Xiu Liang

Tuning plasmonic and chemical enhancement for SERS detection on graphene-based Au hybrids

Various graphene-based Au nanocomposites have been developed as surface-enhanced Raman scattering (SERS) substrates recently. However, efficient use of SERS has been impeded by the difficulty of tuning SERS enhancement effects induced from chemical and plasmonic enhancement by different preparation methods of graphene. Herein, we developed graphene-based Au hybrids through physical sputtering gold NPs on monolayer graphene prepared by chemical vapor deposition (CVD) as a CVD-G/Au hybrid, as well as graphene oxide-gold (GO/Au) and reduced-graphene oxide (rGO/Au) hybrids prepared using the chemical in situ crystallization growth method. Plasmonic and chemical enhancements were tuned effectively by simple methods in these as-prepared graphene-based Au systems. SERS performances of CVD-G/Au, rGO/Au and GO/Au showed a gradually monotonic increasing tendency of enhancement factors (EFs) for adsorbed Rhodamine 6G (R6G) molecules, which show clear dependence on chemical bonds between graphene and Au, indicating that the chemical enhancement can be steadily controlled by chemical groups in a graphene-based Au hybrid system. Most notably, we demonstrate that the optimized GO/Au was able to detect biomolecules of adenine, which displayed high sensitivity with a detection limit of 10(-7) M as well as good reproducibility and uniformity.

A sensitive SERS substrate based on Au/TiO2/Au nanosheets

Sensitive SERS substrates based on Au/TiO2/Au nanosheet have been prepared by physically sputtering Au nanoparticles onto fabricated TiO2 nanosheets. The Au/TiO2/Au nanosheets show much stronger SERS signal as compared to normal Au/Ti substrates by increasing surface area and effectively inducing plasmonic coupling between adjoining Au nanoparticles. In addition, influence factors such as concentration of probe solution and deposition time of gold nanoparticles were discussed. This study provides an easy-prepared and label-free substrate for the detection of biomolecule.

Controlled assembly of one-dimensional MoO3@Au hybrid nanostructures as SERS substrates for sensitive melamine detection

A new, feasible, and highly efficient platform was developed using self-assembly of (3-aminopropyl) diethoxymethylsilane (ATES) to create metallic oxide-based noble metal nanoparticle hybrid materials. The ex situ approach for fabricating 1D MoO3-nanowires (MoO3-NWs) or ZnO-nanorods (ZnO-NRs) @Au-nanoparticles (Au-NPs) hybrid surface-enhanced Raman scattering (SERS) substrates not only provides a simple route for the decoration of noble metal nanoparticle on metallic oxide nanomaterials, but also realizes the controllable assembly/co-assembly of pre-synthesized nanoparticles with distinctively different sizes, compositions, shapes, and properties. Control over the self-assembly synthesis for morphology and SERS activity was demonstrated by several parameters: (i) coupling agent of ATES, (ii) ATES content, (iii) Au-NPs content, (iv) Au-NP sizes, and (v) other metallic oxide such as ZnO-NRs. The finite difference time domain (FDTD) method was employed to visualize the enhancement mechanism distribution of the hybrid substrate. Furthermore, the optimized MoO3-NW@Au-NPs SERS substrate manifests high SERS sensitivity to melamine with a limit of detection (LOD) of 0.1 ppb (0.08 nM) and excellent uniformity (RSD = 9.26%). The LOD was much lower than the maximal residue limit (MRL) of 2.5 ppm in food prescribed by the US Food and Drug Administration (FDA).

An ultrasensitive near-infrared satellite SERS sensor: DNA self-assembled gold nanorod/nanospheres structure

Coupled plasmonic assemblies have recently attracted tremendous research interest in the field of Surface Enhanced Raman Scattering (SERS) due to their unique optical and biocompatible properties. Using DNA to connect different parts of assembled plasmonic nanostructures has been a simple but useful method to achieve the expected nanocomposites. This work prepared a satellite SERS substrate based on gold nanorod/gold nanosphere structures, in the hope of providing a novel SERS sensor for biomedicine related applications. A seed growth method was adopted to fabricate gold nanorods, using a region specific method to connect the gold nanorod core with the gold nanosphere satellites. The fabricated nanocomposites were further self-assembled with p-mercaptobenzoic acid (p-MBA) molecule layers as Raman reporters for SERS experiments. The obtained satellite nanostructure could produce “hot spots” between the gold nanorods and gold nanospheres to improve the SERS sensitivity and also to function as a key factor to tune the localized surface plasmon resonance (LSPR) absorption band to the near-infrared region. Finally, the optimized satellite SERS sensor was applied in the detection of Crystal Violet (CV) with a limit of detection as low as 10−11 M, proving that the self-assembled nanocomposite could act as an effective substrate for single molecule detection.

An ultrasensitive “turn-off” SERS sensor for quantitatively detecting heparin based on 4-mercaptobenzoic acid functionalized gold nanoparticles

An ultrasensitive “turn off” Surface Enhanced Raman Spectroscopy (SERS) sensor was developed for the detection of heparin based on the anti-aggregation of 4-mercaptobenzoic acid stabilized gold nanoparticles. In this paper, protamine, a small protein molecule with positive charges, could induce the aggregation of 4-MBA functionalized gold nanoparticles via surface electrostatic interaction. However, in the presence of heparin, the aggregation of 4-MBA functionalized gold nanoparticles decreased due to the fact that heparin has a strong affinity toward protamine, further causing the loss of the Raman enhanced effect. The normalized SERS intensity of the Raman reporter was proportional to the concentration of added heparin and a good linear detection range was obtained from 0.05 to 20 ng mL−1 (R2 = 0.999) with a calculated detection limit of 0.03 ng mL−1. Moreover, the developed highly selective method is also successfully demonstrated in fetal bovine serum. Our method is specific, simple and cheap, which could be applied to other further research studies of heparin.

A computational study on surface-enhanced Raman spectroscopy of para-substituted Benzenethiol derivatives adsorbed on gold nanoclusters

We presented a computational study on para-substituted Benzenethiol (x-BT, x=H, F, Cl, Br, OH, SH, SeH, NH2, CH3) derivatives interacting with gold cluster for chemical effects related to surface-enhanced Raman spectroscopy (SERS). Density functional theory (DFT) calculations were performed on a series of bridge-type and vertex type x-BT/Au13 complexes for geometric, electronic and excitation properties to determine the key factor in spectral enhancement. Results indicated that off-resonance enhancement factors of bridge-type and vertex-type complexes exhibited different dependency on substitutions, which was greatly influenced by molecule-cluster transitions instead of properties such as interaction energy and charge transfer due to same origination for off-resonance and resonance chemical enhancement.

Large-scale preparation of flexible and reusable surface-enhanced Raman scattering platform based on electrospinning AgNPs/PCL nanofiber membrane

A flexible and reusable SERS substrates were prepared by electrospinning Ag nanoparticles (AgNPs) in reversed micelle into poly(e-caprolactone) (PCL) nanofibers, forming nanocomposite membrane with high detection sensitivity. The AgNPs/PCL nanofiber membrane was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and surface-enhanced Raman scattering spectroscopy (SERS). SEM and TEM image show that AgNPs dispersed homogeneously in PCL nanofibers. The fabricated substrates based on AgNPs/PCL nanofiber membrane have shown reusability and excellent detection reproducibility in SERS under a low concentration of R6G (10−12 M) and crystal violet (10−11 M), and the detection limit of melamine is as low as 5 ppb, which is far below the safety of FDA and US. Moreover, the substrate showed hydrophobicity and low loss of stress after etching.