Electrochemical application of cobalt nanoparticles-polypyrrole composite modified electrode for the determination of phoxim

Part I：In this study, the author synthesized reduced graphene oxide-carbon nanotube and iron nanoparticle composite (RGO-CNT-FeNPs) modified glassy carbon electrode (GCE) and employed it for the electrocatalytic oxidative determination of nitrite. The synthesized composite was characterized by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) and electrochemical impedance spectroscopy (EIS). The nitrite oxidation peak current (Ip) at RGO-CNT-FeNPs/GCE is significantly higher than that observed at graphene oxide dispersed carbon nanotube (GO-CNT) and unmodified GCEs. In addition, the overpotential for the oxidation of nitrite at RGO-CNT-FeNPs/GCE has been greatly reduced to +770 mV, (vs. Ag/AgCl reference electrode) which is 240 mV less positive than bare GCE. The prepared RGO-CNT-FeNPs/GCE has been demonstrated for the efficient amperometric sensing of nitrite. The sensor detects nitrite in a wide linear range of 1×10-7 M to 1.68×10-3 M, with very low detection limit of 7.56 ×10-8 M. The good synergy among RGO, CNT, and FeNPs endows superior electrocatalytic ability towards the oxidation of nitrite than the individual components. The good recoveries achieved for the determination of nitrite in various water samples reveal the promising practicality of the proposed sensor. Moreover, the sensor displays an acceptable repeatability and reproducibility results along with good storage stability and excellent operational stability. Part II：Herein the author reported electrochemical amperometric sensors for the sensitive determination of Diuron and fenuron by graphene oxide-carbon nanotubes (GO-CNT) hybrid modified electrode. GO-CNT hybrid was prepared by simple sonication approach and its surface morphology was characterized by scanning electron microscopy (SEM). The electrocatalysis of GO-CNT modified glassy carbon electrode (GCE) was studied towards determination of pesticides diuron (DU) and fenuron (FU). Cyclic voltammogram of GO-CNT modified GCE towards DU showed an irreversible anodic peak at the potential of + 800 mV which is 80 mV less positive potential than that at DMF dispersed CNT. Moreover the peak current of DU at the GO-CNT hybrid is greatly enhanced to 2.25 fold than the peak current at the DMF-CNT modified GCE. Amperometric sensor has been fabricated, which possess wide linear range of 9 μM to 0.38 mM with acceptable of 64.51 ?A mM-1cm-2. Similarly, the DU oxidation peak current obtained at GO-CNT hybrid modified GCE is 2.4 fold higher than the current observed at DMF-CNT modified GCEs. Amperometric sensor was constructed, which exhibited good linear range of 0.9 μM to 47 μM and high sensitivity of 1.19 μA μM-1 cm-2. Besides, the practical applicability of the sensor has been demonstrated in various water samples collected from agricultural areas. The good recovery results revealed that the GO-CNT modified GCE acquire versatile ability to determine both DU and FU in real samples. In addition, the sensor offers good repeatability and reproducibility results. Part III：In this study, the author report electrochemical oxidation of tryptophan at a poly(3,4-ethylendioxythiophene)-functionalized multi-walled carbon nanotube (f- MWCNT) and Brilliant blue (BB) composite film modified glassy carbon electrode (GCE). PEDOT was polymerized on the GCE by cyclic voltammetry (CV) in 0.1 M H2SO4, and dropped 4mL f-MWCNT on the PEDOT modified GCE. Then BB was polymerized on the f-MWCNT-PEDOT/GCE in pH 1.5 H2SO4 by cyclic voltammetry. The surface morphology of PEDOT-f-MWCNT/BB was studied using field emission scanning electron microscope (FESEM). The interfacial electron transfer phenomenon at the modified electrode was studied using electrochemical impedance spectroscopy (EIS). Tryptophan showed a well defined anodic peak at 0.78 V (vs. Ag/AgCl electrode). The peak current increased linearly with tryptophan concentration. The amperometric determination of tryptophan at the composite film modified electrode showed linear range from 10 μM to 570 μM (R2 = 0.9985). It also exhibits good sensitivity of 448 μA mM-1, detection limit of 0.5 ?M. This result shows that the proposed composite electrode may be developed for potential application in real sample analysis.

Polysulfone Nanocomposite Membranes with improved hydrophilicity

In present work, reduced graphene oxide nanosheets (rGO) decorated with trimetallic three‐dimensional (3D) Pt−Pd−Co porous nanostructures was fabricated on glassy carbon electrode (Pt−Pd−Co/rGO/GCE). First, GO suspension was drop‐casted on the electrode surface, then GO film reduction was carried out by cycling the potential in negative direction to form the rGO film modified GCE (rGO/GCE). Then, electrodeposition of the cobalt nanoparticles (CoNPs) as sacrificial seeds was performed onto the rGO/GCE by using cyclic voltammetry. Afterward, Pt−Pd−Co 3D porous nanostructures fabrication occurs through galvanic replacement (GR) method based on a spontaneous redox process between PtCl2, PdCl2, and CoNPs. The morphology and structure of the Pt−Pd−Co/rGO porous nanostructure film was characterized by scanning electron microscopy, energy dispersive spectroscopy and X‐ray diffraction method. The performance of the prepared electrode was investigated by various electrochemical methods including, cyclic voltammetry and electrochemical impedance spectroscopy. The electrocatalytic activity of the as‐prepared modified electrode with high surface areas was evaluated in anodic oxidation of ethylene glycol. The study on electrocatalytic performances revealed that, in comparison to various metal combinations in modified electrodes, trimetallic Pt−Pd−Co/rGO/GCE exhibit a lower onset potential, significantly higher peak current density, high durability and stability for the anodic oxidation of ethylene glycol. The excellent performances are attributed to the rGO as catalysts support and resulting synergistic effects of the trimetallic and appropriate characteristics of the resulted 3D porous nanostructures. Moreover, the influence of various concentrations of ethylene glycol, the potential scan rate and switching potential on the electrode reaction, in addition, long‐term stability have been studied by chronoamperometric and cyclic voltammetric methods.

This study presents a simultaneous, rapid, and highly sensitive electrochemical detection of 2,4- and 2,5-dinitrophenol isomers using an iron bismuth oxide (FeBiO3)-modified glassy carbon electrode (GCE). The FeBiO3-modified GCE exhibited superior electrochemical performance over bare GCE, Bi2O3-modified GCE, and Fe2O3-modified GCE, as demonstrated by electrochemical impedance spectroscopy (EIS) with the potassium ferricyanide (Fe(CN6)3-/4-) standard. The synthesised electrocatalyst FeBiO3 exhibited strong absorption both in the visible and UV regions of the spectrum, attributed to band gap transitions and charge transfer transitions linked to its Bi2O3 and Fe2O3 components. The morphology and the structure of the FeBiO3 were characterised using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD), while Fourier transform infrared spectroscopy (FTIR) was employed to analyse the functional groups attached to the surface of FeBiO3. Cyclic voltammetry (CV) clearly demonstrated an increased current and improved peak resolution for the reduction of 2,4- and 2,5-dinitrophenols at the FeBiO3/GCE, compared to the bare GCE. The mechanism shows that the reduction of 2,4- and 2,5-dinitrophenols occurs through interactions with the surface oxygen functional groups of FeBiO3, resulting in the stepwise reduction of their NO2 groups to hydroxyamino compounds and then to nitrosophenols. Using square wave voltammetry (SWV), the reduction peak currents were significantly improved, enhancing detection sensitivity beyond that reported in previous works. A linear range from 1 µM to 1 mm for the dinitrophenol isomers was established, with detection limits of 0.07 µM for 2,4-dinitrophenol and 0.09 µM for 2,5-dinitrophenol. The FeBiO3/GCE electrode showed interferences with mono-nitrophenols and mono-/di-chlorophenols in solution but maintained excellent long-term stability and reproducibility, allowing it to effectively detect dinitrophenol isomers in drinking water, tap water, and wastewater samples.

Molecular imprinted polypyrrole modified glassy carbon electrode for the determination of tobramycin

Graphene-supported nanomaterials as electrochemical sensors: A mini review

Simultaneous determination of paracetamol and ciprofloxacin in biological fluid samples using a glassy carbon electrode modified with graphene oxide and nickel oxide nanoparticles

This study aimed to compare the spectroscopy, morphological, electrocatalytic properties, and antibacterial activities of cobalt nanoparticles (CoNPs) with nickel nanoparticles (NiNPs). Cobalt nanoparticles and NiNPs were prepared via a chemical reduction approach and characterized utilizing transmission electron microscopy (TEM), energy-dispersive X-ray (EDX), and X-ray diffraction (XRD) techniques. The result from XRD and TEM analysis revealed that the synthesized nanoparticles exhibit face-centered cubic with smooth spherical shape, having average particles size of 12 nm (NiNPs) and 18 nm (CoNPs). The electrochemical properties of the nanoparticles were examined via cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques. The CV results showed that GCE-Ni (35.6 μA) has a higher current response compared to GCE-Co (10.5 μA). The EIS analysis revealed that GCE-Ni (1.39 KΩ) has faster electron transport capability compared to GCE-Co (2.99 KΩ) as indicated in their Rct values. The power density of the synthesized nanoparticles was obtained from their "knee" frequency (f°) values, with GCE-Ni (3.16 Hz) having higher f° values compared to GCE-Co (2.00 Hz). The antibacterial activity of the nanoparticles was evaluated against multidrug-resistant Escherichia coli O157, Escherichia coli O177, Salmonella enterica, Staphylococcus aureus, and Vibrio cholerae. The result from the antibacterial study revealed that at low concentrations both CoNPs and NiNPs have significant antibacterial activities against E. coli O157, E. coli O177, S. enterica, S. aureus, and V. cholerae. NiNPs showed better antibacterial activities at low concentrations of 61.5, 61.5, 125, 61.5, and 125 µg/mL compared to CoNPs with minimum inhibitory concentrations of 125, 125, 250, 61.5, and 125 µg/mL against E. coli O157, E. coli O177, S. enterica, S. aureus, and V. cholerae, respectively. These promising antibacterial activities emphasize the potential of CoNPs and NiNPs as effective antibacterial agents, which could aid in the development of novel antibacterial medicines.

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jan 2022Theory-Driven Design of Electrocatalysts for the Two-Electron Oxygen Reduction Reaction Based on Dispersed Metal Phthalocyanines Yang Wang†, Zisheng Zhang†, Xiao Zhang, Yubo Yuan, Zhan Jiang, Hongzhi Zheng, Yang-Gang Wang, Hua Zhou and Yongye Liang Yang Wang† Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 , Zisheng Zhang† Department of Chemistry, Guangdong Provincial Key Laboratory of Catalysis, Southern University of Science and Technology, Shenzhen 518055 , Xiao Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027 , Yubo Yuan Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 , Zhan Jiang Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 , Hongzhi Zheng Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 , Yang-Gang Wang Department of Chemistry, Guangdong Provincial Key Laboratory of Catalysis, Southern University of Science and Technology, Shenzhen 518055 , Hua Zhou X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439 and Yongye Liang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 Guangdong-Hong Kong-Macao Joint Laboratory for Photonic-Thermal-Electrical Energy Materials and Devices, Southern University of Science and Technology, Shenzhen 518055 https://doi.org/10.31635/ccschem.021.202000590 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The two-electron electrochemical reduction of oxygen is an appealing approach to produce hydrogen peroxide. Metal and heteroatom-doped carbon (M–X/C) materials have recently been recognized as compelling catalysts for this process, but their performance improvement is generally hindered by the ill-defined structures of active sites. Herein, we demonstrate a theory-driven design of catalysts for oxygen reduction reactions based on molecularly dispersed electrocatalysts (MDEs) with metal phthalocyanines on carbon nanotubes. Density functional theory calculations suggest that nickel phthalocyanine (NiPc) favors the formation of *H2O2 over *O, thus acting as a selective catalyst for peroxide production. NiPc MDE shows high peroxide yields of ∼83%, superior to the aggregated NiPc and pyrolyzed Ni–N/C catalysts. The performance is further enhanced by the introduction of the cyano group (CN). NiPc–CN MDE exhibits ∼92% peroxide yields and good stability. Our studies provide a new perspective for the development of heterogeneous electrocatalysts for hydrogen peroxide production from metal macrocyclic complexes. Download figure Download PowerPoint Introduction Driving economically important chemical reactions with renewable electricity offers an intriguing opportunity to replace current energy-intensive processes.1–3 For example, the electrochemical oxygen reduction reaction (ORR) through the two-electron (2e−) pathway is considered an environmentally benign alternative to the industrial anthraquinone method to produce hydrogen peroxide (H2O2), which is widely used as a green oxidizer in bleaching, waste water treatment, and the chemical industry.4–6 An ideal electrocatalyst should possess high activity toward the 2e− pathway to the peroxide product and suppress the competing 4e− process to water. Platinum (Pt) or palladium (Pd) alloys with mercury (Hg) have been demonstrated as selective electrocatalysts for the 2e− pathway in ORR.7,8 However, due to the toxicity of Hg and the limited reserve of noble metals, these elements are not preferred for practical applications. Carbon-based materials doped with earth-abundant elements, however, are compelling candidates as efficient and affordable electrocatalysts.9–14 Carbon nanotube (CNT), graphene, and activated carbon with oxygen-containing functional groups were reported to be selective in 2e− ORR.15–18 Embedding coupled boron–nitrogen (BN) domains into graphitic carbon showed enhanced selectivity and activity for reducing O2 to HO2− compared with the catalysts with individual B or N doping.19 In addition to metal-free catalysts, metal and heteroatom-doped carbon (M–X/C) catalysts with isolated heteroatom-coordinated metal moieties, one type of single-atom catalysts (SACs), have also been exploited for H2O2 production.20–23 A series of M–N/C (M = Mn, Fe, Co, Ni, and Cu) catalysts with proposed M–N4 active sites were synthesized to investigate their performance in ORR. The Co–N/C catalyst showed preference for the 2e− pathway in acidic condition.24 Moreover, SACs with transition-metal centers coordinated by different heteroatoms, such as O and S, were also reported to show high selectivities for the 2e− reduction pathway in ORR.25–28 However, the lack of well-defined structures and the copresence of various types of active sites prevent understanding the structure–performance relationships and catalyst design principles in these SAC catalysts. Metal macrocyclic complexes, such as metal phthalocyanines (MPcs) and porphyrins with well-defined M–N4 moieties, have been attractive electrocatalysts since the report of cobalt phthalocyanine as an active ORR catalyst.29–31 For instance, iron phthalocyanine (FePc) has been reported to be efficient in catalyzing ORR through the 4e− pathway to water.32,33 However, the performances of metal complexes in heterogeneous form are often limited by their low electric conductivity.34–36 Hybridizing metal macrocyclic complexes with nanocarbon materials were found to promote their catalytic performances.35,37,38 In the carbon dioxide reduction reaction, achieving molecular dispersion on conducting supports is beneficial to reveal the intrinsic performance of molecular catalysts and establish catalyst design principles.39,40 In addition, previous reports of heterogeneous molecular ORR catalysts mainly focused on optimizing the performance toward the 4e− pathway with little exploration of the 2e− pathway to peroxide production.30,41 In this work, we present a theory-driven design of electrocatalysts based on an molecularly dispersed electrocatalyst (MDE) consisting of dispersed MPcs on CNTs for electrochemical production of peroxide. From density functional theory (DFT) calculations, we identify nickel phthalocyanine (NiPc) as a selective catalyst for 2e− ORR with experimental peroxide yields of ∼83% in the form of MDE, in contrast to FePc MDE that is selective for 4e− ORR. Achieving molecular dispersion of NiPc with well-defined Ni–N4 sites is important to the high peroxide selectivity as proven by the lower peroxide yields of the physically mixed NiPc and CNT (containing aggregated NiPc) and a pyrolyzed Ni–N/C SAC. Moreover, molecular engineering of NiPc MDE with the introduction of cyano groups (CNs) to the Pc ligand (NiPc–CN MDE) further enlarges the free-energy preference to the 2e− pathway and enhances the selectivity for the electrochemical production of peroxide. NiPc–CN MDE exhibits a high peroxide yield of ∼92% in the potential range of 0.70–0.20 V versus a reversible hydrogen electrode (RHE). Experimental Methods Preparation of MPc MDEs The preparation of MPc MDEs was based on a reported procedure with the control of the ratio between MPcs and CNTs.40 NiPc and FePc were obtained from commercial sources, and NiPc–CN was synthesized according to a reported method.40 Briefly, 30 mg purified CNTs were dispersed in 25 mL of N,N-dimethylformamide (DMF) with the assistance of sonication, in which a calculated amount of MPcs in 5 mL of DMF was added to obtain a well-mixed suspension. The mixture was further sonicated for 30 min and then stirred at room temperature for 20 h. Subsequently, the precipitate was collected by centrifuge and washed with DMF (three times) and ethanol (twice). Finally, the collected precipitate was lyophilized to yield the final product. Electrochemical measurements About 4 mg of MPc MDEs and 10 μL of 5 wt % Nafion solution were dispersed in 990 μL ethanol under ultrasonication to form a homogeneous ink. About 13 μL catalyst ink was loaded onto the glassy carbon (GC) disk electrode (5.5 mm in diameter) of a rotating ring-disk electrode (RRDE) to achieve a catalyst loading of ∼0.2 mg cm−2. The ink of NiPc + CNT was prepared by dispersing 2.8 mg of NiPc, 1.2 mg of CNT, and 10 μL of 5 wt % Nafion solution in 990 μL ethanol under ultrasonication, then loaded onto the GC electrode. The RRDE experiments were conducted with a four-electrode system using a saturated calomel electrode (SCE) as the reference electrode (calibrated with a homemade RHE), a graphite rod as the counter electrode, and the catalyst-modified GC disk electrode as the working electrode. Meanwhile, the Pt ring electrode was kept at 1.5 V (vs RHE, the same for following potentials unless otherwise stated) for all experiments. The disk and ring electrodes were rotated at a speed of 1600 rpm (Pine research). Electrolytes (0.1 M KOH) were saturated with O2 by bubbling for 30 min prior to each experiment, and a flow of O2 was maintained over the electrolyte during the reaction. Linear sweep voltammetry (LSV) was conducted by scanning the disk electrode potential with a scan rate of 5 mV/s. For the stability test, the disk electrode potential was kept at 0.5 V. Experiments were also performed under an argon environment to record the background currents of the disk and ring electrodes, which were subtracted from the currents under O2. The peroxide yield and electron transfer number (n) were determined by the following equations: Peroxide yield ( HO 2 − ) = 200 × ( I r / N ) I d + ( I r / N ) % n = 4 × I d I d + ( I r / N ) where Ir is ring current, Id is disk current, and N is current collection efficiency of the Pt ring electrode (0.28, calibrated with K3[Fe(CN)6]). Computational Methods DFT calculations of gas-phase MPc molecules catalyzing ORR were conducted using the Gaussian 09 program.42 B3LYP functional43 with D3 correction (Becke–Johnson damping)44 was adopted for calculation.45 The all-electron 6-31G* basis set (for H, C, N, and O)46–48 and the Stuttgart–Dresden (SDD) basis set containing all double-ξ valence with effective core potentials (ECPs)49 (for Ni and Fe) were used. The geometric structures were all optimized at 298.15 K and under 1 atm. The harmonic vibrational frequencies were computed with no imaginary frequency found for all reaction intermediates. The Gibbs free energies of high- and low-spin forms of all intermediates were calculated with the harmonic potential approximation to determine the ground states. The electrocatalytic mechanisms were investigated with the computational hydrogen electrode (CHE) model.50 Additional details of computational methods are available in the Supporting Information. Results and Discussion Theoretical calculations of MPcs catalyzing ORR To understand how the central metals in MPc molecules affect the product selectivity in ORR, DFT calculations of the free-energy changes of ORR through the 2e− and 4e− pathways were conducted on FePc and NiPc at 1.23 V versus RHE. The calculated free-energy diagrams suggest distinctly different ORR behaviors of FePc and NiPc (Figure 1a). On FePc, O2 is first adsorbed on the Fe center, followed by a proton-coupled electron transfer (PCET) process to form *OOH with an uphill free-energy change. The divergence of the 2e− and 4e− pathways came from the preference of *OOH reduction with a protonation mechanism to *H2O2 or an O–O cleavage to *O. The downhill free-energy change to form *O and the large free-energy increase required for the generation of *H2O2 indicate a high preference for the 4e− reduction pathway on FePc, consistent with high selectivities of O2 reduction to water/hydroxide of Fe macrocyclic complexes and Fe-based SACs in previous reports.34,51 By contrast, the *OOH intermediate (generated from O2 through an *O2 intermediate with two uphill free-energy changes) on NiPc shows a slight downhill free-energy change to generate *H2O2, while the formation of *O in the 4e− reduction pathway is energetically uphill (Figure 1a). In contrast to FePc, the reversed trend in free-energy changes to form *H2O2 and *O on NiPc suggests the preference for the 2e− reduction pathway. Therefore, NiPc molecules are predicted to be selective electrocatalysts for 2e− ORR to peroxide product (Figure 1b). Figure 1 | Theoretical calculations of ORR catalyzed by MPcs. (a) Calculated free-energy diagrams of ORR through the 2e− and 4e− reduction pathways on NiPc and FePc at 1.23 V. (b) Schematic presentation of ORR selectivity on NiPc and FePc based on DFT calculations. Download figure Download PowerPoint ORR performance of MPc MDEs and aggregated MPcs Dispersed NiPc and FePc molecules were supported on the CNTs via π–π interactions to fabricate MPc MDEs according to our previous method40 to examine the calculated trends in ORR. The metal contents in MDEs were controlled to be ∼0.7 wt % ( Supporting Information Table S1), which were measured by inductively coupled plasma mass spectrometry (ICP-MS). The electrochemical behaviors of NiPc MDE and FePc MDE were first investigated in O2-saturated 0.1 M KOH electrolytes with the RRDE setup (0.2 mg cm−2 catalyst loading). The MDEs were drop-coated on the disk electrode as the working electrode to reduce O2, while the ring electrode (Pt) was maintained at 1.5 V to detect the produced peroxide. FePc MDE shows more positive onset potential (0.94 V at −0.025 mA, corresponding to a current density of ∼−0.1 mA cm−2) than that of NiPc MDE (0.79 V) (Figure 2a). The current of FePc MDE is saturated to −1.41 mA at ∼0.58 V. The saturation current for NiPc MDE is about −0.63 mA at the same potential. The peroxide yield and n of MPc MDEs calculated from the disk and ring currents are depicted in Figure 2b and Supporting Information Figure S1, respectively. NiPc MDE shows good peroxide yields of ∼83% in the potential range of 0.70–0.53 V, which decline at more negative potentials. Correspondingly, n of NiPc MDE is below 2.34 in the potential range of 0.70–0.53 V, which gradually increases to 2.83 from 0.53 to 0.20 V ( Supporting Information Figure S1). In contrast, low peroxide yields of ∼1% together with n above 3.97 in the potential range of 0.70–0.20 V are observed with FePc MDE (Figure 2b and Supporting Information Figure S1), confirming its strong preference toward the 4e− reduction pathway. These results indicate that the preferred ORR pathways of NiPc MDE and FePc MDE are the 2e− and 4e− reduction pathways, respectively, which are consistent with the DFT calculations (Figure 1a). Figure 2 | ORR performance of MPc-based electrocatalysts on RRDE. (a) Disk and ring currents of NiPc, NiPc + CNT, NiPc MDE, and FePc MDE in O2-saturated 0.1 M KOH electrolytes on RRDE test rotating at 1600 rpm. (b) Calculated peroxide yields of NiPc MDE, NiPc + CNT, and FePc MDE. Download figure Download PowerPoint The effects of aggregation state of NiPc were further investigated. The NiPc molecules directly deposited on substrates easily formed aggregates due to their strong intermolecular interactions ( Supporting Information Figure S2). Due to the poor electric conductivity and limited exposure of active sites of aggregated NiPc, the neat NiPc electrode shows minimal activity in ORR (Figure 2a). Therefore, we physically mixed NiPc with CNTs (denoted as NiPc + CNT) to enhance the conductivity. LSV shows that the physically mixed NiPc + CNT possesses higher activity than that of neat NiPc (Figure 2a). Although NiPc + CNT exhibits even more positive onset potential than NiPc MDE, the low peroxide yields of NiPc + CNT (under 60% in the potential range of 0.70–0.20 V) suggests much less preference toward the 2e− pathway compared with NiPc MDE (Figure 2b). Topological defects and N-dopants have been considered as active sites for ORR.11 However, Pc MDE (prepared by anchoring Pc molecules on CNTs) without metal centers shows inferior ORR activity and selectivity for the 2e− pathway compared with NiPc MDE ( Supporting Information Figure S3). These results suggest the critical role of dispersed Ni centers rather than the N-dopants or topological defects in the selective electrocatalysis of 2e− ORR. To investigate the origin of the different ORR behaviors of neat NiPc, NiPc + CNT, and NiPc MDE, their structures were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The TEM ( Supporting Information Figure S4a) and SEM ( Supporting Information Figure S4b) images of NiPc MDE show the bundles of multiwalled CNTs and excluded the formation of nano- or microsized NiPc aggregates. The isolated bright spots in the high-angle annular dark-field (HAADF) image of NiPc MDE obtained with a Cs-corrected scanning TEM (STEM) indicate the existence of single-Ni sites, suggesting the molecular dispersion of NiPc on CNTs ( Supporting Information Figure S5). No appreciable signature peaks of NiPc molecules are observed in the Raman spectrum of NiPc MDE ( Supporting Information Figure S6), which could be due to the low content of NiPc in NiPc MDE. On the contrary, physically mixed NiPc + CNT contains microsized NiPc aggregates, as revealed by SEM and confirmed by energy-dispersive spectrometry (EDS) mapping of the Ni signals ( Supporting Information Figures S4c and S7). Electrochemical impedance spectroscopy (EIS) was further conducted to gain insights into the ORR kinetics of the NiPc catalysts in aggregated and dispersed states. The Nyquist plots in Supporting Information Figure S4d show that the charge transfer of NiPc MDE for ORR is more favorable than that of NiPc and NiPc + CNT. The neat NiPc exhibited the largest charge-transfer resistance, in agreement with its low activity from LSV (Figure 2a). It should be noted that DFT calculations with individual NiPc molecule-catalyzing ORR suggest high selectivity toward the 2e− transfer pathway, which is only observed in dispersed NiPc as in NiPc MDE but not in aggregated NiPc as in NiPc + CNT. These results emphasize the importance of correlating free-energy diagrams calculated with individual catalyst molecule with the electrocatalytic performance of dispersed molecular catalysts as opposed to aggregated molecules. Comparison with the pyrolyzed Ni–N/C SAC SACs have gained extensive attention recently due to their superior electrocatalytic properties. In electrocatalytic applications, SACs are generally fabricated by pyrolyzing metal salts and N-containing organic precursors at high temperatures. However, these pyrolyzed SACs often contain parasitic active sites due to the insufficient structural control during the high-temperature synthesis.52 For comparison, a nickel SAC with Ni–Nx structures (denoted as Ni–N/C) was synthesized by pyrolyzing a Ni-containing zeolitic imidazolate framework (ZIF) precursor according to the reported method with minor modifications.53 The Ni content of the Ni–N/C catalyst was also controlled to be ∼0.7 wt % (measured by ICP-MS) to compare with NiPc MDE. TEM images of Ni–N/C suggest the absence of metallic Ni particles in the catalyst ( Supporting Information Figure S8). The X-ray diffraction (XRD) pattern of Ni–N/C only shows broad features attributable to graphitic carbon ( Supporting Information Figure S9),53 further indicating the absence of metallic Ni or crystalline Ni-containing compounds. The Fourier-transformed extended X-ray adsorption fine structure (FT-EXAFS) curve of Ni–N/C exhibits a peak at ∼1.4 Å (without phase correction) corresponding to Ni–N coordination, but little signal at ∼2.1 Å corresponding to Ni–Ni coordination ( Supporting Information Figure S10), confirming the presence of single Ni sites in Ni–N/C. Although no Ni particles are observed, parasitic active sites such as N-doped carbon sites could still be present in the Ni–N/C catalyst,54,55 which are known to be active for 4e− ORR (Figure 3a). The ORR performance of the Ni–N/C catalyst was further characterized in the RRDE setup with identical conditions as NiPc MDE. Given the LSV curves (Figure 3b), Ni–N/C possesses more positive onset potential (0.83 V) than that of NiPc MDE (0.79 V). The peroxide yields of Ni–N/C are under 43% with n larger than 3.1 in the potential range of 0.70–0.20 V (Figure 3c and Supporting Information Figure S11). The much worse selectivity of the Ni–N/C catalyst toward the 2e− reduction pathway than that of NiPc MDE is attributed to the structural heterogeneity in the pyrolyzed Ni–N/C catalyst. Additionally, the stability tests were carried out at the constant potentials of 0.50 V for the disk electrode to conduct ORR and at 1.50 V for the ring electrode to detect generated peroxide. As shown in Figure 3d, Ni–N/C shows obvious decay of both the disk and ring currents in the first suggesting the of the pyrolyzed By contrast, NiPc MDE exhibits much stability without appreciable decay of the disk and ring currents during the Therefore, the molecularly dispersed and well-defined Ni–N4 sites in NiPc MDE a ORR catalyst for the 2e− reduction pathway than the pyrolyzed Ni–N/C. NiPc MDE be a catalyst system to establish the between the active structure and electrocatalytic Figure | Comparison of electrocatalytic ORR performance between NiPc MDE and Ni–N/C. (a) Schematic presentation of ORR with NiPc MDE and Ni–N/C. (b) Disk and ring currents of NiPc MDE and Ni–N/C in O2-saturated 0.1 M KOH Calculated peroxide yields of NiPc MDE and Ni–N/C. tests of NiPc MDE and Ni–N/C under the constant potentials of 0.50 V for the disk electrode and 1.50 V for the ring electrode. Download figure Download PowerPoint engineering of NiPc MDEs for ORR A of the MDE system is the of the of the active sites and the electrocatalytic performance through molecular we to further the performance of NiPc MDE for 2e− ORR for peroxide production with the introduction of groups to the Pc with Figure for molecular as an ORR catalyst by DFT calculations. The free-energy diagrams suggest to NiPc, NiPc–CN shows strong preference toward the 2e− reduction pathway, and the of *H2O2 over *O is from with NiPc to with NiPc–CN (Figure indicating that NiPc–CN be a more selective catalyst for 2e− ORR. Figure 4 | ORR electrocatalysis with NiPc–CN MDE. (a) Calculated free-energy diagrams of ORR through the 2e− and 4e− reduction pathways on NiPc and NiPc–CN at 1.23 V. shows the molecular structure of (b) Disk and ring currents of NiPc and NiPc–CN MDEs in O2-saturated 0.1 M KOH Calculated peroxide yields and n of NiPc and NiPc–CN test of NiPc–CN MDE under the constant potentials of 0.50 V for the disk electrode and 1.50 V for the ring electrode. Download figure Download PowerPoint NiPc–CN MDE was synthesized ( Supporting Information Figure and characterized with the RRDE The LSV curve shows more positive onset potential of NiPc–CN MDE V) than that of NiPc MDE (0.79 V) (Figure Moreover, the saturation current for NiPc–CN MDE in the potential range of V together with constant ring The peroxide yields of NiPc–CN MDE were calculated to be ∼92% from to 0.20 V, superior to NiPc MDE with peroxide yields below V (Figure The enhanced peroxide yields of NiPc–CN MDE are consistent with the preference toward the 2e− reduction pathway from DFT calculations with the introduction of (Figure NiPc–CN MDE also shows good stability in ORR with little decay in the disk and ring currents and the peroxide yields during the (Figure The performance of NiPc–CN MDE for oxygen reduction to peroxide is the for the reported and metal catalysts, high peroxide selectivity of ∼92% at a potential in conditions ( Supporting Information Table S2). by catalyst design with DFT calculations, we identify NiPc MDE as a good ORR catalyst for the 2e− reduction pathway and further enhance its performance through molecular The enhanced NiPc–CN MDE with shows superior selectivity with peroxide yield of ∼92% in the potential range of 0.70–0.20 V. The molecularly dispersed and well-defined Ni–N4 sites on CNTs NiPc MDEs with higher 2e− selectivities than the aggregated NiPc and pyrolyzed Ni–N/C catalysts. These results also that the MPc MDE system as electrocatalysts for the of the between active structures and electrocatalytic performances of molecular catalysts and Supporting Information Supporting Information is available and Figures and and of is no of to Information was supported by Guangdong-Hong Kong-Macao Joint Laboratory for Photonic-Thermal-Electrical Energy Materials and the from Zhejiang University, and Guangdong Provincial Key Laboratory of TEM and were measured with maintained by The were obtained with the of the Advanced Photon a Department of Energy of Science for the of Science by Argonne National Laboratory under The computational is supported from the for Computational Science and the in Energy Materials for It for to Zheng Wang Zhang Wang Pt on for O2 Reduction to H2O2 in Wang of Peroxide from and Jiang Wang Design for Electrochemical Oxygen Reduction toward H2O2 through in the Electrochemical of and by Zheng Zhou with

The measurement of antibiotics in environmental water systems is increasingly becoming a top priority for global environmental watchdog organisations, as the looming threat of emerging contaminants moves to centre stage. A novel chemical sensor based on polyamic acid (PAA) semiconducting polymer and cobalt nanoparticles (CoNP) was developed and used to demonstrate proof of concept evidence for measurement of norfloxacin at trace concentrations in aqueous systems. Polyamic acid and cobalt nanoparticles were both chemically synthesised and characterised using Fourier transform infrared spectroscopy, transmission electron microscopy, cyclic voltammetry and small angle x‐ray scattering. The polyamic acid and cobalt nanoparticles were electrodeposited onto screen printed carbon electrodes to produce novel composite sensors (SPCE/PAA/CoNP). The polymer composite chemical sensors were applied to the detection of norfloxacin in the micromolar concentration using square wave voltammetry, with a sensitivity of 18.0±6.59 μA/mM, calculated from the slope of the calibration curve and a limit of detection (LOD) of 0.979 ±0.419 mM, with LOD=3.3(Sy/S). The SPCE/PAA/CoNP sensor response to norfloxacin as measured by electrochemical impedance spectroscopy, yielded a sensitivity of 17.6±8.72 Ω/mM as and a limit of detection (LOD) of 0.228±0.0935 mM.

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Overoxidized polypyrrole (OvoxPpy) doped with gold-nanoparticles (GNP) on a glassy carbon electrode (GCE) was used as a platform to construct a novel label-free impedimetric immunosensor for anti-transglutaminase (anti-tTG). Field emission scanning electron microscopy results confirmed the deposition of GNP with good surface coverage across the OvoxPpy substrate. The average diameter of the nanocomposite was 100 nm. The platform (GNP|OvoxPpy||GCE) was conductive and exhibited reversible electrochemistry (E°’ = 395 mV) in (pH 7.4) PBS. The electrochemical characterization of the platform was studied by cyclic voltammetry (CV) and square wave voltammetry (SWV). OvoxPpy||GCE had a peak separation, ΔE p , value of −80 mV. The GNP|OvoxPpy||GCE showed enhanced interfacial charge transfer with ΔE p , being −30 mV. The SWV results corroborated CV findings which show enhanced peak current through GNP. The immunosensor was prepared by immobilizing 40 μL tTG (0.3 mg mL−1) onto the platform by drop coating. Electrochemical impedance spectroscopy (EIS) measurements indicated that the immunosensor system (BSA|tTG|GNP|OvoxPpy||GCE) markedly improved the conductivity and response of the anti-tTG immunosensor. The impedimetric immunosensor exhibited a charge transfer resistance (Rct)-dependent dynamic linearity of 10−6 to 10−4 M for target anti-tTG (r = 0.9808), and detection limit of 5.22 × 10−6 M.