Bare AgNPs Research Articles

Boosting bacteria differentiation efficiency with multidimensional surface-enhanced Raman scattering: the example of Bacillus cereus.

Surface-enhanced Raman scattering (SERS) is a powerful tool for constructing biomolecular fingerprints, which play a vital role in differentiation of bacteria. Due to the rather subtle differences in the SERS spectra among different bacteria, artificial intelligence is usually adopted and enormous amounts of spectral data are required to improve the differentiation efficiency. However, in many cases, large volume data acquisition on bacteria is not only technical difficult but labour intensive. It is known that surface modification of SERS nanomaterials can bring additional dimensionality (difference) of the SERS fingerprints. Here in this work, we show that the concept could be used to improve the bacteria differentiation efficiency. Ag NPs were modified with 11-mercaptoundecanoic acid, 11-mercapto-1-undecanol, and 1-dodecanethiol to provide additional dimensionality. The modified NPs then were mixed with cell lysate from different strains of Bacillus cereus (B. cereus). Even by applying a simple PCA process to the resulting SERS spectra data, all the three modified Ag NPs showed superior differentiation results compared with bare Ag NPs, which could only separate Staphylococcus aureus (S. aureus) and B. cereus. It is believed that the multidimensional SERS could find great potential in bacteria differentiation.

Localized Surface Plasmon Resonance Decorated with Carbon Quantum Dots and Triangular Ag Nanoparticles for Chlorophyll Detection

This paper demonstrates carbon quantum dots (CQDs) with triangular silver nanoparticles (AgNPs) as the sensing materials of localized surface plasmon resonance (LSPR) sensors for chlorophyll detection. The CQDs and AgNPs were prepared by a one-step hydrothermal process and a direct chemical reduction process, respectively. FTIR analysis shows that a CQD consists of NH2, OH, and COOH functional groups. The appearance of C=O and NH2 at 399.5 eV and 529.6 eV in XPS analysis indicates that functional groups are available for adsorption sites for chlorophyll interaction. A AgNP–CQD composite was coated on the glass slide surface using (3-aminopropyl) triethoxysilane (APTES) as a coupling agent and acted as the active sensing layer for chlorophyll detection. In LSPR sensing, the linear response detection for AgNP–CQD demonstrates R2 = 0.9581 and a sensitivity of 0.80 nm ppm−1, with a detection limit of 4.71 ppm ranging from 0.2 to 10.0 ppm. Meanwhile, a AgNP shows a linear response of R2 = 0.1541 and a sensitivity of 0.25 nm ppm−1, with the detection limit of 52.76 ppm upon exposure to chlorophyll. Based on these results, the AgNP–CQD composite shows a better linearity response and a higher sensitivity than bare AgNPs when exposed to chlorophyll, highlighting the potential of AgNP–CQD as a sensing material in this study.

Influence of glucose, sucrose, and dextran coatings on the stability and toxicity of silver nanoparticles

Aqueous colloids, consisting of 15–30 nm-sized silver nanoparticles (Ag NPs), were prepared using the reducing and stabilizing abilities of glucose, sucrose, and dextran. The long-term stability of coated Ag NPs increases from glucose over sucrose to dextran, i.e., with the increase of the molecular weight of carbohydrate molecules. The density functional theory (DFT) calculations of the partial atomic (Mulliken) charges and adsorption energies are applied to explain the enhanced stability of coated Ag NPs. All coated Ag NPs have a significantly broader concentration range of nontoxic behavior toward pre-osteoblast cells than bare Ag NPs prepared using sodium borohydride. The carbohydrate-coated Ag NPs display the same level of toxic ability against Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria as bare Ag NPs. The differences in toxicity mechanism of the coated and bare Ag NPs are a consequence of the absence and presence of co-occurring Ag+ ions in examined dispersion, respectively.

Microwave-assisted continuous flow phytosynthesis of silver nanoparticle/reduced graphene oxide composites and related visible light catalytic performance

The creation of an environmentally friendly synthesis method for silver nanomaterials (AgNPs) is an urgent concern for sustainable nanotechnology development. In the present study, a novel straightforward and green method for the preparation of silver nanoparticle/reduced graphene oxide (AgNP/rGO) composites was successfully developed through the combination of phytosynthesis, continuous flow synthesis and microwave-assistance. Oriental persimmon (Diospyros kaki Thunb.) extracts were used as both plant reducing and capping agents for fast online synthesis of AgNP/rGO composites. The experimental parameters were optimized and the morphologies of the prepared materials were investigated. The characterization results reveal that spherical AgNPs were quickly synthesized and uniformly dispersed on rGO sheets using the proposed online system. Fourier transform infrared spectroscopy analysis confirmed that phenols, flavonoids, and other substances in the plant extracts played a decisive role in the synthesis of AgNP/rGO composites. Using sodium borohydride (NaBH4) degradation of p-nitrophenol (4-NP) as a model, the catalytic activity of the prepared AgNP/rGO materials was evaluated. The complete degradation of 4-NP was achieved within 12 min through the use of AgNP/rGO materials, and the composite had a much better catalytic activity than the bare AgNPs and rGO had. Compared with the conventional chemical method, our online method is facile, fast, cost-efficient, and environmentally friendly.

Ag/Pd bimetal nanozyme with enhanced catalytic and photothermal effects for ROS/hyperthermia/chemotherapy triple-modality antitumor therapy

We prepared an Ag/Pd bimetallic nanozyme with the peroxidase-like activity as a nanocarrier to load doxorubicin (DOX). It combines photothermal conversion ability and catalytic production of hydroxyl radicals (HO•) to achieve enhanced antitumor effects. The temperature rise under near-infrared (NIR) laser irradiation was recorded to demonstrate the photothermal conversion ability of the Ag/Pd bimetallic nanoparticles (AgPd NPs). The peroxidase-like activity of the AgPd NPs was visually characterized by a color rendering experiment of TMB. Compared with bare Ag NPs, bimetallic AgPd NPs exhibit better photothermal conversion capability (η = 40.97%) and catalytic ability resulting from alloying with palladium. The antitumor effect was evaluated by cell experiments and tumor-bearing mice experiments, and it was confirmed that AgPd@BSA/DOX has excellent antitumor activity in vitro and in vivo. These results indicate that the AgPd@BSA/DOX can effectively catalyze hydrogen peroxide to generate hydroxyl radicals and induce hyperthermia under the irradiation of the NIR laser while releasing the chemotherapy drug DOX to achieve triple modality treatment. Intrinsic enzymatic activity and photothermal property of AgPd@BSA/DOX can be synergized for efficient antitumor treatment, which provided a novel strategy for cancer therapy.

Silver nanoparticles for selective detection of phosphorus pesticide containing π-conjugated pyrimidine nitrogen and sulfur moieties through non-covalent interactions

Bare silver nanoparticles were employed as a chemical sensor for selective detection of diazinon pesticide in fruit and vegetable samples. The sensing mechanism for detection of diazinon is based on the color change of AgNPs from yellow to pinkish-red followed by the red shift of LSPR absorption band in UV–Vis region after the addition of analyte in to the NPs solution. The color change and shift of LSPR band of AgNPs are observed only with diazinon molecule due to the π-conjugated pyrimidine nitrogen and sulfur moieties as well as specific orientation of diazinon molecule favored the non-covalent interactions with NPs. The control experiments with different pesticides and density functional theory (DFT) studies were also performed to validate the interactions between diazinon with AgNPs. Good linear range of 20–600ngmL−1 with limit of detection of 7ngmL−1 was obtained for determination of diazinon using AgNPs as a chemical sensor. Bare AgNPs was successfully exploited for quantitative determination of diazinon in fruit and vegetable samples (apple, grapes, beans and potato). The advantages of the present method are simple, rapid, selective as well as economic for the detection of pesticide in vegetable and fruit samples.

Synthesis of Au@Ag core-shell nanostructures with a poly(3,4-dihydroxy-L-phenylalanine) interlayer for surface-enhanced Raman scattering imaging of epithelial cells.

Poly(3,4-dihydroxy-L-phenylalanine) (polyDOPA) is a stable and biocompatible reducing agent. A versatile strategy is described here for the synthesis of core-shell Au@Ag nanostructures containing a polyDOPA interlayer. The latter provides abundant sites for deposition of nanocomposites, to immobilize molecules and to grow shells. The Au@polyDOPA@Ag nanoparticles are shown to generate strong and stable surface-enhanced Raman spectroscopy (SERS) signals compared to bare AuNPs and bare AgNPs. Folic acid was then immobilized on Au@polyDOPA@Ag nanoparticles and then applied to SERS imaging of human lung adenocarcinoma cell line A549 by the specific recognition of the folic acid receptor. The folic acid-conjugated SERS tags were promising to be nanoplatforms for imaging of cancer cells. Graphical abstract An Au@Ag core-shell nanostructures SERS nanotag with a polyDOPA interlayer was fabricated and then applied to SERS imaging of epithelial cells. (DOPA: 3,4-Dihydroxy-[L-phenylalanine]; FA: folic acid; 4-MBA: 4-mercaptobenzoic acid).

Coarsening-resistant Ag nanoparticles stabilized on amorphous TiOx nanoparticles

Bare Ag nanoparticles (∼10 nm) and Ag nanoparticles (1–20 nm) on the surfaces of larger TiOx nanoparticles were prepared by laser ablation of microparticle aerosols (LAMA). The behaviors of the nanoparticles during high temperature annealing were then studied with ex situ and in situ transmission electron microscopy. For the ex situ heating experiments, Ag and Ag-on-TiOx NPs were collected onto gold TEM grids and subjected to annealing treatments at 500 °C in argon, vacuum, and air. At this temperature, bare Ag NPs on carbon TEM supports coarsened rapidly in both air and argon atmospheres. In contrast, Ag-on-TiOx NPs that were heated to 500 °C in flowing argon or in a vacuum did not coarsen significantly and were remarkably stable. Ag-on-TiOx NPs that were heated to 500 °C in air, however, behaved quite differently. The TiOx crystallized upon heating and a significant loss of Ag were observed from the surfaces of the TiOx, likely due to sublimation. These results demonstrate that the surface defect structure and chemistry of the oxide support strongly influence the thermal stability of Ag NPs produced by LAMA.

Facile One-Pot Green Synthesis and Antibacterial Activities of GO/Ag Nanocomposites

New materials with good antibacterial activity synthesized by simple, environmentally friendly method have attracted numerous research interests. Taking advantage of Ag and graphene oxide (GO), the GO/Ag nanocomposites were prepared by a facile, green “one-pot” reaction to achieve superior antibacterial properties. AgNO3 is as a precursor of Ag nanoparticles (Ag NPs), and the grape seeds extract as an environmentally friendly reducing and stabilizing agent. The as-prepared GO/Ag nanocomposites were characterized by various technologies. The results indicated that the silver ion was reduced by the grade seeds extract and Ag NPs were decorated on the surface of GO nanosheets successfully. The average size of the Ag NPs anchored on the GO surface was 40–50 nm. In this study, we prepared GO/Ag nanocomposites with different Ag NPs to GO ratios and carefully investigated their antibacterial activities. We found that the GO/Ag nanocomposites show better antibacterial activity when the concentration of AgNO3 is 2 mM. GO/Ag nanocomposites have a minimum inhibitory concentration 23.8 μg/mL, lower than bare Ag NPs 25.5 μg/mL. We try to explain the antibacterial mechanism based on optical microscopy observation.

Influence of silver nanoparticles on heavy metals of pore water in contaminated river sediments

Despite the increasing knowledge on the discharge of silver nanoparticles (AgNPs) into the environment and their potential toxicity to microorganisms, the interaction of AgNPs with heavy metals remains poorly understood. This study focused on the effect of AgNPs on heavy metal concentration and form in sediment contaminated with heavy metals from the Xiangjiang River. The results showed that the concentration of Cu, Zn, Pb and Cd decreased and then increased with a change in form. The changes in form and concentrations of heavy metals in pore water suggested that Cu and Zn were more likely to be affected compared to Pb and Cd. The concentrations of Hg in sediment pore water in three AgNPs-dosed containers, increased greatly until they reached their peaks at 4.468 ± 0.133, 4.589 ± 0.235, and 5.083 ± 0.084 μg L−1 in Bare AgNPs, Citrate AgNPs and Tween 80 AgNPs, respectively. The measurements of Hg concentrations in the sediment pore water, combined with SEM and EDX analysis, demonstrated that added AgNPs stabilized in pore water and formed an amalgam with Hg0, which can affect Hg transportation over long distance.

Thiolated-2-methacryloyloxyethyl phosphorylcholine protected silver nanoparticles as novel photo-induced cell-killing agents

Event Abstract Back to Event Thiolated-2-methacryloyloxyethyl phosphorylcholine protected silver nanoparticles as novel photo-induced cell-killing agents Arunee Sangsuwan1, Hideya Kawasaki2, 3 and Yasuhiko Iwasaki2, 3 1 Kansai university, Graduate School of Science and Engineering, Japan 2 Kansai university, Faculty of Chemistry, Materials and Bioengineering, Japan 3 Kansai university, ORDIST, Japan Introduction: The usefulness of silver nanoparticles is well known due to their various advantages and extensive applications. Thus, the synthesis of metal nanoparticles with predictable and well-defined structures is of interest in medical and diagnostic applications[1]. However, AgNPs are of limited use in medicine because of their toxicity, which may damage healthy surrounding tissue. Research is continuously being conducted to resolve this issue. In this study, thiolated-2-methacryloyloxyethyl phosphorylcholine (MPC-SH) protected silver nanoparticles (MPC-AgNPs) were prepared as cell-killing agents under UV irradiation. The viability of cells in contact with MPC-AgNPs was relatively high and MPC-AgNPs also exhibit a cell-killing effect under UV irradiation. Materials and Methods: Thiol-terminated MPC (MPC-SH) was synthesized from the modified method of Goda and co-workers[2]. MPC-SH was mixed with silver nitrate and formic acid in deionized water. The solution was stirred at 70 °C for 3 hours. The reaction mixture was dialyzed to purify MPC-AgNPs. MPC-AgNPs were then centrifuged at 6,000 rpm for 15 minutes and lyophilized. The cytotoxicity of MPC-AgNPs was detemined by using HeLa cells and half maximal inhibitory concentration (LC50) of the nanoparticles was determined. Moreover, photo-induced toxicity of MPC-AgNPs was investigated under 356 nm photo irradiation. Results: The MPC-AgNPs can be prepared via strong thiolmetal coordination. Immobilized MPC on AgNPs was confirmed by XPS and IR analyses. The presence of AgNPs in the form of Ag0 was verified by XRD, and the average diameter of particles was determined as 13.4 ± 2.2 nm by TEM and 18.4 nm (PDI 0.18) by DLS. MPC-AgNPs showed toxicity at concentrations of 5 μg/mL or higher as shown in Fig.1. The LC50 of MPC-AgNPs was about 14.7 μg/mL, which shows a greater biocompatibility than AgNO3, bare AgNPs, chitosan-coated AgNTs, and polyvinylpyrrolidone-coated AgNPs, as determined by previous studies[3],[4]. Photo-induced toxicity of MPC-AgNPs on HeLa cells significantly decreased the cell viability to about 50% in a particle concentration at 0.5 μg/mL, and at a higher concentration of 1 μg/mL, the HeLa cells died completely from the photo-induced toxicity of MPC-AgNPs as shown in Fig.2. Discussion: MPC improves both biocompatibility and dispersity of nanoparticles. MPC-AgNPs used in our research were small size particles, which are conducive to high cytotoxicity, in accordance with the results of previous studies. In addition, the particles exhibit a highly potent cell-killing effect under UV irradiation; they lost SPR after irradiation, resulting in a decreasing SPR band and particle size, as measured by UV-visible spectrophotometry and DLS, respectively. Conclusion: The present study demonstrates that MPC-AgNPs can be prepared via strong thiolmetal coordination and the average diameter of particles was determined as 13.4 ± 2.2 nm by TEM and 18.4 nm (PDI 0.18) by DLS. MPC improves both biocompatibility and dispersity of nanoparticles. Our findings suggest that the cell death mechanism is possible because particles lose areas of their surface and particles losing MPC are toxic. Finally, biocompatible MPC-AgNPs are exhibited after activation by light, which makes them potentially useful as photo-induced cell-killing agents. Grant-in-Aid for Scientific Research on Innovative Areas ‘‘Nanomedicine Molecular Science’’ (No. 2306) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

Graphene oxide-silver nanocomposite as a promising biocidal agent against methicillin-resistant Staphylococcus aureus.

BackgroundMethicillin-resistant Staphylococcus aureus (MRSA) has been responsible for serious hospital infections worldwide. Nanomaterials are an alternative to conventional antibiotic compounds, because bacteria are unlikely to develop microbial resistance against nanomaterials. In the past decade, graphene oxide (GO) has emerged as a material that is often used to support and stabilize silver nanoparticles (AgNPs) for the preparation of novel antibacterial nanocomposites. In this work, we report the synthesis of the graphene-oxide silver nanocomposite (GO-Ag) and its antibacterial activity against relevant microorganisms in medicine.Materials and methodsGO-Ag nanocomposite was synthesized through the reduction of silver ions (Ag+) by sodium citrate in an aqueous GO dispersion, and was extensively characterized using ultraviolet-visible absorption spectroscopy, X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, and transmission electron microscopy. The antibacterial activity was evaluated by microdilution assays and time-kill experiments. The morphology of bacterial cells treated with GO-Ag was investigated via transmission electron microscopy.ResultsAgNPs were well distributed throughout GO sheets, with an average size of 9.4±2.8 nm. The GO-Ag nanocomposite exhibited an excellent antibacterial activity against methicillin-resistant S. aureus, Acinetobacter baumannii, Enterococcus faecalis, and Escherichia coli. All (100%) MRSA cells were inactivated after 4 hours of exposure to GO-Ag sheets. In addition, no toxicity was found for either pristine GO or bare AgNPs within the tested concentration range. Transmission electronic microscopy images offered insights into how GO-Ag nanosheets interacted with bacterial cells.ConclusionOur results indicate that the GO-Ag nanocomposite is a promising antibacterial agent against common nosocomial bacteria, particularly antibiotic-resistant MRSA. Morphological injuries on MRSA cells revealed a likely loss of viability as a result of the direct contact between bacteria and the GO-Ag sheets.

Green photochemical synthesis of fluorescent carbon spheres in-situ enwrapped around Ag nanoparticles

Biocompatible carbon-spheres-based nanocomposites exhibit great potential in biomedical and clinical applications. In this contribution we report the first green photochemical synthesis of carbon spheres through in-situ enwrapping around silver nanoparticles (CS-Ag NPs). Since mesoporous carbon spheres can provide the location for combining Ag NPs and other agents, one-step synthesis of glutathione-stabilized CS-Ag NPs could be readily realized by photoreduction. TEM characterization of CS-Ag NPs nanocomposites illustrates that Ag NPs were superbly wrapped inside the carbon spheres and also adhered to the surfaces of the carbon spheres. These porous CS-Ag NPs show excellent fluorescence and effective antibacterial efficiency, exhibiting ideal lengthened activities against Escherichia coli and Staphylococcus aureus compared with bare Ag NPs. The relevant rationale behind it could be attributed to the fact that CS-Ag NPs nanocomposites can provide some excellent niches for the durable and slow release of silver ions. This raises the possibility of promising applications of CS-Ag NPs nanocomposites as excellent antibacterial agents for the efficient monitoring of some disease-related bacteria.

Enhanced stability and antibacterial efficacy of a traditional Chinese medicine-mediated silver nanoparticle delivery system.

Silver nanoparticles (AgNPs) are widely used as antibacterial products in various fields. Recent studies have suggested that AgNPs need an appropriate stabilizer to improve their stability. Some antibacterial traditional Chinese medicines (TCMs) contain various reductive components, which can not only stabilize AgNPs but also enhance their antimicrobial activity. In this study, we developed a series of novel AgNPs using a TCM extract as a stabilizer, reducing agent, and antimicrobial agent (TCM-AgNPs). A storage stability investigation of the TCM-AgNPs suggested a significant improvement when compared with bare AgNPs. Further, conjugation of TCMs onto the AgNP surface resulted in stronger antimicrobial potency on antibacterial evaluation using Pseudomonas aeruginosa, Staphylococcus epidermidis, and Staphylococcus aureus with minimum inhibitory concentration 50% (MIC50) ratios (and minimum bactericidal concentration 90% [MBC90] ratios) of AgNPs to respective TCM-AgNPs as assessment indices. Among these, P. cuspidatum Sieb. et-conjugated AgNPs (P.C.-AgNPs) had the advantage of a combination of TCMs and AgNPs and was studied in detail with regard to its synthesis and characterization. The extraction time, reaction temperature, and concentrations of AgNO3 and Polygonum cuspidatum Sieb. et extract were critical factors in the preparation of P.C.-AgNPs. Further, the results of X-ray diffraction and Fourier transform infrared spectroscopy indicated successful preparation of P.C.-AgNPs. In representative studies, P.C.-AgNPs showed a well-defined spherical shape, a homogeneous small particle size (36.78 nm), a narrow polydispersity index (0.105), and a highly negative zeta potential (−23.6 mV) on transmission electron microscopy and dynamic light scattering. These results indicate that TCM-AgNPs have a potential role as antibacterial agents in the clinic setting.

Comparative toxicity of silver nanoparticles on oxidative stress and DNA damage in the nematode, Caenorhabditis elegans

This study examined the effects of polyvinylpyrrolidone (PVP) surface coating and size on the organismal and molecular toxicity of silver nanoparticles (AgNPs) on the nematode, Caenorhabditis elegans. The toxicity of bare AgNPs and 8 and 38nm PVP-coated AgNPs (PVP8-AgNPs, PVP38-AgNPs) were compared. The toxicity of AgNO3 was also tested because ion dissolution and particle-specific effects are often important characteristics determining Ag nanotoxicity. Comparative toxicity across AgNO3 and the three different types of AgNPs was first evaluated using a C. elegans mortality test by a direct comparison of the LC50 values. Subsequently, mutant screening followed by oxidative stress, mitochondrial toxicity and DNA damage assays were carried out at equitoxic (LC10 and LC50) concentrations to further assess the toxicity mechanism of AgNO3 and AgNPs. AgNO3 and bare AgNPs had similar toxicities, whereas PVP coating reduced the toxicity of the AgNPs significantly. Of the PVP-AgNPs, the smaller NPs were more toxic. Different groups of mutants responded differently to AgNO3 and AgNPs, which indicates that their toxicity mechanism might be different. AgNO3 and bare AgNPs induced mitochondrial membrane damage. None of the silver materials tested caused detectable polymerase-inhibiting DNA lesions in either the nucleus or mitochondria as measured by a quantitative PCR assay, but AgNO3, bare AgNPs and PVP8-AgNPs induced oxidative DNA damage. These results show that coatings on the AgNPs surface and the particle size make a clear contribution to the toxicity of the AgNPs, and oxidative stress-related mitochondrial and DNA damage appear to be potential mechanisms of toxicity.

Incompatibility of silver nanoparticles with lactate dehydrogenase leakage assay for cellular viability test is attributed to protein binding and reactive oxygen species generation

A growing number of studies report that conventional cytotoxicity assays are incompatible with certain nanoparticles (NPs) due to artifacts caused by the distinctive characteristics of NPs. Lactate dehydrogenase (LDH) leakage assays have inadequately detected cytotoxicity of silver nanoparticles (AgNPs), leading to research into the underlying mechanism. When ECV304 endothelial-like umbilical cells were treated with citrate-capped AgNPs (cAgNPs) or bare AgNPs (bAgNPs), the plasma membrane was disrupted, but the LDH leakage assay failed to detect cytotoxicity, indicating interference with the assay by AgNPs. Both cAgNPs and bAgNPs inactivated LDH directly when treated to cell lysate as expected. AgNPs adsorbed LDH and thus LDH, together with AgNPs, was removed from assay reactants during sample preparation, with a resultant underestimation of LDH leakage from cells. cAgNPs, but not bAgNPs, generated reactive oxygen species (ROS), which were successfully scavenged by N-acetylcysteine or ascorbic acid. LDH inhibition by cAgNPs could be restored partially by simultaneous treatment with those antioxidants, suggesting the contribution of ROS to LDH inactivation. Additionally, the composition of the protein corona surrounding AgNPs was identified employing liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. In sum, the LDH leakage assay, a conventional cell viability test method, should be employed with caution when assessing cytotoxicity of AgNPs.

Ag@C Core–Shell Colloidal Nanoparticles Prepared by the Hydrothermal Route and the Low Temperature Heating–Stirring Method and Their Application in Surface Enhanced Raman Scattering

Silver nanoparticles (AgNPs) with diameters in the range 40–80 nm have been synthesized by the hydrothermal route and the low temperature heating–stirring method. The influences of reaction time, reactant concentration, and temperature on the AgNP growth have been systemically studied. Experimental surface enhanced Raman scattering (SERS) results show that AgNPs prepared under different temperature and time exhibit a large difference in enhanced SERS signals for rhodamine 6G (10–6 M). Ag@C core–shell NPs have been proved to be formed by using the above two methods, and the carbon shell thickness is gradually increased with increasing reaction time and temperature. It is found that Ag@C NPs with a thick shell (more than 3 nm) have a very low SERS activity, while those with an ultrathin film (less than 1 nm) have a high SERS activity, indicating that carbon shell thickness is a key factor affecting the SERS, which has also been evaluated by finite-difference time-domain simulation. The existence of an ultrathin carbon shell around the AgNP can decrease its surface electric property; then Ag@C NP aggregates are easily formed which may produce the higher hot spots than the bare AgNPs. In addition, this kind of Ag@C NPs exhibits a long SERS-active shelf life (6 months), because the carbon shell can protect AgNPs from oxidation.