Transformation of Ag nanocubes into Ag-Au hollow nanostructures with enriched Ag contents to improve SERS activity and chemical stability

Synthesis of Au@Ag core-shell nanocubes with finely tuned shell thicknesses for surface-enhanced Raman spectroscopic detection

In this work, we describe a facile method for generating monodisperse Au@Ag core-shell nanocubes with well-controlled size and fine-tuned Ag shell thicknesses. In this synthesis method, Au nanocubes were prepared via the seed-mediated growth method. Then, Au@Ag nanocubes with the core-shell structure were prepared separately by reducing AgNO3 with AA using as-prepared Au nanocubes as seeds. The thickness of Ag shells could be finely tuned from 3.6 nm to 10.0 nm by varying the concentration of the AgNO3 precursor. By investigating the localized surface plasmon resonance (LSPR) properties of Au@Ag nanocubes in relation to the thickness of the Ag shell, we found that the intensity of the characteristic peak of Ag gradually increases and that of Au gradually decreases as the thickness of the Ag shell increases. Additionally, surface-enhanced Raman scattering (SERS) properties of Au@Ag core-shell nanocubes were evaluated using rhodamine 6G (R6G) as the probe molecule. Interestingly, Au@Ag nanocubes exhibit efficient SERS intensities compared to the Au nanocubes, and Ag shell with a thickness of about 8.4 nm exhibits the optimal SERS activity. In addition, our results also demonstrated that Au@Ag nanocubes with an Ag shell thickness of 8.4 nm exhibited high SERS sensitivity and are capable of probing the analyte down to 10-12 M. The results obtained here suggest that Au@Ag core-shell nanocubes might serve as a nanoprobe for SERS-based analytical and biosensing applications.

Ratiometric SERS quantification of SO2 vapor based on Au@Ag-Au with Raman reporter as internal standard

Practical gas sensing application requires sensors to quantify target analytes with high sensitivity and reproducibility. However, conventional surface enhanced Raman scattering (SERS) sensor lacks reproducibility and quantification arising from variations of "hot spot" distribution and measurement conditions. Here, a ratio-dependent SERS sensor was developed for quantitative label-free gas sensing. Au@Ag-Au nanoparticles (NPs) were filtered onto anodic aluminum oxide (AAO) forming Au@Ag-Au@AAO SERS substrate. 4-MBA was encapsulated in the gap of Au@Ag-Au and served as the internal standard (IS) to calibrate SERS signal fluctuation for improved quantification ability. Combined with headspace sampling method, SO2 residue in traditional Chinese medicine (TCM) can be extracted and captured on the immediate vicinity of Au@Ag-Au surface. The intensity ratio I613 cm-1/I1078 cm-1 showed excellent linearity within the range of 0.5 mg/kg-500 mg/kg, demonstrating superior quantification performance for SO2 detection. Signals for concentration as low as 0.05 mg/kg of SO2 could be effectively collected, much lower than the strictest limit 10 mg/kg in Chinese Pharmacopoeia. Combined with a handheld Raman spectrometer, handy and quantitative TCM quality evaluation in aspect of SO2 residue was realized. This ratiometric SERS sensor functioned well in rapid on-site SO2 quantification, exhibiting excellent sensitivity and simple operability.

Octahedron in a Cubic Nanoframe: Strong Near-Field Focusing and Surface-Enhanced Raman Scattering

Here, we describe the synthesis of a plasmonic particle-in-a-frame architecture in which a solid Au octahedron is enclosed by a Au cubic nanoframe. The octahedra are positioned inside and surrounded by outer Au cubic nanoframes, creating intra-nanogaps within a single entity. Six sharp vertexes in the Au octahedra point toward the open (100) facets of the cubic nanoframes. This allows not only efficient interactions with the surroundings but also tip-enhanced electromagnetic near-field focusing at the sharp tips of the octahedra, combined with intraparticle coupling. The solid core-frame shell structure enhances near-field focusing, giving rise to a heightened concentration of "hot spots". This effect enables highly sensitive detection of 2-naphthalenethiol and thiram, indicating these substrates for use in surface-enhanced Raman spectroscopy-related applications.

Bimetallic AuNR@AgNCs for ultrasensitive surface-enhanced Raman scattering sensing of dithianon in apple juice

In this study, a bimetallic surfaced-enhanced Raman spectroscopy (SERS)-active substrate consisting of AuNR@AgNCs was proposed for the rapid detection of dithianon. Due to the significant synergistic enhancement of the core-shell nanocuboids, the obtained AuNR@AgNC substrate exhibited excellent SERS performance. The simulation findings supported the practical SERS results and demonstrated that interactions were mainly maintained by the nitrile functional group. The AuNR@AgNCs could be used to detect dithianon with an LOD value of 20 nM. Moreover, dithianon in river water and apple juice could be detected with recovery in the satisfactory ranges of 97.41%-98.35% and 97.77%-98.70%, respectively, by using this substrate under optimal conditions, indicating that the AuNR@AgNC substrate could serve as an excellent SERS detection platform for pesticide residues in fruit.

Plasmonic Alloys Enhanced Metabolic Fingerprints for the Diagnosis of COPD and Exacerbations

Accurate diagnosis of chronic obstructive pulmonary disease (COPD) and exacerbations by metabolic biomarkers enables individualized treatment. Advanced metabolic detection platforms rely on designed materials. Here, we design mesoporous PdPt alloys to characterize metabolic fingerprints for diagnosing COPD and exacerbations. As a result, the optimized PdPt alloys enable the acquisition of metabolic fingerprints within seconds, requiring only 0.5 μL of native plasma by laser desorption/ionization mass spectrometry owing to the enhanced electric field, photothermal conversion, and photocurrent response. Machine learning decodes metabolic profiles acquired from 431 individuals, achieving a precise diagnosis of COPD with an area under the curve (AUC) of 0.904 and an accurate distinction between stable COPD and acute exacerbations of COPD (AECOPD) with an AUC of 0.951. Notably, eight metabolic biomarkers identified accurately discriminate AECOPD from stable COPD while providing valuable information on disease progress. Our platform will offer an advanced nanoplatform for the management of COPD, complementing standard clinical techniques.

Surface-enhanced Raman scattering of a gold core-silver shell-sponge substrate for detection of thiram and diquat

Aiming at the difficulty of traditional pesticide sampling, a low-cost and convenient flexible surface enhanced Raman scattering (SERS) gold core-silver shell-sponge (Au-Ag-sponge) substrate was synthesized by chemical reduction. The SERS substrate consisted of Au-AgNPs and a melamine sponge. The sponge had a rich open pore structure, which could well "capture" Au-AgNPs, generating a large number of "hot spots". The SERS enhancement activity of the flexible substrate was characterized with rhodamine 6G (R6G) Raman probe molecules. The substrate showed good activity to 10-12 M rhodamine 6G with an enhancement factor (EF) of 7.72 × 106. Applying this substrate to the qualitative and quantitative detection of pesticide residues, the results showed that the Raman intensity was well related to the concentration of pesticide solution with the range of 0.1-10 mg L-1 of thiram and 1-10 mg L-1 of diquat. Furthermore, the substrate was analyzed by finite difference time domain (FDTD) simulation and the results were in good agreement with the experimental results. The reason for the difference in Raman signals of pesticide molecules on the same substrate was the different binding modes of Au-AgNPs on the sponge. Finally, we pointed out the advantages of flexible substrates in the field of pesticide residues, as well as future opportunities and challenges.

Sculpting Windows onto AuAg Hollow Cubic Nanocrystals

Using surfactants in the galvanic replacement reaction (GRR) offers a versatile approach to modulating hollow metal nanocrystal (NC) morphology and composition. Among the various surfactants available, quaternary ammonium cationic surfactants are commonly utilised. However, understanding how they precisely influence morphological features, such as the size and void distribution, is still limited. In this study, we aim to uncover how adding different surfactants-CTAB, CTAC, CTApTS, and PVP-can fine-tune the morphological characteristics of AuAg hollow NCs synthesised via GRR at room temperature. Our findings reveal that the halide counterion in the surfactant significantly controls void formation within the hollow structure. When halogenated surfactants, such as CTAB or CTAC, are employed, multichambered opened nanoboxes are formed. In contrast, with non-halogenated CTApTS, single-walled closed nanoboxes with irregularly thick walls form. Furthermore, when PVP, a polymer surfactant, is utilised, changes in concentration lead to the production of well-defined single-walled closed nanoboxes. These observations highlight the role of surfactants in tailoring the morphology of hollow NCs synthesised through GRR.

Silver-Gold Alloy Nanoparticles (AgAu NPs): Photochemical Synthesis of Novel Biocompatible, Bimetallic Alloy Nanoparticles and Study of Their In Vitro Peroxidase Nanozyme Activity

Facile synthesis of metal nanoparticles with controlled physicochemical properties using environment-friendly reagents can open new avenues in biomedical applications. Nanomaterials with controlled physicochemical properties have opened new prospects for a variety of applications. In the present study, we report a single-step photochemical synthesis of ~5 nm-sized silver (Ag) and gold (Au) nanoparticles (NPs), and Ag-Au alloy nanoparticles using L-tyrosine. The physicochemical and surface properties of both monometallic and bimetallic NPs were investigated by analytical, spectroscopic, and microscopic techniques. Our results also displayed an interaction between L-tyrosine and surface atoms that leads to the formation of AgAu NPs by preventing the growth and aggregation of the NPs. This method efficiently produced monodispersed NPs, with a narrow-sized distribution and good stability in an aqueous solution. The cytotoxicity assessment performed on breast cancer cell lines (MCF-7) revealed that the biofriendly L-tyrosine-capped AgNPs, AuNPs, and bimetallic AgAu NPs were biocompatible. Interestingly, AgAu NPs have also unveiled controlled cytotoxicity, cell viability, and in vitro peroxidase nanozyme activity reliant on metal composition and surface coating.

A characteristic bacterial SERS marker for direct identification of Salmonella in real samples assisted by a high-performance SERS chip and a selective culture medium

Salmonella should be absent in pharmaceutical preparations and foods according to the regulations. However, up to now, rapid and convenient identification of Salmonella is still full of challenge. Herein, we reported a label-free surface-enhanced Raman scattering (SERS) method for direct identification of Salmonella spiked in drug samples based on a characteristic bacterial SERS marker assisted by a high-performance SERS chip and a selective culture medium. The SERS chip being fabricated through in situ growth of bimetallic Au-Ag nanocomposites on silicon wafer within 2 h, featured a high SERS activity (EF > 10⁷), good uniformity and batch-to-batch consistency (RSD < 10 %), and satisfactory chemical stability. The directly-visualized SERS marker at 1222 cm^-1 originated from bacterial metabolite hypoxanthine was robust and exclusive for discrimination of Salmonella with other bacterial species. Moreover, the method was successfully used for direct discrimination of Salmonella in mixed pathogens by using a selective culture medium, and could identify Salmonella contaminant at ∼1 CFU spiked level in a real sample (Wenxin granule, a botanical drug) after 12 h of enrichment. The combined results showed that developed SERS method is practical and reliable, and could be a promising alternative for rapid identification of Salmonella contamination in pharmaceutical and foods industries.

Silver Nanocubes: From Serendipity to Mechanistic Understanding, Rational Synthesis, and Niche Applications

Silver has long been interwoven into human history, and its uses have evolved from currency and jewelry to medicine, information technology, catalysis, and electronics. Within the last century, the development of nanomaterials has further solidified the importance of this element. Despite this long history, there was essentially no mechanistic understanding or experimental control of silver nanocrystal synthesis until about two decades ago. Here we aim to provide an account of the history and development of the colloidal synthesis of silver nanocubes, as well as some of their major applications. We begin with a description of the first accidental synthesis of silver nanocubes that spurred subsequent investigations into each of the individual components of the protocol, revealing piece by piece parts of the mechanistic puzzle. This is followed by a discussion of the various obstacles inherent to the original method alongside mechanistic details developed to optimize the synthetic protocol. Finally, we discuss a range of applications enabled by the plasmonic and catalytic properties of silver nanocubes, including localized surface plasmon resonance, surface-enhanced Raman scattering, metamaterials, and ethylene epoxidation, as well as further derivatization and development of size, shape, composition, and related properties.

Detecting nanoparticles by "listening"

In the macroscopic world, we can obtain some important information through the vibration of objects, that is, listening to the sound. Likewise, we can also get some information of the nanoparticles that we want to know by the means of "listening" in the microscopic world. In this review, we will introduce two sensing methods (cavity optomechanical sensing and surface-enhanced Raman scattering sensing) which can be used to detect the nanoparticles. The cavity optomechanical systems are mainly used to detect sub-gigahertz nanoparticle or cavity vibrations, while surface-enhanced Raman scattering is a well-known technique to detect molecular vibrations whose frequency generally exceeds terahertz. Therefore, the vibrational information of nanoparticles from low-frequency to high-frequency could be obtained by these two methods. The size of the viruses is at the nanoscale and we can regard it as a kind of nanoparticles. Rapid and ultrasensitive detection of the viruses is the key strategies to break the spread of the viruses in the community. Cavity optomechanical sensing enables rapid, ultrasensitive detection of nanoparticles through the interaction of light and mechanical oscillators and surface-enhanced Raman scattering is an attractive qualitatively analytical technique for chemical sensing and biomedical applications, which has been used to detect the SARS-CoV-2 infected. Hence, investigation in these two fields is of vital importance in preventing the spread of the virus from affecting human's life and health.

Principles and Applications of Resonance Energy Transfer Involving Noble Metallic Nanoparticles

Over the past several years, resonance energy transfer involving noble metallic nanoparticles has received considerable attention. The aim of this review is to cover advances in resonance energy transfer, widely exploited in biological structures and dynamics. Due to the presence of surface plasmons, strong surface plasmon resonance absorption and local electric field enhancement are generated near noble metallic nanoparticles, and the resulting energy transfer shows potential applications in microlasers, quantum information storage devices and micro-/nanoprocessing. In this review, we present the basic principle of the characteristics of noble metallic nanoparticles, as well as the representative progress in resonance energy transfer involving noble metallic nanoparticles, such as fluorescence resonance energy transfer, nanometal surface energy transfer, plasmon-induced resonance energy transfer, metal-enhanced fluorescence, surface-enhanced Raman scattering and cascade energy transfer. We end this review with an outlook on the development and applications of the transfer process. This will offer theoretical guidance for further optical methods in distance distribution analysis and microscopic detection.

SERS Multiplex Profiling of Melanoma Circulating Tumor Cells for Predicting the Response to Immune Checkpoint Blockade Therapy

Immune checkpoint blockade (ICB) therapy has achieved remarkable success in many cancers including melanoma. However, ICB therapy benefits only a small proportion of patients and produces severe side effects for some patients. Thus, there is an urgent need to identify patients who are more likely to respond to ICB therapy to improve outcomes and minimize side effects. To predict ICB therapy responses, we design a surface-enhanced Raman scattering (SERS) assay for multiplex profiling of circulating tumor cells (CTCs) under basal and interferon-γ (IFN-γ) stimulation. Through simultaneous ensemble and single-cell measurements of CTCs, the SERS assay can reveal tumor heterogeneity and offer a comprehensive CTC phenotype for decision-making. Anisotropic gold-silver alloy nanoboxes are utilized as SERS plasmonic substrates for improved signal readouts of CTC surface biomarkers. By generating a unique CTC signature with four surface biomarkers, the developed assay enables the differentiation of CTCs from three different patient-derived melanoma cell lines. Significantly, in a cohort of 14 melanoma patients who received programmed cell death-1 blockade therapy, the changes of CTC signature induced by IFN-γ stimulation to CTCs show the potential to predict responders. We expect that the SERS assay can help select patients for receiving ICB therapy in other cancers.

Mechanisms for self-templating design of micro/nanostructures toward efficient energy storage

The ever-growing demand in modern power systems calls for the innovation in electrochemical energy storage devices so as to achieve both supercapacitor-like high power density and battery-like high energy density. Rational design of the micro/nanostructures of energy storage materials offers a pathway to finely tailor their electrochemical properties thereby enabling significant improvements in device performances and enormous strategies have been developed for synthesizing hierarchically structured active materials. Among all strategies, the direct conversion of precursor templates into target micro/nanostructures through physical and/or chemical processes is facile, controllable, and scalable. Yet the mechanistic understanding of the self-templating method is lacking and the synthetic versatility for constructing complex architectures is inadequately demonstrated. This review starts with the introduction of five main self-templating synthetic mechanisms and the corresponding constructed hierarchical micro/nanostructures. Subsequently, the structural merits provided by the well-defined architectures for energy storage are elaborately discussed. At last, a summary of current challenges and future development of the self-templating method for synthesizing high-performance electrode materials is also presented.

Tailoring Morphology and Elemental Distribution of Cu-In Nanocrystals via Galvanic Replacement

The compositional and structural diversity of bimetallic nanocrystals (NCs) provides a superior tunability of their physico-chemical properties, making them attractive for a variety of applications, including sensing and catalysis. Nevertheless, the manipulation of the properties-determining features of bimetallic NCs still remains a challenge, especially when moving away from noble metals. In this work, we explore the galvanic replacement reaction (GRR) of In NCs and a copper molecular precursor to obtain Cu-In bimetallic NCs with an unprecedented variety of morphologies and distribution of the two metals. We obtain spherical Cu₁₁In₉ intermetallic and patchy phase-segregated Cu-In NCs, as well as dimer-like Cu-Cu₁₁In₉ and Cu-In NCs. In particular, we find that segregation of the two metals occurs as the GRR progresses with time or with a higher copper precursor concentration. We discover size-dependent reaction kinetics, with the smaller In NCs undergoing a slower transition across the different Cu-In configurations. We compare the obtained results with the bulk Cu-In phase diagram and, interestingly, find that the bigger In NCs stabilize the bulk-like Cu-Cu₁₁In₉ configuration before their complete segregation into Cu-In NCs. Finally, we also prove the utility of the new family of Cu-In NCs as model catalysts to elucidate the impact of the metal elemental distribution on the selectivity of these bimetallics toward the electrochemical CO₂ reduction reaction. Generally, we demonstrate that the GRR is a powerful synthetic approach beyond noble metal-containing bimetallic structures, yet that the current knowledge on this reaction is challenged when oxophilic and poorly miscible metal pairs are used.

3D Spectroscopic Tracking of Individual Brownian Nanoparticles during Galvanic Exchange

Monitoring chemical reactions in solutions at the scale of individual entities is challenging: single-particle detection requires small confocal volumes, which are hardly compatible with Brownian motion, particularly when long integration times are necessary. Here, we propose a real-time (10 Hz) holography-based nm-precision 3D tracking of single moving nanoparticles. Using this localization, the confocal collection volume is dynamically adjusted to follow the moving nanoparticle and allow continuous spectroscopic monitoring. This concept is applied to study galvanic exchange in freely moving colloidal silver nanoparticles with gold ions generated in situ. While the Brownian trajectory reveals particle size, spectral shifts dynamically reveal composition changes and transformation kinetics at the single-object level, pointing at different transformation kinetics for free and tethered particles.

Surface etching-dependent geometry tailoring and multi-spectral information of Au@AuAg yolk-shell nanostructure with asymmetrical pyramidal core: The application in Co2+ determination

In this paper, a novel Au@AuAg yolk-shell heterogeneous nanostructure is designed as plasmonic spectroscopic sensor based on surface etching for ultrasensitive detection of trace cobalt ions (Co²⁺). Due to the surface diffusion of gold atoms, the Ag at one end of the core gold nanobipyramids (Au NBPs) is retained, and Au@AuAg yolk-shell nanostructure with asymmetric core is prepared. The alloy shell is coupled to Au NBPs and the interface of asymmetric Ag respectively, the two local surface plasmon resonance bands will have obvious reverse changes depending on the surface morphology of the shell. By using this distinct plasmon response generated by Co²⁺ induced surface etching, which is driven by discrepancy of double-peaks, a sensing method has been established to realize multi-information spectral detection of Co²⁺. There is a good linear relationship between the intensity ratio and the Co²⁺ concentration in the range of 1-100 nM, in which the limit of detection is 0.2 nM. This method further improves the sensing capability by combining multiple pieces of strongly changing spectral information, and demonstrates great advantages and potential of Au@AuAg yolk-shell heterogeneous nanostructure as a multi-information plasmonic sensor based on etched shell surface for trace detection.

An specific photoelectrochemical sensor based on pillar[5]arenes functionalized gold nanoparticles and bismuth oxybromide nanoflowers for bovine hemoglobin recognition

In this work, the ultrasensitive photoelectrochemical (PEC) sensor for the detection of bovine hemoglobin (BHb) was developed based on water-soluble pillar[5]arenes (WP5) functionalized gold nanoparticles (Au NPs) and bismuth oxybromide (BiOBr) nanoflowers (Au@WP5/BiOBr). The photoelectrical signal of dopamine (DA) was decreased after adding the different concentrations of BHb due to the formation of hydrogen bond between the COOH groups of BHb molecules and the NH₂ group of DA, which could achieve the indirect detection of BHb. Benefiting from the photo-generated electron-holes of BiOBr nanoflowers, the localized surface plasmon resonance (LSPR) effect of Au NPs, the host-guest interaction of WP5 between and DA, the PEC sensor showed a specificallyrecognize toward BHb with a wide detection range of 1.0 × 10^-11-1.0 × 10^-1 mg/mL and a detection limit of 4.2 × 10^-12 mg/mL (S/N = 3). Additionally, the proposed PEC sensor also displayed good stability, remarkable selectivity and provided a promising strategy of design pillar[5]arenes functionalized photoelectric activity nanomaterials for PEC sensing application.

A Low-Temperature Annealing Method for Alloy Nanostructures and Metasurfaces: Unlocking a Novel Degree of Freedom

The material and exact shape of a nanostructure determine its optical response, which is especially strong for plasmonic metals. Unfortunately, only a few plasmonic metals are available, which limits the spectral range where these strong optical effects can be utilized. Alloying different plasmonic metals can overcome this limitation, at the expense of using a high-temperature alloying process, which adversely destroys the nanostructure shape. Here, a low-temperature alloying process is developed where the sample is heated at only 300 °C for 8 h followed by 30 min at 450 °C and Au-Ag nanostructures with a broad diversity of shapes, aspect ratios, and stoichiometries are fabricated. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses confirm the homogeneous alloying through the entire sample. Varying the alloy stoichiometry tunes the optical response and controls spectral features, such as Fano resonances. Binary metasurfaces that combine nanostructures with different stoichiometries are fabricated using multiple-step electron-beam lithography, and their optical function as a hologram or a Fresnel zone plate is demonstrated at the visible wavelength of λ = 532 nm. This low-temperature annealing technique provides a versatile and cost-effective way of fabricating complex Au-Ag nanostructures with arbitrary stoichiometry.

Noble Metal Promoted TiO2 from Silver-Waste Valorisation: Synergism between Ag and Au

Wastewaters from precious metal industries contain high amounts of noble metals, but their efficient recycling is hindered by the wastewater complex composition. Here, we propose an innovative approach for the efficient recovery of noble metals contained in these metal-enriched wastewaters as precursors for the synthesis of noble metal nanoparticles (NPs) and supported metal catalysts. Silver NPs were synthesized from Ag-enriched wastewater and then deposited on TiO2 to prepare photocatalysts. Then, further promotion of the photocatalytic activity of Ag-modified TiO2 was achieved by the addition of as little as 0.5 wt.% of Au. STEM-EDS analyses proved that Au NPs were located on Ag or AgOx nanoparticles. The contact between the two metal-containing NPs results in charge transfer effects, appreciable both in terms of oxidation states determined by XPS and of optical properties. In particular, the plasmon band of Au NPs shows photochromic effects: under UV light irradiation, bimetallic samples exhibit a blue-shift of the plasmon band, which is reversible under dark storage. The activity of the materials was tested towards ethanol photodegradation under UV light. Adding 0.5 wt.% Au NPs resulted in a promoted activity compared to Ag-TiO2, thus showing synergistic effects between Au and Ag. Ethanol was completely converted already after 1 h of UV irradiation, acetaldehyde was formed as the main oxidation product and fully degraded in less than 180 min. Notably, bimetallic samples showed ethylene formation by a parallel dehydration mechanism.

Long Spiky Au-Ag Nanostar Based Fiber Probe for Surface Enhanced Raman Spectroscopy

The detection performances of noble metal-based surface enhanced Raman spectroscopy (SERS) devices are determined by the compositions and geometries of the metal nanostructures, as well as the substrates. In the current study, long spiky Au-Ag alloy nanostars were synthesized, and both core diameters and spike lengths were controlled by Lauryl sulfobetaine concentrations (as the nanostructure growth skeleton). The long spiky star geometries were confirmed by transmission electron micrograph images. Elements energy dispersive spectrometer mapping confirmed that Au and Ag elements were inhomogeneously distributed in the nanostructures and demonstrated a higher Ag content at surface for potential better SERS performance. Selected synthesized spiky nanostars were uniformly assembled on multi-mode silica fiber for probe fabrication by silanization. The SERS performance were characterized using crystal violet (CV) and rhodamine 6G (R6G) as analyte molecules. The lowest detection limit could reach as low as 10^-8 M, with a 6.23 × 10⁶ enhancement factor, and the relationship between analyte concentrations and Raman intensities was linear for both CV and R6G, which indicated the potential qualitative and quantitative molecule detection applications. Moreover, the fiber probes also showed good reproducibility and stability in the ambient atmosphere.

A sandwich SERS immunoassay platform based on a single-layer Au-Ag nanobox array substrate for simultaneous detection of SCCA and survivin in serum of patients with cervical lesions

The evaluation of tumor biomarkers in blood specimens is vital for patients with cervical lesions. Herein, an ultrasensitive surface enhanced Raman scattering (SERS) platform was proposed for simultaneous detection of cervical-lesion-related serum biomarkers. Raman reporter labeled Au-Ag nanoshells (Au-AgNSs) acted as SERS tags and an Au-Ag nanobox (Au-AgNB) array substrate prepared by the oil-water interface self-assembly method was used as a capture substrate. This single-layer Au-AgNB array substrate was proved to have exceptional uniformity by atomic force microscopy and SERS mapping. Numerous "hot spots" and specific adsorption surfaces offered by the Au-AgNB array substrate were confirmed by the finite difference time domain method, which could generate a SERS signal in electromagnetic enhancement. Binding of antigens between antibodies on Au-AgNSs and the Au-AgNB array substrate led to the formation of a sandwich-structure by the two metal nanostructures. Consequently, an ultralow detection limit of 6 pg mL^-1 for squamous cell carcinoma antigen (SCCA) and 5 pg mL^-1 for survivin in a wide linear logarithmic range of 10 pg mL^-1 to 10 μg mL^-1 was acquired. High selectivity and reproducibility with relative standard deviations of 7.701% and 6.943% were detected. Furthermore, the simultaneous detection of the two biomarkers in practical specimens was conducted, and the results were consistent with those of the enzyme-linked immunosorbent assay. This platform exhibited good robustness in the rapid and sensitive detection of SCCA and survivin, which could be a promising tool in early clinical diagnosis for different grades of cervical lesions.

A SERS-LFA biosensor combined with aptamer recognition for simultaneous detection of thrombin and PDGF-BB in prostate cancer plasma

An innovative surface-enhanced Raman spectroscopy and lateral flow assay (SERS-LFA) biosensor combined with aptamer recognition had been developed for the convenient, rapid, sensitive and accurate detection of thrombin and platelet-derived growth factor-BB (PDGF-BB) associated with prostate cancer simultaneously. During the biosensor operation, thrombin and PDGF-BB in the sample were recognized and combined by thiol-modified aptamers immobilized on Au-Ag hollow nanoparticles (Au-Ag HNPs) surface and biotinylated aptamers immobilized on the test lines of the biosensor. Thus, thrombin and PDGF-BB were simultaneously captured between detection aptamers and capture aptamers in a sandwich structure. Finite difference time domain simulation confirmed that 'hot spots' appeared at the gaps of Au-Ag HNPs dimer in the enhanced electromagnetic field compared to that of a single Au-Ag HNP, indicating that the aggregated Au-Ag HNPs owned a good SERS signal amplification effect. The detection limits of thrombin and PDGF-BB in human plasma were as low as 4.837 pg ml^-1and 3.802 pg ml^-1, respectively. Moreover, the accuracy of the biosensor which was applied to detect thrombin and PDGF-BB in prostate cancer plasma had been verified. This designed biosensor had broad application prospects in the clinical diagnosis of prostate cancer.

Facile synthesis of silver/gold alloy nanoparticles for ultra-sensitive rhodamine B detection

The synthesis of Ag/Au nanoparticles (NPs) in a controlled manner has been a challenge for a long time. The aim of this report is to present a systematic study on the fabrication, characterization of Ag/Au alloy NP-based surface-enhanced Raman spectroscopy (SERS) substrates. Silver (Ag) and gold (Au) colloidal NPs were prepared by chemical reduction route of the corresponding metal salts by trisodium citrate (TSC). Ag/Au alloy nanoparticles with varying molar fractions are prepared in aqueous solution by the simultaneous reduction of AgNO3 and HAuCl4 by TSC. The composition of Ag and Au in the alloy samples was controlled by tuning the molar ratio of Ag+/Au3+ in the mixture solution. The morphologies of the different products were characterized by TEM, and the size of obtained samples was in the range of 40 to 60 nm. The resulting samples were denoted as AgNPs, AuNPs, Ag3Au, AgAu, and AgAu3 NPs. In order to compare the optical property of the Ag/Au alloy and Ag/Au mixture, we mixed the pure Ag and Au NPs with different ratios to obtain the aggregated nanoparticles. Ag/Au alloy NPs were demonstrated as an ultrasensitive SERS substrate for the detection of rhodamine B (RhB) molecules. The concentration of RhB ranged from 10-11 to 10-5 M. The effect of the Au content on the optical and SERS properties of the Ag/Au alloys was studied. The obtained results show that the Au content in the Ag/Au alloys play an important role in the physical properties of Ag/Au alloy NPs. The SERS spectra of RhB from the as-prepared Ag/Au alloy NP substrates indicated the superior enhancement with high reproducibility and sensitivity compared to those of Ag or Au samples. Interestingly, the highest SERS activity was achieved for the Ag3Au sample with an enhancement factor larger than 1010 for 10-11 M RhB and a limit of detection (LOD) at 10-11 M, as well as good long-term stability after storage for 1 year. As far as we know, this is the highest sensitivity record of RhB by SERS detection. Furthermore, the composition-dependent SERS activity was explained in detail. These advantages demonstrated the potential for growing Ag/Au alloy NP-based SERS substrates in food safety and bioanalysis.

AgNPs and MIL-101(Fe) self-assembled nanometer materials improved the SERS detection sensitivity and reproducibility

Recently, in the research of Surface-enhanced Raman scattering (SERS) technology, it is found that the preparation of enhanced substrate is particularly important. In this work, the most commonly used methods were used to synthesize AgNPs and MIL-101(Fe), and AgNPs/MIL-101(Fe) nanocomposite was obtained through self-assembly of the two substances. Four different probe molecules were detected with the self-assembled substrate and compared with the results of same probe molecules with AgNPs and MIL-101(Fe) as SERS substrate separately, it was found that AgNPs/ MIL-101 (Fe) nanocomposites had a strong enhancing effect as SERS substrate. The Enhancement Factor (EF) value of 10^-6 mol/L Rhodamine 6G (R6G) was calculated as 2.09 × 10⁹, and the Raman intensities of the peak relative standard deviation (RSD) of R6G Raman attribution was calculated as 7.55%. The time stability of the material was studied and it was found that the reduced Raman signal and poor reproducibility were due to the AgNPs placement time. AgNPs/ MIL-101 (Fe) nanocomposites were used as SERS substrate to detect Paraquat with a minimum concentration of 10^-12 mol/L. The signal values of Paraquat Raman detected at 10^-6 mol/L in different pH environments were relatively stable.

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.

Bioinspired protein corona strategy enhanced biocompatibility of Ag-Hybrid hollow Au nanoshells for surface-enhanced Raman scattering imaging and on-demand activation tumor-phototherapy

Silver-based hybrid nanoprobes for surface-enhanced Raman scattering (SERS) imaging show their tremendous potential for precise biological detection and mediated phototherapy. However, the severe toxicity induced by Ag to normal mammalian cells hinders its further application. Herein, we presented a versatile bioinspired protein corona strategy through assembling bovine serum albumin (BSA) protected Raman tag DTTC-conjugated Ag-hybrid hollow Au nanoshells (hollow AgAu-DTTC-BSA), which their silver ion release and reactive oxygen species (ROS) generation are significantly suppressed, enabling no damage to normal cells and tissues, but can be reactivated on-demand under laser-irradiation at the tumor site. These nanoshells could also produce strong localized surface plasmon resonance for efficient-stable photothermal effect and enhanced SERS activity under laser irradiation, approved by both theoretical and experimental calculations. Furthermore, the biocompatible hollow AgAu-DTTC-BSA could detect both primary tumor tissues and tiny liver metastases (~0.18 mm) in orthotopic/subcutaneous CT26 colon tumor-bearing mice models. We also demonstrate their excellent therapeutic efficacy for colorectal solid neoplasms by accurate SERS imaging-guided photothermal therapy, simultaneously assisted with toxic Ag ion and ROS. These results suggest that hollow AgAu-DTTC-BSA is promising imaging assisted photothermal agents for solid tumor theranostics and enhancing the potential of Ag-based nanoparticles for practical treatment.

A SnO2 shell for high environmental stability of Ag nanowires applied for thermal management

Since silver nanowires (AgNWs) show high infrared reflectance many studies present their applicability as thermal management products for various wearable textiles. However, their use for practical purposes is only partially evaluated, without focusing on improving their low atmospheric and liquid stability. This report describes a new approach for the topic and proposes a facile method of Ag nanowire passivation with a SnO2 layer for high environmental stability and retention of high infrared reflectance. The one-step passivation process of AgNWs was carried out in the presence of sodium stannate in an aqueous solution at 100 °C, and resulted in the formation of core/shell Ag/SnO2 nanowires. This study presents the morphological, chemical, and structural properties of Ag/SnO2NWs formed with a 14 nm thick SnO2 shell, consisting of 7 nm rutile-type crystals, covering the silver metallic core. The optical properties of the AgNWs changed significantly after shell formation, and the longitudinal and transverse modes in the surface plasmon resonance spectrum were red shifted as a result of the surrounding media dielectric constant changes. The passivation process protected the AgNWs from decomposition in air for over 4 months, and from dissolution in a KCN solution at concentrations up to 0.1 wt%. Moreover, the report shows the microwave irradiation effect on the shell synthesis and previously synthesised Ag/SnO2NWs. The post-synthesis irradiation, as well as the SnO2 shell obtained by microwave assistance, did not allow long-term stability to be achieved. The microwave-assisted synthesis process was also not fast enough to inhibit the formation of prismatic silver structures from the nanowires. The Ag/SnO2NWs with a shell obtained by a simple hydrolysis process, apart from showing high infra-red reflectance on the para-aramid fabric, are highly environmentally stable. The presented SnO2 shell preparation method can protect the AgNW's surface from dissolution or decomposition and facilitate the designing of durable smart wearable thermal materials for various conditions.

A novel surface-enhanced Raman scattering probe based on Au nanoboxes for dynamic monitoring of caspase-3 during cervical cancer cell apoptosis

The highly sensitive and reliable detection, imaging, and monitoring of changes of intracellular caspase-3 are critical for understanding the cell apoptosis and studying the progression of caspase-3-related cervical cancer.

Highly selective and sensitive tool for the detection of Hg(II) using 3-(Trimethoxysilyl) propyl methacrylate functionalized Ag-Ce nanocomposite from real water sample

Mercury and its derivates cause distinct toxicity and it is detrimental to the ecosystem where the excessive concentration contributes towards the environmental pollutants. The current study reported a colorimetric method for the detection of Hg(II) ion with high specificity and selectivity using Ag-Ce nanocomposite (NC) functionalized by 3-(Trimethoxysilyl) propyl methacrylate. The synthesized Ag-Ce NC was characterized by using double beam UV-visible spectrophotometer, zeta sizer, EDS, TEM, FT-IR, XRD and particle size analyzer. The synthesized particle possessed an average particle size of 27 ± 1 nm and zeta potential of -39.32 ± 3 mV. The brownish yellow colored Ag-Ce NC changed to colorless in presence of Hg(II) where the colorimetric detection was extremely specific and superior towards Hg(II) ion on comparing the tests with other metal ions. An excellent linear correlation was observed between absorbance (395 nm) and Hg(II) concentrations (1 nM-10 μM) (R² = 0.988) with LOD of 0.03 nM. A cotton swab based probe was prepared for selective, elegant and low cost colorimetric method to detect Hg(II). The parametric study was performed for optimizing the suitable condition. The colorimetric probe developed by this study for Hg(II) detection using Ag-Ce NC shows excellent practical applicability.

Exploring the Chemical Reactivity of Gallium Liquid Metal Nanoparticles in Galvanic Replacement

Micron/nanosized particles of liquid metals possess intriguing properties and are gaining popularity for applications in various research fields. Nevertheless, the knowledge of their chemistry is still very limited compared to that of other classes of materials. In this work, we explore the reactivity of Ga nanoparticles (NPs) toward a copper molecular precursor to synthesize bimetallic Cu-Ga NPs. Anisotropic Cu-Ga nanodimers, where the two segregated domains of the constituent metals share an interface, form as the reaction product. Through a series of careful experiments, we demonstrate that a galvanic replacement reaction (GRR) between the Ga seeds and a copper-amine complex takes place. We attribute the final morphology of the bimetallic NPs, which is unusual for a GRR, to the presence of the native oxide shell around the Ga NPs and their liquid nature, via a mechanism that resembles the adhesion of bulk Ga drops to solid conductors. On the basis of this new knowledge, we also demonstrate that sequential GRRs to include more metal domains are possible. This study illustrates a new approach to the synthesis of Ga-based metal nanoparticles and provides the basis for its extension to many more systems with increased levels of complexity.

Efficient and tunable shape selective synthesis of Ag/CeO2 nanostructures modified highly stable SERS substrate for ultrasensitive detection of pesticides on the surface of an apple

Detection of pesticide residues from fruits and vegetables is of significant importance to ensuring human health and environmental safety. An efficient and tunable shape-selective synthesis of Ag/CeO₂ nanostructures as an active flexible SERS substrate for the detection of thiram on an apple surface via a paste, peel off, and paste again process was performed. The well-controlled formation of silver assembled CeO₂ microspheres constituting nanospheres and nanospindles with an average size of approximately 56 and 32 nm with anisotropic structures has been confirmed through morphological and crystallographic analysis. Interestingly, CeO₂ (111) was strongly anchored in the Ag (111) matrix, which provides a more adequate pathway for rapid ion-electron transportation, as observed from the structural and chemical composition analysis. The detection of thiram on the surface of an apple using our proposed nanospindle SERS active substrate achieves a wide detection range from 10^-2 to 10^-9 M with a correlation coefficient of 0.9929 and a low detection limit of 27 nM at S/N = 3. In addition, the charge transfer mechanism between the Ag/CeO₂ nanostructures and thiram molecules has also been proposed. We believe that the present work could provide novel ways to develop SERS active substrates for highly efficient onsite detection of pesticides on fruits in the near future.

Gold nanocage-based surface-enhanced Raman scattering probes for long-term monitoring of intracellular microRNA during bone marrow stem cell differentiation

The ability to monitor the differentiation of living stem cells is essential for understanding stem cell biology and the practical application of stem cell therapies. However, conventional methods of analyzing biomarkers related to differentiation still require a large number of cells or cell lysates. This requirement leads to the unavoidable loss of cell sources and hinders the real-time monitoring of cellular processes. In this study, we report an ultrasensitive surface-enhanced Raman scattering (SERS) method for the long-term detection and imaging of miR-144-3p in osteogenic differentiation of BMSCs, by using target miRNA-induced gold nanocage (GNC)-hairpin DNA1 (hpDNA1)-hpDNA2-GNC assembly in living cells. The finite-difference time domain method demonstrated that the electromagnetic intensities of the dimer and polymer of the GNCs were significantly enhanced compared to that of GNCs only, which theoretically confirmed the rational design of the SERS strategy. The hpDNA-conjugated GNC probes were prepared and used to recognize the target and distinguish from other miRNAs. This method enabled excellent sensitivity and high selectivity toward miR-144-3p with a limit of detection of 13.6 aM and a broad range from 100 aM to 100 pM in cell lysates. Then, we used transmission electron microscopy images, fluorescence microscopy images, and dark-field microscopy images to study the internalization of the probes in BMSCs. A Cell Counting Kit-8 experiment indicated that the probes were not cytotoxic in a certain concentration range. BMSCs were treated with an osteogenic inductor so that they would subsequently differentiate into osteocytes. Upon cellular uptake of these nanoprobes, we observed intense and time-dependent SERS responses from the important osteogenic biomarker miR-144-3p, only in BMSCs undergoing osteogenic differentiation and living undifferentiated BMSCs but not in osteoblasts. Finally, the accuracy of SERS has been proved by a quantitative real-time polymerase chain reaction experiment. The above results demonstrated that our nanoprobes are capable of long-term tracking of the dynamic expression of miR-144-3p (21 days) in the differentiating BMSCs. SERS has broad application prospects in the long-term detection of stem cell differentiation, and identification and isolation of specific cell types as well as in biomedical diagnosis.

Gold nanocages for effective photothermal conversion and related applications

Gold nanocages are highly effective in converting light to heat, making them versatile for an array of photothermal applications.

Ultrasensitive SERS detection of rhodamine 6G and p-nitrophenol based on electrochemically roughened nano-Au film

Quantitative analysis of organic pollutants in environmental water is an important issue for ecological environment and human health. In this paper, the quantitative analysis of rhodamine 6G (R6G) and p-nitrophenol (PNP) is performed by the surface enhanced Raman scattering (SERS) technology. The enhancement of Raman signals is achieved on the surface of an electrochemically roughened nano-Au film. The SERS performance depends on the microstructure of roughened nano-Au films, which is affected by the thickness of Au films and electrochemical roughening parameters. The structure-dependence of SERS performance is validated by finite element simulation of local electromagnetic field distribution. An obvious SERS effect of R6G with an enhancement factor of 10⁸ is obtained on the roughened nano-Au film. A sensitive SERS detection of R6G with a linear range of 10^-9-10^-5 M and a detection limit of 10^-11 M is realized. Moreover, a wide linear range of 10^-9-10^-3 M is obtained for the detection of PNP. The roughened nano-Au film is an effective substrate for the SERS detection of organic pollutants with high reproducibility and good stability. Therefore, the electrochemical technology in this study is expected to be a very promising method for the fabrication of high-performance SERS substrate.

Native MicroRNA Targets Trigger Self-Assembly of Nanozyme-Patterned Hollowed Nanocuboids with Optimal Interparticle Gaps for Plasmonic-Activated Cancer Detection

The modernized use of nucleic acid (NA) sequences to drive nanostructure self-assembly has given rise to a new class of designed nanomaterials with controllable plasmonic functionalities for broad surface-enhanced Raman scattering (SERS)-based bioanalysis applications. Herein, dual usage of microRNAs (miRNAs) as both valuable cancer biomarkers and direct self-assembly triggers is identified and capitalized upon for custom-designed plasmonic nanostructures. Through strict NA hybridization of miRNA targets, Au nanospheres selectively self-assemble onto hollowed Au/Ag alloy nanocuboids with ideal interparticle distances (≈2.3 nm) for optimal SERS signaling. The intrinsic material properties of the self-assembled nanostructures further elevate miRNA detection performance via nanozyme catalytic SERS signaling cascades. This enables fM-level miR-107 detection limit within a clinically-relevant range without any molecular target amplification. The miRNA-triggered nanostructure self-assembly approach is further applied in clinical patient samples, and showcases the potential of miR-107 as a non-invasive prostate cancer diagnostic biomarker. The use of miRNA targets to drive nanostructure self-assembly holds great promise as a practical tool for miRNA detection in disease applications.

In Situ Observation of Dynamic Galvanic Replacement Reactions in Twinned Metallic Nanowires by Liquid Cell Transmission Electron Microscopy

Galvanic replacement is a versatile approach to prepare hollow nanostructures with controllable morphology and elemental composition. The primary issue is to identify its fundamental mechanism. In this study, in situ liquid cell transmission electron microscopy was employed to monitor the dynamic reaction process and to explore the mechanism of galvanic replacement. The detailed reaction process was revealed based on in situ experiments in which small Au particles first appeared around Ag nanowires; they coalesced, grew, and adhered to Ag nanowires. After that, small pits grew from the edge of Ag nanowires to form tubular structures, and then extended along the Ag nanowires to obtain hollowed structures. All of our experimental observations from the viewpoint of electron microscopy, combined with DFT calculations, contribute towards an in-depth understanding of the galvanic replacement reaction process and the design of new materials with hollow structures.

Glutathione Induced Transformation of Partially Hollow Gold-Silver Nanocages for Cancer Diagnosis and Photothermal Therapy

Gold-silver nanocages (GSNCs) are widely used in cancer imaging and therapy due to excellent biocompatibility, internal hollow structures, and tunable optical properties. However, their possible responses toward the tumor microenvironment are still not well understood. In this study, it is demonstrated that a kind of relatively small sized (35 nm) and partially hollow GSNCs (absorbance centered at 532 nm) can enhance the intrinsic photoacoustic imaging performances for blood vessels around tumor sites. More importantly, the high concentration of glutathione around the tumor cells' microenvironment may induce the aggregation, disintegration, and agglomeration of these GSNCs sequentially, allowing significant shifts in the absorbance spectrum of GSNCs to the near-infrared (NIR) region. This enhanced absorbance in the NIR region entails the significant photothermal therapy (PTT) effect. In vivo experiments, including photoacoustic microscopy (PAM) for cancer diagnosis and PTT in tumor model mice, also show coincident consequences. Taken together, the slightly hollow GSNCs may assist PAM-based tumor diagnosis and induce a tumor targeted PTT effect. This work paves a new avenue for the development of an alternative tumor diagnostic and therapeutic strategy.

Tweaking the Interplay among Galvanic Exchange, Oxidative Etching, and Seed-Mediated Deposition toward Architectural Control of Multimetallic Nanoelectrocatalysts

Nanoscale galvanic exchange confined by metallic nanoparticles is an intriguing structure-remodeling process that transforms geometrically simple solid nanoparticles into multimetallic hollow nanoparticles with increased structural complexity and compositional diversity. Using liquid polyols with intrinsic reducing capabilities as the reaction medium for nanoparticle-templated galvanic exchange represents an interesting paradigm shift, allowing us to interface galvanic exchange with oxidative etching and seed-mediated deposition without introducing any additional oxidizing or reducing agents. By kinetically maneuvering the interplay among galvanic Cu-Pt exchange, oxidative Cu etching, and seed-mediated Pt deposition, we have been able to selectively transform AuCu₃ alloy nanoparticles into two architecturally distinct multimetallic heteronanostructures, namely, Au-Pt alloy skin-covered spongy nanoparticles and Pt nanodendrite-covered hollow nanoparticles, both of which exhibit unique structural features highly desirable for high-performance electrocatalysis. Using the formic acid oxidation and hydrogen evolution reactions in acidic electrolytes as model electrocatalytic reactions, we show that the multimetallic nanoparticles derived from AuCu₃ alloy nanoparticles through polyol-mediated galvanic exchange reactions markedly outperform the commercial Pt/C benchmark catalysts in terms of both activity and durability. This work not only provides important mechanistic insights on how galvanic exchange dynamically interplays with other redox processes to rigorously dictate the versatile structural transformations of multimetallic nanoparticles but also sheds light on the detailed structure-property relationships underpinning the intriguing electrocatalytic behaviors of architecturally complex multimetallic heteronanostructures.

Quantitative Evaluation of Surface-Enhanced Raman Scattering Nanoparticles for Intracellular pH Sensing at a Single Particle Level

Intracellular pH is one of the key factors for understanding various biological processes in biological cells. Plasmonic gold and silver nanoparticles (NPs) have been extensively studied for surface-enhanced Raman scattering (SERS) applications for pH sensing as a local pH probe in a living cell. However, the SERS performance of NPs depends on material, size, and shape, which can be controlled by chemical synthesis. Here, we synthesized 18 types of gold and silver NPs with different morphologies such as sphere, rod, flower, star, core/shell, hollow, octahedra, core/satellites, and chainlike aggregates, and quantitatively compared their SERS performance for pH sensing. The SERS intensity from the most commonly utilized SERS probe molecule ( para-mercaptobenzoic acid, p-MBA) for pH sensing was measured at the single nanoparticle level under the same measurement parameters such as low laser power (0.5 mW/μm²), short integration time (100 ms) at wavelengths of 405, 488, 532, 584, 676, and 785 nm. In our measurement, the Ag chain, Ag core/satellites, Ag@Au core/satellites, and Au core/satellites nanoassemblies showed efficient pH sensing at the single particle level. By using p-MBA-conjugated Au@Ag core/satellites, we performed time-lapse pH measurements during apoptosis of HeLa cells. These experimental results confirmed that the pH measurement using p-MBA-conjugated Au@Ag core/satellites can be applied for long-term measurements of intracellular pH during cellular events.

Facile synthesis of silver-rich Au/Ag bimetallic nanoparticles with highly active SERS properties

A one-pot route to the synthesis of Au/Ag bimetallic nanoparticles with a high SERS activity for detection of proteins at low concentration.

Nanomaterial-based SERS sensing technology for biomedical application

Over the past few years, nanomaterial-based surface-enhanced Raman scattering (SERS) detection has emerged as a new exciting field in which theoretical and experimental studies of the structure and function of nanomaterials have become a focus.