Jinhuai Liu

Three-dimensional and time-ordered surface-enhanced raman scattering hotspot matrix

The fixed or flexible design of plasmonic hotspots is a frontier area of research in the field of surface-enhanced Raman scattering (SERS). Most reported SERS hotspots have been shown to exist in zero-dimensional point-like, one-dimensional linear, or two-dimensional planar geometries. Here, we demonstrate a novel three-dimensional (3D) hotspot matrix that can hold hotspots between every two adjacent particles in 3D space, simply achieved by evaporating a droplet of citrate-Ag sols on a fluorosilylated silicon wafer. In situ synchrotron-radiation small-angle X-ray scattering (SR-SAXS), combined with dark-field microscopy and in situ micro-UV, was employed to explore the evolution of the 3D geometry and plasmonic properties of Ag nanoparticles in a single droplet. In such a droplet, there is a distinct 3D geometry with minimal polydispersity of particle size and maximal uniformity of interparticle distance, significantly different from the dry state. According to theoretical simulations, the liquid adhesive force promotes a closely packed assembly of particles, and the interparticle distance is not fixed but can be balanced in a small range by the interplay of the van der Waals attraction and electrostatic repulsion experienced by a particle. The trapping well for immobilizing particles in 3D space can result in a large number of hotspots in a 3D geometry. Both theoretical and experimental results demonstrate that the 3D hotspots are predictable and time-ordered in the absence of any sample manipulation. Use of the matrix not only produces giant Raman enhancement at least 2 orders of magnitude larger than that of dried substrates, but also provides the structural basis for trapping molecules. Even a single molecule of resonant dye can generate a large SERS signal. With a portable Raman spectrometer, the detection capability is also greatly improved for various analytes with different natures, including pesticides and drugs. This 3D hotspot matrix overcomes the long-standing limitations of SERS for the ultrasensitive characterization of various substrates and analytes and promises to transform SERS into a practical analytical technique.

Ultrasensitive SERS Detection of TNT by Imprinting Molecular Recognition Using a New Type of Stable Substrate

We report herein a method for the ultra-trace detection of TNT on p-aminothiophenol-functionalized silver nanoparticles coated on silver molybdate nanowires based on surface-enhanced Raman scattering (SERS). The method relies on π-donor-acceptor interactions between the π-acceptor TNT and the π-donor p,p-dimercaptoazobenzene (DMAB), with the latter serving to cross-link the silver nanoparticles deposited on the silver molybdate nanowires. This system presents optimal imprint molecule contours, with the DMAB forming imprint molecule sites that constitute SERS hot spots. Anchoring of the TNT analyte at these sites leads to a pronounced intensification of its Raman emission. We demonstrate that TNT concentrations as low as 10(-12)u2009M can be accurately detected using the described SERS assay. Most impressively, acting as a new type of SERS substrate, the silver/silver molybdate nanowires complex can yield new silver nanoparticles during the detection process, which makes the Raman signals very stable. A detailed mechanism for the observed SERS intensity change is discussed. Our experiments show that TNT can be detected quickly and accurately with ultra-high sensitivity, selectivity, reusability, and stability. The results reported herein may not only lead to many applications in SERS techniques, but might also form the basis of a new concept for a molecular imprinting strategy.

Electrochemical Detection of Arsenic(III) Completely Free from Noble Metal: Fe3O4 Microspheres-Room Temperature Ionic Liquid Composite Showing Better Performance than Gold

In recent decades, electrochemical detection of arsenic(III) has been undergoing revolutionary developments with higher sensitivity and lower detection limit. Despite great success, electrochemical detection of As(III) still depends heavily on noble metals (predominantly Au) in a strong acid condition, thus increasing the cost and hampering the widespread application. Here, we report a disposable platform completely free from noble metals for electrochemical detection of As(III) in drinking water under nearly neutral condition by square wave anodic stripping voltammetry. By combining the high adsorptivity of Fe3O4 microspheres toward As(III) and the advantages of room temperature ionic liquid (RTIL), the Fe3O4-RTIL composite modified screen-printed carbon electrode (SPCE) showed even better electrochemical performance than commonly used noble metals. Several ionic liquids with different viscosities and surface tensions were found to have a different effect on the voltammetric behavior toward As(III). Under the optimized conditions, the Fe3O4-RTIL composites offered direct detection of As(III) within the desirable range (10 ppb) in drinking water as specified by the World Health Organization (WHO), with a detection limit (3σ method) of 8 × 10(-4) ppb. The obtained sensitivity was 4.91 μA ppb(-1), which is the highest as far as we know. In addition, a possible mechanism for As(III) preconcentration based on adsorption has been proposed and supported by designed experiments. Finally, this platform was successfully applied to analyzing a real sample collected from Inner Mongolia, China.

Sea-urchin-like Fe3O4@C@Ag particles: an efficient SERS substrate for detection of organic pollutants

Ag-coated sea-urchin-like Fe3O4@C core-shell particles can be synthesized by a facile one-step solvothermal method, followed by deposition of high-density Ag nanoparticles onto the carbon surface through an in situ growth process, respectively. The as-synthesized Ag-coated Fe3O4@C particles can be used as a surface-enhanced Raman scattering (SERS) substrate holding reproducible properties under an external magnetic force. The magnetic function of the particles allows concentrating the composite particles into small spatial regions, which can be exploited to decrease the amount of material per analysis while improving its SERS detection limit. In contrast to the traditional SERS substrates, the present Fe3O4@C@Ag particles hold the advantages of enrichment of organic pollutants for improving SERS detection limit and recycled utilization.

Detection and Direct Readout of Drugs in Human Urine Using Dynamic Surface-Enhanced Raman Spectroscopy and Support Vector Machines

A new, novel, rapid method to detect and direct readout of drugs in human urine has been developed using dynamic surface-enhanced Raman spectroscopy (D-SERS) with portable Raman spectrometer on gold nanorods (GNRs) and a classification algorithm called support vector machines (SVM). The high-performance GNRs can generate gigantic enhancement and the SERS signals obtained using D-SERS on it have high reproducibility. On the basis of this feature of D-SERS, we have obtained SERS spectra of urine and urine containing methamphetamine (MAMP). SVM model was built using these data for fast identified and visual results. This general method was successfully applied to the detection of 3, 4-methylenedioxy methamphetamine (MDMA) in human urine. To verify the accuracy of the model, drug addicts urine containing MAMP were detected and identified correctly and rapidly with accuracy more than 90%. The detection results were displayed directly without analysis of their SERS spectra manually. Compared with the conventional method in lab, the method only needs a 2 μL sample volume and takes no more than 2 min on the portable Raman spectrometer. It is anticipated that this method will enable rapid, convenient detection of drugs on site for the police.

Sensitive and selective SERS probe for trivalent chromium detection using citrate attached gold nanoparticles

In this article, we have demonstrated a sensitive and selective surface enhanced Raman spectroscopy (SERS) probe, based on citrate-capped gold nanoparticles (AuNPs), for trivalent chromium (Cr(3+)) detection. After introducing Tween 20 to a solution of citrate-capped AuNPs, the as-prepared Tween 20/citrate-AuNP probe could recognize Cr(3+) at a 50 × 10(-9) M level in an aqueous medium at a pH of 6.0. Tween 20 can stabilize the citrate-capped AuNPs against conditions of high ionic strength. Due to the chelation between Cr(3+) and citrate ions, AuNPs undergo aggregation. As a result, it formed several hot spots and provided a significant enhancement of the Raman signal intensity through electromagnetic (EM) field enhancements. A detailed mechanism for tremendous SERS intensity change had been discussed. The selectivity of this system toward Cr(3+) was 400-fold, remarkably greater than other metal ions.

Capillarity-constructed reversible hot spots for molecular trapping inside silver nanorod arrays light up ultrahigh SERS enhancement

SERS hot spots with outstanding enhancement ability can spontaneously form in a reversible and reproducible way by the self-approach of flexible silver nanorods driven by the capillary force of solvent evaporation, and at the same time the target analytes can be trapped in the top-closed silver nanorods in the solvent evaporation process. The Raman intensity of the top-closed vs. top-opened nanorod arrays was a factor of 100–1000 higher for SERS reporters such as p-aminothiophenol or crystal violet. Furthermore, triplicate measurements on long nanorod arrays at the same position show a variation of the Raman intensity <10%, demonstrating a good reproducibility of the enhancement. Moreover, we found that the self-approach is highly dependent on the nanorod length and the molecules with different adsorptivity have different SERS performance in the solvent evaporation process. This solvent evaporation-controlled self-approach is an extremely simple and efficient strategy for the spontaneous formation of Raman hot spots with outstanding enhancement ability. These characteristics promise a generic platform for molecule trapping and SERS sensing with high sensitivity and reproducibility, which can help to transform SERS into a practical analytical technique.

Three-Dimensional Surface-Enhanced Raman Scattering Hotspots in Spherical Colloidal Superstructure for Identification and Detection of Drugs in Human Urine

Rapid component separation and robust surface-enhanced Raman scattering (SERS) identification of drugs in real human urine remain an attractive challenge because of the sample complexity, low molecular affinity for metal surface, and inefficient use of hotspots in one- or two-dimensional (2D) geometries. Here, we developed a 5 min strategy of cyclohexane (CYH) extraction for separating amphetamines from human urine. Simultaneously, an oil-in-water emulsion method is used to assemble monodisperse Ag nanoparticles in the CYH phase into spherical colloidal superstructures in the aqueous phase. These superstructures create three-dimensional (3D) SERS hotspots which exist between every two adjacent particles in 3D space, break the traditional 2D limitation, and extend the hotspots into the third dimension along the z-axis. In this platform, a conservative estimate of Raman enhancement factor is larger than 10(7), and the same CYH extraction processing results in a high acceptability and enrichment of drug molecules in 3D hotspots which demonstrates excellent stability and reproducibility and is suitable for the quantitative examination of amphetamines in both aqueous and organic phases. Parallel ultraperformance liquid chromatography (UPLC) examinations corroborate an excellent performance of our SERS platform for the quantitative analysis of methamphetamine (MA) in both aqueous solution and real human urine, of which the detection limits reach 1 and 10 ppb, respectively, with tolerable signal-to-noise ratios. Moreover, SERS examinations on different proportions of MA and 3,4-methylenedioxymethamphetamine (MDMA) in human urine demonstrate an excellent capability of multiplex quantification of ultratrace analytes. By virtue of a spectral classification algorithm, we realize the rapid and accurate recognition of weak Raman signals of amphetamines at trace levels and also clearly distinguish various proportions of multiplex components. Our platform for detecting drugs promises to be a great prospect for a rapid, reliable, and on-spot analyzer.

Designing and fabricating of surface-enhanced Raman scattering substrate with high density hot spots by polyaniline template-assisted self-assembly

In general, the procedures for producing a high density hot spots structure should be stable, inexpensive, and easy to make. It still remains a grand challenge to assemble silver or gold nanoparticles (Au NPs) with well-defined hot spots for SERS detection. In this study, we present a very simple method for designing and fabricating a surface-enhanced Raman scattering (SERS) substrate with high density hot spots, using large area positively charged polyaniline (PANI) nanofibers as template to assemble negatively charged Au NPs. In order to obtain the optimized SERS-active substrates, different experiments to synthesize diverse Au/PANI with different sizes of Au NPs from about 50, 30 to 15 nm were carried out. The results revealed that the PANI nanofibers were fully coated by the ~15 nm Au NPs, forming a high density Au/PANI SERS substrate. The results evidence that we can obtain stable and sensitive SERS measurements.

Speedy and surfactant-free in situ synthesis of nickel/Ag nanocomposites for reproducible SERS substrates

Ni nanowires with high purity were synthesized by a wet chemical reduction method in an external magnetic field without any assistance from surfactants and templates. Different morphologies of these nanostructures were obtained through changing the experimental conditions. Ni nanowires acted as templates for the deposition of metallic silver over the nanowire. The Ni/Ag nanocomposites were fabricated in situ by a simple and rapid redox-transmetalation reaction. The as-synthesized Ni/Ag nanocomposites were reproducible and stable for use as a surface-enhanced Raman scattering (SERS) substrate. The reproducible and stable properties will promote practical application of these substrates in the SERS field. The SERS effect of the nanocomposites were evaluated by the use of R6G and 4-ATP as the Raman probes and the results demonstrate that these nanocomposites exhibited strong SERS effects and could be reused.