Quartz Crystal Microbalance Research Articles

Study on the interactions and interfacial behaviors of cellulose nanocrystal/β-lactoglobulin complexes for pickering emulsions

Pickering emulsions stabilized by polysaccharides and protein complexes have attracted increasing attention. Exploring their interfacial activities and adsorption behaviors at the oil–water interface is of great significance for deepening our understanding of the emulsification mechanism and improving the properties of polysaccharide/protein-stabilized emulsions. Here, cellulose nanocrystals (CNCs) with rod-like structures and spherical β-lactoglobulin (β-LG) proteins were selected as representative stabilizers. We initially investigated the interaction between the 2 at pH 2.0–7.0 through a multiscale approach. Then, oil–water interfacial tension tests were used to assess the interfacial activities of CNCs and β-LG, revealing that β-LG exerted primary influence over the interfacial behavior in their complexes. Meanwhile, quartz crystal microbalance with dissipation was used to monitor the interfacial adsorption behaviors of CNCs, β-LG, and CNC/β-LG complexes, demonstrating that the interfacial layer formed by the CNC/β-LG complexes on the oil film (∼696.7 ng/cm2) was thicker and more stable compared with that formed by individual CNCs (∼48.4 ng/cm2) or β-LG (∼87.1 ng/cm2). Finally, we applied the CNC/β-LG complexes as stabilizers in Pickering emulsions and the emulsions prepared with the complexes at concentrations above 0.8 wt% exhibited storage stability for up to 28 d which confirmed their exceptional emulsifying ability and practical value. This study provides profound insights into the interfacial behavior of polysaccharide–protein complexes in Pickering emulsions and is expected to facilitate the design of novel food-grade emulsifiers.

Sensing performance of Schiff base-calix[4]arene including amide units for detection of pharmaceutically active compounds on QCM sensor system

This study aimed to investigate the sensing performance of a Quartz Crystal Microbalance (QCM) sensor coated with a calix[4]arene derivative (calixarene phenol amide, CPhA), containing both Schiff base and amide groups against various pharmaceutically active compounds (PhACs) such as ibuprofen, diphenhydramine, naproxen, and ascorbic acid in aqueous media. CPhA was successfully coated on the QCM crystal surface, and its surface properties were studied using Atomic Force Microscopy (AFM) and contact angle measurements. Sensing studies showed that the CPhA-coated QCM sensor detected more PhACs than the compound 4 (unfunctionalized derivative)-coated sensor. The highest frequency variation and binding ratio values were achieved with the QCM sensor with a relatively low coating amount. The CPhA-coated QCM sensor achieved higher adsorption capacity and was significantly more sensitive to all PhACs than the compound 4-coated one. The fabricated sensor showed remarkable reproducibility for all PhACs except naproxen (NAP). The fabricated sensor exhibited a remarkable sensing performance for detecting PhACs, which is promising for developing novel calix[4]arene-coated QCM sensors for PhACs.

Photolytic Mass Loss of Humic Substances Measured with a Quartz Crystal Microbalance.

Laboratory studies have shown that photolytic mass loss can be a significant sink for secondary organic aerosol (SOA). Here, we use a quartz crystal microbalance to measure mass loss of Suwannee River Humic Acid (SRHA) and Suwannee River Fulvic Acid (SRFA), surrogates for SOA, exposed to 254, 300, and 405 nm radiation over the course of 24 h. We find that the photolytic mass loss rates of these materials are comparable to those for laboratory-generated limonene and toluene SOA material from the study of Baboomian et al, ACS Earth Space Chem. 2020, 4, 1078. Scaling our results to ambient conditions, we estimate that humic substances in aerosols can lose as much as 8% by mass in the first day of exposure in the atmosphere, equivalent to 0.025% of J NO2 , the photolysis rate of nitrogen dioxide. By using zero air instead of nitrogen, we also find that the presence of oxygen accelerates the photolytic mass loss rate by a factor of 2 to 4 at all wavelengths suggesting a potential role for reactive oxygen species. UV photolysis of an aqueous SRFA solution demonstrated both photobleaching at UV wavelengths and photoenhancement at visible wavelengths. Ultrahigh-resolution mass spectrometric analysis showed that condensed-phase SRFA photolysis led to decreased intensity in the 100-300 m/z range while aqueous SRFA photolysis resulted in an increase in intensity in the same range. This work reaffirms that photolytic mass loss is a potentially significant sink for SOA, but only on the time scale of a day or two and demonstrates that SRHA and SRFA are suitable surrogates for atmospheric SOA with respect to photolytic mass loss.

Electrochemical and surface characterisation of poly(3,4-ethylenedioxythiophene) dodecylbenzenesulfonate layers

Poly(3,4-ethylenedioxythiophene) (PEDOT) films were electrochemically synthesised with sodium dodecylbenzenesulfonate (DBS) and chloride acting as dopant anions within the polymer matrix. Upon redox switching of the PEDOT/DBS film between conducting and non-conducting states, the DBS anion remained within the polymer and cation insertion and expulsion occurred, as confirmed by Electrochemical Quartz Crystal Microbalance (EQCM) measurements. Electrolytes composed of alkali metal cations of varying masses (Li+, Na+, K+) were employed to investigate the cation insertion/expulsion processes, thereby resulting in varying mass changes being observed upon film redox switching. The charging and discharging of bulky anion doped polymer films presented higher capacitance upon charging and lower capacitance when discharging, which is expected during doping and de-doping as confirmed by AC impedance. In this work, the main results obtained by chemical-physical characterisation are presented and critically discussed, with regard to the possible use of a viable conducting polymer as a drug delivery vehicle.

Revealing Ion Adsorption and Charging Mechanisms in Layered Metal-Organic Framework Supercapacitors with Solid-State Nuclear Magnetic Resonance.

Conductive layered metal-organic frameworks (MOFs) have demonstrated promising electrochemical performances as supercapacitor electrode materials. The well-defined chemical structures of these crystalline porous electrodes facilitate structure-performance studies; however, there is a fundamental lack in the molecular-level understanding of charge storage mechanisms in conductive layered MOFs. To address this, we employ solid-state nuclear magnetic resonance (NMR) spectroscopy to study ion adsorption in nickel 2,3,6,7,10,11-hexaiminotriphenylene, Ni3(HITP)2. In this system, we find that separate resonances can be observed for the MOF's in-pore and ex-pore ions. The chemical shift of in-pore electrolyte is found to be dominated by specific chemical interactions with the MOF functional groups, with this result supported by quantum mechanics/molecular mechanics (QM/MM) and density functional theory (DFT) calculations. Quantification of the electrolyte environments by NMR was also found to provide a proxy for electrochemical performance, which could facilitate the rapid screening of synthesized MOF samples. Finally, the charge storage mechanism was explored using a combination of ex-situ NMR and operando electrochemical quartz crystal microbalance (EQCM) experiments. These measurements revealed that cations are the dominant contributors to charge storage in Ni3(HITP)2, with anions contributing only a minor contribution to the charge storage. Overall, this work establishes the methods for studying MOF-electrolyte interactions via NMR spectroscopy. Understanding how these interactions influence the charging storage mechanism will aid the design of MOF-electrolyte combinations to optimize the performance of supercapacitors, as well as other electrochemical devices including electrocatalysts and sensors.

Mechanistic Aspects of Selenate Reduction on CU(UPD) on AU Electrodes

Recent findings in our laboratories have shown that copper1and cadmium2 underpotential deposited on polycrystalline Au electrodes, can mediate the reduction of in 0.1 M HClO4 aqueous solutions, to yield irreversibly adsorbed elemental Se allowing its detection down to the nM range. Current efforts are focused on gaining a better understanding of the the factors that govern this unique electrocatalytic phenomenon by employing a combination of electrochemical, microgravimetric, and optical techniques, using Cu as the UPD species. Correlations have been sought between the rates of this process and the coverage of Cu(UPD), the concentration of and, to a more limited extent, the applied potential in 0.1 M HClO4 solutions. Experimental conditions were selected to unveil the functional dependence of the kinetics on each of the aforementioned factors, while keeping the others constant, in solutions both quiescent and in the presence of convective flow. A number of interesting observations were made based on the data collected. In particular, shown in Panel C, Fig. 1, are plots of the charge associated with the oxidation of adsorbed Cu and Se, QCu and QSe, respectively (see Panel C), obtained from the area under the peaks shaded in light blue and yellow in Panels A and B, Fig, 1, respectively, vs (see below). These data were recorded during linear potential scans following polarization of the electrode at Ehold = 0.325 V for various times, thold (see Panels A-C, Fig. 1), where the linear character of the data in solutions is consistent with Cu(UPD) proceeding under strict diffusion control. As indicated by the QSe data, the electrode displayed electrocatalytic activity for reduction only for QCu > ca. 80 μC cm2-. Additional insight was obtained from measurements of the electrode weight as a function of using a quartz crystal microbalance (see Panel D, Fig.1), which yielded evidence for adsorption only for QCu > ca. 80 μC cm2-, pointing to the formation of a surface-bound adduct, as a necessary step for reduction to ensue. On this basis, we propose that for QCu > ca. 80 μC cm2 the mechanism for reduction involves, as a first step, the reversible formation of the adduct, Eq. (1) followed by its irreversible reduction, generating adsorbed elemental Se, Cu|Se(ads), Eq. (2).Atomically resolved images of Cu(UPD) on Au(111) obtained with a scanning tunneling microscope for the closely related sulfate ion, strongly suggest that the active site involved adsorption on the empty Au sites of the so-called √3´x√3 superstructure (see Insert in Panel A, Fig. 1). A similar conclusion was drawn based on measurements performed with rotating Cu ring-polycrystalline Au disk electrodes, RRDE, at constant Cu(UPD) coverage and applied potential for various solution compositions. Quantitative analyses of all the data, which included numerical simulations, yielded kf/kr, and kET values in the range (2.4 – 3.23) ´ 106 cm3 mol-1 and (2.5 – 9) ´ 10-3 s-1, respectively. Acknowledgments Support for this work was provided by NSF CHEM 1808592

(Invited) Molecularly Imprinted Polymer “Antibodies” to Recognize Biological Species and Bioactive Molecules

Diagnostics, healthcare, and lifestyle are key drivers of sensors and assays that allow for measuring at home or at the point of care. Lateral-flow assays relying on antibodies are probably the most widely known examples for this. Despite their undisputed qualities, there is a strong drive to replace antibodies by artificial mimics that come at lower cost and are more stable. Molecularly imprinted polymers (MIPs) tick those boxes: they result from template-directed polymerization and, thus, from straightforward synthesis, and have appreciable binding properties [1]. Traditional imprinting approaches, however, suffer from limited reproducibility [2]. Solid-phase synthesis of MIP nanoparticles followed by enriching high-affinity fractions through sequential elution from the synthesis column can overcome such limitations [3].The approach has turned out very versatile for addressing analyte species almost any size. Originally, it has been focusing on proteins, probably because it follows the logic of interactions between antibodies and antigens. There are two different strategies for achieving that goal: using the entire protein molecule as the template for imprinting, or only a part of it. The former strategy, for instance, allows for establishing both direct and competitive sensing assays for human serum albumin (HSA) and insulin, respectively. Both aim at mass-sensitive sensing with quartz crystal microbalances (QCMs) as the transducers. Hence it is interesting to replace and/or complement the organic polymers with silica or titania particles to increase sensitivity: the number of available binding sites on MIP surface is limited by film and sensor dimensions. Using heavier particles hence increases the frequency shift per binding event. In the same manner, it turned out possible to access cardiac biomarkers, such as Gallectin3 (Gal3).Considering that protein molecules are large and structurally flexible makes it interesting to use only epitopes on their surfaces for imprinting. This leads to (surprisingly) high selectivity: In the case of a Dengue virus structural protein, imprinting with an epitope comprising 11 amino acids indeed leads to binding between the MIP nanoparticles and the respective protein. Changing only one amino acid reduces the sensor responses by a factor of more than two orders of magnitude thus revealing excellent selectivity bordering to specificity. Following that logic further, it is even possible to synthesize MIP nanobodies for small molecules, such as salbutamol. In such cases one usually applies a linker molecule to ensure synthesis. The resulting particles again highly selectively bind their target compound and are also useful for competitive assay formats.Going the other direction in terms of size, it is possible to use entire bacteria, such as S. epidermidis, as the template [4]. In contrast to the previous MIP nanobody approaches, this yields a variety of different nanoparticles that bind to different positions on the bacteria surface. The nanoparticle ensemble selectively interacts with its target species, even though the exact binding sites are unknown. Thus, when using them to incubate a different bacterium, such as E. coli, this reduces the fluorescence intensity by more than 80%. In other words: this means one can synthesize artificial antibodies for species without knowing the exact chemical details.[1] Bui BTS, Haupt K, Anal Bioanal Chem 398 (2010) 2481-92, doi: 10.1007/s00216-010-4158-x[2] Unger C, Lieberzeit PA, Reactive Funct Polym 161 (2021) 104855, doi: 10.1016/j.reactfunctpolym.2021.104855[3] a) Canfarotta F, Poma A, Guerreiro A, Piletsky S (2016) Solid-phase synthesis of molecularly imprinted nanoparticles. Nat Protoc 11:443–455. https://doi.org/10.1038/nprot.2016.030; b) Xu J, et al., Biomacromolecules 17 (2016) 345-353, doi: 1021/acs.biomac.5b01454 [4] Hlaoperm C et al. Sensors 23 (2023) 3526, doi: 10.3390/s23073526

(Invited) Scanning Gel Electrochemical Microscopy: Towards Biological Applications?

Scanning electrochemical probe techniques have been developed in the last 30 years for measuring spatially localized electrochemistry at micro and nano scale. Here, we will present a recently developed tool, namely Scanning Gel Electrochemical Microscopy, for imaging the electrochemical reactivity of surfaces and patterning surfaces by local electrodeposition. The concept is based on a gel probe that is in soft contact with the sample, allowing electrochemical measurements to be spatially localized in the contact area with gel as electrolyte.1 The physical resolution, or the pixel size, can thus be tuned by pressing or pulling the probe after touching the sample.2 So far, two types of gel probes have been developed: Type I by electrodeposition of chitosan on micro-disk electrodes,1-3 and Type II by “electrodeposition + pulling” on sharpened metal wires.4 Local chronoamperometry, potentiometry and cyclic voltammetry have been carried out, either for imaging or for patterning (complex-shaped) surfaces.1-4 The gel has an advantage of localizing and immobilizing the electrolyte, which “frees” the sample from solution in scanning electrochemical microscopy (SECM) and reduces the wetting in scanning electrochemical cell microscopy (SECCM) as supported by quartz crystal microbalance (QCM) measurements in our recent work.5 Moreover, the excellent biocompatibility of the chitosan-based gel (which can easily immobilize drugs, proteins or bacteria) and the soft contact make SGECM a promising technique for biological applications. In this presentation, we will discuss about the feasibility of measuring different possible responses when a gel probe is in contact with a bio-entity (bacteria, bio-film, etc.) in-situ in its culture media: (1) Transport of redox species through the bio-entity, either from the gel to the culture media or the opposite way; (2) Production of redox mediators by the bio-entity that would be captured by the gel probe; (3) Physical and chemical response of the bio-entity induced by the gel probe. These possibilities will be examined by electrochemical modelling. The recent development on polyvinylalcohol-based gel probes will also be introduced. The work is partially supported by ANR-NRF project MEACT (ANR-20-CE09-0028).

Development of an Impedance and QCM-Based Electrochemical Biosensor for the Detection of Colon Cancer-Secreted Protein-2 (CCSP-2), a Potential Colorectal Cancer Biomarker

Research on colon cancer diagnosis is advancing due to the application of electrochemical biosensors. Early screening can significantly reduce the overall mortality rate from colorectal cancer (CRC). Colon cancer-secreted protein-2 (CCSP-2) is specifically overexpressed and secreted by cancerous cells and adenomas found in the colon. Utilizing a cost-effective and non-invasive electrochemical immunosensor for CCSP-2 detection can aid in the early diagnosis of disease recurrences in individuals with colon cancer. In this work, we designed a simplified electrochemical immunosensor consisting of a functionalized gold (Au) electrode using cysteine-modified recombinant protein G (RPGCys) to immobilize the CCSP-2 antibody. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) have been used to analyze the electrochemical response caused by the binding between the target CCSP-2 and anti-CCSP-2, and there are significant changes observed for charge transfer resistance (Rct) in EIS and current density in CV. The quantification of mass changes attributed to binding events within the sensor will be determined through the application of quartz crystal microbalance (QCM). Angle-resolved X-ray photoelectron spectroscopy (ARXPS) and atomic force microscopy (AFM) will be used for the characterization of the sensor surface. The sensor's sensitivity and specificity will be assessed through the determination of the limit of detection and comprehensive testing of the probe with varied proteins and cell lines sourced from both cancerous and non-cancerous patient samples. This electrochemical probe is expected to enable the rapid and direct detection of early-stage colorectal cancer.

Poprhyrinoids Based Sensors for Volatilomics

Metabolomics can be defined as the systematic study of the unique chemical fingerprints that specific cellular and physiological processes leave behind. For the scope, it encompasses a set of analytical methods to detect low molecular weight metabolites in biological samples that are linked to specific health or disease conditions. The collections of compounds form the so-called metabolic profiles usually containing hundreds of different compounds. Ultimately, metabolomics studies are aimed at finding the alteration of the metabolic profile due to specific conditions or diverse pathologies from cancer, to infective diseases.The volatilomics is the case of metabolomics concerned with the small metabolites that are volatile or semi-volatile. The human volatilome can be studied in different human samples such as breath, skin, urines, saliva, and feces.The study of volatile compounds is an important topic of analytical chemistry. Thus, several analytical instruments and methods are available for the investigation of the volatilome5. In general, the scope of the analysis is the detection of the quality and the quantity of the molecules contained in a sample. To achieve this scope these instruments are made by a step of separation and a detector.Analytical machines are generally bulky, costly, and they require specialized staff to be operated and to interpret the data. For these reasons, volatilomics is greatly fascinated by the possibility to replace the analytical machines with sensors. Sensors are devices small enough to be directly connected to electronic platforms.In sensors the separation stage of the analytical instruments is replaced by the affinity of the sensor respect to the molecules composing the sample. however, differently form mass spectrometers. sensors always produce a signal that is related to the totality of the molecules at which it is exposed. Thus, sensors may be selective, when the affinity towards one molecular species overcome the affinity towards all the others, or non-selective when the affinities are comparable. Thus, in principle to replace a GC, an ensemble of sensors each selective to a different species is necessary.The development of sensors for volatilomics found an unexpected inspiration from nature, and more specifically, from olfaction. One of the most striking characteristics of olfaction is the capability to sense many different odors with a limited set of different receptors. It has been surmised that human, with only 388 different receptors, can detect more than one trillion of odor stimuli. The principle of odor identification in olfaction is ruled by the so-called combinatorial selectivity. since a single compound may be sensed by more receptors and more receptors may sense the same compound, the identification of an odor is accomplished by the pattern of responses of the whole set of receptors.Porphyrinoids have been found to be an exceptional material for electronic noses because theie molecular structure made of modular blocks such as the central metal atom, the aromatic ring, and the peripheral motifs can be modified giving rise to a large variety of sensitivity patterns.Porphyrins have been extensivelly used in volatilomics, in the first implementation, as the functional layer of quartz microbalances. In 2003 an array made of porphyrins and corroles demonstrated that lung cancer could be diagnosed from the analysis of breath. This first evidence was followed by other applications aimed at diagnosis tuberculosisfrom breath, lung cancerand kidney cancerfrom urines and COVID-19 from serum. Additionally, these sensors were used to study malariaand stem cellsproliferation in murine models.Recently, quartz microbalance sensors were challenged by the possibility to develop porphyrins-based impedance sensors. The introduction of these sensors is expected to reduce the costs of production and to result into more simple and widely usable devices. For instance, a capacitive sensor array made of porphyrinoids coated silica particles was used to detect COVID-19 from the VOCs released by serum, and Porphyrins-doped PEDOT was implemented in facemasksand used to diagnose chronic kidney diseases.In the presentation, the rationale of these developments will be presented, and somne noteworthy case studies will be discussed.

Cyclic Electrogravimetry Reveals Details of Copper Electrodeposition in the Presence of Additives

Copper electroplating was studied based on cyclic voltammetry combined with a new type of acoustic microgravimetry. The instrument operates similarly to a quartz crystal microbalance with dissipation monitoring (EQCM-D), meaning that it reports shifts in resonance frequency, Δf, and half bandwidth, ΔΓ, on several overtones. Compared to the existing instruments, the time resolution was improved by a factor of 10 down to 10 ms using multifrequency lockin amplification. The frequency noise after accumulation was reduced below 10 mHz ≙ 0.12 ng/cm2 ≙ 0.14 pm (assuming ρ = 8.96 g/cm3) by modulating the electrode potential and averaging of Δf and ΔΓ. Mostly, Δf and ΔΓ are in line with the Sauerbrey prediction (Δf/n ≈const. and ΔΓ/n < -Δf/nwith n the overtone order). Small unsystematic deviations between overtones due to compressional wave effects are seen but no evidence for strong roughness or viscoelasticity. Further, the Faraday efficiency is almost unity, meaning that Δf is close to the expectations derived from the electrical current. The improved sensitivity enables to determine the time derivative of the frequency shift, which is equal to an apparent mass transfer rate that can be compared to the current directly. Again, small deviations are seen for bulk deposition and stripping. These go back to weak roughness effects most pronounced in the initial phase of bulk deposition and, also, to the presence of a Cu(I) intermediate during dissolution. When adding benzotriazole (BTA) or thiourea (TU) as plating additives, the gravimetric and the voltammetric response changes. For BTA, the deviations between mass transfer rate and current vanish indicative of decreased roughness. For TU additional features appear, related to a potential-dependent adsorption of TU and to increased underpotential deposition (UPD). In the presence of TU, the electrode-electrolyte interface displays viscoelasticity, evident by the bandwidth increases during deposition.[1] Caption: Currents, time derivatives of the frequency shift, and bandwidth shift obtained during copper electrodeposition without additives (A), in the presence of 10 µM benzotriazole (B), and in the presence of 10 µM thiourea (C). Reprinted with permission from Reference [1].

Stable Cycling of Ultrahigh Mass Loaded MnO2 for Low-Cost Sodium-Ion Batteries

Achieving cost-effective and sustainable solutions for large-scale energy storage is critical for advancing the global clean energy transition. The intermittent nature of green energy underscores the inadequacy of current large scale energy storage technologies like pumped hydroelectricity which is susceptible to geographic and water resource constraints, making batteries a prime candidate for this role. Although lithium-ion batteries (LIBs) with high energy and power density are a viable solution, the high cost and uneven distribution of lithium reserves limits the widespread application of LIBs for grid energy storage. Considering these developments and the challenges posed by limited lithium reserves, sustainable and low-cost sodium-ion batteries (SIBs) emerge as a promising alternative.Among the various battery electrode materials, manganese dioxide (MnO2) stands out as a promising choice for large-scale energy storage applications due to its earth abundance, cost-effectiveness and non-toxic nature. Although MnO2 is known as a pseudocapacitive material with superior cycling stability in aqueous electrolytes, its dissolution in non-aqueous electrolytes has restricted its widespread use in long lifetime batteries. In this study, we demonstrate that an ether-based electrolyte solution (1M NaClO4 in diglyme) is able to achieve stable cycling of electrodeposited ε-MnO2 as a cathode for SIBs, reaching a over 75% capacity retention over 1000 cycles. This response surpasses conventional ester-based electrolytes (58% of 1M NaClO4 in EC/PC). A comparative cycling stability study, coupled with electrochemical quartz crystal microbalance characterization, validates the efficacy of the ether-based electrolyte in mitigating MnO2 dissolution. Furthermore, the integration of 3D printed graphene aerogel (3D GA) as a scaffold for electrodeposited MnO2 leads to enhanced electrochemical performance of the MnO2 electrode (3D MnO2/GA), resulting in a high areal capacity of 4.4 mAh cm-2 at a current density of 10 mA cm-2. The 3D MnO2/GA electrode possesses scalable electrochemical performance with mass loadings from 20 mg cm-2 to 80 mg cm-2. Using this electrode, a high mass loaded sodium-ion battery (SIB) device was fabricated by using utilizing 58 mg cm-2 3D MnO2/GA as the cathode and 16 mg cm-2 dip-coated TiO2@Cu foam as the anode. This MnO2 | TiO2 device delivered a notable areal capacity of 6.2 mWh cm-2 and a high areal power density of 70.7 mW cm-2. Moreover, the SIB device demonstrates 85% capacity retention after 500 cycles, underscoring the stable cycling of this high-energy and power-density SIB device.

Square Wave Electrogravimetry Monitors Transient Submonolayer Adsorption of Redox-Active Molecules with a Precision of Less Than 1% of a Monolayer

Square wave voltammetry on electrolytes containing the reversible redox pairs flavin adenine dinucleotide (FAD) and methyl viologen (MV) was complemented by a new form of acoustic microgravimetry. The analytes FAD and MV have promising applications in bacterial fuel cells and redox flow batteries. The instrument operates similarly to a quartz crystal microbalance with dissipation monitoring (EQCM-D). It reports shifts in resonance frequency and half bandwidth on several overtones. Compared to the existing instruments, the time resolution was improved by a factor of 40 down to 2.5 ms by multifrequency lockin amplification, which is important for the analysis of the transients in electrochemistry. The frequency noise after accumulation was reduced below 10 mHz ≙ 0.12 ng/cm2 ≙ 1.2 pm (assuming ρ = 1 g/cm3) by employing modulation and averaging. Square wave modulation of the electrode potential is superior to linear ramps (cyclic voltammetry) because it reduces the contribution of non-Faraday currents and, also, improves sensitivity. The shifts of frequency,Δf, and half bandwidth, ΔΓ, are in line with the Sauerbrey prediction (Δf/n ≈const. and ΔΓ/n < -Δf/nwith n the overtone order). Small deviations (Figure B and D) presumably are caused by the layer’s softness. The apparent shear modulus, G app, of this layer is in the GPa range. The frequency difference, Δf/n|CT (Figure E), amounts to an apparent mass transfer rate. Plots of Δf/n|CT versus potential sometimes show curves similar to the electric current as observed for e. g. MV. For FAD, the results obtained depended on pH and the apparent mass transfer rates are much different from the current (Figure C and E). This difference is most likely caused by adsorbed molecules that are intermediates of the underlying two-electron process. The explanation is further corroborated by the response time of the QCM signals being much larger than the RC time of double layer recharging as determined with EIS. An interpretation in terms of adsorption/desorption is more plausible than an interpretation in terms of changed viscosity in the diffuse double layer.[1] Caption: Currents and frequency shifts obtained on a solution containing FAD. Df/n|av evidences the preferred adsorption of the intermediate state (FADH).

Enhanced Neurotransmitter Detection with Biorecognition Element-Functionalized Organic Electrochemical Transistors

The United States National Academies of Science and Engineering have issued a grand challenge to “reverse-engineer the brain”. Addressing this challenge requires a more complete understanding of how connections between individual neurons and groups of neurons are formed and maintained, as well as a more thorough elucidation of the signaling processes that drive neural network optimization. However, to achieve these goals, it is first necessary to develop the appropriate fundamental tools and protocols for adaptive biointerfacing, bottom-up neuroscience, and neuromodulation.To address this challenge, our research team has developed neurotransmitter-specific biosensors based on organic electrochemical transistors (OECT), which are electrolyte-gated devices that permit large signal amplification, ionic-to-electronic signal transduction, and controllable variable resistance. Given that their porous channel materials enable volumetric interaction with the adjacent electrolyte, OECT offer increased detection sensitivity compared to other electrolyte-gated devices such as field-effect transistors (FET). Although these OECT devices have demonstrated utility in a wide range of applications, from neuromorphic computing to bioelectronic devices, they are still in an early stage of development.We report enhanced electrochemical sensing of dopamine, mediated by functionalizing the OECT gate electrode with dopamine-specific biorecognition elements (BRE, i.e., aptamers). To optimize these devices, our team tested a variety of electrode functionalization approaches (e.g. alkane-thiol or silane chemistry, layer-by-layer, and covalent modification). The operational principle of BRE-functionalized OECT gate electrode biosensing is based on changes in gate electrode surface potential caused by target-binding to the BRE. When the target binds to the BRE, the event alters the capacitance at the surface of the functionalized gate electrode. The change in surface potential at the gate electrode shifts the applied potential on the OECT channel, which alters the doping state of the OECT channel. The channel materials are mixed ionic/electronic conductors, so changes in the doping state of the channel will directly affect the transconductance of the channel, which alters the OECT transfer characteristic and thereby permits sensing. The large transconductance of the OECT channel allows small changes in doping state to amplify the signal caused by binding of the target to the surface-attached BRE.In this study, we directly compare the dopamine biosensing performance of BRE-functionalized gold electrodes tested in two different sensing configurations:(i) traditional amperometric biosensor with a BRE-functionalized working electrode(ii) OECT with BRE-functionalized gate electrodeWe confirm the operational principles outlined above, and quantitatively assess the biosensing performance through a variety of analytical and electroanalytical techniques, such as cyclic voltammetry, open-circuit potentiometry, electrochemical impedance spectroscopy (EIS), square-wave and differential pulse voltammetry (DPV), and electrochemical quartz crystal microbalance (EQCM). We also compare the observed concentration-dependent changes in OECT transfer characteristics to the changes predicted by a theoretical analysis of surface potential.Finally, we will present the steps we have taken to implement these devices within organ-on-a-chip systems, both for traditional neurotransmitter biosensing and to demonstrate the utility of our devices for studying the formation and longitudinal evolution of neural networks within in vitro neuronal colonies.

Molecular Simulations Study of the Ionic Adsorption on Oscillating Carbon Electrodes

Molecular dynamics simulations are a method of choice to study, at microscopic scales, the adsorption of electrolyte ions onto the electrodes of capacitive storage systems such as supercapacitors1.Here, we use all-atom molecular dynamics simulations to model a pure ionic liquid electrolyte ([EMIM][TFSI], 1-ethyl-3-methylimidazolium trifluoromethanesulfonylimide) in contact with graphite electrodes (Figure: Supercapacitor composed of graphite electrodes (in light blue) in contact with a pure ionic liquid electrolyte ([EMIM][TFSI], atoms colored according to the atom type. The arrows illustrate the oscillations of electrodes used to probe the influence of such a motion on the interfacial liquid.)When applying a potential difference between the two carbon electrodes, a migration of counter ions is observed, ultimately leading to a compensation of the charges carried by the electrode atoms. The adsorption dynamics depend on several parameters (electrostatic interactions, coordination number, viscosity of the electrolyte, size of the ions etc.) and influence the power of these capacitive systems2. In order to improve the performance of supercapacitors, it is then important to describe and understand interfacial properties at a molecular scale.Experimentally, an Electrochemical Quartz Crystal Microbalance (EQCM) can be used to study ion flow on top of the electrode. The frequency variations recorded by the EQCM are linked to variations in the mass adsorbed on the quartz surface, thus making it possible to study the dynamics of ion adsorption during the charge of a supercapacitor.3 However, variations in quartz frequency can influence the stability of the electrode-electrolyte interface, thus modifying ionic interactions and adsorption dynamics at the electrode surface4.Here, thanks to molecular simulations, we have access to the adsorption rate of ions, to their re-orientation, to the variation of charge of the electrode atoms during adsorption etc. This allows us to study in detail the mechanisms governing the electrolyte-electrode interface during the charging of supercapacitors as part of EQCM. And study the extent to which perturbations caused by quartz oscillations in the ionic liquid affect electrode-electrolyte interfacial properties. References 1 K. Xu, H. Shao, Z. Lin, C. Merlet, G. Feng, et al.. ENERGY & ENVIRONMENTAL MATERIALS, 3, 235-246 (2020) 2C. Péan, B. Rotenberg, P. Simon, M. Salanne, ELECTROCHIM. ACTA, 206, 504-512 (2016) 3Y-C. Wu, J. Ye, G. Jiang, K. Ni, N. Shu, P-L. Taberna, Y. Zhu, P. Simon, ANGEW. CHEM. INT. ED., 60, 13317-13322 (2021) 4Nikolai V. Priezjev, MICROFLUIDICS AND NANOFLUIDICS, 14, 225-233 (2013) Figure 1

Kinetics of Charge Reversal in the Double Layer Studied with an Electrochemical Quartz Crystal Microbalance (EQCM-D)

Normal pulse voltammetry on inert electrolytes was combined with a new electrochemical quartz crystal microbalance (EQCM) with millisecond time resolution, which allows to probe the kinetics of charge reversal in the diffuse double layer (DL) after a step in electrode potential. The QCM responds to the charge reversal because the local viscosity-density product depends on the type and the concentration of the ions. The time resolution was improved to 1 ms using multifrequency lockin amplification, which allows to study kinetic effects in dynamic electrochemistry, which were previously not accessible with gravimetry. The noise was lowered to less than 10 mHz by modulating the electrode potential and averaging Δf and ΔΓ over many modulation cycles.The overtone scaling was found to be the same as in gravimetry (-Δf/n » const. and ΔΓ/n < -Δf/n with n the overtone order), which, however, is not caused by a deposition of a rigid film in the Sauerbrey sense. A Sauerbrey-type QCM response is also caused by changes in Newtonian viscosity inside a layer with a thickness far below the penetration depth of the shear wave (such as the double layer).The frequency response in the electrolytes studied correlated well with the viscosity B-coefficients of the respective ions as employed in the Dole-Jones equation. The time constants inferred from gravimetry were similar to the time constants (the RC-times) from electrochemical impedance spectroscopy (EIS). There were small differences, which presumably go back to slow rearrangements inside the Helmholtz layer, which are not seen in EIS.The QCM response to double layer recharging in inert electrolytes is of great importance for the interpretation of QCM results on processes involving Faradayic charge transfer. Such a response is omnipresent as a background and might be misinterpreted as a gravimetric signal.[1] References[1] Leppin, C.; Peschel, A.; Meyer, F. S.; Langhoff, A.; Johannsmann, D. Kinetics of Viscoelasticity in the Electric Double Layer Following Steps in the Electrode Potential Studied by a Fast Electrochemical Quartz Crystal Microbalance (EQCM). Analyst 2021, 146 (7), 2160–2171. Caption: Shifts in frequency and bandwidth in response to potential steps (left). The gravimetric signal followed the steps with a delay of a few milliseconds. The amplitude and the response time depend on cation type (right). The counter ion was NO3 -. Both the amplitude and the response time correlate well with the viscosity B-coefficient. Figure 1

Electrochemical Quartz Crystal Microbalance Study on the Electrodeposition of Fe, Co and Fe-Co Alloys

Cobalt-iron alloys are interesting soft magnetic materials which are used for example in magnetic sensors and transducers. They can be obtained by different techniques such as PVD, magnetron sputtering, or casting. One easy and not very expensive way to produce these alloys is the electrochemical deposition technique, which was also chosen in this study.This contribution will describe the electrodeposition of Co, Fe and Fe-Co alloys (Fe70Co30) from an aqueous sulphate-based electrolyte containing boric acid as a buffer and sodium citrate and citric acid besides the metal sulphates. Potentiostatic step experiments and cyclic voltammetry were performed in parallel to electrochemical quartz crystal microbalance. Thus, we could identify the potential at which the deposition of the metals sets in, the mass of the deposited species as well as the current efficiency. We could also extract the partial current due to hydrogen evolution reaction and the partial current due to individual metal or their alloys from the total current density. The morphology and structure of the deposited films were investigated by means of SEM and XRD, respectively. The grain size of the deposits was calculated from the XRD data using Scherrer equation. Addition of citric acid to the electrolyte results in the smaller grain size of the deposited Fe-Co films.

Molecular Insights into Redox-Active Polymer Interfaces: Solvation and Ion Valency Effects on Metal Oxyanion Selectivity

Chemical separations are responsible for 10-15% of the world’s energy consumption. Minimizing energy and materials inputs in selective separations is imperative for a sustainable future. Ion-electrosorption mediated by redox-active metallopolymer interfaces has the unique advantage of selectively capturing and releasing metal oxyanions in a switchable manner by adjusting the applied potential, without any regenerants. Electrosorption addresses the need for selective separation approaches with low chemical and energy inputs. Previous studies on ferrocene metallopolymers have demonstrated the role of polymer structure and applied potential on selectivity—however, the ubiquitous role of solvation in redox-polymers has remained unexplored. Here, we investigate how solvation and ion valency influence selectivity of ReO4 - vs MoO4 2- for two redox-metallopolymers, poly(vinyl ferrocene) (PVFc) and poly(3-ferrocenylpropyl methacrylamide) (PFPMAm). Both polymers display time-dependent Re/Mo selectivity, with PVFc having higher selectivity compared to PFPMAm. Operando neutron reflectometry, ellipsometry, and electrochemical quartz-crystal microbalance show that both PVFc and PFPMAm swell in the presence of ReO4 - (with PFPMAm having higher solvation), but do not swell in contact with MoO4 2-. We find that the less solvated anion (ReO4 -) is preferably adsorbed by the more hydrophobic redox-polymer (PVFc). We expect that a deeper understanding of solvation and valency effects on selectivity mechanisms in redox interfaces will expedite the development of targeted selective ion-electrosorption systems. Figure 1

Sensing with Molecularly Imprinted Membranes on Two-Dimensional Solid-Supported Substrates.

Molecularly imprinted membranes (MIMs) have been a focal research interest since 1990, representing a breakthrough in the integration of target molecules into membrane structures for cutting-edge sensing applications. This paper traces the developmental history of MIMs, elucidating the diverse methodologies employed in their preparation and characterization on two-dimensional solid-supported substrates. We then explore the principles and diverse applications of MIMs, particularly in the context of emerging technologies encompassing electrochemistry, surface-enhanced Raman scattering (SERS), surface plasmon resonance (SPR), and the quartz crystal microbalance (QCM). Furthermore, we shed light on the unique features of ion-sensitive field-effect transistor (ISFET) biosensors that rely on MIMs, with the notable advancements and challenges of point-of-care biochemical sensors highlighted. By providing a comprehensive overview of the latest innovations and future trajectories, this paper aims to inspire further exploration and progress in the field of MIM-driven sensing technologies.

Sol-Gel Transition of a Thermo-Responsive Polymer at the Closest Solid Interface.

Thermo-responsive polymers are applied as surface modifications for the temperature switching of hydrophilic and hydrophobic properties through adsorption and grafting on solid substrates. The current understanding of the influence of polymer chains bound to the solid surface on the transition behavior of thermo-responsive polymers is rather restricted. In this study, we aim to elucidate the effect of the bound polymer chains at the interface on the thermo-responsive sol-gel transition behavior of aqueous methylcellulose (MC) solutions by employing a quartz crystal microbalance (QCM) to evaluate the shear modulus near the solid interface. When the sample thickness was thinner on the order of the millimeter scale, the sol-gel transition temperature evaluated by the cloud point decreased because the condensation of MC near the solid interface promoted the sol-gel transition. On the other hand, focusing on the closest solid interface on the nanometer scale by QCM, the sol-gel transition temperature increased when approaching the solid interface. Adsorption and interfacial interactions reduced the chain mobility and restrained the sol-gel transition by preventing MC chain aggregation. We demonstrated the physical properties evaluation at the closest interface between the thermo-responsive polymer and solid substrate by combining a simple analytical model of QCM and controlling the analytical depth of the QCM sensors. In conclusion, the mobility change of the bound polymer chains at the solid interface caused by adsorption and interfacial interactions must be considered when a thermo-responsive polymer is applied as in adsorbed or thin films on solid substrates for the functionalization of biomaterials.