Simultaneous ionic cobalt sensing and toxic Congo red dye removal: a circular economic approach involving silver-enhanced fluorescence

A highly fluorescent quinone-capped silver hydrosol (AgOSA) was obtained using salicylaldehyde and an ionic silver solution. Such metal-enhanced fluorescence was efficiently quenched with Congo red dye (CR), producing CRAgOSA, due to the strong silver-sulfur interaction, replacing the capping of quinone (oxidized salicylaldehyde). The introduction of cobalt ions restored the fluorescence by engaging CR (CoCRAgOSA). Cobalt-induced fluorescence enhancement was 8.3 times higher than that of AgOSA due to the freeing of CR and the release of self-quenching of excess quinone molecules in CoCRAgOSA. The mammoth and selective fluorescence enhancement with ionic cobalt assisted in designing a turn-on ionic cobalt sensor with a limit of detection (LOD) of 9.4 × 10-11 M and a linear detection range (5 × 10-5 to 10-9 M). Moreover, toxic CR dye was eliminated by quinone-capped silver nanoparticles and Co2+ due to chemisorption. Not only the fluorimetric sensing of ionic cobalt but also the colorimetric sensing of Hg2+ was designed due to the simultaneous aggregation of AgNPs and complexation with CR induced by Hg2+ (LOD 1.36 × 10-5 M and linear detection range from 1.00 × 10-4 to 5 × 10-7 M). We applied our sensing method to estimate ionic cobalt and mercury in natural samples. The experiment was a unique case of circular economy, where a toxic dye was used for making a nanosensor.

Recent applications of coinage metal nanoparticles passivated with salicylaldehyde and salicylaldehyde-based Schiff bases

Salicylaldehyde (SD) and its derivatives are effective precursors for generating coinage metal (gold, silver, and copper) nanoparticles (NPs). These NPs have a variety of potential environmental applications, such as in water purification and sensing, and those arising from their antibacterial activity. The use of SD and its derivatives for synthesizing coinage NPs is attractive due to several factors. First, SD is a relatively inexpensive and readily available starting material. Second, the synthetic procedures are typically simple and can be carried out under mild conditions. Finally, the resulting NPs can be tailored to have specific properties, such as size, shape, and surface functionality, by varying the reaction conditions. In an alkaline solution, the phenolate form of SD was converted to its quinone form, while ionic coinage metal salts were converted to zero-valent nanoparticles. The capping in situ produced quinone of coinage metal nanoparticles generated metal-enhanced fluorescence under suitable experimental conditions. The formation of iminic bonds during the formation of Schiff bases altered the properties (especially metal-enhanced fluorescence) and applications.

Metal-enhanced sensing platform for the highly sensitive detection of C-reactive protein antibody and rhodamine B with applications in cardiovascular diseases and food safety

The potential applications of metal-enhanced fluorescence (MEF) devices include biosensors for the detection of trace amounts in biosciences, biotechnology, and pathogens that are relevant to medical diagnostics and food control. In the present study, the silver (Ag) film thickness (56 nm) of an MEF system was calibrated to maximize the depth-to-width ratio (Γ) of the surface plasmon resonance (SPR) active metal from reflectance dip curves. Upon plasmon coupling with thermally evaporated Ag, we demonstrated a 2.21-fold enhancement compared to the pristine flat substrate with the coefficient of variation (CV) ≈0.22% and the limit of detection (LOD) 0.001 mg L-1 of the concentration of an Alexa Fluor 488-labeled anti-C-reactive protein antibody (CRP@Alexa fluor 488). The structure was developed to simplify the in situ generation of biosensors for the surface-enhanced Raman spectroscopy (SERS) to determine Rhodamine B (RhB) with a highly robust performance. The procedure presented a simple and rapid sample pretreatment for the determination of RhB with a limit of quantification of 10-10 M and a satisfactory linear response (0.98). The results showed the excellent performance of the surface plasmon coupled emission (SPCE), which opens up possibilities for the accurate detection of small-volume and low-concentration target analytes due to the improved sensitivity and signal-to-noise ratio (SNR).

Metal-Enhanced Fluorescence Study in Aqueous Medium by Coupling Gold Nanoparticles and Fluorophores Using a Bilayer Vesicle Platform

Gold nanoparticles (AuNPs) display excellent plasmonic properties, which are expected to assist fluorescence enhancement for dyes, and the phenomenon is known as "metal-enhanced fluorescence" (MEF). In this study, we demonstrate AuNP-induced MEF for a modified bipyridine-based construct 4-(pyridine-2-yl)-3H-pyrrolo[2,3-c]quinoline (PPQ) when it binds with biologically important Zn2+. Importantly, this phenomenon is observed under aqueous conditions in a biocompatible bilayer vesicle platform. When PPQ binds with Zn2+ to form the complex in the presence of appropriate AuNPs, MEF is evident once compared with the fluorescence intensity in the absence of AuNPs. Among the three different sizes of AuNPs used, the enhancement is observed with an average diameter of 33 nm, whereas 18 and 160 nm do not show any enhancement. A possible mechanism is ascribed to the radiating plasmons of the AuNPs, which can couple with the emission frequencies of the fluorophore under a critical distance-dependent arrangement. We witness that the enhancement in fluorescence is accompanied with a reduction in lifetime components. It is proposed that the mechanism may be predominantly derived from the enhancement of an intrinsic radiative decay rate and partly from the localized electric field effect. Overall, this work shows a rational approach to design fluorophore-metal configurations with the desired emissive properties and a basis for a useful nanophotonic technology under biological conditions.

Tuning the luminescence properties of lanthanide coordination polymers with Ag@SiO2 nanoparticles

A series of core-shell Ag@SiO₂ nanoparticles with different core diameters and shell thicknesses have been prepared by a modified-Stöber method. They provide a facile route to tune the luminescence intensities, lifetimes and quantum efficiencies of lanthanide coordination polymers in the solid powder state. The coordination polymers [Tb₂(p-PTA)₃(H₂O)₂]_n, [Tb₂(o-PTA)₃(H₂O)₂]_n, [Eu₂(p-PTA)₃(H₂O)₂]_n and [Eu₂(o-PTA)₃(H₂O)₂]_n (PTA = phthalic acid) are synthesized and subsequently bound to the surface of Ag@SiO₂ nanoparticles. The luminescence intensities of the lanthanide complexes are enhanced as high as 10.8 times. The enhancement times depend on the core diameter and shell thickness of the Ag@SiO₂ nanoparticles. Importantly, by simply controlling the ratios between the complexes and the nanoparticles, the luminescence intensities, lifetimes and quantum efficiencies of the lanthanide complexes can be tuned in wide ranges. Typically, the luminescence lifetime of [Eu₂(p-PTA)₃(H₂O)₂]_n powder increases from 451 μs to 783 μs when 300 μL Ag@SiO₂ solution is added. Meanwhile, the luminescence quantum efficiency of the complex increases from 32.1% to 40.9%. The change of the luminescence properties of the lanthanide coordination polymers can be ascribed to the surface plasmon resonance effect of the Ag@SiO₂ nanoparticles as well as the decrease of the nonradiative decay rates of the complexes.

Fluorescence enhancement via varied long-chain thiol stabilized gold nanoparticles: A study of far-field effect

Metal enhanced fluorescence of carbon dots has been reported in aqueous solution. Moderately fluorescing carbon dots (λ_ex=360nm and λ_em=440nm) of 6-8nm diameters (CDA) have been synthesized from freshly prepared aqueous ascorbic acid solution under modified hydrothermal treatment. The CDA fluorescence is quenched at the close proximity with gold nanoparticles (AuNPs). Here, a substrate specific near-field electric field distribution is pronounced. Anticipating distance dependent fluorescence enhancement phenomenon, long-chain aliphatic thiol capped AuNPs are introduced to improve fluorescence of moderately fluorescing CDAs. The long-chain aliphatic thiols act as spacers between CDA and AuNP. Interestingly, the fluorescence of CDA is observed to be enhanced successively as the chain lengths of aliphatic thiols are increased. Fluorescing CDA, upon excitation, transfers energy to the nearby AuNP and a plasmon is induced. This plasmon radiates in the far-field resulting in fluorescence enhancement of CDAs. Such an interesting enhancement in emission with metallic gold is termed as gold enhanced fluorescence. This far-field effect for fluorescence enhancement of CDA particles becomes a general consensus in solution with varied long-chain aliphatic amine ligand capped silver nanoparticles (AgNPs). Finally, consequence of far-field effect of fluorescence enhancement has been observed while derivatized AuNP and AgNP are introduced into the CDA solution simultaneously which is described as reinforced fluorescence enhancement due to coupled plasmonic radiation.

Microwave-assisted one-pot synthesis of anisotropic gold nanoparticles with active high-energy facets for enhanced catalytic and metal enhanced fluorescence activities

Rhombic dodecahedron Au nanoparticles synthesized via a microwave assisted green route with high energy {110} facets are highly efficient for catalysis and metal enhanced fluorescence activities.

Temperature dependence of metal-enhanced fluorescence of photosystem I from Thermosynechococcus elongatus

We report the temperature dependence of metal-enhanced fluorescence (MEF) of individual photosystem I (PSI) complexes from Thermosynechococcus elongatus (T. elongatus) coupled to gold nanoparticles (AuNPs). A strong temperature dependence of shape and intensity of the emission spectra is observed when PSI is coupled to AuNPs. For each temperature, the enhancement factor (EF) is calculated by comparing the intensity of individual AuNP-coupled PSI to the mean intensity of 'uncoupled' PSI. At cryogenic temperature (1.6 K) the average EF was 4.3-fold. Upon increasing the temperature to 250 K the EF increases to 84-fold. Single complexes show even higher EFs up to 441.0-fold. At increasing temperatures the different spectral pools of PSI from T. elongatus become distinguishable. These pools are affected differently by the plasmonic interactions and show different enhancements. The remarkable increase of the EFs is explained by a rate model including the temperature dependence of the fluorescence yield of PSI and the spectral overlap between absorption and emission spectra of AuNPs and PSI, respectively.

Bloch Surface Wave-Coupled Emission at Ultra-Violet Wavelengths

The interaction of fluorophores with nearby metallic structures is now an active area of research. Dielectric photonic structures offer some advantages over plasmonic structures, namely small energy losses and less quenching. We describe a dielectric one-dimensional photonic crystal (1DPC), which supports Bloch surface waves (BSWs) from 280 to 440 nm. This BSW structure is a quartz slide coated with alternating layers of SiO2 and Si3N4. We show that this structure displays BSWs and that the near-UV fluorophore, 2-aminopurine (2-AP), on the top surface of the structure couples with the BSWs. Fluorophores do not have to be inside the structure for coupling and show a narrow angular distribution, with an angular separation of wavelengths. The Bloch wave-coupled emission (BWCE) radiates through the dielectric layer. These BSW structures, with useful wavelength range for detection of intrinsic protein and cofactor fluorescence, provide opportunities for novel optical configurations for bioassays with surface-localized biomolecules and for optical imaging using the coupled emission.

Recent progress on colloidal metal nanoparticles as signal enhancers in nanosensing

Colloidal metal nanoparticles present very special optical and electromagnetic properties at the nanoscale range. Such plasmonic properties have derived in a huge research field that encompasses the understanding of nanoparticle formation mechanisms for the ultimate goal of developing novel materials for real-life applications. Plasmonic sensing is experiencing a rapid transition by taking advantage of the characteristic properties of colloidal metal nanoparticles. However, a rational design of novel nanoplasmonic substrates, which gathers as much as the required properties for a substrate to be a 'good' sensor is critical through the development of applications that can be effectively transferred as applied technologies. Also, the chosen sensing technique is a key factor when planning the design of a new plasmonic-based sensor. Several factors such as composition, shape, size, particle interactions or stability among others will define the final quality of the nanomaterial as sensing platform. Herein, we review the latest and most promising state-of-the art of nanoplasmonic-based sensors in four differentiated areas regarding the surface-enhanced spectroscopy detection technique being LSPR-, SERS- and SEIRA-, and SEF based platforms.

Synergism of gold and silver invites enhanced fluorescence for practical applications

Synergism of gold and silver improves fluorescence behavior of gold–silver bimetallic clusters with practical applications.

Fluorescence Spectroscopy with Metal-Dielectric Waveguides

We describe a hybrid metal-dielectric waveguide structures (MDWs) with numerous potential applications in the biosciences. These structures consist of a thin metal film coated with a dielectric layer. Depending on the thickness of the dielectric layer, the modes can be localized near the metal, within the dielectric, or at the top surface of the dielectric. The optical modes in a metal-dielectric waveguide can have either S (TE) or P (TM) polarization. The dielectric spacer avoids the quenching, which usually occurs for fluorophores within about 5 nm from the metal. Additionally, the resonances display a sharp angular dependence and can exhibit several hundred-fold increases in intensity (E²) at the silica-air interface relative to the incident intensity. Fluorophores placed on top of the silica layer couple efficiently with the metal, resulting in a sharp angular distribution of emission through the metal and down from the bottom of the structure. This coupling occurs over large distances to several hundred nm away from the metal and was found to be consistent with simulations of the reflectivity of the metal-dielectric waveguides. Remarkably, for some silica thicknesses, the emission is almost completely coupled through the structure with little free-space emission away from the metal-dielectric waveguide. The efficiency of fluorophore coupling is related to the quality of the resonant modes sustained by the metal-dielectric waveguide, resulting in coupling of most of the emission through the metal into the underlying glass substrates. Metal-dielectric waveguides also provide a method to resolve the emission from surface-bound fluorophores from the bulk-phase fluorophores. Metal-dielectric waveguides are simple to fabricate for large surface areas, the resonance wavelength can be adjusted by the dielectric thickness, and the silica surface is suitable for coupling to biomolecules. Metal-dielectric waveguides can have numerous applications in diagnostics and high-throughput proteomics or DNA analysis.

Imine (–CHN–) brings selectivity for silver enhanced fluorescence

Strong silver and gold stimulated fluorescence enhancement of alkaline salicylaldehyde solution have been observed. Ammonia or primary amine quantitatively eliminates gold enhanced fluorescence, keeping silver enhanced fluorescence unaffected.

Orange-red silver emitters for sensing application and bio-imaging

Strongly fluorescent HFL-containing Ag@Au particles are synthesized via a modified hydrothermal technique. This solution is used for sulfide sensing and cell imaging.

Surface plasmon resonance mediated photoluminescence properties of nanostructured multicomponent fluorophore systems

The interaction between light and matter is the fundamental aspect of many optoelectronic applications. The efficiency of such devices is mainly dictated by the light emitting properties of fluorophores. Unfortunately, the intensity of emission is adversely affected by surface defects, scattering and chemical instability. Therefore, enhancing the luminescence of fluorophores is necessary for better implementation of nanocomposites in biological and optical applications. There are many interesting phenomena which can be observed if the characteristics of the fluorophores and metal nanoparticles are integrated. Photoluminescence (PL) by fluorophores can be enhanced or quenched by the presence of neighboring plasmonic metal nanostructures. An unambiguous study of the mechanism behind the enhancement and the quenching of emission is necessary to obtain new insight into the interactions between light and metal-fluorophore nanocomposites. In this review the core aspect of combining plasmonic metal nanostructures with fluorophores is discussed by considering various functional roles of plasmonic metals in modifying the PL properties reported by various research groups. A few representative applications of SPR mediated luminescence are also discussed.

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.