Grating-like SERS substrate with tunable gaps based on nanorough Ag nanoislands/moth wing scale arrays for quantitative detection of cypermethrin

Cotton swabs wrapped with three-dimensional silver nanoflowers as SERS substrates for the determination of food colorant carmine on irregular surfaces

A lightweight, portable, low-cost, and accessible cotton swab was employed as surface enhanced Raman spectroscopy (SERS) matrix template. The silver nanoflowers were in situ grown on the surface of cotton swabs to form three-dimensional Ag nanoflower@cotton swabs (AgNF@CS) SERS substrate with high-density and multi-level hot spots. The SERS performance of AgNFs@CS substrates with various reaction time was systematically studied. The optimal AgNF-120@CS SERS substrate exhibits superior detection sensitivity of 10-10 M for methylene blue, good signal reproducibility, high enhancement factor of 1.4 × 107, and excellent storage stability (over 30 days). Moreover, the AgNF-120@CS SERS substrate also exhibits prominent detection sensitivity of 10-8 M for food colorant of carmine. Besides, the portable AgNF-120@CS SERS substrate is also capable of detecting food colorant residues on irregular food surfaces.

Aramid nanofiber membrane decorated with monodispersed silver nanoparticles as robust and flexible SERS chips for trace detection of multiple toxic substances

Aramid nanofibers (ANFs) as an innovative nanoscale building block exhibit great potential for novel high-performance multifunctional membranes attributed to their extraordinary performance. However, the application of aramid nanofibers in the field of surface enhanced Raman scattering (SERS) sensing has been rarely reported. In this work, aramid nanofibers derived from commercial Kevlar fibers were synthesized by a facile dimethyl sulfoxide/potassium hydroxide (DMSO/KOH) solution treatment. The monodispersed silver nanoparticle-decorated aramid nanofiber (m-Ag@ANF) membranes were constructed by an efficient vacuum filtration technique. Taking advantages of unique intrinsic properties of ANF, the m-Ag@ANF substrates exhibit good flexibility, excellent mechanical properties and prominent thermal stability. Besides, due to the abundance of positively charged amino-group on the ANF substrates, the negatively charged m-AgNPs were uniformly and firmly deposited on the surface of ANF substrate through electrostatic interactions. As a result, the optimal flexible m-Ag-9@ANF SERS substrate exhibits high sensitivity of 10-9 M for methylene blue (MB) and excellent signal reproducibility (RSD = 6.37 %), as well as outstanding signal stability (up to 15 days). Besides, the 2D Raman mapping and FDTD simulations further reveal prominent signal homogeneity and strong electric field distribution for flexible m-Ag-9@ANF SERS substrate. Finally, it is demonstrated that the flexible m-Ag-9@ANF SERS substrate can also be used for detection of toxic molecules on irregular surfaces by a feasible paste-and-read process. The m-Ag@ANF paper exhibits potential applications as a flexible, low-cost, robust and stable SERS sensing platform for trace detection of toxic materials.

Development of Copper Nanoclusters-Based Turn-Off Nanosensor for Fluorescence Detection of Two Pyrethroid Pesticides (Cypermethrin and Lambda-Cyhalothrin)

Fluorescent copper nanoclusters (Cu NCs) were synthesized by using Withania somnifera (W. somnifera) plant extract as a biotemplate. Aqueous dispersion of W. somnifera-Cu NCs displays intense emission peak at 458 nm upon excitation at 350 nm. This fluorescence emission was utilized for the detection of two pyrethroid pesticides (cypermethrin and lambda-cyhalothrin) via "turn-off" mechanism. Upon the addition of two pyrethiod pesticides independently, the fluorescence emission of W. somnifera-Cu NCs was gradually decreased with increasing concentrations of both pesticides. It was noticed that the decrease in emission intensity at 458 nm was linearly dependent on the logarithm of both pesticides concentrations in the ranges of 0.01-100 μM and of 0.05-100 μM for cypermethrin and lambda-cyhalothrin, respectively. Consequently, the limits of detection were found to be 27.06 and 23.28 nM for cypermethrin and lambda-cyhalothrin, respectively. The as-fabricated W. somnifera-Cu NCs acted as a facile sensor for the analyses of cypermethrin and lambda-cyhalothrin in vegetables (tomato and bottle gourd), which demonstrates that it could be used as portable sensing platform for assaying of two pyrethroid pesticides in food samples.

Tuning SERS Signal via Substrate Structuring: Valves of Different Diatom Species with Ultrathin Gold Coating

The discovered light modulation capabilities of diatom silicious valves make them an excellent toolkit for photonic devices and applications. In this work, a reproducible surface-enhanced Raman scattering (SERS) enhancement was achieved with hybrid substrates employing diatom silica valves coated with an ultrathin uniform gold film. Three structurally different hybrid substrates, based on the valves of three dissimilar diatom species, have been compared to elucidate the structural contribution to SERS enhancement. The comparative analysis of obtained results showed that substrates containing cylindrical Aulacoseira sp. valves achieved the highest enhancement, up to 14-fold. Numerical analysis based on the frequency domain finite element method was carried out to supplement the experimental results. Our results demonstrate that diatom valves of different shapes can enhance the SERS signal, offering a toolbox for SERS-based sensors, where the magnitude of the enhancement depends on valve geometry and ultrastructure.

Numerical Analysis of the Light Modulation by the Frustule of Gomphonema parvulum: The Role of Integrated Optical Components

Siliceous diatom frustules present a huge variety of shapes and nanometric pore patterns. A better understanding of the light modulation by these frustules is required to determine whether or not they might have photobiological roles besides their possible utilization as building blocks in photonic applications. In this study, we propose a novel approach for analyzing the near-field light modulation by small pennate diatom frustules, utilizing the frustule of Gomphonema parvulum as a model. Numerical analysis was carried out for the wave propagation across selected 2D cross-sections in a statistically representative 3D model for the valve based on the finite element frequency domain method. The influences of light wavelength (vacuum wavelengths from 300 to 800 nm) and refractive index changes, as well as structural parameters, on the light modulation were investigated and compared to theoretical predictions when possible. The results showed complex interference patterns resulting from the overlay of different optical phenomena, which can be explained by the presence of a few integrated optical components in the valve. Moreover, studies on the complete frustule in an aqueous medium allow the discussion of its possible photobiological relevance. Furthermore, our results may enable the simple screening of unstudied pennate frustules for photonic applications.

Protein capture and SERS detection on multiwavelength rainbow-trapping width-graded nano-gratings

Surface-enhanced Raman scattering (SERS) substrates with multiwavelength rainbow-trapping properties hold the potential for a one-size-fits-all platform for rapid and multiplexed disease detection. We present the first report on the utilization of rainbow-trapping width-graded nano-gratings, a new class of chirped metamaterials, to detect protein biomarkers. Using cytochrome c (Cc), a charged analyte with inherent difficulty in adsorbing onto sputtered silver films, we investigated methods of binding Cc on the silver nano-grating in order to improve the SERS signal strength at both 532 and 638 nm excitation. Cc was not detectable on the Ag nano-gratings without surface functionalization at 1μM concentration. Upon charge reversal functionalization of the Ag nano-gratings, 1μM Cc was detectable albeit not reliably. By further crosslinking 1μM Cc to the functionalized Ag nano-gratings, the analyte-capture detection scheme greatly improved the SERS signal strength and reliability at both excitation wavelengths and allowed for quantification of their coefficients of variation with values down to 27%.

Self-Assembled Black Phosphorus-Based Composite Langmuir-Blodgett Films with an Enhanced Photocurrent Generation Capability and Surface-Enhanced Raman Scattering Properties

In this work, Langmuir-Blodgett (LB) composite thin films were successfully prepared using black phosphorus nanosheets (BPNS) and dye molecules. Black phosphorus (BP) was first exfoliated in isopropanol solution to form BPNS, and then, BPNS were modified with 4-azidobenzoic acid (Az-BPNS) to improve their stability. The characterization results showed that the synthesized Az-BPNS-dye LB films have a uniform and ordered structure. In addition, the synthesized Az-BPNS-dye LB films exhibit excellent photoelectrochemical performance, and Az-BPNS-methylene blue (MB) produces higher photocurrent compared to Az-BPNS-Neutral red (NR) films. The current work shows an effective way to prepare functionalized BP-based materials and provide evidence for their application in optoelectronic devices.

Fabrication and SERS performance of silver nanoarrays by inkjet printing silver nanoparticles ink on the gratings of compact disc recordable

Silver nanoarrays were fabricated by inkjet printing silver nanoparticles ink on the gratings of compact disc recordable (CD-R). Rhodamine 6G (R6G) was chosen as a probe molecule to evaluate their surface-enhanced Raman scattering (SERS) performance. The finite-difference time domain (FDTD) solution was used to simulate local electric field distribution of silver nanoparticles on the grating surface and flat surface, respectively. It was found that the Ag/grating substrate possessed higher enhancement ability than the Ag/flat due to the high-density hot spots of periodic structure of the grating. The silver nanoarrays substrate exhibited high stability and the characteristic peaks of R6G can be still well observed after eight months. The substrate also exhibited a good spot-to-spot reproducibility with an RSD of 10.21% by eight points. SERS mappings of R6G adsorbed on silver nanoarrays were tested under the ultra-fast Raman imaging mode, and the relative standard deviation (RSD) values of uniformity were calculated to be 8.35% and 11.53% at 610 cm^-1 band measured by 2500 and 6480 points, respectively. In addition, the as-prepared silver nanoarrays was successfully applied to the detection of melamine in adult milk powder solution directly. A good linear relationship with the correlation coefficient of 0.9968 between peak intensity and concentration was obtained from 1.2 to 100 mg/L.

Improved Charge Transfer and Hot Spots by Doping and Modulating the Semiconductor Structure: A High Sensitivity and Renewability Surface-Enhanced Raman Spectroscopy Substrate

Here, we develop a new method to improve the surface-enhanced Raman spectroscopy (SERS) activity of ZnO using Mg doping combined with noble metals. Highly aligned silver nanoparticles (AgNPs) decorated on an array of Mg-doped ZnO (MZO@Ag) were fabricated. Using rhodamine 6G as the probe molecule, SERS indicated that the MZO@Ag substrate possesses perfect sensitivity, homogeneity, and chemical stability. The enhancement mechanism of this substrate was analyzed in detail, and finite-difference time-domain (FDTD) simulations were used to examine "hot spot" distribution which generated gaps between the balls, the rods, and the stems. FDTD simulation calculated ( E/ E₀)⁴ to be 2.5 × 10⁶. Furthermore, the prepared substrates could degrade the target molecules in situ irradiated by visible light irradiation over the course of 40 min and then efficiently recover detectability through a recycling process. Our substrates were easy to fabricate, self-cleaning, and reusable. They are expected to provide new opportunities for the use of SERS in biological sensors, biomedical diagnostics, and food safety.

Preparation of a Novel SERS Platform Based on Mantis Wing with High-Density and Multi-Level "Hot Spots"

The recent development of SERS substrates based on irregular nanostructures for directly molecule recognition has aroused increasing attention. By combining the irregular flake-like nanostructures of mantis wings, high SERS performance of Ag nanofilms, and the chemical stability of Au nanoparticles (NPs), an ultra-sensitive and flexible SERS substrate based on Au NPs functionalized Ag nanofilms-mantis wings (Au-Ag-M.w.) hybrid system is successfully fabricated. When 4-aminothiophenol is selected as the probe molecule, the limit of detection (LOD) is as low as 10-13 M and the relative standard deviation (RSD) is lower than 7.15%. This novel SERS platform exhibits high SERS performance in terms of sensitivity, reproducibility and practicability mainly because there are high-density and multi-level "hot spots" in the appropriate nanogaps. Meanwhile, it also systematically compares the differences of the SERS performance of Cu and Ag decorated M.w. hybrids and how these differences can alter their response. Moreover, the proposed substrate is employed to rapidly detect the pesticide residues on apple peels and the LOD for cypermethrin is estimated at 10-10 mg/mL. Therefore, this novel SERS substrate has great potential in rapid sampling of pesticide residues on real samples and expands the investigation to other natural materials for fabricating various SERS platforms.

Biomimetic synthesis of Ag-coated glasswing butterfly arrays as ultra-sensitive SERS substrates for efficient trace detection of pesticides

In this work, we report a biomimetic synthesis route of 3D Ag nanofilm/glasswing butterfly wing hybrids (Ag-G.b.) by magnetron sputtering technology. The 3D surface-enhanced Raman scattering (SERS) substrate is fabricated from an original chitin-based nanostructure, which serves as a bio-scaffold for Ag nanofilms to be coated on. The novel crisscrossing plate-like nanostructures of 3D Ag-G.b. nanohybrids with thick Ag nanofilms provide a substantial contribution to SERS enhancement. Measuring the SERS performance with crystal violet (CV), the Ag-G.b. nanohybrids with the sputtering time of 20 min (Ag-G.b.-20) shows the highest enhancement performance with an enhancement factor (EF) of up to 2.96 × 10⁷. The limit of detection (LOD) for CV was as low as 10^-11 M, demonstrating the ultrahigh sensitivity of the Ag-G.b.-20 substrate. In addition, the Ag-G.b.-20 substrate has an outstanding reproducibility across the entire area with the maximum value of relative standard deviation (RSD) of less than 10.78%. The nanohybrids also exhibit a long-term stability regarding Raman enhancement, as suggested by a duration stability test over a period of 60 days. Importantly, the high-performance Ag-G.b.-20 substrate is further applied as an ultra-sensitive SERS platform for the trace detection of acephate, showing its great potential application in biochemical sensing and food security.

Graphene-Ag nanoparticles-cicada wings hybrid system for obvious SERS performance and DNA molecular detection

In recent years, biomaterials have increasingly attracted attention on surface-enhanced Raman spectroscopy (SERS) due to their well Raman performance while metal particles are combined with biological substrates. Therefore, we propose an environmentally friendly substrate based on silver-plated cicada wings with seamless graphene layer (Gr-AgNPs-C.w.), which can be prepared with a simple and inexpensive method. Compared with AgNPs-C.w., Gr-AgNPs-C.w. hybrids show better SERS performance with high sensitivity, good uniformity and good stability with R6G detection. The minimum detected concentration can reach 10^-15 M, and the value of R² can reach 0.996, respectively. Theoretical simulation demonstrates the situation of electromagnetic field through COMSOL software. In addition, due to the affinity of graphene for biomolecules, we can successfully detect the DNA biomolecules through a simple process. Therefore, this cheap and efficient natural SERS substrate has great potential for a considerable number of biochemical SERS applications and can broaden the way in which multiple SERS platforms derived from other natural materials are prepared.

Three-Dimensional Hierarchical Reticular Nanostructure of Fulfora candelaria Wing Decorated by Ag Nanoislands as Practical SERS-Active Substrates

Although surface-enhanced Raman scattering (SERS) technology has been widely explored nowadays in various fields, the fabrication of practical SERS-active substrates with prominent recognition ability for various analyte molecules is still defective. Natural Fulfora candelaria wing (FCW) with three-dimensional (3D) hierarchical reticular nanostructure was selected as a new bioscaffold for rough silver (Ag) nanoislands to be assembled on to prepare a practical SERS substrate (Ag/FCW substrate). By adjusting the sputtering time of metal Ag, the morphology of the substrates could be easily tuned to control the formation and distribution of “hot spots”. Three-dimensional finite-difference time-domain (3D-FDTD) simulation indicated that the excellent SERS performance under optimal morphology was ascribed to the local enhanced electric field in rough Ag surface and effective “hot spot” areas. The SERS measurement results show that the optimal Ag/FCW substrates had high SERS performance in terms of Raman signal sensitivity, reproducibility, uniformity and recognition ability for various analyte molecules. Coupled with flexibility of the biological substrates and the cost effectiveness, the sensitive SERS detection of varied analytes based on Ag/FCW substrates offered great potential for practical applications.