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This review systematically summarizes the preparation strategies of gold nanocluster composites, analyzes their properties, and discusses their applications in sensing, bioimaging, and catalysis.

In this work, a tyrosine-decorated gold nanocluster (Tyr-GNC)–cerium(III) complex was established for the fluorescent detection of dopamine (DA). Tris–HCl buffer was used for synthesizing Tyr-GNC; ...

Plasmonic core-satellite nanostructures assembled from simple building blocks have attracted extensive attention since they were reported by the way of DNA-directed assembly in 1998, because of their unique enhanced and synergistic optical properties and widespread potential applications in biosensing, imaging, drug delivery, and diagnostics. In this review, we introduce the synthetic methods of core-satellite nanostructures, emphazising the bottom-up synthesis method, including DNA, molecular, protein, peptide, amino acids, metal ion-assisted assembly, electrostatic adsorption assembly, clicked-to-assembly, and in situ deposition. Than we review and discuss their morphology classification, and summarize influencing factors of morphology. This is followed by overviews on optical properties, including localized surface plasmon resonance, surface-enhanced Raman scattering, surface-enhanced fluorescence and quenching, and applications in the biomedical field. Finally, the challenges and prospects of these kinds of nanostructures are discussed.

Since the colorimetric method has the characteristics of being simple and low cost, the fluorescence spectrum has the characteristics of a strong signal, and Surface-enhanced Raman scattering (SERS) detection has the characteristics of high sensitivity and strong specificity, people usually use these three methods for detection, but the detection of a single sample takes more time. If multiple samples can be tested at the same time, the detection efficiency and sensitivity can be improved, and the selectivity and reliability will be greatly improved. Multiplex sensing also provides a new direction for researchers. To fully understand the research of multiplex sensing based on the plasmonic optics of noble metal nanostructures, this review summarizes all the results previously reported in this field. It also discusses the principles of various detection methods and the biochemical application of multiple detections and finally summarizes the challenges and prospects.

In this paper, the fluorescence quenching characteristics of Au-Ag-Pt core-shell nanorods have been studied.Due to nonradiative energy transformation, the fluorescence emission intensity of bovine serum albumin (BSA) could be greatly quenched.It has been found that the quenching effect of Au-Ag-Pt core-shell nanorods could be optimized by adjusting the concentration of chloroplatinic acid.Based on the fluorescence quenching properties of Au-Ag-Pt core-shell nanorods, Au-Ag-Pt trimetal fluorescence quenching nanoprobe has been prepared, and the specificity of alpha-fetoprotein (AFP) detection has also been realized.In order to guarantee the sensing specificity, the surface modification including carboxyl replacement, carboxyl activation and antibody connection have been performed on Au-Ag-Pt core-shell nanorods.By using the principle of specific combination of antigen and antibody, the specific detection of AFP has been realized with a lower detection limit of 4.0 pg/mL, and the linear detection range spans a scope from 0.03 to 0.5 ng/mL.Interference experiments and the actual samples detection results show that the Au-Ag-Pt trimetal core-shell nanorod probes have good anti-interference and repeatability.

The localized surface plasmon resonance (LSPR) properties of Pt-coated Au nanoellipsoid have been studied using the quasi-static calculation. Because of the Pt coating, both the transverse and longitudinal LSPR absorption peaks (denoted as AuT and AuL) corresponding to Au-Pt interface of Au nanoellipsoid get intense greatly in the visible and infrared region. Due to the longer relaxation time and higher conduction electron density, two new LSPR absorption peaks corresponding to transverse and longitudinal resonances of outer Pt shell surface (denoted as PtT and PtL) take place in the ultraviolet region. The intensity and wavelength of these four LSPR peaks are sensitive to the Pt coating thickness, and could be further fine tuned by changing the aspect ratio of inner Au nanoellipsoid and environmental dielectric constant. Thus we can obtain four intense LSPR peaks from ultraviolet to near infrared wavelength region. It is interested to find that the AuT and AuL peaks always get intense as the Pt thickness is increased. However, only on condition that environment has a small dielectric constant, the PtT and PtL peaks get intense as the Pt thickness is increased. The PtT and PtL peaks always red shift as the Pt thickness is increased. However, only on condition that environment has a large dielectric constant, the AuT and AuL peaks red shift as the Pt thickness is increased. By increasing the aspect ratio, the four LSPR peaks could be separated from each other and distribute over a wider wavelength range. These calculation results provide well potential application for multichannel plasmonic sensing.

Plasmonic light absorption properties of bimetallic Au–Pt core–shell nanorod are investigated theoretically. The plasmonic absorption intensity comparison between the longitudinal peak corresponding to outer Pt surface (denoted as band $$\hbox {Pt}_{\mathrm {L}})$$ and the transverse peak corresponding to Au–Pt interface (denoted as band $$\hbox {Au}_{\mathrm {T}})$$ is sensitive to the environmental dielectric constant. By increasing the environmental dielectric constant, the band $$\hbox {Pt}_{\mathrm {L}}$$ fades down, whereas the band $$\hbox {Au}_{\mathrm {T}}$$ gets intense. So the absorption discrepancy between $$\hbox {Pt}_{\mathrm {L}}$$ and $$\hbox {Au}_{\mathrm {T}}$$ bands could be greatly enhanced by increasing the environmental dielectric constant, which is more sensitive to the environmental refractive index than single plasmonic band. This refractive index sensing based on two bands’ absorption discrepancy could be further improved by increasing the Pt coating thickness or the aspect ratio of inner Au nanorod. The refractive index sensing based on red shift of the longitudinal peak corresponding to Au–Pt interface is also competitive, which can also be improved by increasing the Pt coating thickness or aspect ratio of inner Au nanorod. A mechanism based on media polarization-related electric field discontinuity and distribution of surface charge density was investigated to illuminate the absorption intensity-dependent refractive index sensing. In this Au–Pt core–shell nanorod, the $$\hbox {Pt}_{\mathrm {L}}$$ band fades down, whereas the $$\hbox {Au}_{\mathrm {T}}$$ band gets intense as the environmental dielectric constant is increased. So the absorption discrepancy between $$\hbox {Pt}_{\mathrm {L}}$$ and $$\hbox {Au}_{\mathrm {T}}$$ bands could be greatly enhanced by increasing the environmental dielectric constant, which is more sensitive to the environmental refractive index than single plasmonic band.

Ag triangular nanoplates have been widely used as the SERS substrate due to their anisotropic morphology and sharp corners. In this work, SERS activity of Ag triangular nanoplates on glass and paper substrates was investigated with different concentration and size. A tiny triangular nuclei based synthesis method was employed to produce Ag triangular nanoplates with different size. And the extinction coefficients of Ag triangular nanoplates with different size were quantified by inductively coupled plasma mass spectrometry (ICP-MS) and transmission electron microscopy (TEM). With definite concentration, Ag triangular nanoplates with small size showed stronger SERS intensity on glass substrate at high concentrations, while large Ag triangular nanoplates exhibited better SERS effect on paper substrate at low concentrations. The possible reasons for distinct SERS activity of Ag triangular nanoplates on glass and paper were discussed with scanning electron microscope (SEM) characterization. These results exhibited important implications for fabricating Raman substrates using Ag triangular nanoplates and could be applied to the SERS sensing with flexile substrate.

Noble metal nanostructures exhibit strong surface-enhanced Raman scattering (SERS) activity and attract much attention due to their enhancement of electromagnetic field that is induced by localized surface plasmon resonance (LSPR). It is well known that fabrication of gold nanoparticles with sharp corners or deposition of Ag on gold nanoparticles is the effective way to improve the SERS activity. In this paper, we investigated the SERS performance of gold nanorods (Au NRs) and gold nanobipyramids (Au NBs). Compared with Au NRs, Au NBs had two pentagonal pyramids of which the apexes were much sharper than those of Au NRs and hence possessed greater SERS activity due to the lightning-rod effect. In addition, Au NRs-Ag core-shell nanoparticles (Au NRs@Ag nanoparticles) and Au NBs-Ag core-shell nanoparticles (Au NBs@Ag nanoparticles) were successfully synthesized and their SERS activities were compared. It was observed that the Ag deposition processes were different between Au NRs and Au NBs. During the Ag deposition process, Au NRs gradually changed from nanorods to irregular morphologies (spherical, triangular pyramid, octahedron and so on); the shape of Au NBs changed from bipyramids to rice and adopted a final shape of cylindrical rod. Interestingly, the Au NRs@Ag nanoparticles had higher SERS activity than the corresponding Au NBs@Ag nanoparticles. The mechanism could be attributed to the Ag depositing-induced different shell shapes and alignment fashions of the nanoparticles.

In this work, the novel tetrapod gold nanostars with four specific numbers of sharp tips were synthesized with good uniformity, high yield and chemical stability. The size of tetrapod gold nanostars has been increased effectively by increasing the amount of HAuCl4 or DEA added to the reaction solution, and the corresponding in-plane localized surface plasmon resonance (LSPR) band can shift from 700 to 1100 nm. In order to combine the advantages of different metals, the Ag-coated tetrapod gold nanostars were prepared with different Ag coating thicknesses by increasing the volume of AgNO3. With the increase of Ag coating thickness, the morphology of the nanoparticles has changed from gold nanostars with four sharp tips to bimetallic cuboids with four sharp tips and finally to crore-shell cuboids completely. The surface-enhanced Raman scattering (SERS) activity of Ag-coated tetrapod gold nanostars has also been improved with the different Ag coating thicknesses. Combined the tip effect and the enhancement effect of Ag, the SERS activity has the strongest enhancement when the four sharp tips of tetrapod gold nanostars are not completely coated by the Ag layer but still exposed to the outsides. For tetrapod gold nanostars with larger size to grow Ag-coated tetrapod gold nanostars, the bimetallic cuboids with four sharp tips nanostructure have the biggest SERS enhancement factor (EF) of 1.73 × 106 for Rhodamine 6G (R6G), making it an ideal candidate in SERS-based sensing application.

The identification and detection of cancer biomarkers in early stages is an important issue for the therapy of cancer. However, most methods are time-consuming and have limited sensing sensitivity and specificity. In this work, we prepared a novel plasmonic multilayered core-shell-satellite nanostructure (Au@Ag@SiO2-AuNP) consisting of a gold nanosphere with a silver coating core (Au@Ag), an ultrathin continuous silica (SiO2) shell, and a high coverage of gold nanosphere (AuNP) satellites. The Au@Ag core is a prominent surface enhanced Raman scattering (SERS) platform, and the thin SiO2 layer exhibits a long-range plasmon coupling between the Au@Ag core to the AuNP satellites, further leading to enhanced Raman scattering. Meanwhile, the outer AuNP satellites have a high biocompatibility and long-term stability. Combining the above advantages, the well-designed metallic nanoassemblies would be a promising candidate for SERS-based applications in biochemistry. For specific detection of alpha-fetoprotein (AFP), we utilized the SERS-active core-shell-satellite nanostructures modified with AFP antibody as immune probes and nitrocellulose membrane (NC) stabilized captured anti-AFP antibodies as solid substrate. To improve the detection performance, we further systematically optimized the parameters, including the silver coating thickness of the Au@Ag core and the density and size of the satellite AuNPs. Under the optimized conditions, AFP could be detected by the SERS-based sandwich immunoassay with an ultralow detection limit of 0.3 fg/mL, and the method exhibited a wide linear response from 1 fg/mL to 1 ng/mL. The limit of detection (LOD) was considerably lower than conventional methods in the literature. This work relies on the unique Au@Ag@SiO2-AuNP nanostructures as the immune probe develops a new outlook for the application of multilayered nanoassemblies and demonstrates the great potential in early tumor marker detection.

In this work, we prepared graphene oxide (GO)-decorated silver-coated gold nanobones (AuNBs@Ag@GO) for obtaining optimal surface-enhanced Raman scattering (SERS) activity. 4-Mercaptobenzoic acid (4-MBA) was adsorbed on the interface between gold and silver to provide more space for GO decoration on the outer surface of silver. Silver was deposited on the middle and end surfaces of the gold nanobones (AuNBs) with negative curvature, which resulted in stubby bimetallic nanocuboids inducing transverse surface plasmon resonance (SPR); the intermediate SPR merged into a new plasmon resonance. The SERS activity of silver-coated gold nanobones (AuNBs@Ag) changed non-monotonically with an increase in the silver coating, which reached up to 8 times larger than that of bare AuNBs. This is better performance than that of silver-coated gold nanorods (AuNRs@Ag). GO hybridized with AuNBs@Ag by electrostatic adherence could further improve the SERS activity of AuNBs@Ag, and the concentration of GO was adjusted for the best SERS performance, which was 4 times higher than that of naked AuNBs@Ag. AuNBs@Ag@GO can be used as a SERS substrate to detect glucose in urine. The mechanism is that the higher concentration of liquid glucose induced a large density of coverage, forming a glucose partition layer on the AuNBs@Ag@GO substrate: with an increase in the glucose concentration, the thickness of the glucose partition layer increased. This reduced the intensity of incident light and resulted in a decrease in the Raman signal of 4-MBA. Moreover, the strong SERS activity of the AuNBs@Ag@GO substrate can provide a wider space to achieve a lower limit of detection (LOD) value for glucose. The intensity of the characteristic Raman peak at 1585 cm−1 linearly decreased with an increase in the glucose concentration in the range of 10–107 nmol L−1 with a low LOD value (S/N = 3) of 2.61 nmol L−1. At the same time, it is confirmed that the SERS substrate can be selective and reliable and shows promising potential and broad applications in glucose detection.

A surface-enhanced Raman scattering (SERS)-based sandwich type immunoassay was developed for the detection of α-fetoprotein (AFP) with high sensitivity, reproducibility and selectivity. In the immunoassay, an antibody immobilized nitrocellulose (NC) membrane was employed to capture AFP and antibody conjugated silica coated gold/silver core–shell nanostars (AuNS@Ag@SiO2) were used as SERS probes. AuNS@Ag was synthesized by the direct deposition of Ag on the AuNS. The Ag shell grew outward from the center towards the tips and adopted a polyhedral shape. The AuNS@Ag nanoparticles were shown to have higher SERS activity than gold nanostars (AuNSs), and their LSPR wavelengths could be finely tuned across a wide spectrum ranging from visible to near-infrared by simply adjusting the Ag shell thickness. The silica coating improved colloidal stability of the nanostructure and provided functionality for antibody conjugation. By using 4-mercaptobenzoic acid (4-MBA) as a Raman reporter molecule, the proposed SERS sensor is able to detect AFP with a low limit of detection (LOD) of 0.72 pg mL−1 and a wide and clinically relevant linear detection range from 3 pg mL−1 to 3 μg mL−1. Hence, this SERS-based immunoassay provides a strategy for the detection of trace AFP and has potential to assist in the monitoring of prognosis for hepatocellular carcinoma (HCC) patients.

In this work, we develop a simple and selective sensing method for the detection of mercury ions based on surface plasmon resonance (SPR) spectrum change of Au-Ag core-shell triangular nanoplates. When the concentration of mercury is increased, the etching-induced change of particle size and shape also leads to the decrease of the absorption peak at the fixed wavelength, until a spectrum dip takes place. This spectral change of “peak-to-dip” greatly enlarges the detection range of mercury ions, which could be fine tuned by changing the initial thickness of the Ag coating. Under optimal conditions, the decrease of the logarithmic absorption intensity has a good linear response with the concentration of mercury ions increasing from 10 to 1000 μM, and the limit of detection (LOD) is 0.88 μM. Interference studies and real samples test indicate that, this new sensing method has a good selection for mercury ions and can be practically used in lake water. This work shows the surface etching-induced SPR shift can also leads to the intensity change with “peak-to-dip” fashion, which greatly enlarge the concentration range of the detection and could be widely applied in the spectroscopy sensing based on SPR.

By using gold nanorods with silver coating as the sacrificial templates, we prepared spiky yolk-shell AuAg bimetallic nanorods with uniform interior gap via galvanic replacement reaction. The length and number of Au tips of the spiky yolk-shell AuAg nanorods can be tuned simultaneously by altering HAuCl4 volume. The influence of HAuCl4 volume and the sliver layer thickness on the SERS activity of spiky yolk-shell AuAg nanorods are studied. When the sliver layer is thin, the interior gap has not been shielded completely and the outer shell has obvious tips, thus the surface-enhanced Raman scattering (SERS) activity has the strongest enhancement with an enhancement factor (EF) of 4.9 × 105. The spiky yolk-shell AuAg nanorods with the strongest SERS activity are used as SERS substrates to detect thiram. The results demonstrate that the SERS intensity increases linearly with the logarithmic concentration of thiram in the range of 10−3 M to 10−7 M. The detection limit is as low as 97 nM, which is lower than the maximum pesticide residue limit (29 µM) in fruits stipulated by the US Environmental Protection Agency (EPA). Therefore, the spiky yolk-shell AuAg bimetallic nanorods have important practical application value in pesticide detection.

In this paper, the optical properties and local electric field distribution of transverse plasmon mode of a single pentagonal gold nanotube are studied for the first time by the discrete dipole approximation (DDA). The theoretical simulation results of pentagonal gold nanotube are closer to the actual experiment compared with circular section. We find that the transverse plasmon peaks can be exponentially red shift from visible to infrared region via controlling the inner diameter. In addition, the transverse plasmon peak firstly blue shifts and then red shifts in the visible region with the increase of outer diameter. Further analysis shows that the spectra red shift with the increase of outer diameters when scattering is dominant. Local electric field analysis reveals that thin tube wall is easier to excite higher-order plasmons modes. Localized surface plasmon resonance (LSPR) peaks of gold nanotube mainly come from dipole resonance and quadrupole resonance. The surface charges of inner and outer tube walls are changed by tuning the inner diameter and outer diameter parameters of pentagonal gold nanotube. The selective controlling transverse plasmon spectra of gold nanotube are realized, which is of great significance to the study of optical properties of gold nanotube and the application of molecular detection and biological imaging.

In this paper, the longitudinal plasmon mode optical properties and localized electric field distribution of a single pentagonal gold nanotube are investigated for the first time by the discrete dipole approximation. It is found that pentagonal gold nanotube has stronger electric field distribution compared with circular gold nanotubes when the incident wavelength is located at the plasmon resonance peak. Additionally, we observed that the longitudinal plasmon resonance peak can blue shift nonlinearly with increasing wall thickness, but red shifts linearly with the increase of the length of the pentagonal gold nanotube. The localized electric field analysis reveals that the longitudinal plasmon peak of the pentagonal gold nanotube originates from the dipole resonance mode. The local electric field intensity is controlled by the wall thickness and length. Notably, the effect of wall thickness on the longitudinal plasmon resonance and electric field enhancement can be attributed to the change of the plasmon coupling position and intensity. This work has enriched the theoretical research of pentagonal gold nanotubes and provided ideas for the preparation of high sensitivity nanoprobes biosensors.

This review (with 106 references) summarizes the latest progress in the synthesis, properties and biomedical applications of gold nanotubes (AuNTs). Following an introduction into the field, a first large section covers two popular AuNTs synthesis methods. The hard template method introduces anodic alumina oxide template (AAO) and track-etched membranes (TeMs), while the sacrificial template method based on galvanic replacement introduces bimetallic, trimetallic AuNTs and AuNT-semiconductor hybrid materials. Then, the factors affecting the morphology of AuNTs are discussed. The next section covers their unique surface plasmon resonance (SPR), surface-enhanced Raman scattering (SERS) and their catalytic properties. This is followed by overviews on the applications of AuNTs in biosensors, protein transportation, photothermal therapy and imaging. Several tables are presented that give an overview on the wealth of synthetic methods, morphology factors and biological application. A concluding section summarizes the current status, addresses current challenges and gives an outlook on potential applications of AuNTs in biochemical detection and drug delivery.Graphical abstract.

Colorimetric sensing methods based on non-spherically symmetric gold (Au) nanoparticles have become a powerful tool in the field of biomedical detection due to their intriguing plasmonic properties. In this study, Au nanobipyramids (Au NBPs) were used as colorimetric sensing probes to detect ferrous ions (Fe2+) through tip etching. The quick etching of Au NBPs along the longitudinal direction by superoxide radicals generated by the reaction of Fe2+ and H2O2 led local surface plasmon resonance (LSPR) to blue shift and produced vivid color change that could be used for visual inspection. Under the optimal reaction conditions, the peak shift of the Au NBPs and the logarithm of the concentrations of Fe2+ had a linear relationship in the range of 10 nM to 10 μM, with a very low detection limit of 1.29 nM. During the etching process, a different end shape of the Au nanoparticles results in a different process for the morphology transition, which makes the degree of spectral change and detection sensitivity significantly different. In the presence of trace amounts of Fe2+ (

Au@AuAg yolk-shell nanorods with tunable and uniform interior gap were synthesized through galvanic replacement reaction, where Au@Ag core-shell nanorods served as sacrificial templates and HAuCl4 solution served as reductant. The effects of HAuCl4, Ag shell thickness and aspect ratio (AR) of Au nanorods on the morphology of Au@AuAg yolk-shell nanorods had been investigated systemically. The results clearly indicated that AuAg alloy shell thickness of Au@AuAg yolk-shell nanorods could be increased from 3.6 to 10.0 nm by varying the amount of HAuCl4. Meanwhile, the shape of AuAg alloy shell could be tuned by changing the shape of Ag coating. With the increasing of Ag coating thickness, the interior gap could be finely tuned in the range from 2.6 to 8.1 nm. The uniformity of interior gap could be improved by increasing the AR of Au nanorods. All these tunable geometries can further affect the plasmonic spectral properties of Au@AuAg yolk-shell nanorods. Because of the appearance of interior gap, the longitudinal localized surface plasmon resonance (LSPR) peak of Au@AuAg yolk-shell nanorods was located between that of bare Au nanorods and Au@Ag core-shell nanorods without interior gap. The increase of outer AuAg shell thickness can weaken the coupling between the inner and outer surface of the AuAg shell and lead to the decrease of AR, so the transverse and longitudinal LSPR bands gather together. The decrease of Ag coating thickness can enhance the coupling between inner Au nanorod and outer AuAg shell, which results in the red shift of the longitudinal LSPR band. This paper provides a method for studying the plasmonic coupling between two metal surfaces with a metal layer or a dielectric layer, which is also a new approach for regulating the plasmonic spectral properties of bimetallic nanoparticles. The controllability of Au@AuAg yolk-shell nanorods in both the interior gap and outer alloy shells makes them have potential applications in biomedicine, catalysis, nanoreactors, and energy storage.

The refractive index sensitivity of Au nanorod with Pt coating has been studied theoretically. Calculation results based on quasi-static approximation show that Au–Pt core–shell nanorod exhibits higher refractive index sensing ability than bare Au nanorod without Pt coating. The refractive index sensitivity could be improved by increasing the aspect ratio of the inner Au nanorod or increasing the Pt coating thickness, and the maximum sensitivity can exceed 561 nm per refractive index unit. Because of the ultra-narrow plasmonic bandwidth , the Au–Pt core–shell nanorod also has large value of figure of merit. By increasing the Pt coating thickness or decreasing the aspect ratio, the figure of merit could be effectively improved to 7.45. The physical origin has also been investigated based on the geometry-dependent relaxation time and density of surface charges. These excellent refractive index sensing performances of Au–Pt core–shell nanorods present well potential for plasmonic sensing applications.

The super-multibranched Au Ag bimetallic nanostructures are synthesized due to the aggregation of Au nanostars in the process of silver coating. The super-multibranched bimetallic nanostructures with different silver coating thickness are obtained by changing the concentration of silver nitrate and ascorbic acid. It has been found that the formation of these nanostructures is due to the stacking of several nanostars during the process of silver coating. By comparing the silver coating process of gold nanostars with different branch lengths, we found that the nanostars with longish branches are easy to aggregate and form the super-multibranched nanostructures in the process of silver coating. In the Au Ag bimetallic nanostructures, the silver layer is mainly covered on the surface of the cores and the thickness increases with the increasing of the AgNO3, which leads to the change of the surface-enhanced Raman scattering (SERS) activity. It has been found that the SERS activity is stronger when the silver layer is thin and the Au branches are still exposed to the outside of the Ag shell. The sample with the strongest SERS activity has been used to detect thiram with different concentrations. The Raman intensity increases linearly with the logarithmic concentration of thiram ranging from 10−3 to 10−7 M with a detection limit of 6.3 × 10−7 M. These experimental results show that the super-multibranched bimetallic nanostructures have a broad application prospect in molecular detection and biologic sensing based on SERS.

The surface-enhanced Raman signals of 4-Aminobenzenethiol (4-ABT) adsorbed on the surface of triangular Au-AuAg hierarchical-multishell nanostructure have been investigated. Here, the approach to produce core-cavity-shell sandwich nanostructures presented as Au-AuAg is the same as preparing metal nanoparticles with hollow morphology, in which the galvanic replacement reaction takes place between silver and chloroauric acid. In this paper, we directly mix 4-ABT with gold nanoparticles and drop it on glass slides to study the effect of nanoparticles on signal enhancement of Raman spectrum, avoiding the cumbersome process of preparing metal-molecular-metal three-layer structure as reported. A significant increase in the SERS intensity of b2 mode around 1140 cm−1 was observed, which could quantify the concentration of 4-ABT indirectly. In a certain range, the Raman intensity gradually increases with the increasing intermediate gap, which has a strong relationship with dipole plasmon hybridization of core-dielectric-shell sandwich nanostructure. Moreover, Raman spectrum results show that the Au-AuAg substrate can produce signal intensity about 3.8 × 102 times stronger than that of 4-ABT alone and the detection limit was as low as 0.1 μM in solution.

In this work, a combined seed mediated and galvanic replacement strategy has been proposed to design and fabricate two unique nanostructures, including Au@AuAg yolk-shell nanostructures with tunable interior gap and Au multi-spiky nanostructures with tunable spike length. The interior gap of the Au@AuAg yolk-shell nanostructures can be controlled by varying either the amount of AgNO3 in the first step or the amount of HAuCl4 in the second step. The SERS activity of Au@AuAg yolk-shell nanostructures was found to increase with increase in the interior gap. Owing to the build-in electromagnetic hot spots induced by plasmonic coupling effect of the Au triangular nanoplate core and AuAg shell especially the intensive coupling between the three sharp tips of the core and the concave of the inner wall of the shell, Au@AuAg yolk-shell nanostructures exhibit significantly higher SERS enhancement compared to their original Au triangular nanoplates. The best enhancement has been achieved when the interior gap of Au@AuAg yolk-shell nanostructures is 10 nm with the enhancement factor of 3.2 × 106 for rhodamine 6G, making them promising candidates for SERS-based detection.

A simple and rapid colorimetric approach for Hg2+ detection is achieved by etching silver-coated gold nanobipyramids (Au NBs@Ag nanoparticles) based on the redox reaction between Hg2+ and silver nanoshell of the Au NBs@Ag nanoparticles. The decrease of the silver nanoshell thickness leads to first blue shift and then red shift of the longitudinal localized surface plasmon resonance (LSPR) peak and accompanies a vivid color change. The blue shift is more suitable for colorimetric detection which has the obvious multicolor change from yellow-green to green and then to blue-gray; and could be employed to quantitatively detect the concentrations of Hg2+. This nanoprobe with the longitudinal LSPR peak at 730 nm exhibits a linear range of 0.1–20 μM with a limit of detection (LOD) of 22 nM based on blue shift and has the most obvious color change with the lowest LOD of 0.8 μM by direct observation. In this case, the Au NBs@Ag nanoparticles are gradually etched from the nanorods to nanorices as the Hg2+ concentration increases. Compared with other techniques, this method could avoid tedious procedures and is time-saving. Furthermore, this nanoprobe has good selectivity, and is well demonstrated in real samples with satisfied results.

In this work, the effect of core size on the fluorescence emission properties of CdTe@CdS core@shell quantum dots was investigated under the condition of different shell thickness and pH values. Both CdTe and CdTe@CdS core@shell QDs were synthesized in aqueous phase by a simple method, whose optical features were characterized by absorption and fluorescence spectra. High resolution transmission electron microscopy (HRTEM) and X-Ray Diffraction (XRD) were utilized to study the structure and composition of the as-prepared QDs. We can conclude that a smaller initial core exhibits a greater potential for optimizing both fluorescence intensity enhancement and red shift. What's more, there is an optimal shell thickness for the strongest fluorescence intensity while a thicker shell contributes to a larger fluorescence red shift. The optimal pH value is 10.5 for the fluorescence intensity enhancement of the core@shell QDs and a higher pH value is an advantage to expedite the growth rate of CdS shell.

Surface-enhanced Raman scattering (SERS) has been widely used in biomedical sensing with the advantages of high sensitivity and label-free. However, the fabrication of SERS substrates with good Raman activity, reproducibility, and low cost is still under development in practical applications. This paper presents a practicable method for fabricating Au NP/paper strips by using inkjet printing and seed mediated growth. Small gold seed synthesized by borohydride reduction was used as ink and printed on the filter paper. The printed gold seed grew in situ in the growth solution and formed the gold nanoparticle (Au NP)/paper strips. The fabricated paper strip was characterized by diffuse reflectance spectroscopy and scanning electron microscopy (SEM). The diffuse reflectance spectra indicated that the Au NP/paper strips had two local surface plasmon resonance (LSPR) peaks: the short one at around 540 nm and the long one located in the range of 640–840 nm. And the long LSPR peak firstly shifted to red then to blue with the increased concentrations of silver ions in growth solution. From the SEM images, the shape of grown Au NPs was diverse, including sphere, rod, ellipsoid, dimer, trimer, and big aggregates. We thought the short peak came from the LSPR of nanospheres and the transvers LSPR of rod and ellipsoid like particles, while the long peak mainly came from the plasmonic coupling of dimer along the inter-particle axis. The obtained Au NP/paper strip with the long peak located around 650 nm had the highest SERS activity, which could be attributed to the plasmon resonance induced local field enhancement and nanogap effect. Also, the SERS performance results indicated the printed SERS strips exhibited satisfied uniformity and stability, demonstrating the potential of Au NP/paper strip in real-world applications.

In this report, a simple and effective approach for colorimetric detection of Hg2+ based on surface etching and aggregation effect of cysteine-modified Au-Ag core-shell nanorods has been investigated. When the addition of Hg2+ has a low concentration under 60 μM, electrostatic interaction-induced intense aggregation of colloidal Au-Ag core-shell nanorods takes place. Thus the longitudinal plasmonic absorption peak decreases rapidly, which also leads to the colloidal color become shallow. Whereas when the addition of Hg2+ has a high concentration greater than 60 μM, the adherent cysteine molecules break away from the surface of nanorods due to the intense Hg-S bond. Then the bare nanorods have been etched under the action of Hg2+. The decrease of the Ag shell results in the red shift of the longitudinal absorption peak, which further leads to the color change of the colloids. The sensing based on particle aggregation-induced absorption decrease has a linear response for Hg2+ from 1 to 60 μM with a theoretical detection limit of 0.273 μM. The sensing based on etching effect-induced red shift has a logarithmic response for Hg2+ from 60 to 250 μM with a theoretical detection limit of 1.065 μM. Interference test and real samples detection results show that Hg2+ could be specifically detected by using this probe based on Au-Ag core-shell nanorods.

This study demonstrates a novel strategy for colorimetric and surface enhanced Raman scattering (SERS) dual-mode sensing of mercury (Hg2+) based on rhodanine-stabilized gold nanobipyramids (Au NBs). The Au NBs are first modified by rhodanine through gold–thiol (Au–S) affinity interactions. Next, the addition of Hg2+ into the rhodanine-stabilized Au NBs induces the formation of a partition layer with tunable thickness on the surface of the Au NBs, resulting in the redshift of the longitudinal localized surface plasmonic resonance (LSPR) wavelength of the Au NBs, which further leads to the colloidal color changing from blue grey to red. The sensing based on the partition layer-induced absorption spectrum redshift of the longitudinal LSPR has a linear response for the concentration of Hg2+ from 5.0 × 10−7 to 6.0 × 10−5 M with a limit of detection (LOD) of 2.0 × 10−7 M measured by an absorption spectrometer. The LOD for Hg2+ is 2.0 × 10−6 M by the naked eye. Meanwhile, the partition layer on the surface of the Au NBs draws the Raman reporter molecule (4-mercaptobenzoic acid (4-MBA)) away from the surface of the Au NBs and decreases the LSPR phenomenon of the Au NBs, which leads to the SERS intensity of 4-MBA decreasing with the addition of Hg2+. The sensing based on the partition layer-induced SERS intensity decrease of 4-MBA has a linear response for the logarithm of Hg2+ concentration from 1.0 × 10−10 to 1.0 × 10−5 M with a LOD of 5.0 × 10−11 M. Therefore, the rhodanine-stabilized Au NBs can be used not only as a naked-eye sensor of Hg2+, but also as a highly selective SERS probe. Furthermore, other cations do not interfere with this dual-mode sensor, and the applicability of the sensor is well demonstrated in real samples with satisfactory results. Compared with the methods in the literature, which generally exploit the aggregation or etching of nanoparticles, this method depends on the formation of a partition layer on the surface of the nanoparticles and provides a new strategy for optical sensing that relies on the change in dielectric environment near the surface of the nanoparticles.

The effect of silicon-coated gold nanoparticles with different gold core diameter and silica shell thickness on the fluorescence emission of CdTe quantum dots (QDs) was investigated. For gold nanoparticles with a diameter of 15nm, silica coating can only results in fluorescence recover of the bare gold nanoparticle-induced quenching of QDs. However, when the size of gold nanoparticle is increased to 60nm, fluorescence enhancement of the QDs could be obtained by silica coating. Because of the isolation of the silica shell-reduced quenching effect and local electric field effect, the fluorescence of QDs gets intense firstly and then decreases. The maximum fluorescence enhancement takes place as the silica shell has a thickness of 30nm. This enhanced fluorescence from silicon-coated gold nanoparticles is demonstrated for sensing of Hg2+. Under optimal conditions, the enhanced fluorescence intensity decreases linearly with the concentration of Hg2+ ranging from 0 to 200ng/mL. The limit of detection for Hg2+ is 1.25ng/mL. Interference test and real samples detection indicate that the influence from other metal ions could be neglected, and the Hg2+ could be specifically detected.

We present a wavelength shift-based fluorescence detection method for ascorbic acid (AA) using CdTe quantum dots (QDs). Upon addition of ascorbic acid, the fluorescence emission peak of CdTe QDs red shifts linearly with the increase of the AA concentration, and the red shift speed could be improved by increasing the diameter of the QDs. The mechanism has been attributed to the AA-induced size change of the QDs. This AA-dependent red shift of fluorescence emission has been used to detect and quantify AA. Dynamic range and detection limit of this fluorescence probe are found to be 10–250 μmol/L and 1.3 μmol/L, respectively. Interference studies indicate that the AA could be specifically detected by using this wavelength shift-based fluorescence probe, and the QDs with larger diameter have better anti-interference performance. This fluorescence detection method is based on the wavelength shift of the emission peak, which is different from those reported traditional emission intensity-based fluorescent probes. Thus this fluorescence probe has the advantages of higher accuracy and convenient operation, which provides an effective strategy for design of wavelength shift-based fluorescence sensing method.

In this paper, we developed a simple, rapid, selective and colorimetric method to detect iodide (I−) by etching silver-coated gold nanobipyramids (Au NBs@Ag nanostructure) in the presence of Cu2+. The detection is based on the reaction that Cu2+ could oxidize I− into I2, which sequentially etches the Ag shell of Au NBs@Ag nanostructure. When the Ag coating is thick and the Au NBs are buried in Ag nanorods, the decrease of the Ag shell thickness and aspect ratio results in the blue-shift and intensity decrease of the longitudinal localized surface plasmon resonances (LSPR) peak. The blue-shift and intensity decrease of the longitudinal LSPR peak have been employed to quantificationally detect the concentration of I−. The linear range of the detection could be improved by increasing the concentration of Cu2+ and reaction time. Under the optimized conditions, both the blue-shift and intensity decrease of the longitudinal LSPR have linear correlation with the concentration of I− in the range of 1.0–15 μM. When the Ag coating is very thin, the etching of the Ag shell results in the red shift of the LSPR in the visible range, which further leads to the color of the colloid change from blue to pink. The etching-induced color change of the colloid also depends on the concentration of I−, which enables the visual detection of I−. Compared with other techniques, this method could facilitate multi-model quantitative detection of I− without any sophisticated instrument. Interference studies and real samples test show that I− could be specifically detected in dried kelp using this bimetallic optical nanoprobe.

Gold nanobipyramids (Au NBPs) are thought to have the same localized surface plasmon resonance (LSPR) peaks as gold nanorods but with narrow bandwidths in experimental spectra. However, the actual LSPR of Au NBPs is still confused because of an extra peak always appearing in simulated spectra without definite attribution. In this study, the plasmonic properties of Au NBPs were investigated based on the discrete dipole approximation (DDA). The simulated electric field enhancement confirmed that the extra LSPR peak came from the multipole plasmon resonance of Au NBPs. According to the polarized charges distribution, we defined the multipole plasmon resonance order of the extra LSPR peak as l = 5. Then, the effects of tip shape and size on the multipole plasmon resonance (l = 5) were investigated. The above results will help to understand multipole plasmon resonance in Au NBPs and design the nanobipyramids with fascinating properties.