Shape controlled synthesis of concave octahedral Au@AuAg nanoparticles to improve their surface-enhanced Raman scattering performance

In this work, a seed mediated strategy has been proposed to design and fabricate uniform octahedral shaped gold@gold-silver nanoparticles (Au@AuAg NPs) with unique concave structure and an AuAg alloy shell. The morphology and Au/Ag ratio of the Au@AuAg nanostructures can be delicately controlled by varying the concentration of reagents, namely the Au nanorod (NR) seeds, HAuCl₄ and AgNO₃ precursor. Besides, the investigation of the growth mechanism revealed that the morphology of the product also can be controlled by tuning the growth time. Furthermore, uniformly arranged assemblies of concave octahedral Au@AuAg NPs were prepared through a solvent evaporation self-assembly strategy and employed as surface-enhanced Raman scattering (SERS) substrates, effectively applied to the analysis of R6G for the examination of SERS performance. Satisfyingly, owing to the synergistic effect between the Au and Ag elements and concave structure, concave octahedral Au@AuAg NPs exhibit significantly higher SERS enhancement compared with traditional octahedral Au NPs, which have an enhancement factor of ∼1.3 × 10⁷ and a detection limit as low as 10⁻¹⁰ M. Meanwhile, the SERS substrate reveals an excellent uniformity and reproducibility of the SERS performance. This work opens a new avenue toward bimetallic NPs with concave structure, which have broad application prospects in optics, SERS detection and other fields.

In this work, a seed mediated strategy has been proposed to design and fabricate uniform octahedral shaped gold@gold-silver nanoparticles (Au@AuAg NPs) with unique concave structure and an AuAg alloy shell.     

Ultrasensitive flower-like TiO2/Ag substrate for SERS detection of pigments and melamine

TiO₂ flower like nanomaterials (FLNMs) are fabricated via a hydrothermal method and Ag nanoparticles (NPs) are deposited via electron beam evaporation. Several biological pigments (CV, R6G and RhB) are selected as target molecules to investigate their surface enhanced Raman scattering (SERS) property. The results demonstrate ultrasensitivity and high reproducibility. They reveal that the limit of detection (LOD) is 4.17 × 10⁻¹⁶ M and the enhancement factor (EF) is 2.87 × 10¹⁰ for CV, and the LOD is 5.01 × 10⁻¹⁶ M and 7.94 × 10⁻¹⁴ M for R6G and RhB, respectively. To assess the reproducibility on TiO₂/Ag FLNMs SERS substrates, they are tested with 1.0 × 10⁻¹³ M of CV, 1.0 × 10⁻¹³ M of R6G and 1.0 × 10⁻¹¹ M of RhB, respectively. The relative standard deviations (RSD) are less than 12.93%, 13.52% and 11.74% for CV, R6G and RhB, respectively. In addition, we carry out melamine detection and the LOD is up to 7.41 × 10⁻¹⁰ M, which is over 1000 times lower than the severest standards in the world. Therefore, the obtained TiO₂ FLNMs have potential application in detecting illegal food additives.

A newly TiO₂/Ag flower like nanomaterials (FLNMs) are fabricated via a hydrothermal method and electron beam evaporation.Pigments and melamine are selected to investigate its SERS performance. The results demonstrate ultrasensitivity and high reproducibility.     

Controllable preparation of sea urchin-like Au NPs as a SERS substrate for highly sensitive detection of the toxic atropine

Branched Au nanoparticles (Au NPs) can significantly enhance the Raman signal of trace chemical substances, and have attracted the interest of researchers. However, there are still challenges to accurately prepare the morphology of branched Au NPs. In this work, we have successfully prepared sea urchin-like Au NPs and Au nanowires by using the seed-mediate growth method, with cetyltrimethylammonium bromide (CTAB) and glutathione as ligands, and ascorbic acid as a reducing agent. Using Au NPs with a tetrahexahedron (THH) morphology as seeds, and by simply changing the concentration of glutathione, we explored the growth process of sea urchin-like Au and Au nanowires. At low concentrations of glutathione, Au NPs will preferentially grow along the edges and corners of the THH Au seed, forming a core/satellite structure. As the concentration of glutathione increases, Au NPs will grow along the direction of glutathione, forming sea urchin-like Au NPs. To further increase the concentration of glutathione, we will prepare Au nanowires. In addition, we use the prepared Au NPs as a substrate material for surface-enhanced Raman (SERS) high-sensitivity detection. By using 4-aminothiophenol (4-ATP) as the test molecule, we evaluated the SERS effect of the prepared Au NPs with different morphologies. The results showed that sea urchin-like Au NPs have the best enhancement effect. The lowest concentrations of Rhodamine 6G and 4-ATP were 10⁻¹⁰ M and 10⁻¹² M, respectively, using sea urchin Au NPs as the base material. Furthermore, we conducted a highly sensitive SERS detection of the poison atropine monohydrate, and the lowest detected concentration was 10⁻¹⁰ M.

Using glutathione as a ligand, sea urchin-like gold nanoparticles with controllable surface morphology were prepared by the seed growth method for SERS to detect the poison atropine.     

Catalytic and anti-bacterial properties of biosynthesized silver nanoparticles using native inulin

Silver nanoparticles (Ag NPs) were green synthesized using native inulin as the reducing and capping agent with varied incubation temperatures, incubation times and Ag⁺ concentrations. The biosynthesized Ag NPs were characterized using UV-visible spectroscopy, Field Emission Transmission Electron Microscopy (FE-TEM) and X-ray powder diffraction. The UV visible spectra of the Ag NPs revealed a characteristic surface plasmon resonance peak at 420 nm. FE-TEM showed that the biosynthesized Ag NPs were spherically shaped and monodispersed nanoparticles. The sizes were 18.5 ± 0.9 nm and 20.0 ± 1.2 nm for the Ag NPs synthesized at 80 °C and 100 °C for 2 h using 0.1% inulin and 2 mM Ag⁺. Their PDIs were 0.180 ± 0.05 and 0.282 ± 0.13, respectively. Improving the incubation temperature, incubation time and silver nitrate concentration promoted Ag NP synthesis. The prepared Ag NPs were effective in the catalytic reduction of 4-NP and in inhibiting the growth of bacteria. The inhibition zone could reach 10.21 ± 2.12 mm and 9.92 ± 0.50 mm for Escherichia coli and Staphylococcus aureus. The kinetic rate constant (k_app) could reach 0.0113 s⁻¹, and the maximum inhibitory zones were 10.21 ± 2.12 mm and 9.92 ± 0.50 mm, respectively, for the two microorganisms. This biosynthesis illustrates that native inulin could be a potential candidate in the green fabrication of Ag NPs, and this is promising in catalytic and bacteriostatic fields.

Silver nanoparticles (Ag NPs) were green synthesized using native inulin as the reducing and capping agent with varied incubation temperatures, incubation times and Ag⁺ concentrations.     

Tunable LSPR of silver/gold bimetallic nanoframes and their SERS activity for methyl red detection

Ag/Au bimetallic nanostructures have received much attention in surface-enhanced Raman scattering (SERS). However, the synthesis of this nanostructure type still remains a challenge. In the present research, Ag/Au nanoframes were synthesized via a simple room temperature solution phase chemical reduction method using pre-synthesized triangular Ag nanoplates as templates in the presence of appropriate amounts of HAuCl₄. Controlling experimental parameters was applied for understanding of the growth mechanism. The galvanic exchange reaction resulted in a uniform deposition of the Au shell on the Ag nanoplates and the Ag core was removed which generated triangular hollow nanoframes. It is found that the amount of HAuCl₄ added to the growth solution played a key role in controlling the Ag/Au nanoframes. The resultant silver/gold nanoframes with average size of 50 nm were applied in detecting methyl red (MR) in the solution-phase using an excitation wavelength laser of 532 nm. The SERS signal was greatly enhanced owing to the tunable plasmonic peaks in the visible region (400–650 nm). The limit of detection (LOD) of MR in diluted solution was 10⁻⁶ M. The enhancement factor (EF) was about 8 × 10⁴ toward 10⁻⁵ M of MR. Interestingly, the linear dependence between the logarithm of the SERS signal intensity (log I) and the logarithm of the MR concentration (log C) occurred in the range from 10⁻⁶ to 10⁻⁴ M. Our work promises the application of Ag/Au nanoframes as a chemical sensor in detecting MR molecules at low concentration with high performance.

Ag/Au nanoframes were synthesized via room temperature solution phase chemical reduction using pre-synthesized triangular Ag nanoplates as templates in the presence of appropriate amounts of HAuCl₄, and were applied in detecting methyl red.     

The structural transition of bimetallic Ag–Au from core/shell to alloy and SERS application

It is well-known that Ag–Au bimetallic nanoplates have attracted significant research interest due to their unique plasmonic properties and surface-enhanced Raman scattering (SERS). In recent years, there have been many studies on the fabrication of bimetallic nanostructures. However, controlling the shape, size, and structure of bimetallic nanostructures still has many challenges. In this work, we present the results of the synthesis of silver nanoplates (Ag NPls), and Ag–Au bimetallic core/shell and alloy nanostructures, using seed-mediated growth under green LED excitation and a gold salt (HAuCl₄) as a precursor of gold. The results show that the optical properties and crystal structure strongly depend on the amount of added gold salt. Interestingly, when the amount of gold(x) in the sample was less than 0.6 μmol (x < 0.6 μmol), the structural nature of Ag–Au was core/shell, in contrast x > 0.6 μmol gave the alloy structure. The morphology of the obtained nanostructures was investigated using the field emission scanning electron microscopy (FESEM) technique. The UV–Vis extinction spectra of Ag–Au nanostructures showed localized surface plasmon resonance (LSPR) bands in the spectral range of 402–627 nm which changed from two peaks to one peak as the amount of gold increased. Ag–Au core/shell and alloy nanostructures were utilized as surface enhanced Raman scattering (SERS) substrates to detect methylene blue (MB) (10⁻⁷ M concentration). Our experimental observations indicated that the highest enhancement factor (EF) of about 1.2 × 10⁷ was obtained with Ag–Au alloy. Our detailed investigations revealed that the Ag–Au alloy exhibited significant EF compared to pure metal Ag and Ag–Au core/shell nanostructures. Moreover, the analysis of the data revealed a linear dependence between the logarithm of concentration (log C) and the logarithm of SERS signal intensity (log I) in the range of 10⁻⁷–10⁻⁴ M with a correlation coefficient (R²) of 0.994. This research helps us understand better the SERS mechanism and the application of Raman spectroscopy on a bimetallic surface.

It is well-known that Ag–Au bimetallic nanoplates have attracted significant research interest due to their unique plasmonic properties and surface-enhanced Raman scattering (SERS).     

Counterion coupled (COCO) gemini surfactant capped Ag/Au alloy and Ag@Au core–shell nanoparticles for cancer therapy

Hybrid silver (Ag)–gold (Au) nanoparticles (NPs) with different sizes and compositions were synthesized. Ag/Au alloy and Ag@Au core–shell type NPs were prepared from Ag and Au with various ratios using the COCO gemini surfactant, 1,6-bis (N,N-hexadecyldimethylammonium) adipate (COCOGS), 16-6-16 as a stabilizer. The formation of the Ag/Au alloy and Ag@Au core–shell was confirmed by UV-visible absorption spectroscopy, high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX) and selected area electron diffraction (SAED) patterns. Depending on the composition of the Ag/Au alloy NPs, the λ_max values varied from 408 nm to 525 nm. FTIR measurements were used to evaluate the adsorption of the COCO gemini surfactant (16-6-16) on the Ag/Au alloy and Ag@Au core–shell surface. In this present work, we study how to achieve the stability and activity of the COCO gemini surfactant (16-6-16) capped Ag/Au alloy and Ag@Au core–shell NPs for developing novel anti-cancer agents by evaluating their potentials in the Hep-2 cell line model. Thus the developed core–shell NPs were possibly involved in inducing cytotoxicity followed by inhibition of cell proliferation to the cancer cells with apoptosis induction. The developed core–shell NPs might serve as highly applicable agents in the development of next-generation cancer chemotherapeutic agents.

In this work hybrid silver (Ag)–gold (Au) nanoparticles (NPs) with different sizes and compositions were synthesized and applied for anticancer evaluations and which is effectively involved in cancer cell apoptosis through DNA damage.     

Facile synthesis of high-performance SiO2@Au core–shell nanoparticles with high SERS activity

This study proposes a facile and general method for fabricating a wide range of high-performance SiO₂@Au core–shell nanoparticles (NPs). The thicknesses of Au shells can be easily controlled, and the process of Au shell formation was completed within 5 min through sonication. The fabricated SiO₂@Au NPs with highly uniform size and SERS activity could be ideal SERS tags for SERS-based immunoassay.

This study proposes a facile and general method for fabricating a wide range of high-performance SiO₂@Au core–shell nanoparticles (NPs).

The design and controlled fabrication of Au nanocomposites have attracted extensive attention because of their outstanding chemical and optical properties and wide applications in various fields, such as catalysis,¹ drug delivery,² photothermal cancer therapy,³ sensing,⁴ and surface-enhanced Raman scattering (SERS).⁵ However, small Au nanocomposites tend to aggregate, which seriously affects their stability and usability. The combination of silica nanoparticles (SiO₂ NPs) and Au shells provides a good alternative to Au nanocomposites.^6,7 These SiO₂ NPs are ideal core materials due to their high stability, easy preparation, uniform spherical shape, and large particle size range.^8,9Many synthesis methods have been explored for the fabrication of SiO₂@Au core–shell NPs; these methods include electroless plating,¹⁰ self-assembly,¹¹ layer-by-layer synthesis,¹² and seed growth.¹³ The seed growth method is the most commonly used to coat the Au shell on the surface of the SiO₂ core and involves two steps: deposition of nucleus seeds on the functionalized SiO₂ surface and Au shell growth. Although this method is beneficial for the synthesis of nanostructures with narrow size distribution, it exhibits two major shortcomings. First, the surface of SiO₂ NPs must be functionalized with various organosilanes containing amino (–NH₂) or mercapto (–SH) groups for adsorption or deposition of metal seeds on the SiO₂ NPs before subsequent growth of Au shells.^14,15 However, full surface amino/mercapto modification is often difficult to achieve; in this regard, dense metal seed layer formation on the surface of SiO₂ NPs cannot be achieved, eventually affecting the uniform and complete Au shell coating. Second, the formation of complete Au shell on the SiO₂ NPs is frequently achieved using a slow-growth approach through slow or multiple addition of HAuCl₄ to the seed-coated SiO₂ NPs suspension containing reducing agents.^16,17 The application of these slow-growth methods is restricted by its complex procedure and time-consuming preparation. Thus, a facile method must be developed for synthesis of Au coated SiO₂ NPs with controllable Au shell, good dispersibility, and fast preparation.In this work, we report a sonochemically assisted seed growth method for facile synthesis of monodisperse SiO₂@Au core–shell NPs for the first time. Cationic polyethyleneimine (PEI) was used to form a cationic thin interlayer with numerous primary amine groups for easy adsorption of dense Au seeds on the silica surface and keeping the nanostructure stability during shell growth. Sonication was used instead of traditionally used mechanical stirring to shorten the reaction time. The entire reaction process for Au shell formation was completed within 5 min. Moreover, the thickness of the Au shell was easily controlled outside the silica cores of different sizes. To the best of our knowledge, the proposed method is the most convenient synthesis route for preparation of high-performance SiO₂@Au core–shell NPs to date. Our results further demonstrate that the fabricated SiO₂@Au NP could be an ideal SERS tag for SERS-based lateral flow immunoassay (LFA). The method was validated for detection of human immunoglobulin M (IgM) and showed a detection limit as low as 0.1 ng mL⁻¹. The details of the experiments including SiO₂@Au NPs preparation, SERS-based LFA strip preparation, SERS detection protocol, and sensitivity test were provided in the ESI^† section.The synthesis principle of monodisperse SiO₂@Au NPs is presented in Fig. 1a. SiO₂ NPs were first prepared by using a modified Stöber method as the core. The SiO₂ NPs were ultrasonically treated with PEI solution to form PEI-coated SiO₂ NPs (SiO₂@PEI). The positively charged PEI effectively attached to the negatively charged SiO₂ NPs and formed a stable polymer layer via electrostatic self-assembly. SiO₂–Au seed NPs were prepared by adsorbing small Au NPs (3–5 nm) on the PEI layer of SiO₂ NPs densely and firmly through covalent binding between the –NH₂ groups of PEI and Au NPs. Finally, monodisperse SiO₂@Au NPs were quickly obtained through the reduction of HAuCl₄ by hydroxylamine hydrochloride (NH₂OH·HCl) under the stabilization of PVP. The uniform Au shells outside the SiO₂ NPs were formed within 5 minutes through the isotropic growth of all Au seeds under sonication.Open in a separate windowFig. 1Synthesis principle of SiO₂@Au NPs (a). TEM images of (b) SiO₂ NPs, (c) SiO₂–Au seed NPs, (d) SiO₂@Au NPs and their corresponding elemental mapping in (g), (h), and (i) respectively. (e) HRTEM picture and (f) bright-field TEM image of a single SiO₂@Au NP.The morphology of the as-synthesized products in different stages were characterized through transmission electron microscopy (TEM). The as-prepared SiO₂ NPs were uniform in size and had a diameter of approximately 140 nm (Fig. 1b). After coating the SiO₂@PEI NPs with Au seeds, many small seeds homogeneously adhered to the surface of the silica core (Fig. 1c). The dense Au seeds acted as randomly oriented crystalline sites for subsequent seed-mediated growth of the Au shell. Fig. 1d and e show the low- and high-magnification TEM images of the final SiO₂@Au core–shell NPs, respectively. Continuous and rough edges were detected around the SiO₂@Au NPs. The HRTEM image (Fig. 1e) indicated that large adjacent Au NPs covered the entire surface of the SiO₂ NPs, forming a complete and rough Au shell. The average particle size increased from 140 nm to 190 nm after the Au shell formation, indicating that the thickness of the Au shell was approximately 25 nm. Additionally, the SEM images (Fig. S1^†) showed that the SiO₂@Au NPs were successfully fabricated on a large scale and exhibited a rough surface and uniform size. The elemental composition of SiO₂@Au NPs was also confirmed through X-ray mapping (Fig. 1f–i). The results indicated that a layer of Au shell was uniformly coated on the surface of the SiO₂ NPs. The zeta potentials of SiO₂, SiO₂@PEI, SiO₂–Au seeds, and SiO₂@Au NPs in aqueous solution were found to be −46.7, +41.9, −7.4, and −21.1 mV, respectively (Fig. S2^†). The significant change in the zeta potential revealed the successive completion of PEI coating, Au seed adsorption, and Au shell formation. Fig. 2a shows the typical XRD patterns of the as-synthesized SiO₂–Au seed (blue line) and SiO₂@Au NPs (red line). The specific XRD pattern of Au is characterized by five peaks positioned at 2θ values of 38.3°, 44.3°, 64.5°, 77.4°, and 81.6°, which correspond to the reflections of the (111), (200), (220), (311), and (222) crystalline planes of Au (JCPDS no. 04-0784), respectively.^18,19 The intensity of the diffraction peaks of SiO₂@Au NPs increased when the Au shells were coated. No peaks of SiO₂ and PEI were detected in the XRD pattern because of their amorphous form.²⁰Open in a separate windowFig. 2Typical XRD patterns (a) and UV-visible spectra (b) of the as-synthesized products. Fig. 2b illustrates the UV-vis spectra of the as-synthesized products dispersed in deionized water in different stages. SiO₂ and SiO₂@PEI NPs had no obvious absorption peaks in the UV-vis spectra (curves a and b). SiO₂–Au seed NPs displayed a clear absorption peak at about 568 nm (curve c), which confirms the formation of the Au seed layer onto the surface of SiO₂ NPs. As the Au shell formed, the UV-vis spectral peak obviously red-shifted, and the intensity increased significantly (curve d). This result could be due to the strong interaction between and the coupling of the large adjacent Au NPs of the Au shells outside the SiO₂ NPs.²¹The strategy for Au shell formation is essentially seed-mediated growth. Thus, the surface morphology of SiO₂@Au NPs can be easily controlled by adjusting the Au³⁺ concentration by using a constant amount of SiO₂–Au seed. Fig. 3a–d shows the representative TEM images of SiO₂@Au NPs synthesized with different concentrations of HAuCl₄ while the other parameters remained constant. As the concentration of the HAuCl₄ increased from 0.01 mM to 0.04 mM, the Au seeds absorbed outside the SiO₂ NPs gradually increased in size and finally intersected with each other and formed a continuous and Au shell of a different thickness.Open in a separate windowFig. 3TEM images of SiO₂@Au NPs synthesized with different HAuCl₄ concentrations: (a–d) 0.01, 0.02, 0.03, and 0.04 mM HAuCl₄. (e) UV-vis spectra of SiO₂@Au synthesized with different HAuCl₄ concentrations: curves (a–e) 0, 0.01, 0.02, 0.03, and 0.04 mM HAuCl₄ and the corresponding Raman spectra of DTNB (f). Fig. 3e shows the UV-vis spectra of the synthesized SiO₂–Au seed and SiO₂@Au NPs with different Au shell thicknesses. The absorption peak of the obtained products red shifted gradually from 568 nm to 700 nm, and the peak width became broader with increasing concentration of HAuCl₄. Thus, the absorption peak of SiO₂@Au NPs can also reflect the formation and thickness of the Au shell. Fig. 3f shows the SERS activity of SiO₂@Au NPs prepared with different HAuCl₄ concentrations. 5,5-Dithiobis-(2-nitrobenzoic acid) (DTNB) was used as Raman molecule because it contains a double sulfur bond, which can be chemically coupled to the Au shell to form Au–S chemical bond and could produce strong and concise SERS peaks located at 1062, 1148, 1331, and 1556 cm⁻¹.^22,23 Moreover, DTNB molecules can provide free carboxyl groups as sites to conjugate antibodies.²⁴ As shown in the Raman spectra in Fig. 3f, the SiO₂–Au seed showed fairly weak SERS ability (spectra a), whereas the SiO₂@Au NPs exhibited gradually enhanced SERS activity as the HAuCl₄ concentration increased (spectra b–d). However, the overgrowth of the Au shell decreased the SERS activity of SiO₂@Au NPs (spectra e). This phenomenon could be attributed to the fully continuous Au shell formation, which reduced the nanogaps and hot spots on the surface of SiO₂@Au NPs. Hence, we chose SiO₂@Au NPs prepared with 0.02 mM HAuCl₄ as the optimal material for SERS application because of their nearly complete Au shell and optimal enhancement effect.PEI can be self-assembled on the surface of SiO₂ NPs of any size under sonication conditions. Thus, our proposed PEI-assisted seed growth method is a general route for preparing monodisperse SiO₂@Au core–shell particles with different sizes, ranging from nanoscale to microscale levels. Fig. 4a–c shows the TEM images of single SiO₂–Au seed NPs with different sizes (70–300 nm), and Fig. 4d–f clearly shows their corresponding fabricated SiO₂@Au NPs, respectively. The TEM images of multiple SiO₂@Au NPs with different sizes are displayed in Fig. S3.^† All of the obtained SiO₂@Au NPs possess homogeneous nanostructure, uniform Au nanoshell, and good dispersity. We further examine the dependence of SERS activity on the SiO₂@Au NPs size up to 350 nm. Fig. S4^† shows a set of SERS spectra of DTNB (10⁻⁵ M) adsorbed on the SiO₂@Au NPs of different sizes. The SERS intensity presented in the figure is the average intensity from 10 spots for each sample. Obviously, all the SiO₂@Au NPs exhibited excellent SERS abilities, and the signal intensities were gradually enhanced as the particle size increased. In fact, the Au shells of SiO₂@Au NPs were made of large sized Au NPs. This experimental result indicates that the larger the size of the Au NPs of shell, the higher the SERS activity achieved.Open in a separate windowFig. 4(a–c) TEM images of single SiO₂–Au seed with different sizes: (a) 70, (b) 150, and (c) 300 nm and their corresponding fabricated SiO₂@Au NPs (d), (e), and (f), respectively.For the determination of the SERS sensitivity of the 80 nm SiO₂@Au NPs, a series of DTNB ethanol solution (with concentration ranging from 10⁻⁴ M to 10⁻¹¹ M) was prepared. Each tube of DTNB solution was mixed with 10 μL of SiO₂@Au NPs (1 mg mL⁻¹) and sonicated for 1 h. After separation and washing, the final precipitate was dropped on a Si chip and analyzed with Raman signals. The spectra and calibration curve of DTNB absorbed on the SiO₂@Au NPs are shown in Fig. 5a and b, respectively. The SERS signal significantly decreased as the concentration of DTNB decreased, and the main Raman peak at 1331 cm⁻¹ remained evident at DTNB concentrations as low as 10⁻¹⁰ M. Thus, the limit of detection (LOD) of DTNB is 10⁻¹⁰ M. These results indicate that the SiO₂@Au NPs have good potential to be active SERS substrate for greatly enhancing the SERS signal of molecules adsorbed on them.Open in a separate windowFig. 5(a) SERS spectra of DTNB measured with different concentrations on the SiO₂@Au NPs. (b) Calibration curve for DTNB at a concentration range of 10⁻⁴ M to 10⁻¹¹ M obtained using SERS intensity at 1331 cm⁻¹. The error bars represent the standard deviations from five measurements.Upon modification with Raman report molecules and detection antibodies, the monodisperse SiO₂@Au NPs must be efficient SERS tags for highly reproducible SERS immunoassays due to the integration of high SERS activity of the Au nanoshell and the homogeneity and stability of SiO₂ NPs (Fig. 6a). SERS-based LFA strip is a recently reported analytical technique to overcome the shortcomings of conventional lateral flow assay, such as poor sensitivity and semiquantitative ability on the basis of colorimetric analysis.^25–27 The fundamental principle of SERS-based strip is the use of functional SERS tags instead of Au NPs. High-sensitivity and quantitative detection can be achieved by Raman spectroscopy because the intensity of the SERS signal is directly proportional to the number of SERS tags on the test line.Open in a separate windowFig. 6(a) Synthesis route for SiO₂@Au SERS tags. (b) Schematic of SERS-based LFA strips for quantitative detection of human IgM. Fig. 6b represents the operating principle of the monodisperse SiO₂@Au NPs (80 nm) based SERS-LFA strip. Human IgM was selected as the model target antigen to explore the sensitivity of the proposed method. The representative SERS-LFA strip is composed of a sample loading pad, a conjugate pad, a NC membrane containing test line and control line, and an absorption pad. In our system, goat anti-human IgM antibody-labeled SiO₂@Au/DTNB NPs were dispensed onto the glass fiber paper as the conjugate pad, and the goat anti-human IgM antibody and donkey anti-goat immunoglobulin G (IgG) antibody were dispensed onto the NC membrane to form the test line and control line, respectively. When the sample solution containing the target human IgM passed through the conjugation pad, immunocomplexes (human IgM/SERS tags) were formed and continued migrating along the NC membrane until they reach the test line where they were captured by the previously immobilized anti-human IgM antibodies. Excess antibody-conjugated SiO₂@Au tags continued to migrate to the control line and were immobilized by the donkey anti-goat IgG antibody. Consequently, two dark bands appeared in the presence of the target human IgM (positive), whereas only the control line turned to a dark band in the absence of human IgM (negative). Quantitative detection of human IgM could be realized by detecting the SERS signal on the test line.Human IgM was diluted within 10 000 ng mL⁻¹ to 0.1 ng mL⁻¹ as the sample solution, and PBST solution (10 mM PBS, 0.05% Tween-20) was used as blank control. As shown in Fig. 7a, the color of SERS tags captured by the test line was visualized and gradually decreased with decreasing human IgM concentration. The LOD of colorimetric method for detection of human IgM was found to be 10 ng mL⁻¹. Quantitative analysis was also conducted by measuring the characteristic Raman signals of the SERS tags on the test lines, and the Raman spectra are displayed in Fig. 7b. The Raman spectra were analyzed by plotting the intensity at 1331 cm⁻¹ of DTNB as a function of the logarithm of the target human IgM concentration to generate a calibration curve (Fig. 7c). The LOD of the SERS-LFA strips based on the SiO₂@Au tags is 0.1 ng mL⁻¹, which was calculated as a 3 : 1 threshold ratio with respect to the blank control measurement. Using SiO₂@Au tags-based SERS LFA strip offers a 100-fold improvement in the detection limit compared with colorimetric analysis. Basing on these results, we demonstrated the high efficiency and great potential of monodisperse SiO₂@Au NPs as suitable SERS tags for SERS-based LFA strips. The specificity of the SERS-LFA strips was tested by a high concentration (1 μg mL⁻¹) of other proteins including human IgG and BSA. Fig. S5^† shows the result of the specificity test. IgG and BSA did not show significant interference signals both in visualization and Raman spectrum analyses, whereas 100 ng mL⁻¹ human IgM exhibited a strong signal. Hence, the SiO₂@Au tags-based SERS-LFA strip has good selectivity.Open in a separate windowFig. 7(a) Photographs of SERS-based LFA strips in the presence of different concentrations of human IgM. (b) SERS spectra measured in the corresponding test lines. (c) Plot of the Raman intensity at 1331 cm⁻¹ as a function of the logarithmic concentration of human IgM. The error bars represent the standard deviations from five measurements.In summary, this work proposes a sonochemically assisted seed growth method for facile synthesis of monodisperse SiO₂@Au core–shell NPs with a complete Au shell. This method is a general route for preparing SiO₂@Au particles with sizes ranging from nanoscale to microscale levels. High-performance SiO₂@Au NPs were obtained from the intermediate product (SiO₂–Au seed) within 5 min through sonication. The obtained SiO₂@Au NPs were highly uniform in size and shape and exhibited satisfactory SERS activity. Hence, these NPs could be ideal SERS tags for various SERS based immunoassays. The small SiO₂@Au NPs (80 nm) with light weight and good dispersibility were also successfully applied to SERS-based LFA strip for human IgM rapid detection, with limit of detection as low as 0.1 ng mL⁻¹. We expect that high-performance SiO₂@Au NPs SERS tags can be used for actual detection.     

Interfacial effect of dual ultra-thin SiO2 core–triple shell Au@SiO2@Ag@SiO2 for ultra-sensitive trinitrotoluene (TNT) detection

Nanostructured hybrid Au@SiO₂@Ag@SiO₂ was developed, which greatly enhanced the surface plasmon resonance effect due to the interfacial effect of dual ultra-thin SiO₂ in which the double-superimposed long-range plasmon transfer between Au and Ag and determinand molecules. In addition, the interfacial effect between the inner and outermost silica layer can contribute to the presence of an amplified electric field between Au core and Ag shell, which prevents aggregation and oxidation of nanoparticles. At the same time, the influence of the amount of silica in SiO₂ shells on the Surface Enhanced Raman Scattering (SERS) was explored by controlling the experimental conditions. In our experiments, the ultrathin silica coating Au@SiO₂@Ag@SiO₂ showed the best SERS performance, generating an analytical enhancement factor (AEF) of 5 × 10⁶. At the same time, nanoparticles modified by 4-aminothiophenol (4-ATP) can detect 2,4,6-TNT as low as 2.27 × 10⁻⁶ ppb (10⁻¹⁴ M) and exhibit excellent versatility in the detection of nitroaromatics. The results demonstrated that the interfacial effect of double-layer dielectric silica achieved the localized surface plasmon resonance enhancement effect in Au@SiO₂@Ag@SiO₂.

The ultra-sensitive detection of trinitrotoluene (TNT) demonstrates that interfacial effect of double-layer dielectric silica achieves the LSPR enhancement effect in Au@SiO₂@Ag@SiO₂.     

Synthesis of silver sulfide nanoparticles and their photodetector applications

Silver sulfide nanoparticles (Ag₂S NPs) are currently being explored as infrared active nanomaterials that can provide environmentally stable alternatives to heavy metals such as lead. In this paper, we describe the novel synthesis of Ag₂S NPs by using a sonochemistry method and the fabrication of photodetector devices through the integration of Ag₂S NPs atop a graphene sheet. We have also synthesized Li-doped Ag₂S NPs that exhibited a significantly enhanced photodetector sensitivity via their enhanced absorption ability in the UV-NIR region. First-principles calculations based on a density functional theory formalism indicated that Li-doping produced a dramatic enhancement of NIR photoluminescence of the Ag₂S NPs. Finally, high-performance photodetectors based on CVD graphene and Ag₂S NPs were demonstrated and investigated; the hybrid photodetectors based on Ag₂S NPs and Li-doped Ag₂S NPs exhibited a photoresponse of 2723.2 and 4146.0 A W⁻¹ respectively under a light exposure of 0.89 mW cm⁻² at 550 nm. Our novel approach represents a promising and effective method for the synthesis of eco-friendly semiconducting NPs for photoelectric devices.

Silver sulfide nanoparticles (Ag₂S NPs) are currently being explored as infrared active nanomaterials that can provide environmentally stable alternatives to heavy metals such as lead.     

Explosive vapour/particles detection using SERS substrates and a hand-held Raman detector

We developed and optimized surface-enhanced Raman spectrometry (SERS) methods for trace analysis of explosive vapour and particles using a hand-held Raman spectrometer in the field. At first, limits of detection (LODs) using SERS methods based on a colloidal suspension of gold nanoparticles were measured under alkaline conditions and are as follows: pentaerythritol tetranitrate (PETN) (1.5 × 10⁻⁶ M, 6.9 ng), 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX), 8.1 × 10⁻⁶ M, 35 ng; urea nitrate (UN), 9.2 × 10⁻⁴ M, 165 ng; 2,4,6-trinitrotoluene (TNT), 1.1 × 10⁻⁷ M, 0.35 ng. We developed SERS substrates that demonstrate the wide applicability of this technique for use in the field for explosive vapour and particles adsorbed on a surface based on Au nanoparticles that were optimal for the detection of the target materials in solution. Au nanoparticles were modified onto quartz fibres or a polyurethane sponge for vapour/particles detection. SERS detection of vapours of 2,4-dinitrotoluene (2,4-DNT) and 1,3-dinitrobenzene (1,3-DNB) was shown by sampling vapours onto Au-modified quartz fibres followed by hand-held Raman analysis with estimated minimum detection levels of 3.6 ng and 54 ng, respectively. The detection of 2,4-DNT using sponge-based SERS decorated with Au nanoparticles was also demonstrated; however, the sensitivity was lower than that observed using quartz fibres. The detection of TNT on a surface was performed by utilizing quartz-fibres precoated with alumina and modified with Au nanoparticles, and the detection of 10 μg (0.53 μg cm⁻²) of TNT was demonstrated.

We developed and optimized surface-enhanced Raman spectrometry (SERS) methods for trace analysis of explosive vapour and particles using a hand-held Raman spectrometer in the field.     

Ag nanoparticles on ZnO nanoplates as a hybrid SERS-active substrate for trace detection of methylene blue

Decorating two-dimensional (2D) nanomaterials with nanoparticles provides an effective method to integrate their physicochemical properties. In this work, we present the hydrothermal growth process of 2D zinc oxide nanoplates (ZnO NPls), then silver nanoparticles (AgNPs) were uniformly distributed on the surface of ZnO NPls through the reduction procedure of silver nitrate with sodium borohydride to create a metal–semiconductor hybrid. The amount of AgNPs on the ZnO NPls'' surface was carefully controlled by varying the volume of silver nitrate (AgNO₃) solution. Moreover, the effect of AgNPs on the surface-enhanced Raman scattering (SERS) property of ZnO NPls was thoroughly investigated by using methylene blue (MB) as the target molecule. After calculation, the maximum enhancement factor value for 10⁻⁴ M of MB reached 6.2 × 10⁶ for the peak at 1436 cm⁻¹ and the limit of detection was 10⁻⁹ M. In addition, the hybrid nanosystem could distinguish MB with good reproducibility over a wide range of concentrations, from 10⁻⁹ to 10⁻⁴ M. The SERS mechanism is well elucidated based on the chemical and electromagnetic mechanisms related to the synergism of ZnO and Ag in the enhancement of Raman signal. Abundant hot spots located at the gap between adjacent separate Ag nanoparticles and ZnO nanoplates which formed a strong local electromagnetic field and electron transfer between ZnO and Ag are considered to be the key factors affecting the SERS performance of our prepared ZnO/Ag substrates. In this research, we found high sensitivity of ZnO nanoplates/Ag nanoparticles in detecting MB molecules. This unique metal–semiconductor hybrid nanosystem is advantageous for the formation of Raman signals and is thus suitable for the trace detection of methylene blue.

Decorating two-dimensional (2D) nanomaterials with nanoparticles provides an effective method to integrate their physicochemical properties.     

Bioscaffold arrays decorated with Ag nanoparticles as a SERS substrate for direct detection of melamine in infant formula

Three-dimensional (3D) plasmonic structures have been intensively investigated as high performance surface enhanced Raman scattering (SERS) substrates. Here, we demonstrate a 3D biomimetic SERS substrate prepared by deposition of silver nanoparticles (Ag NPs) on the bioscaffold arrays of cicada wings using laser molecular beam epitaxy. This deposition method can offer a large number of nanoparticles with average diameter of ∼10 nm and nanogaps of sub-10 nm on the surface of chitin nanopillars to generate a high density of hotspots. The prepared 3D Ag/cicada SERS substrate shows a limit of detection (LOD) for Rhodamine 6G as low as 10⁻⁷ M, high enhancement factor of 1.09 × 10⁵, and excellent signal uniformity of 6.8%. Moreover, the molecular fingerprints of melamine in infant formula can be directly extracted with an LOD as low as 10 mg L⁻¹, without the need for functional modification. The prepared SERS-active substrate, due to its low cost, high-throughput, and good detection performance, can be widely used in applications such as food safety and environmental monitoring.

Three-dimensional (3D) plasmonic structures have been intensively investigated as high performance surface enhanced Raman scattering (SERS) substrates.     

Optimization of a hybrid plasmonic configuration: particle on a corrugated film and its SERS application

Hybrid SERS configurations, which combine manufactured metallic chips with nanoparticles, have emerged as powerful and promising SERS substrates because they not only provide cost-effective and high-yield manufacture, but also demonstrate excellent sensitivity and outstanding reproducibility. Herein, a plasmonic hybrid structure, a particle on an Au film over nanoparticles (particle-AuFON) configuration, was studied for SERS application. In a previous study, we constructed a hybrid substrate by grafting Au@Ag core–shell NPs onto the AuFON structure. In this study, the hybrid substrate is designed and simulated to optimize electromagnetic enhancement while also affording exceptional uniformity, repeatability and stability, which are essential factors in SERS applications. This hybrid substrate provides good SERS performance with a detection limit of 1 × 10⁻¹⁰ M, which is 100-fold improvement compared to AuFON substrate or Au@Ag NPs. The excellent signal enhancement originates from the hotspot improvement and densification, as visualized by the FDTD calculations. Additional hotspots were created at the gaps between the Au@Ag NPs and the AuFON, thus improving the density of hotspots. Moreover, the intensity of the hotspots was improved due to EM coupling between the original hotspots and additional hotspots. To validate the feasibility of this hybrid substrate in SERS-based detection, melamine was detected as an example. The detection limit was 10 nM, which was much lower than the maximum limit of melamine in infant formula (1 ppm) legislated by the governments of both the United States and China. A calibration curve was plotted between the SERS intensity and melamine concentration with a correlation coefficient of 0.98. This hybrid SERS substrate shows great potential in SERS-based sensing and imaging, as it provides high sensitivity and outstanding reproducibility with a simple fabrication procedure, facilitating the cost-effective and high-yield manufacture of SERS substrates.

A plasmonic hybrid structure of particles on a Au film over nanoparticles (particle-AuFON) configuration was studied for application in SERS. It showed great potential in SERS-based sensing and it provides outstanding uniformity, repeatability and stability.     

Halogen bond triggered aggregation induced emission in an iodinated cyanine dye for ultra sensitive detection of Ag nanoparticles in tap water and agricultural wastewater

Aggregation induced emission (AIE) has emerged as a powerful method for sensing applications. Based on AIE triggered by halogen bond (XB) formation, an ultrasensitive and selective sensor for picomolar detection of Ag nanoparticles (Ag NPs) is reported. The dye (CyI) has an iodine atom in its skeleton which functions as a halogen bond acceptor, and aggregates on the Ag NP plasmonic surfaces as a halogen bond donor or forms halogen bonds with the vacant π orbitals of silver ions (Ag⁺). Formation of XB leads to fluorescence enhancement, which forms the basis of the Ag NPs or Ag⁺ sensor. The sensor response is linearly dependent on the Ag NP concentration over the range 1.0–8.2 pM with an LOD of 6.21 pM (σ = 3), while for Ag⁺ it was linear over the 1.0–10 μM range (LOD = 2.36 μM). The sensor shows a remarkable sensitivity for Ag NPs (pM), compared to that for Ag⁺ (μM). The sensor did not show any interference from different metal ions with 10-fold higher concentrations. This result indicates that the proposed sensor is inexpensive, simple, sensitive, and selective for the detection of Ag NPs in both tap and wastewater samples.

Based on AIE triggered by halogen bond (XB) formation, we established an ultrasensitive and selective sensor for picomolar detection of Ag nanoparticles (Ag NPs).     

Efficient photocatalysis with graphene oxide/Ag/Ag2S–TiO2 nanocomposites under visible light irradiation

Lack of visible light response and low quantum yield hinder the practical application of TiO₂ as a high-performance photocatalyst. Herein, we present a rational design of TiO₂ nanorod arrays (NRAs) decorated with Ag/Ag₂S nanoparticles (NPs) synthesized through successive ion layer adsorption and reaction (SILAR) and covered by graphene oxide (GO) at room temperature. Ag/Ag₂S NPs with uniform sizes are well-dispersed on the TiO₂ nanorods (NRs) as evidenced by electron microscopic analyses. The photocatalyst GO/Ag/Ag₂S decorated TiO₂ NRAs shows much higher visible light absorption response, which leads to remarkably enhanced photocatalytic activities on both dye degradation and photoelectrochemical (PEC) performance. Its photocatalytic reaction efficiency is 600% higher than that of pure TiO₂ sample under visible light. This remarkable enhancement can be attributed to a synergy of electron-sink function and surface plasmon resonance (SPR) of Ag NPs, band matching of Ag₂S NPs, and rapid charge carrier transport by GO, which significantly improves charge separation of the photoexcited TiO₂. The photocurrent density of GO/Ag/Ag₂S–TiO₂ NRAs reached to maximum (i.e. 6.77 mA cm⁻²vs. 0 V). Our study proves that the rational design of composite nanostructures enhances the photocatalytic activity under visible light, and efficiently utilizes the complete solar spectrum for pollutant degradation.

The photocatalytic reaction efficiency of GO/Ag/Ag₂S–TiO₂ nanorod arrays is 600% higher than that of a pure TiO₂ sample under visible light.     

Explosives sensing using Ag–Cu alloy nanoparticles synthesized by femtosecond laser ablation and irradiation

Herein we demonstrate the synthesis of Ag–Cu alloy NPs through a consecutive two-step process; laser ablation followed by laser irradiation. Initially, pure Ag and Cu NPs were produced individually using the laser ablation in liquid technique (with ∼50 femtosecond pulses at 800 nm) which was followed by laser irradiation of the mixed Ag and Cu NPs in equal volume. These Ag, Cu, and Ag–Cu NPs were characterised by UV-visible absorption, HRTEM and XRD techniques. The alloy formation was confirmed by the presence of a single surface plasmon resonance peak in absorption spectra and elemental mapping using FESEM techniques. Furthermore, the results from surface enhanced Raman scattering (SERS) studies performed for the methylene blue (MB) molecule suggested that Ag–Cu alloy NPs demonstrate a higher enhancement factor (EF) compared to pure Ag/Cu NPs. Additionally, SERS studies of Ag–Cu alloy NPs were implemented for the detection of explosive molecules such as picric acid (PA – 5 μM), ammonium nitrate (AN – 5 μM) and the dye molecule methylene blue (MB – 5 nM). These alloy NPs exhibited superiority in the detection of various analyte molecules with good reproducibility and high sensitivity with EFs in the range of 10⁴ to 10⁷.

Herein we demonstrate the synthesis of Ag–Cu alloy NPs through a consecutive two-step process; laser ablation followed by laser irradiation.     

In situ preparation of Ag nanoparticles on silicon wafer as highly sensitive SERS substrate

An intensive surface enhanced Raman scattering (SERS) effect is realized by ordered Ag nanoparticles (NPs) in situ grown on silicon wafer directly using (3-aminopropyl) trimethoxysilane (APS) as both the surface modifier and reducing agent. The as-prepared ordered Ag NPs based SERS substrate shows excellent performance in detecting glycerin (an important integration in liquid super lubricating system) as well as conventional Rhodamine 6G (R6G, a kind of dye organic pollutant). The enhancement factor (EF) achieves 4-fold for glycerin and 10-fold for R6G (allowing for detecting as low as 10⁻¹¹ M aqueous R6G), confirming the high sensitivity. The limited relative standard deviations (RSD) of the enhancement factors are within 15% for both glycerin and R6G, indicating the excellent uniformity. This remarkable progress is ascribed to the advantages of APS in improving adsorption and modulating distribution of Ag NPs on silicon, which results in a large local electric field to enhance the Raman signals. The SEM and UV-visible absorption spectrum characterization verified the contribution of APS in SERS improvement by investigating the influence of APS content and reduction time during the preparation process. All these advances imply that the SERS substrates prepared by Ag NPs in situ grown on silicon wafer have great potential application in real-time interface state tracing and sensitive detection.

An intensive surface enhanced Raman scattering (SERS) effect is realized by ordered Ag nanoparticles (NPs) in situ grown on silicon wafer directly using (3-aminopropyl) trimethoxysilane (APS) as both the surface modifier and reducing agent.     

Materials characterization of TiO2 nanotubes decorated by Au nanoparticles for photoelectrochemical applications

The structural and chemical modification of TiO₂ nanotubes (NTs) by the deposition of a well-controlled Au deposit was investigated using a combination of X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Scanning Transmission Electron Microscopy (STEM), Raman measurements, UV-Vis spectroscopy and photoelectrochemical investigations. The fabrication of the materials focused on two important factors: the deposition of Au nanoparticles (NPs) in UHV (ultra high vacuum) conditions (1–2 × 10⁻⁸ mbar) on TiO₂ nanotubes (NTs) having a diameter of ∼110 nm, and modifying the electronic interaction between the TiO₂ NTs and Au nanoparticles (NPs) with an average diameter of about 5 nm through the synergistic effects of SMSI (Strong Metal Support Interaction) and LSPR (Local Surface Plasmon Resonance). Due to the formation of unique places in the form of “hot spots”, the proposed nanostructures proved to be photoactive in the UV-Vis range, where a characteristic gold plasmonic peak was observed at a wavelength of 580 nm. The photocurrent density of Au deposited TiO₂ NTs annealed at 650 °C was found to be much greater (14.7 μA cm⁻²) than the corresponding value (∼0.2 μA cm⁻²) for nanotubes in the as-received state. The IPCE (incident photon current efficiency) spectral evidence also indicates an enhancement of the photoconversion of TiO₂ NTs due to Au NP deposition without any significant change in the band gap energy of the titanium dioxide (E_g ∼3.0 eV). This suggests that a plasmon-induced resonant energy transfer (PRET) was the dominant effect responsible for the photoactivity of the obtained materials.

The structural and chemical modification of TiO₂ NTs by the deposition of a well-controlled Au deposit (0.01 mg cm⁻¹) was investigated using a combination of microscopic (SEM, STEM), analytical measurements (XPS, SERS, UV-Vis, XRD) and photoelectrochemical investigations.     

Peptide-directed Pd-decorated Au and PdAu nanocatalysts for degradation of nitrite in water

In this work, a palladium binding peptide, Pd4, has been used for the synthesis of catalytically active palladium-decorated gold (Pd-on-Au) nanoparticles (NPs) and palladium–gold (Pd_xAu_100−x) alloy NPs exhibiting high nitrite degradation efficiency. Pd-on-Au NPs with 20% to 300% surface coverage (sc%) of Au showed catalytic activity commensurate with sc%. Additionally, the catalytic activity of Pd_xAu_100−x alloy NPs varied based on palladium composition (x = 6–59). The maximum nitrite removal efficiency of Pd-on-Au and Pd_xAu_100−x alloy NPs was obtained at sc 100% and x = 59, respectively. The synthesized peptide-directed Pd-on-Au catalysts showed an increase in nitrite reduction three and a half times better than monometallic Pd and two and a half times better than Pd_xAu_100−x NPs under comparable conditions. Furthermore, peptide-directed NPs showed high activity after five reuse cycles. Pd-on-Au NPs with more available activated palladium atoms showed high selectivity (98%) toward nitrogen gas production over ammonia.

In this work, a palladium binding peptide, Pd4, has been used for the synthesis of catalytically active palladium-decorated gold (Pd-on-Au) nanoparticles (NPs) and palladium–gold (Pd_xAu_100−x) alloy NPs exhibiting high nitrite degradation efficiency.