Electrical monitoring of polyelectrolyte multilayer formation by means of capacitive field-effect devices

The semiconductor field-effect platform represents a powerful tool for detecting the adsorption and binding of charged macromolecules with direct electrical readout. In this work, a capacitive electrolyte–insulator–semiconductor (EIS) field-effect sensor consisting of an Al-p-Si-SiO₂ structure has been applied for real-time in situ electrical monitoring of the layer-by-layer formation of polyelectrolyte (PE) multilayers (PEM). The PEMs were deposited directly onto the SiO₂ surface without any precursor layer or drying procedures. Anionic poly(sodium 4-styrene sulfonate) and cationic weak polyelectrolyte poly(allylamine hydrochloride) have been chosen as a model system. The effect of the ionic strength of the solution, polyelectrolyte concentration, number and polarity of the PE layers on the characteristics of the PEM-modified EIS sensors have been studied by means of capacitance–voltage and constant-capacitance methods. In addition, the thickness, surface morphology, roughness and wettabilityof the PE mono- and multilayers have been characterised by ellipsometry, atomic force microscopy and water contact-angle methods, respectively. To explain potential oscillations on the gate surface and signal behaviour of the capacitive field-effect EIS sensor modified with a PEM, a simplified electrostatic model that takes into account the reduced electrostatic screening of PE charges by mobile ions within the PEM has been proposed and discussed.

Synthesis and characterization of high-integrity solid-contact polymeric ion sensors

High-integrity solid-contact (SC) polymeric ion sensors have been produced by using spin casting and electropolymerization techniques in the preparation of the SC employing the conductive polymer, poly(3-octylthiophene) (POT). The physical and chemical integrity of the POT SCs have been evaluated using scanning electron microscopy (SEM), atomic force microscopy (AFM), secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (XPS). Furthermore, the electrochemical stability of SC polymeric ion sensors has been investigated using electrochemical impedance spectroscopy (EIS). The results of this study demonstrate that electropolymerization and spin casting methods also comprising annealing of the synthesized SC film are capable of producing SCs that are relatively free of imperfections such as pores and pinholes. This leads to electrochemically stable and robust polymeric ion sensors where the SC/sensor interface is resistant to the formation of a detrimental water layer that normally gives rise to spurious ion fluxes and a degradation in the sensitivity and selectivity of the SC polymeric ion sensor.     

Fibronectin adsorption onto polyelectrolyte multilayer films

The Layer-by-layer deposition of positively and negatively charged macromolecular species is an ideal method for constructing thin films incorporating biological molecules. We investigate the adsorption of fibronectin onto polyelectrolyte multilayer (PEM) films using optical waveguide lightmode spectroscopy (OWLS) and atomic force microscopy (AFM). PEM films are formed by adsorption onto Si(Ti)O2 from alternately introduced flowing solutions of anionic poly(sodium 4-styrenesulfonate) (PSS) and cationic poly(allylamine hydrochloride) (PAH). Using OWLS, we find the initial rate and overall extent offibronectin adsorption to be greatest on PEM films terminated with a PAH layer. The polarizability density of the adsorbed protein layer, as measured by its refractive index, is virtually identical on both PAH- and PSS-terminated films; the higher adsorbed density on the PAH-terminated film is due to an adsorbed layer of roughly twice the thickness. The binding of monoclonal antibodies specific to the protein's cell binding site is considerably enhanced to fibronectin adsorbed to the PSS layer, indicating a more accessible adsorbed layer. With increased salt concentration, we find thicker PEM films but considerably thinner adsorbed fibronectin layers, owing to increased electrostatic screening. Using AFM, we find adsorbed fibronectin layers to contain clusters; these are more numerous and symmetric on the PSS-terminated film. By considering the electrostatic binding of a segmental model fibronectin molecule, we propose a picture of fibronectin adsorbed primarily in an end-on-oriented monolayer on a PAH-terminated film and as clusters plus side-on-oriented isolated molecules onto a PSS-terminated film.     

Reversible switching of protein uptake and release at polyelectrolyte multilayers detected by ATR-FTIR spectroscopy

The reversible switching of uptake and release of the proteins lysozyme (LYZ, IEP = 11.1) and human serum albumin (HSA, IEP = 4.8) at the surface attached polyelectrolyte multilayer (PEM) consisting of poly(ethylene-imine) (PEI) and poly(acrylic acid) (PAC) is shown. Protein adsorption could be switched by pH setting due to electrostatic interaction. Adsorption of positively charged LYZ at PEM-6 took place at pH = 7.3, where the outermost PAC layer was negatively charged. Complete desorption was obtained at pH = 4, where the outermost PAC layer was neutral. Additionally the charge state of the last adsorbed PAC layer in dependence of the pH of the medium could be determined in the ATR-FTIR difference spectra by the ν(COO⁻) and ν(C=O) band due to carboxylate and carboxylic acid groups. Adsorption of negatively charged HSA at PEM-7 was achieved at pH = 7.3, where the outermost PEI layer was positively charged. Part desorption was obtained at pH = 10, where the outermost PEI layer was neutral. PEM of PEI/PAC may be used for the development of bioactive and bionert materials and protein sensors.     

Optical sensors with molecularly imprinted nanospheres: a promising approach for robust and label-free detection of small molecules

Molecularly imprinted nanospheres obtained by miniemulsion polymerization have been applied as the sensitive layer for label-free direct optical sensing of small molecules. Using these particles as the sensitive layer allowed for improving response times in comparison to sensors using MIP layers. As a model compound, well-characterized nanospheres imprinted against l-Boc-phenylalanine anilide (l-BFA) were chosen. For immobilization, a simple concept based on electrostatic adsorption was used, showing its applicability to different types of surfaces, leading to a good surface coverage. The sensor showed short response times, good selectivity, and high reversibility with a limit of detection down to 60 μM and a limit of quantitation of 94 μM. Furthermore, reproducibility, selectivity, and long-term stability of the sensitive layers were tested. The best results were achieved with an adsorption on aminopropylsilane layers, showing a chip-to-chip reproducibility of 22%. Furthermore, the sensors showed no loss in signal after a storage time of 1 year.     

Modulation of hemocompatibility of polysulfone by polyelectrolyte multilayer films

Polyelectrolyte multilayer (PEM) films have been recently applied to surface modification of biomaterials. Cellular interactions with PEM films consisted of weak polyelectrolytes are greatly affected by the conditions of polyelectrolyte deposition, such as pH of polyelectrolyte solution. Previous studies indicated that the adhesion of several types of mammalian cells to PAH/PAA multilayer films was hindered by low pH and high layer numbers. The objective of this study is to evaluate whether the hemocompatibility of polysulfone can be modulated by deposition of poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA) multilayer films. PAH/PAA multilayer films with different layer numbers were assembled onto polysulfone at either pH 2.0 or pH 6.5. The number of platelet adhesion and the morphology of adherent platelets were determined to evaluate hemocompatibility of modified substrates. Compared to non-treat polysulfone, the PEM films developed at pH 2.0 decreased platelet adhesion, while those built at pH 6.5 enhanced platelet deposition. Platelet adhesion was found positively correlated to polyclonal antibodies binding to surface-bound fibrinogen. The extent of platelet spreading was increased with layer numbers of PEM films, suggesting that the adherent platelets on thick PEM films were prone to activation. In conclusion, PAH/PAA films with few layers developed at pH 2.0 possessed better hemocompatibility compared to other substrates.     

Application of sensitive hydrogels in chemical and pH sensors

In the present work, pH-sensitive poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) blends as well as hydrogels based on poly(N-isopropylacrylamide) (PNIPAAm), which are sensitive to organic solvent concentration in aqueous solutions, were used in silicon micromachined sensors. A sensitivity of approximately 15 mV/pH was obtained for a pH sensor with a 50 μm thick PVA/PAA hydrogel layer in a pH range above the acid exponent of acrylic acid (pK_a=4.7). The output voltage versus pH-value characteristics and the long-term signal stability of hydrogel-based sensors were investigated and the measurement conditions necessary for high signal reproducibility were determined. The influence of the preparation conditions of the hydrogel films on the sensitivity and response time of the chemical and pH sensors is discussed.     

Multilayer sol-gel membranes for optical sensing applications: single layer pH and dual layer CO(2) and NH(3) sensors

Organo-silica sol-gel membranes have been prepared and demonstrated in a single layer format for pH measurement and multiple-layer format for both carbon dioxide and ammonia. The sensors are simple and versatile since the same chemistry and membranes are used for each sensor. The sensors use hydroxypyrenetrisulfonic acid (HPTS) as the indicator immobilized in a base-catalyzed sol-gel containing poly(dimethyl)siloxane, aminopropyltriethoxysilane (APTES) and tetraethylorthosilicate (TEOS). This indicator gel is over coated with a hydrophobic sol-gel to reduce cross reactivity to pH when either CO(2) or NH(3) are examined. The gels are very stable and the sensors retain response up to a 12-month period. Sensors can be stored in buffer or dry without loss of function and have response times to that are comparable to literature values.     

Microcoated wire sensors for the determination of anticancer drugs cyclophosphamide and ifosphamide in the presence of their degradates

The construction and electrochemical response characteristics of poly (vinyl chloride) and poly (vinyl chloride) carboxylate membrane sensors for the determination of cyclophosphamide and ifosphamide are described. Based on the formation of an ion-pair complex between the drug cation and sodium tetraphenylborate, two poly (vinyl chloride) sensors, namely a cyclophosphamide membrane sensor and ifosphamide membrane sensor were fabricated. They show a linear response for both drugs over the concentration range 10⁻²–10⁻⁴ M with cationic slopes of 56 and 54.6 mV per concentration decade, for sensor 1 and sensor 2, respectively. Based on the interaction between the drug solution and the dissociated COOH groups in the poly (vinyl chloride) carboxylate, sensor 3 was fabricated. It shows a linear response for both drugs over the concentration range 10⁻³–10⁻⁵ M with a cationic slope of 49.7 mV per concentration decade. The direct potentiometric determination of cyclophosphamide and ifosphamide in their pharmaceutical preparations using the proposed sensors gave average recoveries of 101.3±0.6, 100.8±10.7 and 102.0±11.0% for the sensors 1, 2 and 3, respectively, which compares reasonably well with the data obtained using the British Pharmacopoeial method (1993). Sensors 1 and 2 were also used to follow up the stability of the drugs studied in the presence of their degradates. These degradation products have no diverse effect on the responses of sensors 1 and 2.     

Electrochemical Properties of Highly Sensitive and Selective CuO Nanostructures Based Neurotransmitter Dopamine Sensor

In this article, electrochemical properties of CuO nanostructures based dopamine (DA) sensor was investigated. The morphology, structure, optical, and compositional properties of the CuO nanostructures were characterized by using SEM, XRD, UV‐Vis, and XPS techniques. The electrochemical properties were investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques. The CV results indicate that biosensors based on CuO nanostructures exhibit a high selectivity and sensitivity of 0.1975 μA μM^–1 toward DA and effectively avoids the interference of ascorbic acid (AA) and uric acid (UA). The obtained EIS spectra for CuO sensors were analysed using an electrical equivalent circuit to understand the bulk and surface response via the capacitive and resistive parameters. The EIS measurement also leads to the direct determination of parameters like series resistance and ion diffusion phenomena at electrode‐electrolyte interface. The experimental CV and EIS results along with their analysis will have a significant impact on understanding the mechanism of high sensitivity and selectivity performance of CuO based sensors. This study may also lay the basis for efficient characterization of biosensors by coupling both the CV and EIS characterization techniques.     

Preparation and Enantiospecific Binding of Chiral Polyelectrolyte Multilayers: An In-Situ ATR-FTIR Study

Summary: Two chiral polyelectrolyte multilayers (PEM) composed of poly(L-lysine) (PLL) and poly(vinylsulfate) (PVS) as well as poly(ethyleneimine-maltose) (PEI-m) and poly(vinylsulfate) and a nonchiral PEM composed of poly(ethyleneimine) (PEI) and poly(vinylsulfate) were deposited on a silica surface using the layer by layer method. For both PEM enantiospecific interaction towards one enantiomer of either L-/D- glutamic acid (L-/D-GLU) or L-/D-ascorbic acid (L-/D-ASC), respectively, was checked under variation of the concentration. Both deposition and enantiospecific interaction were studied by attenuated total reflection Fourier transform (ATR-FTIR) spectroscopy. Preliminary results show a significant enantiospecific preference of D- GLU over L-GLU at PEM of PLL/PVS and of D-ASC over L-ASC at PEM of PEI-m/PVS and no such preference for nonchiral PEM of PEI/PVS. PEM of PLL/PVS shows higher enantiospecifity with increasing L-/D-GLU concentration.     

Electrochemical Aptasensor Based on ZnO Modified Gold Electrode

We developed an electrochemical thrombin aptasensor based on ZnO nanorods functionalized by electrostatically adsorption of 30‐mer DNA aptamers. The sensor surface was characterized by AFM and SEM. The surface layer assembling was optimized using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) with ferricyanide ions as redox markers. The peak current of the ferricyanide and the charge transfer resistance gradually decreased with increasing concentration of thrombin in the range from 3 pM to 100 nM due to formation of aptamer‐thrombin complexes and slower diffusion of the marker ions through the surface layer. At optimal conditions, a limit of detection (LOD) of 3 pM for EIS measurements and 10 pM for CV response was calculated from the S/N=3.     

Polymeric encapsulated membrane for optrodes

Highly efficient and simple reagent immobilisation procedure for an optrode membrane has been developed. The preparation procedure combines two well-known physical immobilisation procedures, viz. adsorption and encapsulation techniques, applied sequentially which, in turn, produce a combined optical sensing layer. The sensitive polymeric encapsulated membrane (PEM) contains the polymeric resin, which can adsorb a large amount of indicator dye, encapsulated within porous polymeric structure. The procedure has been tested in the preparation of a pH sensor.     

Polyelectrolyte Multilayer Films Containing Ferrocene: Effects of Polyelectrolyte Type and Ferrocene Contents in the Film on the Redox Properties

Electrochemical properties of Fc‐PEM films have been studied by changing the chemical structure of the polymer chains and the content of Fc moiety in the film systematically. We have prepared a series of PEM films by a layer‐by‐layer deposition of polycations, Fc‐modified poly(allylamine) (Fc‐PAA) and poly(ethyleneimine) (Fc‐PEI), and polyanionic poly(vinyl sulfate) (PVS) on the surface of a gold electrode. The redox properties of the Fc‐PAA/PVS and Fc‐PEI/PVS films depended significantly on the content of Fc moiety in the polymer chains and on the polymer type. Fc‐ PAA and Fc‐PEI polymer chains can penetrate 3 or 4 PAA/PVS bilayers inserted between the redox polymers and electrode. The Fc‐PAA film‐modified electrode can be used for electrocatalytic oxidation of ascorbic acid.     

Silver and lead all-plastic sensors—polyaniline vs. poly(3,4-ethyledioxythiophene) solid contact

Silver and lead selective all-plastic ion-selective electrodes were obtained using poly(vinyl chloride)-based membranes and either poly(3,4-ethylenedioxythiophene) or polyaniline dispersion cast on an insulating plastic support as transducer and electrical lead. The effect of interactions of applied conducting polymer with analyte ions on potentiometric responses was evaluated and correlated with changes in elemental composition and element distribution within the ion-selective membrane and the conducting polymer transducer revealed in course of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) experiments. In the case of silver selective electrodes, potentiometric responses obtained are much dependent on the oxidation state of the polymer placed beneath the ion-selective membrane. For semi-oxidized polymer (poly(3,4-ethylenedioxythiophene)-based electrodes, linear responses with detection limit equal to 10^−5.4 M were obtained. For a more oxidized polyaniline (of higher conductivity), although the electrodes were pretreated exactly in the same way and tested in parallel, super Nernstian potential slope was recorded within the AgNO₃ activities range form 10⁻⁶ to 10⁻⁷ M. These responses were consistent with results of LA-ICP-MS, revealing profoundly higher silver signals intensities for poly(3,4-ethylenedioxythiophene) underlying silver selective membrane. It seems highly probable that silver is accumulated in this polymer layer as Ag⁰ due to spontaneous redox reaction leading to oxidation of the polymer; however, this process requires also the presence of silver ions at the interface. In fact, when reduced (deprotonated) polyaniline was used as transducer, the potentiometric responses of the sensor were, within the range of experimental error, the same as obtained for poly(3,4-ethylenedioxythiophene)-based sensor. On the other hand, for lead(II) selective sensors, the difference in responses of electrodes prepared using poly(3,4-ethylenedioxythiophene) or polyaniline was less pronounced, which is in accordance with the elemental composition of these sensors.     

Layer-by-layer adsorption of polyelectrolyte and surfactant and adsorption of their complexes on solid surface

The capillary electrokinetics method (measurement of streaming potential and current in a capillary with a radius of 5–7 μm made of fused quartz) is employed to study the structure formation at interfaces between quartz and solutions containing a cationic polyelectrolyte (poly(diallyldimethylammonium chloride) with molecular mass M = 100000−200000) and an anionic surfactant (sodium dodecyl sulfate). The kinetics of surface layer formation is studied upon the layer-by-layer adsorption of the components and the adsorption of their complexes at the same component ratios. It is established that the formation time and the electrokinetic potentials of the surface layers are almost independent of the procedure of their formation. In the case of the layer-by-layer adsorption, the first layers of the polyelectrolyte appear to be virtually undeformed, thus indicating that molecules with a planar conformation prevail in the adsorption layer. Surfactant adsorption enhances the deformation (layer loosening), which decreases with time (layer aging). Layers formed from the complexes have a denser (less deformable) structure. Variations in the electrokinetic potentials of the layers during the long-term pumping of a background electrolyte solution through a capillary witnesses the prevailing desorption of the anionic surfactant, with the desorption being noticeably more pronounced for the layers resultant from the adsorption of the complexes.     

Dynamics of ion exchange between self-assembled redox polyelectrolyte multilayer modified electrode and liquid electrolyte

A probe beam deflection (PBD) study of ion exchange between an electroactive polymer poly(allylamine)-bipyridyl-pyridine osmium complex film and liquid electrolyte is reported. The PBD measurements were made simultaneously to chronoamperometric oxidation-reduction cycles, to be able to detect kinetic effects in the ion exchange. Layer-by-layer (LbL) self-assembled redox polyelectrolyte films with osmium bipyridyl complex covalently attached to poly(allylamine) (PAH-Os) and poly(styrene sulfonate) (PSS) have been built by alternate electrostatic adsorption from soluble polyelectrolytes. The ionic exchange during initial conditioning of the film ("break-in") undergoing oxidation-reduction cycles and recovery after equilibration in the reduced state have shown an exchange of anions and cations with time lag between them. The effect of the nature of cation on the ionic exchange has been investigated with dilute HCl, LiCl, NaCl, and CsCl electrolytes. The ratio of anion to cation exchanged at the film-electrolyte interface has a strong dependence on the nature of charge in the topmost layer, that is, when negatively charged PSS is the capping layer, a larger proportion of cation exchange is observed. This demonstrates that the electrical potential distribution at the redox polyelectrolyte multilayer (PEM)/electrolyte interface determines the ionic flux in response to charge injection in the film.     

Highly improved carbon dioxide sensitivity and selectivity of black phosphorene sensor by vacancy doping: A quantum chemical perspective

The adsorption and sensing properties of a carbon dioxide (CO₂) molecule on the pristine (BP) and vacancy-doped (DP) black phosphorusmono layers have been investigated using the periodic density functional theory at Heyd-Scuseria-Ernzerhof (HSE06)/triple-zeta valence polarization (TZVP). For both sensors, the most stable structures among the recognized possibilities preferred a linear configuration for carbon dioxide, with a shorter equilibrium distance (2.13 Å) on the defect-containing surface. Although carbon dioxide was weakly physiosorbed on both phosphorene sensors (up to −2.52 kcal/mol), the defect-engineered material presented highly improved sensitivity (by a factor of 6.6) to CO₂ compared to the pristine layer. The former was also a (2.6 times) better work function sensor of carbon dioxide. At the same time, recovery was extremely fast (lasting for 70 ps at most) at room temperature. The selectivity coefficient of carbon dioxide was also strikingly high (64.0). The improved nanosensor would be a step forward in the rational design of highly sensitive and reusable detectors of carbon dioxide.     

In situ ATR-FTIR and SFM Studies on the Influence of Adsorption time on Deposition and Nanostructure of Poly(ethyleneimine)/Poly(acrylic acid) Multilayers

Summary: The deposition and the nanostructure of polyelectrolyte multilayers (PEM) of branched poly(ethyleneimine)/poly(acrylic acid) (PEI/PAC) was studied in dependence of the adsorption time (t_ADS) of the individual steps. PEM were reproducibly deposited applying up to z = 20 adsorption steps at the fixed pH combination of 10/4 and polyelectrolyte concentration c_PEL = 0.005 M in a flow cell using an automated valve system. in situ ATR-FTIR spectroscopy and SFM were used for quantitative determination of deposited amount and thickness, respectively. A linear relation between PEL band integrals and thickness of thin PEM films was found. Varying t_ADS from 0.5 to 5 min in each of the adsorption steps resulted in a steep rise of the deposited PEM amount. For t_ADS > 5 min the deposition did only marginally increase. Evidence for the release of outermost located PEI upon PAC immersion (even step) and of outermost PAC upon PEI immersion (odd step) was obtained. SFM images on consecutively deposited PEM-6 showed a slight increase in structure size and roughness for increasing t_ADS. These studies help to prepare polyelectrolyte based films with controlled thickness for the interaction with biofluids in the biomedical and food field.     

Surface plasmon resonance sensors for real-time detection of cyclic citrullinated peptide antibodies

Surface plasmon resonance (SPR) sensors have been used for detection of various biomolecules because of their simplicity, high specificity and sensitivity, real-time detection, low cost, and no requirement of labeling. Recently, molecularly imprinted polymers that are easy to prepare, less expensive, stable, have talent for molecular recognition and also are used for creation selective binding sites for target molecule on the SPR sensors. Here, we show that preparation of cyclic citrullinated peptide antibody (anti-CCP) imprinted SPR sensor to detect CCP antibodies. For this purpose, anti-CCP/AAm pre-complex was synthesized by interacting acrylamide (AAm) monomer with anti-CCP. Then, anti-CCP imprinted (anti-CCP/PAAm) SPR sensor was obtained by reacting with anti-CCP/AAm pre-complex in the presence of the crosslinker, and initiator/activator pair. Besides this, non-imprinted (PAAm) SPR sensor was also prepared without using anti-CCP template. The SPR sensors were characterized and then adsorption-desorption studies were performed with pH 7.0 phosphate buffer (10 mM) and acetic acid (10%) with Tween 20 (1%) in pH 7.0 phosphate buffer. Selectivitiy of sensors was investigated by using immunoglobulin M (IgM) and bovine serum albumin (BSA). To determine the adsorption model of interactions between anti-CCP solutions and anti-CCP/PAAm SPR sensor, different adsorption models were performed. The calculated maximum reflection, detection limit, association and dissociation constants were 1.079 RU/mL, 0.177 RU/mL, 0.589 RU/mL and 1.697 mL/RU, respectively. Repeatability experiments of anti-CCP/PAAm SPR sensor was performed four times with adsorption-desorption-regeneration cycles without any performance losing. Results showed that anti-CCP/PAAm SPR sensor had high selectivity and sensitivity for detection of CCP antibodies.