Although plasmonic nanofibrous mats have received much attention for SERS detection in recent years, plasmonic nanofibrous mats with superhydrophobicity are rarely reported due to the hydrophilic characteristics of Au and/or Ag nanocrystals. The analyte solution usually spreads randomly over a large area based on this hydrophilic characteristic of Au and/or Ag nanocrystals, which severely restricts the improvement of SERS detection sensitivity. Here, we prepared superhydrophobic polymer nanofibers decorated with noble metal nanoparticles with largerly improved sensitivity for SERS detection. The superhydrophobic characteristic of the polymer/noble metal hybrid nanofibrous mat can overcome the random spreading of the analyte aqueous solution over the substrate and enriches the droplet containing analyte molecules on a very small area where a strong plasmonic electric-field was used to carry out SERS detection. About a 14-fold decrease in spot area on our superhydrophobic substrate leads to a corresponding 8-fold improvement in the lowest detection concentration for rhodamine 6G molecules, as compared to a non-superhydrophobic substrate.

Concave nanocrystals usually exhibit a large electromagnetic-field enhancement and superior catalytic performance due to their sharp corners, negative curvature and high-index facets. Conventional gold bipyramids (AuBPs) possess intriguing plasmonic properties which are attractive for various applications while the surface curvature of the reported bipyramids has not been fine-tuned to concave or convex structures to date. Additionally, the longitudinal surface plasmon resonance (LSPR) wavelengths of conventional AuBPs are mostly located in the range of 650–1350 nm and the sizes of these nanoparticles are usually not beyond 350 nm, which are not facilitated to some nano-focusing and nanophotonic applications. Herein, we reported a facile and robust approach for fabricating concave AuBPs (CAuBPs) with multiple high-index facets which are distinct from the conventional AuBPs and nanojavelin structures. The length of the as-prepared CAuBPs can even extend up to 800 nm. The CAuBP nanoparticles exhibit a strikingly pronounced broader plasmonic tuning range (even exceeding 1800 nm) and provide much higher electromagnetic-field enhancements in comparison to the conventional AuBPs, which broaden the promising applications of CAuBPs for many single-particle analyses. More importantly, the surface-enhanced Raman scattering (SERS) signals of CAuBPs on the single-particle or aqueous solution both displayed an enhanced intensity compared to conventional AuBPs. The CAuBP nanoparticles also exhibited improved catalytic activity due to the incredible abundance of uncoordinated atoms as active sites.

Most as-reported nanostructures through galvanic replacement reactions are still symmetric hollow structures, until now. Asymmetric structures fabricated through a galvanic replacement reaction have been rarely reported. However, asymmetric heterostructures can generally lead to new intriguing properties through asymmetric synergistic coupling. Here, we report a simple synthesis of an asymmetric one-ended AgPd bimetal on Au nanorods (AuNR) by combining a galvanic replacement reaction with an Ostwald ripening process. The morphological evolution from a nanodumbbell to a dandelion structure is thoroughly investigated. The unique asymmetric AgPd–AuNR heterostructures possess the required plasmonic performance and avoid strong damping caused by the poor plasmonic metal Pd, resulting in a superior photothermal heating performance and enhanced SERS sensitivity for in situ monitoring of a catalytic reaction compared with the symmetric counterparts.

Electrospun polymer/noble metal hybrid nanofibers have developed rapidly as surface-enhanced Raman scattering (SERS)-active substrates over the last few years. However, polymer/noble metal nanofibers with plasmon-enhanced fluorescence (PEF) activity have received no attention to date. Herein, we show a general and facile approach for the preparation of polyacrylonitrile (PAN)/noble metal/SiO2 nanofibrous mats with PEF activity for the first time by combining electrospinning and controlled silica coatings. These PEF-active nanofibrous mats can selectively improve the fluorescence intensity of conjugated polyelectrolytes (CPEs). Importantly, the CPE solution in the presence of a PAN/noble metal/SiO2 nanofibrous mat showed dramatic fluorescence quenching towards picomolar (pM) amounts of heavy metal ions, while the fluorescence of the CPE solution without the nanofibrous mat had no apparent quenching towards micromolar (μM) amounts of metal ions. The combination of the distance-dependent fluorescence enhancement performance of metal NPs and the ionic characteristics of the CPE solution makes the polymer/noble metal nanofibers promising substrates for greatly improving the detection sensitivity towards metal ions. We believe that this work provides a general strategy for preparing plasmon band-tuned PEF-active substrates with advantages including good selectivity, remarkable sensitivity and recyclability, which make them a preferable choice for practical sensing applications.

A facile and rapid approach for detecting low concentration of iron ion (Fe3+) with improved sensitivity was developed on the basis of plasmon enhanced fluorescence and subsequently amplified fluorescence quenching. Au1Ag4@SiO2 nanoparticles were synthesized and dispersed into fluorescein isothiocyanate (FITC) solution. The fluorescence of the FITC solution was improved due to plasmon enhanced fluorescence. However, efficient fluorescence quenching of the FITC/Au1Ag4@SiO2 solution was subsequently achieved when Fe3+, with a concentration ranging from 17 nM to 3.4 μM, was added into the FITC/Au1Ag4@SiO2 solution, whereas almost no fluorescence quenching was observed for pure FITC solution under the same condition. FITC/Au1Ag4@SiO2 solution shows a better sensitivity for detecting low concentration of Fe3+ compared to pure FITC solution. The quantized limit of detection toward Fe3+ was improved from 4.6 μM for pure FITC solution to 20 nM for FITC/Au1Ag4@SiO2 solution.

SERS has been applied to monitor different types of catalytic reactions on the surface of metal NPs in recent years, which requires bifunctional metal structures with plasmonic properties as well as catalytic activity. Monitoring catalytic reactions with SERS technology in previous reports was usually performed in metal solution, using metal electrodes or on a glass/silica substrate with an immobilized metal. Here, we report a general approach for preparing novel SERS-active substrates which are used to monitor catalytic reactions. The polyacrylonitrile (PAN)/Ag–M (M = Au or Pd) bimetallic nanofibrous mats are prepared through galvanic replacement reactions of Ag with Au or Pd on the surface of electrospun PAN/Ag nanofibers. The composition of Ag and Au or Pd could be tuned by changing the concentration of the metal solution used for replacement. The PAN/Ag0.60Au0.40 and PAN/Ag0.90Pd0.10 bimetallic nanofibrous mats exhibit both excellent SERS and catalytic activities. The reduction of 4-nitrothiophenol (4-NTP) to the corresponding to 4-aminothiophenol (4-ATP) on PAN/Ag0.60Au0.40 and PAN/Ag0.90Pd0.10 bimetallic nanofibrous mats are monitored by SERS technology. The SERS signals of the reaction intermediate, 4,4′-dimercapto-azobenzene(4,4′-DMAB), are observed as the reaction proceeds with time.

A facile approach to prepare electrospun poly(vinly alcohol) (PVA) nanofibers with high concentration of gold nanoparticles (Au NPs) on the fibers, had been developed. These PVA/Au nanofibers could be used as flexible surface-enhanced Raman scattering (SERS) substrates. Relatively high concentration of PVA aqueous solution (10 wt %) was used as the stabilizing agent for gold salt precursor, as well as the starting solution for electrospinning. This method was demonstrated to be effective to prepare high-concentration-gold nanoparticles without aggregation and precipitation by reducing high concentration of gold salt in the presence of PVA aqueous solution. SEM and TEM images showed that both the amount and the size of Au NPs which embedded in PVA nanofibers, increased with increasing the gold salt content, while the gap between the adjacent NPs decreased. Raman spectra showed an apparent enhancement in the signal of 4-mercaptobenzoic acid (4-MBA) molecules pre-absorbed from its ethanol solution onto the PVA/Au nanofibers. The high SERS activity to 4-MBA in solution with a relatively low concentration (10−6 M), could be mainly attributed to the reduced gap of Au NPs.

SERS has been applied to monitor different types of catalytic reactions on the surface of metal NPs in recent years, which requires bifunctional metal structures with plasmonic properties as well as catalytic activity. Monitoring catalytic reactions with SERS technology in previous reports was usually performed in metal solution, using metal electrodes or on a glass/silica substrate with an immobilized metal. Here, we report a general approach for preparing novel SERS-active substrates which are used to monitor catalytic reactions. The polyacrylonitrile (PAN)/Ag–M (M = Au or Pd) bimetallic nanofibrous mats are prepared through galvanic replacement reactions of Ag with Au or Pd on the surface of electrospun PAN/Ag nanofibers. The composition of Ag and Au or Pd could be tuned by changing the concentration of the metal solution used for replacement. The PAN/Ag0.60Au0.40 and PAN/Ag0.90Pd0.10 bimetallic nanofibrous mats exhibit both excellent SERS and catalytic activities. The reduction of 4-nitrothiophenol (4-NTP) to the corresponding to 4-aminothiophenol (4-ATP) on PAN/Ag0.60Au0.40 and PAN/Ag0.90Pd0.10 bimetallic nanofibrous mats are monitored by SERS technology. The SERS signals of the reaction intermediate, 4,4′-dimercapto-azobenzene(4,4′-DMAB), are observed as the reaction proceeds with time.