Single Entity Behavior of CdSe Quantum Dot Aggregates During Photoelectrochemical Detection.

We demonstrate that colloidal quantum dots of CdSe and CdSe/ZnS are detected during the photooxidation of MeOH, under broad spectrum illumination (250 mW/cm2). The stepwise photocurrent vs. time response corresponds to single entities adsorbing to the Pt electrode surface irreversibly. The adsorption/desorption of the QDs and the nature of the single entities is discussed. In suspensions, the QDs behave differently depending on the solvent used to suspend the materials. For MeOH, CdSe is not as stable as CdSe/ZnS under constant illumination. The photocurrent expected for single QDs is discussed. The value of the observed photocurrents, > 1 pA is due to the formation of agglomerates consistent with the collision frequency and suspension stability. The observed frequency of collisions for the stepwise photocurrents is smaller than the diffusion-limited cases expected for single QDs colliding with the electrode surface. Dynamic light scattering and scanning electron microscopy studies support the detection of aggregates. The results indicate that the ZnS layer on the CdSe/ZnS material facilitates the detection of single entities by increasing the stability of the nanomaterial. The rate of hole transfer from the QD aggregates to MeOH outcompetes the dissolution of the CdSe core under certain conditions of electron injection to the Pt electrode and in colloidal suspensions of CdSe/ZnS.

Stochastic electrochemistry and photoelectrochemistry of colloidal dye-sensitized anatase nanoparticles at a Pt ultramicroelectrode.

We report the stochastic interactions between dye sensitized anatase nanoparticles, suspended in a colloid, and a Pt ultramicroelectrode (UME) that result in step-wise behavior in the current vs. time response. The stochastic currents are observed in the dark and under illumination. In the dark, the currents are anodic, consistent with the oxidation of the dye N719 at the Pt surface. The electrochemical behavior of the dye was investigated in MeOH and MeCN with a quasireversible cyclic voltammogram (CV) observed at 1 V s-1. The anodic currents observed in the dark due to nanoparticles (NPs) at the Pt surface are consistent with the CVs in MeOH and MeCN. Under illumination cathodic steps are observed and assigned to the reduction of the oxidized form of the dye generated after electrons are injected into the TiO2 NPs. The colloidal behavior is a strong function of the history of the colloid with illumination time increasing the size of the agglomerates and with larger agglomerates being less photoelectrochemically active. Agglomerates of ca. 100 nm in diameter are proposed to be photoactive entities with a higher probability of detection that contribute to the staircase photocurrent response.

Observation of individual semiconducting nanoparticle collisions by stochastic photoelectrochemical currents.

We describe a method to detect individual semiconducting nanoparticles (NPs) using the photoelectrochemical (PEC) current measured at an ultramicroelectrode (UME). We use photooxidation of MeOH by TiO2 NPs as a model system of photocatalysis in solution. NPs suspended in MeOH under constant illumination produce valence-band holes that oxidize MeOH. The electrons are collected at the UME, and the current-versus-time data show discrete current changes that are assigned to particle-by-particle interactions of the NPs with the UME. The stepwise changes in the photocurrent denote irreversible attachment of NPs to Pt UMEs (<30 µm diameter). We found that accumulation of electrons in the conduction band by the NPs is not enough to explain the stochastic PEC currents. We propose that the observed anodic steps have a PEC nature and are due to photooxidation of MeOH by the NPs at the electrode surface.

Sensitive determination of triacetone triperoxide explosives using electrogenerated chemiluminescence.

Sensitive and selective detection and quantification of high explosive triacetone triperoxide (TATP) with electrogenerated chemiluminescence (ECL) at a glassy carbon electrode in water-acetonitrile solvent mixture were reported. In the presence of ruthenium(II) tris(bipyridine), TATP or hydrogen peroxide derived from TATP via UV irradiation or acid treatment produced ECL emissions upon cathodic potential scanning. Interference from hydrogen peroxide on TATP detection was eliminated by pretreatment of the analyte with catalase enzyme. Selective detection of TATP from hexamethylene triperoxide diamine (HMTD, another common peroxide-based explosive) was realized by comparing ECL responses obtained from the anodic and the cathodic potential scanning; TATP produced ECL upon cathodic potential scanning only, whereas HMTD produced ECL upon both cathodic and anodic potential scanning. The hydroxyl radical formed after the electrochemical reduction of TATP was believed to be the key intermediate for ECL production, and its stability was strongly dependent on the solution composition, which was verified with electron paramagnetic resonance spectroscopy. A detection limit of 2.5 µM TATP was obtained from direct electrochemical reduction of the explosive or hydrogen peroxide derived from TATP in 70/30% (v/v) water-acetonitrile solutions, which was ~400 times lower than that reported previously based on liquid chromatography separation and Fourier transform infrared detection.

Electrogenerated chemiluminescence determination of C-reactive protein with carboxyl CdSe/ZnS core/shell quantum dots.

Electrogenerated chemiluminescence (ECL) of water-soluble core/shell CdSe/ZnS quantum dots (QDs) coated with carboxylated polyethylene glycol polymers ("Qdot 625") was investigated in aqueous solutions using 2-(dibutylamino)ethanol (DBAE) and tri-n-propylamine (TPrA) as ECL coreactants. In both cases, ECL emissions at glassy carbon (GC) electrode appeared at the same potential of approximately 0.80 V vs. Ag/AgCl (3.0 M KCl), which was approximately 200 and approximately 150 mV more positive compared with the oxidation potentials for DBAE (approximately +0.60 V vs. Ag/AgCl) and TPrA (approximately +0.65 V vs. Ag/AgCl), respectively. The ECL intensity, however, was significantly affected by the type and the concentration of the ECL coreactant used as well as the nature of the working electrode. Under the present experimental conditions, ECL from DBAE was approximately 17 times stronger than that from TPrA. The maximum ECL was obtained at GC electrode when [DBAE] approximately = 53 mM, where a ratio of 11:3:1 in ECL intensity was evaluated for GC, Au, and Pt electrodes, respectively. The ECL emission of the Qdot 625/DBAE system had an apparent peak value of approximately 625 nm that matched well the fluorescence data. The QD as a label for ECL-based immunoassays of C-reactive protein (CRP) was realized by covalent binding of avidin on its surface, which allowed biotinylated anti-CRP to be attached and interacted with solution-phase CRP and the anti-CRP linked to micro-sized magnetic beads. The newly formed sandwich type aggregates were separated magnetically from the solution matrix, followed by the ECL generation at partially transparent Au nanoparticle-coated ITO electrode or Au/CD electrode in the presence of DBAE. Much stronger ECL responses were observed from the Au/CD electrode, at which a dynamic range of 1.0-10.0 microg mL(-1) CRP and a limit of detection of 1.0 microg mL(-1) CRP were obtained, respectively.

Sensitive determination of hexamethylene triperoxide diamine explosives, using electrogenerated chemiluminescence enhanced by silver nitrate.

Sensitive detection and quantification of hexamethylene triperoxide diamine (HMTD), which is one of commonly used explosives by terrorists, was presented on the basis of electrogenerated chemiluminescence (ECL) technology coupled with silver nitrate (AgNO3) enhancement in acetonitrile at a platinum electrode. Upon anodic potential scanning, HMTD irreversibly oxidized at approximately 1.70 V vs Ag/Ag+ (10 mM) at a scan rate of 50 mV/s, and the ECL profile was coincident with the oxidation potential of HMTD in the presence of ruthenium(II) tris(bipyridine) (Ru(bpy)3(2+)) luminophore species, which showed a half-wave potential of 0.96 V vs Ag/Ag+. The addition of small amounts of AgNO3 (0.50-7.0 mM) into the HMTD/Ru(bpy)3(2+) system resulted in significant enhancement in HMTD ECL production (up to 27 times). This enhancement was determined to be largely associated with NO3(-) and was linearly proportional to the concentrations of NO3(-) and Ag+ in solution. Homogeneous chemical oxidations of HMTD by electrogenerated NO3* and Ag(II) species proximity to the electrode were proposed to be responsible for the ECL enhancement. On the basis of cyclic voltammetry (CV) and CV digital simulations, standard potential values of 1.79 V vs Ag/Ag+ (or 1.98 V vs NHE) and 1.82 V vs Ag/Ag+ (or 2.01 V vs NHE) were estimated for Ag(II)/Ag(I) and NO3*/NO3(-) couples, respectively. A detection limit of 50 microM of HMTD was achieved with the current technique, which was 10 times more sensitive than that reported previously, which was based on a high-performance liquid chromatography/Fourier transform infrared (HPLC/FT-IR) detection method.