Fluorescence Quenching/Enhancement Surface Assays: Signal Manipulation Using Silver-coated Gold Nanoparticles

A Review on the Development of Gold and Silver Nanoparticles-Based Biosensor as a Detection Strategy of Emerging and Pathogenic RNA Virus

The emergence of highly pathogenic and deadly human coronaviruses, namely SARS-CoV and MERS-CoV within the past two decades and currently SARS-CoV-2, have resulted in millions of human death across the world. In addition, other human viral diseases, such as mosquito borne-viral diseases and blood-borne viruses, also contribute to a higher risk of death in severe cases. To date, there is no specific drug or medicine available to cure these human viral diseases. Therefore, the early and rapid detection without compromising the test accuracy is required in order to provide a suitable treatment for the containment of the diseases. Recently, nanomaterials-based biosensors have attracted enormous interest due to their biological activities and unique sensing properties, which enable the detection of analytes such as nucleic acid (DNA or RNA), aptamers, and proteins in clinical samples. In addition, the advances of nanotechnologies also enable the development of miniaturized detection systems for point-of-care (POC) biosensors, which could be a new strategy for detecting human viral diseases. The detection of virus-specific genes by using single-stranded DNA (ssDNA) probes has become a particular interest due to their higher sensitivity and specificity compared to immunological methods based on antibody or antigen for early diagnosis of viral infection. Hence, this review has been developed to provide an overview of the current development of nanoparticles-based biosensors that target pathogenic RNA viruses, toward a robust and effective detection strategy of the existing or newly emerging human viral diseases such as SARS-CoV-2. This review emphasizes the nanoparticles-based biosensors developed using noble metals such as gold (Au) and silver (Ag) by virtue of their powerful characteristics as a signal amplifier or enhancer in the detection of nucleic acid. In addition, this review provides a broad knowledge with respect to several analytical methods involved in the development of nanoparticles-based biosensors for the detection of viral nucleic acid using both optical and electrochemical techniques.

Dansyl-Labelled Ag@SiO2 Core-Shell Nanostructures-Synthesis, Characterization, and Metal-Enhanced Fluorescence

The present work describes synthesis, characterization, and use of a new dansyl-labelled Ag@SiO2 nanocomposite as an element of a new plasmonic platform to enhance the fluorescence intensity. Keeping in mind that typical surface plasmon resonance (SPR) characteristics of silver nanoparticles coincide well enough with the absorption of dansyl molecules, we used them to build the core of the nanocomposite. Moreover, we utilized 10 nm amino-functionalized silica shell as a separator between silver nanoparticles and the dansyl dye to prevent the dye-to-metal energy transfer. The dansyl group was incorporated into Ag@SiO2 core-shell nanostructures by the reaction of aminopropyltrimethoxysilane with dansyl chloride and we characterized the new dansyl-labelled Ag@SiO2 nanocomposite using transmission electron microscopy (TEM) and Fourier-transform infrared spectroscopy (FTIR). Additionally, water wettability measurements (WWM) were carried out to assess the hydrophobicity and hydrophilicity of the studied surface. We found that the nanocomposite deposited on a semitransparent silver mirror strongly increased the fluorescence intensity of dansyl dye (about 87-fold) compared with the control sample on the glass, proving that the system is a perfect candidate for a sensitive plasmonic platform.

Macroscopic two-dimensional monolayer films of gold nanoparticles: fabrication strategies, surface engineering and functional applications

In the last few decades, two-dimensional monolayer films of gold nanoparticles (2D MFGS) have attracted increasing attention in various fields, due to their superior attributes of macroscopic size and accessible fabrication, controllable electromagnetic enhancement, distinctive optical harvesting and electron transport capabilities. This review will focus on the recent progress of 2D monolayer films of gold nanoparticles in construction approaches, surface engineering strategies and functional applications in the optical and electric fields. The research challenges and prospective directions of 2D MFGS are also discussed. This review would promote a better understanding of 2D MFGS and establish a necessary bridge among the multidisciplinary research fields.

Fluorometric determination of the CCAAT/enhancer binding protein alpha by using gold nanoparticles and a labeled protein-binding DNA

A method is described for the determination of the CCAAT/enhancer binding protein alpha (C/EBPα) which is a regulator in adipocyte differentiation. The method is based on quenching of the red fluorescence (with excitation/emission maxima at 548/562 nm) of Cy3-labeled DNA if it becomes adsorbed on positively charged gold nanoparticles (AuNPs). Fluorescently labeled dsDNA that can bind C/EBPα is introduced as a fluorescent probes. The dsDNA is electrostatically adsorbed on the positively charged AuNPs to quench their fluorescence. In the presence of C/EBPα, it will bind dsDNA which then diffuses away. The fluorescence of the AuNPs becomes restored. The fluorescent signal increases linearly in the 0.05 to 600 ng·mL^-1 μM C/EBPα concentration range, and the detection limit is 29 pg·mL^-1. The method is specific and was applied to analyze cell lysates and in-situ. Graphical abstractSchematic representation of a fluorometric method for determination of the CCAAT/enhancer binding protein alpha (C/EBPα). Fluorescently labeled dsDNA that can bind C/EBPα is introduced as a fluorescent probes. The dsDNA is electrostatically adsorbed on the positively charged AuNPs to quench their fluorescence. In the presence of C/EBPα, it will bind dsDNA which then diffuses away. The fluorescence of the AuNPs becomes restored.

Direct Evidence of Lack of Colocalisation of Fluorescently Labelled Gold Labels Used in Correlative Light Electron Microscopy

Fluorescently labelled nanoparticles are routinely used in Correlative Light Electron Microscopy (CLEM) to combine the capabilities of two separate microscope platforms: fluorescent light microscopy (LM) and electron microscopy (EM). The inherent assumption is that the fluorescent label observed under LM colocalises well with the electron dense nanoparticle observed in EM. Herein we show, by combining single molecule fluorescent imaging with optical detection of the scattering from single gold nanoparticles, that for a commercially produced sample of 10 nm gold nanoparticles tagged to Alexa-633 there is in fact no colocalisation between the fluorescent signatures of Alexa-633 and the scattering associated with the gold nanoparticle. This shows that the attached gold nanoparticle quenches the fluorescent signal by ~95%, or less likely that the complex has dissociated. In either scenario, the observed fluorescent signal in fact arises from a large population of untagged fluorophores; rendering these labels potentially ineffective and misleading to the field.

Exploiting fluorescence for multiplex immunoassays on protein microarrays

Protein microarray technology is becoming the method of choice for identifying protein interaction partners, detecting specific proteins, carbohydrates and lipids, or for characterizing protein interactions and serum antibodies in a massively parallel manner. Availability of the well-established instrumentation of DNA arrays and development of new fluorescent detection instruments promoted the spread of this technique. Fluorescent detection has the advantage of high sensitivity, specificity, simplicity and wide dynamic range required by most measurements. Fluorescence through specifically designed probes and an increasing variety of detection modes offers an excellent tool for such microarray platforms. Measuring for example the level of antibodies, their isotypes and/or antigen specificity simultaneously can offer more complex and comprehensive information about the investigated biological phenomenon, especially if we take into consideration that hundreds of samples can be measured in a single assay. Not only body fluids, but also cell lysates, extracted cellular components, and intact living cells can be analyzed on protein arrays for monitoring functional responses to printed samples on the surface. As a rapidly evolving area, protein microarray technology offers a great bulk of information and new depth of knowledge. These are the features that endow protein arrays with wide applicability and robust sample analyzing capability. On the whole, protein arrays are emerging new tools not just in proteomics, but glycomics, lipidomics, and are also important for immunological research. In this review we attempt to summarize the technical aspects of planar fluorescent microarray technology along with the description of its main immunological applications.

Surface enhanced Raman scattering and plasmon enhanced fluorescence in zinc-tellurite glass

We report significant enhancements in Er(3+) luminescence as well as in Raman intensity in silver nanoparticles embedded zinc-tellurite glass. Surface enhanced Raman scattering effect is highlighted for the first time in tellurite glass containing silver NPs resulting in an enhanced Raman signal (~10 times). SAED manifest the growth of Ag(0) nanoparticles along the (111) and (200) crystallographic planes having average diameter in the range 14-36 nm. Surface plasmon resonance bands are observed in the range 484-551 nm. Furthermore, four prominent photoluminescence bands undergo significant enhancements up to 3 times. The enhancement is majorly attributed to the local field effect of silver NPs.

Silver cluster-biomolecule hybrids: from basics towards sensors

We focus on the functional role of small silver clusters in model hybrid systems involving peptides in the context of a new generation of nanostructured materials for biosensing. The optical properties of hybrids in the gas phase and at support will be addressed with the aim to bridge fundamental and application aspects. We show that extension and enhancement of absorption of peptides can be achieved by small silver clusters due to the interaction of intense intracluster excitations with the π-π* excitations of chromophoric aminoacids. Moreover, we demonstrate that the binding of a peptide to a supported silver cluster can be detected by the optical fingerprint. This illustrates that supported silver clusters can serve as building blocks for biosensing materials. Moreover, the clusters can be used simultaneously to immobilize biomolecules and to increase the sensitivity of detection, thus replacing the standard use of organic dyes and providing label-free detection. Complementary to that, we show that protected silver clusters containing a cluster core and a shell liganded by thiolates exhibit absorption properties with intense transitions in the visible regime which are also suitable for biosensing applications.

RECENT ADVANCES IN METAL-ENHANCED OPTICAL PROPERTIES

Noble metal nanoparticles exhibit strong surface plasmon resonance (SPR) and have been utilized in many chemical, biological and electronic applications. Recent advances on metal-enhanced optical properties demonstrated that the quantum yield and photo-stability of the fluorophores can be significantly enhanced when they are in the proximity of the metal surface, that will benefit many fluorescence-based applications. In this review article we first discuss the fundamental concepts of metal-enhanced optical properties and the recent achievements of metal-enhanced fluorescence of organic fluorophores and quantum dots, as well as metal-enhanced phosphorescence of organic molecules and upconversion nanoparticles that have long life times. Finally, we present recent applications of metal-enhanced optical properties in biosensing and bioassays, photodynamic therapy and optoelectronics.

Plasmonic platforms of self-assembled silver nanostructures in application to fluorescence

Fluorescence intensity changes were investigated theoretically and experimentally using self-assembled colloidal structures on silver semitransparent mirrors. Using a simplified quasi-static model and finite element method, we demonstrate that near-field interactions of metallic nanostructures with a continuous metallic surface create conditions that produce enormously enhanced surface plasmon resonances. The results were used to explain the observed enhancements and determine the optimal conditions for the experiment. The theoretical parts of the studies are supported with reports on detailed emission intensity changes which provided multiple fluorescence hot spots with 2-3 orders of enhancements. We study two kinds of the fluorophores: dye molecules and fluorescent nanospheres characterized with similar spectral emission regions. Using a lifetime-resolved fluorescence/reflection confocal microscopy technique, we find that the largest rate for enhancement (~1000-fold) comes from localized areas of silver nanostructures.

Molecular fluorescence enhancement on fractal-like structures

In this report we discuss strong fluorescence enhancements on electrochemically grown silver nanostructures examined through fluorescence lifetime imaging microscopy (FLIM). Silver fractal-like nanostructures were deposited on three different substrates: glass, plastic, and silicon. For all of the surfaces the same dye was tested, DyLight 649, deposited in the form of a model immunoassay through excitation from a 635 nm pulsed solid-state laser. The brightness improvement in hot spots exceeded 300 fold, which is about two times higher than was observed previously on similar surfaces. The strongest enhancements correspond to the shortest lifetimes, indicating a strong interaction between excited molecules and silver nanostructures. Additionally, the photostability of the fluorescence dye was dramatically increased in the presence of electrochemically deposited silver nanostructures. The production of silver fractals is easy, very controllable, and can be applied to any surface. We therefore believe that silver fractal-like nanostructures can be used successfully in ultrasensitive assays and fluorophore trace detection.