An Unprecedented Metal Distribution in Silica Nanoparticles Determined by Single-Particle Inductively Coupled Plasma Mass Spectrometry.

Metal-containing nanoparticles are now common in applications ranging from catalysts to biomarkers. However, little research has focused on per-particle metal content in multicomponent nanoparticles. In this work, we used single-particle inductively coupled plasma mass spectrometry (ICP-MS) to determine the per-particle metal content of silica nanoparticles doped with tris(2,2'-bipyridyl)ruthenium(II). Monodispersed silica nanoparticles with varied Ru doping levels were prepared using a water-in-oil microemulsion method. These nanoparticles were characterized using common bulk-sample methods such as absorbance spectroscopy and conventional ICP-MS, and also with single-particle ICP-MS. The results showed that averaged concentrations of metal dopant measured per-particle by single-particle ICP-MS were consistent with the bulk-sample methods over a wide range of dopant levels. However, the per-particle amount of metal varied greatly and did not adhere to the usual Gaussian distribution encountered with one-component nanoparticles, such as gold or silver. Instead, the amount of metal dopant per silica particle showed an unexpected geometric distribution regardless of the prepared doping levels. The results indicate that an unusual metal dispersal mechanism is taking place during the microemulsion synthesis, and they challenge a common assumption that doped silica nanoparticles have the same metal content as the average measured by bulk-sample methods.

Graphene/gold nanoparticle composites for ultrasensitive and versatile biomarker assay using single-particle inductively-coupled plasma/mass spectrometry.

An ultrasensitive and versatile assay for biomarkers has been developed using graphene/gold nanoparticles (AuNPs) composites and single-particle inductively-coupled plasma/mass spectrometry (spICP-MS). Thrombin was chosen as a model biomarker for this study. AuNPs modified with thrombin aptamers were first non-selectively adsorbed onto the surface of graphene oxide (GO) to form GO/AuNPs composites. In the presence of thrombin, the AuNPs desorbed from the GO/AuNPs composites due to a conformation change of the thrombin aptamer after binding with thrombin. The desorbed AuNPs were proportional to the concentration of thrombin and could be quantified by spICP-MS. By counting the individual AuNPs in the spICP-MS measurement, the concentration of thrombin could be determined. This assay achieved an ultralow detection limit of 4.5 fM with a broad linear range from 10 fM to 100 pM. The method also showed excellent selectivity and reproducibility when a complex protein matrix was evaluated. Furthermore, the diversity and ready availability of ssDNA ligands make this method a versatile new technique for ultrasensitive detection of a wide variety of biomarkers in clinical diagnostics.

Aggregation-based determination of mercury(II) using DNA-modified single gold nanoparticle, T-Hg(II)-T interaction, and single-particle ICP-MS.

An ultrasensitive assay is described for the detection and determination of Hg2+(aq) in water samples based on single-particle inductively-coupled plasma/mass spectrometry (spICP-MS). In the presence of Hg2+(aq), AuNPs modified with a segment of single-stranded DNA aggregate due to the formation of the well-known thymine (T)-Hg2+-T complex. Single particle (sp) ICP-MS is used quantify the degree of aggregation by the overall decrease in number of detected AuNPs or NP aggregates. Compared with most other Hg2+ assays that use the same principle of aggregation-dispersion with DNA modified AuNPs, this method has a much lower detection limit of (0.031 ng L-1, 155 fM) and a wider (10,000-fold) linear range (up to 1 µg L-1). The method also showed good practical potential because of its minimal interference from the water sample matrix. Graphical abstractSchematic representation of Hg2+ determination by using modified AuNP probes measured by spICP-MS. AuNPs pulses detected in ICP-MS is relative to the aggregation status of AuNPs based on thymine-Hg2+-thymine interaction.

Effects of a nanoscale silica matrix on the fluorescence quantum yield of encapsulated dye molecules.

The effects that nanometer-sized matrices have on the properties of molecules encapsulated within the nanomatrix are not fully understood. In this work, dye-doped silica nanoparticles were employed as a model for studying the effects of a nanomatrix on the fluorescence quantum yield of encapsulated dye molecules. Two types of dye molecules were selected based on their different responses to the surrounding media. Several factors that affect fluorescence quantum yields were investigated, including aggregation of dye molecules, diffusion of atmospheric oxygen, concentration of dye molecules, and size of the nanomatrix. The results showed that the silica nanomatrix has a varied effect on the fluorescence quantum yield of encapsulated dye molecules, including enhancement, quenching and insignificant changes. Both the properties of dye molecules and the conditions of the nanomatrix played important roles in these effects. Finally, a physical model was proposed to explain the varied nanomatrix effects on the fluorescence quantum yield of encapsulated dye molecules.

A target-induced fluorescent nanoparticle for in situ monitoring of Zn(II).

A target-induced fluorescent silica nanoparticle has been developed for the identification, enrichment and in situ determination of trace amounts of zinc(II). The nanoparticle combines the advantages of target-induced fluorescent compounds and the small size of the nanomaterial to produce a new, smarter nanosignaling material that is capable of selectively enriching a target and detecting a specific binding process in one step. As the target analyte, Zn(II), changes the fluorescence characteristics of the nanoparticle and effectively 'turns on' the fluorescence signal, no separation step is needed to confirm or quantify the binding process. The designed nanoparticle was characterized by several aspects prior to monitoring of Zn(II) in situ. The interferences from common metal ions were studied in detail. The photostability and reversibility of the sensing materials were investigated as well. The ability of this nanoparticle to detect the target Zn(II) provides a great advantage for in situ monitoring targets in biological samples under the fluorescence microscope.

A turn-on fluorescent nanoprobe for selective determination of selenium(IV).

A turn-on fluorescent nanoprobe was developed for selective determination of selenium(IV). A trace amount of selenium, as an essential nutrient, plays an important role in human health. It has been proven that a selenium deficiency will result in serious health problems. The developed nanoprobe is capable of in situ detection of selenium with target-induced signaling, and no separation step is needed. The nanoprobe consists of a silica nanoparticle core and a coating layer containing selenium(IV)-induced fluorescent molecules, 3,3'-diaminobenzidine (DAB). The nanoprobes have no fluorescence signals if they are not exposed to selenium(IV). However, the nanoprobes will be "turned on", with fluorescence, when they bind to the targets of selenium(IV). With this strategy, the selenium(IV) are first collected and enriched on a small domain of the nanoprobes. Then, with an excitation at 420 nm, the nanoprobes emit fluorescence signals at 530 nm. The fluorescence intensity is proportional to the selenium concentration. A fluorescence microscope was used to monitor the process of enriching and collecting of the selenium(IV) by the nanoprobes. The optimal conditions for the determination of selenium(IV) using the nanoprobe were investigated including pH, solvent, and linear range. The interference from common metal ions was studied as well. This study is expected to shed light on how to design turn-on fluorescent nanoprobes for in situ monitoring of a wide variety of targets in biological processes.

Nanoscale effects of silica particle supports on the formation and properties of TiO2 nanocatalysts.

Small TiO2 crystals in the anatase phase are in high demand as photocatalysts. Stable TiO2 crystals in the anatase phase were obtained using a silica nanoparticle as a support. The focus of this study was to investigate the nanoscale effect of the silica support on the formation and properties of small anatase crystals. The experiments were carried out using powder X-ray diffraction, differential thermal analysis, transmission electron microscopy, and energy dispersion spectroscopy. The results showed that the size of the silica support played a crucial role in crystallization of TiO2 and regulation of TiO2 properties, including phase transition, crystal size, thermodynamic property and catalytic activity. A nanoscale curvature model of the spherical silica support was proposed to explain these size effects. Finally, the developed TiO2 catalysts were applied to the oxidation of methanol using a high-throughput photochemical reactor. The size effect of the silica supports on the TiO2 catalytic efficiency was demonstrated using this system.

Structure change associated with the [M(II/III) 1,4,7-triazacyclononane-N,N',N''-triacetate (TCTA)](-/0) electron transfers (M = Mn, Fe, and Ni): crystal structure for [Fe(II)(H2O)6][Fe(II)(TCTA)]2.

Heterogeneous electron-transfer rate measurements using the scanning electrochemical microscope are reported for the [M(TCTA)](-/0) couples (M = Mn, Fe, and Ni) in aqueous solution. Solution IR spectroscopy indicates that N(3)O(3) coordination is preserved for each couple within the pH range of 2-4, and susceptibility measurements indicate little or no interference from spin-state changes at room temperature. Marcus-Hush expressions were used to quantitatively relate structural differences between oxidation states to measured standard heterogeneous electron-transfer rate constants. Good correlation was obtained for the Fe couple, and structural changes associated with the Mn and Ni couples were estimated. In addition, the structure of the Fe(II) complex was determined by X-ray crystallography. The molecule [Fe(H(2)O)(6)][Fe(TCTA)](2) is trigonal, space group P3(1)/c (no. 159) with a = b = 12.530(3) Å, c = 12.656(4) Å, and Z = 2. A notable feature of the structure is that the [Fe(TCTA)](-) complex is distributed between two different geometries, one being rigorously trigonal prismatic and the other having a 26° antiprismatic twist.

Silica nanoparticles for template synthesis of supported Pt and Pt-Ru electrocatalysts.

A simple method was developed for the template synthesis of silica-supported Pt and bimetallic Pt-Ru nanocatalysts. The synthesis used amine-functionalized silica nanoparticles as both template and support for the formation of metal nanoclusters with a precise composition, narrow size distribution, uniform shape and close spatial association. The nanocatalysts were compared with a commercial Pt-C catalyst used in fuel cell applications. Electrochemical measurements of methanol oxidation demonstrated higher catalytic activity for silica-supported Pt nanocatalysts and a reduced CO poisoning effect from co-templating of Pt and Ru.

Developments and Applications of Electrogenerated Chemiluminescence Sensors Based on Micro- and Nanomaterials.

A variety of recent developments and applications of electrogenerated chemiluminescence (ECL) for sensors are described. While tris(2,2'-bipyridyl)-ruthenium(II) and luminol have dominated and continue to pervade the field of ECL-based sensors, recent work has focused on use of these lumophores with micro- and nanomaterials. It has also extended to inherently luminescent nanomaterials, such as quantum dots. Sensor configurations including microelectrode arrays and microfluidics are reviewed and, with the recent trend toward increased use of nanomaterials, special attention has been given to sensors which include thin films, nanoparticles and nanotubes. Applications of ECL labels and examples of label-free sensing that incorporate nanomaterials are also discussed.

Ultratrace determination of inorganic selenium without signal calibration.

To more readily evaluate the complex biogeochemistry of selenium, a flow-through electrochemical method was developed that can accurately determine Se(IV) concentrations in aqueous samples to part-per-trillion levels without signal calibration. Stripping methods were used in conjunction with a high-efficiency, flow-through cell. The cell was designed with a novel gold working electrode that was separated from a porous counter electrode by a Nafion membrane. Because this design permitted exhaustive deposition of selenium from the sample stream as well as efficient coulometric stripping, determinations obeyed Faraday's law over a reasonably wide range of operating conditions. The method had a minimum quantitation limit of approximately 8 ng and a maximum limit of 800 ng for Se(IV). It was reliable for sample volumes as small as 0.5 mL and as high as 20 mL, thereby allowing determinations from part-per-million to just below part-per-billion levels. Interferences from Cu(II) and arsenate were evident, but only when these species were present at concentrations exceeding 10 mg.L-1. Overall, the method had a performance comparable to hydride-generation atomic absorption spectrometry but with less cumbersome equipment and freedom from calibration.

A sensitive sandwich DNA array using fluorescent nanoparticle probes.

An ultrasensitive sandwich DNA array using highly fluorescent and photostable dyedoped silica nanoparticles is described. Compared to traditional sandwich arrays in which fluorophores have been used to signal target DNA molecules, the developed nanoparticle probes provide a much stronger fluorescent emission. Signal amplification of the dyedoped silica nanoparticles originates from the large number of dye molecules doped inside each individual nanoparticle. In addition, the silica matrix of the nanoparticles protects dye molecules from photobleaching. Thus, the dye-doped nanoparticles provide a constant fluorescent signal that is sufficient for detection of trace amounts of target DNA. By immobilizing a complementary DNA sequence to the target onto the nanoparticle surface, a fluorescent nanoparticle-DNA probe is formed. These nanoparticle probes are then used as superemitting reagents to perform a typical sandwich assay. By using a high-resolution fluorescent microscope, individual nanoparticle-DNA probes that have been hybridized to capture target strands can be observed clearly at low target DNA concentrations. More important, the number of the nanoparticle-DNA probes hybridized to the target DNA is proportional to the target DNA concentration in solution. By counting the number of localized fluorescent "spots" on the array, the target DNA concentration can be determined. In this chapter, detailed methods used to synthesize nanoparticle-DNA probes, fabricate the sandwich array, prepare the substrate, and quantitatively determine DNA concentration are described.

Field screening of waterborne petroleum hydrocarbons by thickness shear-mode resonator measurements.

An inexpensive, field-portable sensor for direct, aggregate determination of aqueous petroleum hydrocarbons (PH) down to sub-ppm levels was developed. The basis of this sensor was an unusual, highly nongravimetric frequency response of 10 MHz (series fundamental) AT-cut quartz crystals when coated with rubbery silicone films. The response depended linearly and reliably on the total concentration of dissolved hydrocarbons over a range of 0.01-100 mg x L(-1) or up to aqueous solubility limits. Calibration sensitivities were measured individually for laboratory-prepared solutions of BTEX (benzene, toluene, ethylbenzene, and xylene isomers) and C6-C8 aliphatic components. Each component demonstrated a method detection limit (MDL) in the low-to sub-ppm range (benzene 10 mg x L(-1), n-hexane 0.54 mg x L(-1)) for light coatings of a commercially available poly-(dimethylsiloxane) gum (OV-1, > 10(6) g x mol(-1)) and lower MDLs for heavier coatings. Pairwise responses for the aliphatic and benzenoid standards were additive, indicating that aggregate determinations of mixtures (especially light fuels) were possible. Natural matrix interferences caused by sample turbidity and ionic strength were overcome by simple preparative methods. Fuel-spiked natural waters were determined with respect to standards and verified by gas chromatography. A 0.19 mg x L(-1) MDL for gasoline was obtained for heavy OV-1 films. Field determinations of groundwater surrounding a leaking underground fuel tank demonstrated that the sensor and method were useful for on-site PH screening. Large differences between the equilibration times of aliphatic and benzenoid components also indicated one avenue for BTEX speciation with the device.

Oxidatively Induced Isomerization of Square-Planar [Ni(1,4,8,11-tetraazacyclotetradecane)](ClO(4))(2).

Previous proton NMR and electronic spectroscopy studies have demonstrated that the square-planar complex of Ni(II) and 1,4,8,11-tetraazacyclotetradecane (cyclam) exists in two stable isomeric conformations, namely the R,S,R,S form or trans-I isomer and the R,R,S,S form or trans-III isomer. Electrochemical analysis of the trans-I isomer of Ni(II)-cyclam has demonstrated rapid conversion to the trans-III cyclam conformation following oxidation to Ni(III). The mechanism and kinetics for this oxidatively-induced isomerization were studied by the technique of cyclic voltammetry (CV) with simulation of CV traces by finite-difference computations. The most crucial mechanistic indicator was found to be the transient trans-I-Ni(III) species. This intermediate was detected by CV over a pH range 2-4 and was found to have an apparent half-life of ca. 400 ms at room temperature. Remarkably, this life-time was approximately a billionfold shorter than the corresponding trans-I-Ni(II) species. Measurements made at varied solution temperature and pH demonstrated that the oxidatively induced isomerization followed an apparent square-scheme, where the trans-I/III isomerization process of Ni(III) was independent of pH. This finding precluded a base-catalyzed isomerization process that has been previously identified for the Ni(II) system. Arrhenius plots of the forward isomerization rate constant allowed the extraction of activation parameters for the Ni(III) process. These parameters are discussed with respect to possible rate-determining steps.