4-MBA Molecules Research Articles

Surface-enhanced raman scattering properties of Au@TiO2 plasma exciton array structures

Surface Enhanced Raman Scattering (SERS) is an ultrasensitive spectroscopic analysis technique widely used for molecular detection. SERS is characterized by rapid analysis, trace detection, ultra-high sensitivity, non-destructive, etc., and can analyze the concentration, composition, and structure of the detected substances. It has a significant potential for application in many fields, such as biomedicine, trace detection, food safety, and environmental protection. However, fabrication of homogeneous, stable, and ultrasensitive SERS substrates remains a challenge for the practical application of SERS technology. Precious metals and semiconductors have been proven attractive and versatile for SERS detection. In this work, three-dimensional Au@TiO2 substrates were prepared using a facile strategy, and two different types of Raman probe molecules were used to demonstrate the versatility of the design. Using 4-MBA as the probe molecule, the enhancement factor of the TiO2 substrate could reach 1.52×104, and the detection limit was as low as 10-16 M. The results indicate that the Au@TiO2 substrate is simple to prepare, inexpensive, and serves as an excellent SERS substrate structure. 4-MBA and R6G dual probe molecules demonstrate the universality of the Au@TiO2 nanocavity array structure. In the meantime, the results of the simulations match the experimental results. It also demonstrates that the three-dimensional Au@TiO2 has the potential to be an excellent SERS substrate.

Tuning SERS performance through the laser-induced morphology changes of gold nanostructures

Differently shaped periodic gold nanostructures on the surface of a thin metal film are fabricated utilizing the direct laser writing technique. The variations in the energy of single femtosecond pulses lead to the formation of morphologically distinct hollow nanostructures. The arrays that excite hybridized plasmonic modes are characterized by electron microscopy and assessed for surface-enhanced Raman spectroscopy (SERS). Plasmonic properties are investigated through SERS using the adsorbed 4-mercaptobenzoic acid (4-MBA) molecules. The development of sharp tips in the nanostructures is initiated by the higher-energy laser pulses, while the structure formation principle itself is altered by the thickening of the film. The change in the structure shape and gold layer thickness tune the SERS signal and the enhancement factor (EF), leading to an EF of 107. Such a factor enables the future prospects of the arrays to be applied for single-molecule detection.

Investigation of selective SERS enhancement mechanism of Au nanospheres and Au nanorods based on 2T2D-SERS correlation spectroscopy

The selective enhancement mechanism in surface-enhanced Raman scattering (SERS) is demonstrated. Two different types of single nanoparticles (Au nanosphere and Au nanorod) were used to investigate the role of the localized surface plasmon resonance (LSPR) in SERS spectra by using the two-trace two-dimensional (2T2D) correlation spectroscopy. The SERS intensities of three probe molecules, 4-mercaptobenzoic acid (4-MBA), 4-aminothiophenol (4-ATP), and 4-bromobenzenethiol (4-BBT), respectively, were enhanced but slightly different when adsorbed on Au nanospheres and Au nanorods. 2T2D correlation SERS spectra clearly showed that even with the same shape of Au nanoparticles, the main factors influencing the SERS enhancement can vary depending on the specific type of SERS tags used. Such subtle difference could not be clearly identified by the conventional spectral analysis. This result sheds light on potential applications of 2T2D correlation spectroscopy. For 4-MBA molecules, the a1 and b2 modes are mainly affected by the Au nanospheres and Au nanorods. For 4-ATP molecules, the a1 and b2 modes related to C–S stretching combined with C–C stretching band are mainly affected by Au nanorods and Au nanospheres. For 4-BBT molecules, the a1 and b2 modes of C–C (aromatic ring) stretching band are mainly affected by Au nanorods and Au nanospheres. This study offers valuable insights into the relationship between nanoparticle shape and SERS enhancement.

Revealing Adsorption Mechanism of p-Mercaptobenzoic Acid with TiO2 Surfaces Using Electric Field-Enhanced Semiconductor SERS

p-Mercaptobenzoic acid (4-MBA) is a typical molecular probe for a surface-enhanced Raman scattering (SERS) study of the enhancement performance of semiconductor nanoparticles. Understanding the molecular adsorption mechanism of 4-MBA on a semiconductor surface is crucial to reveal the enhancement mechanism of semiconductor SERS. Herein, two types of submicrometer-sized TiO2 particles with amorphous (denoted as a-TiO2) and anatase structures (denoted as c-TiO2) were fabricated, and their potential as SERS-active substrates with high electric-field enhancement was explored based on the near-field scattering theory and finite-element method simulation. The electric field-enhanced semiconductor SERS provide a better vision for us to study the adsorption modes of molecules on the TiO2 surface. On this basis, adsorption behaviors of 4-MBA on a-TiO2 and c-TiO2 particles were systematically studied by the semiconductor SERS and density functional theory. The results demonstrated that the adsorption mechanism of 4-MBA with TiO2 surfaces is highly dependent on the exposure of acid sites of TiO2 surfaces. 4-MBA adsorbs preferentially on Brønsted acid sites of a-TiO2 through a carboxyl group, in contrast on Lewis acid sites of c-TiO2 through a sulfhydryl group. Furthermore, 4-MBA molecules may form multilayer adsorption on TiO2 surfaces through the hydrogen bond and/or π–π stacking interaction. Research results not only provide a new insight to re-evaluate the chemical enhancement mechanism for TiO2–4-MBA systems but also provide a theoretical guidance for the modification of TiO2 surface with organic molecules containing carboxyl and sulfhydryl groups.

Stable and Reusable Lace-like Black Silicon Nanostructures Coated with Nanometer-Thick Gold Films for SERS-Based Sensing.

We propose a simple, fast, and low-cost method for producing Au-coated black Si-based SERS-active substrates with a proven enhancement factor of 106. Room temperature reactive ion etching of silicon wafer followed by nanometer-thin gold sputtering allows the formation of a highly developed lace-type Si surface covered with homogeneously distributed gold islands. The mosaic structure of deposited gold allows the use of Au-uncovered Si domains for Raman peak intensity normalization. The fabricated SERS substrates have prominent uniformity (with less than 6% SERS signal variations over large areas, 100 × 100 μm2). It has been found that the storage of SERS-active substrates in an ambient environment reduces the SERS signal by less than 3% in 1 month and not more than 40% in 20 months. We showed that Au-coated black Si-based SERS-active substrates can be reused after oxygen plasma cleaning and developed relevant protocols for removing covalently bonded and electrostatically attached molecules. Experiments revealed that the Raman signal of 4-MBA molecules covalently bonded to the Au coating measured after the 10th cycle was just 4 times lower than that observed for the virgin substrate. A case study of the reusability of the black Si-based substrate was conducted for the subsequent detection of 10-5 M doxorubicin, a widely used anticancer drug, after the reuse cycle. The obtained SERS spectra of doxorubicin were highly reproducible. We demonstrated that the fabricated substrate permits not only qualitative but also quantitative monitoring of analytes and is suitable for the determination of concentrations of doxorubicin in the range of 10-9-10-4 M. Reusable, stable, reliable, durable, low-cost Au-coated black Si-based SERS-active substrates are promising tools for routine laboratory research in different areas of science and healthcare.

Fabrication of black phosphorous quantum dots and Ag nanoparticles co-sensitized TiO2 nanorod arrays as powerful SERS substrate

Surface enhanced Raman scattering (SERS) substrate based on plasma metal nanoparticles and non-metal quantum dots co-sensitized semiconductor has been used in limit trace detection test. In this research, BPQDs (black phosphorus quantum dots) and Ag nanoparticles co-sensitized TiO 2 nanorod arrays were successfully fabricated as powerful SERS substrate. For BPQDs/Ag/TiO 2 , its detection limit of 4-mercaptobenzoic acid (4-MBA) molecules could achieve even at as low as 1.0 × 10 −12 M, and the enhancement factor (EF) is up to 2.5 × 10 5 , exhibiting an excellent SERS sensitivity. Furthermore, a good uniformity is also observed during the detection process, the relative standard deviation of 4-MBA Raman fingerprint peak intensity at 1587 cm −1 is only 7.3 %. Furthermore, BPQDs/Ag/TiO 2 substrate is demonstrated to have good repeatability and stability. The synergistic effect of BPQDs, Ag nanoparticles and TiO 2 nanorod array is mainly responsible for improved SERS activity, namely, high-density hot spot, optical capture expansion and enhanced charge transfer. • BPQDs/Ag/TiO 2 was firstly fabricated as powerful SERS substrate. • SERS detection limit of BPQDs/Ag/TiO 2 substrate toward 4-MBA could achieve 1.0 × 10 −12 M. • The study provides an efficient approach to extend the photocarriers lifetime of BPQDs. • The possible electron generation and transport modes of BPQDs/Ag/TiO 2 was investigated.

A new semiconductor-based SERS substrate with enhanced charge collection and improved carrier separation: CuO/TiO2 p-n heterojunction

In this paper, CuO/TiO2 p-n heterojunction was developed as a new surface enhanced Raman scattering (SERS) substrate to magnify Raman signal of 4-mercaptobenzoic acid (4-MBA) molecule. In the heterojunction-molecule system, CuO as an “electron capsule” can not only offer more electrons to inject into the surface state energy level of TiO2 and consequently bring additional charge transfer, but also improve photogenerated carrier separation efficiency itself due to strong interfacial coupling in the interface of heterojunction, which together boost SERS performance of the heterojunction substrate. As expected, owing to the enhanced charge collection capacity and the improvement of photogenerated carrier separation efficiency derived from internal electric field and strong interface coupling provided in the interface of heterojunction, this substrate exhibits excellent SERS detection sensitivity towards 4-MBA, with a detection limit as low as 1 × 10−10 mol/L and an enhancement factor of 8.87 × 106.

Third-order nonlinear optical studies of Bis(4-methylbenzylammonium) tetrachloridocuprate metal-organic crystal with optical limiting behavior: Experimental and theoretical investigations

The metal-organic nonlinear optical (NLO) compound Bis(4-methylbenzylammonium) tetrachloridocuprate (4MBA) was synthesized and grown by the slow evaporation method. X-ray crystal analysis revealed the crystal structure and conformation. Density functional theory (DFT) at B3PW91/LanL2DZ basis set level was applied and computed the optimized geometry, electronic structure, vibrational spectra, frontier molecular orbitals (FMO), molecular electrostatic potential (MEP), and natural bonding orbitals (NBO). UV–Vis-NIR spectroscopic measurements revealed the optical transparency range from 310 to 1100 nm and excited-state properties. The thermal behavior and stability of the compound were analyzed using TGA/DTA, and the suitability of the optical material is weighed. The dielectric parameters and AC conductivity were investigated as a function of frequency and temperature. Second-harmonic generation (SHG) efficiency was measured using the Kurtz and Perry powder approach and found 4MBA to be 0.8 times the KDP. The nonlinear refractive index (n2), nonlinear optical absorption (β) and third-order nonlinear optical susceptibility (χ(3)) measured values are 1.548 × 10−9 cm2 W−1, 0.0538 × 10−4 cm W−1, and 6.049 × 10−8 esu respectively, and shows that the 4MBA molecule is a third-order nonlinear optical material. The optical limiting threshold value is determined as 7.632 × 103 W cm2 for 532 nm continuous-wave laser, representing that the 4MBA is a strong candidate for the optical limiting application. The static and dynamic nonlinear optical parameters were computed with three different functions including B3PW91, M06 and PBE0 using LanL2DZ basis set. The outcomes revealed that the 4MBA molecule under study might be a promising NLO material for frequency generators and optical limiters applications.

Tailored FTO/Ag/ZIF-8 structure as SERS substrate for ultrasensitive detection

In this work, a series of F-doped SnO2/Ag/zeolite imidazole framework (FTO/Ag/ZIF-8) sandwich structure have been successfully fabricated via a magnetic sputtering method and serve as surface-enhanced Raman scattering (SERS) substrate. The magnetic sputtering time of Ag was adjusted to obtain the optimal SERS substrate. The commonly used 4-mercaptobenzoic acid (4-MBA) molecules was selected for the SERS experiment. When the sputtering time of Ag nanoparticles (NPs) was 120 s, the FTO/Ag/ZIF-8 substrate showed the maximum SERS performance. In the system, the electromagnetic mechanism (EM) and charge-transfer (CT) enhancement mechanism have synergistic effect on the SERS phenomenon. Ag NPs was used to generate electromagnetic hot spots, which was beneficial to the EM mechanism. ZIF-8 could adsorb and capture more 4-MBA probe molecules to the hotspots. At the same time, CT happened between Ag, ZIF-8, and 4-MBA probe molecules, which was attribute to the CM mechanism. The enhancement factor (EF) of the composite SERS substrate was as high as 7.67 × 106. The detection limit of the substrate can reach 10-9 M of 4-MBA probe molecules. Moreover, the SERS templates showed good stability, the SERS signals almost unchanged after naturally kept for 6 months. Besides, due to the high sensitivity and good stability of the substrates, this work might broaden the potential practical application of SERS.

Gap-enhance Raman tags (GERTs) competitive immunoassay based Raman imaging for the quantitative detection of trace florfenicol in milk

Surface Enhanced Raman Spectroscopy (SERS) is a highly sensitive detection method, however, it is still challenging to directly detect florfenicol due to its low Raman activity. To solve this problem, we present a novel sensor for selective detection of florfenicol residue in milk using gap-enhance Raman tags (GERTs) competitive immunoassay. In this nanosensor, Raman reporters 4-mercaptobenzoic acid (4MBA) were embedded in the 1 nm gap of Au core-gap-Au shell nanoparticles, which can protect 4MBA molecules from unwanted external environments and provide a high enhancement. The parameters affecting the detection sensitivity were discussed and optimized. The limit detection of florfenicol in milk can reach 0.1 μg/kg. To evaluate the performance of GERTs immunoassay, Ag@4MBA tags immunoassay was conducted and the results were compared. Moreover, Raman mapping was investigated to improve the quantitative prediction model with R2 of 0.981 achieved, by eliminating the signal fluctuations caused by the inhomogeneous distribution of GERTs.

4-Mercaptobenzoic Acid Adsorption on TiO2 Anatase (101) and TiO2 Rutile (110) Surfaces

The adsorption of 4-mercaptobenzoic acid (4-MBA) on anatase (101) and rutile (110) TiO2 surfaces has been studied using synchrotron radiation photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy techniques. Photoelectron spectroscopy results suggest that the 4-MBA molecule bonds to both TiO2 surfaces through the carboxyl group, following deprotonation in a bidentate geometry. Carbon K-edge NEXAFS spectra show that the phenyl ring of the 4-MBA molecule is oriented at 70° ± 5° from the surface on both the rutile (110) and anatase (101) surfaces, although there are subtle differences in the electronic structure of the molecule following adsorption between the two surfaces.

Deepening of nanograting structures on Si by a two-step laser spatial-selective amorphization strategy combined with chemical etching

Femtosecond laser induced periodic surface structures (LIPSSs) have been implemented on various materials. However, its further development in manipulation of optical properties is hindered by the shallow depth. Herein, the paper proposes a novel method of two-step spatial-selective amorphization for producing the deep regular nanogratings on silicon by femtosecond laser and chemical etching with HF acid. In the first step, the spatially periodic amorphization of silicon surface is fabricated by the surface plasmon excitation of the incident femtosecond laser, the chemical etching of which produces the shallow gratings with a period of 765 nm, or so-called prefabricated-LIPSSs (pre-LIPSSs). During the second step, the spatial-selective amorphization of sample takes place by exploiting the local field enhancement of laser irradiation within the grooves. After chemical etching, the deep local field enhancement assisted LIPSSs (LFE-LIPSSs) are formed, with a three-fold larger depth of 155 nm but an invariable period. Comparing with the pre-LIPSSs, the LFE-LIPSSs show 45% increase in anti-reflection performance at around 700 nm and surface-enhanced Raman scattering (SERS) anisotropy ratio is increased about four times with 4-MBA molecules as probe molecules. This work provides a promising approach to fabricate deeper LIPSSs for more effectively controlling the optical properties of materials by femtosecond laser.

Surface structure effect of F<sub>4</sub>TCNQ/MoS<sub>2</sub> nanocomposite heteromaterials on SERS

Surface-enhanced Raman scattering (SERS) has been widely used in food and drug detection, biological and medical sensing. In recent years, the study of non-metallic SERS substrates has gradually become a hot field of SERS. Here, we investigated the modulation effects on SERS activities of 2,3,5,6-tetrafluoro-7,7,8,8-tetrachyanoquindimethylene (F<sub>4</sub>TCNQ) grown on molybdenum disulfide (MoS<sub>2</sub>) films. The different nanostructures of F<sub>4</sub>TCNQ can have an effect on the bound ability of charges transferred from the surface of MoS<sub>2</sub>, which changes the electron density distribution on the surface of the F<sub>4</sub>TCNQ/MoS<sub>2</sub> nanocomposite material. Therefore, the interface exhibited different charge localization in the F<sub>4</sub>TCNQ/MoS<sub>2</sub> nanocomposite. The charge transfer efficiency between the substrate and with the adsorbed probe molecules leads the substrate to show a different SERS sensitivity. The enhancement factor of 4-mercaptobenzoic acid (4-MBA) molecules on the most optimized 7-min F<sub>4</sub>TCNQ/MoS<sub>2</sub> nanocomposite substrate can reach 6.9×10<sup>4</sup>, and the detection limit concentration is as low as 10<sup>-6</sup>M. The results of research on F<sub>4</sub>TCNQ/MoS<sub>2</sub> nanocomposite provides an effective optimization scheme of energy level regulation for SERS based on the chemical enhancement mechanism, and opens up a new way to further exploit its functional applications.

Hexagonal arrays of plasmonic gold nanopyramids on flexible substrates for surface-enhanced Raman scattering

Surface-enhanced Raman spectroscopy (SERS) with pyramidal gold nanostructures increases the signal of Raman active analytes, since hotspots form at the edges and tip of a nanopyramid under illumination. 2D hexagonal arrays of pyramidal nanostructures with a quadratic base are fabricated through cost-effective nanosphere lithography and transferred onto elastomeric polydimethylsiloxane. By making use of the {111} crystal plane of a silicon (100) wafer, an inverted pyramidal array is etched, which serves as the complementary negative for the gold nanostructures. Either a continuous gold thin-film with protruding pyramids or separate isolated nanopyramids are produced. Three basic fabrication strategies are presented. The SERS enhancement is verified by Raman mapping of 4-mercaptobenzoic acid (4-MBA) molecules. Fabrication on a flexible substrate paves the way for future applications on curved surfaces or in situ tunable resonances.

An ultrasensitive surface-enhanced Raman scattering sensor for the detection of hydrazine via the Schiff base reaction

The development of convenient assays for the determination of hydrazine (N2H4) has drawn significant attention due to the high toxicity of this substance. Herein, we developed a concise, rapid and ultrasensitive surface-enhanced Raman scattering (SERS) sensor for N2H4 detection based on alpha-cyclodextrin-silver nanoparticles (α-CD-AgNPs) modified by 4-mercaptobenzaldehyde (4-MBA). The 4-MBA molecules can specifically capture the N2H4 molecules and undergo a Schiff base reaction. As a result, this induces the aggregation of nanoparticles and generates a new characteristic peak at 1529 cm–1 that is attributed to CN and CC vibrations. Compared with noble metal nanoparticles, 4-MBA not only formed AgS bonds but could also be fixed in the cavity of cyclodextrin to produce a more stable and stronger SERS signal. The SERS intensity at 1529 cm–1 and the logarithm of the concentration of N2H4 presented a good linear relationship from 10–9 to 10–7 M with an unprecedented limit of detection (LOD) of 38 pM. The proposed SERS sensor exhibited satisfactory selectivity and reproducibility and was applied to detect N2H4 in real and complex water samples. We expect this assay to be a promising alternative tool for the on-site detection of N2H4.

Reliable SERS detection of pesticides with a large-scale self-assembled Au@4-MBA@Ag nanoparticle array

The fabrication of sensitive and reliable interfacial plasmonic platform for measuring chemical contaminants in various phases is an exciting topic in the food industry and for environment monitoring. In this study, a high-performance surface-enhanced Raman spectroscopy (SERS) analytic platform was developed through self-assembly of the gold@4-mercaptobenzoic acid@silver nanoparticles (Au@4-MBA@Ag NPs) at the cyclohexane/water interface. By addition of the inducer ethanol, the Au@4-MBA@Ag NPs in aqueous phase was effectively migrated to the biphasic interface, forming a large-scale close-packed nanoparticle array. The average gap between adjacent nanoparticles was smaller than 3 nm, where intensive SERS “hot spots” were created for high-sensitive detection. Furthermore, using the sandwiched 4-MBA molecule as the internal standard to correct the Raman signal fluctuations, the point-to-point and batch-to-batch reproducibility of Au@4-MBA@Ag array were improved with lower relative standard deviation (RSD) values of 8.84% and 14.97%, respectively, and pesticides (thiram and thiabendazole) analysis in both aqueous and organic phases were achieved with higher accuracy (R2 of 0.986 and 0.990) as compared with those without 4-MBA correction (R2 of 0.867 and 0.974). The high-throughput fabrication of the self-assembled nanoparticle array is a promising approach for development of a sensitive and reliable SERS platform for chemical contaminants monitoring in multiphase.

SPR/SERS dual-mode plasmonic biosensor via catalytic hairpin assembly-induced AuNP network

Highly sensitive and reliable detection of disease-related nucleic acids is still a big challenge in liquid biopsy because of their homologous sequences and low abundance. Herein, a novel surface plasmon resonance/surface-enhanced Raman scattering (SPR/SERS) dual-mode plasmonic biosensor based on catalytic hairpin assembly (CHA)-induced Au nanoparticle (AuNP) network was proposed for highly sensitive and reliable detection of cancer-related miRNA-652. The biosensor includes capture DNA-functionalized AuNPs (Probe 1), H1 and 4-mercaptobenzoic acid (4-MBA) co-modified AuNPs (Probe 2), and 6-carboxyl-Xrhodamine (ROX)-labeled H2 (fuel strands). The Probe 1-Probe 2 networks were formed via the target-triggered CHA reactions, which resulted in the color change of dark-field microscopy (DFM) images and enhanced SERS effect. The SPR sensing was achieved by extracting the integral optical density of dark-field color in DFM images, and the SERS sensing was realized by the ratiometric SERS signals of ROX and internal standards 4-MBA molecules. After characterizing the feasibility and optimality of the sensing strategy, the good performance of biosensors on sensitivity, specificity and uniformity was approved. The practicability of biosensors was confirmed by detecting miRNA-652 in human serum, and both the SPR and SERS assays showed good linear calibration curves and low limit of detections (LODs) of 42.5 fM and 2.91 fM, respectively, with the recovery in the range of 94.67–111.4%. These two modes show complementary advantages, and the combined SPR/SERS dual-mode can provide more options for detection and double check the results to improve the accuracy and reliability of assays, which holds a great application prospect for cancer-related nucleic acids detection in early disease stage.

Dendritic Forest-Like Ag Nanostructures Prepared Using Fluoride-Assisted Galvanic Replacement Reaction for SERS Applications.

Dendritic forest-like Ag nanostructures were deposited on a silicon wafer through fluoride-assisted galvanic replacement reaction (FAGRR) in aqueous AgNO3 and buffered oxide etchant. The prepared nanostructures were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, inductively coupled plasma–optical emission spectroscopy, a surface profiler (alpha step), and X-ray diffraction. Additionally, the dendritic forest-like Ag nanostructures were characterized using surface-enhanced Raman scattering (SERS) when a 4-mercaptobenzoic acid (4-MBA) monolayer was adsorbed on the Ag surface. The Ag nanostructures exhibited intense SERS signal from 4-MBA because of their rough surface, and this intense signal led to an intense local electromagnetic field upon electromagnetic excitation. The enhancement factor for 4-MBA molecules adsorbed on the Ag nanostructures was calculated to be 9.18 × 108. Furthermore, common Raman reporters such as rhodamine 6G, 4-aminothiolphenol, 5,5′-dithiobis-2-nitrobenzoic acid, and carboxyfluorescein (FAM) were characterized on these dendritic forest-like Ag nanostructures, leading to the development of an ultrasensitive SERS-based DNA sensor with a limit of detection of 33.5 nM of 15-mer oligonucleotide.

MXene (Ti3C2T )-Ag nanocomplex as efficient and quantitative SERS biosensor platform by in-situ PDDA electrostatic self-assembly synthesis strategy

A novel MXene (Ti 3 C 2 T x ) -Ag nanoparticles (NPs) hybrid biosensor prepared by in-situ electrostatic self-assembly method and could apply for label-free, repeatable and sensitive quantitative detection of biomolecules based on SERS technology. • A novel SERS biosensor based on Ti3C2Tx-Ag NPs hybrid through electrostatic self-assembly method has been fabricated. • The prepared platform exhibits excellent SERS performance, long-term stability and good uniformity. • The sensor can be used for serum sample detection. • The biosensor can be applied for label-free, repeatable and sensitive quantitative detection of biomolecules based on SERS. Recent years, two-dimensional transition metal carbonitrides, MXene, have emerged as SERS substrates due to their unique optical and electrical properties. In order to combine the advantages of metal nanoparticles and MXene for SERS substrates, this paper prepared an MXene (Ti 3 C 2 T x ) -Ag nanoparticles (NPs) hybrid biosensor through in-situ electrostatic self-assembly method, in which the citrate-coated Ag NPs are loaded on the negatively surface of Ti 3 C 2 T x flakes through positively charged PDDA polymer. The characterizations show that a large number of Ag NPs uniformly adsorb on both side of Ti 3 C 2 T x nanosheets. The prepared platform exhibits excellent SERS performance, long-term stability and good uniformity by detecting 4-MBA molecules. The electromagnetic field distribution was simulated to theoretically show the Raman enhancement mechanism of this platform. In order to explore the possible application of the sensor in clinical diagnosis and monitoring of biological environment, the platform was used for biomolecule detection. The detection limit of adenine molecules can be as low as 10 −8 M. For dopamine molecules with concentration ranging from 5 × 10 -6 M to 5 × 10 −8 M, the SERS results show a linear relationship between peak intensity and concentration of dopamine molecules. The biosensor was applied for serum detection. The results suggest that MXene-PDDA-Ag NPs hybrid platform could be used as a sensitive and uniform biosensor for label-free quantitative detection of bio-molecular based on SERS method, which could be applied in the field of biotechnology.

Au@ZIF-8 SERS paper for food spoilage detection

Putrescine and cadaverine are important volatile indicators for the evaluation of food spoilage. In this study, a metal-organic framework (MOF)-coated surface-enhanced Raman scattering (SERS) paper platform for the detection of putrescine and cadaverine is developed. Au@ zeolite imidazolate framework-8 (ZIF-8) SERS paper is fabricated by the coating of ZIF-8 layer on a Au nanoparticle-impregnated paper that is prepared by dry plasma reduction. The Au@ZIF-8 SERS paper is characterized by scanning electron microscope, energy-dispersive X-ray spectroscopy, X-ray diffraction, and N2 sorption isotherm. The ZIF-8 layer enables the accumulation of gaseous molecules and also provides enhancement of SERS signals. The fluorescence, SERS, and simulation results prove the improved detection ability of the Au@ZIF-8 platform for the volatile molecules. For the selective detection of putrescine and cadaverine, the Au@ZIF-8 SERS paper is functionalized with 4-mercatobenzaldehyde (4-MBA). The 4-MBA molecule acts as a Raman reporter and also a specific receptor for the volatile amine molecules. Using the intensity ratiometric detection of 4-MBA-functionalized Au@ZIF-8 SERS paper, putrescine and cadaverine are quantitatively detected with detection limits of 76.99 and 115.88 parts per billion, respectively. Furthermore, the detection of volatile amine molecules released from spoiled salmon, chicken, beef, and pork samples is demonstrated. It is anticipated that the MOF-coated SERS paper platforms will be applicable not only in food safety but other applications including disease diagnosis and environmental monitoring.