Shape-Controllable Gold Nanoparticle-MoS2 Hybrids Prepared by Tuning Edge-Active Sites and Surface Structures of MoS2 via Temporally Shaped Femtosecond Pulses

Controllable Fabrication of Hydrophilic Surface Micro/Nanostructures of CFRP by Femtosecond Laser

Carbon fiber reinforced polymer (CFRP), a highly engineered lightweight material with superior properties, is widely used in industrial fields, such as aerospace, automobile, and railway transportation, as well as medical implants and supercapacitor. This work presents an effective surface treatment method for the controllable fabrication of hydrophilic surface micro/nanostructures of CFRP through femtosecond laser processing. Selective removal of the epoxy resin and leaving the carbon fibers exposed are achieved when CFRP is weakly ablated by a femtosecond laser. The diameters and structures of the carbon fibers can be controlled by adjusting the laser processing parameters. Three-dimensional surface micro/nanostructures are processed when CFRP is strongly ablated by a femtosecond laser. Meanwhile, the transformation of the sp2 orbitals to sp3 orbitals of graphitic carbons of carbon fibers is induced by a femtosecond laser. Moreover, the investigation of surface roughness and wettability of femtosecond laser-processed CFRP indicates increased roughness and excellent hydrophilicity (a contact angle of 28.1°). This work reveals the effect of femtosecond laser processing on the regulation of the physicochemical properties of CFRP, which can be applicable to surface treatment and performance control of other fiber-resin composites. The excellent hydrophilicity will be conducive to the combination of CFRP with other materials or to reducing the friction resistance of CFRP used in medical implants.

Resonance-Assisted Surface-Enhanced Raman Spectroscopy Amplification on Hierarchical Rose-Shaped MoS2/Au Nanocomposites

Surface-enhanced Raman spectroscopy (SERS) has emerged as a highly sensitive trace detection technique in recent decades, yet its exceptional performance remains elusive in semiconductor materials due to the intricate and ambiguous nature of the SERS mechanism. Herein, we have synthesized MoS2 nanoflowers (NFs) decorated with Au nanoparticles (NPs) by hydrothermal and redox methods to explore the size-dependence SERS effect. This strategy enhances the interactions between the substrate and molecules, resulting in exceptional uniformity and reproducibility. Compared to the unadorned Au nanoparticles (NPs), the decoration of Au NPs induces an n-type effect on MoS2, resulting in a significant enhancement of the SERS effect. This augmentation empowers MoS2 to achieve a low limit of detection concentration of 2.1 × 10-9 M for crystal violet (CV) molecules and the enhancement factor (EF) is about 8.52 × 106. The time-stability for a duration of 20 days was carried out, revealing that the Raman intensity of CV on the MoS2/Au-6 substrate only exhibited a reduction of 24.36% after undergoing aging for 20 days. The proposed mechanism for SERS primarily stems from the synergistic interplay among the resonance of CV molecules, local surface plasma resonance (LSPR) of Au NPs, and the dual-step charge transfer enhancement. This research offers comprehensive insights into SERS enhancement and provides guidance for the molecular design of highly sensitive SERS systems.

Functionalized MoS2: circular economy SERS substrate for label-free detection of bilirubin in clinical diagnosis

A one-pot hydrothermal synthesis of Fe-doped MoS₂ nanoflowers (Fe-MoS₂ NFs) has been developed as a surface-enhanced Raman spectroscopy (SERS) substrate. The Fe-MoS₂ NFs display high reproducibility, stability, and recyclability, which is beneficial for the development of the sustainable ecological environment. The SERS substrate provides a high enhancement factor of 10⁵, which can be ascribed to the inducing defects by doping Fe that can improve the charge transfer between probe molecules and MoS₂. The Fe-MoS₂ NFs have been used to detect bilirubin in serum. The Fe-MoS₂ NF SERS substrate exhibits a linear detection range from 10^-3 to 10^-9 M with a low limit of detection (LOD) of 10^-8 M. The substrate displays an excellent selectivity to bilirubin in the presence of other potentially interfering molecules (dextrose and phosphate). These results provide a novel concept to synthesize ultra-sensitive SERS substrates and open up a wide range of possibilities for new applications of MoS₂ in clinical diagnosis.

Lys-AuNPs@MoS2 Nanocomposite Self-Assembled Microfluidic Immunoassay Biochip for Ultrasensitive Detection of Multiplex Biomarkers for Cardiovascular Diseases

The progression of cardiovascular diseases is accompanied by myocardial injury and necrosis, heart failure, and inflammatory response. Accordingly, ultrasensitive and rapid detection of multiple biomarkers plays a vital role in clinical diagnosis and timely treatment. Here, we developed a novel Lys-AuNPs@MoS₂ nanocomposite self-assembled microfluidic immunoassay biochip with digital signal output and applied it to the simultaneous detection of multiple serum biomarkers including inflammatory factors and cardiovascular biomarkers, PCT, CRP, IL6, cTnI, cTnT, and NT-BNP, with high throughput and sensitivity. The digital output signal was collected in the solid phase on the chip surface with two-dimensional distribution of targets. Lys-AuNPs@MoS₂ nanocomposites self-assembled biochips could simultaneously detect all six biomarkers in 60 samples in 40 min with detection limit of a few to tens of pg/mL for all serum biomarkers. The microfluidic biochip based on Lys-AuNPs@MoS₂ nanocomposites provides a promising method in applications for clinical diagnosis.

Effect of Al2O3 Passive Layer on Stability and Doping of MoS2 Field-Effect Transistor (FET) Biosensors

Molybdenum disulfide (MoS2) features a band gap of 1.3 eV (indirect) to 1.9 eV (direct). This tunable band gap renders MoS2 a suitable conducting channel for field-effect transistors (FETs). In addition, the highly sensitive surface potential in MoS2 layers allows the feasibility of FET applications in biosensors, where direct immobilization and detection of biological molecules are conducted in wet conditions. In this work, we report, for the first time, the degradation of chemical vapor deposition (CVD) grown MoS2 FET-based sensors in the presence of phosphate buffer and water, which caused false positive response in detection. We conclude the degradation was originated by physical delamination of MoS2 thin films from the SiO2 substrate. The problem was alleviated by coating the sensors with a 30 nm thick aluminum oxide (Al2O3) layer using atomic layer deposition technique (ALD). This passive oxide thin film not only acted as a protecting layer against the device degradation but also induced a strong n-doping onto MoS2, which permitted a facile method of detection in MoS2 FET-based sensors using a low-power mode chemiresistive I-V measurement at zero gate voltage (Vgate = 0 V). Additionally, the oxide layer provided available sites for facile functionalization with bioreceptors. As immunoreaction plays a key role in clinical diagnosis and environmental analysis, our work presented a promising application using such enhanced Al2O3-coated MoS2 chemiresistive biosensors for detection of HIgG with high sensitivity and selectivity. The biosensor was successfully applied to detect HIgG in artificial urine, a complex matrix containing organics and salts.

Activating the Basal Plane of 2H-MoS2 by Doping Phosphor for Enhancement in the Photocatalytic Degradation of Organic Contaminants

The 2H phases of MoS₂ (2H-MoS₂) monolayers present a wealth of new opportunities in photocatalysis owing to their photoinduced catalyzing ability and excellent charge carrier mobility. However, the complete release of their catalytic activities is restricted by their inert basal planes. Although the inert base planes of 2H-MoS₂ are known to be activated by atomic doping, the operational principle of the exotic atoms remains vague. In this study, the unutilized inert base sites of MoS₂ were activated via an oxygen-aided P-substituted method (denoted as POMS). Molecular structural tests and analyses of POMS indicated that the inert MoS₂ substrate is activated when the inerratic crystal phases transform to amorphous phases in the P-doping process. The fully activated inert base planes provide sufficient reaction sites for photo-oxidized water contaminants. The designed POMS presented superior activity in organic degradation and completely removed sulfamethoxazole within 20 min. Uncovering these operational principles provides a theoretical basis for designing effective catalysts.

Electrochemical Determination of Hydroxyurea in a Complex Biological Matrix Using MoS2-Modified Electrodes and Chemometrics

Hydroxyurea, an oral medication with important clinical benefits in the treatment of sickle cell anemia, can be accurately determined in plasma with a transition metal dichalcogenide-based electrochemical sensor. We used a two-dimensional molybdenum sulfide material (MoS₂) selectively electrodeposited on a polycrystalline gold electrode via tailored waveform polarization in the gold electrical double layer formation region. The electro-activity of the modified electrode depends on the electrical waveform parameters used to electro-deposit MoS₂. The concomitant oxidation of the MoS₂ material during its electrodeposition allows for the tuning of the sensor’s specificity. Chemometrics, utilizing mathematical procedures such as principal component analysis and multivariable partial least square regression, were used to process the electrochemical data generated at the bare and the modified electrodes, thus allowing the hydroxyurea concentrations to be predicted in human plasma. A limit-of-detection of 22 nM and a sensitivity of 37 nA cm⁻² µM⁻¹ were found to be suitable for pharmaceutical and clinical applications.

Nb2Se2C: a new compound as a combination of transition metal dichalcogenide and MXene for oxygen evolution reaction

Transition metal dichalcogenides and carbonitrides (TMDs and MXenes) have attracted great attention in electrochemistry due to their tunable electronic structures. Herein, a new compound of Nb2Se2C is designed as a "TMD-MXene"-like material. It exhibits better oxygen evolution reaction performance than most other reported TMDs, MXenes, and commercial electrocatalysts due to the enriched active sites and excellent conductivity.

Gold nanorod/molybdenum sulfide core-shell nanostructures synthesized by a photo-induced reduction process

Coupling of plasmonic nanostructures and semiconductors gives promising hybrid nanostructures that can be used in different applications such as photosensing and energy conversion. In this report, we describe an approach for fabricating a new hybrid material by coupling a gold nanorod (Au NR) core and amorphous molybdenum sulfide (MoS_x) shell. The Au NR/MoS_x core-shell structure is achieved by exploiting the hot electrons generated in the plasmonic excitation of Au NRs to drive the reduction of [MoS₄]^2-, which is pre-adsorbed on the Au NR surface, producing a thin MoS_x layer. This approach allows us to control the thickness of the MoS_x coating layer on the Au NR surface. The resultant Au NR/MoS_x hybrid is characterized by absorption spectroscopy, scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectroscopy elemental mapping, x-ray diffraction and x-ray photoelectron spectroscopy.

Hybridizing Plasmonic Materials with 2D-Transition Metal Dichalcogenides toward Functional Applications

Recently, 2D transition metal dichalcogenides (TMDs) have become intriguing materials in the versatile field of photonics and optoelectronics because of their strong light-matter interaction that stems from the atomic layer thickness, broadband optical response, controllable optoelectronic properties, and high nonlinearity, as well as compatibility. Nevertheless, the low optical cross-section of 2D-TMDs inhibits the light-matter interaction, resulting in lower quantum yield. Therefore, hybridizing the 2D-TMDs with plasmonic nanomaterials has become one of the promising strategies to boost the optical absorption of thin 2D-TMDs. The appeal of plasmonics is based on their capability to localize and enhance the electromagnetic field and increase the optical path length of light by scattering and injecting hot electrons to TMDs. In this regard, recent achievements with respect to hybridization of the plasmonic effect in 2D-TMDs systems and its augmented optical and optoelectronic properties are reviewed. The phenomenon of plasmon-enhanced interaction in 2D-TMDs is briefly described and state-of-the-art hybrid device applications are comprehensively discussed. Finally, an outlook on future applications of these hybrid devices is provided.

Plasmonic-induced overgrowth of amorphous molybdenum sulfide on nanoporous gold: An ambient synthesis method of hybrid nanoparticles with enhanced electrocatalytic activity

Hybrid materials of earth abundant transition metal dichalcogenides and noble metal nanoparticles, such as molybdenum sulfide (MoS_x) and gold nanoparticles, exhibit synergistic effects that can enhance electrocatalytic reactions. However, most current hybrid MoS_x-gold synthesis requires an energy intensive heat source of >500 °C or chemical plating to achieve deposition of MoS_x on the gold surface. Herein, we demonstrate the direct overgrowth of MoS_x over colloidal nanoporous gold (NPG), conducted feasibly under ambient conditions, to form hybrid particles with enhanced electrocatalytic performance toward hydrogen evolution reaction. Our strategy exploits the localized surface plasmon resonance-mediated photothermal heating of NPG to achieve >230 °C surface temperature, which induces the decomposition of the (NH₄)₂MoS₄ precursor and direct overgrowth of MoS_x over NPG. By tuning the concentration ratio between the precursor and NPG, the amount of MoS_x particles deposited can be systematically controlled from 0.5% to 2% of the Mo/(Au + Mo) ratio. Importantly, we find that the hybrid particles exhibit higher bridging and an apical S to terminal S atomic ratio than pure molybdenum sulfide, which gives rise to their enhanced electrocatalytic performance for hydrogen evolution reaction. We demonstrate that hybrid MoS_x-NPG exhibits >30 mV lower onset potential and a 1.7-fold lower Tafel slope as compared to pure MoS_x. Our methodology provides an energy- and cost-efficient synthesis pathway, which can be extended to the synthesis of various functional hybrid structures with unique properties for catalysis and sensing applications.

Pressure-induced SERS enhancement in a MoS2/Au/R6G system by a two-step charge transfer process

Pressure-induced surface-enhanced Raman spectroscopy (PI-SERS) represents a new frontier in the research field of SERS. However, relatively few studies have focused on PI-SERS due to many difficulties, such as easy aggregation of nanoparticles, and difficulty in understanding the interaction mechanisms between probe molecules and the SERS substrate at high pressure. Here we developed an efficient semiconductor-metal SERS substrate (MoS₂/Au) to study PI-SERS. Different from the previously reported monotonous decrease in Raman intensities upon compression, an anomalous Raman enhancement of R6G molecules adsorbed on the MoS₂/Au substrate was observed up to 2.39 GPa, at which the degree of charge transfer (ρ_CT) between the R6G molecules and the MoS₂/Au substrate reaches a maximum. By comparison, it is proposed that the decoration of Au on the SERS system could bring about a two-step charge transfer (CT) process, introduce localized surface plasmon resonance (LSPR), and thus favor the PI-SERS enhancement. Moreover, this charge transfer also causes obvious changes in the optical behaviors of R6G molecules upon compression. This brings new insights into the SERS study and also offers new ideas for the development of SERS application in high pressure studies.

Rapidly fabricating a large area nanotip microstructure for high-sensitivity SERS applications

Here, we propose a novel nanotip microstructure which can be easily fabricated through a simply Reactive Ion Etching (RIE) process combined with anodic aluminum oxide (AAO) membranes. When combined with Ag coating and annealing on the surface of micro-sized nanotip arrays, the as-formed Ag-nanoparticles (Ag-NPs)/Si-nanotip hybrid structure exhibited a significantly high enhancement factor and highly sensitive surface enhanced Raman scattering (SERS) for rhodamine 6G molecules. The nanotip microstructure showed a sharp curvature with an apex diameter which significantly affected the SERS results. The Ag-NPs/Si-nanotip hybrid structure verified a very prominent "hot spot" effect that exists around the nanotip structures, which contributed mainly to an enhanced SERS signal with an enhancement factor (EF) of 1.6 × 10⁶. Moreover, the Ag-NPs/Si-nanotip hybrid structure demonstrated superior sensitivity, with obvious featured Raman peaks even when the concentration was as low as 10^-10 M. Our work demonstrated a feasible way to prepare a novel nanotip microstructure with a highly localized surface plasmon resonance response which could be feasibly applied for highly sensitive and reproducible SERS applications.

SERS based detection of multiple analytes from dye/explosive mixtures using picosecond laser fabricated gold nanoparticles and nanostructures

Surface enhanced Raman spectroscopy (SERS) is a cutting edge analytical tool for trace analyte detection due to its highly sensitive, non-destructive and fingerprinting capability. Herein, we report the detection of multiple analytes from various mixtures using gold nanoparticles (NPs) and nanostructures (NSs) as SERS platforms. NPs and NSs were achieved through the simple approach of laser ablation in liquids (LAL) and their morphological studies were conducted with a UV-Visible absorption spectrometer, a high resolution transmission electron microscope (HRTEM) and a field emission scanning electron microscope (FESEM). The fabricated NPs/NSs allowed the sensitive and selective detection of different mixed compounds containing (i) rhodamine 6G (Rh6G) and methylene blue (MB), (ii) crystal violet (CV) and malachite green (MG), (iii) picric acid (explosive) and MB (dye), (iv) picric acid and 3-nitro-1,2,4- triazol-5-one (explosive, NTO) and (v) picric acid and 2,4-dinitrotoluene (explosive, DNT) using a portable Raman spectrometer. Thus, the obtained results demonstrate the capability of fabricated SERS substrates in identifying explosives and dyes from various mixtures. This could pave a new way for simultaneous detection of multiple analytes in real field applications.

Enhancing charge transfer with foreign molecules through femtosecond laser induced MoS2 defect sites for photoluminescence control and SERS enhancement

Defect/active site control is crucial for tuning the chemical, optical, and electronic properties of MoS2, which can adjust the performance of MoS2 in application areas such as electronics, optics, catalysis, and molecular sensing. This study presents an effective method of inducing defect/active sites, including micro/nanofractured structures and S atomic vacancies, on monolayer MoS2 flakes by using femtosecond laser pulses, through which physical-chemical adsorption and charge transfer between foreign molecules (O2 or R6G molecules) and MoS2 are enhanced. The enhanced charge transfer between foreign molecules (O2 or R6G) and femtosecond laser-treated MoS2 can enhance the electronic doping effect between them, hence resulting in a photoluminescence photon energy shift (reaching 0.05 eV) of MoS2 and Raman enhancement (reaching 6.4 times) on MoS2 flakes for R6G molecule detection. Finally, photoluminescence control and micropatterns on MoS2 and surface-enhanced-Raman-scattering (SERS) enhancement of MoS2 for organic molecule detection are achieved. The proposed method, which can control the photoluminescence properties and arbitrary micropatterns on MoS2 and enhance its chemicobiological sensing performance for organic/biological molecules, has advantages of simplicity, maskless processing, strong controllability, high precision, and high flexibility, highlighting the superior ability of femtosecond laser pulses to achieve the property control and functionalization of two-dimensional materials.

3D SERS substrate based on Au-Ag bi-metal nanoparticles/MoS2 hybrid with pyramid structure

It is very vital to construct the dense hot spots for the strong surface-enhanced Raman scattering (SERS) signals. We take full advantage of the MoS₂ edge-active sites induced from annealing the Ag film on the surface of the MoS₂. Furthermore, the composite structure of Au-Ag bi-metal nanoparticles (NPs)/MoS₂ hybrid with pyramid structure is obtained by the in situ grown AuNPs around AgNPs, which serves the optimal SERS performance (enhancement factor is ~9.67 × 10⁹) in experiment. Due to the introduction of AuNPs with the simple method, the denser hot spots contribute greatly to the stronger local electric field, which is also confirmed by the finite-different time-domain (FDTD) simulation. Therefore, the ultralow limit of detection (the LOD of 10^-13 and 10^-12 M respectively for the resonant R6G and non-resonant CV), quantitative detection and excellent reproducibility are achieved by the proposed SERS substrate. For practical application, the melamine molecule is detected with the LOD of 10^-10 M using the proposed SERS substrate that has the potential to be a food security sensor.

Electrons dynamics control by shaping femtosecond laser pulses in micro/nanofabrication: modeling, method, measurement and application

During femtosecond laser fabrication, photons are mainly absorbed by electrons, and the subsequent energy transfer from electrons to ions is of picosecond order. Hence, lattice motion is negligible within the femtosecond pulse duration, whereas femtosecond photon-electron interactions dominate the entire fabrication process. Therefore, femtosecond laser fabrication must be improved by controlling localized transient electron dynamics, which poses a challenge for measuring and controlling at the electron level during fabrication processes. Pump-probe spectroscopy presents a viable solution, which can be used to observe electron dynamics during a chemical reaction. In fact, femtosecond pulse durations are shorter than many physical/chemical characteristic times, which permits manipulating, adjusting, or interfering with electron dynamics. Hence, we proposed to control localized transient electron dynamics by temporally or spatially shaping femtosecond pulses, and further to modify localized transient materials properties, and then to adjust material phase change, and eventually to implement a novel fabrication method. This review covers our progresses over the past decade regarding electrons dynamics control (EDC) by shaping femtosecond laser pulses in micro/nanomanufacturing: (1) Theoretical models were developed to prove EDC feasibility and reveal its mechanisms; (2) on the basis of the theoretical predictions, many experiments are conducted to validate our EDC-based femtosecond laser fabrication method. Seven examples are reported, which proves that the proposed method can significantly improve fabrication precision, quality, throughput and repeatability and effectively control micro/nanoscale structures; (3) a multiscale measurement system was proposed and developed to study the fundamentals of EDC from the femtosecond scale to the nanosecond scale and to the millisecond scale; and (4) As an example of practical applications, our method was employed to fabricate some key structures in one of the 16 Chinese National S&T Major Projects, for which electron dynamics were measured using our multiscale measurement system.

Optical Field Enhancement in Au Nanoparticle-Decorated Nanorod Arrays Prepared by Femtosecond Laser and Their Tunable Surface-Enhanced Raman Scattering Applications

Various Au nanostructures have been demonstrated to have an enhanced local electric field around them because of surface plasmons. Herein, we propose a novel method for fabricating Au nanoparticle-decorated nanorod (NPDN) arrays through femtosecond laser irradiation combined with Au coating and annealing. The nanorod cavities strongly confined light and produced an enhanced optical field in response to Au nanoparticles (NPs) introduction. The nanogap and diameter of the fabricated Au NPs significantly affected the surface-enhanced Raman scattering (SERS) performance and could be simultaneously tuned with thickness-controllable Au films and substrate morphologies. The resulting Au NPDN substrate was observed to have efficient "hot spots" for tunable SERS applications. We experimentally determined that the enhancement factor of the Au NPDN substrate reached up to 8.3 × 10⁷ at optimal parameters. Moreover, the Au NPDN substrate showed superior chemical stability, with the greatest intensity deviation of 3.2% on exposure to air for 2 months. This work provides a promising method to fabricate tunable plasmonic surfaces for highly sensitive, reproducible, and chemically stable SERS applications.