Engineering microscale two-dimensional gold nanoparticle cluster arrays for advanced Raman sensing: An AFM study

Computational Investigation of Advanced Refractive Index Sensor Using 3-Dimensional Metamaterial Based Nanoantenna Array

Photonic researchers are increasingly exploiting nanotechnology due to the development of numerous prevalent nanosized manufacturing technologies, which has enabled novel shape-optimized nanostructures to be manufactured and investigated. Hybrid nanostructures that integrate dielectric resonators with plasmonic nanostructures are also offering new opportunities. In this work, we have explored a hybrid coupled nano-structured antenna with stacked multilayer lithium tantalate (LiTaO₃) and Aluminum oxide (Al₂O₃), operating at wavelength ranging from 400 nm to 2000 nm. Here, the sensitivity response has been explored of these nano-structured hybrid arrays. It shows a strong electromagnetic confinement in the separation gap (g) of the dimers due to strong surface plasmon resonance (SPR). The influences of the structural dimensions have been investigated to optimize the sensitivity. The designed hybrid coupled nanostructure with the combination of 10 layers of gold (Au) and Lithium tantalate (LiTaO₃) or Aluminum oxide (Al₂O₃) (five layers each) having height, h₁ = h₂ = 10 nm exhibits 730 and 660 nm/RIU sensitivity, respectively. The sensitivity of the proposed hybrid nanostructure has been compared with a single metallic (only gold) elliptical paired nanostructure. Depending on these findings, we demonstrated that a roughly two-fold increase in the sensitivity (S) can be obtained by utilizing a hybrid coupled nanostructure compared to an identical nanostructure, which competes with traditional sensors of the same height, (h). Our innovative novel plasmonic hybrid nanostructures provide a framework for developing plasmonic nanostructures for use in various sensing applications.

Responsivity of Fractal Nanoparticle Assemblies to Multiple Stimuli: Structural Insights on the Modulation of the Optical Properties

Multi-responsive nanomaterials based on the self-limited assembly of plasmonic nanoparticles are of great interest due to their widespread employment in sensing applications. We present a thorough investigation of a hybrid nanomaterial based on the protein-mediated aggregation of gold nanoparticles at varying protein concentration, pH and temperature. By combining Small Angle X-ray Scattering with extinction spectroscopy, we are able to frame the morphological features of the formed fractal aggregates in a theoretical model based on patchy interactions. Based on this, we established the main factors that determine the assembly process and their strong correlation with the optical properties of the assemblies. Moreover, the calibration curves that we obtained for each parameter investigated based on the extinction spectra point out to the notable flexibility of this nanomaterial, enabling the selection of different working ranges with high sensitivity. Our study opens for the rational tuning of the morphology and the optical properties of plasmonic assemblies to design colorimetric sensors with improved performances.

Synthesis of wool roll-like silver nanoflowers in an ethanol/water mixture and their application to detect traces of the fungicide carbendazim by SERS technique

The Raman signal enhancement ability of the surface-enhanced Raman scattering (SERS) technique is largely determined by the SERS substrate, which is usually a collection of precious metal (such as silver or gold) nanoparticles. For use in the SERS substrate, anisotropic metal nanoparticles, e.g. flower-like, will be preferred over the isotropic ones since they will give higher Raman enhancement. The problem is that it is very difficult to fabricate anisotropic metal nanoparticles as small as the isotropic ones that are best suited for use as SERS substrates. This study deals with the synthesis of wool roll-like silver nanoflowers (AgNFs) in a mixed ethanol/water solution instead of the usual aqueous solution when reducing silver nitrate with ascorbic acid in the presence of citric acid, which acts as a structure-directing agent. The size of the wool roll-shaped AgNFs was reduced from about 700 nm when the solution was purely aqueous to about 280 nm when in the mixed solution the ethanol/water volume ratio was 75/25. Thanks to the size reduction of AgNFs, the enhancement factor of SERS substrates made from them has increased dramatically, from 2.7 × 106 when the size of AgNFs is 700 nm to 5.4 × 109 when their size is 280 nm (the calculation is based on rhodamine 6G Raman and SERS spectroscopy). The application of the above AgNFs to recording the SERS spectrum of carbendazim (CBZ), a typical fungicide, at low concentrations has also shown that the smaller the size of the AgNFs, the higher the intensity of the CBZ characteristic bands. The wool roll-shaped AgNFs with a size of 280 nm allowed CBZ to be detected down to a concentration of 0.01 ppm (4.2 × 10-8 M) with a detection limit of 3.2 ppb (13.4 × 10-9 M).

Exploiting SERS sensitivity to monitor DNA aggregation properties

In the last decades, DNA has been considered far more than the system carrying the essential genetic instructions. Indeed, because of the remarkable properties of the base-pairing specificity and thermoreversibility of the interactions, DNA plays a central role in the design of innovative architectures at the nanoscale. Here, combining complementary DNA strands with a custom-made solution of silver nanoparticles, we realize plasmonic aggregates to exploit the sensitivity of Surface Enhanced Raman Spectroscopy (SERS) for the identification/detection of the distinctive features of DNA hybridization, both in solution and on dried samples. Moreover, SERS allows monitoring the DNA aggregation process by following the temperature variation of a specific spectroscopic marker associated with the Watson-Crick hydrogen bond formation. This temperature-dependent behavior enables us to precisely reconstruct the melting profile of the selected DNA sequences by spectroscopic measurements only.

Electrotunable Nanoplasmonics for Amplified Surface Enhanced Raman Spectroscopy

Tuning the properties of optical metamaterials in real time is one of the grand challenges of photonics. Being able to do so will enable a class of adaptive photonic materials for use in applications such as surface enhanced Raman spectroscopy and reflectors/absorbers. One strategy to achieving this goal is based on the electrovariable self-assembly and disassembly of two-dimensional nanoparticle arrays at a metal | liquid interface. As expected, the structure results in plasmonic coupling between NPs in the array but perhaps as importantly between the array and the metal surface. In such a system, the density of the nanoparticle array can be reversibly controlled by the variation of electrode potential. Theory suggests that due to a collective plasmon-coupling effect  less than 1 V variation of electrode potential can give rise to a dramatic simultaneous change in optical reflectivity from ∼93% to ∼1% and the amplification of the SERS signal by up to 5 orders of magnitude. This is experimentally demonstrated using a platform based on the voltage-controlled assembly of 40 nm Au-nanoparticle arrays at a TiN/Ag electrode in contact with an aqueous electrolyte. We show that all the physics underpinning the behavior of this platform works precisely as suggested by the proposed theory, setting the electrochemical nanoplasmonics as a promising direction in photonics research.

Label-free highly sensitive probe detection with novel hierarchical SERS substrates fabricated by nanoindentation and chemical reaction methods

Nanostructures have been widely employed in surface-enhanced Raman scattering (SERS) substrates. Recently, in order to obtain a higher enhancement factor at a lower detection limit, hierarchical structures, including nanostructures and nanoparticles, appear to be viable SERS substrate candidates. Here we describe a novel method integrating the nanoindentation process and chemical redox reaction to machine a hierarchical SERS substrate. The micro/nanostructures are first formed on a Cu(110) plane and then Ag nanoparticles are generated on the structured copper surface. The effect of the indentation process parameters and the corrosion time in the AgNO₃ solution on the Raman intensities of the SERS substrate with hierarchical structures are experimentally studied. The intensity and distribution of the electric field of single and multiple Ag nanoparticles on the surface of a plane and with multiple micro/nanostructures are studied with COMSOL software. The feasibility of the hierarchical SERS substrate is verified using R6G molecules. Finally, the enhancement factor using malachite green molecules was found to reach 5.089 × 10⁹, which demonstrates that the production method is a simple, reproducible and low-cost method for machining a highly sensitive, hierarchical SERS substrate.

Exploring the Potentiality of a SERS-Active pH Nano-Biosensor

The merging of the molecular specificity of Raman spectroscopy with the extraordinary optical properties of metallic nanoarchitectures is at the heart of Surface Enhanced Raman Spectroscopy (SERS), which in the last few decades proved its worth as powerful analytical tool with detection limits pushed to the single molecule recognition. Within this frame, SERS-based nanosensors for localized pH measurements have been developed and employed for a wide range of applications. Nevertheless, to improve the performances of such nanosensors, many key issues concerning their assembling, calibration and stability, that could significantly impact on the outcome of the pH measurements, need to be clarified. Here, we report on the detailed characterization of a case study SERS-active pH nanosensor, based on the conjugation of gold nanoparticles with the pH-sensitive molecular probe 4-mercaptobenzoic acid (4MBA). We analyzed and optimized all the aspects of the synthesis procedure and of the operating conditions to preserve the sensor stability and provide the highest responsiveness to pH. Exploiting the dependence of the SERS spectrum on the protonation degree of the carboxylic group at the edge of the 4MBA molecules, we derived a calibration curve for the nanosensor. The extrapolated working point, i.e., the pH value corresponding to the highest sensitivity, falls at pH 5.6, which corresponds to the pKa value of the molecule confined at the nanoparticle surface. A shift of the pKa of 4MBA, observed on the molecules confined at the nanostructured interface respect to the bulk counterpart, unveils the opportunity to assembly a SERS-based pH nanosensor with the ability to select its working point in the sensitivity region of interest, by acting on the nanostructured surface on which the molecular probe is confined. As a proof-of-concept, the nanosensor was successfully employed to measure the extracellular pH of normal and cancer cells, demonstrating the capability to discriminate between them.

Folate-based single cell screening using surface enhanced Raman microimaging

Recent progress in nanotechnology and its application to biomedical settings have generated great advantages in dealing with early cancer diagnosis. The identification of the specific properties of cancer cells, such as the expression of particular plasma membrane molecular receptors, has become crucial in revealing the presence and in assessing the stage of development of the disease. Here we report a single cell screening approach based on Surface Enhanced Raman Scattering (SERS) microimaging. We fabricated a SERS-labelled nanovector based on the biofunctionalization of gold nanoparticles with folic acid. After treating the cells with the nanovector, we were able to distinguish three different cell populations from different cell lines (cancer HeLa and PC-3, and normal HaCaT lines), suitably chosen for their different expressions of folate binding proteins. The nanovector, indeed, binds much more efficiently on cancer cell lines than on normal ones, resulting in a higher SERS signal measured on cancer cells. These results pave the way for applications in single cell diagnostics and, potentially, in theranostics.