AgNP Suspensions Research Articles

Transport and fate of silver as polymer-stabilised nanoparticles and ions in a pilot wastewater treatment plant, followed by sludge digestion and disposal of sludge/soil mixtures: A case study

A case study of transport and changes in properties of polymer-stabilised Ag NPs is presented in this paper investigating their interaction in different treatment steps within a fully realistic pilot wastewater treatment plant (WWTP), in anaerobic digested sludge, and in soil/sludge mixtures. The fate of the same Ag NPs was tracked in these environments, hence taking the history of the Ag NPs into account. The results show that most of the Ag NPs end up in the sludge (80–100%), also after anaerobically digestion. Furthermore, the fraction of silver in the supernatant was very low after 48 h incubation with silver-containing digested sludge mixed with different soil types. However, when Ag NPs were added directly to the sludge/soil mixture, soluble silver was present in the supernatant with sandy soil, but not with clayey soil. In all, generated findings show that risk assessments and toxicological studies of Ag NPs suspensions must take into account possible chemical and particle transformations upon environmental entry, as silver in general become less soluble when transported to WWTPs and interacting with sludge, and soil.

Influence of Water Hardness on Silver Ion and Silver Nanoparticle Fate and Toxicity Toward Nitrosomonas europaea.

This study investigated the influence of water hardness (Mg2+ and Ca2+) on the fate and toxicity of 20 nm citrate silver nanoparticles (AgNPs) and Ag+ toward Nitrosomonas europaea, a model ammonia-oxidizing bacterium. Nitrification inhibition of N. europaea by 1 ppm AgNPs and 0.5 ppm Ag+ was reduced from 80% and 83%, respectively, in the absence of Mg2+ to 2% and 33%, respectively, in the presence of 730 μM Mg2+. Introduction of Mg2+ resulted in the rapid aggregation of the AgNP suspensions and reduced the 3 h Ag+ dissolution rates from 30%, in the absence of Mg2+, to 9%, in the presence of 730 μM Mg2+. Reduced AgNP dissolution rates resulted in decreased concentrations of silver that were found adsorbed to N. europaea cells. Increasing AgNP concentrations in the presence of Mg2+ increased the observed inhibition of nitrification, but was always less than what was observed in the absence of Mg2+. The presence of Mg2+ also reduced the adsorption of Ag+ to cells, possibly due to multiple mechanisms, including a reduction in the negative surface charge of the N. europaea membrane and a competition between Mg2+ and Ag+ for membrane binding sites and transport into the cells. Ca2+ demonstrated similar protection mechanisms, as Ag+ toxicity was reduced and AgNP suspensions aggregated and decreased their dissolution rates. These results indicate that the toxicity of Ag+ and AgNPs to nitrifying bacteria in wastewater treatment would be less pronounced in systems with hard water.

Continuous Production of Tailored Silver Nanoparticles by Polyol Synthesis and Reaction Yield Measured by X-ray Absorption Spectroscopy: Toward a Growth Mechanism

Two complementary topics are discussed: (i) the continuous production of silver nanoparticles (AgNPs) and (ii) the measurement of reaction yield by X-ray absorption near-edge spectroscopy (XANES). The continuous polyol synthesis of AgNPs in grams amount and in the size range of 7-104 nm was carried out in the segmented flow tubular reactor (SFTR). Particle size was tailored by controlling the synthesis parameters such as temperature, reactant concentrations, and polyvinylpyrrolidone (PVP) molecular weight. The continuous production was tested for 4 h, and the product has shown constant particle size distribution over the whole production time. Reliable continuous production of 2.3 g h(-1) of similar to 100 nm AgNPs can be achieved with a lab-scale SFTR. The produced particles were fully characterized with respect to size, size distribution, and chemical purity. To better understand the growth mechanism, synchrotron-radiation high-resolution X-ray diffraction, and X-ray absorption spectroscopy data were collected directly on the AgNPs suspension. In particular, from}CANES experiments, the conversion yield of Ag+ to Ag-0 was measured. The results are consistent with a two-step process where PVP controls the particle nucleation while growth is ensured by thermally induced ethylene glycol oxidation.

Photochemical Transformation and Photoinduced Toxicity Reduction of Silver Nanoparticles in the Presence of Perfluorocarboxylic Acids under UV Irradiation

The impact of perfluorocarboxylic acids (PFCAs) with carbon chain length C2 to C8 on the dissolution, aggregation, reactive oxygen species (ROS) generation, and toxicity of citrate-coated AgNPs was investigated under UV irradiation. The presence of PFCAs decreased dissolution, aggregation, ROS generation, and toxicity of AgNPs because the negatively charged PFCAs sorbed on AgNP surface enhanced their stability. Both dissolution and aggregation rate of AgNPs decreased with chain length of PFCAs under UV irradiation, primarily because PFCAs with longer chain length sorbed on AgNP surface could form thicker coatings. The dissolution of AgNPs followed pseudo-first-order kinetics, and the rate constant decreased from 0.58 h(-1) with C2 to 0.30 h(-1) with C8. The hydrodynamic diameters of AgNPs linearly increased under UV irradiation with aggregation rates ranged from 72.1 to 143.5 nm/h. O2(•-) generation was observed in AgNP suspension with quantum yield of 0.12%, but was completely suppressed by PFCAs because they inhibited the interaction between photoelectrons and O2. A linear correlation was established between dissolved Ag(+) and bacterial survival rates of AgNPs with and without PFCAs under UV irradiation. This study highlights the necessity of considering coexisting organic contaminants when investigating the environmental behaviors of AgNPs.

Silver percutaneous absorption after exposure to silver nanoparticles: A comparison study of three human skin graft samples used for clinical applications

Silver nanoparticles (AgNPs) are increasingly applied to a wide range of materials for biomedical use. These enable a close contact with human skin, thanks to the large release of silver ions that is responsible for a broad spectrum of antimicrobial activity. Silver can permeate the skin; however, there are no data available on silver permeation through skin grafts commonly used in burns recovery. The aim of our study was to evaluate silver penetration using fresh, cryopreserved, and glycerolized human skin grafts after exposure to a suspension of AgNPs in synthetic sweat using a Franz diffusion cell apparatus for 24h.Silver permeation profiles revealed a significantly higher permeation through glycerolized skin compared with both fresh and cryopreserved skin: 24-h silver flux penetration was 0.2ngcm−2h−1 (lag time: 8.2h) for fresh skin, 0.3ngcm−2h−1 (lag time: 10.9h) for cryopreserved skin, and 3.8ngcm−2h−1 (lag time: 6.3h) for glycerolized skin.Permeation through glycerolized skin is significantly higher compared to both fresh and cryopreserved skin. This result can generate relevant clinical implications for burns treatment with products containing AgNPs.

Storage Wars: how citrate-capped silver nanoparticle suspensions are affected by not-so-trivial decisions

A critical but often overlooked component of silver nanoparticle (AgNP) suspensions involves their behavior following short- and long-term storage. The current study investigates the integrity of citrate-capped AgNP suspensions, nominally 20 nm in average diameter, in a series of distinct storage conditions, based on possible combinations of reasonable decisions researchers make, both nanoparticle-based (AgNP and relative citrate concentration) and environmental-based (solution oxygenation and ambient light or dark). AgNP integrity was determined by monitoring single particle stability, aggregation/agglomeration, and oxidation for 104 days. We demonstrate that AgNP suspensions lose their physical and chemical integrity by two distinct processes: (1) oxidation only (light-independent) and (2) oxidation followed by photo-reduction (light-dependent), following initial dilution from a concentrated (and newly synthesized) AgNP stock solution. Optical spectroscopy indicates that the effects of oxidation are readily observed while the effects of photo-reduction are less obvious, leading to a greater increase in average particle diameter, the formation of new, metallic nanoparticles, and the oxidation of the parent citrate capping agent. In general, the overall integrity of citrate-capped AgNP suspensions are best maintained when these solutions are purged with nitrogen gas and stored in the dark at the highest AgNP and citrate concentrations. This study outlines a strategy for both assessing and monitoring the integrity of AgNP suspensions in an effort to harmonize long-term experiments and promote inter-laboratory consistency.

Silver Nanoparticle Behavior, Uptake, and Toxicity in Caenorhabditis elegans: Effects of Natural Organic Matter

Significant progress has been made in understanding the toxicity of silver nanoparticles (Ag NPs) under carefully controlled laboratory conditions. Natural organic matter (NOM) is omnipresent in complex environmental systems, where it may alter the behavior of nanoparticles in these systems. We exposed the nematode Caenorhabditis elegans to Ag NP suspensions with or without one of two kinds of NOM, Suwannee River and Pony Lake fulvic acids (SRFA and PLFA, respectively). PLFA rescued toxicity more effectively than SRFA. Measurement of total tissue silver content indicated that PLFA reduced total organismal (including digestive tract) uptake of ionic silver, but not of citrate-coated Ag NPs (CIT-Ag NPs). The majority of the CIT-Ag NP uptake was in the digestive tract. Limited tissue uptake was detected by hyperspectral microscopy but not by transmission electron microscopy. Co-exposure to PLFA resulted in the formation of NOM-Ag NP composites (both in medium and in nematodes) and rescued AgNO3- and CIT-Ag NP-induced cellular damage, potentially by decreasing intracellular uptake of CIT-Ag NPs.

Inhibition effect of silver nanoparticles on herpes simplex virus 2

The herpes simplex virus 2 (HSV-2) is one of the most important sexually transmitted pathogens, and can facilitate the spread of human immunodeficiency virus. The currently available antiviral drugs have certain limitations. Nanosilver has received increasing attention recently with respect to its antibacterial and antiviral properties. The purpose of this study was to determine the inhibiting effect and mechanism of silver nanoparticles (Ag-NPs) on HSV-2. The cytotoxicity of Vero cells induced by different Ag-NP concentrations was investigated by using the methyl thiazolyl tetrazolium (MTT) assay. The inhibiting effect of Ag-NPs on HSV-2 at various times was also evaluated by using a plaque assay. The toxicity of 100 μg/mL Ag-NPs on Vero cells was very low. The mixture of Ag-NP suspension and HSV-2 prior to infecting cells could significantly inhibit the production of progeny viruses. Ag-NPs also inhibited the replication of HSV-2 for 24 h before infecting cells with HSV-2. Therefore, 100 μg/mL Ag-NPs could completely inhibit HSV-2 replication. Ag-NPs at nontoxic concentrations were capable of inhibiting HSV-2 replication when administered prior to viral infection or soon after initial virus exposure. This suggests that the mode of action of Ag-Nps occurs during the early phases of viral replication.

Cationic-anionic polyelectrolyte interaction as a tool to graft silver nanoparticles on hydroxyapatite crystals and prevent cytotoxicity

This paper reports an easy, green and low cost method to support silver nanoparticles (AgNPs) onto functionalized hydroxyapatite (HA) crystals. The process involves low molecular weight poly(ethylenimine) (PEI) as a reducing and stabilizing agent of AgNPs and polyacrylic acid (PAA) as a functionalizing agent of hydroxyapatite (HAPAA). With respect to HA, the negatively charged surface of HAPAA significantly improves the interaction with the positively charged surface of PEI-stabilized AgNPs. AgNPs content up to about 6.5 wt% in the composite HAPAA–AgNPs crystals has been obtained by varying the volume of the AgNPs suspension submitted to interaction with HAPAA. The cytotoxicity of the crystals, evaluated through in vitro tests on MG63 osteoblast-like human cells, increases on growing Ag content, but it is reduced by the presence of PAA. The HAPAA–AgNPs crystals display similar dose-dependent antibacterial activity towards gram-positive and gram-negative bacteria. The comparison of data indicates that an AgNPs content of about 2 wt% allows for combining absence of cytotoxicity with a significant long-standing antibacterial activity towards both Staphylococcus aureus and Escherichia coli.

Green Synthesis of Silver Nanoparticles: Structural Features and In Vivo and In Vitro Therapeutic Effects against Helicobacter pylori Induced Gastritis.

This study evaluates in vivo and in vitro anti-Helicobacter pylori (H. pylori) efficacy of silver nanoparticles (Ag-NPs) prepared via a cost-effective green chemistry route wherein Peganum harmala L. seeds extract was used as a reducing and capping agent. The structural features, as elucidated by surface plasmon resonance spectrophotometry, transmission electron microscopy, and powder X-ray diffraction spectroscopy, revealed the Ag-NPs synthesized to be polydispersed in nature and spherical in shape with 5–40 nm size. A typical Ag-NPs suspension (S5), with size being 15 nm, when tested in vitro against forty-two local isolates and two reference strains, showed a considerable anti-H. pylori activity. In case of in vivo trial against H. pylori induced gastritis, after oral administration of 16 mg/kg body weight of S5 for seven days, a complete clearance was recorded in male albino rates. In comparative time-killing kinetics, S5 exhibited dose- and time-dependent anti-H. pylori activity that was almost similar to tetracycline and clarithromycin, less than amoxicillin, but higher than metronidazole. Furthermore, S5 was found to be an equally effective anti-H. pylori agent at low (≤4) and high pH with no drug resistance observed even up to 10 repeated exposures while a significant drug resistance was recorded for most of the standard drugs employed. The present results revealed the potential of the synthesized Ag-NPs as safer bactericidal agents for the treatment of H. pylori induced gastritis.

Effects of ionization on the toxicity of silver nanoparticles to Japanese medaka (Oryzias latipes) embryos

Increase in the use of manufactured nanomaterials (NMs) has led to concerns about the environmental impacts. Especially, hazard of metal-based NMs is more severe due to ions released from surface by water quality parameters and physicochemical properties after entering into the water environment. However, little is known about the effects of ionization on the toxicity of metal-based NMs in the water environment. To address this question, we prepared the suspensions of silver nanoparticles (AgNP) at 25 μg L−1 containing different concentrations of Ag+ (5, 10, 20, 45, and 75% Ag+ to total Ag), and evaluated their toxicity to Japanese medaka (Oryzias latipes) embryos. Higher Ag+ ratios in the AgNP suspension, suggesting the lower number of particles, led to the higher adverse effects on embryos and sac-fries. In addition, histopathology analysis revealed that AgNPs penetrated through chorion of eggs and skin membrane, and were distributed into the tissues. The results imply that the ionization could decrease the toxicity of metal-based NMs in the water environment.

Colorimetric Detection of Trivalent Chromium in Aqueous Solution Using Tartrate-Capped Silver Nanoparticles as Probe

This study describes a simple and highly selective method for the colorimetric detection of trivalent chromium (Cr3+) using tartrate-capped silver nanoparticles (AgNPs) as probe. The addition of tartrate to the initially prepared AgNPs gives tartrate-stabilized AgNPs ascribing to the electrostatic repulsion of the highly negatively charged tartrate ions covered on the surface of AgNPs. It is found that, in the presence of Cr3+ in aqueous solution, the aggregation of tartrate-stabilized AgNPs occurs. The color of AgNPs suspension changes from yellow to pink and the surface plasmon absorption band broadens and red shifts, which could be applied for the colorimetric detection of Cr3+ in aqueous solution. The utilization of tartrate-stabilized AgNPs as probe substantially increases the selectivity and sensitivity for colorimetric detection of Cr3+. Control experiments with the addition of over 14 other metal ions, such as Pb2+, Zn2+, Cr2O7(2-), Cd2+, Co2+, Cu2+, Al3+, Ni2+, Mn2+, Ba2+, Fe3+, Ca2+, Mg2+, Sr+ do not result in a distinct change in the color or in the spectrum of the suspension, indicating that these metal ions do not interfere with the colorimetric detection of Cr3+. Under the conditions employed here, A502/A393 (ratio of absorption value at 502 nm to 393 nm) is linear with the concentration of Cr3+ within a concentration range from 0.1 to 1.17 microM with a detection limit of 0.06 microM. This study may offer a simple, rapid and sensitive approach to colorimetric detection of Cr3+ in aqueous solution.

Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag+) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag+ release, especially for smaller sized AgNP suspensions (∼10nm) at low pH (∼6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag+ and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1mmol/L) and organic matter (<5mg/L) levels.

Silver nanoparticles induce cytotoxicity, but not cell transformation or genotoxicity on Balb3T3 mouse fibroblasts

AbstractSilver nanoparticles (Ag NPs) are one of the most common nanomaterials present in nanotechnology-based products. Here, the physical chemical properties of Ag NPs suspensions of 44 nm, 84 nm and 100 nm sizes synthesized in our laboratory were characterized. The NM-300 material (average size of 17 nm), supplied by the Joint Research Centre Nanomaterials Repository was also included in the present study. The Ag NPs potential cytotoxicity was tested on the Balb3T3 cell line by the Colony Forming Efficiency assay, while their potential morphological neoplastic transformation and genotoxicity were tested by the Cell Transformation Assay and the micronucleus test, respectively. After 24 h of exposure, NM-300 showed cytotoxicity with an IC50 of 8 µM (corresponding to 0.88 µg/mL) while for the other nanomaterials tested, values of IC50 were higher than 10 µM (1.10 µg/mL). After 72 h of exposure, Ag NPs showed size-dependent cytotoxic effect with IC50 values of 1.5 µM (1.16 µg/mL) for NM-300, 1.7 µM (1.19 µg/mL) for Ag 44 nm, 1.9 µM (0.21 µg/mL) for Ag 84 nm and 3.2 µM (0.35 µg/mL) for Ag 100 nm. None of the Ag NPs tested was able to induce either morphological neoplastic transformation or micronuclei formation.

Green synthesis of silver nanoparticles by microorganism using organic pollutant: its antimicrobial and catalytic application

A novel approach for the green synthesis of silver nanoparticles (AgNPs) from aqueous solution of AgNO3 using culture supernatant of phenol degraded broth is reported in this work. The synthesis was observed within 10h, and AgNPs showed characteristic surface plasmon resonance around 410nm. Spherical nanoparticles of size less than 30nm were observed in transmission electron microscopy. X-ray diffraction pattern corresponding to 111, 200, 220, and 311 revealed the crystalline nature of the as-formed nanoparticles. It was found that the colloidal solution of AgNP suspensions exhibited excellent stability over a wide range of ionic strength, pH, and temperature. The effect of pH and ionic strength indicated that stabilization is due to electrostatic repulsion arising from the negative charge of the conjugate proteins. The AgNPs showed highly potent antimicrobial activity against Gram-positive, Gram-negative, and fungal microorganisms. The as-prepared AgNPs showed excellent catalytic activity in reduction of 4-nitrophenol to 4-aminophenol by NaBH4. By manufacturing magnetic alginate beads, the reusability of the AgNPs for the catalytic reaction has been demonstrated.

Removal of silver nanoparticles by coagulation processes

Commercial use of silver nanoparticles (AgNPs) will lead to a potential route for human exposure via potable water. Coagulation followed by sedimentation, as a conventional technique in the drinking water treatment facilities, may become an important barrier to prevent human from AgNP exposures. This study investigated the removal of AgNP suspensions by four regular coagulants. In the aluminum sulfate and ferric chloride coagulation systems, the water parameters slightly affected the AgNP removal. However, in the poly aluminum chloride and polyferric sulfate coagulation systems, the optimal removal efficiencies were achieved at pH 7.5, while higher or lower of pH could reduce the AgNP removal. Besides, the increasing natural organic matter (NOM) would reduce the AgNP removal, while Ca2+ and suspended solids concentrations would also affect the AgNP removal. In addition, results from the transmission electron microscopy and X-ray diffraction showed AgNPs or silver-containing nanoparticles were adsorbed onto the flocs. Finally, natural water samples were used to validate AgNP removal by coagulation. This study suggests that in the case of release of AgNPs into the source water, the traditional water treatment process, coagulation/sedimentation, can remove AgNPs and minimize the silver ion concentration under the well-optimized conditions.

Detection and characterization of silver nanoparticles in chicken meat by asymmetric flow field flow fractionation with detection by conventional or single particle ICP-MS

A method of analysis of silver nanoparticles (AgNPs) in chicken meat was developed. The homogenized chicken meat sample, which was spiked with AgNPs, was subjected to enzymolysis by Proteinase K for 40 min at 37 °C. Transmission electron microscopy and inductively coupled plasma mass spectrometry (ICP-MS) in single particle mode were used to characterize the number-based size distribution of AgNPs in the meat digestate. Because similar size distributions were found in the meat digestate and in the aqueous suspension of AgNPs used for spiking the meat, it was shown that no detectable dissolution of the AgNPs took place during the sample preparation stage. The digestate was injected into the asymmetric flow field flow fractionation (AF(4)) -ICP-MS system, which enabled fractionation of nanoparticles from the remaining meat matrix, and resulted in one large peak in the fractograms as well as two smaller peaks eluting close to the void volume. The recovery of silver contained in the large AgNP peak was around 80%. Size determination of AgNPs in the meat matrix, based on external size calibration of the AF(4) channel, was hampered by non-ideal (early elution) behavior of the AgNPs. Single particle ICP-MS was applied for determination of the number-based particle size distribution of AgNPs in collected fractions. The presented work describes for the first time the coupling of AF(4) and ICP-MS for AgNP separation in a food matrix.

Comparing the effects of nanosilver size and coating variations on bioavailability, internalization, and elimination, using Lumbriculus variegatus

As the production and applications of silver nanoparticles (AgNPs) increase, it is essential to characterize fate and effects in environmental systems. Nanosilver materials may settle from suspension; therefore, the authors' objective was to utilize environmentally relevant bioassays and study the impact, bioaccumulation, tissue distribution, uptake, and depuration of AgNPs on a sediment-dwelling invertebrate, Lumbriculus variegatus. Hydrodynamic diameters of uncoated 30-nm, 80-nm, and 1500-nm AgNP powders and a polyvinyl pyrrolidone (PVP) AgNP suspension were measured utilizing dynamic light scattering in freshwater media (0-280 µS/cm). Aggregation for 30 nm, 80 nm, and 1500 nm silver increased with conductivity but was minimal for PVP silver. Lumbriculus variegatus were exposed to AgNPs or silver nitrate (AgNO3 ) spiked into sediment (nominally 100 mg/kg) and water (PVP 30 nm and 70 nm Ag, nominally 5 mg/L). Uptake was assessed through inductively coupled plasma mass spectroscopy (ICP-MS) and hyperspectral imaging. Particle sizes were examined through field flow fractionation-ICP-MS (FFF-ICP-MS) and ICP-MS in single particle mode (SP-ICP-MS). Lumbriculus variegatus were also depurated for 6 h, 8 h, 24 h, and 48 h to determine gut clearance. Bioaccumulation factors of sediment-exposed L. variegatus were similar regardless of particle size or coatings. The FFF-ICP-MS and SP-ICP-MS detected AgNPs for up to 48 h post depuration. The present study provides information on bioaccumulation and interactions of AgNPs within biological systems.

Just add water: reproducible singly dispersed silver nanoparticle suspensions on-demand

Silver nanoparticles (AgNPs) are of interest due to their antimicrobial attributes, which are derived from their inherent redox instability and subsequent release of silver ions. At the same time, this instability is a substantial challenge for achieving stable long-term storage for on-demand use of AgNPs. In this study, we describe and validate a “just add water” approach for achieving suspensions of principally singly dispersed AgNPs. By lyophilizing (freeze drying) the formulated AgNPs into a solid powder, or cake, water is removed thereby eliminating solution-based chemical changes. Storing under inert gas further reduces surface reactions such as oxidation. An example of how to optimize a lyophilization formulation is presented, as well as example formulations for three AgNP core sizes. This “just add water” approach enables ease of use for the researcher desiring on-demand singly dispersed AgNP suspensions from a single master batch. Implementation of this methodology will enable studies to be performed over long periods of time and across different laboratories using particles that are identical chemically and physically and available on-demand. In addition, the approach of freeze drying and on-demand reconstitution by adding water has enabled the development of AgNP reference materials with the required shelf-life stability, one of the principal objectives of this research.

Surface-enhanced Raman effect in hybrid metal–semiconductor nanoparticle assemblies

Hybrid metal–semiconductor nanoparticles consisting of silver nanoparticle cores (AgNPs) coated with a layer of CdSe quantum dots (QDs) have been studied by Raman spectroscopy. The hybrid nanoparticles were prepared via electrostatic interaction by mixing aqueous suspensions of QDs and AgNPs, where opposite charges on the AgNPs and QDs surfaces were induced by opportunely selected capping agents. Assemblies of such hybrid nanoparticles show an increased intensity of the Raman spectrum of up to 500 times, when compared to that of the sole QDs. This enhancement is attributed to the SERS effect (Surface-enhanced Raman scattering). Such enhancement of the Raman modes suggests several opportunities for further research, both in imaging and sensing applications.