Empirical structural design of core@shell Au@Ag nanoparticles for SERS applications

Self Cite

In this work, we reported the synthesis of monodisperse, quasispherical Ag nanoparticles (NPs) with sizes of 40–300 nm by using ascorbic acid reduction of a silver–ammonia complex onto preformed, 23 nm Ag-NP seeds in the aqueous solution with an optimal pH of about 9.6 at room temperature. The as-prepared Ag NPs with such a large size span (from 40 to 300 nm) and high quality by one-pot seeded growth method are reported for the first time to the best of our knowledge. It is found that the key in the present seed-mediated growth method is to introduce a proper amount of ammonia water for the formation of a stable complex with a silver precursor (silver–ammonia complex) while maintaining the pH value of the growth solution simultaneously. By using rhodamine 6G molecules as probes, the surface-enhanced Raman scattering (SERS) activities of the as-prepared Ag NPs in ethanol solution are highly dependent on the sizes of Ag NPs at the fixed 633 nm laser and at the fixed particle number, which show a volcano-like curve. Moreover, 125 nm Ag NPs bear the largest SERS activity among them. Furthermore, Ag NPs with narrow distributions in shape and size (say, less than 10%) can achieve the uniformity and reproducibility of their SERS signals in solution; their relative standard deviations can be as low as 5% in space and temporal scale.

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Size Control Synthesis of Monodisperse, Quasi-Spherical Silver Nanoparticles To Realize Surface-Enhanced Raman Scattering Uniformity and Reproducibility

Xing

Xiahou

Zhang

et al. 2019

Self Cite

“…The exact mechanism behind this removal is still unclear, but we hypothesize that the higher affinity of the silver atoms to the Au surface displaces the physisorbed DMAP molecules, similar to what was observed for the displacement of covalently attached molecules from AuNP by sodium borohydride. 42 The effect of having cleaner SERS spectra became readily apparent when trying to discriminate ELVs derived from B16F10 melanoma cells and ELVs from red blood cells. With the Au@AgNPs a sensitivity and specificity >90% were obtained, which is a marked improvement over the DMAP−AuNPs for which a sensitivity and specificity of only 60−80% were found.…”

Section: ■ Discussionmentioning

confidence: 99%

Improved Label-Free Identification of Individual Exosome-like Vesicles with Au@Ag Nanoparticles as SERS Substrate

Fraire

Stremersch

Bouckaert

et al. 2019

Exosome-like vesicles (ELVs) are nanovectors released by cells that are endowed with a variety of molecules, including proteins, nucleic acids, and chemicals that reflect the molecular signature of the producing cell. Given their presence in many biofluids, they form an easily accessible biomarker for early disease detection. Previously we demonstrated the possibility of identifying individual ELVs by analyzing their molecular signatures with surface-enhanced Raman scattering (SERS) after functionalization of ELVs with 4-(dimethylamino)pyridine (DMAP)-stabilized gold nanoparticles (AuNP). Although this strategy was capable of distinguishing ELVs from different cellular origins, the quality of the SERS spectra was suboptimal due to high background coming from the DMAP stabilizing molecules at the AuNP surface. In this study we demonstrate that it is possible to eliminate interfering SERS signals from stabilizing molecules at the AuNP surface by overgrowing in situ the ELV-attached AuNPs with a silver layer so as to form a core–shell nanoparticle (Au@AgNPs) directly at the ELV surface. As such it represents the first known strategy to generate clear SERS spectral fingerprints of delicate biological structures without interference of linker molecules that are needed to ensure colloidal stability of the plasmonic NP and to allow them to associate to the ELV surface. This new strategy using core–shell plasmonic NPs as SERS substrate showed higher near-field enhancements than previous approaches, which resulted in SERS spectra with improved signal-to-noise ratio. This allowed us to discriminate individual vesicles derived from B16F10 melanoma cells and red blood cells (RBC) with an unprecedented sensitivity and specificity >90%. Importantly, thanks to the higher near field enhancement the acquisition time could be reduced by 20-fold in comparison to previously reported strategies, paving the way toward high-throughput label-free single ELV identification.

“…To date, Au@Ag bimetallic NPs for SERS detection can be classified into two structures, that is, Au/Ag-adhesive type and Au/Ag-gapped type. The silver shell of the former was directly deposited on a bare gold core 14 or a Raman reporter (such as 4-mercaptobenzoic acid, 8 4-aminothiophenol 12 )-attached gold core. There were no apparent gaps between the two metals.…”

Section: ■ Introductionmentioning

confidence: 99%

Gold Nanorod Array-Bridged Internal-Standard SERS Tags: From Ultrasensitivity to Multifunctionality

Mei

Wang

et al. 2019

Bimetallic gold core−silver shell (Au@Ag) surfaceenhanced Raman scattering (SERS) tags draw broad interest in the fields of biological and environmental analysis. In reported tags, silver coating tended to smooth the surface and merge the original hotspot of Au cores, which was disadvantageous to signal enhancement from the aspect of surface topography. Herein, we developed gold nanorod (AuNR)-bridged Au@Ag SERS tags with uniform three-dimensional (3D) topography for the first time. This unique structure was achieved by selecting waxberry-like Au nanoparticles (NPs) as cores, which were capped by vertically oriented AuNR arrays. Upon selective surface blocking with thiol− ligands, Ag NPs were controlled to anisotropically grow on the tips of the AuNRs, producing high-density homo-(Ag−Ag) and hetero-(Au−Ag) hotspots in a single NP. The 3D hotspots rendered this NP extraordinary SERS enhancement ability (an analytical enhancement factor of 3.4 × 10 6) 30 times higher than the counterpart with a smooth surface, realizing signal detection from a single NP. More importantly, multiplexing signals ("blank" or multiplex "internal standard") can be achieved by simply changing thiol−ligands, as exemplified in the synthesis of NPs with 8 signatures. Furthermore, the multifunctionality has been demonstrated in living cell/in vivo imaging, photothermal therapy, and SERS substrates for ratiometric quantitative analysis, relying on the inherent internal standard signal. The prepared Au@Ag NPs have great potential as standard tools in many SERS-related fields.