The rapidly increasing use of silver nanoparticles (Ag NPs) in consumer products and medical applications has raised ecological and human health concerns. A key question for addressing these concerns is whether Ag NP toxicity is mechanistically unique to nanoparticulate silver, or if it is a result of the release of silver ions. Furthermore, since Ag NPs are produced in a large variety of monomer sizes and coatings, and since their physicochemical behavior depends on the media composition, it is important to understand how these variables modulate toxicity. We found that a lower ionic strength medium resulted in greater toxicity (measured as growth inhibition) of all tested Ag NPs to Caenorhabditis elegans and that both dissolved silver and coating influenced Ag NP toxicity. We found a linear correlation between Ag NP toxicity and dissolved silver, but no correlation between size and toxicity. We used three independent and complementary approaches to investigate the mechanisms of toxicity of differentially coated and sized Ag NPs: pharmacological (rescue with trolox and N-acetylcysteine), genetic (analysis of metal-sensitive and oxidative stress-sensitive mutants), and physicochemical (including analysis of dissolution of Ag NPs). Oxidative dissolution was limited in our experimental conditions (maximally 15% in 24 h) yet was key to the toxicity of most Ag NPs, highlighting a critical role for dissolved silver complexed with thiols in the toxicity of all tested Ag NPs. Some Ag NPs (typically less soluble due to size or coating) also acted via oxidative stress, an effect specific to nanoparticulate silver. However, in no case studied here was the toxicity of a Ag NP greater than would be predicted by complete dissolution of the same mass of silver as silver ions.

The solubility of Ag NPs can affect their toxicity and persistence in the environment. We measured the solubility of organic-coated silver nanoparticles (Ag NPs) having particle diameters ranging from 5 to 80 nm that were synthesized using various methods, and with different organic polymer coatings including poly(vinylpyrrolidone) and gum arabic. The size and morphology of Ag NPs were characterized by transmission electron microscopy (TEM). X-ray absorption fine structure (XAFS) spectroscopy and synchrotron-based total X-ray scattering and pair distribution function (PDF) analysis were used to determine the local structure around Ag and evaluate changes in crystal lattice parameters and structure as a function of NP size. Ag NP solubility dispersed in 1 mM NaHCO(3) at pH 8 was found to be well correlated with particle size based on the distribution of measured TEM sizes as predicted by the modified Kelvin equation. Solubility of Ag NPs was not affected by the synthesis method and coating as much as by their size. Based on the modified Kelvin equation, the surface tension of Ag NPs was found to be ∼1 J/m(2), which is expected for bulk fcc (face centered cubic) silver. Analysis of XAFS, X-ray scattering, and PDFs confirm that the lattice parameter, a, of the fcc crystal structure of Ag NPs did not change with particle size for Ag NPs as small as 6 nm, indicating the absence of lattice strain. These results are consistent with the finding that Ag NP solubility can be estimated based on TEM-derived particle size using the modified Kelvin equation for particles in the size range of 5-40 nm in diameter.

We describe the fabrication of a label-free, chip-based biosensor based on the localized surface plasmon resonance (LSPR) of gold nanorods. Gold nanorods were chemisorbed onto a mercaptosilane-modified glass substrate, followed by conjugation of biotin to the nanorods. Streptavidin binding to biotin was monitored by the wavelength shift of the LSPR peak in the UV-vis extinction spectrum of the immobilized gold nanorods due to the change in local refractive index at the gold nanorod surface induced by streptavidin binding. The limit of detection of the sensor is 0.005 microg/mL (94 pM) in PBS and 1 microg/mL (19 nM) in serum, and the dynamic range spans 94 pM to 0.19 microM. The advantages of the nanorod-based sensor over an LSPR sensor that we had previously fabricated from gold nanospheres (Nath, N.; Chilkoti, A. Anal. Chem. 2002, 74, 504-509; J. Fluoresc. 2004, 14, 377-389; Anal. Chem. 2004, 76, 5370-5378) are the significantly lower detection limit and the internal self-reference that the signal of the nanorod sensor provides based on the measurement of peak wavelength shift.

We present the development of an analytical model that can be used for the rational design of a biosensor based on shifts in the local surface plasmon resonance (LSPR) of individual gold nanoparticles. The model relates the peak wavelength of light scattered by an individual plasmonic nanoparticle to the number of bound analyte molecules and provides an analytical formulation that predicts relevant figures-of-merit of the sensor such as the molecular detection limit (MDL) and dynamic range as a function of nanoparticle geometry and detection system parameters. The model calculates LSPR shifts for individual molecules bound by a nanorod, so that the MDL is defined as the smallest number of bound molecules that is measurable by the system, and the dynamic range is defined as the maximum number of molecules that can be detected by a single nanorod. This model is useful because it will allow a priori design of an LSPR sensor with figures-of-merit that can be optimized for the target analyte. This model was used to design an LSPR sensor based on biotin-functionalized gold nanorods that offers the lowest MDL for this class of sensors. The model predicts a MDL of 18 streptavidin molecules for this sensor, which is in good agreement with experiments and estimates. Further, we discuss how the model can be utilized to guide the development of future generations of LSPR biosensors.

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We report the use of individual gold nanorods as plasmonic transducers to detect the binding of streptavidin to individual biotin-conjugated nanorods in real time on a surface. Label-free detection at the single-nanorod level was performed by tracking the wavelength shift of the nanorod-localized surface plasmon resonant scattering spectrum using a dark-field microspectroscopy system. The lowest streptavidin concentration that was experimentally measured was 1 nM, which is a factor of 1000-fold lower than the previously reported detection limit for streptavidin binding by biotinylated single plasmonic nanostructures. We believe that the current optical setup is able to reliably measure wavelength shifts as small as 0.3 nm. Binding of streptavidin at 1 nM concentration induces a mean resonant wavelength shift of 0.59 nm suggesting that we are currently operating at close to the limit of detection of the system.

The persistence of silver nanoparticles in aquatic environments and their subsequent impact on organisms depends on key transformation processes, which include aggregation, dissolution, and surface modifications by metal-complexing ligands. Here, we studied how cysteine, an amino acid representative of thiol ligands that bind monovalent silver, can alter the surface chemistry, aggregation, and dissolution of zero-valent silver nanoparticles. We compared nanoparticles synthesized with two coatings, citrate and polyvinylpirrolidone (PVP), and prepared nanoparticle suspensions (approximately 8 μM total Ag) containing an excess of cysteine (400 μM). Within 48 h, up to 47% of the silver had dissolved, as indicated by filtration of the samples with a 0.025-μm filter. Initial dissolution rates were calculated from the increase of dissolved silver concentration when particles were exposed to cysteine and normalized to the available surface area of nanoparticles in solution. In general, the rates of dissolution were almost 3 times faster for citrate-coated nanoparticles relative to PVP-coated nanoparticles. Rates tended to be slower in solutions with higher ionic strength in which the nanoparticles were aggregating. Xray absorption spectroscopy analysis of the particles suggested that cysteine adsorbed to silver nanoparticles surfaces through the formation of Ag(+I)sulfhydryl bonds. Overall, the results of this study highlight the importance of modifications by sulfhydrylcontaining ligands that can drastically influence the long-term reactivity of silver nanoparticles in the aquatic environment and their bioavailability to exposed organisms. Our findings demonstrate the need to consider multiple interlinked transformation processes when assessing the bioavailability, environmental risks, and safety of nanoparticles, particularly in the presence of metalbinding ligands.

A method for synthesizing hollow nanoscopic polypyrrole and poly(N-methylpyrrole) capsules is described. The method employs gold nanoparticles as templates for polymer nucleation and growth. Etching the gold leaves a structurally intact hollow polymer capsule with a shell thickness governed by polymerization time (ca. 5 to >100 nm) and a hollow core diameter dictated by the diameter of the template particle (ca. 5−200 nm). Transport rates of gold etchant through the polymer shell to the gold core were found to depend on the oxidation state of the polymer, those rates being a factor of 3 greater for the reduced form of the polymer. We show for the first time that not only is the particle a useful template material but also that it can be employed to deliver guest molecules into the capsule core. For example, ligands attached to the gold surface prior to poly(N-methylpyrrole) formation remained trapped inside the hollow capsule following polymer formation and gold etching.