Facile tuning of tip sharpness on gold nanostars by the controlled seed-growth method and coating with a silver shell for detection of thiram using surface enhanced Raman spectroscopy (SERS)

Developing SERS substrates based on individual gold and silver metals, either with rough surfaces or bare nanoparticles, has certain limitations in practical analysis applications. In order to improve the range of applications of the noble metallic substrates, a comprehensive approach has been proposed for preparing non-traditional SERS nano-substrates by combining tip-enhanced gold nanostars and Raman signal amplification of the silver layer. This preparation process is conducted in two steps, including tuning the sharpness and length of tips by a modified seed growth method followed by coating the silver layer on the formed star-shaped nanoparticles. The obtained AuNS-Ag covered with an average size of around 100 nm exhibited interesting properties as a two-component nano-substrate to amplify the activities in SERS for detecting thiram. The controllable and convenient preparation route of gold nanostars is based on the comproportionation reaction of Au seed particles with Au(iii) ions, achieved by governing the stirring times of the mixture of the Au seed and the growth solution. Thus, the citrate-seed particles decreased in size (below 2 nm) and grew into nanostars with sharp tips. The thickness of Ag covering the Au particles' surface also was appropriately controlled and the tips were still exposed to the outside, which is a benefit for matching with the source excitation wavelength to achieve good SERS performance. The Raman signals of thiram can be instantly and remarkably detected with the enhancement of the substrates. Thiram can be determined without any pretreatment. It was found that the limit of detection for thiram is 0.22 ppm, and the limit of quantification is 0.73 ppm. These experimental results shed some light on developing the SERS method for detecting pesticide residue.

Developing SERS substrates based on the star-like morphology of gold nanoparticles covered by a silver layer to overcome limitations in practical analysis application.

Au Nanoflowers for Catalyzing and In Situ Surface-Enhanced Raman Spectroscopy Monitoring of the Dimerization of p-Aminothiophenol

In this work, we demonstrated a facile approach for fabrication of Au nanoflowers (Au NFs) using an amino-containing organosilane, 3-aminopropyltriethoxysilane (APTES), as a shape-directing agent. In this approach, the morphology of the Au particles evolved from sphere-like to flower-like with increasing the concentration of APTES, accompanied by a red shift in the localized surface plasmon resonance peak from 520 to 685 nm. It was identified that the addition of APTES is profitable to direct the preferential growth of the (111) plane of face-centered cubic gold and promote the formation of anisotropic Au NFs. The as-prepared Au NFs, with APTES on their surface, presented effective catalytic and surface-enhanced Raman scattering (SERS) performances, as evidenced by their applications in catalyzing the dimerization of p-aminothiophenol and monitoring the reaction process via in situ SERS analysis.

Shape-Controllable Gold Nanoparticle-MoS2 Hybrids Prepared by Tuning Edge-Active Sites and Surface Structures of MoS2 via Temporally Shaped Femtosecond Pulses

Edge-active site control of MoS₂ is crucial for applications such as chemical catalysis, synthesis of functional composites, and biochemical sensing. This work presents a novel nonthermal method to simultaneously tune surface chemical (edge-active sites) and physical (surface periodic micro/nano structures) properties of MoS₂ using temporally shaped femtosecond pulses, through which shape-controlled gold nanoparticles are in situ and self-assembly grown on MoS₂ surfaces to form Au-MoS₂ hybrids. The edge-active sites with unbound sulfurs of laser-treated MoS₂ drive the reduction of gold nanoparticles, while the surface periodic structures of laser-treated MoS₂ assist the shape-controllable growth of gold nanoparticles. The proposed novel method highlights the broad application potential of MoS₂; for example, these Au-MoS₂ hybrids exhibit tunable and highly sensitive SERS activity with an enhancement factor up to 1.2 × 10⁷, indicating the marked potential of MoS₂ in future chemical and biological sensing applications.

Synthesis of Multibranched Gold Nanoechinus Using a Gemini Cationic Surfactant and Its Application for Surface Enhanced Raman Scattering

High-yield multibranched Au nanoechinus possessing lengthy and dense branched nanorods on the surface were synthesized using a seed-mediated surfactant-directed approach in the presence of gemini cationic surfactant N,N,N'N'-tetramethyl-N,N'-ditetradecylethane-1,2-diaminium bromide (C14C2C14Br2), HAuCl4, AgNO3, and ascorbic acid. C14C2C14Br2 surfactant provides a versatile template in designing the unique morphology of Au nanoechinus with the assistance of AgNO3. UV-vis spectroscopic analysis proves that Au nanoechinus possess a unique intense broad localized surface plasmon resonance (LSPR) peak, which extends from 400 to 1700 nm in the NIR region making a highly potential platform for biomedical applications. Systematic time-dependent TEM, UV-vis-NIR, and XRD analysis were performed to monitor the morphological evolution of multibranched Au nanoechinus. It was found that the surface of branched nanorods of Au NE preferentially grew along (111) crystal planes. Furthermore, as-synthesized Au nanoechinus shows excellent SERS enhancement ability for dopamine inside HeLa cells.

Stable ligand-free stellated polyhedral gold nanoparticles for sensitive plasmonic detection

Ligand-free stellated gold nanoparticles (AuStNPs) with well-defined octahedral (O(h)) and icosahedral (I(h)) core symmetries were prepared using hydrogen peroxide as a reducing agent. Only three reagents: gold precursor (HAuCl4), H2O2 and NaOH were required to form colloidally and chemically stable AuStNPs with a zeta-potential between -55 and -40 mV indicative of excellent charge stabilization. The size and degree of stellation of AuStNPs can be controlled by several synthetic parameters so that the localized surface plasmon resonance (LSPR) can be varied from ca. 850 nm in near-infrared (NIR) to ca. 530 nm. In particular, AuStNP size and LSPR tuning can be conveniently accomplished by iodide variation. The size distribution of AuStNPs was improved by nucleation with ascorbic acid, and the AuStNP size and degree of branching could be readily modified using arginine. AuStNPs are advantageous for SPR sensing, as it was demonstrated in the sensitive detection of not only thiols, such as ampicillin, but also iodide with the detection limit of 3.2 pM (0.4 ng L(-1)). The reported ligand-free stable AuStNPs thus should be very useful for biodiagnostics based on SPR sensing and potentially for SERS and hyperthermia therapy.

Compositional Analysis of Ternary and Binary Chemical Mixtures by Surface-Enhanced Raman Scattering at Trace Levels

Surface-enhanced Raman scattering has been proven a powerful means in the fast detection and recognition of chemicals at trace levels, while quantitative analysis especially the compositional analysis of trace chemical mixtures remains a challenge. We report here a "triangle-rule" based on the principal component analysis (PCA) of surface-enhanced Raman scattering spectra, to calculate the composition of individual component of ternary chemical mixtures at trace levels, which can be simplified into the "balance-rule" for binary mixtures. We demonstrated the validity of the triangle-rule and balance-rule in estimating the composition of ternary and binary mixtures of methyl orange, methylene blue, and crystal violet with different molecular structures, and the validity for ternary and binary mixtures of three isomers of monochlorobiphenyl with very similar molecular structures. This idea might be also applicable to mixtures of more components at the trace levels.