Highly selective sensing of dopamine using carbon nanotube ink doped with anionic surfactant modified disposable paper electrode

An unconventional, flexible, disposable paper-based selective sensing platform for dopamine in the presence of ascorbic acid, suitable for wearable electronics, has been described for the first time in this work. The carbon nanotube ink-modified paper (CNIMP) in the presence of an anionic surfactant was able to discriminate effectively between dopamine and ascorbic acid thereby alleviating the difficulties associated with the sensing of dopamine in the presence of high concentration of ascorbic acid which undergoes oxidation at similar potential. The CNIMP electrode provided a large surface area in addition to its flexibility and disposability which was 25 times higher compared to a glassy carbon electrode of the same geometric area. The conductivity of the CNIMP electrodes as measured by four probe conductivity measurements was reasonably high of the order of 1.7 × 10^?2 S cm^?1 facilitating its usefulness for the development of flexible sensors. The microscopic features of the electrodes showed the dense coverage and even distribution of carbon nanotubes on the surface with the surfactant molecules uniformly encapsulated on the surface.     

Analysis of stability of nanotube dispersions using surface tension isotherms

In this paper, we present the analyses of surface tension of surfactant-stabilized dispersions of carbon nanotubes. This method allows one to study interactions of carbon nanotubes with surfactants at different levels of nanotube loading when optical methods fall short in quantifying the level of nanotube separation. Sodium dodecyl sulfate was used as a stabilizing agent to uniformly disperse single-walled carbon nanotubes in an aqueous media. We show that surface tension is very sensitive to small changes of nanotube and surfactant concentrations. The experimental data suggest that, at moderate concentrations, surfactant displaces carbon nanotubes from the air-water interface and the nanotubes are mostly moved into the bulk of the liquid. By analyzing the surface tension as a function of surfactant concentration, we obtained the dependence of critical micelle concentration on nanotube loading. We then constructed the adsorption isotherm for dodecyl sulfate on carbon nanotubes and bundles of carbon nanotubes. The results of these experiments enabled us to extend the phase diagram of the produced dispersions to a broader range of surfactant and nanotube concentrations.     

Role of adsorbed surfactant in the reaction of aryl diazonium salts with single-walled carbon nanotubes

Because covalent chemistry can diminish the optical and electronic properties of single-walled carbon nanotubes (SWCNTs), there is significant interest in developing methods of controllably functionalizing the nanotube sidewall. To date, most attempts at obtaining such control have focused on reaction stoichiometry or strength of oxidative treatment. Here, we examine the role of surfactants in the chemical modification of single-walled carbon nanotubes with aryl diazonium salts. The adsorbed surfactant layer is shown to affect the diazonium derivatization of carbon nanotubes in several ways, including electrostatic attraction or repulsion, steric exclusion, and direct chemical modification of the diazonium reactant. Electrostatic effects are most pronounced in the cases of anionic sodium dodecyl sulfate and cationic cetyltrimethylammonium bromide, where differences in surfactant charge can significantly affect the ability of the diazonium ion to access the SWCNT surface. For bile salt surfactants, with the exception of sodium cholate, we find that the surfactant wraps tightly enough such that exclusion effects are dominant. Here, sodium taurocholate exhibits almost no reactivity under the explored reaction conditions, while for sodium deoxycholate and sodium taurodeoxycholate, we show that the greatest extent of reaction is observed among a small population of nanotube species, with diameters between 0.88 and 0.92 nm. The anomalous reaction of nanotubes in this diameter range seems to imply that the surfactant is less effective at coating these species, resulting in a reduced surface coverage on the nanotube. Contrary to the other bile salts studied, sodium cholate enables high selectivity toward metallic species and small band gap semiconductors, which is attributed to surfactant-diazonium coupling to form highly reactive diazoesters. Further, it is found that the rigidity of anionic surfactants can significantly influence the ability of the surfactant layer to stabilize the diazonium ion near the nanotube surface. Such Coulombic and surfactant packing effects offer promise toward employing surfactants to controllably functionalize carbon nanotubes.     

Structure and properties of aqueous dispersions of sodium dodecyl sulfate with carbon nanotubes

The dispersing action of the surfactant (sodium dodecyl sulfate, SDS) on the carbon nanotubes (CNT) in aqueous medium has been studied. Electron microscopy, molecular docking, NMR and IR spectroscopies were applied to determine the physical-chemical properties of CNT dispersions in SDS—water solutions. It was established that micellar adsorption of the surfactant on the surface of carbon material and solubilization of SDS in aqueous medium contribute to improving CNT dispersing in water solutions. It was shown that the non-polar hydrocarbon radicals of a single surfactant molecule form the highest possible number of contacts with the graphene surface. Upon increase of the SDS in solution these radicals form micelles connected with the surface of the nanotubes. At the sufficiently high SDS concentration the nanotube surface becomes covered with an adsorbed layer of surfactant micelles. Water molecules and sodium cations are concentrated in spaces between micelles. The observed pattern of micellar adsorption is somewhat similar to a loose bilayer of surfactant molecules.     

Influence of size and functionality of polymeric nanoparticles on the adsorption behavior of sodium dodecyl sulfate as detected by isothermal titration calorimetry

Isothermal titration calorimetry was used to monitor the adsorption of the surfactant sodium dodecylsulfate (SDS) on different sized pure and carboxy functionalized polystyrene nanoparticles prepared by the mini-emulsion process. The ITC experiment gives, additionally to the CMC values, information about the interaction of the surfactant molecules to the particle’s surface due to the particle surface properties. The adsorption heat depends on the chemical composition of the polymer as well on the particle size. It also provides information about the surface coverage with surfactant and the number of additional adsorbed molecules per particle until full coverage by surfactant is obtained. The surfactant adsorption increases from 0.3 molecules per nm² for 50 nm to 8.5 molecules per nm² for carboxy functionalized particles with diameters larger than 160 nm. The area A _Surf-dens after the adsorption process gives information about the packing density of surfactant molecules on the particles in dependence of carboxy groups: an increasing number of carboxylic groups decreases the area occupied per SDS molecule. The adsorption process was also monitored by zeta potential measurements, where an increasing potential during the adsorption was detected.     

碳纳米管/ZnO纳米复合体的制备和表征

通过将不同直径的ZnO纳米颗粒与碳纳米管连接制备了碳纳米管/ZnO纳米复合体. 将团聚的ZnO纳米颗粒分散并用表面活性剂CTAB使纳米颗粒带正电. 化学氧化碳纳米管使其带负电. ZnO/CTAB微团通过碳管表面羧基与CTAB的静电作用与碳纳米管连接形成纳米复合体. 研究了复合体形成的不同实验条件, 表征了碳纳米管/ZnO纳米复合体的结构并研究了纳米复合体的光学特性. 研究表明, 与碳纳米管连接的ZnO纳米颗粒是互不连接的并保持量子点的特性. 光致发光研究表明ZnO纳米颗粒的激发在纳米复合体中有淬灭.     

Dynamics of surfactant-suspended single-walled carbon nanotubes in a centrifugal field

A hydrodynamic model is used to describe the motion of surfactant-suspended single-walled carbon nanotubes in a density gradient, while being subjected to a centrifugal field. The number of surfactant molecules adsorbed on each nanotube determines its effective density and, hence, its position in the gradient after centrifugation has been completed. Analysis of the spatial concentration distributions of CoMoCAT nanotubes suspended with 2 w/v% sodium cholate yielded 2.09, 2.14, and 2.08 surfactant molecules adsorbed per nanometer along the length of the (6,5), (7,5), and (8,7) nanotubes, respectively. The estimates are commensurate with experimental values reported in the literature and can be used to predict the fate of sodium cholate-suspended nanotubes in the separation process. Since the density of the surfactant-nanotube assembly is highly sensitive to the number of adsorbed molecules, a perturbation would cause it to be enriched at a different location in the gradient. The level of sensitivity is also reflected in the 95% confidence levels that are reported in this work.     

Directed self-assembly of surfactants in carbon nanotube materials

The self-assembly of surfactant molecules on crossing carbon nanotubes has been investigated using a bead-spring model and implicit solvent dissipative particle dynamics simulations. Adsorption is directed to the nanotube crossing by its higher hydrophobic potential which is due to the presence of two surfaces. As a consequence of the tendency of surfactant molecules to self-assemble into micelles, the adsorbed molecules form a "central aggregate" at the crossing, thus, confining the molecules to the immediate vicinity of the crossing. Adsorption on the remaining nanotube surface becomes significant only at higher surfactant concentrations, where the molecules self-assemble to hemimicelles which grow continuously to full micelles upon increase of the (bulk) surfactant concentration. Our results allow two conclusions for the rational design of nanostructured materials: (i) the size of the central aggregate can not be much larger than that of a bulk micelle and (ii) control of the adsorbed structures is conveniently possible via the (bulk) surfactant concentration.     

Dispersing carbon nanotubes using surfactants

We review the recent advances in dispersing single-wall carbon nanotubes (SWNTs) using amphiphilic surfactants in aqueous solutions. Three aspects are discussed. (1) On the organization of surfactant molecules with SWNTs, new insights at the microscopic level arise from electron microscopy and detailed computer simulation studies. (2) Quantitative measurements, such as molecular interactions between functional groups and SWNTs, the coverage of surfactant on SWNTs in solution, the charge state of the SWNT/surfactant complex, and the degree of dispersion are critical for better understanding dispersion mechanisms and for the further development of dispersion strategies. (3) The thermodynamic driving forces and the role of metastability in the structure of surfactant dispersed SWNT suspensions are analyzed. An outlook on practical and fundamental issues is also presented.     

Surfactant-directed polypyrrole/CNT nanocables: synthesis, characterization, and enhanced electrical properties.

We describe here a new approach to the synthesis of size-controllable polypyrrole/carbon nanotube (CNT) nanocables by in situ chemical oxidative polymerization directed by the cationic surfactant cetyltrimethylammonium bromide (CTAB) or the nonionic surfactant polyethylene glycol mono-p-nonylphenyl ether (Opi-10). When carbon nanotubes are dispersed in a solution containing a certain concentration of CTAB or Opi-10, the surfactant molecules are adsorbed and arranged regularly on the CNT surfaces. On addition of pyrrole, some of the monomer is adsorbed at the surface of CNTs and/or wedged between the arranged CTAB or Opi-10 molecules. When ammonium persulfate (APS) is added, pyrrole is polymerized in situ at the surfaces of the CNTs (core layer) and ultimately forms the outer shell of the nanocables. Such polypyrrole/CNT nanocables show enhanced electrical properties; a negative temperature coefficient of resistance at 77-300 K and a negative magnetoresistance at 10-200 K were observed.     

Photochemical approach toward deposition of gold nanoparticles on functionalized carbon nanotubes

The development of new methods for the facile synthesis of hybrid nanomaterials is of great importance due to their importance in nanotechnology. In this work, we report a new method to deposit Au nanoparticles on the surface of single-walled carbon nanotubes (SWCNTs). Our approach consists of a one pot synthesis in which Au nanoparticles are generated in the presence of a photoreducing agent (Irgacure-2959) and carboxyl or polymer-functionalized SWCNTs (f-SWCNTs). We have observed that when carbon nanotubes are functionalized with polymers containing pendant amino groups, the latter can act as specific nucleation sites for well-dispersed deposition of Au nanoparticles. The surface coverage of the Au nanoparticles can be observed by transmission electron spectroscopy. These observations are compared to that of carboxyl functionalized SWCNTs, in which less surface coverage was observed. The f-SWCNT/Au nanocomposites were also characterized by UV-vis, infrared, and Raman spectroscopy and thermogravimetric analysis (TGA). This facile and effective route can be implemented to deposit gold nanoparticles on other surface-functionalized carbon nanotubes.     

The crystalline structures of carboxylic acid monolayers adsorbed on graphite

X-ray and neutron diffraction have been used to investigate the formation of solid crystalline monolayers of all of the linear carboxylic acids from C(6) to C(14) at submonolayer coverage and from C(8) to C(14) at multilayer coverages, and to characterize their structures. X-rays and neutrons highlight different aspects of the monolayer structures, and their combination is therefore important in structural determination. For all of the acids with an odd number of carbon atoms, the unit cell is rectangular of plane group pgg containing four molecules. The members of the homologous series with an even number of carbon atoms have an oblique unit cell with two molecules per unit cell and plane group p2. This odd-even variation in crystal structure provides an explanation for the odd-even variation observed in monolayer melting points and mixing behavior. In all cases, the molecules are arranged in strongly hydrogen-bonded dimers with their extended axes parallel to the surface and the plane of the carbon skeleton essentially parallel to the graphite surface. The monolayer crystal structures have unit cell dimensions similar to certain close-packed planes of the bulk crystals, but the molecular arrangements are different. There is a 1-3% compression on increasing the coverage over a monolayer.     

The role of the medium in electrochemical functionalization and dispersion of carbon nanotubes

An electrochemical method for dispersion of single-walled carbon nanotubes (SWNTs) is described. The technique is based on grafting of oxygen-containing functional groups to the nanotube surface during electrolysis in aqueous and nonaqueous potassium bromide solutions. A dependence of the degree of functionalization of nanotubes on the solvent was revealed experimentally. Nanotubes treated in DMSO have about 14 carbon atoms per oxygen atom from functional groups (cf. nearly four C atoms per oxygen atom in the nanotubes treated in aqueous solutions). The corresponding maximum specific capacities of the electrodes are nearly 10 and 60 F g⁻¹. The samples treated in solutions of KBr in DMSO have about 300 carbon atoms per bromine atom on the nanotube surface (cf. only 30 carbon atoms in the samples treated in aqueous solution). A mechanism of electrochemical modification of SWNTs is proposed. Its key step is production of atomic oxygen that oxidizes the nanotube surface with the formation of functional groups.     

DNA‐Directed Attachment of Carbon Nanotubes for Enhanced Label‐Free Electrochemical Detection of DNA Hybridization

Multi‐walled carbon nanotubes (MWNTs) were used as nanowires, which combined DNA molecules to a carbon paste electrode (CPE). The attachment of MWNT on the electrode surface was controlled by a hybridization assay between adenine and thymine containing oligonucleotides. The appearance of guanine oxidation signal after hybridization with target DNA greatly simplified the specific sequence DNA detection mechanism. Combination of sidewall‐ and end‐functionalization of MWNT provided a significant enhancement in the voltammetric signal of guanine oxidation in comparison with the signals obtained from only end‐oxidized MWNT modified CPE and a bare CPE. A control experiment involving adenine containing polynucleotide (poly(A)) instead of adenine probe modified MWNT was performed. The effect of target and noncomplementary DNA concentration on the guanine signal was also monitored. Discrimination against single‐base mismatch and noncomplementary DNA was achieved by surfactant containing washing solution. The promising conductivity of carbon nanotubes, and the creation of a larger surface area for DNA immobilization by sidewall‐ and end‐oxidation of MWNT provided a detection limit down to 10 pg/mL, which is compatible with the demand of the genetic tests.     

Structure of DNA-cationic surfactant complexes at hydrophobically modified and hydrophilic silica surfaces as revealed by neutron reflectometry

In this article, we discuss the structure and composition of mixed DNA-cationic surfactant adsorption layers on both hydrophobic and hydrophilic solid surfaces. We have focused on the effects of the bulk concentrations, the surfactant chain length, and the type of solid surface on the interfacial layer structure (the location, coverage, and conformation of the DNA and surfactant molecules). Neutron reflectometry is the technique of choice for revealing the surface layer structure by means of selective deuteration. We start by studying the interfacial complexation of DNA with dodecyltrimethylammonium bromide (DTAB) and hexadecyltrimethylammonium bromide (CTAB) on hydrophobic surfaces, where we show that DNA molecules are located on top of a self-assembled surfactant monolayer, with the thickness of the DNA layer and the surfactant-DNA ratio determined by the surface coverage of the underlying cationic layer. The surface coverages of surfactant and DNA are determined by the bulk concentration of the surfactant relative to its critical micelle concentration (cmc). The structure of the interfacial layer is not affected by the choice of cationic surfactant studied. However, to obtain similar interfacial structures, a higher concentration in relation to its cmc is required for the more soluble DTAB surfactant with a shorter alkyl chain than for CTAB. Our results suggest that the DNA molecules will spontaneously form a relatively dense, thin layer on top of a surfactant monolayer (hydrophobic surface) or a layer of admicelles (hydrophilic surface) as long as the surface concentration of surfactant is great enough to ensure a high interfacial charge density. These findings have implications for bioanalytical and nanotechnology applications, which require the deposition of DNA layers with well-controlled structure and composition.     

Lipid bilayers covalently anchored to carbon nanotubes

The unique physical and electrical properties of carbon nanotubes make them an exciting material for applications in various fields such as bioelectronics and biosensing. Due to the poor water solubility of carbon nanotubes, functionalization for such applications has been a challenge. Of particular need are functionalization methods for integrating carbon nanotubes with biomolecules and constructing novel hybrid nanostructures for bionanoelectronic applications. We present a novel method for the fabrication of dispersible, biocompatible carbon nanotube-based materials. Multiwalled carbon nanotubes (MWCNTs) are covalently modified with primary amine-bearing phospholipids in a carbodiimide-activated reaction. These modified carbon nanotubes have good dispersibility in nonpolar solvents. Fourier transform infrared (FTIR) spectroscopy shows peaks attributable to the formation of amide bonds between lipids and the nanotube surface. Simple sonication of lipid-modified nanotubes with other lipid molecules leads to the formation of a uniform lipid bilayer coating the nanotubes. These bilayer-coated nanotubes are highly dispersible and stable in aqueous solution. Confocal fluorescence microscopy shows labeled lipids on the surface of bilayer-modified nanotubes. Transmission electron microscopy (TEM) shows the morphology of dispersed bilayer-coated MWCNTs. Fluorescence quenching of lipid-coated MWCNTs confirms the bilayer configuration of the lipids on the nanotube surface, and fluorescence anisotropy measurements show that the bilayer is fluid above the gel-to-liquid transition temperature. The membrane protein α-hemolysin spontaneously inserts into the MWCNT-supported bilayer, confirming the biomimetic membrane structure. These biomimetic nanostructures are a promising platform for the integration of carbon nanotube-based materials with biomolecules.     

Direct electrochemistry of heme proteins: effect of electrode surface modification by neutral surfactants

Direct electrochemical studies on horse heart myoglobin and horseradish peroxidase (HRP) have been carried out using tin-doped indium oxide (ITO) and surfactant modified glassy carbon working electrodes. These proteins show very slow electron transfer kinetics at metal or untreated electrodes. Moreover, small amounts of surface-active impurity were drastically affects the electrode reaction of these proteins. The results showed that modification of the electrode surface with neutral surfactants significantly improves the electrochemical response of myoglobin as well as of HRP. The electrode response was found to depend on the structure of the surfactants. The amount of surfactant required per unit area of the electrode surface to promote the maximum electron transfer rate constants was found to be constant. This indicated that the surfactant molecules interacted with the electrode surface in a specific manner and anchored the protein molecules to align in the suitable orientation. The hydrophobicity of the surfactants rather than their charge was found to be crucial in promoting the electrode response of these proteins at the glassy carbon electrode.     

Directing and sensing changes in molecular conformation on individual carbon nanotube field effect transistors

This study explores how to populate the surface of the carbon nanotubes with functional molecules that can be toggled back-and-forth between different molecular conformations. The molecules synthesized for this study are tagged with a photoswitchable headgroup and a functional group which directs the assembly on the surface of the carbon nanotubes. Single-walled carbon nanotube field effect transistors switch between high and low conductance as the molecules are switched with light between open and closed conformations. These devices detect the photoswitching of approximately 104 molecules.     

Uniform directional alignment of single-walled carbon nanotubes in viscous polymer flow

In this work, we probed the effects of shear flow on the alignment of dispersed single-walled carbon nanotubes in polymer solutions. Two different systems were compared: Single-walled carbon nanotubes dispersed using an anionic surfactant and single-walled carbon nanotubes dispersed using an anionic surfactant and a weakly binding polymer. It was determined that the addition of the weakly binding polymer increased the degree of dispersion of the carbon nanotubes and the ability to induce their alignment when subjected to shear forces.     

Electrochemical surface plasmon resonance investigation of dodecyl sulfate adsorption to electroactive self-assembled monolayers via ion-pairing interactions

The redox-induced assembly of amphiphilic molecules and macromolecules at electrode surfaces is a potentially attractive means of electrochemically modulating the organization of materials and nanostructures on solid substrates via ion-pairing interactions or charge-transfer complexation. In this regard, we have investigated the potential-induced adsorption and aggregation of dodecyl sulfate, a common anionic surfactant, at a ferrocenylundecanethiolate (FcC11SAu) self-assembled monolayer (SAM)/aqueous solution interface by electrochemical surface plasmon resonance (ESPR) spectroscopy. The surfactant anions adsorb onto the electroactive SAM by specific ion-pairing interactions with the oxidized ferricinium species. The ferricinium charge density (QFc+) obtained by cyclic voltammetry and surface coverage measured by SPR indicate that the dodecyl sulfate forms an interdigitated monolayer, where half of the surfactant molecules have their sulfate headgroups paired to the surface and half have their headgroups exposed to the aqueous solution. The surface coverage of dodecyl sulfate was found to depend on both the ferricinium surface concentration and the surfactant aggregation state in solution. A maximum coverage of dodecyl sulfate on the ferricinium surface is obtained below the critical micelle concentration (cmc), in contrast to dodecyl sulfate adsorption to SAM surfaces of static positive charge. This marked difference in adsorption behavior is attributed to the dynamic generation of ferricinium by potential cycling and the specific nature of the ion-pairing interactions versus pure electrostatic ones. The results presented point to a new way of organizing molecules via electrical stimulus.