Zhen Rong

A rapid SERS method for label-free bacteria detection using polyethylenimine-modified Au-coated magnetic microspheres and Au@Ag nanoparticles

A rapid, sensitive, and label-free SERS detection method for bacteria pathogens is reported for the first time. The method, which is based on the combination of polyethylenimine (PEI)-modified Au-coated magnetic microspheres (Fe3O4@Au@PEI) and concentrated Au@Ag nanoparticles (NPs), was named the capture-enrichment-enhancement (CEE) three-step method. A novel Fe3O4@Au microsphere with monodispersity and strong magnetic responsiveness was synthesized as a magnetic SERS substrate and amino functionalized by PEI self-assembly. The negatively charged bacteria were quickly captured and enriched by the positively charged Fe3O4@Au@PEI microspheres, and the bacteria SERS signal was synergistically enhanced by using Fe3O4@Au@PEI microspheres and Au@Ag NPs in conjunction. The CEE three-step method proved useful in tap water and milk samples, and the total assay time required was only 10 min. Results further demonstrated that the CEE three-step method could be a common approach for detecting a wide range of bacteria, as verified by its detection of the Gram-positive bacterium E. coli and Gram-positive bacterium S. aureus at a detection limit of as low as 103 cells per mL. Therefore, our CEE three-step method offered the significant advantages of short assay time, simple operating procedure, and higher sensitivity than previously reported methods of SERS-based bacteria detection.

Polyethylenimine-interlayered silver-shell magnetic-core microspheres as multifunctional SERS substrates

The fabrication of an ideal noble metal modified magnetic microsphere as a high performance SERS substrate that possesses good dispersibility, strong magnetic responsiveness, and high sensitivity is still a challenge. Herein, we report a novel route for fabricating Ag-coated magnetic core–shell microspheres (Fe3O4@PEI@Ag) with most of the desired advantages by using polyethyleneimine (PEI) as an interlayer. The size and coverage level of the Ag-NPs shell on Fe3O4@PEI@Ag microspheres were easily controlled by varying the amount of AgNO3. Meanwhile, the magnetic core endowed the Fe3O4@PEI@Ag microspheres with superior magnetic nature, which enabled convenient separation and further enhanced Raman signals due to enrichment of targeted analytes and abundant interparticle hotspots created by magnetism-induced aggregation. Considering these features, Fe3O4@PEI@Ag is expected to be a versatile SERS substrate, which was verified by the detection of adsorbed PATP molecules and human IgG with a detection limit as low as 10−11 M and 10−14 g mL−1, respectively. Therefore, the novel Fe3O4@PEI@Ag microsphere has an enormous potential for practical SERS detection applications, especially in the field of quantitative detection of target proteins.

Plasmonic Ag Core–Satellite Nanostructures with a Tunable Silica-Spaced Nanogap for Surface-Enhanced Raman Scattering

Plasmonic Ag core-satellite nanostructures were synthesized by utilizing the ultrathin silica shell as a spacer to generate a tunable nanogap between the Ag core and satellites. To synthesize the nanoparticles, Ag nanoparticles (Ag NPs) with a diameter of ∼60 nm were synthesized as cores, on which Raman dyes were adsorbed and then tunable ultrathin silica shells from 2.0 to 6.5 nm were coated, followed by the deposition of Ag NPs as satellites onto the silica surface. The relationships between the SERS signal and the important parameters, including the satellite diameter and the nanogap distance, were studied by experimental methods and theoretical calculations. The maximum SERS intensity of the core-satellite nanoparticles was over 14.6 times stronger than that of the isolated Raman-encoded Ag/PATP@SiO2 NP. The theoretical calculations indicated that the local maximum calculated enhancement factor (EF) of the hot spots with a 2.0 nm nanogap was 9.5 × 10(5). The well-defined Ag core-satellite nanostructures have a high structural uniformity and an anomalously strong electromagnetic enhancement for highly quantitative SERS, leading to a better understanding of hot spot formation and providing new insights into the optimal design and synthesis of the hot SERS nanostructures in a controlled manner.

A graphene-interlayered magnetic composite as a multifunctional SERS substrate

Investigations in the combination of graphene with noble-metal nanostructures mostly focus on the fabrication of stacked hybrid films or nanoparticle (Au or Ag NPs) decorated graphene sheets, while few are carried out in graphene built-in monodispersed microspheres. Herein, we propose a novel hierarchical composite with a silver-shelled Fe3O4 microsphere as the initial core and graphene oxide (GO) as the multifunctional inserted layer. The flexible GO provides abundant nucleation sites for further in situ growth of Au nanoparticles. Moreover, it introduces considerable chemical enhancement and enhances surface adsorption toward aromatic molecules. Considering the synergistic properties of versatile GO with noble-metal nanostructures, the synthesized Fe3O4@Ag-rGO-Au microspheres were expected to have excellent surface-enhanced Raman scattering (SERS) activity, which was verified by the detection of the sulfhydryl-contained PATP and aromatic DTTC, with respective detection limits of 10−11 M and 10−10 M. The enhancement factor (EF) was calculated to be 2.83 × 106 for totally symmetric (a1) vibrations located at 1077 cm−1 of PATP. Additionally, its fast magnetic response enables rapid separation from solution, which is a key factor for many practical applications. Therefore, the well-dispersed hybrid microspheres have enormous potential in sensing applications as well as in effective immobilization and enrichment of biomolecules and aromatic pollution.

Silver coated magnetic microflowers as efficient and recyclable catalysts for catalytic reduction

This study proposes an effective route for synthesis of three-dimensional (3D) flower-like, Ag-coated magnetic (Fe3O4@SiO2@Ag) microcomposites with well-controlled sizes and shapes. The fabricated microflowers consist of a micro-scale Fe3O4@SiO2 core, which provides sufficient magnetic response properties and good dispersibility, and a highly-branched Ag shell, which is characterized by a large specific surface area and multidimensional catalytically active sites. The Ag seeds on the Fe3O4@SiO2 microspheres play a key role in the formation of a flower-like structure and can be produced massively and reproducibly through electroless plating. Furthermore, the surface morphologies of the microcomposites can be well controlled by changing the experimental parameters. Four kinds of composites with well-tuned surface morphologies were synthesized to systematically investigate the effect of surface nanostructures on the performance of catalytic reduction reactions. Highly-branched microflowers exhibit significantly higher catalytic activity than non-branched or little-branched structures toward the reduction of 4-nitrophenol and methylene blue. The flower-like catalysts, which exhibit excellent magnetic properties, can be easily recycled and retain >93% conversion for at least six cycles. Hence, Fe3O4@SiO2@Ag microflowers can be efficient and recyclable catalysts for various catalytic reductions.

Dual dye-loaded Au@Ag coupled to a lateral flow immunoassay for the accurate and sensitive detection of Mycoplasma pneumoniae infection

We present an attractive model of surface-enhanced Raman scattering-based lateral flow immunoassay (SERS-LFIA) for the sensitive and accurate detection of Mycoplasma pneumoniae (MP) infection in human serum. The SERS-LFIA strip uses Au@Ag nanoparticles (Au@Ag NPs) loaded with two layers of Raman dye 5,5′-dithiobis-(2-nitrobenzoic acid) (DTNB) as SERS tags. The advantages of the dual dye-loaded SERS tags (Au/DTNB@Ag/DTNB) are the high sensitivity and the bioconjugation flexibility of the detection antibody. As determined from our SERS-LFIA strip, human IgM was quantified by monitoring the SERS signal on the test line. The limit of detection for human IgM was 0.1 ng mL−1, which was 100 times more sensitive than that by using the colorimetric method. Our assay results for 20 MP-specific IgM positive serum specimens showed 100% accuracy and detection rate, whereas the parallel enzyme-linked immunosorbent assay only showed 85% detection rate. The SERS-LFIA strip also exhibited high specificity and potential clinical applications. Therefore, our SERS-based LFIA strip has strong potential for practical applications in the sensitive and rapid detection of MP.

Study on the SERS substrate composed of Au@Ag core-shell nanoparticles linked to SiO2 spheres

ABSTRACT We have developed a sensitive surface-enhanced Raman scattering (SERS) substrate composed of 60-nm Au@Ag core-shell nanoparticles linked to 360-nm SiO2 sphere base, which experimentally demonstrated that the detection limits for p-aminothiophenol (PATP), Rhodamine 6G (R6G) and 5,5-dithiobis(2-dinitrobenzoic acid) (DTNB) could be 10−13, 10−13 and 10−12 M, respectively. The fabrication method of the SERS substrate and the experimental investigation on the enhancement performance of the substrate are presented in detail in this article. The results show that the 360 nm SiO2-60 nm Au@Ag nanocomposites could be used as an excellent substrate for SERS detection of small organic molecules.