A novel SERS-based immunoassay for adipokines in patient’s serum dependent on dual-functional Fe3O4@SiO2@Ag triple core-shell microparticles (TCSMPs) was developed. The construction and morphology of the Fe3O4@SiO2@Ag TCSMPs were investigated by XRD, SEM, and TEM. Particularly, the size and quantity of Ag NPs and its density on the surface of SiO2 shell can be controlled by adjusting the concentration of AgNO3 in the precursor solution. Consequently, the highest SERS activity of Fe3O4@SiO2@Ag TCSMPs can be obtained by choosing the sample with the proper coating density of Ag NPs. Meanwhile, the Fe3O4 core endowed the TCSMPs with superior magnetic nature, which enabled their convenient separation. With the two crucial features, the Fe3O4@SiO2@Ag TCSMPs were eventually applied in the SERS-based immunoassay for adiponectin and leptin in PBS solution and patient’s serum, respectively. The detection linear ranges of adiponectin and leptin in PBS solution were from 25 μg mL−1 to 25 pg mL−1 and 200 μg mL−1 to 20 pg mL−1 with the detection limits as low as 25 and 20 pg mL−1, respectively. More significantly, the trace adiponectin (0.934 ng mL−1) and leptin (2.636 pg mL−1) in obesity patient’s serum, which had been diluted by three orders of magnitude, were successfully detected using such an immunoassay. Therefore, this method has an enormous potential for accurately testing clinical obesity and diabetes.A novel SERS-based immunoassay for adipokines in patient’s serum dependent on dual-functional Fe3O4@SiO2@Ag triple core-shell microparticles (TCSMPs) was developed. The highest SERS activity of Fe3O4@SiO2@Ag TCSMPs can be obtained by choosing the sample with the proper coating density of Ag NPs. Meanwhile, the Fe3O4 core endowed the TCSMPs with superior magnetic nature, which enabled their convenient separation. The detection linear ranges of adiponectin and leptin in PBS solution were from 25 μg mL−1 to 25 pg mL−1 and 200 μg mL−1 to 20 pg mL−1 with the detection limits as low as 25 and 20 pg mL−1, respectively. More significantly, the trace adiponectin (0.934 ng mL−1) and leptin (2.636 pg mL−1) in obesity patient’s serum, which had been diluted by three orders of magnitude, were successfully detected using such an immunoassay.Download high-res image (160KB)Download full-size image

An Ag@MSiO2@Ag three core–shell architecture was synthesized by a facial hydrothermal method. The features of the sample were characterized by SEM, TEM, and AFM images, EDS analyses and absorption spectra. This novel nanostructure exhibited excellent SERS properties due to the formation of hot spots around the inner and outer Ag NPs, which were identified by theoretical calculations. A detection limit of the analyte molecule was obtained as low as 10−11 M by using this SERS nanostructure. Moreover, the homogeneity of SERS signals from the three core–shell nanostructure was checked by Raman mapping. Our studies show that the unique Ag@MSiO2@Ag three core–shell nanostructure has significant potential to realize a SERS substrate with both sensitivity and stability, which are important in SERS-based immunoassay.

Based on a sandwich structure consisting of nano-Si immune probes and a SiC@Ag SERS-active immune substrate, a kind of ultra-sensitive immunoassay protocol is presented to detect tumor markers in human serum. The nano-Si immune probes were prepared by immobilizing the detecting antibodies onto the surfaces of SiO2-coated Si nanoparticles (NPs) which were modified with 3-(aminopropyl)trimethoxysilane, and the SiC@Ag SERS-active immune substrates were prepared by immobilizing the captured antibodies on Ag film sputtered on SiC sandpaper. To the best of our knowledge, it is the first time that Si NPs are directly used as Raman tags in an immunoassay strategy. And, the SiC@Ag SERS-active substrates exhibit excellent surface enhanced Raman scattering (SERS) performances with an enhancement factor of ∼105, owing to the plasmonic effect of the Ag film on the rough surface of the SiC sandpaper. In our experiments, the sandwich immunoassay structure has been successfully applied to detect prostate specific antigen (PSA), α-fetoprotein (AFP) and carbohydrate antigen 19-9 (CA19-9) in a human serum sample and the limit of detections are as low as 1.79 fg mL−1, 0.46 fg mL−1 and 1.3 × 10−3 U mL−1, respectively. It reveals that the proposed immunoassay protocol has demonstrated a high sensitivity for tumor markers in human serum and a potential practicability in biosensing and clinical diagnostics.

An immunoassay with high sensitivity based on surface enhanced Raman scattering (SERS) was realized by using a SiO2@Ag immune probe and Ag-decorated NiCo2O4 nanorods (NNRs) immune substrate. The SiO2@Ag immune probe was prepared by immobilizing Raman reporter molecules and anti-alpha fetoprotein (AFP) onto the SiO2@Ag hybrid structure. It was discovered that SiO2 colloidal microspheres attached with an appropriate amount of Ag nanoparticles (NPs) maintained the highest SERS intensity. Nanogaps between adjacent Ag NPs were proved to be the primary “hot spots” responsible for the significant Raman enhancement. Then, the immune substrate was synthesized by magnetron sputtering Ag NPs onto a NNRs coated carbon fiber cloth and linking them with anti-AFP. Such a novel substrate exhibited strong SERS activity and excellent reproducibility, due to the large amount of homogeneous Ag aggregates on the one-dimensional NNRs. Finally, a sandwich structure consisting of the SiO2@Ag immune probe and the Ag-decorated NNRs substrate was used to detect AFP and a detection limit as low as 2.1 fg mL−1 was obtained. It is imagined that the proposed strategy will show great advantages in sensitive immunoassays.

Polyhedral gold nanocrystals (Au NCs) with quasi-spherical, octahedral and triangle-like morphologies were synthesized using a hydrothermal method, and ordered packing structures were formed by water droplet evaporation-induced deposition. The self-assembled structures of the polyhedral Au NCs exhibit shape-directive arrangement during the building block orientation process, and the structure-dependent plasmonic characteristics of the self-assembled NCs were analysed numerically using the finite element method (FEM). The surface-enhanced Raman scattering (SERS) spectra of the self-assembled structures were measured by choosing 4-mercaptobenzoic acid as the Raman reporter, using an excitation wavelength of 785 nm. Both the theoretical and experimental results show that the self-assembled structures of polyhedral Au NCs have high electric field enhancements and excellent SERS performances, in particular, the octahedron self-assembled structures generate higher plasmonic enhancemence efficiency compared to other close-packed configurations. The superior SERS behaviours can be explained based on the electromagnetic enhancement mechanism and the plasmonic antenna effect of the interstitial hot spots, confirming that the self-assembly structures of polyhedral Au NCs offer an alternate way in the design of plasmonic enhancement substrates, with potential applications in bio-sensing and medical detection.

A surface-enhanced Raman scattering (SERS) film consisting of mesoporous silica (MSiO2) coated Ag nanoparticles (NPs) was achieved. The as-prepared hybrid NPs were uniform in size and formed large amount of aggregates in the film. “Hot spots” were supposed to appear in the MSiO2 shells with an average size as small as 15 nm. Such a novel core–shell structure therefore induced the enhancement of SERS intensity compared to the film of bare Ag NPs and polymer film of Ag-CMC. The homogeneity and stability of SERS signals from the Ag@MSiO2 film were also tested. A relative standard deviation of SERS intensity lower than 20% from Raman mapping and a stable SERS signal with excitation power of 100 mW were observed, which were both better than the other two films. Moreover, the obtained Ag@MSiO2 film was applied to detect thiram pesticides and a detection limit as low as 1×10−8 M was reached, which indicates the advantages of the Ag@MSiO2 film in biosensor.

Well-shaped Ag@Au hexagonal nanorings (Ag@Au HNRs) were successfully synthesized with the assistance of Ag hexagonal nanoplates as structure-directing templates in a galvanic replacement route, and their optical characteristics were studied in detail. During the synthetic process, the morphology of the Ag@Au HNRs was found to be strongly depended on the dosage of HAuCl4 added into the template solution. Both the experimental and calculated absorption spectra showed that the red-shift of localized surface plasmon resonance (LSPR) peaks of the Ag@Au HNRs obeyed a modified Lorentz function with increasing dosage of HAuCl4 solution, which was also explained by the plasmon hybridization of the Ag@Au HNRs. Moreover, the surface-enhanced Raman scattering (SERS) characteristics of Ag@Au HNRs were investigated by using 4-mercaptobenzoic acid (4MBA) as a Raman reporter molecule. And the as-prepared Ag@Au HNRs exhibit excellent SERS performances with an enhancement factor ∼106, especially for the Ag@Au HNR prepared at 160 μl HAuCl4 solution due to its strongest plasmon electric-field enhancement. It reveals that the as-synthesized Ag@Au HNRs possess great potential for the application in ultrasensitive biosensors.

Novel Au@Ag core–shell nanocubes (NCs) were successfully prepared by the controlled epitaxial growth of Ag shells onto Au nanoellipsoids (NEs) in the presence of surfactants. The growth mechanism of the Au@Ag core–shell NCs was systematically investigated by analyzing their morphology, optical properties, and crystallography. The localized surface plasmon resonance (LSPR) characteristics and the electric field distribution of the Au@Ag core–shell NCs were studied using the finite element method (FEM) based on the plasmon hybridization theory. Compared with pure Ag NCs, the absorption spectrum of the Au@Ag core–shell NCs exhibits a red shift and a weak shoulder near 550 nm, and the notable enhancement of electric field occurs around the corners along the long-axis of the Au ellipsoidal core because of plasmonic resonant coupling. Surface-enhanced Raman scattering (SERS) of the Au@Ag core–shell NCs labeled with 4-mercaptobenzoic acid molecules reveals that the bimetallic core–shell NCs possess efficient SERS activity with an enhancement factor EF = 2.27 × 106, thus confirming the possibility of using the Au@Ag core–shell NCs as a stable probe for SERS-based biosensing applications.

Single-crystal tetragonal α-MnO2 nanorods with different amounts of gold nanoparticles (NPs) attached were successfully prepared by a facile sputtering deposition technique. Initially, the morphology and crystal structure of the bare α-MnO2 nanorods synthesized via a hydrothermal approach were investigated. Then, the amount of gold NPs at different sputtering times was analyzed. It was confirmed that the amount of the decorated gold NPs increased with the lengthening of the sputtering time until they completely covered the α-MnO2 nanorods. Theoretical calculation results indicated the advantages of the composite structure by showing the enhanced electromagnetic fields around both the bare α-MnO2 nanorods and the gold NP decorated ones. The surface-enhanced Raman scattering (SERS) efficiency of these nanocomposites was evaluated using methylene blue and 4-mercaptobenzoic acid as Raman probe molecules. It was found that the SERS intensity of the substrates strongly depended on the degree of aggregation of the gold NPs. Uniform SERS signals across the entire surface of these samples were obtained. Moreover, a typical chemical toxin, methyl parathion, was effectively detected over a broad concentration range from 1 × 10−3 to 100 ppm using the gold NP decorated α-MnO2 nanorods, suggesting this hybrid structure is highly valuable for further applications on the rapid detection of organic environmental pollutants.

Several polyhedral gold nanocrystals (Au NCs) whose geometric morphology belongs to the Platonic solid and Archimedean solid groups are synthesized by modulating the cetyltrimethylammonium bromide (CTAB) concentration in the designed hydrothermal processes. The mechanism of growth of these polyhedral Au NCs is revealed via crystallographic analyses, which suggest that their morphologies mainly depend on the diminishing of the surface energy of the specific facets in the CTAB-directive shape-control process. Furthermore, the localized surface plasmon resonance (LSPR) properties of the as-prepared polyhedral Au NCs are numerically assessed using the finite element method (FEM). The calculations show that these polyhedral Au NCs exhibit geometry-dependence plasmonic properties: specifically, the enormous local field enhancements occur around their vertices and corners. The SERS spectra of 4-mercaptobenzoic acid molecules adsorbed on these polyhedral Au NCs display superior SERS activity with an enhancement factor of 105 to 106 at the excitation wavelength of 785 nm. Particularly, the truncated tetrahedron and bitetrahedron show higher SERS enhancement factors due to their LSPR being closer to the excitation wavelength of 785 nm. Thus, the facile surfactant-assisted synthesis offers the opportunity to design and optimize the shape-dependent SERS activity of polyhedral Au NCs with respect to possible applications in bio-sensing and imaging.

A new nanostructure, silica-coated Ag nanorods (NRs) aggregate with 4-mercaptobenzoic acid molecules (4MBA-Ag NRs@SiO2), was prepared by a seed-mediated growth method and a modified Stöber method. In the synthetic process, the optimal 4MBA-Ag NRs@SiO2 was obtained by varying conditions such as surfactant concentration and centrifugation cycles. The morphologies and the optical properties of the 4MBA-Ag NRs@SiO2 were investigated in detail by transmission electron microscope, UV-Vis spectrometer and Raman spectrometer. The experimental results show that the 4MBA-Ag NRs@SiO2 prepared with 0.05 M CTAB has the aggregate morphology of single-crystalline Ag NRs, typical localized surface plasmon resonance (LSPR) characteristics and a high enhancement factor of surface-enhanced Raman scattering (SERS). Based on the SERS activity of 4MBA-Ag NRs@SiO2 and Ag NRs, an immune probe and an immune substrate immobilized with anti-prostate specific antigen (anti-PSA) antibody have been fabricated and used to construct a sandwich structure for a PSA immunoassay; the detection limit of PSA reached 0.3 fg ml−1. It is demonstrated that the as-prepared 4MBA-Ag NRs@SiO2 has great potential in biosensing applications.

We proposed an efficient spaser based on gold–silver core–shell nanorods (NRs) encapsulated by an outer silica shell doped with a gain medium. The optical characteristics of the spaser were numerically simulated based on the finite element method (FEM). The results showed that the localized surface plasmon resonance (LSPR) amplification characteristics of the spaser strongly depend on the thickness of silver shell, the aspect ratio of the inner gold NRs, and the polarization direction of the incident light. And, the maximum absolute value of optical cross-section of the spaser can reach 21,824 μm2, which is about 1115, 523, and 18 times higher than that of spasers based on the gold NRs, the silver NRs, and the silver–gold core–shell NRs, respectively. The ultra-strong surface plasmon amplification characteristics of the spaser have potential applications in optical information storage, high sensitivity biochemical sensing, and medical engineering.

An immunoassay based on surface enhanced Raman scattering (SERS) has been developed using immuno-silver nanocubes (NCs) and silicon nanowires with high sensitivity. The features of the samples were characterized by XRD profiles, absorption spectra, SEM, TEM, and AFM images, EDS analyses, and SERS spectra. It was found that the obtained silver NCs maintained higher SERS activity than silver nanospheres. After the silver NCs were modified by an antibody and employed in immunoassay with silicon nanowire arrays as the substrate, the antigen concentration-dependent SERS spectra and dose–response calibration curves were obtained. The detection limit of a prostate-specific antigen in this immunosystem was as low as 2 × 10−14 g ml−1, attributing to the significant electromagnetic coupling effect generated between the densely packed silver NCs and the SERS-active silicon nanowire arrays. It can be inferred that this proposed strategy will show great advantages in sensitive immunoassay and other biochemical examinations.

Hollow sea-urchin gold nanoparticles (HSU-GNPs) were successfully prepared through a novel one-step galvanic replacement strategy, and their corresponding optical properties was studied in detail. During the synthesis process, the sizes of the interior hollows of the HSU-GNPs could be changed by adjusting the amount of silver nitrate added into hydrogen tetrachloroaurate trihydrate solution. The absorption spectra of the HSU-GNPs showed that the localized surface plasmon resonance (LSPR) peaks were red-shifted with increasing size of the interior hollows in the HSU-GNPs. When the added amount of silver nitrate was up to 6 μl, the LSPR peak of the synthesized HSU-GNP reached 726 nm as a maximum red-shift. Furthermore, the absorption spectra of the HSU-GNPs with different morphologies were theoretically simulated by the finite element method, which was consistent with the experimental results and explained the origin of the red-shift of the LSPR peaks. In addition, the surface-enhanced Raman scattering (SERS) of the sea urchin gold nanoparticles were also investigated using 4-mercaptobenzoic acid as a Raman reporter molecule. Both the experimental and calculated results showed that the HSU-GNPs had stronger SERS enhancement than the solid sea-urchin gold nanoparticles. In particular, the HSU-GNPs prepared by adding 6 μl silver nitrate exhibited a maximum SERS enhancement factor, EF = 1.1 × 109, due to the LSPR peak at 726 nm which is near to the excitation wavelength, 785 nm. This feature is significant for designing a biosensor with a super-high sensitivity based on the morphology of the HSU-GNPs.

•The detection limit of apoB reaches 2 fg/mL (3.878×10−18 mol/L) based on SERS immunoassay.•Nano-Ag immune substrate with great SERS enhancement was prepared.•Anti-apoB caused further aggregation of AuNPs to produce stronger SERS enhancement.•The sandwich structure with bimetal nanoparticle layers exhibits high immunoassay sensitivity.A super-high-sensitivity immunoassay based on surface-enhanced Raman scattering (SERS) was implemented using the nano-Au immune probes and nano-Ag immune substrate. Ultraviolet–visible extinction spectra, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images, and SERS spectra were used to characterise the nano-Au immune probes and the nano-Ag immune substrate. The nano-Ag immune substrate was prepared by the in situ growth of Ag nanoparticles and the subsequent linkage of these nanoparticles with anti-apolipoprotein B on a silicon wafer. The nano-Ag immune substrate exhibited strong SERS activity, excellent reproducibility, and high biospecificity. The nano-Au immune probes were prepared by immobilising 4-mercaptobenzoic acid (4MBA) molecules as a Raman reporter and anti-apolipoprotein B onto the surfaces of Au nanoparticles. It was found that 4MBA induced the aggregation of Au nanoparticles, resulting in the generation of vast hot spots. Moreover, the nano-Au immune probes exhibited strong SERS activity and high biospecificity. A sandwich-type immunoassay structure consisting of the nano-Au immune probes and nano-Ag immune substrate was used to detect the concentration of apolipoprotein B, where the detection limit was as low as 2 fg/mL (3.878×10−18 mol/L). Taken together, the experimental results indicate that the proposed immunoassay protocol has a great potential application in biological sensing and clinical diagnostics.

The purification, stability, and surface modification of Au/Ag core–shell nanorods (Au/Ag NRs) in a biological buffer solution were systematically studied for the first time. In this study, Au/Ag NRs were synthesized by chemically reducing silver on the surface of gold nanorods using cetyltrimethylammonium bromide as surfactant and then purified by centrifugation washing. Based on the analysis of UV–Vis absorption spectra, TEM images, Raman spectra, and the ξ-potential, it was observed that after the second washing step, the Au/Ag NRs displayed good stability and high surface-enhanced Raman scattering (SERS) enhancement. When the as-prepared Au/Ag NRs were centrifuged more than twice, a structural transition in the surfactant layer was manifested with a sudden increase in the Raman signal intensities at 760 and 1,455 cm−1. Moreover, 4-mercaptobenzoic acid (4MBA) was used as a Raman reporter molecule to investigate the SERS characteristics of the purified Au/Ag NRs. The Raman signal intensity was enhanced with increasing the concentration of 4MBA and reached its highest intensity at the saturation concentration of 1.0 µM 4MBA in a 5 ml solution of the purified Au/Ag NRs. To prevent significant aggregation of the 4MBA-tagged Au/Ag NRs, a poly(styrenesulfonate) (PSS) layer was assembled on the nanorod surfaces by electrostatic adsorption for further surface modification, which made the 4MBA-tagged Au/Ag NRs suitable for the labeled biosensing. Subsequently, the characteristics of the PSS-coated Au/Ag NRs were demonstrated for the potential applications of label-free biosensing.

•The selective synthesis of cubic and hexagonal NaYF4 crystals was successfully performed.•The crystal phase conversion was observed by adjusting the NaF/Re3+ ratio.•Intense ultraviolet and weak violet upconversion emissions were obtained in the hexagonal NaYF4 crystals.The selective synthesis of cubic and hexagonal NaYF4 crystals was successfully performed by a facile citric acid assisted hydrothermal method. The crystal phase conversion was observed through tuning the added amount of fluoride. A possible growth mechanism was proposed for the formation of hexagonal NaYF4 microcrystals (MCs). Under 980 nm excitation, intense ultraviolet (UV), blue, and weak violet upconversion (UC) emissions were obtained in the hexagonal NaYF4:20%Yb3+, 0.5%Tm3+ MCs. The 5-photon UC emissions from the 1I6 level of Tm3+ ions were much stronger than the 4-photon UC emissions from the 1D2 level and the 3-photon UC emissions from the 1G4 level. The enhancement of UV UC emissions was attributed to higher crystallization degree and less luminescence quenching centers.The selective synthesis of cubic and hexagonal NaYF4 crystals was successfully performed by a facile citric acid assisted hydrothermal method. The crystal phase conversion was observed through tuning the added amount of fluoride. A possible growth mechanism was proposed for the formation of hexagonal NaYF4 microcrystals (MCs). Under 980 nm excitation, intense ultraviolet (UV), blue, and weak violet upconversion (UC) emissions were obtained in the hexagonal NaYF4:20%Yb3+,0.5%Tm3+ MCs. The 5-photon UC emissions from the 1I6 level of Tm3+ ions were much stronger than the 4-photon UC emissions from the 1D2 level and the 3-photon UC emissions from the 1G4 level. The enhancement of UV UC emissions was attributed to higher crystallization degree and less luminescence quenching centers.

Single-crystal tetragonal α-MnO2 nanorods with different amounts of gold nanoparticles (NPs) attached were successfully prepared by a facile sputtering deposition technique. Initially, the morphology and crystal structure of the bare α-MnO2 nanorods synthesized via a hydrothermal approach were investigated. Then, the amount of gold NPs at different sputtering times was analyzed. It was confirmed that the amount of the decorated gold NPs increased with the lengthening of the sputtering time until they completely covered the α-MnO2 nanorods. Theoretical calculation results indicated the advantages of the composite structure by showing the enhanced electromagnetic fields around both the bare α-MnO2 nanorods and the gold NP decorated ones. The surface-enhanced Raman scattering (SERS) efficiency of these nanocomposites was evaluated using methylene blue and 4-mercaptobenzoic acid as Raman probe molecules. It was found that the SERS intensity of the substrates strongly depended on the degree of aggregation of the gold NPs. Uniform SERS signals across the entire surface of these samples were obtained. Moreover, a typical chemical toxin, methyl parathion, was effectively detected over a broad concentration range from 1 × 10−3 to 100 ppm using the gold NP decorated α-MnO2 nanorods, suggesting this hybrid structure is highly valuable for further applications on the rapid detection of organic environmental pollutants.

Hollow sea-urchin gold nanoparticles (HSU-GNPs) were successfully prepared through a novel one-step galvanic replacement strategy, and their corresponding optical properties was studied in detail. During the synthesis process, the sizes of the interior hollows of the HSU-GNPs could be changed by adjusting the amount of silver nitrate added into hydrogen tetrachloroaurate trihydrate solution. The absorption spectra of the HSU-GNPs showed that the localized surface plasmon resonance (LSPR) peaks were red-shifted with increasing size of the interior hollows in the HSU-GNPs. When the added amount of silver nitrate was up to 6 μl, the LSPR peak of the synthesized HSU-GNP reached 726 nm as a maximum red-shift. Furthermore, the absorption spectra of the HSU-GNPs with different morphologies were theoretically simulated by the finite element method, which was consistent with the experimental results and explained the origin of the red-shift of the LSPR peaks. In addition, the surface-enhanced Raman scattering (SERS) of the sea urchin gold nanoparticles were also investigated using 4-mercaptobenzoic acid as a Raman reporter molecule. Both the experimental and calculated results showed that the HSU-GNPs had stronger SERS enhancement than the solid sea-urchin gold nanoparticles. In particular, the HSU-GNPs prepared by adding 6 μl silver nitrate exhibited a maximum SERS enhancement factor, EF = 1.1 × 109, due to the LSPR peak at 726 nm which is near to the excitation wavelength, 785 nm. This feature is significant for designing a biosensor with a super-high sensitivity based on the morphology of the HSU-GNPs.

Well-shaped Ag@Au hexagonal nanorings (Ag@Au HNRs) were successfully synthesized with the assistance of Ag hexagonal nanoplates as structure-directing templates in a galvanic replacement route, and their optical characteristics were studied in detail. During the synthetic process, the morphology of the Ag@Au HNRs was found to be strongly depended on the dosage of HAuCl4 added into the template solution. Both the experimental and calculated absorption spectra showed that the red-shift of localized surface plasmon resonance (LSPR) peaks of the Ag@Au HNRs obeyed a modified Lorentz function with increasing dosage of HAuCl4 solution, which was also explained by the plasmon hybridization of the Ag@Au HNRs. Moreover, the surface-enhanced Raman scattering (SERS) characteristics of Ag@Au HNRs were investigated by using 4-mercaptobenzoic acid (4MBA) as a Raman reporter molecule. And the as-prepared Ag@Au HNRs exhibit excellent SERS performances with an enhancement factor ∼106, especially for the Ag@Au HNR prepared at 160 μl HAuCl4 solution due to its strongest plasmon electric-field enhancement. It reveals that the as-synthesized Ag@Au HNRs possess great potential for the application in ultrasensitive biosensors.

Novel Au@Ag core–shell nanocubes (NCs) were successfully prepared by the controlled epitaxial growth of Ag shells onto Au nanoellipsoids (NEs) in the presence of surfactants. The growth mechanism of the Au@Ag core–shell NCs was systematically investigated by analyzing their morphology, optical properties, and crystallography. The localized surface plasmon resonance (LSPR) characteristics and the electric field distribution of the Au@Ag core–shell NCs were studied using the finite element method (FEM) based on the plasmon hybridization theory. Compared with pure Ag NCs, the absorption spectrum of the Au@Ag core–shell NCs exhibits a red shift and a weak shoulder near 550 nm, and the notable enhancement of electric field occurs around the corners along the long-axis of the Au ellipsoidal core because of plasmonic resonant coupling. Surface-enhanced Raman scattering (SERS) of the Au@Ag core–shell NCs labeled with 4-mercaptobenzoic acid molecules reveals that the bimetallic core–shell NCs possess efficient SERS activity with an enhancement factor EF = 2.27 × 106, thus confirming the possibility of using the Au@Ag core–shell NCs as a stable probe for SERS-based biosensing applications.

A new nanostructure, silica-coated Ag nanorods (NRs) aggregate with 4-mercaptobenzoic acid molecules (4MBA-Ag NRs@SiO2), was prepared by a seed-mediated growth method and a modified Stöber method. In the synthetic process, the optimal 4MBA-Ag NRs@SiO2 was obtained by varying conditions such as surfactant concentration and centrifugation cycles. The morphologies and the optical properties of the 4MBA-Ag NRs@SiO2 were investigated in detail by transmission electron microscope, UV-Vis spectrometer and Raman spectrometer. The experimental results show that the 4MBA-Ag NRs@SiO2 prepared with 0.05 M CTAB has the aggregate morphology of single-crystalline Ag NRs, typical localized surface plasmon resonance (LSPR) characteristics and a high enhancement factor of surface-enhanced Raman scattering (SERS). Based on the SERS activity of 4MBA-Ag NRs@SiO2 and Ag NRs, an immune probe and an immune substrate immobilized with anti-prostate specific antigen (anti-PSA) antibody have been fabricated and used to construct a sandwich structure for a PSA immunoassay; the detection limit of PSA reached 0.3 fg ml−1. It is demonstrated that the as-prepared 4MBA-Ag NRs@SiO2 has great potential in biosensing applications.