Thickness of a metallic film, in addition to its roughness, plays a significant role in SERS activity

Flexible nanoimprinted substrate integrating piezoelectric potential and photonic-plasmonic resonances

Flexible substrates for sensing provide adaptable, lightweight, and highly sensitive platforms for detecting different substances. The flexibility of these substrates allows for seamless integration with complex shapes and dynamic surfaces, enabling monitoring in challenging conditions using methods such as surface-enhanced Raman spectroscopy (SERS). Here we outline a flexible metamaterial array sensor formed from plasmonic silver-coated nanoimprinted piezoelectric polyvinylidene fluoride film. We show that nanoscale array features can be prepared on the surface of the piezoelectric film using a facile nanoimprinting procedure. These nanoimprinted features act as polarization channels that enable plasmonic resonances, enhancing the SERS signal strength and improving reproducibility. We combine this effect with the inherent piezoelectric properties of polyvinylidene fluoride to further enhance the Raman signal strength upon mechanical deformation. Our results demonstrate a significant enhancement of the SERS signal when probed at a wavelength of 532 nm, achieving over an order of magnitude increase in signal strength for a range of analytes. This lightweight and flexible SERS substrate holds significant potential for applications in medical diagnostics, environmental monitoring, and trace detection, offering a highly sensitive and reproducible analytical platform.

Investigating Raman peak enhancement in carboxyl-rich molecules: insights from Au@Ag core-shell nanoparticles in colloids

Surface-enhanced Raman scattering (SERS) is a promising analytical technique with applications in environmental monitoring, healthcare, and the biopharmaceutical industry. While SERS has been successfully applied to molecules such as 4-mercaptobenzoic acid and other thiol- and amine-containing compounds, there is limited research on its detection capabilities for molecules rich in carboxyl groups or unsaturated bonds, such as citric acid. This study investigates the SERS enhancement of Au@Ag core-shell nanoparticles in response to citric acid and other molecules with carboxyl and unsaturated bonds. We compare the SERS behavior of nanoparticles in freshly prepared and aged sodium citrate solutions to identify differences in Raman peak enhancement. Our findings show that the Au@Ag core-shell nanoparticles exhibit significant SERS enhancement when exposed to citric acid and other related compounds. The enhancement varies based on the age of the sodium citrate solution, which influences the structural properties of the nanoparticles. This work opens avenues for further research and applications in biological monitoring, environmental testing, and the pharmaceutical industry.

Impact of Graphene Oxide on SERS Enhancement of Arylated Gold Nanospheres: Mechanistic Insight

The performance of gold nanospheres as substrates for surface-enhanced Raman spectroscopy (SERS) investigation has been compromised by their low adsorption efficiency, high colloidal dispersibility, and diminishing hot spots. However, gold nanosphere substrates modified using aryldiazonium gold(III) chemistry via durable gold-carbon bonds are promising for SERS enhancement due to their controlled organic layer density. In this study, arylated gold nanospheres AuNSs-COOH have shown SERS enhancement when incorporated into graphene oxide (GO) to form nanocomposites (NCs) labeled AuNSs-COOH/GO (AuNCs). Our investigation using X-ray photoelectron spectroscopy (XPS) surface analysis showed that the gold-aryl nanospheres reached their maximum SERS enhancement with an optimal coating. The evaluation included the Au 4f chemical environment and compact graphitic layers for the SERS substrate optimization. The fabricated AuNC substrates demonstrated superior efficiency and reproducibility. A broad linear range of 10-3-10-7 M 4-nitrophenol detection was obtained with exceptional repeatability, as evidenced by the relative standard deviation (RSD) of 9.32%. A detailed investigation of the energy profiles, particularly the valence band maximum (VBM) and band gap values of the substrate and analyte, depicted the electromagnetic (EM) and charge-transfer-induced enhancement and the role of GO inclusion in substrate efficiency in SERS enhancement mechanisms. The finite-difference time domain (FDTD) simulation results revealed that AuNCs incorporated with graphitic nanostructures exhibited the most substantial SERS effect through an EM field enhancement mechanism. This study demonstrated significant SERS enhancement using gold-aryl nanospheres when modified with GO, in contrast to the typical reliance on anisotropic nanostructures.

Face-to-face Assembly Strategy of Au Nanocubes: Induced Generation of Broad Hotspot Regions for SERS-Fluorescence Dual-Signal Detection of Intracellular miRNAs

While designing anisotropic noble metal nanoparticles (NPs) can enhance the signal intensity of Raman dyes, more sensitive surface-enhanced Raman scattering (SERS) probes can be designed by oriented self-assembly of noble metal nanomaterials into dimers or higher-order nanoclusters. In this study, we engineered a self-assembly strategy in living cells for real-time fluorescence and SERS dual-channel detection of intracellular microRNAs (miRNAs), using Mg2+-dependent 8-17E DNAzyme sequences as the driving motors, gold nanocubes (AuNCs) as the driver components, and three-branched double-stranded DNA as the linking tool. The assembly selects adenine in DNA as a reporter molecule, simplifying the labeling process of Raman reporter molecules and reducing the synthesis process. In addition, adenine is stably distributed between the faces of AuNCs and the wide hotspot region gives good reproducibility of the adenine SERS signal. In this strategy, the SERS channel was consistently stable and more sensitive compared to the fluorescence channel. Among them, the detection limit of the SERS channel was 2.1 pM and the coefficient of variation was 1.26% in the in vitro liquid phase and 1.49% in MCF-7 cells. The strategy successfully achieved accurate tracking and quantification of miRNA-21 in cancer cells, showing good reproducibility in complex samples as well as cells. The reported strategy provides ideas for exploring intracellular specific triggering of nanoparticles for precise control of self-assembly.

Long-Focusing Device for Broadband THz Applications Based on a Tunable Reflective Biprism

THz radiation has assumed great importance thanks to the efforts in the development of technological tools used in this versatile band of the electromagnetic spectrum. Here, we propose a reflective biprism device with wavelength-independent long-focusing performances in the THz band by exploiting the high thermo-mechanical deformation of the elastomer polydimethylsiloxane (PDMS). This deformation allows for achieving significant optical path modulations in the THz band and effective focusing. The surface of a PDMS layer is covered with a gold thin film acting as a heater thanks to its absorption of wavelengths below ~500 nm. An invariance property of the Fresnel integral has been exploited to experimentally verify the THz performances of the device with an ordinary visible laser source, finding excellent agreement with the theoretical predictions at 1 and 3 THz. The same property also allowed us to experimentally verify that the reflective biprism focus has a longitudinal extension much greater than that exhibited by a benchmark convex cylindrical mirror with the same optical power. The device is thermo-mechanically stable up to a heating power of 270 mW, although it might be potentially exploited at higher powers with minor degradation of the optical performances.

Differences between surfactant-free Au@Ag and CTAB-stabilized Au@Ag star-like nanoparticles in the preparation of nanoarrays to improve their surface-enhanced Raman scattering (SERS) performance

In this study, we assessed the controlled synthesis and efficacy of surface-enhanced Raman scattering (SERS) on two distinct types of star-like Au@Ag core-shell nanoarrays. These nanoarrays were designed based on gold nanostars (AuNSs), which were synthesized with and without CTAB surfactant (AuNSs-CTAB and AuNSs-FS, respectively). The AuNS-FS nanoparticles were synthesized via a novel modification process, which helped overcome the previous limitations in the free-surfactant preparation of AuNSs by significantly increasing the number of branches, increasing the sharpness of the branches and minimizing the adsorption of the surfactant on the surface of AuNSs. Furthermore, the differences in the size and morphology of these AuNSs in the created nanoarrays were studied. To create the nanoarrays, a three-step method was employed, which involved the controlled synthesis of gold nanostars, covering them with a silver layer (AuNSs-FS@Ag and AuNSs-CTAB@Ag), and finally self-assembling the AuNS@Ag core-shelled nanoparticles via the liquid/liquid self-assembly method. AuNSs-FS@Ag showed higher ability in forming self-assembled nanoarrays than the nanoparticles prepared using CTAB, which can be attributed to the decrease in the repulsion between the nanoparticles at the interface. The nano-substrates developed with AuNSs-FS@Ag possessed numerous "hot spots" on their surface, resulting in a highly effective SERS performance. AuNSs-FS featured a significantly higher number of sharp branches than AuNSs-CTAB, making it the better choice for creating nanoarrays. It is worth mentioning that AuNSs-CTAB did not exhibit the same benefits as AuNSs-FS. The morphology of AuNSs with numerous branches was formed by controlling the seed boiling temperature and adding a specific amount of silver ions. To compare the SERS activity between the as-prepared nano-substrates, i.e., AuNS-CTAB@Ag and AuNS-FS@Ag self-assembled nanoarrays, low concentrations of crystal violet aqueous solution were characterized. The results showed that the developed AuNSs-FS@Ag could detect CV at trace concentrations ranging from 1.0 ng mL-1 to 10 ng mL-1 with a limit of detection (LOD) of 0.45 ng mL-1 and limit of quantification (LOQ) of 1.38 ng mL-1. The nano-substrates remained stable for 42 days with a decrease in the intensity of the characteristic Raman peaks of CV by less than 7.0% after storage. Furthermore, the spiking method could detect trace amounts of CV in natural water from the Dong Nai River with concentrations as low as 1 to 100 ng mL-1, with an LOD of 6.07 ng mL-1 and LOQ of 18.4 ng mL-1. This method also displayed good reproducibility with an RSD value of 5.71%. To better understand the impact of CTAB stabilization of the Au@Ag star-like nanoparticles on their surface-enhanced Raman scattering (SERS) performance, we conducted density functional theory (DFT) calculations. Our research showed that the preparation of AuNSs-FS@Ag via self-assembly is an efficient, simple, and fast process, which can be easily performed in any laboratory. Furthermore, the research and development results presented herein on nanoarrays have potential application in analyzing and determining trace amounts of organic compounds in textile dyeing wastewater.

Chemical Sensor Nanotechnology in Pharmaceutical Drug Research

The increase in demand for pharmaceutical treatments due to pandemic-related illnesses has created a need for improved quality control in drug manufacturing. Understanding the physical, biological, and chemical properties of APIs is an important area of health-related research. As such, research into enhanced chemical sensing and analysis of pharmaceutical ingredients (APIs) for drug development, delivery and monitoring has become immensely popular in the nanotechnology space. Nanomaterial-based chemical sensors have been used to detect and analyze APIs related to the treatment of various illnesses pre and post administration. Furthermore, electrical and optical techniques are often coupled with nano-chemical sensors to produce data for various applications which relate to the efficiencies of the APIs. In this review, we focus on the latest nanotechnology applied to probing the chemical and biochemical properties of pharmaceutical drugs, placing specific interest on several types of nanomaterial-based chemical sensors, their characteristics, detection methods, and applications. This study offers insight into the progress in drug development and monitoring research for designing improved quality control methods for pharmaceutical and health-related research.

3D silver metallized nanotrenches fabricated by nanoimprint lithography as flexible SERS detection platform

We report the development of highly sensitive substrates with great potential as Surface-enhanced Raman scattering (SERS) spectroscopy detection platforms, consisting of nanoimprint lithography (NIL) fabricated nanotrenches in plastic and covered by nanostructured silver (Ag) films with thicknesses in the 10-100 nm range deposited by direct current (DC) sputtering. The Ag film thickness was increased by using sequential deposition times and its contribution to the obtained enhancement factor was determined. The morphological and structural properties of the metalized nanotrenches were assessed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. Crystal violet (CV) was used as analyte to test the SERS activity of the substrates prepared with or without the nanoimprinted pattern. Our original approach was to determine the resulted SERS enhancement from the synergy of three key aspects: the Ag metallization of cheap, flexible substrates, the effect of increasing the Ag film thickness and the periodic nanotrenches imprinted by NIL as substrate. We found a dramatical contribution in the SERS signal of the periodical Ag nanopattern in comparison to the Ag film quantified by a calculated enhancement factor (EF) up to 10⁷ in case of the SERS detection platform with a 25 nm Ag layer on top of the periodic nanotrenches. The contribution of plasmonic nanostructures contained in the Ag films as well as the contribution of the periodical nanopatterned trenches was assessed, as a cumulative effect to the first contribution. This substrate showed a considerably lower limit of detection (LOD) for SERS, down to 10 pM, much better uniformity as well as more reproducible signals in comparison with the other thicknesses of the metallic film.

Hybrid Surface-Enhanced Raman Scattering Substrates for the Trace Detection of Ammonium Nitrate, Thiram, and Nile Blue

We report the fabrication and performance evaluation of hybrid surface-enhanced Raman scattering (SERS) substrates involving laser ablation and chemical routes for the trace-level detection of various analyte molecules. Initially, picosecond laser ablation experiments under ambient conditions were performed on pure silver (Ag) and gold (Au) substrates to achieve distinct nanosized features on the surface. The properties of the generated surface features on laser-processed portions of Ag/Au targets were systematically analyzed using UV-visible reflection and field emission scanning electron microscopy studies. Later, hybrid-SERS substrates were achieved by grafting the chemically synthesized Au nanostars on the plain and laser-processed plasmonic targets. Subsequently, we employed these as SERS platforms for the detection of a pesticide (thiram), a molecule used in explosive compositions [ammonium nitrate (AN)], and a dye molecule [Nile blue (NB)]. A comparative SERS study between the Au nanostar-decorated bare glass, silicon, Ag, Au, and laser-processed Ag and Au targets has been established. Our studies and the obtained data have unambiguously determined that laser-processed Ag structures have demonstrated reasonably good enhancements in the Raman signal intensities for distinct analytes among other substrates. Importantly, the fabricated hybrid SERS substrate of "Au nanostar-decorated laser-processed Ag" exhibited up to eight times enhancement in the SERS intensity compared to laser-processed Ag (without nanostars), as well as up to three times enhancement than the Au nanostar-loaded plain Ag substrates. Additionally, the achieved detection limits from the Au nanostar-decorated laser-processed Ag SERS substrate were ∼50 pM, ∼5 nM, and ∼5 μM for NB, thiram, and AN, respectively. The estimated enhancement factors accomplished from the Au nanostar-decorated laser-processed Ag substrate were ∼106, ∼106, and ∼104 for NB, thiram, and AN, respectively.

Fabrication of highly sensitive and uniform Ag/PS/PDMS SERS substrate and its application forin-situdetection

In this study, we developed a flexible and transparent silver/polystyrene/polydimethylsiloxane (Ag/PS/PDMS) substrate with both high density of hot spots and satisfactory uniformity using a cost-effective approach. Via template-guided self-assembly, PS beads were arranged regularly in nanobowls of a square array on PDMS, whose surface structure was transferred from a commercial complementary metal oxide semiconductor chip. Roughness was introduced onto the PS bead surface by nitrogen plasma treatment, followed by sputtering of Ag which generated many hot spots. Differential roughness on the PS bead surface greatly influenced the morphology of the Ag/PS/PDMS substrate. A meat-ball like surface structure was formed with a plasma etching time of 5 min, whose growth mechanism was proposed based on the scanning electron microscope analysis. The high sensitivity and desirable uniformity of the meat-ball like Ag/PS/PDMS substrate were demonstrated by using crystal violet as a Raman reporter, exhibiting an enhancement factor of 2.7 × 10⁷and a relative standard deviation of 5.04%. Thiram of a lower concentration than the maximum residue limit on the cucumber surface could easily be detectedin situby the proposed substrate, demonstrating its great potential forin-situfood safety analysis.

Rapid Quantitative Determination of Multiple Pesticide Residues in Mango Fruits by Surface-Enhanced Raman Spectroscopy

Imidacloprid, acephate, and carbaryl are common insecticides that are extensively used in planting mango, a well-known fruit in Vietnam, to ease mango hopper issues. The accurate detection of pesticide residues is critical for mango export to meet quality criteria. This study developed a novel SERS platform by using polydimethylsiloxane (PDMS) to simulate the rose petal structure incorporated with a silver coating layer and silver nanoparticles (AgNPs) to detect imidacloprid, acephate, and carbaryl in mango fruits. In this paper, the rose petal PDMS/Ag-AgNPs replica was considered the most efficient substrate for SERS measurement with an EF of 4.7 × 107. The Raman spectra of the three insecticides obtained from the PDMS/Ag-AgNPs substrate were clearly observed with their characteristic peaks of 1105 cm−1 for imidacloprid, 1083 cm−1, and 1579 cm−1 for acephate, and 727 cm−1 and 1378 cm−1 for carbaryl. The application of PDMS/Ag-AgNPs substrate in quantitative analysis of the three pesticides in mango fruit was evaluated. As a result, the limit of detection was 0.02 mg/kg for imidacloprid, 5 × 10−5 mg/kg for acephate, and 5 × 10−3 mg/kg for carbaryl. The SERS result also revealed that the pesticide residues in the mango sample were within an acceptable limit. This suggested the possibility of the rose petal PDMS/Ag-AgNPs replica for rapid quantification of pesticide residues not only in mango fruit but also in many other agricultural products.

How gap distance between gold nanoparticles in dimers and trimers on metallic and non-metallic SERS substrates can impact signal enhancement

The impact of variation in the interparticle gaps in dimers and trimers of gold nanoparticles (AuNPs), modified with Raman reporter (2-MOTP), on surface-enhanced Raman scattering (SERS) intensity, relative to the SERS intensity of a single AuNP, is investigated in this paper. The dimers, trimers, and single particles are investigated on the surfaces of four substrates: gold (Au), aluminium (Al), silver (Ag) film, and silicon (Si) wafer. The interparticle distance between AuNPs was tuned by selecting mercaptocarboxylic acids of various carbon chain lengths when each acid forms a mixed SAM with 2-MOTP. The SERS signal quantification was accomplished by combining maps of SERS intensity from a Raman microscope, optical microscope images (×100), and maps/images from AFM or SEM. In total, we analysed 1224 SERS nanoantennas (533 dimers, 648 monomers, and 43 trimers). The average interparticle gaps were measured using TEM. We observed inverse exponential trends for the Raman intensity ratio and enhancement factor ratio versus gap distance on all substrates. Gold substrate, followed by silicon, showed the highest Raman intensity ratio (9) and dimer vs. monomer enhancement factor ratio (up to 4.5), in addition to the steepest inverse exponential curve. The results may help find a balance between SERS signal reproducibility and signal intensity that would be beneficial for future agglomerated NPs in SERS measurements. The developed method of 3 to 1 map combination by an increase in image transparency can be used to study structure-activity relationships on various substrates in situ, and it can be applied beyond SERS microscopy.

Charge transport variation from Bloch-Grüneisen to Mott variable range hopping and transport change due to hydrogenation in Palladium thin films

We report a systematic investigation of the differences in charge transport mechanism in ultra-thin nano-island like films of palladium with thickness varying between 5 nm and 3 nm. The thicker films were found to be metallic in a large temperature range with a dominant Bloch-Grüneisen mechanism of charge transport arising due to electron-acoustic phonon scattering. These films were also found to exhibit an additional electron-magnon scattering. At temperatures below 20 K, the two films displayed a metal-insulator transition which was explained using Al'tshuler's model of increased scattering in disordered conductors. The thinner films were insulating and were found to exhibit Mott's variable range hopping mechanism of charge transport. The thinnest film showed a linear decrease of resistance with an increase in temperature in the entire temperature range. The island-like thin films were found to display very different response to hydrogenation at room temperature where the metallic films were found to display a decrease of resistance while the insulating films were found to have an increase of resistance. The decrease of resistance was ascribed to a hydrogen induced lattice expansion in the thin films that were at the percolation threshold while the resistance increase to an increase in work function of the films due to an increased adsorption of the hydrogen atoms at the surface sites of palladium.

Improved Sensitivity of Surface-Enhanced Raman Scattering with Gold Nanoparticles-Insulator-Metal Sandwich Layers on Flat Sapphire Substrate

Surface-enhanced Raman scattering (SERS) as a high sensitivity analytical method for molecule detection has attracted much attention in recent research. In this work, we demonstrated an improved SERS substrate, which has the gold nanoparticles randomly distributed on a SiO₂ interception layer over a gold thin film layer on the flat sapphire substrate (AuNP/SiO₂/Au/Sapphire), over the dispersed gold nanoparticles on a silicon substrate (AuNP/Si), for detection of R6G (1 × 10⁻⁶ M) in a Raman microscope. The fabrication of sandwich layers on top of the sapphire substrate involves evaporation of a gold mirror as thick as 100 nm, plasma enhanced chemical vapor deposition of the silica insulator layer 10 nm thick, and evaporation of a thin gold layer 10 nm thick for forming gold nanoparticles. For comparison, a gold thin film with a thickness of 5 nm and 10 nm was evaporated on a silicon substrate, respectively (AuNP/Si), as the reference SERS substrates in the experiment. The AuNP/SiO₂/Au/Sapphire substrate demonstrated improved sensitivity in detection of molecules in Raman microscopy, which can enable the molecules to be recognizable at a low laser power as 8.5 × 10⁻³ mW, 0.017 mW, 0.085 mW, and 0.17 mW for ultrashort exposure time. The simulation of AuNP/SiO₂/Au/Sapphire substrate and AuNP/Si substrate, based on the finite-difference time-domain (FDTD) method, explained the improved sensitivity for detection of R6G molecules from the view of classical electromagnetics, and it suggested the optimized size for the gold nanoparticles and the optimized laser wavelength for Raman microscopy for further research.

Optimization of gold nanorod arrays for surface enhanced Raman spectroscopy (SERS) detection of atrazine

Recently, there has been increasing concern over the widespread use of the herbicide atrazine which has been reported to have problematic side effects on local ecosystems. This has highlighted the need for rapid and accurate point-of-need assessment tools for analytical determination of herbicides in ground and surface waters. Surface enhanced Raman spectroscopy (SERS) is a sensitive vibrational spectroscopy technique which has recently been employed for the analysis of a variety of analytes in water, ranging from pharmaceuticals to pesticides. In this work, SERS sensors constructed using gold nanorod (AuNR) arrays are optimized and then utilized for the rapid and sensitive detection of atrazine. In this study, the effect of relative humidity on the self-assembly of gold nanorods into arrays was explored, and the SERS performance was assessed using para-aminothiophenol as a SERS probe. Once the SERS performance of the substrates was deemed optimal, the detection of atrazine was highlighted. This work represents the first time that relative humidity has been explored as an optimization strategy for controlled alignment of gold nanorods for SERS analysis of atrazine.

Development of Surface-Enhanced Raman Scattering (SERS)-Based Surface-Corrugated Nanopillars for Biomolecular Detection of Colorectal Cancer

In this paper, a nanobiosensor with surface-enhanced Raman scattering (SERS) capability is introduced for highly sensitive miRNA detection in colorectal cancer. This sensor was designed and fabricated by employing a nanoshielding mechanism from nanopolystyrene beads to resist reactive ion etching and allow anisotropic electrochemical etching, producing high-aspect-ratio, surface-corrugated nanopillars (SiNPs) on a silicon wafer to create extensive hot spots along the nanopillars for improved SERS signals. SERS enhancements were correlated with nanorange roughness, indicating that hot spots along the pillars were the crucial factor to improve the SERS effect. We achieved the detection capability of a trace amount of R6G (10-8 M), and the SERS signal enhancement factor (EF) was close to 1.0 × 107 on surface-corrugated gold SiNPs. miRNA samples were also demonstrated on this sensor with good sensitivity and specificity. The target molecule miR-21-Cy5 was easily monitored through Raman spectrum variation with a PCR-comparable concentration at around 100 pM with clear nucleotide-specific Raman signals, which is also suitable for biomolecule sensing.

How levelling and scan line corrections ruin roughness measurement and how to prevent it

Surface roughness plays an important role in various fields of nanoscience and nanotechnology. However, the present practices in roughness measurements, typically based on some Atomic Force Microscopy measurements for nanometric roughness or optical or mechanical profilometry for larger scale roughness significantly bias the results. Such biased values are present in nearly all the papers dealing with surface parameters, in the areas of nanotechnology, thin films or material science. Surface roughness, most typically root mean square value of irregularities Sq is often used parameter that is used to control the technologies or to link the surface properties with other material functionality. The error in estimated values depends on the ratio between scan size and roughness correlation length and on the way how the data are processed and can easily be larger than 10% without us noting anything suspicious. Here we present a survey of how large is the problem, detailed analysis of its nature and suggest methods to predict the error in roughness measurements and possibly to correct them. We also present a guidance for choosing suitable scan area during the measurement.

Gold nanoparticle assembly on porous silicon by pulsed laser induced dewetting

This work reports the influence of the substrate in the pulsed laser-induced dewetting (PLiD) of Au thin films for the fabrication of nanoparticle (NP) arrays. Two substrates were studied, i.e., polished silicon and porous silicon (PS), the latter being fabricated via electrochemical anodization in HF-containing electrolytes. The effect of both PLiD and substrate preparation parameters was explored systematically. On polished silicon substrates, it has been shown that uniform, randomly arranged NPs between 15 ± 7 nm and 89 ± 19 nm in diameter are produced, depending on initial thin film thickness. On PS however, there are topographical features that lead to the formation of ordered NPs with their diameters being controllable through laser irradiation time. The presence of surface pores and the appearance of surface ripples under low HF concentrations (<9.4 wt%) during electrochemical anodization results in this unique dewetting behaviour. Through AFM analysis, it has been determined that the ordered NPs sit within the valleys of the ripples, and form due to the atomic mobility enabled using the PLiD approach. This work has demonstrated that the utilization of topographically complex PS substrates results in size controllable and ordered NPs, while the use of polished Si does not enable such control over array fabrication.

Are plasmonic optical biosensors ready for use in point-of-need applications?

Plasmonics has drawn significant attention in the area of biosensors for decades due to the unique optical properties of plasmonic resonant nanostructures. While the sensitivity and specificity of molecular detection relies significantly on the resonance conditions, significant attention has been dedicated to the design, fabrication, and optimization of plasmonic substrates. The adequate choice of materials, structures, and functionality goes hand in hand with a fundamental understanding of plasmonics to enable the development of practical biosensors that can be deployed in real life situations. Here we provide a brief review of plasmonic biosensors detailing most recent developments and applications. Besides metals, novel plasmonic materials such as graphene are highlighted. Sensors based on Surface Plasmon Resonance (SPR), Localized Surface Plasmon Resonance (LSPR), and Surface Enhanced Raman Spectroscopy (SERS) are presented and classified based on their materials and structure. In addition, most recent applications to environment monitoring, health diagnosis, and food safety are presented. Potential problems related to the implementation in such applications are discussed and an outlook is presented.

Microfluidic Designing Microgels Containing Highly Concentrated Gold Nanoparticles for SERS Analysis of Complex Fluids

Surface-enhanced Raman scattering (SERS) is one of the most promising methods to detect small molecules for point-of-care analysis as it is rapid, nondestructive, label-free, and applicable for aqueous samples. Here, microgels containing highly concentrated yet evenly dispersed gold nanoparticles are designed to provide SERS substrates that simultaneously achieve contamination-free metal surfaces and high signal enhancement and reproducibility. With capillary microfluidic devices, water-in-oil-in-water (W/O/W) double-emulsion drops are prepared to contain gold nanoparticles and hydrogel precursors in innermost drop. Under hypertonic condition, water is selectively pumped out from the innermost drops. Therefore, gold nanoparticles are gently concentrated without forming aggregates, which are then captured by hydrogel matrix. The resulting microgels have a concentration of gold nanoparticles ≈30 times higher and show Raman intensity two orders of magnitude higher than those with no enrichment. In addition, even distribution of gold nanoparticles results in uniform Raman intensity, providing high signal reproducibility. Moreover, as the matrix of the microgel serves as a molecular filter, large adhesive proteins are rejected, which enables the direct detection of small molecules dissolved in the protein solution. It is believed that this advanced SERS platform is useful for in situ detection of toxic molecules in complex mixtures such as biological fluids, foods, and cosmetics.

Gold Nanofilm-Coated Porous Silicon as Surface-Enhanced Raman Scattering Substrate

Metallic film-coated porous silicon (PSi) has been reported as a lucrative surface-enhanced Raman scattering (SERS) substrate. The solution-based fabrication process is facile and easy; however, it requires additional reducing agent and extra chemical treatment, as well as hinders the suitability as a reproducible SERS substrate due to irregular hot spot generation via irregular deposition of metallic nanocrystallites. To address this issue, we report a unique one-step electronic beam (e-beam) physical vapor deposition (PVD) method to fabricate a consistent layer of gold (Au) nanofilm on PSi. Moreover, to achieve the best output as a SERS substrate, PSi prepared by electrochemical etching was used as template to generate an Au layer of irregular surface, offering the surface roughness feature of the PSi–Au thin film. Furthermore, to investigate the etching role and Au film thickness, Au-nanocrystallites of varying thickness (5, 7, and 10 nm) showing discrete surface morphology were characterized and evaluated for SERS effect using Rhodamine 6G (R6G). The SERS signal of R6G adsorbed on PSi–Au thin film showed a marked enhancement, around three-fold enhancement factor (EF), than the Si–Au thin film. The optimal SERS output was obtained for PSi–Au substrate of 7 nm Au film thickness. This study thus indicates that the SERS enhancement relies on the Au film thickness and the roughness feature of the PSi–Au substrate.

Enhancement of Single Molecule Raman Scattering using Sprouted Potato Shaped Bimetallic Nanoparticles

Herein, for the first time, we report the single molecule surface enhanced resonance Raman scattering (SERRS) and surface enhanced Raman scattering (SERS) spectra with high signal to noise ratio (S/N) using plasmon-active substrates fabricated by sprouted potato shaped Au-Ag bimetallic nanoparticles, prepared using a new one-step synthesis method. This particular shape of the nanoparticles has been obtained by fixing the amount of Au and carefully adjusting the amount of Ag. These nanoparticles have been characterized using scanning electron microscopy, extinction spectroscopy, and glancing angle X-ray diffraction. The single molecule sensitivity of SERS substrates has been tested with two different molecular Raman probes. The origin of the electromagnetic enhancement of single molecule Raman scattering in the presence of sprouted shape nanoparticles has been explained using quasi-static theory as well as finite element method (FEM) simulations. Moreover, the role of (i) methods for binding Raman probe molecules to the substrate, (ii) concentration of molecules, and (iii) Au-Ag ratio on the spectra of molecules has been studied in detail.

Hierarchic Interfacial Nanocube Assembly for Sensitive, Selective, and Quantitative DNA Detection with Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS)-based sensing is promising in that it has potential to allow for highly sensitive, selective, and multiplexed detection and imaging. However, the controlled assembly and gap formation between plasmonic particles for generating strong SERS signals in a quantitative manner is highly challenging, especially on biodetection platforms, and particle-to-particle variation in the signal enhancement can vary by several orders of magnitude in a single batch, largely limiting the reliable use of SERS for practical sensing applications. Here, a hierarchic-nanocube-assembly based SERS (H-Cube-SERS) bioassay to controllably amplify the electromagnetic field between gold nanocubes (AuNCs) is developed. Based on this strategy, H-Cube-SERS assay allows for detecting target DNA with a wide dynamic range from 100 aM to 10 pM concentrations in a stable and reproducible manner. It is also found that the uniformly formed AuNCs with flat surfaces are much more suitable for highly sensitive, reliable, and quantitative biodetection assays due to faster DNA binding kinetics, sharper DNA melting transition, wider hot spot regions, and less dependence on light polarization direction than spherical Au nanoparticles with curved interfaces. This work paves the pathways to the quantitative and sensitive biodetection on a SERS platform and can be extended to other particle assembly systems.

Limits of the Effective Medium Theory in Particle Amplified Surface Plasmon Resonance Spectroscopy Biosensors

The resonant wave modes in monomodal and multimodal planar Surface Plasmon Resonance (SPR) sensors and their response to a bidimensional array of gold nanoparticles (AuNPs) are analyzed both theoretically and experimentally, to investigate the parameters that rule the correct nanoparticle counting in the emerging metal nanoparticle-amplified surface plasmon resonance (PA-SPR) spectroscopy. With numerical simulations based on the Finite Element Method (FEM), we evaluate the error performed in the determination of the surface density of nanoparticles σ when the Maxwell-Garnett effective medium theory is used for fast data processing of the SPR reflectivity curves upon nanoparticle detection. The deviation increases directly with the manifestations of non-negligible scattering cross-section of the single nanoparticle, dipole-dipole interactions between adjacent AuNPs and dipolar interactions with the metal substrate. Near field simulations show clearly the set-up of dipolar interactions when the dielectric thickness is smaller than 10 nm and confirm that the anomalous dispersion usually observed experimentally is due to the failure of the effective medium theories. Using citrate stabilized AuNPs with a nominal diameter of about 15 nm, we demonstrate experimentally that Dielectric Loaded Waveguides (DLWGs) can be used as accurate nanocounters in the range of surface density between 20 and 200 NP/µm², opening the way to the use of PA-SPR spectroscopy on systems mimicking the physiological cell membranes on SiO₂ supports.

Correlation between surface scaling behavior and surface plasmon resonance properties of semitransparent nanostructured Cu thin films deposited via PLD

The surface scaling behavior of nanostructured Cu thin films, grown on glass substrates by the pulsed laser deposition technique, as a function of the deposition time has been studied using height–height correlation function analysis from atomic force microscopy (AFM) images. The scaling exponents α, β, 1/z and γ of the films were determined from AFM images. The local roughness exponent, α, was found to be ∼0.86 in the early stage of growth of Cu films deposited for 10 minutes while it increased to 0.95 with a longer time of deposition of 20 minutes and beyond this, it was nearly constant. Interface width w (rms roughness) scales with depositing time (t) as ∼ t^β, with the value of the growth exponent, β, of 1.07 ± 0.11 and lateral correlation length ξ following ξ = t^1/z and the value of 1/z = 0.70 ± 0.10. These exponent values convey that the growth dynamics of PLD Cu films can be best described by a combination of local and non-local models under a shadowing mechanism and under highly sticking substrate conditions. From the scaling exponents and power spectral density function, it is concluded that the films follow a mound like growth mechanism which becomes prominent at longer deposition times. All the Cu films exhibited SPR properties where the SPR peak shifts towards red with increasing correlation length (ξ) whereas bandwidth increases initially with ξ and thereafter decreases gradually with ξ.

The surface scaling behavior of nanostructured Cu thin films, grown on glass by the PLD technique, as a function of deposition time has been studied using height–height correlation function analysis from AFM images.

APTES Duality and Nanopore Seed Regulation in Homogeneous and Nanoscale-Controlled Reduction of Ag Shell on SiO2 Microparticle for Quantifiable Single Particle SERS

Noble-metal nanoparticles size and packing density are critical for sensitive surface-enhanced Raman scattering (SERS) and controlled preparation of such films required to achieve reproducibility. Provided that they are made reliable, Ag shell on SiO₂ microscopic particles (Ag/SiO₂) are promising candidates for lab-on-a-bead analytical measurements of low analyte concentration in liquid specimen. Here, we selected nanoporous silica microparticles as a substrate for reduction of AgNO₃ with 3-aminopropyltriethoxysilane (APTES). In a single preparation step, homogeneous and continuous films of Ag nanoparticles are formed on SiO₂ surfaces with equimolar concentration of APTES and silver nitrate in ethanol. It is discussed that amine and silane moieties in APTES contribute first to an efficient reduction on the silica and second to capping the Ag nanoparticles. The high density and homogeneity of nanoparticle nucleation is further regulated by the nanoporosity of the silica. The Ag/SiO₂ microparticles were tested for SERS using self-assembled 4-aminothiophenol monolayers, and an enhancement factor of ca. 2 × 10⁶ is measured. Importantly, the SERS relative standard deviation is 36% when a single microparticle is considered and drops to 11% when sets of 10 microparticles are considered. As prepared, the microparticles are highly suitable for state-of-the-art quantitative lab-on-a-bead interrogation of specimens.

Urea and creatinine detection on nano-laminated gold thin film using Kretschmann-based surface plasmon resonance biosensor

This work investigates the surface plasmon resonance (SPR) response of 50-nm thick nano-laminated gold film using Kretschmann-based biosensing for detection of urea and creatinine in solution of various concentrations (non-enzymatic samples). Comparison was made with the presence of urease and creatininase enzymes in the urea and creatinine solutions (enzymatic samples), respectively. Angular interrogation technique was applied using optical wavelengths of 670 nm and 785 nm. The biosensor detects the presence of urea and creatinine at concentrations ranging from 50–800 mM for urea samples and 10–200 mM for creatinine samples. The purpose of studying the enzymatic sample was mainly to enhance the sensitivity of the sensor towards urea and creatinine in the samples. Upon exposure to 670 nm optical wavelength, the sensitivity of 1.4°/M was detected in non-enzymatic urea samples and 4°/M in non-enzymatic creatinine samples. On the other hand, sensor sensitivity as high as 16.2°/M in urea-urease samples and 10°/M in creatinine-creatininase samples was detected. The enhanced sensitivity possibly attributed to the increase in refractive index of analyte sensing layer due to urea-urease and creatinine-creatininase coupling activity. This work has successfully proved the design and demonstrated a proof-of-concept experiment using a low-cost and easy fabrication of Kretschmann based nano-laminated gold film SPR biosensor for detection of urea and creatinine using urease and creatininase enzymes.

Surface potential and morphology mapping to investigate analyte adsorption effects on surface enhanced Raman scattering (SERS)

We demonstrate the power of Kelvin probe force microscopy (KPFM) in enabling a comprehensive study of enhancement mechanisms of surface enhanced Raman scattering (SERS) through the correlation of surface electrical and topographical effects. Local electric fields generated on Au/ZnO nanohybrid films impact analyte adsorption, while roughness is linked to hotspot generation. Optimizing the interplay between these two effects yields SERS enhancement factors (EFs) of 10⁶, enabling ppb detection of polycyclic aromatic hydrocarbons (PAHs) in water.

Dealloyed Intra-Nanogap Particles with Highly Robust, Quantifiable Surface-Enhanced Raman Scattering Signals for Biosensing and Bioimaging Applications

Uniformly controlling a large number of metal nanostructures with a plasmonically enhanced signal to generate quantitative optical signals and the widespread use of these structures for surface-enhanced Raman scattering (SERS)-based biosensing and bioimaging applications are of paramount importance but are extremely challenging. Here, we report a highly controllable, facile selective-interdiffusive dealloying chemistry for synthesizing the dealloyed intra-nanogap particles (DIPs) with a ∼2 nm intragap in a high yield (∼95%) without the need for an interlayer. The SERS signals from DIPs are highly quantitative and polarization-independent with polarized laser sources. Remarkably, all the analyzed particles displayed the SERS enhancement factors (EFs) of ≥1.1 × 108 with a very narrow distribution of EFs. Finally, we show that DIPs can be used as ultrasensitive SERS-based DNA detection probes for detecting 10 aM to 1 pM target concentrations and highly robust, quantitative real-time cell imaging probes for long-term imaging with low laser power and short exposure time.

Gold@silver bimetal nanoparticles/pyramidal silicon 3D substrate with high reproducibility for high-performance SERS

A novel and efficient surface enhanced Raman scattering (SERS) substrate has been presented based on Gold@silver/pyramidal silicon 3D substrate (Au@Ag/3D-Si). By combining the SERS activity of Ag, the chemical stability of Au and the large field enhancement of 3D-Si, the Au@Ag/3D-Si substrate possesses perfect sensitivity, homogeneity, reproducibility and chemical stability. Using R6G as probe molecule, the SERS results imply that the Au@Ag/3D-Si substrate is superior to the 3D-Si, Ag/3D-Si and Au/3D-Si substrate. We also confirmed these excellent behaviors in theory via a commercial COMSOL software. The corresponding experimental and theoretical results indicate that our proposed Au@Ag/3D-Si substrate is expected to develop new opportunities for label-free SERS detections in biological sensors, biomedical diagnostics and food safety.

Texture orientation of silver thin films grown via gas-timing radio frequency magnetron sputtering and their SERS activity

Here we have demonstrated the special technique so called gas-timing (GT) rf magnetron sputtering which allow us to control a texture orientation of Ag thin films without applying any additional energy sources.

Few molecule SERS detection using nanolens based plasmonic nanostructure: application to point mutation detection

Self-similar chain based nanolens plasmonic devices were fabricated for detecting single point mutations.

Effect of Pore Size and Film Thickness on Gold-Coated Nanoporous Anodic Aluminum Oxide Substrates for Surface-Enhanced Raman Scattering Sensor

A sensitive surface enhanced Raman scattering chemical sensor is demonstrated by using inexpensive gold-coated nanoporous anodic aluminum oxide substrates. To optimize the performance of the substrates for sensing by the Surface-enhanced Raman scattering (SERS) technique, the size of the nanopores is varied from 18 nm to 150 nm and the gold film thickness is varied from 30 nm to 120 nm. The sensitivity of gold-coated nanoporous surface enhanced Raman scattering sensor is characterized by detecting low concentrations of Rhodamine 6G laser dye molecules. The morphology of the SERS substrates is characterized by atomic force microscopy. Optical properties of the nanoporous SERS substrates including transmittance, reflectance, and absorbance are also investigated. Relative signal enhancement is plotted for a range of substrate parameters and a detection limit of 10⁻⁶ M is established.