biotargets while rejecting macromolecular interferences. Moreover, the hydrogel exhibits excellent self-healing property, with the ECL intensity recovered rapidly in 10 min after cutting. The hydrogel ECL system is successfully applied in sensing glutathione (GSH) in serum, confirming the applicability of the hydrogel based anti-biofouling ECL sensing system in sensing complex biological samples. This research may inspire the development of novel anti-biofouling and self-healing ECL biosensors for biosensing applications.

Electrochemical synthesis of tin plasmonic dendritic nanostructures with SEF capability through in situ replacement

Dendrite nanostructures with noble metals, such as Au and silver, act as plasmonic substrates with excellent potential in enhanced fluorescence technology. However, tin dendritic nanostructures are poorly investigated. In this study, we proposed a method of in situ electrochemical synthesis replacement to fabricate highly branched tin dendritic nanostructures on aluminum substrates. The surface enhanced fluorescence performance of the tin dendrites was tested for the detection of rhodamine 6G as probe molecules, and the result showed that the enhancement factors can reach to 36.5-fold that of an aluminum substrate. The fabricated tin dendrites have numerous nanogaps between the stratified and adjacent ones, thereby creating many plasmon-active “hotspots” dedicated to enhanced fluorescence. Electrical field simulation results for the tin dendritic nanostructures proved that its nanogaps can enhance the nearby local electromagnetic field. As a result, tin dendritic nanostructures exhibit outstanding surface enhanced fluorescence and promising application in biomolecule detection and sensor devices.

Dendrite nanostructures with noble metals, such as Au, silver and tin, act as plasmonic substrates with excellent potential in enhanced fluorescence technology.

SERS-fluorescence-superresolution triple-mode nanoprobe based on surface enhanced Raman scattering and surface enhanced fluorescence

Multifunctional nanoprobes play important roles in cell imaging and sensing. Here, we present a novel optical nanoprobe based on surface enhanced Raman scattering (SERS) and surface enhanced fluorescence (SEF), which can realize the SERS-fluorescence and superresolution triple-mode imaging of cancer cells. Compared with other previously reported multifunctional nanoprobes, the proposed nanoprobe holds two exquisite properties. The first one is that, in addition to normal SERS and fluorescence imaging, the nanoprobe can also be used for single molecule localization microscopy (SMLM) imaging, which helps compensate for the diffraction limited spatial resolution of normal SERS and fluorescence imaging. The second one is that, other than simple fluorescence, SEF is used in the nanoprobe to produce a stronger signal for fluorescence imaging and, more importantly, better photo-switching for SMLM imaging. In the experiment, we optimized the structure of the nanoprobe to obtain the best SEF effect. With the optimal structure, the triple-mode imaging of a breast cancer cell line (SKBR3) is realized. Since such triple-mode imaging of cancer cells has never been achieved before, we believe that the presented nanoprobe holds great potential for cancer cell targeting or the investigation of cell-nanomaterial interactions.

High Contrast Surface Enhanced Fluorescence of Carbon Dot Labeled Bacteria Cells on Aluminum Foil

Surface enhanced fluorescence (SEF) is observed with very high contrast (100-200) from single E. coli bacteria cells labeled with Carbon nanodots (CDs), on aluminum foil and aluminum film. Likely, it is the first application of organic CDs in SEF. SEF with 633 nm excitation delivered a much higher contrast than SEF with 532 nm excitation. Contrast is the ratio of the fluorescent intensities of labeled CDs to unlabeled (control) cells. High contrast with CDs is also observed on the gold film, silicon, and glass. Enhancement factor (EF) is the ratio of the signal on the metal substrate to the signal on the glass. Single E. coli cells, labeled with commercial graphene quantum dots (GCDs), demonstrated higher EFs (44 on gold, 35 on Al film), but at least one order of magnitude lower contrast (7-10 on aluminum and gold) than cells labeled with organic CDs. Therefore, organic CDs can be a good choice for cell imaging/labeling, capable of achieving a signal to noise (standard deviation of the control) as high as 700 on Al film. Overall, aluminum foil and film are highlighted as inexpensive but efficient substrates for Metal Enhanced Fluorescence, particularly MEF of bacterial cells stained with CDs.

Mass Cytometry and Single-Cell RNA-seq Profiling of the Heterogeneity in Human Peripheral Blood Mononuclear Cells Interacting with Silver Nanoparticles

Understanding the interactions between nanoparticles (NPs) and human immune cells is necessary for justifying their utilization in consumer products and biomedical applications. However, conventional assays may be insufficient in describing the complexity and heterogeneity of cell-NP interactions. Herein, mass cytometry and single-cell RNA-sequencing (scRNA-seq) are complementarily used to investigate the heterogeneous interactions between silver nanoparticles (AgNPs) and primary immune cells. Mass cytometry reveals the heterogeneous biodistribution of the positively charged polyethylenimine-coated AgNPs in various cell types and finds that monocytes and B cells have higher association with the AgNPs than other populations. scRNA-seq data of these two cell types demonstrate that each type has distinct responses to AgNP treatment: NRF2-mediated oxidative stress is confined to B cells, whereas monocytes show Fcγ-mediated phagocytosis. Besides the between-population heterogeneity, analysis of single-cell dose-response relationships further reveals within-population diversity for the B cells and naïve CD4⁺ T cells. Distinct subsets having different levels of cellular responses with respect to their cellular AgNP doses are found. This study demonstrates that the complementary use of mass cytometry and scRNA-seq is helpful for gaining in-depth knowledge on the heterogeneous interactions between immune cells and NPs and can be incorporated into future toxicity assessments of nanomaterials.

Integration of patterned photonic nitrocellulose and microfluidic chip for fluorescent point-of-care testing of multiple targets

With the unique capability of enhancing fluorescence, photonic material is integrated into microfluidic chip for point-of-care testing of multiple targets.

Recent advances in synthetic methods and applications of silver nanostructures

As the advanced functional materials, silver nanoparticles are potentially useful in various fields such as photoelectric, bio-sensing, catalysis, antibacterial and other fields, which are mainly based on their various properties. However, the properties of silver nanoparticles are usually determined by their size, shape, and surrounding medium, which can be modulated by various synthesis methods. In this review, the fabrication methods for synthesizing silver nanoparticles of different shapes and specific size are illustrated in detail. Besides, the corresponding properties and applications of silver nanoparticles are also discussed in this paper.

Plant-Mediated Synthesis of Silver Nanoparticles: Their Characteristic Properties and Therapeutic Applications

Interest in "green nanotechnology" in nanoparticle biosynthesis is growing among researchers. Nanotechnologies, due to their physicochemical and biological properties, have applications in diverse fields, including drug delivery, sensors, optoelectronics, and magnetic devices. This review focuses on the green synthesis of silver nanoparticles (AgNPs) using plant sources. Green synthesis of nanoparticles is an eco-friendly approach, which should be further explored for the potential of different plants to synthesize nanoparticles. The sizes of AgNPs are in the range of 1 to 100 nm. Characterization of synthesized nanoparticles is accomplished through UV spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, and scanning electron microscopy. AgNPs have great potential to act as antimicrobial agents. The green synthesis of AgNPs can be efficiently applied for future engineering and medical concerns. Different types of cancers can be treated and/or controlled by phytonanotechnology. The present review provides a comprehensive survey of plant-mediated synthesis of AgNPs with specific focus on their applications, e.g., antimicrobial, antioxidant, and anticancer activities.

Plasmon-Enhanced Photocleaving Dynamics in Colloidal MicroRNA-Functionalized Silver Nanoparticles Monitored with Second Harmonic Generation

The photocleaving dynamics of colloidal microRNA-functionalized nanoparticles are studied using time-dependent second harmonic generation (SHG) measurements. Model drug-delivery systems composed of oligonucleotides attached to either silver nanoparticles or polystyrene nanoparticles using a nitrobenzyl photocleavable linker are prepared and characterized. The photoactivated controlled release is observed to be most efficient on resonance at 365 nm irradiation, with pseudo-first-order rate constants that are linearly proportional to irradiation powers. Additionally, silver nanoparticles show a 6-fold plasmon enhancement in photocleaving efficiency over corresponding polystyrene nanoparticle rates, while our previous measurements on gold nanoparticles show a 2-fold plasmon enhancement compared to polystyrene nanoparticles. Characterizations including extinction spectroscopy, electrophoretic mobility, and fluorimetry measurements confirm the analysis from the SHG results. The real-time SHG measurements are shown to be a highly sensitive method for investigating plasmon-enhanced photocleaving dynamics in model drug delivery systems.

Improved maximum entropy method for the analysis of fluorescence spectroscopy data: evaluating zero-time shift and assessing its effect on the determination of fluorescence lifetimes

A new algorithm based on the Maximum Entropy Method (MEM) is proposed for recovering both the lifetime distribution and the zero-time shift from time-resolved fluorescence decay intensities. The developed algorithm allows the analysis of complex time decays through an iterative scheme based on entropy maximization and the Brent method to determine the minimum of the reduced chi-squared value as a function of the zero-time shift. The accuracy of this algorithm has been assessed through comparisons with simulated fluorescence decays both of multi-exponential and broad lifetime distributions for different values of the zero-time shift. The method is capable of recovering the zero-time shift with an accuracy greater than 0.2% over a time range of 2000 ps. The center and the width of the lifetime distributions are retrieved with relative discrepancies that are lower than 0.1% and 1% for the multi-exponential and continuous lifetime distributions, respectively. The MEM algorithm is experimentally validated by applying the method to fluorescence measurements of the time decays of the flavin adenine dinucleotide (FAD).

A novel 2D infinite M3L2 cage-based Cd(ii) microporous coordination polymer with a tripodal carboxylic acid ligand and solvent-dependent luminescence properties

A novel M₃L₂ cage-based microporous coordination polymer has an obvious, surface-enhanced luminescence in the solvents CH₂Cl₂ and CHCl₃.

Synthesis of Ag@Silica Nanoparticles by Assisted Laser Ablation

This paper reports the synthesis of silver nanoparticles coated with porous silica (Ag@Silica NPs) using an assisted laser ablation method. This method is a chemical synthesis where one of the reagents (the reducer agent) is introduced in nanometer form by laser ablation of a solid target submerged in an aqueous solution. In a first step, a silicon wafer immersed in water solution was laser ablated for several minutes. Subsequently, an AgNO3 aliquot was added to the aqueous solution. The redox reaction between the silver ions and ablation products leads to a colloidal suspension of core-shell Ag@Silica NPs. The influence of the laser pulse energy, laser wavelength, ablation time, and Ag(+) concentration on the size and optical properties of the Ag@Silica NPs was investigated. Furthermore, the colloidal suspensions were studied by UV-VIS-NIR spectroscopy, X-Ray diffraction, and high-resolution transmission electron microscopy (HRTEM).

Monitoring the Photocleaving Dynamics of Colloidal MicroRNA-Functionalized Gold Nanoparticles Using Second Harmonic Generation

Photoactivated drug delivery systems using gold nanoparticles provide the promise of spatiotemporal control of delivery that is crucial for applications ranging from regenerative medicine to cancer therapy. In this study, we use second harmonic generation (SHG) spectroscopy to monitor the light-activated controlled release of oligonucleotides from the surface of colloidal gold nanoparticles. MicroRNA is functionalized to spherical gold nanoparticles using a nitrobenzyl linker that undergoes photocleaving upon ultraviolet irradiation. The SHG signal generated from the colloidal nanoparticle sample is shown to be a sensitive probe for monitoring the photocleaving dynamics in real time. The photocleaving irradiation wavelength is scanned to show maximum efficiency on resonance at 365 nm, and the kinetics are investigated at varying irradiation powers to demonstrate that the nitrobenzyl photocleaving is a one-photon process. Additional characterization methods including electrophoretic mobility measurements, extinction spectroscopy, and fluorimetry are used to verify the SHG results, leading to a better understanding of the photocleaving dynamics for this model oligonucleotide therapeutic delivery system.

MoS2-based nanoprobes for detection of silver ions in aqueous solutions and bacteria

Silver as an extensively used antibacterial agent also poses potential threats to the environment and human health. Hence, in this work, we design a fluorescent nanoprobe by using rhodamine B isothiocyanate (RhoBS) adsorbed MoS2 nanosheets to realize sensitive and selective detection of Ag(+). On the surface of RhoBS-loaded MoS2 nanosheets, Ag(+) can be reduced to Ag nanoparticles, which afterward could not only lead to the detachment of RhoBS molecules and thus their recovered fluorescence but also the surface-enhanced fluorescence from RhoBS remaining adsorbed on MoS2. Such an interesting mechanism allows highly sensitive detection of Ag(+) (down to 10 nM) with great selectivity among other metal ions. Moreover, we further demonstrate that our MoS2-RhoBS complex could act as a nontoxic nanoprobe to detect Ag(+) in live bacteria samples. Our work resulted from an unexpected finding and suggests the promise of two-dimensional transition-metal sulfide nanosheets as a novel platform for chemical and biological sensing.

Fluorescence quenching N,N-bis(2,6-dimethylphenyl)-3,4:9,10-perylenetetracarboxylic diimide (BDPD) laser dye by colloidal silver nanoparticles

The fluorescence quenching N,N-bis(2,6-dimethylphenyl)-3,4:9,10-perylenetetra-carboxylic diimide (BDPD) by colloidal silver nanoparticles (AgNPs) was studied in methanol and ethylene glycol by steady state fluorescence measurements. The Stern-Volmer quenching rate constant (Ksv) was calculated as 8.1 × 10(8) and 8.22 × 10(8) M(-1) in methanol and ethylene glycol respectively. Taking the fluorescence lifetime of BDPD in the absence of silver nanoparticles as 3.2 ns, the values of the fluorescence quenching rate constants (kq = Ksv/τ) are calculated as 2.54 × 10(17) and 2.56 × 10(17) M(-1) s(-1) in methanol and ethylene glycol respectively. From the data, fluorescence resonance energy transfer and / or electron transfer processes play a major role in the fluorescence quenching of BDPD by AgNPs in methanol and low concentrations of Ag NPs in ethylene glycol. The static quenching rate constant in ethylene glycol was calculated by modified Stern-Volmer equation as V = 8.86 × 10(9) M(-1). For dynamic quenching, the radius of quenching sphere volume r values were found to be 68.3 and 70.6 nm in ethanol and ethylene glycol, respectively. For static quenching in ethylene glycol the effective radius of quenching sphere action (kinetic radius) was calculated as r = 152 nm.

Green synthesis and characterization of silver nanoparticles using alcoholic flower extract of Nyctanthes arbortristis and in vitro investigation of their antibacterial and cytotoxic activities

Here we report the synthesis of silver nanoparticles using ethanolic flower extract of Nyctanthes arbortristis, UVvisible spectra and TEM indicated the successful formation of silver nanoparticles. Crystalline nature of the silver nanoparticles was confirmed by X-ray diffraction. Fourier Transform Infra-Red Spectroscopy analysis established the capping of the synthesized silver nanoparticles with phytochemicals naturally occurring in the ethanolic flower extract of N. arbortristis. The synthesized silver nanoparticles showed antibacterial activity against the pathogenic strain of Escherichia coli MTCC 443. Furthermore, cytotoxicity of the silver nanoparticles was tested on mouse fibroblastic cell line (L929) and found to be non-toxic, which thus proved their biocompatibility. Antibacterial activity and cytotoxicity assay carried out in this study open up an important perspective of the synthesized silver nanoparticles.

Selectively assaying CEA based on a creative strategy of gold nanoparticles enhancing silver nanoclusters' fluorescence

Herein, we have successfully built up connections between nanoparticles and nanoclusters, and further constructed a surface-enhanced fluorescence (SEF) strategy based on the two types of nanomaterials for selectively assaying carcinoembryonic antigen (CEA). Specifically, silver nanoclusters provided the original fluorescence signal, while gold nanoparticles modified with DNA served as the fluorescence enhancer simultaneously. On the basis of this proposed nano-system, the two nanomaterials were linked by CEA-aptamer, thus facilitating SEF occurring. Nevertheless, more competitive interactions between CEA and CEA-aptamer emerged once CEA added, leading to SEF failed and their fluorescence decreased. Significantly, this creative method was further applied to detect CEA, and showed the linear relationship between the fluorescence intensity and CEA concentrations in the range of 0.01-1 ng mL(-1) with a detection limit of 3 pg mL(-1) at a signal-to-noise ratio of 3, demonstrating its sensitivity and promising towards multiple applications. On the whole, this approach we established may broaden potential ways of combining nanoparticles and nanoclusters for detecting trace targets in bioanalytical fields.

Enhanced infrared LSPR sensitivity of cap-shaped gold nanoparticles coupled to a metallic film

We report on optical properties of gold deposited on SiO2 nanospheres randomly adsorbed on a thin gold layer. Extinction peaks with optical density of more than 2 are observed in the visible as well as near-IR regimes. The peak wavelength of the latter was affected exquisitely by the thickness of the top layer. A helium ion microscope (HIM) was used for careful observation of morphological transformation accompanying the change in the deposition thickness. Growth of grain structures into a capped-dimer structure was accompanied by slight blue-shift of the visible peak and significantly greater red-shift of the near-IR peak. Our finite-difference time-domain (FDTD) calculations show that these peaks in the visible and near-IR can be respectively attributed to dipole modes associated with transverse and longitudinal oscillations of free electrons in the gold-capped dimer. To investigate the refractive index sensitivity of these peaks, we used two approaches: immersion in solutions of varying refractive index and coating with an organic layer. With the first approach that characterizes the bulk sensitivity, the visible peak shows sensitivity of 122 nm/RIU, while the near-IR peak shifts at the rate of 506 nm/RIU. With the second approach that reflects the local sensitivity, the surface was saturated with alkaline phosphatase (ALP), whose subsequent reaction led to formation of a thin insoluble organic layer, causing a relatively small blue-shift, under 7 nm, of the visible peak and much larger red-shift, over 50 nm, of the near-IR peak when measured in buffer. When the same reaction was measured at end points in the air, the shift was as large as 444 nm for the near-IR peak.

Large enhancement of quantum dot fluorescence by highly scalable nanoporous gold

Dealloyed nanoporous gold (NPG) dramatically enhances quantum dot (QD) fluorescence by amplifying near-field excitation and increasing the radiative decay rate. Originating from plasmonic coupling, the fluorescence enhancement is highly dependent upon the nanopore size of the NPG. In contrast to other nanoengineered metallic structures, NPG exhibits fluorescence enhancement of QDs over a large substrate surface.

Double-layered nanoparticle stacks for surface enhanced infrared absorption spectroscopy

We demonstrate that double-layered stacks of gold and insulator nanoparticles arranged on a flat gold surface dramatically enhance the sensitivity in absorption infrared microscopy. Through morphological variations of the nanoparticles, the frequency of the plasmon resonances can be tuned to match the frequency of the molecular vibration in the mid-infrared region. The results show that the nanostructures enhance the absorption signal of the molecules by a factor of up to ~2.2 × 10(6), while preserving their characteristic line-shape remarkably well.

Highly sensitive detection of superoxide dismutase based on an immunoassay with surface-enhanced fluorescence

Herein, a novel highly sensitive enhanced-fluorescence immunoassay for detection of superoxide dismutase (SOD) is established by combining surface-enhanced fluorescence (SEF) with immuno-magnetic separation. Based on a sandwich-type immunoassay, analytes in samples are first captured by magnetic beads coated with a monoclonal antibody and then "sandwiched" by another monoclonal antibody on silver nanoparticles labeled with fluorescein-labeled oligonucleotides in the presence of a magnet. Subsequently, the immune complex is enriched by exposure to a magnetic field. Lastly, the fluorescence intensity is measured according to the number of dissociated fluoresceins. The increased fluorescence intensity permits highly sensitive detection of SOD in a linear range of 10-8 × 10(5) pg mL(-1), with a detection limit of 4 pg mL(-1) at a signal-to-noise ratio of 3. Significantly, this method was validated for detection of SOD in human serum, human urine, and cosmetic samples. Moreover, the reliability and accuracy of results obtained by the enhanced-fluorescence method was confirmed by the analysis of high performance liquid chromatography (HPLC).

Single-step nanoplasmonic VEGF165 aptasensor for early cancer diagnosis

Early cancer diagnosis is very important for the prevention or mitigation of metastasis. However, effective and efficient methods are needed to improve the diagnosis and assessment of cancer. Here, we report a single-step detection method using a nanoplasmonic aptamer sensor (aptasensor), targeting a vascular endothelial growth factor-165 (VEGF(165)), a predominant biomarker of cancer angiogenesis. Our single-step detection is accomplished by (1) specific target recognition by an aptamer-target molecule interaction and (2) direct readouts of the target recognition. The readout is achieved by inactivation of surface plasmon enhancement of fluorescent probes preattached to the aptamers. Our aptasensor provides the appropriate sensitivity for clinical diagnostics with a wide range of linear detection from 25 pg/mL to 25 μg/mL (=from 1.25 pM to 1.25 μM), high specificity for VEGF(165) against PDGF-BB, osteopontin (OPN), VEGF(121), NaCl, and temporal/thermal/biological stability. In experiments with 100% serum and saliva from clinical samples, readouts of the aptasensor and an ELISA for VEGF(165) show good agreement within the limit of the ELISA kit. We envision that our developed aptasensor holds utilities for point-of-care cancer prognostics by incorporating simplicity in detection, low-cost for test, and required small sample volumes.

Plasmonic approach to enhanced fluorescence for applications in biotechnology and the life sciences

One of the most rapidly growing areas of physics and nanotechnology is concerned with plasmonic effects on the nanometer scale; these have applications in sensing and imaging technologies. Nanoplasmonic colloids such as Ag and Au have been attracting active interest, and there has been a recent explosion in the use of these metallic nanostructures to modify the spectral properties of fluorophores favorably and to enhance the fluorescence emission intensity. In this feature article, we summarize our work over a range of nanoplasmonics-assisted biological applications such as flow cytometry, immunoassays, cell imaging and bioassays where we use custom-designed plasmonic nanostructures (Ag and Au) to enhance fluorescence signatures. This fluorophore-metal effect offers unique advantages in providing improved photostability and enhanced fluorescence signals. We discuss the plasmonic enhancement of lanthanide fluorophores whose long and microsecond lifetimes offer the advantage of background-free fluorescence detection, but low photon cycling rates lead to poor brightness. We also show that plasmonic colloids are capable of enhancing the emission of fluorescent nanoparticles, including upconverting nanocrystals and lanthanide nanocomposites.

Functionalized gold nanoparticle supported sensory mechanisms applied in detection of chemical and biological threat agents: a review

There is a great necessity for development of novel sensory concepts supportive of smart sensing capabilities in defense and homeland security applications for detection of chemical and biological threat agents. A smart sensor is a detection device that can exhibit important features such as speed, sensitivity, selectivity, portability, and more importantly, simplicity in identifying a target analyte. Emerging nanomaterial based sensors, particularly those developed by utilizing functionalized gold nanoparticles (GNPs) as a sensing component potentially offer many desirable features needed for threat agent detection. The sensitiveness of physical properties expressed by GNPs, e.g. color, surface plasmon resonance, electrical conductivity and binding affinity are significantly enhanced when they are subjected to functionalization with an appropriate metal, organic or biomolecular functional groups. This sensitive nature of functionalized GNPs can be potentially exploited in the design of threat agent detection devices with smart sensing capabilities. In the presence of a target analyte (i.e., a chemical or biological threat agent) a change proportional to concentration of the analyte is observed, which can be measured either by colorimetric, fluorimetric, electrochemical or spectroscopic means. This article provides a review of how functionally modified gold colloids are applied in the detection of a broad range of threat agents, including radioactive substances, explosive compounds, chemical warfare agents, biotoxins, and biothreat pathogens through any of the four sensory means mentioned previously.

Electrochemical and optical characterization of triarylamine functionalized gold nanoparticles

This paper describes the synthesis, structural analysis, and investigations of the optical and electrochemical properties of some gold nanoparticles (AuNPs) which consist of a triarylamine ligand shell attached to small gold cores (Au-Tara). The triarylamine chromophores were attached to small 4-bromobenzenethiol covered gold nanoparticles (ca. 2 nm in diameter) by Sonogashira reaction. This procedure yields triarylamine redox centers attached via π-conjugated bridging units of different length to the gold core. The AuNPs were analyzed with (1)H NMR spectroscopy, diffusion ordered NMR spectroscopy (DOSY), thermogravimetric analysis (TGA), and scanning transmission electron microscopy (STEM). Cyclic voltammetry (CV) technique was used to determine the composition of the redox active particles via the Randles-Sevcik equation. The optical and electrochemical properties of the Au-Tara nanoparticles and of their corresponding unbound ligands (Ref) were investigated with UV/vis/NIR absorption spectroscopy, Osteryoung square wave voltammetry (OSWV), and spectroelectrochemistry (SEC). These data show that the assembling of triarylamines in the vicinity of a gold nanoparticle can change the optical and electrochemical properties of the triarylamine redox chromophores depending on the kind and length of the bridging unit. This is due to gold core-chromophore and chromophore-chromophore interactions.

Probing the plasmonic near-field of gold nanocrescent antennas

We present an investigation of the plasmon-induced electromagnetic near-field around gold nanocrescent (NC) antennas which exhibit localized surface plasmon resonances (LSPRs) in the infrared. To probe the near-field behavior, we monitored the LSPR shift of NCs to adsorption of dielectric layers of varying thickness. The experimental results are analyzed using theoretical simulations, and the EM field decay lengths for the NCs are determined. We discuss how the structural properties of NC antennas influence the near-field properties and compare the results with the near-fields of other metal nanostructures. We show that the near-field distribution around NCs depends strongly on the structural parameters of the NC and that its spatial extent can be tuned to large distances (>700 nm) from the nanostructure surface. In addition, we discuss NC antenna structural changes associated with exposure to ethanol and buffer solutions and the impact on LSPR properties.

Surface plasmon enhanced fluorescence of cationic conjugated polymer on periodic nanoarrays

The fluorescence from conjugated polymer assembled onto lithographically fabricated gold nanoarrays using genetically engineered peptides as molecular linkers is studied. A 16-fold increase in the photoluminescence of the conjugated polymer is observed when assembled on the optimized nanostructures due to surface plasmon enhanced fluorescence. This is achieved using a water-soluble cationic conjugated polymer, poly[(9,9-bis(6'-((N,N,N-trimethylammonium)hexyl)-2,7-fluorene)-co-4,7-di-2-thienyl-2,1,3-benzothiadiazole] dibromide (PFDBT-N(+)), systematically tuning the vertical distance of PFDBT-N(+) from the gold nanopillar surface using solid-specific peptide linkers and horizontally optimizing the localized surface plasmon resonance by varying the geometric arrangements of the patterned metal nanoarrays. The diameter and tip-to-tip spacing of the nanopillars along with vertically tuning the distance of PFDBT-N(+) from the nanopillar affected the observed fluorescence enhancements. The collective optical properties of conjugated polymers combined with the photonic properties of nanoparticles provide a new means in the development of metal enhanced hybrid nanomaterials for biotechnology.

Optimized nanospherical layered alternating metal-dielectric probes for optical sensing

Multishell nanospheres have been proposed as a class of layered alternating metal-dielectric probes (LAMPs) that can greatly enhance sensitivity and multiplexing capabilities of optical molecular imaging . Here we theoretically demonstrate that the interplasmonic coupling within these spheres and hence their spectral responses can be tuned by a rational selection of layer thicknesses. As a proof-of-concept, layered Mie theory calculations of near- and far-field characteristics followed by a genetic algorithm-based selection are presented for gold-silica, silver-silica and copper-silica LAMPs. The results demonstrate that the optical tunability available allows for design of application (excitation wavelength)-specific probes of different sizes. The tunability further increases with number of layers and within a particular allowable probe size provides for structures with distinct resonances at longer wavelengths. The concept of scaling internal field resonances is also shown theoretically and the range over which the magnitudes can be tuned are presented.