Mahdi Hesari

Highly Efficient Electrogenerated Chemiluminescence of Au38 Nanoclusters

An investigation of mechanisms for the near-infrared (NIR) electrogenerated chemiluminescence/electrochemiluminescence (ECL) of Au38(SC2H4Ph)24 (Au38, SC2H4Ph = 2-phenylethanethiol) nanoclusters both in annihilation and coreactant paths is reported. Essentially, no ECL emission was produced in the annihilation route over the potential range of the accessible redox states of Au38, because of the short lifetime and/or low reactivity of the electrogenerated Au38 intermediates necessary for ECL. Highly efficient light emission with a nominal peak wavelength of 930 nm in the NIR region was observed in the anodic region upon addition of tri-n-propylamine (TPrA) as the coreactant. The ECL mechanisms were elucidated by means of ECL-potential curves and spooling ECL spectroscopy. It was discovered that the Au38(+*) (and also Au38(3+*)) were electrogenerated as the major excited species in the light emission processes. Benzoyl peroxide was also used as a coreactant in the cathodic potential range from which benzoate radicals, with a high oxidizing power, were formed. These radicals accepted electrons from the electrogenerated Au38(2-) HOMO, resulting in the Au38(-*) excited state that emitted light at 930 nm. The photoluminescence of the various Au38 charge states, namely, Au38(2-), Au38(-), Au38(0), Au38(+), Au38(2+), and Au38(4+), electrogenerated in situ, indicated no significant difference in the emission peak wavelength. This information allowed a careful mapping of the relevant ECL mechanisms. It was found that the ECL efficiency could reach an efficiency of 3.5 times as high as that of the Ru(bpy)3(2+)/TPrA system.

Electron transfer catalysis with monolayer protected Au25 clusters

Au₂₅L₁₈ (L = S(CH₂)₂Ph) clusters were prepared and characterized. The resulting monodisperse clusters were reacted with bis(pentafluorobenzoyl) peroxide in dichloromethane to form Au₂₅L₁₈⁺ quantitatively. The kinetics and thermodynamics of the corresponding electron transfer (ET) reactions were characterized via electrochemistry and thermochemical calculations. Au₂₅L₁₈⁺ was used in homogeneous redox catalysis experiments with a series of sym-substituted benzoyl peroxides, including the above peroxide, bis(para-cyanobenzoyl) peroxide, dibenzoyl peroxide, and bis(para-methoxybenzoyl) peroxide. Peroxide dissociative ET was catalyzed using both the Au₂₅L₁₈/Au₂₅L₁₈⁻ and the Au₂₅L₁₈⁺/Au₂₅L₁₈ redox couples as redox mediators. Simulation of the CV curves led to determination of the ET rate constant (k(ET)) values for concerted dissociative ET to the peroxides. The ET free energy ΔG° could be estimated for all donor-acceptor combinations, leading to observation of a nice activation-driving force (log k(ET)vs.ΔG°) relationship. Comparison with the k(ET) obtained using a ferrocene-type donor with a formal potential similar to that of Au₂₅L₁₈/Au₂₅L₁₈⁻ showed that the presence of the capping monolayer affects the ET rate rather significantly, which is attributed to the intrinsic nonadiabaticity of peroxide acceptors.

Highly Efficient Dual-Color Electrochemiluminescence from BODIPY-Capped PbS Nanocrystals

Electrochemiluminescence (ECL) of a hybrid system consisting of PbS nanocrystals (NCs) and a BODIPY dye (BDY) capping ligand was discovered to produce highly efficient dual emissions with tri-n-propylamine as a coreactant. By means of spooling ECL spectroscopy, the strong dual ECL emission peaks of 984 and 680 nm were attributed to the PbS and BDY moieties, respectively, and found to be simultaneous during the light evolution and devolution. The ECL of the PbS was enhanced via NC collisions with the electrode and reached an efficiency of 96% relative to that of Ru(bpy)3(2+), which is the highest among the semiconductor NCs.

NIR electrochemiluminescence from Au25− nanoclusters facilitated by highly oxidizing and reducing co-reactant radicals

The well-defined electrochemical features and optical properties of the negatively charged Au25 clusters (Au25−) provide opportunities for a photoelectrochemical study by means of electrochemiluminescence (ECL) technique. Under annihilation conditions where the Au25− is electrochemically pumped to its various oxidized and reduced forms showed no appreciable ECL light emission, due presumably to the short lifetime of the electrogenerated intermediates and their reactivity. Interestingly, in either Au25−/tri-n-propylamine (TPrA) or Au25−/benzoyl peroxide (BPO) co-reactant systems, the correspondingly highly reducing and oxidizing intermediates electrogenerated from TPrA and BPO lead to light emission at 950 and 890 nm in near-infrared (NIR) region. The ECL in the presence of various concentrations of TPrA (6.3, 12.5, 25, 50, 100 and 200 mM) and BPO (2.5, 5, 25 and 50 mM) was explicitly investigated. Along with the concentration dependence study, spooling ECL spectroscopy provided insight into the ECL mechanisms. Notably, while the Au25−* is the main light emission source with BPO, ECL in the presence of TPrA is attributed to emissions from the Au25−*, Au250* and Au25+* that are tuneable by means of the applied potential and TPrA concentration.

Reductive Deprotection of Monolayer Protected Nanoclusters: An Efficient Route to Supported Ultrasmall Au Nanocatalysts for Selective Oxidation

Bulk gold has long been considered too inert to be a catalyst until the discovery that Au nanoparticles (AuNPs) supported on metal oxides such as TiO 2 , CeO 2 and Fe 2 O 3 could be very active for CO oxidation. [ 1 ] Supported Au and other NPs have now been successfully shown to catalyze various chemical reactions. [ 2 ] AuNP-catalyzed oxidation reactions, in particular, have attracted special attention because the reactions can lead to a range of important value-added oxygenated chemical products and pharmaceuticals, and also because oxidation (or epoxidation) of various alkenes, arenes and alcohols are proven to be effectively catalyzed by AuNPs. [ 3 ]

An efficient electrochemical synthesis of diamino-o-benzoquinone: mechanistic and kinetic evaluation of the reaction of azide ion with o-benzoquinone.

An efficient method for the synthesis of diamino-o-benzoquinone based on the Michael reaction of electrochemically generated o-benzoquinone with azide ion is described, as well as an estimation of the homogeneous rate constant (k(obs)) of the reaction of o-benzoquinone with azide ion by the digital-simulation method.

A Grand Avenue to Au Nanocluster Electrochemiluminescence

In most cases of semiconductor quantum dot nanocrystals, the inherent optical and electrochemical properties of these interesting nanomaterials do not translate into expected efficient electrochemiluminescence or electrogenerated chemiluminescence (ECL) because of the surface-state induction effect. Thus, their low ECL efficiencies, while very interesting to explore, limit their applications. As their electrochemistry is not well-defined, insight into their ECL mechanistic details is also limited. Alternatively, gold nanoclusters possess monodispersed sizes with atomic precision, low and well defined HOMO-LUMO energy gaps, and stable optical and electrochemical properties that make them suitable for potential ECL applications. In this Account, we demonstrate strong and sustainable ECL of gold nanoclusters Au25z (i.e., Au25(SR)18z, z = 1-, 0, 1+), Au38(SR)24, and Au144(SR)60, where the ligand SR is 2-phenylethanethiol. By correlation of the optical and electrochemical features of Au25 nanoclusters, a Latimer-type diagram can be constructed to reveal thermodynamic relationships of five oxidation states (Au252+, Au25+, Au250, Au25-, and Au252-) and three excited states (Au25-*, Au250*, and Au25+*). We describe ECL mechanisms and reaction kinetics by means of conventional ECL-voltage curves and novel spooling ECL spectroscopy. Notably, their ECL in the presence of tri-n-propylamine (TPrA), as a coreactant, is attributed to emissions from Au25-* (950 nm, strong), Au250* (890 nm, very strong), and Au25+* (890 nm, very strong), as confirmed by the photoluminescence (PL) spectra of the three Au25 clusters electrogenerated in situ. The ECL emissions are controllable by adjustment of the concentrations of TPrA· and Au25-, Au250, and Au25+ species in the vicinity of the working electrode and ultimately the applied potential. It was determined that the Au25-/TPrA coreactant system should have an ECL efficiency of >50% relative to the Ru(bpy)32+/TPrA, while those of Au250/TPrA and Au25+/TPrA reach 103% and 116%, respectively. Au25-* is the main light emission source for Au25z in the presence of benzoyl peroxide (BPO) as a coreactant, with a relative efficiency of up to 30%. For Au38, BPO leads to the Au38-* excited state, which emits light at 930 nm. In the Au38/TPrA coreactant system, we find that highly efficient light emission at 930 nm is mainly from Au38+* (and also Au383+*), with an efficiency 3.5 times that of the Ru(bpy)32+/TPrA reference. We show that the ECL and PL of the various Au38 charge states, namely, Au382-, Au38-, Au380, Au38+, Au382+, and Au384+, have the same peak wavelength of 930 nm. Finally, we demonstrate ECL with a peak wavelength of 930 nm from the Au144/TPrA coreactant system, which is released from the electrogenerated excited states Au144+* and Au1443+*. In our opinion, these gold nanoclusters represent a new class of effective near-IR ECL emitters, from which applications such as bioimaging, biological testing, and medical diagnosis are anticipated once they are made water-dispersible with hydrophilic capping ligands.

Thermodynamic and Kinetic Origins of Au250 Nanocluster Electrochemiluminescence

Au clusters with protecting organothiolate ligands and core diameters less than 2 nm are molecule-like structures, suitable for catalysis, optoelectronics and biology applications. The spectroscopy and electrochemistry of Au25(0) (Au25[(SCH2CH2Ph)18], SCH2CH2Ph = 2-phenylethanethiol) allowed us to construct a Latimer-type diagram for the first time, which revealed a rich photoelectrochemistry of the cluster and the unique relationship to its various oxidation states and corresponding excited states. The occurrence of cluster electrochemiluminescence (ECL) was examined in the presence of tri-n-propylamine (TPrA) as a co-reactant and was discovered to be in the near-infrared (NIR) region with peak wavelengths of 860, 865, and 960 nm, emitted by Au25(+*), Au25(0*), and Au25(-*), respectively. The light emissions, with an efficiency up to 103% relative to that of the efficient Ru(bpy)3(2+)/TPrA system, depended on the kinetics of the reactions between the electrogenerated TPrA radical and Au25(z) (z = 2+, 1+, 1-, and 2-) in the vicinity of the electrode or the bulk Au25(0). These thermodynamic and kinetic origins were further explored by means of spooling ECL and photoluminescence spectroscopy during a sweep of the potential or at a constant potential applied to the working electrode. NIR-ECL emissions of the cluster can be tuned in wavelength and intensity by adjusting the applied potential and TPrA concentration based on the above discoveries.

Near-infrared electrochemiluminescence from Au25(SC2H4Ph)18+ clusters co-reacted with tri-n-propylamine

The oxidation of Au25(SC2H4Ph)18+C6F5CO2− clusters along with tri-n-propylamine (TPrA) as a co-reactant produces very strong near-infrared electrochemiluminescence with emission peak wavelengths (872 and 945 nm) and intensities depending on the TPrA concentration and working electrode potential.

Electrochemical study of iodide in the presence of barbituric acid. Application to coulometric titration of barbituric acid

Abstract Electrochemical oxidation of iodide in the presence of barbituric acid has been studied using cyclic voltammetry and controlled-potential coulometry. The results indicate that the resulting iodine takes part in halogenation reaction and reacts with barbituric acid. According to obtained results, a new coulometric titration method with potentiometric end-point detection for the determination of barbituric acid is presented. In the presented method, 1–200 μmol of barbituric acid was successfully determined.