Mingli Wang

Density-matrix evaluation of the enhancement to resonant Raman scattering and fluorescence of molecules confined in metallic nanoparticle dimers

In the present work we study the surface-enhanced resonant Raman scattering (SERRS) and fluorescence (SEF) spectra of a general model molecule confined in metallic dimers consisting of Ag, Au and hybrid AuAg nanoparticles (NPs). The electromagnetic (EM) enhancement factors were simulated by the generalized Mie scatting method and the scattering cross section of the molecules were obtained by density-matrix calculations. The influence of the size of the NPs and the separation between the dimer on the Raman scattering and fluorescence were systematically studied and analyzed in detail. It was found that the SERRS mainly related to EM enhancement and the SEF depended on the competition between EM enhancement and quantum yield, both of which could be controlled by tuning the radius and separation of the metallic dimers. The optimal radius of the NPs for SERRS were found to be around 30u2009nm for AgNPs, 40u2009nm for AuNPs and 50u2009nm for hybrid AuAgNPs. The strongest Raman enhancement as predicted by the theoretical simulations were 6.2u2009×u20091010, 1.5u2009×u2009107 and 5.2u2009×u2009108 for the three types of structures, respectively. These results could offer valuable information for the design of metallic substrates for surface enhanced Raman and fluorescence measurements.

A general method for large-scale fabrication of Cu nanoislands/dragonfly wing SERS flexible substrates*

Noble metal nanorough surfaces that support strong surface-enhanced Raman scattering (SERS) is widely applied in the practical detection of organic molecules. A low-cost, large-area, and environment-friendly SERS-active substrate was acquired by sputtering inexpensive copper (Cu) on natural dragonfly wing (DW) with an easily controlled way of magnetron sputtering. By controlling the sputtering time of the fabrication of Cu on the DW, the performance of the SERS substrates was greatly improved. The SERS-active substrates, obtained at the optimal sputtering time (50 min), showed a low detection limit (10−6M) to 4-aminothiophenol (4-ATP), a high average enhancement factor (EF, , excellent signal uniformity, and good reproducibility. In addition, the results of the 3D finite-difference time-domain (3D-FDTD) simulation illustrated that the SERS-active substrates provided high-density hot spots, leading to a large SERS enhancement.

Au-Decorated Dragonfly Wing Bioscaffold Arrays as Flexible Surface-Enhanced Raman Scattering (SERS) Substrate for Simultaneous Determination of Pesticide Residues

Rapid sampling and multicomponent analysis are vital in pesticide residue detection. In this work, we proposed a SERS platform to detect three kinds of pesticides on apple peels simultaneously by a straightforward “press and peel off” method. The flexible Au/dragonfly wing (Au/DW) substrate was obtained from sputtering Au nanoislands on DW bioscaffold arrays by a simple direct current (DC) magnetron sputtering system. The high-performance substrate exhibited a low limit of detection (LOD) to 4-aminothiophenol (4-ATP) (10−9 M), outstanding reproducibility (less than 12.15%), good stability and suitability in multifold pesticide residues detection. Considering its excellent sample collection efficiency, the Au/DW substrate was employed to solve critical pesticide residue problems for detection of acephate (APT), cypermethrin (CPT), tsumacide (MTMC) and their multiple components on apple peels. The results show that the LOD was 10−3 ng/cm2 for APT obtained on the apple surface with a calculation equation of y = 0.26x + 6.68 and a determination coefficient (R2) of 0.970. Additionally, the LOD values for CPT and MTMC were 10−3 ng/cm2 and 10−4 ng/cm2, respectively. The finding in this work may provide a promising biomimetic SERS platform for on-spot detection of other organic pollutants in the food industry and inenvironmental protection.

Rapid, simple and quantitative detection of metolcarb residues in apples by surface-enhanced Raman scattering

A rapid and simple detection method of metolcarb residues in apples with spectral analysis technology was achieved drawing support from the high sensitive and flexible silver/dragonfly wing (Ag/DW) surface-enhanced Raman scattering (SERS) substrates. The three steps “spray”, “press” and “separate” greatly simplified the procedures of extraction and sampling of pesticide molecules, resulting in the entire detection process was completed just in a few minutes. Importantly, the Ag nanoislands offered strong electromagnetic (EM) field enhancement near metallic nanostructures and significantly improved the sensitivity and reproducibility of the Raman signals. Meanwhile, surface plasmon coupling at the nanogaps between adjacent nanoislands created abundant “hot spots”, which became enormous enhancement necessary for high sensitivity SERS detection of metolcarb. Taking the apple peels as carriers, the trace detection of metolcarb residues on them was realized, whose detection limit reached 1×10-9 g/cm2. In addition, the linear relationship (R2 = 0.98666) between the logarithmic concentrations of metolcarb residues and the logarithmic peak areas at 1581 cm-1 was established, which was the more accurate reference for the prediction of the unknown concentration of metolcarb residues. In order to carry out the actual emulation, we studied metolcarb in mixed solution, and its obvious characteristic peaks were observed. These results indicated that SERS technology coupled with “spray-press-separate-test” method had the potential to qualitatively and quantitatively analyse metolcarb residues on complex apple peels.A rapid and simple detection method of metolcarb residues in apples with spectral analysis technology was achieved drawing support from the high sensitive and flexible silver/dragonfly wing (Ag/DW) surface-enhanced Raman scattering (SERS) substrates. The three steps “spray”, “press” and “separate” greatly simplified the procedures of extraction and sampling of pesticide molecules, resulting in the entire detection process was completed just in a few minutes. Importantly, the Ag nanoislands offered strong electromagnetic (EM) field enhancement near metallic nanostructures and significantly improved the sensitivity and reproducibility of the Raman signals. Meanwhile, surface plasmon coupling at the nanogaps between adjacent nanoislands created abundant “hot spots”, which became enormous enhancement necessary for high sensitivity SERS detection of metolcarb. Taking the apple peels as carriers, the trace detection of metolcarb residues on them was realized, whose detection limit reached 1×10-9 g/cm2. In addit...