Progress in the Development of SERS-Active Substrates Based on Metal-Coated Porous Silicon

Porous silicon-based sensing and delivery platforms for wound management applications

Wound management remains a great challenge for clinicians due to the complex physiological process of wound healing. Porous silicon (PSi) with controlled pore morphology, abundant surface chemistry, unique photonic properties, good biocompatibility, easy biodegradation and potential bioactivity represent an exciting class of materials for various biomedical applications. In this review, we focus on the recent progress of PSi in the design of advanced sensing and delivery systems for wound management applications. Firstly, we comprehensively introduce the common type, normal healing process, delaying factors and therapeutic drugs of wound healing. Subsequently, the typical fabrication, functionalization and key characteristics of PSi have been summarized because they provide the basis for further use as biosensing and delivery materials in wound management. Depending on these properties, the rise of PSi materials is evidenced by the examples in literature in recent years, which has emphasized the robust potential of PSi for wound monitoring, treatment and theranostics. Finally, challenges and opportunities for the future development of PSi-based sensors and delivery systems for wound management applications are proposed and summarized. We hope that this review will help readers to better understand current achievements and future prospects on PSi-based sensing and delivery systems for advanced wound management.

Plasmonic surface-enhanced Raman scattering nano-substrates for detection of anionic environmental contaminants: Current progress and future perspectives

Surface-enhanced Raman scattering spectroscopy (SERS) is a powerful technique of vibrational spectroscopy based on the inelastic scattering of incident photons by molecular species. It has unique properties such as ultra-sensitivity, selectivity, non-destructivity, speed, and fingerprinting properties for analytical and sensing applications. This enables SERS to be widely used in real-world sample analysis and basic plasmonic mechanistic studies. However, the desirable properties of SERS are compromised by the high cost and low reproducibility of the signals. The development of multifunctional, stable and reusable nano-engineered SERS substrates is a viable solution to circumvent these drawbacks. Recently, plasmonic SERS active nano-substrates with various morphologies have attracted the attention of researchers due to promising properties such as the formation of dense hot spots, additional stability, tunable and controlled morphology, and surface functionalization. This comprehensive review focused on the current advances in the field of SERS active nanosubstrates suitable for the detection and quantification of anionic environmental pollutants. The common fabrication methods, including the techniques for morphological adjustments and surface modification, substrate categories, and the design of nanotechnologically fabricated plasmonic SERS substrates for anion detection are systematically presented. Here, the need for the design, synthesis, and functionalization of SERS nano-substrates for anions of great environmental importance is explained in detail. In addition, the broad categories of SERS nano-substrates, namely colloid-based SERS substrates and solid-support SERS substrates are discussed. Moreover, a brief discussion of SERS detection of certain anionic pollutants in the environment is presented. Finally, the prospects in the fabrication and commercialization of pilot-scale handheld SERS sensors and the construction of smart nanosubstrates integrated with novel amplifying materials for the detection of anions of environmental and health concern are proposed.

Paper-Based Substrate for a Surface-Enhanced Raman Spectroscopy Biosensing Platform-A Silver/Chitosan Nanocomposite Approach

Paper is a popular platform material in all areas of sensor research due to its porosity, large surface area, and biodegradability, to name but a few. Many paper-based nanocomposites have been reported in the last decade as novel substrates for surface-enhanced Raman spectroscopy (SERS). However, there are still limiting factors, like the low density of hot spots or loss of wettability. Herein, we designed a process to fabricate a silver-chitosan nanocomposite layer on paper celluloses by a layer-by-layer method and pH-triggered chitosan assembly. Under microscopic observation, the resulting material showed a nanoporous structure, and silver nanoparticles were anchored evenly over the nanocomposite layer. In SERS measurement, the detection limit of 4-aminothiophenol was 5.13 ppb. Furthermore, its mechanical property and a strategy toward further biosensing approaches were investigated.

Trace level detection of explosives and pesticides using robust, low-cost, free-standing silver nanoparticles decorated porous silicon

We report results from our extensive studies on the fabrication of ultra-thin, flexible, and cost-effective Ag nanoparticle (NP) coated free-standing porous silicon (FS-pSi) for superior molecular sensing. The FS-pSi has been prepared by adopting a simple wet-etching method. The deposition time of AgNO₃ has been increased to improve the number of hot-spot regions, thereby the sensing abilities are improved efficiently. FESEM images illustrated the morphology of uniformly distributed AgNPs on the pSi surface. Initially, a dye molecule [methylene blue (MB)] was used as a probe to evaluate the sensing capabilities of the substrate using the surface-enhanced Raman scattering (SERS) technique. The detection was later extended towards the sensing of two important explosive molecules [ammonium nitrate (AN), picric acid (PA)], and a pesticide molecule (thiram) clearly demonstrating the versatility of the investigated substrates. The sensitivity was confirmed by estimating the analytical enhancement factor (AEF), which was ∼10⁷ for MB and ∼10⁴ for explosives and pesticides. We have also evaluated the limit of detection (LOD) values in each case, which were found to be 50 nM, 1 µM, 2 µM, and 1 µM, respectively, for MB, PA, AN, and thiram. Undeniably, our detailed SERS results established excellent reproducibility with a low RSD (relative standard deviation). Furthermore, we also demonstrate the reasonable stability of AgNPs decorated pSi by inspecting and studying their SERS performance over a period of 90 days. The overall cost of these substrates is attractive for practical applications on account of the above-mentioned superior qualities.

Plasmonic Nanohole Arrays on Top of Porous Silicon Sensors: A Win-Win Situation

Label-free optical sensors are attractive candidates, for example, for detecting toxic substances and monitoring biomolecular interactions. Their performance can be pushed by the design of the sensor through clever material choices and integration of components. In this work, two porous materials, namely, porous silicon and plasmonic nanohole arrays, are combined in order to obtain increased sensitivity and dual-mode sensing capabilities. For this purpose, porous silicon monolayers are prepared by electrochemical etching and plasmonic nanohole arrays are obtained using a bottom-up strategy. Hybrid sensors of these two materials are realized by transferring the plasmonic nanohole array on top of the porous silicon. Reflectance spectra of the hybrid sensors are characterized by a fringe pattern resulting from the Fabry-Pérot interference at the porous silicon borders, which is overlaid with a broad dip based on surface plasmon resonance in the plasmonic nanohole array. In addition, the hybrid sensor shows a significant higher reflectance in comparison to the porous silicon monolayer. The sensitivities of the hybrid sensor to refractive index changes are separately determined for both components. A significant increase in sensitivity from 213 ± 12 to 386 ± 5 nm/RIU is determined for the transfer of the plasmonic nanohole array sensors from solid glass substrates to porous silicon monolayers. In contrast, the spectral position of the interference pattern of porous silicon monolayers in different media is not affected by the presence of the plasmonic nanohole array. However, the changes in fringe pattern reflectance of the hybrid sensor are increased 3.7-fold after being covered with plasmonic nanohole arrays and could be used for high-sensitivity sensing. Finally, the capability of the hybrid sensor for simultaneous and independent dual-mode sensing is demonstrated.

Immersion-plated palladium nanoparticles onto meso-porous silicon layer as novel SERS substrate for sensitive detection of imidacloprid pesticide

Herein, we demonstrate the effectiveness of surface-enhanced Raman scattering (SERS) to detect trace concentration of potentially harmful imidacloprid pesticide. To achieve this ultimate objective, a rapid and highly effective methodology for the fabrication of active and stable porous silicon (PSi) plated palladium nanoparticles (PdNPs) SERS substrates by an electrochemical anodization and immersion plating routes was applied. The PSi layers were fabricated by the electrochemical anodization of a silicon wafer in ethanoic fluoride solution, followed by uniformly deposition of PdNPs via a simple immersion plating technique. The structural features and morphology of fabricated frameworks of PSi-Pd NPs have been investigated by field emission scanning electron microscopy (FE-SEM) with energy dispersive X-ray (EDX), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectra. The PSi substrate demonstrates a meso-porous morphology with good distribution, good pore density and average pore sizes around 20 nm. The SERS performance of Si-Pd NPs and PSi-Pd NPs substrates has been examined taking imidacloprid (an insecticide) as a target analyte. The SERS signal of imidacloprid using PSi-Pd NPs substrate exhibited immense enhancement compared to the Si-Pd NPs substrate. The active substrate revealed excellent detectable performance with a concentration as low as 10^-9 M imidacloprid and an enhancement factor (EF) of 1.2 × 10⁵. This large EF is fundamentally ascribed to the combined effect of the electromagnetic improvement and charge transfer mechanisms. Additionally, no aging effect was observed for the present substrates kept in air for two weeks. Striking enhancement in Raman spectral signals obtained with the current PSi-Pd NPs substrates can provide a simple and smooth platform towards the sensitive detection of various target analytes.

Combined negative dielectrophoresis with a flexible SERS platform as a novel strategy for rapid detection and identification of bacteria

Surface-enhanced Raman spectroscopy (SERS) is a vibrational method successfully applied in analytical chemistry, molecular biology and medical diagnostics. In this article, we demonstrate the combination of the negative dielectrophoretic (nDEP) phenomenon and a flexible surface-enhanced Raman platform for quick isolation (3 min), concentration and label-free identification of bacteria. The platform ensures a strong enhancement factor, high stability and reproducibility for the SERS response of analyzed samples. By introducing radial dielectrophoretic forces directed at the SERS platform, we can efficiently execute bacterial cell separation, concentration and deposition onto the SERS-active surface, which simultaneously works as a counter electrode and thus enables such hybrid DEP-SERS device vibration-based detection. Additionally, we show the ability of our DEP-SERS system to perform rapid, cultivation-free, direct detection of bacteria in urine and apple juice samples. The device provides new opportunities for the detection of pathogens.

Raman Signal Enhancement Tunable by Gold-Covered Porous Silicon Films with Different Morphology

The ease of fabrication, large surface area, tunable pore size and morphology as well surface modification capabilities of a porous silicon (PSi) layer make it widely used for sensoric applications. The pore size of a PSi layer can be an important parameter when used as a matrix for creating surface-enhanced Raman scattering (SERS) surfaces. Here, we evaluated the SERS activity of PSi with pores ranging in size from meso to macro, the surface of which was coated with gold nanoparticles (Au NPs). We found that different pore diameters in the PSi layers provide different morphology of the gold coating, from an almost monolayer to 50 nm distance between nanoparticles. Methylene blue (MB) and 4-mercaptopyridine (4-MPy) were used to describe the SERS activity of obtained Au/PSi surfaces. The best Raman signal enhancement was shown when the internal diameter of torus-shaped Au NPs is around 35 nm. To understand the role of plasmonic resonances in the observed SERS spectrum, we performed electromagnetic simulations of Raman scattering intensity as a function of the internal diameter. The results of these simulations are consistent with the obtained experimental data.

Hybrid Porous Silicon Biosensors Using Plasmonic and Fluorescent Nanomaterials: A Mini Review

During the last two decades, porous silicon (PSi) has been proposed as a high-performance biosensing platform due to its biocompatibility, surface tailorability, and reproducibility. This review focuses on the recent developments and progress in the area related to hybrid PSi biosensors using plasmonic metal nanoparticles (MNPs), fluorescent quantum dots (QDs), or a combination of both MNPs and QDs for creating hybrid nanostructured architectures for ultrasensitive detection of biomolecules. The review discusses the mechanisms of sensitivity enhancement based on Localized Surface Plasmon Resonance (LSPR) of MNPs, Fluorescence Resonance Energy Transfer (FRET) in the case of MNPs/QDs donor-acceptor interactions, and photoluminescence/fluorescence enhancement resulting from the embedded fluorescent QDs inside the PSi microcavity. The review highlights the key features of hybrid PSi/MNPs/QDs biosensors for dual-mode detection applications.

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.

Plasmonic biosensors fabricated by galvanic displacement reactions for monitoring biomolecular interactions in real time

Optical sensors are prepared by reduction of gold ions using freshly etched hydride-terminated porous silicon, and their ability to specifically detect binding between protein A/rabbit IgG and asialofetuin/Erythrina cristagalli lectin is studied. The fabrication process is simple, fast, and reproducible, and does not require complicated lab equipment. The resulting nanostructured gold layer on silicon shows an optical response in the visible range based on the excitation of localized surface plasmon resonance. Variations in the refractive index of the surrounding medium result in a color change of the sensor which can be observed by the naked eye. By monitoring the spectral position of the localized surface plasmon resonance using reflectance spectroscopy, a bulk sensitivity of 296 nm ± 3 nm/RIU is determined. Furthermore, selectivity to target analytes is conferred to the sensor through functionalization of its surface with appropriate capture probes. For this purpose, biomolecules are deposited either by physical adsorption or by covalent coupling. Both strategies are successfully tested, i.e., the optical response of the sensor is dependent on the concentration of respective target analyte in the solution facilitating the determination of equilibrium dissociation constants for protein A/rabbit IgG as well as asialofetuin/Erythrina cristagalli lectin which are in accordance with reported values in literature. These results demonstrate the potential of the developed optical sensor for cost-efficient biosensor applications. Graphical abstract.

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.

Print metallic nanoparticles on a fiber probe for 1064-nm surface-enhanced Raman scattering

This Letter presents 1064-nm surface-enhanced Raman scattering (SERS) on an optical fiber probe, or 1064-nm-SERS-on-fiber. Metallic nanoparticles are printed on an optical fiber probe by using optothermal surface bubbles under ambient conditions. An optothermal surface bubble is a laser-induced micro-sized bubble that is formed on a solid-liquid interface. The SERS activity of the optical fiber probe for 1064-nm Raman microscopy is tested with rhodamine 6G in aqueous solution. The 1064-nm-SERS-on-fiber can reduce the fluorescent background noise that commonly exists in other Raman systems. It can also compensate for the decreased Raman signal due to the use of an infrared Raman laser. The 1064-nm-SERS-on-fiber will find potential applications in low-background-noise biosensing and endoscopy.

Electrospinning Nanoparticles-Based Materials Interfaces for Sensor Applications

Electrospinning is a facile technique to fabricate nanofibrous materials with adjustable structure, property, and functions. Electrospun materials have exhibited wide applications in the fields of materials science, biomedicine, tissue engineering, energy storage, environmental science, sensing, and others. In this review, we present recent advance in the fabrication of nanoparticles (NPs)-based materials interfaces through electrospinning technique and their applications for high-performance sensors. To achieve this aim, first the strategies for fabricating various materials interfaces through electrospinning NPs, such as metallic, oxide, alloy/metal oxide, and carbon NPs, are demonstrated and discussed, and then the sensor applications of the fabricated NPs-based materials interfaces in electrochemical, electric, fluorescent, colorimetric, surface-enhanced Raman scattering, photoelectric, and chemoresistance-based sensing and detection are presented and discussed in detail. We believe that this study will be helpful for readers to understand the fabrication of functional materials interfaces by electrospinning, and at the same time will promote the design and fabrication of electrospun nano/micro-devices for wider applications in bioanalysis and label-free sensors.

Nanosilicon-Based Composites for (Bio)sensing Applications: Current Status, Advantages, and Perspectives

This review highlights the application of different types of nanosilicon (nano-Si) materials and nano-Si-based composites for (bio)sensing applications. Different detection approaches and (bio)functionalization protocols were found for certain types of transducers suitable for the detection of biological compounds and gas molecules. The importance of the immobilization process that is responsible for biosensor performance (biomolecule adsorption, surface properties, surface functionalization, etc.) along with the interaction mechanism between biomolecules and nano-Si are disclosed. Current trends in the fabrication of nano-Si-based composites, basic gas detection mechanisms, and the advantages of nano-Si/metal nanoparticles for surface enhanced Raman spectroscopy (SERS)-based detection are proposed.

Effect of Graphene Coating on SERS Performance of Plasmonic Nanostructures Based on Silvered Porous Silicon

The degradation of the Surface-enhanced Raman scattering (SERS) signal under laser illumination during spectral measurements is typically observed. To improve the stability of the SERS spectra intensity, hybrid plasmonic structures involving graphene-based protective layers were formed. An observation of the time evolution of the characteristic peak from analyte molecules indicates that the graphene coating can be used to improve the stability of the SERS signal.

Near-Infrared Surface-Enhanced Raman Scattering on Silver-Coated Porous Silicon Photonic Crystals

Surface-enhanced Raman scattering (SERS) with near-infrared (NIR) excitation offers a safe way for the detection and study of fragile biomolecules. In this work, we present the possibility of using silver-coated porous silicon photonic crystals as SERS substrates for near-infrared (1064 nm) excitation. Due to the deep penetration of NIR light inside silicon, the fabrication of photonic crystals was necessary to quench the band gap photoluminescence of silicon crystal, which acts as mechanical support for the porous layer. Optimal parameters of the immersion plating process that gave maximum enhancement were found and the activity of SERS substrates was tested using rhodamine 6G and crystal violet dye molecules, yielding significant SERS enhancement for off-resonant conditions. To our knowledge, this is the first time that the 1064 nm NIR laser excitation is used for obtaining the SERS effect on porous silicon as a substrate.

Surface Enhanced Raman Spectroscopy of Lactoferrin Adsorbed on Silvered Porous Silicon Covered with Graphene

We registered surface enhanced Raman scattering (SERS) spectra of the human lactoferrin molecules adsorbed on a silvered porous silicon (por-Si) from 10-6⁻10-18 M solutions. It was found that the por-Si template causes a negative surface potential of silver particles and their chemical resistivity to oxidation. These properties provided to attract positively charged lactoferrin molecules and prevent their interaction with metallic particles upon 473 nm laser excitation. The SERS spectra of lactoferrin adsorbed from 10-6 M solution were rather weak but a decrease of the concentration to 10-10 M led to an enormous growth of the SERS signal. This effect took place as oligomers of lactoferrin were broken down to monomeric units while its concentration was reduced. Oligomers are too large for a uniform overlap with electromagnetic field from silver particles. They cannot provide an intensive SERS signal from the top part of the molecules in contrast to monomers that can be completely covered by the electromagnetic field. The SERS spectra of lactoferrin at the 10-14 and 10-16 M concentrations were less intensive and started to change due to increasing contribution from the laser burned molecules. To prevent overheating the analyte molecules on the silvered por-Si were protected with graphene, which allowed the detection of lactoferrin adsorbed from the 10-18 M solution.

Femtosecond Laser-Induced, Nanoparticle-Embedded Periodic Surface Structures on Crystalline Silicon for Reproducible and Multi-utility SERS Platforms

Fabrication of reproducible and versatile surface-enhanced Raman scattering (SERS) substrates is crucial for real-time applications such as explosive detection for human safety and biological imaging for cancer diagnosis. However, it still remains a challenging task, even after several methodologies were developed by various research groups, primarily due to (a) a lack of consistency in detection of a variety of molecules (b) cost-effectiveness of the SERS substrates prepared, and (c) byzantine preparation procedures, etc. Herein, we establish a procedure for preparing reproducible SERS-active substrates comprised of laser-induced nanoparticle-embedded periodic surface structures (LINEPSS) and metallization of silicon (Si) LINEPSS. LINEPSS were fabricated using the technique of femtosecond laser ablation of Si in acetone. The versatile SERS-active substrates were then achieved by two ways, including the drop casting of silver (Ag)/gold (Au) nanoparticles (NPs) on Si LINEPSS and Ag plating on the Si LINEPSS structures. By controlling the LINEPSS grating periodicity, the effect of plasmonic nanoparticles/plasmonic plating on the Si NPs embedded periodic surface structures enormously improved the SPR strength, resulting in the consistent and superior Raman enhancements. The reproducible SERS signals were achieved by detecting the molecules of Methylene Blue (MB), 2,4-dinitrotoluene (DNT), and 5-amino-3-nitro-l,2,4-triazole (ANTA). The SERS signal strength is determined by the grating periodicity, which, in turn, is determined by the input laser fluence. The SERS-active platform with grating periodicity of 130 ± 10 nm and 150 ± 5 nm exhibited strong Raman enhancements of ∼10⁸ for MB and ∼10⁷ for ANTA molecules, respectively, and these platforms are demonstrated to be capable, even for multiple usages.

Detection of Chloroalkanes by Surface-Enhanced Raman Spectroscopy in Microfluidic Chips

Optofluidics, a research discipline combining optics with microfluidics, currently aspires to revolutionize the analysis of biological and chemical samples, e.g., for medicine, pharmacology, or molecular biology. In order to detect low concentrations of analytes in water, we have developed an optofluidic device containing a nanostructured substrate for surface enhanced Raman spectroscopy (SERS). The geometry of the gold surface allows localized plasmon oscillations to give rise to the SERS effect, in which the Raman spectral lines are intensified by the interaction of the plasmonic field with the electrons in the molecular bonds. The SERS substrate was enclosed in a microfluidic system, which allowed transport and precise mixing of the analyzed fluids, while preventing contamination or abrasion of the highly sensitive substrate. To illustrate its practical use, we employed the device for quantitative detection of persistent environmental pollutant 1,2,3-trichloropropane in water in submillimolar concentrations. The developed sensor allows fast and simple quantification of halogenated compounds and it will contribute towards the environmental monitoring and enzymology experiments with engineered haloalkane dehalogenase enzymes.