We report the modulation of the specific metal gation properties of a peptide and demonstrate a highly selective sensor for copper(II) ion. The neuropeptide oxytocin (OT) is reported for its high affinity towards Zn2+and Cu2+ at physiological pH. The binding of the metal ions to OT is tuned by altering the pH of the medium. OT was self-assembled on glassy carbon electrode using surface chemistry, and electrochemical impedance spectroscopy (EIS) was used to probe the binding of Cu2+. Our results clearly indicate that at pH 10.0, the binding of Cu2+ to OT is increased compared to that at pH 7.0, while the binding to Zn2+ becomes almost negligible. This proves that the selectivity of OT towards each of the ions can be regulated simply by controlling the pH of the medium and hence allows the preparation of a sensing device with selectivity to Cu2+.
Inorganic materials such as semiconductors, oxides, and metals are ubiquitous in a wide range of device technologies owing to the outstanding robustness and mature processing technologies available for such materials. However, while the important contribution of inorganic materials to the advancement of device technologies has been well established for decades, organic-inorganic hybrid device systems, which merge molecular functionalities with inorganic platforms represent a newer domain that is rapidly evolving at an increasing pace. Such devices benefit from the great versatility and flexibility of the organic building blocks merged with the robustness of the inorganic platforms. Given the overwhelming wealth of literature covering various approaches for modifying and using inorganic devices, this feature article selectively highlights some of the advances made in the context of diversification of devices by surface chemistry. Particular attention is given to oxide-semiconductor systems and metallic surfaces modified with organic monolayers. The inorganic device components, such as semiconductors, metals, or oxides, are modified by organic monolayers, which may serve as either active, static, or sacrificial components. We portray research directions within the broader field of organic-inorganic hybrid device systems that can be viewed as specific examples of the potential of such hybrid device systems given their comprehensive capabilities of design and diversification. Monolayer doping techniques where sacrificial organic monolayers are introduced to semiconducting elements are reviewed as a specific case, together with associated requirements for nanosystems, devices, and sensors for controlling doping levels and doping profiles at the nanometric scale. Another series of examples of the flexibility provided by the marriage of organic functional monolayers and inorganic device components is represented by a new class of biosensors, where the organic layer functionality is exploited in a functioning device for sensing. Considerations for relying on oxide-terminated semiconductors rather than the pristine semiconductor material as a platform both for processing and sensing are discussed. Finally, we cover aspects related to the use of various theoretical and computational approaches to model organic-inorganic systems. The main objectives of the topics covered here are (i) to present the advances made in each respective domain, and (ii) provide a comprehensive view of the potential uses of organic monolayers and self-assembly processes in the rapidly evolving field of molecular-inorganic hybrid device platforms and processing methodologies. The combination of directions highlighted here provides a perspective, on a future, not yet fully realized, integrated approach where organic monolayers are combined with inorganic platforms in order to obtain versatile, robust, and flexible systems with enhanced capabilities.
Hybrid nanomaterials having tunable properties that can be reversibly conducted by external stimuli, in a particular light, are of great importance since they enable synergetic behavior between their components and enable the design of stimuli responsive “smart” materials and surfaces. Here we describe the formation of organic–inorganic hybrid nanoparticles that photochemically aggregate and their effect on the electronic properties of a semiconducting surface, as a function of external irradiation. The inorganic component consists of 3 nm gold nanoparticles while the organic component is a covalently attached, photochromic spiropyran derivative. Aggregation/deaggregation patterns in solution were obtained and analyzed by UV–vis spectroscopy and transmission electron microscopy upon photoswitching. The assembly of spiropyran-modified gold nanoparticles on an Si/SiO2 surface proved useful in phototuning the electronic properties of semiconductors measured by contact potential difference.
Zinc and copper are essential metal ions for numerous biological processes. Their levels are tightly maintained in all body organs. Impairment of the Zn2+ to Cu2+ ratio in serum was found to correlate with many disease states, including immunological and inflammatory disorders. Oxytocin (OT) is a neuropeptide, and its activity is modulated by zinc and copper ion binding. Harnessing the intrinsic properties of OT is one of the attractive ways to develop valuable metal ion sensors. Here, we report for the first time an OT-based metal ion sensor prepared by immobilizing the neuropeptide onto a glassy carbon electrode. The developed impedimetric biosensor was ultrasensitive to Zn2+ and Cu2+ ions at physiological pH and not to other biologically relevant ions. Interestingly, the electrochemical impedance signal of two hemicircle systems was recorded after the attachment of OT to the surface. These two semicircles suggest two capacitive regions that result from two different domains in the OT monolayer. Moreover, the change in the charge-transfer resistance of either Zn2+ or Cu2+ was not similar in response to binding. This suggests that the metal-dependent conformational changes of OT can be translated to distinct impedimetric data. Selective masking of Zn2+ and Cu2+ was used to allow for the simultaneous determination of zinc to copper ions ratio by the OT sensor. The OT sensor was able to distinguish between healthy control and multiple sclerosis patients diluted sera samples by determining the Zn/Cu ratio similar to the state-of-the-art techniques. The OT sensor presented herein is likely to have numerous applications in biomedical research and pave the way to other types of neuropeptide-derived sensors.
In this work, we demonstrate the tunability of electronic properties of Si/SiO2 substrates by molecular and ionic surface modifications. The changes in the electronic properties such as the work function (WF) and electron affinity were experimentally measured by the contact potential difference technique and theoretically supported by density functional theory calculations. We attribute these molecular electronic effects mainly to the variations of molecular and surface dipoles of the ionic and neutral species. We have previously shown that for the alkylhalide monolayers, changing the tail group from Cl to I decreased the WF of the substrate. Here, we report on the opposite trend of WF changes, that is, the increase of the WF, obtained by using the anions of these halides from Cl– to I–. This trend was observed on self-assembled alkylammonium halide (−NH3+ X–, where X– = Cl–, Br–, or I–) monolayer-modified substrates. The monolayer’s formation was supported by ellipsometry measurements, X-ray photoelectron spectroscopy, and atomic force microscopy. Comparison of the theoretical and experimental data suggests that the ionic surface dipole depends mainly on the polarizability and the position of the counter halide anion along with the organization and packaging of the layer. The described ionic modification can be easily used for facile tailoring and design of the electronic properties Si/SiO2 substrates for various device applications.
Copper ions play a major role in biological processes. Abnormal Cu2+ions concentrations are associated with various diseases, hence, can be used as diagnostic target. Monitoring copper ion is currently performed by non-portable, expensive and complicated to use equipment. We present a label free and a highly sensitive electrochemical ion-detecting biosensor based on a Gly-Gly-His tripeptide layer that chelate with Cu2+ ions. The proposed sensing mechanism is that the chelation results in conformational changes in the peptide that forms a denser insulating layer that prevents RedOx species transfer to the surface. This chelation event was monitored using various electrochemical methods and surface chemistry analysis and supported by theoretical calculations. We propose a highly sensitive ion-detection biosensor that can detect Cu2+ ions in the pM range with high SNR parameter.
Carbon nanotubes (CNTs) and semiconductor nanocrystals (SCNCs) are known to be interesting donor–acceptor partners due to their unique optical and electronic properties. These exciting features have led to the development of novel composites based on these two nanomaterials and to their characterization for use in various applications, such as components in sensors, transistors, solar cells and biomedical devices. Two approaches based on covalent and noncovalent methods have been suggested for coupling the SCNCs to CNTs. Most covalent conjugation methods used so far were found to disrupt the electronic structure of the CNTs or interfere with charge transfer in the CNT–SCNC interface. Moreover, it offers random and poorly organized nanoparticle coatings. Therefore, noncovalent methods are considered to be ideal for better electronic coupling. However, a key common drawback of noncovalent methods is the lack of stability which hampers their applicability. In this article, a method has been developed to couple semiconductor seeded nanorods onto CNTs through π–π interactions. The CNTs and pyrene conjugated SCNC hybrid materials were characterized by both microscopic and spectroscopic techniques. Fluorescence and photocurrent measurements suggest the proposed pi-stacking approach results in a strong electronic coupling between the CNTs and the SCNCs leading to better photocurrent efficiency than that of a covalent conjugation method reported using similar SCNC material. Overall, the CNT–SCNC films reported in the present study open the scope for the fabrication of optoelectronic devices for various applications.
Polycrystalline SnO2 1D nanostructures with diameters of 150–200 nm and consisting of uniformly sized single-crystallites of ~ 10 nm in size were produced via aqueous sol-gel and hard template assisted methods. Stable sols were prepared from tin oxalate precursor, citric acid and H2O2. Solution chemistry and parameters influencing the stability of the sol and morphological structure of the resulting materials are addressed. Morphological and structural characterization revealed uniform and porous structure of the nanostructured SnO2. Hollow nanotubular structures were produced from solutions of lower concentration. Results of XPS analysis revealed that a sub-stoichiometric phase with surface composition of SnO1.8 was obtained indicating the formation of oxygen vacancies. The optical properties were investigated and optical band gap energy for the powder samples was estimated to be 3.61 eV. An environmentally friendly aqueous sol-gel route developed to prepare homogeneous nanostructures of 1D SnO2 will enable facile fabrication of various oxide based nanostructured materials.
We describe the detailed microscopic changes in a peptide monolayer following kinase-mediated phosphorylation. A reversible electrochemical transformation was observed using square wave voltammetry (SWV) in the reversible cycle of peptide phosphorylation by ERK2 followed by dephosphorylation by alkaline phosphatase. A newly developed method for analyzing local roughness, measured by atomic force microscope (AFM), showed a bimodal distribution. This may indicate either a hole-formation mechanism and/or regions on the surface in which the peptide changed its conformation upon phosphorylation, resulting in increased roughness and current. Our results provide the mechanistic basis for developing biosensors for detecting kinase-mediated phosphorylation in disease.
The manipulation of biological substrates is becoming more popular route toward generating novel computing devices. Physarum polycephalum is used as a model organism in biocomputing because it can create “wires” for use in hybrid circuits; programmable growth by manipulation through external stimuli and the ability withstanding a current and its tolerance to hybridization with a variety of nano/microparticles. Lettuce seedlings have also had previous interest invested in them for generating plant wires, although currently there is little information as to their suitability for such applications. In this study both P. polycephalum and Lettuce seedlings were hybridized with gold nanoparticles — functionalized and unfunctionalized — to explore their uptake, toxicological effects and, crucially, any alterations in electrical properties they bestow upon the organisms. Using various microscopy techniques it was shown that P. polycephalum and lettuce seedlings are able to internalize nanoparticles and assemble them in vivo, however some toxicological effects were observed. The electrical resistance of both lettuce seedlings and P. polycephalum was found to decrease, the most significant reduction being with lettuce seedlings whose resistance reduced from 3MΩΩs to 0.5MΩΩs. We conclude that gold is a suitable nanomaterial for biohybridization specifically in creating conductive pathways for more efficient biological wires in self-growing hybrid circuitry.
Poly(4-vinylpyridine) swollen in pyridine displays changes in electrical conductivity in response to white light and to low level thermal perturbation; protonation of the side-chain nitrogen is believed to play a role. Here we present spectroscopic evidence that the proton donor is the methyne group CH on the polymer chain.
In this contribution we describe the formation of gold nanoparticles (AuNP) and polyaniline (PANI) AuNP-PANI nanocomposite via in-situ enzymatic polymerization. The method consists of electrostatic adsorption of anilinium monomers on AuNPs citrate stabilized surface of 50 nm diameters, followed by oxidation with horseradish peroxidase (HRP) enzyme and its cofactor H2O2. All reaction steps were monitored by UV-Vis-NIR spectroscopy including in-situ detection of the polymerization process. UV-Vis-NIR, Cyclic voltammetry (CV) and surface enhanced Raman scattering (SERS) measurements supported the formation of a nanoshell of PANI on the AuNP core. Two templates for anilinium assembly were compared revealing a strong dependence of the enzymatic kinetics on the template. The kinetic study had shown that the rigid template of the AuNP contributes to higher reaction rate on the AuNP compared with the more flexible polyanion template. The mild reaction’ condition enables an easy and precise method for obtaining PANI nano-shell on anionic templates for advanced bioelectronic applications.
An electrochemical biosensor has been developed for ultrasensitive, label-free determination of protein kinase activity. The sensor is composed of a unique peptide monolayer on a gold electrode. It identifies the order change in the monolayer upon phosphorylation, via square wave voltametry (SWV) measurements. Disorder caused by the introduction of the phosphate groups onto the middle of the peptide sequence results in pinhole formation and therefore an increase in the electrochemical signal. The measured sensitivity was 100 nM of kinase and the dynamic range was 100 nM up to 11 μM. Sensitivity was an order of magnitude higher, and the dynamic range wider by two orders of magnitude, as compared to our previously reported impedimetric method, in which the sensitivity was 1 μM, and the dynamic range was 1-20 μM.
We present an integrated approach for highly sensitive identification and validation of substrate-specific kinases as cancer biomarkers. Our approach combines phosphoproteomics for high throughput cancer-related biomarker discovery from patient tissues and an impedimetric kinase activity biosensor for sensitive validation. Using non-small-cell lung cancer (NSCLC) as a proof-of-concept study, label-free quantitative phosphoproteomic analysis of a pair of cancerous and its adjacent normal tissues revealed 198 phosphoproteins that are over-phosphorylated in NSCLC. Among the differentially regulated phosphorylation sites, the most significant alteration was in residue S165 in the Hepatoma Derived Growth Factor (HDGF) protein. Hence, HDGF was selected as a model system for the electrochemical studies. Further motif-based analysis of this altered phosphorylation site revealed that extracellular-signal-regulated kinase 1/2 (ERK1/2) are most likely to be the corresponding kinases. For validation of the kinase–substrate pair, densely packed peptide monolayers corresponding to the HDGF phosphorylation site were coupled to a gold electrode. Phosphorylation of the monolayer by ERK2 and dephosphorylation by alkaline phosphatase (AP) were detected by electrochemical impedance spectroscopy (EIS) and surface roughness analysis. Compared to other methods for quantification of kinase concentration, this label-free electrochemical assay offers the advantages of ultra-sensitivity as well as higher specificity for the detection of cancer-related kinase–substrate pair. With implementation of multiple kinase–substrate biomarker pairs, we expect this integrated approach to become a high throughput platform for discovery and validation of phosphorylation-mediated biomarkers.
Functionalized nanoparticle networks offer a model system for the study of charge transport in low-dimensional systems as well as a potential platform to implement and test electronic functionalities. The electrical response of a nanoparticle network is expected to sensitively depend on the molecular inter-connects, i.e. on the linker chemistry. If these linkers have complex charge transport properties, then phenomenological models addressing the large scale properties of the network need to be complemented with microscopic calculations of the network building blocks. In this study we focus on the interplay between conformational fluctuations and electronic $π$-stacking in single molecule junctions and use the obtained microscopic information on their electrical transport properties to parametrize transition rates describing charge diffusion in mesoscopic nanoparticle networks. Our results point out at the strong influence of mechanical degrees of freedom on the electronic transport signatures of the studied molecules. This is then reflected in the varying charge diffusion at the network level. The modeling studies are complemented with first charge transport measurements at the single-molecule level of $π$-stacked molecular dimers using state of the art mechanically controllable break junction techniques in a liquid environment.
We report on the fabrication and characterization of porous-silicon/conjugated-polymer hybrids, created by combining a host columnar matrix of mesoporous silicon and a network of organic nanowires made from poly(N-vinylcarbazole) (PVK). A uniform and homogeneous filling of the pores by the polymers was accomplished by electrochemical polymerization of organic monomers inside the pores by using cyclic voltammetry. Spectroscopic measurements showed that polymerization inside the confined environment of the nanometric pores results in a tighter and denser packing of the polymer due to a change of the polymerization process from the vinyl groups to the conjugated carbazole groups, giving rise to a redshift of the absorption spectra and better electrical conductivity. Current-voltage characterization of the hybrids under dark conditions and under illumination were investigated. We demonstrate a simple method to control the band alignment between the organic polymer and the porous silicon, altering it from a type-I to a type-II interface by changing the doping polarity of the silicon substrate (from p-type to n-type, respectively). An efficient photoinduced charge separation was observed for the type-II interface (n-type porous-silicon–polymer interface), while no such effect was observed for the type-I organic–inorganic interface.
There is continuing interest in determining essential structural features of polymer gels, which display photoelectric and/or thermoelectric behavior. One such gel is the blend, poly(4-vinylpyridine-co-butyl methacrylate)/poly(4-vinylpyridine), dissolved in liquid pyridine. Following extended aeration of a three-component mixture, which serves as a model for the gel side chain interactions, crystallization of a new molecule, 4-isopropylpyridine hydroxide (IPPOH), occurs. X-ray diffraction, DFT modeling, and spectroscopy were used to determine the structural, electronic, and luminescent properties of the crystal. The crystal structure reveals molecules forming head-to-tail, hydrogen-bonded chains without base stacking or marked interchain interaction. The molecular chains are characterized by moderately long-lived, blue-violet luminescence excited in the near-UV. Because these photoluminescent properties resemble those of the gel from which the crystals are derived, we may posit similar structural features in the gel for which direct structural analysis is not available.
Sensing from the ultraviolet to the infrared is important for a number of scientific and industrial applications. Poly(4-vinyl pyridine) swollen in liquid pyridine functions as a photoconductive gel sensitive to irradiation in the ultraviolet. By blending poly(4-vinyl pyridine) with poly(4-vinyl pyridine-co-butyl methacrylate), we have now succeeded in expanding the range of wavelength sensitivity of the gel to cover the whole visible spectrum. Furthermore, addition of a small amount of 4-hydroxypyridine to the polymer blend results in unusually high thermal sensitivity (TCR = (0.1–0.16)/1 °C). Spectroscopic measurements show that the combined processes of proton transfer and electron transfer, occurring in a DC electric field, contribute to the gel properties. The optimized system has potential application as a simple and inexpensive active layer in organic photovoltaic cells as well as a thermal sensor.
In this work we show dipole-assisted photogated switching by covalent grafting of photoactive molecules to conducting polymers. Photochromic spiropyran molecules were covalently attached to polyaniline (PANI) nanowires via N-alkylation reaction to the quinoic part of PANI. Upon irradiation with ultraviolet light spiropyran transformed to a large dipole containing molecule, merocyanine form. We show that this transformation leads to a substantial (ca. 2 orders of magnitude) increase in conductance of the photochromic PANI nanowires, which were evident by an increase in field-effect mobility and calculated band gap narrowing of the system. Finally, this transformation was found to be fully reversible with no significant photofatigue.