Surface-enhanced Raman scattering from Au nanorods, nanotriangles, and nanostars with tuned plasmon resonances.

Electromagnetic theory predicts that the optimal value of the localized plasmon resonance (LPR) wavelength for the maximal SERS enhancement factor (EF) is half the sum of the laser and Raman wavelengths. For small Raman shifts, the theoretical EF scales as the fourth power of the local field. However, experimental data often disagree with these theoretical conclusions, leaving the question of choosing the optimal plasmon resonance for the maximal SERS signal unresolved. Here, we present experimental data for gold nanorods (AuNRs), gold nanotriangles (AuNTs), and gold nanostars (AuNSTs) simulating 1D, 2D, and 3D plasmonic nanostructures, respectively. The LPR wavelengths were tuned by chemical etching within 580-1020 nm at a constant concentration of the particles. The particles were functionalized with Cy7.5 and NBT, and the dependence of the intensity at 940 cm-1 (Cy7.5) and 1343 cm-1 (NBT) on the LPR wavelength was examined for laser wavelengths of 633 nm and 785 nm. The electromagnetic SERS EFs were calculated by averaging the product of the local field intensities at the laser and Raman wavelengths over the particle surface and their random orientations. The calculated SERS plasmonic profiles were redshifted compared to the laser wavelength. For 785 nm excitation, the calculated EFs were five to seven times higher than those for 633 nm excitation. With AuNR@Cy7.5 and AuNT@ Cy7.5, the experimental SERS was 35-fold stronger than it was with NBT-functionalized particles, but with AuNST@Cy7.5 and AuNST@NBT, the SERS responses were similar. With all nanoparticles tested, the SERS plasmonic profiles after 785 nm excitation were slightly blue-shifted, as compared with the laser wavelength, possibly owing to the inner filter effect. After 633 nm excitation, the SERS profiles were red-shifted, in agreement with EM theory. In all cases, the plasmonic EF profiles were much broadened compared to the calculated ones and did not follow the four-power law.

SERS and Indicator Paper Sensing of Hydrogen Peroxide Using Au@Ag Nanorods.

The detection of hydrogen peroxide and the control of its concentration are important tasks in the biological and chemical sciences. In this paper, we developed a simple and quantitative method for the non-enzymatic detection of H2O2 based on the selective etching of Au@Ag nanorods with embedded Raman active molecules. The transfer of electrons between silver atoms and hydrogen peroxide enhances the oxidation reaction, and the Ag shell around the Au nanorod gradually dissolves. This leads to a change in the color of the nanoparticle colloid, a shift in LSPR, and a decrease in the SERS response from molecules embedded between the Au core and Ag shell. In our study, we compared the sensitivity of these readouts for nanoparticles with different Ag shell morphology. We found that triangle core-shell nanoparticles exhibited the highest sensitivity, with a detection limit of 10-4 M, and the SERS detection range of 1 × 10-4 to 2 × 10-2 M. In addition, a colorimetric strategy was applied to fabricate a simple indicator paper sensor for fast detection of hydrogen peroxide in liquids. In this case, the concentration of hydrogen peroxide was qualitatively determined by the change in the color of the nanoparticles deposited on the nitrocellulose membrane.

Sensor System Based on a Piezoelectric Resonator with a Lateral Electric Field for Virus Diagnostics.

A sensor system based on a piezoelectric resonator with a lateral electric field in the frequency range 6-7 MHz of the electric field for virus detection is described. Through use of the transmissible virus causing gastroenteritis in pigs and specific antibodies, the possibility of detecting the virus in suspension in real time was determined. It was found that the frequency dependence of the real and imaginary parts of the electrical impedance of such a resonator loaded with a virus suspension changes significantly after the addition of specific antibodies to the suspension. No changes are observed if the antibodies are not specific. Thus, the results obtained illustrate the possibility of detecting viruses in situ, directly in the liquid phase, if the change in the real or imaginary parts of the electrical impedance after the addition of antibodies is used as an analytical signal. The possibility of virus detection in the presence of foreign viral particles has been illustrated.

Advantages of Highly Spherical Gold Nanoparticles as Labels for Lateral Flow Immunoassay.

The use of lateral flow immunoassays (LFIAs) for rapid on-site testing is restricted by their relatively high limit of detection (LoD). One possible way to decrease the LoD is to optimize nanoparticle properties that are used as labels. We compare two types of Au nanoparticles: usual quasispherical gold nanoparticles (C-GNPs), obtained by the Turkevich-Frens method, and superspherical gold nanoparticles (S-GNPs), obtained by a progressive overgrowth technique. Average diameters were 18.6-47.5 nm for C-GNPs and 20.2-90.4 nm for S-GNPs. Cardiomarker troponin I was considered as the target analyte. Adsorption and covalent conjugation with antibodies were tested for both GNP types. For C-GNPs, the minimal LoD was obtained with 33.7 nm nanoparticles, reaching 12.7 ng/mL for covalent immobilization and 9.9 ng/mL for adsorption. The average diameter of S-GNPs varied from 20.2 to 64.5 nm, which resulted in a decrease in LoD for an LFIA of troponin I from 3.4 to 1.2 ng/mL for covalent immobilization and from 2.9 to 2.0 ng/mL for adsorption. Thus, we obtained an 8-fold decrease in LoD (9.9 to 1.2 ng/mL) by using S-GNPs. This effect can be related to more effective antibody immobilization and improved S-GNP optical properties. The obtained results can improve LFIAs for various practically significant analytes.

Petal-like Gap-Enhanced Raman Tags with Controllable Structures for High-Speed Raman Imaging.

Surface-enhanced Raman scattering (SERS) is widely used for in vitro and in vivo bioimaging applications. However, reproducible and controllable fabrication of SERS tags with high density of electromagnetic hot-spots is still challenging. We report an improved strategy for the synthesis of core/shell Raman tags with high density of hot-spots and high immobilization of reporter molecules. The strategy is based on simultaneous growth and functionalization of an Au shell around Au nanospheres coated with 4-nitrobenzenethiol (NBT). The amount of added 4-NBT is key factor to control the structure SERS response of the resulting particles. Specifically, we demonstrate the formation of gap-enhanced Raman tags (GERTs) with a smooth solid shell (sGERTs), petal-like GERTs (pGERTs), and mesoporous Au particles (mGERTs) filled with Raman molecules. In contrast to NBT molecules, similar thiols such as 1,4-benzenedithiol (BDT) and 2-naphtalenethiol (NT) do not support the formation of pGERTs and mGERTs. To explain this finding, we proposed a growth mechanism based on the unique chemical structure of NBT. The SERS response of optimized pGERTs is 50 times higher than that from usual sGERTs, which makes pGERTs suitable for single-particle spectroscopy. We demonstrate successful application of pGERTs for high-speed cell imaging using 10 ms accumulation time per pixel and a total imaging time of about 1 min. Because of the high SERS response and unique porous structure, these nanoparticles have great potential for bioimaging and other applications.

Enzymatic formation of gold nanoparticles by submerged culture of the basidiomycete Lentinus edodes.

We report for the first time that the medicinal basidiomycete Lentinus edodes can reduce Au(III) from chloroauric acid (HAuCl4) to elemental Au [Au(0)], forming nanoparticles. Several methods, including transmission electron microscopy, electron energy loss spectroscopy, X-ray fluorescence, and dynamic light scattering, were used to show that when the fungus was grown submerged, colloidal gold accumulated on the surface of and inside the mycelial hyphae as electron-dense particles mostly spherical in shape, with sizes ranging from 5 to 50nm. Homogeneous proteins (the fungal enzymes laccase, tyrosinase, and Mn-peroxidase) were found for the first time to be involved in the reduction of Au(III) to Au(0) from HAuCl4. A possible mechanism forming Au nanoparticles is discussed.