Self-assembly of metal nanoparticles has attracted considerable attention because of its unique applications in technologies such as plasmonics, surface-enhanced optics, sensors, and catalysts. However, fabrication of ordered nanoparticle structures remains a significant challenge. Thus, developing an efficient approach for the assembly of large-scale Au nanoparticles films for theoretical studies and for various applications is highly desired. In this paper, a facial approach for fabricating a monolayer film of Au nanoparticles was developed successfully. Using the surfactant polyvinylpyrrolidone (PVP), a large-scale monolayer film of well-ordered, uniform-sized Au nanoparticles was fabricated at the air/water interface. The film exhibited a two-dimensional (2D) hexagonal close-packed (HCP) structure having interparticle gaps smaller than 2 nm. These gaps generated numerous uniform “hot spots” for surface-enhanced Raman scattering (SERS) activity. The as-prepared monolayer film could be transferred to a solid substrate for use as a suitable SERS substrate with high activity, high uniformity, and high stability. The low spot-to-spot and substrate-to-substrate variations of intensity (<10%), the large surface enhancement factor (∼106), and the high stability (∼45 days) make the substrate suitable for SERS measurements. Transfer of the monolayer film onto a glassy carbon electrode produced an Au electrode with clean, well-defined nanostructure suitable for electrochemical SERS measurements. The adsorption process of ionic liquids on the electrode with the monolayer film is similar to that on bulk metal electrodes. The present strategy provides an effective way for self-assembly of Au nanoparticles into well-defined nanostructures that may form optimal reproducible SERS substrates for quantitative analysis. It also provides an electrode with clean, well-defined nanostructure for electrochemical investigations.

The assembly of simple nanostructures has already attracted significant interest in the field of plasmonic devices and other relevant areas. From the viewpoint of theoretical simulation and practical application, the precise control of nanoparticles still remains a significant challenge. Herein, a strategy was successfully developed to fabricate in situ a polymer-encapsulated Au nanoparticle dimer based on the C–C coupling reaction of p-aminophenylacetylene (p-APAC). The balance between the polymerization processes and the coupling reaction resulted in Au nanoparticle assemblies with different configurations, such as monomer, dimer, and multimer, depending on the concentration of p-APAC. The gap distance of about 1.8 nm was well consistent with the length of the coupling products of p-APAC, i.e. the gap distance was about double the length of a single p-APAC molecule. The observation of a longitudinal peak in the UV-vis spectrum demonstrated that the aspect ratio of the Au nanoparticles was about 2.5, indicating the formation of Au dimers with reasonable yield. Moreover, the thickness of the polymer shell was well-controlled via changing the concentration of p-APAC. The gap of the dimer resulted in a very large coupling effect of the localized surface plasmon resonance (LSPR), and the surface enhanced Raman spectroscopy (SERS) signal of the molecules was accordingly enhanced in the gap areas, which served as the hot spots. Based on the characteristic spectral feature of the coupling products, the single Au nanoparticle dimer was positioned via SERS mapping. The large LSPR coupling effect in the gap area allowed the conversion of p-nitrothiophenol (PNTP) to dimercaptoazobenzene (DMAB) with high efficiency. Thus, it was confirmed that the SPR-catalyzed coupling reaction preferentially occurred on a hot spot area. The proposed approach is expected to be developed into a promising tool for precisely controlling the gap distance of a nanoparticle assembly, and it may serve as a simple model for theoretical consideration in understanding the SERS mechanism(s).

Rapid separation and detection of analytes have been the focus of a growing body of investigation for potential applications including food safety and environment science. However, the development of a robust analytical technique for simultaneous rapid separation and on-line detection remains a formidable challenge. Herein, we report a rational design based on the combination of high performance liquid chromatography (HPLC) and surface-enhanced Raman spectroscopy (SERS) for the rapid separation and on-line detection of multi-analytes. In particular, a plasmonic nanoparticle-modified capillary (NPMC) is fabricated through a self-assembly process and connected to a HPLC effluent-end port. After separation by HPLC, the analytes are adsorbed onto plasmonic nanoparticles in the capillary and then detected by SERS. The resulting HPLC-SERS coupled detection system can simultaneously achieve rapid separation and provide on-line molecular structural information of multi-analytes. In addition, we also demonstrate the on-line detection of a pesticide molecule (thiram) in an orange using this combined system. Importantly, the detection limit can be down to 10−7 mol L−1. These findings indicate that our coupled HPLC-SERS system offers a promising analytical technique in modern analytical science and technology.

Surface enhanced Raman spectroscopy (SERS) has been considered as a promising tool for detecting targets with single molecule sensitivity. However, the SERS detection on targets without a specific adsorption group has still remained a significant challenge. In this paper, we reported a facile strategy to fabricate a PDMS film-coated Au nanoparticle monolayer film (Au MLF) composite substrate for improving SERS detection of aromatic molecules in water and in the atmosphere. Toluene, benzene and nitrobenzene were used as the targets to evaluate the performance of the composite substrate. The results indicated that the PDMS film played the vital role to capture and preconcentrate these targets for improving the capability in SERS detection of these targets. The performance was critically dependent on the hydrophobicity, functional groups and the permeability of the targets. This composite substrate was more favorable for the detection of toluene and nitrobenzene than benzene. The limit of detection (LOD) for toluene and nitrobenzene was decreased by about two orders of magnitude on the PDMS-Au MLF compared to that on the naked Au MLF, and by one order of magnitude for benzene. It was estimated to be 0.5 ppm, 0.6 ppm and 78 ppm for toluene, nitrobenzene and benzene, respectively. The results demonstrated that this approach could be developed as a promising tool to detect numerous targets which were non-specifically adsorbed onto metallic nanostructures. It opened a window towards the general application of SERS for in situ monitoring of pollutants in water and in the atmosphere.

Surface plasmon plays an important role in surface catalysis reactions, and thus the tuning of plasmon on metal nanostructures and the extension of plasmon induced surface catalysis reactions have become important issues. Au nanoparticle monolayer film was fabricated by the assembling of Au nanoparticles at the liquid–air interface with numerous “hot spots” for strong surface plasmon coupling. A facile approach was developed to achieve the decarboxylation reaction driven by appropriate surface plasmon on the Au nanoparticle monolayer film surface, and surface enhanced Raman spectroscopy (SERS) has been developed as a sensitive tool for the in situ monitoring of the plasmon induced surface reaction. The effects of the power and wavelength of the laser and solution pH on the decarboxylation reaction were investigated. With laser illumination, para-mercaptobenzoic acid (PMBA) was transformed to thiophenol (TP), and the decarboxylation was enhanced on increasing the laser power and illumination time. The results revealed that the carboxylate groups of the adsorbed PMBA molecules were removed to produce TP, which were still adsorbed onto Au surfaces. The solution pH values exhibited a significant influence on the decarboxylation reaction. In air and neutral solution, decarboxylation proceeded at a slow rate to transform PMBA to TP, while it was absent in acidic solution. The deprotonated carboxylate group accelerated the decarboxylation for producing TP with a fast rate in alkaline solution. As a comparison, a similar plasmon driven decarboxylation reaction was observed on a Ag nanoparticle monolayer film surface. These results suggested that the transformation from PMBA to TP molecules on an Au nanoparticle film surface under laser illumination was associated with a surface-catalyzed reaction driven by local surface plasmon.

•In situ SERS was developed to probe electric double layer and pzc of RTILs.•The minimum of camel-shaped capacitance curves was assigned to pzc.•SERS was well correlated with the capacitance–potential curves.•The pzc was depended on the length of alkyl chain of imidazolium based RTILs.The electrochemical differential capacitance and in situ surface enhanced Raman spectroscopy (SERS) have been combined to investigate the structure of electric double layer (EDL) at a series of imidazolium based ionic liquids/Ag electrode interface. By associating with the change in spectral feature of the imidazolium ring, the potential at local minimum between the two maxima in the camel-shaped differential capacitance curves was assigned to the potential of zero charge (pzc). The pzc shifted positively with increasing the length of the alkyl chain substituted to the imidazolium ring, and the approximate linear relation was observed between the pzc values and the carbon numbers of the alkyl chain. The values of the minimum capacitance at the pzc decreased with increasing the alkyl chain length, which was mainly contributed by the smaller values of dielectric constant (ε) for the RTILs with long alkyl chain. By employing in-situ SERS, the potential dependent spectral feature could be served as the criteria for resolving the adsorption behavior of the cations. With the movement of the potential from relative positive to negative potential, the cation adsorption configuration changed from vertical to nearly flat on. The reorientation could be considered as the evidence for estimating the pzc values. The spectroscopic investigation revealed that the onset potentials for the reorientation were shifted to positive direction as increasing the length of the alkyl chain. The tendency in changing the potential at the minimum capacitance or the reorientation was contributed to the π-electron interaction of imidazolium ring with the Ag electrode, in which the adsorption of cation acted like the behavior of a specific adsorbed anion. The weaker the interaction (long alkyl chain) was, the more the pzc shifted to positive potential direction. The above facts indicated that the classical differential capacitance curves and in situ SERS technique were well correlated for obtaining a deeper insight into the interfacial structure of RTILs at Ag electrodes.

•Chromium selenidostannates 1 and 2 were firstly prepared using ionothermal methods.•1 and 2 are the first examples of ionothermally synthesized TM-chalcogenidostannates.•The chromium selenidostannates exhibit optical band gaps of 2.08 and 2.19 eV.Ternary chromium selenidostannates [Cr(tepa)(OH)]2Sn2Se6·H2O (tepa = tetraethylenepentamine) (1) and [Cr(peha)]2(Sn2Se6)Cl2 (peha = pentaethylenehexamide) (2) were successfully synthesized by the reaction of CrCl3·6H2O, Sn, Se and tepa or peha in ionic liquid 1-ethyl-3-methylimidazolium chloride at 160 °C. In the ionic liquid, the Cr3 + ion forms [Cr(tepa)(OH)]2 + or [Cr(peha)]3 + chelating complex cations, which lead to the formation of chromium selenidostannates 1 and 2. But the reactions in pure tepa or peha under solvothermal conditions produced amorphous powders. 1 and 2 are the first examples of TM-complex contained chalcogenidostannates synthesized by ionothermal technique. Both 1 and 2 form three-dimensional networks via intermolecular NH⋯Se, OH⋯Se and NH⋯O or NH⋯Cl interactions, respectively. 1 and 2 exhibit possible semiconducting properties with the band gaps at 2.08 and 2.19 eV, respectively.Chromium selenidostannates [Cr(tepa)(OH)]2Sn2Se6·H2O (1) and [Cr(peha)]2(Sn2Se6)Cl2 (2) were firstly prepared under ionothermal conditions. 1 and 2 are the first examples of ionothermally synthesized chalcogenidostannates containing TM-complexes.