Silver-Gold Nanocomposite Substrates for Metal-Enhanced Fluorescence: Ensemble and Single-Molecule Spectroscopic Studies

Improved imaging and preservation of lysosome dynamics using silver nanoparticle-enhanced fluorescence

The dynamics of living cells can be studied by live-cell fluorescence microscopy. However, this requires the use of excessive light energy to obtain good signal-to-noise ratio, which can then photobleach fluorochromes, and more worrisomely, lead to phototoxicity. Upon light excitation, noble metal nanoparticles such as silver nanoparticles (AgNPs) generate plasmons, which can then amplify excitation in direct proximity of the nanoparticle's surface and couple to the oscillating dipole of nearby radiating fluorophores, modifying their rate of emission and thus, enhancing their fluorescence. Here, we show that AgNPs fed to cells to accumulate within lysosomes enhanced the fluorescence of lysosome-targeted Alexa488-conjugated dextran, BODIPY-cholesterol, and DQ-BSA. Moreover, AgNP increased the fluorescence of GFP fused to the cytosolic tail of LAMP1, showing that metal enhanced fluorescence can occur across the lysosomal membrane. The inclusion of AgNPs in lysosomes did not disturb lysosomal properties such as lysosomal pH, degradative capacity, autophagy and autophagic flux, and membrane integrity, though AgNP seemed to increase basal lysosome tubulation. Importantly, by using AgNP, we could track lysosome motility with reduced laser power without damaging and altering lysosome dynamics. Overall, AgNP-enhanced fluorescence may be a useful tool to study the dynamics of the endo-lysosomal pathway while minimizing phototoxicity.

Wavelength-Dependent Metal-Enhanced Fluorescence Biosensors via Resonance Energy Transfer Modulation

Fluorescence can be enhanced or quenched depending on the distance between the surface of a metal nanoparticle and the fluorophore molecule. Fluorescence enhancement by nearby metal particles is called metal-enhanced fluorescence (MEF). MEF shows promising potential in the field of fluorescence-based biological sensing. MEF-based biosensor systems generally fall into two platform categories: (1) a two/three-dimensional scaffold, or (2) a colloidal suspension. This review briefly summarizes the application studies using wavelength-dependent carbon dots (UV-VIS), noble metals (VIS), and upconversion nanoparticles (NIR to VIS), representative nanomaterials that contribute to the enhancement of fluorescence through the resonance energy transfer modulation and then presents a perspective on this topic.

Silver Nanoprism-mediated Protein Estimation - An Ultrasensitive Platform for Rapid Estimation of Protein Concentration

Estimation of protein concentration in the range of nanogram level (ng/mL) is a big challenge for conventional protein estimation methods. The highly dipole sensitive spectroscopic properties of Silver nanoprism (AgNPR) has been utilized to develop a rapid and highly sensitive method for the estimation of globular protein concentration at ng/mL (or ppb) range. We have applied a unique molecular doping approach to introduce protein in the interstitial space of the Ag fcc(111) crystal planes within AgNPR structure. The presence of the doped protein induces deformation in the crystal plane arrangement of AgNPR that results in a quantitative red shift of the dipole resonance peak (D-peak) of AgNPR under UV-vis spectroscopy. The proposed method allows detection of a protein concentration range of as low as 1-20 ng/mL- that is better than the sensitivity limit of conventional protein estimation techniques. This method has been successfully applied for commonly used proteins like haemoglobin (Hb), Bovine serum albumin (BSA), Trypsin (TRYP) and Lysozyme (LYS) with a very low limit of detection (LOD) within 2-6 ng/mL. The lowest LOD value was shown by Hb as 2.08 ng/mL. The method has further been validated by measuring Casein concentration from milk with an accuracy of 99% and 95% recovery for the concentration of 3.1 and 31 ng/mL respectively. Transmission emission microscopy (TEM) images show that the doped protein has been found to alter the size and shape of the AgNPR as a function of the dopant concentration by creating systematic deformation. This method does not require any alteration of the reaction temperature and solely depends on the physical interaction of doped protein with its neighbouring crystal structure of the nanoplanar geometry.

Ultrasensitive Detection of Exosome Using Biofunctionalized Gold Nanorods on a Silver-Island Film

Exosomes are often a promising source of biomarkers for cancer diagnosis in the early stages. Therefore, it is important to develop a sensitive and low-cost detection method. Here, we introduce a new substrate using gold nanorods (GNRs) on a silver-island film that produces a 360-fold AF647 molecule fluorescence enhancement compared to glass. The amplified fluorescence was proven theoretically by using finite difference time-domain simulation (FDTD). Utilizing the enhanced fluorescence from the substrate, GNRs attached with the biomolecules and created a sandwich immunoassay that can significantly detect human CD63 antigen on the exosome. By applying the method, the detection limit of mouse IgG goes down to 0.3 ng/mL, which is considerably better than the existing methods. Moreover, the sensitivity and accuracy for clinical plasma from six patients confirm its diagnostic feasibility. The proposed substrate can be uniformly extended to the identification of other biomarkers by modifying the antibodies on the surfaces of the GNRs.

Fluorescence activation, patterning and enhancement with photogenerated radicals, a prefluorescent probe and silver nanostructures

We designed a switchable fluorophore activated by UVA light and a radical initiator, for optical lithography with concomitant metal-enhanced fluorescence by silver nanoparticles.

Gated Resonance Energy Transfer (gRET) Controlled by Programmed Death Protein Ligand 1

The resonance energy transfer (RET) between an excited fluorescent probe molecule and a plasmonic nanoparticle (AuNP) has been investigated to evaluate the effect of protein molecules on the RET efficiency. We have found that the energy transfer to a functionalized AuNP can be modulated by a sub-monolayer film of programmed death-ligand 1 (PD-L1) protein. The interactions of PD-L1 with AuNP@Cit involve incorporation of the protein in AuNP shell and formation of a submonolayer adsorption film with voids enabling gated surface plasmon resonance energy transfer (SPRET). A model of the gated-RET system based on the protein size, estimated using Fisher-Polikarpov-Craievich density approximation, has been developed and can be utilized for other proteins, with minimum data requirement, as well. The value of the equilibrium constant K_L determined for the Langmuir isotherm is high: K_L = 1.27 × 10⁸ M^-1, enabling highly sensitive control of the gated-RET by PD-L1. Thus, with the gated-RET technique, one can determine PD-L1 within the dynamic range, extending from 1.2 to 50 nM. Moreover, we have found that the Gibbs free energy for PD-L1 binding to AuNP@Cit is -46.26 kJ/mol (-11.05 kcal/mol), indicating a strong adsorption with supramolecular interactions. The proposed gated-RET system, with the fluorescence intensity of the fluorophore probe molecule modulated by plasmonic quenching with AuNP and shielding of energy transfer by the adsorbed PD-L1 can be further developed for determination of PD-L1 in pharmaceutical formulations for immune checkpoint control in cancer therapy.

Optical writing and single molecule reading of photoactivatable and silver nanoparticle-enhanced fluorescence

We designed a hybrid nanoparticle-molecular system composed of silver nanostructures (AgNP) and a fluorogenic boron dipyrromethene (BODIPY) that can be selectively activated by UVA or UVC light in the presence of an appropriate photoacid generator (PAG). Light irradiation of the PAG encourages the release of p-toluenesulfonic, triflic or hydrobromic acid, any of which facilitate optical 'writing' by promoting the formation of a fluorescent species. Metal-enhanced fluorescence (MEF) by AgNP was achieved through rational design of the nano-molecular system in accordance with the principles of radiative decay engineering. In addition to increasing signal to noise, AgNP permitted shorter reaction times and low irradiance - all of which have important implications for applications of fluorescence activation in portable fluorescence patterning, bioimaging and super-resolution microscopy. Single molecule fluorescence microscopy provided unique insights into the MEF mechanism which were hidden by ensemble-averaged measurements, demonstrating that single molecule 'reading' is a valuable tool for characterizing particle-molecule interactions such as those responsible for the relative contributions of increased excitation and plasmophoric emission toward overall MEF. This work represents a step forward in the contemporary design of synergistic nano-molecular systems, and showcases the advantage of fusion between classic spectroscopic techniques and single molecule methods in terms of improved quantitative understanding of fluorophore-nanoparticle interactions, and how these interactions can be exploited to the fullest extent possible.

Optical Diagnostic Based on Functionalized Gold Nanoparticles

Au nanoparticles (NPs) possess unique physicochemical and optical properties, showing great potential in biomedical applications. Diagnostic spectroscopy utilizing varied Au NPs has become a precision tool of in vitro and in vivo diagnostic for cancer and other specific diseases. In this review, we tried to comprehensively introduce the remarkable optical properties of Au NPs, including localized surfaces plasmon resonance (LSPR), surface-enhanced Raman scattering (SERS), and metal-enhanced fluorescence (MEF). Then, we highlighted the excellent works using Au NPs for optical diagnostic applications. Ultimately, the challenges and future perspective of using Au NPs for optical diagnostic were discussed.

Topological Luminophor Y2O3:Eu3++Ag with High Electroluminescence Performance

Improving luminescent intensity is a significant technical requirement and scientific problem for the luminescent performance of fluorophor materials through the ages. The process control and luminescence performance still limit the developments of luminescent intensity even though it can be improved partly by covering or magnetron sputtering of precious metals on the surface of the fluorophore materials. On the basis of the improvement of luminescence center radiative transition rate by surface plasma resonance and Y₂O₃:Eu³⁺ microsheet phosphors, a fundamental model for topological luminophor Y₂O₃:Eu³⁺+Ag was designed through referencing the concepts of topological materials to enhance luminescent performance by composite luminescence, which is composed of Eu³⁺ centric electroluminescence and surface plasma-enhanced photoluminescence by Ag. The topological luminophor Y₂O₃:Eu³⁺+Ag was successfully synthesized with an asymmetric-discrete Ag nanocrystal topological structure on the surface just via illumination. Experimental results suggest that the luminescence performance of topological luminophor Y₂O₃:Eu³⁺+Ag increased by about 300% compared to that of Y₂O₃:Eu³⁺ phosphors in the same conditions. The design of a topological luminophor provides a new approach to further improve the luminescent intensity of phosphors.

Integrated dielectrophoretic and surface plasmonic platform for million-fold improvement in the detection of fluorescent events

We present an integrated dielectrophoretic (DEP) and surface plasmonic technique to quantify ∼1 pM of fluorescent molecules in low conductivity buffers. We have established a DEP force on target molecules to bring those molecules and place them on the nanometallic structures (hotspots) for quantification through surface plasmonic effects. Our results show that the DEP is capable of placing the fluorescent molecules on the hotspots, which are depicted as a significant reduction in the fluorescence lifetime of those molecules. To efficiently integrate the DEP and plasmonic effects, we have designed and utilized pearl-shaped interdigitated electrodes (PIDEs) in experiments. These electrodes generate 2-3 times higher DEP force than traditional interdigitated electrodes. Therefore, high-throughput assays can be developed. The nanometallic structures were strategically fabricated in the periphery of PIDEs for smooth integration of DEP and plasmonic detection. With the introduction of DEP, about 10⁶-fold improvement was achieved over existing plasmonic-based detection. Therefore, this simple addition to the existing surface plasmonic-based detection will enable the disease related protein detection.

Controllable metal-enhanced fluorescence in organized films and colloidal system

In recent years, considerable efforts have been devoted to better understand the unique emission properties of fluorophores enhanced by the localized surface plasmon resonance of metal nanoparticles (NPs), due to the widespread applications of fluorescence techniques. It is demonstrated by experiment and theoretical calculation that the enhancement efficiency strongly depends on the morphology of the metal NPs, the spectral overlap between metal and fluorophores, the separation distance between them, and other factors. Among these aspects to be considered are suitable spacer material and assembling methods to control the spatial arrangement of plasmonic NPs and fluorophore with proper optical properties and interactions. In this contribution, we provide a brief overview on recent progress of metal-enhanced fluorescence in organized films and colloidal systems.

Selective dopamine chemosensing using silver-enhanced fluorescence

Condensation product of salicylaldehyde and 1,3 propylenediamine becomes a diiminic Schiff base, which is oxidized by AgNO3 in alkaline solution, and in turn, stable Ag(0) is produced at room temperature. Under this condition, the solution exhibits intense silver nanoparticle enhanced fluorescence (SEF) with the λ(em) at 412 nm. Dopamine is selectively detected down to the nanomolar level via exclusive fluorescence quenching of the SEF. Dopamine-infested solution regains the fluorescence [i.e., SEF in the presence of Hg(II) ions]. Thus dopamine and Hg(II) in succession demonstrate "turn off/on" fluorescence due to the change in the scattering cross section of Ag(0) and gives a quantitative measure of dopamine in real samples. The proposed method is free from interferences of common biocompetitors.

Photoproduced fluorescent Au(I)@(Ag2/Ag3)-thiolate giant cluster: an intriguing sensing platform for DMSO and Pb(II)

Synergistic evolution of fluorescent Au(I)@(Ag2/Ag3)-thiolate core-shell particles has been made possible under the Sun in presence of the respective precursor coinage metal compounds and glutathione (GSH). The green chemically synthesized fluorescent clusters are giant (∼600 nm) in size and robust. Among all the common water miscible solvents, exclusively DMSO exhibits selective fluorescence quenching (Turn Off) because of the removal of GSH from the giant cluster. Again, only Pb(II) ion brings back the lost fluorescence (Turn On) leaving aside all other metal ions. This happens owing to the strong affinity of the sulfur donor of DMSO for Pb(II). Thus, employing the aqueous solution containing the giant cluster, we can detect DMSO contamination in water bodies at trace level. Besides, a selective sensing platform has emerged out for Pb(II) ion with a detection limit of 14 × 10(-8) M. Pb(II) induced fluorescence recovery is again vanished by I(-) implying a promising route to sense I(-) ion.

Surface plasmon-enhanced photoluminescence of DCJTB by using silver nanoparticle arrays

It is demonstrated that photoluminescence of DCJTB can be enhanced by surface plasmons occurred in silver nanoparticle arrays on glass substrates fabricated by using nanosphere lithography (NSL) combined with reactive ion etching (RIE). By changing the size of the seed polystyrene nanosphere with fixed thickness of SiO(2) film as a buffer layer between silver nanoparticles and fluorescent dye, we systematically studied the interaction between surface plasmons in Ag nanostructures and fluorescent dye by measuring the photoluminescence and time-resolved photoluminescence (TRPL) of the samples. As compared with pure DCJTB, it is observed that PL enhancement as high as 9.4 times and life time shortening from 0.966 ns shortened to 0.63 ns can be achieved with polystyrene nanosphere 430 nm in diameter. The physical origin due to plasmonic excitation has been clarified from 3D finite element simulations, as well as the assistance of UV-visible reflectance spectrum.

Plasmonic amplifiers: engineering giant light enhancements by tuning resonances in multiscale plasmonic nanostructures

The unique ability of plasmonic nanostructures to guide, enhance, and manipulate subwavelength light offers multiple novel applications in chemical and biological sensing, imaging, and photonic microcircuitry. Here the reproducible, giant light amplification in multiscale plasmonic structures is demonstrated. These structures combine strongly coupled components of different dimensions and topologies that resonate at the same optical frequency. A light amplifier is constructed using a silver mirror carrying light-enhancing surface plasmons, dielectric gratings forming distributed Bragg cavities on top of the mirror, and gold nanoparticle arrays self-assembled into the grating grooves. By tuning the resonances of the individual components to the same frequency, multiple enhancement of the light intensity in the nanometer gaps between the particles is achieved. Using a monolayer of benzenethiol molecules on this structure, an average SERS enhancement factor <EF> ∼10⁸ is obtained, and the maximum enhancement in the interparticle hot-spots is ∼3 × 10¹⁰, in good agreement with FDTD calculations. The high enhancement factor, large density of well-ordered hot-spots, and good fidelity of the SERS signal make this design a promising platform for quantitative SERS sensing, optical detection, efficient solid state lighting, advanced photovoltaics, and other emerging photonic applications.

Gold nanoparticle-enhanced luminescence of silicon quantum dots co-encapsulated in polymer nanoparticles

The preparation of two-component polymer composite nanoparticles encapsulating both Si quantum dots (SiQDs) and Au nanoparticles (AuNPs) by a single step miniemulsion polymerization of divinylbenzene is described. This simple and robust method affords well-defined polymer composite nanoparticles with mean diameters in a range of 100-200 nm and with narrow polydispersity indices as determined by dynamic light scattering and transmission electron microscopy. The successful encapsulation of AuNPs within poly(divinylbenzene) was confirmed by UV-visible spectroscopy and from TEM images. Plasmon-enhanced fluorescence of the luminescence of the SiQDs by AuNPs encapsulated within the polymer composite nanoparticles was evaluated by confocal microspectroscopy, and luminescence enhancements of up to 15 times were observed. These observations indicate that the luminescence of the SiQDs is enhanced by the proximity of the AuNPs. The polymer composite nanoparticles were successfully ink-jet printed onto a glass substrate, which demonstrates that these composites are processable in printing applications.