Surface Enhanced Raman Spectroscopy Performance Research Articles

Controlled Growth of Silver Nanoparticles on Cotton Substrate for Enhanced SERS, Catalytic, and Antibacterial Performance

AbstractIn this study, we report the growth of surfactant‐free silver nanoparticles (NPs) on cotton through a facile chemical reduction method, iteratively applied to gradually increase and control the concentration of silver NPs. Our substrates showed enhanced performance in surface enhanced Raman spectroscopy (SERS), catalytic, and antibacterial properties, all attributable to the high concentration of NPs on the cotton and their surfactant‐free nature. While the antibacterial properties improve with increasing growth cycles, SERS and catalytic properties first increase from the first to fourth growth cycles and then decrease for the fifth growth cycle. For substrate with optimum deposition for SERS, rhodamine 6G (R6G) analyte down to nanomolar concentrations were detectable, with a linear correlation of intensity versus concentration and exhibit excellent uniformity in the SERS signal. Furthermore, the catalytic activity of our substrates against the degradation of methyl orange (MO) was impressive with a rate constant of 0.0866 min−1 for the first growth cycle, which increased by ~3.4‐fold to 0.2979 min−1 for the fourth growth cycle. Finally, the substrates showed excellent antibacterial activity towards both Gram‐negative (E. coli) and Gram‐positive (S. aureus) bacteria. For both bacteria, the zone of inhibition increased with increasing growth cycles.

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 10-10M for methylene blue, good signal reproducibility, high enhancement factor of 1.4 × 107, and excellent storage stability (over 30days). Moreover, the AgNF-120@CS SERS substrate also exhibits prominent detection sensitivity of 10-8M 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.

Plasma-assisted synthesis of gold-silver core-shell nanoparticles and their enhanced catalytic dye degradation and surface enhanced Raman spectroscopy performance

The plasma-liquid interaction (PLI) technique has been employed as a green approach for the synthesis of gold-silver core-shell nanoparticles (Au@AgNPs) using a two-step method. In the first step, the Au seed particles were grown by the PLI technique. Subsequently, the formation of Ag shell was achieved by adding AgNO3 to the solution containing Au seeds and placing it under the atmospheric pressure plasma. The growth of Ag shell on Au seeds was optimized and secondary nucleation was avoided by tuning the discharge current, precursor concentration, and stabilizer (d-fructose) concentration. The shell thickness of Ag was also changed by changing the concentration of AgNO3 in the solution. UV-Vis spectroscopy, scanning electron microscopy, and energy-dispersive x-ray spectroscopy were used to analyze the synthesized nanoparticles. Furthermore, the particles were employed for catalysis and surface enhanced Raman spectroscopy (SERS)-based molecular trace detection. The catalytic efficiency of the NPs was investigated for the reduction of four molecules, i.e., 4-nitrophenol, methyl orange, Congo red, and rhodamine B. The SERS-based trace detection was studied for rhodamine 6G using the substrates of synthesized NPs. In both cases, core-shell nanoparticles showed far superior performance compared to the seed Au nanoparticles.

Deep Learning Enabled SERS Identification of Gaseous Molecules on Flexible Plasmonic MOF Nanowire Films.

Through the capture of a target molecule at the metal surface with a highly confined electromagnetic field induced by surface plasmon, surface enhanced Raman spectroscopy (SERS) emerges as a spectral analysis technology with high sensitivity. However, accurate SERS identification of a gaseous molecule with low density and high velocity is still a challenge due to its difficulty in capture. In this work, a flexible paper-based plasmonic metal-organic framework (MOF) film consisting of Ag nanowires@ZIF-8 (AgNWs@ZIF-8) is fabricated for SERS detection of gaseous molecules. Benefiting from its micronanopores generated by the nanowire network and ZIF-8 shell, the effective capture of the gaseous molecule is achieved, and its SERS spectrum is obtained in this paper-based flexible plasmonic MOF nanowire film. With optimal structure parameters, spectra of gaseous 4-aminothiophenol, 4-mercaptophenol, and dithiohydroquinone demonstrate that this film has good SERS performance, which could maintain obvious Raman signals within 30 days during reproducible detection. To realize SERS identification of gaseous molecules, deep learning is performed based on the SERS spectra of the mixed gaseous analyte obtained in this flexible porous film. The results point out that an artificial neural network algorithm could identify gaseous aldehydes (gaseous biomarker of colorectal cancer) in simulated exhaled breath with high accuracy at 93.7%. The integration of the flexible paper-based film sensors with deep learning offers a promising new approach for noninvasive colorectal cancer screening. Our work explores SERS applications in gaseous analyte detection and has broad potential in clinical medicine, food safety, environmental monitoring, etc.

Reliable and easy-to-use SERS spectroscopy probe using a tapered opto-fluidic photonic crystal fiber.

Surface enhanced Raman spectroscopy (SERS) is one of the most sensitive biosensing techniques that offers label free detection for a variety of applications. Generally, SERS spectroscopy is performed on nano-functionalized planar substrates with plasmonic structures or colloidal nanoparticles. Recently, photonic crystal fibers (PCFs) have gained great interest for SERS based bio sensing applications due to the immense advantages such as improved sensitivity, flexibility and remote sensing capability that it offers compared to the planar substrates. However, the use of PCF based biosensors demand the alignment of it under a microscope, which can affect the reliability of SERS measurements and could be restrictive for practical end use applications. Herein, we aim to develop a tapered suspended core PCF fiber (Tapered-SuC-PCF) that represents an improvement in coupling efficiency and measurement reliability as well as it opens the way to the development of an easy-to-use bio-sensing probes with a plug and play option with conventional Raman spectrometers. We have fabricated several samples of the optimized tapered-SuC-PCF and demonstrated its superior SERS performance compared to standard SuC-PCF fibers with 2 µm core diameter. An excellent SERS measurement reliability is demonstrated using such a fiber in a plug and play type system demonstrating its versatility for practical end use applications.

Exploring the potential of branched submicron channels-based structures in ultra-thin Ag films for SERS molecular sensing

Noble metal-based nanostructures are actively studied due to their promising prospects for fabricating efficient substrates for Surface Enhanced Raman Spectroscopy (SERS)-based molecular sensing. In this work, we report the gross potential of SERS substrates based on branched submicron channels in ultra-thin Ag films. These films were fabricated by magnetron sputtering and then annealed in high vacuum conditions to induce branched channels growth by thermal dewetting. Different sets of nominally equal samples were tested during a determined period of time, using confocal Raman spectroscopy and methylene blue (MB) as Raman-active molecule, to assess their performance for SERS in ambient aging conditions. Micro-Raman intensity mapping studies demonstrated the emergence of “hot spots” enhancing the Raman signal in the branched submicron channels. A good mass sensitivity and fast spectral acquisition were achieved using these nanostructures, observing an exceptional spectral resolution and identifying all main vibrational states of MB in a few seconds for samples with a MB surface mass density of about 2 ng/mm2. A good spectral resolution was achieved even using shorter measurement times from 1.00 to 0.05 s, suggesting the potential for fast SERS determinations. Samples showed good reproducibility in the Raman spectral response during the tested period, demonstrating the marginal impact of the aging effects on the Raman signal enhancement and ensuring a time-stable SERS performance in the short term. Our results show that the proposed nanostructures are promising candidates for the development of substrates with competitive-sensitivity, time-stability, and fast SERS response, representing a simple and attractive alternative for efficient SERS-based molecular sensing.

Facile one−step synthesis of Ag anchoring on Cu2O for enhanced SERS activity: Efficient charge transfer and adsorption

There is a keen research interest to develop inexpensive and ultrasensitive substrate materials for practical sensing application of surface enhanced Raman spectroscopy (SERS). Aiming to utilize synergistic enhancement contributions originated from plasmon adsorption and charge transfer (CT), it is highly desirable to construct hybrid substrates of noble metals and semiconductors. In this work, Ag nanoparticles−decorated Cu2O hybrids (Ag−Cu2O) were synthesized through a simple one−step growth method, i.e., reducing AgNO3 and CuCl2 by ascorbic acid (AA) in the presence of PVP. A systematic study has been carried out to characterize the material properties and SERS performance of Ag−Cu2O hybrids prepared under different molar ratios of precursors, growth temperatures and addition speed of reducing agent. Ag−Cu2O hybrids, composed of well−distributed Ag nanoparticles on Cu2O nanoplates, showed enhanced SERS performance in comparison with pure Cu2O. When 4−nitrobenzenethiol (4−NBT) was used as probe molecule, the optimal sample showed a calculated enhancement factor of 1.4 × 105 and a low detection of limit (LOD) of 1 × 10−10 M, which is four orders of magnitude lower than that of pure Cu2O. Ag−Cu2O hybrid substrates can also be used to detect dye molecues and the LODs for rhodamine 6G and crystal violet were 1 × 10−8 M and 1 × 10−9 M, respectively. It is feasible that the observed enhancement of SERS activity were benefited from synergistic contributions of the abundant “hot spots”, rich photo−induced charge transfer pathways, improved adsorption ability, and suppressed recombination of e−h pairs in Ag−Cu2O hybrids. This work provides a simple and controllable approach to fabricate metal/semiconductor hybrids as SERS sensing platform.

Molecular Cooperativity in the Intense Raman Scattering on the Surface of an Organic Molecular Microcrystal

AbstractMetal free substrates as promising surface enhanced Raman spectroscopy (SERS) platforms have recently gained significant attention owing to their high spectral stability, ease of synthesis, environmental stability, biocompatibility, and target molecule specificity. In this context, molecular crystals of π‐conjugated small molecules can represent a class of prominent candidates, the SERS properties – especially analyte specificity – of which can be tuned based on molecular bonding and interaction. Herein it is suggested and demonstrated that microcrystals of a small organic molecule such as those of π‐conjugated terephthalic acid (TA) can be used as novel platforms for SERS. TA microcrystals show unprecedented Raman signal enhancements up to 1.50 × 106 for the analyte molecule rhodamine‐6G (R6G), 3.23 × 104 for rhodamine B (RhB), and 7.17 × 104 for the eosin yellowish (EY) at 785 nm laser stimulation. Experimental results also show that molecular cooperativity in the Raman signal enhancement or reduction of binary mixtures over TA microcrystals can be analyte‐specific. The density functional theory (DFT) based electronic structure of the crystals, photo‐induced charge transfer transitions, and the concept of hybrid orbitals through noncovalent interactions in the combined system provide insights into the cooperative, molecule‐specific intense SERS performance. These findings represent a substantial exploratory breakthrough in the relatively nascent domain of organic SERS.

Localized surface plasmon resonance enhanced charge transfer effect in MoO2/ZnSe nanocomposites enabling efficient SERS detection and visible light photocatalytic degradation

The detection of pharmaceutical residues in the environment based on surface enhanced Raman spectroscopy (SERS) technology has become a popular tool due to its superior molecular specificity and high sensitivity. However, the practical applicability of semiconductor-based SERS sensors for trace pharmaceutical contaminants is limited due to their weak and unstable SERS performance. Herein, a donor-bridge-acceptor (D-B-A) system of MoO2 and ZnSe was fabricated to enhance the separation efficiency of electron-hole pairs in ZnSe and the charge transfer between MoO2 and ZnSe, which causes the MoO2/ZnSe nanocomposites to exhibit excellent SERS sensitivity and photocatalytic activity for the detection of multi-component contaminants and the antibiotic. The LOD of ciprofloxacin hydrochloride (CIP HCl) on MoO2/ZnSe nanocomposites is 8.30 × 10−8 M, and the degradation rate of CIP HCl is 80 % after visible light irradiation for 180 min. Our research effectively improves the SERS sensitivities and expands the practical SERS application of plasma-nonmetal/semiconductor composites, and avoids secondary contamination in the detection process. The enhancement mechanism of SERS and the interpretation of photocatalytic phenomena provide promising guidance for providing a low-cost and stable composite SERS platform in the detection and treatment of emerging contaminants.

Layered double hydroxide nanosheets-built porous film-covered Au nanoarrays as enrichment and enhancement chips for efficient SERS detection of trace styrene

Styrene, a prevalent volatile organic compounds (VOCs), is very harmful to atmosphere and humans. Consequently, the development of efficient detection technologies for styrene is of high importance, which is still in challenge! In this work, we crafted a layered double hydroxide (LDH) porous film-coated gold nanoarray, designed to act as a surface enhanced Raman spectroscopy (SERS) chip for the efficient and portable detection of gaseous styrene. This chip features a covering layer composed of cross-linked LDH nanosheets, notable for their porous structure and high specific surface area. When the covering layer is 100–300 nm in thickness, this composite chip has significant enrichment effect and strong SERS performance to gaseous styrene with a lowest detectable concentration below 1 ppb (4.64 ×10-3 mg/m3), and can response within 10 s, showing the rapid response and high sensitivity. Additionally, the chip has strong anti-interference capabilities and maintains excellent response to styrene, even in mixed benzene-VOCs gases. The exceptional SERS performances of this chip is ascribed to its LDH covering layer-induced styrene-enrichment and structurally-enhanced SERS performances. This study provides a simple route and practical chip for the rapid and ultrasensitive SERS-based detection of gaseous styrene, which is also potentially beneficial for the detection of other gaseous VOCs.

Plasmonic Bullseye Nanocavities for Broadband Light Localization and Multi‐Wavelength SERS

AbstractPlasmonic nanostructures capable of broadband light trapping and field enhancement have promising applications in a wide range of fields. This study presents a platform for broadband, polarization‐independent field enhancement in the visible regime through the use of width‐graded nanocavities in a bullseye configuration. The fabrication procedure utilizes electron beam lithography (EBL) to achieve fine control over the nanocavity geometry and template stripping to enable rapid and low‐cost production. The utility of these devices as substrates for multi‐wavelength surface enhanced Raman spectroscopy (SERS) is demonstrated through molecular detection in a 10 μM solution at two excitation wavelengths. The impact of bullseye geometry on both the broadband spectral response and multi‐wavelength SERS performance is examined. The measured SERS enhancement factor (EF) is shown to depend primarily on the plasmonically active surface area of the device, regardless of the local electromagnetic field strength within the nanocavities. These results highlight not only the utility of the width‐graded bullseye as a broadband platform for SERS and other applications but also provide design guidelines to optimize the enhancement factor and broadband performance of similar devices.

Shrinking gap between nanoparticles in gold nanofilms to enhance Surface-Enhanced Raman Spectroscopy performance investigated by both experimental and theoretical methods

Sensitive and reliable interfacial self-assembled nanomembranes are widely used for the detection of chemical contaminants in food and environment. This study developed a high-performance Surface-Enhanced Raman Spectroscopy (SERS) analytical platform by self-assembling Au nanoparticles (AuNPs) at the three-phase liquid–liquid interface. The assembled nanoparticles arrays were immersed in halogen ions solution and the nanoparticle gaps were further shrunk through electrostatic interactions, resulting in a large-scale array of closely packed nanoparticles. The average gap between adjacent nanoparticles was less than 3 nm, where dense SERS ‘hot spots’ were generated and used for highly sensitive detection. The homogeneity of the self-assembled nanoparticle arrays under different conditions was still guaranteed despite the reduced integration time and the number of times in the mapping face sweep of the Au arrays. The improved high-throughput fabrication of self-assembled nanoparticle arrays in this way holds great promise for the development of a sensitive and reliable SERS platform for chemical contaminant monitoring.

One-step surfactant-free photoreduction synthesis of single-crystal silver triangular nanoprisms by surface modified chemically patterned ferroelectric crystals for SERS application

Anisotropic metal nanoparticles with plasmonic and photocatalytic action exhibit significant applications for SERS detection and green transformation. Current synthetic approaches using multiple chemicals cause substantial resource consumption and surface chemical residues that hinder analyte adsorption and thus prevent efficient action. Herein, we present a green production process using surface modified ferroelectric templates to grow triangular Ag nanoprisms for surface enhanced Raman spectroscopy (SERS). Surface modification of ferroelectric templates with positively charged ions suppresses the Ag nucleation on template surface and enables the reaction-controlled process to promote anisotropic photoreduction for the growth of Ag nanoprisms. Under optimal experimental conditions, numerous Ag nanoprisms with the (111) plane can be photoreduced with sharp tips and edges and greatly strengthen the excited electric field for Raman signal enhancement. The synthesized Ag nanoprism array presents superior SERS performance, including enhancement factor of 3.78 × 108, limit of detection as low as 4.95 × 10-9 M, and Raman signal enhancement by 8.63 times. The practical application of Ag nanoprisms is effectively demonstrated by the SERS detection of antibiotics. The reaction control by utilization of surface-modified ferroelectric templates provides a promising prospect for the green synthesis of anisotropic metal nanomaterials and the technical development of green applications.

Application of serum SERS technology based on thermally annealed silver nanoparticle composite substrate in breast cancer.

Liquid biopsy is currently a non-destructive and convenient method of cancer screening, due to human blood containing a variety of cancer-related biomolecules. Therefore, the development of an accurate and rapid breast cancer screening technique combined with breast cancer serum is crucial for the treatment and prognosis of breast cancer patients. In this study, the surface enhanced Raman spectroscopy (SERS) technique is used to enhance the Raman spectroscopy (RS) signal of serum based on a high sensitivity thermally annealed silver nanoparticle/porous silicon bragg mirror (AgNPs/PSB) composite substrate. Compared with RS, SERS reflects more and stronger spectral peak information, which is beneficial to discover new biomarkers of breast cancer. At the same time, to further explore the diagnostic ability of SERS technology for breast cancer. In this study, the raw spectral data are processed by baseline correction, polynomial smoothing, and normalization. Then, the relevant feature information of SERS and RS is extracted by principal component analysis (PCA), and five classification models are established to compare the diagnostic performance of SERS and RS models respectively. The experimental results show that the breast cancer diagnosis model based on the improved SERS substrate combined with the machine learning algorithm can be used to distinguish breast cancer patients from controls. The accuracy, sensitivity, specificity and AUC values of the SVM model are 100%, 100%, 100% and 100%, respectively, as well as the training time of 4ms. The above experimental results show that the SERS technology based on AgNPs/PSB composite substrate, combined with machine learning methods, has great potential in the rapid and accurate identification of breast cancer patients.

Performance evaluation of novel Ag@GO-biomaterial SERS substrates for the ultrasensitive detection of neomycin in foods

Many biomaterials are hydrophobic, making them attractive materials for the fabrication of surface enhanced Raman spectroscopy (SERS) substrates. Herein, we prepared a series of novel SERS substrates by loading graphene oxide (GO) and silver nanoparticles onto four different hydrophobic biomaterials (rose petals, lotus leaves, cicada wings or dragonfly wings). The obtained Ag@GO-biomaterial SERS substrates offered outstanding SERS performance. These substrates utilized both the electromagnetic mechanism and chemical mechanism to boost the SERS signal, as well as the concentrating effect owing to the hydrophobicity of biomaterials. The Ag@GO-rose petal SERS substrate showed the strongest signal enhancement capability, offering an LOD of 4.52 × 10-11 mol L-1 for the detection of the neomycin. Results encourage the wider use of hydrophobic biomaterials in SERS substrate construction and hazard monitoring.

Spontaneous synthesis of Ag nanoparticles decorated ZnAl layered double hydroxides for synergistically improved SERS activity

Surface enhanced Raman spectroscopy (SERS) is a rapid and ultrasensitive analytical technique. For practical applications, it is highly desirable to explore metal/semiconductor hybrid substrates, which utilize the synergistic enhancement effects of plasmons and charge transfer (CT). Herein, we successfully fabricated Ag nanoparticles decorated ZnAl LDHs nanosheets (Ag-ZnAl) by a facile one-pot solvothermal growth method and investigated the SERS performance. When 4-nitrobenzenethiol (4-NBT) was used as the probe molecule, the detection limit reaches as low as 1 × 10−10 M and the enhancement factor is 1.7 × 105. A systematic experimental study revealed that the unique structure of LDH facilitates formation of densely distributed Ag NPs with good dispersibility, providing abundant “hot spots” for electromagnetic enhancement. Moreover, Ag-ZnAl hybrid has a higher concentration of oxygen vacancies, which can promote photo-induced CT for chemical enhancement. Furthermore, first principles calculations demonstrated that Ag-ZnAl hybrid has improved adsorption ability of 4-NBT through the formation of Ag-S and NOH bonds, which can facilitate charge transfer process and increase Raman intensity of 4-NBT. This work provides an effective strategy to design easy-to-fabricated metal/semiconductor hybrids as SERS sensing platform.

Rapid and ultralow level SERS detection of ethylparaben using silver nanoprisms functionalized sea urchin-like Zinc oxide nanorod arrays for food safety analysis

Surface enhanced Raman spectroscopy (SERS) offers a tremendous opportunity to sense food additives in a fast, flexible and ultrasensitive mode. In the present study, we have designed silver nanoprism (AgNPr) functionalized Zinc Oxide (ZnO) hetero architectures for SERS sensing of food preservatives such as ethylparaben (EP). ZnO nanorods (NRs) were found to be formed which organized into sea urchin-like morphology via a facile hydrothermal process. While varying the growth time from 3 to 72 h, the nanorod length was found to be enhanced from 500 to 1900 nm range. The effect of morphology of as-fabricated substrates was well correlated with absorption and vibrational modes. Moreover, silver nanoprisms with peculiar triangular morphology with a size range of 20–80 nm were synthesized for functionalizing ZnO arrays; whose incorporation was further confirmed by EDS and XPS analysis. SERS studies demonstrated that AgNPr/ZnO NRs assembly exhibited synergistic Raman signal enhancement, which was modulated by the length of the nanorods. The highest SERS activity was exhibited by substrates with ∼1500 nm nanorod length. The reproducibility and reusability of optimized AgNPr/ZnO NRs substrates were demonstrated as well. The developed sensor exhibited highly specific real-time sensing of EP with sensitivity down to pico-molar levels; could be considered a good candidate for rapid, in situ identification of food preservatives for food safety applications. • Hierarchical zinc oxide-silver nanoprism arrays were designed as SERS sensors. • Morphological properties of sensor was well correlated with SERS performance. • Nanorod lengths of ∼1500 nm demonstrated highest signal enhancement. • Food additive like ethylparaben (EP) was detected down to pico-molar levels. • The optical sensing of EP was evidenced with high sensitivity and reusability.

Tunable SERS activity of Ag@Fe3O4 core-shell nanoparticles: Effect of shell thickness on the sensing performance

Surface enhanced Raman spectroscopy (SERS) with enormous advantages has emerged as a powerful tool for both highly sensitive structural detection of analytes and imaging applications. Among various multifunctional nanomaterials for SERS substrate, nanocomposites of magnetic and plasmonic materials deliver both SERS effect and magnetic manipulation capacity, and thereby, have practical advantages for trace analysis in a broad range of fields. Herein, we investigate the SERS performance of different-sized Ag@Fe3O4 core-shell nanoparticles (NPs) obtained by controlling the Fe precursor concentration in a facile one-pot solvothermal synthesis method. The surface morphology, composition, structure, optical and magnetic properties are characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), UV-Vis spectroscopy, and electrochemical techniques. The small shell thickness and high shell porosity of the smallest NPs significantly improve the mass diffusion and therefore SERS performance, making it an ideal substrate for highly sensitive detection of methylene blue (MB) and crystal violet (CV) organic pollutants. Quantitative SERS measurements using the Ag@Fe3O4 NPs are successfully demonstrated with MB as probe analytes. Due to the presence of a porous Fe3O4 shell, the Ag@Fe3O4 NPs showed outstanding SERS sensing performance with a linear concentration range from 10−7 to 10−10 M and the limit of detection (LOD) of 3 × 10−10 M. Moreover, the Ag@Fe3O4 core-shell nanocomposite exhibits high reproducibility and reliability and can be a promising candidate for SERS sensing systems.

Superhydrophobic 3D-Assembled Metallic Nanoparticles for Trace Chemical Enrichment in SERS Sensing.

The performance of surface-enhanced Raman spectroscopy (SERS) is determined by the interaction between highly diluted analytes and boosted localized electromagnetic fields in nanovolumes. Although superhydrophobic surfaces are developed for analyte enrichment, i.e., to concentrate and transfer analytes toward a specific position, it is still challenging to realize reproducible, uniform, and sensitive superhydrophobic SERS substrates over large scales, representing a major barrier for practical sensing applications. To overcome this challenge, a superhydrophobic SERS chip that combines 3D-assembled gold nanoparticles on nanoporous substrates is proposed, for a strong localized field, with superhydrophobic surface treatment for analyte enrichment. Intriguingly, by concentrating droplets in the volume of 40µL, the sensitivity of 1nm is demonstrated using 1,2-bis(4-pyridyl)-ethylene molecules. In addition, this unique chip demonstrates a relative standard deviation (RSD) of 2.2% in chip-to-chip reproducibility for detection of fentanyl at 1µgmL-1 concentration, revealing its potential for quantitative sensing of chemicals and drugs. Furthermore, the trace analysis of fentanyl and fentanyl-heroin mixture in human saliva is realized after a simple pretreatment process. This superhydrophobic chip paves the way toward on-site and real-time drug sensing to tackle many societal issues like drug abuse and the opioid crisis.

Long Spiky Au-Ag Nanostar Based Fiber Probe for Surface Enhanced Raman Spectroscopy.

The detection performances of noble metal-based surface enhanced Raman spectroscopy (SERS) devices are determined by the compositions and geometries of the metal nanostructures, as well as the substrates. In the current study, long spiky Au-Ag alloy nanostars were synthesized, and both core diameters and spike lengths were controlled by Lauryl sulfobetaine concentrations (as the nanostructure growth skeleton). The long spiky star geometries were confirmed by transmission electron micrograph images. Elements energy dispersive spectrometer mapping confirmed that Au and Ag elements were inhomogeneously distributed in the nanostructures and demonstrated a higher Ag content at surface for potential better SERS performance. Selected synthesized spiky nanostars were uniformly assembled on multi-mode silica fiber for probe fabrication by silanization. The SERS performance were characterized using crystal violet (CV) and rhodamine 6G (R6G) as analyte molecules. The lowest detection limit could reach as low as 10−8 M, with a 6.23 × 106 enhancement factor, and the relationship between analyte concentrations and Raman intensities was linear for both CV and R6G, which indicated the potential qualitative and quantitative molecule detection applications. Moreover, the fiber probes also showed good reproducibility and stability in the ambient atmosphere.