Noiseless Performance of Prussian Blue Based (Bio)sensors through Power Generation

In contrast to “self-powered” (bio)­sensors aiming to generate maximum energy output, we propose the systems with the lowest potential difference between the working and the counter electrodes, which in galvanic mode would provide achievement of the best analytical performance characteristics. Prussian Blue based (bio)­sensors known to operate at 0.00 V versus Ag|AgCl reference, in the short-circuit regime generate the current proportional to analyte concentration. Sensitivity and dynamic range of Prussian Blue based (bio)­sensors in power generation mode are, respectively, even slightly higher and wider compared to the same (bio)­sensors operated in the conventional three-electrode regime powered by a potentiostat. Selectivity of the (bio)­sensors in power generation mode is similarly high relative to both oxygen, allowing H₂O₂ detection by its reduction, and reductants. Among the most important advantages of the proposed power generation mode is an order of magnitude decreased noise compared to performance in a conventional three-electrode setup powered by a potentiostat. Noiseless performances of Prussian Blue based (bio)­sensors would open new horizons for electrochemical analysis

Ultramicrosensors based on transition metal hexacyanoferrates for scanning electrochemical microscopy

We report here a way for improving the stability of ultramicroelectrodes (UME) based on hexacyanoferrate-modified metals for the detection of hydrogen peroxide. The most stable sensors were obtained by electrochemical deposition of six layers of hexacyanoferrates (HCF), more specifically, an alternating pattern of three layers of Prussian Blue and three layers of Ni–HCF. The microelectrodes modified with mixed layers were continuously monitored in 1 mM hydrogen peroxide and proved to be stable for more than 5 h under these conditions. The mixed layer microelectrodes exhibited a stability which is five times as high as the stability of conventional Prussian Blue-modified UMEs. The sensitivity of the mixed layer sensor was 0.32 A·M−1·cm−2, and the detection limit was 10 µM. The mixed layer-based UMEs were used as sensors in scanning electrochemical microscopy (SECM) experiments for imaging of hydrogen peroxide evolution

Evidence for polarization-induced phase transformations and degradation in CH3_3NH3_3PbI3_3

In solar cells, hybrid halide perovskites operate under constant bias, thus their stability towards electric field-induced degradation is of key importance. Here we report on evidence of previously unidentified electric field-induced transitions and degradation path of CH$_3$NH$_3$PbI$_3$ (MAPbI$_3$) using elemental and phase mapping. Thin films of MAPbI$_3$ were deposited onto 1–2 µm-pitch interdigitated electrodes and subjected to direct current (DC)-polarization. The MAPbI$_3$ layer polarized with < 0.8 V/µm DC electric field undergoes pronounced ion redistribution to methylammonium-rich MAPbI$_{3−y}$ (y < 0.6) and iodine-rich MA$_{1−x}$PbI$_3$ (x < 0.3) regions. Polarization-induced loss of both methylammonium and iodine provokes degradation of MAPbI$_3$. Using nanofocus grazing-incidence wide-angle X-ray scattering (GIWAXS), we unambiguously showed that the bias voltage induces the transformation of β-MAPbI$_3$ to metastable δ-MAPbI$_3$ polymorph via alignment of polar organic cation with the electric field. This transformation is partially reversible upon field removal. However, once formed, δ-MAPbI$_3$ disrupts the morphology of pristine film and undergoes decomposition to β-MAPbI$_3$ (β-MAPI) and PbI$_2$. With the aforementioned compositional and phase changes, only MA-rich part serves as the charge separation layer, while the I-rich excitation is blocked with the PbI$_2$ barrier serving as holes trapping layer. These observations reveal the intermediate steps in electric-field-driven degradation of halide perovskites and show the role of polar cations in the process, which is instructive for further material design with higher stability metrics

Reagentless Polyol Detection by Conductivity Increase in the Course of Self-Doping of Boronate-Substituted Polyaniline

We report on the novel reagentless and label-free detection principle based on electroactive (conducting) polymers considering sensors for polyols, particularly, saccharides and hydroxy acids. Unlike the majority of impedimetric and conductometric (bio)­sensors, which specific and unspecific signals are directed in the same way (resistance increase), making doubtful their real applications, the response of the reported system results in resistance decrease, which is directed oppositely to the background. The mechanism of the resistance decrease is the polyaniline self-doping, i.e., as an alternative to proton doping, an appearance of the negatively charged aromatic ring substituents in polymer chain. Negative charge “freezing” at the boron atom is indeed a result of complex formation with di- and polyols, specific binding. Changes in Raman spectra of boronate-substituted polyaniline after addition of glucose are similar to those caused by proton doping of the polymer. Thermodynamic data on interaction of the electropolymerized 3-aminophenylboronic acid with saccharides and hydroxy acids also confirm that the observed resistance decrease is due to polymer interaction with polyols. The first reported conductivity increase as a specific signal opens new horizons for reagentless affinity sensors, allowing the discrimination of specific affinity bindings from nonspecific interactions