Graphdiyne (GDY) is recently synthesized two-dimensional carbon allotrope with hexagonal rings cross-linked by diacetylene through introducing butadiyne linkages (−C≡C–C≡C−) to form 18-C hexagons and is emerging to be fundamentally interesting and particularly useful in various research fields. In this study, we for the first time find that GDY can be used as an electrode material with reactivity tunable by electronic states and surface chemistry of GDY. To demonstrate this, GDY is oxidized into graphdiyne oxide (GDYO) that is then chemically and electrochemically reduced into chemically reduced GDYO (cr-GDYO) and electrochemically reduced GDYO (er-GDYO), respectively. Electrode reactivity of GDY and its derivatives (i.e., GDYO, cr-GDYO, and er-GDYO) is studied with hexaammineruthenium chloride ([Ru(NH3)6]Cl3) and potassium ferricyanide (K3Fe(CN)6) as redox probes. We find that electron transfer kinetics of the redox probes employed here at GDYs depends on the density of electronic state (DOS) and the synergetic effects of the surface chemistry as well as the hydrophilicity of the materials, and that the electron transfer kinetics at cr-GDYO and er-GDYO are faster than those at GDY and GDYO, and quite comparable with those at carbon nanotubes and graphene and its derivatives (i.e., GO, cr-GO, and er-GO). These properties, combined with the unique electronic and chemical structures of GDY, essentially enable GDY as a new kind of electrode material for fundamental studies on carbon electrochemistry and various electroanalytical applications.

Zeolitic imidazole frameworks (ZIFs) are an emerging class of functional porous materials with promising biomedical applications such as molecular sensing and intracellular drug delivery. We report herein the first example of using nanoscale ZIFs (i.e., ZIF-90), self-assembled from Zn2+ and imidazole-2-carboxyaldehyde, to target subcellular mitochondria and image dynamics of mitochondrial ATP in live cells. Encapsulation of fluorescent Rhodamine B (RhB) into ZIF-90 suppresses the emission of RhB, while the competitive coordination between ATP and the metal node of ZIF-90 dissembles ZIFs, resulting in the release of RhB for ATP sensing. With this method, we are able to image mitochondrial ATP in live cells and study the ATP level fluctuation in cellular glycolysis and apoptosis processes. The strategy reported here could be further extended to tune nanoscale ZIFs inside live cells for targeted delivery of therapeutics to subcellular organelles for advanced biomedical applications.

Here we report for the first time that ion current rectification (ICR) can be observed at the micrometer scale in symmetric electrolyte solution with polyimidazolium brush (PimB)-modified micropipets, which we call micrometer-scale ion current rectification (MICR). To qualitatively understand MICR, a three-layer model including a charged layer, an electrical double layer, and a bulk layer is proposed, which could also be extended to understanding ICR at the nanoscale. Based on this model, we propose that when charges in the charged layer are comparable with those in the bulk layer, ICR would occur regardless of whether the electrical double layers are overlapped. Finite element simulations based on the solution of Poisson and Nernst–Planck equations and in situ confocal laser scanning microscopy results qualitatively validate the experimental observations and the proposed three-layer model. Moreover, possible factors influencing MICR, including the length of PimB, electrolyte concentration, and the radius of the pipet, are investigated and discussed. This study successfully extends ICR to the micrometer scale and thus opens a new door to the development of ICR-based devices by taking advantage of ease-in-manipulation and designable surface chemistry of micropipets.

Real-time monitoring of respiratory rate (RR) is highly important for human health, clinical diagnosis, and fundamental scientific research. Exhaled humidity-based RR monitoring has recently attracted increased attention because of its accuracy and portability. Here, we report a new design of an exhaled humidity sensor for the real-time monitoring of the RR based on a synthetic redox conducting supramolecular ionic material (SIM). The humidity-dependent conducting SIM is prepared by ionic self-assembly in aqueous solutions of electroactive 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and 1,10-bis(3-methylimidazolium-1-yl) decane (C10(mim)2). By taking full advantage of the high hygroscopicity and water stability arising from the ionic and hydrophobic interactions between two building blocks (i.e., ABTS and C10(mim)2), the SIM-based humidity sensor exhibits both high sensitivity (less than 0.1% relative humidity) and fast response time (∼37 ms). These excellent properties allow this humidity sensor to noninvasively monitor the RRs of not only humans but also rats that have a much faster RR and much smaller tidal volume than humans. Moreover, this sensor could also be efficiently used for the real-time monitoring of the recovery process of rats from anesthesia.

Ion current rectification (ICR), which is the departure of the experimentally measured current-voltage curves from ohmic behavior, has recently drawn intensive interests from both a fundamental point of view and microfluidic-based applications. In this case, the rectification factor (RF), which is defined as the absolute value of the quotient between the currents recorded for one voltage polarity and the currents recorded for the same absolute value of voltage at the opposite polarity, is one of the most important parameters to describe the rectification behavior. Herein, we interestingly observed that RF is strongly dependent on the bias voltage from both the experimental and theoretically simulated results, and this dependence exhibits nonlinear deviation with increasing the rectification degree. At the same surface charge density, the linear deviation increases with decreasing the electrolyte concentration. When the electrolyte concentration remains the same, the linear deviation increases with increasing the surface charge density. The simulated results demonstrate that the electro-osmotic flow is not the dominating factor to this nonlinear phenomenon. This nonlinear deviation is considered to possibly originate from the inhomogeneous conductivity and the nonlinear change of conductance with the bias potential. This study is not only useful to understanding the ICR behavior at a conical glass nanopipette, but also potentially offers a new parameter to describe the rectification degree with this nonlinear relationship.

The development of stable and reproducible methods for in vivo electrochemical monitoring of neurochemicals is of great physiological importance. In this study, we demonstrate ferricyanide-filled cylindrical carbon fiber microelectrodes (CFEs) of high stability and low polarized potential for in vivo electrochemical analysis. We first studied the voltammetric behavior of cylindrical CFEs by using a model system consisting of two separated cells each containing potassium ferricyanide (K3Fe(CN)6) or potassium ferrocyanide (K4Fe(CN)6). We observed that E1/2 values of the system were dependent on the ratio of the lengths of the cylindrical CFEs and of the concentrations of the redox species on both poles. Based on this property, we prepared the ferricyanide-backfilled cylindrical CFEs, and found that this kind of electrode exhibits a more stable current response and a lower polarized potential than the CFEs backfilled with KCl or Ru(NH3)6Cl3. Animal experiments with the ferricyanide-backfilled cylindrical CFEs demonstrate that this kind of electrode could be used for in vivo monitoring of neurochemical release with a high stability under some physiological conditions.

Developing water-stable and adaptive supramolecular materials is of great importance in various research fields. Here, we demonstrate a new kind of water-stable, adaptive, and electroactive supramolecular ionic materials (SIM) that is formed from the aqueous solutions of imidazolium-based dication and dianionic dye (i.e., 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), ABTS) through ionic self-assembly. The formed SIM not only shows good thermostability and unique optical and electrochemical properties that are raised from precursors of the SIM, but also exhibits good water-stability, salt-stability, and adaptive encapsulation properties toward some heterocyclic cationic dye molecules. UV–vis and FT-IR results demonstrate that this encapsulation property is essentially based on the electrostatic interactions between the guest dye molecules and ABTS in the SIM. The application of the SIM prepared here is illustrated by the development of a new electrochemical sensor for NADH sensing at a low potential. This study not only opens a new avenue to the preparation of the supramolecular materials, but also provides a versatile platform for electrochemical (bio)sensing.Keywords: adaptive encapsulation; biosensor; electrochemistry; supramolecular ionic materials; water-stable;

This study demonstrates a rapid visualization assay for on-spot sensing of alcohol content as well as for discriminating methanol-containing beverages with solvent stimuli-responsive supramolecular ionic material (SIM). The SIM is synthesized by ionic self-assembling of imidazolium-based dication C10(mim)2 and dianionic 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) in water and shows water stability, a solvent stimuli-responsive property, and adaptive encapsulation capability. The rationale for the visualization assay demonstrated here is based on the combined utilization of the unique properties of SIM, including its water stability, ethanol stimuli-responsive feature, and adaptive encapsulation capability toward optically active rhodamine 6G (Rh6G); the addition of ethanol into a stable aqueous dispersion of Rh6G-encapsulated SIM (Rh6G-SIM) destructs the Rh6G-SIM structure, resulting in the release of Rh6G from SIM into the solvent. Alcohol content can thus be visualized with the naked eyes through the color change of the dispersion caused by the addition of ethanol. Alcohol content can also be quantified by measuring the fluorescence line of Rh6G released from Rh6G-SIM on a thin-layer chromatography (TLC) plate in response to alcoholic beverages. By fixing the diffusion distance of the mobile phase, the fluorescence line of Rh6G shows a linear relationship with alcohol content (vol %) within a concentration range from 15% to 40%. We utilized this visualization assay for on-spot visualizing of the alcohol contents of three Chinese commercial spirits and discriminating methanol-containing counterfeit beverages. We found that addition of a trace amount of methanol leads to a large increase of the length of Rh6G on TLC plates, which provides a method to identify methanol adulterated beverages with labeled ethanol content. This study provides a simple yet effective assay for alcohol content sensing and methanol differentiation.

We have demonstrated a new strategy to improve the fluorescence detection limit by enhancing the energy transfer efficiency between carbon structures and fluorescent dyes using polyimidazolium-functionalized carbon nanostructures as a low background signal platform. Based on this, a highly sensitive method for thrombin was proposed with a detection limit as low as 2.79 pM without any amplification.

Accurately characterizing the product of photodecomposition of ferrocene derivatives remains a longstanding challenge due to its structural complexity and strong dependence on the solvent and the substituent. Herein, photodecomposition of ferrocenedicarboxylic acid (FcDC) in methanol is found for the first time to form an electroactive infinite coordinate polymer (ICP) with uniform size, good water stability and photostability, and excellent electrochemical activity. The possible mechanism for the ICP formation is proposed based on the fission of the Fe-ring bond and deprotonation of FcDC under light irradiation. The dissociated Fe2+ is first oxidized to Fe3+ that consequently coordinates with the deprotonated ferrocene dicarboxylate to produce ICP nanoparticles. This work not only provides a new insight into the product formation of the photodecomposition of ferrocene derivatives but also offers a mild and simple route to the synthesis of electroactive ICPs.Keywords: bioelectrochemistry; ferrocenedicarboxylic acid; infinite coordination polymer; laccase; photodecomposition;

This study demonstrates a facile yet effective strategy for amperometric assay of electrochemically inactive heparin based on an anion-exchange mechanism with polyimidazolium (Pim) as the synthetic receptor. The rationale for the amperometric heparin assay is essentially based on the different binding affinity of the synthetic Pim receptor toward electrochemically active ferricyanide (Fe(CN)63–) and electrochemically inactive heparin. To accomplish the amperometric assay, Pim is first synthesized and used as the artificial receptor to recognize the anions (i.e., Fe(CN)63– and heparin). The stronger binding affinity of the synthetic Pim receptor toward heparin than toward Fe(CN)63– essentially validates the amperometric heparin assay through an anion-exchange mechanism with the decrease in the redox peak current of Fe(CN)63– adsorbed onto the Pim film as the signal readout. The anion exchange between Fe(CN)63– and heparin on the Pim receptor is verified by cyclic voltammetry and Fourier transform IR and UV–visible spectroscopies. The ratio of the current decrease shows a linear relationship with heparin concentration with a concentration range from 0.5 to 10 μM. With animal experiments by dosing intraperitoneally and collecting the serum sample, the method is demonstrated to be potentially useful for investigating heparin metabolism in the biological system. This study not only provides a simple yet effective route to a heparin assay but also opens a new way to developing amperometric methods for electrochemically inert species by fully utilizing the supramolecular principles.

In this study we demonstrate a new colorimetric method for real-time pyrophosphatase (PPase) activity assay based on reversible tuning of the dispersion/aggregation states of gold nanoparticles (Au-NPs) by controlling the coordination of Cu2+ between cysteine and pyrophosphate ion (PPi) with PPase. The addition of Cu2+ to the cysteine-stabilized Au-NP dispersion results in the aggregation of Au-NPs, while the further addition of PPi to this aggregation turns the aggregated Au-NPs into their dispersed state because of the higher coordination reactivity between Cu2+ and PPi than that between Cu2+ and cysteine. The subsequent addition of PPase to the PPi-triggered dispersed Au-NPs restores the aggregation state of Au-NPs because PPase catalyzes the hydrolysis of PPi into orthophosphate and thus consumes PPi in the reaction system. In this study, we utilize this reversibility of the change between the aggregation/dispersion states of Au-NPs for real-time colorimetric monitoring of PPase activity by continuously measuring the ratio of absorbance at the wavelength of 650 nm (A650) to that at 522 nm (A522) in the time-dependent UV–vis spectra of Au-NP dispersions containing different activities of PPase. To calculate the kinetics of the PPase-catalyzed hydrolysis of PPi, the A650/A522 values are converted into PPi concentrations to obtain the time-dependent changes of PPi concentrations in the dispersions containing different activities of PPase. The initial reaction rates (v0) are thus achieved from the time-dependent logarithm of PPi concentrations with the presence of different PPase activities. Under the experimental conditions employed here, the v0 values are linear with the PPase activity within a range from 0.025 to 0.4 U with a detection limit down to 0.010 U (S/N = 3). Moreover, the colorimetric method developed here is also employed for PPase inhibitor evaluation. This study offers a simple yet effective method for real-time PPase activity assay.

Simple and effective measurement of Mg2+ in the brain of living animals is of great physiological and pathological importance. In this study, we report a facile yet highly selective colorimetric method for effective sensing of cerebral Mg2+. The method is based on rational design of surface chemistry of gold nanoparticles (Au-NPs) with functional molecules including 1,4-dithiothreitol (DTT) and cysteine, enabling the fine tuning of the surface chemistry of Au-NPs in such a way that the addition of Mg2+ into the Au-NPs dispersion could selectively trigger the change of the dispersion/aggregation states of Au-NPs. The strong chelation interaction between Mg2+ and the hydroxyls in 1,4-dithiothreitol and the co-existence of cysteine on the surface of Au-NPs could, on one hand, enable the selective colorimetric detection of Mg2+ and, on the other hand, avoid the aggregation of Au-NPs induced by DTT itself. As a result, the addition of Mg2+ into the dispersion of the Au-NPs containing both cysteine and DTT results in the changes in both the color and the UV-vis spectra of the Au-NPs dispersion. The signal readout shows a linear relationship of Mg2+ within the concentration range from 1 μM to 40 μM with a detection limit of 800 nM (S/N = 3). Moreover, the assay demonstrated here is free from the interference of some physiological species commonly existing in rat brain. Although Ca2+ could interfere with the detection of Mg2+ because of its strong chelation with DTT, it could be selectively masked by masking agent (i.e., ethyleneglcol-bis (2-aminoethylether) tetraacetic acid). By combining the microdialysis technique, the basal dialysate level of Mg2+ is determined to be 299.2 ± 41.1 μM (n = 3) in the cerebral systems. The method essentially offers a new method for the detection of Mg2+ in the cerebral system.

This study effectively demonstrates a strategy to enable the ferricyanide-based second-generation biosensors for selective in vivo measurements of neurochemicals, with glucose as an example. The strategy is based on regulation of redox potential of ferricyanide mediator by carefully controlling the different adsorption ability of ferricyanide (Fe(CN)63-) and ferrocyanide (Fe(CN)64-) onto electrode surface. To realize the negative shift of the redox potential of Fe(CN)63-/4-, imidazolium-based polymer (Pim) is synthesized and used as a matrix for surface adsorption of Fe(CN)63-/4- due to its stronger interaction with Fe(CN)63- than with Fe(CN)64-. The different adsorption ability of Fe(CN)63- and Fe(CN)64- onto electrodes modified with a composite of Pim and multiwalled carbon nanotubes (MWNTs) eventually enables the stable surface adsorption of both species to generate integrated biosensors and, more importantly, leads to a negative shift of the redox potential of the surface-confined redox mediator. Using glucose oxidase (GOD) as the model biorecognition units, we demonstrate the validity of the ferricyanide-based second-generation biosensors for selective in vivo neurochemical measurements. We find that the biosensors developed with the strategy demonstrated in this study can be used well as the selective detector for continuous online detection of striatum glucose of guinea pigs, by integration with in vivo microdialysis. This study essentially paves a new avenue to developing electrochemical biosensors effectively for in vivo neurochemical measurements, which is envisaged to be of great importance in understanding the molecular basis of physiological and pathological events.

Direct selective determination of cysteine in the cerebral system is of great importance because of the crucial roles of cysteine in physiological and pathological processes. In this study, we report a sensitive and selective colorimetric assay for cysteine in the rat brain with gold nanoparticles (Au-NPs) as the signal readout. Initially, Au-NPs synthesized with citrate as the stabilizer are red in color and exhibit absorption at 520 nm. The addition of an aqueous solution (20 μL) of cysteine or aspartic acid alone to a 200 μL Au-NP dispersion causes no aggregation, while the addition of an aqueous solution of cysteine into a Au-NP dispersion containing aspartic acid (1.8 mM) causes the aggregation of Au-NPs and thus results in the color change of the colloid from wine red to blue. These changes are ascribed to the ion pair interaction between aspartic acid and cysteine on the interface between Au-NPs and solution. The concentration of cysteine can be visualized with the naked eye and determined by UV–vis spectroscopy. The signal output shows a linear relationship for cysteine within the concentration range from 0.166 to 1.67 μM with a detection limit of 100 nM. The assay demonstrated here is highly selective and is free from the interference of other natural amino acids and other thiol-containing species as well as the species commonly existing in the brain such as lactate, ascorbic acid, and glucose. The basal dialysate level of cysteine in the microdialysate from the striatum of adult male Sprague–Dawley rats is determined to be around 9.6 ± 2.1 μM. The method demonstrated here is facile but reliable and durable and is envisaged to be applicable to understanding the chemical essence involved in physiological and pathological events associated with cysteine.

This study demonstrates a new electrochemical impedance spectroscopic (EIS) method for measurements of the changes in membrane permeability during the process of cell anoxia. Madin-Darby canine kidney (MDCK) cells were employed as the model cells and were cultured onto gelatin-modified glassy carbon (GC) electrodes. EIS measurements were conducted at the MDCK/gelatin-modified GC electrodes with Fe(CN)63−/4− as the redox probe. The anoxia of the cells grown onto electrode surface was induced by the addition of carbonycyanide p-(trifluoromethoxy) phenylhydrazone (FCCP) into the cell culture, in which the MDCK/gelatin-modified GC electrodes were immersed for different times. The EIS results show that the presence of FCCP in the cell culture clearly decreases the charge-transfer resistance of the Fe(CN)63−/4− redox probe at the MDCK/gelatin-modified GC electrodes, and the charge-transfer resistance decreases with increasing time employed for immersing the MDCK/gelatin-modified GC electrodes into the cell culture containing FCCP. These results demonstrate that the EIS method could be used to monitor the changes in the cell membrane permeability during the FCCP-induced cell anoxia. To simulate the EIS system, a rational equivalent circuit was proposed and the values of ohmic resistance of the electrolyte, charge-transfer resistance and constant phase elements for both the gelatin and the cell layers are given with the fitting error in an acceptable value. This study actually offers a new and simple approach to measuring the dynamic process of cell death induced by anoxia through monitoring the changes in the cell membrane permeability.

We report here a new voltammetric method for the sensitive and selective determination of Hg2+ based on rational covalent functionalization of graphene oxide with cysteamine to form cysteamine-functionalized graphene through nucleophilic ring-opening reaction between the epoxy of graphene oxide and the amino group of cysteamine in KOH solution.