A simplified methodology: pH sensing using an in situ fabricated Ir electrode under neutral conditions

Herein, a simplified fabrication method for the producing of a pH-sensitive iridium electrode is developed. The in situ electrochemical fabrication of an iridium oxide film is optimized and shown to be achievable under neutral conditions rather than the acidic conditions hitherto employed. The formation of a pH sensitive Ir(III/IV) hydrous film is confirmed via XPS. The amperometric pH-sensing properties of this electrochemically generated material were investigated using square wave voltammetry. In the pH range 2–13, the iridium oxide redox signal has a pH dependency of 86.1 ± 1.1 mV per pH unit for midpoint potentials with uncertainties being ± 0.01–0.05 pH. Finally, the newly developed pH sensor was used to measure the pH of a natural water sample with excellent results as compared to a conventional glass pH probe.

     

Do carbon nanotubes catalyse bromine/bromide redox chemistry?

The redox chemistries of both the bromide oxidation and bromine reduction reactions are studied at single multi-walled carbon nanotubes (MWCNTs) as a function of their electrical potential allowing inference of the electron transfer kinetics of the Br₂/Br⁻ redox couple, widely used in batteries. The nanotubes are shown to be mildly catalytic compared to a glassy carbon surface but much less as inferred from conventional voltammetry on porous ensembles of MWCNTs where the mixed transport regime masks the true catalytic response.

Schematic of a carbon nanotube impact in bromide solution.

The bromine–bromide redox couple plays an essential role in diverse energy storage devices including hydrogen–bromine, zinc–bromine, quinone–bromine, vanadium–bromide and bromide–polysulphide flow batteries.^1–5 The Br₂/Br⁻ redox couple is attractive as a cathode reaction due to its high standard potential, large solubility of both reagents, high power density and cost efficiency.⁶ The performance of such devices is generically limited by the thermodynamics and kinetics of the redox couple comprising the battery with fast (‘reversible’) electron transfer is essential. In many cases, including the Br₂/Br⁻ couple the electrode reaction involves more than one electron as given in the stoichiometric reaction:2Br⁻ − 2e⁻ ⇄ Br₂; E⁰ = 1.08 V vs. SHEwith, at high bromide concentrations, the possibility of the follow up chemical reaction⁷Br₂ + Br⁻ ⇄ Br₃⁻Since electrons are usually transferred sequentially this implies that the mechanism is multistep with any of the individual mechanistic steps in principle being rate limiting. For this reason catalysts are commonly required to enhance the electrode kinetics at otherwise favourable electrode materials. One type of catalyst which has seen wide usage, including for the Br₂/Br⁻ couple^8,9 are carbon nanotubes (CNTs) with suggested advantages which include high surface area and the inherent porosity of CNT composites.¹⁰ The deployment of CNTs as a porous composite presents a further level of complexity to the electrode reaction beyond its multistep character because of the ill-defined mass transport within the porous layer. In particular ascertaining the intrinsic electron transfer kinetics and hence the level of catalysis, if any, is essentially impossible since these are masked in the voltammetric response by diffusional mass transport effects.^11–14 Specifically the transport within the porous structure of CNT layers is dominated by thin-layer and other^15,16 effects which give the illusion of electrochemical reversibility. In order to unscramble possible electro-catalysis of the bromine/bromide couple a different approach is needed.In the following we study both the electro-oxidation of bromide (BOR) and the electro-reduction of bromine (BRR) at single MWCNTs via ‘nano-impact (aka ‘single entity’) electrochemistry’^17–20 in aqueous solution. In this approach a micro-wire electrode at a fixed potential is inserted in a suspension of CNTs in the solution of interest. From time to time a single CNT impacts the electrode, adopts the potential of the latter for the duration of the impact which in the case of CNTs can vary from 1–100 of seconds^21–23 and sustained catalytic currents flow if the oxidation/reduction of interest is faster at the nanotube in comparison with the micro-wire electrode. The catalytic currents are studied as a function of potential revealing the electron transfer kinetics. Fig. 1 shows the concept of the experiment.Open in a separate windowFig. 1Schematic representation of ‘nano-impact’ electrochemistry on a carbon micro wire electrode for the oxidation of aqueous bromide from which the kinetics of the BOR are inferred. Analogous experiments but showing negative impact currents allow the inference of the kinetics of the BRR.The BOR and BRR were studied first, however, voltammetrically at an unmodified glassy carbon (GC) electrode as shown in Fig. 2 (black line) using 5.0 mM solutions of either NaBr or Br₂ in 0.1 M HNO₃. The midpoint potential was 0.82 V versus the saturated calomel electrode (SCE) consistent with the literature values for the formal potential of the Br₂/Br⁻ couple.²⁴ The voltammograms were analysed to give transfer coefficients of 0.45 ± 0.01 and 0.33 ± 0.01 (ESI, Section 2^†) for the BOR and BRR respectively. Both processes were inferred to be diffusional and the diffusion coefficients D_Br⁻ and D_Br2 were calculated to be 2.05 (±0.04) × 10⁻⁵ cm² s⁻¹ and 1.50 (±0.04) × 10⁻⁵ cm² s⁻¹ (ESI, Section 3^†) using the Randles–Ševčík equation for an irreversible reaction the values are consistent with literature reports.²⁴ Then the electrodes were modified with 30 μg of MWCNTs consisting of ca. 125 monolayers (the calculation is given in the ESI, Section 9^†) of MWCNTs assuming that they are closely packed across the area of the GC electrode, and the resulting voltammograms are shown in Fig. 2 (red line). In comparison with the unmodified electrode, enhanced currents are seen for the Br₂/Br⁻ couple which partly reflects the enhanced capacitance of the interface reflecting in turn the large surface area of the deposited nanotubes (ca. 60–120 cm²). Larger signals are also seen indicating a thin layer contribution from the material occluded within the porous layer which also leads to the apparently quasi-reversible shape of the voltammograms obtained for both reactions. A log–log plot of peak current (I_p) vs. scan rate (ν) showed a gradient value of 0.68 (±0.01) and 0.66 (±0.03) for the BOR and BRR (ESI, Section 4^†) confirming a mixed mass transport regime^12,14 with a combination of semi-infinite diffusion and thin layer behaviour. The transition from the fully irreversible to the apparent quasi-reversible character is sometimes confused with electro-catalysis attributed to the CNTs rather than thin-layer diffusion. In order to ascertain the true catalytic response, single entity electrochemistry was measured to obtain the BOR and BRR responses at single CNTs.Open in a separate windowFig. 2Cyclic voltammograms at pristine GC (black line) and 30 μg MWCNTs dropcast on GC (red line) at a scan rate of 0.05 V s⁻¹ (a) for the bromide oxidation reaction (BOR) in 5.0 mM NaBr in 0.1 M HNO₃, (b) for the bromine reduction reaction (BRR) in 5.0 mM bromine in 0.1 M HNO₃.For single entity measurements, a clean carbon wire (CWE, length 1 mm and diameter 7 μm) working electrode was used. Chronoamperograms were recorded at a constant applied potential of 0.2 V vs. SCE and 1.3 V vs. SCE for the BOR and BRR respectively (5.0 mM solutions). These values were selected in the light of Fig. 2 to provide a large overpotential for each reaction. Clear oxidative and reductive current steps were observed (Fig. 3). These were ascribed to the arrival of a MWCNT at the electrode surface and the resulting catalytic electron transfer for the duration of the impact. No steps were observed in the absence of MWCNTs (ESI, Fig. S4^†). The average residence time of the MWCNT was 1.2 (±0.5) seconds and the frequency of the collisions was 0.3 (±0.1) impacts per second. The average impact current for the BOR at 1.3 V vs. SCE was 2.8 (±0.2) nA (65 impacts) and for the BRR at 0.2 V vs. SCE it was 3.8 (±0.1) nA (70 impacts). The impact currents were assumed to be entirely faradaic since control experiments in 0.1 M HNO₃ solution in the presence of 100 μg of MWCNTs (in the absence of Br⁻ and Br₂) showed no obvious impacts as shown in ESI Section 10.^†Open in a separate windowFig. 3Chronoamperograms showing the impact step current (a) for the BOR in 5.0 mM NaBr in 0.1 M HNO₃ at 1.3 V vs. SCE, (b) for the BRR in 5.0 mM bromine in 0.1 M HNO₃ at 0.2 V vs. SCE.Further, impacts for both the BOR and BRR were observed at various potentials (ESI, Section 11^†) and analysed to obtain the average faradaic current at each potential. The average impact step current was plotted against the applied potential (Fig. 4). Two sigmoidal curves were obtained reflecting the current–potential response for either the bromide oxidation (BOR) or the bromine reduction (BRR). The curves reflect the average voltammograms (current–potential characteristics) for the Br₂/Br⁻ redox reaction at single carbon nanotubes. The shape of the two sigmoidal curves reflects the onset of electrolysis followed by a diffusion controlled plateau at high over-potentials.²⁵ Mass transport corrected Tafel analysis (Fig. 4; inset) showed the transfer coefficients β to be ca. 0.42 and α to be ca. 0.20 from the impacts for the BOR and BRR respectively (ESI, Section 6^†). The length distribution of the MWCNTs was calculated (ESI, Section 6^†) from the currents recorded at potentials corresponding to the plateau in Fig. 4 assuming that the reactions are (Fickian) diffusion controlled at the potentials used and by modelling the CNTs as cylindrical electrodes²¹ assuming a nanotube radius of 15 (±5) nm and the diffusion coefficients reported above. Chronoamperometry was also conducted for the BOR and BRR in the absence of MWCNTs at 1.3 V and 0.2 V vs. SCE respectively to confirm that no impact currents were contributed by the redox species in the electrolyte (ESI, Section 5^†). Alongside, chronoamperograms in 0.1 M HNO₃ and 100 μg show that the impact current was contributed only by the Br⁻ and Br₂ redox reaction and the results are shown in the ESI, Section 10.^†Open in a separate windowFig. 4Average step currents observed as a function of applied potential (a) for the BOR in 5.0 mM NaBr in 0.1 M HNO₃ at, (b) for the BRR in 5.0 mM Bromine in 0.1 M HNO₃; insets in both the cases show mass transport corrected Tafel analyses.The lengths were found to be 5.4 (±3.4) μm (BOR) and 5.9 (±1.3) μm (BRR) and are given in Fig. 5 (see ESI, Section 7^† for calculations). These values were compared with previously reported dark-field optical microscopy data and good agreement was observed with the literature value of 5.3 (±2.1) μm.²⁶ The observed consistency provides strong support for the choice of modelling the single entity voltammetry by analogy with that of a cylindrical electrode.Open in a separate windowFig. 5The length of MWCNTs calculated from the impact currents for the BOR (at 1.3 V vs. SCE) and BRR (at 0.2 V vs. SCE).It is evident that the single entity measurements allow a clear analysis of the catalytic behaviour of the carbon nanotubes by providing a well-defined diffusional regime conducive to the extraction of the electrode kinetics of both the bromide oxidation and the bromine reduction process. In contrast, electrodes were formed by ensembles of carbon nanotubes in the form of a porous layer where the mixed transport regime is not amenable to ready modelling and the dissection of thin-layer effects from the measured voltammetry. The electron transfer kinetics for both the BOR and BRR at single MWCNTs was then obtained via full simulation of the two single entity ‘voltammograms’ using the above measured diffusion coefficients and again treating the impacted MWCNT as a cylindrical electrode with uniform diffusional access and further assuming Butler–Volmer kinetics. For the BOR, one electron transfer was considered as given below,For the BRR the two electron transfer was modelled as,Br₂ + 2e⁻ → 2Br⁻The set of parameters used for the analysis are given in the ESI, Section 8.^† By using the transfer coefficients deduced from Fig. 4, the only unknown is the standard electrochemical rate constant k which is determined by fitting the impact voltammogram measured relative to a formal potential for the Br₂/Br⁻ couple of 0.82 V vs. SCE obtained from the voltammogram at pristine GC. Fig. 6 shows the fitting for the BOR and the BRR with rate constants k_BOR of 1.0 (±0.1) × 10⁻³ cm s⁻¹ and k_BRR of 5.0 (±0.1) × 10⁻⁴ cm s⁻¹ respectively. The transfer coefficients and rate constants obtained from impacts were compared to the voltammograms obtained at pristine GC for the BOR and BRR and are given in Open in a separate windowFig. 6DIGISIM simulated curves (black line) for average impact currents obtained at different potentials (red circles) (a) for the BOR with a rate constant (k_BOR) of 1.0 (±0.1) × 10⁻³ cm s⁻¹; (b) BRR with a k_BRR of 5.0 (±0.1) × 10⁻⁴ cm s⁻¹.Transfer coefficients and rate constants for the BOR in 5.0 mM NaBr in 0.1 M HNO₃ and the BRR in 5.0 mM bromine in 0.1 M HNO₃ obtained at the glassy carbon macroelectrode GC, and single MWCNT impact current

Ammonium pyrrolidinedithiocarbamate-modified activated carbon micro-column extraction for the determination of As(III) in water by graphite furnace atomic absorption spectrometry

A simple and selective method using ammonium pyrrolidinedithiocarbamate modified activated carbon (APDC-AC) as solid phase extractant has been developed for speciation of As(III) in water samples. At pH 1.8–3.0, As(III) could be adsorbed quantitatively by APDC-AC, and then eluted completely with 2.0 mL of 0.1 mol L⁻¹ HNO₃, while As(V) could almost not be retained at pH 1–7. Effects of acidity, sample flow rate, concentration of elution solution and interfering ions on the recovery of As(III) have been systematically investigated. Under the optimal conditions, the adsorption capacity of APDC-AC for As(III) is 7.3 mg g⁻¹. The detection limit (3σ) of As(III) is 0.05 ng mL⁻¹ for graphite furnace atomic absorption spectrometry (GFAAS) with enrichment factor of 50, and the relative standard deviation (RSD) is 4.1% (n = 9, C = 5 ng mL⁻¹). The method has been applied to the determination of trace As(III) in water, and the recoveries of As(III) are 100 ± 10%. Correspondence: Yiwei Wu, Department of Chemistry and Environmental Engineering, Hubei Normal University, Huangshi 435002, P.R. China     

Silver deposition directed by self-assembled gold nanorods for amplified electrochemical immunoassay

A novel electrochemical immunoassay was developed based on the signal amplification strategy of silver deposition directed by gold nanorods (AuNRs), which was in-situ assembled on the sandwich immunocomplex. The superstructure formed by the self-assembly of AuNRs provided abundant active sites for the nucleation of silver nanoparticles. In this pathway, the stripping current of silver was greatly enhanced. Using human immunoglobulin G (HIgG) as a model analyte, the ultrasensitive immunoassay showed a wide linear range of six orders of magnitude from 0.1 fg mL⁻¹ to 100 pg mL⁻¹, with the low detection limit down to 0.08 fg mL⁻¹. The practicality of this electrochemical immunoassay for detection of HIgG in serum was validated with the average recovery of 93.9%. In addition, this enzyme-free immunoassay also has the advantages of acceptable reproducibility and specificity, and thus this immunosensing protocol can be extended to the detection of other low-abundant protein biomarkers.     

富氮空心蠕虫状碳材料的合成及其苯甲醇非金属选择性催化氧化的高活性

Carbon materials have become one of the research hotspots in the field of catalysis as a typical representative of non-metallic catalytic materials. Herein, a facile synthetic strategy is developed to fabricate a series of hollow carbon nanoworms (h-NCNWs) that contain nitrogen up to 9.83 wt% by employing graphitic carbon nitride (g-C₃N₄) as the sacrificing template and solid nitrogen source. The h-NCNWs catalysts were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscope (HR-TEM), N₂ adsorption-desorption, Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric (TG), Raman spectra, and X-ray photoelectron spectroscopies (XPS). The catalytic activities of the h-NCNWs catalysts for selective oxidation of benzyl alcohol with O₂ were also evaluated. The characterization results revealed that the h-NCNWs catalysts displayed a unique hollow worm-like nanostructure with turbostratic carbon shells. The nitrogen content and shell thickness can be tuned by varying the relative ratio of resorcinol to g-C₃N₄ during the preparation process. Furthermore, nitrogen is incorporated to the carbon network in the form of graphite (predominantly) and pyridine, which is critical for the enhancement of the catalytic activity of carbon catalysts for the selective oxidation of benzyl alcohol. At a reaction temperature of 120 ℃, a 24.9% conversion of benzyl alcohol with > 99% selectivity to benzaldehyde can be achieved on the h-NCNWs catalyst prepared with a mass ratio of resorcinol to g-C₃N₄ of 0.5. However, the catalytic activities of the h-NCNWs catalysts were dependent on the amount of N dopants, in particular graphitic nitrogen species. The conversion of benzyl alcohol markedly decreased to 13.1% on the h-NCNWs catalyst prepared with a mass ratio of resorcinol to g-C₃N₄ of 1.5. Moreover, the h-NCNWs catalyst showed excellent stability during the reaction process. The conversion of benzyl alcohol and the high selectivity to aldehyde can be kept within five catalytic runs over the h-NCNWs0.5 catalyst. These results indicate that rationally designed carbon materials have great potential as highly efficient heterogeneous catalysts for oxidation reactions.     

重金属污泥高温处理磨细粉对改性硫氧镁水泥基材料性能的影响

为探究重金属污泥高值资源化利用途径, 研究了含重金属污泥高温处理渣料磨细粉(简称磨细粉)对改性硫氧镁(MMOS)水泥基材料的工作性能、力学性能、早期自由收缩变形及其重金属浸出行为的影响. 结果表明: 随着磨细粉掺量的增加, MMOS水泥基材料的净浆流动度呈现逐渐增大趋势; 早期(3d)力学性能则呈现明显的降低趋势, 90d后磨细粉掺量对MMOS水泥基材料的力学性能的影响程度降低; 磨细粉可以显著降低MMOS水泥基材料早期的自由收缩变形值, 其中含10%磨细粉的MMOS水泥基材料早期的自由收缩变形比未掺加磨细粉MMOS水泥基材料的自由收缩变形值降低了57.1%. 微观分析结果表明: 掺加30%磨细粉后MMOS水泥基材料中水化产物5Mg(OH)2?MgSO4?7H2O(517相)的相对含量降低, MMOS水泥基材料硬化体的平均孔径提高了31.0%, 表明磨细粉降低了MMOS水泥基材料的力学性能. 当磨细粉掺量增加至40%时MMOS水泥基材料硬化体中重金属Ni、Cr、Zn和Cu的浸出浓度指标均能满足GB 30760-2014的浸出要求.