While ion to electron transducing layers for the fabrication of potentiometric membrane electrodes for the detection of cations have been well established, similar progress for the sensing of anions has not yet been realized. We report for this reason on a novel approach for the development of all-solid-state anion selective electrodes using lipophilic multiwalled carbon nanotubes (f-MWCNTs) as the inner ion to electron transducing layer. This material can be solvent cast, as it conveniently dissolves in tetrahydrofuran (THF), an important advantage to develop uniform films without the need for using surfactants that might deteriorate the performance of the electrode. Solid contact sensors for carbonate, nitrate, nitrite, and dihydrogen phosphate are fabricated and characterized, and all exhibit comparable analytical characteristics to the inner liquid electrodes. For example, the carbonate sensor exhibits a Nernstian slope of 27.2 ± 0.8 mV·dec(-1), a LOD = 2.3 μM, a response time of 1 s, a linear range of four logarithmic units, and a medium-term stability of 0.04 mV·h(-1) is obtained in a pH 8.6 buffered solution. Water layer test, reversibility, and selectivity for chloride, nitrate, and hydroxide are also reported. The excellent properties of f-MWCNTs as a transducer are contrasted to the deficient performance of poly(3-octyl-thiophene) (POT) for carbonate detection. This is evidenced both with a significant drift in the potentiometric measures as well as a pronounced sensitivity to light (either sunlight or artificial light). This latter aspect may compromise its potential for environmental in situ measurements (night/day cycles). The concentration of carbonate is determined in a river sample (Arve river, Geneva) and compared to a reference method (automatic titrator with potentiometric pH detection). The results suggest that nanostructured materials such as f-MWCNTs are an attractive platform as a general ion-to-electron transducer for anion-selective electrodes.
Rapid and sensitive detection of tumor biomarkers plays a critical role in the early diagnosis of cancer and better understanding of disease progression. We report here a novel wireless electrochemiluminescence (ECL) strategy for visualizing prostate-specific antigen (PSA) on the basis of electrical switch control of ECL generation on bipolar electrodes (BPEs). The visual device comprises a two-channel microfluidic chip with two indium tin oxide (ITO) bands with a gap of 200 mm. The gap between the two ITO bands in one channel could be regarded as an electrical switch, the conductivity of which controls ECL generation on the ITO bands in the other channel, which are used as bipolar electrodes (BPEs). The electronic conductivity of the electrical switch could be tuned by PSA guided silver particles deposition via an immunosandwich assembly and a silver enhancement strategy. At the "on" state of the electrical switch, PSA induced deposition of silver particles forms an electronic circuit between the adjacent BPEs and makes them behave like a continuous H-shaped BPE, which results in only one ECL signal.Meanwhile, the external voltage for driving the oxidation reactions of Ru(bpy) 3 2+ and TPA is significantly reduced compared with the "off" state. This important characterization of the electrical switch could eliminate the background signal and enable a sensitive measurement of PSA by observing the ECL lightspots on BPEs, providing a simple and sensitive visual means of detecting cancer biomarkers. Besides, this two-channel design avoids the chemical interference between sensing and reporting reactions. Combining the advantages of BPE and the high visual sensitivity of the electrical switch, it could be easily expected to achieve sensitive screening of other biomarkers.
Here we developed a novel hybrid bipolar electrode (BPE)-electrochemiluminescence (ECL) biosensor based on hybrid bipolar electrode (BPE) for the measurement of cancer cell surface protein using ferrocence (Fc) labeled aptamer as signal recognition and amplification probe. According to the electric neutrality of BPE, the cathode of U-shaped ITO BPE was electrochemically deposited by Au nanoparticles (NPs) to enhance its conductivity and surface area, decrease the overpotential of O2 reduction, which would correspondingly increase the oxidation current of Ru(bpy)3(2+)/tripropylamine (TPA) on the anode of BPE and resulting a ∼4-fold enhancement of ECL intensity. Then a signal amplification strategy was designed by introducing Fc modified aptamer on the anode surface of BPE through hybridization for detecting the amount of mucin-1 on MCF-7 cells. The presence of Fc could not only inhibit the oxidation of Ru(bpy)3(2+) because of its lower oxidation potential, its oxidation product Fc(+) could also quench the ECL of Ru(bpy)3(2+)/TPA by efficient energy-transfer from the excited-state Ru(bpy)3(2+)* to Fc(+), making the ECL intensity greatly quenched. On the basis of the cathodic Au NPs induced ECL enhancing coupled with anodic Fc induced signal quenching amplification, the approach allowed detection of mucin-1 aptamer at a concentration down to 0.5 fM and was capable of detecting a minimum of 20 MCF-7 cells. Besides, the amount of mucin-1 on MCF-7 cells was calculated to be 9041 ± 388 molecules/cell. This approach therefore shows great promise in bioanalysis.
Voltammetric thin layer (∼200 nm) ionophore-based polymeric films of defined ion-exchange capacity have recently emerged as a promising approach to acquire multi-ion information about the sample, in analogy to performing multiple potentiometric measurements with individual membranes. They behave under two different regimes that are dependent on the ion concentration. A thin layer control (no mass transport limitation of the polymer film or solution) is identified for ion concentrations of >10 μM, in which case the peak potential serves as the readout signal, in analogy to a potentiometric sensor. On the other hand, ion transfer at lower concentrations is chiefly controlled by diffusional mass transport from the solution to the sensing film, resulting in an increase of peak current with ion concentration. This concentration range is suitable for electrochemical ion transfer stripping analysis. Here, the transition between the two mentioned scenarios is explored experimentally, using a highly silver-selective membrane as a proof-of-concept under different conditions (variation of ion concentration in the sample from 0.1 μM to 1 mM, scan rate from 25 mV s to 200 mV s, and angular frequency from 100 rpm to 6400 rpm). Apart from experimental evidence, a numerical simulation is developed that considers an idealized conducting polymer behavior and permits one to predict experimental behavior under diffusion or thin-layer control.
The work dramatically improves the lower detection limit of anion selective membranes at environmental pH by using local acidification to suppress hydroxide interference at the membrane surface. Three separate localized acidification strategies are explored to achieve this, with ionophore-based membrane electrodes selective for nitrite and dihydrogen phosphate as guiding examples. In a first approach, a concentrated acetic acid solution (ca. 1 M) is placed in the inner filling solution of the PVC-based membrane electrode, forcing a significant acid gradient across the membrane. A second strategy achieves the same type of passive acidification by using an external proton source (fast diffusive doped polypropylene membrane) placed in front of a potentiometric solid contact anion selective electrode where the thin layer gap allows one to observe spontaneous acidification at the opposing detection electrode. The third approach shares the same configuration, but protons are here released by electrochemical control from the selective proton source into the thin layer sample. All three protocols improve the limit of detection by more than 2 orders of magnitude at environmental pH. Nitrite and dihydrogen phosphate determinations in artificial and natural samples are demonstrated.
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