Minmin Xu

Supersaturation-Dependent Surface Structure Evolution: From Ionic, Molecular to Metallic Micro/Nanocrystals

Deduced from thermodynamics and the Thomson-Gibbs equation that the surface energy of crystal face is in proportion to the supersaturation of crystal growth units during the crystal growth, we propose that the exposed crystal faces can be simply tuned by controlling the supersaturation, and higher supersaturation will result in the formation of crystallites with higher surface-energy faces. We have successfully applied it for the growth of ionic (NaCl), molecular (TBPe), and metallic (Au, Pd) micro/nanocrystals with high-surface-energy faces. The above proposed strategy can be rationally designed to synthesize micro/nanocrystals with specific crystal faces and functionality toward specific applications.

Tunable fabrication on iron oxide/Au/Ag nanostructures for surface enhanced Raman spectroscopy and magnetic enrichment.

A facile approach was developed to prepare novel multifunctional Fe(2)O(3)/Au/Ag nanostructures integrated with isolated functions involving magnetic and optical properties. The Fe(2)O(3)/Au/Ag hybrid nanoparticles with different thicknesses of Ag shell were prepared by adjusting the amount of the AgNO(3). Surface structures were varied from the rough with pinhole to smooth and pinhole free surfaces with increasing amounts of AgNO(3). The surface plasmon resonance was tuned in a very wide region from that of Au to Ag. Surface enhanced Raman scattering (SERS) effects were also investigated, employing thiophenol (TP) and aminothiophenol (PATP) as probe molecules. It was revealed that the SERS intensity was strongly depended on the molar ratio of Ag and Au. With an increase in the Ag molar fractions, SERS signals were enhanced to the maximum due to the surface plasmon resonance of the pinhole structure. The magnetic enrichment for on line SERS monitoring the molecules with low concentration was performed based on the magnetic core and the SERS activity of the bimetallic shells. This enrichment procedure improved efficiently the limits of the SERS detection. It was shown that the multicomponent nanoparticles have potential applications in the fields of optical devices and magnetic separation.

Plasmon-induced decarboxylation of mercaptobenzoic acid on nanoparticle film monitored by surface-enhanced Raman spectroscopy

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.

Rapid separation and on-line detection by coupling high performance liquid chromatography with surface-enhanced Raman spectroscopy

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.

Ultra-sensitive magnetic immunoassay of HE4 based on surface enhanced Raman spectroscopy

Human epididymis protein 4 (HE4), as a serological marker, has been proposed to be the most promising tumor marker in ovarian cancer diagnosis. An approach based on surface enhanced Raman spectroscopy (SERS) and a magnetic immunoassay technique was developed successfully for rapid detection and separation of HE4 with high sensitivity and selectivity. The detection was involved in the construction of a unique sandwich structure using a bottom-up method, which consisted of HE4 antibody and SERS reporter coated Au nanoparticles (A) and target HE4 antigen and HE4 antibody-modified magnetic core–shell Fe3O4@Au nanoparticles (B). The sandwich structure was effectively enriched by using a magnet for SERS detection. This approach exhibited an extremely high specificity in the detection of HE4 due to the strong specific interaction between the antibody and the corresponding antigen. The results revealed that the limit of detection (LOD) of the present approach was as low as 100 fg mL−1 and demonstrated a linear relationship between SERS intensities and lg c in a concentration range of 1 pg mL−1 to 10 ng mL−1. Accompanied by the magnetic enrichment procedure after the assembling of the sandwich structure, almost all of the HE4 protein was removed. The immuno Fe3O4@Au nanoparticles were regenerated by releasing the HE4 from the sandwich structure into the acidified methanol solution, and it could be used for magnetic enrichment and SERS detection for at least five times. Moreover, two kinds of immuno nanoparticles (A and B) could be developed as reagent kits in the clinical diagnosis of ovarian cancer.

Surface enhanced Raman spectroscopic studies on magnetic Fe3O4@AuAg alloy core-shell nanoparticles.

A facile approach has been developed to fabricate multifunctional Fe3O4@AuAg alloy core-shell nanoparticles, owning the magnetism of the core and the surface enhanced Raman spectroscopy (SERS) activities of the alloy shell. By changing the amount of HAuCl4 and AgNO3, Fe3O4@AuAg alloy nanoparticles with different component ratios of Au and Ag were successfully prepared. The surface plasmon resonance of the composition was linearly tuned in a wide range by varying the molar fraction of Ag and Au, suggesting the formation of AuAg alloy shell. SERS and magnetic enrichment effects were investigated by using thiophenol (TP) as the probe molecule. The SERS intensity was strongly dependent on the molar ratios of Au and Ag and the excitation line. Enrichment for the molecules with low concentration and on line SERS monitoring experiments were performed through combining the magnetism of the core and the SERS effect of the alloy shell. The results revealed that the magnetic enrichment efficiency was dramatically increased due to the strong magnetism of Fe3O4 core. In addition, the Fe3O4@AuAg nanoparticles were also used in the microfluidic chip to continuously detect different flowing solution in the channel. The detection time and amount of analyte were successfully decreased.

Improving the SERS detection sensitivity of aromatic molecules by a PDMS-coated Au nanoparticle monolayer film

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.

Ni@Au nanoparticles for surface enhanced Raman spectroscopy based ultrasensitive magnetic immunoassay on aflatoxin B1

The performance of a surface enhanced Raman spectroscopy (SERS) based magnetic immunoassay is critically dependent upon the properties of the magnetic nanoparticles, in which the plasmon enhanced optics and magnetism are integrated together. Tuning SERS activity and magnetism together still remains a significant challenge. Herein, a facile approach for the fabrication of Ni@Au magnetic nanoparticles was developed as the immune substrate for a competitive magnetic immunoassay. Immune Ni@Au nanoparticles and 4-mercaptobenzoic acid (MBA)-labelled immune Au nanoparticles (immune Au-MBA) were employed for detection of aflatoxin B1 (AFB1) through a SERS based competitive magnetic immunoassay. In the immune system, competitive binding with immune Au nanoparticles appeared between the free AFB1 and coating antigen modified Ni@Au nanoparticles; the concentration of AFB1 was determined by comparing the extent of the decrease in the SERS intensity of MBA labels. Based on the relationship between SERS intensity and AFB1 concentrations, the inhibitory concentration 50 (IC50) was determined to be about 27.1 fg mL−1 (around 0.03 ppt) with a reasonable correlation coefficient of 0.997 and the limit of detection was about 0.05 fg mL−1. The observation of unobvious cross-reactions suggested the high specificity of this strategy. By comparing to the traditional determination techniques, the present approach based on Ni@Au nanoparticles exhibited the highest sensitivity. In the spiking experiments, the recoveries ranged from 87.4% to 111.7% after the addition of standard AFB1 at different concentrations in fresh maize samples. The results were also verified by the commonly accepted LC-MS technique. It was revealed that the competitive magnetic immunoassay exhibited the distinct advantage of high sensitivity. The proposed approach is expected to be developed into a promising tool for quasi-quantitative detection of the trace residues of AFB1 in food.

Magnetic separation of clenbuterol based on competitive immunoassay and evaluation by surface-enhanced Raman spectroscopy

The elimination of β-agonist has attracted considerable interest due to its harmfulness to human health when it existed in pork. Here, a strategy based on immuno-magnetic nanoparticles has been successfully developed for the selective and successive magnetic separation of two kinds of β-agonists, clenbuterol (CL) and salbutamol (SAL). The calibration curve of competitive immunoassay was determined for the estimation of the final concentration of targets after the separation, in which the limit of detection (LOD) and half maximal inhibitory concentration (IC50) were about 17 fg mL−1 and 193 pg mL−1, respectively. The specific interaction between the target and the complementary antibody attached to Fe3O4@Au nanoparticles resulted in the aggregation of Fe3O4@Au nanoparticles carried with targets. The magnetic collection of the aggregation of Fe3O4@Au nanoparticles decreased the concentration of targets significantly. The results revealed that the final concentration of remaining targets was lower than the LOD. This strategy was employed to separate CL and SAL molecules in mixed solutions simultaneously or successively with high efficiency. The results demonstrate that it provides a selective and effective approach for the removal of harmful residues in practical samples.

Stacking faults enriched silver nanowires: Facile synthesis, catalysis and SERS investigations

A facile approach based on seed-mediated method for synthesis of stacking faults enriched Ag nanowires (SFEANWs) was successfully developed. SFEANWs were formed and attached onto the seed (α-Fe2O3/Au) surfaces through the reduction of AgNO3 by ascorbic acid (AA) in the presence of sodium polyacrylate (PAANa). Their length can be tuned with different concentrations of AgNO3 or PAANa. According to the effects of seeds and PAANa, the plausible growth mechanism of SFEANWs was discussed. The catalytic activity of SFEANWs comparing with fivefold twinned Ag nanowires (FFTANWs) was evaluated through reducing p-nitrophenol by NaBH4. The activation energy of the classical reaction catalyzed by SFEANWs was calculated through the Arrhenius equation. In addition, these SFEANWs exhibited excellent surface enhanced Raman scattering (SERS) activities due to the hot spots located in the cross of the twist wires. The detection limit of by SERS for 1,4-benzenedithiol (1,4-BDT) was estimated about 10(-7) mol/L.