Surface Enhanced Raman Spectroscopy Substrates Research Articles

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.

Surface-enhanced Raman spectroscopy biosensor based on silver nanoparticles@metal-organic frameworks with peroxidase-mimicking activities for ultrasensitive monitoring of blood cholesterol

Nanozymes have received much attention due to their advantages of lower cost, higher stability, and simple preparation when compared to conventional enzymes. Herein, we prepared a peroxidase-mimicking nanozyme by the growth of silver nanoparticles (AgNPs) on the surface of a metal-organic framework (MOF) called MIL-101 (Fe), obtaining the AgNPs@MOF. The AgNPs@MOF was applied as surface-enhanced Raman spectroscopy (SERS) substrate and simultaneously served as peroxidase mimics to oxidize the without Raman-active leucomalachite green into the Raman-active malachite green. Based on the excellent peroxidase-mimicking activity and high SERS enhancement of AgNPs@MOF, a highly sensitive and specific SERS sensing platform was proposed to monitor cholesterol. Under the optimized conditions, the established biosensor achieved a broad dynamic detection range of 1.0–100 μM and a relatively low limit of detection down to 0.36 μM. More importantly, this reported sensing platform was employed to detect cholesterol in human serum with excellent accuracy and precision, providing great potential in health management and clinical diagnosis.

Rapid and flexible construction of inverted silicon architectures with nanogaps as high-performance SERS substrates

Three-dimensional (3D) plasmonic nanostructures act as excellent surface-enhanced Raman spectroscopy (SERS) substrates for detecting trace analytes. Nevertheless, an efficient 3D architecture and the optimal fabrication route remain to be uncovered to further raise the performance of 3D SERS substrates and expand application horizons. This work reports a cost-effective and flexible route to fabricate 3D inverted silicon (Si) architectures with nanogaps that are difficult to achieve by conventional lithography towards SERS applications. Finite-difference time-domain simulations demonstrated that the distribution of electrical field depended significantly on the architecture and layout of inverted pyramids. Several parameters including volume ratio of HF/HNO3 mixtures, indentation force and etching time were evaluated to determine optimal fabrication process of inverted Si architecture. Based on an electrochemical dissolution model and molecular dynamic simulations, the rapid selective etching can be ascribed to nanocrystals in amorphous Si (a-Si) regions, and distorted Si beneath a-Si layer. SERS detection illustrated an impressive sensitivity and excellent reproducibility, and the practical applicability of as-prepared SERS substrates was demonstrated by detecting malachite green residues from different water environments as well as fish tissues and scales. The proposed selective etching provides a brand-new route for manufacturing high-performance 3D architecture substrates for SERS applications.

Rapid Assay for the Therapeutic Drug Monitoring of Edoxaban.

Edoxaban is a direct oral anticoagulant (DOAC) that has been recently indicated for the treatment of pulmonary embolism (PE) in SARS-CoV-2 infections. Due to its pharmacokinetic variability and a narrow therapeutic index, the safe administration of the drug requires its therapeutic drug monitoring (TDM) in patients receiving the treatment. In this work, we present a label-free method for the TDM of edoxaban by surface enhanced Raman spectroscopy (SERS). The new method utilises the thiol chemistry of the drug to chemisorb its molecules onto a highly sensitive SERS substrate. This leads to the formation of efficient hotspots and a strong signal enhancement of the drug Raman bands, thus negating the need for a Raman reporter for its SERS quantification. The standard samples were run with a concentration range of 1.4 × 10−4 M to 10−12 M using a mobile phase comprising of methanol/acetonitrile (85:15 v/v) at 291 nm followed by the good linearity of R2 = 0.997. The lowest limit of quantification (LOQ) by the SERS method was experimentally determined to be 10−12 M, whereas LOQ for HPLC-UV was 4.5 × 10−7 M, respectively. The new method was used directly and in a simple HPLC-SERS assembly to detect the drug in aqueous solutions and in spiked human blood plasma down to 1 pM. Therefore, the SERS method has strong potential for the rapid screening of the drug at pathology labs and points of care.

Magnetic-Core/Gold-Shell Nanoparticles for the Detection of Hydrophobic Chemical Contaminants.

Magnetic-core/gold-shell nanoparticles (MAuNPs) are of interest for enabling rapid and portable detection of trace adulterants in complex media. Gold coating provides biocompatibility and facile functionalization, and a magnetic core affords analyte concentration and controlled deposition onto substrates for surface-enhanced Raman spectroscopy. Iron oxide cores were synthesized and coated with gold by reduction of HAuCl4 by NH2OH. MAuNPs were grafted with polyethylene glycol (PEG) and/or functionalized with 4-mercaptobenzoic acid (4-MBA) and examined using a variety of microscopic, spectroscopic, magnetometric, and scattering techniques. For MAuNPs grafted with both PEG and 4-MBA, the order in which they were grafted impacted not only the graft density of the individual ligands, but also the overall graft density. Significant Raman signal enhancement of the model analyte, 4-MBA, was observed. This enhancement demonstrates the functionality of MAuNPs in direct detection of trace contaminants. The magnetic deposition rate of MAuNPs in chloroform and water was explored. The presence of 4-MBA slowed the mass deposition rate, and it was postulated that the rate disparity originated from differing NP-substrate surface interactions. These findings emphasize the importance of ligand choice in reference to the medium, target analyte, and substrate material, as well as functionalization procedure in the design of similar sensing platforms.

High-Throughput Fabrication of Triangular Nanogap Arrays for Surface-Enhanced Raman Spectroscopy.

Squeezing light into nanometer-sized metallic nanogaps can generate extremely high near-field intensities, resulting in dramatically enhanced absorption, emission, and Raman scattering of target molecules embedded within the gaps. However, the scarcity of low-cost, high-throughput, and reproducible nanogap fabrication methods offering precise control over the gap size is a continuing obstacle to practical applications. Using a combination of molecular self-assembly, colloidal nanosphere lithography, and physical peeling, we report here a high-throughput method for fabricating large-area arrays of triangular nanogaps that allow the gap width to be tuned from ∼10 to ∼3 nm. The nanogap arrays function as high-performance substrates for surface-enhanced Raman spectroscopy (SERS), with measured enhancement factors as high as 108 relative to a thin gold film. Using the nanogap arrays, methylene blue dye molecules can be detected at concentrations as low as 1 pM, while adenine biomolecules can be detected down to 100 pM. We further show that it is possible to achieve sensitive SERS detection on binary-metal nanogap arrays containing gold and platinum, potentially extending SERS detection to the investigation of reactive species at platinum-based catalytic and electrochemical surfaces.

Sensitive determination of Norfloxacin in milk based on β-cyclodextrin functionalized silver nanoparticles SERS substrate

The norfloxacin (NFX) residue in milk will increase human resistance to drugs and pose a threat to public health. In this work, a highly sensitive method for detection of NFX was developed based on surface enhanced Raman spectroscopy (SERS) using β-cyclodextrin functionalized silver nanoparticles (β-CD-AgNPs) as substrate. The unique spatial size and hydrophilicity of β-CD on the surface of AgNPs could selectively capture the target molecule (NFX) through some weak interactions, including hydrogen-bond interaction, electrostatic interaction, etc. The interactions were characterized by the UV–Vis absorption spectroscopy, fluorescence spectroscopy, Zeta potential and DLS. The Raman signal of NFX is largely enhanced when anchored by β-CD on the surface of AgNPs due to SERS effect. Through a series of experiments and analysis, the limit of detection (LOD) in standard solution and spiked milk were calculated to be 3.214 pmol/L and 5.327 nmol/L. The correlation coefficients (R2) were 0.986 and 0.984, respectively. For milk sample determination of NFX, the recovery was 101.29% to 104.00% with the relative standard deviation (RSD) from 2.986% to 9.136%. To sum up, this developed SERS strategy is sensitive and specific to detect NFX in milk, it has practical application value and prospects.

Ultrasensitive SERS Analysis of Liquid and Gaseous Putrescine and Cadaverine by a 3D-Rosettelike Nanostructure-Decorated Flexible Porous Substrate.

Putrescine and cadaverine are toxic biogenic amines in spoiled food, which poses a serious threat to food security. In this work, we reported a highly sensitive three-dimensional (3D)-rosettelike surface-enhanced Raman spectroscopy (SERS) substrate functionalized with a p-mercaptobenzoic acid (p-MBA) monolayer to detect liquid and gaseous putrescine and cadaverine in pork samples. The SERS substrate was made by a combination of the merit of the 3D morphology of ZnO nanorod arrays on a flexible porous poly(vinylidene fluoride) (PVDF) membrane and the in situ chemical growth of Au nanoparticle seeds on Au film-coated ZnO nanorods, which produced a 3D-rosettelike BigAuNP/Au/ZnO/P heterostructure with abundant SERS-active hot spots that significantly enhanced the localized surface plasmonic resonance (LSPR) effect and charge-transfer (CT) effect of Raman enhancement. This SERS substrate showed high sensitivity, reproducibility, stability, and uniformity. With the p-MBA molecular monolayer as the sensing interface, our SERS substrate realized the highly sensitive and quantitative detection of liquid putrescine and cadaverine within 10 min, with a limit of detection (LOD) of 3.2 × 10-16 and 1.6 × 10-13 M, respectively. Additionally, the sensor showed efficient SERS responses to gaseous amine molecules at low concentrations (putrescine: 1.26 × 10-9 M, cadaverine: 2.5 × 10-9 M). Further, the sensor was successfully applied to determine the total content of putrescine and cadaverine. Moreover, the practicability of this SERS sensor was verified by the measurement of liquid and gaseous amines in pork samples, and it showed great potential applications for sensitive detection of food spoilage.

Sensitive, selective and rapid detection of 4,4′-methylenedianiline by surface-enhanced Raman spectroscopy using flower-like gold-silver nanoalloy embedded nickel-cobalt layered double hydroxide composites

In this paper, an ultra-sensitive and selective surface-enhanced Raman spectroscopy (SERS) substrate was designed and fabricated for rapid determination of 4,4′-methylenedianiline (4,4′-MDA) based on flower-like gold-silver nanoalloys embedded nickel-cobalt layered double hydroxide composites (NiCo/Au2Ag1). Flower-like NiCo-LDH with thin nanosheets were prepared via the hydrothermal method and the gold-silver nanoalloys were in-situ embedded on the surface of NiCo-LDH. The ultrahigh SERS activity of NiCo/Au2Ag1 was originated from the synergistic effect of electromagnetic and chemical mechanism induced by AuAg nanoalloys and NiCo-LDH, respectively. In addition, the flower-like of NiCo-LDH can provide more sites for AuAg and target molecules loading, further making highly sensitive SERS activity. The minimum detectable concentration for 4-mercaptobenzoic acid (4-MBA), crystal violet (CV) and Rhodamine 6 G (R6G) are 1.0 pg L−1 and 1.0 ng L−1 with enhancement factor (EF) of 1.2 × 1010, 1.7 × 108 and 1.1 × 108, respectively. The theoretical simulations and experiment results can further confirm the high SERS performance of NiCo/Au2Ag1 and superiority of NiCo-LDH compared to Ni(OH)2 and Co(OH)2. Good selectivity and anti-interference for 4,4′-MDA indicates great potentials in contaminants screening and detection. Subsequently, the NiCo/Au2Ag1 composites were used to determine group 2B carcinogen of 4,4′-MDA in water samples and indoor dust. The fabricated NiCo/Au2Ag1 substrate show good satisfied linearity (5.0–750.0 μg L−1) with low limits of detection (LOD) of 1.5 μg L−1, and the practical applications in environmental samples were obtained with good rapidity and accuracy. The developed NiCo/Au2Ag1 offers a novel strategy for fabricating SERS substrates with high sensitivity and selectivity in rapid monitoring of environmental pollutants.

Surface-enhanced Raman spectroscopic analysis of centrifugally filtered HBV serum samples

BackgroundRaman spectroscopy is an effective tool for detecting and discriminating centrifugally filtered hepatitis B virus serum and centrifugally filtered control serum. ObjectivesThe purpose of current study is to separate high molecular weight fractions from low molecular weight fractions present hepatitis B serum to increase the disease diagnostic ability of surface enhanced Raman spectroscopy (SERS). MethodsClinically diagnosed centrifugally filtered serum samples of hepatitis B patients are subjected for surface enhanced Raman spectroscopy (SERS) in comparison with centrifugally filtered serum samples of healthy individuals by using silver nanoparticles (Ag-NPs) as SERS substrates. Some SERS spectral features are solely observed in centrifugally filtered serum samples of hepatitis B and some SERS spectral are solely observed in centrifugally filtered serum samples of healthy individuals. The diagnostic ability of SERS is further enhanced with different statistical techniques like principal component analysis (PCA), partial least square discriminant analysis (PLS-DA) and partial least square regression analysis (PLSR) have applied. ResultsThe disease biomarkers of hepatitis B are more pronounced after their centrifugation as compared with uncentrifuged form. Statistical tools like principal component analysis (PCA) and partial least square discriminant analysis (PLS-DA) clearly differentiated centrifugally filtered serum samples of hepatitis B from centrifugally filtered serum samples of healthy individuals. Furthermore, partial least square regression analysis (PLSR) has been applied for predicting unknown viral load of centrifugally filtered serum sample of hepatitis B. ConclusionSERS technique along with chemometric tools have successfully differentiated centrifugally filtered serum samples of hepatitis B from centrifugally filtered serum samples of healthy individuals. The centrifugal filtration process has increased the differentiation accuracy of PLS-DA in terms of percentage 98% and regression accuracy of PLSR regression analysis in terms of RMSEP (0.30 IU/mL) of this diagnostic method as compared with that of uncentrifuged method.

Abnormally Weak Surface-Enhanced Raman Scattering Activity of Tip-Rich Au Nanostars: The Role of Interfacial Defects.

Designing and regulating the geometry of a given plasmonic metal (Au, Ag, etc.) has become one of the most efficient approaches to achieve highly active surface-enhanced Raman spectroscopy (SERS) substrates, but this work demonstrates that plain efforts on this may not be enough. Here, we report that the often-neglected inner crystal defects also have huge impacts on the SERS activity, through a case of Au nanostars (NSs) with good SERS geometry but rich in defects. The results suggest that the interfacial defects (twin boundaries and superlattices) in the NSs aggravate the electronic oscillation damping via reducing the free path of electron scattering. This eventually results in weak local electromagnetic fields near the NS surfaces (or weak SERS activity of the NSs). This study has demonstrated the huge impact of interfacial defects on SERS activity and thus has a significant guideline for the design and fabrication of efficient SERS substrates.

Development of Gold Nanoparticle-Based SERS Substrates on TiO2-Coating to Reduce the Coffee Ring Effect.

Hydrophilic surface-enhanced Raman spectroscopy (SERS) substrates were prepared by a combination of TiO2-coatings of aluminium plates through a direct titanium tetraisopropoxide (TTIP) coating and drop coated by synthesised gold nanoparticles (AuNPs). Differences between the wettability of the untreated substrates, the slowly dried Ti(OH)4 substrates and calcinated as well as plasma treated TiO2 substrates were analysed by water contact angle (WCA) measurements. The hydrophilic behaviour of the developed substrates helped to improve the distribution of the AuNPs, which reflects in overall higher lateral SERS enhancement. Surface enhancement of the substrates was tested with target molecule rhodamine 6G (R6G) and a fibre-coupled 638 nm Raman spectrometer. Additionally, the morphology of the substrates was characterised using scanning electron microscopy (SEM) and Raman microscopy. The studies showed a reduced influence of the coffee ring effect on the particle distribution, resulting in a more broadly distributed edge region, which increased the spatial reproducibility of the measured SERS signal in the surface-enhanced Raman mapping measurements on mm scale.

Facile fabrication of superhydrophobic gold loaded nanoporous anodic alumina as surface-enhanced Raman spectroscopy substrates

A facile method of creating a sensitive and inexpensive superhydrophobic nanoporous anodic alumina (NAA) based surface-enhanced Raman spectroscopy (SERS) substrate is reported. A superhydrophobic NAA was created by coating polydimethylsiloxane on NAA via polymer evaporation technique which further coated with gold to fabricate NAA-based superhydrophobic SERS substrate. NAA and nanopatterned aluminum with varying pore properties were used for the SERS studies using rhodamine 6 G as the model analyte. The limit of detection was calculated for the SERS substrate and found to be as low as 146.3 pM. The analytical enhancement factor was found to be 6.9 × 105 successfully demonstrating the potential use of NAA-based superhydrophobic substrate as a SERS substrate. The substrates displayed good spatial reproducibility with a relative standard deviation of 12.62%, demonstrating the potential use of such substrates in chemical and biological sensing applications. The method reported is general and provides a simple and cost-effective approach for generating efficient SERS platforms for trace molecular sensing.

Fine fabrication of TiO2/MoOx nano-heterojunctions and investigating on the improved charge transfer for SERS application

Metal-oxide surface-enhanced Raman spectroscopy (SERS) substrates have attracted enormous attentions for trace-level molecule detection, and efficient photo-induced charge transfer is crucial to further enhance the SERS sensitivity. Herein, TiO2@MoOx nanorods are finely prepared to construct the TiO2/MoOx nano-heterojunctions, and then they are implemented as an efficient charge transfer substrate for SERS application. The SERS performance of the substrate is evaluated by using a 532-nm laser as the excitation source and R6G molecules as the Raman reporter. Experimental results show that R6G's Raman signal intensity is gradually improved with the increase of the nano-heterojunction numbers on TiO2@MoOx nanorods. A limit of detection as low as 10−8 M (R6G@4.0 × 10−13 M/cm2) and Raman enhancement factor of 1.445 × 108 (R6G@10−8 M) can be achieved on the most optimal TiO2@MoOx nanorods. Moreover, the mechanism of the SERS enhancement is investigated by analyzing the TiO2 and MoOx band diagrams. It evidences that TiO2@MoOx forms staggered gap heterojunction. Under the laser irradiation, the photo-generate electron-hole pairs are separated in the depletion region and then free electrons move toward the MoOx surface. Thereafter, the electrons are further transferred to the molecules enhancing the SERS performance. Finally, to verify the aforementioned explanation, the surface potential of a single TiO2@MoOx nanorod is measured by Kelvin prober force microscope, it exhibits that the surface potential distribution shifts to a higher level under the light illumination, confirming the accumulation of free electrons on the MoOx surface, and then enhancing charge transfer between the molecules and heterojunctions.

Assembly of long silver nanowires into highly aligned structure to achieve uniform “Hot Spots” for Surface-enhanced Raman scattering detection

Silver nanowires (AgNWs) as a promising surface-enhanced Raman spectroscopy (SERS) substrate could be used in the analytical science due to its high sensitivity. However, it is difficult for the randomly-distributed silver nanowires to offer uniform “hot spots” to achieve the SERS signal reproducibility of small molecules detection. Herein, the evaporation-induced aggregation had been used to assemble long silver nanowires into highly aligned structure to achieve uniform “hot spots” for SERS detection. The normal glass slide with well-aligned silver nanowires could act as a high sensitivity and excellent reproducibility SERS substrate to provide a versatile platform for detecting analytes. Rhodamine 6G (R6G) is used to evaluate the sensitivity and reproducibility of these AgNWs SERS substrates. Even the low concentration of the R6G was 10-10 mol/L, the SERS features of R6G could be still observed clearly, and the uniform distribution of enhancement factor (EF) was higher than 0.8 × 104 accounting for about 75 % in the observed mapping area. Moreover, the relative standard deviation (RSD) of SERS intensity at the band of 610 cm−1 was used to estimate the signal reproducibility, and the calculated RSD value of aligned AgNWs substrate was about 3.6%, which was much higher than that of the randomly distributed AgNWs (26.8%) because of the highly aligned structure of silver nanowires with abundant and uniform inherent “hot spots”. In addition, potential SERS detection of other small molecule, e.g. melamine was also demonstrated in the micromolar range.

Surface-enhanced Raman spectroscopy of polymerase chain reaction (PCR) products of Rifampin resistant and susceptible tuberculosis patients

BackgroundRaman spectroscopy is an effective tool for detecting and discriminating microorganisms that is robust, reliable, and rapid. ObjectivesTo develop a polymerase chain reaction technique (PCR) based on Surface Enhanced Raman Spectroscopy (SERS) technique with principal component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA) were used to assess diagnostic capability of SERS for distinguishing between tuberculosis (TB) positive rifampin resistant and tuberculosis (TB) positive rifampin susceptible samples. MethodsSilver nanoparticles (Ag NPs) were used as SERS substrates and technique was used to distinguish TB positive rifampin (RIF) resistant and TB positive rifampin (RIF) susceptible patients on the basis of characteristic SERS spectral features of their respective PCR products. SERS spectra were acquired from 52 samples of PCR products including 22 samples of TB positive rifampin susceptible, 30 samples of TB positive rifampin resistant and negative control samples. All these samples were collected from individuals of same age. Furthermore, multivariate data analyses techniques such as PCA and PLS-DA were used to assess diagnostic capability of SERS for distinguishing between TB positive rifampin resistant and TB positive rifampin susceptible samples. ResultsPCA is found helpful for successful differentiation among these two groups of spectral data sets. Moreover, PLS-DA provides this classification quantitatively by predicting the class of SERS spectral data set with 73% area under curve, 96% sensitivity, 95.6% specificity and 95% accuracy. ConclusionSERS can be employed for the rapid distinguishing between TB positive rifampin resistant and TB positive rifampin susceptible samples.

Improved SERS detection of anionic pollutant with Ag Nanoparticles-decorated ZnAl layered double hydroxide nanosheets array

Enrichment is a valuable strategy to further improve the detection limit. In this work, ZnAl-layered double hydroxide (ZnAl-LDH) nanosheets array in situ synthesized on Al foil was used as the enrichment frame for surface-enhanced Raman spectroscopy (SERS) substrate to further improve the detection limit of anionic pollutant. High density of Ag nanoparticles (Ag-NPs) with an average diameter of 29 nm sputtered onto the surface of the ZnAl-LDH could create localized electromagnetic field enhancement for SERS detection. The as-prepared Ag-NPs@ZnAl-LDH composite substrates showed improved sensitivity to anionic pollutants methyl orange and erythrosin B, demonstrating the promising potentials to improve the detection limit of anion analytes based on SERS spectroscopy.

Optimizing and improving the growth of Ag nanoparticles decorated on the silicon pyramid for surface-enhanced Raman spectroscopy

This study demonstrates a simple method to fabricate the silicon pyramid on the silicon substrate via a chemical etching method. The chemical etching process with different reaction conditions plays an essential role in controlling the growth of uniform and large silicon pyramids. The silicon pyramid can provide suitable geometric structures to deposit silver nanoparticles with better anti-aggregation stability to apply surface-enhanced Raman spectroscopy (SERS) substrates via the ion-sputtering method. The appropriate silicon pyramid and silver nanoparticles sizes were optimized to produce the most significant SERS effect in the rhodamine 6G molecule. The silicon pyramid exhibited better morphology for the deposition of uniform-sized silver nanoparticles, facilitating the fabrication of high-performance SERS substrate to detect different drugs. The fabrication of silver nanoparticles decorated on the silicon pyramid is facile, of high reproducibility, which shall be advantageous in applications for detecting other molecules via SERS.

Ultrasensitive SERS substrates based on Au nanoparticles photo-decorated on Cu2O microspheres for the detection of rhodamine B and methylene blue

Integrating noble-metal and semiconductor nanomaterials for Raman signal enhancement is of great significance in the field of surface enhanced Raman spectroscopy (SERS). In this work, a SERS-active substrate composed of Au nanoparticles (NPs) on Cu2O microspheres is produced by the polyol and photochemical methods for the effective detection of organic dye molecules, rhodamine B (RhB) and methylene blue (MB). The formation of Au/Cu2O nanostructure is confirmed by X-ray diffraction, field emission scanning electron microscopy , elemental mapping and UV-Vis spectroscopy. The proposed heterogeneous nanomaterials have high SERS activity by the synergistic interaction among Au NPs, Cu2O microspheres and organic dye molecules through electromagnetic and chemical mechanisms. The photochemical parameter for the growth of Au NPs and Cu2O microspheres is optimized by elevating the SERS signal intensity to high extent. The measured analytical enhancement factors (AEFs) are as high as 2.55 × 1012 for RhB and 1.2 × 1011 for MB. The corresponding limits of detection (LOD) are 2.36 × 10-13 M for RhB and 3.40 × 10-12 M for MB. The reproducibility and the uniformity with the relative standard deviation (RSD) < 10% confirm the superiority of the proposed SERS substrate. The real-sample analysis of RhB in the chili solution and MB in the pond water gives the satisfactory recovery values > 87.8%. This reliable and facile production approach is practically applicable to produce SERS-active substrates in various real applications.

Gold nanoparticles-based SERS nanosensor for thiram and chloramphenicol monitoring in food samples: Insight into effects of analyte molecular structure on their sensing performance and signal enhancement

Electrochemically synthesized gold nanoparticles (e-AuNPs) were deposited on aluminum substrates to fabricate effective surface enhanced Raman spectroscopy (SERS) sensors for thiram and CAP monitoring in food samples, between which the thiram sensor showed better performance in terms of sensibility, reliability and practicability. Regarding that the differences in SERS performance of the two sensors were closely associated to the distinction in molecular structure of the two analytes, we investigated the effects of analyte molecular structure on the sensing performance, especially the enhancement factors (EFs), of the sensors. As the enhancement of SERS signal is explained by both chemical and electromagnetic mechanisms (CM and EM), the investigation was also carried out in two directions. On one hand (CM), the adsorption of the analytes on the surface of e-AuNPs was studied by using both Langmuir and Freundlich isotherm models to qualitatively evaluate their adsorption capacity on e-AuNPs. On the other hand (EM), SERS measurements of thiram and CAP were performed on a set of substrates, on which the interpaticle distance between e-AuNPs were controlled by pH adjustment before deposition, to study their adsorption ability in the hotspots. The obtained results showed that thiram adsorption obeyed Langmuir model, thus, thiram residues adsorbed as a monolayer on the e-AuNPs surface at the same adsorbing site. In contrast, CAP adsorption obeyed Freundlich one with multilayer adsorption and two potential adsorbing sites. In addition, with sulfur-containing structure, small and simple molecule, capability of S-S bond cleavage and smaller steric hindrance, thiram exhibited better adsorption ability not only on the surface of e-AuNPs but also in the nano-gaps between them. Therefore, on SERS substrates that e-AuNPs were deposited before the addition of analyte, analyte molecular structure still shows great effects on sensing performance of the sensors, especially on the enhancement factors, in both CM and EM.