Quantifying the Polymeric Capping of Nanoparticles with X-Ray Photoelectron Spectroscopy.

X-ray photoelectron spectroscopy was used to characterise silver nanoparticles capped with poly(ethylene) glycol (PEG) in a room-temperature ionic liquid (RTIL), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4 ]). The amounts of oxygen and silver present in nanoparticles capped with different molecular weight thiolated PEG chains were monitored, and the number of thiolated PEG chains per nanoparticle was calculated, an in situ characterisation not previously possible.

Nafion particles doped with methyl viologen: electrochemistry.

Nafion sub-micro particles doped with methyl viologen (MV2+) are synthesized using the re-precipitation method and characterized by scanning electron microscopy and UV-Vis spectroscopy. The electrochemical behavior of MV2+ incorporated into Nafion particles was investigated at both the ensemble and single particle levels. The charge transferred to single MV-Nafion particles was observed using the nano-impact method and shown to be quantitative. Finally, the charge transferred via individual MV-Nafion particle was substantially enhanced in the presence of permanganate. The mechanism of the catalytic nano-impact reaction of mediated permanganate reduction by single MV-Nafion particle is revealed as proceeding via MV+˙ reduced from MV2+ incorporated in Nafion particles through one electron transfer followed by the reduction of permanganate.

Immobilised Electrocatalysts: Nafion Particles Doped with Ruthenium(II) Tris(2,2'-bipyridyl).

Nafion particles doped with ruthenium(II) tris(2,2'-bipyridyl) are synthesized by using a re-precipitation method. Characterization including SEM sizing and quantification of Ru(bpy)32+ in the Nafion particles using UV/Vis spectroscopy was conducted. The synthesized Ru-Nafion particles were investigated electrochemically at both ensemble and single particle levels. Voltammetry of the drop-cast Ru-Nafion particles evidences the successful incorporation of Ru(bpy)32+ into the Nafion particle but only a small fraction of the incorporated Ru(bpy)32+ was detected due at least in part to the formation of the likely agglomerated and irregular "mat" associated with the dropcast technique. In contrast, nano-impact experiments provided a quantitative determination of the amount of Ru(bpy)32+ in single Ru-Nafion particles. Finally, oxidation of solution-phase oxalate mediated by Ru(bpy)32+ within individual Nafion particles was observed, showing the electrocatalytic properties of the Ru-Nafion particles.

Single Oxidative Collision Events of Silver Nanoparticles: Understanding the Rate-Determining Chemistry.

The oxidative dissolution of citrate-capped silver nanoparticles (AgNPs, ∼50 nm diameter) is investigated herein by two electrochemical techniques: nano-impacts and anodic stripping voltammetry. Nano-impacts or single nanoparticle-electrode collisions allow the detection of individual nanoparticles. The technique offers an advantage over surface-immobilized methods such as anodic stripping voltammetry as it eliminates the effects of particle agglomeration/aggregation. The electrochemical studies are performed in different electrolytes (KNO3 , KCl, KBr and KI) at varied concentrations (≤20 mm). In nano-impact measurements, the AgNP undergoes complete oxidation upon impact at a suitably potentiostated electrode. The frequency of the nanoparticle-electrode collisions observed as current-transient spikes depends on the electrolyte identity, its concentration and the potential applied at the working electrode. The frequencies of the spikes are significantly higher in the presence of halide ions and increase with increasing potentials. From the frequency, the rate of AgNP oxidation as compared with the timescale the AgNP is in electrical contact with the electrode can be inferred, and hence is indicative of the relative kinetics of the oxidation process. Primarily based on these results, we propose the initial formation of the silver (I) nucleus (Ag+ , AgCl, AgBr or AgI) as the rate-determining process of silver oxidation on the nanoparticle.

Dynamics of Silver Nanoparticles in Aqueous Solution in the Presence of Metal Ions.

Using a combined UV-vis, DLS, and electrochemical approach, this work experimentally studies the physical origin of the observed colorimetric sensitivity of aqueous silver nanoparticles toward divalent metal ions. In the presence of Pb2+, AgNPs are slow to reversibly form agglomerates (the time scale of the reverse deagglomeration process is of the order of hours). This agglomeration is shown to be induced by complex formation between Pb2+ and citrate groups localized on the AgNPs, reducing surface charges (zeta-potential) and hence electrostatic repulsion between the AgNPs. Other divalent metal ions including Ca2+, Cd2+, Zn2+, Ni2+, Co2+, and Sn2+ are also studied, and the resulting sizes of the AgNPs clusters and the extents of the UV-vis spectrum red-shift in λmax have a strong positive correlation with the metal-ligand (citrate) complex formation constant (Kf). This work thus serves as a guide for the selection of capping agents on the basis of Kf and demonstrates the correlation between sizes and spectrophotometric as well as electrochemical responses of the AgNPs clusters. Importantly, we give further physical insights into the size-dependent properties of AgNPs and emphasize the difference between theoretical and experimental values of extinction coefficients, where the latter is affected by the angle-dependent scattering intensities and the measurement technique used.

Fluorescence Electrochemical Microscopy: Capping Agent Effects with Ethidium Bromide/DNA Capped Silver Nanoparticles.

Fluorescence microscopy and electrochemistry were employed to examine capping agent dynamics in silver nanoparticles capped with DNA intercalated with ethidium bromide, a fluorescent molecule. The capped NPs were studied first electrochemically, demonstrating that the intercalation of the capping agent promotes oxidation of the silver core, occurring at 0.50 V (vs. Ag, compared with 1.15 V for Ag NPs capped in DNA alone). Second, fluorescence electrochemical microscopy revealed that the electron transfer from the nanoparticles is gated by the capping agent, allowing dynamic insights unobservable using electrochemistry alone.

Potassium (De-)insertion Processes in Prussian Blue Particles: Ensemble versus Single Nanoparticle Behaviour.

Potassium (de-)insertion from Prussian blue (PB) is investigated at the single and multi-particle scale. The electrochemical behaviour is found to differ between the two measurement types. At the single particle level, oxidation of the PB nanoparticles with concomitant K+ deinsertion occurs more readily than the associated reduction, relating to K+ insertion. In contrast, the cyclic voltammetry of PB in a composite electrode containing conductive additives and polymeric binder suggests the opposite behaviour. Implications for assessing battery materials are discussed.

Electrochemical Detection of Ultratrace (Picomolar) Levels of Hg2+ Using a Silver Nanoparticle-Modified Glassy Carbon Electrode.

Ultratrace levels of Hg2+ have been quantified by undertaking linear sweep voltammetry with a silver nanoparticle-modified glassy carbon electrode (AgNP-GCE) in aqueous solutions containing Hg2+. This is achieved by monitoring the change in the silver stripping peak with Hg2+ concentration resulting from the galvanic displacement of silver by mercury: Ag(np) + 1/2Hg2+(aq) → Ag+(aq) + 1/2Hg(l). This facile and reproducible detection method exhibits an excellent linear dynamic range of 100.0 pM to 10.0 nM Hg2+ concentration with R2 = 0.982. The limit of detection (LoD) based on 3σ is 28 pM Hg2+, while the lowest detectable level for quantification purposes is 100.0 pM. This method is appropriate for routine environmental monitoring and drinking water quality assessment since the guideline value set by the US Environmental Protection Agency (EPA) for inorganic mercury in drinking water is 0.002 mg L-1 (10 nM).

Electrochemistry of single droplets of inverse (water-in-oil) emulsions.

We demonstrate the feasibility of electrochemically detecting individual water droplets dispersed in an oil phase (inverse emulsions) via the use of a redox probe confined in the droplet phase. The water droplets were tagged with potassium ferrocyanide, and were injected into an electrolyte cyclohexene/dichloromethane oil solution. Via simple cyclic voltammetry scans it is shown that single water droplets from a water-in-oil emulsion can be detected provided that rapid anion transfer from the oil to the water phase maintains electro-neutrality in the droplet.

Understanding nanoparticle porosity via nanoimpacts and XPS: electro-oxidation of platinum nanoparticle aggregates.

The porosity of platinum nanoparticle aggregates (PtNPs) is investigated electrochemically via particle-electrode impacts and by XPS. The mean charge per oxidative transient is measured from nanoimpacts; XPS shows the formation of PtO and PtO2 in relative amounts defined by the electrode potential and an average oxidation state is deduced as a function of potential. The number of platinum atoms oxidised per PtNP is calculated and compared with two models: solid and porous spheres, within which there are two cases: full and surface oxidation. This allows insight into extent to which the internal surface of the aggregate is 'seen' by the solution and is electrochemically active.

Catalytic activity of catalase-silica nanoparticle hybrids: from ensemble to individual entity activity.

We demonstrate the electrochemical detection and characterization of individual nanoparticle-enzyme hybrids. Silica nanoparticles were functionalized with catalase enzyme and investigated spectroscopically and electrochemically. The catalytic activity of the hybrids towards hydrogen peroxide decomposition was comparable to the activity of a freely diffusing enzyme in solution, exhibiting a Michaelis-Menten constant of KM = 74 mM and a turnover number of kcat = 8 × 107 s-1 per NP. The fast turnover number of the hybrid further enabled the electrochemical detection of individual nanoparticle-enzyme hybrid via a novel method: the hydrogen peroxide substrate was generated at a microelectrode which enabled enzymatic activity exclusively within the diffusion layer of the electrode. The method is the first electrochemical approach for measuring hybrid nanoparticles, at the single entity level.

DNA capping agent control of electron transfer from silver nanoparticles.

Silver nanoparticles capped with either DNA or citrate are investigated electrochemically using stripping voltammetry and nano-impacts. Whilst the citrate capped particles are readily oxidised to silver cations at 0.7 V, the DNA capped particles undergo electron transfer from the silver core to the electrode in two distinct potential ranges -0.8 to 1.1 V and 1.125 to 1.2 V, and only undergo complete oxidation at the higher potential range. These potentials reflect the oxidation of guanine and adenine respectively, with a potential sufficient to oxidise both base pairs being necessary to observe full silver oxidation. The DNA thus serves as a tunnelling barrier to electrically insulate the particle, and allows for selective oxidation to occur by controlling the potential applied.

Particle-impact analysis of the degree of cluster formation of rutile nanoparticles in aqueous solution.

Cluster formation can profoundly influence the bioavailability and (bio)geochemical activity of nanoparticles in natural aquatic systems. While colloidal properties of nanoparticles are commonly investigated using light-scattering techniques, the requirement to dilute samples can affect the fundamental nature and extent of the cluster size. Hence, an alternative in situ approach that can cover a much higher and wider concentration range of particles is desirable. In this study, particle impact chronoamperometry is employed to probe the degree of cluster formation of Alizarin Red S modified rutile nanoparticles of diameter ca. 167 nm in conditions approximating those existing in the environment. Random collisions of individual clusters of the modified rutile particles with a stationary electrode result in transient current signals during a chronoamperometric measurement, indicative of the reduction of the adsorbed Alizarin Red S dye molecules. The results from the particle-impact analysis reveal that the nanoparticles are heavily clustered with an average 91 monomeric particles per cluster. As the spherical equivalent size of the clusters (ca. 754 nm in diameter) is considerably larger than that from nanoparticle tracking analysis (ca. 117 nm), the present work highlights the impact of the dilution on the fundamental nature of the colloidal suspension and introduces the electrochemical determination of the size distribution of inert mineral nanoparticles in highly concentrated media.

Quantifying Single-Carbon Nanotube-Electrode Contact via the Nanoimpact Method.

A new methodology is developed to enable the measurement of the resistance across individual carbon nanotube-electrode contacts. Carbon nanotubes (CNTs) are suspended in the solution phase and occasionally contact the electrified interface, some of which bridge a micron-sized gap between two microbands of an interdigitated gold electrode. A potential difference is applied between the contacts and the magnitude of the current increase after the arrival of the CNT gives a measure of the resistance associated with the single CNT-gold contact. These experiments reveal the presence of a high contact resistance (∼50 MΩ), which significantly dominates the charge-transfer process. Further measurements on ensembles of CNTs made using a dilute layer of CNTs affixed to the interdigitated electrode surface and measured in the absence of solvent showed responses consistent with the same high value of contact resistance.

The Role of Entropy in Nanoparticle Agglomeration.

Agglomeration processes in non-interacting particle systems can be understood from a thermodynamic point of view. If the enthalpy of agglomeration is negligible, the distribution of agglomeration states adopts the state of highest entropy. Herein, we provide the exact analytical solution to the mole fractions of agglomerates comprising i monomers, xi =2-i .

Exploring nanoparticle porosity using nano-impacts: platinum nanoparticle aggregates.

The porosity of platinum nanoparticles (PtNPs) is explored for the first time using tag-redox coulometry (TRC). This is achieved by monitoring the reduction of the 4-nitrobenzenethiol (NTP)-tagged PtNPs on carbon electrodes via both immobilisation and nanoimpacts. The average charge per impact is measured and attributed to the reduction of NTP adsorbed on individual PtNPs. The number of NTP molecules and thus the "active surface area" of the PtNPs is calculated and compared with two models: fully solid and porous nanoparticles, and the extent of the particle porosity is revealed. This allows a fuller understanding of the (electro-)catalytic behaviour of nanoparticles by providing insight into their porosity and "true/active surface areas".

Electrode-particle impacts: a users guide.

We present a comprehensive guide to nano-impact experiments, in which we introduce newcomers to this rapidly-developing field of research. Central questions are answered regarding required experimental set-ups, categories of materials that can be detected, and the theoretical frameworks enabling the analysis of experimental data. Commonly-encountered issues are considered and presented alongside methods for their solutions.

Fluorescence Monitored Voltammetry of Single Attoliter Droplets.

The lipid soluble fluorophore Nile Red (9-diethylamino-5-benzo[α]phenoxazinone) is used to fluorescently and electrochemically label an organic-in-water emulsion, where the organic phase is an ionic liquid [P6,6,6,14][FAP]/toluene mixture. The optical detection of the individual droplets is enabled facilitating the in situ tracking and sizing of the suspended particles (average diameter = 530 nm, interquartile range = 180 nm). Through the use of a combined thin-layer optical/electrochemical cell, the irreversible accumulation of the droplets at an optically opaque carbon fiber electrode (diameter ∼7.5 µm) can be monitored. Potentiostatic control of the system enables the fluorescence of the surface bound particles to be electrochemically switched via control of the redox state of the dye. Subsequent measurements of the individual particle fluorescence intensities as a function of the applied electrode potential enables construction of an effective, dynamically recorded cyclic voltammogram of an individual particle. The confined volume voltammetry (∼tens of attoliters) yields insight into the asymmetry of the kinetics of the redox switching process, where it is proposed that the reformation of the fluorescent Nile Red becomes chemically "gated" in the organic phase.

Tracking of photochemical Ostwald ripening of nanoparticles through voltammetric atom counting.

We report the tracking of atom count in individual nanoparticles during photochemical Ostwald ripening. The nano-impact technique, in conjunction with UV-Vis and TEM analysis, is used to follow the photochemical formation of silver nano-prisms from spherical seed particles. A mechanism of photochemical Ostwald ripening is deduced and key growth stages are identified.